Phase inversion and mass transfer during liquid–liquid dispersed flow through mini-channel

The present study aims to identify means of process intensification during liquid–liquid flow through a mini-channel. During liquid–liquid flow, depending on the flow conditions either the organic or the aqueous phase can be dispersed and with increase in flow velocity the dispersed phase can spontaneously invert to form the continuous phase or vice-versa. The present study aims to investigate the phenomena of phase inversion and its influence on mass transfer during toluene/acetic acid-water flow in a 1.98 mm glass mini-channel. It is observed that for organic phase as dispersed regime, higher mass transfer efficiency is achieved when the liquid–liquid mixture is in the phase inversion zone which marks the transition from organic to aqueous phase dispersion. The mixture velocities as well as the inlet concentration of diffusing species influence mass transfer characteristics in this zone. The results have indicated some interesting observations which can be exploited for process intensification in monolith and micro-reactor.     

Membrane contactor for reactive extraction of succinic acid from aqueous solution by tertiary amine

A hollow fibre membrane contactor (HFMC) made of microporous, hydrophobic, polypropylene (PP) fibres was used for removing dissolved succinic acid (SA) from aqueous streams. The Liquicel^® HFMC module was operated in the liquid–liquid extraction (LLE) mode with aqueous SA as the feed solution. The aqueous feed was prepared by dissolving measured amounts of SA in deionised water, with concentrations varying from 5000 to 59 000 ppm. Two different types of organic extractant solutions were chosen to extract SA from aqueous streams: (a) 30% tripropylamine (TPA) dissolved in 1-octanol and (b) 30% trioctylamine (TOA)–TPA mixture in a 2:8 weight ratio and dissolved in 1-octanol. The aqueous feed was circulated through the lumens of the hollow fibres, and the extractant solution was passed through the shell side (i.e., inside the shell) of the HFMC module. Both liquids flowed counter-currently within the HFMC module and were recirculated continuously. Conditions such as SA-water flow rate, organic phase flow rate, and initial SA-water concentration were selected as the operating variables. The complexation reaction of SA with the amine was assumed at the aqueous–organic interface on the lumen side. The complex thus formed first diffused through the membrane pores filled with the organic phase (hereafter “organic-filled membrane pores”) and was subsequently swept by the organic liquid flowing through the shell side of the module. The HFMC was observed to be highly efficient in removing SA from water, and a removal efficiency of more than 95% was obtained. A mathematical analysis was performed by considering the transport of SA molecules through the bulk aqueous phase and within the organic-filled membrane pores. The results of the model simulation were shown to be in agreement with the experimental data.     

Process intensification and waste minimization in liquid–liquid–liquid phase transfer catalyzed selective synthesis of mandelic acid

Conversion of biphasic reactions into triphasic reactions can lead to process intensification, waste minimization and selectivity enhancement. Unlike liquid–liquid (L–L) PTC, the Liquid–Liquid–Liquid phase transfer catalysis (L–L–L PTC) offers high order of intensification of rates of reaction and catalyst reuse. The rate of reaction is remarkably enhanced by the catalyst-rich middle phase, which is the main reaction phase. Separation of catalyst can be done easily and the separated catalyst can be reused several times by using L–L–L PTC. This leads to waste minimization and other benefits of Green Chemistry. Mandelic acid and its derivatives are used for their dual activities as antibacterial and anti-aging agents. In this work, mandelic acid was produced by L–L–L PTC reaction of dichlorocarbene with benzaldehyde. Dichlorocarbene was generated in situ by the reaction of chloroform and sodium hydroxide in the presence of poly ethylene glycol (PEG) 4000 as the catalyst. The selectivity to mandelic acid was 98%. The reaction mechanism and kinetics model were established to validate the experimental data.     

Effect of a dispersed immiscible liquid phase on the hydrodynamics of a bubble column and ebullated bed

Secondary undesired reactions in ebullated bed resid hydroprocessors can generate an additional dispersed liquid phase, referred as mesophase, which is denser and more viscous than the continuous liquid phase and affects the operation and transport phenomena of the fluidized bed. This study investigates the effect of a dispersed immiscible liquid phase on the overall phase holdups, bubble properties, and fluidization behavior in a bubble column and ebullated bed. The experimental system consisted of biodiesel as the continuous liquid phase, glycerol as the dispersed liquid phase, 1.3 mm diameter glass beads, and nitrogen. The addition of dispersed glycerol reduced the gas holdups in the bubble column for the studied gas and liquid superficial velocities. Dynamic gas disengagement profiles reveal a rise in the large bubble population and reductions to the small and micro bubble holdups when increasing the glycerol concentration. Liquid–liquid–solid bed expansions at various liquid flowrates confirm particle agglomeration in the presence of a more viscous dispersed liquid phase. Overall phase holdups in a gas–liquid–liquid–solid ebullated bed were obtained while varying the gas and liquid flowrates as well as the glycerol concentration. A coalesced bubble flow regime was observed in the bed region without glycerol whereas the addition of glycerol resulted in the dispersed bubble flow regime due to particle clustering and a greater apparent particle size. The resulting bubble flow regime increased the bed and freeboard region gas holdups due to enhanced bubble break-up. Observations of the fluidized bed behavior following the addition of the dispersed glycerol are also discussed.     

