Ozone Mass Transfer in the Mixing Zone of a Confined Plunging Liquid Jet Contactor

In a Confined Plunging Liquid Jet Contactor (CPLJC) a jet of liquid is introduced into an enclosed cylindrical column (downcomer) that generates fine gas bubbles that are contacted with the bulk liquid flow. The region where the liquid jet impinges the receiving liquid and expands to the wall of the downcomer is called the Mixing Zone (MZ). In the MZ, the energy of the liquid jet is dissipated by the breakup of the entrained gas into fine bubbles, and the intense recirculation of the two-phase mixture. The study presented here was undertaken to quantify the ozone-water mass transfer performance of the MZ through the determination of the volumetric mass transfer coefficient, k_La (s^?1), and to produce a model for predicting k_La based on the specific energy dissipation rate. It was found experimentally that k_La in the MZ increased with increasing superficial gas velocity. A maximum experimental k_La value of 0.84 s^?1 was achieved which compares well to other contactors used in water treatment. Such a large k_La value combined with the small volume of the reactor, favorable energy requirements and safety features of the system, suggests that the CPLJC provides an attractive alternative to conventional ozone contactors. The relatively large mass transfer rates were found to be a function of the high gas holdup and fine bubble size generated in the MZ, which results in an almost froth-like consistency. A model based on the specific energy dissipation rate of the water jet, E (kg · m^?1· s^?3), and MZ bubble size was used to predict k_La in the MZ. Using E, the number average bubble size was predicted which was then used to calculate the liquid phase mass transfer coefficient k_L. The bubble size was also used with the predicted mixing zone gas holdup to calculate the specific interfacial area, a (m^?1), which was then combined with k_L to determine a predicted value of k_La. The average deviation between experimental and predicted k_La was 6.2%.     

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.     

Gas absorption with or without chemical reaction in laminar non-newtonian falling liquid films

An analytical solution is presented for gas absorption with or without a first-order or zero-order chemical reaction in a laminar non-Newtonian power-l model falling liquid film. For physical absorption, the first ten eigenvalues, series coefficients and related quantities are computed accurately by a quasinumerical method which shows considerable improvement over previous investigations. The range of applicability of the penetration theory solution is also established to indicate in what regions will the finite film thickness and complete velocity profile be important in determining the absorption rate. It is found that the range of dimensionless axial contact length X* in which the penetration theory is valid diminishes rapidly with increasi values of the power-law index n. For chemical absorption, the solution can be obtained by a linear superposition principle in terms of a “transie part” in which the effect of hydrodynamics within the liquid film is of importance and a “steady part” in which the reaction rate is controlling. In the “transient part” solution, the first ten eigenvalues and related quantities are reported for a variety of values of n and the dimensionl reaction rate parameter k_l* or k₀*. Certain asymptotic solutions from the penetration theory are also given and their range of applicab estimated. For any given n, it is estimated that only when k₁* or k₀* is less than approximately 10 will the finite film thickness and velocity profile have any effect on the absorption rate as compared to that calculated from the penetration theory with chemical reaction. The non-Newt character of the liquid film also has a significant influence on the absorption rate. At a fixed X*, the absorption enhancement due to reaction is when n = ∞ and is smallest when n = 0. The solutions obtained in this work are useful either for predicting absorption rates or for deter molecular diffusivity (and reaction rate constant) of gases in non-Newtonian falling liquid films.     

Mass transfer characteristics in a gas–liquid reciprocating plate column. II. Interfacial area

This paper is the second part of a continuing study on mass transfer in a reciprocating plate column. The first part dealt with k_La. The bubble size distribution, the Sauter mean diameter and the interfacial area are the subject of this paper. The bubble size increases slightly with gas flow rate and decreases with agitation intensity above a “critical” level. The interfacial area increases with increasing agitation and aeration intensities, while the liquid flow rate and coalescing properties of the liquid have no significant effect. The specific interfacial area is correlated in terms of the superficial gas velocity and the maximum power consumption. The correlations obtained for k_La and a were used to calculate k_L. It was found that k_L depends on the agitation intensity and the bubble size.     

