Ind. Eng. Chem. Res. 1989,28, 1564-1567
1564 I
1
Hawkins and Wanke, 1979; Mars and van Krevelen, 1954).
Conclusions The oxidation of diethyl sulfide over Pt/A1203catalyst is zeroth order at low concentrations (5-250 ppm) and temperatures to 300 "C. Acknowledgment I thank the National Research Council and the U.S. Army CRDEC for financial assistance on this project. Registry No. Pt, 7440-06-4; diethyl sulfide, 352-93-2. 17
1 .R
I .9
2.0
2.1
l / T (li) X 1000
Figure 4. Arrhenius plot of the zeroth-order rate constant. K = 78.8 exp(-19117/RT) mol of EtzS/(s-g of catalyst); r2 = 0.9992.
form of the design equation for a zeroth-order reaction is (Froment and Bischoff, 1979; Smith, 1981) W / F = X/Ko (1) where W is the weight of the catalyst (g), F is the sulfide flow rate (mol/s), X is the fractional conversion (dimensionless),and KOis the zeroth-order rate constant (mol/s.g of catalyst). From eq 1, note that a plot of W / F versus X will yield a straight line, with the slope of the line equal to l/K@ This is consistent with the data reported in Figure 2 and 3, indicating that the reaction is zeroth order with respect to diethyl sulfide over the range of conditions evaluated in this study. There are a number of mechanisms which may be proposed that will explain the observed zeroth-order kinetics. One possible explanation is that the reactant sulfide is strongly and rapidly adsorbed onto the catalytic sites, which is consistent with data repored by Pope et al. (1978). Based on this assumption, the reaction would be limited by the rate at which active sites become available. The availability of active sites would then be determined by the rate at which the sulfur compound can be oxidized or the rate at which product SO2 is desorbed. The temperature dependence of the zeroth-order rate constant is illustrated in Figure 4. The rate constant, KO, could be represented by an Arrhenius equation and was calculated to be 78.8 exp(-19117/RT) mol/(s.g of catalyst). The units of activation energy are calories/mole, and T is in kelvins. It is interesting to note that the activation energy of 19 kcal/mol is consistent with a value of approximately 20 kcal/mol which has been reported for hydrocarbon oxidation (Golodets, 1983; Gangwal et al., 1988;
Literature Cited Froment, G. F.; Bischoff, K. B. Chemical Reactor Analysis and Design; Wiley: New York, 1979. Gangwal, S. K.; Mullins, M. E.; Spivey, J. J.; Caffrey, P. R.; Tichenor, B. A. Kinetics and Selectivity of Deep Catalytic Oxidation of n-Hexane and Benzene. Appl. Catal. 1988,36, 231-247. Golodets, G. I. Heterogeneous Catalytic Reactions Involving Molecular Oxygen; Elsevier: New York, 1983. Halachev, T.; Ruckenstein, E. Poisoning and Promoting Effects of Additives on Catalytic Behavior of Metal Clusters. J. Catal. 1982, 73, 171-186. Hawkins, J. R.; Wanke, S. E. The Oxidation of Ethylene Over a Supported Platinum Catalyst. Can. J. Chem. Eng. 1979, 57, 621-626. Kummer, J. T. Laboratory Experiments Evaluating the Effects of S and Cu on a Pt-A1203Auto Exhaust Catalyst. J. Catal. 1975, 38, 166-171. Mars, P.; van Krevelen, D. W. Oxidations Carried Out by Means of Vanadium Oxide Catalysts. Chem. Eng. Sci. (Spec. Suppl.) 1954, 3, 41-59. Pope, D.; Walker, D. S.; Moss, R. L. Evaluation of Platinum-Honeycomb Catalysts for the Destructive Oxidation of Low Concentrations of Odorous Compounds in Air. Atmos. Environ. 1978, 10,192-1927. Rossin,J. A. Effects of Pretreatment Conditions on the Activity and Poisoning of a 1% Pt/AlZO9Catalyst. J. Mol. Catal. 1989, in press. Smith, J. M. Chemical Engineering Kinetics; McGraw-Hill: New York, 1981. Spivey, J. J. Complete Catalytic Oxidation of Volatile Organics. Ind. Eng. Chem. Res. 1987,26, 2165-2180. Summers, J. C.; Baron, K. The Effects of SOz on the Performance of Noble Metal Catalysts in Automobile Exhaust. J.Catal. 1979, 57, 380-389.
