In the oil segment of the global energy industry, feedstocks are moving toward the heavier end. The extraction, transport and processing of lighter crudes, using a stable of proven technologies, is no longer possible. These light oil supplies have become exhausted or more difficult to obtain, causing strain on the crude oil supply chain. Explosive economic growth in India, China and other emerging countries requires large volumes of conventional crude oils as well as increasing supplies of heavy oils.
Heavy crude oils have unique physical properties that often present costly refining challenges. Efficient processing solutions require a thorough understanding of the oils characteristics and operating behaviors. Companies involved in heavy or extra-heavy oil refining must evaluate these characteristics and choose the best processing options for these oils.
Heavy oil trends
With increasing global demand and elevated prices for crude oil, heavier oils are still economical to produce using advanced technologies, despite the challenges associated with their production and processing. Their discounted market prices, relative to West Texas Intermediate (WTI), Brent and other benchmark light oils, can provide a financial incentive for refineries capable of processing these heavy oils. Conventional heavy oil reserves are plentiful in the Orinoco region of Venezuela, while the largest concentration of extra-heavy unconventional oil is found in the rich oil sands of western Canada.
Global heavy oil reserves have been estimated at more than twice those of conventional light oils, at six trillion barrels. This massive volume makes heavy oil an important energy resource, especially as conventional oil sources continue to diminish. US refineries import the majority of their heavy oil feedstocks from Canada. Amid global political volatility, Canadas heavy oil resources provide a politically and logistically secure supply of energy, despite the difficulty involved in their extraction, transport and refining.
There is a global need for alternatives to light, sweet, conventional oils. The industry has adopted methods to recover and manage increasingly heavier oils that require additional processing steps. These heavy and extra-heavy oils have complex characteristics that must be identified to be efficiently processed. The knowledge of how to process these oils is important for designing and operating the process correctly, for reducing downtime, for increasing efficiency, for optimizing profitability, for promoting higher environmental standards and for effectively eliminating safety concerns.
Heavy oil characteristics
Heavy oil is generally defined as a viscous crude oil with an API gravity between 10° and 22°, and a viscosity of less than 10,000 centipoise. Extra-heavy oils are unconventional oils with an API gravity below 10°; they are bitumen-like substances with extremely low flowrates at the reservoir. In many instances, they have been considered bottom of the barrel, as compared to conventional petroleum sources with lower viscosity and higher API gravity. Their heaviness is generally attributed to high-molecular-weight compounds, including asphaltenes, which contribute greatly to feed viscosity and coking tendency. They contain relatively small amounts of paraffinic components and are more naphthenic and aromatic in nature. While heavy and extra-heavy oils are present in many global regions, the largest concentration of supply is found in North and South America.
In addition to high viscosity, high pour point and low API gravity, heavy and extra-heavy oils are characterized by higher levels of sulfur, nitrogen and heavy metals, including mercury (Hg). They have low hydrogen-to-carbon ratios and high carbon residue. These feedstocks often contain elevated amounts of particulates and water and are generally high in acids, particularly naphthenic acids.
Issues posed by heavy oils
Heavy and extra-heavy crude oils present a number of challenges, beginning with their extraction and continuing through refinery transportation and processing (Fig. 1). As oil sands recovery has progressed, the surface extraction of oil sands with strip-mining technology has been superseded by several thermal methods, with the most predominant being steam-assisted gravity drainage (SAGD). SAGD and similar technologies have allowed heavy oil to be recovered at lower depths as surface deposits have been depleted.
| Fig. 1. Heavy oils pose a number of |
problems for upstream and downstream operations.
Newer methods have reduced the environmental impact of extraction while enabling access to a higher percentage of known subsurface reserves. SAGD technologies rely heavily on natural gas to raise steam. As a result, this extraction process generates a higher concentration of carbon dioxide, raising environmental concerns. However, low gas prices have helped make SAGD an attractive extraction process.
Due to the viscosity of heavy crude oils, especially bitumen-type material from Canada, these oils are not shipped via pipeline unless they are first blended with a diluent. This blend is often referred to as dilbit, a naphtha/bitumen blend, and it is needed to facilitate flow. Bitumen properties and ambient temperatures at origin and along the pipeline route determine the percentage of naphtha that is utilized. Natural gas condensate can also be used as a diluent.
Heavy oil can be diluted for pipeline transport with a synthetic crude oil (SCO), which is produced by partially upgrading the heavy oil through prerefining distillation processes to lower the viscosity. This synthetic oil/bitumen blend, known as synbit, adds to the cost of the heavy oil. As the quality and attributes of dilbit and synbit diluents can vary considerably from one upgrading facility or supplier to another, their removal and recycle present additional factors that must be understood to effectively refine heavy oils.
