Production of an isocyanate by way of a gas phase reaction in several gas phase reaction arrangements operated in parallel

Abstract

The present invention relates to the production of an isocyanate by reacting a primary amine with phosgene by way of a gas phase reaction in several gas phase reaction arrangements connected in parallel. Excess phosgene not reacted in the gas phase reaction is returned, as a return flow, to a phosgene gas generation device to form a phosgene gas stream and a state parameter of the formed phosgene gas steam is set by means of a setting parameter of the return flow. This concept enables stable reaction conditions of the gas phase reaction, in particular a stable stoichiometric phosgene excess in the gas phase reaction arrangements, to be ensured.

Description

[0001] Production of an isocyanate by means of a gas phase reaction in several gas phase reaction arrangements operated in parallel

[0002] The present invention relates to a process for producing an isocyanate by reacting a primary amine with phosgene in a gas-phase reaction. The present invention also relates to a plant for carrying out said process, a computer system for controlling the stoichiometric phosgene excess, a computer program product, and a computer-implemented process for adjusting the stoichiometric phosgene excess. Specifically, the present invention relates to the production of an isocyanate in several gas-phase reaction arrangements connected in parallel. In this process, excess phosgene not converted in the gas-phase reaction is recycled as a recycle stream into a phosgene gas generation device to form a phosgene gas stream, and a state parameter of the resulting phosgene gas stream is adjusted via an adjustment parameter for the recycle stream.With this concept, stable reaction conditions of the gas phase reaction, in particular a stable stoichiometric phosgene excess in the gas phase reaction arrangements, can be ensured.

[0003] Isocyanates, especially diisocyanates such as toluene diisocyanate, are produced on an industrial scale and are primarily used as starting materials for the production of flexible polyurethane foams, particularly for the production of upholstery and mattresses, but also as insulation or protective materials. One method for producing isocyanates that has long been described in the prior art is the reaction of primary amines with phosgene in a gas-phase reaction. This process is characterized by the fact that, under the selected reaction conditions, at least the reaction components, primary amine and phosgene, are gaseous.Production by means of a gas-phase reaction has numerous advantages over conventional liquid-phase production processes, such as increased reaction yields, simpler processing to the desired isocyanate, lower investment costs for plants using this technology, as well as energy and raw material savings and lower emissions of carbon dioxide, nitrogen oxides and other gases that are environmentally harmful.

[0004] However, the gas-phase reaction also presents manufacturers with particular challenges. The isocyanates produced are thermally unstable at the reaction temperatures of up to 500 °C required for the gas-phase reaction and form undesirable byproducts such as isocyanurates, biurets, allophanates, carbodiimides, or ureas, both through thermal decomposition and through further reactions. As a result of byproduct formation, solids accumulate at various locations in the production plant during ongoing production, e.g., on the walls of gas-phase reactors or in mixing nozzles for injecting liquids or gases, and increasingly block these. Therefore, it is of great interest to ensure smooth production processes by reducing the formation of such byproducts as much as possible.For this purpose, it is currently preferred in the prior art, for example, to cool the gaseous reaction product mixture formed in the gas-phase reaction to temperatures below 150 °C by contacting it with quench liquids that are inert under the reaction conditions, with as complete a condensation of the isocyanate formed as possible, in order to prevent decomposition and subsequent reactions. For example, WO 2018 / 224529 A1 relates to a process for preparing isocyanates by phosgenating primary amines in the gas phase, wherein a crude gaseous reaction product mixture obtained from the gas-phase reaction is cooled and partially condensed by contacting it with at least one stream of a quench liquid. WO 2018 / 224529 A1 describes the use of a stoichiometric excess of phosgene, based on primary amine groups, which is advantageous for the gas-phase reaction.It is also mentioned that it is possible to free a gaseous stream obtained after scrubbing from excess phosgene used in excess of stoichiometric amounts, e.g. by means of a cold trap or by absorption in an inert solvent.

[0005] WO 2022 / 106716 A1 describes a process for producing isocyanates and / or polyisocyanates by reacting the corresponding amines with phosgene, comprising the steps of a) feeding a first reactant stream containing the amine in gaseous or liquid form into a reactor, b) feeding a second reactant stream containing the phosgene in gaseous form into the reactor, c) mixing and reacting the first and second reactant streams in the reactor to obtain a reaction mixture, d) cooling the reaction mixture by indirect heat transfer or by quenching with cooled reaction product to obtain a liquid reaction product, e) separating the cooled reaction mixture into a liquid reaction product containing isocyanate and a gaseous reaction product, f) purifying the liquid reaction product to obtain liquid isocyanate,and h) separating the gaseous reaction product into a first gaseous product stream containing essentially HCl and a second liquid or gaseous product stream containing essentially phosgene, wherein no liquid solvent is used in steps a) to f) and h).

[0006] WO 2017 / 055311 A1 describes a continuously operated process for the production of isocyanates by reacting the corresponding amines with phosgene, in which, when the desired production capacity is changed, starting from an initial state with a certain production capacity and ending in a final state with a different production capacity, in the transition period between the initial and final state

[0007] - the instantaneous phosgene excess at the time of the transition period at which the change in the amine mass flow is started is at least as large as, and preferably greater than, the phosgene excess during the period of production with the production capacity in the initial state before the start of the transition period, and that

[0008] - the average phosgene surplus during the transition period is greater than the phosgene surplus during the period of production with the production capacity in the initial state before the start of the transition period.

[0009] WO 2011 / 003532 A1 describes a process for producing isocyanates by reacting primary amines with phosgene in stoichiometric excess in the gas phase, in which a) the amine is reacted with phosgene in a reactor above the boiling point of the amine, whereby a liquid stream containing the isocyanate and a gas stream containing HCl and phosgene are obtained, and b) the gas stream containing HCl and phosgene obtained in step a) is first separated into a gas stream containing HCl and a liquid stream containing phosgene, and c) the phosgene-containing liquid stream obtained in step b) is then at least partially converted into a gaseous stream containing phosgene, and d) the gaseous stream containing phosgene obtained in step c) is recycled to the reaction in step a),wherein e) the pressure of the gaseous stream containing phosgene obtained in step c) is higher than the pressure of the liquid stream containing phosgene obtained in step b).

[0010] Despite known possibilities for reducing the formation of by-products, the processes known in the prior art still have the disadvantage that decomposition and by-products in the form of residues, especially solid ones, can never be completely avoided, especially not their settling on the reactor walls of commonly used gas-phase reactors. This so-called "caking" can suddenly break off from the reactor walls, thereby creating a drastic pressure drop in the gas-phase reactor. Amine gas generation devices used to generate the amine reaction gases used in the gas-phase reaction, in particular, react very sensitively to such a pressure drop, resulting in a very brief, violent discharge of amine reaction gas into the gas-phase reactor.If several gas-phase reactors are operated in parallel and supplied with gaseous phosgene from a common phosgene gas generation plant via a distribution system, a pressure drop in one of the gas-phase reactors will also affect the other gas-phase reactors via the distribution system. This creates unstable operating and reaction conditions, which in turn promote the formation of by-products and further destabilize the production process. For example, a previously precisely adjusted, very short residence time of the gaseous reactants in a reaction zone of the gas-phase reactor (the gas-phase reaction is very fast and exothermic) before a reaction is terminated by cooling in a quench zone can be compromised. Both excessively short and excessively long residence times can lead to a greatly increased formation of by-products. The desired stoichiometric excess of phosgene can also be compromised.Maintaining a previously determined stoichiometric excess of phosgene as precisely as possible, i.e. ensuring a defined, previously determined molar ratio of phosgene to primary amino groups, whereby this molar ratio is greater than 1 (i.e., phosgene is used in excess of stoichiometric amounts relative to primary amino groups), has proven to be of great importance for a trouble-free reaction.

[0011] The object of the invention is therefore to develop a process and a plant for carrying out the process for producing an isocyanate by reacting a primary amine with phosgene by means of a gas-phase reaction, with which the described disadvantages of the processes and plants used in the prior art are avoided or at least reduced. Furthermore, a suitable computer-implemented control system should be provided. Thus, the process proposed according to the invention and the plant for carrying out the process should be able to be carried out with greater operational stability compared to processes and plants known from the prior art, in particular with regard to maintaining a desired stoichiometric phosgene excess as consistently as possible in all gas-phase reactors.In particular, the pressure drops caused by breaking off solid residues of by-products and subsequent products deposited on the walls of gas phase reactors should be compensated in order to ensure more stable reaction conditions of the gas phase reaction compared to known processes and plants, in particular with regard to a constant stoichiometric phosgene excess in all gas phase reactors.

[0012] To solve this problem, the invention provides the following:

[0013] In a first aspect, the present invention relates to a process for producing an isocyanate by reacting a primary amine with phosgene by means of a gas-phase reaction in a plant comprising m gas-phase reaction arrangements (100-1, 100-2, ..., 100-m), where m is a natural number in the range from 2 to 6. Said process comprises at least the following steps:

[0014] (i) generating m amine reaction gas streams (Al, A-2, ..., Am), each comprising a primary amine; (ii) generating a phosgene gas stream (P) comprising phosgene at a pressure (pP) in a phosgene gas generating device (104) (= device for generating gaseous phosgene from liquid phosgene and phosgene solutions) and dividing the phosgene gas stream (P) comprising phosgene into m phosgene reaction gas streams (Pl, P-2, ... Pm);

[0015] (iii) Combining one of the amine reaction gas streams (Al, A-2, ..., Am) and one of the m phosgene-comprising phosgene reaction gas streams (Pl, P-2, ... Pm) in each case in one of the gas phase reaction arrangements (100-1, 100-2, ..., 100-m), wherein (at least) the m phosgene reaction gas streams (Pl, P-2, ... Pm) pass through one of m phosgene flow control valves (PV-1, PV-2, ..., PV-m) before entering the m gas phase reaction arrangements (100-1, 100-2, ..., 100-m) in each case to adjust the stoichiometric phosgene excess, and carrying out the gas phase reaction to obtain m isocyanate-comprising isocyanate crude product fluid streams (IC-1, IC-2, ..., IC-m) and m Residual fluid streams (Rl, R-2, ..., Rm), wherein the m residual fluid streams (Rl, R-2, ..., Rm) each comprise excess phosgene not converted in the gas phase reaction;

[0016] (iv) returning the unreacted excess phosgene to the phosgene gas generating device (104), namely by

[0017] (iv-1) optionally, combining the m residual fluid streams (Rl, R-2, ..., Rm) into n residual fluid streams (Rl, R-2, ... Rn), where n is a natural number in the range from 1 to m-1 (and in particular 1),

[0018] (iv-2) introducing each of the residual fluid streams into a respective phosgene absorption device (103-1, 103-2, ... 103-m or 103-n), each having a phosgene absorption device collection region (115-1, 115-2, ... 115-m or 115-n), wherein an absorbent fluid stream (AbsM-1, AbsM-2, ..., AbsM-m or AbsM-n), each containing an absorbent (AbsM), is also introduced into each of the phosgene absorption devices (103-1, 103-2, ... 103-m or 103-n), with absorption of the excess phosgene not converted in the gas phase reaction by the absorbent (ie by the uptake of the phosgene [= absorbate] into the free volume of the absorbent [= absorbent]), wherein in each phosgene absorption device collection area (115-1, 115-2, ... 115-m or 115-n) there is an absorbed phosgene and the recycle medium comprising the absorbent (Return-1, Return-2, Return-m orRecycle-n) collects, from which, if necessary after combining several recycle media (Recycle-1, Recycle-2, ..., Recycle-m or Recycle-n), a (single) recycleable recycle stream (Recycle) comprising phosgene is fed, and.

[0019] (iv-3) introducing the recycle stream (Return) at a flow rate (f-Return) into the phosgene gas generation device (104) with desorption of the phosgene to be recycled from the absorption medium (i.e. with transition of phosgene dissolved in the absorption medium into the gas phase) to obtain desorbed (= transferred into the gas phase) phosgene and a liquid desorption medium (DesM) (= the liquid phase largely freed from phosgene, the chemical composition of which is essentially the same as that of the absorption medium (AbsM) used in (iv-2)), wherein the phosgene gas stream (P) generated in the phosgene gas generation device (104) comprises the desorbed phosgene and a phosgene fresh fluid stream (Fresh) additionally introduced into the phosgene gas generation device (104) at a flow rate (f-Fresh), and wherein to ensure the stoichiometric phosgene excess in each of the m Gas-phase reaction arrangements (100-1, 100-2, ...), 100-m)) a state parameter (X) of the phosgene gas stream (P) is set (= regulated) via a setting parameter (Y), wherein (at least) the pressure (pP) serves as the state parameter (X) and wherein the setting parameter (Y) comprises at least one setting parameter (Yl) of the recirculation stream (Rück), namely in such a way that when a change is detected in (at least) a pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m), the flow rate (f-Rück) of the recirculation stream (Rück) is used as the setting parameter (Yl).In a second aspect, the present invention relates to a computer system for controlling the stoichiometric phosgene excess in the process according to the invention for producing an isocyanate, comprising: an interface unit configured to read in (periodically or continuously) data suitable for detecting a change in (at least) one pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m), and a processor configured to, upon a detected change in the pressure (pP-1, pP-2, ... pPm), use the read-in data to determine an adjusted setpoint (Y)SOLL for the setting parameter (Y) (ieat least for (Yl), if necessary also for (Y-2)) and to transmit the adjusted setpoint (Y)SOLL to an actuator in communicative connection with the processor for manipulating the setting parameter (Y) in order to counteract the detected change in pressure (pP-1, pP-2, ... pPm) (and thus the change in the stoichiometric phosgene excess).

[0020] In a third aspect, the present invention relates to a plant for producing an isocyanate by reacting a primary amine with phosgene in a stoichiometric excess of phosgene by means of a gas phase reaction, wherein the plant is designed to carry out the process according to the invention and further comprises the computer system according to the invention.

