An analysis of chemical reactor stability and control-XV The multicomponent spray reactor

The objective of this paper is to present a continuous stirred-tank model for processes in which a liquid is sprayed into a gas. In this model a uniform spray of droplets enters the reactor along with one or more gaseous reactants. Both the droplets and the gas are taken to be well-mixed in the reactor. The droplets evaporate and the resulting vapors combine homogeneously with the gaseous reactants. In the multicomponent case the droplets contain a mixture of two or more miscible components. Steady state and transient equations are derived, and the numerical methods of solution are discussed. Steady states are calculated for two-component systems of heptane-octane and hydrazine-water and for a three-component system of heptane-octane-nonane, with oxygen as the gaseous reactant in each case. In addition, a study is made of several parameters of the heptane-octane system. Stability is determined by calculating transients for selected perturbations from the steady states. Incorporated in the reactor model is an analytic model for the fluxes of vapors from a multicomponent droplet which exhibits the augmentation of transfer of one vapor due to the net flux of the remaining vapors. Also presented is an approximate model for the multicomponent spray reactor which gives good results for small droplets and requires only a fraction of the effort needed to solve the complete set of equations.     

Population balance modeling and simulation of liquid–liquid–liquid phase transfer catalyzed synthesis of mandelic acid from benzaldehyde

Mandelic acid has cosmetic, pharmaceutical, and antibacterial activities and is used in urinary antiseptic medicines. An attractive process for the production of mandelic acid is through reaction between benzaldehyde, sodium hydroxide, and chloroform in the presence of polyethylene glycol 4000 as a phase transfer catalyst. The liquid–liquid phase transfer catalyzed (L–L PTC) reaction can be intensified by converting it into three‐liquid phases (L–L–L PTC). We address the modeling of a well‐stirred reactor for the foregoing process, in which organic droplets surrounded by a thin film of catalyst‐rich phase are suspended in the aqueous phase. A population balance model is formulated for the L–L–L PTC reaction and solved by Monte Carlo simulation using interval of quiescence technique. Transport processes and intrinsic reaction kinetics are extracted from the experiments. This population balance model serves to assess and interpret the relative roles of various processes in L–L–L PTC reaction, such as diffusive transport, reaction, and interaction between dispersed phase droplets. The model is expected to be an effective tool for reactor design and scale up. © 2012 American Institute of Chemical Engineers AIChE J, 2012     

Liquid-liquid two phase flow of millichannel with a dynamic mixer

A novel millimeter-sized reactor, named millireactor in the present study, is proposed for oil droplet formation aiming at heterogeneous enzymatic hydrolysis. The reactor consists of a circular ditch with a small stirring bar and a straight channel. To obtain optimal operation conditions and a reactor configuration for droplet formation, the stable droplet formation region was explored by mapping analysis, and the relationship between the specific interfacial area of droplets and other physical parameters were investigated by dimension analysis. It was found that the shear stress parameter and interfacial tension played important roles in the droplet formation. Considering parameters such as the length of the stirring bar, rotating speed, diameter of the stirring section, gap width, interfacial tension, viscosity and residence time in the stirring section, a correlation equation including three dimensionless numbers was obtained. The optimal design guide for obtaining high specific area of oil droplets involves increasing the volume of the stirring section and reducing the gap width as narrow as possible. The reactor performance was evaluated comparing the amount of fatty acid produced in the millireactor versus the batch reactor. The productivity of the millireactor was higher than the batch reactor for the first 10 min.     

Kinetics of homogeneous 5‐hydroxymethylfurfural oxidation to 2,5‐furandicarboxylic acid with Co/Mn/Br catalyst

2,5‐furandicarboxylic acid (FDCA) is a potential non‐phthalate based bio‐renewable substitute for terephthalic acid‐based plastics. Herein, we present an investigation of the oxidation rate of 5‐hydroxymethylfurfural (HMF) to FDCA in acetic acid medium using Co/Mn/Br catalyst. Transient concentration profiles of the reactant (HMF), intermediates [2,5‐diformylfuran (DFF), 5‐formyl‐2‐furancarboxylic acid (FFCA)], and the desired product (FDCA) were obtained for this relatively fast reaction in a stirred semi‐batch reactor using rapid in‐line sampling. Comparison of the effective rate constants for the series oxidation steps with predicted gas–liquid mass transfer coefficients reveals that except for the FFCA → FDCA step, the first two oxidation steps are subject to gas–liquid mass transfer limitations even at high stirrer speeds. Novel reactor configurations, such as a reactor in which the reaction mixture is dispersed as fine droplets into a gas phase containing oxygen, are required to overcome oxygen starvation in the liquid phase and further intensify FDCA production. © 2016 American Institute of Chemical Engineers AIChE J, 63: 162–171, 2017     

