Method and reactor for producing phosgene

Abstract

The present invention relates to a method for producing phosgene by gas phase reaction of carbon monoxide and chlorine in the presence of a catalyst in a reactor, the reactor comprising a plurality of contact tubes arranged parallel to one another, the contact tubes being filled with a catalyst and at least one fluid heat exchange medium flowing around the contact tubes, a feed stream of a mixture of a chlorine input stream and a carbon monoxide input stream being introduced into the contact tubes and reacting to form a product gas mixture containing phosgene, characterized in that the product gas mixture is discharged from the contact tubes at the outlet end of the contact tubes. The method of the present invention is characterized in that the gas phase reaction is carried out in the reactor so that the position of the highest temperature (hot spot) in the contact tube moves along the longitudinal axis of the contact tube at a predetermined migration speed, the migration speed of the hot spot in the longitudinal direction of the contact tube being 1 to 50 mm / day. The present invention also relates to a reactor for carrying out the method.

Description

[0001] The present invention relates to a process and a reactor for producing 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 for the production of intermediates and end products in almost all branches of chemistry. In particular, phosgene is a widely used reagent for industrial carbonylations, for example in the production of isocyanates or organic acid chlorides. The largest area of ​​application is the production of diisocyanates for polyurethane chemistry, in particular toluene diisocyanate or diphenylmethane 4,4-diisocyanate.

[0003] Phosgene is produced on an industrial scale in the catalytic gas phase reaction of carbon monoxide and chlorine in the presence of a catalyst (e.g. an activated carbon catalyst) according to the following reaction equation:

[0004]

[0005] The reaction is highly exothermic, and the reaction enthalpy is ΔH is -107.6 kJ / mol. The reaction is usually carried out in a shell and tube reactor by the method described in "Phosgene" chapter (5th edition, A19 volume, page 413 and thereafter, VCH Verlagsgesellschaft mbH, Weinheim, 1991) of Ullmann's Encyclopedia of Industrial Chemistry. In this method, a particle size of 3 to 10 mm, preferably 3.5 to 7 mm, is used in a catalyst tube with a conventional inner diameter of up to 100 mm, usually 35 to 70 mm and preferably 39 to 45 mm. The elongated catalyst tube has a longitudinal axis, wherein the length of the catalyst tube in the industrial-scale phosgene production is measured in the direction of the longitudinal axis to be usually 1.5 m to 12 m. In addition to the short portion at the beginning and end of the tube, the catalyst tube is usually filled with catalyst material so that the bed height of the catalyst material substantially corresponds to the length of the catalyst tube. The reaction usually starts at a temperature of 40 to 120° C. in the inlet area where the reactants enter the catalyst material. In the flow direction of the reactants, as the distance from the gas inlet increases, the temperature in the tube along the longitudinal axis of the tube increases rapidly, and usually reaches the highest temperature in the front half of the tube (from the inlet of the gaseous reactants), which is usually 400° C. or higher, and may be up to 600° C. This point of the highest temperature along the longitudinal axis of the catalyst tube is also called a "hot spot". Thereafter, the temperature further drops rapidly along the catalyst tube, because most of the chlorine used has been converted at the hot spot, and there is less and less chlorine available for forming phosgene further along the catalyst tube. In the reaction, an excess of carbon monoxide is usually used to ensure that all chlorine is converted and to produce most of the chlorine-free phosgene over the length of the catalyst tube, because chlorine can produce undesirable side reactions in the subsequent use of phosgene.

[0006] The reaction can be carried out at ambient pressure, but is usually carried out at elevated pressures of 3 to 7 bar. In this pressure range, the phosgene formed can be condensed downstream of the reactor using cooling water or other (e.g. organic) heat carriers, so that the condenser can be operated in a more economically viable manner.

[0007] Conventional industrial-scale reactors for producing phosgene are described, for example, in the applicant's international patent application WO 03 / 072237 A1.

[0008] An important problem in the design of phosgene reactors is to remove the reaction heat formed. For this reason, the heat carrier flows around the catalyst tubes of the shell and tube reactor to remove the reaction heat generated by the reactor. It has been found that the removal of heat is improved in the case of cross flow around the catalyst tubes. Therefore, it is customary to install baffles in the reactor to enable the heat carrier to cross flow around the catalyst tubes by means of the tortuous flow pattern of the heat carrier. The heat carrier can be a liquid that boils or does not boil under given reaction conditions. In the case of evaporative cooling, as described in European patent application EP 01 34 506 A2, it is preferred to use a liquid that boils usually at 150 to 320° C. under given pressure conditions, which is guided through a typical water-cooled heat exchanger in the loop. It is also possible to combine liquid cooling and evaporative cooling in one reactor.

[0009] Due to the cooling of the catalyst tube and the above-mentioned temperature distribution along the longitudinal direction of the catalyst tube, a temperature distribution also occurs in the cross section of the catalyst tube. Usually, the highest temperature is reached in the center of the catalyst tube, and the temperature drops toward the inner wall of the catalyst tube. In the bed, a temperature distribution at right angles to the longitudinal axis is formed in the cross section, which usually has the form of an inverted parabola with a maximum value in the center. The outward transport of the reaction heat by the cooling medium depends on the following: wall heat transfer from the reaction mixture and catalyst material at the inner wall of the catalyst tube, heat conduction of the shell material of the catalyst tube, and heat transfer of the cooling medium toward the outer wall of the catalyst tube. The key factors here are the thermal conductivity of the catalyst tube material, as well as the temperature, flow rate and Reynolds number of the reaction gas and the cooling medium, and the thermal conductivity of the catalyst material present in the catalyst tube.

