Crystallizers increasing crystal size

At lower temperatures, the crystals increase in size, and form networks that trap the liquid and hinder its ability to flow. The pour point is attained which can, depending on the diesel fuel, vary between -15 and -30°C. This characteristic (NF T 60-105) is determined, like the cloud point, with a very rudimentary device (maintaining a test tube in the horizontal position without apparent movement of the diesel fuel inside). 

Bikerman [179] has argued that the Kelvin equation should not apply to crystals, that is, in terms of increased vapor pressure or solubility of small crystals. The reasoning is that perfect crystals of whatever size will consist of plane facets whose radius of curvature is therefore infinite. On a molecular scale, it is argued that local condensation-evaporation equilibrium on a crystal plane should not be affected by the extent of the plane, that is, the crystal size, since molecular forces are short range. This conclusion is contrary to that in Section VII-2C. Discuss the situation. The derivation of the Kelvin equation in Ref. 180 is helpful. 

In most covalent compounds, the strong covalent bonds link the atoms together into molecules, but the molecules themselves are held together by much weaker forces, hence the low melting points of molecular crystals and their inability to conduct electricity. These weak intermolecular forces are called van der WaaFs forces in general, they increase with increase in size of the molecule. Only... 

Optimizing the Cr layer also controls the crystal size and morphology. It was reported in 1986 (89,90) that the Cr underlayer thickness has a great influence on the coercivity of the Co—Ni—Cr layer. In most of the Hterature it can be found that with increasing Cr thickness the increases. Under ideal conditions and the right material combinations coercivities above 240 kA/m have been prepared. 

Figure 1 shows the decomposition sequence for several hydrous precursors and indicates approximate temperatures at which the activated forms occur (1). As activation temperature is increased, the crystal stmctures become more ordered as can be seen by the x-ray diffraction patterns of Figure 2 (2). The similarity of these patterns combined with subtie effects of precursor crystal size, trace impurities, and details of sample preparation have led to some confusion in the Hterature (3). The crystal stmctures of the activated aluminas have, however, been well-documented by x-ray diffraction (4) and by nmr techniques (5).

The gaseous ammonia is passed through electrostatic precipitators for particulate removal and mixed with the cooled gas stream. The combined stream flows to the ammonia absorber where the ammonia is recovered by reaction with a dilute solution of sulfuric acid to form ammonium sulfate. Ammonium sulfate precipitates as small crystals after the solution becomes saturated and is withdrawn as a slurry. The slurry is further processed in centrifuge faciHties for recovery. Crystal size can be increased by employing one of two processes (99), either low differential controUed crystallization or mechanical size enlargement by continuous compacting and granulation. 

In the manufacture of explosives, sodium nitrate is used mainly in blasting agents. In slurries and emulsions, sodium nitrate improves stabiUty and sensitivity. It also improves the energy balance because sodium nitrate replaces water, so that more fuel can be added to the formulation. Sodium nitrate reduces crystal size of slurries, which in turn increases detonating speed. In dynamites sodium nitrate is used as an energy modifier. Typical content of sodium nitrate is 20—50 wt % in dynamites, 5—30 wt % in slurries, and 5—15 wt % in emulsions. Sodium nitrate is used also in permissible dynamites, a special type of dynamite for coal (qv) mining. 

Pitches can be transformed to a mesophase state by further chemical and physical operations. Heat treatment of conventional pitches results in additional aromatic polymeriza tion and the distillation of low molecular weight components. This results in an increase in size and concentration of large planar aromatic molecular species whereupon the precursor pitch is transformed to a mesophase state exhibiting the characteristics of nematic Hquid crystals (1). Additional heat treatment converts the mesophase pitch to an infusible aromatic hydrocarbon polymer designated as coke. 

