Liquid-Film Mass-Transfer Coefficient in a Column Equipped with

The aim of the present work is to examine liquid film and to identify a relation that allows accurately evaluating the liquid film contribution to ove...
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Ind. Eng. Chem. Res. 1997, 36, 3792-3799

Liquid-Film Mass-Transfer Coefficient in a Column Equipped with Structured Packings Elisabetta Brunazzi and Alessandro Paglianti* Department of Chemical Engineering, Industrial Chemistry and Materials Science, University of Pisa, 56126 Pisa, Italy

The effective mass-transfer efficiency in a column equipped with structured packings was investigated experimentally. The packings analyzed in the present work were Sulzer Mellapak 250Y, made of corrugated plastic or metal sheets, and Sulzer BX, made of plastic gauze. In the literature many works have been published on this topic, but they have mainly been oriented toward evaluating overall mass-transfer efficiency or the gas-side coefficient, leaving out closer examination of the liquid-side mass-transfer coefficient. The aim of the present work is to examine liquid film and to identify a relation that allows accurately evaluating the liquid film contribution to overall mass-transfer efficiency. Experimental results showed that column packing height affects the liquid-side mass-transfer coefficient at the usual industrial-scale liquid and gas flow rates. This result agrees with theoretical analysis on stable liquid rivulet flow. Introduction Structured packings represent a very positive innovation compared with irregular packings. Columns working with this type of packings combine low-pressure drop with excellent mass-transfer efficiency, and thus their use allows reducing both operating costs and column dimensions (Brunazzi et al., 1996). Notwithstanding these positive characteristics, few models are available to design the equipment. The first problem to solve in designing the equipment is to evaluate the effective area available to the mass transfer. Some semiempirical models have been published for this purpose. Most of them agree in identifying the effective surface area of the structured packings as a fraction of the geometric surface (Brunazzi et al. (1995a); Nardini et al. (1996); Bravo et al. (1992); Billet and Shultes (1993)). On the contrary, Henriques de Brito et al. (1994) suggested that, depending on the liquid and gas flow rates, the effective mass-transfer area can be considerably larger than the geometric one. Brunazzi et al. (1995a) limited the number of empirical parameters used to evaluate the surface available to the mass transfer and showed that it can be theoretically evaluated if the liquid holdup of the column is known. Their model, developed on the Mellapak 250Y packing type, has also been tested on gauze packings (Nardini et al. (1996)) with good results. It has been shown that this model allows one to predict not only the masstransfer efficiency (Brunazzi et al. (1995a)) but also the pressure drop in gauze and sheet packings (Brunazzi and Paglianti (1997)). For this reason computation of the surface area taking part in the mass-transfer process is based on this model in the present work. Unlike for gas-phase mass-transfer coefficients, little data are available in the open literature for computing the liquid-side mass-transfer coefficient, probably, as suggested by Laso et al. (1995), because suppliers wish to preserve their proprietary know-how. To our knowledge, the little data published so far regard columns equipped with Mellapak (Laso et al., 1995) and Montz A2 (Weiland et al., 1993) structured packings. * Author to whom correspondence should be addressed. Fax/Tel.: +39.50.511266/511225. E-mail: [email protected]. S0888-5885(97)00045-6 CCC: $14.00

Figure 1. Schematic diagram of a structured packing.

This contribution analyzes the behavior of two of the most commonly used packings, Mellapak 250Y and BX, both made by Sulzer. Experimental Apparatus The test rig consisted of a Pyrex glass column with 2 m height and 100 mm inner diameter. Experiments were carried out by varying packing height in the range of 0.42-1.89 m. In the present work, plastic BX packings and both stainless steel and plastic Mellapak 250Y packings were tested. From the geometric point of view (see Figure 1), the packings are made of corrugated gauzes or sheets arranged in parallel, successive layers having an opposite angle of corrugation. Flow channels resulting from this arrangement are inclined by an angle of 60° (the BX type) or 45° (the Mellapak type) to the horizontal. The particular form of the packings makes it possible to obtain a high-specific surface, 500 m2/m3 (BX type) or 250 m2/m3 (Mellapak 250Y type), with truly minimal reduction in free volume. Different sets of experiments were carried out during the work. Since we aimed at a correct evaluation of the liquid film resistance, a first set of experiments, as suggested by Billet (1989), concerned the desorption of CO2 from water into air. In order to verify whether the developed relation was suitable to predict the performance of packed columns in a common absorption © 1997 American Chemical Society

