Concentration driven processes

Dialysis and diffusion dialysis are membrane processes that consumes generally not much energy. The energy consumption Ep is determined by the pumps to circulate the feed and permeate (dialysate) stream along the membrane (eq. VIII - 87). If concentration polarisation becomes severe higher cross-flow velocities may be required to increase the solute mass transfer coefficient and consequently the energy consumption will increase. 

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So, Sulfolane and Carom, ca 1997, are two current rival processes. Sulfolane has a slight advantage over Carom ia energy consumption, while Carom has 6—8% less capital for the same capacity Sulfolane unit. In 1995, Exxon (37) commercialized the most recent technology for aromatics recovery when it used copolymer hoUow-fiber membrane ia concentration-driven processes, pervaporation and perstraction, for aromatic—paraffin separation. Once the non aromatic paraffins and cycloparaffins are removed, fractionation to separate the C to C aromatics is relatively simple. 

In the late 1980s, it was observed that the same membranes that had been used in thermal membrane distillation could be used in a concentration-driven process called OMD [36]. Both these processes show great similarity in employing hydrophobic membranes, where a liquid-vapor interface is formed on both sides of the membrane pores. The source for the driving force (vapor pressure difference) is different in both cases. It is a temperature difference in the case of thermal membrane distillation, while it is a concentration difference in the case of OMD [37]. 

Pervaporation is a special case of gas separations in that it is a concentration driven process. The feed mixture is supplied as a liquid to the membrane and the permeate is recovered as a vapor on the low pressure side of the membrane. Pervaporation finds application in dehydration as well as the separation of a variety of liquid mixtures. A discussion of pervaporation is beyond the scope of this review. 

Diffusion That form of mass transport in which motion occurs in response to a gradient in concentration or composition, itself caused by a gradient of the chemical potential fi. Diffusion is ultimately an entropy-driven process. 

Diffusion in general, not only in the case of thin films, is a thermodynamically irreversible self-driven process. It is best defined in simple terms, such as the tendency of two gases to mix when separated by a porous partition. It drives toward an equilibrium maximum-entropy state of a system. It does so by eliminating concentration gradients of, for example, impurity atoms or vacancies in a solid or between physically connected thin films. In the case of two gases separated by a porous partition, it leads eventually to perfect mixing of the two. 

FORMATION. Aqueous solutions of highly surface-active substances spontaneously tend to reduce interfacial energy of solute-solvent interactions by forming micelles. The critical micelle concentration (or, c.m.c.) is the threshold surfactant concentration, above which micelle formation (also known as micellization) is highly favorable. For sodium dodecyl sulfate, the c.m.c. is 5.6 mM at 0.01 M NaCl or about 3.1 mM at 0.03 M NaCl. The lower c.m.c. observed at higher salt concentration results from a reduction in repulsive forces among the ionic head groups on the surface of micelles made up of ionic surfactants. As would be expected for any entropy-driven process, micelle formation is less favorable as the temperature is lowered. 

Kadler K, Hojima Y, Prockop DJ (1987) Assembly of collagen fibrils de novo by cleavage of the type I pC-collagen with procollagen C-proteinase. Assay of critical concentration demonstrates that collagen self-assembly is a classical example of an entropy-driven process. J Biol Chem 262 15696-15701... 

Membrane distillation offers a number of advantages over alternative pressure-driven processes such as reverse osmosis. Because the process is driven by temperature gradients, low-grade waste heat can be used and expensive high-pressure pumps are not required. Membrane fluxes are comparable to reverse osmosis fluxes, so membrane areas are not excessive. Finally, the process is still effective with slightly reduced fluxes even for very concentrated solutions. This is an advantage over reverse osmosis, in which the feed solution osmotic pressure places a practical limit on the concentration of a salt in the feed solution to be processed. 

Pervaporation is a concentration-driven membrane process for liquid feeds. It is based on selective sorption of feed compounds into the membrane phase, as a result of differences in membrane-solvent compatibility, often referred to as solubility in the membrane matrix. The concentration difference (or, in fact, the difference in chemical potential) is obtained by applying a vacuum at the permeate side, so that transport through the membrane matrix occurs by diffusion in a transition from liquid to vapor conditions (Figure 3.1). Alternatively, a sweep gas can be used to obtain low vapor pressures at the permeate side with the same effect of a chemical potential gradient. 

Concentrated solar irradiation could be used to drive the thermal stages (desorption of products and regeneration of the catalyst) of this process. The advantage of this potentially solar-driven process is that it converts methane to hydrogen and valuable olefins without production of... 

For several decades, Ostwald s work was the foundation of crystallization theory. In 1950, an American chemist, Victor Kuhn LaMer (1895-1966), advanced our understanding of crystallization of colloidal sols. LaMer assumed that crystallization was a diffusion-driven process (LaMer, 1952 Boistelle and Astier, 1988), as described in previous chapters. LaMer s model is described in three steps, presented in Figure 4.1. In the initial steps of the reaction, the concentration of the molecular materials builds until a critical concentration is reached. This critical concentration, called supersaturation, can be written as... 

Permeation is a general term for the concentration-driven membrane-based mass transport process. Application of a pressure difference, an electric held, or temperature considerably intensibes the process, but these special methods are beyond the scope of this overview. Three groups of liquid membranes are usually considered bulk liquid membrane (BLM), supported liquid... 

The SGMD is a temperature driven process, and it involves (a) evaporation of water at the hot feed side, (b) transport of water vapor through the pores of hydrophobic membrane, (c) collection of the permeating water vapor into an inert cold sweeping gas, and (d) condensation outside the membrane module. A decrease in driving force has been observed due to polarization effects of both temperature and concentration [80,82]. To calculate both heat and mass transfer through microporous hydrophobic membrane as well as the temperature and concentration polarization layer, the theoretical model suggested by Khayet et al. [58] can be written as... 

High thermal energy consumption comparing with pressure-driven processes unless cheap energy source or waste heat is utilized Cannot be applied for wastes with volatile radioactive compounds The necessity of pretreatment and periodical scale removal Process limited by concentration (low conductivity for diluted solutions) Fouling is a problem in higher concentrations... 

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