OUT OF SIGHT INSIGHTS

Solving Process MysteriesUsing a Simulation Lens

Kenneth R. Cox

Department of Chemical and Biomolecular Engineering

Rice University, Houston, Texas USA

6 October 2005

The Lure of the Great Mystery

“There is nothing more stimulating

than a case where everything goes

against you”

�

�

�

�Sherlock Holmes

6 October 2005 Rice University/Houston,Texas/USA Page 2

The Classic Whodunnit

* No Witness

* Puzzle to be solved

* Clues not obvious

* Several suspects

* Eureka moment

* Order restored

6 October 2005 Rice University/Houston,Texas/USA Page 3

Typical Oilfield Brine System

�

�

�

�Morgenthaler, Khatib, French, and Cox (1991)

6 October 2005 Rice University/Houston,Texas/USA Page 4

Assembling the Clues

T

P

P

T

from Evidence

CarbonDioxide

Oil

ScaleCarbonate Water

Scene ofthe Crime

Reconstructed

CarbonDioxide

Water

6 October 2005 Rice University/Houston,Texas/USA Page 5

Le Chatlier’s Principle

Ca2+(aq)+CO2−3 (aq) ⇋ CaCO3(s)

CO2(g) ⇋ CO2(aq)

CO2(aq)+H2O(aq) ⇋ H2CO3(aq)

H2CO3(aq) ⇋ H+(aq)+HCO−

3 (aq)

HCO−

3 (aq) ⇋ H+(aq)+CO2−3 (aq)

6 October 2005 Rice University/Houston,Texas/USA Page 6

Pressure Dependence of Calcite Solubility

�

�

�

�Cox and Moore(1985)

6 October 2005 Rice University/Houston,Texas/USA Page 7

Hydrogen Ions are Components, too

CO2(g) ⇋ CO2(aq)

CO2(aq)+H2O ⇋ H+(aq)+HCO−

3 (aq)

HCO−

3 (aq) ⇋ H+(aq)+CO2−3 (aq)

H2O ⇋ H+(aq)+OH−(aq)

PCO2 = HCO2[CO2]

K1 =[H+][HCO−

3 ]

[CO2]

K2 =[H+][CO2−

3 ]

[HCO−

3 ]

Kw = [H+][OH−]

6 October 2005 Rice University/Houston,Texas/USA Page 8

Hydrogen Ions are Components, too

[CO2−3 ]

[CO2]=

K1K2

[H+]2

6 October 2005 Rice University/Houston,Texas/USA Page 9

The Moment of Insight

“Exactly! It is absurd – improbably

– it cannot be. So I myself have

said. And yet, my friend, there it is!

One cannot escape from the facts.”

�

�

�

�Hercule Poirot

6 October 2005 Rice University/Houston,Texas/USA Page 10

Evaporation of Water with Temperature

6 October 2005 Rice University/Houston,Texas/USA Page 11

Solubility of Water in Carbon Dioxide

�

�

�

�Cox and Moore(1985)

6 October 2005 Rice University/Houston,Texas/USA Page 12

An Education Challenge

“The world is full of obvious things

which nobody by any chance ever

observes”

�

�

�

�Sherlock Holmes

6 October 2005 Rice University/Houston,Texas/USA Page 13

Relative Volatility of Gases

Plot the equilibrium partial pressure

of carbon dioxide over a 0.125 N

aqueous ammonia solution at 60◦C.

Use the molar ratio of CO2 to NH3

as the x-axis. Vary this molar ratio

from 0.1 to 0.9. Use the best

available method in HYSYS.

6 October 2005 Rice University/Houston,Texas/USA Page 14

Relative Volatility of Gases

Use the best available physical property method to

confirm the experimental partial pressure data in the

table below

6 October 2005 Rice University/Houston,Texas/USA Page 15

A Surprising Result?

6 October 2005 Rice University/Houston,Texas/USA Page 16

What is going on?

NH3(g) ⇋ NH3(aq)

NH3(aq)+H2O ⇋ NH+4 (aq)+OH−(aq)

H2O ⇋ H+(aq)+OH−(aq)

PNH3 = HNH3[NH3]

KNH3 =[NH+

4 ][OH−]

[NH3]

Kw = [H+][OH−]

[NH3]total = [NH3]+ [NH+4 ]

6 October 2005 Rice University/Houston,Texas/USA Page 17

What is going on?

