Example-Toluene Hydrodealkylation Process

In the HDA process, benzene is produced by toluene hydrodealkylation, a mildly exothermic reaction (ArH950 K = -50450J/mol) with moderate activation energy ( a = 217 600 J/mol)  

In this study, the following reaction rate expression is used  

By neglecting the secondary reactions and assuming perfect separation, the following equations can be derived  

Fj and ykj denote the mole flow rate of stream j and the molar fraction of the k component (H-hydrogen, T-toluene) in stream j, respectively X is the reaction conversion. 

In addition, the conversion X depends on the reactor volume according to the characteristic equations of the adiabatic tubular reactor. 

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Be able to use the process simulators systematically during process creation, following sequences similar to those illustrated later in this chapter for a toluene hydrodealkylation process. The reader will learn to simulate portions of the process (the reactor section, the distillation section, etc.) before attempting to simulate the entire process with its recycle loops. Many examples and exercises enable the reader to master these techniques. 

This example follows the simulation of the complete toluene hydrodealkylation process at the end of Section 4.3 and is presented without a solution because it is the basis for Exercise 5.4. To recycle the biphenyl to extinction, the flowsheet in Figure 4.20 is modified to eliminate the last distillation column, and unreacted toluene from the second column is recycled with biphenyl. This is accomplished by the reversible reaction... 

Since toluene is nothing more than benzene with a methyl group attached, creating one from another is relatively easy. Benzene, toluene, and for that matter, xylenes too, are coproduced in the processes just described—coke making, cat reforming, and olefin plants operations. The ratio of benzene to the other aromatics production is rarely equal to the chemical feedstock requirements.. fo.r the three. One method for balancing supply and demand is toluene hydrodealkylation (HDA). This process accounts for 10—15% of the supply of benzene in the United States and is a good example of what can be done when one or more coproducts are produced in proportions out of balance with the marketplace. 

For the hydrodealkylation process, the reactor effluent. Stream 9, is cooled and separated in a two-stage flash operation. The liquid. Stream 18, contains essentially benzene and toluene. The combined vapor stream, Streams 8 and 17, contain essentially methane and hydrogen. In Example 2.3. methods to separate the hydrogen in these two streams are considered and are used to screen potential changes in the recycle structure of the HDA process. 

These same notions can be extended to an entire plant in which several unit operations are connected together. The HDA process for hydrodealkylation of toluene to form benzene is a good example of where an eigenstructure can be found that provides a more easily and simply controlled plant. See Fig. 8.15. Assuming that the toluene feed rate to the unit is fixed, this plant has 22 valves that must be set. There are 11 inventory loops (levels and pressures), so they require 11 valves. One possible conventional control structure is shown in Fig. 8.15. 

The hydrodealkylation of side-chain aromatics to nonsubstituted parents is a major process in petrochemistry. A typical example is the conversion of toluene to benzene ... 

We adopt here the principle of learning by example . The methodology will be illustrated by developing flowsheet alternatives for the process known as the Hydrodealkylation of Toluene, commonly abbreviated as HDA. This example, already... 

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