The present invention relates to a process for the manufacture of ethylene oxide with in particular a very high degree of purity.

It is known to manufacture ethylene oxide according to a process comprising several successive stages, in particular (i) a stage of synthesis of the ethylene oxide carried out by bringing ethylene into contact with oxygen in the presence of a catalyst, so as to form a gaseous reaction mixture comprising-ethylene oxide, (ii) a stage of absorption carried out by bringing the gaseous reaction mixture into contact with water, so as to form a dilute aqueous ethylene oxide solution, (iii) a stage of desorption carried out by entrainment with steam of the dilute aqueous ethylene oxide solution, so as to form a mixture of water enriched in ethylene oxide and to separate, from the said mixture, an aqueous solution depleted in ethylene oxide, and (iv) a stage of purification of the ethylene oxide by distillation of the mixture of water enriched in ethylene oxide, so as to isolate and recover a purified ethylene oxide. Such a process is, for example, disclosed in International Patent Application

WO 03/055869.

It is also known that, during the stage of synthesis, the gaseous reaction mixture formed comprises ethylene oxide, impurities originating from side reactions, in particular carbon dioxide and aldehyde impurities, such as formaldehyde and < ^' ., acetaldehyde, unreacted reactants, such as ethylene and oxygen, and optionally inert gases, such as nitrogen, argon, methane and ethane. It is this gas mixture which is subsequently subjected to the successive stages of absorption, then of desorption and finally of purification of the ethylene oxide. The stage of purification, carried out by one or more distillations, makes it possible to separate the ethylene oxide from the final impurities, in particular aldehyde impurities. However, this final stage is generally difficult to carry out due to the similar boiling points between ethylene oxide and the aldehyde impurities, so that the ethylene oxide thus isolated may still comprise not insignificant amounts of aldehyde impurities. British Patent Application GB 891 823 and United States Patent US 3 212 113 provide processes for the post-treatment of ethylene oxide using an alkali metal borohydride carried out in particular by an additional distillation or employing neutralization using an organic carboxylic acid, followed by filtration to remove the alkali metal salts of the

acid and the borates. However, such processes exhibit the disadvantage of adding an additional stage to the process for the manufacture of the ethylene oxide, hi addition, it is recognized that this post-treatment generally results in the formation of gelatinous products arising in particular from a polymerization of the ethylene oxide. Furthermore, for several years, it has appeared advantageous to use, during the stage of synthesis of the ethylene oxide, catalysts of high selectivity, as disclosed in United States Patent US 4 012425. However, it turned put that, in order to obtain a good compromise between high selectivity and high yield during the synthesis of the ethylene oxide, it is recommended to carry out the synthesis at a relatively high temperature. The result of this is that the impurities, in particular aldehyde impurities, are then formed in greater amounts during the synthesis of the ethylene oxide and that it follows that the final stage of purification of the ethylene oxide becomes even more difficult to carry out, even with the optional stage of post- treatment provided. It is an object of the present invention to provide a process which makes it possible to solve the above mentioned problems and in particular to manufacture an ethylene oxide with a high purity, in particular without adding an additional stage, such as a post-treatment. The process provided makes it possible in particular to employ a catalyst of high selectivity for the stage of synthesis of the ethylene oxide and to carry out this stage at a relatively high temperature while controlling the impurities, in particular aldehyde impurities.

Thus, the invention relates to a process for the manufacture of ethylene oxide successively comprising:

(a) a stage of synthesis of the ethylene oxide carried out by bringing ethylene into contact with oxygen in the presence of a catalyst, so as to form a gaseous reaction mixture comprising ethylene oxide and impurities, in particular aldehyde impurities,

(b) a stage of absorption carried out by bringing the gaseous reaction mixture into contact with water, so as to form a dilute aqueous ethylene oxide solution,

(c) a stage of desorption carried out by entrainment with steam of the dilute aqueous ethylene oxide solution, so as to form a mixture of water

enriched in ethylene oxide and to separate, from the said mixture, an • aqueous solution depleted in ethylene oxide which is preferably, returned to the stage (b) of absorption, and

(d) a stage of purification of the ethylene oxide by distillation of the mixture of water enriched in ethylene oxide, so as to isolate and recover a purified ethylene oxide, which process is characterized in that an alkali metal borohydride is employed after the stage (a) of synthesis but before or during the stage (d) of purification.

