Membrane distillation, alternative reverse osmosis

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. 

In cases of extreme temperamre instability, evaporative methods of concentration have not been feasible because distillation temperatures at even the lowest feasible pressures are still too high (e.g., distilling water at 25°C may be possible but this temperature may be still too high for the extended time cycle required). Alternative means of concentration are reverse osmosis for aqueous systems and some membrane systems with molecular weight cutoffs compatible with the compound in question. Obviously, these systems preclude crystallization during operation. An example of an antibiotic with severe temperamre restrictions is presented in Example 11-1. 

Membrane Separations. Separation processes using polymeric membranes (30) have become important techniques because of their simplicity and low consumption of energy in comparison to alternatives such as distillation. Membranes for ultrafiltration are porous, and no diffusive transport actually occurs through the polymer itself. However, for separation at the molecular level, diffusion through the polymer provides a possible mechanism for selective passage of the desired small molecule. Reverse osmosis for desalination of water can occur by this mechanism, and large commercial processes using this technique are now in operation. 

Modern distillation plants have unit capacities of about 25,000 t/d. The energy consumption is approximately 95 KWh/to distillate in the form of saturated steam of a pressure of about 2.2 bar and 4 to 5 KWh/to distillate in form of power for the pump drives. All distillation processes produce a distillate containing only 20 to 30 ppm salts (total dissolved solids). Reverse osmosis (RO) will be the alternative to distillation. The modules which are successfully employed are of the "spiral wound" or the "hollow fiber" configuration. Under favorable conditions, the economic service lifetime of the modules resp. the membranes is about 5 years and warranties for such a figure are given by manufacturers. 

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. 

Solvent fermentation produces a dilute solution of butanol, acetone, or isopropanol. The recovery of these products by distillation is an energy-intensive process. Unless the final product concentrations are elevated, the development of alternative methods for product recovery may improve the economics of this fermentation. Techniques for the alternative methods could include the use of membranes (reverse osmosis, perstraction, pervaporation, and membrane evaporation). 

Membrane separations began in the 1960s as an alternative means to distillation for the desalination of salt (i.e. sea) and brackish waters. This was called reverse osmosis because it works by applying a transmembrane pressnre greater than the natural osmotic pressure between the two solutions (seawater, say, and desalted water). 

---