This article was originally published in Electrochemical and Metallurgical Industry Publication of October 1907. Information within this article is therefore correct as of 1907. The publication of this material aims to provide historical insight on the subject and its place in industry.
Atmospheric air is a mixture of about 23% by weight of oxygen and 77% by weight of nitrogen. To make the constituents of air available for industrial use two ways are open. One is to combine the nitrogen and the oxygen so as to form nitrogen oxide which can be worked up into nitric acid or nitrates. The second way is to separate the nitrogen from the oxygen in the air and make both constituent gases individually available for various applications. Both methods require extreme temperatures; the first method an extremely high temperature, such as is obtainable from the electric arc; the second an extremely low temperature, since the separation of air into its constituents depends on the liquefaction of the air and its subsequent fractional evaporation.
An extensive plant for producing oxygen from liquid air on an industrial scale is now being erected in Buffalo by the Linde Air Products Co. Dr. Linde's system of air liquefaction and subsequent rectification being applied. For the annexed illustration of the plant and the information given in the following notes, we are obliged to Mr Cecil Lightfoot, the consulting engineer and general manager of the Company.
The action of the Linde apparatus for producing liquid air is based on the refrigerating effect resulting from an expansion of air from a higher to a lower pressure and which is due to the performance of internal work. The action of any desired number of expansions is accumulated and intensified by causing each preceding expansion to act as a fore-cooler for the air for the following expansion.
Figure 1. Apparatus for making liquid air
Since the refrigerating action depends upon the difference of pressure before and after expansion, while the work of compression corresponds to the ratio of these pressures, it is advantageous to select a large difference in pressures but a small ratio of pressures. In the Linde apparatus air is expanded from an initial pressure of about 200 atmospheres to about 50 or 20 atmospheres, so that the difference of the pressures varies between 150 and 180 and the ratio is between 4 and 10.
As will be seen in Fig. 1 a triple coil is provided, which is composed of copper tubes placed one within the other. This is called the heat interchanger. Air compressed to 200 atmospheres flows downward through the innermost coil, at the lower extremity of which it is allowed to expand to an intermediate pressure of 20 to 50 atmospheres. The expanded air is then returned through the annular space between the innermost and middle coils to the top, when it is again compressed up to 200 atmospheres pressure and the cycle is repeated. Immediately behind the first regulating valve A is placed a second valve B, through which, when the operation of the machine has been brought to a state of equilibrium, a small quantity of air is allowed to escape at atmospheric pressure, a corresponding amount being introduced into the cycle from the surrounding atmosphere. Part of this air leaves the second regulating valve in the liquid state collects in the vessel C; the remaining portion is returned to the atmosphere through the annular space between the middle and outer coils. The liquid air is drawn from the collector by means of the small cock D.
In the larger installations the necessary compression of the air is effected by means of a high-pressure air pump, which is usually arranged for two-stage compression, working in conjunction with a single-cylinder low-pressure air pump. The high-pressure cylinder F of the former draws the partly expanded air from the heat interchanger at an intermediate pressure of about 50 atmospheres, and compressing it up to 200 atmospheres pressure, delivers it to the interchanger again through the cooler E, The air which is to be added to the cycle as "make-up" is supplied by the low-pressure air pump H, which draws it from the atmosphere, compresses it to a pressure of 4 atmospheres and delivers it to the low-pressure cylinder G of the high pressure air pump, where it is compressed to a pressure of 50 atmospheres. At this pressure it enters the high-pressure cylinder, together with the partly expanded air from the heat interchanger, as described above.
Low-pressure compressors are not usually supplied with the smaller sizes of air-liquefying plants, the lower pressure of the cycle being in such cases maintained at 20 atmospheres and the low-pressure cylinder G drawing the "make-up" air direct from the atmosphere.
Regulation of the several pressures is performed with the aid of pressure gauges by means of regulating valves in the heat interchanger. Safety valves are provided to prevent the maximum pressures being exceeded.
In electric-furnace operations when electric energy is expensive it is advantageous to preheat the charge or to start at once with a molten charge so as to produce only the highest temperatures from electrical energy, in order to save in the consumption of the expensive electrical energy. In the liquefaction of air where the object is to get a very low-temperature it is advantageous in quite an analogous way to pre-cool the air. For this purpose a fore-cooler is provided by means of which the compressed air is reduced in temperature to 5° or 10° above zero Fahrenheit, by filling the receptacle provided for the purpose with a suitable freezing mixture, such as ice and salt. This is practical in smaller illustrations.
