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Membrane gas absorption technology is considered as one of the promising alternatives to conventional techniques for CO₂ separation from the flue gas of fossil fuels combustion. As a hybrid approach of chemical absorption and membrane separation, it may offer a number of important features, including operational flexibility, compact structure, linear scale up and predictable performance. The main challenge is the additional membrane mass transfer resistance, especially when this resistance increases due to the absorbent intruding into the membrane pores.

In this thesis, the experimental was set up to investigate how the operating parameters affect the absorption performance when using absorbent in hollow fiber contactor, and to obtain the optimal range of operation parameters for the designated membrane gas absorption system . During 20 days’ continuous experiment, we observed that the CO₂ mass transfer rate decreases significantly following the operating time, which is attributed to the increase of membrane mass transfer resistance resulting from partial membrane wetting.

To better understand the wetting evolution mechanism, the immersion experiments were carried out to assume that the membrane fibers immersed in the absorbents would undergo similar exposure as those used in the membrane contactor. Various membrane characterization methods were used to illustrate the wetting process before and after the membrane fibers were exposed to the absorbents. The characterization results showed that the absorbent molecules diffuse into the polypropylene (PP) polymer during the contact with the membrane, resulting in the swelling of the membrane. In addition, the effects of operating parameters such as immersion time, CO₂ loading, as well as absorbent type on the membrane wetting were investigated in detail. Finally, based on the analysis results, methods to smooth the membrane wetting were discussed. It was suggested that improving the hydrophobicity of PP membrane by surface modification may be an effective way to improve the membrane long-term performance.

Modification of the polypropylene membrane by depositing a rough layer of PP was carried out in order to improve the non-wettability of membrane. The comparison of long-term CO₂ absorption performance by PP membranes before and after modification proves that the modified polypropylene membranes retained higher hydrophobicity than the untreated polypropylene membrane. Therefore modification is likely to be more suitable for use in membrane gas absorption contactors for CO₂ separation, particularly over long operation time.

In recent years, membrane gas absorption technology has been considered as one of the promising alternatives to conventional techniques for CO₂ capture due to its favorable mass transfer performance. As a hybrid approach of chemical absorption and membrane separation, it exhibits a number of advantages, such as operational flexibility, compact structure, high surface-area-to-volume ratio, linear scale up, modularity and predictable performance. One of the main challenges of membrane gas absorption technology is the membrane wetting by absorbent over prolonged operating time, which may significantly decrease the mass transfer coefficients of the membrane module.

In this thesis, the experimental was set up to investigate the dependency of CO₂ removal efficiency and mass transfer rate on various operating parameters, such as the gas and liquid flow rates, absorbent type and concentration and volume fraction CO₂ at the feed gas inlet. In addition, the simultaneous removal of SO₂ and CO₂ was investigated to evaluate the feasibility of simultaneous desulphurization and decarbonization in the same membrane contactor. During 14 days of continuous operation, it was observed that the CO₂ mass transfer rate decreased significantly following the operating time, which was attributed to partial membrane wetting.

To better understand the wetting mechanism of membrane pores during their prolonged contact with absorbents, immersion experiments for up to 90 days were carried out. Various membrane characterization methods were used to illustrate the wetting process before and after the membrane fibers were exposed to the absorbents. The characterization results showed that the absorbent molecules diffused into the polypropylene polymer during the contact with the membrane, resulting in the swelling of the membrane. In addition, the effects of operating parameters such as immersion time and absorbent type on the membrane wetting were investigated in detail. Finally, based on the analysis results, methods to smooth the membrane wetting were discussed. It was suggested that improving the hydrophobicity of polypropylene membrane by surface modification may be an effective way to improve the long-term operating performance of membrane contactors. Therefore, the polypropylene hollow fibers were modified by depositing a thin superhydrophobic coating on the membrane surface to improve their hydrophobicity. The mixture of cyclohexanone and methylethyl ketone was considered as the best non-solvent to achieve the fiber surface with good homogeneity and acceptably high hydrophobicity. In the long-period operation, the modified membrane contactor exhibited more stable and efficient performance than the untreated one. Hence, surface treatment provides a feasibility of improving the system stability for CO₂ capture from the view of long-term operation.

En av de tekniker som under senare framhållits som ett lovande alternativ till konventionell CO₂-avskiljning är membran-gas-absorptionstekniken på grund av god prestanda vad gäller masstransport. Det blandade angreppssättet med både kemisk absorption och membranseparation har en rad fördelar, såsom driftflexibilitet, kompakt konstruktion, högt yt-volymsförhållande, linjär uppskalning, modularitet och förutsägbar prestanda. En av de viktigaste utmaningarna för membran-gas-absorptionstekniken är vätningen av membranet med absorbenten under långa drifttider, vilket väsentligt kan minska membranmodulens masstransportkoefficienter. 

I avhandlingen har en rad olika driftparametrars påverkan på CO₂-reningsgraden och massöverföringshastigheten undersökts. Driftparametrar inkluderar gas- och vätskeflöden, typ av absorbent och koncentration och volymfraktion av CO₂ vid gasinloppet. Avskiljning av SO₂ och CO₂ har dessutom undersökts för att utvärdera möjligheten att samtidigt, i samma membranenhet, avlägsna svavel och kol. Under 14 dagars kontinuerlig drift konstaterades det att massöverföringshastigheten för CO₂ minskade avsevärt med drifttiden, vilket hänfördes till partiell vätning av membranet.  

För att bättre förstå mekanismerna för vätning av membranporer under långvarig kontakt med absorbenter genomfördes doppningsexperiment i upp till 90 dagar. Olika metoder för karakterisering av membran användes för att illustrera vätningsprocessen före och efter det att membranfibrerna exponerades för absorbenterna. Resultaten av karakteriseringen visade att absorbentmolekylerna spreds in i polypropenpolymeren under kontakten med membranet, vilket ledde till att membranet svällde. Dessutom undersöktes effekterna av driftsparametrar såsom nedsänkningstid och typ av absorbent i detalj. Slutligen, på grundval av analysresultaten, diskuterades metoder för att underlätta vätningen av membran. Att förbättra polypropylenmembranets hydrofobicitet genom modifiering av ytan föreslogs kunna vara ett effektivt sätt att förbättra den långsiktiga driftprestandan för membranenheter. Därför modifierades de ihåliga fibrerna av polyproylen med ett tunt lager av en superhydrofob beläggning på membranets yta för att förbättra hydrofobiciteten. En blandning av cyklohexanon och metyletylketon ansågs vara det bästa icke-lösningsmedlet för att få en fiber yta med god homogenitet och acceptabelt hög hydrofobicitet. Under lång driftperiod, uppvisade den modifierade membranenheten stabilare och effektivare prestanda än den obehandlade. Därför erbjuder ytbehandling en möjlighet till att förbättra systemets stabilitet för CO₂-avskiljning när det gäller långsiktig drift.