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CAREERS If* CHEMISTRY AM® CHEMICAL ENGiiSEERBNG
The Process Design Engineer WALTER E. LOBO, M W. Kellogg Co., Jersey City, N. J.
I m a g i n a t i o n , versatility, a n d c o o p e r a t i v e talent must s u p p l e m e n t g r o u n d i n g in t h e f u n d a m e n t a l s of unit o p e r a t i o n s HOSE of us w h o have had t h e opportunity of interviewing a n d hiring many young chemical engineers know that a large n u m b e r of t h e m say that they a r e interested in process design work. T h e y seem to think that, inexperienced as they are, process design engineering is a good place to start. Therefore, it should b e worth while to look into t h e qualities, t h e b a c k g r o u n d , a n d t h e experience which such a n engineer should have. W e shall limit ourselves to chemical engineers, w h e t h e r with a bachelor's, master's or doctor's degree in chemical engineering. T o b e successful in this field a m a n should b e thoroughly grounded in the fundamentals of unit operations. H e should h a v e a good understanding of thermodynamics a n d of physical chemistry. T h e s e , with some knowledge of industrial chemistry, will prepare him for process work in a variety of industries. Next, w e come to t h e m a n himself. T h e qualities n e e d e d for success in this field are little different from those needed in any other. Perhaps there should be a little m o r e emphasis on a general awareness of progress in other fields than that r e q u i r e d by, say, a research worker. Imagination, versatility, a n d the ability t o break d o w n the problem to its fundamentals are of inestimable value. Keeping u p w i t h technical literature and association w i t h chemical engineers in other industries will promote new ideas, a n d will facilitate their application to t h e improvement of t h e processes under study. A constant desire should exist to better t h e m e t h o d s being used, to improve t h e process, a n d to overcome its inadequacies and shortcomings—all processes have them. These qualities, w e might say, are professional attributes.
may see things from sibly better angle. He along well with those for engineering work operative effort. F e w , t h e work of one m a n .
another and posmust be able t o get working with him, is primarily a coif any, projects are
Cooperative Projects Perhaps the best w a y to describe the work of an experienced process design engineer would b e to describe a project which was started during t h e war and which was brought t o fruition shortly thereafter. T h e development of the "tonn a g e oxygen" process is a typical example of a cooperative effort of process men, design men, operating m e n , and mechanical men, most of whom were chemical engineers. It was early recognized that the most favorable a p p r o a c h to t h e problem lay along t h e route of the well known m e t h o d of fractionating liquid air. H o w ever, current practice was unsuitable for a n u m b e r of reasons, among t h e m being t h e large, bulky e q u i p m e n t a n d t h e dep e n d e n c e on an outside source of chemicals. Expert knowledge of the m a n y problem" which w o u l d b e encountered was not at hand and means to obtain such knowledge were necessary. A w i d e spread development program was launched. T h e first problem was the process flowsheet or cycle to be employed. Process m e n w e r e set to work calculating cycles and studying t h e m from operability and efficiency angles. O t h e r s went t o work to
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Development W o r k to Be D o n e T h e result of all this preliminary s t u d y was that development experimental w o r k was found to b e necessary in certain fields. Once the most favorable cycle was ascertained on paper, it was necessary to develop equipment to p e r m i t its use in the models proposed. There were no c o m pact, lightweight, h i g h - s p e e d compressors or expanders. T h e r e w e r e no suitable, r e l atively small, efficient heat exchangers. There were no fractionating towers w h i c h were capable of producing a good yield of high-purity oxygen in the short h e i g h t available. T h e r e w a s little reliable k n o w l edge on the equilibrium relationships of the three-component system, n i t r o g e n , argon, and oxygen. There w a s still a lot to learn about the properties of low t e m perature insulations. M a n y universities a n d industrial c o n cerns collaborated in t h e great effort t o obtain t h e fundamental and design d a t a . Many test units w e r e built and tested. Small packed fractionating columns g a v e
TV"/"ALTER E. L O B O has been in c h a r g e of t h e VV chemical engineering division of t h e M. W . Kellogg C o . since 1939. D u r i n g t h e w a r h e directed the company's d e v e l o p m e n t project for t h e National Defense Research C o m m i t t e e , which involved the design, building, and operation of small a n d large oxygen plants. H e originally j o i n e d Kellogg in 1929 a s assistant job e n g i n e e r , then w a s p u t in charge of furnace design work, later of the technical d a t a and t o w e r design groups. An M I T m a n , he d i d graduate work at L o u i s i a n a State University and for several years was in t h e r a w sugar a n d plantation white sugar business in C u b a a n d Colombia before joining Kellogg.