The process is advantageously carried out continuously. The borohydride can in particular be employed one or more times during the process, after the stage (a) of synthesis and before or during the stage (d) of purification. More particularly, it can be employed during the stage (b) of absorption and/or during the stage (c) of desorption and/or between the stages (a) and (b) and/or between the stages (b) and (c) and/or between the stages (c) and (d) but before the stage (d) of purification. It is preferable to employ the borohydride during the stage

(b) of absorption and/or during the stage (c) of desorption and/or between these two stages and more particularly during the stage (b) of absorption. Alternatively it is possible to employ the borohydride during stage (d). In other words it is possible to employ the borohydride treatment on a product stream that has not been substantially stripped of water. For example the borohydride can be used up to the point just before the final ethylene oxide distillation column i.e. at the outlet of the ethylene oxide dryer.

The borohydride can be introduced directly or indirectly into at least one of the aqueous mixtures or solutions employed or formed after the stage (a) of synthesis but before or alternatively during the stage (d) of purification. Thus, it can be introduced directly or indirectly into the dilute aqueous ethylene oxide solution formed in the stage (b) of absorption. It can also be introduced directly or indirectly into the mixture of water enriched in ethylene oxide formed in the stage (c) of desorption and/or into the aqueous solution depleted in ethylene oxide formed in the stage (c) when the said solution is advantageously returned to the stage (b) of absorption. The borohydride can also be introduced directly or indirectly into the mixture of water enriched in ethylene oxide after the stage (c) of desorption but before the stage (d) of

purification, in particular when the said mixture is subjected to one or more intermediate treatments before the stage (d) of purification, for example an intermediate gas/liquid phase change treatment or an intermediate treatment in which dissolved gaseous by-products, such as carbon dioxide, are desorbed. It is preferable to introduce the borohydride directly or indirectly into the mixture of water enriched in ethylene oxide formed in the stage (c) or, more particularly still, into the dilute aqueous ethylene oxide solution formed in the stage (b) of absorption and/or into the aqueous solution depleted in ethylene oxide formed in the stage (c) of desorption when the said solution is advantageously returned to the stage (b) of absorption. The borohydride can be employed in or introduced into at least one of the aqueous mixtures or solutions employed or formed after the stage (a) of synthesis and before or during the stage (d) of purification so that it is in contact with the ethylene oxide and the impurities, in particular aldehyde impurities, especially formaldehyde and acetaldehyde, for a period of time of at least 10 seconds, preferably of at least 30 seconds, and in particular at a temperature ranging from 10 to 15O ⁰ C, preferably from 20 to 130 ⁰ C. The contact time can range from 10 seconds to a period of time equivalent to the (mean) residence time of the ethylene oxide and of the impurities, in particular aldehyde impurities, in one of the stages (b), (c) or (d), for example a period of time ranging from 0.5 to 600 minutes, preferably from 1 to 500 minutes, in particular from 2 to 300 minutes.

An effective amount of the borohydride, so as in particular to reduce the impurities, especially aldehyde impurities, present with the ethylene oxide, preferably formaldehyde and acetaldehyde, can be employed in or introduced into at least one of the aqueous mixtures or solutions employed or formed after the stage (a) of synthesis and before or during the stage (d) of purification. The amount of borohydride employed or introduced can correspond in particular to a proportion of 5 to 500 parts by weight per million (ppm) of borohydride, preferably of 10 to 250 ppm of borohydride, with respect to the aqueous mixture or solution employed or formed after the stage (a) of synthesis and before or during the stage (d) of purification.

The borohydride can also advantageously be employed or introduced in an effective amount, such that the proportion by weight of impurities, in particular

aldehyde impurities, especially formaldehyde and acetaldehyde, with respect to the ethylene oxide can be reduced by a factor of at least 2, preferably of at least 3, in particular of at least 4, especially of at least 5, between the gaseous reaction mixture formed in the stage (a) of synthesis and the mixture of water enriched in ethylene oxide employed in the stage (d) of purification. Thus, by way of example, the mixture of water enriched in ethylene oxide can comprise, when it is employed in the stage (d) of distillation, a proportion of aldehyde impurities, in particular of formaldehyde and acetaldehyde, with respect to the said mixture of less than or equal to 80 ppm, preferably of less than or equal to 50 ppm, in particular of less than or equal to 30 ppm, especially of less than or equal to 20 ppm.

The borohydride can be chosen from for example lithium borohydride, sodium borohydride and potassium borohydride, preferably sodium borohydride and potassium borohydride. It corresponds to the general formula:

MBH ₄ in which M represents an atom of an alkali metal, preferably lithium, sodium or potassium, B represents the boron atom and H represents the hydrogen atom.

