CHE 425 Engineering Economics and Design Principles 1
CHAPTER 4 Understanding Process Conditions 2
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
PURPOSE OF CHAPTER 4 Analyze reasons for selecting specific conditions: Temperatures Pressures Compositions Analyze process conditions that require special consideration 4
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 40-260 C Pressures between 1-10 bar Conditions outside the above favored temperature and pressure ranges are identified as conditions of special concern 5
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
PRESSURE (cont.) A decision to operate outside the pressure range of 1-101 10 bar must be justified 7
TEMPERATURE (cont.) 8
TEMPERATURE (cont.) 9
TEMPERATURE (cont.) A decision to operate above 400 C must be justified 10
TEMPERATURE (cont.) Below 40 C 40-260 C Above 260 C Require refrigeration Okay Require special heaters 11
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
Do we ever operate outside these limits? Tables 4.1 4.3 Reactors and Separators Table 4.4 Other Equipment DME Process pretty much inside limits 13
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
Table 4.1 15
Table 4.1 (cont.) 16
Table 4.2 17
Table 4.2 (cont.) 18
Table 4.3 19
Table 4.3 (cont.) 20
Table 4.4 21
Table 4.4 (cont.) 22
Table 4.4 (cont.) 23
Example 4.2 24
Example 4.2 (cont.) 25
Example 4.2 (cont.) 26
Example 4.2 (cont.) 27
Process Conditions Matrix (PCM) 28
Process Conditions Matrix (PCM) (cont.) 29
Evaluation of Reactor R-101 Three conditions of concern High Temperature High Pressure Non-stoichiometric Feed Conditions 30
Additional Information about toluene HDA reaction 31
Reactor Analysis 32
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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
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
Example 4.3 37
Example 4.3 (cont.) 38
Example 4.4 39
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
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