Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979 WT-2575 W - I 5 0 SYSTEM
of a given polymer may not be a strong function of flocculation rpm and/or time for the ranges examined.
I
u sa c
e D:
FLOC TIME (mi")
2 40w
d
;e
20
0
I
IO
STOCK C O N C l i w m l
?A 78
IS IS
10 0
333 310
0 RUN 0 RUN
7C 7D
IS
363
40
IO IO
RUN 7E
IS
23
1
5
FLOC RPM
A RUN e RUN A
61
,
IS 20 25 POLYMER CONCENTRATION lwpPrnl
343 310
3s
30
I
40
Figure 5. Effect of flocculation time and stirrer speed on optimum dosage of WT-2575 cationic polyelectrolyte for W-150 emulsion system.
lustrate a lack of clear choice among the six polymers for the oil/water emulsion studies. However, although it is not possible to single out any one polymer as being clearly superior to the rest, WT-2575 showed the largest turbidity reduction and WT-3000 had the lowest optimum dose. Some additional experiments were performed with WT-2575 which yielded the highest turbidity reduction (see Table 11). A run was repeated for a flocculation rpm of 25, and another run using a flocculation time of 40 min with all other parameters remaining unchanged. These results, as shown in Figure 5, are not significantly different from the others, indicating that flocculating effectiveness
Conclusions Various polyelectrolytes including cationic, anionic, and nonionic polymers were tested to determine their flocculating effectiveness for an oil-water emulsion. Based upon laboratory jar test results, the following conclusions were drawn. 1. Several of the polymers tested (6 out of 12) were capable of reducing turbidity by about 20-30% a t their respective optimum dosages. These polymers were WT-2575, WT-2635, WT-2640, WT-2690, and WT-3000. 2. Optimum dosages for these six polymers ranged from about 6 to 30 wppm with the exception of WT-3000, which had an optimum dosage value of about 0.6 wppm. Acknowledgment The authors express their gratitude to the sponsoring companies of the University of Tulsa Environmental Protection Projects program for partial financial support. They also wish to thank Sun Oil Company, Amoco Production Company, Calgon Corporation, and Dearborn Chemical Company for donating various chemicals. Literature Cited "Standard Methods for the Examination of Water and Waste Water", 13th ed, American Public Health Association, 197 1.
Received f o r review March 27, 1978 Accepted September 22, 1978
Intrinsic Kinetics of Hydrogenation of Benzene on Nickel Catalysts Supported on Kieselguhr John K. Marangotis,' Basil G. Manttouranls, and Anastasios
N. Sophos
Laboratory of Chemical Process Engineering, Department of Chemical Engineering, National Technical University, Athens, Greece
Intrinsic rates of benzene hydrogenation have been measured on a commercial NVkieselguhr catalyst. The rates have been correlated according to the power law, r B = k ~ p , s o 3 2 ~ ~and B 0 2also 1 , according to the model r B = [ k K ~ . p ~ /+ ( 1K B - P ~[PH,/(~H, )] pB)], which postulates reaction between hydrogen gas and adsorbed benzene molecules. The apparatus was an isothermal, tubular, differential reactor. Temperature was varied from 67 to 90 'C, hydrogen partial pressure between 1000 and 3000 torr, and benzene partial pressure from 70 to 286 torr. Specific reaction rates k and adsorption parameters KB have been correlated by the Arrhenius law.
+
Benzene hydrogenation to cyclohexane in the gas phase is an industrially important reaction which has been extensively investigated in the laboratory as a model reaction for studies in catalysis. One of the industrially interesting catalysts is nickel supported on kieselguhr in various amounts. Investigations on the rate of hydrogenation of benzene on this important catalyst are reviewed here with the aim to correlate the data with a single reaction model in order to establish the intrinsic reaction kinetics. New data obtained by the present authors extend the range of total gas pressure, shedding new light on the effect of process parameters.
Experimental Section Rates of hydrogenation of benzene in the vapor phase were measured in a tubular flow, differential reactor under 0019-7890/79/ 1218-0061$01.00/0
isothermal conditions. The catalyst used was 0.87 g of dried crushed particles, 0.20-0.25 mm in size, of commercial catalyst 42-1 donated by I.C.I., Ltd. According to the manufacturer, it contains 67% nickel on kieselguhr with a bulk density of 1.2 kg/L; it has a true density of 3.37 kg/L. The catalyst bed was diluted with 19.2 g of glass beads of the same particle size to a bed length of 35 mm in a bed diameter of 21.5 mm. Benzene was Merck-Rein grade; hydrogen was Air Liquide electrolytic grade. No effort was made to purify materials further, since a parallel objective was to study deactivation and regeneration of the catalyst. The reactor effluent was cooled to -78 "C and the condensate sample was weighed and analyzed refractometrically and chromatographically. Qnly benzene and cyclohexane were detected in the samples. The temperature in the bed was CC: 1979 American Chemical Society
62
Ind. Eng. Chem. Prod. Res. Dev., Vol. 18, No. 1, 1979
72
26 1
i
I
OW 0
50
100
150
200
250 ?
