Seader & Henley, Separation Process Principles f01_11

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

Solubility curves in crystallization

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

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)

Swenson-walker scraped surface crystallizers f17_15

Crystallization theory - Reaching supersaturation Kelvin Eqn. 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

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)

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

Crystallization theory Crystal growth Mass flux from bulk to surface i: k y from typical correlations y A is surface concentration Surface reaction is c-dependent: k S from reference

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

Model for mixed suspension-mixed product removal crystallizer (MSMPR) Crystal population-density obtained experimentally using screens. Sieve fractions are weighed, between two sieves where L AV =(L 1 +L 2 )/2 and ΔL = L 1 -L 2 (upper screen lower).

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)

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 n 0 : Average particle size (50% smaller, 50% larger): Predominant particle size: Nucleation rate:

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 B 0 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 B 0 Continue until desired W f distribution or L d is obtained

Example: Growth and nucleation in MSMPR