Electrical measurements in bipolar trickle reactors

Current potential curves for the total current flowing through the reactor and for the current passing through a single ring of the column packing have been measured using solutions containing the ferroferricyanide couple. The theoretical formulation of current-potential plots has been extended to incorporate a fast reversible reaction in the presence of diffusion polarization. A method for deriving the film thickness and mass transfer limiting current from these plots has been provided.List of symbols a integration constant in Equation 6 - b integration constant in Equation 8 - E applied potential (V) - E _r potential of the ring electrode with respect to the feeder electrode at the entry position of the reactor (V) - E ₁,E ₂ reversible potentials of an anodic and cathodic reaction, respectively - F Faraday constant - h film thickness (cm) - I current passing through segmented rings (mA) - I _F Faradaic current per unit length of wetted perimeter (A cm^–1) - I _NF non-Faradaic current per unit length of wetted perimeter (A cm^–1) - I _T total current per unit length of wetted perimeter (A cm^–1) - L half-length of Raschig ring (cm) - i _D limiting mass-transfer controlled current (A cm^–1) - i _D limiting mass transfer controlled current at the end of the rings (A cm^–2) - i_{D, 1mM} limiting mass transfer controlled current for 1 mM of redox couple - i_o1, i_o2 exchange current for two reactions (one anodic and the other cathodic) - n number of electrons transferred in an electrochemical reaction - n ₁,n ₂ number of electrons transferred in two reactions (one anodic and the other cathodic) - n _c number of mmol of ferro-ferricyanide - n _r number of graphite Raschig rings in a single layer of a packed column - r reaction rate (mol cm^–2 s^–1) - R gas constant (8.314JK^–1mol^–1) - r _o,r _i radii of the outer and inner perimeter of the ring (cm) - (Re)_f film Reynolds number (dimensionless) - T temperature (K) - v volumetric liquid flow rate (cm³ min^–1) - x axial co-ordinate along Raschig ring (cm) - ₁, ₂ transfer coefficients for two reactions (one anodic and the other cathodic) (dimensionless) - fraction of the end areas of the rings which overlap (dimensionless) - electrode overpotential (V) - _T total overpotential for half of a bipolar ring (V) - v kinematic viscosity (cm² s^–1) - solution resistivity ( cm) - _s potential in the solution phase (V)     

Scaling down trickle bed reactors

The scaling down of trickle bed reactors for catalyst testing in deep hydrodesulphurisation (HDS) is evaluated. A multiphase micro-reactor system has been built specifically for HDS, consisting of a set of six 2 mm diameter packed beds with particles of approximately 100 μm. To confirm plug-flow behaviour (for integral conversion) and to guarantee the measurement of true kinetics, the hydrodynamics have to be investigated.

For this purpose, a ‘cold-flow’ set-up of the same dimensions as the HDS reactors, has been built. A liquid feed with a dye tracer pulse as well as gas are fed to a glass column, packed with glass particles. From the recorded outlet concentration, the influence of the gas- and liquid-flow rate on the mean residence time and residence-time distribution (RTD) have been determined.

This hydrodynamics investigation describes the deviation from plug flow in micro-scale packed beds. The results show that the deviations from plug flow are minimal. The effect of the gas-flow rate on the liquid-residence time is more pronounced in micro-packed beds than that in trickle beds with larger particles.

The RTD study presented here provides valuable insight into the behaviour of scaled-down kinetic-test facilities.     

Lateral mixing in trickle bed reactors

Knowledge of lateral mixing is essential to understand heat and momentum transfer parameters in both single-phase liquid and two-phase gas-liquid co-current down flow through packed bed columns. The reactors through which gas and liquid concurrently flow downwards through a bed of catalytic packing are called trickle bed reactors. Experimental data on lateral mixing coefficients from both the heat transfer and radial liquid distribution studies are obtained over a wide range of flow rates of gas and liquid using glass spheres (4.05 and 6.75 mm), ceramic spheres (2.59 mm), and ceramic raschig rings (4 and 6.75 mm) as packing materials covering trickle flow, pulse flow, and dispersed bubble flow regimes. In the present work, an expression for estimation of lateral mixing coefficient (αβ)_L is derived using the data on radial liquid distribution studies. The agreement between the values of (αβ)_L obtained from heat transfer studies and from radial liquid distribution studies using the experimental data shows that there exists an analogy between the heat transfer and radial liquid distribution in packed beds. Since (αβ)_L is an important variable for estimation of various heat and mass transfer parameters, a correlation for (αβ)_L based on present heat transfer study is proposed. The agreement between the (αβ)_L values estimated from the proposed correlation and experimental values is satisfactory with a standard deviation (s.d.) of 0.119.     

