Maximising stripping section performance
The design of a crude unit stripping section affects diesel and gasoil yields, energy usage, and unit reliability.
EDWARD HARTMAN and BRYAN WHITE
Process Consulting Services
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Maximising stripping section performance in crude and vacuum units is traditionally used as a tool to enhance recovery of diesel, atmospheric gasoil, and vacuum gasoil. Refinery economics have long favoured projects that aim to increase profitability through yield and/or charge rate improvements. In one documented case, revamping a crude unit stripping section, combined with flow scheme changes and modifications to the diesel fractionation section internals, allowed an Australian refiner to raise diesel production by 14.5 vol% on crude.1 This article focuses on the benefits of optimisation of stripping sections for both revamps and grassroots projects and the impact on reducing energy consumption. In today’s environment of low refining margins, emphasis on energy efficiency and reduction of greenhouse gases emission, and extremely constrained capital budgets, optimisation of stripping sections presents refiners with a cost effective and focused means to improve their competitiveness.
Common stripping section problems
Stripping sections in crude atmospheric and vacuum towers are often troubled by process, hydraulic, and mechanical design errors or inadequacies that result in poor stripping performance and low vaporisation of atmospheric and vacuum residues. At first, the process design for a stripping section must consider the type of crudes that will be processed, as well as cut point and product yield requirements. Many older units were designed with only three or four stripping trays and low steam rates of less than 3 lb/bbl of residue. While these factors may not carry a large penalty for light crudes, they significantly reduce diesel and vacuum gasoil yields for heavy crudes.2,3
The hydraulic design of the stripping section affects tray efficiency in several ways. Modern crude atmospheric towers normally have a smaller diameter cylindrical shroud near the bottom of the column that serves as a stripping section. In older crude towers, it is common to find the stripping trays located in the same diameter as the flash zone. Either way, the resulting cross-sectional area is often too large to promote good vapour and liquid contact, which leads to low tray efficiency and fouling on tray decks. In vacuum towers, the majority of stripping sections are located in smaller diameter vessel sections which frequently are still too large, resulting in poor tray efficiency and fouling.
Another fundamental error in the design of atmospheric and vacuum towers stripping sections is to use the same open area for all stripping trays. With this approach, open area is usually set by the highest vapour load, which occurs at the top stripping tray. The lower trays operate with gradually less vapour with the bottom tray handling only stripping steam. The strip-out ratio (ratio between total vapour generated in the stripping section and stripping steam on a volumetric basis) is a function of crude blend, flash zone pressure and temperature conditions, number of stripping trays, and steam to residue ratio. The strip-out ratio is approximately 2:1 in crude towers and 4:1 in vacuum towers. Imparting the same tray design and open area throughout the stripping section results in weeping and poor vapour-liquid contact in the lower trays.
Based on the authors’ experience, the combination of these hydraulic design errors results in low stripping section tray efficiencies, in the 10% range or even lower. These low efficiencies reduce diesel and vacuum gasoil yields at constant flash zone conditions in the crude and vacuum columns, respectively. In order to maintain yields and compensate for poor stripping section efficiencies, higher duties are needed in the crude and vacuum heaters.
Fouling in stripping sections typically occurs due to deposition of asphaltene compounds on tray decks and inside downcomers. This deposition tends to be more pronounced with heavy crudes prone to lower thermal stability and when running incompatible crude blends of aromatic and paraffinic crudes. Some crudes are also known for having high amounts of solids and sediment. For example, solids in some shale crudes have been measured as high as 350 lb per thousand barrels. In addition to causing emulsion problems in desalters and fouling in preheat train exchangers and crude heaters, solids have also caused plugging of crude tower stripping section trays. Poorly designed trays with excessively large diameters and small open areas aggravate fouling and plugging. In the most severe instances of fouling, refiners are forced to reduce stripping steam rates and even take unscheduled shutdowns.
Tray features and details such as type of valve, opening size on the tray decks, weir height, and downcomer clearance also impact fouling tendency in the stripping section. For example, moveable valves or fixed valves with small openings, as well as small diameter sieve holes, tend to foul and plug quickly. High outlet weirs promote accumulation of fouling material on tray decks. Small downcomer clearances can lead to downcomer plugging. Additionally, the inherent liquid flow pattern of conventional circular trays leads to stagnant pools of liquid on the deck (see Figure 1). Downcomer bottom edge length is normally between 50% and 60% of tower diameter. As liquid exits the downcomer and flows towards the outlet weir, it does not spread evenly across the deck. This uneven flow pattern is especially poor on single pass trays and results in stagnant areas near the periphery where more fouling materials tend to accumulate. The same problem occurs to a lesser extent on two and four pass tray designs.
Improving tray mechanical integrity is paramount to achieving good stripping section performance and maintaining that performance between turnarounds. Damaged stripping sections can cause crude and vacuum distillate yield losses as high as 4% and 3% on whole crude, respectively.4 Crude and vacuum towers commonly have episodes of high bottoms liquid level and ‘wet’ steam, especially during unit start-up. In vacuum towers, introduction of wet steam can be very destructive due to the expansion of condensate from liquid to vapour phase under vacuum pressure. Trays designed with normal metal thickness and frictional assembly methods do not resist uplift forces resulting from these operational upsets. The result is often partially damaged trays or all stripping trays laying in the tower bottom head. With this kind of disastrous damage, the trays do not contribute to atmospheric or vacuum residue vaporisation, no matter how well intended the process and hydraulic designs.
Stripping section improvements
Improvements to crude atmospheric and vacuum tower stripping sections can be applied in grassroots and revamp projects. These projects should focus on maximising tray count and efficiency, minimising their fouling tendency, and upgrading mechanical resistance against operational upsets. Stripping steam rate should also be reviewed and adjusted if necessary. The goal is to optimise the ratio of steam to residue and to increase vaporisation of light material, considering unit constraints such as overhead condenser capacity and the effect on tower operating pressure.
An effective way to achieve these goals in crude atmospheric stripping sections is to locate the trays inside a shroud constructed as parallel walls. This design results in rectangular active areas with no stagnant zones. Shroud width and number of tray passes are selected to obtain a high weir liquid loading. The resulting liquid velocity profile on the tray decks is analogous to a plug flow regime, which reduces fouling potential. Outlet weirs have low height and are also configured to minimise fouling. Downcomers are properly sized and with a high clearance to avoid stagnation and plugging. Finally, active panels are fitted with ¾” or 1” sieve holes and the open area of each tray is adjusted according to the increasing vapour rate from bottom to top. The overall effect of this design approach in projects executed by Process Consulting Services (PCS) has been tray efficiencies of 35% or higher with minimal fouling tendency.
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