Ind. Eng. Chem. Res. 1993,32, 1411-1418
1411
SEPARATIONS Mass-Transfer Characteristics of Some Structured Packings Ralph H. Weiland’ and Kelly R. Ahlgren Department of Chemical Engineering, Clarkson University, Potsdam, New York 13676
Matthew Evans Department of Chemical Engineering, University of Newcastle, Newcastle, NS W 2308, Australia
Gas- and liquid-side mass-transfer coefficients and gas-liquid interfacial areas for Goodloe packing and ChemPro’s Montz A2 packing (similar to Sulzer BX) are reported, along with holdup data. Measurements were made in a 150mm diameter column packed to an average depth of 1m. Results are presented in terms of regression equations for the Sherwood number for mass transfer as a function of the Reynolds number and the effective interfacial area as a function of column F factor.
Introduction Structured packings for mass-transfer operations offer exceedingly low pressure drops, very high vapor-liquid contact areas, and good turndown characteristics. They are used increasingly in those applications where maintaining low pressure drop is critical. However, despite the fact that structured packings are not a new innovation (for example, Goodloe packing has been around for over 30 years; Bragg, 1957),only very little has been reported on their mass-transfer properties in terms of the gas- and liquid-side mass-transfer coefficients and the interfacial areas active for gas-liquid contact. These parameters are more fundamental than such quantities as the height equivalent to a theoretical plate (HETP) and the overall height of a transfer unint (HTU),which incorporate masstransfer resistances from both phases; mass-transfer film coefficients refer to a single phase. They depend only on the physical and transport properties of that phase and its hydrodynamics and are very important when the truly rigorous design methods based on mass transfer rates are used. In this work, we report on the mass-transfer and holdup characteristics of Goodloe (Glitsch, Inc.) and Montz A2 (ChemPro, Inc.) packings and the liquid holdup data on a Gempak packing (Glitsch, Inc.). The mass-transfer properties and their dependence on gas and liquid flow rates were derived from measurements of chemically reactive gas absorption rates; holdup data were obtained using a standard tracer technique. Severalworkers have reported on some of the operating characteristics of these packings. McNulty et al. (1982) have reported on the holdup characteristics of Koch Flexipac packings. Begovich et al. (1976) and Choi et al. (1976)determined the flooding characteristics of Goodloe packing. The work of both groups indicates that the gas mass velocity at flooding is about half the value given by themanufacturer’s correlation (Glitsch,Inc., 1981). Bragg (1957) has reported on the results of distillation tests run in 25,78, and 103 mm diameter columns having packed depths from 600 to 900 mm using the benzene/l,2dichloroethane system at atmospheric pressure and total
* Address correspondence to this author at the Department
of Chemical Engineering, University of Newcastle.
reflux. Results were given in terms of HETP as a function of the reflux and vapor flow rates. Unfortunately, considerableinformation is needed to convert HETP data into mass-transferCoefficients and, in any case, these kinds of experiments can provide at best only a value for the ; experiments are vapor-phase coefficient ~ G U distillation usually unsuited to generating information on liquid-side coefficientsand interfacial areas separately. The fact that Bragg also failed to use a liquid distributor does not enhance the reliability of his results. Ayala et al. (1977) measured the overall, liquid-phasebased, mass-transfer coefficient, &a, for Goodloe packing using the COz-air-water system in small-scale columns (64 mm diameter X 1070 mm high, and 152 mm diameter X 810 mm high) with about 5 % COz in the inlet gas. (For a sparingly soluble gas such as COZ, mass transfer is controlled almost entirely by the liquid-phase resistance, so the overall coefficient, K o ~ aand , the film coefficient, k ~ aare , nearly equal.) They reported values of K o ~ ain the range 2-8 X 10-4 s-1 over a range of superficial liquid velocities from 7.5 to 30 mm/s. The data exhibited scatter of about 50 % ;this may have been due to the difficulty of measuring very small changes in gas-phase COZconcentration between the column inlet and outlet since, for a low-solubility gas such as COZ in water, it is almost impossible to avoid nearly saturating the liquid near the bottom of the column. Therefore, it is possible that these data are not a reliable indicator of even the correct trend. The most usefulmass-transfer study of Goodloe packing was carried out by Kanak (1980) and was based on the absorption of krypton gas into a freon. From his data, a k ~ and a correlated Kanak calculated values of k ~ and these data in the forms of the Sherwood numbers ShGa and ShLa. No attempt was made to determine the effects separately of operating variables on the interfacial areas active for mass transfer. Kanak’s mass-transfercorrelation is needlessly complicated by the fact that he determined empirically a separate correlation for the characteristic flow dimension of the packing. This characteristic length was based on liquid holdup, which was itself a function of flow rates and fluid properties; thus, his mass-transfer correlation is convoluted by use of a further correlation. The following equations summarize his results:
0888-5885/93/2632-1411$04.00/00 1993 American Chemical Society
1412 Ind. Eng. Chem. Res., Vol. 32, No. 7, 1993 ShGU = 2.3RebWSScz3
(1)
ShLa = 8 . 0 R e ~ 6 0 s R e ~ 4 6 S ~ ~ 2
No information appears to be available on ChemPro Montz A2 packing per se in the technical literature. However, this packing is very similar to Sulzer BX (Sulzer Brothers), for which data have been reported by Bravo et al. (1985), and Gempak-2B, which has been studied by Chen and Kitterman (1983), although Gempak is manufactured from sheet metal containing tiny slits rather than from a woven metal fabric or cloth. Chen and Kitterman (1983)reported that Gempak was easilywetted, with a stable liquid film being produced even at very low liquid flow rates. Bravo et al. (1985) developed correlations for ShG and kL for Sulzer BX packing. The k~ correlation was based on the Higbie penetration model with the renewal time equal to the time during which liquid can flow unidirectionally before being interrupted by a packing corrugation. Their correlations are
kL = 2[DLUL,eff/(?rS)l'/2
(4)
in which ReG = (DeqPG/PG)(Ge,eff+ L,etf), Gs,eff = G$(0.90 s ) l Aeoi sin 60°), Ls,eff= [3L$(2f'Acol)1 [ P A c o l p ~ / ( 3 ~ d1/3, is the column cross-sectional area (m2),P is the available perimeter per unit cross-sectionalarea (594.2 m-1 for Sulzer BX), S is the side dimension of the corrugation (8.9 mm for Sulzer BX), and other symbols have their Nomenclature definitions. They assumed that the specific, gasliquid, interfacial area was equal to the specific area of the dry packing (492 m-1 for Sulzer BX). The correlations for the gas-film coefficient were based on distillation data for o/p-xylenes,ethylbenzene-styrene, methanol-ethanol, and ethylene-propylene glycols. Since most distillations are largely controlled by mass-transfer resistance in the vapor phase, we expect the ShG correlation to be reliable. However, in view of the fact that the liquid-fib coefficient, k ~ was , also extracted from these distillation data, the correlation for k~ is probably only an order of magnitude estimate. Bravo et al. (1986) have also presented a correlation for pressure drop across corrugated sheet structured packings.
Experimental Section Equipment. The equipment was very simple, consisting of a 150 mm diameter Plexiglas column packed with either seven sections of Montz A2 packing (152 mm per section) or six sections of Goodloe packing (170 mm per section). The column was fitted with an orifice-typeliquid distributor containing 36 perforations of 2.38 mm (3/ls in.) diameter weep holes and 12 19 mm diameter (3/4 in.) gas risers. Type-K thermocouples and sample ports were installed in the column feed and bottoms lines. Liquid feed was pumped from storage and metered with rotameters; superficial liquid velocitiesranged from 0.0028 to 0.011 m/s (equivalent to liquid flows between 0.8 and 3.2 gpm) giving a 4-fold change in Reynolds number. Bottoms liquid was removed via a flexible vinyl tube whose position was used to control the liquid level in the base of the column. Provision was made to cool the liquid using refrigeration coils immersed in the feed tank. Column air was provided by a Spencer VB-030-E vortex blower; flow control was had by releasing part of the blower
Table I. Physical Characteristics of Packing8 Studied Goodloe Montz B2 Gempak wire diameter (pm) 114 492 492 specific area (m-9 1920 0.83 0.83 void fraction 0.95 316-stainless 316-stainless material 316-stainless 6 6 height of elements 5 (mm) 11 11 crimp/cormgation 8 pitch (mm) layered, slit, structure spirally wound, layered, crimped woven and perforated knitted fabric fabric sheet
