Aspen Simulation Talk

8/3/2019 Aspen Simulation Talk

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Process Simulation usingASPEN PLUS

Karthikeyan MarimuthuGraduate Research Assistant

January 20, 2011


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O utline

Introduction

Motivation

Principles of flow sheet simulation

Thermodynamic Models

Examples

Conclusion


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Introduction

Complex Ammonia Plant A Simple flow sheet of Ammonia Plant

Process Flow Sheets are thel anguage of ChemicalProcesses and Plants 1

1. Seider et al (2004)

Modeling converts the flow sheet in to equations.Software Simulations solve the equations.


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Process Modeling

ModelingModeling is an art of deriving inter relationshipsbetween process variables.

Model

A process model is a set mathematical equations thatexpress the behavior of the system under the variousassumptions.

Examples1) Expected travelling time from Guindy to A.C.Tech when theChief Minister is expected to arrive Governor bungalow.

2) The rate of diffusion of a drug molecule inside blood.


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Notion of a Model

This slide is taken from the lecture notes of Dr. Niket Kaisare, CH 512, IIT Madras


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Model

Deterministic

Steady State

Linear

Non Linear

Dynamic

LumpedParameter

Linear

Non linear

DistributedParameter

Linear

Non linearStochastic

EmpiricalCorrelations

Difference

Equations

Classification of Models

This model classification is biased towards deterministic modeling in

Chemical Engineering. O therwise, one can classify models in manyways.


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Model Development Procedure1) Define the objectives of the model?

The purpose of the model?Type of the model?

2) Identify and classify different variables

Model inputs are not same as the process inputs

3) Flow diagram and the control volume (helps forstep 1 and 2)


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Model Development Procedure4) Conservation of Momentum/Mass/Energy/Species

ncedisappeara

/generationof rate

output

of rate -

input

of rate

onaccumulati

of rate

s

!

5 ) Additional Constitutive RelationshipsMass Transfer/Thermodynamics : Equation of state, Equilibriumrelationships (VLE), etcReaction Engineering : Reaction KineticsHeat Transfer : Empirical equations for heat/mass transfercoefficients, etcFluid Mechanics : Coefficient of discharge, friction factor, dragcoefficient, etcDesign : Pump specification, Valve behavior etc


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Model Development Procedure6) Check for the consistency

7) Solve the model equationsCheck whether analytical solution is possible ( Mostly notpossible).

Identify an appropriate numerical scheme to solvedifferential equation/algebraic equations.

Model should be mathematically consistent.Units of the variables should be consistent.How many variables are needed to be specified (Degrees

of Freedom) .

8) ValidationCheck whether the results are physically realizable.Check whether the results matches the experimental data.


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Example Flash Drum

V v P y j

V LT L

x j

F v

v y j

F L x j

F 0 P 0T 0

0 x0j

Feed

Gas

liquid

Binary Flash Drum


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Flash Drum1) O bjectives of the model?

For a given input mixture, find out amount and concentrationof outlet.Deterministic Steady state and Dynamic model

2) Identify and classify different variables

3) Flow diagram and the control volume

Input : Feed composition (x 0j ), Pressure inside the vessel(P), Inlet temperature and temperature inside the vessel(Isothermal expansion ).

O utput : Final liquid and gas compositions.


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Steady State Model4) Steady State Model

ncedisappeara

/generationof rate

output

of rate -

input

of rate

onaccumulati

of rate

s

!

0 0

L Lvv F F F V V V !00O ver all B alance

Component B alance

jav L

L L jav

v

vv jav x M

F y

M F

x M

F V V V !0

0

00


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Steady State ModelEnergy B alance

L Lvv F h F H F h V V V !000

Vapor Liquid Equilibrium

P T y f H

T x f hT x f h

j

j j

,,

,,,,

3

2010

!

!!

P T x f y j j ,,!This is very important and we will discuss this in

detail.


