| Summary: | 碩士 === 國立臺南大學 === 綠色能源科技研究所碩士班 === 101 === Concentration polarization, a retarding effect upon hydrogen permeation across a membrane, will affect the performance of hydrogen separation. To recognize the concentration polarization behavior of hydrogen-rich gas in high-permeance membrane tubes, three palladium-based membranes with two pure palladium (Pd) membranes and one Pd-Cu alloy membrane were tested in the first part. Three important parameters, consisting of the H2 partial pressure difference, the flow rate at the exit of the retentate side, and the H2 concentration in the feed gas mixture (H2-N2) are taken into account. A comparison to pure H2 permeation suggests that the concentration polarization is clearly exhibited when the gas mixture is fed. Decreasing H2 concentration in the feed gas tends to deteriorate the polarization phenomena. The sensitivity of polarization to H2 concentration is higher when its concentration in the feed gas is higher. For the gas mixtures with 50 and 75 vol% of H2, the permeances of the membranes are reduced by 1-2 orders of magnitude. The permeances of the membranes are also affected a bit when the H2 partial pressure difference or the flow rate is altered.
In the second part, to figure out the mass transfer phenomena of H2 in membrane tubes, H2 permeation and recovery characteristics of two high-permeance Pd membranes are investigated. Four important factors influencing H2 permeation, namely, the H2 partial pressure difference, H2 concentration, the flow rate at the exit of the retentate side, and membrane temperature, are taken into account. The experimental results suggest that decreasing H2 concentration, flow rate, and temperature reduce the permeances of the membranes and H2 recovery, even though the H2 pressure difference is identical. The dimensionless permeance, a permeance ratio between H2-N2 gas mixture and pure H2 as feed gases, is used to evaluate the extent of concentration polarization. Within the investigated ranges of the four factors, the dimensionless permeances of the two membranes are in the ranges of 0.022-0.206 and 0.042-0.359, respectively, revealing that the concentration polarization diminishes the permeance of the membranes down to the level of two orders of magnitude. Nevertheless, over 46% of H2 is recovered.
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