Process Simulation
•
Once we have established the block flow and started filling in the main
vessels, heat exchangers, pumps, etc. we want to develop a mass and
energy balance for the process so we can start evaluating the process in
more detail
•
The simulation is also the starting point for equipment design, as it will
set the flow rates and duties for process equipment
•
In most companies, mass and energy balances are developed using a
process simulator such as AspenPlus, ChemCad, ProII or UniSim. Each
program has its own idiosyncracies, but they have many common
features. Examples will be given in both AspenPlus and UniSim.
•
Note that the simulation should come after you know what’s in the PFD,
but often it’s an iterative process to get both
Process Simulation
• Structure of process simulators
• Components and physical property models
• Modeling reactors
• Modeling separations
• User models
• Recycles & convergence
Structure of Process Simulators
•
The user manipulates the
program through a GUI that is set
up to look similar to a PFD
•
The executive program
determines the calculation
sequence and calls the other
subroutines
Equipment
sub-routines
E
xe
cut
ive
P
rogra
m
Graphical User
Interface (GUI)
Thermodynamics
sub-routines
Convergence &
optimization
sub-routines
Physical
property data
Cost data
Example: UniSim Simulation of GE
LM6000 Engine
Using the GUI: Basis Environment
Basis Environment
Enter
components
Enter
reactions
Specify
stoichiometry
Select
property
package
Basis Environment
Enter
reactions
Using the GUI: Object Palette
Object Palette
User can
select
operations
from the
palette and
drag and
drop to the
PFD
Unit
operations
Separator
models
Dynamics functions
Adjust, Set,
Recycle
Spreadsheet
General
reactors
Using the GUI: Workbook View
Click here
Brings up all the basic
stream data such as
temperature, pressure,
flow rates, etc. in one
screen
Windows Can Be Configured to Show
PFD & Workbook
Editing the Flowsheet in the GUI
Right click on any vessel or stream
icon and you get a menu that allows
you to select from similar icons,
hide the stream or operation, rotate
it, rename it and generally tidy up
the drawing to look more like a
proper PFD
Sub-Flowsheets
Sub-flowsheet
You can define a sub-flowsheet
and use it as a way of grouping
several operations away from
the main flowsheet. This is
particularly useful when you
need several unit operations to
model a single piece of process
equipment.
Generating Mass & Energy Balance
Reports
Report manager is on the Tools menu
Define a report
Select all streams,
conditions and
composition only
Process Simulation
• Structure of process simulators
• Components and physical property models
• Modeling reactors
• Modeling separations
• User models
• Recycles & convergence
Entering Components: Pure Components
• Pure components
• Component library has thousands of pure components
• Mostly organic compounds, but some inorganic compounds
• Rules for selecting pure components
• Always include any compound that has a specified limit in the product
• Always include any compound that has a specified limit in any process feed
• Always include anything formed in side reactions or consecutive reactions
• Always include anything with significant HS&E concerns
• Usually include anything that is present at >2% (by mole or mass)
• Usually do not include isomers unless required by the process
• Usually try to have < 40 pure components
Pseudocomponents
• Petroleum fractions can contain ~ 10
4
to 10
6
components, many isomers, many compounds that
cannot be isolated and identified
• Instead, use a pseudocomponent that represents all the
compounds that boil in a given temperature range
V
o
lu
m
e
%
d
is
tilled
Temperature (F)
0
50
100
Crude Oil Boiling Curve
Pseudocomponents
• Example: this pseudocomponent represents all
compounds that boil between 300F and 350F, making up
roughly 8 vol% of the feed
• Simulators have default pseudocomponents, but user may
need to add more around critical cut points
V
o
lu
m
e
%
d
is
tilled
Temperature (F)
0
50
100
Crude Oil Boiling Curve
Solids and Salts
• Solids
• Some simulators recognize solid phase pure components when they are
formed
• Phase equilibrium with solid phase is often not well predicted: check the
model carefully against the literature
• Solid phases of mixed composition usually have to be defined as user
components (e.g.: cells, catalysts, coal, paper fibers, etc.)
• Some of the simulation programs have good models for solid handling
operations, including modeling the effect of particle size distribution
• Salts
• Ionic compounds in the presence of water must be treated as electrolytes
and require special phase equilibrium models
User Components
• Users occasionally need to add components that
are not included in the component library
• Examples:
• Complex molecules for pharmaceutical APIs
• Specialty chemicals
• Proprietary compounds
• Advanced solvents
• Electrolytes
Defining User Components
In the Basis environment, select
Hypo Components
Create Hypo
Component
Enter or estimate
properties
Defining User Components Using
UNIFAC Groups
Select UNIFAC groups
to build up the molecular
structure. The program
will then estimate
properties using group
contribution methods
Physical Property Models
• All the simulation programs have a range of physical
property models
• Model selection depends on the system chemistry – see
Chapter 4
• Be careful: if the physical property database does not have
the model parameters then they may be estimated using
methods such as UNIFAC, but estimated parameters should
be confirmed experimentally
• Models are often inaccurate when predicting LLE, SLE, SSE
• When user components are present, models will be near
useless unless some experimental data is fitted
Phase Equilibrium Model Selection
•
Chapter 4 has a chart to help with model selection:
Hydrocarbon C5 or lighter H2 present Polar or Hydrogen bonding Sour Water H2 present Electrolytes T < 250 K P < 200 bar T < 250 K γi experimental data P < 350 bar P < 4 bar T < 150ºC Two Liq phases Need more experimental data
Select model that gives best fit to
data Use G-S Use B-W-R or L-K-P Use P-R or R-K-S Use R-K-S Use G-S 0<T<750K Use G-S or P-R Use NRTL or UNIQUAC Use Wilson, NRTL or UNIQUAC Use electrolyte Use sour water system Use UNIFAC to estimate interaction parameters Start Y Y Y Y Y Y Y Y Y Y Y N N N N N N N N N N N Y N Y N Y N
