WO2000029366A1

WO2000029366A1 - Process for the purification of methyl acetate

Info

Publication number: WO2000029366A1
Application number: PCT/US1999/027295
Authority: WO
Inventors: Charles Emory Outlaw; Carl Franklin Fillers; Brian Travis Smith; Karen Hunter Maness; David Justin Olsen
Original assignee: Eastman Chemical Company
Priority date: 1998-11-18
Filing date: 1999-11-17
Publication date: 2000-05-25

Abstract

The present invention relates to a process for purifying methyl acetate. More specifically, the present invention provides a process for removing water, and methanol from methyl acetate comprising contacting the methyl acetate stream with a molecular sieve having a pore size from about 4 angstroms to about 15 angstroms and collecting the purified methyl acetate. The present invention further provides a process for producing methyl acetate having a high purity.

Description

PROCESS FOR THE PURIFICATION OF METHYL ACETATE

Field of the Invention

The present invention relates to a process for purifying methyl acetate. In addition, the present invention relates to a process for preparing methyl acetate.

The Background of the Invention

Methyl Acetate is conventionally used as a solvent or a chemical intermediate. Methyl acetate may be prepared by a reaction of methanol and acetic acid in the presence of an acid catalyst. This reaction synthesis results in a product predominantly containing methyl acetate, water and unreacted methanol. The methyl acetate may be purified prior to use if a higher level of purity is desired.

Many processes for purifying methyl acetate have been proposed. While many of these processes produce methyl acetate having a relatively high degree of purity, there are few economically viable processes that can be used to produce methyl acetate having a purity greater than 97 % by weight based on the total weight of the product. For example, many of the purification processes that produce high purity methyl acetate use multiple fractional distillation columns. Hence, there is a need for an economically viable process for purifying methyl acetate and producing high purity methyl acetate.

More and more methyl acetate applications are requiring highly pure methyl acetate having very low concentrations of methanol and water. For instance, the food industry requires highly pure methyl acetate for such applications as artificial sweeteners. Likewise, the electronics industry requires methyl acetate solvents that are virtually free of water, methanol and trace metals.

More recently, there has been heightened concern over the use of methanol in consumer products, such as cosmetic products. The United States Environmental Protection Agency (EPA) defines methanol as a volatile organic compound (VOC) and regulates the presence of methanol in cosmetic applications such as hair spray. In addition, methanol is characterized as a Hazardous Air Pollutant (HAP) which can place additional limits on the use of methanol. Hence, it is often desirable to remove substantially all methanol present in the methyl acetate streams prior to use in these consumer applications.

It is, therefore, apparent that there is a need for an economic process for purifying methyl acetate. Furthermore, it is apparent that there is a need for a process for preparing methyl acetate having a high purity.

SUMMARY OF THE INVENTION

The present invention provides a process for purifying methyl acetate. More specifically, the present invention provides a process for removing water, methanol and acetic acid from methyl acetate. The present invention further provides a process for producing methyl acetate having a high purity. In one embodiment, the present invention is directed to a process for purifying a crude methyl acetate stream comprising contacting the methyl acetate stream with a molecular sieve having a pore size from about 4 angstroms to about 15 angstroms and collecting the purified methyl acetate. The molecular sieve removes water and methanol from the methyl acetate in a simple, economic process.

In another embodiment, the present invention is also directed to a process for preparing methyl acetate having a high purity from a reaction of methanol and acetic acid, the process comprising the steps of:

(a) selecting a design for a single reactive distillation column and a residence time thereof to provide intimate contact sufficient to enable the acetic acid to be used both as a reactant and as an extractive agent within said column; (b) countercurrently flowing approximately stoichiometrically equivalent quantities of acetic acid and methanol through the single reactive distillation column in the presence of a catalyst so as to provide intimate contact in said column between the acetic acid and methanol and between the acetic acid and methyl acetate/methanol azeotrope, and between the acetic acid and methyl acetate/methanol azeotrope, the residence time in the column being sufficient to accomplish high reactant conversion and to obtain a methyl acetate stream;

(c) continuously removing methyl acetate stream from the top of the single column and continuously removing water from the bottom of the column;

(d) contacting the methyl acetate stream with a molecular sieve having a pore size from about 4 angstroms to about 15 angstroms; and

(e) collecting the methyl acetate. An advantage of the present invention is that water and methanol can be removed from a crude methyl acetate stream in a simple, economic purification process. An additional advantage of the present invention is that it provides an economical process for preparing methyl acetate having a high purity. These and other features, and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the of the invention as set forth hereinafter.

