METHOD AND RECIPE FOR THE PRODUCTION OF PHOSGENE

Description

[0001] The invention relates to a process and a reactor 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.

[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 ⇄ COCl₂. 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). In this process, granular catalyst with a particle size in the range of 3 to 10 mm, preferably 3.5 to 7 mm, is used in contact tubes with a typical inner diameter of up to 100 mm, typically between 35 and 70 mm and preferably between 39 and 45 mm.The elongated contact tubes have a longitudinal axis, with the length of the contact tubes, measured along this axis, typically ranging from 1.5 m to 12 m in industrial-scale phosgene production. The contact tubes are usually filled with catalyst material, except for short sections at the beginning and end, so that the height of the catalyst material is essentially equal to the length of the contact tubes. The reaction typically begins at temperatures of 40 to 120 °C at the point where the reactants enter the catalyst material. In the direction of reactant flow, the temperature in the tubes rises rapidly along the longitudinal axis with increasing distance from the gas inlet opening, typically reaching a temperature maximum of 400 °C or more, and potentially up to 600 °C, within the first half of the tube (measured from the gaseous reactant inlet).This point of highest temperature along the longitudinal axis of a contact tube is also known as the "hot spot." The temperature then drops rapidly along the further length of the contact tube because a large portion of the chlorine gas used has already reacted at the hot spot, and less and less chlorine gas is available for phosgene formation further along the tube. Carbon monoxide is typically used in excess during the reaction to ensure that all the chlorine is converted along the length of the contact tubes and that largely chlorine-free phosgene is produced, since chlorine can lead to undesirable side reactions when phosgene is subsequently used.

[0004] The reaction can be carried out without pressure, but is usually performed at an overpressure of 3 to 7 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 ones, thus enabling more economical operation of the condenser.

[0005] A typical large-scale reactor for the production of phosgene is described, for example, in the applicant's international patent application WO 03 / 072237 A1.

[0006] A key challenge in the design of phosgene reactors is the removal of the heat generated by the reaction. The contact tubes of the tube bundle reactor are surrounded by a heat transfer fluid, which carries the heat away from the reactor. It has been shown that heat removal is improved by a crossflow across the contact tubes. Therefore, baffles are typically installed in the reactor, creating a meandering flow pattern for the heat transfer fluid, thus enabling a crossflow of the heat transfer fluid across the contact tubes. The heat transfer fluid can be a liquid that boils or does not boil under the given reaction conditions.In the case of boiling cooling, as described, for example, in European patent application EP 01 34 506 A2, a liquid that boils at typically 150 to 320 °C under the given pressure conditions is preferably used and circulated through a heat exchanger, which is usually water-cooled. Liquid cooling and boiling cooling can also be combined in a reactor.

[0007] Due to the cooling of the contact tubes, in addition to the longitudinal temperature profile described above, a cross-sectional temperature profile also develops. Typically, the highest temperatures are reached in the center of the contact tube, and the temperature decreases towards the inner wall. In the packed bed, a cross-sectional temperature profile then forms perpendicular to the longitudinal axis, typically resembling an inverted parabola with a maximum at the center. The removal of the heat of reaction by the cooling medium is determined by the wall heat transfer from the reaction mixture and catalyst material at the inner wall of the contact tube, the thermal conductivity of the contact tube's outer wall material, and the heat transfer to the cooling medium at the outer wall. The thermal conductivity of the contact tube material, the temperature, flow velocity, and Reynolds number of the reaction gas play a particularly important role here.The cooling medium plays a role, but so does the thermal conductivity of the catalyst material located in the contact tube.

[0008] In large-scale reactors, significant differences in heat transfer coefficients occur across the reactor cross-section at the interfaces between the contact tubes and the heat transfer fluid. These differences are caused, for example, by the deflection of the heat transfer fluid from transverse to longitudinal flow, but also by pressure losses of the heat transfer fluid flowing in the shell. The heat transfer coefficients of areas with good heat transfer and areas with poor heat transfer within the reactor's reaction cross-section can differ by a factor of two. Consequently, the contact tubes in the areas with poor heat transfer are cooled less effectively. Furthermore, the thermal stress on the wall material of the contact tubes is increased if the catalyst material used has high thermal conductivity, as is the case, for example, with activated carbons.Particularly in the hot spot area, which, as described above, can reach temperatures of up to 600 °C in the center of a contact tube, significantly elevated wall temperatures of the contact tubes can occur despite cooling. Increased thermal stress on the contact tubes can also occur during partial load operation, as larger hot spots develop due to poorer heat dissipation. In conjunction with the chlorine-containing atmosphere, prolonged exposure of the contact tube walls to high temperatures can therefore lead to corrosion. The effects depend primarily on the wall material used. The susceptibility to such corrosion damage is lowest for contact tubes made of nickel-based alloys and increases with materials such as stainless steels, duplex steels, and carbon steels. The corrosion rate in chlorine atmospheres as a function of temperature is generally known for various materials (see, for example, [reference]). "Materials Selector for Hazardous Chemical, Vol. 3, Hydrochloric Acid, Hydrogen Chloride and Chlorine", MIT Publication MS-3, Elsevier Science).

