Crystallization in the production

Various crystalline materials with desired properties have been synthesized, and this has driven the utilization of single crystals in the production of semiconductor, opto-electronic, piezoelectric, and pyroelectric materials. 

The primary drying time of the product, which can be defined as the time elapsed from the moment when the vacuum is created in the chamber to the disappearance of the last ice crystals in the product, is obviously a dependent parameter as the rate of ice sublimation is controlled by the same factors as product temperature. This is also the case for the secondary drying time of the product, i.e., the time elapsed from the moment when ice sublimation is complete to the end of the cycle. During this period, water is desorbed from the product at a rate dictated both by technical factors and by product characteristics. 

Fe202 content Composition of hexagonal prisms of the batch FeASPO crystals in the product... 

The nucieation rate must be just sufficient to generate one nucleus for each crystal in the product. If C is the mass production rate of crystals, the required nucieation, in number per unit time and volume of mother liquor, is, from Eqs. 

The total mass of crystals per volume of liquid is same in both crystallizer and product magmas. For a volume V of product, by a particle balance, the number of crystals in the product equals the number of crystals per unit time. The mass of a single product crystal is... 

The product classification in the elutriation leg is operated under flow rate control without automatic feedback from the product CSD. The rate of slurry discharge is set by the level control loop, responding to the differential pressure generated by the varying slurry-gas interface. A progressive cavity pump is the final control element to achieve the level control while providing minimal degradation of the crystals in the product slurry. 

A clear-liquor advance operation is one in which the overflow liquid is continuously removed from the tank. The overflow is not actually clear but contains small crystals that have not settled in the overflow section. This method is a simple way of controlling the CSD, because the residence times of the clear mother-liquor overflow and crystal are different. The flows are separated and the excess nuclei are removed with the overflow. The nuclei in the overflow can be used, for example, in the following crystallization unit. The larger the clear-liquor flow for a given feed flow, the longer the residence time of the crystals in the product stream. 

A crystallizer equipped with the devices for product classification and fines destruction can efficiently improve the particle size and particle size distribution. With the fines destruction, a portion of the small particles can be eliminated in such a way that the number of crystals in the crystallizer can be controlled within the required range. When this method is complemented with classified product removal, the size of the crystals in the product is controlled on the other hand, an undesired increase in the amount of small crystals is prevented. With this method, it is possible to obtain crystals of a uniform and large size. 

E. Curcio, G. Di Profio, and E. Drioli, Recovery of fumaric acid by membrane crystallization in the production of L-mahc acid, Sep. Purif. Technol. 33 (2003) 63-73. 

The independence of the properties of crystalline end products on the duration of room temperature ageing of reaction mixtures indicates that the formation of precursors for the subsequent growth of ZSM-5 zeolites does not occur during the room-temperature ageing in the current system and, at the same time, this confirms the hypothesis that, under such conditions, the active growth precursors can only be formed at reaction temperature. Since in SDA-free crystallization systems, the formation of new nuclei is greatly suppressed in the presence of seed crystals [51], the size of discrete crystals in the product depend only on the amount and size of the seeds present in the reaction mixture. 

Let each seed crystal grow by a certain amount. If we assume that the growth rate G is size-independent (assumption (1)), then the net growth in size in the MSMPR crystallizer for all crystals is Arp = Gtres cm for the given residence time. The mass density per unit liquid volume for crystals in the product stream exiting the crystallizer (population density function i(rp)) is... 

Ethoxylation reactions are normally initiated with sodium or potassium hydroxide as catalyst. When the reaction is complete, the catalyst may be removed by filtration or liquid-liquid extraction, or may simply be neutralized and left in the product. The presence of neutralized catalyst may cause problems in certain applications, or may sometimes be detectable as turbidity or even crystals in the product. Normally, if the catalyst is removed, inorganic residues are reduced to the low parts per million level. If the catalyst is simply neutralized, the resulting salt is present in the 0.5-1.0% range. 

Parallel-feed operation is illustrated in Fig. 3.12c. Fresh feed is added to each stage, and product is withdrawn from each stage. The vapor from each stage is still used to heat the next stage. This arrangement is used mainly when the feed is almost saturated, particularly when solid crystals are the product. 

D-fructose, C HijOo. Crystallizes in large needles m.p. 102-104 C. The most eommon ketose sugar. Combined with glucose it occurs as sucrose and rafftnose mixed with glucose it is present in fruit juices, honey and other products inulin and levan are built of fructose residues only. In natural products it is always in the furanose form, but it crystallizes in the pyranose form. It is very soluble in... 

