RECRYSTALLIZATION OF BISPHENOL A BY AZEOTROPICALLY DRYING THE SOLVENT

Abstract: This invention discloses a high yield azeotropic separation procedure for isomers of bisphenol A. Specifically, the o,p′-isomer is sepa­ rated from the p,p′-isomer by removal of a cosolvent thus producing crystallization. Water and an organic solvent not miscible with water are used as cosolvents.

Description: Bisphenol A is an important commercial intermediate in the manufacture of epoxy, polycarbonate, phenyloxy, polysulfone and certain polyester resins. It is also used in flame retardants and rubber chemicals. A derivation of bisphenol A is possible by a condensation reaction of phenol and acetone catalyzed by HCL at 65.6°C (150°F). The p,p'-isomer of bisphenol is commercially favored over the o,p'-isomer and for that reason separation of these two isomers is appropriate. Commercial market expectations of lower o,p'-isomer concentrations have required need of better separation procedures. Current bisphenol A purification processes include crystallization from a solution of wet bisphenol in toluene as disclosed in US-A- 4,354,046; adductive crystallization with phenol as shown in US-A- 4,156,098 and 2,791,616; solvent leaching of solid bisphenol as in US-A- 3,493,622 and 3,207,795; recrystallization by cooling of water-wet melt disclosed in US-A- 3,326,986; recrystallization by cooling of a solution in various organic solvents as in US-A- 4,242,527; and recrystallization by adding cosolvents to a solution of bisphenol in an organic solvent as disclosed in US-A- 3,919,330 and 4,192,955. From US-A- 4,294,993 a method for purification of bisphenol A is known to remove impurities such as o,p'-isomer. A bisphenol A adduct with phenol is formed and treated with toluene or with a mixture of toluene and water at elevated temperature to form a homogeneous liquid-liquid mixture and then cooling the mixture to 20°-35°C to precipitate solid bisphenol A in a purified state. AU-B- 489 747 refers to the production of bisphenol A by condensation of phenol and acetone in the presence of an acid catalyst and a boron compound dehydrating agent. Water is added to the reaction mixture comprising toluene or 1,2-dichloroethane as solvent medium and then neutralized. The organic solvent is removed from the system by azeotropic distillation until the reaction mass reached an end temperature of 105° to 110°C. The mixture is then cooled so that bisphenol crystallizes. The reference does not mention o,p'-isomer nor its removal, only recovery of a purified bisphenol A from the reaction mixture is disclosed. It is the object of the present invention to provide an improved process of separating o,p'-isomers of bisphenol A from the p,p'-isomers of bisphenol A. This object is attained by the process for purification of bisphenol A by (a) mixing bisphenol A with water and an organic solvent which is not miscible with water, (b) heating the resulting mixture with agitation to between 68°C and the melting point of bisphenol A under pressure sufficient to vaporize only an azeotropic ratio of water and the organic solvent at the temperature employed, (c) removing the azeotropic vapors which form from the heated mixture, (d) cooling the remaining mixture to effectuate formation of crystalline solids in the mixture and separating the crystalline solids from the mother liquor, (e) recovering a solid product and further, optionally subjecting the so recovered solids to at least one of the steps selected from (f) washing the crystalline solids with a solvent and/or (g) drying the crystalline solids, characterized by separating o,p'-isomers of bisphenol A from p,p'-isomers of bisphenol A to recover a solid product having less o,p'-isomers than in the bisphenol A fed to step (a) wherein the azeotropic vapors are removed from the mixture to reduce the total volume of water in the liquid state of the mixture until crystallization initiates in the liquid at the temperature of the mixture and the crystalline solids are separated from the mother liquor prior cooling the mixture to below 64°C. This invention discloses a novel and unobvious separation process which utilizes a different principle from the above procedures: crystallization of bisphenol A isomers through removal of a cosolvent by azeotropic distillation. The solubility of p,p'-bisphenol in toluene at temperature as high as 98°C is only 8.6 percent by weight. With water present at an amount equal to 10 percent of the weight of bisphenol, formation of a stable liquid solution containing equal quantities of bisphenol and toluene to temperatures as low as 80°C is possible. This phenomenon is likely due to a synergistic solvent effect related to the depression of the bisphenol freezing point by a small amount of water: 10 percent water in o,p'-bisphenol will depress its freezing point from 108°C to 70°C, and 17 percent water in p,p'-bisphenol will depress its freezing point from 157°C to 90°C. This data on the solubility of bisphenol in water and toluene, separately and together, sparked curiosity concerning the behavior of a bisphenol/water/toluene solution as the water is removed by azeotropic distillation. This invention is the result of investigating that curiosity; a high yield one step separation process by azeotropic distillation of the primary isomers of bisphenol A. The two primary isomers of bisphenol A are separated by means of a novel and unobvious