Pressuring and Depressuring

One incident has been reported (364) where an 8-ft column equipped with valve trays was pressured from the top while it was circulating liquid. This caused the top 12 trays to be deflected 6 in downward in the middle and loosened manway covers. The loose covers were later displaced by upflowing vapor when normal operation commenced this in turn caused premature flooding. The corrective action was to pressure the column from the bottom. 

Location of relief valves, bursting disks, and m or vents should be carefully reviewed. Normally, these should be at the top of superatmospheric columns (or in their overhead systems). Conversely, the vacuum-breaking device should normally be at the bottom of the column. This is important for avoiding damage to trays and for achieving the required relief rates. Detailed discussion is in Sec. 9.5. 

When the column separates combustible or hazardous materials, it is usually purged with an inert gas prior to startup to remove air. It may 

Drain and vent valves must be opened intermittently so that no dead pockets are left unpurged. When purging nears completion, analyses are performed at each vent or drain. Purging continues until the concentration of air, combustibles, or toxic gas does not exceed a de- 

The purging steps should he logged, including times at which each vent and drain was opened, and any analytical results (153). Coloring a column process and instrumentation diagram (P ID) at the completion of each pm-ging step is also helpful. 

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For glassy polymers, sorption isotherms are more complex and hysteresis oetween the pressurization and depressurization steps may... 

It is common to use compressed gas to force the liquid N2 through the transfer tube. If transfers are frequent, pressurization and depressurization of the container leads to waste and contamination of the liquid. To avoid these drawbacks, one can leave the container sealed by a blowoff valve (release for an overpressure of about 0.2 atm). A valve in the transfer tube allows liquid N2 to be drawn under this pressure when desired. 

In the PSA process, the reformer product gas is passed through an adsorption column, where impurities are preferentially adsorbed. After a set time, the feed gas to the reactor column is diverted to a parallel column. At this stage, the first column is depressurized, allowing the bed regeneration. It is a cyclic process, the two columns being alternately pressurized and depressurized [15]. 

As regards the sample, it is compulsory to use a treatment-resistant packaging, such as a plastic multi-layer envelope, or aluminium foil. When a rigid material (glass or metals) is preferred, a compressibility space must exist inside the packaging, otherwise the pressure is not transferred and the material could break. The container should be filled as much as possible, to reduce the empty space in order to minimize the stress imposed during the steps of pressurization and depressurization. 

In a typical experiment, 4.63 g. (0.025 mole) of niobium(V) fluoride is mixed with 0.348 g. (0.012 mole) of -200-mesh silicon and loaded into the reactor inside a glove box under dry nitrogen. The assembled reactor is connected to a helium tank and repeatedly pressurized and depressurized to rid the system of air. The entire system is pressurized to 50 p.s.i., the Monel reactor valve is closed, and the reaction chamber is placed in a vertical furnace at 350°C. The pressure in the external lines is left at 50 p.s.i. to prevent entry of air. 

Figure 2.7 (a) Apparatus for charge-based measurement of thin film d33j coefficient, (b) measured electrical signal on pressurizing and depressurizing cavities, after [26]. 

Shims Shims are used to prevent modules from moving back and forth during pressurization and depressurization. Such movement could wear on the internal O-ring seals. Shims are plastic spacer rings similar to washers. They are typically 0.20-inches thick, and can be purchased from the manufacturer of the pressure vessel or fashioned from polyvinylchloride (PVC) pipe. Shims fashioned from PVC pipe must be cut parallel and free of burrs to work correctly. They are installed between the face of the lead membrane module and the adapter hub (see Figure 6.16) after all the... 

Shims are installed at the feed end of the module/pressure vessel assembly (refer to figures 4.18 and 4.19). Because pressure vessels are constructed with slight variations in length (known as "freeboard"), membrane modules can slide during pressurization and depressurization. Shims are installed between the face of the lead module and the adapter hub to prevent this motion. Membrane modules should be pushed completely against the thrust ring prior to installation of the shims. Shims are washer-like plastic rings that may be purchased from the pressure vessel manufacturer or fashioned out of PVC (must be free of burrs and be cut parallel to work properly). 

Periodic changes in operation can help to reduce accumulation of foulants on the membrane surface. This can be accomplished with alternate pressurization and depressurization, changing the flow direction at a given frequency, or backflushing (Figure 9.18). 

The pressure used in temperature-swing regeneration is usually that of the ndsorption step. This essures that there is negligible pressure drop across closed adsorber valves so that cross-contamination is minimized. It also means there is no need for separate steps to pressurize and depressurize the adsorbers. Occasionally, it may be worthwhile to use a lower pressure for regeneration, either to achieve better desorption or because the fluid is only available ai and/or deliverable to rhet pressure. 

In pressurization and depressurization steps, the amount of gas leaving or entering is determined by the pressure change which was set at a constant rate. The flow rate during the adsorption step determines the conversion range and was set to ensure a low conversion. The purge gas flow rate is expressed relative to the adsorption gas flow rate. 

Once a vapor slug passes through a deep liquid layer, the vapor space previously occupied by the slug is left empty and turns into a vapor gap under the bottom tray. Fast drainage of a deep liquid layer can also cause a vapor gap or induce excessive pressuring and depressuring rates. The consequences are described in Sec. 12.9. The description includes an experience where a vapor gap was initiated by vapor slugging. 

According to the previous chapters the reader may come to the conclusion that countercurrent columns are dominant as has been shown for mass transfer equipment used in the areas of rectification, absorption, and extraction. However, this is not true because the continuous transport of solid granular material is much more difficult in comparison to a fluid. Therefore, nearly all adsorbers are fixed beds which are operated batchwise. As a rule, at least two fixed beds are installed in continuously operated industrial processes. The first bed is used for the adsorption step whereas in the second the adsorbates is removed or desorbed at the same time. The duty of the two beds is changed when the adsorption capacity is exhausted. Sometimes several beds are arranged to cany out pressurization and depressurization steps. 

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