Module 3: Liquid Fossil Fuel (Petroleum) Lecture 17: Evaluation of crude

Transcription

1 1 P age Module 3: Liquid Fossil Fuel (Petroleum) Lecture 17: Evaluation of crude

2 2 P age Keywords: Evaluation, characterization factor, TBP, ASTM, EFV 3.2 Evaluation of crude The assessment of a crude oil mainly involves the chemical evaluation of crude oil feedstocks by petroleum testing laboratories. Each crude oil type has unique characteristics and no crude oil is identical to the other. The results of crude oil assay testing provide extensive and detailed analytical data for a particular crude oil which are necessary for refinery. In practice it is difficult and very expensive to carry out full laboratory analysis of every type of crude oil at the refinery. This has resulted in development of a number of computing methods that can predict the properties of petroleum fractions by routine laboratory analyses. For application of these methods, information about distillation characteristics, density, sulfur content, viscosity etc. of petroleum fractions are required. According to U.S Bureau of Mines, eight bases of crude oil are designated depending on the distillation characteristics. The characteristics of a crude oil is expressed according to key fraction no. 1, which boils at 482 to F at atmospheric pressure and key fraction no. 2, which boils at 527 to F at 40 mm pressure in a standard Hemple distillation apparatus and condition. Table 1. shows different crude oil bases with their characterization factors and API gravity. The bases of the crude oil are designated as paraffin, paraffin-intermediate, paraffinnaphthene etc, depending on the nature of the low boiling and high boiling fractions of crude. For example, in paraffin-naphthene base, the first word of the base name, such as paraffin denotes the nature of the low boiling fraction and the second word, naphthene indicates the nature of the higher boiling fraction of the crude oil.

3 3 P age Characterisation factor of a crude oil, defined by Universal Oil Product (U.O.P) is expressed as Where, K is the characterization factor, T B is the molal average boiling point in o R and S is the specific gravity at 60 0 F. Bases of crude oil can be indicated primarily by this factor. base crude oils show the range of characterization factor as 12.9 to 12.15, for intermediate base crude oils the range is 12.1 to 11.5 and for naphthene base crude oil it is to These ranges are determined based on the properties of crude oil, such as, viscosity, aniline point, molecular weight, critical temperature, percentage of hydrocarbon etc. Table 1. Different bases of crude oil with their properties Base of crude oil API gravity at 60 0 F Approx. U.O.P characterization factor Low boiling part High boiling part Key fraction 1 Key fraction 2 Low boiling High boiling to to to to to to to to to to to to

4 4 P age A crude oil is termed as sour crude when it contains dissolved hydrogen sulfide in it, 0.05 cu ft of hydrogen sulfide per 100 gallon of crude oil. At this level, the crude oil becomes dangerously toxic. The crude oils which contain disulfides, mercaptans, thiophenes in a sufficient amount, are said to be high sulfur crude. Sometimes high sulfur crude oils are erroneously termed as sour crude. Such as, high sulfur crude oils of California, Venezuela and Mexico do not contain any dissolved hydrogen sulfide in them; hence they should not be termed as sour crude. The vaporization characteristics of petroleum mixtures are determined by a batch distillation curve where percentage distilled is plotted against respective temperature at which it is distilled. Theoretically, a true boiling point (TBP) distillation is that where a very close separation is made so that each component in the mixture is separated at its own boiling point and the quantity present in the original mixture. Fig. 1 represents the TBP curve of a mixture containing two components A and B which are present in the volume percent 30 and 70 respectively and their boiling points are t A and t B respectively (t A < t B ). Fig. 1. True boiling point (TBP) distillation of a two component mixture

5 5 P age The stepwise plot shows the theoretical TBP plot with perfect fractionation while smooth curve represents the actual curve with imperfect fractionation i.e incomplete separation of the two components. Vaporisation characteristics of petroleum fractions are determined by means of a simple distillation with a little fractionation. This distillation is done by following the methods and apparatus designated by ASTM (American Society for Testing and Materials). This type of distillation is called ASTM distillation and the method of this distillation is termed as ASTM D158 in ASTM standard. In this method 100 or 200 ml sample is distilled in a batch mode in specified condition and apparatus. Fig. 2a and 2b show typical ASTM distillation curve for three components compared to the typical TBP curve for the same three components and the same curve for a complex system respectively. (a) (b) Fig. 2. (a) Comparison of TBP and ASTM distillation curves for a three component mixture (b) for a complex system

6 6 P age Equilibrium flash vaporization (EFV) is a type of separation of components of a petroleum mixture where, the mixture under pressure is suddenly vaporized or flashed in a still and the mixture is separated into a vapour and a liquid portion which remain at equilibrium. In fact, this type of flash is observed in the refinery when crude oil containing appreciable amount of light components under pressure is piped to separators at lower pressure and allowed to vaporize suddenly through a pressure reducing valve. If a number of samples of the same composition are flashed at the same pressure but at different temperatures between the bubble point and dew point temperatures of the feed, an EFV curve can be generated. Fig. 3 shows the comparison of the slopes of TBP, ASTM and EFV. It has been shown that 10-70% slope of EFV> 10-70% slope of ASTM> 10-70% slope of TBP. Fig. 3. Comparison of the slopes of TBP, ASTM and EFV distillation curves

7 7 P age The physical properties of petroleum oil vary gradually throughout the range of compounds by which the oil is constituted. Distillation is a means of arranging these compounds in accordance to their boiling points. The properties such as, viscosity, specific gravity, colour etc are found to vary at each drop or fraction of the mixture distilled. The rate at which these properties change can be shown by mid-percent curve. Actually, the viscosity or specific gravity of a petroleum fraction is an average of that property of many drops which constitute the fraction. The concept of mid-percent curve is that, if each drop is equally different from the last drop and it s succeeding one, then the property of the whole fraction is determined by the property of the drop which comes at exactly 50% distilled. This would be the condition when a mid-percent curve is a straight line. Although mid-percent curves are never exactly straight line, but they may be straight for a short range of percent distilled and this short range, the average property of the fraction is exactly the same as that of mid-percent fraction. There are some properties which are not additive, such as, viscosity, API gravity, colour, flash point etc., for which this mid-percent curve is not suitable. But the properties which are additive, such as, specific gravity, aniline point, percent sulfur etc. can be utilized nicely in mid-percent curve to determine the average property of the whole fraction. Although viscosity is not an additive property, but it has been seen that, for a wide fraction of oil, the average viscosity is almost exactly the same as that of the mid-point fraction.

8 8 P age Reference: 1. Petroleum refinery engineering, W. L. Nelson, 4 th edition, McGraw-Hill Book Company, Distillation, M. Van Winkle, McGraw-Hill Book Company, Evaluation of crude oil quality, D. Stratiev, R.Dinkov, K. Petkov, K. Stanulov, Petroleum & Coal, ISSN , 52(1) 35-43, 2010.