seader & henley, separation process...
TRANSCRIPT
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Seader & Henley, Separation Process Principles
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Crystallization (12.11, p.817)
• Solid-liquid separation where solid particles are formed from a homogenous liquid phase
– Ice crystals in freezing water
– Snow flakes from vapor
– Solid particles from a liquid melt (e.g. molten metals, lava)
– Solid crystals (e.g. salt) from aqueous solution • Typical process involves cooling a concentrated solution at a temperature past
the solute solubility limit • Another approach is to add an antisolvent that is miscible with the liquid, but
cannot dissolve the solute (e.g. benzoic acid from ethanol using water) – Supersaturation required for crystallization ---- Controlled by nucleation and growth
– Rate of cooling or antisolvent addition can be used to control the rate of crystal formation
• Yield, purity, uniform particle size, and desired shapes (e.g. needles vs. cubes) – Permit easy powder flows, low caking in packaged product form
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Solubility curves in crystallization
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Crystallizer equipment
Types:
1. Supersaturation by cooling with negligible evaporation – Solubility must have a strong T dependence
2. Supersaturation by evaporation with little or no cooling – No T dependence needed
3. Supersaturation by combined cooling and evaporation (vacuum)
Suspending growing crystals & controlling how the liquid contacts them
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Crystallizer equipment
Configurations:
1. Tank crystallizers – Hot saturated solutions cools in open tank
– Difficult to control nucleation and, hence, crystal size
2. Scraped surface crystallizers – Cooling surface scraped periodically to remove crystals (e.g. ice cream)
3. Circulating-liquid evaporator-crystallizer – Supersaturation generated by evaporation (thermal)
4. Circulating-liquid vacuum crystallizer – Supersaturation generated by evaporation (thermal + vacuum)
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Swenson-walker scraped surface crystallizers
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Crystallization theory - Reaching supersaturation
a = undersaturated b = equilibrium between saturated solution and visible crystals c = supersaturation where crystals grow, but do not nucleate c-e = supersaturation temperature difference d = spontaneous nucleation of small crystals d-f = maximum (limiting) supersaturation Temperature difference
Kelvin Eqn.
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Crystallization theory - Nucleation
• Solubility & crystal size – Small particles, greater surface energy, greater solubility.
• Homogeneous nucleation – Molecules cluster together to form small particles.
– Small particles aggregate to form a nucleus, which can grow
• Contact (heterogeneous) nucleation – E.g. new nuclei formed at reactor walls, by agitator blades, and/or by colliding
crystals
• Commercial setting – Supersaturation is low, agitation is needed to suspend crystals, contact
nucleation is predominant (little homogeneous crystallization)
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Crystallization theory
• Primary – Molecules form a cluster, cluster can grow into a particle, particles
can grow and become a nucleus…
• Secondary – Nucleation caused by presence of existing crystals
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Crystallization theory – Crystal growth
• Mass flux from bulk to surface i: – ky from “typical” correlations
– y’A is surface concentration
• Surface reaction is c-dependent: – kS from reference
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Crystallization theory – Crystal growth
• ΔL law of growth (McCabe) ‒ ΔL is increase in linear length, proportional to
all crystals
– G is a constant (e.g. mm/h)
– Total growth, ΔL, is same for all crystals
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Model for mixed suspension-mixed product removal crystallizer (MSMPR)
• Crystal population-density – obtained experimentally using screens. Sieve fractions are weighed, between two sieves where LAV=(L1+L2)/2 and ΔL = L1-L2 (upper screen – lower).
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Model for mixed suspension-mixed product removal crystallizer (MSMPR)
• Population material balance
– Crystals collected from CSTR (ΔnΔL) in Δt.
– Composition of effluent = that in crystillzer (i.e. composition leaving stage = stage)
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Model for mixed suspension-mixed product removal crystallizer (MSMPR)
• Population material balance
– Particle size and nucleation rate, plot of n vs. L gives G and n0:
– Average particle size (50% smaller, 50% larger):
– Predominant particle size:
– Nucleation rate:
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Model for mixed suspension-mixed product removal crystallizer (MSMPR)
• Population material balance
– Predicting cumulative wt fraction obtained at opening L:
– Process design:
• Experimental G and B0 are obtained by population material balance for a given set of conditions
• Additional experiments performed to determine effect of τ and mixing on G and B0
• Continue until desired Wf distribution or Ld is obtained
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Example: Growth and nucleation in MSMPR
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Example: Growth and nucleation in MSMPR