THE DISTILLATION AND RECTIFICATION OF ALCOHOLS BY THE RATIONAL USE OF LOW TEMPERATURES.
By RAOUL PICTET.
The industrial problem of the rectification of alcohols is based entirely upon the properties of volatile liquids, upon the laws of the maximum tensions of the vapors of these liquids, and upon the influence of temperature upon those different elements which find themselves in presence of each other in an alembic.
If we desire to follow, in their least details, all the phenomena which succeed one another in a rectifying column, and which are connected with one another by a continuous chain of reciprocal influences, the problem becomes exceedingly complex.
PICTET'S APPARATUS FOR THE RECTIFICATION OF ALCOHOL BY COLD.
In order that the new applications of the mechanical theory of heat may be readily understood, we shall divide this problem into a series of propositions, which we shall examine separately, and which collectively constitutes in its general features the methodical rectification of liquids.
I. Knowing the maximum tensions of pure water and pure alcohol, can we calculate directly the tensions of the vapors of any mixture whatever of alcohol and water?
Yes, we can calculate this tension by a general formula, provided we take into account the affinity of water for alcohol, which increases the value of the total latent heat of evaporation of the liquid. The results of the calculation are fully confirmed by experience. We thus establish the following laws:
a. For any temperature whatever, the maximum tension of the vapors of a mixture of water and alcohol is always comprised between that of pure water and that of pure alcohol.
b. The tension of the vapors of a mixture of water and alcohol approaches the tension of alcohol so much the nearer in proportion as the proof is higher; and, reciprocally, if water is in excess, the tension of the vapors approaches the tension of the vapors of water.
c. The curves of the maximum tensions of vapors formed by all mixtures of alcohol and water are represented by the same general formula, one factor only of which is a function of the richness of the alcoholic solution.
It results, then, from these laws that we may determine with the greatest exactness the richness of a solution containing alcohol and water, if we know the tension of the vapors that it gives off at a certain temperature. Such indications are confirmed by the centigrade alcoholmeter.
We see likewise that, for these solutions of alcohol and water, the laws of Dalton are completely at fault, since the total pressure of the vapors is never equal to the sum of the tensions of the two liquids, water and alcohol.
II. Being given a solution of water and alcohol, mixed in equal volumes, what will be the quality of the vapors emitted from it?
In other terms, do the vapors which escape from a definite mixture of water and alcohol also contain volumes of vapor of water and alcohol in the same proportion as the liquids?
We have discovered the following laws:
d. The quality of the vapors emitted by a mixture of water and alcohol varies according to the alcoholic richness of the solution, but is not in simple proportion thereto.
e. The quality of the vapors emitted by a definite mixture of water and alcohol varies according to the temperature.
f. In a same solution of water and alcohol, it is at low temperatures that the vapors emitted by the mixture contain the largest proportion of alcohol.
g. The more the temperature rises the more the tensions of the two liquids tend to become equalized.
We have been able to verify these different laws experimentally, and to find an interesting confirmation of our general formula of maximum tensions, in the following way:
Let us take a test tube containing a 50 per cent. solution of alcohol and water, plunge it into water of 20°C., and put its interior in hermetic communication with the receiver of a mercurial air-pump.
We vaporize at 20° a certain quantity of the liquid, and the vapors fill the known capacity of the pump. The pressure of the gases in the interior is ascertained by a pressure gauge, and this pressure should be constant if care is taken to act upon a sufficient mass of liquid and with moderate speed. When the receiver of the air-pump is full of vapors, communication between it and the test-tube is shut off, and communication is effected with a second test-tube, like the first, plunged into the same water at 20°. Care must be taken beforehand to create a perfect vacuum in this test-tube.
On causing the mercury to rise into the space that it previously occupied, the vapors are made to condense in the second test-tube at the same temperature as that at which they were formed.
We immediately ascertain that the pressure-gauge shows an elevation of pressure; moreover, the proof of the condensed alcohol has very perceptibly risen.
If, instead of causing these vapors to condense in the second test-tube, we leave the first communication open, the vapors recondense in the first test-tube without any elevation of pressure; and we do not see the least trace of liquid forming in the second test tube.
This difference of pressure in the two foregoing experiments must be attributed, then, to the specific action of the water on the vapors of alcohol. Now we can calculate the difference of the work of the pump, and put at 1 kilogramme of condensed liquid the difference of mechanical work represented in kilogrammeters. What is remarkable is that this difference is absolutely the equivalent of the heat disengaged when the condensed liquid and the old liquid are remixed; there is a complete identity. Thus the affinity of the water for the alcohol modifies the tension of the vapors which form or condense upon the free surface of the mixture. The two phenomena are closely connected by the law of equivalence.
It results from all the laws that we have cited that by properly regulating the tensions of the vapors of a mixture of alcohol and water, and the temperature of the liquid, we shall be able to obtain a liquid of a desired richness by the condensation of these vapors.
