Environ. Sci. Technol. 1989, 23, 309-314
Effect of Liquid Maldistribution on the Performance of Packed Stripping Columns Peter Harrlott* School of Chemical Engineering, Cornel1 University, Ithaca, New York 14853
When stripping moderately volatile organic compounds from dilute aqueous solution in a packed tower, the apparent height of a transfer unit is often much greater than predicted from published correlations. The increase over the expected value is due mainly to the inherently uneven liquid distribution in the packing, which has a large effect on mass transfer at low stripping factors. An empirical correlation of the increase in HOL with the stripping factor fits the data for several systems.
Introduction Volatile organic compounds can be stripped from water by countercurrent contact with air in a packed column. Often the liquid phase has the controlling resistance, and the performance of the column can be predicted fairly well by using published correlations for liquid-phase mass transfer. However in a number of cases, anomalously low mass-transfer coefficients were reported, and it was not clear whether these were due to experimental error or to mass-transfer resistance in the gas phase, backmixing of the gas, or uneven distribution of the liquid. The differences in packing size, packing density, and liquid distribution make it difficult to compare isolated results from different studies and determine the reason for the low coefficients. Some recent studies provide data for multicomponent systems, so that mass-transfer coefficients for different solutes can be compared at identical flow conditions. This makes it easier to correlate the data and determine the relative importance of the factors that could lead to low coefficients. In this paper, recent data for stripping chlorinated solvents from water are reviewed, and the possible reasons for low mass-transfer coefficients are discussed. The major factor is believed to be maldistribution of the liquid, and a simple model is used to illustrate the effect of maldistribution on the apparent height of a mass-transfer unit. Air Stripping Data An extensive study was carried out by Gossett and coworkers (1) to measure the equilibrium constants and the mass-transfer coefficients for stripping five chlorinated organic solvents from dilute aqueous solutions. A 44.5-cm column was used with 2.44 m of packing, and the masstransfer coefficients were determined from liquid samples taken at 0.3-m intervals between the top of the packing and the support plate. The packings included 5 / 8 - , 1-,1.5-, and 2-in. plastic Pall rings, and tests were made at temperatures from 10 to 30 "C. Gossett's data for volumetric coefficient were converted to height of a transfer unit by the relationship HOL = U L / K L ~ (1) The stripping factors were calculated from the slope of the equilibrium line and the molar flow ratios or from the ratio *Present address: Dept. of Energy, Pittsburgh Energy Technology Center, P.O. Box 10940, Pittsburgh, PA 15236-0940. 0013-936X/89/0923-0309$01 S O / O
of superficial velocities and a modified Henry's law constant, H'. S = m V / L = H'UG/UL (2) The values of HOL for three packings are plotted against the stripping factor in Figures 1-3, where S is varied in a different way for each set of data. The effect of solute type at constant gas and liquid flows is shown by the lower line in Figure 1. For 1-in. Pall rings with uL = 0.92 m/min and UG = 7.3 m/min, HOL is -0.7 m for trichloroethylene, l,l,l-trichloroethane, and perchloroethylene, but HOL is considerably higher for chloroform and methylene chloride. The C1 compounds have aqueous-phasediffusivities about 10-30% greater than the Cz compounds, and the liquid phase is expected to have the controlling resistance, so the high HoL values for CHCl:, and CHzCl2were unexpected. Some