Mass Transfer Studies in Stirred AirLift Reactor

In the present work, water and three phase compositions of Solka-Floc, a cellulose fiber for simulating the biomass in bacteria, yeast, and fungal fermentation were studied in a 1.4 m³ stirred airlift reactor. The fractional dispersed phase holdup and the overall volumetric mass transfer coefficients were measured. The dispersed phase riser gas holdup and overall volumetric mass transfer coefficients both increased with increasing riser superficial dispersed phase velocity (0.02-0.1 ms^-1) and agitator speed in the range of 0-5 rs^-1. An increase in the Solka-Floc concentration (1-3% w/v) was found to reduce ε_GR and K_La_L. Empirical correlations have been developed for fractional dispersed phase gas holdup and overall volumetric mass transfer coefficients.     

Hydrodynamic Modelling of Internal Loop Airlift Reactor Applying Drift‐Flux Model in Bubbly Flow Regime

A new model for the liquid circulation rates in airlift reactor (ALR) is presented. The model is based on the energy balance for the flow loop (riser, turn riser‐downcomer, downcomer, and turn downcomer‐riser) coupled with a drift flux theory of two‐phase flow gas‐liquid system, considering a bubbly flow regime. The predicted values of the liquid circulation rates by the developed model are compared with experimental results performed in a 22 dm³ internal loop airlift reactor and with the results obtained in the literatures. The proposed model predicted the experimental results very well. Slip velocity relationship based on the drift flux model was proposed; including the gas holdup, bubble size and the liquid physical properties. The predicted slip velocity was similar to that obtained from the literature. The study revealed that appropriate arrangements of internal bioreactor parts can positively influence the liquid circulation velocity at the same energy consumption. The proposed models are useful in the design; scale up and characterization of the internal loop airlift reactors, and provides a direct method of predicting hydrodynamic behaviour in gas‐liquid airlift reactors.     

EFFECTS OF GEOMETRY ON HYDRODYNAMICS IN EXTERNAL-LOOP AIRLIFT REACTORS

Experimental investigations were carried out in model external-loop airlift reactors. Two reactors of laboratory scale (riser liquid height ranged between 1.16-1.56 m, riser diameter 0.03 m, A_D/A_R ratio between 0.111-1,000, total liquid volume V_T = (1.189-2.446).10^-3m³) and pilot-plant scale (riser liquid height of 4.4 and 4.7 m, respectively, riser diameter 0.200 m, A_D/A_R ratio of 0.1225 and 0.040 m, total liquid volume, V_T = (0.144-0.170) m³) were used.

The influences of reactor geometry characterized by some parameter as: A_D/A_R ratio, liquid height in riser and downcomer and liquid height in gas separator, together with the amount of introduced air, on the basic hydrodynamic design parameters: gas holdup and liquid circulation velocity were analysed.

The influence of gas sparger design on gas holdup and liquid velocity was found to be negligible.

The experimental liquid circulation velocity was correlated using a simplified form of the energy balance in airlift reactors, valid for external-loop airlift reactors with almost complete phase separation at the top.

An original dimensionless correlation for gas holdup prediction involving superficial velocities of gas and liquid, cross sectional areas, dispersion height, riser diameter, as well as Froude number, was obtained.     

VOLUMETRIC MASS TRANSFER COEFFICENTS OF GAS LIQUID JET LOOP REACTORS BY OXIDATION OF HYDRAZINE

Oxidation of hydrazine in the presence of a homogeneous catalyst (CuTeSP) has been proved as a suitable reaction for the determination of volumetric mass transfer coefficients k_La of stirred tank reactors. This method is applied to a gas liquid jet loop reactor (JLR).At power inputs of N/V_R≤ 8 kW/m³ k_La values of about 0·1 and 0·4s^-1 were achieved. The evaluated values agree well with those determined by a stationary physical method.     

