July, 1922
T H E JOURNAL OF INDUSTRIAL A N D ENGINFERIATG CHEMISTRY
647 -
Study of C.hemica1 Engineering by the Unit-Operation
Method' . -
By W. I(. Lewis and R. T. Haslam SCHOOL OF CHEMICAL ENGINEERING PRACTICE, MASSACHUSETTS INSTITUTE OF
The School of Chemical Engineering Practice, as established by ;the Massachusetts institute of Technology, gives the student practice i n applying previously learned theory to the "unit operations" into which the field of chemical engineering is dioided. The two primary unit operations are flow of fluids and flow of heat, while the secondary unit operations are eoaporation, distillation, filtratign, extraction, combustion, drying, etc. I t is felt that not only is it necessary for the student to have a knowledge of the fundamentals of the science but that he also must have training in the use of this knowledge. Instruction in the application of knowledge is eflectioely gioen by using industrial plants as laboratories of chemical engineering, since the work is not stereotyped and may be made to deoelop resourcefulness and judgment in the student.
B
EFORE considering the study of chemical engineering by the unit-operation method as carried out in the School of Chemical Engineering Practice of the Massachusetts Institute of Technology, i t will be necessary to give a brief description2 of this School, together with the methods employed in teaching chemical engineering by experimental work on full scale operating equipment.
SCHOOL OF CHEMICAL ENGINEERING PRACTICS At three industrial centers, the Massachusetts Institute of Technology has established field stations of the School of Chemical Engineering Practice, each station having access to two or more industrial chemical plants. Here the student studies the practice of his profession and carries out quantitative tests on the process and equipment to determine their efficiency and how they may be improved. As the attendance is limited, there are never more than twelve students a t a time a t one station and these men are under close supervision of a member of the Institute Faculty and his assistant who devote their entire time to the educational work of the station. After graduating from the usual four-year course a t the Institute or any other school of recognized standing and therefore being well grounded in the principles of general inorganic and organic chemistry, analytical chemistry, physical chemistry, and chemical engineering, the student spends six months in the School of Chemical Engineering Practice, where the work is confined t o teaching him how t o use this knowledge in the plant. By means of experimental work on full scale equipment and the application or comparison of theory and practice there are fixed more firmly in the student's mind the fundamental factors of chemical engineering and this knowledge is rendered more available for future use. Although the plants which cooperate with the Institute cover a wide field of industrial chemical processes, namely, the manufacture of sulfite and soda pulp, writing-paper, caustic soda, chlorine, heavy chemicals and acids, rubber, sugar, iron, steel, gas, coke, benzene, ammonia, soap, etc., the majority of the time in the Practice School is not spent on the study of industrial chemistry but on the study of chemical engineering which the manufacture of these materials illustrates. PRIMARY AND
SECONDARY OPERATIONS
The field of chemical engineering has been divided into unit 1 Presented before the Section of Chemical Education at the 63rd Meeting of the American Chemical Society, Birmingham, Ala., April 3 to 7, 1922. For a more complete description of the School of Chemical Engineeriog Practice see THISJOURNAL, 18 (1921), 4 6 8 ; 9 (1917), 1087.
