Translation of MAT Kinetic Data to Model Industrial Catalytic Cracking Units

Transcription

1 Michoacan University of St Nicholas of Hidalgo From the SelectedWorks of Rafael Maya-Yescas May, 2004 Translation of MAT Kinetic Data to Model Industrial Catalytic Cracking Units Rafael Maya-Yescas Elizabeth León-Becerril Daniel Salazar-Sotelo Available at:

2 Translation of MAT Kinetic Data to Model Industrial Catalytic Cracking Units By Rafael Maya-Yescas*, Elizabeth León-Becerril, and Daniel Salazar-Sotelo Bench- and laboratory-scale reactors are required to infer kinetic data for catalytic cracking units. One of the most common methods is the microactivity test (MAT, ASTM D-3902±92), that emulates the catalyst-to-oil ratio using a fixed-bed reactor and a semibatch accumulator of liquids. Translation of data obtained from MAT tests in order to infer kinetic parameters to model continuous industrial units is, consequently, difficult and uncertain. In this work, the extraction of kinetic data obtained in a MAT reactor is analyzed. Estimation of a kinetic rate equation to evaluate instantaneous conversion in MAT reactors is performed. The activation energy obtained is kinetic and can be used during the modeling of riser reactors. It was possible, also, to infer values of the remaining catalytic activity after each experiment; these values were used to adjust a hyperbolic deactivation function, useful to model industrial riser reactors. 1 Introduction ± [*] Dr.-Ing. R. Maya-Yescas (rmaya@imp.mx), Dr.-Ing. E. León-Becerril, Dr.-Ing. D. Salazar-Sotelo, Chem. Eng. Programa de Tratamiento de Crudo Maya, Instituto Mexicano del Petróleo, Lµzaro Cµrdenas , MØxico, D.F. Mexico. Figure 1. Schematic of a laboratory-scale device. One of the most important processes in petroleum refining is the catalytic cracking of heavy hydrocarbons to produce aggregated value liquid fuels (mainly gasoline and liquid petroleum gas). Catalytic cracking reactions take place in risers, which are transported solid bed devices. These reactors are currently under research because of the complexity derived from hydrodynamic and kinetic factors involved. Moreover, in order to model these devices it is necessary to develop kinetic schemes; however, complex reaction paths, catalyst deactivation and nonisothermal behavior make difficult to propose accurate schemes. One alternative is to infer kinetic parameters using laboratory- or bench-scale reactors. One of the most common methods is the microactivity test (MAT, ASTM D-3902±92) device [1]. This unit works under isothermal conditions and it is possible to change injection times (t S ) and/or weight hourly space velocity (WHSV). Along experimental times, liquid products are received in a semibatch accumulator (see Fig. 1). This feature makes difficult the interpretation of results in order to model continuous industrial units, a topic that has been under discussion for a while [2±4]. On the other hand, it is important to note that expert' operators of MAT reactors are able to infer the responses of industrial cracking reactors. This situation is due to the fact that catalyst holdup, injection time and amount of feedstock were designed to emulate industrial C/O ratios. Therefore, there are some thumb rules that can be obtained by analyzing the transient behavior of the semibatch liquids accumulator of MAT laboratory devices. Moreover, when experiments are performed at constant WHSV and varying t S, there is a change in catalyst activity [4]. This change will affect the averaged results in the semibatch accumulator. In contrast, during kinetic parameter estimation, conversion and yield data have to be measured in stream m R (see Fig. 1), which leaves the reactor at final reaction conditions (instantaneous), instead of data from the semibatch accumulator that have values averaged over the time t s. In this work, a methodology to calculate instantaneous data at the reactor outlet is proposed; results are compared to the usual averaged ones. 2 Methodology 2.1 Experimental Estimation of standard conversions, both instantaneous and averaged, during laboratory experiments at three different t S and three different reaction temperatures (T rx ) were analyzed. One typical industrial feedstock and one commercial catalyst were used. The nine experiments were performed at the same WHSV = 16 (see Tab. 1) by triplicate. In all the MAT experiments, m 1 = g/s was used, as suggested by the method ASTM D 3907±92. Averaged data for the remaining nonreacted feedstock (y fsa ) and yield to coke specific to catalyst weight (x CSC ) were collected from the standard MAT data (see Tab. 1). Chem. Eng. Technol. 2004, 27, No. 7 DOI: /ceat WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 777

