Page 97 - The engineering of chemical reactions

P r o b l e m s 8 1
(f) Zeolites are crystalline synthetic clays that are porous and contain anionic groups in their
lattice. The largest use of zeolites industrially is in the soap and detergent business, where
a few percent of the Na salt of Zeolite A is added to most detergents. Why?

2.2 In ethanol production by fermentation of corn, hydrolyzed ground corn in a water suspension
containing 50% corn by weight is mixed with sufficient enzyme to produce ethanol by the
approximate reaction

C6H1206 + 2C2H50H + 2C02
When the solution reaches 12% ethanol, the yeast dies and the reaction stops.
(a) How many bushels of corn are required to produce 500 tons of pure ethanol per day? Assume
1 bushel of corn weighs 56 lb.
(b) The process requires 72 hours to go to completion. What size batch-reactor tank or tanks
are required for this process?

2.3 The onset of World War II caused major expansion and technology innovation in the petroleum
and petrochemical industries, with major increases in demands for (1) branched aliphatic
and aromatic hydrocarbons, (2) high-purity toluene, (3) butadiene, and (4) nylon. What uses
produced each of these demands? [Some hints: The jet engine was important only after 1950,
rubber plantations were located in Malaysia, and cotton becomes weak and mildews when wet.]

2.4 You wish to design a plant to produce 100 tons/day of ethylene glycol from ethane, air, and water.
The plant has three reactor stages, ethane dehydrogenation, ethylene oxidation, and ethylene
oxide hydration.
(a) What are the reactions?
(b) Both dehydrogenation and hydration have nearly 100% selectivity (with recycle of unre-
acted reactants), but ethylene to ethylene oxide has only 70% selectivity with an old catalyst
and 90% selectivity with a new and expensive catalyst. How many tons/day of ethane do
we need to supply to this plant with each of these catalysts?
(c) Ethylene to EO has a heat of reaction of -25 kcalfmole, and the undesired byproducts are
exclusively CO2 and H20. [You can look up the heat of combustion of ethylene.] What is
the rate of heat removal in watts with the two catalysts?
(d) If all this heat is used to produce low-pressure steam (from 25°C at 1 atm), approximately
how many tons of steam per day can be produced?
(e) Could the heat from this reaction be used to provide heat in the other two reactions? Where
else could it be used?
(f) Sketch a flow diagram of this plant including reactors and separation units.

2.5 We want to hydrolyze 500 lb per day of an ester at an initial concentration of 5 molar (the
ester has a molecular weight of 120) in aqueous basic solution in a batch process, and we need
product that is 99% hydrolyzed. In benchtop experiments in a flask we find that 50% of the
ester hydrolyzes in 15 min for initial ester concentrations of either 1 or 5 molar. We also find
that, when we react for 8 h, all of the ester has hydrolyzed. It takes 1 h to empty the reactor and
refill and heat it to start another batch.
(a) What size reactor will we need?
(b) What size reactor will we need if we can tolerate 90% conversion?