OIL SPILL MAGNETISATION RECOVERY METHOD

Introduction

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. 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.

Background

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.

Oil spill clean-up through magnetic separation

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.

Design of Deployment and Recovery System using Electromagnetic Field for Maximum Efficiency

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:

1. Magnetic Pickering Emulsions In principle, Pickering emulsions are stabilised by the presence of solid particles that reside on the interface between continuous and dispersed phases. The contact angle made by the magnetising particle at the interface determines whether the emulsion is water in oil (w/o) or oil in water (o/w) emulsion (Chen & Pan, 2013). In instances where super-paramagnetic particles are used, the stable droplets can be controlled physically by use of magnetic fields. Such droplets can also be burst to release their contained phases at critical applied magnetic field strengths (Li, et al., 2012).
2. Magneto-rheological Fluid In this technique, magnetic microparticles with suitable oleophilic surfactants are used to render the collected oil phase magnetic (Agarwal & Joshi, 2010). The particles used are usually micron-sized and may settle out of solution due to the effect of gravity. Unlike the previous technology, micron-sized particles are larger, and the main disadvantage is that they are not single domains and as a result, oppositely directed fields partially cancel the magnetic field inside and outside the particle (Rahman, 2009; Guidi, et al., 2015; Chen & Pan, 2013). This demerit can be overcome if strong external magnetic fields are used. Such magnets destroy domain walls to allow the large micron-sized particles to have a large uniform magnetic field when magnetised to saturation (Gui, et al., 2013).
3. Magnetic Nanoparticle Suspension (Ferrofluid) In this technique, single-domain magnetic nanoparticles can be coated with either hydrophilic stabilising surfactant and dispersed into water to render it magnetic or oleophilic stabilising surfactant to prevent agglomeration of magnetic particles which then can be dispersed into the oil phase to make it magnetic (Calcagnile, et al., 2012; Agarwal & Joshi, 2010; Rusu & Gasparotti, 2012). The magnetic nanoparticles have a diameter smaller than 10 nanometres to prevent gravity-induced agglomeration. In fact, they are dispersed through Brownian motion. Nanoparticles with minimal environmental damage such as iron (III) oxide and iron (II) oxide are used due to their active ferrous properties (Souza, et al., 2010; Viswanathan, 2011). Additionally, by controlling magnetic volume fraction of disseminated magnetic nanoparticles, the magnetisation of the resulting suspension can be monitored which makes this technique very efficient, attractive and straightforward for oil spill recovery.

Step 3: First magnetic separation

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.

Step 4: Second magnetic separation procedure

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.

Step 5: Storage and Disposal

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.

Numerical and Financial Study of the Design

1. Numerical study Orthogonal thinking has characterised magnetic separation of oil and water for the past six years. While this technology is still advancing, researchers have varied the traditional approaches in two main ways. First, magnets are oriented perpendicularly to the streams’ flow but not parallel to it (Chen & Pan, 2013; Thanikaivelan, et al., 2012). Second, the magnets are immersed in the stream instead of positioning them outside such mixtures. As mentioned earlier, magnetic field in a permanent magnet or a solenoid is usually stronger at its edges than its sides. Therefore, magnetic bar tips attract the Ferro-fluid much more powerfully than the sides (Viswanathan, 2011; Zhu, et al., 2010). The separation design, in this case, uses a solenoid which, when optimised produced more magnetic power than the conventional permanent magnets. The system above employs a unique configuration of magnets known as the Halbach array for the extraction of oil collected by the tubular solenoids. Figure 2: Halbach’s array. Source: (Viswanathan, 2011) The magnets are arranged in a manner that allows the net effect of one side to be reduced – close to zero while it is doubled on the opposite side (Agarwal & Joshi, 2010; Guidi, et al., 2015). This effect significantly increases the overall oil spill recovery efficiency.
2. Financial Study According to Oil and Gas UK (2012), the costs associated with cleaning up an oil spill are strongly influenced by the context of the spill. As such, the factors that influence the cost of clean-up exercise are as follows: the location, type of product spilled, sensitive areas threatened or affected, local and national laws, liability limits, clean-up strategies and the timing of the spill. These factors affect the overall financial expenses. According to Etkin (2013), hydrophobic magnetic powders are simple and inexpensive to prepare in large quantities. As mentioned earlier, this technology has been employed in small scale, and there is no substantial evidence of the financial costs associated with it in large scale applications. Oil and Gas UK (2012) points out to the fact that spills that occur near the shoreline are more expensive to clean up than those that occur offshore or deep onshore. Oil separation by magnetisation is considerably cheaper than other methods in shoreline locations due to its ability to completely separate the two liquid phases (Chen & Pan, 2013). However, the system is expensive when employed in small scale since there are many associated costs. For example, storage costs in deep onshore, electricity generation costs, set up and instrument costs and the expenses associated with running the system (Etkin, 2013). Unlike other clean-up strategies such as burning and Corexit spray, magnetic separation is significantly expensive (Oil and Gas UK, 2012).

Conclusion

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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