阅读说明：本技术 用于使用烷基碘保护床系统制备碳酸亚乙酯和乙二醇的方法 (Process for the production of ethylene carbonate and ethylene glycol using an alkyl iodide guard bed system ) 是由 R·G·H·L·巴斯廷格斯 J·R·布莱克 V·波约维奇 W·E·埃万斯 于 2018-12-06 设计创作，主要内容包括：本发明涉及一种用于生产乙二醇和/或碳酸亚乙酯的方法,所述方法包含使包含烷基碘杂质的再循环气体物流的至少一部分与定位在环氧乙烷反应器上游的保护床系统接触以产生处理的再循环气体物流,其中所述保护床系统包含有包含处于氧化铝上的银的保护床材料；使包含乙烯、氧气和所述处理的再循环气体物流的至少一部分的进料气体物流与环氧化催化剂在所述环氧乙烷反应器中接触以产生包含环氧乙烷的环氧化反应产物；和使所述包含环氧乙烷的环氧化反应产物的至少一部分与水性吸收剂在含碘化物的催化剂存在下在吸收器中接触以产生包含碳酸亚乙酯和/或乙二醇的水性产物物流和包含所述烷基碘杂质的再循环气体物流,其中以供应到所述保护床系统的总再循环气体物流计,供应到所述保护床系统的所述再循环气体物流包含(a)不大于0.6mol％的水；和/或(b)不大于90ppmv的环氧乙烷。(The present invention relates to a process for the production of ethylene glycol and/or ethylene carbonate, said process comprising contacting at least a portion of a recycle gas stream comprising alkyl iodide impurities with a guard bed system positioned upstream of an ethylene oxide reactor to produce a treated recycle gas stream, wherein said guard bed system comprises a guard bed material comprising silver on alumina; contacting a feed gas stream comprising ethylene, oxygen, and at least a portion of the treated recycle gas stream with an epoxidation catalyst in the ethylene oxide reactor to produce an epoxidation reaction product comprising ethylene oxide; and contacting at least a portion of the epoxidation reaction product comprising ethylene oxide with an aqueous absorbent in the presence of an iodide-containing catalyst in an absorber to produce an aqueous product stream comprising ethylene carbonate and/or ethylene glycol and a recycle gas stream comprising the alkyl iodide impurities, wherein the recycle gas stream supplied to the guard bed system comprises (a) no more than 0.6 mol% water, based on the total recycle gas stream supplied to the guard bed system; and/or (b) not more than 90ppmv ethylene oxide.)

1. A process for the production of ethylene glycol and/or ethylene carbonate, said process comprising

Contacting at least a portion of a recycle gas stream comprising alkyl iodide impurities with a guard bed system positioned upstream of an ethylene oxide reactor to produce a treated recycle gas stream, wherein the guard bed system comprises a guard bed material comprising silver on alumina;

contacting a feed gas stream comprising ethylene, oxygen, and at least a portion of the treated recycle gas stream with an epoxidation catalyst in the ethylene oxide reactor to produce an epoxidation reaction product comprising ethylene oxide; and

contacting at least a portion of the epoxidation reaction product comprising ethylene oxide with an aqueous absorbent in the presence of an iodide-containing catalyst in an absorber to produce an aqueous product stream comprising ethylene carbonate and/or ethylene glycol and a recycle gas stream comprising the alkyl iodide impurities,

wherein the recycle gas stream supplied to the guard bed system comprises the total recycle gas stream supplied to the guard bed system

(a) Not more than 0.6 mol% of water; and/or

(b) Not more than 90ppmv ethylene oxide.

2. The method of claim 1, wherein the recycle gas stream supplied to the guard bed system comprises, in a total recycle gas stream supplied to the guard bed system

a) Not more than 0.6 mol% of water; and

b) not more than 90ppmv ethylene oxide.

3. A process according to claim 1 or 2, wherein the recycle gas stream supplied to the guard bed system comprises no more than 0.59 mol% water, preferably no more than 0.55 mol% water, more preferably no more than 0.5 mol% water, more preferably no more than 0.4 mol% water, even more preferably no more than 0.3 mol% water, yet even more preferably no more than 0.2 mol% water, most preferably no more than 0.1 mol% water.

4. A method according to claim 3, wherein the water concentration in the recycle gas stream is controlled by one or more of the type of cooling medium used to cool the recycle gas stream exiting the absorber, the type of cooling means, the temperature of the cooling medium and the amount of cooling medium.

5. Process according to any one of the preceding claims, wherein the recycle gas stream supplied to the guard bed system comprises not more than 70ppmv ethylene oxide, preferably not more than 50ppmv ethylene oxide, more preferably not more than 40ppmv ethylene oxide, more preferably not more than 30ppmv ethylene oxide, more preferably not more than 20ppmv ethylene oxide, more preferably not more than 15ppmv ethylene oxide, even more preferably not more than 10ppmv ethylene oxide, most preferably not more than 5ppmv ethylene oxide, or is substantially free of ethylene oxide, or is free of ethylene oxide.

