How to operate a distillation column

Abstract

The present invention relates to a method for continuously operating a distillation column designed to separate a mixture S essentially comprising a substance A and a substance B having a boiling point significantly higher than that of substance A. In the method according to the invention, the reflux ratio is varied depending on the feed flow, and at the same time the energy input via the heat transport medium is proactively varied by compensating the feed flow via feedforward control (so-called feedforward control). At the same time, the bottom temperature is monitored, and if the bottom temperature drops significantly when the heat transport medium is reduced by the feed flow, the control structure is changed.

Description

[Technical Field]

[0001] The present invention relates to a method for continuously operating a distillation column configured for the separation of a substance mixture S essentially comprising a substance A and a substance B having a significantly higher boiling point than substance A. In the method of the present invention, the reflux ratio is varied depending on the feed flow, and at the same time the energy input via the heat carrier is proactively varied by compensating the feed flow via feedforward control (so-called feedforward control). At the same time, the bottom temperature is monitored, and if the bottom temperature drops significantly when the heat carrier via the feed flow is reduced, the closed-loop control structure is modified. [Background technology]

[0002] In many manufacturing processes, separation problems arise in which a substance mixture of two or more components must be separated into its constituent components. If permitted by the differences in thermal stability, volatility, and boiling points of the individual substances to be separated, such separation problems in industrial production are usually solved by continuous distillation. In the simplest case of a substance mixture S consisting essentially of two components A and B, where B has a higher boiling point than A under the operating conditions selected for the distillation column used, the problem arises: to transfer B to the bottom of the column and A to the top of the column so that each component is almost essentially free of the other. The present invention relates to such a separation problem, i.e., to separate a substance mixture S consisting essentially of the two individual components A and B (except for impurities). Distillation columns suitable for such separation problems (and more demanding separation problems) are basically known in the prior art. The present invention relates to a specific closed-loop control concept for such distillation columns.

[0003] In contrast, U.S. Patent No. 5,999,623 relates to the closed-loop control of a distillation column that separates a feed composed of many different hydrocarbons into a low-boiling overhead fraction and a high-boiling bottom fraction, where the overhead fraction is intended for further material use and the bottom fraction is intended for use as fuel. The hydrocarbon mixtures used as feed here are typically complex mixtures of many different individual components, whose boiling points are "smoothly transitioned." As a result, such mixtures have a wide boiling range formed by the many different boiling points of the individual components. With regard to the separation of such mixtures in a distillation column, this leads to the unique feature that the temperature sensitivity of the column, for example, to changes in reflux, is relatively low and has an essentially linear profile. This means that the temperature change in the distillation column depends, at most, only slightly on the location in the column where the temperature is measured. The closed-loop control concept described in U.S. Patent No. 5,999,623 is adapted to these special boundary conditions.

[0004] The described closed-loop control concept aims to keep the ratio of the bottoms stream withdrawn from the distillation column to the feed stream supplied to the distillation column very constant, and to control the quality of the withdrawn top stream so that its content of (relatively) high-boiling compounds does not become too high, achieving these two objectives so that the distillation column can be operated with maximum energy efficiency. To this end, a number of process parameters are measured and input into a computer, which uses these process parameters and predetermined target values ​​to calculate signals to be sent to control elements (especially valves) for reflux rate, bottoms withdrawal rate, etc. The computer is used to control, among other things, the amount of fuel supplied to the distillation column's evaporator (the evaporator is driven by burning fuel oil) and the absolute reflux rate at the top of the distillation column (which is distinct from the reflux ratio). These two parameters also affect the bottoms discharge rate and the composition of the top product. Considering the above objectives (a very sufficient constant ratio between the bottom stream withdrawn from the distillation column and the feed stream fed to the distillation column, and quality control of the top product), the amount of fuel oil fed and the values ​​for reflux should be minimized so that the distillation column can be operated with maximum energy efficiency.

[0005] For this purpose, firstly, the operating variable of the valve for the fuel oil supply is determined from the difference between the target value for the mass flow rate of the fuel oil and its actual measured value, where the target value for the mass flow rate of the fuel oil is determined by a computer taking into account the mass flow rate of the feed, the mass flow rate of the withdrawn bottom product, as well as the temperature of the feed, the temperature of the top product after its condensation (which is a measure of the composition of the top product under the specific conditions of the separation task described above), and the temperature of the bottom product fraction supplied to the evaporator of the distillation column.

