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Oil tanker cleaning

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Current cleaning method

In most cases sludge in crude carriers is removed manually (see picture above). Manual removal of the sludge is a time consuming operation, complicated by the fact that crude carriers are difficult to operate in. Manual cleaning always implies a special attention to safety of operations.

 

ITW method

ITW has patented a novel technology for asphaltene stabilization. Such technology makes use of chemical additives to be added to hydrocarbons and has proved very effective in many industrial applications. The technology has been suitably modified for the cleaning of crude oil cargo tanks. Asphaltene stabilization achieves an improvement in sludge reuse, in that asphaltene association is reduced, compatibility with the receiving hydrocarbon is enhanced, precipitation does not occur and cracking of asphaltene is facilitated.

According to the ITW approach, sludge is removed by the addition of a chemical additive, see case story, which contains asphaltene stabilizers, patented by ITW. The formulation also includes paraffin solvents and fluidizing agents. The additive is utilized to help sludge penetration, thus favouring its solubilisation into a carrier. Sludge dissolution occurs due to the chemical action of the additive during recycling of the oil phase.

Marine Engineers review Cargo oil sludge minimised by COW additives (MEG has changed its name into ITW)

>>>> read further Marine Engineers Review

Case StoryAn oil tanker (120.000 m3 capacity) needed to be cleaned before dry-docking. Standard oil tanker cleaning procedure involves the use of Crude Oil Washing (COW) followed by water cleaning. However, these operations are not completely effective as tank washing machines have some shadow areas, where the flow has no direct impact on the sludge. Therefore after COW some sludge is left in the bottom of the tanks, and has to be removed manually. ITW additives have been added both in the COW phase and in the water-washing phase. In the water-washing phase a patented hydrocarbon solubilizer has been added. This product is capable of temporarily solubilizing hydrocarbons in water, when the two phases (additivized water and hydrocarbons) are in direct contact with each other (e.g. under agitation).The solubilization is temporary and, after some minutes, the hydrocarbilic and the water phases separate out. The water phase is very clear and no emulsion is formed. The additive also facilitates the separation of oil from water. COW was performed with Bouri crude oil, which is a very severe crude, like the others transported by the ship. Before performing the additivized COW, some standard COW were performed: a) one upper cycle 120°/30° and b) two bottom cycles 0°/30°/0°.

At the end of those cycles the slop tank contained 1300 m3 of Bouri crude and sediments. This is what the standard COW procedure would have achieved in terms of recovered sludge. One further bottom cycle with the additive (injected in the suction of the COW pump) was performed, and stopped after only 12 hours washing (total time for all the cargo tanks).At the end of this additivized cycle, the slop tank contained 1500 m3 of Bouri crude and sediments.

Therefore, the use of ITW stabilizer allowed for a recovery of 200 m3 of sludge in the COW phase. The additivized water-washing phase was performed in the same way as the additivized COW, with water heated at 60 °C. At the end of this phase, further recovery of 130 m3 of oil was achieved. Therefore, following injection of ITW additives, the total amount of recovered sludge was 330 m3. All the 1630 m3 of crude oil + recovered sludge were sent to the refinery for reprocessing. No operational problems were reported at all. The results achieved are extremely positive in terms of recovered sludge.

Above results are even more valuable when taking into account the type of crude oils carried by the ship and hence the kind of sludge generated by them. The carried crudes are mainly Belayim, Bouri, Bu Attifel and Es Sider. Belayim and Bouri are particularly well-known in the petroleum industry as crudes which bear fouling problems. Bouri, in particular, is a very unstable crude, which causes severe fouling problems in refinery plants. Its visbroken TAR instability has been well recognized; some FCCU overhead section restrictions (following coke deposition) have also been reported when processing Bouri gas-oils. Despite Bouri’s characteristics, the additivized COW procedure was extremely successful, which confirms the stabilizing properties of ITW additives.

The payout of the application has been in the range of 16:1, due to the dramatic reduction in oil tankers’ cleaning time, and hence recovered freight costs.

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