Effects of Ethane Partial Pressure on the Apparent Rate Expressions of Oxidative Couplin... Page 1 of 16

Size: px

Start display at page:

Download "Effects of Ethane Partial Pressure on the Apparent Rate Expressions of Oxidative Couplin... Page 1 of 16"

Edwina Freeman
5 years ago
Views:

1 Effects of Ethane Partial Pressure on the Apparent Rate Expressions of Oxidative Couplin... Page 1 of 16 Effects of Ethane Partial Pressure on the Apparent Rate Expressions of Oxidative Coupling of Methane over Na/BaTiO 3 Naser Seyed Matin,* and Reza Ahmadi National Iranian Oil Company(NIOC), Research Institute of Petroleum Industry (RIPI), Qom Road, P. O. Box , Tehran, Iran *Corresponding author: matinn@ripi.ir Abstract The empirical reaction rate expressions have been introduced for Oxidative Coupling of Methane (OCM) in the presence of titanate oxide catalyst. The conversion rate of C and also formation rate of C 2 were obtained on the basis of oxygen, methane and ethane partial pressures. All experiments were carried out at differential conditions. Results indicate that CH4 conversion and C 2 formation rates increase with ethane partial pressure. The selectivity of C 2 increases with C 2 partial pressure. The rate expressions agree well with experimental values and predict the observed trends. Keywords: oxidative coupling of methane, reaction rate, reaction mechanism, kinetic model Introduction The direct catalytic conversion of methane to higher hydrocarbons has been the subject of a number of investigations in recent years [1,2,3]. Despite many studies related to the mechanism of the OCM over various catalysts, however uncertainties concerning important features of the reaction still exist. However the generation of methyl radicals on the surface of catalyst is an important step in the OCM reaction. These radicals emanate into gas phase where most of the coupling occurs. It is widely believed

2 Effects of Ethane Partial Pressure on the Apparent Rate Expressions of Oxidative Couplin... Page 2 of 16 that the gateway to C 2 products in all methane coupling schemes is the homogeneous recombination of methyl radicals [4]. In spite of these molecular features, it is very important to obtain the applicable and amenable rate equations of the reaction for catalyst and reactor design. Generally two types of rate expressions can be established, i.e., empirical and/or mechanistic rate equation. In the mechanistic approach rate constants are related to the catalytic and molecular properties and transport phenomena [5,6,7]. Because of the uncertainty concerning the detail features of the reaction mechanism, there is the necessity of an empirical rate equation for the reactor design purposes is revealed. Stanch et al [8] used the combination of mechanistic and also empirical approach to propose a comprehensive kinetic model for OCM reaction. Their proposed rate expressions were based on C, O 2, H 2 O, CO 2, CO, C 2, C 2 and H 2 partial pressures. For this purpose they had to introduce all of the abovementioned gases in the feed stream with certain amounts to the reactor. However they used a ten step reaction scheme with a reaction rate equation for each step. Consequently their kinetic model is mainly an empirical rate equation with some molecular view. Al-Zahrin et al [9] presented the poisoning effects of CO 2 partial pressure on OCM reaction. Their kinetic model was derived via mechanistic approach, which was included CO 2 partial pressure. Xu et al [10] also considered the effect of CO 2 partial pressure on the methyl radical formation rate. Smith and Galuszka [11] investigated the kinetics of OCM over a Li/Pb/Ca catalyst in the presence of CO 2 in the feed gas. They observed the reduction in C conversion and increase in C 2 selectivity when CO 2 was added to the feed gas. Generally, it is well known that addition of CO 2 lowers the reaction rate of the OCM, although its quantitative effects depends on catalyst used. The effect of C 2 partial pressure in the OCM reaction is the subject of the present study. In our previous study [12] we obtained the mechanistic rate equations for C conversion and ethane formation rates as a function of oxygen and methane partial pressures over the same catalyst (titanate oxide). The power law rate expressions were also introduced in that work for C conversion and C 2, C 2, CO 2 and CO formation rates. In the present work it was to determine the apparent reaction rates affected by C 2 partial pressures with a little sense of molecular view. Then the empirical aspect prevail over mechanistic prospect. The Hougen-Watson type of reaction expressions, for gas-solid phase was considered. In order to take into account the ethane partial pressure in the rate laws ethane was introduced to the reactor in the feed streams. Because of comprising ethane partial pressure in the rate equations, the model equations are complicated. As it was mentioned, this project is not concerned with the molecular and/or mechanistic approach. Experimental Section The atmospheric pressure continuous flow apparatus and Barium Titanium Oxide as catalyst were used in these experiments. The details of experimental setup and procedures for preparation and characterization of the catalyst were described elsewhere [12]. In order to incorporate the ethane partial

