Method for treating shaped catalyst bodies

Abstract

A method for treating shaped catalyst bodies comprising a porous support and silver deposited thereon comprises i) subjecting the catalyst bodies to an attrition treatment to superficially abrade the catalyst bodies, and ii) subjecting the abraded catalyst bodies to a dust removal treatment, wherein steps i) and ii) are performed subsequently or concurrently. The method yields shaped catalyst bodies exhibiting reduced dust formation, which allow for more efficient plant operation due to reduced pressure drop. The method allows for post-treatment of known shaped catalyst bodies rather than requiring modifications of known catalyst production steps, such as carrier production. Further provided are shaped catalyst bodies, an epoxidation reactor, and a process for producing ethylene oxide.

Description

240836W001 1Method for Treating Shaped Catalyst BodiesThe present invention relates to a method for treating shaped catalyst bodies, a method for filling a reactor, shaped catalyst bodies, an epoxidation reactor, and a process for producing ethylene oxide.Shaped catalysts bodies comprising silver deposited on a porous support are typically used for the industrial oxidation of ethylene to ethylene oxide. Ethylene oxide is produced in large volumes and is primarily used as an intermediate in the production of several industrial chemicals. Suitable shaped catalyst bodies may be obtained by impregnating a porous support with a silver impregnation and subjecting the impregnated support to a heat treatment, e.g., calcination.During catalyst loading and during the catalysis process, dust and debris may be generated by abrasive contact of the catalyst bodies with one another. The generated dust and debris are known to increase pressure drop in epoxidation reactors. Only limited characterization of the particles affecting the epoxidation reactor is available. Therefore, the characterization of the dust and debris mostly relies on post-mortem analysis.WO 2019 / 133174 A1 describes the use of surfactants in the production of silver-containing catalysts, which is stated to reduce dust and debris formation so as to allow for simplified catalyst handling post calcination. It is hypothesized that excess silver on the exterior chips off the catalyst during the operation, leading to an accumulation of fines and an increase in pressure drop.WO 2005 / 037427 A1 describes catalyst beds comprising a physical mixture of catalytically active and catalytically inactive shaped bodies, wherein the catalytically inactive shaped bodies have rounded edges on the external friction surfaces. Optionally, the catalytically active shaped bodies likewise have rounded edges. The shaped bodies having rounded edges can be produced in shaping or pressing tools.WO 2012 / 091898 A2 describes a multi-lobed porous ceramic body and a catalyst comprising silver and promoters deposited on the ceramic body. It is described that carrier precursors obtained by extrusion may be tumbled in a container, such as a rotating tube, so as to contact the carrier precursors with one another and to allowing for rounding the edges of the lobes.The known methods and catalysts thus require the adjustment of known production processes by using chemical auxiliaries or by providing modified carriers. It is desirable to provide shaped catalyst bodies which exhibit reduced dust formation, which shaped catalyst bodies are easily obtainable.The present invention provides a method for treating shaped catalyst bodies comprising a porous support and silver deposited thereon, the method comprisingI) subjecting the catalyst bodies to an attrition treatment to superficially abrade the catalyst bodies, and ii) subjecting the abraded catalyst bodies to a dust removal treatment, wherein steps I) and ii) are performed subsequently or concurrently.240836W001 2The invention further pertains to a method for filling a reactor with shaped catalyst bodies, comprising providing shaped catalyst bodies comprising a porous support and silver deposited thereon, treating the shaped catalyst bodies by the above, and filling the shaped catalyst bodies into the reactor, for example into reaction tubes of a multi-tube reactor.It was found that the claimed method yields shaped catalyst bodies exhibiting reduced dust formation, which allow for more efficient plant operation due to reduced pressure drop. The method allows for post-treatment of known shaped catalyst bodies rather than requiring modifications of known catalyst production steps, such as carrier production.Further provided are shaped catalyst bodies obtained by the method of the invention, an epoxidation reactor comprising a bed of shaped catalyst bodies obtained by the method of the invention, and a process for producing ethylene oxide by gas-phase oxidation of ethylene, comprising reacting ethylene and oxygen in the presence of shaped catalyst bodies obtained by the method of the invention.In practice, silver may tend to form unwanted dendrites or needles when impregnated on a porous support. Such dendrites or needles may grow away from surfaces upon which silver is impregnated. The dendritic structures may be fragile or unstable structures, which produce silver-containing dust under mechanical strain. Further, it is believed that excess silver impregnation solution clinging on the surface of the porous support forms brittle clusters on the support surface upon calcination, and is susceptible to attrition and thus, dust formation. In addition, the porous support, in particular at the edges thereof, may segregate dust under mechanical strain.The method comprises subjecting shaped catalyst bodies to an attrition treatment to superficially abrade loosely bound silver structures and / or fragile support structures from the catalyst bodies. Concurrently and / or