[0010] In the case of industrial-scale reactors, at the interface between the catalyst tube and the heat carrier, there are significant differences in the heat transfer coefficient over the reactor cross section, which is caused by, for example, the deflection of the heat carrier from the transverse flow to the longitudinal flow, and also depends on the pressure drop of the heat carrier flowing in the shell space. The heat transfer coefficient of the area with good heat transfer and the area with poor heat transfer in the reaction cross section of the reactor is likely to differ by a factor of 2. Therefore, the cooling efficiency of the catalyst tube is low in the area with poor heat transfer. Another factor that leads to increased thermal stress on the catalyst tube wall material is that the catalyst material used has a high thermal conductivity, for example, activated carbon as a catalyst material is the case. Especially in the hot spot area, as mentioned above, the center of the catalyst tube may have a temperature of up to 600°C, and despite cooling, the wall temperature of the catalyst tube may still increase significantly. In partial load operation, the thermal stress on the catalyst tube may also increase because larger hot spots are formed due to the low heat dissipation efficiency. Therefore, combined with a chlorine-containing atmosphere, corrosion may occur when high temperature acts on the catalyst tube wall for a long time. The effect depends mainly on the wall material used. The sensitivity to this corrosion damage is lowest for catalyst tubes made of nickel-based alloys, and increases from materials such as stainless steel and duplex steel to black steel. It is well known that the corrosion rate in a chlorine atmosphere depends on the temperature for different materials (for example, see "Materials Selector for Hazardous Chemical, Vol. 3, Hydrochloric Acid, Hydrogen Chloride and Chlorine", MIT Publication MS-3, Elsevier Science).

[0011] This corrosion damage can lead to leaks between the product side and the coolant side, resulting in safety-critical conditions in the reactor. When water is used as cooling medium, contact with phosgene produces aqueous HCl, which in turn can lead to corrosion damage to other plant components. Especially in the event of corrosion damage or leaks, it is necessary to shut down the production unit and repair or even replace the reactor, which is associated with high costs and capital costs resulting from production downtime.

[0012] Therefore, when designing a reactor, it is usually assumed that the catalyst tubes (depending on the material used) must have a thermal stress limit. For duplex steels often used for catalyst tubes, the thermal stress limit is usually in the range of about 170 to 200° C., while for stainless steels, the thermal stress limit is about 250° C. However, the operating conditions of the reactor are designed to comply with the thermal stress limit of the catalyst tube wall in the above range, which limits the production and capacity of the reactor to the selected design values.

[0013] The throughput of the reactor can be specified here by the so-called area load or phosgene load of the reactor, which is defined as the amount of phosgene produced per unit time (usually in kg / s), based on the cross-sectional area of ​​the catalyst, i.e. the sum of the internal cross-sectional areas of the catalyst tubes filled with catalyst (usually in square meters). Therefore, in order to control the heat of reaction, area loads of 0.5 to 2 kg phosgene / m are usually operated in the prior art. 2 The phosgene area loading is therefore determined substantially on the assumption that the deficient component is converted completely during operation, ie, for example, in the case of an excess of carbon monoxide, it is determined substantially by the chlorine feed.

[0014] The term "reactor" in the present application includes all components of the equipment in which carbon monoxide and chlorine are chemically converted into phosgene. Typically, the reactor herein is a single component defined by a reactor vessel. However, for example, the reactor in the present application context may also include two or more components of a separate reactor vessel with a continuous (series) arrangement. In this case, the area load is based on the total conversion, i.e., based on the phosgene flow meter leaving the last reactor assembly (e.g., the last reactor vessel).

[0015] The technical problem solved by the present invention is to provide a method for producing phosgene, which method can increase the production capacity of the reactor without increasing the risk of damage to the reactor due to increased thermal stress on the catalyst tubes. The present invention also relates to a reactor for implementing the method of the present invention.

[0016] This technical problem is solved by the method according to the invention. Advantageous developments of the method according to the invention are the subject matter of the dependent claims.

[0017] The present invention thus relates to 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 plurality of catalyst tubes arranged parallel to one another, the catalyst tubes being filled with a catalyst and at least one fluid heat carrier flowing around the catalyst tubes, wherein a gas feed stream (or simply "feed stream") of a mixture of a chlorine feed stream and a carbon monoxide feed stream is introduced into the catalyst tubes at the inlet end of the catalyst tubes and allowed to react in the catalyst tubes to produce a product gas mixture comprising phosgene, and the product gas mixture is removed from the catalyst tubes at the outlet end of the catalyst tubes. The process of the invention comprises carrying out the gas phase reaction in the reactor in such a way that the position of the highest temperature in the catalyst tubes (i.e., the so-called hot spot) moves along the longitudinal axis of the catalyst tubes at a specified migration speed, wherein the migration speed of the hot spot in the longitudinal direction of the catalyst tubes is 1 to 50 mm / day.

[0018] As mentioned above, the catalytic gas phase reactions considered in the context of the present invention are characterized by the formation of a characteristic temperature profile with a pronounced temperature peak, also called a hot spot, over the length of the catalyst tube. If the reaction conditions remain unchanged, the position of this hot spot along the longitudinal axis of the catalyst tube does not change, so that the catalyst tube in this area is subjected to high thermal stress for a long time.

