Absorption of carbon dioxide into aqueous solutions of ethylenediamine: Effect of interfacial turbulence

The Chemical EngineenngJournal, 13 (1977) 213-217 @ EIsevier Sequoia S.A., Lausanne. Printed in the Netherlands

Absorption of Carbon Dioxide into Aqueous Solutions of Ethylenediamine : Effect of Interfacial Turbulence E. SADA, H. KUMAZAWA and M. A. BUTT Department of Chemical Eniineering, (Received

6 January

Faculty of Engineering,

1976; in final form 10 January

Nagoya University, Nagoia (Japan)

1977)

Abstract Absorption rates for carbon dioxide into aqueous ethylenediamine solutions were measured in a laminar liquid jet, a wetted-wall column and a quiescent liquid absorber to investigate the chemical absorption mechanism and the reaction kinetics over a wide range of contact times. From the chemical absorption data in the laminar liquid jet with surface-active agent, the value of the rate constant for the second order reaction was derived as 1.75 x IO4 1 mol-’ s-‘. Additional experiments were carried out on the tracer desorption of ethylene from aqueous ethylenediamine solutions saturated with ethylene to check and verify effects due to interfacial turbulence. We conclude that the Marangoni effect cannot be neglected even for such a short contact time as that in a laminar liquid jet.

an interfacial turbulence driven by surface tension gradients3* 4 may appear and thus the reaction rate constant may be overestimated; a Marangoni instability is easily established at the entrance region of any absorption equipment since the surface of the liquid just after its formation is not contaminated. If present, the interfacial turbulence needs to be suppressed. In this paper we attempt to clarify the chemical mechanism and the reaction kinetics over a wide range of contact times for the system carbon dioxide-aqueous ethylenediamine solution. Another aim is to check and verify the interfacial turbulence induced by the Marangoni effect even for such short contact times as those involved in the laminar liquid jet technique, and to develop a procedure for the derivation of the intrinsic reaction rate constant.

2. EXPERIMENTAL 1. INTRODUCTION

the process of chemical absorption the reaction kinetics and mechanism are considered to be of major importance. For instance, a gas absorbed in a liquid medium may react not only with a reactant that is originally present in the liquid but also with a product of this reaction. In this case, for short contact times the chemical absorption process may be regarded as gas absorption with a first-step reaction, whereas after long contact times the rate of diffusion is considerably reduced and the second reaction step may influence the absorption rate. In our previous work’* * the rate of carbon dioxide absorption into aqueous solutions of various ethanolamines over a wide range of contact times was satisfactorily predicted by the penetration theory using a two-step consecutive reaction. Another important problem encountered is the accurate derivation of the reaction rate constant; this has been carried out using chemical absorption theory for the fast reaction regime. However, in some’systems In

213

2.1. Chemical absorption The rates of chemical absorption of carbon dioxide into aqueous ethylenediamine solutions were measured in a laminar liquid jet, a wetted-wall column and a quiescent liquid absorber. In the laminar liquid jet, experiments were carried out with and without a surface-active agent in aqueous ethylenediamine solution to check the interfacial turbulence. In experiments with the wetted-wall column, a surface-active agent was again added to the solution to prevent rippling at the surface of the falling liquid film and to suppress interfacial turbulence. Phensol NP-100 (0.025 vol.%) was added to the absorbent in the laminar liquid jet and wetted-wall column experiments. The surfactant was a pure reagent grade of poly 10 mol oxyethylenenonylphenol ether. In the experiments with the quiescent liquid absorber, 3 wt.% agar-agar was added to eliminate natural convection. All the absorption rates were measured volumetrically at a constant temperature of 25 “C and at atmospheric pressure. Further details of

