NDT.net • April 2004 • Vol. 9 No.04
2nd MENDT Proceedings

Corrosion Problem in Sulfolane Extraction Unit (U 7300)

Abdulmunem AL Harbi, SASREF, Jubail

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Description of Process:

Sulfolane Extraction Unit uses as a separation process an extractive solvent (sulfolane) to separate aromatics from non-aromatics.

Aromatics and non-aromatics mixture is coming from platformer Unit 7200, where aromatics are generated.

In sulfolane Extraction RDC, aromatics are dissolved in sulfolane and this mixture is the extract phase (bottom), while the remaining non-aromatics counter-currently contacted and efficiency of stage is improved by mixing with rotating baffles.

Sulfolane is recovered from raffinate first in a coalescer and then in a water wash column. The final raffinate is pumped to Naphtha Blending as gasline components.

Extract is stripped and enriched in aromatics after recontacting with additional sulfolane. Solvent is recovered under vacuum distillation form extract; extract is separated from water by cooling and clay treated to eliminate olefins and diolefins, sent to Aromatics Fractionation Unit 7400 to separate benzene, toluene and xylenes.

Summary

This unit has a history of severe erosion/corrosion and especially after shutdown March-96; it has been observed an increase in corrosion rate in the SEU (sulfolane extraction unit) especially the fat solvent line to stripper C-7303 ( after upgrading the bottom section of C7302 , March 99) . This loop contents of extractive stripper column(C-7302), reboilers (E7306 A/B), transfer lines (P73214/P73006) and solvent recovery column (C-7303).

The Extraction Stripper Column (C7302) has been lined with Carbon Steel plated at the bottom section in Dec-1989. In Nov-1991 the lining plates started to corrode. The bottom section has been lined with 410 SS in Mar-1996. In Aug-1997 a leak developed on the bottom dome. In Mar-1999 the bottom section 7m from the bottom has been replaced with duplex SS.

The reboilers (E-7306 A/B) have been retubed six times since day one 1985. The last retubing was by Duplex SS at Mar-01.

After those modifications the erosion/corrosion problem has shifted to transfer lines (P73214/P93006) and solvent recovery column. The corrosion rate became higher than before and three pinholes were developed in the transfer lines after that modification.

At the last shutdown the solvent recovery column was opened for inspection and found the inlet and bottom sections have corrosion more than was expected.

The root cause is mostly the accumulation of acidic material in the circulating solvent.

The accumulation of acidic material in the circulating solvent (i.e. chlorine ex feed and suffocate degradation products due to reactions with oxygen) has the combined effect:

  1. Oxygen ingress
  2. Solvent filter not always in operation
  3. Solvent regenerator operated below max. capacity.
  4. Reduction in water bleed ex C-7307 after shutdown in Mar-01

SEU Corrosion/Erosion Problem

In general the following four items are seen as the main causes for SEU corrosion/erosion problems in the SEU
  • Oxygen in the plant ( ingress or via incoming flows )
  • Chlorine in circulating solvent.
  • Accumulation of degradation and corrosion products in the plant.
  • High temperature in reboilers.

Oxygen

Oxygen in contact with suffocate at the condition in the SEU plant leads to rapid degradation of the solvent. The most likely places for oxygen entering the system are the vacuum parts of the unit, dissolved oxygen in the feed and in fresh or wet solvent supply. To check where oxygen ingress takes place it will be required to check all flanges in the vacuum system. This can be done during normal operation (very difficult) or at least this should be checked before the plant is started up after a shutdown or having opened equipment in the vacuum section. After a shutdown it is recommended to do both a vacuum test as well as a pressure test to find leaks.

Chlorine

Chlorine can form HCL in the hotter parts of the unit and will therefore be very corrosive Chlorine can enter the unit with the platformate feed (usually 1 ppmwt or less) or via cooling (sea) water leaks.

Accumulation of degradation and corrosion products

Over the years, degradation and corrosion products will have accumulated in the plant. Experience from other sites that once in a while a very through cleaning of the plant is required. Dirt accumulated in the plant is usually corrosive material and/or act as a kind of catalyst to make more degradation products. It is therefore advised to plan for the next major shutdown to do a though cleaning. In the Stanlow SEU about 10-15 m3 dirt was taken out of the unit.

High steam and reboiler temperature

The temperature has a large effect on the degradation of sulfolane by oxidation. To minimize the temperature effect it is advised to operate always at temperature lower than 175° C on the process side and at a (condensation) temperature of max. 205° C on the steam side of a reboiler. The operating data from the SEU show that most of the time these criteria are met.

RECOMMENDATION

  • Check tightness of vacuum system
  • At start up do pressure test as well as vacuum
  • Check chlorine content a regular basis
  • Check water of V-7301/V-7304 on Na (if there sea water leak)
  • Operate regenerator at max. throughput and drain every 2-3 months
  • Clean SEU equipment and lines in shutdown
  • Max temperature on process-side reboilers is 175° C
  • Steam condensation temperature in reboiler max 250° C

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