computer aided chemical process design: the flowtran...

Computers end Chemical Engineering, Vol. I. pp. 11-21. Pergamon Press, 1977. Printed in Great Britain

COMPUTER AIDED CHEMICAL PROCESS DESIGN: THE FLOWTRAN SYSTEM*

EDWARD M. RosENt and ALLEN C. PAULS Monsanto Company, 800N. Lindbergh, St. Louis, MO 63166, U.S.A.

(Received 28July 1975)

Abstract-A description of Monsanto’s FLOWTRAN (Flowsheet Translator) system is given. A sample chemical process is first used to illustrate the system. Then the manner in which FLOWTRAN’s design addresses some of the commonly encountered problems in chemical process simulations is discussed. Finally some reactions of industrial and academic users to the system are indicated.

Scope-The general field of steady state chemical process simulation has recently been reviewed by Motard et al. [ 141 and a number of the available flowsheet simulation systems have been investigated by Flower & Whitehead[8]. This paper describes Monsanto Company’s system (FLOWTRAN) as well as some user reactions to it from industry and academia. A comprehensive description of FLOWTRAN may be found in Seader et al.[19].

Conclusions and Significance-The FLOWTRAN system has been found to be a viable and economically attractive steady state chemical process simulation system in a number of industrial environments. Although the full impact of its use in the academic community has not yet been fully evaluated, early indications are that it has been useful and productive in process design courses. Key to the success of a process simulation system is its physical property system, g simple user- language and its reliability.

1NTRODUCTlON The use of the digital computer to perform material and energy balance computations for interconnected chemical processing units is now nearly two decades old. Indeed, literature on the subject of chemical process simulation as a design tool began appearing in the late 195Os[12,15] and by 1960 the Machine Computation Committee of AIChE held a Workshop on Heat and Material Balances[l]. In 1968 Evans et a!.[71 reviewed the developments in the field of computer-aided chemical process design and discussed many of the flow sheet systems which had been publically acknowledged at that time. This was updated by Flower & Whitehead in 1973[8]. Among these flow sheet systems is the FLOWTRAN system of the Monsanto Company[F5,9,11, 16,191.

HISTORY Much of the early work in Monsanto Company in the

field of computer aided process design was characterized by efforts to simulate individual chemical processing units such as distillation columns, absorbers and flash tanks. By l%l attempts to tie together simplified models of individual process units into more comprehensive flow sheet simulations was well underway[l7,22]. However, it .was not until late 1964 that a team of six was formed to attempt to define and implement what has now become the FLOWTRAN system.

Initially, efforts were concentrated on physical properties, input/output routines and design of the general

*Presented at the session on “Computer-Aided Design”, 71st National Meeting, AIChE 20-23 February 1972, Dallas, Texas, U.S.A.

Revised May 1975 for presentation at U.S.-Japan joint seminar on “Application of Process Systems Engineering to Chemical Technology Assessment” 23-27 June 1975, Kyoto, Japan.

tTo whom correspondence should be addressed.

purpose system. A user-oriented language was next developed, a number of the basic simulation subroutines (blocks) were implemented and a user’s manual was written. This phase terminated the initial development. The date was April, 1,966 and FLOWTRAN went into general use in Monsanto.

In early 1969, FLOWTRAN was marketed outside of Monsanto using external commercial computers for approximately 70 outside customers and in early 1972, FLOWTRAN was offered for license. Though the service bureau business was terminated in early 1973, the intention to make FLOWTRAN available to Universities through an alliance of chemical engineering educators (the CACHE committee) was announced in December 1973 [ll]. The continuation of licensing of the FLOW- TRAN system was reiterated in early 1974.

THE FLOWTRAN SYSTEM The FLOWTRAN system is a completely integrated set

of programs written entirely in FORTRAN. The use of FORTRAN makes the system portable, easy to modify and maintain. It has, in fact, been implemented on the CDC 6000 series, the GE/Honeywell 600/6080, and the IBM 360 and 370 series. The major components of the system are indicated in Table 1.

Central to the use of FLOWTRAN is the information stream, the design of which is indicated in Fig. 1, and the operation blocks which calculate output streams from input streams. Each of the blocks has available a large array of physical properties, such as vapor pressure, as well as thermodynamic routines to carry out such calculations as a dewpoint or bubblepoint determination. In addition a basic mathematical library to perform such functions as equation solving and interpolation is available. The system also includes preprocessors for FLOW- TRAN input and physical property development. Finally, it contains a fully integrated physical property file.

TO carry out a FLOWTRAN simulation the name and

12 E. M. ROSEN and A. C. PAULS

Table 1. Components of the FLOWTRAN system

(1) FLOWTRAN Process Simulator: This program translates the FLOWTRAN description of a process flow sheet into computer programs which it then executes.

(2) PROPTY Physical Property Program: This program takes raw property data and computes constants for physical property correlations used in the FLOWTRAN simulator.

(3) VLE Phase Equilibria Program: This program takes raw phase equilibrium data and computes parameters for liquid phase activity coefficient correlations used in the FLOW- TRAN simulator.

(4) INF Information Retrieval Program: This program stores the physical property constants from PROPTY in a public or private data file. Subsequently, it retrieves the constants for use by the FLOWTRAN simulator. Data for 180 chemical species (components) are stored in the public data file.

Vector index Usage 1 Flow of component 1, lb mole hr-’ 2 Flow of component 2, lb mole hr6

N Flow of component N, lb mole hr-’ N+l Total flow, lb mole hr-’ N+2 Temperature of stream, “F N+3 Pressure of stream, psia N+4 Enthalpy of stream, Btu hr-’ Ni5 Fraction vapor (molar) N+6 Stream name

Note: The maximum number of components (N) is 25.