Joseph A. Rossin Air Purification Branch US.Army CRDEC Aberdeen Proving Ground, Maryland 21010 Received for review February 13, 1989 Accepted July 3, 1989
Vapor Pressures of the Water-Lithium Bromide-Zinc Bromide-Lithium Chloride System at Low Temperatures The vapor pressures (80-2494 Pa) of the water-lithium bromidezinc bromide-lithium chloride system (salt weight ratios 1.0:1.8:0.26) were measured in the range of temperatures from 279.55 to 348.15 K and in the range of absorbent concentrations from 51.1 to 77.8 wt %. The vapor pressure measurements were made with a static method and a special apparatus devised for low-pressure measurements. The experimental data for lithium bromidezinc bromide-lithium chloride aqueous solutions were correlated by means of a n Antoine-type equation. Maximum and average absolute deviations between the experimental data and the calculated values from this equation were 2.45% and 0.95%, respectively. The water-lithium bromide-zinc bromide-lithium chloride system was proposed in order to improve the performance characteristics and to reduce the corrosion 0888-5885/89/2628-1564$01.50/0
caused by the water-lithium bromide system (Uemura and Hasaba, 1964). The optimum mixing ratio of lithium bromide, zinc bromide, and lithium chloride was deter0 1989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 10, 1989 1565 mined by measuring the crystallization temperature and the corrosion caused by sample solutions. Consequently, the most suitable mixing ratio of LiBr-ZnBr2-LiC1 was found to be 1.0:1.8:0.26 by weight, respectively. The physical properties (density, viscosity, solubility,and vapor pressure) and thermal properties (heat capacity and heat of mixing) for the working medium-absorbent systems are necessary for the research and design of absorption refrigerating machines, absorption heat pumps, and absorption heat transformers. Namely, the purpose of this paper is the application of refrigeration. Accurate vapor pressure data for the working medium-absorbent system at low temperatures are particularly important for the design of the absorber. The physical and thermal properties of this four-component system using water as the working medium and lithium bromide-zinc bromidelithium chloride as the absorbent were measured at various temperatures and absorbent concentrations by Takigawa (1988). The vapor pressures (2060-38300 Pa) of this four-component system were measured in the range of temperatures from 302.95 to 392.45 K and in the range of absorbent concentrations from 5.2 to 77.7 w t %. The vapor pressures of the working fluids using water as the working medium for absorption refrigerating machines and absorption heat pumps have been measured by many investigators (for example, Uemura and Hasaba (1964), Iyoki et al. (1981), Pennington (1955), Bach and Boardman (1967), Boryta et al. (1975), and Matsuda et al. (1980)). However, the vapor pressures of this four-component system at low temperatures have not been measured. Therefore, the vapor pressures (80-2494 Pa) of this fourcomponent system at low temperaturs were measured by a static method in the range of temperatures from 279.55 to 348.15 K and absorbent concentrations from 51.1 to 77.8 wt %.
Experimental Section Experimental Apparatus and Procedure. The vapor pressures of the water-lithium bromide-zinc bromidelithium chloride system at low temperatures were measured with an apparatus similar to that devised by Matsuda et al. (1980) for low-pressure measurements. The schematic diagram of the experimental apparatus used for vapor pressure measurements of this four-component system at low temperatures is shown in Figure 1. I t consists primarily of a sample vessel with a volume of about 0.5 L, a buffer tank with a volume of about 5 L, a constant-temperature bath, two traps, two vacuum pumps, and a U-tube manometer. The volume of the buffer tank was determined as follows (Matsuda et al., 1980): V = uO/bPmi, = 6.7 X 1800/0.03 X 67 = 6000 cm3 (1) where V is the volume of the apparatus (cm3), u is the volumetric air flow rate leaked on the vacuum side in the apparatus (6.7 cm3.Pa/s), 0 is the time of the measurement (1800 s), b is the allowable pressure elevation (3%), and P- is the minimum pressure of the measurement (67 Pa). Therefore, the volume (5 L) of the buffer tank was determined by considering the volume of the sample vessel and the connecting glass tube. The equilibrium still was made of Pyrex glass. The equilibrium still consists of four glass sections: a sample vessel, a buffer tank, a T-shaped piece, and a U-tube manometer. The four glass pieces were connected by ground-glass joints sealed with Dow Corning high-vacuum grease. The constant-temperature bath was maintained to within fO.O1 K. The temperature inside the sample vessel was measured with a standard thermometer. The measurements of pressure were carried out with a U-tube manometer. The manometer liquids were
saH Figure 1. Experimental apparatus for the measurements of vapor pressure: A, buffer tank; B, manometer; C, magnetic stirrer; D, coolant; E, needle valve; F, trap; G , cold trap; H, vacuum pump; I, standard thermometer; J, sample vessel; K, constant-temperature bath.