subsequently, the catalyst bodies are subjected to a forced dedusting step, wherein the dust produced in the attrition treatment and mixed with the catalyst bodies is removed.The attrition treatment is not especially limited and may be selected from any known method which induces abrasion of the catalyst bodies.In one embodiment, attrition treatment comprises agitating the catalyst bodies in a rotary drum or tumbler.A tumbler is a mixer for which the mixing driving force is achieved by rotating a shell, with mixing by free-fall within the rotating shell. The shell rotates around one or more axes. Tumblers typically do not comprise agitators. Tumblers are commercially available in a variety of shapes and sizes. Suitable tumblers include V blenders, double cone blenders, roto cube blenders, drum blenders and bin blenders. Herein, the term "blender" is used synonymously with the term "mixer".While some tumblers such as V blenders, double cone blenders and roto cube blenders have a fixed shell which is filled and emptied in place, other tumblers such as drum blenders and bin blenders (also referred to as (Flexible) Intermediate Bulk Container ((F)IBC) blenders) make use of a removable shell which may be filled and emptied at a location independent of the location of the mixer. Drum blenders comprise one240836W001 3 valve for loading and unloading of materials. Bin blenders comprise discrete valves for loading and unloading of materials.Preferred tumblers include drum blenders and double cone blenders, in particular double cone blenders.Alternatively, attrition treatment may involve agitating the catalyst bodies by vibratory action. To this end, the catalyst bodies are introduced into a vibrating chamber or onto a vibrating surface, where they are agitated and abrade each other.Care should be taken that the attrition treatment is suitable to superficially abrade loosely bound silver structures and / or fragile support structures from the catalyst bodies, but that the treatment is not so harsh as to induce unnecessarily excessive attrition of the catalyst bodies. For example, when attrition treatment comprises agitating the catalyst bodies in a rotary drum or tumbler, the treatment conditions can be controlled via the rotational speed and via the total number of rotations of the rotary drum or tumbler.In one embodiment, attrition treatment comprises agitating the catalyst bodies in a rotary drum or tumbler, wherein the number of rotations is at most 10 per minute, preferably at most 5 per minute, more preferably at most 3 per minute, and the total number of rotations is at most 500, preferably at most 300, more preferably at most 150, most preferably at most 100, such as at most 50.The present method is suitable for commercial applications in which large amounts of catalyst bodies are treated. In one embodiment, the method comprises subjecting at least 500 kg, preferably at least 750 kg, more preferably at least 1 ,000 kg of the catalyst bodies to an attrition treatment.In a particular embodiment, the method comprises subjecting at least 500 kg, preferably at least 750 kg, more preferably at least 1 ,000 kg of the catalyst bodies to an attrition treatment, wherein attrition treatment comprises agitating the catalyst bodies in a rotary drum or tumbler, wherein the number of rotations is at most 10 per minute, preferably at most 5 per minute, more preferably at most 3 per minute, and the total number of rotations is at most 500, preferably at most 300, more preferably at most 150, most preferably at most 100, such as at most 50.The method moreover comprises subjecting the abraded catalyst bodies to a dust removal treatment.Dust is understood to relate to fine particles of solid matter. In the present method, dust is obtained by superficial abrasion of the catalyst bodies during attrition treatment. In one embodiment, dust comprises particles having a diameter of less than 3.5 mm, as determined by passing the particles through a 3.5 mm screen, in particular a diameter of less than 3 mm, as determined by passing the particles through a screen.Dust removal treatment is not especially limited and may be selected from any known suitable method. In one embodiment, the dust removal treatment comprises sieving of the abraded catalyst bodies.Suitable sieving apparatus include vibratory screeners. A vibratory screener sieves (or screens) the material to be classified through a vibrating screen or perforated steel sheet with openings that suit the desired particle diameter.240836W001 4In another embodiment, the dust removal treatment comprises passing a gas stream over the abraded catalyst bodies to entrain the dust so as to obtain a vent gas stream laden with dust particles. In particular, the gas stream may be passed over the abraded catalyst bodies via suctioning or blowing. The gas stream is not particularly limited. Suitable gas streams may comprise air and / or nitrogen, preferably air.In a further embodiment, the dust removal treatment comprises sieving of the abraded catalyst bodies while passing a gas stream over the abraded catalyst bodies to entrain the dust so as to obtain a vent gas stream laden with dust particles. In particular, the gas stream may be passed over the abraded catalyst bodies via suctioning or blowing.The dust produced in the attrition treatment and mixed with the catalyst bodies may also be removed by subjecting the abraded catalyst bodies to wind sifting in a wind sifting separator. In the wind sifting separator, most of the dust is separated from the catalyst bodies by means of moving air and gravity. The wind sifting separator may selected from a conical sifter, zigzag sifter, a spiral wind sifter, a cross-flow sifter or a gas cyclone, in particular a conical sifter.In one embodiment, the wind sifting separator is operated at a volume flow of 500 to 1 ,000 m3per