Pot stills

The simplest form of distiUation involves boihng a multicomponent liquid mixture batchwise in a single-stage still pot. At any instant in... 

As a first step in the calculation, the minimum-reflux ratio should be determined. In Fig. 13-100, point D, representing the distillate, is on the diagonal since a total condenser is assumed and Xo = yo- Point F represents the initial condition in the still pot with coordinates ip, y. Minimum internal reflux is represented by the slope of the line DF,... 

Heat input to the still-pot should remain constant. 

A small amount of dimethylamine is always present in the crude product. This is removed before collection of the product by operating the column under total reflux for 1-2 hours. The vent of the still head should be connected to a hood during this operation. If foaming does not subside after this treatment the use of a 1-1. still pot is indicated. 

Wiped film stills in place of continuous still pots —Centrifugal extractors in place of extraction columns —Flash dryers in place of tray dryers —Continuous reactors in place of batch —Plug flow reactors in place of CFSTRs —Continuous in-line mixers in place of mixing vessels... 

The reaction mixture was removed from the vessel and distilled at a pressure of 30-60 mm, and a bath temperature of 30°C to 50°C until the methanol had all been removed. The extremely viscous tarry residue remaining in the still pot was given a very crude distillation, the distillate boiling at B2°C to 1 32°C/2 mm. In an attempt to purify this distillate by a more careful distillation, 5.3 g of a liquid distilling from 53°C to 150°C/5 mm was collected. At this point, much solid sublimate was noted not only in this distillate but in the condenser of the still. 7 g of the solid sublimate was scraped out of the condenser of the still. Recrystallization of the sublimate from ethyl acetate containing a small amount of petroleum ether gave beautiful crystals melting at 175°C to 177°C (5 g). Infrared analysis confirmed that this compound was hydroquinone (9% conversion). 

V = quantity of vapor, moles W = contents ol still pot, moles... 

W i = Contents of still pot or ketde at any point, 1, after start for components, i, mols Wio = Initial contents of ketde or still pot, mols, for component, i... 

Wo = Mols liquid mixture originally charged to still pot w = Pounds coolant per hour... 

Wade, P. A. et al., J. Amer. Chem. Soc., 1987,109, 5052-5055 A grossly impure sample of the ( ) mixed isomers decomposed vigorously when distilled at 130°C. During steam distillation of the R, S isomer, overheating of the still pot caused vigorous decomposition destruction of the pot residues with nitric acid was also vigorous for both. 

Alkali-washed material, stabilised with 0.25% of pyrogallol, was distilled at 103°C/4 mbar until slight decomposition began. The heating mantle was then removed and the still-pot temperature had fallen below its maximum value of 135°C when the residue exploded violently [1], The presence of solid alkali [2] or 5% of phenolic inhibitor is recommended, together with low-temperature high-vacuum distillation, to avoid formation of acidic decomposition products, which catalyse rapid exothermic polymerisation. 

Wiped film stills in place of continuous still pots... 

The simplest example of batch distillation is a single stage, differential distillation, starting with a still pot, initially full, heated at a constant rate. In this process the vapour formed on boiling the liquid is removed at once from the system. Since this vapour is richer in the more volatile component than the liquid, it follows that the liquid remaining becomes steadily weaker in this component, with the result that the composition of the product progressively alters. Thus, whilst the vapour formed over a short period is in equilibrium with the liquid, the total vapour formed is not in equilibrium with the residual liquid. At the end of the process the liquid which has not been vaporised is removed as the bottom product. The analysis of this process was first proposed by Rayleigh(24). 

Figure 3.14 shows a typical batch distillation column Fresh feed is charged into the still pot and heated until it begins to boil. The vapor works its way up the column and is condensed in the condenser. The condensate liquid runs into... 

The eolunin is run on total reflux until the overhead distillate eomposition of the lightest component (component 1) reaches its specification purity. Then a distillate product, which is the lightest component, is withdrawn at some rate. Eventually the amount of eomponent 1 in the still pot gets very low and the Xdi purity of the distillate drops. There is a period of time when the distillate contains too little of component 1 to be used for that product and also too little of component 2 to be used for the next heavier product. Therefore a slop cut must be withdrawn until Xj)2 builds up to its specifieation. Then a seeond product is withdrawn. Thus multiple produets can be made from a single eolumn. 

Theoretical trays, equimolal overflow, and constant relative volatilities are assumed. The total amount of material charged to the column is M q (moles). This material ean be fresh feed with composition Zj or a mixture of fresh feed and the slop cuts. The composition in the still pot at the begiiming of the batch is Xgoj. The composition in the still pot at any point in time is Xgj. The instantaneous holdup in the still pot is Mg. Tray liquid holdup and reflux drum holdup are assumed constant. The vapor boilup rate is constant at V (moles per hour). The reflux drum, eolumn trays, and still pot are all initially filled with material of eomposition Xg j. 

TIME=TIME+DELTA HB=HB+DHDtDELTA IF(HB.LE.0.)THEN WRJTE(6, ) STILL POT EMPTY ... 

Three products (PI, P2, aod P3) and two slop cuts (SI and S2) are produced. The average composition of the products are 95 mole percent. The PI product is mostly the lightest component (component 1). The P2 product is mostly intermediate component (number 2) with some impurities of both the light and the heavy components. The final product P3 is what is left in the still pot and on the trays. The times to produce the various products and slop cuts are given in the results shown in Table 5.14. The total time for the batch distillation in this example is 6.4 hours. 

During the early part of the distillation when a substantial amount of benzyl bromide is present, the still pot should not be heated above 140° to avoid serious discoloration. When the bulk of the benzyl bromide has been removed, the temperature of the still pot may be raised to 150-160° to facilitate distillation of the product. 

A still pot of at least 100-ml. capacity should be used for the distillation since foaming occurs as the distillation proceeds. 

The checkers used a 50-cm. spinning-band column.4 In order to minimize resinification of the cydohexyl isocyanide, distillation should be as rapid as possible and the temperature in the still pot should not exceed 90°. 

The nucleate type of boiling is far more common than any other type. This form usually occurs in commercial evaporators, still pots, and similar boiling equipment. Much attention has been given to nucleate boiling and a good many data are available. Because the critical temperature difference is the upper limit to nucleate boiling, it too has received notice. The number of data for it is far from ample, because of the burnout difficulty discussed in Sec. IID. 

Caution. Diethyl ether used as a solvent should be free of peroxides before further drying and distillation from lithium tetrahydroaluminate(l-) is attempted. During distillation, caution must be exercised never to permit the still pot to run dry. 

At this point about 150-200 ml. of liquid remains in the still pot. If the distillation is continued, the peppermint-like odor of the product, 2,4,4-trimethylcyclopentanone, can be detected in the distillate. 

Details of the lower end of the column are shown in Fig. 2. S is a rubber stopper to fit the still pot, and L and K are cork stoppers. The outer jacket (G) ends at the bottom of K. M is a stack of asbestos disks which serve as a bumper between the stoppers S and K they are indispensable for operation under reduced pressure. The disks are easily removable and provide for flexibility in the choice of stopper sizes. Still pots of 250 ml. to 12 1. can be attached to this column. 

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