Analysis of Liquid Distribution in a Packed Column on a Pilot Scale

S. M. Pizzo and D. Moraes, Jr.*. Departamento de Engenharia Quı´mica, Universidade Federal de Sa˜o Carlos rodovia Washington Luiz, km 235, Caixa Po...
0 downloads 0 Views 78KB Size
Download PDF
2844

Ind. Eng. Chem. Res. 1998, 37, 2844-2849

Analysis of Liquid Distribution in a Packed Column on a Pilot Scale S. M. Pizzo and D. Moraes, Jr.* Departamento de Engenharia Quı´mica, Universidade Federal de Sa˜ o Carlos rodovia Washington Luiz, km 235, Caixa Postal 676, 13565-905 Sa˜ o Carlos (SP), Brazil

F. A. N. Fernandes† Departamento de Engenharia Quı´mica, UNICAMP, Cidade Universita´ ria Zeferino Vaz, distrito de Bara˜ o Geraldo, DPQ/FEQ/UNICAMP, Caixa Postal 6066, 13083-970 Campinas (SP), Brazil

M. S. Kobayasi and R. J. Pazini‡ Departamento de Engenharia Quı´mica, UNIMEP, rodovia Santa Ba´ rbara-Iracema´ polis, km 1.5, 03450-000 Santa Ba´ rbara D’oeste (SP), Brazil

A simple method of quantification of liquid distribution efficiency was employed to characterize a packed column (diameter, 0.4 m; height, 1.8 m), on a pilot scale, operated with 1-in. plastic Pall rings. Two distribution efficiency factorssglobal and dispersionswere defined from a normal tridimensional surface. The global efficiency is derived from the amplitude, and the dispersion efficiency, from the distribution variance. Two series of tests were run. In the first series, just one pipe was employed to feed the column. In the second series, the tests were run with a pipe lateral distributor representative of commercial application. Without the packing, the distance between the distributor and the column base was varied. The greatest dispersion efficiency values from the first series were smaller than the smallest values from the second series, which showed that the packing per se does not promote good distribution. The efficiency values showed that the smaller the distance between the distributor and the packing surface, the better the initial distribution is. Introduction The efficient use of packed columns is directly related to the liquid distribution in them. Despite the fact that some manufacturers recommend the use of liquid distributors at the top of the columns, as troughs and perforated tubes, it is quite common to find columns that lack this device in research centers (Haure et al., 1992; Metzinger et al., 1992) and in many industrial units, since it is believed that the packing itself promotes the distribution of the liquid. It is fundamental that the distribution of the liquid in contact with the gas be homogeneous. The formation of inoperative or stagnation zones must be avoided in the columns, because the mass-transfer processes take place in the effectively wetted regions of the bed section (Leva, 1953; Treybal, 1980; Kister, 1992). Thus, support plates, besides distributors and redistributors, are designed to allow the passage of the gaseous phase with minimal head loss as well as the liquid spreading among the packing modules (Chen, 1984). Great efforts have been dedicated to the study of the composing elements and of the behavior of packed columns since the 1930’s, aimed at the use of this type of equipment in distillation and gas cleaning processes. With the intent of observing the wettability effect in the liquid-phase spread and distribution, Bemer and * To whom correspondence is addressed. Telephone: 55 16 260 82 64. Fax: 55 16 260 82 66. E-mail: deovaldo@ power.ufscar.br. † Telephone: 55 16 983 56 53. Fax: 55 19 788 47 17. E-mail: [email protected]. ‡ Telephone: 55 19 422 15 15, ext. 1540. E-mail: [email protected].

