Ind. Eng. Chem. Res. 199433, 647-656
647
SEPARATIONS Effective Mass-Transfer Area in a Pilot Plant Column Equipped with Structured Packings and with Ceramic Rings M. Henriques de Brito,t U. von Stockar,’ A. Menendez Bangerterj P. Bomio,t and M. Lasoil Institute of Chemical Engineering, Swiss Federal Institute of Technology (EPFL), CH 1015 Lausanne, Switzerland
The effective area for mass transfer of the Sulzer structured packing Mellapak 125.Y, 250.Y, and 500.Y was measured under industrial-scale operating conditions, with a broad range of gas and liquid flows, in a pilot plant with a column having an internal diameter of 295 mm and a packing height of 420 mm. The experimental procedures and apparatus employed in this work were checked by making additional measurements of the effective area for 25-mm ceramic ring packing; these values were compared with literature data. This contribution shows that the effective mass-transfer area for packings 125.Y, 250.Y, and 500.Y can be considerably higher than the defined geometric area of the packing. The ratio of the effective area to the geometric area is a function of the Reynolds number defined with the liquid characteristic velocity and the wetted perimeter. The additional mass-transfer area beyond the fully wetted packing is attributed to liquid flow instabilities, leading to waves, film detachment, and droplet formation between the sheets. The effect is particularly important in Mellapak with low specific geometric area. 1. Introduction
Structured packings have been widely used in separation columns since they offer an excellent mass-transfer efficiency while affording a lower pressure drop than compared to irregular packings. These facts reduce both fixed and operating costs. Despite the importance of effective area values for the understanding and design of absorption equipment, and despite the increased use of structured packings in industrial separation columns, no investigation reporting actual measurements with structured packings of any kind has been published to our knowledge. Fair and Bravo (1987) suggest a procedure for estimating effective areas of structured packings. This will be discussed in section 3. Therefore, the aim of the present work is to characterize the effective area of the structured packing. Specific effective areas for gas absorbers are measured by combining absorption with a chemical reaction, such that the absorption rate is independent of the mass-transfer resistance in the liquid phase. There are also works in which effective areas were determined simultaneously with the k ~ using a the “Danckwert’s plot method” (Sharma and Danckwerts, 1970). Landau et al. (1977) state that chemical methods are the only ones which “do not require verification by another kind of measurement and which can yield from a single measurement an overall value of the interfacial area”. The review paper of Guillen et al. (1982) expresses the same opinion. Landau et al. (1977) draw attention,
* To whom all correspondence should be addressed.
+ Present address: Rua Paulisthia 575/71,5440-001 Vila Madalena, SBo Paulo, Brasil. * Present address: Woodward-Clyde International, CH 1006 Lausanne, Switzerland. Present address: Sulzer Brothers Limited, CH 8401 Winterthur, Switzerland. 11 Present address: Institut fur Polymere, Swiss Federal Institute of Technology (ETHZ), CH 8092 Zurich, Switzerland.
however, to the necessity of a careful choice of a “suitable reaction” and to the extensive experimental time required. Table 1 lists those chemical systems which have been used for determining the mass-transfer area in packed columns. The sulfite method, employing the oxidation of NazSOs, has been frequently proposed for giving estimates of various mass-transfer parameters, such as effectiveareas and k ~ aaccording , to the range of reaction rate. Linek et al. (1981) recommend that “it is not justifiable to adopt kinetic constants of the reaction reported in the literature; the constants must be determined experimentally separately for each production charge of sulfite”. Delaloye (1986) carried out such experiments with a wetted wall column to verify the kinetics of the reaction, but no reproducible results were obtained. The reaction kinetics may be very sensitive to trace impurities, so that reproducibility is almost impossible. In view of the results, further use of the method is not recommended. The absorption of COz diluted with air into NaOH solutions is frequently used. Sharma and Danckwerts (1970) consider that the reaction is a “very convenient system to use”. The authors do point out that ”this system cannot be easily used when effective areas are higher than 10 cm2/crn3because of the evolution of heat”. However, packed columns present lower values of specific effective areas. The applicability of this reaction for obtaining effective area values is discussed in the Materials and Methods section. 