Page 65 - Chemical Process Equipment - Selection and Design

12. AIR SEPARATION 37

A hydrocaxbon stream containing 60 mol % isoprene is charged generator and a water cooled exchanger E-3 beyond the battery
at the rate of 10,000piph to the main fractionator D-1 at tray 40 limits. A side stream is taken off tray 6, pumped with 5-2 through
from the top. The solvent is acetonitrile with 10 wt % water; it is E-1, and returned onto tray 3 of the tower. Another stream is
charged at the rate of 70,000 pph on tray 11 of D-1. This column has removed from tray 2 with pump 5-4 and cooled in water exchanger
a total of 70 trays, operates at 1Opsig and 100°F at the top and E-2; part of this stream is returned to tray 1, and the rest of it leaves
about 220°F at the bottom. It has the usual provisions for reboiling the plant as product gas oil.
and top reflux. Uncondensed vapors are removed at the top of the column
The extract is, pumped from the bottom of D-1 to a stripper with a one-stage steam jet ejector equipped with a barometric
D-2 with 35 trays. The stripped solvent is cooled with water and condenser.
returned to D-1. An isoprene-acetonitrile azeotrope goes Show the principal controls required to make this plant operate
overhead, condenses, and is partly returned as top tray reflux. The automatically.
net overhead proceeds to an extract wash column D-3 with 20 trays
where the solvent is recovered by countercurrent washing with 12. AIR SEPARATION
water. The overhead from D-3 is the finished product isoprene. The
bottoms is combined with the bottoms from the raffinate wash Make a flowsheet of an air purification and separation plant that
column D-4 (20 trays) and sent to the solvent recovery column D-5 operates according to the following description.
with 15 trays. Atmospheric air at the rate of 6.1 million SCFD is compressed
Overhead from D-1 is called the raffinate. It is washed to 160psig in a two-stage compressor JJ-1 that is provided with an
countercurrently with water in D-4 for the recovery of the solvent, intercooler and a knockout drum. Then it proceeds to a packed
and then proceeds beyond the battery limits for further conversion tower T-1 where it is scrubbed with recirculating caustic soda
to isoprene. Both wash columns operate at substantially solution. Overhead from T-1 is cooled to 14°F in a refrigerated
atmospheric pressure and 100°F. The product streams are delivered exchanger. After removal of the condensate, this stream proceeds
to the battery limits at 100 psig. Uo a dryer system that consists principally of two vessels F-1 and F-2
Solvent recovery column D-5 is operated at 50mmHg packed with solid desiccant.
absolute, so as to avoid the formation of an azeotrope overhead. After being precooled with product oxygen in exchanger E-1
The required overhead condensing temperature of about 55°F is and with product nitrogen in E-2, the air serves as the heating
provided with a propane compression refrigeration system; suction medium in reboiler E-3 of column T-2. Its pressure then is reduced
condition is 40°F and 80 psig, and discharge condition is 200 psig. to lOOpsig, and it is fed to the middle of column T-2. Bottoms of
Vacuum is maintained on the reflux accumulator with a two-stage T-2 is fed to the middle of column T-3. This stream contains 40%
steam ejector:, with a surface interstage condenser and a direct oxygen.
water spray after-condenser. The stripped bottoms of D-5 is cooled Columns T-3 and T-4 operate at 15 and 3Qpsig, respectively.
to 100°F and returned to the wash columns. Some water makeup is Column T-3 is located above T-4. Elevations and pressure
necessary because of leakages and losses to process streams. The differentials are maintained in such a way that no liquid pumps are
solvent recovered overhead in D-5 is returned to the main column needed in the distillation section of the plant.
D-1. Solvent makeup of about 20 pph is needed because of losses in Part of the overhead from T-2 (containing 96% nitrogen) is
the system. condensed in E-4 which is the reboiler for column T-3, and the
Steam is adequate for ahl reboiling needs in this plant. remainder is condensed in E-5 which is the reboiler for T-4. Part of
the condensate from E-4 is returned as reflux to T-2 and the rest of
11. VA~UU~I I ~ ~ ~ L ~ T ~ Q ~ the condensates from E-4 and E-5 serve as top reflux to T-3.
~
Overhead from T-3 contains 99.5% nitrogen. After precooling the
This plant is for the distillation of a heavy petroleum oil. The feed in E-2, this nitrogen proceeds to the battery limits.
principal equipment is a vacuum tower with 12 trays. The top tray is Bottoms of T-3 proceeds to the top of stripper T-4. Vapor
numbered 1. Trays 1, 2, 10, 11, and 12 are one-half the diameter of overhead from T-4 is recycled to the middle of T-3. The bottoms
the other trays;. The lower operates at 50 mm Hg. product (containing 99.5% oxygen) is sent partly to liquid storage
Oil is charged with pump J-1 through an exchanger E-1, and the remainder to precooler E-1 where it is vaporized. Then it is
through a fired heater from which it proceeds at 800°F onto tray 10 compressed to 150 psig in a two-stage compressor JJ-2 and sent to
of the tower. Live steam is fed below the bottom tray. the battery limits. Compressor JJ-2 has inter- and aftercoolers and
Bottoms product is removed with pump J-3 through a steam knockout drums for condensate.