Isothermal trickle-bed reactors

A number of attempts in interpreting trickle-bed performance appeared in the open literature (6-14). These studies did not demonstrate the predictive ability of the proposed reactor models. Some used the reaction data in trickle-beds to evaluate unknown model parameters in order to match calculated and experimental results (7-11). Other studies left certain observed phenomena unexplained (6-12). The objective of this paper is to develop a model for a gas reactant limited reaction in an isothermal trickle-bed reactor. Model parameters are evaluated by independent means and model s predictive ability is tested. 

A Comparison of Current Models for Isothermal Trickle-Bed Reactors... 

Mil.I S AND DUDUKOVlC Isothermal Trickle-Bed Reactors Current Models 39... 

If one has intrinsic and apparent reaction kinetics available, then Equation 10 may be viewed as a three-parameter model (t)ce, b wo> B -d f°r prediction of isothermal trickle-bed reactor performance. However, Biwo depends on two mass transfer coefficients and a priori model parameter evaluation is no simpler than before. 

Equations 10 - 15 are used in a later section to predict the performance of an isothermal trickle-bed reactor using measured values of all the model parameters except the mass transfer coefficients (and hence Biot numbers). The correlations of Goto and Smith (J35) and Turek and Lange (18) were used to obtain values of while the correlation... 

Sylvester and Pitayagulsarn53,54 considered combined effects of axial dispersion, external diffusion (gas-liquid, liquid-solid), intraparticle diffusion, and the intrinsic kinetics (surface reaction) on the conversion for a first-order irreversible reaction in an isothermal, trickle-bed reactor. They used the procedure developed by Suzuki and Smith,51,52 where the zero, first, and second moments of the reactant concentration in the effluent from a reactor, in response to a pulse introduced, are taken. The equation for the zero moment can be related to the conversion X, in the form... 

The catalyst performance tests were carried out in bench scale isothermal trickle-bed reactors. The reactor was loaded with 373 cm catalyst extrudates (0.13 cm diameter). The catalyst was diluted with 0.2-O.3 mm diameter (50-80 mesh) inert glass beads in order to improve the liquid distribution and contacting effidency witbin the reactor. 

A summary of reactor models used by various authors to interpret trickle-bed reactor data mainly from liquid-limiting petroleum hydrodesulfurization reactions (19-21) is given in Table I of reference (37). These models are based upon i) plug-flow of the liquid-phase, ii) the apparent rate of reaction is controlled by either internal diffusion or intrinsic kinetics, iii) the reactor operates isothermally, and iv) the intrinsic reaction rate is first-order with respect to the nonvolatile liquid-limiting reactant. Model 4 in this table accounts for both incomplete external and internal catalyst wetting by introduction of the effectiveness factor r)Tg developed especially for this situation (37 ). 

A few reactor models have recently been proposed (30-31) for prediction of integral trickle-bed reactor performance when the gaseous reactant is limiting. Common features or assumptions include i) gas-to-liquid and liquid-to-solid external mass transfer resistances are present, ii) internal particle diffusion resistance is present, iii) catalyst particles are completely externally and internally wetted, iv) gas solubility can be described by Henry s law, v) isothermal operation, vi) the axial-dispersion model can be used to describe deviations from plug-flow, and vii) the intrinsic reaction kinetics exhibit first-order behavior. A few others have used similar assumptions except were developed for nonlinear kinetics (27—28). Only in a couple of instances (7,13, 29) was incomplete external catalyst wetting accounted for. 

Activation energy, stability in trickle-bed reactors, 76 Activation overpotential, cross-flow monolith fuel cell reactor, 182 Activity balance, deactivation of non-adiabatic packed-bed reactors, 394 Adiabatic reactors stability, 337-58 trickle-bed, safe operation, 61-81 Adsorption equilibrium, countercurrent moving-bed catalytic reactor, 273 Adsorption isotherms, countercurrent moving-bed catalytic reactor, 278,279f... 

