Purification hydrogen

The highest demands made on the purity of the produced hydrogen are for its use in fuel cells, since even the slightest trace of carbon monoxide impedes the functioning of the precious metal catalyst in the fuel cell.10 For the platinum used in hydrogen fuel 

10 There are differing degrees of purity for gases. The classification for hydrogen is based on the number of nines 

The most important purification processes can be differentiated into catalytic, membrane and adsorption processes. While catalytic processes are used only to remove CO, the other processes can also remove other substances depending on the material involved. 

In catalytic processes, such as CO conversion (CO + H20 — C02 + H2), selective methanisation (CO + 3 H2 — CH4 + H20) or selective CO oxidation (CO + Vi 02 — C02), the achievable efficiencies depend on reaction parameters, such as temperature, pressure, volume flow, raw gas concentration and catalyst material. These are capable of achieving contamination levels from 1 % down to a few ppm. The selection of different reaction paths is based on the use of different types of catalyst. 

Membrane processes are based on the selective transmission characteristics of the membrane material for different molecules, whereby the most effective membranes are usually also the most expensive. For example, the purest hydrogen can be captured by palladium membranes with suitable additives, but their low permeability make it necessary to use large membrane surfaces and high pressures, which result in high costs. 

In a conventional flow sheet, at the exit of low-temperature conversion, hydrogen meeting the standard commercial specifications (average 97 per cent volume) can only be obtained by means of supplementan purtficacioo treatments, including the successive removal of the following products  

Carbon monoxide, normally present in small amounts, less than 1 per cent Methane and nitrogen from traces to a few per cent 

Depending on the raw material concerned and the controlled oxidation method employed, and also according to the type of catalyst used for the conversion of CO with steam, acid gas elimination exhibits three different aspects  

The main techniques developed orindustralized in this area chiefly concern absorption by means of solvents. 

The add gases extracted are released by raising the temperature and lowering the pressure. The basic solutions employed to achieve this include the following  

The combined presence of H,S and C02 in a given effluent usually leads to a joint removal operation. This type of treatment stems from the fact that, among the most economical alternatives available, the liquid absorption of acid constituents is the most widespread, and also because, at the technical level, the type of product to be extracted, apart from its acidic character, has little effect on the behavior of the solvent employed. Hence, although, as a rule, hydrogen sulfide is absorbed faster than C02, the separation of the different acid gases is more or less simultaneous. For reasons of environmental protection, this scheme must also be supplemented by the direct conversion of hydrogen sulfide to sulfur, 

5A zeolite and YBa2Cu307-x, O2 desorbed when the magnetic field was turned on, and adsorbed when it was turned off. 

Hydrogen purification was the first large-scale application of PSA technology. The first commercial PSA hydrogen purification unit was installed in conjunction with a steam reformer, in Toronto around 1966. The standard five-step PSA cycle (with a co-current depressurization step) is used. Three or more pressure 

Numerous studies have been undertaken on the use of layered beds consisting of different sorbents for cyclic adsorption/ion exchange (Klein and Ver-meulen, 1975 Frey, 1983 Wankat and Tondeur, 1985 Chlendi and Tondeur, 1995 Watson et al., 1996 Pigorini and LeVan, 1997). For hydrogen purification using layered activated carbon and zeolite, Chlendi and Tondeur (1995) used the 

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A wide range and a number of purification steps are required to make available hydrogen/synthesis gas having the desired purity that depends on use. Technology is available in many forms and combinations for specific hydrogen purification requirements. Methods include physical and chemical treatments (solvent scmbbing) low temperature (cryogenic) systems adsorption on soHds, such as active carbon, metal oxides, and molecular sieves, and various membrane systems. Composition of the raw gas and the amount of impurities that can be tolerated in the product determine the selection of the most suitable process. 

H. A. Stewart and J. L. Heck, Hydrogen Purification By Pressure Swing Adsorption, Union Carbide Corp., Linde Division, New York. 

The sodium chlorate manufacturing process can be divided into six steps (/) brine treatment 2 electrolysis (J) crystallisation and salt recovery (4) chromium removal (5) hydrogen purification and collection and (6) electrical distribution. These steps are outlined in Figure 3. 

Compact brazed aluminum plate-fin heat exchangers can be used in most cryogenic hydrogen purification apphcations. The use of these relatively low cost heat exchangers, combined with low separation energy requirements, results in a highly economical process for hydrogen purification. 

Reflux overhead vapor recompression, staged crude pre-heat, mechanical vacuum pumps Fluid coking to gasification, turbine power recovery train at the FCC, hydraulic turbine power recovery, membrane hydrogen purification, unit to hydrocracker recycle loop Improved catalysts (reforming), and hydraulic turbine power recovery Process management and integration... 

Ethanol is a nontoxic substance with relatively high H2 content, and its advantage is that it can be produced from renewable sources, for example, from various biomasses and wastes. In addition, purification of the produced reforming gas has been of interest to researchers. Hydrogen purification has been studied, for instance, with membranes [19] which can also have catalytic performances. 

The separation factors are relatively low and consequently the MR is not able to approach full conversion. With a molecular sieve silica (MSS) or a supported palladium film membrane, an (almost) absolute separation can be obtained (Table 10.1). The MSS membranes however, suffer from a flux/selectivity trade-off meaning that a high separation factor is combined with a relative low flux. Pd membranes do not suffer from this trade-off and can combine an absolute separation factor with very high fluxes. A favorable aspect for zeoHte membranes is their thermal and chemical stability. Pd membranes can become unstable due to impurities like CO, H2S, and carbonaceous deposits, and for the MSS membrane, hydrothermal stability is a major concern [62]. But the performance of the currently used zeolite membranes is insufficient to compete with other inorganic membranes, as was also concluded by Caro et al. [63] for the use of zeolite membranes for hydrogen purification. 

Fig. 3. Current efficiency for hydrogen separation. Calculated overall energy efficiency vs. current density of hydrogen purification for conditions of Table 1 including reversible work O excluding reversible work.

Palladium-based dense metallic membranes have been known to be completely selective for hydrogen permeation and are used in commercially available small-scale hydrogen purification units (e.g., Johnson Matthey, 2007 REB Research, 2007 Power + Energy, 2007 ATI Wah Chang, 2007). These hydrogen purification units typically use palladium-alloy... 

Lin, H., E.V. Wagner, B.D. Freeman, L.G. Toy, and R.P. Gupta, Plasticization-enhanced hydrogen purification using polymeric membranes, Science, 311, 639-642,2006. 

Sumitomo-BF A PSA hydrogen purification process using a carbon molecular sieve as the selective adsorbent. Developed by Sumitomo, Japan. 

In most hydrogen plants, a pressure-swing adsorption (PSA) system is used for hydrogen purification. In these plants, a major portion of reformer fuel is PSA offgas with a hydrocarbon stream for makeup fuel. 

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