Process for the preparation of maleic anhydride

Abstract

The invention relates to a process for the production of maleic anhydride (MA) by a catalytic partial gas phase oxidation reaction of saturated and / or unsaturated C4 hydrocarbons, characterized in that the gas coolers 5 (tube bundle coolers) downstream of the isothermally controlled reactor unit 1 are at least partially equipped with a catalyst 7 and are thus at least partially used as post-reactor(s) for the oxidative degradation of the undesired by-products formed in the main reactor 2, in particular acrylic acid, and the cooling capacity of the gas coolers is minimized by a maximum of 20% by the installation of the catalyst.

Description

[0001] With an annual worldwide production of well over 2 million t, maleic anhydride (MA) is an important intermediate for the production of polymers, solvents, dyes, etc. (F. Trifiro, RK Grasselli, Topics in Catalysis 57 (14), 1188–1195). MA production is largely achieved by gas-phase oxidation of saturated and / or unsaturated C4 hydrocarbons with oxygen or oxygen-containing gas mixtures, such as air, and has been known for many years. The oxidation can be carried out in various reactor types (fixed-bed, fluidized-bed, or transport-bed reactors) (Ullmann, Encyclopedia of Industrial Chemistry, 2017, electronic edition, chapter "Maleic and Fumaric Acid"). The most commonly used reactor type for MSA synthesis is a salt bath cooled fixed bed reactor, consisting of a large number of reaction tubes (tube bundle reactor) filled with a bed of shaped catalyst bodies.The catalysts used are mostly full catalysts based on vanadyl pyrophosphate (VO)2P2O7, the so-called VPO catalysts.

[0002] The preparation of the VPO catalyst mass typically involves three steps: the synthesis of the precursor compound VO(HPO4)*0.5 H2O, shaping, and a calcination step to convert the precursor compound into the actual catalytically active VPO compound. All of these steps influence the final performance of the catalyst. To control the catalytic properties, most VPO catalysts are additionally doped with promoter compounds, which can, in principle, be added at any time during preparation. Recent studies also demonstrate the important role of porosity, micropores, and macropores, on the activity and selectivity of the MSA catalyst (Y. Dong, FJ Keil, O. Korup, F. Rosowski, R. Horn, Chem. Eng. Science 142 (2016), pp. 299–309).

[0003] The synthesis of the precursor compound is preferably carried out by the so-called organic route by reacting V2O5 with a reducing agent and H3PO4 in an organic solvent such as i-butanol, the latter serving simultaneously as a reducing agent (US 4,132,670). The addition of H3PO3 as an additional reducing agent and P source has also been described (US 4,382,876). Furthermore, entraining agents to facilitate the removal of water, such as cyclohexane, and so-called structure-forming agents from the alcohol group, organic amine, or phosphorus compounds (EP 1117482 B1), can be added during precursor preparation.

[0004] Shaping is typically achieved through an extrusion process or tabletting of the dried precursor compound. The dimensions of the shaped bodies are adapted to the geometry of the reaction tubes in order to minimize or optimize the pressure drop. Typical tube diameters of the reaction tubes arranged in the fixed-bed reactors are 21–25 mm, and the tubes are generally 3–6 m long. The shaped bodies are generally so-called full catalyst bodies, i.e. they consist almost exclusively of catalyst mass, and are used in the form of cylinders, hollow cylinders (US 4,283,307; WO 01 / 68245), trilobes (EP 0593646), and other special shapes (WO 2012 / 069481 A1, WO 2010 / 072723 A2). The dimensions of the molded bodies range from 3 to 10 mm in height and 3 to 10 mm in diameter. The diameters of the cylinder cavities range from 0.5 to 5 mm.

