Pyrophoric Iron Oxidation in Distillation Columns

In petroleum refineries, the equipment most prone to pyrophoric combustion induced fires is the distillation columns in crude and vacuum distillation units. Deposits of iron sulfide are formed from corrosion products that most readily accumulate at the trays, pump around zones, and structured packing. If these pyrophoric iron sulfide (PIS) deposits are not removed properly before the columns are opened up, there is a greater likelihood of PIS spontaneous ignition. The trapped combustible hydrocarbons, coke, etc. that do not get adequately removed during steaming/washing often get ignited, leading to fires and explosions inside the equipment. These fires not only result in equipment damage but can also prove fatal for the personnel who are performing inspection and maintenance work inside the columns.

The accidents due to pyrophoric iron oxidations are entirely avoidable if safe procedures for column handover are followed. The targets of these procedures should be twofold:

1. First, to remove all the combustibles
2. Second, to remove or neutralize pyrophoric iron sulfide deposits

The basic distillation column oil-cleanup procedure is discussed in steps below.

Distillation Column Oil Cleanup Procedure

1. Steaming: The steaming is done after all liquid hydrocarbons have been drained from the column and associated piping. The objective of steaming is to make the column and associated piping free of residual hydrocarbons. The column vent and pump strainers in the side draw piping are de-blinded and steaming is started from utility connections at the bottom of the column. Generally, steaming is continued for about 20 to 24 hrs, ensuring the column top temperature remains more than 100 ^°C throughout the operation.

2. Hot Water Washing: When clear steam is observed exiting the column vents, water washing of the column should be started. With steam still in commission, water is sent to the column, usually via reflux lines, and it is drained from the column bottom, associated pump strainers, etc. The water flow rate should be adjusted so that steam still comes from the vent (i.e. water should not result in condensing of all steam before it reaches the column top). Water flow should be stopped for 2-3 hrs and then resumed. This cycle of steaming and washing should be repeated several times for a total of about 15 to 20 hours. Injection of a turpene-based detergent into the steam can also be considered. The condensate-soap solution can be collected and circulated through the various side cuts.

3. Blinding: When clear water is observed at side draw pump strainers, etc., associated piping should be isolated by installing blinds wherever isolation is possible.

4. Cold Water Washing: The hot water wash should be followed by a cold water wash (i.e. steam should be fully closed). The cold water washing is done for about 20-24 hrs.

5. Chemical Injection for Removal and Neutralization of PIS Deposits: During the cold-water wash or after washing is over, chemical injection for removal of pyrophoric sulfides should be considered. The various options for chemical treatment are discussed below:

• Acid cleaning - This procedure involves pumping in an acid with some corrosion inhibitor. The acid dissolves sulfide scale and releases hydrogen sulfide gas. It is effective and inexpensive, however, disposal of hydrogen sulfide gas can be a problem, as can corrosion (when the system contains more than one alloy). Dilute hydrochloric acid solutions may also be used. The resulting iron chloride turns bright yellow, acting as an indicator for removal of the iron sulfide.

• Acid plus hydrogen sulfide suppressant - Additional chemicals can be added to the acid solution to convert or scrub the hydrogen sulfide gas.

• Chelating solutions - Specially formulated, high pH, chelating solutions are quite effective in dissolving the sulfide deposits without emitting hydrogen sulfide, but this is an expensive application.
• Oxidizing chemicals - Oxidizing chemicals convert sulfide to oxide. Potassium permanganate (KMnO₄) has been used commonly in the past to oxidize pyrophoric sulfide. Generally the potassium permanganate is added to the tower during the cold water washing as a 1% solution. At various intervals, samples are taken and checked for color. The colors of the fresh KMnO₄ and the spent MnO₂ are purple and brown respectively. If the color of the solution becomes brown, additional KMnO₄ is needed. The reaction is judged complete when the solution color remains purple. It takes approximately 12 hours to complete the job.

  The cost of potassium permanganate treatment is more expensive than acid cleaning and traditional oxidizing agents such as sodium hypochlorite or hydrogen peroxide. Nevertheless, it is less corrosive to equipment than acid cleaning and used properly can be safer than other oxidizing agents.

  The following conditions should be avoided when using potassium permanganate:

  · Do not add KMnO₄ to acids or use in a low pH environment

  · Combustible materials should not be allowed to contaminate KMnO₄ stocks

  · Residual MnO₂ may remain in vessels after treatment and cause combustion or flammability issues in equipment with large surface areas such as packed towers

  · KMnO₄ cannot be used in conjunction with most detergents

  · KMnO₄ may have a “bad reputation” in some processing plants, but this is often times the result of misuse by contractors or plant personnel.

Alternative oxidation technologies are being developed with a focus on

• increasing safety in application
• saving water
• eliminating odor problems
• minimizing wastewater problems
• reducing wastes

One such alternative is Zyme-Flow®. Zyme-Flow® offers unique chemistry which is patented and offered by license from United Laboratories International, LLC as Zyme-Flow® and related products. The Zyme-Flow® chemical applications are administered by a highly trained staff of technicians provided by United and sold only by license from United Laboratories International, LLC.

The Zyme-Flow® generic Vapour Phase® method is apparently unique in that the de-oiling and oxidizer composition that is being dispersed actually may be vaporized in the steam (instead of being just atomized).  This allows the Zyme-Flow® composition to travel (in easily measurable concentrations) extensive distances and throughout equipment with high efficiency for contacting and condensing on internal surfaces.  The composition may expend quickly, but the application technicians can measure its progress.  This prevents over-dosing.

A very generic Vapour Phase® procedure may include:

1. Stop feed and de-inventory the unit per normal procedures.
2. Perform initial isolation per the pre-established plan with the Zyme-Flow® specialists.
3. Establish a thorough steam path throughout the equipment.
4. Add Zyme-Flow® to the incoming steam (commonly only 1-3 points are needed even for very large units).
5. Continue the injection and steaming for 8-12 hours.
6. Perform isolation per pre-established plan with the Zyme-Flow® specialists.
7. Possibly perform a final rinse with cold water to cool the columns quickly.
8. The unit is then ready for opening, ventilation, and hot work.

One major advantage for oxidizing pyrophoric iron sulfide is that the distribution dynamics of the Vapour Phase® applications are often more efficient than atomized distribution methods.  This is why Zyme-Flow® is often used for decontamination of flare lines and overhead systems where few injection points can be utilized.   The same dynamics allows for full treatment of most tight packing structures in refinery columns.

In some situations, Zyme-Flow® applications may be combined simultaneously or in sequence with compatible solvent and oxidizer products to target specific challenges such as monomer / polymer coatings and other challenges.    These applications require specialty design consideration from the Zyme-Flow® specialists.  This is especially important in structured packing situations where polymerization tends to coat and protect the pyrophoric deposits from contact with oxidizers until cooling promotes cracking of the polymer.