Radiation-Initiated Side-Chain Chlorination of Toluene. Kinetic

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D = inside diameter of column, meters G’ = mass rate, kg./hour G, = mean mass velocity, kg./sq. meter hour G = inlet mass velocity, kg./sq. meter hour h, = corlvective heat transfer coefficient of gas film, kcal./sq. meter hr. C. H = humidity of gas, kg./kg. dry air J H = (h,/c,G,) Pr2 < J D = (kcp,,Mm/Gm) SC* kb = mass transfer coefficient of gas film, kg./sq. meter hr. mm. Hg k , = mass transfer coefficient of gas film, kg. mole/ sq. meter hr. mm. Hg M , = mean molecular weight of gas Pf = total pressure in column, mm. Hg P = partial pressure of gas, mm. Hg P B M = logarithmic. mean partial pressure of air in gas film, mm. Hg P r = Prandtl number = c , p / x r, = latent heat at t,, kcal./kg. Re = Reynolds number = DG,/p s = sectional area of column, sq. meters s c = Schmidnumber =p/pDL t = temperature of gas, C. t, = temperature of water surface, O C. T = time, hours v = volume of evaporation steam, cu. meters/ hour volume of dryer, cu. meters = evaporation rate, kg./hour = evaporation rate eliminated radiation effect, kg./ hour x = volume of steam in dryer, cu. meters F = viscosity of gas film, kg./hour meter A = thermal conductivity of gas film, kcal./ meter hr.

Literature Cited

Barnet, W. I., Kobe, K. A., Ind. Eng. Chem. 33, 436 (1941). Basel, L., Conn, S., Gray, E., Chem. Eng. Progr. 58, 67 (1962). Bromley, L. A., Wilke, C. R., Ind. Eng. Chem. 43, 1641 (1951). Cairns, R. C., Roper, G. H., Chem. Eng. Sei. 3, 97 (1954). Chu, J. C., Finelt, S., Hoerrner, W., Lin, M. M., Ind. Eng. Chem. 51, 275 (1959). Chu, J. C., Lane, A. M., Conklin, D., Ind. Eng. Chem. 45, 1586 (1953). Gilliland, E . R., Sherwood, T. K., Znd. Eng. Chem. 26, 516 (1934). Lurie, M., Michailoff, N., Ind. Eng. Chem. 28, 345 (1936). McAdams, W. H., “Heat Transmission,” 3rd ed., p. 262, McGraw-Hill, New York, 1954. Perry, J. H., “Chemical Engineers’ Handbook,” 4th ed., p. 10-40, McGraw-Hill, New York, 1963. Rantz, W. E., Marshall, W. R., Chem. Eng. Progr. 48, 141, 173 (1952). Shepherd, C. B., Handlock, C., Brewer, R. C., I d . Eng. Chem. 30, 338 (1938). Tdei, R., Okazaki, M., Kubota, K., Ohashi, K., Kataoka, K., Mizuta, K., J . Chem. Eng. Japan 30, 43 (1966). Vivian, J. E., Behrmann, W. C., A.I.Ch.E. J . 11, 656 (1965). Wasan, D. T., Wilke, C. R., A.I.Ch.E. J . 14, 577 (1968). Wassiljewa, A., Physik. 2. 5, 737 (1904). Wenzel, L., White, R. R., Ind. Eng. Chem. 43, 1829 (1951). Yoshida, T., Hydd6, T., Food Eng. 38, 86 (1966). DESIGN Yoshida, T., Hyddd, T., IND.ENG.CHEM.PROCESS DEVELOP. 2, 52 (1963).

v= w, w,

O C .

3pim = logarithmic mean driving force, mm. Hg P = density of gas film, Kg./cu. meter SUBSCRIPTS RECEIVED for review October 7, 1968 ACCEPTED September 29, 1969

1 = inlet 2 = outlet

RADIAT ION-INIT IAT E D SIDE-CHA IN C H LOR INATION 0F TO L UE NE Kinetic Investigations J .

Y .

Y A N G

A N D

C .

C .

T H O M A S ,

J R .

Western New York Nuclear Research Center, Inc., Power Drive, Buffalo, N . Y . 14214 H .

T .