When (a) there are no external mass-transfer resistances (such as gas-liquid, liquid solid, etc.), (b) catalysts are all effectively wetted, (< ) there is no radial or axial dispersion in the liquid phase, (d) a gaseous reactant takes part in the reaction and its concentration in the liquid film is uniform and in excess, (e) reaction occurs only at the liquid-solid interface, (/) no condensation or vaporization of the reactant occurs, and (g) the heat effects are negligible, i.e., there is an isothermal operation, then a differential balance on such an ideal plug-flow trickle-bed reactor would give... 

We now look at the mathematical equations for a general isothermal steady-state model for the trickle-bed reactor, which takes into account external mass-transfer resistances, i.e., gas-liquid and liquid-solid, axial dispersion, and the intraparticle mass-transfer resistances, along with the intrinsic kinetics occurring at the catalyst surface. Since many practical reactions can be characterized as... 

Hydrocracking tests were conducted in an isothermal gas phase / trickle-bed reactor of 20 mm internal diameter. The catalyst bed, containing 3 g of catalyst extrudate, was diluted with nominally 0.8 mm SiC granulate in the volume ratio 2 1 (diluent catalyst) and was both preceded by and supported on beds of pure SiC granulate which acted as preheat and post reaction temperature trim zones, respectively. 

Large-scale hydroprocessing trickle-bed reactors normally operate under adiabatic conditions therefore, heat effects caused by the reaction must also be included. Shah [61] showed that in this case the critical Bodenstein number for elimination of axial dispersion effects is a function of a heat parameter as well as a modified Damkohler number. For low Damkohler numbers smaller critical Bodenstein numbers than in isothermal reactors are sufficient to eliminate axial dispersion in adiabatic reactors, whereas the inverse is true for large Damkohler numbers. 

The participants had the opportunity to contribute to the NATO-ASI during the Poster Session, Posters covering a wide range of subjects were presented non-isothermal trickle beds, catalyst deactivation, mixing in fluidized beds, control of chemical reactors, maldistribution in chemical reactors, cyclic operation in trickle beds, polymerization reactors, etc. The most relevant contributions of the Poster Session were selected to be included in this NATO-ASI Proceedings,... 

Contrary to what we observed for the hydrogenation of a-methylstyrene on Pd-Al203 in a countercurrent trickle-bed reactor ( ), the temperature control appears very easy in the present investigation. If the liquid and the gas are preheated, the jacket heat transfer is sufficient to ensure the isothermicity of the catalytic bed. Hot spots or dry catalyst particles are not observed, corroborating the assumption that the hot spot formation is related to the drying of some catalyst particles. 

Trickle-bed reactors, wherein gas and liquid reactants are contacted in a co-current down flow mode in the presence of heterogeneous catalysts, are used in a large number of industrial chemical processes. Being a multiphase catalytic reactor with complex hydrodynamics and mass transfer characteristics, the development of a generalized model for predicting the performance of such reactors is still a difficult task. However, due to its direct relevance to industrial-scale processes, several important aspects with respect to the influence of external and intraparticle mass transfer effects, partial wetting of catalyst particles and heat effects have been studied previously (Satterfield and Way (1972) Hanika et. al., (1975,1977,1981) Herskowitz and Mosseri (1983)). The previous work has mainly addressed the question of catalyst effectiveness under isothermal conditions and for simple kinetics. It is well known that most of the industrially important reactions represent complex reaction kinetics and very often multistep reactions. Very few attempts have been made on experimental verification of trickle-bed reactor models for multistep catalytic reactions in the previous work. 

It is possible to obtain kinetically representative data in very small laboratory reactors if the temperature and concentration gradients on the scale of the catalyst particle are absent. The bed needs to be isothermal and the reactor should behave as a plug-flow reactor. These requirements imply using small particles and a sufficient bed length. For flow through such small particles, surface tension forces become much more important, and established trickle-bed correlations cannot be extrapolated. For a reliable design of the reactor bed and for a choice of window of operation, the hydrodynamics of such fixed-bed microreactors have to be investigated. 

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