Text 21

Basic Physico-chemical properties

of petroleum and petroleum products

1. Density

The density of petroleum and petroleum products can be expressed in either absolute or relative values. The relative density is the ratio of the density of a petroleum product at temperature t2 to the density of distilled water at temperature t1. The density of petroleum products is normally measured at 200 C and that of water, at 40 C. Since the latter is taken as unity, the numerical values of the relative and absolute density coincide.

To find the absolute density  [kg/m3 or g/cm3] the mass of a product is divided by its volume, i. e. =m/V.

The density of petroleum and petroleum products depends on the content and composition of light low-boiling [which have a low density] and heavy high-boiling constituents [fractions]. Indeed, among the components having roughly the same boiling point, paraffin hydrocarbons have the lowest density and benzene hydrocarbons have the highest value, with that of naphthalenes being in the middle. This is why density is one of the principal characteristics of petroleum and petroleum products.

The density of petroleum and petroleum products decreases with the increasing temperature and their volume respectively increases. The temperature relationship for density can be expressed by Mendeleev's formula:

dt4 = d204 - a[t-20]

where dt4 is the relative density of a product at temperature t; d204 is the relative density of a product at 200C; a is a temperature correction factor.

2. Molecular Mass

This is one of the basic physico-chemical characteristics of petroleum and petroleum products. The molecular mass of paraffin hydrocarbons can be found approximately by using the formula:

M = 60 + 0.3t + 0.001t2

where t is the average temperature of boiling of a petroleum fraction, 0C; it is calculated as the arithmetic mean of the temperatures at which equal volumes of the liquid, say, 10% fraction, are distilled off.

The relationship between the molecular mass and relative density of petroleum fractions is determined by the following empirical formula:

M = 44.29d1515/1.03- d1515

Using this formula, it is also possible to fine [with a certain approximation] the molecular mass of all classes of hydrocarbons.

3. Boiling Point. Fractional Composition

The boiling point of a liquid is the temperature at which the pressure of vapours is equal to the external pressure; on reaching this point, vaporization, which up to that moment occurred from the surface only, begins in bulk of the liquid [at the bottom and walls of the vessel being heated], where vapour bubbles are formed; this is what is called the boiling proper. If vapours are not removed off the liquid surface during heating, an equilibrium is established between the liquid and vapour phase. Vapours in equilibrium with the liquid are called saturated. At a higher temperature of heating of a liquid, vaporization occurs more intensively, more vapours are formed above the liquid, and the pressure of saturated vapours is higher.

The boiling point of a liquid depends on the external pressure. For instance, water at a pressure of 0.1 MPa boils at 1000C. At a higher pressure, say 0.4 MPa, boiling begins only at 1440C. Thus , the boiling point is higher at a higher external pressure and at a lower external pressure or in vacuum, water boils at a lower temperature. The same effect of pressure is found in other liquids. This phenomenon is utilized in vacuum distillation of fuel oil.

Petroleum and petroleum products can be separated into individual hydrocarbons only with certain difficulties. Usually separation is carried out by distillation which gives simpler mixtures of hydrocarbons than is the original mixture. These mixtures are called fractions. They boil not at a constant temperature, but in a temperature range between the point of the beginning of boiling and that of its end. Depending on the boiling points and contents of various hydrocarbons, a product may have different boiling ranges, i. e. may have a different fractional composition.

All petroleum products obtained from crude petroleum by distillation are essentially fractions that can boil off within particular temperature ranges. For instance, gasoline fractions boil off within 35-2050C, kerosene fraction within 150-315 0C, diesel-fuel fractions within 180-3500C, light oil distillates within 350-420 0C, heavy oil distillates within 420-490 0C, and oil residues at temperatures above 490 0C.

4. Thermal Properties of Petroleum and Petroleum products

These properties are of high practical importance for calculating the heat balance of all processes associated with heating or cooling.

Specific heat is the quality of heat needed to heat up 1 kg of substance by 10C. The approximate values of specific heat, kJ/kg K, are as follows: petroleum 2.1, petroleum vapours 2.1, water 4.19.

With the specific heat of a petroleum product being known , it is possible to calculate the quantity of heat for heating. For this, the specific heat is multiplied by the mass of the product [kg] and by the difference between the final and initial temperature [0C]. The specific heat of petroleum products increases with increasing temperature and is higher for products of lower density.

Specific latent heat of evaporation is the quantity of heat spent to vaporize 1 kg of a liquid at its boiling point [this characteristic is called latent, since the heat is spent for evaporation and the temperature of the product remains constant during heating]. The average values of the latent heat of evaporation at the atmospheric pressure, kJ/kg, are as follows: water 2257, gasoline 293.3-314.3, kerosene 230-251, diesel fuels 209-213, oils 167-209. Thus, the latent heat of evaporation decreases with increasing density and molecular mass of petroleum products, and also with increasing temperature and pressure.

