Nucleation, primary

Primary intermediates Primary nucleation Primary ozomdes Primary plasticizer Primary recycling Primary structure Primary tastes Primatene Mist Primaxin Prime+... 

This assumes that only individual radicals nucleate ("self-nucleation") primary particles. If aggregation of radicals is involved, then the first term on the right of Equation 3 should be modified to bR., where b is the reciprocal of the aggregation number in a primary particle. The absolute number of particles is obtained by integration ... 

ORIGINS OF CRYSTALS IN CRYSTALLIZERS. If all sources of particles are subsumed under the term nucleation, a number of kinds of nucleation may occur. Many of these are important only as methods to be avoided. They may be classified into three groups spurious nucleation, primary nucleation, and secondary nucleation. 

Heterogeneous Nucleation Primary nucleation in the presence of a foreign surface that enhances nucleation by reducing the critical size needed for crystal growth. 

Homogeneous Nucleation Primary nucleation without the presence of preformed nuclei or crystalline surfaces. 

Several mechanisms of nucleation can be distinguished according to conditions in a supersaturated solution [178] nucleation - primary - homogeneous... 

Nucleation, primary The mechanism by which crystallization is initiated often by an added nucleation agent. 

When nucleation takes place without any crystal surfaces, we have primary nucleation. Primary nucleation is said to occur by homogeneous nucleation when no dissolved impurities are present. When primary nucleation occurs due to the presence of dissolved impurities, we encounter heterogeneous nucleation. When nuclei are formed due to the presence of existing macroscopic crystals, interaction with the crystallizer wall, rotary impellers, fluid shear, etc., we have secondary nucleation. Mechanisms of secondary nucleation are not sufficiently clear. An introduction to theories on secondary nucleation is provided by Myerson (1993). Here we will focus on homogeneous nucleation. Note that homogeneous nucleation is rarely achieved or desired in practical crystallization (McCabe and Smith, 1976 Myerson, 1993). 

Nucleation is the initiation of a phase change in a small region, such as the formation of a solid crystal from a liquid solution. It is a consequence of rapid local fluctuations on a molecular scale in a homogeneous phase that is in a state of metastable equilibrium. Total nucleation is the sum effect of two categories of nucleation - primary and secondary. 

Three basic types of nucleation are considered at various dimensions primary nucleation, secondary nucleation, and tertiary nucleation. Primary nucleation is a three-dimensional nucleus newly formed by thermal fluctuations, with six extra nucleus surfaces if the nucleus is considered to be cubic. Secondary nucleation is two-dimensional nucleation on the advancing surface of nucleus, with four extra surfaces produced. Secondary nucleation is much easier than primary nucleation as its free energy barrier is lower, making crystallization kinetics like a self-acceleration process. Tertiary nucleation is one-dimensional nucleation at the step edge of the spreading layer on the advancing smooth surface of nucleus, with only two extra surfaces produced. Tertiary nucleation is so fast that it can rarely be observed. The schematic pictures of different types of nucleation are shown in Figure 13.8. 

Since polymer swelling is poor and the aqueous solubiUty of acrylonitrile is relatively high, the tendency for radical capture is limited. Consequentiy, the rate of particle nucleation is high throughout the course of the polymerization, and particle growth occurs predominantiy by a process of agglomeration of primary particles. Unlike emulsion particles of a readily swollen polymer, such as polystyrene, the acrylonitrile aqueous dispersion polymer particles are massive agglomerates of primary particles which are approximately 100 nm in diameter. 

Polyethylene. This is essentially a closed-ceU insulation manufactured at 448 2 K by an extmsion process. A blowing agent and nucleating agent are employed to control the ceU size, and primary use is in insulating pipelines for hot and chilled water lines, air conditioning, and processing systems. 

Aerosol Dynamics. Inclusion of a description of aerosol dynamics within air quaUty models is of primary importance because of the health effects associated with fine particles in the atmosphere, visibiUty deterioration, and the acid deposition problem. Aerosol dynamics differ markedly from gaseous pollutant dynamics in that particles come in a continuous distribution of sizes and can coagulate, evaporate, grow in size by condensation, be formed by nucleation, or be deposited by sedimentation. Furthermore, the species mass concentration alone does not fliUy characterize the aerosol. The particle size distribution, which changes as a function of time, and size-dependent composition determine the fate of particulate air pollutants and their... 

Both homogeneous and heterogeneous mechanisms requite relatively high supersaturation, and they exhibit a high order dependence on supersaturation. These factors often lead to production of excessive fines ia systems where primary aucleatioa mechanisms are important. The classical theoretical treatment of primary nucleation results ia the expressioa (5) ... 

The metastable limit can provide an empirical approach to modeling primary nucleation. This limit, which was first observed in 1951 (6), must be determined through experimentation, and nucleation rate is correlated with the following equation... 

Several features of secondary nucleation make it more important than primary nucleation in industrial crystallizers. First, continuous crystallizers and seeded batch crystallizers have crystals in the magma that can participate in secondary nucleation mechanisms. Second, the requirements for the mechanisms of secondary nucleation to be operative are fulfilled easily in most industrial crystallizers. Finally, low supersaturation can support secondary nucleation but not primary nucleation, and most crystallizers are operated in a low supersaturation regime that improves yield and enhances product purity and crystal morphology. 

Onset of primary nucleation results in large numbers of small crystals... 

Supersaturation remains high and primary nucleation continues to occur... 

Supersaturation drops to modest value, eliminating primary nucleation... 

Crystals formed by primary nucleation grow to sufficient size to participate in seconday nucleation... 

---