Fuel Oil

Question 1

What is crude oil?

Answer 1

Crude oils are a naturally occurring flammable liquid consisting of a complex mixture of hydrocarbons of various molecular weights as well as other liquid organic compounds. The proportion of hydrocarbons found in crude oil caries greatly amongst different oil fields. In oil field producing light oils, it may be as high as 97% by weight whereas in fields such as the oil sands it may be as low as 50%.
There are 4 different types of hydrocarbons found in crude oil (paraffins, naphthalenes, aromatics, and asphaltics) and the relative percentage varies from oil to oil, with this variation determining the properties of each individual crude oil. The organic compounds found in crude oil contain nitrogen, oxygen, sulfur and trace amounts of metals such as iron, nickel, copper and vanadium, and the exact molecular composition varies widely from oil to oil.
Crude oil varies greatly in appearance, depending on its composition and it is usually black or dark brown (but could be yellowish, reddish, or even green). Can be found in a liquid state in naturally occurring geological reservoirs where it is most often mixed with natural gas and salt water, OR in a semi-solid form where it is mixed with sand and water.

Question 2

How is crude oil classified ?

Answer 2

Generally classified by the geographic location in which it is produced, its API gravity, and its sulfur content.

Question 3

API Weight

Answer 3

Crude oil may be considered light if it has a low density or heavy if it has a high density. Light crude oil is more desirable than heavy oil since it produces a higher yield of gasoline.

Question 4

Sulfur content

Answer 4

Sweet if it contains relatively little sulfur (<1%) or sour if it contains substantial amounts of sulfur (1-2%). Sweet oil commands a higher price than sour oil because it has fewer environmental problems and requires less refining to meet sulfur standards imposed on fuel internationally.

Question 5

Geographic location

Answer 5

Affects transportation costs to the refinery. Barrels from an area in which the crude oil’s molecular characteristics have been determined and the oil has been classified are used as pricing reference throughout the world.

Question 6

Common crude references

Answer 6

1. West Texas Intermediate: a very high-quality sweet, light oil delivered at Cushing, OK for North American oil
2. Brent Blend : comprises 15 oils from fields in the Brent and Ninian systems in the East Shetland Basin of the North Sea. Oil production from Europe, Africa and Middle Eastern oil flowing West tends to be priced off this oil, which forms a benchmark
3. Dubai-Oman: used as a benchmark for Middle East sour crude oil flowing to the Asia-Pacific region
4. Tapis (Malaysia): light Far East oil
5. Minas (Indonesia): heavy Far East oil
6. OPEC Reference basket: weighted average of oil blends from various OPEC countries
7. Midway Sunset Heavy: heavy California oil

Question 7

Crude Oil Distillation products

Answer 7

1. Gasoline
2. Kerosene
3. Liquified Petroleum Gas
4. Distillate fuels
5. Residual fuels
6. Coke and Asphalt
7. Solvents
8. Petrochemicals
9. Lubricants

Question 8

1. Gasoline

Answer 8

The most important refinery product with a blend of hydrocarbons with boiling ranges from ambient temperatures to about 400F

Question 9

2. Kerosene

Answer 9

Refined middle-distillate petroleum product that is used as a jet fuel and in cooking and space heating

Question 10

3. Liquified Petroleum Gas

Answer 10

Consists principally of propane and butane, produced for use as fuel and is an intermediate material in the manufacture of petrochemicals

Question 11

4. Distillate fuels

Answer 11

Diesel fuels and domestic heating oils. Have controlled flash and pour points, are clean burning, leave no deposits in storage tanks, have proper diesel fuel cetane rating for good starting and combustion

Question 12

5. Residual fuels

Answer 12

Many marine vessels, power plants, commercial buildings and industrial facilities use this or a combination of this and distillate fuels for heating and processing

Question 13

6. Coke and Asphalt

Answer 13

Coke is almost pure carbon with a variety of uses from electrodes to charcoal briquettes. Asphalt is commonly used for roads and roofing materials

Question 14

7. Solvents

Answer 14

A variety of products whose boiling points and hydrocarbon composition are closely controlled.
Includes benzene, toluene, and xylene

Question 15

8. Petrochemicals

Answer 15

Many products derived from crude oil refining such as ethylene, propylene, butylene, and isobutylene are primarily intended for use as petrochemical feedstocks in the production of plastics, synthetic fibers, synthetic rubbers, and other products

Question 16

9. Lubricants

Answer 16

Special refining processes produce lubricating oil base stocks. Additives such as demulsifiers, antioxidants, and viscosity improvers are blended into the base stocks to provide the characteristics required for motor oils, industrial greases, lubricants and cutting oils.

