Tải bản đầy đủ - 0trang
2 Extraction of Fossil Fuels
Chemistry of Sustainable Energy
FIGURE 2.4 Types of structural oil traps: (a) anticline and (b) fault. (Skinner, B.J. and S.C.
Porter: The Dynamic Earth. An Introduction to Physical Geology. 4th ed. 2000. Copyright
Wiley-VCH Verlag GmbH & Co. KGaA. Reprinted with permission.)
onshore supplies have dwindled, exploration and recovery have moved offshore, with
concomitant increase in risk and cost. The BP Deepwater Horizon explosion and
oil spill of April 2010 has the distinction of being the largest unintentional oil spill
in our planet’s history. Having killed 11 oil rig workers and spewed an estimated
4.9 million barrels of crude oil into the Gulf of Mexico, the costs of this event are
2.2.2 Nonconventional Sources
As supplies of conventional oil and gas have declined, new methods for accessing
the more elusive hydrocarbons trapped onshore have evolved. Once deemed nonproducible, these nonconventional sources of oil and gas (also called tight oil or gas)
include shale gas, oil shale, tar sands, methane coalbeds, and methane clathrates.
In other words, as the relatively accessible supplies dry up, the petroleum industry
must go to further and further lengths to find and extract petroleum from the planet,
using breakthrough technology in physics, chemistry, and engineering to meet our
18.104.22.168 Shale Oil and Gas
Shale is a fine-grained sedimentary rock that can trap natural gas in natural fractures and pores, or adsorb it onto organic matter and minerals in the shale (Gregory
et al. 2011). The relatively recent development of horizontal drilling and hydraulic
fracturing (“fracking”) techniques have grown exponentially in the extraction of tight
fossil fuels, largely supplanting vertical well drilling (Figure 2.5). The Barnett Shale
in Texas, the Marcellus formation in the northeastern United States, the Bakken
formation in western North Dakota and Eastern Wyoming, the natural gas and “light
tight crude” locked in shale are now recoverable in what were once considered poor
risks for petroleum and natural gas recovery. In fact, shale gas is the fastest-growing
source of natural gas (U.S. Energy Information Administration 2011).
Horizontal drilling is self-explanatory: the vertical drill is sunk to the point at
which a layer of gas-infused shale is reached. The drill is then curved to enter the
shale stratum horizontally and boring is continued. After some length of horizontal bore has been reached, hydraulic fracturing of the shale is used to liberate the
trapped hydrocarbons (Figure 2.6). Slick water—a fracturing fluid made up of sand
(the proppant), water, and a small amount of numerous other chemicals (Table 2.2)—
is pumped at high pressure into the horizontal wellbore to fracture the shale. Natural
gas and light crude trapped in the shale is released and percolates through the sand
back to the surface, where it is captured at the wellhead.
While fracking has proven to be successful at recovering what was once considered to be inaccessible petroleum reserves, controversy abounds. Water use and
water quality are huge issues—millions of gallons of fracking fluid are required
for each well (Manning 2013). Although these fracking fluids are greater than
98% water, the friction reducers, biocides, and corrosion inhibitors are many
(see Table 2.2 with the caveat that not all of these chemicals are necessarily used
simultaneously; the actual mixture depends on the site). The water that percolates
back to the surface (flowback) has a very high total dissolved solids concentration
and consists of salts, sediment, bacteria, and some heavy and light hydrocarbons.
Extensive treatment of the recovered water is required for reuse or recycling, but
only about 35% of the fracking fluid returns to the surface—the remainder remains
Annual Barnett Shale natural gas production by well type
Billion cubic feet (Bcf )
FIGURE 2.5 Growth in horizontal well drilling in the Barnett Shale region. (Adapted
from U.S. Energy Information Administration. 2011. Technology drives natural gas production growth from shale gas formations. Today in Energy, July 12, 2011. http://www.eia.
gov/todayinenergy/images/2011.07.12/barnettbarpII.png. Accessed 11 June 2013.)
