Volumetric Liquid-side Mass Transfer Coefficient Research Articles

Chemical Absorption of Carbon Dioxide into Aqueous Colloidal Silica Solution with Triethanolamine

Carbon dioxide was absorbed into an aqueous nanometer sized colloidal silica solution of 0–31 wt% and triethanolamine of 0–2 kmol/m3 in a flat-stirred vessel with an impeller size of 0.05 m and speed of 50 rpm at 25°C and 101.3 kPa to measure the absorption rate of CO2. The measured rate of CO2 absorption was compared with the values estimated from the model based on the film theory accompanied by chemical reaction. The volumetric liquid-side mass transfer coefficient (kLaL) of CO2 decreased with increasing silica concentration and was expressed as an empirical correlation formula presenting the relationship between kLaL and the rheological behavior of the aqueous colloidal silica solution. Reduction of the measured kLaL was explained by the viscoelastic properties of the aqueous colloidal silica solution.

Absorption of carbon dioxide into aqueous PAA solution containing diethanolamine

Carbon dioxide was absorbed into aqueous polyacrylamide (PAA) solution containing diethanolamine (DEA) of 0–2 kmol/m 3 in a flat-stirred vessel with the impeller of 0.034 m and agitation speed of 50 rpm at 25 °C and 0.101 MPa to measure the absorption rate of CO 2. The volumetric liquid-side mass transfer coefficient ( k L a) was obtained from the dimensionless empirical correlation formula presenting the rheological behavior of aqueous PAA solution. PAA with elastic property of non-Newtonian liquid made the rate of chemical absorption of CO 2 accelerate compared with Newtonian liquid based on the same viscosity of the solution. The estimated value of the absorption rate of CO 2 was obtained from the model based on the film theory accompanied by chemical reaction and compared with the measured value.

Influence of polyethylene oxide on absorption of carbon dioxide into aqueous N-methyldiethanolamine solution

Carbon dioxide was absorbed into aqueous polyethylene oxide (PEO) solution containing N-methyldiethanolamine (MDEA) in a flat-stirred vessel to investigate the effect of non-Newtonian rheological behavior of PEO on the chemical absorption rate of CO2, where the reaction between CO2 and MDEA was assumed to be a first-order reaction with respect to the concentration of CO2 and MDEA, respectively. A unified correlation equation containing the Deborah number, which reflects the viscoelastic properties of a non-Newtonian liquid, was used to obtain the volumetric liquid-side mass transfer coefficient of carbon dioxide in aqueous PEO solution. The elastic properties of PEO accelerated the absorption rate of CO2 compared with that of a Newtonian liquid based on the same values of viscosity.

Solid effects on gas–liquid mass transfer in catalytic slurry system of isobutene hydration over fine ion exchange resin particles

To study solid effects on gas–liquid mass transfer in slurry system accompanied by chemical reaction, the experiments of isobutene hydration over fine cation exchanger resin were performed in three-phase slurry catalytic reactor. The mathematical model for the reactor with multi-parameter estimation procedures was developed to obtain the volumetric liquid-side mass transfer coefficient (kLa). The experimental study with different catalyst particle sizes and catalyst solid loadings has shown that gas–liquid mass transfer can be significantly enhanced by chemical reaction on the surface of fine catalyst particles in the liquid film at low solid concentrations. The kLa increases with decreasing catalyst particles mean size, and with increasing catalyst loading. © 2007 Elsevier B.V. All rights reserved.

Mass transfer characteristics of a novel multi-stage external loop airlift reactor

This paper reports on the experimental investigation carried out to evaluate the interfacial area, a, and liquid-side volumetric mass transfer coefficient, kLa, to characterize a multi-stage external loop airlift reactor (with contraction and expansion disks), which has been conceived, designed, and fabricated as an equipment to remove trace pollutants from wastewater. In addition, it has versatile use as a gas–liquid contactor in chemical process industries. In the experimental range of gas and liquid flows investigated, the maximum interfacial area and volumetric mass transfer coefficient obtained was in the order of 300 m2/m3 and 0.05 s−1, respectively. Correlations developed for predicting the interfacial area and liquid-side volumetric mass transfer coefficient have been found to be encouraging and highly significant from statistical analysis.