BRIEF DESCIPTION OF THE DRAWINGS The various embodiments can be further understood by examination of the attached drawings. These drawings only depict typical embodiments of the invention and should therefore not be considered to be limiting in scope.

Figure 1 is a flow diagram of a process for preparing a methyl acetate stream. Figure 2 is a depiction of a reactor column illustrating the various functional sections of the column.

Figure 3 is a schematic of a purification process using a pressure swing method.

DETAILED DESCRIPTION OF THE INVENTION

Use of Terms

It must be noted that, as used in the specification and the appended claims, the singular forms "a," "an" and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "a molecular sieve" includes a plurality of molecular sieves or mixtures of molecular sieves having the recited characteristics.

Ranges are often expressed herein as from "about" one particular value, and/or to "about" another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent "about," it will be understood that the particular value forms another embodiment. Unless otherwise specified, all percents referred to herein are percent by weight of the entire composition.

The term "methyl acetate stream" is also referred to herein as the "stream". Likewise, reference to "purified methyl acetate stream" is also referred to herein as the "purified stream". The "purified methyl acetate stream" as used herein is defined as having a higher level of purity than the initial methyl acetate stream.

Discussion

It is a feature of the present invention to provide a process for purifying methyl acetate that removes methanol and water from a methyl acetate stream.

It is further a feature of the present invention to provide a process for preparing high purity methyl acetate.

In one embodiment, the present invention is directed to a process for purifying a methyl acetate stream comprising contacting the methyl acetate stream with a molecular sieve having a pore size from about 4 angstroms to about 15 angstroms. A purified methyl acetate stream can then be collected from the molecular sieve using any method known in the art.

The methyl acetate stream comprises methyl acetate, methanol and water. While the major component in the methyl acetate stream is methyl acetate, the stream may comprise large amounts of water and/or methanol. In a preferred embodiment, the methyl acetate stream comprises methyl acetate in an amount of at least about 80% by weight of the stream. In a more preferred embodiment, the methyl acetate is present in the stream in an amount of at least about 90 percent by weight of the stream. In the most preferred embodiment, the methyl acetate stream is present in the stream in an amount of about 97.5 % by weight of the stream.

In a preferred embodiment, the molecular size is a plurality of molecular sieves typically referred to as a molecular sieve bed. The molecular sieve bed is preferably fixed in some way as in a column, to form a fixed molecular sieve bed that the methyl acetate stream can pass through, removing unwanted components of the stream, specifically methanol and water.

The pores in the molecular sieves are from about 4 angstroms to about 15 angstroms, preferably from about 4 angstroms to about 12 angstroms, more preferably about 4 angstroms to about 10 angstroms, and most preferably about 4 angstroms to about 5 angstroms. The molecular sieves may be composed of a plurality of molecular sieves having the same or differing pore sizes.

In a preferred embodiment, the molecular sieve is a crystalline alumino- silicate adsorbent, such as a zeolite. Examples of suitable molecular sieves include ZEOCHEM Molecular Sieves Types 4A, 5 A, and 13X, available from Zeochem, P.O. Box 35904, Louisville, Kentucky 40232.

The methyl acetate stream can be contacted with the molecular sieve bed in liquid or vapor form. When the methyl acetate stream is a liquid, the stream is preferably introduced into the bottom of the column holding the molecular sieve bed and pushed up through the bed and a purified stream is collected from the top of the column. Conventional means can be used to vary the residence time of the stream in the column.

