Zone catalyst loading and process for acetoxylation of olefins using same

Abstract

Disclosed herein is a process for acetoxylating an olefin in a gaseous reaction stream containing the olefin, acetic acid, and an oxygen-containing gas. The method includes passing a reactant gas through at least two fixed catalyst zones arranged in series. The catalyst zones are located in one or more reaction tubes arranged in parallel. The at least two fixed catalyst zones include an inlet catalyst zone comprising an inlet catalyst and an outlet catalyst zone comprising an outlet catalyst, and certain conditions of the inlet catalyst and the outlet catalyst may be satisfied.

Description

Priority claim

[0001] This application claims priority to U.S. Provisional Application No. 63 / 395,485, filed on August 5, 2022, the entire contents and disclosure of which are incorporated herein by reference. Technical Field

[0002] The present invention relates to a zone catalyst loading and a method for acetoxylating olefins using the zone catalyst loading. In particular, the present invention relates to a zone catalyst loading having at least two fixed catalyst zones, wherein at least one of the following conditions is met: (a) the inlet catalyst contains greater than 7 g / L: Pd; (b) the outlet catalyst contains greater than 8.3 g / L Pd; and / or (c) the outlet catalyst contains from 7.5 to 8.1 g / L Pd. Background Art

[0003] The process for acetoxylating ethylene in the gas phase is of particular industrial interest. Acetoxylation can be carried out industrially by catalytic gas phase oxidation of olefins, such as ethylene or propylene, in fixed bed reactors. These reactions can be carried out in shell and tube reactors. Unsaturated esters, such as vinyl acetate, can be prepared by this reaction. Typically, the reaction is carried out by passing a gaseous mixture comprising molecular oxygen, olefins and acetic acid through a reactor. The reaction is typically carried out in a shell and tube reactor in which a plurality of reaction tubes are arranged in parallel and a uniform catalyst charge is located in each of the plurality of reaction tubes. The excess reaction heat involved is removed by means of a heat transfer medium. An example of such a reactor is a boiling water reactor.

[0004] Although there are various methods for regulating the reaction temperature, temperature differences may still exist during the reaction. During the course of the reaction, these temperature peaks (called "hot spots") may bring about a series of undesirable effects. Hot spots may prematurely age the catalyst, reduce selectivity and productivity, and limit space-time yield. Processes for the acetoxylation of olefins, in particular ethylene, in the gas phase give high yields of vinyl acetate and are of great economic importance.

[0005] WO 2008 / 071610 discloses a process and a catalyst system comprising a catalyst which comprises palladium, gold and potassium acetate and is applied to a SiO2 support having a large surface area and can be operated at a space-time yield of more than 800 [g(VAM) / lcat*h], an ethylene selectivity of more than 92% and a low degree of formation of ethyl acetate relative to vinyl acetate.

[0006] US Patent No. 5,179,056 discloses a process for preparing vinyl acetate by reacting ethylene and acetic acid in the presence of an oxygen-containing gas over a highly reactive palladium / gold coated catalyst.

[0007] US Patent No. 6,399,813 discloses a highly active fluidized bed vinyl acetate catalyst on a support composed of inert microspherical particles composed of silica, zirconia or alumina and having a defined pore distribution.

[0008] In view of the great economic importance of the acetoxylation products and the high-performance catalysts known from the prior art, there is a great need to optimize the reaction process with regard to conversion, selectivity and lifetime of the catalyst.

[0009] For example, WO 2007 / 101749 and WO 2006 / 042659 disclose synthesis reactors for preparing vinyl acetate monomer with increased selectivity and productivity, in which gaseous ethylene and acetic acid and also an oxygen-containing gas are reacted catalytically, wherein the synthesis reactors are wall reactors and the catalytic synthesis is carried out in a plurality of reaction spaces and at least one wall of the reaction spaces is coated with a catalyst and at least one wall of the reaction spaces is indirectly cooled.

[0010] U.S. Pat. No. 8,907,123 discloses a method for acetoxylating olefins in a gaseous reaction stream containing olefins, acetic acid and an oxygen-containing gas, the method comprising passing a reaction gas comprising at least one olefin, oxygen and acetic acid through at least two fixed catalyst zones having supported olefin acetoxylation catalysts of different activities arranged in series, wherein the catalyst zones are located in one or more reaction tubes arranged in parallel.

[0011] There remains a need for catalyst zone loadings that produce desirable selectivities, productivity, and space-time yields for the acetoxylation of olefins. These and other problems are addressed in the present specification. Summary of the invention

[0012] In some embodiments, the present disclosure includes a method for acetoxylating an olefin in a gaseous reaction stream containing an olefin, acetic acid, and an oxygen-containing gas, the method comprising: passing the reaction gas through at least two fixed catalyst zones arranged in series, wherein the catalyst zones are located in one or more reaction tubes arranged in parallel; wherein the at least two fixed catalyst zones include an inlet catalyst zone comprising an inlet catalyst and an outlet catalyst zone comprising an outlet catalyst; wherein based on 100% catalyst loading by weight of all catalyst zones, the inlet catalyst zone accounts for from 5% to 70% of the catalyst loading, and the outlet catalyst zone accounts for from 30% to 95% of the catalyst loading; and wherein at least one of the following conditions is met: (a) the inlet catalyst contains greater than 7 g / L: Pd; (b) the outlet catalyst contains greater than 8.3 g / L Pd; and / or (c) the outlet catalyst contains from 7.5 to 8.1 g / L Pd. The inlet catalyst zone can account for from 40% to 70% of the catalyst loading by weight. In some aspects, condition (b) and / or (c) are met and the inlet catalyst comprises from 4 to 15g / L Pd and from 2 to 8g / L Au. In some aspects, the outlet catalyst zone accounts for the catalyst loading from 30% to 50% by weight. In some aspects, condition (a) is met and the outlet catalyst comprises from 4 to 15g / L Pd and from 2 to 8g / L Au. In some aspects, the inlet catalyst zone accounts for the catalyst loading from 5% to 40% by weight. In some aspects, the inlet catalyst has a weight ratio of Au to Pd from 0.3:1 to 1.5:1. In some aspects, the inlet catalyst has a layer thickness from 100 to 300 microns. In some aspects, the inlet catalyst zone contains at least two catalysts in series and the weight ratio of the two catalysts is from 1:5 to 5:1. In some aspects, the outlet catalyst zone accounts for the catalyst loading from 60% to 95% by weight. In some aspects, the outlet catalyst has a weight ratio of Au to Pd of 0.3:1 to 1.5:1. In some aspects, the inlet catalyst has a metal loading different from the outlet catalyst. In some aspects, the inlet catalyst zone comprises a catalyst having a size different from the outlet catalyst zone. In some aspects, the inlet catalyst and the outlet catalyst each comprise less than 2wt.%Pd. In some aspects, the inlet catalyst and the outlet catalyst each comprise from 25 to 55g / L potassium acetate. In some aspects, the olefin is ethylene and the selectivity to vinyl acetate monomer is at least 60%. In some aspects, the olefin is ethylene and the conversion of ethylene is at least 60%. In some aspects, the olefin is ethylene and wherein the selectivity to heavy fractions is less than 10%. In some aspects, the inlet catalyst and the outlet catalyst have a spherical shape. In some aspects, the olefin is ethylene and the oxygen conversion is from 35% to 55%. In some aspects, only condition a) is satisfied.In some aspects, only condition b) is satisfied. In some aspects, only condition c) is satisfied. In some aspects, both conditions a) and b) are satisfied. In some aspects, both conditions a) and c) are satisfied.

