APPLICATION
OF
MACHINE
E. J. HIGGINS, J. W. KELLETT, and 1.
COMPUTATlON
TO
PETROLEUM
RESEARCH
T. U N G
Esso Research and Engineering Co., Linden, N. J.
Engineering Design on a Computer Automatic computation of design calculations is a new and rapidly growing field. Maximum use of the advances in computer technology will open new vistas to the design engineer
A w o m m c computers are not magic wands to be waved. They have been called “thinking machines,” but might better be described as “unthinking machines.” They follow instructions blindly, nothing more and nothing less. To receive the right answers, the right instructions must be given. T o find answers in the planning and design of new refineries for the Jersey interests, and for major improvements and expansions to existing refineries, the engineering divisions of Esso Research and Engineering have used mainly a mediumsized digital computer, the IBM 650. Their system of an “open shop” for the programming of engineering calculations was developed to get the right men to work with the machine: the men who asked the questions-the engineers-and the men who make the machine answer them-the programmers. The mathematician or trained programmer, trying to build a design program from a single or even many sets of design calculations, can incorporate the logic needed to cope with only those contingencies which arise in the calculations before him. Such a program would be limited in scope, and could not handle many situations which would fall within the broader experience of the engineer. On the other hand, when an engineer with limited experience was the only man available for programming, the programs were frequently limited in scope. In some cases they gave erroneous results because the programmer had insufficient appreciation
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712
of the range of problems to be solved. Fortunately, it is easier to teach a n engineer programming than to teach a programmer engineering. Under the “open shop” system a design engineer familiar with calculations in a given field (an expert, if possible) is trained in programming techniques. Once he has learned the few basic principles of the computer, he is ready for extensive consultation with other experts on the calculation. This consultation should be followed by a review of past designs and a look at probable trends for future designs to determine the full range of each variable that may be encountered in future calculations. H e now approaches the critical period for any program: serious, feet-on-thedesk thinking to set up the basic logic for the calculation, to determine what range of variables the deck should handle, and to decide upon the degree of internal optimization that should be built into the deck. The scope and philosophy formulated here will decide whether a useful tool is being created or an engineer is wasting several months of his life. The scope chosen a t this point must usually be a compromise between two competing goals: maximum flexibility and infallibility, and minimum machine running time. Programming a calculation as two or more somewhat similar decks, each covering part of the desired range, will often help to achieve the best compromise. Assistants to engineers and math clerks help to save engineering manpower. The assistants are women with nonengineering degrees, proficient in computer techniques. As programming experts they work with engineering experts who are programming novices. The math clerks are girls with high school diplomas, and a flair for math. They take over a great deal of data compila-
INDUSTRIAL AND ENGINEERING CHEMISTRY
tion, routine programming, and other tasks associated lrith the over-all programming work. However, the engineer remains in full control of the program from start to finish. Automatic coding systems now make open shop programming even more fruitful. Esso Research now uses a large scale computer, the IBM 704. IBM has developed an automatic coding system for the 704 called Fortran. This system is much easier to learn than methods ‘for 650 programming-an invaluable help for the engineer who has to handle large complex problems effectively. Programming and debugging on the 704 take only 50 to 70% of the time formerly required for the 650. For the 650 a n automatic coding system similar to Fortran is now available, called ForTransit.
Working with the Computer Some of the specific calculation pro-
I
symposium articles: Analytical Laboratory Operation and Control, Utilizing Business Machine Punch Card Procedures, and a special feature entitled Application o f Computers to the W o r k of the Analyst.
PROCESS DESIGN S P E C I F I C A T I O N S
PAGE
IVOb
PfiOCa
HEAT
EXCHANGt
SPEC.
59881
EQUIPMtiUT
U N I T NUMBER-FLOW PLAN SYMtJOL SERVICE OF U N I T TYPE AND L O C A T I O N OF EXCHANGER TOTAL SURFACE PER PER U N IU T *N I T NUMBER OF SHELLS SURFACE PER SHELL DUTY REQUIREMENTS PER U N I T TOTAL HEAT TRANSFERRED
E-4000 CRUDE HORIZONTAL SQaFT.