Vapour- and liquid-side volumetric mass transfer coefficients measured in distillation column. Comparison with data calculated from absorption correlations

Volumetric mass transfer coefficients in liquid and vapour phases in distillation column were measured by the method consisting of a fitting of the concentration profile of liquid phase along the column obtained by the integration of a differential model to the experimental one. The mathematical model of distillation process includes mass and energy balances and the heat and mass transfer equations. The film model flux expressions with the convective transport contributions have been considered in the transfer equations. Vapour and liquid phases are supposed to be at their saturated temperatures along the column. Effect of changes of phase flows and physical properties of phases on the mass transfer coefficients along the column and non-ideal thermodynamic behaviour of the liquid phase have been taken into account. The concentration profiles of liquid phase are measured in the binary distillation of the ethanol–water and methanol–ethanol systems at total reflux on metal Pall Rings and Intalox saddles 25 mm in the column with diameter of 150 mm. The distillation mass transfer coefficients obtained by the fitting procedure are compared with those calculated from absorption data using Onda's, Billet's and Linek's correlations. The distillation heat transfer coefficients calculated from the model assuming saturated temperatures in both phases are compared with those calculated from the Chilton–Colburn and penetration model analogy between mass and heat transfer. The results have confirmed an agreement neither between distillation and from absorption correlations calculated mass transfer coefficients nor between analogy and from enthalpy balance calculated heat transfer coefficients. Also the concentration profiles obtained by the integration of the differential model of the distillation column using the coefficients from absorption correlation have differed from the experimental profiles considerably.

A New Correction Factor for Theoretical Prediction of Mass Transfer Coefficients in Bubble Columns

The volumetric liquid-side mass transfer coefficients kLa in 1-butanol, toluene, ethanol and tap water were used in order to show that, when the classical penetration theory is applied to ellipsoidal bubbles, it needs to be corrected. The kLa data were obtained in a 0.102 m diameter bubble column operated in the homogeneous flow regime at pressures varied from 0.1 to 1.0 MPa. The new correction factor (lower than unity) was correlated with the dimensionless Eötvös number, Eo (gravitational force to surface tension force ratio). The kLa data (32 points) are fitted with a mean error of 8.2%.

In Situ Measurement of Steady-State Hydrogen Concentrations during a Hydrogenation Reaction in a Gas-Inducing Stirred Slurry Reactor

A new instrument, the Fugatron, is presented that was developed to make in situ measurements of dissolved hydrogen concentrations in laboratory reactors. The principle of measurement is based on a probe with a tip that consists of a small porous metal substrate coated with a dense layer of fluoropolymer. The probe is exposed to the liquid phase and is internally purged by a constant flow of carrier gas. Driven by the hydrogen concentration gradient between the external and internal side of the composite layer, the hydrogen molecules permeate into the carrier gas stream. The concentration of the permeate in the carrier gas stream is measured by a gas analyzer. The analyzer signal is proportional to the partial pressure of dissolved gas. The steady-state concentration of dissolved hydrogen in the liquid is calculated from the analyzer signal by using solubility data. Calibration methods are described for immersion probes used for in situ measurements in reactors which are operated in batch or continuous mode as well as for flow-through probes used for liquid stream analysis. The sensitivity of the instrument and the reliability of the measurements were tested in different solutions as a function of temperature and pressure. The hydrogenation of an alkyne with a suspended palladium catalyst was chosen as a model reaction for in situ measurements. The hydrogenation experiments were carried out in a 0.5-L stirred-tank reactor with a turbine stirrer in a semibatch mode. Initial hydrogen uptake rates and dissolved hydrogen concentrations during this model reaction were measured as a function of the amount of catalyst in the liquid. The solubility of hydrogen in the pure liquid was determined by a gas absorption measurement at reaction conditions. From these measurements, the volumetric liquid-side mass transfer coefficient kLa at the gas−liquid interface was calculated as a function of catalyst loading. In addition, the kLa value was also determined in the absence of solid particles by the dynamic pressure drop method. Discrepancies that may result from calculating steady-state dissolved hydrogen concentrations using kLa values obtained from measurements conducted either in pure liquids or in liquids that contain suspended catalyst particles are discussed and illustrated by these measurements. The new method allows direct measurement of the concentrations of dissolved gases during a reaction, which can be a decisive factor for the rate as well as for the selectivity of a process. Estimations of the gas concentrations via reaction rate data and cumbersome determinations of kLa values are, therefore, no longer necessary.