[0006] For this purpose, the operating variable of the reflux valve is secondarily determined from the difference between the setpoint value for the reflux and its actual measured value, the setpoint value for the reflux being determined by a computer taking into account the mass flow rate of the feed, the mass flow rate of the withdrawn bottom product, as well as the temperature of the top product withdrawn in gaseous form and the temperature of the top product after its condensation (which is a measure of the composition of the top product under the specific conditions of the separation task described above).

[0007] The closed-loop control concept described here cannot be applied to the separation problem described at the beginning (separation of a substance mixture S consisting essentially of one substance A and one substance B with significantly different boiling points), since the temperature profile of the distillation column here does not have an essentially linear profile, since the boiling points of the individual components are not "smoothly transitioned", and furthermore the temperature of the distillate does not allow any specific conclusions regarding its composition (at least not as in the case of the distillation of hydrocarbon mixtures).

[0008] US Patent No. 5,999,949 relates to a method and apparatus for controlling a process having three variable associated conditions, particularly for controlling a fractionation column. The control unit described includes four control systems: (1) a distribution system, (2) a reflux control system, (3) a pressure control system, and (4) a preventive system. The distribution system's main function is to control the distribution between the distillate and the bottoms. When the feed flow rate increases or decreases, the distribution system ensures that the distillate and bottoms flow rates increase or decrease precisely proportionally. When the feed composition changes, the distribution system can alter the distribution between the distillate and the bottoms to ensure that the product breakdown is maintained. The reflux control system adjusts the reflux flow rate. When the feed flow rate increases or decreases, the reflux flow rate is changed proportionally. A novel feature of this system is that the reflux temperature is used to modify the reflux flow rate if it deviates from a predetermined reference temperature. The system also includes (a) means to prevent the column from completely starving when the feed flow is established, and (b) means to minimize the possibility of column flooding, i.e., filling up with liquid. The pressure control system ensures that the column pressure remains essentially constant. The pressure control system includes a controller that adjusts the heat flow rate. It increases or decreases the heat flow to maintain constant column pressure in response to any initial pressure changes. The system includes a manual override that allows the operator to switch between automatic and manual control. During manual control, an automatic circuit provides easy switchover to automatic control. If the column pressure is incorrect upon switching to automatic control, the heat flow is gradually, rather than abruptly, restored to the appropriate rate, correcting the column pressure. The preventative system ensures that the column material balance is maintained. This is accomplished by continuously monitoring the liquid level at the column bottom. According to the present invention, the liquid level is allowed to fluctuate within predetermined limits (positions 490 and 492 in the figure) without taking any corrective action, as long as there is no risk of exceeding the level limits, as determined by the relationship between the control signal and the liquid level signal as a function of the feed flow rate, the feed composition, or both. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] U.S. Patent No. 4,166,770 [Patent Document 2] U.S. Patent No. 3,905,873 Summary of the Invention [Problem to be solved by the invention]

[0010] From an economic point of view, it is very important to operate the distillation column in a very energy-efficient manner and to maintain the quality of the distillation product at a sufficiently constant and high level. A further object is to configure the operation of the distillation column in a very user-friendly manner, i.e., so that manual intervention by the operator is required very rarely and, if it occurs, without complex operations, and in particular not even under conditions that deviate from normal operation, for example, when the mass flow rate of the feed to the distillation column changes suddenly.

[0011] Achieving these objectives requires efficient closed-loop control of distillation columns used in separation tasks. [Means for solving the problem]

[0012] Thus, the present invention provides:

[0013] 1. A method for continuously operating a distillation column 1000 (under pressure conditions selected for operation of the distillation column) configured for the separation of a mixture of substances S comprising a substance A and a substance B having a boiling point higher than that of substance A, the distillation column comprising: (I) a vertically arranged column body 100 including a stripping section 110 and an upper rectifying section 120, wherein (at least) the stripping section has a temperature measuring device for measuring the stripping section temperature T(AT); (II) a column bottom 130 below the stripping section, which receives a liquid bottom product B1 at a bottom temperature T(B1) up to a bottoms level H(B1); (III) a column top 140 above the rectification section containing the vaporized overhead product A1; (IV) a feed location 150 for the substance mixture S, at which the substance mixture S is fed to the distillation column at a flow rate m(S); (V) a circulation evaporator 200 for heating the column bottom by indirect heating of a first portion B11 of the liquid bottom product B1, the circulation evaporator being supplied with a heat carrier W at a mass flow rate m(W) and having a heat carrier valve 210 for adjusting the mass flow rate m(W); (VI) a withdrawal unit 220 for withdrawing a second portion B12 of the liquid bottom product B1 at a mass flow rate m(B12), the withdrawal unit 220 having a bottoms withdrawal valve 230 for adjusting the mass flow rate m(B12); (VII) a top condenser 300 for condensing the vaporized overhead product A1 to obtain a liquefied stream A2; (VIII) a recycle and withdrawal unit 310 for returning a first portion A21 of the liquefied stream A2 to the distillation column at a mass flow rate m(A21) and withdrawing a second portion A22 of the liquefied stream A2 from the distillation column at a mass flow rate m(A22), the recycle and withdrawal unit 310 having a reflux valve 320 for adjusting the reflux ratio r=m(A21) / m(A22); (IX) a closed-loop control unit including a reflux controller 410, a bottoms level controller 420, a stripping section temperature controller 430, a bottoms temperature limit controller 440, a mass flow controller 450 for the heat carrier medium W, and preferably a flow measuring device 460 for the substance mixture S fed to the distillation column; where (i) For the reflux ratio r, the target value r within the range of r1 to r2 TARGET is defined, and the reflux controller 410 determines the defined value r by taking into account the minimum allowable value r1 for r. TARGET and using values ​​for mass flow rate m(S) (preferably determined by flow measurement device 460) to calculate the setting of reflux valve 320; (ii) For the bottom object level H(B1), set the target value H(B1) within the range of H(B1)1 to H(B1)2. TARGET is defined, and the bottom object level control device 420 sets this target value H(B1) TARGET , the current value H(B1) of the bottom level at a given time CURR , and the values ​​for mass flow rate m(S) are used to calculate the setting of bottoms draw valve 230; (iii) For the stripping section temperature T(AT), set the target value T(AT) within the range of T(AT)1 to T(AT)2. TARGET is defined, and the stripping section temperature controller 430 controls the stripping section temperature to the target value T(AT) TARGET , the current value for the stripping section temperature at a given time T(AT) CURR , and the mass flow rate m(S), to calculate the setting of the heat carrier medium valve 210, and transmit the calculated setting of the heat carrier medium valve 210 to the heat carrier medium valve 210 by the mass flow controller 450; (iv) For the bottom temperature T(B1), set the target value T(B1) within the range of T(B1)1 to T(B1)2. TARGET is defined, and the bottom temperature limiting control device 440 is configured to disable the setting of the heat carrying medium valve 210 according to (iii) when the temperature falls below the temperature T(B1)1, change the setting of the heat carrying medium valve 210 to increase the mass flow rate m(W), and re-enable the setting of the heat carrying medium valve 210 according to (iii) when the bottom temperature T(B1) again falls within the range of T(B1)1 to T(B1)2.

[0014] The reason for this is that it has surprisingly been found that the above-mentioned objectives can be achieved, or at least come close, if a more efficient operation of the column is achieved by changing the reflux ratio as a function of the feed flow (step (i)) and at the same time proactively changing the energy input by the heat carrier (so-called feedforward control) (steps (ii) and (iii)) by compensating the feed flow by means of feedforward control. This requires simultaneously monitoring the bottom temperature and modifying the closed-loop control structure (feedforward control) (step (iv)) if the bottom temperature drops significantly when the heat carrier is reduced as a result of a change in the feed flow.

[0015] These steps (i) to (iv) together constitute a complex multivariable closed-loop control system or a multivariable system, where steps (i) to (iv) interact synergistically, in particular as follows:

[0016] a) Steps (i) and (iii) determine the flow in the column and ensure their balance and equilibrium with one another so that the required distillation quality can be achieved. The high temperature sensitivity of the distillation column compared to other separation tasks (for example, the separation of hydrocarbon mixtures discussed at the beginning) is taken into account by measuring the temperature in the stripping section.

[0017] b) Step (iv) ensures that the column can be robustly operated near the operating range fixed in its design (especially the operating range fixed with respect to the temperature window in which the column is operated). The closed-loop control operation of step (iv) intervenes if the operating or design range is so far removed that reliable operation of the column can no longer be ensured. As soon as the operating range is reached again (as soon as the bottom temperature T(B1) returns to the range T(B1)1 - T(B1)2), operation is resumed in the manner described in point a.