VAPOR CONDENSED OH L I Q U I D VAPORIZED MOLECULAR WEIGHT GRAVITY STEAM CONDENSED I N L E T TEMPERATURE OUTLET TEMPERATURE MAXIMUM F R I C T I O N PRESSUiiE DROP NORMAL OUTLET PRESSURE D E S I G N AND CONSTRUCTION
P A
658 r 00000
M M BTU/HR
1 6 00000
109. 6 0 0 0 0 55.00000 PARALLEL 2
SERIES
77aCiOG00
2 TUBC S i C i ' CRbDE
M LB/HR
SULFUR' 164 00000
Sb5 LF ila H6 3 8 0 0 01
M LB/HR
164a00000 29.50000
API CP CP
-
60
F
501r63000 28.70000
r 32000
56000
a61000
600aOOOOO 460r00000
435*00000
a
M LB/HR M
LB/nR
M LB/HR
-
60 F API M LB/HR F F
388.00000
10.10000
150 e00000
15r10000
D E S I G N PRESSURE TEMPERATURE
PSIG
254.00000
440 0 0 0 0 0 650 e00000
I N L E T NOZZLE IZE OUTLET NOZZLE SSIZE FLANGE R A T I N G
NC CH riE II N ES S
TUBE GAGE TUBE LENGTH TUBE PITCH-SQUARE SUGGESTED NUMBER OF TUBE PASSES SUGGESTED B A F F L E P I T C H ( A T 45 DEG) >,UTSIDE AREA OF TUBES BETWEEN NTERFACES OF TUBE SHEETS
%
FUIILIIUG
PSI PSIG
SHEET OD D E S I G N TEMPERATURE TUBE SIZE9
f? 4 i I
5 H E L L SZDE GAS OIL FOULING CGRR
PROCESS REQUIREMENTS PER U N I T F L U I D CIRCULATED F L U I D Q U A L I T Y - F O U L I N G OR NON-FOULING CORROSIVE OR NON-CORROSIVE
STEAM NON-CONDENSABLE GAS MOLECULAR WEIGHT
BUNDLE 2632.00000 4.00000
SQaFTa
LOG MEAN TEMPaDIFFERtNCE-EFFECilVE HEAT TRANSFER RATEpDUTY AND CLEAN FLOW THRU U N I T - S E R I E S OR P A R A L L E L
CORROSIVE COMPOUNDS TOTAL F L U I D ENTERING LIOUID GRAVITY INLET VISCOSITY OUTLET V I S C O S I T Y
BO TTOM
COOLING REMOVABLE
P
675r00000 . 66 ra OO OO OO OO OO 300
F
300 657.5000O I + 50000 16 00 OO#
INCHES BWG FT
16
INCHES
I 00000 B 7500 41
INCHES
5010000
-
PROCESS REFINERYLOCATION-
PIPESTILL BILLINGS BILLINGS
RESEARCH AND E N G I N E E R I N G COe ZJN E N G I N E E R I N G D I V I S I O N 3 0 X 8 L I N D E N - Na J a RON I C A L L Y . COMPUTED
Figure 1 .
8r00000 8 b00000
Specification sheet for heat exchanger
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714
INDUSTRIAL AND ENGINEERING CHEMISTRY
M A C H I N E C O M P U T A T I O N IN PETROLEUM RESEARCH cedures which have been programmed for the IBM 650 computer were Process design of liquid-liquid heat exchangers Process engineering of light ends fractionators Thermal cracking furnace radiant coil calculation Calculation of motor acceleration characteristics Cost estimation of shop-fabricated vessels Heat exchanger design is described in considerable detail to show some of the factors in programming and using automatic computation for a typical engineering calculation. The range of engineering work adaptable to automatic computation will be shown, more briefly, in the other examples. These will also illustrate other programming considerations like running large-sized problems on a medium-sized computer, and using the convergence technique in thermal cracking furnace calculations. Convergence techniques are very important in a multiple trialand-error calculation of this kind. Little publicized computer applications are demonstrated by the motor acceleration and cost estimating decks. Finally, these decks will show that even with a mediumsized computer, decks can be developed which take into account a large number of process and design variables. Under the control of the engineer using the deck, this gives a great freedom in
developing an optimum design with much less effort than hand calculation. Process Design of liquidliquid Heat Exchangers
Design of heat exchangers makes up an appleciable proportion of refinery design work. ‘ The program described here will do essentially the complete design of a heat exchanger having liquid on both the shell and the tube side. I t was written as part of a plan to permit automatic computation of all major heat exchanger types. The program calculates an over-all heat transfer coefficient by trial and error, using an assumed coefficient supplied by the engineer. If calculated and assumed values do not agree, the dgck autqmatically adjusts its assumption and repeats the entire calculation. After agreement has been reached within a preset range, the deck rounds off calculated values, sizes exchanger nozzles, and punches out the answers. Descriptive material is combined with the answers and printed in the form of a n exchanger specification sheet, shown in Figure 1. Each exchanger calculation takes from 10 to 45 minutes of machine time, plus time for gathering input data and interpreting results. It would take a n engineer with limited experience 1 to 2 days for a similar calculation by hand. A block diagram corresponding to a new exchanger design is shown in Figure
ENGINEERING DESIGN ON A COMPUTER-THE
