CA1121829A - Continuous manufacture of allyl diglycol carbonate - Google Patents

Abstract

-I-ABSTRACT OF THE INVENTION
A continuous process for the manufacture of allyl diglycol carbonate (ADC) from diethylene glycol bis-chloro-formate (Bis CF), allyl alcohol (AA) and alkali metal hydroxide produces a pure product of good quality, high yields and high throughput per unit volume of the reactor, The invention allows the recycle of an aqueous alcohol waste solution in a concentration below the azeotropic point by simple dis-tillation by employing a more concentrated alkali metal hydroxide and an allyl allele aqueous solution. The process comprises at least two reaction zones, a continuous physical separation zone for separating the high purity product and a distillation column for reclaiming the aqueous allyl alcohol solution below the azeotropic concentration.

Description

18~9 Continuous Manufacture of AIIY _iqlYCol Carbonate (IR 2341) Backqround of the Invention Field of the Invention s The present invention relates to a process and apparatus for the continuous manufacture of allyl diglycol carbonate ~ADC) in high purity and at high throughput per unit reactor volume.

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- 2 -State of the Prior Art Allyl diglycol carbonate, (CH2=CH-cH2-O-C-O-CH2-cH2 ) 2 ~

S has gained considerable importance as the preferred monomer for use in preparation of hard safety lenses and plastic sheets which are scratch resistant and tough. Certain important requirements for these end uses are that the monomers be obtained in high purity, consistent in quality and low in 10 chlorides and volatiles. Variations in quality, especially viscosity, between lots can lead to a high rejection rate of polymerized cast sheets and lenses. High chloride content ln starting monomer will impart a tint to the finished article on storage.
An example of the prior art is U, S. Patent No. 2, 370, 569 which describes a batch process for making allyl diglycol carbonate This batch process, however, is subject to variations from batch to batch. Since the preparation of ADC is a highly exothermic reation, a number of difficulties 20 arise in the batch preparation thereof. The reaction mass is often viscous which further decreases the efficiency of heat transfer surfaces. To improve the heat loss from reaction . .`

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mass to cooling fluid, the temperature of the cooling fluid may be reduced. It may not be lowered, however, below the freezing point of the bis-chloroformate and the allyl alcohol solution (about -8~C) or that of the caustic; if the temperature 5 is lowered, a solid phase appears on the cooling surfaces reducing the efficiency. If higher reaction temperatures are used, the relative yield of side reactions are also increased.
' This too leads to impure product or lower yields. Higher temperatures can also increase the rate of ionic polymeriza-10 tion of the monomer. Hence, because of the above-men-tioned problems, the optimum reaction designed for batch operations involves the use of large heat tran~fer areas.
This is a good solution for small scale operations. However, for large scale commercial reactors, i. e., about 1, 000 15 gallons, the available tank surface for heat removal increases less rapidly than the heat load. The additional heat transfer area is provided as coils that impede the flow of viscous reaction mass over surface of other coils. The effectiveness of the heat transfer surfaces decrease as the number of coils 20 is increased. Another possible alternatlve is to lengthen the reaction period; this leads to a low monomer yield also.

i8;~9 The present invention overcomes the problems with batch processes of batch to batch variation, poor monomer yield on large scale" cyclic var~ation and heat load and need to handle larger quantities of obnoxious and potentially 5 harmful raw materials9 by providing a continuous flow of reactants and product throughout the process. The invention allows the recovery of the excess allyl alcohol in a concen-' tration below the azeotrope of the alcohol water mixture, 72. 3% by weight alcohol, for recycle to the reaction zone 10 without a complicated alcohol recovery system such as anextractive or azeotropic distillation. A single simple dis-tillation column is employed to recover most of the excess allyl alcohol in an aqueous solution used in manufacture of the ADC. The apparent disadvantage ln contlnuous processes lS of diluting raw materials with the product which results in lower reactant concentration should lead to lower reaction rates and yield of product. In the instant lnvention, however, this is not the case. On the contrary, it has been found that in practice the yield of product per unit volume 20 of reactor is substantially higher in the continuous process than in the batch process.
Another advantage of the present invention is the recycle of the allyl alcohol in a diluted form. The reaction ~ lBZ'3 to the formation of ADC favors a molar excess of alcohol that is later removed in the puriEication process. This diluted alcohol stream from the purification process and aqueous waste stream from the reaction procedure pose a 5 disposal problem since no simple method of alcohol recovery exists In a batch process, recycle of recovered allyl alcohol containing appreceable (30-60%) quantity of water would lead to hydrolysls of bis-chloroformate, and, as a conse-quence, higher impurity levels in the product and low yield.
10 In the continuous process, since water iB belng added with alkali metal hydroxide all the pure recycle of this water-alcohol stream is acceptable. The present ln~ention utilizes a simple alcohol recovery technique which allows recycle of the aqueous alcohol stream. By recovering the 15 alcohol for reuse, the environmental problems of disposal of a toxic chemical is eliminated Summary of the Invention The present invention concerns an improved process for the continuous manufacture of allyl diglycol carbonate 20 of the general formula:
O O
R~-O-c-~ORl)n-oc-o R2 ' .~ .
.