2. Materials and Methods 2.1. Packings. The structured metal sheet packing Mellapak was supplied by Sulzer Brothers Limited (Winterthur, Switzerland). Performance characteristics of Mellapak are described by Spiegel and Meier (1987). The structured packing Mellapak (see Figure 1)consists of cylindrical elements of height h (see Figure 2a-c) made of corrugated metal, ceramic, or plastic sheets. In this
0888-5885/94/2633-0647$04.50/0 0 1994 American Chemical Society
648 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 Table 1. Measurements of Mass-Transfer Area Using Chemical Methods reference chemical system used Bennett and Goodridge (1970) absorption of trace quantities of radioactive C"O2 in NaOH solutions Bornhiitter and Meramann (1991) absorption of COz into a solution of NaOH Danckwerta and Guillbam (1966) absorption of pure CO2 into different solutions (NadO,; buffer solutions of KzCOs with KHCOs and KCIO,NaOH solutions with NaS."Od Danckwerta and Rizvi (1971) absorption of 0 2 from air into NalsOs with Cos04 88 catalpt Dharwadkar and Sawant (1985) absorption of C02 in air into solutions of NaOH with NazCOa absorption of COz in air into NaOH solutions Cianetto and Sicardi (1972) absorption of @ into different solutions (cuprouschloride and sodium dithionite); absorption of Jhaveri and Sharma (1968) isobutene in aqueous HISO, Joosten and Danckwerta (1973) absorption of pure COz into carbonate buffer solutions Kolev (1973) absorption of Cot in air into solutions of NaOH absorption of 0 2 from air into NazSOa solutions with CcSO, 88 catalyst KrBbch and K W n (1979) Lee and Kim (1982) ahsorption of COSin air into buffer solutions (KzCOs. KHCOa, KCI. and NaOCI) Lineket al. (1984) ahsorption of COSin air into solutions of NaOH Lineket al. (1974) absorption of pure 0 2 into NazSOs solutions with COS04 88 catalyst Merchuk (1980) absorption of COz in air into NaOH solutions Mohunta et al. (1969) absorption of COZin air into a solution of NaOH absorption of pure 0 2 in aqueous solutions of ammonium sulfite Neelakantanet al. (1982) absorption of pure COZinto carbonate buffer solutions with arsenious oxide ions Richards et al. (1964) absorption of pure CO2 into a buffer solution K&Os and KHCOs with arsenious oxide ions Riuuti and Brucato (1989) Sahay and Sharma (1973) absorption from 0 2 from air in dithionite solutions and absorption of lean CO, in NaOH and diethanolaminesolutions Sedelieset al. (1987) absorption of 0 2 into NalSOs solutions Vidwans and Sharma (1967) absorption of pure COZinto aqueous solutions of NaOH and MEA absorption of COSin air into solutions of NaOH and KOH Ycshida and Miura (1963) work, stainless steel Mellapak was used. Besides the corrugations, the metal sheets are embossed and grooved horizontally to promote turbulence and improve spreading of the liquid. The additional area provided by the surface structure is not taken into account in the nominal packing area. The corrugations of the metal sheets are inclined by an angle 8 with respect t o a vertical axis. For the Mellapak-Y type this angle 8 is 45O (see Figure 2c). The sheets are arranged vertically and parallel to each other (Figure Za), so that the corrugations of contiguous sheeta are inclined alternately by +8 and -8 and, therefore, in the case of Mellapak-Y type, they run perpendicular to each other. These corrugations define straight, inclined channels of triangular cross section through which the gas flows (a top view of these channels is given in Figure 2b). The liquid flows down the corrugated sheeta approximately in countercurrent fashion to the gas. TheMellapaktypesueedin thisstudywere 125.Y.250.Y. and 500.Y, corresponding to a nominal specific surface ap of 125,250, and 500 m2/m3, respectively. Measurements were also carried out with 25-mm ceramic Raschig ring packings using the same experimental procedure. These rings have a geometric area of 195-210 m2/m3,the exact figure depending on the packing procedure used. In this work, a value of 200 m2/m3was found. 2.2. The System COrNaOH for Measuring MassTransfer Area. The absorption of COz into NaOH solutions occurs with the following chemical reaction:
CO,
+ 2 NaOH
-
Na&O,
+ H,O
Figure 1. The Sulzer structured
packing
Mellapsk.