Supply trends impact, and will undoubtedly continue to affect, refining capabilities. Traditionally, refineries were designed to accommodate light, sweet crude oilsusually a specific blend that may no longer be available or that is presently cost prohibitive. Most existing refineries were not originally designed to be feedstock flexible or to accommodate heavy feeds.
Heavy oils require additional processing steps to remove impurities and to provide a full spectrum of products from a variety of feedstocks. For this purpose, the addition of hydrogen (H2) in the hydrocracking process is necessary. The crude oil combines with the H2 at high temperature and pressure, in the presence of a catalyst, to saturate aromatic molecules, separating out the lighter streams. Subsequent reactor stages further separate the hydrocarbon components, increasing the yield of low-boiling-point, high-value fluids and middle distillates, while leaving the heavier residue to be converted into coke in a separate operation.
Heavy oils bring additional levels of heavy metals, such as Hg, vanadium, magnesium, nickel and iron. It is imperative to assess and remove certain metals, particularly Hg. Even if Hg levels are low, the presence of this metal in large volumes of liquid hydrocarbons represents significant exposure to processing equipment. Non-removal of Hg can cause metal embrittlement and failure due to corrosion, and it can pose a health hazard to refinery workers. Furthermore, Hg levels in C3C6 product streams from the crude distillation column and in water effluent can poison catalysts.
Other materials commonly found in heavy oils can cause a myriad of operational problems if not eliminated. Calcium (Ca), present as calcium naphthenate, can cause fouling at the desalter, catalyst poisoning and scaling issues that require increased maintenance for heat exchanger tubes and other equipment internals. Ca deposits often require additional processes for elimination, such as demulsification.
Many of the new heavy oil sources have increased sulfur content and high levels of other impurities. Due to the compositional variability of heavy oils from different fields, processing technologies cannot be uniform, but instead must be structured to accommodate the specific characteristics of each oil.
High densities and viscosities require higher temperatures in refining units. In desalters, higher temperatures can compromise the heat energy balances designed for light oils. Additionally, the higher levels of solids found in heavy oils can lead to sludge, which can pose storage problems and have undesirable impacts on wastewater treatment and other offsite operations.
Delayed coker residue from the vacuum-reduced heavy oil presents another potential bottleneck in the refinery. As a batch process, this residue accumulates in the coker drums. If it is not removed in a timely manner, the refinery could back up and be forced to shut down.
Once a refinery has shifted its operational capabilities to produce high-value products from heavy oil, the facility must continue on this path, as significant modifications would be needed to convert the refinery feedstock slate back to light oil. A number of US Gulf Coast refineries have made the switch to heavy oil, and they rely on the delivery of Canadian crude oils. As a result of this scenario, pipelines have been reversed and extended to serve these facilities.
Modeling for heavy oil refining
As the availability of crude oil supplies changes and the characteristics of these feedstocks are more varied, existing refineries are forced to become more flexible in their operational capabilities and to adapt to the diversity of input materials.
Operations planning and optimization are critical in an existing facility, where margins fluctuate depending on processing costs, product yields and the adaptability of each processing unit. As an example, fractionation requires exact measurements of pressure, temperature and volume to maximize yields from feedstocks with varying compositions.
All refineries struggle to process heavy oils economically. Modeling methods that accurately depict the characteristics of these heavy feedstocks allow refiners to adapt their processes accordingly, whether by altering existing operating processes or by designing new equipment.
The ability to predict the performance behavior of heavy oils at each stage of the refining process is important. It is difficult to use traditional modeling software, as these programs were developed to predict the behavior of light, sweet oils. There is no single solution for heavier oil processing to suit all refineries. Each refinery has its own set of parameters governed by its existing configuration, the composition of the oils being processed, the operating conditions within the plant and the desired range of product output.
For example, asphaltenes are prevalent in heavy oils. The use of processes to precipitate them to lower viscosities can assist in refining. When diluted with solvents, including condensates, the dilbits from oil sands are easier to transport via pipeline. Therefore, to optimize processing, refinery operators must be able to predict the amount of asphaltene precipitated as a function of the amount and nature of the solvent and the ranges of temperature and pressure.
Addressing issues jointly
Utilizing field-proven, proprietary modeling software, one company enlisted a consortium of global industry leaders (i.e., major oil companies, Canadian heavy oil producers, engineering firms and national oil companies) to provide a forum for defining and solving major issues in heavy oil processing.