[0018] c) Steps (ii) and (iii) control the ratio of the external and internal mass flow rates. The two closed loops are specifically adapted to each other, and the closed loop control parameters are designed together and in combination with each other.

[0019] d) Taking into account item c, step (i) is also determined thereby, and the internal mass flows are balanced in both the rectifying and stripping sections to adjust the desired distillation quality.

[0020] According to the present invention, distillation column 1000 is configured to separate a mixture of substances S, which comprises a substance A and a substance B, which has a higher boiling point than substance A (under the pressure conditions selected for operation of the distillation column). This, of course, involves the need for the difference in boiling points between A and B to be large enough to allow one substance (i.e., A) to be removed at the top and the other substance (i.e., B) to be removed at the bottom of the distillation column. This is particularly the case when the difference in boiling points between A and B at 1.01 bar is 10° C. or more, more preferably 25° C. or more, and most preferably 50° C. or more. In general, the difference in boiling points between A and B will not exceed 250° C.

[0021] Since substance A has a lower boiling point than substance B under the pressure conditions selected for operating the distillation column, substance A will also be referred to below as low boilers. It is possible to consider that the substance mixture S as well as the low boilers A contain further compounds A' (i.e. further low boilers) which have a boiling point lower than substance B, in particular low-boiling impurities, without going beyond the scope of the present invention. These are removed together with substance A at the top of the distillation column and are therefore part of the top product A1. Similarly, substance mixture S may also contain a relatively high-boiling impurity B' which accumulates at the bottom of the distillation column and becomes part of the bottom product B1. However, the total mass proportion of substances A and B in substance mixture S is at least 90%, preferably at least 95%, more preferably at least 98%, based on the total mass of substance mixture S.

[0022] The term stripping section (AT) in the present terminology is understood to mean the region below the feed point 150 for the substance mixture S to be separated, including the entirety of all separation internals (e.g., trays or one or more packings) present in this region (see also FIGS. 1 and 2, therein 110). The temperature T(AT) of the stripping section is measured in the stripping section thus understood. The most suitable location for this temperature measurement can be determined by a person skilled in the art, if necessary, in simple preliminary tests for the optimization of closed-loop control. The separation internals used are preferably packings, preferably one to three packings, in particular one (exactly one) packing. In this context, it is preferable to measure the temperature T(AT) of the stripping section in the center of the packing (determined in the longitudinal direction of the distillation column) or in a region of up to 20% above or below the packing height (determined in the longitudinal direction of the distillation column). In the case of a stack of packings, this corresponds to the lowest packing.

[0023] The term rectification section (VT) in the terminology of the present invention is understood to mean the region above the feed location 150, including the totality of all separation internals present in this region (see also Figures 1 and 2, therein 120). Here too, structured packing is preferred, more preferably one to three structured packings, in particular one (exactly one) structured packing.

[0024] The construction of such separation internals (whether in the stripping section or the rectifying section) is known to those skilled in the art and therefore no further explanation is required at this point.

[0025] The bottoms level H(B1) in the terminology of the present invention refers to the height of the liquid level of the liquid bottom product B1 measured from the lower boundary of the distillation column (see also Figures 1 and 2). In this specification, the expression "lower boundary of the distillation column" in the general terminology of the present invention refers to the lowest point in the internal volume of the distillation column that comes into contact with the liquid bottom product during operation. The method according to the present invention aims to raise the bottoms level to a defined target value H(B1) TARGET This target value is within the limits of H(B1)1 to H(B1)2. Therefore, the actual value H(B1) CURR is the target value H(B1) TARGET When it deviates from this value, it is readjusted to that value, and does not fluctuate freely between H(B1)1 and H(B1)2 (in this case between positions 490 and 492) as is the case in the closed-loop control system according to the patent specification WO 2005 / 024990 discussed above. In contrast to the closed-loop control system described therein, the bottoms level in the method of the present invention is therefore kept constant (i.e., at the target value H(B1) TARGET (The value is readjusted to that value when it deviates from the value.)

