Process for extracting water and acetic acid from the aqueous ph

Details Process for extracting water and acetic acid from the aqueous ph Process for extracting water and acetic acid from the aqueous ph

1996-04-30

1997-09-16

Drodge, Joseph W.

Liquid purification or separation

Processes

Liquid/liquid solvent or colloidal extraction or diffusing...

2105003, 264344, 536 71, B01D 1104, C08B 102

Patent

active

056676913

DESCRIPTION:

BRIEF SUMMARY
FIELD OF THE INVENTION

The present invention refers to a process for the extraction with liquids under supercritical conditions of the aqueous phase present in the flakes of cellulose acetate precipitated from the solutions of the esterification of cellulose.


BACKGROUND OF THE INVENTION

Generally the aqueous phase embedded in the precipitates of cellulose acetate prevailingly comprises acetic acid (ca. 30%) water (ca. 70%) and small quantities of sulphuric acid.
It is well known that cellulose acetate is produced by esterification of cellulose with acetic anhydride in glacial acetic acid as a solvent and using sulphuric acid as catalyst. Diluted acid is used to hydrolize the ester and the sulphate groups. Cellulose acetate is then precipitated by further addition of the diluted acetic acid.
As an example the cellulose acetate flakes are formed of about 15% of cellulose acetate and about 85% of a solution comprising acetic acid, water, sodium sulphate and small amounts of sulphuric acid. The flakes are usually washed with water a pH of 7 is obtained in the washing liquid.
The washed flakes contain ca. 85% by weight of aqueous solution. The flakes are then pressed to reduce the water content to ca. 50% by weight and then dried to a water content of about 1% by weight (Encyclopaedia of Polymer Science and Engineering, 2nd Edition, Vol. 3, page 158, Wiley-Interscience Publication, 1985). This separation process produces remarkable amounts of waste water which need purification before being emitted into the surrounding.
The prior process also produces a great amount of diluted acetic acid which has to be recovered to render the process economically practicable.
The recovery of acetic acid is based on the azeotropic distillation with organic solvents to separate the acid from water.
It has been found that it is possible to reduce the content of the aqueous phase (mainly water and acetic acid) present in the flakes of cellulose acetate precipitated from the solution of esterification of cellulose (the flakes contain up to about 85% or more of a solution comprising water, acetic acid and small amounts of sulphuric acid) without using the numerous washing steps with water and the subsequent drying step which are representative of the processes of the prior art.


SUMMARY OF THE INVENTION

The process of the present invention comprises extracting with a supercritical fluid the solvents present in the precipitate of cellulose acetate deriving from processes of esterification of cellulose. In particular, as a supercritical liquid, carbon dioxide, at a temperature higher than 32.degree. C. and a pressure higher than 7.3 MPa, is used. Preferably the extraction with supercritical CO.sub.2 is carried out at temperatures between 80.degree. C. and 100.degree. C. and at a pressure from 12 to 17 MPa.
More preferably the extraction is carried out at temperature of about 90.degree. C. and a pressure of 15 MPa.
Besides lessening the problem of the waste water, the process of the present invention allows recovery of concentrated acetic acid from the liquid wherefrom the supercritical carbon dioxide is removed by reduction of pression and temperature operating under conditions outside the supercritical range according to the well-known process of extraction with supercritical fluids (Kirk-Othmer Encyclopaedia of Chemical Technology, 3rd Edition, Supplement Volume, Page 872, 1984).


BRIEF DESCRIPTION OF THE DRAWINGS

Described the preferred embodiment, reference is made to the accompanying drawing:
FIG. 1 illustrates an example of a system for utilizing the process of the present invention.


DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

For example and with reference to the scheme of FIG. 1, the cellulose acetate in flakes containing up to about 85% by weight of a mixture of water (ca. 70%) and acetic acid (ca. 30%) is loaded into the extractor R1 wherein a flow of supercritical CO.sub.2 is circulated to raise the temperature up to the desired value of 80.degree. C.-95.degree. C.
A step of extraction is then carried out

REFERENCES:
patent: 4250331 (1981-02-01), Shimshick
patent: 4770780 (1988-09-01), Moses
patent: 5049235 (1991-09-01), Barcus et al.
"Removal of organic solvents from pharmaceuticals by extraction with supercritical carbon dioxide", Chemical Abstracts, vol. 114, abstract No. 214422k.

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