Literature review of diesel includes...Hydocarbon introduction, Hydrocarbon properties, Diesel production techniques & routes, Selection of best techique for diesel production, Technology provider & licencers.

1. Chapter 2: Literature Review
Hydrocarbons
Hydrocarbons are organic compounds composed entirely of carbon and hydrogen atoms.
There are four major classes of hydrocarbons: paraffins, naphthenes, olefins, and
aromatics. Each class is a family of individual hydrocarbon molecules that share a
common structural feature, but differ in size (number of carbon atoms) or geometry. The
classes also differ in the ratio of hydrogen to carbon atoms and in the way the carbon atoms
are bonded to each other.
Paraffins
Paraffins have the general formula CnH2n+2, where “n” is the number of carbon atoms
(carbon number) in the molecule. There are two subclasses of paraffins: normal paraffins
and isoparaffins. Normal paraffins have carbon atoms linked to form chain-like molecules,
with each carbon except those at the ends – bonded to two others, one on either side.
Isoparaffins have a similar carbon backbone, but they also have one or more carbons
branching off from the backbone.
Naphthenes
Naphthenes have some of their carbon atoms arranged in a ring. The naphthenes in diesel
fuel have rings of five or six carbons. Sometimes two or more rings are fused together,
with some carbons shared by adjacent rings. Naphthenes with one ring have the general
formula CnH2n.
Olefins
Olefins are similar to paraffins but have fewer hydrogen atoms and contain at least one
double bond between a pair of carbon atoms. Olefins rarely occur in crude oil; they are
formed by certain refinery processes. Like paraffins, olefins with four or more carbons can
exist as structural isomers. Olefins with one double bond have the general formula CnH2n,
the same as naphthenes [3].

2. Aromatics
As with naphthenes, some of the carbon atoms in aromatics are arranged in a ring, but they
are joined by aromatic bonds, not the single bonds found in naphthenes. Aromatic
hydrocarbon rings contain six carbon atoms. Benzene is the simplest aromatic compound.
One-ring aromatics have the general formula CnH2n-6. Polycyclic aromatics are compounds
with two or more aromatic rings. These rings are fused together, with some carbons being
shared by adjacent rings. Aromatics and olefins are classified as unsaturated hydrocarbons.
They contain carbon to carbon double bonds or aromatic bonds that can be converted to
single bonds by adding hydrogen atoms to the adjacent carbons.
Hydrocarbon Properties
Boiling Points:
For compounds in the same class, boiling point increases with carbon number. For
compounds of the same carbon number, the increasing boiling point by class is Isoparaffin
> n-paraffin > naphthene > aromatic. The boiling point difference (60° to 80°C or 100° to
150°F) between isoparaffins and aromatics of the same carbon number is larger than the
boiling point difference (about 20°C or 35°F) between compounds of the same class that
differ by one carbon number.
Freezing Point
Freezing points (melting points) also increase with molecular weight, but they are strongly
influenced by molecular shape. Molecules that fit more easily into a crystal structure have
higher freezing points than other molecules. This explains the high melting points of n-
paraffins and unsubstituted aromatics, compared to the melting points of isoparaffins and
naphthenes of the same carbon number.

3. Density
For compounds of the same class, density increases with carbon number. For compounds
with the same carbon number, the order of increasing density is paraffin, naphthene, and
aromatic
Heating Value
For compounds with the same carbon number, the order of increasing heating value by
class is aromatic, naphthene, and paraffin on a weight basis. However, the order is reversed
for a comparison on a volume basis, with aromatic highest and paraffin lowest. Lighter
(less dense) fuels, like gasoline, have higher heating values on a weight basis, whereas the
heavier (more dense) fuels, like diesel, have higher heating values on a volume basis.
Cetane Number
Normal paraffins have high cetane numbers that increase with molecular weight.
Molecules with many short side chains have low cetane numbers.
Isoparaffins 10-80
Naphthenes 40-70
Aromatics 0-60
Aromatics (with 2 or 3 rings) Below 20
Viscosity
Viscosity is primarily related to molecular weight and not so much to hydrocarbon class.
For a given carbon number, naphthenes generally have slightly higher viscosities than
paraffins or aromatics.
Diesel fuel operating properties
Smoke:
The fuel system of a diesel engine is designed and calibrated so that it does not inject more
fuel than the engine can consume completely through combustion. If excess of fuel exists,

