Crude Unit Stripping Sections

Authors: 
Sloley, A. W., Advisian (WorleyParsons Group)
Crude unit main fractionators include stripping sections in three services:

  • Atmospheric residue stripping
  • Vacuum residue stripping
  • Atmospheric preflash stripping

Other types of crude unit main fractionators can have stripping sections, but they are rare and are not discussed here.

The stripping sections of the atmospheric tower and vacuum tower use inert gas stripping to recover light material by vaporization and partial pressure reduction. Stripping in preflash towers may be either inert gas stripping, reboiled stripping, or a combination of both.

Based on a recent survey of crude units, the most common services are atmospheric stripping (nearly always present) and vacuum stripping (in about 55% of vacuum towers).

The stripping section in a crude unit main fractionator is the section below the two-phase feed. The stripping section removes light material from the feed by inert gas stripping. Steam is used for the stripping.

Stripping section objectives vary with the service and unit configuration. Common objectives include flashpoint control and stripping for bulk recovery of light material. Downstream consequences are different for different types of units.

The major factors determining stripping section performance are stripping steam rate and number of stripping stages. The higher the stripping steam rate, the more stripping. The higher the number of theoretical stages, the more stripping.

The effect of one theoretical stage of stripping is large. Two theoretical stages of stripping would be cost effective on most units. Three theoretical stages may or may not be economically attractive. The benefits of four or five stages are minor.

Stripping in the main fractionator can also allow for operation at lower heater outlet temperatures. This keeps the unit yield constant at lower coking rates.

Vapor rates change dramatically across the stripping section. This requires customized tray layouts to maintain tray efficiency. Many towers have very low stripping section efficiency because of ineffective tray layouts.

Stripping is an effective method of improving crude unit operation. Keeping the stripping section working increases yield and extends run-length. Both improve operating profits. Understanding the interaction of mechanical and process factors allows the plant engineer to maintain unit performance.

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