[0026] The target values ​​(for reflux ratio, bottoms level, stripping section temperature, and bottoms temperature) fixed within the scope of the present invention naturally depend on the nature of the separation task and the distillation column available for that purpose. The same applies to the specific values ​​calculated from the target values ​​(for example, the setting of the bottoms withdrawal valve 220). These values ​​therefore cannot be generalized and must be determined by the skilled person in each case for given boundary conditions, which is merely a routine task. Typical values ​​for the preferred use of the separation of benzene as substance A and nitrobenzene as substance B are further disclosed in the detailed description and examples below.

[0027] The accompanying drawings show: [Brief explanation of the drawings]

[0028] [Figure 1] 1 is a schematic diagram of a distillation column 1000 whose operation may be controlled by a method according to the present invention. [Figure 2] FIG. 2 shows the distillation column from FIG. 1 including a process control device according to the present invention. [Figure 3] FIG. 1 shows the benzene content in the bottom product from a distillation column separating benzene from nitrobenzene and the temperature at the center of the column when the mass flow rate of the crude nitrobenzene feed is changed by 20% with a closed-loop control system according to the invention (Example 2) and without a closed-loop control system according to the invention (Example 1). DETAILED DESCRIPTION OF THE INVENTION

[0029] First, an overview of various possible embodiments is provided.

[0030] In a first embodiment of the method of the invention, which can be combined with all other embodiments, the closed-loop control unit comprises a flow measuring device 460 for the substance mixture S fed to the distillation column, by means of which the mass flow rate m(S) is determined in step (i).

[0031] In a second embodiment of the method of the invention, which can be combined with all other embodiments, the heat-carrying medium is steam.

[0032] In a third embodiment of the process according to the invention, which can be combined with all other embodiments, the substance mixture S comprises benzene as substance A and nitrobenzene as substance B.

[0033] In a fourth embodiment of the method according to the invention, which is a specific configuration of the third embodiment, r1=55, r2=65, H(B1)1 = 0.18 × H(130), H(B1)2 = 0.42 × H(130), where H(130) denotes the height of the bottom of the column measured from the lower boundary of the distillation column to the lower end of the stripping section; T(AT)1 = 166°C, T(AT)2 = 172°C, and T(B1)1 = 168°C, T(B1)2 = 173°C.

[0034] The embodiments briefly outlined above and further possible configurations of the invention are described in detail below, and these embodiments can be combined with each other if desired, unless the context makes clear to the contrary.

[0035] The accompanying FIG. 1 shows an example of a distillation column 1000, the operation of which can be advantageously controlled by the method according to the invention. The substance mixture S to be separated is fed in from the side between the rectification section 120 and the stripping section 110. The distillation column can, of course, also have further auxiliary units and peripheral devices known to those skilled in the art (e.g., liquid collectors, liquid distributors, pumps, etc.). The gaseous overhead product A1 is condensed in an overhead condenser 300, where a portion A21 of the condensate is returned to the distillation column as reflux, and another portion A22 is withdrawn from the distillation column as overhead product. The mass flow ratio m(A21) / m(A22) of the reflux to the withdrawn product is called the reflux ratio r. The reflux ratio is set by means of a reflux valve 320. In contrast to FIG. 1, the overhead condenser can also be integrated into the column body 100.

[0036] Liquid bottom product B1 fills the bottom of the column up to a height H(B1) indicated by the dashed line. Distillation column 1000 is heated by circulation evaporator 200, in which a first portion of the discharged bottoms (i.e., B11) is indirectly heated with a heat carrier (W) and returned to bottom 130. Suitable heat carriers are steam, condensate, and further liquid and gaseous heat carriers. The mass flow rate m(W) of the heat carrier is regulated by heat carrier valve 210. A second portion of the bottoms (i.e., B12) is discharged as bottom product, the mass flow rate of which is regulated via bottoms withdrawal valve 230.

[0037] 2 shows a process control device used in the context of the present invention. To simplify the drawing, some reference numerals from FIG. 1 have been omitted here. The substance mixture S to be separated is first detected by a mass flow measuring device 460. This mass flow is used to calculate a target value for the reflux controller 410 via a defined reflux ratio (ranging from r1 to r2). In particular, when designing a distillation column, the reflux ratio r TARGET To ensure reliable operation of the distillation column, a minimum reflux (m(A21) MIN ) should be ensured at all times. This ensures that the distillation column never runs dry. MIN ) is determined by selecting appropriate design parameters in the design of the distillation column.

