Column distillation enriching section

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. 

In operation, a batch of liquid is charged to the pot and the system is first brought to steady state under total reflux. A portion of the overhead condensate is then continuously withdrawn in accordance with the established reflux pohcy. Cuts are made by switching to alternate receivers, at which time operating conditions may be altered. The entire column operates as an enriching section. As time proceeds, composition of the material being distilled becomes less rich in the more volatile components, and distillation of a cut is stopped when accumulated distillate attains the desired average composition. 

The relationships for the section of column above the feed plate, i.e., the enriching section of the column, are exactly the same as those derived previously for the case of the batch distillation column. 

The component balance equations for reflux drum, column enriching section, feed plate, column stripping section and the column base and reboiler are very similar to those derived previously for the binary distillation example CONSTILL, but are now expressed in multicomponent terms as described in Section 3.3.3.4. 

If, from a specified lube distillate we are required to make a given raffinate, R, and extract, E, and are given the reflux ratio for the enriching section of the column, operating points V and 0 for each section may be constructed on the diagram. A line constructed... 

Equation (14.181) may be compared with the corresponding differential equation for the enriching section of a two-stream, close-separation, countercurrent column like a distillation column ... 

Example 8.7 A complex distillation column, equipped with a partial reboiler and total condenser and operating at steady state with a saturated liquid feed, has a liquid side stream draw-of in the enriching section. Making the usual simplifying assumptions ... 

This equation represents a general plate in the enriching section of a binary distillation column. In the usual case, a total condenser is used, so F = L + D, and L/D = R is the recycle ratio, D being the distillate product removed. In the present case, it is the liquid composition that is monitored, so we proceed to eliminate y using the equilibrium relation, Eq. 5.92... 

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. 

Up to this point we have considered conplete distillation columns with at least two sections. Columns with only a stripping section or only an enriching section are also commonly used. These are illustrated in Figures 4-24A and B. When only a stripping section is used, the feed must be a subcooled or saturated liquid. No reflux is used. A very pure bottoms product can be obtained but the vapor distillate will not be pure. In the enriching or rectifying column, on the other hand, the feed is a superheated vapor or a... 

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. 

Gas extraction extends the possibilities of separation processes like distillation, absorption and liquid-liquid extraction to the isolation and purification of components of low volatility. Furthermore, it enables separation of components with very similar properties if used in the countercurrent mode. Process temperatures in gas extraction are determined by the critical temperature of the solvent and not, as is the case of distillation of any kind, by the liquid-vapor transition of the feed mixture. As compared to liquid-liquid extraction, gas extraction makes easily possible to operate a two cascade separation column, applying a stripping and an enriching section. Combined, these possibilites allow gas extraction to be operated at very moderate temperatures and as a separation process for difficult separations. 

Equations for enriching section. In Fig. 11.4-3 a continuous distillation column is shown with feed being introduced to the column at an intermediate point and an overhead distillate product and a bottoms product being withdrawn. The upper part of. the tower above the feed entrance is called the enriching section, since the entering feed of binary components A and B is enriched in. this section, so that the distillate is richer in A than the feed. The tower is at steady state. 

The basic components of a plate distillation column include a feed line, feed tray, rectifying or enriching section, stripping section, downcomer, reflux line, energy-balance system, overhead cooling system, condenser, preheater, reboiler, accumulator, feed tank, product tanks, bottom line, top line, side stream, and an advanced instrument control system. Plate columns hold trays that may be bubble-cap, valve, or sieve. Figure 6-19 shows the basic components of a plate distillation column. 

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