CHEN 4470 – Process Design Practice

1. Mass Integration CHEN 4470 – Process Design Practice Dr. Mario Richard EdenDepartment of Chemical EngineeringAuburn University Lecture No. 8 – Synthesis of Mass Exchange Networks I February 5, 2013

2. Mass Exchange Networks 1:7

3. Mass Exchange Networks 2:7 • What do we know? • Number of rich streams (NR) • Number of process lean streams or process MSA’s (NSP) • Number of external MSA’s (NSE) • Rich stream data • Flowrate (Gi), supply (yis) and target compositions (yit) • Lean stream (MSA) data • Supply (xjs) and target compositions (xjt) • Flowrate of each MSA is unknown and is determined as to minimize the network cost

4. Mass Exchange Networks 3:7 • Synthesis Tasks • Which mass-exchange operations should be used (e.g., absorption, adsorption, etc.)? • Which MSA's should be selected (e.g., which solvents, adsorbents, etc.)? • What is the optimal flowrate of each MSA? • How should these MSA's be matched with the rich streams (i.e., stream parings)? • What is the optimal system configuration?

5. Mass Exchange Networks 4:7 • Classification of Candidate Lean Streams (MSA’s) • NSP Process MSA’s • NSE External MSA’s • Process MSA’s • Already available at plant site • Can be used for pollutant removal virtually for free • Flowrate is bounded by availability in the plant • External MSA’s • Must be purchased from market • Flowrates determined according to overall economics NS = NSP + NSE

6. Mass Exchange Networks 5:7 • Target Compositions in the MSA’s • Assigned by the designer based on different considerations • Physical • e.g., maximum solubility of the pollutant in the MSA • Technical • e.g., to avoid excessive corrosion, viscosity or fouling • Environmental • e.g. to comply with environmental regulations • Safety • e.g. to stay away from flammability limits • Economic • e.g., to optimize the cost of subsequent regeneration of MSA

7. Number of independent subproblems into which the original synthesis problem can be devided. USUALLY Ni = 1 Mass Exchange Networks 6:7 • The Targeting Approach • Based on identification of performance targets ahead of design and without prior commitment to the final network configuration • Minimum Cost of MSA’s • Any design featuring the minimum cost of MSA's will be referred to as a minimum operating cost "MOC" solution • Minimum Number of Mass Exchange Units U = NR + NS – Ni

8. Two of the most important equations to remember in mass integration!! Mass Exchange Networks 7:7 • Corresponding Composition Scales

9. The Pinch Diagram 1:6 • Amount of Mass Transferred by Rich Streams

10. The Pinch Diagram 2:6 • Constructing Rich Composite using Superposition

11. The Pinch Diagram 3:6 • Amount of Mass Accepted by Process MSA’s

12. The Pinch Diagram 4:6 • Constructing Lean Composite using Superposition

13. The Pinch Diagram 5:6 • Constructing the Pinch Diagram • Plot the two composite curves on the same diagram Pinch Point Move the lean composite vertically until the entire stream exists above the rich composite. The point closest to the rich composite is the Pinch.

14. The Pinch Diagram 6:6 • Decomposing the Synthesis Problem • Creates two subregions, i.e. a rich end and a lean end • Above the Pinch • Mass exchange between rich and lean process streams • No external MSA’s required • Below the Pinch • Both process and external MSA’s are used • If mass is transferred across the pinch, the lean composite moves upward, thus: • DON’T TRANSFER MASS ACROSS THE PINCH!

15. Example No. 1 1:14 • Benzene Recovery from Polymer Production

16. Example No. 1 2:14 • Rich Stream Data • Candidate MSA’s • Two process MSA’s • One external MSA

17. Example No. 1 3:14 • The Process MSA’s • Additives (S1) • The additives mixing column can be used as an absorption column by bubbling the gaseous waste into the additives • Liquid Catalytic Solution (S2)

18. Example No. 1 4:14 • The Process MSA’s (Continued) • The External MSA (S3) • Organic oil, which may be regenerated by flash sep. • Operating cost is $0.05/kgmol of recirculating oil

19. Example No. 1 5:14 • The External MSA (S3) (Continued)

20. Example No. 1 6:14 • Constructing the Pinch Diagram • Constructing the rich composite curve

21. Example No. 1 7:14 • Constructing the Pinch Diagram (Continued) • Constructing the lean composite curve

22. Example No. 1 8:14 • Constructing the Pinch Diagram (Continued) • Constructing the lean composite curve

23. Excess Capacity of Process MSA’s (5.2 – 3.8)*10-4 = 1.4*10-4 kgmole benzene/s Pinch Point y = 0.001 x1 = 0.003 x2 = 0.001 External MSA Load 1.8*10-4 kgmole benzene/s Example No. 1 9:14 • Constructing the Pinch Diagram (Continued) • Plot the two composite curves on the same diagram

24. Example No. 1 10:14 • Removing Excess Capacity • Infinite combinations of L1 and x1out capable of removing the excess • Additives column will be used for absorption, thus all of S1 (0.08 kgmole/s) should be fed to this unit.

25. Example No. 1 11:14 • Removing Excess Capacity (Continued) • Graphical identification of x1out

26. Example No. 1 12:14 • Identifying the Optimal Value of ε1 • Pinch diagram for ε1 = 0.002 External MSA Load Increased from 1.8 to 2.3*10-4 kgmole benzene/s Thus optimal value of ε1 is the feasible minimum, i.e. 0.001

27. Example No. 1 13:14 • Remaining Problem (Below the Pinch) • Optimizing the use of external MSA’s

28. Example No. 1 14:14 • Remaining Problem (Below the Pinch) • Optimizing the use of external MSA’s Key Results Optimal flowrate of S3 L3 = 0.0234 kgmol/s Optimal outlet composition of S3 X3out = 0.0085 Minimum TAC $41,560/yr

29. Other Business • Next Lecture – February 7 • Finalize mass exchange network synthesis • SSLW pp. 297-308 • Progress Report No. 1 • Due Friday February 8 • Remember to fill out team evaluation forms