PNH3

[NH3]total=

HNH3

1+ KNH3Kw

10−pH

At high pH:PNH3

[NH3]total⇒ HNH3

At low pH:

log

(

PNH3

[NH3]total

)

⇒ log

(

HNH3Kw

KNH3

)

+ logpH

6 October 2005 Rice University/Houston,Texas/USA Page 18

Bookkeeping of Component Paths

“I suggest you count your bees, you

may find that one of them is

missing”

�

�

�

�Inspector Clouseau

6 October 2005 Rice University/Houston,Texas/USA Page 19

Which Way Does Water Go?

H2, CH4

C2’s, C3’s

H2, CH4

C2’s

C3’s

C2’s

C3’s

dem

etha

nize

r

deet

hani

zer

168 K

302 K

258 K

360 K

230 K 311 K

6 October 2005 Rice University/Houston,Texas/USA Page 20

Relative Volatilities based on Pure Compounds

Liquid

Vapor

T = 25 C T = 25 C

Vapor

Liquid

T

P

T

P

Water 1−Propene

Water 1−Propene

P = 23.776 torrN/V = 1.2808 mol/m^3

P = 8682.7 torrN/V = 580.41 mol/m^3

6 October 2005 Rice University/Houston,Texas/USA Page 21

Water More Volatile than Propylene!

Vapor

T

P

in in

X = 0.0001 X = 0.0001

Vapor

T

P

PropyleneWater

C7= Liquid C7= Liquid

T = 25 C T = 25 C

P C3 = 0.798 torrN/V = 0.0769 mol/m^3

P H2O = 1.427 torrN/V = 0.0431 mol/m^3

6 October 2005 Rice University/Houston,Texas/USA Page 22

Water Less Volatile than Ethylene

Vapor

T

P

in in

X = 0.0001 X = 0.0001

Vapor

T

P

Water

C7= Liquid C7= Liquid

T = 25 C T = 25 C

N/V = 0.0769 mol/m^3 P H2O = 1.427 torr

Ethylene

P C2 = 3.841 torrN/V = 0.2077 mol/m^3

6 October 2005 Rice University/Houston,Texas/USA Page 23

Partial Pressures over 1-Heptene Solvent

6 October 2005 Rice University/Houston,Texas/USA Page 24

Component with Intermediate Volatility

H2, CH4

C2’s, C3’s

H2, CH4

C2’s

C3’s

C2’s

C3’s

dem

etha

nize

r

deet

hani

zer

168 K

302 K

258 K

360 K

230 K 311 K

6 October 2005 Rice University/Houston,Texas/USA Page 25

Formation of Corrosive Phases

HA

H + A+ −HA

H + A+ −

HA

HA

HA

Non−volatile Acid

(eg, Carboxylic Acids) (eg, H2S, CO2)

Volatile Acid

T

P

T

P

6 October 2005 Rice University/Houston,Texas/USA Page 26

Kister’s Leading Causes of Tower Malfunctions

Fractionator malfunctions due to plugging – mostly

scale products and corrosion products

Column Type Rank

Atmospheric Crude Towers 1

Vacuum Towers 5

FCC Main Fractionator 1

Alky Unit Main Fractionator 1

6 October 2005 Rice University/Houston,Texas/USA Page 27

The Role of Physical Property Experts

TOUGH DECISIONS VS.DEPTH OF UNDERSTANDING

1980 - PRESENTA

vg E

xper

tise

of U

sers

Number of Choices

EOS MODELS GE MODELS

✲

✲

✲

✲

✲

✲

✲

✲

✲

✲

✲

✲

✲

✲

✲

✲

✲

✲

✲

✲

✲

✲

✲

✲

✲

✲Peng Robinson

SRK

Kabadi Danner

Lee Kesler

Zukevitch-Joffee

BWR-Lee Starling

PR-MHV2

PR-Wong Sandler

Nothnagel

Ideal Gas

Hayden-O’Connell

NBS/NRC Steam

Boston-Matthias

NRTL

Margules

UNIQUAC

van Laar

Wilson

Bromley-Pitzer

Electrolyte NRTL

Redlich-Kister

UNIFAC

Ideal Liquid

SH-RST

Chien-Null

Pitzer

More than 50 Choices!More than 50 Choices!

6 October 2005 Rice University/Houston,Texas/USA Page 28

Lessons for the Day

• Familiar heuristics fail with electrolyte systems

• Process simulators usually get it right

• Property experts are useful

• Major opportunity to educate young engineers

• Need to analyze and catalog published cases

6 October 2005 Rice University/Houston,Texas/USA Page 29

The Final Word on Mysteries

“The most beautiful experience we

can have is the mysterious. It is the

fundamental emotion which stands

at the cradle of true art and true

science”

�

�

�

�Albert Einstein

6 October 2005 Rice University/Houston,Texas/USA Page 30