Melt crystallization problems

The mapping (7) introduces the unknown interface shape explicitly into the equation set and fixes the boundary shapes. The shape function h(x,t) is viewed as an auxiliary function determined by an added condition at the melt/crystal interface. The Gibbs-Thomson condition is distinguished as this condition. This approach is similar to methods used for liquid/fluid interface problems that include interfacial tension (30) and preserves the inherent accuracy of the finite element approximation to the field equation (27)... 

Detection of Crystallization and Melting. A problem frequently tackled by monitoring WAXS peaks is the detection of crystallization and melting as a function of temperature, time, pressure, or other processing parameters —> As long as some of the characteristic peaks of a polymer material are observable, a fraction of the material is in crystalline state. 

Kobayashi (143) presented the first computer simulations that considered the determination of the crystal radius as part of the analysis but avoided the capillary problem by considering a flat melt-ambient surface, which is consistent with <)>o = 99°. Calculations were performed for a fixed crystal radius, and then the growth rate was adjusted to balance the heat flux into the crystal. Crowley (148) was the first to present numerical calculations of a conduction-dominated heat-transfer model for the simultaneous determination of the temperature fields in crystal and melt and of the shapes of the melt-crystal and melt-ambient surfaces for an idealized system with a melt pool so large that no interactions with the crucible are considered. She used a time-dependent formulation of the thermal-capillary model and computed the shape of an evolving crystal from a short initial configuration. 

Crystallization from solution may be quite selective, making controlled doping of crystals very difficult to control or even impossible. In the melt this problem is much easier to handle and also much larger crystals may be grown. Because of their potential applications, various doped fluorides have been intensively studied ... 

Examples in this chapter include sterile crystallization of a labile compound, yield enhancement by crystallization, yield and selectivity enhancement, removal of low-level impurities via crystallization from the melt, crystal formation in vials in a freeze drier, and non-equilibrium resolution of stereoisomers by crystallization. These examples represent unique crystallization processes designed for specific purposes. One lesson to be learned from examination of these nonmainstream applications is that understanding of principles can lead to inventive solutions to problems. For instance, in Examples 11-2 and 11-3, the solubility difference between starting material and desired product is used to optimize the reaction yield/selectivity by crystallizing the product and protecting it from overreaction. 

Figure ll-12a presents room temperature adsorption isotherms for some representative activated carbon samples on Technical Grade DMSO, showing the difficulty of reducing UV275 to the desired level of <0.10 without additional purification. Figure ll-12a is a schematic of the melt crystallization-carbon column recycle system which was employed to get around this problem. The higher concentration of impurities in the unfrozen melt altered the equilibrium concentration on the activated carbon. In the steady state (Fig. 1 l-12b) a reasonably sized carbon column could produce effluent suitable for further freeze CrystalUzation, and the yield of the total process was close to 100%. 

Crystals Are Sticky. Here the crystals seem covered by a thin layer of oily material, causing them to stick to razor blades, etc. The problem is dealt with in the same way as melting crystals. 

The overabundance of one species of resin acid and the presence of neutral materials in a rosin affect its tendency to crystallize. One of the serious problems of tall oil rosin in the early days was its rapid rate of crystallization, making this type of rosin difficult to melt. A commonly used yardstick for keeping rosin and rosin derivatives from serious crystallization problems is to keep any one species of the isomers below 20% of the total. It is generally observed that gum rosin is the least crystallizable and tall oil rosin the most. Heat treatment or minor chemical modification greatly reduces the crystallization tendency of tall oil rosin. 

Crystal Geometry Principles of Crystallization Equilibria and Yields Nucleation Crystal Growth Crystallization Equipment Applications of Principles to Design MSMPR Crystallizer Crystallization from Melts Symbols Problems References... 

Back mixing, slurry transport, moving parts in the equipment, and scaling problems are the biggest difficulties in continuously running plants. Therefore, a truly ideal continuous countercurrent melt crystallizer that is flexible enough to handle a variety of substances with high efficiency is not, unfortunately, found to date. Due to problems, the semibatch-type of processes dominates in most of the recently built plants. 

From all these data analyses, we can definitely say that the D and H chain stems are distributed statistically randomly in the crystalline lamellae of the D/H cocrystallized blend. This conclusion is quite important in relation with the chain-folding problem, a controversial research theme that had been discussed for a long time (30). The random distribution of the D and H chain stems naturally supports the idea that the D and H chains reenter randomly into and out of the crystalline lamellae as shown in Fig. 5.7. The regular adjacent reentry model is impossible to apply at all as for as the melt-crystallized sample is concerned. 

Hoffman JD, Lauritzen JI (1961) Crystallization of bulk polymers with chain folding theory of growth of lamellar spherulites. J Res Natl Bur Stand 65A 297-336 Hoffman JD, Guttman CM, DiMarzio EA (1979) On the problem of crystallization of polymers from the melt with chain folding. Faraday Discuss Chem Soc 68 177-197 Hoffman JD (1983) Regime in crystallization in melt-crystallized polymers The variable cluster model of chain folding. Polymer 24 3-26... 

The problem is to determine the approximate size of melt crystallizer required to produce 5000 Ib, of naphthalene feom a feed stream containing 80% by weight naphthalene and 20% botzene. Saturated residue is rejected at 4S C. The overall material balance is compt easily, and is shown in Fig. 11.4-6 the phase diagram is given in Fig. 11.2-2. For evaluation purposes, the contitroous melt ctys lizer is conveniently divided into three sections the recovery section where solute is crystallized and mnoved from solution, the rtfining section where reflux melt is recrystallized, and the pur katUm zone where crystals are contacted with reflux, are melted, and the melt removed and/or refluxed. 

The outstanding problems central to polymer crystallization is the nature of the chain conformation at the surface of a polymer crystal and melt-crystallized lamellae. This is central to the development of the subject and certainly the use of small-angle neutron scattering (SANS) has gone some way to resolving some of the issues. We look forward to see what the next 2 years will bring in this area. 

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