(94d) 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