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J. Ind. Eng. Chem.,Vol. 13, No. 2, (2007) 170-175

Effect of Polyethylene Oxide on Absorption of Carbon Dioxide intoAqueous NaOH Solution

Sang-Wook Park, Byoung-Sik Choi, and Jae-Wook Lee*

Division of Chemical Engineering, Pusan National University, Busan 609-735, Korea

*Department of Chemical Engineering, Sogang University, Seoul 121-742, Korea

Received July 11, 2006; Accepted November 7, 2006

Abstract: Carbon dioxide was absorbed into aqueous poly(ethylene oxide) (PEO) solution containing NaOH in

a flat-stirred vessel to investigate the effect of the non-Newtonian rheological behavior of PEO on the chemicalabsorption rate of CO2, where the reaction between CO2and NaOH was assumed to be a first-order reactionwith respect to the concentrations of CO2and NaOH, respectively. A unified correlation equation containing

the Deborah number, which reflects the viscoelastic properties of the non-Newtonian liquid, was used to obtainthe volumetric liquid-side mass transfer coefficient of carbon dioxide in the aqueous PEO solution. The elastic

properties of PEO accelerated the absorption rate of CO2 when compared with that of a Newtonian liquid,based on the same values of viscosity.

Keywords: absorption, carbon dioxide, polyethylene oxide, NaOH, viscoelastic liquid

1)

Introduction

The viscosity of a fluid in hydrodynamic system de-

pends on the shear rate and also the type of fluid, i.e., aNewtonian or non-Newtonian fluid; the mass transfer co-efficient of a solute in one phase is in inverse proportion

to the viscosity of its phase due to the inverse proportionof viscosity to diffusivity. The apparent viscosity of

non-Newtonian fluids is not sufficient to obtain a unifiedcorrelation for the liquid-side mass transfer coefficient(kL) of a gas in the case of gas absorption into non-

Newtonian fluids. Because of the complexities of gas ab-sorption in non-Newtonian media, the correlations ob-

tained by these studies have been limited to just a fewkinds of non-Newtonian fluids, such as carbopol, carbox-ymethylcellulose (CMC), polyacrylate (PA), poly(ethy-

lene oxide) (PEO), polyacrylamide (PAA), and poly-isobutylene (PIB) solutions. If a considerable reduction

of kLa is due to the viscoelasticity of the aqueous sol-ution, then the extent to which data for a viscoelastic sol-

ution, such as PAA, deviate from those for an inelasticsolution, such as CMC, should correlate with some meas-ure of the solutions elasticity. A dimensionless number,

To whom all correspondence should be addressed.

(e-mail: swpark@pusan.ac.kr)

such as the Deborah number (De), which relates the elas-tic properties to the process parameters, is used to corre-

late the volumetric liquid-side mass transfer coefficient(kLa) with the properties of non-Newtonian liquids.Unified correlations have been proposed for kLa in

Newtonian as well as non-Newtonian solutions by in-troducing the dimensionless term, having the form

(1+ n1Den2

)n3

; the values of n1, n2, and n3(Table 1) differdepending on whether the polymers in Table 1 act as re-

duction or increment agents of the absorption rate of gas[1-7]. There is little information about the effects that elastic

properties have on the absorption of gas accompanied by

a chemical reaction in a non-Newtonian liquid. Park andcoworkers presented the effect of the elasticity of poly-isobutylene (PIB) [4] in a benzene solution of polybutene(PB) and PIB on the absorption rate of CO2, and that [5]

in a w/o emulsion composed of an aqueous solution asthe dispersed phase and a benzene solution of PB and

PIB as the continuous phase in an agitation vessel. Theyshowed that PIB accelerated the absorption rate of CO2.The effects of PAA [6] and PEO [7] in aqueous solutions

on the absorption rate of CO2were also investigated. Thepolymers used in these studies acted as accelerators of

the absorption rate of CO2in the non-Newtonian viscoe-

lastic liquid, based on the same viscosity of the solution.

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Effect of Polyethylene Oxide on Absorption of Carbon Dioxide into Aqueous NaOH Solution 171

Table 1.Coefficients of Dimensionless Group in Gas-Liquid Mass Transfer

Investigator n1 n2 n3 polymer contactor

Yagi and Yoshida [1] 2 0.5 -0.67 CMC, PA agitated vessel

Ranade and Ulbrecht [2] 100 1 -0.67 CMC, PAA stirred tank

Nakanoh and Yoshida [3] 0.13 0.55 -1 CMC, PA bubble column

Park and coworkers [4] 100 1 -0.42 PB, PIB agitated vessel

Park and coworkers [5] 2461.3 1 -0.274 PB, PIB agitated vessel

Park and coworkers [6] 54.7 1 -0.45 PAA agitated vessel

Park and coworkers [7] 8.33 1.31 1 PEO agitated vessel

To investigate the effect that the behavior of a non-Newtonian liquid has on the gas absorption in series, an

aqueous PEO solution was used as a viscoelastic materialin this study. The absorption rates of CO2were measured

in aqueous PEO solutions containing NaOH; they were

compared with those estimated by the mass transfer witha chemical reaction, based on film theory. The Deborah

number, which was obtained from a power-law model ofthe relationship between the measured shear stress and

the primary normal stress difference against the shearrate of the aqueous PEO solution, was used to obtain aunified correlation between the values of kLa in water

and in aqueous PEO solutions.

