Page 84 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Clearly these conversions are much lower than for toluene. The single-pass conversion is kept low
because a high hydrogen-to-hydrocarbon ratio is desired everywhere in the reactor so as to avoid or
reduce coking of the catalyst. However, the low overall conversion of hydrogen indicates poor raw
material usage. Therefore, the questions to ask are, Why is the material usage for toluene so much better
than that of hydrogen? and, How can the hydrogen usage be improved? These questions can be answered
by looking at the ease of separation of hydrogen and toluene from their respective streams and leads us to
investigate the recycle structure of the process.

2.4.2 Identification and Definition of the Recycle Structure of the Process

There are basically three ways that unreacted raw materials can be recycled in continuous processes.
1. Separate and purify unreacted feed material from products and then recycle.
2. Recycle feed and product together and use a purge stream.
3. Recycle feed and product together and do not use a purge stream.

Separate and Purify. Through the ingenuity of chemical engineers and chemists, technically feasible
separation paths exist for mixtures of nearly all commercially desired chemicals. Therefore, the decision
on whether to separate the unreacted raw materials must be made purely from economic considerations. In
general, the ease with which a given separation can be made is dependent on two principles.

• First, for the separation process (unit operation) being considered, what conditions (temperature
and pressure) are necessary to operate the process?
• Second, for the chemical species requiring separation, are the differences in physical or chemical
properties for the species, on which the separation is based, large or small? Examples that
illustrate these principles are given below.

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.

Example 2.3

For the separation of methane and hydrogen, first look at distillation:

Normal boiling point of methane = –161°C

Normal boiling point of hydrogen = –252°C

Separation should be easy using distillation due to the large difference in boiling points of the two
components. However, in order to get a liquid phase, we will have to use a combination of high pressure
and low temperature. This will be very costly and suggests that distillation is not the best operation for
this separation.

Absorption