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

1. Influence of Liquid Properties on Effective Mass Transfer Area of Structured Packing Robert E. Tsai January 11, 2008 Research Review Meeting Department of Chemical Engineering The University of Texas at Austin

2. Overview • Introduction: Motivation & Objectives • Materials and Methods • Pilot-scale packed column • Wetted-wall column (WWC) • Experimental Results • Reduced surface tension • Enhanced viscosity • Conclusions

3. Importance of Mass Transfer Area • Packing: promotes gas-liquid mass transfer • Random: less $$ • Structured: lower ΔP, better mass transfer, “cleaner” mechanics • Need for reliable mass transfer models (kL/kG, ae) • Measured performance: kLae or kGae • For industrial CO2 capture (amine absorption), ae particularly important • Absorption rate independent of MTCs but remains directly related to ae

4. Research Motivation • No ae models predictive over range of conditions • Different effects of viscosity and surface tension ae = f(μ,σ) water data may not be reflective of amine conditions!

5. Project Scope • Measurement of ae of Mellapak packings (250 and 500-series) • Fluid property variations • Viscosity (1, 5, 10 cP) • Surface tension (72, 50, 30 dynes/cm) • Geometric variations • Kinetic measurements (WWC) • Test impact of additives on CO2-NaOH rxn. • Semi-empirical model • Predicts ae of sheet-metal packing as function of viscosity, surface tension, liquid load

6. Separations Research Program (SRP) Database • CO2 absorption from air into 0.1 M NaOH • Measured in 16.8” (430 mm) ID column • 10+ random packings • CMR #2, IMTP #40 • 10+ structured packings • Mellapak 250Y, Flexipac 1Y • Hydraulic measurements (ΔP, holdup)

7. Caustic Absorption • ae measured by CO2-NaOH reactive absorption • Inexpensive and non-hazardous • Kinetics have been extensively characterized • Overall rxn: CO2 (aq) + 2 OH- → CO32- + H2O • Pseudo-first-order (low PCO2, excess OH-): (Irreversible)

8. Packed Column Setup Air Outlet Distributor, Demister Packing ~ 10 ft (3 m) DPC PVC: ID ~ 16.8” (430 mm) Blower (Air: 380-400 ppm CO2) 300 or 450 ACFM (1 or 1.5 m/s) Optional Recycle (for mixing) Liquid Pump Storage Tank (Up to 35 gpm/ft2 or 85 m3/m2-h)

9. Packing Area Characterization Series resistance: 1/kG≈ 0 for high gas velocity, dilute NaOH

10. WWC Experimental Setup Condenser CO2 Analyzer (IR) Needle Valve GasOUT yCO2: 500 – 1500 ppm (minimize OH- depletion) Septum WWC Bypass Valve LiqOUT Pump N2 GasIN LiqIN Solution Reservoir Saturator / Temp. Bath Temp. Bath N2 / CO2 Liq. Rate: 2-4 cm3/s (constant) Mass Flow Controllers (5 SLPM)

11. WWC Calculations Experimental kg′: CO2 flux Correlated via SO2-NaOH absorption Literature kg′: Pohorecki and Moniuk (1988): Eqns for kOH-, DCO2 liq, HCO2

12. Reduced Surface Tension Studies

16. Mellapak 250Y/500Y Comparison: σ ~ 72 dynes/cm • af, 250Y~ unity vs. af, 500Y 0.6 • Similar trend for 250 and 500-series prototype packings • Liquid pooling in corrugation troughs, bridging across adjacent sheets • Reduces area available for mass transfer • High structural density more prone – partially offsets advantage of higher ap

17. Mellapak 250Y uL = 36.7 m3/m2-h [Green (2006)]

18. Mellapak 250Y/500Y Comparison: σ ~ 35 dynes/cm • Expect better wetting, but no change in af, 250Y • Same surface coverage at high and low σ • Also applies to 500Y – same texture, shorter crimp • Key effect of reduced σ • Alleviation of liquid menisci/bridging • NOT improved wetting of bulk surface • Significant “restoration” of 500Y area (af, 500Y af, 250Y)

19. Wetting Phenomena • Contact angle (θ): liquid’s propensity to wet • σ and θ relatable for given surface • Dramatic effect predicted in ae models contradicted? • θ may be of limited importance? • Fully wetted surface • Liquid spreading dictated by surface texture • θ same at high/low σ? • Offsetting interfacial energies

20. Contact Angle Measurements • Establish reproducibility of technique • Interfacial energy hypothesis invalidated Non-corrugated Mellapak σ ~ 72 dynes/cm θvariable (drop size, placement) Flat SS σ ~ 72 dynes/cm θ~ 70˚ Flat SS σ ~ 35 dynes/cm θ~ 40˚

21. Enhanced Viscosity Studies

22. Viscosity Enhancement • High MW PEO favorable • Low concentrations • Minor impact on DCO2, HCO2 • Kinetically inert (kOH-) • PEO-300K (POLYOXTM WSR N750) • 1.25 wt % → 10-fold viscosity increase • Newtonian behavior • DCO2: ~7% decrease • HCO2: negligible change

27. Conclusions (σ Studies) • NP-7 / antifoam do not have distinguishable effect on CO2-NaOH kinetics (kg′) • σ has strong effect on performance of low capacity (high surface area) packing • Attributed to capillary phenomena • θ may be of limited significance

28. Conclusions (μL Studies) • High MW PEO minimally impacts kg’ (marginal decrease, corresponding to theory) • af, 1Y: same for baseline, enhanced μL • Interaction of μL, σ effects? • af, 250Y: drastic impact of μL • Systematic error? • Fluid property impact may differ depending on specific packing (i.e., texture)??