Evaluation of Crude Oil

Transcription

1 Evaluation of Crude Oil Evaluation of crude oil is important for refiner because it gives the following types of information: 1. Base and general properties of the crude oil. 2. Presence of impurities such as sulfur, salt, and emulsions which cause general difficulties in processing. 3. Operating o r design data. Primarily this necessitates curves of temperature and gravity vs. per cent distilled. a. Fractionating or true boiling point distillation curve. b. Equilibrium or flash-vaporization curve. c. API or specific gravity curve of each fraction distilled. 4. Curves of the properties of the fractions vs. percent distilled (mid per cent curves) or the average properties of a series of fractions vs. Percentage yield (yield curve) by which common realization of yields can be prepared. Base of crude oil The crude oil can be classified into: 1) Paraffinic base crude (P) 2) Intermediate base crude (IN) 3) Naphthenic base crude (N) Some authors have referred the naphthenic base as asphaltic base. The name naphthenic is more convincing as it represents the homologous series whereas asphalt is a polymeric condensed material having very high carbon to hydrogen ratio of no equable nature of general formula. 1

2 Classification Methods: 1- Mallison Classification according to Composition of Residue ( Residue of crude oil): No. Base of crude oil Paraffin% 1 Paraffinic base (P) paraffin > 5% 2 Intermediate base (IN) 2-5% paraffin 3 Naphthenic base (N) paraffin < 5% 2- The U.S. Bureau of Mines which designated eight different bases of crude oil Notes API No. 1 Kerosene Pressure atm. Boiling point ᵒC Key Fraction API > 40 (Paraffinic Base) 33<API>40 (Intermediate Base) If the pour point is < 3ᵒC, the base is followed by the "Wax Free" term; otherwise it is followed with subscript " Waxy" No. 2 Lube Oil (40-mm)Hg atm ᵒC ᵒC API < 33 (Naphthenic base) API > 30 (Paraffinic Base) API < 22 (Naphthenic base) 22<API<30 (Intermediate Base) 3- Specific Gravity and API Scale Specific gravity and ᵒAPI (American Petroleum Institute) gravity are expressions of the density or weight of a unit volume of material. The specific gravity is the ratio of the weight of a unit volume of oil to the weight of the same volume of water at a standard; both specific gravity and API gravity refer to these constants at 60ᵒF (15.7 ᵒC). 2

3 or ᵒAPI = Corresponding values of API gravity (0 to 100). For blend or mixture of petroleum fractions, the API gravity is equal to the sum of individual API gravity of a component multiplied by corresponding weight fraction in the mixture, (API) mix = (API) a W a + (API) b W b + (API) c W c + Where; W a, W b, W c are the weight fraction in the mixture. 4- Specific Gravity of two cuts. API Fraction 1 API Fraction 1 Crude Base P IN N PIN(mix) INP(mix) 5- Characterization Factor(Watson Factor) (C.F), (K) The most widely used index is characterization factor (Watson, Nelson and Murphy). It was originally defined as: No. K Units of Temperature T b 1 ᵒK 2 ᵒR 3

4 Where; T b : is molal average boiling point ( MABP) SG: is the specific gravity at 60 ₒ F or 15 ₒ C It has since related to viscosity, aniline point, molecular weight, critical temperature, percentage of hydrocarbon etc. If T b in ᵒK units then K-values No. K value Base of Crude Oil Paraffinic Base (P) Intermediate Base (IN) Naphthenic base (N) Naphthenic Intermediate base (NIN) Naphthenic Intermediate base (NIN) If T b in ᵒR units then K-values No. K value Base of Crude Oil 1 If K Paraffinic Base (P) 2 If K< 11.5 Naphthenic Base (N) 3 If K between Intermediate Base (IN) K-value of a mixture is the sum of the components K-value multiplied by their weight fractions in petroleum mixture. K mis = K 1 W 1 + K 2 W 2 + K 3 W 3 + Where; W 1,W 2,W 3 are weight fractions of different components. K 1, K 2, K 3 are respective characterization factors. 4

