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Apr-2008

Improving refinery distillation operations

The outline of a route map or staged approach to unit process improvement follows similar steps to those taken when installing advanced controls on a unit

Bernard Hagger, Foster Wheeler Energy Limited
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Article Summary
Distillation equipment is the heart of most refineries, but its performance is often given much less attention than it deserves. Maximising throughput tends to be given a higher priority than optimising fractionation or energy efficiency. However, as crude oil and energy prices rise, energy efficiency, together with optimising product quality and maximising high-value product yields, has become more important.

In order to improve a refinery’s current operation, it is recommended that a staged route map for improvement be put in place. This will lead to a better understanding of the current operation and improved unit monitoring while providing troubleshooting tools. It will also identify scope for improvement and a series of measures that can be taken to improve the unit operation.

The outline of a route map, or staged approach to unit process improvement (eg, improved distillation), is an approach that coincidentally follows similar steps taken when installing advanced controls on a unit. How far a refiner chooses to go down the road towards installing advanced controls, for example, would depend on individual needs and the benefits that can be delivered.

With the downsizing over time of refinery in-house technical resources, optimising process operation is a key area that often receives less attention than it used to. This may be a false economy. In the event that the refinery does not have the resources to do this optimisation work in-house, it will need to look at bringing in suitably experienced outside resources to help. The optimisation exercise can also offer an excellent training and development opportunity for junior engineers, under the guidance of senior operational or contractor engineers.

First steps to process improvement
The route map has five key stages — refiners can elect to go as far down this road as is appropriate for their own refinery’s circumstances and objectives:
1 Troubleshooting and getting the unit to mass balance
2 Steady-state modelling
3 Analysing the plant operation
4 Investigating possible advanced process control applications (APC)
5 Identifying future value-added improvements and revamps.
A useful byproduct of following an APC route map is that it forces the refiner to operate the facility under automatic control and requires that the facility can be easily and reliably mass balanced.

On that basis, the operator very often has to address and resolve a whole series of underlying instrumentation problems, some of which may have been there for a significant length of time. Plants where APC projects have been implemented have seen many of the benefits realised just by getting the plant to operate reliably under the regulatory controls. It was the same when distributed control systems (DCS) were initially installed, replacing pneumatic control systems. It gave a better focus on control issues and many things that had been long broken were fixed.

Step 1: Troubleshooting and unit mass balance
Step 1: Troubleshooting and unit mass balance
When troubleshooting and getting the unit to mass balance, the following major issues need to be addressed:
•    First, instrumentation control loops that are not on automatic control need to be evaluated. They could be on manual for a variety of reasons, such as unsuitable control schemes, poor 
instrumentation, a fundamental process problem or “custom and practice”
•    Second, a reliable mass balance is required for the unit.
There are a variety of reasons why mass balance for a unit cannot be achieved and a systematic exercise needs to be carried out to identify these.
On a new unit, test runs are generally undertaken and the mass balancing is done to sufficient depth to meet the owner’s objectives and to demonstrate that the unit meets its guarantees. At a later date, or on a unit that has been operating for some time, when the refiner tries to mass balance the unit and simulate it in a process simulator, it is often found that there are problems that need to be addressed.
The first thing to look at is the instrument calibration to ensure this is correct, as this is the most common source of errors. This calibration exercise should look at all the key streams, 
including recycles, reflux streams, lean oil streams and offgases, not just the main input and output streams. These secondary streams are often neglected, but are important in any simulation modelling.

The next issue is flow correction for temperature, pressure and molecular weight or density; as-measured flow rates need correcting for actual conditions. This instrumentation would have originally been set up based on a given set of flow conditions and standard conditions. This needs checking and correcting against the current actual plant operation. This can be very important if, say, a different feed is being run or different product specifications are being targeted, as temperatures and densities can be very different to the original design. If the DCS is not programmed to correct for these differences, a manual calculation needs to be done. Generally, the gas molecular weight or liquid density used can be taken from a daily analysis, selected at a time when the unit is under stable operation.

The next stage is to cross-check liquid flows with tank gauging, or preferably tank dips. These should match the measured flow closely. If it does not, it may indicate further unresolved problems with the measurements. Gas flows are more difficult to verify. It is sometimes possible to do cross-checks by comparing flows between units.

Unfortunately at this stage, the unit may still not mass balance. This may be due to incorrect basic instrument installation or the data in the DCS may have been incorrectly programmed. Even on the best-constructed units, some of the instruments may have developed problems over time, including:
•    Calibration errors, thermocouples that have drifted over time
•    Orifice plates replaced and incorrectly installed (wrong way round?)
•    Meter ranges which are incorrect for the current flows
•    DCS data input errors (wrong data for the installed instrument)
•    Issues that arise during operation with the levels of instrumentation on key streams, or instruments which do not remain in calibration for long.

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