[0009] These corrosion damages can lead to leakage between the product and coolant sides, resulting in safety-critical conditions within the reactor. When water is used as the coolant, contact with phosgene can generate aqueous HCl, which in turn leads to corrosion damage in other plant components. In particular, the occurrence of corrosion damage or leakage necessitates shutting down the production plant and repairing or even replacing the reactor, incurring significant costs due to production downtime and investment.

[0010] Therefore, when designing reactors, it is generally assumed that the contact tubes, depending on the material used, may withstand a maximum temperature load. For duplex steel, which is frequently used for contact tubes, the maximum temperature load is typically in the range of approximately 170 to 200 °C, and for stainless steels, around 250 °C. However, designing the reactor operating conditions to comply with the maximum temperature load of the contact tube walls within the aforementioned range limits the throughput and thus the reactor capacity to the selected design value.

[0011] The reactor's throughput can be specified via the so-called surface load or phosgene load, which is defined as the amount of phosgene produced per unit time (usually expressed in kg / s) relative to the cross-sectional area of ​​the catalyst, i.e., the sum of the internal cross-sectional areas of the catalyst-filled contact tubes (usually expressed in square meters). To control the heat of reaction, surface loads between 0.5 and 2 kg phosgene / m²s are therefore typically used in the prior art. The phosgene surface load is thus essentially determined under the assumption of complete conversion of the component operating at a reduced load, for example, in the case of excess carbon monoxide, primarily through the chlorine feed.

[0012] In this application, the term "reactor" 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 as defined in this application can also comprise two or more components with separate reactor vessels, arranged, for example, in series. In this case, the surface load refers to the total conversion, i.e., the phosgene flow leaving the last reactor component, e.g., the last reactor vessel.

[0013] The present invention addresses the technical problem of providing a method for producing phosgene that allows for an increased reactor throughput without increasing the risk of reactor damage due to increased thermal stress on the contact tubes. The invention also relates to a reactor for carrying out the process according to the invention.

[0014] This technical problem is solved by the method of claim 1. Advantageous further developments of the method according to the invention are the subject of the dependent claims.

[0015] The present invention therefore relates to a process for the production of phosgene by a gas-phase reaction of carbon monoxide and chlorine in the presence of a catalyst in a reactor comprising a plurality of contact tubes arranged parallel to one another, which are filled with the catalyst and around which at least one fluid heat transfer medium flows, wherein a feed gas stream (or simply "feed stream") of a mixture of a chlorine feed stream and a carbon monoxide feed stream is introduced into the contact tubes at an inlet end and allowed to react in the contact tubes to form a product gas mixture containing phosgene, and the product gas mixture is discharged from the contact tubes at an outlet end. The process according to the invention is characterized in that the gas-phase reaction is carried out in the reactor such that the position of the highest temperature in a contact tube, i.e., the so-calledA hot spot is moved at a predetermined migration speed along the longitudinal axis of the contact tube, wherein the hot spot has a migration speed in the longitudinal direction of the contact tubes which is in the range of 1 to 50 mm per day, characterized in that a continuous movement of the hot spot is effected by controlled change of the operating conditions, wherein the operating conditions are effected by partial recirculation of the product gas mixture into the feed stream and / or that a continuous movement of the hot spot is effected by controlled deactivation of the catalyst in the contact tubes.

[0016] As described above, the catalytic gas-phase reactions considered within the scope of the present invention are characterized by the formation of a characteristic temperature profile along the length of the contact tubes, with a pronounced temperature peak, also known as a hot spot. Under unchanged reaction conditions, the position of this hot spot along the longitudinal axis of the contact tubes remains constant, so that the contact tubes in this region are subjected to a prolonged high thermal load.

[0017] The invention is based on the understanding that the design criteria used to date for reactors assume that the highest thermal stress to which a contact tube is exposed in the area of ​​the hot spot acts on the same area of ​​the contact tube for an extended period of time. In contrast, the invention proposes to reduce the thermal stress on the contact tubes by adjusting the operating conditions of the reactor such that the hot spot moves continuously along the longitudinal axis of the contact tube in the direction of the flow of the reaction gases within the contact tube during reactor operation.Since the temperature profile along the length of the contact tubes drops rapidly before and after the hot spot (viewed in the direction of the reaction gas flow), a given section of the contact tube containing the hot spot is only briefly exposed to the highest temperatures, thus reducing the overall thermal stress on these areas. As a result, the reactor capacity can be increased because the wall temperatures can be temporarily higher than permitted by previous design criteria without significantly increasing the risk of corrosion.

[0018] In the method according to the invention, the hot spot can move, for example in a periodic motion, from an area at the inlet end of a contact tube towards the outlet end and back again towards the inlet end. Preferably, however, the direction of the hot spot movement does not reverse, so that in a preferred embodiment of the method according to the invention, the hot spot moves from an area near the inlet end of a contact tube towards the outlet end.

[0019] In the method according to the invention, the hot spot is designed to move continuously. Here, continuous movement or displacement of the hot spot is understood to occur within the typical operating time of an industrial phosgene reactor, which is typically on the order of many weeks and months. On this timescale, movements of the hot spot that are detectable within the limits of measurement accuracy only over a period of several days can still be considered continuous. Similarly, a periodic movement profile of the hot spot in the contact tube, which is associated with regular reversals of the direction of movement, falls under the definition of continuous movement according to the present invention, even if the migration speed is briefly zero at the reversal points of the movement.

