Distillation Column Design and Optimization
Initial Design
Initially the separation train was designed using the shortcut method and an assumed reactor
conversion of 90%. With these values we were able to come up with a working separator and flash
system that achieved a successful separation. The desired separation was to produce a pure benzene
stream for recycle and a pure cumene product. For initial guesses we two US patents and work done by
W.L. Luyben. The main tower separates the benzene needed for recycle and impurities leaving a
stream of benzene and cumene for one of the flash units. Using the trayed tower design we were able
to come up with a packed tower to achieve equal separation and were able to compare the two towers.
The two towers were compared mainly in terms of cost which is discussed further in the next section.
Optimization
The initial optimization was done on number of trays and reflux rate. The best number of trays
was found by increasing the number of trays until the separation reached a level where 99% of the
cumene in the feed to the column was removed. Increasing the number of trays above 14 did not
increase the separation of cumene any so it was concluded that this would be the optimum number of
trays for the column. Reflux ratio was determined in the same manner as the number of trays. We
started low and increased until we saw the best separation we could get without drastically changing
the reflux rate. The reflux rate was determined to be 0.75 for the column. The main focus for the
optimization of the distillation column was cost. Once the column was running to the preferred
separation standards a comparison of cost for trayed and packed columns was done. The trayed column
turned out to be the better economical choice. The trayed tower came out with 14 trays 16 feet in
diameter and 18 inches apart, a tower height of 48 feet, pressures of 45.39 psi at the bottom and 44.09
psi at the top, and a 6 inch weir height. This tower included a condenser and condenser tank, a reflux
pump, and a reboiler as auxiliary units. The total cost of the tower includes the vessel, the trays, and all
of the auxiliary equipment. The total cost for a trayed tower to achieve desired separation was
approximately $5.7 million. The total cost for the packed tower with the same auxiliary equipment
required was approximately $7.9 million. The main distillation column is then followed by two parallel
flash units, one of which separates propane and propylene thus creating a pure, benzene recycle stream
and the other separates cumene and benzene to create a pure cumene stream. The flash column
removing benzene from the cumene stream has a volume of has a volume of 1360 cubic feet and is run
at 254.1 Fahrenheit and 45.39 psi. The flash column removing propane and propylene has a volume of
1420 cubic feet and is run at 247.6 Fahrenheit and 44.09 psi. The two flash columns were chosen
because they cost $345,000 and $358,000 which is much cheaper than another full tower would be and
they achieve very good separation.
Final Design
The separation train begins with a trayed tower and is followed by two flash columns. The
trayed tower has 14 sieve trays separated by 18 inches and a reflux rate of 0.75. The column has a
length of 48 feet and a diameter of 16 feet, giving it a volume of 9650 cubic feet, with a height-todiameter ratio of 3. The column is operated at temperatures of 247.6 and 254.1 degrees Fahrenheit and
pressures of 44.09 and 45.39 psi for the top and bottom respectively. The pressures were chosen as
greater than atmospheric so that if there is an issue with a leak in the tower no oxygen will be able to
enter the system since we have multiple flammable chemicals. Pressure drop came from a heuristic in
our book that trayed towers operated at or above atmospheric pressure can be operated with a
pressure drop of 0.1 psi per tray. The column separates 99.9% of the cumene fed to it and produces the
benzene recycle stream that continues through the flash column for purification. The tower also
contains a condenser and condenser tank, and a reboiler. The tower condenser has a length of 20 feet
and a diameter of 3 feet with a heat exchange area of 5993 square feet. It also contains 600 1 inch
diameter tubes which produce a heat duty of 140.9MMBtu/hr. The shell is operated at 250 F and the
tubes are operated at 95 F with one pass each for both the tubes and the shell. The condenser tank has
a length of 29.5 feet and a diameter of 9.5 feet giving it a volume of 2091.024 cubic feet. The tank is
operated at a temperature of 248 F. The reboiler has a length of 20 feet and a diameter of 3 feet with a
heat exchange area of 26010 square feet. It contains 600 1 inch diameter tubes which produce a heat
duty of 191.1MMBtu/hr. The shell is operated at 254 F and the tubes are operated at 328 F with one
pass for the shell and 2 passes for the tubes. The tower produces two streams, one of which is benzene
and cumene and the other is benzene, propane, and propylene.
