Development of a Mechanistic Kinetic Model of the Higher Alcohol

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Development of a Mechanistic Kinetic Model of the Higher Alcohol Synthesis over a Cs-Doped Zn/Cr/O Catalyst. 1. Model Derivation and Data Fitting Alessandra Beretta, Enrico Tronconi, Pio Forzatti,* and Italo Pasquon Dipartimento di Chimica Industriale e Ingegneria Chimica del Politecnico, Piazza Leonardo da Vinci 32, 20133 Milano, Italy

Emilio Micheli, Lorenzo Tagliabue, and Gian Battista Antonelli SNAMPROGETTI SpA, Via Maritano 26, 20097 San Donato Milanese (MI), Italy

A kinetic model of the synthesis of higher alcohols from CO/H2 mixtures (HAS) has been developed. The model is grounded on the chemistry of the synthesis, and its structure closely reflects previous mechanistic findings in HAS over modified methanol synthesis catalysts. It provides a detailed description of the product distribution, including CO2, methanol, primary and secondary alcohols up to C8, the corresponding aldehydes and ketones, methane, and higher hydrocarbons. Isothermal kinetic data over a Cs-promoted Zn/Cr/O catalyst which represent the effects of space velocity, reaction pressure, and feed composition (H2/CO ratio, CO2 feed content) are successfully fitted using a single set of chemically consistent estimates of the rate parameters. The model is further tested against chemical enrichment experiments in part 2 of the present work. Introduction The traditional interest in the higher alcohol synthesis (HAS) from synthesis gas (Natta et al., 1957) has been focused in the last few years on the accomplishment of high selectivities to branched alcohols. They offer good octane properties that can be exploited for direct gasoline blending (Zhou, 1994). Moreover, isobutanol and 2-methylbutanol have been proposed as precursors for the synthesis of ethers like MTBE and TAME. MTBE is presently the additive of choice for reformulated gasolines (Sanfilippo, 1993), and its demand has recently undergone an impressive growth. Among the known catalysts for higher alcohol synthesis (Forzatti et al., 1991; Hindermann et al., 1993), modified Fischer-Tropsch catalysts, which promote CO insertion mechanisms, are ruled out because they lead to the formation of linear alcohols only. On the contrary, those catalysts which promote condensation mechanisms resulting in the formation of branched oxygenates are presently the object of development and investigation. In this respect, along with the copperbased catalysts active in HAS at lower reaction temperatures and pressures (573-598 K, 7.0-7.5 MPa) (Nunan et al., 1989a,b; Hindermann et al., 1993; Boz et al., 1994) and alternative catalysts claimed to offer high productivities of isobutanol under pressures as high as 25 MPa (Keim and Falter, 1989), the so-called high-temperature modified methanol catalysts are wellknown to lead to a preferential production of 2-methyl primary alcohols as compared to linear alcohols. These catalytic systems also provide product mixtures with high C2+ alcohols/methanol molar ratios, due to the thermodynamic limitations on the methanol synthesis at typical reaction temperatures and pressures (about 673 K and 8.5 MPa) (Forzatti et al., 1991). The chemical routes active over the high-temperature catalysts (K- or Cs-doped Zn/Cr/O systems) and the roles * Author to whom correspondence is addressed. Fax: (+39)2-7063-8173.

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played by a large number of reaction intermediates have been previously studied (Lietti et al., 1988, 1990, 1991a, 1992). In this work, the development of a detailed kinetic treatment of HAS over a 15 w/w % Cs2O Zn/ Cr/O catalyst is addressed in view of its application to the design of industrial processes for isobutanol production. In the past, kinetic models have been proposed for the synthesis of oxygenates over both low-temperature (Smith and Anderson, 1983; Smith et al., 1990; Smith et al., 1991) and high-temperature (Tronconi et al., 1992) modified methanol catalysts. Such works share the adoption of a mechanistic approach as the most suitable and reliable one in relation to the complexity of the reacting system as well as the goal of validating it by reproducing the product mixture distribution for assigned values of the operating conditions. Recently, Breman et al. (1994) have presented an extended kinetic model of HAS over Cs/Cu/ZnO/Al2O3 which improves upon Smith’s model by introducing a wider spectrum of oxygenated products and kinetics for the formation of hydrocarbons. Still, each set of experimental conditions is associated with a specific set of fitting parameters; in this way, prediction of the influence of the operating variables on the product distribution could be attained through empirical dependences of the parameter estimates on the variable settings. More flexible kinetics of HAS globally applicable over ranges of operating conditions have been published, too (Tronconi et al., 1989; Calverley and Smith, 1992). However, they use a simplified approach consisting of modeling the total production rate of higher alcohols, regarded as a single pseudocomponent. No kinetic analysis of HAS has been published so far that can predict the whole distribution of products (both oxygenates and hydrocarbons) as a continuous function of the operating variables using a single set of parameters, as required for design purposes. In part 1 of this paper, a comprehensive description of the reacting system and of its dependence on the major process variables (pressure, space velocity, feed © 1996 American Chemical Society

