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Ind. Eng. Chem. Res. 2005, 44, 1742-1751
Carbon Dioxide Absorption in a Falling Film Microstructured Reactor: Experiments and Modeling M. Zanfir and A. Gavriilidis* Department of Chemical Engineering, University College London, Torrington Place, London WC1 7JE, U.K.
Ch. Wille Clariant GmbH, Division Pigments & Additives, SU Technology & Innovation Management, Process Development/Pilot Plants, Clariant Competence Center Micro Reaction Technology C3-MRT, Industriepark Ho¨ chst G835, D-65926 Frankfurt am Main, Germany
V. Hessel Institut fu¨ r Mikrotechnik Mainz, Carl Zeiss Strasse 18-20, D-55129 Mainz, Germany
Carbon dioxide absorption in sodium hydroxide solution was performed in a microstructured falling film gas-liquid reactor. The liquid phase was distributed on a reaction plate of 64 microchannels of 300 × 100 µm having a length of 66.4 mm, while the gas-phase was guided in a gas chamber with a depth of 5.5 mm or 2.5 mm. Experimental data were obtained keeping a fixed overall liquid flowrate of 50 mL/h, using three different NaOH inlet concentration (0.1, 1, and 2 M) and a fixed inlet molar ratio CO2:NaOH of 0.4, for a range of CO2 concentration of 0.8-100%. A plate with 16 microchannels of 1200 µm × 400 µm was also employed. A twodimensional model was formulated to simulate the reactor, and experimental results were compared to model prediction in terms of carbon dioxide conversion. The model gives good agreement with the experimental data at low inlet NaOH concentration (0.1 and 1 M), while the agreement with the experiments at 2 M NaOH is reasonable only for low CO2 inlet concentration. The model indicates that carbon dioxide is consumed within a short distance from the gas-liquid interface. 1. Introduction Carbon dioxide absorption in alkaline solutions is an illustrative example of an absorption process accompanied by a fast irreversible chemical reaction.1,2 The system has industrial applications such as CO2 removal from synthesis gas in ammonia manufacture,3 CO2 removal from flue gases,4 environmental applications,5 and applications of medical interest.6,7 Moreover, extensively studied kinetics8 eliminate the uncertainties introduced by kinetic parameters and make this system suitable as a test reaction to evaluate the performance of gas-liquid reactors in terms of mass transfer coefficients and interfacial area. Reactor types investigated using CO2 absorption in alkaline solution, usually NaOH or KOH, are packed bed absorbers,5,9,10 rotating packed bed absorbers,11 external loop gas-lift reactors,12 spray-tower-loop absorbers,13 bubble columns,14 and reactors that use thin liquid films either as falling films,15-17 or as horizontal films.18 Falling films are widely used for gas-liquid reactions such as sulfonation, chlorination, ethoxylation, or hydrogenation.19,20 The main characteristic of such reactors is the motion of a thin layer of liquid over wetted surfaces under the action of gravity. Their main advantages include high capability for heat removal and minimization of mass transfer resistance in the liquid phase. For conventional units, achievable interfacial * To whom correspondence should be addressed. E-mail:
[email protected].
area is between 300 and 600 m2/m3.21 Recent developments in microtechnology22 allowed the manufacture of microstructured reactors where film thickness under 100 µm can be achieved, resulting in interfacial area higher than 10 000 m2/m3.23,24 This represents about 1 order of magnitude higher interfacial area than for conventional falling film reactors. The microstructuring of the plates helps film stabilization and avoids Rayleigh-Taylor instabilities which can be encountered in films falling over a flat surface.25 Extensive modeling work of conventional falling film reactors is available in the literature. A review of models for coupled heat and mass transfer in falling film absorption has been presented by Killion and Garimella.26 The authors provide detailed discussion of governing equations, boundary conditions, assumptions, solution methods, results, and validation for gas-liquid reactions in falling film published by various researchers. The models available in the literature for fallingfilm absorption use mostly numerical correlations to estimate the friction factor at the gas-liquid interface as well as coefficients for mass and heat transfer. This simplifies the model but introduces adjustable parameters that make the model system dependent.20 However, complex models that solve together momentum, mass, and energy balances are complicated and they have been solved only for physical absorption.27 Models for strongly exothermic absorption and reaction in falling films are reviewed by Villadsen and Nielsen,28 who support the idea of division of the overall model into submodels for the liquid phase, gas phase, and
10.1021/ie049726k CCC: $30.25 © 2005 American Chemical Society Published on Web 02/22/2005
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interface. It is also pointed out that simplifications of hydrodynamics for both phases must be used with care, especially if film instability or variable film thickness is involved. Davis et al. presented a mathematical model for a falling-film reactor where heat effects have been included.29 van Dam et al. presented a mathematical model which was a reasonable compromise between accuracy and computational effort.30 They used twodimensional models for liquid and gas phases coupled by means of the gas-liquid interface boundary conditions expressed by physical equilibrium and equality of molar fluxes for the absorbed component. The model was used for kinetic studies of sulfur dioxide absorption in organic and aqueous solvents where a pseudo-firstorder reaction with respect to SO2 was present. In the present work, a two-dimensional model is formulated and solved for CO2 absorption in NaOH solution in a falling-film microstructured reactor. Reasonable simplifying assumptions are applied to balance model accuracy and computational effort. Comparison between the model and experimental results is also carried out.