increase in the gas film resistance does occur as S decreases ( m is decreased), but this should be offset by the decrease in HL. (3) HOL = HL + HG/S On the basis of the data for oxygen desorption and ammonia absorption with 1-in. rings, HoL is predicted to be between 0.62 and 0.66 m for all five compounds. The details of this prediction are discussed in the section on gas film resistance. The data for the dashed line of Figure 1were taken at 10 or 30 "C, and the HOL values were corrected to 20 "C assuming KLa increases by 2.4%/"C (2). The values of m were not changed, so there are two values of S for each solute at constant values of V and L. The corrected data then show the effect of about a 3-fold change in m for each solute. Decreasing m has a large effect on HoLfor CHZClz and CHC1, but not much effect for the other solutes, which have higher stripping factors. The increase in HOL at low values of S is similar to that for the 26 "C run. The limiting value of HOL at high values of S is -1.0 m instead of 0.7 m, the difference coming from the higher liquid rate and from the lower temperature (20 "C). Data for 2- and 5/8-in. Pall rings at 25 "C are shown in Figure 2. The liquid velocity was held constant at 0.92 m/min, and the air velocity was varied from 4.56 to 50.2 m/min. The flow rates are below the loading point, and the air rate should have no effect on HL, though HG will increase with air rate. The nearly constant HOL at high values of S is consistent with liquid film control, but the rise in HOL at low S is again much greater than expected from the predicted magnitude of H L and HG. The limiting values for H O L of -0.76 m for 2-in. rings and 0.57 m for 5/8-in. rings are consistent with the data for 1-in. rings and with other studies of mass-transfer coefficients for different packing sizes. Figure 3 shows HOL for a series of runs at constant air rate and varying liquid rate. Although a line with a slope of -0.12 would be a reasonable correlation of all the data, the individual sets of data suggest that HoL varies with a higher power of S or L. The range of liquid flows is not great enough to determine the exponent accurately, but the results are consistent with published data for systems
0 1989 American Chemical Society
Environ. Sci. Technol., Vol. 23, No. 3, 1989
309
0
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A
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4
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cc14
o d d
CCIp;CCI,
al
ad
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'
' ' 0.4 0.6 0.81.0 "
'
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Stripping Factor S
Flgure 1. Height of a transfer unit for l-in. Pall rings (Gossett): (-) uL= 0.92 m/min, uQ = 7.3 m/min, T = 26 O C ; (---) uL= 1.36 m/min, uQ = 10.95 m/min, T = 10 or 30 OC, corrected to 20 OC.
0 A A
CHzC12 CHCIS C H C l i CClp
0'41 0.2
0. I
2
0.6 0.8 I
4
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40
Stripping Factor S
Flgure 2. Height of a transfer unit for 5/8- and 24%Pall rings (Gossett): u L = 0.92 m/min, T = 25 O C , uQ varled.
O 0 43I I- - & - - J
2
3
4
6
8 IO
20
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Stripping Factor S
Flgure 3. Height of a transfer unit for l-in. Pall rings (Gossett): uQ = 25.6 m/min, T = 25 OC, uL varied; symbols same as Figure 2.
showing that H L varies with about L-0.3. The stripping factor was greater than 2 for these tests, so the data do not show the rapid increase in H O L as S approaches and becomes less than 1.0. However, the system with the lowest value of S, CH2C12,does show the most scatter in the data. 310
Environ. Sci. Technol., Vol. 23, No. 3, 1989
01 01
I
02
0304
060810
2
3
4
6
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810
Stripping Factor S Flgure 4. Height of a transfer unit for 0.5411. Berl saddles (Roberts).