[0005] The third step, the calcination of the dried precursor compound to the active VPO catalyst mass, vanadyl pyrophosphate, involves the elimination of the water of hydration. Calcination can be carried out batchwise or continuously in a rotary kiln or belt calciner, or directly in the MSA reactor. The processes described in the patent literature differ in terms of heating rate, gas atmosphere composition, holding times, and cooling rates. The goal is to adjust the calcination conditions to the vanadium oxidation state preferred for the catalyst. Typically, values ​​of +4.0 to +4.3 are used (EP 1117482 B1, US 5137860, WO2010 / 072723 A2).

[0006] So-called MSA coated catalysts have also been described, which can be produced by coating inert support materials with the precursor compound, the hemihydrate VO(HPO4)*0.5H2O (EP 0756519 B1), or according to EP 0917909 B1, by coating inert support material with an already catalytically active VPO catalyst mass. In both variants, the geometry of the shaped catalyst body is determined by the inert support material.

[0007] To control the catalytic properties, most VPO catalysts are additionally doped with promoter compounds. In principle, these can be added at any time during production. They are introduced, for example, in the form of soluble promoter compounds such as acetates, acetylacetonates, oxalates, oxides, etc. Promoter elements include Co, Mo, Zn, Fe, Li, Ni, Zr, Ce, Bi, and Cr. The V / promoter element ratios range from 1:0.2 to 0.001 (US 4,132,670, EP 0458541).

[0008] The reaction of C4 hydrocarbons with air or oxygen-containing gas mixtures in the fixed-bed reactor is carried out at temperatures between 380 and 430 °C, depending on the catalyst used. The usual gas velocities (GHSV) are between 1500 and 2000 h -1The C4-HC concentrations are between 1.5 and 2.2 vol.%. For reasons of catalyst stability with regard to performance and lifetime, volatile P compounds such as trimethyl phosphate in the range of 2-20 ppm and water vapor are added to the reactant gas before it enters the reactor (US 5185455).

[0009] The largest by-product in terms of quantity is the CO / CO2 formed. Other by-products include acrylic acid and acetic acid. Acrylic acid is particularly undesirable, as it can lead to the formation of polymeric deposits in subsequent processing steps and also has a significant negative impact on the heat stability of pure MA, an important quality feature.

[0010] Due to the high heat of the target reaction and the significant, highly exothermic side reaction, the total oxidation to CO / CO2, a temperature profile develops in the catalyst bed despite heat removal through the salt bath, with the temperature maximum (hot spot) occurring within the first third of the catalyst bed. Depending on the reaction conditions and catalyst performance, the hot spot temperatures can be between 20 and 60 °C above the corresponding salt bath temperature, i.e., temperatures between 400 and 460 °C can be reached. Temperatures above 440 °C lead to a significant reduction in MSA selectivity (Carsten Becker, (2002) Dissertation: Catalytic Wall Reactor Concepts for MSA Synthesis and Methanol Steam Reforming, Chapter 8, University of Stuttgart). This temperature sensitivity limits the conversion of the C4-KW used in the commonly used multi-tube reactors to approx.85% and thus also the economic viability of this process.

[0011] The MSA is isolated by cooling the reaction gas mixture leaving the reactor, usually in two gas coolers connected in series, followed by a gas scrubbing step. The gas coolers are typically tube bundle coolers that cool the product-laden reaction gas from the reactor outlet temperature (approx. 380–420 °C) to the temperature specific for the downstream processing (200–170 °C). Gas scrubbing is typically performed with an organic solvent that is highly soluble in maleic anhydride (e.g., dibutyl phthalate, WO 2012 / 081043 A1) or with water to form maleic acid. This is followed by further substeps such as concentration, dewatering, dehydration, and distillation.Particularly in these processing steps, the acrylic acid produced during the oxidation reaction causes problems, since its polymers require the processing equipment to be cleaned at great expense or bypass solutions must be available.