TECHNOLOGY, CAMBRIDGE, MASSACHUSETTS
operations and the particular industries cooperating in the work have been so chosen that in them we find numerous examples of each. A list of these unit operations may be found in THIS JOURNAL, although this list has been slightly enlarged and modified during the past year. The two basic or primary unit operations are, first, the flow of fluids (liquids and gases) and, second, the flow of energy, usually in the form of heat. Those secondary unit operations which are of next importance are evaporation, distillation, drying, combustion, filtration, extraction, etc. BROAD SCOPE OR THE PRIMARY UNIT OPERATIONS The primary unit operations, flow of fluids and flow of heat, are of utmost importance to every chemical engineer. They provide the means for carrying out or controlling almost all industrial chemical processes since the rate of reaction, the yield and the efficiency of a process are usually regulated by the rate at which material or heat energy is put into or taken away from the system. Furthermore, a study of the remaining unit operations is dependent on a knowledge of these two primary unit operations. Examination, in detail, of two of the secondary unit operations reveal: the intimate part the two primary unit operations play. Consider distillation and fractionation. Heat has to be supplied to the wall of the still, passed by conduction through the walls of the still itself and into the liquid which is being distilled. The vapor from the boiling liquid passes up through the fractionating column and the liquid reflux passes down on the absorption of the right amount of heat which must pass serially through the condensing vapor film, through the material of the condenser itself, and finally through the film of the cooling fluid on the outside. Later, the final distillate must be completely condensed by another similar process of heat absorption. Throughout the entire process of distillation and fractionation a n exact knowledge of the flow of fluids and the transfer of heat energy is, therefore, indispensable. This is even more emphasized when we consider the fact that any quantitative study of distillation depends on suitably equating the material and heat input with output. Again, consider the secondary unit operation of drying, and, in particular, a subdivision of this operation, air-drying. Air has to be forced through a heater in order to pick up the right amount of heat t o dry the material properly, the heated air is passed over the material to be dried, the water in the wet material is evaporated by the heat in the air, and the moisture must be carried out of the dryer by the outgoing air. The power required t o force the air through the system increases with increased velocity of the air, but the cost of this is offset by the fact that increased air velocity aids in the transfer of heat from the heater to the air and also aids in the rate a t which water is evaporated from the wet stock. Therefore, i t is seen that both the design of a n economical air-drying apparatus and its control (either from the standpoint of quality or quantity of output) depends on a thorough knowledge of the flow of fluids and the flow of heat. So with the other unit operations of chemical engineering, each may be analyzed and a proper understanding of all will be found t o depend to a considerable extent on a thorough knowledge of the two primary unit operations, the flow of fluids and the flow of heat.
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648
T H E JOURNAL OF INDUXTRIAL A N D ENGINEERING CHEMISTRY CLASSROOM TRAINING IN THE UNITOPERATIONS
As stated above, before the student enters the Practice Sdhool, he must be well grounded on the theoretical side of both the primary and the more important secondary unit operations. This is accomplished a t the Institute by mea5s of a course extending throughout the sen?or year in which these subjects are treated in a quantitative, mathematical way by means of lectures, recitations, and home problems. The numerous problems are broad in scope and tax the student’s imagination and judgment, giving him practice in the quantitative application of the laws and principles taken up in lecture and recitation. The instruction along these lines is thorough and exhaustive, but as in any recitation and problem course most of the necessary data must be supplied by the instructor, and, furthermore, there is a limit beyond which it is difficult for the average student to go without corresponding personal experimentation. Therefore, after the student has completed his four-year undergraduate course in Chemical Engineering which includes the above lecture, recitation, and problem work, and for the completion of which he receives the degree of Bachelor of Science, he then may elect to go into the School of Chemical Engineering Practice, where he carries out personal experimentation in the various unit operations of chemical engineering.
EXPERIMENTAL WORKIN
Vol. 14, No. 7
second function has been ignored and the assumption made t h a t if a student knew the fundamentals he could find out by expe-
THE UNIT OPERATIONS OF CHEMICAL
ENGINEERING
’
During the first week or so a t each station in the School of Chemical Engineering Practice, the student examines in detail the plant layout, construction, and methods of manufacture. He correlates his efforts along these lines by making up a flowsheet of the process or by writing a cbmplete report. Flowsheets are made by the student when the sequence of operation, plant layout, method of handling materials are important; a report of his detailed study is made when construction methods and the materials of construction are more important. After the student has been made familiar with the particular plant and process, he is ready for his unit operation experimental work. The first experimental work is usually carried out by the entire group a t a given station (from six to twelve students). I n this way, the student quickly learns how to organize his plant experimental work, how to attack a plant problem, and comes to appreciate the difficulties of plant research and some of the precautions that must be observed to obtain reliable figures. After two or three “group tests” he is allowed to choose individual “plant investigations” in the various fields of the unit operations. It should be pointed out that in all this experimental work in the plant the Director of the station endeavors to throw on the student the entire burden of designing the test, carrying it out, and working up the results. Some men, owing either to ability or maturity of judgment, are better able to decide what data should be obtained and how, and the significance of poor results, while others with less ability along these lines have to be aided to a greater extent by the station’s staff. Such individual attention is possible, since the average number of students at a station is about ten. I n the‘ period of eight weeks which the student spends a t a station, he carries out experimental work in each of the unit operations assigned to that station, and he then proceeds in a like manner to the next two stations, thus in twentyfour weeks carrying out experimental work in all the more important unit operations under plant conditions, such as exist in seven large chemical industries. I n all this experimental work, it should be emphasized that the underlying motive is the application of theory, as developed in the classroom, to practice, or a comparison of calculated with observed results. Since the function of engineers is to apply science to industry, it is necessary in the training of engineers to do two things: first, teach them the fundamentals of their science, and, second, teach them how to apply it. Too often the
per cent 60, in Nue Gases P E R CENT
EXCESS AIR AND
STACK TEMPERATURES V S . P E R CENT F L U E GAWS
cos I N
rience how to apply them. Not only is this incorrect, but it is usually true t h a t i t is much easier for the average student to absorb knowledge than i t is for him to apply it.