4. the engine will be unable to consume it completely, and incomplete combustion will
produce black smoke. The point at which smoke production begins is known as the smoke
limit. Fuel with a very high cetane number can cause smoking in some engines. The short
ignition delay causes most of the fuel to be burned in the diffusion-controlled phase of
combustion, which can lead to higher PM emissions.
Fuel stability – filter life
Unstable diesel fuels can form soluble gums or insoluble organic particulates. Both gums
and particulates may contribute to injector deposits, and particulates can clog fuel filters.
The formation of gums and particulates may occur gradually during long-term storage or
quickly during fuel system recirculation caused by fuel heating.
Low - Temperature Operability
Low temperature operability is an issue with diesel fuel because it contain straight and
branched chain hydrocarbons (paraffin waxes) that become solid at ambient winter
temperatures in colder geographic areas. Wax may plug the fuel filter or completely gel
the fuel, making it impossible for the fuel system to deliver fuel to the engine.
In a refinery, there are a number of approaches to improve a fuel’s low-temperature
operability, such as:
• Manufacture it from less waxy crudes.
• Dilute it with a fuel with lower wax content (kerosene).
• Treat it with a low-temperature operability additive.
Diesel engines and emissions
Diesel exhaust tends to be high in NOx and particulates, both visible (smoke) and invisible.
Both NOx and particulates are significant environmental pollutants. Unlike the exhaust of
gasoline engines, diesel exhaust contains much less unburned or partially burned
hydrocarbons and carbon monoxide.

5. Nitrogen Oxides: NO and NO2 tend to form in the regions where there is excess oxygen
and the temperature is high. Outside of these regions, either there is insufficient oxygen to
form NOx or temperatures are too low for the reactions to occur quickly enough.
Carbon Monoxide
CO is a result of incomplete combustion. It mostly forms in regions of the cylinder that
are too fuel-rich to support complete combustion. If temperatures are high enough, the CO
can further react with oxygen to form CO2. Because diesel engines have excess oxygen,
CO emissions are generally low.
Particulates
Some of the fuel droplets may never vaporize and/or mix with air, and thus, never burn.
The conversion of fuel to particulates is most likely to occur when the last bit of fuel is
injected in a cycle, or when the engine is being operated at high load and high speed. At
higher engine speeds and loads, the total amount of fuel injected increases and the time
available for combustion decreases. Finally, a poorly operating or mistimed fuel injection
system can substantially increase emissions of particulates.
Sulfur
The sulfur content of diesel fuel affects Particulate Matters emissions because some of it
in the fuel is converted to sulfate particulates in the exhaust.
The U.S. EPA limited the sulfur content of on-road diesel fuel to 15 ppm.
The European Union has limited diesel sulfur content to 50 ppm,
Japan limited sulfur to 10 ppm in 2007.
Ultra-low Sulfur diesel fuel
In the past, diesel engine manufacturers have produced engines to meet the increasingly
stringent emissions standards through improvements to the combustion process itself. In
order to meet additional regulatory standards, most new diesel engines will need to employ
some type of advanced exhaust aftertreatment technology. Because most exhaust
aftertreatment devices are very sensitive to sulfur (some devices can be permanently

