Page 41 - The engineering of chemical reactions

Multiple Reactions 25

The equilibrium in this isomerization reaction is 58% fructose and 42% glucose. An enzyme
was discovered called glucose isomerase, which isomerizes the molecule by exchanging
the end aldehyde group with the neighboring OH group to convert glucose into fructose.
This enzyme cannot isomerize any other bonds in these molecules. This is fortunate because
only this isomer tastes sweet and is digestible, so no manmade catalysts could hope to be
successful in this process.
The high-fructose corn syrup (HFCS) industry produces -10 million tons of fructose
from glucose annually (how many pounds do you purchase per year?), and this is the major
industrial bioengineering processes today, at least in volume.
This process could not work without finding an effective and cheap way to run this
biological reaction and to separate fructose from glucose (sucrose contains 50% glucose).
These were accomplished by finding improved strains of the enzyme, by finding ways to
immobilize and stabilize the enzyme on solid beads to keep them in the reactor, and by
finding adsorbents to separate fructose from glucose.
These are accomplished in large fermenters in chemical plants, mostly in the Midwest.
The cheapest feedstock is starch rather than sugar (other enzymes convert starch to glucose),
and corn from the Midwest is the cheapest source of starch.
Artificial sweeteners have also been developed to give the taste of sweetness without
the calories. These chemicals have sweetness many times that of sugar; so they sell for high
prices as low-calorie sweeteners. Many artificial flavors have also been developed to replace
natural biological flavors. In all cases we search for processes that convert inexpensive raw
materials into chemicals that taste or smell like natural chemicals, either by producing the
same chemical synthetically or by producing a different chemical that can replace the natural
chemical.
In this book we will consider mostly the simpler chemical reaction processes in the
petroleum and commodity chemicals industries because they are more “central” to chemical
engineering. However, the same principles and strategies apply in the pharmaceutical and
food industries, and students may need these principles for these or other applications later
in their careers.

MULTIPLE REACTIONS

Every chemical process of practical interest, such as the isomerization of butylenes written
previously, forms several products (some undesired) and involves multiple reactions.
Consider next the reaction system
N2+02-+2N0
2NO+02 + 2N02
In NO, smog formation (NO, is a mixture of NO, N20, NO2, N204, and N205) the NO is
produced by reaction of N2 and 02 at the high temperatures of combustion in automobiles
and fossil fuel power plants, and NO2 and the other NO, species are produced by subsequent
low-temperature oxidation of NO in air. NO is colorless, but NO2 absorbs visible radiation
and produces brown haze. We write these reactions as a set of two reactions among four
species,
-Al -A2+2A3 =0
-2A3--A2+2A4=0