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ENCH491Advanced Process Control &

Simulation

Dynamic Simulation using Unisim

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ENCH 491 Numerical methods for the simulation of chemicalprocesses, including methods for solution of algebraicequations, solution of large systems of ODEs, modelling andsolution of differential-algebraic equation systems, solutionof partial differential equation systems, optimisation ofdynamic systems (KRM - Fridays, 11-12). Advanced control. To study selected topics in processcontrol beyond the standard undergraduate course withparticular emphasis on multi-variable systems and modelbased control. A further goal is for students to becomeproficient in using an industry standard process simulationpackage (UNISIM) for dynamic analysis and control systemdesign (CJW - Mondays - 2-3 pm, Thursday - 3-4 pm).2/19/2012 ENCH491 Introduction

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Introduction to UNISIMe What is UNISIM?

- It is a ProcessSimulation software package Based on HYSYSDeveloped by Hyprotech Ltd. (nowAspentech) now marketed by Honeywell

- It offers "Integrated Modeling" This helps reduce the gap between Steady State andDynamic simulations

Users experienced with Steady State simulations (ENCH394) are well prepared for Dynamic modeling

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Intro to UNISIM - Steady StatefI In UNISIM, most Dynamic Simulations startwith a Steady State Simulation- Therefore, users must be comfortable with thesteady state environment before they can createdynamic models

- The results from the steady state case can be usedas a starting point for the dynamic model

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Steady State TermsCI Here are several key definitions for terms used

in UNISIM - Steady State:- Material Stream

Represents an actual process stream, containsinformation on flows, conditions and compositions

- Energy Stream Represents a heating or cooling duty in the simulation;they have no mass, conditions or compositions

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Steady State Terms (cont inued)- Unit Operation

Anyone of several unit operation types in HYSYSsimulations; several of the most common include:

- Heaters and Coolers- Valves- Heat Exchangers- Separators (two and three phase)- Columns (Distil lation, Refluxed Absorber, Reboiled Absorber,

Contactor)

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Building in Steady StateII S tead y S tate sim ula tion s are , gen era lly ,

created with the fo llow ing steps:- De fine the S im ula tion Basis

- The simulation basis contains al l information on components,property package, reactions, etc, that will be used in thesimulation

- C rea te and Define a ll known Feed S treams- Add Un it O perations- U se S tream s to connec t opera tions together

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Degrees of Freedom (OOF)II Number o f independent variables that must

be specified in order to so lve the set ofequ atio ns co nstitu tin g the sim ula tio n

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Introduction to Dynamicse Why Do Dynamic Simulations?

- With Dynamic Simulations, you can: Examine the impact of equipment volumes on processbehaviour

Understand how Disturbances will affect the Process Study various control schemes to find the best setup Identify process step response models for modelpredictive control system development

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Introduction to Dynamics (2 )e You can use Dynamic Simulations to

evaluate control schemes To decide which one provides better overall control To reduce bottlenecks and process inefficiencies To reduce equipment sizes, leading to reducedcapita I costs Totune plant controls for optimal performancewithout upsetting the actual plant's operation

Provide models for advanced control schemes Process / operator trainers

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Building in Dynamicsiii M ost H YSY SD ynam ic cases start out as Steady

S tate cases It is easier to add operations, controllers, and streamsin steady state, then transfer the whole simulation overto dynamics

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Steady State -> Dynamics- The Procedure for moving from a Steady Statecase to a Dynamic one follows: Adding Valves to the simulation

- Valves are used to control the flow in streams - they are oftennot required in Steady State cases but are key operations inDynamic cases

Adding Controllers- PID controllers are used to control the simulation

Defining the Pressure-Flow Relationship in the process

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Pressure-Flow DynamicsII The Dynamic Solver in UNISIM is not the same

as the Steady State solverII There are two types of operations in

Dynamics:- Resistance Equation Operations- Pressure Node Operations

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Resistance Equation OperationsII The P-F relationship of these operations can

be defined as:Flow = k * ( p * ~P)1/2

The "k" term is determined from the Steady State Flowand Pressure Drop Examples of this type of operation include:

- Heaters and Coolers- Heat Exchangers

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Pressure Node OperationsIII The other type of dynamic operation is calleda "pressure node operation"

In this type of operation, allowance is made for theaccumulation of material in the operation

- The Pressure-Flow relationship can be defined as:dPjdT = f (V , T , F)

- In words, the pressure in the operation is a function of theVolume, Temperature, and Net Flow of the operation

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Pressure Node Operations (2 )- Examples of Pressure Node Operations include the

following:o Separators(two and three phase)

Columns(Distillation, Refluxed Absorber, Reboiled Absorber andContactor)

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Valve Operations Valve operations were not included amongthe other "Resistance Equation Operations"- The main difference is that the "k" term is

replaced by another term that incorporates the Cvof the valve and its % opening

Valve Pressure-Flow relationships can befound in the UNISIM Manual

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Pressure-Flow Guidelines- The following guidelines are meant to help you setup the P-Fspecifications with as little hassle aspossible One Por Fspecification must be made on everyboundary stream- Use Pspecs on boundary streams attached to valves and

other resistance equation operations- Use Fspecs on boundary streams attached to all other

operations

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P ressure-F low Guidelines (2)- Before the simulation can run dynamically, a

pressure gradient must be established through theentire process

- UNISIM will use the defined P-Fspecifications andthe various operations' equations to solve thesystem

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P ressure-Flow Guidelines (3) The most important thing to remember:

- The flow through the entire process is driven bythe pressure gradient No Gradient = No Flow! Negative Gradient = Negative Flow!

Payspecial attention to the pressure gradientin your dynamic models

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Integrated Modeling UNISIM is one of the only process

simulators available that brings the steadystate and dynamic worlds together

With Integrated Modeling: A steady state case can easily be converted to adynam ic one

- You can use the same program for both SSand Dynamicsimulations

- Steady State cases do not have to be rebuilt in order tocreate the dynamic simulation

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Changing from 55 to Dynamics- The steps for converting a Steady State case toa Dynamic one are as follows: S ize all relevant process equipm ent

- Separators, Reboilers, Condensers, Columns, etc Add the valves and contro llers to control thesimulation

- Valves must be sized, and controllers must be properly setup (correct action, reasonable tuning parameters, etc)

- Use Transfer Blocks to simulate disturbances, if desired Add the Strip Charts to monitor the process

- Monitor key process variables, no more than 6 per chart

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Changing from SS to D ynamic s (2 ) Set up the P/F specs on all boundary streams

- Make sure that the P/F relation is chosen for all"resistanceequation operations"

Run the Dynamics Assistant- This will check your setup and compare against several default

rules Note that if you case is "non-standard" the Assistant will

make suggestions but these do not necessarily have to beimplemented in order for the case to run)} Initially, however, most of your cases will likely be close to

the standard case so pay attention to the Assistant'ssuggestions

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Chang in g from SS to D yn am ics (3 ) Enter the Dynamic operation mode

- Make sure that all operations initialise successfully Save your case at this time

- It is a good idea to save your case quite often when working indynamics as HYSYSis not able to go backwards in simulationtime; the only way to go back is by using a saved case

Start the Integrator- If everything was set up correctly, the simulation should run

without any diff iculty

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