Effect of homogeneity of methanol/water/monomer mixture on the mode of polymerization of MMA: Soap-free emulsion polymerization versus dispersion polymerization

The soap-free emulsion polymerization of methyl methacrylate (MMA) was carried out with varying amount of methanol in the aqueous medium. As methanol content increased, the phase of polymerization mixture (methanol/water/monomer) changed from heterogeneous to homogeneous state and the transition occurred at 30 wt% methanol. In order to identify the mechanism of the polymerization in heterogeneous and homogeneous mixture, the properties of the prepared PMMA particles were analyzed in terms of the effects of methanol content on the conversion at 1 h, initiator concentration and polymerization temperature. With the heterogeneous mixture in the range of 0-20 wt% methanol, the polymerization product and polymerization behavior resembled those typically observed in the soap-free emulsion polymerization. On the other hand, the characteristics of the polymerization products were similar to those typically obtained in the dispersion polymerization under the homogeneous mixture ranging 40-60 wt% methanol. Thus, the homogeneity in the aqueous methanol mixture can be a critical factor in determining the polymerization modes between the soap-free emulsion and dispersion polymerization.     

Polymer property control in a continuous styrene polymerization reactor using model-on-demand predictive controller

The model-on-demand (MoD) framework was extended to the model predictive control (MPC) to design a multiple variable model-on-demand predictive controller (MoD-PC). This control algorithm was applied to the property control of polymer product in a continuous styrene polymerization reactor. For this purpose, a local auto-regressive exogenous input (ARX) model was constructed with a small portion of data located in the region of interest at every sample time. With this model an output prediction equation was formulated to calculate the optimal control input sequence. Jacket inlet temperature and conversion were chosen as the elements of regressor state vector in data searching step. Simulation studies were conducted by applying the MoD-PC to MIMO control problems associated with the continuous styrene polymerization reactor. The control performance of the MoD-PC was then compared with that of a nonlinear MPC based on the polynomial auto-regressive moving average (ARMA) model for disturbance rejection as well as for setpoint-tracking. As a result, the MoD-PC was found to be an effective strategy for the production of polymers with desired properties.     

Polymer-supported metallocene catalysts for gas-phase ethylene polymerization

The use of hydroxylated chloromethylated-styrene/divinylbenzene copolymer as a support for three different catalysts, Cp₂ZrCl₂, [Ind]₂ZrCl₂ and (CH₃)₂Si[Ind]₂ZrCl₂ has been examined for the polymerization of ethylene in gas phase. The gas phase polymerization experiments were performed in a horizontal reactor by using Box-Behnken experimental design [Box and Wilson, 1951] to study the effects of temperature, ethylene partial pressure, and MAO cocatalyst level on polymerization. The measured average catalyst activities were empirically correlated with these three factors. Temperature appears to be the most important factor, which shows a first and second order effect on activity and also interacts with pressure and MAO. The kinetic study shows that these supported catalysts might contain two types of active sites, and the deactivation of sites follows a first order kinetic. This paper is dedicated to Professor Wha Young Lee on the occasion of his retirement from Seoul National University.     

Evaluation of the mixing performance of three passive micromixers

This work presents a numerical investigation on mixing and flow structures in microchannels with different geometries: zig-zag; square-wave; and curved. To conduct the investigation, geometric parameters, such as the cross-section of the channel, channel height, axial length of the channel, and number of pitches, are kept constant for all three cases. Analyses of mixing and flow fields have been carried out for a wide range – 0.267–267 – of the Reynolds number. Mixing in the channels has been analyzed by using Navier–Stokes equations with two working fluids, water and ethanol. The results show that the square-wave microchannel yields the best mixing performance, and the curved and the zig-zag microchannels show nearly the same performance for most Reynolds number. For all three cases, the pressure drop has been calculated for channels with equal streamwise lengths. The curved channel exhibits the smallest pressure drop among the microchannels, while the pressure drops in the square-wave and zig-zag channels are approximately the same.     

