xúc tác reforming
13 Catalytic Reforming The catalytic reforming process consists of a number of reactions which take place on bifunctional catalysts for converting the hydrocarbons contained in naphtha fractions to monocyclic aromatics. Naphthenes with six carbon atom rings are subjected to dehydrogenation. Naphthenes with five carbon atom rings are subjected to isomerization followed by dehydrogenation, usually called dehydroisomerization. The alkanes go through cyclization followed by dehydrogenation, usually called dehydrocyclization. Simultaneously, the hydrocarbons and especially the alkanes undergo parallel, com- peting reactions of isomerization and hydrocracking with conversions sometimes comparable to the reactions producing aromatics. There are two ways in which catalytic reforming may be used. One option is to process the heavy fractions of straight run naphthas in order to increase their octane rating by 40–50 units. The other way is to process a narrow fraction of gasoline such as C 6 -C 8 or C 7 -C 8 . From the obtained reformate (called in this case BTX) are then separated the aromatic hydrocarbons (mainly benzene, toluene and xylenes), for the petrochemical industry. This second process is also called aromatization. Both processing options are performed in the same units, working under simi- lar operating conditions. The presentation that follows will be referring to both options at the same time. The final part of the chapter (Section 13.9) will present the catalytic processes used for converting hydrocarbons obtained from the aromatization, in order to increase the production of those hydrocarbons that present a higher interest for the petrochemical industry: hydrodisproportionation or dealkylation of toluene and isomerization of xylenes. In contrast with this classical image of catalytic reforming, a new process has been developed , the feed of which is the propane-butane fraction. An extra step, First time communicated at the ‘‘American Institute of Chemical Engineers Summer National Meeting’’, Denver, Colorado 21–24 August 1988 [1]. Copyright © 2003 by Taylor & Francis Group, LLC dehydropolymerization, is added before cyclization and aromatization. This new process will be presented at the end of the chapter, but it is too early to estimate its impact on global processing. 13.1 THERMODYNAMICS As mentioned above, catalytic reforming consists of reactions of dehydrogenation, dehydroisomerization, and dehydrocyclization leading to the formation of aromatic hydrocarbons. The thermodynamics of other concurrent reactions, mainly involving alkanes, of isomerization and hydrocracking were examined in previous chapters. Specific to catalytic reforming processes is the introduction of hydrogen together with hydrocarbon feed into the reactor system. Molecular ratios of 2 to 5 hydrogen/hydrocarbons are used in order to decrease the rate of coking of the catalyst. Since hydrogen is a product of the aromatization reactions, its presence in the feed to the reactor displaces the thermodynamic equilibrium of the reactions. This effect is accounted for in the following calculations and discussions. The equilibrium calculations were performed for pressures between 2 and 40 bar in order to compare both older processes working at pressures of around 20–30 bar and newer processes working at much lower pressures. 13.1.1 Dehydrogenation of Cyclohexanes The variations in heat of formation and entropy for the most important reactions of catalytic reforming were calculated based on the thermodynamic constants published by Stull et al. [2] (see Table 13.1). Using the calculation method developed by us and presented in Section.1.2, the equilibrium of dehydrogenation of the cyclohexanes is shown in Figure 13.1. The hydrocarbons having similar values for the equilibrium conversion are grouped together. The data of Figure 13.1 refer to stoichiometric conditions. However, if an excess of hydrogen is present in the system, further calculations will be necessary. The equilibrium constant for the dehydrogenation of cycloalkanes is given by: K p ¼ xð3xÞ 3 1 x p 1 þ3x 3 ð13:1Þ where x is the