Process for recovering .gamma.-butyrolactone from a mixture of heavy organics

Gamma-Butyrolactone (GBL) is recovered from a mixture containing GBL and other heavy organics by a process of azeotropic distillation in the presence of a C.sub.8-C .sub.10 hydrocarbon as an azeotroping agent, wherein an azeotrope of GBL and the hydrocarbon is obtained as a distillate, which forms immiscible GBL-rich and hydrocarbon-rich phases, and the hydrocarbon-rich phase is decanted or isolated from the GBL-rich phase. By this process GBL can be efficiently separated from a large proportion of various compounds having boiling points close to that of GBL, e.g., the methyl-.gamma.-butyrolactones (MeGBL's).

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 This invention relates to an improved process for recovering
 gamma-butyrolactone (.gamma.-butyrolactone) from a mixture of heavy
 organics by means of azeotropic distillation.
 2. Description of the Related Art
 .gamma.-Butyrolactone (GBL) is a commodity of commerce useful in various
 applications such as a chemical intermediate, e.g., in the preparation of
 butyric compounds and polyvinylpyrrolidone, a solvent for various
 polymers, e.g., acrylate and styrene polymers, and in specialty products
 such as paint removers, textile assistants and drilling oils.
 In a number of commercial processes resulting in the synthesis of GBL,
 e.g., the liquid phase oxidation of butane with molecular oxygen and the
 reaction of acetylene and formaldehyde using Reppe chemistry, the GBL is
 obtained in admixture with other relatively heavy, i.e., high boiling
 organic compounds, several of which have boiling points very close to GBL,
 e.g., the methyl-.gamma.-butyrolactones (MeGBL's). In view of this, it is
 difficult to recover GBL in sufficiently high purity from these other
 heavy organics without incurring prohibitively high energy and equipment
 costs so that in many instances, mixtures of heavy organics containing a
 relatively high percentage of GBL are burned for fuel rather than treated
 to recover the GBL. Thus, any expedient which is effective in rendering
 the economic recovery of GBL in high purity from mixtures with heavy
 organics, would be very desirable.
 U.S. Pat. No. 2,875,213 issued Feb. 24, 1959 to Ulvild et al., discloses
 the recovery of GBL from a distillation fraction of the product of liquid
 phase oxidation of n-butane, comprising a major proportion of GBL and a
 minor proportion of glycol esters having boiling points close to that of
 GBL, by solvent extraction with a dual solvent of water and a liquid
 aliphatic hydrocarbon, e.g., n-hexane, to obtain aqueous and hydrocarbon
 phases, and obtaining GBL of high purity from the aqueous phase by
 distillation.
 U.S. Pat. No. 4,851,085, issued Jul. 25, 1989 to De Thomas, discloses a
 process of treating GBL of at least 95% purity with a strong acid, e.g., a
 mineral acid or acidic ion exchange resin, to remove color forming
 impurities for product stabilization.
 U.S. Pat. No. 5,319,111 issued Jun. 7, 1994 to Zimmermann et al. teaches a
 process for the preparation of tetrahydrofuran and GBL from the product of
 the hydrogenation of maleic acid, succinic acid, maleic anhydride,
 succinic anhydride and/or fumaric acid, by treating the crude
 hydrogenation product with a protic acid, e.g., a mineral acid or cation
 exchange resin, and isolating pure tetrafuran and GBL from the reaction
 mixture by distillation.
 BRIEF SUMMARY OF THE INVENTION
 In accordance with this invention, .gamma.-butyrolactone (GBL) is recovered
 from a mixture containing GBL and other heavy organics by a process of
 azeotropic distillation in the presence of a C.sub.8-C.sub.10 hydrocarbon
 as an azeotroping agent. An azeotrope of GBL and the hydrocarbon is
 obtained as a distillate, which forms immiscible GBL-rich and
 hydrocarbon-rich phases. The hydrocarbon-rich phase is then decanted from
 the GBL-rich phase. By this process, GBL can be efficiently separated from
 a large proportion of various compounds having boiling points close to
 that of GBL, e.g., the methyl-.gamma.-butyrolactones (MeGBL's).

