Fuel oil products

Above: Typical modern oil refinery 

Oil Refineries and the Production of Fuel OilAn oil refinery may be considered as a factory that converts crude oil into a range of useable products. It is designed to produce what the market requires in the most economical and efficient manner. The first step in the manufacture of petroleum products is the separation of crude oil into the main fractions by atmospheric distillation. When crude oil is heated, the lightest and most volatile hydrocarbons boil off as vapours first and the heaviest and least volatile last. The vapours are then cooled and condensed back into liquids. This distillation process is carried out in a fractionating column. The column is divided into a series of chambers by perforated trays, which condense the vapours at each stage and allow the liquids to flow into storage tanks. Pre-heating of the crude oil is limited to 350°C to prevent the oil being thermally cracked. 

Atmospheric DistillationThe residue from atmospheric distillation is sometimes referred to as long residue and to recover more distillate product, further distillation is carried out at a reduced pressure and high temperature. This vacuum distillation process is important in maximising the upgrading of crude oil. The residue from vacuum distillation, sometimes referred to as short residue, is used as a feedstock for further upgrading or as a fuel component. Unlike the fractionating column for atmospheric distillation, a system of packed beds instead of trays is used for condensation of the low-pressure vapours. Refineries based just on atmospheric and vacuum distillation are said to be operating “the straight run” process and the fuel oil is basically either long or short run residue. The percentage of residue varies depending on the composition of crude processed. For a typical “light” North African crude the residue is 28%, whilst for a “heavy” Venezuelan crude it is as high as 85%. The proportion of products produced does not always match the product demand and is primarily determined by the crude oil. 

Thermal Cracking

In order to meet the product demand, further refining processes were introduced. Today, a modern refinery, in addition to atmospheric and vacuum distillation, may also consist of secondary refining processes such as cracking, which may be thermal or with a catalyst. A typical modern refinery installation is shown below. Thermal cracking is the oldest and in principle the simplest refinery conversion process. It is carried out over a wide range of temperatures, between 450-750°C and pressures from atmospheric to 70 bar. The temperature and pressure depends on the type of feedstock and the product requirement. At these elevated temperatures, the large hydrocarbon molecules become unstable and spontaneously break into smaller molecules. Another important factor in the process is the residence time. The feedstock can be either the residue from the atmospheric or vacuum distillation units, or a mixture of the two. In modern refineries, there are three major applications of the thermal cracking process: visbreaking, a thermal gas oil unit and coking. Visbreaking is the most significant process with regard to the manufacture of residual fuel oil. It is a mild form of thermal cracking often used for reducing the viscosity of straight-run residual fuels. Normally such fuels are very viscous and, if required for sale as heavy fuel oil, must be blended with a relatively high value distillate to meet the finished product specification. Visbreaking reduces the quantity of distillate required as diluent or “cutter stock” and this material can then be diverted to a more profitable product stream. The main aim of a thermal gas oil unit is to produce and recover the maximum amount of gas oil. In extreme cases, the viscosity of the residue may be higher than that of the feed stock. Coking is a relatively severe form of thermal cracking. It is designed to convert straight-run residues into more valuable products such as naphtha and diesel oil. In addition, gas and coke are produced and thus this process does not feature in the manufacture of residual fuel oils. 

Catalytic CrackingCatalytic cracking has become the major process in the petroleum refining industry for the conversion of heavy hydrocarbon fractions, mainly into high-quality gasoline and fuel oil components. These are lighter, less viscous and more valuable than the feedstock. There are various different catalytic cracker designs however, in all cases the product output can finally be separated to: gases, gasoline blending components, catalytically cracked cycle oils and cycle oil slurry. The cycle oils are very important with respect to residual fuel oil since they are used as cutter stocks to reduce the viscosity of residues. Prior to use as a cutter stock, the cycle oil slurry has to be treated to remove particles of catalyst entrained in it. In a modern refinery, there is a wide range of residues and diluent available for the production of fuel oil. Usually the fuel will consist of visbroken residue diluted with cycle oils and smaller amounts of other distillates. The figure shows the main streams of feedstock, fuel oil diluent and fuel oil residues in a modern refinery. Clearly, if a refinery does not have a thermal cracking facility (visbreaker or thermal gas oil unit) then the fuel oil will be based on long or short residue. Additional to the main residual fuel streams in a modern refinery, it should be appreciated that other developments have taken place to further maximise the production of gasoline, kerosene and diesel from a barrel of oil. One of these is by residue hydroconversion where residual fractions are converted into feedstock, which in turn can be further processed in conventional crackers to yield lighter products. Maximisation of production for the lighter products is carried out at the expense of residual fuel oil. Below:

Below: Simplified diagram showing the straight run refinery process.

 Below:The main HFO refinery streams.

 Comment: Oil Refineries can include: Atmospheric Distillation Vacuum Distillation Thermal/Catalytic Cracking. When used as a cutter stock, cycle oil slurry is also a potential source of catfines in residual fuel oil streams. 

Fuel SpecificationsIn 1982, the Working Group responsible for the development of the international standard issued the Draft Proposal of what became ISO 8217 “Petroleum products - Fuels (class F) - Specifications of marine fuels”. Issued at the same time was the Draft Proposal of what became ISO 8216 “Petroleum products - Fuels (class F) - Classification Part 1 - Marine Fuels”. The first edition of the international standards was published in 1987. In 1989, BS MA 100 was reissued and is identical to the International Standard. In 1996, the

second edition of ISO 8217 was issued and this is denominated as ISO 8217 : 1996 (E). 

