Drop mixing in suspension polymerisation

In suspension polymerisation, monomer is suspended as liquid droplets in a continuous water phase by means of strong agitation and the presence of a suspending agent. As the suspension polymerisation proceeds, the viscosity of a monomer-polymer droplet increases with conversion. Hence, the physical behaviour of the droplet changes during the process. When new dispersible material is added to the existing suspension drops, the new material and existing drops can remain segregated for significant amounts of time. The aim of this project was to study the behaviour of drop mixing when new material is added to the existing suspension polymerisation. This study concentrated on the effect of the dispersed phase viscosity on drop mixing. The results show that viscosity affects drop size and that may then affect the rate of coalescence between drops. A critical drop size exists which determines the coalescence efficiency effect. Above the critical drop size, mixing rate increases as the drop viscosity decreases. While below the critical drop size, drop size of the dispersion determines the coalescence rate; as the drop size increases, coalescence rate also increases. The investigation of the effect of suspending agent shows that Tween 20 is more efficient in stabilising and protecting the drops, based on a weight basis, than PVA as the coalescence rate is lower with Tween 20.     

Particulate stabilisers for drops in suspension polymerisation: Use of a wax model

Finely divided inorganic solids can be used for drop stabilisation in suspension polymerisation. In order to clarify the role of these solids, experiments have been carried out using a model system in which wax drops were dispersed in water in the presence of magnesium hydroxide. Drop sizes were reduced if the hydroxide was precipitated in situ before dispersion of the wax. As the loading of this precipitate increased, the average drop size passed through a minimum. The presence of a surfactant modified the effect of the hydroxide. Electrolyte, which is formed in the precipitation reaction, also influences the drop size. The separate effects of solid hydroxide, surfactant and electrolyte are not additive. There is some evidence for a nucleating effect which may not be desirable in suspension polymerisation.     

Characteristic intervals in suspension polymerisation reactors: An experimental and modelling study

Suspension polymerisation of methyl methacrylate was carried out as a model to elaborate on the evolution of particle size average and distribution in the course of polymerisation. Four characteristic intervals in the evolution of particle size were identified as: transition, quasi-steady-state, growth, and identification stages. The effects of stabiliser and initiator concentrations, monomer hold up, reaction temperature, and agitation speed on the characteristic intervals, as well as the kinetics of polymerisation, were examined. The transition stage, which has been totally ignored in the literature, was found to have significant effect on the evolution of particle size. The transition stage is shortened by increasing the rate of polymerisation in the drops (either by increasing initiator concentration or using a higher reaction temperature). Increasing the impeller speed and stabiliser concentration will also lead to a shorter transition period. However, the delayed adsorption of the stabiliser on the surface of drops will prolong the transition stage. It is shown that the occurrence of the quasi-steady state depends on the polymerisation conditions. The quasi-steady state occurs only if the balance between drop break up and coalescence can be maintained. This requires a high rate of drop break up within a period of time during polymerisation (i.e., a low rate of polymerisation in the drops by using a low initiator concentration and reaction temperature, a high agitation speed and a high stabiliser concentration). The mechanisms underlying the growth stage are explained in terms of the overall rates of drop break up and coalescence in the course of polymerisation reactions. It is also shown that the onset of growth stage cannot be defined in terms of a critical conversation or viscosity, and it depends on the polymerisation conditions including mixing. The growth stage occurs if drops are not sufficiently stable against both break up and coalescence. The onset of the growth stage is advanced with a decrease in the rate of drop break up (e.g., decreasing agitation speed and stabiliser concentration). The growth stage can be totally eliminated from a polymerisation process if dispersions with a static steady state can be formed. That requires a high concentration of stabiliser, or a low concentration of monomer, to be used. A population balance model, which included the transition stage and the delayed adsorption of the stabiliser, was developed that is capable of predicting the evolution of drop size in the suspension polymerisation.     