Hydrodynamic and mass transfer characteristics of bubble and packed bubble columns with downcomer

The hydrodynamic and mass transfer characteristics of bubble and packed bubble columns with downcomer were investigated. The contactor consisted of two concentric columns of 0.11 and 0.2 m i.d., with the annulus acting as the downcomer. The packing used in this investigation was standard 16 mm stainless steel Pall rings. The superficial gas and liquid velocities, V_G and V_L, were varied from 0.01 to 0.09 and 1 × 10^?3 to 8.8 × 10^?3 m s^?1 respectively. Two flow patterns, namely the bubble and pulse flows were observed in the packed bubble column with downcomer, as shown by a flow map. The liquid circulation velocity in both the contactors was observed to be constant throughout the ranges of V_G and V_L covered in this work. The effect of liquid viscosity (0.8 to 9.5 mPa ? s) and surface tension (45 to 72 mN m^?1) on the flow pattern, liquid circulation, gas hold-up and pressure drop was investigated. The pressure drop characteristics across the two contactors have been compared with those across a bubble column. Values of the effective interfacial area, a, and the volumetric mass transfer coefficient, k_L a, were measured by using chemical methods. Values of a as high as 180 and 700 m^?1 and k_L a as high as 0.075 and 0.22 s^?1, in the bubble and packed bubble columns with downcomer, respectively, were obtained. The values of true liquid-side mass transfer coefficient, k_L, were found to be independent of V_G and were of the order of 5.5 × 10^?4 and 3.5 × 10^?4 m s^?1, respectively, in the two contactors.     

降膜微反应器中CO_2化学吸收过程传质行为

Gas phase mass transfer in falling film microreactors (FFMRs) with the absorption of CO₂ into aqueous solutions of NaOH was investigated. The overall gas-phase mass transfer coefficient increases with NaOH concentration, but decreases as the concentration of CO₂ increases. There exists an entrance effect, hindering the mass transfer, which is caused by the dead volume for gas-phase flow in the gas chamber in FFMRs. The entrance effect has a larger impact in a shorter FFMR owing to the relatively large dead volume with respect to that of gas chamber. A decrease in the depth of gas chamber facilitates the mass transfer process. Therefore, the gas-phase entrance or geometry of the gas chamber should be designed appropriately to reduce the entrance effect and improve the mass transfer.     

Gas-side mass transfer coefficients in a falling film microreactor

Gas–liquid mass transfer in a falling film microreactor (FFMR) with 29 microchannels (0.6 mm width each) was investigated. CO₂ was absorbed from a CO₂/N₂ gaseous mixture into a NaOH aqueous solution and the liquid-side mass transfer coefficient and the gas-side mass transfer coefficient were measured. The influence of gas concentration on the value of gas-side mass transfer coefficient has been discussed.     

Chemical hydrodynamics of a downward microbubble flow for intensification of gas‐fed bioreactors

Bioreactors are of interest for value‐upgrading of stranded or waste industrial gases. Reactor intensification requires development of low cost bioreactors with fast gas–liquid mass transfer rate. Here we assess published reactor technology in comparison with a novel downward bubble flow created by a micro‐jet array. Compared to known technology, the advanced design achieves higher volumetric gas transfer efficiency (k_La per power density) and can operate at higher k_La. We measure the effect of four reactor heights (height‐to‐diameter ratios of 12, 9, 6, and 3) on the gas transfer coefficient k_L, total interfacial area a, liquid residence time distribution, energy consumption, and turbulent hydrodynamics. Leading models for predicting k_L and a are appraised with experimental data. The results show k_L is governed by “entrance effects” due to Higbie penetration dominate at short distances below the micro‐jet array, while turbulence dominates at intermediate distances, and finally terminal rise velocity dominates at large distances. © 2017 American Institute of Chemical Engineers AIChE J, 64: 1399–1411, 2018     

Impinging-Jet Ozone Bubble Column Modeling: Hydrodynamics,Gas Hold-up,Bubble Characteristics,and Ozone Mass Transfer

A transient back flow cell model was used to model the hydrodynamic behaviour of an impinging-jet ozone bubble column. A steady-state back flow cell model was developed to analyze the dissolved ozone concentration profiles measured in the bubble column. The column-average overall mass transfer coefficient, k_La (s^?1), was found to be dependent on the superficial gas and liquid velocities, u_G (m.s^?1) and u_L (m.s^?1), respectively, as follows: k_La?=?55.58 · u_G ^1.26· u_L ^0.08 . The specific interfacial area, a (m^?1), was determined as a = 3.61 × 10³ · u_G ^0.902 · u_L ^?0.038 by measuring the gas hold-up (ε _G?=?4.67 · u_G ^1.11 · u_L ^?0.05 ) and Sauter mean diameter, d_S (mm), of the bubbles (d_S?=?7.78 · u_G ^0.207 · u_L ^{? 0.008} ). The local mass transfer coefficient, k_L (m.s^?1), was then determined to be: k_L?=?15.40 · u_G ^0.354 · u_L ^0.118 .     

Vertical two-phase flow: Part II. Holdup and pressure drop

Two-phase gas-liquid flow has been investigated in a 1-inch internal diameter vertical tube coil containing two risers and a downcomer all connected by “U” bends. Pressure drop, holdup and flow pattern data were successfully obtained simultaneously in the three vertical tubes, each 17.30 ft. long, for five different air-liquid systems at about 25 psia and 50°F-80°F over flow ranges of 0–700 lb_m air/min-ft² and 140--25300 lb_m liquid/min-ft². Pressure drops and liquid holdups were plotted against gas volume flowrate with liquid flowrate as a parameter. From these plots it was found that for a combination of an increase in liquid viscosity and density, and a decrease in surface tension, the frictional pressure drop increased in down flow and decreased in upflow. Holdup, on the other hand, increased for both types of vertical flow with respect to the same combination of parameters. The Lockhart-Martinelli scheme was satisfactory in correlating frictional pressure drop and holdup in all the flow regimes except the frothy-slug regime in upflow. In downflow however, the Lockhart-Martinelli scheme met with limited success because of a strong influence of liquid flowrate, physical properties and pipe orientation. Holdup in the falling film and falling bubbly film regimes in downflow were satisfactorily treated by the drift flux approach which emphasizes the relative motion of the two phases.     

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.     

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 k_La increase with increasing gas mass flow rates. The mass transfer coefficient k_L is independent of pressure.     

不同流型下离子液体-MEA复合工质降膜吸收CO2特性

以氨基酸离子液体和乙醇胺混合水溶液为吸收剂研究逆流气体对竖直平板降膜流型转换的影响,考察了3种流型下液体流量和入口温度、气体流量和进口CO₂浓度对CO₂吸收性能的影响。结果表明:随着液体流量的增加,液膜呈现溪流、片状流和完整流3种流型,降膜流型转换临界流量随逆流气体流量增大而增加;溪流和片状流时CO₂吸收速率随液体流量的增加而增加,但在完整流条件下基本不变;完整流下具有较高的CO₂吸收速率,然而溪流下的液相传质系数最高。     

A kinetic model for pyrolysis of cellulose

It has been shown that the pyrolysis of cellulose at low pressure (1.5 Torr) can be described by a three reaction model. In this model, it is assumed that an “initiation reaction” leads to formation of an “active cellulose” which subsequently decomposes by two competitive first-order reactions, one yielding volatiles and the other char and a gaseous fraction. Over the temperature range of 259–341°C, the rate constants of these reactions, k_i (for cellulose → “active cellulose”), k_v (for “active cellulose” → “volatiles”), and k_c (for “active cellulose” → char + the gaseous fraction) are given by k_i = 1.7 × 10²¹e^{? (58,000/RT)} min ^?1, k_v = 1.9 × 10¹⁶e^{? (47,300/RT)} min^?1, and k_c = 7.9 × 10¹¹e^{? (36,600/RT)} min^?1, respectively.     

