Example 2 - HDA Process

Process Description

The second example we are going to study is the Hydrodealkylation of Toluene. The main reaction is

However there is also a side reaction which produces unwanted diphenyl.

Below is a flowsheet of the complete process. There then follows a description of the process giving more information on the role of each part of the equipment.

Figure 1 - HDA Process Without Control

The homogeneous reactions take place in the range 600oC - below this temperature the reaction rate is too slow - to 700oC - above this temperature a significant amount of hydrocracking takes place - and at a pressure of about 35 bar. An excess of hydrogen - a 5/1 ratio - is needed to prevent coking.

The toluene and hydrogen raw-material streams are heated and combined with recycled toluene and hydrogen before they are fed to the reactor. The product stream leaving the reactor contains hydrogen, methane, benzene, toluene and the unwanted diphenyl. We attempt to separate most of the hydrogen and methane from the aromatics by flashing away the light gases.

We would like to recycle the hydrogen leaving in the flash vapour, but the methane, which is produced in the reaction and enters as an impurity in the hydrogen stream, will accumulate in the gas-recycle loop. Hence a purge stream is required to remove both the feed and the product methane from the process.

Not all the hydrogen and methane can be separated from the aromatics in the flash drum and therefore we remove most of the remaining amount in a distillation column - the stabilizer - to prevent them from contaminating our benzene product. The benzene is then recovered in a second distillation column and finally the recycle toluene is separated from the unwanted diphenyl.

To attempt to place control loops on this flowsheet as it stands would be rather complex so once again we will use the hierarchical approach.