[0021] In a fourth aspect, the present invention relates to a computer program product which, when loaded into a memory unit of a processing unit of the computer system according to the invention and executed by the processor, detects, in the method according to the invention, changes in (at least) one pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m) (periodically or continuously) and, upon a detected change in the pressure (pP-1, pP-2, ... pPm), adapts the setpoint (Y)SOLL of the setting parameter (Y) (i.e. at least of the setting parameter (Yl), optionally also of the setting parameter (Y-2)) (i.e. calculates a new setpoint (Y)SOLL) in order to correspond to the change in the pressure (pP-1, pP-2, ... pPm) (and thus the change in the stoichiometric excess phosgene).

[0022] The attached drawings show:

[0023] FIG. 1 shows an overview (without control equipment) of a possible design of the plant for producing an isocyanate using the example of m = n = 2;

[0024] FIG. 2 shows a more detailed representation of a production line;

[0025] FIG. 3 shows an overview of a possible design of the plant for producing an isocyanate with m = 6 and n = 2; and

[0026] FIG. 4 shows a possible design of the plant for producing an isocyanate using the example m = n = 2, including control equipment.

[0027] The fluid flows shown in the drawings are primarily conveyed via pipes. Arrows in the drawings indicate the primary flow direction of the fluid flows. The symbols PP and WT generally represent one or more intermediate pumps PP for conveying fluids, or intermediate heat exchangers WT, through which the fluid flows shown are conveyed.

[0028] In the context of the present invention, a "gas phase reaction arrangement (100-1, 100-2, ... 100-m)" is understood to mean an arrangement which is designed to combine a phosgene reaction gas stream comprising phosgene and an amine reaction gas stream comprising primary amine, with a gas phase reaction of a primary amine with phosgene taking place.

[0029] In the context of the present invention, a "phosgene gas generation device (104)" is understood to mean a technical device for providing the phosgene gas stream (P) which is fed to the gas-phase phosgenation. In the phosgene gas generation device (104), the phosgene gas stream (P) is generated from the fresh phosgene fluid stream (Fresh) and the recycle stream (Recycled) comprising the phosgene to be recycled. This is to be distinguished from a fresh phosgene generation device (109 in FIG. 1), in which phosgene is produced (chemically synthesized).

[0030] In the context of the present invention, a "phosgene absorption device (103-1, 103-2, ... 103-m or 103-n)" is understood to mean a technical device for absorbing the excess phosgene not converted in the gas-phase reaction by an absorbent (i.e., by absorbing the phosgene [= absorbate] into the free volume of the absorbent [= absorbent]). The "phosgene absorption device (103-1, 103-2, ... 103-m or 103-n)" has a "phosgene absorption device collection region (115-1, 115-2, ... 115-m or 115-n)". This is a region in which the absorbent collects with the phosgene absorbed therein; This composition of absorbent and phosgene absorbed therein (= phosgene solution in the absorbent) represents the "recycle medium (Recycle 1, Recycle 2, ..., Recycle m or Recycle n)".A simple example of a phosgene absorption device is an absorption column in which a liquid absorbent, for example ortho-7?o-dichlorobenzene, and gaseous phosgene are brought into contact (preferably countercurrently), whereby the recycle medium (recycle) (solution of phosgene in ortho-7?o-dichlorobenzene) is formed and collected in the phosgene absorption device collection region (which in the case of an absorption column is the column bottom).

[0031] In the terminology of the present invention, a "state parameter (X) of the phosgene gas stream (P)" refers to a physical and / or chemical parameter characteristic of the phosgene gas stream (P), i.e., a parameter that describes the state of the phosgene gas stream in physical and / or chemical terms. A state parameter can therefore be a physical or chemical parameter, for example, a density, temperature, flow rate, concentration, volume, or pressure of a system under consideration. Within the scope of the present invention, at least the pressure (pP) of the phosgene gas stream (P) is considered to be such a state parameter.

[0032] In the terminology of the present invention, a "setting parameter (Y)" refers to a parameter that influences the previously defined state parameter (X), i.e., by means of which the state parameter (X) can be set to a desired value. In principle, any parameter that can be used to adjust the physical state of a system under consideration can be regarded as a setting parameter. Within the scope of the present invention, at least the return flow (Return) is used to adjust the state parameter (X), specifically in the form of its flow rate (f-Return) as the setting parameter (Yl).

[0033] According to the invention, the "pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m)" is examined for changes. Such a change can be directly detected, for example, by continuous or periodic pressure measurement. The principle of pressure measurement using known pressure measuring devices is generally known to those skilled in the art. A phosgene pressure measuring device can, for example, be an electronic, mechatronic, or mechanical pressure gauge or a diaphragm seal. An electronic pressure gauge can be based on a pressure sensor that converts the measured pressure into an electronic signal. The advantage of electronic pressure gauges lies in their highly dynamic behavior under low material stress and the correspondingly high load resistance and long-term stability.A mechanical pressure gauge can incorporate the integration of electronic parts or components at the measuring point. The measured pressure can be displayed on site, but the mechanical pressure gauge can also emit an electrical signal or incorporate an electrical switching function. With mechanical pressure gauges, the measured value can advantageously be reliably read on site even if the power supply fails or the measuring signal is disrupted. By combining mechanical measuring devices with different signals and switches, a wide range of mechatronic pressure gauges can be created. A mechanical pressure gauge can incorporate elastic measuring elements that deform under the influence of pressure. The measuring system of the mechanical pressure gauge can comprise a capsule, diaphragm, or Bourdon tube. Mechanical pressure gauges can be advantageous due to their robustness and ease of use.Alternatively, the phosgene pressure gauge can be a diaphragm seal. A diaphragm seal can be a pressure gauge that enables pressure measurement under difficult measuring conditions. Difficult measuring conditions can include, for example, corrosive, highly viscous, or fibrous media, very high temperatures, poorly located measuring points, hygiene regulations, or even toxic or environmentally harmful measuring media. Selecting a suitable measuring device is a routine task for the specialist and can be done, for example, with the assistance of technical regulations.

[0034] It is also possible to indirectly detect a change in pressure (pPl, pP-2, ... pPm) by observing the effects a pressure change has on other devices. In particular, the position of the phosgene flow control valves (PV-1, PV-2, ..., PV-m) is a suitable indicator of the pressure (pPl, pP-2, ... pPm). For example, if a phosgene flow control valve continues to open or needs to be opened, this is a sure sign of a pressure increase in the downstream gas-phase reaction assembly (the "backpressure" in the gas-phase reaction assembly increases, which must be compensated by a greater degree of opening of the phosgene flow control valve). If a change is detected, "the flow rate (f-Return) of the recycle stream (Return) is used as the adjustment parameter (Yl)."In the described example of a pressure increase, the flow rate (f-back) is increased to ensure a sufficiently high stoichiometric phosgene excess. Should the pressure drop (e.g., due to the sudden detachment of solid deposits), the flow rate (f-back) is reduced to avoid an excessive stoichiometric phosgene excess. For example, if (at least) one of the phosgene flow control valves (PV-1, PV-2, ..., PV-m) is open by 70% or more, preferably by 80% or more, the inventive adjustment (control) of the state parameter (X) of the phosgene gas stream (P) can be carried out via the setting parameter (Y). The percentage "opening degree" of a valve (the phosgene flow control valve as well as any other valve) describes its setting relative to the fully closed state.The opening degree of a fully closed valve is 0% and that of a fully open valve is 100%.

[0035] In the present invention, phosgene is used in a molar ratio of phosgene to primary amino groups of greater than 1, ie, phosgene is used in a stoichiometric excess. This so-called "stoichiometric phosgene excess" is expressed as a percentage of theory. Theoretically, 1 mole of phosgene reacts with 1 mole of primary amino groups. A phosgene excess of x% compared to primary amino groups therefore corresponds to a molar ratio of n(phosgene) / n(primary amino groups) i + — i + —

[0036] (n = amount of substance) of — , for example — = 1.1 at 10% i + — stoichiometric phosgene excess or for example — = 1.4 at

[0037] 40% stoichiometric phosgene excess. A target value (PÜSOLL) is established for the stoichiometric phosgene excess (which must be maintained equally in each of the m gas-phase reaction arrangements (100-1, 100-2, ..., 100-m).

[0038] The "flow rate (f-return)" of the recirculation flow (return) refers to the mass flow (specified as volumetric, molar, or mass flow; also called flow rate or flow rate) of the recirculation flow. The flow rate of the recirculation flow can be measured using one or more recirculation measuring devices. The principle of measuring a flow rate using known measuring devices is generally known to those skilled in the art. A recirculation measuring device can be a flow sensor or flow indicator, whereby the flow meter is suitable for the continuous measurement of a medium flowing through the flow sensor or flow indicator, such as a liquid, gas, or vapor quantity of the flowing medium. A flow sensor can be designed as a time measuring device (measuring a flow rate over a specified period of time). Flow sensors can comprise a primary media connection device, a measuring transducer, and a transmitter.A transducer can measure the fluid flowing through the primary device. A transmitter can then generate an output signal from the transducer's raw signal. A flow sensor can be, for example, a paddlewheel flow sensor, a turbine, a Rotaflow meter, or a magnetic inductive sensor. Flow rates can also be determined using imaging techniques, such as MRI (Magnetic Resonance Imaging).

[0039] First, a brief summary of various possible embodiments of the invention follows:

[0040] In a first embodiment of the process for preparing an isocyanate, which can be combined with all other embodiments except the second embodiment described below, step (i) is carried out in m amine gas generating devices (106-1, 106-2, ..., 106-m).

[0041] In a second embodiment of the process for producing an isocyanate, which can be combined with all other embodiments except the first embodiment described above, in step (i), an amine reaction gas stream (A) is first generated in a (single) amine gas generating device (106), which amine reaction gas stream is then divided into the m amine reaction gas streams (Al, A-2, ..., Am).

[0042] In a third embodiment of the process for producing an isocyanate, which can be combined with all other embodiments, each of the m gas phase reaction arrangements (100-1, 100-2, ..., 100-m) has a reaction device (100a-l, 100a-2, ..., 100a-m) and one of the

[0043] Reaction device has a separation device (100b-l, 100b-2, ..., 100b-m) downstream of the process.

[0044] In a fourth embodiment of the process for producing an isocyanate, which is a particular embodiment of the third embodiment, the reaction device (100a-l, 100a-2, ..., 100a-m) comprises a

[0045] Mixing zone (101-1, 101-2, ..., 101-m) and a reaction zone (102-1, 102-2, ..., 102-m) downstream of the mixing zone, wherein in the mixing zone the phosgene reaction gas stream (Pl, P-2, ..., Pm) and the

[0046] Amine reaction gas stream (Al, A-2, ..., Am) are mixed, and wherein in the reaction zone (102-1, 102-2, ..., 102-m) the gas phase reaction of the primary amine (contained in the respective amine reaction gas stream (Al, A-2, ..., Am)) with phosgene (contained in the respective phosgene reaction gas stream (Pl, P-2, ..., Pm)) takes place to obtain in each case a reaction product mixture (RP-1, RP-2, ... RP-m).

[0047] In a fifth embodiment of the process for producing an isocyanate, which is a special embodiment of the third and fourth embodiments, the separation device (100b-l, 100b-2, ..., 100b-m) comprises a quench zone (107-1, 107-2, ..., 107-m) and a phase separation zone (108-1, 108-2, ..., 108-m) arranged downstream of the quench zone, wherein in the quench zone the reaction product mixture (RP-1, RP-2, ..., RP-m) is cooled by contacting it with a quench liquid to obtain a quench mixture, and wherein in the phase separation zone (108-1, 108-2, 108-m) a phase separation of the resulting quench mixture into the isocyanate crude product fluid stream (IC-1, IC-2, IC-m) and an additional isocyanate comprising the raw residual fluid stream (R-raw-1, R-raw-2, R-raw-m).

[0048] In a sixth embodiment of the process for producing an isocyanate, which is a special embodiment of the fifth embodiment, the crude residual fluid stream (R-raw-1, R-raw-2, ..., R-raw-m) comprising additional isocyanate obtained from the phase separation zone (108-1, 108-2, ..., 108-m) is fed to a washing device (105-1, 105-2, ..., 105-m) arranged downstream of the phase separation zone, in which the additional isocyanate is washed out of the crude residual fluid stream (R-raw-1, R-raw-2, ..., R-raw-m) with a washing liquid to obtain the residual fluid stream (Rl, R-2, ..., Rm).

[0049] In a seventh embodiment of the process for producing an isocyanate, which is a special embodiment of the sixth embodiment, a first partial stream of the liquid desorption medium (DesM) is returned to the phosgene gas generation device (104) via a first heat exchanger, the first partial stream being indirectly heated in the first heat exchanger by a heat transfer medium, and a second partial stream of the liquid desorption medium (DesM) is passed through a second heat exchanger, the second partial stream indirectly heating the phosgene reaction gas stream (P) in the second heat exchanger, and the second partial stream of the liquid desorption medium (DesM) cooled in the process is used as a component of the scrubbing liquid.

[0050] In an eighth embodiment of the process for producing an isocyanate, which can be combined with all other embodiments and is in particular a special embodiment of the seventh embodiment, the phosgene gas generation device (104) has an introduction region (111) for introducing the recycle stream (Recycled) and a phosgene gas generation device collection region (110), wherein the desorbed phosgene and the phosgene obtained from the fresh phosgene fluid stream (Fresh) collect in the phosgene gas generation device collection region (110) to form the phosgene reaction gas stream (P). In a ninth embodiment of the process for producing an isocyanate, which is a special embodiment of the eighth embodiment, a desorption column is used as the phosgene gas generation device (104), which has a sump (112) in which the liquid desorption medium (DesM) collects.