Simultaneous Determination of Vapour- and Liquid-Side Volumetric Mass Transfer Coefficients in Distillation Column

The method for the determination of kLa and kVa in distillation column consists in a fitting of the concentration profile along the column obtained by the integration of a differential model to the experimental one. Preliminary test of the method is presented using concentration profile measured in the binary distillation of the ethanol–water system at total reflux on metal Pall Ring 25 mm. The distillation is modelled as a simultaneous heat and mass transfer process and the mathematical model includes mass and energy balances and the heat and mass transfer equations. By using the Maxwell–Stefan flux expressions the convective transport contributions have been considered in the transfer equations. Vapour and liquid phases are supposed to be at their saturated temperatures along the column. Concentration and temperature dependent physical parameters and the liquid phase thermodynamic non-idealities have been taken into account. The distillation mass transfer coefficients in the liquid and vapour phases obtained by the fitting procedure are compared with those calculated from absorption data using Onda's, Billet's and Linek's correlations. The distillation heat transfer coefficients calculated from the model assuming saturated temperatures in both phases are compared with those calculated from the Chilton–Colburn and penetration model analogy.

Gas-LiquidMassTransferin Continuous Oscillatory Flow BaaledColumnina

Gas-liquid mass transfer in continuous oscillatory flow was conducted in a vertical baffled column. Pure carbon dioxide (C02) was used as the dispersed phase and tap water was used as the continuous phase. The mass transfer rate of C02 measured under continuous operation was expressed in terms of the liquid-side volumetric mass transfer coefficient (kLa) and was calculated using a stationary method. The effects of oscillation frequency, oscillation amplitude, and flow rates on mass transfer were also determined. The results showed that a significant increase in mass transfer could be achieved in oscillatory flow in a baffled column comp.:lfed to that in a bubble column. The mass transfer in continuous oscillatory flow in a baffled column was not affected by the liquid flow rate in the range tested. Then, kLQwas correlated as a function of power density and superficial gas velocity.

Absorption of Carbon Dioxide into Non-Newtonian Liquid. I. Effect of Viscoelasticity

Carbon dioxide was absorbed into benzene solutions of PB (polybutene) and PIB (polyisobutylene) in an agitated vessel to get the liquid-side volumetric mass transfer coefficient (k L a). The effect of liquid-phase viscosity, pseudoplasticity, and viscoelasticity of the benzene solutions of PB and PIB, and speed and size of impeller on the gas-liquid mass transfer have been investigated. The elastic properties, considered in the form of the Deborah number, were found to decrease the volumetric mass transfer coefficient. On the basis of experimental data of k L a, a dimensionless equation for k L a was proposed by using the Deborah number.

Hydraulic and Mass Transfer Characteristics of Packings for Absorption and Distillation Columns. Rauschert-Metall-Sattel-Rings

Hydraulic and mass transfer data including pressure drop, liquid holdup, gas- and liquid-side volumetric mass transfer coefficients and interfacial area are measured for Rauschert-Metall-Sattel-Rings (RMSR) 25, 40 and 50. The volumetric gas- and liquid-side mass transfer coefficients are measured using the absorption systems SO 2 /air-1 M NaOH and O 2 /air-water, respectively. The interfacial area is measured by the C02/air-l M NaOH absorption system. The data are described by 2- or 3-parameter empirical correlations and, with significantly larger scatter, also by well-known models taken from the literature. The results are compared with those for classical 25 and 50 mm metal Pall rings and with the results given by the Norton company for geometrically similar Intalox metal tower packing—packings for which a similar range of applications can be expected. Height equivalent to a theoretical plate (HETP) calculated from the mass transfer characteristics of the RMSR packings are compared with the experimentally measured HETP for the atmospheric distillation of isooctane-toluene, methanol-ethanol and ethanol-water.

