1. education
2. graduate school

Seoul National University
Chemical Process and Product Design
Aspen HYSYS :
Steady states and Dynamic Simulator
(Introduction)
Spring Semester, 2014
TA : Ikhyun Kim
([email protected])
Instructor : En Sup Yoon
What is PSE?
• Process Systems Engineering :
– See the BIG picture in the small pieces
Finding the right piece and seeing how it fits is the key.
Many may look attractive, but they may not answer to
our current needs.
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Finding the right piece?
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What is PSE?
• Broad aim of PSE researches
Develop efficient method &
computer aided tools for
Process Synthesis
Process Optimization
Planning and Scheduling
Process Control
Safety and Reliability
PSE tools & methodologies are routine
in many chemical industries
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Sequential Modular Strategy
for S-S Process Simulation
• Nonlinear algebraic equations:
f (y )  0
l yu
where ∈
is the vector of unknown process variables to be
solved for l, ∈ are vectors of upper and lower bounds on the
process variables and :
⟶
•
Sequential modular strategy is one approach to solving problem
especially tailored to the network structure of process flowsheets
– Typically simultaneous solution of 100s~100,000s of equations
requires an iterative process.
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Sequential Modular Strategy
for S-S Process Simulation
• Gaussian’s elimination:
– Given a linear system,
– Manipulate | to an upper-triangular form
– Then, solve backwards from the
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th
row according to:
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Sequential Modular Strategy
for S-S Process Simulation
• Example of Gaussian’s elimination:
And now…
1,
3,
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Sequential Modular Strategy
for S-S Process Simulation
• Will it help if can we break the problem into a sequence
of smaller problems?
a. If computation time grows super linearly with problem size
then solving a sequence of smaller problems is cheaper than
solving one big problem
b. For example, recall that Gaussian elimination is a cubic
function of the number of equations. If we can break the overall
problem into two subproblems:
2
2
and a lot less effort is expended in achieving a solution
The sequential modular strategy exploits the topology(structure) of
the flowsheet to suggest a partitioning and precedence ordering
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Sequential Modular Strategy
for S-S Process Simulation
• Solving recycle problems
a.
d
b.
‘
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Sequential Modular Strategy
for S-S Process Simulation
•
Solving recycle problems
a.
b.
c.
d.
e.
•
Guess S5
Given S1 and S5, solve A for S2
Given S2, solve B for S3
Given S5`, update guess for S5
Repeat from step2 until converged (e.g., S5~S5`=0)
Problems
– How to select which stream(s) to tear in order to break the cycle
– How to update the guess for the torn stream(s) so that the iterative process
converges rapidly, and when to terminate the iterative process
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Sequential Modular Strategy
for S-S Process Simulation
•
How to update the guess?
a.
Bisection method
-
b.
Newton’s method (Newton-Raphson method)
-
c.
Linearizing the system using Taylor’s expansion
Jacobian matrix of partial dervatives
Successive over-relaxation
-
d.
Intermediate value theorem
when the multiplicity of system > 1
Secant method / Broyden method(Quasi-Newton method)
-
Finite difference approximation
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Sequential Modular Approach
•
Features:
– Process unit models precoded as subroutines and fixed, and a library
made available to the user
– Stream structure fixed (e.g. F, T, P)
– Solution procedures embedded in subroutines with unit model equations
– Inputs and results of unit model calculations (directionality) fixed – given
inputs, solve for the outputs.