[0020] According to the invention, the gas-phase reaction is carried out such that the migration of the hot spots has a migration velocity in the longitudinal direction of the contact tubes which is in the range of 1 to 50 mm per day. Preferably, the migration velocity of the hot spot is in the range of 2 to 25 mm per day.

[0021] The migration speed of a hot spot can be measured, for example, by installing two or more temperature measuring points along the axis of a contact tube. The migration speed of the hot spot can be directly determined from the travel time of a specific temperature value from one measuring point to the next, since not only the hot spot but essentially the entire temperature profile migrates along the tube axis. Because the temperature profile itself does not change significantly, it is not necessary to measure directly at the hot spot. Instead, the migration speed of the hot spot can be determined by measuring the migration of a specific temperature point outside the hot spot. In fact, the measurement accuracy can even be increased by measuring not directly at the hot spot but on a steeper slope before or after the hot spot, as this minimizes the effects of the hot spot's movement.The largest temperature changes are caused by the overall temperature profile.

[0022] According to the invention, the movement of the hot spot along the longitudinal axis of the contact tube at a predetermined migration velocity is carried out in a controlled manner. The term "controlled" is not only to be understood in the narrower control engineering sense of active control, where the migration velocity of the hot spot is measured as a controlled variable and influenced by suitable manipulated variables so that it reaches the desired predetermined setpoint. Within the scope of the present invention, the term "controlled" also includes passive control, in which, for example, based on preliminary tests and / or suitable reactor models, the influence of different operating parameters and catalyst materials on the migration velocity of the hot spot is investigated, and the operating parameters and catalyst are selected such that the desired, i.e., predetermined, migration velocity can be achieved in actual operation.In these cases, an actual measurement of the migration speed can be dispensed with.

[0023] In this sense, a controlled migration of the hot spot in the contact tubes can be achieved in different ways.

[0024] It is known, for example, that the specific operating conditions influence the position of the hot spot along the longitudinal axis of the contact tubes after the reactor has been started up. According to the method according to the invention, a continuous movement of the hot spot can be achieved by controlled changes to the operating conditions. It is known, for example, that phosgene load, excess CO, pressure, coolant temperature, coolant flow rate, and partial recirculation of excess CO (CO recycling) influence the position of the hot spot along the longitudinal axis of the contact tubes after the reactor has been started up. Therefore, within the framework of the method according to the invention, a controlled migration of the hot spot is achieved by controlled changes to the operating conditions.This allows one to exploit the fact that measures which increase the reaction rate shift the location of the hot spot towards the inlet-side end of the contact tubes, for example higher CO excess, higher pressure, higher coolant temperature or lower coolant flow.

[0025] Increasing the CO surplus can be achieved by partially recirculating the product gas mixture into the feed stream. This measure not only influences the operating conditions with regard to the hot spot migration rate but also reduces the specific carbon monoxide consumption. For this purpose, the valuable product phosgene is separated from the product gas mixture, for example, by condensation to liquid phosgene. A portion of the remaining gas stream is then separated and recycled back into the feed stream upstream of the phosgene reactor. The recycled stream often still has a carbon monoxide concentration in the range of 20 to 60 wt% based on the total weight of the recycled stream.

[0026] Since the position of the hot spot can be shifted towards the inlet end or towards the outlet end of a contact tube depending on the specific changes in the operating conditions, periodic movements of the hot spot in a contact tube can also be achieved by controlling the operating conditions.

[0027] Alternatively or in addition to changing the operating conditions, controlled movement of the hot spot can also be achieved by causing continuous movement of the hot spot through controlled deactivation of the catalyst in the contact tubes.

[0028] As the catalyst becomes increasingly deactivated, the temperature profile shifts towards the outlet end of the contact tubes. This is because, due to the partially deactivated catalyst material, the reactant conversion rate in the inlet area of ​​the contact tubes is lower, meaning that further downstream, more reactants are available for reaction than before the catalyst material was partially deactivated. Therefore, a controlled, gradual deactivation of the catalyst material can only shift the hot spot, or the temperature profile, towards the outlet end of the contact tubes.

[0029] According to the invention, the deactivation of the catalyst material is then controlled in such a way that the desired migration speed of the hot spot is achieved.

[0030] According to one variant of the inventive method, a catalyst can be used that undergoes continuous deactivation under operating conditions, for example, a catalyst that exhibits continuous deactivation under the thermal conditions of the catalytic gas-phase reaction. To adjust the desired migration rate, the operating conditions can be adapted according to the desired deactivation rate, provided this is possible without impairing the throughput. Additionally or alternatively, chemical modification of the catalyst is also conceivable in order to generate the desired hot-spot migration rate through the resulting controlled deactivation. For example, the catalyst can be diluted with inert material, thermally treated, or chemically modified.

