Ind. Eng. C h e m . Res. 1989, 28, 1222-1226
1222
Supercritical Regeneration of Activated Carbon Loaded with Benzene and Toluene Chung-Sung Tan* and Din-Chung Liou Department o f Chemical Engineering, National Tsing Hua University, Hsinchu, Taiwan 30043, Republic of China
In this study, the regeneration by supercritical carbon dioxide of activated carbon loaded with benzene and toluene was investigated. From the experimental data, it was found that the adsorptive capacities of the regenerated activated carbon for benzene and toluene after many cycles were still close to those of the virgin carbon and remained stable. The effects of temperature, pressure, and flow rate on regeneration efficiency were also studied. Regeneration was found to be more favorable at higher pressures, but the optimal regeneration temperature was dependent on pressure. A mathematic model was proposed in this study, which was found to agree well with the experimental data. The adsorption rates of the activated carbon regenerated by the supercritical fluid method and the steam method were also compared in this study. I t was observed that the supercritical fluid method was superior to the steam method. toluene was obtained by passing nitrogen through a saturator containing equal amounts (about 100 cm3)of benzene and toluene (Figure 1). The saturated benzene and toluene concentrations were 1.84 X and 1.13 X g/ cm3, respectively. Virgin activated carbon (Degussa, WSIV) was first screened to obtain a 18-20-mesh fraction (the average particle size was 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 5.1 g of activated carbon was packed in an 4.1-cm-i.d. glass tube for adsorption experiments (Figure 1). In order to achieve uniform flow distribution and to avoid possible end effects, glass beads of 0.1-cm diameter were packed in the regions above and below the activated carbon packing both with heights of about 4.0 cm. Platinum wires were used to support the glass beads and the activated carbon particles in the column. To execute the adsorption experiments, the prepared gas stream passed through the carbon column at 308 K. The flow rate was kept constant at 0.25 cm3/s. A portion of the effluent was sent to the GC (Varian 3700, FID detector) to analyze the benzene and toluene concentrations. The experiment was stopped when the benzene and toluene breakthroughs were achieved. The adsorptive capacities of virgin carbon were found to be about 0.39 g of benzenelg of activated carbon and 0.20 g of toluene/g of activated carbon for the gas concentrations used in this study. These values were obtained by integrating the inlet minus the exit mass flows of benzene and toluene in the breakthrough experiments. After the adsorption experiment, the loaded activated carbon was taken out of the adsorber and was packed in a 2.12-cm-i.d. stainless steel 316 tube (regenerator). In order to make the supercritical fluid uniform in the packed bed, glass beads of 0.1-cm diameter were also packed in the regions above and below the activated carbon packing with heights of about 4.0 cm. Then the packed column was put into the regeneration apparatus, which is shown in Figure 2. Carbon dioxide of 99.7% purity was used as the desorbent. It first passed through a silica gel bed in order to remove the 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 could be maintained within 1.0% deviation of the desired value. The temperature was controlled in a constant-temperature bath whose accuracy was within 0.5 K. A preheating coil with 0.3-cm diameter and about 110-cm
The supercritical fluid technology has received widespread attention over the past years. One of its applications is to regenerate activated carbon. Kander and Paulaitis (1983) studied the desorption of activated carbon loaded with phenol by supercritical carbon dioxide. Though they found that supercritical carbon dioxide offered no significant thermodynamic advantages for regenerating carbon loaded with phenol, they believed that it would be a powerful desorbent for other organic compounds that 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 observed that the supercritical regeneration method was economical even though the operating temperature and pressure were above 387 K and 150 atm. A local equilibrium model using the Freundlich isotherm was used and was able to explain the experimental data. Tan and Liou (1988, 1989) studied the desorption by supercritical carbon dioxide of activated carbon loaded with either ethyl acetate or toluene. They found that this regeneration method was better than the steam regeneration method. A linear desorption kinetics model was proposed by these authors and was found to fit experimental data quite well. In all the above mentioned studies, only a single compound adsorbed on activated carbon was considered. Benzene and toluene are important solvents used in petrochemical and polymer industries and are commonly emitted together from industrial plants. To recover them from the effluent gas streams, activated carbon is generally employed. Since benzene and toluene are soluble in supercritical carbon dioxide (Ng and Robinson, 1978; Sebastian et al., 19801, regeneration of activated carbon by supercritical carbon dioxide seems to be a possible method. Hence, the main objective of this paper is to study the regeneration by supercritical carbon dioxide of activated carbon loaded with both benzene and toluene at different operating conditions. In order to interpret the experimental data, a mathematical model is also proposed. Because the steam regeneration method is customarily employed to regenerate activated carbon (Smisek and Cerny, 1970; Ruthven, 1984), the other objective of this study is to compare the supercritical regeneration method to the steam regeneration method.
Experimental Section Benzene and toluene were used as the adsorbates in this study. A gas stream containing saturated benzene and
* To whom correspondence should be addressed. 0888-5885 I89 12628-1222501.50/0 , I
,
@
1989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 8, 1989 1223 EXPERIMENTAL Temp
308K ?IRK
--MODEL
Benzene Toluene A A 0
s
c
E 1
N2 Cylinder
Pieheoling Coil 5 Three Way Volve
7 Adsorber 10 Wet Tesl Meter 8 Gas Chromatograph 11 Hmtmg Tope 9 Collector 12 Constant
L
2 Mass Flow Controller 3 Monometer
..e
-
6 Soturotor
Temperature Bath
n
Figure 1. Schematic diagram of the apparatus used for adsorption experiments. U
4U
100
bU
Time Mmutes
Figure 4. Temperature effect on regeneration a t 87 atm. EXPERIMENTAL Temp. Benzene Toluene
MODEL
::
s 1 COz Cylinder
r-
2 RQgUlOtOr
6 Pressure Guage 7 Regenerotor
I1 Mognetlc S t i r r e r 12 F i l t e r
3 Shea Gel Bed
8 Heating rope
13 Wet Test Meter
L Compressor 5 Surge Tank
9 Metering Valve
1L Constont-Temperature Both
5 E
s
x) Cold Trop
n
Figure 2. Schematic diagram of the apparatus used for supercritical regeneration experiments.
-
0.6
1
A Benzene Toluene
Time, MlnUtrS u
Figure 5. Temperature effect on regeneration a t 100 atm. Term 308K 318K 328K 338K
s
EXPERIMENTAL Benzene Toluene A
-MODEL
*
0
-'
::
'r
.c
E 80
c
01 L
I 1
I
2
I
3
I
L
I
5
I
6
I
7
1
60
8
Number of Cycle
Figure 3. Adsorptive capacities of regenerated activated carbon for the beginning cycles.
length was immersed in the constant-temperature bath in order to let the temperature of the carbon dioxide reach that of the constant-temperature bath. 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 benzene and toluene were collected in a cold trap which contained 1.25 L of ethanol. Samples of 2.0 ILLwere frequently taken out for GC (FID detector) analysis in order to obtain the desorption curves. When the concentrations of benzene and toluene in the cold trap were not changed any longer, the desorption experiment was stopped. It generally took 5 h to complete the experiment. Then, the subsequent adsorption experiment was executed to obtain the adsorptive capacities of benzene and toluene. These capacities were compared with the desorption amounts 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 regeneration experiment were considered
T Imp,Minutes
Figure 6. Temperature effect on regeneration a t 136 atm.
as a cycle. Each cycle included the adsorption taking place at 308 K for saturated benzene and toluene concentrations and the regeneration occurring at different temperatures and pressures. Figure 3 shows that the adsorptive capacities for benzene and toluene after the first cycle dropped about 15% from those for the virgin activated carbon and reached stable values. This phenomenon was also observed by Kander and Paulaitis (1983) and Tan and Liou (1988, 1989). When the carbon dioxide flow rate was fixed at about 4.5 cm3/min (calculated based on the operating temperature and pressure rather than the normal condition of 298 K and 1atm), the effects of temperature on regeneration efficiency at operating pressures of 87, 100, and 136 atm are shown in Figures 4-6, respectively. Though the regeneration period in each experiment was more than 5 h, only the data at the first 100 min are reported in these figures. Nevertheless, it can be seen that more than 50% of 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 the dynamic data was less
1224 Ind. Eng. Chem. Res., Vol. 28, No. 8, 1989 Table I. Density and Viscosity of Carbon Dioxide at Various Conditions 87 atm 100 atm temp, K Pa LLb Pa Ub 308 0.60 4.47 0.71 5.54 318 0.34 2.75 0.49 3.71 328 0.24 2.29 0.32 2.67 338 0.21 2.11 0.27 2.46 g/(cm.s) x
"In g/cm3.
