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THREE CAREER EPISODES 1. DESIGN AND DEVELOPMENT OF A STEADY STATE SYSTEM SIMULATOR 1.1 INTRODUCTION CE 1.1. This project was made in order to get my degree as Chemical Engineer. Me and other two students developed a Steady State System Simulator to be applied in Personal Computers at the University of America in Bogotá, Colombia. The project was lead by the Chief of the Computing Department of ECOPETROL, the Colombian Oil Company that belongs to the Colombian State. 1.2 BACKGROUND CE 1.2. Processes simulation through computing models was clearly becoming one of the major tools to do mass and energy balances, that Chemical Engineers apply to design and optimize processes in the mining and petrochemical industry, allowing not destructive tests and avoiding pilot plants, saving materials, energy, avoiding potential accidents, and not generating wastes. Computers applied to Process Engineering has become one of the main branches of the Chemical Engineering. CE 1.3. As a main objective we developed the Steady State System Simulator SIMPRES to model the following Equipment Processes: 1. Pumps 2. Compressors 3. Heat Exchangers 4. Mixers 5. Splitters 6. Fractionators 7. Reactors 8. Absorption Towers 9. Distillation Towers 10. Flash Distillatory 11. Valves 12. Turbines 13. User’s Module

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THREE CAREER EPISODES

1. DESIGN AND DEVELOPMENT OF A STEADY STATE SYSTEM

SIMULATOR

1.1 INTRODUCTION

CE 1.1. This project was made in order to get my degree as Chemical Engineer. Me and

other two students developed a Steady State System Simulator to be applied in Personal

Computers at the University of America in Bogotá, Colombia. The project was lead by

the Chief of the Computing Department of ECOPETROL, the Colombian Oil

Company that belongs to the Colombian State.

1.2 BACKGROUND

CE 1.2. Processes simulation through computing models was clearly becoming one of

the major tools to do mass and energy balances, that Chemical Engineers apply to

design and optimize processes in the mining and petrochemical industry, allowing not

destructive tests and avoiding pilot plants, saving materials, energy, avoiding potential

accidents, and not generating wastes. Computers applied to Process Engineering has

become one of the main branches of the Chemical Engineering.

CE 1.3. As a main objective we developed the Steady State System Simulator –

SIMPRES to model the following Equipment Processes:

1. Pumps

2. Compressors

3. Heat Exchangers

4. Mixers

5. Splitters

6. Fractionators

7. Reactors

8. Absorption Towers

9. Distillation Towers

10. Flash Distillatory

11. Valves

12. Turbines

13. User’s Module

CE 1 Alvaro H. Pescador 2

CE 1.4 I was in charge of fully developing the simulation modules of Pumps,

Compressors, Heat Exchangers, Valves and Turbines.

CE 1.5. On the other hand, SIMPRES had a bank of 471 compounds including must of

Hydrocarbon used in the Petrochemical Industry both sutured and not sutured, linear

and not linear, as well as inorganic gases such as the H2, CO, CO2, NO, NO2, SO2, SO3,

NH3, and the water.

CE 1.6. The following physicochemical and thermodynamic properties were possible to

compute for mixtures up to 10 compounds, to a given condition of Pressure,

Temperature and composition (P, T, X):

1. Overall Molecular Weigh

2. Density

3. Overall Heat Capacity

4. Enthalpy

5. Boiling point

6. Dwelled point

CE 1.7. I was in charge of developing the Thermodynamic and Physicochemical

algorithms for computing these properties by using State Equations of Redlich Kwong1,

and Peng-Robinson2.

CE 1.8 Overall Project’s Architecture and main Program’s objectives are shown in

Figure 1 in the following page.

1 REID R, PRAUNITZ J. and SHERWOOD T., “The Properties of the Gases and Liquids”, New York,

McGraw Hill, 3a ed, 1977. 2 PENG D.Y., AND ROBINSON D.B., “A New Two Constant Equation of State”, Chemical Engineering

Vol. 15, 1976, p. 59.

CE 1 Alvaro H. Pescador 3

CE 1.8 Figure 1. Structure of the Steady State System Si mulator – SIMPRES

1. Pumps

2. Compressors

3. Heat Exchangers

4. Mixers

5. Splitters

7. Reactors

6. Fractionators

9. Distillation

Towers

8. Absorption /Desortion

Towers

11. Valves

12. Turbines

Calls PROPERTY to

characterize OUTPUT

Streams

10. Flash

13. User´s Module

COMPOUNDS DATA

BANK

Matrix of 471 Compounds

with 14 Physicochemical

parameters of pure

Substances (Appendix 2)

PRINTER

Shows the OUTPUT DATA

through the screen, or by a

Printed form:

Process Topology

Input and Output Equipment’s

Simulation Parameters.

P, T, M, X conditions of the

process Streams.

Warning Messages

READER

Asks and validates INPUT DATA

to the user:

Process Topology

System Unit, British or International

Compounds Codes, up to 471.

Equipment’s Simulation Parameters

P,T,M,X conditions of Input streams

up to 10 compounds.

PROPERTY

By Using Equations of

State Calls The Bank

which have parameters

of 471 pure compounds,

to compute

Physiochemical and

Thermodynamic

properties of Input

and Output Streams.

SIMPRO

Executive Program

Calls PROPERTY

to characterize

Input Streams.

Calls the Equipment

Simulation Modules in

agreement with the

specified Process Topology.

