Improved cleaning method safely removes pyrophoric iron sulfide

Philip A. Vella
Carus Chemical Co.
LaSalle, Ill.

Traditionally, refiners have used mechanical or chemical methods for removing and controlling these contaminants. With increased focus on safety, efficiency, and the environment, however, new methods for equipment cleaning are required.

Potassium permanganate, long used for industrial odor control, is finding increased acceptance among refiners for the destruction of pyrophoric iron sulfide and odorous compounds.

A number of facilities have found that use of potassium permanganate results in increased productivity, improved safety, and significant cost savings.

Driving forces

The world's supply of light crude oils is diminishing, and heavy crudes are replacing them. Because these heavy crudes contain significantly more sulfur, their use increases contamination of process equipment.

One of the more common problems facing refineries that process heavy crude, or any crude containing sulfur, is the formation of pyrophoric iron. It is formed when sulfide-laden hydrocarbon streams contact any source of iron.

During normal plant operations, pyrophoric iron sulfide does not cause a problem. When specific units are opened for inspection and maintenance, however, the potential for a pyrophoric iron sulfide fire is significant.

Significant problems also are caused by sulfides and various organosulfur compounds. In addition to being odorous, the majority of them are also toxic. Because of safety and environmental considerations, these materials must be treated before a refinery unit is placed back in service.

Traditional decontamination methods include the use of steam, caustic soda, and, in some cases, sodium hypochlorite or hydrogen peroxide. Each of these methods has advantages and drawbacks.

Many refiners have used potassium permanganate successfully for the elimination of pyrophoric iron sulfide and odors (e.g., sulfides, mercaptans). The technique has been applied in a number of refinery units, including debutanizers, hydro crackers, cokers, and sour water tanks.

Advantages of this decontamination process include:

• Waste minimization (less waste water is generated)

• Safer, nontoxic by-products (direct discharge to a publicly owned treatment works is possible)

• Shorter downtimes

• Cost savings.

Pyrophoric iron sulfide

Deposits of pyrophoric iron sulfide form in environments deficient in oxygen, and oxidize rapidly upon exposure to air. When dry, these deposits will auto-ignite when exposed to air at ambient temperatures.

Iron and sulfur are the materials found most frequently in these deposits, although combinations of other elements may also exhibit pyrophoric properties. On the other hand, not every deposit that contains iron and sulfur is pyrophoric.

Pyrophoric deposits frequently form in storage tanks and processing units when iron oxide (rust) reacts with sulfur in the product being handled:

 Fe2O3 + 3H2S ' 2FeS + 3H2O + S

Upon exposure to air, the iron sulfide will react with oxygen:

 4FeS + 3O2 ' 2Fe2O3 + 4S + Heat

The reaction between iron sulfide and oxygen is exothermic and can provide enough heat to ignite any combustible material in or around the deposit.

Options for the removal of pyrophoric iron sulfide from equipment can be divided into four categories: acid washing; acid washing combined with chemical suppression; high-pH reagent use; and multistep chemical cleaning (oxidation).

Table 1 [14672 bytes] shows some of the advantages and disadvantages of the common methods for pyrophoric iron sulfide removal. It should be noted that, of the methods listed, only chemical oxidation removes both the pyrophoric iron sulfide and hydrogen sulfide in one step.

The features of several chemical oxidants for eliminating both pyrophoric iron sulfide and odors (sulfides, mercaptans, etc.) are given in Table 2 [31241 bytes].

Acid washing

This procedure involves pumping an acid solution into or through the equipment that needs to be cleaned. The acid reacts with the pyrophoric scale, dissolving it and releasing hydrogen sulfide gas.

The potential difficulty of this method is removal of the H₂S gas. If the concentration of the acid is too high, the evolution of H₂S can be so rapid that controlling the chemical cleaning procedure becomes impossible. If the gas cannot be removed efficiently and safely, the atmosphere can be explosive. In addition, acid cleaning can be corrosive to the equipment.

This method also leads to the problem of H₂S disposal, which may be accomplished a number of ways. The simplest method is to vent the H₂S to an existing flare system and burn it. This requires that the unit be properly equipped and piped to the flare tower. In addition, the accompanying problem of SO2 evolution and emission must be addressed.

Chemical suppression

A second way of dealing with the H₂S generated during acid washing is to scrub the gas with a chemical solution that converts it to a disposable form. One of the more common scrubbing methods uses sodium hydroxide to convert H₂S to sodium sulfide. Although sodium sulfide is less hazardous than H₂S, disposal of this caustic waste can be difficult.

Another method for H₂S control is to add a reagent directly to the acid solution. The reagents used for this purpose are capable of converting H₂S gas to another chemical compound. This compound is either retained in solution or precipitated as the H₂S reacts.

Concerns over this method are the scavenging ability and overall degree of H₂S suppression of the chemical additive. This could necessitate having a back-up system of some kind-either flaring or caustic scrubbing.

Common among these first two methods is the need to safely dispose of the acid and other chemicals used in the cleaning process.

