Ocean transportation remains the most efficient and cost-effective method of transportation of heavy products such crude oil and refined petroleum products (Mullai, 2006). Maritime shipping is cheaper in the transportation of massive quantities of goods as compared to railway or air transport (Soares & Texeira, 2001). According to the World Trade Organisation report of 2015, over 90 percent of world trade was moved through the oceans through export and import. A significant proportion of the goods comprised of crude oil and refined petroleum products. Maritime transportation of goods is fraught with many risks, making environmental studies dissertation help important in understanding and mitigating all these challenges. For instance, ship collision, storms and mechanical failure among other hazards (Rahman, 2009). Although it is the cheapest mode of transportation for crude and refined products, instances of spillage have devastating environmental consequences. Apart from harming the environment, oil spillage has economic repercussions since massive amounts of resources are set aside for clean-up (Souza, et al., 2010). According to Hatton et al. (2013), the earth has vast reserves of natural gas and oil trapped in the subsurface. Occasionally, the reserves may develop cracks which may release little amounts of the products. However, such events are part of nature, and they rarely cause any significant damage to the surroundings (Thanikaivelan, et al., 2012). On the other hand, oil spills resulting from human error cause lots of damage to marine ecosystems and by extension, to the local, national and regional economies through pollution.
Since the advent of marine transport, there have been numerous incidences of oil spillage in the global maritime transportation system. Refined and crude oil spills resulting from tanker ships accidents have destroyed vulnerable ecosystems in many places such as the Gulf of Mexico, France and Alaska (Calcagnile, et al., 2012). Oil spillage has ranged from a few tonnes to several hundreds of thousands of tonnes. According to Agarwal and Joshi (2010), oil spills at sea are usually much more destructive than those on land. The main reason is that ocean oil spills can spread over a large area in a thin oil slick (Zhu, et al., 2010). Some of the biggest oil spills in history include Gulf War oil spill, Atlantic Empress, ABT Summer and Deepwater Horizon. The most recent major oil spill is the Deepwater Horizon oil spill that took place in the Gulf of Mexico in 2010. According to Goerlandt and Montewka (2015), the oil spill is considered the largest accidental maritime spillage in the history of crude marine transportation. There were adverse and extensive damages to wildlife, marine and fishing habitats (Calcagnile, et al., 2012). The tourism industry was also affected by the spillage in some coastal parts of Mexico and the United States. In the last three years, further research into the effects of the spill has revealed that marine life continues to die with the young ones having little chances of survival (Goerlandt & Montewka, 2015). Numerous oil spill clean-up methods were employed to contain the spill. Examples include the use of Corexit dispersant and use of oil-eating microbes (Guidi, et al., 2015). Floating containment booms were used to prevent oil spread to unaffected sites. Sorbents were used to remove remnants of oil from the water. According to the National Oceanic and Atmospheric Administration (NOAA) report of 2014, the oil spill vicinity is among the most productive ocean ecosystems in the world. For this reason, the spill led to the deaths of a vast number of species particularly those that survive at the surface of the water.
The Deepwater Horizon incident led to increased interest in oil spill clean-up techniques. In 2012, Massachusetts Institute of Technology (MIT) researchers found a novice and promising way of separating oil and water through the use of magnets. According to Gui et al. (2013), the damage caused by oil spills is permanent and takes a considerable amount of time to clean up. In the first place, oil floats on water which reduces the surface area interface between air and water to maintain aquatic aeration. This leads to suffocation of living organisms in oceans and other large water bodies (Goerlandt & Montewka, 2015). Next, oil prevents sunlight from passing through which makes it difficult for aquatic life to survive (Knapick, 2012). Therefore, it is imperative that oil spills must be cleaned up immediately to minimise their irreversible impacts to life and the environment. The aim of this review is to discuss oil and water separation through magnetisation. Important aspects such as design, numerical and financial characteristics of the method are discussed.
According to Calcagnile et al. (2012), oil is not a magnetic substance, but by suspending magnetic nanoparticles within crude and refined oil, the product turns into a magnetic liquid known as a Ferro-fluid. According to Knapick (2012), current oil spill technologies entail the use of booms to separate spilt floating oil from maritime waters. Booms are floating containments that enclose spilt oil to enhance the recovery efficiency. Chen and Pan (2013) report that current oil spill recovery techniques have an efficiency of 95 percent in calm waters. In stormy waters, the efficiency can be as low as 40 percent. Other methods use the difference in density between oil and water and have two significant shortcomings. As such, they either take a long time for separation, or they are energy intensive. According to Zhu et al. (2010), the use of magnetic forces to enhance oil recovery efficiency has been explored before the Deepwater Horizon incident, but the implementations were limited due to ti several practical concerns. Before describing the technology, it is important to consider the set of requirements as applied in oil spill separation. First, materials and methodology must be reusable, continuous and environmentally safe. Second, marine spill methodologies should be robust in the coastal environments. Third, techniques should require minimal materials and external energy (Calcagnile, et al., 2012). Lastly, methodologies should work when there is a variable amount of oil and water in any given mixture (Guidi, et al., 2015). In the light of the above discussion, it is evident that current techniques are expensive, energy intensive and have lower efficiency. Some practices such as burning oil patches only exacerbate the situation leading to more harm (Agarwal & Joshi, 2010). Therefore, an emphasis is placed on highly efficient systems that can pull oil out of the ocean before marine wildlife is harmed (Goerlandt & Montewka, 2015). The magnetic oil spill recovery system uses magnetic nanoparticles to bind oil molecules together (Rusu & Gasparotti, 2012). As such, the oil is transformed into a magnetic liquid that is drawn towards magnets. When such particles are sprinkled over oil spills in seas and oceans, they bind lighter oils floating at the surface and heavier oils that may have sunk (Souza, et al., 2010). Afterwards, ships with small magnets can move around spills where the Ferrofluid would be attracted to the magnets and collected.
