Crystallization time

When the reaction mixtures are prepared from colloidal siUca sol or amorphous siUca, additional 2eohtes may form which do not readily crystalline from the homogeneous sodium siUcate—alurninosiUcate gels. The temperature strongly influences the crystallization time of even the most reactive gels for example, zeoHte X crystallizes in 800 h at 25°C and in 6 h at 100°C. 

Figure 4 DSC melting endotherms of P7MB after isothermal crystallization at 135°C, starting from the isotropic melt [10]. The curves correspond to 0, 3,6, and 35 min of crystallization time, from bottom to top. Scanning rate 5°C/min.

We assume, for our analysis on small particles, that the overall crystallization time is limited by the nucleation time for a single germ. 

In this work, we developed the safeguard active-set method by modifying the active-set method for thermodynamic equilibrium in order to include the classical nucleation theory. At tn, assume that the partition ( (r ), M(t ), N(t ) and the crystallization time tciyst(t ) forM(t ) are known. For a new feed vector and RH at Vu compute W(tn+i), M(t i), N(t + )) and tciyst(t +i) as follows ... 

Compute Tnuci.j for i M tn+i) according to the classical nucle-ation theory Tnud,i = l/(Jnud,il (tn+i)), update the expected crystallization time fcryst,i tn+i) for the meta-stable phases M (t +i)... 

But in sonicated condition hexamminenickel(II) ions did not form, because of the degassing effect of the ultrasound leading to the removal of ammonia gas during the sonication process. Because of this reason the turbidity in sonicated sample did not come to the zero value (Table 9.8). Ultrasound was also effective in the reduction of crystallization time by 50% in Ni-chloro and by 25% in Ni-thiocyanato complex, which, however, remained the same in NiSO.4 solution as shown in Table 9.10. 

Figure 5. Pore size distribution of composite under different crystallization time (a) 2 days, (b) 3...

Mossbauer spectra were recorded for the same samples four species for Fe3+ and two for Fe2+ were distinguished. The relation between their relative proportions and crystallization time are shown in Figs. 2A and 2B. RBS spectra are shown in Fig 2C. 

Kumar et al. (298-300) reported a method wherein the crystallization time is significantly reduced. They found that addition of a small amount of oxyanion (e.g., H3PO4) to the TS-1 synthesis gel enhances the nucleation and crystallization... 

Prehydrolysis method. Investigation of influence of added oxyanions such as phosphate, perchlorate, arsenate, chlorate, bromate, etc. on rate of crystallization. The overall crystallization time in the presence of additives reduced by about five times compared to the conventional prehydrolysis method (7,8)... 

Except the particular behaviour of NH4 which is envisaged separately, starting from Li to Cs, the following trends are observed crystallization times and average particles sizes increase while the size distribution becomes less homogeneous (Figure 9). 

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... 

After the solution temperature reached at 50 C 1 gram of the seed crystals were added to the solution, when the crystallization time is defined to be 0. Each crystallization was conducted for three hours. Samples of the solution and the crystalline particles were taken at every 30 minutes and their concentrations or purities of L-threonine were measured by the HPLC at least for two times. 

In Figure 4 the corresponding changes in the crystal purity are shown. For the solutions with the initial supersaturation At=5 C, the purity was always 100% for three hours, while purity decrease started very soon for the solution having higher initial supersaturation such as At=10 C. Since with different initial supersaturations, the crystallization rate are very different, the purity decrease should be compared on the basis of either the supersaturation change or the crystal mass grown in the solution rather than the crystallization time. 

Figure 3(a). Effect of seed density on nucleation at 70 C. Number of crystals 1.2 im) vs crystallization time. 

As an example, we consider crystallization of polyethylene from a melt. As mentioned above, crystallization proceeds with the initial formation of isolated spherulites, which then grow until their mutual impingement with further slow crystallization. Time (t)-dependent measurements [26] of the density of the crystallizing melt at different temperatures are given in Figure 1.3 as a plot of degree of crystallinity versus logarithm of time. 

In the same gel containing octylamine, ZSM-48 also rapidly crystallizes in presence of small amount of Al (Table 4, samples 4 and 5). However, for larger amounts of Al, a poorly crystalline ZSM-48 is obtained after 7 days heating, which for longer crystallization times, completely transforms into ZSM-22, another zeolite involving a linear channel structure (sample 6, Table 4). This iatter is probably formed upon a... 

The above discussion is for the crystallization of one single mineral (i.e., many crystals of the same mineral). If several minerals are crystallizing and they have different crystallization temperature (and hence different crystallization time), then the relation between the degree of crystallization and time would be much more complicated. 

Starting Material Carbonyl Compound Mol. Equiv. Solvent Crystallization Time (h) Yield (%) op (%)... 

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