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effect of packing height must be accurately verified. Indeed, in a recent work on absorption into stable rivulets, Nawrocki and Chuang (1996) showed, both from an experimental and a theoretical point of view, that the flow distance on an inclined plate is considerably important in the mass-transfer process. Experimental absorption results obtained by these authors, using air and water as working fluids, showed that in a log-log plot the relation between the liquid Sherwood number and the inverse of the Graetz number is a straight line. Finally, these experimental observations allow us to assume that

ShL ∝ GzB Figure 2. Schematic diagram of the experimental test rig.

process, a second set of experiments concerned the absorption of chlorinated compounds from waste air. Figure 2 illustrates schematically the experimental loop that was used in the present work. The column could work both cocurrently and countercurrently. For the desorption tests, tank D1 was used to saturate the liquid phase with pure carbon dioxide supplied from a cylinder. During these experiments the solution was fed to the top of the column from tank D1 by means of a displacement pump. After coming into contact with the gas phase, flowing cocurrently, the liquid was discharged from the bottom of the column into tank D2. The desorption efficiency was measured by analyzing the CO2 content in the liquid phase. Samples of the inlet and outlet liquids were taken using appropriate sample valves and analyzed by titration procedures. The absorption of chlorinated compounds was carried out in the same column working countercurrently. The liquid phase and the waste air stream came into contact in the test column. The waste air current was obtained by pumping the chlorinated solvent that was to be absorbed into a vaporization system and mixing it with the air stream supplied by a high-pressure line. To absorb chlorinated solvents, two commercial highboiling liquids, manufactured by Hoechst, were used: Genosorb 300, a mixture of polyethylene glycol dimethyl ethers, with a viscosity of 7.7 cP and a surface tension of 0.038 N/m, and Genosorb 1843, a mixture of polyethylene glycol dibutyl ethers, characterized by a viscosity of about 4 cP and a surface tension of 0.030 N/m. These absorbing liquids were chosen because of their favorable properties (Brunazzi et al. (1995b)). Experiments were carried out at atmospheric pressure and room temperature. Liquid flow rates were varied up to 250 l/h and gas flow rates up to 50 m3/h. Liquid-Side Mass-Transfer Coefficient The biggest problem to be solved in designing an absorption/desorption column working with structured or random packing is to evaluate the wetted area and the mass-transfer coefficients. While the common dumped packings have been thoroughly studied, structured packings, notwithstanding their extensive use, are not yet fully understood. Some relations can be found in the literature that allow the liquid mass transfer coefficient to be evaluated for structured packings (Bravo et al. (1992); Henriques de Brito et al. (1994); Billet and Schultes (1993)), and all of them neglect the dependence on gas velocity and packing height. While the effect of gas velocity is certainly negligible for gas Reynolds numbers below the loading conditions, the

(1)

Since structured packings are composed of inclined sheets, it is reasonable to suppose that eq 1 can be used to evaluate the mass-transfer coefficient. Nevertheless, some modifications are necessary because, with respect to the simple case of the inclined plate, in structured packings many junctions are present. In these junctions some mixing of the liquid phase occurs. Ponter and AuYeung (1982) have shown that when this phenomenon arises, a mixing factor must be introduced. They have also shown that this mixing factor is a function of the liquid Reynolds number and of the Kapitsa number. The equation suggested by Nawrocki and Chuang (1996) can thus be modified to account for mixing effects, assuming the following final form:

GzB ShL ) A C Ka

(2)

where

kLd ShL ) DL

Ka )

σ3FL µ4Lg

δ Gz ) ReLScL H

(3)

