Influence of Liquid Properties Influence of Liquid Properties on Effective Mass Transfer on Effective Mass Transfer Area of Structured PackingArea of Structured Packing

Robert E. TsaiRobert E. Tsai

January 11, 2008January 11, 2008

Research Review MeetingResearch Review Meeting

Department of Chemical EngineeringDepartment of Chemical Engineering

The University of Texas at AustinThe University of Texas at Austin

OverviewOverview

Introduction: Motivation & ObjectivesIntroduction: Motivation & Objectives

Materials and MethodsMaterials and Methods– Pilot-scale packed columnPilot-scale packed column– Wetted-wall column (WWC)Wetted-wall column (WWC)

Experimental ResultsExperimental Results– Reduced surface tensionReduced surface tension– Enhanced viscosityEnhanced viscosity

ConclusionsConclusions

Importance of Mass Transfer AreaImportance of Mass Transfer Area

Packing: promotes gas-liquid mass transferPacking: promotes gas-liquid mass transfer– Random: less $$Random: less $$– Structured: lower Structured: lower ΔΔP, better mass transfer, “cleaner” P, better mass transfer, “cleaner”

mechanicsmechanics

Need for reliable mass transfer models (kNeed for reliable mass transfer models (kLL/k/kGG, a, aee))

Measured performance: kMeasured performance: kLLaaee or k or kGGaaee

For industrial COFor industrial CO22 capture (amine absorption), capture (amine absorption),

aaee particularly important particularly important– Absorption rate independent of MTCs but remains Absorption rate independent of MTCs but remains

directly related to adirectly related to aee

Research MotivationResearch Motivation

No aNo aee models predictive over range of conditions models predictive over range of conditions– Different effects of viscosity and surface tensionDifferent effects of viscosity and surface tension

SolventSolvent

(40 (40 ˚̊C)C)

Viscosity, Viscosity, μμLL

[cP or mPa·s][cP or mPa·s]

Surface tension, Surface tension, σσ

[dynes/cm or mN/m][dynes/cm or mN/m]

WaterWater 0.630.63 69.669.6

7 m MEA7 m MEA

Ldg = 0.4Ldg = 0.4

1.741.74

2.482.48

57.9457.94

--------

5 M AMP5 M AMP ~24~24 33.7833.78

ae = f(μ,σ) water data may not be reflective of amine conditions!

Project ScopeProject ScopeMeasurement of aMeasurement of aee of Mellapak packings of Mellapak packings (250 and 500-series)(250 and 500-series)– Fluid property variationsFluid property variations

Viscosity (1, 5, 10 cP)Viscosity (1, 5, 10 cP)Surface tension (72, 50, 30 dynes/cm)Surface tension (72, 50, 30 dynes/cm)

– Geometric variationsGeometric variations

Kinetic measurements (WWC)Kinetic measurements (WWC)– Test impact of additives on COTest impact of additives on CO22-NaOH rxn.-NaOH rxn.

Semi-empirical modelSemi-empirical model– Predicts aPredicts aee of sheet-metal packing as function of sheet-metal packing as function

of viscosity, surface tension, liquid loadof viscosity, surface tension, liquid load

Separations Research Program (SRP) DatabaseSeparations Research Program (SRP) Database

COCO22 absorption from air into 0.1 M NaOH absorption from air into 0.1 M NaOH

Measured in 16.8” (430 mm) ID columnMeasured in 16.8” (430 mm) ID column

10+ random packings10+ random packings– CMR #2, IMTP #40CMR #2, IMTP #40

10+ structured packings10+ structured packings– Mellapak 250Y, Flexipac 1YMellapak 250Y, Flexipac 1Y

Hydraulic measurements (Hydraulic measurements (ΔΔPP, holdup), holdup)

Caustic AbsorptionCaustic Absorption

aaee measured by CO measured by CO22-NaOH reactive absorption-NaOH reactive absorption– Inexpensive and non-hazardousInexpensive and non-hazardous– Kinetics have been extensively characterized Kinetics have been extensively characterized

Overall rxn: COOverall rxn: CO22 (aq) + 2 OH (aq) + 2 OH-- → CO → CO332-2- + H + H22OO

Pseudo-first-order (low PPseudo-first-order (low PCO2CO2, excess OH, excess OH--):):

]CO][OH[kr 2OH

]CO[kr 21

(Irreversible)

Packed Column Setup Air Outlet

Storage TankLiquid Pump

Packing ~ 10 ft (3 m)

Blower (Air: 380-400 ppm CO2)

Distributor, Demister

DPC

Optional Recycle

(for mixing)

PVC: ID ~ 16.8” (430 mm)

(Up to 35 gpm/ft2 or 85 m3/m2-h)

300 or 450 ACFM(1 or 1.5 m/s)

