4.4.

SPECIFICATION OF COMPONENTS AND PHYSICAL PROPERTY MODELS 167

number of possible compounds can be from 104 to >106 . At the time of writing, there
is no characterization method that can identify all of these compounds, so it would be
impossible to include them all in a model even if the resulting model could be solved.
Instead, a large number of possible compounds with boiling points in a given range are
‘‘lumped’’ together and represented by a single pseudocomponent with a boiling
point in the middle of that range. A set of 10 to 30 pseudocomponents can then be
fitted to any petroleum assay and used to model that oil.
Pseudocomponent models are very useful for oil fractionation and blending prob-
lems. They can also be used to characterize heavy products in some chemical pro-
cesses such as ethane cracking. Pseudocomponents are treated as inert in most of the
reactor models, but they can be converted or produced in yield-shift reactors (see
Section 4.5.1).
Some of the commercial simulation programs use a standard default set of pseu-
docomponents and fit the composition of each to match a boiling curve of the oil that
is entered by the user. This can sometimes lead to errors when predicting ASTM D86
or D2887 curves for products from a feed that has been defined based on a true
boiling point (TBP) curve, or when making many subcuts or cuts with tight distillation
specifications. It is often better to work back from the product distillation curves and
add extra pseudocomponents around the cut points to make sure that the recoveries
and 5% and 95% points on the product distillation curves are predicted properly.
All of the simulators have the option to add pseudocomponents to the default set or
use a user-generated curve.

4.4.3. Solids and Salts

Most chemical and pharmaceutical processes involve some degree of solids handling.
Examples of solids that must be modeled include
& Components that are crystallized for separation, recovery, or purification;
& Pharmaceutical products that are manufactured as powders or tablets;
& Insoluble salts formed by the reaction of acids and bases or other electrolytes;
& Hydrates, ice, and solid carbon dioxide that can form in cryogenic processes;
& Cells, bacteria, and immobilized enzymes in biological processes;
& Pellets or crystals of polymer formed in polymerization processes;
& Coal and ash particles in power generation;
& Catalyst pellets in processes in which the catalyst is fluidized or transported as a
slurry;
& Mineral salts and ores that are used as process feeds;
& Fertilizer products;
& Fibers in paper processing.

Some solid phase components can be characterized as pure components and can
interact with other components in the model through phase and reaction equilibrium.
Others, such as cells and catalysts, are unlikely to equilibrate with other components,
although they can play a vital role in the process.
168 CHAPTER 4 FLOWSHEETING

In Aspen Plus, solid components are identified as different types. Pure materials
with measurable properties such as molecular weight, vapor pressure, and critical
temperature and pressure are known as conventional solids and are present in the
MIXED substream with other pure components. They can participate in any of the
phase or reaction equilibria specified in any unit operation. If the solid phase partici-
pates only in reaction equilibrium but not in phase equilibrium (for example, when
the solubility in the fluid phase is known to be very low), then it is called a conven-
tional inert solid and is listed in a substream CISOLID. If a solid is not involved in
either phase or reaction equilibrium, then it is a nonconventional solid and is assigned
to substream NC. Nonconventional solids are defined by attributes rather than
molecular properties and can be used for coal, cells, catalysts, bacteria, wood pulp,
and other multicomponent solid materials.
In UniSim Design, nonconventional solids can be defined as hypothetical compo-
nents (see Section 4.4.4). The solid phases of pure components are predicted in
the phase and reaction equilibrium calculations and do not need to be identified
separately.
Many solids-handling operations have an effect on the particle size distribution
(PSD) of the solid phase. The particle size distribution can also be an important
product property. Aspen Plus allows the user to enter a particle size distribution as
an attribute of a solid substream. In UniSim Design, the particle size distribution is
entered on the ‘‘PSD Property’’ tab, which appears under ‘‘worksheet’’ on the stream
editor window for any stream that contains a pure or hypothetical solid component.
Unit operations such as yield-shift reactor, crusher, screen, cyclone, electrostatic
precipitator, and crystallizer can then be set up to modify the particle size distribution,
typically by using a conversion function or a particle capture efficiency in each size
range.
When inorganic solids and water are present, an electrolyte phase equilibrium
model must be selected for the aqueous phase, to properly account for the dissolution
of the solid and formation of ions in solution.