In a study by Roberts and co-workers (3, 4), seven halogenated compounds and oxygen were stripped from water in a column packed with 0.5411. ceramic Berl saddles. Runs were made over a 10-fold range of liquid rates and a 16-fold range of air rates, and the packed height was varied to determine the end effects. For the most volatile compounds, the values of K L u , corrected for end effects, were independent of gas rate and increased with the 0.3-0.5 power of the liquid rate. However, KLuvalues for the least volatile compounds increased considerably with gas rate and were either independent of liquid rate or decreased with increasing liquid rate. Use of the two-resistance model of Onda and co-workers (5) gave predicted masstransfer coefficients with an average standard deviative of 21 % , but the error was much greater for solutes of low volatility and at low air rates. Roberts' data were converted to height of a transfer unit and plotted against the stripping factor in Figure 4. This plot includes several points at low S that were omitted from the 1985 paper because of the greater error in determining K L u at low S; The data for oxygen and CC12F2 are not included in Figure 4,because the stripping factors were greater than 10, and the data followed the expected trends. The lower line in Figure 4 correlates the data for a liquid rate of 1.0 L/min (uL = 0.026 m/min). For S > 4, HOLis nearly independent of air rate and is about the same for the different solutes. This is expected, since the liquid resistance is controlling, and the diffusivity changes only about 20% from the smallest to the largest solute. For lower stripping factors, there is a pronounced increase in H O L , similar to that shown in Figures 1 and 2. The data for higher liquid rates should fall on separate lines, with H O L increasing with liquid rate, but there is too much scatter and not enough points at high values of S to define these lines. The dashed line was drawn as an approximate fit to the data at 4 and 10 L/min. The curves in Figure 4 are much like those shown in Figures 1 and 2, with a pronounced increase in HOLas the stripping factor becomes 1.0 or smaller. The ratio of HOL at S = 1 to the value at high S is 1.6-2.0 for all the sets of data. Similar results have been reported in other studies. Ball et al. (6)found low mass-transfer coefficients for bromoform when the stripping factor was less than 3,
but normal values for chloroform and other solvents that had high stripping factors, Cummins and Westrick (7) reported low KLa values for stripping trichloroethylene at air/water ratios of 5,8, or 15, but normal values at higher ratios. The increase in apparent mass-transfer resistance at low stripping factors has been attributed to gas film resistance or to gas backmixing, but it could also be caused by uneven liquid distribution. These possible explanations are discussed in the following sections.
Gas Film Resistance Although the liquid film generally has the controlling resistance to mass-transfer when stripping volatile solutes from water, the gas film resistance becomes more important at low gas rates and might lead to a significant increase in HTUoL as S decreases. The magnitude of the gas film resistance can be estimated from general correlations or from data for gas film controlled systems, such as NH3-air-water, but care must be taken when extrapolating to low gas rates. In the normal operating range k,a varies with about Go.’, but this type of exponential correlation underestimates k,a for very low flow rates, since it does not give a limiting value as G approaches zero. Theoretical and experimental studies of mass transfer in one-phase flow through packed beds by Suzuki (8) show that the transfer coefficient approaches a constant value at low flows, and this value corresponds to a Sherwood number of 5-10. A similar limiting coefficient must exist for mass transf6r in a stripping or absorption column, and a Sherwood number of 5 is used to estimate this minimum gas film coefficient in the following example. Consider Roberts’ data for stripping trichloroethylene (TCE) from water at 20 OC with 1.0 L/min air and 1.0 L/min water in a column packed with 0.5-in. Berl saddles. The flow rates were G = 0.62 lb/h.ft2 and L = 309 Ib/h.ft2 and 0.4 kg/s.m2), giving a stripping factor S = (8.4 X 0.4 and H O L = 1.32 m (Figure 4). From Fellinger’s data for NH3 absorption (9), HG = 1.4 ft at L = 500 lb/h.ft2 and G = 200 lb/h.ft2 (the lowest flow rates used with 0.5-in. saddles.) Correcting for the lower diffusivity for TCE and extrapolating to the lower flow rates assuming HG varies would give HG = 0.59 f t for TCE. This with G0*3L-0.5D-0.67 value may seem reasonable at first, but it corresponds to a gas film volumetric coefficient, k,a, of only 3.83 X s-l, and if the effective mass-transfer area is 5 ft2/ft3(IO), k , = 7.66 lo-* ft/s (2.34 X cm s). Based on the film theory and D = 8.5 X cm /s, the effective filp thickness would be z = D / k , = 3.64 cm, which is 3 times the diameter of the packing! This shows that the data should not be extrapolated to very low flows by using exponents measured at high flows. Instead, a limiting Sherwood number of 5 is assumed, correspondingto a film thickness d,/5 or 0.25 cm, and the predicted value of HG is then 0.021 ft or 0.0088m. Using the Sherwood and Holloway data for O2 desorption (2) and correcting for the lower diffusivity gives a predicted H L of 0.25 m. Combining these terms predicted HOL= 0.25 + 0.0088/0.4 = 0.27 m
