Unit Operations Lecture 10 23 June 2010 1
Last time Column Internals; sizing Batch (Rayleigh) Distillation Evaporation Crystallization Overview 2
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
Equipment Types 4
5
6
7
Equipment Information Perry s 8 th ed. 8
Equipment Information Perry s 8 th ed. 9
Equipment Information Perry s 8 th ed. 10
Equipment Information Perry s 8 th ed. 11
Equipment Types 12
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
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
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 2 2 2 H 2 O Boiling Point Elevation 15
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, 2006 16
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 1100 2800 200 500 Horizontal 1100 2800 200 500 Long tube vertical 1100 4000 200 700 Forced circulation 2300 11000 400 2000 Agitated film 680 2300 120 400 Geankoplis, 2003 T s T p Seader & Henley, 2006 17
Multiple-Effect Evaporator Systems Seader & Henley, 2006 18
Mechanical Recompression McCabe, Smith, Harriott, 4 th ed. 19
Last time Column Internals; sizing Batch (Rayleigh) Distillation Evaporation Crystallization Overview 20
Crystallization Cool or evaporate a solution to cause crystal formation of the product in the desired (and uniform): size, shape, and purity. Seader & Henley, 2006 21
Thermodynamics (review) Solid Liquid Equilibrium Seader & Henley, 2006 22
Thermodynamics Seader & Henley, 2006 23
MgSO 4 7 H 2 O Production Seader & Henley, 2006 24
MgSO 4 7 H 2 O Production Seader & Henley, 2006 25
Crystal habits Growth Crystal Size Distribution (CSD) Crystals 26
Crystals (Bravais, 1848) Seader & Henley, 2006 27
28
Crystal Systems Seader & Henley, 2006 29
NaCl sucrose MgS0 4-7H 2 O 30
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, 2006 31
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 6 3 3 sph D sph 1. 2407 L 3 6 L 0. 806 2 1.2407L 6L 32
Particle Size Measurement Seader & Henley, 2006 33
Particle Size Measurement Screens (US/British Standard Screens) Pictures 34
Particle Size Measurement Coulter Counter Laser light scattering Malvern 35
Crystal Size Distribution (CSD) Screen analysis (for example) Seader & Henley, 2006 100 Differential screen analysis 50 Cumulative screen analysis Cumula ative wt. % 90 80 70 60 40 30 20 10 Aperature opening [mm] Cum. wt. % 1.400 100 1.180 98.14 1.000 91.60 0.850 83.49 0.600 35.5454 0.425 17.39 0.300 6.31 0.212 1.83 0.150 036 0.36 0.106 0.11 0 0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 Screen opening [mm] 36
Questions? 37