PEP Review IX-3-4
Toluene Steam Dealkylation
Published: May 1974
PEP Reports 30 and 30A covered the production of benzene by dealkylation. It was noted that patents proposed dealkylation in the presence of steam, but process developers had clearly concentrated on hydrodealkylation. Recent changes in refinery balances as well as in raw material and product values prompt us to reexamine this choice to determine its continued validity.
The primary reaction in hydrodealkylation of toluene is:
The primary reaction is steam dealkylation is:
Thus, hydrodealkylation consumes 1 mol of H2 per mol of toluene, while steam dealkylation produces 3 mols of H2, per mol of toluene in the principal reaction.
Many dealkylation plants were built when refineries had a surplus of hydrogen from catalytic reforming, and it was assigned fuel value. Moreover, fuel values were low in those days. Because the hydrodealkylation reaction forms CO2, which is probably burned, the net effect is degrading one CH4 group to fuel value, and there is no net cost for the H2 when it enters at fuel value and is subsequently burned.
New demands for hydrogen, particularly for desulfurization and hydro- cracking, have raised its value to its manufacturing cost by alternate means, less any cost of purification.