HW 3 Debriefing

1
HW 3 -Debriefing

• CH EN 5253
• Design II
• Terry A. Ring

2
Reactor/Separator/Recycle

• Reactor
• n-penate ? iso-pentane
• Separator
• Distillation column
• Difficult separation
• a is small (a 1.1)
• Large number of trays
• R large
• Qreboiler and Qcondenser are large
• Recycle Loop
• Where to place Distillation Column in Loop?

3
Case 1
Separation First before Reactor gives Pure
n-pentane to reactor
4
Setting Pressure for Recycle Loop

• Issues to consider
• Pressure at Condenser so that cooling water can
  be used for coolant
• Higher Pressure reduces Vapor flow thus column
  diameter
• Pressure in Reactor
• Le Chatlier (equilibrium) is not effected
• Kinetics are effected
• Ci Pi/(RT)

CW ?T
5
Reactor Product Mole Fraction
Feed is n-pentane
6
Reactor Kinetics
Case 1 Case 2 3
7
Reactor

• Temp 90 C
• Diameter 1 meters
• Length 20 meters

8
Case 1
Separation First before Reactor gives Pure
n-pentane to reactor
9
Distillation Column

• 63 trays
• R29.19
• QRB 6.68e8 kJ/h
• QC 6.74e8 kJ/h

10
Costs (Case 1)
Case I Equipment FOB cost Distillation
Columns (14) 7,712,713.55 Reactor
124,608.39 HX 88,793.49 Pump 7,467.16
TOTAL CAPITAL 7,933,582.59
11
Cost (Case 1)
Stream kJ/h /kg Annual Cost _at_50w/yr
operation Q2 6.74E08 0.0055 13,796,211.26
Q5 1.70E07 0.0055 347,838.65
CW ?T 16.66666667 K CP 4.184
J/(gK) Utilities Units Price/unit Annual Cost
_at_50w/yr operation CW /m3 Q1 6.68E08 0.01
1,046,573.30 Electricity kW /kW-h Annual Cost
_at_50w/yr operation Q3 1.52E-02 0.04 5.12
Total Utilities 15,190,628.33
Reboiler
Condenser
12
Case II
Separation after reactor gives mixed stream to
reactor
13
Reactor Product Mole Fraction
14
Reactor Kinetics
Case 1 Case 2 3
15
Reactor

• Temp 190C
• Diameter 1 meters
• Length 20 meters

16
Distillation Column

• 50 trays
• R35.15
• QRB 8.18e8 kJ/h
• QC 8.33e8 kJ/h

17
Cost (Case 2)
Case II Efficient Distillation Equipment FOB
cost Distillation Columns (18) 7,360,049.85
Reactor 118,545.85 HX 548,024.36
Pump 8,341.45 TOTAL CAPITAL
8,034,961.52
Slightly more than case 1
18
Cost (Case 2)
Stream kJ/h /kg Annual Cost _at_50w/yr
operation Q7 7.46E08 0.0055 15,267,276.79
Q8 4.44E07 0.0055 909,191.30
CW ?T 16.66666667 K CP 4.184
J/(gK) Utilities Units Price/unit Annual Cost
_at_50w/yr operation CW kJ/h /m3 Q6 7.69E08
0.01 1,204,558.75 Electricity kW /kW-h Annu
al Cost _at_50w/yr operation Q4 3.83E-01 0.04
128.72 Total Utilities 17,381,155.56
Reboiler
Condenser
2 million more than case 1
19
Cost Comparison

• Case 1
• FOB equipment
• 7,933,582.59
• Utilities
• 15,190,628.33
• COM
• 19,187,734.41
• BEST

• Case 2
• FOB equipment
• 8,034,961.52
• Utilities
• 17,381,155.56
• COM
• 21,406,531.57

20
Can it be done better?

21
Analysis

• Distillation costs are the largest
• In case 2, why are we distilling to high purity
  in bottom just to mix with impure feed?
• Try an inefficient distillation column in Case 2
  for comparison with Case 1.
• Keep reactor the same as Case 2.

22
Case III
Inefficient Distillation Bottom - impure
Separation after reactor gives mixed stream to
reactor
23
Distillation Column

• 50 trays
• R8.9
• QRB 1.88e8 kJ/h
• QC 2.11e8 kJ/h

Distillation bottoms is same composition as feed
to process.
24
Costs (Case 3)
Equipment FOB cost Distillation Columns (6)
2,453,349.95 Reactor 118,545.85 HX
1,077,748.41 Pump 8,205.70 TOTAL
CAPITAL 3,657,849.91
½ of Case 1
25
Cost (Case 3)
Stream kJ/h /kg Annual Cost _at_50w/yr
operation Q7 1.88E08 0.0055 3,847,738.62
Q8 44124751.6 0.0055 903,218.22
CW ?T 16.66666667 K CP 4.184
J/(gK) Utilities Units Price/unit Annual Cost
_at_50w/yr operation CW /m3 Q6 2.11E08 0.01
330,501.83 Electricity kW /kW-h Annual Cost
_at_50w/yr operation Q4 6.78E-01 0.04 227.72
Total Utilities 5,081,686.39
1/3 of Case 1
26
Cost Comparison

• Case 1
• FOB equipment
• 7,933,582.59
• Utilities
• 15,190,628.33
• COM
• 19,187,734.41

• Case 3
• FOB equipment
• 3,657,849.91
• Utilities
• 5,081,686.39
• COM
• 7,886,487.32
• Best by far

27
Problems

• HX
• Minimum approach temperature negative
• Temperature crossover in HX
• Adjust stream temps to increase ?Tapproach
• Want rating with low pressure drops
• Heat Exchangers are enormous!
• Keep U high so A is lower.
• Increase ?Tlm if possible.
• Distillation column is larger than cost equation
  limits use multiple units of max. size!