[0051] In a tenth embodiment of the process for preparing an isocyanate, which can be combined with all other embodiments, an absorption column is used as the phosgene absorption device (103-1, 103-2, ... 103-m or 103-n), wherein the phosgene absorption device collection region (115-1, 115-2, ... 115-m or 115-n) is arranged in a bottom of the absorption column.

[0052] In an eleventh embodiment of the process for producing an isocyanate, which can be combined with all other embodiments, the setting parameter (Y) additionally comprises a setting parameter (Y-2) of the phosgene fresh fluid stream (Fresh), wherein the state parameter (X) is set (= regulated) in such a way that when a change in (at least) one pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m) is detected, first the flow rate (f-Return) of the recycle stream (Return) is used as the setting parameter (Yl) and then the flow rate (f-Fresh) of the introduced phosgene fresh fluid stream (Fresh) is used as the setting parameter (Y-2).

[0053] In a twelfth embodiment of the process for producing an isocyanate, which is a particular embodiment of the eleventh embodiment, the flow rate (f-Return) of the recycle stream (Return) is adjusted by setting a first adjustment parameter (ZI) of the absorbed phosgene comprised in the recycle medium and a second adjustment parameter (Z2) of the absorbent comprised in the recycle medium.

[0054] In a thirteenth embodiment of the process for producing an isocyanate, which is a particular embodiment of the twelfth embodiment, the flow rate (f-Return) of the recycle stream (Return) is adjusted by setting a phosgene mass (mP-1, mP-2, ..., mPm or mPn) of the absorbed phosgene comprised in the recycle medium (Return-1, Return-2, ..., Return-m or Return-n) as the first adjustment parameter (Zl-1, Zl-2, ..., Zl-n or Zl-m) and an absorbent mass (m-AbsM-1, m-AbsM-2, ... m-AbsM-m or m-AbsM-n) of the absorbent comprised in the recycle medium (Return-1, Return-2, ..., Return-m or Return-n) as the second adjustment parameter (Z2-1, Z2-2, ..., Z2-n or Z2-m).

[0055] In a fourteenth embodiment of the process for producing an isocyanate, which is a particular embodiment of the thirteenth embodiment, the phosgene mass (mP-1, mP-2, ..., mPm or mPn) is adjusted by manipulating the flow rate (f-fresh) of the phosgene fresh fluid stream (Fresh), wherein the absorbent mass (m-AbsM-1, m-AbsM-2, ... m-AbsM-m or m-AbsM-n) is adjusted by manipulating a flow rate (f-AbsM-1, f-AbsM-2, ..., f-AbsM-m or f-AbsM-n) of the absorbent fluid stream (AbsM-1, AbsM-2, ..., AbsM-m or AbsM-n).

[0056] In a fifteenth embodiment of the process for producing an isocyanate, which is a further particular embodiment of the thirteenth embodiment, the recycle medium (Recycle-1, Recycle-2, ..., Recycle-m or Recycle-n) comprises the absorbed phosgene and non-phosgene components, wherein the non-phosgene components comprise the absorption medium, wherein the phosgene mass (mP-1, mP-2, ..., mPm or mPn) is determined in each case by multiplying a fill level (L-recycle-1, L-recycle-2, ..., L-recycle-m or L-recycle-n) of the recycle medium in the phosgene absorption device collection region (115-1, 115-2, ..., 115-m or 115-n) and a concentration (xP-1, xP-2, ..., xPm or xPn) of the absorbed phosgene in the recycle medium (Recycle-1, Back-2, ..., Back-m or Back-n) is calculated, and the absorption medium mass (m-AbsM-1, m-AbsM-2, ... m-AbsM-m or m-AbsM-n) is calculated by multiplying the fill level (L-back-1, L-back-2, ..., L-back-m orL-return-n) of the recycle medium (Return-1, Return-2, ..., Return-m or Return-n) in the phosgene absorption device collection section (115-1, 115-2, ... 115-m or 115-n) and a non-phosgene component concentration (l-(xPl), l-(xP-2), l-(xPm) or l-(xPn)) of the non-phosgene components in the recycle medium (Return-1, Return-2, ..., Return-m or Return-n).

[0057] In a sixteenth embodiment of the process for preparing an isocyanate, which can be combined with all other embodiments,

[0058] - the absorption agent (AbsM) is selected from decahydronaphthalene, toluene, xylene, chlorobenzene, para-dichlorobenzene, orü7?o-dichlorobenzene, chlorotoluene, chloronaphthalene or mixtures thereof; preferably the absorption agent is orü7?o-dichlorobenzene, and / or

[0059] - the primary amine is selected from toluenediamine, diphenylmethanediamine, xylylenediamine, 1,5-pentanediamine, 1,6-hexamethylenediamine, isophoronediamine, diaminodicyclohexylmethane or mixtures thereof; preferably the primary amine is toluenediamine.

[0060] In a seventeenth embodiment of the process for preparing an isocyanate, which can be combined with all other embodiments, a target value (PÜSOLL) in the range from 10% of theory to 400% of theory, preferably from 20% of theory to 150% of theory, is set for the stoichiometric phosgene excess.

[0061] In a first embodiment of the method for controlling the stoichiometric phosgene excess, which can be combined with all other embodiments, a self-learning algorithm is implemented on the processor for automatically comparing changes in (at least) one pressure (pP-1, pP-2, ... pPm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m) with the stoichiometric phosgene excess and, in response thereto, for automatically setting and adjusting the setting parameter (Y), wherein the algorithm is configured to optimize the process for producing an isocyanate by adjusting the setting parameter (Y) (i.e., to counteract deviations from the stoichiometric phosgene excess more quickly).

[0062] The embodiments briefly described above and further possible embodiments of the invention are explained in more detail below. All embodiments described above and the further embodiments of the invention described below can be combined with one another as desired, even across categories, unless the context clearly indicates the opposite to a person skilled in the art or unless expressly stated otherwise. For example, all embodiments or features mentioned in connection with the proposed process for producing an isocyanate can also readily represent embodiments or features of the proposed plant for producing an isocyanate, and vice versa.

[0063] For the sake of simplicity, for those devices of which two or more are or can be provided according to the invention (e.g., the gas-phase reaction arrangements (100-1, 100-2, ..., 100-m) or the amine gas generation devices (106-1, 106-2, ..., 106-m)), only the respective main reference symbol (e.g., "100" or "106", without the trailing numbering) is used in the following text. This is done solely for reasons of linguistic simplification and is not intended to imply that there is only a single such device (if this is even possible within the scope of the invention), unless this is clearly evident from the context (as, for example, in the description of the embodiment using a single amine gas generation device 106 for all m gas-phase reaction arrangements).

[0064] The process according to the invention and the plant according to the invention designed for its implementation serve to produce an isocyanate. The isocyanate is obtained by reacting a primary amine with phosgene in a gas-phase reaction. In the context of the present invention, in principle any primary amine that can be vaporized without decomposition can be reacted. For example, the primary amine can be a primary diamine. The primary amine can be an isomer mixture comprising several isomeric forms of a primary amine. If the primary amine can exist in different isomeric forms, the isomer mixture can in principle comprise all possible isomer distributions.The term gas phase reaction is understood to mean that during the reaction of the primary amine with phosgene to produce the isocyanate, at least the reaction components primary amine, phosgene and isocyanate are present in a gaseous state.

[0065] To enable the gas-phase reaction, in the first process step (i) of the process according to the invention, the m amine reaction gas streams (Al, A-2, ..., Am), each comprising a primary amine, are first generated (see also FIG. 1). An amine reaction gas stream is understood to be a gas stream that comprises the primary amine in the gaseous state. An amine reaction gas stream is generated in an amine gas generation device (106). It is possible to use a separate amine gas generation device (106-1, 106-2, ..., 106-m) for each of the amine reaction gas streams (Al, A-2, ..., Am) to be provided. This is shown in FIG. 1. It is also possible to first generate an amine reaction gas stream (A) in a (single) amine gas generating device (106), which is then divided into the m amine reaction gas streams (Al, A-2, ..., Am).The first variant simplifies the control of the plant, the second variant requires fewer devices. What is more advantageous in a specific case depends on the exact circumstances of the individual case. According to the invention, a plant for carrying out the process according to the invention therefore comprises a plurality of gas-phase reaction arrangements and one or more amine gas generation devices. Preferably, the plant comprises a plurality of amine gas generation devices, with each gas-phase reaction arrangement being assigned an amine gas generation device. In other words, the plant preferably comprises one amine gas generation device per gas-phase reaction arrangement. Each of the plurality of amine gas generation devices can be arranged in the one gas-phase reaction arrangement assigned to it in terms of process technology. Particularly preferably, each of the plurality of amine gas generation devices is part of the one gas-phase reaction arrangement assigned to it in terms of process technology.

[0066] An amine gas generation device can, in principle, be understood as any device suitable for generating an amine reaction gas stream. Methods for generating an amine reaction gas stream are known to those skilled in the art. The conversion of the primary amine into the gas phase can, in principle, take place in all amine gas generation devices known from the prior art. For example, the amine gas generation device can be an evaporation apparatus such as a falling-film evaporator. The amine gas generation device can, for example, be an evaporation apparatus in which a small working volume is passed through a falling-film evaporator at a high circulation rate. For example, evaporation apparatuses can be used in which a small working volume is circulated through at least one micro-heat exchanger or micro-evaporator. Such a use of corresponding heat exchangers for the evaporation of amines is disclosed, for example, in EP 1754698.To minimize the thermal stress on the amine, the evaporation process can be assisted by feeding in an inert gas such as nitrogen, helium, or argon, regardless of the precise design of the evaporation apparatus. The evaporation of the primary amine and, if necessary, its superheating, in particular to a temperature in the range of 225°C to 430°C, preferably 250°C to 420°C, particularly preferably 250°C to 400°C, can be carried out in multiple stages to avoid unevaporated droplets in the generated amine reaction gas stream. Multi-stage evaporation and superheating steps can be carried out, with droplet separators installed between the evaporation and superheating systems and / or the evaporation apparatus also functioning as a droplet separator. Suitable droplet separators are known to those skilled in the art.After leaving the last superheater in the flow direction, the gaseous amine, preheated to its target temperature, can then be introduced into the gas-phase reaction system. Regardless of the detailed design of the amine feed, the risk of renewed droplet formation can be counteracted by technical measures such as adequate insulation to prevent radiation losses.

[0067] In a preferred embodiment, the primary amine is selected from the group consisting of toluenediamine, diphenylmethanediamine, xylylenediamine, 1,5-pentanediamine, 1,6-hexamethylenediamine, isophoronediamine, diaminodicyclohexylmethane, and mixtures thereof. If the primary amine can exist in various isomeric forms, all isomer distributions are encompassed. In principle, mixtures of several primary amines can also be reacted. The primary amine is preferably toluenediamine. Toluenediamine, which is preferably used, comprises in particular 78% to 82% by mass of 2,4-toluenediamine and 18% to 22% by mass of 2,6-toluenediamine, based on the total mass of the 2,4- and 2,6-toluenediamine isomers. Based on the total mass of the toluenediamine, the 2,4- and 2,6-toluenediamine isomers preferably make up from 95 mass% to 100 mass%, particularly preferably from 98 mass% to 100 mass%.Most preferably, the content of toluenediamine isomers with amine groups positioned 7?o relative to one another in the toluenediamine to be used is less than 0.2% by mass, based on the total mass of the toluenediamine to be used. Processes for preparing toluenediamine meeting the aforementioned requirements are known to those skilled in the art.

[0068] Scheme: Chemical structures of 2,4- (left) and 2,6-toluenediamine (right).

[0069] In the second process step (ii) of the process according to the invention, the phosgene-comprising phosgene gas stream (P) is first generated in a phosgene gas generation device (104) and then divided into the m phosgene-comprising phosgene reaction gas streams (Pl, P-2, ... Pm). For this purpose, splitting devices (not shown as separate devices in FIG. 1) are provided. Such a splitting device is designed to split the phosgene gas stream (P) into several phosgene reaction gas streams (Pl, P-2, Pm). One of the phosgene reaction gas streams (Pl, P-2, Pm) is fed to each gas-phase reaction arrangement.

[0070] A phosgene (reaction) gas stream is understood to be a gas stream which comprises phosgene in the gaseous state. A phosgene gas generation device (104) is understood to be a device which is designed to generate a phosgene gas stream from liquid or dissolved phosgene. For example, phosgene present in the phosgene gas generation device can be heated to a temperature in the range from 200°C to 350°C, preferably from 250°C to 325°C, particularly preferably from 250°C to 300°C, for generating the phosgene gas stream and optionally diluted with an inert gas such as nitrogen, helium or argon. The phosgene gas stream (P) leaving a phosgene gas generation device (104) contains portions of freshly produced phosgene (fresh) and portions of recycled phosgene (recycled).The freshly produced phosgene comes from a fresh phosgene production facility (109), in which phosgene is synthesized from chlorine and carbon monoxide in a manner known per se.

[0071] According to a preferred embodiment of the method according to the invention, which is also shown in FIG. 1, it is provided that a phosgene gas generation device (104) is used which has an introduction region (111) for introducing the recycle stream (Return) into the phosgene gas generation device (104) and a phosgene gas generation device collection region (110). Desorbed phosgene and the phosgene obtained from the phosgene fresh fluid stream (Fresh) collect in the phosgene gas generation device collection region (110) to form the phosgene gas stream (P). The pressure of the formed phosgene gas stream (i.e., the phosgene gas stream formed from desorbed phosgene and phosgene obtained from the phosgene fresh fluid stream) is set as a state parameter (X) via the flow rate of the recycle stream upon introduction into the introduction region as a setting parameter (Y1).An introduction region (111) can be understood as a region designed to receive the recycle stream (return) and to introduce the phosgene to be recycled, contained in the recycle stream (return), into the phosgene gas generation device for desorption from the absorbent. The introduction region can, for example, be an inlet to the phosgene gas generation device (104).