DETAILED DESCRIPTION OF THE INVENTION
 In an embodiment the feed mixture subjected to the azeotropic distillation
 of this invention will contain about 1 to about 99 wt. % of GBL and at
 least about 1 wt. % of other relatively heavy organic compounds.
 Preferably the feed mixture will contain about 10 to about 80 wt. % of
 other heavy organic compounds, having an atmospheric boiling point of at
 least about 130.degree. C., preferably about 150.degree. C. to about
 250.degree. C. The organic compounds in the feed mixture may include, for
 example, about 0.1 to about 50 wt. % of MeGBL's and/or about 0.1 to about
 50 wt. % of esters having boiling points within the foregoing ranges. All
 of the foregoing weight percents are based on the total weight of the
 mixture fed to the azeotropic distillation unit.
 The C.sub.8-C.sub.10 hydrocarbon azeotroping agent may be, for example,
 n-octane, n-nonane or n-decane, and is preferably n-octane. The weight
 ratio of azeotroping agent to the GBL-containing mixture fed to the
 azeotropic distillation unit may be, for example, about 1 to about 99 wt.
 %, preferably about 5 to about 30 wt. %. The azeotropic distillation unit
 is preferably a column containing, for example, about 10 to about 100
 trays which operates at continuous steady state at a pressure of, for
 example, about 10 to about 100 psia, a reboiler temperature of, for
 example, about 150 to about 300.degree. C. and a top tray temperature of,
 for example, about 100 to about 200.degree. C.
 The feed mixture flows into the azeotrope distillation column at an
 intermediate tray and hydrocarbon azeotroping agent composed mainly of
 recycled hydrocarbon as discussed hereinafter plus any makeup hydrocarbon
 necessary to maintain the amount in the column at a desired level, is fed
 to the column at a tray below the feed point, e.g. about 1 to 50 trays
 below the feed point, which allows for good physical contact between the
 feed and the azeotroping hydrocarbon. A vaporous azeotrope of GBL and
 hydrocarbon azeotroping agent is withdrawn from the top of the column and
 is condensed into a distillate containing immiscible GBL-rich and
 hydrocarbon-rich phases which are separated in a decanter, preferably
 using chilled water as a coolant. Most of the upper hydrocarbon-rich phase
 in the decanter is recycled to the azeotroping column at a tray below the
 feed point as discussed, and a reflux amount of hydrocarbon only large
 enough to maintain liquid on the top trays of the column is fed to the top
 of the column. The lower GBL-rich liquid phase in the decanter is crude
 GBL containing, for example, about 5 to about 98 wt. % of GBL. Since the
 solubility of hydrocarbon in the GBL-rich phase increases with increasing
 temperature, in the case of n-octane rising from about 1.9 wt. % at
 12.degree. C. to about 2.9 wt. % at 40.degree. C., the temperature of the
 liquid in the decanter should be kept as low as practicable, e.g. from
 about 0 to about 20.degree. C., to avoid the loss of hydrocarbon
 downstream from the azeotroping column.
 The crude GBL from the azeotroping column is found to contain only a minor
 percentage of the MeGBL's present in the feed to the azeotroping column,
 e.g. about 10 to about 50 wt. %. This is an unexpected advantage of the
 process of this invention since the MeGBL's, considered a serious impurity
 in commercial GBL, have boiling points similar to that of GBL, namely
 about 204.degree. C.
