Thermal dilatation
Density depends on temperature, so meaning that the volume occupied by the oil varies as a function of the temperature of the oil. Although it is necessary to fill the tank container as full as possible, to limit any oxidative processes in the oil, the thermal dilatation of the oils does have to be taken into consideration on loading. The change in volume resulting from the change in temperature follows the equation:
ΔV = Va × γ × Δt
in which
ΔV change in volume
Va volume at initial temperature a
γ coefficient of cubic (thermal) expansion
Δt temperature difference in °C
For general calculations, 0.007/°C is an acceptable approximate value. As a rule of thumb, oils may be expected to increase in volume by 1% of their total volume for each 14°C temperature increase. In particular in the case of oils requiring heating, the ullage space must be calculated accordingly.
Phase changes
Solidification and melting temperatures are of considerable significance in the transport of fatty oils and fats. They depend on which fatty acids predominate in the oil or fat.
Fats of a solid consistency primarily contain saturated long-chain fatty acids, e.g. palmitic acid and stearic acid. Some have high melting temperatures, e.g. beef suet. Semisolid and liquid fats consist primarily of the glycerides of unsaturated fatty acids, e.g. oleic acid, linoleic acid and linolenic acid. The predominance of unsaturated fatty acids means that the characteristic melting temperatures of fatty oils are lower than those of fats. The fat of land animals is generally solid, while that of marine animals is liquid.
Where fatty oils are transported in tank containers, it is essential that they remain in the liquid state during loading, the voyage and pumping; chill haze (separation), which begins if cooling causes the temperature of the oil to approach solidification point, is therefore important.
As cooling progresses, the oils become ointment-like and finally solid. This separation and the associated change in consistency from liquid to solid occurs more readily upon cooling, the higher is the oil's solidification point.
Due to the gradual transition from one state to the other which is usual in oils, solidification range would be a better term to use. This phase change is a reversible process, i.e. the oil becomes liquid and clear again when heated (see below under transport temperatures).
Iodine value
Of considerable significance with regard to tank cleaning is the iodine value. This indicates how many grams of iodine are bound by a 100 g sample and is a measure of the degree of unsaturated hydrocarbons in oils and fats. It is thus a measure of how strong a tendency the oil has to oxidation and thus to drying. Drying of the oils is caused by the presence of polyunsaturated glycerides and results from the fact that, in the presence of atmospheric oxygen, these combine in a process resembling polymerization to form macromolecular compounds, such as, in the case of linseed oil, linoxyn, a viscous, solid mass. The capacity of fatty oils to absorb atmospheric oxygen is a significant factor in the transport of fatty oils, in particular with regard to cleaning the tank container. Thus, on the basis of drying capacity, oils are divided into nondrying, semidrying and drying oils.
- Nondrying oils have iodine values below 100, i.e. contact with atmospheric oxygen does not lead to any appreciable drying of the oils, e.g. peanut oil, olive oil, palm oil. The tank containers are easy to clean.
- Semidrying oils have iodine values of between 100 and 130. These oils dry within acceptable limits, e.g. cottonseed oil, sesame oil.
- Drying oils have iodine values of between 130 and 190, i.e. they dry rapidly on contact with atmospheric oxygen. Considerable cargo residues are left on tank container walls by rapid drying-on of the oils and these have to be scraped off, e.g. linseed oil, tung oil, camelina oil. This causes considerable weight losses.
Acid value
The acid value specifies how many mg of potassium hydroxide (KOH) are required to saturate the free organic fatty acids contained in 1 kg fat. Low acid values (around 1%) are not a problem. In the case of tung oil, a maximum of 4% free fatty acids is acceptable. Higher acid values degrade the quality of the oil (souring). The free fatty acid content may increase as a result of high transport temperatures. High fatty acid contents lead to souring of the oils. The free fatty acids are also oil-soluble and may cause discoloration of the oil. For this reason, for certain oils it is favorable to coat the tank walls, e.g. with plastic coatings.
Rancidity
Oils and fats spoil by becoming rancid, which entails changes to odor and taste which can make edible oils and fats inedible, e.g. cottonseed oil has a tendency to turn rapidly rancid. Rancidity is caused by autoxidative processes, which are promoted by light, atmospheric oxygen and humidity/moisture. Thus, the tanks containers must be filled as full as possible, taking into consideration the coefficient of thermal expansion, so that little ullage space is left above the oil and the cargo can move only slightly in rough seas or as a result of shaking during road or rail transport. Tank containers must be closed immediately. Tung and linseed oil are at particular risk.
Isomerization and polymerization
Decomposition (isomerization) is frequently the cause of catalysis during extended voyages and causes the sweet oil cargo to become lard-like. The risk of isomerization is especially pronounced in the case of tung oil, which is highly sensitive. Polymerization is caused in particular by the influence of heat, if, for example, the oil has been overheated. The consequence is thickening or gelling of the oil, which is also the case with oxidation.
Contamination
Oils are sensitive to mixing/contamination, e.g. kapok seed oil, which is the most sensitive of the vegetable oils. Even slight mixing with other goods can cause it to suffer spoilage. The same applies to olive oil, which is a high quality product.
Castor oil must not be contaminated by water; such contamination would make it smell like dirty, stagnant water. Oil of turpentine is sensitive to water and rust, which may cause discoloration. Tung oil becomes cloudy when mixed with water.
Linseed oil residues, which have dried on the tank walls and have not been thoroughly removed may affect the odor of subsequent, odor-sensitive oils. For this reason, before the cargo is accepted, the tank containers have to be thoroughly cleaned and dried. Old cargo residues must be removed. Fitness for loading should be certified by sworn inspectors or by a specialized cleaning company.
Transport temperatures
Some fatty oils do not need to be heated during the voyage because their solidification temperatures are low, e.g. peanut oil, olive oil, castor oil, rapeseed oil, safflower oil, sunflower oil and teaseed oil. Other fatty oils with high solidification temperatures have specific loading, travel and pumping temperatures. Table 24 lists some examples. To make the heated oil pumpable at its destination, it has to be brought to the necessary pumping temperature by heating the tank. This is only possible, however, if the oils are kept liquid during the voyage (above a minimum temperature). The prescribed loading, travel and pumping temperatures must therefore be precisely complied with, since any change in consistency which occurs during transport may prove irreversible. If the oil solidifies in the tanks, it cannot be liquefied again even by forced heating. In the vicinity of the heating coils, the oil melts, scorches and discolors. Heating does not progress, since the cargo is solid and cannot circulate. Heat conduction is slight, on the other hand, and is insufficient to liquefy the oil.
Problems also arise with these oils during pumping in cold weather, as the oil cools too rapidly in the long lines and solid deposits form on the outer walls, which cannot be pumped out. On the other hand, however, the oil must not become too hot, since excessive heat can cause chemical changes and impair or alter its quality. In general, fats may be heated up to 10% above their melting point without suffering losses in quality. It is important to start increasing the temperature to the necessary pumping temperature in good time. The temperature is increased evenly and initially only slowly, by no more than 3°C per day. The more sensitive and viscous the oil, the more carefully must it be heated. Later, when the oil has become thinner and circulation is accordingly ensured, it may be heated by 5°C per day.
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