Hydrodynamics in rectangular air-lift reactors: Scale-up and the scale-up and the influence of gas-liquid separator design

Experiments were conducted on air-lift reactors with different liquid volumes, aspect ratios, and gas-liquid separator configurations, with constant downcomer to riser cross-sectional area ratio and riser/downcomer height. Reactor performance was highly dependent on the gas-liquid separator design. By changing the latter, the mode of operation could be changed from that of a concentric tube reactor (with gas recirculation) to an external loop reactor (without gas recirculation). Liquid velocity and overall gas hold-up data were correlated to the power per liquid volume and “Disengagement Ratio”, a geometric parameter describing the separator's configuration. During scale-up, similar gas hold-ups and liquid velocities were found in laboratory and pilot reactors. A modified air-lift reactor with an enlarged channel for a gas-liquid separator also was studied.     

Air-lift reactors in chemical and biological technology

The principle of the air-lift reactor is finding wide application in the fields of both chemical technology and biotechnology because it offers simple and effective mixing in three-phase processes involving gases, liquids and solids. The air-lift principle offers advantages of no moving parts, high gas absorption efficiency, good heat transfer characteristics and rapid mixing which are applicable in a range of industrial processes. This review considers specific aspects of air-lift reactors emphasizing their function and relevance to particular applications. The two main groups of air-lift reactor, baffled vessels and external loop reactors, have four main component parts in common, the riser, the downcomer, the base and the gas separator, all of which have distinct characteristics. The performance of these four components with respect to flow configurations, fluidization of suspended solids and mass transfer are discussed together with methods of evaluating mass transfer coefficients.     

Influences of top clearance and liquid throughput on the performances of an external loop airlift slurry reactor integrated mixing and separation

A new developed external loop airlift slurry reactor, which was integrated with gas–liquid–solid three-phase mixing, mass transfer, and liquid–solid separation simultaneously, was deemed to be a promising slurry reactor due to its prominent advantages such as achieving continuous separation of clear liquid from slurry and cyclic utilization of solid particles without any extra energy, energy-saving, and intrinsic safety design. The principal operating parameters, including gas separator volume, handling capacity, and superficial gas velocity, are systematically investigated here to promote the capabilities of mixing, mass transfer, and yield in the pilot external loop airlift slurry reactor. The influences of top clearance and throughput of the clear liquid on flow regime and gas holdup in the riser, liquid circulating velocity, and volumetric mass transfer coefficient with a typical high solid holdup and free of particles are examined experimentally. It was found that increasing the gas separator volume could promote the liquid circulating velocity by about 14.0% at most. Increasing the handling capacity of the clear liquid from 0.9 m~3·h~(-1) to 3.0 m~3·h~(-1) not only could increase the output without any adverse consequences, but also could enhance the liquid circulating velocity as much as 97.3%. Typical operating conditions investigated here can provide some necessary data and guidelines for this new external loop airlift slurry reactor to upgrade its performances.     

DIFFUSIVITY DETERMINATION FROM THE RESPONSE TO INJECTION INTO LAMINAR LIQUID FLOW IN A CAPILLARY

A method for the determination of diffusivities in liquids is described; the method is based on measurements of the response to injection in a laminar flow of liquid in a capillary. By injection of δ-type, the component whose diffusivity is to be measured is introduced into the inlet part of a capillary of internal diameter 1.6 mm. The response to the injection is measured at the end of the capillary by a flow-through refractometer, the output signal of which is recorded on tape at 16-s intervals. The output signal is compared with a theroetical model of the time dependence of the response, based on Taylor's analysis of the dispersion of an injected component in laminar flow. The diffusivity is calculated from one of the model parameters, which were evaluated by the method of maximum likelihood. The calibration of the instrument was performed by measuring the diffusivity of KC1 in aqueous solution. The following diffusivities at 25°C were determined: n-butanol 971 μm²/s, dioxane 1093μm²/s, caprolactame 948 μm²/s, Na₂SO₄, 1126 μm²/s, salicylaldoxime 895 μm²/s, cyclohexanone 960 μm²/s, acetone 1316μm²/s, oxine 820.5 μm²/s, all in water; caprolactame in trichlorethylene 1958μm²/s, caprolactame in 10% aqueous (NH₄4)₂,SO₄ solution 1148 μm²/s, salicylaldoxime in chloroform 1878 μm²/S.     