1 Ha < sEi
where the Hatta number and the enhancement factor for instantaneous reaction are defined for a second-order reaction as
(1)
(3)
In highly alkalinesolutions, this reaction becomes pseudofirst order. In mathematical terms, the conditions for pseudo-firstorder kinetics aregiven as (Danckwerts, 1970)
and, respectively,
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 649 corrugated metal sheets
/
(4)
It is well-known that reaction 1 is sufficiently fast as compared to mass transfer that, given the right conditions, the carbon dioxide cannot be transported far away from the interface before it reacts. The criterion to be fulfilled is
Ha>3
(5)
-
If both criteria 2 and 5 are satisfied, the absorption rate becomes independent of mass-transfer parameters and is given by
N = nA, = A, = ( k 2 C w ~ D ~ 0 > 1 / 2 ( C i ~ ~ (6) >
Liquidflow Gasflow
Simplified diagram of structured packing structured packing
The concentration of dissolved C02 at the interface is in equilibriumwith the concentration of the gas phase at the interface, as given by
where:
The physical consequence of eq 8 is that the absorption rate is independent of the mass-transfer coefficients but remains directly proportional to the effective interfacial area. Measuring absorption rates therefore enables determination of A, independently from either ki or ko (for reviews, see: Charpentier, 1981; or Shah, 1979). In most literature references reporting effective interfacial area measurements using this technique, the diffusivities and the rate constant needed in eq 9 were estimated based on the literature. Criteria 2 and 5 are usually checked, but again often using literature values. However, as shown in Table 2 there is quite a disagreement on the chemical rate constant in the literature, and errors in this constant will affect directly the result of the measurement. For this study, it was thus decided to determine t#J experimentally, using the same experimental conditions, the same concentrations, and the same chemicals as in the semi-industrial packed column. Absorption rates were measured in a falling film column of diameter 27 mm. Two film heights, 80 and 52 mm, were used. In order to prevent depletion of hydroxyl ions at the interface, NaOH solutions of between 1.6 and 2 M were used. Lack of OH- ions at the surface, Le., depletion, violates the pseudo-first-ordercondition and thus criterion 2 (Danckwertsand Kennedy, 1958). The liquid flow rate was varied within the relatively narrow bounds that permitted obtainment of a ripple-free film, and the C02 mole fraction was varied between 1 and 0.3, the smallest value enabling precise absorption rate measurements.Full details were given by Henriques de Brito (1992). Figure 3 shows the observed absorption rates as a function of yica and of the film height. From the slopes of the two regression lines, the same value of4 was found. It is reported in Table 2 in comparison with literature data. The fact that 4 depended neither on yich nor on the film height or the flow rate testifies to the fact that
Cross section of pilot plant column column axis
line of descent
e = 45' a = 60'
h = 210 m m h,,, = 11.5
b = 32.5 m m
Packing geometry for Mellapak 250.Y
Figure 2. (a, top) Simplified diagram of structured packing, (b, middle) crass section of structured packing, and (c,bottom) packing geometry for Mellapak 250.Y.
the conditions for a fast, pseudo-first-order reaction were met. The same conclusions were reached from checking criteria 2 and 5. Using a diffusivity ratio for OH-/C02 of 1.7 (Danckwerta, 1970) Ei values ranged between 80 and 280. Based upon simple falling film theory, the upper m/s limit for k~ could roughly be estimated at 1.3 X which makes the Hatta number 2.8. Realistic k~ values are expected to be lower. As the same experimental conditions were also used in the pilot plant column, it is expected that the fast pseudo-
650 Ind. Eng. Chem. Res., Vol. 33,No. 3, 1994 Table 2. Chemical Rate Constants for the Reaction of COz in NaOH Solutions at 295 K
CWH, reference kmol m4 2 Pinsent et al. (1956) Nijsing et al. (1959) 2.07 1.05 Nijsing et al. (1959) 2 Astarita, cited in Tseng et al. (1988) Pohorecki and 2 Moniuk (1988) present work 1.6-2.0
k2,m3 kmol-l s-l 6630 8550 5770 9580
4, mol m-2 s-l 8.3 X 7.6 X
le2
14470
12 X
b
(7.3 f 0.5) X
a These values have been recalculated from original data for comparison. b Not determined as such.
n
t 0 r 4l
cn
f 3 ) "C by a heat exchanger before the column inlet. The feed solution was supplied by a stirred tank reactor having a volume of 2.5 m3. Liquid samples were taken above and below the packing by means of small purpose designed collector cups connected to peristaltic pumps. The carbonate content was determined by automatic titration with 0.1 M HC1. Further details have been given by Henriques de Brito (1992). 2.4. Evaluation of the Results. Equation 8 necessitates the measurement of the concentration of C02 at the gas-liquid interface, where only bulk concentrations can be measured. If in the pilot column, the gas-phase mass-transfer resistance is negligible, then the gas concentration at the interface is approximately the same as the concentration in the bulk. Spiegel and Meier (1987) propose a model for predicting the gas side Sherwood number in structured packings. However, the empirical coefficient contained therein was not published. Fair and Bravo (1990) suggested that with their publication "from a data plot one can infer values in the range 0.018-0.040". Using the value of 0.0338 proposed by Fair and Bravo (1990):
\
I
0
E
Y
0 . . 0 20 1
.
1
40
'
'
60
.
'
80
.
100
Figure 3. Absorption rate versus C02 mole fraction from in a wetted wall column. Film height: 80 mm (+), 52 mm ( 0 ) .Best fit of data: 80 mm (broken line), 52 mm (solid line).
