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Front end filtering of unconventional oil

Processing unconventional oils requires technologies such as electrostatic separation to handle higher amounts of filterable solids.

General Atomics Electromagnetic Systems
Article Summary
Increasing market demand for cleaner fuels, both in quality and quantity, is pushing refineries to address the degrading quality of crude oils as unconventional oil reserves become more predominant with a lower initial cost. To gain economic benefit, refineries need to make better use of lower quality oil resources such as Venezuelan oils, Canadian oil sands, and tight oil (also called shale oil). Oil production in the United States is predominantly made up of tight oil which is projected to increase through the early 2040s.1 In 2017, tight oil made up 54% of total US production.2 In 2040, tight oil is expected to account for nearly 70% of total US production.2 Recent analysis indicates that tight oil formations are located throughout the world and constitute a substantial share of global technically recoverable oil resources.3

Refineries process a variety of crude oils to maintain a positive margin. Major investments by refiners in the past incorporated equipment needed to match the composition of available crude oils to their refineries’ configurations and to maintain the market production levels set by each refinery. Many of these investments were made prior to the advances in technology that enabled production of tight oil. Successful refineries are characterised by their ability to respond to market demands and by their ability to adapt to a changing oil base by overcoming the mismatch between their current capabilities and those needed to process tight oil.

Challenges in processing unconventional oils
While economically viable, utilising unconventional oils presents challenges in downstream maintenance and product quality. Unconventional oils contain higher levels of unwanted components that promote corrosion, erosion and fouling within process equipment, poisoning of catalyst, increased contaminants in end products, and environmental pollution.7 The high toxic nature of arsenic, cadmium, lead and mercury in the environment has made them priority pollutants. To maximise the benefits of unconventional oils, refineries will need new technologies and methods to remove or reduce the unwanted components contained in these oils prior to transforming them into fuels.

The growth in production of light tight oil has made it more readily available to US refiners, which makes it a secure source of domestic energy. However, refineries will need to invest more in transportation and process modifications required to process tight oil. Given its variation in composition4 and large amount of filterable solids,5 tight oil presents a number of challenges throughout the refining process. These challenges cost time and energy and reduce throughput within the refinery which is why tight oil generally costs less then Brent (global crude oil benchmark) or West Texas Intermediate (US crude oil benchmark).

During the processing of tight oil and other unconventional oils, refineries need to address the increase of filterable contaminants/sediments such as heteroatom compounds (incorporating sulphur, nitrogen, and oxygen), metals, solids, and water. One of the significant challenges is the high level of solids entrained within the crude oil, 100-700 ppm. This level of solids is often coupled with a large number of solids below 20μ. These solids tend to destabilise emulsions in desalters and carry over oil coated solids that contribute to preheat fouling along with crude oil distillation unit conversion loss.6

What are solids?
There are two classes of solids mostly referred to in refining: basic sediments and filterable solids. The former are particles greater than 20μ that can settle out in the desalter and in settling tanks for decant oil. Smaller particles referred to as filterable are typically smaller and range below 20μ. The effects of contamination from these solids can be seen downstream of the desalting process and after fluidised catalytic cracking. Fouling, erosion, catalyst poisoning, and inventory contamination erode the profit potential of the refiner when left to conventional processing methods. Entire tank farms may be impacted whereby tank turnaround schedules are shortened due to excess settling, and large investments are required to remove the hazardous waste created in decanting these solids.6

Crude oil composition
Crude oil is a naturally occurring complex hydrocarbon mixture containing organic and inorganic compounds and species which vary depending on geological origin.7 Figure 1 provides a general overview of the constituents that make up crude oil. Hydrocarbons constitute the largest group of organic compounds found in crude oil.8 The main hydrocarbon compounds are:
•    n-paraffins and isoparaffins (alkanes)
•    Olefins (not usually found in crude oils but produced in a number of refining processes)
•    Naphthenes (cycloalkanes)
•    Aromatic and polyaromatic structures.7,8
Crude oil contains varying amounts of heteroatomic (sulphur, nitrogen, and oxygen) compounds, metals, emulsified water, and minerals. Table 1 provides a more detailed breakdown of the constituents of crude oil.

Unconventional oil
Unconventional oil is crude oil produced or extracted using techniques other than the conventional (oil well) method.9 Unconventional oils are more difficult and costly to process since added steps (hydrotreatment, demetallisation, catalytic dewaxing, and desulphurisation) are required within the refinery to convert these oils to lighter products. Unconventional oil feedstocks tend to be heavy conventionally produced crude oils, extra heavy oil that has a low degree of mobility and tar sand bitumen diluted with lighter hydrocarbons or synthetic crude oils to meet pipeline gravity and viscosity specifications.10

There are also lower density crude oils that fall into the unconventional oil category, such as tight oil, that are problematic due to their high filterable solids content and paraffin levels. Although tight oil is considered sweet (low sulphur content), the processing of this type of oil can be problematic. Tight oil can contain over seven times more filterable solids than conventional crude oil.5 Refineries have reported filterable solids in excess of 200 lbs per thousand barrels of tight oil.5 For a refinery processing 100000 b/d, this yields more than 10 tonnes of solids per day entering the refinery with the crude oil.5 The increased level of paraffin waxes creates operational issues by fouling preheat sections of crude oil heat exchangers and other equipment and by remaining on the walls of railcars, tanks, and piping. Paraffin waxes that stick to vessels walls and piping can trap components within the crude oil which creates localised corrosion if not removed. Other issues include increased asphaltenes, crude oil incompatibility, and deficiencies in product qualities (for instance, cold flow properties such as cloud point, pour point, and cold filter plugging point).5

Typically, unconventional oils have a high total acid number (TAN >0.5 mg KOH/g),11 higher density and viscosity, high quantities of filterable solids (sulphur, salts, and metals), unstable asphaltene compounds, and difficult-to-remove chloride salts.10 These constituents are concentrated in the low volatile (bottoms) fractions which are normally alkylated to lighter fractions. They can also be coked, which yields a lower value product than the alkylated products.12

The main characteristics of heavy oils (high viscosity and significant content of heteroatoms) are directly related to the significant presence of higher polar compounds, such as resins and asphaltenes.7 Asphaltenes contain high amounts of molecules of variable aromaticity with different contents of heteroatoms, such as sulphur, and complexed metals, such as nickel and vanadium, which are harmful compounds to refining processes and to the environment.7
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