 In many instances, the GBL subjected to further purification, heating and
 storage downstream from the azeotropic distillation unit develops an
 undesirable color which can be traced to the presence of unsaturated
 impurities in the crude GBL obtained from the distillation. Thus, in
 accordance with another aspect of the invention, the crude GBL obtained
 from the azeotropic distillation unit is subjected to a hydrogenation
 treatment to eliminate or reduce the unsaturation of the unsaturated
 impurities present in the crude GBL. The hydrogenation may be carried out,
 for example, in a pipe reactor packed with a catalyst effective for the
 hydrogenation of carbon-carbon double bonds (C.dbd.C) as are well known in
 the art. A particularly effective catalyst is palladium, e.g. supported on
 carbon. The hydrogenation reaction may be carried out at a hydrogen
 pressure of, for example, about 1 to about 500 psig and a temperature of,
 for example, about 100 to about 300.degree. C. at a residence time
 sufficient to eliminate or reduce the unsaturated impurities to a
 sufficiently low level.
 Whether the crude GBL from the azeotropic distillation unit is subjected to
 a hydrogenation treatment or not, it is generally too impure for
 commercial use, i.e. contains a sufficiently large amount of impurities
 having lower and higher boiling points than GBL such as organic acids,
 ethylene glycol diacetate (EGDA) and 2,5-hexanedione, to cause
 interference with the effectiveness of the GBL for several of its
 applications. It is therefore another aspect of the invention to subject
 the crude GBL, after the hydrogenation step if such hydrogenation is
 deemed necessary or desirable, to a purification treatment such as vacuum
 distillation as a finishing treatnent for the purpose of removing such
 impurities. The vacuum distillation intended to produce a GBL of high
 purity, may be carried out a pressure of, for example, about 10 to about
 700 mm Hg absolute while maintaining the reboiler temperature close to
 about 150.degree. C. and a top end distillate temperature of about 50 to
 about 150.degree. C., in a column containing, for example, about 10 to
 about 100 trays. Preferably, the feed (crude GBL) is introduced at a point
 above the midpoint of the column and the product is withdrawn as a vapor
 sidestream from a point below the midpoint several trays above the
 reboiler to remove traces of heavy ends, i.e., impurities having boiling
 points higher than that of GBL, from the product. The sidestrean product
 generally has a GBL content of over 99 wt. % and lighter impurities, i.e.
 having boiling points below that of GBL, such as lighter organic acids,
 EGDA and 2,5-hexanedione, are removed in the overhead distillate.
 Despite the relatively high purity of the GBL product from the purification
 or finishing column, it has been found that such product may still contain
 various high boiling esters which may interfere with the use of GBL in
 certain applications, e.g. in the electronics and pharmaceutical
 industries. In accordance with another aspect of the invention, all or
 part of the product from the finishing column may be subjected to
 hydrolysis conditions to hydrolyze the high boiling esters contained
 therein and is then subjected to distillation to remove the acid and
 alcohol products of hydrolysis. One method of accomplishing this is to add
 a small amount, e.g. about 1 to 3 wt. % of water to at least part of the
 side-stream product of the finishing column, then contact the
 water-containing product stream with an acidic hydrolysis catalyst, and
 finally recycle the stream containing the hydrolysis products to the
 finishing column. The acidic hydrolysis catalyst may be any of those known
 in the art, preferably a strong acidic cation exchange resin such as that
 sold as Amberlyst 36.RTM. by Rohm and Haas.
 The hydrolysis may be carried out, for example, by passing most of the
 product from the finishing column through a bed of the strong acid cation
 exchange resin at a temperature of about 10 to about 150.degree. C. at
 atmospheric pressure and a space velocity of about 10 to about 2000 grams
 of feed/L of resin per hour.
 As stated, the hydrolysis products which comprise mostly acids and alcohols
 must be removed from the product to maintain its purity. This may be
 accomplished by the distillation of the product containing hydrolyzed
 impurities in a separate column or, more desirably, by recycling the
 hydrolyzed product through the vacuum distillation finishing column. If
 recycling of the hydrolyzed product is carried out, then the ratio of
 recycle to the total sidestream product is, for example, about 1:1 to
 about 10:1.