Theory

The problem to be considered is that of a gaseous spe-cies A (CO2) dissolving into a liquid phase, and then re-acting irreversibly with a species B (NaOH) according to

(1)

where the stoichiometric coefficient () in Eq. (1) forNaOH was 1 [8].

Species B is a nonvolatile solute that was dissolved intothe liquid phase prior to its introduction into the gas

absorber. It is assumed that gas phase resistance to ab-sorption is negligible by using pure species A; thus, the

concentration of species A at the gas-liquid inter facecorresponds to equilibrium with the partial pressure ofspecies A in the bulk gas phase.

The chemical reaction of Eq. (1) is assumed to be sec-ond-order [8] as follows:

(2)

Under the assumptions mentioned above, the con-

servation equations of species A and B, based on the filmtheory with a chemical reaction and boundary conditions,

can be written in dimensionless forms as follows:

(3)

(4)

(5)

(6)

where

, a=CA/CAi, b=CB/CBo, , q

= CAi/CBo, and r = DA/DB, and the subscripts i and o re-fer to the gas-liquid interface and feed, respectively.

The enhancement factor () here is defined as the ratioof molar flux with a chemical reaction to that without a

chemical reaction:

(7)

The value of is used to estimate the absorption rate(RA) of CO2with a chemical reaction, as follows:

(8)

where RAois the absorption rate of CO2in water and VL

is the volume of the liquid phase.

Experimental

All chemicals in this study were of reagent grade andused without further purification. The purities of both

CO2and N2were more than 99.9 %. The polymer used inthis study was poly(ethylene oxide) (PEO) having amean molecular weight of 200,000 (Aldrich Chemical

Company, U.S.A.). NaOH (Aldrich, U.S.A.) was used inreagent grade without purification.

The gas-liquid contactor used to absorb CO2was a stir-red tank made of glass (0.075 m inside diameter, 0.13 min height) incorporating four equally spaced vertical baf-

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Sang-Wook Park, Byoung-Sik Choi, and Jae-Wook Lee172

Table 2.Physicochemical and Rheological Properties of CO2and Aqueous PEO Solution

C(kg/m

3)

Viscosity(Ns/m

2) 10

3Diffusivity(m

2/s) 10

9Solubility(kmol/m

3)

Density(kg/m

3)

Rheological properties

n(-)

K(Ns

nm

2)

b(-)

A(Ns

nm

2)

0 1 1.950 0.039 1000 1.0 0.001 - -

10 3.05 1.875 0.034 1001.4 0.529 0.019 0.103 0.134

20 8.17 1.811 0.029 1003.2 0.485 0.416 0.099 1.255

30 14.9 1.774 0.028 1004.8 0.453 0.895 0.084 2.766

fles; it was operated continuously with respect to the gasand batch-wise with respect to the liquid phase. A

straight impeller (0.034, 0.04, or 0.05 m long, 0.017 mwide) was used as the liquid phase agitator; it was lo-cated at the middle position of the liquid phase. The ab-

sorption rate of CO2 was obtained from the difference be-tween the inlet and outlet flow rates of CO2 at the con-

centrations of PEO of 1030 g/L and NaOH of 02kmol/m

3under typical conditions of an agitation speed of

50 rev/min with an impeller size of 0.034 m at 101.3N/m

2 and 25

oC, following the procedure reported else-

where [7].

Physicochemical and Rheological Properties

The solubility (CAPi) of CO2 in the aqueous PEO sol-

utions was obtained using the pressure measuring meth-od, which measured the pressure difference of CO2 be-fore and after equilibrium in the gas and liquid phases,

similar to the procedure reported elsewhere [9]. The ex-perimental procedure duplicated one reported [7] in

detail. The solubility (CAi) of CO2in aqueous NaOH sol-ution was estimated as follows [8]:

(9)

The density () of the aqueous solution of PEO wasmeasured within 0.1 kg/m

3 by weighing with a pycn-

ometer (Fisher Scientific Co., USA); it was found to be

identical within experimental accuracy to the density ofwater.

The apparent viscosity (L) of the aqueous solution ofPEO and that of water (w) were measured using aBrookfield viscometer (Brookfield Eng. Lab. Inc, USA).

