Crystallization solutions

Crystallization is a commonly used industrial sqiaiation and purification technique. If the desired product is an evaporate (or filtrate) rather than a crystalline phase, then the process enqihasis is primarily that of separation rather than purification. In either case tiiere is a strong interaction and dependence betw both degree of separation and purification and the paniculate nature of the solid phase produced. Fundamental research on crystallization has focused mainly on understanding the variables that influence the structure and size of the crystalline phase, recognizing that better knowledge and control of this aspect would permit improvement of the unit operation of crystallization, both as a separation and purification technique. 

A considerable amount of skepticism naturally is expressed by crystallization practitioners concerning the unbridled use of paiamders generated in well-controlled small-scale equipment to predict performaiKe of industrial-scale ciystallizeis. Matty factors contribote to the nonideat behavior of industrial crystallizers, and not all can be easily explained. 

The ability to predict crystal behavior in complex systems is ahead of our ability to manipulate crystal and liquid residence times. Generalizations rtf die MSMPR equations have been made to predict CSD with aibitraiy process configurations and kinetics however, there is no guaramee that an assumed process residence-time distribution can be physically inqilemented to produce a customized product CSD. Size distributiom in cascaded crystallizers, for exaiiqile, multiple-elfect evaporators, can be computed if kinetic 

In optimizing any separation process it becomes necessary to compote the product recovery efficiency of the operation as a function of design variables. For crystallization operations the theoretical maximum product recovery or yield from the crystallizer is defined by the following general relationship  

Rc a mass of dissolved solute in liquid discharged from the crystallizer per unit mass of discharged 

In solution crystallization a pure substance R is crystallized from a solution and thus separated from a foreign substance F, which remains in solution [Birmingham 2000]. 

5 4 Three-component system (solvent, pure substance, impurity) in a triangular diagram (Z = feed, E = mother liquor). 

From which Equation (2.3.5-4) follows for the mass of isolate pure substance Io-(Xq-X ) + D.X  

Liquid feed 20 to 60% w/w target solute becomes insoluble by producing supersaturation. Operates about the freezing temperature of the solute. Use for heat 

The change in solubility with temperature is usually the most significant characteristic for selecting type of crystallizer . 

If the solute solubility is relatively temperature independent or inversely temperature dependent add heat to remove the solvent (i.e., must use evaporative crystallization) or add an antisolvent to drown out crystals. For evaporation 14 to 20 g vapor evaporated/s nf exchanger area. For the exchangers use 2.5 cm diameter tubes with fluid velocities 1.5 to 3 m/s to minimize plugging. Caution if the vapor pressure rise 3.4 kPa/°C then potential problems with control. 

If the solute solubility shows some temperature dependence, use vacuum cooling plus external heat. 

If the solute solubility is strongly temperature dependent use cooling (e.g. scraped surface) or might use vacuum cooling without external heat that is, preheated feed followed by crystallizer. 

Krimm and Cheam have also accumulated evidence for the nature pf the fold surface by i.r. analysis of mixed crystals containing small amounts of deuterio- 

Models of adjacent re-entry tight folds surfaces for polyethylene single crystals have been made to try and resolve the apparent anomoly of the surface density. The tight-folds model has given a density of 1.0 g cm for the fold surface and this in turn raises the values of the density of the crystal to above the observed value. However, re-calculations suggest that the fold-surface density is in fact 0.75 g cm and this gives no inconsistency between calculated and accepted value. 

Isotactic and syndiotactic poly(methyi methacrylate) form a stereocomplex and this has been investigated by isolating crystalline complexes from dilute solution.  

These had the same crystallographic form as complexes isolated from melt crystallization and from concentrated solutions. The stereocomplex is a 2 1 molar ratio of syndiotactic isotactic. Since the melting point also varied with crystallization temperature, lamella and single-crystal structure have been suggested for the dilute solution complex. 

Small-angle neutron scattering (SANS) studies on melt crystallized linear polyethylene, isotactic polypropylene, and poly(ethylene oxide) have shown that deuteriated molecules in a hydro-chain matrix have the same radius of gyration, and so molecular conformation, as in melt. This is evidently inconsistent with a regular adjacent re-entry folded chain lamellae. Recently Stamm et in comparing the scattering of neutrons at intermediate angles, of 

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Solution crystalli tion (adiabatic evaporation (vacuum cooling))... 

Secondary nucleation is crystal formation through a mechanism involving the solute crystals crystals of the solute must be present for secondary nucleation to occur. Thorough reviews have been given (8,9). 

Crystallization from Solution. Crystallization techniques are related to the methods used to iaduce a driving force for soflds formation and to the medium from which crystals are obtained. Several approaches are defined ia the foUowiag discussion. 

Purification of a chemical species by solidification from a liquid mixture can be termed either solution crystallization or ciystallization from the melt. The distinction between these two operations is somewhat subtle. The term melt crystallization has been defined as the separation of components of a binaiy mixture without addition of solvent, but this definition is somewhat restrictive. In solution crystallization a diluent solvent is added to the mixture the solution is then directly or indirec tly cooled, and/or solvent is evaporated to effect ciystallization. The solid phase is formed and maintained somewhat below its pure-component freezing-point temperature. In melt ciystallization no diluent solvent is added to the reaction mixture, and the solid phase is formed by cooling of the melt. Product is frequently maintained near or above its pure-component freezing point in the refining sec tion of the apparatus. 