6. The process of claim 5, wherein the concentration of ethylene oxide in the recycle gas stream supplied to the guard bed system is controlled by adjusting one or more of the concentration of ethylene oxide in the epoxidation reaction product stream, absorber temperature, absorber pressure, catalyst composition, number of absorber trays, shape of absorber trays, absorbent flow rate, and absorbent composition in the absorber.

7. A process according to any one of the preceding claims, wherein the recycle gas stream supplied to the guard bed system is additionally contacted with a second guard bed system comprising guard bed material capable of removing at least a portion of the iodoethylene impurities from the recycle gas stream,

wherein the treated gaseous feed stream removed from the last guard bed vessel in the series of the first guard bed system is supplied as a gaseous feed to the second guard bed system via a feed line.

8. The process according to claim 7, wherein the guard bed material capable of removing at least a portion of the iodoethylene impurities comprises palladium and gold, preferably supported on silica.

Technical Field

The present invention relates to a process for the production of ethylene oxide, ethylene carbonate and/or ethylene glycol from ethylene, in particular wherein such process uses a guard bed for the removal of alkyl iodide impurities from a recycle gas stream.

Background

Ethylene Glycol (EG) is a valuable industrial compound widely used as a raw material for manufacturing polyester fibers and polyethylene terephthalate (PET) resins; it can also be used in automobile antifreeze fluid, hydraulic brake fluid, airplane deicer and medical products.

Ethylene glycol is typically prepared from Ethylene Oxide (EO). Ethylene oxide is then produced by the silver-catalyzed oxidation of ethylene. More specifically, ethylene and oxygen are passed over a silver oxide catalyst, typically at a pressure of from 10 to 30 bar and a temperature of from 200 ℃ to 300 ℃, to produce a product stream comprising ethylene oxide, carbon dioxide, ethylene, oxygen and water. In one well known process, ethylene oxide is then reacted with a large excess of water in a non-catalytic process to produce a glycol product stream containing approximately 90 wt% monoethylene glycol (MEG), the remainder being mainly diethylene glycol (DEG), some triethylene glycol (TEG) and small amounts of higher homologues. In another well known process, ethylene oxide is reacted with carbon dioxide in the presence of a catalyst to produce ethylene carbonate. The ethylene carbonate is subsequently hydrolyzed to provide ethylene glycol. The reaction via ethylene carbonate significantly improves the selectivity of the conversion of ethylene oxide to monoethylene glycol.

Over the past few decades, much effort has been devoted to developing simplified processes and plants for the production of alkylene glycols from olefins, and in particular ethylene glycol from ethylene. For example, GB2107712 describes a process for the preparation of monoethylene glycol wherein gas from an Ethylene Oxide (EO) reactor is directly supplied to the reactor, wherein ethylene oxide is converted into ethylene carbonate or a mixture of ethylene glycol and ethylene carbonate.

EP 0776890 describes a process in which the gas from an ethylene oxide reactor is supplied to an ethylene oxide absorber in which the absorption solution contains mainly Ethylene Carbonate (EC) and Ethylene Glycol (EG). The ethylene oxide in the absorbing solution is supplied to the carboxylation reactor and is reacted with carbon dioxide in the presence of a carboxylation catalyst. The ethylene carbonate in the absorption solution is then supplied to a hydrolysis reactor with the addition of water and subjected to hydrolysis in the presence of a hydrolysis catalyst.

EP2178815 describes a reactive absorption process for the production of monoethylene glycol wherein gas from an ethylene oxide reactor is supplied to a reactive absorber and ethylene oxide is contacted with an aqueous lean absorbent in the presence of one or more carboxylation and hydrolysis catalysts, and wherein in the absorber a major part of the ethylene oxide is converted to Ethylene Carbonate (EC) or Ethylene Glycol (EG).

In each of these cases, a gas stream will be produced from the absorber that contains gas that has not been absorbed by the recycle absorbent stream. This gas stream is treated in a carbon dioxide absorber and then recombined with any gas that bypasses the carbon dioxide absorber. The combined gases are then recycled at least in part as a recycle gas stream to the EO reactor.

However, it has been found that in those processes in which an iodide-containing carboxylation catalyst is used to carry out the carboxylation reaction in a reactive absorber, decomposition materials and by-products may be present in the recycle gas stream and/or the fat absorbent stream. Examples of such decomposition materials and byproducts include gaseous iodide-containing impurities such as alkyl iodides (e.g., methyl iodide, ethyl iodide, etc.) and ethylene iodide.

