Reactor Modeling Tools - An Overview

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-RXT-817

Reactor Modeling Tools - An
Overview
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Process Engineering Guide:

Reactor Modeling Tools

CONTENTS

1

SCOPE

2

OPTIONS IN REACTOR MODELLING
2.1
2.2
2.3
2.4
2.5
2.6
2.7
2.8
2.9

General
Level of Complexity of Model
Mode of Operation of Model
Deterministic versus Empirical Modeling
Platforms for Model
Steady State versus Dynamic Model
Dimensions Modeled in Reactor
Scale of Modeling for Multiphase Reactors
Writing and Using the Model

APPENDICES

A

CHARACTERISTICS OF DIFFERENT REACTOR MODELS

B

NEEDS FOR MODELLING AT DIFFERENT SCALES IN
HETEROGENEOUS CATALYTIC REACTORS

C

REACTOR MODELS EMPLOYED WITHIN GBHE

DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE


1

SCOPE

This Guide provides a general overview of reactor modeling needed for reactor
design and manufacturing improvement, and the issues that need to be
addressed before starting to write a reactor model.
Appendix C gives a list of the reactor models that are available within GBHE,
with some basic information and contacts. More detailed information on some of
these programs is given in their respective guides.

2

OPTIONS IN REACTOR MODELLING

2.1

General
There are a large number of options and types of reactor model and the
choice of model for a particular reactor will depend on the objectives of the
model. It is normally the case that a given reactor will have a number of
different models with each model set up to achieve various objectives.
The options for reactor modeling are listed below and the issues are dealt
with in more detail in 2.2 to 2.9.
(a)

Level of complexity of model:
(1)
(2)
(3)

(b)

Mode of operation of model:
(1)
(2)
(3)

(c)

Equilibrium model.
Simple kinetic model. Apparent kinetics plus residence time
distribution (and heat input distribution).
Mass transfer/kinetic model. Intrinsic kinetics plus interphase
mass and heat transfer plus flow patterns (use of
computational fluid dynamics).

Design of reactor.
Rating of existing reactor.
Performance evaluation.

Deterministic versus empirical modeling.


(d)

Platforms for model:
(1)
(2)
(3)

Standalone program.
Spreadsheet package.
Flowsheeting package;
(i)
Standard reactor module.
(ii)
User supplied reactor module.

(e)
(f)

Number of dimensions modeled in reactor.

(g)

Scale of modeling for multiphase reactors.

(h)

2.2

Steady state versus dynamic model.

Writing and using the model.

Level of Complexity of Model
It is normal to start with simple models for initial evaluation of new
processes and progress to more complicated models as the knowledge to
build the model and the detail required in the reactor design increases.
The level of complexity will depend on:
(a)

What variables need to be explored?
For example, is the model to be used to determine the size of
reactor for a given conversion, or is it also going to be used to
calculate by-product formation?

(b)

What mechanistic features need to be included?
For example, does it need to determine the reactant residence time
distribution, or will this be assumed? Does it need to calculate a
heat transfer coefficient within the model or will it be calculated offline?

(c)

How much time (and effort) is available for model development?

Three basic levels of complexity are given in 2.2.1 to 2.2.3.


2.2.1 Equilibrium Model
The equilibrium model is used for the evaluation of new process routes in
order to estimate the conversion attainable in the reactor.
Equilibrium calculations can be performed using standalone models e.g.
STANJAN as part of a short cut design study, or are supposedly able to
be carried out within modern flowsheeting packages as part of the
flowsheet model.

2.2.2 Simple Kinetic Model
A simple kinetic model will typically be one dimensional, modeling
apparent kinetics (which includes mass transfer effects) within an imposed
residence time distribution. If the reactor is not adiabatic or isothermal a
heat input distribution is also imposed.
Simple modeling of this type can be performed using spreadsheets e.g.
Excel. However a common problem with these and more complicated
models is 'stiffness'. This arises when the various differential equations in
a simultaneous set have time constants that differ by orders of magnitude.
The effect is that in ordinary integration algorithms the step length for
accuracy and stability is very small and computation would take too long.
Hence more advanced tools or models become necessary. Options are:
(a)

Build kinetic equations into a general model framework e.g.
KINPACK or BATCHCAD.