Zwiderweg (1978) utilized a packed column with 0.2-m diameter, filled with Teflon- or gas-coated Raschig rings, besides water-butanol solutions of different compositions in the feeding. They concluded that the bed length, the irrigation rate, and even the wettability did not influence significantly the behavior of the column. The size of the packing elements was the determinant in the variation of the liquid radial distribution. They proposed a factor N, representing the maldistribution, to quantify the anisotropic characteristic of the liquid flow and also verified that this factor increased at low liquid flow rates. Albright (1984) carried out some computer simulations of liquid distribution through a packed column so as to test several types of distributors, such as drip pans, single sprays, and seven-spot sprays. After using different values for the ratio between the diameter of the column and that of the packing elements, he concluded that every type of packing has a natural distribution of liquid flow. Thus, irrespective of how perfect or imperfect the initial distribution is, the liquid flow will tend toward this natural distribution. He also noticed that the heterogeneity of the flow from any distributor might cause serious degradation of the bed performance, even for optimum-design distributors. Bonilla (1983) and Klemas and Bonilla (1995) presented a model of liquid maldistribution that allowed the design of distributors and established criteria of selection and installation for those components. Zuiderweg et al. (1993) considered the radial mixtures of liquid and vapor and proposed a calculation of the liquid distribution and of the packed column efficiency, for a given initial distribution. Furthermore, Gunn and Al-

S0888-5885(97)00785-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 05/30/1998

Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998 2845

Figure 2. Representation of the tube numbering of the sample collecting module. Figure 1. Schematic representation of packed column (study and collecting modules).

Saffar (1993), based on dispersion models, explained the liquid distribution in a bed made with Raschig rings. Kouri and Sohlo (1996) observed the flow patterns in a 0.5-m-diameter column filled with plastic Pall rings of 25 or 50 mm diameter or with Intalox ceramic saddles. They especially noticed the development of flow on the walls. They concluded that the flow profiles were a function of the initial flow rates and distributions of the liquid and gas as well as of the packing section height. The effect of the liquid maldistribution in packed columns is also described by Killat and Rey (1996), who provided a method to determine this phenomenon on a macroscopic scale. They noticed that the division of the packed bed into zones with predistributors eliminated most of the macroscopic maldistribution, although some heterogeneity in the section may have occurred. The objective of this work was to quantify the liquid distribution in a pilot column filled with 1-in. plastic Pall rings, with and without a distributing device, through the application of a mathematical model to determine efficiency. Materials and Methods Equipment. As shown in Figure 1, the equipment utilized in the experiments was composed of a pumping unit and study/collecting modules. The liquid phase (water) was aspirated from a 250-L reservoir, by a centrifugal pump. Then, it went through a rotameter and returned to the reservoir; a three-way valve with total passage deviated the water flow from the tank to the column studied. The collecting module was composed of a site of 21 clear acrylic tubes. These tubes (4-mm thickness, 800mm height, and 52-mm internal diameter) were disposed in a square pitch and had points at the base for the collection of liquid samples. At the top of the site of tubes there was a middle head, and above it there was a column of the same material as the tubes, with 400-mm internal diameter and 1800mm height. In this study module, the packing investigated (1-in. plastic Pall rings) was introduced. On top of this column there was an upper head for the feeding of water, with orifices distributed in a square pitch identical with that of the collecting tube module. Experimental Methodology. The experiments done only with the packing were based on a complete threelevel statistical planning. The result of each one of these experiments was the mass of water collected at the collecting points located at the bases of all 21 lower tubes, numbered as shown in Figure 2.

