The moderate cooling of fluids by the effect of a current of cold water is an essential condition of the condensers attached to stills, and this part of the subject will be found discussed under Distillation. Much the same principle is employed in refrigerators for cooling brewers' worts, the object being to attain the maximum exposure of the wort in the least possible time and space.
An equally important but far less developed application of cold to solutions is with a view to separating their valuable portion from the accompanying water. On this subject, Prof. Mills remarks that " theoretically it comes to much the same thing whether you get a substance separated out by means of heat or cold. Cooling is effected by a heat engine, but universally the nature of the substance must have a very material influence, and that alone may decide as to whether we ought to apply heat or cold in any particular case. Ellis has proved the economy in the application of cold in the case of soda sulphate, owing to the peculiar property it has of crystallising out in great abundance at low temperature. In this case, the mere application of heat raises no objection, because the sulphate is an object which you can treat as severely as you please by means of heat. This process, however, is very suggestive in other ways, for example, in dealing with organic bodies, which seem to be specially proper substances for this treatment.
Again, in the preparation of solid paraffin wax from the blue oil, cold is constantly employed for the purpose; in fact ever since the greater improvements in the paraffin oil manufacture have been made, this has been systematically applied in order to extract the paraffin. It is surprising that we have not heard more of cold in this way. Why should we not purify such a substance as carbolic acid by dissolving it into some suitable naphtha, and by means of cold separate it again ? By this way we might find some means of preparing pure carbolic acid with greater rapidity. Again, polybasic acids might yield similar results. Why not try the effect of cooling solutions of benzoates and tartrates ? "
More than 30 years ago, Kneller proposed to concentrate syrups by forcing cold air through them, and his plan was much improved by Chevallier. Sugar made in Chevallier's apparatus rivalled that of the vacuum-pan in every respect. A vessel holding 200 gal. of syrup (comprised of 3 parts sugar to 1 of water) is estimated by Wray to turn out 12 tons of sugar daily. The cost of the apparatus is small; the power required is trifling; the ordinary air of the estate could be used in dry weather, and would entail an insignificant expense for drying in damp weather; and the quality of the sugar is unsurpassed. In 1865, Alvaro Reynoso proposed to rapidly cool the syrup in suitable machines, and thus form a confused mass of particles of frozen water (ice) and dense syrup. The mixture is afterwards separated in centrifugals, and the syrup, deprived of ice, is evaporated in vacuo ready for crystallisation. It seems most singular that, in the face of the many drawbacks and great cost incurred by concentration by heat, so little effort is made by sugar-growers to adapt the cooling system to their needs.
Ellis (Jl. Soc. Chem. Ind.) has published the results of his experience in the application of cooling to the recovery of soda sulphate from waste liquors. He found that 100 parts of water at 94° F. can hold in solution 412 parts of soda sulphate crystals (Na2S04+10H2O); at 86° F., 184 parts; at 79° F., 110 parts; at 77° F., 98 parts; at 68° F., 58 parts; at 50° F., 23 parts; and at freezing-point only 12 parts; so that a very slight lowering of temperature in the case of a strong solution gives a very considerable yield of crystals; a solution saturated at 94° F. should yield almost 97 per cent. of the crystals if cooled to 32° F. The waste liquor experimented on was about equivalent to a solution saturated at 65° F., and on cooling it down to 40° F., about 2*5 lb. of the salt were always obtained from 1 gal. of liquor; this salt was tolerably pure, and by washing it with a spray of saturated solution of soda sulphate, a salt almost free from foreign bodies was obtained. Some samples which were analysed contained about 0.2 per cent. of common salt and 0.04 per cent. of iron.
The next point to be considered is how much heat requires to be abstracted from 1 gal. of the liquor at 65° F. in order to reduce its temperature to 40° F. and to obtain the crystals from it. We will take as our thermal unit the quantity of heat required to raise 1 lb. of water through 1° F.; 1 gal. of the liquor weighs about 12.5 lb., and has a specific heat of about 0.85, so that 12.5x0.85x25 = 265.6 thermal units must be extracted in addition to that which is given out by 2.5 lb. of salt while crystallising: this may be taken at 250 thermal units, giving a total of 515.6, to obtain 2.5 lb. of the salt. About 10 times this quantity of heat would require to be supplied to the liquor in order to get the same amount of the salt by evaporating it to dryness. In order to arrive at an idea of the cost of abstracting heat from a solution by artificial means, Ellis consulted Coleman of the Bell-Coleman Refrigerating Company, and he stated that one of his refrigerators which consumed 3 tons of coal in 24 hours could in that time produce cold capable of abstracting 4,000,000 of the above thermal units, which is just about 1/10 of the corresponding heat which the same weight of coal can supply in practice when applied to evaporation.
However, in the case under consideration it would be quite unnecessary to make use of artificial cold for the whole of the reduction of temperature from 65° F. to 46°F., as during a considerable portion of the year, at least half this cooling could be brought about by natural means, and as the yield of crystals is proportionately much greater between 65° F. and 50° F. than between 50° F. and 40° F. it might be found more advantageous not to attempt cooling below 50° F. at all. The liquor contains about 53 parts of salt to 100 parts of water, and by cooling to 50° F. 30 of those parts should be recovered, whereas further cooling to 40° F. would only yield 6-7 parts more of crystals. Ellis was at first inclined to think that the question of the recovery of the salt, economically, could be solved by the use of artificial cold, produced by a mechanical refrigerator of such form as the Bell-Coleman Co. make, but on going into details of cost and working expenses, he feels almost convinced that a similar result could be brought about in another way much more economically, and he proposed the following method for the treatment of this liquor on the large scale. Let us first take a case when the atmospheric temperature is about 50°F. or lower.