Separation of Organic Vapors

With membranes, organic vapors can be separated continuously from large scale production processes. Application areas in polyolefin and polymer production are raw material purification, product purification and finishing. Propylene, ethylene, and hexane, for example are separated from process streams and recycled for further use [24].

Another example of industrial application is the recovery of vinyl chloride monomer from off-gas from PVC. In the conventional recovery process, the off-gas is compressed and subsequently condensed in multistage condensers. The first condenser is operated with a cooling tower water. In the subsequent condenser refrigerants are used. The advent of new membrane technology enables the second and third condensation stages to be replaced by a membrane stage. Depending on the membrane stage layout an increased recovery of vinyl chloride monomer and reduced energy consumption can be achieved [24].

7.8.2.2 Pervaporation

Distillation is energy-intensive for solutions that form azeotropes. In order to separate such components through distillation, it is necessary to eliminate the azeotropic point by adding an entrainer and perform an extractive or azeotropic distillation, which increases the complexity of the separation [2].

The application of vapor permeation and pervaporation membranes has become an increasingly popular solution for the process industry during the last couple of years, because the separation mechanism at membranes is not governed by the thermodynamic equilibrium between vapor and liquid.

In principle, pervaporation is based on the solution-diffusion mechanism. Its driving force is the gradient of the chemical potential between the feed and the permeate sides of the membrane. Membranes can be either hydrophilic or hydrophobic. In general, most membranes are hydrophilic or water permselective due to the smaller molecular size of water. Few membranes are hydrophobic or

Pervaporation

Retentat

Ethanol

Superheater

Ethanol / Water

Condenser

Sweep Gas

Sweep Gas

Condenser

Permeat

Waste Gas

Vacuum Pump

Ethanol / Water

Recycle

Figure 7.24 Typical hybrid purification process with distillation column and pervaporation for the separation of water and ethanol.

Reboiler

Water

Figure 7.24 Typical hybrid purification process with distillation column and pervaporation for the separation of water and ethanol.

permselective for organic components. There are three categories of membrane materials: inorganic, polymeric and composite membrane [25].

Currently the dehydration of solvent mixtures is the biggest field of application of membrane-assisted distillation. Ethanol-water for example forms an azeotrope at approx. 96% ethanol. A typical distillation column with pervaporation for the separation of water and ethanol is shown in Figure 7.24 . So far, over 100 plants worldwide use the pervaporation technique for alcohol dehydration [26]. Additional applications are given by Ohlrogge et al. [24].

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