 The purified final product from for example, the vacuum distillation
 column, from which the hydrolyzed impurities have been removed, is taken
 as a sidestream from the column and generally contains at least about 99
 wt. % of GBL and in many cases as high as about 99.9 wt. % not including
 the MeGBL's. Furthermore, the MeGBL's and high boiling esters present in
 such final product are generally present in much lower quantities than in
 the initial feed to the azeotropic distillation column or in any material
 between such column and the vacuum distillation finishing column.
 In addition to the sidestream product taken from the distillation finishing
 column as described, a small amount of a residue comprising over 95 wt. %
 of GBL with minor percentages of MeGBL's and the products of hydrolysis is
 withdrawn and recycled to the azeotropic distillation column, and an
 overhead stream comprised primarily of substantial amounts of GBL and
 water and minor amounts of MeGBL's and products of hydrolysis, are
 condensed and decanted with most of the overhead in the decanter recycled
 as reflux to the column and a minor amount including the lower layer
 withdrawn and burned for fuel.
 An important source of the GBL-containing feed to the azeotropic
 distillation column is a heavy residue from the purification of the
 products of the liquid phase oxidation of n-butane with molecular oxygen,
 such residue containing from about 1 to about 70 wt. % of GBL, about 1 to
 about 30 wt. % of n-butyric acid (HBu) about 1 to about 20 wt. % of other
 compounds having boiling points near or lower than GBL, and about 1 to
 about 50 wt. % of compounds having boiling points higher than GBL, often
 including about 1 to 20 wt. % of succinic anhydride. This residue is
 generally subjected to a vacuum evaporation step using, for example, a
 falling film evaporator or a rotary vacuum evaporator operating at a
 temperature of, for example, about 50 to about 300.degree. C. and a
 pressure of, for example, about 1 to about 500 mm Hg absolute. The
 evaporation step yields an overhead product containing, for example, about
 10 to about 50 wt. % of GBL, about 1 to about 50 wt. % of butyric acid,
 and about 1 to about 30 wt. % of other compounds having boiling points
 near or lower than GBL; and a residue often containing heavy metals,
 succinic acid and succinic anhydride among other compounds having higher
 boiling points than GBL, which is generally burned for fuel.
 Since succinic anhydride has a melting point of 120.degree. C., it tends to
 solidify and foul equipment downstream of the vacuum evaporation step if
 allowed to remain in the overhead from the vacuum evaporation step in any
 significant quantity. To prevent this, the evaporator is preferably
 operated to allow about 1 to about 20 wt. % of GBL to remain in the
 residue which, in effect, substantially prevents much of the succinic
 anhydride in the feed from vaporizing and leaving the evaporator as part
 of the overhead.
 Since the presence of butyric acid in the GBL-containing feed to the
 azeotropic distillation column would interfere with the separation
 process, it is necessary to remove the butyric acid from the overhead of
 the vacuum evaporator before the GBL-containing composition can be fed to
 the column. This may be accomplished by subjecting the overhead from the
 evaporator to the fractional distillation, e.g., in a HBu removal column
 containing, for example, about 10 to about 100 trays wherein the base
 temperature is in the range, for example, of about 170 to about
 250.degree. C. and the overhead temperature is about 100 to about
 190.degree. C., at atmospheric pressure and a reflux to distillate ratio
 of about 0.5 to about 1.0. The bulk of the HBu in the feed is removed in
 the column overhead which contains, for example, about 10 to about 99 wt.
 % of HBu, whereas most of the GBL in the feed is in the base product which
 contains, for example, about 50 to about 99 wt. % GBL and which is the
 feed to the azeotropic distillation column as previously described.
