Effect of water on the solubilities and mass transfer coefficients of gases in a heavy fraction of fischer-tropsch products

The effects of water on the solubilities, C^*, and volumetric mass transfer coefficients, k_La, for CO, H₂, CH₄ and CO₂ in a heavy fraction of Fischer-Tropsch liquid were examined at elevated pressures and temperatures at different mixing power inputs. For these gases, higher solubilities were measured in the hydrocarbon mixture saturated with water than those obtained in the hydrocarbon free of water. The k_La values for the four gases were slightly affected by the presence of dissolved water in the hydrocarbon mixture; and they were strongly dependent on the power input per unit liquid volume. Two empirical correlations for k_La as a function of turbine speed and pressure are proposed.     

Gas Holdup and Volumetric Mass Transfer Coefficient in a Slurry Bubble Column

The gas holdup, ?, and volumetric mass transfer coefficient, k_La, were measured in a 0.051 m diameter glass column with ethanol as the liquid phase and cobalt catalyst as the solid phase in concentrations of 1.0 and 3.8 vol.‐%. The superficial gas velocity U was varied in the range from 0 to 0.11 m/s, spanning both the homogeneous and heterogeneous flow regimes. Experimental results show that increasing catalyst concentration decreases the gas holdup to a significant extent. The volumetric mass transfer coefficient, k_La, closely follows the trend in gas holdup. Above a superficial gas velocity of 0.04 m/s the value of k_La/? was found to be practically independent of slurry concentration and the gas velocity U; the value of this parameter is found to be about 0.45 s^–1. Our studies provide a simple method for the estimation of k_La in industrial‐size bubble column slurry reactors.     

Gas‐liquid mass transfer in low‐ and medium‐consistency pulp suspensions

The volumetric gas—liquid mass transfer coefficient (k_La) was measured for low‐ and medium‐consistency pulp suspensions using the cobalt‐catalyzed sulfite oxidation technique. Mass transfer rates were measured in a high‐shear mixer for a range of operating parameters, including the rotor speed (N = 10 to 50 rev/s), gas void fraction (X_g = 0.10 to 0.40) and fibre mass concentration (C_m = 0.0 to 0.10). k_La measurements were compared with the macroscale flow regime in the vessel (characterized using photographic techniques) and correlated with energy dissipation, gas void fraction and suspension mass concentration in the mixer. We found that gas‐liquid mass transfer was significantly reduced in pulp suspensions, even for low suspension concentrations. Part of this reduction was associated with dissolved components leached from the fibres into the liquid phase. This could account for reductions in k_La of up to 30% when compared with distilled water. The fibres further reduced k_La, with the magnitude of the decrease depending on the fibre mass concentration. Correlations were developed for k_La and compared with results available in the literature.     

Performance of a UV‐assisted Hydrogen‐peroxide‐fed Spray Tower for Sulfur Dioxide Abatement

Spray towers are widely used for controlling air pollution by gases such as SO₂, CO₂, NO_x, and HCl. Results of sulfur dioxide absorption in a spray tower using solutions of 1 g L^–1 and 2 g L^–1 of hydrogen peroxide are reported. For comparison, a water and sodium hydroxide solution was also used for SO₂ abatement. The results indicate that H₂O₂ may be an important alternative for SO₂ removal in spray towers. A set of experimental removal efficiency data was obtained as a function of gas and liquid flow rates. Volumetric mass transfer coefficients (k_ga) were calculated and an experimental relationship among k_ga, gas, and liquid flow rates was proposed. As a final experiment, an oxidation process assisted by UV radiation using a 1 g L^–1 solution of H₂O₂ was carried out to speed up the SO₂ removal rate. The results obtained in this condition are similar to those achieved with a solution of 2 g L^–1 H₂O₂.     

Gas-liquid mass transfer in “dead-end” autoclave reactors

Gas-liquid volumetric mass transfer coefficients, (k_La), have been obtained for “dead-end” autoclave reactors operated in two different modes: (a) gas introduced into the gas phase, and (b) gas introduced through a dip-tube in the liquid. Three different methods of k_La determination have been compared. Effects of agitation speed, impeller diameter, gas to liquid volume ratio (V_g/V_L), position of the impeller and reactor size on k_La have been investigated. The k_La data were found to be correlated as: k_La = 1.48 × 10^?3 (N)^2.18 (V_g/V_L)^1.88 (d_I/d_T)^2.16 (h₁/h₂)^1.16 The critical speed of surface breakage, at which transition from the surface convection to the surface entrainment regime occurs, was also determined for different impeller positions, impeller diameters and gas to liquid volume ratios.     