It can be employed or introduced in the form of particles, such as granules or powder, or, preferably, in the form of an aqueous solution, in particular stabilized using a base, such as sodium hydroxide or potassium hydroxide. The stage (a) of synthesis of the ethylene oxide is a known stage. It is generally carried out according to a reaction for the catalytic gas-phase oxidation of ethylene by oxygen. It is carried out in particular by bringing ethylene into contact with oxygen in the presence of a catalyst, in particular a silver-based catalyst, preferably in a tubular reactor, at a temperature which can range from 100 to 400 ⁰ C, preferably from 190 to 300 ⁰ C, in particular from 210 to 285 ⁰ C, under an absolute pressure of greater than atmospheric pressure, for example an absolute pressure ranging from 0.5 to 5 MPa. The stage (a) of synthesis of the ethylene oxide is, for example, disclosed in United States Patent US 2 775 510 or in International Patent Application WO 96/33182. The process of the present invention is particularly suitable when the stage (a) is carried out in the presence of a catalyst for the oxidation of ethylene of high selectivity in the conversion to ethylene oxide, such as the catalyst disclosed in United States Patent US 4 012425, which is in particular a supported catalyst based

on silver, on caesium and/or on rubidium. Such a catalyst exhibits the advantage of carrying out the stage (a) of synthesis of the ethylene oxide at a temperature which can range from 200 to 300 ⁰ C, preferably from 210 to 285°C, with both a high selectivity for and a high yield of ethylene oxide. The stage (a) of synthesis results in a gaseous reaction mixture as described above comprising in particular ethylene oxide and impurities, in particular aldehyde impurities, in a proportion which can range from 5 to 50 ppm of formaldehyde and of acetaldehyde in the said mixture.

The stage (b) of absorption of the ethylene oxide in water is also a known stage. It is generally carried out by bringing water into contact with the gaseous reaction mixture comprising ethylene oxide and impurities, in particular aldehyde impurities, such as formaldehyde and acetaldehyde, which mixture is formed in the stage (a) of synthesis of the ethylene oxide. The mixture is preferably brought into contact with the water countercurrentwise, in particular in an absorption tower, especially at a temperature of less than 100 ⁰ C, for example a temperature ranging from 10 to 90°C, preferably from 20 to 8O ⁰ C, under an absolute pressure in particular of greater than atmospheric pressure, for example ranging from 0.2 to 5 MPa, preferably from 0.5 to 3 MPa. Generally, during the stage (b) of absorption in water, a dilute aqueous ethylene oxide solution is formed which has in particular a concentration of ethylene oxide of 2 to 3% by weight and which comprises the impurities, in particular aldehyde impurities, in a relatively high proportion, which can range from 50 to 1000 ppm of formaldehyde and of acetaldehyde in the said solution, or, in contrast, in an extremely reduced proportion when, according to the invention, the borohydride is employed or introduced directly or indirectly during the stage (b). In addition, the dilute aqueous ethylene oxide solution can be at least partially freed from the unreacted gaseous reaction constituents, such as ethylene and oxygen, and from the by-products formed, in particular carbon dioxide. The unreacted gaseous constituents and the by-products thus separated can advantageously be at least partially returned to the stage (a) of synthesis, in particular after having been subjected to a treatment for at least partial removal of the by- products, such as carbon dioxide, for example a decarbonation treatment.

The stage (c) of desorption with steam is also a known stage. It is generally carried out by entrainment with steam of the dilute aqueous ethylene oxide solution