150
2CC
Torr Benzene
The present data when correlated with the power law give fractional values of a which increase with temperature and of b which decrease, respectively. This indicates that the power law may be a simplification of the actual kinetic model. The form that fits the present data best has a = 0.32 and b = 0.21. Other investigators (Kehoe and Butt, 1972; Thomson and Webb, 1968) have correlated their data with an Eley-Rideal type of mechanism of single-site adsorption (eq 2). This model postulates that benzene is adsorbed on catalyst active centers and reacts with hydrogen molecules in the gas phase with a rate ~KB*PB*PH~ 1 + KB.PB
100
2M
300
pB Torr Beizene
measured with five fine TC’s and it was maintained constant within h0.5 “C with appropriate heat exchange. Due to the small size of the catalyst and the flowrates used, external mass transfer resistance was negligible and the catalyst effectiveness factor was close to unity; thus the rates measured represent intrinsic kinetics. Activity of the catalyst decreased rather fast, possibly due to poisoning by a trace of thiophene in benzene. Regeneration of the catalyst to a constant “standard” activity was achieved by heating the bed to 260 “C in a stream of Hz at 3 atm pressure for 3 h. Activity at “standard” conditions was defined as the rate of hydrogenation of benzene at 91.5 f 0.5 “C, partial pressure of benzene 80 torr, and partial pressure of hydrogen 1520 torr. Catalyst activity was never allowed to fall below 80% of “standard”. The rates measured were corrected to a “standard” activity of 100%. The maximum experimental error of rates, reported in Table I, is estimated to be *8%; average error is h5%. Table I shows the range of variables studied. Further experimental details are given by Sophos (1976). Reaction Kinetics Previous investigators (Bond, 1962; Nicolai et al., 1948; Emmett, 1960) correlated their data on hydrogenation of benzene (far away from chemical equilibrium conditions) by the power-law rate formula (eq l),with various values of the exponents a and b
=
50
0
300
Figure 1. Plot of experimental data according to eq 2; unsatisfactory correlation.
rg
-1
(2)
Such a model suggests a zero-order reaction with respect to benzene at high values of p B , an indication which perhaps distinguishes this model from the dual-site adsorption model of Langmuir-Hinshelwood.
Figure 2. Plot of experimental data according to eq 3; satisfactory correlation.
Figure 3. Comparison of experimental rates of benzene hydrogenation with calculated values according to eq 3.
The present data, when correlated according to eq 2 as shown in Figure 1, indicate a zero-order reaction with respect to benzene at high pB values but also demonstrate a dependence on the total pressure of the system (pHT+ pB)which was varied in our experiments. This surprising effect was not noticed by Kehoe and Butt (1972) because their experiments were conducted under a single constant total pressure. This observation led us to a modification of the rate law to include the total pressure. Thus, it is postulated here that adsorbed benzene reacts with hydrogen in the gas phase in proportion to the hydrogen mole fraction rather than in proportion to hydrogen partial pressure rg
=
(3)
Equation 3 introduces the total pressure in the reaction model and accounts for the fractional values of a and b in the power law. Correlation of rates in Table I with eq 3 shown in Figure 2 resulted in the determination of k and KB k = 4.36 X lo5 exp(-12000/RT) mol of B/(g)(h) (4) KB = 1.05 X 10’ exp(-6000/RT) torr-’
(5)
A comparison of experimental rates with calculated ones is shown in Figure 3. The average deviation between calculated and experimental results was h870 and the maximum was f 2 1 % . The negative value of 6000 for the activation energy of benzene adsorption indicates that
Ind. Eng. Chem. Prod.
