Ind. Eng. Chem. Process Des. Dev. 1986, 25, 601-6 12
60 1
Methodology for an Optimal Design of a Photoreactor. Application to Methane Chloro Derivatives Production Ellana R. De Bernardezt and Albert0 E. Cassano*$ INTEC, Casilla de Correo N o 9 1, 3000-Sante
Fe, Argentina
The present work studies the consecutive chlorination of methane in a continuous annular photoreactor. The optimal design for the obtention of methyl chloride or carbon tetrachloride does not present important qualiitive differences from those processes thermally conducted. Instead, in the case of a photoreactor, the intermediate stable species (CH,CI, and CHC13) present distinctive optimization features. In the general problem, the fundamental independent variables are reactant feed molar ratio, mean residence time, and characteristic length of the optical path. The results demonstrate that photochemical reactors have a very particular degree of freedom by a proper combination of the following operating variables: concentration of the absorbing species, spectral distribution of the radiation source output power, and characteristic length of the radiation path. The work finally shows that the best selectivity and production of each one of the intermediate compounds may be obtained with a battery of several reactors of small equivalent diameters.
Chlorinated derivatives of methane have had a sustained economic importance for the chemical industry. A great number of patents in the field indicate the commercial importance of these processes. The reader can refer to Lowenheim and Morand (1975) for further information. Chlorination of methane or methyl chloride can be performed either in the vapor or liquid phase. Undoubtedly the reaction between methane or chloromethane derivatives and chlorine is a chain reaction. The activation step involves the formation of atomic chlorine and can be performed either thermally or photochemically. Earlier papers also studied the possibility of catalytic activation. A complete survey of the literature in the field can be found in Mc Ketta (1979) where more than 100 references are quoted. Both thermal and photochemical methods have been commercially practiced. For thermal activation, temperatures in the range of 400-450 "C are used to obtain an appreciable rate of reaction. The photochemical activation requires radiation in the range of 2500-4500 A. Thermal chlorination is used in large-scale production, but it tends to form impurities such as unsaturated compounds through the combination of chlorination, dehydrochlorination, and dechlorination. It requires a close temperature control to prevent pyrolysis which leads to carbon and tar deposition liberating a great amount of heat with the possibility of explosion. On the contrary, the photochemical method allows us to obtain a product free from impurities, but it seems economically feasible only for medium- or small-scale production (Hirschkind, 1949). Several papers have reported experimental data on the chlorination of methane and methyl chloride regarding the incidence of the reactant feed molar ratio and temperature on the molar product distribution for both activation methods (Mc Bee et al., 1942; Hirschkind, 1949; Lian and Ho, 1978). Optimal values of the operating conditions can be derived from these data, but they are restricted to the experimental conditions in which they were obtained. This is due to the fact that theoretical studies reported in the Research Assistant from CONICET. *Memberof CONICET's Scientificand Technological Research Staff and professor a t U.N.L. Instituto de Desarrollo Tecnoltgico para la Industria Qu-him. Universidad Nacional del-Litoral (U.N.L.) and Consejo Nacional de Investigaciones Cientificas y TBcnicas (CONICET).
*
literature do not consider the reaction mechanism involved; instead, they make use of phenomenological rate constants obtained by fitting experimental data for the overall reactions. These treatments do not seem appropriate for technological applications since they are valid only for a small range of process conditions. To our knowledge, only Kurtz (1972) used the complete reaction scheme for the thermal chlorination of methyl chloride to predict the molar product distribution in a tubular reactor. The resulting theoretical predictions were compared with experimental results, obtaining good agreement. This work reports the modeling of the gas-phase photochemical chlorination of methane in a continuous annular reactor by using the complete chain reaction scheme. The annular reactor was chosen because this type of reactor provides a practical system for medium-to-large-scale operations. Using a rigorous model for the radiation field and the reactor behavior, the reaction was simulated in order to investigate some simplifications in the kinetic sequence. I t was found that the exit concentration of stable species was unaffected when some of the termination steps were eliminated from the reaction network. Operating values of the mean residence time similarly found allowed the application of the micro-steady-state approximation for the highly reactive intermediate species. Then an optimization problem was posed. Since there is a large number of operating variables, a preliminary parametric study was accomplished. It was found that the most important variables are feed composition, radiation output power and its spectral distribution, mean residence time, and characteristic length of the radiation path. The results of the optimization problem indicated the existence of unique and very sensitive parameters in the performance of a photoreactor. At the same time, they provided significant conclusions about operating conditions for a process design where selectivity, production rates and low-energy consumption are the main targets.
The Reactions The mechanism of the reactions involves both chlorine atoms and organic free radicals as alternate chain carriers (Noyes and Leighton, 1941; Benson, 1960; Chiltz et al., 1963; Dainton and Ayscough, 1967). For the photochemical chlorination of methane the mechanism can be written as
0196-430518611 125-0601$01.5010 0 1986 American Chemical Society
602
Ind. Eng. Chem. Process Des. Dev., Vol. 25, No. 3, 1986
initiation step Cl2
@ea
2c1.
