Thermographic Investigations of a Microstructured ... - ACS Publications

A new microstructured thin film reactor that allows for thermographic imaging of the reaction through the thin metal foil along which the liquid flows...
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Ind. Eng. Chem. Res. 2010, 49, 10889–10896

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Thermographic Investigations of a Microstructured Thin Film Reactor for Gas/Liquid Contacting Kristin Hecht* and Manfred Kraut* Institute for Micro Process Engineering (IMVT), Karlsruhe Institute of Technology (KIT), Hermann-Von-Helmholtz-Platz 1, 76344 Eggenstein-Leopoldshafen, Germany

A new microstructured thin film reactor that allows for thermographic imaging of the reaction through the thin metal foil along which the liquid flows has been studied with the exothermic absorption of CO2 into aqueous solutions of NaOH. The usefulness of the new method is demonstrated on the extreme differences in temperature obtained for reaction with dry CO2 compared to reaction with prehumidified CO2. The method is able to follow the reaction in a case where complete conversion is reached before the reactor’s end, meaning that the reaction rate changes drastically from a fast beginning to zero at the end. Concentration and liquid transport coefficient profiles can be derived from the measurements obtained with this method. The information obtained indicates that this method can become a useful tool in the study of gas/liquid reactions because the measurements obtained along the length of the reactor allow for a better analysis than those obtained from an integral analysis of the reactor. Introduction Thin film microreactors are interesting devices for gas/liquid reactions due to their exceptional heat and mass transfer characteristics.1 They have proliferated in use due to their ease of operation and accessible design parameters and have been scaled-up into multiple layer reactors enabling higher throughputs.2 In a thin film microreactor, the liquid flows vertically down through channels that are open on one side to a gas. The flow may be gravity driven as in a falling film reactor, or gravity in conjunction with pressure can drive the flow as in the reactor presented here. A cross-sectional representation is shown in Figure 1. The geometric area of the channels exposed to the gas defines an approximate interfacial area, but the curvature or meniscus formed by the liquid surface can increase the area. Yeong et al.3 measured liquid profiles in round-bottom channels etched in stainless steel for ethanol, isopropanol, and acetone in air. They identified changes in the profile due to liquid film thickness and influenced by the contact angle. Zhang et al.4 investigated the liquid flow and mass transport in a falling film reactor constructed of PMMA with rectangular channels. They modified the channels with a sol-gel coating and found that the rate of absorption of carbon dioxide into water was higher for the modified channels, demonstrating the importance of material properties, especially contact angle, on the performance of microreactors. In a similar experiment, Al-Rawashdeh5 demonstrated the inverse effect by coating the channels with a fluorinated polymer. Claudel et al.6 have characterized interfacial area and mass transport coefficients in a falling film microreactor with roundbottom, etched stainless steel channels in various sizes. They reported interfacial areas between 180 and 200 m2/m3 reactor, corresponding to 6000-9000 m2/m3 liquid, and liquid transport coefficients, kL, ranging from 4 × 10-4 m/s to 10-3 m/s depending on plate geometry and flow rate. Zhang et al.4 measured liquid transport coefficients between 5.83 and 13.4 × 10-5 m/s and demonstrated the influence of surface tension and viscosity. They provided a correlation for kL through a * To whom correspondence should be addressed. E-mail: [email protected] (K.H.), [email protected] (M.K.).

Figure 1. Cross-section of thin film microreactor.

correlation of Sherwood number with Reynolds and Schmidt numbers. They found that the penetration model fits the measured values better than film theory, using the thickness of the entire liquid stream for the thickness of the liquid film in their calculations. In the actual theory, the liquid film applies only to a very thin layer of liquid at the interface, which is assumed to be stagnant and across which the concentration of dissolved gas decreases from the interfacial concentration to the concentration present in the bulk liquid, zero for moderate and fast reactions. Simulations of CO2 absorption into NaOH in falling film microreactors from Zanfir et al.,7 Al-Rawashdeh et al.,5 and Chasanis et al.8 have all indicated that CO2 is completely reacted within only a few micrometers from the gas/ liquid interface. Although an exact determination of this film thickness is an intractable problem, a more realistic film thickness would be expected to improve the results obtained with the film model. Thermographic measurements have been previously conducted on a falling film microreactor by Wille et al.9 These measurements were collected through an IR-transparent glass above the gas chamber. They were able to demonstrate the

