Nucleation fluid-shear

Contact nucleation is the most common mechanism of secondary nucleation. Crystal-crystal-, crystal-impeller, and crystal-wall collisions are involved. Secondary nuclei arise from microabrasion (crystal surface damage) or ordered cluster removal by fluid shear forces, as noted above. Figure 4-12 shows that for a given substance, impeller speed and material of construction can both play a role. 

SECONDARY NUCLEATION. The formation of nuclei attributable to the influence of the existing macroscopic crystals in the magma is called secondary nucleationP Two kinds are known, one attributable to fluid shear and the other to collisions between existing crystals with each other or with the walls of the crystallizer and rotary impellers or agitator blades. 

Fluid-shear nucleation. This type is known to take place under certain conditions and is suspected in others. When supersatured solution moves past the surface of a growing crystal at a substantial velocity, the shear stresses in the boundary layer may sweep away embryos or nuclei that would otherwise be incorporated into the growing crystal and so appear as new crystals. This has been reported in work on sucrose crystallization. It also has been demonstrated in the nucleation of MgS04 7H20, if the solution is subjected to shear at the crystal face at one supersaturation and then quickly cooled to a higher supersaturation and allowed to stand while nuclei grow to macroscopic size. 

Sung, C.Y., Estrin, J. and Youngquist, G.R. (1973) Secondary nucleation of MgSO by fluid shear. AIChEJ, 19, 957-962. 

The heterogeneous nucleation can be treated as secondary nucleation with the mechanism of interphase layer. At the solid surface there are more or less oriented clusters that may be removed by fluid shear back into the bulk of solution 120,43.97,188.2161. These clusters. If they are of the critical size, can survive and form new nuclei. 

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). 

Secondary nucleation is the formation of nuclei attributable to the influence of the existing microscopic crystals in the magma. The first type of known secondary crystallization is attributable to fluid shear, the other due to collisions between already existing crystals with either a solid surface of the crystallizer or with other crystals themselves. Fluid shear nucleation occurs when liquid travels across a Crystal at a high speed, sweeping away nuclei that would otherwise be incorporated into a Crystal, causing the swept-away nuclei to become new crystals. Contact nucleation has been found to be the most effective and common method for nucleation. The benefits include the following... 

Figure 10.9 The axisymmetric free turbulent jet. The initial region of the axisymmetric jet, extending to 5-10 nozzle diameters, consists of an undisturbed cone of nozzle fluid surrounded by the shear layer. For nucleation-controlled growth, particle formation is conlined to the shear layer.

In many, if not most, cases of practical interest, the fluid in which the particles are suspended is in turbulent motion. In Chapter 7. the effects of turbulence on the collision frequency function for coagulation were di.scussed. In the last chapter, nucleation in turbulent How was analy ,ed through certain scaling relations based on the form of the concentration and velocity fluctuations in the shear layer of a turbulent jet. In this section the GDE for turbulent flow is derived by making the Reynolds assumption that the fluid velocity and size distribution function cun be written as the sum of mean and fluctuating components ... 

If nucleation must be avoided in a product, agitation during processing must be kept to an absolute minimum. Sometimes even the shear forces that exist as a supersaturated fluid is passed through a pipe and fittings may be sufficient to initiate nucleation. 

Depending on the system and on the applied shear rate, a has been found to vary between 1 and 4 [33,138,140,144,191,199], Such kinetics suggests metastabiUty reminiscent of equilibrium first-order phase transitions and has been originally interpreted by Berret and coworkers [33,138] in terms of nucleation and one-dimensional growth of a fluid phase containing highly ordered entities. Other mechanisms involving the slow drift of a sharp interface to a fixed position in the gap of the cell have also been advanced to explain this slow kinetics [190,234,235]. 

---