Transient Dynamic and Modeling of Hydrogen Permeation through a Palladium Membrane

• Main Authors: I-han Chiu, 邱奕翰
• Other Authors: Wei-Hsin Chen
• Format: Others
• Language: zh-TW
• Published: 2009
• Online Access: http://ndltd.ncl.edu.tw/handle/64563597566677772045

碩士 === 國立臺南大學 === 綠色能源科技研究所 === 97 === Transient dynamic of hydrogen permeation through a palladium (Pd) membrane is studied in the present study by constructing a Pd membrane reactor. The reactor consists of an inlet unit, a preheating unit, a membrane separation unit and a gas control and monitor unit. Three different pressure differences between the two sides of the membrane are considered; they are 3, 5 and 8 atm. The experimental results indicate that the variation in the hydrogen permeation process is notable at the selected pressure differences. When the pressure difference is relatively low (i.e. 3 atm), the hydrogen permeation process proceeds from a time-lag period, then to a concave up period and eventually to a concave down period. Therefore, the transient hydrogen permeation is characterized by a three-stage mass transfer process. When the pressure difference is increased to 5 atm, the time-lag period disappears, thereby evolving the three-stage mass transfer process into a two-stage one. Once the pressure difference is as high as 8 atm, the transient hydrogen permeation is completely characterized by a concave down curve, yielding a single-stage mass transfer process. Once the hydrogen permeation is in the steady state, over 80% of hydrogen can be recovered from the membrane. The transient processes for hydrogen permeating through the Pd membrane at various membrane temperatures and pressure differences are also modeled to aid in predicting the hydrogen transport behavior. The model of the transient permeation process is established in terms of the quasi-steady time and the steady permeation rate. Meanwhile, four important parameters are taken into account; they are the permeation lag time, the initial permeation rate (the initial jump), the concave up period and the concave down period. A unit step function is embedded in the model to account for the effect of the hydrogen permeation lag. By introducing an adjusting parameter in the arc tangential function, there exists an optimal value of the adjusting parameter to evaluate the transient mass transfer process when the pressure difference is lower (i.e. 3 atm). In regard to the moderate and higher pressure differences (i.e. 5 and 8 atm), it is found that the predictions from the model are in excellent agreement with the experimental results if the adjusting parameter is sufficiently large. Eventually, the steam methanol reforming (SMR) in association with a low-temperature water gas shift reaction and the Pd membrane is studied. Three different pressure differences between the two sides of the membrane are regarded; they are 5, 7.5 and 10.5 atm. When the pressure difference is 5 atm and the reaction temperature is 350℃, it is found that the CO concentration is lower than 50ppm at the permeate site and the methanol conversion is larger than 98%. Therefore, the hydrogen obtained from the reactor can be used as the feedstock of proton exchange membrane fuel cells (PEMFC).