Modelling and Initialisation of a Distillation Modelling and Initialisation of a Distillation

Column for the Separation of Binary SystemsColumn for the Separation of Binary Systems

Simone Christa, Orestis Almpanis-Lekkas, Walter Wukovits

Institute of Chemical Engineering, Vienna University of Technology, Vienna, Austria Contact: s.christa@gmx.net

Introduction

Institute of Chemical Engineering, Vienna University of Technology, Vienna, Austria Contact: s.christa@gmx.net

Model AssumptionsIntroductionDistillation is an important separation process used in

different industrial fields. Increasing requirements on

Model Assumptions• steady state: no material or energy accumulation on the trays

• no reaction termsD&different industrial fields. Increasing requirements on

yields, product purity and economical feasibility

require efficient models to analyse and predict

system performances without time consuming and

no reaction terms

• consideration of ideal trays:

� perfect mixing of the vapour and liquid phase

� thermal equilibrium for every component on each trayF&

R&

system performances without time consuming and

complex investigations on real processes. The target

of this work is to develop a mathematical model of a

distillation column for binary systems. Due to the

� thermal equilibrium for every component on each tray

• no heat loss

xL ⋅&yG ⋅&

F&

Equation Description

pxpy ⋅⋅=⋅⋅ γϕdistillation column for binary systems. Due to the

flexibility in the programming structure and the

flowsheeting capability the implementation of thisFi,

xF ⋅&

1-ni,1-nxL ⋅&

ni,nyG ⋅&

S&

Equilibrium

Component

mass balance

ni,nni,n1-ni,1-n1ni,1Fi,xLyGxLyGxF n ⋅+⋅=⋅+⋅+⋅

++&&&&&

iiii pxpy,0i

⋅⋅=⋅⋅ γϕ

flowsheeting capability the implementation of this

model took place within the environment of the

gPROMS ModelBuilder.1ni,1 ++

⋅ yGn&

ni,nxL ⋅&

B&

mass balance

Summation

condition

Heat balance

1,11

,

1

,== ∑∑

==

nc

i

ni

nc

i

ni yx

lvlv

hLhGhLhGhF ⋅+⋅=⋅+⋅+⋅ &&&&&

Model DescriptionThe steady state column model for the separation of Process Initialisation Procedure

Heat balance nn11-n11F

l

n

v

n

l

n

v

nn hLhGhLhGhF ⋅+⋅=⋅+⋅+⋅−++

&&&&&

The steady state column model for the separation of

binary mixtures is based on mass and energy balances.

It is designed as an n-equilibrium stage model

including a total condenser, a splitter and a reboiler.

Process Initialisation ProcedureThe mass/energy balances and equilibrium conditions form a complex system of non-linear coupled equations. For the

handling of this numerical challenge, an initialisation procedure is developed in regard to both, model robustness and

including a total condenser, a splitter and a reboiler.

For the calculation of the equilibrium state in each

stage, the thermodynamic properties (e.g. enthalpies,

flexibility in user defined specification. The following sequence of initialisation steps proved to be effective:

Initial stateCondenser Condenser Reboiler

Stage equilibriumstage, the thermodynamic properties (e.g. enthalpies,

activities) of Multiflash are used. For the consideration

of the deviation from ideal equilibrium state, the

murphree efficiency and pressure drop can be

Initial stateCondenserequilibrium

Condenserenthalpy balance

Reboilerequilibrium

Stage equilibrium

murphree efficiency and pressure drop can be

defined.Murphreeefficiency

Pressure drop Reflux ratioReboiler enthalpy

balanceStage enthalpy

balance

Simulation ResultsTo validate the model, simulation results of two different binary systems are cross-referenced with Aspen Plus. The reboiler and condenser data are added as stages in the graphics.To validate the model, simulation results of two different binary systems are cross-referenced with Aspen Plus. The reboiler and condenser data are added as stages in the graphics.

a) C H OH / H O mixturea) C2H5OH / H2O mixture

Specifications

Feedflow [kg/h] 89000

Beer-Column profile (temperature and ethanol

fractions)

Rectification-Column profile (temperature and

ethanol fractions)

Feed

Feedflow [kg/h] 89000

Concentration ethanol in

feed [wt %]0.08

Temperature feed [K] 3080,8

0,9

1,0

373

378

fractions)

0,8

0,9

1,0

415

420

ethanol fractions)

