Refinery Feedstocks

James G. Speight

• Publisher: CRC Press
• Year: 2020
• ISBN:
  9780429675645
• Edition: 1
• Topic: Arte
• Subtopic: Técnicas artísticas

Over the last several decades, the petroleum industry has experienced significant changes in resource availability, petro-politics, and technological advancements dictated by the changing quality of refinery feedstocks. However, the dependence on fossil fuels as the primary energy source has remained unchanged.

Refinery Feedstocks addresses the problems of changing feedstock availability and properties; the refining process; and solids deposition during refining. This book will take the reader through the various steps that are necessary for crude oil evaluation and refining including the potential for the use of coal liquids, shale oil, and non-fossil fuel materials (biomass) as refinery feedstocks.

Other features:

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• Describes the various types of crude oil and includes a discussion of extra heavy oil and tar sand bitumen

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• Includes basic properties and specifications of crude oil and the significance in refinery operations

This book is a handy reference for engineers, scientists, and students who want an update on crude oil refining and on the direction the industry must take to assure the refinability of various feedstocks and the efficiency of the refining processes in the next fifty years. Non-technical readers, with help from the extensive glossary, will also benefit from reading this book.

Part 1

Feedstocks – Evaluation and Properties

1 Natural Gas, Crude Oil, Heavy Crude Oil, Extra-Heavy Crude Oil, and Tar Sand Bitumen

1.1 Introduction

For the purposes of this book, a refinery feedstock is defined as a native (naturally occurring) fossil fuel such as natural gas, crude oil, heavy crude oil, extra-heavy crude oil, and tar sand bitumen as well as any carbonaceous material that is destined for further processing. With further processing, the product will be transformed into one or more components and/or finished (saleable) products. In some cases, but excluded from this book, the feedstock is a refinery product (such as a residuum) that is destined for further processing (such as vacuum gas oil) excluding blending.

In the modern refinery, the feedstock is no longer (or very rarely) a single crude oil instead but is typically a blend of two or more crude oils (including heavy crude oil, extra-heavy crude oil, and tar sand bitumen). The refinery process units and the combination of units built and in service at a given refinery location are part of a plan to accommodate a certain slate of crudes based on their properties. The more consistent the supply of crude oil to a specific refinery, the more that refinery can tailor its operation to that specific crude supply. Necessity forces refiners to have to retain some flexibility in the refinery process to handle a wider range of crude types than that preferred. Crude blending works hand in hand with refinery process flexibility in crude types by enabling the ability to mix crudes that may not, as individual feeds, satisfy the operating range of the refinery, but as components of a mixed feed will meet the refinery operating requirements.

For those refineries which have to deal with a varied feedstock slate, pre-refinery blending is an attractive option to obtain either a better (average) quality feedstock or a stable quality feedstock. The blending operation typically takes place in the field where the properties of the blend are matched to meet the pipeline specifications. Because heavy crude oil and extra-heavy crude oil or tar sand bitumen cannot flow from the field to the refinery in their original state and at normal surface temperatures, they are blended with lighter (lower density) crude oils primarily to reduce the viscosity, thereby enabling transportation to a refinery. A secondary objective may be to produce a blend that has significantly higher economic value than the more viscous feedstocks. The blend is usually constructed so that the value of the overall blended volume is greater than the summed value of the initial volumes of individual viscous feedstocks and the light crude oil.

In terms of crude oils, such as opportunity crude oils and their blends, the risks may be high because these members of the crude oil family are usually laden with contaminants such as destabilized asphaltene constituents and high metals content. These contaminants can cause stable oil–water emulsion problems, heat exchanger fouling, and catastrophic coking in the furnace tubes, leading to high maintenance costs and equipment losses. Furthermore, incompatible crude oil blends can result in the flocculation and deposition of asphaltenes.

