Method for the manufacture of phosgene

Abstract

The invention relates to a method for producing phosgene by reacting chloride and CO over an activated carbon catalyst, characterized in that the activated carbon catalyst is dried by reducing its water content. The drying comprises the following steps: a) contacting the catalyst with an inert gas stream, b) determining the residual moisture of the catalyst by measuring the moisture in the exhaust gas stream, c) completing the drying process once the desired moisture content in the exhaust gas stream is reached by switching from the inert gas to the reaction gases, and optionally d) heating the catalyst bed and / or the inert gas during the drying process.

Description

[0001] The invention relates to a process for the production of phosgene by gas phase reaction of carbon monoxide and chlorine in the presence of a catalyst, in particular in the presence of an activated carbon catalyst, in which the catalyst is dried by reducing the water content.

[0002] Phosgene is an important auxiliary material in the production of intermediates and end products in almost all branches of chemistry. In particular, phosgene is a widely used reagent for industrial carbonylation, for example in the production of isocyanates or organic acid chlorides. Its largest application by volume is the production of diisocyanates for polyurethane chemistry, especially toluene diisocyanate or 4,4-diisocyanate diphenylmethane.

[0003] Phosgene is produced on an industrial scale in a catalytic gas-phase reaction of carbon monoxide and chlorine in the presence of a catalyst, for example, an activated carbon catalyst, according to the reaction equation: CO + Cl₂ ⇄ 2COCl₂. The reaction is strongly exothermic with a reaction enthalpy ΔH of -107.6 kJ / mol. The reaction is usually carried out in a tube bundle reactor according to the process described in Ullmann's Encyclopedia of Industrial Chemistry in the chapter "Phosgene" (5th Ed. Vol. A 19, p 413 ff., VCH Verlagsgesellschaft mbH, Weinheim, 1991). This process uses granular catalyst with a particle size in the range of 3 to 5 mm in tubes with a typical inner diameter between 35 and 70 mm, typically between 39 and 45 mm. The reaction begins at temperatures of 40 to 50 °C, but rises to 400 °C and more in the pipes and then drops rapidly again.The reaction typically uses an excess of carbon monoxide to ensure that all the chlorine is converted and largely chlorine-free phosgene is produced, as chlorine can lead to undesirable side reactions when the phosgene is subsequently used. The reaction can be carried out at room pressure, but is usually performed at an overpressure of 200–600 kPa (2–6 bar). Within this pressure range, the phosgene formed can be condensed after the reactor using cooling water or other heat transfer fluids, such as organic heat transfer fluids, thus enabling more economical operation of the condenser.

[0004] Thermal management in the reactor is one of the biggest challenges in phosgene production, essential for a safe and economical process. Generally, various methods are available to dissipate the heat of reaction. The primary influence on thermal management comes from the specific reactor design and catalyst selection, or a specific catalyst design that enables rapid removal of the resulting heat of reaction by reducing heat and mass transport constraints.

[0005] The prior art describes a variety of reactor designs. Generally, the contact tubes of the tube bundle reactor are surrounded by a heat transfer fluid that carries away the resulting heat of reaction from the reactor. It has been shown that a crossflow across the contact tubes improves heat dissipation. To achieve this, baffles are typically installed in the reactor, which, through a meandering flow pattern of the heat transfer fluid, allow a crossflow to the contact tubes.

[0006] For example, a typical large reactor for the production of phosgene is described in the international patent application WO 03 / 072237 A1.

[0007] The reactor throughput can be determined by the so-called surface or phosgene load of the reactor, which is defined as the amount of phosgene converted per unit time (usually expressed in kg / s), based on the cross-sectional area of ​​the catalyst, i.e., the sum of the internal cross-sectional areas of the catalytic contact tubes (usually given in m²). To control the heat of reaction, surface loads between 0.5 and 2 kg phosgene / m² s are therefore typically described in the prior art.

[0008] The term "reactor" in this application encompasses all parts of a plant in which the chemical conversion of carbon monoxide and chlorine gas to phosgene takes place. Often, a reactor in this sense is a single component defined by a reactor vessel. However, a reactor within the meaning of this application may also comprise two or more components with separate reactor vessels, arranged, for example, in series. In this case, the surface loading refers to the total conversion, i.e., the phosgene flow rate leaving the last reactor component, e.g., the last reactor vessel.

[0009] International patent application WO 2010 / 076208 A1 discloses an optimized arrangement of contact tubes that results in uniform heat transfer coefficients across the reactor cross-section at the interface between the contact tubes and the heat transfer fluid. This was achieved by a specific orientation of the heat transfer fluid flow paths in each reactor cross-section. In such a reactor with an optimized heat flow profile, surface loads of up to 2.74 kg phosgene / m²s could be achieved.

[0010] Besides the reactor, the catalyst has a major influence on the efficiency of the respective phosgene production process.