6. damaged by prolonged exposure to fuel sulfur levels as low as 50 ppm), vehicles so
equipped must use ultra-low sulfur diesel (ULSD) fuel. The term “ultra-low sulfur diesel”
may refer to different levels of sulfur in different parts of the world. However, for the
purposes of this review, ULSD refers to diesel fuel containing less than 15 ppm sulfur in
the U.S. and less than 10 ppm sulfur in Europe and the Asia-Pacific region [11].
Blending
The diesel fuel produced by a refinery is a blend of all the appropriate available streams:
straight-run product, FCC light cycle oil, and hydrocracked gas oil. The straight-run diesel
may be acceptable as is, or may need minor upgrading for use in diesel fuel prepared for
off-road use.The refiner must blend the available streams to meet all performance,
regulatory, economic, and inventory requirements.
Diesel Fuel Additives
Types of additives
Diesel fuel additives are used for a wide variety of purposes. Four applicable areas are:
• Engine and fuel delivery system performance
• Fuel handling
• Fuel stability
• Contaminant control
Cetane Number Improvers (Diesel Ignition Improvers)
Cetane number improvers raise the cetane number of the fuel. Within a certain range, a
higher number can reduce combustion noise and smoke and enhance ease of starting the
engine in cold climates. 2-Ethylhexyl nitrate (EHN) is the most widely used cetane number
improver. It is also called octyl nitrate. EHN is thermally unstable and decomposes rapidly
at the high temperatures in the combustion chamber. The products of decomposition help
initiate fuel combustion and thus shorten the ignition delay period from that of the fuel
without the additive. A disadvantage of EHN is that it decreases the thermal stability of
some diesel fuels. This can be compensated for by the use of thermal stability additives.

7. Injector Cleanliness Additives
Ash less polymeric detergent additives can clean up fuel injector deposits and/or keep
injectors clean. These additives are composed of a polar group that bonds to deposits and
deposit precursors, and a non-polar group that dissolves in the fuel. Detergent additives
typically are used in the concentration range of 50 to 300 ppm.
Lubricity Additives
Lubricity additives are used to compensate for the lower lubricity of severely hydrotreated
diesel fuels. They contain a polar group that is attracted to metal surfaces that causes the
additive to form a thin surface film. The film acts as a boundary lubricant when two metal
surfaces come in contact. Mono acids are more effective, therefore lower concentrations
are used (10 to 50 ppm). Most ultra-low sulfur diesel fuels need a lubricity additive to
meet the ASTM lubricity specifications.
Fuel Handling Additives
Antifoam Additives
Some diesel fuels tend to foam as they are pumped into vehicle tanks. The foaming can
interfere with filling the tank completely or result in a spill. Most antifoam additives are
organosilicone compounds and are typically used at concentrations of 10 ppm or lower.
De-Icing Additives
Free water in diesel fuel freezes at low temperatures. The resulting ice crystals can plug
fuel lines or filters, blocking fuel flow. Low molecular weight alcohols or glycols can be
added to diesel fuel to prevent ice formation. The alcohols/glycols preferentially dissolve
in the free water giving the resulting mixture a lower freezing point than that of pure water
[11].

8. Diesel Fuel production routes
1. Fischer Tropsch Process
As the price of crude oil sets a record high, liquid transportation fuels synthesized from
coal, natural gas, and biomass are proposed as one solution to reducing dependency on
imported petroleum and strained refinery capacity. Fischer-Tropsch synthesis is well
suited to producing middle-distillate range fuels like diesel and jet. Synthetic diesel can be
produced from any carbonaceous material, including biomass, biogas, natural gas, coal
and many others. The raw material is gasified into synthesis gas, which after purification
is converted by the Fischer Tropsch process to a synthetic diesel.
In the following simplification, Fischer-Tropsch synthesis occurs through two
simultaneous reactions promoted by the contact of CO and H2 with a catalyst:
2 H2 + CO -CH2- + H2O and
CO + H2O CO2 + H2
Which can be simplified as:
2CO + H2 -CH2- + CO2.
The CO2 byproduct of these reactions can be scrubbed from the “syngas” stream before it
is introduced to the synthesis reactor. Paraffinic synthetic diesel generally has a near-zero
content of sulfur and very low aromatics content, reducing unregulated emissions of toxic
hydrocarbons, nitrous oxides and particulate matter (PM).
The process is typically referred to as biomass-to-liquid (BTL), gas-to-liquid (GTL) or
coal-to-liquid (CTL), depending on the raw material used.
Gas to liquids (GTL) is a refinery process to convert natural gas or other gaseous
hydrocarbons into longer-chain hydrocarbons such as gasoline or diesel fuel. Methane-
rich gases are converted into liquid synthetic fuels via Fischer Tropsch process.

