Page 178 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

2. Suggest two reasons each why distillation columns are run above or below ambient pressure. Be sure
to state clearly which explanation is for above and which is for below ambient pressure.

Suggest two reasons why reactors are run at elevated pressures and/or temperatures. Be sure to state
3.
clearly which explanation is for elevated pressure and which is for elevated temperature.

Give two reasons why operation of a process greater than 250°C is undesirable. Give one reason each
4.
why one would operate a distillation column and a reactor at temperature greater than 250°C.

Define a “condition of special concern.” Define two such conditions, and state one possible
5.
justification for each.

In the food and drug industries, many processes used to produce new active ingredients (drugs) or to
separate and purify drugs and foods occur at vacuum conditions and often at low temperatures (less
6.
than room temperature). What is it about these types of products that requires that these conditions of
special concern be used?

Problems

For the separation of a binary mixture in a distillation column, what will be the effect of an increase
in column pressure on the following variables?
a. Tendency to flood at a fixed reflux ratio
7.
b. Reflux ratio for a given top and bottom purity at a constant number of stages
c. Number of stages required for a given top and bottom purity at constant reflux ratio
d. Overhead condenser temperature

In a new chemical process, a reboiler for a tower requires a heating medium at 290°C. Two possible
8. solutions have been suggested: (a) use high-pressure steam superheated to 320°C, and (b) use
saturated steam at 320°C. Suggest one disadvantage for each suggestion.

As the ambient temperature and humidity increase, the temperature at which cooling water (cw) can
be supplied to any piece of equipment increases. For example, in the winter, cw may be available at
9. 27°C whereas in midsummer it may rise to 34°C. How, if at all, does this affect the pressure at which
a distillation column operates (assuming that the overhead condenser uses cooling water as the heat
exchange utility)?

It is desired to produce a hot vapor stream of benzene to feed a reactor for a certain petrochemical
process. The benzene is available from an off-site storage facility at 1 atm pressure and ambient
temperature (assume 25°C), and the reactor requires the benzene to be at 250°C at 10 atm. Two

possible process schemes are being considered to heat and pressurize the feed: (1) pump the liquid
benzene to pressure and then vaporize it in a heat exchanger, and (2) vaporize the benzene first and
10. then compress it to the desired pressure. Answer the following.
a. Discuss qualitatively which scheme (if either) is better.
b. Confirm your answer to part (a) by comparing the costs using both schemes to feed 1000
kg/h of benzene to the reactor. (Assume that the cost of heating is $15/GJ and that