Suspension crystallization description

Description Suspension crystallization of paraxylene (PX) in the xylene isomer mixture is used to produce paraxylene crystals. The technology uses an optimized arrangement of equipment to obtain the required recovery and product purity. Washing the paraxylene crystal with the final product in a high efficiency pusher-centrifuge system produces the paraxylene product. 

In order to be consistent with normal usage, the particle-size distribution when this parameter is used should be a straight line between approximately 10 percent cumulative weight and 90 percent cumulative weight. By giving the coefficient of variation ancfthe mean particle diameter, a description of the particle-size distribution is obtained which is normally satisfactory for most industrial purposes. If the product is removed from a mixed-suspension crystallizer, this coeffi-... 

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. 

The following description is taken from U.S. Patent 3,116,203. A stirred solution of 75 g of 2-amino.2 -nitrobenzophenone in 700 ml of hot concentrated hydrochloric acid was cooled to 0°C and a solution of 21.5 g of sodium nitrite in 50 ml of water was added in the course of 3 hours. The temperature of the suspension was kept at 2° to 7°C during the addition. The resulting clear solution was poured into a stirred solution of 37 g of cuprous chloride in 350 ml of hydrochloric acid 1 1. The solid which had formed after a few minutes was filtered off, washed with water and recrystallized from ethanol. Crystals of 2-chloro-2 -nitrobenzophenone melting at 76° to 79°C were obtained. 

Statistical mechanics was originally formulated to describe the properties of systems of identical particles such as atoms or small molecules. However, many materials of industrial and commercial importance do not fit neatly into this framework. For example, the particles in a colloidal suspension are never strictly identical to one another, but have a range of radii (and possibly surface charges, shapes, etc.). This dependence of the particle properties on one or more continuous parameters is known as polydispersity. One can regard a polydisperse fluid as a mixture of an infinite number of distinct particle species. If we label each species according to the value of its polydisperse attribute, a, the state of a polydisperse system entails specification of a density distribution p(a), rather than a finite number of density variables. It is usual to identify two distinct types of polydispersity variable and fixed. Variable polydispersity pertains to systems such as ionic micelles or oil-water emulsions, where the degree of polydispersity (as measured by the form of p(a)) can change under the influence of external factors. A more common situation is fixed polydispersity, appropriate for the description of systems such as colloidal dispersions, liquid crystals, and polymers. Here the form of p(cr) is determined by the synthesis of the fluid. 

Myerson (2002), Mullin (2001), and Mersmann (2001) provide excellent descriptions of methods for crystal growth rate measurements. These methods involve measurements of either single crystals or suspensions. Much information can be gained from the traditional technique of measuring ( grab samples or in-line) solute concentration versus time in batch crystallization on a seed bed. Initial and later slopes on such a plot can provide multiple data points of growth rate versus supersaturation. 

The problem becomes complicated and essentially escapes a rigorous theoretical description when the crystals in a stirred suspension are susceptible to secondary nucleation or grow via an agglomeration mechanism. In order to suppress the formation of contact secondary nuclei, the impeller may be covered with a plastic coating, and its speed reduced to the lowest acceptable level. Sometimes a fines destruction loop is the only possibility in order to control an excessive secondary nucleation (Karpinski 1981). 

Pure hard rod suspensions exhibit interesting phase transitions themselves. Upon concentrating a dilute rod suspension for L/D > 3.5 the phase states schematically depicted in Fig. 6.1 will be encountered the isotropic, nematic and smectic liquid states and a crystalline solid state [2-4]. For a description of the various liquid crystalline phases we refer the reader to standard textbooks on liquid crystals such as [5]. We mainly focus on the isotropic and nematic states and it will become clear that adding depletants will strongly affect the isotropic-nematic phase transition. 

The co-precipitation method involves the simultaneous precipitation of a selected pair of metal ions from their mixed aqueous solution by dilute NaOH and/or NaHCOs, Na2C03, or NH4OH solution. The pH of the reaction medium is maintained in the range 8-10, depending on the nature of metal ions. Hydrothermal treatment of the final suspension is usually carried out to obtain well-crystallized samples. Detailed descriptions of the process are available in numerous reports available in the literature [25-29]. 

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