Elsevier

Planetary and Space Science

Volume 191, 15 October 2020, 104975
Planetary and Space Science

Filtration of simulated Martian atmosphere for in-situ oxygen production

https://doi.org/10.1016/j.pss.2020.104975Get rights and content

Highlights

  • In-Situ Resource Utilisation (ISRU) can reduce mass and cost of planetary missions.

  • The Mars Oxygen ISRU Experiment (MOXIE) will filter dust aerosol and produce oxygen.

  • Analysis and experimental study of filtration of simulated Martian atmosphere.

  • Determined filter dust loading as a function of time.

  • Determined filter pressure drop as a function of dust loading.

Abstract

In-Situ Resource Utilisation (ISRU) can reduce the mass and cost of planetary missions. The Mars Oxygen ISRU Experiment (MOXIE) on the Mars 2020 rover Perseverance will demonstrate ISRU on Mars for the first time by producing oxygen from atmospheric carbon dioxide via solid oxide electrolysis. To protect the solid oxide electrolysis subsystem from contamination by dust, a High Efficiency Particulate Air (HEPA) filter is used. However, the performance of HEPA filters in Martian atmospheric conditions is not well understood. The theory of filtration was reviewed in the context of filtration of Mars’ atmosphere, and an experimental investigation was carried out to determine the dust loading rate and pressure drop as a function of dust loading and filtration velocity for a flight-representative pleated and baffled MOXIE HEPA filter using wind tunnels and Martian dust simulant. In simulated atmospheric conditions of 10.3 ​mbar carbon dioxide at room temperature with a horizontal wind speed of 3 ​m ​s−1 and filter inlet face velocity of 7.1 ​cm ​s−1, the dust loading rate was (0.19 ± 0.02) mg ​m−2 h−1. This is likely a lower bound: analytical approaches estimate dust loading rates of up to approximately 20 ​mg ​m−2 h−1. The pressure drop ΔP (mbar) as a function of dust loading m (g ​m−2) and filtration velocity UF (cm ​s−1) was ΔP=am+bUF, where a = 0.0012(1)mbar (g m-2)-1 (cm s-1)-1 and b = 0.063(1) mbar (cm s-1)-1. Due to operation outside the continuum flow regime, pressure drop increased with atmospheric pressure, unlike HEPA filters on Earth where pressure drop is independent of atmospheric pressure. Dust is unlikely to produce a problematic pressure drop for MOXIE, but needs to be considered for large-scale filtration if the benefits of atmospheric ISRU on Mars are to be fully realised.

Introduction

In-Situ Resource Utilisation (ISRU) has the potential to dramatically reduce the mass and cost of planetary missions. For example, producing 1 ​kg of consumable in situ on Mars reduces the payload mass required to be launched from Earth by approximately 10 ​kg (Sanders, 2018). To increase the technology readiness level of ISRU, the Mars 2020 rover Perseverance will carry the Mars Oxygen ISRU Experiment (MOXIE). MOXIE is a small-scale ISRU technology demonstrator that will produce oxygen from Mars’ atmospheric carbon dioxide using solid oxide electrolysis (Hecht and Hoffman, 2015).

Mars’ atmosphere contains a large amount of dust that, if ingested by MOXIE, may contaminate the electrodes of the solid oxide electrolysis subsystem. MOXIE is protected from dust by a High-Efficiency Particulate Air (HEPA) filter. However, as dust accumulates on the filter, the pressure drop across it will increase. Should the pressure drop become sufficiently large, there is a risk that MOXIE’s compressor will not be able to deliver the required mass flow rate of carbon dioxide, limiting the rate of oxygen production.

The dust loading rate and pressure drop versus dust loading relationship of fibrous filters have been extensively investigated in terrestrial conditions, where the continuum and slip flow regimes apply (see, for example, the textbooks of Brown, 1993; Thomas et al., 2017). However, fewer studies have examined low-pressure conditions where the transition and free molecular flow regimes apply (Li et al., 2014), and even fewer have explored Martian conditions, where both a low-pressure carbon dioxide atmosphere and Mars dust simulant are required. Within the context of filtration for ISRU, previous work has focused on cabin air filtration, instead of atmospheric filtration (for example, the Scroll Filter System (SFS), Agui and Perry, 2017; Agui et al., 2017); lunar dust, instead of Martian dust (Agui, 2008); flat filters, instead of pleated filters (Agui, 2016); or electrostatic removal of dust, instead of removal by fibrous filters (Calle et al., 2011).

