1. CHEN 4470 – Process Design Practice Dr. Mario Richard Eden Department of Chemical Engineering Auburn University Lecture No. 5 – Synthesis of Mass Exchange Networks I January 24, 2008 Mass Integration

2. Mass Exchange Networks 1:7

3. Mass Exchange Networks 2:7 What do we know? –Number of rich streams (N R ) –Number of process lean streams or process MSA’s (N SP ) –Number of external MSA’s (N SE ) –Rich stream data Flowrate (G i ), supply (y i s ) and target compositions (y i t ) –Lean stream (MSA) data Supply (x j s ) and target compositions (x j t ) 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) –N SP Process MSA’s –N SE 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 N S = N SP + N SE

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. 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 = N R + N S – N i Number of independent subproblems into which the original synthesis problem can be devided. USUALLY N i = 1

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

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 (S 1 ) The additives mixing column can be used as an absorption column by bubbling the gaseous waste into the additives –Liquid Catalytic Solution (S 2 )

18. Example No. 1 4:14 The Process MSA’s (Continued) The External MSA (S 3 ) –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 (S 3 ) (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. Example No. 1 9:14 Constructing the Pinch Diagram (Continued) –Plot the two composite curves on the same diagram Pinch Point y = 0.001 x 1 = 0.003 x 2 = 0.001 Excess Capacity of Process MSA’s (5.2 – 3.8)*10 -4 = 1.4*10 -4 kgmole benzene/s External MSA Load 1.8*10 -4 kgmole benzene/s

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

25. Example No. 1 11:14 Removing Excess Capacity (Continued) –Graphical identification of x 1 out

26. Example No. 1 12:14 Identifying the Optimal Value of ε 1 –Pinch diagram for ε 1 = 0.002 External MSA Load Increased from 1.8 to 2.3*10 -4 kgmole benzene/s Thus optimal value of ε 1 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 S 3 L 3 = 0.0234 kgmol/s Optimal outlet composition of S 3 X 3 out = 0.0085 Minimum TAC $41,560/yr

29. Next Lecture – January 29 –Finalize graphical mass integration techniques –Introduce algebraic mass integration techniques –SSL pp. 367-383 Other Business