k
The gas film term contributes 8% of the predicted H O L but only 2 % of the measured H O L . Although the estimate of HG is uncertain, it is unlikely that increasing gas film resistance accounts for any significant part of the increase in H O L shown in Figure 4. The data of Gossett are for much higher mass velocities than those of Roberts, and only a slight extrapolation of the NH3 data is needed to predict HG for the organic solutes. For the conditions of Figure l , uL = 0.92 m/min and
uG= 7.3 m/min, the mass velocities are L = 11300 lb/h*ft2 (15.3 kg/s.m2) and G = 108 lb/h.ft2 (0.147 kg/s-m2). From Fellinger’s data for NH3 absorption with l-in. Raschig rings 0.47 f t (0.14 m) at G 200 and L = 4500 lb/h.ft2. (9), HG Correcting for the differences in flow rate and diffusivity, the predicted H G is 0.15 m for CH2C12. Similar calculations for the liquid film were based on oxygen desorption data. The predicted overall heights of a transfer unit for the three least volatile solutes are CH2C12 H O L = 0.50 0.15/0.97 = 0.65 m
+
HoL = 0.54 + 0.16/1.43 = 0.65 m
CHC13 CHClCC12
HOL
= 0.57
+ 0.17/3.44 = 0.62 m
Thus while the percentage of the total resistance in the gas phase does increase as the stripping factor decreases, the change is largely offset by the decrease in HL, and the large change in H O L found at low values of S cannot be attributed to the gas film resistance. Other generalized correlations for mass-transfer coefficients, such as those of Shulman (10) or Onda (5) lead to the same conclusion, that the gas film resistance is relatively small, even for the least volatile solutes, CH2C12 and CHC13. Gossett showed that his data could be correlated by modifying Onda’s equations to increase the gas-phase resistance by a factor of 7.4 while halving the liquid-phase resistance, but there is no evidence for such extreme changes in the correlations.
Backmixing of Gas Some of the increase in H O L at low values of S could be due to backmixing of the gas or to axial dispersion, which are more important at low gas rates and high liquid rates. When the actual velocity of the liquid is much greater than the upward velocity of the gas, gas may be drawn downward in some sections of the column and mixed with upflowing gas. This would decrease the driving force for mass transfer and increase the apparent HOL, since H O L is calculated by assuming plug flow. Axial dispersion due to molecular or eddy diffusion would also lead to a flattening of the concentration profile, a lower driving force for mass transfer, and an increase in the apparent HOL, but without any backward flow of gas. The effect of axial dispersion on H O L is greatest for low values of S and for a column with a large number of transfer units, as shown by the theoretical analysis of Miyauchi and Vermeulen (11). However, published values of the Peclet number for axial dispersion in the gas-phase differ widely (121, and there are no data for very low gas flow rates. Furthermore, the axial dispersion model may not be realistic for packed columns, since departures from plug flow caused by velocity gradients, stagnant pockets, or reverse flow of gas are not considered in the model (13). For these reasons, the axial dispersion model was not used to analyze the data shown in Figures 1-4, and instead, comparison was made to other experimental work on stripping at low gas velocities. The stripping of C02 from water is clearly liquid-phase controlled, but Cooper et al. (14) found up to a 2-fold increase in HOLas the superficial gas velocity was reduced from 1to -0.1 ft/s. This effect was more pronounced at higher liquid rates, and the data were correlated with the ratio of the average linear water and gas velocities. Some of these data are plotted in Figure 5 against the ratio of superficial velocities, which permits easier comparison with other studies for which holdup data are not available. There is a significant increase in H O L for COzwhen uL/uG exceeds 0.2, and at this point, the average linear velocity Environ. Sci. Technol., Vol. 23, No. 3, 1989 311
2 HOL I
m
I
01 0.01
,I
0.02
0.04 006 0 I
I
02
I
I
I
04 0.6
I
1
I
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Figure 5. Effect of velocity ratio on height of a transfer unit for 2-in. rings (Gossett, Cooper).