conversion at equilibrium. For an excess of n moles of hydrogen per mole of cycloalkane, the equilibrium constant becomes: K p ¼ x 0 n þ3x 0 3 1 x 0 p 1 þ3x 0 þ n 3 ð13:2 where x 0 is the equilibrium conversion in the presence of the hydrogen excess. So far this process is known by its trade names, such as ‘‘Cyclar’’ of BP-UOP or ‘‘Aroforming’’ of IFP. Names such as ‘‘dehydropolymerization of lower alkanes’’ and ‘‘catalytic poly-reforming’’ are our suggestions. Copyright © 2003 by Taylor & Francis Group, LLC Since at any given temperature the two equilibrium constants are equal, Eq. (13.1) and (13.2) can be equalized and, denoting x 0 ¼ ax ð13:3Þ gives xð3xÞ 3 1 x 1 1 þ3x 3 ¼ axðn þ3axÞ 3 1 ax 1 1 þ3ax þn 3 Finally, this expression becomes: 1 3ax þn þ 1 3 ¼ 1 3x þ 1 3 að1 xÞ 1 ax ð13:4Þ Equation (13.4) allows one to calculate the correlation between the values of a and n for any value of the conversion x. As a result, the variation of a vs. n may be plotted for various stoichiometric conditions x, as shown in Figure 13.2. Figure 13.2 gives the value of a, for the equilibrium conversion x in stoichio- metric conditions, (as read from the plots of Figure 13.1), at a given temperature and Table 13.1 Thermodynamic Data for Cyclohexanes Dehydrogenation Reaction (ÁH 0 800 Þ r , kcal/mol (ÁS 0 800 Þ r , cal/mol Á grd 52.7 94.744 51.75 94.894 50.86 94.824 50.57 95.254 50.14 95.324 cis 48.39 95.184 trans 49.95 95.404 cis 50.55 96.424 trans 48.76 95.304 cis 48.79 95.114 trans 50.37 95.894 Source: Ref. 2. Copyright © 2003 by Taylor & Francis Group, LLC Figure 13.1 The thermodynamic equilibrium of dehydrogenation of cyclohexanes in stoichiometric conditions. 4. C 8 alkylcyclohexanes $ xylenes þ 3H 2 ,C 9 -C 11 alkylcyclo- hexanes$ alkylbenzenes þ 3H 2 . Figure 13.2 The influence of hydrogen excess upon the equilibrium of dehydrogenation and dehydroisomerization of cycloalkanes. Copyright © 2003 by Taylor & Francis Group, LLC pressure and an excess of n moles of hydrogen. The equilibrium conversion in the presence of an excess of n moles of hydrogen is then calculated using Eq. (13.3). Example. Calculate the cyclohexane-benzene equilibrium at 468 C, 33 bar, and an excess of 5 mol hydrogen per mol feed. ANSWER: In stoichiometric condition, Figure 13.1 gives x ¼ 0:90. For x ¼ 0:90 and n ¼ 5 , Figure 13.2 gives a ¼ 0:93. According to Eq. (13.3), the equilibrium conversion with excess of hydrogen, x 0 , will be: x 0 ¼ a x ¼ 0:93 0:90 ¼ 0:837 13.1.2 Dehydroisomerization of Alkylcyclopentanes Variations of the heats of formation and of the reaction entropies for dehydroiso- merization of some representative alkylcyclopentanes were calculated in a similar manner (see Table 13.2). Using the same methodology as in the case of the cyclohexa nes, the equilibrium for stoichiometric conditions is shown in Figure 13.3. Due to the fact that the stoichiometry of the reaction is identical to that for cyclohexanes, the expression in Eq. (13.4) and consequently the graph in Figure 13.2 for alkylcyclopentanes are Table 13.2 Thermodynamic Data for Dehydro-isomerization of Alkylcyclopentanes Reaction ðÁH 0 800 Þ r , kcal/mol (ÁS 0 800 Þ r , cal/mol Á deg 49.24 85.414 cis 46.16 90.684 trans 48.18 90.524 cis 47.98 90.524 trans 47.44 90.524 46.20 88.344 46.39 88.634 46.02 89.034 45.78 86.994 Source: Ref. 2. Copyright © 2003 by Taylor & Francis Group, LLC Figure 13.3 Thermodynamic equilibrium of dehydroisomerization of alkylcyclopentanes in stoichiometric conditions: also the same. Thus, the equilibrium in the presence of hydrogen excess can be calculated in a similar way. 13.1.3 Dehydrocyclization of Alkanes Variations in the heat of formation and in the entropy for the reaction of dehydro- cyclization were calculated for several typical hydrocarbons using the same source of thermodynamic data [2]. The results are given in Table 13.3. In the calculations for the conversion of octanes, only the methylheptanes were taken into account, since dimethylhexanes are found in much smaller quantities in the straight-run gasoline (see Section 5.1.4). The equilibrium conversion for dehydrocyclization under stoichiometric