JPH07324047A - Treatment of impure ethylene stream in ehtylene hydration - Google Patents

Abstract

PURPOSE:To purify an impure ethylene stream, in producing ethanol by hydration of ethylene, by membrane separation, using a specific aromatic polyimlde membrane, of impurities in a circulating unreacted ethylene stream to separate them from the ethylene. CONSTITUTION:The inert gases such as nitrogen, methane and ethane and >=4C hydrocarbon components in the purge gas of a circulating unreacted ethylene stream are subjected to membrane separation at 20-150 deg.C under a pressure of 0-80kg/cm<2>G using an aromatic polyimide membrane having >=80% of recurring unit of the formula (R is a divalent residue resulted from removing the amino groups of a diamine compound) to separate them from the ethylene. This method has the following advantages: investment cost is low, utility cost can be reduced, ethylene recovery is high, and high-purity recovered ethylene can be obtained esp. through effectively removing C<4+> polymers and from this ethylene, ethanol can be produced.

Description

Detailed Description of the Invention

[0007]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for treating an impure ethylene stream by a membrane separation method using a polymer membrane, and more specifically, for producing ethanol by a hydration reaction of ethylene using a catalyst. In addition, the inert gas such as nitrogen, methane and ethane contained in the purge gas of the circulating unreacted ethylene stream and the hydrocarbon component having 4 or more carbon atoms (hereinafter referred to as C4 ⁺ polymer) are separated from ethylene by the membrane separation method. The present invention relates to a method for purifying an impure ethylene stream.

[0008]

2. Description of the Related Art For example, by using a mineral acid such as phosphoric acid or sulfuric acid, a heteropolyacid such as silicotungstic acid or phosphomolybdic acid, a metal oxide such as tungsten oxide, silica alumina or niobate, or a catalyst such as zeolites, It is known to produce ethanol by the hydration reaction of ethylene. Temperature, pressure and water /
Under the reaction conditions such as the molar ratio of ethylene, the conversion rate of ethylene into ethanol in one pass is only about 6% at most due to the restriction of the reaction equilibrium. Therefore, by operating a large amount of unreacted ethylene as a circulating gas and again supplying it to the reaction, the total yield of ethanol is usually improved to about 96%. The side reaction product of this reaction is mainly diethyl ether, and in addition, a small amount of carbonyl compounds such as acetaldehyde and acetone is produced. Further, about 1 to 2% of ethylene is converted to the above-mentioned C4 ⁺ polymer such as saturated and unsaturated hydrocarbons of di-pentamer of ethylene, and further, a small amount of higher grade corresponding to di-pentamer of ethylene is used. Alcohol is also a byproduct.

C4 ⁺ polymers are those having up to about 10 carbon atoms and are gaseous under normal reaction conditions. However, these low molecular weight polymers are successively produced into high molecular weight polymers, and the main reaction products are It is slightly removed to the outside of the downstream reaction system together with ethanol, and accumulates until an equilibrium state is maintained. Thus, this high molecular weight polymer poses significant operational obstacles. For example, covering the surface of the catalyst causes a decrease in activity and a reduction in catalyst life, as well as the formation of deposits at the inlet of a heat exchanger that cools the effluent from the reaction tower, the compressor and / or Pollutes the distillation column for concentrating and purifying downstream ethanol. Therefore, in order to prevent the formation of such high molecular weight polymers, it is necessary to avoid the situation where a high proportion of low molecular weight polymers are present in the circulating unreacted ethylene stream.

Further, the raw material ethylene contains an inert gas such as methane and ethane. Further, at the start of the reaction, ethylene is introduced after purging the reaction system with nitrogen, so that a small amount of nitrogen remains in the circulating unreacted ethylene stream. These inert gases do not cause serious problems in the reaction operation like C4 ⁺ polymer,
If the amount extracted out of the reaction system is very small, it gradually accumulates or remains in the circulating unreacted ethylene stream, and the ethylene concentration decreases. Since the amount of ethanol produced is proportional to the ethylene concentration, it is important to keep the ethylene concentration high. Although a very small amount of hydrogen is also present as an inert gas in the raw material ethylene, it does not accumulate to the extent that it reduces the ethylene concentration in the circulating unreacted ethylene stream because it is an extremely small amount of hydrogen. Because of the relationship, there is no particular problem in terms of reaction.

Since diethyl ether is in reaction equilibrium with ethanol and water, there are no particular problems in the reaction operation. In addition, carbonyl compounds such as acetaldehyde and acetone, and higher alcohols, by absorption operation using water,
Since it is extracted with the main reaction product, ethanol, out of the downstream reaction system, it does not accumulate, and there is no particular problem in the reaction like diethyl ether. In the known method,
It is usual to discharge a part of the circulating unreacted ethylene stream as a purge gas, purify it, and return it to the reaction system to keep the ethylene concentration in the circulating unreacted ethylene stream at about 85 mol%. Because the reaction will increase with increasing ethylene concentration, the construction cost of the manufacturing equipment and the manufacturing energy cost will decrease, but the increase of impurities in the circulating unreacted ethylene stream is not so large, so the amount of gas to be discharged The reason is that the refining treatment cost and the ethylene loss amount must be increased, and the economic efficiency is deteriorated. Further, the operation method in which the ethylene concentration is significantly lower than 85 mol% not only lowers the production amount because the production amount of ethanol is proportional to the ethylene concentration, but also causes various serious problems described above due to the accumulation of C4 ⁺ polymer. Cause disabilities.