[0013] Additional embodiments and features are described in part in the following description, and in part will become apparent to those skilled in the art after examining the description or can be understood through practice of the invention. The features and advantages of the invention can be realized and obtained by means of the means, combinations, and methods described in the description. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] A further understanding of the nature and advantages of the present invention may be realized by reference to the remainder of the specification and to the drawings, wherein the same reference numerals are used to refer to similar parts throughout the several drawings. In some cases, a sub-label is associated with a reference numeral and is followed by a hyphen to indicate one of multiple similar parts. When reference is made to a reference numeral without specification of an existing sub-label, the sub-label is intended to refer to all such multiple similar parts.

[0015] Figure 1 is a schematic diagram of a method for forming vinyl acetate monomer according to an embodiment of the present disclosure.

[0016] Figure 2 is a diagrammatic representation of two catalyst zones according to an embodiment of the present disclosure.

[0017] Figure 3 is a diagrammatic representation of three catalyst zones according to an embodiment of the present disclosure.

[0018] Figure 4 is a diagram of a plurality of parallel reaction tubes according to an embodiment of the present disclosure. DETAILED DESCRIPTION

[0019] introduction

[0020] As described herein, the present disclosure relates to a zone catalyst loading and a method for acetoxylating olefins using the zone catalyst loading. The method may include acetoxylating olefins in a gaseous stream. The gaseous stream may contain olefins, acetic acid, and an oxygen-containing gas. The method steps may include: 1) passing the gaseous reaction stream through at least two fixed catalyst zones. The at least two fixed catalyst zones are arranged in series. The catalyst zones are located in one or more reaction tubes. The tubes may be arranged in parallel. The at least two fixed catalyst zones may include an inlet catalyst zone comprising an inlet catalyst and an outlet catalyst zone comprising an outlet catalyst. Based on 100% by weight of the catalyst loading of all catalyst zones, the inlet catalyst zone may account for a catalyst loading of from 5% to 70% by weight. In order to form a product, at least one of the following conditions may be met in the following items: (a) the inlet catalyst contains greater than 7g / L: Pd; (b) the outlet catalyst contains greater than 8.3g / LPd; and / or (c) the outlet catalyst contains from 7.5 to 8.1g / L Pd.

[0021] Without being bound by theory, it is believed that by using two or more catalyst zones as described herein, the catalyst working load can be flattened along the length of the reaction tube. A flatter temperature profile can make it possible to better control the heat moving through the reaction tube and to improve heat removal capacity. By using two or more catalyst zones as described herein, the catalyst aging pattern can be made more uniform, thereby producing greater selectivity and / or productivity over time. In terms of metal content, the catalyst selected for the catalyst zone can be a different catalyst, or the size, shape or preparation can be different. In some aspects, the catalyst is selected to balance the metal loss during catalyst life, product yield and manufacturing. These and other details are further described herein.

[0022] Acetoxylation of Olefins

[0023] As described herein, zone catalyst loading can be used in a process for acetoxylating olefins. Exemplary olefins include ethylene and propylene, although other olefins are also contemplated. Acetoxylating ethylene to form vinyl acetate monomer is of particular industrial interest. Vinyl acetate monomer (VAM) is a compound represented by the formula:

[0024] VAM is an important component in various products, including polymers. VAM is also an important intermediate in coatings, textiles, paints, and other applications. For example, the polymer of VAM, polyvinyl acetate, is used in a variety of applications including glues and adhesives.

[0025] The reaction to form VAM includes forming a gaseous reaction stream, for example, a reaction gas. The reaction gas may include ethylene, acetic acid and an oxygen-containing gas such as molecular oxygen. In some aspects, the reaction gas may further include an inert gas. The reaction gas may then pass through at least two fixed catalyst zones, wherein the at least two fixed catalyst zones are arranged in series. The at least two fixed catalyst zones are located in a reaction tube. There may be a plurality of reaction tubes arranged in parallel. Each reaction tube may include at least two identical fixed catalyst zones. The at least two fixed catalyst zones may include an inlet catalyst zone and an outlet catalyst zone. In the presence of more than two catalyst zones, it is understood that the additional catalyst zone is between the inlet catalyst zone and the outlet catalyst zone. Each catalyst zone includes at least one catalyst.

[0026] The amount of catalyst loaded in each catalyst zone can be selected according to the desired catalyst performance. In some aspects, the inlet catalyst zone can account for 5% to 70% of the catalyst loading of the entire reaction tube (for example, all catalysts in all catalyst zones in the reaction tube). In some aspects, the outlet catalyst zone can account for 30% to 95% of the catalyst loading of the entire reaction tube. In some aspects, most of the reaction tubes include roughly the same catalyst zone, thereby allowing slight differences based on catalyst aging in each zone.

[0027] The reaction can be carried out in the gas phase at a temperature of from 100 to 250° C. and at a pressure of from 1 to 25 bar.

[0028] The acetoxylation of ethylene produces a crude vinyl acetate product containing VAM, water, and carbon dioxide, as well as unreacted ethylene and acetic acid used in excess. Ethylene and acetic acid are recycled back to the reactor from the reaction section and purification section of the unit. The product VAM is recovered and purified in the purification section and sent to a storage tank. The wastewater is sent to a treatment facility and the carbon dioxide is discharged to a pollution control device. Inert gases such as nitrogen and argon may accumulate over time and can then be purged from the reaction section to minimize accumulation. A complete description of the VAM production process is provided herein and is described in detail in the accompanying drawings. Figure 1 In the description.

