Fundamentals of Refinery Processing

The basic products from fractional distillation are:

Liquid petroleum gas (LPG) has carbon numbers of 1-5 and a boiling point up to 20 °C. Most of the LPGs are propane and butane, with carbon number 3 and 4 and boiling points -42 °C and -1 °C, respectively. Typical usage is domestic and camping gas, LPG vehicles and petrochemical feedstock.

Naphtha, or full range naphtha, is the fraction with boiling points between 30 °C and 200 °C and molecules generally having carbon numbers 5 to 12. The fraction is typically 15–30% of crude oil by weight. It is used mainly as a feedstock for other processes:

• In the refinery for producing additives for high octane gasoline

• A diluent for transporting very heavy crude

• Feedstock to the petrochemical olefins chain

• Feedstock for many other chemicals

• As a solvent in cleaning

Gasoline has carbon numbers mainly between 4 and 12 and boiling points up to 120 °C. Its main use is as fuel for internal combustion engines. Early on, this fraction could be sold directly as gasoline for cars, but today’s engines require more precisely formulated fuel, so less than 20% of gasoline at the pump is the raw gasoline fraction. Additional sources are needed to meet the demand, and additives are required to control such parameters as octane rating and volatility. Also, other sources such as bioethanol may be added, up to about 5%.

Kerosene has main carbon numbers 10 to 16 (range 6 to 16) boiling between 150 °C and 275 °C. Its main use is as aviation fuel, where the best known blend is Jet A-1. Kerosene is also used for lighting (paraffin lamps) and heating.

Diesel oil, or petrodiesel, is used for diesel engines in cars, trucks, ships, trains and utility machinery. It has a carbon number range of 8 to 21 (mainly 16-20) and is the fraction that boils between 200 °C and 350 °C.

White and black oils: The above products are often called white oils, and the fractions are generally available from the atmospheric distillation column. The remaining fraction below are the black oils, which must be further separated by vacuum distillation due to the temperature restriction of heating raw crude to no more than 370-380 °C. This allows the lighter fractions to boil off at a lower temperatures than with atmospheric distillation, avoiding overheating.

Lubricating oils, or mineral base lubricating oil (as opposed to synthetic lubricants), form the basis for lubricating waxes and polishes. These typically contain 90% raw material with carbon numbers from 20 to 50 and a fraction boiling at 300-600 °C. 10% additives are used to control lubricant properties, such as viscosity.

Fuel oils is a common term encompassing a wide range of fuels that also includes forms of kerosene and diesel, as well as the heavy fuel oil and bunker that is produced at the low end of the column before bitumen and coke residues. Fuel oil is graded on a scale of 1 to 6 where grade 1 and 2 is similar to kerosene and diesel, 3 is rarely used anymore. 4-6 are the heavy

fuels, also called Bunker A, B and C, where B and C are very high viscosity at normal ambient temperatures and requires preheating to about 100 °C and 120 °C respectively, before it flows enough to be used in an engine or burner. Fuel oil grade 4 does not require preheating and is sometimes mixed with off spec products, such as tank residue and interface liquid from multiphase pipelines or with grade 2 fuel oil to achieve low-enough viscosity at ambient temperatures. Fuel oil 6 is the lowest grade, its specification also allows 2% water and 0.5% mineral soil and is consumed almost exclusively by large ships in international waters, where pollutants such as sulfur is less regulated.

Bitumen and other residues like coke and tar has carbon numbers above 70 and boiling points above 525 °C. Low sulfur coke can be used for anodes in the metals industry (aluminum and steel) after processing (calcining). The remainder is a problem fuel, because of high sulfur content and even higher CO2 emissions than coal (typically 15% higher). Bitumen in the form of asphalt boiling above 525 °C is used for roofing and road paving. Asphalt concrete pavement material is commonly composed of 5% asphalt/bitumen and 95% stone, sand, and gravel (aggregates).

Calculation of Fuel Quantity & Density-Volume Correction Factor

To calculate the weight of the fuel, we need to find out the volume and temperature. Having Density and temperature, enter Table 54B to obtain Volume Correction Factor.

Mass = Density x Volume

         = VCF x WCF x Actual Sounded Volume 

Where: 

         Density = Temperature Corrected Density = VCF x WCF 
         Volume = Actual Sounded Volume 
         VCF = 1- {(T-15) * 0.00064} 
        WCF = Density @ 15 deg.C - 0.0011

CCAI related bunker issues

In these days of burning residual fuels in our ships, various types of fuel related problems occur. These can, while being evident, be a considerable 'pain in the neck' for the engine crew and for the operator of the vessel. Hence, the measures to be taken from the owner's side to avoid these hick-ups are to specify as appropriate as possible the grade of fuel required for 'his' engine. And in case of a dispute, to be a subscriber to a recognized fuel analyzing scheme. Sampling procedures for receiving fuel should be accepted by all involved parties.

Let us dwell for a moment on the issue of ordering bunkers and the specification that normally is submitted to ensure the correct grade is received on board. A number of parameters are normally mentioned, such as; density max, viscosity max, sulphur max, poue point etc. There is however a parameter rarely being mentioned in these specifications and that is the CCAI, Calculated Carbon Aromaticity Index, which gives a value on the Ignition Quality for residual fuels, since these grades cannot be verified by methods used for distillates, i.e. Diesel Index, Cetane Index and Cetane Number.

Accepted method for determination of the ignition quality of residual fuels is currently not available. It has, however, been empirically established that there is a relationship between the density, the viscosity and the ignition performance and the Shell-developed CCAI is the one presently most accepted for indicating ignition delay, although there is also a BP- developed Calculated Ignition Index (CII). CCAI gives an idea of how much the ignition is delayed, the higher the index, the longer the delay. The CCAI can be determined, with limited accuracy, by the enclosed nomogram AAAA

The combustion starts with a short delay already when a small amount of the fuel has been injected and therefore the remaining quantity injected burns in a controlled manner. If,however, the delay is long, a large amount is injected before the combustion starts,producing a quick and violent raise of pressure. This produces the characteristic"diesel knock". The problem is generally related to medium speed diesel engines when burning blended fuels less than 220 cSt. and problems seems to appear in the CCAI-span 850-890. See enclosed diagram BBBB If it is required (necessary) to operate the engine within this span the stresses on the engine components might increase considerably and special attention should be paid to:

0  Connecting rod big-end and bearing shells.

0   Main bearing shells

0  Pistons(particularly composite pistons)

0  Piston rings and liners

0  Cylinder head with studs and gaskets

0  Tie bolts

0  Intake and exhaust valves

To alleviate the effect of the ignition delay, the ambition should be to keep the engine load within 50 - 85 % and to maintain the inlet air temperature  as high  as practically possible and through pre-heating prior start-up(the CCAI problems  are accentuated on a cooler engine, hence a known  scenario is the vessel makes it to port but the engine can not be restarted upon departure due to fouled/clogged piston rings, poppet valves and turbocharger).With the violent increase of combustion pressure, when operating on fuels delaying the ignition, the rate of blow-by will increase and it goes without saying that the lub.oil quality must be optimal to cope with the additional load imposed on the bearings.

So, by way of conclusion, if the shipowner is operating engines which  are sensitive to ignition quality he would  be wise to order fuels with a CCAI limit or to set density and viscosity limits which will control the CCAI.

Special care to CCAI is needed when a ship is forced to use low viscosity fuels (below

180 cSt), due to heating limitations. If the density of these fuels is high the CCAI will be too high and ignition problems may be encountered.