Known treatment methods for reducing the C4 ⁺ polymer in the circulating unreacted ethylene stream include deep-chill or low-temperature distillation separation methods (US Pat. No. 3,827,245, Japanese Patent Publication No. 51-9728). And an absorption method using heavy hydrocarbons (JP-A-48-15805) have been proposed. According to U.S. Pat. No. 3,827,245, an ethylene concentration of 85-
Withdraw a portion of the 95 mol% of the circulating unreacted ethylene stream,
This is introduced into a dryer filled with a desiccant such as alumina, and the gas stream is dried and supplied to a cryogenic distillation column.
The operating pressure of the cryogenic distillation column is about 290 PSIG (19.7).
Atmospheric pressure), the top temperature is about -19 ° F (-28.3 ° C), the bottom temperature is about + 260 ° F (+ 126.7 ° C), and the overhead stream is about -22 ° F (-30 ° C) with a condenser. (° C) and reflux at the top of the column. The ethylene concentration of the reflux liquid is 97-
Purified to 99 mol%. A part of the reflux liquid is extracted and recycled as recycled ethylene to the reaction system. This method
Since the degree of purification of ethylene is high, the amount of the unreacted ethylene stream that is circulated can be reduced and the amount of ethylene loss can be reduced accordingly.However, a dryer, a low temperature distillation column, a relatively large capacity refrigerator, a pump, Not only the equipment cost of the reboiler, condenser, etc. will increase, but also electricity for the refrigerator, steam for the reboiler, etc. will be needed, so the utility cost will increase and the economic efficiency will deteriorate.

The known invention of Japanese Examined Patent Publication No. 51-9728 is a method for enhancing the economical efficiency of the above-mentioned cryogenic distillation separation method, in which a part of the circulating unreacted ethylene stream having an ethylene concentration of 90.0 mol% or more is extracted and hydrated. In order to prevent the blockage of the device due to the formation of a substance, after a small amount of ethylene glycol was added, the gas was directly supplied to a low temperature distillation column equipped with a reflux condenser, and the distillation column was close to the critical point of ethylene. Operating under overhead operating conditions, ethylene concentration 95-97 from the top of the overhead cooler
Molten gaseous ethylene recovered is withdrawn and recycled to the reaction system. This method facilitates the separation of impurities by operating near the critical pressure of ethylene, and can be operated at a low temperature of about 0 ° C., so that the operating temperature is higher than that of US Pat. No. 3,827,245. Although it is a little economical because of the high cost, it is still low-temperature distillation, so that the facility costs of utilities such as refrigerators and utility costs have not been significantly improved.

The method disclosed in Japanese Patent Laid-Open No. 48-15805 is a method aimed at improving equipment costs and utility costs by operating the absorption method. According to this method, a part of the circulating unreacted ethylene stream having an ethylene concentration of 85 to 90 mol% is withdrawn and introduced to the bottom of the washing tower. Washing tower pressure 55-85
It is operated at atmospheric pressure and a temperature of 15 to 75 ° C., and the ethylene gas is washed with water to remove oxygen-containing compounds, withdrawn from the upper part and introduced to the bottom part of the absorption tower. Absorption tower pressure is 20-6
The C4 ⁺ polymer in the gas is absorbed and removed using a heavy hydrocarbon consisting of light gas oil, which is operated at 5 atm, and is withdrawn as recovered ethylene substantially free of the C4 ⁺ polymer from the upper part, and is recycled to the reaction system. Circulate. The light gas oil having absorbed the C4 ⁺ polymer is introduced into a stripping column operated at a pressure of 1 to 8 atm, and separated into a waste gas containing ethylene and a C4 ⁺ polymer and a light gas oil containing no C4 ⁺ polymer, The light gas oil containing no C4 ⁺ polymer is recycled to the absorber.

This method is a simpler absorption method than the distillation method, and the equipment cost and utility cost are considerably improved, but since the ethylene concentration contained in the waste gas of the stripping column is considerably high, it is a main raw material. There is a drawback that the recovery rate of certain ethylene is poor and that the ethylene consumption rate is worse than in the cryogenic distillation method or the low temperature distillation method. In addition, inert gases such as nitrogen, methane and ethane are hardly removed by this absorption method. However, when the operation is continued for a long period of time, it is gradually accumulated in the reaction system, and the purity of ethylene in the circulating unreacted ethylene stream is deteriorated.

As described above, the conventional method of removing C4 ⁺ polymer from the circulating unreacted ethylene stream during the production of ethanol by the hydration reaction of ethylene using a catalyst is as follows.
None of them are yet to be fully satisfied in terms of equipment costs, utility costs, ethylene unit consumption, etc.

[0017]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has a low equipment cost, a low utility cost, and a high ethylene recovery rate. Particularly, a C4 ⁺ polymer is effective. It is an object of the present invention to provide a method for purifying an impure ethylene stream in the production of ethanol that can be highly removed to obtain highly purified recovered ethylene.

