PROCESS FOR THE SEPARATION OF THE AQUEOUS PHASE FROM CELLULOSE ACETATE

Abstract: Process for reducing the content of water and acetic acid in the cellulose acetate precipitated from the esterification solutions of cellulose consisting in submitting the precipitated acetate to extraction with a supercritical liquid.

Description: Process for the separation of the aqueous phase from cellulose acetate 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. Generally the aqueous phase embedded in the precipi¬ tates 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 solvent and using sulphuric acid as catalyst. Diluted acid is used to hydrolize the ester and the sulphate groups. Cellulose acetate is then precipitat¬ ed by further addition of diluted acetic acid. As indicative 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 until to obtain a pH of 7 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 surroundings. The process produces also great amounts 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 precipi¬ tated 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. The process of the invention consists in extracting with a supercritical fluid the solvents present in the precipitate of cellulose acetate deriving from processes of esterification of cellulose. Particularly, as supercritical liquid, carbon dioxide at temperature higher than 32°C and pressure higher than 7.3 MPa, is used. Preferably the extraction with supercritical CO~ is carried out at temperatures between 80°C and 100"C and under pressure from 12 to 17 MPa. More preferably the extraction is carried out at temperatures of about 90°C and pressure of 15 MPa. Besides lessening the problem of the waste water, the process of the invention allows to recovery concentrated acetic acid from the liquid wherefro 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 Encyclo¬ paedia of Chemical Technology, 3rd Edition, Supplement Volume, Page 872 r 1984). For example and with reference to the scheme of figure 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 Rl wherein a flow of supercritical C0„ is circulated to raise the temperature up to the desired value of 80°C - 95°C. A step of extraction is then carried out at 85° 95°C under a pressure of 15 MPa. The C0„ flow leaving the extractor Rl is expanded to a pressure outside the range of the supercritical conditions, for example to 5 MPa and then cooled to room temperature; is then sent to separator R2 wherein the carbon dioxide is separated from the aqueous mixture. After ma-ke up for carbon dioxide from container Dl , the flow of carbon dioxide vapours coming from separator R2 is heated by a heat exchanger and then compressed with compressor Cl and thereafter heated to the supercritical conditions and sent to the extractor Rl . The extracted liquid phase contains ca. 5% by weight of carbon dioxide and is separated in continuous and sent, after removal of the residual CO-, to the acetic acid recovery section. In the figure, 1, 2 , 3, 4 and 6 are heat exchangers, 5 is an evaporator, 7 and 8 are pumps. With the process of the present invention it is possible to obtain cellulose acetate with water content of 1-2% by weight and with as little as 10-20 ppm of acetic acid. The following examples are given to illustrate the invention and not to limit the scope of the same. EXAMPLE 1 About 30 Kg of cellulose acetate containing 85% by weight of solvent are charged into extractor Rl . The solvent is formed of about 70% by weight of water and 30% of acetic acid and contains traces of sulphuric acid. About 1200 Kg/h of supercritical carbon dioxide are circulated in continuous through the extractor until to increase temperature to about 95°C. The extraction is then carried out at 95°C and under 15 MPa. The flow of carbon dioxide leaving the extractor Ri is expanded at 50 atm. by a pressure reduction valve, cooled to 25°C by means of a heat exchanger and finally sent to separator R2 wherein it is evaporated and separated from the extracted aqueous solution. After make up for carbon dioxide from Dl container, the flow of carbon dioxide vapour coming from separator R2 is heated and then compressed with compressor Cl_ The liquid carbon dioxide obtained is heated to the supercritical condition and sent to the extractor Rl . The extracted liquid contains 5% carbon dioxide by weight-; it is sent to the recovery of acetic acid, after removal of carbon dioxide. After 10 hours extraction, the amount of cellulose acetate extracted is 4.5 Kg. The humidity content is about 1% by weight. The extraction conditions are as here below reported. Operation : in continuous with continuous flow of extractants Temperature .- 95°C Pressure : 15 MPa Extraction time : 10 h Duration of the cycle: 11 h Extraction rate : about 40 Kg/h/Kg cellulose acetate EXAMPLE 2 About 30 Kg of flakes of cellulose acetate precipi¬ tate containing 85% by weight of a mixture of acetic acid (30%) and water (70%) and impurities of sodium sulphate are loaded in the extractor used in example 1. About 1200 Kg/h of supercritical carbon dioxide are circulated in continuous through the extractor Rl to raise the tempera¬ ture of the ' flakes to about 90°C. The aqueous solution is extracted from the flakes at the above temperature and under a pressure of 15 MPa. The flow of supercritical carbon dioxide leaving the extractor is expanded to 5 MPa and the cooled to 25°C arid sent to a separator wherein the vapours are separated from the aqueous mixture. The carbon dioxide vapours are compressed and then heated according to the cycle reported in example 1. After 7 hours extraction the temperature is raised to ca. 100°C and the extraction continued for additional 3 hours. The total time of extraction was 10 hours. The quantity of cellulose acetate recovered is 4.6 Kg; the humidity content is 1.5% by weight. The extraction rate is about 40 Kg/h/Kg of cellulose acetate. EXAMPLE 3 32.7 g of cellulose acetate having the following composition (% by weight) : water 58.91 acetic acid 24.18 cellulose acetate 16.81 sulphuric acid 0.1 were loaded into 2 cells of a Dionex SSE-703 equipment for supercritical fluid extraction. The temperature of the sample was raised to 80°C. A flow of carbon dioxide under superc itical conditions (12 MPa and 80°C) was circulated for 50 minutes. The total amount of carbon dioxide used was 6.9 litres (standard conditions). The sample was then cooled down to room temperature. The sample weight was 24.9 g and the composition (% by weight) was: cellulose acetate 22.02 water 55.24 acetic acid 22.74 The comparison of the composition prior and after extraction shows that 28.7% of water and 28.1% of acetic acid available in the sample were extracted. The extraction yield, measured as g of total ex¬ tract/litres of carbon dioxide (standard conditions) was 1.13. The' acetic acid water ratio remained practically uneffected . EXAMPLE 4 A sample of 29.10 g of cellulose acetate having the composition set forth in example 3 was subjected to supercritical extraction under the following conditions; Temperature : 90°C CO- pressure: 15 MPa Test time : 120 minutes C0 2 flow : 8.8 1 (standard conditions) After extraction, the sample weight was 25.08 g and its composition (% by weight) was: cellulose acetate: 19.51 warer : 58.93 acetic acid : 21.55 Comparison of the composition before and after extraction shows that 16.6% of available water and acetic acid were extracted. The extraction yield was 0.46. The acetic acid and water were extracted with different rates i.e. 13.9% of available water and 23% of available acetic acid. The selectivity expressed as the ratio of the acid/water ratio (extracted) to the ratio acid/water (unextracted) was 1.86. This example shows that it is possible to extract the liquid phase contained in the cellulose acetate with good yield and with high selectivity.