Chemistry of Sustainable Energy
Gas is treated and stored prior to entering
Flowback water returns to the
surface and is impounded prior
to treatment or reuse
Shale gas development
A vertical well more than a km deep is
gradually turned through 90° angle and
continued up to 3 km horizontally.
Hydraulic fracture occurs with 7–18
million liters of water which has been
mixed with sand and chemical
amendments to protect down-well
equipment and optimize flow conditions.
holds open new
pressure is released
at the well head
Formation gas and
return to surface
fracture in shale
Fracture water is pumped at high
pressure to fracture the formation
1100 m solids and increase permeability. Sand
or other “proppant” flows into the new
fissures to retain the newly introduced
permeability as down-hole pressure
decreases with the flow of the fracture
1500 m water and gas to the surface.
fractures formation and
increases permeability of
FIGURE 2.6 (See color insert.) Hydraulic fracturing in the extraction of shale gas.
(Reprinted with permission from Gregory, K.B., R.D. Vidic, and D.A. Dzombak. 2011. Water
management challenges associated with the production of shale gas by hydraulic fracturing.
Elements 7 (3):181–186. Copyright 2011, Mineralogical Society of America.)
a potential threat to groundwater contamination and there have been reports of
water supplies being literally flammable (Bomgardner 2012). Fracturing shale has
led to increased seismic activity; discharge of nitrogen oxides and volatile organic
compounds at the sites results in decreased air quality (Schmidt 2011), and studies have shown that leakage of methane from fracking sites has led to a greater
carbon footprint than the extraction of coal or conventional natural gas wells
(Howarth et al. 2011; Weber and Clavin 2012). Furthermore, the mining of enormous amounts of fracking sand has led to considerable community controversy
across the United States. Overall, the ecological and health effects of fracturing
warrant serious concern.
22.214.171.124 Heavy Oil
Heavy oil is a complex hydrocarbon mixture that is too viscous to flow at ambient temperatures due to loss of the light hydrocarbon fraction during migration or
microbial decay. Asphaltenes (Figure 2.7) exemplify the type of compounds found
in heavy oil. Given their high molecular weight and ample opportunity for intramolecular attractions and conformational entanglement, it is clear why these are highly
viscous materials. As a result, either heating or dilution with a solvent is required
Example Composition of Fracking Fluid
Iron control agent
Crystalline silica (fracking
Petroleum distillate blend
Petroleum distillate, light
Dimethyl benzyl ammonium
Concentration, % by Mass
Source: FracFocus Chemical Disclosure Registry. http://fracfocus.org/chemical-use/whatchemicals-are-used; accessed 8 July 2013.
to get this material to flow, with most enhanced oil recovery (EOR) methods using
steam injection to force the material to the surface. (In fact, one use of captured CO2
is for enhanced oil recovery.) In the United States, the largest reservoirs of heavy oil
are located in Alaska and California.
Oil shale contains trapped organic matter that has not “cooked” enough at the
needed temperature and pressure. It is generally found all over the world at depths of
<900 m and has a high bitumen (essentially tar) content. In oil shale, the bitumen–
kerogen mixture trapped in the shale is, like heavy oil, immobile. This material is
typically mined and crushed and the pulverized material then heated to 450–500°C
in the absence of air (a process known as retorting) to obtain crude oil. Other methods for extracting oil from oil shale (including in situ retorting while the material is
still underground) are under development, but the environmental impact and costs
remain very high with commercial-scale viability still uncertain.
126.96.36.199 Oil Sands
Heavy oil is, in essence, tar—more accurately referred to as bitumen or asphalt.
