Pressure differences causing vacuum

The gravimetric method has in its favour the fact that all variables adsorbed mass, gas pressure and temperature are measured independently. Apparatus are more complicated and more expensive on account of the additional vacuum microbalance. Balance operating requires some skills. The sample is not in direct contact with the thermostat and must be shielded against false heat radiation. Measures to avoid the influence of eddy gas flow may be required. At low pressure the Knudsen pressure difference causes thermal gas flow which seriously interferes in measurements in the Henry region. 

A further rising of the reservoir causes a compression of the gas in the capillary C (closed). Capillary D is open and connected to the vacuum system. The difference Ah between the two mercury heights corresponds to a pressure difference Ap = pg-Ah (Ah in mm gives numerically Ap in torr) p is the density of mercury. If the compression of the gas in B and C is isothermal, we can write ... 

The pervaporation process to separate liquid mixtures is shown schematically in Figure 9.1. A feed liquid mixture contacts one side of a membrane the permeate is removed as a vapor from the other side. Transport through the membrane is induced by the vapor pressure difference between the feed solution and the permeate vapor. This vapor pressure difference can be maintained in several ways. In the laboratory, a vacuum pump is usually used to draw a vacuum on the permeate side of the system. Industrially, the permeate vacuum is most economically generated by cooling the permeate vapor, causing it to condense condensation spontaneously creates a partial vacuum. 

De-airing removes the air embedded in the body. In order to optimise the de-airing process, the body must first be compacted, i.e. subjected to a higher pressure and then broken down in such a manner to achieve a maximum surface area. Since plastic clay is basically impervious to gas, the only way the embedded air can be released is by the effect of the differential pressure between the residual pressure in the vacuum chamber and the pressure of the air embedded in the body causing it to bursf out. The greater the pressure difference and the thinner the material layer, the easier the body particles can be broken up. Various de-airing systems were developed for this purpose, which can be differentiated as follows ... 

For classical electrophoresis, samples of 0.1-1 cm are loaded into wells formed in gel slabs or layered onto the tops of gel columns, often with the addition of a sucrose solution to increase the density. For CE and CEC, much smaller samples (1-50 nl) are drawn into one end of the capillary (usually the anodic end) from a sample vial, either hydrod3mamically using gravity, positive pressure or a vacuum, or electrokinetically by applying a voltage for a short time when the EOF causes the sample components to migrate into the capillary. The reproducibility of sample injection into capillaries, typically 0.5-3%, is variable, and electrokinetic methods may discriminate between components of a mixture because of differences in electrophoretic mobilities. Time, temperature, pressure drops and sample and running buffer viscosities are all sources of variability, and automated sample injection is preferable to minimize these effects. 

Measurements of filtration rates should be repeated at different pressures or different vacuum levels. This gives information on the influence of pressure on the specific cake resistance. The specific resistance of cakes that are difficult to filter is often pressure-dependent. Thus, use of excessive pressure can result in blocking of the cake, causing filtration to stop. In the case of compressible cakes, information is needed over the whole range of pressures being considered for industrial filters since extrapolation of compressibility beyond the experimentally covered region is always risky. The larger the scale of an experimental filter, the less risky predictions based on the experimental data. 

Figure 4-4 shows a typical system under positive pressure. It differs from the vacuum system in that the material enters from one source and is distributed directly to several tanks. In this case no cyclone separator is used the air laden with solids enters the process bins directly. The decrease in velocity of the stream and its change in direction will cause most of the solids to drop out. For this system each receiver must have a filter to remove the remaining solids. Note that the blower is placed at the air entrance, instead of after the filter as in the vacuum system. Should a bag in the fiber filter break, no dust will get into the blower or its motor. Another advantage is that no contaminants from the atmosphere can enter the system when it is under positive pressure, except through the air inlet system. 

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