Ind. Eng. Chem. Res. 1996, 35, 4619-4625
4619
Desorption of Phenol from Activated Carbon by Hot Water Regeneration. Desorption Isotherms Gorazd Bercˇ icˇ and Albin Pintar Laboratory for Catalysis & Chemical Reaction Engineering, National Institute of Chemistry, P.O. Box 3430, 1001 Ljubljana, Slovenia
Janez Levec* Department of Chemical Engineering, University of Ljubljana, P.O. Box 537, 1001 Ljubljana, Slovenia
Adsorption of phenol from aqueous solutions at high temperatures on activated carbon was studied. The adsorption capacities were determined by the semicontinuous desorption experiments carried out in a fixed bed adsorber, which was operated in liquid-full mode at a pressure of 25 bar and at temperatures up to 190 °C. It was found that in the range of phenol concentrations from 0.005 to 30 g/L the five-parameter Redlich-Petersen adsorption model is equivalent to the six-parameter Fritz-Schlu¨nder adsorption model when describing liquidsolid adsorption on the Filtrasorb (F-400) activated carbon. By hot water regeneration, 95% recovery of initial adsorption capacity of activated carbon was obtained. It is also demonstrated that the proposed experimental method for determination of adsorption capacities at high temperatures can be used as an alternative to the traditional method by taking breakthrough curves. Introduction Wastewaters containing relatively low concentrations of toxic or poorly biodegradable organic contaminants cannot be economically treated by a direct oxidation technique such as wet air oxidation. One can use an activated carbon (AC) adsorption step to remove the organics from a diluted wastewater. Activated carbon is capable of adsorbing a broad spectrum of organics from wastewaters; however, once the bed becomes saturated, the exhausted carbon must be regenerated before reused. Regeneration of AC adsorbers which operate in the liquid phase is mostly performed by use of different solvents and salt solutions and by pH swing (Radeke and Hartmann, 1992). When adsorber is used for purification of drinking water, the total reactivation of spent carbon at higher temperatures is carried out in multiple-hearth furnaces or rotary kilns (Jankovska et al., 1991). Regeneration of AC beds by temperature swing is mostly used for adsorbers which operate in the gas phase and is achieved by means of steam, hot inert gases, or direct heating (Boppart, 1995). A technique which combines adsorption and oxidation processes has been reported recently for the treatment of wastewaters containing relatively low concentrations of toxic organic contaminants (Levec and Pintar, 1995). In this process, the organics are removed from the wastewater by adsorption on the activated carbon. Once the carbon bed is saturated with organics, it is regenerated by the temperature swing method, which is routing hot water at temperatures up to 200 °C (and elevated pressure) through the adsorber and oxidation reactor. This procedure desorbs most of the organics, and subsequently accomplishes their destruction by oxidation either in a trickle bed or in a wet air oxidation reactor. Since this was not of great importance in the past, only a few studies of adsorption and desorption at elevated temperatures were made (Bhatia et al., 1990; Ling and Hsu, 1995; Radeke and Hartmann, 1992). It should be pointed out that all these studies were made at ambient pressure and at temperatures below the normal boiling point of water. * Author to whom correspondence should be addressed.
