Jorge Ancheyta-Juarez, Enrique Aguilar-Rodriguez
Instituto Mexicano del Petroleo
Mexico City
Instituto Mexicano del Petroleo has developed a model of the catalytic reforming process that accurately predicts reformate composition at the higher-severity conditions at which new reformers are being designed.
The new "AA" model is more accurate than previous proposals because it takes into account the effects of temperature and pressure on the rate constants of each chemical reaction.
BACKGROUND
Although naphtha reforming is a well known process, the evolution of catalyst formulation, as well as new trends in gasoline specifications, have led to rapid evolution of the process, including: reactor design, regeneration mode, and operating conditions.
New catalysts have in recent years evolved rapidly to attain higher severity, in terms of lower operating pressure. This change favors reactions that produce higher octane products.
Because of more stringent gasoline specifications designed to reduce the photochemical reactivity of the compounds that occur in reformed naphtha new reformer designs face a compromise between high quality and low reactive compounds content. This compromise must be resolved through the use of mathematical models that accurately represent reforming reactions and permit simulation of the reactor at different operating conditions and with different catalysts.
Mathematical modeling of the reforming process is an increasingly important tool. It is fundamental to the proper design of new reactors and revamp of existing ones. Modeling can be used to optimize operating conditions, analyze the effects of process variables, and enhance unit performance.
Catalytic reforming modeling is currently a major challenge. Engineers must cope with the demands of processing petrochemical naphtha with high BTX (benzene, toluene, and xylenes) content while meeting maximum specifications for photochemically reactive compounds (olefins, benzene, and other aromatics).
A number of kinetic models to represent catalytic reforming have been reported, based on different feedstocks, catalysts, and operating conditions.1 4
AA REFORMING MODEL
Catalytic reforming is carried out in a fixed bed reactor, in which a number of chemical reactions occur. Examples of these reactions include hydrocyclization, hydrocracking, aromatization, and others with an overall endothermic nature.
The new model is based on one by Krane, et al., which utilizes lumped mathematical representation of the reactions that take place.1 These representations are written in terms of isomers of the same nature (paraffins, naphthenes, or aromatics). These groups range from 1 to 10 carbon atoms for paraffins, and from 6 to 10 carbon atoms for naphthenes and aromatics.
The Krane model includes 53 chemical reactions that can be represented as:
- Paraffins
Pn - Nn
Pn- Pn i + Pi
- Naphthenes
Nn An
Nn - Nn i + Pi
Nn - Pn
- Aromatics
An An i + Pi
An Pn
An Nn
where:
P = paraffins, N = naphthenes, A = aromatics, n = number of carbon atoms, and 1
These kinetic equations permit the calculation of conversion for each of the C1 C10 paraffins, C6 C10 naphthenes, and C6 C10 aromatics. The conversion equations are represented by first order kinetic differential equations, Equations 1 3 (Equations).
The subscripts in Equations 1 3 may take the following values: 1
TEMPERATURE CORRECTION
The Krane model satisfactorily describes the reforming process, although its only serious limitation is that it does not include the influence of temperature on the reaction. In other words, the model is limited to the representation of isothermal operation at some point within the experimental temperature range in which Krane fit the parameters (800 9600 F., or 700 7880 K.).
To overcome this limitation, the AA model includes an Arrhenius type variation of the rate constants, which can be expressed by Equation 4. The activation energy values, EAk, for all reactions were taken from those reported by Henningen, et al.5
PRESSURE CORRECTION
Another limitation of the Krane model is that experimental data do not include variations in operating pressure. The model, therefore, is valid only at the base pressure (300 psig).
It is well known that pressure affects the equilibrium conversion of reforming reactions in which a change of volume occurs as a result of the chemical reaction. The AA model, therefore, includes a factor that accounts for the pressure effect on the rate constant.
Krane proposed Equation 6 for variations of the rate constant as a function of operating pressure. Because the pressure effect is different for each type of reaction, the AA model includes the values of ak reported by Jenkins, et al.6
PILOT PLANT TESTS
The AA model was validated using experimental data obtained in a pilot plant described by Schacht, et al.7 The pilot test utilized naphtha with composition as shown in Table 1.
Test runs were performed at 750, 766, and 7870 K. The H2 to hydrocarbon molar ratio was 9, reactor pressure was 500 psig, and the catalyst used was developed in-house.
Results of these tests are shown in Tables 2, 3, and 4.
MODEL VALIDATION
The Krane and AA models were compared with the experimental data. The Krane model considers the values of the kinetic rate constants to be independent of operating pressure and temperature (Equation 6).
Parameters were fitted with the experimental data for three cases:
- AA1 The AA model considering only the effect of pressure on the rate constants (Equation 7).
- AA2 The AA model considering only the effect of temperature on the rate constants (Equation 8).
- AA3 The AA model considering the effects of pressure and temperature on the rate constants (Equation 9).
A more detailed description of the fitting process has been published.8
Tables 2, 3, and 4 show the predictions obtained from the Krane and AA models at, respectively, 750 766, and 7870 K., in terms of PNA composition. Experimental data are also included for comparison.
MODEL COMPARISON
The results shown in Tables 2, 3, and 4 confirm that the Krane model is useful when used for process conditions of about 300 psig and 7660 K. Major deviations occur if the model is used at operating conditions that vary much from these values. The Krane model also produces poor estimates at higher operating temperatures, mainly for paraffins content.
The introduction of the pressure correction only (AAI) does not provide much better results than Krane. Reforming reactions are, therefore, more sensitive to the temperature effect. But the effect of temperature only in the new model (AA2) does not provide enough accuracy.
The inclusion of the effects of both temperature and pressure is necessary to obtain a model that substantially improves the prediction of the concentrations of individual species (Model AA3). Model AA3 produces only minor deviations, as shown in Tables 2, 3, and 4.
The concentration profiles of C6 and C10 paraffins, naphthenes, and aromatics, as predicted by Model AA3, are shown in Figs. 1, 2, and 3.
Analysis of the results show that the AA3 model (Equation 9) predicts the behavior of the catalytic reforming process at the higher severity conditions at which new units are now being designed.
REFERENCES
- Krane, H.G., Groh, A.B., Shulman, B.L., and Sinfelt, J.H., "Reactions in Catalytic Reforming of Naphthas," 5th World Petroleum Congress, Vol. 3, 1960, p. 39.
- Smith, J.M., "Kinetics of Analysis of Naphtha Reforming with Platinum Catalyst," Chem. Eng. Prog., Vol. 55, No. 6, 1959, p. 76.
- Ramage, M.P., Graziani, K,R., and Krambeck, F.J., "Development of Mobil's Kinetic Reforming Model," Chem. Eng. Sci., Vol. 35, 1980, pp. 41 48.
- Marin, G.B., and Froment, G.F., "Reforming of C6 Hydrocarbons on Pt A1203," Chem. Eng. Sci., Vol. 37, No. 5, 1982, pp. 759 73.
- Henningen, H., and Bundgaard Nielson, M., "Catalytic Reforming, British Chem. Eng., Vol. 15, No. 11, 1970.
- Jenkins, J.H., and Stephens, T.W., "Kinetics of Cat Reforming," Hyd. Proc., November 1980.
- Schacht, H.P., Ortega, G.F., and Flores, A.E., "Estudio de reduccion de benceno en las gasolinas de reformacion," Rev. del Inst. Mex. del Pet., Vol. 23, No. 1, 1991, p. 69.
- Ancheyta, J.J., "Modelo Matematico para la Simulacion del proceso de Reformacion Catalitica de Naftas," MSc thesis, Esique National Polytechnic Institute of Mexico (IPN), 1993.