Calls CSL, Convergence

Simulation Loop

if recycling streams are

found, in agreement with

the process Topology

1. Pumps

2. Compressors

3. Heat Exchangers

4. Mixers

5. Splitters

7. Reactors

6. Fractionators

9. Distillation

Towers

8. Absorption /Desortion

Towers

11. Valves

12. Turbines

Calls PROPERTY to

characterize OUTPUT

Streams

10. Flash

13. User´s Module

COMPOUNDS DATA

BANK

Matrix of 471 Compounds

with 14 Physicochemical

parameters of pure

Substances (Appendix 2)

PRINTER

Shows the OUTPUT DATA

through the screen, or by a

Printed form:

Process Topology

Input and Output Equipment’s

Simulation Parameters.

P, T, M, X conditions of the

process Streams.

Warning Messages

READER

Asks and validates INPUT DATA

to the user:

Process Topology

System Unit, British or International

Compounds Codes, up to 471.

Equipment’s Simulation Parameters

P,T,M,X conditions of Input streams

up to 10 compounds.

PROPERTY

By Using Equations of

State Calls The Bank

which have parameters

of 471 pure compounds,

to compute

Physiochemical and

Thermodynamic

properties of Input

and Output Streams.

SIMPRO

Executive Program

Calls PROPERTY

to characterize

Input Streams.

Calls the Equipment

Simulation Modules in

agreement with the

specified Process Topology.

Calls CSL, Convergence

Simulation Loop

if recycling streams are

found, in agreement with

the process Topology

CE 1 Alvaro H. Pescador 4

1.3. PERSONAL WORKPLACE ACTIVITY

CE 1.9. I was in charge to identify the grades of freedom3 for each one of the

Equipment modules in Figure 1. It means that for simulation purposes of Steady State

Operations, the package should ask as few data to the user as it would be

mathematically possible. The P,T,M,X conditions of inputs streams must always be

defined by the user.

CE 1.10. Pumps. It was very important to secure the information introduced to the

system. In an input stream to a pump I must be secure that at the P1,T1,M1,X1 conditions

of a pure compound, or mixture, it is in a liquid state. If not, a warning message is given

to the user.

Figure 2. Simulation Parameters for Pumps

CE 1.11 The output conditions of Temperature, T2 and Enthalpy, H2, are computed, in

agreement to a desirable output pressure, P2. The Work, W, necessary to do the job is

computed by the module. Another option is to supply the Work value, W, and then P2

will be computed by SIMPRES. In any case, the Pump’s efficiency, η, must be

provided by the user.

CE 1.12. Compressors. At P1,T1,M1,X1 conditions of a pure compound, or a mixture,

the input stream must be a gas. If not, a warning message is given to the user. The user

may choose between an adiabatic or a polytrophic compression. In the first case an

efficiency, η, must be provided, in the second is computed by the module as a result, in

agreement to a compression factor, Cf, or a desirable output pressure, P2.

3 PERRY, Robert E and CHILTON, Cecil, “Chemical Engineering Handbook”, Mexico, McGraw-Hill,

1984, Vol. 1, Simulation System Processes p. 2-73.

P P1,T1,M1,X1

P2,T2,M2,X2

W

η

CE 1 Alvaro H. Pescador 5

Figure 3. Simulation Parameters for Compressors

CE 1.13 As output values the simulator find the kind of Eliot compressor needed,

number of Stages, pressure and temperature of the output stream, as well as W, the job

needed to compress the gas, following a polytrophic compression curve. If an adiabatic

(ideal) compression was chosen, only W is computed, as well as the characteristics of

the output stream.

CE 1.14. Heat Exchangers. SIMPRES has two categories of Equipments:

a. Heat Exchangers of Service. They can be either coolers or heaters, condensers or

boilers. To simulate them it is necessary to specify two streams in the equipment’s

topology: those which manage the process fluid, as shown in figure 4. The process

fluid is the fluid which is about to be condensed, cooled, heated or boiled. To

provide the heat, Q, SIMPRES uses water as service stream in the case of coolers

and condensers, and steam in the case of heaters and boilers.

Figure 4. Heat Exchangers of Service

CCP1,T1,M1,X1 P2,T2,M2,X2

W

η Cf

HE

S-2

HE

S-2

HE

S-2HE

C-1

HE

C-1

Condenser Heater

Process Fluid

Q

-Q

CE 1 Alvaro H. Pescador 6

b. Heat Exchangers of Process. They can also be coolers or heaters, condensers or

boilers. To simulate them, it is necessary to specify four streams in the equipment’s

topology: those which manage the process fluid, and those which manages the

service fluid. The hot fluid must be taken as the process fluid, and its stream be

defined first in the input and output topology of the heat exchanger, as shown in

figure 5. Calculations for the energy balance are shown in Appendix 1.

Figure 5. Heat Exchangers of Process

c. Parameters. SIMPRES allows to simulate the following kind of heat exchangers:

1. Double Pipe heat exchanger, fluids in parallel or counter stream (A, UD, DPP,

DPS).

2. 1-1 Heat Exchanger (A, UD, DPP, DPS).