Tar pits, tar pools, and oil sands (often less accurately referred to as “tar sands”)
are scattered about the globe at shallow depths; in some areas, the pitch has seeped
Chemistry of Sustainable Energy
FIGURE 2.7 Some representative asphaltenes.
to the surface. Evaporation of the more volatile components and oxidation of the
residue yields the tarry, carbon-rich fuel source. The Athabasca Valley in Alberta,
Canada, is one of the largest oil sands reserves in the world, with an estimated 170
billion barrels of oil locked in these bitumen-soaked sands (Figure 2.8; Energy
Resources Conservation Board 2011). But bitumen is notoriously difficult to handle
and correspondingly hard to extract. The methods whereby the oil sands are converted into a useable fuel—open pit mining or forcing superheated steam into a
well to force the bitumen to flow to the surface—again have serious environmental
consequences. Recent findings show that levels of toxic polycyclic aromatic hydrocarbons (thiophenes, anthracenes, pyrenes, etc.) are increasing in the environment
of northern Alberta, in parallel with the increase in oil sands production. Levels at
one site are now as much as 23 times higher than the 1960 levels (Kurek et al. 2013).
FIGURE 2.8 (See color insert.) An oil sands development in northern Alberta, Canada.
(Shutterstock Image id 48011344.)
Furthermore, the transport of the crude product from Alberta to refineries in the U.S.
Gulf of Mexico has been proposed via the “Keystone XL” pipeline, an extremely
controversial project due to environmental and safety concerns. As is evident, nonconventional fossil fuel sources are laden with difficult choices.
188.8.131.52 Coal Bed Methane and Methane Hydrates
184.108.40.206.1 Coal Bed Methane
It is only too well known that underground coal mines contain potentially hazardous amounts of methane gas, occasionally exploding with tragic results. Extensive
safety precautions are taken—including ventilation—in order to prevent such tragedies. But as fossil fuel supplies decrease, capturing this firedamp becomes an
attractive alternative as a source of natural gas. Because of its highly porous nature,
coal is an ideal substrate for adsorption of large volumes of methane. In order to
obtain methane from coal beds (primarily bituminous), the water that permeates
the coal beds must first be pumped out. The coal bed methane is then freed from
the coal bed by desorption and the gas flows to the wellhead. (Note that a new
approach to coal bed methane production is under investigation in which pressurized CO2, ideally captured from combustion of fossil fuel, is used to displace the
methane gas as a carbon capture and sequestration strategy; see Section 220.127.116.11.)
The U.S. production of coal bed methane in 2010 was about 1886 billion cubic feet
(roughly 7.5% of all U.S. natural gas marketed in 2010; U.S. Energy Information
18.104.22.168.2 Methane Hydrates
All it takes is water and the microbial decay of organic matter, plus low temperature (4°C) and high pressure (60 bar), to trap methane in an ice cage (the methane
Chemistry of Sustainable Energy
FIGURE 2.9 Methane hydrate. (Adapted from United States Geological Survey. The U.S.
Geological Survey Gas Hydrates Project Gas Hydrates Primer. Retrieved 14 May 2013, from
hydrate; see Figure 2.9). The concept of harvesting significant quantities of methane
from methane hydrates (also known as methane clathrates) is relatively recent. The
permafrost and continental margins (e.g., the Gulf of Mexico in the United States)
are locations where large quantities of methane hydrates are found. Methane hydrate
recovery is a very active area of research.
Methane hydrates lead a schizophrenic life: they are hazards when it comes to
conventional natural gas exploration and recovery in that accidental release of methane from the hydrate (degassing) can result in explosion or environmental catastrophe. At the same time, methane hydrates clearly pose great potential as a production
target. A 2010 report of the National Research Council suggested that methane
hydrate remains a feasible source for commercial production of methane (Committee
on Assessment of the Department of Energy’s Methane Hydrate Research and
Development Program: Evaluating Methane Hydrate as a Future Energy Resource
2010). As with every other nonconventional (and conventional!) source of fossil fuel,
the trick is figuring out a way to extract the fuel from the planetary resource safely
and with minimal environmental impact.