con- ditions is plotted in Figure 13.4. Following the same logic used in obtaining Eq. (13.4), the equivalent expres- sion for dehydrocyclization is: 1 4ax þn þ 1 4 ¼ 1 4x þ 1 4 að1 xÞ 1 ax ð13:5Þ Based on the results of Eq. (13.5), Figure 13.5 allows the calculation of the effect of the hydrogen excess on the thermodynamic equilibrium of the dehydro- cyclization reactions, where a was defined by the same Eq. (13.3). Copyright © 2003 by Taylor & Francis Group, LLC 13.1.4 Isomerization and Hydrocracking Isomerization and hydrocracking reactions compete with aromatization. Consequently, the degree of influence depends on their relative rates. The rate of aromatization of alkanes is the slowest. Therefore their overall conversion to aromatics is influenced to a large extent by the reactions of isomeriza- tion and hydrocracking. The rate of aromatization of the alkylcyclopentanes is much higher. Therefore they are less affected by such reactions. Finally, for alkylcyclohex- anes, the rate of aromatization is very fast, so that the reactions of isomerization and hydrocracking may be completely ignored. The isomerization of alkanes has been examined in Chapter 11. The composition of various heptane isomers may be taken as representative of the higher alkanes. According to Figure 11.4, at the temperatures Table 13.3 Thermodynamic Data for the Dehydrocyclization of Alkanes Reaction ðÁH 0 800 Þ r , kcal/mol ðÁS 0 800 Þ r , cal=mol Á deg 63.77 105.022 60.85 107.872 62.56 109.762 61.93 108.772 60.96 108.192 58.31 106.342 59.24 109.522 61.96 109.122 59.31 107.272 58.66 106.142 58.53 108.412 60.68 108.592 Source: Ref. 2. Copyright © 2003 by Taylor & Francis Group, LLC practiced in catalytic reforming, the equilibrium corresponds to 14% n-heptane, 40% 2- and 3-methylheptanes, the rest being dimethylpentanes and trimethylbu- tanes. In conclusion, isomerization diverts a high proportion of alkanes towards molecular structures that can no longer undergo dehydrocyclization. Furthermore, the reactions of hydrocracking, undergone mainly by the iso- alkanes, lower the conversion to aromatics even more. The thermodynamic equilibrium of the overall reactions is influenced also by the fact that the hydrocracking reactions are highly exothermic while the reactions of aromatization are all highly endothermic. As a result, the hydrocracking reactions lead to an increase of the outlet temperature and therefore an increase in the con- version to aromatic hydrocarbons, especially in the last reactor of the unit. The isomerization equilibrium of the alkylaromatic hydrocarbons produced in the process becomes important, especially for the xylenes, the proportions of which almost always correspond to the thermodynamic equilibrium of their isomerization. Since the equilibrium composition corresponds to approx. 20% ortho-, 20% para-, and 60% meta-xylene, the isomerization of meta-xylene becomes an important com- Figure 13.4 The thermodynamic equilibrium for dehydrocyclization of alkanes in stoichio- metric conditions. Copyright © 2003 by Taylor & Francis Group, LLC mercial process. It will be presented separately, at the end of this chapter (Section 13.9.5). 13.1.5 Conclusions The calculations and results obtained in the previous sections lead to the following conclusions concerning the thermodynamic limitations to the process of catalytic reforming. The temperatures do not change significantly with the process type and range between 410 C and 420 C for the exit from the first react or and about 480–500 C for the exit from the last reactor. In contrast, the pressure in the system varies more significantly. Older units were operated at a pressure between 18 and 30 bar while the units built after 1980 operate at pressures between 3.5 and 7 bar. The molar ratio hydrogen/hydrocarbon varies between 8 and 10 for the older units and was lowered to 2 to 5 for the more modern ones. Considering an effluent temperature of 480 C from the last reactor of the unit and a molar ratio hydrogen/hydrocarbon of 4, the equilibrium conversions were calculated comparatively for two levels of pressure, 20 bar and 2.5 bar. The follow- ing