CHEMICAL ENGINEERING. The branch of engineer ing which relates to the "design, construction, erection and opera tion of industrial plants in which matter undergoes a change of state or composition." This definition, due to the Institution of Chemical Engineers, is rather broad but logical. A chemical en gineer should possess a good general knowledge of chemistry and physics, a special knowledge of physical chemistry and ther modynamics, a thorough grasp of mechanical and electrical en gineering and of those branches of civil engineering which deal with the strength of materials and the theory of design of struc tures. There is little doubt that the material rewards of scientific chemical engineering are greater to-day than in any other branch of engineering, while its national importance cannot be over-esti mated.
Since the ultimate object of the chemical engineer is economic production, industrial economics and law, business management and factory organization must be given a primary place in his training. He must be able to devise simple and effective methods of recording operations and determining actual costs, prepare de signs, specifications and estimates free from ambiguity and error, report efficiently on any problem investigated, obtain technical information readily and compile and index it for future use.
The practice of chemical engineering has naturally preceded the scientific analysis of its principles. Typi cal chemical engineering operations have been performed in certain industries to such an extent as to evolve special types of plant peculiar to those industries. The chemical engineer, however, must understand the underlying scientific principles upon which such operations or processes depend, so as to be able to design an efficient plant for any particular purpose, rather than allow it to be evolved by rule-of-thumb guesswork with expensive large scale experiments and costly alterations in construction before reasonable efficiency is obtained.
The adoption in the factory of one unit of weight and volume together with the decimal system is a first step and facilitates recording, avoids errors, saves time and makes the scientific control of processes comparatively easy.
Chemical engineering operations are of three general types : (a) the conveyance and storage of materials; (b) the production, transfer and conserva tion of heat, and (c) the treatment of materials. In these proc esses an exact knowledge is required to secure the greatest indus trial efficiency, and questions relating to rent, capital cost, re pairs, labour and superintendence, output, material efficiency, character of the market, etc., must be considered. Unfortunately some of these conditions vary from year to year and the chemical engineer must be somewhat of a seer to achieve the greatest success.
is often the most important item of the cost of production; in the factory itself, the cost of carrying materials to a machine is often as great as that of the operation conducted. Methods of weighing and measuring materials in their progress through the factory need careful study, for it is only by maintaining the highest rate of output as a uniform thing, that the best results can be obtained.
In industrial practice fluids may flow in two distinct ways, usually referred to as "stream-line" and "turbulent" flow. The former method offers advantages in certain operations, filtration, classification, separation, etc., while the latter is essen tial to secure the greatest heat transfer. The criterion by which it can be approximately determined whether stream-line or turbu lent flow is occurring in a tube is given by the value of the modulus vd P where v is the velocity of the fluid, d the diameter lu of the tube, P the density of the fluid and its viscosity. The value of the fraction is the same in any system of units and when it exceeds 2,000 the flow is turbulent. The transport of fluids in factories is obviously a simple and cheap process, so that a solid material which is used in solution will be transported more cheaply and conveniently by dissolving it at once and pumping the solo tion to stock tanks, rather than by conveying the solid itself.
The laws governing the flow of semi-fluids such as dry sand, wheat, etc., which occupy an intermediate position between solids and liquids, must be understood. A peculiarity of such materials is that their flow from an orifice is proportional to the cube of the area of the orifice and independent of the head.
production and transfer of heat in volve a knowledge of combustion, the study of fuels, their calo rific value and intensity, while the electrical generation of heat is becoming more and more important in the development of indus trial processes. The design of furnaces of different types, often a matter of rule-of-thumb evolution, involves questions of heat transmission and radiation, the factors of which are very difficult to estimate. The great diminution in density of the hot gases and the increasing importance of radiation as a factor in heat trans mission, as the temperature rises, are points needing the most careful study. The loss of heat from exposed hot surfaces by radiation and air contact may be fairly closely estimated, a fac tor often very important in the operation of a plant. The trans mission of heat through a metal diaphragm from gas to gas, gas to liquid, saturated vapour to liquid, liquid to liquid, gas to solid, etc., varies so enormously as to demand the most careful experi mental research. The transmission of heat from a gas to a gas through copper imm. thick, is of the order of 5 calories per sq.m. of surface per hr. per one degree C. of temperature difference between the gases; under similar circumstances the transmission from gas to non-boiling water is about 20 calories, from liquid to liquid about 35o calories, and from steam to water about 1,500 calories. If the liquid is boiling the heat transfer may be in creased greatly; it is usually at least twice and may be over four times that of non-boiling liquid. The importance in steam boilers and evaporators of keeping the liquid constantly boiling is obvious. Again, an increase of velocity of either fluid will raise the rate of heat transmission considerably by an amount which is capable of estimation. The effects of corrosion, of deposited scale, the influ ence of salts in solution and of precipitates in suspension on the rate of heat transmission are matters of great industrial im portance. When a gas or liquid moves over a solid surface or when a gas moves over a liquid surface there exists a film of the moving fluid over the stationary substance in which there is prac tically no motion. The thickness of this film is mainly responsible for the great differences in heat transmission already mentioned. Much chemical engineering research is necessary to elucidate these questions adequately, and great wisdom in design is neces sary to secure the highest economic efficiency.
