Crystallization crystal size determination

Insulin suspensions. When the hormone is injected as a suspension of insulin-containing particles, its dissolution and release in subcutaneous tissue are retarded (rapid, intermediate, and slow insulins). Suitable particles can be obtained by precipitation of apolar, poorly water-soluble complexes consisting of anionic insulin and cationic partners, e.g the polycationic protein protamine or the compound aminoqui-nuride (Surfen). In the presence of zinc and acetate ions, insulin crystallizes crystal size determines the rate of dissolution. Intermediate insulin preparations (NPH or isophane, lente or zinc insulin) act for 18 to 26 h, slow preparations (protamine zinc insulin, ultralente or extended zinc insulin) for up to 36 h. 

The particle sizes relevant for inorganic pigments stretch between several tens of nanometers for transparent pigment types to approximately two micrometers. For practical applications it is very desirable to determine not only the mean particle size but also the whole distribution. These parameters must not be confused with the crystal size determined by X-ray diffraction, as pigment particles usually are not monocrystals. 

SZl sample, reference materials of this work, is constituted by pure tetragonal ZrOz, with an average crystal size (determined with the classical Scherrer equation) of 85 A (Table 1). The stabilisation of the tetragonal ZrOz depends on the presence of TPAOH in the reaction mixture. In fact, variable amounts of monoclinic phase are detected in the products synthesised in the absence of organic base (SZ2 sample. Table 1). This is probably related to the pH of the sol, but this hypothesis should be confirmed. Basic medium usually enhances the condensation reactions [15], but at the same time the ability of sulphates to form complexes with Zr precursors up to pH = 11 [16] and the low water content in the sol prevent the growth of zirconia particles. 

Ice Crystal Size Determination. Frozen green beans were taken from storage at times which corresponded to the biochemical assay points and put into the Isothermal freeze fixative, Asquith and Reid (1). 

The synthesized materials posses large pore volume and a good thermal and hydrothermal stability. Table 1 shows textural properties of the calcined materials the use of a surfactant leads to higher surface areas and pore volume than those observed for reference C-0 and C-4 samples. On the other hand, the increase in surface area correlates well with the intensity of the carbonate bands observed by DRIFT. Examination of the N2-physisorption isotherms let us assume a non-ordered mesoporous structure. This was confirmed by the lack of a small angle X-ray difliraction peak. By XRD the Ce02 cerianite phase was identified and crystal size determined by the Rietveld method showed that samples C-1 through C-3 are nanometric particles with crystal size of about 4-7 nm while reference samples C-0 and C-4 were much larger (w30 and 19 nm, respectively). 

A usual requirement is control over the concentration of crystals in the discharge slurry. In many cases, however, crystal size is important as well. Crystal concentration is customarily measured as density, if the crystals are uniformly dispersed across the sensitive span of the detector. Crystal size determination unfortunately does not lend itself to on line analysis. 

Crystal size determinations from fractured surfaces are problematic since it is not clear how the fracture propagates with regard to the grains or crystals. Sometimes evaluation is impossible (Fig. 6). 

The general manufacturing scheme for phosphate salts is shown in Figure 11. Condensed phosphates are prepared from the appropriate orthophosphate or mixture of orthophosphates, so the preparation of orthophosphates must be considered first for the manufacture of any phosphate salt. Phosphoric acid is neutralized to form a solution or slurry with a carefully adjusted acid/base ratio according to the desired orthophosphate product. The orthophosphate may be recovered either by crystallization from solution, or the entire solution or slurry may be evaporated to dryness. The dewatering (qv) method is determined by the solubihty properties of the product and by its desired physical properties such as crystal size and shape, bulk density, and surface area. Acid orthophosphate salts may be converted to condensed phosphates by thermal dehydration (calcination). 

