PRESSURE CHEMISTRY. The influence of pressure on chemical transformations has long been recognized, but it is only within the 2oth century that high pressures have been applied to industrial chemical processes. Haber's process for the direct synthesis of ammonia from its elements, which has been developed in Germany by the Badische Anilin and Soda Fabrik, has not only solved the problem of the world's supply of fixed nitrogen but has produced a technique in dealing with gases under conditions of high pressure and temperature, which is now being applied in the field of organic chemistry. Bergius's process for the production of oil from coal involves treatment of the latter with hydrogen at high temperatures and pressures. This process, after over 20 years' sustained research, has been placed on a manufacturing basis, and affords a solution of one of the greatest European problems, namely, the creation of an adequate oil supply.
Le Chatelier's principle states, in effect, that if a system in equilibrium be subjected to a constraint, a change will take place in the system which is in opposition to the constraint. An increase of pressure favours the system formed with a decrease in volume, while a reduction of pressures favours the system formed with an increase in volume. Pressure may consequently be employed in gaseous reactions, which proceed with diminution in volume, to change the equilib rium in favour of the product desired. The attainment of this equilibrium or a close approach to it is brought about by the use of a catalyst, which may be defined as a substance which can accelerate a chemical change without itself undergoing perma nent alteration. The development of ammonia synthesis has shown that a catalyst may be specially sensitive to the presence of certain substances which may either inhibit or enhance its activity. For example, the activity of iron as an ammonia catalyst is partially or totally destroyed by the presence of traces of sulphur, selenium, tellurium and other elements which act as poisons. Catalyst masses must consequently be prepared with great care, and the reacting gases need special purification. In contrast to the poisoning action of the elements mentioned above, the presence of a small proportion of certain substances called promoters, as for example the oxides of the alkalis and alka line earths, greatly enhances the activity of the iron catalyst.
Lowering the temperatures of reaction has the same effect on the ammonia equilibrium as raising the pressure. Haber has cal culated that if the temperature of reaction could be reduced to 300° C, satisfactory yields of ammonia would result at ordinary pressures. No catalyst is known which will bring about reaction at so low a temperature, therefore the industrial procedure is to obtain a catalyst highly reactive at as low a temperature as possible, and to raise the pressure till adequate yields of ammonia result.
Processes.—Nitrogen and hydrogen sub jected to high temperatures and pressures in the presence of a suitable catalyst unite to form ammonia, in accordance with the equation
Variations in the different industrial processes lie principally in the pressures employed, the special form of catalyst, and in the method of preparing the nitrogen hydrogen mixture. The Haber-Bosch, Casale, and Claude pro cesses operate at pressures of about zoo, Boo and 90o atmospheres respectively. The first two processes employ circulatory forms of plant (figs. I and 2), where the nitrogen-hydrogen mixture is passed over the catalyst at a temperature of 500°-600° C. The partial pressure of ammonia in the gases leaving the converter amounts to about 12 atmospheres in the Haber-Bosch, and 16o atmospheres in the Casale process. Ammonia is removed in the former process by solution in water, and in the latter by condensa tion at ordinary temperatures. Unchanged gas is circulated by a pump over the catalyst together with fresh gas mixture added to maintain the pressure. A flow-through form of plant is employed in the Claude process. The catalyst is held in a battery of six con verter tubes, one of which is shown in fig. 3. In the first of these, which contains the old catalyst, any carbon monoxide in the gas is converted to methane. The next two tubes are arranged in parallel to prevent overheating due to excessive reaction, while the last three tubes are in series. The partial pressure of ammonia in the gases leaving each converter tube is about 25o atmospheres. Am monia is removed by simple condensation after each contact with the catalyst. In all, 85% of gas is converted to ammonia. The re sidual gas after reduction to atmospheric pressure is used for other purposes.