Modelling and simulation of suspension polymerization of vinyl chloride via population balance model

A detailed population balance model is presented for suspension polymerization of vinyl chloride in an isothermal batch reactor perfectly mixed on macrolevel. Coalescence and breakage of monomer droplets, as well as mass exchange of species between the droplets induced by collisions, termed micromixing, are also included into the model forming a complex three-scale system. The resulted population balance equation is solved by coupling the deterministic continuous time computation of polymerization reactions inside the droplets with the random coalescence and breakage events of droplets using Monte Carlo simulation. The results obtained by simulation revealed that aggregation, breakage and micro-mixing of species induced by droplet collisions affect the process significantly.     

Two‐phase production of cyclohexene and cyclooctene oxides in water as an approach to pollution prevention

Liquid–liquid two‐phase epoxidation from cyclohexene and cyclooctene in aqueous potassium peroxymonosulfate (commercially available as Oxone^®) solution was studied as an application in pollution prevention. To avoid potential emissions of volatile organic compounds an aqueous solution was employed to replace the usual chlorinated solvents used in epoxide production. A droplet column reactor and stirred tank reactor were used to investigate two‐phase synthesis of epoxide. An aqueous Oxone^® solution was used to oxidize a dispersion of alkene droplets and form epoxide. The study of aqueous epoxidation in both reactors showed that the epoxidation of alkenes can be represented as a first order reaction with respect to alkene. The salting out effect of Oxone^® concentration was studied in both reactors and found to be very similar at optimal conditions. In comparing the two reactors, it was found that the droplet column reactor produces larger quantities of product per unit reactor volume for the same reaction time. The objective of this study is to provide an alternative reactor design and synthesis route that can meet pollution prevention goals. Copyright © 2004 Society of Chemical Industry     

CFD analysis of hydrodynamic, heat transfer and reaction of three phase riser reactor

Catalytic cracking reaction and vaporization of gas oil droplets have significant effects on the gas solid mixture hydrodynamic and heat transfer phenomena in a fluid catalytic cracking (FCC) riser reactor. A three-dimensional computational fluid dynamic (CFD) model of the reactor has been developed considering three phase hydrodynamics, cracking reactions, heat and mass transfer as well as evaporation of the feed droplets into a gas solid flow. A hybrid Eulerian-Lagrangian method was applied to numerically simulate the vaporization of gas oil droplets and catalytic reactions in the gas-solid fluidized bed. The distributions of volume fraction of each phase, gas and catalyst velocities, gas and particle temperatures as well as gas oil vapor species were computed assuming six lump kinetic reactions in the gas phase. The developed model is capable of predicting coke formation and its effect on catalyst activity reduction. In this research, the catalyst deactivation coefficient was modeled as a function of catalyst particle residence time, in order to investigate the effects of catalyst deactivation on gas oil and gasoline concentrations along the reactor length. The simulation results showed that droplet vaporization and catalytic cracking reactions drastically impact riser hydrodynamics and heat transfer.     

Modeling study on the impaction and humidification process in desulfurization activation reactor

In a semi-industrial desulfurization activation reactor, the high-CaO coal ash is directly used as desulfurization sorbent and activated by spray water. As a result of the inertial impactions between ash particles and water droplets, concentrated slurry droplets are formed. In order to numerically investigate the process, a new model of particle-flow-passage is deduced to simulate the impaction humidification process between particles and droplets. Computation results show that the spray water flow rate and droplets size have obvious influence on droplets catching efficiency, and the ash particles are caught primarily in the vicinity of the atomizer nozzle. With the models of gas phase turbulence, heat and mass transfer, and chemical reactions being considered, a comprehensive 3D model for FGD process (Flue Gas Desulfurization) is established, and several operation conditions are simulated by the model. The predicted results such as SO₂ absorption efficiency and the flue gas temperature at the outlet are in good agreement with experimental results.     

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.     