Fig. 1. Information makeup of a stream.

type of each unit operation in a process, the order of calculation, and a name for each unit’s input and output streams is needed. In addition three other categories of information are required:

1. The chemical components used in the process. 2. The unit’s design and operating variables

(parameters) such as numbers of trays in distillation columns and areas of heat exchanges.

3. The composition and condition of each stream which flows into the plant.

From this information the FLOWTRAN system calcu- lates the steady-state operation of the entire plant. The results show the operating conditions for each piece of equipment (e.g. the inlet and outlet temperatures and heat duties of heat exchangers) and the complete chemical composition and condition of every process stream. In addition, the capital and operating costs of each piece of equipment can be obtained along with a profit and loss statement for the entire plant.

EXAMPLE OF A FLOWTRAN SIMULATION

Figure 2 is the flowsheet of a simple isothermal flash operation in which one-half of the bottoms liquid is pumped back and mixed with the feed. To simulate this process, FLOWTRAN units corresponding to each of the operations are assembled as shown in Fig. 3. The unit for an adiabatic flash operation is the AFLSH block (User Abstract shown in Fig. 4) and the PUMP block is used for the pump. The SPLIT block corresponds to the collector and the HEATR block is used for the heat exchanger. In each unit is written an arbitrary unit name (Al, Fl, Hl, Cl, PI, RI) and the unit type (ADD, IFLSH, HEATR, SPLIT,

OVERHEAD PRODUCT

FLASH DRUM FEED I *

RECYCLE BOl-rOMS

1

PRODUCT

Fig. 2. A flash with recycle.

Fig. 3. FLOWTRAN flow sheet.

?iFLSH

Adiabatic Flash

DescrzDtioa: AFISH determines the quantrty and composition of liquid and vapor streams resulting when up to seven feed streams are mixed and flashed adiabatically. The number of prOduct streams my be one or two. The block can be used to simulate a pressure drop across a valve or through a pipeline. If two producr: streams are specxfied and the flash conditions result in a single phase. the appropriate product is set equal to the sum of the feed streams and the other stream is set to zero. You can also specify heat addition to 01 removal from the flash unit.

a: The output gives the unit name, the feed and product stream names, the flash temperatures and pressure, the heat added to or renmvcd from the system and the fraction of feed which leaves as vapor. An option is provided for printing stream flows and physical properties including equilibrium K-values.

Pxmerties Used: Vapor-lrquid equilibria and enthalpies.

Block List:

List type

Unit name

Unit type

Name of 1st feed stream

Name of 2nd feed stream or C

Name of 3rd feed stream or 0

Name of 4th feed stream or 0



fame of 7th feed stream or 0

Name of lquid product stream*

Name of vapor product stream or O*

BLOCK

AFLSH

*If only one product stream is specified, Its phase candi- tion will be determined.

AFISH' BA-102.2

Pa

zzz:e' Lia: PARAM

unit name

Index of first entry 1

1. Flash pressure. psia, if positive. Flash pressure minus the minimum feed pressure, psia. If 0 or negative.

2. Heat added. Btw'hr (If negative, heat removed)

3. Print stream flows, physical properties, K-values: 0 = NO, 1 = Yes

Fig. 4. Sample block abstract.

Computer aided chemical process design 13

Table 2. Major features of the FLOWTRAN system

(1) Ability to handle nonelectrolyte process streams of liquid mixtures that range from ideal to nonideal including three phases.

(2) Pure component physical property fde for 180 components. (3) Simple control over input and output. (4) Free format input to the process simulator. (5) Easy addition of new blocks. (6) Can accept FORTRAN statements to control simulation

logic. (7) Has been widely used in American industry.

PUMP, SCVW-Table 3). Also, note that each stream in the FLOWTRAN flow sheet is given a unique name.

There are two units in Fig. 3 which require special explanation. First, whenever two streams (such as FEED and R03) are combined, an ADD block may be used. Second, it can be seen that because of recycle the composition of the feed to the flash drum depends upon the composition of the bottom stream; that is, the inlet stream composition depends upon the outlet stream. In this situation the stream convergence block (SCVW) is used to determine the composition of the recycle stream at steady-state operation. It does this by iteratively estimating the composition of the recycle stream until the estimate matches the calculated composition within a specified tolerance.

To complete the specification of the problem we will fix the feed stream (FEED) composition as:

Methane-50 lb hr-’ Ethane-100 lb hrr’ Propane-700 lb hr-’ n-Butane-15 lb mole hr~’ I-Butene-21 lb mole hr-’ 1,3-Butadiene-95 lb mole hr-‘.

The temperature will be 85°F and the pressure lOOpsi. The heater outlet temperature will be fixed at 120°F with a 15 psi pressure drop. The flash is at 25 psi.

The complete input to the FLOWTRAN program for this problem is shown in Fig. 5. The input to the FLOWTRAN program is completely “free format” with each item of data separated by a blank. This allows the user the freedom to enter his data without being concerned with specific card columns. The major sections of the program, corresponding to the four types of

TITLE FLASH WITH RECYCLE

FLOWTRAN input information are:

1. FLOWTRAN units-one line is required for each unit. Each line starts with the word “BLOCK” and shows the unit name, the unit type and the names of the inlet and outlet streams with each name separated by one or more spaces.

Since units are calculated in the order in which these lines are arranged, the inlet streams to a unit must be either the outlet streams from preceding units or fresh feed or recycle streams.

The SCVW recycle block lists, as the fifth data item, the name of the first unit in the recycle loop. This is called the “transfer point”. After each unit in the loop has been calculated, the SCVW block will make a new estimate of the recycle stream which is fed back to the beginning of the loop, indicated by the transfer point. The order of calculation is shown in Fig. 6. Unit Al is the transfer point.