The ISO 8217:2010 Standard represents a significant advance over the previous 2005 version. The changes are designed to promote the safer use of marine residual and distillate fuels, recognising the fact that the industry is entering into a new realm of multi-blend products driven by ever increasing environmental regulation. Furthermore, users are provided with better protection against the supply of poor quality fuels. Additionally, the supporting Standard, ISO 8216, dealing with the classification of marine fuels, has similarly been updated to reflect the changes to ISO 8217.

It should be noted that in some countries, gas oil and diesel oil are produced for the local land-based market to a national specification. Included in such a specification is usually a minimum flash point and this value may be below that required by international legislation for normal marine use.

Summary of fuel specifications

New parameters for both distillate and residual fuels are the inclusion of acid number limits as well as a limit on hydrogen sulphide. For distillate grades, oxidation stability and a lubricity requirement have been introduced. For residual marine fuels, Calculated Carbon Aromaticity Index as an indictor of ignition delay, and a limit on sodium content have been introduced. As well as this, limits for ash and vanadium have been tightened and there has been a significant reduction in limits for aluminium plus silicon, also known as cat fines.

Below: INTERNATIONAL STANDARD ISO 8217:2010 (F)Petroleum Products - Fuels (Class F) - Specifications of marine fuels. Requirements for marine distillate and residual fuels 

 

 Fuel Oil - Delivery & Sampling

Click image to enlarge Fuel Oil: ViscosityFuel oil, especially residual fuel grades, has historically been ordered by viscosity, although distillate fuels are often ordered by using other criteria.Whilst viscosity is an important characteristic, it does not give an indication of the other characteristics of the fuel. With viscosity as the sole criterion for ordering, engine operators remain unaware of these other characteristics.

Fuel Oil : CharacteristicsSimilarly, the designers of the machinery and fuel oil systems are not always informed of the fuel characteristics to which they should be designing. It is therefore necessary to define the quality of the fuel over a range of parameters to which the engine plant is sensitive. Specifications exist to assist this process.

Fuel Oil DeliveryThis section deals with delivery of large volumes of fuel oil. It relies heavily on marine practice but the methods and lessons have relevance to land-based applications where current procedures are often less strict, to the detriment of the fuel buyer. Associated with the actual physical delivery of the bunkers is the accompanying paperwork. This can be conveniently sub-divided into two parts:

Pre-delivery documentation Bunker Delivery Receipt (BDR)

Pre-delivery Fuel Oil Documentation The purpose of the pre-delivery document is to record agreement on the operational details of the transfer and to ensure safe transfer of the product. This document originates from the seller’s representative and states the grade or grades with quantity which have been nominated. Ideally the grades will be expressed by reference to ISO 8217, which defines viscosity for residual fuel oil grades at 100°C. In reality, a residual grade may be expressed as a maximum viscosity usually quoted at 50°C. Distillate grades, instead of being referenced as DMA, DMB and DMC, may be referred to as marine gas oil, marine diesel or marine diesel blended.

Having established what is to be transferred, agreement has to be reached on the pumping rate acceptable to the receiving vessel to ensure safe transfer. The next aspect which needs to be considered is witnessing of tanks by sounding or ullage and the approach to this is detailed later in this section. Agreement should be reached on the witnessing of a representative sample and this again is detailed later. Another aspect which should be discussed and agreed is a spill prevention transfer procedure. The key to this is

communication and a check list can be useful to ensure no points are missed.

Bunker Delivery Receipt (BDR)The purpose of the Bunker Delivery Receipt (BDR) is to record what has been transferred. Various factors are recorded including:

Location and time of transfer Details of product delivered Temperature of product delivered Product density at standard reference temperature Sample seal numbers

Care should be taken before signing the BDR. For example, the bunkers should not be signed for in weight form, only for volume at observed temperature. The actual weight can only be calculated after a representative sample of the delivery has been tested for density.

MARPOL Annex VI requires you to retain the BDR for at least 3 years from the date of issue. Letter of Protest Should there be any dispute in the quantity of bunkers delivered, the purchaser or his representative should issue a letter of protest, which is properly signed and stamped by both parties. An example of such a letter is shown below. The letter of protest system can be used by both the barge or supplier as well as the ship owner or engine operator. Bunkering Check List Bunkering is often carried out when the engineering staff are under pressure in both time and manpower. Key checks are often missed and only come to light when it is too late. A few relevant points are detailed below: 1. The purchaser should obtain specification acceptance from the supplier2. Purchaser needs to advise ship’s buyer what grade of fuel will be delivered and how transferred3. Fuels from different deliveries should be segregated as far as practical4. All receiving tanks need to be gauged prior to taking fuel5. Don’t sign any documentation unless you have witnessed the actual event6. Always take up witness offers made by the supplier 7. If the suppliers sampling method is unknown, then sign adding the words “for receipt only - source unknown”8. Always take a fuel sample using a continuous drip method9. Take one sample per barge/ delivery 10. Sign the BDR for volume only, if necessary adding the words “for volume only - weight to be determined after density tests”11. Ensure good records are kept throughout the bunkering12. Keep accurate engine logs in the event of any subsequent problems 13. Keep fuel samples for at least 12 months14. Test all fuel on delivery for Viscosity, Density, Water,Stability, Pour Point and Salt (if water present)

15. Use a laboratory to check results in the event of any discrepancies being indicated by on-site test equipment

Thank you

Contact

TINSEL CARGO & OIL COMPANYCOMMERCE HOUSE3RD FLOOR,  SUITE 311,MOI AVENUE, NAIROBI.P.O. BOX 79456-00200 NAIROBI, KENYATELE FAX:  +254-20-2229781,Cellphone: +254-722-761587,+254-734-939308

Website: www.tinselcargo.comEMAIL:   info@tinselcargo.com