[0031] Controlled deactivation of the catalyst can also be achieved by chemically deactivating it through reaction with components from the gas mixture flowing through the contact tubes. This is accomplished by conducting the catalytic gas-phase reaction in such a way that the catalyst gradually transforms into a catalytically inactive or at least less active species through reaction with components from the gas mixture. The reactants for deactivating the catalyst can be starting materials of the catalytic gas-phase reaction, such as carbon monoxide or chlorine in the case of phosgene production. However, products or byproducts of the catalytic gas-phase reaction can also be used for the targeted deactivation of the catalyst.To reduce the yield, byproducts are preferably used to deactivate the catalyst. The reaction conditions can then be adjusted so that byproducts are formed in the desired quantity for deactivation and thus for the migration rate of the hot spot. Furthermore, additional components can be added to the gas stream that do not participate in the catalytic gas-phase reaction but specifically target the continuous deactivation of the catalyst. In the latter case, the advantage is that the actual catalytic gas-phase reaction and the targeted deactivation of the catalyst can be optimized independently of each other; that is, the migration rate of the hot spot can be optimized without adverse feedback on phosgene formation. Suitable components that can be added to the gas stream include, for example, oxygen, chlorine oxides, and mixtures thereof.Instead of actively adding such deactivating components, their desired concentration in the gas stream can also be achieved by not fully purifying the feed streams that make up the gas stream in upstream process steps. For example, a chlorine gas stream originating from a chlor-alkali electrolysis process may initially contain impurities such as oxygen and chlorine oxides, which are usually removed from the gas stream in a chlorine purification step before further use of the chlorine gas. Within the framework of the process according to the invention, the chlorine purification can be adjusted so that a concentration of impurities required for the targeted deactivation of the catalyst is maintained in the chlorine gas stream. Other suitable components that can be added to the feed stream to continuously deactivate the catalyst include, for example, metal chlorides, such as iron, nickel, or molybdenum chlorides.These can be actively added to the feed stream. Alternatively, the chlorine gas from the feed stream can be deliberately routed through metallic pipes, where a small portion of the chlorine gas forms metal chloride compounds with components of the pipe's inner walls. These compounds are then introduced into the phosgene reactor along with the chlorine gas stream.

[0032] In the process according to the invention, various catalysts can be used. However, in the industrial production of phosgene by catalytic gas-phase reaction, activated carbon is preferably used as the catalyst. In this case, deactivation of the activated carbon catalyst can be achieved, for example, by selecting reaction conditions that lead to an increased reaction of carbon and chlorine, for example, forming carbon tetrachloride, so that catalyst material is continuously discharged in the form of carbon tetrachloride during operation, which is equivalent to a partial deactivation of the total amount of catalyst material originally used. Deactivation of activated carbon as a catalyst can also be achieved, for example, by using a feed stream containing a deactivating component, such as oxygen or chlorine oxides.For example, a chlorine feed stream can be used in which the oxygen content is greater than 10 ppm, preferably greater than 20 ppm, to enable continuous deactivation of the catalyst by burning off the activated carbon with the formation of carbon monoxide / carbon dioxide.

[0033] The operating conditions or the deactivation of the catalyst are to be selected such that the integral material removal of the walls of the contact tubes is a maximum of 0.1 mm per year, preferably a maximum of 0.05 mm per year and particularly preferably a maximum of 0.02 mm per year.

[0034] The integral material removal s up to time t after the start of operation at location x along the longitudinal axis of a contact tube is given as a function of a corrosion rate that depends on the catalyst material and the temperature T. KR as: s x = ∫ t = 0 t K R T x , t dt

[0035] Given a known corrosion rate KR, which are used for numerous materials in reactor construction in literature sources, such as the aforementioned " Materials Selector for Hazardous Chemicals" , is documented, and given a known temporal development of the temperature profile T x,t The integral material removal s can therefore be determined in the contact tube.

[0036] Preferably, in the process according to the invention, the feed stream has a stoichiometric excess of carbon monoxide to chlorine of 0.1 to 50 mol%, so that almost complete conversion of chlorine is ensured. If fluctuating chlorine concentrations in the chlorine feed stream are expected, a higher carbon monoxide excess will be chosen; however, for cost reasons, the excess will generally be chosen to be as low as possible, as long as complete chlorine conversion is still guaranteed.

[0037] 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, for example at an absolute pressure of 3 to 7 bar. The higher the pressure of the resulting reaction mixture at the reactor outlet, the higher the temperature at which the phosgene contained in the reaction mixture can condense. 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.

[0038] According to a particular embodiment of the invention, the reactor is divided longitudinally along the contact tubes into at least two cooling zones, which are separated from each other, for example, by intermediate trays. 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. However, since a complete seal of the contact tube penetrations in the intermediate trays is technically complex, and leaks must usually be expected in practice, the same heat transfer fluid is preferably used in the different cooling zones. In this case, for example, boiling cooling can be carried out in one cooling zone with particularly high heat generation, while liquid cooling is carried out in another cooling zone.In boiling cooling, it is preferable to omit baffles or to use specially designed baffles that prevent the backflow of rising gas bubbles. If the reactor has multiple cooling zones, it also facilitates the setting of controlled operating conditions for a defined hot spot migration rate.

[0039] Various substances and mixtures can be used as fluid heat transfer fluids, chosen for their suitability to dissipate 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.

