GB791784A - Method of recovering the solvent used in the dry-spinning of polyacrylonitrile or copolymers thereof - Google Patents

Abstract

<FORM:0791784/IV (a)/1> In the dry-spinning of a solution of polyacrylonitrile (or of an acrylonitrile copolymer containing at least 85 per cent by weight of acrylonitrile) in an organic solvent, the solvent is recovered from the evaporative medium withdrawn from the spinning zone by scrubbing the medium with a liquid which is a solvent for the polymer and miscible or identical with the solvent used for preparing the spinning solution and which is circulated through a closed circuit comprising in series a zone in which the liquid is contacted in countercurrent with the evaporative medium and a zone in which the liquid, containing solvent extracted from the evaporative medium, is cooled. The solvent may be dimethyl formamide, dimethyl acetamide, butyrolactone glycol carbonate, dimethyl sulphone or a cyclic sulphone. In an example, and as shown in Fig. 1, a spinning solution of polyacrylonitrile in dimethyl formamide is prepared by feeding the polymer and solvent through pipes 2, 3 respectively into a mixing tank 1, and the solution is supplied by the pipe 4 to the spinneret 5 located in the head 6 of the spinning cell 7. Dry nitrogen gas, pre-heated in a heater 11, is supplied through the pipe 10 to the spinning cell at a temperature sufficiently high to maintain the spinning head zone at 180-200 DEG C. The solvent-charged nitrogen passes from the head of the spinning cell through a pipe 12 to the water-cooled condenser 13, where the gas-vapour mixture is cooled to 15 DEG C. and 75 per cent of the total quantity of dimethyl formamide in the nitrogen is condensed. The condensate leaves the condenser through the pipe 14 which joins the pipe 3 through which the condensed solvent is returned to the solution tank 1. The cooled nitrogen containing the remainder of the dimethyl formamide passes through the pipe 15 from the condenser 13 to the scrubbing tower 16 packed with Raschig rings 17 where it is contacted in countercurrent with dimethyl formamide which has been previously cooled to - 20 DEG C. in the cooler 23, whence it is supplied to the top of the scrubber through the pipe 19. The nitrogen emerges from the top of the tower at a temperature of - 5 DEG C. with only a very low dimethyl formamide content and is conducted through a pipe 18 back to the heater 11. The dimethyl formamide discharged from the bottom of the tower 16 collects in the tank 20, whence a portion is pumped by means of pump 21 and pipe 22 to the cooler 23 while the remainder is returned to the pipe 14 through the pipe 24 and (optionally) a dehydrator 25. In another example, the evaporative medium leaving the spinning cell at about 190 DEG C. and saturated with solvent vapour is conducted straight to a packed counter-current scrubbing tower down which flows dimethyl formamide, precooled to 0 DEG C., which absorbs practically the whole of the solvent from the evaporative medium. The latter leaves the top of the scrubber at a temperature of about + 5 DEG C. The scrubbing liquor discharged from the bottom of the scrubber at about 50 DEG C. flows to a cooler supplied with water at 13 DEG C. in which its temperature is reduced to 20 DEG C. and from which it flows to a second cooling zone in which it is cooled to 0 DEG C. by brine at - 10 DEG C. and is recirculated to the scrubbing tower. Some of the solvent extracted from the evaporative medium by the scrubber is continuously withdrawn from the closed circuit through a branch pipe and used in the preparation of a further quantity of spinning solution. In order to prevent decomposition of the solvent and discoloration of the filaments, the moisture content of the air or inert gas entering the spinning cell is preferably reduced to less than 1 gram per cubic metre by eliminating water from the solvent condensed by the cold scrubbing medium by distillation or chemical dehydration means, preferably operated by an automatic control device comprising a moisture-sensitive indicating unit located in the evaporative medium circuit either before the preheater or between the preheater and the spinning cell.