1. Distillation Train
Let us see the train of distillation columns shown in Fig. 1. Since the
unwanted diphenyl is formed by a reversible reaction.
• The diphenyl can be recycled with toluene and it can be allowed to
build up till equilibrium level.
• This alternative would make it possible to eliminate one of the
distillation column although, the flow rate through the reactor
would increase.
• If it is decided to recover the diphenyl, it is expected that the
diphenyl toluene split will be very easy.
• It may be possible to use a side stream column to accomplish
benzene-toluene-diphenyl split i.e. benzene can be recovered as
overhead, toluene as side stream and diphenyl as bottom stream
can
2. If Methane-Benzene split in the stabilizer is easy.
Vapour Recovery System
Some of aromatics will be with vapours in flash drum.
• Condensation
• Absorption
• Adsorption
• Membrane separation
3. Simplified Flows sheet for the separation
Systems
• The flow sheet can be further simplified by lumping the vapors and liquid systems in a
single box.
Fig. 5.
Fig. 6.
Recycle Structure
• Fig. 1 , the flow sheet can be simplified as
4. Recycle Structure of the Flow sheet
A very simple flow sheet as shown in Fig. 6 has been obtained. It
can be used to estimate:
• The recycle flows and their effect on the reactor cost and the
cost of a gas recycle compressor, if any.
• Moreover, we can be tried to understand what design
questions are important to obtain this simplified
representation, without worrying about the additional
complexities, caused by the separation system or the energy
integration network.
• For example, we can study the factors that determine the
number of recycle streams, heat effects in the reactor,
equilibrium limitations in the reactor, etc.
• Thus, the recycle structure of the flow sheet can be studied
before the details of separation system.
5. INPUT/OUTPUT STRUCTURE OF THE SHEET
• This type of flow sheet helps in understanding the design variables
that affect the overall material balance without introducing any
other complications.
• Since raw material costs normally fall in the range of 33 to 85% of
the total product costs, the overall material balances are a
dominant factor in a design.
• Also, one can avoid the waste of time, in investigating the design
variables, in the ranges where the products and by-products are
worth less than the raw materials.
Possible Limitations:
Continuous/batch process
6. Hierarchy of Discussions
With the help of discussions held above, a systematic
approach to process design can be developed by reducing
the design problem to a hierarchy of decisions.
• Batch Vs/ Continuous
• Input output structure of flow sheet
• Recycle structure of the flow sheet
• General structure of the separation system
Vapour recovery
Liquid recovery
• Heat exchanger network
• These steps allow us to calculate equipment sizes and
costs as we proceed through various levels in the
hierarchy.
• If potential profit becomes negative at some level, the
alternative process should be looked into, or the design
problem can be terminated.
