INTRODUCTION Trace metals are metals occurring at 1000mg/kg or less in the earth crust. Depending on their densities, these elements can be classified as light or heavy. Metals with densities greater than 5g/cm 3 are ‘heavy’ metals while those with densities less than 5g/cm 3 are ‘light’ metals (Osuji and Onojake, 2004). Trace elements in petroleum have received increased attention in recent years because of their importance both in genesis of petroleum and its refining. Recent advances in analytical techniques, including inductively coupled plasma emission spectrometry, have led to a resurgence of interest in metal occurrence in petroleum and in the application of transition metal information to natural resource exploration. The presence of metals in oil may be a useful indicator of nearby ore deposits (Hitchon, 1977). The nature of these metals and their abundance in crude oil can give information on the origin, migration, and ma turation of petroleum. Vanadium, nickel, iron and copper are normally found in petroleum as naturally occurring elements associated to the formation process and although present only in small amounts (μg g−1 or ng g−1 levels), they are very important to the petroleum industry . Their determination is of considerable importance, since they have deleterious effects on refinery operation and performance. They may corrode refinery equipment, poison and foul catalysts and/or cause undesirable side reactions in refinery operations (Brandao et al., 2007). In addition, the ash of certain oils and solid bitumen has been proposed as an exploitable metal-containing resource, particularly in the case of heavy oils (Skinner, 1952; Erickson et al., 1954). Despite these potential applications, virtually little systematic investigations of the uses of metal information in resource exploitation have been published, although the use of metal data in petroleum exploration was first suggested many years ago (American Petroleum Institute, I nstitute, Project 43,1947). The current curren t interest in using metal concentrations in crude oil as a proximity indicator for nearby commercial ore deposits once again raises the suggestion that knowledge of the occurrence and an d distribution of transition metals may also have direct application in petroleum exploration (Zhang et al., 1981). Studies of metals in petroleum have focused on heavy oils, primarily because of their high concentrations of transition metals and the fact that these high concentrations poison refining catalysts. As industrial utilization of heavy oils increases, knowledge knowledge of the concentration concentration and chemical occurrence of metals in petroleum has become increasingly critical. Detailed chemical information concerning the molecular association 1
of metals in heavy oil will provide insight into rapid and cost-efficient methods of their removal. As a preliminary step to utilizing metal data in refining research and petroleum exploration, precise determination of elemental concentrations in whole oils and their compound class fractions can be useful in establishing the molecular association of organometallics in petroleum. Eventually information from organometallic compounds in crude oil will be routinely used to supplement that provided from analysis of hydrocarbon fractions. Petroleum consists predominantly of hydrocarbons, and contains measurable quantities of many metals such as Iron (Fe), Zinc (Zn), Nickel (Ni), Vanadium (V), Chromium (Cr), Cadmium (Cd), Copper (Cu), Lead (Pb) Manganese(Mn), Cobalt (Co) etc. These trace metals are simply a reflection of those picked up during migration or in the reservoir, and incorporated directly into oil in form of porphyrin complexes (species) in petroleum source rocks from the biomass and formation during sedimentation (Akinluaet al., 2007). It may also involve digenesis from organic molecules as well as metals derived from different biogenic (biomass) and abiogenic (Weathering of minerals) sources. Metals of proven association with organic matter may be used as reliable correlation tools (Akinlua et al., 2007). Nickel, Vanadium and Cobalt (usually referred to as biophile elements) are such examples. Also, the nature of metals in crude and residual oils is of interest to the refiner as they are the source of environmental pollutants and the cause of corrosion of equipment and poisoning of process catalysts. Nigeria recently has witnessed an increase in oil pollution due to equipment failure, inexperienced operators and sabotage. The increasing rate of corruption, perceived marginalization and unemployment have given rise to many illegal refineries and bunkering in Niger Delta and Oguta is not an exception. These unprofessional that engage in these acts discharge their waste in our environment without considering their effects in our environment. In other to save ourselves and environment there is need to have qualitative and quantitative knowledge of trace metals present in the crude oil and their effects to aquatic and terrestrial lives