10.1021/ie100431r  2010 American Chemical Society Published on Web 06/22/2010

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Figure 2. Disassembled thin film microreactor: heat exchange module (left), reaction module (middle), cover (right).

equipartition of the liquid in the channels. A heat exchanger was used on the reverse side of the liquid channels to control the temperature. A small temperature rise of 1.5 °C was nevertheless measured for the absorption of CO2 into NaOH (2.0 M). This paper presents a specially designed reactor and thermographic imaging technique that uses temperature profiles measured along the length of the reactor to calculate the changing rate of reaction. Contact angle measurements for the specific materials have been performed in order to best estimate the shape and size of the interfacial area. The thermographic measurement technique is demonstrated on the example of the exothermic absorption of CO2 into 2.0 M NaOH. The first experiment described shows the significance of heat loss due to the evaporation of the liquid. This effect may not be neglected when assuming that thermographic measurements can be used to predict the temperature in the liquid film. A second example with a system where conversion reaches completion early in the reactor is used to demonstrate the effectiveness in using the method to derive the reaction rate along the reactor length even when it is rapidly changing. Also, kL values have then been calculated from the measurements, and these values are compared to the values measured in the literature according to the typical analysis, where the reaction rate is assumed to be constant in the entire reactor volume. Experimental Methods The microstructured thin film reactor used in this study has been developed at the IMVT to perform highly exothermic gas-liquid reactions. Details of the reactor design are shown in Figures 2-4. At the heart of the reactor is a 2 mm thick microstructured foil containing 33 channels 0.6 mm wide, 1 mm deep, spaced 0.6 mm apart, with a length exposed to the gas chamber of 197.5 mm. A photograph of the foil is shown in Figure 3. The liquid entrance consists of closed microchannels, which forces the liquid into the microchannels and is intended to provide a good distribution of the liquid. The channels are separated by 10 mm from the glass window by the gas chamber, which has a crosssectional area of 622 mm2. The front of the reactor contains six ports for the gas feeds. The first pair of gas entries is located above the point at which the microchannels are open to the gas chamber. The other entries allow for staging of the gas feed. In this study, all gas has been fed symmetrically through the first

Figure 3. Microstructured thin film foil.

Figure 4. Assembled thin film microreactor for thermographic investigations and thermographic image during operation.

pair of gas entries. It is possible to place a heat exchange module in direct contact with the reverse side of the microstructured foil; however, for the purpose of the reported investigations, the heat exchange module has been removed in order to collect thermographic measurements directly on the reverse side of the microstructured foil. The foil was painted black in order to provide a homogeneous heat emitting surface. In the first experiments, the paint was applied with a paint brush; later, the paint was applied with an airbrush. The reactor can be operated either with gas-liquid separation performed in the bottom of the reactor with liquid exiting through the outlet and gas exiting through the bottommost pair of gas ports, or, as was done in this study, the two-phase flow can exit through a single outlet. The new reactor presented here is called a “thin film microreactor.” Thin film reactors consist of a film of liquid running along a wall in contact with a gas phase. This arrangement allows the temperature in the liquid phase to be controlled via heat exchange medium on the opposite side of the wall. The flow of liquid may be pressure driven, gravity driven, or some combination of both. These reactors can also be referred to as “falling film reactors”. The even distribution of the liquid on the walls of this type of reactor can be a challenge. Wipers are sometimes employed to distribute viscous media, and microstructured plates have more recently been used to distribute the liquid among channels in the wall. Several falling film microreactors are commercially available, including those from the Little Things Factory of Illmenau, Germany; Ehrfeld Mikrotechnik, owned by Bayer; Mikroglas Chemtech

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GmbH; and especially those from the Institut fu¨r Mikrotechnik (IMM) of Mainz, Germany. The IMM type falling film microreactor has been used or copied for many characterization studies. The operation of the IMM reactor includes first flooding the entire reactor with liquid and then removing a portion of this liquid with suction. Operation requires two liquid pumps.2 The reactor presented here does not require flooding and requires only a single pump. Several lengths of IMM falling film microreactors are available. The IMM “FFMR-STANDARD” has a channel length of 7.6 cm and that of IMM “FFMRLARGE” is 25 cm.2 The length of the channels in the reactor presented here lie between these values. The typical microstructured foil in the IMM reactors contains round-bottom etched channels although some studies have used rectangular machined channels. The foils in both the IMM falling film reactor and the IMVT thin film reactor are exchangeable to allow modification of the channel geometry. Thermographic images of the reactor were taken with a VarioCAM HR HEAD camera (InfraTec) and analyzed with IRBIS3 software. The reaction of carbon dioxide (CO2) with 2.0 M sodium hydroxide (NaOH) was used as a model exothermic reaction. For a pH greater than 11, the following mechanism applies:10 OH- + CO2(aq) f HCO3-