2.3.1 Crude Petroleum
Regardless of the source or type of material extracted from the earth, crude petroleum is a mixture of many hundreds of compounds that must be upgraded in order
to obtain marketable products. The plethora of molecules that make up crude oil can
be put into four basic categories (N.B. Bear in mind that these relative amounts vary
depending upon the source of the oil.):
• Saturated hydrocarbons. Approximately 30% (based on molecular
weight) of oil is made up of acyclic aliphatics (alkanes), also known as
• Cycloalkanes. Roughly 49% consists of cyclopentanes, cyclohexanes, and
cycloheptanes, many with short-chain substituents. These are also known
as the napthenes.
• Aromatics. Benzene, toluene, and xylenes (“BTX”), among other aromatics, make up ca. 15%.
• Asphaltics. Residual material with more than 38–40 carbon atoms constitutes about 6% of crude petroleum by molecular weight.
Nitrogen heterocycles, sulfur (as thiols and thiophenes), and oxygen contaminants
(from water to phenols) are also present in small (trace up to approximately 4%)
amounts. Alkenes are present in only very limited amounts due to their natural
It is futile to attempt to identify and purify every molecule making up crude
oil; what is important is separating the mess into cleaner fractions based on similar
physical properties and (as a result) similar end uses—this is the aim of petroleum
refining. Petroleum refining is quite different from isolating a single, identifiable
compound, but the separation techniques for refining petroleum are, for the most
part, identical to those used in any undergraduate organic chemistry laboratory: distillation, extraction, chromatography—all techniques that separate the crude material into fractions (or cuts) based on similar physical properties. Further conversion
of the separated materials by chemical means takes place to convert these fractions
into more useful products. The pathway from crude oil to finished product via the
petroleum refinery is very complex (and very big business indeed), as can be seen in
Figure 2.10. Our focus will be on four main processes: distillation, extraction, cracking, and reforming.
Distillation of crude petroleum is the first step in the process of refining the material.
Given the complexity of the mixture, simple distillation is insufficient to separate
the fractions. Instead, fractional distillation in a distillation tower (Figure 2.11) is
required. As in simple distillation, fractional distillation separates on the basis of
volatility (vapor pressure/boiling point) but the efficiency of separation is greatly
improved by allowing repeated vaporization–condensation cycles on the tower’s
internal surfaces as the crude distillate flows upward through the multiplate fractionating column. Multiplate refers to the fact that the increased surface area—a result
of packing material inside the tower and represented by the squiggly lines in Figure
2.11—increases the number of theoretical plates upon which the vaporization–
condensation equilibrium can be reestablished many times over. The fractions are
split off at varying heights in the tower as the distillation continues, separating the
Lube feedstock (20)
HDS mid-distillate (6A)
SR mid-distillate (6)
SR kerosene (5)
HDS hvy naphtha (4A)
Lt SR naphtha (3)
Atmospheric tower residue (8)
Vacuum residue (21)
clarified oil (27)
Hvy cat-cracked distillate (26)
Hvy vaccum distillate (20)
Lt cat-cracked distillate (24)
Lt thermal cracked distillate (30) (Gas oil)
Lt vacuum distillate (19)
SR gas oil (7)
SR middle distillate
Atmospheric SR kerosene (5)
Heavy SR naphtha (4) Hydrodesulfurization/treating
Light SR naphtha (3)
Light crude oil
petroleum gas (LPG)
FIGURE 2.10 The complexity that is the modern petroleum refining process. (Adapted from U.S. Department of Labor OSHA Technical Manual,
Section IV: Chapter 2. Petroleum Refining Processes. http://www.osha.gov/dts/osta/otm/otm_iv/otm_iv_2.html#3. Accessed 29 August 2012.)
