CHE 425. Engineering Economics and Design Principles. Prof. Adnan Alamer Chemical Engineering Dept., KFUPM.

Transcription

1 CHE 425 Engineering Economics and Design Principles 1

2 CHAPTER 4 Understanding Process Conditions 2

3 The ability to make an economic analysis of a chemical process based on a PFD is not proof that the process will actually work. 3

4 PURPOSE OF CHAPTER 4 Analyze reasons for selecting specific conditions: Temperatures Pressures Compositions Analyze process conditions that require special consideration 4

5 CONDITIONS OF SPECIAL CONCERN It is easier to adjust T and/or P of a stream than to change composition. The following conditions do not cause severe difficulties Temperatures between C Pressures between 1-10 bar Conditions outside the above favored temperature and pressure ranges are identified as conditions of special concern 5

6 PRESSURE Above 10 bar Thicker walled expensive equipment Safety issue 1-10 bar Favorable condition Large equipment Below 1 bar Special construction material Increase in cost 6

7 PRESSURE (cont.) A decision to operate outside the pressure range of bar must be justified 7

8 TEMPERATURE (cont.) 8

9 TEMPERATURE (cont.) 9

10 TEMPERATURE (cont.) A decision to operate above 400 C must be justified 10

11 TEMPERATURE (cont.) Below 40 C C Above 260 C Require refrigeration Okay Require special heaters 11

12 TEMPERATURE (cont.) T < 40º C Refrigeration Use as much Cooling Water as Possible CW RW Operating Costs(Table 3.4) Cooling Water Refrigerated Water $0.16/ GJ $20.60/ GJ 12

13 Do we ever operate outside these limits? Tables Reactors and Separators Table 4.4 Other Equipment DME Process pretty much inside limits 13

14 REASONS FOR OPERATING AT CONDITIONS OF SPECIAL CONCERN Favorable equilibrium conversion Increase in reaction rates k reaction = k e 0 E act RT Maintain a gas phase Improve selectivity 14

15 Table

16 Table 4.1 (cont.) 16

17 Table

18 Table 4.2 (cont.) 18

19 Table

20 Table 4.3 (cont.) 20

21 Table

22 Table 4.4 (cont.) 22

23 Table 4.4 (cont.) 23

24 Example

25 Example 4.2 (cont.) 25

26 Example 4.2 (cont.) 26

27 Example 4.2 (cont.) 27

28 Process Conditions Matrix (PCM) 28

29 Process Conditions Matrix (PCM) (cont.) 29

30 Evaluation of Reactor R-101 Three conditions of concern High Temperature High Pressure Non-stoichiometric Feed Conditions 30

31 Additional Information about toluene HDA reaction 31

32 Reactor Analysis 32

33 33

34 34

35 Thermodynamic Consideration High temperature concern Fig 4.1 implies the rxn is exothermic For exothermic rxn,, as T, Xeq Decrease in Xeq is undesirable Use of high T cannot be justified from thermodynamic point of view 35

36 Thermodynamic Consideration (cont.) High pressure concern Rxn stoichiometry shows that there are equal number of reactant and product moles in the HDA rxn. Thus, no effect of pressure on Xeq No reason to use high P from thermodynamic point of view 36

37 Example

38 Example 4.3 (cont.) 38

39 Example

40 Conclusions T < 40ºC Refrigeration T > 250ºC Fired Heater or Furnace T > 400ºC M.O.C. Issues P < 1 atm Vacuum and Large Equipment P > 10 atm Cost 40

41 SUMMARY Wall Thickness Increases 10 atm 1 atm Requires Refrigeration Requires Fired Heater Requires Special M.O.C. 40º C 250º C 400º C Vacuum Chemical Large Engineering Equipment Dept., KFUPM. 41