logo


Avoid fired heater coking

For many refiners, heater coking in Crude and Vacuum Distillation Units (CDU/VDUs) is a common occurrence. Many units around the world are shut down every two years, every year, or even every six months to deal with chronic heater coking. However, with the right design features driven by a solid understanding of heater coking mechanisms, fired heater run length can be extended beyond five years, even with relatively challenging crudes.

The two primary drivers of heater tube coking in CDU/VDU services are oil film temperature and residence time. Secondary factors such as crude coking tendency, solids content, and blend instability can further accelerate heater tube coking. So, which heater design parameters will maximize heater run length and avoid shutdowns for high heater tube metal temperature or high heater pass pressure drop?

Mass flux is king
Mass flux (lb/s/ft2 or kg/s/m2) is found by dividing the mass flow through a heater tube by the tube’s cross sectional area. High mass flux begets high velocity and suppresses coking in several important ways. First, high mass flux means that the fluid moves through the tube faster, minimizing residence time. Second, high velocity results in high heat transfer coefficient, which minimizes internal oil film temperature. Finally, high mass flux creates high wall shear inside the tube, minimizing build-up of solids or asphaltenes.

Heat flux can surprise
Heat flux (BTU/hr/ft2 or kcal/hr/m2) measures the amount of heat absorbed through a given outside surface area of a heater tube. High heat flux raises tube metal temperature and causes high oil film temperature inside the tube. Popular fired heater design programs use a well-stirred firebox model and calculate peak heat flux by applying a simple multiplier to the average heat flux. In reality, heater design parameters such as firebox height/width ratio, burner type, burner sizing, burner placement, and air/flue gas flow patterns can result in actual peak heat fluxes that are much higher than the “calculated” peak heat flux on the heater datasheet. Localized areas with very high heat flux will coke and suffer from high tube metal temperature.

Of course there are many other variables that must be considered, such as pass arrangement, vertical or horizontal tubes, cylindrical or box or cabin, coil steam, etc. Problems stemming from blend instability are becoming more common as refiners are increasing lymixing light shale crudes with heavy crudes. As the crude begins to vaporize, asphaltenes can precipitate out of unstable mixtures and coat the heater tubes, forming coke and creating hot spots.

Even with challenging crudes, refiners have achieved Crude Heater and Vacuum Heater run length goals through careful design and respect for the basics of coking. Contact Process Consulting Services, Inc. to learn more.

DOWNLOAD LITERATURE

View More

  • Oil sands crude — profits and problems?

    Canadian bitumen production currently runs about 1 MMbpd, with some being sold as Synbit and Dilbit. Over the next 10-12 years output is expected to increase to 3.5 MMbpd and more refiners will begin investing to process it and come to depend on the Synbit and Dilbit for a significant part of their supply. ...

  • Nasty stuff

    Heavy crudes are here to stay. As longs as oil prices remain high, Canadian, Venezuelan, Deep Water Gulf of Mexico, Mexican and other low API gravity crude oils will play an ever more important role in supplying world refineries. And prices promise to remain high because gainsayers notwithstanding, Hubbert ...

  • Designing deepcut vacuum units that really work

    Every barrel of vacuum gas oil (VGO) you can save from being reduced to coke in the delayed coker unit is a barrel more that can go to the FCCU. That’s a good reason to raise HVGO cutpoint. But how to do it? Some people think the job can be done just by running computer models in the engineering ...

  • A time for grass roots thinking ?

    Within the past year or two spiking crude prices and surging refinery margins have led to overheated talk about increasing refinery capacity worldwide. Plans for construction of as many 60 grass roots refineries have been discussed. But stretched out lead times for major equipment and inflated prices, ...

  • A single integrated vacuum system

    Failure to design the vacuum unit as an integrated system will invariably result in unsatisfactory yield and poor product quality (high vanadium, nickel, microcarbon, or asphaltenes), and ultimately, an unscheduled shutdown. To avoid these revamp problems the charge pump, fired heater, transfer line, ...

  • Is pinch enough?

    Back in the late 1970’s and early 1980’s when fuel gas prices were high, energy utilization assumed major importance. A new method of calculating heat exchanger networks was developed. It was called Pinch Technology. Today pinch has been rediscovered by engineers who have access to fast computer ...

  • Opportunity knocks

    A group of interesting articles* deals with opportunity crudes, a mixed breed that includes very heavy, sour and high total acid number types as well as those with unexceptional naphthenic acid content but which do have significant concentrations of aliphatic acids or possess the ability to generate ...

  • Processing heavy Canadian crude

    Reducing crude oil cost is the major incentive driving crude and vacuum unit projects to handle heavy Canadian crudes. But such crudes–Albian Heavy, Christina Lake, MacKay River and others derived from oil sands–today present refiners with a unique set of problems not just because of extra-low ...

  • Why do many crude/vacuum units perform poorly?

    In many cases it’s because the original design was based more on virtual than actual reality. There is no question: computer simulations have a key role to play but it’s equally true that process design needs to be based on what works in the field and not on the ideals of the process simulator. ...

  • Why produce diesel from the vacuum unit?

    Look ahead five years. The economy is likely to keep tightening and the rush to control pollution will inevitably be accompanied by demands for greater energy conservation. Consequence? A growing market for diesel which yields more energy per unit volume. Yet many continue to believe that producing diesel ...