Solvent scrubbing recovery of lactide and other dimeric cyclic esters

An improved process for the recovery of lactide or other dimeric cyclic ester from a gas stream containing the cyclic ester and such hydroxylic impurities as water and open-chain hydroxycarboxylic acids by scrubbing the gas stream with a nonpolar water-immiscible solvent at a temperature at which the cyclic ester is removed from the solvent and any water present in the gas stream is vaporized from the solvent. The cyclic ester is recovered from the solvent in a high state of optical purity.

FIELD OF THE INVENTION 
This invention relates to a solvent scrubbing process for the recovery of 
dimeric cyclic esters from impure reaction product streams. In particular 
the present invention relates to such recovery of the cyclic esters from 
gas product streams also containing water as an impurity by scrubbing with 
a non-polar water immiscible solvent, more particularly counter currently, 
at a temperature at which the cyclic ester is soluble in the solvent and 
any water present in the gas stream is vaporized from the solvent. 
BACKGROUND OF THE INVENTION 
The preparation of dimeric cyclic esters of alpha-hydroxycarboxylic acids 
is an old and much studied process. The preparation is normally conducted 
in two stages involving first preparing an oligomer of the 
hydroxycarboxylic acid, i.e., a relatively short-chain condensation 
polymer thereof, then heating the oligomer to depolymerize it to the 
desired cyclic ester. The preparation of dimeric cyclic esters is 
discussed in Gruter et al., U.S. Pat. No. 1,095,205 (1914); Lowe, U.S. 
Pat. No. 2,668,162 (1954); Bhatia, U.S. Pat. No. 4,835,293 (1989); Bellis 
U.S. Pat. No. 4,727,163 (1988); Muller, Ger. Pat. Application Publication 
Nos. 3632103 and 3708915 (1988). In the preparation of the oligomers from 
the corresponding alpha-hydroxycarboxylic acids the water of condensation 
is difficult to completely remove from the polymer. Water is also formed 
in the depolymerization step so that the cyclic ester depolymerization 
product generally contains water as an impurity. The cyclic ester may also 
contain one or more open-chain hydroxycarboxylic acids as impurities. All 
such hydroxylic impurities are undesirable as they act as chain-stoppers 
in the subsequent polymerization of the cyclic ester to the high molecular 
weight products required for biomedical and other uses. It is therefore 
desired to keep the water and open-chain hydroxycarboxylic acid content of 
the dimeric cyclic ester as low as practicable. 
U.S. Pat. No. 4,835,293, to Bhatia discloses an improved depolymerization 
and product recovery process for the production of dimeric cyclic esters 
such as lactide wherein a stream of an inert gas is employed to strip the 
cyclic ester from the reaction zone along with any water and/or volatile 
hydroxycarboxylic acid also formed therein. The resulting gaseous product 
stream is scrubbed with a polar organic solvent to recover the cyclic 
ester. The solvents include alcohols, ethers, esters and ketones, with use 
of isopropyl alcohol exemplifying the recovery of glycolide from its 
impurities. Isopropyl alcohol as scrubbing solvent solubilizes the 
hydroxycarboxylic acids and any water present, thereby enabling the 
recovery of glycolide directly from the scrubbing medium as a 
substantially insoluble filterable crystalline solid. 
Use of an alcohol, however, as the scrubbing solvent for the recovery of 
glycolide, lactide or other such cyclic ester from a vapor product stream 
is not entirely satisfactory. It as well as water can react in the 
alcoholic solution to form open-chain products, which not only constitute 
a yield loss but further tend to increase the solubility of the cyclic 
ester in the scrubbing solution, further aggravating the yield loss 
problem. 
On the other hand, use of a non-hydroxylic polar scrubbing solvent such as 
acetone, for example, which is non-reactive towards dimeric cyclic esters 
and in which the esters are highly soluble, likewise presents difficulties 
inasmuch as such polar solvent solubilizes the by-product 
hydroxycarboxylic acids as well, so that further processing would be 
required to separate the cyclic ester from the acids. 
Water as a scrubbing solvent is also unsatisfactory in that heat transfer 
to it is much faster then mass transfer; consequently, the cyclic ester 
precipitates as a fog of particles, difficult to capture in the absence of 
specialized and costly equipment. 