WHY’S
The principal objective: better designs
Computers carry out more case studies or more complicated calculations than could be done by hand. The ultimate goal: entirely new designs made possible only through automatic computation
2. Steps are described in Table I. Logic is complex even in this simplified version of the program. Those unfamiliar with programming question why it takes longer to program and “debug” a calculation than to make the same calculation by hand. The answer is that programming entails simultaneously solving all the problems that the program will ever be used to solve. Logic must be set up to handle the entire gamut of situations that might arise in the design. The exchanger program has a number of boundaries built into it. They are maximum or minimum allowable values for the design. Many are specified by the engineer. A number of them will be overridden in order to meet some of the other maximum or minimum specifications (Figure 2 and Table I). Design practices dictate others that are
b The Engineer Specifies Maximum area per shell Minimum and maximum tube side velocity Minimum shell side velocity $faximum tube side pressure drop Maximum shell side pressure drop ~~
~~~~~
built . into the deck. They include maximum and minimum baffle spacing, type of baffle, and tube arrangement. Some can be changed by a minor adjustment of the deck.
Questions the Programmer Must Answer
What should be scope of program? To what extent should program optimize design it calculates? Who should make decisions? programmer? engineer?
Secondary objectives: speed, manpower, accuracy
Jobs get done sooner by speeding up calculations. Engineers are free for other work while computers take over the more routine and repetitious calculations. Consistency of designs increases as computers reduce arithmetic errors, and help ensure that proper and consistent calculation procedures are used.
ESSO RESEARCH’S COMPUTER ORGANIZATION COMPUTING CENTER
ODerations G r o w Handles day-to-day running of problems
Programming Research and Training Group Investigates programming techniques, trains and advises engineers on programming methods
PROGRAMMING GROUPS located in the other divisions of Esso Research. Each group programs calculations required by its own division
This exchanger design deck is useful for both new and existing equipment. Since it does not take much more running time and did not require much more programming, it was economical to make the deck flexible enough to handle both. This deck always uses all the available prmsufe drop it can to get increased turbulence and therefore higher coefficients. This means less heat transfer surface and a cheaper exchanger. More computer time is needed to do this, as t&e computer often goes through a part of the calculation several times. This is justified to obtain more nearly optimum exchangers. The deck does not design the cheapest exchanger possible. Optimization must be limited to a degree where it does not increase machine time too much. In the VOL. 50, NO. 5
M A Y 1958
715
complete exchanger design case, cost estimating and design decks for associated equipment would have to be included, so that the whole system could be optimized as a unit. This is the goal of design using computers, but has not yet been reached. One of the big problems in programming design calculations is handling the decisions which are made during the run. Two types of decisions are involved-those built into the program and those the engineer makes in using it. An example of built-in decisions would be use of all available pressure drop. Another would be choice of sequence of passes to meet maximum surface per shell limitation. The size of the LMTD correction factor, FAT,has been used as the criterion in making this decision. A factor of less than 0.95 indicates the need for a shell in series and greater than this, a pass in parallel. This rule is applicable in a majority of cases and the break point can be altered simply for unusual situations. An example of a decision not built in is when and how much to relax the many maximum and minimum design criteria. Because the reasoning behind relaxing these criteria is complicated, the best approach is to run several cases, using different values. The engineer thereby manipulates the mathematical model,
looks a t the results, picks the optimum values, and sends the problem back for final calculation. Some existing equipment design decks have been modified to permit the engineer to specify more variables to help get a better design.