[0040] The process according to the invention can be carried out in a conventional reactor for the production of phosgene by catalytic gas-phase reaction of carbon monoxide and chlorine, particularly when the migration rate of the hot spot is induced by controlled deactivation of the catalyst. The operating conditions and catalyst properties required to adjust the migration rate, especially the corrosion rate of the catalyst, can be determined by preliminary tests and adjusted accordingly in the reactor. Preferably, however, the reactor also includes a control device for monitoring the migration rate of the hot spot. The control device comprises at least one temperature measuring probe for determining the temperature in at least one contact tube at at least two measuring points spaced apart along the longitudinal axis of the contact tube.The control device also includes an evaluation unit that determines the migration speed of the hot spot from the temperature measurements. This allows the migration speed to be... W from the term □ t , which a certain temperature value requires to travel from one measuring point to the next, and the distance □ x The migration velocity can be directly determined from the measuring points along the longitudinal axis of a contact tube according to the formula: W = □ x / □ t

[0041] According to a further embodiment of the invention, the control of the hot spot's migration velocity along the longitudinal axis of the contact tubes is not based on an actual temperature measurement in at least one contact tube. It is also possible to calculate the instantaneous temperature profile in the contact tubes and its changes, and thus also the hot spot's migration velocity, using a reactor model. The reactor model incorporates the current operating conditions and the influence of their changes on the hot spot's position. Based on preliminary experimental investigations, a kinetic model for the catalyst's change due to the operating conditions, including consideration of any impurities in the feed stream, can be developed. A reactor model usable within the scope of the present invention is described, for example, by Michell et al.In "Selections of carbon catalyst for the industrial manufacture of phosgene", Catal. Sci. Technol., 2012, 2, 2109-2115, a reactor with an activated carbon catalyst is described. The reactor model described therein additionally incorporates a kinetic model for the reaction of the carbon catalyst with chlorine gas and possibly oxygen components in the feed stream. From the described model, it can be deduced that, for example, a precise measurement of the chlorine content in the product gas stream can be used to determine the temperature profile in the contact tubes and to regulate the hot-spot migration velocity within the scope of the present invention.

[0042] The invention also relates to a reactor 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, comprising a plurality of contact tubes arranged parallel to one another with inlet and outlet ends, which are filled with the catalyst and are welded into a tube sheet at their inlet and outlet ends, with the reactants being supplied at the upper end of the contact tubes and the gaseous reaction mixture being discharged at the lower end of the contact tubes, each via a hood, as well as with inlet and outlet connections.Discharge devices for a liquid heat transfer medium in jacket form between the contact tubes, wherein the reactor according to the invention is characterized in that the reactor has a control device for monitoring the migration rate of the position of the highest temperature in the contact tubes (hot spot), wherein the control device for monitoring the migration rate has control means for changing the operating conditions of the reactor and that the control means control the addition of a catalyst-deactivating component to the feed stream and / or control the addition of oxygen or chlorine oxides to the feed stream.

[0043] According to a preferred embodiment, the control device comprises at least one temperature measuring probe for determining the temperature in at least one contact tube at at least two measuring points spaced apart from each other along the longitudinal axis of the contact tube, and an evaluation unit. The temperature measuring probe can, for example, be designed as a multi-thermocouple, wherein numerous measuring elements for temperature detection are arranged along the longitudinal axis of the temperature measuring probe.

[0044] As explained above, to determine the migration speed of the hot spot it is not necessary to measure the temperature at the position of the highest temperature, i.e. at the hot spot itself, but the temperature can be measured at any point on the temperature profile, preferably at a point with a high temperature gradient, i.e. at a point where the temperature changes significantly along the longitudinal axis of the contact tube.

[0045] The evaluation unit for determining the migration speed of the hot spot preferably includes a microprocessor which also includes means for time measurement in order to calculate the migration speed W from temperature transit times according to the formula above.

[0046] The control device for monitoring the migration rate of the hot spot also includes control means for changing the reactor's operating conditions in order to set the migration rate to a desired value or to change the migration rate during operation. For example, if, according to a variant of the method according to the invention, the migration of the hot spot is achieved by actively or passively deactivating the catalyst, the migration rate can also be reduced as the operating time progresses, since the thermal stress on the contact tube walls decreases due to the increasing deactivation of the catalyst.

[0047] In a control loop, the control elements for changing operating conditions correspond to the actuators, which can be used to influence the migration velocity of the hot spot as the controlled variable. These actuators can act on a wide variety of manipulated variables, such as coolant flow rate, coolant temperature, surface load, concentration ratios in the feed stream, etc. The control range is typically limited to prevent a reduction in phosin yield.

[0048] In the case of active catalyst deactivation by adding specific deactivating components to the feed stream, the control devices can also affect the concentration or volumetric flow rate of the corresponding deactivating components. The control devices can affect the addition of oxygen or chlorine oxides to the feed stream.

[0049] The contact tubes of the reactor according to the invention can have a length L in the range of 1.5 to 12 m, preferably from 2.5 to 8 m. Reactor tube lengths in the range of 6 to 6.5 m are particularly preferred. Typically, approximately 25 cm at the beginning and end of each contact tube are free of catalyst, since heat dissipation is insufficient in this area due to the installation situation of the tubes.

