Beating pressure drop with a trash baskets redesign
Unscheduled turnarounds of hydrotreaters can be expensive in terms of catalyst and manpower costs, as well as the value of lost production
Byron G Johnson, Phillips Petroleum Company
Brian M Moyse and Kenneth Lee Smith, Haldor Topsoe
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Reactor fouling can be a serious problem in today’s refining environment. Catalyst manufacturers continue to increase the intrinsic activity of their products, and refiners pack as much catalyst as possible into a given reactor volume. With poorer quality feedstocks and more severe product requirements, the operating cycle or onstream time of the unit becomes a key economic factor in the overall refining scheme. Run lengths or operational cycles on such units should ideally be determined by control of catalytic activity and not by upsets, fouling, pressure drop, poisoning and so on.
In today’s era of low refining margins, economic operations are paramount. In the hydroprocessing area, shutdowns for catalyst replacement are normally planned and are, therefore, part of the normal refining budget. If, however, unscheduled turnarounds and/or limited unit throughputs are thrust upon the refiner due to pressure drop constraints in the reactor, then extra costs for turnarounds, catalyst replacement, lost production etc will be experienced. In some severe cases, this can even represent a severe refinery bottleneck.
This article describes a case of problem-solving using a team approach, involving many disciplines to address one such bottleneck. The unit in question is a naphtha hydrotreater, which was experiencing severe fouling and subsequent pressure drop problems. This not only restricted throughput and necessitated three or four reactor turnarounds per year, but also affected other refinery units.
Ultimately, this unit became the refinery bottleneck, and a solution had to be found. The unit essentially is the pretreater for a platinum reformer (platformer) and, as such, is a very important unit in the refinery. Whenever the naphtha hydrotreater was shut down, it affected the production of platformate and, ultimately, the quantity of gasoline available for marketing.
Phillips Petroleum Company owns and operates three refineries in the USA, at Woods Cross, Utah; Borger, in the Texas Panhandle; and Sweeny, in South Texas. The refinery at Borger is the second largest, with a crude capacity in excess of 120000bpd. The complex contains a number of hydroprocessing units treating streams from natural gas liquids (NGL), naphtha, distillates, light cycle oil (LCO), and a large resid hydrotreating unit.
As part of the refinery complex, a naphtha hydrotreater operates upstream of the platformer. This unit has had a history of short operating cycles, due to unfavourable pressure drop development. But with a team approach, Operations, Maintenance, Research and Development, and Process Engineering, together with catalyst and chemical vendors, worked together to solve these problems. This resulted in increasing operating cycles from a few months to more than one year at full plant operating capacity.
The naphtha unit with the pressure drop problem is Unit 19.1, the naphtha reformer desulphuriser (HDS) for the platformer unit. Unit 19.1 is a typical naphtha unit that runs mostly straight-run naphtha, with some atmospheric resid desulphurisation (ARDS) unit naphtha in the feed. Unblanketed intermediate storage is used between the units. As part of the evaluation, the subject of intermediate storage of naphtha was also considered.
Recognising the fact that naphtha containing olefins and diolefins does have a gum-forming potential, especially if it is exposed to oxygen, required that this possibility should also be considered.
Obviously, it would be desirable to minimise the exposure of naphtha feed to oxygen or air by converting to gas-blanketed tankage. This would have necessitated a major capital project, which was not an option at this point in time. Hydrogen treat gas is added after the heater. Unit 19.1 run length has been limited by pressure drop since the 1980s. Recently, due to increased feed rate and other factors, the typical run length on the unit has been between three and six months before the pressure drop forced the unit to shut down for corrective action, such as a skimming or catalyst replacement.
This pressure drop limitation became more severe as the overall refinery’s and unit throughput has increased. As most of the other refinery limitations have been eliminated over time, this Unit 19.1 pressure drop problem became the real refinery bottleneck with a major impact on the economic performance of the refinery, due, in part, to loss of gasoline production.
Upon preliminary investigation, it was determined that the pressure drop problem was caused by a large amount of carbonaceous-type deposits in the top of the reactor, where the trash baskets were located. A multifunctional team was formed to study this problem and collectively provide solutions to eliminate this pressure drop limitation as a refinery bottleneck. A system-wide and holistic approach was used to analyse and subsequently make the necessary recommendations and plan-of-action to implement the necessary changes to solve this problem.
The multifunctional team included Phillips personnel and outside vendors with a particular expertise. This effort was lead by the Process Engineering group at the Borger refinery and included support and contributions from the refinery’s Operations and Maintenance departments. Corporate Engineering, together with Research and Development, contributed their knowledge and support, where appropriate.
Outside the Phillips organisation, expertise was provided by both Unichem, a division of BJ Services Company and, in particular, Haldor Topsoe Inc. Unichem helped evaluate the antioxidant and antifoulant chemical addition programmes to help deter the formation of polymers and coke in the preheat furnace and heat exchangers.
Haldor Topsoe, with more than 20 years’ experience in graded catalyst beds using high-void topping and ring-shaped catalysts, advised on the possible mechanisms of the pressure drop limitation and on the application of the best catalyst bed grading system to manage these pressure drop mechanisms.
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