The learning objectives of this chapter are that the learner will have a basic
overview of the refinery process as the source of marine fuel oil know the main
components that make up marine fuel oils know the structure of basic hydrocarbons
and know the other common elements that occur in the fuel structure
petroleum crude oil or crude oil as it is commonly known forms over millions of
years in underground porous rock structures this is thought to be due to
the decomposition of organic matter such as animal and plant remains under the
influence of temperature and pressure crude oil is a mixture of a large number
of hydrocarbon compounds each having different molecular structures it is
these structures which dictate the physical and chemical properties of the
compound we will be looking at some of the hydrocarbon structures to see how
these various combinations of hydrogen and carbon influence these properties in
its natural state crude oil has very little practical use but it has huge
potential since the individual compounds that it is made up from are the raw
materials for producing fuels plastics and chemicals in the petrochemical
industry in order to separate the different compounds or products as they
are often referred to the crude oil is subject to a series of processes which
together are commonly known as oil refining the processes involved in oil
refining use both physical and chemical means to achieve the required results
the various hydrocarbon compounds contained within crude oil have
different boiling temperatures this fact is used as the basis for separating the
components of the crude oil through a process called fractional distillation
this is really just the first stage used in modern refining and as you will see
later additional more complex processes are now employed in an effort to
maximize product output the boiling temperature is the temperature at which
a liquid changes state into a vapor or gas this is also the same as the
condensing temperature at which gases and vapors return to the liquid state in
fractional distillation the crude oil is first passed through a cleansing process
to remove water sand and other impurities it is then heated to a
temperature of around 350 to 400 degrees Celsius at this temperature many of the
components of the crew oil will boil resulting in a mixture of
liquid vapor and gas the mixture is then passed into a fractionating column or
tower where the gases and vapors rise towards the top the tower which gets
gradually cooler from bottom to top is fitted with collecting trays position
that suitable points the collecting trays are provided with capped
perforations referred to as bubble caps these direct the gas and vapor through
the liquid in the connecting trays as the components of the mixture reached
the point in the tower where the temperature is just below the condensing
temperature they start to condense on the bubble caps and the liquid product
gathers in the collecting tray from where it can be piped away for storage
and further processing any of the crude components which did not boil plus those
components with the highest boiling points which condense as they enter the
tower gather as liquids in the bottom of the tower as a residue of the process
those components with the lowest boiling points continue up to the top of the
tower and have taken off as gases these gases are often referred to as refinery
gases and include methane ethane butane and propane and some of this is used as
fuel to heat the crude oil to continue the process you will probably recognize
some of these gases as they are bottled under pressure and used as a fuel source
for portable heating and cooking appliances in between the two extremes
at the top and bottom of the fractionating tower the other products
condense at the relevant condensing temperatures and are collected the main
products are indicated in the sketch of a fractionating tower and as you can see
include some of the fuels that you will be familiar with atmospheric fractional
distillation is limited to temperatures of 350 to 400 degrees Celsius due to the
risk of the crude oil cracking prematurely at higher temperatures
therefore other processes have been developed to maximize the product yield
from the original crude oil and to modify some of the product to meet
market demands some of these processes use the residue from atmospheric
distillation as the feedstock while others use some of the lower grade
products as the feedstock these processes
may produce further product by either distillation or by altering the chemical
structure of the more complex hydrocarbon components the most common
of these other processes are shown below you can get a brief explanation of these
processes by clicking on the buttons below vacuum distillation is used to
extract further products from the residue of the atmospheric distillation
process it works on the principle that liquids will boil at lower temperatures
when the pressure is reduced to less than atmospheric pressure the residues
are heated again under vacuum conditions and vaporization of some of the
hydrocarbons occurs the vapor is fed to a vacuum distillation unit where it
condenses that is collected in a similar way to the products from the atmospheric
distillation unit typical products are gas oil and lubricating oil feedstock
this process leaves a lower quality residue than the atmospheric
distillation process cracking is the general term used to describe the
process of breaking down the molecular structure of the more complex heavier
hydrocarbons into lighter more useful forms thermal cracking is one such
process that uses heat to break down the molecular structures of part of the
residue and some of the heavier products from the distillation processes coking
is a severe form of thermal cracking which leaves a solid hydrogen depleted
carbon residue or coke miss breaking is a milder form of thermal cracking where
the residue from the distillation processes is heated to approximately 450
to 550 degrees Celsius and then mixed or quenched with gas oil before being
flashed off in a distillation Tower this process produces various distillates
which can be further processed to meet market demands catalytic cracking is
another process which breaks down the molecular structure of the heavier
hydrocarbons this time using chemical means as well as heat this is achieved
by heating the residues from the distillation processes at pressures
slightly above atmospheric in the presence of a catalyst the catalyst is
continually regenerated for further use aluminium and
silicone-based catalysts are commonly used and as we shall see later in the
module are the source of an unwanted contaminant in some marine fuels hydro
cracking is another form of catalytic cracking process but with hydrogen being
mixed to help form hydrocarbons from the carbon rich residues from other
processes the study of organic chemistry covers any compound which contains
carbon and includes those consisting of mainly hydrogen and carbon the so called
hydrocarbons to gain even a basic understanding of marine fuel and its
properties it is necessary to first consider some basic chemistry of
hydrocarbons in general the molecular structure of the hydrocarbons will