Investigation of the convective motion through a staggered herringbone micromixer at low Reynolds number flow

A computational study is presented of the complex flow through a staggered herringbone micromixer (SHM), which utilises sequences of asymmetrical herringbone grooves in cycles where a set of topologically similar grooves represent a half cycle. It was analysed using finite-element (method) based software to elucidate the fluid flow within the channel and characterise the effect of the grooves at moving fluid across the channel thus creating non-axial fluid movement. Three separate physical systems were modelled: a channel containing a single groove, a half cycle of infinite grooves and an infinite system with one groove per half cycle. A range of groove heights were investigated for the single groove for the Reynolds number range 0-15 to identify the mechanics through which fluid is transported across the channel by the grooves, the effect that inertial and viscous forces have on the process and to identify a groove height range for optimised cross channel fluid transfer. The flow field within the grooves at various heights was analysed and their relationship with non-axial flow within the bulk channel identified. The culminating effect of increasing grooves per half cycle on their ability to transport fluid across the channel is analysed by comparing the entrainment of fluid into and across the groove for both a single and infinite grooves. The maximum increase in fluid entrainment per groove for the addition of extra grooves to a cycle was found to be 14%. The helicity (or swirl) of the flow within the channel is found to be small for all three systems, while increased helicity within the flow was found to correspond to an increase in energy dissipation.     

Mixing efficiency of a multilamination micromixer with consecutive recirculation zones

Adding recirculation zones to a mixer for a microplant is proposed for enhanced mixing efficiency. A multilamination interdigital micromixer has been widely used in microchemical plants for precision or small scale chemical process. The mixing efficiency of this micromixer is relatively low as the mixing of two fluids is executed by the laminar diffusion process. To assist the mixing by fluid action, a series of recirculation zones were added to the mixing chamber. The effectiveness of the recirculation zones on mixing was estimated through a numerical simulation which indicated the dependence on Reynolds number. Mixing efficiency increased at Reynolds number that is relevant to the condition that is prevalent in a microchemical plant. The proposed micromixer was fabricated by the lithography process on the photosensitive glass wafers. The mixing qualities of the fabricated micromixer were measured by two methods; the flow visualization of dilution type experiments and the reactivity measurement. The measurement of color intensity of the mixed fluid followed the predictions by the simulation. For a Reynolds number greater than 400 that was relevant in mixers for microchemical plant, a mixing efficiency higher than 90% was obtained by adding the recirculation zones.     

Monitoring stability of reaction and dispersion states in a suspension polymerization reactor using electrical resistance tomography measurements

A system was constructed for visualizing the dispersion states of liquid monomer and polymer droplets in a suspension polymerization reaction in a non-intrusive and continuous manner using electrical resistance tomography measurements.Using this system, a method was established for keeping reaction states stable and the quality of the particles produced good by varying the mixing conditions, such as the rotational speed of the impeller and the injection amount of dispersion reagents, under the principles that the lower the rotational speed of the impeller, the better; and the smaller the injection amount of dispersing agents, the better.It was found that the amount of additional injection significantly affected both the sharpness and the average value of the particle diameter distribution, based on the data of the tomograms.     

Electrokinetic mixing of two fluids with equivalent conductivity

Electrokinetic (EK) micromixers have been widely studied in the past decade for biochemical applications, biological and chemical analysis, etc. Unfortunately, almost all EK mixers require different electrical conductivity between the two fluids to be mixed, which has greatly limited their wide applications, in cases where the two streams to be mixed have equivalent electrical conductivity. Here we show that mixing enhancement between two fluids with identical conductivity can be achieved in an EK micromixer with conductive sidewalls, where the electric field is in transverse direction of the flow. The results revealed that the mixing became stronger with increased conductivity value. This mixing method provides a novel and convenient strategy for mixing two liquids with the same or similar electrical conductivity in microfluidic systems, and could potentially serves as a powerful tool for sample preparation in applications such as liquid biopsy, and environmental monitoring, etc.     

