Why is online analysis installed after the reactor?

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The sulfur recovery units designed today are equipped with an on-line detector in the gas phase of the condenser at the outlet of the Claus reactor. This detector is interlocked with the air control valve at the inlet of the acidic gas combustion furnace, in order to control and measure the H2S:SO2 ratio at 2:1. The purpose is to maintain this 2:1 ratio of H2S to SO2 as it enters the Claus reactor. But why is this detector installed on the exhaust gas pipeline rather than on the process pipeline at the inlet of the Claus reactor? The highest sulfur conversion rate is achieved in the Claus reaction when the ratio of hydrogen sulfide to sulfur dioxide is 2:1. This ratio is ensured through an appropriate air supply volume to the combustion furnace; however, if the air supply volume is inappropriate (too much or too little), the ratio of hydrogen sulfide to sulfur dioxide entering the reactor will deviate from 2. The reaction between the two to produce sulfur and water in the reactor is reversible; if their ratio is not 2, after passing through two-stage or three-stage reactors, the ratio between them in the process gas will deviate from 2 even more than it did before entering the reactor. (Assuming it is 1.9 or 2.1 before entering the reactor, the corresponding ratio after reaction may be 1.6 or 2.5.) Therefore, online analysis is employed after the reactor in order to more accurately determine whether the air supply volume is appropriate; if the analyzed ratio is 2.1, it indicates that the ratio before entering the reactor is a value less than 2.1 but slightly greater than 2. Doing this serves to amplify the deviation, thereby enabling more precise control of the air flow into the furnace. This post was last edited by wangzhi on 2008-4-2 07:50]