[0018]

In order to achieve the above-mentioned object, the present inventor has conducted a chemical and effective separation method of ethylene and an inert gas such as nitrogen, methane and ethane and a C4 ⁺ polymer. It was examined first from an engineering point of view. The circulating unreacted ethylene flow when producing ethanol by ethylene hydration using a catalyst is operated at a pressure of 50 to 80 kg / cm ² G, and the driving force of the membrane separation method is the pressure difference. As long as the ethylene can be efficiently purified from the unreacted ethylene stream by the membrane separation method, the pressure of the unreacted ethylene stream can be used, so no electric power is required, and if a membrane module for separation is installed, the operating pressure will be lowered sequentially. It does not require any other expensive equipment, and since it only heats the gaseous impure ethylene stream, it significantly reduces the amount of steam used compared to the cryogenic distillation separation. Furthermore, if a separation membrane capable of efficiently separating ethylene from the above-mentioned impurities can be found, the recovery rate of ethylene will be good, not only the ethylene unit consumption will be improved, but also the unreacted circulating ethylene Min ethylene purity flow is improved, ethanol -
We focused on the fact that the yield also increased. In other words, it is an extremely inexpensive and economical ethylene recovery device in terms of the ethylene consumption rate, equipment cost, and utility cost.

Then, as a result of keen search for the membrane permeation performance (permeation coefficient, separation coefficient) of ethylene, inert gas and C4 ⁺ polymer of various polymer membranes, ethylene of the aromatic polyimide membrane represented by the following formula I was obtained. Has a practically sufficient magnitude, the permeation coefficient of ethylene is larger than the permeation coefficient of inert gases and the C4 ⁺ polymer, and the permeation coefficient of ethylene depends on the separation temperature. Although it increases with increase, the permeability coefficient of C4 ⁺ polymer is
We have discovered the surprising fact that increasing the separation temperature does not change much, or vice versa. That is, the aromatic polyimide membrane represented by the following formula I not only increases the amount of ethylene permeation but also C4 when the separation temperature is increased.
^The present invention has been completed by finding that the separation coefficient of the polymer is increased, and that ethylene and impurities can be easily and effectively separated.

That is, according to the present invention, when ethanol is produced by the hydration reaction of ethylene using a catalyst, impurities in the circulating unreacted ethylene stream are removed by the following formula I:

[0021]

[Chemical 1]

[In the formula, R represents a divalent residue of the diamine compound excluding the amino group, and the diamine compound is represented by the following formula:
II:

[0023]

[Chemical 2]

(Wherein R ₁ and R ₂ represent hydrogen or a methyl group) and the following formula III:

[0025]

[Chemical 3]

It represents at least one aromatic diamine compound selected from the group consisting of diamine compounds represented by: ] It is characterized in that it is separated from ethylene by a membrane separation method using an aromatic polyimide membrane having a repeating unit represented by the above formula at a ratio of 80% or more.

Next, the aromatic polyimide film used in the present invention will be described in detail. The aromatic polyimide film used in the present invention is a homogeneous film or asymmetric film having the repeating unit represented by the above formula I in a proportion of 80% or more, particularly 90 to 100%, based on the total main chain bonding units. As the permeable membrane, a separation membrane having a flat membrane shape, a hollow fiber shape, or the like is effective. In particular, the hollow fiber asymmetric aromatic polyimide membrane is particularly effective in terms of downsizing the membrane module and improving gas permeability. The aromatic polyimide represented by the formula I has, for example, a biphenyl tetracarboxylic acid as a main component (preferably,
Aromatic tetracarboxylic acid component containing at least 80 mol%, particularly preferably 90 to 100 mol%,
II and at least one diamine compound selected from the group consisting of aromatic diamine compounds represented by formula III, based on the total diamine components, at least 80 mol%, preferably 85 to 100 mol%, particularly preferably 90 to An aromatic diamine component contained at a ratio of 100 mol% in an organic solvent at a high temperature or at a low temperature in the presence of an imidizing agent,
It can be obtained by a known method of polymerization and imidization.

The above-mentioned biphenyltetracarboxylic acids include 2,3,3 ', 4'- or 3,3', 4.
Mention may be made of 4'-biphenyltetracarboxylic acid or its acid dianhydride, or the lower alcohol ester of the acid. From the durability of the separation membrane, 3,3 ',
An aromatic polyimide represented by the above formula I obtained from an aromatic tetracarboxylic acid component containing 4,4'-biphenyltetracarboxylic dianhydride as a main component and the above diamine component is particularly preferable. In the above-mentioned method for producing an aromatic polyimide, the tetracarboxylic acid component that can be used together with the biphenyl tetracarboxylic acids is pyromellitic acid, 3,3 ', 4,4'-biphenyl ether tetracarboxylic acid, 3 , 3 ′, 4,4′-benzophenonetetracarboxylic acid, or their acid dianhydrides. However, if the content ratio of the biphenyltetracarboxylic acids in the tetracarboxylic acid is too small, the durability is lowered, which is not suitable.