3. Pipes-Shell Heat Exchanger (A, UD, NSP, NSS, DPP, DPS).

4. Coolers and Heaters (Q, OTPF, DPP, DPS).

5. Condensers (Q, X, Tb, Dsub, DPP, DPS).

6. Boilers (Q, X, Tr, Dsob, DPP, DPS).

CE 1.15 The user must provide the parameters in parenthesis. Boldfaced parameters

means simulation can be done by using any of them, the one user may know or

estimate easier. The following is the list of the parameters defined above. As can be

seen, SIMPRES allows to work units in the British System or in the International one,

but once a Unit System is chosen all the units must be worked on it:

t4

3 T3 T1 1 HE

P-1

2

4

t2

Process Fluid: T1 > T3

T1 > t2

CE 1 Alvaro H. Pescador 7

A: Heat Exchange Area (ft2, or m2)

UD: Total Dirty Coefficient of Heat Exchange (Btu / hr ft2 °F, or Kcal / hr m2 °C)

DPP: Lost Pressure in the fluid of Process (Psi, or atm)

DPS: Lost Pressure in the fluid of Service (Psi, or atm)

NSP: Number of Steps, pipe side.

NSS: Number of Steps, shell side.

Q: Heat given by the hot fluid, (Btu/lb, or Kcal / Kg).

OTPF: Output Temperature of the Process Fluid. (°F or °C).

X: Vapor Relation at condenser or boiler output (when a partial condensation or

partial evaporation is desired).

Tb: Specifies condensed at boiling point, (°F or °C).

Dsub: Specifies condensed sub cooled with a determined delta of temperature, under

its boiling point, (°F or °C).

Tr: Specifies total evaporation, at Dwelt point, (°F or °C).

Dsob: Specifies steam overheated with a determined delta of temperature over its

dwelt point, (°F or °C).

CE 1.16. The exchangers 1 to 3 are Heat Exchangers of Process for operations with

out change of phase, while exchangers 4 to 6 are able to simulate operations with or

with out change of phase, and can be defined either as Heat Exchanger of Service or

of Process. In the last case, the module computes the heat transferred with an energetic

balance over the Process Stream (Hot Fluid in the Appendix 1, Eq. A-1.1). The

temperature of the fluid of service can be established then by using the heat balance Eq.,

A-1.3 of the Appendix 1.

CE 1.17. Mixers. A maximum amount of 6 input streams are allowed, and only one

output stream, as shown in Figure 6.

Figure 6. Mixers

M

.

.

.

.

1

n

M

.

.

.

.

1

n

2 ≤ n ≤ 6

CE 1 Alvaro H. Pescador 8

It is not necessary to ask parameters to the user to run this module. By specifying the

codes of input streams (with P,T,M,X conditions defined), and the code of the resulting

or output stream, the mass and energy balance can be done4.

CE 1.18. Splitters. One single input stream is accepted to feed the splitter and a

maximum of six output streams might be defined, as shown in Figure 7:

Figure 7. Splitters

As simulation parameters the module ask the molar fraction of the input stream which

will conform each one of the output streams (e.g. for tree output streams: 0,3 for the

output stream 1, 0,3 for the output stream 2, and 0,4 for the output stream 3). The total

amount of fractions provided is validated and must be equal to one5, if not, a warning

message is given to the user.

4 HIMMELBLAU, David, “Basic Principles and Calculations in Chemical Engineering”, Prentice-Hall,

1974, section 2.2, analysis program to solve mass balance problems. Taking from the Spanish

translation, Mexico, CECSA, 1982, p. 99-104. 5 HIMMELBLAU, David, Ob. Cit, p. 102.

S .

.

.

.

1

n

2 ≤ n ≤ 6

CE 1 Alvaro H. Pescador 9

CE 1.19. Reactors. Only one input stream to the reactor and one output stream from it

are allowed, as shown in Figure 8. The estequeometry coefficients of reactives and

products are asked to the user as input parameters (a cero must be provided for

compounds which does not take part in the reaction).

Figure 8. Reactors

CE 1.20. For the reactants, the fraction that is not consumed during the reaction is

asked, thus, for each one of the reactives, the fraction in the products stream is asked.

Optionally, the enthalpy of the reaction may be provided, and must be introduced as

negative, if the reaction is exothermic. Otherwise, it will be computed by the module.

CE 1.21. In a Steady State System Simulator the reactor module takes care about the

mass and energy balance in agreement with reaction’s estequeometry and does not

model kinetic of reaction. To perform kinetic calculations it would be necessary to

consider the time as a process variable6 and that is beyond the boundaries of Steady

State System Simulation7.

CE 1.22. If in the real process there is more than one stream feeding the reactor, a mixer

must be put before the reactor to run the simulation, and the output stream from the

mixer must feed the reactor.

6 LEVENSPIEL, Octave, “Chemical Reaction Engineering”, Jhon Wiley and Sons Inc, New York, 1980.

Taken form the Spanish Edition, Barcelona, Ed. Reverté, 1986, p. 3-7. 7 PERRY, Robert E and CHILTON, Cecil, “Chemical Engineering Handbook”, Mexico, McGraw-Hill,

1984, Vol. 1, Simulation System Processes p. 2-105.

RR

∆HR aX + bY cZ

CE 1 Alvaro H. Pescador 10

CE 1.23. Fractionation Towers. Six input streams to the tower and six output streams

from it are allowed, as shown in Figure 9.

Figure 9. Fractionation Towers

For the output streams, as simulation parameters SIMPRES ask the distribution factor

for each one of the compounds present in the input stream(s), as well as the desirable

output temperature of the leaving streams. This simulation module can be use to

perform quick mass and energy balance for multi components separation processes in

stages columns, with several feedings and several retirements. As simulation result,

besides the separation, the heat necessary to perform the operation is computed.

CE 1.24 Absorption and Desorption. Six input streams to the tower and six output

streams from it are allowed, as shown in Figure 10. Simulation is done by using the

Wang-Henke method originally developed for distillation8. As shown in Figure 10, at

least one liquid and one gas inputs streams completely defined (P,T,M,X conditions)

should go inside the tower (maximum 6).

