Isothermal membrane distillation

Osmotic distillation (OD), sometimes called isothermal membrane distillation, is a membrane process in which a liquid phase (usually an aqueous solution) containing one or more volatile components flows across one surface of a microporous membrane whose pores are not wetted by the hquid, while the opposing surfece is in contact with a second nonwetting liquid phase (usually an aqueous solution) in which the volatile components are soluble or miscible [35]. The device is similar to the membrane contactor (MC) discussed in Chapter 1, which contains hollow fibre membranes that are hydrophobic (non-wetting. 

Osmotic distillation also removes the solvent from a solution through a microporous membrane that is not wetted by the liquid phase. Unlike membrane distillation, which uses a thermal gradient to manipulate the activity of the solvent on the two sides of the membrane, an activity gradient in osmotic distillation is created by using a brine or other concentrated solution in which the activity of the solvent is depressed. Solvent transport occurs at a rate proportional to the local activity gradient. Since the process operates essentially isothermally, heat-sensitive solutions may be concentrated quickly without an adverse effect. Commercially, osmotic distillation has been used to de-water fruit juices and liquid foods. In principle, pharmaceuticals and other delicate solutes may also be processed in this way. 

In comparison to isothermal membrane processes, little attention has been paid to date to polarisation phenomena in non-isothermal processes. In non-isothermal processes such as membrane distillation and thermo-osmosis, transport through the membrane Occurs when a temperature difference is applied across the membrane. Temperature polarisation will occur in both membrane processes although both differ considerably in membrane structure, separation principle and practical-application. In a similar manner to concentration polarisation in pressure-driven membrane processes, coupled heat and mass transfer contribute towards temperature polarisation. 

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. 

Gas diffusion can be accomplished using an air gap instead of a membrane between the donor and acceptor streams (Fig. 8.23, right). The feasibility of membraneless gas diffusion was demonstrated in the flow injection determination of ammonium in plant digests [267], and this innovation was originally referred to as isothermal distillation. The donor stream provided the alkaline conditions for analyte conversion to ammonia and flowed on an inclined silicone sheet inside the GD unit. Ammonia was released towards the headspace and collected by the acidic acceptor stream that flowed below another silicone sheet parallel to the first one. The air gap between the sheets was the barrier for gas diffusion and could be finely adjusted. The collected analyte was then directed towards the detector. 

In 1979, Baadenhuijsen and Seuren-Jacobs [2] were the first to report on a FI gas diffusion separation system with a semi-permeable dimethylsilicone rubber membrane, used for the determination of carbon dioxide in plasma. In the same year. Zagatto et al.[3] introduced an isothermal distillation FI system in which ammonia diffused from a flowing donor liquid film across an air-gap and absorbed by a flowing acceptor film on the opposite side of the gap. However, later developments on gas diffusion separations mainly followed the approach of Baadenhuijsen and Seuren-Jacobs, obviously due to its simpler design and higher versatility. The first theoretical study on an FI gas-diffusion separation system was attempted by van der Linden [4], who used a tank-in-series model for the mathematical evaluation of the separation process. 

In the vinyl-chloride process, because of the significant differences in the volatilities of the three principal chemical species, distillation, absorption, and stripping are prime candidates for the separators, especially at the high production rates specified. For other processes, liquid-liquid extraction, enhanced distillation, adsorption, and membrane separators might become more attractive, in which case the design team would need to assemble data that describe the effect of solvents on species phase equilibrium, species adsorption isotherms, and the permeabilities of the species through various membranes. 

Membranes can also be used to purify a mixmre and attain composition beyond the azeotropic composition. The pervaporation process features a liquid feed, a liquid retentate, and a vapor permeate. While gas-phase membrane processes are essentially isothermal, the phase change in the pervaporation process produces a temperature decrease as the retentate flows through the unit. Since flux rates decrease with decreasing temperature, the conventional pervaporation unit consists of several membrane modules in series with interstage heating. The vapor permeate must be condensed for recovery and recycle, and refrigeration is usually required. Hybrid systems of distillation columns and pervaporation units are frequently used in situations where distillation alone is impossible or very expensive. An important application is the removal of water from the ethanol-water azeotrope. Chapter 14 will discuss the details of design and control of such processes. 

A semiautomatic method was described for the routine determination of cyanide in water [40]. Membrane diffusion and isothermal distillation are examined for the separ-ation/concentration of cyanide. An air-segmented flow analyzer is used to quantify cyanide. The method based on reaction with picric acid is applicable at cyanide concentrations exceeding 1 mg. The method is suitable for the determination of cyanide in waters in the concentration range of 0.01-10 mg... 

Accordingly we experimentally investigated the influence of either one of these two parameters on membrane transport. The system studied was particularly simple, consisting of a synthetic membrane, the AP-20 Millipore porous partition, sandwitched between two containers filled with distilled water maintained at different temperatures. Isothermal measurements of water transport in the same system, induced by an hydrostatic pressure difference rather than by a temperature gradient, were also effected to com.pare the relative energy barriers. 

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