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Chemical Composition of Marine Fuel Oils

August 25, 2019


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

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