Droplet coalescence model optimization using a detailed population balance model for RDC extraction column

Single droplet experiments in a small lab scale Rotating Disk Contactor (RDC) for two different liquid–liquid systems were used to evaluate the coalescence parameters necessary for column simulations. Five different coalescence models are studied; the models parameters were obtained by an inverse solution of the population balance model using the extended fixed-pivot technique for the discretization of the droplet internal coordinate. The estimated coalescence parameters by solving the inverse problem were found dependent on the chemical test system. The Coulaloglou and Tavlarides model was found to be the best model to predict the experimental data for both test systems. These parameters were used to study the hydrodynamics and mass transfer behavior of pilot plant RDC extraction column using the simulation tool LLECMOD. This is performed for two different liquid–liquid systems as recommended by the European Federation of Chemical Engineering (EFCE) (butylacetate–acetone–water (b–a–w) and toluene–acetone–water (t–a–w)). The simulated Sauter mean droplet diameter, hold-up values and concentration profiles for organic and aqueous phase were found to be well predicted compared to the experimental data.     

Electrosynthesis of thin sol–gel films at a three-phase junction

Thin, functionalised silica films of between 4 and 15 nm thickness were prepared by sol–gel processing at an electrode organic phase aqueous phase junction by slow withdrawal of a conducting support through the liquid–liquid interface. Protons were electrogenerated in the aqueous phase and catalysed the hydrolysis of the sol–gel precursor in the organic phase. The film was characterised using atomic force microscopy, scanning electron microscopy, ellipsometry, infrared spectroscopy, voltammetry and scanning electrochemical microscopy. The thickness, electrodeposited on gold, was determined to be less than 10 nm and the density of imidazolium functional groups was estimated to be 1.2 molecules nm⁻². The film obtained on ITO is approximately twice thicker compared to the gold support.     

Heterogeneous reactive extraction for secondary butyl alcohol liquid phase synthesis: Microkinetics and equilibria

The reaction kinetics for the liquid phase synthesis of a racemic mixture of the secondary butyl alcohols (SBA) from linear butene isomers (1-butene (1B); cis-2-butene (c2B); trans-2-butene (t2B)) and water (W) using a macroporous sulfonic acid ion exchange resin as catalyst were determined experimentally in a multiphase CSTR in the temperature range 39–433 K at 6–8 MPa. This range of pressures is necessary to dissolve butenes in the aqueous phase and to ensure a liquid state of all components. For temperatures higher than 423 K the reaction kinetics for the used catalyst size are influenced by mass transfer resistances within the catalyst matrix. The reaction takes place in the water swollen gel phase of the catalysts microspheres. Due to the large excess of water in the gel phase the compositions in the gel phase, in the macropore fluid, and in the catalyst surrounding aqueous phase are assumed to be identical. According to the literature the reaction is rather catalyzed by hydrated acid protons (specific catalysis) than by polymer-bonded-SO₃H groups (general catalysis). The experimental results can therefore be described sufficiently by a pseudo-homogeneous 3-parameter rate expression in aqueous phase activities. The forward reaction is first-order in butene. The reverse reaction is first-order in secondary butyl alcohol. The activation energy was determined to be 108 kJ/mol. Practically no pressure dependence could be observed for pressures exceeding 6 MPa. The ever-present isomerization of the linear butenes on acid catalysts was found to be remarkably faster than the hydration of butenes to SBA. Therefore, the isomerization is considered to be always in equilibrium during the olefin hydration. The formation of the possible by-product di-sec-butyl ether (DSBE) was never observed to a measurable extent. Simultaneous chemical and phase equilibria were investigated theoretically using the volume translated Peng–Robinson equation of state (VTPR-EoS) in combination with a g^E-mixing rule. Parameters of the used g^E-model were adjusted to experimental ternary liquid–liquid equilibrium (LLE) data.     

