performance break, and results in little or no forgiveness for fouling, higher oil rates, or even long air baffle bypass. Designing with a generous amount of cracked gas adds inherent design margin to the first-stage inter-condenser. Vacuum systems designed for high cracked gas/air rates and adequate MDP margin above the calculated condens- ing pressure and operate at a lower cracked gas/air rate can tolerate some long air baffle bypass. The lower cracked gas rate ‘carries’ less steam in the condenser outlet gas, allow- ing higher temperature before overloading the second- stage ejector (see Figure 12 ). This means the second-stage pressure should be below design, thus providing more mar- gin below MDP. MDP margin allows higher second-stage suction pres- sure without exceeding first-stage MDP. In our example, the estimated first-stage inter-condenser inlet pressure was 84 mmHg absolute with a resultant 87 mmHg first- stage ejector discharge pressure. MDP is 98 mmHg abso- lute for the design vacuum column overhead process load implying an 11 mmHg margin. When the first-stage inter-condenser inlet pressure increases from 84 mmHg absolute to 94 mmHg absolute, higher condensing pres- sure raises the initial steam condensing temperature from 116-120°F at the design cracked rate of 0.3 wt% feed. This increased pressure reduces the water vapour load at the same outlet temperature (see Figure 13 ) or allows a higher temperature, in this example about 5°F for the same vapour load. If the cracked gas rate decreases to 0.1 wt % feed at 94 mmHg absolute condensing pressure, the initial steam condensing temperature increases to 119-121°F. Since CW inlet and outlet temperatures are 90-103°F, these condensing temperature increases have a huge effect on driving forces. Vacuum systems designed at low MDP mar- gin and low cracked gas/air rate cannot tolerate deviation in CW conditions, fouling, or practically any long air baffle bypass without first-stage ejector performance break. Second-stage ejector capacity
30000
0.3 wt% 0.1 wt% 0.2 wt%
25000
20000
15000
10000
5000
0
95
100
105
110
115
Vapour temperature, ˚F
Figure 12 First inter-condenser outlet vapour rate
80000
80 mm 90 mm
70000
60000
50000
40000
0 10000 20000 30000
95
100
105
110
115
Vapour temperature, ˚F
Figure 13 First-stage inter-condenser pressure
Cracked gas – impact on first inter-condenser The amount of cracked gas/air rate plays a significant role in first-stage inter-condenser performance. Using too little cracked gas flow rate in the process design basis to reduce motive steam consumption is short-sighted, risks
The first-stage inter-condenser reduces second- stage ejector process load from roughly 24,000 lb/hr water equivalent shown in Table 1 to 7,850 lb/hr water equivalent based on a gas outlet temperature of 100°F. The majority of the con- densable oil and steam (process plus motive) is condensed in the first-stage inter-condenser. A small amount leaves with the cracked gas. The water equivalent loads are shown in Table 2 . The design second-stage ejector suction pressure is 75 mmHg absolute pressure per the ejector curve shown in Figure 8. If first-stage inter-condenser outlet tempera- ture increases to 105°F, the vapour outlet will increase to roughly 8,600 lb/hr water vapour equivalent (see Table 3 ). This load corresponds to 86 mmHg absolute per the second-stage ejec- tor curve. The increase in second-stage ejector pressure of 11 mmHg consumes all of the system margin. The first-stage ejector discharge pressure will increase to 98 mmHg absolute, presuming
Second-stage ejector components, 100°F gas outlet temperature
Components
lb/hr Molecular,
Water vapour equivalent, lb/hr
% Total load
wt
Process steam Cracked gas/air Condensable oil
4,220 3,000 3,420
18 32
3,930 2,200 1,720 7,850
50.0 28.0 22.0
107
Total
10,640
100.0
Table 2
Second-stage ejector components, 105°F gas outlet temperature
Components
lb/hr
Molecular,
Water vapour equivalent, lb/hr
% Total load
wt 18 32
Process steam Cracked gas/air Condensable oil
5,100 3,000 3,440
4,760 2,150 1,690 8,600
55.4 25.0 19.6
119
Total
11,540
100.0
Table 3
36
PTQ Q1 2023
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