Plasma Enhancement Gases separation via ceramic porous membranes for plasma exhaust processing system of DEMO

https://doi.org/10.1016/j.fusengdes.2021.112484Get rights and content

Highlights

  • Plasma Enhancement Gases (PEGs) are considered in DEMO to mitigate the heat load over plasma facing components.

  • The use of single channel microporous ceramic membranes is investigated for separating PEGs from H2.

  • The work describes a theoretical study which allows the assessment of the microporous ceramic membrane length.

  • Results of simulations for the separation of binary mixtures are presented and discussed.

  • The outcomes of this study are important to correctly assess the conditions of the next experimental campaign.

Abstract

Plasma Enhancement Gases (PEGs), i.e. N2 and noble gases, are used to mitigate the heat load over plasma facing components. Therefore, in DEMO, the tokamak exhaust stream will contain a certain amount of these PEGs which require a dedicated purification and separation unit. In the last years, PEG separation using microporous ceramic membranes has been studied at ENEA Frascati laboratories in the framework of the EUROfusion programme. The experiments conducted with a binary gas mixture have revealed the poor separation capability of the multi-channel membranes as a consequence of their high permeance.

In this activity, the behaviour of a commercial microporous single-channel membrane made of alpha alumina has been theoretically studied to evaluate its efficacy in separating H2 from PEGs. The analysis has considered a total feed flow rate of 20 L min−1 (at STP) having an equimolar composition of H2 with one of the following species: PEG (Ar or N2), D2 or He. The separation capability has been evaluated by assuming the upstream pressure in the range 200−1000 kPa and keeping the downstream pressure at 100 kPa. Considering the values of the permeances assessed in our previous work for single gas tests, the membrane length needed to achieve a given target of separation has been evaluated as a function of the H2 mole fraction in the retentate stream.

Introduction

Plasma Enhancement Gases (PEGs), consisting of noble gases (Ar, Ne, Xe, etc.) and N2, are studied to convert the plasma thermal energy and then to mitigate the heat load over the plasma facing components [[1], [2], [3], [4], [5]]. While in ITER fresh PEGs are fuelled into the plasma and their recovery is not foreseen, the Plasma Exhaust Processing of DEMO is considering several options for separating the PEGs, such as: pressure swing adsorption (PSA), cryogenic trapping, cryogenic distillation, cryogenic absorption, fluorocarbon absorption, adsorption on activated charcoal/zeolite, zeolite membranes, metal organic frameworks (MOFs) and ceramic membranes. Among these techniques, ceramic membranes are increasingly being explored thanks to their wear resistance, well-defined and stable pore structure, chemical inertness and because they do not require the use of cryogenic temperature [6,7].

As reported in [[8], [9], [10]], recent activities have been carried out in ENEA-Frascati to investigate the possible use of ceramic porous membranes in PEGs recovery. The first research assessed the mass transfer properties of ceramic membranes (pore size 0.1 nm to 1 μm) via the Poiseuille and Knudsen models. That work suggested, for the separation of PEGs, the use of membranes of pore size 1 μm operating at room temperature and low pressure [8]. Then, the permeance of commercial ceramic porous membranes was measured via single gas tests and the selectivities of binary mixtures were theoretically evaluated [9]. With this information and based on the DEMO process requirements, a preliminary design of a membrane cascade for the PEGs separation was carried out. In the vein of these results, a further research activity was performed using two commercial membranes (ATECH 02 and ATECH 5 kD) with the aim to experimentally verify their separation capability for binary mixtures of H2 with a PEG [10]. In contrast with the theoretical expectation [8,9], the results of the experimental activity showed a negligible separation efficacy of the two commercial membranes. In fact, during the experiments, it was not possible to establish the required pressure drop across the membrane due to their very high gas permeances. For this reason, in the last experimental campaign, a procedure capable to simulate binary gas tests was adopted and the design of a membrane cascade was updated with the new results [10].

In the present activity, a single channel membrane, characterized by lower gas permeance has been considered. The single channel membrane has the same geometrical characteristics of the previous one (ATECH 02 with top layer pore size of 0.2 μm) and thanks to its lower permeance is expected to exhibit significant improvement in the separation of the binary gas mixtures.

Section snippets

Method

A single-channel ceramic porous membrane produced by Atech Innovation gmbh (Germany) and purchasable by MeTe Membrane Technology s.r.l. (Italy) is considered. This commercial membrane is widely used for the filtration of liquids, has good stability and is not expensive. The characteristics of the membrane are the same of the ATECH 02 studied in our previous works [9,10] i.e., a macro-porous support made up of alpha alumina and three 0.2 μm pores sintered layers made up of alpha alumina with an

Results

Fig. 3, Fig. 4, Fig. 5, Fig. 6 show the behaviour of the permeate and retentate mole fractions of H2 vs. membrane length at plumen = 200–400 – 800–1000 kPa for each binary gas mixture. Particularly, the simulation has considered the same binary mixtures investigated in the previous work [9], i.e. H2/N2, H2/He, H2/D2 and H2/Ar.

From Fig. 3, Fig. 4, Fig. 5, Fig. 6, it can be noted that the membrane length reduces by increasing the feed pressure (plumen). The increase of the pressure from 200 to

Conclusion

A commercial single channel membrane has been considered to simulate the separation of binary mixtures consisting of hydrogen with a PEG (Ar and N2), helium or deuterium. Comparing the results of this theoretical assessment with those of our previous work, the separation of binary mixture is to be more effective using single channel microporous membrane. For a separation target fixed at xRH2 = 0.4, the increase of the upstream pressure (plumen) reduces significantly the length of the membrane:

Declaration of Competing Interest

The authors report no declarations of interest.

Acknowledgments

This work has been carried out within the framework of the EUROfusion Consortium and has received funding from the Euratom research and training programme 2014-2018 and 2019-2020 under grant agreement No 633053. The views and opinions expressed herein do not necessarily reflect those of the European Commission.

References (10)

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    Experimental characterisation of membranes for PEG separation. Tests have been undertaken using a purpose-built experiment to investigate the diffusion and selectivity of gases through porous membranes [62,63]. The obtained selectivity values were still low, so that membrane cascading would be necessary, if this technology were to be chosen.

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