 Another component of the overhead of the HBu removal column is acetic acid
 which may be present in the range, for example, of about 0.1 to about 20
 wt. %. This amount is 2-3 times the amount in the overall feed to the
 process, e.g., the residue stream fed to the vacuum evaporator as
 previously described. The additional acetic acid originates from Michael
 adducts formed in earlier processing steps. Such adducts are known to
 decompose at elevated temperatures, e.g., that at the base of the butyric
 acid removal column, to form acetic acid and olefinic compounds, e.g.,
 acrylic or crotonic acid, or their esters which under certain
 circumstances tend to polymerize in the HBu removal column, the trays of
 which may become plugged or fouled by the resulting polymer. To avoid
 this, a small amount of an inhibitor of the polymerization of acrylic
 acid, e.g. phenothiazine (PTZ) may be added to the top of the column, e.g.
 enough to maintain a concentration of about 50 to about 100 wt. % in the
 reflux.
 Another source of GBL-containing feed to the azeotropic distillation column
 is compositions of varying GBL content resulting from the preparation of
 GBL by the Reppe reaction of acetylene and formaldehyde.
 The purification steps described hereinbefore may be operated as
 continuous, semi-continuous, or batch operations, separately or together.
 However, it is preferred that the azeotropic distillation of the invention
 and any of the other described operations found to be desirable in the
 production of pure GBL, be carried out as a continuous, integrated,
 overall process.
 EXAMPLE
 The following example further illustrates the invention and describes, with
 reference to the drawing, the preparation of specification grade GBL from
 a heavy GBL-containing residue obtained as a result of the purification of
 products of the liquid phase oxidation of n-butane with molecular oxygen,
 including the azeotropic distillation of a GBL-containing stream,
 utilizing n-octane as azeotroping agent. The residue contains 26 wt. %
 GBL, 0.71 wt % MeGBL's, 18 wt. % HBu, 0.21 wt. acetic acid 2.54 wt. %
 succinic acid and the remainder a large variety of compounds, mostly
 unidentified, and is fed through line 1 to rotary vacuum evaporator 2
 which operates at a pressure of 0.75-3 mm Hg absolute at 108-109.degree.
 C. and a residence time of about 77 min. The residue containing 2 wt. % of
 GBL, 5 wt. % of succinic anhydride, and the remainder largely unidentified
 heavy metals and compounds having higher boiling points than GBL, is taken
 off through line 3 and burned for fuel, and the overhead comprising 42 wt.
 % GBL, 1.2 wt. % of MeGBL's, 25 wt. % of HBu, 0.25 wt. % of acetic acid,
 0.85 wt. % of crotonic acid, 0.31 wt. % of furanone, 1.8 wt. % of succinic
 anhydride and the remainder mostly unidentified compounds having boiling
 points near or lower than GBL, is fed through line 4 to an intermediate
 tray of HBu removal column 5 which is a 40 tray, 2 inch Oldershaw column
 operating at a base temperature of about 213.degree. C., an overhead
 temperature of 165.degree. C., a pressure of 1 atmosphere and a reflux
 ratio of 0.58. An amount of polymerization inhibitor phenothiazine (PTZ)
 is added to the top of the column through line 6 to maintain a
 concentration of PTZ in the reflux of about 50 ppm.
 The overhead from HBu removal column 5 containing 66 wt. % of HBu, 0.79 wt.
 % of GBL, 2.81 wt. % of acetic acid, 3.19 wt. % of acrylic acid, 3.23 wt.
 % of crotonic acid with the remainder mostly unidentified compounds having
 boiling points lower than GBL, is withdrawn from line 7 and sent to HBu
 recovery, while the base product from column 5 containing 63 wt. % of GBL,
 2 wt. % of MeGBL's, 0.5 wt. % of furanone, and 0.2-0.5 wt. % of crotonic
 acid, withdrawn through line 7A is combined with the base product from
 vacuum distillation finishing column 22 (hereinafter described) in line
 29, with the combined stream being fed through line 8 to azeotropic
 distillation column 9.