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composition) is developed. A review of the mechanistic research which had been already incorporated in the treatment of Tronconi et al. (1992) and is also at the basis of the present model is first given. Then, significant changes with respect to the original treatment which improve the model affinity to the real chemistry of HAS are discussed. Finally, the model fitting to data obtained under typical synthesis conditions is addressed. In part 2, the chemical consistency of the model is tested to a more stringent degree by applying it to the simulation of chemical enrichment experiments. The technique of perturbing the reacting system by adding reaction intermediates to the feed stream has been already proven in the HAS literature to provide an effective insight into the chain-growth mechanism (Smith and Anderson, 1984; Vedage et al., 1985; Riva et al., 1987; Kiennemann et al., 1991). However, no comparison between experimental and calculated effects of the perturbations has ever been reported to our knowledge. On the other hand, this kind of validation is expected to be an informative tool to verify the adequacy of the present model in reproducing the relative rates of HAS chemical routes. It can also provide guidelines for further improvement of the kinetic model. The final revision of the model, as well as the effect of the reaction temperature, will be addressed in a forthcoming paper. Identification of the Reaction Network The hydrogenation of CO over alkali-doped ZnO/Cr2O3 catalysts leads to the formation of a large number of reaction products with different chemical nature, molecular structures, and carbon atom numbers (Forzatti et al., 1991). CO2 and methanol represent the main products. Linear and branched primary alcohols (among which isobutanol predominates) are also produced to a significant extent, whereas secondary alcohols, ketones, and aldehydes are present in lower amounts. Methyl esters and traces of dimethyl ether (DME) are detected in the reactor outlet stream, too. Methane and higher hydrocarbons are then side products of HAS. The complexity of the product mixture is consequent to the wide variety of catalytic functions that are active under synthesis conditions. They form an interconnected reaction network whose identification has been pursued through the combined analysis of three independent sources of data. 1. Experiments under Synthesis Conditions: Effects of the Operating Variables and Thermodynamic Analysis. HAS runs over various alkalipromoted Zn/Cr/O catalysts were performed at typical industrial conditions to study the effects of the operating variables on the product distribution. Experimental details are given elsewhere (Tronconi et al., 1989, 1992). The experiments indicated that the formation of oxygenates occurs through a kinetically controlled chaingrowth process and is favored by long contact times. It was found that HAS is affected by the feed composition, being optimal when CO2-free syngas with equimolar H2 and CO fractions is fed to the synthesis reactor. Both the production and the selectivity of higher alcohols were observed to decrease significantly in the presence of excess hydrogen and with increasing CO2 feed content (Forzatti et al., 1991). A thermodynamic analysis of the product mixture showed that, opposite to the reactions responsible for the growth of higher alcohols, a number of other HAS

reactions approach the chemical equilibrium (Tronconi et al., 1990). They include methanol synthesis, water gas shift (WGS) reaction, hydrogenation of the carbonyl species to primary and secondary alcohols, ketonization reactions, and esterification of primary alcohols. 2. HAS Experiments at Atmospheric Pressure: Chain-Growth Reactions. The chemical behavior of a variety of oxygenates involved in HAS was studied in detail over promoted and unpromoted Zn/Cr/O catalysts. The study was performed at atmospheric pressure and consisted of temperature-programmed desorption and temperature-programmed surface reaction (TPD and TPSR) measurements as well as of flow runs of selected probe molecules: C1 species (methanol, formaldehyde, formic acid, CO2, methyl formate) and C2+ species including primary alcohols (1-propanol, 1-butanol, isobutanol), aldehydes (propanaldehyde, butanal, isobutanal), ketones (2-butanone, 3-pentanone), and carboxylic acids (propionic, n-butyric, and isobutyric acids) (Lietti et al., 1988, 1990, 1991a). Complementary information was gained by cofeeding probe molecules in pairs (formaldehyde + methanol, formaldehyde + formic acid, methanol + propanol) (Colombo and Passuello, 1994). The transient experiments provided evidence on the nature and stability of the intermediate surface species (e.g., alkoxide, aldehydic, and carboxylate); they confirmed, for instance, that the carbonyl species are the reactive intermediates of the chain-growth reactions, whereas alcohols are consecutive products formed via surface hydrogenation. The steady-state runs were mostly informative on the relative reactivity of the oxygenates in relation to their chemical nature or structure. For instance, the flow experiments demonstrated the lower activity of ketones compared to aldehydes and of the branched species compared to the linear isomers in the chain-growth reactions. This last observation explained the abundance of isobutanol and of other branched oxygenates in the synthesis product mixture; formed at high reaction rate, such products terminate the chain growth so that their concentrations grow significantly with increasing contact time. An overall picture of the chain-growth process was obtained. A first C1 f C2 step involving two C1 reactive species related to formaldehyde was suggested to initiate the chain-growth process (Lietti et al., 1991b). Three classes of reactions, schematically represented in Figure 1, were then identified as responsible for the formation of C2+ oxygenates: (a) The aldol-type condensations between a nucleophilic and an electrophilic species, resulting in the formation of a surface aldol intermediate. This can evolve by either losing (the so-called normal mode, N) or retaining (oxygen retention reversal mode, ORR) the anionic oxygen (Nunan et al., 1989a). The two possible paths lead to different products: for example, if both the reactants are aldehydes (as in Figure 1) the N-mode product is a branched higher aldehyde, while the ORRmode product is a ketone. (b) The so-called R-additions, consisting of the attack of a nucleophilic C1 intermediate to C2+ aldehydes. The R-additions give rise to the formation of higher aldehydes (N mode) or 2-ketones (ORR mode). While at atmospheric pressure only the ORR-mode reaction was observed, both the normal and the reversal modes were proven to be active under synthesis conditions. The nature of the C1 reactive species has not been conclusively identified, yet; however, the experiments with C1 probe molecules of different oxidation states would

