Industrial chemical crystallization unit

01

Pick the crystallizer type for your crystal size

GEA describes solution (mass) crystallization as forming solid crystals from a supersaturated solution to recover the dissolved substance in high purity and the desired granulometry, with achievable particle size from microns to millimetres depending on crystallizer type. It lists cooling, evaporative-cooling, evaporative and reaction crystallization as process options. [1]

GEA characterises its three solution crystallizer types this way: the forced-circulation (FC) crystallizer is simple in design and robust in operation; the draft-tube-baffle (DTB) crystallizer gives limited attrition and efficient fines destruction to produce coarse crystals with a narrow size distribution; and the OSLO fluidized-bed crystallizer grows the largest crystals at elevated retention time. [1]

On its OSLO page GEA explains that clarified solution carrying fine crystals is drawn from the baffle zone, the fines are dissolved by superheating it in an external heat exchanger, and the supersaturated solution is then guided down the draft tube to fluidise a crystal bed, where supersaturation is relieved through crystal growth. [2] GEA runs test centres and pilot plants for product trials. [1]

Give the supplier the product and solvent, feed concentration, impurities and temperature, required crystal size distribution, purity and moisture, capacity, preferred process (cooling, evaporative, reaction), available steam, cooling water and power, materials of construction, and whether samples can be sent for pilot tests.

02

Control supersaturation, slurry density and crystal size

In a Chemical Processing problem-solving column, crystallization consultant Wayne J. Genck lists the key variables that control crystal size and distribution: slurry density, supersaturation, crystallizer geometry including impeller-to-vessel diameter ratio, impeller tip speed, circulation rate, and pressure and temperature. For scale-up he advises keeping supersaturation and slurry density close to the original, scaling the agitator carefully, matching circulation rate and keeping pressure and temperature the same. [3]

Analysing a DTB borax crystallizer whose active volume had been raised 50% without more circulation, the column explains that the higher temperature rise around the draft tube boosted supersaturation and caused severe encrustation through the circuit; a poorly placed feed pipe let fines short-circuit; and abrasive borax crystals eroded pumps, impellers and heat-exchanger tubes. It states: "For many systems, the fines destruction circuit should have less than 1 wt-% solids." [3]

The column also warns that impulse-line differential-pressure and displacer level instruments are prone to plugging in slurry service, and that lower circulation and slow-velocity zones can cause scaling. [3] Editorial calculation: retention time is active slurry volume divided by product slurry flow, so a 20 m³ active volume discharging 5 m³/h gives about 4 hours; volume and flow are assumed for illustration.

03

DTB, MVR, multiple-effect, vacuum and reaction crystallizers from listed suppliers

Listings include DTB and MVR evaporation crystallizers, multiple-effect evaporation crystallizers, vacuum crystallizers and reaction or continuous crystallizers.

04

Check the DTB temperature rise, slurry density and fines removal

Swenson Technology explains on its draft-tube-baffle (DTB) crystallizer page that a large, slow-moving propeller lifts growing crystals to the boiling surface, where supersaturation is “most intense”, and that an annular baffle zone lets a stream of mother liquor carrying fine crystals settle out and leave the body. The fines are destroyed by heating or by mixing with dilute feed or water before the liquor returns to the propeller suction. [4]

Swenson gives figures a buyer can hold a DTB quotation against. Where feed or heated mother liquor mixes with the circulated magma at the eye of the propeller, the temperature rise is approximately 1 °F, which it says limits the supersaturation rate to very low values. It states that crystallizers of this type “typically operate with a suspension of solids ranging from 25-50% apparent settled volume”. In multiple-stage cooling duty, where cooling of the feed in each stage limits natural slurry density to a few percent, the baffle zone lets the operating slurry density be set by controlling the slurry underflow rate and removing the remaining mother liquor from the baffle section. [4]

Editorial calculation: a 1 °F rise equals about 0.56 °C (1 × 5/9). Ask the supplier for its design temperature rise at the propeller or heat exchanger, the design slurry density and how it is measured, and the underflow and clear-liquor flows, then compare them with the figures above. [4]

05

Crystallizer body, draft tube and agitator, baffle zone, heat exchanger, condenser and instruments

The baffle zone and heat exchanger work as a pair for fines destruction: in GEA's OSLO design, fine crystals drawn from the baffle zone are dissolved by superheating in the external exchanger before the liquor returns. [2]

06

Working volume, agitation, temperature control, heat transfer area and materials for a crystallizer quotation

Working volume and circulation have to be sized together: in the Chemical Processing case, raising a DTB unit's active volume by 50% without more circulation raised the temperature rise and caused severe encrustation. [3]

07

Send the product, duty and test data together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Product: [compound; solvent; impurities]
Feed: [flow; concentration; temperature]
Crystals: [size distribution; purity; moisture]
Process: [cooling; evaporative; reaction; capacity]
Utilities: [steam; cooling water; power]
Tests: [samples for pilot trials; acceptance criteria]

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