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 obtaining a high concentration of separated material 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 Cost limits the pressure ratio in a membrane CO2 separation stage to a value of 5; higher pressure ratios eliminate any