– Hence, sequential solution of units from feed streams to product
streams
•
Problems, what about:
– Recycle streams (material or information)
– Downstream (design) specifications
– Extension to custom models or new technologies
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Aspen HYSYS Solvers
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Aspen HYSYS
Key design elements
– Event driven interface
– Modular operations
– Subflowsheet architecture
Multiple environments
– Flowsheet
– Simulation basis
– Oil characterization
Interactive
Flexible
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Insert Figure
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Aspen HYSYS Environments
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Properties
Property methods are a collection of models and methods used to
describe pure component and mixture behavior
•
Choosing the correct physical properties is critical for obtaining
reliable simulation results
1.0
0.8
0.6
0.4
Raoult’s Law
0.2
0.0
0.0
0.2
0.4
0.6
0.8
Liquid Mole Fraction METHANOL
•
1.0
1.0
0.8
0.6
0.4
RK-Soave
0.2
0.0
0.0
0.2
0.4
0.6
0.8
1.0
Vapor Mole Fraction METHANOL
•
Vapor Mole Fraction METHANOL
Use the Properties Specifications form to specify the physical
property methods to be used in the simulation
Vapor Mole Fraction METHANOL
•
1.0
0.8
0.6
0.4
NRTL
0.2
0.0
0.0
0.2
0.4
0.6
0.8
1.0
Liquid Mole Fraction METHANOL
Liquid Mole Fraction METHANOL
Selecting a Process Type will narrow the number of methods
available
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Effect of System Thermodynamics
• Correct choice of physical property models and
accurate physical property parameters are essential for
obtaining accurate simulation results
OVHD
FEED
Specification:
99.5 mole %
acetone
recovery
COLUMN
BTMS
Ideal
Approach
Equation of
State
Approach
Activity
Coefficient
Model
Predicted number of
stages required
11
7
42
Approximate cost ($)
650,000
490,000
1,110,000
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Aspen HYSYS Environments
• Via the two main Aspen HYSYS Environments
Basic Environment
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Simulation Environment
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Aspen HYSYS Architecture
• Basic Environment
–
–
–
–
Components
Property Package (Thermodynamic model)
Hypothetical Components
Reactions
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Aspen HYSYS Architecture
Aspen HYSYS Library Components
– Over 1800 components in main databank
– Search by Simulation name, Full name, Synonym or Formula
– Use property package or family filters
Aspen Properties Database
– Pure component databanks contain over 23000 species
– NIST Pure component data and NIST Thermodata Engine
(TDE) for improved data fitting and estimation
Hypothetical Components
– Minimum data entry is one property (NBP, MW, density…)
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Choosing a Fluid Package
Fluid package sources
– HYSYS
– Aspen Properties
– COMThermo
Property model selection
– Property Wizard
– Aspen HYSYS documentation
Parameters
– Pure component parameters accessed via Component view
– Interaction parameters are available on the Binary Coeffs. tab
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Aspen HYSYS Architecture
• Simulation Environment
– Streams, Unit Operations, Analysis tools, etc.
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Aspen HYSYS Color Scheme
Values (Variables):
Blue: User-specified
• Red: Default value
• Black: Calculated (or fixed) value
•
Streams:
Light Blue: Not Solved
• Dark Blue: Solved
•
Unit Operations
Red: Connection is missing—unable to begin solving
• Yellow: Unable to Solve or Solved with Warnings
• Black: Solved
•
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Process Simulation
• What information do we need to enter?
1. Fluid Package information
a. What components do we have (databank, hypos, assays, etc.)
b. What thermodynamic method we will use (EOS, activity models,
…)
2. Details of your process
a. Unit operations (equations to be solved)
b. Process conditions and equipment specifications (defined
parameters)
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Basic Steps for Simulation
• Create a unit set
• Select the components
Basic
Environment
• Choose a property package (Thermodynamic model)
• Create and Specify the streams
Simulation
Environment
• Install and Define the unit operation prior to the column
• Install and Define the column
• DOF & Specification
Analysis
• Analyzing the Result (Case Study, Verification, Optimization, etc.)
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The Aspen HYSYS Solver…
…is responsible for all steady state calculations in the
Aspen HYSYS program
…is a non–sequential solver: information can flow forward
and backward through the flowsheet
…is interactive and uses a Degrees of Freedom analysis
to trigger solving of unit operations and streams
…tracks all numerical values in Aspen HYSYS according
to their source
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Reactor Models
Reactor
[ Balance Based ]
[ Equilibrium Based ]
[ Kinetics Based ]
Yield Shift Reactor
Equilibrium Reactor
Gibbs Reactor
PFR
CSTR
Conversion Reactor
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Balanced Based Reactors
• Yield Shift Reactor
– Requires a mass balance only, not an atom balance
– No reaction stoichiometry required
– Is used to simulate reactors in which inlets to the reactor are not
completely known but outlets are known
• Conversion Reactor
– Performs mass balance calculations based on reaction
stoichiometry(or conversion) and flashes the outlet stream
– Used when reactions kinetics are unknown or unimportant
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Equilibrium Based Reactors
• Equilibrium Reactor
– Computes combined chemical and phase equilibrium by solving
reaction equilibrium equations
– Useful when there are many components, a few known
reactions, and when relatively few components take part in the
reactions
• Gibbs Reactor
– A Gibbs free energy minimization is done to determine the
product composition at which the Gibbs free energy of the