214

E. SADA, H. KUMAZAWA,

the experimental procedure and conditions may be found in ref. 1. In order to evaluate the experimental enhancement factor, values of the diffusivity and the physical solubility of carbon dioxide in aqueous ethylenediamine solutions are required. However, in view of the chemical reactions between carbon dioxide and ethylenediamine the diffusivity and the physical solubility of carbon dioxide in aqueous ethylenediamine solutions were deduced from the corresponding values for nitrous oxide. This is considered to be valid, as suggested by previous investigators 5@, because of the similarity in mass and molecular interaction parameters of carbon dioxide and nitrous oxide. The solubility of nitrous oxide in aqueous ethylenediamine solution was therefore determined at different concentrations. The experimental apparatus used in this work was the same as described in previous papers> ‘. The full experimental details have been given elsewhere7. The diffusivity of nitrous oxide in aqueous ethylenediamine solutions of varying concentrations was measured from the physical absorption rates in the laminar liquid jet. 2.2. Desorption of tracer gas Experiments were also performed on the tracer desorption of ethylene from aqueous ethylenediamine solutions to search for any interfacial turbulence in such a short contact time as that in the laminar liquid jet. These investigations were carried out with and without a surface-active agent in aqueous ethylenediamine solutions in the presence of a nitrogen or carbon dioxide gas stream. An aqueous ethylenediamine solution was initially saturated :vith ethylene by bubbling it through the ethylenediamine solution for a sufficiently long time. The gas flow rate was measured using a soap film meter and was kept constant for all the runs. The ethylene content in the exit gas stream from the laminar liquid jet apparatus was determined using a gas chromatograph. The surface-active agent and the other experimental conditions were the same as in the chemical absorption experiments.

Fig. 1. Solubility solutions.

of nitrous

oxide in aqueous

AND DISCUSSION

3. I. Physicochemical properties The effect of ethylenediamine concentration on the solubility of nitrous oxide is shown in Fig. 1. These results have been plotted as log (cu,/o)versus the ethylenediamine concentration in the same manner as

ethylenediamine

in our previous work7. The corresponding curve for monoethanolamme shows a linear relationship at low concentrations, whilst at high concentrations (greater than 3 mol 1-l) the trend of the curve shifts from linear to concave. Sada and Kito8 fitted the curve by a straight line which is satisfactory up to a concentration of 3 mol. In the case of ethylenediamine-nitrous oxide, a concave curve is found even for low concentrations. Since this curve could not be represented by a suitable equation, the carbon dioxide solubilities were evaluated from the corresponding measurements with nitrous oxide in the same way as in ref. 6. From physical absorption data in the laminar liquid jet, the diffusivities of nitrous oxide in various concentrations of ethylenediamine were calculated using the penetration theory solution

(1) Table 1 shows the values of the nitrous oxide diffusivity for various concentrations of ethylenediamine. TABLE

1

Diffusivity of nitrous oxide into aqueous ethylenediamine at 1 atm and 25 “C 3. RESULTS

M. A. BUTT

CBO (mol 1-l)

DA

0.618 1.373 1.821 2.350 3.369

1.91 1.67 1.44 1.30 1.09

solutions

x 10s (cm2 S-I)

of

ABSORPTION OF CO2 IN ETHYLENEDIAMINE

215

3.2. Second order reaction rate constant The rate constant of the reaction between carbon dioxide and ethyleilediamine can be derived by comparing the observed absorption rates under the fast reaction regime with the penetration theory solution for a first order reaction’ : 6?A = ~d~cAi(~A~IcBO~-

:

(2)

