Packings vapor entrainment

An alternative explanation, preferred by the author, is in terms of mechanisms postulated by Kurtz et al. (31a). As liquid rate increases, more vapor is entrained down the bed. This drops efficiency. Because structured packings permit far less lateral movement of fluids than random packings, far more vapor will be carried downward. The vapor entrainment will be most detrimental to efficiency when fluid lateral movement is restricted most. This can be expected with narrow flow channels (e.g., wire-mesh structured packings), at high liquid rates and high pressure. 

At high liquid loads (> 15 gpm/ft of bed cross section) and high pressures, vapor entrainment in the liquid may restrict structured packings capacity well before flooding is approached. This phenomenon causes efficiency to rapidly diminish as throughout is raised. Random packings also experience this type of limitation (386), but to a lesser extent, because of the unrestricted lateral movement of vapor and liquid. 

The bottom section of the main column provides a heat transfer zone. Shed decks, disk/doughnut trays, and grid packing are among some of the contacting devices used to promote vapor/liquid contact. The overhead reactor vapor is desuperheated and cooled by a pumparound stream. The cooled pumparound also serves as a scrubbing medium to wash down catalyst fines entrained in the vapors. Pool quench can be used to maintain the fractionator bottoms temperature below coking temperature, usually at about 700°F (370°C). 

Extractive Distillation. In extractive distillation a fraction comprising compounds of similar volatility is vaporized and passed countercurrent to a liquid solvent stream in a packed or bubble cap tower. The operating conditions of temperature and pressure are regulated so that one or more of the components of the mixture are dissolved in the entrainer and removed in a liquid phase extract, while the remaining vapor is taken overhead and condensed or discharged as gaseous effluent. 

Figure 5.B-9 shows effective interfhcial areas that were deduced by Shulman et al.N from experiments on tha sublime ion of naphthalene Rase hi g rings. Many other studies have been inede in na attempt to relate effective area to specific packing surface, as a function or liquid and vapor flow rates. Generally, it is thought that the effective area reachas the specific area near the onset of flooding, but Pig. 3.8-9 shows that this mey not be the case. It also should be noted that the total area available for main transfer includes film surface ripples, entrained liquid within the bed, and vapor bubbling through pockets of liquid held up in the bed. Another important point relative specific surface of differeat packings might indicate relative mass transfer efficiencies, but this Is often sot the case.

Collection of sample vapors in the carrier gas effluent is performed automatically or manually, initiated by the detector signal. In purpose-designed instruments, injection and sample collection are automated for unattended operation. The exit from the detector splitter is usually led out through a side wall of the oven and thermostatted to avoid condensation of the sample. Several methods are used to trap samples, ineluding packed and unpacked cold traps, solution and entrainment traps, total effluent and adsorption traps, Volman traps, and electrostatie precipitators. Figure 11.12 [219-223]. 

The author experienced one troublesome case, which was also reported by Lieberman (237), where liquid overflow through the chimneys caused a severe loss of efficiency in the packed section above. The chimney tray had undersized downpipes that were not liquid-sealed either the undersizing or the lack of seal (or both) could have caused the overflow. Lieberman (237) suggests that the overflow led to entrainment and flooding, hence the loss in efficiency. However, subsequent pressure-drop measurements and other observations provided no supporting evidence for the existence of flooding, and the author believes that vapor maldistribution due to liquid overflow (guideline 14 above) caused the loss in efficiency. 

I-beam interference can be just as troublesome in the space above a chimney tray. In one case history contributed by D. W. Reay (334), this interference is believed to have led to severe vapor maldistribution in a refinery vacuum tower (Fig. 8.66). The maldistributed vapor profile was displayed as a carbon deposit on the siuTace of the bottom packing. The deposit formed an annular ring about 5 ft wide that extended about 1 in into the bed. In that case, liquid was known to overflow the chimneys for several months because of an incorrect location of level tappings. This overflow caused liquid entrainment. Some entrained droplets ultimately carbonized on the base of the bed. Had the vapor profile been uniform, entrainment (and therefore deposit laydown) would have been more uniform. It is believed that vapor from the side chimneys was blocked by the beams and preferentially ascended around the periphery. If liquid overflow (down the risers) had been uneven, the maldistribution could have been further aggravated. 

The results in Table V illustrate the influence of sample weight and heating rate on product yields. The data show that increasing sample size reduces the tar yields and increases the char yields. The drastic decrease in tar yields is primarily due to increased vapor residence time in the reaction bed. If the vapor residence time is reduced in the reaction bed by using a fluidized bed or an entrained flow reactor, then secondary decomposition can be significantly reduced, and the effects of sample weight will not be as drastic. Thus, data collected in the loosely packed fixed-bed reactor of the present study may represent an extreme for an operating industrial reactor. 

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