Hydrogen purification using metallic hydrides

BORZONE E. M.; BARUJ A.; MEYER G. O.

Zaragoza

Congreso; WHEC 2016 - 21st World Hydrogen Energy Conference; 2016

An in-line hydrogen purification process, based onhydride forming materials, is presented. The work is part of a study to assessthe convenience of recovering hydrogen gas from a metallurgic manufacturingplant. The plant currently uses a 24 Nm3/h flow of hydrogen in asuperficial reduction process, during which small amounts of impurities areincorporated to the stream. Such contaminants, as measured by gaschromatography and a dew-point sensor, include up to 1000 ppm of H2O,300 ppm of CO2, 10 ppm of CO and 7 ppm of CH4. Thedew-point is continuously monitored in-line as a measure of the inlet gasquality. In order to reuse this gas, these contaminants must be removed. Otheroptions, such as PSA, are not commercially available for this scale and thus requireadditional R&D effort to be considered. A purification process is implemented at a prototypescale, using a total of 300 g of LaNi5 distributed in two reactionbeds working in opposition. In this schema, one reactor receives the incominggas, absorbing the hydrogen in it, while the other desorbs pure hydrogen aftera brief venting stage in which contaminants are eliminated. Thus a constantflow is processed. Fig. 1 shows theprototype´s layout, in which the two reactors are thermally coupled in order touse the heat of reaction and save energy in comparison with other schemas [1-3],working at room temperature without the need of heat sources or sinks. The reactors?design criteria and construction details are discussed, including aspectsrelated to mechanical stresses, thermal behavior, bed permeability and generalgeometry. Purification experiments are presented, usingpreviously humidified hydrogen. The choice of humidity as contaminant relatesto three key aspects; its mentioned use in the plant, the fact that it can bemeasured in-line, and the difficulty of removing it by this method, moresuitable for less reactive impurities, making the experiments presented aless-favorable scenario. As a test case, using a flow of 100 sccm, the process successfullylowers the humidity content in the gas from 3800 ppm to 190 ppm, with a hydrogenrecovery fraction of 93%. Fig. 2shows the water vapor partial pressure in one reactor during a complete cycleof operation. Its value rises steadily during the absorption stage, as the gascontinuously enters the reactor, then sharply decreases at venting, to finallyshow a low value during the desorption stage. The humidity tail observed comesfrom the vapor adsorbed in the reactor and pipes, and it limits the finalpurity obtained.A lumped computational model is developed to describethe reaction's behavior. The model is validated against measured results withpure H2 at flows from 100 sccm to 2000 sccm. This tool is used tostudy the effect of different working conditions on the recovery fraction.Specifically, the effect of the operating flow, the supply pressure, the material?scomposition within the LaNi5-xSnx family as reference [4],the pressure loss in the filters at the reactor's inlet, the operatingtemperature and different heat transfer conditions are evaluated. At roomtemperature and a supply pressure over 300 kPa, the prototype can recover over98% of the hydrogen at flows up to 800 sccm.References[1]  M.Lototskyy, K.D. Modibane, M. Williams, Ye. Klochko, V. Linkov, B.G. Pollet, J.Alloys Compd. 580 (2013), S382-S385.[2] K.B. Minko, V.I. Artemov, G.G. Yan'kov,Appl. Therm. Eng. 76(2015), 175-184.[3]  See anexample of commercial design at www.bjhaoyun.com. Reaction heat issupplied by external heating and cooling systems.[4]  E.M. Borzone, A. Baruj, M.V. Blanco, G.O.Meyer. Int. J. Hydrogen Energy 38(2013), 7335-7343.