PG sc = p&G
first-order kinetics are preserved with the same value of 4. The C02 mole fraction in the gas, however, was lowered to 1% in order to prevent substantial heat release and to minimize cost. As is shown in Figure 3 , this should not affect 4. 2.3. The Pilot Plant. The experiments were carried out under industrial-scale operating conditions in a pilot plant column with an internal diameter of 295 mm, a packing height of 420 mm (2 packing elements), and with a liquid distributor having a high drip point density of 527 points/m2, thus eliminating possible entrance effects. The pilot plant and associated analytical equipment is described elsewhere (Henriques de Brito (1992). The inlet gas was saturated with water in another packed column to prevent heat effects due to the evaporation of the solvent in the column. The COSwas supplied by a gas manifold (Carbagas AG, Lausanne, Switzerland) connected to CO2 bottles with dip tubes. Liquefied C02 drawn from the bottles was gasified in an evaporator. To measure and guarantee a stable flow of C02, two mass flow meters (Brooks, model 58513, Rosemount AG, Baar, Switzerland) were installed in the pilot plant. The C02 flow could be adjusted up to about 9.2 m3/h. The mixture of C02 with air was homogeneous due to the installation of four static mixers Sulzer SMV (Sulzer Brothers Ltd., in Winterthur, Switzerland) in the pilot plant. The concentration of COP in the inlet and outlet gas stream was measured with an infrared gas analyzer (Binos 1, Leybold-Heraeus, Hanau, Germany). This equipment was calibrated at the beginning of each experiment. The liquid flow was fed into the liquid distributor at the top of the column and had its temperature adjusted to (22
Actually, the original model suggests the interfacial area and not the geometric area in eq lob. For a rough estimation up was used. The specific absorption rate is
where Ay accounts for the concentration gradient in the gas phase. Thus: Y
k
-PG+ @
GMG
The resistance in the gas phase is small if the above ratio is small. This ratio is almost independent of the C02 concentration in the gas flow. For 250.Y, with F-factors of 0.85, 1.1, and 2.1 m0.33kgo.6s-l, the ratios given by eq 12 are respectively 16.4%, 14.196, and 8.6%. Some dependence on packing density would be expected through the factor kG in eq 12. The packing density governs the hydraulic diameter and thereby kG (eq 10). Using eqs 10 and loa-c, it is seen that kG is proportional to the geometric area of the packing raised to the power of 0.2. This means that the Mellapak type should not significantly affect the above values. For 500.Y, for a F-factor of 0.85, the ratio of eq 12 is 14.6%. For 125.Y, for a F-factor of 2.1, the lowest that will be used, the ratio of eq 12 is 9.8%. The above values are very conservative. Dharwadkar and Sawant (1985) estimated 10-15 % resistance to be on the gas side with the C02-NaOH system for their
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 651 Table 3. Average of the Ratio m/aL (Mass Balance Ratio) for two 125.Y packing elements for two 250.Y packing elements for two 500.Y packing elements for three 125.Y packing elements for 25-mmceramic rings with h = 800 mm
average a d a L 1.00 0.83 0.70 0.88 1.19
measurements of effective areas in columns packed with irregular packings. The gas superficial velocity was between 0.09 and 0.17 m/s. Yoshida and Miura (1963) suggested that the gas-phase resistance accounted for less than 5% of the overall resistance in their experiments with COz and NaOH. The gas flows were not published. Both teams of authors did not consider the gas side resistance in their calculations. In the light of the uncertainty of the kG estimation and based on the fact that errors due to a possible gas-phase mass-transfer resistance are within measurement accuracy (see later), the effect was not further considered. As mentioned before, this practice is common. A differential mass balance for the COz must be established and integrated together with eq 8 through the packing height. Assuming a straight operating line, the integration yields:
Table 4. Estimate in Evaluating the Mass-Transfer Area error, % a b 26 15 20 9 17 8 20 4 19 4 65 25 48 21 37 13 65 27 29 4
UL
aG
remark for two 125.Y packing elementa for two 250.Y packing elementa for two 500.Y packing elementa for three 125.Y packing elements for 25-mmceramic rings with h = 800 mm for two 125.Y packing elementa for two 250.Y packing elementa for two 500.Y packing elements Y for three 125.Y packing elements for 25-mmceramic rings with h = 800 mm
a Estimate based on an error propagationstudy and on estimations of the errors in the individual measurements affecting a (Henriquea de Brito, 1992). b Estimate based on the standard deviation in the final a values (Henriques de Brito, 1992). n
0
a)
1I
8
500
/ A
375
(14) where the logarithmic mean for the gas concentration is defined as
Q
5 ._
I
1254
Y
Ya
In Yw