Most refinery processes depend heavily on fractionation of hydrocarbons in the gasoline boiling range or lighter. Engineering calculations for towers in this service have been programmed. Light hydrocarbon mixtures can usually be analyzed into familiar chemical compounds, for which there are available considerable physical and thermodynamic data. However, it is rare to have a component analysis that is accurate and firm. I t is usually someone’s best guess based on less-than-perfect correlations for predicting conversions and product compositions. For this reason, the program employs a n empirical method (7). A rigorous plate-to-plate calculation can rarely be justified, even with computers available. The deck calculates the physical properties of all streams entering and leaving the tower, heat duties for the preheater, condenser, and reboiler, theoretical plate requirement and feed plate location,
DIVIDE ASSUM E Outlet temperature and pressure for first tube length based on inlet data and conversion at the inlet.
CALCULATE Average temperature, pressure, and volume. reaction rate over tube length.
Evaluate
average
DETERMINE Heat required for cracking from a table of heat of cracking us. conversion.
CALCULATE Heat input to the tube length by multiplying the design heat density by the outside surface area of the tubes.
CALCULATE Outlet temperature from heat available, weight flow rate, and specific heat of the fluid passing through the tube length.
COMPARE with assumed.
Repeat
until agreement
is
CALCULATE Molar expansion and pressure drop through the tube length. Repeat until calculated and assumed agree. Repeat all steps for each tube length.
COMPARE Calculated with desired outlet pressure. If they do not agree, assume a new inlet pressure and repeat the entire calculation.
INDUSTRIAL AND ENGINEERING CHEMISTRY
and maximum vapor and liquid hading a t the top and bottom of the tower. The deck was made specific for light hydrocarbon streams normally found in the refinery. Flexibility was obtained by designating the heavier fractions only by normal boiling point. Thermodynamic properties were obtained by correlations based on API gravity and boiling point. Input data required are component analysis of the feed, distillate drum temperature, and product specifications. The computer performs the heat and material balance for the basis chosen. This program requires up to 7 hours of 650 computer time. In spite of a number of difficulties due to the size of the program, such as lack of memory space on the magnetic drum, and card mixups due to multiple handling, the deck has been used extensively. Computer time is about 1 to 20 compared to the time required by hand. Pieces of the deck have been used for other calculations, such as equilibrium flashes, bubble points, and dew points. Thermal Cracking Furnace Radiant Coil Calculation
Coil into a number of “tube length” increments.
716
The Engineer Specifies
Process Engineering of light Ends Fractionators
FURNACE COIL PROCEDURE
Outlet temperature satisfactory.
b
Ratio of top to bottom vapor loading or Fixed preheat temperature or Fixed feed enthalpy or Feed at its bubble point
Calculation of pressure drop through a tubular furnace is time-consuming when vaporization or chemical reaction is occurring in the tubes. This program designs a furnace coil for vapor flow with chemical rraction, such as in a thermal reformer. The procedure requires satisfactory convergence of assumed us. calculated values for four variables-conversion, temperature, and pressure for each tube length, and over-all furnace pressure drop. The highly repetitive nature of the calculation makes it an ideal candidate for a computer, eliminating a boring and exasperating job by hand. This program has been used in the design of several furnaces and has saved up to 3 to 4 weeks of elapsed time. The convergence technique used was a simple one. The next value to be assumed for the variable in question is chosen by taking the deviation of the last two trials and extrapolating linearly to zero. Successive deviations obtained are compared with tolerances entered by the engineer. Convergence occurs when a deviation is obtained which is smaller than the tolerance. Efficient convergence routines become very important where one complete calculation
M A C H I N E C O M P U T A T I O N IN PETROLEUM RESEARCH
TABLE I. 1.
SUMMARY OF PROGRAM FOR LIQUID-LIQUID HEAT EXCHANGER DESIGN
Calculate log mean temperature difference and tube side stream properties, using coefficient assumed by engineer.
2. Calculate surface per shell.
If it is greater than allowable surface per shell, make less by adding streams in parallel or shells in series.
3,
Calculate tube side velocity. If less than minimum, add tube passes, two at a time, until it is greafer.