III. It was likewise indispensable to make sure of one important fact: When the temperature of a liquid like alcohol is considerably lowered, can the distillation of a given weight of this substance be effected with sufficient rapidity for industrial requirements? Repeated experiments with a host of volatile liquids have demonstrated the following laws:
If we introduce a volatile liquid into two spherical receivers connected by a wide tube, and if these be kept at different temperatures after driving out all the air from the apparatus, the liquid distills from the warmer into the cooler receiver, and we ascertain that:
h. The weight of the liquid which distills in the unit of time increases with the deviation of temperature between the two receivers.
i. The weight of the liquid which distills in the unit of time is constant for a same deviation of temperature between the receivers, whatever be, moreover, the absolute temperature of the receivers.
k. The weight of the liquid distilled in the unit of time is proportional to the active surfaces of the receivers; that is to say, to the surfaces which are the seat of passage of heat through their thickness.
l. The least trace of a foreign gas in the vapors left in the apparatus throws the preceding laws into confusion, and checks distillation to a considerable degree, especially at low temperatures.
Thus, water distilling between 100° and 60° will pass over as quickly as that which is distilling between 40° and 0°. Absolute temperature is without influence, provided every trace of air or foreign gas be got rid of.
The distillatory apparatus should be provided with an excellent air-pump, capable of preventing all those entrances of air which are inevitable in practice.
The following is the industrial application that we have endeavored to make of these theoretical views: The rectification of alcohols is one of the most complex of operations; it looks toward several results simultaneously. Alcohol derived from the fermentation of grain, sugar, and of all starchy matters in general, contains an innumerable host of different products, which may be grouped under four principal heads:
1. Empyreumatic essential oils, characteristic of the source of the alcohol, and having a powerful odor which infects the total mass of the crude spirits. 2. A considerable quantity of water. 3. A certain quantity of pure alcohol. 4. A variable proportion of volatile substances, composed in great part of ethers, different alcohols, and bodies as yet not well defined. These latter affect the quality of the alcohol by an odor which is entirely different from that of the essential oils.
The object of rectification is to bring out No. 3 all alone; that is to say, to extract the alcohol in a pure state by ridding it of oils, water, ether, and foreign alcohols.
The alcohol industry never realizes this operation in an absolutely complete manner. All the rectifying apparatus in operation at the present day are based on the use of high temperatures varying between 78.5° and 100°. The successive condensation and vaporization of the vapors issuing from the spirits effect in the rectifying columns a partial separation of these liquids, and there are received successively as products of rectification:
1. Bad tasting alcohols, containing the majority of the ethers and impure alcohols.
2. Fine alcohol.
3. Alcohols contaminated by notable proportions of empyreumatic oils.
Industry knows only one means of obtaining an excellent product, and that is to diminish the quantity of fine alcohol which comes from a same lot of spirits, and to make a large number of successive distillations. Hence the large expenses attending rectification, which produce fine alcohols necessarily at an elevated price. We may remark, in passing, that the toxic action of commercial alcohols is in great part caused by the presence of essential oils, amylic alcohol, and ethers, absolutely pure alcohol, as compared with these, being relatively innocent.
Why is it that our present apparatus cannot produce good results in rectifying alcohol? Because they are limited by the temperature at which they must operate. Between 78° and 100° the tension of the vapors of all the liquids mixed in the spirits is considerable for each of them; they all pass over, then, in certain proportions during the operation of rectification.
We have been led, by examining the theoretical question, to ascertain that the proportion of alcohol which evaporates from a mixture is maximum at low temperatures; consequently, we should seek to establish some arrangement which can realize the following conditions: (1) Render variable, at will, the temperature of the boiling liquid; and (2), render variable the pressure of the vapors which act on the liquid.
Thus, to effect the rectification of alcohol it suffices to cause its ebullition at very low temperatures, and to keep up the ebullition without changing such temperatures when once obtained.
It is exactly these two conditions that we have fulfilled in the apparatus that we have just installed in our factory in Rue Immeubles Industriels, at Paris.
By their arrangement, which is shown in the opposite figure, they form a mechanical system permitting of the rectification of alcohols at temperatures as low as -40° or even -50°. They verify experimentally, by their operation, the theoretical deductions which precede. The boilers, A, which, in an industrial application, may be more numerous, receive their supply of spirits from the country distilleries in the vicinity of the factory. There may even be introduced directly into them vinasses, or washes, that is to say, liquids, such as are obtained by alcoholic fermentation.
Above the boiler rises a rectifying column composed of superposed plates inclined one over the other, and surmounted by a tubular condenser, which serves to effect the retrogression of the first condensation by means of a current of water supplied by the reservoir placed above.
On leaving this condenser, the vapors which have escaped condensation pass into the refrigerator, C, where they are totally condensed by a current of water which goes to the reservoir above.