Packing Area CharacterizationPacking Area Characterization

Series resistance:Series resistance:'gGG k

1

k

1

K

1

Z

y

ylnU

aKoutCO

inCOGS

eG2

2

'g

eGe

k

aKa

1/kG ≈ 0 for high gas velocity, dilute NaOH'gG kK

2

2

2 CO

liqCOOH

i,CO

'g H

D]OH[k

)0P(

Fluxk

Mass Flow Controllers

SolutionReservoir

Septum

Temp.Bath

Pump

Needle Valve

N2 / CO2

N2

Bypass Valve

GasIN

LiqIN

LiqOUT

GasOUT

Condenser

Saturator / Temp. Bath

WWC

CO2 Analyzer (IR)

WWC Experimental Setup

yCO2: 500 – 1500 ppm

(minimize OH- depletion)

Liq. Rate: 2-4 cm3/s

(constant)

(5 SLPM)

WWC CalculationsWWC Calculations

2

2

CO

liqCOOH'g H

D]OH[kk

Experimental kg′:

Pohorecki and Moniuk (1988): Eqns for kOH-, DCO2 liq, HCO2

GG'g k

1

K

1

k

1

CO2 flux Correlated via SO2-NaOH absorption

Literature kg′:

Reduced Surface Tension Reduced Surface Tension StudiesStudies

WWC Data for Baseline & Low Surface Tension Systems

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

130 180 230 280 330 380

PCO2 LM (Pa)

No

rma

lize

d k

g′

Baseline (σ ~ 72 dynes/cm)

NP-7 / antifoam (σ ~ 35 dynes/cm)

Baseline average

NP-7 / antifoam average

(125 ppmv TergitolTM NP-7 / 50 ppmw/v Dow Corning® Q2-3183A antifoam)

Fractional Area Data for 250-Series Packings

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

0 20 40 60 80 100Liquid load (m3/m2-h)

Fra

cti

on

al

are

a

Mellapak 250Y (1 m/s, 72 dynes/cm)

Mellapak 250Y (1.5 m/s, 72 dynes/cm)

250 Prototype (1.5 m/s, 72 dynes/cm)

Mellapak 250Y (1 m/s, 35 dynes/cm)

ap = 250 m2/m3

Fractional Area Data for 500-Series Packings(uG = 1 m/s)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 10 20 30 40 50

Liquid load (m3/m2-h)

Fra

ctio

nal

are

a

Mellapak 500Y (72 dynes/cm)

500 Prototype (72 dynes/cm)

Mellapak 500Y (35 dynes/cm)

Mellapak 500Y (35 dynes/cm)

σ ~ 35 dynes/cm

σ ~ 72 dynes/cm

ap = 500 m2/m3

Mellapak 250Y/500Y Comparison: Mellapak 250Y/500Y Comparison: σσ ~ 72 dynes/cm ~ 72 dynes/cm

aaf, 250Yf, 250Y ~ unity vs. a ~ unity vs. af, 500Yf, 500Y 0.6 0.6

Similar trend for 250 and 500-series Similar trend for 250 and 500-series prototype packingsprototype packings

Liquid pooling in corrugation troughs, Liquid pooling in corrugation troughs, bridging across adjacent sheetsbridging across adjacent sheets

Reduces area available for mass transferReduces area available for mass transfer

High structural density more prone – partially offsets High structural density more prone – partially offsets advantage of higher aadvantage of higher app

Mellapak 250Y

uL = 36.7 m3/m2-h

[Green (2006)]

Mellapak 250Y/500Y Comparison: Mellapak 250Y/500Y Comparison: σσ ~ 35 dynes/cm ~ 35 dynes/cm

Expect better wetting, but no change in aExpect better wetting, but no change in af, 250Yf, 250Y

– Same surface coverage at high and low Same surface coverage at high and low σσAlso applies to 500Y – same texture, shorter crimp Also applies to 500Y – same texture, shorter crimp

Key effect of reduced Key effect of reduced σσ– Alleviation of liquid menisci/bridgingAlleviation of liquid menisci/bridging– NOT improved wetting of bulk surfaceNOT improved wetting of bulk surface

Significant “restoration” of 500Y areaSignificant “restoration” of 500Y area

((aaf, 500Yf, 500Y a af, 250Yf, 250Y))

Wetting PhenomenaWetting Phenomena

Contact angle (Contact angle (θθ): liquid’s propensity to wet): liquid’s propensity to wet– σσ and and θθ relatable for given surface relatable for given surface

Dramatic effect predicted in aDramatic effect predicted in aee models models

contradicted?contradicted?– θθ may be of limited importance? may be of limited importance?