[0012] The residual reaction gas freed from the target product MA, mainly consisting of CO / CO2, residual oxygen and inert gas (e.g. nitrogen) as well as unreacted C4 hydrocarbons, is fed into an exhaust gas incinerator for thermal utilization in most current MA production plants. State of the art

[0013] There has been no shortage of attempts in the past to reduce the significantly high C4 loss and the formation of undesirable acrylic acid through optimized catalysts and / or process modifications. US 3,904,652 proposes recycling the C4-containing scrubber offgas into the reactor in a type of cyclic process, thus recycling the unused C4-HC portion to the gas-phase oxidation. US 6,194,587 B1 claims a process in which at least two isothermally operated fixed-bed reactors are connected in series, with the MA formed in the first reactor being largely removed or isolated from the reaction gas via gas scrubbing. The gas mixture leaving the gas scrubber is then recharged with C4 feed and, if necessary, with oxygen, and then converted back to MA in the second reactor. Such a cascade can also consist of three or more reactors.

[0014] Another process variant involves splitting the cooling circuit in the fixed-bed reactor into two or more cooling segments, a so-called two- or multi-zone reactor. The aim is to increase the yield by conducting the MA synthesis reaction with two or more separate cooling circuits compared to the standard procedure (WO 01 / 68626 A1, EP 3360611 A1). This reaction principle can be further optimized by using a VPO catalyst optimized (in shape and composition) for the reaction process in each zone, which could also be optimized for acrylic acid reduction.

[0015] To reduce acrylic acid formation, the use of tungsten- or molybdenum-doped VPO catalysts is described (EP 3771490 B1, US 5929256). The Mo- or W-containing catalysts are preferably used in so-called two-layer systems, each reaction tube is filled with two (or more) catalyst types, which can differ in terms of quantity, shape, and composition. The W- or Mo-doped catalyst system is preferably used as the second layer, i.e. outside the hot spot area and thus in the somewhat colder part of the reactor. This is intended to counteract a simultaneous loss of selectivity with regard to MA formation. There has also been no shortage of attempts to mitigate the problem with acrylic acid and the resulting economic consequences by optimizing the workup chain.US 6120654 describes a distillation variant which is intended to improve the separation of acrylic acid and thereby achieve a significant increase in the color stability of the MA. Task

[0016] Despite progress in optimizing catalysts and process variants, the issue of "unwanted acrylic acid" has not been satisfactorily resolved. The previously mentioned proposals for avoiding acrylic acid by modifying the main reactor require significant additional investment (e.g., more complex reactors, additional isothermal reactors, and larger blowers) and significantly higher energy consumption (heating energy, higher pressure drops). Furthermore, existing MSA plants can only be retrofitted at considerable expense.

[0017] The task was therefore to find a process variant with which - the concentration of unwanted acrylic acid in the downstream part of the MSA process can be reduced without - larger investments, e.g. in the form of an additional salt bath reactor and - can be integrated into existing MSA systems. Subject of the invention

[0018] The solution to the problem is in Fig. 1 and consists in the use of one or more existing gas cooler units 5 downstream of the main reactor 2 as partially operated secondary reactors, in that parts of the cooling pipes 6a, 6b are equipped with a catalyst 7, the quantity, shape and mode of action of which is adapted so that a) a reduction in cooling capacity due to the partial filling of the cooling pipes is a maximum of 20% (cooling capacity of a cooling pipe without catalyst = 100%), and b) an increase in pressure loss is a maximum of 15% (pressure loss of an unfilled cooling pipe = 100%), c) all tubes within a tube bundle cooler are always filled or unfilled in the same way and d) the catalyst 7 is designed in its mode of operation such that it degrades the organic by-products acrylic acid and acetic acid present in the reaction gas, in particular the acrylic acid, to CO / CO2 at temperatures which are below the reaction gas outlet temperature from the main reactor 2, but does not attack the maleic acid.