Mass Transfer Characteristics of Syngas Components in Slurry System at Industrial Conditions

The gas-liquid mass transfer behavior of syngas components, H2 and CO, has been studied in a three-phase bubble column reactor at industrial conditions. The influences of the main operating conditions, such as temperature, pressure, superficial gas velocity and solid concentration, have been studied systematically. The volumetric liquid-side mass transfer coefficient kLa is obtained by measuring the dissolution rate of H2 and CO. The gas holdup and the bubble size distribution in the reactor are measured by an optical fiber technique, the specific gas-liquid interfacial area aand the liquid-side mass transfer coefficient kL are calculated based on the experimental measurements. Empirical correlations are proposed to predict kL and a values for H2 and CO in liquid paraffin/solid particles slurry bubble column reactors.

Influence of Pressure on the Hydrodynamics and Mass Transfer Parameters of an Agitated Bubble Reactor

The present study deals with the pressure effects on the hydrodynamic flow and mass transfer within an agitated bubble reactor operated at pressures between 105 and 100 × 105 Pa. In order to clarify the flow behavior within the reactor, liquid phase residence time distributions (RTD) for different operating pressures and gas velocities ranging between 0.005 m/s and 0.03 m/s are determined experimentally by the tracer method for which a KCl solution is used as a tracer. The result of the analysis of the liquid-phase RTD curves justifies the tank-in-series model flow for the operating pressure range. Good agreement is obtained between theoretical and experimental results assuming the reactor is operating as perfectly mixed. Two parameters characterizing the mass transfer are identified and investigated in respect to pressure: the gas-liquid interfacial area and volumetric liquid-side mass transfer coefficient. The chemical absorption method is used. 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 kLa increase with increasing gas mass flow rates. The mass transfer coefficient kL is independent of pressure.

Hydrodynamic and mass transfer characteristics of ejector loop reactors

Volumetric liquid-side mass transfer coefficient k L a was measured in the upflow ejector (with and without swirl) and in the cocurrent ejector–distributor bubble column as well as gas and liquid phase backmixing in the column. Water and 0.3 M aqueous solution of Na 2SO 4 were used to simulate coalescent and non-coalescent batches. Different methods were used to measure k L a: dynamic pressure-step (DPM), steady-state physical (SPM), classic sulfite (CSM) and steady-state sulfite method (SSM). Pure oxygen and air were used as the gas phase. Large intensity of liquid mixing in the column caused by the jet with high kinetic energy was confirmed. Gas backmixing in the coalescent batch was well described by the axial dispersion model while gas backmixing in the non-coalescent batch was successfully described by the model of two parallel series of four perfect mixers with backflow, where the respective branches corresponded to the small and large bubbles present in the dispersion, was therefore proposed. Experimental data indicated lower extent of mixing in large bubbles as compared with the smaller ones. Installation of a swirl significantly influenced bubble sizes in the non-coalescent batch. Values of k L a ej in the ejector with a swirl were lower than those for the ejector without a swirl, both in the coalescent and non-coalescent batches. The coefficient k L a col in the column was well correlated with gas holdup for the coalescent batch. A good agreement was observed between the data obtained by DPM and SPM methods. In the non-coalescent batch the chemical methods were used as the DPM method failed being limited by the response rate of oxygen probes used. Both the methods (CSM and SSF) yielded similar values of k L a col in the case of oxygen absorption. Absorption of air yielded significantly lower values k L a col (by factor of two) when a model of a single series of four perfect mixers was used for description of the gas flow in the reactor. The model of two parallel series of four perfect mixers with backflow was therefore used, taking into account particular contributions of small and large bubbles to the overall mass transfer in the column. It has been shown that decisive part of oxygen has been absorbed from the small bubbles.