In addition to understanding the level of risk presented by filter pressure drop increase, experimental characterisation of the MOXIE HEPA filter is important for several reasons. First, it is difficult to predict filter performance analytically without accurate knowledge of the dust Particle Size Distribution (PSD), particularly at the HEPA filter Most Penetrating Particle Size (MPPS) of approximately 0.3 μm. Although models of the dust PSD on Mars are in general agreement above 1 μm, they vary over several orders of magnitude below 1 μm (Esposito et al., 2011). This uncertainty hampers efforts to predict the dust loading rate and the pressure drop versus dust loading relationship. Second, it will not be possible to monitor the dust loading on the MOXIE HEPA filter on Mars by visual inspection, as the intake is covered by a baffle. Third, the risk presented by dust increases in severity for potential full-scale ISRU plants after Mars 2020. Compared to MOXIE, a full-scale ISRU plant will operate for longer, at higher flow rates, and continuously instead of intermittently, introducing the possibility of operation in a wide range of dust particle number densities. Depending upon filter design, this may result in a higher dust loading rate and pressure drop.

This paper addresses two main objectives and seven sub-objectives that relate to the performance of HEPA filters in simulated Martian conditions. The two main objectives are to determine the dust loading rate, dm/dt, and pressure drop as a function of dust loading and filtration velocity ΔPm,UF, for the MOXIE HEPA filter. Dust loading, m, is defined as the mass of dust captured by the filter per unit filter media area. Only suspended dust is considered. Saltated (wind-blown) dust is excluded as it is assumed that MOXIE’s baffle will deflect all saltated particles. The seven sub-objectives are to characterise how the dust loading rate is affected by particle size, filter geometry (pleated or flat), whether or not carbon dioxide is being actively drawn through the filter (active or passive dust loading), whether or not the filter is fitted with a baffle, and filter orientation to the wind; and how the pressure drop is affected by inlet face velocity and wind tunnel pressure.

It was expected that the dust loading rate would be reduced for larger particles and pleated, passive, and baffled filters aligned at an angle of 90 to the wind, and that the pressure drop would increase with dust loading, inlet face velocity, and atmospheric pressure. These hypotheses, and the underlying theory supporting them, are discussed in Sect. 2. The dust loading rate was determined by exposing various filters to simulated Martian conditions and measuring their mass before and after exposure. The pressure drop as a function of dust loading was determined by loading flat filters to various dust loadings, measuring the pressure drop versus inlet face velocity for each one, and measuring their mass before and after loading. Materials and methods are discussed in Sect. 3. Results are reported and compared to theoretical predictions in Sect. 4, before Sect. 5 concludes with implications for MOXIE, Mars 2020, and future atmospheric ISRU on Mars.

Section snippets

Dust loading rate

Dust loading, m, is the mass of dust captured by the filter per unit filter media area. The dust loading rate, dm/dt, can be estimated as:dmdt=πρpnpdp3UIAI6AFwhere ρp is the dust particle density, np and dp are the dust particle number density and diameter, UI is the inlet face velocity, and AI and AF are the areas of the inlet and filter media. Eq. (1) assumes that the particle density, number density, and size are uniform. Therefore, when applied in previous work, typical values for these

Equipment

The Mars Simulation Laboratory at Aarhus University, Denmark, has several wind tunnels to study dust transport. The Aarhus Wind Tunnel Simulator II (AWTSII, the “large wind tunnel”) is a 2.1 ​m diameter, 10 ​m long recirculating wind tunnel (Holstein-Rathlou et al., 2014) that was used to address the first objective by exposing various filters to simulated Martian conditions for a set amount of time. The Aarhus Wind Tunnel Simulator I (AWTSI, the “small wind tunnel”) is a 0.8 ​m diameter, 3 ​m

Dust loading rate

Table 3 shows the dust loading rates for all of the filter configurations and dust simulants tested in the large wind tunnel. Fig. 11 shows the increase in the mass of the flight-representative filter versus time.

The dust loading rate was found by taking the slope of the best-fit linear trend, resulting in (0.19 ± 0.02) mg ​m−2 h−1, which is equivalent to (4.6 ± 0.4) mg m-2sol-1. This dust loading rate was found using an inlet face velocity of 7.1 ​cm ​s−1. Dust loading rates for other inlet

Conclusion

In-situ resource utilisation is a powerful way to reduce the mass and cost of planetary missions. MOXIE will demonstrate ISRU of atmospheric carbon dioxide on Mars 2020, however dust ingestion is a risk. An experimental investigation was carried out to determine the dust loading rate and the pressure drop as a function of dust loading for the MOXIE HEPA filter.

The dust loading rate was found to be (0.19 ± 0.02) mg m−2 h−1 using a flight-representative filter (Sect. 3.1.1). The dust loading rate

Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: A. Wiegmann is the co-founder and CEO of Math2Market GmbH. M. Azimian is an employee of Math2Market GmbH.

Acknowledgements

This study was funded by Europlanet, the Danish Council for Independent Research grant no. 4002-00292B, the UK Space Agency, and the Val O’Donoghue scholarship at Imperial College London. Europlanet 2020 RI has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No. 654208. We thank Morten Bo Madsen for initial funding, Robin Vinther Nielsen and Simon Jacobsen for valuable assistance during the testing, Jeff Mellstrom and Adrian Ponce

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