In a preferred embodiment the stream is vaporized prior to contacting the molecular sieve bed. The stream can be vaporized using any method known in the art that will vaporize substantially all the components in the stream. For instance, the stream is heated above the vaporization points of methyl acetate, methanol and water using any means typically used for vaporizing mixtures. The vaporized stream is contacted with the molecular sieve bed using any process known in the art. Preferably, the vaporized stream is passed down through the fixed molecular sieve bed so that the vaporized stream contacts the molecular sieves. The molecular sieves physically separate the methanol and water from the stream allowing the purified methyl acetate to pass through the molecular sieves and be collected. The contacting step can be performed by any means of contacting the stream with molecular sieves known in the art, including a fixed catalyst bed and a pressure swing absorption. In addition, once the molecular sieves have been used to purify the stream, the molecular sieves can be regenerated by any means known in the art, such as heating or exposing to a hot nitrogen stream or pressure swing absorption.

In a preferred embodiment, the pressure swing absorption method is used to purify the methyl acetate stream and regenerate the molecular sieve. In the pressure swing absorption the adsorption of methanol and water in the molecular sieves takes place at 20-30 psia. The regeneration of the molecular sieves (removal of the adsorbed methanol and water) takes place at 4 psia. This pressure change (swing) is one of the drivers for bed regeneration. A pressure swing process uses two molecular sieve beds operating in series: one bed purifying the methyl acetate stream while the other bed is being regenerated. This allows for there always to be one molecular sieve bed in use. In addition to the pressure swing, a portion of the purified methyl acetate stream vapor, obtained from the bed doing the adsorption, is passed through the bed being regenerated. The purified methyl acetate stream vapor has very little methanol and water, while the "dirty" molecular sieves have a substantial portion of methanol and water. This difference in concentrations of methyl acetate, methanol and water creates a concentration gradient that, along with the pressure change, helps clean the molecular sieve bed. Once the molecular sieve bed has been regenerated, the bed may be reused to purify the stream.

Figure 3 shows a preferred schematic of a purification process using the pressure swing method. A methyl acetate stream 100 is vaporized by a vaporizing means 110 and fed into a first column 114 containing molecular sieves through a feeding means 112. The stream is contacted with the molecular sieves in column 114. The molecular sieves remove methanol and water from stream 100. Purified methyl acetate 116 exits column 114 and is split into two different portions. A first portion 118, which is the major portion, and the second portion, which is the minor portion that is directed to a second column 122. Purified methyl acetate stream 118 is condensed in a condensing means 128 and collected in a collecting means 130, to yield a purified liquid methyl acetate stream which can be removed by removing means 132.

Column 122 is regenerated by the pressure swing method. The methyl acetate 120 aids the regeneration of column 122 and exits column 122 as a stream 124 containing methanol and water in addition to methyl acetate. Stream 124 is condensed, collected and recycled back into methyl acetate stream 100.

The purified methyl acetate stream is collected by any means know in the art. When the stream in liquid form is passed through the molecular sieves, the purified stream is collected in any suitable container. When the stream is in vapor form, the purified vapor collected will preferably be condensed into a liquid. The condensation of the purified vapor can be accomplished by any means known in the art.

Although passing the stream through one molecular sieve is typically sufficient, it may be necessary to pass the stream through a second molecular sieve bed. When this is the case, the stream is contacted with the molecular sieves in the same manner as the first run. In a preferred embodiment, the methyl acetate stream obtains a purity of about at least 99.8 % by weight methyl acetate based on the total weight of the product. In another embodiment, the present invention is directed to a process for preparing methyl acetate from methanol and acetic acid. This process comprises preparing a methyl acetate stream. While the methyl acetate stream produced may be of any purity, in the preferred embodiment, the methyl acetate stream has a purity of at least 95 % by weight, more preferably 97 % by weight and most preferably 97.5 % by weight. The methyl acetate stream is then contacted with molecular sieves to produce a purified methyl acetate stream. The preferred process for preparing the methyl acetate is disclosed in U.S. 4,435,595, which is hereby incorporated by reference. In a preferred embodiment, the process for preparing methyl acetate from methanol and acetic acid comprises the steps of: selecting a design for a single reactive distillation column and a residence time thereof to provide intimate contact sufficient to enable the acetic acid to be used both as a reactant and as an extractive agent within the column; countercurrently flowing approximately stoichiometrically equivalent quantities of acetic acid and methanol through the single reactive distillation column in the presence of a catalyst so as to provide intimate contact in said column between the acetic acid and methanol, between the acetic acid and methyl acetate/methanol azeotrope and between the acetic acid and methyl acetate/methanol azeotrope, the residence time in the column being sufficient to accomplish high reactant conversion and to obtain methyl acetate stream; continuously removing the methyl acetate stream from the top of the single column and continuously removing water from the bottom of the column; contacting the methyl acetate stream with a molecular sieve having a pore size from about 4 angstroms to about 15 angstroms; and collecting methyl acetate.