[0019] The invention is based on the finding that the design standards used for reactors to date assume that the highest thermal stresses to which the catalyst tubes are subjected in the hot spot region affect the same region of the catalyst tubes for a long time. In contrast, the invention proposes to reduce the thermal stresses on the catalyst tubes by adjusting the operating conditions of the reactor so that during operation of the reactor, the hot spot moves continuously along the longitudinal axis of the catalyst tubes in the direction of flow of the reaction gas in the catalyst tubes. Since the temperature distribution drops rapidly in the longitudinal direction of the catalyst tubes upstream and downstream of the hot spot (seen in the direction of flow of the reaction gas), a given region of the catalyst tube where the hot spot is currently present is only briefly exposed to the highest temperature, resulting in a general reduction in the thermal stresses in these regions. Therefore, the capacity of the reactor can be increased, since the wall temperature can be temporarily higher than the temperature allowed according to the existing design standards without significantly increasing the risk of corrosion.

[0020] In the process according to the invention, the hot spot can move (e.g. in a periodic motion) from the region of the inlet end of the catalyst tube in the direction of the outlet end and back in the direction of the inlet end. Preferably, however, there is no reversal of direction in the movement of the hot spot, so that in a preferred embodiment of the process according to the invention, the hot spot moves from the region close to the inlet end of the catalyst tube in the direction of the outlet end.

[0021] It is preferred that the hot spot moves continuously in the process according to the invention, whereby a continuous movement or transfer of the hot spot is understood to mean within the scope of a typical operating cycle of an industrial phosgene reactor, which is usually in the range of weeks or months. For example, on this time scale, a hot spot movement which is detectable within the measurement accuracy only for a few days should still be regarded as continuous. The same is true, for example, of a periodic movement profile of a hot spot in a catalyst tube associated with a regular reversal of the direction of movement, which should be understood as a continuous movement within the scope of the invention, even if the migration velocity is briefly zero at the reversal point of the movement.

[0022] According to the present invention, the gas phase reaction is carried out so that the migration speed of the hot spot in the longitudinal direction of the catalyst tube is 1 to 50 mm / day. The migration speed of the hot spot is preferably 2 to 25 mm / day.

[0023] For example, the migration speed of the hotspot can be measured by installing two or more temperature measurement points along the tube axis of the catalyst tube. The travel time taken for a specific temperature value to move from one measurement position to the next can be used directly to infer the migration speed of the hotspot, since it is not only the hotspot that migrates through the tube axis, but essentially the entire temperature distribution. Since the temperature distribution itself does not change significantly, there is no need to measure directly at the hotspot; instead, the migration speed of the hotspot is found in a corresponding manner by measuring the migration of specific temperature points outside the hotspot. In fact, the measurement accuracy can even be improved if the measurement is not made directly in the hotspot, but on the side that drops sharply before or after the hotspot, because it is here that the temperature changes caused by the movement of the hotspot or the entire temperature distribution are at their maximum.

[0024] According to the present invention, the hot spot moves in a controlled manner along the longitudinal axis of the catalyst tube at a defined migration speed. The term "controlled" should not only be understood as the narrow meaning of active control within the scope of control technology, in which the speed of hot spot migration is measured as a controlled variable and is affected by suitable manipulated variables so that it takes the required defined target value. In the context of the present invention, the term "controlled" also includes passive control, in which, for example, using preliminary experiments and / or suitable reaction models, the influence of different operating parameters and catalyst materials on the hot spot migration speed is checked and operating parameters and catalysts are selected so that the required (i.e. defined) migration speed can be achieved in actual operation. In these cases, any actual measurement of the migration speed can be omitted.

[0025] In this case, a controlled migratory movement of the hot spots in the catalyst tubes can be achieved in different ways.

[0026] For example, it is known that certain operating conditions influence the position of the hot spot established after the reactor is started up along the longitudinal axis of the catalyst tube. In a variant of the process of the invention, a continuous movement of the hot spot can therefore be achieved by a controlled change in the operating conditions. For example, it is known that phosgene load, CO excess, pressure, coolant temperature, coolant flow, partial recirculation of excess CO (CO recycle) have an influence on the position of the hot spot established after the reactor is started up along the longitudinal axis of the catalyst tube. Therefore, in the context of the process of the invention, a controlled change in the operating conditions can achieve a controlled migration movement of the hot spot. Therefore, the fact that the position of the hot spot is moved to the inlet end of the catalyst tube by measures that increase the reaction rate can be utilized, i.e., for example, a higher CO excess, a higher pressure, a higher coolant temperature or a lower coolant flow.

[0027] The increase of CO excess can preferably be achieved by partially recycling the product gas mixture into the feed stream. This measure can not only affect the operating conditions about the hot spot migration speed, but also reduce the carbon monoxide consumption rate. For this purpose, the valuable phosgene product is separated from the product gas mixture, for example, by condensing into liquid phosgene. Then a sub-stream is separated from the remaining gas stream and recycled as a recycle stream into the feed stream upstream of the phosgene reactor. Based on the gross weight of the recycle stream, the recycle stream usually still has a carbon monoxide concentration of 20% to 60% by weight.

[0028] Since the position of the hot spot (depending on specific changes in the operating conditions) can move in the direction of the inlet end or in the direction of the outlet end of the catalyst tube, a periodic movement of the hot spot in the catalyst tube can also be achieved by controlled influencing of the operating conditions.

[0029] As an alternative or in addition to a change in the operating conditions, a controlled movement of the hot spot can also be achieved by controlled deactivation of the catalyst in the catalyst tubes causing a continuous movement of the hot spot.

[0030] An increase in catalyst deactivation is associated with a change in the temperature distribution in the direction of the outlet end of the catalyst tube, since, due to the already partial deactivation of the catalyst material, a lower conversion of the reactants occurs in the inlet region of the catalyst tube and even more reactants are available for conversion downstream than before the partial deactivation of the catalyst material. By means of a controlled, gradual deactivation of the catalyst material, it is thus possible to produce only a movement of the hot spot or a shift in the temperature distribution in the direction of the outlet end of the catalyst tube.