C U L L I N A N

Department of Chemical Engineering, State University of New York at Buffalo,Buffalo,N . Y . 14214

ALTHOUGH there have been significant advances toward the peaceful utilization of atomic energy in many areas of industrial applications, the extent of developments within the chemical industry has been rather disappointing. Silverman (1968) and Ballantine (1968) recently reviewed the problems as well as achievements in the field of radiation chemical processing. The application of radiation energy for the initiation of chemical synthesis 214

Ind. Eng. Chem. Process Des. Develop.,Vol. 9, No. 2, 1970

has made particularly slow progress and more efforts directed toward the development of such applications are definitely needed. The side-chain chlorination of toluene is expected to proceed by free radical chain reactions, resulting in a large number of molecules reacted for a given amount of energy absorbed. A recent report (Collins et al., 1967) based on a literature survey and economic evaluations


The radiation-induced side-chain chlorination of toluene was investigated using accelerated electrons as the radiation source. The reaction was carried out to complete side-chain chlorination within the temperature range of 80' to 150' C. Contamination by-products resulting from substitution and addition a t the aromatic nucleus were small. The conversion of toluene to benzotrichloride follows three consecutive reaction processes and the yield efficiencies are approximately inversely proportional to the square root of the radiation dose intensities. At reasonable dose intensities, the yield efficiencies are sufficiently high to merit considerations for commercial application of a radiation reactor for the production of benzotrichloride.

indicated that the radiation-induced conversion of toluene to benzotrichloride may be developed into a commercially feasible production process, provided the yield efficiencies are sufficiently high. A detailed kinetic investigation of the liquid phase side-chain chlorination of toluene has, therefore, been carried out and the results are reported here. The design of a radioactive reactor for the commercial production of benzotrichloride is reported in a subsequent paper (Cullinan et al., 1970). Experimental

Irradiations. A vertically mounted high voltage engineering Model GS van de Graaff electron accelerator was used as the radiation source. Such a source offers the advantages that the beam current can be precisely and reproducibly contrplled and the radiation dose rate can be varied over an extremely wide range. Irradiations were carried out in a Corning 7740 borosilicate glass reaction vessel (Figure l ) , mounted on an aluminum plate bolted to the accelerator window to maintain a fixed geometry, well centered, under the electron beam. The reaction vessel consisted of a cylindrical cell of 8-cm. length x 6-cm. diameter, with a neck of 5-cm. length x 4-cm. diameter. A medium pore fritted disk was used to disperse the gas, a condenser with water cooling was incorporated to avoid the loss of liquid reactants, and a metering Teflon stopcock was used for sampling the reaction mixture. The high energy electron beam was allowed to enter the irradiation cell through either a 0.002-inch-thick titanium foil or a glass insert tube with 2.5-cm. i.d. and 0.02-inch end wall. These irradiation windows were sealed to the ground flange of the reaction vessel with the aid of Viton rubber gaskets. Monenergetic 1.0-m.e.v. electrons were

Figure 1. Radiation reactor A. B. C.

Glass window Gasket Sampling valve

D. E.

Thermoco3ple well Fritted glass disk

used throughout this study to approximate the beta radiation expected from a i)(JSr-g"Y isotope source. Radiation dose rates were calibrated a t frequent intervals by means of the Fricke dosimeter, and the desired dose rates were controlled both by varying the electron beam current and by interposing multilayers of Nichrome screens between the accelerator window and the reaction vessel. For the chlorination studies, 1 mole of toluene (106 ml.) was used as the starting material and during the course of the reaction the total liquid components remained a t essentially 1 mole even though the total weight of the reaction mixture increased as a function of the degree of chlorination-i.e., the number of gram atoms of chlorine per mole of the organic reactant. For electron irradiation, therefore, it is more convenient t o express dose rates in the units of e.v. mole - I min.-' (1 e.v.g. - 1 = 1.6 x 10 l 4 rad). All irradiations were carried out with dose rates to 1.0 x 10'' e.v. mole-' min. ' in the range of 7.5 x The reaction temperature was controlled by immersing the reaction flask in a circulating diethyl phthalate bath set at 100°C. Because of the high exothermicity of the chlorination, however, the reaction temperature was consistently 20" to 30°C. higher than the control temperature during the initial stages of the reaction. The chlorine gas (Matheson pure grade) was metered from the bottle and passed through a drying tower before entering the reaction vessel. The exit gas escaping from the condenser was passed successively through a glass trap t o retain any carried-over liquid, a drying tower to prevent back-diffusion of water vapor, a series of waterfilled washing flasks to absorb the hydrogen chloride, and a sodium hydroxide solution to retain unreacted chlorine before being vented through an exhaust blower. Thus, the chlorine in contact with the liquid mixture was slightly above the ambient atmospheric pressure. Initial chlorine input rates were in the range of 300 to 2000 ml. min. ', and these flow rates were usually reduced gradually until only slight excess of chlorine left the reaction vessel. The maximum flow rate was limited by a frothing of the liquid react ants. Product Analyses. The reaction mixture composition was analyzed by gas chromatography using a Varian Aerograph Model 1620 C instrument equipped with flame ionization and electron-capture detectors. Component separations were achieved on a 2-meter L-inch 0.d. stainless stet.1 column packed with 5 ' i DC 200 on 60- to 80-mesh Chromosorb Q. The carrier nitrogen flow rate was 30 ml. per minute. Final product purities were examined by programming the column temperature for a rate of increase of 10" per minute over the temperature range 100" to 250" C. The detectable high boiling impurities Ind. Eng. Chem. Process Des. Develop., Vol. 9,No. 2, 1970