Question 17

Refining crude oil

Answer 17

1. Atmospheric Distillation Tower
2. Vacuum distillation tower
3. Catalytic Cracking
   The first step in the refining process is the separation of the crude oil into various fractions by distillation in atmospheric and vacuum towers. The main fractions or “cuts” obtained have specific boiling-point ranges and can be classified (in decreasing volatility) into gasses, light distillates, middle distillates, gas oil, and residuals

Question 18

1. Atmospheric Distillation Tower

Answer 18

At the refinery, the crude feedstock is preheated and pumped to a heater before being fed into the vertical distillation column (fractioning tower) at a location just above the bottom. The tower is a steel cylinder about 120 ft high which contains horizontal steel trays for separating and collecting liquids. The tower is maintained at a pressure slightly above atmospheric and at temperatures ranging from 650F to 700F, as heating crude oil above these temperatures may cause undesirable thermal cracking. All but the heaviest fractions in the crude flash to vapor and as the hot vapor rises in the tower, its temperature is reduced.
Fitted at different levels in the tower are trays. At each tray, vapors from below enter perforations and bubble caps. The bubble caps permit the vapors to bubble through the liquid on the tray, causing some condensation at the temperature of that tray. An overflow pipe drains the condenses liquids from each tray back to the tray below, where the higher temperatures causes re-evaporation. Side pipes on each tray remove the desired fractions.
Heavy fuel or asphalt residue is taken from the bottom. At successively higher points on the tower, the various major products including lubricating oil, heating oil, kerosene, gasoline and uncondensed gases are drawn off.

Question 19

2. Vacuum distillation tower

Answer 19

The atmospheric distillation process does not fully refine the crude, so another process is required to remove more products. The crude could be reheated and run back through the atmospheric distillation tower but this may damage the remaining hydrocarbons in the crude (thermal cracking). However, since the boiling point of a liquid is directly proportional to atmospheric pressure, the residual could be distilled off at a lower temperature if it was done in a distillation tower with a pressure below atmospheric.
The principles and equipment used in vacuum distillation are similar to those used in fractional distillation, except that the columns are larger and the internal designs of the trays are slightly different

Question 20

3. Catalytic Cracking

Answer 20

The leftover crude is further processed by catalytic cracking, a process where complex hydrocarbon molecules are broken apart and rearranged into simpler molecules. This process converts heavy hydrocarbon feedstock into lighter fractions such as kerosene, gasoline, LPG, heating oil and petrochemical feedstock.
Similar process to thermal cracking except that catalysts facilitate the conversion of the heavier molecules into lighter products. Use of a catalyst (a material that assists a chemical reaction but does not take part in it) in the cracking reaction increases the yield of desired products without the need to heavily heat the crude.

Question 21

Fuel Oil Properties

Answer 21

1. Density and Specific Gravity
2. Viscosity
3. Flash point
4. Pour Point
5. Cloud Point
6. Colour
7. Ash Content
8. Water Content
9. Stability and Compatibility
10. Cetane Number

Question 22

1. Density and Specific Gravity

Answer 22

Density is the mass of liquid per unit volume at 15C. Specific gravity is the ratio of the mass of a given volume of the liquid at 60F to the mass of an equal volume of pure water at the same temperature.
Storage of liquids on ship is based on volume so some common comparison of mass to volume is required. This is important to bunker capacities, choice of heating, purifier set-up, and engine fuel injection.

Question 23

2. Viscosity

Answer 23

Kinematic and dynamic.
For fuels to be easily pumped between tanks and used in the engine(s), it must be adequately heated to a desirable viscosity. The transfer pumps in a heavy fuel system are usually designed to operate at a maximum viscosity of 800-1000 cSt and the fuel must be heated to a viscosity of between 10 to 20 cSt for injection into a diesel engine.

Question 24

3. Flash point

Answer 24

The temperature at which vapor is given off which will ignite when an external flame is applied under standardized conditions. This is used to minimize fire risk during normal storage and handling. The minimum flash point for fuel in a machinery space of merchant ships and many land-based installations is 60C

Question 25

4. Pour Point

Answer 25

The lowest temperature at which oil will pour or flow under prescribed conditions when it is chilled. Important in establishing the lowest temperature at which a diesel fuel is still sufficiently fluid to be pumped or transferred. The pour point represents the lowest temperature at which a fuel can be handled without excessive amounts of wax crystals forming out of solution. As a rule of thumb, fuel must be stored at a temperature at least 10C above the pour point. Note: knowing the pour point of distillates (light diesel) is important as it can have an effect on lifeboats, FRCs and emergency generator fuel tanks that may be exposed to the cold.

Question 26

5. Cloud Point

Answer 26

The temperature at which paraffin wax (or other compounds present in the oil) begins to crystallize or separate from solution. Useful for estimating the temperature at which filters in fuel systems (transfer or main engine) might become clogged because of wax separation. If a fuel is below the pour point, wax will begin to separate out and eventually the fuel filters may become blocked. Wax will also build up on tank bottoms and on heating coils, and when heat is reapplied, it can be difficult to get the wax to re-dissolve (because of its insulating nature).

