VOLATILE ORGANIC COMPOUNDS (VOC) REMOVAL BY PERVAPORATION IN A TUBULAR TYPE MEMBRANE MATHEMATICAL MODELLING AND PRELIMINARY TESTS Ramin Nikpour Khoshgrudi a, Aleksandra Ciosek a, Michał Zalewski a, Maciej Szwast a a PW WIChIP: Politechnika Warszawska, Wydział Inżynierii Chemicznej i Procesowej, 00-645 Warszawa, Waryńskiego 1 r.nikpour@ichip.pw.edu.pl Keywords: pervaporation, VOC removal, mathematical modelling in membrane process 1. INTRODUCTION By definition, volatile organic compounds (VOC), refer to materials that contain Carbon atoms and they have the partial vapour pressure at least 0.13kPa in standard conditions (293K, 101kPa), exception metalliferous organic compounds and organic acids [1]. Many industrialized countries are grappling with the problem of water pollution with these compounds which are man-made from various sources such as agriculture operations, industrial factories and also municipal waste [2,3]. Anyway, there are many motivations for more advanced methods to remove VOC from water, to prevent pollution of water resources, more appropriate wastewater treatment plants and protection of health of humans [1]. The main methods for VOC removal from a dilute solution (such as water) are: Biological methods [4,5], adsorption (via activated carbon) [6], Stripping in packing filled columns (towers) [7], catalytic oxidation [3] and eventually pervaporation process as a kind of membranes technologies [8,3]. Each of these methods have some advantages and disadvantages relative to one another. But recently, according to some limitations such as cost and energy requirements in the first four traditional classic methods, the new technology of membranes has found an important place in case of VOC removal from water by pervaporation process [8]. 2. MEMBRANE TECHNOLOGY Membrane technologies are used for separation processes which have been considered their importance in industry purposes since the 1970s simultaneously with the feeling of the energy crisis. These processes are based on differences in some parameters such as molecule size, concentration, electric charge, solubility between various components in mixtures and membrane materials and also partial pressure [2]. Among the advantages of the membranes can be noted: their proven selectivity separation potential, energy effective, environmental friendly, good reliability for operation in remote locations, small footprint, affordability and good economy and continuously operation without need to generation. In addition, the product stream can be reused to reach higher purities. Separation processes based on synthetic mainly polymeric membranes are used in various applications particularly in water treatment plants. There are different types of membrane technologies such as reverse osmosis, nanofiltration, pervaporation (PV), membrane air stripping and membrane distillation. Among them, PV is a promising method for VOC removal from a dilute solution such as water [3]. PV attracted much attention since the early 1980s and there are many reviews in the field of separation and removal of various VOC from water and their modelling over these past
three decades [2]. In this paper the usage of PV processes for the VOC removal from water, its mathematical modelling according to mass balance and permeation flux for the availability desired components (VOC) with analytical and numerical solutions and comparison some experimental results are examined. And finally is tried to find some of the best polymeric materials to produce the composite membrane in this technology. 2.1. Pervaporation Pervaporation (PV) is a type of separation process in which a liquid mixture containing water and VOC is placed in contact with the membrane. Feed flow is exposed to atmospheric pressure at the upstream before the membrane, and permeate flow through the membrane under partial vacuum pressure in the downstream mostly will be included VOC with some water and finally retentate is water virtually free of VOC depending on the efficiency of the membrane system. It can be said that VOC species absorb or dissolve into the polymeric structure of membrane, diffused and permeate through it and then evaporate into the vapour phase at the back of the membrane. Required partial vacuum pressure usually is applied by a vacuum pump or sometimes maybe by a sweep gas. Therefore, the driving force in this process is difference in the vapour pressure as the potential chemical between the feed and permeate flows before and after the membrane [2,9] (Fig. 1). Fig. 1. Scheme of a pervaporation system for VOC removal from water. In act both mass and heat transfer phenomena occur in this process. The term of transfer here can be a simple concept of solubility diffusion mechanism. Such that selectivity determines with the concept of selective diffusion or adsorption. Anyway, synthetic affinity for a liquid to a polymer is more than a gas into that polymer as its solubility is much higher too. So, membranes that are used in PV can be similar to the gas separation membranes in terms of type and even materials [9]. These membranes are generally produced from nonporous hydrophobic polymeric materials such as silicone rubber composite membrane (dense thin layer of silicone rubber or some other polymers on a supporting porous sublayer) [3,10]. In a PV system, applied partial vacuum pressure should be lower than saturation. Therefore, the saturation vapour pressure is one of the important physical properties in this process. Some other main properties are: affinity, diffusivity in water and permeability in membrane [2]. In comparison with other conventional methods, PV can be applied to aqueous streams that are contaminated with VOC, no additional chemical are needed, does not require expensive regeneration, does not have emission problems. Other additional advantages are environmentally friendly, lower costs of operation, ability to recycle and possible to reuse of VOC and eventually this systems are flexible in modular designs and generally are compact [2].