EXPERIMENTAL
I
Pressure
87atm lmatm
120 atm Pa
Ub
0"
Ub
0.75 0.60 0.48 0.36
6.45 5.21 3.83 2.87
0.78 0.66 0.53 0.46
6.74 5.42 4.29 3.51
io4. -MODEL
Pressure 87otm
A 0
than 3.0%, with a maximum deviation of about 5.0%. Figure 4 indicates that the efficiencies for both benzene and toluene decreased with temperature when the pressure was 87 atm. But when the regeneration pressures were raised to 100 and 136 atm, the situation was changed and an optimal regeneration temperature was found for both benzene and toluene, which are illustrated in Figures 5 and 6. This trend was also observed for a single-solute system (Tan and Liou, 1988a,b). This means that, for the benzene and toluene system, the existence of the second compound loaded on activated carbon would not influence the desorption of the first compound. The existence of an optimal operating temperature might be explained in terms of density and viscosity of the fluid. Table I gives the density and viscosity of carbon dioxide a t different operating conditions. Since the concentrations of benzene and toluene in supercritical carbon dioxide were quite small, the density and viscosity of the supercritical mixture could be regarded as those of the pure carbon dioxide. In general, higher density may enhance the solubility of a solute in a supercritical fluid, but higher viscosity may have an adverse effect on diffusion rate. A t lower operating pressures, such as 87 atm, the density effect seems more important; therefore, the optimal regeneration condition is at the lowest temperature. But at higher operating pressures, such as 100 and 136 atm, it seems that the viscosity effect in addition to density is also important; hence, an optimal temperature exists somewhere in the supercritical region. Comparing the desorption breakthrough curves within the first 100 min from Figures 7 and 8, it can be seen that regeneration efficiencies increased with the operating pressure for both benzene and toluene. This trend was also observed for a single-adsorbate system (Tan and Liou, 1988a,b). Hence, the same conclusion can be applied to a two-adsorbate system that the pressure effect may be due to the increase of density. Since the interphase mass-transfer coefficient is a hydrodynamic property, the regeneration may be influenced by the flow rate of the regenerating fluid. Figure 9 illustrates that the desorption amounts for benzene and toluene increased with the flow rate at fixed times. This indicates that the interphase mass-transfer resistance may play an important role during the regeneration.
1
Benrrn Toluene A
A
1CQotm
8
0
1lOatm
e
0
T i m e , Minutes
Figure 7. Pressure effect on regeneration a t 318 K.
-H O G L
EXPERIMENTAL
Benzene Toluene A
136 atm
r # m e Mlnutes
Figure 8. Pressure effect on regeneration a t 338 K.
,
Tim? Minutes
Figure 9. Flow rate effect on regeneration at 318 K and 120 atm. n 2
5
i 1
51
7 I
I
I
1
Steam Generator
L
Heating Tape
7
2
Pressure Gauge
5 Thermocouple
8
Condenser
3
S o f t y Valve
6 Regenerator
9
Collector
Metering Valve
Figure 10. Schematic diagram of the apparatus used for steam regeneration experiments.
