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Level 4: General structure of separation system
Cheng-Ching Yu
Dept of Chem. Eng.National Taiwan University
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Separation Systems• Typical reaction/separation structure
Remark: Focus on reactor exit.
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Remark: 1. Liquid separation system: column,extraction, azeotropic distillation etc.
2. Two phase: use reactor as a phasesplitter or cool down to 35 C.
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Steps: 1. Cool down to 35C2. 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.
Heuristic: phase split is cheap.
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• 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.
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Type: condensationabsorptionadsorptionmembranereaction system (removing CO2, H2S)
Strategy: If vapor flow after the phase split is small, we can consider combining vapor/liquid recovery together.
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• Liquid separation system
• Light ends
Method- drop the pressure for removal- partial condenser- pasteurization column- stabilizer column
Destination- vent- flare- vapor recovery system
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• Azeotropes- recycle the azeotropes- split the azeotrope (need two columns for abinary azeotrope).
• Applicability of distillationWhy- 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
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Basic principle of distillation
Principle
- use boiling point differencefor separation
- described by vapor liquid equilibrium (VLE)
VLE- Txy Diagram
- x1 and y1 in equilibrium
xLK=xHKyLK>yHK
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Effect of Pressure
- keep the pressure low
P2 > P1
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Column Configuration
input heatat higher T
remove heatat lower T
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Column Notation- EMO
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Realistic View
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Minimum Reflux Ratio
Understand the effects of changing reflux rate or boilup rate.
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Total Reflux
Find minimum numberof trays (Nmin)
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Capital cost vs operating cost (energy)
RR= 1.1~1.2 RRmin
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Typical Procedure to Set Column Pressure
Remark: Prefer using cooling water as cooling media.
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Column sequencing
Definitions
- single feed- two products- key components areadjacent in boiling pt.
- column has a condenserand a reboiler
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Direct vs Indirect (LOF vs HOF)
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Two Alternatives for NC= 3
/
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Six Alternatives for NC= 4
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14 Alternatives for NC= 5
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Summary
- # of alternatives increasesdrastically
- how do we screen for the bestfew sequences
- what criteria should we use tochoose between alternative sequences
… 104862
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Heuristics
Ex.A 33% B 33%C 33%(8/2/1)
Ex.A 15% B 15%C 70%(4/2/1)
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Quantitative Measure
Calculate vapor ratesfrom short-cut method:
1. Underwood Eq. (RRmin)2. RR=~1.1 RRmin3. V=(1+RR)D4. Repeat 1-3 for all columns
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Ternary System- boundary when Qdirect=Qindirect
DI
LKIK
HK
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• Complete picture for HDA
light ends (H2/CH4) 3.8%product (Benzene) 69.3% reactant (Toluene) 25.7%heavy ends (Diphenyl) 1.2%
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• Complex column
LOF HOF
SSR SSS
PF
SS
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Azeotropic Systems
Minimum-boiling
repulsion
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Azeotropic Systems-cont
Maximum-boiling
attraction
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Activity Coefficient- Wilson, NRTL, UNIQUAC, UNIFAC
repulsionvs
attraction
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Two-liquid phase
Repulsion force is extremely large
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Two-liquid phase
(a) is a typical system
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Two-liquid phase- effect of temperature
Effects of temperature (for which case?)
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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)
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Pressure Swing
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Entrainer Addition
Remark: 1. Must change gamma or the ratio of them.2. Can possibly induce phase splitting.3. Too many choices!
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Design Procedure
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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)
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References
Books1. 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.
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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