[0029] VAM production process

[0030] Figure 1A process flow diagram of an exemplary vinyl acetate production process 100 of the present disclosure is shown. Component additions and modifications may be made to process 100 without changing the scope of the present invention. In addition, as will be appreciated by those skilled in the art, the description of process 100 and related systems uses streams to describe fluids passing through various pipelines. Whether or not explicitly described, for each stream, the related system has a corresponding pipeline (e.g., a pipe or other path through which the corresponding fluid or other material can easily pass) and optional valves, pumps, compressors, heat exchangers, or other equipment to ensure proper operation of the system.

[0031] In addition, the descriptors used for each stream do not limit the composition of the stream to being composed of the descriptors. For example, an ethylene stream is not necessarily composed only of ethylene. On the contrary, an ethylene stream may comprise ethylene and a diluent gas (e.g., an inert gas). Alternatively, an ethylene stream may consist only of ethylene. Alternatively, an ethylene stream may comprise ethylene, another reactant, and optionally an inert component.

[0032] In the illustrated process 100, an acetic acid stream 102 and an ethylene stream 104 are introduced into a vaporizer 106. Optionally, ethane may also be added to the vaporizer 106. In addition, one or more recycle streams 130, 158 (each further described herein) may also be introduced into the vaporizer 106. Optionally, one or more of the recycle streams 130, 158 may be combined with the acetic acid stream 102 (not shown) prior to introduction into the vaporizer 106. The temperature and pressure of the vaporizer 106 can vary within a wide range. The vaporizer 106 is preferably operated at a temperature of from 100° C. to 250° C., or from 100° C. to 200° C., or from 120° C. to 150° C. The operating pressure of the vaporizer 106 can range from 0.1 MPa to 2.03 MPa, or from 0.25 MPa to 1.75 MPa, or from 0.5 MPa to 1.5 MPa. The vaporizer 106 produces a vaporized feed stream 108. The vaporized feed stream 108 leaves the vaporizer 106 and is combined with an oxygen stream 110 to produce a combined feed stream 112. The combined feed stream 112 is analyzed by a sensor 114 before being fed to a vinyl acetate reactor 116.

[0033] Sensor 114 includes a water sensor for determining the water concentration in the combined feed stream 112. Sensor 114 may optionally also include a temperature sensor, a pressure sensor, a flow sensor, a composition sensor (e.g., a gas chromatograph, an infrared spectrometer, and an oxygen analyzer), etc., and any combination thereof. Each of the individual sensors may exist alone or in multiples. Having multiple specific sensors gives redundancy, thereby minimizing the downtime for sensor replacement and alleviating safety issues caused by sensor failure or misalignment. Although sensor 114 is generally shown as being upstream of vinyl acetate reactor 116, the sensor may be placed at other locations or other suitable locations where calculations can be performed to estimate the conditions (e.g., temperature, pressure, or component concentration) at the reactor inlet.

[0034] The operating conditions in vinyl acetate reactor 116 may be adjusted based on the composition of combined feed stream 112. In general, suitable ranges for operating conditions in vinyl acetate reactor 116 are provided below.

[0035] With respect to the general operating conditions of the vinyl acetate reactor 116, when producing vinyl acetate, the molar ratio of ethylene to oxygen in the vinyl acetate reactor 116 is preferably less than 20:1 (e.g., 1:1 to 20:1, or 1:1 to 10:1, or 1.5:1 to 5:1, or 2:1 to 4:1). In addition, the molar ratio of acetic acid to oxygen in the vinyl acetate reactor 116 is preferably less than 10:1 (e.g., 0.5:1 to 10:1, 0.5:1 to 5:1, or 0.5:1 to 3:1). The molar ratio of ethylene to acetic acid in the vinyl acetate reactor 116 is preferably less than 10:1 (e.g., 1:1 to 10:1, or 1:1 to 5:1, or 2:1 to 3:1). Thus, the combined feed stream 112 comprises ethylene, oxygen, and acetic acid in the stated molar ratios.

[0036] The vinyl acetate reactor 116 may be a shell and tube reactor capable of absorbing heat generated by the exothermic reaction through a heat exchange medium and controlling the temperature therein within a temperature range of 100° C. to 250° C., or 110° C. to 200° C., or 120° C. to 180° C. The pressure in the vinyl acetate reactor 116 may be maintained at 0.5 MPa to 2.5 MPa, or 0.5 MPa to 2 MPa.

[0037] Furthermore, vinyl acetate reactor 116 may be a fixed bed reactor or a fluidized bed reactor, preferably a fixed bed reactor comprising reaction tubes as described herein.

[0038] Reactor 116 may include one or more tubes, for example, a plurality of tubes arranged in parallel. In some aspects, the reactor may include from 2 to 10,000 tubes, for example, from 50 to 10,000 tubes, from 500 to 10,000 tubes, from 1000 to 9500 tubes, or from 3000 to 9000 tubes. Each tube includes at least two fixed catalyst zones as described herein. In some aspects, based on the total number of catalyst tubes, at least 10%, for example, at least 20%, at least 30%, at least 40%, at least 50%, at least 60%, at least 70%, at least 80%, at least 90%, at least 95%, at least 97.5%, at least 99%, at least 99.5%, or 100% of the tubes include the same at least two fixed catalyst zones. Two exemplary but non-limiting catalyst tube configurations are shown in FIG. Figure 2 (Tube 180) and Figure 3 (Tube 190). Figure 2 Tube 180 is shown showing an inlet flow 181 entering an open portion of the tube which may include inert materials. These inert materials may be used similar to guard beds to filter the incoming flow to remove solids or liquids. Inlet catalyst zone 183 contains a fixed inlet catalyst. Outlet catalyst zone 184 contains a fixed outlet catalyst. The reactant gas then passes through inert material 185 which may load the catalyst to prevent it from moving into / through springs 186. The reactant gas then exits tube 180 in the direction of outlet flow 182. Similarly, Figure 3 Another non-limiting zone loaded catalyst configuration is shown wherein tube 190 has an inlet flow 191, an inlet catalyst zone 193 comprising an inlet catalyst, an additional catalyst zone 194 comprising additional catalyst, an outlet catalyst zone 197 comprising an outlet catalyst, an inert material 195, a spring 196, and an outlet flow 192.

[0039] Multiple tube arrangements are shown in Figure 4 middle. Figure 4 A plurality of tubes 200 are shown in parallel, but are not intended to be limiting with respect to the number of tubes. Tubes 200 may be tubes 180, tubes 190, or another tube configuration described herein.