3 Effects of Ethane Partial Pressure on the Apparent Rate Expressions of Oxidative Couplin... Page 3 of 16 pressure in the rate laws, the ethane was added to the feed gas. Experiments were carried out at 775, 800 and C at a total pressure of 1 atm. The usual catalyst charge was 0.03gr which had been diluted by 0.02gr quartz. In order to fit the kinetics regime, the mesh size catalyst with relatively high total flow rates were used in all measurements. The catalyst bulk density was 0.9 g/ml. The total flow rate was approximately 500 ml/min (STP). Methane (99.99%), Oxygen (99.996%) and Ethane (99.5%) partial pressures in the reactor were varied from 0.1 to 0.8 atm, 0.05 to 0.13 atm and to 0.08 atm respectively. Helium (99.999%) was used as a carrier gas to maintain the total pressure at 1 atm. Result and Discussion Table 1 shows the experimental results of feed conversion and products formation rates obtained from different runs. The experimental reaction rate was obtained as follows: R CH4 =X CH4 x CH4 F in /(WRT) (1a) Where X CH4, x CH4, F in, W, R and T are the methane conversion, methane mole fraction in the feed gas, total feed flow rate, catalyst weight, gas constant and temperature in Kelvin respectively. The experimental selectivity for products were calculated by, S C2H4 = (1b) As it can be seen in Figure 1 ethane selectivities decrease with temperature while for ethylene increasing trends are observed. The phenomena may be explained by gas phase reaction of methyl radical [6], C 2 (2a) CH 3 +CH 3 { C 2 H 5 +H (2b)

4 Effects of Ethane Partial Pressure on the Apparent Rate Expressions of Oxidative Couplin... Page 4 of 16 The route 2b is endothermic and has positive activation energy ( 11Kcal/mol). Increasing temperature resulting an increase in the second route 2b and then C 2 H 5 would be converted rapidly to C 2 by reaction with excess O 2 present, C 2 H 5 +O 2 C 2 +HO 2 (3) Above description attributes the C 2 formation to reactions in the gas phase. The decreasing trends of C 2 selectivity with temperature can be explained by competition effects between routes 2a and 2b. Consequently the main route for ethane and ethylene formation should be the gas-phase reactions, as it is mentioned in Ref. [4]. Based on the variations of experimental rates as a function of feed compositions, the following empirical rate expressions were introduced R CH4 = (4) R C2H4 = (5) The estimated model parameters have been given in the tables 1 to 4. Figures 2 and 3 show the predicted and experimental values of C conversion and C 2 formation rates as a function of C, O 2 and C 2 partial pressures. Figure 2a reveals that, the CH4 conversion rates increase with C 2 partial pressure. Figures 2b and 2c show the dependence of the consumption rate of C on the C and O 2 partial pressures, respectively. The proposed model shows a good agreement between experiment and results obtained from the model. As it can be seen in Figure 3a C 2 formation rates increase with C 2 partial pressure especially in high temperature region. Figures 3b and 3c display the C 2 formation rate as functions of C and O 2 partial pressures. It is obvious that the predictability of the model equations is good in the wide range of reactants concentration. The decreasing reaction rates with increasing of reactant i (e.g., C, O 2 or

5 Effects of Ethane Partial Pressure on the Apparent Rate Expressions of Oxidative Couplin... Page 5 of 16 C 2 ) partial pressures show kinetics with negative order with respect to that species. This is evident for some observed rates in the present work. This behavior was verified with inserting the partial pressure of species i in the denominator of rate expressions. It was noted that, the existence of a maximum in the reaction rate diagrams and a gradual shifting of this maximum location toward lower partial pressure values of reactant I implies the adsorption of component i on the catalyst surface [13, 14]. Figures 4a-4c show the model predicted selectivities compared with experimental data for C 2 at 800 o C and 825 o C. Since it is beyond the scope of this study to investigate the reaction mechanism, only the general rate laws were obtained. The proposed rate laws predict the overall trends of reaction in a reasonable manner. Conclusions The apparent rate laws for the OCM over Barium Titanium Oxide catalyst were introduced. The effects of C 2 partial pressure on the formation rates of C 2 were investigated. Generally the C 2 increases methane conversion and ethylene formation rates. It should be noted that, because of the low pellet sizes (mesh 40-50) and within the flow rates used in our experiments, it is not expected to observe the transport limitations in the studied reactions and the reactor was operating under differential conditions. Of course, the transport phenomena can also be taken into account in apparent reaction rate expressions for the definite pellet size, where a pilot plant is intended to be designed. Literature Cited 1. W.Y. Tung and Lance L. Lobban, I&EC, Res., 31(1992) M. Traykova, N. Davidova, J.-S. Tsaih and A. H. Weiss, Appl. Catal. A, 169(1998) D. E. Walsh, D. J. Martenak, S. Han, and R. E. Palermo, I&EC, Res., 31(1992) Y. Feng, J. Niiranen, and D. Gutman, J. Phy. Chem., 95(1991) E. Iwamatsu and K. Aika, J. Catal., 117(1989) Y. Feng, J. Niiranen, and D. Gutman, J. Phy. Chem., 95(1991)6564