(b)

Write bespoke Fortran model with advanced solver.

This may take longer, but gives more flexibility for moving to more
complicated models and integrating to flowsheeting programs.


2.2.3 Mass Transfer/Kinetic Model
The mass transfer/kinetic model has a higher level of complexity than the
simple kinetic model and will include any or all of:
(a)

Intrinsic kinetics plus calculated mass transfer.

(b)

Calculated flow patterns and residence time distributions.

(c)

Calculated heat transfer.

(d)

Two or three dimensional modeling.

There is probably an infinite level of complication that can go into these
models, so it is important to understand the objectives of the model and
the important factors affecting the reactor performance, so that a
reasonable level of complexity can be used.

2.3

Mode of Operation of Model
A reactor model can be used in a variety of modes. These affect which are
the input variables and which are the output variables of the model.

2.3.1 Design of Reactor
The model is set up, for example, to calculate the size of reactor needed
in order to achieve a given conversion, i.e. the model input is the
conversion and the model output is the reactor size. The model needs to
be run in this mode for design optimization.
2.3.2 Rating of Existing Reactor
When a reactor is designed it is usual to calculate how it would perform
under a range of operating conditions. The model is set up, for example,
to calculate the conversion from the input streams and reactor geometry.
The model needs to be run in the rating mode for operation optimization.
This is the easiest way to set up a model.


2.3.3 Performance Evaluation
The model is set up to check or monitor the performance of the reactor
compared with what it is expected to achieve. The model inputs are
therefore the input and output streams and reactor geometry and the
model output is some reactor performance measure e.g. catalyst activity.
It will be normal for the model to be operated in all the different modes
over the life of a project. Since the natural way to write a model is for
rating mode, there is an issue of how to run the model in the design mode
and performance mode. There are three ways in which to do this:
(a)

the model has to be run a number of times changing model input
variables;

(b)

the model is set up with internal iteration loops to automatically
adjust reactor input variables to obtain the reactor output variables
that are set;

(c)

the model is integrated into a flowsheet modeling platform such as
Aspen HYSYS V8, PRO II or SPEED-UP and the convergence
routines within these platforms are used to adjust reactor inputs to
obtain the required output variables.

A lot of time can be saved by considering all the input and output variables
that may be needed when the model is first written, so that changes do not
have to be made later on.

2.4

Deterministic versus Empirical Modeling
A table with the characteristics of different types of model is given
in Appendix A.

2.4.1 Deterministic or Mechanistic Modeling
Deterministic or mechanistic modeling involves building a model
completely from known data obtained from kinetics experiments or
the literature. The model can then be used to predict the effect of
design and operational changes. These models can only be written
for reactors where the technology is fairly well established.


2.4.2 Semi-empirical Modeling
Semi-empirical modeling uses standard equations or relationships,
which are known to fit the reactor performance, but a small number
of constants in the model are fitted by comparing the model
predictions to plant data. It can be used predicatively to some
extent, but care has to be taken and its accuracy questioned for
extrapolation outside the conditions for which the constants were
fitted.
2.4.3 Empirical Modeling
Empirical modeling involves characterizing a reactor from data
obtained from the reactor itself. It can only be used for predicting
performance within the design and operating conditions for which
data has been fitted. Empirical modeling is used for plant control
and can also be used to establish which input variables have an
effect on the output variables. The latter is useful as a prelude to
writing a more complex model to identify all the relationships that a
more advanced model needs to include.
Empirical modeling can use a variety of techniques ranging from
multivariate regression to neural networks. Empirical modeling has
to be used for plants where the technology is an art rather than a
science and is too difficult to be modeled deterministically.

2.5

Platforms for Model
It is important to choose the appropriate platform or platforms upon
which the model is going to be run. For example it is normal to start
off with a model as a standalone model that will be run on its own,
but it will later be required to integrate the model into a flowsheeting
system.

2.5.1 Standalone Model
A standalone model is used at any stage of reactor modeling. If the
model does not take too long to solve, it can be incorporated into a
flowsheeting system - see 2.5.3.


