A multi-cycle pump for efficient separation of CO2 or other trace gas from a mixture of gases

An idealized gas separation pump is described for a membrane filtration system that conserves compressional energy by transferring it from one component to another operating at a different phase of the compression-decompression cycle. An example of the basic principle, we consider a pump that resembles an internal combustion engine with pistons in cylinders and three valves, one for mixed gas input, another for select gas output through a membrane, and a third for non-select gas output to the environment. With the transfer of compressional energy from the decompressing cylinder to the compressing cylinder, the energy required is the sum of frictional losses and the amount needed to compress only the trace gas that is to be separated. Compressional energy for the non-select is recovered. The mechanical energy required is the energy to compress the cylinder, NRT(lnC−1+1∕C)∼3NRT in the isothermal case for number of moles of select gas, N, gas constant R, temperature T, and compression factor C ∼ 50, multiplied by 1 + 2ϵ∕f ∼ 2 for frictional loss fraction ϵ ∼ 6% comparable to that in internal combustion engines and an input select gas molar fraction f ∼ 10% as for CO2 in flue gas. With T = 300 K and N = 2.3 × 104 per ton for CO2, this energy becomes ∼ 6NRT ∼ 94 kWh/tCO2. The advantages of a system like this are that the compression ratio through the membrane can be large and the timescale for compression can be adjusted to maximize the throughput for a given selectivity, overcoming the usual selectivity-permeance trade-off. A disadvantage is that the throughput is generally low for volumetric pumps at practical scales.