As the diamine component, at least one diamine compound selected from the group consisting of diamine compounds represented by the formulas II and III, preferably a mixture of two or more diamine compounds, is used as the total diamine component. With respect to at least 80 mol%, preferably 85-100
It is contained at a mol%, particularly preferably 90 to 100 mol%. As the diamine component, formula II
And when containing two or more diamine compounds selected from the group consisting of diamine compounds represented by Formula III,
At least one of the diamine compounds represented by Formula II and Formula III contained therein, based on the total diamine component,
It is contained in the range of about 20 to 80 mol%,
And, the total amount of the diamine compounds represented by the formula II and the formula III is at least 80 mol% based on all the diamine components,
In particular, 85 to 100 mol%, more preferably 90 to 10
The amount is preferably 0 mol%.

Examples of the aromatic diamine compound represented by the formula II used as the diamine component include diphenylmethane type diamine compounds such as 3,3'-diaminodiphenylmethane and 4,4'-diaminodiphenylmethane. In addition, 2,2-bis [4-aminophenyl] propane,
2,2-bis [3-aminophenyl] propane, 2,2
Examples include 2,2-bisphenylpropane-based diamine compounds such as -bis [3,4-diaminodiphenyl] propane. Furthermore, as the aromatic diamine compound represented by the formula III used as the diamine component, 3,
Examples include diphenyl ether-based diamine compounds such as 3'-diaminodiphenyl ether, 4,4'-diaminodiphenyl ether, and 3,4'-diaminodiphenyl ether.

Other aromatic diamine compounds that can be used as the diamine component together with the compounds represented by formula II and / or formula III include, for example, (a) 1,
4-bis (4-aminophenoxy) benzene, 1,4-
Bis (3-aminophenoxy) benzene, 1,3-bis (4-aminophenoxy) benzene, 1,3-bis (3
-Bisphenoxybenzene-based diamine compounds such as aminophenoxy) benzene, (b) bis [4- (4-aminophenoxy) phenyl] ether, bis [4- (3-aminophenoxy) phenyl] ether and the like A bis [4- (phenoxy) phenyl] ether-based diamine compound,
(C) Bis [4- (4-aminophenoxy) phenyl] methane, 2,2-bis [4- (4-aminophenoxy) phenyl] propane and other bis [4- (phenoxy) phenyl]
Alkane-based diamine compound, (d) o-tolidine, m
A biphenyl-based diamine compound such as tolidine, (e)
Examples thereof include phenylenediamine-based diamine compounds such as m-phenylenediamine and p-phenylenediamine.

In the present invention, in the aromatic polyimide represented by the formula I, the diamine compound forming R in the formula I is 4,4'-diaminodiphenylmethane or 4,4'-diaminodiphenylmethane.
4'-diaminodiphenyl ether, particularly preferably these two kinds of diamine compounds are preferable. The separation membrane composed of the aromatic polyimide represented by the formula I used in the present invention can be produced by a conventionally known method. For example, a thin film (flat film shape or hollow fiber shape) of an aromatic polyimide solution is formed by casting or extrusion, and then the thin film is immersed in a coagulating liquid to coagulate, and the solvent, coagulating liquid, etc. are washed from the coagulating liquid. There is a film forming method of removing and finally heat treating to form a separation film made of aromatic polyimide.

Next, the gas permeation properties of impurities such as ethylene, inert gas and C4 ⁺ polymer into the aromatic polyimide film used in the present invention will be described. Aromatic polyimide has a very high glass transition temperature (Tg) as compared with other polymers, and maintains the form of glassy polymer even at a high temperature of 200 ° C. or higher, and thus has a high heat resistance. It is well known that it is a polymer. The aromatic polyimide film used in the present invention has an aromatic tetracarboxylic acid component and an aromatic diamine component having a benzene ring, as shown in Formula I, and therefore, due to the stacking effect, a polymer gap is formed. Is highly densified. Therefore, it is already known that it is useful as a separation membrane for gases having different molecular diameters such as hydrogen / carbon monoxide, hydrogen / methane or nitrogen / carbon monoxide (T. Nakagawa, Chemical Industry,
June 1993, pages 452-459, JP-A-56-1
57,435, JP-A-57-209,608).

In the above Chemical Industry, pages 452 to 459, the gas permeation properties of the aromatic polyimide film used in the present invention, such as inorganic gas, water vapor, and organic gas and organic vapor having up to 3 carbon atoms, are examined in detail. There is. According to this known document, water vapor shows a constant permeation rate regardless of temperature and is most easily permeated in the measurement range. It has been clarified that the permeation rate of the inorganic gas and the organic gas having up to 3 carbon atoms or the organic vapor increases exponentially as the temperature rises. And the tendency of separation is not necessarily in the order of molecular diameter. Unfortunately, in the present situation where the theoretical relationship between the chemical structure of the aromatic polyimide membrane and the separation tendency has not been clarified, it is necessary to measure the actual gas for each application. According to the quest of the present inventor, the gas permeation tendency of the aromatic polyimide membrane is not always the same between the pure gas and the mixed gas depending on the type of gas or vapor, and the organic gas or carbon atoms having 4 or more carbon atoms or It has become clear that the permeation rate of steam does not always increase with the increase of temperature. Further, it is known that a polyimide film in which the tetracarboxylic acid component is an aliphatic hydrocarbon (aliphatic polyimide film) has better permeability as the hydrocarbon has a larger number of carbon atoms. Therefore, it was considered to be effectively used for separating impurities such as inert gas and C4 ⁺ polymer from ethylene. However, it was found that when used in the impure ethylene stream to be treated in the present invention, the plasticization of the film progressed, and it could not be used in terms of durability.

