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Modelling for ULSD optimisation

On-line coordination and optimisation of refinery process units led to a 10% increase in middle distillate production

Klas Dahlgren, Apex Optimisation/Dynaproc
An Rigden, Chevron & Henrik Terndrup, Apex Optimisation
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Article Summary
The Chevron Pembroke oil refinery is a complex and large (220 000 b/d) processing site. This case study examines the improvements achieved by a project with a high return on investment, which resulted in better operation of the process units involved in middle distillate production and higher ultra low-sulphur diesel (ULSD) output. This article describes how as much as a 10% increase in middle distillate production can be achieved essentially without investment in process units or equipment, mainly through the upgrading of cracked feeds, higher average distillate cut points, optimisation of the process unit and diesel rundown blending. These significant improvements, which are estimated at $10 million per year (minimum), have been realised through a team effort involving various departments of the Pembroke refinery, in particular the following groups of people:

•    Operations organisation, including white oils, black oils and cracking
•    Planning and scheduling teams
•    Process engineering group
•    Control and information system department, where the process control team resides
•    Apex Optimisation, supplier of medium-term closed loop optimisation technology.

The Pembroke refinery blends middle distillates directly from the process unit to hydrotreaters. The day-to-day operation of the two downstream hydrotreating units (HTUs) is challenging as throughput has to be maximised subject to a variety of process constraints and the availability of the various feed components, which include kerosene, several straight-run gas oil streams and FCC product streams such as HHCN and light cycle gas oil (LCGO). The decision-making process for these blends involves several refinery areas and console operators in different control rooms across the site.

Hence, as part of the improvement programme, a new large-scale, multi-unit coordination tool (GDOT) was implemented. The GDOT software supplied by Apex Optimisation is used within Chevron Pembroke for medium-term optimisation problems. This system, which is basically an on-line refinery linear programming (LP) model, runs in closed loop and has been in service since late 2006 at Pembroke with essentially 100% utilisation even during significant changes in crude slates.

This article describes the issues, challenges and constraints that the Pembroke refinery faces when ULSD becomes the most valuable product most of the time.

Like many other ULSD-producing refineries, the Pembroke site blends middle distillates directly from the process unit rundown lines prior to hydrotreating. The main advantages of this approach, compared to a conventional batch blending system, are lower tank storage and manpower requirements, and the swing cuts of the upstream process unit can be optimised in real-time to operate the hydrotreaters at multiple ULSD quality constraints. However, the rundown blending approach also results in a more challenging day-to-day operation of the downstream HTUs, especially if the throughput is to be maximised subject to a variety of process constraints, taking into account the availability of the various feed components.

The Pembroke diesel system has two HTUs, HTU1 and HTU2, which are fed by a rundown blending header. The configuration of the Pembroke refinery diesel system is shown in Figure 1.

The operations department with the console operators is organised into three areas: black oils (crude and vacuum distillation), white oils (hydrotreaters and naphtha processing) and the cracking area. Traditionally, the scheduling department advises the area operators, through a daily schedule, on how to set diesel blending component flow rates and middle distillate component cut points from the crude and vacuum distillation units. Now, using the medium-term optimisation tool, the coordination of the units is done automatically on-line and, instead of fixing diesel component flow rates, the scheduler specifies the product specifications and the key swing cut points to be optimised by the system. 

Maximising ULSD production
Feed quality management is one of the keys to maximising the performance of a HTU, subject to constraints. A highly constrained HTU can be very sensitive to incremental changes in the individual component flows of the feedstock. Therefore, the challenge is not just to push the rate through the unit to the maximum, but to establish the optimum blend that enables throughput to be maximised subject to product quality constraints. A different feedstock composition will significantly change the hydrotreater operation, which will have an impact on the maximum possible feed rate dictated by unit constraints. The following list highlights some of the difficulties with feedstock components and constraints observed at the Pembroke refinery’s HTUs.

• Maximisation of cracked feed (eg, LCGO) results in higher reactor temperatures and high hydrogen consumption
• Maximisation of kerosene and light cracked feeds results in constraints on the operation of the product stripper columns. In the past, this has caused operational problems, including a positive doctor test requiring costly diesel reprocessing, which led operations to put conservative limits on the throughput of the unit
• Maximisation of heavy feedstock, such as the back end swing cuts from the crude and vacuum distillation units, requires high reactor temperatures to meet the sulphur specification, which aggravates HTU heater constraints.

There are essentially 18 
variables available to control the production rates and the qualities of the three middle distillate products of the refinery: kerosene, diesel and gas oil. During the winter period, it is typically best to run at minimum flash points on all three products and at maximum cloud points on gas oil and diesel, while also meeting production rate targets on one or two of the three streams. In the summer period, the 95% point or density typically replace the cloud point as the back end constraint on the diesel.

The sulphur content is controlled within the diesel hydrotreater, but other diesel qualities such as density, cloud point, flash point and distillation must be controlled upstream of the hydrotreater; that is, by the side stream cuts and the feed blends.
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