[0031] According to the invention, the deactivation of the catalyst material is then controlled in such a way that the desired hot spot migration speed is achieved.

[0032] In a variant of the process according to the invention, a catalyst which deactivates continuously under the operating conditions can be used, for example a catalyst which shows continuous deactivation under the thermal conditions of a catalytic gas phase reaction. In order to establish the desired migration speed, the operating conditions can be adjusted according to the desired deactivation rate, with minimal impact on the production volume. Additionally or alternatively, a chemical modification of the catalyst is also conceivable in order to produce the desired hot spot migration speed by the controlled deactivation thus produced. For example, the catalyst can be diluted with an inert material, or subjected to a thermal treatment or chemical modification.

[0033] The controlled deactivation of the catalyst can also be achieved by reacting with the components of the gas mixture flowing through the catalyst tube to chemically deactivate the catalyst in a controlled manner. For this purpose, the catalytic gas phase reaction is carried out in such a way that the catalyst is converted into a material with no catalytic activity or at least reduced activity over time by reacting with the components from the gas mixture. The reactant used to deactivate the catalyst can be a reactant of the catalytic gas phase reaction, that is, carbon monoxide or chlorine in the case of phosgene production. Alternatively, the product or by-product from the catalytic gas phase reaction can be used to controllably deactivate the catalyst. In order to reduce the decline in yield, it is preferred to use a by-product for deactivating the catalyst, in which case the reaction conditions can be adjusted in such a way that the by-product is formed in the desired amount for deactivation and therefore for the migration speed of the hot spot. In addition, additional components that do not participate in the catalytic gas phase reaction but have the specific purpose of continuously deactivating the catalyst can be provided for the gas stream. In the latter case, the advantage is that the catalytic gas phase reaction itself and the controlled deactivation of the catalyst can be optimized independently of each other, which means that the migration speed of the hot spot can be optimized without any adverse feedback affecting the formation of phosgene. Suitable components that can be supplied to the gas stream are, for example, oxygen, chlorine oxides, and mixtures thereof. In addition to actively supplying this deactivated component, its desired concentration in the gas stream can also be achieved by incompletely purifying the feed stream of the gas stream formed in the upstream process step. For example, the chlorine stream from chlor-alkali electrolysis may first still contain impurities, such as oxygen and chlorine oxides, that are usually removed from the gas stream before further using the chlorine in the chlorine purification step. In the method of the present invention, the purification of chlorine can be adjusted to maintain the concentration of impurities required for controlled deactivation of the catalyst in the chlorine stream. In order to continuously deactivate the catalyst, other suitable components that can be provided to the feed stream are, for example, metal chlorides, such as iron chloride, nickel chloride, or molybdenum chloride. They can be actively added to the feed stream. However, the chlorine in the feed stream can also be intentionally passed through a metal pipe, in which a small portion of the chlorine forms a metal chloride compound with the components of the inner wall of the pipe, and then introduced into the phosgene reactor together with the chlorine stream.

[0034] In the process of the invention, different catalysts can be used. However, it is preferred to use activated carbon as a catalyst in the industrial production of phosgene by catalytic gas phase reaction. In this case, the deactivation of the activated carbon catalyst can be achieved, for example, by selecting reaction conditions that lead to an increase in the reaction of carbon and chlorine (e.g., to form carbon tetrachloride), so that the 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 initially used. The deactivation of the activated carbon as a catalyst can also be achieved, for example, by using a feed stream containing a deactivating component (e.g., oxygen or oxychloride). For example, a chlorine feed stream in which the oxygen content is greater than 10 ppm, preferably greater than 20 ppm, can be used so that the catalyst can be continuously deactivated by burning off the activated carbon to form carbon monoxide / carbon dioxide.

[0035] The operating conditions or the deactivation of the catalyst should be selected such that the overall loss of catalyst tube wall material does not exceed 0.1 mm / year, preferably 0.05 mm / year, more preferably 0.02 mm / year.

[0036] The overall loss of material s along the longitudinal axis of the catalyst tube at position x until time t after the start of operation is considered here to be the corrosion rate K which depends on the catalyst material and the temperature T. R Function:

[0037]

[0038] When a known corrosion rate K is given R , as documented in literature sources (e.g., "Materials Selector for Hazardous Chemical" supra) for various materials used for reactor construction, and given the known temperature distribution T in the catalyst tubes x,t As time goes by, it is therefore possible to determine the overall loss of material s.

[0039] Preferably, the carbon monoxide in the feed stream in the process of the invention is in a stoichiometric excess of 0.1 to 50 mol% relative to the chlorine, thereby ensuring almost complete conversion of the chlorine. If fluctuations in the chlorine concentration in the chlorine feed stream are expected, the selected carbon monoxide excess will be relatively high, but in general, for cost reasons, the selected excess will be as small as possible, so that complete conversion of the chlorine is still ensured.

[0040] The feed stream is preferably fed at an absolute pressure of 0.5 to 20 bar. More preferably, the feed stream is fed at an elevated pressure, for example at an absolute pressure of 3 to 7 bar (absolute). The higher the pressure of the reaction mixture formed at the reactor outlet, the higher the temperature at which the phosgene present in the reaction mixture can be condensed. Preferably, the pressure of the reaction mixture at the reactor outlet is still high enough so that the phosgene can be condensed at least partially with cooling water.