discharge to atmosphere (throttling the blower inlet resulted in the air overheating). Air flow rate was measured using a flange-tapped, 25.4 mm diameter, sharp-edged, orifice plate. This orifice was calibrated in situ in a nominal 50 mm diameter line using a Singer dry test meter because the requisite upstream straight run of pipe could not be fitted into the available space. Superficial air velocities were in the range from 0.46 to 1.0 m/s (actual gas flows used were between 18 and 39 SCFM). Solute gases (COZand SOz) m e supplied from cylinders and injected into the air stream 100mm upstream of the orifice plate. Gas temperatures were measured on the inlet and discharge sides of the column, and pressure drop across the packing was monitored. Procedures. Before measurements of any kind were taken, packings were washed in situ with caustic soda and then repeatedly flooded with water, and finally,the column was filled with water to the top and drained. These steps were taken in an effort to produce packing surfaces that were clean and readily wetted. Liquid holdup was measured by injecting 4 mL of a saturated salt solution into the inlet liquid through a tee branch fitted with a rubber septum and located in the feed line and then measuring the electrical conductivity of the solution coming from the bottom of the column. The signal from a flow-through conductivity cell was monitored by a conductivity meter, amplified, and passed through an analog-to-digital converter for recording on a portable PC by a data logging program; further details can be found in Evans (1990). The sampling interval was 1s, providing digital data in a very convenient form for numerical analysis. The quantity of interest in these experiments was the residence time (axial dispersion coefficients were not calculated), and a series of 'blank" runs was performed to determine the holdup in the piping and in the base of the column. The mass-transfer characteristics ~ G U k, ~ and a the interfacial area, a, were determined by measuring the rate of solute gas absorption into a suitable, chemically reactive solution. Absorption rates were determined from a solute balance on the gas phase with gas-phase compositions determined by gas chromatography. Liquid analyses were used only to check material balances (which closed within 10% for valid runs) because of the greater difficulty in measuring liquid compositions accurately. Procedures for the determination of the mass-transfer coefficients, koa and k ~ aand , the interfacial area, a, have been given in some detail in an earlier publication (Scheffe and Weiland, 1987) and have been described in general terms by Danckwerte (1970). It is well-known that these procedures yield mass-transfer coefficientdata for physical mass transfer, i.e., from which all reaction effects have been removed. Exactly the same procedures were followed here, and the interested reader is referred to Scheffe and Weiland (1987) and Ahlgren (1986) for complete details. Absorption of SO2 from air into aqueous caustic soda was used to determine kGu. This is a completely gas-
Ind. Eng. Chem. Res., Vol. 32, No. 7,1993 1413 phase-controlled process that has been used successfully in packed (Vidwansand Sharma, 1966)and trayed (Scheffe and Weiland, 1987)column applications; it presents fewer problems of toxicity and corrosivenessthan other chemical systems used for this purpose, such as iodine and chlorine absorption into caustic soda and ammonia into sulfuric acid solution. The drawback with this system, particularly in highly efficient contacting devices such as we are using here, is that the SO2 tends to be so readily absorbed that the outlet gas SO2 content from columns of reasonable packed height tends to be below chromatographically measurable limits. Thus, our initial experiments, conducted with the full complement of six or seven packing segments in the column, resulted in no measurable concentration of SO2 emergingfrom the column even when inlet SO2 concentrations as high as 16 mol % were used. As a result, the experiments to determine kGa could only be run using a single packing segment. It was found to be necessary to take inlet gas samples directly below the packing itself because significant absorption occurred in the (empty) base of the column. Even with these measures, it was found that up to 97% of the feed SO2 was absorbed. Gas sampleswere withdrawn slowly from a small (20-mm diameter) inverted glass funnel suspended in the gas space immediately below the packing support plate. Although there was no way to ensure that the integrity of samples was not compromised by, for example, absorption into moisture deposited on the inner wall of the sample tube, careful observation of the glass sample device revealed no internal moisture. Furthermore, on those occasions when even a very small amount of liquid entered the sampler, GC analysis of the samples became highly erratic. However, because the mass-transfer measurements were generally quite reproducible, it seems unlikely that a sampling error from the above cause operated. (Such an error ought to depend at least on the sampling rate and probably the column’s fractional approach to flooding.) Nevertheless, gas sampling must be allowed to remain as a possible source of (unknown) error and, if it occurred, one which would lead to high values of the reported kG data. At the other end of the test section, liquid distribution is a potential source of negative error. However, the liquid distributor contained a very high number density of distributor holes-2030 