of the water is more than 2 times that of the gas. The authors attributed the increase in H O L to induced circulation of the gas from top toward bottom, but there was no direct evidence of backmixing. The data for the chlorinated solvents in Figure 5 show similar increases in HOL, but the upswing starts at much lower ratios of uL/uG. Since the extent of backmixing should depend on the gas and liquid velocities and not on the type of solute, the high values of H O L for methylene chloride and chloroform cannot be attributed to backmixing of the gas, though partial backmixing might have some effect on HOL. Direct evidence for gas backmixing at high ratios of UL/UG was obtained by Hatton and Woodburn (12)in their study of C02 absorption in a column packed with 1-in. Raschig rings. They measured axial concentration profiles for the gas, and at R k = 1312 (uL= 3.1 m/min) they found an abrupt concentration drop at the gas inlet, which is consistent with the axial dispersion model. At a lower liquid rate, ReL = 500 (uL = 1.2 m/min), the concentration profile near the inlet was steeper than predicted for plug flow but did not show a discontinuity. Their values of HOL were -50% higher than normal for uL/uG = 1.4-2.2; while this increase can be attributed to backmixing, the effect should be much less at the lower ratios of u L / u ~used in the stripping tests (see Figure 5).
Maldistribution of Liquid Liquid flow in a packed column is inherently nonuniform because of the random arrangement of packing pieces and the limited number of liquid distribution points. Liquid tends to move to preferred channels as it passes down the column, and local flow rates may be several times the average (15). For small columns a large fraction of the liquid may flow along the wall after a short distance from the distributor. Porter and Templeman (16)estimated the wall flow to be 40-60% of the total for a column diameter 10 times the packing size and 10-20% of the total for columns 25 times the packing size. They also reported rapid mixing of wall fluid with the rest of the liquid, but Hoek (15)said there was very slow equilibration between the wall region and the rest of the packing. Thus the quantitative effect of wall flow on mass transfer is uncertain, though wall flow is generally considered detrimental. For large-diameter columns, the wall flow may be relatively small, but channels still develop in the interior of the column, and having regions of high local flow rate could have as much of an adverse effect as high wall flow. 312
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Although channeling probably reduces the area for mass transfer somewhat, the main effect is to change the local ratio of liquid to gas flow, which changes the local value of the stripping factor. Since the driving force for masstransfer increases with S, sections of the column where S is low will tend to have lower solute removal. Sections with low liquid rate and thus high S will have higher than average solute removal, but not enough higher to offset the low removal in other sections. If there is only limited mixing of liquid in different channels, the overall removal in a tall column may be considerablyless than if the liquid was uniformly distributed. The effect of maldistribution could be large when S is close to 1.0, since a local value less than 1.0 could lead to a “pinch as the operating line approached the equilibrium line. If S is quite large, say 4-5, having a distribution of local stripping factors from 3 to 7 will not have much effect, since the solute removal is relatively insensitive to S in this range. The effect of maldistribution can be illustrated with a simple model. For a chosen value of S, half of the column is assumed to have more than half of the liquid, giving a local value Sa that is less than S. The other part of the column has less than half the liquid and has a higher stripping factor, S,. There is assumed to be no mixing of the gas or liquid streams between the two parts of the column until the streams combine at the exit. The number of transfer units for each part of the column is specified, and the percent solute removal is calculated. The final solute concentration is