Gas-liquid mass transfer in trickle-bed reactors: Gas-side mass transfer

New data of gas-liquid mass transfer for cocurrent downflow through packed beds of non-porous particles are presented. Mass transfer parameters for air/carbon dioxide/water and air/carbon dioxide/sodium hydroxide systems were evaluated by least squares fit of the calculated CO₂ concentration profiles in the gas phase to the experimental values. The dependence of k_Ga on gas and liquid flow rates is caused by the dependence of gas-liquid interfacial area, not by the gas-side mass transfer coefficient k_G. In the case of the absorption of dilute carbon dioxide the gas-side resistance is considerably smaller than the liquid-side resistance. In the pulse flow regime, gas-liquid interfacial area calculated from k_La and k_L values obtained by physical, respectively, chemical absorption are lower than the gas-liquid interfacial area evaluated from the measurements under reaction conditions.     

Mass transfer in trickle-bed reactors at low reynolds number

Mass transfer rates were determined in a 3.4 cm i.d. trickle-bed reactor in the absence of reaction by absorption measurements and in presence of reaction. Gas flow rates were varied from 0-100 l/h and liquid flow rates from 0-1.5 l/h. The catalyst particles were crushed to an average diameter of 0.054 and 0.09 cm. Mass transfer coefficients remained unaffected by change in gas flow rate but increased with liquid rate. The data from absorption measurements were evaluated with predictions based upon plug-flow and axial dispersion model. Mass transfer coefficients were found greater in case of axial dispersion model than that of plug-flow model specially at low Reynolds number (Re₁ < 1).Hydrogenation of α-methylstyrene to cumene using a Pd/Al₂O₃ catalyst was taken as a model reaction. Intrinsic kinetic studies were made in a laboratory-stirred-autoclave. Mass transfer coefficients were determined using these intrinsic kinetic data from the process kinetic measurements in trickle-bed reactor. Mass transfer coefficients under reaction conditions were found to be considerably higher than those obtained by absorption measurements.Correlations were suggested for predicting mass transfer coefficients at low Reynolds number.The gas to liquid mass transfer coefficients for lower gas and liquid flow rates were determined in a laboratory trickle-bed reactor. The effect of axial dispersion on mass transfer was considered in order to evaluate the experimental data. Three correlations were formulated to calculate the mass transfer coefficients, which included the effect of liquid loading, particle size and the properties of the reacting substances. The gas flow rate influences the gas to liquid mass transfer only in the region of low gas velocities. In the additional investigations of gas to liquid mass transfer without reaction in trickle-bed reactor, the mass transfer coefficients were determined under reaction conditions and the intrinsic kinetics was studied in a laboratory scale stirred autoclave with suspended catalyst. A few correlations are formulated for the mass transfer coefficients. A comparison with the gas-liquid mass transfer coefficient obtained by absorption measurements showed considerable deviations, which were illustrated phenomenologically.     

Hydrodynamics and mass transfer behavior in multiple draft tube airlift contactors

The draft tube configuration significantly affected the performance of an airlift contactor. The multiple draft tube configuration was demonstrated to give a better gas-liquid mass transfer when compared with a conventional one-draft-tube system. The airlift with a larger number of draft tubes allowed a higher level of bubble entrainment, which rendered a high downcomer gas holdup. This resulted in a higher overall gas holdup in the contactor. Liquid velocity was also enhanced by increasing the number of draft tubes. The ratio between downcomer and riser cross sectional areas, A _d /A _r , had a great effect on the system performance, where a larger A _d /A _r led to a lower downcomer liquid velocity and smaller quantity of gas bubbles being dragged into the downcomer. This resulted in low gas holdup, and consequently, low gas-liquid interfacial mass transfer area, which led to a reduction in the overall volumetric mass transfer coefficient. The presence of salinity in the system drastically reduced the bubble size and subsequently led to an enhancement of gas entrainment within the system. As a result, higher gas holdups and gas-liquid interfacial area were observed, and hence, a higher rate of gas-liquid mass transfer was obtained.     