Question 27

6. Colour

Answer 27

When refining crude oil, colour is used to help determine if the refining process is consistent, and to ensure that the finished product is within specifications. If the refining process is off, the colour of the product will change rapidly and the process can be quickly adjusted to ensure minimal impact.

Question 28

7. Ash Content

Answer 28

All crude oil contains ash and this material is considered to be undesirable and a contaminant. Ash is an incombustible inorganic compound that will not burn, so it ends up as a residue in combustion spaces and on exhaust valves, turbochargers, etc. Some ash forming materials naturally occur in crude oil and some may be introduced in the refining process, or picked up as a contaminant when the crude was being transported and stored. Residual fuels contain more ash than distillate fuels with residuals having a maximum content of 0.0150% by mass and distillates having a maximum content 0.010% by mass. During onboard treatment, ash levels may be reduced by the use of settling tanks and purifiers, which will remove the catalyst fines, sodium and iron.

Question 29

8. Water Content

Answer 29

Water can be introduced into the fuel from a number of sources including tank condensation, tank leakage or deliberate contamination. Generally, fuels contain less than 0.2% water. Excessive water in the fuel represents a triple loss: 1. Loss of specific energy in the fuel which will affect fuel consumption 2. Cost of disposal of water removed by the treatment system 3. Water will damage fuel injection equipment, cause corrosion and lead to failure of exhaust valves and turbochargers. The settling tank will remove water as long as its not emulsified, and a properly functioning centrifuge will as well, however the effectiveness will be less for very viscous fuels

Question 30

9. Stability and Compatibility

Answer 30

Stability is a measurement of the fuel to remain in an unchanged condition despite the circumstances that may tend to cause change. It is the resistance of a fuel oil to breakdown. Once the fuel has chemically broken down, there is no way of satisfactorily reversing the process. Stability can sometimes be improved by the use of fuel additives. While every fuel is manufactured to be stable when standing alone, it does not necessarily follow that two stable fuels are compatible when blended or mixed together. Incompatibility is the tendency of a fuel to produce a deposit when diluted or blended with other fuel oils. Problems of incompatibility between fuels are rare but when they happen, the results are severe. Typically, a sludge will form, resulting in pipe blockage, clogged filters, and inoperable purifiers. In extreme circumstances, the only remedy is manual removal of the sludge build up. It is impossible to be absolutely precise on the probability of compatibility problems between two fuels. A blend is regarded as being stable if it is homogeneous immediately after preparation, remains so in normal storage and at no time produces or tends to produce sludge on a significant scale. Under these circumstances, the fuels forming the blend can be considered as compatible with each other. Note: you cannot possibly know what constituents were used to blend each load of fuel so you should avoid the mixing of fuels from different deliveries.

Question 31

10. Cetane Number

Answer 31

AKA cetane rating, is an indicator of the combustion speed of diesel fuel and the compression needed for ignition. An important factor in determining the quality of diesel fuel since the higher the number, the more easily the fuel will ignite in the engine

Question 32

Contaminants found in Fuel Oil

Answer 32

1. Catalyst Fines 2. Sulphur 3. Vanadium and Sodium

Question 33

1. Catalyst Fines

Answer 33

Particles of spent aluminum and silicon catalyst that arise from the catalytic cracking process in the refinery. The fines, depending on the catalyst used, vary in both size and hardness and if not reduced/removed by suitable treatment, their abrasive nature will damage the engine (particularly fuel pumps, injectors, piston rings, and liners). Can be reduced to an acceptable lever by use of a settling tank and fuel oil purifier

Question 34

2. Sulphur

Answer 34

Typically between 2% and 4%. The level of sulphur has a marginal effect on the specific energy of the fuel. During the combustion process in a diesel engine, the presence of sulphur in the fuel can give rise to corrosive wear. This can be minimized by using suitable operating conditions and an alkaline lubricant for the cylinder liner. Fuel sulphur has a beneficial effect in preventing scuffing of fuel injection components. Very low sulphur distillate fuels are used in environmentally-sensitive areas. A lubricant additive must be used to prevent damage to fuel injection components when operating on these very low sulphur fuels.

Question 35

3. Vanadium and Sodium

Answer 35

Vanadium is a metal present in all crude oils in an oil-soluble form and no economical process exists for removing vanadium from either crude oil or residual fuel. The levels found in fuels depend mainly on the crude oil source, with those from Venezuela and Mexico having the highest levers. The actual level is also related to the concentrating effect of the refinery processes used in the production of the residual. Most residual fuels have vanadium levels of less than 150 mg/kg. Some fuels however have a vanadium lever greater than 400 mg/kg. Sodium content is typically below 50 mg/kg. If not removed in the fuel treatment process, a high level of sodium will give rise to post-combustion deposits in the turbocharger. Although potentially harmful, these can normally be removed by water washing.