Crystallizes in colourless needles m.p. 300° (sublimes). Manufactured by the oxidation of p-xylene and used in the production of Terylene (see also polyesters). U.S. production 1980 2-05 megatonnes. 

Into a 1-litre beaker, provided with a mechanical stirrer, place 36 - 8 g. (36 ml.) of aniline, 50 g. of sodium bicarbonate and 350 ml. of water cool to 12-15° by the addition of a little crushed ice. Stir the mixture, and introduce 85 g. of powdered, resublimed iodine in portions of 5-6 g, at intervals of 2-3 minutes so that all the iodine is added during 30 minutes. Continue stirring for 20-30 minutes, by which time the colour of the free iodine in the solution has practically disappeared and the reaction is complete. Filter the crude p-iodoaniline with suction on a Buchner funnel, drain as completely as possible, and dry it in the air. Save the filtrate for the recovery of the iodine (1). Place the crude product in a 750 ml. round-bottomed flask fitted with a reflux double surface condenser add 325 ml. of light petroleum, b.p. 60-80°, and heat in a water bath maintained at 75-80°. Shake the flask frequently and after about 15 minutes, slowly decant the clear hot solution into a beaker set in a freezing mixture of ice and salt, and stir constantly. The p-iodoaniline crystallises almost immediately in almost colourless needles filter and dry the crystals in the air. Return the filtrate to the flask for use in a second extraction as before (2). The yield of p-iodoaniline, m.p. 62-63°, is 60 g. 

The way the chemist knows that she has methylamine and not ammonium chloride is that she compares the look of the two types of crystals. Ammonium chloride crystals that come from this reaction are white, tiny and fuzzy. The methylamine hydrochloride crystals are longer, more crystalline in nature and are a lot more sparkly. The chemist leaves the methylamine crystals in the Buchner funnel of the vacuum filtration apparatus and returns the filtrate to the distillation set up so it can be reduced one last time to afford a second crop. The combined methylamine hydrochloride filter cake is washed with a little chloroform, scraped into a beaker of hot ethanol and chilled. The methylamine hydrochloride that recrystallizes in the cold ethanol is vacuum filtered to afford clean, happy product (yield=50%). 

Naphthalenediol. This diol is prepared by the alkah fusion of 2-hydroxynaphthalene-6-sulfonic acid (Schaffer acid) at 290—295°C. Schaffer acid is usually produced by sulfonation of 2-naphthol with the addition of sodium sulfate at 85—105°C. This acid is also used as a coupling component in the production of a2o dyes such as Acid Black 26. 2,6-Naphthalenediol is used as a component in the manufacture of aromatic polyesters which, as is also tme of the corresponding amides, display Hquid crystal characteristics (52). 

Naphthalenedicarboxylic Acid. This dicarboxyhc acid, a potential monomer in the production of polyester fibers and plastics with superior properties (105), and of thermotropic Hquid crystal polymers (106), is manufactured by the oxidation of 2,6-dialkylnaphthalenes (107,108). 

Low Soda Hydroxide. Tlie Na20 content of nomial Bayer hydroxide is around 0.2—0.4%, 0.1% of wliich can be removed by thorough wasliing. Tlie remaining soda is trapped within the hydroxide crystal. Experience shows that the occluded soda content is reduced when cry staUization is carried out under low aluniina-supersaturation conditions and at relatively higher temperatures (80—95°C). Soda contents as low as 0.05% Na20 can be obtained by tliis procedure. However, these conditions also reduce hydroxide ield and thus increase the production cost. Low soda aluminum hydroxide is generally employed in the production of aluminas for the ceramics industries. 

Solubility. An important aspect of sihca chemistry concerns the sihca— water system. The interaction of the various forms of sihca with water has geological significance and is apphed in steam-power engineering where the volatilization of sihca and its deposition on turbine blades may occur (see Power generation), in the production of synthetic quartz crystals by hydrothermal processes (qv), and in the preparation of commercially important soluble sihcates, coUoidal sihca, and sihca gel. 

Nitrite is usuaUy one indicator of the infection level in the diffuser. Exposure of nitrite to sulfur dioxide, either as a diffusion additive or later to thin juice, results in the production of potassium imidodisulfonate which precipitates when sugar is later crystallized, a cause of turbid or cloudy sugar. 

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