Solid-liquid contacting effectiveness in trickle bed reactors

Internal and external effective wetting of a porous catalyst in a trickle bed reactor was considered. Experimental tests were carried out based on the analysis of the response curves of the reactor to a step decrease of the tracer inlet concentration.The pore filling of the catalyst pellets was practically total even at very low liquid flow rates, probably due to capillarity.The external effective wetting was interpreted in terms of an apparent internal diffusivity (D_i)_app, determined on the basis of a model which assumes a total external wetting of the catalyst. The values of (D_i)_app increased with the liquid flow rate, tending to the actual internal diffusivity D_i determined in a full reactor. The found values of (D_i)_app/D_i were used to interpret the ratio of the apparent kinetic rate constant determined in a trickle reactor k_app and that for a totally wetted catalyst k_v.The calculated values agree substantially with those proposed by Satterfield[3] from kinetic tests.     

Pressure drop hysteresis in trickle bed reactors

An understanding of the hydrodynamics of trickle bed reactors (TBR) is essential for their design and prediction of their performance. Flow variables, packing characteristics, physical properties of fluids and operation modes influence the behavior of the TBR. The existence of multiple hydrodynamic states or hysteresis (pressure drop, liquid holdup, catalyst wetting, gas-liquid mass transfer) is due to the different flow structures in the packed bed and can be attained by a set of different operating procedures. Experiments were performed to study the effect of liquid and gas velocity, liquid surface tension, liquid viscosity and the particle diameter of the packing on two-phase pressure drop hysteresis. The parallel zone model for pressure drop hysteresis in the trickling flow was used for analysis of experimental data and flow structure. Theoretically predicted pressure drop hysteresis loop is in satisfactory agreement with experimental data.     

Radioisotope tracer study in trickle bed reactors

The residence time distribution (RTD) of liquid phase in trickle bed reactors has been measured for air‐water system using radioisotope tracer technique. Experiments were carried out in a glass column of internal diameter of 0.152 m packed with glass beads and actual catalyst particles of two different shapes. From the measured RTD curves, mean residence time of liquid was calculated and used to estimate liquid holdup. The axial dispersion model was used to simulate the experimental data and estimate mixing index, ie. Peclet number. The effect of liquid and gas flow rates on total liquid holdup and Peclet number has been investigated. Results of the study indicated that shape of the packing has significant effect on holdup and axial dispersion. Bodenstein number has been correlated to Reynolds number, Galileo number, shape and size of the packing.     

Advantages of counter-current operation for hydrodesulfurization in trickle bed reactors

The deep desulfurization of oil fraction is a central matter of concern to every refinery. Hydrogen sulfide is the product of hydrodesulfurization reaction and it is the inhibiter of the reaction. When products inhibit the reaction, the counter-current operation is expected to have an advantage over the co-current operation. Hydrodesulfurization of vacuum gas oil in a trickle bed reactor was simulated for both models of co-current operation and counter-current operation. The models were simulated on high and low gas and liquid velocities. Hydrogen sulfide was affected by mass transfer resistance in both gas-liquid and liquid-catalyst interface. The other component mass transfer resistances were negligible. When the deep desulfurization was required, simulation results showed that the counter-current operation was superior to the co-current operation in organic sulfur conversion     

RTD studies in trickle bed reactors packed with porous particles

The residence time distribution (RTD) for liquid phase in a trickle bed reactor (TBR) has been experimentally studied for air-water system. Experiments were performed in a 15.2 cm diameter column using commerical alumina extrudates with D/d_p ratio equal to 75 to eliminate the radial flow differences. The range of liquid and gas flow rates covered was 3.76 < Re_L < 9.3 and 0 < Re_G < 2.92. The axial dispersion model was used to compute axial dispersion coefficient. The effect of liquid and gas flow rates on total liquid holdup and axial dispersion was investigated. The total liquid holdup has been correlated to liquid and gas flow rates.     

A cellular automata model for liquid distribution in trickle bed reactors

A cellular automata model for liquid distribution studies in trickle bed reactors is presented. It is a potential tool for describing non-uniform distribution of gas and liquid in a trickle bed. This non-uniformity may arise from a wide range of potential sources, such as improper distribution of the feed, random or radial porosity variation, wall effect, partial wetting of the catalyst, and gas-liquid surface tension related effects. Axial and radial dispersion of the liquid flow are inherently included in the model, since the fundamental model probability parameters are directly related to the dispersion coefficients. The present model is extremely fast due to simple single-event modeling, and it is well suited for parallelization. Three examples of the model performance are shown. In the first a liquid jet spreading from a point source is followed, and in the second, effect of radial porosity profile to wall flow is examined. The third example illustrates the potential of the model to predict pulsing flow regime.     

Criteria for axial dispersion effects in adiabatic trickle bed hydroprocessing reactors

This paper first outlines an approximate solution to the governing equations for an adiabatic hydroprocessing trickle bed reactor operating in the presence of axial dispersion. The approximate solution agrees very well with the rigorous numerical solution for Peclet numbers greater than approximately three.Using the approximate solution, criteria for significant axial dispersion effect are obtained. These criteria indicate that at high conversions, an adiabatic operation produces a larger axial dispersion effect than the isothermal operation. At low conversions, opposite results are obtained.The derived criteria are used to evaluate the orders of magnitude of Peclet number required to avoid axial dispersion effect in pilot scale adiabatic reactors for (a) residual hydrodesulfurization (b) hydrocracking of gas oils and (c) denitrogenation of shale oils. The calculations indicate that the axial dispersion effect is of less importance in case (c) than in cases (a) and (b).Finally, the role of heat effects on the axial dispersion in a vapor phase fixed bed adiabatic reactor is evaluated.     