The reaction rate constant (k2) in the reaction of CO2with NaOH was estimated as follows [8]:

(10)

The diffusivity (DAB) of CO2in the aqueous NaOH sol-

ution was estimated as follows [10]:

(11)

The diffusivity (DB) of NaOH in the aqueous NaOH

solution was obtained from the assumption that the ratioof DB to DAB was equal to the ratio in water [11]. Thediffusivities of CO2 and NaOH in water at 25

oC were

taken to be 1.97 10-9

m2/s [8] and 3.24 10

-9m

2/s [11],

respectively. The diffusivity (DA) of CO2 in the aqueousPEO solution was corrected for the viscosity of the aque-

ous PEO solution as follows [12]:

(12)

The material parameters K, n, A, and b in a power-lawmodel such as= kr

nand N1= Ar

bwere obtained from

measurements of the shear stress () and the primarynormal stress difference (N1) for the change of the shear

rate () using a parallel disk type rheometer (Ares,Rheometrics, U.S.A.) having a diameter of 0.05 m and a

gap of 0.001 m. The physical properties, such as the solubility, dif-fusivity of CO2, density, and apparent viscosity, and the

rheological properties, such as the values of K, n, A, andb, of the aqueous PEO solution are given in Table 2.

The value of kLa was obtained using Eq. (13), whichpresents the relationship between kLa and the rheologicalbehavior of the aqueous PEO solution [7] under the con-

ditions of an agitation speed of 50

400 rev/min withimpeller sizes of 0.034, 0.04, and 0.05 m, as follows:

(13)

where De is defined as the ratio of the characteristic ma-terial time () to the characteristic process time (t), asfollows:

(14)

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Effect of Polyethylene Oxide on Absorption of Carbon Dioxide into Aqueous NaOH Solution 173

Figure 1. Comparison of elasticity with viscosity of PEO inNaOH solution (CPEO = 30 kg/m

3; d = 30 mm; N = 50 rpm).

where N is the speed of the impeller; the shear rate is ob-

tained in the case of agitation of a liquid in a cylindricalvessel as follows [13]:

(15)

Results and Discussion

To observe the effect that the elasticity of PEO and thereactivity of NaOH have on the value of RA in thenon-Newtonian liquid containing a reactant (NaOH), the

absorption rate of CO2 was measured in the range ofPEO concentrations of 1030 g/L for various NaOH

concentrations (02 kmol/m3) under the typical con-

ditions of an agitation speed of 50 rev/min with an im-

peller size of 0.034 m. Figure 1 shows the measured values of RA against the

NaOH concentration as circles in the aqueous PEO sol-ution at a fixed concentration of PEO (30 g/L), and as tri-angles in water. As shown in Figure 1, the value of RA

increased upon increasing the NaOH concentration, dueto the increase of (as mentioned below); it approachesthe calculated value very well.

The lines in Figure 1 are the values of RAcalculated us-ing Eq. (8) and values of kLa,, CAiand VL; VLwas 3 10

-4m

3, a was 14.29 m

2/m

3, CAiwas obtained from Eq.

(10) at a given NaOH concentration, and estimated byEqs. (7) and a solution of Eq. (3) and (4) using a numer-

ical analysis of the finite element method and the phys-icochemical properties listed in Table 2. The value of

Figure 2.Effect of PEO concentration on enhancement factor

for various NaOH concentrations.

kLa in the dashed line was obtained from Eq. (13) with = wand De = 0; the value of kLa in the solid line was

obtained using = Land De > 0; the value of kLa in thedotted line was obtained using = Land De = 0. We as-sumed that the aqueous PEO solution with = Land De

= 0 would be an imaginary solution to act as a

Newtonian liquid. If the aqueous PEO solution in thisstudy would have only viscous behavior with a value ofDe of 0, the value of RAshould be that of the dotted line.

The value of RAis larger, however, than that of the imagi-nary solution. This finding means that the effect of theelasticity of the aqueous PEO solution on RAis stronger

than the effect of the viscosity; in other words, the elas-ticity of the aqueous PEO solution accelerates RAbased

on the same viscosity of the solution.Figure 2 shows the measured and calculated values ofplotted against the PEO concentration (030 g/L) us-ing various NaOH concentrations (symbols and solid

lines). The values of remained constant with increas-ing PEO concentration and increased with increasingNaOH concentration; the measured values approached

the calculated values very well. This result means thatis dependent on the reactant reactivity, rather than therheological behavior, of the non-Newtonian liquid.

Figure 3 shows the measured and calculated values ofkLa plotted against the PEO concentration (030 g/L) asopen circles and a solid line, respectively. The calculatedvalues of kLa were obtained from Eq. (13) with = Land De > 0. As shown in Figure 3, kLa decreases upon in-

creasing the PEO concentration; the measured values ap-proach the calculated values very well.

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