crystallization technique. The bisphenol A is dissolved by heating in a mixture of water and toluene. The p,p'-isomer preferentially crystallizes out as the water is removed by boiling as the azeotrope. The toluene layer of the condensed vapors may be, but not necessarily, refluxed to the crystallization vessel. Crystallization is conventionally carried out by cooling a saturated solution, evaporating the major solvent, or changing the nature of a solvent by addition of a cosolvent. By way of distinction, this invention crystallizes by removal of a cosolvent. The major advantages for the disclosed process of this invention are better separation of p,p'-isomer from the unwanted o,p'-isomer and a larger product yield. This invention is used to produce high quality p,p'-isomer bisphenol from bisphenol A by removal of o,p'-isomer bisphenol and other impurities. The previous best purity obtained in a single-step laboratory procedure with a feed containing about 3 percent o,p'-isomer bisphenol was 0.09 percent o,p'-isomer in the product. The process of this invention has produced 0.047 percent o,p'isomer content material with certain larger crystals containing only 0.046 percent o,p'-isomer and yield comparable to a cooling type crystallization. Another use to which this process has been put is in producing a product rich in o,p'-isomer bisphenol by separation of the remaining mother liquor after initial crystallization further cooling and then evaporation of the remaining solvent from the mother liquor. One important step in this invention is the crystallization itself. The solute bisphenol is caused to crystallize by removal of the relatively small proportion of water in the bisphenol/-solvent/water mixture. At atmospheric pressure the water boils off in the range of 85 to 105°C. The mass of water most preferably should be 10 to 20 percent of the mass of bisphenol A used; less water limits the amount of bisphenol A dissolved and more water causes unnecessary expenditure of energy to evaporate it. There are two proposed explanations for the improved effectiveness of this process over previous processes. The first deals with the improvement of solvent selectivity with increasing temperature. Table I shows the change in solubility of both pure isomers with temperature. This drying-type crystallization allows precipitation of p,p'-isomer bisphenol at a higher temperature than does a cooling-type crystallization of past processes. The second proposed explanation for the improved effectiveness of this process over previous processes relates to the proportion of water present during crystallization. Previous research had concluded that decreasing the amount of water present improves the effectiveness of a cooling-type crystallization from toluene. This drying-type crystallization allows separation of the isomers in the presence of a smaller concentration of water than a cooling-type crystallization. An observable effect of this difference in water content is in the phase behavior of the crystallizer contents. In this cooling-type crystallization, two nearly equal volume liquid phases form: a toluene rich upper phase and a wet bisphenol rich lower phase. Upon cooling, crystals form in the wet bisphenol rich phase. Upon heating and subsequent removal of water, the bisphenol is transferred into the toluene rich phase leaving only a very small amount of water which is removed as vaporization proceeds. The crystals actually form out of a toluene solution due to the influence of increasing temperature and decreasing water content. Although toluene is used as the solvent in the examples of this disclosure, other organic solvents not miscible with water can be used. Various solvents can be suitable replacements for toluene such as chloroform, benzene, xylene, ortho-dichlorobenzene, ethylene dichloride, 1,1,1-trichloroethane and methylene chloride. Solvent to bisphenol A ratios of 1 to 1 through 3 to 1 all worked equally well, but the preferred ratio is 2 to 1. Lower ratios would work also, but at some point, purity would be compromised. Higher ratios might produce slightly better product purity at the cost of overall yield. Pressure limits are not precisely fixed. The lower limit would be expected at the point where boiling would be lowered to temperatures into the range of 68 to 72°C and solid bisphenol A would be crystallized out by cooling. Upper pressure limits would normally be expected to be where the temperature required to boil off the solvent/water azeotrope exceeds 157°C, the melting point of the p,p'-isomer of bisphenol A. The azeotropic evaporation rate used in this invention is such that the water is removed in 2 to 3 hours. Excessive boiling rates would be expected to decrease product purity. By the process of this invention, a high purity p,p'-isomer of bisphenol may be crystallized by azeotropically drying a concentrated wet bisphenol A/-toluene solution. Using this novel and unobvious process, a product with reduced content of o,p'-isomer of bisphenol can be produced compared with other known single-stage processes of purification. Another beneficial aspect of the process of this invention is the observed ability to produce larger crystals than previously used processes. This enables production of a crystalline product with superior handling characteristics due to the ease of handling large crystals which flow over themselves readily. These large