Next, the gas permeation performance of the aromatic polyimide membrane used in the present invention in the impure ethylene flow treatment will be described with reference to measurement examples using pure gas and mixed gas. (Measurement Example 1) As an example of the aromatic polyimide film represented by the formula I, as an aromatic tetracarboxylic acid component, 3,
40 mmol of 3 ', 4,4'-biphenyltetracarboxylic dianhydride (hereinafter abbreviated as BPDA) and 4,4'-diaminodiphenylmethane (hereinafter abbreviated as DADM) as an aromatic diamine compound represented by the formula II. ) And 4,4'-diaminodiphenyl ether (hereinafter abbreviated as DADE) using 16 mmol and 24 mmol, respectively, and performing imidization polymerization / film formation (film thickness: 0.12 mm) by a conventional method to obtain asymmetry. Aromatic polyimide film (hereinafter aromatic polyimide film-1
Abbreviated). This was cut into 50 mmφ and attached to a commercially available gas membrane permeation experiment device (effective membrane area: 12.6).
cm ² ), nitrogen, methane, ethane and the like as an inert gas, isobutane, isobutene, trans-2-butene and the like as a C 4 ⁺ polymer and ethylene as a main component,
Using each pure gas, the gas permeation amount was measured at various temperatures and pressures, and the permeation coefficient (cc (STP) ・ cm / cm ²・ sec ・ cmH
g) was asked. From the obtained permeation coefficient (P _i ), the separation coefficient (α ⁱ _C2H4 ) from ethylene of various gases (i component) was obtained using the formula (1) that is generally used. α ⁱ _C2H4 = (P _i / P _C2H4 ) (1)

The following Table 1 shows the permeation performance of various gases of the aromatic polyimide membrane-1, and the following Table 2 shows the separation coefficients of various gases with respect to ethylene of the same membrane determined by the equation (1).

[0037]

[Table 1]

[0038]

[Table 2]

In order to make the results in Table 1 easy to understand, FIG. 1 shows a graph in which the permeation coefficient of various gases of the aromatic polyimide film-1 is plotted against the reciprocal of temperature (Arrhenius plot). As is clear from the results of Tables 1 and 2 and FIG. 1, in this temperature range, all gases except trans-2-butene are less likely to permeate the aromatic polyimide membrane-1 than ethylene. The permeability coefficient of ethylene is 3
Increasing exponentially with increasing temperature from 0 to 70 ° C. Inert gases such as nitrogen, methane and ethane have a smaller permeation coefficient as compared with ethylene, and show almost the same permeation tendency with respect to temperature as ethylene. This shows that ethylene can be concentrated on the permeate side, and when the temperature is raised, the separation rate of these gases and ethylene can be increased and the throughput per unit membrane area can be increased without much change. ing.

On the other hand, it was discovered that the permeation tendency of isobutane, isobutene, trans-2-butene, etc. is significantly different from the gas permeation tendency of the conventionally known aromatic polyimide membrane. That is, the permeation coefficient of isobutane is smaller than that of ethylene, and decreases with increasing temperature. The permeation coefficient of isobutene is smaller than that of ethylene, and when the temperature is increased, it has a minimum value around a temperature of 60 ° C, decreases slightly up to a temperature of about 60 ° C, and increases at higher temperatures. The permeation coefficient of trans-2-butene is larger than that of ethylene, and when the temperature is increased, the permeation coefficient of trans-2-butene is slightly increased and hardly changes. Extrapolating the lines of the change in the transmission coefficient of ethylene and trans-2-butene, the transmission coefficient of ethylene and trans-
The transmission coefficients of 2-butene intersect. In the separation of ethylene from isobutane and isobutene, it is shown that ethylene can be concentrated on the permeate side, and that the higher the temperature, the higher the separation performance and the greater the throughput per unit membrane area. In the separation of ethylene and trans-2-butene, it is shown that ethylene is concentrated on the high pressure side and trans-2-butene is concentrated on the permeation side up to a temperature of about 120 ° C.

Ethylene and inert gas and C4 ⁺ polymer
In order to separate impurities such as ethylene oxide using an aromatic polyimide membrane, ethylene and trans-2-butene are separated from the other gas on the permeate side in the first stage separation operation, and then ethylene is pressurized to high pressure in the second stage. It is shown that the impure ethylene stream can be purified by one-step concentration of ethylene on the permeate side by concentrating on the side or performing membrane separation at a temperature of about 120 ° C. or less. However, surprisingly, as in the measurement example 2 using the mixed gas shown below, in the purification treatment using an aromatic polyimide membrane with an impure ethylene flow, all the C4 containing an inert gas and trans-2-butene was used. ^{The +} polymer is less likely to permeate the aromatic polyimide membrane than ethylene, and in particular, the C4 ⁺ polymer is a major obstacle in the ethylene hydration reaction.
Found that the higher the temperature, the better the separation from ethylene and the more effective ethylene purification on the permeate side.