ii wherein:
(a) Rl is an aliphatic diradieal such as ethylene or propylene;
(b) R2 is an unsaturated aliphatic radical sueh as allyl or methallyl; and (c) n ls an integer of 1 to 3;
which comprises:
(I) eontinuously reaeting a bis-chloroformate having the general structure:

.11 11 Cl-C - (OR1)n-O -C-Cl, with an alcohol having the formula R2OH in the presence of an alkali metal hydroxide in at least two reaction zones of intense mixing conneeted in series at a temperature range of about -5C to +60C and a pH of greater than 7, wherein the first reaetion zone is maintained in the lower part of the temperature range and the seeond and subse-quent reaetion zones are slightly higher in temperature than the progressively maintained first reaetion zone.
20 (II) continuously reeovering the allyl diglyeol carbonate product by (i) continuously feeding the reaction mixture directly ~ I

from the last reaction zone through a liquid-liquid separation zone;
(ii~ continuously subjecting the product stream from the liquid-liquid separation zone to a countercurrent gas S stripping zone maintained at a temperature of from 50 to 150C and a pressure of 5 mm of Hg to atmospheric pressure with the pure product being removed from the bottom of the stripping zone;
(iii) continuously removing the gas stream from the stripping zone, condensing and distilling the con-densate to reclaim the allyl alcohol; and (iv) continuously adding the bis-chloroformate, aqueous alkali metal hydroxide and a portion of the reclaimed allyl alcohol or pure allyl alcohol to the first reaction zone to replace that used up in the reaction effluent.
The product obtained from the stripping column contains at least 88% of the product monomer. The aqueous solution of allyl alcohol that is recycled to the first reaction zone is reclaimed by continuously subjecting the dilute alcohol stream to a distillative unit maintained at a temperature from 20 to 100C and a pressure of 25 mm of Hg to atmo-spheric pressure. The recycle stream from the overhead of the alcohol recovery column contains between 40% by weight 82'~