D e a l i n g with People A process design m a n will have to deal with p e o p l e . H e will have to explain his t h o u g h t s , his ideas to others. Therefore, h e should b e able to express himself properly, both orally and in writing. H e must h a v e confidence in himself and in his work; b u t h e must not suffer from over-confidence or fail to heed t h e counsel and advice of others. H e must realize t h a t other p e o p l e have good ideas, that they
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study a n d analyze all the available d a t a on low-temperature regenerators, heat exchangers, and fractionating equipment. Still others correlated the properties of the materials t o be dealt with; air, oxygen, nitrogen, argon, c a r b o n dioxide, acetylene, a n d water vapor. The properties of insulating materials a n d of m a t e rials of construction at low t e m p e r a t u r e s also had to b e studied. Experts in c o m pressors and expansion engines were called in a n d the problems w e r e outlined t o t h e m .
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excellent perloriiKiiicc. hut their results were far from d u p l i c a t e d when the e q u i p ment was increased in size. Tests on air and water systems finally led to t h e d e velopment of efficient fractionating trays with one-and-a-half-inch tray s p a c i n g something w h i c h , to o a r knowledge, h a d not previoush Keen done. T h e design ol efficient counter-current, multifiuid heat exchangers finally permitU'd the use of cycles with exceedingly close w a r m - e n d temperature a p p r o a c h e s . Soon prototype units were under construction. Eventually, a compact 1,()()(>C F H unit was r e a d y t o test. T h e process engineers were on the job to prove that they had solved the problem of designing a n e w , simpler oxygen cycle. T h e first run was not t o o encouraging; t h e second was less so. T h e third looked rather hopeless. The unit p l u g g e d u p , and changes d i d not seem to h e l p . Finally, analysis showed that two things w e r e giving t r o u h l e ; the air to the plant w a s contaminated by oil from \he compressor, a n d carbon d i oxide was not being completely removed in t h e reversing heat exchangers. T h e first problem was originally eliminated by t h e use of a fine asbestos paper oil filter, a n d finally by the use ol A carbon ring compressor. The second -was eliminated b y a t h e r m o d y n a m i c trick, the so-called " u n balance system." The cycle was p r o v e d and the aim w a s accomplished, although not in the time hoped for. Other units of similar design, some smaller, some larger, w e r e built and tested. All t h a t was necessary was to a p p l y t h e principles which had been rather thoroughly worked out. T h i s was t h e war development; t h e application to peacetime industrial uses followed. T h e approach n o w was somew h a t different. Compactness was not so necessary; cost and economy b e c a m e t h e prime considerations. F u r t h e r study, further development were required. T h e size of the contemplat eel plants was m a i n times that o f the war-developed units. W i t h the increase in size c a m e new p r o b lems of large gas-to-gas heat exchangers a n d still larger fractionating equipment. Longer-time o p e r a t i o n became a icquisite a n d automatic control a necessity. H a z a r d s which m i g h t be accepted in w a r t i m e h a d to b e completely eliminated in industrial use. W h a t Part D o e s Process Engineer Play? What was the tasT< of the process design engineer in this new endeavor? He h a d to d r a w u p h i s process flow-sheet, showing all t h e design conditions of flow rates, temperatures, pressures, a n d heat duties. He h a d to see that all t h e e q u i p m e n t satisfied the conditions to b e imposed u p o n it, from both a process a r d a m e chanical point of view. H e h a d to see t h a t the plant w o u l d get into operation quickly and safely. He h a d to m a k e sure t h a t once i n operation it would c o n t i n u e t o produce the designed quantity a n d quality of p r o d u c t over long p e r i o d s of t i m e at h i g h efficiency. H e h a d t o b e certain that his design represented t h e 3632