crystals contain even higher purity p,p'-isomer bisphenol due to lower content of o,p'-isomer bisphenol. This is probably due to the large crystals longer life, that is, the largest crystals are formed first so that their growth began from a mother liquor containing a smaller concentration of the o,p'-isomer. Table II reveals crystal size distributions for various crystallization procedures. Crystal size is reported as the percent of the total sample weight which is retained on each sieve. The temperature at which the isomer crystals form is higher in this drying-type crystallization than in a conventional cooling-type crystallization. Separation of the crystals from their mother liquor before allowing the suspension to cool significantly is essential to maintaining high product purity as additional material of lower quality will crystallize out of the mother liquor upon cooling. The following examples are illustrations of specific embodiments of the invention. Example 1 In a flask, 400 g of bisphenol A containing 3.1 percent of o,p'-isomer was mixed with 40 g of water and 950 g of toluene, and was heated with agitation to 102°C. The vapor generated by the mixture was removed from the flask and condensed. The remaining mixture was cooled to 75°C and the resulting solids were separated by vacuum filtration. Remaining mother liquor was removed by washing the resulting solids with 200 g of toluene at room temperature. The washed solids were then dried and analyzed; they weighed 332.1 g and contained 0.062 percent o,p′-isomer. Final yield of p,p'-isomer bisphenol to initial feed was 86.4 percent. The mother liquor from the initial mixture was allowed to cool slowly to 25°C, then the remaining liquid was decanted off of the solids formed during this cooling. These secondary recovered solids contained 1.6 percent o,p'-isomer and the liquid contained 1.5 times as much o,p'-isomer as p,p'-isomer. In addition, the secondary solids recovered by further cooling of the mother liquor, representing 10.5 percent of the p,p'-isomer in the feed, are suitable for recycle. Using this process, the solids formed crystals which were of relatively large size, with 45 percent failing to pass through 0.2 mm mesh. Example 2 In order to test the selectivity of dry toluene for the separation of o,p'-isomer bisphenol from p,p'-isomer bisphenol, the following procedure was used. Specifically, a determination was made of the maximum o,p'-isomer ratio attainable in the mother liquor. A 1,000 ml 3-neck spherical flask was equipped with an air driven stirring motor with a paddle agitator, a thermometer to measure the temperatures of both the liquid and the vapor phases, and a distillation condenser to remove condensables in the vapor phase rather than refluxing them. 151 g of bisphenol A containing 3.06 percent o,p'-isomer, 151 g of toluene and 21 g of deionized water were charged to the flask and heated to 98°C followed by cooling to 80°C. The resulting mother liquor was removed using a fitted glass filter on the end of glass tube connected to a vacuum. 140 g of toluene was added and agitated in order to wash the resulting solids. The mixture was then vacuum filtered and the solids dried under vacuum. The removed mother liquor was allowed to cool slowly to room temperature, then the liquid was decanted off the solid material that was formed during cooling. It is observed that upon heating the bisphenol A was melted by 80 to 82°C. At 85°C vapor started to flow to the condenser producing a distillate containing two liquid phases. This mixture distilled off in the range of 85 to 86°C. No additional vapor was observed until the temperature reached 95.5°C whereupon more two phase distillate was taken overhead between 95.5 to 96.5°C. Crystals appeared quickly during this time. Heating was continued to 98°C with no more vapor observed. The crystal slurry was cooled to 80°C before removal of the mother liquor. The cooled mother liquor contained 4.2 percent p,p′-isomer bisphenol and 7.4 percent o,p'-isomer bisphenol. The total mass of crystalline solids recovered, excluding those precipitated from the cooling mother liquor, was 129.1 g for a yield of 88 percent based on initial feed of bisphenol A. Example 3 To further test the selectivity of dry toluene for the separation of o,p′-isomer bisphenol from p,p'-isomer bisphenol, the procedure of Example 2 was repeated. 150 g of bisphenol A, containing 3.06 percent o,p'-isomer, 510 g toluene and 15 g of deionized water were heated with stirring to 105°C. The resulting mixture was cooled to 91°C and vacuum filtered. A 78 g sample of wet crystals was washed with 234 g of toluene by agitating for 10 minutes at room temperature. The crystals were then vacuum filtered and dried overnight in a nitrogen purged vacuum oven. This sample contained 877.2 ppm of o,p'-isomer bisphenol in 67.7 g of dried crystals. The mother liquor was allowed to cool overnight at room temperature and then the mother liquor liquid was removed from the further crystallized solids. The mother liquor liquid contained 0.81 percent o,p′-isomer bisphenol and 0.76 percent p,p'-isomer bisphenol while the mother liquor solids contained 0.77 percent o,p'-isomer bisphenol and 25.97 percent p,p'-isomer bisphenol. Temperature influctions were somewhat less distinct than during Example 2. Vapor was observed between 85°C and 101°C. No additional vapor was observed in the range of 101-105°C. Example 4 To further test the selectivity of dry toluene for the separation of o,p'-isomer bisphenol from p,p'-isomer bisphenol, the procedure of Example 2 was repeated. 