H2 purification

Other methods suggested for H2 purification include adsorption and desorption on active carbon and fractional crystalliza tion (qv) of impurities fromhquid hydrogen. Centrifuging at 1—5.1 MPa (10—50 atm) and 64—110 K while keeping the temperature below the dew point of impurities that collect as a hquid film in the centrifuge has also been suggested. Purities over 99% can be obtained by this method (192). 

Despite the technical success of electrochemical H2 purification, it has not found commercial use. The reason is economic at a total voltage of 200 mV and an electricity cost of 0.05/kWh, the electric cost alone amounts to 2.36 per million BTU, an unacceptable price for commercial-grade H2. The real value in this process is the high purity of the product, and if a need for this purity arises the process will be attractive. At present, electrolytically-produced by-product H2 is often discarded. 

Although SMR is a well-developed technology, there is room for further technological improvement, in particular, with regard to energy efficiency, gas separation, and H2 purification stages. 

Na N ag6l(AlO2)86(SiO2) 06] 12-ring 8.4 Pressure swing H2 purification... 

Figure 7.11 Dual-membrane separation utilizing a highly selective metal or inorganic membrane for H2 purification and a conventional polymer membrane for the CO2/N2 separation.

Plasticization-enhanced H2 purification using polymeric membranes. Science, 311, 639-642. 

Rechargeable metal hydrides have a number of potential applications in the "hydrogen economy" concept and in the present industrial sector. Because general surveys of hydride applications have been recently published(1,22,23), we will not try to review all the potential applications, but rather concentrate on a few that might be particularly applicable to the hydrogen industry storage containers, H2 compressors, H2 purification or separation, and deuterium separation. 

During operation of the loop, small quantities of sulfur were observed in the recycled liquids from the HI cracking coolers and small amounts of H2S were collected in the H2 purification traps. This was due to incomplete separation of the sulfur containing species (SO2 and H2SO4) from the lower phase prior to decomposition. The lower phase concentration and purification step (H3PO4 treatment), which is an integral part of the cycle, will eliminate this problem. This step will be tested in the Bench Scale Unit. 

The purity of H2 produced will affect further purification costs. PSA is commonly used for H2 purification, and it is best suited for H2 concentrations of 50%... 

Alternative technologies to the PSA process for H2 purification include, after the HTS reaction, a low-temperature shift (LTS) reaction followed by C02 scrubbing (e.g., monoethanolamine or hot potash).11 The LTS reaction can increase the H2 yield slightly. However, the product stream, after the HTS, needs to be cooled to about 220 °C. Preferential oxidation (Prox) and/or methanation reaction as shown in Equations 2.6 and 2.7, respectively, removes the traces of CO and C02. The product H2 has a purity of over 97%. 

Figure 3.3. Schematic representation of partial oxidation (POX), autothermal reforming (ATR), and catalytic partial oxidation (CPO) reformers followed by H2 purification steps. HEX, heat exchanger.

The majority of the published literature on improved adsorbents for H2 purification by PSA deals with equilibrium adsorption properties (adsorption capacities of the impurities and their selectivities over H2) of the materials. The adsorbents are generally chosen in such a way that the kinetics of adsorption of the impurities into the adsorbents are relatively fast, primarily being controlled by macro- and mesopore diffusion within the adsorbent particles. The kinetics of adsorption may, however, become an issue for the removal of the trace amounts (ppm) of a relatively weakly adsorbed impurity (N2 or CH4) at the product end of an H2 PSA due to the existence of a very low driving force for the adsorption process. It was suggested that a layer... 

Recent patents by Questair Industries claim to use ultrarapid PSA cycle times using adsorbent laminate modules. Cyclic frequency of 100 cycles per minute (-cycle time = 0.6 s) to possibly 600 cycles per minute (-cycle time = 0.1s) are claimed.46 47 The ultrarapid cycles can potentially increase the BSF further by two orders of magnitude and significantly decrease the adsorber size. It is not known to the authors whether these ultrarapid PSA units have been used for H2 purification applications. 

Measurement of the solubility of solid oxygen in liquid hydrogen (and low temperature gaseous H2) showed exactly what had to be done in 02 removal during the H2 purification process to avoid solid 02-LH2 explosions. Understanding of another oxidant of concern, N20, was also obtained. N20 may be present in hydrogen from electrolytic cells but it can be converted catalytically in H2 to water and N2 which in turn are removed by conventional means. 

Identify promising hydrogen (H2) purification technologies that DOE does not currently fund... 

On the basis of the single reactor productivity, 205 compact MRs are needed for an industrial plant able to produce 10,000 Nm /h of pure hydrogen without requiring any H2 purification unit downstream the reactors system. 

Scura, E, Barbieri, G., De Luca, G. and Drioli, E. (2008) The influence of the CO inhibition effect on the estimation of the H2 purification unit surface. International Journal of Hydrogen Energy, 33,4183-4192. 

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