Nanoparticle formation by highly diffusion-controlled emulsion polymerisation

Emulsion polymerisations of several monomers with different water solubilities including styrene, butyl acrylate, methyl methacrylate, vinyl acetate, and methyl acrylate were carried out under highly diffusion-controlled conditions. The monomer was placed on top of an aqueous solution of an emulsifier and an initiator, while being gently stirred. Polymerisation occurred by diffusion of monomer from the monomer phase via the interface to the micellar solution. Nanoparticles as small as 25 nm were produced as a result of reduced particle growth and delayed depletion of emulsifier micelles. Nanolatexes with relatively high solids content (20%) but with a low surfactant/monomer ratio (1/50) were obtained. The monomers with the highest solubility, with the exception of methyl acrylate, produced the smallest particles. The rate of diffusion-controlled polymerisation was found to be almost proportional to the saturation monomer concentration in the water phase . The results were compared with those obtained with a high rate of agitation which allowed kinetics of polymerisation to become the rate determinant. While particles obtained by kinetics-controlled emulsion polymerisation of the monomers were large, as well as similar in size, particles obtained from diffusion-controlled runs were small, but different in size. On the other hand, particles made by kinetics-controlled emulsion polymerisation had increasing surface coverage (by emulsifier) with monomer solubility in water. Whereas, nanoparticles made by diffusion-controlled emulsion polymerisation reached almost a constant surfactant coverage independent of the monomer type (except for methyl acrylate), a surface coverage as low as 0.20 was found to be sufficient for stabilisation of nanoparticles.     

振动力场作用下聚合物填充体系挤出混合特性

本文利用一种新型毛细管动态流变仪,对碳酸钙填充聚丙烯体系进行了实验性研究。发现质量百分比为８５／１５的ＰＰ／ＣａＣＯ３填充体系,在一较佳振动条件下动态挤出时,振动场的存在对ＣａＣＯ３在ＰＰ熔体中的分散有促进作用,大大改善了混合料的混合程度。     

Drop stabilisation by inorganic solids in suspension polymerisation: modification by electrolytes using a wax model

Finely divided inorganic solids can be used for drop stabilisation in suspension polymerisation. Usually solids are dispersed in water in the presence of electrolytes formed during their preparation. Experiments, using a wax model as described previously,¹ have been carried out by using two inorganic solids, magnesium phosphate and calcium phosphate, in either the presence or absence of electrolytes. The experiments showed that the effect of electrolytes depends on both the inorganic solid which is used and on the nature of the electrolyte which is present. Magnesium phosphate was a destabiliser in the absence of electrolytes but could act as a stabiliser in the presence of sodium sulphate. Calcium phosphate acted as a stabiliser both in the presence and in the absence of electrolytes.     

Suspension and Mixing Characterization of Intermittent Agitation Modes in DASGIP Bioreactors

Intermittent agitation strategies have been increasingly used for a range of process development applications, i.e., to modulate physical cues, to improve stem cell differentiation yields, and to control hydrodynamic shear stresses in microcarrier suspension; however, there is a distinct lack of characterization. Both continuous and intermittent agitation modes in relation to suspension and mixing dynamics within a DASGIP bioreactor were characterized. Suspension dynamics were found to be affected by microcarrier porosity and the degree of settling was found to be more pronounced at the top of the bioreactor. Mixing time characterization showed a marked improvement in mixing efficiency for intermittent agitation, with an overall dependence on the timing of tracer insertion.     

Suspension polymerisation of methyl methacrylate using sodium polymethacrylate as a suspending agent

Aqueous solutions of sodium polymethacrylate (PMA-Na) are used as suspending agents for the suspension polymerisation of methyl methacrylate. These aqueous polyelectrolyte solutions are viscous and they show a non-Newtonian shear thinning behaviour. The Sauter mean drop diameter decreases with increasing continuous-phase viscosity, but increases with increasing stirring speed. The maximum drop sizes follow the same trends. The Sauter mean diameter also decreases, initially, for increasing hold-up, reaches a minimum, and then increases with increasing hold-up. Taylor's theory of viscous break-up seems to be suitable to describe the breakage of the droplets.     

Unsteady Mixing Characteristics in a Vessel with Forward‐Reverse Rotating Impeller

Usually, mixing is carried out in a vessel with four baffles and a single impeller. In some applications, however, the use of a baffled vessel is not recommended. One of the stirring methods used instead is unsteady agitation with forward‐reverse rotating impellers. The aim of this work was to characterize the agitation characteristics in a baffled and an unbaffled vessel with a turbine impeller. Mixing time and mixing power were evaluated in relation to the presence of baffles and the frequency of forward‐reverse rotation. It was found that the frequency of oscillation does not affect either the mixing time and mixing power values or the drag and added mass coefficients. Power requirements and mixing time were higher compared to the steady mixing conditions in a baffled vessel. The results showed that it is not recommended to use baffles because they have no influence on unsteady mixing.     

