Gas–liquid mass transfer in rotating solid foam reactors

Three-phase reactor designs based on rotating solid foams for the application in the fine chemical industry are developed. The aim is to use solid foams both as a catalyst support and stirrer in order to mix the gas and liquid phases and create fine gas bubbles. Gas–liquid mass transfer data are presented for different solid foam stirrer configurations and compared to an optimized Rushton stirrer. Solid foam stirrers were developed in a blade and a block design. Both foam reactor designs work at stirring rates below 600 rpm. Using the foam blade design, gas bubbles are mainly created by the turbulence at the gas–liquid interface. Large bubbles are broken up by the foam blades. Using a foam block design, rotation leads to the structurization of the reactor volume into sections strongly differing in gas holdup, flow behavior and bubble size distribution. This results in a gas–liquid mass transfer, which is 50% higher than the Rushton stirrer used as comparison. The foam stirrer designs can be easily used in ordinary three-phase reactors and show a high potential for further optimization of the gas–liquid flow pattern and therefore for further increase of the rate of mass transfer.     

Gas–liquid mass transfer and bubble size distribution in a multi-Cyclone separator

This article discusses the use of a multi-cyclone separator, which is a simplified form of a degassing hydrocyclone, in the separation of sweeping nitrogen bubbles and dissolved oxygen from water. The motion of the nitrogen bubbles and mass transfer of dissolved oxygen is discussed. It was observed that the Sauter mean diameter and gas volume in the swirling flow region as well as the total gas holdup increased as the volumetric ratio of gas to liquid flow increased. Almost all bubbles were found to exit through the gas outlet, indicating optimum performance of the bubble-separation process. The multi-cyclone separator was found to achieve good performance for the mass transfer of oxygen from water to nitrogen. This work is important in predicting the destination of bubbles and dissolved gas in swirling flow. © 2018 American Institute of Chemical Engineers AIChE J, 65: 215–223, 2019     

Local and global gas–liquid mass transfer in a micro-packed bed reactor utilizing a noninvasive colorimetric technique

Micro-packed bed reactor (μPBR) presents great potential in the field of multiphase reactions due to the features of safety and high efficiency. However, the deeper cognition of mass transfer needs to be taken into consideration that is the foundation of reactor design. In this work, local and global gas–liquid mass transfer in the μPBR were studied utilizing a noninvasive colorimetric technique. In reactor level, the qualitative and quantitative comparisons were conducted; in particle level, liquid flow and mass transfer textures were assessed for the first time. The diversities of local mass transfer characteristics from temporal and spatial dimensions were obtained, and the heterogeneity of local and global mass transfer was revealed. The predicted correlations of $$ in μPBR with churn flow and pseudo-static flow were established with deviations generally within ±18%. This study contributes to improve the understanding of mass transfer and points out the process intensification direction of μPBR.     

Intensification of liquid–liquid two-phase mass transfer in a capillary microreactor system

A convenient strategy to intensify the liquid–liquid mass transfer performance in a capillary microreactor system was developed by narrowing the inlet channel of T-micromixer or adding baffles into the capillary. Various geometrical parameters such as the inlet mode and diameter of the modified T-micromixer, the number of baffles and interval distance between baffles in the modified capillary were investigated to elaborate the mass transfer intensification mechanism. The liquid–liquid two-phase flow patterns in new capillary microreactors were captured by a high-speed camera. Moreover, pressure drops and specific energy dissipation of these modified microreactor systems were studied and a new parameter indicating the ratio of the mass transfer coefficient to the energy dissipation was proposed. This work highlights the modified capillary microreactor systems with embedding baffle units for achieving high mass transfer rates with its advantages over other types of reactors or microreactors considering specific energy dissipation and effective energy utilization efficiency. © 2018 American Institute of Chemical Engineers AIChE J, 65: 334–346, 2019     

Foam generation in a rotor—stator mixer: schaumerzeugung in einem rotor—stator mischer

The foaming process of an aqueous liquid system with surface active agents and thickeners in a rotor-stator mixer has been studied.The foaming capacity of a rotor—stator mixer may be represented by a so-called mixing characteristic. The foamabilities of several liquid systems have been measured as a function of the mixer geometry and the rotational speed.The hydrodynamics in a rotor-stator mixer is characterized by a Newton—Reynolds relationship. The mechanism of foaming and the dependence of several mixing parameters are different for the turbulent and laminar flow regions. The mixing process is evaluated in both regimes. In the transition region from turbulent to laminar the foaming is very poor in comparison with that in the turbulent and laminar flow regimes.     

