Refineries have long been using linear programming to calculate the feed rates, product splits and utility consumptions of the various process units so that the refinery produces the contracted quantities of fuels, to the required specifications, at the lowest cost. This is often referred to as the site's "linear plan".
The linear plan model is a simplified process model focusing on the overall product split, utility demands and side stream production rates, and the interconnections for each process unit on the refinery. The planner uses it to optimise the operation of the refinery. The primary variables available to the planner to "run the plan" are;
The linear plan allows the planner to determine which combination of cheap crude, requiring a lot of energy and chemicals to process it, verses expensive crudes requiring less processing; will produce the fuels at the lowest cost.
From Planning to Scheduling System
The scheduling system extends the use and scope of the linear plan enabling the refinery to determine what crudes to purchase and when to purchase them.
As crude prices for the different types of crude fluctuate the linear plan can identify opportunities to minimise crude purchase costs by identifying the cheapest crudes and blend ratio that must be utilised to meet the production requirements.
The large volumes of crude and refined fuels used in the world require the use of ships for the majority of the transportation. Ships are slow and there are significant costs associated with having a ship berthed at the refinery's jetty. The scheduling system enables the refinery to reduce costs by accounting for the transportation time and ensuring there is sufficient product in the product tanks and sufficient capacity in the crude storage tanks when the tankers arrive so that the tanker can be filled or emptied in the shortest possible time.
Utilising shipping time
Once the refinery has purchased a crude tanker's cargo of crude there will be a shipping time before the tanker reaches the refinery. When planning the purchase of a cargo of crude the refinery must account for this time, the refinery's crude inventory and consumption rates to not only ensure that there is sufficient crude to continue continuous operation during the transit time, but also that the remaining crude in the refinery can be blended to produce the required fuels.
The refinery can utilise crude price fluctuations and the transport time to look for cheaper cargoes of crude and purchase these and sell the first tanker's cargo if it can get a better price. The scheduling system will identify if the new cargoes will reach the refinery in time and if the blend with the crude in the storage tanks is compatible with the production requirements, or if additional crude types need to be purchased, and if so how much and when.
Tank-age and berthing time
While a tanker is berthed at the refinery's jetty the refinery has to pay the shipping owner, therefore the refinery must minimise the on-loading and offloading times and ensure that there is sufficient capacity in the refinery's storage tanks.
One solution is for the refinery to have a large number of storage tanks so that there is always spare capacity, however storage tanks are expensive to build and operate and so refineries minimise the amount of storage they have and use the planning and scheduling system to ensure that tank inventories are suitable for the tanker's arrival.
The increase in computer power and capacity of telecommunications systems have seen the scope of the linear plan extend from that of a single site to being able to schedule production quotas for multiple sites and the required distribution network. This allows large oil companies, with refineries in dispersed geographical locations, to minimise their overall production costs by being able to identify the optimum production rates and shipment strategies for their process units and incorporating regional fuel consumption differences.
Last modified 19 Oct 09