In this paper a unique liquid film characteristic dimension, d, has been used both in the Reynolds and the Sherwood numbers. For this reason, ShL has been defined as a function of the film height, δ, and not, as proposed by Nawrocki and Chuang (1996), based on the width of the rivulet profile (see Appendix A). The last parameter which has to be defined is the flow distance, H. Two different assumptions are possible according to the type of liquid mixing at the junctions. In the case of complete mixing, H is a function of the channel dimension, which is also the hypothesis used so far, whereas in the case of partial mixing, H has to be computed as the distance covered by the liquid phase flowing into the column, and thus it represents the path of the liquid phase from the top to the bottom of the column. In the latter case, in addition to the geometric characteristics of the packings, H can also be related to the packing height, Z. The liquid phase flowing in the column certainly follows the line of steepest descent, which is inclined with respect to the horizontal axis by a maximum of 60° for the Mellapak Y types, with little differences linked to variations in crimp angle, and by 69° for the BX type (R in Figure 1). Finally, the flow distance, H, can be computed as

H)

Z sin R

(4)

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Figure 3. Height of the overall liquid-phase transfer unit vs the liquid Reynolds number (Mellapak packings; column diameter 0.1 m; present experimental data air/CO2/water system).

where R is the slope of the steepest descent line with respect to the horizontal axis. Results and Discussion First of all it is necessary to examine the effect of column height. It is well-known that in columns equipped with common dumped packings, when the mixing in the junctions is not complete, packing height influences the height of a transfer unit and this effect cannot be attributed only to a maldistribution of the liquid phase (Ponter and Au-Yeung (1982); Mangers and Ponter (1980)). It must be pointed out that the available relations used so far to compute the liquid-side masstransfer coefficient do not take this effect into account. The present experimental data are plotted in Figure 3 (vertical bars represent uncertainty in the experimental values). The figure shows two different behaviors. For packing heights lower than 1 m the height of the transfer unit remains approximately constant, whereas for packing heights higher than 1 m a critical value of the Reynolds number can be defined, close to 100, below which the mixing in the junction point is not complete. Only above this critical value is it possible to assume that liquid-side mass transfer is not a function of packing height. The influence of column height was also observed by Henriques de Brito et al. (1994), but these authors assumed that the effect was due to liquid maldistribution. This conclusion could be inaccurate because, if it is true, the drip point density of the distributor should become quite important, whereas, as pointed out by the same authors, when the drip point density is changed from 527 to 117 points/m2, the specific effective area changes by only about 2.5%. Instead, it is possible that this effect is due to a change in the liquid-side masstransfer coefficient because, as shown by Nardini et al. (1996), the liquid holdup, and consequently the surface area, remains approximately constant when the packing height is changed. Since all the structured packings are characterized by a low minimum wetting ratesand this is one of the reasons why they are extensively usedsit is necessary to look for an accurate relation to compute the liquidside mass-transfer coefficient in a low Reynolds number range. Nawrocki and Chuang (1996), by analyzing the rivulet flow, which is the typical flow in structured packings, showed that a close relation exists between the Sherwood number and the inverse of the Graetz

Figure 4. (a) Sherwood number vs inverse of Graetz number (Mellapak packings made of metal (Z ) 0.42, Z ) 1.05, Z ) 1.47, Z ) 1.89) and plastic (Z ) 0.96); column diameter 0.1 m; H computed according to eq 4, air/CO2/water system). (b) Sherwood number vs inverse of Graetz number (Mellapak metal; column diameter 0.1 m; H ) de, air/CO2/water system).

number. Even if their analysis does not accurately correlate their experimental results, they proved that in an inclined plate a relation such as eq 1 can be used to predict the correct value of the liquid-side masstransfer coefficient. As pointed out previously, it is necessary to determine whether partial or complete mixing occurs at the junctions. As pointed out before, if a complete mixing occurs at the junctions, as assumed in the papers so far published, H is only a function of the channel dimensions, whereas if a partial mixing occurs, as proposed in the present work, H is a function of the geometric characteristics of the packing and also of the packing height (see eq 4). Figure 4a shows the results in the case of the latter assumption; Figure 4b shows the results in the former case. Finally, the experimental results suggested that, in structured packings, as in dumped ones, the length of the column is an important parameter that cannot be neglected in column design. To define the three constants that appear in eq 2, we used two different sets of data, relative to desorption of CO2 from distilled water and to absorption of chlorinated solvents. Due to the different viscosities and surface tensions of these liquids, analysis of this set of

Ind. Eng. Chem. Res., Vol. 36, No. 9, 1997 3795

Figure 5. Parity plot of KLae. Comparison between experimental values and computed data obtained by using the present model (error mean square ) 0.14) and models proposed by Bravo et al. (1992) (error mean square ) 1.14) and Laso et al. (1995) (error mean square ) 3.94) (Mellapak 250Y metal; column diameter 0.1 m; air/CO2/water system).