Thus, a need exists for a means that provides for the substantially 
complete recovery of a cyclic ester such as lactide from a vapor stream 
that also contains open-chain acids as well as water. A need also exists 
for such process that also provides for the substantially complete 
recovery of the acid values as cyclic ester. 
SUMMARY OF THE INVENTION 
In the gas-assisted process for depolymerizing an oligomeric 
poly(hydroxycarboxylic acid) to a dimeric cyclic ester, which process 
comprises: 
(1) heating the oligomer in a reaction zone at a suitable temperature and 
pressure and for a time effective to generate the cyclic ester, 
(2) passing a stream of an inert gas through the oligomeric material at a 
rate and in an amount sufficient to sweep the cyclic ester and any water 
present from the reaction zone and to form a gas stream containing the 
cyclic ester and any water entrained therewith, and 
(3) scrubbing the gas stream with a solvent to remove the cyclic ester 
therefrom, the improvement wherein: 
(a) the solvent is nonpolar and water-immiscible, and preferably is a 
solvent for the cyclic ester at least at one temperature and is a 
non-solvent for water at said temperature, 
(b) scrubbing step (3) is conducted at a first temperature which is below 
the temperature of (a) and at which the solvent is liquid, the cyclic 
ester is removed from the gas stream and the temperature is such that any 
water removed from the gas stream forms an aqueous phase separate from the 
solvent phase containing cyclic esters, and 
(c) the solvent phase containing the cyclic ester is recovered. 
In a preferred embodiment, the solvent is selected such that the cyclic 
ester is soluble and water substantially insoluble therein at the first 
temperature and the cyclic ester is a solid substantially insoluble in the 
solvent at a second, lower temperature. The solvent-cyclic ester phase is 
separated from the aqueous phase at the first temperature. The 
solvent-cyclic ester phase is cooled to the second, lower temperature to 
precipitate the cyclic ester and the precipitate is recovered, as by 
filtration or centrifugation. 
In another preferred embodiment, the scrubbing temperature is such that the 
water is thus vaporized at said temperature and removed from the solvent. 
In still another embodiment the scrubbing step is carried out 
counter-currently. 
In other, more specific embodiments the oligomer is a relatively low 
molecular weight polymer of glycolic and/or lactic acid, including aqueous 
lactic acid such as the 80-90% acid available commercially, and the 
scrubbing solvent is an aliphatic, cycloaliphatic, aromatic hydrocarbon or 
halocarbon, preferably boiling in the range of from about 90.degree. to 
about 200.degree. C. 
DETAILED DESCRIPTION OF THE INVENTION 
The invention is applicable to the treatment of impure dimeric cyclic 
esters containing hydroxylic impurities such as water and open-chain 
hydroxycarboxylic acids. It is particularly applicable to the treatment of 
a vapor stream containing the impure cyclic ester, more particularly where 
the impure cyclic ester is a lactide composition. The invention process 
broadly comprises contacting a gas stream containing a dimeric cyclic 
ester as defined above and water, as an impurity, with a non-polar solvent 
as defined above in an amount and at a temperature at which the cyclic 
ester is removed from the gas stream and the water is volatized therefrom, 
thereby effecting separation of the cyclic ester from the water impurity. 
Preferably the solvent is such that the cyclic ester is soluble therein. 
The cyclic ester is then separated from the solvent by any means known to 
the art, including evaporation of the solvent or crystallization of the 
cyclic ester from the solvent followed by filtration or centrifugation. 
The gas stream containing the impure dimeric cyclic ester may be that 
generated in a gas-assisted depolymerization process as described by 
Bhatia in U.S. Pat. No. 4,835,293, which disclosure is incorporated herein 
by reference. In general such gas-assisted depolymerization process 
comprises heating an oligomer of an alpha-hydroxycarboxylic acid, (e.g. 
glycolic, lactic or mixed glycolic and lactic acids), to a temperature at 
which the oligomer is molten and depolymerizable to the corresponding 
dimeric cyclic ester, usually and preferably in the presence of a 
depolymerization catalyst, while passing an inert gas through the molten 
oligomer in an amount and at a rate sufficient to entrain the cyclic ester 
from the reaction mass, preferably as fast as it is formed. The resulting 
gas product stream normally also contains water and other volatile 
materials such as open-chain carboxylic acids. 
