Gas separation

Usually nonporous polymeric membranes are utilized. There, vapours and gases are separated due to their different solubility and diffusivity in polymers. Polymers in glassy state, generally more effective for separation, predominantly differentiate in diffusivity. Small molecules of penetrants move among polymer chains according to the formation of local gaps by thermal motion of polymer segments. Free volume of the polymer, its distribution and local changes of distribution are of the utmost importance. Then diffusivity of a penetrant depends mainly on the size of its molecule.

Porous membranes can also be utilized for the gas separation. The pores diameter must be smaller than the mean free path of gas molecules. Under normal condition (100 kPa, 300 K) it is about 50 nm. Then the gas flux through the pore is proportional to molecules velocity i.e. inversely proportional to square root of the molecule mass. It is known as Knudsen diffusion. Gas flux through a porous membrane is much higher than through nonporous one – 3 to 5 orders of magnitude. Separation efficiency is moderate – hydrogen passes 4 times faster than oxygen. Porous polymeric or ceramic membranes for ultrafiltration serve the purpose. Note, in case the pores are larger than the limit then viscous flow occurs, hence no separation.

3. Make up the plan to the text.

4. Give a brief summary of the text.

Text D

1. Read the text and choose the most suitable title out of the given ones:

a) Filtering Devices.

b) Advantages of Filtering Process.

c) Types of Separation.

The separation of comparatively small amounts of solids from larger amounts of liquids is done by settling, followed by decantation of the supernatant clear liquid, or by the use of a filtering device. The separation of very small amounts of liquids from large amounts of solids is done by draining, or by centrifuging.

Settling out the solid is the cheapest method, and is frequently used. If the operation is extensive, a large capacity in settling tanks is required: these act also as storage tanks. If the liquor is hot, the tank may be insulated by strips of wood. As a rule, this process is too slow; also the mud collected in the bottom of the tank still contains too much water, and is usually sent through a filter, a combination of settling and filtering.

Filtering devices include gravity filters, pressure filters, more commonly called filter-pressers, and suction filters. Several points must be considered when studying a filtration problem: whether the cake or the filtrate is the valuable part, whether or not washing is required, and whether the operation should be continuous or intermittent. These considerations will have weight in the choice of the device.

The suspension to be filtered is called a slurry, if its content in solids is not so high that it does not flow and cannot be pumped; slurries contain from less than 1 per cent to 40 per cent of solids are more properly called sludges. The solid separated in the filter device is called the cake, the clear liquor running off is the filtrate. Filtering devices such as filter-presses and suction filters are really frames for the support of the filtering medium, the filter cloth, which may be cotton duck of various thicknesses, muslin, paper, wool flannel, or metal wire woven into cloth, such as iron wire, and nickel wire.

2. Write out:

а) key words and terms;

b) the sentences expressing the main idea of each paragraph.