The Performance of Structured Packings in Trickle-Bed Reactors

An experimental study was carried out to investigate whether the use of structured packings might improve the mass transfer characteristics and the catalyst effectiveness of a trickle-bed reactor. Therefore, the performances of a structured packing, consisting of KATAPAK elements, and a dumped packing, consisting of small diameter spherical particles, have been compared for both a chemisorption process and a process where a heterogeneously catalysed chemical reaction is carried out. The chemisorption of CO2 in aqueous amine solutions and the hydrogenation of ?-methylstyrene catalysed by palladium on ?-alumina were chosen as model reactions, respectively. The performance of the trickle-bed reactor was quantified by measuring the specific gas-liquid contact area and the volumetric liquid-side mass transfer coefficient in the case of the chemisorption process and the conversion rate in the case of the heterogeneously catalysed chemical reaction. These parameters were measured for both packing types as a function of a number of process parameters. In this paper, the experimental results are presented and a comparison is made of the performances of the two packing types for both types of processes. Both packing types showed similar mass transfer characteristics as well as volumetric conversion rates. However, the structured packing showed a much higher contact efficiency as well as a much higher catalyst effectiveness. A significant improvement is therefore expected when a structured packing is used with a higher specific geometrical area than that applied in this study. Furthermore, the structured packing is favoured in the case of fast exothermic liquid-phase reactions

Gas−Liquid Interfacial Mass Transfer in Trickle-Bed Reactors at Elevated Pressures

A phenomenological description and a semiempirical two-zone model are proposed for the gas−liquid interfacial areas and the volumetric liquid-side mass-transfer coefficients in cocurrent downflow trickle-bed reactors operated at elevated pressure. Gas−liquid interfacial areas, a, and volumetric liquid-side mass-transfer coefficients, kLa, are measured in the trickle flow regime at high nitrogen pressure (0.3−3.2 MPa). Use is made of diethanolamine carbamation in aqueous viscous and organic model solutions in which fast and slow absorptions of carbon dioxide occur. In order to extract genuine mass-transfer parameters, a rigorous thermodynamic model is established to account for liquid and gas nonidealities. The influence of pressure, gas and liquid superficial velocities, liquid viscosity, and packing size on the gas−liquid interfacial mass transfer is examined. At constant gas and liquid superficial velocities, increasing the reactor pressure improves the gas−liquid interfacial mass transfer at the expense of increased two-phase pressure drop and gas holdup. At high pressure, the gas−liquid flow may be viewed as a two-zone flow pattern: (i) a liquid-free gas continuous phase which delineates a macroscopic gas−liquid interface (ii) and a gas−liquid film emulsion comprised of tiny bubbles which form in the films and delineate a microscopic gas−liquid interface. Taylor's theory of fluid−fluid sheared emulsions is used to quantify the microscopic interface via the effect of pressure on the size of bubbles in the trickling film. A bubble Sauter diameter is related to viscous shear stress and surface tension force, the two competing forces that determine bubble size. The model is also extended to estimate volumetric gas−liquid mass-transfer coefficients under high-pressure conditions.

Gas-liquid mass transfer in cyclohexane oxidation process using gas-inducing and surface-aeration agitated reactors

The equilibrium solubilities, C ∗ , and volumetric liquid-side mass transfer coefficients ( k L a) for gaseous nitrogen and oxygen in liquid cyclohexane were obtained in wide ranges of pressures (1–40 bar), temperatures (380–480 K) and mixing speeds (13.3–20 Hz) in gas-inducing (GIR) and surface-aeration (SAR) agitated reactors. The C ∗ values were calculated using a modified Peng-Robinson equation of state and the transient physical gas absorption technique was employed to obtain the mass transfer coefficients. The C ∗ values for both N 2 and O 2 were found to increase with the solute gas equilibrium partial pressure and temperature. Under the same operating conditions, O 2 appeared to be more soluble in cyclohexane than N 2. The k L a values for the two gases were found to increase strongly with mixing speed and slightly increase with pressure in both reactor types. k L a values for both gases appeared to slightly increase with temperature in the SAR while an opposite trend was observed in the GIR. k L a values for both gases in the GIR were higher than those in the SAR at higher mixing speeds (16.6 and 20.0 Hz). It appeared that k L a values in the SAR were controlled by the mass transfer coefficient, k L , whereas those in the GIR were controlled by the gas-liquid interfacial area. a. The effect of gas nature on k L a values in both reactors was insignificant. Two empirical correlations to predict k L a and gas holdup values for N 2 and O 2 in liquid cyclohexane in both reactor types with ±30% accuracy were developed.