[0011] As described in the publication by Mitchell et al., "Selection of carbon catalyst for the industrial manufacture of phosgene," Catal. Sci. Technol., 2012, Vol. 2, pp. 2109–2115, the catalyst is deactivated or burned off during phosgene synthesis, necessitating plant shutdown and catalyst replacement after a certain operating period. This can be caused, firstly, by the oxidation of carbon by traces of oxygen in the supplied chlorine gas. Secondly, at higher temperatures, typically above 300 °C, a chlorine reaction with the activated carbon catalyst can also occur, leading to the formation of volatile carbon tetrachloride (CCl₄). Mitchell et al. evaluated seven commercially available carbon catalysts recommended by various suppliers for phosgene production. The low activity of some catalysts was attributed to their mesoporous nature.

[0012] Furthermore, Christopher J. Mitchell et al. describe in, "Selection of carbon catalyst for the industrial manufacture of phosgene", Catal. Sci. Technol., 2012, Volume 2, p. 2109, that carbon catalysts are used for the production of phosgene.

[0013] In this publication, various carbon catalysts (porous materials) are tested. Two commercially available catalysts, namely Chemviron Solcarb 208C DM and Donau Supersorbon K40, exhibit the best catalytic activity.

[0014] Activated carbons derived from natural sources such as coconut shells, wood, and olive pits are typically used as catalysts. The porous structure of such activated carbons contains primarily micropores (< 2 nm) and macropores (> 50 nm). The main reaction takes place in the micropores, which represent a high surface area, while the macropores are responsible for transporting raw materials within and the product out of the catalyst particle.

[0015] The activity of the catalyst depends on a suitable ratio of transport and reaction pores, and thus of macropores to micropores.

[0016] Activated carbon is characterized by a very high specific surface area. This is mainly due to its porous structure and especially to its micropores (< 2 nm) (US9174205).

[0017] It is known that water can condense in the pores at certain humidity levels. As described in "https: / / www.arnold-chemie.de / zeolithe / temperaturadsorbens-feuchte / ", activated carbon is essentially hydrophobic; however, above a relative humidity of 60%, pore condensation reaches a level that prevents the successful use of the carbon as an adsorbent.

[0018] The adsorption of water vapor onto activated carbon follows the rare isotherm type V, with initially minimal adsorption followed by rapid pore filling. Patent literature, particularly CN204417136, CN107906925, and US2016 / 0137511, describes various devices for drying activated carbon during its production and before its use, for example, as a catalyst in reactors.

[0019] DE 33 27 274 A1 describes a new process for the production of phosgene by reacting chlorine with carbon monoxide in the presence of activated carbon as a catalyst, with simultaneous generation of steam under a two-stage reaction procedure, wherein the reaction is carried out in a first stage at over 250°C with the conversion of 95 to 98% of the chlorine used, the heat generated in this process is used to generate steam at a pressure of 5 to 50 bar and finally the reaction is completed in a second reaction stage at 50 to 100°C.

[0020] US 2020 / 222871 A1 describes a method for cleaning a phosgene-containing device by pressurizing it with ammonia gas to a constant pressure. This effectively decomposes phosgene residues in the device being cleaned.

[0021] WO 2015027686 A1 describes a catalyst for the production of phosgene and a process for producing phosgene using the catalyst. The process comprises: modifying the surface of an activated carbon coating / structural catalyst made of foamed silicon carbide with an alkali metal salt; filling the catalysts with different thicknesses of the activated carbon coating and different amounts of the alkali metal salt in different sections in the axial direction of the multi-tube fixed-bed reactor; and producing phosgene using Cl₂ and CO.

[0022] CN 107906925 A1 describes a regenerative activated carbon drying plant. The plant comprises a spiral conveyor, a hot air flow generation unit, a first separation unit, and a drying and separation unit.

[0023] The reaction tubes for phosgene synthesis are generally filled with activated carbon under ambient air conditions, thus subjecting them to the prevailing humidity. During the filling process, the activated carbon can bind water and potentially condense. When the reactor is started up, this condensed water can react with chlorine and CO to form hydrochloric acid and cause corrosion, or with chlorine to form solid deposits in the reactor outlet and downstream systems. Furthermore, deactivation has been observed when a moist catalyst is used.

[0024] The aim of this application is to overcome the disadvantages of the state of the art.

[0025] The technical problem underlying the present invention is to provide a process for the production of phosgene that prevents corrosion damage in plant components, reduces hydrochloric acid formation, avoids solid deposits at the reactor outlet and provides increased catalyst activity of the catalyst used.

[0026] This technical problem is solved by the method according to claim 1.

