on the volume of gases to be removed from the system, a portion of which comes from air leakage from the atmos- phere. Accurate estimation of air leakage is thus critical for the design of ejectors and vacuum overhead systems. The Heat Exchanger Institute (HEI) has developed standards for estimating the rate of air leakage into commercially tight vacuum systems, specifically outlined in the 5th Edition of their Steam Jet Vacuum Systems standards. Utilising the following Equation 1, air leakage can be calcu- lated without resorting to graphical interpretation:
Comparisons between single-diameter and multi-diameter vacuum dryers
Parameters
Single
Multi
ID, m
1.6
Top
1.6 3.8 3.6
Height (T-T), m
52.33
ID, m
- -
Bottom
Height (T-T), m
7.58
Cost, $
465 k
421 k
Table 3
Air leakage rate, (L) Ib/hr. = a x V^b
Eq 1
a coalescer with a VD presents operational challenges. Coalescers are more effective at lower temperatures (prefer - ably below 60°C), whereas economically, VDs operate better at higher temperatures (above 120°C). Moreover, the need for frequent replacement of coalescer elements every 12-18 months, and pre-filter elements every six months, adds sig - nificantly to operational expenditures. Advanced VD design considerations Vacuum columns are significant Capex contributors in ULSD systems. Adopting multi-diameter towers can opti- mise tower size and cost. Liquid residence time is a critical factor in determining tower dimensions, as vapour load is relatively negligible (see Table 3 ). Some refineries do not account for an instrumented pro - tection system to prevent liquid overfill scenarios; instead, they size the tower’s vapour space from the high-level alarm to the top of the tower to accommodate at least 10-15 minutes of hold-up. In configurations where a flash vessel or a first-stage VD is placed above the second-stage VD, using a hydrostatic loop instead of a level control valve between these two pieces of equipment can significantly reduce size and eliminate the need for separate relief valves for each piece of equipment. Calculation of air leakage The size of the ejector and its steam consumption depend
where: V = system volume (ft 3) and ‘a’ and ‘b’ are coefficients which are a function of process pressure, as shown in Table 4 . Feed to the ULSD VD usually comes from the ULSD stripper. During start-up, shutdown, or any upset of the stripper, there is a possibility of encountering H₂S in the overhead, along with non-condensable hydrocarbons, water, hydrocarbon gases, and possible oxygen from air leakage. These components can lead to corrosion of the vacuum overhead system, particularly the overhead cooler/condenser and the overhead receiver. Some refineries use chemical injections (neutralisers) to mit - igate corrosion. Most refiners prefer overhead exchangers made of alloy material, and overhead vessels equipped with lining. Benchmarking across different facilities also supports the use of higher-grade metallurgy as a preferred option. Conclusion The selection of an appropriate ULSD water removal system is crucial and depends on both the required product specifi - cations and the specific design of the stripper. For product water content specifications below 100 wppm, VDs emerge as the most reliable method, offering significant operational benefits. Operating VDs at temperatures above 120°C, after maximum energy recovery from the ULSD rundown, is eco- nomically beneficial and increases system efficiency. From the comparative analysis, both single-stage and
600
10,000
9,057
544
8,921
9,000
512
8,417
500
8,730
481
7,710
8,000
440
6,869
406
7,000
400
6,000
369
4,905
300
5,000
290
224
4,000
179
200
3,000
144
118
200 wppm 5000 wppm
2,000
100
1,000
333
240
191
576
0
0
60
80
100
120
140
160
Vacuum dryer feed temp (˚C)
Figure 4 Effect of water in ULSD vs VD pressure and temperature
48
PTQ Q4 2024
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