Surface Effects in Gas Absorption . Ivan Metzger Department of Civil Engineering, Newark College of Engineering, 323 High St., Newark, N.J. 07102
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• Gas absorption experimt:nts t:mploying hdium and nitrogen were conducted with water to obtain surface;: renewal frequencies and tilm thicknesses in the presence of 0.5 mg. per liter of alkyl benzene sulfonatc. The relationship betwcen these parameters was the same as in distilled water and in accord with a previously proposed equation for which a constant term was evaluated. The results are suggested for use in the film penetration model along with field data for evaluating aeration under natural stream conditions.
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I n a previous paper (Mt:tzger and 1967) _attenti_on was focused on the absorption coefhctent. Iu ,
D,
(2)
where r is the surface renewal frequency, L the film thickness, and Dm the molecular diffusion coefficient. The following equations were proposed for the surface renewal frequency and the film thickness
C1Cz 31 ~C:; 3 pz·' •E'
r = --·------------Ms
4
(3)
(4) where p is the fluid density, v the kinematic viscosity, E the energy dissipated in the fluid, and M. is the modulus of compression at the surface. C~o C2, C:,, and C4 are constants with subscripts as in the previous development. Equations 3 and 4 lead to r£3 = C- ~!:' • Ms
(5)
with the nun.ber of constants reduced to one, C,. Equation 5 was employed to obtain partial verification of the proposed equations. It predicts that rP is independent of the energy dissipated and should remain constant at a given temperature. This constancy was found experimentally in a serie~ of experiments condLtcted with distilled water. Further, the temperature dependency was in accord with the terms in the numerator of Equation 5. Subsequenl experiments were conducted with surface active substances in an attempt to extend the usefulness;·of Equation :: 1 .:>r the prediction of surface effects in gus absorption. The results and
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HELIUM
. ' • NITROGEN
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CONCENTRATION OF ABS IN MILLIGRAMS PER LITER
0~--~----~---L----~--~----~--~----~ 0 20 40 60 80 100 120 140 160
Figure I. Absorption coefficients for argon
MIXER SPEED IN REVOLUTIONS PER MINUTE
per ~.:ent redu~o:tion inK~. for ea~.:h mixing ~.:ondition. Thevalues of r and L presented will be discussed subsequently. At low and moderate mixing conditions where the surface expansion was negligible true absorption coefficients, K L• are shown. Apparent ab~orption coetllcients, KL ', based on the projected surface area are reported for the intense mixing condition. The per ~o:ent reduction, in KL is highest for moderate mixing conditions and lower for both low •md intense conditions. This is similar to results reported h) Maney and Okun ( 1965) who found no eflel·t at low speed~ with the absorption coetrcient undergoing a 1m1rked depression over a range of intermediate speeds and ~.:onverging to pure water values at high speeds. This trend may indicate that the major effect of surface active substances is on the frequen~o:y of surface renewals (Davidson. Cullen, er a/., 1959) and not on the ditfusivity. If the ditfusivity chunges significantly. an effect should be observed at the lower speeds. A more comprehensive series of experiments with helium and nitrogen was conducted at 25c C. with 0.5 mg. per liter of ABS over a range of mixing speeds. The results are presented in Figure 2 as apparent absorption coefficients. K~. '; however, for speeds under about 140 r.p.m. the surfa~.:e was undisturbed and these values are true absorption coefficient~. K~.. The data are represented by straight lines over most of the range to a speed where the surface begun to ripple. The dep:trture from linearity occurred at a higher speed but at the same K~. value as in previously reported (Metzger and Dobbins, 1967) experiments conducted with distilled water. Analysis of Results The K~. values presented in Figure 2 were obtained from experiments conducted so that at given speeds and temperatures mixing conditions were identical for the measurement of the absorption coefficients for helium and nitrogen. Thus, from Equation 2, the value of the surface renewal frequency, r, and the film thickness, L, at a .given mixing condition, could be computed by inserting the known values of KL and
Figt•re 2. -\bsorption coefficients for helium and nitrogen
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25"C ABS (PRESENT WORK) o 20°C DISTILLED WATER (METZGER AND DOBBINS 1967)
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