Transposition from a batch to a continuous process for microencapsulation by interfacial polycondensation

A novel continuous process is proposed and investigated to produce microcapsules by interfacial polycondensation. Polymeric microcapsules are obtained via a two-step process including an initial emulsification of two immiscible fluids in static mixers and a subsequent interfacial polycondensation reaction performed in two different continuous reactors, the Deanhex heat exchanger/reactor or a classical coiled-tube. This study is carried out through a step by step approach. A model system involving polyurea as the polymeric membrane and cyclohexane as the encapsulated species is chosen. A semi-batch reaction kinetic study is first performed in order to obtain kinetics data of the polycondensation reaction and to highlight hydrodynamic issues that can happen when running the encapsulation reaction in classical stirred tank. Parameters influencing droplets size obtained when carrying out emulsification in static mixers are then investigated. The hydrodynamic of the Deanhex reactor used is also characterized in terms of mixing time and residence time distribution. To validate the innovative continuous process, the emulsion droplets obtained at the static mixer outlet are encapsulated firstly in the Deanhex reactor and secondly in the coiled-tube. The apparent reaction kinetics and microcapsules characteristics corresponding to different operating conditions are discussed.     

Circumferential mixing in one-phase and two-phase Taylor vortex flows

This work deals with the experimental study of the circumferential mixing in the Taylor vortex flow between coaxial cylinders. Each pair of vortices is considered as a closed chemical reactor and the residence time distribution in this reactor is determined by an electrochemical method. The tanks-in-series model with recirculation is applied to describe the flow in the vortex reactor; the circumferential mixing is characterized by the number of tanks in series and by the mixing time. Experiments were carried out with one-phase flow and with two-phase liquid—liquid flow. It is shown that stirring due to the liquid droplets increases the circumferential mixing.     

Intensification of biodiesel synthesis using metal foam reactors

This study presents a technology for continuous and high-efficiency alkali-catalyzed biodiesel synthesis using a metal foam reactor combined with a passive mixer. A metal foam reactor with higher pore density produces smaller droplets that result in higher efficiency of biodiesel synthesis. Compared with conventional stirred reactors, the time for high methyl ester conversion can be shortened remarkably by the use of metal foam reactors. Experimental results reveal that a metal foam reactor of 50 pores per inch exhibits an energy consumption per gram biodiesel of 1.01 J g⁻¹, merely 1.69% and 0.77% of energy consumption of the zigzag micro-channel and conventional stirred reactors, respectively. Moreover, biodiesel yield per reactor for the metal foam reactor is approximately 60 times that of the zigzag micro-channel reactor, thus overcoming the problem of numbering up an excessive number of reactors in the application. These results indicate the great potential of metal foam reactors in small-fuel biodiesel processing plants for distributive applications.     

Calibration of an Aerosol Composition Mass Spectrometer with Sulfuric Acid Water Aerosol

The first quantitative chemical analysis of polar stratospheric cloud particles has recently been performed using a balloon-borne aerosol composition mass spectrometer (ACMS). A similar spectrometer is presently used in a large cryo-chamber experiment to study low temperature aerosols. All experiments require prior to their employment an accurate calibration to convert mass spectrometer signals into molecular species contained in the aerosols. For the calibration, pure H 2 SO 4 /H 2 O droplets are generated having known composition and diameters between 0.4 w m and 1 w m. The size distribution and the number concentration can be controlled. A flow reactor with a rotating inner glass cylinder placed in a H 2 SO 4 /H 2 O bath solution of known concentration is used to condition the droplets. The residence time of the particles in the flow reactor is long enough that the droplets adopt the composition of the bath solution before entering the ACMS. The result is a linear relationship between the mole ratio of the H 2 SO 4 /H 2 O droplets and the mass spectrometer count rate ratio of water to sulfuric acid. The evaluation takes the dissociation of H 2 SO 4 inside the ACMS into account. The calibration error varies between 3 and 4 wt. % H 2 SO 4 for stratospheric particles with a composition of 30-70 wt. % H 2 SO 4 . Besides the calibration of the instrument, the analysis of the aerosols is a valuable diagnostic tool to investigate impurities in the particles.     

Morphologies and properties of NiO particles prepared from NiCl<Subscript>2</Subscript>·6H<Subscript>2</Subscript>O by spray pyrolysis

Nickel oxide particles were prepared by spray pyrolysis of aqueous solution of NiCl₂·6H₂O. In the reactor the salt droplets were first converted to hollow particles by drying and then they were collapsed by oxidation to reduce their size. Each oxide particle was composed of many small nuclei with voids among them due to extremely low rate of sintering. The particle size decreased with the temperature as the sintering and crystallization proceeded. The size as well as the crystallinity of the particles increased with the initial salt concentration. When the salt droplets were preliminarily dried in diffusion dryer before entering the reactor, the collapse of the particles was considerably reduced, resulting in lower hollowness and higher sphericity. Numerical simulation on the drying of the droplets provided insight on the initial stage of spray pyrolysis.     