The first products obtained contain ethers and impure alcohols, which are collected in the reservoir, E.
When the first products have been thus introduced into the reservoir, and it is ascertained by tasting that good alcohol is passing over, the liquid produced is directed into the second boiler, F. The sliding valve, operated by a screw having a very fine pitch, establishes a communication between the refrigerator, C, and the second boiler, F. The office of this valve we shall learn further on. This first rectification is performed in a vacuum, for a system of metallic pipes connects the entire apparatus with an air-pump, O. The temperature at which the liquids shall enter into ebullition in the boilers, A A, may, then, be regulated in advance.
The operations will be carried on with a more or less complete vacuum, according to the nature of the products to be rectified. The distiller will have to be guided in this by practice alone.
The good tasted products are received in boiler No. 2, F, and there the liquids are submitted to the action of an almost absolute vacuum. As we have before said, their temperature falls immediately and spontaneously. The vapors which issue from this liquid contain almost solely pure alcohol. The other substances, which passed over in the first distillation, no longer emit vapors at temperatures ranging between -10° and +5°. Their temperature is shown by a thermometer running into the boiler, F.
These vapors, purified by ebullition at a low temperature, rise into a second rectifying column, G, which terminates in the refrigerator, H, filled with liquid sulphurous anhydride. This refrigerator is like those which we employ in our sulphurous anhydride frigorific apparatus. Under the action of a special pump, M, this liquid produces and maintains a constant temperature of -25° to -30° in the refrigerator. The vapors of alcohol condense therein at this low temperature, and the cold liquid alcohol flows into the lower part of the refrigerator.
By the action of a return cock, a portion of this liquid falls upon the plates of the column, G, and descends, while the vapors are rising therein. The other portion of the liquid obtained flows into the reservoir, K, at the beginning of the operation, and into the reservoir, L, during all the remainder of the rectification. The ice-making machine keeps up of itself alone the two operations.
In fact, the exhaust of the steam engine which actuates the sulphurous anhydride pump is directed into a worm which circulates through the first boiler, A, and the refrigerator, H, of the frigorific machine keeps up the second rectification, which was brought about below the surrounding temperature, and which for this reason takes place without necessitating any combustion of coal. It suffices to cause the current of water which issues from the condenser of the frigorific machine to pass into the worm of the boiler.
We have, then, two results, two like operations, both produced by the working of a single machine. Moreover, these two operations are performed in vacuo, and we know that under these conditions they are effected at lower temperatures. Owing to this fact, likewise, the weight of the water that must be evaporated diminishes just so much. Now, one kilogramme of water requires 636 heat units to cause it to pass from the liquid to the gaseous state, while one kilogramme of alcohol requires only 230 heat units to vaporize it. Thus every decrease of temperature in rectification has for an immediate corollary an important economy of fuel, which is proved by the diminution of radiation, and by the less quantity of water to be distilled.
Between the boilers, A, in which is maintained a temperature bordering on +50° to +60°, and the refrigerator, H, in which is easily obtained a temperature of -30° to -40°, there is at our disposal a range of temperature of nearly 100°, an immense difference compared with that which can be made use of in ordinary apparatus. Thanks to this powerful factor, which is manageable at will, we can take directly from the apparatus alcohols marking 98 and 99 degrees by the centigrade alcoholmeter. Such results are unobtainable by the usual methods.
We have likewise ascertained that at low temperatures the ebullition of alcohol is as active as at near 100°.
For a same range of temperature between the boiler and the refrigerator, the weight of alcohol which distills in an hour is constant. By the operation of the valve, D, it becomes easy to allow all the liquid condensed in the first refrigerator to pass into the second boiler; and thus the second rectification, which is effected in a more perfect vacuum, is supplied with exactness. The object of this valve, then, is to allow the liquid to pass, and yet to cut off the pressure in such a way as to have a double fall of temperature throughout the whole apparatus; from 60° to 20° in the first operation, and from 0° to -40° in the second. We may add that the regulation of the valve is extremely easy, because of the screw which actuates it.
To sum up the commercial advantages that our process procures, we may say that it realizes the following desiderata: 1. With the cost of a single distillation we have, at once, distillation and rectification, or a single expense for two results. 2. With one operation at a low temperature we obtain products which are almost impossible to get even by an indefinite number of rectifications at a high temperature, the temperature having an intrinsic value in the operation. 3. The alcohols obtained are wholesome, and can be put on the market without danger. 4. Their superior quality gives these alcohols an extra value difficult to calculate, but which is very notable. 5. The whole operation being performed in closed vessels, there is absolutely no waste. 6. For the same reason there is scarcely any danger of fire. 7. The management of the works and the service are performed by the pressure of the gases entirely; there are only a few cocks to be turned to perform all the interior maneuvers, empty and fill the vessels, etc. Hence economy in personnel.