2.5.2 Spreadsheet Model
A spreadsheet model can be used at the early stages of design for
simple modeling or scouting and for running empirical models.
However, it has limitations in that it can lead to problems with
iteration and it cannot be interfaced into flowsheeting packages.
Since the structure of spreadsheet models is difficult to check,
spreadsheet models cannot be used with confidence, except by the
person who wrote them and should not be made generally
available.
2.5.3 Flowsheeting Package
When a reactor model is built into a flowsheeting package, see
Appendix C.6, it gives a number of advantages:
(a)

The reactor model can use the flowsheet package to access
physical properties, rather than having to supply them
separately. However, care is needed when using these
packages as there is a tendency for some commercial
software to offer default data without appropriate warnings
when results are given outside the valid range of the data.

(b)

The algorithms within the flowsheeting package can be used
to do iteration around the reactor, for instance in design
mode, to adjust the reactor size to obtain a given
performance.

(c)

A whole system around the reactor, including separation
equipment, can be modeled so that the effects of changes in
reactor performance can be followed through the flowsheet.

(d)

When the financial drivers associated with the reactor and
the reactor system are added, the algorithms within the
flowsheeting package reactor model can be used to do a
design or performance optimization of the reactor system.


There are two ways that the reactor model can be built into the
flowsheeting package:
(1)

Standard Reactor Modules
Flowsheeting packages such as ASPEN and PRO II have standard
reactor modules which can be used to model a reactor. However,
these are generally inflexible and do not accommodate the
peculiarities that need to be modeled to make the reactor model
realistic.

(2)

User Supplied Reactor Module
All sequential modular flowsheeting packages such as GENIE,
FLOWPAK, ASPEN and PRO II allow Fortran models of reactors to
be incorporated into the flowsheet package, see Appendix C.6. A
harness needs to be written around the standalone reactor model in
order to incorporate it into a flowsheet package.
Equation based flowsheeting packages such as SPEED-UP, need
the reactor equations to be written into the package and cannot
accept Fortran subroutines.

2.5.4 Computational Fluid Dynamics (CFD) Packages
CFD packages can be extended to be used as reactor modeling
programs. The CFD package itself gives an accurate residence
time distribution (RTD) of reactants in a reactor and kinetics can be
added to describe the reaction. This approach is applicable when
the RTD is crucial to the reactor performance and idealized RTDs
such as plug flow and well mixed are not appropriate. This is the
case in liquid reactors when the reactions are fast and are therefore
controlled by mixing rate, or in fixed bed reactors where there is
poor flow distribution.


2.6

Steady State versus Dynamic Model
While it may seem to be ideal to write reactor models so that they can be
run either in the steady state or dynamically, this is not usually the case.
There is quite a lot of extra investment required in reactor data, model
writing time and run times as well as problems with robustness for a
dynamic model compared with a steady state model. Dynamic models are
normally written to answer specific questions influencing the design of the
reactor under transient conditions, or to identify control problems. The
effects of reactor start-up and shut down on the rest of the flowsheet can
usually be assessed by running a steady state reactor model at different
points in the startup sequence. The dynamic models that are written
usually use simplified kinetics or contain fewer features than steady state
models, see Appendix C.4.

2.7

Dimensions Modeled in Reactor
Most reactor models are zero or one dimensional models. For example a
stirred tank is a zero dimensional model, while a plug flow reactor is a one
dimensional model.
It is sometimes necessary to go to two or even three dimensions. These
multi-dimensional models are normally only needed as standalone models
and can be used to calculate corrections or other approximations to be
added to a one dimensional reactor model to be used for incorporation
into flowsheeting packages.

Examples are:
(a)

A two dimensional model is needed to determine the heat transfer from
the fluid to the wall of a packed tubular reactor, but once this has been
calculated at two or three different conditions, the information can be
captured in a one dimensional model as a heat transfer coefficient
between a bulk fluid temperature and the wall temperature. Checks may
subsequently be needed using the two dimensional model to calculate
maximum or minimum fluid temperatures.


