252
Ind. Eng. Chem. Process Des. Dev., Vol. 17,No.3, 1978
Kinetics of Hydrogenolysis of Thiophene in Naphtha P. Chakraborty and A. K. Kar' Department of Chemistry, lndian lnstitute of Technology, Kharagpur, lndia
The intrinsic kinetics of the hydrogenolysis of thiophene on a nickel oxide-molybdenum oxide-aluminum oxide catalyst were studied in a differential reactor at a total pressure of 1 atm and temperatures of 237-290 O C . Retardation of the reaction by hydrogen sulfide was significant and the rate of thiophene disappearance was correlated by a Hougen-Watson type of kinetic equation. The activation energy of the process was found to be more or less similar to that obtained from integral reactor data.
Introduction The hydrogenolysis of thiophene has received substantial attention in the past, in part because thiophene is the most refractory sulfur compound present in petroleum fractions. The general consensus of opinion of the previous workers (Griffith et al., 1949; Pease and Keighton, 1933; Satterfield and Roberts, 1967) is that the reaction proceeds on a number of catalysts as follows. CdHdS
+ -
+ 3H2
C ~ H S HZ
C4Hs
+ H2S
C4HlO
(1)
(2)
The opinion is that the first step of the overall reaction, reaction 1, proceeds through a butadiene intermediate. The second step, reaction 2, is not rapid compared with the first step so that the C4 product obtained consists of butane and a mixture of the n-butene isomers. A variety of catalysts have been employed for the reaction including oxides or sulfides of molybdenum (Griffth et al., 1949; Kolboe and Amberg, 1966). Despite the commercial importance of thiophene hydrogenolysis, studies of the kinetics of this reaction have been limited. Most of the work in the literature is on pure compounds or pure compounds dissolved in petroleum fractions. Gasoline is a complex mixture of hydrocarbons containing different types of sulfur compounds. Experiments with one or two pure hydrocarbons and sulfur compounds would not be very significant for the behavior of gasoline. The preliminary kinetic measurement of thiophene hydrogenolysis in shale gasoline (Hammer, 1951) over sulfurized Co-Mo catalyst at a pressure of 15 atm and a temperatures of 300-375 OC revealed that adsorbed hydrogen sulfide has a distinct influence on the reaction rate of thiophene, while the presence of hydrocarbon plays no role. These data are in good agreement with the rate equation of the form (3) assuming the surface reaction to be the rate-controlling step. In the kinetics of thiophene hydrogenolysis on a cobalt molybdate catalyst (Satterfield and Roberts, 1967) in the rate equation they have included the terms for the adsorption of thiophene and hydrogen sulfide but no terms were included for the adsorption of butane or hydrogen. Their contention is that butane has a negligible effect on the reaction rate and variation of the hydrogen partial pressure is too small. Hydrogen sulfide exerts an inhibiting effect on the reaction rate. Though the inhibiting effect of thiophene was less obvious, they had to include a term for thiophene adsorption to obtain satisfactory correlation of data. The best kinetic equation for thiophene disappearance was found to be 0019-7882/78/1117-0252$01.00/0
There has been no kinetic study on the kinetics of thiophene hydrogenolysis over nickel oxide-molybdenum oxide-aluminum oxide catalysts.
Experimental Section Apparatus and Arrangements. The reactor design and experimental arrangements for kinetic measurements were similar to that described in the previous work (Chakraborty and Kar, 1976). The apparatus is comprised of three main units: the feeding system, the reactor, and the product recovery unit. The purified gas was metered by means of a flow meter and naphtha containing thiophene was passed into the vaporizer by the mercury displacement method. The reactor (2 cm in internal diameter and 20 cm long) consisted of a preheater zone filled with broken glass and the catalyst zone. The temperature was measured by a thermocuple and kept constant by means of a variable transformer. The effluent from the reactor was passed successively through an ice-cooled spiral condenser and a series of bubblers containing caustic soda solution to remove liquid product and hydrogen sulfide, respectively. Preparation of the Catalyst. The catalyst NiO-Moo3calcined A1203 from BDH in the ratio of 15:35:60 by weight was prepared by adding a slightly ammoniacal solution of a requisite quantity of ammonium permolybdate to an aqueous solution of nickel nitrate containing A1203 with stirring. The mixture was adjusted to pH 5.5, evaporated to dryness on a water bath, dried at 110 "C, and finally calcined at 500 "C for 6 h in the presence of air. The average diameter of the catalyst particles was 0.026 cm, and 0.8 g of the catalyst was used in the reactor. The surface area, pore volume, and average pore size of the fresh catalyst was found to be 8.0 m2/g, 0.4162 cm3/g, and >500 A, respectively. The corresponding values for the sulfided catalyst were found to be 50.5, m2/g, 0.6921 cm3/g, and >500 A, respectively. Identification and Analysis of Products. The method of analysis and identification of products have been stated elsewhere (Chakraborty and Kar, 1976). Procedure. The differential reactor was operated with low conversion of 5 to 10%. It was observed that with particles 0.026 cm in diameter and a t a flow rate of 0.90 m o l h , both internal and bulk diffusion will be negligible. From integral reactor data (Chakraborty and Kar, to be published), calculation from the ratio of experimental rate constant and theoretical rate constant (Sherwood and Reid, 1958) indicated that more than 95% of the internal surface is available for the reaction.