(1)

OH- + HCO3- f CO32- + H2O

(2)

The overall reaction stoichiometry is 2NaOH + CO2 f Na2CO3 + H2O

(3)

The liquid solution was fed in a pulsation-free, pressure-driven system utilizing a liquid mass flow controller (Brooks 5582, 0-1 kg/h). Pure CO2 was dosed from a cylinder via a gas mass flow controller (Brooks 5850, 0-2 L/min). Except in the experiment explicitly using dry CO2, CO2 was bubbled through water at 60-80 °C, and then fed through a condenser at 15 °C before being introduced into the reactor to attenuate the evaporation of water under the reaction conditions. Liquid samples were analyzed by precipitating CO32- with BaCl2 and titrating the unreacted OH- with HCl using phenolphthalein.11 Contact angles were measured using the sessile drop method. The contact angle measurements apparatus consists of an optical cell (SITEC AG, Maur/Zu¨rich, 740.2086) with sapphire windows for observation of the droplet, gas introduction with pressure regulation (Rotarex/SMT, Genlis, France, SL225-16), and liquid dosing through means of a hand-operated pump (SITEC AG, Maur/Zu¨rich, 750.1400). An Imaging Source C3516-M(KP) (35 mm focal length) lens with a 15 mm extension ring was used with a SONY AVC-D5CE CCD camera. Light (HLV-24SW-NR-3W, CCS Inc., Japan) was passed through a series of lenses to create a parallel beam. Images collected from the apparatus were analyzed with the DropSnake plugin for ImageJ.12,13 A small drop of fluid is released from a syringe suspended over a level, solid surface. The contact angles measured are those spontaneously formed when the drop lands upon the surface. Incremental addition or removal of fluid when the syringe is in contact with the drop results in a virtually unlimited range of angles because the incremental additions alter the contact angle without changing the size of the drop. The solid surfaces measured were prepared as representative conditions of those seen in the actual reactor and should not be interpreted as being accurate for the pure solid materials. The

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Table 1. Contact Angles gas

N2

liquid

2M NaOH

H2O

CO2 1M Na2CO3

glass 15° stainless steel, polished 52° ( 9° 38° ( 7° stainless steel, milled 50° ( 9° 33° ( 5°

35° 48° 44°

H2O

1M Na2CO3

56° 52° ( 5° 48° ( 4° 67° ( 5° 50° ( 2°

solid surfaces used for measurement were: a common glass microscope slide (Menzel-Gla¨ser), polished stainless steel (3 µm abrasive paper; ATM Saphir 550, 120/min), and milled stainless steel (EMCO FB-3, dry, no cooling; 35 mm HSS shell end mill; 160/min, 150 mm/min). The milled surface provides a surface roughness comparable to those of the microchannels. For stainless steel, the surface roughness of the polished surface was measured (Mahr M2) to be 0.012 µm average/0.333 µm maximal over a 5.6 mm length and 0.696 µm average/7.337 µm maximal for the milled sample. Solid samples were cleaned with deionized water and isopropanol and dried prior to measurement. Measurements were collected for 2 M NaOH in a nitrogen atmosphere (1.25 bar), 1 M Na2CO3 in nitrogen (1.25 bar) and carbon dioxide (1.5 bar), and deionized water in nitrogen and carbon dioxide (2 bar). It is not possible to measure NaOH in a carbon dioxide atmosphere because the reaction proceeds quickly. Na2CO3 also reacts with CO2 to form NaHCO3 but much more slowly. The contact between the Na2CO3 and CO2 was limited to the less than 2 min needed for the measurement in order to minimize the effect of this reaction. Results and Discussion The contact angle, a property dependent on the surface energies of the solid, liquid, and gas phases formed at a threephase boundary, plays an important role in determining the profile of the fluid in the channels. Small contact angles indicate a system where the liquid easily wets the solid surface; large contact angles indicate low wettability. Zhang et al.4 have correlated the contact angle with the fluid profile in rectangular channels according to the following equation: b′ )

bπ 90°-θ cos θ 180°

(4)

