Page 100 - The engineering of chemical reactions

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

(c) It is important that the temperature of the reactor and regenerator remain constant, indepen-
dent of reactions and conversions. How might the reactor be controlled to maintain constant
temperatures? There are many options.

2.16 The enthalpies of some hydrocarbon reactions are shown here:

Reaction A HR (kcal/mol)
C2H6 -+ 2C, + 3H2 $20.2
CzH4 -+ 2C, + 2H2 -12.5
C2H2 -+ 2C, + H2 -54.5
C2H4 + H2 -+ C2H6 -7.5
2C2H4 -9 C4Hg -24.7

Estimate the final temperatures if a container of these gases at 25°C suddenly goes to complete
reaction. Use C, = 5 R. [One answer: - 8OOO”C!]

2.17 (a) What safety precaution is usually taken in storing acetylene?
(b) How much excess Hz would have to be used in a gas-phase ethylene hydrogenation process
to have the adiabatic temperature rise not exceed 3OO”C? List at least two reasons why
isobutane is a preferred diluent instead of Hz to prevent thermal runaway in this reaction.

2.18 Polyurethanes are used for most plastic parts in automobiles such as bumpers and nonmetal
body panels. They are made from toluene diisocyanate and ethylene glycol. We are designing
a chemical plant that makes TDIC and need to evaluate the safety of the units. Phosgene is
made by reacting CO and Cl2 in a 20 atm reactor with a residence time of 10 sec. Toluene is
nitrated by reacting with concentrated nitric acid solution at 140°C with a 1 min residence time,
and the aqueous and organic phases are separated. The organic phase is distilled to separate
dinitrotoluene. The dinitrotoluene liquid is hydrogenated with Hz at 5 atm at 120°C with a
liquid residence time of 20 sec. The product is reacted with phosgene in a vapor-phase reaction
at 150°C t = 2 min, and the TDIC is distilled. The TDIC is finally reacted with ethylene glycol
in a reaction injection molding process to form the bumper.
(a) This process consists of 5 reactors. Write down the reactions that occur in each.
(b) Sketch the flow sheet of the 5 reactors and 3 separation units with feeds of CO, Clz, toluene,
Hz, and ethylene glycol.
(c) The process is to produce 10 tons/h of polymer. Assume that each reactor produces 100%
conversion and that the nitration process produces equal amounts of the three nitrates. What
are the sizes of each of these reactors? Approximately how much chemical do they contain?
(d) These 8 units are all connected by valves that will close quickly to isolate all units in the
event of an upset of any kind. Describe the potential safety hazards of each unit and the
possible things that might go wrong. Include runaway reaction and dangers from venting
including chemical toxicity, fire, and explosion. Write -l/4 page of text on each.
(e) Order the units in terms of decreasing hazard. Which units do you request not to be assigned
to?

2.19 How many pentene isomers exist? How many isomerization reactions would describe their
isomerizations?