[0072] In a further embodiment of the process according to the invention, it is provided that a desorption column is used as the phosgene gas generation device (104). A desorption column can be understood to mean a rectification column in which a phase equilibrium prevails between desorption and absorption, the phase equilibrium being on the desorption side. In the desorption column, therefore, desorption of the phosgene to be recycled, which is comprised in the recycle stream, from the absorbent thus (predominantly) takes place, to obtain desorbed phosgene. The desorption column can comprise at least one separation stage, preferably a plurality of separation stages. The desorption column can, for example, be a tray column with horizontal trays installed at regular intervals, each tray corresponding to a separation stage. Trays can consist of circular plates with a plurality of passage openings for the gas stream, so-calledBell caps, slots, or valves. The trays can be installed horizontally in the tray column at regular vertical intervals, for example, at vertical intervals of 0.3 to 1.0 m. In addition to the trays, the desorption column can also contain packing, particularly in the stripping section. The desorption column has a sump (112) in which liquid desorption medium (DesM) collects.

[0073] For example, a desorption column can be used which is fed with the recycle stream via a desorption column inlet, wherein the desorption column inlet is arranged in the introduction region (111) of the desorption column. The phosgene gas generation device collection region (110) is arranged at a top of the desorption column. Thus, the pressure of the phosgene gas stream (P) formed in the phosgene gas generation device collection region (110) arranged at the top of the desorption column can be adjusted as a state parameter (X) via the flow rate of the recycle stream upon introduction into the desorption column inlet as a setting parameter (Y1). This is particularly the case when the stream of phosgene absorbed in the absorbent, which is introduced into the desorption column inlet, is directly evaporated (at least to a considerable extent).

[0074] In the third process step (iii), each of the generated phosgene reaction gas streams (two, P-1 and P-2, are shown as examples in FIG. 1, i.e., m = 2) is combined with one of the generated amine reaction gas streams (A-1 and A-2 in FIG. 1) in one of the gas-phase reaction arrangements. It is preferred that each of the m gas-phase reaction arrangements has a phosgene gas inlet and a separate amine gas inlet, with the respective phosgene reaction gas stream being introduced into the corresponding gas-phase reaction arrangement through the phosgene gas inlet, and the respective amine reaction gas stream being introduced through the separate amine gas inlet. In order to regulate the phosgene reaction gas flow (Pl, P-2, ..., Pm) introduced into the gas phase reaction arrangement, an adjusting means is provided which comprises a phosgene gas valve, the phosgene flow control valve (PV-1, PV-2, ..., PV-m), arranged in particular at the phosgene gas inlet.To regulate the amine reaction gas stream introduced into the gas-phase reaction arrangement, an adjustment means can preferably be used, which comprises an amine gas valve, i.e., an amine flow control valve (AV-1, AV-2, ..., AV-m), arranged in particular at the separate amine gas inlet. The actual adjustment of the amount of gaseous amine reaction gas stream required for a desired amine flow is achieved by supplying energy to the amine gas generation device (106). In particular, the amine gas generation device, and accordingly also the amine reaction gas stream generated therein, which is introduced into the gas-phase reaction arrangement through the amine gas inlet, reacts very sensitively to pressure drops in the gas-phase reaction arrangement.Such pressure drops can, as previously described, be caused in particular by the break-off of subsequent and by-products of the gas-phase reaction adhering to the inner walls of the gas-phase reaction arrangement in the form of solid residues. Separate introduction of the phosgene reaction gas stream and the amine reaction gas stream into each of the multiple gas-phase reaction arrangements provides the particular advantage that, in the event of a pressure drop in the gas-phase reaction arrangement, a disruption of the amine gas generation device or the generated amine reaction gas stream has no effect on the generated phosgene reaction gas stream or its introduction into the gas-phase reaction arrangement. In other words, the plant according to the invention, comprising the adjusting means according to the invention, is capable of compensating for the described pressure drops in the gas-phase reaction arrangement (e.g., a gas-phase reactor), also through the separate introduction of the phosgene and amine reaction gas streams.

[0075] The merging of the phosgene and amine reaction gas streams is carried out while maintaining a stoichiometric excess of phosgene with respect to the amino groups of the primary amine to be reacted. A preferred molar ratio of phosgene to primary amino groups during normal operation (i.e., as soon as the desired flow rates of phosgene reaction gas stream and amine reaction gas stream are reached after start-up of a gas-phase reaction arrangement) of 1.1:1 (corresponding to a stoichiometric phosgene excess of 10% of theory) to 5.0:1 (corresponding to a stoichiometric phosgene excess of 400% of theory), particularly preferably of 1.2:1 (corresponding to a stoichiometric phosgene excess of 20% of theory) to 2.5:1 (corresponding to a stoichiometric phosgene excess of 150% of theory) can be set as the setpoint PÜSOLL during normal operation. When starting up the plant, only phosgene will be added first, followed by the amine.During the start-up phase, the phosgene excess is therefore initially "infinite" and can reach very high values, such as 1900% of theory (corresponding to a molar ratio of phosgene to primary amino groups of 20:1), even after the first addition of the amine. The invention allows the stoichiometric excess of phosgene to be adjusted so that it is as constant as possible, i.e., fluctuates as little as possible (the phosgene excess has a significant influence on the reaction temperature and thus on the reaction rate; the residence time also changes depending on the phosgene excess).

[0076] The gas-phase reaction arrangement comprises, in particular, a gas-phase reactor. The gas-phase reactor can be, for example, a tubular reactor, in particular a vertically arranged tubular reactor with a cylindrical, conical, or cylindrical-conical shape.

[0077] In the gas-phase reaction arrangement, the gas-phase reaction of the primary amine with phosgene to form the isocyanate then proceeds, with an isocyanate crude product fluid stream IC (IC-1 and IC-2 in FIG. 1) and a residual fluid stream R (RI and R-2 in FIG. 1) being obtained from each of the m gas-phase reaction arrangements. The isocyanate crude product fluid stream IC obtained in each case comprises produced isocyanate. The residual fluid stream R, in turn, comprises unreacted, excess phosgene in the gas-phase reaction. The process according to the invention fundamentally provides for recirculating unreacted, excess phosgene to the phosgene gas generation device (104) and thus making it available for the generation of the phosgene-comprising phosgene gas stream (P) and the m phosgene reaction gas streams. In other words, the stoichiometric phosgene excess is cyclized.

[0078] The recycling of the unreacted, excess phosgene comprised in the residual fluid stream to the phosgene gas generation device (104) in the fourth process step (iv) optionally comprises, in a first sub-step (iv-1), combining the m residual fluid streams (Rl, R-2, ..., Rm) to form n residual fluid streams (Rl, R-2, ... Rn), where n is a natural number in the range from 1 to m-1 ("m minus one") and in particular 1. This optional step is not shown in FIG. 1, but may nevertheless be technically and economically expedient depending on the circumstances of the individual case.

[0079] In a second sub-step (iv-2), each of the m or n residual fluid streams is then introduced into a respective phosgene absorption device (103-1, 103-2, ... 103-m or 103-n). Each phosgene absorption device has a phosgene absorption device collection region (115-1, 115-2, ... 115-m or 115-n). To carry out the absorption, an absorbent fluid stream (AbsM-1, AbsM-2, ..., AbsM-m or AbsM-n), each containing an absorbent (AbsM), is also introduced into each of the phosgene absorption devices (103-1, 103-2, ... 103-m or 103-n). In each phosgene absorption device, the excess phosgene not converted in the gas phase reaction is absorbed by the absorption medium to form recycled phosgene.The unreacted, excess phosgene is present in a gas stream together with the hydrogen chloride gas formed as a by-product and is scrubbed from this gas stream in the phosgene absorption unit. The recycle stream (recycle), which comprises recyclable phosgene, is then obtained from the phosgene absorption unit. An absorption medium can in principle be understood to mean any inert solvent suitable for absorbing phosgene. An inert solvent can be understood to mean a solvent that is inert at least toward the primary amine, phosgene, and the isocyanate produced. In other words, the inert solvent is not involved in the gas-phase reaction. The inert solvent is preferably an organic solvent.The organic solvent can comprise, for example, aliphatic hydrocarbons, aromatic hydrocarbons with or without halogen substitution, and mixtures thereof. Absorption of phosgene by the absorption medium is understood to mean uptake of the phosgene into the free volume of the absorption medium present in a different phase than phosgene. For example, phosgene present in a gaseous state can form a gaseous phase in the phosgene absorption device, while absorption medium present in a liquid state forms a liquid—i.e., a different—phase in the phosgene absorption device. During absorption of the phosgene by the absorption medium, the gaseous phosgene is absorbed predominantly or completely into the free volume of the liquid absorption medium. A particularly preferred absorption medium is ortho-7?o-dichlorobenzene.According to one embodiment of the process according to the invention, it can be provided that one absorption column is used as a phosgene absorption device (103-1, 103-2, ... 103-m or 103-n). An absorption column (also called an absorber) is understood to be a process apparatus in which the residual fluid stream is brought into contact with the absorption medium-comprising absorbent fluid stream in order to absorb phosgene from the residual fluid stream in the absorption medium. A phase equilibrium between desorption and absorption can prevail in the absorption column, with the phase equilibrium in the absorption column being on the absorption side. In the absorption column, therefore, the unreacted, excess phosgene contained in the residual fluid stream is (predominantly) absorbed by the absorption medium to obtain recyclable phosgene.For this purpose, the absorption column can have one or more of the following six absorption column sections: a bottom, a gas inlet, a contact zone, an absorption medium feed zone, a droplet separator, and a head. Thus, the absorption column can have a bottom as a phosgene absorption device collection zone (115-1, 115-2, ... 115-m or 115-n), with the recycle medium collecting in the bottom, the recycle medium being withdrawn from the bottom and fed as a recycle stream to the phosgene gas generation device, which serves as a phosgene desorption device. Furthermore, the absorption column can have a gas inlet, the residual fluid stream being fed into the gas inlet, after which the interior is uniformly supplied with the recycle stream by suitable flow guidance (e.g., by means of internals). In addition, the absorption column can have a contact zone in which the absorption takes place.The absorption column can also have an absorption medium feed, into which the absorption medium fluid stream is introduced, after which the interior is uniformly supplied with the absorption medium by suitable flow guidance (e.g., through internals). Furthermore, the absorption column can have a droplet separator and / or an absorption column head, with a hydrogen chloride stream freed from unreacted, excess phosgene leaving the absorption column at the absorption column head. The absorption column can be a packed column, a tray column, a fluidized-bed scrubber, a rotary scrubber, a Venturi scrubber, an immersion scrubber, or a spray scrubber. The absorption column is preferably a packed column. A packed column can be understood as a column filled with packing in loose layers. Packing can be made of a material including stainless steel.

[0080] Regardless of the precise design of the phosgene absorption device (103), it is preferred that the absorption medium (AbsM) be selected from the group consisting of decahydronaphthalene, toluene, xylene, chlorobenzene, para-dichlorobenzene, ortho-dichlorobenzene, chlorotoluene, chloronaphthalene, and mixtures thereof. The absorption medium is preferably chlorobenzene or ortho-dichlorobenzene, particularly preferably ortho-dichlorobenzene.

[0081] The recycle stream (Recycle) obtained from the phosgene absorption device (103) and comprising recirculated phosgene is introduced in the third sub-step (iv-3) at a flow rate (f-Recycle) into the phosgene gas generation device (104), wherein desorption of the recirculated phosgene from the absorption medium takes place to obtain desorbed phosgene. In the context of the present invention, the term "desorption" describes the release of gaseous phosgene from the phosgene obtained in the phosgene absorption device and absorbed in the absorption medium (dissolved in the absorption medium). This can be achieved by simply expelling gaseous phosgene by reducing the pressure and / or increasing the temperature, but also by distillation or rectification with a liquid reflux of condensed phosgene. The pressure during desorption should, in particular, be above the pressure at the inlet to the m gas-phase reaction arrangements.

[0082] In addition, as already mentioned, a fresh phosgene fluid stream (Fresh) is introduced into the phosgene gas generation device (104). The phosgene gas stream (P) generated in the phosgene gas generation device is thus formed by the desorbed phosgene and the phosgene from the additionally introduced into the phosgene gas generation device.

[0083] Phosgene is formed from a fresh fluid stream (freshly obtained phosgene). The phosgene gas generation device can be a rectification column. A rectification column can be, for example, a tray column or a packed column. Rectification is understood to be a thermal separation process that represents an extension of distillation or the series connection of many distillation steps. The main advantages of rectification are that the plant can be operated continuously and that the separation effect is many times greater than that of distillation, since the vapor is in contact with the liquid several times in countercurrent. A rectification column is therefore more energy-efficient, technically less complex, and requires less space than a series connection of single distillations.The contact surface between the vapor and liquid phases in the rectification column is provided, for example, by internals (e.g., bubble-cap trays, random packings, packing). At each of these additional contact surfaces, a mixture with a changed concentration condenses from the vapor mixture according to the phase equilibrium, while a mixture with a higher concentration of the more volatile component evaporates from the liquid phase as a result of the released latent heat of condensation. The temperatures at the separation stages of a rectification column can be influenced at a given pressure by the liquid-to-vapor ratio (F / D ratio). In rectification columns, the F / D ratio is adjusted by the reflux ratio at the top of the column.If a liquid mixture is evaporated in the rectification column, the concentrations of the individual substances in the gas and liquid phase are determined by the temperature and pressure of the rectification column.