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Figure 1. Schematic of the oxygenate chain-growth reactions over a Cs-promoted Zn/Cr/O catalyst. Reactions are not balanced.

indicate a surface formaldehyde as the most probable reactant (Lietti et al., 1991b). (c) The ketonization reactions, leading to the formation of a ketone via condensation of two lower aldehydes with concurrent loss of a carbon atom in the form of CO2 (decarboxylative condensations). 3. Supporting Data from the Literature on HAS over Cu-Based Catalysts. The indications from the experimental study of HAS over alkali-doped ZnO/Cr2O3 catalysts are substantially in line with the literature of HAS over copper-based catalysts. The results previously obtained by Klier and co-workers (Nunan et al., 1988, 1989a) by means of independent investigation techniques (e.g., chemical enrichment experiments with labeled molecules) point out significant similarities between the low-temperature and high-temperature systems in relation to their chemical functions. The differences in the typical product distributions observed over such catalytic systems can be largely explained on the basis of the influence of the reaction temperature on the prevailing thermodynamic constraints of the synthesis (Forzatti et al., 1991; Herman and Lietti, 1994). Development of a Kinetic Model of HAS A quantitative validation of the reaction scheme proposed above was accomplished in the past by means of a kinetic treatment (Tronconi et al., 1992) that included all the major results of the mechanistic study. The model explained the chain-growth process through the combined contributions of R-additions, aldol-type condensations, and ketonizations and diversified the reaction rates according to both the molecular structure of the reactants and the evolution of the intermediates (N or ORR). For the sake of simplicity it was formally assumed that the reactions occurred between alcohols. A satisfactory description of the product distribution

a The symbols identify the reactivities attributed to each species. Those carbonylic species which are not associated with any reactivity are treated in the kinetic model as terminal products.

could be obtained for each single set of operating conditions considered; however, this result was achieved by allowing the estimates of the kinetic parameters to vary with the operating variables. In this work, the treatment of Tronconi et al. (1992) is modified in order to make it fully predictive. A major enhancement involves the adoption of aldehydes and ketones, i.e., the true reactive species, as reaction intermediates in the kinetic scheme: this implies the introduction of rate expressions written as functions of aldehyde and ketone concentrations. In principle, the model rate parameters are thus expected to become representative of the intrinsic kinetic constants and independent of any variable other than the reaction temperature. To better clarify the merit of this change, we note that in the previous treatment, due to the assumption of primary alcohols as the formal reactants of HAS, the simulation of the experimental effect of the H2/CO feed ratio required the empirical correlation of the rate constants to the H2 partial pressure (as shown, e.g., in Figure 6 of Tronconi et al., 1992): this is no longer necessary in the present approach. The related reformulation of the model is discussed in the following. The formalization of the kinetic scheme, the attribution of the species reactivities, and the definition of the rate expressions have been, in fact, substantially revised with respect to the original treatment. 1. Identification of the Reacting Species. In order to constrain the number of variables included in the kinetic model without losing detail in the description of the product mixture, the reacting system has been schematized as consisting of 80 C1-C8 oxygenates (40 carbonyl species and the corresponding 40 primary and secondary alcohols), accounting for almost the total liquid productivity. As mentioned, only the carbonyl species have been treated as intermediates of the chaingrowth process; they are listed in Table 1, where the


