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Background
Today, membranes are used for commercial separations involving: N2 from air, H2 from ammonia in the Haber-Bosch process, natural gas purification, and tertiary-level Enhanced Oil Recovery supply.

Single-stage membrane operations involve a single membrane with one selectivity value. Single-stage membranes were first used in natural gas purification, separating CO2 from methane. A disadvantage of single-stage membranes is the loss of product in the permeate due to the constraints imposed by the single selectivity value. Increasing the selectivity reduces the amount of product lost in the permeate, but comes at the cost of requiring a larger pressure difference to process an equivalent amount of a flue stream. In practice, the maximum pressure ratio economically possible is around 5:1.

To combat the loss of product in the membrane permeate, engineers use “cascade processes” in which the permeate is recompressed and interfaced with additional, higher selectivity membranes. The retentate streams can be recycled, which achieves a better yield of product.

Need for multi-stage process
Single stage membranes devices are not feasible for a high concentration in the permeate stream. This is due to the pressure ratio limit that is economically unrealistic to exceed. Therefore, the use of multi stage membranes is required to concentrate the permeate stream. The use of a second stage allows for less membrane area and power to be used. This is because of the higher concentration that passes the second stage, as well as the lower volume of gas for the pump to process. Other factors, such as adding another stage that uses air to concentrate the stream further reduces cost by increasing concentration within the feed stream. Additional methods such as combining multiple types of separation methods allow for variation in creating economical process designs.

Membrane Use in Hybrid Processes
Hybrid processes have long standing history with gas separation. Typically, membranes are integrated into already existing processes such that they can be retrofitted into already existing carbon capture systems.

MTR, Membrane Technology and Research Inc., and UT Austin have worked to create hybrid processes, utilizing both absorption and membranes, for CO2 capture. First, an absorption column using piperazine as a solvent absorbs about half the carbon dioxide in the flue gas, then the use of a membrane results in 90% capture. A parallel setup is also, with the membrane and absorption processes occurring simultaneously. Generally, these processes are most effective when the highest content of carbon dioxide enters the amine absorption column. Incorporating hybrid design processes allows for retrofitting into fossil fuel power plants.

Hybrid processes can also use cryogenic distillation and membranes. For example, hydrogen and carbon dioxide can be separated, first using cryogenic gas separation, whereby most of the carbon dioxide exits first, then using a membrane process to separate the remaining carbon dioxide, after which it is recycled for further attempts at cryogenic separation.

Cost Analysis of Current Membrane Technology for CCS Relative to Well Established Techniques
Cost limits the pressure ratio in a membrane CO2 separation stage to a value of 5; higher pressure rations eliminate any economic viability for CO2 capture using membrane processes. Recent studies have demonstrated that multi-stage CO2 capture/separation processes using membranes can be economically competitive with older and more common technologies such as amine-based absorption. Currently, both membrane and amine-based absorption processes can be designed to yield a 90% CO2 capture rate. For carbon capture at an average 600 MW coal-fired power plant, the cost of CO2 capture using amine-based absorption is in the $40-100 per ton of CO2 range, while the cost of CO2 capture using current membrane technology (including current process design schemes) is about $23 per ton of CO2. Additionally, running an amine-based absorption process at an average 600 MW coal-fired power plant consumes about 30% of the energy generated by the power plant, while running a membrane process requires about 16% of the energy generated. CO2 transport (e.g. to geologic sequestration sites, or to be used for EOR) costs about $2-5 per ton of CO2. This cost is the same for all types of CO2 capture/separation processes such as membrane separation and absorption. In terms of dollars per ton of  captured CO2, the least expensive membrane processes being studied at this time are multi-step counter-current flow/sweep processes.