[0038] The level at the bottom of the column is controlled in response to the feed flow as well via level controller 420. This is achieved by a disturbance variable feedforward of the feed flow S.

[0039] The temperature T(AT) of the stripping section, which determines the quality of the bottom product, is guaranteed by a temperature controller 430. This controller uses a defined target value and the feed flow S to calculate a target value for the mass flow controller 450 for the heat carrier. Since large changes in the feed flow could result in the amount of heat carrier being too low, which would cause the bottom temperature to fall below the required minimum, a closed-loop temperature control 440 at the bottom of the column ensures the required minimum temperature. This temperature controller intervenes if the temperature falls below the minimum temperature, stopping the closed-loop control according to (iii) and reactivating it once the bottom temperature is again within the range T(B1)1 to T(B1)2.

[0040] The heat carrier W used is preferably steam, but it is equally possible to use other heat carriers, for example thermal oil.

[0041] The process of the present invention is suitable for operating a distillation column in which low boilers A must be separated from high boiling products B.

[0042] An example is the separation of benzene and nitrobenzene required in the context of a process for preparing nitrobenzene by nitrating benzene with nitric acid in the presence of sulfuric acid. The benzene is typically used in stoichiometric excess over nitric acid, particularly in the adiabatically operated nitration processes customary today, and must therefore be removed during workup of the nitration product. Workup of the nitration product is typically achieved by separating the reaction mixture (nitration product) present after nitration, containing nitrobenzene, unconverted benzene, and sulfuric acid, into an aqueous sulfuric acid phase and an organic nitrobenzene phase in a first step, followed by a second step of washing the nitrobenzene phase in one or more stages, and a third step of separating the excess benzene from the washed nitrobenzene phase.

[0043] Excess benzene is typically recycled to the nitration, resulting in the accumulation of impurities (e.g., especially aliphatic organic compounds) that were present in the benzene originally used and that passed through the nitration process essentially unchanged. Since these impurities have lower boiling points than nitrobenzene, they are distilled off together with the benzene. The excessive accumulation of these impurities is prevented by the discharge of a purge stream and / or by their decomposition after some time.

[0044] This distillation task of providing purified nitrobenzene can be advantageously achieved by the process of the present invention. For this particular separation task, target values ​​within the following ranges are preferred: Reflux ratio: r1~r2 corresponds to 55~65; The bottoms levels: H(B1)1 to H(B1)2 correspond to 18% to 42% of the bottoms level H(130) measured from the lower boundary of the distillation column to the bottom end of the stripping section (see also Figures 1 and 2); Stripping section temperature: T(AT)1 to T(AT)2 corresponds to 166°C to 172°C; The bottom temperatures: T(B1)1 to T(B1)2 correspond to 168°C to 173°C. [Example]

[0045] The following example results were achieved using a distillation column to separate excess benzene from nitrobenzene. The basic configuration of the distillation column corresponded to Figure 1. Feed stream S consisted of crude nitrobenzene that had only been separated from the mixed acid and washed. A stream of benzene and other compounds with a boiling point lower than that of nitrobenzene (especially aliphatic organic compounds) was taken off overhead as stream A1. A nitrobenzene stream almost free of low boilers was obtained at the bottom as stream B1.

[0046] In both examples, the feed mass flow rate S was increased by 20% during continuous operation of the distillation column.

[0047] Example 1 (comparison): The feed rate of the heat transfer medium W (steam) was controlled according to the temperature at the center of the tower (tray 20). When this temperature decreased, the steam feed rate was increased. In FIG. 3, the vertical axis shows the feed flow S (thick dashed line, in kg / h), the temperature at the center of the tower T (thick solid line at tray 20, in °C), and the benzene concentration in the bottoms (benzene content c(Bz) expressed as a mass fraction) (thin dotted line, in kg of benzene per kg of bottoms) versus time t (in hours) shown on the horizontal axis.

[0048] Example 2 (according to the invention): The supply rate of the heat transfer medium W (steam) was controlled according to the present invention (see also FIG. 2). Similar to Example 1, FIG. 3 shows the feed stream S (thick dashed line), the temperature T at the center of the tower (thin solid line), and the benzene concentration c(Bz) in the bottoms (thick dotted line). It can be seen that the fluctuations in both the temperature at the center of the tower and the benzene concentration in the bottoms are clearly smaller than in Example 1.