The heat of condensation is the quantity of heat liberated by vapours during their condensation and is numerically equal to the latent heat of evaporation.

The latent heat of fusion is the quantity of heat absorbed during fusion of 1 kg of a solid at the melting point.

The heat of combustion [calorific value] of fuel is the quantity of heat liberated by the fuel on full combustion. A distinction is made between the high and low heat of combustion: the former [Qh} takes into account the heat of condensation of the water present in the fuel and formed during combustion [it is taken conditionally that the combustion products contain liquid water rather than water vapours]. The low head of combustion, Ql, implies that the water of the fuel and the water formed by combustion is removed as vapours with combustion gases [i.e. it is lower than the high heat of combustion by the quantity of heat spent for evaporation of moisture of the fuel and of the water formed through combustion of hydrogen in the fuel].

5. Viscosity [internal friction]

Viscosity is the ability of a liquid [or gas] to resist the motion of a layer relative to other layers. As regards petroleum products, a distinction is made between dynamic, kinematic and relative viscosity.

Dynamic viscosity  is measured in pascal-second [Pa s]. The dynamic viscosity of selected liquids is as follows:

Pa s

Ether [at 180C]

Gasoline [at 200C]

Kerosene [at 200C]

Alcohol [at 180C]

Water [at 200C]

Glycerine [at 180C]

Spindle oil [at 200C]

Cylinder oil [at 200C]

Caster oil [at 180C]

0.000026

0.0045

0.0017

0.00166

0.001006

1.10

0.042

0.35

1.20

An inverse value of dynamic viscosity is called fluidity.

In process calculations and for testing the quality of many petroleum products, use is made of kinematic viscosity , which is the ratio of the dynamic viscosity  to the relative density of a liquid, d, at the same temperature, i.e.

 = /d

Kinematic viscosity  is measured in square metre [square millimetre] per second[m2/s, mm2/s].

In practical calculations, especially for quality control of petroleum products, use is often made of relative viscosity which is the time of efflux of 200 ml of a petroleum product at the testing temperature related to the time of efflux of the same volume of distilled water at 200C [the time of efflux of 200 ml of water at 200C is what is called the water number of a viscosimeter].

Viscosity- temperature relationships. Viscosity becomes lower with increasing temperature and vice versa. The pattern of variation of viscosity with temperature is an important characteristic of petroleum products, especially of lubricating oils. These variations can be determined by various methods, for instance, by the ratio of the viscosity at 50 0C to that at 1000C, which is now specified for many lubricating oils, or by the viscosity index; the latter is found from monograms for the known values of viscosity at 50 0C and 1000C. With a higher ratio of viscosities, the temperature curve of viscosity is steeper and on the contrary with a lower ratio, the curve is less steep and the quality of the oil is better.

6. The Setting and Fusion points

When being cooled, petroleum and petroleum products gradually loss mobility and can set [solidify] notwithstanding the fact that they contain some substances that might be liquid at the temperature considered.

The setting [solidification] point of a petroleum product is the temperature at which the product loses mobility under strictly specified testing conditions. The loss of mobility and freezing of petroleum and petroleum products depend mainly on the content of hydrocarbons which are solid [at the normal temperature]. The higher the content of such hydrocarbons [in dissolved or crystalline state], the more quickly the product loses its mobility during cooling, i.e. the products has a relatively high setting point. Tarry products and asphaltenes can retard somewhat the crystallization of solid hydrocarbons, that is why the setting point of detarred products is always higher than that of the distillates from which they have been obtained.

During cooling to their setting point. white petroleum products pass through a number of intermediate stages- the stage of turbidity [blushing] and that of the beginning of crystallization. The highest temperature at which crystals [say, of benzene, etc.] can be detected in the cooled fuel by naked eye is called the temperature of the beginning of crystallization, or the chilling temperature [point]. The temperature at which crystals of hydrocarbons [mainly of paraffins] start to precipitate and make the product turbid is called the blushing temperature [point]. Along with the temperature of chilling of liquid petroleum products, the temperature of fusion of some products which are solid at normal temperature [paraffin and ceresin] is also practical importance.

The fusion point is the temperature at which a solid product becomes liquid under strictly specified testing conditions.

With these constants being known it is possible to select properly the method of petroleum processing and take the required measures to ensure pipeline transportation, especially in winter time, and also to choose the methods of storage and transportation of solid products having a high chilling point.