conclusions may be drawn: 1. The equilibrium conversions of cyclohexane and alkylcyclohexanes will be higher than 99%, both at lower pressures of 3.5–7.8 bar and at higher pressures of 20 bar. The conversion will drop to 90% for cyclohexane and to 95% for methylcyclohexane at pressures of 30–35 bar. 2. The equilibrium conversions of alkylcyclopentanes are somewhat less favorable. The equilibrium conversion of methylcyclopentane at a pressure of 20 bar reaches only 86% but increases to over 99% at pressures of 3.5–7 bar. The equilibrium conversion of higher alkylcyclopentanes will be almost quantitative at both low pressures and pressures of around 20 bar. 3. The equilibrium conversions of alkanes to aromatics are the least favored. For n-hexane, the conversion will reach only 30% at a pressure of 20 bar, decreasing to 10% at a pressure of 30 bar. The equilibrium conversion of n-hexane would reach 85–90% only at lower pressures of 3.5–7 bar. The conversion of heptanes and octanes to ethylbenzene will correspond to about 85% at a pressure of 20 bar, becoming almost complete at pressures of 3.5–7 bar. The conversions to xylenes and higher hydrocarbons will reach 85% at a pressure of 40 bar, increasing to 92–95% at 20 bar and becoming almost complete in low pressure processes. It should be noted here that the graphs presented in Figures 13.1–13.5 allow an estimation of the thermodynamic limits to conversion, at the exits of any of the three (or four) reactors of the commercial catalytic reforming units. 13.2 THE CATALYSTS The first catalyst used for industrial catalytic reforming consisted of 9% molyb- denum oxide on alumin a gel. The plant started in 1940 unde r the name of ‘‘Hydroforming.’’ Copyright © 2003 by Taylor & Francis Group, LLC The use of platinum instead of molybdenum oxide was patented by V Haensel of UOP in 1949 and the first plant using such a catalyst started working in the same year under the name of ‘‘Platforming.’’ The overwhelming advantages of platinum when compared to molybdenum oxide led to a complete replacement of the latter. The catalyst of platinum on -alumina support underwent various improve- ments, both with respect to the support and by promoting the platinum with other metals such as iridium, palladium, tin, and rhenium. Catalytic reforming catalysts patented by different manufacturers were reviewed by Aalund [3] and presented in the monograph of Little [4]. The dual function character of the catalysts for catalytic reforming is provided by the acid centers of the support, which catalyze the reactions of isomerization and hydrocracking, as well as by the metallic centers—platinum associated with other metals dispersed on the support—which catalyze the dehydrogenation reactions. A more detailed analysis of the reaction mechanisms (see Section 13.3) has to consider the formation of complex metal–acid centers. In order to achieve maximum effi- ciency for the process, a balance must be found between the acidic and dehydrogen- ating functions of the catalyst. The most frequently used support is -alumina (-Al 2 O 3 ), and its appropriate acidic level is achieved through a treatment with HCl or sometimes wi th HF. Hydrochloric acid is prefer red, because the final acidity is easier to control. Sometimes CCl 4 or organic chlorides are used instead of HCl. In the catalytic reforming process, traces of water tend to eliminate the hydro- chloric acid fixed on the support; therefore a certain amount of hydrochloric acid is Figure 13.5 The effect of the hydrogen excess on the dehydrocyclization of alkanes. Copyright © 2003 by Taylor & Francis Group, LLC [...]