21 5

totaled only about 1% of the total product. For analyses of major product compositions a t desired reaction intervals, chromatograms of the samples were taken with the column at a constant temperature of 110°C. The sample to be analyzed was diluted 20-fold with carbon tetrachloride; then 0.5 ~ 1 of. the carbon tetrachloride solution was injected for analysis. Flame ionization detector signal sensitivities were calibrated with synthetic mixtures of toluene and the three side-chain chlorinated products. Relative sensitivities based on the peak area to mole fraction ratio were: toluene 1.0, benzyl chloride 0.98, benzal chloride 0.92, and benzotrichloride 0.89. Results. The complete side-chain chlorination of toluene is known to proceed via stepwise chlorine incorporation to yield benzotrichloride.


C6HjCH?+ C12

C6HjCH2C1+ HC1

(1)

C,HjCHIC1 + C12 5 CsH5CHC12+ HC1

(2)

C6HiCHC12+ C12 5 C6HjCC1?+ HC1

(3)

3.c

2.5

2

Q

t

2.0

f

3 S 0

1.5

LL

0 W

W

a

cl

w n

1.0

0.5 I

0

The detailed mechanism for each of these reactions is rather complex and the over-all reaction rate is highly sensitive to experimental conditions as well as the presence of certain types of impurities. T o facilitate an optimum design for a radiation chemical reactor, the kinetic data are summarized. Effects of Temperature. As indicated by Collins et ul. (1966), the side-chain chlorination of toluene is favored a t elevated temperatures. T o minimize possible contamination by ring addition or substitution products, all experiments were carried out above 80°C. During the initial stages of chlorination, the reaction rate appears to be limited by the chlorine gas input rate and, therefore, is not affected by variations in temperature. For the final conversion of benzal chloride to benzotrichloride, the reaction rate was likewise found to be essentially temperatureindependent within the range of 80” to 150°C. The latter observation is unexpected from a simple consideration of the activation energy. Apparently, the decrease in chlorine solubility a t increasing temperatures tends to negate the effect of specific rate increases. Effects of Chlorine Concentration and Chlorine Gas Input Rate. Side-chain chlorination rates expressed as the degree of chlorination for four runs a t initial chlorine input rates ranging from 300 to 2000 ml. per minute are given in Figure 2. The initial chlorination rate is obviously limited by the rate of chlorine gas input. In fact, under our experimental conditions, the initial reaction rate was independent of the radiation intensity. Similar observations for photoinitiated reactions have been reported by Haring and Knol (1964a,b,c, 1965). As subsequent chlorine substitution rates are much slower, mass transfer limitations no longer exist. An equilibrium chlorine concentration appears to be established and the reaction proceeds through homogeneous kinetics. Previous studies have indicated a square root dependence on the chlorine concentration for both the radiation (Cox and Swallow, 1956, 1958) and the photoinitiated (Miyazaki, 1951, 1952) reactions. For the present study, the reaction rate dependence on the chlorine concentration was not determined, since under our desired reaction conditions the chlorine concentration as governed by the solu216

Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 2, 1970

I

1

I

100

200

30(

REACTION

TIME,

MIN

Figure 2 . Effects of chlorine flow on the chlorination rate Initial chlorine input rates; ml./min.

0 2000

A

1000

0

500

V

300

bility coefficient did not vary appreciably. Initially the chlorine gas was instantaneously consumed, and thus, it appeared that the reaction rate was independent of the chlorine concentration. I t is not certain whether the reaction had taken place a t the gas-liquid interface or in a narrow reaction zone where the chlorine gas bubbles are in equilibrium with the surrounding liquid solution. The results showed, however, that possible chlorine concentration fluctuations did not alter the product distribution pattern. As the degree of chlorination was increased, the chlorination rate became progressively slower and the reaction appeared homogeneous. The reaction rate, however, was not affected by an excess of chlorine gas flow above that incorporated in the reaction products. Although the composition of the gas above the liquid surface was a function of the level of excess chlorine, the finely dispersed inlet chlorine was apparently in much greater surface contact with the liquid. Assuming that the solubility coefficient was not appreciably changed by continuous changes in the liquid mixture composition, the chlorine concentration would remain essentially constant once the chlorine input rate exceeded the reaction rate. Thus, the final stage of the chlorination reaction occurred a t a single chlorine concentration and possible effects of the dissolved gas concentration cannot be established from our results. Effects of Liquid Mixture Composition. Time rate changes of the product mixture distribution for a typical run at the initial chlorine flow of 1200 ml. per minute and the radiation dose rate of 5.0 x 10’’ e.v./mole/minute are shown in Figure 3. For irradiations a t various chlorine flow and radiation intensity conditions, the changes in toluene and benzyl chloride concentrations depended mainly on the chlorine gas input rate, whereas the changes in benzal chloride and benzotrichloride concentrations were

