Petrotos et al. (1998) have investigated the concentration of tomato juice by direct osmosis, using commercial thin film composite RO membrane with uniform active layer thickness and varying membrane-backing material thickness. Membrane thick- ness is found to have a significant role in the formation of overall mass transfer coef-ficient. A sharper reduction of overall mass transfer coefficient with thinner membrane is noticed. The thinner membrane has a greater contribution of the osmotic medium film coefficient in the formation of the overall mass transfer coefficient due to reduction in membrane resistance. An exponential decreasing trend of the flux is observed with increasing the overall membrane thickness (Table 4.8). Similar effect of the thickness of membrane backing material on the osmotic flux is also observed by Loeb et al. (1997).
4.3.2.5.1 Apparatus Design
Different types of membrane modules with specific design are constructed for direct osmosis process. Flat sheet and tubular configurations are most common. A simple schematic of DOC process is shown in Figure 4.16 (Herron et al., 1994).
TABLE 4.8
The Effect of Membrane Thickness on the Performance of the Direct Osmosis Process
Overall Membrane Thickness (μm)
Thickness of Membrane Active
Layer (μm)
Thickness of Membrane Backing Material Layer (μm)
Osmotic Flux (kg/m2 h)
400 140 260 3.1
500 140 360 1.48
600 140 460 0.64
Source: From Petrotos, K.B. et al., J. Membr. Sci., 150(1), 99, 1998.
Product concentrate
Product inlet exchangerHeat
Recirculation loop
Membrane Osmotic
agent (sugar)
Water
Evaporator Water
Concentration cell
FIGURE 4.16 Simplified DOC flow sheet. (Reprinted from Herron, J.R. et al., Medina, Osmotic concentration apparatus and method for direct osmotic concentration of fruit juice, U.S. Patent 5,281,430, January 25, 1994. With permission.)
In this process, juices of high solid contents can be concentrated with minimum fouling because the solids are not convected toward the membrane surface as it oper-ates under low pressure. The fruit juices can be fed to the module in a continuous, multistage process. At each stage, the fruit juice is recycled through a heat exchanger (if refrigeration is desired) and then through the DOC modules. The juice is kept in inert atmosphere to increase the flavor and aroma retention. The osmotic agent solution is fed either once counter currently through the entire process or is recycled continuously through an evaporator. If there is a large quantity of very concentrated solution that is diluted at the plant, and if this solution could be used as the osmotic agent solution, the dilution can be performed within the juice concentration mod-ules. This would result in substantial cost savings for the juice concentration process (Girard and Fukumoto, 2000b). Generally, the osmotic agent solution is diluted and reconcentrated using evaporation. Therefore, instead of juice, the osmotic agent solu-tion is exposed to the high temperature, allowing the improvement of concentrate quality. In continuous process, it is possible to obtain a steady-state osmotic flux.
However, during the concentration process, osmotic flux gradually declines with time. This is due to increase in osmotic pressure of juice as it is concentrated with time. Hence, osmotic pressure differences between the osmotic agent solution and the juice decrease, resulting in flux decline. During the experiment, the flux of a depectinized raspberry juice at 30°C declined from 1.37 to 0.3 L/m2 h, while the osmotic agent solution decreased from 69 to 60 °Bx (Beaudry and Lampi, 1990).
A flat osmotic apparatus with a special configuration to promote turbulence was developed by Petrotos and Lazarides (2001). They reported that with this apparatus an average flux of 4.5 kg/m2 h was obtained during concentration of tomato juice from 5 to 16 °Bx at room temperature and low pressure. It has also been reported that tubular module offers some advantages such as turbulent flow, less pressure drop, larger section of the membrane without fouling, and concentration polarization in concentration of juice of high solid content, leading to an increase in osmotic flux (Jiao et al., 2004).
4.3.3 MeMbrane distillation
4.3.3.1 Basic Principle
MD is a separation process in which a microporous hydrophobic membrane sepa-rates two aqueous solutions, at different temperatures. This process is based on the formation of a vapor on the side of the warmer solution–membrane inter-face, to the transport of vapor phase through the microporous membrane, and to its condensation at the cold membrane solution interface. The driving force for MD is the partial pressure difference induced by the temperature gradient between the two solution–membrane interfaces. Heat is also transported within the vapor across the membrane (Figure 4.17). In this process, the membrane acts as a physical support for the vapor–liquid interface. A liquid–vapor interface is formed at the pore mouths, where liquid and vapor are in equilibrium; inside the membrane pores only a gaseous phase is present through which vapors are transported as long as a partial pressure difference is maintained. This process takes place at atmospheric pressure and at a temperature much lower than its
boiling point, with net flux always from warm solution to cold solution. The MD includes the following characteristics: (1) the membrane should be porous, (2) the membrane should not be wetted by the process liquids, (3) no capillary condensation should take place inside the pores of the membrane, (4) the mem-brane must not alter the vapor–liquid equilibrium of the different components in the process liquids, (5) at least one side of the membrane should be in indirect contact with the process liquid, (6) for each component, the driving force of this membrane operation is a partial pressure gradient in the vapor phase. There are various types of MDs depending on the methods employed to impose a vapor pressure difference across the membrane to drive flux (Figure 4.18). The per-meate side of the membrane may consist of a condensing fluid in direct contact with the membrane (direct contact MD), a condensing surface separated from the membrane by an air gap (air gap MD), a sweeping gas (sweeping gas MD), or a vacuum (vacuum MD). The type of MD employed is dependent upon the permeate composition, flux, and volatility. The direct contact MD configuration is suitable for applications such as desalination or the concentration of aqueous solutions (orange juice), in which water is the major permeate component. For the separation of volatile organic or a dissolved gas from an aqueous solution, either sweeping gas MD or vacuum MD should be used. Air gap MD, which is the most versatile MD configuration, can be applied to almost any application.