optimum tor the specific conditions obtaining. For instance, h e h a d to he certain that he h a d designed for t h e optim u m head pressure, or o p t i m u m temperat u r e differences for t h e given value of steam or electric power. The use of ox>gen for industrial purposes is almost completely d e p e n d e n t on its cost, so that cost must be pared to a m i n i m u m when it includes all such items as p o w e r , labor, maintenance, and depreciation. In the broad p i c t u r e , tin- process n u n has to know how the cost < >t his. product varies as a function i t h e size of the plant, how it varies w i t h t h e cost of utilities, how it varies w i t h p r o d u c t p u r i t y l i e must be able to recognize t h e r e l a t h c importance of the various parts of tinplant in determining plant and product cost. He must know what to make mort efficient and what less as circumstances vary, in order to b e able t o propose the optimum design conditions. N u m e r o u s Facets to t h e J o b Process design engineers may hawm a m parts to play in the d e v e l o p m e n t of a process such as t h a t described above Some ma> specialize in t h e development of e q u i p m e n t , some in the design of special types of heat exchangers o r fractionators. Some may prefer operation a n d the solving of operating difficulties. SOUHmay spend most of their time in arriving: at the o p t i m u m over-all design of plant And some with the most experience may direct a n d guide t h e entire project, coordinating t h e efforts of all so that each will play his part most effectively in t h e earnest g a m e . T h e over-all result is t h e product of many individual efforts a n d n o one can say which, is t h e most important. A simple idea such as t h e "unbalance"" ina> make t h e entire process u n i q u e . A ' h e a p , efficient gas-to-gas heat exchanger may so greatly decrease t h e cost of t h e product that its use can b e considered i n applications not previously thought to b e feasible. T h e combined efforts of m a n y engineers all working toward a c o m m o n goal arc needed to m a k e a process a success. Once the process is developed it imi-ct be sold. Again, t h e process engineer ha> a part to play. H e m u s t investigate t h e possible applications of t h e process or its product. H e must a t t e m p t to devise n e w uses for each. And h e must never b e satisfied that h e has r e a c h e d the optimum— that improvements a r e n o longer possible. In a competitive world, t o remain static is to go b a c k w a r d . C o n s t a n t effort is n e e d e d to k e e p a h e a d , to offer a better proce-ss than o n e s c o m p e t i t o r does. T h e example given ol the t o n n a g e oxygen development is but one of t h e many which might have b e e n cited. Petroleum processing plants, including t h e r m a l a n d catalytic cracking u n i t s , polymerization plants a n d gas r e c o v e r y units, heavy chemical plants, s y n t h e t i c rubber a n d a m monia plants, a n d t h e m a n y processes involved in the p r e p a r a t i o n of p h a r m a ceuticals, fats and soaps, cements, paints, metals, food p r o d u c t s , a n d a n endless host CHEMIC
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ui others, might have been u>ed equally well to illustrate t h e work of t h e chemical plant designer. In e a c h of the»e t h e proce>smg is different b u t t h e s a m e principles apply. T h e fundamentals of c h e m i cal engineering play an important part in. all of then*. T h e weighing of alternatives iijust b e m a d e ; t h e economic evaluation of methods, designs^ and e q u i p m e n t niu>! b e undertaken. T h e help a n d assistance of others must b e utilized a n d t h e coordination of entire groups will b e necessary. T h e process may start in t h e laboratory ane. o \ c r l a p . T h e s e might b e called pro-.es> e q u i p m e n t design and p u r e proce*^ d e sign. Furnace Design Man T h e process e q u i p m e n t doJs£ucr tn.t> h e a specialist. l i e may restrict his effort- t o one or more of t h e following. 