I O

z 0

-0

20

40

60

80

REACTION TIME,

100

120

140

160

MIN

Figure 3. Product distribution as a function of reaction time


Initial chlorine input rate 1200 ml./min. Radiation intensity 5.0 X 5.0'' e.v./mole/min.

0 Toluene v Benzyl chloride 0 Benzol chloride A Benzotrichloride functions almost solely of the radiation intensity. The former observation is due obviously to reaction rates being limited by the mass transfer, and the latter reflects merely that the equilibrium chlorine concentration is not a function of the chlorine gas throughput under the given range of reaction conditions. Component product concentrations can also be represented as functions of the degree of chlorination, and as long as the three consecutive Reactions 1, 2, and 3 have similar kinetic dependencies, these functions will be unaffected by changes in radiation intensity or chlorine flow. I n Figure 4, the data for two runs a t the extreme dose rates of 7.5 x 1016 and 1.0 x lo2"e.v./mole/minute are fitted in a single graph. All other data under a wide variety of conditions showed similar product distribution patterns as well. Also, the data in Figure 4 are in excellent agreement with the findings of Haring and Knol (1964a,b,c, 1965) for side-chain chlorine substitution under various initiating conditions. Order of Reaction. Brief inspections of the product dis-

tribution data similar to those shown in Figure 3 will yield considerable information regarding the kinetic dependencies on the organic substrate concentrations. Since initially the chlorine input rate was reaction rate-limiting, the conversion of toluene to benzyl chloride appeared t o follow pseudo-zero-order reaction kinetics, but more careful considerations of the competition effects revealed, in fact, first-order reaction kinetics. Time rate changes of the benzyl chloride concentration as a result of competing reactions are too complex to be analyzed by simple graphical techniques. Only the conversion of benzal chloride t o benzotrichloride appeared as a simple first-order reaction. Indeed, as shown in Figure 5, when the function 1 - M D (where M D represents the mole fraction of benzotrichloride) was plotted on a log scale as a function of the reaction time, the data for (1 - MD)< 0.7 fitted well to a linear function as expected from first-order kinetics. Since the product distributions as functions of the degree of chlorination all fit into a single pattern as shown in Figure 4,we may further assume that all three Reactions 1, 2, and 3, are first-order with respect to the organic substrate. I n each case, the chlorine concentration is merged as an integral part of the pseudo-first-order rate constant. Since the gas solubility is inversely related t o the reaction temperature, changes in the chlorine concentration tend to balance the effects of reasonable activation energies and, thus, account for the observations that the rate of chlorination was insensitive to temperature changes within the range studied. Effects of Radiation Intensity. Pseudo-first-order reaction rate constants for the conversion of benzal chloride to benzotrichloride can be calculated from the slopes of the linear functions in Figure 5. Similarly, the G-value for benzotrichloride formation a t a given radiation dose rate can be obtained by integration of the rate expression up to 95% reactant conversion. The rate constants and G-values a t various dose intensities as well as those adjusted for contributions by a pure thermal reaction are given in Table I. The adjusted rate constants and G-values are shown in a log-log plot as functions of the radiation dose rate in Figure 6.

I .o Z

2

ta U

0.8

F

Z

W 0

Z

0.6

0 0

I-

z W z 0

a I

0.4

0.2

0 V

0

0.5

1.0 DEGREE

1.5

2.0

2.5

3.0

OF CHLORINATION

Figure 4. Product distribution as a function of extent of chlorination Unfilled points, 7.5 X 10l6 e.v./mole/min. Filled points, 1 .O X 10'" e.v./mole/min.