S0888-5885(96)00208-4 CCC: $12.00
At elevated temperatures isotherms for adsorption of an organic contaminant from aqueous solutions onto a particular adsorbent can be determined by measuring contaminant concentrations in an experimental setup which can operate in either batch or continuous mode (Smisˇek and C ˇ erny, 1970). One of the main difficulties arising when carrying out batch experiments at temperatures above the normal boiling point of water is the filtration of the suspension. If the conditions during sampling/filtration are not the same as during the adsorption, a new equilibrium is established which consequently leads to wrong results. The adsorption capacity obtained by measuring the breakthrough curves can be affected by transport limitations, especially when larger particles are used. The apparatus dead volume is considerably higher compared to low temperature experiments due to employment of preheater, cooler, and pressure reducer; therefore it must be accounted for. At high temperatures and high concentrations its influence on the breakthrough time cannot be neglected, otherwise the calculated capacities would be too high. The aim of this work is to determine the phenol adsorption/desorption isotherms for activated carbon at temperatures up to 190 °C. The new experimental technique employed is simple and avoids the problems usually encountered in the conventional methods and is particularly advantageous at temperatures and pressures above the normal boiling point of water. Once these isotherms are at our disposal, it would be possible to effectively design the oxidation reactor in the advanced adsorption/oxidation wastewater treatment unit. Experimental Section Apparatus. The adsorption isotherm of phenol from aqueous solutions on activated carbon at 25 °C was determined by a standard bottle-point method (Chatzopoulos et al., 1993). The apparatus for measuring the adsorption isotherms by a semicontinuous technique above 100.0 °C is schematically shown in Figure 1. Properties of the column and the activated carbon as well as the operating conditions employed are summarized in Table 1. Before starting an experiment, the © 1996 American Chemical Society
4620 Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996
Figure 1. Schematic drawing of the experimental apparatus: 1, phenol solution; 2, water; 3, three-way ball valve; 4, metering pump; 5, peristaltic pump; 6, oil bath; 7, preheating coil; 8, adsorber; 9, heater; 10, thermoregulator; 11, thermocouple; 12, heat exchanger; 13, filter; 14, manometer; 15, needle valve; 16, refractory index detector; 17, analog to digital converter; 18, personal computer; 19, centrifugal pump.
Figure 2. Influence of equilibration time on measured equilibrium concentrations.
Table 1. Properties and Operating Conditions of Adsorber adsorber length, cm adsorber diameter, cm flow rate during desorption, mL/min bulk density, g/L bed porosity mass of AC in adsorber, g particle effective size, mm particle porosity skeletal density, g/L phenol concentration during bed saturation, g/L operating pressure, bar operating temperature, °C
30 1 3.8-4.2 495 0.325a 5-12 0.6-0.7 0.652b 2100b 0.05-2 25 110-190
a Calculated on the basis of experimental data. b Data from literature (Chatzopoulos et al., 1993; McKay and Al Duri, 1989).
column was carefully packed with activated carbon. It was placed between layers of glass wool and glass spheres (the same size as the carbon particles) on the upper and lower sides, respectively. The carbon bed was first saturated at 25.0 °C in a water bath by pumping down through it a phenol solution of a predetermined concentration. The outlet column concentration was continuously measured by a refractive index (RI) detector and via a HP 3421 data acquisition/control unit monitored on a HP-150 PC. Once saturation of the carbon bed was reached, the column was quickly immersed in an oil bath with a temperature above 100 °C. Since the adsorption/desorption process was carried out in the liquid-full operated carbon bed, it was necessary to adjust the pressure in the column during the heating. The carbon bed achieved the temperature of the oil bath within 5 min, while the liquid-solid equilibrium was accomplished within first 20 min of equilibration in the oil bath; longer equilibration times did not influence the measured concentrations as is clearly evident from the results represented by Figure 2. During further experimental work equilibration was carried out for 30 min. After equilibration time had passed, the equilibrium solution from the bed was displaced by means of pumping (a high pressure metering pump, Beckman 114M) preheated water through the column. The column effluent was fed into the RI detector through a capillary tubing and a tube-and-shell heat exchanger. The detector output signal was continuously monitored and stored on the PC. Although the apparatus was
Figure 3. Measured outlet concentrations as a function of time after step concentration change at different locations in the experimental setup.
carefully designed, dead volumes and axial dispersion were not completely avoided. By means of a stimulusresponse technique using step concentration change at different locations between the adsorber outlet and RI detector, the influence of axial dispersion on measured phenol concentrations was examined. It is evident from Figure 3 that after approximately 6 min the system outlet concentration was equal to the inlet one. During desorption experiments, a maximum-equilibrium peak concentration was also detected after 6 min as can be seen from Figure 7, where typical concentration vs time profiles during the desorption process at different temperatures are plotted. This observation alludes that the observed dispersion is produced mostly by mixing effects in the dead volumes as well as flow through the outlet tubing but positively not by the molecular diffusivity. It is obvious that the dispersion may affect the measured equilibrium concentrations; therefore some experiments were performed in order to determine experimental conditions where this effect is negligible. Typical effluent profiles obtained during preliminary experiments are depicted in Figure 4. It is seen that the maximal measured concentration, i.e., the equilib-
Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 4621
Figure 4. Measured effluent concentration profiles after equilibration carried out in adsorber filled with different amounts of AC.
