High performance polymer membranes for CO2 separation
Introduction
Membrane technology has been recognized as a useful tool in many industrial applications such as water purification [1], gas and vapor separation [2], and fuel cells [3]. It has benefits in terms of low energy consumption and compact process design compared to other separation technologies. Thus, membrane processes has been considered as an environmentally sustainable technology [2, 4].
Membrane gas separation is a pressure-driven process where the pressure difference between the upstream and downstream acts as a driving force for separation [5]. Industrial membrane gas separation processes have been mainly used in nitrogen production, air drying, and natural gas treatment. For CO2 separation, the removal of CO2 as an impurity from methane (CO2/CH4 separation) is the largest application in natural gas processing, landfill biogas recovery, and enhanced oil recovery [4]. Recently, CO2 separation has been investigated as a greenhouse gas treatment process to combat global warming. The major emission source of CO2 is from coal-fired power plants as a result of fuel combustion (post-combustion, CO2/N2 separation) and from the water gas shift (WGS) reaction in the integrated gasification combined cycle (IGCC) process (pre-combustion, H2/CO2 separation).
Post-combustion flue gas is released from power generators at moderate temperatures (50–100°C) and low pressure (lower than 1.5 bar). Thus, at the low feed pressure, highly permeable membranes are required [4]. Meanwhile, pre-combustion CO2 separation processes are usually operated at high pressure (20 bar) and high temperature (up to 900°C). While the pressure-driven gas separation membrane process has the benefit of high flux, the use of membrane materials is quite limited due to high temperature operation [4, 6, 7].
Polymers are dominant membrane materials for gas separation applications due to their easy processability with asymmetric structures so that high flux membrane modules can be prepared for large scale applications. Various polymeric membrane modules have been commercialized to date. However, because of the low gas separation performance of the existing polymeric materials coupled with their poor chemical and thermal stabilities and plasticization, their use in gas separation applications are still limited [2, 5]. This review will introduce the recent advances in high performance polymeric membrane materials for gas separation, particularly focusing on CO2 separation applications.
Section snippets
Solution-diffusion mechanism
Permeability and selectivity are used to quantify gas separation performance. Generally, permeability and selectivity have a trade-off relationship, in which highly permeable materials have a low selectivity and vice versa. Clearly, current membrane performance has a limitation which is the so-called ‘upper bound’ [8•, 9]. For the last two decades, many researches have been focused on surpassing the upper bound by developing new materials considering transport mechanisms.
In gas separation
Sorption enhanced polymer membranes
The solubility coefficient is related to the chemical affinity between a gas and the polymer membrane and it influences the permeability and permselectivity of rubbery polymer membranes, which usually have a low diffusional selectivity. Separating gases by controlling the gas solubility in polymer membranes is an attractive way to improve CO2 separation because rubbery polymer membranes usually have high CO2 solubility and thus, a high solubility–selectivity. Some examples of rubbery polymers
Diffusion enhanced glassy polymer membranes
Gas diffusivity and diffusion selectivity in polymer membranes are especially governed by the chain rigidity and free volume in the polymer structures. In particular, glassy polymer membranes are influenced by diffusion selective gas separation because gas diffusion occurs through the space between polymer chains known as the free volume element or cavities [5, 10, 28].
Typical glassy polymers are known to be non-porous materials with small free volume elements due to their rigid structure
High temperature CO2 separation
Membrane gas separation at high temperatures in pre-combustion processes has been advantageous because it eliminates the cooling step and prevents energy loss. Depending on operation conditions of the WGS reaction, the H2/CO2 mixed gas is fed usually above 300°C which common polymeric materials cannot endure due to degradation. As a result, various palladium-based metals and ceramics have been studied for pre-combustion CO2 separation membranes, but they are difficult to apply for large scale
Conclusions
Recent development of high performance polymer membranes demonstrates the progress made in novel polymer membrane materials for future membrane technology applications for CO2 separation. Research has been intensely focused on membrane materials and membrane processes. Therefore, membrane processes using recently developed high performance polymer membranes can be expected to be applied in large scale CO2 separation applications in the near future. The CO2 separation performance of high
References and recommended reading
Papers of particular interest, published within the period of review, have been highlighted as:
• of special interest
•• of outstanding interest
Acknowledgments
This research was supported by Korea CCS R&D Center (KCRC), funded by the Ministry of Education, Science and Technology in Korea, and by WCU (World Class University) program, funded by the National Research Foundation (NRF) of the Korean Ministry of Science and Technology (No. R31-2008-000-10092-0), which we gratefully acknowledge.
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