Studies in industrial crystallisation

Bain, John (1961) Studies in industrial crystallisation. PhD thesis, University of Glasgow.

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An Investigation was made of the crystallising properties of sodium chloride, sodium thiosulphate alpha-pentahydrate, and barium hydroxide octahydrate, in fluidised bed crystallisers. The operational proceedure was either to grow batches of crystals under known degree of supersaturation to determine rate of growth, or to grow crystals continuously to determine crystal size and quality. It was found that the diffusive presentation of solute to the crystal surface was, in general, slower than the rate of surface reaction. This was probably caused by the low relative velocity of crystal and solution in fluidised bed crystallisation, and by the imperfections in structure produced by the collisions in the fluidised bed. Typical mass transfer coefficients are 80, 4, and 50 gms/cm2 x hr. x gm/cc for the three solutes, in the order given above.The crystals showed a resistance to growth below supersaturations of the order of 0.5 gms. litre, which agrees with the work of Burton and Cabrera, (Disc. of the Far. Soc., No.5, "Crystal Growth", 1945) who showed that a lower metastable limit for growth is possible. The effect of temperature on growth rate was found to vary with the solute. Sodium chloride growth rate increased with temperature, while sodium thiosulphate pentahydrate and barium hydroxide octahydrate growth rates remained almost constant with temperature increase. Under production conditions in the crystalliser, at some specific working temperature, the metastable limits for bulk nucleation are 1.6, 50, and 2.8 gms/litre, supersaturation coefficients of 1.005, 1.048, and 1.025) for the three solutes. Just before supersaturation reaches the metastable limit the crystals become covered with protruberances which probably signify dendritic growth, and poor crystal structure. The measured rates of crystal growth showed an adequate degree of repeatability; and it was possible to scale up results from a one inch diameter batch cooling crystalliser up to a one foot diameter continuous evaporative crystalliser. The product crystal for each solute was greater than mesh under continuous production conditions, with no dust present. The crystals were hard, tended to be spherical, and had a good luster. Production rates of 2 m.n. crystals were of the order of 40 lbs/hr. x ft2 of crystalliser cross-sectional area, provided a suitable height of fluidised bed was available.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Additional Information: Adviser: F Rumford
Keywords: Chemical engineering
Date of Award: 1961
Depositing User: Enlighten Team
Unique ID: glathesis:1961-73541
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 14 Jun 2019 08:56
Last Modified: 14 Jun 2019 08:56

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