Rawlings, J.B., Miller, S.M. and Witkowski, W.R., 1993. Model identification and control of solution crystallization processes A review. Industrial and Engineering Chemistry Research, 32, 1275-1296. 

Add a little caustic potash solution. Crystals of potassium oxalate are deposited. The ester is hydrolysed. 

To a suspension containing 4.86 parts of 4-methylbenzenesulfonyl urethane (MP 80° to 82°C) and 36 parts of anhydrous toluene there are rapidly added 2.5 parts of N-amino-3-azabicyclo(3.3.0)octane (BP/18 mm = 86°C). The reaction mixture is heated under reflux for 1 hour. The resulting ciear solution crystallizes on cooling. The crystals are filtered, washed with 2 parts of toluene, then recrystallized from anhydrous ethanol. There are obtained 3.8 parts of the desired product, MP 180° to 182°C. 

For PVDF, several experiments have shown that at room temperature the p form is thermodynamically the most stable, while the ot form is kinetically the most advantageous. For instance, by solution crystallization (casting from polar hexamethylphosphamide solutions) p form, y form and a form crystals are obtained for low, intermediate and high evaporation rates, respectively [15, 66]. 

A lot of emphasis is laid on three possible basic interferences during the application process (1) single constituents of the sample solution crystallize out, (2) the property of the solvent used for dissolution, and (3) the concentration of the solution generating an imacceptably broad application zone. Such interferences could be avoided with proper planning before application. 

Two kinds of solid solutions crystallize, a solution of metal 1 in metal 2 and vice versa (limited miscibility). 

Describe each of the solutions indicated as saturated, unsaturated, supersaturated, or impossible to tell, (a) More solute is added to a solution of that solute, and the additional solute all dissolves. Describe the original solution. (6) More solute is added to a solution of that solute, and the additional solute does not all dissolve. Describe the final solution, (c) A solution is left standing, and some of the solvent evaporates. After a time, some solute crystallizes out. Describe the final solution, (d) A hot saturated solution is cooled slowly, and no solid crystallizes out. Describe the cold solution, (e) A hot solution is cooled slowly, and after a time some solid crystallizes out. Describe the cold solution. (/) A hot saturated solution is cooled slowly, and no solid crystallizes out. The solute is a solid that is more soluble hot than cold. Describe the cold solution. 

Nadagouda, M.N. and Varma, R.S. (2008) Microwave-assisted shape-controlled bulk synthesis of Ag and Fe nanorods in polyfethylene glycol) solutions. Crystal Growth and Design, 8, 291-295. 

Very shock-sensitive and explodes readily when precipitated from aqueous solution. Crystals obtained by slow evaporation of a carbon tetrachloride extract were less sensitive. 

The residual amine in the filtrate may be isolated in the form of the hydrochloride. The combined solutions are evaporated on a steam bath, 50 ml. of concentrated hydrochloric acid is added, and heating is continued for 2 hours. On cooling, the syrupy solution crystallizes. It is triturated with 50 ml. of ethanol, and the 4-amino-l,2,4-triazole hydrochloride is filtered, washed with a little ethanol, and dried. The yield of the hydrochloride is 10-18 g. (8-15%) the salt melts at 147-148° and may be recrystallized from 95% ethanol, using 10 ml. per gram the melting point is thus raised to 151-152°. 

Fig. 5 Initial fold length L against undercooling AT = To - T for both melt- and solution-crystallized polyethylene, from different solvents [16]. Dashed line gives previous calculations (Model A [9], old model in Fig. 4), solid line shows the results from the new model in Fig. 4 after re-adjusting the energy of fusion per - CH2 - unit from E= 1.07 to E- 1.42 kcal/mol...

The d,L-arabitol pentaacetate was hydrolyzed by refluxing it for three hours with methanol containing an excess (7 moles) of hydrogen chloride. After concentration to one-half its volume the solution crystallized spontaneously. By collecting the product before crystallization was too far advanced and washing it with a little fresh methanol, crystals melting at 105-106° were obtained. This is the melting point of d,L-arabitol as first recorded by Ruff.1 ... 

The triselenadiborolanes 3,5-R2-l,2,4,3,5-Se3B2 R=Et (33), Pr readily formed coordination adducts with two equivalents of pyridine, 3,5-dime-thylpyridine, and 3-chloropyridine.168 With one equivalent of base, only one of the B atoms became coordinated, and surprisingly, the system was not fluxional at room temperature.168 The addition of two equivalents of pyrazole to 33 (Scheme 7) resulted in a brown suspension and a yellow solution. Crystals of a B2N4Se2-bicyclo[2.2.2]octane were formed upon cooling this solution to —80 °C. With bulkier pyrazole derivatives (phenyl-pyrazole), the B2N4Se-bicyclo[2.2.1] heptanes were formed.169... 

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