0

v

Toluene Benzol chloride

hA

0W

Benzyl chloride Benzotrichloride

Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 2, 1970 217

The exponent of the reaction rate dependence on the radiation intensity is 0.43 and that for the G-value is -0.58. These values have been derived by assuming that the observed reaction rate is a sum of the rates of a dark reaction and radiation-induced reaction. Thermal and photochemical side-chain chlorinations of alkyl benzenes appear t o follow different reaction mechanisms. In the I O

Table 1. Rate Constants and G V a l u e s for Benzal Chloride Conversion to Benzotrichloride

Reaction Rate Dose Rate, E,u,,, Constant, See. x 10' Mole Min. Obsd. Adj. 0 7.5 x 10l6 6.1 x 10" 1.5 x 10IR 5.4 x 1 O l 8 1.0 x l o i y 5.0 x 10lY 1.0 x 10'" 8.9 x 10lm

0.81 1.30 1.61 2.11 2.80 3.65 6.65 8.08 5.40

G(Benzotrich1oride) Obsd. Adj.

... 0.49 0.80 1.30 1.99 2.84 5.84 7.21 4.59

... 2.0 x 3.2 x 1.6 x 6.0 x 4.2 x 1.5 x 9.3 x 7.0 x

lo8 io5 10" 10' 10'

io4 loJ 10'

... 7.5 x 1.6 x 1.0x 4.2 x 3.3 x 1.3 x 8.4 x 6.4 x

1oj 1@

io5 io4 10'

10' 10'

io4


' Irradiations through titanium foil u'indou.

, REACTION

TIME,

,

I

MIN

Figure 5 . Rates of benzal chloride conversion to benzotrichloride Radiation intensities, e.v./rnole/rnin.

A

Dark reaction

A 0

7.5 1.5 5.4 1.0

v x

x 10'~

A 1.0 x

X 10"

0 0

x lo'&

10''

5.0 x 10'' 1.0 x lozo

x 10Ib

transition state of the thermally induced chlorination of toluene, the methyl C-H bond is not completely broken and a radical chain mechanism is not involved (Goldwhite, 1960). The linear plot in Figure 6 tends to justify our simple correction for the dark rate. Under our experimental conditions, the electron beam is absorbed in the reaction mixture ununiformly within a rather small volume. The extent of ununiform distribution of intermediates depends largely on a characteristic mixing time relative to the lifetime of the intermediate. The effects of mixing on reaction rates in optically thick photoreactors have been considered analytically by Hill and Felder (1965). I n the case of long-chain reactions where the chain lifetime is large when compared t o the times characteristic of normal mixing processes, the introduction of mixing tends to smooth out intermediate concentration gradients. I n the case of radical-radical terminations, mixing will produce a higher mean radical concentration and therefore a higher reaction rate approaching that for uniform initiation as a limit. For our chlorination studies, turbulent mixing is provided by the chlorine and hydrogen chloride gas dispersion through the liquid mixtures. This may have accounted in part for our observed benzyl chloride formation with yield efficiencies

I

u 0) v)

I--

5

hz

0 0

w

k? a z 0

I-

O

a

w

a

RADIATION

DOSE RATE, ev

mole

-I

min

-I

Figure 6. Order of dependence of chlorination yields on radiation intensity 0 Rate constants for benzal chloride conversion to benzotrichloride

A 218

G-value for benzotrichloride formation

Ind. Eng. Chem. Process Des. Develop., Vol. 9,No. 2, 1970


much higher than those reported by Cox and Swallow (1956, 1958). I n our present study, the reaction rates for irradiations through the titanium foil window were consistently higher than those for irradiations through the glass window at comparable radiation intensities. The high energy electron beam exiting from the accelerator window is continuously scattered in a cone-like geometry. With the titanium window the dose distribution is fairly uniform over the entire surface area of the organic substrate; with the glass window the energy absorption is limited mainly to the area in proximity to the end of the tube. Thus, the lower yields observed in the latter case tend to suggest nonuniform initiation as well as insufficient mixing under our experimental condition. The G-values given in Table I represent only the yields obtainable under the specified reaction conditions. Higher yields are possible if uniform initiation can be achieved. Effects of Surface Impurities. Adverse impurity effects have been observed under a variety of conditions. Commercially available benzyl chloride and benzal chloride often contained undesirable impurities which thwarted efforts to carry out chlorination studies of these compounds. I n general, such impurities tend to catalyze initially the formation of ring substitution and addition products, which in turn inhibit further chlorination both a t the ring and a t the side chain. Possible surface effects are investigated by the addition of 100- to 200-micron diameter glass or strontium silicate beads to the reaction mixture. The only effects observed were those accountable on the basis of the introduction of inhibiting impurities. I n the case of glass beads the adverse effects can be easily avoided by pretreating the beads with hot nitric acid. The strontium silicate beads can be cleaned only under very mild conditions and have not been treated successfully to eliminate the inhibiting effect completely. Possibly, some of the minor ingredients such as vanadium and aluminum included in the fabrication of the strontium silicate beads used in our studies are chlorination inhibitors. These studies have been carried out to establish possible complicating factors in a fluidized radioactive reactor design as discussed in the subsequent paper (Cullinan et al., 1970). Discussion

Kinetic Analysis. As indicated earlier, the side-chain chlorination of toluene takes place through Reactions 1, 2, and 3 and it has been established that each of these reactions is first-order with respect to the organic substrate. The instantaneous rates of component concentration changes are given by the following series of equations:

(4)

(5)

and

(7)

where M A , M B , Me, and M D represent, respectively, the concentration of toluene, benzyl chloride, benzal chloride, and benzotrichloride in mole fraction units. Each component concentration a t any given time may be calculated by integration of above rate expressions and for the initial conditions of M A = 1 the following equations are obtained.