EXPERIMENTATIONS IN PLANT 17s. EXPERIMENTATION IN LABORATORIES
Here the question might well be raised why plant experimental work is considered necessary and why we are not content to have the student carry out experimental work on a laboratory scale only. Consider the primary unit operation, flow of fluids. It is easily possible to arrange a series of laboratory experiments measuring the flow of gas and liquid streams by means of orifices, pitot tubes, etc., and from such work the student obtains information and training of much value. However, the laboratory apparatus is practically all set up for him, even the location a t which the orifice or pitot tube must be inserted is evident from an inspection of the equipment, and the student has no opportunity either to design his test or to exercise any judgment as to the most suitable method. He does not decide what instrument or what particular point for measurement is best suited, nor does he develop the resourcefulness necessary to obtain this measurement under plant conditions of inaccessibility and continuity of production. The particular example of experimentation just given is of the simplest sort and such work can be made much more effective in technical laboratories than experimental work in the more complicated unit operations. As carried out in the School of Chemical Engineering Practice, the student would meet the above problem as a subdivision of a larger problem, and his first decision would be that this particular stream of material had to be measured in order to obtain necessary data for the main problem. Next he would have to decide on his method of measurement, taking into consideration the accuracy needed in connection with the problem, as a whole, and how the apparatus could be installed to give satisfactory results without seriously interfering with plant production. I n plant experimentation,
T H E JOURNAL OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
July, 1922
conditions are such t h a t the student develops technical judgment and mental resourcefulness, whereas in the chemical engineering laboratory the experimental difficulties are much less and fail to develop the student properly. E’urthermore, the student has to find out for himself a t some later date the difficulties of repeating his laboratory methods in the plant and how to work effectively under such changed conditions. Many other points, such as cost of equipment and cost of operating such equipment, might be brought up against confining the chemical engineer’s training to class and laboratory work, but we believe a sufficient number of points have been brought out to show the superior effectiveness of plant experimentation. ILLUSTRATIONS OF
EXPERIMENTAL METHODS AND RESULTS
Having seen how the field of chemical engineering has been subdivided and the general method by which the students attack plant research problems, a few details and illustrative results on several of the unit operations may be of interest. COMBUSTION-In view of the wide use and importance of fuels in all industries, considerable attention is given to the study of the secondary unit operation combustion. I n the Practice School many facilities are available for studying this important phase of chemical engineering, consisting of hand and stoker fired boilers burning coal, coke breeze, fuel oil, blast furnace gas and wood waste, open-hearth furnaces and soaking pits burning producer gas generated in numerous types of producers, coke ovens burning coke oven gas; rotary kilns using powdered coal, etc. This unit operation offers one of the most striking fields for demonstrating the industrial applicability of the principles of physical chemistry, such as the law of mass action, the effect of temperature and the effect of catalysts (incandescent brick surfaces) on the rate of reaction. I n studying combustion, the student also receives much training in the use and importance of the two primary unit operations, flow of heat and flow of fluids. Figs. 1 and 2 show the summary of the results obtained by two students experimenting with a 500-h. p. boiler burning fuel oil. These men studied the effect of varying the amount of excess air, the effect of changing the distribution and mixing of the air and oil, and the effect of load on the efficiency. Table I shows a total heat balance on a set of sixty coke ovens. It should be borne in mind that the primary educational object
Percenfdi in f l u e Gases PER CENT SENSIBLE HEATLOSTI N FLUEGASES FLUEGASES
PIS. PER CENT
COOIN
649
is not to obtain a “heat balance” but to study one of the unit operations. The comparison of the heat input and output merely gives some idea of the completeness of the data, although, of course, a check may be reached by counterbalancing errors. The measurement of the heat quantities shown in Table I gives good training in the study of the flow of fluids and the flow of heat as well as in combustion, although the main benefit of such a test lies in the power of reasoning developed in the design of the test and the practice obtained in organizing for effective work. A comprehensive test on coke ovens gives excellent training in the measurement of material and heat quantities, in gas and fuel analysis, in the study of the laws of heat radiation and convection, and in the use of industrial stoichiometry. Furthermore, to increase the efficiency of teaching in the School of Chemical Engineering Practice, all the work is “motivated,” that is to say, the student is not told to carry out experimentation in the unit operation itself, but some definite and usually attainable objective, the importance and significance of which he understands, is placed before him. TABLE I-HEAT BALANCEON SETOF 60 COKEOVENS INPUT
Per cent Coal, latent.. . . . . . . . . . . . . . . . . . . . 92.82 0.01 Coal, sensible.. Fuel gas latent ................................. 7.15 Fuel gas: sensib . . . . . . . . . . . . . . . . . . . 0.02
.....
