Blending of Pseudocomponents
The pseudocomponents for petroleum assay streams which use the same TBP Cutpoint set are combined (blended) to produce a common set of pseudocomponents unless the assay stream has been designated as an exclude stream.
The blending process proceeds in the following fashion. First, each assay stream is characterized with pseudocomponents which correspond to the cutpoint ranges defined by the TBP Cutpoint set. The average normal boiling point, gravity, and molecular weight are determined for each pseudocomponent by integration of the TBP, gravity, and molecular weight curves over the cutpoint range specified in the TBP Cutpoint set.
The weights, volumes, and number of moles are next totaled for every cutpoint range (pseudocomponent) for every assay stream which uses the TBP Cutpoint set. For example, a TBP cutpoint range may be specified as 150 to 200 degrees F, defining a pseudocomponent with an average normal boiling point of approximately 175 degrees F for all streams which use the TBP Cutpoint set (the actual NBP is determined by integration of the TBP between 150 and 200 F ). The weight, volume, and number of moles for this cutpoint range are totaled for every assay stream which uses the Cutpoint set.
The totals for every cutpoint range are used to calculate the average molecular weight and gravity for every cutpoint range (pseudocomponent). Note that the flowrates of the assay streams have a direct impact on the blend components. That is, a stream with a large flowrate versus the other assay streams will have a larger contribution toward the final properties of the blend components.
The normal boiling point for every blend component is next determined by weight averaging the individual stream values. All other thermophysical properties are derived from the normal boiling point, gravity, and molecular weight of the blend component.
The final step is to represent each of the individual assay streams with the blend components. For assay streams which have a liquid volume percent basis for the distillation data, the volume of each pseudocomponent is maintained at the quantity determined from the TBP curve. The number of moles and weight are adjusted as needed. For assay streams which have a weight basis for the distillation data, the weight for each pseudocomponent is maintained at the quantity predicted by the TBP curve and the volume and number of moles are adjusted.
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