The Gas-Liquid Mass Transfer Coefficient (k La) in the Gas-Liquid Multi-stage Agitated Contactor (MAC)

Data on the volumetric liquid-side gas-liquid mass transfer coefficient, k(L)a, in a Multi-stage Agitated Contractor (MAC) are reported for three gas-liquid systems (air-water, helium-n-octane, and air-Monoethylene Glycol (MEG)). k(L)a (s(-1)) was determined using a dynamic method with moderately soluble gaseous tracers (acetylene for water and MEG and methane for n-octane). The observed response curves could be described accurately by coupling the single phase residence time distribution models for gas and liquid phase (Breman et al., (1,2)), via interfacial gas-liquid mass transfer. All k,a data could be correlated with an average relative deviation of 11.3% by a combination of Calderbanks' relation(3) for k(L) (m s(-1)) and a modified type of Calderbanks' relation(3) for the interfacial area, a (m(-1)), which takes a combined effect of the superficial gas velocity (u(G)) and the liquid viscosity (eta(L)) on a into account: a = 51.1(P/V)(0.286) sigma(-0.19)(u(G)/v(b))((0.127+21.1 eta L)) with g = acceleration of gravity (= 9.81 m(2) s(-1)), P/V = volumetric power input (W/m(-3)), v(b) = bubble rise velocity (m s(-1)) and the experimental conditions ranging from: superficial gas velocity u(G) = 0.01-0.09 m s(-1), stirring speed (N) = 10-36.7 rev s(-1), eta(L) = 0.00041-0.021 N m(-2) s, surface tension (sigma) = 0.02-0.073 N m(-1), liquid density (rho(L)) = 684-1113 kg m(-3), gas density (rho(G)) = 0.16-1.205 kg m(-3) and liquid diffusion coefficient (L-L) = 1.3 x 10(-10)-3.5 x 10(-9) m(2) s(-1). The overall influence of u(G) on k(L)a in this relation is such that k(L)a is hardly affected by u(G) both for water and n-octane, whereas, in contrast, k(L)a is predicted to increase significantly with increasing u(G) for the more viscous monoethylene glycol, The general applicability of the proposed relation for a has to be confirmed by measurements for other liquids and by measurements on a larger scale.

Gas-liquid mass transfer in a slurry reactor operating under olefinic polymerization process conditions

The equilibrium solubilities, C ∗ , and the volumetric liquid-side mass transfer coefficients, k L a, of propylene, ethylene and hydrogen in liquid n-hexane containing up to 30 wt% solid polypropylene powder were obtained in a 4−1 agitated batch reactor operated in surface-aeration mode. The data were collected under pressures between 2 and 55 bar, temperatures from 313 to 353 K, mixing speeds from 13.3 to 20.0 Hz and solid concentrations between 0 and 30 wt%. The gas solubilities were calculated using a modified Peng-Robinson equation of state (PR-EOS) and the mass transfer coefficients were determined using the transient physical gas absorption technique. The solubilities of hydrogen, ethylene and propylene in n-hexane were found to obey Henry's law and the values were not affected by the presence of solids. The gas with the closest solubility parameter to that of liquid n-hexane appeared to have the highest solubility. As expected, the mass transfer coefficients of the three gases in liquid n-hexane with and without solids increased with increasing mixing speed. The k L a values of hydrogen in n-hexane and slurries were found to slightly increase whereas those of ethylene and propylene slightly decrease with increasing the mean partial pressure of the gas component. The temperature appeared to have no effect on k L a values of hydrogen in n-hexane with and without solids while those of ethylene and propylene were slightly increased with temperature. The k L a values for the three gases increased at low solid concentration (10 wt%) and decreased at high solid concentration (30 wt%). A dramatic decrease of k L a values for ethylene and propylene in liquid n-hexane was observed at particular operating conditions ( T = 353 K, N = 13.3 Hz, W s = 30 wt% and P 1, m ≥5 bar). This behavior was attributed to the high slurry viscosity prevailed under these particular conditions. The k L a values of the three gases used in n-hexane were correlated with operating variables using empirical correlations.