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Published byFredrick Hanken Modified over 2 years ago

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Thermodynamics of separation What is the minimum work to separate a mixture into it’s pure components? Ex. Mining, Desalination, Material Purification, Recycling.

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Balance Eq’ns for Mass, Energy & Entropy S irr

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Minimum Work of Separation

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Gibbs Free Energy of Mixing* g o mix = h o mix – T 0 s o mix. g o mix – T 0 s mix = – T 0 (s 12 – x 1 s 1 – x 2 s 2 ) For non-interacting molecules entropy can dominate often resulting in a negative Gibbs Free Energy and hence spontaneous mixing. I.e. g o mix < 0 * at standard conditions

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S = k ln Boltzmann’s entropy equation How many ways can “r” atoms be positioned in a lattice with “n” locations?

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w min = T 0 s mix = k T 0 (ln 12 ) Ex. 4 atoms in 8 locations

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Using Stirling’s Approximation Where x is mol fraction r/n, and R = k N avo ln N! = N ln N - N

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Multi-component System

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“Separation”

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“Extraction”

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Separation Examples From the atmosphere From the Ocean Solutions –Polymer –Water based –Liquid metals (activity coef)

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The minimum work to separate O 2 from the atmosphere In wet air you get 3.97 kJ/mol : compare with Szargut Table from the EngineeringToolBox.com

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energy requirements for mining and milling, possible future trends Chapman and Roberts p 113 & 116 underground ~ 1000/g (MJ/t metal) open pit ~ 400/g (MJ/t metal)

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Sherwood plot showing the relationship between the concentration of a target material in a feed stream and the market value of (or cost to remove) the target material [Grübler 1998].

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Exergy of a Mixture

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CRUST at T o, p o Ore value at mine Pure ore (e.g. Fe 2 O 3 ) Pure metal Metal alloy Mixing in product Mixing in waste stream Further mixing and corrosion Exergy Purification Stages Recycle to pure metal Theoretical Exergy Values for a metal extracted from the earth ’ s crust shown at various stages of a product life cycle (not to scale)

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