Once the collecting tubes were disposed in a square pitch, a Cartesian x-y orientation was then established, with the system origin being tube 11 (at the center). After statistical considerations, the pitch step was established to be 1.5. Thus, for instance, tube 3 was located in position (1.5, 3.0). The study variables were the water flow rate, the feeding point on the upper head of the column, and the employed height of the random packing section. The chosen values were 1, 2, and 3 m3/h for the water flow rate and 30, 60, and 90 cm for the packing bed height. The liquid feeding in the column was done at the center of the upper head and at two different positions, equivalent to the radial distances 7.5 and 15 cm (positions of tubes 1 and 6, respectively). Thus, 27 experiments were done involving possible combinations of the influential variable values. It is important to note that a gate valve was placed in the recycling line of the water tank. The adjusted opening of the valve caused the water to undergo a head loss equal to that of the feeding line of the column. Thus, once the three-way valve was turned on, it was possible to reduce the oscillations of the previously adjusted flow rates. A nonrandom process was adopted to carry out the experiments. Therefore, for every packing section height previously chosen, nine experiments were done, increasing the water feeding flow rate in all radial positions, starting from the central one. Hence, the bed features were preserved, such as the arrangement of the rings and consequently the fraction of voids. The column feeding for each height of the packing section was done through the placement of a tube at approximately 5 cm from the top of the packing section, thus preventing the water from spreading before reaching the bed. The obtained mass data were transformed into heights of liquid columns, using the density values as well as the internal diameter of each tube. Therefore, these columns represented the quality of liquid distribution, attributed to the packing and other analyzed variables. Finally, 21 tests were run with a perforated-pipe distributor. This distributor, shown in Figure 3, is composed of 1-in. PVC tubing containing 12 3-mm orifices. The characteristics of this distributor correspond approximately to those of distributors with industrial application, which display 100 distribution points/m2 of bed section. These experiments were performed in two stages. In the first stage, again, those bed heights mentioned previously were used. The second stage consisted of determining the influence of the distributor on the liquid spread. Thus, the distance between the distributor and the sampling module was varied. Four positions were

2846 Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998

Figure 4. Representation of water distribution in experiment 1 of the first series: central water feeding, 1.0-m3/h flow, and 30cm-high packing section. Figure 3. Representation of the utilized distributing device. Dimensions are in millimeters.

chosen for analysis: the top and bottom of the column and the positions at 54.8 and 84.5 cm from the middle head. The liquid flow rates at the feeding were 0.1, 0.3, and 0.6 m3/h for all tests. It was important to study the liquid distribution in this new range of flow rates, given the increase in the number of processes that operate below that range utilized in the first series (1-3 m3/h or 8-24 m3/h/m2), which is present in most of the distillation processes at atmospheric pressure.

The second efficiency defined is the dispersion efficiency, given by eq 3. In this case, the degree of liquid

EfD ) (1 - c) × 100%

spreading is analyzed in the collecting section. Low dispersion efficiency equals a situation in which the collected water has concentrated in certain tubes, depending on the point from which the feeding was done on the upper head. To locate the regions eventually favored by the irregular distribution of liquid, two other factors of center displacement were also defined, as given by eqs 4 and 5.

Results and Discussion The output of each one of the tests was the mass of water collected at the sampling points located at the base of the sampling module, disposed as shown in Figure 2. These masses of water were normalized; i.e., each mass was divided by the total sum of the masses in each test. Then, these normalized values were used in the Sigmaplot software in order to obtain the parameter values of the normal tridimensional distribution model, given by eq 1, chosen to represent the final liquid distribution.

Z ) b ( a exp

[

] [

]

-c(x + d)2 -c(y + e)2 exp 2 2

(1)

In eq 1, a is the normal curve amplitude, b is the curve displacement in relation to the base z ) 0, c is the distribution variance multiplier, and d and e are the distribution variance displacements in relation to x and y, respectively. z is the normalized height of a liquid column for a given pair (x, y). The signal preceding the parameter a may be positive, as when the distribution displays a peak (maximum point), or negative, denoting the presence of a depression in the final liquid distribution (minimum point). From the physical significance of the parameters defined in eq 1, there may be established two measures of efficiency of liquid distribution in the packed column. The first measure is the global efficiency, given by eq 2, which provides an indication of the distribution

EfG ) (1 - a) × 100%

(2)

quality in terms of the height differences of the water columns obtained. Hence, a high global efficiency corresponds to a liquid distribution without significant differences in the water level of the collecting tubes.