In a preferred embodiment the methyl acetate product has a purity of at least about 97 % by weight methyl acetate based on the total weight of the total product and more preferably at least 99.8% by weight of the total product.

Any known method of purification can be used with the molecular sieves. In a preferred embodiment the methyl acetate stream is vaporized prior to contact with the molecular sieves. A pressure swing method is preferably used to purify the methyl acetate stream and to regenerate the molecular sieves.

Figures 1 and 2 provide a more detailed description of the method for preparing the methyl acetate. Figure 1 depicts a flow diagram of the process used to form the methyl acetate stream of the present invention. The reactor/distillation column 1 is provided in its upper section with an inlet for acetic acid which is provided to the column through feed stream 3. Methanol is fed to the lower part of the reaction section of the reactor/column through feed pipe 5. Sulfuric acid catalyst is fed through line 7 to the lower portion of the extractive distillation section of the column. Steam is applied to the base of the column through line 9.

A vapor sidedraw stream is withdrawn from the middle to upper part of the reaction section of the column through line 11. The vapor stream is passed through wire mesh separator 12, from which entrained sulfuric acid is returned to the column through line 13, and the remaining components of the vapor sidedraw stream are fed through line 14 to a first sidedraw distillation column 15. A bottom stream comprising mostly acetic acid and water is taken from column 15 and is returned through line 17 to reactor/column 1. Methyl acetate and its azeotropes and intermediate boiling compounds are taken overhead and are fed through line 18 to a second sidedraw distillation column 19. Methyl acetate and its azeotropes are taken as the distillate from column 19 and are returned through line 21 to the reactor/column 1. The intermediate boiling compounds are obtained as the underflow from column 19 through line 23.

The reaction mixture from the lower end of the reaction section of reactor/column 1 is fed through line 24 to hold tank 25. The hold tank has a hold- up time of at least about one hour and may be used to increase the reactant conversion for a given number of trays in reactor/column 1. The reaction mixture is returned from hold tank 25 to reactor/column 1 through line 26. The hold tank product vapor is returned from hold tank 25 to reactor/column 1 through line 27.

By-product water, sulfuric acid, and excess methanol, if any, are withdrawn from the base of the column through line 28. The methyl acetate product stream is withdrawn from the top of the column through line 29. A reflux stream is returned to the column through line 30, and a product stream is taken off through line 31.

The flow diagram represented by Figure 1 and described above is, of course, a simplified flow diagram of preferred embodiments of the process of the present invention. The apparatus utilized in the process of the present invention may additionally include vent scrubbers and other well-known apparatus.

Figure 2 is a detail of reactor/column 1 showing the various functional sections within the column. Feedstreams for acetic acid, methanol, sulfuric acid, and steam are again represented by numerals 3, 5, 7, and 9, respectively. The vapor sidedraw stream and the corresponding return streams are indicated by numerals 11 and 13, 17, and 21, respectively. Line 24 represents a liquid sidedraw to the hold tank, and the return lines from the hold tank are indicated by lines 26 and 27. The column bottoms are removed through line 28, and the overhead is removed through line 29, with a reflux stream being returned through line 30.

The area labeled as 33 represents the approximate limits of the methanol/water stripping section of the column. In this section, methanol is stripped from the by-product water. The water is removed from the column through line 28 while the methanol ascends through the column, reacting with acetic acid as it ascends.

The region of the column designated as 35 is the reactive distillation section of the column. In this area, much of the reaction between acetic acid and methanol occurs. The methyl acetate product is flash distilled at each stage and rises through the column. During this process, methyl acetate/water and methyl acetate/methanol azeotropes are inevitably formed. These azeotropes are broken by the acetic acid which acts as an extractive agent as it descends through the column and as it reacts with the methanol.

