Chapter 4, Problem 4.55P

A stream containing H₂S and inert gases and a second stream of pure SO₂ are fed to a sulfur recover)' reactor, where the reaction

2H₂S + SO₂ -» 3S + 2H₂O

takes place. The feed rates are adjusted so that the ratio of H₂S to SO₂ in the combined feed is always stoichiometric.

In the normal operation of the reactor the flow rate and composition of the H₂S feed stream both fluctuate. In the past, each time either variable changed the required SO₂ feed rate had to be reset by adjusting a valve in the feed line. A control system has been installed to automate this process. The H₂S feed stream passes through an electronic flowmeter that transmits a signal Rf directly proportional to the molar flow rate of the stream, h_{. When = 100 kmol/h, the transmitted signal R₍= 15 mV. The mole fraction of H₂S in this stream is measured with a thermal conductivity detector, which transmits a signal /f_a. Analyzer calibration data are as follows:

fla(mV)	0	25.4	42.8	58.0	71.9	85.1
x(mol H2S/mol)	0.00	0.20	0.40	0.60	0.80	1.00

The controller takes as input the transmitted values of Rf and R₃and calculates and transmits a voltage signal j?_c to a flow control valve in the SO₂ line, which opens and closes to an extent dependent on the value of R_c. A plot of the SO₂ flow rate. fi_c, versus R_con rectangular coordinates is a straight line through the points (R_c= 10.0mV.h_c= 25.0kmol/h) and (/<. = 25.0mV.h_c = 60.0kmol/h).

1. Why would it be important to feed the reactants in stoichiometric proportion? (Hint: SO₂ and especially H₂S are serious pollutants.) What are several likely reasons for wanting to automate the SO₂ feed rate adjustment?
2. If the first stream contains 85.0 mole% H₂S and enters the unit at a rate of hf = 3.00 X 10² kmol/h, what must the value of n_t(kmol SO₂/h) be?
3. Fit a function to the H₂S analyzer calibration data to derive an expression for x as a function of R_t. Check the fit by plotting both the function and the calibration data on the same graph.
4. Derive a formula for R_cfrom specified values of Rf and R_a, using the result of Part (c) in the derivation. (This formula would be built into the controller.) Test the formula using the flow rate and composition data of Part (a).
5. The system has been installed and made operational, and at some point the concentration of H₂S in the feed stream suddenly changes. A sample of the blended gas is collected and analyzed a short time later and the mole ratio of H₂S to SO₂ is not the required 2:1. List as many possible reasons as you can think of for this apparent failure of the control system.