In Fig. 3 the hourly costs due to purchase of reactants and disposal
of residues are plotted as a function of 1st stage conversion
for each of the three waste composition scenarios in Table 2. In
the figure, square dots pinpoint the values of 1st stage conversions
for which reactant consumption is minimum, while circle dots
indicate the values of 1st stage conversions for which the economic
optimum is obtained. For all the three waste composition scenarios,
the optimised 1st stage conversion value is in the range 55–
60%. The plots also evidence that deviation from the optimal operating
point implies steeper cost increases when the acid gas concentration
in the untreated flue gas is higher