In the present work, experimental absorption rates obtained using 0.025 vol.% surface-active agent in a liquid jet were compared with the theoretical solution described later. The value of the rate constant for the second order reaction was derived as I .75 x lo4 1mol-’ S -‘. This is very low compared with the value obtained by W&land and Trass6 (1 .O x 10’ 1mol-’ s-l) using a laminar liquid jet without a surface-active agent, but it agrees well with the value obtained by Jensen and Cllristensen” using the quenching method (1.7 x lo4 1 mol-* s-l). The rates of carbon dioxide absorption into aqueous solutions of ethylenediamine using a laminar liquid jet with and without a surface-active agent are shown in Fig. 2 as a plot of enhancement factor Q1versus the diffusion-reaction modulus v@. The broken line represents the penetration theory solution for gas absorption with a first order reaction. Chemical absorption data taken without adding a surface-active agent to the ethylenediamine aqueous solutions fall above the broken line. Furthermore, the experimental enhancement factors for the lowest ethylenediamine concentration without a surface-active agent exceed

those for the instantaneous reaction regime. This enhancement in absorption rate may be attributed to interfacial turbulence, which is discussed in detail in Section 3.4. 3.3. Variation of the enhancement factor Experimental results for the chemical absorption of carbon dioxide in aqueous solutions of ethylenediamine in three different types of absorbers are shown in Fig. 2 as a plot of Cpversus a The broken line and the solid lines represent the penetration theory solutions for, respectively, a first order reaction and a second order reaction of the form A + B + products. The value of DB required in the computation of the solid lines was calculated from the correlation equation of Thomas and Furzer”. The viscosity of the aqueous solution of ethylenediamine (required in the correlation equation) was measured using a standard Ostwald viscometer. The series of experimental points in the three types of absorber coincide well with the solid line. Even for the long contact times attainable with the quiescent liquid technique, there was no significant deviation from the solid line for experimental enhancement factors. It has been noted previously” 2 in the case of absorption of carbon dioxide into aqueous ethanolamine solutions that, after being absorbed into the liquid medium, the gas may react not only with a reactant dissolved in the liquid (A + 2B + R (I)), but also with a product of this reaction (A + 2B -+ R (I) and A + R + products (II)). The rate of gas

IO2 I

0.297 0.292

35.7 24.0

Wetted

Fig. 2. Variation of the enhancement diamine solutions.

Quiescent

factor with the modulus $?Z for the absorption of carbon dioxide into aqueous ethylene-

216 absorption will then only be influenced by the second reaction after an exposure time long enough for the product of the first reaction to form. The rate constant of the second reaction step was assessed to be 6.72 x 10” 1 mol-” s-l irrespective of the kind of ethanolamine2. In the present system, where the rate constant of the first reaction step is much greater, the ratio of the reaction rate constants of the two steps decreases. The second step will occur for a contact time as long as 2 h but from the experimental data it may be concluded that the effect of the second reaction step on the rate of absorption will be negligibly small. 3.4. Desorption of ethylene from aqueous solutions of ethylenediarnine Figure 2 shows that the enhan~ment factors without a surface-active agent fall above the penetration theory solutions for the fast reaction and the instantaneous reaction regimes, whereas those with a surface-active agent are in fairly good agreement with the penetration theory predictions represented by the solid lines. The reaction between carbon dioxide and ethylenediamine is fast enough, even with such a short contact time as in the laminar liquid jet, for the consumption of the reactant liquid and the formation of the reaction products to affect the hydrodynamic conditions at the gas-liquid interface. Both the reaction products (monocarbamate and dicarbamate) are partially dissociated. The presence of ionic products’2 together with a decrease in the concentration of the reactant liquid near the interface result in an increase in surface tension, The surface tension gradient at the interface then becomes negative (i.e. au/a2 < 0), which causes the instability. Recently, Vijayan and Ponter3 and Khutoryanskii et aL4 measured the dynamic surface tension during carbon dioxide absorption into aqueous solutions of various amines using an oscillating jet technique, and they reported that differences in interfacial tension exist for reacting and non-reacting conditions. However, at present it is difficult to observe directly (optically), as in a stagnant po01’~, the dynamics near the gas-liquid interface in a laminar liquid jet. We therefore carried out experiments on the tracer desorption of ethylene from aqueous solutions of ethylenediamine under reacting and non-reacting conditions to verify the presence of interfacial turbulence in the laminar liquid jet. Reacting and nonreacting conditions refer to tracer desorption into carbon dioxide and into nitrogen streams, respectively. This technique was originally developed by Brian et al. I4 to verify the presence of interfacial turbulence in the

