Characterization of crude:

Transcription

1 Crude Oil Properties Characterization of crude: Crude of petroleum is very complex except for the lowboiling components, no attempt is made by the refiner to analyze for the pure components that contained in the crude oil. Relatively simple analytical tests are run on the crude and the results of these are used with empirical correlations to evaluate the crude oils as feedstocks for the particular refinery. Each crude is compared with the other feedstocks available and, based upon the operating cost and product realization, is assigned a value. The useful properties are discussed. API Gravity:The density of petroleum oils is expressed in the United States in terms of API gravity rather than specific gravity; it is related to specific gravity in such a fashion that an increase in API gravity corresponds to a decrease in specific gravity. The units of API gravity are API and can be calculated from specific gravity (sp. Gr.) by the following: API=141.5/Sp.Gr Sp. Gr. of rude oil may vary from less than 10 o API to over 50 o API but most crudes fall in the 20 to 45 o API range. API gravity always refers to the liquid sample at 60 o F (15.6 o C). API gravities are not linear and, therefore, cannot be averaged. For example, a gallon of 20 o API gravity hydrocarbons when mixed with a gallon of 30 o API hydrocarbons will not yield two gallons of 25 o API hydrocarbons, but will give two gallons of hydrocarbons with an API gravity different from 25 0 API. On the other hand Specific gravities of different oils can be averaged. Sulfur Content, wt% [20] Sulfur content and API gravity are two properties which have the greatest influence on the value of crude oil, although nitrogen and metals contents are increasing in importance. The sulfur content is expressed as percent of sulfur by weight and varies from less than 0.1% to greater than 5%. Crudes with greater than 0.5% sulfur generally require more extensive processing than those with lower sulfur content. Pour Point [20] data:text/html;charset=utf 8,%3Ctable%20width%3D%2290%25%22%20border%3D%220%22%20align%3D%22center%22%20cellpadding%3D%220%22%2 1/5

2 The pour point of the crude oil, in o F or o C, is a rough indicator of the relative paraffinicity and aromaticity of the crude. The lower the pour point, the lower the paraffin content and the greater the content of aromatics. Carbon Residue [20] Carbon residue is determined by distillation to a coke residue in the absence of air. The carbon residue is roughly related to the asphalt content of the crude and to the quantity of the lubricating oil fraction that can be recovered. In most cases the lower the carbon residue, the more valuable the crude. This is expressed in terms of the weight percent carbon residue by either the Ramsbottom (RCR) or Conradson (CCR). Salt Content [20] If the salt content of the crude, when expressed as NaCl, is greater than 10 lb/ 1000 bbl, it is generally necessary to desalt the crude before processing. If the salt is not removed, severe corrosion problems may be encountered. If residua are processed catalytically, desalting is desirable at even lower salt contents of the crude. Although it is not possible to have an accurate conversion unit between lb/1000 bbl and ppm by weight because of the different densities of crude oils, 1 lb/1000 bbl is approximately 3 ppm. Characterization Factors [20] There are several correlations between yield and the aromaticity and paraffinicity of crude oils, but the two most widely used are the UOP or Watson characterization factor (KW) and the U.S. Bureau of Mines correlation index (CI). KW= TB 1/3 / G, Where,TB is the mean average boiling point, R and G is the specific gravity at 60 o F. The Watson characterization factor :KW ~15 for highly paraffinic compounds, KW <10 for highly aromatic materials and KW vary from 10.5 to For a highly naphthenic crude it is 10.5 to 12.9 for a paraffinic base crude. The correlation index is useful in evaluating individual fractions from crude oils. The CI scale is based upon straight chain paraffins having a CI value of 0 and benzene having a CI value of 100 and is defined as follows: C.I = 87552/TB G data:text/html;charset=utf 8,%3Ctable%20width%3D%2290%25%22%20border%3D%220%22%20align%3D%22center%22%20cellpadding%3D%220%22%2 2/5

