The recovery of VOC (Volatile Organic Compounds) from air is a major issue in terms of minimizing the environmental impact of numerous industrial processes (chemistry, food, pharmaceutical, ...metallurgy, refrigeration…). Non destructive VOC capture technologies are preferentially used in order to enable the recycling of a large ratio of the emitted compounds. To that respect, condensation is attractive because it offers the possibility to recover the VOC from the air stream under liquid state thanks to a physical, non destructive, separation process. Nevertheless, a very low (cryogenic) condensation temperature is often required in order to achieve that target. In that case, a membrane VOC pre-concentration step can be of major interest in order to increase the VOC content of the condensation unit and possibly improve the energy efficiency of the overall operation. In this study, a systematic analysis of the energy efficiency (overall electrical energy needed per kg of recovered VOC) of a standalone condensation process is compared to a hybrid process based on membrane concentration + condensation. It is shown that the standalone condensation remains more energy efficient for high boiling VOC (e.g. toluene, octane, acetone), while a significant improvement of the energy efficiency is obtained with the hybrid process for intermediate to low boiling temperature VOC (e.g. propane, ethane, ethylene…). A generic map of the most energy efficient VOC recovery process as a function of the VOC boiling temperature is finally proposed and potential improvement of the hybrid approach, based on a retentate recycling strategy is discussed.
•A systematic parametric analysis of hybrid membrane-condensation unit is presented.•Hybrid process energy efficiency depends on the VOC boiling temperature and content.•Standalone condensation remains more energy efficient for high boiling VOC.•Hybrid process is more energy efficient for intermediate to low boiling VOC.•A generic selection map of the most energy efficient process is proposed.
•Using dense fibers, absorbent recompression is not necessary before its recycling.•The required energy of the process is half of that in packed columns.•Without flash recovery, the corresponding H2 ...loss is 4.8%.•Process selectivity is mainly controlled by the absorbent.•Lowering H2 loss requires increased contactor size and liquid energy pumping.
Integrated Gasification Combined Cycle (IGCC) technologies involve the processing of synthesis gas (syngas) produced by carbonaceous fuels gasification. CO2 removal from syngas is a key requirement for combined CO2 capture and hydrogen production in IGCC power plant processes for both power generation and greenhouse-gas emission mitigation. Conventional absorption in packed columns using pressurized physical solvents such as Dimethyl Ether of Polyethylene Glycol (DEPG) (Selexol™) is commonly used for this application. In this work, a dense skin hollow fiber membrane contactor (HFMC) based process for CO2 absorption and desorption using Selexol as a physical absorbent is investigated by simulation and compared to the conventional process. The ability of dense membranes to withstand high transmembrane pressure differentials allows the absorbent to circulate in a closed loop system at a fixed pressure set independently of the syngas pressure. Differing from the conventional process, neither absorbent depressurization before the desorber nor absorbent recompression before the absorber are needed. Under the investigated operating conditions wherein we used polydimetylsiloxane (PDMS), one of the most gas permeable polymeric membrane materials available, this process allowed for recovery of up to 94.6% of CO2, with CO2 and H2 purity of 92.4% and 96.6% respectively. The corresponding energy requirement for the absorption and desorption loop was of 0.19 MJel/kg CO2 which is approximately two times lower than that reported in the literature under comparable gas inlet conditions and separation specifications using packed columns. Without flash recovery, the corresponding H2 loss was of 4.8%. The overall mass transfer coefficient was of 1.2 · 10−5 m/s and 6.8 · 10−6 m/s in the absorber and desorber respectively. Membrane mass transfer lower or comparable to that of the absorbent combined with higher CO2/H2 membrane selectivity is required for H2 loss decrease. Lower H2 loss is achieved at the expense of increased contactor size and liquid energy pumping energy. Finally, perspectives for process optimization are proposed.
•Membrane compete to cryogeny only if selectivity is over 20.•Air drying has a strong impact on membrane energy efficiency.•Attractive performances with selective membranes for 30–70% ...oxygen.•Improved cryogenic processes could offer decisive advantages over membranes.
Oxygen Enriched Air (OEA) is already used for numerous chemical, medical and industrial applications (e.g. combustion enhancement for natural gas furnaces, coal gasification) and more recently attracted attention for hybrid carbon capture processes. Membrane separation has shown growing interest for OEA production, providing an alternative to conventional air separation processes such as cryogenic distillation and pressure swing adsorption. Nevertheless, based on the current polymeric materials performances, membranes are usually considered to be competitive only for medium O2 purity (25–40%) and small scale plants (10–25tons/day).