Figure 5. Measured breakthrough curves after three consecutive regenerations by hot water.
rium concentration, was affected by the dispersion when adsorber was filled with less than 7.5 g of activated carbon. Consequently, in all subsequent runs the adsorber was filled with 9 g of activated carbon. The concentration oscillations were observed when the adsorber effluent was diluted with water in order to fit the detector range. However, placing a 3 cm layer of 1 mm stainless steel spheres into a 1/8 SS tubing, after mixing the effluent with distilled water, smoothed the profiles. Reproducibility of the experimentally measured equilibrium concentrations was found within 3%, regardless of the desorption temperature employed and carbon used (fresh or regenerated) during the saturation of the bed at low temperature. To verify results obtained by the semicontinuous technique, some adsorption capacity measurements of the F-400 activated carbon were performed at elevated temperatures (110-170 °C) by means of the breakthrough curve method. These experiments were carried out in the same apparatus as described above applying the following operating conditions: P ) 25 bar; mAC ) 5 and 7.5 g; Csat. ) 2.5 and 15 gPh/L; Φvol ) 5 and 7 mL/ min. From Figures 5 and 6 it is evident that the adsorption capacity of the active carbon bed was 95% recovered during three successive adsorption/desorption
Figure 6. Recovered adsorption capacity during hot water regeneration.
steps (saturated at 25 °C with solution containing 1 gPh/L at a flow rate of 9.5 mL/h and regenerated with water at 180 °C for 150 min at a flow rate of 4.75 mL/ h). Materials. Adsorption and desorption experiments were conducted on Filtrasorb (F-400) activated carbon (Chemviron). Phenol (HPLC grade) was supplied by Carlo Erba. Aqueous solutions of phenol were prepared from deionized and distilled water. Analysis. In the batch experiments and during measurement of breakthrough curves, phenol concentrations were obtained by means of a total organic carbon (TOC) analyzer (Rosemount/Dorhmann, Model DC-190) or by an HPLC instrument (Thermo Separation Products) equipped with a UV detector (Spectro Monitor 3200) and using a 250 × 4.6 mm column packed with Spherisorb ODS2-10. In the semicontinuous experiments, phenol concentrations were measured with a differential refractometer (Knauer) which was tuned daily with reference phenol solutions to validate and monitor its performance. It was possible to measure phenol concentrations up to 10 g/L without prior dilution. When the effluent concentrations exceeded the calibrated range of the RI detector, the effluent solutions were diluted with a stream of distilled water by means of a pump (Masterflex). In all cases, the effluent solutions, either diluted or not, were fed into the RI detector by a peristaltic pump (Masterflex). Results and Discussion Adsorption Capacity at 25 °C. Adsorption capacity of the activated carbon at 25.0 °C was calculated using the equation
Q)
75.5C 1 + 295.2C0.793
(1)
which represents the Redlich-Petersen (1959) isotherm for the adsorption of phenol on activated carbon F-400. Equation 1 was derived from the data obtained by means of the bottle-point experiments (Lesjak, 1992). Adsorption Capacity at Elevated Temperature. Typical concentration profiles measured during the desorption experiments, performed at five different temperatures, are depicted in Figure 7. The activated carbon bed was saturated at 25.0 °C with a solution
4622 Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 Table 2. Influence of Some Parameters and Assumptions on the Calculated Adsorption Capacity change of calc ads. cap., % variable/term
variation
average
maximal
B FB Q(FB/FPh)CB S mdis Ph
(10% (10% neglected neglected considered
(0.09 (0.66 -0.85 2.5 -0.75
(0.25 (1.8 -2.9 6.5 -2.4
calculated if we know the equilibrium phenol solution concentration (C) at that temperature. During equilibration, a small amount of phenol was lost due to the bed expansion in the heating process. The maximal possible amount of phenol that could have been lost due to the expansion is estimated by means of the relation Figure 7. Typical concentration profiles measured during desorption experiments carried out at different temperatures.
