ACETIC ACID PRODUCTION (MONSANTO PROCESS)
Process Description
The feedstocks for the reaction of interest are methanol, carbon monoxide, and water and utilizes a
rhodium catalyst. The raw materials namely carbon monoxide as gas at room temperature, methanol
and water at about 20oC are fed to reactor R1 through streams S1, S2 and S3 from tank TK2, TK3
and TK1 respectively.The operating pressure of the reactor is 1 bar. The reaction is carried out in a
continuous stirred tank reactor. The gaseous by products such as CO2 and H2 are taken off the top
through stream S10 and directed towards a gas holder GH1. The liquid products are then taken off
the bottom through stream S4 which is passed through a pressure reduction valve V-1 to a flash
vessel FV-1 operating at a pressure of 1.1 bar and temperature of 115oC where light components
(gases) consisting of methyl acetate, methyl iodide, some water, and product acid are removed as
vapor through the top of the vessel through stream S6. The gases are fed forward to a series of
distillation columns for further purification whilst the remaining liquid in the flash vessel containing
the dissolved catalyst comes from the bottom as stream S49 which is pumped to the reactor R1
through stream S51.
The light components which predominantly contains acetic acid enters a multiple -tray distillation
column DC1 operating at a pressure of 1.2 bar and temperature of 112oC. Hydrogen iodide present
in the feed stream is concentrated in the acetic acid solution in the bottom of the column. This
stream S13 is recycled back to the reactor. Carbon monoxide, water, methyl iodide and some
entrained hydrogen iodide comprise the overhead stream. Solution of acetic acid in water containing
some catalyst and by- products is withdrawn from the bottom through stream S7 and introduced into
a second multiple -tray distillation column DC2 operating at a pressure of 1.2 bar and temperature
of 120oC. In this column, water and remaining inert is withdrawn as the overhead through stream
S38 and joins with stream S29 ; the combined stream S48 is recycled to the reactor R1.
Acetic acid is withdrawn from the drying column through stream ST-19 from into a third multiple
tray distillation column DC3. This column is designed to separate the unwanted propionic acid
produced as part of the process from the desired acetic acid produced to obtain a purity of greater
than 99.9%. The operating conditions of distillation column DC-3 are 125OC and 1 atm temperature
and pressure respectively. Acetic acid at 125oC comes out as the overhead product and passes
through a condenser from which it is pumped to storage tank STK1 through stream S9. The
temperature of the liquid acetic acid obtained is 120oC. The major liquid by product of the reaction
is propionic acid (0.1% selectivity) at 0.5986 bar and 141oC; which comes out as the bottom product
from DC-3; and is fed to tank TK5 through stream S11.