3 Table 1. Operating conditions and results at the MAT unit. Experiment T rx (C) t S (s) y fsa x CSC Mathematical Modeling Along t S, liquid products arrive to the accumulator changing the relative mass fraction distribution, because of the changing activity and selectivity of the catalyst inside the reactor [2, 4]. In order to extract the instantaneous composition of the liquids coming from the reactor, it is necessary to know the transient mass balance of the accumulator (M A )as a function of the flow coming from the reactor (m R ) and the flow of gases vented to the gas chromatograph (m Ag ) (Eq. (1)). dm A dt ˆ m R m Ag I:C: MA t ˆ 0 ˆ0 (1) The mass flow m R is related to the injection flow rate as (m R = m I ± m C ), where m C is the mass rate of coke generation that can be calculated from the corresponding MAT data. This is possible because coke will remain adsorbed to the catalyst surface. In order to calculate the standard conversion (v R =1±y fsr ), a mass balance was performed for the cyclic oils accumulated (Eq. (2)). dm fs dt ˆ y fsr m R I:C: Mfs t ˆ 0 ˆ0 (2) Here, (M fs =M A y fsa ). It should be noticed that the instantaneous mass balance in the accumulator depends on the mass fraction inside this accumulator (y fsa ), whereas the conversion reached in the reactor is evaluated using the mass fraction at its outlet (y fsr ). The goal is to evaluate y fsr using the data obtained during the laboratory evaluation, y fsa, at different t s. Once the results at the reactor outlet have been evaluated, it is possible to obtain kinetic rate parameters for the feedstock. Following the classic assumption, feedstock cracking exhibits a second-order reaction rate; the mass balance for the feedstock inside the fixed-bed reactor is shown in Eq. (3). u dy fsr dz ˆ k y2 fsr U I:C: y fsr z ˆ 0 ˆ1 (3) Here, z is the axial reactor coordinate, u is the gas velocity and U is the activity (or deactivation) function. For the used equilibrium catalyst, the initial MAT activity is U 0 = 0.70, mass fraction. The residence time inside the reactor is given by the ratio between gas velocity and axial coordinate. These results were used to evaluate the kinetic rate for feedstock conversion in the MAT reactor and the remaining catalyst activity after each of the nine experiments shown in Tab. 1. In this work, a hyperbolic activity function [2] was used (Eq. (4)). 8 < U 0 x CRC x CSC < x CSC min U ˆ U 0 x 1 a x CSC x CSC min CSC x (4) : CSC min Here, x CRC is the mass of coke adsorbed to the equilibrium catalyst surface specific to the mass of catalyst, x CSC is the instantaneous mass of coke adsorbed to the catalyst surface, a is an activity factor and x CSCmin is the minimum coke amount that provokes the catalyst to show deactivation. 3 Results and Discussion Following the standard MAT procedure [1], the yield to cyclic oils (y fsa ) was obtained for the nine experiments mentioned in Tab. 1. By integration on discrete intervals of the mass balances (Eqs. (1) and (2)), the instantaneous value for the yield to cyclic oils, y fsr, was calculated. Then, on the basis of y fsa and y fsr, instantaneous (v R ) and averaged (v A ) standard conversions were calculated (see Tab. 2). Numerical values for standard conversion differ depending on the data considered; moreover, if averaged data are used for kinetic parameter evaluation, an increasing estima- Table 2. Instantaneous and averaged standard conversions. Experiment t S (s) y fsr v R (wt.-%) v A (wt.-%) WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eng. Technol. 2004, 27, No. 7

