C-4-based process for the production of mma using methacrolein return and recycle

Abstract

Improved processes for the production of methacrylates, especially methacrylic acid and methyl methacrylate (MMA), based on C-4 feedstocks are described. In this case the C4 feedstock is especially isobutene or tertiary butanol. In addition, a wide range of embodiments for the efficient production of these products is provided. The invention especially relates to a process for the production of methacrylic acid and methyl methacrylate by gas phase oxidation of isobutene. In a second gas phase oxidation step, methacrolein is converted to methacrylic acid and unconverted methacrolein is removed from the methacrylic acid. This methacrolein is evaporated in the presence of various gases and vapors in an evaporation device and mixed in gaseous form with the process gas of the first oxidation stage in which the methacrolein was produced. In this way, the recycled methacrolein can be fed back into the process and catalytic partial oxidation.

Description

Technical Field

[0001] This invention describes an improved method for preparing methacrylates, particularly methacrylic acid and methyl methacrylate (MMA), from C-4 feedstocks. In this case, the C4 feedstock is particularly isobutylene or tert-butanol. Furthermore, a wide range of embodiments for the efficient preparation of these products are provided.

[0002] This invention particularly relates to a method for preparing methacrylic acid and methyl methacrylate by gas-phase oxidation of isobutylene. In a second gas-phase oxidation step, methacrolein is converted to methacrylic acid, and unconverted methacrolein is removed from the methacrylic acid. This methacrolein is evaporated in an evaporation apparatus in the presence of various gases and vapors and mixed in gaseous form with the process gas from the first oxidation stage in which methacrolein was prepared. In this way, the recycled methacrolein can be resupplyed to the process and catalytic oxidation sections. Background Technology

[0003] Methyl methacrylate (MMA) is widely used in the preparation of polymers and in copolymers with other copolymerizable compounds. Furthermore, MMA is an important synthetic unit for a variety of specialty esters based on methacrylic acid (MAA), which are prepared by transesterification with suitable alcohols.

[0004] Therefore, there are significant benefits to preparing this important chemical product using a very simple, economical, and environmentally friendly method.

[0005] The preparation of MMA is based on three possible groups of raw materials. One is the C3 unit, and the other is the C4 unit and the C2 unit.

[0006] Unit C3 is the first commercially viable group. Here, MMA is primarily prepared from hydrogen cyanide and acetone via acetone cyanohydrin (ACH), which is formed as a central intermediate. A drawback of this method is the yield of very large quantities of ammonium sulfate, with subsequent processing associated with very high costs. Other methods using raw materials other than ACH have been described in relevant patent literature and have been implemented on a production scale.

[0007] Currently, it is increasingly important to use isobutylene or tert-butanol as C-4-based starting materials for the preparation of methacrylates. These are converted into the desired methacrylate derivatives through multiple process stages. As a third alternative, methyl tert-butyl ether (MTBE) can also be used, which is converted to isobutylene by eliminating methanol.

[0008] In this C4-based method, isobutylene or tert-butanol is typically oxidized to methacrolein in the first stage, and then this methacrolein is reacted with oxygen to produce methacrylic acid. The resulting methacrylic acid is then converted to MMA using methanol. Further details about this method are given in particular in the following literature: Ullmann's Encyclopedia of Industrial Chemistry 2012, Wiley-VCH Verlag GmbH&Co. KGaA, Weinheim, Methacrylic Acid and Derivatives, DOI: 10.1002 / 14356007.a16_441.pub2 and Trends and Future of Monomer-MMA Technologies, SUMITOMO KAGAKU 2004-II. More detailed descriptions of general methods for the preparation of MMA and methacrylic acid, particularly of multi-stage gas-phase methods starting from the C4 unit, can be found in "Many paths lead to methacrylic acid methyl ester" by Krill and Rühling et al., WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, doi.org / 10.1002 / ciuz.201900869.

[0009] Generally, the C4 route begins with the steam cracking product IBEN or TBA, which is oxidized to MAL in the first step via gas-phase oxidation. In a second gas-phase oxidation stage, methacrolein (MAL), obtained as an intermediate, is oxidized to MAA. The gaseous reaction products are cooled and largely condensed in a downstream quenching step. A key feature of this method is that the second reaction stage does not involve the complete conversion of MAL, and the unconverted MAL is recovered in an absorption and deabsorption unit (recycled MAL) so that it can subsequently be fed back as feed for the second reaction stage.

[0010] Isobutylene or tert-butanol can react with atmospheric oxygen in the gas phase over a heterogeneous catalyst to produce methacrolein (MAL), which is then converted to MMA via oxidative esterification with methanol. This method is specifically described in US 5,969,178 and US 7,012,039. This method is also described in a SUMITOMO article (see above), which explicitly mentions the disadvantages of this method, particularly the high energy requirement caused especially by the non-pressurized operating mode. In this method, since the process occurs in the liquid phase, the problem of MAL evaporation is avoided, and therefore MAL does not need to be converted to a gaseous state, thus avoiding the problem of mixing with critical oxygen-containing gases. A solution for optimizing the two-stage isobutylene gas-phase method using MAA as an intermediate stage cannot be derived from this.

[0011] Relatively unsatisfactory yields (especially due to high losses in the oxidation step and the resulting CO2 formation) are also a problem in all these methods. The formation of byproducts must also be noted, which further accompanies this process and necessitates complex process steps for product separation. For example, all methods starting from isobutylene or equivalent C-4-based feedstocks such as TBA or MTBE achieve 80%–90% selectivity for each process stage in gas-phase oxidation on heterogeneous catalyst systems. Thus, a total yield of at most 65%–70% is achieved. Naturally, gas-phase methods are carried out at moderate pressures of 1–2 bar absolute and produce process gases in which product components are present only at approximately 4%–6% vol%. Separation of valuable products from inert gas ballasts is correspondingly energy-intensive and consumes significant amounts of cooling energy and steam in multi-stage distillation post-processing steps.

[0012] Therefore, in general, the following three C4-based MMA preparation routes are well known:

[0013] Method A, the “tandem C4 direct oxidation method,” eliminates the intermediate separation of methacrolein: Here, in the first step, methacrolein is prepared from isobutylene and oxidized to methacrylic acid in method step 2, then in method step 3, MMA is obtained by esterifying methacrylic acid with methanol. This method is also referred to in the literature as the “tandem method” because the process gas from the first stage is directly oxidized to methacrylic acid without the need for separation of the methacrolein intermediate.

[0014] Method B, “C4 Direct Oxidation Alone”: Here, similar to Method A, isobutylene is prepared into methacrolein in the first method step, and is isolated and purified in liquid form in a separate method step, then evaporated and oxidized to methacrylic acid in method step 3, and finally converted to MMA by esterification in method step 4.

[0015] Method C, the "direct methylation method," or direct oxidative esterification method: Here, in the first method step, methacrolein is prepared from isobutylene in the gas phase over catalyst I, and similarly, it is first separated and intermediately purified in method step 2, and then directly oxidized and esterified to MMA in method step 3. Method step 3 is carried out in the liquid phase over suspended catalyst II. Therefore, the combination of gas-phase and liquid-phase method steps is the essential difference between this method and methods A and B, where the two partial oxidations are carried out on different catalysts, each in the gas phase.

[0016] All methods described in the prior art are well documented, for example in (i) IHS Chemical Process Economics Program, Review 2015-05, RJ Chang, Syed Naqvi or (ii) Vapor Phase Catalytic Oxidation of Isobutene to Methacrylic Acid, Stud. Sci. Catal. 1981, 7, 755-767.