holes/m2 (more than 200 impinging liquid streams/ft2)-so the distance needed to reach an “equilibrium” flow distribution within the packing is likely to be quite short. Although possible errors from these two sources are opposite to each other, their magnitudes are unknown; as a result, the gas-film coefficient data reported here are possibly less reliable than what would be obtained using distillation data, for example. The interfacial area and liquid-film coefficient measurements did not suffer from this problem because the full packed depth of the column was always used, absorption rates were very much slower, gas samples did not have to be taken from within the column itself, and there was ample distance for the liquid flow to equilibrate. Interfacial areas were measured using COZabsorption into dilute NaOH from nominal 2% C02 in air under conditionsof pseudo-fiist-orderreaction. This has proven to be a convenient and reliable method by several workers [see Danckwerta and Sharma (196611. Experiments were conducted over the full range of gas and liquid flows available, and corrections were made for the small (less than 5 % ) amount of gas-phaseresistance to mass transfer. Finally, the liquid-film coefficient,kLa, was found from rates of absorption of CO2 into sodium carbonatel
bicarbonate buffer solutions. The carbonate and bicarbonate concentrations were kept at approximately 0.44 and 0.22 N, respectively,and the carbonate to bicarbonate ratio was kept at between 1 and 2 to obtain the desired buffering action. (A buffer is used to increase the capacity of the solution for C02 without much affecting the absorption rate.) C02 is quite sparingly soluble in water, and the buffer solution reacts only very slowly, so one would expect the amount of CO2 picked up to be quite small. This was found to be very much the case so that in the first few experiments (at ambient temperature) the amount of C02 absorbed was too small to measure accurately. Therefore, the solution was cooled to between 2 and 5 “C, giving a 2-fold increase in gas solubility; this was sufficient to give absorption rate measurements of reasonable accuracy and reproducibility. However, it was not possible to observe the effect of gas rate on kLa because of the necessity to run at the lowest practicable gas rate to produce accurately measurable changes in C02 concentration across the column.
Results and Discussion For the purpose of developing empirical correlations in terms of dimensionlessgroups for the gas- and liquid-side mass-transfer coefficients
Sh, = A‘RegReESc; we need to define a characteristic length on which to base the Reynolds number. Goodloe packing, being made from fine-gauge stainless steel wire knitted into a corrugated gauze strip, has such a poorly defined structure that assignment of a physically meaningful characteristiclength is difficult. Kanak (1980), for example, related the characteristic dimension to liquid holdup by developing a separate correlation. However, by doing this, he introduced implicitly a secondary correlation into the original, primary correlation, itself already empirical. This created added complexity for no practical gain. We have chosen the characteristic length to be the diameter of a sphere having the same dry surface area to volume ratio (specific area) as the packing (1920m-I). On this definition, the characteristic length is 0.313 mm. An alternative length can be defined to be the equivalent diameter of the flow channels, which is 4 times the void fraction divided by the specific surface area. This is probably the most appropriate length scale to use for thegas-phase Reynolds number, at least in a corrugated packing. For Goodloe packing, the wire diameter might be an appropriate length to use, since mass transfer takes place from a gas flowing over wires rather than along channels. For the liquidfilm flow, the length should ideally be related to the depth of the liquid film, a very difficult quantity to fix because of the constantly changing angle of corrugated packing surfaces and the almost random orientation of the wires in Goodloe packing. In any case, in the present work a constant length scale has been used for a given packing and the resultling correlations can readily be converted to some other length parameter if desired. Montz A2 packing is also constructed of very fine stainless steel wire, but the wire is woven into a fabric (which has wicklike characteristics) and then stamped into corrugated sheets. These sheete are cut and layered, one upon the other, so that corrugations in adjacent sheets run opposite to each other at an angle of 60”. This
1414 Ind. Eng. Chem. Res., Vol. 32, No. 7, 1993
1
o+ 0
I
1
2
3
1
I
4
6
,
'
'
I
8
1
'
0
e
1
1
2
0 4 ,
1
1
0
4
Gas flowrate ( kg/hr/m2 1
i
Figure 1. Holdup in Goodloe packing. Liquid flow rates are ( X ) 19 700, (A)39 200; (0) 58 OOO; (+) 77 100; (0) 96 200 kg h-l m-2.