Silver-based catalysts, which are commonly used in the conversion (epoxidation) of ethylene to ethylene oxide, are very susceptible to catalyst poisoning, particularly due to gaseous iodide-containing impurities such as alkyl iodides and ethylene iodide. Catalyst poisoning affects the performance, particularly selectivity and/or activity, of the epoxidation catalyst and shortens the length of time that the epoxidation catalyst can remain in the epoxidation reactor before the catalyst has to be replaced with fresh catalyst.

It is therefore desirable to remove as much of such catalyst poisons as possible from the recycle gas stream prior to contact with the epoxidation catalyst. To this end, various so-called "guard bed" systems have been developed, positioned upstream of the EO reactor, as previously disclosed, inter alia, in EP2285795, EP2279182 and EP 2155375. Such guard bed systems typically comprise one or more vessels, each guard bed vessel comprising an inlet, an outlet, and a packed bed ("guard bed") comprising an absorbent ("guard bed material") capable of reducing the amount of iodide-containing impurities in the fluid stream by chemical or physical means, including but not limited to reacting with and absorbing/adsorbing the impurities.

During operation, the guard bed becomes increasingly depleted from the continuous removal of iodide-containing impurities from the recycle gas stream, resulting in a loss of impurity removal capacity compared to the initial capacity of the guard bed. Such a loss of capacity results in the bed passing an unacceptable level of impurities, known as "breakthrough", and if proper adjustments are not made to the guard bed design (i.e., increasing adsorbent volume) to prevent these losses, the guard bed system must be replaced by partially or completely removing the guard bed material and replacing it with fresh or reactivated guard bed material.

In the characterization device, the first guard bed is on-line while the second guard bed remains on standby and the first guard bed is switched on as soon as it needs to be replaced until the second guard bed (the first bed with replacement is on standby) becomes exhausted and the process is repeated.

Typically, in such simple guard bed arrangements, the guard bed material is only partially exhausted when the said ingress of iodide containing impurities through the guard bed vessel has risen to an unacceptable level. In an effort to increase the utilization of expensive guard bed materials, more advanced guard bed system arrangements have recently been developed in which an iodide-contaminated gaseous stream is fed through a guard bed system comprising a connected series of guard bed vessels, and in which the first guard bed vessel in the pipeline is replaced as soon as it becomes depleted, and is subsequently reinserted and used as the last guard bed in the pipeline in a carousel fashion, as disclosed in WO 2017/102694.

There are many factors that affect the ability of the guard bed system to effectively reduce the amount of iodide-containing impurities in the recycle gas stream and thereby prevent EO catalyst poisoning. Among these factors, in addition to the iodoethylene and/or alkyl iodide impurities, other reactants, reaction products, and/or by-products are present in the recycle gas, which by themselves or in combination affect guard bed capacity.

It has now been found that, in particular, when water and/or ethylene oxide exceed a certain content, a reduced ability to remove alkyl iodide impurities from a recycle gas stream of a guard bed system configured for removing alkyl iodide impurities is caused.

Therefore, it is imperative that ethylene oxide/ethylene glycol process design considerations control the content of the components of the recycle gas stream that affect the guard bed capacity, as well as properly compensate for the expected loss of guard bed volume over the life cycle of the epoxidation catalyst.

Accordingly, there is a need for an improved process for the production of ethylene oxide, ethylene carbonate and/or ethylene glycol from ethylene, in particular a process using a guard bed system for the removal of alkyl iodide impurities from a recycle gas stream upstream of an ethylene oxide reactor.

Disclosure of Invention

Accordingly, in a first aspect, the present invention provides a process for the production of ethylene glycol and/or ethylene carbonate, said process comprising

Contacting at least a portion of a recycle gas stream comprising alkyl iodide impurities with a guard bed system positioned upstream of an ethylene oxide reactor to produce a treated recycle gas stream, wherein the guard bed system comprises a guard bed material comprising silver on alumina;

contacting a feed gas stream comprising ethylene, oxygen, and at least a portion of the treated recycle gas stream with an epoxidation catalyst in an ethylene oxide reactor to produce an epoxidation reaction product comprising ethylene oxide; and

contacting at least a portion of an epoxidation reaction product comprising ethylene oxide with an aqueous absorbent in the presence of an iodide-containing catalyst in an absorber to produce an aqueous product stream comprising ethylene carbonate and/or ethylene glycol and a recycle gas stream comprising alkyl iodide impurities,

wherein the recycle gas stream supplied to the guard bed system comprises the total recycle gas stream supplied to the guard bed system

(a) Not more than 0.6 mol% of water; and/or

(b) Not more than 90ppmv ethylene oxide.

Drawings

Figure 1 shows the effect of water in an ethylene epoxidation feed stream on the iodide adsorption capacity of a silver based guard bed.