[0017] Methods A and B share the commonality of using methacrylic acid as the major product, which can be optionally converted to methyl methacrylate in the esterification reaction with methanol. Generally, an activated Brønsted acid catalyst is used for this final esterification step. In a homogeneous variant, a dissolved strong acid can be used. Preferred acids are sulfuric acid, methanesulfonic acid, or acids with a similar pKa. In another variant, an ion exchanger with fixed acid functional groups can be used.

[0018] Method C, or the "direct methylation method," proceeds in a similar manner to Method B, using intermediate separation of liquid methacrolein as described in the first two steps. In this case, MMA is obtained directly from methacrolein in liquid-phase oxidation, without the need for the intermediate step in the production of methacrylic acid.

[0019] Therefore, methods A and B are characterized in that only a portion of the MAL contained in the gas mixture undergoes conversion during the second oxidation stage. Besides carbon monoxide and carbon dioxide, many other reaction products are formed, particularly formaldehyde, acetic acid, acetone, acetaldehyde, and acrolein. In subsequent processing steps, the unconverted MAL must be separated from the desired methacrylic acid product. Typically, this separation is carried out by at least one distillation or extraction step. The MAL itself is absorbed from the process gas and further processed and purified to a certain extent, and separated by desorption and, unnecessarily, at least one distillation operation. Further distillation is optional after absorption and desorption. Due to the presence of volatile but condensable components besides acrolein, crude methacrylic acid with some organic byproducts is obtained, typically a MAL mixture with a concentration >70% by weight, depending on the complexity of the separation. This means that the crude methacrylic acid mixture obtained in liquid form by removing methacrylic acid may contain not only trace components such as methacrylic acid, acetic acid, terephthalic acid, and stabilizers, but also a certain amount of components with similar or lower boiling points compared to MAL. There are various methods for reintroducing liquid-recycled methacrolein into the main gas system (especially upstream of the second reaction stage). The steps of evaporating crude methacrolein and mixing it with other gases and vapors, especially the addition of oxygen-containing gases, particularly the mixing and recycling into the process, are complex and critical operations that are not satisfactorily achieved in the prior art. Some of these minor components have critical flash points and affect the explosive characteristics of the resulting mixture containing oxygen-containing gases. The method of reintroducing recycled methacrolein into the main process, for example, upstream of the second reaction stage, is a particularly safety-critical step in the entire process. Many suggestions have been made in the prior art to address this problem by various methods in order to return incompletely converted methacrolein to the process. However, the prior art is not satisfactory, particularly in the following safety-related aspects: removal after the second oxidation stage, recycling of unconverted methacrolein, its conditioning and evaporation, mixing with inert gases and oxygen-containing gases, and additional conditioning with vapor as necessary to create optimal conditions for the second gas-phase oxidation step.

[0020] In all these processes, there are safety-related difficulties, especially during the removal and separation of the crude methacrolein fraction and its evaporation and conditioning with gas, which must be optimized:

[0021] 1) Methacrolein is typically separated into a liquid form after the removal of methacrylic acid. It must then be converted to a gaseous form by evaporation and introduced into a gaseous reaction system. Liquid-recycled methacrolein reacts sensitively at present temperatures, where polymerization, dimerization, and other undesirable associated effects may occur above specific temperatures and within specific residence times. Therefore, it is necessary to find a route in which, for a given energy input required for evaporation, the liquid-recycled methacrolein is maintained at a concentration such that the residence time can be kept so short that polymerization can be minimized or completely stopped.

[0022] 2) Recycled methacrolein may contain not only trace amounts of components such as methacrylic acid, acetic acid, terephthalic acid, and stabilizers, but also significant amounts of components with similar or lower boiling points. Furthermore, terephthalic acid and commonly used stabilizers are catalyst poisons for the second-stage catalyst because they contribute to increased coking and deposition on the catalyst during long-term operation. This, in turn, negatively impacts catalyst performance and lifespan. Therefore, it is necessary to remove terephthalic acid and stabilizers, such as hydroquinone (HQ), as much as possible before reintroducing recycled methacrolein from upstream into the second-stage gaseous reaction system. A further significant problem arising from components with boiling points higher than methacrolein is the observable deposition and deposition in the evaporation and return areas, particularly on many equipment components and piping used in long-term operation. This leads to operational interruptions and costly and inconvenient downtime for removing these residues. Therefore, the considerable need for improvement is also an object of this invention.

[0023] 3) Depending on the catalyst state, the two temperature-controlled reaction zones have different temperature levels. The gas stream leaving the first reaction zone must be cooled before entering the second reaction zone. This is because, in conventional methods according to existing technology, the salt bath temperature of the first stage (which can be considered a direct indicator of the reaction temperature) is 30°C to 100°C higher than the corresponding salt bath temperature of the second stage. Generally, the hotter process gas in the first stage is cooled by mixing it with a cooler oxygen-containing gas stream containing recycled methacrolein. This procedure increases the oxygen content of the feed gas in the second stage while allowing the recycled methacrolein to be returned to the process. In terms of safe operation, this method reacts very sensitively to all physical parameters and stoichiometry, as well as residence time and the quality of the mixture. Otherwise, higher oxygen concentrations and a certain residence time in a higher temperature zone would increase the risk of "post-combustion effects." Post-combustion is a known phenomenon for some temperature-sensitive substances, such as methacrolein in gaseous form. The corresponding substance tends to decompose into smaller fragments. These, in turn, can lead to spontaneous pressure and / or temperature increases, which are problematic from a safety perspective.

[0024] 4) Last but not least, methacrolein cannot be completely converted in the second reaction zone. Reasonably good selectivity can be achieved at conversion rates of 70%–85%. However, at higher conversion rates, selectivity begins to decline significantly. To achieve high yields of methacrylic acid, methacrolein must be recycled. Recycling consists of (i) cooling and quenching the process gas in a thermostatically controlled condensate of the process liquid and (ii) absorption, desorption, and unnecessary distillation of methacrolein in downstream equipment units. The subsequent reintroduction of methacrolein involves the evaporation of a certain mass of recycled methacrolein and requires mixing the evaporated crude methacrolein with oxygen-containing gases such as recycled gas, air, and unnecessary vapors. In some cases, explosive gas mixtures may form during this recycling step due to localized high temperatures or excessively high fuel concentrations above the lower explosive limit. Particularly in long-term, continuous operation, some equipment components and piping exhibit precipitation, contamination, and deposition of byproducts and polymer materials, with inappropriate distribution observed, which, combined with excessively high oxygen concentrations, leads to critical safety conditions. Finally, this could also lead to overpressure release through the safety vent, or in the worst case, even a localized explosion, or, depending on the severity of the damage, a deflagration.

[0025] Therefore, a new approach to improving process safety and optimizing yield at the industrial scale must take into account each of these four aspects discussed.

[0026] Therefore, for example, in USA 7,799,946 and GB 2,004,886, the recycled methacrolein is first passed through a quench tower (quenched with a process medium), where the heavier components of methacrylic acid are condensed. The methacrolein-containing gas is absorbed in a subsequent tower, and finally desorbed and stripped from the absorbed liquid to obtain a methacrolein-containing gas mixture that can be introduced into a second reactor. Here, this gas mixture is a non-explosive gas with a sufficiently low oxygen concentration. However, even with this method, oxygen is insufficient to adequately and optimally supply the second-stage reaction. Therefore, additional oxygen-containing gas must be supplied again before the reaction in the second stage.