[0040] Reference again Figure 1 Since the process 100 uses one or more recycle streams 130, 156, 168 and may include a diluent, the components in the combined feed stream 112 are not just ethylene, acetic acid, and oxygen. Examples of components in the combined feed stream 112 where the concentrations of the components are present may include, but are not limited to, ethylene, acetic acid, methane, ethane, propane, water, nitrogen, argon, and carbon dioxide.

[0041] Reference again Figure 1, the vinyl acetate in reactor 116 is reacted to produce a crude vinyl acetate stream 118. Depending on the conversion rate and reaction conditions, the crude vinyl acetate stream 118 may contain 5 wt.% to 30 wt.% vinyl acetate, 5 wt.% to 40 wt.% acetic acid, 0.1 wt.% to 10 wt.% water, 10 wt.% to 80 wt.% ethylene, 1 wt.% to 40 wt.% carbon dioxide, 0.1 wt.% to 50 wt.% alkane (e.g., methane, ethane, or a mixture thereof), and 0.1 wt.% to 15 wt.% oxygen. Optionally, the crude vinyl acetate stream 118 may also contain 0.01 wt.% to 10 wt.% ethyl acetate. The crude vinyl acetate stream 118 may contain other compounds, such as methyl acetate, acetaldehyde, acrolein, propane, and inert substances such as nitrogen or argon. Typically, these other compounds are present in very low amounts, except for the inert substances.

[0042] The crude vinyl acetate stream 118 passes through a heat exchanger 120 to reduce the temperature of the crude vinyl acetate stream 118 and then to a separator 122 (e.g., a distillation column). Preferably, the crude vinyl acetate stream 118 is cooled to a temperature of 80° C. to 145° C., or 90° C. to 135° C., before being introduced into the separator 122. Preferably, the liquefiable components are not condensed, and the cooled crude vinyl acetate stream 118 is introduced into the separator 122 as a gas.

[0043] The energy to separate the components in crude vinyl acetate stream 118 may be provided by the heat of reaction in reactor 116. In some embodiments, an optional reboiler dedicated to increasing the separation energy within separator 122 may be present. Separator 122 separates crude vinyl acetate stream 118 into at least two streams: overhead stream 124 and bottoms stream 126. Overhead stream 124 may comprise ethylene, carbon dioxide, water, alkanes (e.g., methane, ethane, propane, or mixtures thereof), oxygen, and vinyl acetate. The bottoms stream may comprise vinyl acetate, acetic acid, water, and possibly ethylene, carbon dioxide, and alkanes.

[0044] Overhead stream 124 is sent to scrubber 128 to remove vinyl acetate in overhead stream 124. Thus, scrubber 128 has gas stream 130 and bottoms stream 132. Vinyl acetate scrubbing may be accomplished by passing overhead stream 124 through a mixture of water and acetic acid.

[0045] Tail gas stream 130 contains ethylene, carbon dioxide, alkane, and oxygen. The condition (e.g., temperature, pressure, and / or component composition) of tail gas stream 130 can be measured using sensor 134. Sensor 134 may include, but is not limited to, temperature sensors, pressure sensors, flow sensors, composition sensors (e.g., gas chromatographs, infrared spectrometers, and oxygen analyzers), etc., and any combination thereof. Each of the individual sensors may exist alone or in multiples. Having multiple specific sensors gives redundancy, thereby minimizing the downtime of sensor replacement and alleviating safety issues caused by sensor failure or misalignment. Although sensor 134 is generally shown as being downstream of scrubber 128 along tail gas stream 130, the sensor may be placed in other locations or other suitable locations where calculations can be performed to estimate the condition (e.g., temperature, pressure, or component concentration) of tail gas stream 130 after scrubber 128.

[0046] Tail gas stream 130 (also referred to as a recycle stream) is passed back to vaporizer 106 through heat exchanger 120 where crude vinyl acetate stream 118 heats tail gas stream 130. Optionally, tail gas stream 130 may be augmented with or otherwise added to it, including other recycle streams from the process and feed streams (not shown). As shown, ethylene feed stream 136 and methane feed stream 138 (or other ballast gas streams) are combined (e.g., mixed or entrained) with tail gas stream 130.

[0047] In addition, other processes (not shown) may be performed on the tail gas stream 130 between the scrubber 128 and the heat exchanger 120. For example, the tail gas stream 130 may remove at least a portion of the carbon dioxide.

[0048] Between the heat exchanger 120 and the vaporizer 106, the exhaust gas flow 130 is analyzed by a sensor 140. Examples of the sensor 140 include, but are not limited to, temperature sensors, pressure sensors, flow sensors, composition sensors (e.g., gas chromatographs, infrared spectrometers, and oxygen analyzers), etc., and any combination thereof. Each of the individual sensors may exist alone or in multiples. Having multiple specific sensors gives redundancy, thereby minimizing downtime for sensor replacement and alleviating safety issues caused by sensor failure or misalignment. Although the sensor 140 is generally shown as being between the heat exchanger 120 and the vaporizer 106, the sensor may be placed at other locations or other suitable locations where calculations can be performed to estimate the conditions (e.g., temperature, pressure, or component concentration) between the heat exchanger 120 and the vaporizer 106.

[0049] Reference again Figure 1, the bottom stream 126 from the separator 122 and the bottom stream 132 from the scrubber 128 can be combined and fed to the crude product tank 142. Typically, one or more streams entering the crude product tank 142 are depressurized to a pressure of 0.1 MPa to 0.15 MPa. When the incoming stream is depressurized, ethylene, carbon dioxide, inert gas (e.g., nitrogen and / or argon), and acetic acid flash to produce a flash gas stream 144. The bottoms of the crude product tank 142 mainly contain vinyl acetate, water, and acetic acid and some ethyl acetate by-products. The bottoms are transported as a vinyl acetate stream 146, which is purified by various processes 148 to produce a purified vinyl acetate product stream 150. Examples of purification processes 148 include, but are not limited to, azeotropic distillation, water stripping, distillation, phase separation, etc., and any combination thereof. Examples of different processing methods and systems are described in US Pat. Nos. 6,410,817, 8,993,796, and 9,045,413 and US Patent Application Publication No. 2014 / 0066649, each of which is incorporated herein by reference.

[0050] Furthermore, the purification process 148 may generate additional streams that, alone or in any combination, may be recycled back to the vaporizer 106 , the tail gas stream 130 , the flash gas stream 144 , and / or other streams within the method 100 .

