Rate-Based Concepts

Best Practices for When and How Mass Transfer is

Applied in a Column Simulation

Peter Poellmann, AQSim Europe OLI Simulation Conference 2014

22 October 2014

http://www.sulzer.com/en/

Products-and-Services/

Separation-Technology/

Distillation-and-Absorption/Distillation

Vapor to condenser

Reflux from

condenser

Liquid feed

Liquid to reboiler

Vapor from

reboiler

Distillation

Column

Manway

Manway

Liquid distributor

Structured packing

Random packing

Trays

Liquid collector with

packing support

Support device

Liquid distributor

Q1

Q2

Qj

QN-1

QN

Stage 1

2

Stage j

N-1

N

F1

F2

Fj

FN-1

FN

W2

W3

Wj+1

WN-1

WN

Wj

U1

U2

Uj

Uj-1

UN-2

UN-1

L1

Lj-1

LN-2

LN-1

V2

V3

Vj+1

VN

Modeling

Distillation

F feed

V vapor

L liquid

Q heat

U liquid side product

W vapor side product

Countercurrent

cascade of stages –

generic model

LN

V1

Q1

QN

2

Stage j

N-1

F

U1

L1

Lj-1

LN-2

LN-1

V2

V3

Vj+1

VN

Modeling

Distillation

F feed

V vapor

L liquid

Q heat

Countercurrent

cascade of stages –

ordinary distillation

column

LN

V1

V1 vapor distillate product

U1 liquid distillate product

L1 reflux

LN bottom product

Q1 condenser duty (-)

QN reboiler duty (+)

1

2

Stage j

N-1

N

F1

FN

L1

Lj-1

LN-2

LN-1

V2

V3

Vj+1

VN

Modeling

Distillation

F feed

V vapor

L liquid

Countercurrent

cascade of stages –

absorption column

LN

V1

FN rich/dirty feed gas

F1 fresh liquid absorption medium

V1 lean/clean off gas

LN product/spent absorption medium

Qn

Stage n

yi,n

HV,n

Tn

pn

yi,n+1

HV,n+1

Tn+1

pn+1

xi,n

HL,n

Tn

pn

xi,n-1

HL,n-1

Tn-1

Pn-1

Wn

Un

Ln

Vn

Fn

zi,n

HF,n

TF,n

pF,n

Equilibrium

Stage

Tn, pn, xi,n and yi,n are

related by Vapor-

Liquid Equilibrium VLE

F feed

V vapor

L liquid

Q heat

U liquid side product

W vapor side product

H enthalpy

T temperature

P pressure

x,y,z liquid, vapor or feed

compositions

i index for component

n index for stage number

Ln-1

Vn+1

=> MESH equations

Problems Usually Encoutered in Dealing

with Distillation Models

• Column solving algorithm usually fails to converge on

infeasible specifications of separation or numerics

• Solver may diverge even on feasible specifications of

the separation job, caused e.g. by numerical trouble,

missing estimates, many stages, large flow feeded

• Feasibility of separation non-trivial to ensure, e.g. in

cases of pinch conditions or distillation boundaries

• Divergence behaviour rarely helpful for correction

• Among different specifications, some may lead to

trouble, while others may run fine

• Still today, automated methods for doing distillation

design, e.g. finding numbers of stages, locations of

feeds or side-stream draw-offs, are not at hand

QV,n

Segment n yi,n

HV,n

TV,n

pV,n

yi,n+1

HV,n+1

TV,n+1

pV,n+1

xi,n

HL,n

TL,n

pL,n

xi,n-1

HL,n-1

TL,n-1

PL,n-1

Ln

Vn

FL,n

xF,i,n

HL,F,n

TL,F,n

pL,F,n

Mass-Transfer

Segment

• Bulk and film regions

are distinguished in

each phase

• Films have contact

along a V-L interface

• Each bulk has its own

feed of material and

heat

• Material and heat fluxes

across interface are

defined

• Exiting streams are not

related by VLE – rather

are conditions at V-L

interface

N material flux

q heat flux

IF vapor-liquid interface

Ln-1

Vn+1

FV,n

yF,i,n

HV,,F,n

TV,F,n

pV,,F,n

QL,,n

qV,n qIF,n qL,n

NV,n NIF,n NL,n

Bulk

vapor

Bulk

liq

uid

Vapor

film

Liq

uid

film

xIF,i,n yIF,i,n

TIF,n

V-L

inte

rface

=> MESHNQ equations

When is Mass-Transfer Applied in a Column

simulation ?

• When a good VLE model of the physicochemical

system is at hand, then the rate-based model is a

useful fine-tuning of the column simulation.

• Primary motivation for applying mass-transfer in a

column simulation is design of the height of the

equipment.