Figure 2 shows the effect of ethylene oxide in an ethylene epoxidation feed stream on the iodide adsorption capacity of a silver based guard bed.

Detailed Description

The present invention provides a method for controlling the capacity of a guard bed system located upstream of a catalytic EO reactor used in a process for producing ethylene glycol and/or ethylene carbonate.

Processes for the production of ethylene glycol and/or ethylene carbonate by epoxidation of ethylene and reactive absorption of ethylene oxide have been described in particular detail in WO2009021830, WO2009140318, WO2009140319, the disclosures of which are incorporated herein by reference. In particular, guard bed systems for this method have been described in detail in WO2008144402, WO2017102694, WO2017102698, WO2017102701 and WO 2017102706.

Typically, the process comprises reacting ethylene with oxygen in the presence of an epoxidation catalyst in an ethylene oxide reactor to form ethylene oxide. In such reactions, oxygen may be provided in the form of oxygen or air, preferably oxygen. Ballast gas, such as methane or nitrogen, is typically supplied to allow operation at high oxygen levels without causing flammable mixtures. Moderators, such as monochloroethane (ethyl chloride), vinyl chloride, or ethylene dichloride, may be provided for ethylene oxide catalyst performance control. Ethylene, oxygen, ballast gas and moderator are preferably supplied from the ethylene oxide absorber (preferably via a carbon dioxide absorber) to the recycle gas, i.e. to the ethylene oxide reactor. The catalyst is preferably finely divided silver and optionally a promoter metal on a support material (e.g. alumina). The reaction is preferably carried out at a pressure greater than 1MPa and less than 3MPa and at a temperature greater than 200 ℃ and less than 300 ℃. The gas composition from the ethylene oxide reactor is preferably cooled in one or more coolers, preferably wherein steam is generated at one or more temperature levels.

The gas composition is then passed to a reactive absorber where it is in intimate contact with a "lean" absorbent. The lean absorbent typically comprises at least 20 wt% water, preferably 20 wt% to 80 wt% water. Preferably, the lean absorbent further comprises ethylene carbonate and/or ethylene glycol. At least a portion, and preferably substantially all, of the ethylene oxide in the gas composition is absorbed into the lean absorbent. According to the invention, the composition is intimately contacted with the lean absorbent in the presence of one or more catalysts that promote carboxylation and hydrolysis. Suitably, the absorber may be of the class of reactive absorbers described in WO2009021830 or WO 2016046100. Preferred homogeneous catalysts known to promote carboxylation include alkali metal iodides, such as potassium iodide; and organic phosphonium halides or ammonium salts such as tributylmethylphosphonium iodide, tetrabutylphosphonium iodide, triphenylmethylphosphonium iodide and tributylmethylammonium iodide. Homogeneous catalysts known to promote hydrolysis include basic alkali metal salts such as potassium carbonate, potassium hydroxide and potassium bicarbonate, or alkali metal oxoates such as potassium molybdate.

Preferred homogeneous catalyst systems include a combination of potassium iodide and potassium carbonate, and a combination of potassium iodide and potassium molybdate. Heterogeneous catalysts that promote carboxylation include quaternary ammonium and quaternary phosphonium iodides immobilized on silica, quaternary ammonium and quaternary phosphonium iodides bound to insoluble polystyrene beads, and metal (e.g., zinc) iodides immobilized on a solid support containing quaternary ammonium or quaternary phosphonium groups, such as an ion exchange resin containing quaternary ammonium or quaternary phosphonium groups. Heterogeneous catalysts which promote hydrolysis include oxometalates immobilized on a solid support, such as molybdates, vanadates or tungstates immobilized on ion exchange resins containing quaternary ammonium or phosphonium groups; or basic anions such as bicarbonate ions immobilized on a solid support, for example bicarbonate immobilized on an ion exchange resin containing quaternary ammonium or quaternary phosphonium groups.

The "rich" absorbent stream is withdrawn from the absorber, preferably by withdrawing liquid from the bottom of the absorber. The fat absorbent stream will contain ethylene carbonate and/or ethylene glycol and any remaining EO (if present) depending on the conditions, the apparatus and the catalyst in the absorber.

Any gases not absorbed in the absorber, including any catalyst decomposition products or by-products, will be removed from the top of the absorber and eventually recycled to the epoxidation reactor. Preferably, at least a portion of the gas to be recycled to the epoxidation reactor will be supplied to a carbon dioxide absorption column, wherein carbon dioxide is at least partially absorbed before the gas thus treated is supplied to the epoxidation reactor.

The inventors of the present invention have found that organic iodide containing impurities, and more specifically ethylene iodide and alkyl iodides such as ethyl iodide and methyl iodide, in particular in the recycle gas need to be reduced to very low levels in order to keep the performance of the epoxidation catalyst unaffected by its presence.

