Question

An absorption column (1st unit) and a stripping tower (2nd unit) are used in a separation process
to separate gas mixtures that contains 35 mol% CO2 and the balance methane (CH4). The gas
mixture is fed to the bottom of absorption column where in the column, the gas mixture is
contacted with a recycled solvent. The contacting solvent stream contains 0.45 mol% dissolved
CO2 in methanol (CH3OH), which is recycled from the bottom of the stripping tower and enters
the absorption column at the top. The product gas stream leaving the top of absorption column
contains 1 mol% CO2 and 99 mol% methanol, where all methanol fed to the column exits in
this stream.
The liquid bottom stream of the absorption column which is rich in CO2 is fed to the top of the
stripping tower while a stream of nitrogen gas at a flow rate of 100 mol/h is fed to the bottom.
90% of carbon dioxide in the liquid solvent fed to the stripping tower is stripped off and exits
the tower together with the entire nitrogen fed. This stream is released to the atmosphere
through a stack. As mentioned above, the liquid stream leaving the stripping tower is the
recycled solvent stream of CO2 –methanol solution fed to the absorption column.
There are two assumptions made in this separation process where firstly, no methanol enters
as vapor phase in both columns as it is assumed to be non-volatile. Another assumption is that
nitrogen is insoluble in methanol. By taking 100 mol/h of gas mixture fed to the absorption
column as the basis of calculation,
a) With the support of a complete process flow chart, determine all flow rates and
compositions entering and leaving the stripping tower.

Answer 1