Cooling and crystallization

The crude terephthalic acid is cooled and crystallized. For polyester grade, it must be purified to 99.9 per cent by subsequent recrystallization. 

To 1,000 g of the crude reaction product a re added 264g of ethylene diamine tetracetic acid. The mixture is preferably heated to incipient boiling to increase the rate of reaction,and then the mixture is allowed to cool and crystallize. The crystals formed are filtered off, washed with the smallest possible amount of ice water, and dried to a constant weight, which is 452 g. 

A mixture of 10.5 g of 1,4steam bath for 4 hours during which 2.0 g of methylmercaptan was collected in a dry ice bath connected to the reaction flask through a water cooled reflux condenser. The reaction mixture was then evaporated at 15 mm pressure to a solid residue which was then dissoived in 80 ml of 50/50 methanol-ethanol. The solution was filtered and evaporated to approximately 50 ml volume and allowed to cool and crystallize, giving a crop melting at 213.5°C to 215°C of 1,4[Pg.743]

A solution comprising 3,24 kg (23.3 mols) of pyridine-2,6-dimethanol in 35 liters of pure pyridine was added to the residue and the mass was heated to 80°C for 2 /a hours. The reaction mass became brown in color. The chlorhydrate of pyridine so formed was cooled and crystallized. The resulting reaction mass was then poured into water. The precipitate obtained was filtered, repeatedly rinsed with water, and dissolved in 400 liters of methanol. The resulting solution was filtered with activated charcoal. From this filtration 50 liters of methanol were distilled at normal pressure and then crystallized. 8.35 kg (15.8 mols) of pyridine-2,6-dimethanol trimethoxybenzoate were obtained, which represented a yield of 68%. 

It is essential to carry out filtration under only slightly reduced pressure in order to minimize evaporation, cooling, and crystallization in the filter plate and funnel. 

The crude TA is cooled and crystallized. The acetic acid and xylene are evaporated and the TA is washed with hot water to remove traces of the catalyst and acetic acid. Some p-formylbenzoic acid is present as an impurity from incomplete oxidation. This is most easily removed by hydrogenation to /7-methylbenzoic acid and recrystallization of the TA to give 99.9% PTA, which is a polyester-grade product, mp > 300°C. 

If the boundary motion is controlled by an independent process, then the boundary motion velocity is independent of diffusion. This can happen if the magma is gradually cooling and crystal growth rate is controlled both by temperature change and mass diffusion. This problem does not have a name. In this case, u depends on time or may be constant. If the dependence of u on time is known, the problem can also be solved. The Stefan problem and the constant-w problem are covered below. 

These examples involve partitioning of elements as liquids cooled and crystallized. Partial melting of a solid rock also results in partitioning of incompatible elements into the liquid phase, which contains no rigid crystalline sites. Separation of the melt then fractionates incompatible elements from the compatible elements left behind in the solid residue. 

This value will be compared with a calculation using data of heat capacities and heat of solution. From Perry s Chemical Engineers Handbook (1984), the heat solution of the heptahydrate is -39.2 Btu/lb and its heat capacity is 0.36 Btu/(lb)(°F). The enthalpy change of the cooling and crystallization process is... 

Preparation of Trinitrostilbene. To 10 grams of TNT dissolved in 25 cc. of benzene in a 100-cc. round-bottom flask equipped with a reflux condenser, 6 cc. of benzaldehyde and 0.5 cc. of piperidine are added, and the mixture is refluxed on the water bath for half an hour. The material, while still hot, is poured into a beaker and allowed to cool and crystallize. The crystals, collected on a filter, are rinsed twice with alcohol and recrystallized from a mixture of 2 volumes of alcohol and 1 of benzene. Brilliant yellow glistening needles, m.p. 158°. 

Dry fractionation involves melting the milk fat, controlled cooling and crystallization of molten milk fat while cooling to or at a desired temperature and separation of the crystals from the liquid phase. The process is attractive because of its simplicity, relatively low costs and ability to select between fractions based on the melting or functional properties of the fats, which is usually the reason for fractionation. It does not involve the use of solvents, detergents or other additives and furthermore, the desirable flavor notes are not lost although they are partitioned differently between the various fractions. 

Crystallization near Tm Subsequent cooling of the Solid Cooling and Crystallization interfere Serious undercooling of the melt, subsequent retarded crystallization... 

For a successful hot filtration the solution must pass through the filter paper and filter funnel into the collection flask as quickly as possible so that cooling and crystallization do not occur. The following points should be noted ... 

If filtration is rapid, keep the filter cone topped up with the hot solution being filtered, since this keeps the filter cone and filter funnel hot. If you allow the filter cone to empty of liquid, cooling and crystallization may occur. 

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