oil.txt

oil.txt - - - - Origin of Oil COMBUSTION FUNDAMENTALS: OIL, ITS ORIGIN AND REFINING

The Origin of Oil
Petroleum is was formed as a result of the laying down of organisms such as
plankton and bacteria millions of years ago on the sea floor. The main
differences between coal and oil are that
Coal was formed from land plant debris decaying under mildly reducing
atmospheres. Oil was formed from sea plants and animals under strongly
reducing conditions.
Coal seams remained where deposited. Oil migrates under the effect of
temperature and pressure so that the location of the deposit is not the
location of the initial debris accumulation.

Formation - Over the years a layer of partially decomposed material was formed
(source rock horizon) which was subsequently covered with layers of mud /
sediment (or strata). Subsequent compaction of this matter forms "kerogen", a
complex mixture of hydrocarbons which under special conditions of high pressure
and temperature produces oil and gas. Temperature, pressure and earth movement
cause the oil to migrate to favourable locations. That is porous sedimentary
rock called "Reservoir Rocks".
Because these substances are less dense than their surrounding rocks they tend
to rise until they reach impermeable rock above where they are trapped
preventing escape. This is called the cap rock.

Refinery Operations
Every crude oil has very different characteristics. It may be heavy (with more
heavier hydrocarbons) or light (more smaller H/C’s), it may be aliphatic (many
long chain molecules) or asphaltene (more ring structures).
It certainly contains a complex mixture of many different chemicals which must
be separated and treated. Refinery operations fall into three categories:
Physical Separation - Distillation, Solvent extraction, desulphurisation.
Breaking Down - Cracking, Visbreaking, Coking
Rebuilding Processes - Reforming
The first process is distillation. The different components of the mixture are
seperated in a distillation column by their boiling range. The crude oil is
vapourised and passed into a vertical column. The vapours rise until they reach
a temperature where they condense onto trays and are taken off. See fig 1. Table
1 gives the approximate boiling range of the smaller hydrocarbons



Approx. boiling pt range (C)Number of C atomsDescription
201-4Gas (LPG)
20-605-6Gasoline
40-1806-10Naphtha
180-26010-14Kerosine
260-34014-20Gas Oil / Diesel
340 plus20 plusFuel oils, waxes, bitumen, coke.


The amount of each fraction depends on the crude. Table 2 gives the composition
of various crudes (expressed on a gravimetric basis)
SourceNaphthaKerosine / Gas oilFuel Oil
UK North Sea 304030
Nigeria253738
Middle East203248
North Africa304030
Venezuela11980


Amongst other factors this depends on the degree of permeability of the cap rock
and the proximity of the oil to ground level. For example in Venezuela, the oil
is relatively close to the surface and hence all the lighter fractions have
evaporated off over the years leaving the heavier residue.
Of course the levels above are not necessarily those demanded by the market
place. The most significant user of petroleum are the transport and industrial
sectors which demand lighter fractions. Very heavy fuel oils are only valuable
in power generation where they compete with the cheaper alternative coal.
The problem is solved by cracking the heavier components. Cracking is the
process of reducing the size of the molecules to form lighter ones.

CRACKING
Thermal Cracking - is the oldest, simplest but least effective method where the
molecules are broken down by the action of heat. When heavy petroleum is heated
above its decomposition temperature, the molecules are broken down and
rearranged. This results in an increased yield of gasoline. Gas and petroleum
coke are also formed.

Catalytic Cracking - is the next most sophisticated method. Thermal cracking of
heavy distillates is not very selective and produces substantial quantities of
gas and fuel oil as well as gasoline. The gasoline is also of not very good
quality. The heavy oil is heated to a lower temperature under pressure with a
catalyst. Catalysts are usually natural or artificial clays (e.g. bentonite,
montmorillonite activated by sulphuric acid or aluminium silicates). More
recently zeolites are used due to higher activity. The catalysts are in the form
of pellets, beads or powder depending on the process.
The result is a higher liquid yield (lower coke and gas) but more importantly a
better quality of gasoline due to higher levels of iso-paraffins and aromatic
H/C’s. All new cracking units for gasoline production from heavy oil are of this
type.
The main effect of the catalyst is to direct the cracking of the alkanes towards
the molecule and to convert alkenes into the corresponding alkanes. Naphthalenes
are converted to the corresponding alkanes and alkenes. Aromatics are largely
inert although a small proportion do get converted to coke. This collects on the
catalyst surface and deactivates it. Catalyst regeneration is vital to the
economics of the process.
Crackers are either of the fixed bed (batch) , moving bed (continuous) or
fluidised bed type. Fluidised beds are preferred nowadays due to uniformity of
temperature and higher heat and mass transfer rates. A typical yield for a
modern plant (10,000 Tonnes per day of feedstock) is 50-60% gasoline of octane
number 90.