Sepa.ra.tlon, Sodium carbonate (soda ash) is recovered from a brine by first contacting the brine with carbon dioxide to form sodium bicarbonate. Sodium bicarbonate has a lower solubiUty than sodium carbonate, and it can be readily crystallized. The primary function of crystallization in this process is separation a high percentage of sodium bicarbonate is soHdified in a form that makes subsequent separation of the crystals from the mother hquor economical. With the available pressure drop across filters that separate Hquid and soHd, the capacity of the process is determined by the rate at which hquor flows through the filter cake. That rate is set by the crystal size distribution produced in the crystallizer. 

An average crystal size can be used to characterize a CSD. However, the average can be determined on any of several bases, and the basis selected must be specified for the average to be usehil. More than 20 different averaging procedures have been proposed, yet none is generally satisfactory or preferred (5). 

The dominant crystal size, is most often used as a representation of the product size, because it represents the size about which most of the mass in the distribution is clustered. If the mass density function defined in equation 33 is plotted for a set of hypothetical data as shown in Figure 10, it would typically be observed to have a maximum at the dominant crystal size. In other words, the dominant crystal size is that characteristic crystal dimension at which drajdL = 0. Also shown in Figure 10 is the theoretical result obtained when the mass density is determined for a perfectiy mixed, continuous crystallizer within which invariant crystal growth occurs. That is, mass density is found for such systems to foUow a relationship of the form m = aL exp —bL where a and b are system-dependent parameters. 

A pair of kinetic parameters, one for nucleation rate and another for growth rate, describe the crystal size distribution for a given set of crystallizer operating conditions. Variation ia one of the kinetic parameters without changing the other is not possible. Accordingly, the relationship between these parameters determines the abiUty to alter the characteristic properties (such as dominant size) of the distribution obtained from an MSMPR crystallizer (7). 

Preferential Removal of Crystals. Crystal size distributions produced ia a perfectiy mixed continuous crystallizer are highly constraiaed the form of the CSD ia such systems is determined entirely by the residence time distribution of a perfectly mixed crystallizer. Greater flexibiUty can be obtained through iatroduction of selective removal devices that alter the residence time distribution of materials flowing from the crystallizer. The... 

Thin films of metals, alloys and compounds of a few micrometres diickness, which play an important part in microelectronics, can be prepared by die condensation of atomic species on an inert substrate from a gaseous phase. The source of die atoms is, in die simplest circumstances, a sample of die collision-free evaporated beam originating from an elemental substance, or a number of elementary substances, which is formed in vacuum. The condensing surface is selected and held at a pre-determined temperature, so as to affect die crystallographic form of die condensate. If diis surface is at room teiiiperamre, a polycrystalline film is usually formed. As die temperature of die surface is increased die deposit crystal size increases, and can be made practically monocrystalline at elevated temperatures. The degree of crystallinity which has been achieved can be determined by electron diffraction, while odier properties such as surface morphology and dislocation sttiicmre can be established by electron microscopy. 

The population balance analysis of the idealized MSMPR crystallizer is a particularly elegant method for analysing crystal size distributions at steady state in order to determine crystal growth and nucleation kinetics. Unfortunately, the latter cannot currently be predicted a priori and must be measured, as considered in Chapter 5. Anomalies can occur in the data and their subsequent analysis, however, if the assumptions of the MSMPR crystallizer are not strictly met. 

Plots of log population density versus crystal size of the type shown in Figure 5.14 enable the crystallization kinetics to be determined. Some early literature data reporting such analyses are summarized in Table 5.2. 

Estimate the maximum product crystal size and determine a simple controlled cooling curve (Jones, 1972, 1974 Mullin and Jones, 1974). 

In this case, the co-solvent dosage rate is programmed in order to control the transient level of supersaturation in an effort to improve on the product crystal size distribution from simply dumping in all the solvent at the start of the batch. An experimental crystallizer within which a programmed microcomputer determines the set point of a variable speed-dosing pump is shown in Figure 7.7. Controlled co-solvent dosing improves the product crystal size, with a consequent increase in the filterability of the product. These process concepts are developed further in Chapter 9. 

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