3 The CI values are not quantitative, but the lower the CI value, the greater the concentration of paraffin hydrocarbons in the fraction; and higher the CI value, greater the concentration of naphthenes and aromatics. Nitrogen Content, wt% High nitrogen content is undesirable in crude oils because organic nitrogen compounds cause severe poisoning of catalysts used in processing and cause corrosion problems. Crudes containing nitrogen more than 0.25% by weight require special processing to remove the nitrogen. [19,20] Distillation Range The boiling range of the crude gives an indication of the quantities of the various products present. The most useful type of distillation is known as a true boiling. point (TBP) distillation and generally refers to a distillation performed in equipment that accomplishes a reasonable degree of fractionation. There is no specific test procedure called a TBP distillation, but the U.S. Bureau of Mines Hempel and ASTM D 285 distillations are the tests most commonly used. Neither of these specify either the number of theoretical plates or the reflux ratio used and, as a result, there is a trend toward using the results of a 15:5 distillation (D 2892) rather than the TBP. The 15:5 distillation is carried out using 15 theoretical plates at a reflux ratio of 5: 1. The crude distillation range has to be correlated with ASTM distillations because product specifications are generally based on the simple ASTM distillation tests like D 86 and D 1160 etc. [20] Metals Content, ppm The metals content metals (nickel, vanadium, and copper) of crude oils vary from a few parts per million to more than 1000 ppm. Minute quantities of some of these metals can severely affect the activities of catalysts and result in a lower value product distribution. Vanadium concentrations above 2 ppm in fuel oils can lead to severe corrosion to turbine blades and deterioration of refractory furnace linings and stacks. Distillation concentrates the metallic constituents of crude in the residues, but some of the organometallic compounds are actually volatilized at refinery distillation temperatures and appear in the higherboiling distillates. The metallic content may be reduced by data:text/html;charset=utf 8,%3Ctable%20width%3D%2290%25%22%20border%3D%220%22%20align%3D%22center%22%20cellpadding%3D%220%22%2 3/5

4 solvent extraction with propane or similar solvents as the organometallic compounds are precipitated with the asphaltenes and resins. API GRAVITY(Indicates the crude sp.gr) (American petroleum Institute) API=141.5/Sp.Gr Heavy oils: Petroleum materials less than 200 o API Heavy oils: API gravity range of 10 o to 15 o API Bitumen : API gravity 5 o to 10 o API Atmospheric residua: 10 o to 15 o API vacuum residua : 2 o to 8 o API UOP Characterization Factor/ Waston Characterization Fator [23] Characterization factors: K=(T B ) 1/3 /d (Correlates boiling point with sp.gr.) where T B is the average boiling point in degrees Rankine (= F + 460) and d is the specific gravity 60 o F/60 o F. (Also Kuop= (Tk) 3 /0.827 G, if T in K) K:additive on a weight basis. Highly paraffin oils:k : 12.5 to 13.0 Cyclic (naphthenes) oils have K : 10.5 to Aromatics: Correlation Index (C.I.) [23] According to the chemical composition of the 250 o C to 300 o C fraction has been used. But the difficulty is that in the fractions boiling above 200 o C (390 o F) the molecules can no longer be placed in one group, because most of them are of a typically mixed nature. More direct chemical information is desirable and can be supplied by means of the correlation index (CI). The CI, developed by the U.S. Bureau of Mines, is based on the plot of specific gravity vs the reciprocal of the boiling point in Kelvin. For pure hydrocarbons,the normal paraffin series is given value of CI=0 and for benzene CI = 100. CI= 473.7d ,640/T K data:text/html;charset=utf 8,%3Ctable%20width%3D%2290%25%22%20border%3D%220%22%20align%3D%22center%22%20cellpadding%3D%220%22%2 4/5

5 Where, T K for a petroleum fraction is the average boiling point (K) and d is the specific gravity CI Values between 0 and 15 : indicates a predominance of paraffin hydrocarbons in the fraction. CI Values 15 to 50 : indicates predominance of either naphthenes or of mixtures of paraffins, naphthenes, and aromatics. CI values more than 50 : indicates a predominance ofaromatic species. data:text/html;charset=utf 8,%3Ctable%20width%3D%2290%25%22%20border%3D%220%22%20align%3D%22center%22%20cellpadding%3D%220%22%2 5/5