Crystallizers agitated suspension

Synowiec, P., Jones, A.G. and Shamlou, P.A., 1993. Crystal break-up in dilute tur-bulently agitated suspensions. Chemical Engineering Science, 48, 3485-3495. 

Wojcik, J. and Jones, A.G., 1998b. Particle disruption of precipitated CaC03 crystal agglomerates in turbulently agitated suspensions. Chemical Engineering Science, 53, 1097-1101. 

Several parameters have been seen to influence the crystallization of ice crystals in subcooled aqueous solutions. The primary factor is the extent of subcooling of the solution. Other factors include the agitation rate, the types and levels of solutes in solution. Huige (6) has summarized past work on conditions under which dusk-shaped and spherical crystals can be found in suspension crystallizers. The effects of heat and mass transfer phenomena on the morphology of an ice crystal growing in a suspension have not been fully understood. 

Agitated tank crystallizer for continuous and discontinuous operation, crystallizer with forced suspension circulation, horizontal crystallizer and multistage, crystallizer with agitator and controlled flow (DTB), vortex crystallizer according to Standard-MESSO. 

For agitated suspensions, however, recent work suggests that micro-attrition of parent crystals can occur in agitated suspensions (Synowiec etal., 1993). Such breakage via micro-attrition will result in many fine crystals generated and a relatively unchanged parent particle (cf. secondary nucleation, see below) with a correspondingly more complex distribution function (Hill and Ng, 1995, 1996). These alternative attrition models are considered in more detail later. 

The experimental results are consistent with crystal attrition occurring via both crystal-impeller impacts and turbulent disruption with no significant effect of crystal-crystal collisions on the number of fine fragments produced in the dilute agitated suspensions. 

The DTB, crystallizer has a relatively slow-speed propeller agitator located within a draft-tube which draws a fine-crystal suspension up to a boiling zone of wide cross-sectional area, as shown in Figure 3.3(i). The fine-crystal magma then passes through an annular zone in which an additional baffle is located. Liquor flow continues upwards at low velocity while crystals settle out and fall to the base of the vessel. Liquor from the external pumped loop provides an up-... 

A number of authors have developed mechanistic descriptions of the processes causing secondary nucleation in agitated crystallizers (Ottens etal., 1972 Ottens and de Jong, 1973 Bennett etal., 1973 Evans etal., 1974 Garside and Jancic, 1979 Synowiec etal., 1993). The energy and frequency of crystal collisions are determined by the fluid mechanics of the crystallizer and crystal suspension. The numbers of nuclei formed by a given contact and those that proceed to survive can be represented by different functions. 

As mentioned above batch crystallizers are usually simple vessels provided with some means of mechanical agitation or particulate fluidization. These have the effect of reducing temperature and concentration gradients, and maintain crystals in suspension. Baffles may be added to improve mixing and heat exchange or vacuum systems may be added, as appropriate. Various design combinations are available and some are illustrated in Figure 7.1. 

Mazzarotta, B., 1992. Abrasion and Breakage Phenomena in Agitated Crystal Suspensions. Chemical Engineering Science, 47, 3105-3111. 

To a suspension of 500 mg of 6a-fluoro-triamcinolone in 75 ml of acetone is added 0.05 milliliters of 72% perchloric acid and the mixture agitated at room temperature for 3 hours. During this period the crystals gradually dissolve and the clear solution is neutralized with dilute bicarbonate and the acetone removed in vacuo. The resulting crystalline suspension is filtered and the crystals washed with water. The dried material is recrystallized from 95% alcohol to give the pure acetonide. 

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