Treatment of Water by Granular Activated Carbon - ACS Publications

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Downloaded by UNIV OF CALIFORNIA SAN DIEGO on August 23, 2015 | http://pubs.acs.org Publication Date: March 15, 1983 | doi: 10.1021/ba-1983-0202.ch009

Controlling Mechanisms for Granular Activated-Carbon Adsorption Columns in the Liquid Phase M. R. ROSENE Calgon Corporation, Pittsburgh, PA 15230

Nonadsorptive phenomena contributed to granular activatedcarbon column performance as measured by the breakthrough curve. A model was developed that examines the impact of three possible mechanisms that control the development of the breakthrough curve and allows the use of the mass transfer zone in the interpretation of breakthrough curves. The developed concepts help provide a sound basis for the interpretation of granular activated-carbon column adsorptive dynamics.

T

HE USE OF GRANULAR ACTIVATED-CARBON (GAC) columns for the removal

of undesirable organics in both industrial and municipal applications has been widely discussed (1-12). Mechanistic studies of column operation have mainly centered on the adsorption process as the rate-limiting step. While this approach is valid for many cases, nonadsorptive phenomena also contribute to column performance as measured by the breakthrough curve. In particular, the impact of the axial or longitudinal dispersion phenomenon has been largely ignored as an important aspect of column operation. Consequently, the need exists for an approach that delineates the conditions under which this axial dispersion phenomenon cannot be neglected and that describes the physical phenomena underlying both the mass transfer limited and axial dispersion mechanisms of GAC column performance through the interpretation of breakthrough data

0065-2393/83/0202-0201$06.00/0 © 1983 American Chemical Society

In Treatment of Water by Granular Activated Carbon; McGuire, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1983.

202

T R E A T M E N T O F W A T E R BY G R A N U L A R A C T I V A T E D C A R B O N

Theory


The G A C kinetic adsorption process in the liquid phase can be considered to consist of several steps. Each step can be argued to have several substeps and even the exact number of steps may be debated, however; the approach here considers only the following: 1. Bulk Diffusion—This step includes the transport of the adsorbate in the bulk liquid, either by mixing or molecular diffusion. 2. External Mass Transfer (Film Diffusion)—This step involves the transport of the adsorbate across the boundary layer at the external surface of the particle. 3. Intraparticle Mass Transfer (Pore Diffusion)—This step concerns the rate at which the adsorbate is transferred from the surface to the particle interior. 4. Micropore Adsorption—This rate involves the adsorption step itself and is generally considered to be very rapid. Of these steps, intraparticle mass transfer is generally recognized as the most common rate-limiting step in adsorption (1-12). A useful parameter for describing the mass transfer within the adsorbent particle is the effective intraparticle diffusivity (D). The basic mathematics of diffusion into a sphere are given by Crank (13) and further modified by Dedrick and Beckmann (2). If a homogeneous diffusion rate is assumed within the particle, then from Dedrick and Beckmann:

where D is the effective diffusivity, t is the time, a is the effective particle radius, and E is the fraction of the equilibrium capacity obtained in time t. Rearranging Equation 1 for y/Dt/a < 0.4 gives: 2


9. ROSENE

Controlling Mechanisms for GAC Columns

203

The effective diffusivity is used to predict the breakthrough curve of a single component from a G A C column when intraparticle mass transfer is the rate-controlling phenomenon. The procedure involves the use of a standard compound (in this case p-nitrophenol) and the concept of the mass transfer zone (MTZ) as approximated here:


(3)

where t is the time to saturation in a column, ty is the time to initial breakthrough, and L is the column length. This approximation is valid when the MTZ is small compared to the total column length. The experimentally determined effective intraparticle diffusivity, calculated from Equation 2 using an experimental procedure described later, for the standard compound is correlated with the width of the M T Z experimentally determined for a set of column conditions where intraparticle mass transfer is the controlling factor. This ratio then, can be used to calculate the MTZ width for other compounds under intraparticle controlled conditions from their determined effective intraparticle diffusivities. Once the effective diffusivity is obtained, the procedure for predicting the breakthrough curve by computer is relatively straightforward. s

The other step that controls adsorption is external mass transfer. This step involves transport of the adsorbate across the boundary layer or film at the external surface of the particle. This phenomenon is a function of the film diffusion rates for the adsorbate of interest as well as factors such as liquid shear forces which affect the thickness of the boundary layer. Liu (14) isolated the contribution of external mass transfer to the kinetics of GAC column operation using short columns and determined that the external mass transfer rate only controls in the early stages of his column runs. This result is not unexpected as the external surface of the adsorbent particle is saturated rapidly relative to the internal surface and intraparticle mass transfer quickly becomes the rate-limiting adsorption step. The prediction of breakthrough curves when the controlling mechanism is external mass transfer could be accomplished in much the same manner as for intraparticle diffusion. Here, however, the mass transfer rate across the boundary layer must be estimated for the conditions to be used in the calculations. This step involves more experimental work since the film boundary response to a change in flow rate would have to be determined for the adsorbent of interest In addition to the impact of the adsorption process on G A C column