0 1978 American Chemical Society
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 3, 1978
t
-E I
'
.L 3s
-
9 1 .c
.
I
P
30-
Y
0.72-0,7L atmos
p,
P H , X~ l O ,' 0.2 - 0% at rnos
E
E, 0
253
25-
mX
I1
IO
2D
P XIO? 1
30
atm
ZO40 ---L
Figure 1. Effect of partial pressure of thiophene on rate: temp, 237 "C; P H = 0.72-0.74 atm; WIF = 0.09-0.94 g.hImo1. During the course of the study, standard runs were made to check the activity of the catalyst. The activity remained fairly constant during the course of the study. The catalyst was reduced with pure Hz at 275 "C for about 6 h and then sulfurized by passing HzS, a t the reaction temperature and 1atm pressure, through the reactor for 3 h at a rate of about 1 L/h. The catalyst was regenerated by oxidizing with air a t 450 "C, reducing with Hz, and sulfurizing with HZS. The naphtha fraction contains very little (0.06%) nonthiophenic sulfur which decomposed completely. The amount of thiophenic sulfur being 0.052% by weight, studies were made with added thiophene. Since the bromine number of the naphtha fraction is only 1.05,no attempt was made to find the rate of hydrogenation of olefin. The reaction rate was computed from the equation
In calculating the reaction rate r T , only the concentration of thiophene has been considered. While conversion is referred to as moles of thiophene reacted per hour to the moles of thiophene fed per hour, the rate of formation is referred to moles of thiophene hydrogenated per hour per gram of the catalyst.
Experimental Results The effect of partial pressure of thiophene, partial pressure of hydrogen, and space velocity on the rate of thiophene disappearance is plotted in Figures 1 , 2 , and 3 respectively. The results are seen to be consistent with expectations and literature reports (Kronig, 1950; Hoag, 1950). As is expected, at the same P T the degree of hydrodesulfurization increased with increase of temperature (Table I). Kinetic Model. In the kinetic study of thiophene hydrogenolysis in shale gasoline over Co-Mo catalyst, it was assumed (Hammar, 1951) that the rate-determining step is the thiophene hydrogenation to dihydrothiophene. The Hougen-Watson type rate equation with the adsorption term of hydrogen sulfide only in the denominator fitted their data. In the study of kinetics of thiophene hydrogenolysis on a Co-Mo catalyst it was found that the rate is retarded by hydrogen sulfide (Satterfield and Roberts, 1967). Though the inhibiting effect of thiophene was less obvious, they included a term for this effect in order to obtain satisfactory correlation of the data.
IS
-
IO
I
I
I
;'I 10 0
'
1.0
W/F,
grn. m o l d ' grn hr
----c
Figure 3. Effect of space velocity on rate: temp, 237 "C; P H = 0.65-0.68 atm; W = 1.5 g. Correlating Equations. In the present investigation, hydrogen sulfide was found to have a retarding effect on the rate of hydrogenolysis of thiophene in naphtha (Figure 4). The hydrodesulfurization kinetics of a number of sulfur compounds (Obolentsev and Mash Kina, 1961),have been interpreted by means of Frost's equation (Frost, 1946), which is of the form (5) where (F/W) is space velocity, cy is a coefficient proportional to the rate constant, /3 is a constant of Frost's equation, including the adsorption coefficient of the substance involved,
254
Ind. Eng. Chern. Process Des. Dev., Vol. 17, No. 3, 1978
Table I. Effect of Temperature on the Rate of Thiophene DisauDearancea Temp, Partial pressure, "C atm. DT x 102
Exptl rate, mol/h.e
0.584 1.48 1.52 1.100 2.19 2.15 2.030 2.93 2.88 3.090 3.68 3.69 3.760 3.89 3.93 260 0.584 1.65 1.68 1.100 2.43 2.40 2.030 3.55 3.31 3.090 4.12 4.52 3.760 4.47 4.56 290 0.584 1.90 1.95 1.100 2.76 2.70 2.030 3.76 3.73 3.090 4.66 4.58 3.760 5.00 5.12 PH = 0.73-0.74 atm; W/F = 0.90-0.94 g-h/g-mol.