The data in Figure 3 appear to follow such functions under conditions where kl >> kp >> k3. T o determine the maximum intermediate concentration allowed by the series of consecutive reactions, we can set dMB/dt = 0 and obtain the reaction time when the maximum Mg is reached. Thus,

t = h ( h l / h z ) ( hl (12) Substituting Equation 12 into Equation 9, one obtains

or

(ME),,,

r'

- r

(14)

where r = k l / k 2 . Similarly, (Mc),, is a function only of the pseudo-first-order rate constants k l , k2, kj. Since it is obvious that k 1 >> k Z >> k S , certain approximation may be made and a relationship analogous to Equation 14, (15) ( M L a x5 1 - s is obtained by setting s = k2/k3. As shown in Figures 3 and 4, maximum benzyl chloride and benzal chloride concentrations are easily determined experimentally. T o estimate the k l / k 2 and k 2 / k 3 ratios by Equations 14 and 15, the graphical interpolation method appears most appropriate. ( M d m a xand ( M c ) , ~ , values as determined by given values of k 1 / k 2 or k 2 / k 3 are listed in Table I1 and presented graphically as curve A in Figure 7. Since the observed benzyl chloride and benzal chloride concentration maxima lie between 0.70 and 0.75, both rate constant ratios are estimated as about Ind. Eng. Chem. Process Des. Develop., Vol. 9,No. 2, 1970

219

I .2

z 2

Ga iZ

w

0 Z

1.1

1.0

0 V

0.9

2n

W

I a

0 8

W

+

0.7

5 r 5

0.6

z

I


I

1

I

Table II. Maximum Intermediate Concentration and Location in Terms of Degree of Chlorination as Functions of Rate Constant Ratio

Rate Constant Ratio 2 3 4 5 6 7 8 9 10 15 20 25 30 35 40

( M B ) ~ ~ ~ , Degree of Chlorination or ( M c ) ~ ~ ~(MH)max (Mc)max 0.500 0.577 0.630 0.668 0.699 0.723 0.743 0.760 0.774 0.824 0.855 0.875 0.888 0.902 0.910

1.oo 1.04 1.06 1.06 1.06 1.07 1.07 1.07 1.07 1.06 1.05 1.05 1.05 1.05 1.04

2.00 2.13 2.16 2.16 2.16 2.16 2.15 2.14 2.14 2.12 2.10 2.09 2.08 2.07 2.07

I

I

I

1

I

of chlorine atoms incorporated per total molecules of organic substrates (MacMullin 1948). For toluene sidechain substitutions the degree of chlorination, x , is given as

x = Mg

-I-2Mc

+ 3Mn

(16) The time rate change of x is obtained by differentiating the above function and substituting the appropriate values from Equations 5 to 10, giving

For x = 0 a t t = 0, Equation 17 is integrated to yield 7 or 8. Thus k l and hS values a t various radiation intensities can be calculated from corresponding ks values in Table I. The rate constant ratios may be calculated a t the intermediate concentration maxima using Equations 5 and 6. In practice, however, it is difficult t o locate the positions of the maxima exactly and consequently large errors may result. I n Figure 4, where the data represent average values of all our chlorination runs, k l / h 2 and k Z / k 3 rate constant ratios may be estimated, respectively, a t ( M B ) ~ ~ ~ and (Mc),, and the results agree within experimental Thus, a graphical representation of the product distribuuncertainties with the previously estimated values of 7 tion as a function of x may be constructed from Equations or 8. 8, 9, 10, 11, and 18, provided reaction rate constants For production process considerations, it is desirable hi, k p , and h3 are known. Such a task, however, is tedious to establish the production distribution as a function of and has not been carried out in a precise manner for the extent of reaction. I n the case of chlorination reactions, the present report. Only approximate solutions have been such functions can be expressed conveniently in terms considered. of the degree of chlorination which is defined as the number 220