Interfacial area and volumetric mass transfer coefficient in a bubble reactor at elevated pressures

The present study deals with the pressure effects on mass transfer parameters within a bubble reactor operating at pressures up to . The gas-liquid systems are N₂/CO₂-aqueous solution of Na₂CO₃-NaHCO₃ and N₂/CO₂-aqueous solution of NaOH. A sintered powder plate is used as a gas distributor. Three parameters characterizing the mass transfer are identified and investigated with respect to pressure: the gas-liquid interfacial area a, the volumetric liquid side mass transfer coefficient k_La and the volumetric gas side mass transfer coefficient k_Ga. The gas-liquid absorption with chemical reaction is used and the mass transfer parameters are determined by using the model reaction between CO₂ and the aqueous solutions of Na₂CO₃-NaHCO₃ and NaOH. For a given gas mass flow rate, the interfacial area as well as the volumetric liquid mass transfer coefficient decrease with increasing operating pressure. However, for a given pressure, a and k_La increase with increasing gas mass flow rates. The mass transfer coefficient k_L is independent of pressure. Furthermore, the pressure increase results in a decrease of k_G and k_Ga for a given gas mass flow rate. The values of the interfacial area, which are obtained from both chemical systems are found to be different. These discrepancies are attributed to the choice of the liquid system in the absorption reaction model.     

Effects of enzyme microcapsule shape on the performance of a nonisothermal packed-bed tubular reactor

The effects of enzyme microcapsule shape (spherical, cylindrical and flat plate) on the performance of a nonisothermal, packed-bed reactor have been modeled as a function of Biot number and Peclet number for mass and heat transfer (Bi_m, Bi_h, Pe_m and Pe_h), and dimensionless heat of reaction α. Under the given simulation conditions, only higher values of Bi_m and Bi_h (>2·5) confirm the influence of microcapsule shape on the reactor performance such that the axial and overall conversion and bulk temperature decrease as follows: spherical > cylindrical > flat plate. In terms of the shape-independent modified Biot number, Bi* = Bi/{(n + 1)/3}, this order is retained for 2 < Bi* < 8. The influence of increasing Pe_m, Pe_h, and α on conversion and bulk temperature also follows the above order. For the flat plate, the exit conversion and temperature are not influenced by Pe_m and Pe_h, that is, mass transfer and thermal backmixing effects, respectively. On the other hand, for the spherical and cylindrical microcapsules, overall backmixing effects are negligible only beyond a critical value of Pe_m (∼7) and Pe_h (∼1·75). The conversion and bulk temperature increase with the increase in α, independent of the microcapsule shape. The spherical and cylindrical microcapsules, unlike the flat plate, cannot be considered isothermal.     

Hydrodynamics,Mass Transfer and Gas Scrubbing in a Co‐current Droplet Column Operating at High Gas Velocities

The main objective of this work was to propose a new process for household fume incineration treatment: the droplet column. A feature of this upward gas‐liquid reactor which makes it original, is to use high superficial gas velocities (13 m s^–1) which allow acid gas scrubbing at low energy costs. Tests were conducted to characterize the hydrodynamics, mass transfer performances, and acid gas scrubbing under various conditions of superficial gas velocity (from 10.0 to 12.0 m s^–1) and superficial liquid velocity (from 9.4·10^–3 to 18.9·10^–3 m s^–1). The following parameters characterized the hydrodynamics: pressure drops, liquid hold‐ups, and liquid residence time distribution were identified and investigated with respect to flow conditions. To characterize mass transfer in the droplet column, three parameters were determined: the gas‐liquid interfacial area (a), the liquid‐phase volumetric mass transfer coefficient (k_La) and the gas‐phase volumetric mass transfer coefficient (k_Ga). Gas absorption with chemical reaction methods were applied to evaluate a and k_Ga, while a physical absorption method was used to estimate k_La. The influence of the gas and liquid velocities on a, k_La, and k_Ga were investigated. Furthermore, tests were conducted to examine the utility of the droplet column for the acid gas scrubbing, of gases like hydrogen chloride (HCl) and sulfur dioxide (SO₂). This is a process of high efficiency and the amount of pollutants in the cleaned air is always much lower than the regulatory European standards imposed on household waste incinerators.     