h. The wind sifting separator is typically operated at flow velocity of at least 25 m per s. These embodiments in particular when the wind sifting separator is a conical sifter.Steps i) and ii) of the present method may be performed subsequently or concurrently, preferably subsequently.In one embodiment, steps i) and ii) of the present method are performed concurrently while passing a gas stream over the abraded catalyst bodies via suctioning or blowing, preferably via suctioning, to entrain the dust so as to obtain a laden gas stream comprising dust particles.The dust that is removed and collected contains valuable raw materials such as silver and is preferably processed to recover and recycle the raw materials.The shaped catalyst bodies comprise a porous support and silver deposited thereon. In one embodiment, the shaped catalyst bodies are catalyst bodies for producing ethylene oxide by gas-phase oxidation of ethylene.The shaped catalyst bodies comprise individual shaped bodies. The size and shape of the individual shaped bodies and thus of the catalyst is preferably selected to allow a suitable packing of the shaped bodies in a reactor tube. In general, the shaped catalyst bodies are comprised of individual bodies having a maximum extension in the range of 4 to 20 mm, such as 5 to 15 mm, in particular 5 to 12 mm. The maximum extension is understood to mean the longest straight line between two points on the outer circumference of the shaped catalyst body.The shape of the shaped catalyst bodies is essentially determined by the shape of the porous support on which silver is deposited. The following discussion regarding the shape of the catalyst bodies is understood to likewise relate to the underlying supports and vice versa.240836W001 5The shape of the shaped catalyst bodies is not especially limited, and may be in any technically feasible form. For example, the porous support may be a solid extrudate or a hollow extrudate, and the shaped catalyst body may be in the shape of a hollow cylinder, for example a ring. In another embodiment, the shaped catalyst body may be characterized by a cylinder shape with multiple passageways extending from a first face side surface to a second face side surface, such as a 5-hole cylinder or a 7-hole cylinder, a wagon wheel shape such as a pentaring, or a multilobe shape.In a cylinder shape with multiple passageways extending from a first face side surface to a second face side surface, the passageways are typically arranged essentially equidistantly to each other. In one embodiment, multiple passageways are arranged essentially equidistantly around a central passageway. In one embodiment, the passageways have an essentially circular cross-section.A wagon wheel shape, which is a hollow cylinder having multiple vanes which extend from the center of the axis of rotation to the cylindrical wall. For instance, a wagon wheel shape in the form of a pentaring is a hollow cylinder having five vanes which extend from the center of the axis of rotation to the cylindrical wall.A multilobe shape is meant to denote a cylinder structure which has a plurality of void spaces, e.g., grooves or furrows, running in the cylinder periphery along the cylinder height. Generally, the void spaces are arranged essentially equidistantly around the circumference of the cylinder. A multilobe shape may comprise passageways extending from a first face side surface to a second face side surface. The passageways are typically arranged essentially equidistantly to each other. In one embodiment, multiple outer passageways, each of which is preferably assigned to one lobe, are arranged equidistantly around a central passageway.Preferably, the porous support is in the shape of a solid extrudate, such as pellets or cylinders, or a hollow extrudate, such as a hollow cylinder, for example a ring. Alternatively, the support may be shaped by tableting.In one embodiment, the porous support is an alumina support. The alumina support typically comprises a high proportion of alumina, i.e. AI2O3, and in particular alpha-alumina, for example at least 50 wt.-%, at least 70 wt.-%, at least 80 wt.-%, or at least 90 wt.-%, preferably at least 95 wt.-%, most preferably at least 97.5 wt.-% or at least 99 wt.-%, based on the total weight of the support. Besides alumina, the support may comprise other components, for example binders such as silicates, or other refractory oxides such as zirconia or titania.The support may comprise trace amounts of further elements, such as sodium, potassium, iron, silica, magnesium, calcium, zirconium in an amount of 20 to 200 mmol / kg, based on the total weight of the support.The support is typically a porous support and preferably has a water absorption in the range of 0.35 to 1.20 mL / g (mL of water / gram of support), preferably 0.40 to 1.00 mL / g, more preferably 0.40 to 0.80 mL / g. Water absorption refers to vacuum cold water uptake measured at a vacuum of 80 mbar absolute.Vacuum cold water uptake is determined by placing about 100 g of support ("initial support weight”) in a rotating flask, covering the support with deionized water, and rotating the rotary evaporator for 5 min at about 30 rpm. Subsequently, a vacuum of 80 mbar is applied for 3 min, the water and the support are transferred240836W001 6 into a glass funnel, and the support is kept in the funnel for about 5 min with occasional shaking in order to ensure that adhering water runs down the funnel. The support is weighed ("final support weight”). The water absorption is calculated by subtracting the initial support weight from the final support weight and then dividing this difference by the initial support weight.In one embodiment, the Hg pore volume of the catalyst bodies is in the range of 0.20 to 1.00 mL / g, as determined by mercury porosimetry. Preferably, the Hg pore volume of the