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Comment on the “Dynamic Simulation of Sorption Enhanced Reaction Processes for Biodiesel Production” Pedro Sa´ Gomes* CAT Catalytic Center, Institute for Technical and Macromolecular Chemistry, Rheinisch-Westfa¨lische Technische Hochschule, RWTH Aachen UniVersity, Worringerweg 1, D-52074 Aachen, Germany
Viviana M. T. M. Silva and Alı´rio E. Rodrigues Laboratory of Separation and Reaction Engineering, Associate Laboratory LSRE/LCM, Department of Chemical Engineering, Faculty of Engineering, UniVersity of Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal Sir: In a recent paper by Kapil, Bhat, and Sadhukhan (“Dynamic Simulation of Sorption Enhanced Reaction Processes for Biodiesel Production”),1 the authors analyze biodiesel production in a simulated moving bed reactor (SMBR). This is a very interesting topic; in fact, there is not much information about biodiesel production with SMBRs, apart from a few patents2,3 that, by the way, the authors should have included in their reference list. Nevertheless, the methodology presented in the paper has various shortcomings, which, and if employed as the authors report, will lead to considerable inaccuracies: (1) The work starts with the section, “Kinetic Modeling of the Esterification Reaction”. Here, the authors used the experimental results of Ni and Meunier4 for the heterogeneous esterification of palmitic acid, dissolved in commercial sunflower oil, with methanol over a silica-supported Nafion resin (SAC13). (a) A general Langmuir-Hinshelwood-Hougen-Watson mechanism is employed to describe the esterification reaction, and six parameters are used to fit the experimental data. Kapil et al.1 concluded that “the proposed kinetic model is a statistically reliable representation of the experimental data”. Looking at Figure 1 in Kapil et al.,1 the authors seem to use seven experimental conversion points to fit a six-parameter model. Nonetheless, and apart from the quality of the fitting, what it is really difficult to understand is which data, from the set of experimental results of Ni and Meunier,4 were used to determine the parameters of the proposed reaction kinetic model. If one checks Ni and Meunier,4 one can observe that, for the experiments catalyzed by the silica-supported Nafion resin (SAC-13), a value of 50% of palmitic acid conversion is never achieved earlier than 5 h, while in Kapil et al.1 (Figure 1), a time period of less than 4000 s is needed to reach this same conversion value. (b) If one simulates the batch esterification using the kinetic rate constants stated in Table 1 from Kapil et al.1 for a initial mixture constituted by 5.70 mol/L of methanol and 0.27 mol/L of palmitic acid in sunflower oil (the experimental conditions from Ni and Meunier), the predicted conversion of the palmitic acid does not match those shown in Figure 1 of Kapil et al.1 It seems that the units of the kinetic constant reported in Table 1 (kf) are not correct: Should the units be mol/(m3 s)? Moreover, does it make sense to attribute units of mol/m3 to the * To whom correspondence should be addressed. E-mail:
[email protected].
species activities? In eq 3 in the Kapil et al. work,1 is not it missing a stoichiometric coefficient? (2) The next step in Kapil et al.’s methodology is the section, “Modeling of SMBR”.1 (a) The authors argue that adsorbent (cationic resin) characteristics are based in Yu et al.,5 considering a linear adsorption isotherm type for all components (methanol, palmitic acid, FAME, and water). However, the work of Yu et al.5 involves the esterification of acetic acid with methanol to produce methyl acetate. Only water and methanol are common species to the biodiesel process. The acetic acid and methyl acetate adsorption constants from Yu et al.5 were used in the Kapil et al.1 work as palmitic acid and FAME, respectively. This approach, which is completely misleading, was not even commented by the authors. (b) Regarding the resins, Kapil et al.1 seem to use a mixture of catalyst/adsorbent: (i) the silica-supported Nafion resin (SAC-13) from Ni and Meunier4 was used as a catalyst, and (ii) the Amberlyst 15 resin from Yu et al.5 was used as an adsorbent. These are both cationic exchange resins, and, thus, one expects that they will both catalyze the esterification as they selectively adsorb the various species, but not to the same extent. However, the authors do not consider the catalytic activity of Amberlyst 15, nor the adsorptive capacity of SAC-13. (c) Moreover, there are questions related with the use of data from various sources: (i) the catalyst density, from Xiu et al.,6 is, in fact, for a commercial nickel-based catalyst and not for SAC-13, and (ii) the adsorbent density, from Yori et al.,7 is for silica beds and not for Amberlyst 15. In addition, the units in the SMBR model do not match: (i) the adsorbed concentration given in eq 5 is presented in units of mol/L, while in the bulk mass balance (eq 7), should be expressed in mol/kg, and (ii) the rate of reaction given by eq 3 is expressed in units of mol/(L s), which is consistent with eq 2 and the kinetic parameter presented in Table 1, but does not match that used in eq 7, where the rate of reaction is expressed in units of mol/(kg s). (3) The remarks in the “Results and Discussion” section of Kapil et al.1 are not conclusive at all, because they are based on simulations performed for a single-cycle time period (just the four initial switching times). The type of observations that the authors try to address are only acceptable if the SMBR process analysis is carried out at the cyclic steady state (CSS), as in most of the state-of-the-art literature.5,8-12 The CSS is reached when the difference between the column concentration profiles for two consecutive cycles are quite small (usually
10.1021/ie100488j 2010 American Chemical Society Published on Web 10/07/2010
Ind. Eng. Chem. Res., Vol. 49, No. 22, 2010
