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Process Simulation
IntroductionPlanning and Scheduling
Oil refineries process crude oil to produce a variety of fuels, the quantities of which "straight from the barrel" are unfortunately out of balance with demand. In addition the combustion properties of straight run fuels are also out of specification, i.e. gasoline octane is too low, sulphur contents are too high, with what is required. After the refinery's main fractionation column has performed the initial split of the crude oil, the refinery utilises various process units to bring the fuels to specification.

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;

1.the crude oil available,
Obviously the cheapest crude will reduce the main raw material cost and there are two main factors determining the price of crude oil;
1.how light it is; that is how much "good stuff", (i.e. LPG, gasoline, kerosene and diesel), vs. how much unwanted heavy "stuff" (e.g. heavy fuel oil) it contains,
2.its sulphur content, i.e. whether it is sweet (low sulphur content) or sour, high sulphur content,

Light, sweet crudes require less processing to produce the low sulphur fuels we currently require, but because of this are the most expensive; whereas heavy, sour crudes are cheap because they require a lot of processing to not only remove the sulphur but to "smash up" the heavies to something useful.

2.how the crude is processed.
Of course the refinery planner is constrained by what process units there are in the refinery. When the refinery is planned or expanded, the company executives in charge will determine what fuels and increasing chemicals, (e.g. propylene, benzene, toluene, xylene,) are to be produced and at what capacities, based on their expectations of the product market. This determines what process units are built. However the process units on a refinery fall into three categories,
1. octane improvement,
2. desulphurisation,
3. residue conversion,
and while most refineries use all three groups, process licensors are continuously developing new technologies for each of these categories.

Each technology requires different quantities of utilities, (e.g. steam and electricity), chemicals (which are consumed), catalysts (which must be regenerated or disposed of), produces different side streams and possibly have different side effects on the fuel. Consequently each process unit will produce fuels with different specifications, at different costs and produce side streams that either require further processing to be used as fuels, or that can be used in other process units or are waste.

The characteristics of these units provide the planner with the flexibility to process different crudes to meet the production requirements.

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.

Increasing Integration
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