Level 4: General structure of separation system Cheng-Ching Yu Dept of Chem. Eng. National Taiwan University ccyu@ntu.edu.tw 02-3365-1759 1
Separation Systems Typical reaction/separation structure Remark: Focus on reactor exit. 2
Remark: 1. Liquid separation system: column, extraction, azeotropic distillation etc. 2. Two phase: use reactor as a phase splitter or cool down to 35 C. 3
Heuristic: phase split is cheap. Steps: 1. Cool down to 35C 2. A. phase split exists == Step 3 B. no phase split (i) pressurize (ii) refrigerated partial condenser (iii) to vapor recovery system 3. Design vapor recovery system and liquid separation systems. 4
Gas recovery systems location: 2 1 3 Location 1: If significant amount of valuable materials are being lost in the purge. Location 2: If materials are deleterious to the reactor operation. Location 3: Both factors are valid. None: Both factors do not exist. 5
Type: condensation absorption adsorption membrane reaction system (removing CO 2, H 2 S) Strategy: If vapor flow after the phase split is small, we can consider combining vapor/liquid recovery together. 6
Liquid separation system Light ends Method -drop the pressure for removal -partial condenser -pasteurization column -stabilizer column Destination -vent -flare -vapor recovery system 7
Azeotropes -recycle the azeotropes -split the azeotrope (need two columns for a binary azeotrope). Applicability of distillation Why -handle wide range of throughput -handle wide range of feed composition -ability to produce high purity product Cases not suited -relative volatility < 1.1 (column can become very expensive) -low molecular weight -high molecular weight heat sensitive materials 8
Basic principle of distillation Principle -use boiling point difference for separation -described by vapor liquid equilibrium (VLE) x LK =x HK y LK >y HK VLE- Txy Diagram - x1 and y1 in equilibrium 9
Effect of Pressure -keep the pressure low P2 > P1 10
Column Configuration remove heat at lower T input heat at higher T 11
Column Notation- EMO 12
Realistic View 13
Minimum Reflux Ratio Understand the effects of changing reflux rate or boilup rate. 14
Total Reflux Find minimum number of trays (N min ) 15
Capital cost vs operating cost (energy) RR= 1.1~1.2 RRmin 16
Typical Procedure to Set Column Pressure Remark: Prefer using cooling water as cooling media. 17
Column sequencing Definitions -single feed -two products -key components are adjacent in boiling pt. -column has a condenser and a reboiler 18
Direct vs Indirect (LOF vs HOF) 19
Two Alternatives for NC= 3 / 20
Six Alternatives for NC= 4 21
14 Alternatives for NC= 5 22
Summary -# of alternatives increases drastically - how do we screen for the best few sequences - what criteria should we use to choose between alternative sequences 10 4862 23
Heuristics Ex. A 33% B 33% C 33% (8/2/1) Ex. A 15% B 15% C 70% (4/2/1) 24
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Quantitative Measure Calculate vapor rates from short-cut method: 1. Underwood Eq. (RRmin) 2. RR=~1.1 RRmin 3. V=(1+RR)D 4. Repeat 1-3 for all columns 26
Ternary System- boundary when Q direct =Q indirect HK I D IK LK 27
Complete picture for HDA light ends (H2/CH4) 3.8% product (Benzene) 69.3% reactant (Toluene) 25.7% heavy ends (Diphenyl) 1.2% 28
Complex column LOF HOF SSR SSS PF SS 29
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Azeotropic Systems Minimum-boiling repulsion 32
Azeotropic Systems-cont Maximum-boiling attraction 33
Activity Coefficient- Wilson, NRTL, UNIQUAC, UNIFAC repulsion vs attraction 34
Two-liquid phase Repulsion force is extremely large 35
Two-liquid phase (a) is a typical system 36
Two-liquid phase- effect of temperature Effects of temperature (for which case?) 37
Breaking Azeotrope We have to change something to overcome azeotrope 1. Change P- pressure swing 2. Change gamma- add entrainer 3. Change y- remove vapor across membrane (pervaporization) 38
Pressure Swing 39
Entrainer Addition Remark: 1. Must change gamma or the ratio of them. 2. Can possibly induce phase splitting. 3. Too many choices! 40
Design Procedure 41
Design Procedure- In practice -leave azeotropes until last -try pressure swing -try extractive distillation using entrainer while not causing phase-split (homogeneous azeotropic distillation) -try adding a (light) entrainer and induce phase-split (heterogeneous azeotropic distillation) 42
References Books 1. Doherty, M. F.; Malone, M. F. Conceptual Design of Distillation Systems, McGraw-Hill, 2001. an up-to-date book on distillation 2. Douglas, J. M. Conceptual Process Design, McGraw-Hill, 1988. a highly readable and useful book in process design 3. Luyben, W. L.; Wenzel, L. A. Chemical Process Analysis, Prentice-Hall, 1988. an introductory book on material & energy balances 4. Smith, R. Chemical Process Design, McGraw-Hill, 1995. philosophy is similar to the Douglas book with somewhat updated materials 5. Smith, R. Lecture Note on Advanced Distillation System Design, Taipei, 1999. 43
References Papers 1. Fien, G-J A.; Liu, Y. A. I&EC Res 1994, 33, 2505. a highly readable review paper on azeotropic distillation 2. Guttinger, T. E.; Morari, M. I&EC Res 1996, 35, 4597. look at stability of homogeneous azeotrpic distillation- typical Morari paper 3. Laroche. L.; Bekiaris, N.; Anderson, H. W.; Morari, M. I&EC Res 1992, 31, 2190. explore design aspect of homogeneous azeotropic distillation 4. Hauan, S.; Westerberg, A. W.; Lien, C. Chem. Eng. Sci. 2000,55,1053. first systematic paper investigate the effect of reaction on RCM 5. Venimadhavan, G.; Malone, M. F.; Doherty, M. F. AIChE J. 1999, 45, 546. also explore the effect of reaction on RCM 44