GRAIN GROWTH AND DENSIFICATION IN PALLADIUM OXIDE PARTICLES DURING SPRAY PYROLYSIS

A model for aerosol-phase densification of particles during their synthesis by spray pyrolysis is presented. The model was used to describe the evolution of the specific surface area of PdO powders synthesized at temperatures between 400 to 800 °C (residence times 27.9 to 22.7 s). Surface areas and grain sizes ranged between 56 m²/g and 4 nm at 400 °C to 3.2 m²/g and 40 nm at 800 °C The characteristic coalescence lime was determined as: τ= 1.7 x 10¹³T d³ exp(1.3 x 10⁵/RT) [for lattice diffusion] and t = 2.7 x 10¹⁹Td⁴exp (1.6 x 10⁵/RT) [for grain boundary diffusion] (τ is in seconds, Tin degrees Kelvin, R is 8.314J/(mol.K)and d is in meters), but the data and model did not allow identification of the specific mechanism. The model provides a general approach for correlating changes in particle surface area with reactor operating conditions that is applicable to a wide variety of materials.     

Sulfuric acid formation over ammonium sulfate loaded V2O5–WO3/TiO2 catalysts by DeNOx reaction with NOx

Operating the SCR DeNO_x reactor at temperatures below 200 °C results in a considerable saving in operating costs. Plant experience shows that on the catalysts in these second generation DeNO_x plants, even for flue gases with SO₂ concentration below 10 mg/m³, over 1–2 years operating time sizeable quantities of ammonium sulfates accumulate. Ammonium sulfates deposited on V₂O₅–WO₃/TiO₂ catalysts react with NO_x to nitrogen and sulfuric acid. Second-order rate constants of this reaction for temperatures of 170 °C have been derived. It could be shown that the sulfuric acid formed on the catalyst is displaced by water vapour and desorbs resulting in gas phase concentrations of up to 6.5 mg acid/m³ flue gas. Plant equipment downstream of the ammonium sulfate containing low temperature DeNO_x catalysts has to be protected against the corrosive action of the sulfuric acid in the flue gases leaving the DeNO_x reactor.     

LIQUID PHASE MIXING IN A SLURRY BUBBLE COLUMN WITH DRAFT TUBE

In this work we characterize liquid phase mixing in a 150 L bubble column with a draft tube (internal airlift configuration) for a water-air-sand system at high solid concentrations. Liquid mixing is assessed by measuring the evolution of sodium chloride concentration after a pulse of concentrated NaCl solution is injected. Tracer concentrations were measured by means of electrical conductivity probes. The experimental set up consists of a 0.29 m internal diameter, 3 m length Plexiglas column with a conical bottom (cone apex angle of 60°) and a concentric draft tube with 0.14 m internal diameter and 2 m length. The gas superficial velocity based on the cross section of the column vaired from 0.057 to 0.22 m/s. Sand particles of 280 μm in average size were used, with slurry concentrations ranging from 120 to 500 kg/m³. From the tracer outputs, circulation time (time between peaks of the response curve) and mixing time (time required to achieve a 95% homogeneous solution) were determined after the pulse had been injected. The experimental data were analyzed by means of a tank in series model with recirculation. The circulation and mixing times were found to increase with solids concentration, and to decrease as the riser gas velocity was increased. The circulation limes were larger in continuous operation than in semibatch mode. The results show that the system has equivalent dispersion coefficients that are one order of magnitude lower than those found in a conventional bubble column.     

Droplet evaporation and solute precipitation during spray pyrolysis

The process of spray pyrolysis was investigated theoretically using a model that describes the evolution of the droplet size, solvent vapor concentration in the carrier gas, and both droplet and gas temperatures along the reactor axis. The model also accounts for solute concentration profiles and solute precipitation in the solution droplets. The model was used to describe the evaporation of sodium chloride aqueous solution droplets in diffusion dryers and hot-wall reactors as a function of reactor residence time, droplet size (a few microns), solution molality (up to 2 M), droplet concentration (10⁶–10⁷ cm⁻³), relative humidity of the carrier gas (0–50%) and reactor wall conditions. Decreasing initial droplet size and solution molality accelerated droplet evaporation and resulted in smaller droplets at the onset of solute nucleation. Decreasing droplet concentration and carrier gas inlet relative humidity as well as increasing wall temperature (up to 350°C) or axial wall temperature gradient (up to 100°C cm⁻¹) increased the droplet evaporation rate, but did not change appreciably the droplet size at the point of precipitation for a given droplet size and solute concentration. Thus, control of droplet size at the onset of solute nucleation by varying process parameters other than the solution concentration and initial droplet size is limited.     

