Liquid vacuum pump seal to reduce contamination in bisphenol-A

An improved process for producing Bisphenol-A which is essentially free of sodium impurity using rotary vacuum filter pumps which utilize a liquid ring seal to effect drawing a vacuum on a Bisphenol-A/phenol slurry, the improvement is in using phenol as the liquid ring seal medium.

FIELD OF THE INVENTION 
This invention relates to a rotary vacuum pump employing a particular pump 
seal liquor in order to eliminate contamination in the production of 
dihydric phenols particularly Bisphenol-A which is used in substantial 
quantities to synthesize polymers particularly aromatic carbonate 
polymers. The novel feature of this invention is employing liquid phenol 
as the vacuum pump seal liquor as will be described hereinafter. 
BACKGROUND OF THE INVENTION 
Bisphenol-A has been an extremely useful chemical for many decades. AS a 
difunctional monomer, it has been used in the preparation of numerous 
polymers. For example Bisphenol-A [2,2'-bis(4-hydroxyphenyl)propane] has 
been utilized in preparing such materials as epoxy resins, 
polyetherimides, polyarylates and, in particular, polycarbonates. In 
certain of these polymer systems, particularly the epoxy systems, the 
purity of the Bisphenol-A (hereinafter referred to as BPA) employed in the 
polymer reaction need not be that high. Epoxy resins only need BPA of 
approximately 95% purity. The impurity which is present in the greatest 
amount in such systems is generally orthopara BPA. However with other 
polymer systems, particularly polycarbonates, the purity of the BPA must 
be substantially higher. Purities of BPA of about 99.50% or higher 
preferably 99.80 or 99.90% or higher are desirable and in many cases 
necessary for the preparation of BPA polycarbonates. Therefore there has 
been substantial attention directed to the preparation and purification of 
BPA. 
The art is replete with references directed to the preparation of BPA. 
Usually this is done by the condensation of phenol with acetone in the 
presence of a catalyst system. Generally the catalyst is an acidic 
catalyst. For many years one of the particularly useful catalyst systems 
in the patent art and employed commercially was hydrochloric acid. 
Although the economics of the process are initially good with respect to 
the conversion of the reactants to BPA, the maintenance of the apparatus 
is costly. The hydrochloric acid is extremely corrosive and ordinary 
metallic reactors and piping must be changed on a frequent basis. 
Obviously glass lined reactors or certain alloyed metals can be employed, 
however, these are quite expensive. In later years there seems to be the 
tendency to use heterogeneous acidic catalyst system wherein the acidic 
catalyzation occurs at the catalyst surface and is actually bound to the 
catalyst. in this manner the "acid" does not flow with the unused 
reactants and BPA. Such catalyst systems are generally sulfonated 
polystyrenes which are substantially crosslinked such as the Amberlites 
and like materials. However, such sulfonated polystrenes, because they 
contain sulfonic pendent groups may form sulfonic acids. The sulfonic 
acids, along with hydrochloric acid, if employed as part of or a 
contaminant in the catalyst system, and any other acids can be very 
corrosive to the equipment, particularly rotary vacuum pumps. 
Consequently, aqueous caustic has been used as the liquid vacuum seal in 
rotary vacuum pumps in order to neutralize the acids in the BPA 
condensation reaction products generally comprised of BPA, phenol, and 
water and other materials. The reaction products are sent to a 
crystallizer system wherein a BPA/phenol crystal slurry is formed. The 
slurry is sent to a rotary vacuum filter, equipped with vacuum pumps which 
pull the slurry through a filter drum. A cake (BPA/phenol adduct) is 
formed on the drum and the extracted liquid often referred to as the 
mother liquor is recycled to the reactor in which BPA is formed. The 
mother liquor may be subjected to dehydration in order to remove water of 
reaction and other unwanted materials before recycling the mother liquor 
back to the reactor. Optionally, the BPA adduct may be redissolved in 
phenol and sent to a second rotary filter (second stage) equipped with 
vacuum pull for further purification. 
After the BPA is prepared, various isolation and purification procedures 
are known. Many of these appear in the relatively voluminous patent art. 
Generally phenol is distilled off to a great extent and/or the initial 
purification by adduct crystallization of the BPA/phenol adduct 
Distillation of BPA itself can also be employed. The purification of the 
BPA can then be further accomplished through the addition of various 
organic solvents such as toluene or methylene chloride so as to remove the 
BPA from various impurities. Additionally water and various glycols such 
as ethylene glycol and glycerin have been used alone or together to 
separate and thus purify the BPA from its impurities. 
Therefore, it is an object of this invention to provide a process whereby 
sodium ions are not introduced into the BPA. 
Another object of this invention is to employ liquid phenol as a seal 
liquor in rotary vacuum pumps. 
The foregoing and other objects of this invention will become apparent from 
the following description and appended claims. 
SUMMARY OF THE INVENTION 
In accordance with the invention, there is a method of reducing 
contamination in BPA upon separating an adduct slurry and a mother liquor. 
In the process of separating the adduct slurry from the mother liquor, 
rotary vacuum filters may be employed. The rotary vacuum filters are 
operated under a vacuum wherein the pressure is controlled at a level 
below atmospheric and operate in a range of 50 mm Hg to about 250 mm Hg 
absolute. In systems employing rotary vacuum filters, the rotary filters 
may be mechanical liquid ring pumps to provide the vacuum source. In such 
systems, aqueous caustic (sodium hydroxide/water) solution has been used 
to provide the liquid seal for these pumps. The fans or blades of the pump 
cannot touch the inside of the pump housing since wear would be extremely 
extensive. The vacuum pump seal liquor is essential to the ability of the 
pumps to pull a vacuum (reduced pressure) down to the operating level. 
Sodium hydroxide solution has been used as the liquid seal in such pumps 
when producing BPA as described heretofore. However, it has been 
determined that small amounts of acidic and/or basic ionic contamination, 
such as sodium, are catalytic in causing undesirable reactions to occur in 
the BPA and/or polymer products produced therewith such as aromatic 
carbonate polymers. 
It has also been determined that the use of aqueous sodium hydroxide 
solutions as pump seal liquor provides for a direct source of process 
contamination with sodium. 
It has now been discovered that by employing phenol as a liquid ring seal 
for a rotary pump, undesirable contamination from sodium hydroxide 
solution is completely eliminated. 
DETAILED DESCRIPTION OF THE INVENTION 
BPA is generally prepared by the reaction of phenol and acetone in the 
presence of an acidic catalyst, such as HCl or an ion exchange resin. 
During the reaction, some unwanted by-products and color are formed which 
affect yield and quality. Also in the filtration of a BPA/phenol adduct 
slurry generally comprising a liquor (often referred to as a mother 
liquor), BPA and phenol, the adduct slurry is deliquored by vacuum 
filtration which slurry comprises BPA and phenol. The mother liquor is 
essentially recycled back to the reaction for producing BPA as described 
heretofore. 
In the filtration system, rotary vacuum filters are used to separate the 
adduct cake from the mother liquor. The process for producing the BPA can 
be any process and is not critical in the practice of this invention. The 
critical feature is the use of phenol as the liquid in a liquid ring seal 
rotary pump. These pumps are employed to provide a vacuum source for 
removal or separation of the mother liquor from the adduct slurry. AS 
stated previously, the use of sodium hydroxide as the liquid ring seal in 
rotary vacuum pumps introduces contamination into the BPA. The use of 
phenol essentially eliminates undesirable sodium contamination from the 
BPA. In addition, it has also been surprisingly discovered that the use of 
phenol results in the elimination of an amount of waste water that would 
come from the aqueous caustic, which waste water must be recovered for 
environmental purposes. This also results in significant reduction in 
process investment and operating costs by eliminating these water sources. 
Another advantage is that it is possible to discontinue the use of sulfuric 
acid which is required to neutralize the aqueous caustic effluents which 
are contaminated with phenol. 
Since phenol is a solid at room temperature, the phenol used as a liquid 
seal is kept at a temperature above its melting point under process 
conditions such that it is in a liquid state, preferably at a temperature 
of at least about 150.degree. C. and more preferably at a temperature of 
about 150 to about 175.degree. F. 
In another embodiment of this invention, a small amount of fresh phenol may 
be continuously fed to the rotary pumps. The replaced or used phenol may 
then be treated for removal of contaminants before recycling the replaced 
phenol back to the reactor for forming BPA. Since phenol does not 
neutralize the acid contaminants as described heretofore when employing 
aqueous caustic, the acid contaminants can be removed by continuously 
feeding fresh phenol to the rotary vacuum pumps. The replaced or used 
phenol will carry off the acid contaminants.