(b)

Two or three dimensional models are needed to model fluid mixing using
computational fluid dynamics, but the results of the mixing modeling can
be used to calculate a residence time distribution, or used to characterize
the reactor as a series of zones, which can then be used in a one
dimensional kinetic model. If the mixing is such that a one dimensional
kinetic model is not adequate, then a two dimensional kinetic model is
needed. However if this is the case for a new reactor, the reactor design is
probably wrong and the mixing intensity needs to be increased.

2.8

Scale of Modeling for Multiphase Reactors
While most reactor models are at the reactor scale, there is often a need
to model at smaller scales. These sorts of models can cover a large
range. A table of how the performance of heterogeneous reactors can be
affected by the properties at different scales and hence the potential need
for modeling at these scales is shown in Appendix B.
The models that are written at the smaller scale are nearly always
standalone models for parts of the reactor that are used to calculate
parameters that are subsequently used in the full reactor model.

2.9

Writing and Using the Model
Writing and using the model will depend on the type of model:
(a)

Spreadsheet Models
Spreadsheet models will normally be written and used by the
process engineer or scientist who is collecting the data or designing
the reactor.

(b)

Standalone Models
Standalone models should ideally be written jointly by the process
engineer or scientist and a mathematician. With simple models, the
mathematical input will be small and can be done by consultation.
As models become more complicated a greater mathematical input
is required. It is best if the process engineer or scientist supplies
the equations and relationships and the mathematician builds the
model.


Mathematicians are able to use the most appropriate algorithms to
solve problems with stiffness and will use standard codes as much
as possible.
(c)

Integrating into Flowsheeting Packages
Integrating user supplied modules into flowsheeting packages can
be done more efficiently by an experienced person. If there is not
an experienced person in the group, help should be sought from an
experienced Process Systems Engineering Specialist.

(d)

Empirical Models
There is a potential danger with empirical modeling that although a
model fits existing data very well, the predictive power of the model
is very poor. This is especially the case with neural network
models. Because they are very flexible, they are able to fit the
errors in data. Advice should be sought from a statistician or
mathematician to get a view on the empirical technique to be used
and the uses to which the model can be put.


APPENDIX C REACTOR MODELS EMPLOYED WITHIN GBHE
C.1

EQUILIBRIUM MODELLING

EXCEL 2007: Multiple Regression Data Analysis Add-In
Platform PC
aspenONE Engineering V8.4
Platform PC
STANJAN
Platform PC
C.2

KINETIC MODELLING

C.2.1 Kinetics Fitting Packages
GNU Octave
Platform PC
BatchCAD 7.0 (Rate)
Platform PC
C.2.2 General Kinetic Models
EXCEL 2007: Multiple Regression Data Analysis Add-In
Platform PC
Can be used to model simple kinetics in idealized residence time distributions
(RTDs) of one or more phases.
Chemical WorkBench
Platform PC
KINPACK
Platform PC – Fortran
Incorporates complicated kinetics in idealized plug flow, stirred pot or batch
reactors.


BatchCAD (REACTION)
Platform PC
Dynamic kinetic model for batch reactors.
CHEMKIN
Platform PC
Gas phase and gas/solid chemical kinetic and equilibrium modeling of aeronomic
(Atmospheric, etc.) Chemistry, combustion and heterogeneous catalysis, for
stirred tanks,
CSTR, shock tube and flames.

C.2.3 Specific Kinetic Models
REFORMER - SimSci-Esscor’s Reformer Reactor Model
REFORMER – Bozedown Model
Platform PC
Dedicated to aromatics reformers. Uses fundamental kinetics, and integrates
reactors into a dedicated flowsheet package including heaters and compressor.
CHEMCAD
Platform PC
Dedicated to benzene hydrogenation reactor.

C.3

KINETIC MODEL WITH MASS AND/OR HEAT TRANSFER

Models are reactor type dependent.
C.3.1 Mass Transfer
Computational Fluid Dynamic (CFD) Codes
Open FOAM
Platform PC
Kinetics can be added to CFD codes in order to model the kinetics in an accurate
real residence time distribution.