[0019] Suitable catalysts or active catalyst masses for the targeted oxidation of acrylic acid are VPO catalysts, which can be prepared, for example, according to EP 1117482 B1 or EP 0917909 B1 and contain one or more dopants from the group Li, Fe, Mo, W, Cr, Co, Ni, Ce, Zr, Zn, Bi, preferably one or more from the group Mo, W, Cr, and can be used in particular in the form of coated three-dimensional structures. Doping can take place during any production step of the catalyst. Doping preferably takes place during the coating of the three-dimensionally structured catalyst support. Particular preference is given to VPO catalyst masses whose promoter content, selected from the group Mo, W, and Cr, totals at least 0.5% by weight (based on pure active catalyst mass).

[0020] A preferred form of the process is that the catalyst 7 is placed only in selected temperature ranges of the tube bundle cooler 5, particularly preferred temperature ranges are between 380 °C and 300 °C.

[0021] Another preferred procedure is to alternate catalyst-filled and unfilled zones in the cooling tubes (6b). This counteracts heating of the reaction gas due to the exothermic decomposition of the organic by-products.

[0022] A preferred procedure is the use of catalysts that have a spatial structure, in particular a channel-shaped structure such as monolithic catalyst supports.

[0023] Another preferred catalyst form is network structures or coarse open-pore foams made of metals, metal alloys or ceramic material (coderite, SiC, etc.) which are coated with catalytically active mass.

[0024] Particularly preferred are structures that additionally direct at least a portion of the gas flow to the cooled inner walls of the cooling tubes, thus reducing the reduction in cooling capacity in these areas. Such structures include, for example, open cross-channel structures / gas mixing elements coated with catalyst mass.

[0025] Another possible procedure is the combination of different spatially structured catalysts, whereby, however, all tubes within a tube bundle cooler are always equipped in the same way.

[0026] The following example illustrates the invention in more detail. This example does not represent a limitation within the meaning of the present invention. The invention may encompass any embodiment familiar to a person skilled in the art. Example: Implementation of the method according to the invention

[0027] Fig. Figure 1 schematically shows the process principle of the process according to the invention. In the oxidation section 1, the reactant gas, consisting of C4-HC / air, is passed through a predominantly isothermal tube-bundle reactor 2 (main reactor) cooled by a salt bath cooler 4 at temperatures between 380 and 420°C. The reaction tubes 3 have a diameter of 21-25 mm and a length of approximately 6 m and are filled with a particulate, annular VPO catalyst. The concentration of C4-HC in the reactant gas is generally between 1.4 and 2 vol%, and the gas velocities (GHSV) are between 1400 and 2000 h -1The reactant gas is conditioned to a constant moisture content (2 - 3 vol.% water) before entering the main reactor, and a phosphorus compound such as trimethyl phosphate (2 - 20 ppm) is added after the start-up process.

[0028] Due to the high heat of the target reaction and the significant side reaction, total oxidation to CO / CO2, a temperature profile develops within the first third of the catalyst bed, despite heat removal through the salt bath. The height and position of the temperature profile, as well as the maximum temperature in the catalyst bed (hot spot), are recorded by several multi-thermocouples evenly distributed across the reactor cross-section, which are centrally positioned in appropriately selected reaction tubes, and used to control the reactor (C4-HC concentration / conversion / temperature). This ensures that the maximum permissible difference between the maximum temperature in the catalyst bed (T Hot-Spot ) and the salt bath temperature (T Salzbad , measured at the inlet of the molten salt into the reactor) is less than 60° C.

[0029] The product gas leaving the main reactor 2, consisting primarily of O2 / N2 / CO / CO2 / MA / H2O, residual unconverted C4-HC, and the undesired by-products acrylic acid and acetic acid, is passed, as usual, through two consecutively arranged gas cooler units 5, which are designed as so-called shell-and-tube coolers (approximately 7 m long tubes, inner tube diameter 25 mm). In the first cooler, the reaction gas is cooled from approximately 420 °C to approximately 350 °C, then passed to the next gas cooler unit, where it is cooled to a final temperature of approximately 200 °C. The reaction gas is then fed to the further processing units 8. In contrast to the original process, in the process according to the invention, in the lower part of the 1st tube bundle cooler (T range 370 - 350 °C), each tube 6a is filled with a 1 m high catalyst bed 7, consisting of 10 cm long cross-channel elements (diameter 23 - 24 mm) coated with active VPO catalyst mass.The 1 m long catalyst bed (10 x 10 cm long elements) has a gas exchange area of ​​approximately 5000 cm per tube. 2 , coated with an approximately 30 µm thick layer of VPO catalyst mass (Mo content: 0.5 wt%). The coated cross-channel elements can be manufactured using a dipping process as described in EP 0917909 B1.

