Membranes for CO2 removal

The main companies producing membranes for CO2 removal are listed in Table 4.6. 

All the plants mentioned are operating with membranes based on hollow fiber polyimides or spiral-wound CA, which is considered proven technology. The environmental aspect related to CO2 as a green house gas has triggered the development of better membranes for CO2 removal—this is more closely discussed in Section 5.2.3. 

The majority of a membrane system cost is associated with the large compressors required to pressurize the permeate stream and feed pretreatment operations required to remove heavy hydrocarbon and water vapor impurities only about 10%-25% of the total system cost is associated with the membrane module [27]. Membranes for CO2 removal applications are typically fabricated in either hollow fiber or spiral wound format. Hollow fiber approach has the advantage of greater membrane area per unit volume and greater operational flexibility and module fabrication cost [28]. 

With the size and growth of the NG market, it is expected that some novel polymeric membranes for CO2 removal from NG will undergo pilot scale testing. In the near term, cellulose acetate-based membranes are expected to maintain their dominant position, with other membrane types increasing their market share due to their higher plasticization resistance properties [14]. 

Commercial membranes for CO2 removal are polymer based, and the materials of choice are cellulose acetate, polyimides, polyamides, polysulfone, polycarbonates, and polyeth-erimide [12]. The most tested and used material is cellulose acetate, although polyimide has also some potential in certain CO2 removal applications. The properties of polyimides and other polymers can be modified to enhance the performance of the membrane. For instance, polyimide membranes were initially used for hydrogen recovery, but they were then modified for CO2 removal [13]. Cellulose acetate membranes were initially developed for reverse osmosis [14], and now they are the most popular CO2 removal membrane. To overcome state-of-the-art membranes for CO2 separation, new polymers, copolymers, block copolymers, blends and nanocomposites (mixed matrix membranes) have been developed [15-22]. However, many of them have failed during application because of different reasons (expensive materials, weak mechanical and chemical stability, etc.). 

Y. Xiao, B. T. Low, S. S. Hosseini, T. S. Chung, The strategies of molecular architecture and modification of polyimide-based membranes for CO2 removal from natural gas - a review. Prog. Polym. Sci., 34, 561-580 (2009). 

A. Callison, G. Davidson, Offshore processing uses membranes for CO2 removal. Oil Gas Journal (2007). 

This section reviews our recent work on new facilitated transport C02-selective membranes for CO2 removal, WGS membrane reactor, and CO2 capture from flue gas/synthesis gas. 

UOP Separex membrane comprising cellulose acetate (CA) polymer has been extensively used for CO2 removal from natural gas and currently holds the membrane market leadership for this appUcation. The UOP Polysep membrane, a polymeric membrane, has been successfully applied to H2 separation processes. 

A C02-CH4 methane process gas stream, similar to a typical high CO2 natural gas has been under test by SEPAREX for CO2 removal in a 2-in. diameter element pilot plant since September 1981. The feed gas contains 30% CO2 and is delivered to the membrane test unit at 250-450 psig under ambient temperature conditions. The objective of the system is to reduce the CO2 level of the methane to less than 3.5%. The membrane system consists of 5 pressure tubes in series, each tube containing three 40-in. long elements. The gas is conditioned to maintain it at a minimum of 20°F above the dewpoint. The system was operated at a variety of flow rates, pressures, recoveries and temperatures. Selected data are presented in Figures 6 through 8. 

The membrane contactor for CO2 removal deserves special attention. It can be used for natural gas treatment, dehydration, and removal of CO2 from flue gas (see Section 4.4.4). A contactor (see Figure 4.22) patented and developed for this purpose by Aker Kvaerner— pUots have been installed and tested both in Norway (at Karstp) and at a gas terminal in Scotland. This module is based on PTFE membranes. A different commercial contactor based on polyimide membranes was recently installed at Santos Gas Plant in Queensland, Australia (December 2003). Santos is the largest gas producer in Australia. 

FIGURE 13.15 Schematic diagram of experimental setup with capillary membrane for simultaneous removal and enrichment of CO2. (From Teramoto, M., Kitada, S., Ohnishi, N., et al., J. Mem. Sci., 234, 83, 2004. With permission.)... 

The selection of the liquid absorbents in membrane contactors is critical. The commonly used absorbents for CO2 removal are amine based (i.e., MEA, DEA, and TEA) [160-163], Recently, membrane contactors using ILs as alternative absorbents for the capture of acid gases have been reported [166-170], The unique properties of ILs (nonvolatile with a high affinity for the acid gas component, thermally and chemically stable [171]) make it very promising as CO2 absorbents in membrane contactors, especially for applications in harsh conditions, such as CO2 separation from precombustion flue gas at elevated temperatures and pressures [169,170]. 

Simons K, Nijmeijer K, Wessling M. Gas-Uquid membrane contactors for CO2 removal. J Membr Sci 2009 340 214-220. 

NG processing is the largest industrial gas separation application for membranes. Membrane market share in 2008 was 5%, with interest in this application growing rapidly [2]. Membranes for CO2 and heavy hydrocarbon removal (C3+) from NG continue to be the two largest membrane gas separation applications, while N2 and H2S removal from NG is in its early stages of commercialization [12], Dehydration of NG using membranes is also attracting interest [13]. 

Figure 4.8 Process flow diagram of two-stage membrane separation for CO2 removal from natural gas stream data correspond to the optimal solution ( / in Figure 4.9.

H2O plasma was used to modify poly(methyl pentene) hollow fiber membranes, making it possible to immobilize an enzyme (carbonic anhydrase) on them. Such treatment significantly improved the respiratory assistance devices for CO2 removal... 

GS for CO2 removal in power generation (Dijkstra and Jansen, 2004 Kaldis et al. 2004 Ho et al., 2006 Mundschau et al., 2006 Tarun et al., 2007). As far as membranes are concerned, each one of these processes could benefit from their integration in the power plants. To better understand the use of membranes in the three technological pathways, some details on the principles of the three main COj capture options are given. Rgure 7.9 shows the conceptual scheme of the three main CO2 capture options. 

Figure 1. Principe of membrane gas absorption (MGA) and Membrane Gas Desorption (MGD) process for CO2 removal.

Several processes need to be carried out before natural gas can be safely released into transmission pipelines. The removal of water is one of these and an improved system based upon membranes, already successfully used for CO2 removal, was tested some years ago by the US Gas Technology Instimte (GTI). The system was expected to result in a 50 70% reduction in size and weight of the dehydration unit. 

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