X, =
mole fraction of hydrogen at reactor inlet 0 = reciprocal of gaseous volume of reactants entering reactor per volume of catalyst per hour, assuming perfect gas behavior at reactor conditions.
Reaction rate constants obtained using this mixed thermal naphtha may be summarized thus :
Hydrofining Therma Ily Cracked Naphthas
C~VJKGATED
DIOLEFINS and sulfur compounds generally impair stability and other performance characteristics of cracked naphthas. They appear in small concentrations, and are difficult to remove without also removing desirable substances such as high octane monoolefins. This problem has caused the petroleum industry to expend considerable research in applying mild hydrogenation for their removal with minimum saturation of olefins. Hydrogenation of a typical thermal naphtha derived from thermally reforming a West Texas virgin naphtha and thermally cracking a typical refinery virgin residuum was investigated extensively on a pilot plant scale. From this, it was found that rate of hydrogenation for mono- and diolefins can be expressed in terms of pseudo reaction rate constants; also, that a relationship between leaded octane number of the hydrogenated product can be expressed in terms of a mathematical equation that contains terms for bromine number and sulfur content along with appropriate constants. This study was conducted using a commercial nickel-tungsten sulfide catalyst and covers temperatures from 400" to 550" F.,pressures from 40 to 200 pounds per square inch gage, liquid feed rates from 3 to 18 volumes of oil per hour per volume of catalyst, and hydrogen rates from 100 to 4000 standard cubic feet per barrel of feed. The equations developed were also applicable to oxide catalysts containing molybdenum. Hydrogenation reactions depended on the ratio of reactants, and temperature had an appreciable effect on selectivity of the over-all hydrogen reaction. Therefore, it was concluded that data obtained would lend themselves to interpretations and correlations based on con-
656
cepts involving reaction kinetics. This system, however, was too complex to express in terms of conventional first- or second-order reaction rate equations. A convenient method of correlating such experimental data is to assume that the reaction system, even though highly complex, follows a pseudo first-order relationship. However, runs in which the hydrogen-oil ratio was varied did not correlate using either this pseudo firstorder approach or an integrated zero order reaction rate equation of the type,
Definite dependence of the hydrogenation reactions on concentrations of hydrogen andl'or olefins strongly suggested that the equations would have to include concentrations of one or both reactants. O n this basis, it \vas possible to develop the following pseudo reaction rate equations that fit experimental data for this particular feed stock.
where
K T , K K = pseudo reaction rate constants for total and diene conversions, respectively fT = fraction of total unsaturates converted, based on bromine number reduction f I c = fraction of conjugated diolefins converted, based on reduction in ultraviolet coefficient at 235 m,u after correction for aromatic absorption Ph, Po = initial partial pressure of hydrogen and total olefins, respec tively
INDUSTRIAL AND ENGINEERING CHEMISTRY
Summary of Reaction Rate Constants Pseudo Reaction Rate Constant, See.-' Temp., Diene, Total, F. KK 'I 2' 550 500 450
0.63 0.33
0.0096
0.17
400
0.067
0.0018 0.0006
0.0045
The rate equations previously described held true for other types of thermally cracked naphthas, although different reaction rate constants were obtained for each of the various stocks treated. One interesting development resulted from using these equations in analyzing data from selectively hydrogenating diolefins-i.e., maximum selective removal of diolefins occurred when the hydrogen rates were equivalent to a gas-oil molar ratio of 1.0. This observation may be substantiated by differentiating the diene rate equation and setting the resulting derivative equal to zero. The regression equation developed for predicting research octane number (with 2 cc. of tetraethyllead added) is Research O.N. 2 cc. TEL = 79.0 0.32 [bromine number 203.1 (sulfur content)]
+
+
At levels of moderate conversion and high selectivity to diolefin hydrogenation inherent in this process, a fair correlation exists between the bromine number and the sulfur content of the treated product. Thus, sulfur data needed for the above equation can be read directly from a single curve. On the basis of data obtained, it was possible to predict inspections---diene, olefin, and sulfur content, and research octane number-on products obtainable by selectively hydrogenating the mixed thermal naphtha under any set of operating conditions within the range studied; also, it was possible to extrapolate outside this range with confidence. This approach has proved useful in subsequent studies of selective hydrogenation and in design studies for commercial plant installations.
E. J. HOFFMANN, E. W. LEWIS, and E. F. WADLEY Humble Oil and Refining Co. Baytown, Tex.
