Page 70 - Gas Purification 5E

60 Gas Purification

potassium carbonate process (see Chapter 3, and was first disclosed by Shoeld (1934). The
rich solution from the bottom of the absorber is split into two streams, one being fed to the
top of the stripping column and one to the midpoint. The top stream flows downward coun-
tercurrent to the stream of vapors rising from the reboiler and is withdrawn at a point which
is above the inlet of the second portion of the rich solution. The liquid withdrawn from the
upper portion of the stripping column is not completely stripped and is recycled back to the
absorber to absorb the bulk of the acid gases in the lower portion of the absorber column.
The portion of solution, which is introduced near the midpoint of the stripping column, flows
through the reboiler and is very thoroughly stripped of absorbed acid gases. This solution is
returned to the top of the absorber where it serves to reduce the acid gas content of the prod-
uct gas to the desired low level. In this system, the quantity of vapors rising through the
stripping column is somewhat less than that in a conventional plant. However, the ratio of
liquid to vapor is lower in both sections because neither carries the total liquid stream.
The obvious drawback of this process modification is that it appreciably increases the ini-
tial cost of the treating plant. The stripping column is taller and somewhat more complex,
and the two streams require separate piping systems with two sets of pumps, heat exchang-
ers, and coolers. Commercial units utilizing a system of this type have been described by
Bellah et al. (1949) and by Estep et al. (1962).
A simpliied form of the split-stream cycle consists of dividing the lean solution before
introduction into the absorber into two unequal streams. The larger stream is fed to the middle
of the absorber, while the smaller stream is introduced at the top of the column. In cases
where gases of high acid-gas concentration ~IE treated, this scheme may be more economical
than the basic flow scheme, as the diameter of the top section of the absorber may be appre-
ciably smaller than that of the bottom section. Furthermore, the lean-solution stream fed to the
middle of the absorber may not have to be cooled to as low a temperature as the stream flow-
ing to the top of the column, resulting in reduction of heat exchange surface.
A split flow cycle specifically designed for the removal of COz from high pressure gas
streams with promoted MDEA solutions is shown in Figure 2-12. This and several other
flow schemes are offered by BASF for use with their activated MDEA process (Gerhardt and
Hefner, 1988; Meissner and Hefner, 1990). In the illustrated configuration, the gas is treated
in a two stage absorber using partially regenerated solution in the bottom stage and com-
pletely regenerated solution in the top stage. Rich solution from the bottom of the absorber
passes through a hydraulic turbine for energy recovery, and is then flashed in the high pres-
sure flash unit where most of the dissolved inert gases are released. The rich solution then
flows to the low pressure flash unit, which operates close to atmospheric pressure. Hot over-
head gas from the thermal stripping column is passed through the solution in the low pres-
sure flash tank to improve the efficiency of COz removal in this vessel. A significant portion
of the C02 contained in the rich solution is stripped in the low pressure flash unit. Partially
regenerated solution from the low pressure flash step is split into two portions. The larger
portion is fed to the bottom stage of the absorber; while the balance flows to a conventional
reboiled stripping column. Completely regenerated solution from the stripping column is fed
to the top absorption stage, completing the cycle.

Cocurrent Absorption

With cocumnt absorbers, the highest gas purity attainable is represented by equilibrium
between the product gas and the product (rich) solution. When an irreversible reaction occurs
in the liquid phase, the equilibrium vapor pressure of acid gas over the solution is negligible