Batch crystallization period

The Investigation was carried out using a seeded, batch crystallization In the absence of nucleatlon. Supersaturated solutions were prepared, seeded and maintained at a constant temperature while crystallization proceeded. Samples were taken periodically to give a solution for analysis and crystals for size analysis and crystal content determination. 

The dependence of average crystal size with time over the initial batch operation period was determined for Runs R0-R3. Hie data was fit to a power-law model of the form... 

For any fixed batch crystallization temperature, the effective nucleation rate passes through a maximum even at high seed densities. It is suggested that the induction period r uired to activate the seed surfaces may be responsible for the lower initial nucleation rate observed when the supersaturation was higher. It is also suggested that agglomeration may have caused the observed phenomenon. 

Synthesis of Zeolite A. Zeolite A was crystallized from a batch of overall composition 2.5 Na20-Al203-1.7 SiO2-150 H20 at 60, 75, and 90°C (10). The same system was seeded with zeolite A crystals of 0.5-5 /zmeter size at an initial conversion level of 25% and crystallized at 60°C. As with the crystallization of zeolite X from seeded systems, the data were treated by ignoring the presence of seed crystals. The crystallization curves are shown in Figure 8. During the induction time period in the unseeded system, crystallization was taking place at a slow rate in the seeded system, both at 60°C. After this slow crystallization period the crystallization rate reached 22% per hour at the 50% conversion level... 

Batch crystallizers can be used in a campaign to produce a particular product and in a second campaign to produce another product. Generally, it is not possible to operate continuous processes in this way. Batch crystallizers can handle viscous or toxic systems more easily than can continuous systems, and interruption of batch operations for periodic maintenence is less difficult than dealing with interruptions in continuous processes. The latter factor may be especially important in biological processes that require frequent sterilization of equipment. Batch crystallizers can produce a narrow crystal size distribution, whereas special processing features are required to narrow the distribu-... 

In the initial stage of a MSMPR precipitator operation, both the solute concentration and the particle number per unit volume increase rapidly. However, due to the rapid growth of these initial nuclei a subsequent rapid desupersaturation occurs. A steady state is established after about 10 mean residence times. On the other hand, in a semi-batch crystallizer, the particle number per unit volume quickly reaches a constant value while the solute concentration continues to decline steadily throughout the feed period as the precipitate growth proceeds. 

For a cooling crystallizer, the various periods of a batch cycle are shown in a plot of temperature versus time (Figure 10.12). These periods include (1) filling the crystallizer (2) cooling to saturation temperature (7 ) (3) crystallization period (4) removing the suspension and (5) cleaning the crystallizer. 

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... 

M Preparation of isopropyiidene peniciiiamine hydrochioride To the filtrate obtained In step (b) is added at 20°C to 25°C a total of 85 g of hydrogen sulfide. The precipitated HgS is filtered off and the filtrate is concentrated under reduced pressure to a volume of 200 to 500 ml. Following e polish filtration, the product-rich concentrate is mixed with 1.5 liters of isobutyl acetate. The mixture is refluxed at about 40 C under reduced pressure in equipment fitted with a water separation device. When no further water separates, the batch is cooled to 30t and filtered. The reactor is washed with 1 liter of acetone, which Is used also to wash the cake. The cake is further washed with 200 ml of acetone. The acetone washes are added to the isobutyl acetate filtrate and the mixture is refluxed for 20 to 30 minutes. After a holding period of one hour at 5°C, the crystals of isopropyiidene penicillamine hydrochloride are filtered and washed with 200 m of acetone. On drying for twelve hours at 25°C this product, containing 1 mol of water, weighs about 178 g (73%). 

Synthesis of Zeolite X. Zeolite X was crystallized from a batch of overall composition 4 Na20-Al203-5 SiO2-200 H20 at 90° C (8). The crystallization curve is shown in Figure 7. After an induction period of 2.4 hours, zeolite X was formed rapidly as a single phase at a conversion... 

Breck (1) was the first to investigate the reaction in the hydrothermal formation of zeolites. He found that there is always some delay before crystallization starts. This so-called induction period can be reduced by raising the temperature or alkalinity of the reaction batch (2). As Sand (8) reported in 1968 in connection with the formation of mordenite, the nature of the Si02 material also has a decisive influence on the reaction and the nature of the zeolite crystals. The induction period as a nucleation phase is discussed by Domine and Quobex (4) in connection with kinetic investigations relating to mordenite formation. 

While in batch operation, the fresh solution is fed into the crystallizer all at once the crystallization process carries on for a certain period, then the suspension is discharged from the crystallizer and is conveyed to the device for solid-liquid separation. For such operation the classifying leg (9), the overflow cofferdam (7), and the pump A are not needed. 

After removal of such tar or gum, the concentrate is further evaporated at a temperature below about 50°C to about one-fourth the volume, i.e., 70 gallons is concentrated to about 15 to 20 gallons. This concentrate is cooled to a temperature of about 0°C to 5°C and allowed to stand for an extended period, such as overnight, whereupon there is a separation of crude crystalline glaucarubin therefrom. The crude crystals thus formed are removed by filtration and the mother liquors again concentrated to about one-half volume and cooled to permit separation of a second batch of crude glaucarubin crystals. The two batches of crude glaucarubin crystals are combined and dried preparatory to further purification. 

The presence of even traces of ethanol in the synthesis batch, stemming from an incomplete hydrolysis of Si(OEt)4 during the ageing period results in a drastical reduction of the ZSM-20 crystallization rate zeolite Beta is then readily formed and achieves a fast growth at the expense of the ZSM-20 "Figure 3".This goes in line with the higher yields of Si-richer Beta observed when this zeolite is intentionally prepared in presence of ethanol (311.This was attributed to the lower solubility of silica in EtOH (311. but it is also probable that the whole system (nature and solubility of ZSM-20 aluminosilicate precursors and intermediates) is perturbed (321. 

When preparing these zeolites, this dual effect must be considered. Any variation from the desired batch composition can present problems if the test is designed to react for a fixed period of time. Variations which slow down the reaction could produce a zeolite which has poor properties simply because it has not completely crystallized. If one has variations which speed up the reaction, one could produce a mixture of zeolite phases. 

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