Abstrakt: |
The monoenic fatty acid methyl ester hydrogenation and isomerization over a supported nickel catalyst was studied at 0.02 ≤ PH2 ≤ 0.50 MPa and 333 ≤ T ≤ 443 K. Batch hydrogenations at constant and variable hydrogen pressure were carried out to investigate the rate-determining steps. On the basis of the Horiuti−Polanyi mechanism, involving a half-hydrogenated surface intermediate, kinetic rate equations were systematically derived following the Langmuir−Hinshelwood−Hougen−Watson approach. Each set of rate equations was tested by means of a χ2 method on its ability to describe the experimental curves simultaneously. The χ2 was minimized by a stepwise optimization of model parameters. Furthermore, we used Bartlett's test to reduce the set of most-likely rate expressions. The statistically most significant model assumes the formation of the half-hydrogenated surface intermediate as the rate-determining step and an equilibrium associative hydrogen adsorption. The rate equations are RO = mc{[−(%@mt;sys@%%@ital@%k%@rsf@%%@sx@%f,O%@be@%O,h%@sxx@%%@mx@% + Kiso&kmacr;iso)CO + &kmacr;isoCE]/[CO + CE + (KS/KM)CS]}KHPH [mol/(m3 s)] and RE = mc{[−(%@mt;sys@%%@ital@%k%@rsf@%%@sx@%f,E%@be@%E,h%@sxx@%%@mx@% + &kmacr;iso)CE + &kmacr;isoKisoCO]/[CO + CE + (KS/KM)CS]}KHPH [mol/(m3 s)] with %@mt;sys@%%@ital@%k%@rsf@%%@sx@%f,O%@be@%O,h%@sxx@%%@mx@% , %@mt;sys@%%@ital@%k%@rsf@%%@sx@%f,E%@be@%E,h%@sxx@%%@mx@% , and &kmacr;iso (units: [kgr/(kgNi s MPa)]) the formation rate constants of the half-hydrogenated intermediate in the hydrogenation of cis-monoene and trans-monoene and in the isomerization path, respectively. Kiso is the equilibrium constant for the isomerization, KS/KM is the ratio of adsorption constants of saturated and monoene components, mc is the catalyst load [kgNi/mr3], CO, CE, and CS are the concentrations of methyl oleate (cis), methyl elaidate (trans), and methyl stearate, respectively [mol/m3], KH is the adsorption constant of hydrogen [MPa-1], and PH2 [MPa] is the hydrogen pressure. The activation energies of %@mt;sys@%%@ital@%k%@rsf@%%@sx@%f,O%@be@%O,h%@sxx@%%@mx@% KH, %@mt;sys@%%@ital@%k%@rsf@%%@sx@%f,E%@be@%E,h%@sxx@%%@mx@% KH, and &kmacr;isoKH are 32.2, 28.1, and 47.2 kJ/mol, respectively. |