Flash pyrolysis of wood in a cyclone reactor

This paper reports the first results of an experimental study of the continuous flash pyrolysis of wood sawdust in a cyclone reactor between 893 and 1330 K. The reaction produces low fractions of char (4%) and the gasification yield increases from 0% at about 800 K to 90% at around 1330 K with a constant volume fraction of CO and H₂ (≈73%) and an increasing fraction of light hydrocarbons (up to 50% mass fraction). The heating value of the gas reaches 19 000 kJ m⁻³ STP for the highest temperatures. The wood particles mainly heated by radiation and solid convection react in less than 1 s while the carrier gas (residence time of the order of 0.05 s) seems to be only weakly heated. The 46.2 × 10⁻⁶ m³ cyclone reactor can operate with excellent stability for wood flow rates up to 0.35 kg h⁻¹ at a wall temperature of 1330 K. The cyclone seems to be very efficient for carrying out reactions of the solid → fluids type but more accurate determination of process parameters such as gas and solid residence times and heat transfer efficiencies are required to gain a better understanding of the behaviour of such a high temperature reactor.     

Scale influence on the hydrodynamics of an internal loop airlift reactor

The overall circulation velocity, the overall riser and downcomer gas hold-ups and the effect of reactor scale on a two-phase circulation regimes were studied in this work in three airlift reactors of different scale. The measurements were carried out in airlift reactor with internal loops (IALRs) with a working volume of 10.5, 32 and 200 l at the range of temperatures 18–21 °C, under atmospheric pressure. Air and water were used as gas and liquid media. The three reactors were of similar geometry, the ratio between riser and downcomer cross-sectional areas, the aspect ratio of the column and the shape of the column bottom were taken as similarity criteria. In order to determine the linear circulation velocities, the magnetic tracer method was used. The riser and the downcomer were studied separately. Based on gas hold-up in both the riser and the downcomer, two regimes (homogeneous bubble (HMG) and heterogeneous churn-turbulent (HTG)) of the two-phase flow were observed. These were defined by Daniels [Chem. Eng. 70 (1995)] and described using the correlation proposed by Chisti [Airlift Bioreactors, Elsevier, London, 1989]. The average of the liquid circulation velocities increased with increasing reactor scale for the same superficial gas velocity. The overall circulation velocity was modelled on the basis of the momentum balance proposed in paper [Chem. Eng. Sci. 52 (1997) 25]. The parameters of both the correlation and the model tend to be constant for larger reactor scales. The value of the driving force (_R−_D) was found to be important only for lower values of gas flow rate, because at higher values, the circulation velocity seemed to be governed only by friction in the reactor vessel.     

铜铈改性镁铝尖晶石作为助催化剂同时脱除FCC烟气中的SOx,Nox

为降低催化裂化烟气中的SO_x、No_x,采用溶胶凝胶法合成了Mg-Al尖晶石,并通过共胶法进行金属氧化物改性制备得到催化裂化烟气脱硫脱硝助催化剂。在小型固定床反应装置上考察了铜铈改性以及烟气中O₂含量对尖晶石脱硫脱硝性能的影响,并在小型提升管装置上考察了铜铈复合改性后尖晶石同时脱硫脱硝的性能。结果表明：经铜铈复合改性后的尖晶石脱硫活性最高,40min内仍能保持90.67%的SO₂脱除率;含铜尖晶石的脱硝性能最优,450℃后NO转化率达100%。烟气中O₂含量对脱硫有利而对脱硝不利,当O₂含量过高时,NO不能有效脱除。原位红外漫反射分析表明：含铜尖晶石良好的脱硝性能是因为其含有的Cu⁺对CO优异的吸附性能,O₂的影响在于其与CO的反应,导致CO消耗。提升管装置评价实验结果表明：主风量为0.6m³/h时,CuCe-MgAl脱硫效率为59.96%,脱硝效率为87.63%。当主风量为0.8m³/h时,脱硫效果提升至74.50%而失去了脱硝功能。     

