Distilling Heads reflux ratio

A mixture of 20 g (0.1 mol) of aluminum isopropoxide, 0.1 mol of an aldehyde or a ketone and 100 ml of dry isopropyl alcohol is placed in a 250 ml flask surmounted by an efficient column fitted with a column head providing for variable reflux. The mixture is heated in an oil bath or by a heating mantle until the by-product of the reaction - acetone - starts distilling. The reflux ratio is adjusted so that the temperature in the column head is kept at about 55° (b.p. of acetone) and acetone only is collected while the rest of the condensate, mainly isopropyl cohol (b.p. 82°), flows down to the reaction flask. When no more acetone is noticeable in the condensate based on the test for acetone by 2,4-dinitrophenylhydrazine the reflux regulating stopcock is opened and most of the isopropyl alcohol is distilled off through the column. The residue in the distilling flask is cooled, treated with 200 ml of 7% hydrochloric acid and extracted with benzene the benzene extract is washed with water, dried and either distilled if the product of the reduction is volatile or evaporated in vacuo in the case of non-volatile or solid products. Yields of the alcohols are 80-90%. 

Otherwise expressed, the number of theoretical plates required for a given separation increases when the reflux ratio is decreased, i.e. when the amount of condensed vapour returned to the column is decreased and the amount distilled off becomes greater. The variation in the reflux ratio is achieved by the use of a suitable take-off head (or still-head), usually of the total condensation variable take-off type. In use, all the vapour is condensed and the bulk of the condensate is returned to the fractionating column, small fractions of the condensate being allowed to collect in a suitable receiver. The design may be appreciated from the line diagram shown in Fig. 2.107 in which the controlled collection of distillate is by the socket-cone screw-operated valve sited just below the condenser drip end. 

In a l-l. three-necked flask equipped with a dropping funnel, a thermometer, and an efiicient fractionation column fitted with either a vapor- or liquid-splitting head (Note 1) is placed 400 ml. of mineral oil. The oil is heated to 240-270 and dicyclopen-tadiene (Note 2) is added at the rate of 5-10 ml. per minute. The reflux ratio and the rate of addition of dicyclopentadiene are adjusted to maintain the distillation head temperature at 40°. The cyclopentadiene is collected in a Dry Ice-acetone receiver (Note 3). 

A mixture of 41 g. (0.25 mole) of trichloroacetic acid and 53 g. (0.375 mole) of benzoyl chloride is placed imder an efficient fractionating column and heated. The trichloroacetyl chloride is removed from the top of the column as rapidly as it is formed. The head temperature should not be allowed to go above 119°. The distillate is redistilled through the same column. During the first part of the distillation, a high reflux ratio should be maintained, since in this way hydrogen chloride present in the material from the first distillate is eliminated. There is obtained 23 g. (51%) of trichloroacetyl chloride boiling at 116-119° at atmospheric pressure. 

A 500-ml. two-necked round-bottomed flask is attached to a distillation column (25 X 2 cm.) packed with Fenske helixes and fitted with a slow variable-reflux-ratio distillation head. One hundred milliliters of dry ethanol is placed in the flask, and 7.7 g. (0.335 mol) of sodium is added slowly. Then 31.5 g. (0.355 mol) of phenol dissolved in 50 ml. of ethanol is added and the mixture is boiled. The ethanol is... 

A distillation head of the type shown in Fig. 6 allows fractions to be removed without disturbing the vacuum, and it cilso allows control of the reflux ratio (Chapter 5) by manipulation of the condenser and stopcock A. These can be adjusted to remove all material that condenses or only a small fraction, with the bulk of the liquid being returned to the distilling column to establish equilibrium between descending liquid and ascending vapor. In this way liquids with small boiling-point differences can be separated. 

With any column it is necessary to return a major portion of the condensate at the head in order to maintain equilibrium. A good general rule is that the reflux ratio should be about equivalent to the number of theoretical plates at total reflux. Thus, a 30-cm Vigreux column having about three theoretical plates should be operated at a reflux ratio of about 3 1. This reflux ratio is maintained by an adjustment of the distilling head (see below). 

Removing a reaction by-product efficiently can be necessary for good product yields and purities. In the semibatch method of preparing pentenoic acid ethyl ester (see Figure 13.9), efficient reaction of allyl alcohol and removal of the byproduct EtOH were necessary [15].The allyl alcohol was introduced subsurface, to prevent loss by flash volatilization in the head space over the reaction surface. EtOH was displaced from the reaction by operating at a temperature significantly above EtOH s boiling point. A low reflux ratio of about 4 1 (moles of liquid returned to the system moles of EtOH collected as distillate [16]) was used to ensure that the EtOH was not returned to the reaction. 

In a simple distillation, the distilling head holds the thermometer and connects the pot to the condenser. In fractional distillation, the distilling head must provide some means to alter the reflux ratio, and some even contain a condenser. 

Extractive Distillation of Liquid-Liquid Extract. The extract portion was fed into a continuous still, 2 inches in diameter, made up of the following sections in ascending order a 100-ml capacity reboiler, a vacuum-jacketed, silvered column 3 feet long containing helipak, an intermediate feed section, and a vacuum-jacketed silvered 15-plate Older-shaw column, another feed section and a 10-plate Oldershaw column, topped off with a solenoid operated liquid dividing reflux head. The extract entered the still through a preheater at the lower feed section. The reflux ratio and boil-up rate were adjusted to fix the bottoms compo-... 

The field of engineering contains many examples of trade-offs. You have seen some of them in previous courses. In distillation there is the classical trade-off between the number of trays (height) and the reflux ratio (energy and diameter). In heat transfer there is the trade-off between heat exchanger size (area) and pressure drop (pump or compressor work) more pressure drop gives higher heat transfer coefficients and smaller areas but increases energy cost. We have mentioned several trade-offs in this book control valve pressure drop versus pump head, robustness versus performance, etc. 

The apparatus must be adequately insulated and the column jacket provided with the requisite heat compensation (section 7.7.3). The use of an automatic column head (section 7.5.3) allows the distillation to be performed with a minimum of attention. In the present case it would be useful to place a contact thermometer, connected to a bell via a relay (section 8.2.2), in the column head. The thermometer would be set to the boiling point of benzene, and when a signal was heard, the initial reflux ratio of 9 would be increased, say, to 20. In the same way it would be progressively raised to the final value of 50. The load may conveniently be regulated by means of a contact manometer actuated by the pressure differential (section 8.4.2). In accordance with the amount of the charge the still pot may be a pilot-plant flask (section 7.6.1) or a round-bottomed flask of 4 to 10 litres capacity, heated by a flask heater (section 7.7.1). 

---