Chemical Decontamination Of Sour Water Tanks

Abstract:

Decontamination in general is defined as the removal of hazardous material from areas where it is not wanted. Decontamination is utilized to reduce the dose that worker may receive from a component or surface, to reduce the potential for airborne Chemical, biological, radiological, and nuclear (CBRN) agents, or to reduce the disposal cost associated with the component or the material. Chemical decontamination can be defined as a method essentially involving the conversion of toxic chemicals into harmless products, either by degradation or detoxification.

Chemical decontamination uses concentrated or dilute reagents in contact with the contaminated item, to dissolve the contamination layer covering the base metal and eventually part of the base metal. Chemical solutions are generally most effective on nonporous surfaces. The choice of the decontamination agents is based upon the chemistry of the contaminant, the chemistry of the substrate, and the ability to manage the waste generated during the process.[5] Chemical decontamination is usually carried out by circulating the selected reagents in the system. However, segmented parts may be decontaminated by immersing them in tank containing the reagent, which is then mostly agitated. Chemical decontamination processes are basically divided into two groups.

. Mild chemicals include noncorrosive reagents such as detergents, complexing agents, dilute acids, and alkalies. Aggressive chemicals include concentrated strong acids and alkalies and other corrosive reagents. The dividing line between these two groups of processes is usually at about 1 to 10% concentration of the active reagent. Mild chemical decontamination techniques have generally been used for items where the objective is to remove contamination without attacking the base material. Their advantages are low corrosion rates and low chemical concentrations, which facilitates the treatment of the spent decontamination solution (secondary waste). Although some low concentration decontamination techniques have low DFs and require long contact times, they may be made more effective by combining with processes using noncorrosive oxidizing or reducing agents, and complexing and chelating agents, and applying them in several stages. In many cases, the effectiveness may also be improved by increasing the treatment temperature usually in the range of 20 to 90°C. The selection of redox and chelating agents will depend on the composition of the surface corrosion products to be removed.

Aggressive chemicals and electrochemical decontamination techniques may involve one or more stages using different chemical solutions. A multistep

Chemical Decontamination Of Sour Water Tanks

process (i.e., the application of a strongly oxidized solution followed by a complexing acid solution) is a common technique for removal of the contaminated oxide layer from metal surfaces, such as stainless steel. Chemical decontamination involves techniques like using chemical solutions, multiphase treatment processes, foam decontamination, chemical gels, decontamination by pastes, decontamination by chemical fog, and gas phase decontamination.

Caustic compounds are used both solely and in solution with other compounds to remove grease and oil films, to neutralize acids, to act as surface passivators, to remove paint and other coatings, to remove rust from mild steel, to act as a solvent for species that are soluble at high pH, and as a means of providing the right chemical environment for other agents, mainly oxidizing ones. Examples include sodium carbonate, trisodium phosphate, and ammonium carbonate.

This research report is aimed at the finding the methods of chemical decontamination of sour water tank in refinery. Due to strict environmental regulations, the trend is changing from older less safe to relatively new, robust methods that provide better protection to working personnel and environment. Older methods concentrated on purging techniques to make it ……..for cleaning purposes. As new environmental regulations have put further restrictions on the free release of these contaminants in environment, so there is need to look for methods that can help us handle this contaminants to avoid safety hazard to workers and environment. The major contaminants that need to be dealt with in a sour water tank include ammonia, hydrogen sulfide and pyrophoric substance.

Chemical Decontamination:

Chemical decontamination of refinery process units reduces the preparation time for entry compared to traditional steaming out and hydrocarbon flushing. The chemical washing procedure rapidly penetrates deposits and suspends them for flushing out of the system, providing cleaner internal conditions. At the same time, residual hydrocarbons are captured and held in a temporary emulsion and transported away. Steaming alone cannot do this.

Goal:

To develop a method (with chemicals) to clean sour water tank with DECONTAMINATION TECHNIQUE using indigenous sources. Also list down the possible number of equipments required to carry out the cleaning.