A study of oxygen absorption kinetics in ionic Cu(I) aqueous solutions

We developed the oxidation reaction of Cu(I) ion in aqueous hydrochloric solution by oxygen in a gas-lift capillary bubble column. The method of gas-liquid absorption with chemical reaction was used. Test data confirmed that the rate constant of the reaction is strongly affected by solution compositions, and that chloride ions induce an inhibition effect. The kinetical orders found are equal to 1 and 2 for oxygen and Cu(I) respectively. The influence of temperature on reaction rate constant showed the existence of a maximum value between 303 and 313K. The effect of gas-liquid of gas-liquid system on the liquid-side mass transfer coefficient k_L was also investigated.     

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.     

Absorption of carbon dioxide into glycidyl methacrylate solution containing the triethylamine immobilized ionic liquid on MCM-41 catalyst

An ionic liquid (TEA-MS41), triethylamine-immobilized on chloropropyl-functionalized MCM-41, was synthesized by a grafting technique through a co-condensation method and used as a catalyst in the reaction of carbon dioxide with glycidyl methacrylate (GMA). CO₂ was absorbed into the heterogeneous system of the GMA solution and dispersed with solid particles of the catalyst in a batch stirred tank with a plane gas-liquid interface at 101.3 kPa. The absorption of CO₂ was analyzed by using mass transfer accompanied by chemical reactions based on film theory. The proposed model fits the measured data of the enhancement factor to obtain the reaction rate constants. Solvents such as N,N-dimethylacetamide, N-methyl-2-pyrrolidinone, and dimethyl sulfoxide influenced the reaction rate constants.     

EXPERIMENTAL STUDY ON GAS-HOLDUP AND GAS-LIQUID INTERFACIAL AREA IN TPCFBs∗

A new fiber optic probe system has been developed for measuring the gas-holdup ?_g and bubble size distribution P_s(d_b) in multiphase reactors. Experiments have been carried out in a gas-liquid-solid three-phase circulating fluidized bed (TPCFB) with 140mm inner diameter and 3 m height, in which air, tap water and 0.4 mm glass beads are used as the gas, liquid and solid phase, respectively. Based on the measured data, the gas-liquid interracial area a, one or the most important parameters concerning the mass transfer, has been derived. The radial profile of the gas holdup and the gas-liquid interracial area, and the influence or main operating conditions have been studied experimentally in TPCFBs.     

Gas-liquid mass transfer in two and three-phase upflows through a vertical tube

Oxygen absorption rates were measured to determine volumetric coefficients of gas-liquid mass transfer coefficients k_La in gas-liquid and gas-liquid-solid upward flows through a vertical tube. The liquid was deionized water or aqueous glycerol solution, and the solids were glass beads or polystyrene beads. The dependencies of k_La on gas velocity, liquid velocity, temperature, solid material, and solid concentration were examined. The experimental results were correlated with empirical equations. The mechanisms of the solid loading effect are discussed.     

Modelling reactive absorption of CO2 in packed columns for post-combustion carbon capture applications

A rate-based process model for the reactive absorption of carbon dioxide (CO₂) from a gas mixture into an aqueous monoethanolamine (MEA) solution in a packed column is developed. The model is based on the fast second-order kinetics for the CO₂-MEA reactions and takes into account the mass transfer resistances. The heat effects associated with the absorption and chemical reaction are included through energy balances in the gas and liquid phases. Appropriate correlations for the key thermodynamic and transport properties and for the gas-liquid mass transfer are incorporated into the model to ensure reliable predictions. The model predictions are validated by simulating a series of experiments conducted in pilot and industrial scale absorption columns with random and structured packings reported in the literature. Comparisons between the simulation results and the experimental data reveal good quality predictions of the gas phase CO₂ and MEA concentrations and the liquid temperature along the column height. The sensitivity studies reveal that the correlations for the gas- and liquid-film mass transfer coefficients given by Onda et al. (1968) provide better predictions than the penetration theory of Higbie (1935) and the correlation of Bravo et al. (1985).     