400 g of bisphenol A containing 3.06 percent o,p'-isomer, 40.2 g deionized water and 933 g toluene were heated together to 103°C while removing the condensed vapors. The mixture was then cooled to 75°C and vacuum filtered. The ratio of wash solvent to bisphenol crystals was decreased from 3/1 ratio of Example 2 to 0.5/1. 342.9 g of crystals was collected and tested. The crystals contained 860 ppm o,p′-isomer. A yield of 90.9 percent p,p'-isomer was produced based upon initial feed of bisphenol A. Certain large crystals, about 1 cm long, were isolated and analyzed separately revealing only 580 ppm o,p'-isomer concentration. The mother liquor was allowed to cool overnight at room temperature and then the mother liquor liquid was removed from the further crystallized solids. The mother liquor liquid contained 1.45 percent o,p'-isomer bisphenol and 0.69 percent p,p'-isomer bisphenol. Boiling of the toluene/water azeotrope started at 85°C and continued throughout the range of 85°C to 104°C. A record of distillate volume collected as a function of liquid temperature for this example is shown in Table III. The volume fraction of the collected distillate aqueous phase was constant at 16 percent over the entire temperature range. The appearance of the crystallizing mixture changes significantly at about 91°C, from two nearly equal volume liquid phases to a large, clear organic phase with very small amounts of clear water in the bottom. The first crystal nuclei were observed at 93.4°C, but the crystal population was very small until the temperature reached 96°C and then large numbers of crystals were formed. Example 5 To illustrate the high o,p'-isomer bisphenol concentration in the mother liquor after initial crystallization of predominantly p,p'-isomer bisphenol, the procedures of Example 2 were utilized here. 2,268 kg (5,000 lbs) of bisphenol A containing 2.6 percent o,p'-isomer, 2,268 kg (5,000 lbs) of toluene and 226.8 kg (500 lbs) of water were mixed. The mixture was heated to 105°C to crystallize the predominate p,p'-isomer material, then cooled to 31°C. The mother liquor was separated and then evaporated to recover solids containing 43 percent o,p'-isomer and 17.4 percent p,p'-isomer. Example 6 To illustrate the continuous operation of this invention, bisphenol A containing 2.66 percent o,p'-isomer was utilized in a feed containing a bisphenol A/toluene/water ratio of 1/2/0.1. This mixture was fed continuously at a volume flow rate equal to 1/2 the crystallizer volume per hour for 6 hours. The feed tank was maintained at 86°C and the crystallizer at 104°C. At the end of the run, the solids were sampled, rinsed with clean toluene and found to contain 475 ppm of o,p'-isomer bisphenol.

Claims: A process for purification of bisphenol A by (a) mixing bisphenol A with water and an organic solvent which is not miscible with water, (b) heating the resulting mixture with agitation to between 68°C and the melting point of bisphenol A under pressure sufficient to vaporize only an azeotropic ratio of water and the organic solvent at the temperature employed, (c) removing the azeotropic vapors which form from the heated mixture, (d) cooling the remaining mixture to effectuate formation of crystalline solids in the mixture and separating the crystalline solids from the mother liquor, (e) recovering a solid product and further, optionally subjecting the so recovered solids to at least one of the steps selected from (f) washing the crystalline solids with a solvent and/or (g) drying the crystalline solids, characterized by separating o,p'-isomers of bisphenol A from p,p'-isomers of bisphenol A to recover a solid product having less o,p'-isomers than in the bisphenol A fed to step (a) wherein the azeotropic vapors are removed from the mixture to reduce the total volume of water in the liquid state of the mixture until crystallization initiates in the liquid at the temperature of the mixture and the crystalline solids are separated from the mother liquor prior cooling the mixture to below 64°C. The method of Claim 1 wherein the solvent is selected from the group consisting of chloroform, benzene, toluene, xylene, ortho dichlorobenzene, ethylene dichloride, 1,1,1-trichloroethane and methylene chloride. The method of Claim 1 wherein the heating temperature is in the range of 85°C to 105°C. The method of Claim 1 wherein the mass of water is between 10 percent to 20 percent of the mass of bisphenol A. The method of Claim 1 wherein the weight ratio of solvent to bisphenol A is between 1/1 and 3/1. The method of Claim 1 wherein the weight ratio of solvent to bisphenol A is 2/1. The method of Claim 1 wherein the mixture is cooled to at least 75°C before the solids are separated. The method of Claim 1 where the heating rate is adequate to remove the water in the range of 2 to 3 hours. The method of Claim 1 further including the steps of; (a) cooling further the separated mother liquor liquid recovered from the solids; (b) collecting further crystallized solids from the cooled liquid; and (c) removing these further crystallized solids.

Inventors: LITTLE, DOUGLAS JAMES
Applicants: THE DOW CHEMICAL COMPANY
Application year: 1986
Main classification: C07C37
IPC codes: C07C37, C07C39, B01D9, C07C27, C07C67