WO1997036030A1

WO1997036030A1 - Method of dope spinning

Info

Publication number: WO1997036030A1
Application number: PCT/GB1997/000892
Authority: WO
Inventors: Roberto Hubert Minis
Original assignee: Viscofil Consolidated Corporation
Priority date: 1996-03-27
Filing date: 1997-03-27
Publication date: 1997-10-02
Also published as: GB2311484A; GB9606486D0

Abstract

A method of spinning a cellulose ester dope comprises extruding the dope into an inert atmosphere and recovering solvent such as acetone from the emerging inert atmosphere by contacting it under controlled conditions with a cooled solvent source to provide a recycled inert gas supply containing a controlled proportion of solvent.

Description

METHOD OF DOPE SPINNING

This invention relates to the spinning of a dope of cellulose ester, and in particular to the recovery of solvent therefrom.

The invention is predominantly concerned with the spinning of cellulose diacetate in the form of a spinning dope comprising a solution in acetone containing a small proportion of water Such acetate spinning is the conventional process for the production of cellulose filaments for use, inter alia, in cigarette and other filters However, the invention can also generally relate to any dope spinning process, for example in the preparation of cellulose ester filaments in the textile industry

In conventional dope spinning, dope is supplied under pressure to a spinneret having a large number of orifices, to produce a series of fine filaments which are dried and further processed as a bundle The filaments emerging from the spinneret are conventionally passed downwards through a tower in a current of air which generally moves at about the same speed. The air is returned to the atmosphere, but must first be stripped of evaporated solvent, generally acetone. This is conventionally achieved by passing the emerging air through adsorbent or absorbent columns, etc However, the use of air, particularly in the presence of an adsorbent such as activated carbon having acetone adsorbed thereon, provides a very high risk of explosion Also, when using air, the acetone concentration should not exceed a certain percentage of the lower explosion limit, thus limiting the concentration of acetone in the drying section For this reason, the plant must be cited in a hazard zone and must be very carefully controlled

It is an object of the present invention to overcome the problems of explosion and air pollution by spinning the dope into an inert gas atmosphere and then stripping the acetone from the inert gas by means of condensation According to the present invention, therefore, there is provided a method of spinning a cellulose ester dope comprising extruding under pressure a dope containing cellulose ester dissolved in a spinning solvent, into a stream of gas in a spinning zone, separating the spun filaments from the gas stream, recovering solvent from the gas stream, and venting at least some of said gas to the atmosphere;

characterised in that the gas is nitrogen or another inert gas which is recycled to the spinning zone, and the solvent recovery includes the steps of

a) continuously passing at least a portion of the gas stream emerging from the spinning zone through a condensation unit controlled at a predetermined temperature so as to capture a required proportion of solvent from the gas stream, and

b) recirculating the thus-treated gas stream to the spinning zone together with any untreated gas stream.

The condensation unit will, preferably, be one in which the gas stream is brought into direct contact with the chilled solvent and may conveniently comprise a bubble column of chilled solvent cooled by means of a heat exchanger, e g a spiral wound tube containing circulating refrigerant, or a spray cooler using chilled solvent, the solvent being chilled by a heat exchanger (e.g. as above) before being sprayed into the solvent vapour - rich gas stream. Other condensation apparatus may be used.

The essence of this invention is the passing of a portion of the gas stream emergr from the spinning zone into the condensation unit. By maintaining the condensa n unit at a temperature considerably lower than the temperature of the incoming gas, it is possible to strip a solvent such as acetone from the gas and to return it to the spinning zone with an appropriately low level of remaining solvent Thus, in brief, the solvent is rather surprisingly removed using acetone itself (and the process can be considered j analogous to a method of drying a gas stream by directly contacting it with chilled water).

It is not necessary to treat the entire gas stream, since the gas supplied to the spinning zone should desirably contain a proportion of acetone and water. Indeed, we have found that gas completely stripped of acetone and water gives unsatisfactory results.

In a preferred embodiment of the invention, the bundle of filaments leaving the spinning zone passes through an incoming "rinse" of clean gas, this inflow being the result of an underpressure caused by extraction of a gas flow from the circulating gas flow. The bundle of filaments thus emerges relatively free from acetone and can be subsequently processed as required.