mercury and isobutyl phthalate with a density of 1034 kg/m3 at 313.15 K. Pressures below 666 Pa were read with a isobutyl phthalate U-tube manometer, and readings above 666 Pa were made with a mercury U-tube manometer. The heights of the mercury and isobutyl phthalate meniscuses in the manometer limbs were measured by means of a cathetometer fitted with a vernier scale, capable of readings to 0.01 mm. Experiments were made using aqueous salt solutions where the ratio of LiBr-ZnBr,-LiC1 was 1.0:1.8:0.26 by weight. A sample solution at various absorbent concentrations was placed in the sample vessel, and the extraneous gases in the apparatus were then removed with a vacuum pump. The sample solution was stirred well with a magnetic stirrer to prevent superheating. After thermal equilibrium was reached, the temperature of the sample solution and the pressure in the apparatus were measured. The pressures and temperatures were measured three times at 5-min intervals for each data point to verify the reproducibility of the apparatus. Absorbent concentration of the sample solution was analyzed after the vapor pressure measurements. The vapor pressure measurements at various temperatures were also made on pure water and n-butyl alcohol to examine the accuracy of the experimental apparatus and procedure. The results agreed well with literature values (Steam Tables, 1980; Timmermans, 1965) with an average absolute deviation of less than 0.60%. Materials. The lithium bromide, zinc bromide, and lithium chloride used in this work were from the Honjo Chemical Co., Ltd. (Japan), analytical reagent grade. The analytical results of the lithium bromide aqueous solution were as follows: concentration, 55.0 wt %; Ca, 0.002 wt %; SO4, 0.005 wt %; Na, 0.0002 wt 9'0; pH, 7.2. The analytical resulb of the zinc bromide aqueous solution were as follows: concentration, 55.0 w t %; Pb, 0.0005 w t %; Ca, 0.0004 w t %; Na, 0.0004 wt %; pH, 1.8. The analytical results of the lithium chloride aqueous solution were as follows: concentration, 40.4 wt %; Na, 0.0016 wt %; Ca, 0.0032 wt %; SO4, 0.007 wt %; pH, 7.0. The n-butyl alcohol used in this work was Wako Pure Chemical Industries Ltd. (Japan) analytical reagent with a minimum
1566 Ind. Eng. Chem. Res., Vol. 28, No. 10, 1989 Table I. Vapor Pressures of the HzO-LiBr-ZnBq-LiC1 System T,K peXnr Pa P ~ ~Pa I, c, % X = 51.1 wt % 294.95 1528 1539 -0.72 298.55 1925 1921 0.21 303.05 2494 2512 4.72
X
335 424 549 1153 1622 2115
286.05 289.55 292.65
337 435 537
299.35 303.45 307.75
802 1038 1344
307.75 313.05 317.65
741 1021 1325
X
X
295.05 297.85 301.15
335 424 546 1155 1629 2113
(K) 29y5
2t3.15,
= 64.8 wt %
344 439 541
-2.08 -0.92 -0.74
3.2
I
I
I
I
3.4
3.6
3.8
4.0
1 4.2
= 65.0 wt %
818 1051 1363
-2.00 -1.25 -1.41
= 69.1 wt %
739 1016 1325
X = 69.2 wt % 325 324 392 391 495 484 131 192 269 1204 1561 2055
X = 77.8 wt 308.05 313.05 317.85 337.95 343.85 348.15
31y5
0 0 0.55 -0.17 -0.43 0.09
0.27 0.49 0
0.31 0.26 2.22
x = 73.9 wt % 298.05 303.35 308.05 332.85 337.65 343.05
333.15
= 60.3 wt %
279.55 282.85 286.45 297.85 303.45 307.85
X
Temperature 43r15
80 115 163 584 824 1046
133 193 264 1177 1527 2027
-1.53 -0.52 1.86 2.24 2.18 1.36
Table 11. Values of A, and B, in Equation 2 n An B, 0 1.51003 X 10’ -3.54234 X 1 -9.517 45 2.314 75 2 2.396 19 X lo-’ -5.93604 X 3 -2.66548 X 10” 6.74740 X 4 1.10471 X -2.87614 X
10 lo-’ lo4
IO4
Antoine-type equation which expresses vapor pressure as a function of temperature and absorbent concentration: 4
log p =
C { A , + [1000B,/(T- 43.15)I)X”
(2)
ne0
%
79 113 159 579 819 1046
1000/( T-43.15 )
Figure 2. Vapor pressures of the H20-LiBr-ZnBrz-LiC1 system: (-) calculated vapor pressures, (0-V) experimental d a t a (0) 51.1 Wt %, (A)60.3 wt %, (0) 64.8 wt %, (V)65.0 wt %, ( 0 )69.1 wt %, (A)69.2 wt %, (m) 73.9 wt %, and (V)77.8 wt %.