0
1. Fromeq6 nT = n H = 1; n D = 2 2. Fromeq7 n T = n H = 1; n D = 2 3. Fromeq7 nT = n H = 1; n D = 1
-1.968
-2.105
2.746
-2.311
2.974
-3.75
2.128
was observed that eq 6 with nT, n H , and n D equal to 1,1,and 2, respectively, and eq 7 with nT, nH, and nD equal to 1,1,1, and 1, 1, 2, respectively, fitted the experimental data satisfactorily. The maximum deviation of the calculated and experimental rate was found to be least for the equation
1
3
Heats of adsorption, Activation kcal/mol energy, E , ET EE kcal/mol
Calcd rate, mol/h.g (from ea 9)
237
5
Table 11. Activation Energy and Heats of Adsorption
5
p T x 102, a t m . -
Figure 4. Effect of hydrogen sulfide on rate: temp, 237 "C; p~ = 0.72-0.74 atm; WIF = 0.90-0.94 gh/g-mol. and y is the degree of conversion. In the hydrogenolysis of thiophene in naphtha over NiO-Moos catalyst in the integral reactor (Chakraborti and Kar, to be published), it was found that the value of (3 which characterizes the stability of the ratio of adsorption coefficient is equal to 1and is the same for all temperatures. This indicates stronger adsorption of the reaction products compared to initial compounds. Previous work suggests (Owens and Anberg, 1961) that butane and butene have a negligible effect on the reaction rate. The variation of hydrogen partial pressure from run to run in the present investigation was too small to allow a meaningful hydrogen adsorption coefficient to be included. Thermodynamic calculations show that reverse reaction is negligible. Thus, for reaction 1,the following rate equations
for a number of different combinations of nT, nH, and n D were tested. Each of these equations was linearized and the rate coefficients were evaluated by the method of least squares. It
The calculated rates of thiophene hydrogenolysis from eq 9 for varying partial pressures of thiophene and hydrogen and for varying space velocity at a temperature of 237 OC are given in Figure 1,2, and 3, respectively, along with the experimental curve. The calculated rates from eq 9 a t different temperatures are given in Table I. The values of apparent activation energy, E , apparent heat of chemisorption of thiophene, ET, and apparent heat of chemisorption of hydrogen sulfide, E,, for the three mechanism obtained from Arrhenius type of plots are given in Table 11. In the Hougen-Watson model, KT and K , are so-called adsorption equilibrium constants for thiophene and hydrogen, respectively. Therefore, ET and E , are the apparent heats of chemisorption of these two species. Both have negative values as required by theory. The apparent activation energy for the reaction rate was found to be nearly 3 kcal/mol. This shows that k' is a product of true rate constant and one or more equilibrium constants. The value compares well with the values of 3.7 kcal/mol (Satterfield and Roberts, 1967) and 3.3 kcal/mol (Watanabe et al., 1967). In the kinetic studies of thiophene hydrogenolysis in naphtha over NiO-Moos, the activation energy measured in an integral reactor (Chakraborty and Kar, to be published) was found to be 3.656 kcal/ mol. Conclusions The kinetics of hydrodesulfurization of thiophene has been studied over NiO-Mo03-Al203 catalyst over a rather narrow range of process conditions. Complex rate equations were found to be necessary to describe the kinetic behavior. It was observed that the product hydrogen sulfide is more strongly adsorbed than the starting compounds; this is also reflected in the rate equation. In the present studies the agreement between the calculated and experimental rate was resonably good in spite of the fact that thiophene hydrogenolysis was done in a complex mixture of hydrocarbons, naphtha, rather than a pure solvent. Nomenclature E = apparent activation energy E, = apparent heat of chemisorption of species i F = flow rate per hour k = reaction rate constant k, = adsorption constant of species i n D = power of denominator in Hougen-Watson rate equations n, = reaction order with respect to species i in numerator of Hougen-Watson rate equations p L = partial pressure of component i r = rate of disappearance in g-mol/(g of catalyst)(h)
Ind. Eng. Chem. Process Des. Dev., Vol. 17, No. 3, 1978 255
w = weight of catalyst, g x = mole fraction conversion
Subscripts H = hydrogen T = thiophene S = hydrogen sulfide
Literature Cited Chakraborty, P., Kar, A. K., lndian J. Techno/., 14, 340 (1976). Frost, A. V., Vestn. Mosk. Gos. Univ., No. 3-4, 111 (1946). Griffith, R. H., Marsh, J. D. F., Newling, W. S.,Proc. Roy. SOC.London, Ser. A, 194 (1949). Hoog, H., J. Inst. Pet., 36, 738 (1950).