• Several mass-transfer devices, e.g. random packing

elements, structured packing, or trays, can be rated

against each other, provided m-t coefficients exist.

• Diameter and height lead to volume of packing, thus

cost comparisons can be done.

• Generally, mass-transfer is applied whenever product

purities or emission limits need to be respected.

• Hydrochloric acid, i.e. Water / HCl

– xy Diagram

– Separation Factor

– Acid concentration column (vacuum distillation)

– Desorption / Specific energy consumption

• Water / HCl / Chlorine

– Solubility of Chlorine in Water

– … in hydrochloric acid

Examples for Significance

of VLE for Distillation

There is no point trying to apply a rate-based model

for distillation, until the VLE is done properly

Hydrochloric Acid / boiling point 1.013 bar

-100

-50

0

50

100

150

200

0 0,05 0,1 0,15 0,2 0,25 0,3 0,35

x_HCl (kmol/kmol)

t_b

ub

(°C

)

Aspen ElecWiz

Aspen EHCLFF

AspenOLI MSE

AspenOLI aq.

OLI reference

HCl Acid / xy Diagram / Pressure 6 bar

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 0,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4 0,45 0,5

x_HCl (kg/kg)

y_H

Cl (k

g/k

g)

Aspen ElecWiz

Aspen EHCLFF

AspenOLI MSE

diag

HCl Acid / Separation Factor / Pressure 6 bar

0,01

0,1

1

10

100

1000

10000

100000

1000000

0 0,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4 0,45 0,5

x_HCl (kg/kg)

alp

ha

Aspen ElecWiz

Aspen EHCLFF

AspenOLI MSE

HCl Acid / Desorption Column

Acid Flowback ~ 18% HCl

Feed Acid 32% HCl by wt.

HCl Gas Product

Motivated by the

separation factor, the

OLI MSE model

should be applied for

desorption of HCl gas

from hydrochloric acid

at elevated pressure.

HCl Acid / Desorption / Reboiler Duty 6 bar

0,7

0,75

0,8

0,85

0,9

0,95

1

1,05

1,1

1,15

1,2

0,9 0,91 0,92 0,93 0,94 0,95 0,96 0,97 0,98 0,99 1

D / D_max

Qr

/ D

(kW

h/k

g)

Aspen ElecWiz

OLI MSE

Desorption of HCl gas

from acid 32% wt.

using 10 equilibrium

stages at 6 bar(a).

Qr reboiler duty (kW)

D product gas flow (kg/h)

D_max assumes

desoption down to

azeotrope

Specification-to-Column-Design Workflow

1. Assume desired separation job as feasible

2. Set up and experiment with equilibrium-staged model, finally put solver

loops (design-specs) in, ensuring product purities met all the time (by

separate investigation, make sure such loops work robustly)

3. Select numbers of stages and feed locations (integer variables), then

specify continuous variables (like, e.g. flow rates, temperatures of feed

streams, pressures)

4. Run equilibrium-stage based, analyse, (if necessary, go back to 3.) – until

staged column configuration and L, G (i.e. operating point) are fixed

5. Select internals, do hydraulic design, i.e. find diameters of every section

6. Set up for rate-based, e.g. numbers of mass-transfer segments, internals,

bed heights, numbers of trays, not to forget numerics of model

7. With solvers (2.) still in, run mass-transfer based model, vary bed heights

or tray numbers for minimal energy consumption

8. Establish a cost function for investment of column, decide about energy

consumption cost, establish a total cost objective function

9. Vary type of internals, calculate volumes of packed sections, evaluate the

total cost objective function (if too large, go back to 7)

10. Design column periphericals, e.g. reboiler, condenser, pumps, valves

Aspects of Applying Mass-Transfer in Column

Simulations

• Have good VLE model at hand, since m-t model is like

fine tuning on eq.-stage based solution

• See that eq.-stage based model converges robustly,

before attemptimg to go for m-t

• Get and accept information on empirical correlations

involved for :

– Mass-transfer coefficients

– Heat-transfer coefficients

– Diffusion coefficients

– Interfacial area

– Pressure drop

Applying Mass-Transfer in Column

Simulations – Tricks of the Trade • While a variation of number of stages is hardly ever found in simulators

(some can vary down), the m-t model allows for some kind of design by

variation of height for fixed number of segments (but see also next point)

• Do not over-do with number of segments – rule of thumb is height of

segment should not exceed 1/10 of nominal size of packing element –

however insert more segments in sections of significant change, look at

profiles and go for smoothness

• Complex columns with pumparounds may converge better overall, after

being cut into pieces made of purely countercurrent or pumped-around

sections, even though a flowsheet tear stream is created

• If a specific operating point will not converge at all, then try to specify

another, easier-to-conerge, maybe even trivial, operating point, and

establish a homotopy from the latter to the former (simulators keep

results obtained in a run as starting values for the next run)

