4 Process Simulation

Process Simulation

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Chemical Engineering Design

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

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Chemical Engineering Design

Process Simulation • Structure of process simulators • Components and physical property models • Modeling reactors • Modeling separations • User models • Recycles & convergence • Optimization © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

Chemical Engineering Design

Structure of Process Simulators Equipment sub-routines

Convergence & optimization sub-routines Physical property data

Executive Program

Thermodynamics sub-routines

Cost data

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Graphical User Interface (GUI)

• 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 Chemical Engineering Design

Example: UniSim Simulation of GE LM6000 Engine

• Features that will be described are common to most other simulators © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

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Using the GUI: Basis Environment

Click here

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Basis Environment

Enter components

Enter reactions

Select property package

Specify stoichiometry

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Basis Environment

Enter reactions

Specify conversion

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Using the GUI: Object Palette

Click here

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Object Palette

Unit operations General reactors Separator models

Spreadsheet

User can select operations from the palette and drag and drop to the PFD

Adjust, Set, Recycle

Dynamics functions © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

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Using the GUI: Workbook View

Click here Brings up all the basic stream data such as temperature, pressure, flow rates, etc. in one screen

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Windows Can Be Configured to Show PFD & Workbook

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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 © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

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Sub-Flowsheets

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. Sub-flowsheet

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Generating Mass & Energy Balance Reports Report manager is on the Tools menu

Define a report

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Select all streams, conditions and composition only

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UniSim Design Stream Report

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Process Simulation • Structure of process simulators • Components and physical property models • Modeling reactors • Modeling separations • User models • Recycles & convergence • Optimization © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

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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

• What is the basis for these rules? © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

Chemical Engineering Design

Pseudocomponents Crude Oil Boiling Curve Volume % distilled

100

50

0 50

1050

Temperature (F)

• Petroleum fractions can contain ~ 104 to 106 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 © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

Chemical Engineering Design

Pseudocomponents Crude Oil Boiling Curve Volume % distilled

100

50

0 50

1050

Temperature (F)

• 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 © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

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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

© 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

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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

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Defining User Components In the Basis environment, select Hypo Components

Create Hypo Component

Enter or estimate properties

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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

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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 © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

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Phase Equilibrium Model Selection • Chapter 4 has a chart to help with model selection: N Y

Start

T < 250 K

Use G-S

Y Y

H2 present

Use P-R or R-K-S

Hydrocarbon C5 or lighter

N Use B-W-R or L-K-P

Y

Use G-S

Y

Sour Water

Y Electrolytes

Y P < 200 bar

N 0 99% • EB recovery in bottoms should be > 99%

© 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

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UniSim Shortcut Model

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Shortcut Column Specifications Note: Toluene mole fraction in bottoms = 1/300 = 0.0033 Ethylbenzene mole fraction in distillate = 1/200 = 0.005

External reflux ratio = 1.15 × minimum reflux (as an initial estimate) © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

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Shortcut Column Results

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UniSim Rigorous Model

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Rigorous Column Specifications

From shortcut model Component recovery can be specified

With good estimate of reflux ratio and number of trays, convergence is fast © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

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Generating Column Profiles It is often useful to plot column composition profiles to see whether the column is efficient

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Column Composition Profiles

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Examples of Bad Profiles

Feed tray too high

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Feed tray too low

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Examples of Bad Profiles

Toluene in bottoms

Reflux too low (toluene recovery 72%)

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Reflux too high (toluene recovery 100%)

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Examples of Bad Profiles

Too few trays: toluene recovery = 24.5% © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

Chemical Engineering Design

Column Sizing in UniSim • Tray sizing is under tools/utilities • Default options (shown) may need changing • Column must be converged with the utility enabled

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Column Sizing Results

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Common Causes of Column Convergence Problems • Infeasible specifications • Make sure specs on distillate or bottoms purity can be achieved (see Section 17.6.2) • Make sure that specifications can mass balance with two products

• Poor initialization • Use shortcut column to confirm R > Rmin, N > Nmin • Remember stage efficiency is typically 0.7 or less • Remember to allow for some pressure drop across the trays

• Poor initial estimates • Most simulation programs default to the Inside-Out algorithm, which is very fast when given good initial estimates. Use simple specs (e.g. distillate flow rate and reflux ratio) to converge an initial simulation, upload the column temperature profile from this as initial estimates and then change to the real specs and the column should converge quickly.

