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Energy efficiency of gas separation by Pressure Swing adsorption

University of British Columbia

Pressure Swing Adsorption (PSA) is a method of separating a mixture of gases into its various components. Cyclic pressure and flow variations, in the presence of a selectively adsorbent material, are used to concentrate one species or group of species at one end of an adsorbent filled vessel, while the other species or group of species is concentrated at the other end. When PSA is used in separating gases, the necessity of gas pressurization and depressurization implies that the process can become very energy intensive. This is especially true in low capacity systems that require small compressors and/or vacuum pumps. There are many ways in which traditional PSA processes have been modified in order to reduce the amount of pressurization energy that is lost. One method is to use high pressure gas from one adsorbent bed to pressurize another adsorbent bed. This "equalization" recovers some of the energy used to initially compress the gas. However, as the gas is throttled from one bed to the other, irreversibilities are introduced into the process. In this thesis, the irreversibilities that are due to throttling are separated from those which are inherent in the PSA process and cannot be removed. The work required to produce a certain amount of gas by various simple PSA cycles is compared to the reversible work required to produce that amount of gas, based on the availability (or exergy) of the gas. The ratio of the reversible work to the actual work required for the PSA cycle is defined as the second law efficiency, and is compared for three cycles: the Four-Step cycle, the Ideal Four-Step cycle, and the Ideal Three-Step cycle. The irreversible expansion of gas through throttling valves is shown to account for the majority of the energy losses of the Four-Step cycle. Useful work (represented by the increase in availability of the product and exhaust) is found to be very small compared with the work required by the cycles. The true bed losses, inherent in the PSA process, are found to be similar in magnitude to the useful work, but much less than the energy lost by the throttling irreversibilities. The work required per mole of product to separate the gases decreases as the pressure ratio increases, and the second law efficiency increases with pressure ratio. For the cycle with no energy recovery, the second law efficiency varies widely with the selectivity ratio. A high selectivity ratio (implying a low separation factor) implies more work is required for the separation and the second law efficiency is lower. For the cycles with full recovery of the expansion energy, the work required and the second law efficiency are relatively independent of the selectivity ratio. The equilibrium based semi-analytical results are confirmed by the use of a numerical "Multiple-Cell" model. This model is also used to show that diffusion does not affect the second law efficiency of a cycle when energy recovery is present.

Thesis/Dissertation

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2009-03-06

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http://hdl.handle.net/2429/5655

Mechanical Engineering

University of British Columbia

UBCV

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