b represents the width of the channel; θ is the contact angle, and b′ is the profile or gas/liquid contact line. Better wettability is viewed as a positive attribute because it increases the interfacial area and also improves the operability of the system. Contact angle measurements are summarized in Table 1. Standard deviations are given when multiple measurements have been taken, a measurement representing the complete process of applying a drop to the solid surface. The radius of the liquid/solid contact line ranged from 3-8 mm for the drops measured; at this size, the contact angle is independent of drop size. NaOH is the best wetting system. Na2CO3 is less wetting that NaOH, and water is less wetting that either reaction mixture. The contact angle of water in N2 agrees well with the measurements of Ponter and Yekta-Fard.14 Most systems demonstrate reduced wettability in a CO2 atmosphere in comparison to N2. The effect of roughness is within the error of the measurements and results, therefore, in no apparent pattern. It is interesting to note that although NaOH/N2 is much more wetting on glass, the product mixture of Na2CO3 in CO2 actually has a higher contact angle on glass compared to stainless

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Figure 5. Temperature profiles measured along length of reactor for reaction with dry gas and reaction with prehumidified gas.

steel, and thus, for this reaction system it is not clear if a glass reactor will demonstrate the advantage of better wettability for NaOH in CO2. Zhang et al.4 have demonstrated this advantage in terms of a higher rate of absorption for the absorption of CO2 into water alone on a glass surface compared to a stainless steel surface. In this work, a contact angle of 50° has been assumed as representative for reaction of 2 M NaOH with CO2. Temperature profiles were measured in the 33-channel foil with and without gas prehumidification. Both measurements were collected for flow rates of 30 mL/min liquid and 1 L/min gas. The resulting temperatures are shown in Figure 5. A temperature rise of 8 °C was observed for the reactor using dry gas, and a temperature rise of 16 °C was observed for reaction with humid gas. For reference, the adiabatic temperature rise calculated from the enthalpies of absorption and reaction from Danckwerts15 (∆Hr ) -26.2 kJ/mol) is 12.5 °C. Calculating the adiabatic temperature rise from thermodynamic properties16 (∆Hr ) -105 kJ/mol) yields a much higher value of 51.7 °C. The temperature profiles were fitted with smoothed curves in order to reduce the influence of the normal variation in the temperature measurements. The humidification of the gas has a considerable effect on the absolute temperature measured. The effect on the conversion was negligible; a conversion of 78% was determined via titration for the dry case, and a conversion of 76% was measured for the prehumidified reaction. Previous work has demonstrated a good agreement between models assuming isothermal reaction operation and experiment results.7,8 Wille et al.9 performed thermographic measurements of the transient operation of an IMM falling film reactor while controlling the temperature at 25 °C. They observed a temperature rise of 1.5 °C and noted it was close to the value estimated from the information given by Danckwerts.15 The measurements presented here indicate a more than 10 °C temperature loss due to evaporation of the liquid. Although the measurements presented here are obtained without an active attempt to control the temperature within the channels, these results connote a need for carefully evaluating specious claims of isothermality even in microreactors because heat losses due to evaporation at the gas/liquid interface can mask the temperature rise that would otherwise be apparent. The presented measurements were collected from a reactor that had been operated for a longer period and had reached a steady temperature. In general, the simplification of isothermal behavior in a microreactor should be verified and not only assumed. A temperature profile measured along the center of the foil for a very low liquid flow rate (5.2 mL/min liquid, 1 L/min gas) is shown in Figure 6. Prehumidified gas was used.

Figure 6. Temperature profile along length of reactor at high conversion.

The conversion should be maximized under these conditions; it was measured to be 100% via titration. Indeed, it can be seen that the temperature does not rise in the last third of the reactor, meaning that the bulk of the conversion occurs in the first half of the reactor. The temperature rise, ∆T, in a segment, i, is calculated as the difference between two adjacent temperature measurements. These calculations are based on the fitted curves. ∆Ti has been assumed to be proportional to the rate of reaction in that portion of the reactor. Thus, the contribution of a segment to the overall conversion has been defined: contributioni )

∆Ti

∑ ∆T

(5) i

i

Any increase in temperature is due to reaction. When the temperature begins to sink, the reaction does not occur at a rate sufficient to replace the heat being lost, and the rate is assumed to be zero. This approach implicitly assumes uniform heat losses along the channel. Temperature values are taken from the center channel of the reactor, where conduction from the symmetrical channels minimize any heat losses at the center. Multiplying the contribution factor by the overall conversion, X, conversion and concentration profiles along the reactor can be obtained. N is the number of temperature point measurements from the beginning of the reactor to the end, and N - 1 is the number of segments into which the reactor is divided. N-1