[0030] In the second downstream gas cooler unit, another catalyst bed (6b) with a total length of 1 m is installed in the cooling pipes, but this time in the gas inlet area of ​​the cooler (T-range 340 - 320 °C), with the difference that the catalytically active VPO catalyst mass has a Mo content of 0.7 wt.% and the catalyst bed is interrupted by an unfilled zone.

[0031] The catalyst configuration, such as catalyst quantity, number of catalyst layers, cell density, etc., is selected such that the additional pressure loss resulting from the partial filling of the gas coolers can be covered by the power of the existing blower, and the conditions in the main reactor are not negatively affected. The additional pressure loss is approximately +10% (pressure loss of an unfilled cooling tube = 100%). The loss of cooling capacity caused by the partial filling of the cooling tubes should also not exceed 10% (cooling capacity of a cooling tube without catalyst = 100%). A further feature of the process according to the invention is that all tubes within a tube bundle cooler are always filled to the same level. If a cooling unit consists of several coolers connected in parallel, all tubes of such a cooling unit must be filled to the same level.

[0032] The further processing steps 8 such as isolation, cleaning, etc. of the MSA are carried out according to the state of the art or with the usual sub-steps, depending on the design of the plant, such as gas scrubbing, concentration, dewatering, dehydration and distillation as well as thermal utilization 9 of the exhaust gas.

[0033] The process according to the invention leads to a significant reduction in unwanted acrylic acid without a noticeable loss in MSA yield, which significantly improves downtimes of the processing units (e.g., distillation), product losses, the heat stability of the MSA, etc., and leads to a considerable increase in economic efficiency (50,000 t plant: savings of between $1 and $2 million). A further advantage of the process according to the invention is that it can be integrated into existing MSA plants with virtually no investment required. List of reference symbols 1 isothermal oxidation unit 2 tube bundle reactor (main reactor) 3 reaction tubes filled with particulate VPO catalyst 4 salt bath coolers with pump 5 gas coolers, designed as tube bundle coolers 6a Cooling pipes partially filled with catalyst 6b cooling tubes partially filled with catalyst in several zones, 7 VPO catalyst 8 Reprocessing part 9 Exhaust gas combustion QUOTES CONTAINED IN THE DESCRIPTION

[0000] This list of documents submitted by the applicant was generated automatically and is included solely for the convenience of the reader. This list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions. Cited patent literature

[0000] US 4132670 [0003, 0007] US 4382876

[0003] EP 1117482 B1 [0003, 0005, 0019] US 4,283,307

[0004] WO 01 / 68245

[0004] EP 0593646

[0004] WO 2012 / 069481 A1

[0004] WO 2010 / 072723 A2 [0004, 0005] US 5137860

[0005] EP 0756519 B1

[0006] EP 0917909 B1 [0006, 0019, 0029] EP 0458541

[0007] US 5185455

[0008] WO 2012 / 081043 A1

[0011] US 3904652

[0013] US 6194587 B1

[0013] WO 01 / 68626 A1

[0014] EP 3360611 A1

[0014] EP 3771490 B1

[0015] US 5929256

[0015] US 6120654

[0015] Zitierte Nicht-Patentliteratur

[0000] F. Trifiro, R.K. Grasselli, Topics in Catalysis 57 (14), 1188 - 1195

[0001] Ullmann, Enzyclopedia of Industrial Chemistry, 2017, elektronische Ausgabe, Kapitel Maleic and Fumaric Acid