Coupling of CFD with PBM for a pilot-plant tubular loop polymerization reactor

A computational fluid dynamics model, coupled with population balance model (CFD–PBM), was developed to describe the liquid–solid two-phase flow in a pilot-plant tubular loop propylene polymerization reactor. The model combines the advantage of CFD to calculate the entire flow field and that of PBM to calculate the particle size distribution (PSD). Particle growth, aggregation and breakage were taken into account to describe the evolution of the PSD. The model was first validated by comparing simulation results with the classical calculated data. Furthermore, four cases studies, involving particle aggregation, particle breakage, particle growth or involving particle growth, breakage and aggregation, were designed to identify the model. The entire flow behavior and PSD in the tubular loop reactor, i.e. PSD, solid holdup and liquid phase velocity distribution, were also obtained numerically. The results showed that the model is effective in describing the entire flow behavior and in tracking the evolution of the PSD.     

Hydrodynamic studies of liquid–liquid slug flows in circular microchannels

The aim of the work presented was to clarify the existence of a wall film and its influence on the hydrodynamics of liquid–liquid slug flow capillary microreactor.The methodology of the laser induced fluorescence (LIF) was adopted for visualisation purposes. The measurement of the light intensity profiles revealed a fully developed wall film for a variety of aqueous–organic two-phase systems in glass and PTFE capillaries of 1 mm internal diameter. In addition an acid as a quenching agent enabled the observation of the internal circulation patterns within the liquid slugs, as the fluorescent dye was deactivated by the acid diffusing in from the dye-free phase. A well-defined internal circulation pattern was always present in the wetting phase, i.e. that forming the wall film, leading to uniform mixing in the slugs of this phase. Stagnant zones and local circulation vortices, indicated by variations in the concentrations of the quenched dye, were observed in the non-wetting dispersed phase. These more complex flow structures varied little with the slug velocity, but were strongly dependent on the physical properties of the liquid–liquid system. To predict slug shape and hydrodynamics within the liquid slugs, CFD simulations were carried out using the volume-of-fluid method (VOF) based on the incompressible Navier–Stokes equation with appropriate boundary conditions between the two phases. The slug generation process was studied in a T-junction with 1 mm internal diameter inlets. The implementation of the wetting contact angle, measured in the visualisation experiments for the various systems, led to realistic slug lengths and shapes. The velocity vector plot indicated a fully developed internal circulation pattern within the simulated slugs. Calculations for a single slug with a non-wetting condition gave rise to a wall film in the simulated system.The results obtained demonstrate the significance of the wall film in the hydrodynamics and mass transfer liquid–liquid slug flow and reveal the presence of hitherto unsuspected complex patterns in place of simple single Taylor vortex flow assumed in the past.     

Kinetics and mass transfer in hydroformylation—bulk or film reaction?

The kinetics of a gas–liquid reaction, alkene hydroformylation was studied in the presence of a homogeneous catalyst in a pressurised laboratory‐scale semibatch reactor. Hydroformylation of propene to isobutyraldehyde and n‐butyraldehyde was carried out at 70–115°C and 1–15 bar pressure in 2,2,4‐trimethyl‐1,3‐pentanediol monoisobutyrate solvent with rhodium catalyst using the ligands cyclohexyl diphenylphosphine. In order to evaluate the influence of mass transfer, experiments were made using varied stirring rate from 100 to 1000 rpm at 100°C and 10 MPa syngas pressure. Only at higher stirrings rates, the reaction took place in the kinetic regime. A reactor model was developed comprising both complex kinetics and liquid‐phase mass transfer. The model was based on the theory of reactive films. The model is able to predict under which circumstances the hydroformylation process is affected by liquid‐phase diffusion of the reactants. Experimental data and model simulations are presented for the hydroformylation of propene in the presence of a homogeneous rhodium catalyst.     

Liquid–liquid slug flow: Hydrodynamics and pressure drop

In this paper, the hydrodynamics and the pressure drop of liquid–liquid slug flow in round microcapillaries are presented. Two liquid–liquid flow systems are considered, viz. water-toluene and ethylene glycol/water-toluene. The slug lengths of the alternating continuous and dispersed phases were measured as a function of the slug velocity (0.03–0.5 m/s), the organic-to-aqueous flow ratio (0.1–4.0), and the microcapillary internal diameter (248 and 498 μm). The pressure drop is modeled as the sum of two contributions: the frictional and the interface pressure drop. Two models are presented, viz. the stagnant film model and the moving film model. Both models account for the presence of a thin liquid film between the dispersed phase slug and the capillary wall. It is found that the film velocity is of negligible influence on the pressure drop. Therefore, the stagnant film model is adequate to accurately predict the liquid–liquid slug flow pressure drop. The influence of inertia and the consequent change of the slug cap curvature are accounted for by modifying Bretherton’s curvature parameter in the interface pressure drop equation. The stagnant film model is in good agreement with experimental data with a mean relative error of less than 7%.     