Tank Service : Sour Water

Chemical Decontamination Of Sour Water Tanks

Tank Type : FRT

Floating Arm : Yes

Capacity : 12100 m3

N2 blanketing : Yes (10kg/cm2)

Contaminants : Hydrogen Sulfide, Ammonia, Pyrophoric content, Hydrocarbon content

Methodology:

Potassium Permanganate Method:

How pyrophoric material is formed and reacts with oxygen?

In the refining of oil, process and storage equipment commonly become contaminated with pyrophoric iron sulfide and various odor causing materials such as hydrogen sulfide and mercaptans. Pyrophoric iron sulphides form when iron is exposed to hydrogen sulphide, or any other compound that contains sulphur, in an oxygen deficient atmosphere. They are found frequently in vessels, storage tanks, and sour gas pipelines. Pyrophoric iron sulphides present a hazard when equipment and tanks are opened for cleaning, inspection, and maintenance. As the iron-sulphide compounds dry out and come in contact with air, they react with the oxygen and spontaneously ignite. Reactive iron sulphides can be deactivated when wetted with oil, therefore, rusted surfaces that are below the oil level are at low risk of causing an explosion.

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.

Chemical and mechanical methods are available to remove iron sulphides. The use of potassium permanganate is gaining acceptance for this purpose because it improves safety, saves significant cost and increases productivity. Other treatments include acid washing, chemical suppression, and the use of high-pH reagents.

Potassium permanganate is a powerful oxidizing agent that reacts with hydrogen sulfide

according to one of two reactions

Under acid conditions, the reaction is

Chemical Decontamination Of Sour Water Tanks

3 H2S + 2 KMnO4  → 3 S + 2 H2O + 2 KOH + 2 MnO2

Under alkaline conditions, the reaction is

3 H2S + 8 KMnO4  → 3 K2SO4 + 8 MnO2 + 2 KOH + 2 H2O

Potassium permanganate, long used for industrial odor control, is finding increasing acceptance for the

destruction of pyrophoric iron sulfide and odor compounds. Depending on the reaction conditions,

potassium permanganate will react with the pyrophoric iron sulfide to form either iron oxides or iron

sulfate.

9FeS + 26KMn04 + 4H20 -----> 3Fe3O4 + 26MnO2 + 26K+ + 9SO42- + 8OH- 

FeS + 2KMnO4 -----> FeSO4 + 2MnO2 + 2K+ 

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.

KMnO4/H2S (lbs of KmnO4per lb of H2S)dose ratio under

different conditions

Theoretical dose ratio Practical dose ratio

Acidic 3.1 3.5 - 5Basic 12.4 12.5 - 15

Chemical Decontamination Of Sour Water Tanks

No interim storage of used solution is required. No gas is generated, as with hydrogen 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.

Chemical Decontamination Of Sour Water Tanks

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.

How to apply the method?

"Isolation" means the process by which a permit space is removed from service and completely protected against the release of energy and material into the space by such means as: blanking or blinding; misaligning or removing sections of lines, pipes, or ducts; a double block and bleed system; lockout or tagout of all sources of energy; or blocking or disconnecting all mechanical linkages. 

The isolation plan should address:

Tank isolation requirements Tank suction and discharge lines Other tank lines, appurtenances and connections

Energy sources - electrical, mechanical and pressure

Cathodic protection systems Vapor recovery systems Tank foam protection system Tank gauging and overfill protection alarm system Interior devices such as mixers, etc. Verification of isolation prior to issuing permits

Evaluation criteria for decontamination technique:

The effectiveness of decontamination is usually measured with a decontamination factor (DF)

defined as

The effectiveness of the decontamination can be expressed as decontamination factor (DF). It is the ratio of contamination level of material before decontamination to the contamination level of material after decontamination.

Chemical Decontamination Of Sour Water Tanks

A decontamination process that removes material will result in a DF greater than 1. The percentage of contamination removed from the surface can be given by

Percent contamination removed = (1-1/DF) × 100

If DF = 10, percent contamination removed = 90%

If DF = 100, percent contamination removed = 99%

DF = Initial Conc. /Residual Conc.