[0027] EP 3 007 69 describes a special distributor configuration at the inlet point of the methacrolein-containing recirculated gas stream and the mixing of this gas stream with the reaction gas of the first stage. This is done to prevent the aforementioned post-combustion effect that leads to the decomposition of methacrolein. EP 3 007 69 further describes a method for mixing the hotter process gas of the first stage with the cooler methacrolein-containing recirculated gas stream. This is done via a special arrangement of distributors or nozzles through a spray injection of the gas stream, which is not necessarily mixed with air, inert gas, and recirculated methacrolein. As a result of this injection, the hotter gas (which has, for example, a temperature of 350°C-400°C) is cooled to a resulting temperature of 200°C-240°C. In this method, the methacrolein-containing gas mixture will contain an oxygen-enriched concentration of up to 13 mol% at a higher methacrolein concentration. This can thus create an explosive gas. Even if the gas mixture is sufficiently cooled below the ignition temperature by spray injection, local hot spots with temperatures above the ignition point cannot be ruled out. Hot spots are observed forming in the distributor due to tar-like deposits and problematic polymer residues. These localized deposits in critical areas lead to contamination and blockage of sections of the piping and the distributor area. While the described implementation of this design is theoretically suitable for the equipment, it is prone to fouling, especially in continuous, permanent operation, necessitating periodic shutdowns to remove these contaminants, which are sometimes tar-like and sometimes powdery. Contaminants can be removed by pyrolysis through combustion, manual removal, or flushing with a suitable medium. Complete cleaning of the equipment is typically only possible after disassembling the distributor assembly, requiring several days of downtime. Contamination in the piping, particularly at the outlet nozzles upstream and inside the distributor area, results in improper distribution of outlet gases containing methacrolein and creates flammable localized gas mixtures; in this respect, operation cannot be considered safe, especially after two weeks or more of operation.

[0028] In US 3,876,693 and US 3,147,084, in each case, the first reactant gas is cooled by a reactor equipped with an additional heat exchanger. This is done to further reduce the temperature of the reactant gas in the first reactor. The safe reintroduction of recycled methacrolein and oxygen is not addressed. However, here, byproducts, especially TPA, may sublimate in the heat exchanger pipes and at least partially clog them. Heat exchange efficiency and reaction run time are thus reduced. Furthermore, the reactor architecture becomes very complex, and the production and maintenance costs of such an equipment-based solution are correspondingly high.

[0029] Another proposal, EP 2 158 954, describes a specific method for recycling unconverted methacrolein. It describes specific equipment for evaporating methacrolein, wherein a stream containing liquid methacrolein is fed to a packed tower, and the resulting MAL-containing gas stream is directly supplied to an oxidation reactor. Higher boiling point components are discharged at the bottom, thus largely avoiding contamination in this equipment. The problems of gas blending (in order to adjust the oxygen content upstream of the second-stage oxidation reactor), particularly blending with oxygen-containing gases, and especially the problem of returning to the higher pressure region of the reactor, have not been satisfactorily resolved. The bottom temperature of the equipment causes further problems in continuous operation, as terephthalic acid and polymer deposits form in the medium. Due to the bottom feed circulation pattern, a considerable portion of the high-boiling point components is sent back to the post-processing, which is associated with higher energy consumption. The composition of the resulting MAL-containing gas stream is largely determined by the pressure and bottom temperature of the apparatus and equipment.

[0030] In summary, these prior art methods pose a considerable technical challenge from the viewpoint of safe and efficient process conditions. When so-called recycled methacrolein, i.e., methacrolein unconverted in the second oxidation stage, is evaporated and injected between the two oxidation reactors, the method operates within a so-called "lean" range. This lean range is very close to the explosive limits of the mixture. The lean range of the mixture is defined by the low concentration of volatile compounds in the gas mixture (which in particular also includes methacrolein). In principle, for the material compositions of the present invention, operation is carried out at less than 2.5% by volume, preferably less than 2.2% by volume, based on the total organic compounds. In principle, the gas mixture becomes less important; the lower the proportion of organic components, the lower the oxygen content in the gas, and the lower the residence time and heat load of the gas mixture. "Less important" here refers to procedures that can avoid the decomposition problems described in the post-combustion case.

[0031] If this critical mixture is fed along with the process gas to the first reactor stage, for example at temperatures typically above 300°C, tarry deposits and problematic components often appear in the feed gas upstream of the second oxidation reactor. This problem is described in many prior art documents, along with corresponding proposed solutions. However, all these solutions share a common characteristic: they can only be implemented through complex and expensive methods. Furthermore, many problems cannot be adequately compensated for, especially in continuous operation.

[0032] Therefore, it can be said at this point that, to date, no method has been described based on C4 feedstock in which the key steps for removing and converting unconverted methacrolein in process gases have been described as sufficiently safe for long-term operation. Therefore, improvement is urgently needed here. Summary of the Invention

[0033] Purpose

[0034] One object of this invention is to optimize C4-based methods for the preparation of methacrylic acid or MMA, particularly those using methacrylic acid as an intermediate or final product. The aim is to obtain high yields from said methods while achieving a long lifespan for the catalysts used, especially in the second oxidation stage. Furthermore, the optimized methods should be able to be carried out at noncritical concentrations of oxygen and volatiles and / or at noncritical temperatures.

[0035] Another objective is to optimize the method so that it can operate with longer processing times between separate retrofit or cleaning downtime. This specifically involves the replacement of equipment components in the so-called distributor section between the two oxidation stages. This involves the following areas of the equipment: where various byproducts and residual reactants, as well as MAL containing oxygen, vapor, and nitrogen, are supplied in gaseous form for the reaction in the second oxidation stage.

[0036] In summary, one object of the present invention is, in particular, to provide an improved C4-based method for preparing methacrylic acid as a final product or intermediate, which can be operated with fewer maintenance-related operational interruptions, as well as higher overall productivity and efficiency, and improved overall operational safety. This method should be particularly effective in avoiding contamination buildup in the evaporation zone of the recycle MAL and in the downstream distributor section.

[0037] These objectives specifically relate to methods involving the two-stage catalytic gas-phase oxidation of isobutylene. To this end, the formation of critical gas mixtures under critical reaction conditions should be avoided in all process steps in the simplest possible manner. This also particularly involves avoiding the formation of polymers and high-boiling-point components in the distributor section. Otherwise, such deposits would lead to increased operational risks and / or increased operational disruptions related to maintenance and cleaning.

[0038] Other purposes not explicitly mentioned may become apparent from the description or embodiments and from the overall context of the application, especially in relation to the prior art.

[0039] Solution

[0040] The aforementioned objective has been achieved by providing a novel method for preparing methacrylic acid and / or methacrylate esters. This novel method comprises the following steps:

[0041] (a) In the first catalytic gas-phase oxidation step, isobutylene, tert-butanol, or a mixture containing isobutylene and / or tert-butanol is converted to obtain a reaction product mainly containing methacrolein.

[0042] (b) In the second catalytic gas-phase oxidation step, the main methacrolein-containing reaction product of step (a) is converted to obtain a gaseous reaction product mainly composed of methacrylic acid and unconverted methacrolein.

[0043] (c) A portion of the unconverted methacrolein accumulated in the gaseous reaction products of step (b) is removed, preliminarily purified, and condensed to form a liquid recycled methacrolein fraction.

[0044] (d) Transferring the recycled methacrolein fraction to a gaseous stream, and

[0045] (e) This gaseous recycle methacrolein stream is converted in a catalytic oxidation step.

[0046] The method according to the invention is particularly characterized in that method step (d) includes the following method aspects:

[0047] i) The liquid recycled methacrolein fraction obtained in step c) is uniformly distributed in droplet form on the surface of the packing material to obtain a liquid film.

