Reduced-pressure cycle

A typical example, from the extensive study by Kamakin on an alumina-silica gel, is shown in Fig. 3.32. When the mercury pressure was reduced to 1 atm at the end of the first cycle, 27 per cent of the intruded mercury was retained by the sample a second intrusion run followed a different path from the first, whereas the second extrusion curve agreed closely with the first. Change in f re structure of the kind described above could perhaps account for the difference between the two intrusion curves, but could not explain the reproducibility of the remainder of the loop. There is no doubt that hysteresis can exist in the absence of structural change. 

None of the selectively adsorbed components is removed from the adsorption vessel until the countercurrent depressurization (blowdown) step. During this step, the strongly adsorbed species are desorbed and recovered at the adsorption inlet of the bed. The reduction in pressure also reduces the amount of gas in the bed. By extending the blowdown with a vacuum (ie, VSA), the productivity of the cycle can be greatiy increased. 

To reduce the work of compression in this cycle a two-stage or dualpressure process may be usedwhereby the pressure is reduced by two successive isenthalpic expansions. Since the isothermal work of compression is approximately proportional to the logarithm of the pressure ratio, and the Joule-Tnomson cooling is roughly proportional to... 

An oversized relief valve may also chatter since the valve may quickly relieve enough contained fluid to allow the vessel pressure to momentarily fall back to below set pressure only to rapidly increase again. Rapid cycling reduces capacity and is destructive to the valve seat in addition to subjecting all the moving parts in the valve to excessive wear. E.xcessive back pressure can also cause rapid cycling as discussed above. 

Vessels can be pressure-purged by adding inert gas under pressure. After this added gas is diffused throughout the vessel, it is vented to the atmosphere, usually down to atmospheric pressure. More than one pressure cycle may be necessary to reduce the oxidant content to the desired concentration. 

The Schlenk tube was connected to a low-pressure hydrogenation apparatus fitted with a gas burette system to measure the hydrogen consumed. The Schlenk tube was flushed through three cycles (reduced pressure/ hydrogen) and then placed under an atmospheric pressure of hydrogen. The burette was filled with 200 mL of hydrogen. 

The unique design of the tube press allows for this cycle to be amended, however, to include air pressing and/or cake washing. With air pressing, once the initial filtration is complete, air is introduced between the membrane and the cake. The pressure cycle is then repeated. Typically an air press will further reduce the moisture content of china clay by 2.5-8 per cent. The final moisture contents with other materials are shown in Figure 7.25. Water washing, which is used for the removal of soluble salts, is similar to air pressing, except that it is water that is introduced between membrane and cake. 

We are now in a cycle characterized by incremental innovation, not therapeutic breakthroughs, and it is reasonable for industry critics to question the industry on the value of its innovations. A wave of patent expiries, high failure rates of developmental products, pressure to reduce or contain healthcare inflation (prices and expenditures), and access to essential medicines in developing countries have resulted in a global environment of increasing hostility. At this point it is difficult to predict the direction in which the environment will move over the next three to five years. 

Figure 1 shows the outline of the experimental apparatus used. The sulfide was packed in No.3 in this figure. The H2 recovery under H2S flow and the sulfur recovery under argon flow were alternately repeated many times. The thermal decomposition for sulfur was carried out under normal or reduced pressure. In this study, the repeat of the experiment associated with the former was called the normal pressure cycle and that associated with the latter was called the reduced pressure cycle, respectively. Moreover, the H2 concentration of off-gas was analyzed by gas chromatography and the behavior of the H2 formation was investigated during the H2 recovery experiment. 

Figure 2. Hydrogen evolution curves for normal and reduced pressure cycles with pyrrhotite (FeS) ( ), normal pressure cycle at 600°C (O), reduced pressure cycle at 550°C.

The concentrate was composed of fine chalcopyrite particles of ca. 50 micron. The maximum H2 concentration in both the normal and reduced pressure cycles was larger than the value obtained with iron sulfide. The results are shown in Figure 3. 

Based on the identification by X-ray diffraction and observation by micrography, the variation was found to be within the non-stoichiometric composition of chalcopyrite in the normal pressure cycle. Despite the decomposition into bornite(Cu FeS ) and pyrrhotite during the reduced pressure cycle, the chalcopyrite was found to be completely restored to its original chalcopyrite form by the succeeding sulfurization. 

The sulfur composition of the pyrrhotite was very low, and FeSi.oi- Since a favorable H2 formation behavior was not obtained from the experiment using synthetic bornite, the excellent results obtained during the reduced pressure cycle were thought to be due to the pyrrhotite. 

The sulfurization of M3S2 to NiS proceeded easily, though the thermal decomposition of NiS into M3S2 was found to be difficult under the reduced pressure. The repeated results for the reduced pressure cycle are shown in Figure 4. It shows that consistent behavior is difficult to obtain. 

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