having 0.5 g/L phenol. The maximal concentrations attained during the course of desorption were assumed to be equal to the equilibrium concentration. Supposing an equilibrium between activated carbon and phenol solution, the total mass of phenol in the bed is equal to the sum of the amount of phenol present in the solution within the interparticle voids, the amount of phenol in the solution within the pores of carbon particles, and the amount of phenol adsorbed on the carbon surface; thus
mPh ) BC + (1 - B)SC + QFB
(2)
where the porosity (S and B) and the bed density are considered to be independent of temperature. Since the adsorption capacity of activated carbon for phenol is relatively high, eq 2 can be written more precisely if the pore volume occupied with phenol is taken into account. The pore volume is decreased for the portion that is occupied with the adsorbed phenol; thus eq 2 becomes
mPh ) BC + ((1 - B)S - QFB/FPh)C + QFB (3) where the density of the adsorbed phenol is assumed to be equal to the density of liquid phenol which can be calculated according to Yaws (1977) with the following equation 2/7
FPh ) (0.4094)0.3246-(1-Tr)
(4)
If the bed of activated carbon, in which the equilibrium was attained at 25.0 °C, is heated up to a temperature T2, a new equilibrium state will be achieved after a while. At this new equilibrium, the phenol mass balance for the carbon bed can also be written in the form of eq 3. Since the total mass of phenol in the bed at T2 is approximately the same as at 25.0 °C, the adsorption capacity of phenol on the activated carbon at T2 is given by
Q)
T1 (mPh
- BC - (1 - B)SC)/(FB - (FB/FPh)C) (5)
In eq 5 all parameters are either measured or calculated using an appropriate correlation; therefore the adsorption capacity at any temperature T2 can be readily
dis mPh ) (B + (1 - B)S)(1 - FH2O(T2)/FH2O(T1))CT2 (6)
It is obvious that eq 6 overestimates the phenol lost since the bed already expands before the equilibrium is reached and before the concentration of the displaced solution (CTB2) has reached the equilibrium value. During preliminary experiments operating conditions of the apparatus were examined in order to ensure reproducible and accurate results. Since adsorption capacities were calculated according to eq 5, we verified the influence of some parameters and assumptions made during calculation by investigation of the influence of each particular parameter and proposition on the calculated adsorption capacities. The results of the parametric sensitivity analysis of eq 5 are summarized in Table 2. As can be seen, the carbon bed parameters (B and FB) have a very small effect on the calculated adsorption capacities. In our case, the reproducibility of the carbon bed packing was within 3%; therefore the errors arising from this part can be neglected. It should be underlined that neglecting the estimated mass of phenol lost during the bed expansion does not contribute appreciably to the error in the calculated capacity; therefore it was not considered in the analysis of the experimental results presented in this work. The desorption experiments were performed at 25 bar in order to prevent evaporation. A few experiments carried out at 50 bar have shown that the experimentally measured equilibrium phenol concentrations are independent of pressure. Adsorption Isotherm. Once the adsorption capacities at various equilibrium concentrations and temperatures are available, it is possible to search for an adsorption isotherm that would describe the experiments appropriately. The adsorption models confronted with the experimental data are given in Table 3, together with some relevant statistical results. The temperature effect on the parameters ki appearing in the adsorption models was accounted for by the exponential relation
ki ) k°i exp(Ai/T)
(7)
The concentration powers were assumed independent of temperature. The experimentally measured equilibrium data were fitted to the proposed adsorption isotherms using the Marquardt optimization technique (Duggleby, 1984). A proportionally weighted fit was used; i.e., the weighted sum of the squares of the residuals was minimized and calculated as
Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 4623 Table 3. Results of Statistical Analysis of Different Adsorption Models (Set of 37 Experimental Points Was Used) adsorption model
k1C 1 + k2C
Langmuir
Q)
Freundlich
Q ) k1Cn1
Redlich-Petersen (1959)
Q)
Q)
Toth (1971)
Fritz-Schlu¨nder (1974)
Q)
normalized RMSa × 10-2
N(10b
reduced normalized RMSc × 10-2
20.74
21
3.5
18.7
26
5.4
6.9
36
1.9
9.6
33
2.1
4.74
36
1.8
k1C 1 + k2Cn1 k1C (1 + k2Cn1)1/n1 k1Cn1 1 + k2Cn2
N Normalized RMS ) ((1/N)∑i)1 (Qi,exp - Qi,calc)2/Qi,exp2)1/2. b Number of points within (10%. c Reduced normalized RMS ) N(10 2 1/2 2 ((1/N(10)∑i)1 (Qi,exp - Qi,calc) /Qi,exp ) .