[0048] ii) Contact the liquid film obtained in step (i) with a gas stream containing 3-21% by volume oxygen and having a temperature 10-150°C higher than that of the liquid methacrolein to obtain a gas mixture containing recycled methacrolein.

[0049] iii) Unnecessarily mix the gas stream obtained according to (ii) with additional steam, and

[0050] iv) The resulting gas stream containing methacrolein, oxygen, steam, nitrogen and other organic compounds containing 1-6 carbon atoms is mixed with the reaction products of the first oxidation stage (a).

[0051] The oxidation steps described in steps (a) and (b) preferably include catalytic gas-phase oxidation in the presence of an oxygen-containing gas mixture and steam. This is particularly preferably carried out in a tubular reactor on a heterogeneous catalyst.

[0052] For step (c), it is particularly preferred to remove at least 80% of the formed methacrylic acid from the unconverted methacrolein. Particularly preferably, the removal and preliminary purification according to step (c) are carried out by means of the following aspects (i) to (iii):

[0053] (i) The reaction mixture is quenched to obtain process gas and process liquid.

[0054] (ii) Removing at least a portion of the byproducts from the process liquid by crystallization, and

[0055] (iii) The absorption and desorption steps are performed sequentially to obtain a liquid recirculated methacrolein fraction and a recirculated gas phase containing nitrogen, oxygen and vapor.

[0056] In this case, according to the invention, although less preferred, it is possible to omit or implement only a single aspect of these aspects.

[0057] The liquid recycle methacrolein fraction obtained in step (c) preferably contains 65%-99% by weight of methacrolein, 0%-5% by weight of methacrylic acid, 0%-5% by weight of water, up to 5000 ppm of stabilizer, terephthalic acid, non-volatile compounds, 0.5%-10% by weight of acetone, and a total of 0%-5% by weight of acetic acid, acetaldehyde, and acrolein. In a preferred embodiment, this liquid fraction (mainly containing methacrolein, particularly heavy volatile fractions such as acetic acid and methacrylic acid) also contains trace amounts of stabilizers, such as hydroquinone or derivatives thereof, or N oxides, such as Tempol or other inhibitors described in the prior art, and mixtures thereof. Thus, the evaporation section can be provided with inhibitors to prevent deposits. However, on the other hand, suitable discharge points are preferably installed in the evaporation zone and in the piping leading to the distributor to clean the equipment continuously or intermittently at predetermined intervals and remove liquid or solid residues or residues in suspension. Therefore, the equipment can remain residue-free during operation and can largely avoid deposits.

[0058] For step (d), it is preferred to spray and distribute the recirculated methacrolein fraction using one or more nozzles in the presence of one or more gas streams with different oxygen contents. This is particularly preferred to form droplets with an average diameter of 100-1000 μm. Furthermore, it is preferred to supply a first gas stream with an oxygen content of less than 9% by volume, and optional to supply a second gas stream with an oxygen content greater than 9% by volume. In a preferred embodiment, the two gas streams with different oxygen contents, resulting in different nitrogen contents, can be supplied separately to the evaporation unit or pre-mixed and combined to form a homogeneous gas mixture. In the simplest case, the gas stream with an oxygen content greater than 9% by volume is compressed air. The optional gas stream with an oxygen content of less than 9% by volume originates from the workup section and contains recirculated gas that has been substantially free of volatile organic compounds, especially methacrolein, during absorption. Of course, this recirculated gas stream still contains carbon monoxide and carbon dioxide components.

[0059] The equipment operates at atmospheric pressure or a low positive pressure of up to 2 bar. Temperature and pressure should be selected here to avoid falling below the dew points of methacrolein and water in the resulting gas mixture. Therefore, only small amounts of the more volatile components condense with a certain amount of water and acetic acid, which can then be removed via the discharge point described above.

[0060] Step (e) also has various preferred embodiments. For example, in step (e)(i), at least 50% of the recycled methacrolein droplets having a droplet size of less than 500 μm can preferably be distributed on the packing material.

[0061] In step (e), a gas stream containing 3-21 vol% oxygen is preferably obtained by mixing the returned process gas obtained in steps (a) and / or (b) with additional air. This returned oxygen-containing gas, which is already free of volatile organic compounds in the absorption step, has not necessarily been treated in post-catalytic combustion to also remove trace amounts of organic compounds or convert them into inert oxidation products such as carbon monoxide and carbon dioxide. In a preferred embodiment, this recirculated gas should have an oxygen content of less than 9 vol% and should be compressed before being returned to the methacrolein evaporator. According to the invention, the return and use of this gas stream as an inert gas in the evaporator is carried out in steps e(i) and e(ii).

[0062] In the evaporation and mixing unit, crude methacrolein, fresh air, and recirculated gas are mixed, and steam is added, so that the resulting gas stream PG8 contains C1-C6 components at a concentration of less than or equal to 2% by volume. These components are present in crude methacrolein in certain proportions, such as trace amounts and, in some cases, significant amounts of acetone, acetaldehyde, acrolein, and similar components. It is crucial for the safe operation of this part of the unit to adjust the resulting gas mixture to a lean region, ensuring that the total amount of organic components does not exceed 2% by volume. The ratio of fuel to total gas and, equally importantly, the ratio of oxygen to fuel are thus defined. Otherwise, the presence of high concentrations of these volatile components could lead to the formation of a combustible mixture or to a mixture in which the flash point of individual components is considered critical.

[0063] The resulting gas stream PG8 typically has an oxygen concentration of >9% by volume. This resulting gas mixture is set at a temperature well below 200°C, preferably below 100°C, in the evaporation and mixing zone. This recycled methacrolein-oxygen is further heated by the process gas from the first oxidation stage up to the distributor section, after which it exits the distributor tip. The resulting gas stream PG3, formed by contacting PG8 with PG2, has an oxygen content of less than 9% by volume. PG3 must meet essential conditions to be used as the feed gas for the second part of the oxidation process.

[0064] The resulting gas stream PG3, formed by contacting PG8 with PG2, has an oxygen content of 6.5 vol% to 8.7 vol% while the molar stoichiometry of oxygen to the total C-1 to C-6 components is 1.4 to 3.0. A range of 2.0 to 2.9 has proven to be a particularly preferred and safe range for the stoichiometry of these components. Equally important for safe operation is the concentration of methacrolein in the gas stream PG3. Typically, the concentration of methacrolein in the gas stream PG3 before entering the catalyst bed is 2.5 vol% to 5 vol%, particularly preferably 3.0 vol% to 4.6 vol%.

[0065] To the applicant's surprise, it has been found that the oxygen-to-fuel ratio and methacrolein concentration in PG3 are highly correlated with the selectivity and conversion rate of the second-stage oxidation, and also have a significant impact on the lifetime of the modified heteropolyacid catalyst used.

[0066] Optimal selectivity is achieved, particularly within the preferred concentration range of 3.0 vol%–4.6 vol%, and with a molar ratio of oxygen to C1–C6 components of 2.0–2.9. Higher oxygen ratios reduce the selectivity for the conversion of methacrolein to methacrylic acid, with little impact on catalyst lifetime. Conversely, setting too low an oxygen excess will impair catalyst lifetime. Catalyst lifetime can be assessed based on how much the salt bath temperature in the second stage must be increased to achieve approximately equal conversions in the second-stage reaction. Typically, a conversion of 65%–85%, preferably 70%–80%, of methacrolein is set in the second reaction stage (for a single-pass through a catalyst bed). Gas velocity, expressed as gas hourly space velocity (GHSV), also affects the overall target performance of the system. GHSV values ​​of 500–1500 (L / s) are typically used, preferably 700–1300.