7. Flash and Ignition Points. Self- ignition temperature. Explosibility

The fire hazard of petroleum products is judged upon by their flash, ignition and self- ignition temperatures [point]. At lower values of these characteristics, a product is more fire- hazardous.

The flash point is the temperature at which a mixture of air and vapours of a product being heated under standard conditions ignites on contact with an ignition source, but the product proper is not ignited and the flame is damped. For light petroleum products [with the flash point not above 500C] the flash point is measured in a closed apparatus and that of heavier products [with the flash point above 70 0C] can be determined in an open vessel. The product to be tested is poured into the apparatus and a thermometer is put inside. With light products, the apparatus is covered by a lid with a window which can closed by a gate. During the test, the window is opened periodically and a burner is brought close to it. In an open apparatus, the burner is moved close to the liquid surface. Tests in an open apparatus give a higher value of a flash point, since the vapour formed are partially dissipated to the surroundings.

In further heating, a petroleum product can ignite at a certain temperature. This temperature is called the ignition point.

There is a certain relationship between the fractional composition of a product and its flash and ignition points: lighter hydrocarbons in its composition lower these points. For instance, gasoline has the flash point below - 500C, whereas the flash point of fuel oil is above 1100C.

According to international recommendations, easily igniting liquids include those flash point is below 610C [in a closed vessel] or 660C [in an open vessel]. These liquids, which can be ignited by a short action or even a small ignition source [say, a spark] and without preliminary heating.

The temperature of self- ignition of a petroleum product is lower at a higher content of heavy hydrocarbons. This is the temperature at which a product ignites spontaneously on contact with the air, i.e. in the absence of flame or spark. Some products, such as fuel oils, goudron, soot and coke, self- ignite quite easily at temperature slightly above 300 0C. Self- ignition usually occurs in untight pipelines and apparatus in which petroleum products are at temperature above their ignition point. It is therefore essential to check the equipment for tightness to prevent self- ignition and fires.

Explosibility. In petroleum processing plants, mixtures of vapours of some products with air may be explosive. Such mixtures may form in open air, in closed premises and inside processing equipment. A mixture of vapours of a product with air become explosive when the concentration of the vapours in mixture exceeds a definite limit. At lower concentrations, the mixture is not explosion hazardous, since the greatest portion of the heat evolved in the ignition zone is spent to heat up the air. A mixture can not explode, too, if it contains little air and therefore there is not enough oxygen to sustain combustion.

The lowest concentration of vapours of a petroleum product [or other substance] in the air at which explosion is probable is called the lower explosive limit and the highest concentration of vapours at which explosion is still possible is respectively the upper explosive limit. The concentration range between the two limit in which explosion can take place on contact with open fire [or spark] is called the explosibility range.

The upper and lower explosive limits and the explosibility are different for various vapours and gases. The explosibility ranges for some vapours and gases obtained at petroleum processing plants [percent] are as follows: gasoline 0.8 to 5.1; kerosene 1.4 to 7.4; propane 2.1 to 9.5; methane 5 to 15; ammonia 15 to 28; ethylene 3 to 32; hydrogen sulphide 4.3 to 46; carbon monoxide 12.5 to 74; hydrogen 4 to 74; and acetylene 2.3 to 81.

The highest permissible concentration of vapours of a product in working premises depends on the composition of that product. The selected products it is as follows [mg/m3]: 100 for gasoline fuels, 300 for gasoline solvents, 5 for benzene and methanol, 50 for toluene and xylene, 10 for pure hydrogen sulphide, 3 for mixture of hydrogen sulphide with C1- C5 hydrocarbons, and 5 for phenol.

Exercises

Answer the following question

1. How can they express the density of petroleum and petroleum products?

2. What is the relative density of a petroleum product ?

3. How can you calculate the absolute density?

4. Is there any the relationship between the density of petroleum products and their boiling point? What is it?

5. What are the relationship between the density of petroleum products and their temperature and volume?

6. What is the boiling point of a liquid?

7. What is the relationship between the boiling point and the external pressure ?

8. What are fractions?

9. What is specific heat?

10. How can you calculate the quantity of heat for heating?

11. What is the specific latent heat of evaporation?

12. How can you understand the term "latent"?

13. How can you distinguish the high and low heat of combustion?

14. What is the viscosity of a liquid?

15. How many types of viscosity of liquid do you know?

16. What is the relationship between the viscosity index and the quality of a oil?

17. What is the setting point of a petroleum product?

18. Why is the setting point of detared products higher than that of the distillates from which they have been obtained?

19. What is fusion point?

20. What is flash point?

21. How can you measure the flash point?

22. What is ignition point?

23. What is temperature of self-ignition?

24. When does the explosibility happen?