Preflash Towers Save Energy

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A refinery designed to run a 30° American Petroleum Institute (API) crude must unexpectedly switch to a 40°API crude. Sometimes, large amounts of natural gas condensate (butane and naphtha) are commingled with the crude supply. Such changes can lead to a reduction in crude running capacity. This happens because:

  • The fractionator tower becomes overloaded and the trays flood.
  • Increased vaporization of the lighter crude through the preheat exchan­ger train and furnace increases pressure drop, which reduces the capacity of the crude charge pump.
  • The furnace duty must increase to vaporize a higher percentage of the lighter crude.
  • The fractionator condensers cannot handle the increased load; conse­quently, the reflux drum runs hotter and excessive wet gas production results.

The answer to these problems is to install a preflash tower. If the unit has a flash drum upstream of its furnace, this drum can be retrofitted as a preflash tower. Figure 1-9 illustrates a preflash tower arrangement used with success at several refineries. Significant features of this arrangement are:
• The preflash tower is located midway in the preheat exchanger train, and its pressure “floats” on the fractionator pressure.
• Only one barrel of reflux for each four or five barrels of net overhead liquid is required. This low reflux ratio is permissible because the preflash overhead liquid product is charged to the crude fractionator, several trays above the heavy naphtha drawoff. This effectively redistills the poorly fractionated preflash liquid.
• Preflash overhead vapors are cooled by preheating crude.

Field tests have shown that the changes depicted in Figure 1-9 result in no measurable loss in separation efficiency between light and heavy naphtha. For a typical low-sulfur crude (35°API), 6-8 vol% of the crude is taken overhead in the preflash tower. The observed reduction in furnace duty was 5%-10%.

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