2. Components-since each of the six chemical components in this process is included in the data bank, only the component names are needed. They are preceded by the words RETR, indicating the data are to be retrieved from the physical property data bank.

3. Parameters-each block has a list of parameters (design and operating variables) which are listed in the User’s Abstract for the block. All parameters are entered on lines which begin with the letters PARAM followed by the name of the unit to which the parameters apply. For example, the PUMP parameter is the outlet pressure (115 psi in this case).

Each parameter line may list as many parameter values as can be fit on one line. The index of the first parameter in the line is entered as the first number after the unit name

Fig. 6. Order of calculation.

PROPS 6 2 2 2 2 PRINT INPUT TABLES RETR METHANE ETHANE PROPANE N-BUTANE 1-BUTENE 1.3~EJUTAOIENE BLOCK Al ADD FEED R03 5.0 SO1 BLOCK F.1 AFLSH SO1 6.0 SO2 OVHD PARAM Fl 1 25 BLOCK Cl SPLIT SO2 ROl BTMS 5*0 PARAM Cl 1 2 0.5 0.5 BLOCK Pl PUMP ROl RO2

PARAM Pi 1 115 BLOCK Hl HEATR R02 R03A PARAM Hl 1 120 15 0 0 1 0 BLOCK RI SCVW R03A 2*0 Al I403 2.0 POUNDS FEED 1 50 100 700 MOLES FEED 4 I5 21 95 TEMP FEED 85 PRESS FEED 100 TEYP R03 120 PRESS R03 100 END CASE END JOB

Fig. 5. FLOWTRAN input for flash with recycle.


Table 3. FLOWTRAN blocks

Category Block Descriptive title Categor) . Block Descriptive title

Flash IFLSH AFLSH BFLSH KFLSH FLSH3

Absorption/ Stripping ABSBR

Extraction EXTRC Heat Exchange EXCHI

CLCNI DESUP HEATR

Miscellaneous ADD Unit Operations MIX

SPLIT EXCH2

BOILR

Distillation

HTR3 EXCH3 FRAKB DISTL DSTWU SEPR AFRAC

Reaction REACT AREAC XTNT

PUMP MULPY GCOMP

Isothermal Flash Adiabatic Flash General Purpose Flash Isothermal Three-Phase Flash Adiabatic Three-Phase Flash

Rigorous Rigorous Liquid-Liquid Shortcut Heat Exchanger Shortcut Cooler Condenser Shortcut Desuper-heater Heat Requirements Stream Addition

Adiabatic Add/Subtract Reactor Chemical Reactor (Extent of Reaction

Model)

Stream Addition with no Phase Change

Stream Split Shortcut Partial-Total Vaporizer-

Condenser Reboiler/Intercooler Simplified

Model Three-Phase Heater/Cooler Shortcut Heat Exchanger Rigorous (Ke Method) Shortcut (Edmister) Shortcut (Winn-Underwood) Constant Split Fraction Separation Rigorous Distillation/Absorption

(Matrix Method) Chemical Reactor

Bounded Wegstein (Acceleration Method)

Control CNTRL Feedback Controller PCVB Multiple Parameter Controller DSPLT Distillate Fraction Controller RCNTL Ratio, Sum & Difference

Feedback Controller

Cost & Sizing CAFLH Flash Drum Estimation CIFLH Cost Estimate

CKFLH Cost Estimate CFLH3 Cost Estimate CFRKB Distillation Column (‘ost Estimate CAFRC Distillation Column Cost Estimate CDSTL Distillation Column Coat Estimate CABSB Packed Absorber Cost Estimate CTABS Tray Absorber Cost Estimate CEXCI Heat Exchanger Cost Estimate CEXC? Heat Exchanger Cost Estimate CEXC3 Heat Exchanger Cost Estimate CHETR Heat Exchanger Cost Estimate CCLNI Shortcut Cooler Condenser Coct Est. CPUMP Pump Cost Estimate CC@MP Compressor Cost Estimate

II Report Writer SUMRY Stream Output Editor Component Physical Properties Table

GAMX Liquid-Activity-Coefficients Table

Centrifugal Pump Size and Power Stream Multiplication by a Parameter Heating and Cooling Curves Centrifugal Compressor Positive Displacement Compressor and Turbine Profit RAWMT Raw Material, By

I BPR0D By-Product PR0DT Produce Stream Value PRBFT Profitability Analysis

followed by numeric values for each parameter. Each item of data is separated by at least one blank. The parameter lines can be in any order (they do not have to be in the same order as the FLOWTRAN units).

4. Input streams--the composition, temperature and pressure of each stream which is fed to the plant are entered on lines which closely resemble the parameter information. The flow rate of each component, in lb hr-’ or lb mole hr-‘, is entered on lines starting with the words POUNDS or MOLES followed by the name of the stream and the number of the first component on the line. As with parameters, the remainder of the line simply has the flow rate of each of the components entered separated by one space. The order in which the components are specified on the RETR control line fixes the index of each component (i.e. in this example methane is component 1, ethane is component 2, etc.).

The stream temperature “F is entered on a line beginning with the letters TEMP and the pressure psi on a line beginning with PRESS. As with the parameters, the stream lines may be mixed in any order.

In addition to specifying the fresh feed streams, it is desirable, but not mandatory, to estimate the composition

of recycle streams. Good initial estimates can greatly speed the calculation. Recycle stream estimates are entered in the same way as fresh feed streams.

The streams are followed by the END CASE and END JOB lines.