Majority of the known metals and metalloids are very toxic to living organisms and even those considered as essential, can be toxic if present in excess. Concentrations of several toxic metal and metalloids have been largely increased as a result of human activities. The recent flooding has shown that not only the areas where these activities are carried out can be polluted. There is the need for us to know what happens in our environment and our neighbors in other to protect ourselves. 2
FORMATION OF CRUDE OIL Petroleum is a fossil fuel derived from ancient fossilized organic materials, such as zooplankton and algae. (Keith A. (2006). Vast quantities of these remains settled to sea or lake bottoms, mixing with sediments and being buried under anoxic conditions. As further layers settled to the sea or lake bed, intense heat and pressure built up in the lower regions. This process caused the organic matter to change, first into a waxy material known as kerogen, which is found in various oil shale around the world, and then with more heat into liquid and gaseous hydrocarbons via a process known as catagenesis. Formation of petroleum occurs from hydrocarbon pyrolysis in a variety of mainly endothermic reactions at high temperature and/or pressure.[Chemical Reaction Model for Oil and Gas Generation 2010 ] There were certain warm nutrient-rich environments such as the Gulf of Mexico and the ancient Tethys Sea where the large amounts of organic material falling to the ocean floor exceeded the rate at which it could decompose. This resulted in large masses of organic material being buried under subsequent deposits such as shale formed from mud. This massive organic deposit later became heated and transformed under pressure into oil (Broad, William J. August 2, 2010). Geologists often refer to the temperature range in which oil forms as an "oil window" — below the minimum temperature oil remains trapped in the form of kerogen, and above the maximum temperature the oil is converted to natural gas through the process of thermal cracking. Sometimes, oil formed at extreme depths may migrate and become trapped at a much shallower level. The Athabasca Oil Sands are one example of this CLASSIFICATION OF CRUDE OIL Crude oil otherwise known as petroleum (from Greek: petra (rock) + Latin: oleum (oil)) is a composition of hydrocarbons (chemicals composed solely of hydrogen and carbon in various molecular arrangements) and other compounds which is usually brown or black in color. Classifications of crude oil is based on the geographical locations, sulfur content and relative weight but the major classification is done based on the location, as the oil comes from various parts of the world and they differ in their physiognomies(characteristics).
PHYSICAL PROPERTIES OF CRUDE OIL
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A crude oil assay is essentially the chemical evaluation of crude oil feedstock by petroleum testing laboratories. Each crude oil type has unique molecular, chemical characteristics. No crude oil type is identical and there are crucial differences in crude oil quality. The results of crude oil assay testing provide extensive detailed hydrocarbon analysis data for refiners, oil traders and producers. Assay data help refineries determine if a crude oil feedstock is compatible for a particular petroleum refinery or if the crude oil could cause yield, quality, production, environmental and other problems. The a ssay can be an inspection assay or comprehensive assay. Testing can include crude oil characterization of whole crude oils and the various boiling range fractions produced from physical or simulated distillation by various procedures. Information obtained from the petroleum assay is used for detailed refinery engineering and client marketing purposes. Feedstock assay data are important tools in the refining process.