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Degrees of FreedomTotal Number of Variables

Number of variables in the system 3NC + 16

Input :0

, F0

, M0

av , x01,

x02,

..xNC,

T0,

h0

& Po

- NC + 6

Gas : v , Fv , Mvav , H,y1, y2, ..y NC - NC +4T & P - 2

Liquid : L , FL, MLav ,h,x 1, x2, ..x NC - NC +4


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Degrees of FreedomTotal Number Equations

O ver all Mass balance 1Energy Balance 1Component Mass Balance NC-1

Mole fraction constraints (Feed, Gas and Liquid) 3Equilibrium Relationships (for all components) NCAverage Molecular weight 3Density of Feed vapor and liquid 3Enthalpies of Feed, Vapor and Liquid 3

Total Number of Equations 2NC + 13


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Degrees of Freedom

Degrees of Freedom = Total Number of Variable Total number of Equations

= 3NC + 16 (2NC+13) = NC + 3

Need to specify NC + 3 variables to solve this problem

Feed Composition of NC-1 components - NC-1Initial Pressure P 0 1Pressure inside the vessel P 1Initial Temperature T 0 1Initial flow rate - F 0 1


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Dynamic ModelUnsteady State Model

ncedisappeara

/generationof rate

output

of rate -

input

of rate

onaccumulati

of rate

s

!

0

L Lvv

L L

F F F dt V d

V V V V

!

00

O ver all B alance


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Dynamic Model

ComponentB

alance

jav L

L L jav

v

vv jav

av L

L j L

x M F

y M F

x M F

dt

M

V xd

V V V

V

!

00

00

Energy B alance

L Lvv

L L F h F H F hdt

hV d V V V

V !000


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Thermodynamic ModelsEquilibrium Relationship

In addition to Mass and Energy balance we needthermodynamic relationship (VLE)

Several Thermodynamic models are available to generate

equilibrium relationship between vapor and liquid phase.

Wrong selection of thermodynamic will result in a poorestimation of output variables.

The problem is further complicated when there are severaloperations happening.


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Mixture

Reproduced from Hill and Justice (2011), CEP.


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K Model

ii K x y !

No universal model to predict the values of K.

Different mixture have different dominant behavior.

Different models have been developed to predict the values of Kof a mixture based on it s dominant behavior.

VLE for a mixture of water and Ethanol is dominated by theliquid phase behavior, while the mixture of hexane and propanedominated by vapor.


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Determination of K

P P

K x y

i

S at ii

i

i

JK

!!

Generalized VLE relation ship Activity Coefficient

Fugacity Coefficient

For Ideal mixture K i = Pisat / P(T)

Not all the mixtures of Ideal

Need to determine Activity coefficient and fugacity coefficients

for a given mixture.

Based on the behavior the mixture, one should use theappropriate thermodynamic model


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Equation of State (PVT relationship) Approach

Ideal gas - Light gases (at low pressure and high temperature)

Real gas Compressibility factor Combustion gases, alkane

mixtures

Cubic Equation of state - Van der WaalsRedlich-Kwong

Soave-Redlich-Kwong (Petroleum)Peng Robinson (Hydro Carbons)


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Activity Coefficient Models

Based on the thermodynamic constraint on the liquid

Based on the Excess Gibbs free energy of a mixture.

Includes the binary interaction parameter.

Examples (Commonly used)

NTRL (BIP)

UNIFAC & UNIQUAC

NRTL


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Comparison of Models

Reproduced from Hill and Justice (2011), CEP.


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Summary of Thermodynamic ModelsModel Category Most Commonly used

models

System Type Examples

Equation of State SRK,PR

Real gas + ideal liquid Petroleum pseudocomponents,Similar hydrocarbonsLight gases

Activitycoefficient(BIP) NRTLWilson Ideal gas + Polar liquid Water +O

rganicsDissimilarHydrocarbons(Benzene and cyclohexnane

Predictive Activity UNIFACUNIQUAC

Ideal gas + Polar liquid Mineral acids andwater

Dissimilar organics(Ester and alcohols)

Electrolyte NRTL Aqueous Electrolyte Water + Acid, base orsolids

Hill and Justice (2011), CEP.


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Validation

Hill and Justice (2011), CEP.


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Example - 1

A liquid mixture of 25 mol % n pentane, 45 mol %n-hexane and 30 mol % n-heptane, initially at 69 deg

C and a high pressure, is partially vaporized byisothermally lowering the pressure to 1 bar. Find therelative amounts of vapor and liquid in equilibriumand their components.