Ind. Eng. Chem. Process Des. Develop., Vol. 9,No. 2, 1970

For the kinetic elucidation of the side-chain chlorination reactions, it will suffice to illustrate such calculations by establishing the values of x required for maximum intermediate M e and Mc concentrations. Since it is known that k l >> kl >> k 3 , these calculations may be facilitated by assuming k l = k l - k S = k 1 - 3/2 k3 and kz = kz - k3 = k L - k 2 - 2k3. Furthermore, occurs a t small values of t when M o is negligible and k3t is close to zero. Thus, substituting Equation 12 into Equation 18 and setting k l / k 2 = r, one obtains

Similarly, (Mc),,, occurs at relatively large t when Ma is negligible and e-k1t is nearly zero. Therefore, by substituting in Equation 18

a thermal spike effect in radiation-induced chlorination. I n a free radical-induced chlorination of alkyl benzenes, the competition between the side-chain substitution and the ring reaction resides mainly at the initiation processes. The hydrogen atom abstraction would be favored by the existence of thermal spikes within the radiation particle tracks. Thus, the radiation process furnishes a further advantage in that an effective reaction temperature higher than the boiling point can be achieved in a liquid system. The extremely high yield efficiencies observed in our investigation require the chlorination to take place by chain processes. The first-order dependence on the organic reactant and near square root dependence on radiation intensity are consistent with the following reaction mechanism for each of the consecutive processes 1 , 2 , and 3.

Radiation

-m C1

(22)


CsHjCHB + C1+ C6HiCH2 + HCL and setting k 2 / k 3= s, one obtains

C6HjCH2 + C1, c1+

For the special case as indicated for our present studies where r = s, the following relationship

is obtained. Values of x a t ( M L J ~and , ~(Mc),, for given k l / k l and k e / k j ratios are listed in Table I1 and graphical representations are shown as curves B and C in Figure 7 . Thus, for the estimated k l / k z and h z / k 3 ratios of 7, should occur a t the degree of chlorination value of 1.07 and (Mc),,, at x = 2.16. These values are in good agreement with our data in Figure 4. Mechanistic Considerations. The toluene chlorination mechanism is discussed briefly only in so far as such information is valuable in selecting optimum conditions for the unit process. I n the liquid phase reaction, a large number of products are possible and these may be divided into two general classes: those arising from substitution or addition of chlorine in the aromatic nucleus, and those resulting by substitution on the side-chain methyl group. The former reaction is initiated by chlorine atom additions in the ring which destroy the aromaticity and tend to be thermally reversible (Rabinowitch and Setser, 1964). The side-chain chlorination takes place, on the other hand, by hydrogen abstraction which requires a somewhat higher activation energy than those for the addition processes (Ashmore et al., 1967). Thus, possible contaminations by ring-chlorinated products may be minimized at sufficiently high temperatures. I n the case of fast electron irradiations, there is the possibility of a thermal spike effect (Magee, 1963) giving rise to a temperature in the reaction zone about 50°C. higher than the ambient. Such an effect arises when the reaction time is in the same order of magnitude as the time required for the energy dissipation within a cluster of molecules in close proximity to the reactive intermediate. Since the observed high boiling products in the present study are much less than similar ones reported for photoinitiation at comparable temperatures, we may take this as a positive indication for

c1+ c1,

+

C,H,CH,Cl+ C1 (25)

For the radiation-induced chlorination, very little is known regarding the initiation Reaction 22. The energy is absorbed initially in the organic substrates and efficient energy, charge, or radical transfer processes are needed to yield the initiating chlorine atom intermediates. Most probably Reaction 22 is unimolecular with respect to chlorine, resulting in a square root dependence for the overall chlorination process as reported in the literature. More detailed information regarding the nature of the initiation reaction would be of practical value toward evaluating the chlorination process under certain circumstances. For a continuous process in which hydrogen chloride is to be isolated as a by-product, the process stream needs to be cooled below the freezing point of benzotrichloride. The addition of an easily separable and low-freezing inert diluent would, therefore, enable higher degrees of conversion. A detailed knowledge of the reaction initiation will facilitate the selection of such diluents without sacrificing the chlorination efficiency. Certainly, much remains to be learned about the side-chain chlorination of toluene. Conclusions

The electron radiation-induced side-chain chlorination of toluene under batch conditions has been investigated in detail. The reaction rate constants for benzal chloride conversion to benzotrichloride under a wide range of radiation intensities are tabulated. Since the above reaction is by far the slowest of the three consecutive processes, it is rate-limiting for complete side-chain chlorination. The G-value for the over-all process under a given set of conditions can be calculated from the appropriate rate constant (Table I ) . The yield efficiencies under favorable conditions are much higher than the requirements specified by Collins et al. (1966) for feasible commercial production of benzotrichloride by radiation initiation. The economics of such a radiation process, however, depends largely on the practicality of designing a reactor to take advantage of the favorable conditions. The factors concerning a reactor design based on the fluidized pebble bed process are discussed in a subsequent paper (Cullinan et al., 1970). Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 2, 1970 221

