1. Mass Balances using Microsoft Excel Spreadsheets
EngrD 219
Friday, 21 Sep 2007

2. Why use a spreadsheet? Easy to modify Perform complex balances
Interconnect many balances / variables
Use iterative logic to solve problems
Generate graphical data easier

3. Intro to Excel

4. Intro to Excel Excellent basic introduction to Excel:

5. Example 1 Product water ethanol Evaporator Grain Extract water ethanol
Waste
water
ethanol
50 kg/min Grain Extract (3wt% EtOH)
Product contains 99% of feed EtOH
Product is 50wt% EtOH

6. Step 1
Enter process specifications into a central area on the flowsheet

7. Step 1

8. Step 2
Draw the flowsheet

9. Step 2

10. Step 3
Label your streams

11. Step 3

12. Step 4 Start with what you know Don’t just type in values!
Use formulae
Use references

13. Step 3

14. Step 4 (cont’d) Example: Grain Extract Total Flow Rate
Type “=C4” into cell E11

15. Step 4

16. Step 4 (cont’d) Example: Grain Extract Total Flow Rate
Type “=C4” into cell E11
Ethanol Flow Rate
Type “=C5*E11” into cell E10

17. Step 4

18. Step 4 (cont’d) Example: Grain Extract Total Flow Rate
Type “=C4” into cell E11
Ethanol Flow Rate
Type “=C5*E11” into cell E10
Water Flow Rate
Type “=(1-C5)*E11” into cell E9

19. Step 4

20. Step 4 (cont’d) Example: Grain Extract Total Flow Rate
Type “=C4” into cell E11
Ethanol Flow Rate
Type “=C5*E11” into cell E10
Water Flow Rate
Type “=(1-C5)*E11” into cell E9
OR Type “=E11-E10” into cell E9

21. Step 4

22. Step 4

23. Step 5,6,7… Work through the balances sequentially
Lets calculate the Waste Stream first

28. Ideas about Waste Stream?

29. Neat Tricks! ‘Ctrl ~’ switches from formula to result
e.g. “200” vs. “=10*10*2”
Formula Auditing Toolbar
Trace Precedents

30. Example 2 Ethanol production from grain 2 units: reactor + filter
Specifications:
feed rate = 100 kg/min
fraction of grain in feed = 0.2
solution to grain ratio in slurry = 0.1
fraction of slurry purged = 0.05
Stoichiometry: 1 kg grain  1 kg EtOH + 1 kg water
Note: recycle involved. Go to Tools, Options, Calculation Tab and turn uncheck “Iterate” box, set Calculation to “Manual”

31. Step 1: List Process Specifications

32. Step 2: Draw Flowchart, Step 3: Label Streams

33. Step 4: Develop Solution
Feed: grain flow rate = “=C4*C5” (from given specification)
water flow rate: = “=C4*(1-C5)” (from given specification)
total flow rate = “=C4” (given specification)
Reactor Feed: grain flow rate = “=C13+H23” (feed + recycle)
water flow rate = “=C14+H24” (feed + recycle)
ethanol flow rate = “=H25” (recycle)
total flow rate = “=C15+H26”
Reactor Effluent: grain flow rate = “=(1-I18)*G13” ((1-conversion)*fed grain)
water flow rate = “=G14+(G13*I18/2)”
EtOH flow rate = “=G15+(G13*I18/2)”
Grain Extract: water flow rate = “=K14-O23”
EtOH flow rate = “=K15-O24”

34. Step 4: Develop Solution
Slurry: grain flow rate = “=K13”
water flow rate = “=O22*C6*L14” (specification)
EtOH flow rate = “=O22*C6*L15” (specification)
Purge: grain flow rate = “=O22*$C$7” (specification)
water flow rate = “=O23*$C$7” (specification)
EtOH flow rate = “=O24*$C$7” (specification)
Recycle: grain flow rate = “=O22-S23” (splitter m.b.)
water flow rate = “=O23-S24” (splitter m.b.)
EtOH flow rate = “=O24-S25” (splitter m.b)

35. When recycle formulas are entered, a circular reference is created: (click cancel to keep formula)

36. Circular Reference:

37. Values generated are not the answer; need to iterate

38. Go to Tools, Options, Calculate, and recheck “Iterate” box
In worksheet, press F9 to recalculate cells (iterates once).
Press F9 until cells converge.