| Hydrogen is not only an important industrial raw material,but also an ideal clean energy source.The preparation,separation and purification of hydrogen is the prerequisite and basis for hydrogen/hydrogen energy utilization.At present,hydrogen is mainly obtained from raw materials such as coal,natural gas(methane),and petroleum(hydrocarbons)through steam reforming and shift reaction,and then separation and purification.These hydrogen production reactions generally need to be carried out at high temperature and under a catalyst,and are limited by thermodynamic equilibrium,and the hydrogen production rate must be increased through continuous hydrogen separation and raw material recycling operations.The hydrogen separation membrane reactor is used to make the hydrogen production reaction and the hydrogen separation and purification process proceed simultaneously,which not only breaks through the thermodynamic equilibrium limitation of the reaction,increases the reaction conversion rate and(hydrogen)yield,but also reduces the reaction temperature,eases the reaction conditions,and can directly Pure hydrogen is produced,thereby simplifying the hydrogen production process and reducing the cost of hydrogen production.In this paper,a low-cost metallic nickel hollow fiber membrane was developed and applied to hydrogen separation,and a highly asymmetric structured metallic nickel hollow fiber membrane was successfully prepared by a one-step method to increase the hydrogen permeability,and the asymmetric metallic nickel membrane was used as a catalytic membrane reactor for the water gas shift(WGS)reaction,the methane steam reforming hydrogen production(MSR)reaction,and the reverse water gas shift(RWGS)reaction.Around the above research content,the following work has been mainly carried out:The metallic nickel(Ni)hollow fiber membranes with thin wall thickness and optimal microstructure were fabricated by the dry-wet spinning and sintering technique,and employed for H2 separation from the model reformate mixtures containing CO2,CO,H2O and H2S at elevated temperatures up to 1000?.The prepared Ni hollow fiber membranes possess 100%H2-permselectivity,only allowing for the hydrogen in the reformate mixtures to permeate through under experimental conditions.In the presence of CO,CO2 and H2O(vapor),the hydrogen recovery from reformate mixtures may be noticeably influenced due to the water gas shift reaction(WGS:CO+H2O?CO2+H2).Multiple cycling operation and long term tests were conducted,indicating that the Ni hollow fiber membranes have good cycling operation performance and high resistance to CO,CO2,H2O and H2S poisoning at high temperatures.The excellent thermal and chemical stability as well as the high permeation performance make the Ni hollow fiber membranes great potentials in advanced applications such as the portable hydrogen sources or the large-scale hydrogen production from coal gasification.Ni hollow fibers with an asymmetric structure consisting of a thin dense skin layer on porous support were fabricated in a single step by a modified phase inversion-sintering technique.The effective thickness of the hollow fibers is estimated to be around 28.5?m,which is much less than the overall wall thickness of the hollow fibers(?400?m).The H2 permeance of the resultant hollow fibers reached(0.28-1.35)×10-4mol?m-2?s-1?Pa-0.5 at 600-1000?,which is one order of magnitude higher than that of the symmetric Ni capillaries and equivalent to that of the Pd-based membranes at lower temperatures(200-450?).The metallic nickel hollow fiber membrane with asymmetric internal support structure is applied to WGS reaction,MSR reaction and RWGS reaction without additional catalyst.The results are as follows:(1)In the reverse water-gas shift(RWGS)reaction,the hollow fibre membrane displays excellent catalytic activity in the RWGS reaction.The Ni membrane not only acts as the separation layer for hydrogen permeation but also acts as an effective catalyst bed.The porous finger-like structure provided rich catalytic active sites for the RWGS reaction and reduced the gas transport resistance,thus leading to a high CO yield and H2 permeation flux.Because of the infinite selectivity of the dense skin layer towards H2 permeation,waste gas mixtures containing H2 from chemical plants can be used as the feedstock to supply pure H2 for syngas production.Due to its flexible processability and simplicity,such a Ni hollow fibre membrane can be easily scaled up for industrialization.The application here is demonstrated based on the transformation of two waste gases of hydrogen and CO2 into more useful chemicals.(2)In the water gas shift(WGS)reaction,studies have found that the increase in the H2O/CO ratio leads to a higher CO conversion rate in the nickel membrane reactor system.When the temperature is higher than 950 oC,the catalytic activity of the porous layer of the membrane reactor and the rapid H2 transfer rate cause the CO conversion rate to be greater than the equilibrium conversion rate.The metallic nickel hollow fiber membrane overcomes the poisoning phenomenon of the traditional hydrogen permeable metal Pd and its alloy membrane in the WGS reaction,and the silicon dioxide membrane affects the hydrogen permeability of the water vapor.(3)In the methane steam reforming hydrogen production(MSR)reaction,the effects of the feed composition in terms of steam-to-methane ratio(S/C)and methane concentration;the operational parameters including temperature,space velocity,and the sweep gas flow rate on the performance of the hollow fibers were investigated.The results reveal that the reaction operational temperature should be above 800 oC and the S/C ratio controlled around 3 so as to achieve both high methane conversion and high H2 production rate.When operated at 1000? and 25937 h-1 methane space velocity,the maximum H2 production rate reached 50.84 mmol?m-2?s-1 while the methane conversion reached at 98.58%.In order to produce pure hydrogen,steam may be used as the sweep fluid instead of inert gases such as nitrogen to prevent the dilution of the permeated gaseous hydrogen.The prepared asymmetric NHFMs also demonstrate high chemical stability in the reformate gases and high resistance to carbon deposition at above 800?,and thus may be a promising way of cost-effective hydrogen production by MSR at high temperatures.The metallic Ni hollow fiber membranes with a dense inner skin and open-channel in the wall were fabricated by a modified phase inversion-sintering technique.A triple-orifice spinneret was used for spinning the hollow fiber precursors using a solvent-ethanol mixture was flowing through the outer orifice as the temporary external coagulant.The effects of the external coagulant compositions and air gap on the morphology of the resultant hollow fibres were systematically investigated.The prepared hollow fibre membranes were characterized with scanning electron microscopy(SEM),porosity and tensile strength measurements and hydrogen permeation tests at high temperatures.The results indicate that when the NMP content in the ethanol based external solvent is higher than 50 wt.%and the air gap is higher than 50 cm,a highly asymmetric structure consisting of a dense layer and porous substrate can be formed in the hollow fibre membranes.Such highly asymmetric hollow fibre membranes have exhibited much higher hydrogen permeation flux,especially when the air gap is 80 cm and the NMP concentration is 80 wt.%,i.e.,a maximum of59.09 mmol?m-2?s-1 at 1000?. |
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