The advantages of MD compared with other traditional technologies of concen-tration of juices are (1) the evaporation surface can be made to various membrane configurations, (2) capable of producing high quality concentrates, (3) low oper-ating temperatures (28°C–48°C), an opportunity to achieve high contents of dry
Warm side film Cold side film
Heat Condensate
Cold side
Tp
permeate Tp,b
Tp,b, Pp,b Tf,b, Pf,b
Tp,m, Pp,m Tf,m,Pf,m Heat
Vapor
Feed Warm side
Microporous hydrophobic membrane
Tf,b Tf,m Tp,m
Pf,m Pp,m
FIGURE 4.17 Schematic of the heat and mass transport profile in membrane distillation.
Tf,b, Tf,m: temperature at the bulk and membrane of the feed side, respectively; Tp,b, Tp,m: temperature at the bulk and membrane of the permeate side, respectively; Pf,m, Pp,m: vapor pressure of the membrane surface at the feed side and permeate side, respectively.
substances (60%–70%), (4) low energy consumption, and (5) corrosion or foul-ing are less. Because the process can take place at normal pressure and at lower temperature, MD could be used to concentrate to high osmotic pressures aqueous solutions of solutes sensitive to high temperatures. Therefore, MD is frequently used for concentrating fruit juices such as apple and orange with better flavor and color (Drioli et al., 1992; Calabro et al., 1994).
4.3.3.2 Apparatus Design
A large variety of membrane modules have been used in MD process. This includes plate-and-frame, spiral wound and hollow fiber, etc. (Carlsson, 1983; Andersson et al., 1985; Calabro et al., 1994; Lagan et al., 2000). A well-designed membrane module should provide high rates of heat and mass transfer between the bulk solution and the solution–membrane interface. Gore and Associates (United States), The Swedish Development Co., and Enka AG (Germany) started testing their own membranes in MD units for commercialization. Gore and Associates (United States) devel-oped a spiral wound module for “Gore-Tex Membrane Distillation.” The Swedish Development Co. used a plate-and-frame module for “SU Membrane Distillation,”
and Enka AG (Germany), developed a hollow fiber module for “Transmembrane Distillation” (Lawson and Lloyd, 1997). For the concentration of orange juice, Drioli et al. (1992) investigated MD process, using a commercial plate PVDF membrane (Millipore Corp.) with a nominal pore size of 0.22 μm and a laminated hydropho-bic microporous membrane (G0712) with a pore size of 0.2 μm (Gelman Science Tech. Ltd). They found that the flux of PVDF membrane was remarkably higher than that of G0712 membrane. Calabro et al. (1994) reported that single strength orange juice (10.5 °Bx) could be concentrated to 31.5 °Bx in an MD laboratory plant (Figure 4.19) using a plate poly-vinylidene fluoride (PVDV) hydrophobic membrane
Aqueous solution
Aqueous
solution Aqueous
solution Direct contact
MD Air gap
MD
Sweep gas MD
Sweep gas
Vacuum MD
Vacuum Airgap
surfaceCold
Membrane Aqueous
solution Aqueous
solution
FIGURE 4.18 Various membrane distillation processes.
with a nominal pore diameter of 0.22 μm, porosity of 75%, thickness of 140 μm, and effective area 20.42 cm2 (Millipore Corp.). Nene et al. (2002) concentrated aclarified sugar cane juice (20 °Bx) by MD process using a plate-and-frame module (polypro-pylene membrane of area 75 cm2, nominal pore size of 0.2 μm). An overall flux of 10 kg/m2 h was obtained in this study.
Curcio et al. (2000) studied the concentration of single strength apple juice up to 64 °Bx by MD using a polypropylene hollow fiber membrane module (ENKA Microdyn MD-020-2N-CP) with a membrane area of 0.1 m2 and nominal pore diame-ter of 0.45 μm. In their study, they obtained an overall permeate flux of 1–1.5 kg/m2 h.
4.3.3.3 Effect of Different Process Parameters