Process of Removing Trace Sulfur Compounds From Crude Oil

Question

You are working in an oil refinery and to meet air quality standards you must remove trace sulfur compounds that come along with the crude oil. These H₂S and SO₂. A process has been suggested to convert these compounds into elemental sulfur and water using the following reaction:

2H₂S(g) + SO₂(g) → 3S(s) + 2H₂O(g)

A drawing of the process is given below. You have two input streams F₁ and F₂ which are combined together and this new stream, F0 is connected to the recycle stream R. The two feed streams are fed at a stoichiometric ratio as shown above but contain a fair amount of CO₂ as is shown. The solid sulfur is completely removed and the condenser completely removes the water. The gas stream leaving the condenser is split into a purge stream and recycle stream.

a.    The flow in F1 is 100.00 lb-moles/sec.
b.    The overall process conversion of H₂S is 95% (actually removed).
c.    The recycle to feed ratio is R/F0 = 15.0

What is the purpose of the purge stream?

 
Summary

The question belongs to Chemical Engineering and it is about an oil refinery where trace sulfur compounds such as H2S and SO2 have to be removed from crude oil. The question is about the purpose of purge stream in the process.