Seeding external

One of the main challenges in batch crystallization is to control the supersaturation and nucleation during the initial stage of the batch run. During this period, very little crystal suspension is present on which solute can crystallize, so that high supersaturation and excessive nucleation often occur. Another difficulty associated with batch crystallization is the determination of the initial condition for the population density function. In an unseeded batch crystallizer, initial nucleation can occur by several mechanisms and usually occurs as an initial shower followed by a reduced nucleation rate. Thus, an initial size distribution exists and one 

One method for overcoming the above mentioned difficulties is the use of external seeds so that initial nucleation can be controlled or minimized. Most crystallization systems exhibit a metastable supersaturation zone where crystal growth continues but the supersaturation is too low for nucleation to take place. Thus, if the supersaturation can be maintained below the metastable zone upper limit after seeding, then only growth on the seed crystals will occur. In this case, the initial condition for the population density will be the initial seed distribution. 

As mentioned before, in an ideal case, the supersaturation level during the externally seeded batch growth process stays below the critical (maximum) supersaturation so that the number of uniformly sized seeds introduced at the onset of the batch crystallization remains unchanged throughout the growth. As a result, a fines-free product of uniform size can be obtained. 

1997) have successfully been used in order to improve the CSD of the product crystals over an unseeded batch crystallization. 

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In the forced-circulation-type crystallizer (Fig. 19-43) primaiy control over particle size is exercised by the designer in selecting the circulating system and volume of the body. From the operating standpoint there is little that can be done to an existing unit other than supply external seed, classify the discharge ciystals, or control the shiny... 

Validation of the experiment without dust recycle and external seeds... 

The parameters for the batch granulation experiment without dust recycle and external seeds V-13 are listed in Tab. 16.15. The measured number and mass density particle size distribution (Fig. 16.34) answer the question as to whether particle growth can be modeled as either size-dependent or size-independent in the estimated range. 

Parameters for the batch granulation experiment with dust recycle and without external seeds V-14 are listed in Tab. 16.17. The objective was to investigate the deposition of the recycled dust. 

The parameters listed in Tab. 16.19 represent experiment V-15. The objective was to analyze the influence of absent seeds on the stability of the semi-batch process. Without external seeds, the number of particles will decrease as particles are discharged. 

Experiment V-15 shows that seeds are necessary for a continuous process. Thus, experiment V-16 describes a continuous experiment with larger monodisperse glass spheres as hold-up, and with smaller monodisperse glass spheres as external seeds (Fig. 16.49), together with recirculation of the dust. The mass flow of the seeds influences the duration until steady state is attained, whilst the size of the seeds influences the particle size distribution of the product [30],... 

Heinrich [30] reported that a rapid achievement of steady state depends on complete substitution of the hold-up particles by external seeds when the hold-up and seed particles are of the same size, and very small. Otherwise, the unsteady start-up phase is of much longer duration and the particle size distribution of the product is wider. 

Fig. 16.49. Illustration of external seeds (inert glass spheres, d32 = 1.655 mm), experiment V-l 6.

Calcium-41 decays by electron capture to with a half-life of only 103 kyr. It has the distinction of being the shortest-lived isotope for which firm evidence exists in early solar system materials, and this fact makes it key for constraining the timescale of last nucleosynthetic addition to solar system matter (in the external seeding scenario). It also makes " Ca exceedingly difficult to detect experimentally, because it can only be found to have existed in the oldest materials and then in only very small concentrations. Fortunately, its daughter potassium is rather volatile and calcium is concentrated in refractory minerals (the C in CAI) leading to large fractionations. Hutcheon et al. (1984) found hints for " Ca in Allende refractory inclusions, but could not clearly resolve excesses above measurement uncertainties. 

Oryzabrans A,B,C, and D Oryza saliva L/Gramineae External seed coats The oryzabrans had a hypoglycemic effect in normal mice and oryzabran A also had this efect in alloxan-induced hyperglycemic mice. Oryzabrans A and B were composed mainly of a-l->6-linked D-glucopyranose residues [91]. 

Antimicrobial base materials formulated with metal and amine salts of anacardic acid, said to be non-irritative, have been proposed for cosmetic use [270]. It is of interest that anacardic acid and 2(E)-hexenal isolated from the cashew apple to which the pendant kernel (the external seed) is attached have been found to exhibit antibacterial activity against the Gram-negative bacterium, Helicobacter pylori, which is considered to cause acute gastritis [271]. The same compounds also inhibit the enzyme urease. 

The screw/helix/spiral motive [11a] has more often been applied. Thus, irradiation of achiral 15, which crystallizes in the chiral space group P2i2i2i yields optically active 16 without external seeding. [24] In seven of ten crystallizations from hexane the (-t-)-enan-tiomer 16 predominated, in three cases the (-)-enantiomer 16. The optical yields (10% ee at 0°C, 75% chemical yield) can be improved by reducing the temperature (40 % ee at -40 °C, 70% yield no reaction at -78 °C however, formation of racemic 16 in solution also at -78 °C). This is again a reminder of the necessity of phase rebuilding and finally phase transformation to give the product lattice in solid-state reactions. Both processes are accompanied by extensive molecular movements (on the scale of the crystal lattice constants) in all known cases. [14, 15] Evidently, as a result of these, the chiral con-formers 15 have - as in solution - the opportunity for racemization and further conformational alterations. Nevertheless, the... 

As discussed earlier, the analytical solutions for the CSD for a batch or semibatch crystallizer are difficult to obtain unless both the initial condition for the CSD and appropriate kinetic models for nucleation and growth are known. An example of such an analytical solution—simple yet not overly restrictive—was given by Nyvlt (1991). It is assumed that the process, in which both external seeding and nucleation take place, occurs at constant supersaturation (G = constant, Bq = constant) in an ideally mixed crystallizer. An additional assumption of size-independent growth allows one to rewrite the time-dependent moments, Eqs. (10.12)-(I0.15), in terms of the physical properties such as the total number (A), length (L), surface area A), and mass of... 

The quality, productivity, and batch-to-batch consistency of the final crystal product can be affected by the conditions of the batch crystallizer. Several factors considered here include batch cycle time, supersaturation profile, external seeding, fouling control, CSD control, growth rate dispersions, and mixing. 

There are two basic practical approaches that can help control CSD in batch cooling crystallization. The first is concerned with the use of seed crystals (external seeding), and the second is concerned with limiting the occurrence of spontaneous nucleation only to the initial stage of the process. Both require the control of the cooling rate via the control of the solution temperature in the crystallizer and the manipulations of the flow rate of cooling agent. 

Larson (1978) considered the operation of a batch, externally seeded evaporative crystallizer with a constant growth rate, (J, in which no spontaneous nucleation was allowed. Applying the population balance Eq. (10.1) to this system gave the following equations for the solvent profile... 

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