Weighted support vector machine for quality estimation in polymerization processes

In this paper, a modified version of the Support Vector Machine (SVM) is proposed as an empirical model for polymerization processes modeling. Usually the exact principle models of polymerization processes are seldom known; therefore, the relations between input and output variables have to be estimated by using an empirical inference model. They can be used in process monitoring, optimization and quality control. The Support Vector Machine is a good tool for modeling polymerization process because it can handle highly nonlinear systems successfully. The proposed method is derived by modifying the risk function of the standard Support Vector Machine by using the concept of Locally Weighted Regression. Based on the smoothness concept, it can handle the correlations among many process variables and nonlinearities more effectively. Case studies show that the proposed method exhibits superior performance as compared with the standard SVR, which is itself superior to the traditional statistical learning machine in the case of high dimensional, sparse and nonlinear data.     

Nonlinear model predictive control of an industrial polymerization process

Nonlinear model predictive control (NMPC) is used to maintain and control polymer quality at specified production rates because the polymer quality measures have strong interacting nonlinearities with different temperatures and feed rates. Polymer quality measures that are available from the laboratory infrequently are controlled in closed-loop using a NMPC to set the temperature profile of the reactors. NMPC results in better control of polymer quality measures at different production rates as compared to using the nonlinear process model with reaction kinetics to implement offline targets for reactor temperatures.     

3种被动式微混合器的性能对比及压损分析

通过数值模拟的方法对3种不同结构的被动式微混合器进行了研究,同时对3种被动式微混合器的混合性能进行了对比,并进一步研究了微混合器的压力损失。结果显示,内肋形微混合器在5种速度条件下的混合性能要优于其他两种微混合器,而且随着流量越来越大,其压力损失也越来越大;内肋形微混合器的压力损失最大,Z形微混合器的次之,Y形微混合器的最小。     

Continuous supercritical hydrothermal synthesis of controlled size and highly crystalline anatase TiO2 nanoparticles

The production of size-controlled and highly crystalline anatase titanium dioxide (TiO₂) nanoparticles was carried out under supercritical hydrothermal conditions (400 °C and 30 MPa) in a continuous flow apparatus with a residence time of 1.7 s. An industrially useful titanium sulfate (Ti(SO₄)₂) solution was used as the starting solution. KOH was used to change TiO₂ solubility and pH and thereby control the particle size. The apparatus comprised two micromixers operating at high temperature. The first mixer was configured to prepare a supercritical aqueous KOH solution from supercritical water (SC-H₂O) and KOH. The second mixer combined this KOH solution with aqueous Ti(SO₄)₂. In situ pH control and homogeneous nucleation were achieved in the second mixer. This two-step high-temperature micromixing process produced reasonably small and homogeneous particles. The particles were characterized by transmission electron microscopy (TEM) on the basis of morphology, average size, and size distribution, together with the coefficient of variation (CV). Powder X-ray diffraction (XRD) was used to determine the crystal structure and crystalline size. The weight loss of material was found through thermogravimetric (TG) measurement. The crystal structure of the product was assigned to the anatase single phase. The average particle size could be adjusted in the range 13–30 nm while maintaining a CV of 0.5 by changing the KOH concentration. At low pH, the powder XRD results for crystallite size were in good agreement with the average particle size measured by TEM, confirming that the products were single crystals of TiO₂ nanoparticles. When the reactor temperature was increased from 400 to 500 °C, the weight loss decreased from 4.5 to 2.5%, keeping the average particle size and high crystallinity of the TiO₂ particles unchanged.     