[0084] According to a further embodiment of the process according to the invention, gas-phase reaction arrangements (100) can be provided, each comprising a reaction device (100a) and a separation device (100b) arranged downstream of the reaction device in terms of process technology. This is illustrated in FIG. 2, wherein, for reasons of clarity, only one of the at least two gas-phase reaction arrangements (100) according to the invention is shown, namely (100-1). In the reaction device (100a-1), the respective phosgene reaction gas stream and the respective amine reaction gas stream are combined, whereby the gas-phase reaction of the primary amine with phosgene takes place to obtain a reaction product mixture.In the separation device (100b-l), the reaction product mixture obtained from the reaction device (100a-l) is cooled by contacting it with a quench liquid to obtain a quench mixture, and a phase separation of the resulting quench mixture into the isocyanate crude product fluid stream and an additional crude residual fluid stream comprising isocyanate is carried out. In a preferred embodiment of the invention, which is also shown in FIG. 2, reaction devices (100a-l, 100a-2, ..., 100a-m) are provided for this purpose, each comprising a mixing zone (101-1, 101-2, ..., 101-m) and a reaction zone (102-1, 102-2, ..., 102-m) downstream of the mixing zone in terms of process technology. In the mixing zone, the corresponding phosgene reaction gas stream and the corresponding amine reaction gas stream are mixed.In the reaction zone, the gas-phase reaction of the primary amine with phosgene takes place to yield the reaction product mixture (RP-1, RP-2, ... RP-m). The reaction product mixture comprises at least the produced isocyanate.

[0085] In a further preferred embodiment of the invention, which is also shown in FIG. 2, such separation devices (100b-1, 100b-2, ..., 100b-m) are also provided, each comprising a quench zone (107-1, 107-2, ..., 107-m) and a phase separation zone (108-1, 108-2, ..., 108-m) arranged downstream of the quench zone in terms of process technology. In the quench zone, the reaction product mixture obtained from the reaction device is cooled by contacting it with a quench liquid to obtain a quench mixture. The quench liquid can be introduced into the quench zone in the form of one or more quench streams. In this case, a quench stream can, for example, comprise a portion of the isocyanate crude product fluid stream which, after receipt of the isocyanate crude product fluid stream, is branched off from the gas phase reaction arrangement and recycled to the quench zone arranged in the gas phase reaction arrangement.The quench liquid can, for example, be injected into the quench zone (e.g., via a mixing nozzle). The resulting quench mixture comprises at least the quench liquid and the produced isocyanate. In the downstream phase separation zone, the quench mixture obtained from the quench zone is then phase-separated into the isocyanate crude product fluid stream and the crude residual fluid stream containing additional isocyanate. The isocyanate crude product fluid stream comprises at least the produced isocyanate.

[0086] FIG. 2 also shows further preferred details of the invention relating to heat transfer processes from one process stream to another. The phosgene gas stream (P) produced, after being divided into P1, P-2, ... Pm, is passed from the phosgene gas generation device (104) through one (or more) heat exchangers (WT2) and introduced into the mixing zone (101) via a phosgene gas valve (= phosgene flow control valve PV-1). In the mixing zone 101, the phosgene reaction gas stream P1 and the amine reaction gas stream A1 are mixed. This results in a mixture of at least gaseous primary amine and gaseous phosgene, which is then passed through a reaction zone 102-1 arranged within the gas-phase reaction arrangement. In the reaction zone 102-1, the gas-phase reaction of the primary amine with phosgene takes place to produce isocyanate.This yields a gaseous reaction product mixture RP-1, which is fed into a quench zone 107-1, also located within the gas-phase reaction arrangement. In quench zone 107-1, the gaseous reaction product mixture RP-1 is brought into contact with a quench liquid and thus cooled. The quench liquid is introduced into quench zone 107-1 in the form of two quench streams. The first quench stream corresponds to a portion of a liquid isocyanate crude product liquid stream IC-1 comprising prepared isocyanate, which, after being obtained from a phase separation zone (108-1) downstream of quench zone 107-1, is branched off and pumped back into quench zone 107-1 through a heat exchanger WT3 by means of a pump PP. The second quench stream in turn corresponds to a liquid washout stream Aus-1, which is obtained from a washout device 105-1 arranged downstream of the phase separation zone 108-1.According to a further embodiment of the process according to the invention, the crude residual fluid stream comprising additional isocyanate obtained from the phase separation zone is fed to a scrubbing device arranged downstream of the phase separation zone. In the scrubbing device, additional isocyanate is scrubbed from the crude residual fluid stream using a scrubbing liquid, thereby obtaining the residual fluid stream. A scrubbing liquid can in principle be understood as any liquid suitable for scrubbing isocyanate from the crude residual fluid stream. The scrubbing liquid can comprise parts of the absorption medium. The scrubbing liquid preferably comprises an inert solvent; more preferably, the scrubbing liquid comprises a halogenated aromatic solvent such as chlorobenzene or dichlorobenzene (in particular ortho-dichlorobenzene).The two liquid quench streams are fed into the quench zone via a mixing nozzle.

[0087] 107-1. A quench mixture obtained from the quench process is then fed into a phase separation zone 108-1, which is also arranged within the gas-phase reaction arrangement. Therein, a phase separation of the quench mixture into a liquid phase and a gaseous phase is carried out. The liquid phase, which comprises at least the quench liquid and liquid isocyanate produced, collects at the bottom of the phase separation zone.

[0088] 108-1, and is discharged there through an outlet arranged at the bottom of the phase separation zone 108-1 as the isocyanate crude product fluid stream IC-1. The gaseous phase is discharged from the phase separation zone 108-1 (e.g., via shut-off valves) and fed into the scrubbing device 105-1 as the gaseous crude residual fluid stream R-crude-1. The gaseous crude residual fluid stream is scrubbed in the scrubbing device 105-1 with a scrubbing liquid composed of the liquid desorption medium (DesM-2; see also the explanations below) withdrawn from the heat exchanger WT2 and optionally (particularly during plant start-up) a branched-off portion of a liquid absorption fluid stream AbsM comprising an absorption medium. Chlorobenzene or dichlorobenzene, in particular ortho-dichlorobenzene, is preferably suitable as the liquid absorption medium.A gaseous residual fluid stream R1 is obtained from the scrubbing device 105-1, which comprises excess phosgene not converted in the gas-phase reaction. The excess phosgene not converted is recycled to the phosgene gas generation device 104. For this purpose, the residual fluid stream R1 is introduced via one (or more) heat exchangers WT4 into a region 114-1 of the phosgene absorption device 103-1 located above the liquid level in the sump. The phosgene absorption device 103-1 is an absorption column, and the phosgene absorption device collection region 115-1 of the phosgene absorption device 103 is the sump of the absorption column. In addition, a portion of the liquid absorbent fluid stream AbsM-1 is introduced into the absorption column.In the absorption column, excess phosgene gas contained in the gaseous residual fluid stream R1 and not converted in the gas-phase reaction is then absorbed by the liquid absorption medium. A liquid recycle medium comprising absorbed phosgene with a mass mP and the absorption medium with a mass m-AbsM collects in the bottom of the absorption column. Hydrogen chloride gas (HCl-1) formed in the gas-phase reaction is discharged through a gas outlet arranged at the top of the absorption column. The hydrogen chloride gas can optionally be converted into chlorine and hydrogen in chemical or electrochemical conversion processes. The chlorine obtained can then be fed to the fresh phosgene generation device 109 to produce phosgene, where it can be converted into fresh phosgene.The liquid recycle medium present in the bottom is discharged through a bottom outlet located in the bottom of the absorption column and pumped back as recycle stream (Return) via a heat exchanger WT5 by means of a pump into a phosgene gas generation device 104, which is a desorption column. For this purpose, the recycle stream is introduced into an introduction region 111, which is a desorption column feed. The recycle stream comprises at least recycle phosgene that has been absorbed by the absorption medium. In the desorption column, the recycle phosgene is desorbed from the absorption medium to form desorbed phosgene gas. In addition, a cold, liquid phosgene fresh fluid stream (Fresh) is introduced into a phosgene gas generation device collection region 110, which is located at the top of the desorption column.The phosgene gas stream P is formed in the phosgene gas generation device collection area 110 from the desorbed phosgene gas and the phosgene gas freshly vaporized from the phosgene fresh fluid stream. This phosgene gas stream P is discharged through a gas outlet arranged at the top of the desorption column. In the bottom of the desorption column (112), a liquid desorption medium (DesM) is also formed. This liquid desorption medium essentially consists of the absorption medium used in the phosgene absorption device 103 (i.e., preferably chlorobenzene or dichlorobenzene, in particular ortho-dichlorobenzene). This liquid desorption medium is discharged through a bottom outlet arranged in the bottom of the desorption column and partially (DesM-2) is conducted by a pump PP into a heat exchanger WT2 arranged downstream of the desorption column 104. In the heat exchanger WT2, an indirect heat transfer from DesM-2 to P takes place, cooling the former and heating the latter.The heated phosgene gas stream P is divided into P1, P-2, ... Pm and passed through the phosgene flow control valve PV-1 into the mixing zone 101-1. This completes the phosgene cycle, through which excess phosgene not converted in the gas-phase reaction is recycled and made available again for generating the phosgene reaction gas stream for the gas-phase reaction. The cooled stream of desorption medium DesM-2 is passed into the scrubbing device 105-1. Before entering the heat exchanger WT2, a portion of the liquid desorption medium (DesM-1) is branched off and returned to the bottom of the desorption column via a heat exchanger WT1. The heat exchanger WT1 serves as an evaporator of the phosgene gas generation device (desorption column) 104. It is preferred that a first partial stream (DesM-1 in FIG. 2) of the liquid desorption medium (DesM) accumulating in the phosgene gas generation device (104) is passed through a first heat exchanger (WT1 in FIG.2) is returned to the phosgene gas generation device (104), wherein the first partial stream is indirectly heated in the first heat exchanger by a heat transfer medium, and a second partial stream (DesM-2 in FIG. 2) of the liquid desorption medium (DesM) is passed through a second heat exchanger (WT2 in FIG. 2), wherein the second partial stream in the second heat exchanger indirectly heats the phosgene reaction gas stream (P) and the thereby cooled second partial stream of the liquid desorption medium (DesM) is used as a component of the scrubbing liquid used in the scrubbing device.

[0089] In a preferred embodiment of the invention, the gas-phase reaction arrangements comprise gas-phase reactors. Such a gas-phase reactor preferably comprises both the mixing zone 101-1 and reaction zone 102-1, as well as the quench zone 107-1 and phase separation zone 108-1. In other words, both the mixing and reaction zone and the quench and phase separation zone are arranged in the gas-phase reactor. The respective phosgene reaction gas stream and the respective amine reaction gas stream can be introduced separately into the mixing zone of the gas-phase reactor. This has the advantage that, in the event of a pressure drop due to the susceptibility of the amine gas generation device to failure, short bursts of amine reaction gas introduced into the gas-phase reactor cannot influence or impair the supply of the respective phosgene reaction gas stream to the gas-phase reactor.If the phosgene and amine reaction gas streams were introduced together, the short bursts of amine reaction gas would be amplified many times over and negatively impact the gas-phase process. Such disruption, or amplification of unavoidable disruptions, can advantageously be prevented by the proposed separate introduction of the phosgene and amine reaction gas streams. The phosgene and amine reaction gas streams are then mixed in the mixing zone, forming a gaseous reaction mixture comprising at least the reaction components primary amine and phosgene. The gaseous reaction mixture can then be passed through a reaction zone of the gas-phase reactor, where the gas-phase reaction of the primary amine with phosgene takes place. The gas-phase reactor is preferably a tubular reactor.The tubular reactor preferably has a round (in particular circularly symmetrical) cross-section in the region of the mixing zone, reaction zone, and quench zone and is either cylindrical in shape or has sections of different cross-sections, so that the tubular reactor in the region of the mixing zone, reaction zone, and quench zone consists of cylinders of different diameters connected by conical transition pieces. The tubular reactor is preferably upright, with the phosgene reaction gas stream and the amine reaction gas stream (and the intermediate and end products formed from them) preferably passing through the tubular reactor from top to bottom. The mixing zone, reaction zone, and quench zone can be arranged in this order from top to bottom in the tubular reactor. If the tubular reactor is arranged upright, the phase separation zone is preferably arranged below the quench zone of the tubular reactor.The phase separation zone of the tubular reactor can also have a round (especially circularly symmetrical) cross-section.

[0090] FIG. 3 shows a plant for carrying out the process according to the invention, carried out in six gas-phase reaction arrangements 100-1, 100-2, 100-3, 100-4, 100-5, and 100-6 (m = 6). The plant and the process carried out therewith basically comprise the features shown or described in FIG. 1 and FIG. 2. However, the plant shown in FIG. 3 and the process carried out therewith differ from those shown in FIG. 1 and FIG. 2 essentially in that the gas-phase reaction is carried out simultaneously in six gas-phase reaction arrangements 100-1, 100-2, 100-3, 100-4, 100-5, and 100-6 (with a basically identical structure to the previously described gas-phase reaction arrangements 100). For this purpose, the plant comprises a first and a second production line connected in parallel.The first production line runs via a first production arrangement 1000-1, which comprises the gas-phase reaction arrangements 100-1, 100-2, and 100-3, a first washout column 105-1, a first absorption column 103-1, and a single desorption column 104. The second production line, in turn, runs via a second production arrangement 1000-2, which comprises the gas-phase reaction arrangements 100-4, 100-5, and 100-6, a second washout column 105-2, a second absorption column 103-2, and a single desorption column 104. Thus, m = 6 and n = 2.

[0091] The parallel connection of the first and second production lines, including the parallel connection of the gas-phase reaction arrangements 100-1, 100-2, 100-3 arranged in the first production line and the gas-phase reaction arrangements 100-4, 100-5, and 100-6 arranged in the second production line, is realized via several splitting devices (S) and combining devices (V) arranged in the system. Thus, several splitting devices S are arranged in the system, which split fluid streams passed through the respective splitting device or divert one or more parts of the passed fluid stream. In addition, several combining devices V are arranged in the system, which combine partial fluid streams passed into the respective combining device into a combined fluid stream. The splitting devices S and the combining devices V are each shown as hatched circles in FIG. 3.