9. The Fischer–Tropsch process starts with partial oxidation of methane (natural gas) to
carbon dioxide, carbon monoxide, hydrogen gas and water. The ratio of carbon monoxide
to hydrogen is adjusted using the water gas shift reaction, while the excess carbon dioxide
is removed with aqueous solutions of alkanol amines (or physical solvents). Removing the
water yields synthesis gas (syngas) which is chemically reacted over an iron or cobalt
catalyst to produce liquid hydrocarbons and other byproducts. Oxygen is provided from a
cryogenic air separation unit.
Biomass to liquid (BtL or BMtL) is a multi-step process of producing liquid biofuels from
biomass. The Fischer Tropsch process is used to produce synfuels from gasified biomass.
Carbonaceous material is gasified and the gas is processed to make purified syngas (a
mixture of carbon monoxide and hydrogen). The Fischer-Tropsch polymerizes syngas into
diesel-range hydrocarbons.
Coal To liquid
Coal liquefaction is a process of converting coal into liquid hydrocarbons. liquefaction
processes generally involve gasification of coal to a mixture of carbon monoxide and
hydrogen (syngas) and then using a process such as Fischer Tropsch process to convert
the syngas mixture into liquid hydrocarbons.
Hydrocracking is a catalytic chemical process used in petroleum refineries for converting
the high-boiling constituent hydrocarbons in petroleum crude oils to more valuable lower-
boiling products such as gasoline, kerosene, jet fuel and diesel oil. The process takes place
in a hydrogen-rich atmosphere at elevated temperatures (260 – 425°C) and pressures (35
– 200 bar). Hydrocracking feeds can range from heavy vacuum gas oils and coker gas oils
to atmospheric gas oils. Products usually range from heavy diesel to light naphtha.
Selection of process
Gas to liquid:
• High water consumption in water-gas shift or methane steam reforming reactions
is another adverse environmental effect.

10. • Process improvements such as intensification of the Fischer Tropsch process,
hybrid liquefaction processes, and more efficient air separation technologies
needed for production of oxygen (e.g. ceramic membrane-based oxygen
separation).
Biomass to liquid: The process uses the whole plant to improve the carbon dioxide
balance and increase yield. Fatty-acid methyl ester (FAME), more widely known as
biodiesel. FAME has a lower energy content than diesel due to its oxygen content, and as
a result, performance and fuel consumption can be affected. It also can have higher levels
of NOx emissions, possibly even exceeding the legal limit. FAME also has lower
oxidation stability than diesel, and it offers favorable conditions for bacterial growth, so
applications which have a low fuel turnover should not use FAME.The loss in power when
using pure biodiesel is 5 to 7%.
Coal to liquid:
As coal liquefaction generally is a high-temperature/high-pressure process, it requires a
significant energy consumption and, at industrial scales (thousands of barrels/day),
multibillion-dollar capital investments. Thus, coal liquefaction is only economically viable
at historically high oil prices, and therefore presents a high investment risk. Most coal
liquefaction processes are associated with significant CO2 emissions from the gasification
process or from heat and electricity inputs to the reactors, thus contributing to global
warming.
Produces low-octane gasoline.
Low efficiency in converting coal to liquid.
Hydrocracking
Hydrocracking is particularly well suited to generating products that meet or exceed all of
the present tough environmental regulations. They can be designed and operated to
maximize the production of a gasoline blending component (called hydrocrackate) or to
maximize the production of diesel oil.
• Its flexibility with respect to production of gasoline and middle distillates,

11. • Better Quality of its products (free of sulfur and nitrogen impurities and consist
mostly of paraffinic hydrocarbons).
• ability to handle a wider range of feedstock like cycle oils from other cracking
units
• Does not yield any coke as by-product
• Better conversion (around 100%) of the gas oil and residues into useful products.
History
Hydrocracking was first developed in Germany as early as 1915 to provide liquid fuels
derived from their domestic coal deposits. The first plant that might be considered as a
commercial hydrocracking unit began operation in Leuna, Germany in 1927.
Between 1925 and 1930, Standard Oil of New Jersey collaborated with I.G. Farben
industries of Germany to develop hydrocracking technology capable of converting heavy
petroleum oils into fuels. Such processes required pressures of 200 – 300 bar and
temperatures of over 375 °C and were very expensive.
In 1939, Imperial Chemical Industries of Great Britain developed a two-stage
hydrocracking process. During World War II (1939 – 1945), two-stage hydrocracking
processes played an important role in producing aviation gasoline in Germany, Great
Britain and the United States.
After World War II, hydrocracking technology became less important. The availability of
petroleum crude oil from the Middle East removed the motivation to convert coal into
liquid fuels. Newly developed fluid catalytic cracking processes were much more
economical than hydrocracking for converting high-boiling petroleum oils to fuels [1].
In the early 1960s, hydrocracking become economical for a number of reasons:
• The automobile industry began manufacturing higher-performing automobiles that
required high-octane gasoline.
• Fluid catalytic cracking expanded rapidly to meet the demand for high-octane
gasoline. However, fluid catalytic cracking, in addition to producing gasoline,

