19-06-2013, 12:30 PM
MEMBRANE DISTILLATION
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Introduction
MD is a thermally driven process, in which water vapour transport occurs through a non wetted porous hydrophobic membrane. The term MD comes from the similarity between conventional distillation process and its membrane variant as both technologies are based on the vapour-liquid equilibrium for separation and both of them require the latent heat of evaporation for the phase change from liquid to vapour which is achieved by heating the feed solution. The driving force for MD process is given by the vapour pressure gradient which is generated by a temperature difference across the membrane. As the driving force is not a pure thermal driving force, membrane distillation can be held at a much lower temperature than conventional thermal distillation. The hydrophobic nature of the membrane prevents penetration of the pores by aqueous solutions due to surface tensions, unless a transmembrane pressure higher than the membrane liquid entry pressure (LEP) is applied. Therefore, liquid/vapour interfaces are formed at the entrances of each pore. The water transport through the membrane can be summarized in three steps:
(1) formation of a vapour gap at the hot feed solution–membrane interface;
(2) transport of the vapour phase through the microporous system;
(3) condensation of the vapour at the cold side membrane–permeate solution interface.
Membrane distillation is a relatively new membrane separation process which might overcome some limitations of the more traditional membrane technologies. In particular high solute concentrations can be reached and ultrapure water can be produced in asingle step. The possibility of an industrial development of this technology is related to the growing commercial availability of membranes of potential interest. When micro porous hydrophobic membrane separates two aqueous solutions at different temperatures, selective mass transfer across the membrane occurs: this process takes place at atmospheric pressure and at temperatures which may be much lower than the boiling point of the solutions.
CONCEPT OF MD
MD is a process mainly suited for applications in which water is the major component present in the feed solution. As stated earlier, MD is a thermally driven process, in which only vapour molecules are transported through porous hydrophobic membranes. The liquid feed to be treated by MD must be maintained in direct contact with one side of the membrane without penetrating its dry pores unless a transmembrane pressure higher than the membrane liquid entry pressure (i.e., breakthrough pressure) is applied. The hydrophobic nature of the membrane prevents liquid solutions from entering its pores due to the surface tension forces. As a result, liquid/vapour interfaces are formed at the entrances of the membrane pores. Various MD modes differing in the technology applied to establish the driving force can be used. The differences between them are localized only in the permeate side as can be seen in fig 1.
Principle of Membrane Distillation
Figure 1. A schematic representation of the membrane distillation process: T1, temperature at the hot side; T0, temperature at the cold side; J, flux of the vapor phase
Up to now membrane distillation has referred to the non-isothermal transport of water, via vapor phase, through the pores of a hydrophobic partition. The system consists of a porous hydrophobic membrane, separating two aqueous solutions of a non-volatile component maintained at different temperatures. Due to the liquid-rejecting properties of the membrane material, the liquid phase is prevented from penetrating the pores, as long as the pressure of liquid does not exceed the minimum entry pressure of the porous partition. Liquid-vapor interfaces are formed on both sides of the membrane pores and, due to the temperature difference, a vapor pressure difference is created between sidesof each pore. Evaporation takes place at the warm interface and, after vapor is transported through the pores, condensation takes place at the cold interface. In this way a water flux occurs through the membrane in the direction from warm to cold. Obviously, for membrane distillation to proceed, it is essential that liquid water is excluded from the pores. In this sense, the role of the membranes is somewhat peculiar, since it acts as a physical support for the liquid-vapor interfaces.