0
2
4
6 8 [Thousands I Gas flowrate lkg/hr/m
I
,
v
I
4
6
I
I
,
8
,
1
,
0
1
, , I 2
1
4
I Thousands 1
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1
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Figure 2. Holdup in Montz A2 packing. Liquid flow rates are as in Figure 1.
arrangement produces channels with alternately triangular and rhomboidal cross sections. The dry specific area is reported by the manufacturer to be 492 m-l (ChemPro Bulletin HV610). The characteristic length for this packing has been calculated (Bravo et al., 1985) from the average of the equivalent diameters of the triangular and rhomboidal cross-sectioned flow channels to be 7.23 mm; this is the value used in our correlations. Except for the random fluctuations in physical properties caused by daily temperature changes, the physical properties groups, SCGand SCL, were not varied in these experiments. Property fluctuations were accounted for by assuming the commonly accepted values of one-third and half for the exponents c and c', respectively, and Sc has been included in the correlating equations for convenience of use. Values for the correlation constants and exponents were obtained from least-squares fitting of the correlating equations to the data. Liquid Holdup. Holdup data for Goodloe, Montz A2, and Gempak packings are presented in Figures 1-3, respectively. The commonly defined types of holdup in packed columns are static and dynamic. Static holdup is the volume of liquid remaining within the packing when there is neither gas nor liquid flowing to the column; dynamic holdup may be thought of as the volume of liquid that drains from the packing when the gas and liquid flows are suddenly shut off. In the present work, we have measured the holdup time using the dynamic tracerresponse method, a technique that gives the total effective holdup. It will not, for example, account for isolated
Gas f l o w r a t e l k g / h r / m 2 )
Figure 3. Holdup in Gempak. Liquid flow rates are as in Figure 1.
pockets of (static) liquid trapped within the column but which are unable to communicate with a liquid flow. In the context of mass transfer where the liquid must be renewed periodically to participate in the separation process, the holdup as measured using a tracer is the one of real interest, and we note that it is not necessarily equal to the sum of the so-called static and dynamic holdups. The data shown indicate that, at a particular Iiquid load, holdup is fairly independent of gas rate until a high gas rate is reached. This is in agreement with the work of others (McNulty and Hsieh, 1982; Sherwood et al., 1975). As the gas rate is increased further, the holdup begins to rise and, ultimately, becomes very large as the column floods and a large fraction of the tower volume becomes filled with liquid. Interfacial Area. Interfacial area (m-1) was found to correlate directly with the gas-side F factor (F = GJI;', where G, is the superficial gas velocity in m/s and PG is the gas density in kg/m3)and to be independent from the liquid flow rate. The regression equations were found to be, for Goodloe packing a =356p2
(7)
and for Montz A2 packing a =265p4 (8) For Goodloe packing, the specific area varied from 455 to 322 m-l as the gas flow varied from 0.37 to 1.0m/s; compare these areas with the specific area of the dry packing, 1920 m-I. For Montz A2 packing, the corresponding variation in interfacial area was from 434 to 242 m-1 over the superficial gas flow range from 0.37 to 1.3 m/s; the area of the dry packing was 492 m-l. A proportionately much greater fraction of the total surface area of the Montz packing was wetted compared with the Goodloe packing, a fact that is undoubtedly due to the wicklike behavior of the wire gauze from which the Montz A2 packing is made. All of the data for the Goodloe and Montz packings are shown plotted in Figures 4 and 5, respectively, along with the 95% confidence limits on the lines of best fit. We note that over a more than 10-foldrange in liquid rate, no trend with this variable was in evidence and it can be concluded that, unlike dumped packings, the liquid rate does not affect interfacial area for the high-performance packings studied here. In Goodloe packing, the liquid flow is predominantly along the wires comprising the packing (ourresultsindicate that only about 20% of the strands carry a liquid flow) and the fraction of wires that carries significant liquid
Ind. Eng. Chem. Res., Vol. 32, No. 7, 1993 1415
t
t 7517
1 0 6 oe F lkg/rn~I~’~
0. 4
i o
12
Figure 4. Correlation for effective interfacial area of Goodloe packing.
425
5 N ‘
m
325
e
4
225
I
I
0 3
0 4
I
I
08 F
I
l
12
l
16
(kghs)1’2
Figure 5. Correlation for effective interfacial area of Montz A2 packing.
