Oil recovery from tank sludge
Thorough tank cleaning means removing all of the sludge and restoring the vessel’s full storage capacity.
MARCELLO FERRARA and CRISTINA FERRARA
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Although somehow neglected by the industry, tanks are vital for production: indeed, no production can occur without proper storage. Tank cleaning normally occurs for maintenance and inspection purposes, and less frequently for recovering storage capacity. However, any sludge at the bottom of the tank equals volume lost in the tank’s useful storage capacity.
When selecting the proper tank cleaning technology, a number of evaluations should be performed: for instance, manpower requirements, the effectiveness of the method, the equipment used, the downtime involved, the waste produced, environmental and safety concerns, and utilities consumed. In general, all of the costs (direct and indirect) involved in the cleaning have to be evaluated.
In an effort to reduce operating costs, many oil companies aim for the lowest price without taking into consideration the full picture, just the cleaning cost. This is particularly true for tank cleaning, wherein many conventional manual cleaning operations are available, most of which offer the same technology.
Conventional tank cleaning methods
Manual cleaning is the oldest and today’s most widely used method of tank cleaning. Often, this is also thought of as the only effective, easy and cheap technology.
But what really happens during manual cleaning? The first step is emptying the tank to the lowest possible level; thereafter, only sludge, deposits and a certain amount of oil will be left in the tank.
The manways need to be opened for man entry, so breathing apparatus is mandatory. The tank is not safe for unprotected entry and a number of additional requirements need to be complied with. Among them, any job must be stopped whenever the lower explosive limit (LEL) exceeds 10%. Thereafter the tank is ventilated until the LEL falls to a lower value.
The tank is cleaned by manual removal of sludge and, when applicable, high pressure water jetting, with the following results:
• The value of the oil is lost together with the sludge
• High amounts of waste (water, sludge, oil) are generated, which need to be disposed of
• There is significant use of equipment and manpower
• HSE concerns apply during job execution
• Downtime is huge.
Last but not least, the manual cleaning job is very hard and manpower cleaning efficiency is very low. Workers are acting in a confined space, wearing heavy personal protective equipment with breathing apparatus, and can hardly move inside the tank as they are walking on a very viscous and sticky medium. In the case of high ambient temperatures, working becomes possible for some minutes only. Workers are therefore allowed a long resting time within their working shift after a certain time inside the tank.
It has to be mentioned that nowadays it is very difficult to find skilled workers for this job which is surely the most arduous in the industry. If this method is chosen for its ‘economy’, the above items must be kept in consideration along with the ‘simple’ cleaning cost. The cost impact might be a multiple of the mechanical cleaning cost and so will affect the economy of the entire project.
To overcome the drawbacks of manual cleaning, some other methods have been developed (‘no man entry’ techniques) but all have their related problems. For instance:
• Cleaning with robots improves safety and sometimes the duration of operations but it does not have any impact on sludge recovery.
• Physical separation methods involve the use of multiple equipment like centrifuges and decanters to enhance sludge recovery. This method effectively separates sediments only from the sludge. No real oil recovery occurs, only the creation of an unstable oil phase which will precipitate again once pumped into another tank.
• In crude oil washing, crude oil is used as a ‘solvent’ for sludge. Here is a simple question: if crude is a solvent for the sludge, why has the sludge precipitated? Crude is not a solvent for the sludge and this method simply moves the sludge from one tank to another (a dispersion method). Integration with a physical separation method adds only the removal of sediments but, again, does not impact real oil recovery.
• Conventional chemical cleaning uses a dispersant to solve the problem. In reality, the problem is transferred to the receiving tank and the chemical does not impact real oil recovery.
To solve all of these problems, ITW has patented a technology for tank cleaning.
ITW tank cleaning technology
ITW has a novel technology for asphaltene stabilisation. According to this approach, the sludge is removed by the addition of a proprietary chemical, without the need to open manways.
The patented chemical contains proprietary asphaltene stabilisers, paraffin solvents and fluidising agents. A different proprietary chemical is used to dissolve polymers when cleaning tanks in the petrochemical industry.
The chemical is circulated inside the tank and, while circulating, it will dissolve and stabilise the sludge. With the technology, the sludge is transformed into a reusable, pumpable liquid product. It is chemically stabilised, there is no danger of subsequent precipitation, and therefore the sludge can be pumped from the tank as a stable liquid and fully reprocessed.
Sediments are left at the bottom of the tank or separated during circulation. Release of sediments from precipitated asphaltene is not possible unless the asphaltene structure becomes disjointed. The concept is illustrated in Figure 1.
According to the method, after the first step of dissolving and stabilising the sludge, a second step of tank decontamination follows in order to allow entry under safe conditions.
This second step normally reduces the LEL to 0%, as well as H2S and benzene levels to 0 ppm. No work interruption is needed as LEL safe values are not exceeded.
As the developer of the technology, ITW can tailor-make chemicals to target the many types of sludge in the oil industry. The following case histories illustrate some applications of the technology.
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