(M-1)
propagation steps CH4 + C1. s CH3* HC1 CH3. + C12 F! CHSCl+ C1. CH,Cl+ C1. F! CH2C1. HC1 CHzC1. + C12 e CH2C12 + C1. CHzCl2 + C1. F! CHClz. + HCl CHC12. Cl2 F! CHC13 + C1.
+
+
+ CHC13 + C1. F! CC13. + HC1 CC13. + Clz F! CC1, + C1.
--
homogeneous termination steps C1. + CH3. CH3C1 C1. + CH2C1. CH2C12 C1. CHC12. CHC1, c1-+ CCl,. cc1, C1. + C1. + M C12 + M CH,-CH3 CH3. + CH3. CH3-CH2Cl CH3. CH2Cl. CH3. CHCly CH3-CHCl2 CH3-CC13 CH3. + CCl,. CH2C1. CH2C1. CH2Cl-CH2Cl CH2Cl. CHC12. CH2Cl-CHC12 CH2C1. CC13. CH2C1-CC13 CHC12. + CHC12. CHC12-CHC12 CHC12. + CC13. CHClZ-CCl3 CCl,. + CC13. CC13-CC13 heterogeneous termination steps CH3. + wall products CH2C1. + wall products CHC12. + wall products CCl,. wall products C1- wall products
+
+ +
+
+ +
(M-4) 04-5) (M-6) 04-7) 04-8) (M-9)
------
(M-10) (M-11) (M-12) (M-13) (M-14) (M-15) (M-16) (M-17) (M-18) (M-19) (M-20) (M-21) (M-22) (M-23) (M-24)
----
(M-25) (M-26) (M-27) (M-28) (M-29)
--t
+
+ +
04-2) (M-3)
In reaction M - l , 3 is the primary quantum yield for the activation step and ea is the local volumetric rate of energy absorption (LVREA), which is a characteristic of the photochemical process. The accepted value of 0 for chlorination chain reactions is 1, independent of wavelength (Claril, 1984). Primary products (chloromethanes) result mainly from chain-propagation reactions while secondary products (chloroethanes) result from the free radical-free radical termination reactions. In photochemical chlorinations, byproducts such as unsaturated compounds are not formed (Hirschkind, 1959). Besides, they are free from carbon and tar deposition. In the homogeneous deactivation of chlorine (reaction M-14), a third body collision is necessary from energetic considerations. This third body may be an inert or some of the reactants in the mixture (Chiltz et al., 1963). A great deal of work done in the field has yielded the values of the kinetic parameters for each elementary reaction. Chiltz et al. (1963) compiled activation energy and frequency factor data measured in methane photochlorination ex-
periments. The values reported are a comprehensive review of those obtained by them and other authors in different laboratories. They correspond to each elementary reaction of the propagation steps and to the homogeneous termination reactions involving equal organic free radicals or an organic free radical and a chlorine atom. Experimental kinetic data for the homogeneous termination reactions involving different organic free radicals are not available in the literature. However, Kurtz (1972) estimated the kinetic constants for these reactions by using a collision frequency average method. The values obtained are of the expected order of magnitude. It has been experimentally shown that the wall of the reaction vessel and the impurities in the reaction mixture, such as oxygen, promote chain termination (Pease and Walz, 1931; Yuster and Reyerson, 1935; Cassano and Smith, 1966). The incidence of termination reactions on the wall may be influenced by the operating pressure, the geometric characteristics of the reactor, and the flow conditions. As may be expected, they are more influential at low pressures, at weak radiation energy densities, and in reactors where the surface-to-volumeratio is high. Turbulent flow and radial mixing may also increase their significance. For the wall termination reactions, the rate constants can be calculated by using the kinetic theory, with recombination coefficients taken from the literature (Noyes, 1951; Chiltz, et al., 1963). All the values used in this work are reported in Table I.
The Reactor Mass balance equations for each species have been written under the following assumptions: (a) negligible thermal effects, (b)steady state, (c) Newtonian fluid, (d) incompressible flow, (e) axial laminar flow, (f) azimuthal symmetry, (g) negligible axial diffusion when compared with the convective flux, and (h) diffusion coefficients calculated by Stefan-Maxwell relationships. The first one is plausible in the photochemical case due to the “equivalent”low overall activation energy. However, in spite of the mild effect that temperature has on photon-activated reactions, the energy balance could be needed to avoid overheating of the reaction mixture since the process is very exothermic. Hence, to preserve selectivity and avoid secondary thermal reactions, the reactor may need to incorporate provisions for cooling purposes. The species dimensionless mass balance equation may be written as 1
a*, 1 1 a Y a*, U ( Y ) - = - --[-1 at Ge Y ay Pei a?
+ ni
-CY