flow is possibly decided by capillarity. The only way for liquid to spread out is by spreading to adjacent wires through the nodes that connect them. A plausible explanation for the lack of a liquid flow effect is that capillary forces at nodes constrain the liquid to flow along certain strands and not along others because additional flow along wires already carrying liquid can be accommodated more easily than the opening up of new wires to the flow through the movement of solid-liquid-gas contact lines. We speculate that this effect is possibly related to the critical contact angle in the particular solid-liquidgas system. For the Montz packing, on the other hand, we already have a wicklike material most of which is readily wetted. From Figure 5 it can be seen that at the lowest gas rate almost 90% of the geometric area is effectively wetted. (The remaining 10% may be wetted, too, but may have insufficient liquid flow to participate much in mass transfer, the very process we are using to infer interfacial area.) There is reason to believe (see below) that the gas flow might be responsible for causing the liquid to flow over only a certain part of the packing surface within a gas flow channel in this highly structured material. All of the packed surface is probably wetted, but the gas flow and the structure conspire to confine the liquid to a preferred (albeit large) region on the solid surface of the channel. Nonetheless, for both packings, we have the somewhat surprising result that the effective interfacial area for mass transfer decreases with increasing gas rate, although the decrease is quite a weak one for Goodloe compared with that for corrugated packing. One of the major differences between Goodloe and corrugated packings is the highly structured nature of the latter, whereas, despite the manufacturing process of knitting, crimping, and winding,
Goodloe packing is much closer to being (but is not completely) stucturally random in that it does not really contain well-defined flow channels. This might tend to suggest that because corrugated packings already possess a large-scale, structural nonuniformity, they are able to accept higher gas flows without flooding, in part because the liquid flows along broad preferred paths, at least at the level of the dimensions of the corrugations. It is, after all, structure that often induces preferential flow, as in the wall region of columns with packing to diameter ratios that are too big, and it is undoubtedly structure that results in low pressure drop for corrugated packings. This is not to suggest that structured packings are characterized by gross liquid channeling but rather that within a (gas) flow channel the liquid becomes confined to only part of the channel as the gas flow increases. (Again, it should be pointed out that even though Montz packing has a wicklike character and most of the packing is probably always wetted, much of this apparently wetted surface will be ineffective for mass transfer if there is only small to negligible liquid turnover on it.) Goodloe packing, being much less organized structurally, shows only about a 10% decrease in interfacial area over the entire gas flow range. The latter is not inconsistent with what has been reported for dumped packings; the Onda et al. (1968) correlation for dumped packings, for example, shows no dependence on gas flow at all. One might argue that if the structure of corrugated packings leads to a negative dependence of interfacial area on gas flow rate, then one ought to see reduced separation efficiency. However, the gas-film coefficent, k G , for mass transfer is an increasing function of gas rate. In distillation the product kw is the important parameter, in most instances being equivalent to the overall coefficient, K w a , because liquid-phase resistance is negligible. Our data indicate that, overall, kGa is an increasing function of gas rate for both packings, just as one would expect. Gas-SideMass-TransferCoefficient. The regression equations were found to be, for Goodloe packing ShG = 0.0567Reh’oSci3 (9) with Reynolds numbers ranging from 94 to 210 and, for Montz A2 packing Sh, = 0.0373Reko2Sci3 (10) in which the Reynolds number range was 236-470. The k G and a parameters were separated using the already established correlations for interfacial area, eqs 7 and 8. The nondimensional correlations for Goodloe and Montz packings are shown graphically in Figures 6 and 7, respectively, along with their 95 % confidence limits. Corresponding comparisons with the correlations of Kanak (1980) and Bravo et al. (1985) for the effect of gas rate on kGa are made on a dimensional basis in Figures 8 and 9. Values of kca found for Goodloe packing are roughly twice the values found for Montz A2; such differences are to be expected because of the more tortuous path of the gas through Goodloe packing, compared with the more open flow channels within the Montz A2 packing. It is more difficult to compare directly the Kanak and present correlating equations because of the different definitions of the length scale in the Reynolds number (Kanak’s Reynolds number contains a liquid flow rate dependent length). However, the graphical comparison of Figure 8 shows our data to be a factor of nearly 3 greater than Kanak’s correlation gives. We have no explanation for this discrepancy but, as discussed above, end effects in our work are an unlikely cause; if sampling errors are the
1416 Ind. Eng. Chem. Res., Vol. 32, No. 7, 1993 20
0 50
1
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0 45
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15
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*
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a , ,
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,
,
,
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Figure 9. Comparison between data and the correlation of Bravo et al. (1985) for the effect of superficial gas velocity on gas-film coefficient for Montz A2 packing.
c
301
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400
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...... 0 0
-.-a. ...G.&. ............ 0
0
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0
t
1
c
1 0 1
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1
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............. 06]a.-. u --_. ..........
l 5
200
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Figure 6. Dependence of nondimensional gas-film coefficient on gas-phase Reynolds number for Goodloe packing. Superficial liquid velocity = 5 mm/s.
150
.
08
ReG
20
,
0 6
OL
225
I I , ,
I
I
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I
1
5
10
15
20
25
30
ReG
Figure 7. Dependence of nondimensional gas-film coefficient on gas-phaseReynoldsnumber for Montz A2 packing. Superficialliquid velocity = 6 mm/s.