There are then two possibilities for evaluating the effective specific mass-transfer area, either by measuring the concentrations in the liquid phase (eq 13) or by measuring the concentrations in the gas phase (eq 14). The indexes L and G were used to distinguish the two equations. Both quantities have been measured in this work. The error in the mass balance is given by the ratio of eqs 13 and 14. The average ratios of all measurements for each packing type are given in Table 3. The mass balance indicates that the error may be of the order of 20%. Nevertheless, it should be borne in mind that the uncertainties associated with QG are higher than those associated with aL (Table 4). Thus, the mass balance check mainly reflects the uncertainties in measuring aG. The results given in this work therefore are based exclusively on aL. All the measurement errors affecting eq 13 have been estimated individually (Henriques de Brito, 1992). For example, the F-factor and the specific load are believed to be known with a precision of *lo% and *l%, respectively. An error propagation study taking into account all of these estimations yielded column a of Table 4. Column b is an estimate of the errors based on the reproducibility of the measurements. The precision of QL is principally influenced by the evaluation of the parameter C#J and the carbonate content. With large F-factors, during the experiments with 125.Y, there was liquid in the outlet gas samples. Droplet formation is impeded less by 125.Y than by denser packings, and some droplet carryover in the gas stream at
1
1250 0
1
2
3
4
F-Factor [m/s dkglm31 Figure 4. Mass-transferarea versus F-factorfor a l l Mellapak types: (a) Mellapak 500.Y, (b) Mellapak 250.Y, (c) Mellapak 125.Y. The liquid flow rates used in these experiments were, in ms m-2 h-l, 12.3 (A), 19.8 (W), 40.1 ( O ) , 48.9 ( O ) , 57.7 ( O ) , 71.5 (A).
the top of the column was observed with this packing. The sampling position had to be changed at a certain stage and was installed over the distributor, reducing the likelihood of contact with the liquid drops. After this change, there was a slight decrease of the concentration measured at the gas outlet by the gas analyzer. 3. Results and Discussions 3.1. Effective Mass-Transfer Area for Mellapak. Figure 4 presents all specific mass-transfer area measurements as a function of the F-factor. The error bars are omitted for clarity. The values for the specificmass-transfer area for packing 500.Y for a given specificliquid load and F-factor are higher than those for 250.Y, which in turn are higher than those for 125.Y. This is in accordance with the expectation that the effective area increases with the geometric area. The effective mass-transfer area depends primarily on the packing and on the specific liquid load. The influence
652 Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994
0
50
100
150
Re, Figure 5. The ratio ada, as function of the Reynolds number. The line is the best fit given by eq 17.
of the F-factor is apparently linked to the packing. For 500.Y (Figure 4a), the effective area appears to increase with the F-factors, whereas a, clearly remains unaffected in packings with less internal surface. This would be expected since a given value of the F-factor lies much closer to the flooding conditions in the 500.Y than in the other packings. The horizontal lines of the mass-transfer area with the F-factor for a given liquid load are in line with the contention that the gas side resistance is unimportant. A systematic variation of the apparent a, values with F could have been interpreted as reflecting the variation of a gasphase mass-transfer resistance with F. Given the absence of such variations, a gas-phase mass-transfer resistance, if it really exists, would have to be independent of F. The results for all three packings can be correlated as a function of only a Reynolds number. The correct characteristic length to be used in this Reynolds number is the wetted perimeter, i.e., the total length of all the lines in Figure 2b. Since in Mellapak, as opposed to random packings, this length is proportional to the specific geometric area of the packing, the liquid Reynolds number may be defined as follows: PLWL
Re, = -
(16)
FLU,
The resulting correlation for all measurements is shown in Figure 5. Its mathematical form is as follows: (17) It must be stressed that the correlation appearing in Figure 5 has not been checked with fluids of different densities and viscosities, since these two variables were constant at 1077 kg m-3 and 1.177 mPa s (Vazquez et al., 1989) during this study. An important consequence of eq 17 is that the ratio of the effective area to the geometric area increases with decreasing packing surface at constant specific liquid load. The specific effective area is dependent on the specific geometric area to the power of 0.7. The experimental evidence clearly indicates that the effective area can considerably exceed the geometric area. The magnitude of this effect is significantly larger than the experimental uncertainty. In view of the pivotal importance of such results, a calibration of the C02-NaOH system using dumped packings (section 3.3) as well as a painstaking experimental check of possible additional factors affecting a, was performed (section 3.2). Considering the geometry of Mellapak, it would seem natural to assume that the maximum possible effective
