Membrane distillation, alternative

Process intensifying methods, such as the integration of reaction and separation steps in multifunctional reactors (examples reactive distillation, membrane reactors, fuel cells), hybrid separations (example membrane distillation), alternative energy sources, and new operation modes (example periodic operations). 

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. 

Membrane processes, in general, are very attractive for their simplicity and flexibility. They are capable of achieving separations at a molecular level. Membrane modules are often compact and easily scaleable. For clarification and concentration, microfiltration, ultrahltration, and reverse osmosis are the current methods of choice. RO has been widely used in the food industries as an attractive alternative to classical evaporahon the only hmitahon being its dependence on osmotic pressure, which practically limits concentration of fluid streams to 25°Bx-30°Bx. Hence, currently it is used more as a preconcentration step. In recent years, membrane processes, notably pervaporahon, membrane dishUahon and osmotic membrane distillation (OMD) [21], have been used either by themselves or in combinahon with other membrane processes to overcome the problems associated with thermal processes. 

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 membrane distillation process can be a feasible alternative to treat contaminated ground waters. 

The most common alternative to distillation for the separation of low-molecular-weight materials is absorption. Liquid flowrate, temperature and pressure are important variables to be set, but no attempt should be made to carry out any optimization at this stage. Other commonly used separation methods are adsorption and membranes. 

Pervaporation is a membrane separation process where the liquid feed mixture is in contact with the membrane in the upstream under atmospheric pressure and permeate is removed from the downstream as vapor by vacuum or a swept inert gas. Most of the research efforts of the pervaporation have concentrated on the separation of alcohol-water system [1-20] but the separation of acetic acid-water mixtures has received relatively little attention [21-34]. Acetic acid is an important basic chemical in the industry ranking among the top 20 organic intermediates. Because of the small differences in the volatility s of water and acetic acid in dilute aqueous solutions, azeotropic distillation is used instead of normal binary distillation so that the process is an energy intensive process. From this point of view, the pervaporation separation of acetic acid-water mixtures can be one of the alternate processes for saving energy. 

Since water is the byproduct, and also has an undesired inhibitory effect on catalyst activity, it must be separated efficiently from the reaction mixture. To achieve this, both conventional reactive distillation and reactive membrane separation are considered as process alternatives. In the latter process, a Knudsen-membrane is applied. Consequently, the mass transfer matrix [/c] has a diagonal structure and the diagonal elements are the Knudsen-selectivities - that is, the square-roots of the ratios of the molecular weights Mr. 

As an alternative to simple distillation, pervaporation could be used [124], This technique makes use of non-porous membranes with a selective layer consisting of hydrophilic or hydrophobic polymer. Those compounds, which are volatile and soluble in the membrane, are evaporated into the vacuum on the permeate side. By this means, selective separation, for example of volatile impurities from volatile auxiliary agents in the ionic liquid, should be possible. 

Is the current separation the most suitable or should other methods of separation be considered For example, distillation is commonly the workhorse in the chemical industry and is often the immediate choice. Other separation methods, such as pervaporation, can be sometimes more effective especially if the volatility differences are small. Pervaporation is a membrane-based process with the difference that the permeate appears as a vapor, thus permitting solute recovery and recycle. For example, benzene can be recovered from hydrocarbon streams using this method in fairly high concentrations and in a usable form ready for recycle. Many alternative separation methods must be considered, and one should not simply bank on past experience or expertise. 

The extrapolation is to what is called pervaporation, where the feed mixture is a liquid, but the permeate vaporizes during permeation, induced by the relatively low pressure maintained on the permeate side of the membrane. Accordingly, the reject or retentate remains a liquid, but the permeate is a vapor. Thus, there are features of gas permeation as well as hquid permeation. The process is eminently apphcable to the separation of organics and to the separation of organics and water (e.g., ethanol and water). In the latter case, either water vapor may be the permeate, as in dehydration, or the organic vapor may be the permeate. The obvious, potential application is to the separation of azeotropic mixtures and close-boiling mixtures—as an alternative or adjunct to distillation or liquid-liquid extraction methods. 

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