204


operation, the potential contributions of fluid flow effects in the packed bed must be considered. For example, an aspect of fluid flow requiring careful consideration is that of axial or longitudinal dispersion in a packed GAC bed. This effect is attributable to backmixing and turbulence within the bed and in some cases results in significant spreading of the mass transfer zone. Ebach (15) investigated the phenomenon of axial dispersion in water flowing through beds of packed solids and established that, for a range of interstitial linear velocities, an empirical correlation for calculating the effective axial dispersion (D ) (along the length of the column) was given by: a

D =

Kd U

a

P

(4)

b

where K and b are constants, d is the effective particle diameter, and U is the effective interstitial linear velocity. This correlation is valid for the range of linear velocities normally encountered in GAC column operations. This expression does not account for any adsorption that may occur on the bed of packed soils. To treat this effect, Lapidus and Amundson (16) developed the mathematics for the adsorptive retention of the axial dispersion wavefront. From their derivation we have: P

(5) where C is the concentration in the effluent, Z is.the column length, U is the interstitial velocity, R is a reduced time variable equal to (Ut/Z), and the quantity (dC/dR) R = y is the slope of the breakthrough curve when C is equal to 50% of the influent leveL Combining Equation 5 and D calculated from Equation 4, it is possible to calculate the slope of the breakthrough curve. A major assumption in the derivation is that adsorption equilibrium is obtained locally within the column. Therefore, this calculation is only valid when the adsorption step itself is not the rate-limiting mechanism in the development of the mass transfer zone. If these conditions hold, it is possible to predict the breakthrough curve for a single component when axial dispersion is the controlling mechanism. From Equation 4, the necessary data to calculate D are the interstitial linear velocity, the effective particle diameter, and the constants K and b which are given by Ebach. Then, by using Equation 5, the slope of the breakthrough curve at the 50% breakthrough level can be calculated by inputting the calculated value for D and the column length. When this procedure is computerized and combined with an isotherm a

a

a


9.

ROSENE


capacity, predicted breakthrough curves for the axial dispersion controlled column runs are easily calculated. We now consider how the MTZ, as expressed in Equation 3, can be used to distinguish between the three potential controlling phenomena discussed (i.e., intraparticle mass transfer, external mass transfer, and axial dispersion). First, consider an experiment in which a GAC column of fixed bed height Bi is first exhausted on an influent stream which is applied at a hydraulic loading rate F . The loading rate is then increased to F , and the experiment is repeated. Under these conditions, the MTZ will exhibit a different behavior depending on the controlling phenomena For example, in the case of intraparticle mass transfer, if we assume that the wavefront is contained, then the MTZ increases roughly in proportion to the increase in flow from Fi to F . In other words, the rate of penetration of the particle is independent of the external flow conditions, and a doubling of the flow doubles the volume between initial breakthrough and saturation. The time required to reduce the influent concentration within the column to zero does not change since that is controlled by the rate in which the adsorbent particles become exhausted. This situation results in the MTZ, as expressed in Equation 3, increasing proportionally to the increase in flow. In the case of external mass transfer, the described experimental conditions result in a different response in the MTZ. Here, as the flow is increased from F to F , velocity shear forces affect the hydrodynamic boundary layer, decreasing the resistance to adsorbate mass transfer. Again, if we assume the wavefront is contained, the time to initial breakthrough decreases or increases with the magnitude depending on the relative impact of the increase in flow versus the increased rate of mass transfer across the boundary layer. The time to saturation decreases in response to the increase in mass transfer rate. The overall effect of these responses on the value of the MTZ is indeterminate with either a net decrease or increase possible. In the case of axial dispersion controlled breakthrough, the adsorption step is not limiting, and the breakthrough curve behaves according to Equations 4 and 5. Under the conditions of this experiment, this situation results in an increase in the magnitude of the axial dispersion D in direct proportion to the increase in flow. It is apparent from Equation 5 that this increase has little effect on the slope of the breakthrough curve since the increase in D is offset by a like increase in U. Thus, with the slope constant and the time to 50% breakthrough reduced, the M T Z increases. This discussion reveals a pitfall into which the unwary can easily stumble. If, for example, a column experiment is run at a fixed bed depth and two different flows, an increase in the MTZ less than proportional to the increase in flow cannot be used as unequivocal evidence for either 2


