Repressuring

Steps. A pressure-swing cycle has at least three steps adsorption, blowdown, and repressurization. Although not always necessary, a purge step is normally used. In finely tuned processes, cocurrent depressurization and pressure-equalization steps are frequendy added. 

The repressurization step returns the adsorber to feed pressure and completes the steps of a PSA cycle. Pressurization is carried out with product and/or feed. Pressurizing with product is done countercurrent to adsorption so that purging of the product end continues indeed it may be merely a continuation of the purge step but with the bed exit valve closed. Pressurizing with feed cocurrent to adsorption in effect begins adsorption without producing any product. 

Pressure equalization steps are used to conserve gas and compression energy. They are appHed to reduce the quantity of feed or product gas needed to pressurize the beds. Portions of the effluent gas during depressurization, blowdown, and purge can be used for repressurization. 

Fig. 15. Four-bed PSA system cycle sequence chart (64). EQ, equalization C D A, cocurrent depressurization C D T, countercurrent depressurization R, repressurization A, cocurrent flow T, countercurrent flow. Courtesy of American Institute of Chemical Engineers.

The flow directions in a PSA process are fixed by the composition of the stream. The most common configuration is for adsorption to take place up-flow. AH gases with compositions rich in adsorbate are introduced into the adsorption inlet end, and so effluent streams from the inlet end are rich in adsorbate. Similarly, adsorbate-lean streams to be used for purging or repressurizing must flow into the product end. 

The effect of subjecting a thick-waded cylinder to a pressure greater than the yield pressure and then releasing the pressure is to put the material adjacent to the bore of the cylinder in compression while the outer layers remain in tension. On subsequent repressurization the cylinder wid, to a first approximation, retrace the unloading path BE (see Eig. 3) so that the cylinder withstands elasticady a pressure equal to that appHed originally. 

Gas injection into a gas cap overlaying an oil reservoir is considered an EOR method. The resulting repressurization of the reservoir promotes additional oil production. Reinjection of natural gas is responsible for a significant fraction of Alaskan North Slope oil production. 

The repressurization step that returns the adsorber to feed pressure and completes the steps of a PSA cycle should be completed with pressure equalization steps to conserve gas and compression energy. Portions of the effluent gas during depressurization, blowdown, and enrichment purge can be used for repressurization to reduce the quantity of feed or product gas needed to pressurize the beds. The most efficient cycle is one that most closely matches available pressures and adsorbate concentration to the appropriate portion of the bed at the proper point in the cycle. 

Applications PSA cycles are used primarily for purification of wet gases and of hydrogen. One of the earhest applications was the original Skarstrom two-bed cycle (adsorption, countercurrent blowdown, countercurrent purge, and cocurrent repressurization) to diy air stream to less than 1 ppm H9O [Skarstrom, ibid.]. Instrument-air diyers stiU use a PSA cy(de similar to Skarstrom s with activated alumina or sihca gel [Armond, in Townsend, The Propei ties and Applications of Zeolites, The Chemical Society, London, pp. 92-102 (1980)]. 

Purging (Oj removal) Alternately depressure and repressure system to test (or header) pressure, until O2 by meter is less than 3.0%. Regularly check O2 meter to ensure its accuracy. Passing leak check at test (or N2 header) pressure. ... 

Drainage of nonvolatile liquid from a vessel without atmospheric venting or gas repressuring. 

In general, vacuum vents and inert or gas repressuring systems are not considered an acceptable alternative to vacuum design for process equipment. Repressuring systems may be provided for process reasons, but they are not considered sufficiently reliable for equipment protection. Vacuum breakers are difficult to maintain tight and may admit air into the equipment. 

A reliable automatic repressuring system (inert gas or hydrocarbon gas) to minimize the possibility of vacuum conditions occurring. 

A board-mounted low pressure alarm, set at a positive pressure, and a remote control valve operator, so that repressuring gas can be admitted from the control center, should the automatic system fail to function. 

Chlorine addition may be a prerequisite to sanitize the RO RW supply line and oxidize any organics (followed by dechlorination using sodium bisulfite after the MM filtration stage). Where chlorine is required, it is usual to provide a 20- to 25-minute contact period by means of a temporary storage tank. This is followed by a repressurization pump system. 

The checkers found that it shortened the reaction time appreciably to repressure the autoclave to 400-500 lb./in.2 whenever the pressure dropped below 350 lb./in.3... 

Although PSA is a batchwise process, by using multiple beds in a sequential manner the overall process is operated in a continuous fashion. Each bed may contain layers of different adsorbent materials selective for specific contaminants in the hydrogen gas stream to be purified. Each bed undergoes a sequence of four basic steps in a PSA cycle adsorption, depressurization, purge at low pressure, and repressurization. This sequence of cyclic operations for each bed is shown schematically for a four-bed PSA process in Figure 8.4 (Yang, 1987 Cassidy, 1980 Miller and Stocker, 1999). 

E = Equalization and cocurrent depressurization PD = Purge and cocurrent depressurization BD = Blowdown and countercurrent depressurization P = Purge R = Repressurization... 

Other Cycle Steps A PSA cycle may have several other steps in addition to the basic adsorption, depressurization, and repressurization. 

LO-FIN [Last out-first in] A version of the PSA process for separating hydrogen from other gases. It includes a unique gas-retaining vessel which preserves the concentration gradient in one stream before using it to repressurize another bed. Developed jointly by Toyo Engineering Corporation and Essex Corporation. 

By means of In-situ observation of crystal growth under very high pressure, a defect healing process was studied with p-xylene crystals In p-, m-xylene mixture. Crystals, partially melted by decrease In pressure, were repressurized to grow and to heal their melted surfaces. It was found that the half-melted crystals rapidly grow and recover their rectangular growth shape within a few minutes. 

Repressuring with hydrogen is required during this period. The amount of repressuring required is dependent upon the free space of the pressure vessel. The submitters report lower yields if the pressure falls below 30 p.s.i. No further repressuring is made at the end of 1 hour. 

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