Experimental investigation of fast-mode liquid modulation in a trickle-bed reactor

Unsteady-state operation of trickle-bed reactors (TBRs) is a promising technique to improve reactor performances especially when mass transfer phenomena are rate controlling. Among the different techniques, fast-mode modulation of the liquid flow rate seems to be one of the most successful. In fact cycling the liquid flow rate at very low frequencies can induce the reactor to work at the high-interaction regime where mass and heat transfer phenomena are strongly enhanced. Fast-mode periodic operation, then, can be considered an extension of the natural high-interaction regime at a mean range of gas and liquid flow rate normally associated with trickling regime in steady-state conditions.Experimental tests have been performed in a TBR employing α-methyl styrene hydrogenation on Pd/C catalyst in unsteady-state conditions by “on-off” fast-mode liquid modulation. Results have been compared with the steady-state experiments at the corresponding average liquid flow rate, revealing a conversion rate improvement up to 60%. All experiments have been performed in isothermal conditions, so conversion improvement can be ascribed only to mass transfer increase and not to thermal effects. The variation of gas and liquid flow rates and liquid cycle parameters presented several important implications about the optimal working conditions.     

Effects of reaction order and convection around gas-bubbles in a gas-liquid reacting system

The mass transfer of gaseous reactant into liquid for chemical reaction is significantly affected by relative flow at the interface of gas and liquid. Two extreme cases are for a bubble behaving like a solid particle due to absorbed surfactant impurities and for a freely-internally circulating bubble with a relative interfacial velocity; the present calculations indicate a ratio of mass-transfer rates of a factor up to 1·4–2·45. The factor decreases with increasing reaction rate, becoming negligible for values of K > 2000 sec^?1. It is larger for a $\text{3}\text{2}$ order reaction than for a 1st order reaction.If there is internal circulation, the relative flow changes depending on whether the bubble is alone or in a rising bubble swarm. For small reaction rates the effect of this change in the mass transfer rate has been calculated to be 7–9% at typical bubble sizes of 0·1–0·2 cm radius. The mass transfer rate for a freely-circulating bubble is about 15% larger for a $\text{1}\text{2}$ order reaction than for a 1st order reaction at steady state. Transient and time averaged values of the Sherwood number were obtained. Shrinkage of bubbles from loss of reactant was also considered.     

Modulation of liquid holdup along a trickle bed reactor with periodic operation

Temporal variations of the liquid holdup in a mini-pilot scale trickle bed reactor cold-mockup, induced by an ON-OFF liquid flow modulation strategy of operation, are explored at different axial positions. The reactor is packed with porous beads of γ-Al₂O₃ and the liquid holdup is approximately estimated with a conductimetric technique, using probes that mimic the packing. The effects of the liquid and gas superficial velocities, the bed depth and the cycling parameters, cycle period and split, on the liquid holdup modulation are examined for a wide range of conditions. For slow and intermediate cycle periods, the liquid holdup time dependence observed during the dry period is represented by an exponential function. The characteristic value of the decay is correlated to the examined variables. The correlation allows reconstruction of the liquid holdup time dependence along the column.     

Periodic Operation of Three — Phase Catalytic Reactors

Periodic operation of three phase reactors has been explored for more than two decades. This type of forcing changes selectivity and can increase either conversion or throughput. Experiments and simulation demonstrate periodic flow interruption or variation enhances reaction rates for concurrent trickle beds provided reactants are in the gas phase or can be volatilized under bed operating conditions. Temperature excursions in trickle beds can be controlled by either flow variation or switching the feed between an inert and a reactant. Several approaches to increasing rate through faster mass transfer using flow pulsing have been studied. Pulsing flow can be induced by low amplitude modulation. Periodic switching of flow direction in airlift reactors increases gas hold up and thereby the mass transfer rate. Periodic operation of three phase reactors, thus, appears to be a fertile area for engineering research.     

Enhancement Factor for Gas Absorption in a Finite Liquid Layer. Part 1: Instantaneous Reaction in a Liquid in Plug Flow

An approximate analysis of gas absorption with instantaneous reaction in a liquid layer of finite thickness in plug flow is presented. An approximate solution to the enhancement factor for the case of unequal diffusivities between the dissolved gas and the liquid reactant has been derived and validated by numerical simulation. Depending on the diffusivity ratio of the liquid reactant to the dissolved gas (γ), the enhancement factor tends to be either lower or higher than the prediction of the classical enhancement factor equation based on the penetration theory (E_i,pen) at Fourier numbers typically larger than 0.1. An empirical correlation valid for all Fourier numbers is proposed to allow a quick estimation of the enhancement factor, which describes the prediction of the approximate solution and the simulation data with a relative error below 5 % under the investigated conditions (γ = 0.3–4, E_i,pen = 2–1000).     