symbols identify the kinds of reactivity attributed to each species. By closely following the indications of the mechanistic study with probe molecules, classes of reactants have been defined for each reaction included in the kinetic scheme. In the case of aldol condensations, both aldehydes and ketones containing up to eight carbon atoms have been assumed as possible nucleophilic reactants, if the carbon bonded to the carbonyl group belongs either to a methyl or to a methylenic group. The exclusion of the R-branched species from the nucleophilic reactants is mainly due to the observations concerning the reactivity of isobutanal that exhibited a negligible nucleophilic character (Lietti et al., 1990). The class of electrophilic reactants has been restricted to selected C1-C5 carbonyl species, both linear and branched; other aldehydes and ketones have been excluded since they would originate condensation products with long and complex molecular structures that were never detected in significant amounts under synthesis conditions. Concerning R-additions, the related nucleophilic C1 species has been identified with formaldehyde and a suitable electrophilic character attributed to higher aldehydes only. In fact, a contribution of ketones to this class of reactions was never observed. Finally, for each ketone a reversible formation route via ketonization of lower aldehydes has been considered, the only exceptions being the ketones with double branching adjacent to the carbonyl group (e.g., 2-4dimethyl-3-pentanone). Their formation would require the condensation of two branched aldehydes, whose occurrence was found negligible under synthesis conditions perhaps due to steric hindrance and to the low intrinsic reactivity of the branched molecules. 2. Kinetic Parameters and Rate Expressions. As shown in Table 2, kinetic parameters have been defined for characterizing reactions and reactivities involved in the synthesis of C2+ oxygenates. An intrinsic rate constant has been associated with each main chemical route of the chain growth, namely, the C1-C2 step (k1-2), the normal R-additions (kR), the normal aldol condensations between two linear aldehydes (kβ), and the ketonization reactions (kΓ). Relative reactivity parameters have then been introduced to account for the chemical nature of the reactants: the parameters kket,n and kket,e represent the ratios of reactivity of ketones to that of aldehydes in the respective cases of nucleophilic and electrophilic behavior. The molecular structure of the electrophilic agent has also been taken into account: the parameter kiso is defined as the relative reactivity of branched aldehydes with respect to linear aldehydes. The chain-growth pathway has been diversified, too: four relative factors (ORR1-ORR3) express the ratios of the ORR- to N-rate of specific aldol condensations between higher aldehydes and formaldehyde; an additional parameter (ORR4) has been introduced for the cases of aldol condensations between higher aldehydes and of R-additions. The rate expressions for aldol condensations and R-additions have been derived on the basis of a Langmuir-Hinshelwood mechanism, accounting for saturation of surface sites and competitive adsorption of water, as reported in Table 3. A common adsorption equilibrium constant for all the oxygenates (KOXY) and a specific adsorption constant for water (KH2O) have been introduced to this purpose. In the case of ketonization reactions, reversible kinetics with first-order dependence on ketone concentrations were adopted after the

Table 2. Kinetic Parameters Associated with the Oxygenate Chain-Growth Reactionsa

a They include intrinsic rate constants and relative reactivity parameters.

results of the thermodynamic analysis. Arbitrarily, positive rates were attributed to reverse ketonizations (ketone decomposition to aldehydes). The equilibrium constant for each ketonization reaction was estimated from literature data (Reid et al., 1987). Concerning the C1-C2 step, the lack of specific mechanistic evidence made uncertain the a-priori definition of a rate expression; namely, both our previous mechanistic data and the present kinetic data neither allow a positive identification of the C1 reactive species nor would suggest the existence of competitive routes to the formation of the C2 intermediate, as inferred, e.g., by Calverley and Smith (1992) from the influence of the feed composition (CO/CO2/CH3OH) on the synthesis of higher alcohols over a K-promoted Cu/ZnO/Cr2O3 catalyst. Accordingly, the kinetics of this reaction have been formulated and adjusted empirically in order to obtain the best model fit to the data. As shown in Table 3, a first-order dependence on the concentration of formaldehyde, a mild dependence on H2 partial pressure, and the adoption of a specific inhibition term accounting for competitive adsorption of water turned out to be the most proper choices. These are in line with the expected role of formaldehyde in the formation of the first C-C bond (Nunan et al., 1988; Lietti et al., 1991b) and with the known inhibiting effect of water on the formation of C2+ oxygenates (Frolich and Cryder, 1930). Formaldehyde formation from CO and H2 has been assumed at equilibrium, the equilibrium constant being estimated from literature data (Walker, 1964; Reid et al., 1987). This is justified by the evidence that both methanol synthesis and all the hydrogenations of higher carbonyl species are thermodynamically controlled under HAS conditions (Tronconi et al., 1990).

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Table 3. Rate Expressions of the HAS Reactions Included in the Kinetic Modela

species included in the kinetic scheme, as presented in the following. (a) Forty differential mass balances are written for the oxygenate chain-growth intermediates listed in Table 1. Each balance accounts for all the reactions wherein the single carbonyl species is expected to participate according to its specific reactivities but excludes those which lead to the formation of species not included in the list. (b) Differential balances are defined also for methane and for the C2+ hydrocarbon pseudocomponent. (c) Forty algebraic equations account for the hydrogenation equilibria between carbonyl species and alcohols. At each cross section of the reactor they allow one to calculate the concentration of primary and secondary alcohols from the concentrations of the corresponding aldehydes and ketones, respectively. (d) Algebraic global balances for CO, H2O, H2, and CO2, taking into account the contribution of the watergas shift reaction (assumed at equilibrium), are also included. An exemplification of the model equations is proposed in the appendix, where the model equations for acetaldehyde and ethanol are reported in full. Data Fitting

a C ) concentration of the ith species (ald. ) aldehyde, ket. ) i ketone). COXY ) total concentration of carbonylic species. KEQ ) equilibrium constant of the ketonization reaction.

