Effect Of He And CO On Hydrogen Permeability Of Ultra - Thin Palladium Membrane

Under the current design, much deuterium and tritium will be vented off along with the tokamak exhaust. In order to improve the utilization of deuterium and tritium and avoid causing environmental pollutions, it is quite necessary to apply Tokamak Exhaust Processing (TEP) so as to retrieve the deuterium and tritium. The current studies on TEP all employ palladium-based membrane separators to separate and retrieve most of the unreacted deuterium-tritium fuel exists in simple substance. This method has characteristics like high hydrogen selectivity, high hydrogen permeability, good chemical stability, etc. In recent years, there have been more researches on ultra-thin(the membrane’s thickness is below 5μm) palladium membranes. This is because by reducing the thickness of palladium membranes, the flux of hydrogen permeation could be effectively increased and the amount of palladium used could be reduced that can lower the cost, which is of significance to engineerization. However, as the thickness of palladium membranes keeps going down to a certain degree, the permeation rate of hydrogen will be higher than its radial migration rate in the gaseous phase. Then the impurity gas will enrich on the surface of the membrane to forming a boundary layer, which will result in the concentration polarization and lead to the reduction of effective driving force of hydrogen permeation. Although the Helium and other inert gas components in the exhaust do not affect with the surface of palladium membrane, but the concentration polarization effect cannot be ignored. Therefore, the study on concentration polarization in ultra-thin palladium membrane for the purification of Q2/He is necessary. In addition, the inhibition of CO is also a focus of researcher’s attention.In previous studies, CO is considered to affect the performance of the membrane’s permeability by competitive adsorption with hydrogen in the palladium membrane surface. However, in the separation process in ultra-thin palladium membrane, the influence of concentration polarization also needs to consider. According to the above two points, the effect of He and CO on ultrathin palladium membrane were investigated in detail. The definition of the concentration polarization coefficient (CPC) and competitive adsorption coefficient (CAC) were carried out to describe inhibition degree.The results of pure hydrogen permeation experiment:The pressure exponent was 0.5, which indicate that the diffusion rate of hydrogen atoms in the body of palladium membrane is the rate controlling step for hydrogen permeation.The average values of K in 623 K,648 K and 673 K were 1027.9,1137.6 and 1254.9, respectively. The selectivity of palladium membrane was excellence with the pressure difference range 150～450kPa and in the temperature range 623-673 K.The investigation of the influence of helium concentration, operating pressure, material flow velocity and temperature on the concentration polarization effect were carried out. The results show that the helium concentration changed from 70% to 15%, CPC declined about 8%. CPC sharply increased with increasing DF or Pret, which indicate that the CPC has high sensitivity with pressure. Improve the material flow velocity can effectively reduce the concentration polarization of palladium membrane hydrogen permeation ability of inhibition. The CPC increased by 10% with the temperature changing from 623 K to 673 K.The inhibition of CO on the separation process was also studied. The results are:CO has very obvious effect on hydrogen permeation of palladium membranes. The evaluation of the a parameter and the Langmuir’s absorption constant was carried out, the results are 0.4 and 1.316 respectively. The inhibition effect of CO was weaker with lower CO concentration and Pret, and it reduced by 30% when temperature increased from 623 K. to 673 K. After the separation of CO/H2, the pressure exponent changed from 0.5 to 1.0, which means the dissociative adsorption is the rate controlling step. The concentration polarization effect cannot be ignored with the DF higher than 130kPa and the competitive adsorption effect of CO will be aggravated by the concentration polarization.According to the above results, the inhibition effect of CO changed more evidently with the variation of temperature and CO concentration, however the concentration polarization effect has high sensitivity with pressure. Therefore, we can reduce the competitive adsorption effect of CO on the palladium membrane by changing the operating temperature and pretreatment method, and reduce the concentration polarization effect by optimizing operation pressure of system.In summary, in this paper, the study on the effect of concentration polarization of Helium and inhibition of CO in ultra-thin palladium membrane has made some progress, and it provides support for the design and optimization of TEP system.