Sizing these items need not be precise as long as they are on the same basis. The
simple equation for heat transfer:
Q ¼ U A Δt m
(10)
will suffice for this purpose. The value of U may be taken from Fig. 12.
Step 8. Note the end temperature of the crude, and using this enthalpy and the total
enthalpy in the crude at the flash zone of the main tower, determine the duty
required by the fired heater in each case.
Step 9. Cost out the equipment required for each scheme. This includes all the heat
exchangers (including trim and final coolers) and the fired heater. These can
usually be obtained from equipment vendors on a unit size (e.g., $ per sqft) basis.
Estimate the utility requirements for the equipment in each scheme.
Step 10. Tabulate the results starting with the highest capital cost scheme as a base
case, and calculate a simple incremental return on investment based on savings
for the remaining schemes. The scheme with the highest positive ROI should be
the system selected.
In almost all modern crude distillation units, there are facilities to remove free
salt. These desalting facilities are proprietary units which consist of freshwater
injection into the crude feed and subsequent separation of the water, with the salt
now in solution, from the oil. This separation takes place at a fixed temperature at
the appropriate point in the crude feed preheat train. Demulsifying chemicals, or
(more usually) electrostatic precipitators, or both are used to enhance this
700
0/hd cond.
Stream 1
0/hd vapour
100°F
200°F
360°F
360°F
487°F
704°F
300°F
300°F
558°F
240°F
280°F
198°F
160°F
229°F
Stream 3
Stream 4 Stream 5
488°F
60°F
Feed
600
500
400
300
200
100
0
0
1 0
Duty
16.10 mm
BTU/h
7.121
mm BTU/h
1 ST Exch.
Stream 1
condenser
2 ND Exch.
Stream
3
4 TH Exch.
Stream
4
5 TH Exch.
Hot Stream
5
487°F
Feed enthalpy curve (liquid)
3 RD Exch.
Cooled
Stream
5
15.42
mm BTU/h
43.16
mm BTU/h
4.15
mm BTU/h
20
30
40
50
60
70
Enthalpy (mm BTU/h)
80
90
100
110
120
130
Temperature °F
Fig. 14 Calculation of the crude preheat train
Atmospheric and Vacuum Crude Distillation Units in Petroleum Refineries
157
simple equation for heat transfer:
Q ¼ U A Δt m
(10)
will suffice for this purpose. The value of U may be taken from Fig. 12.
Step 8. Note the end temperature of the crude, and using this enthalpy and the total
enthalpy in the crude at the flash zone of the main tower, determine the duty
required by the fired heater in each case.
Step 9. Cost out the equipment required for each scheme. This includes all the heat
exchangers (including trim and final coolers) and the fired heater. These can
usually be obtained from equipment vendors on a unit size (e.g., $ per sqft) basis.
Estimate the utility requirements for the equipment in each scheme.
Step 10. Tabulate the results starting with the highest capital cost scheme as a base
case, and calculate a simple incremental return on investment based on savings
for the remaining schemes. The scheme with the highest positive ROI should be
the system selected.
In almost all modern crude distillation units, there are facilities to remove free
salt. These desalting facilities are proprietary units which consist of freshwater
injection into the crude feed and subsequent separation of the water, with the salt
now in solution, from the oil. This separation takes place at a fixed temperature at
the appropriate point in the crude feed preheat train. Demulsifying chemicals, or
(more usually) electrostatic precipitators, or both are used to enhance this
700
0/hd cond.
Stream 1
0/hd vapour
100°F
200°F
360°F
360°F
487°F
704°F
300°F
300°F
558°F
240°F
280°F
198°F
160°F
229°F
Stream 3
Stream 4 Stream 5
488°F
60°F
Feed
600
500
400
300
200
100
0
0
1 0
Duty
16.10 mm
BTU/h
7.121
mm BTU/h
1 ST Exch.
Stream 1
condenser
2 ND Exch.
Stream
3
4 TH Exch.
Stream
4
5 TH Exch.
Hot Stream
5
487°F
Feed enthalpy curve (liquid)
3 RD Exch.
Cooled
Stream
5
15.42
mm BTU/h
43.16
mm BTU/h
4.15
mm BTU/h
20
30
40
50
60
70
Enthalpy (mm BTU/h)
80
90
100
110
120
130
Temperature °F
Fig. 14 Calculation of the crude preheat train
Atmospheric and Vacuum Crude Distillation Units in Petroleum Refineries
157