[0067] For steps (e) and (iv), it has proven advantageous and is therefore preferred that the gas mixture obtained in said step be introduced into the gas mixture obtained in step (a) using a gas distributor in such a way that the temperature at the outlet point of the gas distributor is below 250°C. The distributor is designed in a technically advantageous manner such that the gas mixture conditioned with oxygen-containing gas and vapor has a temperature gradient within the conduit and feed. In principle, a lower evaporation temperature is set in the methacrolein evaporation zone. However, they should be high enough to ensure complete evaporation of MAL and avoid temperatures below the dew point. On the other hand, they should be low enough to prevent the formation of polymer deposits. This is possible at temperatures between 50°C and 150°C, depending on the chosen pressure, but preferably below 100°C. The distributor can be designed to guide the resulting MAL mixture containing oxygen-containing gas and inert gas into a reactor region having one or more central pipelines, from which the gas mixture is then supplied to multiple pipelines. In industrial reactors, a preferred embodiment is one where multiple nozzles and outlet pipes are connected to the main distributor pipe. In a preferred embodiment, these outlet pipes or nozzles are arranged at equal intervals along the distributor, but with varying free pipe diameters or lengths to ensure the most uniform gas distribution possible. Ideally, the free pipe diameter of the outlet furthest from the main pipeline should be larger than that of the outlet closest to the main pipeline. Figure 4 Such an implementation scheme is illustrated schematically in the diagram.

[0068] A fundamental characteristic of equipment for distributing recirculated MAL gas is the varying temperatures of the gas exiting the distributor, particularly where it mixes with the process gas from the first reactor stage. Here, the principle applies to gas temperatures at outlets farther from the main pipe, for example, being higher than those closer to the main pipe. In principle, the distributor and outlet should be designed such that the gas temperature at the outermost outlet point does not exceed 260°C. Preferably, the design ensures that the highest temperature at the outermost outlet point is less than or equal to 240°C. Naturally, a lower limit exists for the temperatures to be observed, taking into account the presence of trace amounts of high-boiling-point components, particularly those caused by terephthalic acid (“TPA”). TPA sublimates / condenses at temperatures between 180°C and 220°C, meaning this must be considered when designing the distributor. If these temperatures are below these levels, deposits will form and operational interruptions will occur.

[0069] As is known to those skilled in the art, mixing can be carried out in a countercurrent or cocurrent manner, and both variations are conceivable in principle. A preferred embodiment is to mix the various gas streams in a countercurrent manner.

[0070] It is further advantageous and preferred to remove the high-boiling-point contaminants present in the process liquid obtained in steps (d) and (e) and discharge them.

[0071] Furthermore, it is advantageous to subject the methacrylic acid obtained in step (b) and / or step (e) to a purification step. This is particularly preferred to be carried out to obtain pure methacrylic acid with a purity greater than 99% by weight.

[0072] Methacrylic acid can be further purified—regardless of its purity—and then undergo an esterification step, such as esterification with methanol to obtain MMA.

[0073] Compared with methods known in the prior art, the method according to the present invention has the following particular advantages:

[0074] 1) The method is remarkably energy-efficient, especially since no additional energy is required for the evaporation of recycled MAL.

[0075] 2) It can surprisingly effectively prevent polymerization, especially of MAL. This is achieved, in particular, because at the critical point the liquid can only form for a very short time, if any, such as in the form of droplets or liquid films.

[0076] 3) Surprisingly, because the reaction system operates at low pressure, no special equipment is required to introduce gaseous or liquid flow.

[0077] 4) There is almost no critical point for the potential critical mixing process to occur in the production equipment. This is possible as a result of the fact that the recirculated MAL is first mixed with oxygen and then directly introduced into the distributor.

[0078] 5) The method according to the invention is also particularly effective in intermediate cooling, because the reaction gas in the first stage is cooled in an effective and desirable manner in a distributor with a large amount of cooler gas.

[0079] 6) The method according to the invention has significant advantages over the prior art in terms of feedstock efficiency and selectivity, as well as the overall yield achieved. In particular, precise monitoring and adjustment of the oxygen-to-methacrylaldehyde ratio (molar) within the range of 2.0-2.6 and the oxygen content in the total gas being less than 9 vol% and greater than 7 vol% achieves an optimal trade-off between high selectivity and long service life of the catalyst used in the second catalyst stage. Overall, it achieves lower specific costs for C4 feedstocks such as isobutylene or MTBE and also increases specific catalyst utilization, which can be expressed as the amount of methacrylic acid per ton of catalyst. The method of the invention significantly reduces the amount of costly byproducts such as acrolein, acetic acid, and carbon monoxide.

[0080] In addition to the method described according to the present invention, the present invention also provides corresponding production equipment for preparing methacrylic acid and / or methacrylates. Such production equipment includes at least the following devices:

[0081] (a) At least one supply device for at least one C4 compound,

[0082] (b) A first reactor containing a first oxidation catalyst,

[0083] (c) Second reactor, containing a second oxidation catalyst,

[0084] (d) at least one separation device, and

[0085] (e) At least one evaporation device.

[0086] The production equipment also has the following characteristics:

[0087] (1) The evaporation equipment (e) is equipped with packing and, unnecessarily, at least one discharge valve below the packing for removing liquid.

[0088] (2) The evaporation equipment (e) is connected to at least one liquid methacrolein supply equipment via at least one distribution device.

[0089] (3) The evaporation equipment (e) is connected via an inlet to at least one air supply equipment and at least one oxygen-deficient gas mixture supply equipment.

[0090] (4) The evaporation equipment is connected to the second reactor (c) via a gas conduit.

[0091] (5) The gas conduit is introduced into the second reactor (c) via an inlet including a gas distributor.

[0092] (6) Unless necessary, the gas conduit is also connected to at least one steam supply device.

[0093] Therefore, the following design of the production equipment is preferred: wherein the separation equipment (d) includes at least one quenching step, at least one crystallization step and at least one absorption / desorption step.

[0094] In contrast, the distribution device of aspect (2) preferably includes multiple nozzles.

[0095] In other embodiments where the inlets for air and oxygen-deficient gas are arranged above the distribution device and the packing is arranged below the distribution device, it has proven to be particularly advantageous.

[0096] Finally, the present invention also provides the use of the production equipment described above for the preparation of methacrylic acid and / or methacrylates, preferably MMA. Attached Figure Description

[0097] Figure 1A schematic process flow diagram is depicted. Although the depiction is based on the invention and is intended to illustrate the invention, it is not intended to limit the invention in any way.