Figure 6. Parity plot of KLae. Comparison between experimental values and computed data obtained by using the present model (error mean square ) 0.08) and the model proposed by Bravo et al. (1992) (error mean square ) 0.68) (Mellapak 250Y plastic; column diameter 0.1 m; air/CO2/water system).

data allowed us to quantify the influence of the Kapitsa number (see Appendix B). Figure 5 shows a comparison between the measured and computed values of KLae. In the present model the surface available for the mass transfer was computed as suggested by Brunazzi et al. (1995a) and by Nardini et al. (1996) (see Appendix A). This figure shows that the present model is able to predict the experimental trend of the product KLae under changing working conditions, such as superficial liquid and gas flow rates and column height. The figure also shows that both the model proposed by Bravo et al. (1992) and the model by Laso et al. (1995) are not able to predict the influence of column height since a large spread of data is noticeable. Figure 6 shows that the present model can also be used to predict the experimental value of the product KLae if the column is filled with the plastic type of Mellapak 250Y. The same analysis that was performed in a column equipped with Mellapak 250Y packings was repeated

Figure 7. Parity plot of KLae. Comparison between experimental values and computed data obtained by using the present model (error mean square ) 0.10) and the model proposed by Bravo et al. (1992) (error mean square ) 0.38) (BX plastic; column diameter 0.1 m; air/CO2/water system) [BX metal; column diameter 0.05 m; air/CO2/water system; experimental data by Quaretta (1995)].

for the plastic packing of the BX type. The results are shown in Figure 7. The comparison was also extended to some experimental data produced by Quaretta (1995) using the BX metal packing to investigate the same desorption system. As for the Mellapak 250Y, the agreement between measured and computed values is quite satisfactory in this case as well. Except for the BX metal packing, the experimental data shown so far were obtained in present work. To test the present model, the small amount of data available in the literature was considered. To our knowledge, only two sets of data have been published. One is by Laso et al. (1995) and reports some experimental data on the desorption of O2 using 295 mm inner diameter columns equipped with three different kinds of Mellapak packing, 125Y, 250Y, and 500Y; the other is by Weiland et al. (1993) and reports some data on the absorption of CO2 in a 150 mm inner diameter column equipped with Montz A2 packing, which is similar to the BX type. Figure 8 shows the comparison between the present model, the model suggested by Bravo et al. (1992), and the experimental data published by Laso et al. (1995). It will be noted that the present model is able to predict the measured values with greater accuracy than the model suggested by Bravo et al. (1992). A very important characteristic of the present model is that if used together with the mechanistic model suggested by Brunazzi et al. (1995a) for the computation of the wetted surface, it can also be used to predict the mass transfer in structured packings of different types. The only characteristic that has to be held constant is the superficial manufacture, while both the channel inclination and the value of the geometric surface can be changed. Figures 9 and 10 compare the computed and the measured data obtained by Laso et al. (1995) using columns equipped with Mellapak 125Y and Mellapak 500Y, respectively. These packings differ from the 250Y type only in the different geometric surface value but not with regard to the surface manufacture. Indeed, the Mellapak 125Y has a geometric surface equal about to 125 m2/m3, the 250Y is characterized by a geometric surface of 250 m2/m3, and the 500Y is characterized by 500 m2/m3. For both packings the

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Figure 8. KLae vs superficial liquid velocity. Comparison between experimental and computed data obtained by using the present model (error mean square ) 0.27) and the model proposed by Bravo et al. (1992) (error mean square ) 0.64) (Mellapak 250Y metal; column diameter 0.295 m; air/O2/water system; experimental data by Laso et al. (1995)).