(3)

R)

x(d3) + (3e) 2

2

θ ) arccos(e/3R)

(4) (5)

Equation 4 is the representation of the displacement radial position. It refers to the radial position (tube 11 as the origin) of the peak or depression (maximum and minimum points) of the distribution in each experiment. Equation 5 provides the angle measured from the positive axis of the abscissas counterclockwise, in relation to the peak or depression. Since the collecting tubes were made of clear acrylic, it was possible to observe the formation of liquid distribution columns during the experiments. Figure 4 represents the water distribution resulting from the first experiment, with water being fed at the center of the upper head at a 1.0-m3/h flow rate on a 30-cm-high packing section. On the conditions in Figure 4 there was an irregular conical distribution of water, i.e., the central positions were privileged to the detriment of the more peripheral ones, denoting the role played by the feeding point. Another example of that dependency is shown in Figure 5, which represents experiment 14, for a 2.0-m3/h feeding rate at a point 7.5 cm from the center and with 60-cm packing. The presence of regions not reached by the water in Figure 5 indicates that the packing, by itself, does not fulfill the role of efficient distributor on those conditions. During the experiments it was noticed that the nonhomogeneous characteristic of the liquid distribution was gradually abated as the higher packing sections and higher water flow rates were utilized, besides employing the central column feeding. That may be seen in Figure 6, representing experiment 21, in which a 3.0-m3/h water flow rate and a 90-cm-high bed were used. The values of the global and dispersion distribution efficiencies, as well as those values for the displacement

Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998 2847 Table 1. Values of Distribution Efficiencies and Center Displacement Factors for the First Series of 27 Tests experiment

bed height (cm)

flow rate (m3/h)

feeding point (cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27

30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 30.0 60.0 30.0 60.0 60.0 60.0 60.0 60.0 60.0 60.0 90.0 90.0 90.0 90.0 90.0 90.0 90.0 90.0 90.0

1.0 2.0 3.0 1.0 2.0 3.0 1.0 2.0 3.0 1.0 2.0 3.0 1.0 2.0 3.0 1.0 2.0 3.0 1.0 2.0 3.0 1.0 2.0 3.0 1.0 2.0 3.0

center center center 7.5 7.5 7.5 15.0 15.0 15.0 center center center 7.5 7.5 7.5 15.0 15.0 15.0 center center center 7.5 7.5 7.5 15.0 15.0 15.0

Figure 5. Water distribution in experiment 14 of the first series: central water feeding, 2.0-m3/h flow, and 60-cm-high packing section.

Figure 6. Water columns obtained in experiment 21 of the first series: central feeding, 3.0-m3/h water flow, and 90-cm-high packing.

of the center from the first series of 27 tests, are shown in Table 1. The correlation coefficients (r2) for the adjustment of eq 1 varied between 0.95 and 0.98. It may be noticed from the values shown in Table 1 that the efficiencies increase as the flow rate increases, probably due to the decrease in the liquid-phase channeling. The efficiencies also increase as the packed-bed height is increased, especially in the 30-60-cm range. The increase of the efficiencies is smaller in the 6090-cm range, corroborating perhaps the existence of a natural distribution of liquid flow from the 90-cm bed on (see Albright, 1984).

efficiency (%) global dispersion 69.47 79.16 83.50 68.83 78.22 78.97 56.77 62.17 65.45 79.47 83.35 85.55 76.53 81.51 83.00 57.72 64.78 66.32 81.89 86.04 87.95 80.03 80.86 83.56 66.64 68.22 70.12

17.25 42.97 53.64 12.17 41.45 43.32 54.95 56.44 62.44 44.06 51.61 57.91 44.69 59.90 65.44 53.76 66.13 70.40 47.66 56.13 62.67 53.17 64.23 71.99 68.34 70.04 70.95

displacement of the center radius (cm) angle (deg) 0.152 0.242 0.154 0.337 0.333 0.344 1.034 0.955 0.929 0.057 0.079 0.081 0.784 0.763 0.796 1.043 0.945 0.937 0.158 0.153 0.087 0.730 0.815 0.820 0.925 0.916 0.881

46 49 25 179 176 174 157 160 161 18 22 10 173 174 174 162 164 162 1 8 21 177 179 176 154 156 156