Problem Statement


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Solution

Assumptions

- Ideal mixture , y i /x i = Pivap /P

- Vapor pressure can be determined using Antonie equation(ln(Psat )= A B/(T + C))

- Basis 1 mol/time units

b ar 389.0& b ar 024.1 b ar ,721.2 765 !!!vapvapvap P P P

Equilibrium Relationships

3844.0where

0109.1where

740 6.2where

7777

6666

5555

!!

!!!!

K x K y

K x K y

K x K y


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Solution

Constraint on the Mole fractions

1

1

765

765

!!

y y y

x x x

Component B alances

30.0

4 5.0

2 5.0

77

66

55

!

!!

V y L x

V y L x

V y L x

O ver all B alance

1!V L

8 equations and 8 unknowns Relatively easy to solve


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W hat if

The Mixture is non ideal

- Need to use an appropriate thermodynamic model

- May need to solve non linear algebraic equations ( EspeciallyEO S method)

- Developing an appropriate Numerical Method is veryimportant

- Convergence is critical.

- Need to write your own software code to solve the problem.


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W hat if

If your unit (Flash vessel) is connected to other units

-You are required to do the mass balance for over all system

- Not an easy task

- Need to write a big & generalized code

- You are required to replace one unit - Generalized code may

not work.

ASPEN Solves for you


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ASPEN Process Simulator

In 1970s the researchers at MIT s Energy laboratory developed aprototype for process simulation.

They called it Advanced System for Process Engineering (ASPEN)

This software has been commercialized in 1980 by the foundationof a company named ASPEN Tech .

B uild in Thermodynamic models, model library for distillation

columns, separators, heat exchangers, reactors, etc.

In build property data bank .


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Process SimulatorsSteady State Simulators

ASPEN Plus, (We will study this alone in this work shop)ASPEN Hysys,HYSYSUNISIM(Honeywell),

Pro II (Simulation Sciences)

Dynamic SimulationASPEN dynamicsHYSIS

B atch ProcessesBATCH PlusSUPERPRO Designer.


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How to StartAny Process Simulators does not model the process for you.

You should know how to solve the problem .

You Should know what thermodynamic model should be used.

Simulator will generate equations based on how to you define theproblem and then solve them.

Using the instructions manual start with a simple problem (Forexample Flash vessel).

O nce you start solving the problem, you will learn all the availablesoftware features .


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Example Main W indow

You can selectalready developedflow sheet

To start with freshflow sheet


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User Interface

Status of theflow sheet

EquipmentChoices

Streams thatconnect

differentequipments


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Setting up a Simulation

Separators is checked

Flash Drum

O utput FlowStream - Liquid

Input FlowStream

O utput FlowStream - Vapor

Click on this and drag tothe main window

Note the stream numbers


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Enter the details

Click on this and drag tothe main window

Click on any stream,you will get the below data browser window Click on Components

Red indicated incomplete


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Enter the details

Can see the component Window &Click on the space indicated


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Enter the details

Components Data base.Type the componentname & click find now

Search Results. Select yourcomponent and add.

You can see the new spacefor the next component.Repeat the sameprocedure.


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Enter the details

All components areentered

Can see the colorchange from Red toBlue also there is tick O K.

Click next button, itwill take you to nextstep


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Enter the details

Changing the unitsystems to S I andclick next


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Enter the details

Process Type TheCategory yourcomponent (Select)

Base Method Type of Thermodynamic Modelthat you think is the best


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Enter the details

Ideal mixture isselected. Youchoose whatyou think isright!


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Enter the details

Input conditions areentered


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Enter the details

Input is complete Simulation is ready torun. Click next to run.

Flash conditions


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Enter the details

Simulation isCompleted


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Enter the details

Click on Resultsand Summary, thenStreams ( You can

also generate theoutput summary).


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Enter the details

Final Results!!! You aredone!!!

Change the thermodynamicmodel and check whether theresults are changing!!


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ConclusionA complex flow sheet can also be made and simulated as we have

learned.

Good modeling skill and basic knowledge in Simulations would berecommended to use ASPEN Plus.

Before using ASPEN, it is recommended to simulate a simple flowsheet using your own code (Matlab).


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THANK YO

U

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