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Hybrid hydrogen - carbon dioxide separation systems

Final Report Summary - HY2SEPS (Hybrid hydrogen - carbon dioxide separation systems)

The main goal of the HY2SEPS project was the development of a hybrid membrane pressure swing adsorption (PSA) H2/CO2 separation process, which will be a part of a fossil fuel de-carbonisation process used for the pre-combustion CO2 capture. A typical PSA waste gas stream is not usually recycled since it has to be recompressed to the PSA feed pressure for recovering only a small fraction of the recycled hydrogen. It cannot be used for CO2 sequestration since it contains significant amounts of H2 and CH4.

To achieve the goal of enhancing overall H2 recovery and providing an H2-free CO2 stream ready for capture and sequestration, the following tasks have been identified:
- Generation of transport and adsorption data for H2 / CO2 multicomponent mixtures (CH4, H2O, CO) for well characterised membrane and sorbent materials.
- Development and improvement of membrane and PSA separation models.
- Design and optimisation of membrane, PSA and hybrid separation systems using the improved models developed.
- Component design for the manufacture of a lab-scale hybrid separation system prototype.
- Assessment of the hybrid separation process sustainability and impact on the environment based on a life cycle analysis approach.

The following innovations can be considered as an outcome of this project:
- Experimental tests indicated that H2 recovery of the PSA standalone system was improved from about 65 % to more than about 75 %.
- Process design simulations indicated that the PSA-membrane hybrid system can increase the H2 (99.99 % plus) recovery by over 10 % compared to that of the PSA standalone system. Furthermore, it is possible to capture 40 % of the total CO2 in a stream having about 80 % purity. lt is also possible to capture more than 90 % of the CO2 produced in a stream having purity about 60 %.
- Development of improved membrane materials (CO2 / H2 separation factor up to 18 and corresponding CO2 permeation flux up to about 20 mmol.m-2.s-1).
- Identification of membrane materials with permselective performance that is not significantly influenced by the presence of humidity.
- Two modi?ed adsorbents were prepared. One with enhanced selectivity towards CO2 and another with enhanced selectivity towards nitrogen. The first one has CO2 capacity about 6.7 mol.Kg-1 at 1 bar, and the second one N2 capacity about 0.18 mol/kg at 0.26 bar.

Work performed during HY2SEPS focused on the following:

- The synthesis of the second membrane material has been investigated in detail. Furthermore, in order to improve the permselective properties of the first membrane increase of the CO2 permeation flux and in a decrease of the CO2 / H2 separation factor. The effect of feed pressure on membrane performance has been examined for the first time. ln all cases increase of the feed pressure improved both the separation factor and the CO2 permeation flux. The upscale of the first membrane material has also been studied. lt has been possible to increase the membrane length from about 3.5 cm to 10-12 cm.

A commercial activated carbon adsorbent has been selected as the first adsorbent material to be studied. The adsorption capacities of all components of interest have been measured using a magnetic suspension microbalance. lt was possible to successfully measure the diffusion coefficients of all gasses only when the pulse-response and / or the diluted breakthrough techniques were used. A multicomponent adsorption Virial model was able to successfully predict the adsorption behaviour of the full set of gasses tested. The transport and adsorption properties of the second commercial available sorbent were also determined. The second sorbent was in reality a set of two different sorbents. Based on the outcome of these studies two different sorbents were selected for modification and improvement. The transport and adsorption properties of each one have been determined. The first one showed enhanced selectivity towards CO2 and the other one enhanced selectivity towards N2.

Tests on a PSA pilot plant using commercially available sorbents provided guidance about how to increase the PSA efficiency by adjusting the different cycle times. The membrane and PSA standalone system models have been formulated and tested in selected cases. These models will be used for the design of the standalone processes as well as for the design of the hybrid separation system. Emphasis was given on modelling simulation and optimisation for design of a novel membrane-PSA hybrid system. A model for the hybrid system (PSA- membrane) has been formulated, the appropriate strategies and computational tools for the rational design and optimisation of the hybrid process were developed and the suitable methodologies for model-based process control have been identified. Overall, the investigations illustrated the typical trade-offs between operating and capital costs and formed a platform, upon which the optimal design and control of the hybrid system will be based.

Simulations have shown that a hybrid system can increase the hydrogen recovery up to 10 %. It is also possible to get CO2 stream of almost 80 % purity coming out. However, in that case only about 40 % of the total CO2 is captured. It is possible to increase the fraction of CO2 captured to more than 90 % without sacrificing the H2 purity and recovery if the CO2 concentration of that stream is reduced to about 60 %. This clearly suggests that the hybrid performs much better than a standalone PSA hybrid in terms of hydrogen recovery and also has potential for producing a combined waste stream (including membrane and PSA) more pure in CO2.

Finally, the work related to the assessment of hybrid process sustainability has been finalised. Like every chemical process the proposed hybrid process also impacts the environment. An LCA study is conducted to quantify the value of these impacts. This is compared with the stand alone PSA separation system achieving same hydrogen purity. Global warming potential (GWP) is calculated for hybrid and the stand alone PSA as a function of membrane area and compared. The results show that cumulative GWP for the hybrid process is always less than the stand alone PSA system and thus the hybrid process environmental impact is less.

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