12. produces a by-product high-boiling oil called cycle oil that is very difficult to
recycle for further cracking. However, hydrocracking can crack that cycle oil.
• The switch from railroad steam engines to diesel engines and the introduction of
commercial jet aircraft in the 1950's increased the demand for diesel oil and for jet
fuel. The flexibility of hydrocracking to produce either gasoline, jet fuel or diesel
oil made it desirable for petroleum refineries to install hydrocrackers.
• Zeolite-based catalysts, developed and commercialized during the period from
about 1964 to 1966, performed much better than the earlier catalysts. Most
importantly, they permitted operation at lower pressures than possible with the
earlier catalysts. The higher performance and lower operating pressures made
possible by the new catalysts resulted in significantly more economical
hydrocrackers.
Hydrocracking enjoyed rapid growth in the United States during the late 1960s and the
early 1970s. By the mid-1970s, hydrocracking had become a mature process and its
growth began to moderate. From then on, hydrocracking growth in the United States
proceeded at a slow pace. However, at the same time, hydrocracking enjoyed significant
growth in Europe, the Asia-Pacific region and the Middle East.
As of 2001, there were about 155 hydrocracker units operating worldwide and processing
about 4,000,000 barrels (550,000 metric tons) per day of feedstock. As of 2009, The
feedstock processing capacity of the hydrocrackers in the United States was 1,740, 000
barrels (238,000 metric tons) per day.
Flow schemes
Various licensors have slightly different names for their hydrocracker flow schemes, but
in general, they can be grouped into major two categories: single stage and two stage.

13. Single stage once-through hydrocracking
Figure shows a schematic of a single stage, once-through hydrocracking unit, which is the
simplest configuration for a hydrocracker. The feed mixes with hydrogen and goes to the
reactor.
Fig. 2.1: Single stage once through hydrocracking
The effluent goes to fractionation, with the unconverted material being taken out of the
unit as unconverted material. This type of unit is the lowest cost hydrocracking unit, can
process heavy, high boiling feed stocks and produces high value unconverted material
which becomes feed stock for FCC units, ethylene plants or lube oil units. In general, the
conversion of the feed stock to products is 60–70 vol%, but can range as high as 90 vol%.
Single stage with recycle hydrocracking
The most widely found hydrocracking unit is the single stage with recycle in which the
unconverted feed is sent back to the reactor section for further conversion. Figure depicts
this type of unit. It is the most cost-effective design for 100% (or near 100%) conversion
and is especially used to maximize diesel product. The effluent from the reactors goes
through a series of separators where hydrogen is recovered and, together with makeup
hydrogen, is recycled to the reactors. The liquid product is sent to fractionation where the
final products are separated from unconverted oil. In once-through units, the unconverted

14. oil is sent out of the unit. In this units designed to operate with recycle, the unconverted
oil combines with the fresh feed, as shown in Figure.
Fig. 2.2: Single stage with recycle hydrocracking
The reactor operates at temperatures varying from 570O
F to 800O
F (300– 425O
C) and
hydrogen pressures between 1,250 and 2,500 psig (85–170 bar). Under these conditions,
in addition to heteroatom elimination, significant hydrogenation occurs, and some
cracking also takes place.
Two stage recycle hydrocracking
The two stage hydrocracking process configuration is also widely used, especially for
large throughput units. In two stage units, the hydrotreating and some cracking takes place
in the first stage.
Fig. 2.3: Two stage Recycle hydrocracking