[0051] Optionally (not shown), a portion of the tail gas stream 130 may be combined with (eg, mixed or entrained with) the flash gas stream 144 .

[0052] At least a portion of the carbon dioxide in the flash gas stream 144 (optionally having been combined with a portion of the tail gas tail stream 130) is removed before being recycled back to the vaporizer 106. As shown, the flash gas stream 144 first passes through a CO2 scrubber 152 and then passes through a CO2 absorber 156 to produce a CO2-removed overhead stream 158. Between the CO2 scrubber 152 and the CO2 absorber 156, ethylene can be added to the flash gas stream 144 from the ethylene stream 154.

[0053] In the CO2 absorber 156, the top stream 158 from which CO2 is removed is analyzed by a sensor 160. Examples of the sensor 160 include, but are not limited to, temperature sensors, pressure sensors, flow sensors, composition sensors (e.g., gas chromatographs, infrared spectrometers, and oxygen analyzers), etc., and any combination thereof. Each of the individual sensors may exist alone or in multiples. Having multiple specific sensors gives redundancy, thereby minimizing downtime for sensor replacement and alleviating safety issues caused by sensor failure or misalignment. Although the sensor 160 is generally shown downstream of the CO2 absorber 156, the sensor 160 may be placed at other locations or other suitable locations where calculations can be performed to estimate the conditions (e.g., temperature, pressure, or component concentration) downstream of the CO2 absorber 156.

[0054] Reference again Figure 1 , the top stream 158 of removing CO2 can then pass through heat exchanger 162 and be fed in vaporizer 106. In addition, tail stream 164 from flash gas stream 144 and / or the top stream 158 of removing CO2 is used to purge inert materials in the system. This tail stream 164 can be sent to ethylene recovery process 166. Ethylene recovery process 162 produces ethylene discharge stream 168 and recycle stream 172. The example of ethylene recovery process 166 can include but is not limited to washing system, membrane recovery method, etc., and any combination thereof. Ethylene recovery process 166 can produce discharge stream 168 and additional one or more streams 172, and the one or more streams bring the recovered ethylene to other methods or recycle back to this process 100.

[0055] Therefore, after the ethylene recovery process 166, the ethylene exhaust stream 168 is analyzed by a sensor 170. Examples of the sensor 170 include, but are not limited to, temperature sensors, pressure sensors, flow sensors, composition sensors (e.g., gas chromatographs, infrared spectrometers, and oxygen analyzers), etc., and any combination thereof. Each of the individual sensors may exist alone or in multiples. Having multiple specific sensors gives redundancy, thereby minimizing downtime for sensor replacement and alleviating safety issues caused by sensor failure or misalignment. Although the sensor 170 is generally shown along the ethylene exhaust stream 168, the sensor 170 may be placed at other locations or other suitable locations where calculations can be performed to estimate the condition (e.g., temperature, pressure, or component concentration) of the ethylene exhaust stream 168.

[0056] Catalyst loaded in one or more zones

[0057] like Figure 2As shown, tube 180 includes an inlet catalyst zone 183 and an outlet catalyst zone 184. The inlet catalyst zone may contain one or more catalysts, which are referred to as inlet catalysts. Similarly, the outlet catalyst zone may contain one or more catalysts, which are referred to as outlet catalysts. Figure 3 As shown, it is also contemplated that one or more catalyst zones may be included between the inlet catalyst zone and the outlet catalyst zone, referred to as one or more additional catalyst zones, such as a first additional catalyst zone, a second additional catalyst zone, etc. Each catalyst zone may contain a single catalyst.

[0058] In some aspects, the inlet catalyst zone accounts for from 5% to 70%, e.g., from 10% to 70%, from 15% to 70%, from 20% to 70%, from 25% to 70%, from 30% to 70%, from 35% to 70%, from 40% to 70%, from 45% to 70%, from 50% to 70%, from 50% to 65%, from 15% to 65%, from 15% to 60%, from 15% to 55%, from 20% to 50%, from 20% to 40%, from 20% to 35%, or from 20% to 30% of the catalyst loading, based on 100% catalyst loading by weight of all catalyst zones.

[0059] In some aspects, the outlet catalyst zone accounts for from 30% to 95%, e.g., from 35% to 90%, from 35% to 80%, from 40% to 80%, from 40% to 75%, from 40% to 70%, from 45% to 70%, from 50% to 70%, from 55% to 70%, from 55% to 95%, from 55% to 90%, from 55% to 85%, from 55% to 80%, from 60% to 80%, from 65% to 80%, or from 70% to 80% of the catalyst loading, based on 100% catalyst loading by weight for all catalyst zones.

[0060] In some aspects, there may be at least one additional catalyst zone. In such aspects, based on the catalyst loading of all catalyst zones, at least one additional catalyst zone may account for from 1% to 25%, such as from 5% to 20%, or from 10% to 15% of the catalyst loading.

[0061] In some aspects, the inlet catalyst zone accounts for more (e.g., greater than 50% by weight) of the catalyst loading compared to the outlet catalyst zone, based on the weight of all catalyst loadings in all catalyst zones. In other aspects, the inlet catalyst zone accounts for less (e.g., less than 50% by weight) of the catalyst loading compared to the outlet catalyst zone, based on the weight of all catalyst loadings in all catalyst zones.

[0062] In some aspects, the inlet catalyst, outlet catalyst, and / or additional catalyst include palladium (Pd). In some aspects, the inlet catalyst includes from 4 to 15 g / L, for example, from 5 to 13 g / L, from 5 to 11 g / L, from 5 to 10 g / L, from 5 to 9 g / L, from 5.5 to 8.5 g / L, from 6 to 8.5 g / L, or from 6.5 to 8.5 g / L of Pd. In terms of weight percentage, the catalyst may include less than 2 wt.%, for example, less than 1.75 wt.%, less than 1.5 wt.%, or less than 1.25 wt.% of Pd. In terms of range, the catalyst may include from 0.1 to <2 wt.%, for example, from 0.5 to 1.75 wt.%, from 0.75 to 1.5 wt.%, or from 0.8 to 1.15 wt.% of Pd.

[0063] In some aspects, inlet catalyst, outlet catalyst, and / or additional catalyst include gold (Au). Inlet catalyst can include from 2 to 8g / L, for example, from 2.5 to 7.5g / L, from 3 to 7g / L, from 3 to 6g / L, from 3 to 5g / L, from 3.25 to 5g / L, from 3.5 to 4.75g / L or from 3.5 to 4.5g / L Au. In some specific aspects, in terms of weight percentage, catalyst can include less than 1.5wt.%, for example, less than 1.25wt.%, less than 1wt.% or less than 0.75wt.% Au. In terms of scope, catalyst can include from 0.1 to 1.5wt.%, for example, from 0.2 to 1wt.%, from 0.3 to 0.75wt.% or from 0.4 to 0.75wt.% Au.

