Osmotic pressure effect

Osmotic pressure effects can be substantial. For example, the waters of the oceans contain dissolved salts at a total ionic molarity of about 1.13 M. The osmotic pressure of ocean water can be calculated ... 

For purposes of illustration, the following discussion, unless otherwise specified, is limited to single-solute aqueous feed solutions, cellulose acetate membranes, and reverse osmosis systems for which osmotic pressure effects are essentially negligible. 

Breakdown of the protective layer by osmotic pressure effects. 

Elementary and advanced treatments of such cellular functions are available in specialized monographs and textbooks (Bergethon and Simons 1990 Levitan and Kaczmarek 1991 Nossal and Lecar 1991). One of our objectives in this chapter is to develop the concepts necessary for understanding the Donnan equilibrium and osmotic pressure effects. We define osmotic pressures of charged and uncharged solutes, develop the classical and statistical thermodynamic principles needed to quantify them, discuss some quantitative details of the Donnan equilibrium, and outline some applications. 

The osmotic pressure of brackish water is approximately 11 psi per 1000 ppm salt, so osmotic pressure effects do not generally limit water recovery significantly. Limitations are generally due to scaling. Typical water recoveries are in the 70-90 % range, which means the brine stream leaving the system is up to 10 times more concentrated in calcium, sulfate and silica ions present in the feed. If scaling occurs, the last modules in the system must be replaced first. 

There are two reasons for steric interactions (1) osmotic pressure effect due to the high concentration of chain elements in the region of the overlap as shown in Fig. 10.13, and (2) a steric effect due to the fewer possible conformations of the adsorbed molecule in the region of the overlap. These two aspects correspond to the enthalpy and entropy effects of steric stabilization. It has been found for some types of steric stabilization that increasing the temperature destabilizes the system even though for others increasing the temperature stabilizes the sys-... 

Fig. 3 Steric stabilization of lyophobic colloidal particles. The particles repel one another because of volume restriction and osmotic pressure effects.

Transmembrane pressure It is the average driving force for permeation across the membrane. Neglecting osmotic pressure effects for most MFAJF applications, it is defined as the difference between the average pressure on the feed (or retentate) side and that on the permeate (or shell side). 

Osmotic pressure effects were first observed in cases where they are very strong, i.e. for charged systems (ionized proteins, Donnan effect 1911). However, before being able to study solutions of neutral polymers, the experimentalist had to wait for the manufacture of efficient semi-permeable membranes (collodion). Since that time (1920) up to the present (1988), this technique has been applied with great success to the study of polymers. 

Membrane separations involve the selective solubility in a thin polymeric membrane of a component in a mixture and/or the selective diffusion of that component through the membrane. In reverse osmosis (3) applications, which entail recovery of a solvent from dissolved solutes such as in desalination of brackish or polluted water, pressures sufficient to overcome both osmotic pressure and pressure drop through the membrane must be applied. In permeation (4), osmotic pressure effects are negligible and the upstream side of the membrane can be a gas or liquid mixture. Sometimes a phase transition is involved as in the process for dehydration of isopropanol shown in Fig. 1.8. In addition, polymeric liquid surfactant and immobilized-solvent membranes have been used. 

The Resistance in Series Model describes the flux of a fouled membrane. This is given in equation (3.4). The resistances Ra>, Ri> and Rc denote the additional resistances which result from the exposure of the membrane to a solution containing particles or solute. Rcp is the resistance due to concentration polarisation, Ri> the internal pore fouling resistance, and Rc the resistance due to external deposition or cake formation. These resistances are usually negligible in RO, where the osmotic pressure effects become more important (Fane (1997)). However, the osmotic pressure can also be incorporated into Rcp. 

The choice of membrane for fouling and rejection studies is crucial. Ko and Pellegrino (1992) pointed out that some membranes exhibit low fouling regardless of their rejection. For other membranes, their flux is controlled b5" osmotic pressure effects, which is indicative of rejection. Laine et al. (1989) pointed out that the most important membrane characteristic is probably hydrophilicity. 

Overall, the flux ratios (J/Jwo, solution flux after filtration of 120 mL of solution relative to the pure water flux before the experiment) correspond well to the salt rejection. This indicates a concentration polarisation and osmotic pressure effect due to the accumulation of ions at the membrane surface and an increase in cell concentration. This flux decline was fully reversible. The flux of the CA-UF membrane consistently increases after salt filtration, probably due to an increased hydrophilicity after ion adsorption in the membrane. Rejection of calcium is stable and generally does not increase with the concentration in the feed cell. 

The CA-UF membrane has the lowest rejection of organics, and the ion rejection is slightly increased compared to the absence of organics. This may indicate interactions between the retained organics and the cations. This membrane shows a very interesting flux behaviour, with no decline at all over the experiments and a higher pure water flux after the experiments. This indicates the lack of concentration polarisation or osmotic pressure effect at the low salt rejection. The smooth membrane surface would also influence this. The adsorption of ions or organics render the membrane more hydrophilic. 

It is surprising that the TFC-SR membrane does not show this osmotic pressure effect as much as the other membranes, as this membrane also retains a considerable amount of salt (but less sodium). It was calculated in Table 7.22 that sodium is the main contributor to osmotic pressure. However, the flux behaves as if sodium and calcium were virtually absent (compare with TFC-S membrane in absence of sodium and calcium in Figure 7.17). 

To investigate the lower osmotic pressure effect on the TFC-SR membrane, an experiment was carried out in the absence of stirring, which would increase concentration polarisation effects. The results are shown in Figure 7.19. 

The lack of stirring indeed showed that concentration polarisation (which is increased in the absence of stirring) increases flux decline tremendously, even though the membrane appeared less sensitive to osmotic pressure effects. With the flux decline at unstirred conditions, a very significant deposit was... 

The waste streams generated require treatment, which adds to process costs. The sodium concentrations in Table 8.18 are very high due to the use of a bacl ound electrolyte in the experiments, but they reflect the increase in salinity of the concentrates when NF is used. The NF recovery is limited by this salt content due to possible precipitation and osmotic pressure effects. A production of a 250 m /day waste stream would appear to be excessive. 

A colUgative method, such as osmotic pressure, effectively counts the number of molecules present and provides a number-average molar mass defined by... 

---