Thus, caution is advised in producing the blend because of the potential for blend instability though instability and/or incompatibility of the constituents and the potential, phase separation of one or more constituents of the blend or the deposition of asphaltene constituents, often referred to as fouling (Table 1.1) (Chapters 2 and 5) (Mushrush and Speight, 1995; Del Carmen García and Urbina, 2003; Bai et al., 2010; Speight, 2014a, 2015d; Ben Mahmoud and Aboujadeed, 2017; Speight, 2017; Kumar et al., 2018).

TABLE 1.1
The Constituents of Feedstocks That Can Cause Fouling (Sediment Formation) in the Refinery

Property	Comments
Asphaltene constituents	Separates from oil when gases are dissolved
	Thermal alteration can cause phase separation
Heteroatom constituents	Provide polar character to the oil
	Preferential reaction with oxygen
	Preferential thermal decomposition and molecular alteration
Aromatic constituents	May be incompatible with paraffin medium
	Phase separation of paraffins
Non-asphaltene constituents	Thermal alteration causes changes in polarity

More specifically, fouling as it pertains to refinery feedstocks is deposit formation, encrustation, deposition, scaling, scale formation, slagging, and sludge formation, which has an adverse effect on refinery operations. It is the accumulation of unwanted material within a processing unit or on the solid surfaces of the unit to the detriment of unit functionality. For example, when it does occur during refinery operations, the major effects include (i) loss of heat transfer as indicated by charge outlet temperature decrease and pressure drop increase, (ii) blocked process pipes, (iii) under-deposit corrosion and pollution, and (iv) localized hot spots in reactors, all of which culminate in production losses and increased maintenance costs. Typically, the fouling material consists of organic and/or inorganic materials deposited by the feedstock that is deposited by the occurrence of instability or incompatibility of the feedstock (one crude oil) with another during and shortly after a blending operation (Speight, 2014a). Thus, the complexity of the feedstock introduced into a refinery requires careful monitoring and the ability of the refiner to predict the behavior of the blend during the refining processes.

Predictability of the behavior of the feedstock during refining can be assessed by the development of models, but it must be remembered that models are paper calculations and may not always reflect the true behavior of the feedstock under the temperature and pressure conditions to which the feedstock is subjected during refining. For example, a model may predict excellent feedstock refinability in a particular process but in reality, fouling occurs thereby reducing the efficiency of the refining process.

In order to reduce fouling – while paper studies may solve only a part of the problems of identifying the presence and behavior of the foulants – it is necessary to perform a thorough assessment of the feedstock. This can only be achieved by knowing the feedstock and its behavior in a series of prescribed test methods that enable to refiner to predict the behavior of the feedstock during the refining process (Chapters 2–5). It is also necessary to be able to assess the behavior of the products in the same mix as unreacted and reacted feedstock, i.e., feedstock constituents that are intermediate between the original feedstock constituents and the final products (Mushrush and Speight, 1995).

Finally, an important aspect of designing a refinery for any carbonaceous feedstock (or two or more carbonaceous feedstocks) is the composition of the feedstocks. For example, a heavy oil refinery would differ somewhat from a conventional refinery, and a refinery for tar sand bitumen would be significantly different from both (Speight, 2014a, 2017, 2020). Furthermore, the composition of biomass is variable which is reflected the range of heat value (heat content, calorific value) of biomass, which is somewhat lesser than for coal and much lower than the heat value for crude oil, generally falling in the range 6,000–8,500 Btu/lb (Speight, 2020). Moisture content is probably the most important determinant of heating value. Air-dried biomass typically has about 15%–20% moisture, whereas the moisture content for oven-dried biomass is around 0%. Moisture content is also an important characteristic of coals, varying in the range of 2%–30%. However, the bulk density (and hence energy density) of most biomass feedstocks is generally low, even after densification, about 10% and 40% of the bulk density of most fossil fuels.

It is the purpose of this chapter to present the feedstocks that are currently sent to refineries, and these are (i) members of the natural gas family, (ii) members of the crude oil family, (iii) extra-heavy crude oil, and (iv) tar sand bitumen as well as other feedstocks such as coal liquids, shale oil, and biomass which, while not in popular use at this time, could be well refinery feedstocks of the future.

