Economics of control

Since the conversion limit even in improved Claus plants is not readily raised above about 97%, the emphasis in emission control has now passed to using tail gas-treating plants to attain overall conversion efficiencies of 99% or more. The economics of control at Claus plants are probably more favorable than in any other case requiring control of dilute gas streams (i.e., those containing less than about 2—3% sulfur dioxide). 

Back in Chapter 5 we discussed a method for quantitatively incorporating the economics of control into conventional steady-state design economics. For a given process and control system, the idea is to specify upper and lower limits on product quality and determine what fraction of the time the process is within these specification limits. The capacity of the plant is equal to this fraction times the product flow rate. Periods of off-specification production must be handled by reworking or disposing of the material, which carries some economic cost. The steady-state economic calculations are made using the on-specification capacity. 

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