PTQ Q1 2023 Issue

0% 5% 10% 15% 20% 30% 45% 40% 35% 25% 50%

44% of data set applied 0.004 hr ft deg F/Btu OAFF










0.006 Limited on U design

Figure 4 OAFF specified range between 0.001 and 0.008, with six cases where a limitation on maximum U design applied

Fouling can also vary depending on where it occurs. In some services, it is at the entry section of the tube bundle. For others, it occurs in the coldest regions where velocity is lowest and mole fraction of non-condensible the great- est. And then, there are other services where fouling is throughout the shellside. The bespoke photos in Figure 1 are from different vac- uum distillation ejector system intercondensers and depict the differing type of fouling formation that may occur. The fouling, as indicated, can and will vary, and may be extreme. Applying an appropriate fouling factor to the shellside design is extremely important in order to assure opera- tional reliability throughout the onstream periods between planned turnarounds. Tubeside fouling The cooling water source and the filtration systems or backflushing procedures differ from location to location and refiner to refiner. Tubeside fouling is a serious issue and must be considered thoroughly, along with mitigation measures, to reduce its impact. Understanding the fouling tendency of crude oil pro- cessed in a VDU and the extent to which tubeside fouling can develop is important for assuring refinery economics are realised while avoiding an ejector system performance break. Impact of excessive fouling When actual fouling is beyond the design basis, it will result in U working < U design . Fouling has surpassed the OAFF used in design, and that in itself impedes heat transfer such that the overall actual heat transfer rate is less than the design basis. When this happens, it is important to under- stand how condenser performance will respond. Problems usually arise in hot summer months when an excessively fouled condenser has the warmest cooling water inlet tem- peratures and must serve as the heat sink for the discharge of a preceding ejector. It is paramount to understand that an intercondenser will always reject the heat from the ejec- tor that precedes it. However, the essential determinant for satisfactory ejector system performance is at what inter- condenser operating pressure will that occur. Is the oper- ating pressure above or below the maximum discharge capability of a preceding ejector?

To understand how an intercondenser responds when U working < U design , if water flow rate and temperature are design values, the VDU is operating with design overhead loads. Should U working , for example, be 75% of U design , con- denser pressure must rise. The condenser pressure must rise until LMTD is approximately 133% of the design LMTD (1/0.75). The duty is essentially unchanged as the VDU is operating at design charge rate. Area is fixed as the con - denser is installed. If U working drops 25%, then LMTD must adjust upward by 33%. To illustrate the point a first intercondenser design basis is 85 mm Hg abs operating pressure and the preceding ejector has a maximum discharge pressure of 90 mm Hg abs. The weighted LMTD of the heat release graph is 16.8°F at design of 85 mm Hg abs. Importantly, the steam initial dew point is 115.5°F. When U working is 75% of U design , the sole response of the intercondenser is to increase operating pressure until weighted LMTD is approximately 22°F or 33% greater. To reject the heat with U working at 75% of U design , operat- ing pressure rose 18 mm Hg to elevate the steam dew point from 115.5 to 121.9°F, thereby increasing LMTD by approximately 33%. Under such a circumstance where first intercondenser pressure must rise to 103 mm Hg abs due to excessive fouling, the first-stage ejector will break operation, dissi - pate its shockwave, and VDU column overhead pressure will rise to 25 to 35 mm Hg abs. A pressure unacceptable to the refiner due to lost yield. Surprisingly, in winter, when the cooling water inlet temperature is much colder than design, even though U working is 75% of U design , VDU overhead pressure may be perfectly fine. This is because the colder water tempera - ture served to elevate the LMTD, which compensated for U working < U design . An important concept to consider is that excessive over- head hydrocarbon loading will cause the ejector system first intercondensers to exhibit suppressed overall heat transfer rates that might lead one to judge that the condenser is fouled or poorly designed. Large IOC refiners have trended excessive slop level and observed U working decrements to conclude that excessive hydrocarbon loading will suppress condenser heat transfer, causing operating pressure to rise, thus a similar consequence as fouling.


PTQ Q1 2023

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