Page 79 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Addition of Feeds Required to Stabilize Products or Enable Separations. Generally, product
specifications are given as a series of characteristics that the product stream must meet or exceed.
Clearly, the purity of the main chemical in the product is the major concern. However, other
specifications such as color, density or specific gravity, turbidity, and so on, may also be specified. Often
many of these specifications can be met in a single piece or train of separation equipment. However, if the
product stream is, for example, reactive or unstable, then additional stabilizing chemicals may need to be
added to the product before it goes to storage. These stabilizing chemicals are additional feed streams to
the process. The same argument can be made for other chemicals such as solvent or catalyst that are
effectively consumed in the process. If a solvent such as water or an organic chemical is required to make
a separation take place—for example, absorption of a solvent-soluble chemical from a gas stream—then

this solvent is an additional feed to the process (see Appendix B, Problem 5—the production of maleic
anhydride via the partial oxidation of propylene). Accounting for these chemicals both in feed costs and in
the overall material balance (in the product streams) is very important.

Inert Feed Material to Control Exothermic Reactions. In some cases, it may be necessary to add
additional inert feed streams to the process in order to control the reactions taking place. Common
examples of this are partial oxidation reactions of hydrocarbons. For example, consider the partial
oxidation of propylene to give acrylic acid, an important chemical in the production of acrylic polymers.
The feeds consist of nearly pure propylene, air, and steam. The basic reactions that take place are as
follows:

All these reactions are highly exothermic, not limited by equilibrium, and potentially explosive. In order
to eliminate or reduce the potential for explosion, steam is fed to the reactor to dilute the feed and provide
thermal ballast to absorb the heat of reaction and make control easier. In some processes, enough steam
(or other inert stream) is added to move the reaction mixture out of the flammability limits, thus
eliminating the potential for explosion. The steam (or other inert stream) is considered a feed to the
process, must be separated, and leaves as a product, by-product, or waste stream.

Addition of Inert Feed Material to Control Equilibrium Reactions. Sometimes it is necessary to add
an inert material to shift the equilibrium of the desired reaction. Consider the production of styrene via the
catalytic dehydrogenation of ethyl benzene:

This reaction takes place at high temperature (600–750°C) and low pressure (<1 bar) and is limited by
equilibrium. The ethyl benzene is co-fed to the reactor with superheated steam. The steam acts as an inert
in the reaction and both provides the thermal energy required to preheat the ethyl benzene and dilutes the
feed. As the steam-to-ethyl benzene ratio increases, the equilibrium shifts to the right (LeChatelier’s
principle) and the single-pass conversion increases. The optimum steam-to-ethyl benzene feed ratio is