Magnetohydrodynamics of trickle bed reactors: Mechanistic model, experimental validation and simulations

An isothermal one-dimensional two-fluid magnetohydrodynamic (MHD) model based on the volume average mass, charge, and momentum balance equations and the Maxwell's equations coupled via the Lorentz force and Ohm's law was developed for the prediction of the two-phase pressure drop and the total liquid holdup in trickle bed reactors experiencing a homogeneous transverse magnetic field. The slit model approximation and the drift flux Kozeny-Carman approach were extended for the derivation of appropriate drag force closures required in the conservation equations, respectively, in the trickle flow regime and in the dispersed bubble flow regime. The expression of liquid-solid drag was adapted to take into account the influence of the magnetic field on the laminar term and the damping of turbulent/inertial term via the Hartman number and the liquid-to-bed electrical resistance ratio. Associating these drag forces with the proposed model resulted in a fully predictive MHD approach for trickle beds. Several model limiting formulations were derived for an electrically conducting fluid flowing downwards with a stagnant gas (pure trickle flow) to yield liquid holdup, as well as for single-phase upward conditions to yield the single-phase pressure drops.     

Numerical approach to predict wetting and catalyst efficiencies inside trickle bed reactors

Computational fluid dynamics is used to describe both wetting efficiency in simple configurations of stacked solid particles and catalyst efficiency inside such partially wetted catalyst particles. The volume of fluid (VOF) model used to describe hydrodynamics leads to realistic and promising results for surface wetted ratio. Catalyst efficiency simulations have been performed for different shapes of particles and different Thiele modulus values for a first order reaction rate. Results show that for all the shapes studied, catalyst efficiencies of partially wetted particles can be calculated by simply using a modified Thiele modulus defined as ratio of the actual Thiele modulus $\phi$ to the wetting efficiency f.     

Further studies on the epoxidation of propylene in a bipolar trickle bed

Further studies on the epoxidation of propylene via electrogenerated hypobromite in a trickle bed have been carried out with the aim of increasing the space-time yield. Factors which have been studied in more detail include electrode material, applied voltage, sense of polarity, cell packing arrangements, electrolyte flow rate and sodium bromide concentration. By suitable choice of conditions a significant increase in space-time yield can be obtained, up to a limit set by the depletion of propylene, without a large increase in energy consumption.     

Liquid flow texture analysis in trickle bed reactors using high-resolution gamma ray tomography

Trickle bed reactor performance and safety may suffer from radial and axial liquid maldistribution and thus from non-uniform utilization of the catalyst packing. Therefore, experimental analysis and fluid dynamic simulation of liquid–gas flow in trickle bed reactors is an important topic in chemical engineering. In the present study for the first time a truly high-resolution gamma ray tomography technique was applied to the quantitative analysis of the liquid flow texture in a laboratory cold flow trickle bed reactor of 90 mm diameter. The objective of this study was to present the comparative analysis of the liquid flow dynamics for two different initial liquid distributions and two different types of reactor configurations. Thus, the hydrodynamic behavior of a glass bead packing was compared to a porous Al₂O₃ catalyst particle packing using inlet flow from a commercial spray nozzle (uniform initial liquid distribution) and inlet flow from a central point source (strongly non-uniform initial liquid distribution), respectively. The column was operated in downflow mode at a gas flow rate of 180 L h⁻¹ and at liquid flow rates of 15 and 25 L h⁻¹.     

Effect of partial wetting on liquid/solid mass transfer in trickle bed reactors

The wetting efficiency of liquid trickle flow over a fixed bed reactor has been measured for a wide range of parameters including operating conditions, bed structure and physico-chemistry of liquid/solid phases. This data bank has been used to develop a new correlation for averaged wetting efficiency based on five different non-dimensional numbers. Finally liquid/solid mass transfer has been determined in partial wetting conditions to analyse what are the respective effects of wetting and liquid/gas flow turbulence. These effects appear to be separated: wetting being acting on liquid/solid interfacial area while the liquid/solid mass transfer coefficient is mainly connected to flow turbulence through the interstitial liquid velocity. A correlation has been proposed for liquid/solid mass transfer coefficient at very low liquid flow rate.     

Mathematical simulations of the performance of trickle bed and slurry reactors for methanol synthesis

Three- and two-phase reactor models were developed to simulate the performance of trickle bed and slurry reactors for methanol synthesis. The combination of orthogonal collocation and quasi-linearization was used to solve the trickle bed reactor model incorporating resistance to interparticle and intraparticle diffusion and resistance to mass transfer between gas and liquid phases. Model parameters were estimated independently from either published correlations or literature data. The model predicts significant resistance to intraparticle diffusion on the performance of trickle bed reactors. However, comparisons between pilot size trickle bed and slurry reactors illustrate the superior performance of trickle bed reactors over the slurry reactors for methanol synthesis even with diffusion limitations.