Visbreaking - is the process used if to crack the remaining very heavy residues
left from the refining process. These substances are very viscous and are not
easily transported. They may be diluted with high value gas oil to reduce
viscosity but this is expensive. However, it is possible to subject this
substance to a mild cracking which breaks down enough of the heavier compounds
to lower boiling point, less viscous ones to greatly reduce the need for gas
oil. Care must be taken to minimise the formation of excessive coke.
Coking - is undertaken in certain refineries (patents CONOCO) to break down the
final residuals to coke. Under severe temperature conditions of thermal
cracking, the liquid feed is converted to gas, naphtha, fuel oil, gas oil and
coke. Conditions can be adjusted to either produce gas oil or coke as the main
product. Petroleum coke produced from petroleum of low sulphur is extremely
valuable for the production of electrodes for aluminium and steel manufacture.
Otherwise it is used to raise steam in the refinery.

REFORMING
Once the basic products are distilled and cracked, they still may not have the
ideal formulation. Reforming may be used to adjust their composition.

Thermal / Catalytic reforming - of gasoline is similar to thermal cracking in
principle and is used to improve of octane number. Temperatures and pressures
are generally higher than for thermal; cracking. Products of the process are
gasoline, residual oil and gases. The amount of gasoline is dependent on
temperature but also on the catalyst. Catalysts not only accelerate the process
but increase the yield of reformate. The most effective is platinum on purified
alumina. Companies have developed their own special catalysts. Can be fixed or
moving bed type.

Hydroforming (Hydrocracking) - is the process whereby the reduction in H to C
ratio of a high boiling point product. As the number of C atoms in the molecule
increases so it hydrogen content falls. Heavy, high boiling point fractions may
be cracked in the presence of high pressure hydrogen with catalyst to result in
saturated, lower boiling point products. Temperatures of 480-540C are used at
pressures of 15 - 20 atm. High duty equipment is necessary for these conditions
but also a hydrogen plant is required.
Platinum may be used if the feed has been pretreated to remove sulphur
(Platforming). Platinic Chloride is the best but must be replaced periodically
for regeneration. Catforming allows occsional regeneration in situ



COMBUSTION FUNDAMENTALS: LIQUID FUELS

Consider the energy density of some typical fuels:
FuelCalorific Value (MJ / Nm3)
Bituminous Coal 41,600
Lignite Coal19,000
Kerosine / Diesel36,700
Gasoline / Petrol31, 320
Heavy / Residual Fuel Oil 40,000
Natural Gas33.8
Producer Gas6.0


Liquid fuels combine the flexibility of transporting a fluid fuel with the
massive energy densities of coal. It is not surprising that the dominant market
for liquid fuels is transportation. As with other fuels, CV, sulphur content,
ultimate analysis are relevant properties. However, there are a number of
properties of liquid fuels which are unique and depend on the fuel utilisation.

MOTOR FUELS
The fuel type depends on the engine: Spark Ignition (Gasoline), Compression
Ignition (Diesel). Both require very different properties.
Spark Ignition Engines
These run with a premixture of fuel and air at nominally stoichiometric ratio.
The principle requirements are a very high volatility to allow the fuel to
vaporise and a high resistance to autoignition.
Volatility - High volatility is achieved by ensuring that the fuel contains a
mixture of the lowest boiling point H/C’s with s final boiling point of say
215C. It is made by blending virgin gasoline, reformate and cracked light
oils. Butane is added up to 10% to facilitate cold start in engines. The
quantity of these gases may be measured by the Reid vapour pressure of the
gasoline. The higher the volatile, the easier the engine will cold start. Most
marketed gasolines are more volatile than is absolutely necessary to improve
cold start and reduce the need of choking. There is an upper limit of
volatility governed by the tendency of the fuel to give rise to vapour lock.
IN some countries, the temperature difference between summer and winter is so
great that two (or more) grades of gasoline are required.
Knock - A high resistance to autoignition is vital. The efficiency of a spark
ignition engine is governed by the compression ratio. This is limited by the
onset of detonation of the fuel before the spark or knock. If the mixture is
ignited before the spark or before the oncoming combustion wave, large and
unstable pressure oscillations result (knock) which damage the engine and
reduce efficiency. The requirement is essentially for the fuel to have a high
SIT. The suitability of a fuel to be used in a gasoline engine is
characterised by its Octane number.
Octane Number - It is vital that the fuel does not auto-ignite before the flame
front reaches it. This is dependent on the spontaneous ignition temperature. SIT
is lowest for large alkanes and higher for aromatics, i.e. aromatics are the
best. An "Octane" number is defined for each fuel as the percentage of
iso-octane in an iso-octane heptane mixture which gives the same knocking
tendency as the fuel. There are two ways of increasing octane number: increase
aromatic content (e.g. benzene) or use additives such as tetra - ethyl - lead
(TEL) which suppress auto-ignition. It is cheaper to produce a high octane
gasoline by the use of TEL instead of increasing the aromatic content. 4* petrol
has an octane number of 98, 2* = 90.
Other factors play a role in the fuel’s performance make up. Sulphur results in
emissions of sulphurous oxides and also plays a role in corrosion, additives
such as detergents help to keep the engine clean and therefore running better
for longer.