The entire input is preceded by TITLE, PROPS and PRINT control lines. These lines list information which is used throughout the simulation. The TITLE line is printed out as a heading on the output report. The PROPS line specifies the number of components in the simulation (6 in this case) and the physical property representation selected (Table 4). Keywords on the PRINT line control input and output. The keyword INPUT, for example, produces a report of the imput data.

The output report is in four parts: unit and flow tables, unit report, stream report and history.

The unit and flow tables are shown in Fig. 7 and are printed in response to the TABLES specification on the PRINT card. These tables make it easy to check whether the units have been correctly specified. This output is optional.

The unit report, Fig. 8, shows the names of the input and output streams and the operating conditions for each

Computer aided chemical process design

Table 4. Physical property options

Vapor Vapor Liquid Code pressure fugacity coef. fugacity coef.

1 Antoine Ideal gas Vapor pressure, Chao-Seader if super critical

2 Cavett Redlich-Kwong Redlich-Kwong and Poynting equations; Chao-Seader if super critical

3 Chao-Seader (* components, T,, < 500°F)

4 Grayson-Streed (* components, T,, < 900°F)

5 Option 2 except Prausnitz-Shair for super critical N2, CO, A, 02, NO, CH,

7

Liquid activity coef.

Ideal solution

Regular solution

Wilson

Van Laar

Renon

Renon with regular solution for un- specified pairs

UNIT TABLE

UNIT NAME UNIT TYPE

:: ADO AFLSH

s: SPLIT PUMP

Hl HEATA Rl SCVY

FLOW TABLE

STREAM NAME FROM TO

BTMS Cl FEED Al OVHO Fl ROl Cl Pl I?02 Pl Ml R03 I?1 RO3A I41 i: so1 Fl so2 :: Cl

INPUT STREAMS

FEE0

OUTPUT STREAMS

BTHS OVHD

Fig. 7. Unit and flow tables.

unit. The units are arranged in the same order as they were calculated.

The stream output report, Fig. 9, lists the streams in alphabetical order. The composition units may be controlled by keywords on the PRINT card (default is lb mole hr-‘).

During the course of a simulation it sometimes happens that an error condition arises. The purpose of the history is to give the user a complete picture of the course of the calculations and to highlight any error conditions. Each time a unit is computed in a simulation a message is written in the history. Normally the messages serve to contirm routine operation of the blocks, as is shown for the first and last iterations in Fig. 10. However, if an error

occurs (for example, if the pump outlet pressure is specified to be less than the inlet pressure) a warning message will be written in the history.

FL0wrRAN OF’ERATION UNW

The use in FLOWTRAN of operation units in which stream outputs are calculated from stream inputs subject to selected design parameters was a very natural one. At the time of development this allowed FLOWTRAN to use much of the technology developed for stand alone unit operations. Specifically calculating the output streams from the input streams means that each of the items in the output stream vector shown in Fig. 1 must be determined before the block’s calculation is considered complete.

As FLOWTRAN evolved, it was found that this format was very flexible and eventually the unit types shown in Table 3 emerged. By and large the unit operation blocks do calculate output streams from input streams but in a number of instances the design parameter (which corresponds simply to one degree of freedom in the operation block’s equations) is an output stream item. For example, in the compressor block the exit pressure is a design parameter and the horsepower requirement is calculated. Alternatively the horsepower could have been the design parameter and the exit pressure could have been calculated. Indeed many of the blocks are design oriented rather than rating oriented.

INFOKMATION FEEDBACK-THE CONTROL UNIT

When the design parameters used in the operation units do not correspond to the desired design values, FLOW- TRAN allows the use of “control units”. These units monitor any item in a stream and allow the manipulation of the parameters of any upstream unit to achieve the desired value in the monitored stream. For example, consider the distillation column in Fig. 11 in which one component flow rate in SO02 and one component flow rate in SO03 are desired to have certain specified values. A control unit (block PCVB) monitors the actual value in each of these streams and manipulates the reflux ratio and fraction overhead parameters in the distillation unit until


10/13/15 FLASH WITH RECYCLE

Al (ADD) T = 98.05 FI p = 100.00 PSIAr V/F = 0.0193 HOLS/MOL FEEOS = FEED a03 PRODUCT = SO1

Fl (AFLSH) 1 = 35.50 FI P = 25.00 PSIA* V/F f 0.2308 MOLWMOL FEEDS = so1 BOTTOMS = SO2 * OVERHEAD = OVHD hEAT IN = 0.0 BTU/HI?

Cl - SPLIT - INPUT I SO2 OUTLETS = ROl BTMS FRACTION * 0.5000 0.5000

Pl - PUMP - INLET = ROl , OUTLET = R02 FLOweGPM = 16.55 , DELTA P,PSI 8 90.00 FLUID HP = 0.87 t BRAKE HP = 2.94 9 ELEC KY = 2.63 PUMP EFF = 0.2957 . DRIVER EFF = 0.8318

Ill - HEATR - INLET = RO2 . OUTLET s R03A OUTLET TEMP E 120.00 0EG F . PRESSURE DROP = 15.00 PSI DUTY = 0.24450 06 BTWHR

Fig. 8. Unit report.