Res. Dev., Vol. 18, No. 1, 1979 63
Table I. Experimental Data for Hydrogenation of Benzene on Ni/Kieselguhr (Catalyst I.C.I. 42-1 )
__
I _ _
I
_ I _ _ _ _
space velocity, 102(NB/w), benzene N H , mol of 1 0 2 N ~mol , mol of conversion, P B, t o m k / h o f B / h B/(g)(h) XB, % ___ _______ feed rates
bed temp, 'C ___ 67.6 f 0.5 67.6 i 0.5 67.6 i 0.5 17.5 f 0.5 77.5 f 0.5 77.5 f 0.5 90.5 t 1.5 90.5 i 1 . 5 90.5 f 1.5 67.6 f 0.5 67.6 i 0.5 77.5 i 0.5 77.5 t 0.5 90.5 c 1.5 90.5 f 1.5
.__
PH,, torr 1050 20 2105 f 20 2988 t 20 1054 + 20 2113 i 20 2988 t 20 1052 c 20 2115 c 20 2972 20 2110 f 20 2110 r 20 2110 f 20 2110 f 20 2110 f 20 2110 t 20
_____lll_____
70t 3 70c 3 70t 3 70t 3 70+ 3 70i 3 70i 3 70i 3 70i 3 185 i 3 286 f 3 181 i 3 280 t 3 187 f 3 280 f 3
+_
+_
1.345 1.330 1.415 1.461 1.342 1.822 1.643 2.464 2.488 1.162 1.148 1.591 1.366 1.906 1.640
10.34 4.98 3.37 10.72 4.87 4.67 11.82 8.67 6.95 10.38 15.63 13.83 16.65 17.40 22.41
11.89 5.72 3.87 12.32 5.60 5.37 13.59 9.97 7.99 11.93 17.97 15.90 19.14 20.00 25.76
-_
3.20 7.47 11.6 4.93 13.4 15.5 9.27 15.9 23.4 4.53 3.41 5.65 4.88 10.1 8.15
lo+,,
mol of B/(h)(g of cat.) 3.81 4.27 4.50 6.07 7.50 8.31 12.6 15.9 18.7 5.40 6.12 8.99 9.34 20.3 21.0
unable to deduce the values of K R and its correlation. The data of Nicolai et al. (1948) for hydrogenation of benzene to cyclohexane were also successfully treated according to eq 3 despite the fact that the authors did not specify the units of the rates measured. Their catalyst was nickel (unspecified amount) deposited on kieselguhr, prepared in the laboratory, and the correlation for k is
k = 1.41 X 10" exp(-lOOOO/RT) (units unknown) ( 7 ) It should be pointed out that the activation energy is roughly the same in all three investigations as depicted in Figure 4. Some relevant data on a Ni/Si02 catalyst have been reported by Aben et al. (1970). Unfortunately, the reported information is insufficient to enable us to make a comparison with the kieselguhr catalyst. Those authors worked at higher pressures of hydrogen (up to 28 atm) and reported that adsorption of hydrogen (not of benzene) on the catalyst surface was the most significant factor.
\. icc
a-
23r
2
-
3 .\3
,
~
3 ~ :
-
Figure 4. Arrheiiius plot of specific reaction rates, comparison of all d a t a on Ni/kieselguhr catalysts.
benzene is probably chernisorhed on the nickel surface, as suspected by Bond. Treatment of the data by Kehoe and Butt according to eq 3 resulted in estimating h
k = 5.75
X
105exp-l2400/RT) mol of B/(g)(h)
(6)
which gives a catalyst activity about 25% lower than catalyst I.C.I. 42-1, most likely hecause of the fact that catalyst Harshaw Ni-0304 I' which they iised contained less nickel (58% Ni). On the other hand. since the data of Kehoe and Butt were ohtained mostly in the zero-order ben7ene kinetics range (high values of KR.pR)we were
Nomenclature a, b = exponents k = specific reaction rate, mol of benzene/(g of cat.)(h) KR = adsorption equilibrium coefficient, torr-' N H or ~ N g = feed rates of H2or benzene, mol/h p H 2 = hydrogen partial pressure, torr p B = benzene partial pressure, torr rg = rate of benzene hydrogenation, mol of B/(g)(h) u = weight of catalyst, g x B = henzene conversion, % L i t e r a t u r e Cited Aben, P C , Platteeuw. J C , Stouthamer. B Recl Trav Chim Pavs-Bas ,~ 89, 449 (1970). Bond, G. C.. "Catalysis by Metals", p 315, Academic Press, London. 1962. Emmett, P.. Ed., "Catalysis", Vol. 7, pp 183-191, Reinhold, New York, N.Y., 1960. Kehoe, J. P. G.,Butt, J. B., J . Appl. Chem. Biotechnol., 22, 23 (1972). Nicolai, J., Martin, R., Jungers, J. C., Bull. SOC.Chim Belg., 57, 555 (1948). Sophos. A. N., Diploma Thesis. Chemical Engineering Department, National Technical University of Athens, 1976. Thomson, S. J., Webb, G., "Heterogeneous Catalysis", p 87, Oliver and Boyd, Edinburgh, 1968.
R p c c i c d f o r recieu April 17, 1978 Accepted September 22, 1978