 Since the mass of the material fed to HBu removal column 5 is about four
 times the amount of overhead withdrawn from the column, it can be
 calculated that the amount of acetic acid leaving the column is more than
 twice as much as that entering the column. Moreover, significantly greater
 amounts of olefinically unsaturated compounds acrylic acid, crotonic acid
 and furanone leave the column than enter it. An assumption can therefore
 be made that the additional acetic acid and unsaturated compounds are
 formed as a result of the thermal decomposition of Michael adducts in the
 feed which occurs at a temperature of about 213.degree. C. at the base of
 the column. This is confirmed by the occurrence of fouling of the top of
 the column by a substance resembling polymers of acrylic and/or crotonic
 acids. Such fouling is minimized or prevented by the addition of PTZ
 inhibitor to the column through line 6 as previously described.
 Azeotropic distillation column 9 is constructed of 2 inch Oldershaw
 sections totaling 40 trays. The GBL-containing feed to column 9,
 previously described, is fed to tray 20 of the column, while the bulk of
 azeotroping agent n-octane containing 1.2 wt. % of dissolved GBL, which is
 the top layer in decanter 10 (hereinafter described) is recycled by way of
 lines 11 and 12 onto tray 25 of column 9, with a small amount of n-octane,
 i.e., only large enough to maintain liquid on the top trays of the column,
 being fed as reflux through lines 11 and 13 to the top of the column. The
 weight ratio of n-octane to feed entering column 9 is in the range of 18:1
 to 23:1 and the column operates at atmospheric pressure and temperatures
 of 217.degree. C. in the reboiler, 132.degree. C. where the recycled
 n-octane is introduced, 127.degree. C. at tray 20 where the feed enters
 and 125.degree. C. at the top of the column. The trays above the octane
 recycle point are filled mostly with octane and those below the feed point
 are filled primarily with high boiling material.
 The distillate from the top of column 9 including an azeotrope of GBL and
 n-octane, flows through line 14 to decanter 10, where it is cooled by
 chilled water, with the distillate settling into two phases, the top phase
 being primarily n-octane containing 1.2 wt. % of dissolved GBL which is
 fed back to column 9 as recycle and reflux through line 11 and then
 through lines 12 and 13 respectively, as described, and the bottom phase
 being a crude GBL containing 79-83 wt. % GBL, 0.4-0.9 wt. % MeGBL's,
 0.5-0.7 wt. % of furanone, 0.9-1.3 wt. % of crotonic acid, 1.9-2.5 wt. %
 of n-octane, and the remainder various mostly unidentified compounds. The
 substantially lower weight percent of MeGBL's in the crude GBL product
 from azeotropic distillation column 9 as compared with that in the feed to
 such column indicates the effectiveness of the azeotropic distillation
 process of this invention in reducing the content of the MeGBL's in the
 GBL product despite the closeness of the boiling points of GBL and the
 MeGBL's.
 A small amount of residue from azeotropic distillation column 9, containing
 26.8 wt. % of GBL, 2.8 wt. % of MeGBL's, 7.5 wt. % succinic anhydrids and
 the remainder various mostly unidentified compounds, is withdrawn through
 line 15 and recycled to vacuum evaporator 2.
 The crude GBL product from azeotropic distillation column 8 obtained as the
 lower liquid layer in decanter 10 is fed through line 16 to the top of
 hydrogenation reactor 17 which is a 2 foot section of 2 inch stainless
 steel pipe packed with a 20 inch bed of 0.5% Pd on carbon hydrogenation
 catalyst, operating at a hydrogen pressure of 85 psig, a temperature of
 152-154.degree. C., a hydrogen feed rate of 194 SCFI/1000 lb feed fed
 through line 17A to the bottom of reactor 17, a hydrogen vent rate of 32
 SCF/1000 lb feed flowing through line 17B from the top of reactor 17 and a
 hydrogen consumption rate of 191 SCF/1000 lb feed. The hydrogenated
 product is withdrawn from the bottom of reactor 17 through line 18 and
 contains 84 wt. % of GBL, 0.5 wt. % of MeGBL's, 7 wt. % of saturated
 organic acids, 0 wt. % of furanone, 0-0.02 wt. % of crotonic acid, 1.7-2.5
 wt. % of n-octane and the remainder various other compounds, most of which
 are unidentified as to formula but have been determined to have boiling
 points lower than that of GBL. The substantial elimination of furanone and
 crotonic acid from the hydrogenated product is an indication that most of
 the other olefinically unsaturated impurities, including those which cause
 the development of undesirable color in the GBL product, have also been
 eliminated.
