Handling of Liquefaction Tail Gas

The chlorine remaining in liquefaction tail gas is an intolerable emission. If recovered, it can be a useful economic asset, especially in the larger plants, and many processes have been considered or used for its recovery [77]. TTiere are other sources of dilute chlorine 

Some smaller plants do not find it worthwhile to recover small quantities, and even in large plants it is necessary to provide a backup to any recovery process. The ultimate receptacle for residual chlorine then becomes a scrubber that removes it fiom the gas and converts it to the more easily handled hypochlorite. Section 9.1.10.3 discusses chlorine vent scrubbers. 

The other options include enhanced recovery of elemental chlorine and its conversion to another product. We might include, under enhanced recovery, the addition of higher-severity stages to a liquefaction plant. While this is more correctly a reduction in the amount of chlorine present in the tail gas rather than its recovery, it is a viable retrofit technique and one that has been used to replace some of the older recovery plants. A unit quantity of refrigeration becomes more expensive, hydrate accumulation may be more troublesome, and some of the problems of deep liquefaction are exacerbated. TTiese are the normal effects of extending the process, and they add nothing new to the technical discussion on liquefaction. 

One technique for addition of a stage to handle relatively small amounts of gas is to compress it to a higher pressure and return it to a separate tube bundle in a liquefier that otherwise operates at lower pressure. Another option not yet in known commercial practice but which may appear in the future is to concentrate the chlorine by using gas-separation membranes [78]. The richer gas would return to a liquefier for additional recovery. This is discussed in Section 17.4.2. 

Another technique is an absorption/stripping process in which the chlorine is absorbed in a solvent at high pressure and then recovered from solution by stripping at higher temperature and lower pressure. This is the subject of Section 9.1.9.1. 

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