8 WANG and HENKE, “Tridiagonal Matrix for Distillation”, Hydrocarbon Processing, Vol. 6, p. 155,

1966.

FC FC

.

.

.

.

.

.

.

.

.

. .

.

Q

.

.

.

.

.

.

FT

CE 1 Alvaro H. Pescador 11

Figure 10. Absorption Towers

CE 1.25. The user must provide the following parameters:

1. Tower’s Pressure

2. Number of Stages, N

3. Top Temperature

4. Bottom Temperature

5. Number of vapor retirements

6. Number of liquid retirements

7. Number of Heat Exchangers (in any stage, maximum 6)

8. If there are lateral feedings, the feeding stage.

9. If there are lateral retirements, the retirement stage, the Flow and a code for the

retirement stream.

10. If there are heat exchangers, the stage number and the calorific charge.

CE 1.26. Parameters 3 and 4 are the simulation key, since they set the tower’s

temperature range, and are used to create polynomials to compute the Enthalpy and the

Equilibrium constants, Keq., which will varies between these range. The module

computes mass transfer in each equilibrium stage and as final result provides the

(P,T,M,X) condition of the liquid L', and vapor stream, G', leaving the tower.

AA

.

.

.

.

.

.

.

.

L

G

AA

.

.

.

.

.

.

.

.

AA

.

.

.

.

.

.

.

.

L

GL'

G'

CE 1 Alvaro H. Pescador 12

CE 1.27 Distillation Towers. Distillation Columns for separation of multi components

can be run in SIMPRES by using the shortcut method of Smith-Brinkley9 or the

rigorous of Wang Henke. In the first case, only one feeding stream is allowed, F,

(P,T,M,X conditions known). The output streams are the Distillated, D, and the

Residue, R, at the bottom of the column, as shown in Figure 11.

Figure 11. Distillation Towers

CE 1.28 To do the simulation the following parameters must be provided:

1. Tower’s Pressure

2. Number of Stages, N.

3. Number of the feeding stage

4. Reflux relation: L / D

5. Compound’s distribution factors to the top stream. These are the key simulation

parameters since they set the Distillated flow and the temperature profile range.

CE 1.29 The boiler at the bottom of the model must be numbered as stage one. The

method is not restrictively applicable to a column with a partial condenser due to it does

not make a difference between the top stream composition and the one of the Reflux, L.

The effect of a condenser can be estimated by increasing N in 1, as another stage. The

9 SMITH AND BRINKLEY, American Institute of Chemical Engineering, Journal 6, 446, 1960.

HE

C-1

HE

B-1

DT

D

R

F

L

HE

C-1

HE

B-1

DT

D

R

F

L

CE 1 Alvaro H. Pescador 13

final values of TN+1 and T1 are the Dwelt point of the computed top’s product and the

boiling point of the computed bottom’s product, respectively. The presence of the

boiler and the condenser are included in the model: they DO NOT have to be specified

apart from the model.

CE 1.30 By the Wang Henke Method, six input streams are allowed as well as 6 out put

streams: the Distillated, the Residue and four lateral retirements. The following

parameters must be provided by the user:

1. Tower’s Pressure

2. Number of Stages, N

3. Distillated Flow, D

4. Reflux relation: L / D

5. Top Temperature

6. Bottom Temperature

7. Number of vapor retirements

8. Number of liquid retirements

9. Number of Heat Exchangers (in any stage, maximum 6)

10. Distillated Vaporization Ratio (from 0 to 1)

11. Distillated stream code.

12. If there are lateral feedings, the feeding stage.

13. If there are lateral retirements, the retirement stage number, flow, and code of

the retirement stream.

14. Heat exchangers: the stage number and the calorific charge.

CE 1.31. Flash. One feeding stream is allowed, F, (P,T,M,X conditions known) being

the output streams the Gas phase, G, and the Liquid phase, L, at the bottom of the

column, as shown in Figure 12.

CE 1 Alvaro H. Pescador 14

Figure 12. Flash

Flash module can be used to split a stream in its liquid and vapor phases

instantaneously, in one single stage. Top Stream leaves the splitter as a gas at its dwelt

point, while bottom stream leaves it as liquid at its bubble point.

CE 1.32. Valves. One input stream to the valve and one output stream from it, are

allowed, as shown in figure 13. As simulation parameter the pressure drop caused by the

valve is asked to the user. Temperature of the output stream is computed through an

adiabatic flash.

Figure 13. Valves

CE 1.33. Turbines. One input stream to the turbine and one output stream from it, are

allowed, as shown in figure 14. Either P2 or W is asked to the user as simulation

parameter. In any case as an additional parameter the isentropic efficiency, η, also must

be provided by the user:

FLFLF

G

L

FLFLF

G

L

VV

∆P

R

P1 P2 P2 < P1

CE 1 Alvaro H. Pescador 15

Figure 14. Turbines

following the first law of the Thermodynamics10, the internal energy of the gas slows

down in agreement to the Job provided by the system, therefore the output temperature

of the gas might be very low, allowing the turbine to act as a refrigerant. The output

temperature is estimated as is the expansion would be made at constant entropy. In

agreement with the second law of Thermodynamics this is an idealization (a reversible

an adiabatic process)11. The real Job WR is finally computed based upon the isentropic

efficiency.

