Membranes for ethylene/ethane separation
Promotion date: December 13.
Promotor: Prof.dr. Kitty Nijmeijer
The feasibility of membranes in a hybrid membrane-distillation plant is investigated, aiming to reduce the energy consumption of energy intensive, cryogenic distillation. Membrane based separation can be an interesting alternative, as it has a low energy consumption and footprint. However, polymer membranes suffer from the intrinsic drawback that separation is based on diffusion and solubility differences, which are exceptionally small in case of C2H4 and C2H6.
The economic evaluation of membranes in a hybrid membrane-distillation plant was presented. An optimized process required a membrane selectivity of 30 and C2H4 permeance of 2.8 ∙ 10-5 mol/(m2 s kPa) to break even. Financial savings of 16% were possible with a membrane selectivity of 300.
N2, CO2, C2H4 and C2H6 induce swelling and sorption effects on thin PPO films. C2H6 showed the highest swelling. Penetrant-polymer interactions were examined: N2, CO2,C2H4 and C2H6 induced swelling and sorption phenomena, were investigated on thin P84, PPO, SPPO and PDMS films.
Preparation and characterization of mixed matrix membranes (MMMs) took place. Three different commercial metal organic frameworks (MOFs) were selected (i.e. Cu3BTC2, FeBTC and MIL-53) and incorporated in a P84 polymer matrix. Incorporation of Cu3BTC2 showed the most promising results to increase the C2H4/C2H6 selectivity, and was further investigated.
Investigation on the transport mechanism of C2H4 through Cu3BTC2 MMMs, took place.The C2H4 and C2H6 solubility coefficient was found to increase with increasing Cu3BTC2 loading, while the permeability coefficient was found to remain constant. Consequently, the diffusion coefficient was calculated. Combination of results suggest that C2H4 is immobilized inside the Cu3BTC2 MOF.
Implications of all results were discussed: Polymer swelling was found to be dependent on penetrant-polymer interactions. Crosslinking could be utilized to reduce polymer swelling, while simultaneously introducing functional groups could improve the selectivity. Layer by layer technology seems an interesting method to produce defect free thin membranes, allowing for higher permeances while the selectivity can be kept constant.
Was your research application oriented or fundamental in nature?
It was a mix of both. This mix is necessary to develop new breakthrough technologies, which was – and still is - the aim behind this membrane research project.
In oil cracker processes lots of energy are used to separate the valuable gasses ethylene and ethane. From these valuable products other materials can be fabricated. The aim here was to investigate the potential value of membrane processes to attain good bulk separation results, to a level of approximately thirty/seventy purity ratios. For higher purity, distillation or adsorption processes will always be necessary, I believe.
Our focus at the Membrane Technology Group is unique, building in absorbing reactants in selective membranes. We managed to produce and characterize these membranes for this application. At a fundamental level we investigated the way supply streams passed through the membrane structures. It showed that the ethylene was adsorbed more quickly by the membrane, which is what we hoped for. However some end products are not released quickly enough from the membranes, causing a fall back in the overall performance of the membranes.
In the world, a lot of research groups are working on the topic of developing the absorbing reactants separating the gas fractions of ethane and ethylene. Our hope is the selectivity reactants will improve considerably in the near future. Here in Twente, we can combine the advantage of high selective absorbants with cheap, durable polymer membranes as a matrix. This will bring the efficiency or our membrane separation approach closer to commercial applications in the middle term future, I am sure of. In my thesis work I was able to define some of the dominant process mechanisms taking place here, and I could contribute to the understanding of them.
Did your work lead to some nice publications?
Two articles appeared in the Journal of Membrane Science, one in Industrial Engineering and Chemistry Research. Two articles are under review at this moment.
In what way did you develop personally, as a researcher and scientist?
Perseverance is one of the key factors towards success. In scientific research a lot of steps are involved. It is an illusions all of these steps will work out well right from the start. I learned that from apparently failing experiments some important conclusions and information can still be drawn, leading to new ways to approach the questions under research.
Practically speaking, I also learned a great deal on skills like presentation talks and writing reports and articles. These are now beyond compare as to when I was a master student. In my daily work I act much more independently now. I left the wait-and-see mentality behind me, taking initiative to solve matters right away, for example by consulting colleagues and technicians on the subjects I am working on. Everyone can contribute something, I learned.
What are your future plans?
I currently work at a membrane company housed in Enschede, called Pentair. Although the field of research seemed similar at first sight, the topic of my work there is quite different, studying membranes in process and drink water systems. I choose this company on purpose, as I am very interested in industry working environments, from the moment I had an internship at Shell, as a master student. I like to get to know the working atmosphere there.
Working on a different theme of research also appeals to me. When I started my PhD work I didn’t have any knowledge concerning gas separations. Now again I start from scratch, at water separation technologies in industry. I am looking forward to that.