Sucrose crystallization rates

Zaorska, H. Influence of non-sugar and colouring substance quantity on the sucrose crystallization rate in impure solutions, in Industrial Crystalllzation TS (eds. de Jong. E.J., Janclc, S.J.), p. 517, North-Holland, Amsterdam 1979... 

Figure 16.5. Supersaturation behavior, (a) Schematic plot of the Gibbs energy of a solid solute and solvent mixture at a fixed temperature. The true equilibrium compositions are given by points b and e, the limits of metastability by the inflection points c and d. For a salt-water system, point d virtually coincides with the 100% salt point e, with water contents of the order of 10-6 mol fraction with common salts, (b) Effects of supersaturation and temperature on the linear growth rate of sucrose crystals [data of Smythe (1967) analyzed by Ohara and Reid, 1973],...

Factors that influence growth of sucrose crystals have been listed by Smythe (1971). They include supersaturation of the solution, temperature, relative velocity of crystal and solution, nature and concentration of impurities, and nature of the crystal surface. Crystal growth of sucrose consists of two steps (1) the mass transfer of sucrose molecules to the surface of the crystal, which is a first-order process and (2) the incorporation of the molecules in the crystal surface, a second-order process. Under usual conditions, overall growth rate is a function of the rate of both processes, with neither being rate-controlling. The effect of impurities can be of two kinds. Viscosity can increase, thus reducing the rate of mass transfer, or impurities can involve adsorption on specific surfaces of the crystal, thereby reducing the rate of surface incorporation. 

Effect of Sucrose Polyesters and Sucrose Polyester-Lecithins on Crystallization Rate of Vegetable Ghee... 

Bennett, M. C. Fentiman, Y. L., Growth Rate of Sucrose Crystals Related to Krypton Surface of Seeds. In Industrial Crystallization The Institute of Chemical Engineers London, U.K. 1969, pp. 217-226. 

In Section 15.2.2 it was argued that the linear growth rate of a-lactose hydrate crystals is very much smaller than that of sucrose crystals, because in the former case fierce competition with 3-lactose occurs, and nothing like that can occur for sucrose. Can you now give additional causes for the crystallization rate of a lactose solution being far smaller than that of a sucrose solution at the same supersaturation, especially at low temperature Tip also consult Section 2.2. 

In this equation, A and k are the crystallization rates at temperatures T and Tp respectively, and Ci and C2 are constants. The role of viscosity in the crystallization of amorphous sucrose was suggested by the observation that crystallization is enhanced by a lowered Ts resulting from moisture adsorption.574 7 Also, the crystallization rate of amorphous nifedipine exhibited a temperature dependence best represented by the WLF equation. The increase in the crystallization rate caused by the decrease in Tg under higher humidity conditions was also described by the WLF equation, as shown in Fig. 150.604... 

Effective Nucleation Rates. A simple approach to handling the problem of upward curvature of the semilogarithmic size plot at small size is to use linear extrapolation. Figure 4.11 shows an example of such data for the sucrose-water system. Previously, we saw data for sucrose crystallization in Figure 4.7. These data obeyed the MSMPR formalism since small-size population densities were not measured. The data shown in Figure 4.11, also for the sucrose-water system, were measured using a Coulter counter, which allowed the smaller sizes to be determined. Since the linear portion of the plot represents the product sized crystals, a linear extrapolation to zero size yields an effective zero-size population density. Therefore, an effective nucleation rate can also be determined. This approach does not require knowledge of the cause of the curvature. 

A typical semilogarithmic population density size plot obtained from a 1400-1 pilot scale sucrose crystallizer is shown in Figure 4.12. It was found that a single growth rate distribution is insufficient to the data. Surprisingly, a combination of two growth rates was all that was necessary, i.e., the sum of two exponentials reproduced the experimental data. This is particularly useful since it now permits the use of effective rates that can be determined unambiguously. 

Aso Y, Yoshioka S (2006) Molecular mobility of nifedipine-PVP and phenobarbital-PVP solid dispersions as measured by C-NMR spin-lattice relaxation time. J Pharm Sci 95 318-325 Aso Y, Yoshioka S, Kojima S (2000) Relationship between the crystallization rates of amorphous nifedipine, phenobaibital, and flopropione, and their molecular mobility as measured by their enthalpy relaxation and H NMR relaxation times. J Pharm Sci 89 408-416 Aso Y, Yoshioka S, Zhang J, Zografi G (2002) Effect of water on the molecular mobility of sucrose and poly(vinylpyrrolidone) in a colyophUized formulation as measured by C-NMR relaxation time. Chem Pharm Bull 50 822-826... 

The effect of the crystal habit-forming impurities, raffinose, dextrose, and potassium chloride on the growth of sucrose crystals from various seeds has been studied. A mechanism for the oxidation of o-galactose by Nessler s reagent in alkaline media via the enediol has been proposed on the basis of kinetic measurements. The reaction is zero-order with respect to Hg" and first-order with respect to galactose. The rate is inversely proportional to the concentration of iodine ion. 

FIG. 6 Plots of the non-steady-state nucleation rate for increasing sizes of critical nucleus. The time lag (t) before any appreciable nucleation takes place is indicated on the plot for the case of the nucleus. On occasion, this lag can be a substantial proportion of the overall delay before nucleation—for concentrated sucrose crystallization, it has been estimated at 100 hr. Even when the time lag has elapsed, there may be further significant delays as the rate of time-dependent nucleation, J(t), gradually builds up to the steady state value, Jq. 

The effects of surfactants on the crystallization patterns of other food components were also studied. Surfactants affect the crystallization of sugars (sucrose, sorbitol, glucose, fructose, etc) from aqueous solutions. Surfactants affect yields of crystallization, rates of precipitation and crystallization, mode of crystallization, crystal stmctures, and crystal morphologies and habits. Surfactants also affect crystal size and amounts of impurities that cocrystallize. The effects are tremendous, and an almost endless number of effects have been described. In many cases, the studies are descriptive and the mechanisms involved in these effects are stiU obscure. 

---