[0050] Inside the reactor, a bundle, that is, a multitude of contact tubes, is arranged parallel to each other in the longitudinal direction of the reactor.

[0051] The contact tubes are made of a corrosion-resistant material, for example, stainless steel, preferably duplex steel 1.4462, stainless steel 1.4571, or stainless steel 1.4541, or also of nickel-based alloys or nickel. Preferably, the tube sheets or even the entire reactor are also made of the aforementioned materials, in particular duplex steel or stainless steel. However, the reactor jacket and reactor sheets can also be made of more cost-effective metals and metal alloys, for example, carbon steel. Components that come into contact with reactants can then be clad with a protective layer of higher-grade materials.

[0052] 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 a tube inner diameter in the range of up to 100 mm, typically between 35 and 70 mm and preferably between 39 and 45 mm.

[0053] 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.

[0054] Both ends of the reactor are enclosed by hoods. The reaction mixture is fed to the contact tubes through one hood, while the product stream is drawn off through the hood at the other end of the reactor.

[0055] In the hood into which the reaction mixture is fed, a gas distributor is preferably arranged to equalize the gas flow, for example in the form of a plate, in particular a perforated plate.

[0056] Preferably, deflection plates are arranged perpendicular to the longitudinal direction of the reactor in the space between the contact tubes. The deflection plates can, for example, be designed such that successive deflection plates have opposing circular segment-shaped recesses facing the inner wall of the reactor to ensure a meandering flow of the fluid heat transfer medium. In another embodiment, the tube bundle can also be divided into two bundles, with each deflection plate having two opposing circular segment-shaped recesses, and the immediately following deflection plate having a through-opening in a central area of ​​the reactor. The deflection plates cause the heat transfer medium circulating in the reactor interior, in the space between the contact tubes, to be deflected such that the contact tubes are subjected to a transverse flow of heat transfer medium, thereby improving heat dissipation.The number of baffles is preferably about 6 to 35. Preferably, the baffles are arranged equidistant from one another; however, it is particularly preferred that the lowest and the uppermost baffles are each located further from the tube sheet than the distance between any two successive baffles, preferably by a factor of about 1.5. In the area of ​​the through-openings, the reactor is untubed, meaning it is essentially free of contact tubes. In one embodiment, individual contact tubes may be arranged in the through-openings of the baffles. In another embodiment, the through-openings are completely free of contact tubes. Preferably, all baffles leave the same number of through-openings. The area of ​​each through-opening is preferably 5 to 20%, more preferably 8 to 14% of the reactor cross-section.

[0057] Preferably, the deflection plates are arranged around the contact tubes without a seal, allowing a leakage flow of up to 40 vol.% of the total heat transfer fluid flow. For this purpose, gaps of 0.1 to 0.6 mm, preferably 0.2 to 0.4 mm, are provided between the contact tubes and deflection plates. It is advantageous to design the deflection plates to be liquid-tight, except for the areas of the through-openings to the reactor inner wall, so that no additional leakage occurs there.

[0058] The deflection plates can be made of a corrosion-resistant material, preferably stainless steel, in particular duplex steel, preferably with a thickness of 8 to 30 mm, more preferably 10 to 20 mm. However, since the deflection plates do not come into contact with reactants and the contact tubes are usually guided through the openings of the deflection plates with some play, the deflection plates can also be made of more cost-effective materials such as carbon steel.

[0059] According to a preferred embodiment, the jacket of the reactor according to the invention is divided into at least two cooling zones separated by intermediate shelves. The intermediate shelves are preferably made of a higher-grade material, since the openings in the intermediate shelf, through which the contact tubes are guided, should be sealed as tightly as possible with the outer jacket of the contact tubes by means of rolling.

[0060] The contact tubes are filled with a solid catalyst, preferably activated carbon. The catalyst bed 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 bed, where the solid catalyst is assumed to be a solid body. The porosity of the catalyst bodies themselves, which can be, for example, 50%, is not taken into account.

[0061] The invention is explained in more detail below with reference to schematic drawings and an exemplary embodiment: The drawings show Figure 1 is a schematic representation of a reactor suitable for carrying out the process according to the invention; Figure 2 is a detailed view of the reactor described in Figure 1. Figure 1 area of ​​the reactor designated II; Figure 3 a temperature profile across the cross-section of a contact tube of the reactor Figure 1 along a line III-III of the Figure 2 Figure 4 shows a temperature profile along the longitudinal axis of a contact tube of the reactor. Figure 1 at different times; and Figure 5 shows a temperature profile measured at location x as a function of time.

[0062] Figure 1 Figure 1 shows a phosgene reactor 10 suitable for carrying out the process according to the invention, with a substantially cylindrical reactor jacket 11. The figure shown in Figure 10 shows a phosgene reactor 10 suitable for carrying out the process according to the invention, with a substantially cylindrical reactor jacket 11. Figure 1The reactor 10, shown in longitudinal section, has a bundle of contact tubes 12 which are mounted parallel to each other in the longitudinal direction of the reactor 10 and sealed in upper and lower tube sheets 13, 14. At both ends of the reactor, an upper hood 15 with an inlet nozzle 16 and a lower hood 17 with an outlet nozzle 18 are provided. A gas distributor 19 is arranged in the upper hood 15 for equalizing and distributing the gas flows across the reactor cross-section. The contact tubes 12 open into the upper hood 15 via inlet nozzles 21 and into the lower hood 17 via outlet nozzles 22.