1. F u r n a c e design, involving sisch p r o b lems as p e t r o l e u m heaters a n d cracking furnaces, fired kilns a n d ovens, and fired reactors: 2. Heat exchanger design, involving coolers, condensers. shell-and-tube exchangers, evaporators, a n d crystallizers: o. T o w e r design, involving fractionators. absorbers, extraction, v a c u u m and extractive distillation towers of t h e b u b b l e tray, baffle^ perforated plate, packed, or spray type; 4. Fluid flow problems, involving sizing of pipes, nozzles a n d lines, ejectors, a n d relief valves; 5. Instrumentation a n d control work F o r the purpose of illustration, let us consider t h e specific case of a furnace d e sign m a n w h o is asked to design a simple crude t o p p i n g furnace. H e receives certain d a t a , of which t h e following might he ty pica!: Furnace t h r o u g h p u t Inlet t e m p e r a t u r e Outlet t e m p e r a t u r e Outlet pressure Character of furnace c h a r g e Percentage ~ vaporized Fuel Heating v a l u e of fuel
10.000 barrel* d a y 4 5 0 °= F . 7S0 F . 3 0 p.s.i.g. 3 7 . 3 c API Mid-Continent crude oil S0.5 Natural gas 1,000 Blt.u. int. (LHV)
ft.
T h e r e w o u l d probably also b e given t o him a b r e a k d o w n of t h e various petroleum fractions w h i c h are t o b e obtained From the unit, t h e specific ( o r A P I ) gravity e i N:D
E N G I N E E R I N G
MEW
< « u h . tin; q u a u t i t ) of e a c h s t r r a n i , a n d it>» molecular weight.
T h e first step would be the calculation of the furnace heat d u t y by t h e use of e i t h e r enthalpy charts or specific and latent heats. T h e next step w o u l d b e to d e c i d e upon t h e over-all operating conditions, such as percentage of excess air to b e used a n d t h e outlet flue gas temperat u r e , both of which determine t h e furnace efficiency. W i t h this figure, t h e quantity of fuel to be fired and the q u a n t i t y of the reMilting flue gas may b e calculated. Next, t h e t u b e size and length would he chosen, a-» well as the type of flow, that is, whether t h e tubes are to be arranged in a single stream, in series, or in two or more multiple streams in parallel. F o r instance, experience might indicate that two separ a t e streams, each using 5Va inch O . D . by 5 inch I.D. tubes. 40 feet long, might he economical. D e s i g n i n g the F u r n a c e T h e radiant section, in which the fuel is liberated, is t h e first portion of the furn a c e to b e designed. Here, empirical relationships are usually used to d e t e r m i n e the a m o u n t of heat absorbed by t h e radiant section tubes when firing under certain conditions of excess air, fuel, a n d over-all heat transfer rate. T h e latter might, in this case, he taken as 10,000 B.t.u. per h o u r per s q u a r e foot of over-all circumferential radiant tube surface. With these a s s u m e d conditions, t h e radiant section d u t y would b e determined, a n d from a k n o w l e d g e of t h e radiant r a t e a n d the n u m b e r of square feet of heating surface p e r t u b e , t h e n u m b e r of radiant tubes req u i r e d might b e calculated. I n accurate design work it will b e necessary to lay o u t these t u b e s , showing their location in a sketch of t h e radiant section box, and t h e n with dimensions thus o b t a i n e d the designer can recheck t h e heat absorption. P'inally, by a radiant section h e a t balance, t h e t e m p e r a t u r e of t h e flue gases leaving t h e section can be obtained. T h e design of the convection section is the next step. T h e width of the bank, t h a t is, t h e n u m b e r of tubes across, must b e set. a n d t h e mass velocity of t h e flue gases tlirough the minimum free area of t h e t u b e bank determined. T h e n the numb e r of t u b e s is calculated by considering heat transfer by radiation from the brickw o r k , heat transfer by radiation from the hot flue gases, heat transfer by convection from the hot flue gases, and t h e film coefficient of the fluid within the t u b e s . The convection b a n k is usually d i \ i d e d into t w o or more sections, t h e logarithmic mean t e m p e r a t u r e difference for each being calc u l a t e d by trial and error a n d heat bala n c e s b e t w e e n the flue gas and oil sides. Upon completion of the calculation of t h e t u b e surface, a furnace sketch must be d r a w n . T h e n u m b e r a n d type of burners m u s t b e specified: t u b e materials must be c h e c k e d a n d materials of construction dec i d e d u p o n . T h e pressure d r o p on the oil side must b e calculated by a stepwise trial-and-error method. Flue gas draft V O L U M E
