Unit Operations Lecture June 2010

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Noah Miles
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1 Unit Operations Lecture June

2 Last time Column Internals; sizing Batch (Rayleigh) Distillation Evaporation Crystallization Overview 2

3 Evaporation Evaporate a solvent from a solution to concentrate the solution; recover the solvent, or as a preparation for further downstream processing (like crystallization or distillation) Typical is removal of water or other solvents from: o Aqueous sugar (or fructose) solutions o Salt solutions o Acid/basic solutions (i.e., sulfuric acid or NaOH) o Glycerol (glycerine) solutions o Glue o Milk o Fruit juices o Vegetable extracts o Pharmaceuticals o Fine chemicals Things to consider: o Solute concentration (initial and final) o Solubility of the solute o Temperature sensitivity (time/temperature) o Foaming/frothing o Required ed T and P o Scale formation / Materials of Construction (MOC) 3

4 Equipment Types 4

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8 Equipment Information Perry s 8 th ed. 8

9 Equipment Information Perry s 8 th ed. 9

10 Equipment Information Perry s 8 th ed. 10

11 Equipment Information Perry s 8 th ed. 11

12 Equipment Types 12

13 Boiling Point Elevation Solute Soueinsoluble, e,colloidal o suspension: s :No boiling gpoint change geof the esolvent Solute soluble: osolute non volatile boiling point elevation (colligative) i osolute volatile boiling point elevation/or decrease 13

14 Solute Souesoube, soluble, non volatile: voa Boiling Point Elevation ocolligative property, i.e. only depends on the number of dissolved particles but not their composition; (from thermodynamics or charts) ofor ideal dilute solution the boiling point rise is given by: T K b x dissolved R T hh 2 s vap s x dissolved where: K b R h s T s vap x dissolved is the so called ebullioscopic constant is the gas constant is the boiling point of the solvent is the enthalpy of (heat of) vaporization of the solvent is the mole fraction of dissolve particles (moles of dissolved particles per mole of solution, i.e. solvent for dilute solutions) [i.e. NCl NaCl would have 2 moles of dissolved d particles per mole of salt, since it fully dissociates] i 14

15 Solute Souesoube, soluble, non volatile: Solute soluble, and slightly volatile: odetermine boiling point elevation from Txy diagram or p sat H O xh O H OP H 2 O Boiling Point Elevation 15

16 Evaporator Model Assumptions: Feed (thin-liquor) only has one volatile solvent Only vaporization energy comes from latent heat of vaporization i of the steam Contents are well mixed Heat transfer driving force is difference between the steam temperature and the boiling solution temperature TT T s T p The P is found by knowing the boiling point elevation of the solution and that T v T p T e Evaporator is well insulated solution temperature Seader & Henley,

17 Evaporator Model Mass balance (mass units used) m f mv m p Mass balance on solute w f m f w m v v w p m Energy balance on liquid solution m f H f Q m H Energy from steam heat Q m sh vap v v p m p H p Heat transfer across the heat exchanger Q U A Q k A T s T p Typical HT Coefficients U (overall) Exchanger W/m 2 K btu/hft 2 o F Short tube vertical Horizontal Long tube vertical Forced circulation Agitated film Geankoplis, 2003 T s T p Seader & Henley,

18 Multiple-Effect Evaporator Systems Seader & Henley,

19 Mechanical Recompression McCabe, Smith, Harriott, 4 th ed. 19

20 Last time Column Internals; sizing Batch (Rayleigh) Distillation Evaporation Crystallization Overview 20

21 Crystallization Cool or evaporate a solution to cause crystal formation of the product in the desired (and uniform): size, shape, and purity. Seader & Henley,

22 Thermodynamics (review) Solid Liquid Equilibrium Seader & Henley,

23 Thermodynamics Seader & Henley,

24 MgSO 4 7 H 2 O Production Seader & Henley,

25 MgSO 4 7 H 2 O Production Seader & Henley,

26 Crystal habits Growth Crystal Size Distribution (CSD) Crystals 26

27 Crystals (Bravais, 1848) Seader & Henley,

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29 Crystal Systems Seader & Henley,

30 NaCl sucrose MgS0 4-7H 2 O 30

31 Includes (for example): Plates Needles Prisms Can depend d on things like growth rates, nucleation, and impurities Batch of crystals described by: Average particle size Characteristic dimension Sphericity Crystal Size Distribution (CSD) Sphericity () Crystals Habits surface area sphere ( with same volume) ) surface area particle ( same volume) 6 D sph VP SA P 1 1 sphere non spherical particle Seader & Henley,

32 Sphericity (exercise) Find the sphericityof it a cubic particle with a side of 5 mm. 6 V P Dsph SAP V cube SA cube L 3 6L 2 V cube V sphere L D sph D sph L 3 6 L L 6L 32

33 Particle Size Measurement Seader & Henley,

34 Particle Size Measurement Screens (US/British Standard Screens) Pictures 34

35 Particle Size Measurement Coulter Counter Laser light scattering Malvern 35

36 Crystal Size Distribution (CSD) Screen analysis (for example) Seader & Henley, Differential screen analysis 50 Cumulative screen analysis Cumula ative wt. % Aperature opening [mm] Cum. wt. % Screen opening [mm] 36

37 Questions? 37

EVAPORATION YUSRON SUGIARTO

EVAPORATION YUSRON SUGIARTO Evaporation: - Factors affecting evaporation - Evaporators - Film evaporators - Single effect and multiple effect evaporators - Mathematical problems on evaporation Principal