 The hydrogenated product from reactor 17 flowing through line 18 is
 combined with the hydrolyzed product from hydrolyzer 19 (discussed
 hereinafter) in line 20 to produce a stream containing 97.5 wt. % GBL, 0.5
 wt. % MeGBL's, 1.75 wt. % water, 0.17 wt. % of high boiling esters and the
 remainder small amounts of other compounds including acid and alcohol
 hydrolysis products formed in hydrolyzer 19. Such stream is fed through
 line 21 to tray 35 of vacuum distillation finishing column 22 which is a
 50 tray, 2 inch Oldershaw column operating at a pressure of 100 mm Hg
 absolute and a temperature of 98.degree. C. at the top tray and a pressure
 of 148 mm Hg absolute and a temperature of 148.degree. C. at the bottom
 tray. The feed point at tray 35 is at a pressure of 111 mm Hg absolute and
 a temperature of 138.degree. C. The main product from the column is
 withdrawn as a sidestream through line from tray 10 which is at a pressure
 of 131 mm Hg absolute and a temperature of 143.degree. C., and is
 specification grade GBL containing 99.5 wt. % of GBL, 0.45 wt. % of
 MeGBL's, 0.02 wt. % of water and 0.04 wt. % of high boiling esters, i.e.,
 having boiling points higher than that of GBL.
 To accomplish the reduction of high boiling esters in the product from 0.17
 wt. % present in the feed to finishing column 22 to 0.04 wt. % in the
 product from the column, about 1/4 of the product stream in line 23 is
 withdrawn as final product, while 3/4 is mixed with 2.5 wt. % of water and
 is passed through line 24 into the top of hydrolyzer 19, containing 75
 grams of dry Amberlyst 36 acidic cationic exchange resin having a volume
 of 0.10 L as hydrolysis catalyst, the hydrolyzer operating at about
 70.degree. C. and a space velocity of 900 g/l-hr. The hydrolyzed product
 from which substantially all high boiling esters have been removed, but
 which now contains hydrolysis acids and alcohols, is then recycled through
 20 to finishing column 22 where the hydrolysis products are substantially
 removed. The hydrolysis and recycling of part of the sidestream product to
 finishing column 22 at a fairly high ratio of hydrolyzed and recycled
 product to withdrawn product, e.g. 3:1, accomplish a substantial steady
 state reduction of high boiling esters present in the feed to finishing
 column 22 by means of hydrolysis and the removal of the resulting
 hydrolysis products.
 In addition to the main sidestream product, overhead vapors from the top of
 finishing column containing 27.6 wt. % GBL, 0.14 wt. % of MeGBL's, 68 wt.
 % of water, 2.5 wt. % of n-octane and the remainder other compounds
 including some hydrolysis product formed in hydrolyzer 19, are condensed
 in line 25, the resulting liquid distillate collected in decanter 26 and
 the bulk of the distillate returned as reflux to the top of finishing
 column 22 through line 27 at a reflux/distillate ratio of 81.4. That
 portion of the distillate not returned to the column as reflux is
 withdrawn from decanter 26 through line 28 and is burned for fuel.
 Also obtained from finishing column 22 is a residue stream comprising 98.2
 wt. % of GBL, 0.5 wt. % of MeGBL's, 0.02 wt. % of water, and 1.3 wt. % of
 other compounds, some of which are acid and alcohol hydrolysis products
 formed in hydrolyzer 19. The residue is recycled through line 29 as feed
 to azeotropic distillation column 9.