5 6- Correlation Index(C.I) Like (C.F) related to boiling point and gravity Where; T b is the molal average boiling point ( ₒ K) SG is the specific gravity at 60 ₒ F No. CI value Base of Crude Oil 1 If C I= 0-15 Paraffinic Base (P) 2 If C I= Intermediate Base (IN) 3 If C I>50 Naphthenic base (N) 7- Viscosity Gravity Constant(VGC) No. VGC Base of Crude Oil 1 < 0.82 Paraffinic Base (P) Intermediate Base (IN) 3 > 0.9 Naphthenic base (N) 5

6 The presence of impurities in crude Oil 1- Sulphur Difficulties with oils that contain sulfur compounds arise in only three main ways: corrosion, odor and poor explosion characteristics of gasoline fuels. a) Corrosion: corrosion by finished products presents little difficulty because most products are used at low temperatures. The main bulk of the corrosive sulfur compounds can by removed by treatment with alkalis or the sweetening treatments. In presence of air and moisture the sulphur gases produced during the burning of oil may cause corrosion, as in steel stacks, ducts, and engine exhaust pipes and mufflers. Real difficulties arise when high sulfur oils are heated to temperature 300 ᵒF or higher for copper, or 400 ᵒF for steels. b) Odor: Odor is most obnoxious with low boiling or gaseous sulphur compounds, as H 2 S or SO 2 in flue gases, mercaptans up to even six carbons atoms (B.P. of about 400 ₒ F). Sulfides up to 8 carbons atoms (about 350 ₒ F), and among disulfides only methyl disulfide (B.P. 243 ₒ F). This odor is not obnoxious in sweetened products except in certain extremely high- sulfur gasoline. Percentage of S in crude oil ranges from nearly 0.1 for high API- gravity crude oils as high 5 percentage in a few very heavy crude oils. Generally crude with greater than 0.5% S require more extensive processing than those with lower sulfur content. 2- Salts: Salt carried into the plant in brine associated with crude oils is a major cause of the plugging of exchangers and coking of pipe still tubes. If salt content expressed as NaCl, is greater than 10 lb/1000 bbl, it is 6

7 generally necessary to desalt the crude before processing. 3- Carbon Residue: The less the value of carbon residue the more valuable the crude. 4- Nitrogen Compounds: The nitrogen compounds in petroleum are not of major importance, but they do tend to cause a reduction in the activity of the catalysts used in catalytic cracking and they may assist in the formation of so- called "gum" in distillated or diesel fuel oil. Crude containing in amount above 0.25 % by weight require special processing to remove the nitrogen. 5- Hydrocarbons Gaseous The amount of gaseous hydrocarbons dissolved in crude oil is almost totally a function of the degree of weathering that the oil has undergone or the pressure at which it is collected. The percentage of involved when the dissolved gases are lost cannot be stated with accuracy but it is about 6- Metallic Content (Ni, V, Cu) The metal content in crude oil can vary from a few ppm to more than 1000 ppm, disadvantages affect activities of catalyst, corrosion, deterioration of refractory furnace lining and stacks. Can be reduced by solvent extraction with C3. Analysis of Crude Petroleum 1) Distillation Curves: When a refining company evaluates its own crude oils to determine the most desirable processing sequence to obtain the required products, its own laboratories will provide data concerning the distillation and processing of the oil and its fractions. The first step in 7

8 refinery is distillation in which the crude oil separated into fractions according to its boiling point. There are at least four types of distillation curves or ways of relating vapor temperature and percentage vaporized. Types of Distillation Curve 1- True Boiling point (TBP) Distillation. 2- ASTM Distillation. 3- Semi-fractionating Distillation. 4- Equilibrium Flash Vaporization (EFV). a) True-boiling-point (T.B.P) Distillation: (Fractional, run only on crude oil, batch) Distillation characteristics of a crude are assessed performing a preliminary distillation called "True Boiling Point" analysis (TBP). This test enlightens the refiners with all possible information regarding the percentage quantum of fractions, base of crude oil and the possible difficulties beset during treatment operation etc. This type of distillation as in figure (1-A,1-B,1-C and 1-D) is commonly used due to the accuracy of the results obtained by this method which is very close to that obtained via real distillation or industrial distillation. In this distillation, there is a fractionation column located between the condenser and the flask. In general, this type of distillation is carried out by two steps: firstly, under atmospheric pressure until 300 C (1% distilled very 2 min), secondly under vacuum pressure (to prevent cracking process and to reduce the boiling point) at 40mmHg (1% distilled every3-5 min). In this process, the vapor press. temp. is plotted vs. distilled(%) to get TBP curve. 8