[0001] Y. Dong. F.J. Keil, O. Korup, F. Rosowski, R.Horn, Chem. Eng. Science 142 (2016) S. 299 - 309

[0002] Carsten Becker, (2002) Dissertation: Catalytic wall reactor concepts for MSA synthesis and methanol steam reforming, Chapter 8, University of Stuttgart

[0010]

Claims

[1] The invention relates to a process for the preparation of maleic anhydride (MA) by a catalytic partial gas phase oxidation reaction of saturated and / or unsaturated C4 hydrocarbons, characterized by that the gas coolers (tube bundle coolers) downstream of the isothermally controlled reactor unit 1 are at least partially equipped with a catalyst 7 and are thus at least partially used as post-reactor(s) for the oxidative degradation of undesired by-products, in particular acrylic acid. [2] A process for producing MSA according to claim 1, characterized by that the partial catalyst filling of the gas cooler tubes 6 reduces the original cooling performance by a maximum of 20%, preferably by a maximum of 10%, particularly preferably by a maximum of 5%. [3] Process for the preparation of MSA according to claim 1-2, characterized bythat the partial catalyst filling increases the original pressure loss in the gas cooler 5 by a maximum of 15%, preferably by a maximum of 10%, particularly preferably by a maximum of 5%. [4] Process for the preparation of MSA according to claims 1-3, characterized by that catalyst-filled zones and unfilled zones 6b alternate in the gas cooler tubes, but all cooling tubes within a cooling unit are always filled in the same way. [5] Process for the preparation of MSA according to claims 1-4, characterized by that the catalyst 7 is placed only in selected temperature ranges of the tube bundle cooler 5, particularly preferred temperature ranges are between 380 °C and 300 °C. [6] Process for the preparation of MSA according to claims 1-5, characterized bythat the catalysts 7 used in the downstream gas coolers have a spatial structure, preferably channel-shaped structures (monolithic structure), particularly preferred are structures which direct part of the gas flow to the inner walls of the cooling tubes (e.g. open cross-channel structures). [7] Process for the preparation of MSA according to claims 1-6, characterized by that the catalysts 7 used in the gas coolers have a mesh structure and / or coarse open-pore foam structure, which are made from appropriately structured carrier bodies made of metals, metal alloys or ceramic material (coderite, SiC, etc.) via a coating with active catalyst mass. [8] Process for the preparation of MSA according to claims 1-7, characterized bythat the catalysts 7 used in the downstream gas coolers are so-called VPO catalysts, the active mass of which contains one or more dopants from the group consisting of Li, Fe, Mo, W, Cr, Co, Ni, Ce, Zr, Zn, Bi, preferably one or more from the group consisting of Mo, W, and Cr, and is used in particular in the form of coated three-dimensional structures. Particular preference is given to VPO catalyst masses whose promoter content, selected from the group consisting of Mo, W, and Cr, is at least 0.5 wt.% in total. [9] Process for the preparation of MSA according to claims 1-8, characterized bythat the gas phase oxidation of saturated and / or unsaturated C4 hydrocarbons takes place in an isothermally controlled tube bundle reactor 2 filled with particulate VPO catalyst, which is followed by one or more gas cooling units 5 (tube bundle coolers) which are at least partially equipped with a catalyst 7 and thus accelerate the oxidative degradation of the acrylic acid formed in the main reactor 2 in temperature ranges below 380°C, wherein the catalyst 7 has a spatially structured geometry and whose active mass is a VPO catalyst mass which contains one or more dopants preferably from the group Li, Fe, Mo, W, Cr, Co, Ni, Ce, Zr, Zn, Bi, particularly preferred VPO catalyst masses whose dopant contents, selected from the group Mo, W and Cr, are in total at least 0.5 wt.%.

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