Diffusion Through Selectively Permeable Membranes

Diffusion Through Selectively Permeable Membranes Essay

The processes of separation of liquid and gas mixtures play an important role in many branches of the national economy. To carry out the processes of separation of liquid mixtures, such methods can be applied as distillation, rectification, extraction, adsorption, etc. However, one of the universal methods is separation using semipermeable membranes, also called the membrane method.
In the food industry, membrane methods are used to separate azeotropic mixtures, purify and concentrate solutions, purify or isolate high-molecular compounds from solutions, containing low-molecular components, and the like. The most extensive use of membrane processes is found in the treatment of water and aqueous solutions, as well as the treatment of the waste water. Calculations and the accumulated large material show that the use of semipermeable membranes in existing traditional industries can give a significant economic effect, which opens wide opportunities for creating fundamentally new, simple, low-energy and environmentally friendly technological schemes (especially in combination with such widely used separation methods as rectification, adsorption, extraction, etc.).

Process of Membrane Separation of Mixtures

The process of membrane separation of mixtures is carried out using semipermeable membranes. There are many membrane processes based on different principles of separation, which are used for the separation of objects of different sizes – from particles to molecules. Despite these differences, all membrane processes have something in common. The membrane is the heart of every membrane process, it can be considered as a selectively permeable barrier between the two phases. A separation is achieved due to the fact that at the feed phase one component (the starting mixture) is going through the membrane faster than the other component or components. The process of separation can pass so completely that practically no impurities of those mixture components can leave in the filtrate (permeate). The non-permeated mixture of components in the form of concentrate is removing. The process of membrane separation has two main parameters: permeability and selectivity. The permeability, or the specific productivity, equal to the mass flow of permeate through a unit surface of the membrane, determines the rate of the membrane separation process.

Membranes

The membrane is a semipermeable partition that allows certain components of liquid or gas mixtures to pass through. Membranes must satisfy some basic requirements: they must have a high separating ability (selectivity), a high specific productivity (permeability), a chemical resistance to the action of the components, and must have a mechanical strength, sufficient for their preservation during installation, transportation, and storage. In addition, the properties of the membrane should not vary significantly during their work.

Various membranes (cellulose acetate, polyamides, polysulfone), ceramics, glass, metal foil, etc. are used to produce membranes. Depending on the mechanical strength of the materials used, the membranes are subdivided into porous and non-porous or diffusive.
Porous membranes have found a wide application primarily in the processes of the reverse osmosis, micro- and ultrafiltration. They have both anisotropic and isotropic structure. Membranes with an anisotropic structure have a surface as a finely porous layer of 0.25-0.5 μm thick (called active, or selective), which represent a barrier for the selection. The components of the mixture are separated by this layer, which is located on the side of the separated mixture. A large pore layer, approximately 100-200 μm in thickness is a substrate that increases the mechanical strength of the membrane. Membranes with an anisotropic structure are featured by a high specific productivity and a slower blockage of pores during their work. The service life of these membranes is mainly determined by the resistance of the membrane material in the processed environment.

Diffusive membranes are usually used to separate liquid mixtures by the evaporation through a membrane, or the dialysis. Diffusive membranes are practically non-porous. They present quasi-homogeneous gels through which the solvent and solutes penetrate under the action of the molecular diffusion. The rate, at which individual components pass through the membrane, depends on the activation energy when the transferred particles interact with the membrane material, as well as on the mobility of the individual links of the membrane matrix and on the properties of the diffusing components of the separated mixture. It should be noted that the diffusion rate is higher, the weaker the individual links of the polymer chain in the layer, that is, the more the membrane swells. The rate of molecules passage through the diffusion membrane is usually directly proportional to the diffusion ratio, which is determined by the size of molecules and their shape. Therefore, diffusion membranes are most rational to use for the separation of components that have practically identical properties but differ in molecules size and shape. The permeability of diffusion membranes almost does not decrease with time. Diffusion membranes have great hydrodynamic resistance, so they should be used in the form of ultrathin films.

The reverse osmosis (or hyperfiltration) is a continuous process of molecular separation of solutions by their filtration under pressure through semipermeable membranes that completely or partially trap molecules or ions of a dissolved substance. When the pressure is applied, the solvent is transferred in the opposite direction from the solution to the pure solvent through the membrane and the sufficient purification is provided. The required pressure, which exceeds the osmotic pressure of the dissolved substance in the solution, depends on the salt concentration. For the industrial use of membrane separation processes, semipermeable membranes are required, featured by the high separating ability (selectivity) and the high specific productivity (permeability). In the process of separation of solutions by means of semipermeable membranes, the solvent preferably passes through the membrane.