Yield of a Crystallization Process

EXAMPLE 12.11-1. Yield of a Crystallization Process A salt solution weighing 10000 kg with 30 wt % Na2C03 is cooled to 293 K (20°C). The salt crystallizes as the decahydrate. What will be the yield of Na2C03 IOH2O crystals if the solubility is 21.5 kg anhydrous Na2C03/100 kg of total water Do this for the following cases. 

Example 1 Yield from a Crystallization Process A 10,000-lh batch of a 32,5 percent MgS04 solution at 120°F is cooled without appreciable evaporation to 70°F, What weight of MgS04-7H20 crystals will be formed (if it is assumed that the mother liquor leaving is saturated) ... 

The problem of whether an amorphous material may or may not be considered a form of a solid substance has been discussed [18a]. Many compounds yield stable noncrystalline phases either as the exclusive product of a crystallization process, or in a mixture with crystals or as a consequence of the treatment of otherwise crystalline phases [18]. The fundamental drawback when dealing with amorphous phases is the dearth of techniques for the thorough characterization of an amorphous phase or for the discrimination between different amorphous phases. In general, diffraction techniques are of little help when dealing with amorphous materials and one has to rely mainly on spectroscopic means (see below). Clearly, if one takes the fulfilment of Bragg s law as the pre-condition for the existence of a crystalline phase, and hence for the existence of a molecular crystal polymorph, amorphous materials are not to be considered. 

The objectives in the operation of a crystallization process are to meet the product specification, i.e., a narrow CSD, maximum crystal purity, high yield, and an acceptable crystal morphology. Moreover, the manufacturer s requirements for economic and trouble-free operation should be met. The CSD and crystal morphology are important parameters for downstream operations of crystallization such as filtration and drying. 

In some cases, it is necessary to add antisolvent in order to increase the yield of the crystallization processes. There is a risk of spontaneous nucleation around the location of the addition of the antisolvent in the reactor because there supersaturation reaches its maximum values. 

Table 10.7 shows that the temperature must be decreased to low values to obtain a reasonable yield from the crystallization process. It should also be noted that cooling to 40°C should be possible against cooling water, and perhaps even down to 30°C. Any cooler than this, and refrigeration of some kind is required. This increases the cost of the cooling significantly. 

The solid product is seldom pure when obtained from a chemical reaction, being contaminated with various impurities, reagents and byproducts. For purification, the process of crystallization, sometimes called recrystallization, is generally employed When dealing with large quantity formulas, the utmost care should be taken to obtain the maximum yield of a pure crystallized compound. 

Two methods are employed industrially to produce crystalline fructose, aqueous crystallization and alcoholic crystallization. Yields of fructose crystallized from water syrups are only of the order of 50%, due to the very high water solubility of the sugar, while the high viscosity of the concentrated solution results in long crystallization times, typically 50 hours or more (2). The second process requires the addition of lower alcohols (eg. ethanol) to a concentrated fructose syrup, generally 90% total solids or more, at temperatures of 50 C to 80""c and then cooling to cause crystallization. Fructose yields are from 70 to 80% and the total time involved is 8 to 12 hours (3). However, large quantities of... 

It Is occasionally seen In a crystallization process that a certain gas Is blown to react with some components of the solution and to yield a crystalline product. The admittance of the gas Into the crystallizer was also expected to promote an agitation effect. 

In the Sulzer-MWB process the naphthalene fractions produced by the crystallization process are stored in tanks and fed alternately into the crystallizer. The crystallizer contains around 1100 cooling tubes of 25-mm diameter, through which the naphthalene fraction passes downward in turbulent flow and pardy crystallizes out on the tube walls. The residual melt is recycled and pumped into a storage tank at the end of the crystallization process. The crystals that have been deposited on the tube walls are then partly melted for further purification. Following the removal of the drained liquid, the purified naphthalene is melted. Four to six crystallization stages are required to obtain refined naphthalene with a crystallization point of 80°C, depending on the quality of the feedstock The yield is typically between 88 and 94%, depending on the concentration of the feedstock fraction. 

By necessity, the treatment of solid state kinetics has to be selective in view of the myriad processes which can occur in the solid state. This multitude is mainly due to three facts 1) correlation lengths in crystals are often much larger than in fluids and may comprise the whole crystal, 2) a structure element is characterized by three parameters instead of only by two in a liquid (chemical species, electrical charge, type of crystallographic site), and 3) a crystal can be elastically stressed. The stress state is normally inhomogeneous. If the yield strength is exceeded, then plastic deformation and the formation of dislocations will change the structural state of a crystal. What we aim at in this book is a strict treatment of concepts and basic situations in a quantitative way, so far as it is possible. In contrast, the often extremely complex kinetic situations in solid state chemistry and materials science will be analyzed in a rather qualitative manner, but with clearcut thermodynamic and kinetic concepts. 

Co-product AS is recovered in a crystallization process that is controlled to yield large, uniformly sized crystals. Usually by-product AS contains small amounts of organic contaminants. As a result, the crystallization process generates smaller and finer particles that are harder to handle243. 

If large crystals are desired, some modifications will promote the growth of a smaller number of large crystals. An ampule of at least 2.5 cm. i.d. is desirable. Before the reaction zone is heated above 800°C., the growth zone is heated to 950°C. for 3 hours to minimize seed sites. The growth zone is then allowed to cool to 800°C., the reaction zone is heated to 900°C., and transport is allowed to proceed at 900 —> 800°C. for 70 to 80 hours. The yield of transported crystals depends upon tube dimensions, purity of reactants, and duration of the transport process. 

Less stable structural isomers of a fluxional process are isolated thanks to the compensatory effect of external interactions in the solid Ordered and disordered crystals of the same molecules may show different crystallographic symmetry, e.g. crystallize in different space groups The same (chemically relevant) molecular ion is crystallized in different crystals with a different type/number of counterions The same network ligand yields different... 

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