4 tion error is introduced (see Fig. 2). Also, it is possible to note that t S has great effect on the difference between instantaneous and averaged values; the difference between values at 57 seconds exhibit lower slope than those measured at 75 seconds. Therefore, in order to solve the mass balances in the most accurate way, it is necessary to essay more experiments at different t S. 25 As it is possible to note, MAT data reflect averaged trends related to the C/O ratio, mainly. However, if the aim is to estimate kinetic parameters and the remaining catalytic activity, it is necessary to know instantaneous data from the reactor outlet stream, not from the semibatch accumulator, which are averaged. In order to obtain those instantaneous data, it is necessary to evaluate the mass balances for the accumulated liquids and for the desired yield. The same is true for gaseous products. Solving the model for the fixed-bed reactor (Eq. (3)), it is possible to evaluate the activation energy for feedstock conversion using an Arrhenius plot (see Fig. 3) as E A /R g = ( ± 1.33) K. Activation energy is a kinetic property of the system. It depends on the intrinsic kinetic scheme instead of reactor configuration. Since the Arrhenius plot shows almost parallel lines, it is possible to infer that the activation energy obtained is the intrinsic one. Now, knowing the value for the remaining catalytic activity for these experiments, it is possible to separate the intrinsic value for the frequency factor. Because of the cost of the experiments, this activity was determinated only for the experiments at 37 seconds. Then it was extrapolated to the other six experiments using the deactivation function described below. The best value for the frequency factor was k 0 = s ±1, numerical values for the remaining catalytic activity are shown in Tab. 3. χ A - χr, wt% 20 Table 3. Values of U for the cracking in a MAT reactor (mass fraction). T rx (C) t S (s) s 75 s Temperature, ºC Figure 2. Instantaneous minus averaged values for conversion. 37 s s s Once U values are known, it is possible to fit these data to the desired deactivation function (Eq. (4)); parameters obtained are a = g ±1 ck g cat and x CSCmin = g ck g ±1 cat. As it is possible to see, the function proposed fits experimental data very accurately (see Fig. 4). Now, the activation energy and the activities obtained evaluating Eq. (4) in terms of coke yield can be used in a model of a riser reactor. They are not influenced by the semibatch accumulation time in the MAT device. The difference will be in the absolute value of the k 0 factor, which could change in a different kind of reactor [5]. Since catalytic cracking leads multiple reactions, this approach is valid only to calculate the averaged feedstock cracking reaction rate, which is different to those considered for each individual product. ln k y = x y = x y = x ln k1 (37) ln k1 (57) ln k1 (75) / T, K -1 Figure 3. Arrhenius plot for feedstock conversion in the fixed bed. Predicted Φ, wt fraction s 57 s 75 s Observed Φ, wt fraction Figure 4. Predicted values for deactivation function (Eq. (4)). Chem. Eng. Technol. 2004, 27, No WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 779

5 4 Conclusions MAT [1] data were used to estimate instantaneous conversion at the reactor outlet in this experimental device. These data have to be collected at different t S in order to evaluate mass balances for the semibatch accumulator and the desired products. Numerical results for averaged and instantaneous evaluations are very different. In case of evaluation of kinetic parameters, instantaneous values must be used; otherwise, average results are enough to infer the effect of the C/O ratio. The difference among averaged and instantaneous values increases proportionally to t S. Therefore, in order to obtain accurate instantaneous results, it is necessary to perform as much as possible experiments at different and shorter t S. By using instantaneous data it was possible to infer kinetic parameters, frequency factors and activation energies, and also the remaining catalytic activity after each experiment. This activity was used to fit a hyperbolic deactivation function. These results are of kinetic nature and will preserve if used in a model for a different kind of reactor, an industrial riser, for example; the only parameter that will change is the frequency factor. Acknowledgements The authors gratefully acknowledge the economic support from the Research Program ªTratamiento de Crudo Mayaº at the Instituto Mexicano del Petróleo. They also thank Mr. RubØn Gonzµlez-Serrano for the realization of laboratory tests. References Received: November 7, 2003 [CET 1971] [1] Standard Method for Testing Fluid Catalytic Cracking (FCC) Catalysts by Microactivity Test (ASTM D-3907±92), Philadelphia, rev [2] G. F. Froment, K. B. Bischoff, Chem. Eng. Sci. 1962, 17, 105. [3] S. M. Jacob, B. Gross, S. E. Voltz, V. W. Weekman (jr.), AIChE J. 1978, 22, 701. [4] C. P. Kelkar, M. Xu, R. J. Madon, Ind. Eng. Chem. Res. 2003, 42, 426. [5] G. F. Froment, K. B. Bischoff, Chemical Reactor Analysis and Design, 2nd ed., John Wiley & Sons, New York WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eng. Technol. 2004, 27, No. 7