CE 1.34 User’s Module. Modules designed by the user may have up to 6 input and 6

output streams, as shown in Figure 15:

Figure 15. User’s Modules

up to 18 parameters may be introduced by the user for simulation purposes. Figure 14

must be regarded as a Control Volume12 of a Steady State Process. Module’s purpose is

to establish output stream conditions in agreement with an energy and mass balance of

10 SONNTANG AND VAN WYLEN, “Introduction to the Thermodynamics: Classical and Statistical”,

New York, John Wyley and Sons Inc., 1971. Taken from the Spanish translation, Mexico, Limusa,

1979, p. 156-160. 11 SONNTANG AND VAN WYLEN, Op. cit, p. 255-268. 12 SONNTANG AND VAN WYLEN, Op. cit, p. 38-39.

T

W

TT

W

P2 P1

P2 < P1

η

WR = W * η

UM . . .

n

.

.

.

.

1

n

1 ? n ? 6

-

W

1

. UM

.

.

.

n

.

.

.

.

1

n

UM . . .

n

.

.

.

.

1

n

.

.

.

.

1

n

1 ≤ n ≤ 6

-Q

W

1

.

CE 1 Alvaro H. Pescador 16

the Control Volume, bearing in mind that heat leaving the system Q, must be considered

as negative while introduced to the system as positive. The opposite happens with the

Job W, should the equipment would need or make any Job.

CE 1.35. User’s module can access the PROPERTY library (Figure 1, page 3) of

SIMPRES which allows to estimate physic and thermodynamic properties of Input and

Output streams, in the following ways:

1. Estimates all the properties

2. Computes Bubble Point, °K

3. Computes Dwelt Point, °K

4. Runs Isotermic Flash (outcome: stream vaporization ratio, from 0 to 1, and heat

needed) .

5. Runs Adiabatic Flash (outcome: stream vaporization ratio, from 0 to 1).

6. Computes Gas Compressibility Factor: Z, and gas density (gr / mol-gr).

7. Estimates Enthalpy, Kcal / Kmol

8. Estimates Heat capacity, Kcal / Kmol °K

9. Estimates Equilibrium Constants of pure compounds, Keq.

10. Computes Density, gr / mol-gr

11. Computes Average Molecular Weight, gr / mol-gr

12. Computes output temperature °K for a given enthalpy, (by using adiabatic flash

routine).

CE 1.36. The Simulator SIMPRES was exhaustively tasted for individual equipments,

simple processes and complex processes. Complex processes are those having recycling

streams with equipments conforming loops indented or one internal and the other

external as the one shown on Figure 16.

CE 1 Alvaro H. Pescador 17

Figure 16. Simulation Model for a Ciclohexane Production Plant

CE 1.37. The equipments 2, 3 and 4 are conforming a loop since there is a recycling

stream coming from the equipment 4, to the equipment 2. This loop is internal to the

one conformed by equipments, 5, 6, 7, 8 and 1 which integrate an external loop, since

there is a recycling stream coming from the equipment 8 to the equipment 1, whose

output stream feeds the equipment 2. Equipments 10 and 11 are conforming a simple

loop.

CE 1.38. In order to solve this model it is necessary to know P,T,M,X conditions of the

Streams 1 and 3, and to suppose initial values for the P,T,M,X conditions of the

Recycling Streams 2, 5 and 15. After this, SIMPRES ask to the user simulation

Process Equipments

1) Mixer, M-1 7) Splitter, S-1

2) Heat Exchanger of Process, HE P-1 8) Compressor, C-1

3) Heat Exchanger of Service, HE S-1 9) Valve, V-1

4) Reactor, R-1 10) Heat Exchanger of Process, HE P-2

5) Heat Exchanger of Service, HE S-2 11) Fractionation Tower, F-1

6) Flash, F-1 12) Heat Exchanger of Service, HE S-3

1 R-1

FL-1

FT-1

HE

S-1

HE

S-2

HE

S-3

HE

P-1

HE

P-2

C-1S-1

M-1

3

2

4

5

6

7 8

9

10

11

12

13

14

15

16

17 18

19

1 2 3

4

5

6

V-1

78

9 10

11

12

1 R-1

FL-1

FT-1

HE

S-1

HE

S-2

HE

S-3

HE

P-1

HE

P-2

C-1S-1

M-1

3

2

4

5

6

7 8

9

10

11

12

13

14

15

16

17 18

19

1 2 3

4

5

6

V-1

78

9 10

11

12

CE 1 Alvaro H. Pescador 18

parameters for each one of the Equipments involved in the process in agreement with

the topology specified, from 1 to 12. The results were compared with those provided by

Commercial Simulators such as PRO-II and HYSIM13, both of them word wide used by

Oil Companies including ECOPETROL, finding similar outcomes.

CE 1.39. Finally, it is important to see that there are some differences between a

Process Diagram and a Simulation Model, as can be seen in Figures 17 and 18.

Figure 3. Phenol Production from Iso Propyl Benzene – Process Diagram

13 HIPROTECH, Hyprotech Headquarters, 119-14th Street N.W. Suite 400, Calgary, Alberta, T2N 1Z6.

P-1

HE

C-1

HE

B-1

HE

C-4

HE

B-4

HE

C-2

HE

B-2

HE

C-3

HE

B-3

HE

S-3

R-1

HE

S-1

C-1 HE

S-2

DT 1

DT 2

DT 4

DT 3

P-1P-1

HE

C-1

HE

B-1

HE

C-4

HE

B-4

HE

C-2

HE

B-2

HE

C-3

HE

B-3

HE

S-3

HE

S-3

R-1

HE

S-1

HE

S-1

C-1C-1C-1 HE

S-2

DT 1

DT 2

DT 4

DT 3

CE 1 Alvaro H. Pescador 19

Figure 4. Phenol Production from Iso Propyl Benzene – Simulation Model

CE 1.40. As can be seen, Distillation Towers in the simulation model does not have any

condenser or boiler, due to they are included as stages in the simulation model as was

explained in CE 1-29. On the other hand, two new equipments appears in the simulation

model that were no present in the process diagram, a Mixer and a Splitter, because is the

way to establish in the model that some streams are being join to others, or are being

separated in the process.

