Still pot heating

S = still pot heating and control, 4—9a = distillation apparatus with heating jacket, still pot, Claisen still head, adapter and receiver, 10, II = bubble counters with boiling capillary, 12a, IS = manometers with inclined tube, 17,19 = photo-cells, 14—16 — recorder with thermostat and amplifier, 20 = pressure controller... 

For packed columns, the load is relatively easy to control via the pressure drop whereas concentric-tube columns with pressure drops of the order of 10 torr required a novel control system. Fischer and Weyand [10] solved the problem by electronically measuring the reflux dropping rate. Depending on the desired load the still pot heating is controlled linear-proportional to the difference of actual and preprogrammed drop number. Thus it is possible to cover loads from 50 to 250 ml/h exactly and absolutely reproducibly, the pressure in the still pot ranging from 760 to 10 torr. 

Heat input to the still-pot should remain constant. 

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. 

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... 

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. 

Each still consists of an iron pot covered with a leaden lid to which are luted, say, two earthenware arms each of which is connected with a train of about five stoneware aludels or udells supported on a wooden framework as illustrated in Fig. 13. Each udell has a stoneware stopper below so as to permit condensed water (with the 3 halogens in solution) to be drained off as required. Manganese dioxide is added to the liquid. The still is heated by an open Are, and iodine and steam are evolved. The liquor from the still is a troublesome waste product. The udells are emptied when required. The bromine is in too small a quantity to pay to collect. The iodine from the udells requires further purification. 

It is important to give the still pot an initial charge which substantially exceeds the volume of the upper reservoir so that the still pot will not go dry. It is veiy important to avoid letting the heated still pot go dry. The heated flask may crack and the solid contents in the pot may decompose to contaminate the system. In some cases an explosion may occur (e.g. when LiAIH4 is used see below). A metal catch pan should be positioned under each distillation apparatus to contain any spills which may occur upon solvent removal. Each still should also be vented to a fume hood. 

Gentle heating of a 1 1 chloroform solution of the stannane led to a mild explosion. Similar explosions had been experienced at the conclusion of fractional distillation of other bis(dialkylamino)stannanes where the still pot temperature had risen to about 200° C. 

Batch distillation (Fig. 16) is often preferable to continuous distillation when small quantities of feed material are processed. A liquid feed is charged to a still pot and heated until vaporization occurs. Vapor leaves the top of the column, and after condensation, part is removed as product and the rest returned to the column as reflux. As distillation proceeds, the contents of the still pot and the overhead product become richer in less volatile components. When operated at a fixed reflux ratio, an overhead product cut is collected until the product composition becomes unaccceptable. As an alternative, the reflux ratio can be gradually increased to hold the product composition constant as the cut is taken. For a fixed rate of heat addition to the still pot, the latter option results in a steadily declining product flow rate. After the first cut, subsequent... 

The apparatus originally attached to the three necks of the reactor is quickly replaced by stoppers (rubber or ground glass) and a Vigreux fractionating column adapted for vacuum distillation. The pressure within the system is slowly reduced to about 25 mm. After the dissolved hydrogen chloride and the unreacted ethanol have been removed, the contents of the still pot are heated slowly. A fraction obtained over the range from room temperature up... 

The number of theoretical plates may be determined by allowing a mixture of about 15 mole per cent of the more volatile component and 85 mole per cent of the less volatile component of a suitable mixture (Table 1-10) to reflux through the column (total reflux) until equilibrium is reached. This is judged by the constancy of some physical property (usually the refractive index) of very small fractions drawn from the head of the column. When equilibrium is reached, a sample is withdrawn from the still pot. There are several ways of removing the sample. One of the simplest is to use a flask with a side arm and to withdraw a sample through this opening. It is often possible to drop the heating bath and then rapidly remove and stopper the flask before the material contained in the column drains into it. 

The still pot is then heated so that the liquid boils gently, and a steady reflux is returned from the head of the column. The jacket temperature is adjusted to correspond to the vapor temperature at the head if a thermometer is used, or it is adjusted to adiabatic operation if a differential thermocouple is employed. The boil-up rate (throughput) is adjusted to a value which is appropriate for the column being used (Table 1-12) by regulating the amount of heat supplied to the still pot. The column is allowed to achieve equilibrium before any material is withdrawn. This is usually determined by the constancy of the vapor temperature or of the refractive index of the material at the column head and usually requires from one-half to several hours. The time necessary for establishing equilibrium is usually greatest for the columns with the highest number of theoretical plates. 

The still pot is usually attached to the column by a standard taper joint. Leakage often occurs at this point after the grease has been softened by the heated vapors. This may be avoided by the use of Teflon sleeves instead of grease. For prolonged distillations, such as... 

Round-bottom or pear-shaped flasks are normallv used as still pots, and these may be heated using an oil bath, an air bath, or a heating mantle. The heating mantles are the least desirable of the three, because they supply an excessive amount of heat to the sides of the... 

In operation, the sample container is attached to the joint on the Till pot, and the entire column is evacuated down to the stopcock on the sample container. The still pot is cooled in liquid nitrogen, and the sample is allowed to distill into it. The stopcock on the still pot is closed, and a Dewar is placed around the latter. A small amount of heat is then supplied by the heater, and the pressure in the column is allowed to increase. When the desired operating pressure, which should be fairly high in order to decrease the vapor velocity in the column, is reached, the coolant is supplied to the condenser at such... 

An air bath is occasionally used, since it is clean and easily adaptable to a variety of sizes and shapes of flasks. Its principal use is in heating still pots in fractional distillation because it will heat both the flask and the connection to the column. It is simply an electrically heated coil placed near the bottom of a glass or metal container into which the flask may be inserted (Fig. 1-37). 

---