Table 1. Values of h Parameters for Eq 6, at 298 K
2. Reaction System
Figure 1. Schematic of microstructured falling film reactor with its components.
i 1 2 3 4 G
hi, m3 Kmol-1
component Na+
0.1171 0.0756 0.1372 0.1666 -0.0183
OHHCO3CO32CO2 (l)
The reaction system considered refers to the absorption of CO2 from a N2-CO2 mixture within a solution of NaOH. The reaction steps occurring during absorption of CO2 into aqueous solutions of hydroxides can be expressed by the following equations:1,8
The rate of reaction 3 is significantly higher than that of reaction 2. Hence, reaction 2 governs the overall rate of the process and follows second-order power law kinetics.8
CO2 (g) S CO2 (l)
(1)
r ) kOH- cOH- cCO2
CO2 (l) + OH - w HCO3-
(2)
HCO3- + OH - S CO32- + H2O
(3)
The rate constant, as shown by Pohorecki and Moniuk,8 depends not only on temperature but also on the solution ionic strength according to
log(kOH-) ) 11.916 -
The overall reaction can be written as
2NaOH + CO2 f Na2CO3 +H2O
(4)
Reaction 1 represents the process of physical dissolution of gaseous CO2 into the liquid solution. As the rate of this process is comparatively high, equilibrium at interface is described by Henry’s law.2
cCO2|interface ) HPCO2
(5)
where H is the equilibrium solubility of CO2 in the liquid phase. The solubility of a gas into an electrolyte is influenced by the ionic strength of the solution.2 Schumpe31 studied the effects of dissolved salts on the solubilities of gases and proposed an empirical model to correlate the solubility with the content of ions in a solution as
log
( ) H
Hwater
∑i (hi + hG)ci
)-
(6)
The parameters hi are characteristic to each ion present in the solution, while hG refers to the absorbed gas in the liquid phase and are given in Table 1.31 Data of CO2 solubility in water at temperatures from 273 K to the critical point are available in the literature.32 At 293 K, the equilibrium solubility of CO2 in water is 0.039 Kmol/(m3 atm).
2382 + 0.221I - 0.016I2 T
(7)
(8)
The solution ionic strength, I, can be calculated from the ion concentrations, ci, in the solution and their valence zi as
I ) 0.5
∑i cizi2
(9)
The absorption rate of CO2 in the solution can be calculated by solving simultaneously equations 5-9. 3. Reactor Design and Experimental Conditions The falling film microreactor used in this work has been described in previous publications22,33 and is shown in Figure 1. The gas-liquid contact is facilitated by means of a metal plate with multiple open top microchannels. The liquid phase is distributed in the microchannels through a slit. Thus, the liquid splits into substreams entering each channel at the top of the plate flowing down as a liquid film to a withdrawal zone at the bottom. Two different geometries of the reaction plate have been utilized. The first one contains 64 microchannels of width and depth of 300 µm × 100 µm; this reactor configuration will be referred to as FF I. The second reaction plate contains 16 microchannels of width and depth of 1200 µm × 400 µm and will be referred as FF II. The length of microchannels exposed to the gas flow chamber is 66.4 mm. For each plate, a
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Figure 2. Domains of the mathematical model with their coordinate system.
constant overall liquid flowrate has been utilized; 50 mL/h for FF I and 200 mL/h for FF II. The liquid flowrates have been chosen to give similar residence times for the liquid phase in the two plates. The liquid phase is a solution of NaOH with a molar concentration of 0.1, 1, and 2 M. The temperature of the plate is kept at 20 °C by water that recirculates through a heat exchanger at the back of the plate. The gas-phase can be guided co- or counter-currently related to the liquid phase. The gas flow chamber has a depth of 5.5 mm or 2.5 mm. The gas phase consists of a mixture of N2 and CO2 with a concentration of CO2 between 0.8 and 100%, flowing at almost atmospheric pressure. The reactor top plate contains a glass window, for flow inspection. In all experiments, the liquid volume flow in the microreactor was controlled using a Knauer HPLC pump. Temperature was monitored by resistance thermometers, and the gas flow was set by Bronkhorst mass flow controllers. Reactor operation was initiated by closing both gas inlet and outlet, as well as liquid outlet, and flooding the reactor with the liquid phase (filling the whole gas chamber with liquid). After flooding, the liquid HPLC pump was set at the required flowrate (i.e., 50 mL/h), and simultaneously a Masterflex peristaltic suction pump was connected to the liquid outlet, so that the excessive liquid can be removed fast. The suction pump flowrate was then set at the same value as the inlet pump, and steady state for the liquid flow was achieved, the liquid phase flowing only through the designated channels. At this point, the gas flow was initiated and the suction pump flowrate was adjusted in order to avoid the presence of gas bubbles in the outlet liquid. This procedure ensured a stable steady-state operation, a uniform distribution of the liquid, and no flooding or drying on the liquid side for all NaOH solutions utilized. Once the liquid sample was collected, its OH- content was analyzed by HCl titration.
ficulties. Consequently, a number of simplifications can bring an acceptable compromise between the model complexity and accuracy. Along these lines several aspects are discussed below. Although CO2 absorption and the chemical reaction are exothermic, experimental and theoretical studies related to temperature measurements of a film formed over a hollow sphere revealed that for absorption of pure CO2 in a film thickness of 170 µm of 2 M NaOH, the temperature rise is only 5.5 K.35 In this work, smaller film thicknesses are obtained, while heat exchange medium (water is flowing on the back of the plate) removes any heat produced. Furthermore, thermographic imaging measurements of the falling film reactor showed that the temperature rise is below 1 K.36 Hence the reactor is modeled as isothermal. 4.1. Hydrodynamics. 4.1.1. Liquid Phase. For laminar flow the velocity vector has only one component, the axial velocity vl. Considering that during the process the liquid density and liquid flowrate do not vary significantly and that the pressure drop can be neglected (Reynolds number