Statistical assessment of gas/liquid mass transfer in slurry-phase propylene polymerization process

A statistical approach was developed to investigate the effects of pressure, temperature, mixing speed and solid concentration on k_La for gaseous hydrogen, ethylene, and propylene in liquid n-hexane containing solid polypropylene powder in a 4-litre agitated reactor. The solubilities of the gases appeared to follow Henry's Law. Statistical correlations to predict k_La were proposed and response surfaces were constructed. k_La values appeared to reach a maximum around 15 mass% and sharply decrease above 30 mass%. The effects of pressure and temperature on k_La were found to depend on the gas-liquid system and operating conditions used.     

Mass transfer characteristics of horizontal agitated contactors

The values of effective interfacial area, a, and liquid side mass transfer coefficient, k_La, were obtained in 0.385, 0.57 and 1.0 m i.d. horizontal agitated contactors by using chemical methods. Propeller type of impellers were employed for agitation. Two modes of operation were studied: (i) surface aeration and (ii) sparging of gas. The impeller speed was varied from 50 to 700 rev/min, and the nozzle gas velocity was varied from 40 to 160 m/s. The effects of impeller spacing, liquid submergence and nozzle gas velocity on the values of a and k_La were investigated. A scale-up criterion is discussed.     

Performance of an RTL contactor for gas-liquid systems: Effective interfacial area and volumetric mass transfer coefficient by oxidation of sodium sulphite solution

Effective interfacial area a and volumetric liquid-side mass transfer coefficient k_La of an RTL contactor were obtained at different stirring speeds by absorption of oxygen from air into 0.8 kmol/m³ sodium sulphite solution, in the presence of Co⁺⁺ ions. The values of a and k_La ranged from 80 to 150 m²/m³ and 0.0003 to 0.00053 s^?1, respectively, when stirrer speed was increased from 8 to 40 rpm. When k_L alone was evaluated, it was found to be practically constant, irrespective of stirring speed.     

Mass transfer characteristics of horizontal agitated contactors

The values of effective interfacial area, a, and liquid side mass transfer coefficient, k_La, were obtained in 0.385, 0.57 and 1.0 m i.d. horizontal agitated contactors by using chemical methods. Propeller type of impellers were employed for agitation. Two modes of operation were studied: (i) surface aeration and (ii) sparging of gas. The impeller speed was varied from 50 to 700 rev/min, and the nozzle gas velocity was varied from 40 to 160 m/s. The effects of impeller spacing, liquid submergence and nozzle gas velocity on the values of a and k_La were investigated. A scale-up criterion is discussed.     

Gas‐Liquid Mass Transfer in a Slurry Bubble Column at High Slurry Concentrations and High Gas Velocities

The volumetric mass transfer coefficient k_La in a 0.1 m‐diameter bubble column was studied for an air‐slurry system. A C₉‐C₁₁ n‐paraffin oil was employed as the liquid phase with fine alumina catalyst carrier particles used as the solid phase. The n‐paraffin oil had properties similar to those of the liquid phase in a commercial Fischer‐Tropsch reactor under reaction conditions. The superficial gas velocity U_G was varied in the range of 0.01 to 0.8 m/s, spanning both the homogeneous and heterogeneous flow regimes. The slurry concentration ?_S ranged from 0 to 0.5. The experimental results obtained show that the gas hold‐up ?_G decreases with an increase in slurry concentration, with this decrease being most significant when ?_S < 0.2. k_La/?_G was found to be practically independent of the superficial gas velocity when U_G > 0.1 m/s is taking on values predominantly between 0.4 and 0.6 s^–1 when ?_S = 0.1 to 0.4, and 0.29 s^–1, when ?_S = 0.5. This study provides a practical means for estimating the volumetric mass transfer coefficient k_La in an industrial‐size bubble column slurry reactor, with a particular focus on the Fischer‐Tropsch process as well as high gas velocities and high slurry concentrations.     