Kinetics of formation of hydrocarbons have also been included in the model. Even if minor HAS byproducts, hydrocarbons are highly undesired. Their production is accompanied by a strong release of heat that can lead to an unstable behavior of the synthesis reactor, especially at high reaction temperatures and pressures. Furthermore, they can build up in the synthesis loop. For the purposes of kinetic description, the hydrocarbon mixture has been formally simplified to two components: methane (the most abundant hydrocarbon over ZnCrO-based systems) and a pseudospecies with the same molecular formula of propene and a concentration equal to the overall concentration of C2+ hydrocarbons (they are mostly olefins with an average carbon number close to 3 under typical synthesis conditions). The kinetic expressions adopted in the model to describe their formation are also listed in Table 3. The kinetics of methanation have been already proposed in the literature and successfully applied to simulate the formation of methane over a commercial HAS catalyst (Tronconi et al., 1987); the same kinetic expression (but with a specific rate parameter) has been extended to the case of the C2+ pseudocomponent. 3. Reactor Model. Application of literature criteria (Mears, 1970) to the experimental kinetic data as well as diagnostic measurements supports the following simplifying assumptions in the description of the reacting system: (i) the synthesis reactor behaves as a pseudohomogeneous plug-flow reactor, and (ii) the temperature is constant both in the radial and in the axial direction of the reactor. The analysis of interphase and intraphase mass-transfer resistances has been reported elsewhere (Tronconi et al., 1992). The reactor model results in the set of gas-phase mass balances for the

The kinetic parameters were estimated by global nonlinear regression, fitting the model to a set of experimental data consisting of 11 HAS runs over a Csdoped Zn/Cr/O catalyst. The experiments were carried out at a temperature of 405 °C and cover a wide operational field (GHSV ) 8000-20 000 h-1, P ) 7.010.0 MPa, feed composition H2/CO ) 0.5-4.0, and CO2 content ) 0-6% v/v). More details on catalyst preparation, data, and experimental arrangements were given by Tronconi et al. (1992). Model responses in the regression procedure were the outlet concentrations of 20 oxygenated species selected as the most significant HAS reaction products, along with the concentrations of CO2, the concentration of methane, and the overall concentration of the C2+ hydrocarbons. In Figure 2 experimental and calculated product mixture compositions are compared for reference operating conditions; the match is apparently good, with the model being able to reproduce in detail the features of the product mixture composition. Comparable results have been obtained over the whole set of experiments. Figure 3 shows the global match of measured and calculated concentrations for the species adopted as model responses. It is worth noting that the data presented in Figure 3 account for the effects of all the operating variables but the reaction temperature: the model is able to reproduce such effects on a continuous basis by means of one single set of kinetic parameters. Recalling that a major goal of this work was the development of an entirely predictive description of the dependence of the product distribution on the feed composition, we report as an example in Figure 4 the comparison between measured and calculated effects of the H2/CO feed ratio. Experimental data and calculations both agree on indicating that the formation of C2+ oxygenates is maximum in correspondence to a value of H2/CO close to 1, while it decreases significantly after further enriching the feed stream in hydrogen. The model reproduces such a trend by accounting for the surface saturation due to the adsorption of water (Table 3), whose concentration increases with increasing hydrogen partial pressure in agreement with the water-


Figure 2. Experimental and calculated distribution of the main HAS products over a Zn/Cr/O + 15% Cs2O w/w catalyst in a reference run: T ) 405 °C, P ) 8.6 MPa, GHSV ) 8000 h-1, H2/ CO ) 1/1, CO2 ) 0%. C1OH ) methanol; C2OH ) ethanol; C3OH ) 1-propanol; C4OH ) 1-butanol; C5OH ) 1-pentanol; 2mC3OH ) 2-methyl-1-propanol; 2mC4OH ) 2-methyl-1-butanol; 2mC5OH ) 2-methyl-1-pentanol; 3mC4OH ) 3-methyl-1-butanol; 2but ) 2-butanone; 3pent ) 3-pentanone; 3m2but ) 3-methyl-2-butanone; 2m3pent ) 2-methyl-3-pentanone.

Figure 3. Test of the HAS kinetic model. Experimental versus calculated outlet concentrations of oxygenates and hydrocarbons. T ) 405 °C, P ) 7.0-1.1 MPa, GHSV ) 8000-20 000 h-1, feed composition H2/CO ) 0.5-4.0, CO2 ) 0-6%.

gas shift equilibrium. Methanol formation is maximum for a H2/CO ratio equal to 2, in line with the stoichiometry of the reaction, and the data are well explained and reproduced by the assumption of chemical equilibrium included in the model. The model reproduces also satisfactorily the growing concentration of hydrocarbons (not represented in Figure 4) with growing values of H2/ CO. The optimal parameter estimates are listed in Table 4, together with their 95% confidence limits. The estimates are in good agreement with the general knowledge of the HAS mechanism: for instance, the model correctly envisions the C1 f C2 reaction (see reaction 1 in the appendix) as the rate-determining step and the C2 f C3 reaction (see reactions 24 + 25 in the appendix) as the fastest step in the chain growth. The estimated reactivity of ketones as nucleophilic reactants is lower than the reactivity of aldehydes; the presence of branchings is calculated to lower significantly the