area would correspond to a totally wetted packing (unfortunately, it is not possible to visualize the flow inside the packing, since it is opaque). The evidence, however, clearly suggests that additional mass-transfer area exists beyond the geometric nominal value. Although the nature of this additional area can only be elucidated by further research, it is conceivable that instabilities in the liquid flow (like ripples or waves, detachment, and subsequent fragmentation of the film into copious liquid showers, etc.) lead to an increased effective area. Such liquid flow instabilities develop more extensively with the larger film running lengths and therefore should be more noticeable with the spacing between corrugations is larger, Le., the coarser the packing. This is indeed the trend that is experimentally observed: the fractional additional area is largest for the coarsest packing 125.Y (Figure 4c). The possibility of a,/a, ratios in excess of unity is not mentioned explicitly in the literature, although Bravo and Fair (1982) note that in dumped packings “the value of the effective area is composed not only by the wetted area over the packing but also by the area provided by suspended and falling droplets, gas bubbles within liquid puddles, ripples on the liquid film surface, and any contribution from film falling on the walls of the column”. In a more recent work, the same authors (Fair and Bravo, 1987) attempt to estimate the effective interfacial area of structured packings and come up with ada, ratios as high as 1.2 for Mellapak 250.Y. It should perhaps be noted that ae/apratios above unity are common in gas/liquid contactors of completely different kinds, such as spray towers, where this ratio is actually infinite. 3.2. The Influence of the Drip Point Density and Packing Height. The liquid distributor in the present work originally had a drip point density of 527 points/m2. This was in an effort to minimize entrance effects. The value corresponds to the upper limit of density used for research liquid distributors, which should be higher than 300 points/m2 according to Perry et al. (1990). Some experiments with 3 packing elements of 125.Y were carried out with 28 of the 36 available drip points blocked. The resulting drip point density was 117 points/ m2, which is still higher than the value recommended for industrial operations, quoted at 100 points/m2 by Nutter Engineering (Perry’s Chemical Engineering Handbook, 1990) and at 130 points/m2 by Fair and Bravo (1990). With the new drip point density of 117 points/m2 and using 3 elements of 125.Y,thevalue of the specificeffective area, a L , for a specific liquid load of 19.8 m3/(m.*h) and F-factor of 2.65 was 156 f 4 m2/m3. With the original drip point density, the value of the specific effective area was 160 f 7 m2/m3. This shows that the number of drip points had a negligible effect on the effective area in the range of drip point densities used. For packing 125.Y, the ratio of the specific effective area to geometric area was the highest of all the Mellapak types. This packing was therefore chosen to studywhether a higher packing height would reduce the specific effective area values by a constant factor, which would reveal that “dead area” is measured, thus yielding artificially high a, values. Table 5 compares data from three packing elements with two packings for an F-factor of 2.65. This represents a height increase of 50 % (from 420 to 630 mm). The values of all other experimental variables were maintained as before. Clearly, there is a reduction of the effective area values of packing 125.Y with packing height. But the fact that the fractional reduction also markedly depends on the
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 653 Table 5. Influence of the Packing Height for Packing 125.Y. effective mass-transfer area with an F factor = 2.65, m2/m3 3 packings 2 packings h = 630 mm h = 420 mm 160 f 7 207 f 18 185 6 222 f 42 220 11 235 f 13 250 f 6 265 f 45
B, m3/(m2h) 19.8 40.1 57.7 71.5
* *
250 T
reduction, % 23 17 6 6
a The errors shown come directly from the calculations of the standard deviation of a set.
300
a
.-
,,I
v
%
L
1
E
-
0 0
20
40
60
80
B [m3 m-' h-' ]
.\\\\.
Kolev (1973) Yoshida and Miura (1963) Tnis work F-Factor = 0.32
F-Factor [mlsdkglm3] Figure 6. Mass-transfer area versus F-factor for 25-mm ceramic rings, The liquid flow rates were, in m3 m-2 h-l, 12.3 ( O ) , 19.8 (m),
40.1 (O), 57.7 (a).The F-factors at flooding conditions for these liquid flow rates are, respectively (ms-' kp0.5 m-1.6): 1.76,1.50,1.07, and 0.8 (Eckert, 1970).
