Kinetics of the Liquid Phase Chlorination of n-Dodecane Michael P. Ramage and Roger E. Eckert’ School of Chemical Engineering, Purdue University, West La fayette, Indiana 4 7907
A kinetic model for the photoinitiated liquid phase chlorination of ndodecane is presented. Reaction orders for chlorine, dodecane-dodecyl chloride, and light intensity and rate constants for monochlorination and dichlorination of C12H26 to nC12H25CI and s ~ c C , ~ H ~ were ~ C determined I from experimental data. The model agrees well with a theoretical model derived in this paper. It is shown that the model predicts the selectivity behavior of the process at conversion levels and reaction rates severalfold higher than those used in the kinetic analysis.
Since the turn of the century, the chemical literature has reported hundreds of studies on the vapor phase chlorination of low molecular weight (c1-C~)paraffinic hydrocarbons (Bratolyubov, 1961). In recent years, the focus of the chlorination literature has shifted to the heavier n-paraffins (C,o+) where the corresponding monochlorides can be alkylated onto benzene to produce linear alkylate suitable for use by detergent manufacturers. However, while it is advantageous to chlorinate the lighter alkanes in the vapor phase, chlorination of the heavier paraffins in the liquid phase is preferred so that high-temperature side reactions such as dehydrohalogenation and pyrolysis are avoided. By liquid phase chlorination, we mean contacting gaseous chlorine with liquid hydrocarbon. As with any chemical process, it is desirable to develop a mathematical model of the chlorination process so that the process can be scaled up and optimized with minimal experimentation. For the liquid phase chlorination of heavier paraffins, development of a rigorous model is no easy task. Since gaseous chlorine must transfer to the liquid hydrocarbon phase before reaction can take place, there is a coupling of chemical and physical phenomena. This coupling was first reported by van de Vusse (1966) where it was shown experimentally that mass transfer limitations in the liquid phase chlorination of n-decane affect the yield of total monochlorides. Recently, Ramage and Eckert (1973) showed that in addition to its effect on the yield of total monochlorides, mass transfer limitations also affect the yield ratio of secondary to primary monochlorides in the liquid phase chlorination of n-dodecane. Ding et al. (1974) have also reported that multiple steady states can exist in liquid phase chlorination due to the high activation energy and heat of reaction. A t the present time, a rigorous model describing the chemical and physical laws of the liquid phase chlorination process has not been reported. The first step in such a development is to obtain the intrinsic kinetics for the chlorination. Two kinetic studies of liquid phase chlorination of the heavier alkanes have been reported in the literature. Stauff (1942) found that for the photochlorination of nhexadecane, the formation rate of total monochlorides could be represented by the following power law model
where Z is the volumetric light intensity. Barilli et al. (1970) studied the chlorination of n-decane and found that the ratio of the rate constant for dichlorination to that for monochlorination is 0.88. In both of these studies, the reaction orders on chlorine concentration, alkane concentration, and light intensity were assumed and the rate constants fitted. However, it has been shown theoretically by Ramage (1971), that a power law model of the form 214
Ind. Eng. Chem., Fundam., Vol.
14, No. 3: 1975
yrn
= k,(C12)’(alkane)m(Z)h
is an asymptotic solution of the true kinetics and that 0 5 j 5 1, 0 Im I1, and 0 I h 5 0 . 5 depending on which steps of the free radical mechanism are controlling. Furthermore, the two prior studies only considered the formation rates of total monochlorides. The most biodegradable detergent is obtained using the primary monochloride. Since the formation for the primary isomer requires the primary free radical as an intermediate, its formation rate is much different from those of the secondary isomers which go through the more stable secondary free radical. Also, as previously pointed out, mass transfer limitation can affect the yield ratio of secondary/primary monochlorides. Therefore the above two studies are not adequate for the development of a rigorous liquid phase chlorination model. In the present study, we determine rate constants and reaction orders for the photochlorination of n-dodecane. The formation rates of primary and secondary monochlorides and also the corresponding rates of dichlorination are reported. Photochlorination was chosen because high reaction rates in the liquid phase are obtained as reported by Hutson et al. (1972).