High-pH solvents

Another approach to removing pyrophoric iron sulfide scale is to employ specially formulated high-pH solvents. These chemicals effectively dissolve the FeS scale while retaining most of the H₂S in solution.

This method offers several advantages: it is very effective, it emits little H₂S, and it does not form precipitates. But, although H₂S emissions are low, a back-up system still is required. In addition, the formulation of the high-pH solvent is expensive and may not be cost-effective for every application.

Oxidizing agents

The application of chemical oxidation eliminates the formation of H₂S. Oxidation of the pyrophoric iron sulfide results in the formation of various forms of iron oxide, which can be removed, if needed, by acid washing (hence the term "multistep").

The chemicals used for oxidative cleaning include sodium hypochlorite, hydrogen peroxide, and potassium permanganate. These chemicals are all effective, but each has drawbacks.

The use of sodium hypochlorite is inexpensive, but numerous side reactions can occur. Because organic compounds are always present in refinery equipment, there is a potential to form nitrogen mustard gas (dichlorodiethyl sulfide) when sodium hypochlorite is used. In addition, the nature of the chemical (a skin and lung irritant) demands additional safety procedures.

Hydrogen peroxide is less reactive with organics than chlorine and significantly reduces the chance of forming unwanted organic by-products. The reaction of hydrogen peroxide with pyrophoric iron sulfide, however, can be exothermic, and a considerable amount of oxygen can evolve. If the reaction is not adequately controlled, an explosive situation may develop. In addition, the concentration of hydrogen peroxide used requires special materials of construction and safety procedures.

Potassium permanganate is unique among the oxidants listed in that it can oxidize pyrophoric iron sulfide and other sulfide compounds, but it is safe to use and easy to apply. Under normal conditions it is applied as a 1-4% solution that is relatively harmless if it contacts skin. It requires no special materials of construction, and does not form harmful or potentially explosive by-products.

At the discharge end, potassium permanganate's purple color gives a positive visual indication when the pyrophoric iron sulfide has been destroyed. The by-product of the reaction is manganese dioxide. This material is biologically inert and can be directly discharged to a waste water facility. If the manganese dioxide must be removed from the equipment or discharge stream, simple reducing agents such as sodium thiosulfate or citric acid can be used.

A West Coast U.S. refinery has been using potassium permanganate exclusively for equipment decontamination for the past several years. One reason for its use was an experience the refinery had when using sodium hypochlorite to clean a sour water tank.

Sour water usually is generated during high temperature petroleum processing such as hydrotreating, cracking, or distillation. The water typically is stored in a sour water tank that can contain organic sludge, dissolved organics, and pyrophoric iron sulfide on the inner surface.

In this refinery's sour water tank, the concentrations of ammonia and sulfide were, respectively, 15% and 12%. Ethanolamines, used to remove hydrogen sulfide from refinery gases, also were present.

The recommended procedure was to inject a 0.1% sodium hypochlorite solution into the tank. But, because of worker complacency, a 7% solution was used. The result was formation of a brown gaseous cloud of unknown composition. Because of the combination of low pH and the presence of ammonia and hypochlorite, the cloud could have contained nitrogen trichloride, an extremely hazardous material.

The exterior temperature of the tank reached 140° F. and carbon monoxide was detected, indicating that combustion was occurring. Subsequently, it was determined that the atmosphere within the tank was just below the lower explosive limit.

In addition to the potentially explosive situation, the use of sodium hypochlorite posed other problems. Because diethanolamine was present in the water, the potential for the formation of nitrogen mustard gas, an extremely toxic compound, was high. Also, if the procedure were performed correctly, the volumes of chlorinated water produced would be large, creating a storage and disposal problem. (The volume would have overloaded the refinery's waste water treatment plant.)

Based on this experience, an alternative to bleach was investigated. Hydrogen peroxide was considered, but because of the evolution of oxygen, the potential for an exothermic reaction, and the safety concerns associated with using a 35-50% peroxide solution, it was not chosen. The company determined that, for most applications, potassium permanganate would be the best choice.

Hydrocracker treatment

One of the first applications of potassium permanganate at this West Coast refinery was decontamination of a hydrocracker. Past decontaminations involved the use of steam for deodorization and subsequent removal of the pyrophoric iron sulfide. The steam treatment took, on average, 1-4 days and had no effect on the pyrophoric iron sulfide. Loss of revenue due to down time ranged from $50,000 to $1,000,000.

Potassium permanganate was added to the unit as a 0.1-1.0% solution. The unit was filled with the solution and either circulated with pumps or agitated with air, nitrogen, or steam.

At various intervals, samples were taken and checked for color. If the color of the solution was brown, additional potassium permanganate was added. If the color was purple (indicative of permanganate), the reaction was complete and the unit was drained and opened.

The spent solution from this cleaning contained oxidized organics, iron oxides, and manganese dioxide. Because of the nontoxic nature of these by-products, the refinery was able to send the solution directly to the waste water treatment plant.