An experimental study for property control in a continuous styrene polymerization reactor using a polynomial ARMA model

By using a multivariable nonlinear model predictive controller (NLMPC), the control experiments for the monomer conversion and the weight-average molecular weight are conducted in a continuous styrene polymerization reactor. Instead of a complex first-principles model, a polynomial auto-regressive moving average model (ARMA) is used to describe the nonlinear behavior of the polymerization reactor. The pseudorandom multilevel input signals mounted on the jacket inlet temperature and the feed flow rate are applied to the polymerization reaction system to identify a polynomial ARMA model. In the experiments of identification and control, the monomer conversion and the weight-average molecular weight are measured by on-line densitometer and viscometer with appropriate correlations. The on-line measurements are found to be in good agreement with the off-line analysis by the gravimetry and the gel permeation chromatography. Since a polynomial ARMA model is expected to give a higher order objective function of input variables, we employ the extended Kalman filter based NLMPC scheme to reduce the computational requirement in the control experiments. The NLMPC based on the polynomial ARMA model is found to perform satisfactorily for the control of the polymer properties during a grade-transition period as well as under the steady-state operation.     

Poly(carbyne)s via reductive C1 polymerization

C1 polymerizations enable the synthesis of densely functionalized, persubstituted polymers that are challenging to access using conventional methods. One class of C1 polymers are the poly(carbyne)s, which are unique in that they can adopt branched or linear structures, or combinations thereof. Herein, we report the synthesis of new poly(carbyne)s, including those that feature side chains with carbonyl‐containing functional groups. The polymers were obtained by exposing solutions of monomers outfitted with trihalomethyl or trimethyl orthoformate groups to metallic lithium, and reaction performance was improved when electron transfer agents (e.g. naphthalene) were included. The mechanisms of the polymerizations were also deconvoluted and found to depend on the monomer employed.     

Continuous hydrothermal synthesis of Ca1−xSrxTiO3 solid-solution nanoparticles using a T-type micromixer

In this work, a continuous hydrothermal synthesis method in supercritical water was applied to environmentally benign production of Ca_1−xSr_xTiO₃ (x = 0.0–1.0) solid-solution nanoparticles as key materials in conducting, electric, magnetic, and photocatalytic fields. A T-type micromixer (330 μm id) was introduced for rapid heating of stating solutions of Ca(NO₃)₂, Sr(NO₃)₂, and TiO₂ sol using turbulent flow mixing with preheated NaOH aqueous solution and also for exact control of reaction temperature. At 673 K and 30 MPa for 5.0 s mean residence time, Ca_1−xSr_xTiO₃ solid-solution nanoparticles having crystallite diameters of around 20 nm with monomodal diameter distributions were obtained without byproducts and production of CaTiO₃ and SrTiO₃ separately over the whole composition range. Effects of NaOH molality, Ca and Sr compositions in the starting solutions, and mean residence time on the reaction were examined. The results showed that TiO₂ sol dissolution and Ca_1−xSr_xTiO₃ precipitation were almost finished within 0.25 s mean residence time, and after that Ostwald ripening proceeded.     

THE EFFECT OF IMPERFECT MIXING ON POLYMER QUALITY IN LOW DENSITY POLYETHYLENE VESSEL REACTORS

A mathematical model is developed for the calculation of polymer quality in low density polyethylene vessel reactors, taking into account mixing limitations at the initiator feed point. Model predictions show that imperfect mixing in the reactor can produce considerable variations in polymer molecular weight distribution. The effect of the most important process conditions, input feed temperature, solvent concentration, monomer flow rate and initiator type, on the final polymer quality is analyzed. The advantages of the design solution which divides the reactor in more compartments in series are also discussed.     