[0027] The invention therefore relates to a process for the production of phosgene by reacting chlorine and CO on an activated carbon catalyst, characterized in that the activated carbon catalyst is dried by reducing the water content, wherein the drying comprises the following steps: a) Contacting the catalyst with an inert gas stream b) Determining the residual moisture of the catalyst by determining the moisture in the exhaust gas stream c) Terminating the drying process after reaching the desired moisture in the exhaust gas stream by switching from the inert gas to the reaction gases and optionally d) heating the catalyst bed and / or the inert gas during the drying process. where drying takes place in the phosgene production reactor and the residual moisture of the catalyst is determined by measuring the dew point of the exhaust gas stream at the reactor outlet, and drying is carried out at least down to a dew point of -20°C.

[0028] In other words, the invention relates to a process for the production of phosgene by reacting chlorine and CO on an activated carbon catalyst, characterized in that the activated carbon catalyst is dried by reducing the water content before being brought into contact with the reaction gases chlorine and CO, wherein the drying comprises the following steps: a) Contacting the catalyst with an inert gas stream b) Determining the residual moisture of the catalyst by determining the moisture in the exhaust gas stream c) Terminating the drying process after reaching the desired moisture in the exhaust gas stream by switching from the inert gas to the reaction gases Cl and CO and optionally d) heating the catalyst bed and / or the inert gas during the drying process. where drying takes place in the phosgene production reactor and the residual moisture of the catalyst is determined by measuring the dew point of the exhaust gas stream at the reactor outlet, and drying is carried out at least down to a dew point of -20°C.

[0029] Surprisingly, it was found that the catalyst drying process described above largely prevents corrosion damage to the reactor and downstream plant components while simultaneously achieving higher catalyst activity. Furthermore, significantly fewer solid deposits form at the reactor outlet, allowing for longer cleaning intervals or faster cleaning procedures. While the actual drying step does require additional time and may extend regular plant downtime for maintenance and repairs, intensive catalyst drying is typically omitted or only performed at the catalyst supplier's facility at the end of the synthesis process.Surprisingly, it has been shown that the effects achieved through drying, namely a significantly increased catalyst activity and an extended reactor lifetime coupled with reduced cleaning effort during shutdown, more than compensate for the additional drying step and thus justify the additional effort.

[0030] The process for producing phosgene by reacting chlorine and CO on an activated carbon catalyst is characterized by the reduction of the water content of the activated carbon, with the drying process comprising several steps.

[0031] In step a) of the drying process according to the present invention, the catalyst is brought into contact with an inert gas stream.

[0032] In general, any gas that is inert with respect to the catalyst can be used for this purpose. In the context of the present invention, "inert" means that the gas itself does not react with the catalyst and thus no structural or chemical changes are caused by bringing the catalyst into contact with the inert gas.

[0033] Preferably, the inert gas used is Ar, CO₂, CO, N₂, and air or mixtures thereof. More preferably, the inert gas used is CO, N₂, and air or mixtures thereof. Particularly preferably, N₂ and air or mixtures thereof are used. For example, air is used.

[0034] In general, the inert gas used can be of any quality, as long as it fulfills the purpose of drying the activated carbon catalyst by absorbing water. Such a gas is referred to as a dry inert gas within the scope of the present invention. Preferably, the inert gas used has a water content of 0.5 mg / m³ to 300 mg / m³, more preferably a water content of 0.5 mg / m³ to 120 mg / m³, more preferably a water content of 0.5 mg / m³ to 40 mg / m³, and particularly preferably a water content of 0.5 mg / m³ to 10 mg / m³. For example, a water content of 12 mg / m³, 9 mg / m³, 7 mg / m³, 5 mg / m³, 3 mg / m³, 2 mg / m³, or 1 mg / m³ is acceptable.

[0035] Contacting within the scope of the present invention means contact between the solid catalyst phase and the gas phase of the inert gas, wherein contacting includes any form of contact between the two phases such as flowing past or through the catalyst bed as well as flowing through the catalyst particles.

[0036] It is particularly advantageous if the dried catalyst no longer comes into contact with the ambient atmosphere between the container in which the catalyst is brought into contact with an inert gas stream.

[0037] Drying takes place in the phosgene production reactor.

[0038] In general, the contact of the catalyst with an inert gas stream, also referred to as drying or drying process within the scope of the present invention, can take place at various times in relation to the phosgene production process, for example, after the installation of a fresh, unused catalyst in a phosgene reactor before commissioning, or after a plant shutdown of the already used catalyst installed in the phosgene reactor before recommissioning, for example, after a planned (maintenance work) or unplanned (defects in plant components) plant shutdown.

[0039] Preferably, drying takes place after the installation of a new catalyst in the phosgene reactor and before commissioning.

[0040] In general, any gas flow rate sufficient to ensure effective removal of water from the catalyst and thus effectively reduces moisture can be used. Within the scope of the present invention, the gas flow rate is the volumetric flow rate of the gas.

