Page 66 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

13.

Under normal conditions, valves a to c are open and valves d and e are closed. Answer the
following:

a. Explain, carefully, the sequence of opening and closing valves required in order to change out the
valve stem on the control valve (valve b).

b. What changes, if any, would you make to Figure P1.13 if the process stream did not contain a
process chemical but contained process water?

c. It has been suggested that the bypass valve (valve d) be replaced with another gate valve to save
money. Gate valves are cheap but essentially function as on-off valves. What do you recommend?

d. What would be the consequence of eliminating the bypass valve (valve d)?

Often, during the distillation of liquid mixtures, some noncondensable gases are dissolved in the feed
to the tower. These noncondensables come out of solution when heated in the tower and may
accumulate in the overhead reflux drum. In order for the column to operate satisfactorily, these
vapors must be periodically vented to a flare or stack. One method to achieve this venting process is
14.
to implement a control scheme in which a process control valve is placed on the vent line from the
reflux drum. A pressure signal from the drum is used to trigger the opening or closing of the vent line
valve. Sketch the basic control loop needed for this venting process on a process flow diagram
representing the top portion of the tower.

Repeat Problem 14, but create the sketch as a PI&D to show all the instrumentation needed for this
15.
control loop.

Explain how each of the following statements might affect the layout of process equipment:

a. A specific pump requires a large NPSH.

b. The flow of liquid from an overhead condenser to the reflux drum is gravity driven.

c. Pumps and control valves should be located for easy access and maintenance.
16.