Лако-красочные материалы — производство

This subject is discussed in the section entitled “Vacuum Distillation in the Laboratory and Plant” (page 341).

1. Steam Distillation

Distillation with steam in plant operations is a relatively expensive process because it requires large volumes of steam and cooling water. For laboratory work, however, it is a very suitable method for the smooth separation of organic products, which are volatile with steam, from non-volatile inorganic impurities and high molecular tarry by­products. The separation of isomers is also possible in those instances where only one of the isomers is volatile with steam (e. g., o — and p- nitrophenols, page 147). Volatile acids and basic compounds can be separated from each other, and from neutral products, by successive steam distillations from acid and alkaline solution.

If the steam is taken from a steam line, it is advisable to insert, be­tween the line and the distillation flask, a safety flask fitted with a verti­cal tube. This safety flask serves both to collect the condensed water carried along with the steam and to permit control of the pressure (2, in Fig. 23a, page 141). If the steam is produced in the laboratory, the steam kettle is provided with a safety tube.

A long-necked round-bottomed flask is most suitable for the distilla­tion flask. It is mounted at an angle so that the liquid thrown up when steam is introduced hits the side of the flask and not its neck. (The tubu­lated receivers formerly used in distillation from retorts are especially suitable. The tube serves for introducing the steam, and the neck for tak­ing off the distillate.) The tube through which the steam enters should extend to the bottom of the distillation flask. The tube carrying the dis­tillate to the condenser should be wide and beveled at the bottom, so that drops of the liquid* are not carried over into the condenser, but run back into the flask. The flask should not be more than one third

full, so that the liquid is prevented from bumping or foaming over. If for any reason the distillation flask must be set upright, or if a kettle must be used, then it is essential that the delivery tube be provided with a trap to catch entrained liquid. The distillation flask should be heated during a steam distillation or it will collect too large a volume of water. A hot air bath is a good method of heating for this purpose.

Since the heat of vaporization of water is very large, intensive cool­ing is required if a steam distillation is to be done at a rapid rate. In distilling materials which are liquid at room temperature, it is advisable to use two water cooled condensers arranged one after the other. For the distillation of materials which solidify, only one water cooled con­denser is used, possibly in conjunction with an air cooled condenser. These condensers should be thin-walled and have wide inner tubes not tapered at the end. Any solid distillate forming in the condenser can be remelted by occasionally interrupting the flow of cooling water. It may also be possible to run the condenser warm enough to prevent solidifi­cation of the distillate, and to use as a receiver a round-bottomed flask cooled by a stream of water. For distilling high melting materials which unavoidably solidify in the condenser, one can only use a straight wide condenser which is equipped with a suitably bent, thick wire, or glass, push-rod (cf. 6 in Figure 23a, and 5 in Figure 23b, page 141).

Considerable quantities of volatile materials may be occluded and held back if a large amount of tarry material separates out in the dis­tillation flask. In such cases, it is advisable to remove the tar after it has cooled, break it up as finely as possible, then return it to the flask for further distillation.

Separation of a mixture of substances by fractional steam distilla­tion is not possible to any degree unless the difference in volatilities with steam is very large. Sometimes it is possible to separate compounds differing in basicity or acidity. For example, a mixture of amines may be subjected first to steam distillation from a solution which is quite strongly acid, to distill out the least basic amine. Then the mixture is made less acidic and the distillation is continued to remove an amine which is somewhat more basic than the first one. Finally, distillation from neutral or alkaline solution is carried out to carry over the most strongly basic compound.

The distillation of compounds which are only slightly volatile with steam can be accelerated appreciably by superheating the steam. Of course, nothing would be gained by introducing superheated steam into an aqueous solution or suspension, since a temperature above the boil­ing point of the mixture could not be attained in any case. Hence, the

substance to be distilled must be used in the dry state, or if water is present, this must be removed previously. The distillation flask is heated to the same temperature as the steam, preferably with an oil bath. Where strong superheating is used, the connection between the steam heater and the tube through which the steam enters the flask cannot be made with rubber tubing. Only cork, or metal packed with asbestos, should be used.