Chlorine processing compression

Bromine also can be removed from the chlorine gas after the cells. This is done by partial condensation in a process that also serves to chill the gas before compression. Section 9.1.6.4A on chlorine processing describes the use of compressor suction chillers to remove high boilers including bromine from the gas. It also warns of the danger of accumulation of hazardous quantities of NCI3. 

In a plant producing liquid chlorine, the compressed gas goes next to the liquefaction system. Rather than impose a pressure drop between the processes, the gas is allowed to flow freely into liquefaction. A valve on the uncondensed gas venting from the liquefaction unit (Section 9.1.7.2) controls the pressure on both systems. When chlorine is sent to another process without liquefaction, it would be possible to withdraw it on downstream pressure control and let the compressor outlet pressure fluctuate. This approach leads to variability in the differential pressure across the compressor recycle valve. Fluctuations in this flow can cause fluctuations in the compressor suction pressure and therefore in the cellroom chlorine header. It is better to control the compressor outlet pressure itself, even at the cost of another pressure control loop at the destination. Section 11.3.2.6 describes instrumentation hardware and the problems of transferring chlorine to more than one destination. 

There may be more oxygen in membrane cell chlorine than in the gas from the other processes. This can be a problem if the compressed chlorine is fed directly to chlorination processes. An offsetting advantage is that there is essentially no hydrogen in the chlorine gas. 

Air systems include compressed air as a plant utility and refined versions for more specialized uses. These include instrument air and breathing air. Of particular significance in a chlor-alkali plant is a supply of dry air for use in the chlorine processing section. An alternative to dry air is nitrogen, which also serves as an inert gas in the hydrogen plant and sometimes in chlorine liquefaction and tail-gas handling. For convenience, we include the discussion of nitrogen in the section on air systems. 

After leaving the electrolyzers, the chlorine gas is dried and compressed (liquefied), during which the hydrogen content in the chlorine gas will rise no sudden rises are to be expected in this stage of the process, hence, hydrogen measurements can have a response time of minutes here. 

The dry chlorine may be compressed and cooled in order to liquefy it. Liquefaction is essential to the merchant production of chlorine. It is also sometimes practised in order to enhance the purity of the chlorine sent to further processing. Liquefaction,... 

O Brien, T.F. White, I.F. (1998) Process engineering considerations in chlorine compression and liquefaction. In Modern Chlor-Alkali Technology, Vol. 7 (ed. S. Sealey), pp. 202-213. Society of Chemical Industry, London and Royal Society of Chemistry, Cambridge. 

Process pumps convey a variety of liquids required in processes involving acids, caustics, liquid chlorine, liquid vinyl chloride, and other hazardous fluids. Compressors at this location convey gaseous chlorine, anhydrous hydrogen chloride, refrigerants, and compressed air. 

---