Crystallizers agitated vessels

Alternative microcrystallizer configurations are being developed that seek to avoid the mixing problems asociated with conventional agitated vessels and offer the potential of consistent precipitation of high quality crystal products. 

Jones, A.G. and Mullin, J.W., 1973. Crystallization kinetics of potassium sulphate in a draft-tube agitated vessel. Transactions of the Institution of Chemical Engineers, 51, 362-368. 

Tanimoto, A.K., Kobayashi, K. and Fujita, S., 1964. Overall crystallization rate of copper sulfate pentahydrate in an agitated vessel. International Chemical Engineering, 4(1), 153. 

As with nucleation, classical theories of crystal growth 3 20 2135 40-421 have not led to working relationships, and rates of crystallisation are usually expressed in terms of the supersaturation by empirical relationships. In essence, overall mass deposition rates, which can be measured in laboratory fluidised beds or agitated vessels, are needed for crystalliser design, and growth rates of individual crystal faces under different conditions are required for the specification of operating conditions. 

In this study, D-SCMC seed crystals were put in a racemic SCMC supersaturated solution in a batchwise agitated vessel and growth rates in longitudinal and lateral directions and the optical purity of D-SCMC crystals were measured. The growth rates and optical purity were discussed considering surface states of grown crystal observed by a microscope. The kinetics of crystal growth were measured and a model of inclusion of impurity was proposed. 

The crystallizer was an agitated vessel with an Inside diameter of 9.0 cm and a volume of about 1 1. It was equlppet with four vertical baffles, a water jacket to keep the solution temperature constant, and a nozzle through which nitrogen gas was Introduced In several experiments to suspend a speed crystal more effectively In the solution. Agitation was accomplished with a 5.0 cm stainless steel marine propeller having three blades driven by a variable speed motor. 

Aluminum sulfate is manufactured by the reaction of aluminum hydroxide or other aluminum raw materials, such as bauxite or kaolin, with sulfuric acid at ca. 170°C in a pressure-resistant agitator vessel. The melt obtained after concentration contains ca. 57% aluminum sulfate (ca. 13 moles of crystallization water). Double salts of aluminum sulfate with potassium, ammonium or sodium sulfate (alums e.g. potassium alum KA1(S04)2 12H2O) have been largely supplanted by aluminum sulfate. 

Lindrud et al. (2001), Johnson and Prud homme (2003)]. With proper design, mixing to the molecular level can be accomplished in less time than the nucleation time, thereby achieving a primarily nucleation-based process for the production of uniform, fine particles. After the nuclei leave the mixing zone, additional crystallization continues in a standard agitated vessel on a well-defined initial number of nuclei with a well-defined size and shape. 

The impinging jet device has also been reported to be useful for other problems in pharmaceutical crystallization. As noted earlier some solutes precipitate as a liquid phase ( oiling out ), with subsequent transformation to a solid. In a conventional agitated vessel, these oils have a tendency to accumulate on the walls... 

Figure 17.9 Comparison of single crystal growth cell, fluidized bed agitated vessel growth rates for ammonium alum at J2"C ...

In operations such as crystallization or solid-catalysed liquid reactions, it is necessary to suspend solid particles in a relatively low viscosity liquid. This can be achieved in mechanically agitated vessels where the mixer is used to prevent sedimentation of the solids and to provide conditions suitable for good liquid-solid mass transfer and/or chemical reaction. If agitation is stopped the solids will settle out or float to the surface, depending upon the relative densities of the solid and liquid phases. The suspension of solids in mixing vessels and the design of mixing vessels for solid-liquid reactions are treated in Chapters 16 and 17 respectively. 

It is often much more convenient, and more useful for crystallizer design purposes, to measure crystal growth rates in terms of mass deposited per unit time per unit area of crystal surface rather than as individual face growth rates. This may be done in agitated vessels or fluidized beds, e.g. by measuring the mass deposition on a known mass of sized seed crystals under carefully controlled conditions. 

It is possible to determine overall crystal growth rates by adding a known mass of sized seeds to a supersaturated solution in an agitated vessel, following a similar procedure to that outlined above for the fluidized bed method. To correlate the data, however, it is necessary to estimate the particle-fluid slip velocity as a function of impeller speed in the agitated vessel using relationships of the type described in section 9.4.1. 

An example of the comparison of growth rate data obtained in both fluidized bed and agitated vessel crystallizers, using ammonium alum, has been reported by Nienow, Bujac and Mullin (1972). 

A rapid method for overall crystal growth rate estimation may be made by suspending a batch of seed crystals in a supersaturated solution kept at constant temperature, and following the decay of supersaturation over a period of time. A mass of seed crystals of known size and surface area is added to the solution in a closed system, e.g. in a fluidized bed or an agitated vessel. 

For crystals larger than about 60 pm in agitated vessels, it is difficult to estimate the relative crystal-solution velocity (section 9.4.1), and hence Rcp, but an order of magnitude estimate of the dissolution mass transfer coefficient may be made from the Levins and Glastonbury (1972) equation ... 

Crystallization and dissolution data obtained from agitated vessel studies may be analysed by the methods discussed above, but a survey of the literature related to the subject of solid-liquid mass transfer in agitated vessels shows that there is an extremely wide divergence of results, correlations and theories. The difficulty is the extremely large number of variables that can affect transfer rates, the physical properties and geometry of the system and the complex liquid-solid-agitator interactions. 

Simple agitated vessels, such as those commonly used in solution crystallization (section 8.4), rarely find application in melt crystallization processes. One of the... 

Simple batch cooling crystallization can suffer from a serious defect if the hot feed solution is charged into an agitated vessel and uncontrolled cooling is commenced immediately. The large temperature difference between the solution and the surface of the cooling element, e.g., coil or jacket, often causes encrustation (scale) to form which not only reduces the effective... 

The exact scale-up of crystallizers is not possible because it would be necessary to preserve similar flow characteristics of both liquid and solid phases together with identical temperatures and supersaturations in all equivalent regions. The scale-up of simple agitated vessels containing a liquid phase alone has long been recognized as a difficult problem. The two dimensionless numbers most frequently encountered in the analysis of stirrers and agitators are the Reynolds number, Re, and the Froude number, Fr ... 

The terminal velocities of crystals suspended in agitated vessels may be predicted from the equations... 

Figure 9.19. Suspension of crystals in a baffled, turbine-agitated vessel. Value of the constant S for equation 9.102. The broken line indicates data of Zweitering (1958). (After Nienow, 1968)...

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