to just below the azeotropic concentration of alcohol.
Detailed DescriPtion of the Invention It has now been discovered that allyl diglycol carbonate can be continuously produced at a high throughput 5 per unit reactor volume, safeLy and in high purity in an efficient process by employing at least two intensely mixed reaction zones and physical separation units such as a liquid-liquid centrifuge and stripping columns to isolate pure product and allow the recycle of the excess allyl alcohol.
10 The ADC is generated by continuously reacting a bis-chloro-formate, an alkali metal hydroxide and a pure or aqueous solution of a mono-hydric unsaturated alcohoi in at least two reaction zones of intense mixing in a temperature range of -5 to +60C and a pH greater than 7, The invention will be better understood by reference to the drawing. The accompanying drawing is a schematic plan view of one form of apparatus suitable for practicing this invention.
The reactants are continuously added from storage 20 tanks (2, 4 and 6) at controlled fLow rates via metering devices (8, 10 and 12) to first reactor (14j. The diagram shows the recycled alcohol stream 59 belng added to the pure alcohol in storage tank 4. However, it can be stored _ 9 _ and added to the reactor separately, The concentration of caustic and the start-up are modified to provide the same concentrations of water in feed and in reactor as those where no recycled alcohol is used. A major portion of the reaction 5 (normally 60 to 95%) conversion of the bis-chloroformate takes place in reactor (14) with the temperature of the reaction mixture being maintained within + 1C of that desired (normally -5 to 15C). The reaction mixture from reactor (14) then flows continuously to the second reactor (16) where a minor 10 portion of the reaction occurs and where the temperature is between -5 to +15C, preferably maintained at about 5 to 15C, since slight heating occur~ during the intense agitation and from reaction exotherm, Although two reactors are sufficient to substantially complete the reaction, the embodi-15 ment in the drawing is provided with three reactors to increaseresidence time or to reduce residual chlorides in the organic phase below a specified maximum, Hence, the reaction mixture from reactor 16 then flows continuously to the third reactor 18 where the remainder of the reaction occurs at a 20 temperature between +5 and 30C, preferably in the range of 10-20C. Each of the reactors (14, 16 and 18) is provided with efficient agitation means (5J 9 and 11) respectively to insure thorough mixing of the reactants and 11;~18'~9 cooling means (15, 17 and 19) to remove the heat of reaction, heat of the alkali metal hydroxide dilution and heat added due to the intense agitation. The diagram shows the method of heat removal as acketed cooling, although, any suitable S method of heat removal may be employed such as coils, external heat exchangers, etc. In order to distribute the heat load more evenly, the alkali metal hydroxide may be fed into two or more reactors such as from storage tank 7 through metering device 13 to reactor 16. Reactor 16 is also equipped 10 with a pH meter (3) to regulate the pH of the reaction mixture in the range of 8 to 14 and preferably ~10. Such a system insures the complete conversion of the bis-chloroformate to the desired carbonate product and is important since the - mono-hydric unsaturated alcohol may contain water, thereby 15 improving the product quality and yield considerably. The system also achieves excellent temperature control resulting in consistent product quality and improved safety.
The reaction mixture from reactor 18 may contain salts as a solid phase. In that instance, water is added to 20 dissolve the salts in dissolving means 20 that has agitating means 21 therein, If, on the other hand, the starting caustic is dilute, it is not necessary to add water; the reaction mass can be sent directly to the liquid-liquid separation unit 22 This flow determinatlon is regulated by the two-way valve 12.
Hence, the reaction mixture from reactor 18 is sent either to dissolving means 20 or directly to liquid-liquid separation unit 22. Hence, the reaction mixture from reactor 18 is then 5 continuously fed through separation unit 22 such as a liquid-liquid centrifuge or a gravity settling unit where the product is separated as stream 40 as the organic phase; the aqueous stream 41 transferred to holding vessel 44 for recovery. The organic phase 40 is then treated countercurrently with steam 10 or an inert gas or vapor (e. g. nitrogen) under reduced pres-sure in a stripping column 24. The column 24 is a conven-tional column containing plates or packing with either counter-current or crosscurrent movement of the stripping medium 25 to the crude product stream 40. The volatile 15 components of the organic phase 40 which include excessive reactant alcohol and reaction by-products are removed with the carrier` gas or vapor via line 34 to a suitable condenser 26. When steam is used as the carrying gas it is also con-densed in 26. The pressure in the column is maintained at 20 the desired low value by a vacuum pump 49 or other suitable means such as steam ejectors. The overhead condensate from 26 is sent to a liquid-liquid separation unit 45 where a separation of the organic and aqueous phase is made.

z9 The organic phase contains mainly diallyl carbonate (DAC) formed during the ADC reaction along with allyl alcohol.
Employing steam during stripping leaves an aqueous phase 46 containing allyl alcohol which is transferred to holding 5 vessel 44 for recovery. The pure desired product ls removed via 30 to a storage means that may be cooled. The tempera-ture range in the stripping column is normally in the range of 50 to 150C. The operating pressure in the column is maintained between 5 mm of Hg to atmospheric pressure, preferably 20 to 150 mm of Hg absolute, The organic stream 47 from the overhead is sent to a recovery unit 4~ for recovery of the allyl alcohol and the reaction by-product diallyl carbonate.
Recovery of the dilute solution of allyl alcohol and 15 water is undertaken in column ~2 from holding vessel 44.
The aqueous alcohol stream is heated to the desired tempera-ture by heat exchanger 51 and fed to a conventional dis-tillation column 52 containing plates or packing. ~ost of the allyl alcohol is recovered as an aqueous solution in the 20 overhead of the distillation column 52 in the concentration of 40% to slightly below the azeotropic concentration, The constant boiling mixture is 72. 3% by weight allyl alcohol at atmospheric pressure. The alcohol rich stream exits ~1~18~'~

