Vapor pressure reverse osmosis

Membrane Pervaporation Since 1987, membrane pei vapora-tion has become widely accepted in the CPI as an effective means of separation and recovery of liquid-phase process streams. It is most commonly used to dehydrate hquid hydrocarbons to yield a high-purity ethanol, isopropanol, and ethylene glycol product. The method basically consists of a selec tively-permeable membrane layer separating a liquid feed stream and a gas phase permeate stream as shown in Fig. 25-19. The permeation rate and selectivity is governed bv the physicochemical composition of the membrane. Pei vaporation differs From reverse osmosis systems in that the permeate rate is not a function of osmotic pressure, since the permeate is maintained at saturation pressure (Ref. 24). 

The most common membrane systems are driven by pressure. The essence of a pressure-driven membrane process is to selectively permeate one or more species through the membrane. The stream retained at the high pressure side is called the retentate while that transported to the low pressure side is denoted by the permeate (Fig. 11.1). Pressure-driven membrane systems include microfiltration, ultrafiltration, reverse osmosis, pervaporation and gas/vapor permeation. Table ll.l summarizes the main features and applications of these systems. 

Method D in Table 1 represents a case where dry support films were always used because of the need to employ a vacuum and because of the very nature of plasma deposition processes. Yasuda (12) showed that a wide variety of gas phase reactants could be used in this technique. Not only conventional vinyl monomers were used but also any organic compounds with adequate vapor pressure. Further, copolymers could be prepared by introduction of a second reactant such as nitrogen. Wydeven and coworkers (13,14) showed the utility of this method in preparing reverse osmosis membranes from an allylamine plasma. 

By this process of reverse osmosis salts can be removed at very high values of osmotic pressure by exposing the solution to a thin vapor gap supported by capillarity. The process needs for its economical operation a gel which will remain permeable, while supporting the high air gap pressure. 

Figure 34.15 Reverse osmosis characteristics of composite membranes prepared by plasma polymerization of 4-picoline with and without N2 at two vapor pressures porous polysulfone film as the substrate 1.2% NaCl at 1200 psi.

Separation of cthanol/watcr mixtures Pervaporation is a developing membrane process which has elements of reverse osmosis and gas separation. In pervaporation, a liquid mixture is brought in contact with one side of a membrane while the permeate is removed as a vapor from the other side. The driving force is the difference tween the partial pressure of the feed stream and the vapor pressure on the permeate side. 

A number of alternatives to reverse osmosis are being considered. Two promising alternatives are membrane distillation [97] and forward osmosis [98]. Membrane distillation relies on vapor pressure differences across a membrane, arising from a temperature difference, to drive water transport. The process utilizes low temperature heat sources and operates at low pressure which can reduce operating costs relative to reverse osmosis. Forward osmosis relies on water permeation across a water selective membrane to a draw solution - the reverse of reverse osmosis. The water must then be separated from the draw solution but this may be less expense than reverse osmosis because the process operates at low pressure. 

The mixing of fresh water from estuaries with seawater has the potential to produce more than 2.5 TW of power globally [138]. Recovering this energy has been discussed for the past half century using desalination processes operated in a reverse mode. Proposed alternatives include the use of reverse electro-dialysis (RED) [139], pressure retarded osmosis PRO [140-141], and vapor pressure differences [142]. 

The schematic diagram of the pervaporation apparatus is shown in Figure 3.12. The same permeation cell as the reverse osmosis static cell can be used for the experiment. The feed liquid mixture in the permeation cell (2) is either open to the atmosphere or under the pressure applied from a nitrogen cylinder (1). Vacuum is applied on the permeate side of the membrane by a vacuum pump (6). The permeate vapor is condensed and collected in a cold trap (5) cooled with liquid nitrogen. After a steady state is reached, the line is switched to the second cold trap. The permeation rate is determined by weighing the sample collected in the cold trap during a prescribed period. The sample is also subjected to analysis. 

Let us now review reverse osmosis transport. Both feed and permeates are in the liquid phase, and we do not need to convert the concentration to partial vapor pressure to calculate the chemical potential difference as the driving force. Instead, Equations 5.241 and 5.242 are used as given. Let us define component B as the solvent and component A as the solute. Then Equations 5.242 and 5.241 can be written as... 

Surface adsorption potential Vapor pressure of a liquid, volatility of a solute Water removed by vaporization from a solution or moist solid Water removed from a solid by sublimation excess aonimulation of a species at the interface of phase 1 and phase 2 gas-solid, liquid-solid different volatilities of bulk liquids and solutes evaporation of water sublimation of water adsorption, chromatography (gas-solid, liquid-solid) (Table 1, Sections 3.3.7.G, 4.1.5) distillation, stripping (Table 1, Sections 4.1.1, 4.1.2) evaporation, drying (Table 1) freeze-drying (Table 1) gas separation by surface diffusion (Section S.4.2.4), preferential sorption and capillary transport in reverse osmosis (Table 2) pervaporation (Table 2, Section 6.3.3.4) (plus membrane permeability) membrane distillation (Song et ai, 2008) ... 

System type (4) Two miscible phases flow countercurrently in two regions of the device separated by a membrane. One of the phases may be generated from one feed phase by the application of pressure energy. Examples include reverse osmosis, ultrafiltration, microflltration, gas permeation, pervaporation. Examples where the other phase is introduced from outside are electrodialysis, dialysis, sweep vapor/ liquid based system. 

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