205

2

2

x

2

a

a


206


external mass transfer phenomena or axial dispersion. A second set of experimental conditions is necessary to resolve this question. In this experiment, the flow at F is held constant and the bed depth is increased from B to B . Under these conditions, intraparticle mass transfer exhibits no change in the MTZ since the interval between t and t is determined by the effective intraparticle diffusivity, and increases in t are exactly offset by an increase in L. The case of external mass transfer is nearly identical. The mass transfer rate across the boundary layer is constant since the flow is constant and the interval between t and % is again a constant. Therefore, the MTZ does not change. In the case of axial dispersion, however, the impact of the column length is described by Equation 5. By using this expression, the slope of the breakthrough curve is found to be proportional to the square root of the increase in bed depth. This finding means that the MTZ shows an increase directly related to the increase in bed depth from B to B . Table I summarizes the effect of the controlling mechanism on the MTZ for the two experimental test conditions discussed. Y

x

2

b

s


s

s

x

2

Results and Discussion Consider how the model just described can be used to interpret the results of experimental column runs. Figure 1 presents two column experiments for phenol removal. Both columns were run with the same empty bed contact time (EBCT) but at surface loadings which differed by sixteenfold (0.3 gpm/ft vs. 4.8 gpm/ft ). When the controlling mechanism is intraparticle mass transfer, these two curves plotted as percent breakthrough versus time would be expected to coincide. However, as can be seen, the curves do not overlay. Past experience showed that phenol adsorption at a surface loading of 4.8 gpm/ft is intraparticle mass transfer controlled (12); thus, the marked decrease in the MTZ with the increase in flow from 0.3 gpm/ft is an indication that a change of controlling mechanism occurred A question nevertheless remains as to which al2

2

2

2

Table I. Effect of Controlling Mechanism on M T Z Experimental Conditions Increase bed depth constant Constant

flow/

flow/

Intraparticle (Pore)

External (Film)

Increase MTZ

Decrease or increase M T Z

Increase MTZ

No change in MTZ

No change in M T Z

Increase MTZ

Axial

increase bed depth



9.

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207

TIME

Figure 1. Effect offlow rate on phenol breakthrough (100 mg/L). Key: O, 4.8 gpm/ft*; and #, 0.3 gpm/ft . 2

ternate mechanism does control under the lowflowconditions. Based only on the data of Figure 1, an argument could be made for either external mass transfer or axial dispersion. The definitive experiment would be to conduct another run at the same surface loading and a different bed depth. If the controlling mechanism is external mass transfer, no change should be expected in the MTZ. By contrast, if axial dispersion is the controlling factor, then a decrease in the bed depth would result in a reduction in the MTZ. Figure 2 presents the results of just such an experiment The original curve has been replotted for comparison, and the second curve shows the breakthrough for a bed depth 0.37 times that for the first curve. Calculation of the M T Z for the second curve reveals a decrease in direct relation to the reduction in bed depth. Therefore, this case can be ascribed to a shift of mechanistic control from intraparticle mass transfer to axial dispersion as the flow rate is reduced. As discussed earlier, computerized predictions of single component breakthrough curves can be made for any of the three mechanisms presented, assuming the proper input data are available. For intraparticle



208


mass transfer, the key input is the effective intraparticle diffusivity for the adsorbate-adsorbent pair of interest The data can be obtained experimentally by using a modification of the high-pressure minicolumn (HPMC) technique described by Rosene et al. (17). The experiment is conducted by running the column at sufficiently high surface loadings to obtain immediate breakthrough of the adsorbate concentration in the column effluent of 90% of the influent value. This value allows approximation of the concentration throughout the entire column as the average of the influent and effluent values. Integration of the breakthrough curve gives the carbon loading as a function of time. These data, when entered into Equation 2 yield a value for the effective diffusivity. Table II presents effective intraparticle diffusivity data determined by this technique for p-nitrophenol in combination with three adsorbents at two concentration levels. The first three runs on carbon A show the technique gives reproducibility within 10%. The average of these three runs is a value of 5.9 X 10~ cm /s. Reducing the concentration to 50 ppm 9

2

TIME

Figure 2. Phenol breakthrough (100 mg/L, 0.3 gpm /ft ) vs. bed depth. Key: O, 0.37 original bed depth; and #, original bed depth. 2


9.

ROSENE


209

Table II. Effective Intraparticle Diffusivities p-Nitrophenol Concentration (mg/L) Average Inlet


Carbon A A A A B C

100 100 100

0.597 0.589 0.565 0.584 0.122 0.087

94 97 96

50 50 50

D (cm /s)