Compression Ignition Engines
These engines work with a non premixed, variable air fuel ratio. Importantly,
they do not rely on the fuel being prevapourised as with spark ignition devices
and do not require an external ignition source (a spark) but rely on the
autoignition of the fuel and air mixture as it is brought up to above it SIT.
The main technical requirement is therefore that the fuel should have a low SIT.

Cetane Number - If a high SIT fuel is used then a version of knock is
experienced. As the gases are compressed in the compression cycle there is a
delay before autoignition temperature is reached. If this delay is too great a
large percentage of the fuel charge has already been injected resulting in a
sudden detonation and a rough, bumpy cycle. The situation is far worse for high
speed diesels. The best fuels thus have the lowest SIT temperature - Completely
the opposite of spark ignition engines!
The ideal Diesel fuels are long chain aliphatic (alkane) compounds such as
cetane (C16H34). A cetane number is thus defined based on a cetane /
methyl-napthalene mixture which has the same ignition delay time as the fuel. As
the speed of the engine reduces, the requirement of a high cetane number
reduces. For high speed diesels (rpm>1500) the value should be greater than 45.
For low speed Diesels 25 to 30.
Other factors to be considered with diesel fuels are Viscosity (for good fuel
atomisation) and also pour point and cloud point.
Pour Point and Cloud Point - are relevant for fuel oils in cold surroundings.
The pour point is defined as the temperature 2.8C (5F) above which the oil
ceases to flow when cooled under prescribed conditions. Obviously, this is vital
to winter driving conditions. Cloud point is the temperature at which waxy
crystals are formed in the fuel making it cloudy. This is relevant since these
crystals can block filters long before the pour point is reached.



AVIATION FUELS
If the aircraft uses a piston engine then the relevant properties are similar to
those above. Modern aviation turbines use kerosine which has other relevant
properties. These focus on two areas: safety and integrity of the engine.
Flash Point - This is a very important parameter for safety. For any liquid fuel
to burn it must first vaporise and mix with air (oxygen). Before combustion,
this vapour must be produced by natural evaporation. If evaporation is too low,
then a flammable mixture will not be generated. This evaporation is governed by
temperature. Flash point is the temperature at which the oil evolves just
sufficient vapour to allow a flame and is tested by the formation of a momentary
flame (or flash) when sparked. By the same token, the fire point is the
temperature when at which the oil vapours will continue to burn once ignited.
This is higher than the flash point. Fuels with flash points below 23C (e.g.
gasolines) are considered dangerous and highly flammable.
Smoke Point and Char Value - are used to indicate the burning quality of the
fuel. If the flame contains high soot and smoke levels it is very luminous. This
can result in the combustion chambers being overheated due to excessive
radiation and is potentially dangerous. The smoke point is the maximum flame
height in millimeters to which a kerosine will burn in a standard candle flame
apparatus without emitting smoke. Char value is similar, it is the amount in mg
/ kg of char produced when a kerosene is burned under prescribed conditions in a
standard lamp.