The following examples are intended to provide exemplification of the 
invention and are not intended to limit the invention. 
EXAMPLE 1 
A two stage rotary vacuum pump system is employed to obtain a BPA/phenol 
cake. After each stage, a BPA/phenol cake is obtained by crystallization 
and vacuum filtering a BPA/phenol adduct (slurry). After the first stage, 
the cake is dissolved in phenol and the liquid system is then sent to a 
second stage or second rotary vacuum pump. In each stage, the rotary 
vacuum pump has a sodium hydroxide (caustic) liquid vacuum pump seal. 
Three different flow rates are evaluated and, in each case, the BPA/phenol 
cake after the second stage is analyzed for sodium (Na) contamination in 
the BPA/phenol cake. The results obtained are as follows: 
TABLE 1 
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Flow Rate of Caustic Seal Liquor 
Na in BPA/phenol Cake 
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5 Kg/hr. 30 ppb 
10-11 Kg/hr. 50-60 ppb 
15 Kg/hr. 90 ppb 
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Kg-Kilograms 
Ppbparts per billion 
The sodium hydroxide solution employed as a liquid seal is a 6.0 weight % 
aqueous sodium hydroxide. 
EXAMPLE 2 
Example 1 is repeated except that in place of the caustic liquid seal ring, 
liquid phenol is employed herein. The liquid phenol is at a temperature of 
about 160.degree. F. 
It is found that when analyzing for sodium in the BPA/phenol cake no sodium 
was detected at the parts per billion level. 
What is claimed is set forth in the claims appended to the application. 
Variations in this invention may be made without departing from the scope 
of the claims appended hereto.