specific liquid load suggests that it cannot be attributed simply to a constant additional area acting as an "entrance effect". Rather, it is reasonable to suppose that increasing the specific liquid loads decreases the influence of channeling effects, thus reducing the difference. The same conclusion was reached by Henriques de Brito (1992) who compared the measured results with a prediction based on a numerical model of possible entrance effects. Bravo and Fair (1982) predicted in their correlation for irregular packings that the effective area would be proportional to the packing height to the power of -0.4. An increase of 50%of the packing height would thus reduce the effectivearea by about 155%. Consideringthat irregular packings can be denser than 125.Y and that denser packings may be less sensitive to an increase of the packing height, the reduction of effective area values presented by structured packing is thus not high. The results in Table 5 show that the reduction was less than 17%, except for the smallest liquid load. 3.3. Effective Mass-Transfer Area for 25-mmCeramic Rings. In order to verify the validity of the results with structured packings, the effective mass-transfer area was also measured for 25-mm ceramic rings for which similar results can be found in the literature for comparison. Figure 6 plots the specificmass transfer, aL,evaluated from measured data according to eq 13as a function of the F-factor. A first important observation is that all values for effective areas were below the packing's geometric area of 200 m2/m3. There is a clear dependence of the specific mass-transfer area on the F-factor even for F-factors far from full capacity conditions as shown in Figure 6. Such a dependence is not mentioned in the literature, in which the gas flows used are rarely cited. A possible influence of the gas-phase mass-transfer coefficient &a is difficult to evaluate. The literature does
-
Figure 7. Mass-transfer area versus specific liquid load for 25-mm rings. Points: this study, with F-factors (in ms m-2 h-l) of 0.32 (m) and0.47 (0).Lines: measurementapublished by Sahay and Sharma (1973) (solid black), Kolev (1973) (gray), and Yoshida and Miura (1963) (hashed). not consider this mass-transfer resistance. According to section 2.4, the influence of this resistance should be within experimental uncertainty. When trying to compare these results with published data, it is important to use only literature references reporting measurements of effective mass-transfer areas done in actual columns packed with 25-mm ceramic rings. Work on wetted area and on indirect determinations of wetted or interfacial area probably do not constitute a reliable basis for quantitative comparison. A careful analysis of the literature compiled in Table 1 revealed that the only reports on chemically determined masstransfer area in columns pertaining to 25-mm ceramic rings are those by Sahay and Sharma (19731, Kolev (19731, and Yoshida and Miura (1963). The curves were reproduced in Figure 7 after extrapolating for liquid loadings higher than 30 m3/(m2h), or in the case of Kolev (1973),the data was recalculated for the experimental conditions used in the present study. The widely used correlation of Onda et al. (1967, 1968), which is based on a large number of experimental data including 25-mm ceramic Raschig rings, was also examined and compared with the above data. The predictions of this correlation for the system used in the present work agrees to better than 10%with the values obtained by Sahayand Sharma (1973)and by Kolev (1973) (Figure 7) in the whole range of liquid loads. The measurements of this work are somewhat higher than those reported in the literature (Figure 7). For a gas velocity of 0.3 m/s, the curves reflecting the literature values obtained by the chemical method are between 5 % and 20% lower than the present results. The difference is within the experimental uncertainty, but it appears to be systematic. In order to understand the differences between the chemical absorption measurements, it is important to examine the experimental conditions used by the various authors. Table 6 presents the column characteristics and the range of flow rates applied by the four works used for comparison. The lower results obtained by Yoshida and Miura (1963) may be due to the unreasonably low ratio of column
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994
654
Table 6. Experimental Conditione from t h e Literature column characteristics ratio ref no.a height, mm diameter, mm d/d, h/d 1 2 3
this work a
870 1900 762 800
200 190 121 295
8 7.6 4.8 11.8
4.8 10.0 6.3 2.7
B, m3/(m2h)
range WG,m/s
40
0.3
not given
not given not given
30 12-71
0.3-1.1
method used chemical absorption chemical absorption chemical absorption chemical absorption
Ref no.: 1, Sahay and Sharma (1973); 2, Kolev (1973); 3, Yoshida and Miura (1963).
diameter to packing size they used. Low column diameters with respect to packing size is known to distort the performance through wall effects. Vidwans and Sharma (1967) have shown that the effective interfacial area of 3/s-in.ceramic rings increases by 15-20% when the column diameter is increased from 43.7 to 100 mm and recommended a minimum value of 8 for the ratio did,. On this account, the solid line on Figure 7 is clearly the more reliable reference for comparison. The high aspect ratio (hid) employed in all literature studies might also have influenced the data. A large packed height as compared to column diameter enhances maldistribution of liquid and thus reduces the effective interfacial area. This has been proven explicitly by Yoshida and Koyanagi (1958)who assessed the effective interfacial area of 25-mm ceramic rings using an indirect physical method. Increasing the packed height of a 120 mm diameter column from 203 to 406 mm resulted in an average decrease of a, of about 22 % . Since the aspect ratios of the packed beds used in the literature studies reported in Table 6 were larger than for the present study by a factor ranging from 1.8 to 3.7, this effect may well be the major reason for the observed differences. The data of Kolev (1973) and of Yoshida and Miura (1963) might also be low compared with the present work due to the possibility that they might have used lower gas flow rates than the 0.3 m/s used for comparison. The same remark holds for the general correlation of Onda et al. (1967, 19681, which does not contain a term reflecting the influence of the F-factor. In summary, the effective interfacial areas of 25-mm ceramic Raschig rings measured in this study agree to within experimental uncertainty with comparable literature sources, and the slight but consistent deviation to higher values can be explained by several factors, most notably by the higher aspect ratios of the packed beds used in the literature studies. The high ae/up ratios obtained for the structured packings (section 3.1) must therefore be accepted as real. Even if one assumed that the discrepancy between the literature and this study resulted from a systmatic measuring error and if hence the a, results on the structured packing were corrected by lowering them between 5 and 20% the ue/apratios would still exceed unity by so much that the evidence could not be explained away as an artifact. 3.4. Comparison of Effective Areas between Structures and Irregular Packings. A comparison of effective areas predicted by correlation 17 for structured packings of up = 200 m-1 with our measured values for random packings of the same up appears in Figure 8. Clearly, the former offer higher effective areas than the latter. In other words, the ratios of effective to geometric area tend to be higher in structured packings, to the point of even exceeding unity. Ratios of ada, in excess of unity have even been reported for irregular packings. Bornhutter and Mersmann (1991) have measured ratios for modern irregular packings of large size up to 1.5. According to Linek et al. (1984), hydrophilized plastic Pall rings of 25, 35, and 50 mm do
400
1
0
i
20
B
40 60 80 [m3m-2h" ]
100
Figure 8. Comparison of effective mass-transfer area for Mellapak line and 25-mm ceramic rings (a, = 200 m2/m3and F-factor = 0.4).