[0041] In a specific embodiment of the present invention, the reactor is divided into at least two cooling zones in the longitudinal direction of the catalyst tube, which are separated from each other by, for example, an intermediate plate. Different heat carriers can be used in each cooling zone, and the selection of the heat carrier can match the thermal conditions in each cooling zone. However, since the complete sealing of the catalyst tube holes in the intermediate plate is technically complicated, it is usually necessary to expect leakage in practice, so it is preferred to use the same heat carrier in different cooling zones. In this case, for example, evaporative cooling can be performed in a cooling zone with particularly high heat release, and liquid cooling can be performed in another cooling zone. In the case of evaporative cooling, it is preferred not to provide baffles or specially designed baffles to prevent the backflow of rising bubbles. When the reactor has multiple cooling zones, it is also helpful to establish controlled operating conditions for the defined hot spot migration speed.

[0042] The fluid heat carrier used can be different substances and substance mixtures suitable for removing the heat of reaction, depending on, for example, their heat capacity or their enthalpy of vaporization. Typically, the liquid heat carrier used is, for example, water, dibenzyltoluene (Marlotherm) or monochlorobenzene.

[0043] The method of the present invention, in particular when the migration rate of the hot spot is caused by the controlled deactivation of the catalyst, can be carried out in a conventional reactor for producing phosgene by the catalytic gas phase reaction of carbon monoxide and chlorine. The operating conditions and catalyst properties (especially the corrosion rate of the catalyst) required to adjust the migration rate can be determined by preliminary experiments and appropriately adjusted in the reactor. However, preferably, the reactor in this case also has a control device for monitoring the migration rate of the hot spot. The control device here includes at least one temperature measuring probe for measuring the temperature at at least two measuring points spaced apart along the longitudinal axis of the catalyst tube in at least one catalyst tube. The control device also includes an evaluation unit, which determines the migration rate of the hot spot based on the temperature measurement. For example, the migration rate W can be directly derived from the travel time Δt used for a specific temperature value to move from one measuring point to the next measuring point and the distance Δx between the measuring points along the longitudinal axis of the catalyst tube according to the following formula:

[0044] W=Δx / Δt

[0045] In another embodiment of the present invention, the control of the migration speed of the hot spot along the longitudinal axis of the catalyst tube is not based on the measurement of the actual temperature in at least one catalyst tube. This is because the current temperature distribution in the catalyst tube and the changes therein can also be calculated from the reactor model, so the migration speed of the hot spot can also be calculated. The input of the reactor model includes the influence of the current operating conditions and their changes on the hot spot position. Using preliminary experimental studies, a kinetic model can be established for the changes in the catalyst caused by the operating conditions (including taking into account any impurities in the feed stream). The reactor model suitable for the present invention is recorded by Michell et al. in, for example, "Selections of carbon catalyst for the industrial manufacture of phosgene", Catal. Sci. Technol., 2012, 2, 2109-2115, which is directed to a reactor with an activated carbon catalyst. The input of the reactor model recorded therein also includes a kinetic model for the reaction of the carbon catalyst with chlorine and any oxygen component in the feed stream. It can be inferred from the recorded model that, for example, the temperature distribution in the catalyst tube can be determined using an accurate measurement of the chlorine content in the product gas stream and the speed of hot spot migration can be adjusted in the context of the present invention.

[0046] The invention also relates to a reactor for producing phosgene by gas phase reaction of carbon monoxide with chlorine in the presence of a catalyst, in particular an activated carbon catalyst, comprising a plurality of catalyst tubes having an inlet end and an outlet end, which are arranged parallel to one another and filled with catalyst and are welded at their inlet and outlet ends to form a tube sheet, the reactants being supplied at the upper end of the catalyst tubes and the gaseous reaction mixture being discharged at the lower end of the catalyst tubes, in each case through a hood, and having a feed and discharge device for introducing a fluid heat carrier into the shell space between the catalyst tubes, wherein the reactor of the invention has a control device for monitoring the migration speed of the highest temperature (hot spot) position in the catalyst tubes.

[0047] In a preferred embodiment, the control device has at least one temperature measuring probe for determining the temperature in at least one catalyst tube at at least two measuring points spaced apart along the longitudinal axis of the catalyst tube, and an evaluation unit. The temperature measuring probe can be implemented, for example, as a multi-thermocouple, in which a plurality of measuring elements for detecting the temperature are arranged along the longitudinal axis of the temperature measuring probe.

[0048] As mentioned above, determining the migration rate of the hot spot does not require measuring the temperature at the location of the highest temperature (i.e. the hot spot itself); instead, the temperature can be measured anywhere in the temperature distribution, preferably at points with high temperature gradients, i.e. at points where the temperature changes significantly along the longitudinal axis of the catalyst tube.

[0049] The evaluation unit for determining the migration speed of the hot spot preferably comprises a microprocessor which also comprises time measurement means to calculate the migration speed W from the temperature travel time according to the above formula.

[0050] The control device (for monitoring the migration speed of the hot spot) preferably also has control means for changing the operating conditions of the reactor in order to adjust the migration speed to a desired value or to change the migration speed during operation. For example, if in a variant of the process according to the invention the migration of the hot spot is achieved by active or passive deactivation of the catalyst, then it is also possible to reduce the migration speed as the operating time increases, since the thermal stress on the catalyst tube wall decreases due to the increasing catalyst deactivation.

[0051] In a closed-loop control loop, the control device for changing the operating conditions corresponds to the drive device, by which the migration speed of the hot spot can be influenced as a controlled variable. The drive device can act on a variety of different manipulated variables, such as coolant flow, coolant temperature, area load, concentration ratio in the feed stream, etc. Usually, the range of the closed-loop control is limited to prevent any reduction in the phosgene yield.