Improvement in membrane materials permeability and permselectivity (O2 over N2) is often reported to be a critical issue in order to increase the attainable O2 purity and to make the process more energy efficient. Recently, several membrane materials have been reported to show performances far above the permeability/selectivity trade-off of dense polymers. In this study, the potential of current and prospective membrane materials to achieve OEA production thanks to a single stage process is analysed through a rigorous simulation approach. The two processes (membrane and cryogenic distillation) are finally critically compared in terms of energy efficiency (kWh/ton O2), depending on O2 purity and on membrane material selectivity levels.
Membrane processes have been initially seldom considered within a post-combustion carbon dioxide capture framework. More traditional processes, particularly gas-liquid absorption in chemical ...solvents, are often considered as the most appropriate solution for the first generation of technologies. In this paper, a critical state of the art of gas separation membranes for CO2 capture is proposed. In a first step, the key performances (selectivity, permeability) of different membrane materials such as polymers, inorganic membranes, hybrid matrices and liquid membranes, including recently reported results, are reviewed. In a second step, the process design characteristics of a single stage membrane unit are studied. Purity and energy constraints are analysed as a function of operating conditions and membrane materials performances. The interest of multistage and hybrid systems, two domains which have not sufficiently investigated up to now, are finally discussed. The importance of technico-economical analyses is highlighted in order to better estimate the optimal role of membranes for CCS applications.
Les procédés de séparation par membrane ont été, dans un premier temps, rarement préconisés pour le captage du dioxyde de carbone en post-combustion, les procédés d’absorption gaz-liquide dans un solvant chimique étant considérés comme la technologie la plus mature et la plus adaptée pour assurer cette opération. Une revue critique des possibilités et limites des membranes pour le captage du CO2 par perméation gazeuse est présentée. Les performances de différents types de matériaux membranaires (polymères denses, matériaux inorganiques, matrices hybrides, membranes liquides) sont discutées sur le plan de la sélectivité et de la perméabilité, en prenant en compte les résultats expérimentaux les plus récents dans ce domaine. Les caractéristiques d’une unité de perméation gazeuse mono-étagée fonctionnant en régime permanent, déterminées par simulation, sont ensuite détaillées, en particulier par rapport aux contraintes de pureté et de dépense énergétique de l’opération, en lien avec les performances du matériau. Finalement, les possibilités des dispositifs membranaires multi-étagés et des procédés hybrides associant une unité de perméation gazeuse et un autre procédé, encore peu étudiées dans la littérature sont évoquées, ainsi que les besoins d’analyse technico-économiques afin de mieux identifier la place et le rôle optimal des membranes pour le captage du CO2.
Numerous studies have been reported on CO2 facilitated transport membrane synthesis, but few works have dealt with the interaction between material synthesis and transport modelling aspects for ...optimization purposes. In this work, a hybrid fixed-site carrier membrane was prepared using polyallylamine with 10 wt% polyvinyl alcohol and 0.2 wt% graphene oxide. The membrane was tested using the feed gases with different relative humidity and at different CO2 partial pressures. Selected facilitated transport models reported in the literature were used to fit the experimental data with good agreement. The key dimensionless facilitated transport parameters were obtained from the modelling and data fitting. Based on the values of these parameters, it was shown that the diffusion of the amine-CO2 reaction product was the rate-controlling step of the overall CO2 transport through the membrane. It was shown theoretically that by decreasing the membrane selective layer thickness below the actual value of 1 µm to a value of 0.1 µm, a CO2 permeance as high as 2500 GPU can be attained while maintaining the selectivity at a value of about 19. Furthermore, improving the carrier concentration by a factor of two might shift the performances above the Robeson upper bound. These potential paths for membrane performance improvement have to be confirmed by targeted experimental work.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, UILJ, UKNU, UL, UM, UPUK
•Thanks to dense membrane fiber, absorbent recompression is not required.•Specific energy requirement is 20 to 35% lower than that of packed column.•The process can offer absorption intensification ...factor of about 1.68.•Process selectivity is mainly controlled by the absorbent.•Without flash recovery, CH4 loss is around 8%.
