The absorber-stripper configuration is common for gas
plants. If we are to apply advanced process control (APC)
systems on a gas plant, it better to address the control
problemmaximizing recovery of propane or propylene
(C3) while minimizing C2 at propane
contamination limits. Absorber/strippers work like the
rectifying and stripping sections of any distillation column, but the presence of lean-oil makes it
more complex. The absorber preferentially absorbs and returns
C3 to the stripper, whereas the stripper
preferentially evaporates C2 in the absorber.
However, it is impossible to absorb C3 without some
C2, and likewise, it is impossible to boil
C2 without evaporating some C3. That
creates a large recycle of C2 and C3
between the two columns. Separation gets better with increased
recycle as long as the columns do not flood.