Figure 4. Effect of H2/CO feed ratio on the distribution of the main HAS products. Symbols ) experimental results. Lines ) model calculations. T ) 405 °C, CO2 ) 0%, P ) 8.6 MPa, GHSV ) 8000 h-1.

electrophilic reactivity of aldehydes. Furthermore, the estimated values of the ORR factors reflect the expected decreasing trend of reactivity of aldehydes with increasing carbon atom number. Conclusions A detailed mechanistic model of higher alcohol synthesis has been presented. The model incorporates a complex reaction network, which is closely in line with the mechanistic findings of previous experimental studies. The formalization of the kinetic scheme is based on the evidence that aldehydes and ketones are the true reacting species in HAS, while primary and secondary alcohols are equilibrium constrained products of the hydrogenation of the corresponding carbonyl species. A predictive description of the reacting system as a function of feed composition, contact time, and total pressure has been obtained. Both the successful fit of experimental data and the apparent mechanistic consistency of the parameter

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Table 4. Optimal Parameter Estimates and 95% Confidence Limits

KEQ2-Pent

CH3CO(CH2)2CH3 + CO + H2 798 CH3CHO + CH3(CH2)2CHO (4) KEQ2-Hex

CH3CO(CH2)3CH3 + CO + H2 798 CH3CHO + CH3(CH2)3CHO (5) KEQ3m-2But

CH3COC(CH3)HCH3 + CO + H2 798 CH3CHO + CH3C(CH3)HCHO (6) KEQ3m-2Pent

CH3COC(CH3)HCH2CH3 + CO + H2 798 CH3CHO + CH3CH2C(CH3)HCHO (7) KEQ3m-2Hex

CH3COC(CH3)H(CH2)2CH3 + CO + H2 798 CH3CHO + CH3 -(CH2)2C(CH3)HCHO (8) KEQ3m-2Hept

CH3COC(CH3)H(CH2)3CH3 + CO + H2 798 estimates support the adequacy of the model in its various aspects, namely, the kinetic scheme, the rate expressions, and the definition of species reactivities. Although this could be regarded as a satisfactory conclusion of the kinetic investigation, we cannot neglet that the high complexity of the proposed model provides an intrinsic flexibility to its responses. Therefore, the good match with the experiments could derive more from the mathematical structure of the model rather than from its physicochemical consistency with the described reacting system. On the other hand, a complete confidence in the significance of the kinetic study should be established before addressing process design work, since this may easily involve extrapolation beyond the boundaries of the investigated experimental region. In the second part of this work, then, we proceed to test further the model through a novel approach based on the kinetic analysis of chemical enrichment experiments.

CH3CHO + CH3 -(CH2)3C(CH3)HCHO (9) KEQ4m-2Pent

CH3COCH2C(CH3)HCH3 + CO + H2 798 CH3CHO + CH3C(CH3)HCH2CHO (10) KEQi ) ketonization equilibrium constant for ketone i, estimated from Reid et al. (1987). Reactions involving acetaldehyde as a nucleophilic reactant: kβ

CH3CHO + HCHO + H2 98 CH3CH2CHO + H2O (11) kβORR1

CH3CHO + HCHO + H2 98 CH3CH2CHO + H2O (12) kβ

Appendix In the following the derivation of the mass balance for acetaldehyde is reported. The list of reactions wherein acetaldehyde is involved both as a product and as a nucleophilic and electrophilic reactant is detailed (see Table 1). Kinetic constants and rate expressions are defined for each reaction (see Tables 2 and 3). Finally, the differential mass balance for acetaldehyde is obtained assuming a pseudohomogeneous plug flow reactor model. The algebraic equation for the estimate of ethanol concentration is also reported. The relevant nomenclature is given at the end of the appendix. Reactions involving acetaldehyde as a product: k1-2

2HCHO + H2 98 CH3CHO + H2O

(1)

KEQAcetone

CH3COCH3 + CO + H2 798 2CH3CHO (2) KEQ2-But

CH3COCH2CH3 + CO + H2 798 CH3CHO + CH3CH2CHO (3)

CH3CHO + CH3CHO + H2 98 CH3(CH2)2CHO + H2O (13) kβORR4

CH3CHO + CH3CHO + H2 98 CH3COCH2CH3 + H2O (14) kβ

CH3CHO + CH3CH2CHO + H2 98 CH3(CH2)3CHO + H2O (15) kβORR4

CH3CHO + CH3CH2CHO + H2 98 CH3CH2COCH2CH3 + H2O (16) kβ

CH3CHO + CH3(CH2)2CHO + H2 98 CH3(CH2)4CHO + H2O (17) kβORR4

CH3CHO + CH3(CH2)2CHO + H2 98 CH3CH2CO(CH2)2CH3 + H2O (18)