(Measurement Example 2) Using the same aromatic polyimide membrane and the same gas membrane permeation experimental apparatus as those used in Measurement Example 1, no circulation was carried out in the production of ethanol by the hydration reaction of ethylene using a catalyst. Impurity ethylene gas having the same composition as the reaction ethylene stream was supplied at about 3 NL / h. High side pressure is 10
Kg / cm ² G, permeation side pressure is 0 Kg / cm ² G, temperature is 60 to 120
The gas permeation amount and the separation factor of impurities were measured while changing the temperature range. The results are shown in Table 3 below. The separation factors in the separation of the mixed gas are ethylene and the gas (i component) high-pressure side supply gas composition (C _FC2H4 , C _Fi ), high-pressure side outlet gas composition (C _RC2H4 , C _Ri ) and permeation side gas composition. (C _PC2H4 ,
C _Pi ) was analyzed by gas chromatography and determined using the following equation (2). α ⁱ _C2H4 = (C _Pi / C _PC2H4 ) / [(C _Fi + C _Ri ) / (C _FC2H4 + C _RC2H4 )] (2)

[0043]

[Table 3]

As is clear from Table 3, the separation coefficient of the C4 ⁺ polymer including the inert gas and trans-2-butene contained in the impure ethylene gas is less than 1 and the temperature is 60 to 60 ° C. It was found that the higher the temperature was 120 ° C., the more effectively ethylene was concentrated on the permeate side.

Next, the one-step treatment method of the impure ethylene stream in the method of the present invention will be described with reference to FIG. 2 (solid line). A part of the circulating unreacted ethylene stream when ethanol is produced by the hydration reaction of ethylene using a catalyst is washed with water (not shown) by a known method, and then dehumidified if necessary. Since the gas is usually washed with water to recover the entrained ethanol, it contains saturated steam under the operating conditions of washing. When ethylene becomes a low temperature, it forms a molecular compound (ethylene hydrate) with solid water. Therefore, when the temperature is lowered due to adiabatic expansion during depressurization operation, troubles such as device clogging occur. Therefore, in the known method, drying with a dryer, addition of ethylene glycol, or heating is performed to prevent the formation of ethylene hydrate.

In the method of the present invention, in addition to the conventional dehumidification method or heating method, a commercially available water vapor selectively permeable polymer membrane such as polyvinyl alcohol membrane, chitosan membrane or polyimide membrane is used for dehumidification. Effectively prevent ethylene hydrate formation by pretreatment (not shown) such as dehumidification method that selectively separates water vapor from impure ethylene stream on permeate side (low pressure side) using membrane module can do. This pretreated ethylene stream is fed from line 1 to the membrane module M1 for separating impurities.

In the membrane module M1 made of an aromatic polyimide membrane, an inert gas such as nitrogen, methane and ethane and impurities such as C4 ⁺ polymer are concentrated on the high pressure side and discharged from the line 2. Selectively permeate ethylene to the low pressure side,
The ethylene stream from which impurities have been separated is returned to the reaction system through line 3 as recovered ethylene. The operating pressure on the high-pressure side of the membrane module M1 may be ² kg / cm ² G or more, but since the pressure of the reaction system may be introduced as it is, the pressure is 30 to 80 kg.
/ cm ² G is preferred. The pressure on the permeate side may be lower than the pressure on the high pressure side, but in view of the separation efficiency and the suction pressure of the ethylene feeding compressor of the reaction system, 1 70 kg / cm ²
It is preferably G. As described above, the higher the operating temperature, the more effective it is because of the gas permeable property of the aromatic polyimide membrane.
It is preferable to operate at 0 to 150 ° C, particularly preferably 50 to 120 ° C.

In order to improve the recovery rate of ethylene, it is possible to effectively achieve it by narrowing down the withdrawal amount from the line 2, but the purity of recovered ethylene is slightly lowered. A method of maintaining the recovery rate of ethylene and the purity of recovered ethylene more favorably is effectively achieved by combining a membrane module made of an aromatic polyimide membrane in successive stages. Next, an example of the two-step processing will be described based on FIG. 2 (broken line). That is, by the separation operation in the membrane module M1, ethylene is selectively permeated to the low pressure side and returned to the reaction system as recovered ethylene from the line 3. The exhaust gas on the high-pressure side of the membrane module M1 containing a large amount of impurities to be discharged from the line 2 is introduced from the broken line 4 to the next membrane module M2 made of an aromatic polyimide film to further increase the impurities on the high-pressure side. It is concentrated and discharged as waste gas from the broken line 5, and ethylene is selectively permeated to the low pressure side. The ethylene stream that has permeated to the low pressure side of the membrane module M2 usually has a high impurity content, so it is normally a broken line 6
From the following, a membrane module M consisting of the following aromatic polyimide membrane
Introduce to 3. Impurities are concentrated on the high pressure side in the membrane module M3 and discharged as a waste gas from the broken line 7. Ethylene is selectively permeated to the low pressure side, and is returned to the reaction system through the broken line 8 as recovered ethylene.