100.00
OUTPUT Coke, latent. . ........................................ Coke, sensible. . . . . . . . . .....
......................... . . . . . 0.22 ..... 1.29 .......................... ................... ................... .................... .................................. 3.06 -
Tar, latent.. . . . . . . . Tar, sensible. . . . . . . . . . Stack, sensible. . . . . . . . Foul gas wate Foul gas wate Radiation., . Unaccounted
Per cent 61.5 3.63 21.04 1.24 4.84
100.00
ExTRAcTIoN-In this field there are a number of phases; let us consider the absorption of a gas by a stream of liquid traveling in opposite direction to the gas stream. The underlying principles of countercurrent extraction are studied thoroughly in class by the student before going into the Practice School, so this theory is applied to plant practice under various conditions. For example, the student applies these principles to such problems as the absorption of sulfur dioxide by milk of lime to form bisulfite cooking liquor used in the manufacture of sulfite wood pulp, t o the absorption of hydrogen chloride gas by water in the manufacture of muriatic acid, t o the scrubbing of light oil from coke oven gas by paraffin oil, and to the absorption of ammonia from coke oven gas by water, Knowing the equation expressing the theoretical performance of such a countercurrent extraction system, we can measure the efficiency of the extraction tower by means of a “tower coefficient,” The “tower coefficient” enables the student to compare quantitatively the efficiency of various forms of tower packing, and to forecast the effect on output of a change in the temperature of the gas or absorbing liquid, of increasing the size of the tower, and of increasing or decreasing the partial pressure of the solute in the entering gas stream. Some of the “tower coefficients” determined by the students which were used by them in forecasting results under changed conditions are given in Table 11. The determination of these coefficientsalso shows the student how small towers may be used experimentally in determining the proper size of a new installation of commercial towers. These coefficients are all expressed as lbs. of solute absorbed per min. per cu. ft. of inside tower space per mm. of Hg driving pressure of the solute (partial pressure of solute in gas minus partial pressure of solute in solvent).
THE JOURNAL OF INDUSTRIAL AND ENGINEERING CHEMISTRY
650
TABLE I1
TOWER COEFFICIENT x lG-0
., ..,, ..., . .. .. .. .. .. .. .,.. Benzene absorbed in paraffin oil, slat tower. Ammonia absorbed in water, slat tower.. . . . . . . . . . . .
Sol absorbed in milk of lime, tile tower. SO1 absorbed in milk of lime, plate tower
. I , ,
85 2160 2240 216
DRYING-Drying, an important secondary unit operation, is given much attention, and experimental work has been carried out by the students on coal-fired, semi-indirect rotary coal dryers, steam-heated rotary sugar dryers, steam drum dryers, can dryers for drying pulp and paper, French dryers used for drying flake soap, etc. The student is taught to find the largest resistance to the flow of heat or to the diffusion of water, to con-
Vol. 14, No. 7
centrate on this particular resistance and reduce it by one means or another, thereby bringing the dryer to the maximum possible capacity for the machine or stock in question. For example, a test by some students on a can dryer drying paper showed that the major resistance to the flow of heat from the steam inside the drum to the paper was tho resistance between the outside face of the drum and the underside of the stock. The student could, therefore, direct his attention to the most probable point for a successful attack. Again, in the air-drying of sugar a similar method of attack showed the importance of increasing the air velocity, which made possible a considerable increase in output from the same dryer.