catalyst bodies is in the range of 0.3 to 0.9 mL / g, more preferably 0.4 to 0.8 mL / g.Mercury porosimetry may be performed using a Micrometrics AutoPore IV 9500 mercury porosimeter (140 degrees contact angle, 485 dynes / cm Hg surface tension, 60000 psia max head pressure). The Hg porosity is determined according to DIN 66133 herein, unless stated otherwise. It is believed that a Hg pore volume in this range allows for a favorable duration of exposure of the obtained ethylene oxide to the catalyst bodies.The support preferably has a BET surface area of 0.5 to 3.0 m2 / g, more preferably 1 .0 to 2.5 m2 / g, most preferably 1 .2 m2 / g to 2.0 m2 / g. The BET method is a standard, well-known method and widely used method in surface science for the measurements of surface areas of solids by physical adsorption of gas molecules. The BET surface is determined according to DIN ISO 9277 herein, unless stated otherwise.In one embodiment, the shaped catalyst bodies comprise at least 10 wt.-% silver, preferably at least 12 wt.-% silver, based on the weight of the catalyst bodies. In particular, the shaped catalyst bodies comprise, such as 12 to 40 wt.-% silver, more preferably 15 to 35 wt.-% silver, based on the weight of the catalyst bodies.Silver may be deposited on the support by contacting it with a silver solution formed by dissolving a silver salt, or silver compound, or silver complex in a suitable solvent.In particular, the shaped catalyst bodies subjected to the inventive method may be obtained by a process comprisingI) impregnating an alumina support as described above with a silver impregnation solution; andII) subjecting the impregnated support to a heat treatment; wherein steps I) and II) are optionally repeated, and at least one silver impregnation solution comprises rhenium, tungsten, cesium, potassium, and optionally sodium.In order to obtain shaped catalyst bodies having high silver contents, steps I) and II) can be repeated several times. In that case it is understood that the intermediate product obtained after the first (or subsequent up to the last but one) impregnation / heat treatment cycle comprises a part of the total amount of target Ag and / or promoter concentrations. The intermediate product is then again impregnated with the silver impregnation solution and calcined to yield the target Ag and I or promoter concentrations. It is also possible to establish the desired composition of the catalyst bodies by applying only one impregnation.Any silver impregnation solution suitable for impregnating a refractory support known in the art can be used. Silver impregnation solutions typically contain a silver carboxylate, such as silver oxalate, or a combination240836W001 7 of a silver carboxylate and oxalic acid, in the presence of an aminic complexing agent like a Ci-Cio-alkylenediamine, in particular ethylenediamine. Suitable impregnation solutions are described in EP 0 716 884 A2, EP 1 115 486 A1, EP 1 613 428 A1 , US 4,731 ,350 A, WO 2004 / 094055 A2, WO 2009 / 029419 A1 , WO 2015 / 095508 A1 , US 4,356,312 A, US 5,187,140 A, US 4,908,343 A, US 5,504,053 A, WO 2014 / 105770 A1 and WO 2019 / 154863.The shaped catalyst bodies may comprise promoters. The one or more promoters can be deposited on the support either prior to, coincidentally with, or subsequent to the deposition of the silver, but, preferably, the one or more promoters are deposited on the support coincidentally or simultaneously with the silver. When the catalyst bodies comprise silver, rhenium and a co-promoter for rhenium, it may be advantageous to deposit the co-promoter prior to or simultaneous with the deposition of silver, and to deposit rhenium after at least a portion of the silver has been deposited.Suitable promoters include rare earth metals, magnesium, rhenium and alkali metals (lithium, sodium, potassium, rubidium and cesium), or compounds thereof, and, optionally, one or more co-promoters, such as, for example, sulfur, molybdenum, tungsten and chromium, or compounds thereof. Among the promoter components that can be incorporated, rhenium and the alkali metals, in particular, the combination of light and higher alkali metals, such as a combination of lithium with potassium, rubidium and cesium, are preferred. Most preferred among the higher alkali metals is cesium, which may be most preferably used in a mixture together with for example potassium and / or lithium. The co-promoters for use in combination with rhenium can include one or more of sulfur, molybdenum, tungsten, and chromium.Promoting amounts of alkali metal or mixtures of alkali metal can be deposited on a support using a suitable solution. Alkali metals are generally used as compounds of the alkali metals dissolved in a suitable solvent for impregnation purposes. The support may be impregnated with a solution of the alkali metal compound(s) before, during or after impregnation of the silver in a suitable form has taken place. An alkali metal promoter may even be deposited on the support after the silver component has been reduced to metallic silver.The promoting amount of alkali metal utilized will depend on several variables, such as, for example, the surface area and pore structure and surface chemical properties of the support used, the silver content of the catalyst bodies and the particular ions and their amounts used in conjunction with the alkali metal cation.The amount of alkali metal promoter present in the catalyst bodies is generally in the range of from 100 to 3,000 ppm by weight of the metal relative to the weight of total catalyst bodies. This amount includes alkali metals inherently comprised in the support, e.g., as trace amounts of sodium or potassium.The support can also be impregnated with rhenium ions, salt(s), compound(s), and / or complex(es). This may be done at the same time that the alkali metal promoter