a weighted average of the values for the two streams and can be used to calculate an apparent mass-transfer coefficient or the apparent height of a transfer unit, HOL. For a small column, the two parts might be the region near the wall, which has much of the liquid flow, and the central core. For a large column, there would be several high-flow regions near the preferred channels for liquid flow, but they can be lumped together for a simple model. The difference in liquid flow will tend to cause differences in gas flow, since gas flow will be greatest in regions of high void fraction. However, gas flow rate has little affect on the mass-transfer coefficient for liquid film controlled systems. A local increase in the gas flow rate increases the stripping factor for that part of the column, and for this model, it doesn’t matter whether the different stripping factors are caused by changes in liquid rate or by changes in both liquid and gas rates. Calculations using this model were made by assuming three or five transfer units in both parts of the column. For a column with one transfer unit, there would be much less effect of liquid maldistribution, since the exit streams would be far from equilibrium. More dramatic effects would be calculated for a column with 6-10 transfer units, but redistribution plates would probably be used with such tall columns. Furthermore, some mixing of the streams in the fast and slow regions does occur even without redistribution plates, so a large value of NoL for the model would be unrealistic. The following equation was used to calculate the solute removal for each part of the column with N*OL= 3 or 5 and to calculate the apparent value of NOL from the average removal. s-1 The results are reported as a ratio of transfer unit heights. (5)
' ' ' " 0.5 06 0.8 I
IOL
I
5
3
2
-
5
S
Flgure 8. Effect of maMistribution on height of a transfer unit. (I) N", = 3, L,,, - Lmln = 0.5L; (11) N*oL = 3, L,, LmIn = 1.0 L; (111) N",, = 5,L,, - L, = 1.0 L.
-
..
1.5
0.50.6 0.8 I
2
3
5
S
Figure 8. Increase in H, for low stripping factors. Data from Figures 1, 2, and 4.
X
Figure 7. Operating lines for uneven liquid dlstribution at S = 1.0, N", = 3.
Figure 6 shows how HOL changes with S for a mild and a moderately severe maldistribution. For case I, the maximum and minimum liquid rates were 1.25 and 0.75 times the average. For a stripping factor of 1.0, this maldistribution increases H O L only 10%. Even for S as low as 0.6, H O L is increased only 20%. For case 11, the maximum and minimum flows were set at 1.5 and 0.5 times the average, with 3 / 4 of the flow at the high rate. For a nominal stripping factor of 1.0, the local stripping factors were 0.667 and 2.0, giving calculated solute removals of 61% and 87%. The operating lines for this example are shown in Figure 7. Because most of the liquid is in the section with low removal, the overall removal is only 67 90,and the apparent HoL is 1.44 times the real value. If the column had five transfer units, the effect of maldistribution is more severe, and the apparent H O L is 1.9 times the real value at S = 1.0. The effect of liquid flow rate on the local mass-transfer coefficients was also considered. If the liquid film is controlling, HoL varies with about or NoLvaries with L-0.3. Then for the distribution of case 11, N*oL would be 2.66 for the fast-flow region and 3.69 for the slow-flow region. Using these values gave almost the same result as assuming N*OL = 3.0 for both parts. The experimental results in Figures 1-4 were replotted in Figure 8 as HoL/H*oL,where H*oL is the limiting value of HOLat high S. The curves are bunched closely together and are very similar to the calculated curves for cases I1 and I11 in Figure 6. Even though a very simple model was used, the general agreement with the experimental results and the evidence against substantial gas-phase resistance or backmixing show that uneven liquid distribution is the main cause of high H O L a t low values of S. The effect of channeling should be different for different packings, flow rates, and column sizes, and it is surprising that the data for packing sizes from 'Iz to 2 in. and D l d , from 8.7 to 28