Gas–liquid mass transfer in periodically operated trickle-bed reactors: Influence of the liquid-phase physical properties and flow modulation

The influence of periodic operation on trickle-bed reactor (TBR) hydrodynamics and gas–liquid mass transfer was investigated. Two-phase pressure drop, dynamic liquid hold-up and gas–liquid mass transfer coefficient (k_La) were determined at various liquid flow rates and for different modes of liquid flow variation (increasing and decreasing liquid flow rate). The results reveal the considerable influence of type of liquid flow rate modulation on k_La values (deviations of up to 80% in k_La). Simulation studies on gas-limited reaction in a periodically operated TBR indicate that an enhancement in conversion of about 14% can be expected from an appropriate selection of the operating mode, thus clearly demonstrating the quantitative process intensification feasible through increased gas–liquid mass transfer.     

Mass transfer with heterogeneous and first order homogeneous reaction using the film theory

The problem of mass transfer with both first order irreversible homogeneous and an arbitrary heterogeneous reaction has been analyzed using the film theory, subject to the assumption of a linear profile of reactant between the bulk phase and the heterogeneous reaction surface. The enhancement factor is shown to depend on the product of two dimensionless groups. One is analogous to the usual group arising from mass transfer and first order irreversible reaction, and the other is a Damköhler number which describes the transport of the reactant to and reaction on the surface. The result is discussed in the context of solubilization of an insoluble liquid amphiphile by an aqueous surfactant solution. A maximum in the flux enhancement is observed because of the resistances associated with transport of the reactant species to the reaction surface.     

Multiple steady states in two-phase reactors under boiling conditions

In this paper, we analyze the nonlinear behavior of two-phase reactors under boiling conditions. First we focus on a simple nth-order reaction of the form A→B, which allows a rigorous analytical treatment. Three necessary conditions for the existence of multiple steady states have been identified: the reactant A has to be the light-boiling component, the difference in boiling point temperatures between the reactant A and the product B has to be sufficiently large, and the order of the reaction has to be less than some physical parameter α. This parameter α can be interpreted as a measure for the phase-equilibrium-driven self-inhibition of the reaction mechanism. Thus, we have found an elegant explanation for the occurrence of multiplicities. Analytical and therefore general quantitative criteria identifying the regions of multiplicity for the model system are presented. Practical relevance of our results is demonstrated by means of two examples, the Monsanto process for the production of acetic acid and the ethylene glycol reactive distillation system.     

Catalyst particle effectiveness with unsymmetrical boundary conditions

In trickle-bed reactors part of the surface of the catalyst particles may be covered by liquid and part by gas. Under such conditions conventional expressions for effectiveness factors are not applicable because the surface concentration is not uniform. In this paper more general equations for both particle and overall effectiveness factors are derived. The first development is for slab geometry where the two plane surfaces of equal areas have different reactant concentrations. This is followed by treatment of a cubic particle where one or more faces are covered by liquid. Approximate methods are then presented for evaluating the effectiveness factor for any fractional liquid coverage, f. Comparison of the results at permissible values of f indicates that a simple weighting-factor method gives good results. The method is illustrated by comparing predicted and experimental reaction rates for a case where experimental results are available—the hydrogenation of α-methyl styrene.     

A general enhancement factor model of the physical absorption of gases in multiphase systems

A general mass transfer enhancement factor model (GEFM) has been developed based on the enhancement mechanism proposed in this paper taking both shuttle effect and hydrodynamic effect into account to calculate the overall enhancement factor due to the presence of dispersed particles. GEFM can describe the phenomena of the enhancement factor increasing with partition coefficient (m), surface covering fraction (α) and dispersed phase volume fraction (φ) increase, leveling off at higher φ, changing of enhancement factor (E) at different conditions. Moreover, GEFM can give the enhancement factor not only of multiphase system relative to the pure liquid phase (even at quite high φ), but also in the same system under different conditions (e.g. stirrer speed, viscosity). The enhancement factors predicted from GEFM are consistent with the experimental observations well.     

MASS TRANSFER WITH CHEMICAL REACTION IN PARTIALLY WETTED FLAT PLATE CATALYST PELLETS: RIVULET FLOW ANALYSIS

Mass transfer with chemical reaction is analyzed in a system formed by a flat plate solid catalyst, partially wetted by a flowing rivulet of a liquid in contact with a stagnant pure gas. The paper solves the fluid dynamic problem of the liquid phase first, and afterwards incorporates the mass transfer and the chemical reaction. The system is assumed to be isothermal and at steady state, with a first order kinetics whose limiting reactant is in the gas phase. This work studies the influence of the gas-liquid surface tension, the liquid reactant flow rate, the liquid viscosity and the angle of inclination of the solid, upon the wetting factor. The model proposed also predicts the effect of these parameters and the Thiele modulus on the overall effectiveness factor and the molar flux of the limiting gaseous reactant at the catalytic solid-liquid interface in a direct way. This approach makes the wetting factor a non-manipulated variable.     