10/13/75 FLASH WITH RECYCLE

STREAM NAHEI

1 LIET~~ANE 2 ETWbNE 3 PROPANE 4 N-BUTANE 5 l-BUTENE 6 1,3-BUTAOIENE

TOTAL LBHOC/HR TOTAL LWHR 1000 BTWHR DEGREES F PSIA DENSITY, LWFT~ HOLE fRAC VAPOR

STREAM NAME;

1 METHANE 2 ETHANE 3 PROPANE 4 N-BUTANE 5 1-BUTENE 6 1,3-WTADIENE

TOTAL LBNOL/HR TOTAL LB/HR 1000 GTU/UR DEGREES F PSIA DENSITY, LB/FT3 HOLE FRAC VAPOR

BTMS LbMOL/HR

0.06315 0.40248 5.54239 10.7226 14.0981 64.9872 95.8160 5186.77 -045.44

35.50 25.000

39.0724 0.0

a03 ROJA LBMOL/HR LBNOL/WR 0.06315 0.06315 0.40247 0.40248 5.54218 5.54239 10.7226 10.7226 14.0981 14.OPRl 64.9871 64.9072 95.8156 95.8160 5186.75 5106.77 -600.96 -600.96 120.00 120.00

100.000 100.000 35.1713 35.1713 0.0 0.0

Fig. 9. Str ‘earn output 1

FEED LBNOL/HA

x81; I;.6752 15.0000 21.0000 95.0000 153.318 8038.45

-1062.00 85.00

100.000 0.0 0.0272

the desired flows are achieved. The desired values are parameters in the PCVB block. The PCVB block is entirely flexible in manipulating parameters of units which may be far upstream of the distillation column.

STREAM TEARING-THE CONVERGENCE UNIT

As has been noted above, FLOWTRAN requires that the user specify the order of computation of the units used to simulate the process. In the case recycle streams are present he must decide where to “tear” his system and make an estimate of the tom streams. He may then use a convergence unit[lO] as was indicated in the example. These units calculate new output streams (estimated values) based on their input streams (calculated values) (Fig. 12). The procedure continues until the input stream equals the output stream.

Although there has been considerable literature on the

ovno LRMOL/HR

3.05367 2.92330 10.3326 4.27735 6.90183 30.0127 57.5014 2851.67

27.91 35.50

25.000 0.2430 1.0000

249.133

so1

13225.2

LBNOL/HR

-1662.96

3.17997 3.72827 21.4174 25.7226 35.0981 159.907

98.05 100.000 0.0 0.0193

,eport.

ROI LBNOL/HR 0.06315 0.40248 5.54239 10.7226 14.0981 64.9672 95.8160 5146.77 -045.44

35.50 25.000

39.0724 0.0

191.632 10373.5

so2

-1690.87

LBNOL/HR

35.50

0.12630 0.80497 ll.OB4G 21.4453 28.1963 129.974

25.000 39.0724 0.6

R02 LBMOL/HR

0.06315 0.40240 5.54239 10.7226 14.0981 64.9872 95.8160 5106977 -845.44

35.50 115.000 39.0724 0.0

automatic tearing of chemical processes [ 14,181 FLOW- TRAN’s philosophy has been that the user can generally select the tearing points with efficiency since his knowledge of the process can guide him to select the most suitable streams. Indeed if the streams selected lead to poor convergence, then it is easily possible to tear other streams and use their convergence properties. Automatic tearing algorithms, if fact, are still being studied. The recent work of Upadhye & Grens[21], for example, has shown that the criterion for best tear for direct substitution is whether the tear belongs to a “nonredundant” family.

Upadhye and Grens divide different tears (called decompositions in their work) into three different families; non-redundant, redundant and mixed. Families are related by means of the Replacement Rule. This rule states that all input streams to a unit may be replaced by all the output streams (fresh feeds can be omitted). A

Computer aided chemical process design

10/13/75

FLASH mITti RECYCLE

l * 10/13/75 HH.MH.S.5 Al (ADO) 7 - 85.00 Ft P = 100.00 PSIAq V/F = 0.0272 Fl (AFLSHO 1 = 29.58 Ft P = 25.00 PSIAe V/F = 0.2162

P = 0.0 BTU/W Cl - SPLITS SO2 INTO 2 OUTLETS Pl - OUTLET PRES= 115,OPSIA, FLOW= Hl - TEHP= 120.OF, DELP= -15,OPSI.

*I?1 -SCVY-IlER= I* TIME= OSECt RO3AfNEU) R03 (OLD) ERROR

1 0,69lD-01 0.0 0,069' 2 0.402 i:: 0.402" 3 4.46 4.4619 4 6.45 0.0 6.452' 5 8.75 0.0 0.749. 6 40.0 0.0 39.955' 7 60.1 0.0 60.089" 8 120. 120. 0.0 9 100. 100. 0.0

10-0.3700 06 0.0 **,***

11 0.6380-02 0.0 0.008

. .

.

:: (ADO) 7 B 98.05 F, P * 100.00 pSIA+ V/F = 0.0193 (AFLSH) 1 I 35.50 F, P = 25.00 PSIAr V/F = 0.2306

- spL*Tso = 0.0 GTWHR

Cl SO2 INTO 2 OUTLETS Pl - OUTLET PRES= 115.0PSIA1 FLOV= 95.GLB-CIOL/HR

Hl - TEHP= lZO.OF, OELP= -15,OPSI. G= 0.2440 06BTU/HR **Rl -SCVY CONVERGED-I= 5vT3 05ECsMAX ERR= 0.00004

bO.lLE-MOL/HR G= 0.1670 ObETU/HR MAX ERROR=bO,OGGb9

R03AfNEH) R03 (OLD) ERROR 1 0.631D-01 0.6310-01 0.000 2 0.402 0.402 0.000 3 5.54 5.54 0.000 4 10.7 10.7 0.000 5 14.1 14.1 0.000 6 65.0 65.0 0.000 7 95.8 95.8 0.000 8 120. 120. 0.0 9 100. 100. 0.0

10-0.6010 06-0.6010 06 0.000 11 0.0 0.0 0.0 **END OF HISTORY

Fig. 10. History for first and last iterations.

, -so03

FEED

1

I___---- Parameter 2

Fig. 11. Use of a control unit in design.

