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3 Membranes Technologies for Efficient CO2 Capture–Conversion
Sonia Remiro‐Buenamañana, Laura Navarrete, Julio García‐Fayos, Sara Escorihuela, Sonia Escolástico, and José M. Serra
Instituto de Tecnología Química (Universitat Politècnica de València‐Consejo Superior de Investigaciones Científicas), Av. de los Naranjos s/n, Valencia, E‐46022, Spain
3.1 Introduction
The growing interest of the scientific community toward global warming and its consequences over environment has led to investigate new energy resources in order to decrease the dependence on fossil fuels. Combustion processes, mainly represented by power generation and industry sectors, account for more than 50% of CO2 emissions, increasing at a rate of 2.5 per year with a worldwide amount of 33.1 GtCO2 during 2018 [1]. Therefore, these processes are the main source of the total CO2 emitted to the atmosphere. As part of the greenhouse effect mitigation, efforts have been made toward decreasing these atmospheric CO2 emissions. The most considered actions are to capture the CO2 from point source emissions [2] and to use it as a feedstock to valuable chemicals and fuels [3].
These proposed actions for enabling the CO2 capture and conversion can be implemented by applying the so‐called membrane technology. Membranes are materials acting as semipermeable barriers between two different phases, and because of their intrinsic properties, they are able to selectively transport a particular component (Figure 3.1). The different characteristics between retentate and permeate will work as the driving force and the membrane properties will set the separation degree. The transport mechanism of the membrane will be determined by the membrane composition, structure (homogeneous or heterogeneous), morphology, origin (natural or synthetic), and operation conditions [4]. Most of the membranes are based on the pore dimension principle, where the pore size will act as the cutoff of the compounds that are able to pass through the membrane, blocking in the retentate larger species [5]. Hence, as a general classification, membranes can be divided into microporous membranes and dense membranes. In addition, selective membranes without electrostatic interaction make up a significant proportion of membrane technology. A selective membrane only allows certain molecules to be transported along the membrane to the permeate side. Finally, for ion‐selective membranes, the transport of ions is ascribed to the compensation of charges in the system by charge‐carrying species [6]. Furthermore, membranes possess some advantages such as low energy consumption, mild reaction conditions, ability to continuously perform separation processes, generally facile scaling up, and among others [4].
Figure 3.1 Gas transport through a membrane.
This chapter will take the reader along the most relevant membranes for the aim of CO2 carbon/capture applications, and these can be divided into polymeric, ionic, protonic, and electrochemical membranes. Metallic membranes are not included in this chapter because of several reasons, such as their production costs and CO poisoning. Some studies report the retarding effects of CO on H2 permeation through Pd‐based membranes. This detriment on H2 diffusion through the active sites of Pd became more significant at temperatures around 250 °C and/or higher CO concentration [7–9]. In the majority of industrial applications for CO2 capture or storage, CO may appear as a subproduct. Hence, the use of Pd membranes for H2/CO2 separation processes may be limited.
Regarding the different applications, membrane reactors have demonstrated to be promising candidates to tackle climate change, decreasing the levels of CO2 by using it in capture–conversion technologies to obtain valuable chemicals [10, 11]. The present chapter describes a range of approaches toward CO2 capture–conversion in the context of catalytic membranes. Because of the extension of the field, this chapter will briefly summarize different types of gas separation and gas absorption membranes that are currently being investigated for CO2 applications.
3.2 Polymer Membranes
Polymer materials for gas separation membranes have been widely used for different applications in the past three decades. Although the first recorded description of a semipermeable membrane was in 1748 [12], followed by the observation of the permeation of H2 through balloons in 1831 [13], it was not until the late 1970s when several experiments demonstrated the great commercial potential of polymeric gas separation membranes [14].
Polymer membranes are dense (nonporous) membranes, which follow the solution–diffusion model as a transport mechanism [15]. In this model, gas is transported through a dense polymer membrane in three steps: (i) dissolving into the face of the membrane that is exposed to high gas pressure, (ii) diffusion through the polymer, and finally (iii) desorbing from the face of the membrane that is exposed to low pressure. The rate‐limiting step for gas separation polymer membranes is the second step, diffusion of gas through the polymer material [15]. As a consequence, permeability can be expressed as the product of diffusion coefficient and solubility coefficient.
(3.1)
Diffusion coefficient is related to the kinetic terms, and it reflects the mobility of the individual molecules in the membrane material. In other words, it depends on the molecular size of the target gas (step ii). On the other hand, solubility coefficient links the concentration of a component in the fluid phase with its concentration in the membrane polymer phase and reflects the number of molecules dissolved in the membrane material (step i and iii). It depends on molecular interaction; hence, it is an equilibrium term [15].
Regarding the selectivity