Induced pulsing in trickle beds—characteristics and attenuation of pulses

Gas/liquid down-flow in packed beds is studied, under periodic liquid feeding (at sufficiently high frequencies to be classified as “fast” mode of pulsing), in a range of mean liquid and gas flow rates within the steady “trickling flow regime”. The aim is to identify periodic feeding conditions resulting in improved fluid-mechanical characteristics (e.g. uniform fluids distribution) and possibly enhanced transport rates in this flow regime, which is common in industrial processes. From instantaneous, cross-sectionally averaged holdup measurements, at various locations along the packed bed, quantitative information is obtained on the axial propagation and attenuation of induced pulses. A phenomenological treatment of the pulse decay process facilitates data interpretation and leads to the determination of a characteristic attenuation factor for the various conditions tested. Key parameters of the process studied include, in addition to dynamic holdup, pressure drop, pulse celerity and intensity, as a function of fluid feed rates (G,L) and liquid cyclic frequency. Under the conditions of these tests, and for fixed mean rates G,L, the time averaged holdup and the pulse celerity are practically constant along the bed; furthermore, these quantities as well as the pressure drop do not seem to be affected by the imposed cyclic liquid feeding frequency. An expression to tentatively correlate pulse celerity data is recommended.The computed attenuation factors indicate that there is a rather narrow band of mean gas and liquid rates (along the so-called “pseudo-transition” boundary to pulsing flow) where pulse decay is at a minimum. Based on these results as well as on pulse intensity vs. bed length data, recommendations are made on preferred conditions for induced pulsing (from the fluid-mechanical standpoint) which would maximize expected benefits.     

Nearly irreversible, fast heterogeneous reactions in premixed flow

When heterogeneous chemical reaction is sufficiently fast, transport of reactants becomes limiting. In a fixed bed reactor, macroscopic concentration gradients cannot be eliminated as a factor limiting the rate of reaction, possibility coupling to the mesoscopic mass transfer of reactants to the surface of the catalyst as limiting, if the reaction does not occur inside a porous support. A theory for strictly irreversible binary reaction is developed that shows the possibility of regimes of kinetic asymmetry in which a crossover point occurs internally in the reactor, demarking a region of high supersaturation in which the surface is effectively depleted of one reagent, followed by a region of rapidly decaying supersaturation, where the surface is effectively depleted of the other reagent. The parametric dependence of this crossover point is given in terms of a transcendental equation which depends on operating parameters (superficial velocity and inlet concentrations) and ratios of transport properties of the reagents. These solutions are corroborated by full nonlinear numerical computations of the boundary value problem, for the case when asymmetric mass transfer coefficients admit the possibility that the mode of operation switches from relative surface depletion of one reactant to depletion of the other in a binary reaction. It is shown that X indicates the region of greatest molecular efficiency in the reactor, and if operating parameters are so chosen such that X is large, high conversion is achieved in the precrossover region. The modified Thiele modulus analysis of the numerical solutions identifies the crossover point and the region of greatest product formation.     

Parallel hydrogenation for the quantification of wetting efficiency and liquid–solid mass transfer in a trickle‐bed reactor

A novel method for the measurement of wetting efficiency in a trickle‐bed reactor under reaction conditions is introduced. The method exploits reaction rate differences of two first‐order liquid‐limited reactions occurring in parallel, to infer wetting efficiencies without any other knowledge of the reaction kinetics or external mass transfer characteristics. Using the hydrogenation of linear‐ and isooctenes, wetting efficiency is measured in a 50‐mm internal diameter, high‐pressure trickle‐bed reactor. Liquid–solid mass transfer coefficients are also estimated from the experimental conversion data. Measurements were performed for upflow operation and two literature‐defined boundaries of hydrodynamic multiplicity in trickle flow. Hydrodynamic multiplicity in trickle flow gave rise to as much as 10% variation in wetting efficiency, and 10–20% variation in the specific liquid–solid mass transfer coefficient. Conversions for upflow operation were significantly higher in trickle‐flow operation, because of complete wetting and better liquid–solid mass transfer characteristics. © 2010 American Institute of Chemical Engineers AIChE J, 2011.     