fall so close together in Figure 8. In the design of stripping columns, allowance should be made for the effect of uneven liquid distribution if the stripping factor is 3 or less. An air rate that makes S 3-5 is said to give the minimum column volume for typical cases (7), but the cost of air compression may make the optimum value of S less than 3. Furthermore, a design based on the major solute could result in S of 1or less for trace components that are not as volatile. If the HoLdata are taken from pilot plant tests at high S or predicted from published correlations, which are generally based on data at high S , the curves of Figure 8 could be used to get a correction factor. Alternatively, the design could be based on the following empirical equation, which gives a reasonable fit to the data in Figure 8. HOL/H*OL
=1
+ 0.9/S2
(6)
More experimental work is needed to show how the correction factor varies with packing size and shape, the ratio of packing size to column diameter, and the type of liquid distributor. The effect of maldistribution should be evident when operating absorption columns at liquid rates not much above the minimum value. The key parameter is the absorption factor, A, which is the reciprocal of S , and the correlation shown in Figure 8 would be expected to apply with A as the abscissa. The only data found for absorption at low values of A are for the carbon dioxide-water system (12),and the effect of uneven liquid flow is hard to evaluate because of gas backmixing at the very high ratios of uL/uG that were used. Uneven liquid distribution also has an effect on the performance of packed distillation columns, which are often operated at only slightly more than the minimum reflux rate and are thus quite sensitive to liquid distribution. A two-section model with no lateral mixing was used by Yuan and Spiegel (17) to predict the effect of maldistribution on the height of a theoretical stage. Most of their results are for a large number of stages (5-25) and small values of the maldistribution parameter, X = (L,,, - L,i,)/L = 0-0.2. Although the trends are similar, the predictions are not directly comparable with those of this work. They also measured the effect of a deliberately introduced maldistribution, but it was not possible to esEnviron. Sci. Technol., Vol. 23, No. 3, 1989
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Environ. Sei. Technol. 1989, 23, 314-320
timate the effect of the inherent maldistribution.
79-01-6;CH&C13, 71-55-6;CC12=CC12, 127-18-4;CC4, 56-23-5; CHBr,, 75-25-2;C02, 124-38-9.
Glossary a
C1 c2
A dP
D G H' HG HL HOL
H*OL 2:a
L m NOL
N*OL ReL S sa,sb UG UL
V
X
Y z
x
mass-transfer area inlet concentration of solute outlet concentration of solute absorption factor packing size diffusivity mass velocity of gas Henry's law constant based on volumetric concentrations height of a gas-phase transfer unit height of a liquid-phase transfer unit overall height of a transfer unit based on the liquid-phase HoL for uniform distribution gas film volumetric mass-transfer coefficient overall volumetric mass-transfer coefficient molar flow rate of liquid or mass velocity of liquid slope of equilibrium line on a y - x plot number of transfer units number of transfer units in local regions of the column Reynolds number for liquid stripping factor local value of the stripping factor superficial velocity of gas superficial velocity of liquid molar flow rate of gas liquid-phase concentration Vapor-phase concentration effective film thickness maldistribution parameter
Literature Cited (1) Gossett, J. M.; Cameron,C. E.; Eckstrom, B. P.; Goodman, C.; Lincoff, A. H. Air Force Eng. Service Lab. Report ESL-TR-85-18,June 1985. (2) Shenvood,T. K.; Holloway, F. A. L. Trans. Am. Inst. Chem. Eng. 1940, 36, 39. (3) Roberts, P. V.; Hopkin, G. D.; Munz, C.; Riojas, A. H. Environ. Sei. Technol. 1985, 19, 164. (4) Roberts, P. V.; Hopkin, G. D.; Munz, C.; Riojas, A. H.
OWRT Report, Dec 1982. ( 5 ) Onda, K.; Takeuchi, H.; Okumoto, Y. J. Chem. Eng., Jpn. 1968, 1, 56.
(6) Ball, W. P.; Jones, M. D.; Kavanaugh, M. C. J.-Water Pollut. Control Fed. 1984, 56, 127. (7) Cummins,M. D.; Westrick, J. J. R o c . ASCE Environ. Eng. Conf. 1983, 442. (8) Suzuki, M. J. Chem. Eng., Jpn. 1975, 8, 163.
