Basic Properties of Petroleum and Chemical Products 1 Density of Hydrocarbon Gases

3. Basic Properties of Petroleum and Chemical Products

The gases from most petroleum liquids are heavier than both air and inert gas and can therefore result in the layering of the gases, which in turn can give rise to hazardous conditions.

The density of the undiluted gas from a high vapour pressure distillate, such as motor gasoline, is likely to be about twice that of air and about 1.5 times that of a typical crude oil. These density differences diminish as the gases are diluted with air. Flammable mixtures usually contain at least 90% by volume of air and consequently have densities almost indistinguishable from that of air.

The table below gives gas densities relative to air for the three pure hydrocarbon gases, Propane, Butane and Pentane, which represent roughly the gas mixtures that are produced respectively by crude oils, motor or aviation gasoline and by natural gasoline. These figures are not significantly changed if inert gas is substituted for air.

Density relative to air Gas Pure hydrocarbon 50% by volume

hydrocarbon / 50% by volume air

Lower flammable limit mixture

Propane 1.55 1.25 1.0

Butane 2.0 1.5 1.0

Pentane 2.5 1.8 1.0

3.2 Vapour Pressure

All crude oils and the usual petroleum products are essentially mixtures of a wide range of hydrocarbon compounds (i.e. chemical compounds of Hydrogen and Carbon). The boiling points of these compounds range from -162⁰C (Methane) to well in excess of +400⁰C and the volatility of any particular mixture of compounds depends primarily on the quantities of the more volatile constituents (i.e. those with a lower boiling point).

The volatility (i.e. the tendency of a crude oil or petroleum product to produce gas) is characterised by the vapour pressure. When a petroleum mixture is transferred to a gas free tank or container it commences to vaporise, that is, it liberates gas into the space above it.

3.2.1 Non-Volatile Petroleum

A non-volatile product is one with a flashpoint of 60⁰C or above, as determined by the closed cup method of testing. These liquids produce, when at any normal ambient temperature, equilibrium gas concentrations below the lower flammable limit. They include distillate fuel oils, heavy gas oils, and diesel oils.

However, if a cargo is being handled at a temperature within 10⁰C of its flashpoint it should be considered volatile as lighter factions may be given off and a flammable gas/air mixture may form above the liquid. Therefore, for example, a cargo with a flashpoint of 80⁰C should be considered volatile if being handled at a temperature of 70⁰C and above.

Since less stringent precautions are appropriate for non-volatile liquids, it is essential that under no circumstances is a liquid capable of giving a flammable gas/air mixture ever inadvertently included in the non-volatile category.

3.2.2 Volatile Petroleum

A volatile product is one with a flashpoint below 60⁰C as determined by the closed cup method of testing. Some petroleum liquids in this category are capable of producing an equilibrium gas/air mixture within the flammable range when in some part of the normal ambient temperature range, while most of the rest give equilibrium gas/air mixtures above the upper flammable limit at all normal ambient temperatures. Examples of the former are Jet Fuels and Kerosenes and of the latter, gasolines and most crude oils.

The choice of 60⁰C as the flashpoint criterion for the division between non-volatile and volatile liquids is to some extent arbitrary. The dividing line must therefore be chosen to make allowance for such factors as the misjudging of the temperature, inaccuracy in the flashpoint measurement and the possibility of minor contamination by more volatile materials. The closed cup flashpoint figure of 60⁰C makes ample allowances for these factors and is also compatible with the definitions adopted internationally by the IMO and by a number of regulatory bodies throughout the world.

3.3 Flammability 3.3.1 General

In the process of burning, hydrocarbon gases react with the Oxygen in the air to produce carbon dioxide and water. The reaction gives enough heat to form a flame, which travels through the mixture of hydrocarbon gas and air. When the gas above a liquid hydrocarbon is ignited, the heat produced is usually enough to evaporate sufficient fresh gas to maintain the flame and the liquid is said to burn. In fact, it is the gas that is burning and is being continuously replenished from the liquid.

3.3.2 Flammability Classification of Petroleum

The basic principle in determining flammability is to consider whether or not a flammable equilibrium gas/air mixture can be formed in the space above the liquid when the liquid is at ambient temperature.

Generally, it is sufficient to group petroleum liquids into two categories entitled non-volatile and volatile, defined in terms of flashpoint as follows:

3.3.3 Flammable Limits

A mixture of hydrocarbon gas and air cannot be ignited and burn unless its composition lies within a range of gas in air concentrations known as the Flammable Range.

The lower limit of this range, known as the Lower Flammable Limit (LFL), is that hydrocarbon concentration below which there is insufficient hydrocarbon gas to support and propagate combustion.

The upper limit of the range, known as the Upper Flammable Limit (UFL), is that hydrocarbon concentration above which there is insufficient air to support and propagate combustion.

The table below gives the flammable limits for these three gases. It also shows the amount of dilution with air needed to bring a mixture of 50% by volume of each of these gases in air down to its LFL. This type of information is very relevant to the ease with which vapours disperse to a non-flammable concentration in the atmosphere.

In practice the lower and upper flammable limits of oil cargoes carried in tankers can, for general purposes, be taken as 1% and 10% by volume respectively.