Configurations and dynamics of single air/alcohol gas-liquid compound drops in vegetable oil

Gas-liquid compound drops can be used as a low energy liquid-in-liquid disperser, mixer and transporter in place of the energy intensive mechanical agitator. This paper presents an experimental investigation of single air/methanol and air/ethanol gas-liquid compound drops (alcohol-floated bubbles) in sunflower oil at room temperature. Dispersed methanol (ethanol) droplets rise and dissolve slowly in vegetable oil. The air bubbles can float such droplets and increase their rise velocities. The shape of alcohol droplet on the bubble surface was changed to be an alcohol film covering around the bubble and was continuously thrown into the sunflower oil in the form of a toroidal-shaped film, which increases a huge contact area between the alcohol and the oil. A double tube nozzle was used to inject the air bubble (internal tube) and the alcohols (external tube) into the oil. The images showing the configurations of the compound drops during their rise are shown and discussed. The experimental results of the rise velocity and the drag coefficient of the compound drops are presented and compared to those of the single air bubbles and the single alcohol droplets. Rise velocities and drag coefficients predicted by theory of immobile and mobile surface air bubble in sunflower oil are also included for comparison.     

Promotion of Chemical Reactions by a Grid in a Shear Mixing Layer

The effects of a turbulence‐generating grid on fluid mixing and a passive chemical reaction are experimentally investigated in a liquid shear mixing layer under a nonpremixed condition. The grid is installed at three streamwise locations to find the optimal location to promote the chemical reaction. The results show that the grid generates disturbances at small scales that enhance fluid mixing and the chemical reaction. However, the turbulence intensity and mass diffusion in the mixing layers with the grid decrease rapidly and become even smaller than those in the mixing layer without the grid in a downstream region. Therefore, in the present study, the chemical production is maximized when the grid is installed at where the flow is turning to a developed mixing layer.     

普钙生产新型混合反应装置

由一个喷射管和一个折流室构成的用于普钙生产的新型混合反应器,它有结构简单、操作方便、质轻体小、成本低、应用效果好等优点。     

Suspension polymerisation of methyl methacrylate using ammonium polymethacrylate as a suspending agent

Ammonium polymethacrylate (APMA) was used as a suspending agent for the suspension polymerisation of methyl methacrylate. Aqueous APMA solutions were found to exhibit non-Newtonian behaviour. Their high viscosity decreased with increasing pH. Experimental data for drop break up were in agreement with Taylor's theory of viscous shear break up. It was also found that APMA interferes with the polymerisation kinetics.     

The Role of Mixing in Ozone Dissolution Systems

The mixing efficiency of four alternative ozone dissolution systems, including conventional bubble diffusion and pipeline injection/diffusion reactors, were compared by measuring the variability of ozone residual measurements at the outlet of each reactor. The standard deviation and coefficient of variation of a time series of residual measurements were used to provide a quantitative measure of the level of mixing in the reactor. The results indicate that properly designed pipeline dissolution systems provide significantly improved mixing over oxygen-fed or air-fed bubble diffusion systems. Minimum mixing criteria for bubble diffusion systems cited in the literature may underestimate the level of mixing required to achieve stable ozone residuals downstream of the dissolution chamber of conventional bubble diffusion chambers.     

Mixing Time Scale Determination in Microchannels Using Reaction Calorimetry

Mixing time scales are derived from heat flux profiles for an instantaneous and exothermic reaction in a commercially available microreactor. A continuous reaction calorimeter, based on numerous heat flux sensors, is used to record spatially resolved heat flux profiles in steady state. Total volumetric flow rate is varied at constant flow rate ratio and the region of main reaction progress is shifted within the microreactor according to the advancement of the mixing process. Secondary flow patterns, induced by Dean mixing elements within the microchannel at higher flow rates, enhance the mixing. Results display a decrease in mixing time at increased flow rates and energy dissipation rate. Additionally, the passive micromixer is evaluated regarding its efficiency.     

Drop stabilisation by inorganic solids in suspension polymerisation: Effect of inorganic particle size using a wax model

Inorganic solids can be used for drop stabilisation in suspension polymerisation and, in some cases, the particle size can affect the drop stability significantly. Three theoretical models, each based on different theories for this drop stabilisation by inorganic solids, have been formulated. Experiments have been carried out using a wax model. These experiments were similar to those described previously [Brooks, B. W., Bygate. F. & Lane, A. C, Brit. Polym. J., 20 (1988) 19-24; Brooks, B. W., Trans. I. Chem. E., 57 (1979) 210-12]. Particulate calcium phosphate, in different sizes, was used as the stabiliser. The theoretical models have been compared with the experimental results and it was found that two of the models bore some resemblance to the experimental results.     

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.     

Cell models for suspension viscosity

Cell models for the viscosity μ of a suspension of spherical particles in Newtonian fluid of viscosity μ₀ are discussed. Various boundary conditions on the outer boundary of the spherical cell lead to different results: the viscosity predicted for a zero velocity perturbation boundary condition is higher than those predicted with zero tangential stress perturbation. However, in all cases Einstein's prediction μ/μ₀=1+2.5φ+? holds in the limit of small solid volume fraction φ?1.