[0063] The reactant gas mixture is introduced via the inlet nozzle 16 and distributed via the gas distributor 19 and onto the inlet end 21 of the contact tubes 12. In the example shown, the contact tubes 12 are made of duplex steel 1.4462 and have a typical length L of approximately 6 m, which essentially corresponds to the packing height of the catalyst located in the contact tubes. The contact tubes each have an inner diameter D of 39.3 mm and are filled with cylindrical activated carbon catalyst particles 23 (cf. Fig. 2 ) of 4 mm diameter and 5 mm length. After flowing through the contact tubes 12, the reaction mixture flows from the outlets 22 into the lower hood 17 and is discharged via the outlet nozzle 18.

[0064] A jacket chamber 25 is provided between the contact tubes 12 themselves, or between the contact tubes 12 and an inner wall 24 of the reactor, through which a liquid heat exchanger can flow. For this purpose, a fluid heat transfer medium (not shown) is introduced countercurrently to the gas flow of the reaction gases at the lower end of the reactor 10 via an inlet nozzle 26. The heat transfer medium is guided through the reactor in a meandering flow by means of deflector plates 27 arranged perpendicular to the longitudinal direction of the reactor, which alternately leave openings 28 in the edge region of the reactor, and exits the jacket chamber 25 of the reactor 10 via an outlet nozzle 29. The reactor 10 is untubed in the areas of the openings 28, since in these areas, the transition of the coolant flow from a transverse to a longitudinal flow would result in insufficient cooling of the contact tubes.

[0065] In the illustrated example, reactor 10 has one cooling zone. In alternative embodiments, however, the reactor can also have several, for example two, separate cooling zones separated from each other by intermediate trays. In this case, the cooling zones can be cooled with different heat transfer fluids. Preferably, however, the same heat transfer fluid is used in adjacent cooling zones, since the openings in the intermediate trays for the passage of the contact tubes are very difficult to seal completely. Even when using the same heat transfer fluid, different cooling schemes can be employed. For example, a liquid coolant can be used that dissipates heat in the first cooling zone by means of evaporative cooling, while in the second cooling zone, heat dissipation occurs through pure liquid cooling.

[0066] In Figure 1A control device 30 for monitoring the migration rate of the hot spot's position is also schematically depicted. The control device 30 comprises at least one temperature measuring probe 31, which is inserted into a contact tube 12 via an upper port 32 or a lower port 33. The temperature measuring probe can also be divided, so that one part of the probe is inserted into the contact tube 12 from below and the other from above.

[0067] Figure 2 shows an enlarged section of an area of ​​the reactor cross-section of the Figure 1 , who in Figure 1The diagram is labelled II. Sections of three adjacent contact tubes 12 are visible, surrounded by a jacket 25 through which the fluid heat transfer medium flows. The temperature measuring probe 31 is located in the middle contact tube 12. Besides the deflecting plates 2, the diagram also shows, for orientation, the directional arrows indicating the longitudinal axis of the contact tubes 12 (directional arrow x) and a direction in the cross-section of the contact tubes, perpendicular to the longitudinal direction (directional arrow y). As can be seen particularly in the detailed drawing of the Figure 2The temperature measuring probe 31 is preferably designed as a multi-thermocouple, wherein numerous measuring elements 34 are arranged along the longitudinal axis of the temperature measuring probe 31 to determine a temperature profile in the contact tube. Typically, the distances between the individual measuring elements are in the range of 50–100 mm, with a denser arrangement of the measuring elements being preferred, particularly in the area of ​​the hot spot, to increase the accuracy of the control of the hot spot's migration speed. Each measuring element 34 provides a temperature value Tx i for a point x along the longitudinal axis of the contact tube, which corresponds to the distance of the respective measuring element from the inlet end 21 of the contact tube 12 (in the Figure 1 and 2 (schematically indicated by the arrow x). In Figure 2For example, a temperature value Tx 2 is assigned to a measuring element 34'. When temperature values ​​Tx 1, Tx 2, Tx 3, Tx 4 are mentioned below, it is not necessarily meant that these are temperature values ​​of immediately successive measuring elements 34; however, it is true that a temperature value Tx i+1 is measured by a measuring element that is further away from the inlet end 21 than a measuring element that measures the temperature value Tx i.

[0068] As in Figure 1As schematically indicated by the data line 36, the temperature measuring probe 31 transmits the temperature data Tx 1 , Tx 2 , Tx 3 , Tx 4 , ... to the control unit 30. The control unit 30 has an evaluation unit 35 which determines the migration speed of the hot spot in the contact tube 12 from the data supplied by the temperature measuring probe, the known distance of the measuring elements 34 of the temperature measuring probe from the inlet end 21 and thus also the distance of the measuring elements to each other, and an internal or external clock of the evaluation unit 35. If the determined migration speed deviates from a predetermined target migration speed, the evaluation unit 35 can act on suitable control variables to adjust the migration speed of the hot spot according to the specification.

