CRUDE ASSAY AND PRODUCT YIELDS

 

◆ CRUDE ASSAY AND PRODUCT YIELDS ◆

1. CRUDE ASSAY AND TBP CURVE

In order to obtain a general data of producing various refining products from a crude oil, an assay analysis will provide general data on the oils. On the preliminary planning stage of a refinery construction, ASTM Distillation test D-86 and D-1160 are used as a Crude Assay, since it is cheaper and easier to run than a comprehensive assay analysis using True Boiling Point(TBP) distillation, ASTM Test Method D-2892. Since a crude distillation is based on TBP curve, it is desirable to have an assay data with TBP test, ASTM D-2892. If the apparatus of TBP test is not available, then the method of converting ASTM distillation curve to TBP curve is available in the API Technical Data Book or in the Edrnister's book, "Applied Hydrocarbon Thermodynamics. However, an error of converting ASTM to TBP might be sometimes more than 15 °F. Crude assay must also include the light end analysis in either volume or weight percent. API gravity and viscosity as well as sulfur distribution curves against liquid volume percent should be given on an assay.

2. PSEUDO COMPONENT BREAKDOWN

Crude oil is made up of an almost infinite number of discrete hydrocarbons, ranging from methane to heavy molecules in the range of carbon number over 70. Hence, component by component breakdown of crude is not feasible and not practical. Pseudo component as a petroleum fraction in the given boiling range is, therefore, defined and driven from a TBP curve. In general, intervals of TBP cut points of each pseudo component are 25 °F for the TBP range from 50 °F to 700 °F, 50 °F for the TBP range from 700 to 900 °F, and 100 °F for the TBP range over 900 °F. Characterization of each pseudo component is then determined by the methods and correlations given by API Technical data Book.

3. DISTILLATE PRODUCT BREAKUP AND ITS YIELD

Distillate products are normally specified by either TBP cut points on crude or by ASTM distillation end points. In case of TBP cut points specified, the expected volume % yield of each distillate product is read directly from the TBP distillation curve. In case of ASTM end points given, the end point is to be converted to TBP cut point by the correlation shown on the figure 4. TBP breakup points for products are widely varied and decided by overall refinery requirements. As an illustration, Table 2 shows the expected product yields taken from Figure 3, Arabian Mixed Crude. First column on this table shows the given TBP cut points and the second column indicates the corresponding accumulative liquid volume percent obtained directly from Figure 3. On the third column, the expected product yield is computed by taking interval cut volume percent. The final design product yield might be slightly differ from these expected yields to meet the product specifications such as ASTM gap or ASTM end point. Further detail will be discussed below under the topic of process control variables. The columns fourth through eleventh are estimated normal boiling point, density, mole weight, and molal flow rate for each product, which are determined by using the empirical correlations described in API technical data book. These properties and flow rate will be used in the following part IIl, the empirical design method.

Table 2 Expected Product Yields and Properties

Figure 3  TBP Curve Arabian Light Crude Oil

Figure 4   Relationship Between ASTM and TBP Initial and Final Boiling Points [출처] ◆ CRUDE ASSAY AND PRODUCT YIELDS ◆|작성자 엔지니어