What is it?
The term "Oil Sludge/Tank Bottom Sludge" is used, in general, to designate material generated during the transportation, storage, refining and production of crude oil. Tank bottom sludge is normally a combination of hydrocarbons (oil), sediment (sand & rust), paraffin, wax and water and may include harmful by products such as sour gas, xylene, polycyclic aromatic hydrocarbons, benzene, heavy metals, toluene, and ethyl benzene.
Paraffin-based crude oil sludge forms when the molecular orbitals of individual straight-chain hydrocarbons are blended by proximity, producing an induced dipole force that resists separation. These dipole forces are called London Dispersion Forces, or Van der Waal bonds, and are responsible for like molecular aggregation. As the ‘heavier’ straight-chain hydrocarbons flocculate (heavier meaning predominantly the C20+ hydrocarbon molecules), they tend to fall out of suspension within a static fluid, where they accumulate on the tank floor as a viscous gel.
Over time, this gel stratifies, as the volatile components within the gel are ‘flashed’ from the gel with changes in temperature and pressure. This departure of the volatile components results in a concentration increase of the heavier fractions within the sludge, resulting in increased density and viscosity and decreased mobility.
Alternatively, sludge could be defined as any material which will not flow under gravity to a given suction point.
How Sludge Affects Asset Owners
There are a number of issues surrounding the build-up of sludge. Tank bottom sludge doesn’t only affect storage tanks but the infrastructure that supports them.
Storage Capacity
Whenever crude oil is left in tanks prior to refining or movement onto ships, the heavy ends begin to split and settle on the tank floor. Over time the repeated build-up of these heavy ends starts to reduce the overall capacity of the tank.
In favourable conditions the sludge will dissolve in the crude oil as it is pumped into and out of the tank or during the blending phase. However, the heavy ends do not dissolve and settle out.
The reduction in capacity although small to start with will gradually increase without a pre-planned preventive maintenance regime. As the capacity of the tanks begins to dwindle so too does the throughput of the oil. In most circumstances additional tankage has to be called upon.
Pipeline & Pumping Restrictions
Whenever crude oil is pumped either from a production well, from a storage tank to a ship or between tanks it leaves a residue on the pipe. If the sludge is particularly viscus the constituents of the oil known as asphaltenes clump together and solidify with pressure and temperature changes.
Over time, that build up grows and hardens on the inside of the pipe, resulting in reduced flow or complete blockage of producing well, pumps, pipelines and separators. In the worst cases the pipeline can rupture causing untold environmental damage.
The problem is doubled when you consider that unlined steel pipework will degrade over time through a process called scaling. The rate of scaling is exacerbated by the water levels found in the crude oil. Scale inhibitors help to reduce the effects but are costly to install and maintain. Periodic flushing and cleaning will ensure that the pipelines stay free from any blockages and will alert the asset owner to any issue surrounding the integrity of the pipeline. Alternatively lined pipework would add an additional level of protection along with a pre-planned preventative maintenance schedule.
Corrosion
Corrosion is one of the biggest worldwide problems in the oil and gas industry. In 1996, the total corrosion-related costs in USA refineries were about $3.7 billion and each year the cost increases.
Crude oils contain water, as well as different water soluble inorganic and organic compounds. During the storage and transportation of crude oil, the aqueous solutions settle to the bottom of the tanks. The composition of the aqueous sediment layer depends on many factors, such as the properties of the oil, extraction technology, etc.
The level, composition, concentration, and pH of the water solutions are unpredictable in most cases. In some countries the crude oil stored in tanks can contain ten to ninety percent water. After dewatering and desalting, most storage tanks contain up to 2% water. The level of SAS (sediment aqueous solutions) in production storage tanks increases with time, depending on the tank design. The level of SAS can reach 1 m or more.
The corrosion rate of tank bottoms depends on the composition, concentration, and pH of the aqueous solutions. In a typical aqueous solution corrosion rates can vary from 0.05 to 0.30 mm per year and if sediment water contains sulfide, sulphur dioxide and carbon dioxide components, the pitting corrosion rate can increase to 5 mm per year.
Over the last few years there have been numerous documented cases of tank and pipework failures due to corrosion attributed to crude oil.
In the case of storage tanks it would be regarded as best practice to line the bottom metre of the tank with an epoxy based lining system. This acts as the first defence to the corrosion properties of crude oi and adds significant environmental protection.
Waste Disposal
The two attributes that make oil sludge such a problem are that it is classified as hazardous waste (usually due to the oil content) and in most cases, it is also classified as a liquid, which means, it cannot be disposed of directly to a landfill.
Typically, oil sludge can’t be incinerated because it contains too much oil and water, making it almost impossible to incinerate. The oil sludge can’t be filtered because the solids content is too high, and attempts at filtering will just clog the filtration systems. The oil sludge can’t be pumped to a waste water treatment facility because of the high oil and solid content, and the waste has too high COD/BOD. Because it is a liquid, the oil sludge can’t be disposed of, in traditional hazardous waste landfills, only solids can go to landfills.
Due to the fact that there simply haven’t been many good options for dealing with this type of waste, it has been accumulating at an alarming rate.
Uncontrolled handling of these sludges often leads to environmental pollution and also affects the aesthetic quality. Recycling of sludges in an environment friendly manner is one of the appropriate solutions of sludge management problem. The treatment technologies developed can be grouped as physical remediation, chemical remediation and biological remediation.
In some countries refineries can generate up to twenty thousand cubic meters of sludge per year.
However, what’s not realised in a number of cases is what is actually being disposed of. The average sample of crude sludge will contain upwards of forty percent oil, water, bitumen, wax and other sediments and particles. Modern technology can recover a substantial amount of the entrained oil and newer more effective chemicals can extract the oil from the water and sediments and convert the bitumen and wax back into oil.
As of 2015 there are an estimated nine billion tonnes of oil sludge waste sat in tanks, lagoons and pits throughout the world.
By utilising modern technology and the latest chemicals, the actual volume of sludge to be disposed of can be reduced to a tenth of what it was. Not only is this a direct saving for the client the biggest benefactor is the environment.
The modern technology and chemicals alluded to in this piece will be covered in more detail in our next blog.
