Decarbonisation Technology – August 2021

and the reduction in the number of membrane modules needed to perform the desired separation. Advantages A few key benefits of membrane systems are: • The membrane system is a passive solution with no moving/rotating parts. Considerable reduction in maintenance and operating costs.

Module housing

Permeate ow

Feed ow

Residue ow

Feed ow

Collection pipe

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Permeate ow after passing through membrane

Membrane

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Figure 2 Expanded view of MTR’s spiral wound membrane module

by exploiting the differences in permeability through the company’s robust, high-flux polymeric membrane. The membrane consists of a very thin, highly selective top layer and a tough, relatively open microporous support layer (see Figure 1 ). The top layer performs the separation, while the porous support layer provides mechanical strength. A non-woven fabric serves as the backing material for the membrane structure. For use in CO 2 recovery processes, MTR incorporates membrane into spiral-wound membrane modules. These modules consist of a densely packed sandwich of membrane envelopes and spacers in a spiral wound configuration around a central collection pipe (see Figure 2 ). Mesh spacer materials create channels through which the feed gas and permeate vapours travel with minimum pressure drops. As a feed gas stream containing organic vapour passes across the membrane surface, CO 2 passes preferentially through the membrane and enters the permeate channel. The permeate vapour spirals inward through the permeate channel to the central collection pipe. A pressure difference is maintained across the membrane between feed and permeate stream to provide the driving force for permeation. The pressure difference can be obtained by compressing the feed or using a high-pressure feed stream and maintaining permeate at lower pressure by connecting it to a lower pressure point. This pressure difference directly affects the rate at which CO 2 permeates the membrane. The more significant the pressure difference, the greater the flux of CO 2 through the membrane

• High on-stream factor in excess of 99%. • Modular design of the spiral wound membrane allows for future design flexibility. Additional gas can be processed with the addition of future modules. Process designs Two main membrane process designs – single- stage and multi-stage – are utilised for CO 2 separation from natural gas. Single-stage plants, which are simple, contain no rotating equipment and require minimal maintenance, are preferred for tiny gas flows. In such plants, methane loss to the permeate reject stream is often >15%. If there is no fuel use for this permeate gas, this stream must be flared, representing a significant revenue loss. For gas wells that produce <1 MMSCFD, one-stage membrane units may make sense economically with their low capital and operating costs. As the natural gas stream increases in size, the methane loss from a one-stage system and the resultant loss in revenue soon make the choice of a one-stage system unattractive. Usually, the permeate gas is recompressed and passed through a second membrane stage, which reduces the methane loss to a few per cent. A two-stage system minimises losses compared to a single-stage scheme, resulting in improved recoveries and higher CO 2 concentration in the product stream. A two-stage membrane design is discussed below in detail. Two-stage membrane process A two-stage membrane process scheme is depicted in Figure 3 . The feed gas first passes through a pretreatment section consisting of (a)