In particular, the amount of alkyl iodide present in the partially treated recycle gas stream is preferably no greater than 6ppbv, more preferably no greater than 5ppbv, even more preferably no greater than 3ppbv, even more preferably no greater than 2ppbv, and most preferably no greater than 1 ppbv. In addition, the amount of iodoethylene present in the treated recycle gas stream is preferably no more than 20ppbv, preferably no more than 15ppbv, preferably no more than 10ppbv, more preferably no more than 5ppbv, even more preferably no more than 4ppbv, even more preferably no more than 3ppbv, and most preferably no more than 1 ppbv. Similarly, the total amount of alkyl iodide and ethylene iodide present in the treated recycle gas stream supplied to the epoxidation reactor is preferably not more than 26ppbv, preferably not more than 20ppbv, preferably not more than 16ppbv, preferably not more than 13ppbv, preferably not more than 10ppbv, more preferably not more than 7ppbv, even more preferably not more than 5ppbv, most preferably not more than 2 ppbv.

Such very low levels of iodide impurities in the recycle gas stream supplied from the EO absorber to the Ethylene Oxide (EO) reactor are obtainable by using one or more guard bed systems positioned upstream of the EO reactor. Within such guard bed systems, the recycle gas stream passes through one or more, preferably two or more guard bed vessels and contacts the guard bed material in each guard bed vessel, thereby at least partially removing impurities, typically one or more iodide impurities. Depending on the impurity content of the gaseous feed, impurities will be removed in the first guard bed vessel and possibly any further guard bed vessels. The treated gaseous feed will be removed from the guard bed system. The treated gaseous feed will have a reduced impurity content.

As used herein, at least one of the guard bed materials is silver on an alumina-based support material. Guard bed material of this type is particularly suitable for secondary recyclingThe ring gas stream removes alkyl iodide impurities, specifically methyl iodide and ethyl iodide. Suitably, the guard bed material capable of removing one or more alkyl iodide impurities from the recycle gas stream comprises an alumina support material, and silver deposited on the alumina support material in an amount of from 2 wt% to 10 wt%. A small amount of potassium carbonate (K)₂CO₃) For passivating the alumina and increasing the iodine uptake. Preferably, the first support material comprises gamma-alumina. Suitable support materials may have a surface area of greater than 20m, relative to the weight of the support material, on the same basis²Per g, or at least 25m²Per g, or at least 50m²Per g, or at least 75m²Per g, or at least 100m²Per g, or at least 125m²In terms of/g, or up to 1200m²In terms of/g, or up to 500m²In terms of/g, or up to 300m²In g, or up to 200m²/g, or up to 175m²Per g, or 20m²G to 1200m²Per g, or 50m²G to 500m²Per g, or 75m²G to 300m²Per g, or 100m²G to 200m²Per g, or 125m²G to 175m²(ii) in terms of/g. As used herein, "surface area" is understood to mean the surface area of a support material as measured according to the b.e.t. (Brunauer, Emmett and Teller) method as described in detail in Brunauer, s., Emmet, p.y., and Teller, e., (journal of the american chemical society (j.am.chem.soc.), 60, page 309-. Preferably, the alumina support material is a spherical support material and has a diameter of less than 2mm, or 1.8mm or less, or 1.6mm or less, or 1.5mm or less, or 1.3mm or less, or 1.0mm or less, or a diameter of from 0.25mm to less than 2mm, or from 0.5mm to less than 2mm, or from 0.75mm to less than 2mm, or from 1mm to less than 2mm, or from 0.25mm to 1.5mm, or from 0.5mm to 1.5mm, or from 0.75mm to 1.5mm, or from 1mm to 1.5 mm.

The one or more guard bed vessels containing silver on an alumina-based guard bed material are preferably operated at a temperature of at least 100 ℃, more preferably at least 115 ℃, most preferably at least 120 ℃. In this embodiment, the one or more guard beds are preferably operated at a temperature of at most 145 ℃, more preferably at most 140 ℃, even more preferably at most 135 ℃, most preferably at most 130 ℃.

In some cases, the recycle gas stream is passed through at least two guard bed systems, wherein a first guard bed system is configured to remove one or more alkyl iodide impurities (such as methyl iodide and ethyl iodide) as described above to provide a partially treated recycle gas stream, wherein the partially treated recycle gas stream is subsequently provided to a second guard bed system configured to remove one or more ethylene iodide impurities to provide a further treated recycle gas stream. Thus, in one embodiment, the recycle gas stream supplied to the guard bed system is additionally contacted with a second guard bed system comprising guard bed material capable of removing at least a portion of the iodoethylene impurities from the recycle gas stream, wherein the treated gaseous feed stream removed from the last guard bed vessel of the series of first guard bed systems is supplied to the second guard bed system as a gaseous feed via the feed line.

