Jan-2013

High-capacity tray for debottlenecking a crude distillation unit

Use of ultra-high-capacity trays in the most constrained section of a CDU enabled over a 50% increase in throughput at less cost than other debottlenecking options

KAUSHIK MAJUMDER, Shell Projects & Technology
GIUSEPPE MOSCA, Sulzer Chemtech
KENT MAHON, Refining NZ

Article Summary

The main fractionator of the crude distillation unit (CDU-1) in the Whangarei refinery of The New Zealand Refining Company (NZRC) was retrofitted with high-capacity internals to increase the unit throughput from 
8500 t/d to 13 000 t/d. Ultra-high-capacity Shell ConSep* trays were applied in the most capacity-constrained HGO pumparound (mid circulating reflux) section of the column, as no other first-
generation, high-capacity tray was found adequate to debottleneck this section. By the application of these trays, capex savings of the order of $5.5-6 million were achieved compared to other conventional debottlenecking options. This was the first application of ConSep trays in a CDU main fractionator, and the post-revamp test run established realisation of the expected performance.  

Whangarei refinery targeted expanding its refining capacity through the Point Forward Project.¹ The project involved increasing the throughput of the CDU-1 from 8500 t/d to 13 000 t/d, thereby increasing the distillate component to downstream processing and generating additional long residue to replace imported long residue for loading the vacuum distiller. Figure 1 shows a simplified process flow diagram of CDU-1.

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:
•  Replacement of the existing column internals with high-capacity internals including the ConSep tray for the most capacity-constrained HGO pumparound section
•  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 to be $6 million higher than for option 1
•  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 to be $5.5 million higher than for option 1.

On the basis of a comparison of the revamp options, NZRC decided to proceed with the ConSep tray alternative owing to this option’s lowest capex and most favourable economics.

Shell ConSep tray technology
The ConSep tray utilises 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 tray combines the features of a contacting deck and a separator deck in a single tray. The basic features of the tray are shown in Figure 2. The functioning of the contacting deck, which in fact is a normal tray, is limited by three hydraulic mechanisms: jet flooding, downcomer choking and downcomer backup. The use of a separator deck influences all three mechanisms:²

•  The jet flooding limit is extended as the entrained liquid is efficiently separated from the vapour to prevent carry-over of liquid to the tray above
•  The liquid entering the main downcomer is largely coming from the separator deck, where it is well degassed. As a result, the downcomer liquid handling capacity is substantially increased
•  To eliminate downcomer backup limitation, the separator deck is designed with low-pressure drop swirl tubes. The contacting deck is also designed with a relatively high open area.
Figure 3 shows the expected capacity gain of the ConSep tray over conventional trays and packing.3
The flow parameter (φ) is defined as:
φ = L  √ Pv
       V      PL 

where Lv represents the liquid-to-vapour mass flow ratio and PvPL represents the ratio of vapour-to-liquid density.

Typically, the ConSep tray is capable of offering a 40-50% capacity advantage over a wide range of first-generation, high-capacity trays. In most revamps, the column retrofitted with the ConSep tray becomes limited by other factors such as availability of feed and/or constraints on auxiliary equipment, including reboiler, condenser, pumps and so on, even before the full potential of the ConSep tray is realised. Table 1 shows some applications of this tray along with benefits achieved and constraints faced.

Modifications of main fractionator (C-150)
During the feasibility study, the HGO pumparound section of the column was found to be severely limiting for the targeted throughput of 13 000 t/d. This section was already fitted with Shell Calming Section (CS)* trays. Since the first generation of high-capacity internals was found inadequate to debottleneck this section, ultra-high-capacity ConSep trays were selected. The trays were designed to achieve 33% more capacity compared to the CS tray.

The HGO pumparound section consisted of three contacting trays with a tray spacing of 500mm. A one-for-one tray replacement with ConSep trays was selected. Figure 4 shows a schematic drawing of the HGO pumparound section fitted with these trays. For the remaining sections of the column, the following internals were suggested:
•  Stripping section: Shell HiFi* trays
•  Wash section: MellapakPlus 252Y** packing
•  All other sections: Shell CS trays.

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