Fluids mixing in devices with connected-groove channels

A novel connected-groove micromixer (CGM) has been designed, fabricated, and investigated thoroughly. Connected grooves in this device, crossing multiple sides of the microchannel induced an intensely transverse field of fluids, and thus generating rapid mixing than patterned grooves on a single side alone. The fabrication of a CGM was facilitated to overcome the complication of fabricating the sidewall and bottom grooves in a channel simultaneously; a CGM hence became highly efficient and compact. We propose here CGM of two types—CGM-1 and CGM-2—and compare their mixing performance with a slanted-groove micromixer (SGM) numerically and experimentally for Re over a wide range (1-100). Numerical analysis demonstrated that a CGM provided intense transverse components in the field and great mixing efficiency; in particular, CGM-2 with co-rotating flows encompassing the mechanisms of cutting and blending of fluids had a mixing performance over 50% better than an SGM for Re=1-100. To systematically analyze the mixing by experiments, mixing of slightly viscous fluids, highly viscous fluids, and bio-fluids were adopted, respectively. The mixing experiments of slightly viscous dye solutions on the basis of the color uniformity of mixture showed that mixing lengths of both CGM were smaller than that of SGM. Based on the mixing results of highly viscous fluids, CGM-1 with sidewall grooves had a shorter pitch of spiral flow and more helical turns than an SGM. With a confocal microscope we explored the mixing sections of fluorescent proteins (B-phycoerythrin, BPE; Allophycocyanin alpha subunit, ApcA) inside a micromixer to confirm the numerical results.     

THE EFFECT OF IMPERFECT MIXING ON POLYMER QUALITY IN LOW DENSITY POLYETHYLENE VESSEL REACTORS

A mathematical model is developed for the calculation of polymer quality in low density polyethylene vessel reactors, taking into account mixing limitations at the initiator feed point. Model predictions show that imperfect mixing in the reactor can produce considerable variations in polymer molecular weight distribution. The effect of the most important process conditions, input feed temperature, solvent concentration, monomer flow rate and initiator type, on the final polymer quality is analyzed. The advantages of the design solution which divides the reactor in more compartments in series are also discussed.     

An overlapping crisscross micromixer

We propose a grooved micromixer incorporating an overlapping crisscross inlet port that is located at the intersection of two patterned channels crossing one above another. Both numerical analysis and experimental verification of the flow structure of this design have substantiated the superior mixing features over the existing herringbone mixer. Because of the symmetric feature of this microstructure, fabrication becomes simplified through assembling two identical PDMS-based slabs oppositely. Both experimental results for flow visualization and numerical simulation reveal significant cross flow at the intersection of the two channels and vertical tumbling of the flow. This activated flow feature supplies downstream fluids with vertical momentum to enhance the chaotic advection and enlarges the interfacial area between the two mixing fluids. All these features improve the mixing performance of this novel overlapping crisscross micromixer by 46% compared with the mixing indices of the staggered herringbone mixer for the longitudinal distance . Furthermore, on modulating the ratio of volumetric flow rates between the two inlet streams, an excellent mixing function with a specific prearranged concentration of a mixture and a decreased pressure loss are achieved. The divided ratios Q_t/Q_l, defined as diverted flow rate over forward flow rate, are between 1.86 and 2.88 for fluid A and between 0.52 and 0.67 for fluid B with a variable initial flow rate ratio.     

Multiobjective Optimization of a Micromixer with Convergent–Divergent Sinusoidal Walls

A multiobjective optimization of a micromixer with convergent–divergent sinusoidal walls has been conducted using flow and mixing analyses, surrogate modeling, and multiobjective genetic algorithm. The ratios of amplitude to wavelength of the sinusoidal walls, throat width to depth of the convergent–divergent sections, and diameter of the inner circular wall to wavelength were chosen as the design variables for optimization. The full-factorial method was used to discretize the design space. The mixing index and nondimensional pressure loss were selected as objective functions. Radial basis neural network functions were used to train the objective functions. The optimization was carried out at a Reynolds number of 30. A concave Pareto-optimal front representing the trade-off between the two objective functions was obtained. The analysis of representative designs along the Pareto-optimal front showed significant variation in the ratio of throat width to depth of the convergent–divergent sections, whereas the ratio of amplitude to wavelength of the sinusoidal walls maintained a nearly constant value. The concept of mixing effectiveness was used to select the most efficient designs considering both the mixing performance and pressure drop.