[0052] In the case of active deactivation of the catalyst by adding specific deactivating components to the feed stream, the drive device can also act on the concentration or volume flow of the respective deactivating component. For example, the control device can act on the addition of oxygen or chlorine oxide to the feed stream.

[0053] The catalyst tubes of the reactor according to the invention can have a length L of 1.5 to 12 m, preferably 2.5 to 8 m. Particular preference is given to reactor tube lengths of 6 to 6.5 m. Typically, about 25 cm are free of catalyst at the beginning and end of the catalyst tubes, since, due to the installation position of the tubes, heat removal in this region is insufficient.

[0054] In the reactor, a bundle (ie, a plurality) of catalyst tubes are arranged parallel to each other in the longitudinal direction of the reactor.

[0055] The catalyst tubes are made of corrosion-resistant materials, for example stainless steel, preferably duplex steel 1.4462, stainless steel 1.4571 or stainless steel 1.4541, or of a nickel-based alloy or nickel. The tube sheet or the entire reactor is also preferably made of the above-mentioned materials, in particular of duplex steel or stainless steel. However, the reactor housing and the reactor tray can also be made of less expensive metals and metal alloys, for example of black steel. The components that come into contact with the reactants may then be coated with a protective layer of a high-value material.

[0056] The wall thickness of the individual catalyst tubes is preferably from 2.0 to 4.0 mm, in particular from 2.5 to 3.0 mm, and the inner tube diameter is up to 100 mm, typically from 35 to 70 mm and preferably from 39 to 45 mm.

[0057] The catalyst tubes are fixed at either end in a fluid-tight manner into the tube sheet, preferably welded. The tube sheet is likewise made of a corrosion-resistant material, preferably stainless steel, in particular duplex steel, more preferably the same material as the catalyst tubes. The sealing of the tube sheet is preferably carried out by welding. For example, at least two layers of weld seams can be provided for each tube, which are produced at an offset angle of, for example, 180°, so that the start and end points of the individual layers do not overlap.

[0058] The reactor is surrounded at both ends by external hoods. The reaction mixture is supplied to the catalyst tubes through one hood; the product stream is discharged through the hood at the other end of the reactor.

[0059] In the hood supplied with the reaction mixture, a gas distributor is preferably provided in order to homogenize the gas flow, for example in the form of a plate, in particular a perforated plate.

[0060] The baffle is preferably arranged in the gap between the catalyst tubes, at right angles to the longitudinal direction of the reactor. The baffle can be formed, for example, in such a way that the continuous baffle has circular arc-shaped cutouts facing each other toward the inner wall of the reactor to ensure the tortuous flow of the fluid heat carrier. In another embodiment, the tube bundle can also be divided into two bundles, in which case, a baffle has two circular arc-shaped cutouts facing each other in each case, and the corresponding baffle immediately following it has a channel opening in the central area of ​​the reactor. The function of the baffle is to deflect the heat carrier circulating inside the reactor in the gap between the catalyst tubes, so that the heat carrier is transversely crossed around the catalyst tubes, thereby improving heat dissipation. The number of baffles is preferably about 6 to 35. The baffles are preferably arranged equidistantly from each other; however, more preferably, the distance between the bottom and top baffles is farther from the tube sheet than the distance between two adjacent baffles, for example, about 1.5 times the distance. In the channel opening area, the reactor has no tubes, that is, there are basically no catalyst tubes. In one embodiment, a separate catalyst tube can be arranged in the channel opening of the deflection area. In another embodiment, the channel openings are completely free of catalyst tubes. Preferably, all baffles have the same channel openings. The area of ​​each channel opening is preferably 5% to 20%, in particular 8% to 14%, of the reactor cross section.

[0061] Preferably, the baffles are not arranged in a sealed manner around the catalyst tubes and allow a leakage flow of up to 40% by volume of the total heat carrier flow. For this purpose, a gap of 0.1 to 0.6 mm, preferably 0.2 to 0.4 mm, is provided between the catalyst tubes and the baffles. Advantageously, the baffles are arranged in a liquid-tight manner toward the inner wall of the reactor, except in the region of the channel openings, so that no additional leakage flow occurs there.

[0062] The baffles can be formed of corrosion-resistant materials, preferably stainless steel, especially duplex steel, preferably with a thickness of 8 to 30 mm, preferably 10 to 20 mm. However, since the baffles are not in contact with the reactants and the catalyst tubes usually pass through the openings of the baffles with a certain gap, the baffles can also be made of cheaper materials such as black steel.

[0063] In a preferred embodiment, the shell space of the reactor according to the invention is divided into at least two cooling zones separated by an intermediate plate. The intermediate plate is preferably made of a higher quality material, since the openings in the intermediate plate through which the catalyst tubes are guided should form a very tight seal with the outer shell of the catalyst tubes by rolling.

[0064] The catalyst tubes are filled with a solid catalyst, preferably activated carbon. The catalyst bed in the catalyst tubes preferably has an interstitial volume of 0.33 to 0.6, in particular 0.33 to 0.45. The interstitial volume is based on the catalyst bed, wherein the solid catalyst is considered to be a fully activated body. The porosity of the catalyst body itself (which can be 50%, for example) is not taken into account.

[0065] The present invention is described in detail below through schematic diagrams and embodiments:

[0066] The accompanying drawings show:

[0067] Figure 1 is a schematic diagram of a reactor suitable for implementing the method of the present invention;

[0068] Figure 2 yes Figure 1 A detailed view of the reactor region identified by II;

[0069] Figure 3 is along Figure 2 II-III line Figure 1 Temperature profile of the reactor catalyst tube cross section;

[0070] Figure 4 is along Figure 1 A temperature distribution diagram of the longitudinal axis of the catalyst tube of the reactor at different times; and

[0071] Figure 5 The temperature distribution measured at position x varies with time.