formed in the stage (b). More particularly, the solution is brought into contact with steam, preferably countercurrentwise to the latter, in particular in a desorption tower, in particular at a temperature ranging from 50 to 150°C, preferably from 80 to 130 ⁰ C, under an absolute pressure which can range from 0.1 to 1 MPa, preferably from 0.1 to 0.5 MPa. Generally, a mixture of water enriched in ethylene oxide is recovered, in particular at the top, especially in the form of a gas mixture, and an aqueous solution depleted in ethylene oxide is separated, in particular at the bottom, which solution can advantageously be returned to the stage (b) of absorption. The mixture and the solution which are formed during the stage (c) of desorption can comprise impurities, in particular aldehyde impurities, in a proportion which is still relatively high with respect to the ethylene oxide and greater than that described above in the stage (a) or, preferably, in an extremely reduced proportion when, according to the invention, the borohydride is employed or introduced directly or indirectly during the stage (b) and/or (c) or between these two stages. The stage (d) of purification of the ethylene oxide by distillation is also known, except for the fact that the mixture of water enriched in ethylene oxide formed in the stage (c) and employed in the stage (d) may comprise an already very low amount of aldehyde impurities when the borohydride is used after the stage (a) of synthesis but before the stage (d) of purification. The stage (d) makes it possible, by one or more successive distillations of the mixture of water enriched in ethylene oxide, to essentially remove the remaining water, optionally dissolved gaseous by-products; such as carbon dioxide, and the final traces of the aldehyde impurities, in particular the final traces of the formaldehyde and acetaldehyde. The alkali metal borohydride can be added during stage (d) i.e. prior to further purification of ethylene oxide that has been substantially stripped of water. The stage (d) of purification of the ethylene oxide is disclosed, for example, in United Stages Patents US 3 418 338 and US 4 134 797, in European Patents EP 139 601 and EP 322 323 and in International Patent Applications WO 96/16953 and WO 03/55869. The ethylene oxide can thus be purified and isolated according to the invention very easily with very low contents of aldehyde impurities without employing a post-treatment or additional operations, such as distillations, or formation of gelatinous products. Such an advantage results from the fact that most of the aldehyde impurities have been removed from the

ethylene oxide before or during the stage (d) of purification by virtue of the use of the borohydride before or during this final stage. The ethylene oxide thus purified and isolated can have a very high degree of purity, for example of greater than 99.5%, preferably of greater than 99.9%, in particular of greater than 99.95%, even when the catalyst used in the stage (a) of synthesis of the ethylene oxide is of high selectivity and is a supported catalyst based on silver, on caesium and/or on rubidium.

It is an advantage of the present invention that by reducing the amount of aldehyde impurities in the product stream, the process can be run at higher temperatures. This reduces the necessity to cool the reactor using for example cold kerosene. Consequently the coolant, for example, kerosene that is used to cool the reactor can be circulated at a higher temperature and can be used to generate steam which has significant financial benefits.

A further aspect of the present invention comprises cooling the gaseous reaction mixture produced in stage (a) by means of a coolant such as kerosene and subsequently using the coolant to generate steam.

In addition, the reduction in aldehydes generally reduces the level of organic ^• acids in the product stream and hence the amount of caustic soda needed to neutralise the acids. The mixture of water enriched in ethylene oxide can be subjected to one or more intermediate treatments between the stages (c) and (d). Thus, before the stage (d) of purification, the mixture can be subjected to an intermediate gas/liquid phase change treatment, so as to convert the said mixture from a gas phase to a liquid phase, for example by cooling and condensing of the said mixture. It is also possible to subject the mixture to an intermediate treatment for desorption of dissolved gaseous by-products, such as carbon dioxide, preferably by entrainment with steam, in particular when the said mixture exists in the liquid form, for example after having been subjected to the above intermediate phase change treatment. It is also possible to subject the mixture to an intermediate treatment for reabsorption with water of the ethylene oxide, preferably by bringing the said mixture into contact with water, in particular countercurrentwise, so as to form the mixture of water enriched in ethylene oxide in the form of a liquid mixture substantially freed from dissolved gases, such

as carbon dioxide. It is also possible to subject the mixture to an intermediate progressive condensation treatment, for example by one or more successive partial condensations, followed by a total condensation of the ethylene oxide. The borohydride can be employed or, in particular, introduced directly or indirectly during at least one of these intermediate treatments.

The following examples illustrate the present invention. Example 1 ( ^" comparative)

Ethylene oxide is manufactured continuously according to a process comprising the following successive stages. The stage (a) of synthesis of the ethylene oxide is carried out by a catalytic reaction for the oxidation of ethylene in a tubular reactor of the vertical shell-and-multitube exchanger type, the tubes of the reactor comprising a supported catalyst based on silver. The reactor is fed continuously with a gas stream comprising, by volume, 28.2% of ethylene, 6% of oxygen, 5% of carbon dioxide, 4.7% of nitrogen, 5.5% of argon, 0.3% of ethane, 4.8 parts by volume per million (vpm) of ethyl chloride and the remainder as methane, under an absolute pressure of 2.06 MPa. A gaseous reaction mixture comprising ethylene oxide and aldehyde impurities, in particular formaldehyde and acetaldehyde, is formed in the reactor, which mixture exits from the reactor at a temperature of 235°C.