The efficiencies diminish as the feeding goes from the center to the periphery of the upper head. As a first step in the efficiency analysis of liquid distributors and redistributors for packed columns, a perforated-pipe distributor was built using materials which were easy to find and handle, such as PVC tubing and accessories (crosses, elbow joints, and tees) of 1.0in. nominal diameter. Twelve 3-mm orifices evenly spaced were drilled in the distributor branches. Then, 21 tests of the second series were run, utilizing the laboratory-built distributor. The results of the observed distribution are shown in Table 2. Again, the correlation coefficients (r2) for the adjustment of eq 1 varied between 0.95 and 0.98. It may be noticed that the efficiencies are higher than those resulting from the tests in which only one distribution point was used. It is probable that the greater number of distribution points compensated the reduction in the range of flow rates applied. In general, the efficiencies increased with an increase in the bed height. The global efficiency increased slightly when the flow rate was raised from 0.1 to 0.3 m3/h and remained stable or diminished a little from 0.3 to 0.6 m3/h. The dispersion efficiency tended to decrease as the flow rate increased. Finally, with the empty column and variation of the distance between the distributor and the middle head and the flow rate, it was possible to arrive at an “optimum distribution point”. This optimum point prevented the liquid from spreading in such a way that there was liquid flow on the walls or that the most peripheral regions of the bed were not wetted. At any rate, it was noticed that the initial water distribution was better when the distributor was placed at the bottom of the column, which might correspond to a shorter distance between the distributor and a hypothetical packing layer. The liquid columns obtained in some tests of the second series are shown in Figures 7-9, corresponding to experiments 1, 12, and 20, respectively.

2848 Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998 Table 2. Distribution Efficiency Values and Displacement Factors of the Center for the 21 Tests of the Second Series experiment 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21

bed height (cm)

flow rate (m3/h)

distributor position (cm)

90.0 90.0 90.0 60.0 60.0 60.0 30.0 30.0 30.0

0.1 0.3 0.6 0.1 0.3 0.6 0.1 0.3 0.6 0.1 0.3 0.6 0.1 0.3 0.6 0.1 0.3 0.6 0.1 0.3 0.6

top top top top top top top top top top top top 54.8 54.8 54.8 84.5 84.5 84.5 bottom bottom bottom

Figure 7. Water columns obtained in experiment 1 of the second series: 0.1-m3/h flow and 90-cm-high packing with the distributor located at the top of the column.

efficiency (%) global dispersion 91.4 95.5 94.2 90.0 92.0 91.8 86.6 88.8 91.8 78.4 88.7 87.0 70.3 83.7 87.6 83.1 86.6 88.0 79.8 85.5 87.2

91.6 86.5 88.8 91.5 88.5 88.4 91.5 89.0 81.9 90.2 77.8 82.3 93.0 89.8 94.9 87.3 84.0 78.1 94.0 89.7 89.0

displacement of the center radius (cm) angle (deg) 0.530 0.210 0.130 0.360 0.240 0.260 0.310 0.220 0.110 0.380 0.150 0.130 0.390 0.190 0.140 0.220 0.060 0.140 0.480 0.200 0.110

0 28 56 178 119 104 128 131 177 86 118 76 88 84 72 40 60 61 56 42 47

of orifices per unit of length in the lateral branches of the distributor. Therefore, the surfaces of final distribution presented a point of minimum. By comparing the values presented in Tables 1 and 2, it is clear that a better water distribution may be ascribed to the use of the distributing device. Despite the perceivable differences in the liquid column height in Figures 7-9, they are considerably smaller than those shown in Figures 4-6. Therefore, it is possible to characterize, in a comprehensive way, the liquid distribution resulting from the influence of the analyzed variables (water flow rate, packing section height, and feeding point), for each experiment. Conclusion

Figure 8. Water columns obtained in experiment 12 of the second series: 0.6-m3/h flow and no packing with the distributor located at the top of the column.

Figure 9. Water columns obtained in experiment 20 of the second series: 0.6-m3/h flow and no packing with the distributor located at the bottom of the column.

