Page 90 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Do Unreacted Raw Material Streams Need to Be Purified Prior to Recycling? The next issue is
whether the components need to be separated prior to recycle. For example, if distillation is used to
separate products from unused reactants, and if two of the reactants lie next to each other in a list of
relative volatility, then no separation of these products is necessary. They can be simply recycled as a
mixed stream.

Is Recycling of an Inert Warranted? We next consider components in the feed streams that do not react,
that is, are inert. Depending on the process, it may be worth recycling these streams. For example,
consider the water feed to the absorber, Stream 8, in the acetone production process (Appendix B, Figure
B.10.1). This water stream is used to absorb trace amounts of isopropyl alcohol and acetone from the
hydrogen vent, Stream 5. After purification, the water leaves the process as a waste water stream, Stream
15. This water has been purified in column T-1103 and contains only trace amounts of organics. An
alternative process configuration would be to recycle this water back to the absorber. This type of
pollution prevention strategy is discussed further in Chapter 25.

Can Recycling an Unwanted Product or an Inert Shift the Reaction Equilibrium to Produce Less of
an Unwanted Product? Another example of recycling an inert or unwanted product is to use that material

to change the conversion and selectivity of an equilibrium reaction. For example, consider the production
of synthesis gas (H and CO) via the partial oxidation (gasification) of coal:
2

Coal, shown here simply as a mixture of carbon and hydrogen, is reacted with a substoichiometric amount
of pure oxygen in a gasifier, and steam is added to moderate the temperature. The resulting mixture of
product gases forms the basis of the synthesis gas. The carbon dioxide is an unwanted by-product of the

reaction and must be removed from the product stream, usually by a physical or chemiphysical absorption
process. A viable process alternative is recycling a portion of the separated carbon dioxide stream back
to the reactor. This has the effect of pushing the equilibrium of the water-gas shift reaction to the left, thus
favoring the production of carbon monoxide.

Is Recycling of an Unwanted Product or an Inert Warranted for the Control of Reactor Operation?
As we have mentioned previously, for highly exothermic reactions such as the partial oxidation of organic
molecules, it is sometimes necessary to add an inert material to the reactor feed to moderate the
temperature rise in the reactor and/or to move the reacting components outside of the explosive
(flammability) limits. The most often used material for this purpose is steam, but any inert material that is
available may be considered. For example, in the coal gasification example given earlier, steam is used
to moderate the temperature rise in the reactor. For the case of recycling carbon dioxide to affect the
water-gas shift reaction, there is another potential benefit. The recycling of carbon dioxide reduces the
amount of steam needed in the feed to the reactor, because the carbon dioxide can absorb heat and reduce
the temperature rise in the reactor.

What Phase is the Recycle Stream? The phase of the stream to be recycled plays an important role in