Chemisorption of CO2 in a gas–liquid vortex reactor: An interphase mass transfer efficiency assessment

To develop cost-effective CO₂ capture technology process intensification will play a vital role. In this work, the capabilities of a gas–liquid vortex reactor (GLVR) as novel process intensification equipment are evaluated by studying its interphase mass transfer parameters to build up the fundamentals for its future application to for example, CO₂ capture. The NaOH-CO₂ chemisorption system and Danckwerts' model are applied to obtain the effective interfacial area and liquid-side mass transfer coefficient. Results show that the gas–liquid contact in the GLVR is capable of both generating a large interfacial area in a small reactor volume and creating a region with high-energy dissipation to improve mass transfer. A comparison of the volumetric mass transfer coefficients with data reported in literature for conventional and intensified reactor types confirms a superior mass transfer efficiency and, most importantly, a favorable energetic efficiency of the GLVR.     

Liquid–liquid two-phase mass transfer characteristics in a rotating helical microchannel

In this work, the mass transfer characteristics of two immiscible fluids were investigated in a rotating helical microchannel with hydraulic diameter of 932 μm. Aqueous phosphoric acid solution and 80% tri-n-butyl phosphate (TBP) in kerosene were selected for the investigation of mass transfer performance in quartz glass/high density polyethylene (HDPE) microchannel. High dispersion between the two immiscible fluids can be obtained in the microchannel due to the intensifying action of centrifugal force, and the majority of the droplets with average diameter of 20–100 μm were produced in the microchannel. The flow rate and rotation speed were found to have great effects on the extraction efficiency and average residence time. The empirical correlation of average residence time based on experimental data was developed by theoretical analysis and data fitting method, and a mathematical model of the mass transfer coefficient in dispersed phase was proposed.     

Micromixing in a gas–liquid vortex reactor

The gas–liquid vortex reactor (GLVR) has substantial process intensification potential for multiphase processes. Essential in this respect is the micromixing efficiency, which is of great importance in fast reaction systems such as crystallization, polymerization, and synthesis of nanomaterials. By creating a vortex flow and taking advantage of the centrifugal force field, the liquid micromixing process can be intensified in the GLVR. Results show that introducing a liquid into a gas-only vortex unit results in suppression of primary and secondary gas flow. The Villermaux–Dushman protocol is applied to study the effects of the gas flow rate, liquid flow rate, and liquid viscosity based on a segregation index. Based on the incorporation model and reaction kinetics, the micromixing time of the GLVR is determined to be in the range of 10⁻⁴ ~ 10⁻³ s, which is comparable to the highly efficient rotating packed bed and substantially better than a static mixer.     

Characterization of liquid–liquid mass transfer performance in a novel pore-array intensified tube-in-tube microchannel

Liquid–liquid mass transfer performance of a novel pore-array intensified tube-in-tube microchannel (PA-TMC) was investigated with the water-benzoic acid-kerosene system. Both mass transfer efficiency (E) and volumetric mass transfer coefficient (K _La) are found to increase simultaneously with the flow rate, but decrease with total number of pores and rows. However, E increases but K _La decreases with the annular length. In particular, the pore size shows an optimal value at 0.3 mm due to the interplay problem of the radial adjacent pores. Computational fluid dynamics simulations reveal that the high kinetic energy generated by the pore-array section plays a significant role in mass transfer process. The artificial neural network model is established to correlate K _La with the investigated parameters of PA-TMC. The comparison of K _La with other types of contactors indicates that PA-TMC has superior mass transfer performance and high throughput for a broad industrial application.     

