Spin bath degassing
During the spinning process, CS2 is released. Some of this gas reacts chemically to form H2S.
These two gases, together with air and CO2, are dissolved in the spin bath or form gas bubbles in the released spin bath solution. Some of the gases are exhausted from the spin bath bottom tank. The sulphur compounds in the waste gas are recovered in recovery plants.
Some of the gases contained in the spin bath react with other substances to form solid sulphide compounds that cause serious contamination of the spin bath circuit. It is therefore recommended to install a full-flow degassing system where the separated, highly concentrated gas is subsequently fed to a gas recovery unit.
In Detail
Two-stage degassing systems where most of the gas bubbles and dissolved gases are removed under vacuum. A steam jet ejector and a water ring vacuum pump generated the necessary vacuum. The extracted gas is a rich gas containing high concentrations of H2S and CS2. The concentration is above the lower explosion limit and therefore highly flammable.
The spin bath to be degassed is pumped from the degassing bottom tank to a two-stage degassing unit. In the first degassing column (1), the spin bath is fed through an injector nozzle (2) to form a fine spray mist that percolates through the column. The upper degassing column acts as both a degassing stage and a direct-contact condenser with vacuum. As the pressure is very low, some of the gases contained in the spin bath spray escape already at this stage. The pressure is however high enough to ensure that the mixed vapour produced by the steam jet ejector is nearly fully condensed.
The partially degassed spin bath solution whose temperature is increased by approx. 0.5°C collects at the bottom of the first column and is subsequently sprayed through spray nozzles into the second degassing column (3). In this column, there is even a higher vacuum, so that the solution begins to boil and cool down by approx. 0.3°C.
The partial pressure of the separated gases is kept low by the expansion steam flowing in the same direction, so that less than 10 per cent of the gases initially contained remain in the spin bath as it exits the column. The expansion steam and the gases separated in the lower degassing stage are extracted by the steam jet ejector and compressed to the pressure level pertaining in the upper stage where steam condenses.
The water-vapour saturated gases in the upper section of the first degassing column are extracted by a vacuum pump (4). To purify the spin bath droplets, the gas jet is first fed through a scrubber (5), compressed by a vacuum pump to a pressure of approx. 1 bar and subsequently fed to the recovery system.