3. MATHEMATICAL MODELLING As mentioned in the previous part, the partial pressure difference is the driving force in this process, but it is useful to note that difference in concentration of each component between two sides of membrane or indeed between two phases of liquid and vapour, can be used in theory and mathematical analysis with a good approximation as the another main chemical potential difference of desired component as the follow [2]: Now with this assumption, we begin to provide a mathematical model for a tubular type of PV membrane such as hollow fiber capillary or spiral wound type membranes (of course with some other simplifying assumptions). At first, we consider the following element of a tubular type membrane with length of Δz and thickness Δr : (1) Fig. 2. Elements of a tubular membrane module. Mass balance in element: INPUT OUTPUT + GENERATION = ACCUMULATION (2) (3) where (3) (4)
and D is mass diffusion coefficient and then according to from the fluid mechanic and finally by some mathematical operations: (5) or (6) In the steady-state, the changes of concentration versus time was negligible to simplify the model. Boundary conditions: B.C.1 all r and z B.C.2 all r (7) B.C.3 all z B.C.4 all z For analytical solution the obtained model, at first we use the method of separation of variables to convert this model to two separated soluble equations: (8) (9) The second equation is easy to solve and its general answer is: (10) but for the first equation, the Fourier series as the one of the main important of power series solution of differential equations method is used in our mathematical operations. (12) i represents the roots of the equation. Which H is a real number constant in general answer of second equation (10) and A, B are the constant coefficients of each two answers R 1 and R 2 for eq.(9). That accordance with the method of Fourier series and putting different values of m in and main equation, and finally by consideration the boundary conditions, the analytical solution of model will be achieved. (11) (13) (14) (15) (16)
4. EXPERIMENTAL WORK 4.1. Materials In this experiment the composite membrane consists of a support layer and one or more selective layers was used. Support layer is a hollow fiber capillary type to provide the required mechanical strength. Used capillary membranes are from Polymemtech Co (Poland). These tubes are made of polypropylene and its characteristics are: Table 1. Properties of membranes from Polymemtech Co. Properties Value Unit Outer diameter 2,6 mm Internal diameter 1,8 mm Average pore size 0,3 µm Porosity 50-60 % Length 650 mm But for selective layers, we used Mixed Matrix material. In order to make this composite membrane, dip coating method was applied to coat an appropriate selective layer on the above support layer. Also selective layer contains suspension solid particles of the zeolite A3 inside PEBAX 2533 polymer solution in 2-butanol. Various modifications were achieved corresponding to different concentrations of polymer in solution and density of zeolite in the slurry. Each modified capillary was checked by electron microscope too. 4.2. Membrane module As shown in Fig.4, a bundle of membranes is put inside a special housing which should be fully seal in inlet and outlets openings and whole of its shell. Fig. 4. Picture of a module of membrane produced in laboratory. Since in each membrane module, we need one inlet and two outlet so one of the ways in inlet side (central hole) was clogged too. Also, these experiments repeat by a flat sheet membrane module that is able to replace in our equipment system installation. 4.3. Equipment installation According to diagram which is shown in Fig. 1, our equipment system is installed in laboratory and in a pilot scale for experimental works. Main used devices in this system are: PV membrane module (both tubular capillary and flat sheet type that can be replaced), vacuum pump, heater, condenser, decanter and other necessary piping and instruments.
