Porous Silicon Membranes for Removing Carbon Dioxide From Natural Gas

Tu Xiang Zheng

30 years ago I was assigned to prepare porous silicon membranes for study of removing carbon dioxide from natural gas based on Knudsen diffusion. Natural gas mainly consists of 79-84 mol % methane and 5-8% mol % Carbon dioxide. To meet specifications carbon dioxide must be removed before a natural gas can be delivered to the pipeline.

Knudsen’s diffusion occurs in a porous membrane, whose pore sizes are smaller than the mean free path of the gas molecules. The mass flux of a gas through the porous membrane can be expressed as:

J_k = D_k (∂ρ/∂L)              (1)

Where J_k is mass flux of the gas through the porous membrane, D_k is the Knudsen coefficient, ρ is the density of the gas, and L is the thickness of the membrane.

The Knudsen coefficient is defined as

D_k = d_p /3 (8R_gT/πM_g)^1/2         (2)

Where d_g is the diameter of the pores, R is the gas constant (8.3144J/mol k in SI units), M_g is the molecular weight of the gas (in units of kg/mol) and T has units of k.

Hence, for Knudsen diffusion, the square root of the inverse ration of the molecular weights of the gases will determine the mass flux of the gases through the porous membrane. As shown in the following table, the square root of the ratio of the molecular weights between the methane and the carbon dioxide is 1.66 that represents the mass flux ration between the carbon dioxide and the methane through the porous silicon membrane.

The porous silicon was prepared by anodization of silicon wafers in concentrated HF solutions. The used silicon wafers were p-type silicon wafer (0.01-0.001 Ω-cm), polished on one side and oriented along the crystalline direction. The used HF solutions were composed of HF wt 49% and ethanol and the anodization took place in a double tank cell. The anodization was carried out in the dark at a constant current density from 18 to 36 mA/cm². The obtained porous silicon membrane had a pore size from 6 to 10 nm and a porosity of about 50 %.

In order to form a porous silicon membrane first, applied current density was 36mA/cm² and anodization time was 30 minutes. As a result, a porous silicon layer with a thickness of about 30 microns was created. Then, the applied current density was abruptly increased in order to enable the porous silicon layer detached from the silicon wafer. The abrupt increase applied current density led to a high porosity layer and high released gas pressure. So the porous silicon layer was easy being detached from the silicon wafer.

As an alternative, the porous silicon membranes can be obtained by combination of forming and etching of the porous silicon. First, a thick porous silicon layer was formed in a silicon wafer and then etched in a diluted KOH solution. Secondly, a thin porous silicon layer was formed by etching the leaved silicon layer in the silicon wafer.

After fabrication of the porous silicon membrane, the sample was rinsed gently with ethanol. Then a final rinse was carried out with hexane in order to minimize the possibility of shattering of the membranes due to strong capillary forces and thermal stresses exerted when ethanol evaporates from the pores. Finally, the membranes were dried in a nitrogen flow.