Claims

1. 1. A method for continuously operating a distillation column configured for the separation of a mixture of substances S comprising a substance A and a substance B having a boiling point higher than that of substance A, said distillation column comprising: (I) a vertically disposed column body including a stripping section and an upper rectifying section, the stripping section having a temperature measuring device for measuring the stripping section temperature T(AT); (II) a column bottom below the stripping section, which contains a liquid bottom product B1 at a bottom temperature T(B1) up to a bottoms level H(B1); (III) a column top above the rectifying section containing the evaporated overhead product A1; (IV) a feed point for the substance mixture S, where the substance mixture S is fed to the distillation column at a flow rate m(S); (V) a circulation evaporator for heating the column bottom by indirect heating of a first portion B11 of the liquid bottom product B1, the circulation evaporator being supplied with a heat carrier medium W at a mass flow rate m(W) and having a heat carrier medium valve for adjusting the mass flow rate m(W); (VI) a withdrawal unit for withdrawing a second portion B12 of the liquid bottom product B1 at a mass flow rate m(B12), the withdrawal unit having a bottoms withdrawal valve for adjusting the mass flow rate m(B12); (VII) an overhead condenser for condensing the vaporized overhead product A1 to obtain a liquefied stream A2; (VIII) a recycle and withdrawal unit for returning a first portion of the liquefied stream A2 to the distillation column at a mass flow rate m(A21) and withdrawing a second portion of the liquefied stream A2 from the distillation column at a mass flow rate m(A22), the recycle and withdrawal unit having a reflux valve for adjusting the reflux ratio r = m(A21) / m(A22); (IX) a closed-loop control unit including a reflux controller, a bottoms level controller, a stripping section temperature controller, a bottoms temperature limit controller, and a mass flow controller for the heat carrier medium W; where (i) The reflux ratio r is set to a target value r within the range of r1 to r2. ＴＡＲＧＥＴ is defined, and the reflux control device determines the defined value r by taking into account the minimum allowable value r1 for r. ＴＡＲＧＥＴ and using the values ​​for mass flow rate m(S), calculate the setting of the reflux valve; (ii) Regarding the bottom object level H(B1), a target value H(B1) within the range of H(B1)1 to H(B1)2 ＴＡＲＧＥＴ is defined, and the bottom object level control device sets this target value H(B1) ＴＡＲＧＥＴ , the current value of the bottom level at a given time H(B1) ＣＵＲＲ and using the values ​​for the mass flow rate m(S), calculate the setting of the bottoms draw valve; (iii) The stripping section temperature T(AT) is set to a target value T(AT) within the range of T(AT)1 to T(AT)2. ＴＡＲＧＥＴ is defined, and the stripping section temperature control device controls the stripping section temperature to the target value T(AT) ＴＡＲＧＥＴ , the current value for the stripping section temperature at a given time T(AT) ＣＵＲＲ and the mass flow rate m(S) to calculate a setting for the heat-carrying medium valve, and transmit the calculated setting for the heat-carrying medium valve (210) to the heat-carrying medium valve (210) by the mass flow controller (450); (iv) The bottom temperature T(B1) is set to a target value T(B1) within the range of T(B1)1 to T(B1)2. ＴＡＲＧＥＴ wherein the bottom temperature limiting control device is configured to disable the setting of the heat carrying medium valve according to (iii) when the temperature falls below the temperature T(B1)1, change the setting of the heat carrying medium valve so that the mass flow rate m(W) increases, and re-enable the setting of the heat carrying medium valve according to (iii) when the bottom temperature T(B1) again falls within a range of T(B1)1 to T(B1)2.

2. 2. The method of claim 1, wherein the closed-loop control unit comprises a flow measurement device (460) for the substance mixture S fed to the distillation column, whereby the mass flow rate m(S) is determined in step (i).

3. 3. The method according to claim 1, wherein the heat-carrying medium is steam.

4. 4. The method according to claim 1, wherein the substance mixture S comprises benzene as substance A and nitrobenzene as substance B.

5. r1=55, r2=65, H(B1)1 = 0.18 × H(130), H(B1)2 = 0.42 × H(130), where H(130) represents the height of the bottom of the column measured from the lower boundary of the distillation column to the lower end of the stripping section; T(AT)1 = 166°C, T(AT)2 = 172°C, and 5. The method of claim 4, wherein T(B1)1 = 168°C and T(B1)2 = 173°C.

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