Membranes for hydrogen separation

By: Poonam Shah | Views: 1135 | Date: 10-Apr-2012

The need for hydrogen will increase greatly in the future as a raw material for the chemical industry and as clean fuels in cars and electric industry (e.g. fuel cells). Currently, hydrogen is produced mainly by the reforming of fossil fuels and coal gasification. However, hydrogen is there mixed with large quantities of non-desired components such as light hydrocarbons, CO and CO2 from fossil fuels. The purification or separation of hydrogen from these industrial gases by means of membrane has several adva

The need for hydrogen will increase greatly in the future as a raw material for the chemical industry and as clean fuels in cars and electric industry (e.g. fuel cells). Currently, hydrogen is produced mainly by the reforming of fossil fuels and coal gasification. However, hydrogen is there mixed with large quantities of non-desired components such as light hydrocarbons, CO and CO2 from fossil fuels. The purification or separation of hydrogen from these industrial gases by means of membrane has several advantages, including low energy consumption and cost saving. In general, membranes can be classified as organic and inorganic based on their material composition, as porous and dense or as symmetric and asymmetric based on their structure etc. Flux, selectivity, chemical stability and mechanical strength are the important parameters for the membrane performance.

Although organic membranes have an advantageously low price and good scalability, they cannot be used at high temperatures or in chemically aggressive environments containing e.g. HCl, SOx, and their poor mechanical strength hinders their high pressure application. Dense metal membranes, usually made of palladium or its alloys, have very high selectivity for hydrogen (~100%) based on the solution-diffusion mechanism, but a deadly sensitivity to CO and H2S, in terms of coal gas application. Proton conductors, such as doped BaCeO3, have a very high selectivity in the water vapor atmosphere, because only protons can migrate through these materials. However, H2 flux through the proton conducting membranes is relatively low (~10-8 mol/cm2•s), and their chemical stability in the presence of certain species (e.g. CO2, H2S) is another major concern.

Furthermore, energy consumption is disadvantageous because they must be operated at high temperatures (e.g. 800-1000°C) in order to obtain high flux. Inorganic porous membranes can be used in many industrial applications at high temperatures (>200°C), and they have high flux and very good selectivity. Two of the most promising porous materials for membrane are zeolite and silica: the pores in the zeolites membrane are a part of the crystal structure, and hence have uniform dimensions. Many zeolites are thermally stable above 500°C. Zeolite membranes are generally formed on porous supports by hydrothermal synthesis, and hence the membranes have a lot of defects, lowering the selectivity. The most critical barrier for zeolite applications is the difficulty in producing in a large scale. Microporous silica membranes have high hydrogen permeance and high selectivity and excellent capacity to scale up. Hereby, silica-based membranes are promising candidates for hydrogen separation at elevated temperatures, although the steam/water stability of these membranes may be an issue.

Generally, the porous ceramic membranes for gas separation consist of several layers macroporous (dp > 50 nm) support is often several millimeters thick, giving the mechanical strength to the system; mesoporous (2 nm < dp < 50 nm) intermediate layer of less than 100 μm thickness is the bridge of the gap between the large pores of the support and the small pores of the thin microporous layer (dp < 2 nm); the top layer is the actual functional part for gas separation. Ceramic top layer providing high hydrogen flux and selectivity is very suitable to hydrogen separation

R. de Vos reported about crack-free amorphous silica layers by dip-coating in a clean room, with high H2 permeance (2×10-6 mol/m2•s•Pa at 200°C) and very low CO2 and CH4 permeance (10 and 50×lower, respectively, at 200°C), the details of each layer are listed in Table 1, and they correspond to the characteristics of the membranes described in this report.

Layer

Component

Thickness

Sintering temperature

Pore size in diameter

Macroporous

α-Al2O3

~2.00 millimeter

1100°C

80 nm

Mesoporous

γ-Al2O3

~200 nm

650°C

2~3 nm

Microporous

Amorphous silica

~30 nm

400/600°C

~0.5 nm

Table 1.1 Layer properties of a typical silica membrane for hydrogen separation.

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