Purge enrichment

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. 

Use of a steel chisel to open a drum of carbide caused an incendive spark which ignited traces of acetylene in the drum. The non-ferrous tools normally used for this purpose should be kept free from embedded ferrous particles [1], If calcium carbide is warm when filled into drums, absorption of the nitrogen from the trapped air may enrich the oxygen content up to 28%. In this case, less than 3% of acetylene (liberated by moisture) is enough to form an explosive mixture, which may be initiated on opening the sealed drum. Other precautions are detailed [2], Use of carbon dioxide to purge carbide drums, and of brass or bronze non-sparking tools to open them are advocated [3],... 

As an example. Fig. 18 shows CP/MAS NMR spectra recorded during the investigation of surface ethoxy species (7S) formed on acidic zeolite HY ( si/ Ai = 2.7) by a SF protocol. Figure 18a shows the CP/MAS NMR spectrum recorded after a continuous injection of C-1-enriched ethanol, CHI CHzOH, into the MAS NMR rotor reactor containing calcined zeolite HY. The ethanol was injected at room temperature for 10 min. Subsequently, the loaded zeolite was purged with dry nitrogen (200 mL/min) at room temperature for 2h. 

Fig. 27. CF MAS NMR spectra recorded at 723 K during the conversion of pure C-enriched methanol on zeolite CsOH/Cs,NaX fa), after purging with dry nitrogen (b), and during the conversion of mixtures of toluene and C-enriched methanol with molar ratios of 3 1 (c) and 1 1 (d). Reproduced with permission from (23f). Copyright 2000 Elsevier Science.

To unambiguously elucidate the reactivity of surface methoxy species, the preparation of pure methoxy species on the catalyst surface is an important prerequisite. This preparation can be achieved by a SF protocol, which starts with a flow of C-enriched methanol into acidic zeolites at room temperature, followed by a purging of the catalyst with dry nitrogen at room temperature and subsequently at higher temperatures (74,262. The latter step progressively removes the surplus of methanol and DME, together with water produced by the conversion of methanol. 

Fig. 33. Y HPDEC/MAS NMR (left) and CP/MAS NMR (right) spectra of zeolite HY (na/ Wai = 2.7) recorded during the formation of methoxy species (56.2 ppm) at various temperatures. The spectra were obtained after a continuous injection of C-enriched methanol into the MAS NMR rotor reactor at room temperature for 20 min (a) and after a subsequent purging with dry nitrogen (200 mL/ min) at 298 K (b), 373 K (c), 413 K (d), 433 K (e), 453 K (1), and 473 K (g). The temperature treatments were performed for 2h at each step. Asterisks denote spinning sidebands. Reproduced with permission from (262). Copyright 2003 Elsevier Science.

Fig. 34. SF CP/MAS NMR spectra recorded at 433 K after stopping the conversion of C-enriched methanol (tt, / = 40gh/mol) on zeolite HY (nsi/tiAi = at 423 K and purging the catalyst with dry carrier gas (a). Spectra (b) and (c) were obtained 10 min and 1.0 h, respectively, after starting the flow of C-enriched methanol (Wj F = 40 g h/mol) at 43 3 K. Asterisks denote spinning sidebands. Reproduced with permission from (74). Copyright 2001 American Chemical Society.

The purge-and-trap method (see Section 6.4) is a common method to enrich volatile organic compounds from water samples. In your apparatus, you purge a 1 L water sample with a gas (air) volume flow of 1.5 L gas per minute at a temperature of 25°C. The compounds that you are interested in include tetrachloroethene, chlorobenzene and methyl-t-butylether (MTBE). Calculate the time required to purge 90% of each compound from the water. Any comments How much time would you save if you would increase the temperature from 25°C to 35°C What could be a problem when raising the temperature too much You can find all necessary data in Appendix C and in Table 6.3. 

The effect of feed composition on enrichment at maximum recovery is shown in Table 2. Both high enrichment and good recovery are obtainable with the leaner feed composition, a result tfiich is counter to intuition. The explanation lies in the fact that light gas losses in the purge and blowdown gas decrease as the feed becomes leaner in the light gas. 

E enrichment of ligh gas in product stream H molar purge-to-total product ratio i cell number... 

Is the recycle affecting the purge streams If so consider raw material pretreatment. Nonreactive materials in the feed stream are responsible for the purge and reducing their quantity would reduce the purge. For example, if oxygen-enriched air is used instead of air in a typical partial oxidation process, the quantity of the purge and the associated waste can be reduced. 

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