Clean Synthesis of Fatty Acids in an Intensive Lipase-Catalysed Bioreactor

The operation and performance of a laboratory-scale catalytic bioreactor used for the hydrolysis of mixed natural oils into free fatty acids in the presence of free lipase (triacylglycerol ester hydrolase, EC 3.1.1.3) is described. The novelty of the reactor system is the intensification of the reaction kinetics by electrostatic spraying of the lipase as a dispersion of very fine droplets into the oil phase. The effect of phase ratio and field strength on the kinetics were measured for a simple batch type spray reactor. Coalescence rates within the electrostatic spray reactor were shown to be significantly higher than conventional stirred systems. The kinetic data were compared with a control experiment in which the free lipase suspension and oil were contacted by mechanical mixing in the absence of any externally applied electrical field. © 1997 SCI.     

Continuous preparation of porous organofunctionalized (Hetero-)polysiloxane spheres

When in contact with water, organoalkoxysilanes and other alkoxy metal compounds react to organofunctionalized polysiloxanes or heteropolysiloxanes. A new method for the continuous preparation of porous organopolysiloxane spheres was developed. Droplets of the precursor mixture are injected into the vertical reactor column. Movement of the gelating droplets is controlled by flow of the aqueous reaction medium. Advantages are adjustable residence time, minimized coalescence and low reactor height. An example shows typical product characteristics like narrow particle size distribution and macroporosity.     

Effect of impeller diameter on mean droplet diameter in circular loop reactor

Experiments in liquid-liquid dispersion were performed using a circular loop reactor designed for suspension polymerization. In order to vary widely differences in physical properties such as the viscosity, density, and interfacial tension between the two liquids concerned, polystyrene pellets were dissolved into the dispersed phase which consisted of a styrene monomer liquid. The diameter distributions and mean diameters of droplets formed were measured by stepwise changes in the impeller diameter. From these results, the effect of the impeller diameter on the breakup of droplets was determined. Correlations relating the mean droplet diameter to the operational conditions were derived.     

Seeded emulsion polymerization of n-butyl acrylate utilizing miniemulsions

Nucleation of polymer particles in the seeded emulsion polymerization of n-butyl acrylate (BuA) was studied through experiments designed to control the amount of new particles formed. The results show that for the batch and semicontinuous seeded polymerization of BuA, a small amount of new particles was formed in the system in which the monomer was added neat, whereas a singificant amount of new particles was formed when the monomer was added as a miniemulsion. This suggests that new particles formed in the miniemulsion process were from nucleation of the monomer droplets. These experiments also showed that monomer-droplet nucleation decreased with increasing seed concentration in the reactor. For the seeded semicontinuous polymerizations, monomer-droplet nucleation decreases with decreasing BuA miniemulsion feed rate. The results also show that monomerdroplet nucleation takes place whenever miniemulsion droplets exist in the reactor. This study suggests that miniemulsions can be used to control the particle size distribution of a polymer latex system.     

Online Monitoring of Vinyl Chloride Polymerization in a Microreactor Using Raman Spectroscopy

A novel capillary‐based microfluidic device has been designed to follow the vinyl chloride polymerization reaction. Monodisperse droplets of 200 μm diameter could be obtained by means of a co‐flow generation system, each one being considered as a polymerization reactor. Monomer droplets were visualized in a microchannel with a high‐speed camera. At the end of the reaction, PVC grains were observed with a scanning electron microscopy technique. Real‐time non‐invasive Raman measurement was performed on stationary vinyl chloride monomer droplets and provided values of effective reaction orders and rate constants. This microdevice allowed reaction investigation under difficult conditions of pressure and temperature with a minimal amount of reagents.     

Comparison of Storage Methods for Microfluidically Produced Water‐in‐Oil Droplets

Microfluidically produced water‐in‐oil droplets are an important platform for biochemical research. To investigate the structural integrity of droplets during transfer and storage processes, different methods were compared. Storage as isolated droplets inside plastic tubing or a designed microfluidic chamber led to moderate decreases in droplet volume but only slight changes in monodispersity, whereas bulk storage in an Eppendorf cup led to the complete loss of monodispersity. It is further demonstrated that on‐chip storage of the droplets in a fluidic microcavity array avoids coalescence and enables a reduction in volume with the concurrent increase in the concentration of entrapped proteins, which is relevant for applications in life science.