P-1

HE

S-1

C-1

1

HE

S-2R-1

HE

S-3

M-1

S-1

DT 1

DT 2 DT 3

DT 4

P-1P-1

HE

S-1

C-1C-1C-1

1

HE

S-2R-1

HE

S-3

HE

S-3

M-1

S-1

DT 1

DT 2 DT 3

DT 4

CE 1 Alvaro H. Pescador 20

1.4 SUMMARY

CE 1.41. SIMPRES is used at the Computing Laboratory of the Engineering Faculty at

the University of America since 1991, by Chemical Engineering students along the

whole career, in the following subjects:

1. Mass Balance

2. Energy Balance

3. Mechanic of Fluids

4. Thermodynamics I, II

5. Physical Chemistry I, II

6. Heat Transfer I, II

7. Mass Transfer I, II, III

8. Design of Reactors

9. Design of Plants I, II

10. Computing Applied to Chemical Engineering

CE 1.42. The Project was evaluated as a merit one by the University of America, see

Appendix 3. Then, I went to a National Congress of Chemical Engineering in 1991 on

behalf of the University, and gave a lecture about the Project, see certification in

Appendix 4. After several months, SIMPRES was granted with the National Award to

the best Degree Project by the Colombian Professional Council of Chemical

Engineering, see certification in Appendix 5.

CE 1.43. I contributed to this project having the original idea and convincing my

partners that it was possible to do it, even tough the complexity known to built a

simulator14. I not only search for literature or thank up Programs to simulate some of the

EQUIPMENT MODULES (CE 1.4) and Equations of State in the PROPERTY Module

as explained in CE 1.7, but wrote the document and the SIMPRES User’s Manual in a

friendly way15, despite research’s complexity.

14 PEERY, Robert E and CHILTON, Cecil, “Chemical Engineering Handbook”, Mexico, McGraw-Hill,

1984, Vol. 1, Simulation System Processes p. 2-99 to 2-104. 15 MENDOZA, J., PESCADOR, A. AND RODRÍGUEZ, J., “Diseño y Construcción de un Simulador de

Procesos en Estado Estacionario”, 3 Vol., Bogotá, Universidad de América, 1990.

CE 1 Alvaro H. Pescador 21

CE 1.44. ABREVIATIONS

A: Heat Exchange Area (ft2, or m2)

D: Distillated

DPP: Lost Pressure in the fluid of Process (Psi, or atm)

DPS: Lost Pressure in the fluid of Service (Psi, or atm)

Dsob: Specifies steam overheated with a determined delta of temperature over its dwelt

point, (°F or °C).

Dsub: Specifies condensed sub cooled with a determined delta of temperature, under its

boiling point, (°F or °C).

F: Feeding Stream

G: Gas Stream

L: Liquid Stream

N: Number of Stages in a separation tower.

NSP: Number of Steps, pipe side in a Heat Exchanger

NSS: Number of Steps, shell side in a Heat Exchanger

OTPF: Output Temperature of the Process Fluid. (°F or °C).

P,T,M,X: Pressure, Temperature, Mass Flow, and molar composition of a Process Stream.

Q: Heat given by the hot fluid, or to a System. (Btu/lb, or Kcal / Kg).

R: Residue, in a Distillation Tower

Tb: Specifies condensed at boiling point, (°F or °C).

Tr: Specifies total evaporation, at Dwelt point, (°F or °C).

UD: Total Dirty Coefficient of Heat Exchange (Btu / hr ft2 °F, or Kcal / hr m2 °C)

W¨: Job (done by the system, red, given to the system, blue). (Btu/lb, or Kcal / Kg).

X: Vapor Relation at condenser or boiler output (when a partial condensation or partial

evaporation is desired).

Z: Gas compressibility factor

CE 1 Alvaro H. Pescador 22

BIBLIOGRAPHY

HIMMELBLAU, David, “Basic Principles and Calculations in Chemical Engineering”,

Prentice-Hall, 1974, section 2.2, analysis program to solve mass balance problems.

Taking from the Spanish translation, Mexico, CECSA, 1982, p. 99-104.

HIPROTECH, Hyprotech Headquarters, 119-14th Street N.W. Suite 400, Calgary, Alberta,

T2N 1Z6.

KERN, Donald, “Heat Transfer Processes”, New York, Mc Graw Hill, 1984, p. 122.

LEVENSPIEL, Octave, “Chemical Reaction Engineering”, Jhon Wiley and Sons Inc, New

York, 1980. Taken form the Spanish Edition, Barcelona, Ed. Reverté, 1986, p. 3-7.

LUTHE, OLIVERA, SHULTZ, “Numeric Methods”, 1986, México, Limusa, 1986, p. 63,

70, 144.

MENDOZA, J., PESCADOR, A. AND RODRÍGUEZ, J., “Diseño y Construcción de un

Simulador de Procesos en Estado Estacionario”, 3 Vol., Bogotá, Universidad de

América, 1990.