The effect of scale and interfacial tension on liquid–liquid dispersion in in-line Silverson rotor–stator mixers

The effect of scale, processing conditions, interfacial tension and viscosity of the dispersed phase on power draw and drop size distributions in three in-line Silverson rotor–stator mixers was investigated with the aim to determine the most appropriate scaling up parameter. The largest mixer was a factory scale device, whilst the smallest was a laboratory scale mixer. All the mixers were geometrically similar and were fitted with double rotors and standard double emulsor stators. 1 wt.% silicone oils with viscosities of 9.4 mPa s and 339 mPa s in aqueous solutions of surfactant or ethanol were emulsified in single and multiple pass modes. The effect of rotor speed, flow rate, dispersed phase viscosity, interfacial tension and scale on drop size distributions was investigated.     

Solid–liquid mass transfer studies in trickle bed reactors

Solid–liquid mass transfer was studied in a trickle bed reactor packed with copper cylindrical particles. Diffusion-controlled dissolution of copper in acidified dichromate solution was used to study the solid–liquid mass transfer. The effect of liquid and gas throughputs and particle diameter on solid–liquid mass transfer was investigated.     

Hydrodynamics and local mass transfer characterization under gas–liquid–liquid slug flow in a rectangular microchannel

Gas–liquid–liquid three-phase slug flow was generated in a glass microreactor with rectangular microchannel, where aqueous slugs were distinguished by relative positions to air bubbles and organic droplets. Oxygen from bubbles reacted with resazurin in slugs, leading to prominent color changes, which was used to quantify mass transfer performance. The development of slug length indicated a film flow through the corner between bubbles and the channel wall, where the aqueous phase was saturated with oxygen transferred from bubble body. This film flow results in the highest equivalent oxygen concentration within the slug led by a bubble and followed by a droplet. The three-phase slug flow subregime with alternate bubble and droplet was found to benefit the overall mass transfer performance most. These results provide insights into a precise manipulation of gas–liquid–liquid slug flow in microreactors and the relevant mass transfer behavior thereof.     

Mass transfer in gas–liquid stirred reactor with various triple-impeller combinations

The gassed power demand and volumetric mass transfer coefficient (kLa) were investigated in a fully baffled, dished-base stirred vessel with a diameter of 0.30 m agitated by five triple-impeller combinations. Six types of impellers (six-half-elliptical-blade disk turbine (HEDT), four-wide-blade hydrofoil impeller (WH) pumping down (D) and pumping up (U), parabolic-blade disk turbine (PDT), and CBY narrow blade (N) and wide blade (W)) were used to form five combinations identified by PDT + 2CBYN, PDT + 2CBYW, PDT + 2WHD, HEDT + 2WHD and HEDT + 2WHU, respectively. The results show that the relative power demand of HEDT+2WHU is higher than that of other four impeller combinations under all operating conditions. At low superficial gas velocity (uG), kLa differences among impeller combinations are not obvious. However, when uG is high, PDT+2WHD shows the best mass transfer performance and HEDT+2WHU shows the worst mass trans-fer performance under al operating conditions. At high uG and a given power input, the impel er combinations with high agitation speed and big projection cross-sectional area lead to relatively high values of kLa. Based on the experimental data, the regressed correlations of gassed power number with Froude number and gas flow number, and kLa with power consumption and superficial gas velocity are obtained for five different impeller combinations, which could be used as guidance for industrial design.     

Investigations of mass transfer with chemical reactions in two-phase liquid–liquid systems

A new method based on experimental determination of the product distribution of a set of complex test reactions has been introduced and applied to study mass transfer in liquid–liquid systems. The test reactions consist of two parallel reactions, one of them being instantaneous and the second fast relative to mass transfer. Two reactants are transferred from the dispersed, organic phase (phase volume 1% vol) to the continuous aqueous phase, where the third reactant is present. Experiments were carried out in a batch system agitated with either a six-blade paddle impeller or a high-shear rotor–stator LR4 Silverson mixer to disperse drops and increase the mass transfer rate. The product distribution and the drop size distribution were measured using gas chromatography–mass spectroscopy and Malvern MasterSizer, respectively with pH variation recorded during the process. The results show that the focused supply of energy in the Silverson mixer is effective for the short term irreversible drop break-up process producing smaller droplets than the six-blade paddle impeller. However for the long term mass transfer process the paddle impeller is more effective due to more uniform supply of energy and better mixing throughout the tank compared to the more localized mixing of the Silverson.     