Pressure feed [mbar] 1000

Feed

preheater

Temperature [K] 368

Pressure [mbar] 2000 0,5

0,6

0,7

0,8

368

373

eth

an

ol m

ole

fra

ctio

n [

-]

tem

pe

ratu

re [

K]

0,5

0,6

0,7

0,8

405

410

eth

an

ol m

ole

fra

ctio

n [

-]

tem

pe

ratu

re [

K]

Mixer Pressure [mbar] 2000

Recycle

heater

Temperature [K] 308

Pressure [mbar] 1000 0,2

0,3

0,4

358

363

eth

an

ol m

ole

fra

ctio

n [

tem

pe

ratu

re [

K]

0,2

0,3

0,4

390

395

400

eth

an

ol m

ole

fra

ctio

n [

tem

pe

ratu

re [

K]

0,0

0,1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

353

number of stages

0,0

0,1

1 2 3 4 5 6 7 8 9 10 11 12

385

390

number of stages

Column specificationsNumber of

trays [-]

Feed tray

location [-]

Reflux ratio

(mole based)

[-]

Distillate

rate [kg/h]

Condenser

pressure

[mbar]

Pressure

drop [mbar]

Murphree

efficiency

[-]

Activity

coefficient

model

ASPEN temperature gPROMS temperature gPROMS vapour fr.

ASPEN vapour fr. ASPEN liquid fr. gPROMS liquid fr.

ASPEN temperature gPROMS temperature gPROMS vapour fr.

ASPEN vapour fr. ASPEN liquid fr. gPROMS liquid fr.

Beer-Column 13 3 1 19822 1000 200 1 NRTL

Rectification-Column 10 7 9 9360 3500 500 1 NRTL

b) CH3OH / H2O mixture

Conclusion & Outlook� Good accordance between the simulation results of gPROMS and

Specifications

Feed

Feedflow [kg/h] 10000

Concentration methanol in feed [wt %] 0.05

� Good accordance between the simulation results of gPROMS and

ASPENplus

� The initialisation procedure is robust for different user defined

specifications

FeedTemperature feed [K] 293

Pressure feed [mbar] 1013

Feed preheaterTemperature [K] 313

Pressure [mbar] 1013 specifications

� Further validation of different binary systems

1,0 375

Column profile (temperature and methanol fractions)

Feed preheaterPressure [mbar] 1013

References0,7

0,8

0,9

1,0

365

370

375

-]

Column specifications

Number of trays [-] 10A. Friedl, lecture notes for “Thermische Verfahrenstechnik 1“: course 159.731, version

3, Vienna University of Technology, 2000.

R.H. Perry, D.W. Green & J.O. Maloney, Perry‘s Chemical Engineers‘ Handbook: 7th ed.,0,3

0,4

0,5

0,6

350

355

360

me

tha

no

lmo

lefr

act

ion

[-

tem

pe

ratu

re [

K]

Number of trays [-] 10

Feed tray location [-] 6

Reflux ratio (mole based) [-] 4.36

R.H. Perry, D.W. Green & J.O. Maloney, Perry‘s Chemical Engineers‘ Handbook: 7th ed.,

New York: VCH, 1997.

K. Sattler, Thermische Trennverfahren: Grundlagen, Auslegung, Apparate, Cambridge:

VCH Verlagsgesellschaft mbH, 1988.

A. Schönbucher, Thermische Verfahrenstechnik: Grundlagen und

0,0

0,1

0,2

0,3

335

340

345

me

tha

no

l

tem

pe

ratu

re [

K]

Reflux ratio (mole based) [-] 4.36

Distillate rate [kg/h] 473

Pressure drop [mbar] 0

VCH Verlagsgesellschaft mbH, 1988.

A. Schönbucher, Thermische Verfahrenstechnik: Grundlagen und

Berechnungsmethoden für Ausrüstungen und Prozesse. Berlin: Springer, 2002.

1 2 3 4 5 6 7 8 9 10 11 12

number of stages

ASPEN temperature gPROMS temperature gPROMS vapour fr.

ASPEN vapour fr. ASPEN liquid fr. gPROMS liquid fr.

Pressure drop [mbar] 0

Activity coefficient model NRTL

Advanced Process Modelling Forum 2014 – London, United Kingdom, 2 – 3 April 2014Advanced Process Modelling Forum 2014 – London, United Kingdom, 2 – 3 April 2014