Interfacial Area and Liquid‐Side Volumetric Mass Transfer Coefficient in a Downflow Bubble Column

An experimental investigation was made to measure interfacial area, a, and liquid‐side volumetric mass transfer coefficient, k_La, in a downflow bubble column by chemical methods viz., absorbing CO₂ in aqueous sodium hydroxide and sodium carbonate/bicarbonate buffer solution respectively. The effect of gas and liquid flowrate and nozzle sizes on a and k_La were investigated. The experimental data obtained in the present system were analyzed and correlations were developed to predict a and k_La in terms of superficial gas velocity. The variation of a and k_La with specific power input were shown in graphical plot and compared with other gas‐liquid systems.     

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 k_La 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 k_L are calculated based on the experimental measurements. Empirical correlations are proposed to predict k_L and a values for H₂ and CO in liquid paraffin/solid particles slurry bubble column reactors.     

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.     

Fischer-Tropsch reactor selection

A comparison of slurry versus fixed-bed reactor design principles for methanol and Fischer-Tropsch distillate production.Notation a gas-liquid interfacial area, m^–1 - CCat catalyst concentration, kg mole/m³ - C _HG hydrogen concentration in gas phase, kg mole/m³ - C ^* _HL hydrogen concentration, liquid, in equilibrium with gas, kg mole/m³ - C _HL hydrogen concentration in the liquid phase, kg mole/m³ - D I.D. of reactor, m - GHSV Gas hourly space velocity, Nm³ (H₂ + CO)/[h · m³ reactor volume], (reactor volume is expanded slurry height times cross section area) - H solubility coefficient of hydrogen =C _HG/C ^* C _HL - l Inlet ratio of CO/H₂ - k _L liquid side mass transfer coefficient, m/s - k _H effective reaction rate constant for hydrogen consumption, s^–1 (note that to agree with space velocity in Nm³/[s · kgCat],k _H =k _H ·CCat wherek _H is in m³/[kg · s] - L Length of expanded slurry bed, m - P pressure, kPa - r rate of hydrogen consumption,r =k _H ·C _HL, kg moles/[m³ · s] - SV Space velocity in actual m³ inlet gas/[s · m³] - T temperature, K - U Usage ratio of CO/H₂ - X _H hydrogen fractional conversion per pass (IfU=l,X _H =X _CO) - contraction factor, = [m³/s(X _{H 2+CO} = 1)-m³/s(inlet)]/[m³/s(inlet)] - * contraction factor modified for H₂ conversion, * = · (1 +U)/(1 +l) - _L fractional liquid hold-up     

Gas‐Liquid Mass Transfer in Pulp Retention Towers

The volumetric gas‐liquid mass transfer rate, k_La, was measured under batch conditions in a 0.28 m diameter laboratory‐scale retention column. Tests on water, and on unbleached kraft (UBK) pulp suspensions (mass fractions, C_m from 0.013 to 0.09) were made with air or nitrogen sparged through the column at superficial gas velocities between 0.0015 to 0.05 m/s. k_La varied with suspension mass concentration and superficial gas velocity, initially decreasing with increasing mass concentration, reaching a minimum between C_m = 0.03 and 0.06, and then increasing. The minimum in k_La coincided with a change in hydrodynamics within the column, from bubble column behaviour below C_m = 0.03 to porous solid behaviour above C_m = 0.06.     

A contribution to the measurement of oxygen mass transfer in a laboratory pressure reactor

A method was used to measure the liquid‐side volumetric coefficient of oxygen mass transfer (k_La) in closed, semi‐batch pressure reactors used in hydrometallurgical laboratories. In this method, the oxygen pressure was monitored as oxygen was continuously sparged into a pressure vessel containing a sodium sulfite solution. A material balance equation was derived for oxygen in the vessel and the experimental data were fitted to this equation. From the constant parameters of the equation, k_La was calculated. The solution in the vessel also contained an appropriate amount of cobalt catalyst so that oxygen was consumed rapidly by oxidation of sulfite to sulfate. Under these conditions, the oxygen concentration in the bulk liquid phase could be assumed to be equal to zero. Values of k_La determined by the method under various conditions were reproduced within 12% deviation from the average values. k_La was found to increase moderately with temperature in the range of 25 to 75 °C, with an activation energy of 33.09 ± 1.33 kJ mol⁻¹. The presence of hydrophobic or hydrophilic solids was found to have a deleterious effect on k_La. © 2000 Society of Chemical Industry     