(9) Perry, R. H.; Green, D. W. Chemical Engineering Handbook, 6th ed.; McGraw-Hill: New York, 1984; pp 18-29. (10) Shulman, H. L.; Ulrich, C. F.; Proulx, A. Z.; Zimmerman, J. 0. AIChE J. 1955, 1, 263. (11) Miyauchi, T.; Vermeulen, T. Ind. Eng. Chem. Fundam. 1963, 2, 113. (12) Hatton, T. A,; Woodburn, E. T. AIChE J. 1978,24, 187. (13) Buchanan, J. E. AZChE J. 1971,17, 746. (14) Cooper, C. M.; Christl, R. J.; Peery, L. C. Trans. Am. Inst. Chem. Eng. 1941,37, 979. (15) Hoek, P. J.; Nesselingh, J. A,; Zuiderweg, F. J. Chem. Eng. Res. Des. 1986, 64, 431. (16) Porter, K. E.; Templeman, J. J. Trans. Inst. Chem. Eng. 1968, 46,
86.
(17) Yuan, H.; Spiegel, L., Chem. Ing. Tech. 1982, 54, 774.
Received for review December 23, 1987. Accepted September Registry No. CH2C12,75-09-2;CHC13,67-66-3;CHC1=CCl2,
15, 1988.
Natural Trace Metal Concentrations in Estuarine and Coastal Marine Sediments of the Southeastern United States Herbert L. Windom,",+ Steven J. Schropp,' Fred D. Calder,' Joseph D. Ryan,$ Ralph G. Smith, Jr.,t Louis C. Burney,' Frank G. Lewis,$ and Charles H. Rawllnsont Skidaway Institute of Oceanography, P.O. Box 13687,Savannah, Georgia 31416,and State of Florida Department of Environmental Regulation, 2600 Blair Stone Road, Tallahassee, Florida 32301
IOver 450 sediment samples from estuarine and coastal
marine areas of the southeastern United States remote from contaminant sources were analyzed for trace metals. Although these sediments are compositionally diverse, As, Co, Cr, Cu, Fe, Pb, Mn, Ni, and Zn concentrations covary significantly with aluminum, suggesting that natural aluminosilicate minerals are the dominant natural metal bearing phases. Cd and Hg do not covary with aluminum apparently due to the importance of the contribution of natural organic phases to their concentration in sediments. It is suggested that the covariance of metals with aluminum provides a useful basis for identification and comparison of anthropogenic inputs to southeastern US. coastal/estuarine sediments. By use of this approach sediments from the Savannah River, Biscayne Bay, and Pensacola Bay are compared.
Introduction Estuarine and coastal marine sediments are sinks for many materials transported from the land. Many sub'Skidaway Institute of Oceanography. t
State of Florida Department of Environmental Regulation.
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Environ. Sci. Technol., Vol. 23, No. 3, 1989
stances that occur naturally, such as trace metals and nutrients, may be mobilized as a result of natural processes as well as by man's activities and thus may become enriched in coastal and estuarine sediments. Before anthropogenic contributions to these sediments can be assessed, contributions due to natural processes must be estimated. The concentrations of trace metals in natural estuarine and coastal marine sediments are dominantly determined by inorganic detrital, rather than organic and nondetrital materials. The inorganic detritus is the result of chemical and physical weathering of the continents and is composed mostly of a limited number of silicate minerals, such as quartz, feldspars, micas, pyroxenes, amphiboles, and clay minerals, and smaller amounts of metal oxide and sulfide phases. In some coastal areas, such as those of Florida, carbonate minerals represent the major component of estuarine and coastal sediments. Of the materials contained in natural sediments, quartz, feldspars, and carbonates are relatively metal poor as compared to the other phases and thus serve to dilute sediment metal concentrations. Anthropogenic trace metal contributions to estuarine
0013-936X/89/0923-0314$01.50/0
0 1989 American Chemical Society