A suitable guard bed material for removing the iodoethylene impurities from the recycle gas stream is a palladium/gold based material, preferably supported on silica. Thus, in one embodiment, the guard bed material capable of removing at least a portion of the iodoethylene impurities comprises palladium and gold, preferably supported on silica. The use of such guard beds in processes for the preparation of ethylene carbonate and/or ethylene glycol has been described in detail in WO 2017102701. In this embodiment, the guard bed vessel or vessels comprising the palladium/gold based material are preferably operated at a temperature of at least 65 ℃, more preferably at least 70 ℃, most preferably at least 83 ℃. In this embodiment, the one or more guard bed vessels are preferably operated at a temperature of at most 95 ℃, more preferably at most 90 ℃, even more preferably at most 87 ℃, most preferably at most 85 ℃.

Preferably, the gaseous feed to be treated is a recycle gas from a reactive absorber that has not been treated in a carbon dioxide absorber. Positioning the guard bed system at this stage in the process may have the protection of the CO₂An additional advantage of the absorber from any potential effects that may be caused by impurities that are removed by the guard bed system.

The feed line optionally contains one or more heating or cooling devices, such as heat exchangers, to change the temperature of the gaseous feed to an optimal temperature for the guard bed system.

In any suitable system, each bed of guard bed material may be contained in a guard bed vessel. Preferred systems include an axial fixed bed, wherein the gas to be treated is in contact with a bed of guard bed material in axial flow, and a radial fixed bed, wherein the gas to be treated is supplied from an inlet to the outside of the fixed bed and passes through the fixed bed to the center of the guard bed vessel and then to an outlet. Radial fixed beds are preferred because such beds will typically have a lower pressure drop.

In a preferred embodiment, two or more guard bed systems arranged in series are used, each guard bed system comprising one or more guard bed vessels arranged in sequential order. Herein, each guard bed vessel comprises an inlet, a bed of guard bed material and an outlet, wherein the inlet of each guard bed vessel is attached by means of a valve to the feed line and the outlet of its preceding guard bed vessel of the sequential order, wherein the outlet of each guard bed vessel is attached by means of a valve to the discharge line and the inlet of its succeeding guard bed vessel of the sequential order, and wherein the last guard bed vessel of the sequential order is the succeeding guard bed vessel of the sequential order. In operation, once the amount of impurities in the gaseous feed exiting the first guard bed vessel in the series approaches a predetermined level, the guard bed vessel is removed from the gaseous feed stream by operating the valve. The gaseous feed stream continues through the second guard bed vessel and any subsequent guard bed vessels. The guard bed material in the first guard bed vessel is then replaced by fresh or reactivated material. Once the guard bed material in the first guard bed vessel is replaced, the flow of gaseous feed through the guard bed vessel is restored by operating the valve. However, it was restored such that the first guard bed vessel was now the last guard bed vessel in the series in contact with the gaseous feed. After a further period of time, again determined by monitoring the level of impurities in the gaseous stream, the same procedure is applied to a second guard bed vessel in series (which is first contacted with the gaseous feed at this stage), and so on. A guard bed system of this type is described in detail in WO 2017/102694. A particular advantage of operating one or more guard bed systems in this rotating manner is that a very high proportion of the catalyst poisons present in the recycle gas is removed, while at the same time the guard bed system is used in a reliable, efficient and economical manner.

In any embodiment, the pressure in each guard bed system will be determined by the pressure of the gas circuit throughout the system. Preferred operating pressures are in the range of 1 to 4MPa (gauge). More preferably the operating pressure is in the range of from 2 to 3MPa (gauge).

The theoretical capacity of the silver/potassium-based guard bed material was calculated from the reaction stoichiometry, assuming the formation of AgI and KI. The control type is

Herein, ρ_BedShows the density of the guard bed (in kg/m)³In units) and MW_AgAnd MW_KThe molar weights of silver and potassium, respectively (in mol/g).

The present inventors have found that the capacity of guard beds used to remove iodide impurities from recycle gas streams rapidly decreases if certain operating conditions of the ethylene oxide/ethylene glycol process are not properly controlled, thus requiring the addition of large volumes of excess guard bed material to compensate for such capacity losses and/or high guard bed replacement frequencies, both of which are economically unattractive options.

The inventors of the present invention have developed design and operating strategies that minimize the amount of guard bed material that must be used to maintain the ethylene oxide/ethylene glycol process in operation by protecting the ring oxidation catalyst from alkyl iodides. Generally, this involves controlling the concentration of those components of the recycle gas stream that adversely affect the ability of the guard bed to be at the minimum achievable level. Furthermore, it involves the addition of well-defined amounts of guard bed material (bed size) to compensate for any expected capacity reduction while controlling the amount of detrimental recycle gas components within specific ranges.