9 Fig.1-A. Illustrate Simplified Shape of True boiling point distillation apparatus Fig.1-B. Illustrate Simplified Shape of True boiling point distillation apparatus 9

10 Fig.1-C. Illustrate Picture of True boiling point distillation apparatus Fig.1-D: illustrate True boiling point Curve of Crude Oil 10

11 b) Equilibrium or Flash Vaporization (EFV) Distillation Is a single stage separation technique. As in figure (2), a liquid mixture feed is pumped through a heater to raise the temperature and enthalpy of the mixture. It then flows through a valve and the pressure is reduced, causing the liquid to partially vaporize. Because the vapor and liquid are in such close contact up until the "flash" occurs, the product liquid and vapor phases approach equilibrium. Or the feed material is heated as it flows continuously through a heating coil. As vapor is formed it kept cohesively with liquid at some temperature and a sudden release of pressure quickly flashes or separates the vapor from the mixture without any rectification. By successive flash evaporation like this the stock can be progressively distilled at different increasing temperatures. A curve of percentage vaporized vs. temperature may be plotted. Fig.3-A. Illustrate Simplified Shape of Flash Vaporization distillation apparatus Fig.3-B. Illustrate Simplified Shape of Flash Vaporization distillation apparatus 11

12 c) ASTM or no fractionating or Engler distillation (no fractional, run on fractions) It is supposed to be like EFV, a non-fractionating distillation system, distinguishing itself as differential distillation. It is a simple distillation carried out with standard ASTM flasks 100,200,500 ml flasks. The data obtained is similar to TBP data. In this type of distillation (in fig.3-a) there is on fractionation column located between the condenser and the flask. On the other hand, the raised vapor will not be fractionated in this process. This distillation is used with fractions having short range of the boiling point.fig.3-b illustrate cut points. Fig.3. Illustrate Simplified Shape of ASTM distillation apparatus 12

13 Fig.3-B: illustrate Cut Temperatures for Crude Oil Fractions d) Humpel Distillation(Semi fractional). In this type of distillation, there will be some fractionating process on the raised vapor via package located between the condenser and the flask. Fig.4. Illustrate Temperature versus volume distilled for TBP, ASTM, and EFV 13

14 Fig.5. Illustrate Crude Distillation Curve 2) Mid Percent Curves: The physical properties of an oil found to vary gradually throughout the range of compounds that constitute the oil. Distillation is a means of arranging of compounds these chemical compounds in order of their boiling points. The properties such as color, specific gravity, and viscosity are found to be different for each drop or fraction of the material distilled. The rate at which these properties change from drop to drop may plot as mid per cent curves. In reality, the specific gravity or viscosity of a fraction is an average of the properties of the many drops that constitute the fraction. If each drop is equally different from the last drop and from the succeeding one, then the drop that distills at exactly half of the fraction has the same property as the average of all the drops. This would be the condition for a mid per cent that is a straight line, but they are substantially straight through any short range of percentage. For a short range of percentage the average property is 14

15 equal to the property at the mid-point of the fraction. The arithmetical average of the properties of these small fractions is the property of the total or large fraction, or even the entire sample. Integral- averaging by adding together the properties of a series of short fractions and dividing by number of fractions can not be used on properties that are not additive. Specific gravity (not API gravity) is an example of an additive property, e. g. 10 volumes of an oil of specific gravity 0.8 when mixed with an equal volume of 0.9 specific gravity oil yields a mixture that has a specific gravity Non additive Properties API Gravity Viscosity Color Flash Point Additive Properties Boiling Point (T.B.P) Vapor Pressure Specific Gravity Aniline Point Sulfur Content 15