Kinetics of esterification of benzyl alcohol with acetic acid catalysed by cation-exchange resin (Amberlyst-15)

Esterification of benzyl alcohol with acetic acid catalysed by Amberlyst-15 (cation-exchange resin) was carried out in a batch reactor in the liquid phase in the temperature range 328–359 K and at 1 atm. The reaction rate increased with increase in catalyst concentration and reaction temperature. Resin particle size and stirrer speed had virtually no effect on the rate under the experimental conditions. The rate data were correlated with a kinetic model based on homogeneous reaction. The apparent activation energy was found to be 73.3 kJ mol^?1 for the formation of benzyl acetate.     

Kinetics and phase equilibria of competing aldolization and elimination in a three-phase reactor

Butyraldehyde was aldolized with formaldehyde over a weakly basic anion-exchange resin catalyst in aqueous solvent in a batch reactor operating at atmospheric pressure and at temperatures 50–70°C. The reaction mixture was a liquid–liquid–solid system, an emulsion, the phase equilibria of which were studied through chemical analysis of the organic and aqueous phase as well as of the mixed emulsion. Simplified rate equations were derived starting from molecular reaction mechanisms on the catalyst surface. A liquid–liquid reactor model for the fitting of the experimental results was developed on the basis of the rate equations and the phase equilibria. The model described very well the experimental data.     

Mass transfer of long chain fatty acids through liquid–liquid interface stabilized by porous membrane

Long chain fatty acids are usually undesirable contaminants in food oils and removal of these acids is an important task for the oil industry. In this paper the mechanism of transfer of fatty acids from liquid organic phase through flat porous membrane into water was investigated. The membrane pores initially were filled with the same organic solvent without fatty acid. It was demonstrated that the rates of transfer from octane into aqueous solution at pH 4 are inversely proportional to the distribution coefficients (K_d) and are mostly determined by diffusion in aqueous unstirred layers. Increase of pH of aqueous solutions from four to 12 results in a 20-fold increase of transfer rate for octanoic acid, and more than 1000-fold increase for oleic acid. Time lag values necessary for the kinetics of fatty acid transfer to reach the steady state were very small for the transfer from octane, but they were much higher in the case of viscous mineral oil. These last values were in good agreement with Barrer’s equation for diffusion through two unstirred layers. In both cases, there is no significant interface resistance for the transfer of relatively long chain fatty acids from organic solvent into water. Ion exchange processes take place in the aqueous phase and the thickness of the corresponding reaction layer decreases to zero at high pH. This final state is equivalent to the reaction, taking place at the interface. This method could be used for kinetic separation of short and long chain fatty acids at acidic pH and non-selective removal of different fatty acids into alkaline aqueous solutions. The membrane-based process does not need elevated temperature and probably is less energy consuming than distillation.     

Experimental model validation for n-propyl propionate synthesis in a reactive distillation column coupled with a liquid–liquid phase separator

In the synthesis of some organic esters, reactive distillation coupled with a liquid–liquid phase separator is often used to increase the product purity or to recover the reactants. In this article, we present a comprehensive experimental and theoretical study on the heterogeneously catalysed synthesis of n-propyl propionate by reactive distillation and a subsequent liquid–liquid phase separator. The experiments were performed in a pilot-scale reactive distillation column. Data-reconciliation tests proved that the experimental results obtained comprise a complete, reliable set of composition and temperature profiles along the pilot-scale reactive distillation column and can be used for further model validation. A nonequilibrium-stage model was applied to predict the experimental results. Simulation studies demonstrated that the composition and temperature profiles in the rectifying section of the column were highly sensitive to the composition of the reflux stream entering the column. Deviations between the experimental and predicted composition profiles in the rectifying section were identified. An explanation for the deviations is given in this article.     

A LIMITATION OF REUSING THE CATALYST IN TRI-LIQUID-PHASE CATALYTIC SYSTEMS

This work demonstrates important factor influencing the reusability of the phase transfer catalyst in the third liquid phase in addition to the role of the possible loss of catalyst due to the dissolution of the catalyst into the aqueous and organic phases. When the catalyst might react with the byproducts, in addition to reacting with the organic substrate and aqueous nucleophile, it would lose its catalytic activity. The substitution reaction between the organic substrate and an aqueous nucleophile (sodium phenolate) with tetra-n-butylammonium bromide as a phase-transfer catalyst was employed as a model reaction and was performed in a batch reactor. Three organic substrates, including allyl bromide, n-butyl bromide, and ethyl 2-bromoisobutyrate, were tested. Each of the third liquid phases formed in these tri-liquid-phase catalytic systems was utilized three times to observe the change in the activity of the catalyst. The catalyst in the third liquid phase can be reused without any loss of its catalytic activity when allyl bromide or n-butyl bromide is utilized as the organic substrate; however, the catalytic activity declines when ethyl 2-bromoisobutyrate is the organic reactant. Therefore, the organic reactant plays a crucial role in determining whether the catalyst can be reused or not.     