The inventors of the present invention have found that the performance of guard bed materials, particularly in guard bed systems configured for the removal of alkyl iodide impurities, is adversely affected by the presence of excess water in the recycle gas stream supplied to the guard bed system, wherein the water reduces the ability to remove iodide impurities in a substantially linear dependence. Generally, in the process of the present invention, the water concentration in the recycle gas stream is controlled by one or more of the type of cooling medium used to cool the recycle gas stream exiting the absorber, the type of cooling means, the temperature of the cooling medium and the amount of cooling medium. If cooling water is applied (e.g. 40 ℃), this will result in a relatively high water concentration, whereas a reduced water concentration can be obtained by using a higher cost chiller operating on a low pressure stream, preferably generated elsewhere in the process.

In the present invention, the recycle gas stream supplied to the guard bed system may comprise water, and the amount of water in the recycle gas stream may be up to 0.6 mol%. In a preferred embodiment, the composition of the recycle gas stream supplied to the guard bed system is controlled such that it comprises no more than 0.59 mol% water, more preferably no more than 0.55 mol% water, more preferably no more than 0.5 mol% water, more preferably no more than 0.4 mol% water, more preferably no more than 0.3 mol% water, even more preferably no more than 0.2 mol% water, most preferably no more than 0.1 mol% water. By controlling the water concentration in the recycle gas stream within these limits, excessive adjustments to guard bed size and/or replacement frequency are avoided, thereby helping to optimize the design and operation of the total ethylene oxide/glycol process as described herein, and to minimize capital and operating costs.

The inventors of the present invention have additionally found that the performance of a guard bed system is significantly affected by the presence of excess Ethylene Oxide (EO) in the recycle gas stream supplied to the guard bed system, wherein the iodide reduction capability of the guard bed system decreases rapidly with increasing ethylene oxide concentration in the recycle feed gas stream. The excess ethylene oxide in the recycle gas stream is derived from ethylene oxide produced in the ethylene oxide reactor that has not been fully absorbed by the recycle absorbent stream in the EO absorber or the reactive absorber. In the present invention, the recycle gas stream supplied to the guard bed system may comprise ethylene oxide, and the amount of ethylene oxide in the recycle gas stream may be up to 90 ppmv. Thus, in one embodiment of the invention, the composition of the recycle gas stream supplied to the guard bed system is controlled such that it comprises no more than 90ppmv ethylene oxide. In a preferred embodiment, the recycle gas stream supplied to the guard bed system comprises no more than 70ppmv ethylene oxide, more preferably no more than 50ppmv ethylene oxide, more preferably no more than 40ppmv ethylene oxide, more preferably no more than 30ppmv ethylene oxide, more preferably no more than 25ppmv ethylene oxide, more preferably no more than 20ppmv ethylene oxide, even more preferably no more than 15ppmv ethylene oxide, yet even more preferably no more than 10ppmv ethylene oxide, most preferably no more than 5ppmv ethylene oxide. In one embodiment, the composition of the recycle gas stream supplied to the guard bed system is controlled such that it contains substantially no ethylene oxide, or no ethylene oxide.

Generally, the concentration of ethylene oxide in the recycle gas stream can be controlled by adjusting the concentration of ethylene oxide in the epoxidation reaction product stream, the temperature and/or pressure in the absorber, the catalyst composition, the number and/or design of absorber trays, the flow of absorbent in the absorber, and/or the absorbent composition in the absorber.

By maintaining the concentration of water and/or ethylene oxide below the above defined limits, any loss of capacity of the guard bed system for removing alkyl iodide impurities from the recycle gas stream throughout the catalyst life cycle is maintained at a level that avoids the design of the process for which it is intended. More specifically, by operating the process as substantially described herein, the amount of additional guard bed material applied to protect the epoxidation catalyst from iodide impurities is minimized while maintaining desired production parameters. Accordingly, one of ordinary skill in the art having the benefit of this disclosure will be able to select appropriate operating conditions to maintain at least one of water and ethylene oxide as defined herein, and to obtain maximum product selectivity and yield, and minimum capital and operating costs for the guard bed and ethylene oxide reactor.

In a preferred embodiment of the invention, the composition of the recycle gas stream supplied to the guard bed system is controlled such that the concentration of water and ethylene oxide therein does not exceed the maximum content as defined herein.

Thus, in a preferred embodiment, the recycle gas stream comprises the total recycle gas stream supplied to the guard bed system

(a) Not more than 0.6 mol% water, more preferably not more than 0.59 mol% water, more preferably not more than 0.55 mol% water, more preferably not more than 0.5 mol% water, more preferably not more than 0.4 mol% water, even more preferably not more than 0.3 mol% water, yet even more preferably not more than 0.2 mol% water, most preferably not more than 0.1 mol% water; and

(b) not more than 90ppmv ethylene oxide, more preferably not more than 70ppmv ethylene oxide, more preferably not more than 50ppmv ethylene oxide, more preferably not more than 40ppmv ethylene oxide, more preferably not more than 30ppmv ethylene oxide, more preferably not more than 20ppmv ethylene oxide, even more preferably not more than 15ppmv ethylene oxide, yet even more preferably not more than 10ppmv ethylene oxide, most preferably not more than 5ppmv ethylene oxide.

