Reverse osmotic pressure

Concentration polarization caused by macromolecules, which may induce a reversible osmotic pressure that disappears after the filtration pressure is released, and the adsorption on the membrane pores of solid materials or inside the membrane pores of solid materials, which are rid of by rinsing the membrane after the filtration process, are occurrences that both contribute to the reversible resistance to permeation, 7 rev On the other hand, the solids that are deposited on the membrane surface or inside the pores, which are removed only by chemical cleaning of the membrane, constitute the irreversible fouling, Rmev-... 

Ultrafiltration Experiments. There is no boundary layer theory for ultrafiltration of proteins in tubes for the general case of applied pressures comparable to reverse osmotic pressures. The process involves complete rejection of protein, and most... 

C, which produced the reverse-osmotic pressure and the consequent... 

The pressure difference between the high and low pressure sides of the membrane is denoted as AP the osmotic pressure difference across the membrane is defined as Att the net driving force for water transport across the membrane is AP — (tAtt, where O is the Staverman reflection coefficient and a = 1 means 100% solute rejection. The standardized terminology recommended for use to describe pressure-driven membrane processes, including that for reverse osmosis, has been reviewed (24). 

Reverse osmosis processes for desalination were first appHed to brackish water, which has a lower I DS concentration than seawater. Brackish water has less than 10,000 mg/L IDS seawater contains greater than 30,000 mg/L IDS. This difference in IDS translates into a substantial difference in osmotic pressure and thus the RO operating pressure required to achieve separation. The need to process feed streams containing larger amounts of dissolved soHds led to the development of RO membranes capable of operating at pressures approaching 10.3 MFa (1500 psi). Desalination plants around the world process both brackish water and seawater (15). 

Feed characteri2ation, particularly for nondesalination appHcatioas, should be the first and foremost objective in the design of a reverse osmosis plant. This involves the determination of the type and concentration of the main solutes and foulants in the stream, temperature, pH, osmotic pressure, etc. Once the feed has been characteri2ed, a reaHstic process objective can be defined. In most cases, some level of pretreatment is needed to reduce the number and concentration of foulants present in the feed stream. Pretreatment necessitates the design of processes other than the RO module, thus the overaH process design should use the minimum pretreatment necessary to meet the process objective. Once the pretreatment steps have been determined and the final feed stream defined, the RO module can be selected. 

Reverse Osmosis. Osmosis is the flow of solvent through a semipermeable membrane, from a dilute solution to a concentrated solution. This flow results from the driving force created by the difference in pressure between the two solutions. Osmotic pressure is the pressure that must be added to the concentrated solution side to stop the solvent flow through the membrane. Reverse osmosis is the process of reversing the flow, forcing water through a membrane from a concentrated solution to a dilute solution to produce pure water. Figure 2 illustrates the processes of osmosis and reverse osmosis. 

Reverse osmosis is created when sufficient pressure is appHed to the concentrated solution to overcome the osmotic pressure. This pressure is... 

Concentration of Seawater by ED. In terms of membrane area, concentration of seawater is the second largest use. Warm seawater is concentrated by ED to 18 to 20% dissolved soHds using membranes with monovalent-ion-selective skins. The EDR process is not used. The osmotic pressure difference between about 19% NaCl solution and partially depleted seawater is about 20,000 kPa (200 atm) at 25°C, which is well beyond the range of reverse osmosis. Salt is produced from the brine by evaporation and crystallisa tion at seven plants in Japan and one each in South Korea, Taiwan, and Kuwait. A second plant is soon to be built in South Korea. None of the plants are justified on economic grounds compared to imported solar or mined salt. 

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). 

You should remember that RO uses a semi-permeable membrane. As such, the membrane is permeable to only very light molecules like water. Under atmospheric condirtions the fresh water flows into the solution which is called osmotic flow. But for purification purposes, this is no use, and hence we employ the reverse of osmotic flow. For this to happen, we need to apply external pressure in excess of osmotic pressure. The osmotic pressure is given by ... 

Reverse Osmosis. The process of osmosis is used by plants to obtain food and moisture from the soil. The density of the sap in the roots of the plant is greater than that of the soil water surrounding it. The root wall provides a semipermeable membrane, and the difference in suction across it is the osmotic pressure. 

In reverse osmosis, the osmotic pressure is increased manually to get the water to flow from a high-density area through a semipermeable membrane to the lower-density weaker solution. The water will pass through the membrane and leave the solids behind. A pressure of about 2.76 MPa will extract 90% or more of the dissolved absorbed solids further refinement may be achieved through a base exchange process. 

The most important application of semi-permeable membranes is in separations based on reverse osmosis. These membranes generally have pores smaller than 1 nm. The pressure across the semi-permeable membranes for reverse osmosis is generally much larger than those for ultrafiltration, for example. This is because reverse osmosis is usually used for small molecules which have a much higher osmotic pressure, because of the higher number density, than the colloids separated in ultrafiltration. As a result reverse osmosis membranes have to be much more robust than ultrafiltration membranes. Since the focus of our discussion in this chapter will be on reverse osmosis based separations, we will describe these membranes in greater detail. 

Reverse osmosis, (a) When the external pressure (P) is less than the osmotic pressure (71-) (P < it), normal osmosis occurs, (b) When the external pressure exceeds the osmotic pressure, water flows in the opposite direction, producing reverse osmosis. Reverse osmosis can be used to obtain fresh water from seawater. 