[0072] Figure 1 A phosgene reactor 10 suitable for carrying out the process according to the invention is shown, which has an essentially cylindrical reactor housing 11 . Figure 1The reactor 10 shown in the longitudinal section has a bundle of catalyst tubes 12, which are sealed and fixed in parallel to each other in the longitudinal direction of the reactor 10 in the upper and lower tube plates 13, 14. An upper cover 15 with an inlet short pipe 16 and a lower cover 17 with an outlet short pipe 18 are respectively arranged at both ends of the reactor. The upper cover 15 is provided with a gas distributor 19 for homogenizing and distributing the gas flow over the cross section of the reactor. The catalyst tubes 12 are connected to the upper cover 15 through the inlet end 21 and to the lower cover 17 through the outlet end 22.

[0073] The reactant mixture is introduced via an inlet stub 16 and distributed between the inlet ends 21 of the catalyst tubes 12 by means of a gas distributor 19. In the embodiment shown, the catalyst tubes 12 consist of 1.4462 dual-phase steel and have a typical length L of about 6 m, corresponding essentially to the bed height of the catalyst present in the catalyst tubes. The catalyst tubes each have an inner diameter D of 39.3 mm and are filled with cylindrical activated carbon catalyst particles 23 having a diameter of 4 mm and a length of 5 mm (see Figure 2 After flowing through the catalyst tube 12 , the reaction mixture flows out from the outlet end 22 into the lower cover 17 and is discharged through the outlet short pipe 18 .

[0074] Between the catalyst tubes 12 themselves, and between the catalyst tubes 12 and the inner wall 24 of the reactor, shell spaces 25 are provided, through which a liquid heat exchange medium can flow. For this purpose, a fluid heat carrier (not shown) is introduced at the lower end of the reactor 10 via an inlet stub 26 in countercurrent to the gas flow of the reaction gases. The heat carrier is guided through the reactor in a meandering flow by means of baffles 27 arranged at right angles to the longitudinal direction of the reactor, each of which has a distinct alternating channel opening 28 in the edge region of the reactor, and leaves the shell space 25 of the reactor 10 again via an outlet stub 29. The reactor 10 has no tubes in the region of the channel openings 28, since, due to the change in the coolant flow from a transverse flow to a longitudinal flow, it would be possible for the catalyst tubes in these regions to be cooled only inadequately.

[0075] In the illustrated embodiment, the reactor 10 has one cooling zone. In an alternative embodiment, the reactor may alternatively have two or more (e.g., two) separate cooling zones, which are separated from each other by an intermediate plate. In this case, the cooling zones can be cooled with different heat carriers. However, it is preferred to use the same heat carrier in adjacent cooling zones, because it is very difficult to completely seal the opening in the intermediate plate when passing through the catalyst tube. However, even if the same heat carrier is used, different cooling schemes may be used. For example, a liquid coolant may be used to remove heat by evaporative cooling in the first cooling zone, and heat may be removed by pure liquid cooling in the second cooling zone.

[0076] Figure 1A schematic diagram of a control device 30 for monitoring the moving speed of the hot spot position is also shown. The control device 30 includes at least one temperature measuring probe 31, which is introduced into the catalyst tube 12 through an upper short tube 32 or a lower short tube 33. The temperature measuring probe can also be implemented in a split manner, so that part of the probe is introduced into the catalyst tube 12 from the bottom and part of the probe is introduced from the top.

[0077] Figure 2 Shows Figure 1 A magnified detail of the reactor cross-sectional area, Figure 1 denoted by II in the figure. The cross section of three catalyst tubes 12 arranged side by side is clearly visible, surrounded by a shell space 25 through which the fluid heat carrier flows. The temperature measuring probe 31 is located in the middle catalyst tube 12. In addition to the baffle 2, a directional arrow indicating the longitudinal axis of the catalyst tube 12 (directional arrow x) and the direction in the cross section of the catalyst tube perpendicular to the longitudinal direction (directional arrow y) are also shown. In particular, as shown in Figure 2 As can be seen in the detailed figure, the temperature measuring probe 31 is preferably implemented as a multi-thermocouple, wherein a plurality of measuring elements 34 are arranged along the longitudinal axis of the temperature measuring probe 31 in order to determine the temperature profile in the catalyst tube. Typically, the distance between the individual measuring elements is in the range of 50-100 mm, and in particular in the hot spot area the measuring elements are preferably arranged more closely to improve the accuracy of the closed-loop control of the hot spot migration speed. Each measuring element 34 gives a temperature value Txi of a point x along the longitudinal axis of the catalyst tube, and the point x corresponds to the distance of the corresponding measuring element from the inlet end 21 of the catalyst tube 12 (in Figure 1 and Figure 2 Schematically shown by arrow x in FIG. Figure 2 In the example, the temperature value Tx2 is assigned to the measuring element 34'. When the temperature values ​​Tx1, Tx2, Tx3, Tx4 are mentioned below, this does not necessarily mean that these are temperature values ​​from immediately adjacent consecutive measuring elements 34, but the case is that the temperature value Tx i+1 The temperature value Tx is measured by the distance ratio from the inlet end 21 i The measuring element is measured further away from the measuring element.