For most industrial plants, steam is customarily employed to regenerate activated carbon to recover organic solvents. It is therefore of interest to compare the regeneration efficiencies using saturated or superheated steam and supercritical carbon dioxide as the desorbents. The apparatus used for the steam regeneration method is illustrated in Figure LO. The adsorption and desorption procedures were the same as described for the supercritical regeneration method. Three sets of the temperature and pressure of steam, 440 K, 6 atm, 460 K, 11.5 atm, and 510 K and 5.5 atm, were used. The flow rate varied from 35.0
Ind. Eng. Chem. Res., Vol. 28, No. 8, 1989 1225 Table 11. Simulated Values of k R and kT" 87 atm temp, K 308 318 328 338
kB
kT
4.16 4.11 1.37 1.03
1.05 1.00 0.28 0.21
100 atm kB 4.69 4.60 1.71 1.42
120 atm kT
kB
1.24 1.18 0.43 0.34
4.99 4.84 2.10 1.81
136 atm kT 1.37 1.30 0.58 0.45
kB 5.28 5.04 2.30 2.05
kT 1.47 1.35 0.69 0.51
"The units of kT and kT are cm3/(s.mol). C02 2.L-Steam 22 Stwm steam
-
318K 12Oatm LLOK 6atm L60K 115atm 5 1 0 ~5 5 0 t m
Because of lack of information on effective diffusivities of benzene and toluene in supercritical carbon dioxide and adsorption isotherms under supercritical operations, the mass balances in the activated carbon particle using the
BQnZQnQTOIUQnQ A A 0
0
0
20-
I
1.0
asT/at = -kTS& The initial conditions are
0.8
att=O
0.6 0.L 0.2
0 0
200
I 630
Loo
1
1
800
I
J
Figure 11. Comparison of the breakthrough curves of activated carbon regenerated by supercritical fluid and steam methods.
to 47.5 g/min depending on operating conditions, and the regeneration period lasted at least 2 h. Figure 11 shows the adsorption breakthrough data for the activated carbon regenerated by these two methods, while the supercritical method was operated at 318 K and 120 atm. In this figure, it can be seen that a significant difference in breakthrough times between these two methods existed. The use of saturated and superheated steam practically provided the same breakthrough data. This is also the case for the supercritical regeneration method when the operating temperatures and pressures were other than 318 K and 120 atm. The rapid appearance of the breakthrough time using the steam regeneration method was probably due to the increase of the transport resistances in activated carbon caused by the condensed water. From this comparison, we may conclude that the supercritical regeneration method is superior to the steam method from the viewpoint of the adsorptive capacities in a certain period of time.
Model Description Suppose the axial dispersion effect can be neglected; the mass balances of benzene and toluene in the bulk phase in the column may be written as
- + u - = -(1at az acB
acB
acT
8SB
dt
+u = -(14 dt at az The initial and boundary conditions are att=O acT
€
-
(2)
=0
(3)
CT = 0
(4)
CB
atz=O
8ST
= SB,O
ST
= ST,O
att=O
1000
T i m e , Minutes
t
SB
With eq 7 and 8, the concentrations of benzene and toluene at the exit of the column can be obtained by solving eq 1-10 numerically. The finite difference approximation used by Sutikno and Himmelstein (1983) was employed in this study, which gives the following expression:
The superscript n represents the nth increment of time, and the subscript i represents the ith increment of distance. When At I0.01 and Az I0.005, stable numerical solutions were obtained. The total amount of desorbed benzene and toluene could then be calculated by integrating the exit concentrations with respect to time. The desorption rate constants, k B and kT, were evaluated by fitting the numerical solutions with the experimental data. The iteration was required, and the IMSL subroutine zxss~was employed to achieve this fit. With the calculated rate constants (listed in Table 11),the stimulated results were found to match well with the experimental data for all operating conditions, which are shown in Figures 3-8. The maximum deviation was no more than 5.070,and the average deviation was about 3.0%. These agreements indicate that it is plausible to interpret the regeneration data by using the proposed model.
Conclusions In this study, the regeneration of activated carbon loaded with benzene and toluene by supercritical carbon dioxide was investigated. In the adsorption experiments, a gas stream containing saturated benzene and toluene passed through a column packed with activated carbon at 308 K. After adsorption equilibria were achieved, the supercritical carbon dioxide entered into the column to regenerate the adsorbed benzene and toluene.
I n d . E n g . C h e m . Res. 1989, 28, 1226-1231
1226
From the experimental data, it was found that the adsorptive capacities of the regenerated activated carbon for benzene and toluene after many cycles were still close to those of the virgin carbon and remained stable. The effects of temperature, pressure, and flow rate on regeneration efficiency were also studied. At higher pressures the regeneration was found to be more favorable. But as the temperature effect is concerned, an optimal temperature was observed when the pressure was above 100 atm. Because of the regeneration efficiency varied with the flow rate, the interphase mass-transfer resistance may play an important role under supercritical operations. A mathematic model, assuming the regeneration rate depended on both the benzene and toluene concentrations on activated carbon, was proposed in this study, which was found to agree well with the experimental data. The adsorption rates of benzene and toluene on the activated carbon regenerated by the supercritical fluid method and the steam method using saturated and superheated steams were also compared in this study. It was observed that the supercritical fluid method offered a better regeneration efficiency than the steam method.
Acknowledgment Financial support from the National Science Council of ROC and Asia Chemical Corporation in ROC is gratefully acknowledged.
Nomenclature CB,CT = concentration of benzene and toluene, respectively, mol/cm3 k g , kT = desorption rate constants, cm3/(s.mol)
Sg, ST = loaded benzene and toluene on activated carbon,
respectively, mol/cm3 SB,o, ST,O= initially loaded benzene and toluene on activated
carbon, respectively, mol/cm3 T = temperature, K t = time, s z =
axial position in the column, cm
Greek Symbols = void fraction in the packed column
t
p p
= viscosity, g/(cm-s)
= density, g/cm3 Registry No. C, 7440-44-0; COz, 124-38-9; benzene, 71-43-2; toluene, 108-88-3.
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, DC. Kander, R. G.; Paulaitis, M. E. In Chemical Engineering and Supercritical Conditions; Penninger, J. M. L., Gray, R. D., Davidson, P., Eds.; Ann Arbor Science: Ann Arbon, MI, 1983; p 461. Ng, H. J.; Robinson, J. J. Chem. Eng. Data 1978, 23, 325. Ruthven, D. M. Principles of Adsorption & Adsorption Processes; Wiley: New York, 1984. Sebastian, H. M.; Simnick, J. J.; Lin, H. M.; Chao, K. C. J. Chem. Eng. Data 1980, 25, 246. Smisek, M.; Cerney, S. Active Carbon; Elsevier: New York, 1970. Sutikno, T.; Himmelstein, K. J. Ind. Eng. Chem. Fundam. 1983,22, 420.
Tan, C . S.; Liou, D. C. Ind. Eng. Chem. Res. 1988a, 27, 988. Tan, C. S.; Liou, D. C. Sep. Sci. Technol. 1989, 24, 111.
Received for review September 26, 1988 Revised manuscript received April 11, 1989 Accepted May 1, 1989
Solubility of L-Isoleucine in and Recovery of L-Isoleucine from Neutral and Acidic Aqueous Solutions Ronald C. Zumsteint Department of Chemical Engineering, North Carolina State University, Raleigh, North Carolina 27695-7905
Ronald W. Rousseau* School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100
Recovery and purification of L-isoleucine from fermentation media include several crystallizationrecrystallization steps. Solubilities, which were found to be different from those reported by earlier workers, are therefore important in designing these steps and in analyzing crystallizer performance. The effect of temperature on the solubility of L-Ile a t the isoelectric point was determined, as was the influence of p H as adjusted by the addition of HC1. Increasing the acid content until there was approximately 1 mol of HCl/mol of L-Ile raised the solubility of L-Ile to a maximum value. Subsequent addition of chloride ions, whether added with more HCI or with inorganic salts, decreased the solubility. Solubility data for many amino acids in aqueous solutions consist of measurements taken when isolation and detection methods were crude (Greenstein and Winitz, 1961). Isolation was difficult because amino acid preparation involved either chemical synthesis, which resulted in racemic mixtures, or protein hydrolysis, which often resulted in a mixture of several amino acids. Complete resolution of racemic mixtures was difficult, as was the Current address: Ethyl Corporation, P.O. Box 341, Baton Rouge, LA 70821. * To whom correspondence should be addressed.
0888-5885/89/2628-1226$01.50/0
separation of a specific amino acid from a mixture of several other amino acids. The determination of purity was based partially on optical rotation measurements, which could be indecisive in this respect. More recently, biosynthesis methods have been found that produce isomerically pure compounds (Meister, 1965). Also, chromatographic techniques have been developed that give an accurate determination of amino acid purity (Pfeifer et al., 1983). L-Isoleucine (L-Ile) is an example of the above situation. It is one of the essential amino acids that has been commercially produced by fermentation (Shimura, 1972). 0 1989 American Chemical Society