[0098] F1: Air

[0099] F2: Recirculated gas

[0100] F3: Isobutylene and / or tert-butanol gaseous feed stream

[0101] F4: Steam

[0102] F5: Methanol

[0103] R1: First-stage reactor

[0104] R2: Second-stage reactor

[0105] R3: Esterification reactor

[0106] G1: Gas distributor

[0107] G2: MAL Evaporator

[0108] SU1: Sudden cooling

[0109] SU2: Crystallization

[0110] SU3: Absorption

[0111] SU4: Desorption

[0112] SU5: Extraction

[0113] SU6: Purified methacrylic acid

[0114] SU7: MMA purification

[0115] OU: Exhaust Gas Unit

[0116] WS: Solid Waste

[0117] WG: Exhaust Gas Flow

[0118] DP1: Emissions 1

[0119] DP2: Emissions 2

[0120] PG: Process Gas

[0121] PS: Process material flow

[0122] Figure 2 An example illustrates a specific embodiment of the present invention:

[0123] (1) The reactor in the first stage

[0124] (2) The reactor in the second stage

[0125] (3) Gas distributor

[0126] (4) Sudden cooling / Sudden cooling tower

[0127] (5) Crystallization

[0128] (6) Absorption Tower

[0129] (7) Desorption tower

[0130] (8) Condenser

[0131] (9) Methacrylaldehyde evaporator

[0132] Figure 3 An example illustrates a specific implementation scheme for a MAL evaporator:

[0133] (102) Area with nozzles

[0134] (103) Free space between packing and nozzle

[0135] (104) Main packing

[0136] (105) Lower packing

[0137] Figure 4 An example illustrates a specific implementation scheme for a gas distributor:

[0138] (108) Distributor: Main road entrance

[0139] (109) Distributor: Intermediate section

[0140] (110) Distributor: Tail section

[0141] (111) Distributor: Outlet pipe

[0142] The confirmation of process material flows PS1-PS10 and process gases PG1-PG7 will be based on Figure 1-4 The context and the following description of the examples will become clear. Detailed Implementation

[0143] Example 1

[0144] In corresponding Figure 1 In this setup, a process gas feed (PG1) is introduced into the reactor (R1), the feed containing isobutylene (F3) as a starting material, air (F1), a recirculating gas stream (F2) containing oxygen and an inert gas, and steam (F4). The gas stream has an overall molar ratio of isobutylene to oxygen to water of 1:2:1.5. The temperature in the reactor is maintained at 350°C, and the pressure at the reactor inlet is maintained at a positive pressure of 1.2 bar. This results in a 1,000-hour timeframe. -1The gas hourly space velocity (GHSV) was determined. The reaction was carried out in a tube bundle reactor using a molybdenum oxide-based catalyst prepared according to US2007 / 0010394. A resulting process gas (PG2) was obtained at a temperature of 343 °C. This process gas had the following composition: 4.8 vol% methacrolein, 0.74 mol% CO, 0.21 mol% methacrylic acid, 0.21 mol% acetic acid, 0.12 mol% acetone, 0.21 mol% acetaldehyde, 0.04 mol% acrolein, 0.03 mol% formaldehyde, 0.03 mol% acrylic acid, 250 ppm isobutylene, and 3.5 vol% oxygen. The isobutylene conversion was 99.6%, and the yield of methacrolein was 79.6%.

[0145] In the next step, the process gas (PG2) is contacted and mixed with an additional process gas (PG6) at a volume flow ratio of 1:0.8 (PG6):(PG2). Mixing is performed via a gas distributor (G1). The additional process gas (PG6) is partially recycled methacrolein, which is evaporated in a MAL evaporator (G2) and mixed with further recycled gas (unnamed), air (F1), and steam (F4) at a temperature of 80°C. The process gas (PG6) has the following composition: 1.41 mol% organic components, containing 1.2 mol% methacrolein, 0.03 mol% methacrylic acid, 0.09 mol% acetone, 0.05 mol% acetaldehyde, 0.04 mol% acrolein, 17 mol% oxygen, and 13 mol% water. The process gas (PG3) generated during this mixing and contact with the reactor gas (PG2) has a temperature of 245°C and a molar ratio of methacrolein to oxygen to water of 1:2.5:4.1. The process gas (PG3) contains a total of 4.38 mol% organic components, primarily composed of: 3.44 mol% methacrolein, 0.31 mol% CO, 0.12 mol% methacrylic acid, 0.12 mol% acetic acid, 0.15 mol% acetone, 0.15 mol% acetaldehyde, 0.06 mol% acrolein, 0.02 mol% formaldehyde, and 0.01 mol% acrylic acid. This process gas (PG3) is then passed through a second reactor (R2). Reactor (R2) is partially filled with a catalyst, which is a mixture containing phosphomolybdate. The catalyst was prepared according to US 2007 / 0010394, and the reaction was initially carried out at 292°C (salt temperature) for 1.000 h. -1The flow rate (GHSV) was measured. The conversion of methacrolein in reactor R2 was approximately 78%, and the selectivity of methacrylic acid averaged 85% during operation (variable range of + / -1% for various operating measurements and gas analyses).

[0146] To ensure continuous conversion throughout the thousands of hours of operation, the reactor's salt bath temperature was periodically increased slightly. To maintain a constant conversion rate, the temperature was increased by approximately 1°C per month, reaching a final temperature of 302°C after 7000 operating hours. The reaction gas (PG4) present downstream of reactor R2 had the following composition: 0.7 vol% methacrolein, 1 mol% CO, 2.28 mol% methacrylic acid, 0.31 mol% acetic acid, 0.12 mol% acetone, 0.21 mol% acetaldehyde, 0.04 mol% acrolein, 0.03 mol% formaldehyde, 0.03 mol% acrylic acid, and 6.5 vol% oxygen. The process gas (PG4) containing methacrylic acid and MAL had a temperature of approximately 306°C and was reduced to 230°C using a gas cooler, then further cooled to 80°C in a quench tower (SU1). At the bottom of the quench tower, mainly methacrylic acid, acetic acid, TPA, acrylic acid, and other components with boiling points higher than methacrolein are absorbed or condensed along with some process water.

[0147] The reaction gas containing methacrolein is cooled to below 30°C in a quench tower (SU1). In the quench tower (SU1), a gas mixture (PG5) is formed and further cooled to 13°C using multiple external coolers. Most of the methacrylic acid and most of the water condense in the quench tower. The quench liquid (PS1) formed in this process is collected at the bottom of the tower and contains 35% by weight of methacrylic acid, 0.2% of methacrolein, 1000 ppm of TPA (terephthalic acid), and water. The liquid is cooled to 13°C before being introduced into a TPA crystallizer (SU2). In this crystallizer (SU2), supersaturated TPA crystallizes out, and the liquid is filtered through a TPA filtration unit. The TPA is then recovered as waste solids (WS). The filtrate is then passed to a methacrolein recovery tower (SU4), now substantially free of TPA.

[0148] The gas mixture (PG5) from the quench tower (SU1) is then fed to the bottom of the methacrolein absorber (SU3). In this absorber (SU3), a portion of the process feed (PS3) is used as the absorbent medium for methacrolein. The process feed (PS3) originates from downstream desorption (SU4) and is cooled to 17°C before being introduced to the top of the methacrolein absorber. Most of the methacrolein in the gas mixture (PG5) is absorbed by the process feed (PS3) operating in counter-current mode to form a liquid process feed (PS2). The gas mixture exiting the top of absorber SU3 (which contains most of the unabsorbed or condensed gas phase from the MAL absorber) is scrubbed, then heated in multiple demisters, and further heated to 300°C by heat from the combustion system to form process gas (PG7). Finally, process gas PG7 is fed into a combustion system (OU, exhaust gas unit) equipped with a standard Pt-Pd combustion catalyst, where all organic components are combusted. The combustion system operates in a regenerative manner, meaning that waste heat from the reaction is used to preheat the inlet gas. A portion of the exhaust gas from the combustion system is compressed in the exhaust gas compressor and recycled back into the oxidation reaction system. This gas phase is referred to below as recirculated gas (F2). The remainder is emitted into the atmosphere as exhaust gas (WG).