products is at a minimum
– Do not require reactions stoichiometry
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Kinetics Based Reactors
• CSTR
– Use when reaction kinetics are known and when the reactor
contents have same properties as outlet stream
– Can model equilibrium reactions simultaneously with rate-based
reactions
• PFR
– Handles only rate-based reactions
– A cooling stream is allowed
– You must provide reactor length and diameter
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Heat of Reaction
• Heat of reaction need not be provided for reactions
• Heat of reaction are typically calculated as the difference
between inlet and outlet enthalpies for the reactor
• If you have a heat of reaction value that does not match the
value calculated by simulator, you can adjust the heats of
formation of one or more components to make the heat of
reaction match
• Heat of reaction can also be calculated or specified at a
reference temperature and pressure in an Conversion
Reactor
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Columns in Aspen HYSYS
A column is a specialized sub-flowsheet in Aspen HYSYS
Column
subflowsheet
Main simulation environment
Advantages:
– Isolated column solver
– Optional use of different fluid packages
– Construction of custom templates
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Column Basics
Specifications
– Pressure Profile required
– The number of additional column operating specifications
depends on the complexity, Degrees of Freedom of the system,
usually 0-3
– Degrees of Freedom can be tracked on Monitor and Specs
page
– Active Specs can be entered on Monitor or Specs pages
– Estimates can be entered to help with convergence
Results
– Monitor page contains most results, including convergence
– Column Profiles are available on Performance page
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Converging a Column
1. All feed streams must be fully solved
2. Never specify product streams directly
3. Activate specs to satisfy Degrees of Freedom analysis
4. Make sure all active specs have a value
5. Balance specifications along the entire tower
6. Click Run to run column solver; reset when necessary
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Pre-built Columns (Templates)
Absorber: contains only a tray section  Degrees of Freedom (DOF) =
zero, no additional operating specification can be given
Reboiled absorber: contains a tray section and a reboiler  DOF = 1,
one additional operating specification has to be given
Refluxed absorber: contains a tray section and a top condenser
– With a total/full reflux condenser  DOF = 1
– With a partial condenser  DOF = 2
Distillation column: contains a tray section, condenser and reboiler
– With a total/full reflux condenser  DOF = 2
– With a partial condenser  DOF = 3
Side operations add additional Degrees of Freedom
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Recycles
What is a Recycle operation?
– mathematical / logical unit operation
Assumed
Calculated
R
When to use a Recycle operation?
– Required when downstream material stream(s) mix with
upstream material stream(s) and when there is mass I/O across
the flowsheet
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Adding Recycle Operations (1)
Procedure 1
1.
Solve flowsheet without recycled stream
2.
Add Recycle, and only attach the calculated stream
(calculated = estimated)
3.
Connect assumed stream to flowsheet
1
2
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Adding Recycle Operations (2)
Procedure 2
1.
Guess (estimate) assumed stream
2.
Solve flowsheet up to calculated stream
3.
Add and connect recycle operation
1
3
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Sensitivities in Recycle Operation
Sensitivities used in Recycle operation are multipliers to
internal convergence tolerances in Aspen HYSYS
Aspen HYSYS internal tolerances are:
Vapor Fraction
Temperature
Pressure
Flow
Enthalpy
Composition
0.01
0.01
0.01
0.001
1.00
0.0001
Actual Tolerance = Sensitivity * Internal tolerance
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Sensitivities
Given a molar flow rate of 100 lbmole/hr
Internal tolerance = 0.001
Sensitivity = 10
Absolute tolerance = 100 lbmole/hr * 0.001 * 10
Absolute tolerance = 1 lbmole/hr
Recycle is converged if 99 < molar flow < 101
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Tear Locations
To minimize the number of tear locations, add recycles
– Downstream of gathering points (mixer)
– Upstream of distribution points (column, tee, separator)
To minimize the number of recycle variables (T, P, etc.)
– Choose a tear location that maximizes number of fixed variables
– Add recycle operations at separator inlets
– Compressor after cooler outlets
Choose a stable tear location
– To minimize effect of fluctuations
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Adding Recycles
Which are the physical recycle streams? 6 and 7
Which are the possible tear streams? 6 and 7; 2 and 4; 3
Which is the best choice for the tear stream?
The best tear stream choice is stream 3; if this stream is used, you
only need to converge one recycle instead of two
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Advanced Modeling
Exercise 1–A
Recycle required? If so, how many? Possible location(s)?
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Advanced Modeling
Exercise 1–A
Recycle required? No – closed loop (no I/O in flowsheet)
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Advanced Modeling (2)
Exercise 1–B
Recycle required? If so, how many? Possible location(s)?
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Advanced Modeling (2)
Exercise 1–B
One stream is on the tube side th
e other on the shell side
There is no mixing of fluids
Recycle required? No – downstream material does not mix upstream
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Advanced Modeling (3)
Exercise 1–C
Recycle required? If so, how many? Possible location(s)?
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Advanced Modeling (4)
Exercise 1–D
Recycle required? If so, how many? Possible location(s)?
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