Ind. Eng. Chem. Res., Vol. 35, No. 7, 1996 2151 kβkiso

CH3CHO + CH3C(CH3)HCHO + H2 98 CH3C(CH3)H(CH2)2CHO + H2O (19) kβORR4

CH3CHO + CH3(CH2)3CHO + H2 98 CH3CH2CO(CH2)3CH3 + H2O (20) kβORR4

CH3CHO + CH3C(CH3)HCH2CHO + H2 98 CH3C(CH3)HCH2COCH2CH3 + H2O (21) kβkket,e

CH3CHO + CH3COCH3 + H2 98 CH3C(CH3)HCH2CHO + H2O (22) kβkket,e

CH3CHO + CH3COCH2CH3 + H2 98 CH3CH2C(CH3)HCH2CHO + H2O (23) Reactions involving acetaldehyde as a electrophilic reactant: kR

HCHO + CH3CHO + H2 98 CH3CH2CHO + H2O (24) kRORR4

HCHO + CH3CHO + H2 98 CH3COCH3 + H2O (25) kβ

CH3CH2CHO + CH3CHO + H2 98 CH3CH2C(CH3)HCHO + H2O (26) kβORR4

CH3CH2CHO + CH3CHO + H2 98 CH3COC(CH3)HCH3 + H2O (27) kβ

CH3(CH2)2CHO + CH3CHO + H2 98 CH3CH2C(C2H5)HCHO + H2O (28)

kβkket,nORR4

CH3CO(CH2)2CH3 + CH3CHO + H2 98 CH3CO(CH2)4CH3 + H2O (35) kβkket,n

CH3COCH2CH3 + CH3CHO + H2 98 CH3COC(CH3)HCH2CH3 + H2O (36) kβkket,nORR4

CH3COCH2CH3 + CH3CHO + H2 98 CH3COC(CH3)HCH2CH3 + H2O (37) kβkket,n

CH3CH2COCH2CH3 + CH3CHO + H2 98 CH3CH2COC(CH3)HCH2CH3 + H2O (38) kβkket,nORR4

CH3CH2COCH2CH3 + CH3CHO + H2 98 CH3COC(CH3)H(CH2)2CH3 + H2O (39) kβkket,n

CH3CH2CO(CH2)2CH3 + CH3CHO + H2 98 CH3CH2C(CH3)HCO(CH2)2CH3 + H2O (40) kβkket,nORR4

CH3CH2CO(CH2)2CH3 + CH3CHO + H2 98 CH3COC(CH3)H(CH2)3CH3 + H2O (41) kβkket,n

CH3COC(CH3)HCH3 + CH3CHO + H2 98 CH3(CH2)2COC(CH3)HCH3 + H2O (42) kβkket,n

CH3COC(CH3)HCH2CH3 + CH3CHO + H2 98 CH3(CH2)2COC(CH3)HCH2CH3 + H2O (43) kβkket,n

CH3C(CH3)HCOCH2CH3 + CH3CHO + H2 98 CH3C(CH3)HCOC(CH3)HCH2CH3 + H2O (44) Reaction rates:

r1 ) k1-2CHCHOPH20.5/(1 + KH2OCHCHO)

kβ

CH3(CH2)3CHO + CH3CHO + H2 98 CH3(CH2)2C(C2H5)HCHO + H2O (29) kβ

CH3(CH2)4CHO + CH3CHO + H2 98 CH3(CH2)3C(C2H5)HCHO + H2O (30) kβkket,n

CH3COCH3 + CH3CHO + H2 98 CH3CO(CH2)2CH3 + H2O (31) kβkket,nORR4

CH3COCH3 + CH3CHO + H2 98 CH3CO(CH2)2CH3 + H2O (32) kβkket,n

CH3COCH2CH3 + CH3CHO + H2 98 CH3(CH2)2COCH2CH3 + H2O (33) kβkket,nORR4

CH3COCH2CH3 + CH3CHO + H2 98 CH3CO(CH2)3CH3 + H2O (34)

r2 ) kΓ(CAcetone - CAcetone,EQ.)/(1 + K*H2OCH2O) r3 ) kΓ (C2-But - C2-But,EQ.)/(1 + K*H2OCH2O) r4 ) kΓ (C2-Pent - C2-Pent,EQ.)/(1 + K*H2OCH2O) r5 ) kΓ (C2-Hex - C2-Hex,EQ.)/(1 + K*H2OCH2O) r6 ) kΓ (C3m-2But - C3m-2But,EQ.)/(1 + K*H2OCH2O) r7 ) kΓ (C3m-2Pent - C3m-2Pent,EQ.)/(1 + K*H2OCH2O) r8 ) kΓ (C3m-2Hex - C3m-2Hex,EQ.)/(1 + K*H2OCH2O) r9 ) kΓ (C3m-2Hept - C3m-2Hept,EQ.)/(1 + K*H2OCH2O) r10 ) kΓ (C4m-2Pent - C4m-2Pent,EQ.)/(1 + K*H2OCH2O) where Ci,EQ. ) equilibrium concentration of ketone i in the corresponding ketonization reaction (see rate ex-

2152


pression (4) in Table 3).