[0069] In Figure 1Arrows 37 and 38 schematically illustrate the effects on different control variables. Arrow 37 symbolizes that the control device 30 can influence the properties of the feed stream 39 (for example, its temperature and composition) and / or an optional addition of a supplementary feed stream 40 containing components that at least partially deactivate the catalyst. Arrow 38 symbolizes that the control device 30 influences the properties of the coolant stream 41, for example, its temperature and / or, by controlling a coolant pump 42, its volumetric flow rate.

[0070] Figure 3 shows a temperature profile along a Figure 2The diagram shows the diameter line III-III of a cross-section through a contact tube 12 perpendicular to the longitudinal axis of the contact tube. A substantially parabolic temperature profile can be observed inside the contact tube 12 with diameter D, with the highest temperature being reached at the center of the contact tube. The temperature then decreases towards the cooled tube wall of the contact tube. Within the tube wall, the temperature drops substantially linearly to the temperature of the heat transfer fluid boundary layer at the outer wall of the contact tube 12. Within the coolant boundary layer, a further substantially linear temperature drop occurs, caused by external heat transfer, to the bulk temperature of the fluid heat transfer medium.

[0071] Figure 4Figure 1 shows the migration of a typical temperature profile of a contact tube used in industrial phosgene synthesis, achieved, for example, by targeted deactivation of the catalyst within the contact tube. Two temperature profiles, Tt1 and Tt2, are shown, recorded at different times, t1 and t2, respectively. It can be seen that the shape of the temperature profile does not change significantly at different times, but is primarily shifted along the longitudinal axis x of the contact tube. Therefore, the migration velocity of the hot spot HS does not necessarily have to be measured at the maximum of the temperature profile. If the same temperature T is measured at different times and at different measuring points, the migration velocity of the hot spot can be determined, as explained above, using the formula... W = □ x / □ t , with □ x = x3 -x2 and □ t= t 2 -t 1 .

[0072] Figure 5shows the time course of the temperature at a single measuring point (here the Mac measuring point x 3 from Figure 4 ). From the temperature profile over time, the integral material loss at this point can be determined if the corrosion rate KR (T) is known.

Claims

1. A process for producing phosgene by gas phase reaction of carbon monoxide and chlorine in the presence of a catalyst in a reactor comprising a multitude of catalyst tubes arranged parallel to one another that are filled with the catalyst and around which at least one fluid heat carrier flows, in which a feed stream of a mixture of a chlorine feed stream and a carbon monoxide feed stream is guided into the catalyst tubes at an inlet end of the catalyst tubes and is allowed to react in the catalyst tubes to give a phosgene-comprising product gas mixture, and the product gas mixture is removed from the catalyst tubes at an outlet end of the catalyst tubes, which comprises performing the gas phase reaction in the reactor in such a way that the position of the highest temperature in a catalyst tube (hotspot) moves along the longitudinal axis of the catalyst tube at a defined speed of migration, where the hotspot has a speed of migration in longitudinal direction of the catalyst tubes in the range from 1 to 50 mm per day, where continuous movement of the hotspot is brought about by controlled variation of the operating conditions, where the operating conditions are brought about via partial recycling of the product gas mixture into the feed stream, and / or that continuous movement of the hotspot is brought about by controlled deactivation of the catalyst in the catalyst tubes.

2. The process according to claim 1, wherein the position of the hotspot moves continuously in the direction of the outlet end of the catalyst tubes.

3. The process according to claim 1, wherein a catalyst subject to controlled deactivation under the operating conditions is used.

4. The process according to claim 3, wherein the catalyst is continuously chemically deactivated, especially by addition of oxygen to the feed stream.

5. The process according to any of the preceding claims, wherein the feed stream has a stoichiometric excess of carbon monoxide to chlorine of 0.1 to 50 mol%.

6. The process according to any of the preceding claims, wherein the feed stream is fed in at an absolute pressure in the range from 0.5 to 20 bar.

7. The process according to any of the preceding claims, wherein the at least one fluid heat carrier flows around the catalyst tubes in separate cooling zones.

8. A reactor (10) for production of phosgene by gas phase reaction of carbon monoxide and chlorine in the presence of a catalyst comprising a multitude of catalyst tubes (12) arranged parallel to one another that are filled with the catalyst and are welded into one tube sheet (13, 14) at each end, with supply of the reactants at an inlet end (21) of the catalyst tubes (12) and discharge of the gaseous reaction mixture at an outlet end (22) of the catalyst tubes (12), in each case via a hood (15, 17), and with feed and drain devices (26, 29) for a fluid heat carrier into a shell space (25) between the catalyst tubes (12), wherein the reactor (10) has a control device (30) for monitoring the speed of migration of the position of the highest temperature in the catalyst tubes (hotspot), where the control device, for monitoring of the speed of migration, has control means for varying the operating conditions of the reactor, and that the control means control the addition of a catalyst-deactivating component to the feed stream and / or control the addition of oxygen or chlorine oxides to the feed stream.

9. The reactor according to claim 8, wherein the control device (30) has at least one temperature measurement probe (31) for determining the temperature in at least one catalyst tube (12) at at least two measurement sites (Tx1, Tx2, Tx3, Tx4) spaced apart along the longitudinal axis of the catalyst tube and an evaluation unit (35).