A refined model for ozone mass transfer in a semibatch stirred vessel

The mathematical model proposed by Anselmi et al. (1984) for a semibatch stirred gas‐liquid contactor is refined to describe the mass transfer of ozone absorption and decomposition in aqueous solution with the decomposition rate expression of general reaction orders (not necessarily integers). Three system equations are employed to describe the ozone concentrations in the bulk liquid (C_ALb), the hold‐up gas (C_AGi), and the outlet gas in the free volume above the liquid surface (C_AGe), respectively. The effect of ozone decomposition on the mass transfer, which is reflected by the enhancement factor (E_r) defined as the ratio of mass absorbed per unit area in time t with chemical reaction (r) to that without chemical reaction or of the purely physical absorption, is considered in the refined model. Furthermore, the refined model also takes into account the variation of E_r with C_ALb, which changes with time during the course of gas‐liquid contacting. Thus this analysis extends the applicability of the model of Anselmi et al. (1984) and is of special importance for ozone mass transfer in the cases of basic solutions and of low mass transfer coefficients, in which the effect of decomposition on absorption is significant, and in the system with variable liquid phase ozone concentration.     

EFFECT OF STATIC LIQUID HEIGHT ON GAS-LIQUID MASS TRANSFER IN A DRAFT-TUBE BUBBLE COLUMN AND THREE-PHASE FLUIDIZED BED

Experiments are performed under batch-liquid operating conditions to investigate the effect of static liquid height on the gas-liquid mass transfer coefficient (K_La) in a draft-tube bubble column (DTBC) and a draft-tube three-phase fluidized bed (DTFB). In addition, the effects of column diameter, gas-distributor, and draft-tube diameter are studied. The results indicate that for a given system with a porous plate gas-distributor at low superficial gas velocities (<70 m/hr), increasing static liquid height decreases K_La. At high gas velocities, K_La is independent of the static liquid height. For systems with a perforated gas-distributor, there is no effect of static liquid height on K_La. The formation of small dispersed bubbles at low gas velocities in the porous plate distributor system accounts for the considerably high K_La values and the observed effect of liquid height. On the other hand, the formation of large spherical-cap bubbles and the bubble coalescence at high gas velocities reduce the performance of the porous plate distributor system to that of the perforated one.     

Automated measurements of gas‐liquid mass transfer in micropacked bed reactors

Gas‐liquid mass transfer in micropacked bed reactors is characterized with an automated platform integrated with in‐line Fourier transform infrared spectroscopy. This setup enables screening of a multidimensional parameter space underlying absorption with chemical reaction. Volumetric gas‐liquid mass‐transfer coefficients (k_La) are determined for the model reaction of CO₂ absorption in a methyl diethanolamine/water solution. Parametric studies are conducted varying gas and liquid superficial velocities, packed bed dimensions and packing particle sizes. The results show that k_La values are in the range of 0.12～0.39 s^?1, which is about one‐to‐two orders of magnitude larger than those of conventional trickle beds. An empirical correlation predicts k_La in micropacked bed reactors in good agreement with experimental data. © 2017 American Institute of Chemical Engineers AIChE J, 64: 564–570, 2018     

Mass transfer to volumetric electrodes with two-phase flow

Experimental studies of mass transfer were conducted in stacked screens with a gas-liquid mixture flowing through the bed. Depending on the gas and liquid flow rates and on the geometric characteristics of the screens, different flow regimes are obtained. In the heterogeneous flow regime the gas phase controls mass transfer, meanwhile in the transition and bubbling flow regimes the influence of the liquid flow prevails. Appropriate dimensionless groups correlate the mass transfer coefficients with the pertinent variables for the different regimes.Nomenclature A electrode area (cm²) - A ₁ surface area of one screen (cm²) - c _o bulk concentration (mol cm^–3) - D diffusivity (cm²s^–1) - d particle or wire diameter (cm) - F Faraday's constant - i limiting current (A) - k mass transfer coefficient (cm s^–1) - N distance between wires (cm) - Re _g Reynolds number for gas flow,Re _g=u _g R _h v _g ^–1 - Ré _g Reynolds number for gas flow,Re ₁=u ₁ R _h v ₁ ^–1 - Re ₁ Reynolds number for liquid flow,Re' ₁=u ₁ dv ₁ ^–1 - Ré ₁ Reynolds number for liquid flowRe ₁=u ₁ R _h v ₁ ^–1 - R _h hydraulic radius of screen bed (cm) - S _c Schmidt number,Sc=v ₁ D ^–1 - Sh Sherwood number,Sh=kdD ^–1 - Sh ₀ Sherwood number without gas,Sh ₀ =kdD ^–1 - u _g superficial gas velocity (cm s^–1) - u ₁ superficial liquid velocity (cm s^–1) - screen thickness (cm) - porosity - v kinematic viscosity (cm²s^–1) - specific area (cm^–1)     