6 Effects of Ethane Partial Pressure on the Apparent Rate Expressions of Oxidative Couplin... Page 6 of V. T. Amorebieta and A. J. Colussi, J. Phy. Chem., 92(1988) Z. Stanch, L. Mleczko, and M. Baerns, I&EC, Res., 36(1997) S. Al-Zahrani, Qi Song, and Lance L. Lobban, I&EC, Res., 33(1994) M. Xu, C. Shi, X. Yang, M. P. Rosynek, and J. H. Lunsford, J. Phy. Chem., 96(1992) K. J. smith and J. Galuszka, I&EC, Res., 33(1994) N. Seyed Matin, R. Ahmadi, Z. Shafeei, S. Sadraei, S. Zarrinpashneh, A. Mirkohkhah, and R. Behradi, NIOC-RIPI, Tech. Rep., Proj. No , 1999(Persian) 13. S. Bebelis, A. Zeritis, C. Tiropani, and S. G. Neophyytides, I&EC, Res., 39(2000) H. S. Fogler, Elements of Chemical Reaction Engineering, Prentice Hall Int. Ser. 3th. Edition, Ch. 10 Table (1)-Rate expressions exponents. i l i m i n i r i Table (2)-Kinetic parameters of rate expressions. 775 o C 800 o C 825 o C

7 Effects of Ethane Partial Pressure on the Apparent Rate Expressions of Oxidative Couplin... Page 7 of 16 k 11 [mol.min -1.gr -1.atm -(l +n ] E E E5 K 31 [mol.min -1.gr -1.atm -m 3 ] Table (3)- Kinetic parameters of rate expressions Temperature ( o C) K ij (atm -1 ) K E1 K E1 2.40E E1 K E E3 2.19E1 K E E K E E Table (4)- k=a e -Ea/RT, gr cat =0.03 k 11 k 31 Ln(A) E a (Kcal.mol -1.gr -1 cat )

8 Effects of Ethane Partial Pressure on the Apparent Rate Expressions of Oxidative Couplin... Page 8 of 16 Figure (1)

9 Effects of Ethane Partial Pressure on the Apparent Rate Expressions of Oxidative Couplin... Page 9 of 16 Figure (2a) Figure (2b)

10 Effects of Ethane Partial Pressure on the Apparent Rate Expressions of Oxidative Coup... Page 10 of 16 Figure (2c)

11 Effects of Ethane Partial Pressure on the Apparent Rate Expressions of Oxidative Coup... Page 11 of 16 Figure (3a)

12 Effects of Ethane Partial Pressure on the Apparent Rate Expressions of Oxidative Coup... Page 12 of 16 Figure (3b)

13 Effects of Ethane Partial Pressure on the Apparent Rate Expressions of Oxidative Coup... Page 13 of 16 Figure (3c)

14 Effects of Ethane Partial Pressure on the Apparent Rate Expressions of Oxidative Coup... Page 14 of 16 Figure (4a)

15 Effects of Ethane Partial Pressure on the Apparent Rate Expressions of Oxidative Coup... Page 15 of 16 Figure (4b)

16 Effects of Ethane Partial Pressure on the Apparent Rate Expressions of Oxidative Coup... Page 16 of 16 Figure (4c)

A First Course on Kinetics and Reaction Engineering Example 1.2

A First Course on Kinetics and Reaction Engineering Example 1.2 Example 1.2 Problem Purpose This example shows how to determine when it is permissible to choose a basis for your calculations. It also illustrates how to use reaction progress variables and the initial