[0041] Preferably, the volume flow rate of the inert gas used for drying is in the range of 0.05 to 1.0 Nm³ / h / pipe; more preferably, the volume flow rate of the inert gas used for drying is in the range of 0.05 to 0.8 Nm³ / h / pipe; and most preferably, the volume flow rate of the inert gas used for drying is in the range of 0.1 to 0.7 Nm³ / h / pipe. For example, 0.55 Nm³ / h / pipe.

[0042] In general, contact between the catalyst and the inert gas stream can be achieved in various ways. Preferably, contact between the catalyst and the inert gas stream is carried out continuously for the entire duration of the drying process with a nearly constant volume flow rate.

[0043] In another preferred embodiment of contacting the catalyst with the inert gas stream, the inert gas stream is brought into contact with the catalyst for a certain period of time, preferably between 1 min and 240 min, more preferably between 10 min and 180 min, and particularly preferably between 20 min and 150 min, for example for 90 min, at a nearly constant volume flow rate. Then the gas supply is interrupted and the catalyst-filled reactor is subjected to a vacuum. The applied vacuum is preferably maintained for a period of time between 1 min and 240 min, more preferably between 10 min and 180 min, and particularly preferably between 20 min and 150 min, for example for 60 min.This is followed by a renewed exposure to an inert gas stream, for example, air, for a specific period, preferably between 1 and 240 minutes, more preferably between 10 and 180 minutes, and particularly preferably between 20 and 150 minutes, for example, for 80 minutes, with a nearly constant volume flow rate. This alternation between contacting the catalyst with an inert gas stream followed by vacuum can be repeated several times. Preferably, this alternation between gas stream and vacuum is repeated up to 50 times, more preferably up to 20 times, and particularly preferably up to 10 times, for example, between 3 and 5 times.

[0044] Both contacting the catalyst with a continuous inert gas flow for the entire duration of the drying process with a constant volume flow, and alternating between a constant inert gas flow and vacuum, can be used to accelerate the drying process at elevated temperatures.

[0045] According to step d) of the present invention, the catalyst bed and / or the inert gas can optionally be heated during the drying process.

[0046] Drying is preferably carried out at an elevated temperature in the range of 60 to 170°C. More preferably at an elevated temperature in the range of 80 to 160°C, and particularly preferably at an elevated temperature in the range of 90 to 155°C, for example at 152°C. It is initially irrelevant whether the catalyst is heated to a temperature in the range of 80°C to 160°C or whether the inert gas used is preheated to a temperature of, for example, 60°C to 130°C.

[0047] In general, a combination of both heating methods—namely, heating the catalyst bed itself and heating the inert gas stream—is also conceivable. Preferably, only one of the heating methods is used. Heating the catalyst bed is particularly preferred.

[0048] The catalyst bed is preferably heated using the reactor cooling system. Generally, the achievable temperature level depends on the cooling medium used in the reactor. Various substances and mixtures can be used as fluid heat transfer fluids, which are suitable for dissipating the heat of reaction, for example, due to their heat capacity or their enthalpy of vaporization. Typically, a liquid heat transfer fluid is used, such as water, dibenzyltoluene (Marlotherm), or monochlorobenzene.

[0049] Preferably, the catalyst bed is heated using the reactor cooling system to a maximum temperature that is 5°C below the boiling point of the cooling medium at normal pressure, particularly preferably to a maximum temperature of 8°C below the boiling point of the cooling medium at normal pressure, and especially preferably to a maximum temperature of 10°C below the boiling point of the cooling medium at normal pressure, for example 12°C below the boiling point of the cooling medium at normal pressure.

[0050] According to step b) of the present invention, the residual moisture of the catalyst is determined by determining the residual moisture in the exhaust gas stream.

[0051] The residual moisture content of the catalyst is determined by measuring the dew point in the exhaust gas stream.

[0052] Dew point measurements can be performed using mobile handheld devices from CS-Instruments (DP500 / DP510) or EXTECH (HD550). These devices are equipped with a data logger and cover a measuring range of -80 °C to 50 °C. They measure the current temperature and relative humidity and then calculate the dew point.

[0053] Depending on the design, moisture measurement can be performed continuously (i.e., permanently) or discontinuously in the form of spot checks. Continuous measurement is not preferred. The drying process can take several days, as the dry gas initially becomes saturated with water as it flows through the bulk material.

[0054] Under specified storage and handling conditions, the fresh coal intended for use as a phosgene catalyst contains up to 25% water by mass. However, depending on local humidity and potentially incorrect handling during storage or installation, water contents of up to 30% by mass can occur in the fresh coal.

[0055] According to step c) of the present invention, the drying process is terminated after the desired moisture content in the exhaust gas stream has been reached.

[0056] The desired moisture content of the catalyst is particularly preferably achieved at a dew point of -25°C measured in the exhaust gas stream, and especially preferably at a dew point of -40°C.