By using potassium permanganate, the refinery reduced the cleaning time to less than 12 hr. And the ability to send the used solution directly to the sewer bypassed the need for intermediate storage tanks that had been used with the previous treatment method.

The application of potassium permanganate was easy and required no special equipment or materials of construction. Overall, this procedure was safer, more environmentally friendly, and more cost effective than other chemical treatment methods.

Coker treatment

Based on this experience with potassium permanganate, the refiner decided to use it to decontaminate a coker.

The cleaning proceeded in two phases. First, a surfactant solution was used to remove the majority of the free oil on the equipment. After rinsing, potassium permanganate was used to oxidize sulfide, organics, and pyrophoric iron sulfide.

The time allotted for the initial cleaning phase was found to be inadequate. The process fell about 36 hr behind schedule.

Losses due to coker downtime were estimated to be $500,000-1,000,000/day. To attempt to make up the delay during Phase 2, the following options were outlined:

1. Do not use potassium permanganate to decontaminate the process equipment. Although this would put the timetable back on schedule, the sulfides, mercaptan, and pyrophoric iron sulfide would not be removed. This option was not considered viable.

2. Drain the unit, flush it with fresh water, and fill it with a 0.1% potassium permanganate solution. After decontamination, drain the unit for inspection and mechanical maintenance. Although this option would fulfill the decontamination requirements, it would add at least 24 hr to the turnaround, resulting in a total time delay of more than 60 hr. Because of its effect on project economics, this option was not chosen.

3. Add a 0.1% potassium permanganate to the unit directly, without a clean water flush. It had been determined that the presence of residual surfactant would increase potassium permanganate requirements. This would increase the cost of the operation, as well as the total amount of dissolved solids in the spent solution. The chemical costs could be offset by savings in time, however, and the solids issue was not a problem because the treatment plant would be able to handle the increased loading.

After careful consideration, Option 3 was implemented. The time required for decontamination was less than originally expected, and this portion of the turnaround was returned to its original timetable.

A second refinery has four sour water tanks in service. Two of them had not been cleaned for 11 years and contained about 5 ft of sludge and an additional 6 ft of sour water. In total, each tank contained about 600,000 gal of liquid to be treated. In addition to the liquid, the inside walls of the tank were coated with pyrophoric iron sulfide, which would require treatment after the liquid was removed.

Two of the four tanks had been cleaned the previous year. At this time, the material in the tanks was oxidized and dewatered, and the solids were sent to a hazardous waste disposal site. The total cost of this process was more than $1.00/gal. The refinery needed a more cost-effective process for decontaminating its sour water tanks.

When the option of using hydrogen peroxide arose, it was immediately rejected, primarily for safety reasons. Workers had used peroxide in the past and did not feel comfortable using the product. Minor spills on clothing or skin could cause serious injury-effects not encountered with potassium permanganate.

Having had experience with potassium permanganate in the past, the refiner decided to use it on these tanks.

Treatment of the liquid was a relatively straightforward operation. A 3-4% permanganate solution was prepared (20,000 gal). This solution was fed to a static mixer concurrently with sour water in a 1:1 ratio. The treated solution was then transferred to holding tanks to await further processing on site.

Pyrophoric iron sulfide in the tank was treated subsequently by spraying potassium permanganate into the tank until a purple residual was observed. At that time, the tank entered and the sludge was broken up and treated with a potassium permanganate solution applied at moderate pressure. Residual from this portion of the clean up also was sent to the coker.

At this refinery, the material was not discharged to the waste water treatment plant, but rather fed to the coker in small quantities.

Using potassium permanganate, the cost of handling and disposing of the treated material on site was less than $0.10/gal.

Costs

Potassium permanganate is more expensive than traditional oxidizing agents such as sodium hypochlorite or hydrogen peroxide. Current permanganate costs are $1.50-1.80/lb, while conventional chemicals cost about 30% less.

It is difficult to predict the cost of decontamination because the amount of organic compounds present is unknown. The reduced treatment time and decreased waste handling requirements associated with permanganate, however, almost always gives it a cost advantage.

In one application, a refiner was given an estimated chemical cost of $50,000 for permanganate treatment. This increase of about $15,000 over conventional chemical costs was more than compensated for by a reduction in unit downtime of 1 day.

Advantage summary

In the application of potassium permanganate for pyrophoric iron sulfide and odor destruction, the following advantages over other oxidation and destruction technologies have been observed:

• No interim storage of used solution is required.

• No gas is generated, as with peroxide.

• The concentration is self-limiting. (It cannot form highly concentrated, dangerous solutions, and there is reduced risk of exothermic reaction.)

• No undesirable and hazardous side reactions occur (such as formation of chlorine, mustard gas, etc.).

• No special materials of construction are needed.

• Application is safe, easy, fast and requires simple equipment.

• Purple color provides positive visual identification of completed reaction.

• No harmful effects on biological systems occur.

• Downtime is shorter (hours instead of days), resulting in potential savings of $50,000-1,000,000/day.

The Author