is added, or before or later; or at the same time that the silver is added, or before or later. Rhenium, alkali metal, and silver may be in the same impregnation solution.The preferred amount of rhenium, calculated as the metal, present on the support ranges from 100 to 3,000 ppm by weight relative to the weight of total catalyst bodies. The references to the amount of rhenium240836W001 8 present on the catalyst bodies are expressed as the metal, irrespective of the form in which the rhenium is actually present.Examples of suitable rhenium compounds used in making the inventive catalyst include the rhenium salts such as rhenium halides, the rhenium oxyhalides, the rhenates, the perrhenates, the oxides and the acids of rhenium. A preferred compound for use in the impregnation solution is the perrhenate, preferably ammonium perrhenate. However, the alkali metal perrhenates, alkaline earth metal perrhenates, silver perrhenates, other perrhenates and rhenium heptoxide can also be suitably utilized.Cesium may suitably be provided as cesium hydroxide. Rhenium and tungsten may suitably be provided as an oxyanion, for example, as a perrhenate or tungstate in salt or acid form.During heat treatment of the impregnated support, liquid components of the silver impregnation solution evaporate, causing a silver compound comprising silver ions to precipitate from the solution and be deposited onto the porous support. At least part of the deposited silver ions is subsequently converted to metallic silver upon further heating.The heat treatment may also be referred to as a calcination process. Any calcination processes known in the art for this purpose can be used. Suitable examples of calcination processes are described in US 5,504,052 A, US 5,646,087 A, US 7,553,795 A, US 8,378, 129 A, US 8,546,297 A, US 2014 / 0187417 A1, EP 1 893 331 A1 or WO 2012 / 140614 A1. Heat treatment can be carried out in a pass-through mode or with at least partial recycling of the calcination gas.Heat treatment is usually carried out in a furnace. The type of furnace is not especially limited. For example, stationary circulating air furnaces, revolving cylindrical furnaces or conveyor furnaces may be used. In one embodiment, heat treatment constitutes directing a heated gas stream over the impregnated bodies. The duration of the heat treatment is generally in the range of 5 min to 20 h, preferably 5 min to 30 min.The temperature of the heat treatment is generally in the range of 200 to 800 °C, preferably 210 to 650 °C, more preferably 220 to 500 °C, most preferably 220 to 350 °C. Preferably, the heating rate in the temperature range of 40 to 200 °C is at least 20 K / min, more preferably at least 25 K / min, such as at least 30 K / min. A high heating rate may be achieved by directing a heated gas over the impregnated refractory support or the impregnated intermediate catalyst bodies at a high gas flow.A suitable flow rate for the gas may be in the range of, e.g., 1 to 1 ,000 Nm3 / h, 10 to 1 ,000 Nm3 / h, 15 to 500 Nm3 / h or 20 to 300 Nm3 / h per kg of impregnated bodies. In a continuous process, the term "kg of impregnated bodies” is understood to mean the amount of impregnated bodies (in kg / h) multiplied by the time (in hours) that the gas stream is directed over the impregnated bodies. It has been found that when the gas stream is directed over higher amounts of impregnated bodies, e.g., 15 to 150 kg of impregnated bodies, the flow rate may be chosen in the lower part of the above-described ranges, while achieving the desired effect.Preferably, heating takes place in a step-wise manner. In step-wise heating, the impregnated bodies are placed on a moving belt that moves through a furnace with multiple heating zones, e.g., 2 to 8 or 2 to240836W001 95 heating zones. Heat treatment is preferably performed in an inert atmosphere, such as nitrogen, helium, or mixtures thereof, in particular in nitrogen.Further provided are shaped catalyst bodies obtained by the method as described above.There are several test methods suitable for determining the attrition resistance of shaped catalyst bodies. These methods are a measure of the propensity of the material to produce fines in the course of transportation, handling, and use on stream.Such test methods by design examine only a small sample of supports or catalysts. The material that has been subjected to the test method is not envisioned for subsequent use in a production process. To allow for meaningful results within a short period of time, the test methods deliberately involve harsh attrition conditions. Such harsh conditions are not suitable as treatment conditions for a catalyst intended for subsequent use in a catalytic process.One such method is the attrition index as determined in accordance with ASTM D4058-96 (Standard Test Method for Attrition and Abrasion of Catalysts and Catalyst Carriers), which is a measurement of the resistance of a material (e.g., extrudate or catalyst particle) to attrition wear due to the repeated striking of the particle against hard surfaces within a specified rotating test drum and is incorporated herein by reference. This test method is generally applicable to tablets, extrudates, spheres, granules, pellets as well as irregularly shaped particles typically having at least one dimension larger than about 1 .6 mm and smaller than about 19 mm, although attrition measurements can also be performed on larger size materials. Variable and constant rate rotating cylinder abrasimeters designed according to ASTM D4058-96 are readily available. Typically, about 100 g of the material to be tested is placed in the drum of the rotating test cylinder and rolled at from about 55 to about 65 RPM for about 30 minutes (about 1 ,800 rotations in total). Afterwards, the material is removed from test cylinder and screened on a 20-mesh (850 