The operating conditions of the membrane module M1 are the same as in the one-step treatment described above. The operating pressure on the high-pressure side of the membrane module M2 may be ² kg / cm ² G or more, but since the gas flow on the high-pressure side of the membrane module M1 may be introduced as it is, it is normally operated at a pressure of 30 to 80 kg / cm ² G. To be done. Membrane module M
The permeation side pressure of 2 may be lower than the high pressure side pressure,
From the relationship between the separation efficiency and the operating pressure in the next membrane module M3, 10 □ 70 kg / cm ² G is preferable. Membrane module M
The operating pressure on the high-pressure side of 3 may be ² kg / cm ² G or more, but since the gas flow on the permeate side of the membrane module M2 may be guided as it is, it is normally operated at a pressure of 10 70 kg / cm ² G. Is preferred. The operating pressure on the permeation side of the membrane module M3 may be lower than the pressure on the high pressure side. However, in consideration of the separation efficiency and the suction pressure of the low pressure ethylene supply or recovery compressor of the reaction system, 1 ■ 60 kg / cm ² G preferable.

The operating temperature of the membrane modules M2 and M3 is
Similar to the membrane module M1, from the heat resistance of the aromatic polyimide membrane, the temperature is 20 to 150 ° C., particularly preferably 50 to 120.
℃. In this way, by combining the membrane modules made of an aromatic polyimide membrane in successive stages to purify the impure ethylene stream, the ethylene recovery rate and the purity of the recovered ethylene can be favorably maintained. Further, the sequential step treatment in which a membrane module composed of an aromatic polyimide membrane is combined in three steps, four steps and a sequential step is effective for maintaining good ethylene recovery rate and recovered ethylene purity. Since installing a large number of modules increases the equipment cost, it is preferable to use a combination of two to three stages for the sequential treatment. FIG. 3 shows an example of a three-stage treatment method for an impure ethylene stream. So far, an example of a method for treating a circulating unreacted ethylene stream has been described, but a method for recovering ethylene by a membrane separation method using an aromatic polyimide membrane for the waste gas of the stripping column in the conventional absorption process is also the present invention. Included in.

The present invention will be further described with reference to examples and comparative examples.

[Examples 1 to 3] An asymmetric aromatic polyimide film was imidized and polymerized into a film by a conventional method using approximately equal moles of an acid component and a diamine component having the charging ratios shown in Table 4 below. It was cut into 50 mmφ and attached to the same gas membrane permeation experimental device as in Measurement Example 1, and the same impure ethylene gas as in Measurement Example 2 was supplied in the same amount and pressure conditions as in Measurement Example 2 and at the temperature
The gas permeation amount and the separation factor of impurities were measured at 20 ° C. The results are also shown in Table 4 below.

[0052]

[Table 4]

[0053]

Example 4 A part of the circulating unreacted ethylene stream used for the ethylene hydration reaction was converted into an impure ethylene stream (ethylene concentration 8
Aromatic polyimide asymmetric hollow fiber membrane having the same composition as the aromatic polyimide membrane-1 used in Measurement Example 1 0.2 m ² (hollow fiber outer diameter 0.4 mm × hollow fiber inner diameter 0) 0.2 mm x hollow fiber effective length 230 mm) built-in stainless steel membrane module (outer diameter 60 mmφ x total length 3)
40mm), after heating to a temperature of 118 ° C using a steam, supply at 153.1 NL / h, high pressure side pressure 50kg / cm
² G, permeation side pressure 3 kg / cm ² G was separated under a separation condition, and an inert gas and a waste gas containing a large amount of C4 ⁺ polymer (ethylene concentration 54.0 mol%) were 13.9 NL / h from the high pressure side. Discharged at. From the permeate side, recovered ethylene having an ethylene concentration of 90.5 mol% was obtained at 139.2 NL / h. The ethylene recovery rate at this time was 94.4%.

The recovered ethylene concentration of the method of the present invention is 90.
At 5 mol%, the recovered ethylene concentration in the absorption method of the comparative example was 88.
It is better than 2 mol%, and the ethylene purity of the reaction system is improved accordingly, so that the ethanol production amount can be increased. Further, since the concentration of ethylene in the waste gas can be lowered by concentrating the impurities in the waste gas to a high concentration, the ethylene recovery rate of the method of the present invention is 94.4% (ethylene loss rate 5.6%). Become. Compared with the ethylene recovery rate of the comparative example of 89.8% (ethylene loss rate of 10.2%), the ethylene loss amount of the method of the present invention is about 1/2 of that of the comparative example.
Therefore, the method of the present invention is economically superior.

[0055]

[Comparative Example] A part of the circulating unreacted ethylene stream (ethylene concentration 87.2 mol%) used for the ethylene hydration reaction was withdrawn, and the absorption tower (tower was operated at a pressure of 21 kg / cm ² G and a temperature of 38 ° C). (Diameter 284 mm x tower height 7500 mm) 3 at the bottom
Light gas oil supplied at 54.4 Nm ³ / h and circulated from the next stripping tower is lowered from the upper stage at 342 liters / h to mainly absorb ethylene and C4 ⁺ polymer, and the ethylene concentration from the top of the absorption tower is reduced. 88.2 mol% of recovered ethylene was obtained at 314.5 Nm ³ / h. Light gas oil which mainly absorbed ethylene and C4 ⁺ polymer was heated to 120 ° C by steam with a pressure of 7 kg / cm ² G in a heat exchanger,
Stripping tower operated at the following pressure of 3 kg / cm ² G (tower diameter upper 298 mm × tower diameter lower 700 mm × tower height 28
50 mm), and a waste gas (ethylene concentration 79.2 mol%) was discharged at 39.9 Nm ³ / h from the top of the stripping tower. Light gas oil was extracted from the bottom of the stripping tower using a pump, cooled with industrial water in a heat exchanger, and then circulated in an absorption tower. The ethylene recovery rate at this time is
It was 89.8%.

