Problems for Module III
III.1 CO2 in stack gases is usually removed via a closed circuit amine absorption-desorption process
shown in the Figure. The CO2 rich stack gas is fed to an absorption tower where a CO2 lean aqueous
amine solution absorbs the CO2. The CO2 lean gas is discharged to the atmosphere. The CO2 rich
amines stream from the absorber is preheated in a process-to-process heat exchanger and fed to a
stripping column that uses steam reboiler to strip out the absorbed CO2. The hot vapors from the
stripper are condensed with the condensed liquid being refluxed back to the column (total reflux)
and the non-condensable CO2 with some water vapor being vented as vapor distillate.
a) For this process, design a basic conventional regulatory control system with the gas feed as the
throughput manipulator (TPM).
b) If the bottleneck constraint is the flooding limit on the stripping column, then the stripper boilup
cannot exceed a maximum limit. On hitting this capacity constraint, all the CO2 absorbed in the
absorber cannot be boiled off in the stripper and an override system is needed that would cut the
gas-feed. Design appropriate overrides on top of the regulatory control system synthesized in part
(a) for accomplishing the same.
c) Use the stripper vapor boilup as the TPM and configure a control system that does not require any
overrides for handling the above bottleneck constraint.
CO2 Lean
Gas Out
CO2 Out
Stripper
Absorber
Cooler
PPHE
CO2 Rich
Gas In
Steam
CO2 Rich
Amines
Figure.
CO2 Lean
Amines
CO2 sequestration process
III.2 The process flowsheet producing cumene from benzene and propylene is shown in Figure. The
reactions are
MAIN:
Benzene + Propylene → Cumene
SIDE:
Cumene + Propylene → Diisopropyl Benzene (DIPB)
TRANSALKYLATION:
DIPB + Benzene ↔ 2 Cumene
a) Rationally draw all independent valves for the process.
b) For given throughput, reactor pressure and column pressures, what is the steady state operating
degrees of freedom for the process. State natural specifications for these degrees of freedom.
c) Using the independent control valves, propose a plant-wide control structure for the process with
the fresh limiting reactant feed as the throughput manipulator.
d) If the reactor cooling duty maxing out is the bottleneck constraint, devise appropriate overrides for
handling the constraint as throughput is increased.
e) Revised the control structure with the reactor cooling duty as the TPM.
Process Description:
The reaction chemistry is as above. Fresh benzene (pure, liquid), fresh propylene (liquid containing 5%
inert propane) and recycle benzene are mixed. The mixed stream is vaporized in the vaporizer,
preheated in a feed effluent heat exchanger, heated to the reaction temperature in a furnace and then
fed to the catalytic reactor. The highly exothermic main and side reactions occur in the reactor, which is
a cooled shell and tube heat exchanger with catalyst loaded tubes. The reaction heat is removed as
steam. Almost complete conversion of propylene occurs in the reactor. The reactor effluent is
depressurized across the PRV (pressure reducing valve) and cooled in the cooler. The gases (unreacted
propylene and propane) are removed as fuel gas vapor distillate from a purge column. The purge
column bottoms is fed to the recycle column that separates the unreacted benzene from the reaction
products (cumene and DIPB). The distillate stream, which is essentially pure benzene, is recycled while
the bottoms is fed to the product recovery column that separates cumene and DIPB as the distillate and
bottoms respectively. The heavy DIPB is mixed with a small amount of recycle benzene and sent to an
adiabatic transalkylator after preheating. The reversible transalkylation reaction occurs in the
transalkylator to convert the DIPB back to cumene. For high equilibrium conversion of DIPB, the
benzene to DIPB ratio is maintained large. The transalkylator effluent contains cumene, the extra
benzene and the unconverted DIPB. It is fed to the recycle column for separating the three components.
At steady state, the net DIPB formation by the side reaction equals the net DIPB consumption by
transalkylation. In other words, the DIPB is allowed to accumulate in the DIPB recycle loop to an extent
such that the net DIPB generation rate is zero. The DIPB is thus recycled to extinction.