Theory The reaction rate constants for production of secondary and primary total monochlorides are given in the following schematic reaction paths (omitting chlorine used and HCl formed at each step)
The free radical mechanism (Dewar, 1949) for the photoinitiated chlorination of dodecane or one of the dodecyl chlorides in eq 3 and 4 and the rate expressions for the elementary steps are given in Table I. Ultraviolet light in the spectra range form 2700 to 4200 A can be used to generate free radicals in the initiation step (Gibson and Bayliss, 1933). Calvert and Pitts (1966) report the quantum yield of the initiation (4) is less than 1.0 since the absorbed energy may be lost through re-emission, collision, etc. and furthermore @ is independent of experimental conditions. Chain initiation and termination at the wall of a glass reactor occur in vapor phase chlorination (Bernstein et al., 1969; Bratolyubov, 1961), but in liquid phase chlorination, these effects are not likely to be important due to low radical diffusion rates (Calvert and Pitts, 1966). If liquid phase chlorination is carried out in the presence of solvents such as aromatics
containing electron donor substituents, these solvents combine with the chlorine radicals thus altering the reaction mechanism given in Table I (Russell, 1957,1958). Chlorination Rates. The kinetics for the formation rate of dodecyl chloride or polychloride are quite complex due to the nature of the free radical chain mechanism. Since the rate equations for the elementary steps of the mechanism given in Table I include free radical concentrations which are not experimentally measurable in conducting the reaction, an experimental value for the free radical rate constants is not available. However, an approximate analytical solution can be obtained if two assumptions are made. The first, the pseudostationary state assumption, states that the concentration of each of the free radicals (C1- and R.) does not change with time. Even though justification for such an assumption is difficult to prove, previous investigators have found satisfactory agreement between predicted results and experimental data (Wang et al., 1963; Calvert and Pitts, 1966). Recently, Blakemore and Corcoran (1969) have given quantitative proof to the validity of this assumption for the batch pyrolysis of n-butane, a system consisting of five radical intermediates. Giddings and Shin (1962) have shown theoretically for the free radical reaction between hydrogen and bromine that the pseudostationary state assumption is valid except for extremely fast reactions above 1000°C. From a qualitative standpoint, if the duration of the induction period is small compared to the duration of the overall reaction and if the rates of destruction of the radicals are many orders of magnitude faster than the overall reaction rate, then this assumption can be considered valid (Hirschfelder, 1957). The second assumption is the so called “cage effect” (Franck and Rabinowitch, 1934) for reactions taking place in liquid phase systems. When a chlorine molecule dissociates to two chlorine free radicals in the liquid hydrocarbon, the radicals are trapped in a cage of the solvent molecules. It is well known that the reactivity of free radicals is very high and that the diffusion rate in liquid systems is very low (Calvert and Pitts, 1966). Most chlorine radicals react before they escape the “diffusion cage.” They can react with (a) a dodecane or dodecyl chloride molecule (propagation step), (b) a free radical generated in the propagation step, or (c) recombine to form molecular chlorine. Since the concentration of radicals generated in propagation steps is extremely low (Blakemore and Corcoran, 1969), reactions (a) and (c) have the highest probability of occurring. Therefore, the dominant termination step in liquid phase paraffin chlorination is chlorine radical recombination due to the high local concentrations of chlorine radicals. Utilizing these two assumptions and the rate expressions given in Table I, the formation rate of a dodecyl chloride can be obtained. Solving for the rate of change of chlorine radicals and dodecyl or dodecyl chloride radicals while applying the pseudostationary state assumption
ki(C1*)(R*)- I:,(R*)’
(6)
If one solves eq 6 for [kz(Cl.)(RH)]and substitutes into eq 5, the result is (2’31)
-
I z , ( C ~ . ) ’- 2ki(C1.)(R*) - I:,(R*)? = 0 (7)
The “cage theory” assumption leads to
Table I. Reaction Mechanism for the Photoinitiated Chlorination of n-Dodecane Elementary
Initiation Propagation Termination
c12 C1-
-
+ RH R* + C12
2c1*
Cl’ + C1* Cl* + RRe + R*
k J (C 1. )
HC1 + RRC1 + C1. C12 RC1 R2
(2dI) kz(Cl*)(RH) k3(Ro)(C12) I? i ( c l ‘ )2 kj(Cl*)(R*) /?,(R*)2
k ;( C 1.1 (R * )
( 8)
--
-
* >>
Rate
and
k,(Cl-)’
>>
Iz,(Ra)?
( 9)
Combining eq 8 and 9 with eq 7 , the chlorine radical concentration is given by (, 10)
Substituting eq 7, 8, and 9 into eq 5, one obtains the dodecy1 or dodecyl chloride free radical concentration. k 2 (RH) ( 2 dl) (11) ‘R 1 = [k,]0’5/?,( Cl,) Therefore, the formation rate of a dodecyl chloride is given by d(RC1) - k2(RH)(2dl)“’ (12) dl [/?,]0*5 and it is seen that under the two previously stated assumptions, the formation rate of a dodecyl chloride is independent of molecular chlorine concentration and has power law behavior with respect to the independent variables. Chlorination Selectivity. For the liquid phase chlorination of n-dodecane, it is desirable to know both the yield ratio of secondary/primary monochlorododecane, 7,and the yield of total monochlorides, ym. The functional form of rate expressions for the formation of all dodecyl chlorides in the liquid phase chlorination system is given by eq 12. Yield ratio and the yield of total monochlorides in the presence of dichlorination can be obtained by integrating these equations (Ramage and Eckert, 1973)
The rate constant ratios given in eq 13, 14, and 15 have never been determined experimentally. An estimate of values can be obtained from the work of Fredericks and Tedder (1960,1961). A t 3 5 , 7 8 , and 146’ these investigators determined experimentally the degree to which one chloride atom at various positions on a monochlorobutane molecule reduces the reactivity of the other hydrogen atoms in the molecule with respect to further chlorination. They Ind. Eng. Chem., Fundam., Vol. 14, No. 3, 1975
215
Table 11. Estimated Rate Constant Ratios for n-Decane and n-Dodecane Chlorination as Calculated by Ramage and Eckert (1973) from Results of Fredericks and Tedder (1960,1961) Rate constant ratio
12 -
)I
-Dodecane
Decane,
.