An alternative method of purifying water is by reverse osmosis. Under normal conditions, if an aqueous solution is separated by a semi-permeable membrane from pure water, osmosis will lead to water entering the solution to dilute it. If, however, sufficient pressure is applied to the solution, i.e. a pressure in excess of its osmotic pressure, then water will flow through the membrane from the solution the process of reverse osmosis is taking place. This principle has been... 

It is a necessary consequence of the reversibility of osmotic processes that the osmotic pressure is independent of the nature of the septum used to measure it. For, suppose there are two semiperineable septa [a] and [/3], and let the osmotic pressures of a solution when separated from pure solvent under a given pressure by these septa be Pa and Pp. Then if we separate a volume 8V of solvent through [a], the work Pa V is spent on the system, and if the solvent is readmitted through [3] the work PpSV is done by the system. The isothermal cycle being now completed, we have ... 

The thermodynamic aspect of osmotic pressure is to be sought in the expenditure of work required to separate solvent from solute. The separation may be carried out in other ways than by osmotic processes thus, if we have a solution of ether in benzene, we can separate the ether through a membrane permeable to it, or we may separate it by fractional distillation, or by freezing out benzene, or lastly by extracting the mixture with water. These different processes will involve the expenditure of work in different ways, but, provided the initial and final states are the same in each case, and all the processes are carried out isothermally and reversibly, the quantities of work are equal. This gives a number of relations between the different properties, such as vapour pressure and freezing-point, to which we now turn our attention. 

Allow the added solvent, of volume V, to pass reversibly through a into A, against the osmotic pressure P of the solution (with the pure solvent under the pressure of its own vapour). 

A liquid solution may be separated into its constituents by crystallising out either pure solvent or pure solute, the latter process occurring only with saturated solutions. (At one special temperature, called the cryohydric temperature, both solvent and solute crystallise out side by side in unchanging proportions.) We now consider what happens when a small quantity of solute is separated from or taken up by the saturated solution by reversible processes. Let the saturated solution, with excess of solute, be placed in a cylinder closed below by a semipermeable septum, and the w7hole immersed in pure solvent. The system is in equilibrium if a pressure P, equal to the osmotic pressure of the saturated solution when the free surface of the pure solvent is under atmospheric pressure, is applied to the solution. Dissolution or precipitation of solute can now be brought about by an infinitesimal decrease or increase of the external pressure, and the processes are therefore reversible. If the infinitesimal pressure difference is maintained, and the process conducted so slowly that all changes are isothermal, the heat absorbed when a mol of solute passes into a solution kept always infinitely... 

Then if any two phases are separately in equilibrium with a third phase, they are also in equilibrium when placed in contact, so that if any one phase (e.y., the vapour) is taken as a test-phase, and the other phases are separately in equilibrium with this, the whole system will be in equilibrium. Under the conditions imposed, it is sufficient that the vapour pressure, or osmotic pressure, of each component has the same value at all the interfaces, for we may consider each component separately by intruding across the interface a diaphragm permeable to that compo- -nent alone. Then if the vapour, or osmotic pressures, are not equal at the third interface to their values at the first and second interfaces, i.e., at the interfaces on the test-phase, we could carry out a reversible isothermal cycle in which any quantity of a specified component is taken from the test-phase to the phase of higher pressure, then across the interface to the phase of lower pressure, and then back to the test-phase. In this cycle, work would be obtained, which however is impossible. Hence the two phases which are separately in equilibrium with the test-phase are also in equilibrium with each other. This may be called the Law of the Mutual Compatibility of Phases (cf. 106). 

Osmosis is the flow of solvent through a semipermeable membrane into a solution the osmotic pressure is proportional to the molar concentration of the solute. Osmometry is used to determine the molar masses of compounds with large molecules, such as polymers reverse osmosis is used in water purification. 

Reverse osmosis can be used to purify water, because the liquid passing through the semipermeable membrane is pure solvent. A water purifier that uses reverse osmosis requires semipermeable membranes that do not rapture under the high pressures required for reverse osmosis. Recall that seawater has an osmotic pressure of nearly 28 atm and that red blood cells rupture at 7 atm. Nevertheless, membranes have been developed that make it feasible to purify water using this technique. Reverse osmosis currently supplies pure drinking water to individual households as well as entire municipalities. 

Equation (20-80) requires a mass transfer coefficient k to calculate Cu, and a relation between protein concentration and osmotic pressure. Pure water flux obtained from a plot of flux versus pressure is used to calculate membrane resistance (t ically small). The LMH/psi slope is referred to as the NWP (normal water permeability). The membrane plus fouling resistances are determined after removing the reversible polarization layer through a buffer flush. To illustrate the components of the osmotic flux model. Fig. 20-63 shows flux versus TMP curves corresponding to just the membrane in buffer (Rfouimg = 0, = 0),... 

A phenomenon that is particularly important in the design of reverse osmosis units is that of concentration polarization. This occurs on the feed-side (concentrated side) of the reverse osmosis membrane. Because the solute cannot permeate through the membrane, the concentration of the solute in the liquid adjacent to the surface of the membrane is greater than that in the bulk of the fluid. This difference causes mass transfer of solute by diffusion from the membrane surface back to the bulk liquid. The rate of diffusion back into the bulk fluid depends on the mass transfer coefficient for the boundary layer on feed-side. Concentration polarization is the ratio of the solute concentration at the membrane surface to the solute concentration in the bulk stream. Concentration polarization causes the flux of solvent to decrease since the osmotic pressure increases as the boundary layer concentration increases and the overall driving force (AP - An) decreases. 

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