© 2007 G.P. Towler / UOP. For educational use in conjunction with Sinnott & Towler Chemical Engineering Design only. Do not copy

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Complex Columns: AspenPlus PetroFrac Model of Crude Distillation

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Other Separation Models • Some simulation programs include models for other separations such as extraction, crystallization, solids separations, etc. • All simulators have a “Component Splitter” model – Allows user to specify recovery of each component – Can be used to model any kind of separation process

© 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

Chemical Engineering Design

Process Simulation • Structure of process simulators • Components and physical property models • Modeling reactors • Modeling separations • User models • Recycles & convergence • Optimization © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

Chemical Engineering Design

User Models • User may need to add custom models to the simulation – Detailed reactor models – Novel unit operations

• Most simulators support two ways of doing this: – Spreadsheet tool – Custom model operation

© 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

Chemical Engineering Design

UniSim Spreadsheet • The UniSim spreadsheet can be used to build simple user models of operations that are not on the palette • Allows import and export from cells to streams • Functionality is basic • AspenPlus has full MS Excel © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

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UniSim User Unit Operation Define connections to PFD

Select from palette

Enter code

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Process Simulation • Structure of process simulators • Components and physical property models • Modeling reactors • Modeling separations • User models • Recycles & convergence • Optimization © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

Chemical Engineering Design

Processes With Recycle Feed B Feed A

1 3

2

4

Recycle of B

Reactor

6

Lights 5

7

8

Product

• How do we break the recycle loop to solve in sequential mode? © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

Chemical Engineering Design

Possible Tear Strategy Iterate to convergence

Estimate Feed B Feed A

Update

1

2

4

3a

3b

Recycle of B

Reactor

6

Lights 5

7

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8

Product

Chemical Engineering Design

Tearing at the Reactor Outlet Feed B Feed A

1 3

2

4

Recycle of B

Reactor

6

Lights 5a

5b

7

8

Product

• Which tear point is likely to converge better? © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

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Convergence Problems • Results that are unconverged or “converged with errors” cannot be used for design • If convergence is slow then: – Check specifications are feasible • Use hand calculations or simplified models

– Try increasing number of iterations – Try a different algorithm • Default method is usually Bounded Wegstein – can change bounds on acceleration parameter – see Ch4 • Try Newton method if there are many recycles or specifications to meet

– Try to find a better initial estimate • Use hand calculations or a simplified model to initialize the problem

– Try to find a better tear stream – Creep up on the solution © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

Chemical Engineering Design

Model Simplification Techniques • Complex models with many rigorous columns and recycles can be difficult to converge • A simplified model can be used to initialize tear streams in the complex model • Models can be simplified by: – Using fewer components – Using simpler unit operations (e.g. replace columns with separators) – Eliminating complex user models (replace reactor models with Yield or Conversion reactor) – Reducing the number of specifications (allow some variables to remain not quite converged) © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

Chemical Engineering Design

Gas Recycle Make-up gas

Purge

Feed Reactor Product

• Don’t forget the purge stream • No purge, no converge! © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

Chemical Engineering Design

Process Simulation • Structure of process simulators • Components and physical property models • Modeling reactors • Modeling separations • User models • Recycles & convergence • Optimization © 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

Chemical Engineering Design

Setting Constraints Using Controllers An “Adjust” controller can be used to control the air flow to give a target turbine inlet temperature

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“Adjust” Specifications

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“Adjust” Solving Parameters The parameters tab can be used to set bounds to give the desired solution

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Flowsheet Optimization • Most of the simulators allow optimization inside the program • AspenPlus manual recommends: • 1. Converge the flowsheet first • 2. Carry out a sensitivity analysis and only optimize the variables that have high impact on the objective function • 3. During the sensitivity analysis, see if the optimum is broad or sharp

• Off-line optimization (or near optimization) is usually much easier – see Ch12.

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Chemical Engineering Design

Tips for Process Simulation • For a good (i.e. useful) process simulation, you must have: – Good component properties – A good phase equilibrium model – Flowsheet design that respects the 2nd law of thermodynamics

• It is not essential to have – Reaction kinetics – Detailed models of every unit operation

• Benchmark the simulation against lab, pilot plant or operating plant data whenever possible to increase your confidence that what you see in the virtual world agrees with reality

© 2012 G.P. Towler / UOP. For educational use in conjunction with Towler & Sinnott Chemical Engineering Design only. Do not copy

Chemical Engineering Design

Questions ?

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Chemical Engineering Design