[0056]

The gas permeation mechanism in gas separation using a membrane is called a dissolution diffusion mechanism. Since the aromatic polyimide film used in the present invention has a high degree of benzene ring in its polymer structure, the polymer gap is highly densified due to its stacking effect. Therefore, a gas having a large molecular diameter is more hindered from diffusion than a gas having a small molecular diameter, and thus the aromatic polyimide membrane is effective for separating gases having different molecular diameters. However, since the presence of the imide group promotes the dissolution of highly polar gas, it is considered that, for example, highly polar water vapor having a relatively small molecular diameter exhibits high permeability. When the molecular diameters are almost the same, molecules with higher polarity are more likely to pass through the aromatic polyimide film.
That is, ethylene has a higher permeability than ethane, or isobutene has a higher permeability than isobutane. This is considered to be because the gas permeation property of the aromatic polyimide film is that the molecular diameter and polarity of the permeated gas influence the solubility in the aromatic polyimide film and the diffusion rate in the aromatic polyimide film.

It has been discovered that trans-2-butene has a significantly different permeation tendency between the pure gas and the mixed gas. It is considered that this is because the dissolution rate of the permeated gas changes due to the plasticization of the aromatic polyimide membrane interface by the dissolved gas. That is, in the case of pure gas of trans-2-butene, the molecular diameter on the short side is not much different from ethylene,
Since the polarity is much higher than that of ethylene, it is considered that the dissolution at the membrane interface is synergistically promoted and the high permeability is exhibited.
On the other hand, in the case of the mixed gas, the concentration of trans-2-butene is 1% or less, so that ethylene or other C
The interaction with 4 ⁺ polymer and the solubility at the film interface are different from those of pure gas. As a result, in mixed gas such as impure ethylene, C4 ⁺ including trans-2-butene is included.
The polymer appears to have exhibited a lower permeability than ethylene. That is, the separability of the aromatic polyimide membrane used in the method of the present invention is considered to have shown a unique action only in the separation method of the impure ethylene stream such as the circulating unreacted ethylene stream used for the ethylene hydration reaction. To be

[0058]

[Effect] According to the method of the present invention, since a specific aromatic polyimide membrane is used, it is only necessary to equip a membrane module as compared with a conventional ethylene recovery method by a cryogenic distillation method or an absorption method. , Equipment cost is 1/5 to 1 of cryogenic distillation method
/ 3, about 1/2 to 2/3 of the absorption method, no electric power is used, and steam only needs to heat impure ethylene.
In addition to about 20 to 1/10, the ethylene recovery rate is superior to that of the absorption method, and the ethylene loss amount is small, so that the ethylene basic unit in ethanol production can be improved, and thus ethanol can be produced at a lower cost. And the like, a remarkably remarkable effect is obtained as compared with the conventional ethylene recovery method of this kind.

[0059]

[Brief description of drawings]

FIG. 1 is a graph obtained by plotting (Arrhenius plot) the permeation coefficient of various gases of an aromatic polyimide membrane used in the present invention against the reciprocal of temperature.

FIG. 2 is a conceptual diagram of a membrane separation process flow of the present invention.

FIG. 3 is a conceptual diagram showing another example of the membrane separation process flow of the present invention.

Claims (6)

[Claims] 1. When producing ethanol by a hydration reaction of ethylene using a catalyst, impurities contained in a circulating unreacted ethylene stream are represented by the following formula I: ## STR1 ## [In the formula, R represents a divalent residue of the diamine compound excluding the amino group, and the diamine compound is represented by the following formula:
II: [Chemical formula 2] (Wherein R ₁ and R ₂ represent hydrogen or a methyl group) and the following formula III: ## STR3 ## It represents at least one aromatic diamine compound selected from the group consisting of diamine compounds represented by: ] The process for treating an impure ethylene stream, which comprises separating ethylene from an ethylene by a membrane separation method using an aromatic polyimide membrane having a repeating unit represented by 80% or more. 2. The processing method according to claim 1, wherein the operating temperature is 20 to 150 ° C. 3. The processing method according to claim 1, wherein the operating pressure is 0 to 80 kg / cm ² G. 4. The treatment method according to claim 1, wherein the membrane module in the membrane separation method has one stage or a combination of membrane modules having two to three stages in successive stages. 5. The ethylene concentration in the impure ethylene stream is
It is 40-95 mol%, The processing method of Claims 1-4. 6. The impurities contained in the impure ethylene stream are impurities containing nitrogen, diethyl ether and a saturated or unsaturated hydrocarbon having 1 to 10 carbon atoms.
4. The processing method according to 4.