4. I f tube side velocity is less than maximum, add two more tube passes. If tube velocity is now greater than maximum, and
A.
There are more than two passes now, reduce number of tube passes by two.
6.
There are only two passes now, start the calculation again with a 5% lower assumed coefficient, and prime the computer t o indicate that excess surface is needed t o meet tube velocity limitation. This procedure of adding tube passes until maximum velocity is exceeded and then backing off two passes makes sure that maximum tube velocity is used.
5.
Calculate mass flow rate, inside heat transfer coefficient, and tube side pressure drop. If tube side pressure drop is greater than maximum allowable, reduce number of passes, two at a time, until less than maximum. If it is still above maximurn with two tube passes, start calculation again with a 5% lower assumed coefficient and prime the computer to indicate that excess surface is needed.
may involve hundreds of internal trialand-error calculations. Calculation of Motor Acceleration Characteristics
Catalytic cracking plant blowers and compressors require special attention to starting characteristics. These motors are often the major load on the substations feeding them. Large voltage
b
Questions on Initial Startup
Will voltage drop be so great as to affect any other loads on the same substation? Will the reduced voltage cover the torque requirement and leave an excess for accelerating the machinery? or will the motor have to be unloaded before starting? If the motor can be started under load, will it reach operating speed in a reasonable time?
6. Calculate tube wall resistance and shell side coefficient. If the shell side coefficient is less than zero-;.e., assumed over-all coefficient is greater than calculated tube side Coefficient less tube wall resisfance and fouling factors-begin calculation again with 5% lower assumed coefficient. 7. Calculate exchanger geometry, shell side fluid properties, and velocity. If velocity is less than minimum allowable, reduce baffle pitch until velocity is above minimum or baffle pitch has reached minimum allowable. 8. Calculate shell side Coefficient and pressure drop. If shell side pressure drop is greater than maximum allowable, increase baffle pitch until if is below specified maximum. If this is not possible without exceeding maximum baffle pitch, begin calculation again with 5% lower assumed coefficient. Prime computer to indicate that excess surface is needed to meet the shell side pressure drop specifications. 9. Calculate over-all coefficient. I f it is less than 100% o f assumed value, calculate a new assumed coefficient and begin again. 10. If calculated coefficient is greater than 705% of the assumed, and computer is not primed to print out that the exchanger is oversurfaced, calculate a new assumed coefficient and begin again. I.f calculated coefficient is
( A ) greater than 105% of assumed and exchanger i s oversurfaced t o meet either pressure drop or tube side velocity, or less than 105% of assumed, round off results, and read in the nozzle-sizing deck.
(6)
11.
Calculate nozzle sizes by trial-and-error procedure. Punch out results.
drops result when they‘ are started. When driving high-inertia loads, acceleration times are long. The deck will answer a number of questions which arise in engineering a large motor drive. Added to the engineering questions that arise in starting a motor, there is also the problem of “voltage dips” or losses which are due to trouble elsewhere in the system. Calculations which will answer these questions have been programmed for the IBM 650 computer for three-phase induction motors. A range of 500 to
I
10,000 hp., and 2300 to 13,200 volts has been covered. Motors with smaller
b
Questions o n Reacceleration
How much will the motor slow down because of momentary voltage loss? will the motor develop enough torque to bring the load back U P to speed? Will reduced voltage and torque make reacceleration longer than a normal starting operation and hence motor damage?
THE CALCULATION
I
INPUT DATA
COMPUTER CALCULATES
Motor characteristics Gear and load data Source voltage during startup Source voltage during reacceleration Duration of voltage dips
Motor current and torque at 10 speeds from zero to full load Total elapsed time to reach full load speed Amount motor slows at end of voltage dip Motor currents and torque and elapsed time during reacceleration for various speeds
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VOL. 50, NO. 5
MAY 1958
717
horsepower and lower operating voltage can be tried, but the results may not be as accurate. The computer calculates how long it takes the motor to go through a small change in speed, assuming the net torque remains constant over this range. This calculation is repeated over the entire range, and incremental times are added to get cumulative starting time. Motor terminal voltage is calculated at each speed. This information can be used to determine the effect on other loads on the same substation. The program also compares motor torque and load torque. If load torque exceeds 95% of motor torque a t any speed, time calculations stop. The actual acceleration calculation takes about 10 minutes, as compared to 2 days by hand. Machine calculations are more accurate.
