+ School of Mechanical and Manufacturing Engineering,. "School of Chemical Engineering .... However one of the inherent trade-offs in MD, is that whilst higher ...
SOLAR HEATED MEMBRANE DISTILLATION G.L.Morrison+, Sudjito+, A.G. Fane* and P.Hogan* School of Mechanical and Manufacturing Engineering, "School of Chemical Engineering and Industrial Chemistry University of New South Wales. Sydney Australia +
ABSTRACT The possibility of using membrane distillation for solar powered desalination is described. A simulation model of a membrane distillation system has been developed, and combined with the TRNSYS solar simulation program to evaluate the process of a solar powered membrane distillation system. A preliminary design for a 50 litres/day pilot plant is presented. KEYWORDS Distillation, Membrane, Solar, Thermal
INTRODUCTION Solar powered distillation plants have been developed on the principles of solar stills, solar multiple effect distillation (SMED), solar multistage flash distillation (SMSF), solar photovoltaic powered reverse osmosis (SPRO), and solar photovoltaic powered electrodyalisis (SPED). Basin type solar stills received a considerable amount of research and development between 1950 and early 1970's. This system is suitable for small capacity plants such as a household or a small community in rural/arid zones, but it is not economically viable for large capacity plants due to its low efficiency. The other four systems compete to gain acceptance in applications, which are mostly in the Middle East. In contrast to the solar still, these four systems are usually applicable for large capacities only. A costing of SMSF and SPRO plants in Australia, showed that the product cost increases dramatically for plant capacity less than 300 m3/day. A study for a SPRO plant with a capacity of 50 m3/day concluded that such a system was not economically viable unless the cost of photovoltaic panels decreased to about 5 percent of the cost at that time (about A$ 20/watt). Membrane distillation (MD), a new process in membrane technology, has some features that make it suitable for solar powered distillation plants. The process can operate at temperatures of 50 to 90 C and it is possible to recover latent heat in the product stream. The process also operates at atmospheric pressure and hence the associated components are not expensive. The application of MD for water desalination has been studied by a number of workers [1-3]. A preliminary cost calculation for small capacity MD plants [3] showed production costs marginally higher than conventional RO plants with the same capacity. However, the MD plant has practical advantages for application in rural/remote areas where electricity is not available. Compared to solar stills, MD systems offer better heat utilization due to heat recovery and better system efficiency. This paper presents a study of a small solar powered MD plant to produce potable water from brackish water in rural/arid zones.
SOLAR POWERED MEMBRANE DISTILLATION (SPMD) PROCESS Membrane distillation is a thermally driven membrane process in which a hydrophobic microporous membrane separates hot and cold streams of water. The hydrophobic nature of the membrane prevents the passage of liquid through the pores whilst allowing the passage of water vapour. The temperature difference produces a vapour pressure gradient which causes water vapour to pass through the membrane and condense on the colder surface. The result is a distillate of very high purity which, unlike conventional distillation, does not suffer from entrainment of species which are non-volatile. A flow diagram of the SPMD process is shown in Fig. 1. A hot and cold stream are contacted counter-currently in the MD module, at which point mass and energy are transferred from the feed to the permeate stream. The module contains hollow fibre membranes tightly packed in a shell in order to enhance the heat transfer characteristics of the shell side (7). The streams are recontacted in the main heat recovery heat exchanger (HX1) where energy is transferred back from the permeate to the feed. To provide process continuity the feed and permeate must be returned to their original temperatures before being recycled to the MD module. The feed stream is reheated using energy from a solar collector, which in this case is isolated from the feed circuit. Water is taken from the feed tank heated by the collector and returned to the tank, allowing the flow conditions through the collector to be set independently of those chosen for the MD circuit. The tank enables the system to store solar collector output when the collector output temperature is to low for efficient membreane operation. The primary energy sink is a second heat exchanger HX2 that is cooled by accumulated product or cold excess feed. If cold feed is used in HX2 the make up water is taken from the outlet of HX2 as shown in Fig. 1. As the process continues the accumulating product water is removed from the permeate stream. The feed stream is depleted of water by the distillation, and becomes more concentrated in the retained impurities. This build-up must be limited by a controlled bleed stream. MD Module Simulation Model Many membrane configurations have been investigated for MD modules [1-3]. In this study