Because a relatively small amount of fresh gas is introduced into the system at this point, it will be clear that an equivalent amount of gas must be removed from the system after the acetone recovery step. Since the main acetone recovery step is operated under conditions which do not remove all the acetone (so that it can be recycled to the spinning zone as explained above) the surplus gas must be further stripped of acetone before venting to the atmosphere. This is simply achieved by a second condensation unit, this time using a very low temperature, typically from -0 to -70°C. so as to provide a gas flow to the atmosphere containing, typically, less than 3 kg/h as defined by the German T A Luft regulations.

A preferred embodiment of the invention will now be described with reference to the accompanying figure which comprises a flow chart of the system.

Referring to this chart, a spinning tower A is provided with an input bl for dope and an output b2 for a bundle of spun filaments. A fan Dl is connected to another outlet just above the bottom of the spinning tower A to extract gas, thus creating an underpressure at the exit point of the bundle. The tower A is further supplied with an inlet c l for fresh gas to provide a gas lock, the gas and filaments moving in counter current to ensure that acetone is entrained with the emergent gas flow and not with the bundle of filaments.

The fan Dl is arranged to recycle the gas via a heat-exchanger G and a mixer H to the top of the tower A for re-use. A valve V is provided to divert a chosen proportion of the gas stream, typically 5-80, e.g. 20-80% by volume, to a recovery unit El incorporating chilled acetone of which the temperature can be controlled by a heat exchanger (generally using a circulating refrigerant) Means are provided in El to remove the condensed acetone to storage.

Gas emerging from the recovery unit El passes via a fan D2 to be combined with the recirculating, untreated, gas flow to the heat-exchanger G, via run c7

A take off point above fan D2 allows for surplus gas from the gaslock to be passed through a second recovery unit E2 linked to a secondary acetone return pump F2, the emerging gas being lead to the atmosphere via run c5, typically, at an acetone concentration of about 30 g/m .

An inlet for fresh gas c2 is supplied upstream of fan Dl in order to fill the system during the starting-up procedure. Also, the main exit line from the tower A is provided with an oxygen meter K to check for the presence of any dilution of the inert gas with air.

Claims

CLAIMS:

1 A method of spinning a cellulose ester dope comprising extruding under pressure a dope containing cellulose ester dissolved in a spinning solvent, into a stream of gas in a spinning zone, separating the spun filaments from the gas stream, recovering the solvent from the gas stream; and venting at least some of said gas to the atmosphere;

characterised in that the gas is nitrogen or another ineπ gas which is recycled to the spinning zone, and the solvent recovery includes the steps of

a) continuously passing at least a portion of the gas stream emerging from the spinning zone through an condensation unit controlled at a predetermined temperature so as to capture a required proportion of solvent from the gas stream; and

b) recirculating the thus-treated gas stream to the spinning zone together with any untreated gas stream

2 A method according to Claim 1, in which from 5% to 80% by volume of the gas stream is passed through the condensation unit

3 A method according to Claim 1 or Claim 2, in which the solvent is maintained at a temperature of from -35 to +40°C

4 A method according to any of Claims 1 to 3, in which the gas stream passed through the spinning zone is maintained at from 30 to 130°C

5. A method according to any of Claims 1 to 4, in which the gas stream is pumped out of the spinning zone as the spun filaments exit so as to create an underpressure and a fresh supply of the gas is introduced, thereby providing a gas lock to prevent emergence of solvent-laden gas. and an equivalent quantity of the treated gas emerging from the condensation unit is passed to the atmosphere via a second condensation recovery unit controlled at a predetermined temperature to capture acetone residues to provide the exit gas with an acetone content below the level required by local regulations.

6 A method according to Claim 5, in which the second bath is controlled at a temperature of from 0 to -70°C

7 A method according to any of Claims 1 to 6, in which the gas recirculated to the spinning zone contains from 15 to 1500 g solvent per cubic metre

8 A method according to any of Claims 1 to 6, in which the gas recirculated to the spinning zone contains from 150 to 250 g solvent per cubic metre

9 A method according to Claim 7, in which the gas recirculated to the spinning zone has a solvent level of from 15 to 25% of that of the gas leaving the spinning zone

10. A method according to any of the preceding claims in which the condensation unit comprises a bubble column of chilled solvent cooled by a heat exchanger

1 1 A method according to any of Claims 1 to 9, in which the condensation unit comprises a spray cooler using solvent chilled by contact with a heat exchanger before being sprayed into the solvent vapour-rich gas stream.

12 A method according to any of Claims 1 to 1 1, in which the solvent is acetone

13 A method according to Claim 12, in which the cellulose ester is a diacetate