[0057] Once the desired moisture level is reached, the change from the respective drying conditions to the respective reaction conditions is made by replacing the inert gas(es) with the reaction gases, preferably CO and Cl2.

[0058] Preferably, in the process according to the invention, the feed stream has a stoichiometric excess of carbon monoxide to chlorine of 0.001 to 50 mol%, so that an almost complete conversion of chlorine is ensured. If fluctuating chlorine concentrations are expected in the chlorine feed stream, a higher excess of carbon monoxide is preferred, but in general, for cost reasons, the excess is chosen to be as low as possible, as long as complete chlorine conversion is still ensured.

[0059] The feed stream is preferably introduced at an absolute pressure in the range of 0.5 to 20 bar. Particularly preferably, the feed stream is introduced at an overpressure, e.g., at an absolute pressure of 3 to 7 bar. The higher the pressure of the resulting reaction mixture at the reactor outlet, the more effectively the phosgene contained in the reaction mixture can be condensed. Preferably, the pressure of the reaction mixture at the reactor outlet is still high enough that the phosgene can be at least partially condensed with cooling water.

[0060] Generally, there are no restrictions regarding the catalyst, as long as it is suitable for the production of phosgene. All catalysts for the production of phosgene known in the prior art can be used, such as DONAUCARBON. Different catalysts can be used, for example, SiC catalysts.

[0061] Activated carbon catalysts are preferably used.

[0062] The DONAUCARBON catalyst is still preferred.

[0063] Alternatively, it is preferred that the catalyst preferably comprises a porous material made of carbon, micropores, and mesopores, wherein the micropores have a pore diameter of less than 2 nm and the mesopores have a pore diameter in the range of 2 to 50 nm; wherein the volume of the mesopores of the porous material is at least 0.45 ml / g. Preferably, the micropore volume is determined according to DIN 66135-2, the mesopore volume is determined according to DIN 66134, and the volume of the mesopores of the porous material is determined using the dual-isothermal non-local density functional theoretical (NLDFT) advanced pore size distribution (PSD) technique.

[0064] Preferably, the ratio of the volume of the mesopores of the porous material to the volume of the micropores of the porous material is at least 1:1, particularly preferably in the range of 1.1:1 to 6:1, particularly preferably in the range of 1.15:1 to 5:1, and particularly preferably in the range of 1.2:1 to 4:1. It is preferred that the volume of the mesopores of the porous material and the volume of the micropores of the porous material are determined using dual-isothermal NLDFT advanced PSD technology.

[0065] Preferably, the ratio of the volume of the mesopores of the porous material to the total pore volume of the porous material is at least 0.5:1, particularly preferably in the range of 0.5:1 to 0.9:1, particularly preferably in the range of 0.55:1 to 0.85:1, particularly preferably in the range of 0.6:1 to 0.8:1, and particularly preferably in the range of 0.65:1 to 0.8:1. It is preferred that the volume of the mesopores of the porous material and the total pore volume of the porous material are determined using dual-isothermal NLDFT Advanced PSD technology.

[0066] Preferably, the volume of the mesopores of the porous material is at least 0.5 ml / g.

[0067] Regarding the total pore volume of the porous material, it is preferable that it be in the range of 0.5 to 2.25 ml / g, particularly preferably in the range of 0.55 to 1.75 ml / g, and most preferably in the range of 0.65 to 1.70 ml / g. It is preferred that the total pore volume of the porous material be determined using dual-isothermal NLDFT Advanced PSD technology.

[0068] Preferably less than or equal to 40%, particularly preferably less than or equal to 30%, particularly preferably less than or equal to 25%, preferably less than or equal to 20%, particularly preferably less than or equal to 15%, preferably less than or equal to 10%, particularly preferably less than or equal to 5%, particularly preferably less than or equal to 2.5%, particularly preferably less than or equal to 1% of the total pore volume of the porous material is located in mesopores with a pore diameter of more than 20 nm.

[0069] It is preferred that the volume of the mesopores of the porous material is in the range of 0.50 to 0.54 ml / g, particularly preferably in the range of 0.51 to 0.53 ml / g, and that the ratio of the volume of the mesopores of the porous material to the total pore volume of the porous material is in the range of 0.70:1 to 0.75:1, particularly preferably in the range of 0.72:1 to 0.74:1. It is preferred that the volume of the mesopores of the porous material and the total pore volume of the porous material are determined using dual-isothermal NLDFT advanced PSD technology.

[0070] Alternatively, it is preferred that the volume of the mesopores of the porous material is in the range of 0.64 to 0.70 ml / g, particularly preferably in the range of 0.65 to 0.67 ml / g, and that the ratio of the volume of the mesopores of the porous material to the total pore volume of the porous material is in the range of 0.72:1 to 0.78:1, particularly preferably in the range of 0.73:1 to 0.76:1. It is preferred that the volume of the mesopores of the porous material and the total pore volume of the porous material are determined using dual-isothermal NLDFT advanced PSD technology.