[0092] Thus, the phosgene gas stream P is split into a first and a second production stream by a first splitting device S1, which is downstream of a single desorption column 104. The first production stream is fed into a first partial splitting device S2-1, where it is split into three phosgene reaction gas streams P1, P-2, and P-3. The second production stream is in turn fed into a second partial splitting device S2-2, where it is split into three phosgene reaction gas streams P-4, P-5, and P-6. Each of the gas phase reaction arrangements 100-1, 100-2, 100-3, 100-4, 100-5 and 100-6 is supplied with exactly one phosgene reaction gas stream P1, P-2, P-3, P-4, P-5, and P-6 (in this assignment), i.e., a first phosgene reaction gas stream P1 is supplied to a first gas phase reaction arrangement 100-1, a second phosgene reaction gas stream P-2 is supplied to a second gas phase reaction arrangement 100-2, etc.In addition, gas phase reaction arrangements 100-1, 100-2 and 100-3 arranged in the first production arrangement 1000-1 are obtained.

[0093] Crude residual fluid streams R-raw-1, R-raw-2, and R-raw-3 are combined in a first partial combining device Vl-1 to form a first combined crude residual fluid stream, which is fed into the first scrubbing column 105-1. Crude residual fluid streams R-raw-4, R-raw-5, and R-raw-6 obtained from the gas-phase reaction devices 100-4, 100-5, and 100-6 arranged in the second production device 100-2 are in turn combined in a second partial combining device Vl-2 to form a second combined crude residual fluid stream, which is fed into the second scrubbing column 105-2.

[0094] A first residual fluid stream R1 is obtained from the first washout column 105-1 arranged in the first production line and is fed to the first absorption column 103-1. A second residual fluid stream R-2 is obtained from the second washout column 105-2 arranged in the second production line and is fed to the second absorption column 103-2. Furthermore, the absorption medium-comprising fluid stream AbsM is split in a third splitting device S3 into a first absorption medium partial fluid stream AbsM-1, which is fed to the first absorption column 103-1, and a second absorption medium partial fluid stream AbsM-2, which is fed to the second absorption column 103-2.

[0095] A first partial recycle stream, Recycle-1, and a first hydrogen chloride stream, HCl-1, are obtained from the first absorption column 103-1, and a second partial recycle stream, Recycle-2, and a second hydrogen chloride stream, HCl-2, are obtained from the second absorption column 103-2. The first partial recycle stream, Recycle-1, and the second partial recycle stream, Recycle-2, are combined to form a combined recycle stream (Recycle) in a second combining device V2 downstream of the absorption columns 103-1 and 103-2. The combined recycle stream (Recycle) is then introduced into the single desorption column 104. In addition, a single fresh phosgene fluid stream (Fresh), which is produced in a single fresh phosgene production device 109, is introduced into the desorption column 104. In the desorption column, the phosgene gas stream P is again formed, which is discharged and fed to the first splitting device S1.The method according to the invention provides for a state parameter (X) of the.

[0096] Phosgene reaction gas stream via a setting parameter (Y), namely at least via a setting parameter (Yl) of the recycle stream (Return). The setting of the state parameter (X) of the

[0097] Phosgene gas flow (P) via the setting parameter (Yl) of the

[0098] The feedback current (return) can be adjusted by means of a

[0099] adjustment process. For this purpose, the adjustment means can comprise adjustment units that are connected to one another by signal technology. For example, the adjustment means can comprise a central control unit, an evaluation unit, and a measuring unit as adjustment units. Signal connections can generally be wireless or wired. As part of the adjustment process, the pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m) can be measured at at least one phosgene measuring point within the system using the measuring unit of the adjustment means. Measurements of the pressure (pP-1, pP-2, ... pPm) or, in general, of a state of a system under consideration can generally be carried out in one or more measuring stages or with one or more measuring devices connected in series.Connecting multiple measuring devices in series (a redundant measurement) offers the advantage that the measurement errors of all measurements are compensated, meaning a more accurate measurement can be achieved than with a single-stage measurement process or a single measuring device. Measurements can generally be performed directly or indirectly. For example, an indirect measurement can involve calculating an indirectly measured quantity to be determined based on a directly measured quantity ("soft sensor"). This is advantageous, for example, when direct measurement of the quantity to be determined is not possible.

[0100] A change in at least one pressure (pP-1, pP-2, ... pPm) can also be determined indirectly, in particular, by observing the effects that such a change has on other devices. Thus, the position of the phosgene flow control valves (PV-1, PV-2, ..., PV-m) is a particularly suitable indicator for the pressure fp-P-1, pP-2, ... pPm. If, for example, the opening degree of a phosgene flow control valve must be continually increased, this is a sure sign of a pressure increase in the gas phase reaction arrangement downstream in the flow direction: If the "backpressure" to be overcome in the gas phase reaction arrangement becomes increasingly greater, for example due to the formation of solid deposits, the opening degree of the phosgene flow control valve must be continually increased to compensate for this. Conversely, a sudden drop in pressure (e.g.by the detachment and falling off of solid deposits) can be counteracted by reducing the degree of opening of the phosgene flow control valve. A necessary change in a phosgene flow control valve is therefore a reliable indication of a change in the pressure downstream (pP-1, pP-2, ... pPm). Such a pressure change is counteracted according to the invention by using the flow rate (f-Return) of the recycle stream (Return) as the setting parameter (Yl) of the recycle stream (Return) for the state parameter (X), i.e. at least for the pressure (pP) of the phosgene gas stream (P). In the event of an increase (however determined) in at least one pressure (pP-1, pP-2, ... pPm), the flow rate (f-Return) and thus the pressure (pP) of the phosgene gas stream (P) is increased in order to ensure a sufficiently high stoichiometric phosgene excess. In the event of a drop (however determined) in at least one pressure (pP-1, pP-2, ...pPm), the setting parameter (Y), i.e. at least the setting parameter (Yl), is adjusted. In the example case of a pressure drop, the flow rate (f-Back), and thus the pressure (pP) of the phosgene gas stream (P), is reduced in order to avoid an unnecessarily high stoichiometric phosgene excess. In concrete terms, the procedure can be such that when a change in at least one pressure (pP-1, pP-2, ... pPm) is detected for the pressure (pP), a new setpoint (Y)SOLL is calculated for the setting parameter (Y) (i.e. at least for (Yl), if necessary also for (Y-2)), so that at least the flow rate f-Back, and if necessary also the flow rate f-Fresh, are adjusted.

[0101] To measure the state parameter of the phosgene gas stream as the system under consideration at at least one phosgene measuring location, one or more phosgene measuring devices belonging to the measuring unit can be arranged at the phosgene measuring locations. From the measurement of the state parameter at at least one phosgene measuring location, state information, in particular the pressure (pP) at the at least one phosgene measuring location, can be obtained. The obtained state information can then be transmitted to an evaluation unit connected by signal technology to the one or more phosgene measuring devices. The obtained state information can then be evaluated in the evaluation unit, in particular by means of the computer system according to the invention. The evaluation can then produce evaluated information, on the basis of which an action instruction is created by the evaluation unit and transmitted to the central control unit.For example, the evaluation may show that the state parameter does not correspond to a desired state parameter (in particular because the pressure present after passing through one of the phosgene flow control valves (pP-1, pP-2, ..., pPm) has changed) and therefore needs to be adjusted. In this case, the evaluation unit can be used to create an action instruction that is transmitted to the central control unit and states that and how the state parameter of the phosgene gas flow needs to be adjusted. The central control unit can then be used to adjust (set) the setting parameter of the recirculation flow. To do this, at least one measurement of the state of the recirculation fluid flow can be carried out at at least one recirculation measuring point using a measuring unit, according to the measuring principle previously explained for the state parameter of the phosgene gas flow.For this purpose, one or more feedback measuring devices belonging to the measuring unit can be used. The at least one measurement at the at least one feedback measuring location can yield setting information about the state of the feedback stream (setting parameter) at the at least one feedback measuring location, which is then transmitted to the evaluation unit. The obtained setting information can be evaluated by the evaluation unit. For example, the evaluation of the setting information can show that the state of the feedback stream at the at least one feedback measuring location (actual value) does not correspond to a desired state (setpoint value) of the feedback stream, wherein the desired state of the feedback stream is suitable for setting the state parameter of the phosgene gas stream, i.e., for setting a desired state of the phosgene gas stream.In this case, as previously explained for the state parameter, an action instruction can be generated using the evaluation unit, which can then be transmitted to the central control unit. The central control unit can then evaluate the received action instruction and adjust the desired state of the recycle stream. By setting the desired state of the recycle stream at the recycle control location as a setting parameter, the state parameter of the phosgene gas stream is adjusted. For example, via the aforementioned adjustment process with the aid of the aforementioned adjustment means, the pressure (pP) of the phosgene gas stream can be adjusted as a state parameter (X) by setting a necessary flow rate of the recycle stream as a setting parameter (Yl).

[0102] According to the invention, the adjustment of the state parameter (X) of the phosgene gas stream is a closed-loop control of the state parameter (X) of the phosgene gas stream via the adjustment parameter (Y), wherein the adjustment parameter (Y) is at least one adjustment parameter (Y1) of the recycle stream (Return), namely its flow rate (f-Return). The adjustment process is thus a closed-loop control process, and the adjustment means is a closed-loop control means for implementing the closed-loop control process. The closed-loop control or the closed-loop control process and the closed-loop control means for implementing the closed-loop control process are particularly suitable for ensuring stable reaction conditions of the gas-phase reaction in the gas-phase reaction arrangement, in particular with regard to maintaining the stoichiometric phosgene excess constant.

[0103] A state variable of a system under consideration can in principle be controlled at the same location at which a state variable of the system under consideration is also measured. In other words, control locations can in principle be located at the same locations as one or more measuring locations. In order to control a state parameter of the phosgene gas flow via a setting parameter of the recirculation flow, a central control unit can be provided which controls one or more measuring devices and one or more controllers. A controller can in principle bring about a change in the state of a system under consideration, such as a guided fluid flow. The controller can, for example, be a flow rate controller which controls a flow rate of a guided fluid flow, or a level controller which controls a fill level of a medium in a facility. For example, changes in the flow rate of a controlled fluid flow orChanges in the medium level in a specific device controlled by a level controller can be implemented in the form of a ramp. In other words, the intensity of a flow rate or level change can be increased or decreased as slowly as possible. For example, by increasing the flow rate in the form of a ramp, the overflow of a guided liquid can be prevented in a device into which the liquid is fed at the controlled flow rate, although the device itself is not designed to compensate for a sudden increase in the flow rate.

[0104] A controller can be a continuous linear controller, a nonlinear controller, or a discontinuous controller. A continuous linear controller can be a P controller, I controller, D element, PI controller, PD controller, PD2 element with conjugate complex zeros, PID controller, a state controller with state or output feedback, or a controller for multivariable systems. A nonlinear controller can be a fuzzy controller, an adaptive controller, or an extreme value controller. A discontinuous controller can be a two-point controller, a three-point controller, or a multi-point controller. A controller can generally be implemented as an analog controller, digital controller, compact controller, pneumatic controller, or universal controller. The controller is preferably a universal controller.A universal controller can be understood as a controller that can be operated as a P, PI, PD, or PID controller thanks to the flexible adjustment of its controller parameters (proportional coefficient, reset time, and derivative time). A universal controller can have standardized input and output signals, so-called standard signals. The input signal can come from a measuring system, and the output signal can act on an actuator, allowing devices from different manufacturers to be operated together in a control loop. The standard signal can be an electrical standard signal. Preferably, the electrical standard signal is a current signal. Current signals can be advantageous for long signal lines because they are less susceptible to external interference, and a voltage drop along the signal line has no effect on the current signal.

[0105] To regulate a pressure of the phosgene gas stream (P) via a flow rate of the recycle stream, the central control unit can, for example, control one or more recycle controllers and one or more phosgene pressure controllers. The central control unit is preferably a universal controller that is signal-connected to one or more measuring devices and one or more controllers. Accordingly, a pressure of the phosgene reaction gas stream can be regulated via a universal controller that is connected to one or more phosgene pressure measuring devices, phosgene pressure controllers, recycle measuring devices, and recycle controllers. A phosgene pressure controller can be understood as a controller that is suitable for regulating a pressure of the phosgene gas stream (P) at at least one phosgene control location. A recycle controller, in turn, can be understood as a controller that is suitable for regulating a flow rate of the recycle stream at at least one recycle control location.

[0106] The process according to the invention and the plant for its implementation are particularly suitable for generating a stable phosgene reaction gas stream in each of the gas-phase reaction arrangements with a constantly adjustable state parameter X (the pressure pP). This allows stable reaction conditions for the gas-phase reaction to be generated after the phosgene reaction gas streams have been introduced into the m gas-phase reaction arrangements. As a result, the process according to the invention and the plant for its implementation can be operated with greater operational stability than known processes or plants, in particular with a stoichiometric excess of phosgene as constant as possible in each of the m gas-phase reaction arrangements, which also results in higher product selectivity and space-time yield.These advantages arise in particular from the combination of the inventive adjustment, the recycling of unreacted, excess phosgene, and the separate introduction of phosgene and amine reaction gas streams into the gas-phase reaction arrangement. Thus, the plant and process according to the invention not only make it possible to reduce the overall quantities of solid by-products and subsequent product residues formed in the gas-phase reaction compared to known processes and plants, but also simultaneously compensate for pressure drops resulting from the breaking off of fundamentally unavoidable residue quantities from the walls of the gas-phase reaction arrangement.

[0107] According to the invention, each of the m or n phosgene absorption devices has a phosgene absorption device collection region (115-1, 115-2, ... 115-m or 115-n) in which a recycle medium collects. The recycle medium comprises both absorbed phosgene and the absorbent. The recycle stream is fed from the recycle medium collected in the phosgene absorption device collection region. In one embodiment of the invention, a single (i.e., n = 1) downstream absorption device (103) is assigned to the m gas-phase reaction arrangements, the recycle stream (recycle) being fed from the single absorption device (103).

