Distillation: processes – separatory – With measuring – testing or inspecting
Patent
1997-02-05
1999-11-09
Manoharan, Virginia
Distillation: processes, separatory
With measuring, testing or inspecting
159DIG19, 203 15, 203 16, 203 60, 203 96, 203 97, 203 98, 203 99, 203DIG19, 562608, 202158, B01D 336, B01D 342, C07C 5146
Patent
active
059806966
DESCRIPTION:
BRIEF SUMMARY
This invention relates to separation of water from a liquid phase medium containing water and at least one other component.
The invention has particular application to the separation of water from an aqueous feed stream containing an aliphatic carboxylic acid such as acetic acid.
A specific application of the invention is in a process for the production of an aromatic dicarboxylic acid such as terephthalic acid in which, to remove water generated in the reaction producing the dicarboxylic acid, an acetic acid/water vapor stream is drawn off from the reactor overheads and subjected to distillation to separate the water from the acetic acid. The dehydrated acetic acid is then recycled at least in part to the oxidation reactor. Traditionally fractional distillation has been used for this task since the manufacture of terephthalic acid is a process which, when operated at elevated pressures (typically in excess of 20 bara), produces significant waste heat which is available for use as reboil heat for the distillation column.
However, with the advent of interest in lower pressure processes for the manufacture of terephthalic acid combined with a drive for more efficient heat recovery schemes, heterogeneous azeotropic distillation has been recognised as offering potential capital and variable cost benefits over fractional distillation.
Heterogeneous azeotropic distillation for acetic acid/water separation is disclosed in U.S. Pat. No. 2,050,234, U.S. Pat. No. 4,250,330 and GB-A-1576787. As stated in GB-A-1576787, a significant advantage of azeotropic distillation is low reflux ratio and hence reduced heat energy requirement for distillation. Reflux ratio is dependent on the particular entrainer selected for the azeotropic distillation. In terms of low reflux ratios, n-butyl acetate (boiling point: about 126.2.degree. C.) must be considered. This particular entrainer is favoured in GB-A-1576787 for the separation of acetic acid and water. Less desirable from the standpoint of reflux ratio is the lower boiling point entrainer isobutyl acetate (boiling point: about 117.degree. C.) which is favoured by U.S. Pat. No. 4,250,330; even less desirable in this context is n-propyl acetate (boiling point: about 101.degree. C.) which was found to be useful as an entrainer in the 1930's prior to the recognition of n-butyl acetate as a superior entrainer (see U.S. Pat. No. 2,050,234 which is concerned with the production of substantially pure "glacial" acetic acid from aqueous acetic acid). N-butyl acetate is advantageous because of the higher amount of water entrained in the azeotrope which allows a lower reflux ratio when organic phase reflux only is used.
In the kind of application that the present invention is specifically concerned with, it is not necessary for the distillation to be carried out in such a way as to produce substantially anhydrous acetic acid. Because the acetic acid is to be recycled at least in part to the oxidation reactor, it is expedient to produce a partially dehydrated acetic acid product typically containing of the order of 5% by weight water based on the combined acetic acid/water content. U.S. Pat. No. 4,250,330 and GB-A-1576787 both contemplate the possibility of producing a bottoms product containing some water; U.S. Pat. No. 4,250,330 mentions a water content of no more than 10% (and preferably no more than 5%) by weight using isobutyl acetate entrainer whilst GB-A-1576787 gives a specific Example in which, using n-butyl acetate entrainer, the water content in the dehydrated acetic acid product is 6.3% by weight.
In GB-A-1576787 a bottoms product containing water is obtained using n-butyl acetate entrainer and, apart from using an excessive number of trays in the bottom part of the distillation column, slippage of entrainer into the bottoms product can only be avoided by employing a feed stream which has a high water content and/or by using a reflux combining both organic (entrained and aqueous phases. Thus, in the Example given in GB-A-1576787, the water content of the feed stream supplied vi
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Othmer, D.F., Azeotropic Separation, Chemical Engineering Progress, vol. 59, No. 6, pp. 67-78.
Parten William David
Ure Alan MacPherson
E. I. Du Pont de Nemours and Company
Krukiel Charles E.
Manoharan Virginia
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