CONVERGENCE

ESTIMATED CALCULATED

Fig. 12. The convergence unit.

redundant decomposition arises when a stream may be estimated from a particular tear in more than one way. A nonredundant family is one which contains no redundant decompositions. A redundant family contains only redundant decompositions while a mixed family contains both redundant and nonredundant decompositions. (Alter-

17

, nately a nonredundant family is one that contains those decompositions which minimize the openings of the loops in the system.)

Consider, for example, the process suggested by Cavett[2] shown in Fig. 13 and Table 5. This has been studied by Upadhye and Grens as well as Shacham & Mot&rd[20], and Crowe & Nishio[6]. Here four flash drums are interconnected with three recycle streams, Rl, R2 and R3. Upadhye and Grens point out that decomposition (Rl, R2, R3) is a member of a nonredundant family. (Use of the Replacement Rule shows that (Zl, R3) and (Zl, S3) are also members.) However, although (Zl, 22) is

Fig. 13. Cavett problem to study tearing.

CACE Vol. I. No. 1-B

E. M. ROSEN and A. C. PAULS

Table 5. Data and solution for example of Fig. 13

Component Feed

N 358.2 co* 4965.6 KS 339.4 Methane 2995.5 Ethane 2395.5 Propane 2291.0 Iso -Butane 604.1 N-Butane 1539.9 Is0 -Pentane 790.4 N-Pentane 1129.9 N-Hexane 1764.7 N-Heptane 2606.7 N-Octane 1844.5 N-Nonane 1669.0 N-Decane 831.7 N-Undecane 1214.5 Temp. (“F) 120 Pres. (psi) 49

lb mole hr. ’ K-values at solution Flash Flash Flash Flash

Pl P2 1 2 3 4

358.2 0.0009 5.94 24.6 149.7 620.8 4964.7 4.73 1.51 4.64 21.1 72.3

334.7 5.22 0.89 2.03 8.28 27.1 2996.3 0.17 3.09 10.3 52.9 200.1 2883.2 14.5 1.00 2.66 11.2 39.3 1899.1 404.2 0.502 0.943 3.29 10.8

198.2 410.3 0.310 0.445 1.34 4.22 299.4 1247.4 0.246 0.342 0.99 3.07

37.2 753.9 0.155 0.164 0.417 1.22 34.8 1095.7 0.126 0.132 0.327 0.944 10.5 1754.6 0.064 0.051 0.107 0.290 3.72 2603.2 0.035 0.022 0.039 0.101 0.55 1844.0 0.017 0.008 0.013 0.033 0.12 1668.9 0.009 0.004 0.005 0.012 0.015 831.7 0.005 0.002 0.002 0.004 0.007 1214.5 0.003 O.ooO8 0.0009 0.002

100 85 100 120 96 85 800 13 800 270 49 13

itself a nonredundant decomposition it is a member of a mixed family. Use of the Replacement Rule gives (Sl, S3, R2, R2) in the same family which is redundant.

FLOWTRAN was used to follow the convergence properties of the (RI, R2, R3) decomposition (FLOW- TRAN input shown in Fig. 14) and the (Zl, 22) decomposition (FLOWTRAN input shown in Fig. 1.5). Following Shacham and Motard the error for iteration i was taken (over all components j) as

l i = max )ei,J. (1)

The solution by direct substitution is shown in Fig. 16 on a

plot of error vs iteration. The maximum eigenvalue for the linearized systems

resulting from each decomposition was calculated from

* =lim%.! m I_ 4 ’

where i was taken as 40. As can be seen the mixed family decomposition has a A, = 0.940 and the nonredunant family decomposition has a A,,, = 0.913 and consequently converges faster.

Acceleration in FLOWTRAN is by means of a bounded Wegstein method[l3] where for each component

and

10/13/?5

TITLE TnREE STREAM TEAR - Rlt RZt A3 - ZERO START

PROPS 16 2 2 3 2 PRINT INPUT RETR l N2 *CO2 “H2S *Cl l C2 l C3 l IC4 “NC4 .IC5 *NC5 *NC6 *NC7 *NC6 *NC9

*NC10 *NC11

BLOCK A01 ADD FEED Rl R2 4*0 21 BLOCK FL2 IFLSH 21 52 51 PARAM FLZ i 120 270 i BLOCK A02 ADO SP R3 5.0 22 BLOCK FL3 IFLSH 22 53 H21 PARAM FL3 1 96 49 1 BLOCK FL4 IFLSH 53 P2 R3A PARAM FL4 1 85 13 1 BLOCK FL1 IFLSH Sl RlA Pl PARAM FLI 1 100 Boo 1 BLOCK Cl SCVW RI1 RPA R3A A01 Rl R2 R3 PARAM cl 1 0 0 75 1

uOLES FEED 1 358.2 4965.6 339.4 2995.5 2395.5 2291. 604.1 1539.9 790.4 1129.9 1764.7 2606.7 1844.5 1669.0 831.7 1214.5

TEMP FEED 120 PRESS FEED 49 TEMP Rl 120 PRESS Rl 49 TEMP R2 120 PRESS R2 49 TEHP R3 120 PRESS H3 49 END CASE END JOB

Fig. 14. FLOWTRAN input for Rl, R2, R3 tear of Cavett problem.


