Study of Gas Permeability and Separation Behaviour for Removal of High Content C02 from CH4 by Using Membrane Modelling

The removal of C02 from natural gas down to the pipeline quality is an important
step before the natural gas can be sold to the end users. Typical natural gas
treatment's specification requires that the composition of C02 in the treated gas
cannot be more than 2 mole%. Currently, amine scrubbing units like Benfield
process is extensively used to treat low content of C02 in the natural gas. For high
concentration of C02 in the natural gas stream, the use of amine unit is economically
restricted, as the recirculation rate needs to be increased to cater the need. The use of
membrane separator to treat high content C02 natural gas has matured over the years
and is said to work best at high C02 inlet partial pressure, as this results in increased
permeation of the acid gas across the membrane. However, in the meantime to
achieve low sale gas specification, the natural gas recovery in a membrane separator
remains a question that needs to be explored.
This modelling work comprises the study of gas permeability of pure C02 and CI-4
and the separation behaviour of C02 I CI-4 mixture under different process
influences. The purpose is to predict the capability of membrane separator in
separating high content C02 in the natural gas by using mathematical modelling.
Data for y -alumina and acetate cellulose membranes, as cited from various
references were used in this modelling work. The accuracy of the models developed,
which incorporates the main transport mechanisms due to viscous, Knudsen and
surface diffusion, was tested using experimental data cited.
Simulation results show that the permeability of C02 and CH4 depend strongly on
the pore size of the membrane, temperature and feed composition of the mixture.
The effect of pressure on gas permeability is only apparent at small pore size.
It was found that surface diffusion predominates the other transport mechanisms at
small pores, and it poses the most selective transport mechanism to separate the C~
from CH4. However, when the pore size increases, surface diffusion starts to lose its
effect as the gas molecules continue to diffuse via Knudsen diffusion mechanism.
The results showed that Knudsen diffusion eventually increases the permeability of the gas molecules, but sacrifice in term of separation selectivity was observed at
higher pore size. The contribution of viscous diffusion is not apparent as overall.
The permeability of pure gas is inversely proportional to the system temperature, and
directly proportional to the operating pressure at small pores only. The variation of
surface diffusion due to the effects of both pressure and temperature is profound at
small pore regions. The permeability of C02 and C~ in C02 I C~ mixture will
approach the pure gas permeability as their feed composition increases.
The investigation of separation behaviour of this binary system revealed that the
performance of the single - stage alumina membrane separator is constant over a
range of possible operating pressures. However, the separation factor decreases when
the temperature increases. It showed that the separation factor of this binary system
can be enhanced to be maximum at temperature near 80°C for separation that takes
place in small pore region of the r -alumina membrane used. The separation factor is
also a strong function of both the feed composition of C02 and the separation stage
cut. Membrane separator becomes more efficient in term of selectivity and removal
efficiency at high feed composition of C02 and higher stage cut. The high stage cut
used to obtain sharp separation, however decreases the attractiveness of this binary
separation due to increased loss of natural gas to the impurities stream.