Crystallizers operation

Liquid-liquid and liquid-solid equilibria also find industrial applications in liquid-liquid extraction and fractional crystallization operations. 

Several features of the hypothetical system in Eigure 2 can be used to illustrate proper selection of crystallizer operating conditions and limitations placed on the operation by system properties. Suppose a saturated solution at temperature is fed to a crystallizer operating at temperature T. Because the feed is saturated, the weight fraction of in the feed is given as shown in Eigure 2. The maximum crystal production rate from such a process depends on the value of and is given by... 

The specific enthalpies ia equation 9 can be determined as described earUer, provided the temperatures of the product streams are known. Evaporative cooling crystallizers operate at reduced pressure and may be considered adiabatic (Q = 0). As with of many problems involving equiUbrium relationships and mass and energy balances, trial-and-error computations are often iavolved ia solving equations 7 through 9. 

The morphology (including crystal shape or habit), size distribution, and purity of crystalline materials can determine the success in fulfilling the function of a crystallization operation. 

Many industrial crystallizers operate in a weU-mixed or nearly weU-mixed manner, and the equations derived above can be used to describe their performance. Furthermore, the simplicity of the equations describing an MSMPR crystallizer make experimental equipment configured to meet the assumptions lea ding to equation 44 useful in determining nucleation and growth kinetics in systems of interest. 

A pair of kinetic parameters, one for nucleation rate and another for growth rate, describe the crystal size distribution for a given set of crystallizer operating conditions. Variation ia one of the kinetic parameters without changing the other is not possible. Accordingly, the relationship between these parameters determines the abiUty to alter the characteristic properties (such as dominant size) of the distribution obtained from an MSMPR crystallizer (7). 

For systems following invariant growth the crystal population density in each size range decays exponentially with the inverse of the product of growth rate and residence time. For a continuous distribution, the population densities of the classified fines and the product crystals must be the same at size Accordingly, the population density for a crystallizer operating with classified-fines removal is given by... 

Although many commercial crystallizers operate with some form of selective crystal removal, such devices can be difficult to operate because of fouling of heat exchanger surfaces or blinding of screens. In addition, several investigations identify interactions between classified fines and course product removal as causes of cycling of a crystal size distribution (7). Often such behavior can be rninirnized or even eliminated by increasing the fines removal rate (63,64). 

FIG. 18-63 Calculated product -size distribution for a crystallizer operation at different fine-crystal-separation sizes. 

Attention can also be paid to the other end of the size distribution - that of the large crystals. By allowing the largest crystals to remain in the vessel longer than average, the mean product crystal size is similarly enhanced, but may lead to unstable crystallizer operation (Randolph and Larson, 1988). 

Crystallization process control is desirable from a number of standpoints. The primary objective is often to meet customer requirements by achieving consistent product quality to a desired specification of crystal size, size distribution and purity. Secondly, process requirements often dictate maintenance of stable crystallizer operation, the avoidance of fines and encrustation, and the minimization of subsequent downstream processing. 

Since the frequency can be measured within an accuracy of 1 Hz, a 5-MHz EQCM can detect lOng/cm. This corresponds to 10% of a monolayer of Pb atoms. Much higher sensitivity can be achieved with thinner quartz crystals operated at higher frequencies, as the sensitivity increases with/g. 

The concept of minimum reflux as related to column-crystallizer operation is presented by Brodie (op. cit.) and is applicable to all types... 

This volume contains a selection of chapters presented at the symposium organized into four areas basic studies, crystallizer operation and control, crystallization of organic molecules and biomolecules, and crystallization and precipitation of inorganic compounds. In addition, an overview chapter is included, which reviews important areas in the field. 

The design and operation of industrial crystallizers is where developments in the laboratory are confirmed and their practical significance determined. In recent years, crystallization processes involving specialty chemicals and pharmaceuticals have increased. This has led increased interest in batch crystallization operation, optimization and desigrt At the same time, the advent of powerful computers and their routine avaUabilily has stimulated interest in the area of on-line control of crystallization process (both batch and continuous). Progress in batch crystallization is surrunarized in a number of recent papers and reviews 173-801. In this section I will discuss two areas which I think will have an impact in the next decade. 

Crystallization processes can be used for separation, purification, or concentration of a solute, or because a particular product needs to be used in solid form, or as a component of an analytical procedure. Common requirements for accomplishing these functions are that the crystals must be produced with a particular size distribution and having a specified shape and purity. Almost all crystallizer operating problems are defined in terms of the product not meeting one of these criteria. 

Table III. Stationary values of G and for a DTB crystallizer operated with a fines r oval system and a size dependent growth regime. The values are given before and 35000 second after a step in the heat input of the crystallizer from 120 to 170 kW. The simulation has been performed with kOO gridpoints...

Xylene [106-42-3] can be purified by crystallization or adsorption. When a typical reformate-derived Cg aromatic mixture is cooled, y>-xylene crystallizes first. Most plants employing crystallization operate at —60 to —75° C, depending on feed composition (37). The process is limited by a eutectic temperature below which o- or / -xylene also crystallize. The solubility ofy>-xylene in the remaining Cg aromatic mixture over the range of —60 to —75°C is 9.6 to 6.2%. 

The triple-point crystallization of carbon dioxide is illustrated in Figure 7, which shows a schematic carbon dioxide phase diagram expanded about the triple-point and a closed-cycle triple-point crystallizer operating with pure carbon dioxide. The operation of this closed-cycle unit is identical to that of a unit in the stripping section of a continous crystallizer cascade, except that in the cascade vapor would pass to the unit above, and liquid would pass to the unit below. 

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