a hollow fibre module was used as shown in Fig. 2. A simulation model of the MD process has been developed, based on the theory of Schofield [7]. The MD model has been integrated with the TRNSYS solar package to produce a full system simulation model. The membrane model computes mass transfer, temperature and pressure gradients in the fibre using an iterative solution procedure along the feed stream direction. Membrane Characteristics The simulation model has been used to provide design information with respect to the MD module, by predicting the system performance for a range of operating conditions. Figure 3 shows the benefits of operating at high feed temperatures and high liquid flow rates. This information is based on a pilot module which has a 0.17m2 membrane area consisting of 1100 fibres (0.17m length, 0.3mm i.d. and 0.6mm o.d.) with a pore size of 0.22 micron and 70% porosity. The nonlinear increase in flux with increasing temperature reflects the exponential increase in the vapour pressure which provides the driving force. The effect of a higher liquid flowrate is to increase the heat transfer coefficient, and thus reduce the effect of temperature polarization. This means that the temperatures at the membrane surface more closely approximate that of the bulk streams, and thus the trans-membrane temperature difference is greater. This produces a greater driving force, and consequently enhances the flux. Accompanying the increases in flux is a decrease in the heat loss factor. This represents the fraction of the total heat transferred across the membrane that does
not contribute to the flux. This occurs primarily by conduction through the membrane structure, and through the air and water vapour in the pores. Heat transfer by conduction increases approximately linearly with temperature gradient, unlike the vapour pressure driving force and thus the mass flux which increases exonentially. This means that although more heat is lost by conduction at higher temperature difference, it is a lower proportion of the total heat transfer. Figure 4 shows the effect of increasing the membrane area with constant stream inlet temperatures and flowrates. The increase in area in this case is achieved by increasing the length of the fibres. The overall effect of larger membrane area is that a greater amount of heat is transferred from feed to permeate. As the approach temperature difference becomes smaller the driving force across the membrane is reduced, and the flux drops. For the conditions shown in Fig. 4 the drop in flux is offset by the increase in area and so the production rate (kg/hr) is observed to climb. As the area becomes larger the rate of increase in production rate diminishes until the point where a further increase in area has negligible effect. One of the main advantages of the MD process is its ability to recover the latent heat of vaporisation. The proportion of heat transferred during distillation that can be recovered depends primarily on the approach temperatures of the liquid streams. Large membrane areas and lower flowrates both lead to increased contact time in the module, which gives rise to closer approach temperatures and thus more recoverable heat. However one of the inherent trade-offs in MD, is that whilst higher flowrates and smaller membrane areas yield higher flux, the possibility of heat recovery is reduced. The conditions of choice then become a function of the application.
SYSTEM DESIGN SENSITIVITY The pilot plant (Fig. 1) has been modelled by combining the membrane module simulation program with the TRNSYS solar simulation program [8]. The operating conditions, module specifications, and solar collector type were kept constant during the sensitivity study. The module specifications chosen were as for Fig. 4. Heat exchanger size, defined by the product of the heat transfer coefficient and area (UA), was varied from 25 to 550 W/K for HX1. The performance measure chosen was the productivity, defined as the kg water produced per unit of external energy provided (kg of permeate per MJ energy from the solar collector). Figure 5 indicates that increasing the size of the main heat exchanger reduces the need for external energy, as would be expected. This trend is shown for two sizes of the cooling heat exchanger, HX2 The capacity of the plant is affected by the efficiency of the collector in converting the incident radiation into sensible heat. Figure 6 shows the system performance for a flat plate and evacuated tube collector over a range of feed temperatures. The performance of a system with a flat plate collector drops off above 70°C due to lower collector output at high temperatures. In contrast a system utilizing evacuated tubes increases in efficiency past 100°C. The storage tank for solar thermal input is included to avoid inefficient operation of the module at times when the collector outlet temperature is low. Large storage volumes reduce the mass transfer, because the average feed delivery temperature decreases as the storage volume is increased. A small storage volume allows energy from the early morning and during transient periods to be collected and stored over periods of 30 to 60 minutes without significantly reducing the feed delivery temperature during high irradiation periods.