[0064] To calculate the weight percent of the components on the catalyst, a density of 630 g / L may be used.

[0065] In some aspects, the inlet catalyst, outlet catalyst, and / or additional catalyst comprises Au and Pd. The ratio of Au to Pd can range from 0.3:1 to 1.5:1 based on g / L of Au to Pd, for example, from 0.4:1 to 1.3:1, from 0.45:1 to 1.2:1, from 0.45:1 to 1:1, from 0.5:1 to 0.75:1, or from 0.5:1 to 0.6:1. In some aspects, Pd is present in a greater amount than Au in terms of g / L and wt.%.

[0066] In some aspects, the inlet catalyst, outlet catalyst, and / or additional catalyst comprises potassium acetate (KOAc). KOAc can be present in the largest amount compared to Pd and Au. KOAc can be present from 25 to 55 g / L, for example, from 30 to 50 g / L, from 35 to 45 g / L, from 37.5 to 42.5 g / L, or about 40 g / L. In terms of weight percentage, KOAc can be present from 3.0 to 10.0 wt.%, 4.0 to 9.0 wt.%, from 4.5 to 8.5 wt.%, from 5.0 to 7.5 wt.%, or from 5.5 to 7.0 wt.%.

[0067] In some respects, inlet catalyst, outlet catalyst and / or additional catalyst can contain refractory carrier, comprise metal oxide, such as silicon dioxide, silicon dioxide-alumina, titanium dioxide or zirconium oxide.In some respects, refractory carrier is silicon dioxide.Refractory carrier can be coated with catalyst component then, for example the layer of Pd, Au and KOAc.The scope of the thickness of coating on refractory carrier can be from 100 to 300 microns, for example from 115 to 200 microns, from 120 to 175 microns, from 125 to 150 microns, from 200 to 300 microns or from 225 to 275 microns.In some respects, the thickness of coating can be selected based on the size of catalyst.The size of catalyst refers to the diameter of catalyst in the longest direction.Catalyst diameter can be measured according to ASTM UOP947-96 (1996).

[0068] In some aspects, the inlet catalyst and / or the outlet catalyst comprises the composition shown in Table 1 below.

[0069] In some aspects, the inlet catalyst zone may include catalyst B as an inlet catalyst, and the outlet catalyst zone may include catalyst A, B, C, D, E or F as an outlet catalyst. In some aspects, the inlet catalyst zone may include catalyst C as an inlet catalyst, and the outlet catalyst zone may include catalyst A, B, C, D, E or F as an outlet catalyst. In some aspects, the inlet catalyst zone may include catalyst D as an inlet catalyst, and the outlet catalyst zone may include catalyst A, B, C, D, E or F as an outlet catalyst. In some aspects, the inlet catalyst zone may include catalyst A, B, C, D, E or F as an outlet catalyst, and the outlet catalyst zone may include catalyst E as an outlet catalyst. As disclosed in Table 1 above, the range of Au content can be from 2 to 8 g / L. It is clearly contemplated within the present disclosure that Au content can be selected from the sub-ranges disclosed herein. Similarly, other options of catalysts, including carriers, Au / Pd ratios and the amount of KOAc, can be selected as disclosed herein.

[0070] Inlet catalyst, outlet catalyst and / or any extra catalyst refractory carrier can have such cross-sectional shape, and it includes but is not limited to trilobate, four-leaf shaped, four-leaf clover shaped, saddle shaped, groove shaped, ridge shaped, multi-pointed star shaped, grooved ring, hollow cylinder, gear shaped, spoke wheel shape, porous pellet or monolithic body, T-shaped fin shape, Raschig ring shape, spherical or other shapes.In some respects, shape is spherical.The scope of spherical size can average out to from 5 to 7mm.In some respects, inlet catalyst and outlet catalyst each have different sizes and different layer thicknesses.For example, when the spherical carrier is of a size of 5mm, the layer is thicker than the layer on 6mm or 7mm shape carrier.

[0071] The activity and selectivity of the catalysts were measured over a period of up to 200 hours at 9.8 bar absolute and 4000-8000 standard m 3 / (m 3 The catalyst was tested in a flow tube (reactor length 1200 mm, inner diameter 19 mm) controlled by oil at a space velocity (GHSV) of 1.200 % by volume, using the following gas composition: 60% by volume of ethylene, 19.5% by volume of argon, 13% by volume of acetic acid and 7.5% by volume of oxygen. The catalyst system was tested in a temperature range from 130° C. to 180° C. (gas inlet temperature upstream of the catalyst bed). In order to characterize the course of the reaction, the temperature profile was measured by a multipoint temperature sensor in the catalyst bed. The reaction products and unreacted starting materials were analyzed by online gas chromatography at the output of the reactor. The space-time yield of the catalyst system, in grams of vinyl acetate monomer per hour and per liter of catalyst (g(VAM) / l of cat.*h), was determined as a measure of the catalyst activity. The selectivity was determined via the ratio of vinyl acetate formed to ethylene reacted.

[0072] In addition to the determination of the reaction product in the gas phase, the liquid reaction product was concentrated in a container kept at from 10° C. to 15° C., and the concentrate obtained was analyzed by means of gas chromatography.

[0073] The acetoxylation of olefins (e.g., ethylene) can achieve considerable ethylene conversion and considerable selectivity and productivity to vinyl acetate monomer. For purposes of the present invention, the term "conversion" refers to the amount of ethylene converted into compounds other than ethylene in the feed. The conversion is expressed as a percentage based on the ethylene in the feed. The conversion can be at least 30%, such as at least 40%, or at least 60%. Although catalysts with high conversions (e.g., at least 60%) are desirable, in some embodiments, low conversions may be acceptable at high selectivity to VAM. Of course, it is fully understood that in many cases, the conversion can be compensated by appropriate recycle streams or by using larger reactors / more reactor tubes, but it is more difficult to compensate for poor selectivity.

