988
Ind. Eng. Chem. Res. 1988,27,988-991
Desorption of Ethyl Acetate from Activated Carbon by Supercritical Carbon Dioxide ChungSung Tan* and Din-Chung Liou Department of Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 30043, Republic of China The regeneration of activated carbon loaded with ethyl acetate by supercritical carbon dioxide was investigated in this study. I t was found that the adsorptive capacities after several regeneration cycles were still close to that of virgin carbon and remained stable. T h e effects of temperature, pressure, and flow rate on regeneration efficiency were also investigated. Regeneration was more favorable a t higher pressures, but optimal temperatures were found to depend on pressure. A one-parameter mathematic model assuming linear desorption kinetics was proposed which agreed well with the experimental data. Supercritical fluid extraction has received widespread attention over the past years. The applications of this technology in different areas have been reviewed in detail by Gangoli and Thodos (1977), Williams (1981), and Paulaitis et al. (1983). One of these applications is to regenerate adsorbents, such as activated carbon (Paulaitis et al., 1983; McHugh and Krukonis, 1986). Kander and Paulaitis (1983) studied the desorption of activated carbon loaded with phenol by supercritical carbon dioxide. These authors correlated their equilibrium data by a thermodynamic model using the Toth isotherm. They also concluded that the supercritical carbon dioxide offers no significant thermodynamic advantage for regenerating carbon loaded with phenol; however, it may be a powerful desorbent for other organic solutes which are not adsorbed strongly on activated carbon. DeFilippi et al. (1980) studied the regeneration of activated carbon loaded with pesticides by supercritical carbon dioxide. They pointed out that the supercritical regeneration method was economical even though the operating temperature and pressure were above 387 K and 150 atm, respectively. A local equilibrium model using the Freundlich isotherm was proposed and was able to explain the experimental data. Ethyl acetate (EA) is an important solvent used in petrochemical and polymer industries and is commonly emitted from industrial plants. To recover it and/or to reduce its concentration in effluent streams from environmental concern, activated carbon is customarily employed. Ethyl acetate is soluble in supercritical carbon dioxide and is not strongly adsorbed on activated carbon as compared with phenol; therefore to regenerate activated carbon by supercritical carbon dioxide seems to be a promising method according to the observation of Kander and Paulaitis (1983). The main objective of this paper is to study the regeneration of activated carbon loaded with ethyl acetate by supercritical carbon dioxide at different operating conditions. A mathematical model describing the desorption phenomena in a packed column is also proposed. Experimental Section Ethyl acetate (EA) was used as the adsorbate in this study since it is frequently used as a solvent in petrochemical and polymer industries and is often a constituent of industrial wastes. An aqueous solution containing 2600 mg/L of EA was employed through this study by adding EA with 99.7% purity into deionized water. Virgin activated carbon (Merck 2514) was first screened to obtain a 18-20-mesh fraction (the average particle size
* To whom correspondence should be addressed. 0888-5885/88/262~-0988$01.50/0
is 0.1 cm). This fraction was boiled in deionized water to remove fines and was then dried in an oven at 393 K. After drying, about 13 g of activated carbon was packed in an 2.12-cm-i.d. stainless steel 316 tube with a height of about 8.6 cm. In order to achieve uniform flow distribution and to avoid possible end effeds, glass beads of 0.1-cm diameter were packed in the regions above and below the activated carbon packing with heights of about 4.5 and 2.6 cm, respectively. Platinum wires were used to support the packing in the column. In the adsorption experiments, the prepared aqueous EA solution was pumped through the carbon column at 308 K. The flow rate was kept constant at 15 cm3/min. A portion of the effluent was sent to a GC (Varian 3700, FID detector) for analyzing EA concentration. About 3 L of 2600 mg/L of EA solution passed through the column to achieve EA breakthrough. All of the effluent was collected to check the overall mass balance of ethyl acetate. The difference in the overall mass of EA measured in the original and effluent solutions agreed well with that obtained by integrating the breakthrough curve within 5%. This difference could be regarded as the adsorptive capacity of the activated carbon which was about 0.15 g of EA/g of activated carbon. This adsorption amount is consistent with that obtained by an independent batch experiment where a certain amount of EA was added to a bottle containing deionized water and virgin activated carbon. The bottle was agitated vigorously by a mechanical mixer. The samples were taken for analysis until no further reduction of EA concentration. The column packed with the activated carbon which needed to regenerate and the glass beads were then put into the desorption apparatus which is shown in Figure 1. Carbon dioxide with 99.7% purity was used as the desorbent. It first passed through a silica gel bed in order to remove possible water vapor and then was compressed and sent to a surge tank by a diaphragm compressor (Superpressure Inc.) with a minimum charge pressure of 47.6 atm. In each desorption experiment, the pressure was controlled within 1.0% deviation from the desired value. The temperature was controlled in a constant-temperature bath whose accuracy was within 0.5 K. A preheating coil of 0.3-cm diameter and about 110-cm length was immersed in a constant-temperature bath. From the preliminary studies, it was found that the temperature of carbon dioxide at the exit of the packed bed was the same as that of the constant-temperature bath by using a thermocouple inserted at the exit of the packed bed. This indicated that the preheating coil was long enough to make carbon dioxide reach the desired temperature. It was also found that the pressure drop across the preheating coil was Q 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 6, 1988 989 130 Experimental I
-u
J I I M I
A t
1
D a t a at 07 1 a i m
300K
A
. -c c E
4
OCO2 cylinder @Regulator OSiiica gel bed Qcompressor @Surge tank @Ethyl acetate s l u t i o n
@Tubing pump @Three way valve @Pressure guage @Extractor OHeating tape QCollector
:o
QMeterlng valve 64Cold trap OMagnetlc stirrer @Filter W e t test meter W o n s t a n t temperature bath
23
0 0
10
Figure 1. Schematic diagram of apparatus used for adsorption and supercritical regeneration experiments.
20
40
30
Time,
50
60
Mirru!es
Figure 3. Temperature effects on regeneration at 87.1 atm. "L"l
1
1001 Expermental Data a t 129 3 o t m
300K o 308K A
0 A
s
I
1
1
2
3
L
5
I
6
7
8
311K 318K 328K 338K
I
9
Number Of Cycle
Figure 2. Adsorptive capacities of regenerated activated carbon during the beginning cycles.
negligible. The gas coming out of the extractor was expanded across a metering valve. The flow rate in the extractor was then determined by measuring the volume of the expanded gas as it passed through a cold trap and a wet test meter. The accuracy for the volume measurement was within 1.0%. The desorbed EA was collected in a cold trap which contained 1 L of 2-propanol. Samples of 2.0 p L were frequently taken out for GC (FID detector) analysis in order to obtain desorption breakthrough curves. After about 350 L of carbon dioxide had passed through the column bed,the desorption experiment was stopped. Then the subsequent adsorption experiment was executed to obtain the adsorptive capacity. This capacity was compared with the desorption amount in the previous desorption experiment. The agreements were satisfactory with a deviation of less than 3.0%. Experimental Results and Discussion In the present study, each adsorption experiment and the subsequent desorption experiment were considered as one cycle. In Figure 2, each cycle included an adsorption step taking place at 308 K for 2600 mg/L of EA solution and a desorption step occurring at 308 K and 95.2 atm. The total volume of carbon dioxide used for regeneration was about 350 L, lasting around 8 h. The average residence time of carbon dioxide in the packed activated carbon bed was about 3.0 min. The results illustrate that the adsorptive capacity of the regenerated activated carbon drops slightly during the first three cycles. But after the third cycle, adsorptive capacity becomes quite stable and is approximately 87% of that by virgin activated carbon. This phenomenon was also observed by Kander and Paulaitis (1983).
0
20
40
60
80
102
Time, Minutes
Figure 4. Temperature effects on regeneration at 129.3 atm. Table I. Density and Viscosity of Carbon Dioxide at Various Conditions at 87.1 atm at 129.3 atm density, viscosity, density, viscosity, temp, K g/cm3 g/(cm)(s)lO' g/cm3 g/(cm)(s)104 300 0.77 6.58 0.84 8.34 0.77 7.09 308 0.60 4.47 313 0.48 3.48 0.70 6.36 0.62 5.66 318 0.34 2.75 0.48 4.37 328 0.24 2.09
When the carbon dioxide regeneration flow rate was fixed at 4.53 cm3/min (calculated based on operating temperature and pressure rather than normal condition of 298 K and 1atm), the effects of temperature at operating pressures of 87.1 and 129.3 atm on regeneration efficiency are shown in Figures 3 and 4, respectively. These two pressures were chosen because the optimal operating conditions were found in different phases which will be stated below. Though the regeneration period in each experiment was more than 5 h, only the regeneration data at the beginning times were reported in these figures. Nevertheless it can be seen that more than 50% regeneration for most of the runs could be achieved within the first hour. The reproducibility tests were executed at several operating conditions, and it was found that the average deviation of dynamic data was less than 3.0% with a maximum deviation of about 5.0%. Figure 3 indicates that the regeneration efficiency decreases with temperature at 87 atm; the most efficient operation was found in the liquid phase (300 K) rather than in the supercritical region (308,318,328, and 338 K). But when the regeneration pressure was raised to 129.3
990 Ind. Eng. Chem. Res., Vol. 27, No. 6, 1988 'xperimental Data at 328H and 87 1 a t m
7 3 5 cmlmin 1 7 4 cm'lmin v 201 c r r l min 0 2
c
c
0 3
E C
e
e D
0
t&
0 ;
/'
1
I
I
12
22
30
Time
43
50
0
10
V nu'es
Figure 5. Pressure effects on regeneration at 308 K.
atm, the more efficient operation was found ip the supercritical region instead of in the liquid phase which is shown in Figure 4. Table I gives the density and viscosity of carbon dioxide at different operating conditions. In general, a higher density may enhance the solubility of a solute in a supercritical fluid, but higher viscosity may have an adverse effect on the diffusion rate. A t a lower operating pressure, 87.1 atm, the density effect seems to dominate; therefore, the optimal regeneration condition is in the liquid phase. Due to the limitation of the constanttemperature bath, temperatures lower than 300 K (room temperature) could not be obtained in this study. Presumably at lower temperatures, a higher regeneration efficiency may be obtained because of higher density. But at a higher operating pressure, 129.3 atm, it seems that both the density and viscosity effects are important; hence, an optimal temperature lies somewhere in the supercritical region. At the present time, a general rule for obtaining the optimal temperature, however, has not been found yet. As compared the desorption breakthrough curves within the beginning 60 min from Figures 3 and 4, it obviously can be seen that, the higher the operating pressure, the higher the regeneration efficiency is. The same observation can be seen in Figure 5 which shows the effect of pressure on regeneration efficiency at 308 K and 4.53 cm3/min. This pressure effect may be due to the increase of density. Numerous studies have indicated that the solubility of a solute in a supercritical solvent increases with density (McHugh and Paulaitis, 1980; Penninger et al., 1985; McHugh and Krukonis, 1986, Tan and Weng, 1987). Therefore, the desorption may be easier to occur at higher fluid density or at higher regeneration pressure. Since the interphase mass-transfer coefficient is a hydrodynamic property, desorption is influenced by the flow rate of the regenerating fluid. Figure 6 illustrates the importance of the interphase mass-transfer resistance during the regeneration stage. At lower flow rates of 4.89 and 7.35 cm3/min, the desorption amounts were smaller than those at the higher flow rates. Figure 6 shows that the desorption amounts at these two flow rates were quite close, but this only occurred at the beginning times. When the times were larger than 40 min, the desorption amount at 7.35 cm3/min was found to be larger than that at 4.89 cm3/min. Therefore, it may be concluded that to increase the flow rate of carbon dioxide, less desorption time is required. Mathematical Model Development Since mathematical models for adsorption in a packed column have been studied extensively (Hashimoto and
30
20
40
50
Time, Minutes
Figure 6. Flow rate effects on regeneration at 328 K and 87.1 atm.
Smith, 1973; Ruthven, 1984), and the main issue of this study is to examine the regeneration efficiency by supercritical carbon dioxide, hence, attention is only paid to develop a model during the regeneration stage. The development is based on the mass conservation of EA. Suppose the axial dispersion effect can be neglected; the mass balance in the bulk phase in the column may be written by €
ac + u ac = -(1et az
as
dt
The initial and boundary conditions are
c=o c=o
t = O
(2)
(3) Because of the lack of information on supercritical fluidsolid mass-transfer coefficient, effective diffusivity of EA in supercritical carbon dioxide, and isotherms, the mass balance in the activated carbon is expressed in terms of linear desorption kinetics which may be written by t = O
as = -kS at
The initial condition is t=O
(4)
s=so
The concentration at the exit of the column then can be obtained by solving eq 1-5 and is expressed by
C, = +So[
enp( -k( t -
$) - exp(-kt)) ]
(6)
The total amount of desorbed EA can then be calculated by integrating C, with respect to time.
Discussion The desorption rate constant, k,was evaluated by fitting eq 6 with the experimental data. The IMSL subroutine ZXSSQ was employed to achieve this fit. With the calculated k , the simulated results matched well with the experimental data for all operating conditions; some are shown in Figures 7 and 8. The maximum deviation is no more than lo%, and the average deviation is about 6.0%. These agreements indicate that it is plausible to explain the mass transport process in the regeneration stage by the one-parameter model. If the desorption rate constant follows the Arrhenius law, the following linear relation should exist: E In k = In ko - (7) RT
Ind. Eng. Chem. Res., Vol. 27, No. 6, 1988 991 5
0
871 a l m
Experimental Data at 871atm and 53cmVmin
.