FUEL OILS
Fuel oils fall into a number of categories ranging from light fuel oils (gas
oil) which is very similar to Diesel oil through to heavy fuel oil and residual
fuel oil. They are discriminated on the basis of their boiling range in the
distillation column and are referred to by the ASTM standard number. e.g. No.1
fuel oil, No.2 etc.
Lighter oils are used for heating and power in remote areas where gas and
electricity are not available. Heavier oils are used in steam raising and large
marine engines. In general they are not the preferred fuel in any situation due
to price. It is far more economic to crack them to lighter oils for transport if
at all possible.
The heavier oils are essentially the residue of the distillation process and are
very variable and subject to few specifications. They are characterised by
progressively higher viscosities, sediment and sulphur and ash.
Ash levels - tend to be far lower than for example coal (normally 0.2% up to 1%
for the residual fuel oils). This creates few problems in terms of ash handling
but can cause major problems if certain compounds are present. Petroleum ash
tend to have high levels of Vanadium and sodium. This results in the formation
of ash which is of very low melting point (some as low as 250C up to 680C). This
causes problems since this is in the range of operation of modern superheaters.
A semi-molten ash particle becomes sticky and adheres to the tube. These soon
build up resulting in an insulating layer and therefore increased temperature.
This damages the tubes and reduces their life very significantly. Another
problem is that the sodium / vanadium ashes are corrosive and enhance oxidation
thus reducing life further. Ash deposition can reduce the life of a superheater
from 18 months to 1 or 2 months in severe cases. in some cases can attack
refractories. Additives may be employed to reduce the problems of corrosion and
ash build up. These basically change the nature of the ash produced allowing
more effective cleaning and sootblowing. Examples are dolomite, alumina and
magnesia.
Viscosity - Fuel oils, especially the heavier ones have very high viscosities.
This creates problems of pumping but also of atomisation. Often preheating of
the oil is required to reduce viscosity. This adds significantly to the cost of
the plant. There is an optimum temperature since increasing it has the effect of
improving fuel delivery and atomisation but also of reducing density therefore
resulting in lesser delivery of oil.
Fuel Composition - has a great effect on fuel performance. These fuels have a
very high C/H ratio resulting in sooty, highly luminous flames. This can be an
advantage for heat transfer applications. A major problem with some oils,
particularly cracked oils, is gum and sludge formation. They do not store well
and oxidizable components produce sediment or gum which blocks filters and cause
problems. Perhaps the most significant component of oils to affect their
behaviour is sulphur. Fuel oils may have sulphur contents up to 4.5% depending
on the crude oil. This is far higher than for example coal and is further
aggravated since with coal much sulphur is trapped with the ash. This causes
further problems of corrosion and pollution.

ORIMULSION
A relative newcomer to the fuel market. Orimulsion originates in the Orinoco
basin in Venezuela. This oil field is extremely close to ground level and as a
result of years of evaporation only the very heaviest of compounds remain which
are solid at normal temperatures (asphalt and bitumen). This is very difficult
to extract using normal procedures but methods have been developed using
detergents. Water with a low grade surfactant (detergent) is pumped into the
deposit at very high pressure. The bitumen emulsifies to form a liquid emulsion
which is then pumped to the surface for use. The simplicity of the technique and
the accessibility of the reserves make the product extremely cheap. Its price
has been therefore linked to coal prices (not oil) and it is marketed as "liquid
coal".
Orimulsion has similar properties to heavy fuel oil except that it has a lower
CV (due to the water). It is being burned at Pembroke power station in the UK as
a demonstration unit.



A COMPARISON BETWEEN HEAVY FUEL OIL AND COAL FURNACES
HEAVY FUEL OILCOAL
PROS
- Higher calorific value - smaller furnace - Require 50% less mass of oil
than coal for the same rating.PROS
- Lower sulphur content and some sulphur contained in ash - Less FGD
equipment
- Can be burned with less excess air - smaller fan and furnace. Lower fan
costs. - Lower Vanadium and sodium content - Fewer problems with ash
sticking and corrosion on boiler tube
- Less ash content - smaller ash disposal
equipment- Cheaper
- Lighter fuel oil burners may be designed for much lower loads. Coal
plant is necessarily always big - Because the furnace is operating leaner
due to the excess air requirement there is less chance of local rich
pockets and soot formation.
- Oil is easier to ignite than coal and since start up is easier, furnaces
may be banked at a faster rate to meet demand. - Storage relatively
simple.
- Easier storage
- Oil is easier to atomise and contains less soild carbon. The char
particles left after devolatilisation are much smaller and easier to burn
out. NB: This char burn-out is still the longest step in the reaction.
- Since residence time is reduced, the furnace can be made smaller.



HEAVY FUEL OIL
CONS
- More expensive - Especially true for the lighter oils (not true for
orimulsion)COAL
CONS
- Grinding of coal expensive and complex
- Handling complex due to the requirement to heat the oil before
combustion.- Larger furnace
- Higher Vanadium causes problems with deposition and corrosion.- Less
easy to bank and load change
- Higher SOx emitter. Higher sulphur content than coal plus coal can
retain 10% of sulphur in ash.- Greater ash handling costs
- Problems due to sooting and acid smuts around the heat exchanger.-
Transportation- complex, bulky machinery required.


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