a
N
SR )
(Qi,exp - Qi,calc)2Wi ∑ i)1
(8)
where the weighting factor, Wi, was set proportional to 1/Qi,exp2. This weighting ensures that during the parameter optimization procedure all experimentally obtained points contribute proportionally despite the extent of the measured adsorption capacity. The results shown in Table 3 suggest that two-parameter adsorption models such as Langmuir and Freundlich do not describe the experimental facts quite well. On the other hand, three- and four-parameter models are much better, especially the Redlich-Petersen and FritzSchlu¨nder models which are almost equivalent. In fact, as they are used here they can be considered as fiveand six-parameter models, respectively, because they include the temperature dependency of ki parameters. From Figure 8, where differences between measured and calculated adsorption capacities for both RedlichPetersen and Fritz-Schlu¨nder adsorption models at different measured adsorption capacities are plotted, one can conclude that points obtained with the RedlichPetersen adsorption model are distributed more homogeneously along the axis which represents measured adsorption capacities. Once the experimental data point which belongs to the adsorption capacity of 0.05 g/gAC is treated as an outlier, the differences vanish as one may also suspect from the reduced root-mean-square deviation (RMS) listed in Table 3. Moreover, during the optimization procedure the Redlich-Petersen adsorption model exhibited the fastest convergence and the calculated values of the parameters showed much smaller deviation than the parameters obtained when the experimental data were fitted with the Toth and Fritz-Schlu¨nder adsorption models. The calculated deviations of these parameters are summarized in Table 4. Considering the statistical results and due to the smaller number of parameters in the Redlich-Petersen equation for the representation of the experimental results than with the Fritz-Schlu¨nder adsorption model, the Redlich-Petersen adsorption model was chosen for
Figure 8. Deviation between calculated and measured adsorption capacities for two most favorable adsorption isotherms at experimentally measured adsorption capacities. Table 4. Calculated Deviations of Parameters in the Three Most Agreeable Adsorption Models adsorption isotherm
k°1
dev of params expressed as % of calcd param value A°1 k°2 A°2 n1
n2
Toth (232 (8.06 (6.8 (13.8 (23 Fritz-Schlu¨nder (45 (8.2 (46.5 (8.5 (5.7 (5.1 Redlich-Petersen (2.5 (0.7 (2.7 (1.0 (0.4
the prediction of adsorption capacity of activated carbon F-400 at elevated temperatures. Thus, in Figure 9 the Redlich-Petersen adsorption isotherm is represented by the equation
Q)
3.058 × 10-7e5763/TC 1 + 6.563 × 10-6e5256/TC0.788
(9)
and compared to the experimental data points. From Table 5, where the adsorption capacities calculated
4624 Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996
perature of water and with solutions containing high concentrations of the adsorbate. Acknowledgment The financial support of this work from the Slovenian Ministry of Science and Technology under Grant J27031 is gratefully acknowledged. The authors also thank Chemoservice srl (Gorizia, Italy) for samples of Chemviron F-400 activated carbon. Nomenclature
Figure 9. Comparison between measured adsorption capacities and those calculated by the Redlich-Petersen adsorption model. Table 5. Comparison of Adsorption Capacities Calculated by Means of eq 9 with Measured Data Obtained by a Method of Breakthrough Curves adsorption capacity, g/gAC temp, °C
calcd
measd
109.5 128.8 147.9 168.8
2.5 g/L Phenol 0.197 0.172 0.147 0.118
0.191 0.169 0.145 0.114
109.3 129.5 148.5 168.8
15 g/L Phenol 0.305 0.280 0.257 0.232
0.294 0.272 0.251 0.223