r36 ) kβkket,nC2-ButCC2/(1 + K*H2OCH2O + KOXYCOXY)2

r11 ) kβ CC1/(1 + K*H2OCH2O + KOXY)2

r37 ) kβkket,nORR4C2-ButCC2/ (1 + K*H2OCH2O + KOXYCOXY)2

r12 ) kβORR1CC2CC1/(1 + K*H2OCH2O + KOXYCOXY)2 r13 ) kβCC2CC2/(1 + K*H2OCH2O + KOXYCOXY)2

r38 ) kβkket,nC3-PentCC2/(1 + K*H2OCH2O + KOXYCOXY)2

r14 ) kβORR4CC2CC2/(1 + K*H2OCH2O + KOXYCOXY)2

r39 ) kβkket,nORR4C3-PentCC2/

r15 ) kβCC2CC3/(1 + K*H2OCH2O + KOXYCOXY)

r16 ) kβORR4CC2CC3/(1 + K*H2OCH2O + KOXYCOXY) r17 ) kβCC2CC4/(1 + K*H2OCH2O + KOXYCOXY)

(1 + K*H2OCH2O + KOXYCOXY)2

2

r40 ) kβkket,nC3-HexCC2/(1 + K*H2OCH2O + KOXYCOXY)2

2

r41 ) kβkket,nORR4C3-HexCC2/

2



r42 ) kβkket,nC3m-2ButCC2/ (1 + K*H2OCH2O + KOXYCOXY)2

r19 ) kβkisoCC2C2m-C3/(1 + K*H2OCH2O + KOXYCOXY)2

r43 ) kβkket,nC3m-2PentCC2/



r21 ) kβORR4CC2C3m-C4/(1 + K*H2OCH2O + KOXYCOXY)2 r22 ) kβkket,eCC2CAcetone/(1 + K*H2OCH2O + KOXYCOXY)2

r44 ) kβkket,nC2m-3PentCC2/ (1 + K*H2OCH2O + KOXYCOXY)2 Mass balance for acetaldehyde:

r23 ) kβkket,eCC2C2-But/(1 + K*H2OCH2O + KOXYCOXY)2 r24 ) kRCC1CC2/(1 + K*H2OCH2O + KOXYCOXY)

44

) d(1/GHSV) 2

r25 ) kRORR4CC1CC2/(1 + K*H2OCH2O + KOXYCOXY)

r27 ) kβORR4CC3CC2/(1 + K*H2OCH2O + KOXYCOXY)2 r28 ) kβCC4CC2/(1 + K*H2OCH2O + KOXYCOXY)2 r29 ) kβCC5CC2/(1 + K*H2OCH2O + KOXYCOXY)2

CC2OH ) KHYDCC2PH2

2

r31 ) kβkket,nCAcetoneCC2/(1 + K*H2OCH2O + KOXYCOXY)2 r32 ) kβkket,nORR4CAcetoneCC2/ (1 + K*H2OCH2O + KOXYCOXY)2 r33 ) kβkket,nC2-ButCC2/(1 + K*H2OCH2O + KOXYCOXY)2 r34 ) kβkket,nORR4C2-ButCC2/ (1 + K*H2OCH2OKOXYCOXY)

1 + KHYDPH2

Initial condition: CC2 ) 0 at the inlet reactor section. νi ) acetaldehyde stoichiometric coefficient in the ith reaction. KHYD ) acetaldehyde hydrogenation equilibrium constant, estimated from Reid et al. (1987). Notice that reaction rates r31, r32, r38, and r39 have to be accounted twice in the mass balance of acetaldehyde due to the symmetrical molecular structures of acetone and 3-pentanone. Acetaldehyde-ethanol hydrogenation equilibrium:

r26 ) kβCC3CC2/(1 + K*H2OCH2O + KOXYCOXY)2

r30 ) kβCC6CC2/(1 + K*H2OCH2O + KOXYCOXY)

νiri ∑ i)1

dCC2

2

Nomenclature: C1 ) formaldehyde; C2 ) acetaldehyde; C3 ) propanal; C4 ) butanal; C5 ) pentanal; 2mC3 ) 2-methylpropanaldehyde (isobutanaol); 3m-C4 ) 3-methylbutanal; 2-But ) 2-butanone; 2-Pent ) 2-pentanone; 3-Pent ) 3-pentanone; 2-Hex ) 2-hexanone; 3-Hex ) 3-hexanone; 3m-2But ) 3-methyl-2-butanone; 3m-2Pent ) 3-methyl-2-pentanone; 2m-3Pent ) 2-methyl-3-pentanone; 3m-2Hex ) 3-methyl-2-hexanone; 3m2Hept ) 3-methyl-2-heptanone; 3m-2Pent ) 4-methyl2-pentanone; C2OH ) ethanol. Literature Cited

2

r35 ) kβkket,nORR4C2-PentCC1/ (1 + K*H2OCH2O + KOXYCOXY)2

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Received for review October 11, 1995 Accepted April 2, 1996X IE9506173

X Abstract published in Advance ACS Abstracts, May 15, 1996.

Development of a Mechanistic Kinetic Model of the Higher Alcohol

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