Particle Formation and Growth

The reaction engineering model links the penetration theory to a population balance that includes particle formation and growth with the aim of predicting the average particle size. The model was then applied to the precipitation of CaC03 via CO2 absorption into Ca(OH)2aq in a draft tube bubble column and draws insight into the phenomena underlying the crystal size evolution. 

Particle Formation, Electron microscopy and optical microscopy are the diagnostic tools most often used to study particle formation and growth in precipitation polymerizations (7 8). However, in typical polymerizations of this type, the particle formation is normally completed in a few seconds or tens of seconds after the start of the reaction (9 ), and the physical processes which are involved are difficult to measure in a real time manner. As a result, the actual particle formation mechanism is open to a variety of interpretations and the results could fit more than one theoretical model. Barrett and Thomas (10) have presented an excellent review of the four physical processes involved in the particle formation oligomer growth in the diluent oligomer precipitation to form particle nuclei capture of oligomers by particle nuclei, and coalescence or agglomeration of primary particles. 

However, since virtually no published data are available on the mechanism of continuous particle formation and growth, particlarly under the influence of mechanical agitation, continuous precipitation polymerization remains a vast engineering wilderness waiting for experimental and theoretical explorations. 

On the other hand, very little is known about the mechanism of continuous precipitation polymerization. In particular, the mechanism of continuous particle formation and growth and the effects of starvation feeding on reaction rate and copolymer composition are areas of particular interest. 

The precipitation polymerization literature is reviewed with particular attention to the influence of particle formation and growth, autoaccelerating polymerization rates, and copolymer composition drift on polymer reactor design. 

We have seen in Chapter 8 that reactions in the aqueous phase present in the atmosphere in the form of clouds and fogs play a central role in the formation of sulfuric acid. Thus, an additional mechanism of particle formation and growth involves the oxidation of SOz (and other species as well) in such airborne aqueous media, followed by evaporation of the water to leave a suspended particle. 

The theory for particle formation and growth as it has been developed indicates the need for certain kinds of information on a variety of systems. The important parameters not sufficiently available in the literature are jcr and the interfacial tension,... 

Emulsion polymerization takes place over a number of steps, where various chemical and physical events take place simultaneously during the process of particle formation and growth. Figure 1 depicts the generally accepted scheme for the kinetics of emulsion polymerization. 

Experimental investigations that deal in detail with particle formation in emulsion copolymerization are scarce. Nomura et al. [78] studied the kinetics of particle formation and growth in the emulsion copolymerization of VDC and MMA using NaLS as the emulsifier and KPS as the initiator. The number of polymer particles produced was determined using particle diameters measured by both electron microscopy (TEM) and dynamic light scattering (DLS) for comparison. They found that where Sq and Iq are the initial... 

The free radicals produced from the fraction of initiator dissolved in the water phase are responsible for particle formation and growth in the emulsion polymerization of St initiated by AIBN. The free radicals produced in pairs in the polymer particles play almost no role in the polymerization inside the polymer particles because pairs of radicals produced within a volume as small as a monomer-swollen latex particle or a monomer-swollen micelle are very hkely to recombine. 

The residence time in the first stage should be long enough to overcome the retarding effects of traces of oxygen and/or impurities in the feed-stream upon particle formation and growth. 

Observation (i) above can be understood in terms of droplet nucleation and the lack of competition between nucleation and growth. A mechanistic understanding of observation (ii) above was provided by Samer and Schork [64]. Nomura and Harada [136] quantified the differences in particle nucleation behavior for macroemulsion polymerization between a CSTR and a batch reactor. They started with the rate of particle formation in a CSTR and included an expression for the rate of particle nucleation based on Smith Ewart theory. In macroemulsion, a surfactant balance is used to constrain the micelle concentration, given the surfactant concentration and surface area of existing particles. Therefore, they found a relation between the number of polymer particles and the residence time (reactor volume divided by volumetric flowrate). They compared this relation to a similar equation for particle formation in a batch reactor, and concluded that a CSTR will produce no more than 57% of the number of particles produced in a batch reactor. This is due mainly to the fact that particle formation and growth occur simultaneously in a CSTR, as suggested earlier. 

In spite of different mechanisms of particle formation and growth, Eq. 3 predicts larger particles with increasing amounts of main polymer, and with lower surface coverage by individual dispersant molecules (S/M ), but smaller particles with increasing amounts of dispersant. Also, S/M increases with the solvency of the dispersion medium, and hence the particle radius should be smaller in media that are good solvents for the dispersant. 

The reason for osdllations in conversion and surface tension become dear whea one considers particle formation and growth phenomena. If a single CSTR is started empty or by adding initiator to a full vessel of inactive emulsion, a conversion overshoot occurs. The first free radicals generated are almost entirdy utilized to ibrm new particles. Since these partides do not grow rapidly to the steady-state size distribution, radical... 

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