PLANT E – Distillation Column
Process
Distillation is a process used extensively in the chemical and petrochemical industries to separate liquid
mixtures into their various components. The size and design of the column or columns and the operating
pressure and temperature depend on many factors, such as the number of components in the feed, and the
desired purity of the components to be separated.
Distillation is achieved because of the different boiling points of the materials that are to be separated.
Distillation creates a composition profile in the column. The lower boiling (lighter) components tend to
concentrate in the vapor phase and the higher boiling (heavier) in the liquid phase. At zones in the column,
the vapor and liquid phases are essentially the same temperature and pressure dependent on the efficiency
of phase contact. Various kinds of devices such as random packing, structured packing, bubble cap trays,
or sieve trays, etc. are used to bring the liquid and vapor phases into intimate contact and thus to enhance
the separation.
The feed material is introduced at one or more points along the column. Because of density differences,
liquid runs down the column while vapor flows up. Liquid reaching the bottom of the column is partially
vaporized in a heated reboiler to provide boil-up back to the column. Liquid may be withdrawn from the
base of the column as bottoms product. Vapor reaching the top of the column is cooled and condensed to
liquid in the overhead condenser. Part of the liquid is returned to the column as reflux (downcoming
liquid). The remainder of the condensed liquid is withdrawn as distillate, or overhead product. The
column section above the feed point is often called the rectifying section and that below the feed point is
called the stripping section. In complex distillation operations, there can be multiple feed and draw off
points, as well as multiple columns linked in a variety of ways.
Lower pressures, i.e. vacuum, lower the boiling points of the components thus allowing separation to occur
at lower temperatures. This is helpful if some materials are sensitive to high heat. Also, it can be less
costly since lower pressure and temperature steam can be used at the heat source to the reboiler. Vacuum
may also affect the difference in boiling curves of the compounds, thus improving ease of separation.
This plant uses a packed distillation column under vacuum to separate an antifreeze-water mixture. The
operating conditions and expected separation performance are based on computer simulations, the packing
is a designed wire mesh Sulzer Inc. M25Y. A 40% water / 60% ethylene glycol (by weight) mixture is
pumped by Pump P-506 from the feed tank (D-506) through two preheaters (E-502 and E-505) into the
distillation tower (T-500). The internals of the column consist of two beds of structured packing, one
above the feed point and one below. The bottoms of the tower are heated by thermosiphon reboiler (E-501)
using 15 psig steam. (A thermosiphon circulates fluid based on density differences between the incoming
liquid and the outgoing vapor/liquid mixture). The bottoms product is drawn from the reboiler inlet line
with pump P-500, cooled by the cross-exchanger E-502, and returned to the feed tank.
The vapor from the top of the column is condensed in exchanger E-500 and continuously collected in the
reflux drum D-507. Condensate is pumped from D-507 with P-507 and split; a portion returns to the top
section of the column as reflux, and the remainder recycles back to the feed tank. The column operates
under vacuum by venting the reflux drum through condensing coils contained in a chilled water bath (E504) and then to vacuum pump VP-504. Any water condensed in the coils is collected in knock-out drum
D-504. Cooling for E-504 is provided by chilled water circulated by a packaged chiller and pump unit
(CH-504).
Primary controls for the system consist of feed flow, feed temperature, base temperature, reflux flow, and
system pressure. Feed temperature is controlled by varying steam flow to the preheater at TCV-505. Feed
temperature should not exceed 135F to avoid flashing when operating at 120 mm Hg. At lower pressures,
the temperature should be lowered accordingly. The flashing feed may cause damage to the column
internals. Feed flow is controlled with FCV 500-1. Reflux is flow controlled with FCV 500-2. Base
temperature is controlled with steam to the reboiler at TCV-501. System pressure is controlled at D-504 by
automatically adjusting instrument air flow into the vacuum pump suction through PCV-504.
Utility water is used to cool the condenser E-500 and to cool steam condensate at E-503. Steam is pressure
controlled and supplied from steam generator SG-500.
Because the column and reboiler are designated and stamped as ASME Section VIII pressure vessels, they
are required to have pressure relief protection. The rupture disc (PSE-500) on the column head will relieve
to prevent system pressure from exceeding that allowed by ASME Code. Note however that there is no
credible way (short of a fire engulfing the column) to create pressure as high as the rupture disk burst
pressure in the column. Overpressure is limited by the set pressure of the steam generator as well as by
PSV-501 on the steam supply header to the reboiler.
Precautions should be taken to prevent leaving steam or hot process fluid lined up to an exchanger while
the cold side fluid is blocked in. PSV-502 protects the tube side of E-502 and E-505 from being
overpressured if heat were to accidentally be left on the shell side with blocked in tubes.
Distillate coming off the top of the column will pass through a RO unit to further remove any entrained
salt. From the RO unit the water will then flow to a expansion tank were treatment of the water can occur
prior to use.
Instrumentation
The plant is controlled by modicon PLC with WonderWare or Delta V providing the operational interface.
In addition to the controls described above under PROCESS, this unit is fully field bus instrumented to gain
an understanding of distillation principles and to aid in its stable operation.
Additional controls include the column base level control (LIC 500/SC 500) and the reflux drum level
control (LIC 507/SC 507). Other conditions monitored through the DCS include feed tank level, bottoms
product flow and temperature, column temperature profile, column differential pressure, condensed
overheads temperature, pressure and flow, and make-up air flow to the vacuum pump. Gauges on
equipment and lines are also provided for safety and troubleshooting purposes.
Analytical
Conductivity will be measure at each step, outlet of distillation, outlet of RO unit and outlet of treatment to
allow the student a hand on feel of water purification.
Experimental
Plant E will be used to present the following:
1) Basic principles of distillation, 2) normal operation of a distillation system including start-up and
shutdown procedures, and 3) the adverse effects on distillation control due to upsets such as loss of reflux,
changes to feed rate, loss of vacuum, changes to base temperature, etc..
At suggested operating conditions, computer simulations predict that this column is capable of producing
an overhead product that is nearly pure water and contains less than 0.05% ethylene glycol by weight.
Remember that this is based on normal feed compositions or 60% glycol / 40% water. Experiments will
demonstrate the effect on the separation efficiency of variables such as temperature, pressure, and reflux
flow.
PLANT E – Distillation Column