define both the chemical and physical properties of the fuel this in turn will
dictate the various quality aspects of the fuel we will look at fuel quality in
a later chapter marine fuels are usually a mixture of a number of compounds from
one or more of three basic hydrocarbon groups these three groups are the
paraffins or alkanes the nav teams or cyclo alkanes and the aromatics or
irene's although these are the three main
hydrocarbon groups compounds from other hydrocarbon groups may also be present
but we will not consider them here the alkane series is a group of hydrocarbons
sometimes referred to as the paraffins alkane molecules consist of hydrogen and
carbon atoms joined together to form chains this group has the general
formula for molecular structure and content C n h2 M plus 2 where C is
carbon H is hydrogen and M is the number of carbon atoms in the molecule
molecules with low n values are gases at atmospheric conditions the simplest
molecule of this group is the gas methane ch4 where N equals 1 the next in
the series with 2 carbon atoms is ethan c2h6
then with 3 carbon atoms propane c3h8 eight and finally with four carbon atoms
butane c4h10 all of these are gases but as n increases the alkanes exist as
liquids and at higher end values solid waxes at atmospheric pressures and
temperatures a molecule is made up of atoms which are bonded together
hydrocarbon molecules are said to be saturated if all of the atoms are joined
with single bonds the alkanes are all saturated since each carbon atom has
only single bonds with other carbon atoms and the hydrogen atoms the alkanes
have a high proportion of hydrogen to the carbon content and therefore have
better ignition and combustion quality than other hydrocarbon molecules of
similar carbon content as the chains become longer the molecules are heavier
and have higher boiling points which mean they will condense at different
levels in the fractionating column the longer chains can be broken down into
smaller molecules during the various cracking processes which you are now
familiar with the Nath teams like the alkanes are also
saturated compounds but as the alternate name cyclo alkane suggests the molecules
have a ring structure not a straight chain structure this group has the
general formula for molecular structure and content as shown where C is carbon H
is hydrogen and M is the number of carbon atoms in the molecule as with
long-chain alkanes as the rings become bigger the nav teens are heavier and
have higher boiling points which mean they were also condense at lower levels
of the fractionating column as you will have hopefully realized the products
from the simple distillation processes are actually mixtures of compounds with
similar boiling points and will include compounds from different hydrocarbon
groups we can define aromatic hydrocarbons for
our purpose as those compounds which have at least one benzene ring in the
molecular structure unlike the other two groups of hydrocarbons that we have
looked at the aromatics are classed as unsaturated in simple terms this means
that some of the atoms have either double or triple bonds an example of
this is shown in the diagram of the simple benzene ring other aromatic
molecular structures can be multiple rings sharing common carbon atoms or
hydrocarbon branches or other elements attached to the molecular structure in
place of a hydrogen atom all of the aromatics have relatively high carbon to
hydrogen ratios with the heavier ones having high boiling points and poor
ignition and combustion quality we will knock at ignition and combustion quality
at a later stage asphaltenes are particularly complex
hydrocarbon molecules very heavy and often containing other elements such as
sulfur vanadium and nickel the asphaltene content of fuel can dictate
some of the performance characteristics and it is therefore important to include
it here asphaltenes in residual fuel oil can be the cause of stability problems
as we will see later they can also badly affect the combustion performance of the
fuel and therefore increase the unburned fuel carried out with the exhaust gases
the refining process produces a wide range of fuels which may be gases
liquids and even solids we are only interested in those liquid fuels which
are used on board the ship that these can be put into three groups marine
gouse oils marine diesel oils and marine residual fuel oils as you will now
appreciate each of these is a mixture of compounds which are products of the
refinery process marine gas oil usually consists of only distillate fuel whereas
marine diesel oil is mainly distillate but may contain a small amount of
heavier or residual fuel the compounds in each have similar boiling points and
can be from any or all of the hydrocarbon groups that we have looked
at marine gas oil and marine diesel oil are referred to as marine distillate
fuels and you will see later that they are grouped together for the purpose of
standards and specification marine gas oil can be considered as a higher
quality grade than marine diesel oil marine distillate fuel or marine diesel
contains approximately 75% saturated hydrocarbons and 25% unsaturated
hydrocarbons the number of carbon atoms in each of the molecules would normally
be between 10 and 20 marine diesel is often a blend of gas oil and heavier
distillate fuels from the refining process there are four different grades
of marine distillate fuels as you will see in the next chapter generally the
only pretreatment needed before use in a diesel engine would be removal of any
water or dirt this may be minimal for high grade diesel
Marine residual fuel as the name suggests is part of the residue from the
oil refining process it consists of a high proportion of long-chain and carbon
rich hydrocarbons in addition to other non hydrocarbon elements as the refining
process has become more intense the quality of the residue has deteriorated
to make the residue usable as a fuel it is often necessary to improve it by
mixing it with some distillate fuel this may be heavy gas oil or similar this
process of improving the quality by mixing with a lighter fuel oil is
referred to as blending or cutting residual fuel oil needs to be heated for
storage and transfer it also needs to be cleaned and pre treated prior to use as
fuel for a diesel engine this can include preheating filtration and
centrifuging the non hydrocarbon elements contained in marine fuel oils
will depend largely on the original source of the crude oil and the actual
refining processes that the crude oil passes through the non hydrocarbons may
take the form of elements which are part of the molecular structure of the
various compounds which make up the crude oil they may be liquid or solid
contaminants either in the original crude or from the refining process they
can also result from transport storage or treatment processes some of the
common non hydrocarbon elements and contaminants that are found in marine
fuels are listed below sulfur vanadium sodium aluminium and
silicon nickel zinc phosphorus calcium water and ash we will consider these non
hydrocarbons and the effect they have on the properties and quality of the fuel
in a later chapter