[0071] Preferably, the volume of the micropores of the porous material, preferably determined by Dual-Isotherm NLDFT Advanced PSD technique, is at most 0.7 ml / g, particularly preferably at most 0.6 ml / g.

[0072] With regard to the BET-specific surface area of ​​the porous material, it is preferable that it be at least 500 m² / g, particularly preferably in the range of 500 to 2500 m² / g, particularly preferably in the range of 550 to 1800 m² / g, particularly preferably in the range of 600 to 1500 m² / g.

[0073] Preferably, the total specific surface area of ​​the porous material, measured according to Dual-Isotherm NLDFT Advanced PSD technique, is at least 600 m² / g, particularly preferably in the range of 650 to 2000 m² / g, and particularly preferably in the range of 700 to 1800 m² / g.

[0074] Preferably, the specific surface area of ​​the porous material, measured using Dual-Isotherm NLDFT Advanced PSD technology, is in the range of 70 to 250 m² / g, particularly preferably in the range of 80 to 170 m² / g.

[0075] Preferably, the ratio of the specific surface area of ​​the porous material formed by the mesopores to the total specific surface area of ​​the porous material is in the range of 0.07:1 to 0.40:1, particularly preferably in the range of 0.07:1 to 0.20:1.

[0076] It is preferred that the porous material is a pyrolyzed carbon aerogel.

[0077] It is preferred that the porous material be an activated pyrolyzed carbon aerogel.

[0078] Preferably, the porous material consists of 99 to 100 percent by weight, particularly preferably 99.5 to 100 percent by weight, and particularly preferably 99.9 to 100 percent by weight.

[0079] Preferably less than or equal to 0.5 percent by weight of the porous material consists of oxygen.

[0080] Preferably, the porous material consists of less than or equal to 0.5% by weight, and particularly preferably less than or equal to 0.1% by weight, of hydrogen. Preferably, less than or equal to 0.01% by weight of the porous material consists of nitrogen.

[0081] Preferably, the ash content of the porous material is less than or equal to 0.1 wt%, particularly preferably less than or equal to 0.08 wt%, particularly preferably less than or equal to 0.05 wt%, particularly preferably less than or equal to 0.03 wt%, particularly preferably less than or equal to 0.025 wt%, preferably less than or equal to 0.01 wt%, particularly preferably less than or equal to 0.0075 wt%, particularly preferably less than or equal to 0.005 wt%, particularly preferably less than or equal to 0.001 wt%, based on the weight of the porous material, as calculated from the total reflectance X-ray fluorescence data.

[0082] Preferably, the porous material has an impurity content of elements with atomic numbers from 11 to 92, measured by total reflection X-ray fluorescence (TXRF), of less than 500 ppm, particularly preferably less than 300 ppm, particularly preferably less than 200 ppm, particularly preferably less than 100 ppm.

[0083] In general, the process according to the invention can be carried out in any tube bundle reactor suitable for the production of phosgene by gas phase reaction of carbon monoxide and chlorine in the presence of a carbon catalyst.

[0084] A typical phosgene reactor that can be used for the process of the present invention is disclosed, for example, in international patent application WO 03 / 072273. The reactor has a bundle of contact tubes that are sealed in upper and lower tube sheets parallel to each other in the longitudinal direction of the reactor. Hoods are provided at both ends of the reactor, in which gas distributors are arranged. A liquid heat exchanger medium is used in the space between the contact tubes, and baffles are arranged horizontally. The reactor is de-tubed in the area of ​​the openings, since insufficient cooling of the contact tubes would be possible in these areas due to the transition of the coolant flow from a transverse to a longitudinal flow. Nozzles or partial annular channels are provided for the supply and discharge of the heat exchanger medium. Optionally, a compensator can be used on the reactor shell to compensate for thermal loads.

[0085] A suitable reactor can be divided longitudinally along the contact tubes into at least two cooling zones, which can be separated from each other, for example, by intermediate plates. Different heat transfer fluids can be used in the different cooling zones, the selection of which can be adapted to the thermal conditions in the respective cooling zones. Preferably, the same heat transfer fluid is used. In this case, for example, boiling cooling can be used in one cooling zone with particularly high heat generation, while liquid cooling is used in another cooling zone. With boiling cooling, it is preferable not to provide baffle plates or to use specially designed baffle plates that prevent backflow of rising gas bubbles.

[0086] Various substances and mixtures can be used as fluid heat transfer agents, chosen for their suitability for dissipating the heat of reaction, for example, due to their heat capacity or their enthalpy of vaporization. Typically, a liquid heat transfer agent is used, such as water, dibenzyltoluene (Marlotherm), or monochlorobenzene.

[0087] The contact tubes of the reactor can have a length L in the range of 1.5 to 12 m, preferably from 2.5 to 8 m.