The flash column separating out propane and propylene has a length of 25 feet and a diameter
of 8.5 feet giving it a volume of 1418.63 cubic feet. It is operated at 246 F and 44.09 psi. It produces
one stream of liquid benzene of approximately 99.9% purity and another stream of mixed propane and
propylene. The pure benzene stream is recycled and mixed with the benzene and propylene inlets.
The flash column separation benzene from cumene has a length of 12 feet and a diameter of 12
feet giving it a volume of 1357 cubic feet. It is operated at 254 F and 45.39 psi. It produces a liquid
stream of cumene that is approximately 99.9% pure and a 99.9% benzene stream that is vapor that will
be condensed and returned to the fresh benzene stream to lower the amount of new benzene feed
required. A second flash drum and condenser were added because aspen would not allow the fresh
benzene stream to be lowered after initial startup and could not handle the amount of benzene that would
have been entering the reactor. The thought process behind this was that we would be able to have a
larger amount of the benzene come out of the initial separation and then lower the amount of benzene
being pumped into the system. If this was the case we would not have a need for the second flash drum
and condenser.
Finally, the material selected for all equipment is type 304 stainless steel because propylene and
benzene are very reactive with oxidizing agents.
Cost Equations
1.
2.
3.
4.
5.
6.
7.
8.
Purchased Cost of Trays 𝐶𝐵𝑇 = 468 ∗ exp⁡(0.1739 ∗ 𝐷𝑇𝑟𝑎𝑦)
Purchased Cost of Tower 𝐶𝑇 = 𝑁𝑇 𝐹𝑁𝑇 𝐹𝑇𝑇 𝐹𝑇𝑀 𝐶𝐵𝑇 , NT= number of trays
2.25
𝐹𝑁𝑇 = 1.0414𝑁𝑇
𝐹𝑇𝑇 = 1.0
𝐹𝑇𝑀 = 1.189 + .0577 ∗ 𝐷𝑇𝑟𝑎𝑦
Tower Vessel Cost log(𝑃𝑢𝑟𝑐ℎ𝑎𝑠𝑒𝑑⁡𝐶𝑜𝑠𝑡) = 3.5 + .45 ∗ log(𝑉) + .11 ∗ [log(𝑉)]^2
Packing Cost log(𝑃𝑢𝑟𝑐ℎ𝑎𝑠𝑒𝑑⁡𝐶𝑜𝑠𝑡) = 3 + .97 ∗ log(𝑉) + .0055 ∗ [log⁡(𝑉)]^2\
𝐼𝑛𝑠𝑡𝑎𝑙𝑙𝑒𝑑⁡𝐶𝑜𝑠𝑡 = 𝑃𝑢𝑟𝑐ℎ𝑎𝑠𝑒𝑑⁡𝐶𝑜𝑠𝑡 ∗ 4 correction for stainless steel
Equations 1-5 are from Product and Process Design Principles by Seider, Saeder, Lewin, and
Widagdo.
Equations 6-8 are from the class handout.
Tower
Volume
9646 ft^3
Reflux Ratio
0.75
Length
48 ft
Distillate Rate 6500 lbmol/hr
Diameter
16ft
Bottoms Rate 6500 lbmol/hr
Temperature
247.6 F top
Boilup Ratio
2.372
254.1 F bottom Number of Trays
14
Pressure
44.09 psi top
Weir Height
6 inches
45.39 psi bottom Tray Spacing
18 inches
Split Fractions
Distillate
Bottoms
Benzene
0.50925659
0.49074341
Propane
1
trace
Propylene
1
trace
Cumene
trace
0.999962782
DIB
trace
1
BZ Flash
Feed (mole frac) Outlets (mole frac)
BZREC Cumene
Benzene
0.963
1
Propane
trace
2 ppb
Propylene
trace
trace
Cumene
0.037
1
DIB
16 ppm
415 ppm
Length
12ft
Diameter
12ft
Heat Exch Area
1360 ft^3
Temperature
254.1 F
Pressure
45.39 psi
Prop Flash
Feed (mole frac) Outlets (mole frac)
BZREC Propane
Benzene
0.999
1
Propane
970 ppm
0.834
Propylene
193 ppm
0.166
Cumene
1 ppm
1 ppm
DIB
trace
trace
Length
25 ft
Diameter
8.5 ft
Heat Exch Area
1420 ft^3
Temperature
247.6 F
Pressure
44.09 psi