On the other hand, with larger installations, the preliminary cooling of the air is preferably brought about by a small belt-driven refrigerating machine on the ammonia compression system. The adoption of fore-cooling is advisable in all cases, but especially when the cost of power is an item of great importance.
Between the compressor and the fore-cooler, apparatus is provided for separating water from the compressed air. This apparatus is provided with a suitable drain cock. Further extraction of the aqueous vapors from the compressed air with the smaller installations is performed in the fore-cooler. In larger plants the drying process is effected after the last stage of compression by means of chloride of calcium placed in the dryer supplied with the machine.
If the liquid air is to be used for the production of pure oxygen and pure nitrogen it is subjected to a process of rectification which is very similar to the process employed in spirit distilleries for the separation of alcohol and water. In this way it is possible to get oxygen 95% pure, and if the output is reduced by 20% the purity of the oxygen may be brought up to as much as 98 or 99%. The chief advantages of this method of producing pure oxygen are claimed to be low expenses, simplicity and safety in work, and the freedom of the oxygen from water vapor, chlorine etc. The residual gases entirely consist of the ordinary nitrogen constituents of the atmosphere.
An interesting application of nitrogen produced from air in this way is in the production of calcium cyanamide. Five Linde plants are already at work in Europe on a very extensive scale for this purpose alone, whilst other and still larger installations are now in course of construction, so that the industry appears to be already well established. It will be remembered that for the production of cyanamide, calcium carbide is treated in a nitrogen atmosphere at an elevated temperature. It is, therefore, necessary to first get the nitrogen from air. The question then comes up what to do with the oxygen, and while there are, of course, many applications like its use in blow-pipes etc. the interesting suggestion has been made to use the oxygen for enriching atmospheric air in oxygen and then to treat the latter by electric arc discharges to produce nitrogen oxides for the subsequent manufacture of nitric acid and nitrates. In this way the two methods which have so far been found practical for the fixation of atmospheric nitrogen would work side by side; the nitrogen obtained from the liquid air would be used in the calcium cyanamide manufacture and the oxygen would be used in the nitric acid manufacture.
Figure 3. Part of main engine room, showing the 4-stage air compressor, the 2 fore-coolers and 2 interchangers
Altogether, about 100 liquid air plants on Dr. Linde's system have already been supplied, of which nearly half are employed either for the production of oxygen or of nitrogen in the pure state, or of both.
Of course, liquid air is useful for many other purposes, especially for all researches or processes in which extremely low-temperatures are necessary, since liquid air forms perhaps the most convenient agent for the production and maintenance of such temperatures. Liquid air is also used in surgery, particularly in treatment of certain affections of the skin.
Liquid air boils at atmospheric pressure at 3120 F. As the more volatile nitrogen evaporates the color of the liquid assumes a bluish tinge, the color of liquid oxygen. In the liquid state, air occupies 1/800 of the space occupied by the same weight of air in the gaseous form at normal temperature and atmospheric pressure.
Vacuum vessels are necessary for the storage of liquid air, oxygen and other gases which only liquefy at low temperatures. Such vessels are generally constructed either cylindrical or globular in shape. As shown in figure 2 they consist of two glass vessels, one enclosed inside the other and united at the neck. The space between the two vessels is thoroughly exhausted and sealed under a high permanent vacuum. Liquid stored in a vacuum vessel of this construction is very perfectly insulated from the effect of external heat and evaporates slowly.
Approximately the rate of evaporation of liquid air in vacuum vessels of this construction is from 5% to 15% of the original volume per 24 hours, according to the size of the vessel. As evaporation only takes place from the surface of the liquid it is obvious that the size of the vessel is an important factor in determining the length of time for which liquid air can be stored. It should also be noted that if allowed to rest in a quiescent state evaporation progresses much less rapidly than if the vessel containing the liquid air is subject to vibration.
Vacuum vessels of 2 litres capacity and upwards should not be tilted in order to pour out liquid, because of the severe strain thus thrown on the glass at the neck of the vessel on account of the contraction which takes place. This is due to the sudden cooling of the glass at the neck when the liquid comes into contact with it. A small hand-pump is a convenient device for emptying vessels without tilting or inverting them.