Goldwhite, H., J . Chem. Educ. 37, 295 (1960). Haring, H. G., Knol, H. W., Chem. Proc. Eng. 45, 560 (1964a). Haring, H. G., Knol, H. W., Chem. Proc. Eng. 45, 619 (1964b). Haring H. G., Knol, H. W., Chem. Proc. Eng. 45, 690 (1964~). Haring, H. G., Knol, H. W., Chem. Proc. Eng. 46, 38 (1965). Hill, F. B., Felder, R. M., A.I.Ch.E. J . 11, 873 (1965). MacMullin, R. B., Chem Eng. Progr. 44, 183 (1948). Magee, J. L., Discussions Faraday SOC.36, 232 (1963). Miyazaki, S., J . Chem. SOC.Japan, Pure Chem. See. 72, 1067 (1951). Miyazaki, S., J . Chem. SOC.Japan, Pure Chem. See. 73, 223 (1952). Rabinowitch, B. S., Setser, D. W., Advan. Photochem. 3, 1 (1964). Silverman, J., Nuclear News 1 1 (6), 41 (1968).

Nomenclature

M A ,M B , M c , M D , = mole fraction of toluene, benzyl chloride, hl, k2, k3 = y = s =

x =

benzal chloride, and benzotrichloride, respectively reaction rate constants, set.-' ratio of k i / k , ratio of k 2 / k 3 degree of chlorination


literature Cited

Ashmore, P. G., Dainton, F. S.,Sugden, T. M., “Photochemistry and Reaction Kinetics,” pp. 64-88, Cambridge University Press, London, 1967. Ballantine, D. S., Trans. Am. Nuclear Soc. 11, 84 (1968). Collins, C. H., Cullinan, H. T., Thomas, C. C., Jr., Weller, S.,“Development of a Radioactive Pebble Bed Chemical Reactor System,” USAEC Rept. NYO-3718-1 (1966). Cox, R. A., Swallow, A. J., Chem. Ind. (London) 1956, 1277. Cox, R. A., Swallow, A. J., J . Chem. Soc. (London) 1958, 3727. Cullinan, H. T., Rhambia, H., Yang, J. Y., Thomas, C. C., Jr., IND.ENG.CHEM.PROCESS DESIGNDEVELOP. 9,222 (1970).

RECEIVED for review November 29, 1968 ACCEPTED September 15, 1969 Research supported by the Division of Isotopes Development, US. Atomic Energy Commission, under contract AT(30-1)-3864.

RADIATION-INITIATE D SIDE-CHAI N C HLOR INAT10 N 0F TO LUENE Continuous Reactor Design H .

T .

C U L L I N A N

A N D

H .

R H A M B I A

Department of Chemical Engineering, State University of New York at Buffalo, Buffalo, N . Y . 14214 J.

Y .

Y A N G

A N D

C .

C .

T H O M A S ,

JR.

Western New York Nuclear Research Center, Inc., Power Drive, Buffalo, N . Y . 14214 A theoretical investigation of the design of a chemical reactor for the continuous beta-radiation-induced side-chain chlorination of toluene to produce benzotrichloride is described, The reactor configuration considered is a liquid-phase fluidized bed containing active beta-source particles maintained in a fluidized condition by the upward reactants feed and recycle fhw. A model is developed based on empirical expanded bed characteristics, gross G-value kinetics, empirical radiation energy utilization efficiency, and a n axial dispersed plug flow equation. Results for energy utilization efficiency, reactor geometry, and source strength requirements are presented. The problem of source attrition is discussed.

THEintrinsic rate of the radiation-induced side-chain chlorination of toluene to produce benzotrichloride compares favorably with that of the photoinitiated reaction (Yang et al., 1970), a well-characterized commercial process. The practicality of radiation processing would depend on the successful design of a reactor with a relatively low initial capitalization as well as a high production efficiency. Betaemitting isotopes with short yet reasonable radiation energy ranges appear most suitable for use in the design 222

Ind. Eng. Chem. Process Des. Develop., Vol. 9, No. 2, 1970

of a reactor with minimal shielding and safety requirements. The possibility of using low penetrating betaemitters in intimate contact with reactant mixtures within the reaction zone is enhanced by the recent availability of immersible sources. The problem then devolves to one of finding optimal geometrical conditions so that the deposition of energy within the reaction zone is as efficient as possible. ”Sr - ” Y sources either in the form of strontium

Radiation-Initiated Side-Chain Chlorination of Toluene. Kinetic

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