COMSOL Multiphysics
Platform PC
C.3.2 Liquid/Gas Reactors
CHEMCAD
Platform PC
COMSOL Multiphysics
Platform PC
XREACT
Platform PC
Models kinetics and mass transfer with real RTDs of both phases using RTD
information from an associated CFD package. One, two or three dimensions.

C.3.3 Fluid/Solid Catalytic Reactors
These models are currently all process specific, but it should be possible to
amend an existing model for a new application.
ADv5
Platform PC
Design and performance of steam raising shift and methanol converters with
catalyst in tubes or on the shellside. Kinetics includes pore diffusion and heat
transfer coefficients are calculated on both shell and tube sides.

OTHERS
KHIMERA
Platform PC
Chemical kinetics simulation software tool developed by Kintech Lab
Wolfram SystemModeler
Platform PC
Modeling and simulation software based on the Modelica language

C.4

DYNAMIC MODELS

SPEED-UP
Platform PC
General dynamic modeling platform.

Computational Fluid Dynamics Packages
AUTODESK CFD Fluid Flow Simulations
Platform PC
Can be used for dynamic modeling of kinetics and fluid flow.

Russian Reactor Model
Dynamic model of fixed bed reactor for modeling reactor dynamics or catalyst
deactivation.

C.5

SMALLER SCALE MODELS

These are reactor type dependent.
C.5.1 Liquid/Gas Reactors
Micro-CFD
Models the mass transfer around individual bubbles or droplets.
C.5.2 Fluid/Solid Catalytic Reactors
EFFPAK
Platform PC
Calculates kinetics, mass transfer and heat transfer within porous catalyst
particles given pellet transport properties.


DIFFCALC
Platform PC
Will calculate particle diffusion properties from structural parameters of particle.

C.6

FLOWSHEETING PACKAGES

These will provide the capability for solving a reactor integrated into a flowsheet.
C.6.1 Sequential Modular
Aspen Plus, Aspen HYSYS, Aspen Custom Modeler
Standard modules supplied in the packages can be used for simple models. User
supplied subroutine can be built into packages, but need own internal solver.

C.6.2 Equation Based
SPEEDUP
Platform PC
Package solves reactor at the same time as it solves the flowsheet using the
same solving routine.
The reactor can be split into a number of sections within the package. Can be
used as a platform for XREACT - see C.3.2.

C.7

BATCH REACTORS - HEAT TRANSFER PROGRAMS

ProSim BatchReactor
Platform PC
Cooling an agitated batch reactor with an external jacket with a non-isothermal
service fluid in turbulent flow.
Heating an agitated batch reactor with an external jacket with steam.


STM LIMP
Heating an agitated batch reactor with limpet coils with steam
PRO/II
is a steady-state process simulator (process simulation) for process design and
operational analysis for process engineers in the chemical, petroleum, natural
gas, solids processing, and polymer industries.
It includes a chemical component library, thermodynamic property prediction
methods, and unit operations such as distillation columns, heat exchangers,
compressors, and reactors as found in the chemical processing industries. It can
perform steady state mass and energy balance calculations for modeling
continuous processes.


DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE
This Process Engineering Guide makes reference to the following documents.

PROCESS ENGINEERING GUIDES
GBHE-PEG-RXT-800
GBHE-PEG-RXT-801
GBHE-PEG-RXT-802
GBHE-PEG-RXT-803
GBHE-PEG-RXT-804
GBHE-PEG-RXT-805
GBHE-PEG-RXT-806
GBHE-PEG-RXT-807
GBHE-PEG-RXT-808
GBHE-PEG-RXT-809
GBHE-PEG-RXT-810
GBHE-PEG-RXT-811

How to use the Reactor Technology Guides
Chemical Process Conception
Residence Time Distribution Data
Data and Computer Programs for Calculating
Chemical Reaction Equilibria
Physical Properties and Thermochemistry for
Reactor Technology
Solid Catalyzed Gas Phase Reactor Selection
Fixed Bed Reactor Scale-up Checklist
Reactor and Catalyst Design
Solid Catalyzed Reactions
Homogeneous Reactors
Gas - Liquid Reactors
Novel Reactor Technology


Reactor Modeling Tools - An Overview

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