In accordance with the present invention, a gas stream generated in the 
above referenced depolymerization process is scrubbed with a non-polar 
organic solvent as defined above in order to remove the cyclic ester from 
the gas stream and thereby separate the cyclic ester from any water 
present in the stream. The scrubbing solvent may be any normally liquid 
substance that is non-polar, is a non-solvent for water at the operating 
temperature and has a normal boiling point of at least about 90.degree. 
C., preferably at least about 130.degree. C., more preferably at least 
about 150.degree. C. but practically speaking not greater than about 
230.degree. C., preferably not greater than about 200.degree. C. 
"Non-solvent" for water at the operating temperature refers to a solvent 
from which water will flash off and pass out of the scrubbing zone as a 
vapor stream. Preferably the solvent is selected such that the cyclic 
ester is soluble at the operating temperature and substantially insoluble 
at temperatures substantially below the operating temperature, e.g. at 
room temperatures to facilitate the recovery of the cyclic ester 
therefrom. 
Suitable to this purpose are aliphatic, cycloaliphatic, aromatic 
hydrocarbon and halocarbon solvents exemplified by heptane, decane, 
decene, methylcyclohexane, toluene, o-,m- and p-xylene, cumene 
(isopropylbenzene), ethylbenzene, o-, m- and p-diethylbenzene, n-, sec- 
and isobutylbenzene, m-propyltoluene, p-propyltoleune, 
1,2,4-trimethylbenzene (pseudocumene), chlorobenzene, o-, and 
m-dichlorobenzene, 1,2,4-trichlorobenzene and mixtures of one or more 
thereof. The aromatics are preferred for their greater solvency for the 
cyclic esters. 
The operating temperature, that is, the temperature at which the scrubbing 
medium is maintained during the cyclic ester stripping operation, can vary 
widely depending on the particular cyclic ester and solvent involved. The 
temperature of the liquid scrubbing medium should be at least about as 
high as the melting point of the cyclic ester being recovered to reduce 
the possibility of the scrubber becoming clogged with solids. The 
temperature should also be sufficiently high to drive the water overhead 
and preferably solubilize the cyclic ester substantially completely. At 
the same time it should be lower than the boiling point of the cyclic 
ester as well as of the scrubbing solvent to avoid loss of the ester in 
the aqueous phase overhead. Preferably, it will be 15.degree. below the 
solvent's boiling point, more preferably at least 25.degree. below the 
boiling point. The temperature will normally be at least 70.degree. and 
not more than about 180.degree. C., more usually from about 90.degree. to 
150.degree. C. 
The pressure throughout the scrubbing step may vary from sub-atmospheric to 
atmospheric and super-atmospheric. Conveniently and preferably it will be 
about atmospheric pressure. 
The process of the present invention is applicable to the recovery of 
dimeric cyclic esters having the following formula: 
##STR1## 
where each R group is independently H or a C.sub.1 -C.sub.6 hydrocarbyl or 
substituted hydrocarbyl radical. Preferably each R group is H or a C.sub.1 
-C.sub.3 alkyl group, more preferably H or methyl. Typical dimeric cyclic 
esters include glycolide (R.sub.1 .dbd.R.sub.2 .dbd.H, lactide (R.sub.1 
.dbd.H, R.sub.2 .dbd.CH.sub.3 tetramethylglycolide, sym-diethylglycolide, 
the dimeric cyclic ester of alpha-hydroxyvaleric acid and the like. 
Preferred cyclic esters are glycolide, lactide (including L-, D- and 
meso-lactide) and mixtures of glycolide with one or more of the isomeric 
lactides. 
The steps of polymerizing the alpha-hydroxycarboxylic acid to an oligomer 
and of depolymerizing the oligomer to a cyclic ester are ordinarily and 
preferably conducted in the presence of a catalyst. The catalyst can be 
any of those known in the art for promoting condensation of the 
alpha-hydroxycarboxylic acid to oligomers and for promoting cyclic ester 
formation. The catalysts are generally metals or compounds of metals of 
groups IV, V and VIII of the Periodic Table. Preferred catalysts are 
metals of groups IV, notably Sn as the metal (powdered), oxide, halogenide 
or carboxylate, or V, notably Sb, usually as the oxide Sb.sub.2 O.sub.3. 
