Development

In 1917 PA Kober published a paper in which he described his observation that 'a liquid in a collodion bag, which was suspended in the air, evaporated, although the bag was tightly closed'. Kober was not the first researcher to observe this phenomenon, but the first to realize its potential for the separation of liquid mixtures which otherwise are difficult to separate, and to separate them under moderate conditions. He introduced the terms 'Pervaporation', and 'Perstil-lation', and the first term is now in use to describe in general a process in which one component out of a fluid mixture selectively permeates through a dense membrane, driven by a gradient in partial vapour pressure, leaving the membrane as a vapour, and being recovered in a condensed form as a liquid.

In the years following Kober's publication a number of papers were published describing membranes and processes for pervaporation. Especially during the 1950s, the interest focused on pervaporation membranes and processes for the separation of different classes of hydrocarbons and of isomeres and numerous patents were granted. Membrane materials disclosed were natural and synthetic rubbers, cellulose esters and ethers, and several treated and untreated polyolefines. None of this early membrane, however, was used in any industrial process, owing to insufficient flux and selectivity.

Pervaporation, vapour permeation and gas permeation are very closely related processes. The driving force is always a gradient in partial vapour pressure, and transport through the membrane can best be described by a so-called 'Solution-Diffusion-Mechanism'. In this mechanism it is assumed that a component of the feed having a high affinity to the membrane is easily and preferentially absorbed and dissolved in the dense membrane. Following a concentration gradient it migrates through the membrane by a diffusion process and is desorbed at the downstream side of the membrane. The separation characteristic of the membrane is thus governed primarily by the solubility of components in the membrane material and, to a lesser extent, by its diffusivity which even may counteract against the solubility separation.

In pervaporation and vapour permeation processes the partial vapour pressures of the components at the feed side are fixed by composition and temperature of the feed; they can be influenced only by increasing the temperature. Therefore, the driving force for the transport of matter through the membrane is applied by reducing the partial vapour pressure at the permeate side.

Different means have been proposed in order to effect this reduction of the permeate side partial vapour pressure:

• The permeate side of the membrane is swept with an inert gas in which the partial vapour pressure of the critical (preferential permeating) component is kept sufficiently low. If the gas stream cannot be wasted it has to be reconditioned and recycled.

• All permeating vapour is removed by means of a vacuum pump. The vapour may be condensed after recompression at the downstream side of the pump.

• The permeated vapour is condensed at sufficiently low temperatures. As the condenser surface will be installed at a certain distance to the permeate side of the membrane all non-condensable gases have to be removed from the permeate compartment in order to minimize permeate side pressure losses.

In most industrial installations the last has been proven to be the most effective and economical process.

In a pervaporation process the feed is applied as a liquid and all partial vapour pressures of the components in the feed mixture are at saturation level. Within the limits of membrane stability and process requirements, temperature and pressure on the feed side are free adjustable parameters.

In vapour permeation a vaporous feed mixture is applied, with at least the partial vapour pressure of the preferential permeating component at or close to saturation conditions. Temperature and pressure of the feed are linked by vapour-liquid equilibrium and can be chosen within these limits only.

In gas permeation all partial vapour pressures at the feed side are below saturation and the permeate can no longer be condensed. By increasing the total feed side pressure the driving force for the transmembrane transport can be adjusted.

Pervaporation treatment liquid feed mixtures is insofar unique compared with other membrane processes as the transport of matter across the membrane is coupled with a phase change from liquid to vapour. The heat of evaporation is extracted from the liquid feed and transported through the membrane, too. As a consequence the temperature of the feed is reduced, which reduces driving force and transmembrane flux. Different means such as heated modules have been proposed to replace the lost heat of evaporation. In general, the total membrane area is split into a number of segments (stages) arranged in series with intermediate heat exchangers between each two segments or stages.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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