[0108] According to an alternative embodiment, several absorption devices, in particular m, connected in parallel, are connected downstream of the m gas-phase reaction devices. In this case, one of the several absorption devices is assigned to one of the several gas-phase reaction devices or to a plurality of the several gas-phase reaction devices. The recycle stream is fed from the several absorption devices. In the case of several absorption devices, at least one combining device (not shown as a separate device in FIGS. 1 and 2) is provided downstream of the absorption devices. The combining device is designed to combine partial recycle streams obtained from the several absorption devices to form the recycle stream.

[0109] Thus, a single downstream phosgene gas generation device is assigned to a single absorption device, or several absorption devices connected in parallel are assigned to a single downstream phosgene gas generation device.

[0110] It is preferred that the setting parameter (Y) additionally comprises a setting parameter (Y-2) of the phosgene fresh fluid stream (Fresh), wherein the state parameter (X) is set (= regulated) in such a way that when a change is detected in (at least) one pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m), first - as described above - the flow rate (f-Return) of the recycle stream (Return) is used as the setting parameter (Yl) and then the flow rate (f-Fresh) of the introduced phosgene fresh fluid stream (Fresh) is used as the setting parameter (Y-2). In this case, the flow rate (f-Rück) of the recycle stream (Rück) is preferably adjusted by setting a first adjustment parameter (ZI) of the absorbed phosgene comprised in the recycle medium and a second adjustment parameter (Z2) of the absorption medium comprised in the recycle medium.An adaptation parameter is understood to be a parameter that is suitable for adapting the physical state of a system under consideration. The first adaptation parameter can, in principle, be any possible state variable of the absorbed phosgene contained in the recycle medium. For example, the first adaptation parameter can be a density, temperature, mass, volume, or pressure of the absorbed phosgene contained in the recycle medium. The second adaptation parameter of the absorption medium contained in the recycle medium can, in turn, be any possible state variable of the absorption medium contained in the recycle medium. For example, the second adaptation parameter can be a density, temperature, mass, volume, or pressure of the absorption medium contained in the recycle medium.Preferred is an embodiment in which the flow rate (f-Rück) of the recycle stream (Rück) is adjusted by setting a phosgene mass (mP-1, mP-2, ..., mPm or mPn) of the phosgene in the.

[0111] Recycle medium (Recycle-1, Recycle-2, ..., Recycle-m or Recycle-n) respectively comprised absorbed phosgene as a first adjustment parameter (Zl-1, Zl-2, ..., Zl-n or Zl-m) and an absorption medium mass (m-AbsM-1, m-AbsM-2, ... m-AbsM-m or m-AbsM-n) of the absorption medium respectively comprised in the recycle medium (Recycle-1, Recycle-2, ..., Recycle-m or Recycle-n) as a second adjustment parameter (Z2-1, Z2-2, ..., Z2-n or Z2-m).

[0112] It can be provided that the phosgene mass (mP-1, mP-2, ..., mPm or mPn) is adjusted by manipulating a flow rate (f-fresh) of the phosgene fresh fluid stream, and the absorbent mass (m-AbsM-1, m-AbsM-2, ..., m-AbsM-m or m-AbsM-n) is adjusted by manipulating a flow rate of the absorbent fluid stream (AbsM-1, AbsM-2, ..., AbsM-m or AbsM-n). It is also possible for the phosgene mass to be calculated by multiplying a fill level of the recycle medium in the phosgene absorption device collection area by a concentration of the absorbed phosgene in the recycle medium. In addition, it can be provided that the absorption medium mass is calculated by multiplying the said filling level of the recycle medium in the phosgene absorption device collection area and a non-phosgene component concentration of non-phosgene components comprised in the recycle medium.The recycle medium comprises both the absorbed phosgene and non-phosgene components. Non-phosgene components can be understood as all components contained in the recycle medium that are not phosgene. The non-phosgene components include, in particular, the absorption medium and, in particular, not the absorbed phosgene contained in the recycle medium.

[0113] As already mentioned, a further subject of the invention is a computer system for controlling the stoichiometric phosgene excess in the process according to the invention for producing an isocyanate, comprising: an interface unit configured to read in (periodically or continuously) data suitable for detecting a change in (at least) one pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m), and a processor configured to, upon a detected change in the pressure (pP-1, pP-2, ... pPm), use the read-in data to determine an adjusted setpoint (Y)SOLL for the setting parameter (Y) (ieat least for (Yl), if necessary also for (Y-2)) and to transmit the adjusted setpoint (Y)SOLL to an actuator in communicative connection with the processor for manipulating the setting parameter (Y) in order to counteract the detected change in pressure (pP-1, pP-2, ... pPm) (and thus the change in the stoichiometric phosgene excess).

[0114] The term "processor" as used in the terminology of the present invention corresponds to the common understanding in the art and may refer in particular to any logic circuit configured to perform basic operations of a computer or computer system and / or generally to a device configured to perform calculations or logical operations. In particular, the processor may be configured to process basic instructions that control the computer system.As an example, the processor may include at least one arithmetic logic unit (ALU), at least one floating-point unit (FPU), such as a math coprocessor or a numeric coprocessor, a plurality of registers, in particular registers configured to provide operands to the ALU and to store operation results, and a memory, such as L1 and L2 cache memory. In particular, the processor may be a multi-core processor. In particular, the processor may be or include a central processing unit (CPU). Additionally or alternatively, the processor may be or include a microprocessor, so that specifically the elements of the processor may be contained in a single integrated circuit (IC) chip.Additionally or alternatively, the processor may be or include one or more application-specific integrated circuits (ASICs) and / or one or more field-programmable gate arrays (FPGAs) or the like.

[0115] It is preferred that a self-learning algorithm is implemented on the processor for automatically comparing changes in (at least) one pressure (pP-1, pP-2, ... pPm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m) with the stoichiometric phosgene excess and, in response thereto, for automatically setting and adjusting the setting parameter (Y), wherein the algorithm is configured to optimize the process for producing an isocyanate by adjusting the setting parameter (Y) (i.e., to counteract deviations from the stoichiometric phosgene excess more quickly).

[0116] A further subject of the present invention is a computer program product which, when loaded into a memory unit of a processing unit of the computer system according to claim 19 or 20 and executed by the processor, in a method according to one of claims 1 to 18, detects changes in (at least) one pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m) (periodically or continuously) and, upon a detected change in the pressure (pP-1, pP-2, ... pPm), adapts the setpoint (Y)SOLL of the setting parameter (Y) (i.e., at least of the setting parameter (Yl), optionally also of the setting parameter (Y-2)) (i.e., calculates a new setpoint (Y)SOLL) in order to correspond to the change in the pressure (pP-1, pP-2, ... pPm). (and thus the change in the stoichiometric phosgene excess).The invention also relates to a computer-implemented method for controlling the stoichiometric excess of phosgene in the process according to the invention, comprising:.

[0117] - receiving in real time data suitable for detecting a change in (at least) one pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m), and

[0118] - when a change in pressure is detected (pP-1, pP-2, ... pPm):

[0119] Adjusting the setting parameter (Y) (ie at least the

[0120] setting parameter (Yl), if necessary also the setting parameter (Y-2)) in order to counteract the observed change in pressure (pP-1, pP-2, ... pPm) (and thus the change in the stoichiometric phosgene excess).

[0121] The plant for producing an isocyanate by reacting a primary amine with phosgene in a stoichiometric excess of phosgene by means of a gas-phase reaction, which is a further subject of the present invention, is designed to carry out the process according to the invention. The plant therefore comprises at least the devices described as necessary in connection with the process according to the invention, i.e., at least one, preferably m, amine gas generation devices, one phosgene gas generation device, m gas-phase reaction arrangements, m or n phosgene absorption devices, and adjustment means for adjusting a state parameter (X) of the m phosgene reaction gas streams via an adjustment parameter (Y), wherein the adjustment parameter (Y) is at least one adjustment parameter (Y1) of the recycle stream, and the adjustment means comprise at least the m phosgene flow control valves (PV-1, PV-2, ...)., PV-m) and also the computer system according to the invention. Each amine gas generation device is designed to generate an amine reaction gas stream comprising primary amine. The phosgene gas generation device is designed to generate a phosgene gas stream and divide it into m phosgene reaction gas streams. The gas phase reaction arrangements are designed to combine the generated amine reaction gas streams and the generated phosgene reaction gas streams during the gas phase reaction of the primary amine with phosgene, and to provide isocyanate crude product fluid streams and residual fluid streams obtained therefrom, comprising produced isocyanate, wherein the residual fluid streams comprise excess phosgene not reacted in the gas phase reaction.Each phosgene absorption device is designed to receive an absorption medium fluid stream comprising the absorption medium and the residual fluid stream, further designed to absorb the unreacted, excess phosgene by the absorption medium to obtain phosgene to be recycled, and to provide a recycle stream comprising phosgene to be recycled. The phosgene gas generation device is designed to receive a fresh phosgene fluid stream and the recycle stream comprising phosgene to be recycled, and to desorb the phosgene to be recycled from the absorption medium to obtain desorbed phosgene. The phosgene gas stream is formed by the desorbed phosgene and phosgene obtained from the fresh phosgene fluid stream.The adjustment means for adjusting a state parameter (X) of the phosgene reaction gas stream via an adjustment parameter (Y) of the recycle stream is particularly suitable for ensuring stable reaction conditions of the gas phase reaction, in particular a stable stoichiometric phosgene excess, in the gas phase reaction arrangement.

[0122] Features, advantages and properties of the system according to the invention correspond to the features, advantages and properties of the method according to the invention, and vice versa.

[0123] FIG. 4 shows a plant for carrying out the process according to the invention with the features shown and described in FIG. 1 and FIG. 2. FIG. 4 also shows an exemplary adjustment means arranged in the plant for adjusting a pressure pP of the phosgene reaction gas stream P as state parameter X via the flow rate f-Return of the recycle stream Rück and the flow rate f-Fresh of the fresh phosgene stream Frisch as adjustment parameters Y1 and Y2. For this purpose, the control means comprises a central control unit, which is a universal controller UC, which is signal-connected to an evaluation unit, which comprises the computer system according to the invention (not shown in FIG. 4), a measuring unit, which comprises a plurality of measuring devices, and to a plurality of controllers. Measuring devices and controllers are shown as labeled circles in FIG. 4. Solid arrows indicate the flow andDirection of fluid flows within the system, dashed arrows indicate control loop information running between the universal controller UV and the individual measuring devices and controllers.

[0124] As shown in FIG. 4, the flow rate f-fresh of the fresh phosgene fluid stream Fresh is manipulated via a fresh flow rate controller FO. This adjusts a phosgene mass mP of the absorbed phosgene contained in the recycle medium as the first adjustment parameter ZI. The phosgene mass mP can be calculated by multiplying a fill level L-back of the recycle medium and a concentration xP of the absorbed phosgene in the recycle medium. The fill level L-back depends on a first fill level L-back-1 of a first sump 115-1 of the first absorption column 103-1 and a second fill level L-back-2 of a second sump 115-2 of the second absorption column 103-2. The concentration xP of the absorbed phosgene in the recycle medium in turn depends on the first concentration xP-1 of the absorbed phosgene in the first recycle medium and on the second concentration xP-2 of the absorbed phosgene in the second recycle medium.The flow rates of the recycle media Recycle-1 and Recycle-2 are controlled by the associated flow rate controllers F11 and F21. A1 and A2 denote phosgene analyzers.

[0125] In addition, the flow rate of the first absorption medium partial fluid stream AbsM-1 is manipulated by a first flow rate controller F12, and the flow rate of the second absorption medium partial fluid stream AbsM-2 is manipulated by a second flow rate controller F22. This adjusts an absorption medium mass m-AbsM of the absorption medium contained in the recycle medium as a second adjustment parameter Z2. The absorption medium mass m-AbsM can in turn be calculated by multiplying the fill level L-back of the recycle medium by a non-phosgene component concentration l-(xP) of the non-phosgene components in the recycle medium.

[0126] The calculation is based on the objective that a difference A mP between an actual value (PV) and a setpoint value (SP) of the phosgene mass mP in the return medium (RM) should be as small as possible, and the following calculation principles: mp — Trip py Trip P 0 (Equation 1),

[0127] PRM,PV ~ PRM,SP (equation 3 identical densities, temperature independent),

[0128] On P — A rück 0 p RM x L rück PV x P PV L r ack,sp X P,SP (Equation 4),

[0129] If the actual value xp-PV corresponds to the setpoint xP-SP, then both an absorption medium control loop and a decoupled phosgene control loop act as level controllers. To this end, a first level controller LI, which is arranged at the sump 215-1 of the first absorption column 103-1, and a second level controller L2, which is arranged at the sump 115-2 of the second absorption column 103-2, move the levels L-back-1 and L-back-2 in a direction necessary for control. If it turns out that the levels L-backl and L-back-2 are too low, they are increased, and so on. If, in turn, the actual value L-back-PV corresponds to the setpoint L-back-SP, then the absorption medium and phosgene control loops both act as concentration controllers, i.e. they move the concentrations xP-1 and xP-2 in a direction necessary for control.

[0130] By adjusting the phosgene mass mP as the first adjustment parameter ZI and the absorption medium mass m-AbsM as the second adjustment parameter Z2, the flow rate f-Rück of the recycle stream Rück is adjusted. The flow rate f-Rück of the recycle stream Rück is tapped by means of a recycle controller FC at the inlet of the single desorption column 104. By adjusting the flow rate f-Rück of the recycle stream Rück, the pressure pP of the gas formed in the single desorption column 104

[0131] Phosgene reaction gas stream P is adjusted using the pressure regulator PO.