[0074] Selectivity is expressed as a mole percentage based on converted ethylene. It should be understood that each compound converted by ethylene has an independent selectivity, and the selectivity is independent of the conversion rate. For example, if 60 mole % of the converted ethylene is converted into vinyl acetate monomer, the vinyl acetate monomer selectivity is considered to be 60%. In one embodiment, the catalyst has a selectivity of at least 60% to vinyl acetate monomer, such as at least 70%, at least 80%, or at least 90%. Preferably, the selectivity to vinyl acetate monomer is at least 80%, such as at least 85% or at least 88%. Preferred embodiments of the process also have a low selectivity to undesirable products, such as heavy fractions. The selectivity to these undesirable products is preferably less than 10%, such as less than 5% or less than 2%.

[0075] The selectivity and conversion associated with the reaction are a function of several variables, including reactor temperature, component concentrations, and catalyst condition. Catalyst deactivation, which often occurs over time due to accumulation of tar and polymeric materials on the catalyst surface and / or structural changes in the catalyst metal, can adversely affect the reaction process, particularly in terms of selectivity. These changes in reactor performance can ultimately result in changes in the composition of the liquid stream entering the purification section of the vinyl acetate plant.

[0076] Typically, the performance of zone loading can be evaluated by at least one of the following three metrics: oxygen conversion, space-time yield, and temperature of the entire reactor. In some aspects, two of these metrics are used.

[0077] Similar to the measurement of ethylene conversion, oxygen conversion refers to the amount of oxygen provided to the reaction tube that is converted. In some aspects, the oxygen conversion ranges from 35% to 55%, such as from 40% to 55%, or from 45% to 50%.

[0078] Space-time yield refers to the amount of product produced per packed volume bed per unit time. In some aspects, the space-time yield (STY) is at least 800 g VAM / L / hour and can range from 800-1600 g VAM / L / hour, such as from 900 to 1500 g VAM / L / hour, or from 1000 to 1500 g VAM / L / hour.

[0079] The temperature throughout the reaction tubes can range from 145°C to 190°C, for example, from 150°C to 180°C, from 155°C to 175°C, or from 160°C to 170°C. Without being bound by theory, it is believed that the temperature can vary based on the life of the catalyst, wherein newer catalysts typically have lower temperatures than older catalysts. Compared with higher temperatures, lower temperatures throughout the reaction tubes may indicate that the deactivation of the catalyst is slower. Lower temperatures can also indicate lower carbon dioxide selectivity, which corresponds to producing more products. Since ethylene is a relatively expensive component, operating at lower temperatures can burn less ethylene and can improve process efficiency. The temperature can be balanced with conversion, selectivity, and the cost of replacing the catalyst.

[0080] In some aspects, the use of zone catalyst loading disclosed herein can advantageously achieve a desirable pressure drop throughout the reaction tube. In some aspects, the pressure drop can range from 12 to 30 psi, such as from 15 to 25 psi, or from 17.5 to 22.5 psi. The pressure drop can be measured by measuring the absolute pressure at the reactor inlet and the reactor outlet and then calculating the difference.

[0081] When a range of values ​​is provided, it is understood that unless the context clearly indicates otherwise, each intermediate value (to one-tenth of the unit of the lower limit) between the upper limit and the lower limit of the range is also specifically disclosed. Each smaller range between any of the values ​​or intermediate values ​​in the range and any other value or intermediate value in the range is covered. The upper and lower limits of these smaller ranges may be independently included in the range or excluded from the range, and each range in which either, neither, or both of the limits are included in the smaller range is also covered by the present invention, subject to any explicitly excluded limits in the range. When the range includes one or both of the limits, the range excluding either or both of those included limits is also included.

[0082] As used herein and in the appended claims, the singular forms "a," "an," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a process" includes a plurality of such processes, and so forth.

[0083] In addition, when used in this specification and the following claims, the words "comprise", "comprising", "include", "including" and "includes" are intended to expressly indicate the presence of stated features, integers, components, or steps, but they do not exclude the presence or addition of one or more other features, integers, components, steps, functions, or groups.

[0084] Example

[0085] Embodiment 1: A method for acetoxylating olefins in a gaseous reaction stream containing the olefin, acetic acid and an oxygen-containing gas, the method comprising: passing the reaction gas through at least two fixed catalyst zones arranged in series to form acetoxylated olefins, wherein the catalyst zones are located in one or more reaction tubes arranged in parallel; wherein the at least two fixed catalyst zones include an inlet catalyst zone comprising an inlet catalyst and an outlet catalyst zone comprising an outlet catalyst; wherein based on 100% catalyst loading by weight of all catalyst zones, the inlet catalyst zone accounts for from 5% to 70% of the catalyst loading, and the outlet catalyst zone accounts for from 30% to 95% of the catalyst loading; and wherein at least one of the following conditions is met: (a) the inlet catalyst contains greater than 7 g / L: Pd; (b) the outlet catalyst contains greater than 8.3 g / L Pd; and / or (c) the outlet catalyst contains from 7.5 to 8.1 g / L Pd.

[0086] Embodiment 2: The method of embodiment 1, wherein the inlet catalyst zone accounts for from 40% to 70% by weight of the catalyst loading.

[0087] Embodiment 3: The method of embodiment 1 or 2, wherein conditions (b) and / or (c) are satisfied and wherein the inlet catalyst comprises from 4 to 15 g / L Pd and from 2 to 8 g / L Au.

[0088] Embodiment 4: The method as described in any of the preceding embodiments, wherein the outlet catalyst zone accounts for from 30% to 50% by weight of the catalyst loading.

[0089] Embodiment 5: The method as described in any of the preceding embodiments, wherein condition (a) is met and wherein the outlet catalyst contains from 4 to 15 g / L Pd and from 2 to 8 g / L Au.

[0090] Embodiment 6: The method of embodiment 1, wherein the inlet catalyst zone accounts for from 5% to 40% by weight of the catalyst loading.

[0091] Embodiment 7: The method of embodiment 6, wherein the inlet catalyst has a weight ratio of Au to Pd from 0.3:1 to 1.5:1.

[0092] Embodiment 8: The method as described in embodiment 6 or 7, wherein the inlet catalyst has a layer thickness from 100 to 300 microns.

[0093] Embodiment 9: The method of any one of embodiments 6-8, wherein the inlet catalyst zone contains at least two catalysts in series and wherein the weight ratio of the two catalysts is from 1:5 to 5:1.

[0094] Embodiment 10: The method of any one of Embodiments 6-9, wherein the outlet catalyst zone accounts for from 60% to 95% by weight of the catalyst loading.

[0095] Embodiment 11: The method as described in any one of Embodiments 6-10, wherein the outlet catalyst has a weight ratio of Au to Pd of 0.3:1 to 1.5:1.