[0149] The process feed stream (PS2) from the bottom of the methacrolein absorber (SU3) and the liquid phase from the TPA filter are each fed into the MAL recovery tower (SU4) to recover unconverted methacrolein. This tower operates under reduced pressure of 350 mbar (absolute pressure). The liquid temperature at the bottom is approximately 80°C, and the vapor temperature at the top is approximately 40°C; this corresponds to the vapor temperature upstream of the condenser. As described above, a portion of the liquid from the bottom (PS3) is reused as the absorbent medium. The remainder of the bottom feed (PS3), containing 33% by weight of methacrylic acid, 80 ppm of methacrolein, and 250 ppm of TPA, is fed to the methacrylic acid post-treatment system. Most of the methacrolein in the vapor phase is distilled off at the top of the tower, cooled to 30°C, and yields a liquid containing methacrolein. A portion of the liquid from the condenser is returned to the tower as reflux. The remaining portion of the liquid phase (PS4) is fed as a recirculation feed into the methacrolein evaporator (G2).

[0150] The methacrolein recirculation stream (PS4) contains 85 wt% methacrolein, 5 wt% acetone, 3 wt% acrolein, and 0.5 wt% methacrylic acid, along with an inhibitor. The recirculation stream is cooled to 3°C for intermediate storage before being introduced into the methacrolein evaporator (G2). The methacrolein recirculation stream (PS4) is then pumped into a liquid distributor equipped with an atomizer system. This generates droplets that are sprayed onto the packing material in the methacrolein evaporator (G2). This produces a homogeneous, uniform surface distribution for rapid, complete evaporation. The supplied air (F1) and recirculated gas (F2) are compressed and cooled to 90°C. The resulting gas phase composition after mixing F2 and F1 subsequently contains 15 vol% oxygen. The compressed gas mixture is then introduced into the top of the methacrolein evaporator (G2). Evaporation is carried out at a positive pressure of 1.2 bar, yielding an oxygen-containing gas at a temperature of 80°C, which is higher than the dew point of MAL at the selected pressure. It is important that evaporation occur at or above the dew point of the organic mixture during the evaporation process. This ensures that as much of the organic component as possible is completely evaporated as desired. During the evaporation operation, some organic components with boiling points higher than methacrolein (such as inhibitors, methacrylic acid, or terephthalic acid) may enter the gas phase incompletely or only in trace amounts. Therefore, the methacrolein evaporator is periodically cleaned with water and discharged from the bottom discharge point (DP1) and a second discharge point (DP2) of the methacrolein evaporator (G2). The thus cleaned methacrolein phase (PG6) is then finally introduced into the gas distribution device, namely the distributor (G1). Methacrolein, acrolein, acetone, and other low-boiling-point components are evaporated and introduced as a gas phase at high speed into the distributor (G1), unnecessarily along with additional steam (F4). The distributor guides and distributes the gas mixture directly below the outlet zone of the first reactor (R1). During the direct contact between the gas distribution equipment and the piping connected thereto with the hot reactive gas (PG2) in the first reaction zone, the gaseous recirculated methacrolein stream (PG6) is combined with the reactive gas (PG2) to form a resulting gas stream (PG3) with a temperature of 238°C. This temperature measurement is related to the mixing temperature at the outlet pipe near the distributor section.

[0151] Continuously evaluate and examine the results of operations and production processes to assess conversion rate and selectivity, particularly the selectivity of the second reaction stage.

[0152] Continuous evaluation of productivity and selectivity was completed after 7000 hours of operation; results are provided in Table 1. A constant high selectivity of approximately 84%–85% was achieved throughout the period; the decrease in catalyst activity, assessed by increasing the salt bath temperature, was moderate to low. The catalyst remained fully active after 7000 hours of operation, achieving 78% conversion, with sufficient reserve for continued operation. No shutdowns due to critical equipment conditions were required, and no afterburning or spontaneous pressure increases in critical equipment components that could potentially cause broaching of safety devices (e.g., rupture discs) were observed.

[0153] Example 2

[0154] The isobutylene oxidation was carried out in a manner similar to that of Example 1, but with a modified composition of a gas mixture (PG6) containing recycled methacrolein. Feed streams F1, F2, F4, and PS4 were adjusted so that the process gas (PG6) had the following composition: 1.40 mol% methacrolein, 0.03 mol% methacrylic acid, 0.09 mol% acetone, 0.05 mol% acetaldehyde, 0.04 mol% acrolein, 17.1 mol% oxygen, and 13.5 mol% water. As in Example 1, process gas PG6 at 80°C was introduced into distributor (G1) and mixed with process gas (PG2) to form process gas (PG3) at 240°C. However, this process gas (PG3) had a ratio of methacrolein, oxygen, and water of 1 / 2.8 / 4.1 and contained a total of 4.44 mol% organic components. The organic components consist of the following: 3.31 mol% methacrolein, 0.30 mol% CO, 0.12 mol% methacrylic acid, 0.12 mol% acetic acid, 0.15 mol% acetone, 0.15 mol% acetaldehyde, 0.06 mol% acrolein, 0.02 mol% formaldehyde, and 0.01 mol% acrylic acid.

[0155] The continuous evaluation of productivity and selectivity was also completed after 7000 hours of operation; the results are also provided in Table 1. Here, a constant high selectivity of approximately 83% was also achieved throughout the time period, which is slightly lower than in Example 1. The decrease in catalyst activity, which can be evaluated by increasing the salt bath temperature, is again moderate to low. The catalyst remains fully active after 7000 hours of operation, achieving 77%-80% conversion, with sufficient reserve for continued operation. Unlike Example 1, the increased oxygen concentration cannot reliably rule out pressure increase events. These indicate the presence of afterburning phenomena that could lead to the depletion of safety devices (e.g., rupture discs). Therefore, it cannot be ruled out that using the described operating mode will exhaust the entire service life of the catalyst.

[0156] Example 3

[0157] The isobutylene oxidation was carried out in a manner similar to that of Example 1, but with a modified composition of a gas mixture (PG6) containing recycled methacrolein. Feed streams F1, F2, F4, and PS4 were adjusted so that the process gas (PG6) had the following composition: 1.40 mol% methacrolein, 0.03 mol% methacrylic acid, 0.09 mol% acetone, 0.05 mol% acetaldehyde, 0.04 mol% acrolein, 10 mol% oxygen, and 13.5 mol% water. As in Example 1, process gas PG6 at 80°C was introduced into distributor (G1) and mixed with process gas (PG2) to form process gas (PG3) at 240°C. However, this process gas (PG3) had a methacrolein, oxygen, and water ratio of 1 / 2 / 4.1 and contained a total of 4.44 mol% organic components. The organic components consist of the following: 3.31 mol% of methacrolein and 0.30 mol% of CO, 0.12 mol% of methacrylic acid, 0.12 mol% of acetic acid, 0.15 mol% of acetone, 0.15 mol% of acetaldehyde, 0.06 mol% of acrolein, 0.02 mol% of formaldehyde and 0.01 mol% of acrylic acid.

[0158] The continuous evaluation of productivity and selectivity was also completed after 7000 hours of operation; the results are also provided in Table 1. Throughout the time period, the selectivity for methacrylic acid after the second oxidation stage decreased significantly from 84.6% (after 1000 hours of operation) to 81.8% (after 7000 hours of operation). Unlike Example 2, safe operation was consistently ensured by the low oxygen concentration; pressure surge events could be reliably eliminated. However, overall, the low oxygen concentration meant that the catalyst life exhibited in Example 1 according to the invention was not exhausted in this example.

[0159] Table 1: Results

[0160]

[0161] Example 4

[0162] The correlation between the composition and safety of process gas PG3 can be demonstrated using explosion tests. For this purpose, the gas mixtures PG3 listed in Table 1 (corresponding to Examples 1-3) were investigated in terms of their flammability. Tests were conducted in a 10 L electrically heated explosion sphere at a temperature of 240 °C and a pressure of 2.18 bara; measurements were performed according to the "bomb method" specified in standard DIN EN 1839 "Determination of the explosion limits of gases and vapors". For all metered components, the maximum deviation in gas metering was maintained at 0.1 vol% (absolute). Test results are provided in Table 2.

