Page 89 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Solving for the only unknown gives x = 1.36 kmol/h. Thus, the toluene recycle, Stream 11, will be
increased from 35.7 to 37.06 kmol/h, an increase of 4%, while the increases in Streams 4 and 6 will be
approximately 0.1%. Based on this result, Alternative A will probably be less expensive than Alternative
B.

2.4.3 Other Issues Affecting the Recycle Structure That Lead to Process Alternatives

There are many other issues that affect the recycle structure of the PFD. The use of excess reactant, the
recycling of inert materials, and the control of an equilibrium reaction are some examples that are
addressed in this section.

How Many Potential Recycle Streams Are There? Consider first the reacting species that are of value.
These are essentially all reactants except air and maybe water. Each reacting species that does not have a
single-pass conversion > 99% should be considered as a potential recycle stream. The value of 99% is an
arbitrarily high number, and it could be anywhere from 90 to > 99%, depending on the cost of raw
materials, the cost to separate and recycle unused raw materials, and the cost of disposing of any waste
streams containing these chemicals.

How Does Excess Reactant Affect the Recycle Structure? When designing the separation of recycled
raw materials, it is important to remember which reactant, if any, should be in excess and how much this
excess should be. For the toluene HDA process, the hydrogen is required to be in excess in order to
suppress coking reactions that foul the catalyst. The result is that the hydrogen:toluene ratio at the inlet of
the reactor (from Table 1.5) is 735.4:144, or slightly greater than 5:1. This means that the hydrogen

recycle loop must be large, and a large recycle compressor is required. If it were not for the fact that this
ratio needs to be high, the hydrogen recycle stream, and hence the recycle compressor, could be
eliminated.

How Many Reactors Are Required? The reasons for multiple reactors are as follows.
• Approach to Equilibrium: The classic example is the synthesis of ammonia from hydrogen and
nitrogen. As ammonia is produced in a packed bed reactor, the heat of reaction heats the
products and moves the reaction closer to equilibrium. By adding additional reactants between
staged packed beds arranged in series, the concentration of the reactants is increased, and the
temperature is decreased. Both these factors move the reaction away from equilibrium and
allow the reaction to proceed further to produce the desired product, ammonia.
• Temperature Control: If the reaction is mildly exothermic or endothermic, then internal heat
transfer may not be warranted, and temperature control for gas-phase reactions can be achieved
by adding a “cold (or hot) shot” between staged adiabatic packed beds of catalyst. This is
similar to the ammonia converter described earlier.
• Concentration Control: If one reactant tends to form by-products, then it may be advantageous to
keep this reactant at a low concentration. Multiple side feeds to a series of staged beds or

reactors may be considered. See Chapter 20 for more details.
• Optimization of Conditions for Multiple Reactions: When several series reactions
(A→R→S→T) must take place to produce the desired product (T) and these reactions require
different catalysts and/or different operating conditions, then operating a series of staged
reactors at different conditions may be warranted.