Vertical two-phase flow part I. Flow pattern correlations

Two-phase gas-liquid flow has been investigated in a 1-inch internal diameter vertical tube coil containing two risers and a downcomer all connected by “U” bends. Flow pattern data were obtained in the three vertical tubes, each 17.30 ft. long, for five different air-liquid systems at about 25 psia over flow ranges of 0–700 lb_m air/min-ft² and 140–25300 lb_m liquid/min-ft². Liquid phase viscosities ranged from 1 to 12 cp. A flow pattern classification with six regimes including coring-bubble, bubbly-slug, falling film, falling bubbly-film, froth and annular flow regimes was established for downflow. Flow patterns in the bends were also classified. Data from the present investigation were used to formulate an empirical flow pattern graphical correlation for both upflow and downflow which is based upon the coordinates (R_v)^1/2 and Fr_TP/A, where R_v is the delivered gas-to-liquid volume ratio, Fr_TP is the mixture Froude number, and A = μ_s/(S_L^σ_s³)^1/4 in which μ_s, S_L, σ_s are specific viscosity, specific density and specific surface tension respectively of the liquid with reference to water. The correlation was satisfactorily tested with independent literature data for upflow systems, including air-water, steam-water at various pressures, nitrogen-mercury and air-heptane, and data from flowing gas-oil wells. No independent literature data appear to be available for testing the correlation for downflow systems, but it is anticipated that the correlation will prove to be generally applicable. The coring phenomenon in downward bubble flow was examined by means of high speed motion photography and is explained by the development of a lift force on a bubble.     

Mass Transfer Study of Newtonian Fluids with Different Viscosity under Low-Frequency,High-Power Ultrasound Irradiation

In this article, the effects of liquid properties and operating conditions on gas–liquid mass transfer under ultrasound irradiation and mechanical stirring were studied and compared. Response surface methodology (RSM) was utilized for the design of experiments and evaluation of the influence of operating parameters. The maximum value of volumetric mass transfer coefficient (k_La) was found to be 0.0714?s^?1 when the ultrasonic horn was located horizontally just above the gas sparger in the tank. Ultrasonic power and the position of ultrasonic horn were found to be the most significant parameters that influence k_La. Also, three empirical correlations were developed to estimate k_La considering liquid viscosity as one of the main parameters, and their estimations were compared to those estimated using existing correlations in the literature.     

Modeling Hydrodynamics And Mass Transfer Parameters In A Continuous Ozone Bubble Column

A computer model based on the establishment of mass balance equations and on the model of fluids flow “stirred tank in series” was developed in order to calculate the ozone transfer coefficient k_La and kinetic constant k_c of ozone consumption by water. On the basis of experimental data, the correlation for gas holdup ε_g and bubble diameter d_vs, were proposed and used to calculate the specific interfacial area a. The liquid-phase mass transfer coefficient k_L for ozone was evaluated from a and the k_La data.     

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.     

Einfluss mikrostrukturierter statischer Mischer auf den Gas/Flüssig‐Stofftransport in einem engen Rechteckkanal

Experimental results of the volumetric mass transfer coefficient k_La_Ph in a microstructured rectangular channel (Miprowa®) with static mixers are presented. The physical absorption of CO₂ in H₂O was identified as suitable measuring method. The results include a gas‐liquid flow map and the identification of different flow regimes as well as first systematic measurements of the k_La_Ph value as a function of various process settings like gas and liquid flow rate and gas holdup. A first comparison of Miprowa® with established gas‐liquid contact devices like stirred tank and bubble column is given.     

Gas holdup,bubble size distribution,and mass transfer in an airlift reactor with ceramic membrane and perforated plate distributor

The airlift reactor is one of the most commonly used gas–liquid two-phase reactors in chemical and biological processes. The objective of this study is to generate different-sized bubbles in an internal loop airlift reactor and characterize the behaviours of the bubbly flows. The bubble size, gas holdup, liquid circulation velocity, and the volumetric mass transfer coefficient of gas–liquid two-phase co-current flow in an internal loop airlift reactor equipped with a ceramic membrane module (CMM) and a perforated-plate distributor (PPD) are measured. Experimental results show that CMM can generate small bubbles with Sauter mean diameter d₃₂ less than 2.5 mm. As the liquid inlet velocity increases, the bubble size decreases and the gas holdup increases. In contrast, PPD can generate large bubbles with 4 mm < d₃₂ < 10 mm. The bubble size and liquid circulation velocity increase as the superficial gas velocity increases. Multiscale bubbles with 0.5 mm < d₃₂ < 10 mm can be generated by the CMM and PPD together. The volumetric mass transfer coefficient k_La of the multiscale bubbles is 0.033–0.062 s⁻¹, while that of small bubbles is 0.011–0.057 s⁻¹. Under the same flow rate of oxygen, the k_La of the multiscale bubbles increases by up to 160% in comparison to that of the small bubbles. Finally, empirical correlations for k_La are obtained.