PROTOTYPE PERFORMANCE Figure 7 shows the simulated variation in production rate over an eight hour day-time operation, using radiation data for Sydney in summer. This information is based on a module with the same number and type of fibres as described previously. Comparative data for the actual pilot plant under the same conditions are also graphed (with energy input from a heating element). The chosen operating conditions were for an average feed temperature of 70°C and flowrates of 30kg/hr. The simulation gives a reasonably accurate picture of the production rate trend, although the figure shows that it typically predicts 10% higher than determined experimentally. This may be due to the heat losses or to imperfect flow distribution in the membrane module. CONCLUSIONS A distillation process using solar thermal energy to power a membrane distillation system has been shown to be feasible. Heat recovery is the most critical factor governing the cost of solar powered membrane distillation plants as the solar collectors are likely to be the major cost item. High heat recovery can be achieved either by using low mass flow rates of the feed and permeate, or using long membrane fibres. Evacuated tube collectors are more suitable for membrane distillation than flat plate collectors, since the membrane works best at feed temperatures of 60 to 90°C for a non-pressurized system. The capital cost of the unit is very sensitive to the extent to which heat is recovered, especially above a heat recovery factor of 0.8. The minimum capital cost requires 60 to 80% heat recovery. To obtain this level of energy conservation the process conditions that must be used are generally those that result in fairly low MD fluxes. These are low flowrates and large heat tranfer areas, which lower the driving force. For a domestic sized plant of 50kg/day the optimum configuration appears to be a solar collector area of around 3m2, a membrane area of 1.8m2 and a total heat exchange area of 0.7 m2. The capital cost for this unit is conservatively estimated at $3500 (Aust.). ACKNOWLEDGEMENTS The authors would like to thank Memtec Ltd, for material support and advice, and NERDDC for financial support.
REFERENCES 1. Anderson S.I. et al., "Design And Field Tests Of A New Membrane Distillation Desalination", Desalination Vol.56 (1985) pp.345-354. 2. Sarti G.C., "Low Energy Cost Desalination Processes Using Hydrophobic Membranes", Desalination Vol.56 (1985) pp.277-286. 3. Schofield R.W., Fane A.G., And Fell C.J.D., "The Efficient Use Of Energy In Membrane Distillation", Desalination Vol.64 (1987) pp.231-143. 4. Schofield,R.W. P.A.Hogan, A.G.Fane and CJ.D.Fell, "Developments in Membrane Distillation", Proceedings ICOM '90, p728-730. 5. Schneider,K andT.S.Van Gassel, "Membrandestillation", Chem.Ing.Tech. (1984), pp514-521.
6. Schofield,R.W. A.G.Fane and C.J.D.Fell, "The Efficient Use of Energy in Membrane Distillation", Desalination, 64(1987) pp231-243. 7 Schofield R.W., "Membrane Distillation", PhD. Thesis School of Chemical Engineering and Industrial Chemistry, UNSW, 1989. 8. KlineS.A. et al'TRNSYS 12.2 Users Manual", University of Wisconsin Solar Energy Laboratory, 1988. MD. NODULE
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Fig. 1
Solar Heated Membrane and Heat Recovery System
Permeate out(TPO, mp-t-m)
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.T Feed out (TFO, mf-m) Fig. 2
Schematic of a Hollow Fibre Membrane Module
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