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Velocity water x feedrate

Home Forums Coking Technical Heaters & Furnaces Velocity water x feedrate

This topic contains 3 replies, has 2 voices, and was last updated by  Mike Kimbrell 5 years, 10 months ago.

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  • #25163

    Hedewandro Lucredi

    We inject velocity water in our heaters (+/- 0,5 % of heater feed rate). Our mass velocity is +/-380. What is the velocity water flowrate recommended ? In which process condition can I cut the velocity water in the heater ? We are in the final campaign to decoking the heaters and a feed flowrate opened 100 % because the inlet pressure of the heater path. So, we cut the water injection.

  • #25200

    Mike Kimbrell

    It is typical to use 1% by weight velocity media, steam or water. If you use the Bechtel distillate recycle technology, a lower velocity media can be used. As you say, adding velocity steam, condensate or BFW increases the pressure drop through the furnace. At higher than design flow rates, the velocity media can be reduced to manage the pressure drop through the furnace. If you have the ability to shift the location of the velocity media injection to further down the coil, that can help manage the pressure drop through the furnace. The last 5 rows of tubes normally have the highest exposure to coking and have the highest need for higher velocities. High pressure drop through the furnace is an indication of coke deposition and that the heater should be decoked.

    A mass velocity of 350 lb/ft2-sec is a typical minimum value and provides a cold oil velocity of approximately 6 ft/sec for a feed that has a feed specific gravity of 0.935, more dense feeds will have a higher mass flux. Values of as high as 450 lb/ft2-sec have been used for the design flow rate.

    Coke deposition for a particular feed is dependent on the film temperature of the hydrocarbon in the tube at the tube wall and the residence time at coking temperatures or higher. Heater fouling rate increases exponentially with temperature and linearly with residence time. High velocities minimize the film thickness and the temperature of the film, which minimizes heater fouling rates.

    Velocity media is also needed to keep the velocity in the heater transfer line high enough to minimize the coke deposition between the heater and the coke drums. At high enough velocities, coke forms fairly uniformly around the circumference of the transfer line piping. At low velocities, coke forms in the stagnant zones and is non-uniform. This can cause high back pressure on the heater which will increase the heater fouling rate. When reducing velocity media, check the velocity in the transfer line as well so that velocity stays above the minimum.

  • #25205

    Hedewandro Lucredi

    Thanks for your answer. Which condition can I run the heaters without velocity media? In our case is water and because we are in the end of campaign for decoking we cut the water in one heater path. We measured by infrared the coil temperature and there is no huge change. Is it possible to operate a campaign with no water ? Do you have any experience or know any DCU which operate in this conditions (no velocity media) ?

  • #25208

    Mike Kimbrell

    Yes, with high enough velocity from hydrocarbons additional velocity media does not change the film thickness or heat transfer coefficient so that the additional velocity media is not necessary for the heater tubes. The Bechtel distillate recycle technology takes advantage of this and requires a lower velocity media rate to prevent heater fouling.

    I am aware of a case where a Coker was operating at much higher than design rates and the velocity steam was removed to reduce the pressure drop through the heater. In this case, the velocity in the transfer line from the heater to the coke drums was high enough that coke formation was still uniform around the circumference of the pipe.

    I am also aware of another Coker that reduced velocity media to reduce pressure drop through the heater; however, the velocity in the transfer line was low enough to allow the liquid to settle and coke which resulted in excessive coking in the transfer line. This transfer line coking raised the pressure in the heater coil and increased the heater fouling rate.

    For a low recycle ratio Coker operating near the design rate for the unit, I would always have some velocity media in the heater coils.

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