I
0
Figure 10. Dependence of nondimensional liquid-film coefficient on liquid-phase Reynolds number for Goodloe packing. Superficial gas velocity = 0.37 mls.
I
10
F .......... .............
c
0 0
t.. ...... 0 25
I . .
0 50
........................... 0 75
100
............
J
125
Gs ( m / s )
Figure 8. Comparison between data and Kanak's (1980)correlation for the effect of superficial gas velocity on gas-film coefficient for Goodloe packing.
reason, they would be expected to lead to considerably more scatter than is seen in the data. Nevertheless, SO2 absorption into microscopic liquid droplets adhering to the inner wall of the sampling device would lead to high values of kG; in view of the difficulty in obtaining intracolumn samples that is generally experienced in this type of work, degeneration of the samples on passage through the sampler cannot be discounted altogether. Our data are also a factor of about 2 higher than the correlation of Bravo et al. for Sulzer BX packing. Liquid-Side Mass-Transfer Coefficient. Correlations for the nondimensional liquid film coefficient, ShL, are, for Goodloe packing
1
C I
10
I
I
20
I
I
30
l
l
40
Ret
Figure 11. Dependence of nondimensional liquid-fii coefficient on liquid-phase Reynoldsnumber for Montz A2 packing. Superficial gas velocity = 0.26 m/s.
Sh, = 3.4RezmScz2 and for Montz A2 packing
(11)
Sh, = 5.2RezMScy2 (12) Just as for the gas-sidecoefficients,these correlations were obtained by removing the interfacial area as determined from eqs 7 and 8. Comparisons of these regression equations with all of the experimental data for each of these packings are shown in Figures 10and 11,respectively, where the 95 % confidence limits are also plotted. Comparisons with the correlations of Kanak (1980)and Bravo
Ind. Eng. Chem. Res., Vol. 32, No. 7,1993 1417 0.030
O OZ5
packings. For the first time, liquid-film coefficients obtained using a liquid-phase dominated processhave been reported.
~
t
/
Acknowledgment This work was supported in part by grants from Dow Chemical U.S.A. and United Technologies Corp.
2 1
Nomenclature
0 005
c
0 000 0 0000
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I
0 0025
I
I
0 0050 L,
0 0075
0 0100
0 0125
irnlsl
Figure 12. Comparisonbetweendata and Kanak's (1980)correlation for the effect of superficial liquid velocity on liquid-film coefficient for Goodloe packing. 0 075
/
0 065
1
'Od
0.035 0.025 0.015
t1
(8-1)
t
0 005 b 0 0000
a = specific surface area (m-l), or exponent in correlating equation A = constant in correlating equation b = exponent in correlating equation c = exponent in correlating equation D = diffusion coefficient of solute gas (m2 8-1) D , = equivalent diameter, characteristic length scale of packing (m) F = gas-side F factor (U@Z2)(kg112m-112 s-W) g = gravitational acceleration (ms-2) G = gas velocity (m s-1) kG = gas-film coefficient for mass transfer (kmolm-35-1 atm-1) kL = liquid-film coefficient for mass transfer (m 8-1) KOGU= overall mass-transfer coefficient based on gas phase (kmol m-2 s-1 atm-1) KOLU= overall mass-transfer coefficient based on liquid phase
0.
08
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" 0 0 .
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I
I
I
I
0 0025
0 0050
0 0075
0 0100
0 0125
L,
im/sl
Figure 13. Comparison between data and the correlation of Bravo et al. (1985)for the effect of superficial liquid velocity on liquid-film coefficient for Montz A2 packing.