... equations The developed reaction models were shown to be satisfactory for reactor design and for optimization purposes The kinetics of catalytic reforming was treated as a heterogeneous catalytic process in 1967 by Raseev and Ionescu [51] within a study of the catalytic reforming of C6 -hydrocarbons in a bench unit with a plug flow, isothermal reactor connected to a gas chromatograph The reactions considered... was one of the first calculation methods, it is still used today for model development The models are used to improve the performance of commercial catalytic reforming units [41,42] The approaches of Smith and Krane do not take into account that the catalytic reforming process takes place in adiabatic conditions Therefore, the equations have to be solved simultaneously with those describing the evolution... Figure 13.13 Continuous bench unit for the study of catalytic reforming corresponds to only 0.084 cyclohexane Furthermore, the equilibrium of dehydrogenation of cyclohexane to benzene is less favorable than for the higher homologs (see Figure 13.1) These two effects explain why it is difficult to obtain high yields of benzene when subjecting to catalytic reforming naphthas rich in alkanes The type of ring... conditions are more severe than in classic reforming, the catalyst requires continuous regeneration, either by continuously passing the catalyst through a regenerator (Cyclar) [33,34], or via the cyclic operation of the reactors (Aroforming) [35] Details on the commercial implementation of these processes are given in Section 13.10 13.4 THE KINETICS OF CATALYTIC REFORMING 13.4.1 The Influence of Diffusion... preparation Additional details concerning the structure, characterization and testing of the reforming catalysts are given in the monographs edited by Antos et al [189–191] Copyright © 2003 by Taylor & Francis Group, LLC Figure 13.11 13.3 The effect of promoters on platinum efficiency (From Ref 12.) REACTION MECHANISMS Catalytic reforming was shown to consist of a number of reactions catalyzed by the two functions... to include zeolites in the preparation of the supports for catalytic reforming catalysts However, the multiple possibilities offered by zeolytes and the advances in their technology may increase their use in preparing more efficient supports for bifunctional catalysts Currently, -alumina is used almost exclusively as support for commercial reforming catalysts Their preparation involves strict, proprietary... of various metals added to platinum is not clear Existing publications tend to justify their presence mainly by their Copyright © 2003 by Taylor & Francis Group, LLC Table 13.4 Composition of Some Catalytic Reforming Catalysts, Patented in the U.S Patent Components Platinum Rhenium Chloride Fluoride Patent U.S pat 2,752,289 Typical Range example 0.01–1 — — 0.1–3 0.3 — 0.45 0.3 Components U.S pat 4,312,788... cyclohexane suggests that methylcyclopentane appears from the isomerization of cyclohexane, rather than by cyclization of n-hexane At the beginning of this chapter we suggested the names ‘ catalytic polyforming’’ and ‘ catalytic dehydropolyaromatization’’ for the process with commercial Copyright © 2003 by Taylor & Francis Group, LLC Figure 13.15 The conversion of n-heptane to C6 -hydrocarbons on a Pt/Re... catalysts such as RG422 and RG423, as well as polymetallic catalysts with platinum, iridium, and rhenium There are several ways of expressing the behavior in time, i.e., the performance stability of a catalytic reforming catalyst Usually, the stability is expressed by the decrease in time of the octane rating of the reformate Sometimes it is expressed by the increase of temperature required for maintaining... diameter 1.5 m (From Ref 4.) shape and size of the catalyst granules are determined as a function of the specific process operating conditions 13.4.2 The Reaction Kinetics The first kinetic models for catalytic reforming were proposed by Smith [39] and by Krane et al [40] and were reviewed by Raseev et al [23–25] Smith [39] expressed the rate of reaction by the equations: r¼ dNi dVR ð13:6Þ where Ni is the . 13 Catalytic Reforming The catalytic reforming process consists of a number of reactions which take place on bifunctional. of the three (or four) reactors of the commercial catalytic reforming units. 13.2 THE CATALYSTS The first catalyst used for industrial catalytic reforming consisted of 9% molyb- denum oxide on alumin. rhenium. Catalytic reforming catalysts patented by different manufacturers were reviewed by Aalund [3] and presented in the monograph of Little [4]. The dual function character of the catalysts for catalytic