through the top of the column and is condensed in heat exchanger 54 and collected in holding vessel S6. Part of the aqueous-alcohol stream is recycled to the column through line 58 while the remainder of the liquid contained in 56 is 5 recycled in stream 59 back to holding tank 4 for reuse in the manufacturing procedure. The diagram of the invention shows the recycle stream being added to the pure alcohol stream, however, this does not have to be the case. An individual holding tank containing the aqueous alcohol stream along 10 with a meterins device may be desired to more accurately control the amount of alcohol being added to reactor (14) since during the initial process ~tart up the concentration of the alcohol will be changing before equilibrium conditions are established. A reboiler 60 is included at the base of 15 the alcohol recovery column to add heat to establish a vapor flow through the stripping section of the column. Stream 64 is continuously withdrawn and discarded. Typically this stream may contain from 0. 5 to 0. 005% by weight alcohol depending on the number of theoretical stages contained in 20 the distillation column.
The bis-chloroformate used in the instant process should have an assay of at least 98. 0% preferably at least 99%. ~ow assaying chloroformates may be used if an ~ 18291 lnferior quality ADC is acceptable. Alkali metal hydroxide or potassium hydroxide solutions assaying at about 10 to 50% and preferably 30-50% by weight are desired as the , starting materials. The allyl alcohol should have an assay 5 of 97% and preferably 99% pure in the undiluted concentra-tion. Generally, the reactants are mixed in the ratio of 24 moles of alkall metal hydroxide to 25 moles of alcohol to 10 moles of bis-chloroformate. The molar ratio may be slightly different for optimum operating conditions depend-10 ing on the allyl diglycol carbonate being synthesized. Thestripping medium can be steam, nitrogen or other inert gases, but it is preferable to use steam because steam allows high capacity with a small vacuum pump and reduced costs. Conventional centrifuges of either the basket, bowl 15 or disc type can be employed in the system. The centrifuge used ln the examples infra ls the liquld-liquid centrifuge of the desludging disc type.
In order to carry out the ADC reaction in an efficient manner and to insure complete conversion of the chloro-20 formate, lt is necessary to bring the aqueous and organicphase into intimate contact by intense agitation of the reaction mixture in the reaction zones. This intense egitatlon resul~s ID the modiEication oi the reaction mass.

An added advantage of this intense mixing is the enhance-ment of the heat transfer rate from the aqueous organic system to the coolant in the coil and jacket, Hence, efficient dissapation of the heat of reaction with conse-quently improved temperature control and safety of operation are provided for in this invention.
The types of impellers that can be used for the intense agitation are a flat blade turbine, a pitch blade turbine or a marlne type propeller. The size of the reactor and the dimensions of the impeller determine the speed of agitation. For example, in a ten gallon reactor of standard configuration wherein liquid depth to reactor diameter ratio equals 2 to 1, if the diameter of the impeller is 4 to 5 inches, 1, 000 to 1, 400 revolutions per minute (RPM) give sufficient agitation. Conventional agitation would provide for up to 5 horse power (HP) per 1, 000 gallons. In the present invention, by using a reactor of standard config-uration, it has been found that an impeller tip speed of at least 1, 000 feet per minute and a power input in the range of 50 to 200 horse power per 1, 000 gallons is necessary to provide the intense mixing for high conversion and fast reaction, The arrangement of the reactors and the feed system enables independent control of the raw material flow rates into the reactor Also, in the reactors, the products of reaction themselves serve as a heat sink for the heat 5 generated by the reaction. By the efficient deployment of heat transfer surfaces in the present system, heat is immediately absorbed from the system. Such an arrange-ment significantly improves the production capacity (through-put per unit volume of reactor) of the reactor system over lO the ones of the prior art. By the judicial choice of the reactant concentrations in the first reaction zone, the con-version of the chloroformate to the ADC can be promoted while simultaneously minimizing chloroformate hydrolysis, Ieading to high yield of product based on chloroformate 15 consumed; thus making the present process commercially attractive.
The following examples are set forth to merely illustrate the invention but are not intended to limit the practice of this invention thereto. Flow rates are in parts 20 by weight per hour (pph) unless otherwise indicated.