[0078] like Figure 1As schematically indicated in FIG. 1 by a data link 36, the temperature measuring probe 31 transmits temperature data Tx1, Tx2, Tx3, Tx4, ... to the control device 30. The control device 30 has an evaluation unit 35, which uses the data provided by the temperature measuring probe, the known distance of the measuring element 34 of the temperature measuring probe to the inlet end 21 and the distance of the measuring elements from each other, and an internal or external clock of the evaluation unit 35 to determine the migration speed of the hot spot in the catalyst tube 12. When the determined migration speed differs from the defined target migration speed, the evaluation unit 35 can manipulate suitable manipulated variables in order to adjust the migration speed of the hot spot according to the set value.

[0079] Figure 1 A schematic diagram of the manipulation of different manipulated variables is shown, represented by arrows 37 and 38. Arrow 37 is intended to indicate that the control device 30 can manipulate the properties of the feed stream 39, such as its temperature and composition, and / or any optionally added feed stream 40 containing a component that at least partially deactivates the catalyst. Arrow 38 indicates that the control device 30 manipulates the properties of the coolant stream 41, such as its temperature, and / or controls its volume flow rate via a coolant pump 42.

[0080] Figure 3 The diameter line III-III (eg, Figure 2 1 ). It can be seen that the temperature distribution in the catalyst tube 12 with diameter D is essentially a parabolic variation, wherein the highest temperature is reached in the center of the catalyst tube. The temperature then decreases toward the cooled tube wall of the catalyst tube. In the tube wall, the temperature decreases essentially linearly to the temperature of the heat carrier interface at the outer wall of the catalyst tube 12. In the coolant interface, there is a further essentially linear temperature drop caused by external heat exchange, down to the overall temperature of the fluid heat carrier.

[0081] Figure 4 The migration of a typical temperature profile of a catalyst tube from an industrial phosgene synthesis is shown, which is achieved, for example, by controlled deactivation of the catalyst in the catalyst tube. Shown are two temperature profiles Tt1 and Tt2 recorded at different times t1 and t2. It can be seen that the shape of the temperature profile does not change significantly at different times, but moves mainly along the longitudinal axis x of the catalyst tube. Therefore, the migration speed 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 measuring points at different times, the migration speed of the hot spot is found, as described above, by the formula W=Δx / Δt, where Δx=x3-x2 and Δt=t2-t1.

[0082] Figure 5 The display shows the Figure 4The temperature distribution with respect to time (with a known corrosion rate K) R (T)) can be used to determine the overall removal of material at that point.

Claims

1. A method for producing phosgene by gas phase reaction of carbon monoxide and chlorine in the presence of a catalyst in a reactor, the reactor comprising a plurality of catalyst tubes arranged in parallel to each other, the catalyst tubes being filled with a catalyst and around which at least one fluid heat carrier flows, wherein a feed stream of a mixture of a chlorine feed stream and a carbon monoxide feed stream is introduced into the catalyst tubes at the inlet end of the catalyst tubes and allowed to react in the catalyst tubes to produce a product gas mixture comprising phosgene, and the product gas mixture is removed from the catalyst tubes at the outlet end of the catalyst tubes, the method comprising carrying out the gas phase reaction in the reactor in such a way that the position of the highest temperature in the catalyst tubes - the hot spot - moves along the longitudinal axis of the catalyst tubes at a specified migration speed, wherein the migration speed of the hot spot in the longitudinal direction of the catalyst tubes is 1 to 50 mm / day, The position of the hot spot therein moves continuously in the direction of the outlet end of the catalyst tube.

2. A method according to claim 1, wherein the continuous movement of the hot spot is obtained by controlled variation of operating conditions.

3. The process according to claim 2, wherein the operating conditions are obtained by recycling part of the product gas mixture to the feed stream.

4. The process according to claim 1 or 2, wherein the continuous movement of the hot spot is achieved by controlled deactivation of the catalyst in the catalyst tubes.

5. The process according to claim 4, wherein a catalyst is used which is subjected to controlled deactivation under the operating conditions.

6. The method of claim 5, wherein the catalyst is continuously chemically deactivated.

7. The process of claim 6 wherein the catalyst is continuously chemically deactivated by adding oxygen to the feed stream.

8. The process of claim 1 or 2, wherein the feed stream has a stoichiometric excess of carbon monoxide relative to chlorine of 0.1 to 50 mol%.

9. The process according to claim 1 or 2, wherein the feed stream is fed at a pressure of 0.5 to 20 bar absolute.

10. The process according to claim 1 or 2, wherein at least one fluid heat carrier flows around the catalyst tubes in a separate cooling zone.

11. A reactor (10) for producing phosgene by gas phase reaction of carbon monoxide with chlorine in the presence of a catalyst, comprising a plurality of catalyst tubes (12) arranged parallel to one another, the catalyst tubes (12) being filled with catalyst and welded at each end to form a tube sheet (13, 14), the reactants being supplied at the inlet end (21) of the catalyst tubes (12) and the gaseous reaction mixture being discharged at the outlet end (22) of the catalyst tubes (12), in each case through hoods (15, 17), and having feed and discharge means (26, 29) for introducing a fluid heat carrier into the shell space (25) between the catalyst tubes (12), wherein the reactor (10) has a control device (30) for monitoring the migration speed of the highest temperature position in the catalyst tubes - the hot spot.

12. A reactor according to claim 11, wherein the control device (30) has at least one temperature measuring probe (31) for determining the temperature in at least one catalyst tube (12) at at least two measuring points (Tx1, Tx2, Tx3, Tx4) spaced apart along the longitudinal axis of the catalyst tube; and an evaluation unit (35).

13. A reactor according to claim 11 or 12, wherein the control device for monitoring the migration rate also has control means for changing the operating conditions of the reactor.

14. The reactor according to claim 13, wherein the control device controls the addition of a catalyst deactivating component to the feed stream and / or controls the addition of oxygen or oxychloride to the feed stream.

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