∑ contribution X

(6)

CNaOH ) (1 - X)CNaOH0

(7)

Xi )

i

i)0

CNa2CO3 )

XCNaOH0 2

(8)

Conversion and concentration profiles calculated from the dry and humid temperature profiles are shown in Figure 7. They differ only slightly. Profiles calculated for the high conversion case are shown in Figure 8. The profiles change rapidly in the first half of the reactor, but are completely flat by the last third. This analysis can be extended to calculate the liquid transport coefficient along the reactor length according to the two film theory, depicted in Figure 9. It is assumed that the gas consists of pure CO2 at a uniform pressure, P; then, the concentration

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simpler model is appropriate for a first analysis of the thermographic measurements. The enhancement factor, E, is the ratio between the rate of transport through the liquid film under actual conditions (with reaction) and the rate of transport for pure physical absorption. An enhancement factor for an instantaneous reaction, Ei, can be derived by assuming that the reaction reaches completion at a certain depth within the liquid film. The concentration of both species is 0 at this plane, and the reaction rate is determined by the rates of diffusion for each species:

(

Ei ) 1 + Figure 7. Profiles of conversion and concentration obtained with dry CO2 and prehumidified CO2.

DNaOHCNaOH 2DCO2C*CO2

)

(11)

The reaction between NaOH and CO2 to form Na2CO3 is a fast, second-order reaction with a rate constant of 104 L/mol · s:15 -r ) kCNaOHCCO2εL

(12)

where r is the reaction rate and εL is the liquid volume fraction. At steady-state, the rate of reaction is equal to the rate of transport through the liquid film. Combining eqs 10 and 12 along with eq 9 results in an expression for kL: kL )

Figure 8. Profiles of conversion and concentrations of NaOH and Na2CO3 along the length of the reactor with high conversion.

ΦHkCNaOHεL PaEkCNaOHεL - ΦHaE

(13)

Every term in the expression for kL is known except for E, the actual enhancement factor. The Hatta number, Ha, is the ratio between the maximum conversion possible in the film and the maximum transport through the film. It is dependent on kL. Ha )

√DCO kCNaOH 2

kL

(14)

Hatta in combination with Ei provides critical information regarding where the reaction takes place, for example in the liquid film alone (Ha > 3) or in both the film and the bulk (0.3 < Ha < 3). It is still possible to determine the real enhancement factor without making a simplifying assumption regarding the location of the reaction by using an approximate expression relating E to Hatta and Ei, such as that from van Krevelen and Hoftijzer17

 

Ha

Figure 9. Two film theory.

E)

of dissolved CO2 at the gas/liquid interface, C*CO2, can be described according to Henry’s law: P ) HC*CO2

(9)

The Henry constant has been calculated according to Danckwerts’s recommendations for electrolytic solutions.15 According to the two film theory, the transport of CO2 through the liquid film is described by Φ ) kLaE(C*CO2 - CCO2)

(10)

According to the Higbie penetration model and Danckwerts surface renewal model, kL is proportional to the square root of the diffusion coefficient. According to Nernst and the two film theory, they are proportional. However, the other models include additional parameters, and since the more complex models typically differ little from the simpler two film model, this

tanh

Ei - E Ei - 1

Ei - E Ha Ei - 1

(15)

When substituting eqs 13 and 14 into 15, an expression for E in terms of known variables can be derived, and E can then be calculated. The derivation of kL from absorption or reaction measurements is typically performed on the basis of the whole reactor volume because in most cases only outlet concentrations and not the concentration profiles within the reactor may be measured. As demonstrated, thermographic measurements along the length of the reactor enable concentration profiles to be inferred, albeit with several assumptions and their intrinsic uncertainties. Calculating Ei, E, kL, and Ha as average values from the inlet and outlet concentrations yields the values given in Table 2. When Ei is much greater than Ha, as in the first two cases, then the reaction is pseudo-first-order, meaning that the reaction is limited by the absorption and transport of CO2 and that the

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Table 2. Average Values for the Enhancement Factor for an Instantaneous Equation, Actual Enhancement Factor, Liquid Transport Coefficient, and Hatta Number measurement