Fully wetted surfaceFully wetted surface

Liquid spreading dictated by surface textureLiquid spreading dictated by surface texture

– θθ same at high/low same at high/low σσ??Offsetting interfacial energiesOffsetting interfacial energies LV

SLSV

σ

σσθcos

Contact Angle MeasurementsContact Angle Measurements

Non-corrugated Mellapak

σ ~ 72 dynes/cm

θ variable

(drop size, placement)

Flat SS

σ ~ 72 dynes/cm

θ ~ 70˚

Flat SS

σ ~ 35 dynes/cm

θ ~ 40˚

Establish reproducibility of techniqueEstablish reproducibility of technique

Interfacial energy hypothesis invalidatedInterfacial energy hypothesis invalidated

Enhanced Viscosity StudiesEnhanced Viscosity Studies

Viscosity EnhancementViscosity Enhancement

High MW PEO favorableHigh MW PEO favorable– Low concentrationsLow concentrations

– Minor impact on DMinor impact on DCO2CO2, H, HCO2CO2

– Kinetically inert (kKinetically inert (kOH-OH-))

PEO-300K (POLYOXPEO-300K (POLYOXTMTM WSR N750) WSR N750)– 1.25 wt % 1.25 wt % →→ 10-fold viscosity increase 10-fold viscosity increase

Newtonian behaviorNewtonian behavior

DDCO2CO2: ~7% decrease: ~7% decrease

HHCO2CO2: negligible change: negligible change

WWC Data for Baseline & High Viscosity Systems

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

150 200 250 300 350 400

PCO2 LM (Pa)

No

rma

lize

d k

g′ Baseline (μ ~ 0.73 cP)

Baseline (μ ~ 0.74 cP)

PEO-300K (μ ~ 7.5 cP)

PEO-300K (μ ~ 7.9 cP)

Baseline average

PEO-300K average(approx. 1.25 wt %)

Predicted average (polymer)

Predicted average ("normal" D/μ relation)

Fractional Area Data for Flexipac 1Y

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0 10 20 30 40 50

Liquid load (m3/m2-h)

Fra

cti

on

al

are

a

1.5 m/s, 0.8 cP, 72 dynes/cm

1 m/s, 0.8 cP, 72 dynes/cm

1.5 m/s, 5.5 cP, 57 dynes/cm (POLYOX)

1 m/s, 5.5 cP, 57 dynes/cm (POLYOX)

Chen (5 m K+/2.5 m PZ): 2.8 cP, 40 dynes/cm

ap = 413 m2/m3

Mellapak 250Y Fractional Area Data(uG = 1.5 m/s)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

0 10 20 30 40 50 60 70 80

Liquid load (m3/m2-h)

Fra

cti

on

al

are

a

Baseline (Bringmann): μ ~ 0.75 cP

Sucrose (Bringmann): μ ~ 5 cP

"Adjusted" sucrose (Bringmann-Tam)(kOH-,norm ~ 10.2)POLYOX: μ ~ 5 cP, σ ~ 60 dynes/cm

"Adjusted" sucrose (Bringmann-Rocha)(kOH-,norm ~ 6.4)

Mellapak 250Y Fractional Area Data(uG = 1.5 m/s)

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

0 10 20 30 40 50 60 70 80Liquid load (m3/m2-h)

Fra

cti

on

al

are

a

Baseline (Bringmann): μ ~ 0.75 cP

Sucrose (Bringmann): μ ~ 2.3 cP

"Adjusted" sucrose (Bringmann-Tam) (kOH-,norm ~ 4.4)

POLYOX: μ ~ 2.5 cP, σ ~ 60 dynes/cm

"Adjusted" sucrose (Bringmann-Rocha)(kOH-,norm ~ 2.7)

Chen (Flexipac AQ, 5 m K+ / 2.5 m PZ): 2.8 cP, 40 dynes/cm

Conclusions (Conclusions (σσ Studies) Studies)

NP-7 / antifoam do not have distinguishable NP-7 / antifoam do not have distinguishable effect on COeffect on CO22-NaOH kinetics (k-NaOH kinetics (kgg′′))

σσ has strong effect on performance of low has strong effect on performance of low capacity (high surface area) packingcapacity (high surface area) packing– Attributed to capillary phenomenaAttributed to capillary phenomena

θθ may be of limited significance may be of limited significance

Conclusions (Conclusions (μμLL Studies) Studies)

High MW PEO minimally impacts kHigh MW PEO minimally impacts kgg’ ’

(marginal decrease, corresponding to theory)(marginal decrease, corresponding to theory)

aaf, 1Yf, 1Y: same for baseline, enhanced : same for baseline, enhanced μμLL

– Interaction of Interaction of μμLL, , σσ effects? effects?

aaf, 250Yf, 250Y: drastic impact of : drastic impact of μμLL

– Systematic error?Systematic error?– Fluid property impact may differ depending on Fluid property impact may differ depending on

specific packing (i.e., texture)??specific packing (i.e., texture)??