The experiments are carried out to reduce a VOC species such as isopropanol from water that its concentration in the feed are different from 100 400 mg/l and in various temperatures. 4.4. Comparing the results and discussion We carry out these experiences to prepare and find some better composite membranes for VOC removal from water by PV and this efforts are continued. The preliminary results - as other authors have also reported [11] - show that the use of our capillary composite membranes in a stacked system based on PV, have a good performance for the separation of isopropanol as a kind of VOC from the feed water. VOC concentration decreases with a significant efficiency in outlet and according to the proposed model, this concentration gradient is radial and proportional to the passing through the thickness of the membrane and also simultaneously decreases with going forward along the length of the modulus z. These experiments are continuing to achieve the best options for materials and manufacturing composite membranes in this issue and the corresponding results will be reported in the future. 5. CONCLUSIONS In this study a mathematical model for VOC removal by pervaporation process was presented based on chemical potential of concentration of a VOC species as the driving force and by mass balance around the element of membrane according to first law of thermodynamics. All initial results and other reports are corroborant our theory topics and mathematical modeling. As expected, temperature and vacuum pressure are effective in permeate flux and its concentration. Also by repeating of tests separate other VOC species from water, can observe that flow rate of permeate and its selectivity are dependent on material of membrane. Therefore, this investigation can be useful to find properly material to produce composite membrane to remove each component from water by PV. Although, this technology maybe has some disadvantages including the need for periodic washing the membranes to maintain the treatment efficiency compared some other methods, but according to many other advantages such as less space and competitive price, its uses are increasing progress and this method is constantly being upgraded. However, the path ahead will be long to achieve further improvements. NOMENCLATURE J i - permeation flux, mol s -1 m -2 Ov K i - constant of overall mass transfer, m s -1 q - molar density in feed, mol m -3 (C i ) L - concentration in liquid phase, mol m -3 (C i ) V - concentration in vapor phase, mol m -3 m - permeation flux, mol s -1 m -2 C VOC - VOC concentration, mol m -3 or mgr lit -1 t - time, s A - membrane surface, m 2 V - velocity, m s -1 D - mass diffusion, m 2 s -1 H - real number constant REFFERENCES
[1] Bloemen H.J.Th., Burn J., Chemistry and analysis of volatile organic compounds in the environment. Chapman & Hall, Great Britain, London. 1993, 1, 4. [2] Peng M., Vane L.M., Liu S.X., Recent advances in VOCs removal from water by pervaporation, Journal of Hazardous Materials, 2003, 98, 69 90. [3] Jingli X., Ito A., Removal of VOC from water by pervaporation with hollow-fiber silicone rubber membrane module, Desalination and Water Treatment, 2012, 17, 135-142. [4] Berenjian A., Chan N., Jafarizadeh Malmiri H., Volatile Organic Compounds removal methods: A review, American Journal of Biochemistry and Biotechnology, 2012, 8, 4, 220-229. [5] Chu-Chin H., Removal mechanisms of VOCs in an activated sludge process, J. Hazardous Materials, 2000, 79, 173-187. [6] Navarri P., Marchal D., Ginestet A., Activated carbon fibre materials for VOC removal, Filtration&Separation, 2001, 38, 34-40. [7] Linek V., Sinkule J., Janda V., Design of packed aeration towers to strip volatile organic contaminants from water, Elsevier Science, Water research, 1998, 32, 1264-1270. [9] Ghoreyshi A.A., Peyvandi K., Jahanshahi M., Modeling of volatile organic compounds removal from water by pervaporation process, Desalination, 2008, 222, 410-418. [9] Mulder M., Basic Priciples of Membrane Technology, Second Edition, Kluwer Academic Publishers: Dordrecht, The Nederlands, 1996. [10] Gittel T., Hartwig T., Schaber K., Separation of organic compounds from surfactant solutions by pervaporation. The influence of a Micellar phase of mass transfer, Z. Phys Chem., 2005, 219, 1243-1259. [11] Cséfalvay E., Szitkai Z., Mizsey P., Fonyó Z., Experimental data based modelling and simulation of isopropanol dehydration by pervaporation, Desalination, 2008, 229, 94 108.