The extractive distillation section of the column is indicated by numeral 37. In this region, which is rich in acetic acid, formation of product methyl acetate from the reactants continues to occur, and the breaking of the aforementioned azeotropes by the extractive action of the acetic acid also continues. As indicated earlier, the primary method of removal of methanol from the methyl acetate/methanol azeotrope is by the reaction of acetic acid with methanol.

The area designated as 39 is the methyl acetate/acetic acid rectification section of the column in which methyl acetate product is separated from acetic acid reactant. The acetic acid descends through the column while the methyl acetate product is taken overhead through line 29.

In one particular embodiment, for a given production rate, methanol stripping section 33 comprises 12 reverse flow-valve trays spaced 18 inches apart; reactive distillation section 35 comprises 60 reverse flow-bubble cap trays spaced 24 inches apart; extractive distillation section 37 comprises 10 crossflow-valve trays spaced 18 inches apart. Of course, such an arrangement as outlined above is not critical to the operation of the process of the present invention but is merely an example of a system which has been found by the inventors to operate efficiently. The present invention will be more readily understood by reference to the following examples. There are, of course, many other forms of the invention which will become obvious to one skilled in the art, once the invention has been fully disclosed, and it will accordingly be recognized that these examples are given for the purpose of illustration only and are not to be construed as limiting the scope of the invention in any way.

EXAMPLES The following test procedures were used in evaluating the analytical properties of the mixtures herein and in evaluating the physical properties of the mixtures of the examples.

Water Determination: Karl Fisher Standard Test Method E1064-92, E203- 96; D1364-95

Acid Determination: E301-94 Standard Test Method for Total Acidity in Volatile Organic Acids; and D1613-96 Standard Test Method for Acidity in Volatile Solvents and Chemical Intermediates Used in Paint, Varnish, Lacquer, and Related Products Gas Chromato graph y results were run on a Hewlett Packard 5890 and used a DB 1701 capillary column.

Example 1 Water and methanol were removed from a methyl acetate stream by contacting the methyl acetate stream with a molecular sieve bed in the following manner. 50 cc (31-35 grams) of the appropriate molecular sieve was placed in a jacketed 16 inch resin column approximately 16 inches in length and 1 inch in diameter. As shown in Table 1, four different types of molecular sieves were used in Example 1, ZEOLITE 3 A, 4A, 5 A and 13X. Glass wool was used at the top and the bottom of the resin column to maintain the position of the molecular sieve bed. A methyl acetate stream having a purity of about 97.5 % methyl acetate was pumped into the bottom of the column at a rate that resulted in a half hour residence time (or 2 bed volumes/hr). Samples were taken as indicated in Table 1. Water was removed by all the molecular sieves. Methanol was removed by sieves with pore sizes of 4 angstroms and greater. Both methanol and water were removed using sieves with pore sizes of 4 angstroms and greater.

TABLE 1

REMOVAL OF METHANOL AND WATER BY

MOLECULAR SIEVES¹

Note, % for water and acetic acid are true %. For all other % do not include water and acetic acid(g.c)

Example 2 Example 2 shows the purification of a methyl acetate stream using molecular sieves to produce a high purity methyl acetate for commercial use. A stream of methyl acetate (97.5 % by weight methyl acetate) was vaporized and was passed through a fixed molecular sieve bed to remove impurities (see Table 2 for initial and final concentrations). A Pressure Swing Adsorption apparatus was used to allow for continuous methyl acetate processing and molecular sieve regeneration.

The Pressure Swing apparatus consisted of a feed vaporizer, two fixed beds containing size 4-A type molecular sieve from Zeochem, a vacuum pump, condensers, and liquid accumulators. The two molecular sieve beds were operated in series with one bed purifying methyl acetate while the other bed was being regenerated. The purified methyl acetate vapor exiting the molecular sieve beds was condensed and allowed to flow to an accumulator pot for product analysis. Regeneration of the molecular sieve beds was accomplished by first lowering pressure to 4 psia and then passing a small portion of the purified methyl acetate vapor over the contaminated bed. The driving force for bed regeneration was the reduced pressure and the concentration gradient between the purified methyl acetate and the contaminated molecular sieve. The vapor exiting the bed during regeneration was condensed, allowed to flow to an accumulator pot, and then pumped back to an existing process for recovery of the methyl acetate. The pressure swing apparatus was operated continuously by alternating which bed was purifying the methyl acetate and which bed was being regenerated. The test results demonstrated that molecular sieves could be used to produce high purity methyl acetate for commercial sales by removing methanol and water from a methyl acetate feed. A Pressure Swing apparatus of the type described above can be obtained through Vogelbusch USA, INC. in Houston, Texas.