[0096] Embodiment 12: The method of any of the preceding embodiments, wherein the inlet catalyst has a different metal loading than the outlet catalyst.

[0097] Embodiment 13: The method of any of the preceding embodiments, wherein the inlet catalyst zone comprises a catalyst having a size different from that of the outlet catalyst zone.

[0098] Embodiment 14: The method of any of the preceding embodiments, wherein the inlet catalyst and the outlet catalyst each contain less than 2 wt. % Pd.

[0099] Embodiment 15: The method of any one of the preceding embodiments, wherein the inlet catalyst and the outlet catalyst each contain from 25 to 55 g / L potassium acetate.

[0100] Embodiment 16: The process of any of the preceding embodiments, wherein the olefin is ethylene and wherein the selectivity to vinyl acetate monomer is at least 60%.

[0101] Embodiment 17: The process of any of the preceding embodiments, wherein the olefin is ethylene and wherein the conversion of ethylene is at least 60%.

[0102] Embodiment 18: The process of any one of the preceding embodiments, wherein the olefin is ethylene and the selectivity to heavy ends is less than 10%.

[0103] Embodiment 19: The method as described in any of the preceding embodiments, wherein the inlet catalyst and the outlet catalyst have a spherical shape.

[0104] Embodiment 20: The process of any one of the preceding embodiments, wherein the olefin is ethylene and the oxygen conversion is from 35% to 55%.

[0105] Embodiment 21: The method as described in Embodiment 1, wherein only condition a) is satisfied.

[0106] Embodiment 22: The method as described in Embodiment 1, wherein only condition b) is satisfied.

[0107] Embodiment 23: The method as described in Embodiment 1, wherein only condition c) is satisfied.

[0108] Embodiment 24: The method as described in Embodiment 1, wherein conditions a) and b) are satisfied.

[0109] Embodiment 25: The method as described in Embodiment 1, wherein conditions a) and c) are satisfied.

[0110] Although the present invention has been described in detail, modifications within the spirit and scope of the present invention will be apparent to those skilled in the art. It should be understood that the various aspects of the present invention and the various embodiments and portions of the various features described above and / or in the appended claims may be combined or interchanged in whole or in part. In the above description of the various embodiments, those embodiments that mention another embodiment may be appropriately combined with other embodiments, as understood by those of ordinary skill in the art. In addition, it will be understood by those of ordinary skill in the art that the above description is by way of example only and is not intended to limit the present invention. All U.S. patents and publications cited herein are incorporated by reference in their entirety.

Claims

1. A process for acetoxylating an olefin in a gaseous reaction stream containing the olefin, acetic acid and an oxygen-containing gas, the process comprising: passing the reaction gas through at least two fixed catalyst zones arranged in series to form an acetoxylated olefin, wherein the catalyst zone is located in one or more reaction tubes arranged in parallel; wherein the at least two fixed catalyst zones include an inlet catalyst zone comprising an inlet catalyst and an outlet catalyst zone comprising an outlet catalyst; wherein the inlet catalyst zone accounts for from 5% to 70% of the catalyst loading, and the outlet catalyst zone accounts for from 30% to 95% of the catalyst loading, based on 100% catalyst loading by weight of all catalyst zones; and wherein at least one of the following conditions is met: (a) the inlet catalyst contains greater than 7 g / L Pd; (b) the outlet catalyst contains greater than 8.3 g / L Pd; and / or (c) The outlet catalyst contains from 7.5 to 8.1 g / L Pd.

2. The method of claim 1, wherein: The inlet catalyst zone comprises from 40 to 70% by weight of the catalyst loading.

3. The method according to claim 1 or 2, wherein: Conditions (b) and / or (c) are satisfied and wherein the inlet catalyst comprises from 4 to 15 g / L Pd and from 2 to 8 g / L Au.

4. A method as claimed in any one of the preceding claims, wherein: The outlet catalyst zone comprises from 30 to 50% by weight of the catalyst loading.

5. A method as claimed in any one of the preceding claims, wherein: Condition (a) is satisfied and wherein the outlet catalyst comprises from 4 to 15 g / L Pd and from 2 to 8 g / L Au.

6. The method of claim 1, wherein: The inlet catalyst zone comprises from 5% to 40% by weight of the catalyst loading.

7. The method of claim 6, wherein: The inlet catalyst has a weight ratio of Au to Pd from 0.3:1 to 1.5:

1.

8. The method according to claim 6 or 7, wherein: The inlet catalyst has a layer thickness of from 100 to 300 micrometers.

9. The method according to any one of claims 6 to 8, wherein: The inlet catalyst zone contains at least two catalysts connected in series and wherein the weight ratio of the two catalysts is from 1:5 to 5:

1.

10. The method according to any one of claims 6 to 9, wherein: The outlet catalyst zone comprises from 60% to 95% by weight of the catalyst loading.

11. The method according to any one of claims 6 to 10, wherein: The outlet catalyst has a weight ratio of Au to Pd of 0.3:1 to 1.5:

1.

12. A method as claimed in any one of the preceding claims, wherein: The inlet catalyst has a different metal loading than the outlet catalyst.

13. A method as claimed in any one of the preceding claims, wherein: The inlet catalyst zone comprises a catalyst having a different size than the outlet catalyst zone.

14. A method as claimed in any one of the preceding claims, wherein: The inlet catalyst and the outlet catalyst each contain less than 2 wt. % Pd.

15. A method as claimed in any one of the preceding claims, wherein: The inlet catalyst and the outlet catalyst each comprise from 25 to 55 g / L potassium acetate.

16. A method as claimed in any one of the preceding claims, wherein: The olefin is ethylene and wherein the selectivity to vinyl acetate monomer is at least 60%.

17. A method as claimed in any one of the preceding claims, wherein: The olefin is ethylene and wherein the conversion of ethylene is at least 60%.

18. A method as claimed in any one of the preceding claims, wherein: The olefin is ethylene and wherein the selectivity to heavy ends is less than 10%.

19. A method as claimed in any one of the preceding claims, wherein: The inlet catalyst and the outlet catalyst have a spherical shape.

20. A method as claimed in any one of the preceding claims, wherein: The olefin is ethylene and the oxygen conversion is from 35% to 55%.

21. The method of claim 1, wherein: Only condition a) is satisfied.

22. The method of claim 1, wherein: Only condition b) is met.

23. The method of claim 1, wherein: Only condition c) is satisfied.

24. The method of claim 1, wherein: Conditions a) and b) are met.

25. The method of claim 1, wherein: Conditions a) and c) are met.

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