Claims

Patent claims 1. A process for producing an isocyanate by reacting a primary amine with phosgene in a stoichiometric excess of phosgene by means of a gas phase reaction in a plant comprising m gas phase reaction arrangements (100-1, 100-2, ..., 100-m), where m is a natural number in the range from 2 to 6, the process comprising: (i) generating m amine reaction gas streams (Al, A-2, ..., Am), each comprising a primary amine; (ii) generating a phosgene gas stream (P) comprising phosgene at a pressure (pP) in a phosgene gas generating device (104) and dividing the phosgene gas stream (P) comprising phosgene into m phosgene reaction gas streams (Pl, P-2, ... Pm); (iii) Combining one of the amine reaction gas streams (Al, A-2, ..., Am) and in each case one of the m phosgene-comprising phosgene reaction gas streams (Pl, P-2, ... Pm) in each case in one of the gas phase reaction arrangements (100-1, 100-2, ..., 100-m), wherein the m phosgene reaction gas streams (Pl, P-2, ... Pm) pass through one of m phosgene flow control valves (PV-1, PV-2, ..., PV-m) for adjusting the stoichiometric phosgene excess before entering the m gas phase reaction arrangements (100-1, 100-2, ..., 100-m), and carrying out the gas phase reaction to obtain m isocyanate-comprising isocyanate crude product fluid streams (IC-1, IC-2, ..., IC-m) and m residual fluid streams (Rl, R-2, ..., Rm), wherein the m residual fluid streams (Rl, R-2, ..., Rm) each comprise excess phosgene not reacted in the gas phase reaction; (iv) returning the unreacted excess phosgene to the phosgene gas generating device (104), namely by (iv-1) optionally, combining the m residual fluid streams (Rl, R-2, ..., Rm) into n residual fluid streams (Rl, R-2, ... Rn), where n is a natural number in the range from 1 to m-1, (iv-2) introducing each of the residual fluid streams into a respective phosgene absorption device (103-1, 103-2, ... 103-m or 103-n) each having a phosgene absorption device collection region (115-1, 115-2, ... 115-m or 115-n), wherein in each of the Phosgene absorption devices (103-1, 103-2, ... 103-m or 103-n) are additionally introduced in each case an absorption medium fluid stream (AbsM-1, AbsM-2, ..., AbsM-m or AbsM-n), each containing an absorption medium (AbsM), with absorption of the excess phosgene not converted in the gas phase reaction by the absorption medium, wherein in each phosgene absorption device collection region (115-1, 115-2, ... 115-m or 115-n) an absorbed phosgene and the recycle medium comprising the absorption medium (Recycle-1, Recycle-2, ..., Recycle-m or Recycle-n) are collected, from which, if necessary after combining several recycle media (Recycle-1, Recycle-2, ..., Recycle-m or Recycle-n), a recycle-comprising phosgene recycle stream (Recycle) is fed, and (iv-3) introducing the recycle stream (Recycle) at a flow rate (f-Recycle) into the phosgene gas generation device (104) with desorption of the phosgene to be recycled from the absorption medium to obtain desorbed phosgene and a liquid desorption medium (DesM), wherein the phosgene gas stream (P) generated in the phosgene gas generation device (104) comprises the desorbed phosgene and a phosgene fresh fluid stream (Fresh) additionally introduced into the phosgene gas generation device (104) at a flow rate (f-Fresh), and wherein, to ensure the stoichiometric phosgene excess in each of the m gas-phase reaction arrangements (100-1, 100-2, ..., 100-m)) a state parameter (X) of the phosgene gas stream (P) is set via a setting parameter (Y), wherein the pressure (pP) serves as the state parameter (X) and wherein the setting parameter (Y) comprises at least one setting parameter (Yl) of the recirculation stream (Rück), namely such that when a change in a pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m) is detected, the flow rate (f-Rück) of the recirculation stream (Rück) is used as the setting parameter (Yl).

2. The method according to claim 1, wherein step (i) is carried out in m amine gas generating devices (106-1, 106-2, ..., 106-m).

3. The process according to claim 1, wherein in step (i) an amine reaction gas stream (A) is first generated in an amine gas generation device (106), which amine reaction gas stream is then divided into the m amine reaction gas streams (Al, A-2, ..., Am).

4. A method according to any one of claims 1 to 3, wherein each of the m gas phase reaction arrangements (100-1, 100-2, ..., 100-m) comprises a reaction device (100a-l, 100a-2, ..., 100a-m) and one of the Reaction device has a separation device (100b-l, 100b-2, ..., 100b-m) arranged downstream of the process.

5. The process according to claim 4, wherein the reaction device (100a-1, 100a-2, ..., 100a-m) comprises a mixing zone (101-1, 101-2, ..., 101-m) and a reaction zone (102-1, 102-2, ..., 102-m) downstream of the mixing zone, wherein the phosgene reaction gas stream (Pl, P-2, ..., Pm) and the amine reaction gas stream (A1, A-2, ..., Am) are mixed in the mixing zone, and wherein the gas phase reaction of the primary amine with phosgene takes place in the reaction zone (102-1, 102-2, ..., 102-m) to obtain a reaction product mixture (RP-1, RP-2, ..., RP-m).

6. The process according to claim 4 or 5, wherein the separation device (100b-1, 100b-2, ..., 100b-m) comprises a quench zone (107-1, 107-2, ..., 107-m) and a phase separation zone (108-1, 108-2, ..., 108-m) arranged downstream of the quench zone, wherein in the quench zone the reaction product mixture (RP-1, RP-2, ..., RP-m) is cooled by contacting it with a quench liquid to obtain a quench mixture, and wherein in the phase separation zone (108-1, 108-2, ..., 108-m) a phase separation of the resulting quench mixture into the isocyanate crude product fluid stream (IC-1, IC-2, ..., IC-m) and an additional crude residual fluid stream comprising isocyanate (R-raw-1, R-raw-2, ..., R-raw-m).

7. The process according to claim 6, wherein the crude residual fluid stream (R-raw-1, R-raw-2, ..., R-raw-m) comprising additional isocyanate obtained from the phase separation zone (108-1, 108-2, ..., 108-m) is fed to a respective washing device (105-1, 105-2, ..., 105-m) arranged downstream of the phase separation zone, in which washing device the additional isocyanate is washed out of the crude residual fluid stream (R-raw-1, R-raw-2, ..., R-raw-m) with a washing liquid to obtain the respective residual fluid stream (Rl, R-2, ..., Rm).

8. The method according to claim 7, wherein a first partial stream of the liquid desorption medium (DesM) is returned to the phosgene gas generation device (104) via a first heat exchanger, the first partial stream being indirectly heated in the first heat exchanger by a heat transfer medium, and a second partial stream of the liquid desorption medium (DesM) is passed through a second heat exchanger, the second partial stream indirectly heating the phosgene reaction gas stream (P) in the second heat exchanger, and the second partial stream of the liquid desorption medium (DesM) cooled in the process is used as a component of the scrubbing liquid.

9. The method according to any one of claims 1 to 8, wherein the phosgene gas generating device (104) has an introduction region (111) for introducing the recycle stream (Return) and a phosgene gas generating device collection region (110), wherein the desorbed phosgene and the phosgene obtained from the phosgene fresh fluid stream (Fresh) collect in the phosgene gas generating device collection region (110) to form the phosgene reaction gas stream (P).

10. The process according to claim 9, wherein a desorption column is used as the phosgene gas generating device (104), which desorption column has a sump (112) in which the liquid desorption medium (DesM) collects.

11. The process according to any one of claims 1 to 10, wherein an absorption column is used as the phosgene absorption device (103-1, 103-2, ... 103-m or 103-n), wherein the phosgene absorption device collection region (115-1, 115-2, ... 115-m or 115-n) is arranged in a bottom of the absorption column.

12. The method according to one of claims 1 to 11, wherein the setting parameter (Y) additionally comprises a setting parameter (Y-2) of the phosgene fresh fluid stream (Fresh), wherein the state parameter (X) is set such that when a change in a pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m) is detected, first the flow rate (f-Return) of the recycle stream (Return) is used as the setting parameter (Yl) and then the flow rate (f-Fresh) of the introduced phosgene fresh fluid stream (Fresh) is used as the setting parameter (Y-2).

13. The method according to claim 12, wherein the flow rate (f-Return) of the recycle stream (Return) is adjusted by setting a first adjustment parameter (ZI) of the absorbed phosgene comprised in the recycle medium and a second adjustment parameter (Z2) of the absorbent comprised in the recycle medium.

14. The method according to claim 13, wherein the flow rate (f-Return) of the recycle stream (Return) is adjusted by setting a phosgene mass (mP-1, mP-2, ..., mPm or mPn) of the absorbed phosgene comprised in the recycle medium (Return-1, Return-2, ..., Return-m or Return-n) as the first adjustment parameter (Zl-1, Zl-2, ..., Zl-n or Zl-m) and an absorption medium mass (m-AbsM-1, m-AbsM-2, ... m-AbsM-m or m-AbsM-n) of the absorption medium comprised in the recycle medium (Return-1, Return-2, ..., Return-m or Return-n) as the second adjustment parameter (Z2-1, Z2-2, ..., Z2-n or Z2-m).

15. The method according to claim 14, wherein the phosgene mass (mP-1, mP-2, mPm or mPn) is adjusted by manipulating the flow rate (f-fresh) of the phosgene fresh fluid stream (Fresh), and wherein the absorbent mass (m-AbsM-1, m-AbsM-2, ... m-AbsM-m or m-AbsM-n) is adjusted by manipulating a flow rate (f-AbsM-1, f-AbsM-2, ..., f-AbsM-m or f-AbsM-n) of the absorbent fluid stream (AbsM-1, AbsM-2, ..., AbsM-m or AbsM-n).

16. The method according to claim 14, wherein the recycle medium (Recycle-1, Recycle-2, ..., Recycle-m or Recycle-n) comprises the absorbed phosgene and non-phosgene components, wherein the non-phosgene components comprise the absorption medium, wherein the phosgene mass (mP-1, mP-2, ..., mPm or mPn) is calculated in each case by multiplying a fill level (L-recycle-1, L-recycle-2, ..., L-recycle-m or L-recycle-n) of the recycle medium in the phosgene absorption device collection area (115-1, 115-2, ..., 115-m or 115-n) and a concentration (xP-1, xP-2, ..., xPm or xPn) of the absorbed phosgene in the recycle medium (Recycle-1, Recycle-2, ..., Recycle-m or Recycle-n). and wherein the absorption medium mass (m-AbsM-1, m-AbsM-2, ... m-AbsM-m or m-AbsM-n) is determined in each case by multiplying the fill level (L-rück-1, L-rück-2, ..., L-rück-m or L-rück-n) of the recycle medium (Rück-1, Rück-2, ..., Rück-m or Rück-n) in the phosgene absorption device collecting area (115-1, 115-2, ... 115-m or115-n) and a non-phosgene component concentration of the non-phosgene components in the recycle medium (Recycle-1, Recycle-2, ..., Recycle-m or Recycle-n).

17. Method according to one of claims 1 to 16, in which - the absorption agent (AbsM) is selected from decahydronaphthalene, toluene, xylene, chlorobenzene, para-dichlorobenzene, orü7?o-dichlorobenzene, chlorotoluene, chloronaphthalene or mixtures thereof; preferably the absorption agent is ort7?o-dichlorobenzene, and / or - the primary amine is selected from toluenediamine, diphenylmethanediamine, xylylenediamine, 1,5-pentanediamine, 1,6-hexamethylenediamine, isophoronediamine, diaminodicyclo- hexylmethane or mixtures thereof; preferably the primary amine is toluenediamine.

18. A process according to any one of claims 1 to 17, wherein a target value in the range from 10% of theory to 400% of theory is set for the stoichiometric excess of phosgene.

19. A computer system for controlling the stoichiometric phosgene excess in a process for producing an isocyanate according to any one of claims 1 to 18, comprising: an interface unit configured to read in data suitable for detecting a change in a pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m), and a processor configured to calculate an adjusted setpoint (Y)SOLL for the setting parameter (Y) with the read-in data when a change in the pressure (pP-1, pP-2, ... pPm) is detected, and to transmit the adjusted setpoint (Y)SOLL to an actuator in communicative connection with the processor for manipulating the setting parameter (Y) in order to adapt the detected change in the pressure (pP-1, pP-2, ... pPm).

20. Computer system according to claim 19, wherein a self-learning algorithm is implemented on the processor for automatically comparing changes in a pressure (pP-1, pP-2, ... pPm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m) with the stoichiometric phosgene excess and, in response thereto, for automatically setting and adjusting the setting parameter (Y), wherein the algorithm is configured to optimize the process for producing an isocyanate by adjusting the setting parameter (Y).

21. Plant for producing an isocyanate by reacting a primary amine with phosgene in a stoichiometric excess of phosgene by means of a gas phase reaction, wherein the plant is designed to carry out a process according to one of claims 1 to 18 and further comprises the computer system according to claim 19 or 20.

22. A computer program product which, when loaded into a memory unit of a computing unit of the computer system according to claim 19 or 20 and executed by the processor, in a method according to one of claims 1 to 18, detects changes in a pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) present after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m) and, when a change in the pressure (pP-1, pP-2, ... pPm) is detected, adapts the setpoint (Y)SOLL of the setting parameter (Y) in order to counteract the change in the pressure (pP-1, pP-2, ... pPm).

23. A computer-implemented method for controlling the stoichiometric excess of phosgene in a process for producing an isocyanate according to any one of claims 1 to 18, comprising: - receiving in real time data suitable for detecting a change in a pressure (pP-1, pP-2, ... pPm) of the m phosgene reaction gas streams (Pl, P-2, ... Pm) after passing through the m phosgene flow control valves (PV-1, PV-2, ..., PV-m), and - if a change in pressure (pP-1, pP-2, ... pPm) is detected: adjust the setting parameter (Y) to counteract the change in pressure (pP-1, pP-2, ... pPm) detected.

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