1.25 1.74 2.45 0.86 0.61 0
purity of 99.8 w t YO. The major impurity of n-butyl alcohol was water. All the reagents were used without further purification. The absorbent concentrations of the lithium bromidezinc bromide-lithium chloride aqueous solutions were determined by Fajans’ method (Takagi, 1976) using dichlorofluorescein as an adsorption indicator and by a chelate method (Ueno, 1976) using eriochrome black T as an indicator.
Results and Discussion The experimental results of 33 measurements for this four-component system at various temperatures and absorbent concentrations are shown in Table I along with vapor pressure values calculated by eq 2 derived below. These experimental results were used to determine the constants for a empirical formula with a least-squares method. The experimental results were plotted in the form of log p vs 1000/(T - 43.15) in Figure 2 for eight of the lithium bromide-zinc bromide-lithium chloride aqueous solutions. In this figure, the solid lines indicate the calculated values from eq 2. The log p vs 1000/(T - 43.15) relationship for the absorbent concentration over the temperature and pressure ranges measured was linear. The experimental data were correlated by means of the
where p is the vapor pressure in Pa, T is the absolute temperature in K, and X is absorbent concentration in w t % of aqueous solution. Values of the constants A , and B, in eq 2 are shown in Table 11. The percent deviation at a given temperature and absorbent concentration is defined as lOO(P,, - P4)/Pe, or t (Table I). The percent average absolute deviation is defined as 100[CN(IPeXp Pdl/Pew)/iVJ.Maximum and average absolute deviations between the experimental data and the calculated values from eq 2 were 2.45% and 0.95%, respectively. The calculated vapor pressures of this four-component system obtained in this work were compared with extrapolated values from the literature (Takigawa, 1988) measured at temperatures between 302.95 and 392.45 K. The results are shown in Figure 3. In this figure, the vapor pressure is plotted as ordinate and the absorbent concentration as abscissa. The solid and broken lines in Figure 3 indicate the calculated vapor pressures from eq 2 and the extrapolated vapor pressures from the literature (Takigawa, 1988), respectively. The calculated vapor pressures obtained in this work are somewhat higher, especially at lower temperatures. Therefore, without using the extrapolated values from the experimental data in the range of temperatures from 302.95 to 392.45 K, it is necessary to measure the vapor pressure at low temperatures directly.
Conclusions In the present paper, the vapor pressures of the fourcomponent system using water as the working medium and lithium bromide-zinc bromide-lithium chloride as the
I n d . Eng. Chem. Res. 1989,28, 1567-1570
1567
n = integer exponent in eq 2 t = deviation, %
N = number of experimental data points Subscripts exp = experimental data cal = calculated value from eq 2
min = minimum pressure of measurement Registry No. LiBr, 7550-35-8; LiC1, 7447-41-8;ZnBr2,769945-8.