78”
146”
0.071
0.077
0.929 13.1 0.91 0.94 0.90
0.923 12.0 0.91 0.94 0.90
0.083 0.917 11.1 0.91 0.93 0.90
100”
35’
I? J k m k,/k, f?, / R lZ,/l?, kdD/l?, l?,/fZ,
0.88
MANOMETER
4 O F F GAS AGITATOR THERMOMETER
I
I
LIQUID SAMPLING
r
--
L
A
N2
CJZ
ULTRAVIOLET L I G H T SOURCE l3660A 1
Figure 1. Schematic diagram of liquid phase chlorination apparatus.
found that the chloride atom reduced only the reactivity of the hydrogen atoms attached to the carbons which are C Y , p, and y to the chlorine atom. The effect of the y-carbon is small. Using the results of Fredericks and Tedder, the rate constant ratios for n-dodecane chlorination were estimated in Table 11. For n-decane chlorination, the ratio k d / k , was also estimated and agrees with the experimental value of 0.88 found by Barilli et al. (1970). In the above theoretical development, a rate expression for the formation rate of dodecyl chloride in liquid phase chlorination has been derived along with an estimate of the rate constant ratios. There is no previous experimental evidence to validate either the rate expression or the rate constant ratios. In the remainder of this paper we present our experimental determination of the rate expressions (including reaction orders for chlorine, dodecane, and light) and rate constants for the liquid phase chlorination of ndodecane as described by eq 3 and 4.
Experimental Section Apparatus. The reactor system is represented schematically in Figure 1. The reactor was a Pyrex 250-ml, 2l/4-in. i.d., creased, 4-neck, flat-bottom Woulff bottle equipped with a gas inlet, thermometer, agitator with a l’h-in. flat blade impeller, and condenser. Liquid samples could be ob216
Ind. Eng. Chem., Fundam., Vol. 14, No. 3. 1975
tained through a sample port since the reactor was operated a t a positive pressure due to the off-gas scrubbers. The feed gases were scrubbed in concentrated H2SO4 to remove moisture and the liquid metallic chlorides which form in the chlorine cylinder. The reaction temperature was controlled (&tzoC) by circulating constant-temperature water through a Pyrex 6lk-h. i.d. flat-bottom cooling batch. A B-100A Blak-Ray ultraviolet lamp manufactured by Ultra-Violet Products of San Gabriel, Calif., was used as the light source. This lamp is composed of a 100-w highpressure mercury vapor type light source and a heat-resistant filter; 97.5% of the filtered light has a wavelength of 3660 A. The remaining light is made up of the following wavelengths: 3341 A (LO%), 4047 8, (1.3%), and 4077 8, (0.2%). The top of the lamp (4lh-in. in diameter) was positioned 6 in. beneath the flat bottom of the reactor and 5 in. beneath the bottom of the cooling batch. Two levels of light intensity were used for the homogeneous experiments. Blackened three-pound coffee cans with a piece of 4 in. long straight tubing soldered perpendicularly into the bottom of each were inverted over the light source. The tubing used was 0.13 and 0.19 in. i.d. The ratio of light intensity entering the reactor from the 0.19-in. aperture to that a t and the ratio from the 0.13-in. full strength was 3.9 X aperture to that at full strength was 6.4 X (Ramage, 1971). Materials. ”Pure grade” (99 mole %) n-dodecane was obtained from the Phillips 66 Petroleum Co. The purity was checked by gas chromatography. Primary monochlorododecane and a sample of sec-monochlorododecanes of known isomer distribution were donated by Procter and Gamble Co. Spectro grade (>99%) carbon tetrachloride was used as a liquid phase diluent. A cylinder containing 150 lb. of “high purity” liquefied chlorine (99.5 vol %) was obtained from The Matheson Co. Oxygen was the primary contaminate in the chlorine. Since oxygen is a free radical inhibitor, it had to be removed from the chlorine. This was accomplished by first bleeding gas from the cylinder until the tank pressure equaled the vapor pressure of chlorine a t room temperature. This was followed by bleeding 15 additional pounds of gas from the cylinder. After this procedure, the oxygen content of ,the chlorine was less than 40 ppm (Bernstein, 1969). Nitrogen (
