8% 16% 14% 12% 10% 6% 18%
0% 2% 4%
<40%
50-60%
70-80%
90-100%
110-120%
130-140%
>150%
40-50%
60-70%
80-90%
100-110%
120-130%
140-150%
Figure 5 Excess area provided in those cases where 0.004 OAFF was specified for the 200 systems analysed
One refiner shared trend data highlighting that exces - sive overhead hydrocarbons to the ejector system lowered U working by 15 to 35%, depending on how hydrocarbon loading exceeded the design basis for the ejector sys - tem. There are also instances where independent refiners addressed high hydrocarbon loading to the ejector sys - tem and adjusted first-stage ejector discharge capability to yield superior performance than when high levels of two-phase hydrocarbon loading were in the VDU over - head stream to an ejector system. Importantly, such cases are not fouling per se, but they exhibit fouling-like behav - iour as if that were the root cause of poor ejector system performance. Industry-specified fouling factor An analysis of fouling factors specified over the past 30 years for ejector system condensers was undertaken. More than 200 systems were evaluated to assess the fouling factor specified by industry, the U design , and the resultant amount of excess area the fouling factor provided. In this analysis, only the primary condenser was considered. The primary condenser is the condenser within an ejector sys - tem that is first to handle the vacuum column overhead hydrocarbon and non-condensible gases. That may be a precondenser ahead of the first-stage ejector or first intercondenser after the first-stage ejec - tor (see Figure 4 ). The most prevalent OAFF specified by industry is 0.004 hr ft² °F/Btu. The OAFF specified ranged between 0.001 and 0.008, with six cases where a limitation on maximum U design applied; for example, U design could not exceed 80 Btu/hr ft2 °F. An interesting and deeper analysis is that for nearly 50% of the examples where 0.004 OAFF applied, the amount of safety factor determined by percent excess area varied greatly. Figure 5 highlights the extent of excess area that 0.004 OAFF provided, and it varied greatly. The important takeaway for industry is that 0.004 OAFF, for example, is a traditional fouling factor for this type of application; it provides varying degrees of actual safety fac - tor on area. For designs with an increased risk of U working being less than U design , 0.004 OAFF will offer less safety factor. For cases where load composition to a condenser is low non-condensible and low hydrocarbon vapour, thus the
condenser is handling mostly steam, 0.004 OAFF will offer a considerable safety factor, something on the order of 100 to 150% excess area. On the other hand, as hydrocarbon loading increases or the amount of non-condensible gases increases, the degree of safety provided by 0.004 is less.
Component
Mass flow rate in #/hr
Steam
30,000 30,000 30,000 30,000 30,000 8,000 4,000 4,000 4,000 8,000 12,000 30,000
Hydrocarbon
Non-condensible gases
2,000 4,000 6,000 4,000 4,000 4,000 OAFF hr ft 2 ºF/Btu 0.004 0.004 0.004 0.004 0.004 0.004 % excess area provided 112 97 85 90 80 50
It is always important to consider an OAFF relative to an anticipated overall heat transfer rate, especially when han - dling condenser mass flow rate with hydrocarbon as the predominant component, be they vapours or vapours and liquid. When the overall heat transfer rate is comparatively low, such as U design at 50, for example, 0.004 OAFF embed - ded in U design provides just 25% excess surface area. This analysis and thorough discussion about the foul - ing factor must take place in the FEED phase of a project. Once in the EPC phase, it is difficult to modify a procure - ment specification that drives higher an EPC’s procurement costs. There are other techniques for building in conserva - tism, such as designing for 110 to 125% of the design mass flow rates. Nonetheless, as the preceding pictures suggest, appropriate OAFF is critical to achieving reliable ejector sys - tem operation between planned shutdowns or turnarounds. Case study A refiner operating a world-scale fuels refinery experi - enced elevated vacuum distillation column pressure that led to an unacceptable reduction in yield. The increased vacuum column pressure lowered the recovery of more valuable vacuum gas oils, concurrently increasing less val - ued vacuum tower bottoms. The VDU overhead ejector system was supplied in the mid-1980s and was revamped 20 years later as part of a clean fuels upgrade to produce low-sulphur fuels. As a result of the revamped vacuum col - umn design, overhead pressure was reduced 5 mm Hg to
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PTQ Q1 2023
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