Existing oil spill clean-up technology such as belt skimmers can collect water and oil mixtures that can be pumped onboard into a treatment facility (Hatton, et al., 2013). Magnetic material is added to the mix. In the above method, either water or oil can be the target of magnetisation to separate the magnetic phase from the non-magnetic phase. Magnetic separation is continually repeated until the magnetic phase is satisfactorily separated from the non-magnetic phase. The cleaned water can be pumped back into the ocean if it is the non-magnetic phase. The magnetic phase is injected into another magnetic separation system to separate the magnetic particles from the magnetic phase (Thanikaivelan, et al., 2012). Since the number of magnetic particles had been predetermined before practice, the process for separating magnetic particles from the magnetic phase continues until a sufficient amount of magnetic material is recovered (Chen & Pan, 2013). If the magnetic phase was water, it could be pumped back into the ocean, if it was oil, it could be stored onboard or pumped through a pipeline to a storage facility or a refinery. Step 1: Collecting oil-water mixture Due to the possibility of loss of the particles in vast ocean waters, this system requires mixtures to be collected and then separated onboard. As such, the current boom and skimmer technology have high recovery efficiencies in calm waters. The mixtures are pumped to the oil recovery systems in offshore locations or onboard ships where the magnetic separation of water and oil are performed (Zhu, et al., 2010). The practice in not undertaken in the open waters due to several reasons. The most notable include potential environmental pollution and damage resulting from dispersion of magnetic nanoparticles into water and loss of recyclable magnetisation materials due to water and wind currents (Calcagnile, et al., 2012).
The second phase entails magnetisation of either water or oil phases using chemical techniques. In principle, either phase can be magnetised to allow for separation, but magnetising oil has a merit of using lesser quantity of magnetic particles (Gui, et al., 2013). Also, magnetising oil leaves water particle-free thereby minimising water pollution. In fact, water is pumped back to the ocean without the concern for the efficiency of the magnetic particle removal. The above system employs several chemical technologies to magnetise either liquid phases for oil spill recovery (Viswanathan, 2011). The correct methodologies for given situation are determined by the properties of oil-water mixtures. For example, temperature, the thickness of oil slick and density of the mix. However, due to the advantage of reducing costs, magnetisation material is added to oil instead of water (Knapick, 2012). The following oil magnetisation technologies are used in the above system:
The first step entails the separation of magnetic crude from the non-magnetic water. In this case, the oil-water mixture is transported to a treatment apparatus that uses an electromagnetic field to enhance separation efficiency. The system uses strong electromagnets made by wrapping a coil around an iron core (Calcagnile, et al., 2012). To increase the strength, more turns are added to the core, and the current is varied to alter the strength of the electromagnet. There are several advantages of electromagnets over permanent magnets. The main advantage is that an electromagnets magnetic field can be manipulated to suit the conditions of the oil-water mixtures to allow for maximum recovery efficiencies (Chen & Pan, 2013; Agarwal & Joshi, 2010).
Second, electromagnets can be tens of times stronger than permanent magnets. Although modern magnets made from iron and neodymium are adamant, electromagnets can be twenty times stronger. In this case, the magnetic phase is attracted to the cylindrical pole of the magnet that is present just above the oil-water interface. A single magnetic flux configuration (Halbach array) is used to increase the efficiency of separation of the products (Gui, et al., 2013; Knapick, 2012; Souza, et al., 2010). Since magnetic strength is strongest at the tips, the tips of magnets are used to attract oil particles from the mixture (Knapick, 2012). Following this procedure, the apparatus separates the magnetic oil from the non-magnetic water.
The primary objective of this step is to remove magnetic nanoparticles from magnetic liquid phase which is the oil. After separation of the oil and water, magnetic particles still present in the oil can be recovered for reuse. The high gradient magnetic separator (HGMS) is used to retrieve magnetic particles from the oil (Gui, et al., 2013; Li, et al., 2012; Hatton, et al., 2013). Retrieval of the magnetic nanoparticles facilitates continuous clean-up of oil spills.
The cleaned water is pumped back to oceans while the oil can be stored on-board or pumped through a pipeline to a depot or a refinery.
In summary, the oil recovery efficiency of the current oil spill technologies – skimmers and booms – can be enhanced significantly by using the new magnetic separation technique. In ideal situations, the collected water-oil mixture is magnetised in a confined area using iron oxides that are environmentally safe. Oleic acid and other safe surfactants render this technology suitable as compared to other techniques such as chemical sprays and burning that worsen the damage. Also, onboard separation improves separation efficiency and prevents pollution of water through loss of the nanoparticles. As this technology evolves, the prospects of an effective oil spill clean-up method raise. In the next few decades, it may offer the ultimate solution to oil spill clean-up.
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