Example I
Diethylene glycol bis-chloroformate, allyl alcohol and 40% aqueous caustic soda were continuously added to reactor (14) at the rates of 136. 7 gm/min (18. 1 pph), 85. 8 gm/min (11. 3 pph) and 88. 6 gm/min (11. 7 pph), respectively.
Simultaneously 30% aqueous caustic soda was added to reactor 16 at 71. 2 gm/min (9. 4 pph). The temperatures in the reactors were maintained in the range of 0-5C and only on an occasion were outside this range. Organic 10 stream (40) from separator flow at the rate of 166. 9 gm/min (22. 1 pph) and contained 129. 6 gm/min (17. 1 pph) of ADC
monomer. This represents a 79. 8% yield based on the bis-chloroformate. Chloride levels in the final product were - 11-14 parts per million. Good product quality including 15 monomer content would be obtained after stripping, ExamPle 2 For comparative purposes a batch operation of the prior art was run. In this batch reaction, 40. 2 lbs. of bis-chloroformate and 25. 3 Ibs. of allyl alcohol were 20 mixed in the reactor. To this 41. 8 Ibs. of 40% NaOH was added maintaining the temperature in the range 0-5C.
The reaction mass was stirred for another 15 minutes for the completion of reaction, After stirring, water was added 8~'9 to dissolve the salts and the or~anic phase was separated and assayed from the aqueous phase after settling. The yield was 79 8% basedon the bis-chloroformate. Total time to complete the reaction was approximately 170 5 minutes. It was determined that each gallon of reactor volume produced 0. 84 lbs. of ADC monomer per hour. The worlc was conducted in a reactor with an effective volume of 16 gallons. See Table A.
Example 3 This example was provided to show the feasibility of recycle of an aqueous allyl alcohol solution to the contin-uous process.
Diethylene glycol bis-chloroformate, 82. 9% allyl alcohol in water, and 50% aqueous caustic soda were metered to reactor (14) continuously at the rates of 135. 4 gm/min (17. 9 pph), 102. 1 gm/min (13. 5 pph) and 70. 6 gm/
min (9. 3 pph), respectively. 30% aqueous caustic soda was metered to reactor (16), simultaneously, at the rate of 70. 0 gm/min (9. 3 pph). Temperatures were maintained at 20 0-5C in the reactors. Total molar ratios to each reactor remained approximately the same as in Example 1 for com-parison. Caustic concentration to reactor (14) was adjusted to 50% to compensate for water entering with alcohol.

11~1829 Organic product (stream 40) from separator ~lowed at 162. 5 gm/min (21. 5 pph) and contained 130, 9 gm/min (17. 3 pph) ADC monomer representing an 80. 6% yield based on the bis-chloroformate. Product was stripped to acceptable 5 chloride and ADC monomer levels.

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Claims (7)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. An improved process for the continuous manufacture of allyl diglycol carbonate of the general formula:

wherein:
(a) R1 is an aliphatic diradical;
(b) R2 is an unsaturated aliphatic radical; and (c), n is an integer of 1 to 3;
which comprises:
(I) continuously reacting a bis-chloroformate having the general structure with an alcohol having the formula R2OH in the presence of an aqueous alkali metal hydroxide in at least two reaction zones of intense mixing connected in series at a temperature range of about -5°C
to +60°C and a pH of greater than 7, wherein the first reaction zone is maintained in the lower part of the temperature range and the second and subsequent reaction zones are progressively maintained in the upper part of the temperature range;
(II) continuously recovering the allyl diglycol carbonate product by (i) continuously feeding the reaction mixture directly from the last reaction zone through a liquid-liquid separation zone;
(ii) continuously subjecting the product stream from the liquid-liquid separation zone to a countercurrent gas stripping zone maintained at a temperature of from 50 to 150°C and a pressure of 5 mm of Hg to atmospheric pressure with the product being removed from the bottom of the stripping zone;
(iii) continuously removing the gas stream from the stripping zone, condensing and distilling the condensate to reclaim the allyl alcohol;
and (iv) continuously adding the bis-chloroformate, aqueous alkali metal hydroxide and a portion of the reclaimed allyl alcohol or pure allyl alcohol to the first reaction zone to replace that used up in the reaction effluent.

2. The process of Claim 1 wherein the throughput per unit volume in the system is 30 to 80 parts by weight per hour.

3. The process of Claim 1 wherein R2 is allyl.

4. The process of Claim 1 wherein R2 is methyl allyl.

5. The process of Claim 1 wherein R1 is ethylene.

6. The process of Claim 1 wherein R1 is propylene.

7. The process of Claim 1 wherein the reaction zones have a liquid depth to reaction diameter ratio of 1 to 2 and the intense mixing is provided by an impeller with tip speed of at least 1,000 feet per minute and a power input in the range of 50 to 200 horsepower per 1,000 gallons.