A LIMITATION OF REUSING THE CATALYST IN TRI-LIQUID-PHASE CATALYTIC SYSTEMS

This work demonstrates important factor influencing the reusability of the phase transfer catalyst in the third liquid phase in addition to the role of the possible loss of catalyst due to the dissolution of the catalyst into the aqueous and organic phases. When the catalyst might react with the byproducts, in addition to reacting with the organic substrate and aqueous nucleophile, it would lose its catalytic activity. The substitution reaction between the organic substrate and an aqueous nucleophile (sodium phenolate) with tetra-n-butylammonium bromide as a phase-transfer catalyst was employed as a model reaction and was performed in a batch reactor. Three organic substrates, including allyl bromide, n-butyl bromide, and ethyl 2-bromoisobutyrate, were tested. Each of the third liquid phases formed in these tri-liquid-phase catalytic systems was utilized three times to observe the change in the activity of the catalyst. The catalyst in the third liquid phase can be reused without any loss of its catalytic activity when allyl bromide or n-butyl bromide is utilized as the organic substrate; however, the catalytic activity declines when ethyl 2-bromoisobutyrate is the organic reactant. Therefore, the organic reactant plays a crucial role in determining whether the catalyst can be reused or not.     

Phase equilibria of mixtures containing CO2 and organic acids using the UMR-PRU model

The UMR-PRU model, which has been successfully tested in the past to the predictions of different type of phase equilibrium and thermodynamic properties in binary and multicomponent systems, is applied in this work to phase equilibria in mixtures containing CO₂ and organic acids. New interaction parameters are determined by fitting only binary vapor–liquid equilibrium data and then they are used to predict the vapor–liquid, solid–gas and solid–liquid–gas equilibria in CO₂/organic acid systems. Furthermore, the UMR-PRU model with the newly derived interaction parameters is applied to the prediction of the phase equilibrium in ternary mixtures consisting of CO₂, organic acids and water. Satisfactory results are obtained in all cases.     

Evaluation of published kinetic models for tert-amyl ethyl ether synthesis

The published kinetic models for liquid phase synthesis of tert-amyl ethyl ether (TAEE) by addition of ethanol to isoamylenes on acidic ion exchange resins were evaluated by comparison with own experimental data. Fixed bed and batch reactor experiments were carried out in liquid phase on Amberlyst-35 ion exchange resin as catalyst. Among the published kinetic models, our experimental data fits the best with the model published by [J.A. Linnekoski, A.O. Krause, L.K. Rihko, Kinetics of the heterogeneously catalyzed formation of tert-amyl ethyl ether, Ind. Eng. Chem. Res. 36 (1997) 310–316.]. In the simulation study for the fixed bed reactor experiments, the influences of axial mixing, liquid–solid mass transfer and internal diffusion steps on the overall process kinetics were theoretically evaluated. The results evidenced that on the working temperature domain, significant kinetic limitations by internal diffusion can appear for catalyst pellets size over 1 mm. The external mass transfer step has a weak influence on the process kinetics and can be important only at lower limit of the flow rates domain. Our computations evidenced also a negligible influence of the axial mixing on the reactants conversion in the experimental fixed bed reactor, on the working domain investigated.     

Numerical simulation of influence of mass transfer on phase transfer catalytic reaction

Mass transfer with chemical reaction by liquid/liquid phase tranfer catalysis (LLPTC) for an isothermal batch reactor was analyzed. The results for the phase transfer catalyzed reaction system can be generally described by a pseudo first-order hypothesis, whereas the reaction system can be controlled by simultaneous mass transfer of the catalysts between two liquid phases and chemical reaction in the organic phase. The mass transfer limitation is mainly from the mass transfer step of QX from the organic phase to the aqueous phase. The concept of catalyst-effectiveness vs. physically meaningful parameters in a liquid/liquid phase transfer catalyzed reaction is introduced. The catalyst effectiveness is increased as the mass transfer factors increase, the ratio of reaction rate coefficients of aqueous forward reaction to organic increases, and the equilibrium constant in the aqueous solution increases.