Crystallizers circulating liquor

An Oslo surface-cooled crystallizer is illustrated in Fig. 18-71. Supersaturation is developed in the circulated liquor by chilling in the cooler H. This supersaturated liquor is contacted with the suspension of ciystals in the suspension chamber at E. At the top of the suspension chamber a stream of mother hquor D can be removed to be used for fines removal and destruction. This feature can be added on either type of equipment. Fine ciystals withdrawn from the top of the suspension are destroyed, thereby reducing the overall number of ciys-tals in the system and increasing the particle size of the remaining product ciystals. 

Figure 3.3 Agitated crystallizers (/) Swenson DTB Draft tube and baffle), (ii) Forced circulation (a) with and (b) without external heat exchange, in) Krystal Oslo a) circulating liquor b) circulating slurry...

Circulating liquor crystallisers and circulating magma crystallisers are used for the large-scale production of a wide range of crystal products. 

Circulating liquor Production of uniform crystals (smaller size than circulating magma). High throughputs. Gypsum, inorganic salts, sodium and potassium nitrates, silver nitrates... 

In circulating-liquor crystallizers, such as the Krystal-Oslo unit, supersaturation is created in what is essentially clear liquor. This clear liquor is then transported to the crystal bed where the supersaturation is released. 

In a circulating-liquor crystallizer, as the liquor flashes in the vapor head, it becomes supersaturated (path a-b). Ideally, the liquor will remain at a constant supersaturation (b-c) as it is transported to the seed bed. Spontaneous nucleation, however, will tend to lower the supersaturation (b-d). The remaining supersaturation is released on the crystals in the seed bed (d-e). As hot feed is mixed with the recirculating liquor, the mixture becomes unsaturated (e-a). The cycle recommences as the liquor is transferred to the vapor head. 

In circulating liquor crystallizers, only the liquor is circulated through the heating or cooling equipment the crystals are retained in suspension in the crystallizing zone by... 

The decrease in temperature due to flashing is typically less than 4 to 6°F, and the increase in solute concentration in me circulating liquor is often around 1 to 3 g/L solvent. Care must be taken to ensure that the liquid velocities in the tapered cross-section of the lower body allow classification of the solids. One must know the settling rates and morphologies of the crystals for proper design and operation. An unclassified operation will perform as an FC unit. 

Many of these classes are self-explanatory, but some require definition. For example, the term controlled refers to supersaturation control. The term classifying refers to the production of a selected product size by classification in a fluidized bed of crystals. In a circulating-liquor crystallizer the crystals remain in the crystallization zone only the clear mother liquor is circulated, e.g., through a heat exchanger. In the circulating-magma crystallizer the crystals... 

Figure 8.45. Oslo-Krystal evaporator types a) waisted, (b) and (c) monolithic. A and B, circulating liquor outlet and inlet C, vapour outlet D, crystal magma outlet E, liquor overflow. After Bamforth, 1965)...

There are two basic types, the waisted (a) and the monolithic (b and c). One advantage of the latter is that the conical downcomer, which joints the vaporizer and crystallizer sections and contains highly supersaturated (metastable) solution, is insulated from external conditions by mother liquor within the crystallizer. The circulating liquor inlet and outlet, A and B, are connected through a heat exchanger and pump. Liquor velocities of 1.5-2ms are commonly used through the tubes to minimize crystal depositions. 

Amongst the first quantities that may have to be specified, or calculated, in a design procedure are the crystal production rate, a clearly defined mean product crystal size, the crystallizer working volume, liquor circulation rate, magma density, mass of crystals in the suspension and the residence times of both crystals and liquor. Several of these quantities are interrelated and will not need separate calculation. Some are directly linked to the effective overall crystal growth rate, a sensitive system-specific quantity. 

Note Purchaser should inform the manufacturer about the solubility of the (dissolved) sohd in the circulating liquor at different temperatures tendency to crystallise out and abrasive nature of the crystals. This is very important for selecting the MOC and size of the tubes in the calendria and the type of impellers of the circulation pump. 

The FC crystallizer contains an external circulation loop through a heat exchanger and may also contain an evaporation zone within the main vessel with crystals removed at its base, as shown in Figure 3.3 (ii). Since both crystals and liquor circulate through the pump, secondary nucleation rates and crystal breakage are high. The product crystals are typically in the size range 200-500 pm. 

Ammonium sulfate is produced as a caprolactam by-product from the petrochemical industry, as a coke by-product, and synthetically through reaction of ammonia with sulfuric acid. Only the third process is covered in our discussion. The reaction between anunonia and sulfuric acid produces an ammonium sulfate solution that is continuously circulated through an evaporator to thicken the solution and to produce ammonium sulfate crystals. The crystals are separated from the liquor in a centrifuge, and the liquor is returned to the evaporator. The crystals are fed either to a fluidized bed or to a rotary drum dryer and are screened before bagging or bulk loading. 

After the end of the fermentation (28 hours) the culture broth is filtered off by suction over a large suction filter. The mycel residue is washed with water several times. The filtrate is extracted three times, each time with 10 liters of methyl isobutyl ketone. The extract is concentrated under vacuum in a circulating evaporator and in a round flask carefully dried under vacuum. The residue is crystallized from acetone/isopropyl ether. The melting point is 157°-158°C (fermentation yield = 60%). The pure product yield obtained after a second crystallization and chromatography of the mother liquor on silica gel amounts to 53% of the theoretical. 

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