Aug-2021

A novel approach of CO2 removal from natural gas

A new process combining membranes followed by liquefaction provides a significant advantage in CO2 removal from natural gas.

Mahin Rameshni and Stephen Santo, Rameshni & Associates Technology & Engineering
Priyanka Tiwari, Sachin Joshi, Kaaeid Lokhandwala and Daaniya Rahman
Membrane Technology & Research

Article Summary

In recent years, there have been many developments in LNG, shale gas and others where it is required to remove CO2 from natural gas. The recovered CO2 is compressed and reinjected to reduce CO2 emissions. Producing high-purity dry CO2 is always a challenge with the conventional process. Dry high-purity CO2 is usually required to either reinject or for other usages.

The conventional method has been to design an amine unit configuration with a selective solvent for CO2 removal. The capital and the operating costs are relatively high because it requires considerable solvent circulation and extensive equipment. Also, for offshore processing of natural gas with high CO2, conventional processes such as amine are complex, bulky and energy-intensive.

Two-stage membrane system technology is well known to recover natural gas processing offshore, but it requires recycle compression. The size of recycle compression is in excess due to the residual CO2.

A new process combining membranes followed by liquefaction provides a significant advantage in CO2 removal from natural gas while recovering high-purity dry CO2 as a by-product. Membrane Technology & Research (MTR) patented such a process with a combination of membrane and CO2 liquefaction to separate the CO2 from natural gas.

MTR and Rameshni & Associates Technology & Engineering (RATE) are working together, where MTR designs the membrane systems and RATE designs the CO2 liquefaction system downstream of the membranes. The advantage of using the liquefaction system downstream is to reduce the overall recycling of CO2 to feed the membranes. The advantage of using membranes is to maximise hydrocarbon recovery in the first stage and avoid losing ethane and heavy hydrocarbons to liquefaction. This has the significant benefit of not forming ethane and CO2 azeotrope, which is difficult to break in standalone liquefaction systems. The combination of membranes and liquefaction system can produce up to 99%+ pure dry CO2.

This joint article discusses the advantage of this scheme vs the conventional method. In addition, special compact equipment with higher operating efficiency, used in CO2 liquefaction to reduce plot space, is highlighted along with costs.

CO2 recovery by membrane separation
Key to the CO2 recovery process is a size-selective composite membrane that is more permeable to CO2 than the hydrocarbons present in natural gas (C1 and C2+). MTR can achieve efficient separation 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 CO2 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, CO2 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 CO2 permeates the membrane. The more significant the pressure difference, the greater the flux of CO2 through the membrane 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.
- 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 CO2 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 CO2 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) filter coalescer, (b) carbon bed, (c) particulate filter, and (c) heater. The heated gas is then routed to the first membrane stage, which separates the inlet gas into two streams: