Parallelism and differences of pervaporation and vacuum membrane distillation in the removal of VOCs from aqueous streams (original) (raw)
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Recent advances in VOCs removal from water by pervaporation
Journal of Hazardous Materials, 2003
Pervaporation (PV) is a separation process in which minor components of a liquid mixture are preferentially transported by partial vaporization through a non-porous permselective (selectively permeable) membrane. PV is an emerging technology in environment cleanup operations, especially in the removal of volatile organic compounds (VOCs) from industrial wastewaters or contaminated groundwaters. Current state of PV membrane development in VOC removal and improvement in process engineering, and better understanding of the interactions between VOCs and membrane materials are reviewed. Among PV process parameters documented here are process temperature, permeate pressure, feed concentration, and feed flow rate. The effects of these parameters on PV selectivity and permeation flux have been studied extensively and these studies have borne fruit in a better understanding of many aspects of PV processes. The challenge in implementing PV in practical operations lies in the further enhancement of membrane quality for specific VOCs as well as improved management and control of possible adverse hurdles coming from real systems.
Extraction of organic components from aqueous streams by vacuum membrane distillation
Journal of Membrane Science, 1993
The removal of volatrle organic compounds from aqueous streams by vacuum membrane distrllatlon (VMD) has been analyzed. VMD is an evaporation process which takes place through microporous hydrophobrc membranes; at low pressure the mass transfer through the membrane is generally domrnated by the Knudsen mechanism, while the process selectivity is essentially determined by the hquldvapor equrhbrium condrtlons existing at the mterface. Dilute aqueous mmtures containmg ethanol or methylterbutyl ether have been expenmentally mvestigated, in a wide range of operating conditions. The role of concentration-polarixation phenomena on the separation factor was also mvestrgated. A detailed model of the transport phenomena involved in the process is developed and compared with the expenmental data. A VMD system IS finally designed for the purification of waste waters and the related treatment costs are evaluated.
Removal of VOCs from aqueous solutions using pervaporation process
2012
The contamination of groundwater and surface water by Volatile organic compounds (VOCs) is a problem at many industrial sites. VOCs are present in effluents from industries such as petroleum refineries and chemical plants. Acrylonitrile, which is toxic to humans, is one of the VOCs used in large quantities as an important industrial material for production of synthetic polymers; consequently, it is present in many industrial effluents. More stringent requirements for the removal of VOCs from wastewater in recent years have increased the need to develop new technologies for removal of VOCs from dilute streams. Membrane pervaporation is an attractive and potentially cost-competitive alternative to traditional methods for removing low concentration of VOCs from wastewater. In this study, the batch removal of acrylonitrile, as a VOC, from aqueous solutions using pervavoration process under different experimental conditions was investigated. The influences of temperature, initial concent...
Potentials of pervaporation to assist VOCs’ recovery by liquid absorption
Chemical Engineering Science, 2009
Gas treatment by liquid absorption is a well-known process to remove volatile organic compounds (VOCs) from industrial waste gases. Usually the liquid is an organic solvent of high boiling point; however, after VOCs' absorption it must be regenerated for the possible reuse and this step is classically achieved by heating the liquid. The paper presents the work directed to investigate an alternative regeneration step based on a liquid-vapour membrane separation, i.e. pervaporation. Because most of the energy required in pervaporation processes is consumed to remove the minor component from the initial mixture by selective permeation through the membrane, one can expect a significant energy cut in the operational costs linked to the regeneration of the liquid if the pervaporation step can substitute the heating one. The results reported here show that the technological possibility to use pervaporation is first governed by the stability of the membrane in the absorption liquid. The viability of the overall process is actually controlled by the mutual affinity between the VOCs, the solvent phase and the polymeric material. As a matter of fact, whereas VOCs have to exhibit strong affinities to both the solvent and the membrane material, the polymer has to be well resistant and even repellent to the solvent to avoid the possible sorption in the membrane that would drastically depress the pervaporation efficiency. In other words the membrane transport properties must be specific for the VOCs. This goal was reached following several experimental approaches, going from membrane modifications to the selection of suitable heavy protic solvents. Hence it has been shown for the case of dichloromethane (DCM) that low molecular weights polyalcohols (e.g. glycols) appeared to be suitable media to allow in particular the specific transport of DCM. On the other hand, polydimethylsiloxane (PDMS) based membranes were selected for their stability in these polyglycols and for their marked affinity for DCM. The simulation of the hybrid gas treatment process at pilot-scale was also achieved by a simple model relying on experimental data for both vapour liquid equilibria and permeation flux. A simple comparison of the energy needed to regenerate the heavy solvent by each possible step has also been made.
Pervaporation is the partial vaporization of a liquid mixture through a nonporous membrane and useful in breaking azeotropic and close boiling binary systems. Gas permeation is facilitated by the partial pressure gradient across a similar dense membrane and applied for air separation, purification of natural gas and biogas and even olefin-paraffin mixtures. These two processes are dealt with in terms of principle, mechanisms of separation, synthesis and characterization of appropriate membranes followed by case studies conducted in the laboratory which have lead to successful results. Some of the experimental data include dehydration of liquid propellants and separation of acid gases CO2 and H2S by gas separation. State-of-the art of both processes reveal that they have been commercialized in western countries for alcohol dehydration and natural gas sweetening, but could not make much headway in developing countries due to huge capital investment, operating cost and lack of awarenes...
Vacuum membrane distillation: Experiments and modeling
AIChE Journal, 1997
Vacuum membrane distillation is a membrane-based separation process considered here to remove volatile organic compounds from aqueous streams. Microporous hydrophobic membranes are used to separate the aqueous stream from a gas phase kept under vacuum. The evaporation of the liquid stream takes place on one side of the membrane, and mass transfer occurs through the vapor phase inside the membrane. The role of operative conditions on the process performance is widely investigated in the case of dilute binaly aqueous mixtures containing acetone, ethanol, isopropanol, ethylacetate, methylacetate, or methylterbutyl ether. Temperature, composition, flow rate of the liquid feed, and pressure downstream the membrane are the main operative variables. Among these, the vacuum-side pressure is the major design factor since it greatly affects the separation eficiency. A mathematical model description of the process is developed, and the results are compared with the experiments. The model is finally used to predict the best operative conditions in which the process can work for the case of benzene removal from waste waters.
Separation of organic compounds from gaseous mixtures by vapor permeation
Separation and Purification Technology, 2019
Gas separation technology is a mature topic, while the vapor permeation (VP) process still needs some development. It can be presumed that VP will become extensively applied in the future, thanks to its economic and ecological advantages. Despite single vapor permeability chiefly being reported, less information is available on the binary VOC/N 2 mixture, while a multicomponent mixture is examined only rarely. Some of the newly developed membrane materials offer significant potential. Nevertheless, the long-term stability of membranes is still uncertain and more pilot plan tests are required. Our review compares the advantages and disadvantages of commonly tested membranes as well as newly developed membrane materials. Furthermore, interesting results are highlighted to encourage further research and the future needs and prospects of VP. Both academic and industrial approaches are discussed, with an emphasis on the new trends since 2000.
Pervaporation as a Successful Tool in the Treatment of Industrial Liquid Mixtures
Polymers
Pervaporation is one of the most active topics in membrane research, and it has time and again proven to be an essential component for chemical separation. It has been employed in the removal of impurities from raw materials, separation of products and by-products after reaction, and separation of pollutants from water. Given the global problem of water pollution, this approach is efficient in removing hazardous substances from water bodies. Conventional processes are based on thermodynamic equilibria involving a phase transition such as distillation and liquid–liquid extraction. These techniques have a relatively low efficacy and nowadays they are not recommended because it is not sustainable in terms of energy consumption and/or waste generation. Pervaporation emerged in the 1980s and is now becoming a popular membrane separation technology because of its intrinsic features such as low energy requirements, cheap separation costs, and good quality product output. The focus of this ...
Journal of Membrane Science, 1996
Vapour permeation is a potentially suitable technology for the recovery of organic solvents from waste air streams. New solvent stable capillary membrane modules that are currently emerging on the market provide large membrane areas for an acceptable price and enhance the competitiveness of this process. Most membranes used in vapour permeation are silicone coated composites. Polydimethylsiloxane (PDMS) provides good separation capabilities and is highly permeable. The permeabilities for solvents and permanent gases show an inverse temperature dependence due to the different enthalpies of sorption and diffusion. Selectivities of silicone coated composite membranes are lower than that of pure PDMS but still high enough to enable a high degree of enrichment. The investigation of systems with two solvent components in air shows that selectivities and permeabilities are only slightly lower than in the case when only one solvent component is present. Coupling effects like preferential sorption are, therefore, not very strong. The experimental results have been used as basis for an economical process optimization. Comparison with other waste air cleaning technologies shows that in the range of medium to high solvent concentrations and low to medium feed volume fluxes vapour permeation can be an economical alternative to the conventional processes.