CO2 removal from raw biogas is required in order to meet standards for the gas grid or as a vehicle fuel. Among biogas upgrading available techniques, pressurized water absorption using packed column (PWA) is one of the most well-established technique. In this work, a novel absorption/desorption loop using dense based hollow fiber membrane contactor (HFMC) process and water as absorbent is proposed and investigated by simulation. Thanks to the ability of dense membranes to withstand a high transmembrane pressure, neither water depressurization before the desorber nor water recompression before the absorber is needed. 1D modeling based on a resistances in series approach is used for the modeling of both absorption and desorption units. HFMC process performances are compared to state of the art packed column (PWA) process reported in literature. Using commercially available dense based HFMC, the process is able to recover 96.6% of CO2 and reach biomethane purity of 98%. The corresponding energy requirement is of 0.17 kWh/Nm3 of raw biogas, which is 20 to 35% lower than that reported for packed column based process, under comparable gas inlet conditions and product specifications. HFMC process offers absorption intensification factor of about 1.68, corresponding to a volumetric reduction of about 68% of the absorption unit. Under the investigated operating conditions and due of the preponderant liquid side mass transfer resistance, process selectivity is mainly controlled by the absorbent selectivity. Without flash recovery, CH4 loss is of about 8%. No significant methane loss reduction is obtained from increasing membrane selectivity from 17 to 60 with membrane mass transfer coefficient of 5.10-4 and 5.10-5 m/s respectively. Perspectives for further process optimization are exposed.
Membrane processes have been shown to offer interesting separation performances for postcombustion carbon capture and storage (CCS) from coal power plant flue gases. A 90% CO2 purity and recovery is ...typically required for that application. This set of constraints does not apply within a carbon capture and use (CCU) scenario, because the objective is to ensure carbon dioxide transformation at minimal cost. In this study, a dense polymeric glassy membrane (Matrimid) and a chemically reacting fixed site carrier membrane (FSCM) are investigated in order to achieve a partial carbon dioxide capture and concentration, which could be of interest in order to intensify the carbon dioxide transformation step. The importance of humidity in the carbon capture performance with solution–diffusion membranes is highlighted: flue gas compression significantly changes the inlet water content of the membrane module, and water decreases the membrane separation performances, increases the specific energy requirement, and decreases the membrane surface area because of an internal sweep effect. On the basis of a case study, the performance of a FSCM is shown to be close to the solution–diffusion membrane performance in terms of surface area and energy requirement. However, the FSCM offers two major advantages compared to the solution–diffusion membrane: compatibility to wet flue gases and high CO2 purity. A FSCM single-stage module, operated with a 50% CO2 capture ratio, is suggested to offer the best trade-off for CCU applications with a high carbon dioxide purity (95%) and a low specific surface area and energy requirement.
Membrane contactors have received increased attention since the 1980s and are already used for different industrial applications. A very large number of studies have been reported, more specifically ...to achieve intensified gas–liquid mass transfer, almost exclusively in water or aqueous solutions. In contrast, the potentialities of membrane contactors for gas–liquid processes based on non-aqueous physical solvents are essentially unexplored. This study intends to discuss the difficulties associated with the specific physical solvent context and explore the potentialities of membrane contactors for both absorption and regeneration steps. Theoretical arguments show that dense membranes based on superpermeable and mechanically resistant polymers could offer promising performances, owing to their capacity to simultaneously prevent wetting effects, sustain a high transmembrane pressure and offer process intensification possibilities. Moreover, a significant improvement in terms of energy efficiency is theoretically achievable for the regeneration step. A preliminary proof of concept study, which supports these potentialities, is presented and the research needs for this new approach in order to possibly achieve applications at industrial scale are discussed.
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•Gas absorption in physical solvents requires dense skin composite or self-standing membranes.•Large intensification potentialities of the absorption process with physical solvents.•Absorption and regeneration are validated by a proof of concept study.•Improved energy efficiency is possible.•Studies on dense self-standing hollow fibers are needed.
Due to its low regeneration energy demands relative to MEA, ammonia is one of the most attractive solvents for post-combustion CO2 capture processes. Nevertheless, additionally to a lower kinetic ...constant, a high ammonia slip takes place when the absorption process is performed in a packed column. In this study, the feasibility of an ammonia based CO2 capture process using hollow fiber membrane contactors is investigated. CO2 absorption experiments in ammonia have been performed with porous polypropylene membranes (Oxyphan) and with two different dense skin composite hollow fibers: tailor made (Teflon AF2400) and commercial (TPX). It is shown that microporous membranes do not offer stable performances, due to salt precipitation and pore blocking. Contrarily however, dense skin membranes show stable and attracting performances, whatever the operating conditions: reduced ammonia slip and intensified CO2 mass transfer are obtained compared to packed column. The potentialities of dense skin membrane contactors, particularly based on fluorinated polymers, are discussed with regard to both increased CO2 mass transfer performances and mitigation of ammonia volatilization compared to conventional gas/liquid contactors.
•Absorption performances not stable with microporous membrane contactors.•In situ precipitation in dense rubbery polymers.•Dense skin composite fibers offer stable absorption performances.•Large intensification factor compared to packed columns.•Ammonia slip mitigation compared to packed columns.
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