pm) sieve, designated as retained material. The percentage (by weight) of the original material sample that remains on the 20-mesh sieve is referred to as the "percent retained.” The calculated loss on attrition amounts to the percentage of the weight of tested material minus the weight of retained material to the weight of tested material.In one embodiment, the shaped catalyst bodies obtained according to the method of the invention exhibit an attrition index as determined in accordance with ASTM D4058-96 of less than 20% by weight, less than 18% by weight, less than 16% by weight, less than 14% by weight or less than 12% by weight.Generally, the shaped catalyst bodies obtained according to the method of the invention exhibit an attrition index as determined in accordance with ASTM D4058-96 of more than 5% by weight, more than 8% by weight or more than 10% by weight. This reflects that the attrition treatment of the present method is a gentle treatment intended to superficially abrade loosely bound silver structures and / or fragile support structures from the catalyst bodies, while maintaining the structural integrity of the catalyst bodies which may be compromised under harsher attrition conditions.In one embodiment, the shaped catalyst bodies obtained according to the method of the invention exhibit an attrition index as determined in accordance with ASTM D4058-96 in the range of more than 5 to less240836W001 10 than 20% by weight, such as more than 5 to less than 18% by weight, or more than 8 to less than 16% by weight, such as more than 8 to less than 14% by weight, or more than 10 to less than 12% by weight.Further provided is a process for producing ethylene oxide by gas-phase oxidation of ethylene, comprising reacting ethylene and oxygen in the presence of shaped catalyst bodies obtained by the method as described above.The epoxidation can be carried out by all processes known to those skilled in the art. It is possible to use all reactors which can be used in the ethylene oxide production processes of the prior art; for example externally cooled shell-and-tube reactors (cf. Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, vol. A-10, pp. 117-135, 123-125, VCH-Verlagsgesellschaft, Weinheim 1987) or reactors having a loose catalyst bed and cooling tubes, for example the reactors described in DE 34 14 717 A1 , EP 0 082 609 A1 and EP 0 339 748 A2.The epoxidation is preferably carried out in at least one tube reactor, preferably in a shell-and-tube reactor. On a commercial scale, ethylene epoxidation is preferably carried out in a multi-tube reactor that contains several thousand tubes. The catalyst bodies are filled into the tubes, which are placed in a shell that is filled with a coolant. In commercial applications, the internal tube diameter is typically in the range of 20 to 40 mm (see, e.g., US 4,921 ,681 A) or more than 40 mm (see, e.g., WO 2006 / 102189 A1).To prepare ethylene oxide from ethylene and oxygen, it is possible to carry out the reaction under conventional reaction conditions as described, e.g., in DE 25 21 906 A, EP 0 014 457 A2, DE 23 00 512 A1 , EP 0 172 565 A2, DE 24 54 972 A1 , EP 0 357 293 A1 , EP 0 266 015 A1 , EP 0 085 237 A1 , EP 0 082 609 A1 and EP 0 339 748 A2. Inert gases such as nitrogen or gases which are inert under the reaction conditions, e.g. steam, methane, and also optionally reaction moderators, for example halogenated hydrocarbons such as ethyl chloride, vinyl chloride or 1 ,2-dichloroethane (ethylene dichloride) can additionally be mixed into the reaction gas comprising ethylene and molecular oxygen.The oxygen content of the reaction gas is advantageously in a range in which no explosive gas mixtures are present. A suitable composition of the reaction gas for preparing ethylene oxide can, for example, comprise an amount of ethylene in the range from 10 to 80% by volume, preferably from 20 to 60% by volume, more preferably from 25 to 50% by volume and particularly preferably in the range from 25 to 40% by volume, based on the total volume of the reaction gas. The oxygen content of the reaction gas is advantageously in the range of not more than 10% by volume, preferably not more than 9% by volume, more preferably not more than 8% by volume and very particularly preferably not more than 7.5% by volume, based on the total volume of the reaction gas.The reaction gas preferably comprises a chlorine-comprising reaction moderator such as ethyl chloride, vinyl chloride or 1,2-dichloroethane (ethylene dichloride) in an amount of from 0 to 15 ppm by volume, preferably in an amount of from 0.1 to 8 ppm by volume, based on the total volume of the reaction gas. The remainder of the reaction gas generally comprises hydrocarbons such as methane and also inert gases such as nitrogen. In addition, other materials such as steam, carbon dioxide or noble gases can also be comprised in the reaction gas.240836W001 11The optimal concentration of reaction moderator depends on plant conditions and on the type of catalyst used. It has been considered necessary to individually optimize the moderator concentration for different types of catalysts depending on the specific composition. Selectivity may vary considerably with relatively small changes in moderator concentration.The concentration of carbon dioxide in the feed (i.e. the gas mixture fed to the reactor) typically depends on the catalyst selectivity and the efficiency of the carbon dioxide removal equipment. Carbon dioxide concentration in the feed is preferably at most 3 vol.-%, more preferably less than 2 vol.