CVD of boron and dichloroborane formation in a hot-wire fiber growth reactor

Chemical vapor deposition (CVD) of boron by hydrogen reduction of BCl₃ on a hot tungsten substrate was investigated in a parallel flow reactor. Effect of substrate temperature (1100–1250°C) on the relative rates of formation of BHCl₂ and boron was observed by the on-line analysis of the reactor effluent stream composition using an FT-IR spectrophotometer. It was concluded that BHCl₂ was majorly formed in the gas phase within the thermal boundary layer adjacent to the substrate with possible contribution of surface reactions at higher temperatures. Comparison of results obtained in the impinging jet and parallel flow reactors indicated the significance of diffusion resistance in the parallel flow system. Tubular flow reactor experiments indicated that BHCl₂ formation reaction started at temperatures as low as 350°C and reached equilibrium in less than a second at temperatures over 420°C.     

Effect of a gas–liquid separator on the hydrodynamics and circulation flow regimes in internal‐loop airlift reactors

The role of the gas–liquid separator on hydrodynamic characteristics in an internal‐loop airlift reactor (ALR) was investigated. Both gas holdup and liquid velocity were measured in a 30 dm³ airlift reactor with two different head configurations: with and without an enlarged separator. A magnetic tracer method using a neutrally buoyant magnetic particle as flowfollower was used to measure the liquid velocity in all sections of the internal‐loop airlift reactor. Average liquid circulation velocities in the main parts of the ALR were compared for both reactor configurations. At low air flow rates the separator had no influence on gas holdup, circulation velocity and intensity of turbulence in the downcomer and separator. At higher superficial air velocities, however, the separator design had a decisive effect on the hydrodynamic parameters in the downcomer and the separator. On the other hand, the gas holdup in the riser was only slightly influenced by the separator configuration in the whole range of air flow. Circulation flow regimes, characterising the behaviour of bubbles in the downcomer, were identified and the effect of the separator on these regimes was assessed. © 2001 Society of Chemical Industry     

Bubble size in a forced circulation loop reactor

BACKGROUND: The bubble size distribution in gas‐liquid reactors influences gas holdup, residence time distribution, and gas‐liquid interfacial area for mass transfer. This work reports on the effects of independently varied gas and liquid flow rates on steady‐state bubble size distributions in a new design of forced circulation loop reactor operated with an air–water system. The reactor consisted of a cylindrical vessel (～26 L nominal volume, gas‐free aspect ratio ≈ 6, downcomer‐to‐riser cross‐sectional area ratio of 0.493) with a concentric draft tube and an annular riser zone. Both gas and liquid were in forced flow through a sparger that had been designed for minimizing the bubble size. RESULTS: Photographically measured bubble size distributions in the riser zone could be approximated as normal distributions for the combinations of gas and liquid flow rates used. This contrasted with other kinds of size distributions (e.g. bimodal, Gaussian) that have been reported for other types of gas‐liquid reactors. Most of the bubbles were in the 3 to 5 mm diameter range. At any fixed low value of aeration rate (≤1.8 × 10^?4 m³s^?1), increase in the liquid flow rate caused earlier detachment of bubbles from the sparger holes to reduce the Sauter mean bubble size in the riser region. CONCLUSION: Unlike in conventional bubble columns where bimodal and Gaussian bubble size distributions have been reported, a normal bubble size distribution is attained in forced circulation loop reactors with an air–water system over the entire range of operation. Copyright © 2007 Society of Chemical Industry     

气升式环流反应器数值模拟研究进展

介绍了计算流体力学在气升式环流反应器(二相、三相)中的应用,汇总并介绍了气升式环流反应器的气含率和循环液速的二维与三维数值模拟结果,提出今后进一步的研究方向。