Eia

Eb

kL × 104 (m/s)c

Had

(1) dry CO2 (2) humidified CO2 (3) maximum conversion

17.96 17.88 15.37

1.31 1.31 3.39

40 39 3.65

1.37 1.42 13.7

a Enhancement factor for an instantaneous reaction. b Actual enhancement factor. c Liquid transport coefficient. d Hatta number.

concentration of NaOH can be considered constant because it diffuses quickly to the surface, preventing depletion. Pseudofirst-order reactions can be used to characterize microreactors due to the fact that in this regime the enhancement factor is roughly equal to the Hatta number, simplifying the analysis. The determined values of kL are higher than values published by Claudel et al.6 and Zhang et al.,4 but a higher conversion was used in the thermographic measurements in order to show a clear temperature profile, while for characterization of a gas/ liquid contactor, a low conversion is desirable in order to minimize the effects of concentration and temperature differences. This example, however, demonstrates a real difficulty in characterizing microstructured contactors: conversions are often high; equilibrium is quickly reached, and previously reported data for the reaction of CO2 with NaOH cannot be utilized without models capable of interpreting measurements from high conversions. Numbers obtained from the integral analysis of the maximum conversion case differ substantially from those in the previous cases although the reaction conditions should be similar. It is interesting to note that the kL value calculated in this case is similar to those measured by Claudel et al.6 The Hatta number from the integral analysis is significantly larger, indicating a closer approach to an instantaneous reaction that occurs at the interface or within the film although not to such an extent that E is equal to Ei. The numbers of this analysis are certainly less reliable than those obtained from the integral analysis of the previous two cases because the change in the rate of absorption is significant. The average absorption rate is much lower than the rate for conditions representing the reaction conditions, and kL is therefore underestimated. Applying the same analysis as for the whole reactor but instead to small slices of the reactor is possible from the concentration profiles calculated from the thermographic measurements. The results of this analysis are shown in Figures 10-13. Ha, E, and kL shown in Figures 10 and 11 match the average values calculated in Table 2. Values of Ha and E are somewhat constant and nearly equal, indicating a pseudo-firstorder reaction as also determined in the integral analysis. In this case, the assumption made in the integral analysis that the rate of reaction is constant throughout the reactor does not introduce a large error. It can still be seen that the absorption rate falls toward the end of the reactor. The values of the high conversion case, shown in Figures 12 and 13, differ substantially from the average values. The kL values from this analysis are substantially higher than the average values, and the corresponding Hatta numbers are much lower. Instead of indicating a reaction occurring only in the film as in the integral analysis, the Hatta numbers place the majority of the reaction occurring in the liquid film with a minimal concentration of CO2 reaching the liquid bulk, similar to the other two experiments with lower conversions. E and Ha are approximately equal in this analysis, indicating a pseudofirst-order reaction rather than something closer to an instantaneous reaction at the interface. The integral analysis introduces

Figure 10. Enhancement factor and Hatta number along reactor profile for experiments with dry and humidified CO2.

Figure 11. Liquid transport coefficients along reactor profile for experiments with dry and humidified CO2.

a significant error by assuming a constant reaction rate throughout the reactor, but the analysis enabled by the thermographic measurements provides reasonable values of E and Ha despite the changing rate of reaction. The curves shown in Figures 12 and 13 are smoother than those in Figures 10 and 11. The small variations in the temperature measurement, which were not apparent in the original data, contribute to variation in the calculated values of E and Ha. This variation was reduced in Figures 12 and 13 by improving the uniformity of the paint layer using an airbrush. The kL value is not constant but steadily decreases along the length of the reactor in every analysis. This stems from the fact that kL is calculated from the absorption rate, which also decreases along the reactor length in each of the examples presented here. However, the analysis of each slice takes into account the differences in the rate of absorption due to the changes in the concentrations. Further modeling is needed to reconcile these observations. Danckwerts15 has perspicaciously

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result. Ziegenbalg et al. have demonstrated that bas relief structures in the floors of the channels increase conversion, although the experiments are difficult to interpret without data describing the effect of these structures on the interfacial area. The influence of these structures may be useful in breaking up this rigid film through changes in the flow patterns, much like Marangoni effects. • Departure from stagnant, semi-infinite behavior: This is obviously applicable to experiments with high conversions. However, this is considered in the analysis, and it does not explain the differences observed between the average values and the values determined along the length. Conclusions

Figure 12. Enhancement factor and Hatta number along reactor profile for near complete conversion.

