Application of Ultra High Capacity Shell ConSep* Tray for De-Bottlenecking of a Crude Distillation Unit

NZRC's Whangarei refinery targeted expanding the refining capacity through ‘Point Forward Project'. The project involved increasing the throughput of the Crude Distillation Unit (CDU-1) from 8500 ton/day to 13000 ton/day, thereby increasing distillate component to downstream processing and generating additional long residue to replace imported long residue for loading the vacuum distiller.

The main fractionator of CDU-1 was fitted with bubble-cap trays and valve trays. Shell Global Solutions International (SGSi) carried out the feasibility study for the expansion of CDU-1. Several options were studied to debottleneck the main fractionator:

1.      Replacement of the existing column internals with high capacity internals in the capacity constrained sections

2.      Installation of a new crude pre-fractionator column to separate off light naphtha and reduce the load to the main fractionator. The capex for this option was found USD 6 million higher than for option 1.

3.      Installation of a new heavy end column to recover the Heavy Gas Oil (HGO), dropped into the long residue to offload the main fractionator. The capex for this option was found USD 5.5 million higher than for option 1.

Given the above, it was decided to retrofit the main fractionator with high capacity internals.

During the feasibility study, the HGO pump-around section of the column was found severely limiting for the targeted throughput of 13000 ton/day. This section was already fitted with Shell CS** trays (Calming Section trays). Since the first generation of high capacity internals were found inadequate to debottleneck this section, ultra high capacity Shell ConSep trays were selected. The trays were designed to achieve 33% more capacity compared to the first generation of high capacity trays. Shell ConSep trays utilize the principle of de-entrainment by centrifugal forces to remove the gravitational limitation of jet flood. Separation of the entrained liquid before entering the next tray allows very high vapour velocities to be achieved in the column. The Shell ConSep tray combines the features of a contacting tray and a centrifugal separator in a single stage.

The HGO pump-around section consisted of 3 contacting trays with a tray spacing of 500 mm. A one-for-one tray replacement with ConSep trays was selected. As this was the first application of Shell ConSep trays in this service, a detailed study was carried out to address the risks associated with this application and the mitigations were applied in the design.

For the remaining sections of the column, the following internals were suggested:

–        Stripping section : Shell HiFi*** trays

–        Wash section : Mellapak**** Plus 252Y packing

–        All other sections : Shell CS trays

From conception to implementation, the project took 4 years and the revamp was implemented during a normal planned shutdown in October 2009 without affecting the unit availability.

The ConSep trays were manufactured by Sulzer Chemtech. To ensure proper performance of the trays in a relatively new application, very rigorous quality control steps were followed at the manufacturing site and detailed mock-up of tray components was carried out at refinery site prior to installation in the column.

The performance test run of CDU-1 was conducted in September 2010. The crude throughput was maintained at 13000 ton/day, though the crude blend used during the test run was marginally heavier than the one considered for the revamp design. Based on the review of the operating experience since start-up and the test run results, it could be concluded that the revamp targets of CDU-1 main fractionators was achieved. No hydraulic constraint was experienced in achieving the design intake of 13000 ton/day and required product quality was achieved.

The performance of Shell ConSep trays in the HGO pump-around section met the target of capacity enhancement without any draw back compared to the pre-revamp conditions. During the test run, the trays were operating at 10% lower than the design capacity even at the design intake of 13000 T/D due to heavier crude feed and lower feed temperature. However, the built-in capacity margin enabled stable operation for the trays at much above the capacity limit of the first generation of high capacity trays.

The options to debottleneck columns already equipped with the first generation of high capacity trays are limited. Shell ConSep trays provide a very attractive solution to such cases. In this revamp project, use of only 3 Shell ConSep trays in the most capacity constrained section of the column made it possible to retrofit the existing column and turned the capex option more attractive over the other debottlenecking options. In this paper, the revamp project will be discussed in details from the process study to the hydraulic design, the modifications of the main fractionator, and the achieved results.

(*) Shell ConSep is a Shell trademark

(**) Shell CS a Shell trademark

(***) Shell HiFi a Shell trademark

(****) Mellapak is a Sulzer-Chemtech trademark