Model Exp
..
__
0
300K
-_
A
318K 328K
........
0
1
I
, ,”
I
0
‘\
I
0
c
3
10
30
20
I
32
50
40
l/Temperaiure x l o 3 I
Time, U n u t e s
Figure 7. Comparison of simulated results with experimental data at 87.1 atm.
1
I
3.1
I
Experimental D a t a a t 1293atm
k
’
Figure 9. Plot In k vs 1/T.
An one-parameter mathematical model using linear desorption kinetics is proposed. The simulated results by this model match well with the experimental data for all operating conditions.
Acknowledgment Financial support from the National Science Council of E
ROC is gratefully acknowledged. Nomenclature
0
0
1
I
I
I
10
20
30
40
Time,
,
50
Minutes
Figure 8. Comparison of simulated results with experimental data at 129.3 atm.
From the slope of the plot of In k vs 1/T, the desorption activation energy can be calculated. This plot is shown in Figure 9 from which the activation energies are found to be 11.07 and 4.6 kcal/mol for 87.1 and 129.3 atm, respectively. It notes here that 4.6 kcal/mol is quite close to the amount of the heat released during the adsorption stage (around 4 kcal/mol). The less desorption activation energy at higher pressure may be evidence that operations a t higher pressures are more favorable for regeneration. By looking at eq 6, it can be seen that with a longer activated carbon column, a higher exit concentration of EA can be obtained. This indicates that a higher regeneration efficiency could be achieved using a longer activated carbon column.
Conclusions Regeneration of EA-loaded activated carbon by supercritical carbon dioxide has been experimentally studied and mathematically analyzed. The adsorptive capacity of the regenerated activated carbon is found to be close to that of the virgin carbon and stable after several regeneration cycles. The effects of temperature, pressure, and flow rate of carbon dioxide on regeneration efficiency are examined. It is found that the higher operating pressures are more favorable for regeneration; this is probably due to the increase of density. Besides density, the viscosity may also play an important role. These two properties may determine the optimal operating conditions. A general d e , however, has not been found yet.
C = concentration of EA, mol/cm3 C, = exit concentration of EA, mol/cm3 E = activation energy, kcal/mol k = desorption rate constant ko = desorption rate constant at reference state S = loaded EA an activated carbon, mol/cm3 So = initially loaded EA on activated carbon, mol/cm3 T = temperature, K t = time, s z = axial position in column, cm Greek Symbol E
= void fraction in the packed column Registry No. EA,141-78-6; C,7440-44-0; COP,124-38-9.
Literature Cited DeFilippi, R. P.; Krukonis, V. J.; Robey, R. J.; Modell, M. “Supercritical Fluid Regeneration of Activated Carbon for Adsorption of Pesticides”. Report, 1980; EPA Washington, D.C. Gangoli, N.; Thodos, G. Ind. Eng. Chem. Prod. Res. Dev. 1977,16, 208. Hashimob, N.;Smith, J. M. Ind. Eng. Chem. Fundam. 1973,12,353. McHugh, M. A.; Krukonis, V. J. Supercritical Fluid Extraction, Principles and Practice; Butterworth Stoneham, MA, 1986. McHugh, M. A.; Paulaitis, M. E. J. Chem. Eng. Data 1980,25,326. Kander, R. G.;Paulaitis, M. E. in Chemical Engineering ‘at Supercritical Fluid Conditions; Paulaitis, M. E., Penniger, J., Gray, R., Davidson, P., Eds.; Ann Arbor Science: Ann Arbor, MI, 1983; p 461. Paulaitis, M. E.; Krukonis, V. J.; Kurnik, R. T.; Reid, R. C. Rev. Chem. Eng. 1983,1(2), 179. Penninger, J. M. L.; Radosz, M.; McHugh, M. A,; Krukonis, V. J. Supercritical Fluid Technology; Elsevier: New York, 1985. Ruthven, D. M. Principles of Adsorption & Adsorption Processes; Wiley: New York, 1984. Tan, C. S.; Weng, J. Y. Fluid Phase Equilib. 1987, 34, 37. Williams, D.F. Chem. Eng. Sci. 1981, 36, 1769. Received for review October 1, 1987 Revised manuscript received January 20, 1988 Accepted February 3, 1988