et al. (1985) are made in Figures 12 and 13, where kLa is shown as a direct function of superficial liquid velocity. (It is noted that because we were unable to detect absorption with enough accuracy at high gas rates, the data on these plots were all gathered at the small, constant gas flow rates shown in the figures. Gas flow is not expected to affect the liquid film coefficient, k ~unless , it influences turbulence levels or flow patterns in the liquid film.) Over the range of liquid velocities used in these experiments, we did not detect a statistically significant dependence of the liquid-film coefficient on this variable. As can be seen from Figure 12, our data agree quite well with the magnitudes of kLa reported by Kanak, but not the trend. Finally, our results differ significantly in both the magnitude of kLa and its dependence on liquid rate from the purely theoretical equation of Bravo et al. (1985). It is interesting to note that this theoretical equation predicts no effect of gas rate on kL, a prediction we substantiate experimentally but which is in disagreement with the findings of Kanak for Goodloe packing. Conclusions. We have presented new mass-transfer and holdup data for two examples of high-performance packings. Film coefficients for mass transfer and the interfacial areas effective for gas-liquid contact were measured using well-established, chemically reactive, gas absorption systems. Results obtained are self-consistent but, in several respects, they are in disagreement with the limited results presented by Kanak (1980) and Bravo et al. (1985). Of particular interest, we have found that interfacial areas are a very substantial fraction of (but are certainly not equal to) the specific surface areas of the dry
L = liquid velocity (m 8-11 Re = Reynolds number (U&p/p) Sc = Schmidt number (pIpD) ShG = gas-phase Sherwood number for mas8 transfer (kGD,RTIDd SILL= liquid-phase Sherwood number for mass transfer (kOeqlDL) U = phase velocity (m 8-11 Greek Symbols CL = phase shear viscosity (Pa s) A = 3.14159... p = phase density (kg m-3) Subscripts eff = effective G = refers to gas phase L = refers to liquid phase s = superficial
Literature Cited Ahlgren, K. R. Mass Transfer Characteristics of Structured Mesh Packings. M.S. Thesis, Clarkson University, Potadam, NY, 1986. Ayala, J. S.;Brian, B. W.; Sharon, A. C. Floodingand Mass Transfer in Goodloe-Packed Columna; USDOE Report ORNL/MIT-253; Oak Ridge National Laboratories: Oak Ridge, TN, 1977. Begovich, J. M.; Watson, J. S.Flooding Characteristics of Coodloe Packing; USDOE Report ORNL/TM-6212; Oak Ridge National Laboratories: Oak Ridge, TN, 1976. Bragg,L. B. Goodloe Column Packing-a New Knit Packing Material Ind. Ena. for Vapor-Liquid Contacting- Operations. - Chem. 1967. 49,1os2-1066. Bravo. J. L.: Rocha. J. A.: Fair.J. R. MassTransfer in GauzePackmess. Hydrocarbon hocess. 1985, 64 (l),91-95. Bravo, J. L.; Rocha, J. A.; Fair, J. R. Pressure Drop in Structured Packings. Hydrocarbon Process. 1986, 65 (3), 46-49. Chen, G.K.; Kitterman, L.; Shieh, J. High-Efficiency Packing for Product Separation. Chem. Eng. Prog. 1983, 79 (9) 46-49. Choi, W. M.; Michel, R. C.; Varlet, J. P. Flooding Characteristics of Packed Columns with High Efficiency;USDOE Report ORNL/ MIT-223;Oak Ridge National Laboratories; Oak Ridge, TN, 1976. Danckwerts, P. V. Gas Liquid Reactions; McGraw-Hill Book New York, 1970.
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1418 Ind. Eng. Chem. Res., Vol. 32,No. 7, 1993 Danckwerts, P. V.; Sharma, M. M. The Absorption of Carbon Dioxide into Solutions of Alkalis and Amines (with Some Notes on Hydrogen Sulfide and Carbonyl Sulfide). Chem. Eng. (Rugby, Engl.) 1966,244. Evans, M. Liquid Holdup in Structured Packings. B. E. Thesis, University of Newcastle, Newcastle, NSW, Australia, 1990. Glitsch, Inc. “Goodloe”; Bulletin 520A,Dallas, TX, 1981. Kanak, B. E. Analysisof Gas Absorption System with Soluble Carrier Gas and Volatile Solvent. M.S. Thesis, USDOE, Oak Ridge National Laboratories, University of Tennessee, 1980. McNulty, K. J.; Hsieh, C.-L. Hydraulic Performance and Efficiency of Koch Flexipak Structured Packings. Presented at the Annual Meeting, AICW, Los Angeles, CA, Nov 14-19,1982.
Onda, K.; Takeuchi, H.; Okumoto, Y. Mass Transfer coefficients between Gas and Liquid Phases in Packed Columns. J. Chem. Eng. Jpn. 1968,1 , 56-62. Scheffe, R. D.; Weiland, R. H. Mass Tranefer Characteristica of Valve Trays. Ind. Eng. Chem. Res. 1987,26,228-236. Sherwood, T.K.;Pigford, R. L.; Wilke, C. R. Mass Transfer; McGraw-Hill: New York, 1975;p 603. Vidwans, R. D.; Sharma, M. M. Gas Side Mass Transfer Coefficients in Packed Columns. Chem. Eng. Sci. 1967,22,673-684. Received for review July 27, 1992 Revised manuscript received March 9,1993 Accepted March 18,1993