The cocurrent downflow contactor–a novel reactor for gas-liquid-solid catalysed reactions

The Cocurrent Downflow Contactor (CDC) has been developed as a mass transfer and reactor device, with and without addition of tangential (swirl) flow, giving gas hold-up (E_g) values of 0.5–0.75, interfacial areas in the range 1000–6000 m²m^?3 liquid and k_La values in the range of 0.15–1.55 s^?1 for absorption using the O₂/H₂O system. It has been studied as a catalytic slurry reactor for the hydrogenation of (i) itaconic acid and (ii) triglycerides catalysed by Pd and Ni catalysts. The reactions were observed to be largely surface-reaction rate controlled, due to the very efficient mass transfer (k_La up to 11.75 s^?1 under reaction conditions) and application of swirl flow-enhanced reaction rates. The CDC has recently been found to be capable of operating as a fixed bed reactor, thus eliminating a downstream catalyst separation problem (therefore more cost effective), and is superior in its mass transfer characteristics to other known devices. Scale-up can be undertaken without loss of performance efficiency.     

Bubble shape,gas flow and gas–liquid mass transfer in pulp fibre suspensions

Gas–liquid mass transfer in pulp fibre suspensions in a batch‐operated bubble column is explained by observations of bubble size and shape made in a 2D column. Two pulp fibre suspensions (hardwood and softwood kraft) were studied over a range of suspension mass concentrations and gas flow rates. For a given gas flow rate, bubble size was found to increase as suspension concentration increased, moving from smaller spherical/elliptical bubbles to larger spherical‐capped/dimpled‐elliptical bubbles. At relatively low mass concentrations (C_m = 2–3% for the softwood and C_m ? 7% for the hardwood pulp) distinct bubbles were no longer observed in the suspension. Instead, a network of channels formed through which gas flowed. In the bubble column, the volumetric gas–liquid mass transfer rate, k_La, decreased with increasing suspension concentration. From the 2D studies, this occurred as bubble size and rise velocity increased, which would decrease overall bubble surface area and gas holdup in the column. A minimum in k_La occurred between C_m = 2% and 4% which depended on pulp type and was reached near the mass concentration where the flow channels first formed.     

Molecular structure of metallocene catalyst Cptt2ZrCl2 and structural analysis of PE catalyzed by Cptt2ZrCl2

Cp^tt₂ZrCl₂ (Cp^tt = η⁵?^tBu₂C₅H₃) was synthesized by the reaction of LiCp^tt with ZrCl₄ and characterized by X‐ray crystallographic studies. It was used as catalyst for ethylene polymerization. Structural analysis was carried out on the polyethylene (PE) catalyzed by Cp^tt₂ZrCl₂ via wide‐angle X‐ray diffraction (WAXD) and small‐angle X‐ray scattering (SAXS). The degree of crystallinity (W_c,x) was calculated by WAXD. The semiaxises of the particles (a, a, b) of PE were determined by SAXS and it could be found that the crystalline particles of PE are mainly rod shaped determined by the characteristic function v₀ (r). The radius of gyration R_g, crystalline thickness L_c, the thickness of noncrystalline region L_a, long period L, electron‐density difference between the crystalline and noncrystalline regions η_c ? η_a, and the invariant Q are determined by SAXS. The results also indicate that a transition zone exists between the traditional “two phases” with a clear dimension of 1.3 nm. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 96: 169–175, 2005     

Practical identifiability analysis in dynamic gas–liquid reactors: Optimal experimental design for mass-transfer parameters determination

A dynamic gas–liquid transfer model without chemical reaction based on the unsteady film theory is analysed in order to confirm the possible identifiable parameters of the model from a given set of experimental data. The structural identifiability analysis of the model using the macroscopic concentrations at the gas and liquid phase shows that the identifiable parameters of the model are the gas hold-up, ?, the Henry's constant, H, the reciprocal of the diffusion time, D/δ², and the volumetric mass-transfer coefficient, k_La. A procedure for the optimal experimental design is proposed based on the analysis of the Fisher information matrix of the model. The analysis concludes that the measure of the dynamics of the concentration just in the liquid phase leads to important systematic errors in the determination of k_La. The importance of the concentration measurement simultaneously in the gas and the liquid phase for the parameter estimation is demonstrated and discussed.