-%, most preferably less than 1 volrelative to the total volume of the feed. An example of carbon dioxide removal equipment is provided in US 6,452,027 B1 .The above-described constituents of the reaction mixture may optionally each have small amounts of impurities. Ethylene can, for example, be used in any degree of purity suitable for the gas-phase oxidation according to the invention. Suitable degrees of purity include, but are not limited to, "polymer-grade” ethylene, which typically has a purity of at least 99%, and "chemical-grade” ethylene which typically has a purity of less than 95%. The impurities typically comprise, in particular, ethane, propane and / or propene.The reaction or oxidation of ethylene to ethylene oxide is usually carried out at elevated catalyst temperatures. Preference is given to catalyst temperatures in the range of 150 to 350 °C, more preferably 180 to 300 °C, particularly preferably 190 to 280 °C and especially preferably 200 to 280 °C. The present invention therefore also provides a process as described above in which the oxidation is carried out at a catalyst temperature in the range 180 to 300 °C, preferably 200 to 280 °C. Catalyst temperature can be determined by thermocouples located inside the catalyst bed. As used herein, the catalyst temperature or the temperature of the catalyst bed is deemed to be the weight average temperature of the catalyst particles.The process is preferably carried out at pressures in the range of 5 to 30 bar. All pressures herein are absolute pressures, unless noted otherwise. The oxidation is more preferably carried out at a pressure in the range of 5 to 25 bar, such as 10 bar to 24 bar and in particular 14 bar to 23 bar. The present invention therefore also provides a process as described above in which the oxidation is carried out at a pressure in the range of 14 bar to 23 bar.The physical characteristics of the shaped catalyst bodies, especially the BET surface area and the pore size distribution, may have a significant impact on the catalyst selectivity. This effect is especially pronounced when the catalyst is operated at very high work rates, i.e., high levels of olefin oxide production.The process according to the invention is preferably carried out under conditions conducive to obtain a reaction mixture containing at least 1.6 vol.-% of ethylene oxide. In other words, the ethylene oxide outlet gas phase volume fraction (ethylene oxide gas phase volume fraction at the reactor outlet) is preferably at least 1.6 vol.-%. The ethylene oxide outlet volume fraction is more preferably in the range of 1.8 to 3.4 vol.-%, most preferably in the range of 2.0 to 3.0 vol.-%.The oxidation is preferably carried out in a continuous process. If the reaction is carried out continuously, the GHSV (gas hourly space velocity) is, depending on the type of reactor chosen, for example on the size / cross-sectional area of the reactor, the shape and size of the catalyst, preferably in the range from 800240836W001 12 to 10,000 / h, preferably in the range from 2,000 to 8,000 / h, more preferably in the range from 2,500 to 6,000 / h, most preferably in the range from 4,500 to 5,500 / h, where the values indicated are based on the volume of the catalyst.In one embodiment of the process, the EO-space-time-yield measured is greater than 180 kgEo / (m3cath), preferably greater than 200 kgEo / (m3cath), such as greater than 220 kgEo / (m3cath), greater than 240 kgEo / (m3cath). Preferably, the EO-space-time-yield measured is less than 350 kgEo / (m3cath), more preferably the EO-space-time-yield is less than 310 kgEo / (m3cath), most preferably the EO-space-time-yield is less than 290 kgEo / (m3cath).The preparation of ethylene oxide from ethylene and oxygen can advantageously be carried out in a recycle process. After each pass, the newly formed ethylene oxide and the by-products formed in the reaction are removed from the product gas stream. The remaining gas stream is supplemented with the required amounts of ethylene, oxygen and reaction moderators and reintroduced into the reactor. The separation of the ethylene oxide from the product gas stream and its work-up can be carried out by customary methods of the prior art (cf. Ullmann's Encyclopedia of Industrial Chemistry, 5th edition, vol. A-10, pp. 117-135, 123- 125, VCH-Verlagsgesellschaft, Weinheim 1987).The invention is described in more detail by the subsequent examples.Example 1About 2,000 kg of an ethylene oxide catalyst comprising 26.5 wt.-% of silver deposited on an alpha-alumina carrier were divided into two samples of about 1 ,000 kg. The alpha-alumina carrier (> 99% alpha-alumina) had a total pore volume of 0.53 mL / g, a BET surface area of 2.05 m2 / g, and a tetralobe shape with five passageways with outer diameter of 9.0 mm, length of 9.8 mm, the inner diameter of central passageway of 1.3 mm and the inner diameter of outer passageways of 1.55 mm. The two samples were subjected to attrition treatment in a rotating drum rotating around one axis over 15 min and 45 min, respectively, at 2 rotations per minute (30 and 90 rotations in total, respectively). The abraded catalyst bodies were subjected to dust removal treatment by sieving over a mesh of 3 mm. The results are shown in the table below.Example 2An epoxidation reaction was conducted in a vertically-placed test reactor constructed from stainless steel with an inner-diameter of 44 mm and a length of 12.80 m. The reactor was equipped with a thermocouple of an outer diameter of 8 mm positioned in the center of the reactor. Reactor temperature was regulated using pressurized water contained in the reactor mantle. Catalyst temperatures were measured using the240836W001 13 thermocouple at five different positions equally distributed along the reactor length. Untreated catalyst (comparative example), and post-treated catalyst according to Test No. 1 of Example 1 (inventive example) were each charged into the reactor so as to provide a catalyst bed with a height of 11 .9 m. 0.65 m of inert ceramic balls were packed on top of each catalyst bed. Each catalyst was started up to produce ethylene oxide according to

