| dc.description.abstract | Trickle bed reactors are widely used in petroleum industries and to a limited extent in chemical industries. In these reactors, a liquid reactant (or a liquid containing a reactant) trickles over a fixed bed of catalyst while a gaseous reactant (or gas containing a reacting component) flows either cocurrently or countercurrently. The object of the present work is to study the hydrogenation of aniline in a trickle bed reactor and to propose models to describe the behaviour of the trickle bed reactor, taking into account convective transport effects.
Cylindrical pellets (3 mm × 5 mm) of 30 percent nickel with clay as inert binder are used as catalyst for the hydrogenation reaction. An all glass apparatus is employed for the experimental investigations. The experiments are based on statistical design methods so that comparison between the effects of different variables can be made more precisely.
Initial studies have shown that under the experimental conditions studied, hydrodynamic and gas phase diffusion effects are insignificant, whereas gas–liquid–solid transport effects and pore diffusion effects are significant. The reaction is observed to be zero order with respect to aniline. A rate model is written by taking into account these two transport effects and considering a power law type kinetic equation. For first order kinetics, the rate equation is linear in concentration and simple in form, while for other orders it is implicit and non linear. Therefore, the experiments are designed first to check if the data fit a first order rate model before proceeding with non linear estimation methods.
Other terms in the overall rate equation in the first order case are grouped into a single term. The temperature dependence of this term is assumed to be of Arrhenius type. Factorial experiments are designed with temperature and concentration of hydrogen as factors. The data show that logarithm of the rate of reaction is independent of any interaction between temperature and concentration. Parameters of the rate model are estimated by standard regression techniques.
The estimated coefficient of logarithm of hydrogen concentration in this equation is 1 ± 0.08. The confidence intervals include unity but no other probable values of the apparent order of reaction (e.g., 0, 0.5, 1.5, etc.). This clearly shows the reaction to be first order with respect to hydrogen. The rate model with the estimated parameters is:
k = 6.08 × 10 exp(–6400/T)
This equation is found to adequately describe the experimental data from the factorial experiments as well as from experiments with wider ranges of temperature and partial pressure of hydrogen (hydrogen diluted with nitrogen). The adequacy is proven by statistical methods.
Intrinsic kinetics are deduced from the values of the apparent rate constants computed from the experimental data, the estimated overall mass transfer coefficient, and the effectiveness factor. The intrinsic activation energy thus computed is 35.80 kcal/g mol, approximately three times the apparent activation energy, showing the strong significance of transport effects on the kinetic process.
In general, the operation of a trickle bed reactor is influenced by the effects of the velocity field on transport and kinetic processes in the reactor. An attempt is made in the present work to analyse reactor behaviour by considering convective effects. Two flow regimes-laminar and creeping boundary layer flow-are considered.
The proposed model assumes that in a trickle bed reactor, gaseous reactant dissolved in the liquid stream is transported across diffusion boundary layers repeatedly formed and destroyed around catalyst pellets, and then undergoes chemical reaction with the liquid reactant on the pellet surface. The boundary layer equations for flow around a single pellet are applied to a packed bed using the free surface model of Happel.
According to the free surface model, each particle in a multiparticle system is surrounded by a fluid envelope unaffected by other particles. This envelope is called a “unit cell.” For spherical particles, the unit cell is a spherical envelope; for cylindrical particles, a cylindrical envelope. In the present work, cylindrical particles are considered, as solutions for convective transfer with surface reaction on such particles are not widely explored despite their industrial relevance.
For creeping flow conditions (Re << 1), the velocity boundary layer is assumed to be uniformly distributed, and the free cross section available for flow corresponds to void fraction. The creeping flow boundary layer equations are solved to obtain average surface concentration. For Reynolds numbers greater than 1 (laminar region), the free cross section available for flow is reduced due to a vortex region downstream of the particle. This is accounted for by introducing a pseudo void fraction. With this correction, the laminar boundary layer equations are solved numerically to obtain concentration distribution on the pellet surface.
The influence of modified Reynolds number and temperature (or rate constant) on surface concentration is evaluated from physical data for the hydrogenation of aniline over 3 mm nickel pellets for both flow regimes. | |