Claims: CLAIMS 1) Process for separating the aqueous phase comprising water and acetic acid contained in the cellulose acetate precipitated from esterification solutions of cellulose, characterized in that the aqueous phase is separated by extraction with a superc itical liquid. 2) Process according to claim 1, wherein the extraction is carried out with supercritical carbon dioxide. 3) Process according to claims 1 and 2 wherein the aqueous phase present in the cellulose acetate contains up to 85% or more of a mixture prevailingly formed of water and acetic acid. 4) Process according to claims from 1 to 3 wherein the extraction is carried out until to reduce the content of water in the acetate to 1-2% by weight. 5) Process according to claims 2 and 3, wherein the extraction with carbon dioxide is carried out at tempera¬ tures comprised between 80° and 100°C and under a pressure from 12 to 17 MPa. 6) Process according to claim 5 in which the extraction is carried out at about 90°C and 15 MPa. 7) Cellulose acetate obtained with the process of the claims from 1 to 6.

Inventors: AL GHATTA HUSSAIN, GIOVANNINI ARIANNA, AL GHATTA, HUSSAIN, GIOVANNINI, ARIANNA
Applicants: M & G RICERCHE SPA, M. & G. RICERCHE S.P.A.
Application year: 1992
Main classification: B01D11
IPC codes: B01D11, C08B3