Page 59 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Figure 1.11 Possible Equipment Arrangement for the Reactor and Feed Section of DME Facility, Unit
200

3. The elevation of all major equipment is established. In general, equipment located at grade

(ground) level is easier to access and maintain, and is cheaper to install. However, there are
circumstances that dictate that equipment be elevated in order to provide acceptable operation.
For example, the bottoms product of a distillation column is a liquid at its bubble point. If this
liquid is fed to a pump, then, as the pressure drops in the suction line due to friction, the liquid
boils and causes the pumps to cavitate. To alleviate this problem, it is necessary to elevate the
bottom of the column relative to the pump inlet, in order to increase the Net Positive Suction
Head Available (for more detail about NPSH see Chapter 18). This can be done by digging a
A
pit below grade for the pump or by elevating the tower. Pump pits have a tendency to
accumulate denser-than-air gases, and maintenance of equipment in such pits is dangerous due
to the possibility of suffocation and poisoning (if the gas is poisonous). For this reason, towers
are generally elevated between 3 to 5 m (10 and 15 feet) above ground level by using a “skirt.”
This is illustrated in Figure 1.12. Another reason for elevating a distillation column is also
illustrated in Figure 1.12. Often a thermosiphon reboiler is used. These reboilers use the
difference in density between the liquid fed to the reboiler and the two-phase mixture (saturated
liquid-vapor) that leaves the reboiler to “drive” the circulation of bottoms liquid through the
reboiler. In order to obtain an acceptable driving force for this circulation, the static head of the
liquid must be substantial, and a 3–5 m height differential between the liquid level in the
column and the liquid inlet to the reboiler is typically sufficient. Examples showing when
equipment elevation is required are given in Table 1.12.

Figure 1.12 Sketch Illustrating Reasons for Elevating Distilling Column