Distillate withdrawal

In Distefano s method, Eqs. (13-149) to (13-161) are solved with an initial condition of total reflux at Lq equal to D R + 1) from the specifications. At t = 0, Lq is reduced so as to begin distillate withdrawal. The computational procedure is then as follows ... 

Fig. 2 Composition profiles for distillate withdrawal at controlled reflux ratio, following the initial startup at total reflux. Number of plates = 7.

In view of the precision demanded of an analytical distillation it is necessary to examine which of the following two courses is the most favourable to take off the distillate as a constant fraction of the condensate, or to operate intermittently, i.e. to return the condensate to the column entirely as reflux for a certain period and next to remove it completely as distillate for another period. In the latter case the reflux ratio is the quotient of the periods of reflux return and of distillate withdrawal. 

With a filter-tip pipet, transfer the dried organic solution into a clean 5.0-mL conical vial, rinse the sodium sulfate with about 0.2 mL of diethyl ether, and add this rinse to the vial. Equip the vial for simple distillation. Remove the solvent by simple distillation withdraw the distillate from the Hickman stillhead as needed. Alternatively, use rotary evaporation or other techniques to concentrate the solution. The final traces of solvents may be removed by attaching the vial to a vacuum source. After the crude solid residue has dried, determine its melting range, which may be wide. ... 

Computer studies have shown that there is a program of distillate withdrawal which will recover the maximum amount of product of specified average composition in a specified time interval. Figure 11.28 shows how this program falls between that of constant distillate rate and constant distillate composition. In effect, the final bottoms composition will be lowest because both y and D/V are low when distillation is terminated. 

Hydrolysis (or saponification) of n-butyl acetate. Boil 4-5 g. of n-butyl acetate (Section 111,95) with 50 ml. of 10 per cent, sodium hydroxide solution under reflux until the odour of the ester can no longer be detected (about 1 hour). Set the condenser for downward distiUation and coUect the first 10 ml. of distillate. Saturate it with potassium carbonate, aUow to stand for 5 minutes, and withdraw all the Uquid into a small pipette or dropper pipette. AUow the lower layer of carbonate solution to run slowly into a test-tube, and place the upper layer into a small test-tube or weighing bottle. Dry the alcohol with about one quarter of its buUr of anhydrous potassium carbonate. Remove the alcohol with a dropper pipette and divide it into two parts use one portion for the determination of the b.p. by the Siwoloboff method (Section 11,12) and convert the other portion into the 3 5-dinitrobenzoate (Section III, 27) and determine the m.p. 

Assemble the apparatus shown in Fig. 1 V, 67, 1 this is self-explanatory. The distilling flask has a capacity of 250 ml. and the beaker contains 150 ml. of 10 per cent, sodium hydroxide solution. All corks must fit well and should be coated with paraflSn wax (by dipping into molten wax, and allowing to drain). Place half of the yield of the dry phenyldiazonium fluoborate in the distilling flask. Heat the solid gently with a small luminous flame at one point near its surface until decomposition begins withdraw the flame and allow the reaction to continue... 

Distillation appHcations can be characterized by the type of materials separated, such as petroleum appHcations, gas separations, electrolyte separations, etc. These appHcations have specific characteristics in terms of the way or the correlations by which the physical properties are deterrnined or estimated the special configurations of the process equipment such as having side strippers, multiple product withdrawals, and internal pump arounds the presence of reactions or two Hquid phases etc. Various distillation programs can model these special characteristics of the appHcations to varying degrees and with more or less accuracy and efficiency. 

It would be possible to remove all of the heat by pumping cold reflux from the distillate drum to the top of the tower and thus eliminate the cost of the pumparound circuit. Where more than one sidestream is withdrawn, however, it is usually economical to withdraw part of the heat in a pumparoimd reflux system farther down the tower. The following economic factors affect the choice ... 

Aqueous Drawoffs from Hydrocarbon Vessels - Water or aqueous materials that are withdrawn continuously or intermittently from vessels where they directly contact hydrocarbons (e.g., process water from distillate drums, and spent wash water or spent caustic solution from settlers) must be disposed of in such a way that entraimnent or inadvertent withdrawal of hydrocarbon will not create a hazard. Disposal is therefore a function of hydrocarbon category, as follows ... 

The Alphabutol process (Figure 7-8) operates at low temperatures (50-55°C) and relatively low pressures (22-27 atm). The reaction occurs in the liquid phase without a solvent. The process scheme includes four sections the reactor, the co-catalyst injection, catalyst removal, and distillation. The continuous co-catalyst injection of an organo-hasic compound deactivates the catalyst downstream of the reactor withdrawal valve to limit isomerization of 1-hutene to 2-hutene. Table 7-2 shows the feed and product quality from the dimerization process. 

Procedure B (carbonate-free sodium hydroxide). Dissolve 50 g of sodium hydroxide in 50 mL of distilled water in a Pyrex flask, transfer to a 75 mL test-tube of Pyrex glass, and insert a well-fitting stopper covered with tin foil. Allow it to stand in a vertical position until the supernatant liquid is clear. For a 0.1 M sodium hydroxide solution carefully withdraw, using a pipette fitted with a filling device, 6.5 mL of the concentrated clear solution into a 1 L bottle or flask, and dilute quickly with 1 L of recently boiled-out water. 

Intelligent engineering can drastically improve process selectivity (see Sharma, 1988, 1990) as illustrated in Chapter 4 of this book. A combination of reaction with an appropriate separation operation is the first option if the reaction is limited by chemical equilibrium. In such combinations one product is removed from the reaction zone continuously, allowing for a higher conversion of raw materials. Extractive reactions involve the addition of a second liquid phase, in which the product is better soluble than the reactants, to the reaction zone. Thus, the product is withdrawn from the reactive phase shifting the reaction mixture to product(s). The same principle can be realized if an additive is introduced into the reaction zone that causes precipitation of the desired product. A combination of reaction with distillation in a single column allows the removal of volatile products from the reaction zone that is then realized in the (fractional) distillation zone. Finally, reaction can be combined with filtration. A typical example of the latter system is the application of catalytic membranes. In all these cases, withdrawal of the product shifts the equilibrium mixture to the product. 

Modify the program to allow for continuous feeding of fresh feed into the still during the process of distillation. The feed rate should be adjusted to equal the rate of withdrawal of distillate. What is the effect of feeding on the distillation ... 

For reactions at atmospheric pressure, standard laboratory glassware such as round-bottomed flasks or simple beakers from 0.25 to 2 L can be used. A protective mount in the ceiling of the cavity enables the connection of reflux condensers or distillation equipment. An additional mount in the sidewall allows for sample withdrawal, flushing with gas to create inert atmospheres, or live monitoring of the reaction with a video camera. Most of the published results in controlled MAOS have been obtained from reactions in sealed vessels, and thus in the following mostly accessories for sealed-vessel reaction conditions are described. 

A mixture of water and ethyl alcohol containing 0.16 mole fraction alcohol is continuously distilled in a plate fractionating column to give a product containing 0.77 mole fraction alcohol and a waste of 0.02 mole fraction alcohol. It is proposed to withdraw 25 per cent of the alcohol in the entering stream as a side stream containing 0.50 mole fraction of alcohol. 

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