It is important to note that, contrary to what happened during the experiments done with the packing only, the peripheral collecting tubes were privileged in the liquid distribution, due to a greater concentration

Despite the simplicity of the model presented, eq 1 and the equations derived from the characterization proposal of the liquid distribution, i.e., eqs 2-5, they proved to be very useful in determining the distribution efficiencies of the experiments done. It was verified, from the results presented previously, that on the experimental conditions employed, the packing without a distributor was not capable of distributing the liquid efficiently, which may be verified through the values of efficiency and center displacement factors shown in Table 1. The homogeneity of that distribution may be obtained by necessarily employing the proper liquid distributors and redistributors for this use at the top and between the packing units, besides employing distributing plates. This hypothesis is confirmed by observing the results obtained in the experiments in which the distributing device was utilized (see Table 2). This distributor was responsible for the improvement of the distribution efficiencies on the analyzed conditions. Therefore, special attention should be paid to the project of a distributor, since it affects significantly the operation and the efficiency of a packed column. A possible approach to the project may be found in the work of Otis (1982). Following up this work, other packing elements, smaller than 1 in., will be utilized in the analysis of the column behavior for different ratios between the column diameter and the packing diameter.

Ind. Eng. Chem. Res., Vol. 37, No. 7, 1998 2849

Other distributors will also be employed, with different constructive characteristics, such as orifice diameter, distance between orifices, and branch disposition, to try to improve the final outcome of the distribution. Nomenclature Z ) height of liquid column, normalized x ) position of the collecting tube on the x axis y ) position of the collecting tube on the y axis a ) normalized amplitude coefficient of the liquid distribution, adjustable parameter b ) maximum or minimum height of the liquid distribution, adjustable parameter c ) dispersion coefficient of the liquid distribution, adjustable parameter d ) displacement coefficient of the liquid distribution in relation to the x axis, adjustable parameter e ) displacement coefficient of the liquid distribution in relation to the y axis, adjustable parameter EfG ) global efficiency of the distribution, % EfD ) dispersion efficiency of the distribution, % R ) displacement radius, fraction value θ ) displacement angle, deg

Literature Cited Albright, M. A. Packed Tower Distributors Tested. Hydrocarbon Process. 1984, Sept, 173. Bemer, G. G.; Zuiderweg, F. J. Radial Liquid Spread and Maldistribution in Packed Columns under Different Wetting Conditions. Chem. Eng. Sci. 1978, 33, 1637.

Bonilla, J. A. Don’t Neglect Liquid Distributors. Chem. Eng. Prog. 1993, 89, 47. Chen, G. K. Packed Column Internals. Chem. Eng. 1984, 91, 40. Gunn, D. J.; Al-Saffar, H. B. S. Liquid Distribution in Packed Columns. Chem. Eng. Sci. 1993, 48, 3845. Haure, P. M.; Hudgins, R. R.; Silveston, P. L. Investigation of SO2 Oxidation Rates in Trickle-Bed Reactors Operating at Low Liquid Flow Rates. Can. J. Chem. Eng. 1992, 70, 600. Killat, G. R.; Rey, T. D. Properly Assess Maldistribution in Packed Towers. Chem. Eng. Prog. 1996, 92, 69. Kister, H. Z. Distillation design; McGraw-Hill Inc.: New York, 1992. Klemas, L.; Bonilla, J. A. Accurately Assess Packed-Column Efficiency. Chem. Eng. Prog. 1995, 91, 27. Kouri, R. J.; Sohlo, J. Liquid and Gas Flow Patterns in Random Packings. Chem. Eng. J. 1996, 61, 95. Leva, M. Tower packings and packed tower design; The United States Stoneware Company: Akron, OH, 1953. Metzinger, J.; Hasokowati, W.; Hudgins, R. R.; Silveston, P. L.; Gangwal, S. Application of a Periodically Operated Trickle Bed to Sulfur Removal from Stack Gas. Chem. Eng. Sci. 1992, 47, 3723. Otis, R. J. Pressure Distribution Design for Septic Tank Systems. J. Environ. Eng. Div. 1982, 108, 123. Treybal, R. E. Mass-transfer operations; McGraw-Hill Inc.: New York, 1980. Zuiderweg, F. J.; Kunesh, J. G.; King, D. W. A Model for the Calculation of the Effect of Maldistribution on the Efficiency of a Packed Column. Chem. Eng. Res. Des. 1993, 71, 38.

Received for review November 10, 1997 Revised manuscript received March 23, 1998 Accepted March 24, 1998 IE970785Q