The increasing use of hydrogen in chemical industries and oil refining and clean technologies puts pressure on hydrogen sources, hydrogen production capacities and hydrogen supplies. H2 is used for example for fuel desulfurization, production of ammonia NH3, methanol and other alcohol, urea, hydrochloric acid HCl, in Fischer-Tropsch reactions, i.e. conversion of CO and H2 into liquid hydrocarbon, as a reducing agent in metallurgy and for adding value to petroleum products and oils by hydrogenation. More than 41 million tons of H2 are produced annually, of which 80% by steam reforming, partial oxidation and auto thermal reforming of natural gas. Renewable hydrocarbons and biogas are also used as starting sources.
Methane steam reforming (see relation 1 below) is performed at high temperature, typically between about 800° C. and about 900° C. The resulting H2 and CO gas mixture is cooled down to a temperature in a range comprised between about 350° C. and 450° C. upon exiting a first reactor, and introduced in a second reactor where a water gas shift reaction (WGS) takes place (relation 2 below):CH4+H2O→3H2+CO  (relation 1)CO+H2O⇄H2+CO2  (relation 2)
Then H2 (40 mol %) is mixed with CO2 (55 mol %), CO (3 mol %) and H2S (1-3 mol %) from the original hydrocarbons source.
A number of methods are known to separate and purify H2, such as: i) cryogenic distillation, allowing a purity up to 95%, ii) separation using a polymer membrane, allowing a purity up to 98%, iii) adsorption on a molecular sieve or pressure swing adsorption (PSA), allowing a purity up to about 99.9%, and iv) separation using a metal membrane, allowing a purity of more than to about 99.95%.
In a number of applications such as in H2 supply of fuel cells and specific files of specialized chemical industry, H2 with a purity above 99.999% is needed. For example, fuel cells must be supplied with H2 containing less than 100 ppm carbon monoxide or sulfur. In order to achieve such of purity levels, additional purification stages are needed after the stages of H2 production. H2 purification using metal membranes allow achieving high purity levels and thus look promising for such applications.
Purification by adsorption on a molecular sieve or pressure swing adsorption (PSA) is the most widely used method on production sites. In this method, each adsorbent bed goes through adsorption, depressurization, purging at low pressure and pressurization steps in a continuous operation of the pressure swing adsorption (PSA) unit. The gas flow and distribution among the beds is monitored by a complex network of valves and tubing, which makes the system delicate and the method expensive. Moreover, a dead load in the tubing and valves of the system significantly reduces the yield and efficiency of the method. Finally, the method requires cooling down the gas exiting the water gas shift (WGS) reactor from about 400° C. to 40° C. before going through the pressure swing adsorption (PSA) unit, which results in high energy losses.
In contrast, if the H2 separation is done at high temperature, the purification could be integrated within the water gas shift (WGS) reactor or even within a steam reforming reactor, which would allow avoiding cooling of the gas, and therefore allow significant energy savings and method simplification.
H2 separation at high temperature is possible using membranes that are permeable to hydrogen. In such a method, molecular H2 at a temperature between about 450° C. and 500° C. is adsorbed at the surface of a membrane and dissociated into atomic H before diffusing within the membrane. Under the effect of the concentration gradient, the atomic H crosses the membrane and recombines on its opposite surface to form H2. As the membrane is impermeable to the other species, i.e. CO, CO2, etc. . . . , the membrane thus allows separating and purifying gas H2 (see FIG. 1). In theory, in absence of openings in the membrane, an infinite selectivity can be achieved. H2 separation occurs in a passive way, i.e. in absence of any mobile element, which makes the method very easy to operate and reliable. Such a method is very flexible and can easily be integrated with different types of reactors.
As hydrogen is soluble in palladium, palladium can be used for separation of hydrogen from other gases that are not soluble in palladium. However, at a high concentration of hydrogen in palladium, a phase transformation occurs which renders the membrane fragile. Moreover, Pd reacts with H2S present in the mixture of gases being separated and forms palladium sulfildes, causing a significant drop of H2 solubility and therefore an efficiency drop of the separation method.
In order to overcome these problems, palladium-based alloys are used with different temperatures at which H2 causes the above-mentioned phase transformation to increase the resistance to poisoning.
There are different methods available to prepare palladium-based alloys, namely metallurgical methods (vacuum arc melting, casting), physical vapor deposition (PVD) methods (magnetron sputtering, pulse laser deposition), electrochemical and electroless deposition. Generally, metallurgical methods are used for the preparation of stand-alone membranes, while the other methods mentioned above are preferred when preparing supported membranes on porous substrates. Metallurgical methods rely on long heat treatments at high temperatures to achieve homogenous alloys. Alloys can be formed in one single step with PVD techniques, however scaling up is not straightforward. Electroless deposition consists in chemically reducing target metal salts ions that deposit on the surface of a substrate. The reduction is done in sequence, and followed by a thermal treatment to favor diffusion of the metallic ions and alloy formation. Alternatively, metallic powders of pure elements may be mixed and pressed on the substrate before applying a thermal treatment forming an alloy. In both cases, the duration and temperature of the thermal treatment are adapted according to a desired alloy composition.
There is still a need in the art for a method and system for fabrication of hydrogen-permeable membranes.