Our Research Focus: Déjà Vu

Article by Humbul Suleman AMIChemE and Rizwan Nasir

Humbul Suleman and Rizwan Nasir ask if VHS tapes can help to develop better membranes for CO2 removal?

INSPIRED by VHS tapes of the past, this feature looks at work on developing membranes to remove carbon dioxide from industrial emissions.

Many people can still remember the first VHS tapes introduced in the late 1970s. They remained a hallmark for tape media recording for more than two decades until replaced by CDs, DVDs, and the internet. However, a few know that the first commercial membrane for gas separation was also developed in the same decade1. Interestingly, both products were made from the same material, cellulose acetate, a bioplastic which has many commercial applications even today.

Although early membranes were pretty simple, manufacturing VHS tapes was quite an intricate process. Micro-sized magnetic particles were stuck (using a special glue) on a cellulose acetate strip with certain strength and pattern, as shown in Figure 1. These specially-processed magnetic particles recorded the video, while the cellulose acetate supported them.

For most of the 20th Century, gas separation membranes remained confined to labs and research centres, as they were not technically advanced enough to replace other gas separation technologies. Meanwhile their liquid-separation counterparts found exhaustive applications in water treatment, beer filtration, drug delivery, dialysis and so on.

Figure 1: Typical VHS tape construction

The present

Today, gas separation membranes are technologically developed. They are slowly replacing conventional scrubbers and liquid-absorption towers for separating industrial gases, and CO2 removal is not an exception. With wide applications from natural gas cleaning and biogas upgrading, to ammonia and methanol manufacture, different types of membranes are finding use in CO2 separation processes. This includes polymeric, inorganic, facilitated transport, blended membranes and many others, of which, polymeric membranes have gained a slight technical edge over others, thanks to their chemical structure and early development.

How a gas separation membrane works

In a typical membrane separation process, a mixture of gases passes over a semi-permeable membrane that permits selective passage of a particular gas under a driving force. This can be achieved by either creating nano-sized pores in the membrane or by dissolving the gas molecules in the membrane, allowing them to slowly diffuse through it, as shown here.

As the name implies, polymeric membranes provide a polymer support for holding any special particles coated on them. Since their structures consist of long molecular chains, they have a flexible molecular arrangement that allow small gas molecules like CO2 to easily seep through them. Hence, we can fine-tune them for singling out CO2 from a gaseous mixture2.

Recently, we performed an extensive literature review of these types of membranes for CO2 removal3 and found that there are still many interesting research areas for future development. We believe that mixed-matrix membranes (MMMs) are an excellent bifurcation of the polymeric membrane class. Their exclusive formulation and molecular structure allow a rapid separation of CO2 from other gases, and simultaneously achieve a highly pure CO2 product, a characteristic not reported so far for any other type of membrane.

Surprisingly, the principle once used to develop VHS tapes is still used today in fabricating the mixed matrix membranes for CO2 separation

Surprisingly, the principle once used to develop VHS tapes is still used today in fabricating the mixed matrix membranes for CO2 separation. Special additives with a high affinity for CO2 (derived from organic chemicals) are mechanically dispersed and coated on a thin membrane base layer, analogous to the micro-magnetic particles on VHS tapes. These membranes are then specially treated to strengthen their structure. When in use, the polymer base provides structural support, while the coated materials tremendously enhance the passage of CO2 and separate it from a gaseous mixture, as shown in Figure 2. Sometimes, we add fillers to improve membrane stability and reduce costs4.

With excellent results reported in academic literature, we think MMMs for CO2 separation are lab proven yet far away from commercialisation. Currently, the biggest challenge in our view remains with the pilot-scale testing of these membranes. Many parameters, like behaviour of MMMs in the presence of other minor gas impurities, membrane life, and stability over time are not well known .

Models that can replicate the lab-scale conditions to industrial cases offer a quick route to understanding these unknown parameters. Open literature does report a few good membrane models for MMMs fabricated for CO2 removal6. Sadly, they cannot predict the membrane formulation for an industrial application, as they fail to account for all the required process parameters in a single framework.

Figure 2: (A) mixed matrix membrane with a support layer [5]; (B) Separation concept of a mixed matrix membrane

Article By

Humbul Suleman AMIChemE

Senior Lecturer in Chemical Engineering at Teesside University, UK

Rizwan Nasir

Assistant Professor at University of Jeddah, Saudi Arabia

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