Modeling ofN2 Separation From Natural Gas Using Alumina Membrane

Purification of natural gas is very crucial step for upgrading the current value of natural
gas, which contain about 5-20% of nitrogen. Petroleum refineries currently employ
cryogemcs, pressure swing adsorption (PSA), and membrane systems for hydrogen
recovery. Each of these technologies has limitations: cryogenics is generally used only in
large-scale facilities with liquid hydrocarbon recovery, because of its high capital cost;
and PSA typically recovers less of the feedstream and is limited to modest temperatures.
Due to this reasons, alternative approach using membrane technology is to be explore.
The purpose of this study is to develop a mathematical model for predicting the
permeability and separation of nitrogen from natural gas. The parameters of concerns are
operating pressure and temperature, feed composition and membrane pore size.
Simulated results shows that increase in temperature gives rise to a strong increase in
membrane segmental motions, causing the pore diameter to become larger resulting the
selectivity will be lower. The effective diffusivity is either determined experimentally
without knowledge of the porosity or tortuosity or use of predictive method for ordinary
molecular diffusivity. It considers the normal molecular diffusivity and the Knudsen
diffusivity. Operating conditions can be very important; an excellent membrane will
achieve a poor performance if the operating conditions have not been properly selected.
Selectivity is controlled by the properties of the active layer, and the porous supporting
layer improves mechanical properties of the membrane only. The membrane used in this
study is alumina. The equation used to calculate the permeability for pure gases consider
the effect of viscous, Knudsen and surface diffusion. The total permeability of gases will
increase as the membrane pore size increased. At 323.25 K, and pressure 60 bar, the
permeability of methane at rp =0.2nm is 4.8313xl0"11 molls and the value increase to
1.76xl0.10 molls at rp=2nm.The effect of pressure and temperature were also studied,
where increase in feed pressure will in result increase the total permeability. The effect of
temperature shows the opposite trend where the total permeability decreased as the
temperature increased.The permeability of the binary mixture of methane and nitrogen is very close to
permeability of pure methane. Pure methane attained the highest permeability. As the
membrane pore size increase, more gas molecules can diffuse through the membrane and
as a result, the total permeability of gases increased. Due to increase value in
permeability of both methane and nitrogen, the separation process will be less selective
and thus it decrease with increase in pore size. As the flow rate at the retentate side is
higher, the percentage removal will decrease. Thus in order to achieve high nitrogen
removal, the stage cut must be low.