In the treatment of solid materials, size re duction by crushing, grinding and impact must be well under stood. The relative cost of the different methods and machines and the different character of the final products must be known, for the size and uniformity of the solid material treated in a chemical operation has, in practically every case, a profound in fluence on the cost of production of the final product. The sepa ration of solid particles from each other and from liquids involved in the processes of sifting—electrical, magnetic, hydraulic and air separation, classification, flotation, filtration, centrifuging, etc.— of ten determines the profitable treatment of the material. New discoveries in flotation and similar methods of separation have in recent years proved of the highest value.
A study of the power required for different proc esses is of importance and many prevent false conclusions. In size reduction for example, the power required is approximately proportional to the increased surface produced, so that a simple calculation will readily show how costly it is to produce the finest powders from solid materials. Many processes, though well understood, are difficult and costly; their avoidance or replace ment by some other method at another stage of the process are important points in the economic possibilities of manufacture. Again, it is a common experience in technical manufacture, that a method by which one` process is rendered easy may cause a later process to fail and give rise to a faulty product.
Processes of sep aration in which a change of physical state takes place are pecul iar subjects of study to the chemical engineer. Leaching and ex tracting, evaporation, distillation and condensation, drying, crystal lization, absorption and adsorption, etc., are processes requiring the highest research. Processes of extraction which formerly took
hours are now being carried out more efficiently in one hour as a result of chemical engineering study. Evaporation is now being done in many works at one-sixth of its former cost, while distilla tion and condensation in certain industries are now carried on with a fuel saving of nearly So% in comparison with earlier methods. Wet Distillation.—The process of "wet" distillation, i.e., dis. tillation by means of direct steam, is used in many industries for separating a particular substance, essential oil, hydrocarbon, etc. If such substance does not mix with water, then each develops that particular vapour pressure which it would have if heated sep arately to the same temperature, and the sum of the vapour pres sures is the pressure in the still. A study of the vapour pressure curves of each substance and the conditions of decomposition of the particular one, will give the most profitable still pressure at which to work. By this alone, materials which at ordinary pres sure require 200 parts of steam to distil one part of substance, may be treated under suitable pressure by 3o parts of steam. Again, by a reduction of pressure before condensation has taken place, a separation of the particular substance will take place and the steam may be economically "boosted" back into the still.
drying of gases, liquids and solids is a common requirement of many industries. The study of drying and drying plant may mean a change from comparative failure to great success. The drying of gases by chemical reagents such as calcium chloride, which of ter absorbing moisture is itself dried by hot air or flue gases, cooled and again used, is well understood, but the use of absorption processes for this purpose is a devel opment in industry of the last few years. The "gels" (silica gel, alumina gel, etc.) are treated in a similar way to the calcium chloride previously mentioned, but are free from any risk of fail ure through overheating, and have many other advantages.
efficiency of the gas mask in warfare is par alleled by the importance of the same means in peace to recover benzene and other solvents from air and gas effluents, to remove sulphur from oil and to perform many other chemical functions in a physical way. The absorption of water by "silica gel" is so energetic, and the substance can be so readily regenerated that its general use for refrigeration may be only a matter of time. The chemical engineering problems involved in this process are both interesting and profitable. The treatment of the air of factories to render it constant in temperature and humidity also offers a large field to the chemical engineer. (See ADSORPTION.) Chemical Processes.-The treatment of materials in chemical reactions such as roasting, calcining, destructive distillation, elec trolysis, catalysis, hydrolysis, fermentation, etc., demands care ful consideration of physical, chemical and engineering factors of increasing exactitude. An examination of these processes on a laboratory scale must precede their study on a manufacturing scale. The chemist does the former and the chemical engineer the latter. Additional factors involving time of reaction, strength of materials, presence of traces of impurity, cost of materials and plant, continuous or discontinuous operation, the handling and dis posal of by-products, etc., arise. Increase in efficiency can be obtained by changing from intermittent to continuous working.
design of plant involves the preparation of flow sheets showing the flow and expenditure of material, energy, time and labour, the experimental study of the nature and prop erties of the materials involved in plant construction and a con tinuous search for new materials to meet the conditions of service better than those at present in use. The name of a material, un fortunately, is no indication that it is suitable for a chemical engineering purpose. "Chemical cast iron" is a distinct kind of cast iron, characterised by the amount of silicon, sulphur and carbon content and its treatment. Other materials, alloys, bricks, compositions, cements, etc., must be specified for their work both as to composition and history if success is to be made certain. In recent years, great advances have been made in the production of chemical engineering materials of construction, silica-ware, stoneware, glass, alloy steels, alloys, etc., all of which tend to cause modification of design. The effect of heat on the strength of materials is becoming better known every year and the pro duction of non-scaling steels which are as strong at a white heat as ordinary mild steel at the ordinary temperature will make many developments possible which have hitherto failed.
lay-out and construction of factories involves a knowledge of several branches of civil engineering and cannot be neglected, for the erection of buildings, heavy plant and chimneys on marshy soil is a common necessity in industry. The construction of foundations, roadways, railways, etc., must also be understood. In the provision of power, heat and light, the relative value and use of the different methods must be con sidered for a sound view to be taken as to whether steam, elec tricity, gas or water-power shall be used. High-pressure steam providing power by means of turbines which exhaust into receivers from which evaporating steam may be drawn, presents the most efficient power plant in many chemical industries, while electricity generally provides the best method of power distribution, and occasionally of heating, in process work. The use of producer or water gas for heating and other purposes must not be neglected nor the value of compressed air and hydraulic water for many purposes. Finally the chemical engineer must be familiar with the Factory Acts, trade union law and all those legal questions which factory managers have to face. (J. W. Hi.)