The gas mixture is cooled to 80°C and is then subjected to the stage (b) of absorption with water, carried out by bringing the said mixture into contact with water descending countercurrentwise in an absorption tower at a mean temperature of 30 ⁰ C and under an absolute pressure of 1.75 MPa. A dilute aqueous ethylene oxide solution comprising 2.5% by weight of ethylene oxide exits via the bottom of the tower. The aqueous solution is subsequently heated to 96 ⁰ C and is then subjected to the stage (c) of desorption with steam carried out by bringing the said solution into contact with steam rising countercurrentwise in a desorption tower at a temperature of 100 to 115°C and under an absolute pressure of 0.15 MPa. An aqueous solution depleted in ethylene oxide exits at the bottom of the tower and is returned to the stage (b) at the top of the absorption tower, after having been cooled beforehand to a temperature of 25 ⁰ C. During this time, a mixture of water enriched in ethylene oxide is recovered at the top of the desorption tower in the form of a gas mixture which is

subsequently cooled and condensed, so as to form a mixture of water enriched in ethylene oxide in the liquid form comprising 45% by weight of ethylene oxide.

This mixture is then subjected to an intermediate treatment for desorption with steam of the dissolved gaseous by-products, such as carbon dioxide. The intermediate treatment is carried out in a chamber by bringing the mixture of water enriched in ethylene oxide into contact with steam rising countercurrentwise at a temperature of 4O ⁰ C and under an absolute pressure of 0.25 MPa. A gas stream essentially comprising the dissolved gaseous by-products, in particular carbon dioxide, and steam exits at the top of this chamber. A mixture of water enriched in ethylene oxide is recovered via the bottom of the chamber in the liquid form, which mixture is essentially freed from carbon dioxide and has a content of ethylene oxide which is substantially unchanged with respect to that of the mixture before the intermediate treatment.

The mixture of water enriched in ethylene oxide in the liquid form, having a total content of formaldehyde and acetaldehyde of 110 ppm, is subsequently subjected to the stage (d) of purification by distillation. The stage is carried out in a fractionation region comprising a distillation column, followed by an additional fractionation region comprising two distillation columns arranged in series, so as to essentially remove the water, the impurities, in particular aldehyde impurities, in particular formaldehyde and acetaldehyde, and certain heavy products originating from a partial polymerization of the ethylene oxide. The operation is carried out as in the Example in International Patent Application WO 03/55869. A purified ethylene oxide with a total content of formaldehyde and acetaldehyde of 7 ppm results from this stage and from these fractionations. Example 2

The operation is carried out exactly as in Example 1, except that an aqueous sodium borohydride solution is introduced into the aqueous solution depleted in ethylene oxide which is drawn off during the stage (c) of desorption and returned to the stage (b) of absorption. The amount of sodium borohydride introduced corresponds to 30 ppm of borohydride with respect to the dilute aqueous ethylene oxide solution formed during the stage (b) of absorption. The sodium borohydride is thus brought into contact with the ethylene oxide and the aldehyde impurities, in

particular formaldehyde and acetaldehyde, for a mean period of time of 300 minutes. The result is that the mixture of water enriched in ethylene oxide employed in the stage (d) of purification has a very reduced total content of formaldehyde and acetaldehyde of 15 ppm, before beginning this final stage. A purified ethylene oxide with an extremely low content of aldehyde impurities, in particular with a total content of formaldehyde and acetaldehyde of less than 5 ppm, is thus easily obtained, this being achieved without creating problems with the formation of gelatinous products. It also becomes possible to subject this mixture of water enriched in ethylene oxide and depleted in aldehyde impurities to a simplified stage (d) of purification, in particular devoid of an additional post-treatment.

Example 3

The operation is carried out exactly as in Example 2, except that, during the stage (a) of synthesis, instead of using the supported catalyst based on silver, the supported catalyst based on silver and on caesium is used, and that the gaseous reaction mixture exits from the reactor at a temperature of 253 ⁰ C instead of 235 ⁰ C.

The amount of sodium borohydride introduced corresponds to 90 ppm (instead of 30 ppm) of borohydride with respect to the dilute aqueous ethylene oxide solution formed in the stage (b) of absorption. The result is that the mixture of water enriched in ethylene oxide employed in the stage (d) of purification has a very reduced total content of formaldehyde and acetaldehyde of 17 ppm, before beginning this final stage. A purified ethylene oxide with an extremely low content of aldehyde impurities, in particular with a total content of formaldehyde and acetaldehyde of less than 5 ppm, is thus easily obtained, this being achieved without disrupting the purification of the ethylene oxide with the formation of gelatinous products. It then becomes possible to also subject this mixture of water enriched in ethylene oxide and depleted in aldehyde impurities to a simplified stage (d) of purification, in particular devoid of an additional post-treatment.