[0163] As the comparison of pressures achievable within the first 60 seconds after ignition shows, at an oxygen concentration of 9.5 mol% (according to Example 2, Comparative Example), a pressure just below 6 bar can be reached, indicating the presence of a deflagration reaction. When using lower oxygen concentrations of 8.5 mol% (similar to Example 1, according to an embodiment of the invention) and 6.8 mol% (similar to Example 3, Comparative Example), the pressure remains almost constant relative to the start-up pressure within the first 60 seconds, indicating that these operating parameters ensure safe equipment operation.

[0164] Table 2: Results of the Explosive Test (Test conditions: Temperature = 240℃, Pressure = 2.18 bara)

[0165]

Claims

1. A method for preparing methacrylic acid and / or methacrylates, comprising the following steps: (a) In the first catalytic gas-phase oxidation step, isobutylene, tert-butanol, or a mixture containing isobutylene and / or tert-butanol is converted to obtain a reaction product mainly containing methacrolein. (b) In the second catalytic gas-phase oxidation step, the main methacrolein-containing reaction product of step (a) is converted to obtain a gaseous reaction product mainly composed of methacrylic acid and unconverted methacrolein. (c) A portion of the unconverted methacrolein accumulated in the gaseous reaction products of step (b) is removed, preliminarily purified, and condensed to form a liquid recycled methacrolein fraction. (d) Transferring the recycled methacrolein fraction to a gaseous stream, and (e) Convert this gaseous recycle methacrolein stream in a catalytic oxidation step. Its features Method step (d) includes the following method aspects: i) Distribute the liquid recycled methacrolein fraction obtained in step c) evenly as droplets on the surface of the packing material to obtain a liquid film. ii) Contact the liquid film obtained in step (i) with a gas stream containing 3-21% by volume oxygen and having a temperature 10-150°C higher than that of the liquid recycled methacrolein fraction to obtain a gas mixture containing recycled methacrolein, and iii) The resulting gas stream containing methacrolein, oxygen, steam, nitrogen and other organic compounds containing 1-6 carbon atoms is mixed with the reaction products of the first oxidation stage (a).

2. The method according to claim 1, characterized in that... The oxidation steps described in steps (a) and (b) comprise catalytic gas-phase oxidation in a tubular reactor over a heterogeneous catalyst in the presence of an oxygen-containing gas mixture and steam.

3. The method according to claim 1 or 2, characterized in that... In step (c), at least 80% of the methacrylic acid formed is removed from the unconverted methacrolein.

4. The method according to claim 1 or 2, characterized in that... Based on the removal and preliminary purification in step (c), the following steps are performed: (i) The gaseous reaction products from step (b) are rapidly cooled to obtain process gas and process liquid. (ii) Removing at least a portion of the byproducts from the process liquid by crystallization, and (iii) Perform absorption and desorption steps sequentially to obtain a liquid recirculated methacrolein fraction and a recirculated gas phase containing nitrogen, oxygen and vapor.

5. The method according to claim 1, characterized in that... Prior to steps (d) and (iii), the gas mixture containing recycled methacrolein is mixed with fresh air, steam, and the pre-purified recycled gas from step (c), wherein the mixing is carried out in an evaporation and mixing apparatus according to features 1(a)-1(e), such that... a. The mixture obtained prior to mixing with the reaction product of the first oxidation stage (a) contains C1-C6 components at a concentration of less than or equal to 2% by volume. b. The mixture obtained prior to mixing with the reaction product of the first oxidation stage (a) has an oxygen concentration of >9% by volume. c. The resulting mixture after mixing with the reaction product of the first oxidation stage (a) has an oxygen content of less than 9% by volume.

6. The method according to claim 5, characterized in that... The resulting gas stream PG3, formed by contacting PG8 with PG2, has an oxygen content of 6.5 vol%–8.7 vol% and a molar stoichiometry of 1.4–2.8 for the total amount of oxygen and C-1 to C-6 components.

7. The method according to claim 1 or 2, characterized in that... In method step (d), the recycled methacrolein fraction is sprayed and distributed through one or more nozzles in the presence of one or more gas streams with different oxygen contents, wherein the formed droplets have an average diameter of 100-1000 μm and one gas stream has an oxygen content of less than 10% by volume.

8. The method according to claim 1 or 2, characterized in that... The liquid recycled methacrolein fraction obtained in step (c) contains 65%-99% by weight of methacrolein, 0%-5% by weight of methacrylic acid, 0%-5% by weight of water, up to 5000 ppm of stabilizer, terephthalic acid, non-volatile compounds, 0.5%-10% by weight of acetone, and a total of 0%-5% by weight of acetic acid, acetaldehyde, and acrolein.

9. The method according to claim 1 or 2, characterized in that... In step (d)(i), at least 50% of the recycled methacrolein droplets having a droplet size of less than 500 μm are distributed on the packing material.

10. The method according to claim 1 or 2, characterized in that... The high-boiling-point contaminants present in the process liquid obtained in steps (d) and (e) are removed and discharged.

11. The method according to claim 1 or 2, characterized in that... The gas stream containing 3-21% oxygen by volume is obtained in step (d)(ii) by mixing the returned process gas obtained in step (a) and / or step (b) with additional air.

12. The method according to claim 1 or 2, characterized in that... The gas mixture obtained in step (d) and (iii) is introduced into the gas mixture obtained in step (a) in such a way that the temperature at the outlet of the gas distributor is below 250°C.

13. The method according to claim 1 or 2, characterized in that... The methacrylic acid obtained in step (b) and / or step (e) is subjected to a purification step to obtain pure methacrylic acid with a purity of greater than 99% by weight.

14. The method according to claim 1 or 2, characterized in that... The methacrylic acid is purified and then subjected to an esterification step.

15. The method of claim 1 or 2, wherein the gas stream obtained according to (ii) is mixed with additional steam.

16. Production equipment for preparing methacrylic acid and / or methacrylate esters, including: (a) At least one supply device for at least one C4 compound, (b) A first reactor containing a first oxidation catalyst. (c) Second reactor, containing a second oxidation catalyst, (d) At least one separation device, and (e) At least one evaporation device, Its features are: (1) The evaporation equipment (e) is equipped with packing material and, unnecessarily, at least one drain valve below the packing material for removing liquid. (2) The evaporation equipment (e) is connected to at least one liquid methacrolein supply equipment via at least one distribution device. (3) The evaporation equipment (e) is connected via an inlet to at least one air supply device and at least one oxygen-deficient gas mixture supply device. (4) The evaporation equipment is connected to the second reactor (c) via a gas conduit. (5) The gas conduit is introduced into the second reactor (c) via an inlet including a gas distributor.

17. The production equipment according to claim 16, characterized in that... The separation device (d) includes at least one quenching step, at least one crystallization step, and at least one absorption / desorption step.

18. The production equipment according to claim 16 or 17, characterized in that... The distribution device includes multiple nozzles.

19. The production equipment according to at least one of claims 16-17, characterized in that... Within the evaporation equipment, inlets for air and for oxygen-deficient gas are located above the distribution equipment, and the packing material is located below the distribution equipment.

20. Use of the production equipment according to at least one of claims 16-19 for the preparation of methacrylic acid and / or methacrylic esters.

21. The use according to claim 20, wherein the methacrylate is methyl methacrylate (MMA).

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