[0088] A suitable reactor for the process according to the invention can be equipped with 1000 to 10000 contact tubes and be cylindrical, with an inner diameter of preferably 0.3 to 6 m, more preferably 2 to 5 m, in particular 2.5 to 4 m.

[0089] The reactor contains a bundle, i.e., a large number of contact tubes, arranged parallel to each other in the longitudinal direction of the reactor.

[0090] Each contact tube preferably has a wall thickness in the range of 2.0 to 4.0 mm, in particular 2.5 to 3.0 mm, and an inner tube diameter in the range of 20 to 90 mm, preferably in the range of 30 to 50 mm.

[0091] The contact tubes are made of corrosion-resistant material, e.g., stainless steel, preferably duplex steel 1.4462, stainless steel 1.4571 or stainless steel 1.4541, or also of nickel alloys or nickel. Preferably, the tube sheets or the entire reactor are formed from the aforementioned materials, in particular from duplex or stainless steel.

[0092] However, the reactor casing and base can also be made from cheaper metals and metal alloys such as carbon steel. Components that come into contact with reactants can then be coated with a protective layer of higher-grade materials.

[0093] The contact tubes are fluid-tightly attached at both ends to tube sheets, preferably by welding. The tube sheets are also made of a corrosion-resistant material, preferably stainless steel, in particular duplex steel, and most preferably of the same material as the contact tubes. The seal to the tube sheets is preferably achieved by welding. For example, at least two layers of welds can be provided per tube, which are produced at an angle, for example offset by 180°, so that the beginning and end of the respective layers do not overlap.

[0094] Both ends of the reactor are enclosed by hoods. One hood directs the reaction mixture to the contact tubes, and the product flow is removed through the hood at the other end of the reactor.

[0095] The hood, into which the reaction mixture is fed, preferably contains gas distributors to distribute the gas flow evenly, for example in the form of a plate, in particular a perforated plate.

[0096] The contact tubes are filled with the solid catalyst. The catalyst filling in the contact tubes preferably has a void volume of 0.33 to 0.6, particularly 0.33 to 0.45. The void volume refers to the catalyst filling, assuming the solid catalyst is a solid body. The porosity of the catalyst bodies themselves, which can be, for example, 50%, is not taken into account.

[0097] Preferably, the surface load according to the invention is in the range of 0.5 kg phosgene / m²s to 6 kg phosgene / m²s, particularly preferably in the range of 0.7 kg phosgene / m²s to 5 kg phosgene / m²s, even better in the range of 0.7 kg phosgene / m²s to 4 kg phosgene / m²s, and particularly preferably in the range of 0.8 kg phosgene / m²s to 3.5 kg phosgene / m²s.

[0098] In existing reactors, increasing the surface loading can be achieved by adjusting the operating parameters, particularly by increasing the reactant flow rate. However, newly designed reactors can be engineered from the outset for optimized operation at the intended surface loading.

[0099] Increasing the surface loading can be achieved by reducing the number of contact tubes in the reactor while simultaneously increasing their length. For example, halving the number of contact tubes with the same diameter doubles both the surface loading and the tube length. The resulting reactors are therefore slimmer, meaning they have a smaller diameter for a comparable gas pressure equivalent (GHSV), which is advantageous for both production and cooling of the contact tubes. While the higher gas velocity and greater filling length increase the pressure drop in the contact tubes, they also lead to a better distribution of the feed flow across all contact tubes.

[0100] The surface loading can also be achieved with the same phosgene capacity and catalyst quantity, and with an unchanged number of tubes, by reducing the diameter of the individual contact tubes and again by correspondingly lengthening the contact tubes.

[0101] Of course, combinations of both measures are also conceivable, i.e., reducing the number of pipes as well as reducing the pipe diameter of the individual pipes.

[0102] Preferably, in the process according to the invention, the feed stream has a stoichiometric excess of carbon monoxide to chlorine of 0.001 to 50 mol%, so that almost complete conversion of chlorine is ensured. If fluctuating chlorine concentrations are expected in the chlorine feed stream, a higher excess of carbon monoxide is preferred, but in general, for cost reasons, the excess is chosen to be as low as possible, as long as complete chlorine conversion is still ensured.

[0103] The feed stream is preferably supplied at an absolute pressure in the range of 0.5 to 20 bar. Particularly preferably, the feed stream is supplied at an overpressure, e.g., at an absolute pressure of 3 to 7 bar. The higher the pressure of the resulting reaction mixture at the reactor outlet, the more effectively the phosgene contained in the reaction mixture can be condensed. Preferably, the pressure of the reaction mixture at the reactor outlet is still high enough that the phosgene can be at least partially condensed with cooling water.