HOW MEMBRANE DISTILLATION IS DIFFERENT THAN OTHER PROCESSES
Membrane separation processes have become one of the emerging technologies in the last few decades especially in the separation technology field. They offer a number of advantages over conventional separation methods in a wide variety of applications such as distillation and evaporation. Membrane processes can be easily scaled up due to their compact and modular design; they are able to transfer specific components selectively; they are energy efficient systems operating under moderate temperature conditions ensuring gentle product treatment. Membrane distillation is an alternative to the traditional evaporative distillation systems used for desalination or water purification processes. On the other hand, membrane distillation can be compared with other membrane techniques, e.g. reverse osmosis. Reverse osmosis is industrially used in desalination processes (nearly 30 per cent of desalinated water in the world is produced by this membrane technique), production of ultrapure water and food concentration (juices, sugars and milk, for example). RO is a pressure driven membrane process based on the solution-diffusion of the solvents, mainly water, across the membrane. Reverse osmosis efficiency is strongly affected by the osmotic pressure of the highly concentrate feed solutions (the osmotic pressure of seawater is about 25 bar) and by the concentration polarization phenomena that occurs on the pressurized membrane-solution interface. In addition, the membrane rejection is generally of the order of 98-99 per cent, and some salts can diffuse in the permeate. In contrast, membrane distillation is a thermally driven membrane process where efficiency shows a slight decrease with increasing salt (or other inorganic solutes) concentration, because of a decrease in vapor pressure. In principle, MD can also produce ultrapure water from feeds at quite high concentrations where RO cannot practically operate. In addition, the quality of the permeate (the separation efficiency) is virtually independent on the feed concentration. Mass transport in fact takes place in the vapor phase; non-volatile solutes are completely rejected by the membrane and only volatile solutes can be transported.
EFFECT OF PARAMETERS ON THE MD PROCESS
Feed concentration
Permeate flux decreases with an increase in feed concentration. This phenomenon can be attributed to the reduction of the driving force due to decrease of the vapour pressure of the feed solution and exponential increase of viscosity of the feed with increasing concentration.
The contribution of concentration polarization effects is also known, nevertheless, this is very small in comparison with temperature polarization effects. As it is well known, MD can handle feed solutions at high concentrations without suffering the large drop in permeability observed in other pressure-driven membrane processes and can be preferentially employed whenever elevated permeate recovery factors or high retentate concentrations are requested (i.e. concentration of fruit juices).
Feed temperature
Various investigations have been carried out on the effect of the feed temperature on permeate flux in MD. In general, it is agreed upon that there is an exponential increase of the MD flux with the increase of the feed temperature. As the driving force for membrane distillation is the difference in vapour pressure across the membrane, the increase in temperature increases the vapour pressure of the feed solution, thus results an increase in the transmembrane vapour pressure difference. It is worth quoting that working under high feed temperatures was offered by various MD researches, since the internal evaporation efficiency (the ratio of the heat that contributes to evaporation) and the total heat exchanged from the feed to the permeate side is high. Nevertheless, the increase in quality losses and formation of unfavorable compounds (i.e. hydroxymethyl furfural and furan) in fruit juices due to high operation temperatures restricts the temperature levels. Temperature polarization effect also increases with the increase in feed temperature.
Permeate temperature
The increase in permeate temperature results in lower MD flux due to the decrease of the transmembrane vapour pressure difference as soon as the feed temperature kept constant. It is generally agreed upon that the temperature of cold water on the permeate side has smaller effect on the flux than that of the feed solution for the same temperature difference. This is because the vapour pressure increases exponentially with feed temperature.
Permeate flow rate
The increase in permeate flow and/or stirring rate reduces the temperature polarization effect. Consequently, the temperature at the gas/liquid interface approaches to the bulk temperature at the permeate side. This will tend to increase driving force across the membrane; resulting an increase in MD flux . It is important to note that as the permeate used in the MD is distilled water and in the OD is hypertonic salt solution; the extent of the effect of flow rate is more prominent in the latter configuration. This is because of the contribution of concentration polarization effects on permeate side in OD.
CONFIGURATIONS OF MD
Various MD configurations can be used to drive flux . The difference among these configurations is the way in which the vapour is condensed in the permeate side. Figure 2 illustrates the four commonly used configurations of MD described as follows:
1. In direct contact membrane distillation (DCMD), water having lower temperature than liquid in feed side is used as condensing fluid in permeate side. In this configuration, the liquid in both sides of the membrane is in direct contact with the hydrophobic microporous membrane.
CONCLUSION
Membrane distillation is a most effective separation process than any other processes in certain areas because it gives high concentration at low pressure and temperature, has integration with other membrane operations implies more efficiency, good and excellent mechanical properties and chemical resistance and 100%(theorotical)rejection of ions, macromolecules, colloids and non volatiles
As a promising alternative to replace other separation processes, MD has gained much interest for its lower energy requirement in comparison with conventional distillation, lower operating pressures and higher rejection factors than in pressure driven processes such as NF, and RO. Although MD has been known for more than 40 years, a number of problems exist when MD is considered for industrial implementation. Most of the conducted MD studies are still in the laboratory scale. In recent years, some pilot plant studies have been proposed for desalination.