PENG D.Y., AND ROBINSON D.B., “A New Two Constant Equation of State”, Chemical

Engineering Vol. 15, 1976, p. 59.

PIGORIR, A, DE PASCALE, T and MILANESI, F, Chemical Engineering, July 11 of

1976, “A Numerical Method to calculate pipes-case exchangers”, New York, p. 67.

PERRY, Robert E and CHILTON, Cecil, “Chemical Engineering Handbook”, Mexico,

McGraw-Hill, 1984, Vol. 1, Simulation System Processes p. 2-105.

REID R, PRAUNITZ J. and SHERWOOD T., “The Properties of the Gases and Liquids”,

New York, McGraw Hill, 3a ed, 1977.

SMITH AND BRINKLEY, American Institute of Chemical Engineering, Journal 6, 446,

1960.

SONNTANG AND VAN WYLEN, “Introduction to the Thermodynamics: Classical and

Statistical”, New York, John Wyley and Sons Inc., 1971. Taken from the Spanish

translation, Mexico, Limusa, 1979, p. 38-39, 156-160, 255-268.

WANG and HENKE, “Tridiagonal Matrix for Distillation”, Hydrocarbon Processing, Vol.

6, p. 155, 1966.

CE 1 Alvaro H. Pescador 23

APPENDIX 1. – SIMULATION ROUTINE FOR HEAT EXCHANGERS

Having the input conditions of both streams and a desirable output temperature for one of

them, the output temperature of the other stream, and the Heat exchanged, can be computed

in agreement to the following equations16:

Q1 = M * C * (T2 – T1) Eq. (A-1.1)

Q2 = m * c * (t1 – t2) Eq. (A-1.2)

Where:

Q1: Heat given by the hot fluid, in Btu/lb, or Kcal / Kg

M: Mass flow of the hot fluid, in lb/hr, or Kg/hr

C: Heat Capacity of the hot fluid, in Btu/lb °F, or Kcal / Kg °C

T2: Output temperature of the hot fluid (°F, or °C)

T1: Input temperature of the hot fluid (°F, or °C)

Q2: Heat taken by the cold fluid, in Btu/lb, or Kcal / Kg

m: Mass flow of the cold fluid, in lb/hr, or Kg/hr

c: Heat Capacity of the cold fluid, in Btu/lb °F, or Kcal / Kg °C

t2: Output temperature of cold fluid (°F, or °C)

t1: Input temperature of the cold fluid (°F, or °C)

As we ideally know, Q1 is equal to Q2, or in other words:

M * C * (T2 – T1) = m * c * (t1 – t2) Eq (A-1.3)

Then, the output temperature of the cold fluid can be obtained, if I have T2, and vice versa,

the output temperature of the hot fluid can be find if I have t2. The heat transferred can also

be know through Eq. A-1.1 or A-1.2. The Area needed to design the Exchanger can be

calculated, in general, by graphical methods, such as the one explained by Kern17, bearing

in mind that:

Q = U * A * MLTD Eq. (A-1.4)

Where,

U: Total Coefficient of Heat Exchange (Btu / hr ft2 °F, or Kcal / hr m2 °C)

A: Heat Exchange Area (ft2, or m2), and

(T1 – t2) – (T2 – t1)

MLTD = ---------------------------- Eq (A-1.5)

Ln ((T1 – t2) / T2 – t1))

16 KERN, Donald, “Heat Transfer Processes”, New York, Mc Graw Hill, 1984, p. 122. 17 KERN, Op cit, p. 121-127.

CE 1 Alvaro H. Pescador 24

In the case of double pipe exchangers, MLTD is computed like this for a counter

stream arrangement which optimizes the heat exchanged, Q, and minimizes the

necessary Area, A, which was SIMPRES default option.

On the other hand, in most of the simulation cases the User does not know the output

temperature of any of the fluids. Then, the temperature of the Hot fluid can be computed by

using the following equation, Kern (Ob cit, p. 120):

(1 – R) T1 + (1 – e (UA/mc) (R – 1) ) R t1

T2 = ------------------------------------------------ Eq (A-1.6)

1 - R e (UA/mc) (R - 1)

Where,

R = mc / MC Eq (A-1.7)

If the user specifies parallel flow, T2 is computed by Equation 8 (Kern, Op cit, p.120):

(R + e (UA/mc) (R + 1) ) T1 + (e (UA/mc) (R + 1) – 1) Rt1

T2 = ----------------------------------------------------------- Eq (A-1.8)

(R + 1) e (UA/mc) (R + 1)

CE 1.13. In any case, the temperature of the cold fluid is computed applying the heat

balance Equation (Eq. A-1.3). Double pipe exchangers are useful when the Area needed

for the heat transfer is little: not bigger than 200 ft2 (around 30 m2). Simulation of this kind

of exchangers assumes that none of the streams changes its phase during the process,

because all these equations apply for sensible heat only. If there is vaporization or

condensation during the process, a warning message is sent to the user, recommending him

to use either the routine for boilers, or the one of condensers.

CE 1.14 Most industrial application requires bigger amounts of Area due to bigger

requirements of heat to be exchanged, like those allowed by pipes shell exchangers. Now

will see how SIMPRES Equipment’s Simulation Routine were built, taking the case of

pipes shell exchangers as an example.

Input Data and Targets: As in any case simulation purposes is equipment’s detailed

design, the user must estimate general simulation parameters, such as A and U, the steps

number of the shell side, and the steps number of the pipes side, in the case of a heat

exchanger. It is recommended to the user to provide a pressure lost through each side of the

equipment, as in real processes happens. If this is unknown a default lost of 10 psi is given

to each stream.

