Hydrogen separation reliability

The exhaust gases contain CH4, N2, H2O, tar, acidic and basic compounds (NH3, HCN, H2S) considered as impurities. Tar conversion has to be controlled to maximize the reliability of mechanical equipments and to assure the operation of the successive clean-up catalytic steps for final hydrogen separation and purification [64]. This step involves the utilization of additional steam and selective catalysts, affecting the overall efficiency of the process [65]. The operation with oxygen instead of air may improve the efficiency of the process but it suffers the costs associated with air liquefaction process, necessary for O2/N2 separation. 

In the past, the available membranes lost a significant fraction of their selectivity when operated at these high temperatures. They also became plasticized by absorbed heavy hydrocarbons in the feed gas. As a consequence, a number of early hydrogen-separation plants installed in refineries had reliability problems. The development of newer polyimide and polyaramide membranes that can safely operate at high temperatures has solved most of these problems and the market for membrane-based hydrogen-recovery processes in refineries is growing. 

An essential element of the Hysep technology is the use of thin film palladium composite membranes to enable low cost and reliable hydrogen separation. The supported palladium layer in the Hysep module has a thickness as low as 3-9 pm, a substantial improvement over current commercial available palladium membranes, which are based on self supporting metal foils with a thickness of 20-100 pm. 

In the following, after a brief desolption of the conventional separation processes (PSA and ayogenic) a comparison among these processes and membrane systems for hydrogen separation is reported, also introducing some project considerations such as process flexibility, reliability, ease of response to the variations, expansion capability and versatility [76]. 

In the last three decades, membrane-based gas separation has become a well-established unit operation for a variety of important applications. Gas separation membranes operate in the chemical industry in hydrogen separation, monomer recovery, the enrichment of nitrogen from air, and natural gas treatment. Recovery of hydrogen from the purge gas of an ammonia reactor was established around 1980. Vinylchloride monomer recovery from the PVC production process was started up about 10 years later. Recently, the recovery of propylene from polypropylene purge bins became feasible. The removal of sour gases from natural gas as well as biogas operates reliably, today. ... 

To establish the suitability of a membrane material for hydrogen separation, it is important to consider that around 90% of the current global H2 production is obtained by gas synthesis through steam reforming of methane or other hydrocarbons and alcohols. Therefore, a suitable Hj-separation membrane should fulfil the requirements to reliably operate in a reforming reactions environment and, at the same time, to ensure high performance in terms of permeability and selectivity. 

Although the development of new metals as alternatives to Pd in hydrogen separation membranes is very promising, many aspects concerning the stability and reliability of these innovative membranes have yet to be defined. The main problems to be solved are embrittlement under hydrogenation cycles and permeance/selectivity performances appropriate to the hydrogen separation processes of practical interest. Therefore, only a few application studies are reported here. 

Although the results of the research on these new materials for hydrogen separation membranes are very promising, no important applications are reported at the present time. The aspects of stability and reliability still have to be addressed. Examples of applications are reported for the refractory metals which exhibit a good resistance to aggressive environments such as hydroiodic acid. A study has also demonstrated that Nb or Ta dense membranes could be applied for hydrogen recovery from the decomposition of HI in the iodine-sulphur process for thermochemical hydrogen production. 

Although theoretical estimates for neutral branching fractions of reactions such as 10 have been attempted, their reliability is suspect.32 In the absence of measurement, modelers have typically assumed equal branching fractions between channels in which one and two hydrogen atoms are separated from the molecular skeleton (e.g. 0.50 for reactions 10a and 10b). New assumptions based on recent experimental work have also been made.33... 

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