Flammable limits % volume hydrocarbon in air Gas

Upper Lower

Number of dilutions by air to reduce 50% by volume mixture to LFL

Propane 9.5 2.2 23

Butane 8.5 1.9 26

Pentane 7.8 1.5 33

3.3.4 Effect of Inert Gas on Flammability

When an inert gas, typically flue gas, is added to a hydrocarbon gas/air mixture, the result is to increase the lower flammable limit hydrocarbon concentration and to decrease the upper flammable limit concentration. These effects are illustrated in the figure below, which should be regarded only as a guide to the principles involved.

Flammability Composition Diagram – hydrocarbon gas/air/inert gas mixtures Every point on the diagram represents a hydrocarbon gas/air/inert gas mixture, specified in terms of its hydrocarbon and Oxygen content. Hydrocarbon gas/air mixtures without inert gas lie on the line AB, the slope of which reflects the reduction in Oxygen content as the hydrocarbon contents increases. Points to the left of the line AB represent mixtures with their Oxygen content further reduced by the addition of inert gas.

The lower and upper flammability limit mixtures for hydrocarbon gas in air are represented by the points C and D. As the inert gas content increases, the flammable limit mixtures change as indicated by the lines CE and DE, which finally converge at the point E. Only those mixtures represented by points in the shaded area within the loop CED are capable of burning.

On the diagram below, changes of composition due to the addition of either air or inert gas are represented by movements along straight lines directed either towards the point A (pure air), or towards a point on the Oxygen content axis corresponding to the composition of the added inert gas. Such lines are shown for the gas mixture represented by the point F.

It is evident from the figure that, as inert gas is added to hydrocarbon gas/air mixtures, the flammable range progressively decreases until the Oxygen content reaches a level, generally taken to be about 11% by volume, where no mixture can burn. A figure of 8% by volume of Oxygen allows for a safety margin.

When an inerted mixture, such as that represented by the point F, is diluted by air its composition moves along the line FA and therefore enters the shaded area of flammable mixtures. This means that all inerted mixtures in the region above the line GA go through a flammable condition as they are mixed with air, for example, during a gas freeing operation.

Those below the line GA, such as that represented by point H, do not become flammable on dilution. It should be noted that it is possible to move from a mixture such as F to one such as H by dilution with additional inert gas (i.e. purging to remove hydrocarbon gas).

3.4 Flashpoint

The flashpoint is the lowest liquid temperature at which a flame being repeatedly and momentarily applied to the surface of the liquid initiates a flash of flame across the surface of the liquid which is being gradually heated in a special pot. The flash of flame indicates the presence of a flammable gas/air mixture above the liquid.

For all oils, except some residual fuel oils, this gas/air mixture corresponds closely to the lower flammable limit of the mixture.

3.5 Persistent and Non-Persistent Oil

Generally, persistent oils do not dissipate quickly and therefore pose potential threats to natural resources when released to the environment. Such threats have been evident in the past in terms of impact to wildlife, smothering of habitats and oiling of amenity beaches. Cleanup techniques in response to persistent oils depend on the nature of the oil and the environment in which the oil has been spilled and include for example, the use of booms and skimmers for containment and recovery, the application of dispersants and manual cleanup of foreshores and coastlines.

In contrast, when released to the environment, non-persistent oils will dissipate rapidly through evaporation. In light of this, spills of these oils rarely require a response but when they do, cleanup methods tend to be limited. Impact from non-persistent oils may include, for example, effects on paint coatings in marinas and harbours and at high concentrations, acute toxicity to marine organisms.

The 1992 Civil Liability Convention Fund applies only to ‘persistent hydrocarbon mineral oil’, and a certificate is required to be carried by vessels carrying more than 2,000 tons of persistent oil in bulk as cargo.

The US Code of Federal Regulations distinguishes between persistent and non-persistent oils and oil products. Tankers carrying non-persistent oils are considered ‘clean tankers’ and those carrying persistent oils are considered ‘dirty tankers’.

3.5.1 Persistent Oil

The definition of persistent oil is, in fact, based upon the definition of a non-persistent oil; in other words, anything which is not non-persistent, is persistent. Persistent oil does not meet the distillation criteria for non-persistent oils, and is classified as follows:

• Group II A specific gravity less than 0.85;

• Group III A specific gravity between 0.85 and less than 0.95;

• Group IV A specific gravity of 0.95 up to and including 1.0;

• Group V A specific gravity greater than 1.0.

Examples of persistent oil are:

Crude oil;

Lubricating oil;

Fuel oil;

Heavy diesel oil.

3.5.2 Non-Persistent Oil

Non-persistent oil has the following distillation criteria:

• At least 50% of which by volume, distils at a temperature of 340⁰C (645⁰F); and

• At least 95% of which by volume, distils at a temperature of 370⁰C (700⁰F).

Examples of non-persistent oils are:

LNG and LPG;

Gasoline – Mogas and Avgas;

White Spirit;

Kerosene;

Distillates – Gas oil, Heating oil, Auto diesel;

Gasoline blending components – Naphtha.

In view of the fact that pollution liability and the method of dealing with a spill is significantly different between persistent and non-persistent oil, the Master should know which type of oil the vessel is carrying, and he should if necessary request from the cargo suppliers a written confirmation of the type of cargo. The information should in fact be contained in the Certificate of Quality, although this is not always the case.