The bulk of synthetic industrial diamond production consists of the smaller crystal sizes up to 0.7-mm particle size (25 mesh). This size range has wide utihty in industry, and a significant fraction of the world s need for diamond abrasive grit is now met by synthetic production yielding thousands of kilograms per year. Because the raw materials are plentiful, synthetic production could, if necessary, supply the world demand for diamond abrasive. Development work continues in order to improve size and utility of the manufactured product and to realize the full potential of diamonds at minimum cost. An appreciable increase in performance has been obtained by coating the diamonds with a thin layer of nickel or copper, before incorporating them into wheels. The thin layer of metal apparendy improves adhesion and heat transfer. 

Figure 15 shows how the population density function changes with the addition of classified-fines removal. It is apparent from the figure that fines removal increases the dominant crystal size, but it also increases the spread of the distribution. 

Although many commercial crystallizers operate with some form of selective crystal removal, such devices can be difficult to operate because of fouling of heat exchanger surfaces or blinding of screens. In addition, several investigations identify interactions between classified fines and course product removal as causes of cycling of a crystal size distribution (7). Often such behavior can be rninirnized or even eliminated by increasing the fines removal rate (63,64). 

Crystal Formation There are obviously two steps involved in the preparation of ciystal matter from a solution. The ciystals must first Form and then grow. The formation of a new sohd phase either on an inert particle in the solution or in the solution itself is called nucle-ation. The increase in size of this nucleus with a layer-by-layer addition of solute is called growth. Both nucleation and ciystal growth have supersaturation as a common driving force. Unless a solution is supersaturated, ciystals can neither form nor grow. Supersaturation refers to the quantity of solute present in solution compared with the quantity which would be present if the solution were kept for a veiy long period of time with solid phase in contac t with the solution. The latter value is the equilibrium solubility at the temperature and pressure under consideration. The supersaturation coefficient can be expressed... 

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. 

A secondary particle formation process, which can increase crystal size dramatically, is crystal agglomeration. This process is particularly prevalent in systems exhibiting high levels of supersaturation, such as from precipitation reactions, and is considered along with its opposite viz. particle disruption in Chapter 6. Such high levels of supersaturation can markedly accentuate the effects of spatial variations due to imperfect mixing within a crystallizer. This aspect is considered further in Chapter 8. 

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. 

Geothite is produced by air oxidation of alkaline suspension of ferrous hydroxide (Sada etal., 1988). As a starting material for ferrous oxide, the preparation of fine particles with prescribed size, size distribution and shape is required in its application to magnetic materials for recording tapes and disks. With increasing oxidation rate, the crystal size decreases and the size distribution becomes sharper (Sada etal., 1988). The first step of the reaction. 

The size of crystal increases with time gradually approaching an asymptotic value. The higher the stirring rate, the larger the primary crystal sizes. 

The data plotted in the figure clearly support the predicted positive dependence of crystal size on agitation rate. Precipitation in the crystal film both enhances mass transfer and depletes bulk solute concentration. Thus, in the clear film model plotted by broken lines, bulk crystal sizes are initially slightly smaller than those predicted by the crystal film model but quickly become much larger due to increased yield. Taken together, these data imply that while the initial mean crystal growth rate and mixing rate dependence of size are... 

The optimal network increases total residence time by 48 per cent when compared with an equivalent MSMPR of the same volume and throughput. This increase would translate into a similar increase in mean crystal size and a 78 per cent increase in yield. Exactly the same residence time as for the single crystallizer have been reported from simple cascade configurations previously designed for stage-wise crystallization processes for slight improvements in... 

It was shown in Chapter 7 that the performance of continuous crystallizers is determined by the characteristics of a feedback loop relating the output performance expressed as crystal size distribution and to the feed concentration and residence time. Thus, an increase in crystallizer residence time, or decrease in feed concentration, reduces the working level of supersaturation. This decrease in supersaturation results in a decrease in both nucleation and crystal growth. This in turn leads to a decrease in crystal surface area. By mass balance, this then causes an increase in the working solute concentration and hence an increase in the working level of supersaturation and so on. There is thus a complex feedback loop within a continuous crystallizer, as considered in Chapter 7 and illustrated in Figure 8.11. 

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