In combination with controlling the concentration of recycle gas stream components that are detrimental to guard bed capacity at levels as defined herein, guard beds in guard bed systems are typically designed to have an acceptable fixed replacement frequency. As mentioned above, it is desirable from a capital and operating cost perspective that the guard bed replacement frequency never reach unacceptably high rates.

In the context of the present invention, this involves adding an additional volume of guard bed material (bed size) to compensate for the expected reduction in capacity during commercial use, compared to the reference conditions of the test (e.g. using microreactors and/or pilot plant experiments), within the specific boundaries of the harmful recycle gas components as defined herein, in order to reach a predetermined preferred guard bed replacement frequency. This therefore allows the guard bed to remain on-line when a certain number of operating days is planned.

The invention is further illustrated by the following examples.

Examples of the invention

Micro-reactor experiments

The effect of the presence of water and ethylene oxide in an ethylene oxidation recycle gas stream on the ability of a silver/alumina guard bed to remove alkyl iodides from the recycle gas stream was investigated using a microreactor experiment in which an iodide-containing ethylene epoxidation reaction feed gas was directed through the tubes of a heated guard bed and the iodide content of the outlet gas was monitored periodically. These microreactor experiments closely modeled most of the key operating parameters of commercial units, including feed mixture composition, bed temperature, reaction operating rate and pressure.

In all experiments, an 1/4' inside diameter stainless steel U-shaped microreactor tube was loaded with 5.27g of 5% Ag/0.5% K on alumina (Sasol, 1.0mm, surface area 160 m)²G, volume filling density 0.76g/cm³). Introducing a feed gas composition comprising 25 vol% C₂H₄/7.5vol％O₂/1.3vol％CO₂3ppmv vinyl chloride plus about 5000ppbv methyl iodide and about 5000ppbv ethyl iodide, with the nitrogen ballast gas comprising the remainder of the stream.

The effect of Ethylene Oxide (EO) on the iodide capture performance of the guard bed material in the recycle gas stream was investigated by varying the concentration of ethylene oxide added to the feed gas between 0ppmv, 20ppmv, 44ppmv and 103 ppmv.

The effect of water in the recycle gas stream on the iodide capture performance of the guard bed material was investigated by varying the concentration of water added to the feed gas between 0 vol%, 0.3 vol% and 0.6 vol%.

The performance of the guard bed to capture iodide from the feed stream was measured as the percentage of guard bed capacity utilized under breakthrough of ethyl iodide or methyl iodide fed to the guard bed (where capture of 1mmol of iodide (I) in 1mmol of silver (Ag) feed gas per guard bed represents 100% capacity utilization).

Figure 1 shows the effect of low water vapor content on iodide adsorption capacity. As can be seen in this figure, the water effect is nearly linear with about a 10% loss in iodide adsorption capacity for every 0.3 mol% increase in water in the gas feed.

As an example of a scale-up of a pilot plant design to a commercial design, it can be seen from the curves shown in fig. 1 that, for example, a pilot plant water content of 0.1 mol% in the gas feed corresponds to a stoichiometric alkyl iodide guard bed capacity of about 96%, while an exemplary 0.5 mol% water in the recycle gas stream in a commercial plant corresponds to a capacity of about 84% utilization. The ratio of the two capacities at breakthrough, i.e., 96/84 ═ 1.14, was multiplied by the factor that needs to be applied to adjust the bed size to compensate for the capacity reduction relative to the pilot plant produced from 0.5 mol% water in the commercial plant guard bed feed gas.

Figure 2 shows the effect of ethylene oxide in the recycle gas stream on the iodide adsorption capacity of the silver based guard bed. As can be seen in this figure, the ethylene oxide effect is non-linear with rapid loss of iodide adsorption capacity even at low levels of ethylene oxide.

As an example of a scale-up of a pilot plant design to a commercial design, from the curves shown in fig. 2, it follows that a pilot plant ethylene oxide content in the recycle gas feed of, for example, 8ppmv would correspond to a stoichiometric alkyl iodide guard bed capacity of about 88%, while an exemplary 10ppmv ethylene oxide content in the recycle gas stream in a commercial plant would correspond to a capacity of about 87% utilization. Thus, multiplication by the factor that needs to be applied to adjust the bed size to compensate for the reduced capacity relative to the pilot plant produced by 10ppmv ethylene oxide in the commercial plant guard bed feed gas is equivalent to 88/87-1.02.