4.4.4. User Components

The process simulators were originally developed for petrochemical and fuels appli-
cations; consequently, many molecules that are made in specialty chemical and
pharmaceutical processes are not listed in the component data banks. All of the
simulators allow the designer to overcome this drawback by adding new molecules
to customize the data bank.
In UniSim Design, new molecules are added as hypothetical components. The
minimum information needed to create a new hypothetical pure component is the
normal boiling point, although the user is encouraged to provide as much information
as is available. If the boiling point is unknown, then the molecular weight and density
are used instead. The input information is used to tune the UNIFAC correlation to
predict the physical and phase equilibrium properties of the molecule, as described in
Chapter 8.
4.5. SIMULATION OF UNIT OPERATIONS 169

User-defined components are created in Aspen Plus using a ‘‘user-defined compo-

nent wizard.’’ The minimum required information is the molecular weight and
normal boiling point. The program also allows the designer to enter molecular
structure, specific gravity, enthalpy, and Gibbs energy of formation, ideal gas heat
capacity, and Antoine vapor pressure coefficients, but for complex molecules usually
only the molecular structure is known.
It is often necessary to add user components to complete a simulation model. The
design engineer should always be cautious when interpreting simulation results for
models that include user components. Phase equilibrium predictions for flashes,
decanters, extraction, distillation, and crystallization operations should be carefully
checked against laboratory data to ensure that the model is correctly predicting
the component distribution between the phases. If the fit is poor, the binary inter-
action parameters in the phase equilibrium model can be tuned to improve the
prediction.

4.5. SIMULATION OF UNIT OPERATIONS

A process simulation is built up from a set of unit operation models connected by mass
and energy streams. The commercial simulators include many unit operation sub-
routines, sometimes referred to as library models. These operations can be selected
from a palette or menu and then connected together using the simulator graphical
user interface. Table 4.2 gives a list of the main unit operation models available in
Aspen Plus and UniSim Design. Details of how to specify unit operations are given in
the simulator manuals. This section provides general advice on unit operations
modeling and modeling of nonstandard unit operations.

4.5.1. Reactors
The modeling of real industrial reactors is usually the most difficult step in process
simulation. It is usually easy to construct a model that gives a reasonable prediction of
the yield of main product, but the simulator library models are not sophisticated
enough to fully capture all the details of hydraulics, mixing, mass transfer, catalyst and
enzyme inhibition, cell metabolism, and other effects that often play a critical role in
determining the reactor outlet composition, energy consumption, rate of catalyst
deactivation, and other important design parameters.
In the early stages of process design, the simulator library models are usually used with
simplistic reaction models that give the design engineer a good enough idea of yields and
enthalpy changes to allow design of the rest of the process. If the design seems econom-
ically attractive, then more detailed models can be built and substituted into the flow-
sheet. These detailed models are usually built as user models, as described in Section 4.6.
Most of the commercial simulation programs have variants on the reactor models
described in the following sections.
170 CHAPTER 4 FLOWSHEETING

Table 4.2. Unit Operation Models in Aspen Plus1 and UniSim DesignTM

Unit Operation Aspen Plus Models UniSim Design Models

Stream mixing Mixer Mixer
Component splitter Sep, Sep2 Component Splitter
Decanter Decanter 3-Phase Separator
Flash Flash2, Flash3 Separator, 3-Phase Separator
Piping components
Piping Pipe, Pipeline Pipe Segment, Compressible Gas Pipe
Valves & fittings Valve Valve, Tee, Relief Valve
Hydrocyclone HyCyc Hydrocyclone
Reactors
Conversion reactor RStoic Conversion Reactor
Equilibrium reactor REquil Equilibrium Reactor
Gibbs reactor RGibbs Gibbs Reactor
Yield reactor RYield Yield-shift Reactor
CSTR RCSTR Continuous Stirred Tank Reactor
Plug-flow reactor RPlug Plug-flow Reactor
Columns
Shortcut distillation DSTWU, Distl, SCFrac Shortcut column
Rigorous distillation RadFrac, MultiFrac Distillation, 3-Phase Distillation
Liquid-liquid extraction Extract Liquid-Liquid Extractor
Absorption and stripping RadFrac Absorber, Refluxed Absorber, Reboiled Absorber
Fractionation PetroFrac 3 Stripper Crude, 4 Stripper Crude, Vacuum Resid
Rate-based distillation RATEFRACTM Column, FCCU Main Fractionator
Batch distillation BatchFrac

Heat transfer equipment

Heater or cooler Heater Heater, Cooler
Heat exchanger HeatX, HxFlux, Hetran, HTRI-Xist Heat Exchanger
Air cooler Aerotran Air Cooler
Fired heater Heater Fired Heater
Multi-stream exchanger MheatX LNG Exchanger
Rotating equipment
Compressor Compr, MCompr Compressor
Turbine Compr, MCompr Expander
Pump, hydraulic turbine Pump Pump
Solids handling
Size reduction Crusher
Size selection Screen Screen
Crystallizer Crystallizer Crystallizer, Precipitation
Neutralization Neutralizer
Solids washing SWash
Filter Fabfl, CFuge, Filter Rotary Vacuum Filter
Cyclone HyCyc, Cyclone Hydrocyclone, Cyclone
Solids decanting CCD Simple Solid Separator
Solids transport Conveyor
Secondary recovery ESP, Fabfl, VScrub Baghouse Filter
User models User, User2, User3 User Unit Op