Mass transfer from a contaminated fluid sphere

The unsteady mass transfer from a contaminated fluid sphere moving in an unbounded fluid is examined numerically for unsteady‐state transfer. The effect of the interface contamination and the flow regime on the concentration profiles, inside and outside a fluid sphere, is investigated for different ranges of Reynolds number (0 < Re < 200) and Peclet number (0 < Pe < 10⁵), viscosity ratio between the dispersed phase and the continuous phase (0 < κ < 10), and the stagnant‐cap angle (0° < θ_cap < 180°). It was found that the stagnant‐cap angle significantly influences the mass transfer from the sphere to a surrounding medium. For all Peclet and Reynolds numbers and κ, the contamination reduces the mass transfer flux. The average Sherwood number increases with an increase of stagnant‐cap angle and reaches a maximum equal to the average one for a clean fluid sphere at low viscosity ratio and large Peclet numbers. A predictive equation for the Sherwood number is derived from these numerical results. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Simultaneous measurement of gas hold-up and mass transfer coefficient by tracer dynamic technique in “Emulsair” reactor with an emulsion-venturi distributor

Hydrodynamics and gas-liquid mass transfer have been investigated in an “Emulsair” reactor with cocurrent downflow of gas and liquid. This consists of a cylindrical tank with conical bottom topped by an emulsion-venturi as the gas-liquid distributor in which the gas is self-aspired by action of the kinetic energy of the liquid recirculation. An original tracer dynamic technique using the CO₂-N₂/water system that enables the simultaneous measurement of overall gas hold-up and overall volumetric mass transfer coefficient has been developed and validated using conventional techniques, such as volume expansion for gas hold-up and dynamic oxygenation for mass transfer. It has been shown that gas hold-up and K_La_L can be deduced from the moments of experimental response curves using a CO₂ pulse in the gas feed. Experimental results have proved that hydrodynamics and mass transfer in the Emulsair reactor are strongly influenced by the flow regime in the divergent. Two different regimes have been observed depending on the liquid recirculation flow rate: annular and homogeneous bubbling flows. In both regimes, self-aspired gas flow rate, gas hold-up and K_La_L have been reported to increase with the liquid flow rate. The operation effectiveness, estimated from the gas-to-liquid flow rate ratio, has been shown to pass through a maximum around 0.59 as a function of liquid recirculation. A comparison with the literature proved that this maximum is higher than those observed for other kinds of gas-liquid reactors equipped with a venturi. Correlations for mass transfer estimation have been derived and are in agreement with the literature.     

Design calculations for countercurrent gas-liquid reactors

A mathematical model has been developed for a countercurrent differential contactor in which a gas-liquid reaction occurs. The local mass transfer rates near the gas-liquid interface are computed from a stagnant film model that includes a second-order reaction term. The contactor model accounts for enhancement of the gas absorption rate as well as the contributions to liquid reactant conversion from both the boundary layer region and the bulk liquid.Orthogonal collocation based on Lobatto polynomials has been used to solve the nonlinear equations for both the local mass transfer rates and the macroscopic differential mass balances. Parametric studies indicate that the number of transfer units, the chemical reaction kinetics and the absorption factor all play important roles in determining reactor performance. Collocation techniques are also employed to solve the design equations that include liquid-phase axial dispersion in the countercurrent gas-liquid reactor. These techniques have been found to be sufficiently efficient and robust that they should be useful for reactor design and parameter estimation applications.