The absorption of gases in liquids
INTRODUCTION For over 100 years chemical engineers have been interested in the factors which govern the rate of absorption of gases by liquids. In about 1830, for instance, \Villiam Gossage filled a derelict windmill with brushwood and ran water over it in order to absorb hydrochloric acid vapour from the manufacture of alkali. This perhaps marks the invention of the first practical absorption tower. The principle, incidentally, was protected by a British patent of 1836. In the chemical and process industries there are many occasions on which a gas must be absorbed by a liquid. A few examples will illustrate their diversity: Absorption of nitrous gases in water or nitric acid, during nitric acid manufacture. Absorption of chlorine by alkaline solutions to give hypochlorites. Absorption of oxygen by fermentation broths. Removal of carbon dioxide from water-gas by absorption in water or in solutions of alkalis or amines. Removal of carbon monoxide from water-gas by absorption in solutions of copper complexes. Absorption of hydrogen by petroleum fractions in processes to remove sulphur as hydrogen sulphide. Absorption of chlorine and propylene in water to give the chlorhydrin. In order to carry out the absorption process efficiently and compactly, it is generally necessary to provide a !arge surface of contact between gas and liquid, and to keep both phases in vigorous motion to promote stirring. Some types of equipment used in various circumstances are: Packed columns, in which the liquid runs as a firm over the extensive surface of the packing, while the gas is blown through the open space. Spray columns, in which the liquid rains down through the gas in the form of drops. Stirred vessels, in which the gas is entrained and dispersed in the liquid in the form of bubbles. · Gas absorption is an example of a topic which does not fall entirely within the province of either the ehernist or the conventional engineer, because it requires the simultaneous consideration of molecular diffusion, fluid mechanics and chemical reaction kinetics. Gas absorption has thus become a typical, indeed a classical, chemical engineering topic and it is surprising that in spite of all the work clone and papers published in the field since the work ofWhitman in 19231, we arestill far from a thorough understanding of the process.