present ratios of the effective area to the geometric area of up to 1.2, and Sahay and Sharma (1973) have measured effective areas of 25-mm stainless steel Pall rings 1.16times higher than the specific geometric area of the packing. Several other studies also indicate the possibility of a,la, ratios in excess of unity on theoretical or semitheoretical grounds (Schulthes (1990),Kolev (19731,Ondaet al. (19671, Shi and Mersman (1985)). This discussion suggests that the effective interfacial area in randomly packed columns can be augmented by such effects as liquid flow instabilities, waves, film detachment, and droplets, but to a much lesser extent than in structured packings. 4. Conclusions
The effective interfacial area of several types of structured packings and of 25-mm ceramic Raschig rings has been measured under conditions pertinent for industrial operations. All three types of Mellapak studied can provide an effective mass-transfer area higher than the geometric area defined by the packing surface up to a factor of 2, depending on the liquid and gas flowsand on the geometric area of the packing. Although the mechanism responsible for it remains to be studied in detail, it is suggested that it is due to liquid flow instabilities between the sheets, such as waves, film detachment, and droplets. This phenomenon is especially prevalent in the packings of relatively low geometric area often used in large-scalemasstransfer equipment and is less pronounced in packings with high up. In measurements carried out with 25-mm ceramic rings, no values of a, were found to exceed up. The measured values agreed to within experimental uncertainty with those literature references reporting comparable column configurations. The fact that the experiments of this study consistently yielded slightly higher values can be explained by several factors that might have lowered the results reported in the literature with respect to the measuring conditions employed here. At any rate, these differences
Ind. Eng. Chem. Res., Vol. 33, No. 3, 1994 655 are too small to be able to explain away the high values of a, found in structured packings.
Nomenclature a = specific area, mVm3 A = area, m2 B = specific liquid load, m3/(m2h) b = base of triangular channel, m C = carbonate or C02 concentration in the solution, mol/m3 d = diameter, m D = diffusivity, m2/s F-factor = WG(~G)'/~, m-113 kg112 s-1 G = gas flow rate, mol/s H = Henry constant, Pa m3 mol-l h = packing height, m h, = corrugation height, m kG = gas-phase mass-transfer coefficient, m/s kL = liquid-phase mass-transfer coefficient, m/s kt = second-order reaction rate constant, m3 kmol-l s-l M = molecular weight, g/mol n = specific absorption rate, mol/(m2s) N = absorption rate, mol/s p = pressure, Pa S = cross sectional area, m2 w = superficial velocity, m/s y = gas-phase mole fraction
AY
=Y-Yi
Greek Letters a = inclination of line of steepest descent, deg p = viscosity, kg/ms p = density, kg/m3 C$ = normalized mass-transfer flux, defined by eq 9, mol/(m2 8)
6 = angle of corrugation with respect to column axis, deg
Dimensionless Numbers Re = Reynolds number
Sc = Schmidt number Sh = Sherwood number Ei = enhancement factor for instantaneous reactions (eq 4) Ha = Hatta number (eq 3) Subscripts b = bulk C02 = carbon dioxide e = effective
G = gas h = hydraulic i = interface L = liquid m = logarithmic mean OH = hydroxyl ions p = packing a = at beginning of packing o = at end of packing Superscript
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Receiued for reuiew November 11, 1993 Accepted December 1, 1993.
52.
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* Abstract published in Advance ACS Abstracts, February 1, 1994.