Figure 9. KLae vs superficial liquid velocity. Comparison between experimental and computed data obtained with the present model (error mean square ) 0.43). (Mellapak 125Y metal; column diameter 0.295 m; air/O2/water system; experimental data by Laso et al. (1995)).

suggested model predicts the magnitude of the experimental data. It must be underscored that the errors induced in computing the mass transfer for the 125Y type at high liquid flow rate and low F-factor are probably due to experimental uncertainties. As previously pointed out, the present model can also be used to predict the performances of columns equipped with completely different kinds of structured packing. While the surface of the Mellapak type is an embossed and grooved layer to promote turbulence, the BX type is made of a perforated gauze. For this reason the parameters of eq 2 take on different values (see Appendix B). The BX packing has been studied by Weiland et al. (1993), and the comparison between their experimental and the computed data is shown in Figure 11. Although the experimental data display a large spread, it can be observed that the magnitude of computed KLae agrees with the experimental one. The experimental data on the desorption of CO2 from water into air are important only for testing the available models that can be used to compute liquid-side mass transfer. The final part of the present work was dedicated to verifing if the present model can also be

Figure 10. KLae vs superficial liquid velocity. Comparison between experimental and computed data obtained with the present model (error mean square ) 0.28). (Mellapak 500Y metal; column diameter 0.295 m; air/O2/water system; experimental data by Laso et al. (1995)).

Figure 11. KLae vs superficial liquid velocity. Comparison between experimental and computed data obtained with the present model (error mean square ) 0.43) (Montz A2 metal; column diameter 0.15 m; air/CO2/water system; experimental data by Weiland et al. (1993)).

applied to more complex absorption systems. Two commercial absorption liquids with different viscosities and different surface tensions were tested. The physical properties of the two liquids are shown in Appendix B. The experimental data that will be presented in the following are relative to the absorption of chlorinated solvents. The analyzed systems are the absorption of 1,1,1-trichloroethane, tetrachloromethane, and 1,2dichloroethane using Genosorb 300 or Genosorb 1843 as absorbing liquids. Liquid flow rates were varied in the range 1.2-21.6 m3/(m2‚h), gas flow rates were varied between 0.44 and 1.4 m3/(m2‚s), and the column was 1.89 m high. Experiments were carried out by changing the type of chlorinated solvent to be absorbed, in order to vary the relative weight of the liquid-side masstransfer resistance with respect to the overall resistance. In fact, at these working conditions, while the liquid film mass-transfer resistance is certainly negligible for the 1,2-dichloroethane system, being about 5% of the total resistance, it is no longer true for the tetrachloromethane system, for which the fractional contribution is about 30%. For this reason the experimental data of the overall gas film mass-transfer coefficient were compared with the computed values defined, according

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Figure 12 shows parity plots of the product KGae made with the two different types of liquids. The figure compares the present model and the models suggested by Bravo et al. (1992), Billet and Schultes (1993) (constants suggested by the authors for the Ralu pack 250YC packing, similar to Mellapak 250Y, and appearing in the predicting relations where used), and Laso et al. (1995). Figure 12a shows that the present model seems to be able to predict the experimental data with high accuracy for all the analyzed systems. Figure 12b-d show the comparison between the experimental data and the predicted values obtained using the models of Bravo et al. (1992), Billet and Schultes (1993), and Laso et al. (1995), respectively. Both Bravo et al. and Billet and Schultes tend to overestimate most of the experimental values of the product KGae. Both models induce a large scattering between computed and measured values, especially for changing types of solute and absorbing liquid. By contrast, the model suggested by Laso et al. produces a less scattered trend but tends systematically to underestimate the experimental trend, notwithstanding the correction factor (DL/DO2,L)0.5 introduced in the present work to account for the different diffusivity of the dissolved gas in the liquid phase. Conclusions

Figure 12. Parity plot of KGae. Comparison between experimental measurements with (a) present model (error mean square ) 0.19) and models proposed by (b) Bravo et al. (1992) (error mean square ) 1.20), (c) Billet and Schultes (1993) (error mean square ) 0.82), and (d) Laso et al. (1995) (error mean square ) 0.49) [(f) 1,2dichloroethane/air/Genosorb 1843, (b) 1,1,1-trichloroethane/air/ Genosorb1843, (0) 1,2-dichloroethane/air/Genosorb 300, (O) 1,1,1trichloroethane/air/Genosorb 300, (