This contribution presents a new reactor and method. A microstructured thin film reactor modularly designed to accommodate thermographic imaging through the reverse side of the liquid distribution foil has been studied with the exothermic reaction of CO2 into an aqueous solution of NaOH. This new method enables the tracking of the reaction along the axis of the reactor. The latest results suggest that assumptions of isothermal operation, even in microreactors, should be carefully evaluated on a case by case basis. The new method has been applied to situations where the reaction rate changes rapidly along the reactor length, namely when full conversion is reached well before the end of the reactor. A quantitative analysis of the temperature profile is now possible, and conversion and concentration profiles can thence be derived as a function of reactor length. The profiles have been used to determine liquid transport coefficients along the reactor length. The average values are slightly higher than those presented in the literature. This is supposed to be a result of the relatively large changes in concentration and temperature in the reactor. The liquid transport coefficient was observed to decrease along the reactor length. This observation can be explained by the influence of entrance effects, rigid film, and departure from stagnant, semiinfinite behavior. This analysis demonstrates the difficulties encountered in obtaining good design data for microreactors but also demonstrates the potential of this method to improve the general understanding of gas/liquid processes. Literature Cited

Figure 13. Liquid transport coefficient along reactor profile for near complete conversion.

described several phenomena that contribute to the same observation in wetted wall columns: • Entrance effect: The small gap distributing the liquid at the reactor entrance causes the fluid to initially have a higher velocity that the terminal velocity approached further down the column, increasing the initial rate of absorption. Danckwerts suggests designing reactors to avoid this effect, but this effect surely becomes more important and abatement more difficult in microreactors and should rather be studied so that microstructured devices can be better modeled and designed. • Rigid film: Although the diffusivities of the respective species are high according to the calculations, they can build up at the interface reducing the rate of absorption at the bottom of the reactor. At high conversions the concentration gradients driving diffusion additionally decrease. Poor mixing within the liquid bulk would produce the same

(1) Hessel, V.; Angeli, P.; Gavriilidis, A.; Lo¨we, H. Gas-Liquid and Gas-Liquid-Solid Microstructured Reactors: Contacting Principles and Applications. Ind. Eng. Chem. Res. 2005, 44, 9750–9769. (2) Vankayala, B. K.; Lo¨b, P.; Hessel, V.; Menges, G.; Hofmann, C.; Metzke, D.; Krtschil, U.; Kost, H. Scale-up of Process Intensifying Falling Film Microreactors to Pilot Production Scale. Int. J. Chem. Reactor Eng. 2007, 5, A91. (3) Yeong, K. K.; Gavriilidis, A.; Zapf, R.; Kost, H.; Hessel, V.; Boyde, A. Characterisation of Liquid Film in a Microstructured Falling Film Reactor Using Laser Scanning Confocal Microscopy. Exp. Therm Fluid Sci. 2006, 30, 463–472. (4) Zhang, H.; Chen, G.; Yue, J.; Yuan, Q. Hydrodynamics and Mass Transfer of Gas-Liquid Flow in a Falling Film Microreactor. AIChE J. 2009, 55, 1110–1120. (5) Al-Rawashdeh, M.; Hessel, V.; Lo¨b, P.; Mevissen, K.; Scho¨nfeld, F. Pseudo 3-D Simulation of a Falling Film Microreactor Based on Realistic Channel and Film Profiles. Chem. Eng. Sci. 2008, 63, 5149–5159. (6) Claudel, S.; Nikitine, C.; Boyer, C.; Font, P. In Proceedings of the 2005 AIChE Spring Meeting, 8th International Conference on Microreactor Technology (IMRET 8), April 10-14, 2005; Omnipress: Atlanta, GA, 2005. (7) Zanfir, M.; Gavriilidis, A.; Wille, C.; Hessel, V. Carbon Dioxide Absorption in a Falling Film Microstructured Reactor: Experiments and Modeling. Ind. Eng. Chem. Res. 2005, 44, 1742–1751. (8) Chasanis, P.; Lautenschleger, A.; Kenig, E. Numerical Investigation of Carbon Dioxide Absorption in a Falling-Film Micro-Contactor. Chem. Eng. Sci. 2010, 65, 1125–1133.

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ReceiVed for reView February 26, 2010 ReVised manuscript receiVed May 28, 2010 Accepted June 1, 2010 IE100431R