Mixed matrix membranes preparation for post-combustion capture

Written by Dr Maria-Chiara Ferrari, Lecturer in Carbon Capture, University of Edinburgh

Principal Investigator on Call 1 Project: Mixed matrix membranes preparation for post-combustion capture

Atmospheric temperature increase during recent years has been associated with the growing levels of greenhouse gases (GHG) emissions which in turn is associated with the increase in the worldwide demand of electricity. Unfortunately this trend is expected to continue during the next few decades and, CO₂ is one of the main contributors to GHG emissions. One way to reduce CO₂ emissions to the atmosphere is carbon capture and storage (CCS). Several technologies have been proposed to capture CO₂ from power plant emissions, including absorption, adsorption, cryogenic distillation, and membrane separation. Traditionally, for post-combustion processes, packed columns have been used for absorption but this technique is energy consuming and suffers from various problems including flooding, foaming and/or solvent degradation. Membrane separation however, presents many advantages, like compactness, and easy integration in already installed facilities.

Membrane gas separation is a pressure driven process where the partial pressure difference between the upstream and the downstream acts as a driving force for the transport through the membrane. Thus, as the post-combustion flue gas is released at low pressure (lower than 1.5 bar), selective and highly permeable membranes are required to have high efficiency in the separation process. 

A new type of membrane material emerging with the potential for future applications is mixed matrix materials composed of homogeneously interpenetrating polymeric and inorganic particle matrices.

The aim of the project is to develop a fundamental understanding of gas transport in mixed matrix membranes, through evaluating the behaviour and performance of the composite membranes and their individual component materials. This will be undertaken in an effort to further our understanding of the interactions between organic and inorganic materials and therefore be able to design a membrane that can be utilised in an economically feasible post-combustion carbon capture process.

Dense neat membranes of Matrimid, Ultem (two glassy polymers), PDMS and PEBAX (two rubbery polymers) have been elaborated in the lab.  For that, the polymer was dissolved with an adequate solvent and then the viscous mixture was casted on glass plate with a casting knife. These flat membranes were free defects with a thickness around 30 µm (except for PDMS with a thickness of 100 µm).  They were characterized by pure gas permeabilities (CO₂ and N₂). The results of the permeability and the selectivity (i.e. the ability of the membranes to separate two gases) were in the range of the literature.

We are currently working on the addition of inorganic fillers in these membranes. This step is crucial in the elaboration of MMM. In fact, a poor adhesion between the fillers and the polymer matrix can induce presence of defects as holes which brings no selectivity.

Two mixed matrix membranes were obtained based on Matrimid with 13X and on PDMS with 13X. They had a thickness of 30 µm with Matrimid and 175 µm with PDMS. A selectivity of CO₂ under N₂ was found superior to 1, which confirms the absence of holes. Now, they have to be analysed more in detail.

The first months of the project have also been devoted to the development of a new permeation rig that will double our ability to characterise membranes. The system is almost finished and when completed, it will be possible to determine the permeability of CO₂ and N₂ and the selectivity up to 30 bars and 200°C.The next steps will be, firstly, the preparation of others MMM with new fillers (Zeolites, Silica, provided by Johnson Matthey) and the other polymers (PEBAX and Ultem). Secondly, these membranes will be characterised at high pressure and high temperature with the new rig.