crude oil1 (1)
Crude oil (0)
Chemistry of Sustainable Energy
Methane, ethane, propane, butanes
Flared, or natural gas
Light naphtha (straight run gasoline)
(C5 to C9)
(C5 to C10)
Mostly C4–C10 alkane
and cycloalkane with
some aromatics. Used
for fuel and chemical
Kerosene—jet, tractor, and heating fuel
(C9 to C16)
(C14 to C20)
(C20 to C50)
(C20 to C70)
May be cracked
to lighter fractions
FIGURE 2.11 Schematic of a distillation tower.
crude oil into several cuts, from gases (butane and lower-molecular-weight alkanes)
to residual heavy oils (see Figure 2.11). It is important to note that the fractions thus
obtained are not discrete products with a sharp boiling point. Instead, each fraction consists of an array of compounds exhibiting a band of boiling point ranges.
Ultimately, the higher-boiling fractions are transferred to a new tower where a
vacuum distillation is carried out to separate even more material from less-volatile
The asphaltic residues can also be further fractionated by solvent extraction with
liquified propane or butane, solubilizing the nonpolar solutes. Even more separation
can take place by adsorbing the solubilized material onto some adsorbent (e.g., clay,
alumina, or silica) and extracting different fractions based on the partitioning ability
of an added solvent. Thus, dichloromethane will extract a different type of material
than methyl ethyl ketone, for example. The extracted materials can be used for heavy
lubricating oils, greases, and so on once the solvent is removed (Speight 2007).
Chemistry of Sustainable Energy
Once the crude petroleum has been separated into its various fractions (so-called
straight-run fractions), the next step consists of chemical conversion into materials that exhibit more desirable properties: primarily lower boiling range fractions
that make for good transportation fuels. This conversion is known as cracking, of
which there are three main types: (a) thermal cracking, (b) catalytic cracking, and (c)
hydrocracking. Cracking is intense chemistry: while the principles of basic organic
chemistry apply, cracking begins with mixtures, treats them to harsh conditions, and
ends with different mixtures. A complete understanding of these processes at the
molecular level can only be based on simpler models.
22.214.171.124.1 Thermal Cracking
Thermal cracking is precisely what it sounds like: using high temperature (450–
450°C) and pressure (690−6900 kPa) to crack molecules into smaller fragments. The
high temperatures and pressures under neutral conditions used in thermal cracking
lead to fragmentation and product reforming by homolytic (radical) mechanisms.
Thus, for example, hexanes can be converted into ethene and the n-butyl radical
under these extreme conditions (see Figure 2.12). Control of the cracking products
is an obvious issue. As the temperature is increased, more and more low-molecularweight alkenes are formed. The pressure at which the cracking takes place as well as
the length of time the feedstock is heated can also influence the product distribution.
Thermal cracking was the historical method for petroleum conversion but has now
been largely replaced by catalytic cracking.
126.96.36.199.2 Catalytic Cracking
Catalytic cracking (also known as fluidized catalytic cracking, or FCC) is milder
and more selective than thermal cracking. FCC takes higher-boiling fractions and
converts them into higher-value saturated compounds (branched paraffins and naphthenes) and aromatics. Most catalysts used for FCC are based on natural or synthetic cage-like aluminosilicates known as zeolites, the “crown jewels of catalysis”
(Bartholomew, 2006). Zeolites have the general formula Mv(AlO2)x(SiO2)y•zH2O,
with the aluminum and silicon oxide species sharing oxygen atoms in tetrahedral AlO4 and SiO4 building blocks for the zeolite unit cell and the metal cation
residing in an exchangeable cationic site in the center of the cage (see Figure 2.13)
(Bartholomew and Farrauto 2006). Activation of the zeolite catalyst into an acidic
form facilitates the conversion of higher-molecular-weight compounds into lower
ones via carbocation intermediates. Replacement of the aluminum with a rare earth
element (e.g., La, Ce, Pr, Nd, Sm, Eu, or Gd) imparts greater stability to the catalyst,
FIGURE 2.12 Hypothetical homolytic thermal cracking mechanism.
+ H 2C