Method of crystallizing tetrabromobisphenol A

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Reexamination Certificate

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Reexamination Certificate

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06383127

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to a method of crystallizing tetrabromobisphenol A (hereinafter TBA) widely used as a flame retardant for synthetic resins. More particularly, it relates to a method for effectively and efficiently obtaining TBA crystals of high quality in a continuous manner from a methanol solution of TBA.
BACKGROUND OF THE INVENTION
TBA is generally produced by brominating bisphenol A (hereinafter BPA) with bromine in an organic solvent, such as a lower alkyl alcohol or a halogenated hydrocarbon. In industrial scale production, methanol is chosen as a solvent for ease of handling and relatively suppressed formation of impurities. Bromine is used in a slight excess over a theoretical amount. TBA in the methanolic reaction solution is commonly recovered as crystals by addition of a poor solvent, such as water.
JP-B-41-3376 (The term “JP-B” as used herein means an “examined Japanese patent publication”) discloses a process of producing TBA crystals comprising allowing bromine in excess over a theoretical amount to react BPA in methanol as a preferred alcohol, raising the temperature of the reaction mixture, allowing by-produced hydrogen bromide and residual bromine to react with methanol to produce methyl bromide, which is recovered by distillation, and then adding water to the methanolic TBA solution to crystallize TBA in a batch system.
JP-B-53-20494 proposes a similar technique, in which the reaction mixture obtained by bromination of a methanol solution of BPA is heated, and evolved hydrogen bromide and residual bromine are recovered as methyl bromide. In this process, the inventors use sulfuric acid as a poor solvent for batchwise crystallization of TBA in a methanol solution, asserting that TBA crystals obtained by addition of sulfuric acid are less colored than those obtained by addition of water.
JP-A-2-270833 (The term “JP-A” as used herein means an “unexamined published Japanese patent application) teaches a process comprising heating the reaction mixture obtained by bromination of BPA in methanol to recover residual bromine as methyl bromide, reducing unrecovered residual bromine with sodium sulfite, and then crystallizing TBA in a usual manner, such as addition of water.
In these related arts the excess bromine is allowed to react with methanol, a solvent, to produce methyl bromide, which is recovered. Although methyl bromide was formerly a useful by-product finding use as a fumigant, etc., it has recently been accused of causing destruction of the ozonosphere. Therefore, co-production of methyl bromide is now to be avoided. Additionally the related arts adopt, in common, a batch system for crystallizing TBA by addition of water or sulfuric acid, achieving low production efficiency and insufficient crystal quality.
There have been proposed processes of producing TBA without co-producing methyl bromide. For example, JP-A-2-196747 discloses a process in which the residual bromine of the reaction mixture obtained by brominating BPA in methanol is reduced to hydrogen bromide with an aqueous hydrazine solution, and water is then added to crystallize TBA in a batch system. Involving no production of methyl bromide, this process can be seen as environment friendly. The residual bromide can completely be reduced to hydrogen bromide with hydrazine. However, conducted in a batch system, the TBA crystallization by addition of water is unsatisfactory in production efficiency and quality.
JP-A-3-246245 proposes a process comprising reducing the bromination reaction mixture of BPA with an aqueous hydrazine solution and batchwise crystallizing TBA in methanol by addition of water at a temperature of −15 to 15° C. to obtain TBA crystals reduced in both hydrolyzable bromine content and inorganic bromide ion content. The process, being a batch system, too, has low production efficiency. In addition, because the crystallization is at low temperature, the resulting crystals are small only to need an increased load in filtration and washing.
JP-A-4-9346, which is basically the same as JP-A-3-246245, teaches that TBA crystals reduced in contents of hydrolyzable bromine and inorganic bromide ion which are impurities can be obtained by limiting the amount of water and the rate of dropping addition of water in crystallization to 30 to 100% by weight and 5 to 40% by weight per hour, respectively, based on the reaction solvent. This process possesses the drawback of having a low recovery as well as low production efficiency due to the batch system.
All these conventional processes of recovering TBA crystals without co-producing methyl bromide adopt a batch system for crystallizing TBA by addition of water or sulfuric acid to a methanol solution of TBA. The low production efficiency of batchwise crystallization and the accompanying process and operation complexity have made the processes still unsatisfactory for industrial scale production. Further, the quality of TBA crystals obtained are not satisfactory. It has therefore been keenly demanded to develop an effective and efficient method for recovering high-quality TBA crystals from a methanolic TBA solution by using water which is easier to handle than sulfuric acid as a poor solvent.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method for effectively and efficiently obtaining TBA crystals of high quality in a continuous manner from a methanol solution of TBA.
As a result of investigation, the present inventors have surprisingly found that the problems associated with conventional techniques of TBA crystallization from a methanol solution by addition of water can be solved by a method in which crystallization by addition of water is carried out continuously but in two divided stages.
They have additionally found that high quality TBA crystals can be obtained with satisfactory crystal growth by continuously and simultaneously feeding a bromine-containing methanolic TBA solution, a reducing agent and water separately into a crystallizer without being accompanied by incorporation of bromine into the TBA crystals. This is a totally unexpected finding, considered from the state-of-the-art belief that it is impossible to crystallize TBA directly from a bromine-containing methanol solution of TBA. The present invention has been completed based on these findings.
The present invention provides:
(1) A method of crystallizing TBA which comprises continuously and simultaneously feeding a methanolic TBA solution and water separately to a first crystallizer to obtain a slurry in which a part of the TBA is crystallized and continuously and simultaneously feeding the slurry and water separately to a second crystallizer to crystallize substantially the whole amount of the TBA (first aspect);
(2) A method of crystallizing TBA which comprises continuously and simultaneously feeding a methanolic TBA solution containing bromine, a reducing agent, and water separately to a crystallizer to cause the TBA to crystallize and to cause bromine to be reduced simultaneously (second aspect); and
(3) A method of crystallizing TBA which comprises continuously and simultaneously feeding a methanolic TBA solution containing bromine, a reducing agent, and water separately to a first crystallizer to obtain a slurry in which a part of the TBA is crystallized and continuously and simultaneously feeding the slurry and water separately to a second crystallizer to crystallize substantially the whole amount of the TBA (third aspect).
DETAILED DESCRIPTION OF THE INVENTION
The methanolic TBA solution (or TBA solution in methanol) used in the present invention is not limited in composition but usually contains 10 to 30% by weight of TBA and 70 to 90% by weight of methanol, based on the amount of the methanolic TBA solution. The solution may contain small amounts of water and brominated organic matter.
The methanolic TBA solution suited to the present invention is a liquid prepared by brominating BPA in a methanolic solvent with bromine and treating excess bromine with a reducing agent, such as hydrazine. The term “meth

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