Particularly preferred catalysts are Sn(II) carboxylates, exemplified by 
Sn bis(2-ethylhexanoate), commonly referred to as stannous octoate. 
The catalyst is employed in catalytically effective amounts, which can vary 
widely depending upon reaction conditions. The optimum catalytically 
effective amounts for any particular system can readily be determined 
through trial runs. 
The gaseous agent for entraining/carrying/ sweeping the cyclic ester and 
the impurities out of the reaction mixture and out of the depolymerization 
reactor may be any substance that is gaseous, stable and non-reactive at 
the operating temperatures and pressures and is inert to the starting 
material, reaction mass components and reaction products. It may be 
normally gaseous, such as nitrogen, argon, carbon monoxide or dioxide or 
low molecular weight hydrocarbon. It may be normally non-gaseous but 
gaseous at reaction temperature and pressure. Preferred is nitrogen for 
its inertness and ready availability. Preferably the inert gas will be 
preheated to the operating temperature and will be injected below the 
surface of the reaction mass in the reaction zone; for example, below the 
agitator of a stirred tank reactor or at the bottom of a vertically 
disposed reactor. 
The flow rate of the gas should be sufficiently high so as not to limit the 
cyclic ester stripping rate. If the flow rate is too low, the conversion 
to cyclic ester may be adversely affected and its production rate limited 
since the gas functions importantly to carry the cyclic ester as vapor out 
of the reactor. 
The depolymerizer reactor design is not critical provided the reactor has 
means for introducing an oligomeric feed stream, means for introducing a 
gaseous cyclic ester stripping agent into the reaction zone such that the 
stripping agent directly and intimately contacts the oligomeric 
composition so as to give high gas-liquid interfacial contact and has 
means for removing a gaseous stream containing cyclic ester. Thus, the 
reactor may be a stirred tank equipped with gas-sparging means, preferably 
one which admits the gas directly under the agitator. The reactor may also 
be a packed or sieve-plate column, or the reactor may be of any other 
design known in the art for effecting intimate gas-liquid contact. 
Preferably, the depolymerization step is conducted in a continuous manner 
with the oligomer being fed continuously to the reactor at a controlled 
rate such that hold-up of polymeric material within the reactor is 
minimized. Continuous depolymerization of the oligomer minimizes 
degradation of the oligomer into any unwanted by-products and maximizes 
conversion of the oligomer into the desired cyclic ester. Thus, treating a 
gas stream from such a continuous depolymerization process by the present 
invention method would yield a still higher quality cyclic ester product.

EXAMPLES 
The following examples were conducted in an apparatus comprising a 
gas-assisted depolymerization unit in association with a counter current 
scrubbing unit. 
The depolymerization unit consisted essentially of a stirred 1000 ml tank 
having a gas inlet terminating at a point below the stirrer blade and a 
gas exit line leading to a 1" by 16" scrubbing column and entering the 
column just below its midpoint. The column surmounted a 1000 ml first 
receiver vessel and was connected at its upper end to a 4" head leading to 
downwardly arranged water cooled condenser emptying into a vented second 
reciever. The first receiver was fitted with an external tubular means 
leading via a pump to the top of the column so that liquid from the first 
receiver could be circulated up to the top of the column and allowed to 
flow downwardly through the column in contact with upcoming vapor stream. 
The column was packed with short sections of glass tubing below the gas 
inlet point and with glass beads above it. The stirred tank, the gas inlet 
and outlet, the column, the first receiver and the liquid circulating line 
were all fitted with thermocoupled heating means (mantles and tapes) for 
maintaining temperatures throughout the system. The head at the top of the 
column was unheated. Temperatures where noted are in degrees Celsius. 
EXAMPLE 1 
A. Lactic Acid Oligomer Preparation 
750 grams of 88% L-lactic acid containing 2.5 g of stannous octoate was 
gradually heated under agitation while a stream of N.sub.2 gas preheated 
to 100.degree. C. was passed through it at a rate of 1500 standard cubic 
centimeters per minute (sccm). The temperature reached 92.degree. in about 
30 minutes and water started coming over. After 120 min at 92.degree. the 
temperature was raised to 120.degree. and held there for 160 min longer 
before being increased to 140.degree.. After 75 min at 140.degree. when 
lactide began to be evolved, the temperature was rapidly increased to 
170.degree. and held for about 5 min. A total of 190.1 g of water was 
collected during the above heating period. 
