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Jan-2011

Improving crude unit operation

Most crude and vacuum units present an opportunity for improvement using modern process control systems

Tim Olsen and Gary Hawkins
Emerson Process Management
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Article Summary
The reliability and operating flexibility of a refinery’s crude unit has a material impact on the profitability of the entire refinery due to its importance to the overall process by distributing intermediate streams as feedstock to the downstream workhorse conversion units that create higher value products. Many refinery managers and operations personnel want the crude unit to just run and not cause any trouble. However, it can be a source of increased energy usage, reduced productivity, lower intermediate product quality, high maintenance costs and increased safety risks. This is because crude units are often among the most neglected parts of any refinery, with all the infrastructure issues of ageing facilities. Today, nearly every crude unit faces major operating challenges of energy inefficiency, poor unit utilisation and asset unreliability, as well as safety, health and environmental issues.

Unexpected pump failures, accelerated coking on fired heater tubes, overhead column vapour not condensing and unreliable measurement of high-viscosity fluids are among the everyday occurrences. Energy issues are often related to heat exchanger tube fouling and poor heat transfer efficiency, an inability to optimise fired heaters and inadequate temperature measurements needed to optimise crude unit operation.

But  improvement is possible. In the words of one official of a large US refinery: “Selective application of advanced technology can help a crude unit overcome built-in deficiencies and actually improve its performance to the point of becoming an industry top tier benchmark.”
Besides their improvements in accuracy and long-term stability, modern microprocessor-based field instruments provide lots more information on their status and condition than the simple process 
measurements of older instrumentation. These technologies enable more efficient operations, decreasing costs and increasing profit levels.

Emerson calls this the Smart Refinery — a place where personnel are better informed about process performance, faulty field assets and the root causes of existing or imminent problems. In the Smart Refinery, console operators receive information relating to 
critical control applications, maintenance technicians are made aware of impending maintenance needs, and the safety engineer is notified of safety instrumented system (SIS) issues.

Effective crude unit operations depend on achieving excellence in the areas of energy efficiency, day-to-day reliability, asset utilisation and safety. Each of these can be improved by implementing modern technologies that are often lacking in older units.

Modernisation
The crude unit can be eliminated as a sore spot for the refinery and made a star performer by modernising control systems and adopting digital field architecture. Investments in technology enable efficiency, reducing the impact of crude unit disruptions on the downstream refinery operations. Existing facilities can start small and gain experience through upgrade programmes at a measured pace, with benefits from early installations paying for the later stages.

Replace older legacy control systems
Modern automation technology is faster, more powerful and with features that enable information management beyond the simple process variable, setpoint and output. Embedded advanced process control (APC) tools are easy to use and maintain, require no additional hardware or software, and can shorten implementation schedules dramatically.

Digital automation architecture gives refiners the ability to safely operate closer to process and equipment constraints without violating them, while monitoring the condition of many assets in real-time. Information from this smart technology helps personnel address the long-standing uncertainty as to whether observed problems are the result of instrument degradation or, indeed, are process problems.

For refineries with frequent crude oil switching, disturbances in crude unit operations often propagate into bigger issues downstream. It only makes sense to diminish such disturbances at their source to achieve effective and reliable operations throughout the refinery.

Managing the change from one incoming crude to another requires a regulatory control layer with the dynamic performance necessary to respond quickly and precisely to setpoint changes. Yet, the importance of the regulatory control layer is often overlooked in older systems. For example, energy consumption and material balance must be re-established following a switch to bring the intermediate products within specifications.  Advanced process controls shift the burden of managing crude switching from the operator to a fractionator optimisation package that minimises the chance for human error, hastens the transition time and mitigates crude unit disruption during switching.

It is also important to effectively use the levels in crude unit process vessels as a disturbance buffer. This may require a shift in thinking for operators to allow intermediate levels to fluctuate from 25–75% full and not try to maintain a given setpoint such as 50%. If disturbances are absorbed in the crude unit, the intermediate feedstocks can be processed more effectively in downstream units.

Install digital instrumentation
Digital instrumentation improves asset reliability and utilisation of crude unit production equipment. Smart field devices produce a great deal of useful information that is easily accessed and stored by advanced asset management software. With the availability of near real-time information on the condition of measurement instruments, final control elements and other assets, refiners can predict when these assets will require attention and fix them before the process is adversely affected. In addition, unnecessary maintenance is avoided on those assets that do not need immediate work. This predictive maintenance strategy keeps the unit operating and minimises spurious process tripping. As a result, unscheduled slowdowns and shutdowns are reduced or eliminated, and the unit 
operates reliably within safe limits, thereby improving profitability.
Not all changes require large capital investments to deliver fast and lasting returns. For example, adding missing measurement points to aid optimisation, adding smart, microprocessor-based instruments for asset health information and advanced process control can be implemented on key operating parameters at minimal expense to produce a significant ROI in a short time.
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