TITLE TWO STREAM TEAR - Zl*Z2 - ZERO START

PROPS 16 2 2 3 2 PRINT INPUT QETR l N2 *CO2 l H2S *Cl l C2 l C3 .1C4 *NC4 *It5 *NC5 “NC6

*NC10 *NC11 BLOCK FL2 IFLSti 71 S2 51 PARAY FL2 1 120 ii0 -1

BLOCK FLY IFLSH Sl Rl Pl PARAM FL1 1 100 800 1 BLOCK FLY IFLSH 22 53 R2 PARAM FL3 1 96 49 1 BLOCK FL4 IFLSW 53 P2 R3 PARAW FL4 18513 1 BLOCK AD1 ADD FEED Rl R2 4*0 ZlA BLOCK AD2 ADD S2 R3 5’0 Z2A BLOCK Cl SCVW ZlA 22A 0 FL2 21 Z2 0 PARAM Cl 1 0 0 75 1 MOLES FEED 1 358.2 4965.6 339.4 2995.5 2395.5 2291. 604.1 1539.9 790.4

1129.9 1764.7 2606.7 1844.5 1669.0 831.1 1214.5 TEMP FEED 120 PRESS FEED 49 TEHP Zl 120 PRESS 21 49 TEWP 22 120 PRESS L2 49

END CASE END Jo13

Fig. 15. FLOWTRAN input for Zl 22 stream tear of Cavett problem.

10/13/75

*NC? *NC8 *NC9

10 20 30 40 50 60

ITERATION NUMBER

Fig. 16. Convergence of the Cavett problem.

where

a = f(x”+l) -f(L) .x,+1 -L

If q is calculated to be outside the range

O>qr-5

then 4 is set to 0 (direct substitution) which increases the stability of the iteration. Convergence in FLOWTRAN takes place when lx. -f(x,)l/x. < 0.0005 for each component.

Acceleration was attempted every iteration on the (Rl, R2, R3) and (Zl, 22) decompositions with the result that the error bounced around. However, applying the accelerator only every 4th iteration to the (Rl, R2, R3) decomposition gave convergence in 23 iterations (Fig. 16). Applying acceleration only every 5th iteration to the (Zl, 22) gave convergence in 48 iterations but in a very

irregular manner. In these experiments no automatic way of determining when to accelerate was used as was done by Crowe and Nishio in their application of the general dominant eigenvalue method.

THE FLOWTRAN PREPROCESSOR

As shown in Fig. 5 the user of FLOWTRAN uses a simplified “language” to indicate to the system what his flow sheet configuration lookes like, what parameters he desires for his operations units and what chemical components and physical properties he wishes to use. The preprocessor takes this information and converts it into a FORTRAN program made up principally of a series of CALL statements. This FORTRAN program is then compiled and linked together with those routines which must be loaded for the particular simulation. This procedure has a number of considerable advantages:

1. Only those routines which are required for the particular simulation need be loaded at any one time. A great deal of space can thus be saved.

2. FORTRAN statements can be used interdispersed among the operations block cards.

3. FORTRAN source decks for new blocks or new versions of existing programs can be added to the particular simulation run.

In practice the FLOWTRAN preprocessor has been a great labor saving device. Rearranging or adding units to a simulation, changing physical property choices, adding a component or changing feeds or parameters becomes a very simple task.

THE PHYSICAL PROPERTY PREPROCESSORS AND THE PHYSICAL PROPERTY FILE

Probably the most time consuming and difficult portion of a simulation is the gathering and development of adequate physical property data. One of the features which particularly distinguishes FLOWTRAN is its physical properties,system. Some of the features of this system are:

1. A physical properties file containing physical property data on 180 inorganic and organic compounds.


2. An automatic retrieval of necessary data upon recognition of a component name in a FLOWTRAN simulation deck.

3. Use of advanced correlations and equations so that properties of all classes of compounds can be described over broad ranges of temperature and pressure.

4. Capability of accurately handling systems whose liquid mixtures range from ideal (o-, m-, p-xylene) moderately non-ideal (methanol, propanol) strongly non- ideal (ethanol, water) to partially immiscible (methyl-ethyl- ketone, water).

A list of the physical property options available in FLOWTRAN which can be specified on the PROPS card was indicated in Table 4. To use these options parameters of these correlations must be known and be given to the FLOWTRAN system. For example, to use the Antoine equation for vapor pressure, In p” = A - B/(T + C), the parameters, A, B and C must be known. If these values are not known a pure component data reduction program, PROPTY, is used to process raw vapor pressure vs temperature data to give the required parameters.

In general to estimate the required thermodynamic parameters for compounds not included in the physical property tile the user must supply the following data as a minimum: molecular weight, normal boiling point, critical temperature and pressure, a latent heat, a density and structural groups for the estimation of gas ideal heat capacity. The parameters (output of PROPTY) may be placed directly into the physical property data file.

If vapor-liquid equilibria data are available, then activity coefficients based on the van Laar equation, e.g., can be obtained by least squares fitting the data in a vapor-liquid equilibria preprocessor called VLE. Both the pure component preprocessor and the vapor-liquid equilibria preprocessor are designed to accept a wide range of data and units.

The capability of handling highly non-ideal solutions in FLOWTRAN may give rise to a range of problems not encountered in simulations where only hydrocarbons, for example, are found. For example, if the non-ideality is great enough two liquid phases may appear in equilibrium with a single vapor phase. This means that ordinary bubblepoint and flash calculations must be capable of handling this situation. It is especially important when energy balances are involved since the enthalpy difference between a two phase and three phase system may be considerable. FLOWTRAN allows the user to decide if three phases may be involved and to insert a block which will tell the user if three phases are present.

Consider for example, a mixture of ethyl alcohol, benzene and water at 760 mm Hg (composition 0.23, 0.27 and 0.50) whose condensation curve is shown in Fig. 17[10]). At 65°C two phases exist but at 63.7”C three phases exist. Note how rapidly the amount of vapor present changes with temperature. This situation is practically always present in three phase condensation curves.