[0185] and then operated according to

[0186] of example 3.2 of EP 3 866 972 B1 for about 5 months. Afterwards, the catalysts were discharged, separated from inert balls, and the separated catalyst analyzed for amounts of dust (fraction < 3 mm) and shaped bodies by passing the discharged catalyst over a mesh of 3 mm. Collected weights are summarized in the Table below.

Claims

240836W001 14Claims1 . A method for treating shaped catalyst bodies comprising a porous support and silver deposited thereon, the method comprisingI) subjecting the catalyst bodies to an attrition treatment to superficially abrade the catalyst bodies, and ii) subjecting the abraded catalyst bodies to a dust removal treatment, wherein steps I) and ii) are performed subsequently or concurrently.

2. The method according to claim 1 , wherein the shaped catalyst bodies are catalyst bodies for producing ethylene oxide by gas-phase oxidation of ethylene.

3. The method according to claim 1 or 2, wherein attrition treatment comprises agitating the catalyst bodies in a tumbler or by vibratory action.

4. The method according to any one of the preceding claims, wherein the dust removal treatment comprises sieving of the abraded catalyst bodies.

5. The method according to any one of claims 1 to 3, wherein the dust removal treatment comprises passing a gas stream over the abraded catalyst bodies to entrain the dust so as to obtain a vent gas stream laden with dust particles, preferably via suctioning or blowing.

6. The method according to any one of the preceding claims, wherein the dust removal treatment comprises subjecting the abraded catalyst bodies to wind sifting in a wind sifting separator.

7. The method according to any one of the preceding claims, wherein the shaped catalyst bodies are comprised of individual bodies having a maximum extension in the range of 4 to 20 mm.

8. The method according to any one of the preceding claims, wherein dust comprises particles having a diameter of less than 3.5 mm.

9. The method according to any one of the preceding claims, wherein the Hg pore volume of the shaped catalyst bodies is in the range of 0.20 to 1.00 mL / g, as determined by mercury porosimetry.

10. The method according to any one of the preceding claims, wherein the shaped catalyst bodies comprise at least 10 wt.-% silver, based on the weight of the shaped catalyst bodies.11 . The method according to any one of the preceding claims, wherein the shaped catalyst bodies are in the shape of a hollow cylinder such as a ring, a cylinder shape with multiple passageways, a wagon wheel shape such as a pentaring, or a multilobe shape.240836W001 1512. A method for filling a reactor with shaped catalyst bodies, comprising providing shaped catalyst bodies comprising a porous support and silver deposited thereon, treating the shaped catalyst bodies by a method in accordance with any one of the preceding claims, and filling the shaped catalyst bodies into the reactor.

13. Shaped catalyst bodies obtained by the method according to any one of claims 1 to 11 .

14. An epoxidation reactor comprising a bed of shaped catalyst bodies obtained by the method according to any one of claims 1 to 11 .

15. A process for producing ethylene oxide by gas-phase oxidation of ethylene, comprising reacting ethylene and oxygen in the presence of shaped catalyst bodies obtained by the method according to any one of claims 1 to 11.

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