Literature Cited
55
60
65
70
75
Absorbent concentration (wt Yo)
Bach, R. 0.;Boardman, W. W. Vapor Pressure of Aqueous Lithium Iodide Solutions. ASHRAE J . 1967,11,33-36. Boryta, D. A.; Maas, A. J.; Grant, C. B. Vapor Pressure-Temperature-Concentration Relationship for System Lithium Bromide and Water (40-7070 Lithium Bromide). J . Chem. Eng. Data 1975,20,316-319. Ivoki. S.: Hanafusa. Y.: Koshivama. H.: Uemura. T. Studies on the Water-Lithium Bromide-lithi& Thiocyanate Absorption Refrigerating Machine. Reito 1981,56,661-671. Matsuda, A,; Munakata, T.; Yoshimaru, T.; Kubara, T.; Fuchi, H. Measurement of Vapor Pressures of Lithium Bromide-Water Solutions. Kagaku Kogaku Ronbunshu 1980,6,119-122. Pennington, W. How to Find Accurate Vapor Pressures of LiBr Water Solutions. Refrig. Eng. 1955,63,57-61. Steam Tables;The Japan Society of Mechanical Engineers: Tokyo, 1980. Takagi, S. Teiryo Bunseki no Jikken to Keisan; Kyoritau Shuppan: Tokyo, 1976. Takigawa, T. Master's Thesis, Kansai University, Osaka, Japan, 1988. Timmermans, J. Physico-Chemical Constants of Pure Organic Compounds; Elsevier: Amsterdam, 1965. Uemura, T.; Hasaba, S. Studies on the Lithium Bromide-Water Absorption Refrigerating Machine. Technol. Rep. Kansai Univ. 1964,6,31-55. Ueno, K. Kireito Tekiteihou; Nankodo: Tokyo, 1976. I
Figure 3. Comparison of vapor pressures for HzO-LiBr-ZnBrz-LiC1 system: (-) calculated values from eq 2,(- - -) extrapolated values from the literature (Takigawa, 1988).
absorbent were measured at low temperatures. Experimental data were correlated by means of an Antoine-type equation. The calculated values from this equation were in good agreement with the experimental data. Maximum and average absolute deviations between the experimental data and the calculated values from this equation were 2.45% and 0.95%,respectively. The vapor pressure data for this four-component system at low temperatures are very useful for the design of the absorber of absorption refrigerating machines, absorption heat pumps, and absorption heat transformers. Nomenclature V = volume of the apparatus, cm3 X = absorbent concentration, w t % T = absolute temperature, K p = vapor pressure, Pa A,, B, = constants in eq 2 u = volumetric air flow rate leaked on the vacuum side in the apparatus, cm3-Pa/s 0 = time of measurement, s b = allowable pressure elevation, %
.
I
Shigeki Iyoki,* Shozo Iwasaki, Tadashi Uemura Department of Chemical Engineering Faculty of Engineering Kansai University Yamate-cho, Suita, Osaka 564,Japan Received for review December 20, 1988 Revised manuscript received June 28, 1989 Accepted July 18,1989
Laboratory Evaluation of Clays in the Treatment of Benzene-Toluene-Xylene Feedstocks Clays are used to treat BTX (benzene-bluene-xylene) feedstocks in refineries. During clay treatment, the olefin content of BTX as measured by bromine index is reduced. A BTX aging test is described which can differentiate the olefin removal performance of clays under accelerated aging conditions. The purpose of this paper is to describe a laboratory test for the comparative evaluation of clays in BTX clay treatment under accelerated aging conditions. Test equipment, procedures, and measures of clay performance will be described. Data will be presented to demonstrate test reproducibility, the effect of process variables, and the ability of the test to differentiate olefin removal performance of clays in the treatment of BTX feedstocks. BTX is an acronym for benzene-toluene-xylene. Petroleum-sourced BTX is produced by catalytic reforming and pyrolysis. The BTX produced is used as an octane booster in gasoline and as a feedstock for the production of plastics and fibers (Ransley, 1978). Clay treatment of BTX is practiced when it is desired
to clean up a BTX stream. During clean-up, the olefin content of BTX is reduced. Previous Work Olefin Removal Mechanisms. Reidel (1954)stated that BTX impurities polymerize during clay treatment and are subsequently removed by downstream distillation. Kawakami et al. (1972)concluded that primarily heptene dimers were formed by polymerization when heptene-2-spiked toluene was passed in the liquid phase over a clay at 200 "C. These investigators published a gas chromatograph (GC) trace with four unknown "heavy" peaks. Their unpublished mass spectroscopy data led them to conclude that some of these peaks corresponded
0888-5885/89/2628-1567$01.50/0 0 1989 American Chemical Society