• „No estimates are better than bad estimates“ – they should be inside the

range of variation the model will probably take – it can be severly

disturbed, if estimates are infeasible, e.g. forgotten from earlier runs

Typical Applications with Mass-Transfer from

the Experience of the Author

• Adiabatic absorption of HCl from gases in

countercurrent or recirculated packed beds, for

keeping emission limits, e.g. TA Luft

• Non-adiabatic absorption of greater amounts of HCl

from synthesis or other flue gases, for production of

concentrated acid, and keeping emission limits at the

same time

• Simultaneous removal of HCl and Chlorine from flue

gases of chlorinated chemical waste incinerations

• Judgement of random or structured packed bed

efficiencies during desorption of HCl gas from acid

• Checking preconditions for aerosol formation in HCl

absorbers after incineration units

Discussion

• Questions

• Other experiences

Backup

HCl Acid / Azeotrope / Temperature HCl-water azeotropic T-p

0

50

100

150

200

250

0 2 4 6 8 10 12 14 16 18 20

pressure (bar)

tem

pera

ture

(°C

)

Aspen ElecWiz

Aspen EHCLFF

OLI MSE

Ullmann

HCl Acid / Azeotrope / Composition HCl-water azeotropic x-p

0,075

0,1

0,125

0,15

0,175

0,2

0,225

0,25

0 2 4 6 8 10 12 14 16 18 20

pressure (bar)

HC

l m

ass f

racti

on

(kg

/kg

)

Aspen ElecWiz

Aspen EHCLFF

OLI MSE

Ullmann

Römpp

Gmelin's

Separation Factor / Definition

αij = Ki / Kj = yi/xi / yj/xj

The separation factor is a key figure for the

change of composition of a chemical mixture

by a technical process, by separation in

particular. The ratio is usually chosen for a

value of greater than unity. A large value

denotes good separability of components i

and j. In terms of distillation, the separation

factor is defined via the compositions in liquid

and vapor phases. A value of unity indicates

the separation technique is infeasible. This is

the case at the azeotropic point, where

distillation fails.

HCl Acid / xy Diagram / Vacuum 0.1 bar

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 0,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4 0,45 0,5

x_HCl (kg/kg)

y_H

Cl (k

g/k

g)

Aspen ElecWiz

Aspen EHCLFF

AspenOLI MSE

diag

HCl Acid / Separation Factor / Vacuum 0.1 bar

0,0001

0,001

0,01

0,1

1

10

100

1000

10000

100000

1000000

0 0,05 0,1 0,15 0,2 0,25 0,3 0,35 0,4 0,45 0,5

x_HCl (kg/kg)

alp

ha

Aspen ElecWiz

Aspen EHCLFF

AspenOLI MSE

HCl Acid / Concentration Column

Waste Water ~ 1% HCl (vacuum)

Feed Acid 18% HCl by wt.

Product Acid 22% HCl

Motivated by the

separation factor, the

OLI MSE model

should be applied for

concentrating

hydrochloric acid

under vacuum.

Water / Chlorine Solubility of Chlorine in Water

0

2000

4000

6000

8000

10000

12000

14000

16000

0 10 20 30 40 50 60 70 80 90

t / °C

pp

m C

l2

Aspen ElecWizard

Oli aqueous

Oli MSE

Schönfeld (1855)

Perry (50th edn.)

Winkler (1907)

Water / HCl / Chlorine / 25°C Solubility of Chlorine in Hydrochloric Acid (25 °C)

*) Oliveri-Mandala data hold for 20 °C

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

0 5 10 15 20 25 30 35

HCl (mass-%)

Cl2

(p

pm

)

Aspen ElecWizard

Oli aqueous

Oli MSE

Oliveri-Mandala (1920)

Sherrill-Izard (1928)

Curda-Holas (1964)

Water / HCl / Chlorine Solubility of Chlorine in Hydrochloric Acid

solid lines calculated using AspenPlus standard electrolyte model (2006.5)

0

5000

10000

15000

20000

25000

30000

0 10 20 30 40 50 60 70 80 90

t (°C)

pp

m C

l2

0 % HCl

10 % HCl

20 % HCl

32 % HCl

Perry (50th edn.) - 0%

Curda-Holas (1964) - 10%

... - 20%

... - 32%

Water / HCl / Chlorine Solubility of Chlorine in Hydrochloric Acid

solid lines calculated by Oli MSE model

0

5000

10000

15000

20000

25000

30000

0 10 20 30 40 50 60 70 80 90

t (°C)

pp

m C

l2

0 % HCl

10 % HCl

20 % HCl

32 % HCl

Perry (50th edn.) - 0%

Curda-Holas (1964) - 10%

... - 20%

... - 32%