CE 1 Alvaro H. Pescador 25

Having these data, plus the P,T,M,X characteristics of the input streams, both the output

temperatures and the Pressures of the hot and the cold fluids must be computed.

Mathematic Analysis

As in pipes shell exchangers the flow is a combination of parallel and counter stream, the

MLTD computed by Eq. (A-1.5) must be normalized by a temperature factor, FT which

varies in agreement to the steps numbers:

CMLTD = FT * MLTD (Eq. A-1.9)

Kern found this factor by using graphics for the 1-2, 2-4, 3-6 … till 6-12 arrangements. To

do it, is necessary to read S in the abscises. The value of S can be found by using Eq. A-

1.10:

S = (t2 – t1) / (T1 – t1) Eq. (A-1.10)

Once calculated through the Eq (A-1.7) R can be used as entrance value to the graphic

polynomials, (Kern, Op. cit p. 933-938). Then, FT, which varies between 0,5 and 1, can be

found in the ordinates. The question was: how to program Kern’s graphical method?.

By searching literature I found a paper at the Chemical Engineering Magazine of tree

Italian Engineers, who published a numerical method that allows to find the Media

Logarithmic Temperature Difference already normalized, CMLTD, as follows18:

A

CMLTD = ------------------------ Eq. (A-1.11)

β Ln (B+A)/(B-A)

Where,

A = ( (T1 – T2)2 + (t1 – t2)

2 ) ½ Eq (A-1.12)

B = ( (T1 – t2) 1/ά + (T2 – t1)

ά Eq (A-1.13)

The values of β and ά appears in Table 1. β is the step number of the shell side.

18 PIGORIR, A, DE PASCALE, T and MILANESI, F, Chemical Engineering, July 11 of 1976, “A Numerical

Method to calculate pipes-case exchangers”, New York, p. 67.

CE 1 Alvaro H. Pescador 26

Table 1. β and ά to compute CMLTD by Pigrori and others

Β ά

1 1.000

2 2.000

3 2.595

4 3.000

5 3.355

6 3.586

7 3.700

After doing tastes, the authors found the following deviations from Kern’s graphical

method, as shown in Table 2.

Table 2. Deviation of Pigrori and others Method

from the Traditional of Kern

Arrangement Deviation

1-2 0.157

2-4 0.045

3-6 0.085

4-8 1.597

5-10 0.143

6-12 1.958

It is important to see that in the equations A-1.12 and A-1.13, T2 and t2 are unknown values

for the simulation model. Therefore, it will be necessary to use a convergence method to

compute the value of one of them, lets say T2. Once this value is found, t2 can be known

from the heat balance (Eq. A-1.3).

In SIMPRES I used a Direct Convergence Method of the last supposed value19. Given the

Eq. A-1.1, for real conditions Eq. A-1.4 turns into Eq. A-1.14:

Q = U * A * FT * MLTD Eq (A-1.14)

Substituting Eq. A-1.3 and Eq. A-1.7 on Eq. A-1.14, we can obtain Eq. A-1.15, which then

can be used to obtain T2S in the Eq. A-1.16

Fcali = U * A * FT * (1 – 1/R) / (M * C) Eq (A-1.15)

19 LUTHE, OLIVERA, SHULTZ, “Numerical Methods”, 1986, México, Limusa, 1986, p. 63, 70, 144.

CE 1 Alvaro H. Pescador 27

T1 – ( (T1 – t1) * e Fcali – 1)

T2Si = ---------------------------------- Eq (A-1.16)

e Fcali - 1/R

Where,

Fcal: Normalization Factor calculated at the iteration, i

T2Si: Output Temperature of the hot fluid at the iteration, i

Output Data

In the equations A-1.15 and A-1.16 all the values are known after the first iteration, for

which one it is assumed FT = 1; T2 = (T1 + t1)/2, and t2 = t1 + T2/2. The iterations are made

until convergence is reached or until a iterations limit of 20 is exceeded, taking the last

computed value in such case.

Then, The Output pressure is established for each one of the Streams in agreement to input

data pressure lost parameters provided by the user, or the default option (10 psi). Finally,

the module Calls PROPERTY in order to compute the physicochemical and

thermodynamic properties of the output streams (as described in Figure 1).

CE 1 Alvaro H. Pescador 28

APPDENIX 2 BANK COMPUONDS DATA PARAMETERS

The following parameters are given for 471 Compounds, both inorganic and organic.

Organic or hydrocarbons maybe lineal or branched, saturated or non saturated.

1. Compound’s Code

2. Compound’s Name

3. Molecular Weight (gr / mol-gr).

4. Critical Temperature (°K)

5. Critical Pressure (atm)

6. Critical Volume (cc / gr-mol)

7. Z Critic

8. Critical Density (mol-gr / cc)

9. Normal boiling point (°K)

10. Pitzer Acentrical factor

11. Solubility Hildelbrand Parameter

12. Molecular Volume at 298 °K (cc / mol-gr)

13. Constant A to compute the heat capacity in Eq A-2.1, T in °K

14. Constant B to compute the heat capacity in Eq A-2.1, T in °K

15. Constant C to compute the heat capacity in Eq A-2.1, T in °K

16. Constant D to compute the heat capacity in Eq A-2.1, T in °K

Cp (cal / mol °K) = A + B*T + C*T2 + D*T3 Eq (A-2.1)