Page 40 - The engineering of chemical reactions

24 Reaction Rates, the Batch Reactor, and the Real World

V;Aj =O .
2
j=l
El
The quantity vj is the stoichiometric coefficient of species j, and by convention Vj x 0 for
a reactant and vj > 0 for a product.
All reactions must satisfy mass conservation. The reaction A + B must be an
isomerization reaction because the molecular weights of A and B must be identical. Also,
one should add to these relations the requirements that the number of atoms of each element
must be conserved, but this is usually intuitively obvious for most reaction systems. We do
this whenever we balance a chemical equation.

High-fructose corn syrup

There is at least one important reversible isomerization reaction in chemical engineering, the
conversion of glucose into fructose. All soft drinks are essentially sugar water with a small
amount of flavoring added (usually synthetic), and you like to drink them basically because
they taste sweet. The natural sugar in sugar cane and sugar beets is basically sucrose, a
disaccharide consisting of one molecule of glucose and one molecule of fructose. Plants
form these molecules as food reserves for themselves, and we harvest the plants and extract
sucrose in water solution from them. We have bred strains of cane and beets that produce
much more sucrose than they would by natural selection.
Sucrose is rapidly dissociated into glucose and fructose by the enzymes in your mouth
and in your stomach, and your taste receptors sense sweetness. A problem (for Coca Cola)
is that fructose tastes five times as sweet as glucose; so 40% of the sucrose they purchase
is wasted compared to pure fructose. A problem for you is that both sugars have the same
calories, and the soft drink companies want to advertise lower calories for an acceptable
sweetness.
Within the past 20 years chemical engineers figured out how to run the reaction to
convert glucose into fructose
glucose 2 fructose
which when drawn out looks as shown in Figure 2-2.

H 0 Figure 2-2 Isomerization reaction that converts
‘C4 CH,OH glucose into fructose. An enzyme causes this reac-
tion to run selectively near room temperature. This
process, the largest bioprocess in the chemical indus-
H-A-OH L-0 try, makes high-fructose corn syrup for the beverage
industry.
H O - A - H --) HO-L-H
H - A - O H - H-A-OH
H-A-OH H - A - O H