Chapter 4, Problem 4.73P

Chlorobenzene (C^HsCl), an important solvent and intermediate in the production of many other chemicals, is produced by bubbling chlorine gas through liquid benzene in the presence of ferric chloride catalyst. In an undesired side reaction, the product is further chlorinated to dichlorobenzene, and in a third reaction the dichlorobenzene is chlorinated to trichlorobenzene.

The feed to a chlorination reactor consists of essentially pure benzene and a technical grade of chlorine gas (98 wt% Cl₂, the balance gaseous impurities with an average molecular weight of 25.0). The liquid output from the reactor contains 65.0 wt% C₆H₆, 32.05? C₆H₅C1, 2.5% C₆H₄C1₂. and 0.5% C6H3CI3. The gaseous output contains only HC1 and the impurities that entered w ith the chlorine.

1. You wish to determine (i) the percentage by which benzene is fed in excess, (ii) the fractional conversion of benzene, (iii) the fractional yield of monochlorobenzene, and (iv) the mass ratio of the gas feed to the liquid feed. Without doing any calculations, prove that you have enough information about the process to determine these quantities.
2. Perform the calculations.

Why would benzene be fed in excess and the fractional conversion kept low?

What might be done with the gaseous effluent?

It is possible to use 99.9% pure (“reagent-grade”) chlorine instead of the technical grade actually used in the process. Why is this probably not done? Under what conditions might extremely pure reactants be called for in a commercial process? (Hint: Think about possible problems associated with the impurities in technical grade chemicals.)