reflect price escalation and keep the deck useful. This program will prepare cost estimates for horizontal drums, vertical drums, straight-sided towers, towers with two diameters and one transition section, or towers with three diameters and two transition sections. It may be used for towers with more than one corrosion allowance or shell material. I t is limited to vessels with diameters u p to 14 feet, since larger sizes usually require field fabrication. Spheres and specialized reactors are beyond the scope of this program. The computer estimates the cost of a straight-sided vessel. I t alters the routine and estimates the transition sections. Internals *and erection costs round out the calculation. Time on the 650 is 4 minutes or less, saving up to several hours by hand. The big advan-
of the cases run would not have been hand-calculated. This corresponds to running one extra case for every three ordinarily done. If this incremental case results in a saving of only lyOin the investment for equipment for which the decks were used, the saving is over $100,000 a year. I n some instances extra cases cannot be justified by either the data or the potential gain. The big savings occur where decks are available or can be prepared in a short time to optimize the plant as a whole. Geffing Jobs Done Sooner. I n getting the elapsed time down, completion dates on several projects have been moved u p as much as 3 to 4 weeks. However, there is an increasing tendency to use the computer to get the best design in the available time. The advantages of using the computer are falling more into the field of optimization. Freeing Engineers for Other Work.
Automatic computation has freed enough manpower from hand calculation to make up for the men now programming. The present programming effort will make additional manpower available for creative work. However, the value of manpower savings is far less than those due to better designs and moving up completion dates.
H O W THE PROGRAM OPERATES Looks up the allowable stress of the material for the given conditions Calculates shell thickness in accordance with engineering procedures Calculates shell weight Calculates cost by referring to cost os. weight and thickness curves stored in the computer program 9 Calculates transition sections after all straight-sided sections have been estimated. 0 Includes cost of internals, attachments, lining, and erection 0 0 0 0
Increasing Consisfency of Designs.
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This is difficult to evaluate now, but in time, benefits should be strongly felt. Conclusions
Cost Estimation of Shop-Fabricated Vessels
Cost is the major criterion in setting u p optimum designs and layouts. Allocation of funds for investment in new facilities or modernization projects must depend upon reasonably precise up-todate cost estimates. The procedures in this area are tedious and subject to error. The cost estimating deck falls on the spectrum midway between pure technical computing and straight data processing. Stored cost data, an integral part of the deck, must be revised frequently to
tage is that it takes over the job of tedious hand calculation from the cost estimator. Progress Toward Goals Beffer Designs. Engineering on process units is proceding on firmer bases. This improvement is obtained by the use of more rigorous calculation procedures and more general use of the case study approach than could be justified using hand calculations. .4 survey was made of the use of ten basic engineering decks. About 25%
Strings of Seven Compressors Each Special computer decks prepared Optimization studies reduced plant investment one million dollars Design could be relied on with more confidence Plant could operate with any one of the compressors out of service Decks available for designing and optimizing similar proposed plants in short order with minimum engineering manpower
0 Gas Compression Plant-Two
0 Naphtha Fractionating Unit
Determined optimum recovery and purity for each component Determined best of nine processing sequences Optimized towers as to operating pressures and reflux ratio Reduced investment by 15y0 of total
71 8
INDUSTRIAL AND ENGINEERING CHEMISTRY
I
1. Careful evaluation must be made before programming anything. Some problems are not adaptable to the computer because they require the exercise of too much judgment. Some are too small or occur too seldom. Others may be too big for a given computer. 2. Setting u p proper logic and scope for the calculation is vital. Thiq must be done by an engineer familiar with calculation procedure as well as with programming. 3. Nonengineering assistance is useful in many phases of computation and programming. 4. Automatic computation of design calculations is a new and rapidly growing field with few guideposts. Imagination and flexibility of thinking are required to make maximum use of advances in computer technology. literature Cited (1) Maxwell, J. B., “Data Book on Hydrocarbons,” 1st ed., Van Nostrand, New York, 1950. RECEIVED for review August 12, 1957 ACCEPTED February 1, 1958 Division of Petroleum Chemistry, Symposium on Application of Machine Computation to Petroleum Research, 132nd Meeting, ACS, New York, N. Y . , September 1957.
