Summary of gas separation

Pervaporation is a membrane process in which a pure liquid or liquid mixture is in contact with the membrane on the feed or upstream side at atmospheric pressure and where the permeate is removed as a vapour because of a low vapour pressure existing on the permeate or downstream side. This low (partial) vapour pressure can be achieved by employing a carrier gas or using a vacuum pump. The (partial) downstream pressure must be lower than the saturation pressure at least. A schematic drawing of this process is shown in figure VI-.21. 

Essentially, the pervaporation process involves a sequence of three steps  

Pervaporation is a complex process in which both mass and heat transfer occurs. The membrane acts as a barrier layer between a liquid and a vapour phase implying that a phase transition occurs in going from the feed to the permeate. This means that the heat of vaporisation of the permeating components must be supplied. Because of the existence of a liquid and a vapour pervaporation is often considered as a kind of extractive distillation process with the membrane acting as a third component. The separation principle in distillation is based on the vapour-liquid equilibrium whereas separation in pervaporation is based on differences in solubility and diffusivity. The vapour-liquid equilibrium influences 

The low solubility of gases in po) Tneric materials (at T Tg) can be described by Henry s law. The much higher solubility of liquids implies that Henry- s law is no longer obeyed, and the Flory-Huggins theory is commonly used to provide an adequate description of the solubility of liquid mixtures and pure liquids into a polymeric material (see also chapter V). 

The permeability of a given component i from a mixture of components i and j can be expressed as a function of the diffiisivity (D) and the solubility (S). With liquids the main difference from gases is that the diffiisivity and the solubility are not constants but are strongly dependent on the feed composition  

---