NOTES AND CORRESPONDENCE Petroleum Hydrocarbons Editor of the Journal of Industrial and Engineering Chemistry: While reading the recent interesting paper by Dr. Van H. Manning on “The Pioneer’s Field in Petroleum Research,” I observed the following statement: “Our knowledge of the constituent hydrocarbons making up lubricating oils is well nigh a blank, and so much depends on the quality of those oils.” For more than a year, I have been industriously engaged with the aid of several assistants on the continuation of the work described in the preliminary paper I presented before the Petroa t the Rochester Meeting in May leum Section of the SOCIETY 1921, and have separated from representative samples of the world’s supply of petroleum the undistillable lubricant and asphaltic hydrocarbons. Within a few months I expect to publish a n account of the separation of these hydrocarbons, and their identification by specific gravity, molecular weight, and analysis, together with their index of refraction, and viscosity, and with the relations in general of specific gravity and viscosity. CHARLES F. MABERY APPLIEDS C I E N C E CLEVELAND, OHIO May IR, 1922
C A S E SCHOOL O F
The Plant Manager and the Chemist Editor of the Journal of Industrial and Engineering Chemistry:
T
HE ideal chemist is the catalytic agent in a plant, furnish-
ing and examining ideas to improve or cheapen manufacture, to produce new and desirable products, and to standardize production. Upon him rests the selection of raw materials to build up into the finished line upon which the reputation of the house rests. He is the judge of the standard to be attained by the finished product, and upon his ability to read aright the desirability of the finished article rests the whole force of the sales department. He is the leaven working through the foremen in the plant, developing a cooperative spirit of constructive criticism leading to the development and improvement of the different steps of manufacture. The chemist and his “Lab” and his assistants are the media for the investigation and the rejection or the translation into practice of the ideas not only of his own force but of the plant manager and the other members of the manufacturing force. No person is too exalted to have his ideas carefully tested before attempting to use such ideas in manufacture, and no employee is too humble not to have his thoughts receive careful attention.
Now as a plant manager, I find that the chemist is far from the ideal. The fundamental failure of the chemist is first in a lack of knowledge-of thoroughness of education in chemical and related lines. The chemist must, first of all, know chemistry. He must be familar with modern thought and development, particularly in the chemistry involved in the type of manufacture in which he is employed. He must be a good, rapid, accurate analyst because practically all investigative and research work rests on the bed-rock of accurate analysis. One might write a volume on the failure of our universities to realize this fundamental truth in industry. And not only should a chemist be thoroughly familiar with analytical methods, he must in industry be able to secure results rapidly, for information which is quickly available is of greatly enhanced value. But above all, his conclusions drawn from analysis must be correct. Any plant manager who has given work to chemists or commercial laboratories is familiar with the appalling lack of agreement in simple analytical determinations. Far too often, the plant manager knows the chemist is wrong and still oftener he suspects gross inaccuracy and carelessness. These poor results are not confined to graduates of secondary schools, and it would appear high time that somebody of authority should pass on a commercial laboratory’s ability t o do correct work in the industrial lines in which it claims to be expert. The second great failure of the chemist has been in salesmanship. The chemist must sell himself and his ideas to the plant manager, to the superintendent, and to the various foremen. All too often, meritorious ideas are abandoned because the man actually carrying out the operations has not had the matter thoroughly clarified to him and does not give it a fair trial. If the chemist has good salesmanship, every man from the manager down will always feel that his suggestions have potential merit and no idea will get across in production without such faith. Brains must have the cordial cooperation of labor. When the chemist has the plant foremen as eager helpers, it is safe to say that he is not only a good chemist but a reasonable human being with a proper respect for men working in subordinate positions. This leads to my third criticism that too often the chemist is seeking authority in the plant rather than responsibility. The chemist who must have personal authority to get work done by employees in a factory is about a certain failure. What a plant needs in a chemist is one who takes responsibility for progress in manufacture and does not require authority in order to secure all the cooperation he needs. To secure such help the chemist must remember that any man who is doing his work well even in a menial position takes pride in that work and is anxious to do it better but resents any trace of overbearing authority. The Golden Rule will pay a chemist greater dividends in a factory than
