1. CH EN 5253 – Process Design II
Heat Integration
February 22, 2019

2. Books
Product and Process Design Principles: Synthesis, Analysis and Evaluation
by Warren D. Seider , J. D Seader, Daniel R. Lewin, and Soemantri Widagdo
Chapter 9 ( 3rd Edition)
Chemical Engineering Design: Principles, Practice and Economics of Plant and Process Design
by Gavin Towler, and Ray Sinnott
Section 3.5 ( 2nd Edition)

3. Minimum Utility Target
Temperature Interval (TI) Method
Graphical Method: Composite heating and cooling curves
Linear Programming (LP) method

4. inequality constraints
Linear Programming
objective function
equality constraints
inequality constraints
w.r.t. design variables
The ND design variables, d, are adjusted to minimize f{x} while satisfying the constraints

5. EXAMPLE LP – GRAPHICAL SOLUTION
A refinery uses two crude oils, with yields as below.
Products
Volumetric Yields
Max. Production
Crude #1 (%)
Crude #2 (%)
(bbl/day)
Gasoline 70 31
6,000
Kerosene 6 9
2,400
Fuel Oil 24 60
12,000
The profit on processing each crude is:
Crude #1: $2/bbl
Crude #2: $1.4/bbl
What is the optimum daily processing rate for each grade?
What is the optimum if 6,000 bbl/day of gasoline is needed?

6. EXAMPLE LP –SOLUTION (Cont’d)
Step 1. Identify the variables.
Let x1 and x2 be the daily production rates of Crude #1 and Crude #2.
Step 2. Select objective function. We need to maximize profit:
Step 3. Develop models for process and constraints.
Only constraints on the three products are given:
Step 4. Simplification of model and objective function.
Equality constraints are used to reduce the number of independent variables (ND = NV – NE). Here NE = 0.

7. EXAMPLE LP –SOLUTION (Cont’d)
Step 5. Compute optimum.
Inequality constraints define feasible space.
Feasible Space

8. EXAMPLE LP –SOLUTION (Cont’d)
Step 5. Compute optimum.
Constant J contours are positioned to find optimum.
x1 = 0, x2 = 19,355 bbl/day
J = 27,097
J = 20,000
J = 10,000

9. EXAMPLE LP –SOLUTION (Cont’d)
A refinery uses two crude oils, with yields as below.
Volumetric Yields
Max. Production
Crude #1
Crude #2
(bbl/day)
Gasoline 70 31
6,000
Kerosene 6 9
2,400
Fuel Oil 24 60
12,000
The profit on processing each crude is: $2/bbl for Crude #1 and $1.4/bbl for Crude #2.
What is the optimum daily processing rate for each grade? 19,355 bbl/d
What is the optimum if 6,000 bbl/day of gasoline is needed?
0.7*x1+0.31*x2=6,000, equality constraint added *19,355=6,000

10. Application of Linear Programming in HEN

11. Linear Programming (LP) method
Qsteam - R = ….. (LP.1)
R1 - R = ….. (LP.2)
R2 - R = ….. (LP.3)
R3-R4+75= ….. (LP.4)
R4-Qcw-15= ….. (LP.5)
Qsteam ,Qcw , R1 , R2 , R3 ,R4  Constraints
Qsteam=0, R3=-50

12. Linear Form
All values are positive

13. Solution Region R2 R1 R4 R3 Feasible Space Q_CW
Qsteam ,Qcw , R1 , R2 , R3 ,R4  Constraints

14. Solution Solution When Qsteam is minimum, Qcw is also minimum
R1 =80
R2 =82.5
R3 =0
R4 =75
Qcw =60

15. Heat Integrated Distillation Trains

16. Effect of ΔTmin on total cost ( Summary)
True pinch is approached
Area of heat transfer  
Utility  minimum
ΔTmin  
Area of heat transfer  0
Utility  maximum
Trade-off between Capital cost and Utility cost

17. Distillation Columns
Energy balance
F HF+ Qreb= D HD+B HB+Qcond

18. Distillation Columns Q  Qreb  Qcond F HF-D HD-B HB 0
If Q is reduced
Utility cost reduced
number of trays / height of packing increased
Tradeoff between operating cost and capital cost

19. Heuristic “Position a Distillation Column Between Composite Heating and Cooling Curves”
When utility cost is high
Adjust pressure level to position T-Q between hot and cold composite curves
Hot streams to reboiler
Cold stream to Condenser
Close boiling point species

20. Heuristic “Position a Distillation Column Between Composite Heating and Cooling Curves”
Difficult to use both hot and cold streams
Hot utility to reboiler
Cold streams to condenser

21. Multi-effect Distillation
When position a Distillation Column Between Composite Heating and Cooling Curves not possible
Feed split
Two towers operating at different pressures

22. Heat Integration for Direct Distillation Sequence

23. Adjust Pressure in C2 for ΔTmin
Good when utility cost high
Down side
Purchase cost for two towers
pump cost
process complexity

24. Variation on two-effect distillation
Feed Splitting (FS)
Light Splitting/
forward heat-integration (LSF)
Light Splitting/
reverse heat-integration (LSR)

25. Heat Pumps How do they work?
Carnot Efficiency
ηmax= 1-Tc/Th
Endoreversible
η =1-√(Tc/Th)
Same as Air Conditioner
Convert low temperature heat to high temperature heat.
Must add work as heat can not go up hill.

26. Heat Pumps in Distillation

27. Heat Pumps/Heat Engines Heurisitcs
When positioning heat engines, to reduce the cold utilities, place them entirely above or below the pinch
When positioning heat pumps, to reduce the total utilities, place them across the pinch.

28. Heat Pump Location

29. Heat Engine Location Tp