16 Fig.6 U.S Bureau of Mines crude petroleum analysis (From Gary and Handwerk, 2001) 16

17 Fig.7 Boiling point at 760 mmhg versus Boiling point at 40 mmhg (From Gary and Handwerk, 2001). Fig.8 TBP and gravity- mid percent curves. Hasting Field, Texas crude: gravity 31.7 o API; sulfur, 0.15 wt%.(from Gary and Handwerk, 2001). 17

18 3) Yield Curves: If a property is not additive, the property of various ranges of fractions can be determined experimentally by blending and plotting the property value as obtained as a function of yield or amount of blended material. Fig.9: Yield Curve for Crude oil Fractions Average Boiling Point There are different types of boiling points and functional values Average boiling point Physical property for which it is distinct Volume Average Boiling Use for liquid viscosity, Specific gravity. Point Weight Average Boiling Critical temperature. Point Molal Average Boiling Characterization factor, thermal expansion of Point liquids. 18

19 Mean Average Boiling Point Cubic Average Boiling Point Molecular weight, Specific gravity, Heat of Combustion, Specific heat. For additive properties; viscosities are additive when expressed on cubic average. 1- Volume Average Boiling Point (VABP) For short cut Boiling Point (having short B.P) 2- Weight Average Boiling Point (WABP) 3- Molal Average Boiling Point (MABP) 19

20 4- Mean Average Boiling Point Mean Avg B.P=(Molal avg. B.P + Cubic avg. B.P)/2 ; Va= Vol. Fraction 5- Cubic Average Boiling Point 6- Slope of Distillation Curve 20

21 Fig. 10 Molal average boiling point of petroleum fractions Fig.11 Mean average boiling point of petroleum fractions 21

22 Fig.12 Relationships between the slopes of various distillation curves Calculation of (K) (Characterization Factor) for the Crude Oil T b or MABP temperatures in Rankin degrees( ᵒR) 1. Calculate the TVABP using 10, 30, 50, 70 and 90 if there are available data or (20, 50, and 80 or 30, 50 and 70) volume % TBP temperature If such data is not available. All temperatures in ᵒF Hint: if there are atmospheric pressure and vacuum pressure., then it must correct the true boiling tempertures (TBP) under vacuum pressure to atmospheric pressure. 2. Calculate (the 10 to 90% slope) of the whole curve: or (the 20 to 80% slope): or ( the 30 to 70% slope) 3. Using a proper correction factor, convert TVABP to TMABP: (TMABP=TVABP- T). To calculate T: Some times given as value such : 120 ᵒF or 90 ᵒF or...etc. Some times given as relation such: Some times given as figures such figure( 10) 22

23 4. Constract a spg mid percent curve and evaluate the spg for the whole crude. 5. K is found as a function of TMABP and spg. Ex.1 Evaluate the base of crude oil, which has T b =450 ᵒF and Spg avg.= 0.85? Ex.2 For 100 barrels (API Gravity 36.3) of Whole crude Oil, the TBP Data TBP, ºF : ;<: Vol. % ;8 1. Evaluate this Crude Oil? 2. Calculate the weight of this Crude Oil. Solution 23

24 24

25 Ex. 3 For crude oil given in table below: 1. Draw an assay curve. 2. Evaluate the crude oil given in table below by K-factor method, ΔT using fig.(10). 3. Evaluate the same crude oil by Correlation Index(C.I) method. Vol. %. TBP (ºF) ºAPI

26 Ex. 4 ( Use of Gravity Mid percent Curve) (Nelson p/106) Compute the spg of a 41.4 API ( spg) mixed base crude oil from the spg mid percent Fraction No Range of % Spg Fraction No Range of % spg Solution: Spg of C.O = ( ) = Computed Sp.gr = , Actual Sp.gr = reasonable check (good for most engineer design work), See fig (7). Fig (7): Gravity mid per cent curve. (From, Nelson, W. L, 1958) 26

27 Finished Products and Cut Points 27