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Oil sludge collector and tank cleaning equipment

Oily sludge is produced and collected alongside with oil recovery, transportation and processing. Generally oil sludge can be classified into 3 main groups: ground, bottom and tank types. The first type of sludge results from soil contamination during production activity, or in emergency cases. Bottom sludge is formed due to sediment of oil spills to the bottom of water basins, while tank sludge is formed during storage and transportation of oil products in reservoirs of various designs.

Simplified oil sludges represent multi-component stable aggregative physico-chemical systems consisting of oil products, water and mineral additives (sands, clay, metal oxides, etc.). The primary reason for sludge formation is physico-chemical reaction between oil products and water, air oxygen and mechanical additives. As a result basic oil products are partially oxidized, generating retinoid compounds. Ingress of water and mechanical impurities into oil product leads to water-oil emulsification and formation of mineral dispersions. Since sludge is the result of interaction with specific environment and within specified time period, there can be no types of sludge identical in composition and physico-chemical characteristics. Content percentage of oil products, water and mechanical impurities (sand, clay, rust etc. particles) ranges in wide range: hydrocarbons 5-90%, water 1-52%, solids 0,8-65%. Due to such dispersion in sludge content the range of its physico-chemical characteristics is just as wide. Sludge density varies from 830 to 1700 kg/m3, its setting point from -3°C to +40..50°C.

On entry into oil product water emulsifies into ‘water-in-oil’ emulsions, stabilized by the presence of natural emulsifying agents like pyrobitumen, tar and paraffin.

The reason for stability of "water-in-oil" emulsions" is the presence of a structural-mechanical barrier on the surface of emulsion droplets. This barrier acts as a double electric layer on the interphase surface. Concentration of these substances in oil product volume increases together with the growth of their molecular weight (transition to tailing fractions). Apart from emulsification the processes of transportation and storage of oil products invoke the formation of polydisperse systems with liquid hydrocarbons interacting with solids from mechanical impurities.

Long storage of oil sludge makes it break down into several layers with their own specific features.

Surface layer is watery oil product belonging to ‘water-in-oil’ emulsion types, with fine-dispersed mech. additive content of 5%. This layer consists of 70-80% of oils, 6-25% of pyrobitumen, 7-20% of tars and 1-4% of paraffin. Water content does not exceed 5-8%. Quite often organic component of the newly formed upper layer is very similar to charge oil stock in its content and characteristics.

Medium layer is of relatively small volume, made up of 'water-in-oil’ emulsion type. In this layer water content ranges from 70 to 80% with 1,5 to 15% of mechanical additives.

The following layer fully consists of settled saline water with density range of 1,01- 1,19 g/cm3.

Finally, bottom layer (bottom mush) is usually a solid phase with 45% of organic matter, 52-88% of mechanical additives, including ferrum oxides. A hydrated mass, bottom mush has water content reaching up to 25%.

Methods of sludge processing can vary according to sludge physico-mechanical characteristics. Collected oil sludge of watery-viscous consistency undergoes breaking down into oil product, water and mechanical solids. This processing stage aims at recovery of good quality oil products and their proper application further on. There are two basic methods of phase separation of watery-viscous sludge: mechanical and chemical. A more complex method can be used for finer treatment of oil products. A wide range of equipment is employed in breaking down processes, including separators, centrifuges, and hydrocyclones of various design.

Technical sludge treatment solutions offered by ENCE GmbH

ENCE GmbH offers various complex systems and auxiliary equipment designed for sludge removal from oil storage tanks (pits), as well as for cleaning of reservoirs with max volume up to 50, 000 m3. As a rule oil and gas processing plants of the Republic of Kazakhstan require more than just separate units and appliances recommended by this or that manufacturer. In every case concrete solutions have to be found to comply with the optimal ‘efficiency-price-quality’ criterion.

The main advantages of such systems are:

  1. Recirculation of market condition oil from oil sludge results in returns on investment within 1-2 years in spite of initially high cost of the system.
  2. Min. content of oil impurities in solid residue allows its usage in road construction, etc.
  3. Oil content at the level of 1,5-2,5% in aqueous phase shortens the time of biological treatment of water to the minimum.
  4. System processing rates reach up to 10,000- 15,000 tons of oil sludge treated annually.

Process layout description

Basic process system includes:

  • Systems (units) for oil sludge removal from pits and tanks;
  • Primary oil sludge processing systems;
  • Oil separation systems;

Systems (units) for oil sludge removal from pits and tanks

The following equipment is offered for oil sludge removal purposes: submersible pumps mounted on special pontoons (floating decks) with heating system covering up to 50-70 m 2 of surface, designed to make a sump pit where ‘sludge icebergs’ can be directed to (forming at +20...30 °C).

Oil sludge mass can be collected and moved into primary processing station from pit bank by an excavator with a long shovel boom (up to 18 m). A specialized bucket with rotating drum is mounted on the excavator, with steam feed coil for heating of oil sludge during its agitation.



Cutting of sludge surface and its division into separate lumps can be achieved with the help of WOMA high pressure units that can move along the pontoons or pit banks.



Oil sludge "icebergs" can be moved by skimmers, booms or specialized amphibian vehicles. Sludge is heated on the way to the sump pit; in this heated state it can be easily taken in by pumps.



A specialized hydro tractor is employed for tank cleaning; it is delivered inside the tank through service hatches and assembled inside the tank in the course of 4-5 labor hours. A special pump that is also brought inside the tank is used for transfer of thinned sludge into primary processing station.



Hydro Tractor with various types of integral implements

Oil sludge collecting unit with remote control

After it had been pumped out of collectors oil sludge passes on into primary processing station where most heavy particles are removed, e.g. stones etc. A centrifuge then separates sludge into 3 phases: oil, water and solids.

A heat exchanger is installed into recirculation system. Flow capacity of the system with this kind of medium equals 4-5 m3/h. Process flow scheme can be visualized in the following way:

Slum pretreatment system

The system allows to effectively heat, mix, circulate and decolorize sludge. Due to usage of steam heating coils sludge can be effectively heated to the required temperature to ensure optimal separation in decanter. Agitators mix oil sludge, filtering out solids, thus preventing unwanted sediment formation. Efficiency upgrade can be achieved by using a heat-exchanger for oil sludge that requires additional heating for better-quality separation. This allows heating increase without production tie-ups.

When heated and homogenized, oil sludge is pumped at controlled speed into a three-phase decanter through mixing pipework and a chemical dosing system.

Chemical dosing system

Chemical flocculation agents are supplied into centrifugal feed pipeline through mixing pipework as well. This ensures adequate intermixing of sludge with chemical agents. The dosing system is comprised of 2 tanks, since a certain amount of time is required for effective mixing of powder and water. It takes at least an hour for flocculation agents to react in the right way. While one of the tanks is mixing, the 2nd one feeds the sludge pipeline of the decanter. The tanks are sized so that they can be emptied once in an hour. During this time feeding can be switched to the full tank where flocculation agents are well mixed.

Tricanter (Three-Phase Decanter)

Static decanting is the process of separation of substances suspended in water inside a tank. The decanter allows oil sludge to be separated into 3 phases: water, oil and solids.

ENCE GmbH, Switzerland / ENCE gmbH, Schweiz / ЭНЦЕ ГмбХ, Швейцария © ENCE GmbH