[0104] The inventive process described above is particularly easy to implement if the reactor already has a provision for purging with a gas to remove phosgene from the reactor system, as is proposed, for example, in: Phosgene Safety Practice for design, production and processing - International Isocyanate Inc.; WO2019 / 048371; Phosgene Safe Practice Guidelines - American Chemistry Council [https: / / www.americanchemistry.com / Phosgene-Safe-Practice-Guidelines / ], WO2019 / 048371). Figure 1 Phosgene concentration at the reactor outlet for dried and undried activated carbon. The activated carbon after drying shows an increase in activity of up to 10%. Figure 2HCl concentration at the reactor outlet for dried and undried activated carbon. The increased HCl formation of the undried carbon at the beginning of the reaction is evident here, which leads to corrosion problems, especially upon contact with water (e.g., adsorbed water in the lower layers of a technical reaction tube). Example implementation: Example 1 (comparative example):

[0105] Approximately 0.4 g of a commercial activated carbon catalyst (Donaucarbon) in the form of granules between 1 and 2 mm in size, which had previously been saturated with water through storage in air, were incorporated into a reaction tube with a diameter of 5.4 mm. The catalyst bed was approximately 32 mm high. The reaction tube is heated by a surrounding copper block.

[0106] The reaction tube is supplied with 1.99 NI / h CO, 1.83 NI / h Cl₂, and 26.68 NI / h N₂, each from gas cylinders. The reaction of the reactants in the reaction tube takes place at 100°C and approximately 5 bara for 6 hours. At the reactor outlet, the reaction gases are subjected to an IR measurement, and the amount of phosgene formed in the mixture is measured. The phosgene concentration profile is shown in Fig. 1 shown. After a short start-up phase, a constant phosgene concentration of 3.04 vol% on average is established. Example 2 (according to the invention):

[0107] The experiment described in Example 1 was repeated, with the activated carbon bed being dried with a nitrogen flow of 2 NI / h for 16 h at 150°C after filling the reactor and before starting the reaction. Dry nitrogen, designated 5.0, was used from a cylinder. A control measurement showed a dew point of -66°C for this nitrogen. The exhaust gas stream had a dew point of -48°C after the 16 h drying period. After the start-up time, a constant phosgene concentration of 3.59 vol% was achieved in the dried carbon.

[0108] The also in Fig. 1 The phosgene concentration shown is higher over the entire duration of the experiment compared to the undried charcoal from Example 1. Therefore, the activated charcoal shows an increase in activity of up to 10% after drying.

[0109] In Fig. 2In the graph where the HCl concentration at the reactor outlet is plotted against the duration of the experiment, it becomes clear that the undried coal according to Example 1 shows an increased HCl formation at the beginning of the reaction, which leads to corrosion problems, especially when in contact with water (e.g., adsorbed water in the lower layers of a technical reaction tube).

Claims

1. A process for producing phosgene by conversion of chlorine and CO over an activated carbon catalyst, wherein the activated carbon catalyst is dried by reducing the water content, where the drying comprises the following steps: a) contacting the catalyst with an inert gas stream b) determining the residual moisture content of the catalyst by determining the moisture content in the offgas stream c) ending the drying process after attainment of the desired moisture content in the offgas stream by switching from the inert gas to the reaction gases and optionally d) heating up the catalyst bed and / or the inert gas during the drying process, where the drying is effected in the phosgene production reactor and the residual moisture content of the catalyst is determined by measuring the dewpoint of the offgas stream at the reactor outlet and the drying is conducted at least down to a dewpoint of -20°C.

2. The process according to claim 1, wherein the drying is effected after catalyst installation and before startup or after plant shutdown and before restarting.

3. The process according to any of claims 1 to 2, wherein the drying is effected after catalyst installation and before startup of the phosgene reactor.

4. The process according to any of claims 1 to 3, wherein the inert gas is nitrogen or air.

5. The process according to any of claims 1 to 4, wherein the inert gas used for drying has at least one dewpoint of -35°C.

6. The process according to any of claims 1 to 5, wherein the offgas stream of the inert gas used for drying is released into the atmosphere.

7. The process according to any of claims 1 to 6, wherein the catalyst is brought to a temperature of 80°C to 150°C before and / or during the drying.

8. The process according to any of claims 1 to 6, wherein the inert gas used for drying is preheated to a temperature of 60°C to 130°C.

9. The process according to claim 8, wherein the inert gas used for drying is preheated to the temperature of 60°C to 130°C in mobile heating units.

10. The process according to any of claims 1 to 7, wherein the catalyst is preheated to the desired temperature with the aid of the reactor cooling system.

11. The process according to claim 10, wherein the heating of the catalyst with the aid of the reactor cooling system not more than up to a temperature 10°C below the boiling temperature of the cooling medium of the reactor cooling system at standard pressure.

12. The process according to any of claims 1 to 11, wherein the volume flow rate of the inert gas used for drying is in the range of 0.05-1.0 m3 (STP) / h / tube, preferably 0.2-0.4 m3 (STP) / h / tube.

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