Table 2 Results of Pilot Test to Evaluate the Use of Molecular Sieves to Produce a High Purity Methyl Acetate Product for Commercial Use from a Methyl

Acetate Feed

Claims

CLAIMSWe Claim:

1. A process for purifying a first methyl acetate stream comprising: contacting the first methyl acetate stream with a molecular sieve having a pore size from about 4 angstroms to about 15 angstroms; and collecting the resultant second methyl acetate.

2. A process as recited in claim 1, further comprising vaporizing the first methyl acetate stream prior to contacting the first methyl acetate stream with the molecular sieve.

3. A process as recited in claim 2, wherein said contacting step is performed by passing the vaporized first methyl acetate through a plurality of molecular sieves.

4. A process as recited in claim 3, wherein the plurality of molecular sieves are in a column having a top portion and a bottom portion and wherein the first methyl acetate stream is introduced into the top portion of the column and the resultant second methyl acetate is collected form the bottom portion of the column

5. A process as recited in claim 1, further comprising regenerating the molecular sieves.

6. A process as recited in claim 5, wherein the molecular sieves are regenerated by a pressure swing process.

7. A process as recited in claim 1, wherein the molecular sieve is a zeolite.

8. A process as recited in claim 1, wherein the molecular sieve has a pore size from about 4 angstroms to about 12 angstroms.

9. A process as recited in claim 1, wherein the molecular sieve has a pore size from about 4 angstroms to about 10 angstroms.

10. A process as recited in claim 1, wherein the molecular sieve has a pore size from about 4 angstroms to about 5 angstroms.

11. A process as recited in claim 1, wherein the first methyl acetate stream comprises methyl acetate, methanol and water, and wherein the contacting step removes water and methanol.

12. A process as recited in claim 1, wherein the resultant second methyl acetate stream collected comprises methanol in an amount less than about 0.1 % by weight of the second methyl acetate stream and water in an amount less than about 0.1 % by weight of the second methyl acetate stream.

13. A process as recited in claim 1, wherein the resultant second methyl acetate has a purity of at least about 99.8 % by weight of the second methyl acetate.

14. A process for preparing methyl acetate from methanol and acetic acid, said process comprising the steps of: selecting a design for a single reactive distillation column and a residence time thereof to provide intimate contact sufficient to enable the acetic acid to be used both as a reactant and as an extractive agent within the column; countercurrently flowing approximately stoichiometrically equivalent quantities of acetic acid and methanol through the single reactive distillation column in the presence of a catalytic so as to provide intimate contact in said column between the acetic acid and methanol, between the acetic acid and methyl acetate/methanol azeotrope, and between the acetic acid and methyl acetate/methanol azeotrope, the residence time in the column being sufficient to accomplish high reactant conversion and to obtain first methyl acetate stream; continuously removing first methyl acetate stream from the top of the single column and continuously removing water from the bottom of the column; contacting the first methyl acetate stream with a molecular sieve having a pore size from about 4 angstroms to about 15 angstroms; and collecting the resultant second methyl acetate stream.

15. A process as recited in claim 14, wherein the purified methyl acetate stream comprises at least 97.5 % by weight methyl acetate based on the total weight of the methyl acetate stream.

16. A process as recited in claim 14, wherein the purified methyl acetate stream comprises at least 99.8 % by weight methyl acetate based on the total weight of the methyl acetate stream.

17. A process as recited in claim 14, wherein the molecular sieve has a pore size from about 4 angstroms to about 5 angstroms.

18. A process as recited in claim 14, wherein the molecular sieve is a zeolite.