E. SADA, H. KUMAZAWA, M. A. BUTT 12

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Fig. 3. &sorption of ethylene from ethylenediamine at various concentrations.

solutions

carbon dioxide-monoethanolamine system using a short wetted-wall column. Figure 3 shows the experimental results as a plot of desorption rate of ethylene versus 1/fi The rate of desorption under reacting conditions increases with an increase in ethylenediamine concentration which implies that the desorption rate increases with an increase in chemical reaction rate and thus with an increase in induced surface tension gradient. However, the desorption rate under non-reacting conditions remains unchanged with an increase in ethylenediamine concentration; this also agrees well with the theoretical prediction for the absorption rate of ethylene into water. Furthermore, the desorption rate was lower in the presence of a surface-active agent (0.025 vol.%) but it agreed well with that under non-reacting conditions. In some absor tion processes there may be interfacial resistance ’ P*16 due to the addition of a surfaceactive agent. However, since the desorption rate on adding surface-active agent under reacting conditions showed good agreement with the theoretical prediction, the interfacial resistance effect may be neglected in the present system. Hence, in the carbon dioxide-ethylenediamine system, the Marangoni effect on the rate of absorption cannot be neglected even for short contact times. The

I

ABSORPTION

217

OF CO2 IN ETHYLENEDIAMINE

Marangoni effect may arise from the presence of ionic products and the decrease in concentration of the reactant liquid. The heat of reaction causes an increase in the interfacial temperature. For example, Ponter et al.” using an infrared radiometer detected an increase of 4 “C in interfacial temperature when carbon dioxide was being absorbed into 1 M monoethanolamine solution. Such an interfacial temperature increase reduces the surface tension because ao/aT < 0 and it has a negative effect on the instability. At the same time, convection currents due to density differences will be produced, but in the laminar liquid jet the gas-liquid exposure .^ time is too short for the convection currents to mevail’;‘. However, the addition of trace amounts of surfaceactive agent to the reactant liquid effectively suppresses the interfacial turbulence driven by surface tension gradients. The chemical reaction rate constant can thus be satisfactorily derived from chemical absorption data under the fast reaction regime as described in Section 3.2. For the wetted-wall column experiments, the enhancement factors fit the curves predicted by the penetration theory, as shown in Fig. 2, because the reactant liquid already contains 0.025 vol.% surface-active agent.

Greek symbols a! Bunsen absorption coefficient, cm3 of gas/cm3 of solution e krcnot surface tension of the liquid, dyn cm-r enhancement factor 0” Subscripts A dissolved gas A B liquid phase reactant E ethylene i gas-liquid interface W water 0 initial value REFERENCES

(1976)

c D 12

k M N 4 e 43 T

concentration in the liquid phase, mol I-’ diffusivity in the liquid phase, cm* s-r height of the wetted-wall column or length of the jet, cm reaction rate constant, 1 mol” s-’ Ire/4 desorption rate, mol cm-* s-*

54 (1976)

and M. A. Butt, Grn. J. Chem. Eng.,

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cBO/cAi

rate of gas absorption,

22

196.

2 E. Sada, H. Kumazawa

7 NOMENCLATURE

and M. A. Butt, A.I.Ch.E.J.,

1 E. Sada, H. Kumazawa

mol s-r or cm3 s-l

DB/DA

contact time, s temperature, “C liquid flow rate, cm3 s-r

15 W. J. Thomas, R. Khanna and E. W. Palmer, 3 (1972) 112. 16 W: J. Thomas, R. Khanna and E. W. Palmer, 6 (1973) 165. 17 A. B. Ponter, J. L. Anderson and S. Vijayan,

fi’ng., XVII (1975) 23.

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