Process intensification in vapor–liquid mass transfer: The state-of-the-art

The concept of process intensification(PI) has absorbed diverse definitions and stays true to the mission—"do more with less", which is an approach purposed by chemical engineers to solve the global energy environment problems. To date, the focus of PI has been on processes mainly involving vapor/liquid systems. Based on the fundamental principles of vapor–liquid mass transfer process like distillation and absorption, there are three strategies to intensify interphase mass transfer: enhancing the overall driving force, improving the mass transfer coefficient and enlarging the vapor–liquid interfacial area. More specifically, this article herein provides an overview of various technologies to strengthen the vapor–liquid mass transfer, including application of external fields, addition of third substances, micro-chemical technology and usage of solid foam, with the objective to contribute to the future developments and potential applications of PI in scientific research and industrial sectors.     

Vapour–liquid slip in a parallel-plate electrochemical fluorination reactor

A mathematical model was used to study the effect of slip between the gas and liquid phases on the performance of an electrochemical fluorination reactor. The model incorporates two-phase flow with differential material, energy and pressure balances. The effect of slip on the temperature, pressure, gas fraction and current distribution in the reactor is presented under relatively severe operating conditions. In addition, the effect of slip on the cell voltage, current efficiency and energy usage is shown at different flow rates over a wide current range. It was found that slip of the gas past the liquid is insignificant under normal operating conditions, but it is significant at high cell currents and low flow rates. Under these more severe operating conditions, slip significantly reduces the cell voltage, and hence the energy usage, since less gas resides in the reactor.     

Liquid–liquid two-phase flow and mass transfer characteristics in packed microchannels

In this work, the flow hydrodynamic characteristics and the mass transfer performance of immiscible fluids in the packed microchannels are investigated experimentally. Water–kerosene system is used for visually identifying the flow hydrodynamic characteristics in PMMA microchannels, and water–succinic acid–n-butanol is chosen for investigating mass transfer performance in stainless steel microchannels. Quartz sand micro-particles are used as packing particles. In packed microchannels, high liquid–liquid dispersions can be obtained, and the diameter of droplets produced in the packed microchannel can be even less than 10 μm. It ensures better mixing performance and larger effective interfacial area of two immiscible fluids, and improves the mass transfer performance obviously. Compared to the extraction efficiency (46–61%) in the non-packed microchannel, it can reach 81–96% in the packed microchannel. The effects of packing length, micro-particle size on liquid–liquid dispersions and extraction efficiency are investigated. The pressure drop and the specific energy dissipation in the packed microchannels are also discussed.     

Study on liquid–liquid two-phase mass transfer characteristics in the microchannel with deformed insert

In this work, the liquid–liquid two-phase mass transfer characteristics in the microchannel with deformed insert were studied. The experiment used di-(2-ethylhexyl) phosphoric acid/kerosene-Cu²⁺as the mass transfer evaluation system. The effects of some key factors such as the total flow velocity,channel inner diameter, channel length, insert diameter, extractant concentration on the extraction efficiency and mass transfer coefficient were systematically investigated. Compared with a ...     

An optimization method for enhancement of gas–liquid mass transfer in a bubble column reactor based on the entropy generation extremum principle

In this study, an optimization method is proposed to enhance the gas–liquid mass transfer in bubble column reactor based on the entropy generation extremum principle. The mass transfer–induced entropy generation can be maximized with the increase of mass transfer rate, based on which the velocity field can be optimized. The oxygen gas–liquid mass transfer is the major rate–limiting step of the toluene emissions biodegradation process in bubble column reactor, so the entropy generation due to oxy...