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losses must be figured and t h e required stack height and diameter set. T h e overall task involves t h e application of m a n y different phases of heat transfer a n d fluid
flow. T h e purely process design m a n may not, except in a general way, concern himself with the detailed design of equipment. H e may deal with t h e over ill process, determining t h e following: 1. Over-all and individual material balances, involving the yield and quality of all products to and from the various pieces of equipment; 2. Process Hov\ -sheet, including all operations to he performed in the plant, as well as all operating conditions, such a^ temperature and pressure, in all equipment: •3. Heat balances on all equipment, and calculation of heating, cooling. A\U\ exchanger duties, reflux ratios, etc*.; 4. Tray-to-tray calculations to set satisfactory operating conditions and equipment necessary to effect desired separations or recoveries; 5. Internal flowing quantities and the setting of p u m p capacities and suction and discharge conditions; 6. Optimum plot plan and plant layout. 7. General process specifications; 8. Guarantees or anticipated performance: 9. Economic evaluation of t h e process Designing a C r u d e Distillation Unit As a typical example, in this case w e might consider t h e operations required in the design of a simple c r u d e distillation unit, the furnace for which has just b e e n discussed. By means of true-boiling-point and crude-evaluation curves, it is possible to decide on the amount of each product stream which may he w i t h d r a w n , and t h e respective characteristics of each. The equilibrium flash curve will b e used to determine the furnace outlet conditions and the amount of oil which will b e vaporized in t h e flash zone of t h e c r u d e topping tower. All tower temperatures must t h e n be set. usually f' more or less empirical relationships involving the distillation properties of the products. T h e overhead temperature corresponds t o the? dew point of t h e overhead stream at the tower t o p conditions, the bottom a n d side stream temperatures corresponding to the b u b b l e points of the respective streams. Experience is usually t h e basis in cases of this type for setting the top reflux ratio, as well as t h e reflux ratio at any point in t h e tower. T h e over-all heat balance is t h e n made, considering t h e heat entering t h e tower from the furnace a n d t h e heat leaving it in the product streams. Later, it may he found necessary to increase t h e furnace load if insufficient heat has been provided to satisfy' the reflux ratio required for satisfactory separation between t h e various cuts. Once the heat balance is satisfactory, the internal vapor and liquid loads flowing up and d o w n t h e tower at various points must be calculated. These will be used to set t h e tower diameter. T h e n u m ber of trays may either b e set by experience or by certain empirical methods
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whu-h \\.i\f been developed. If sidestrcdin strippers arc to be employed,, they must be designed in a manner somewhat similar to that used for the main fractionating tower. T h e process flow-sheet will he developed by proper consideration of the -desirability of heat exchange b e t w e e n the incoming cold crude and t h e out going hot product streams. A reflux condenser may be used to supply the first portion ol preheat to the crude, after whicl i additional heat exchangers from the large side streams may he utilized. Fiiua.ll> a hottoms-to-crude exchanger may Ix- emplo\ ed. When the fuel cost is high i t may be desirable 1 to obtain the maximum amount of preheat possible, where .as in other cases it may be economical t o try only lor a moderate amount of preheat by beat exchange throwing more lo.id on the direct-fired heater. The relative economics of the two types of surface siunild be considered, and the effect on ec >st of decreasing logarithmic mean temprr-ature difference between hot and cold st Teams a^ more h< at is obtained must he taken into account. At this point good judgment, experience, and approximate cost curves play ;» great part in obtain iug a good .Hid economical process design. In order to complete the tas!
