Fractionators enriching section

Fig. 2-23. Charge reboiler fractionator (enrichment section only).

Several descriptions have been pubUshed of the continuous tar stills used in the CIS (9—11). These appear to be of the single-pass, atmospheric-pressure type, but are noteworthy in three respects the stills do not employ heat exchange and they incorporate a column having a bubble-cap fractionating section and a baffled enrichment section instead of the simple baffled-pitch flash chamber used in other designs. Both this column and the fractionation column, from which light oil and water overhead distillates, carboHc and naphthalene oil side streams, and a wash oil-base product are taken, are equipped with reboilers. 

As discussed in Section 15.5.2, the separation of two or more sublimable substances by fractional sublimation is theoretically possible if the substances form true solid solutions. Gillot and Goldberger(10°) have reported the development of a laboratory-scale process known as thin-hlm fractional sublimation which has been applied successfully to the separation of volatile solid mixtures such as hafnium and zirconium tetrachlorides, 1,4-dibromobenzene and l-bromo-4-chlorobenzene, and anthracene and carbazole. A stream of inert, non-volatile solids fed to the top of a vertical fractionation column falls counter-currently to the rising supersaturated vapour which is mixed with an entrainer gas. The temperature of the incoming solids is maintained well below the snow-point temperature of the vapour, and thus the solids become coated with a thin film (10. im) of sublimate which acts as a reflux for the enriching section of the column above the feed entry point. 

The properties of a fractionating column which are important for isotope separation are (1) the throughput or boil-up rate which determines production (2) HETP (height equivalent per theoretical plate) which determines column length (3) the hold-up per plate which determines plant inventory and time to production (4) the pressure drop per plate which should be as small as possible. The choice of a particular column is invariably a compromise between these factors. The separation in a production column is of course less than it would be at total reflux (no product withdrawal). The concentration at any point in the enriching section can be calculated from the transport equation (see, e.g., London 1961)... 

For fractionation column tray efficiency determination, use the alpha ratio (the light key K value divided by the heavy key component K value). Please note that the fractionation column involves both the enriching section and the stripping section, as compared to absorber- or stripping-type columns. 

The fraction of desired isotope in material at the product end of the plant continues to increase, reaching the steady-state value yp at time f2. Product withdrawal is then started at such a rate P f) as to keep product composition constant at yp. Feed is supplied at the rate P(r) + H Cr). At first, product withdrawal is at a rate below the steady-state value P because the compositions elsewhere in the enriching section have not yet reached steady-state values. As time goes on, P(f) approaches P asymptotically. 

In an exact treatment, values of Ci and C5 in the stripping section would differ sli tly from the enriching section because of the slightly different flow profile. In the present approximate treatment, the constants are to be evaluated for the total reflux case in which the flow patterns in both sections are the same. If the net flow rate is a small fraction of the circulation rate, studies by Parker [PI] and others have shown that the effect on Ci and C5 of the changed flow pattern with net flow is small. 

This synthesis is usually carried out in the presence of alkaline catalysts. The catalyst is neutralized and at least partially remains in the final product. Primary Cg-Cig long-chain alcohols are generally used for ethoxylation. The alcohols may be of synthetic (Ziegler or oxo alcohols), vegetable and animal origin. Other hydroxo compounds can be subjected to ethoxylation, for example diols, such as 2,4,7,9- tetramethyl-5-decyne-4,7-diol ( Surfinol ), vegetable sterols, lanoline fractions enriched by sterols and fatty alcohols [34]. Ethoxylated alcohols and mercaptanes modified by a subsequent addition of propylene oxide or ethoxylated and propoxylated alcohols mentioned before (section 1.2.2) are of some industrial importance. 

During distillation the light isotopic spedes is preferentially enriched in the vapour phase according to the Rayleigh fractionation law (Section 4.2.2). This process applies to the evaporation and condensation of meteoric water and accounts for the marked fractionation of and 6D in rainwater and in ice. 