B. Gas-Assisted Depolymerization and Counter Current Scrubbing Removal of 
L-Lactide from the Gas Product Stream. 
The polylactic acid (oligomer) from (A) was heated to 215.degree. C. while 
a stream of N.sub.2, preheated to 140.degree. C. was passed through it at 
a rate of 1500 sccm. Cumene was fed into the N.sub.2 stream at a rate of 
0.4 cc/min. over a 3 hr period. During this time the gas feed line to the 
column was held at 104.degree.-110.degree., the first receiver at 
110.degree.-120.degree., the circulating line at 99.degree.-100.degree., 
the column section below the gas feed line at 103.degree.-105.degree. and 
the section above the gas feed line at 114.degree.-130.degree. C. 
(sufficiently high to ensure that any water in the column would be in the 
vapor state but not so high as to completely vaporize the cumene). The 
temperature in the unheated column head was 92.degree.-96.degree.. 
After the first and second hours of operation about 200 cc of cumene was 
added each time to the first receiver to ensure the presence of sufficient 
cumene for counter current flow down through the column. 
The lactide that accumulated in the first receiver as a cumene solution was 
recovered by cooling the solution to room temperature. The resulting 
off-white precipitate was filtered, washed twice with isopropyl alcohol 
and dried under reduced pressure to yield highly pure white crystalline 
L-lactide (52 g) having an optical rotation of -297 versus -300 reported. 
The cumene filtrate was water-free but yellow in color, indicating the 
presence of unsaturated decomposition products formed in the 
depolymerization step. The residue in the cracking pot was amber in color 
and weighed 366 g. The second receiver contained a two-phase mixture of 
water and cumene. 
EXAMPLE 2 
This example illustrates a process wherein aqueous lactic acid is 
dehydrated, converted to oligomer, the oligomer is depolymerized to 
lactide in a stream of carrier gas and the lactide depolymerization 
product is counter currently stripped from the gas product stream and 
recovered as a bottoms solution in the stripping solvent while water is 
removed as overhead. 
The procedure of Example 1 was repeated except that the L-lactic 
acid-stannous octoate composition was heated in the depolymerization unit 
described above such that the lactic acid dehydration and oligomerization 
steps were conducted as essentially described in Section A above and the 
depolymerization step was conducted essentially as in Section B above. The 
optical rotation of the L-lactide product was -294.degree.. 
It will be noted that feeding the scrubbing solvent in the gaseous 
entraining agent as exemplified above aids into stripping the cyclic ester 
from the oligomeric material as well as providing recirculating liquid for 
countercurrently stripping the cyclic ester from the gas product stream. 
However, it can be omitted from the gas feed stream provided the first 
receiver contains sufficient scrubbing solvent to be recirculated up to 
and down the column so as to countercurrently contact the upcoming gas 
stream containing the cyclic ester and its hydroxylic impurities. 
EXAMPLE 3 
L-lactic acid oligomer was prepared from 752.4 grams of 88% L-lactic acid 
essentially as described in Example 1, part A above; 195 grams of water 
were collected. The oligomer was heated under agitation at 
216.degree.-230.degree. C. for about 2.5 hours with a stream of N.sub.2 
passing through it at a rate of 0.35 standard cubic foot per minute, 
during which time the gas stream was passed into a 1000 ml receiver 
containing charge of toluene as the scrubbing medium and surmounted by a 
water-cooled condenser. An additional 300 ml of toluene was added to the 
scrubber after 1 hour followed by 500 ml of toluene 0.75 hour later to 
compensate for vaporization of solvent from the receiver. The temperature 
of the gas stream ranged from a brief (0.25 hour) low of 95.degree. at the 
beginning through a long (almost 2-hour) range of 128.degree.-156.degree. 
C., mostly above 140.degree. C., to a brief (about 0.25 hour) final low 
of 121.degree. C. The temperature of the toluene scrubbing medium during 
this time ranged from an initial 83.degree. C. through a high of 
127.degree. C. to a final 93.degree. C. 
The toluene solution was cooled to below room temperature and filtered. The 
filter cake (199.5 g) was washed with toluene and dried under reduced 
pressure (183.7 g dry weight). The dry product had a purity of 97.8 
determined by differential scanning calorimetry (DSC).