The wide range of K values encountered in non-ideal flash calculations often require considerable numerical precautions in an otherwise routine calculation. For example, the flash equation is well known to be

where (Y = V/F and zi is the feed composition for component i.

601 , t I , I I , , I 1 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.6 0.9 1.0

Fig. 17. Condensation curve for et&l alcohol, benzene and water.

Now if one were to calculate the denominator as 1-cu(l-K,) rather than (I-a)+(~l( then when a =l one could run into considerable difficulty if Ki were small enough so that on your computer 1 - Ki = 1.

EXPERIENCES IN INDUSTRY

Through a licensing program FLOWTRAN has been implemented for industrial use in the oil, chemical, petrochemical, pharmaceutical, and soap and detergent industries. Access to FLOWTRAN on computers desig- nated by the licensee has been both through low speed teletype-like terminals as well as medium speed batch terminals. Initially, industrial usage generally starts with single block applications in which confidence in FLOW- TRAN’s reliability and flexibility is gained. Subsequently, licensee’s have been able to modify FLOWTRAN to meet their specific needs, add in new blocks and build on complementary software without consultation after their initial training and familiarization.

Use of FLOWTRAN, industrially, has improved design bases, has resulted in R & D cost savings, has improved process knowledge, has helped in debottlenecking, has improved product yield, has reduced operating costs and has resulted in engineering cost savings. In addition, it has been used to generate physical property data for other design programs.

EXPERIENCESINEDUCATION

In 1969 the National Academy of Engineering’s Commission on Education established the CACHE (Computer Aids for Chemical Engineering Education) committee. In cooperation with this committee Monsanto implemented the FLOWTRAN system at United Comput- ing Services in August 1974. In October 1974 the user text describing the system became available[19]. As of early 1975 some 21 educational institutions were using FLOWTRAN for educational purposes. Also a user group has been formed within CACHE. Seminars on FLOWTRAN have been presented through universities as well as in AIChE’s Today Series. Initial usage in the classroom has generally been in design courses.

CONCLUSIONS

The experiences with FLOWTRAN both within Mon- santo and among the licensee’s of FLOWTRAN generally point to three areas which appear to be especially important in the design of a chemical process simulation system.

1. The physical property system should be broadly based, easily used, easily checked and easily modified. A data bank of commonly used materials can pay rich dividends in user’s time and avoidance of costly errors.

2. A simple user’s language which requires no


programming and little data preparation are prerequisites for wide adoption.

3. A high degree of reliability of each routine, considerable error checking facilities and dedicated maintenance of the system are absolute requirements for giving the required credibility to the system.

REFERENCES

1. Agenda for AIChE Machine Computations Committee Work- shop on Heat and Material Balances, 27 Sept. (l%O).

2. R. H. Cave& Application of numerical methods to the convergence of simulated processes involving recycle loops. American Petroleum Institute Preprint, No. 04-63 (1%3).

3. R. H. Cavett, FLOWTRAN physical properties. Paper presented at 49th NGPA Annual Convention, 17-19 March, 1970, Denver, CO.

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15.

4. Chemical and Engineering News Staff, Process designers get more software. Chem. Engng News 48, 14 (1970).

5. Chemical Week Staff, To each engineer, his own computer. Chem. Week 106, 11 (1970).

6. C. M. Crowe & M. Nishio, Convergence promotion in the simulation of chemical processes-the general dominant eigenvalue problem. AIChE .I. 21, 528 (1975).

7. L. B. Evans, D. G. Steward & C. R. Sprague, Computer aided chemical process design. Chem. Engng Progress 64, 39-46 (1%8).

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17.

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19.

process analysis and design tool. Paper presented to the American Chemical Society, Los Angeles, CA (1971). E. J. Henley & E. M. Rosen, Material and Energy Balance Computations. Wiley, New York (1969). Institute News Monsanto aids CACHE. Chem. Engng Progress 70, 75 (1974). M. G. Kesler & M. M. Kessler, Engineering a process with a computer. World Petrol 29 (60) (1958). H. C. Kliesch, A study of convergence accelerator algorithms used in steady state process simulation. M.S. Thesis in Chemical Engineering, Tulane University, New Orleans, LA (1967). R. L. Motard, M. Shacham & E. M. Rosen, Steady state chemical process simulation. AIChE J. 21, 417 (1975). M. F. Nagiev, Chem. Engng Prop. 53,297 (1957); The Theory of Recycle Processes in Chemical Engineering. Pergamon, Oxford (1964). R. L. Rorschach & R. E. Harris, Process simulation made by computer. Oil & Gas J. 68, 33 (1970). E. M. Rosen, A machine computation method for performing material balances. Chem. E&g Progress 58, 69 (1%2). _ D. F. Rudd & C. C. Watson, Strategy ofProcess Engineering. Wiley, New York (1%8). J. D. Seader, W. D. Seider & A. C. Pauls, FLOWTRAN Simulation: An Introduction. CACHE Corooration. Ulrich’s

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8. J. R. Flower & B. D. Whitehead; Computer-aided design: a survey of flowsheeting programs-I. The Chem. Engr, No.

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Bookstore, Ann Arbor, Michigan (1974). _ M. Shacham & R. L. Motard, Application of the theory of linear recycle systems. Paper presented 78th AIChE National Meeting, Salt Lake City, 18-21 Aug. (1974). R. S. Upadhye & E. A. Grens, Selection of decompositions for process simulation. AIChE J. 21, 137 (1975).

272, Part II, No. 273 (1973). 22. M. A. Vela, Use fractions for recycle balances--I & II. 9. R. E. Harris, FLOWTRAN: an approach to a computer aided Petroleum Refiner 40(5), 247; (6), 189 (1961).

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