Gas‐Liquid Mass Transfer in a Bench‐Scale Trickle Bed Reactor used for Benzene Hydrogenation

The gas‐liquid mass transfer coefficients (MTCs) of a trickle bed reactor used for the study of benzene hydrogenation were investigated. The Ni/Al₂O₃ catalyst bed was diluted with a coarse‐grained inert carborundum (SiC) particle catalyst. Gas‐liquid mass transfer coefficients were estimated by using a heterogeneous model for reactor simulation, incorporating reaction kinetics, vapor‐liquid equilibrium, and catalyst particle internal mass transfer apart from gas‐liquid interface mass transfer. The effects of liquid axial dispersion and the catalyst wetting efficiency are shown to be negligible. Partial external mass transfer coefficients are correlated with gas superficial velocity, and comparison between them and those obtained from experiments conducted on a bed diluted with fine particles is also presented. On both sides of the gas‐liquid interface the hydrogen mass transfer coefficient is higher than the corresponding benzene one and both increase significantly with gas velocity. The gas‐side mass transfer limitations appear to be higher in the case of dilution with fine particles. On the liquid side, the mass transfer resistances are higher in the case of dilution with coarse inerts for gas velocities up to 3 · 10^–2 cm/sec, while for higher gas velocities this was inversed and higher mass transfer limitations were obtained for the beds diluted with fine inerts.     

Analysis of plug flow reactors with variable mass density

The design of plug flow reactors with variable mass density is examined. Equations which include a two‐term constitutive equation for the reaction rate are derived for the flow of liquids and for the flow of ideal gases in steady plug flow reactors. It is shown that the addition of the second term in the constitutive equation can have a significant effect on the calculation of the reactor volume needed to carry out a specific conversion of the reactant. Published experimental plug flow reactor data support the observation that a reaction rate constitutive equation with two terms can provide a good representation of the experimental data for variable mass density reactors. © 2014 American Institute of Chemical Engineers AIChE J 60: 4185–4189, 2014     

Trickle bed reactors: Effect of liquid flow modulation on catalytic activity

The effect of slow ON-OFF liquid flow modulation on the oxidation of aqueous solutions of ethanol using a 0.5% Pd/Al₂O₃ commercial egg-shell catalyst was investigated in a laboratory trickle bed reactor (TBR). In this mode of operation, the catalyst was cyclically exposed to oxidative and reductive environments.The study was carried out under different gas and liquid flow rates, cycle periods and splits. Cycling results have been compared with the steady-state experiments performed at the corresponding average liquid flow rate. Significant improvements over the continuous operation were obtained when the catalyst was exposed to a short surplus of oxygen (to minimize deactivation by overoxidation in the kinetic regime) after a longer time of working in the mass transfer limited regime. According to the results presented here, it is recommended to work with high liquid flow rates and moderate gas flow rates to ensure complete wetting of the catalyst during the ON cycle and to minimize the overoxidation process during the OFF cycle.     

Three-dimensional-printed holistic reactors with fractal structure for heterogeneous reaction

Holistic catalytic reactors with fractal structures have attracted growing attention in heterogeneous reactions because of their advantages of improved mass transfer and easy separation. Herein, a cost-efficient and straightforward design strategy that combined three-dimensional printing and electroless deposition was presented to construct dynamic, holistic stirred reactors with the fractal structure. The conversion factor α of the fractal impeller is 65.01 mmol m⁻² h⁻¹ with a large volume of reactant (80 ml), which is 1.7 times that of the normal impeller with the same loading of Ag catalysts. Experimental results and simulation analysis demonstrate that the fractal impeller significantly improves the catalytic performance by enhancing mass transfer and spatial dispersion in the reaction. Moreover, the holistic impeller could be reused 10 times without obvious loss of catalytic performance, and easily separated from the reaction system. The structural design of fractal reactors will open the way for high-efficiency dynamic heterogeneous catalytic reactors.     

Hydrodynamics and volumetric gas-liquid mass transfer coefficient of a stirred vessel equipped with a gas-inducing impeller

Hydrodynamic and mass transfer characteristics of a gas-liquid stirred tank provided with a radial gas-inducing turbine were studied. The effect of the rotation speed and the liquid submergence on global hydrodynamic and mass transfer parameters such as the critical impeller speed, the induced gas flow rate, the gas holdup, the power consumption and the volumetric gas-liquid mass transfer coefficient were investigated. The experiments are mainly conducted with air-water system. In the case of critical impeller speed determination, two liquid viscosities have been used. The volumetric gas-liquid mass transfer coefficient k_La has been obtained by two different techniques. The gas holdup, the induced gas rate and the volumetric gas-liquid mass transfer coefficient are increasing functions with the rotation speed and decreasing ones with the liquid submergence. The effects of these operating parameters on the measured global parameters have been taken into account by introducing the dimensionless modified Froude number and correlations have been proposed for this type of impeller.