EFFECT OF TRACE METALS ON THE ENVIRONMENT There are around thirty chemical elements that play a pivotal role in various biochemical and physiological mechanisms in living organisms, and recognized as essential elements for life. Majority of the known metals and metalloids are very toxic to living organisms and even those considered as essential, can be toxic if present in excess. Concentrations of several toxic metals and metalloids have been largely increased as a result of human activities. The metals can enter the environment by accidental oil spills or through combustion products of the refined crude oil (Osuji and Onojake 2004). They can disturb important biochemical processes, constituting an important threat for the health of plants and animals. Plants and animals absorb these elements from soils, sediments and water by contact with their external surfaces, through ingestion and also from inhalation of airborne particles and vaporized metals [Mudgal V, Madaan N, Mudgal 2010]. The requirement for ingestion of trace metals such as Fe and Cu ions to maintain normal body functions such as the synthesis of metallo-proteins is well established. However, cases of excess intake of trace metal ions are credited with pathological events such as the deposition of iron oxides in Parkinson's disease [Candelaria M et al 2006]. In addition to aiding neurological depositions, these redox active metal ions have been credited with enhancing oxidative damage, a key component of chronic inflammatory disease [Pineda C, et al 2006 ] and a suggested initiator of cancer [Ahankour F 2008]. As inflammation is a characteristic feature of a wide range of diseases, further potential pathological roles for metal ions are emerging as exemplified by 4
premature ageing [Carballo N 2008]. For the maintenance of health, a great deal of preventative measures is in place to avoid ingestion of potentially toxic metal ions. From monitoring endogenous levels of metal ions in foods and drinks to detecting contamination during food preparation. European countries spend significant resources to avoid metal intake by the general population [Kozaki K 2002]. From a therapeutic viewpoint, considerable research and development efforts are being exerted to decorporate metal ions from the body. SIGNIFICANCE OF METALS IN CRUDE OIL Metals are present in crude oil at concentration levels from few parts-per million (ppm) in heavy crudes to parts-per billion (ppb) in light crudes. The negative effects of metals include the catalyst poisoning and fouling, corrosion of equipment and particulate emissions into the environment, and the contamination of petroleum-related products. On the other hand, the metals are often used as tracers in geochemical prospecting. Their isotopic ratios are used for oil-oil or oil source rock correlation or for identification of the source rock depositional environment and in the quantification of the oils thermal maturity and biodegradation levels (Caumette et al., 2009). Ratios such as vanadium to vanadium plus nickel and iron to vanadium are suggested as being useful for oil type characterizations. Transition elements concentrations and ratios can serve as excellent oil-oil correlation parameters. Generally, vanadium and nickel content increases with asphaltic content of crude oil (API gravity is an indicator). The lighter oils contain less metal (Caumette et al., 2009). In diesel fuels, vanadium can form low melting compounds (e.g. V 2O5, melts at 6910C) and cause several corrosive attack on all the high temperature alloys used in the gas turbine blades and diesel engine valves, vanadium in fuel should be < 2ppm; at 10ppm vanadium, the corrosive rate is as thrice as high, and at 30ppm vanadium, it is thirteen times much. However, if sufficient magnesium is present in the fuel, it will combine with vanadium to form higher melting compounds and reduce the corrosion. Sodium and potassium combine with vanadium to form low melting eutectics (5650C). Hence, sodium and potassium in the gas turbine fuels must be limited (Boldt and Hall, 1977). If not removed in the fuel treatment process, a high level of sodium will give rise to post combustion deposits in the turbocharger. These can normally be removed by water washing (Reynolds, 2001). Calcium is not harmful from a corrosion stand points, actually it helps to inhibit the corrosive action of vanadium. However, calcium can form hard bonded deposits which are not readily removed. Lead can cause corrosion and spoil the beneficial inhibiting effect of magnesium additives on the vanadium corrosion (Boldt and Hall, 1977). Combine with sodium and vanadium complexes, sulfur forms 5
deposits on the external surfaces of super heater tubes, causing equipment corrosion and loss of thermal efficiency (Boldt and Hall, 1977).The harmful effect of heavy metals, on human health, environment and refinery equipment call for the assessment of the concentration of heavy metals in organic fractions of Nigeria crude oils. This will give an indication to the fractions of Nigeria crude oils. This will give an indication to the fraction that is heavily burden with these metals, and therefore, appropriate measures can be taken in handling such fraction in order to avoid problems associated with the heavy metals such as catalyst poisoning, clogging of refinery linings and equipment as well as other problems that are directly concern to human health and environment. This study is aimed at investigating and comparing the concentration levels of Iron (Fe), Zinc (Zn), Nickel (Ni), Vanadium (V), Chromium (Cr), Cadmium (Cd), Copper (Cu), Lead (Pb) and Cobalt (Co) in Nigerian crude oils(Oguta oil field). This will help in assessing the Nigeria’s crude oil impact on the environment and its economic potentiality .
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