E

46.6 46.5 47.8

2

6.2X10 6.0X10 5.4X10" 5.9X10 2.0X10" 1.0X10

-9

-9

9

-9

10

- 1 0

showed no significant change in the diffusivity, as expected from the homogeneous diffusion model. By contrast, the effect of different adsorbents under the same conditions showed marked differences. Both carbon B and carbon C (experimental carbons) had much lower effective diffusivities, 2.0 X 10" cm /s and 1.0 X 10" cm /s , respectively. An example of how the computer prediction method can be used is illustrated in Figures 3 and 4. Here, two column experiments for the removal of p-nitrophenol by carbon A are presented. The column bed depths are identical in each case, but the surface loading in Figure 3 is 8.7 gpm/ft while in Figure 4 it is 2.9 gpm/ft . The actual data are represented by the individual points, and the dashed and solid lines represent the intraparticle diffusion and axial dispersion model predictions, respectively. At the higher flow, intraparticle diffusion is seen to be the controlling factor; however, when the flow is reduced to the lower rate, the curve does not adopt the sharp S-shape predicted by the intraparticle diffusion model but is much broader as expected from axial dispersion control. Agreement between the intraparticle model prediction and the experimental data in Figure 3 is excellent. The fit to the axial model is fairly good and is quite gratifying since the prediction is based on only an isotherm capacity and Ebach's correlation. 10

2

2

10

2

2

Conclusions The model presented examines the impact of each of three possible mechanisms that control the development of a GAC column effluent breakthrough curve. The model allows the use of the MTZ in the interpretation of breakthrough curves as a means of determining the dominant mechanism. In a number of cases, the model has provided logical explanations for what otherwise would have been very puzzling results. These concepts should help to provide a sound basis for the interpretation of G A C column adsorptive dynamics.


In Treatment of Water by Granular Activated Carbon; McGuire, M., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1983. 2

> Η

Η

η

>

53

r>

? Ζ G

Ο

53 D3 *

Ο

Η

m

Η

>

Η 53 PI

to '

ο

Ο

53 03

Figure 3. Computer predictions for p-nitrophenol breakthrough (100 mg/U 8.7 gpm/ft ). Key: — axial; and m σ η --·--, intraparticle. >



100

200

VOLUME

300

400

2

500

Figure 4. Computer predictions for p-nitrophenol breakthrough (100 mg/L, 2.9 gpm/ft ). Key: - - -, intraparticle; and —, axial.

0


CO

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TREATMENT OF WATER BY GRANULAR ACTIVATED CARBON


Literature Cited 1. Weber, W. J., Jr.; Morris, J. C. J. SanitaryEng.Div. 1963, 89, (542), 31-59. 2. Dedrick, L.; Beckmann, R. B. Chem.Eng.Progr., Symp. Ser. 1967, 63 (74), 6878. 3. Knoblauch, K.; Jüntgen, H.; Peters, W. Chem.Ing.Tech. 1969, 41, 798-805. 4. Knoblauch, K.; Jüntgen, H. Chem. Ing. Tech. 1970, 42(2), 77-81. 5. Mattson J. S.; Kennedy, F. W. J. Water Pollution Control Fed. 43, pp. 22102217 1971, 43, 2210—17. 6. Spiridakis N. J.; Brown, L. F. presented at the AIChE 67th Annual Meeting, Washington, D.C., Dec 1974. 7. Suzuki M.; Kawazoe, K. J. Chem.Eng.Jpn.1975, 8, 379-82. 8. Suzuki, M.; Kawazoe, K. J. Chem.Eng.Jpn. 1974, 7, 346-50. 9. Suzuki, M.; Kawai, T.; Kawazoe, K. J. Chem.Eng.Jpn. 1976, 9, 203-8. 10. Hashimoto, K.; Miura, K.; Nagata, S. J. Chem. Eng. Jpn. 1975, 8, 368-73. 11. Nagy, L. G.; Fόti, G.; Kuty, H.; Schay, G. "Equilibrium and Kinetic Studies of Liquid Adsorption on Porous Activated Carbon," Proceedings of the Inter national Conference on Colloid and Surface Science, Wolfram, E.; Ed. Akad. Kiado: Budapest, Hungary 1975, pp. 107-115. 12. Zogorski, J. S.; Faust, S. D. AIChE Symposium Series Water-I: Physical, Chemical Wastewater Treatment, 1976, pp. 54-65. 13. Crank, J. "Mathematics of Diffusion"; Claredon Press, Oxford, England, 1956. 14. Liu, K. Ph. D. Dissertation, University of Michigan, Ann Arbor, Mich., 1980. 15. E. A. Ebach, Ph.D. Dissertation, University of Michigan, Ann Arbor, Mich., 1957. 16. Lapidus L.; Amundson, N. R. J. Phys. Chem. 1952, 56, 984-88. 17. Rosene, M. R.; Deithorn, R. T.; Lutchko, J. R.; Wagner, N. J. In "Activated Carbon Adsorption of Organics from the Aqueous Phase", Suffet, I. H.; McGuire, M. J. Eds; Ann Arbor Science: Ann Arbor, Mich., 1980; Vol. 1, Chapter 15. RECEIVED for review August 3, 1981. ACCEPTED for publication March 18, 1982.


Treatment of Water by Granular Activated Carbon - ACS Publications

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