2. An engineer suggests that an air stream containing 0.5 mol % hydrogen sulphide (H2S) can be contacted with an aqueous solution of 1wt% sodium hydroxide (NaOH) in a small counter-current staged column. The H2S will react with NaOH by the reaction H2S+2NaOHNa2S+2H2O The Na2S is non-volatile and the reaction is very rapid and irreversible so that at equilibrium the concentration of H2S in the gas phase is essentially zero. Consider the mole ratio of sulphur in the water, X, to be the total free dissolved hydrogen sulphide (H2S) plus the fixed sulphur as Na2S. Atomic weights are: H=1,S=32,Na=23,O=16,N=14 a. Convert all concentrations into mole fractions. What is the minimum flow rate of the aqueous solution of 1 wt\% NaOH required to remove all but 2ppm(w/w) of H2S from the air stream? [10 marks] b. The flow rate of water used is twice that found in (a). Use Figure 2.1 (or your own graph paper or Excel) to construct a McCabe-Thiele diagram of the process. What is the number of theoretical equilibrium stages required? Include your diagram with your answer sheet. [6 marks] c. If the mass transfer rate is slow so that the stage efficiency is 90%, how many stages are needed at the same flow rates as part (b)? [5 marks] d. If a counter-current packed column is used instead of a plate column, what is the number of transfer units, NOG, for this process? The flow rates of the air and gas streams are the same as in part (b)