The company is using the latest modeling technologies to predict how heavy oils will perform at various refinery processing stages. This software has identified several areas in need of improvement, and new and expanded models have been developed to increase prediction and plant optimization accuracy through advanced process simulation.
These efforts have been directed at multiple procedures: Preparing crude oil feed, investigating viscosity and thermal conductivity, maintaining H2 balance and solubility as part of the upgrading process, removing Hg and other contaminants, and performing molecular-based characterization.
Knowledge of viscosity is critical to understanding heavy oil properties and potential product yields. The newest method of liquid viscosity prediction advances the accepted Twu correlation, improving the predictive accuracy of viscosities in the 100 cSt100,000 cSt range and closely estimating temperature dependence, especially for low temperatures. Utilizing data from more than 125 heavy oil assays provided by the consortium, the accuracy of viscosity prediction in simulations is greatly improved.
In addressing liquid-phase thermal conductivity in heavy oils, there is limited industry data. However, in recognizing the comparable characteristics of solvent refined coal II, for which some data is available, accurate correlations have been established. Recent testing has determined a strong performance using the Sato-Riedel method, requiring only an estimate of critical temperature to produce the best level of accuracy.
Hydroprocessing is required to remove impurities in heavy oils. Hydrogenation increases the yield and converts low-value feedstocks into higher-value end products. With the addition of H2 in the process, refinery optimization relies on H2 management and its solubility in hydrocarbons. Laboratory measurements for H2, hydrogen sulfide and ammonia vapor-liquid equilibrium with defined hydrocarbons have been used to fit equation-of-state binary interaction parameters to improve the accuracy of predicting the elements solubility in hydrocarbon mixtures.
Hg is a contaminant in heavy oils that poses multiple challenges during refining. It can poison catalysts, contaminate wastewater, destroy process equipment and impair processes, and be a human health hazard. An accurate understanding of Hgs solubility in hydrocarbons is necessary for its mitigation.
Similarly, naphthenic acid is a prevalent and increasingly corrosive element in heavier oils. The most problematic acids are those with a molecular weight having a boiling point of 430°F750°F. The acid concentration, density and viscosity of the oil must be assessed to predict its corrosion potential.
Kinematic viscosity predictability
The procedure for characterization using data from consortium members focuses on extrapolating the molecular weights of critical heavy oil components with normal boiling points exceeding 1,000 K. The importance of this procedure cannot be understated when designing process components, such as heat exchangers.
Using a newer correlation developed specifically for the attributes of heavy oil and kinematic viscosity, results can be compared to measured data and to data derived from the older API procedure 11A4.2. The new correlation exposes a significant difference in accuracy in heat exchanger sizing.
Table 1 shows predicted kinematic viscosities in centistokes at various temperatures using API procedure 11A4.2, compared to the heavy oil correlation shown in Fig. 2. In Table 2, the comparison shows that the duty would be almost 250% oversized. The newer heavy oil method predicts duty much closer to the measured data, especially as temperatures rise.
| Fig. 2. A correlation developed for attributes |
of heavy oil and kinematic viscosity.
The nature of refining is continually changing with increasing global demand and feedstock supplies that are becoming heavier and more difficult to process. In addition, increasing government oversight and environmental constraints on emissions heavily impact refining operations.
To maximize profitability, refiners are faced with decisions to alter their operations to suit changing feedstock slates. Operational challenges are extremely difficult, if not impossible, to address without knowledge of the specific characteristics of different crude oils.
Heavy oils have become economical feedstocks and are adding to global hydrocarbon supplies. However, they require additional additives to combat high viscosity and high mass density. At the refinery, they need extra treatment phases that include adding H2 during processing to eliminate or mitigate impurities and to alter their molecular structures.
The compositions of heavy oils vary, adding complexity to their processing. While models have been used for years to understand and optimize the processing of light oils, increased demand for heavy oil has led to the development of models to address the unique attributes of these oils.
To optimize processing and reduce design and operating costs, refiners should use accurate simulation models that are specifically tailored for heavy crude oils. Without this accuracy, operating and capital costs will escalate, and performance will diminish, at a time when refiners are seeking advantages to increase profitability. HP
||Joseph McMullen is the product marketing manager for the SimSci brand within Invensys. He earned a BS degree in chemical engineering in 2000 and an MBA degree in 2004, both from Villanova University in Pennsylvania. Since starting with Invensys in 2001 as a senior technical support specialist, Mr. McMullen has held multiple roles in product management and transitioned to product marketing in 2011. He regularly discusses simulation topics via his blog, http://simulationots.blogspot.com/ |