according to the proposed adsorption isotherm (eq 9) are compared to those determined by the breakthrough method at two different concentrations and at four different temperatures, we may conclude that the results obtained with the semicontinuous method are in good agreement with the values determined by means of the conventional breakthrough method. On the basis of the above discussion it can be concluded that the semicontinuous method offers fast, “tailing-free”, and accurate determination of the adsorption isotherms at elevated temperatures. The main drawback of this technique is its relativeness, which means that for the determination of adsorption capacities at higher temperatures an adsorption isotherm at saturation temperature is needed in advance; the latter is easily obtained by means of a widely used bottle-point method. Conclusions It is demonstrated here that the regeneration of activated carbon saturated by phenol may be effectively performed by hot water. It was found experimentally that more than 95% of the initial adsorption capacity of the activated carbon can be recovered during hot water regeneration. The adsorption isotherms developed in this work enable us to predict the adsorption capacity of activated carbon F-400 at temperatures up to 190 °C and phenol concentrations up to 30 g/L. The employed semicontinuous method shows as a promising technique especially when experiments must be performed at temperatures above the normal boiling tem-
A ) constant in eq 7, K AC ) activated carbon C ) concentration, g/L GAC ) granular activated carbon k°i ) preexponential factor in eq 7, (L/g)1/n ki ) parameter in adsorption models, (L/g)1/n mPh ) mass of phenol in the bed of AC, g mdis Ph ) mass of disappeared phenol according to eq 6, g n ) exponent of concentration term in adsorption models N ) number of experimental points P ) pressure, bar SR ) sum of residuals according to eq 8 Q ) adsorption capacity, gPh/gAC t ) time, min T ) temperature, °C Tr ) reduced temperature Wi ) weighting factor, (gAC/g)2 Greek Letters B ) bed porosity S ) solid porosity FB ) bulk density of GAC bed, g/L φvol ) volumetric flow rate, mL/min Subscripts B ) bulk conditions calc ) calculated des ) conditions during desorption eq ) equilibration conditions exp ) experimental i ) experimental point i Ph ) phenol sat. ) conditions during saturation S ) solid phase
Literature Cited Bhatia, S. K.; Kalam, A.; Joglekar, H. S.; Joshi, J. B. Effective Diffusivity of Phenol in Activated Carbon. Chem. Eng. Commun. 1990, 98, 139. Boppart, S. Get the Most from Activated-Carbon Systems. Environ. Eng. World 1995, May/June, 12. Chatzopoulos, D.; Varma, A.; Irvine, R. L. Activated Carbon Adsorption and Desorption of Toluene in the Aqueous Phase. AIChE J. 1993, 39, 2027. Duggleby, R. G. Regression Analysis of Nonlinear Arrhenius Plots: An Empirical Model and a Computer Program. Comput. Biol. Med. 1984, 14, 447. Fritz, W.; Schlu¨nder, E. U. Simultaneous Adsorption Equilibria of Organics Solutes in Dilute Aqueous Solutions on Activated Carbon. Chem. Eng. Sci. 1974, 29, 1279. Jankovska, H.; Swiatkowski, A.; Chroma, J. Active Carbon; Ellis Horwood Ltd. and Wydawnictwa Naukowo-Techniczne: Warsaw, 1991. Lesjak, M. Regeneration of Activated Carbon by Catalytic Oxidation in Liquid Phase. B.D. Thesis, University of Ljubljana, 1992. Levec, J.; Pintar, A. Catalytic oxidation of aqueous solutions of organics. An effective method for removal of toxic pollutants from waste waters. Catal. Today 1995, 24, 51.
Ind. Eng. Chem. Res., Vol. 35, No. 12, 1996 4625 Ling, H. S.; Hsu, F. M. Liquid-Phase Adsorption of Organic Compounds by Granular Activated Carbon and Activated Carbon Fibers. Ind. Eng. Chem. Res. 1995, 34, 2110. McKay, G.; Al Duri, B. Prediction of Multicomponent Adsorption Equilibrium Data Using Empirical Correlations. Chem. Eng. J. 1989, 41, 9. Radeke, K. H.; Hartmann, G. On the Temperature Dependence of Adsorption of Organic Materials from Aqueous Solution. Adsorpt. Sci. Technol. 1992, 8 (3), 153. Redlich, O.; Petersen, D. L. A Useful Adsorption Isotherm. J. Phys. Chem. 1959, 63, 1024. Smisˇek, M.; C ˇ erny, S. Active Carbon; Elsevier Publishing Company: Amsterdam, The Netherlands, 1970. Toth, J. State Equations of the Solid-Gas Interphase Layers. Acta Chim. Acad. Sci. Hung. 1971, 69, 311.
Yaws, C. L. Physical Properties; Chemical Engineering and McGraw-Hill Publishing Co.: New York, 1977.
Received for review April 12, 1996 Revised manuscript received August 26, 1996 Accepted September 5, 1996X IE960208M
X Abstract published in Advance ACS Abstracts, October 15, 1996.