15. The effluent from the first stage is separated and fractionated, with the unconverted oil
going to the second stage. The unconverted oil from the second stage reaction section goes
back to the common fractionator. A simplified schematic of a two stage hydrocracker is
shown in Figure. The catalysts in the first stage are the same types as those used in the
single stage configuration. The catalyst in the second stage is operating in near absence of
ammonia, and depending on the particular design, in the absence or presence of hydrogen
sulfide. The near absence of NH3 and H2S allows the use of either noble metal or base
metal sulfide hydrocracking catalysts.
Isocracking
Isocracking involves the same process as hydrocracking but at a lower temperature and
pressure. This specific form of hydrocracking was developed in the late 1950s by Chevron
in order to convert crude oil to high octane gas. This process is the most widely used form
of hydrocracking in industry because it produces a higher yield of less contaminated oil
and fuels. Also, the products of isocracking contain low amounts of aromatics, which are
difficult to burn and can be carcinogenic. The products produced also have very low
amounts of sulfur and nitrogen after the isocracking process. An application of the
isocracking process in industries is converting heavy fuels that are typically only used in
ships and power plants into a usable, lighter, and less contaminated fuel.
Chevron’s Lummus Global’s hydrocracking process was named Isocracking because of
the unusually high ratio of isoparaffins to normal paraffins in its light products. A high
percentage of isoparaffins increases light naphtha product octane numbers and produces
outstanding middle-distillate cold flow properties—kerosene/jet fuel freeze point and
diesel pour point.
Isocracking Configurations:
Several popular configurations are used in the Isocracking process:
A Two-stage Isocracking unit (see Fig.) is used when maximizing transportation fuel
yield is the primary goal. In this case the unconverted first-stage product is recycle

16. hydrocracked in a second stage. This configuration can be designed for maximum yield of
middle distillates or naphtha, depending on product values. The ratio of kerosene/jet to
diesel or middle distillate to naphtha can be varied over a wide range by either changing
product fractionator operation or using alternative second-stage catalysts.
Fig. 2.4: Two stage with recycle Isocracking Configurations.
Product Quality from Isocracking
Isocracking removes heavy aromatic compounds and creates isoparaffins to produce
middle distillates with outstanding burning and cold flow properties.
1) Kerosene with low freeze points and high smoke points
2) Diesel fuels with low pour points and high cetane numbers
3) Heavy naphthas with a high content of single-ring hydrocarbons
4) Light naphthas with a high isoparaffin content
5) Heavy products that are hydrogen-rich for feeding FCC units, ethylene plants, or
lube oil dewaxing and finishing facilities.
Isocracking Catalysts
Hydrocracking catalysts for upgrading raw (nonhydrotreated) feedstocks contain a
mixture of hydrous oxides for cracking and heavy metal sulfides for hydrogenation.

17. The simplest method for making hydrocracking catalysts is impregnation of the heavy
metals into the pores of the hydrous oxide which has already been formed into the final
catalyst shape. The support material can contain a number of components—silica,
alumina, magnesia, titania, etc. These are all oxides which can exist in a very high surface
area form. The ratio of silica to alumina affects the acidity of the final catalyst and,
therefore, it’s cracking activity. High-silica catalysts have high acidity and high cracking
activity; high-alumina catalysts have low acidity and low cracking activity.
Hydrocracker licensors and catalyst manufacturers
Licensors
Hydrocracking licensing started in 1960. Chevron, UOP, Unocal, Shell and Exxon were
active from the beginning. Since that time, some 250 hydrocrackers have been licensed
worldwide. As of the beginning of 2001, 154 hydrocrackers were in operation.
Through the years, the licensing ‘landscape’ has changed. Currently, the active licensors
are Chevron, EMAK (ExxonMobil-Akzo Nobel-Kellogg), IFP and UOP.
Catalyst suppliers
Catalysts used in hydrocrackers are pre-treating catalysts and cracking catalysts.
Following is a list of the current major suppliers of pre-treating catalysts:
Advanced Refining Technology (in conjunction with Chevron), Akzo Nobel, Criterion,
Haldor Topsoe, Axens/Procatalyse (in connection with IFP) and, UOP. The major
cracking catalyst suppliers are: Akzo Nobel, Chevron, Criterion and
Zeolyst,Axens/Procatalyse (in connection with IFP) [1]