Finding the right blend for effective tank cleaning

The chemical and physical properties of the previous cargo determine the tank cleaning process because all of the residues remaining on board (ROB) after the previous cargo is discharged must be completely removed before the cargo tank can even be considered as being load ready. When the ROB has been completely removed, the cargo tank may be considered visually clean, but thereafter the quality specifications of the next loaded cargo dictate any additional tank cleaning that may be required prior to the start of loading. In a growing number of cases, the suitability of any additional tank cleaning can only be measured by means of chemical analysis, typically a wall-wash inspection. Assuming the cleanliness of the cargo tank meets these specifications, the vessel may then be considered chemically clean.

This is accepted, but how valid is it? All of the published tank cleaning guidelines have some references to the difference between visually clean and chemically clean, but there is very often no mention of the significant challenge faced when a vessel needs to upgrade from a visually clean standard. The reason for this is quite simple: there is no cure-all answer that applies to all cargo tank coatings and previous cargoes.

Cleantech engineering group OTI Greentech decided the best way for it to penetrate the tank cleaning market with its environmentally friendly product in the shortest possible time was to focus on two key areas: meeting the Methanol Standard, and meeting the requirements of the vegetable oil trades

One challenge is cleaning to a methanol standard from clean products, bearing in mind that bulk volumes of methanol tend to be shipped in zinc silicate tonnage. E-14 Marine was developed to remove hydrocarbon residues retained in the surface of zinc silicate paint films, and so expedite the objective of achieving hydrocarbon-free wall-wash samples. In practice, E-14 Marine has been found to outperform many market leading chemicals designed to achieve the same end result.

Another challenge is cleaning from vegetable oils, bearing in mind that the majority of effective vegetable oil cleaners contain highly alkaline additives that are corrosive and hazardous. E-24 Marine was developed specifically to be pH neutral, yet still have an equivalent or superior cleaning effect compared with the market-leading commercial products.

The company spent two years testing, refining and developing its products by working with L&I Maritime Ltd. The result was the Ecosolut 14 Marine range, a marinised version of its well-proven Ecosolut series.

Maersk and Waterfront agreed to trial the products, and this culminated in orders. OTI Greentech then signed a distribution agreement for the innovative tank cleaning products with Uniservice Germany Marine Products. Uniservice Germany is a licensee of Uniservice in Geneva, and is represented in over 785 ports worldwide. Also affiliated with Uniservice Germany are Oslo Ship Service SA and Hansa Safety BV in Rotterdam. In addition, a distribution agreement was signed with OSEC (Oil Spill Environmental Cleaning), a specialist in tank cleaning and oil disposal, with offices in Greece and the southeastern Mediterranean region.

An especially tenacious cargo is paraffin wax. This is obtained from the petroleum industry, mainly by de-waxing light lubricating oil stocks. The product does not consist of one single chemical compound but is a mixture of solid straight-chain hydrocarbons, whose chain length depends on the boiling range of the oil distillate from which they are obtained. Its consistency may vary from solid to liquid, depending on the surroundings, since the melting point ranges from around 48^oC to 66^oC.

Due to their insolubility in water and regardless of the type of cleaning agent used, paraffin wax and related products are impossible to remove from tank walls, lines and pumps without heating the cleaning solution to above the melting point.

These important factors have to be carefully taken into consideration to secure safe, fast and efficient tank cleaning. It is not sufficient to clean only the tanks: all parts of a vessel that come into contact with such cargoes must be cleaned to the same standard, including pumps and line systems. Failure here due to cold areas (below the melting point of wax) may result in clogged pumps and lines during cleaning and draining operations.

When heated to above the melting point, paraffin wax and related products are relatively easy to remove with the aid of an effective cleaner. Unitor markets suitable products, such as Aquatuff and Aquabreak PX, which are rich in selected surface active ingredients. By keeping the temperature of the wash water at >10^oC above the melting point of the wax, hot water does the main job by reducing the viscosity of the wax and liquefying it, while surfactants reduce the surface tension between water and wax, ensuring effective dispersion. In other words, such a solution ‘lifts’ wax off the tank walls and into the wash water as small droplets that will be kept floating in the water without danger of recoating the tank walls.

By still keeping the water temperature at >10^oC above the melting point while the tank is being drained, liquid wax droplets will continue to float in the water without danger of recoating pumps, hoses and lines.

If a cleaning operation does not give satisfactory results first time, it is best to first try increasing the temperature of the wash water, before considering increasing the concentration of the cleaning agent or substituting it for another product.

Occasionally, a solvent is needed to remove traces or spots, but in general, cleaning with hot water and a selected water-based product is all that is needed to achieve economical, high-performance cleaning.

Due to the presence of caustic ingredients, Aquatuff cannot be applied on zinc silicate coatings. Aquabreak PX is based on the same technology as Aquatuff, but is safe on most materials. These chemicals are distributed by Wilhelmsen Ship Services. TST