1.2 The Natural Gas Family

For the purposes of this text, the natural gas family (sometimes referred to as fossil gases) is a naturally occurring gas mixture consisting primarily of hydrocarbon derivatives (predominantly methane) methane, but commonly including varying amounts of other higher-molecular-weight alkane derivatives, and sometimes a small percentage of carbon dioxide, hydrogen sulfide, or helium. The gas is typically formed over millions of years when layers of decomposing plant and animal matter are exposed to heat and pressure under the surface of the Earth.

1.2.1 Natural Gas

Natural gas is the gaseous mixture that is predominantly methane but does contain other combustible hydrocarbon compounds as well as non-hydrocarbon compounds (Figure 1.1) (Mokhatab et al., 2006; Speight, 2014a, 2019a). Natural gas is colorless, odorless, tasteless, and shapeless. In the natural state, it is not possible to see or smell natural gas. In addition to composition and thermal content (Btu/scf, Btu/ft³), natural gas can also be characterized on the basis of the mode of the natural gas found in reservoirs where there is no or, at best only minimal amounts of, crude oil.

FIGURE 1.1 Range of composition of natural gas. (Please use tear sheet from: Speight, J.G., and Ozum, B. 2002. Petroleum Refining Processes. Marcel Dekker Inc., New York, Table 2.2, Page 34.)

Other constituents are paraffinic hydrocarbon derivatives such as ethane (CH₃CH₃), propane (CH₃CH₂CH₃), and the butanes (C₄H₁₀). Many natural gases contain nitrogen (N₂) as well as carbon dioxide (CO₂) and hydrogen sulfide (H₂S). Trace quantities of argon, hydrogen, and helium may also be present. Generally, the hydrocarbon derivatives having a higher molecular weight than methane, carbon dioxide, and hydrogen sulfide are removed from natural gas prior to its use as a fuel. However, since the composition of natural gas and refinery gas is never constant, there are standard test methods that can be used to determine the suitability of natural gas (and refinery gas) for further use and indicate the processes by which the composition of natural gas can be prepared for use (Mokhatab et al., 2006; Speight, 2014a, 2015b, 2019a).

1.2.2 Crude Oil-Related Gas

The generic term natural gas applies to gas commonly associated with petroliferous (crude oil-producing, crude oil-containing) geologic formations. Natural gas generally contains high proportions of methane (CH₄), and some of the higher-molecular-weight paraffins (C_nH_2n+2) generally containing up to six carbon atoms may also be present in small quantities. The hydrocarbon constituents of natural gas are combustible, but non-flammable non-hydrocarbon components such as carbon dioxide, nitrogen, and helium are often present in minor proportion and are regarded as contaminants. In addition to the gas found associated with crude oil in reservoirs, there are also reservoirs in which natural gas may be the sole occupant. And, just as crude oil can vary in composition, natural gas from different reservoirs also varies in composition.

Natural gas is often located in the same reservoir as with crude oil, but it can also be found trapped in gas reservoirs and within coal deposits. The occurrence of methane in coal seams is not a new discovery, and methane (called firedamp by the miners because of its explosive nature) was known to coal miners for at least 150 years (or more) before it was rediscovered and developed as coalbed methane (Speight, 2013a). The natural gas can originate by thermogenic alteration of coal or by biogenic action of indigenous microbes on the coal. There are some horizontally drilled coalbed methane wells, and some coalbed methane wells that receive hydraulic fracturing treatments. However, some coalbed methane reservoirs are also underground sources of drinking water, and as such, there are restrictions on hydraulic fracturing operations. The coalbed methane wells are mostly shallow, as the coal matrix does not have the strength to maintain porosity under the pressure of significant overburden thickness.

In addition to defining natural gas as associated and non-associated, the types of natural gas vary according to composition. There is dry gas or lean gas, which is mostly methane, and wet gas, whi...