Kolboe, S., Amberg, C. H., Can. J. Chem., 44, 2623 (1966). Kronig, W., "Die Katalytische Druckhydrierung Von Kohlen, Teeren and Mineralolen", Springer-Verlag. Berlin, 1950. Obolentsev, R. D., Mashkina, A. V., Gidrogenoliz Seraorg, Soedinenii Nefti, Gos Nauch-Tekhn. Izd. Neft. i Gorno-Toplivoi Lit., Moscow (1961). Owen. P. J., Amberg, C. H., Adv. Chem. Ser., No. 33, 182 (1961). Pease, R. N., Keighton, W. B., Jr., hd. Eng. Chem., 25, 1012 (1933). Satterfield, C. N., Roberts, G. W., AIChEJ., 14, 159 (1967). Sherwood. T. K., Reid, R. C., "The Properties of Gases and Liquids", p 264, McGraw-Hill, New York, N.Y., 1966. Watanabe. T., Echigoya, E., Morikawa, K., Sekiyn Gakkai Shi, 10(12), 882 (1967).
Received for review December 30,1976 Accepted January 9,1978
Process Studies on the Conversion of Methanol to Gasoline Clarence D. Chang,' James C. W. Kuo, William H. Lang, Solomon M. Jacob, John J. Wise, and Anthony J. Silvestri Mobil Research and Development Corporation, Princeton and Paulsboro, New Jersey 08540
Process studies on the conversion of methanol to high quality gasoline are described. Exploratory process variable studies were carried out in single-pass fixed bed reactors. Process design and catalyst aging studies were conducted using a two-stage fixed bed reactor system with light gas recycle to the second reactor. The thermochemistry of the reaction is discussed.
Introduction The production of gasoline from coal and other nonpetroleum carbon sources will likely become a necessity in the United States before the turn of the century. Until now, only two coal conversion processes have attained any measure of commercial significance. These are the Bergius and FischerTropsch processes. In the Bergius process (Kirk-Othmer, 1972), finely divided coal is slurried with recycle oil containing a small amount of iron catalyst and hydrogenated at 900 O F and 3000-10 000 psi. The product is a synthetic crude. This process was practiced extensively in Germany during World War 11, achieving a peak annual production of four million tons of oil by 1944. However, no Bergius plants survive today. The Fischer-Tropsch process (Kirk-Othmer, 1972) is an indirect method for hydrocarbon manufacture from coal, involving coal gasification and the subsequent catalytic conversion of synthesis gas to hydrocarbons at moderate pressure and temperature. Iron-based catalysts are used. The Fischer-Tropsch process is commercially practiced a t Sasolburg, South Africa. The major shortcoming of the Bergius and Fischer-Tropsch processes is poor gasoline selectivity and quality. Products encompass a wide spectrum of molecular weights ranging from methane to heavy residue in the Bergius process and from methane to waxes and large amount of hydrocarbon oxygenates in the Fischer-Tropsch process. Downstream processing entails elaborate separation steps. Naphthas from these processes are generally low in octane and need extensive upgrading for automotive use. Mobil Research is developing a new process for converting coal or natural gas to high quality gasoline with high selectivity (Meisel et al., 1976; Chang and Silvestri, 1977). As shown in Figure 1, coal or natural gas can be converted to synthesis gas. After purification, the synthesis gas can be converted to methanol. By using the Mobil process, crude methanol can 0019-7882/78/1117-0255$01.00/0
then be converted to gasoline and water with small amounts of LPG and high Btu fuel gas as by-products. It is not necessary to remove the water or the small amounts of oxygenates present in the crude methanol. All the upstream steps from coal or natural gas to crude methanol are established technologies (Kirk-Othmer, 1972) while the last step is a simple catalytic step. Methanol can also be directly blended into gasoline or used by itself as an automotive fuel (Reed and Lerner, 1973). However, methanol's affinity for water, its corrosiveness, toxicity, low volumetric energy content, and unusual volatility, present formidable obstacles to its use either alone or as a component of gasoline (Lindquist and Ingamells, 1974). Large investments would be required to modify engines, storage and distribution facilities, and to develop new fuel and lube additive formulations (Kant, 1974). We calculate that it is more economical to convert methanol into gasoline than to use it alone or blend it with conventional gasoline. Methanol synthesis is known to proceed with high selectivity (Danner, 1970). As shown later, high selectivities also typify the newly discovered methanol-to-gasoline conversion represented by the reaction The first reported observation of hydrocarbon formation from methanol may be credited to Mattox (1962), who found minor amounts of c 2 - C ~olefin during methanol dehydration to dimethyl ether over NaX zeolite. Similar results have been reported later by various investigators using various catalysts (Heiba and Landis, 1964; Venuto and Landis, 1968; Topchieva et al., 1972; Swabb and Gates, 1972).Higher yields of hydrocarbons, including substituted aromatics, were observed by Pearson (1974) from methanol dehydration over P205 a t elevated temperature. No processes have been previously reported for the selective direct synthesis of high octane gasoline from methanol.
0 1978 American Chemical Society
