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

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

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 (k L /k G, a e ) Measured performance: k L a e or k G a e For industrial CO 2 capture (amine absorption), a e particularly important –Absorption rate independent of MTCs but remains directly related to a e

Research Motivation No a e models predictive over range of conditions –Different effects of viscosity and surface tension Solvent (40 ˚C) Viscosity, μ L [cP or mPa·s] Surface tension, σ [dynes/cm or mN/m] Water m MEA Ldg = 0.4 Ldg = M AMP ~ a e = f(μ,σ)  water data may not be reflective of amine conditions!

Project Scope Measurement of a e 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 CO 2 -NaOH rxn. Semi-empirical model –Predicts a e of sheet-metal packing as function of viscosity, surface tension, liquid load

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

Caustic Absorption a e measured by CO 2 -NaOH reactive absorption –Inexpensive and non-hazardous –Kinetics have been extensively characterized Overall rxn: CO 2 (aq) + 2 OH - → CO H 2 O Pseudo-first-order (low P CO2, excess OH - ): (Irreversible)

Packed Column Setup Air Outlet Storage Tank Liquid Pump Packing ~ 10 ft (3 m) Blower (Air: ppm CO 2 ) Distributor, Demister DPC Optional Recycle (for mixing) PVC: ID ~ 16.8” (430 mm) (Up to 35 gpm/ft 2 or 85 m 3 /m 2 -h) 300 or 450 ACFM (1 or 1.5 m/s)

Packing Area Characterization Series resistance: 1/k G ≈ 0 for high gas velocity, dilute NaOH

Mass Flow Controllers Solution Reservoir Septum Temp. Bath Pump Needle Valve N 2 / CO 2 N2N2 Bypass Valve Gas IN Liq IN Liq OUT Gas OUT Condenser Saturator / Temp. Bath WWC CO 2 Analyzer (IR) WWC Experimental Setup y CO2 : 500 – 1500 ppm (minimize OH - depletion) Liq. Rate: 2-4 cm 3 /s (constant) (5 SLPM)

WWC Calculations Experimental k g ′: Pohorecki and Moniuk (1988): Eqns for k OH-, D CO2 liq, H CO2 CO 2 fluxCorrelated via SO 2 -NaOH absorption Literature k g ′:

Reduced Surface Tension Studies

Mellapak 250Y/500Y Comparison: σ ~ 72 dynes/cm a f, 250Y ~ unity vs. a f, 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 a p

Mellapak 250Y u L = 36.7 m 3 /m 2 -h [Green (2006)]

Mellapak 250Y/500Y Comparison: σ ~ 35 dynes/cm Expect better wetting, but no change in a f, 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 (a f, 500Y  a f, 250Y )

Wetting Phenomena Contact angle (θ): liquid’s propensity to wet –σ and θ relatable for given surface Dramatic effect predicted in a e 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

Contact 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 technique Interfacial energy hypothesis invalidated

Enhanced Viscosity Studies

Viscosity Enhancement High MW PEO favorable –Low concentrations –Minor impact on D CO2, H CO2 –Kinetically inert (k OH- ) PEO-300K (POLYOX TM WSR N750) –1.25 wt % → 10-fold viscosity increase Newtonian behavior D CO2 : ~7% decrease H CO2 : negligible change

Conclusions (σ Studies) NP-7 / antifoam do not have distinguishable effect on CO 2 -NaOH kinetics (k g ′) σ has strong effect on performance of low capacity (high surface area) packing –Attributed to capillary phenomena θ may be of limited significance

Conclusions (μ L Studies) High MW PEO minimally impacts k g ’ (marginal decrease, corresponding to theory) a f, 1Y : same for baseline, enhanced μ L –Interaction of μ L, σ effects? a f, 250Y : drastic impact of μ L –Systematic error? –Fluid property impact may differ depending on specific packing (i.e., texture)??