S-Sj. A hot vapor stream containing 0.4 mole fraction ammonia and 0.6 mole fraction water is to be enriched in a distillation column consisting of enriching section and total condenser. The saturated vapor at 6.8 atm pressure (100 psia) is injected at a rate 100 moles/hour at the bottom of the column. The liquid distillate product withdrawn from the total condenser has a composition 0.9 mole fraction NH3. Part of the distillate is returned as reflux, so that 85% of the NHj charged must be recovered as distillate product. 

The use of a rectifying column between the heated vessel and condenser permits a greater control on the distillation process and allows separation from the solids and fractionation of the solvent mixture in a single unit. The temperature of the process is controlled such that only a portion of the condensate is continuously withdrawn such that the column acts as an enriching section. As time proceeds the composition of the material under distillation increases in the less volatile components. Fractionation is achieved by switching to different receiving vessels during the distillation process. 

If there are nondistributing HNKs present, a pinch point of constant conposition will occur at minimum reflux in the enriching section above where the HNKs are fractionated out. With nondistributing LNKs present, a pinch point will occur in the stripping section. For the enriching section in Figure 7-2. the mass balance for conponent i is... 

D12. A distillation system is a packed column with 1.524 m of packing. A saturated vapor feed is added to the column (which is only an enriching section). Feed is 23.5 mole % water with the remainder nitromethane. F = 10 kmol/h. An L/V of 0.8 is required, = Xp = 0.914. Find HETP and water mole fraction in bottoms. Water-nitromethane data are given in Problem 8.El. 

FIGURE 12.5. McCabe-Thiele diagram for complete fractionation process made up of stripping and enrichment sections. (From ref. 1, reprinted with permission of Martinus Nijhoff Publishers.)... 

To analyze the enriching section of a fractionating tower using enthalpy-concentration data, we make an overall and a component balance in Fig. 11.6-2,... 

The feed to be fractionated, which can be a liquid, a vapor, or even a combination of both, enters the column at some central location rather tiian at one of the ends of the cascade as had previously been the case. This results in a division of the column into two parts, the rectification or enriching section above the feed tray, and the stripping or exhausting section below it. The upper section serves to enrich the vapor in the more volatile components, a portion of which is ultimately withdrawn as liquid "overhead product" or "distillate." In the lower section, residual volatile components are progressively stripped off the liquid and conveyed upward as vapor, while the downward flow of liquid becomes enriched in fhe heavier or less-volatile components. 

Consider the fractionator of Fig. 9.17. A heat balance for the top n trays of the enriching section (envelope III) which includes the heat loss is [9]... 

Consider the enriching section of the fractionator of Fig. 9.17. In the absence of heat losses, which can be (and usually are) made very small by thermal insulation for reasons of economy if for no other, Eq. (9.61) can be written... 

If it is sufficiently important, therefore, one can be persuaded that, for alt but exceptional cases, the ratio L/G in the enriching section of the fractionator is essentially constant. The same reasoning can be applied to any section of a fractionator between po mts of addition or withdrawal of streams, although each section will have its own ratio. 

Consider next a section of the fractionator below the point of introducing the feed, shown schematically ii Fig. 9.36a. The trays are again theoretical trays. The rates of flow L and G are each constant from tray to tray, but not necessarily equal to the values for the enriching section. A total material balance... 

Starting with the feed-tray liquid as computed from the enriching section, which shows no heavy components, the mole fractions are all reduced proportionately so that their sum, plus the mole frafctions of the missing components (as shown in the feed-tray composition as computed from the exhausting section) is unity. The bubble point is then recalculated. The justification for this... 

As in the enriching section, we now start making a construction of staircases identifying various stages. However, we need to focus first on the mode of operating with the reboiler. In the reboiler, the fraction ((W - W j,) /W of the downcoming liquid flow rate is evaporated this fraction is called the ercternal reboil ratio (or boilup ratio) the remaining fraction Wttb/W f) becomes the bottom liquid product... 

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