Evaluation of Oil Stocks

The routine or control tests mentioned in Chap. 3 are not entirely satisfactory for plant design or plant operation, and hence special distillations, vaporizations, and equipment have been developed by which the yields of the various products contained in a charge stock can be eval- uated. Admittedly many evaluation methods are approximate. They must be so because so many combinations of yields are possible and the market demand varies so rapidly that more precise methods would lead to endless testing. Nevertheless, the somewhat gross methods outlined here must always be used with caution, and they should be augmented at the first opportunity with more complete tests. 

Large refiners usually obtain more complete data than suggested here, by means of pilot- or semiplant-scale equipment in which the essential physical and chemical processes of the operation are duplicated in laboratory equipment. Smaller operators sometimes find that commercial-scale experiments are cheaper. Although pilot plants can usually provide more complete and accurate operating information than commercial operations, pilot plant operation is in some ways more difficult than commercial operation, and thus carelessness may result in misleading data. 

Presence of impurities such as sulfur, salt, and emulsions which cause genera] difficulties in processing. 

Operating or design data. Primarily this necessitates curves of temperature and gravity vs. per cent distOled. 

Curves of the proi erties of the fractions vs. per cent distilled (mid per cent curves) or the average properties of a series of fractions vs. percentage yield (yield curves), by which realizations of yields can be prepared. Among common property curves are 

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True-boiling-point Analysis of Crude Oil. The property curves that have been discussed heretofore are of general usefulness. In the following pages the necessary laboratory procedure, the construction of the curves, and the evaluations of several stocks will be discussed. 

Although distillation and elemental analysis of the fractions provide a good evaluation of the qualities of a crude oil, they are nevertheless insufficient. Indeed, the numerous uses of petroleum demand a detailed molecular analysis. This is true for all distillation fractions, certain crude oils being valued essentially for their light fractions used in motor fuels, others because they make quality lubricating oils and still others because they make excellent base stocks for paving asphalt. 

Standardisation of EPDM characterisation tests (molecular composition, stabiliser and oil content) for QC and specification purposes was reported [64,65]. Infrared spectroscopy (rather than HPLC or photometry) is recommended for the determination of the stabiliser content (hindered phenol type) of EP(D)M [65]. Determination of the oil content of oil-extended EPDM is best carried out by Soxhlet extraction using MEK as a solvent [66], A round robin test was reported that evaluated the various techniques currently used in the investigation of unknown rubber compounds (passenger tyre tread stock formulations) [67]. 

Analytical techniques to evaluate potential lubricating oil stocks are essential to a refiner s planning program. When sulfuric acid was the sole chemical used to refine burning oils and lubricants, it was a simple matter to carry out acid-treating experiments on a small scale in the laboratory, and results so obtained were quite reliable. With the advent of solvent treating, analytical techniques were developed which consisted of single or multiple-batch laboratory extractions in conjunction with correlations based on plant experience (17). 

Furby (12) has developed a method for evaluating stocks in the lubricating oil range that results in a breakdown of components into asphaltenes, resins, wax, and dewaxed oil and provides a yield-viscosity index relationship for the dewaxed oil. The author has found such analyses very useful and inexpensive for evaluating a large number of potential lubricating oil stocks. Furby s method utilizes petroleum ether to precipitate asphaltenes, a fuller s earth-petroleum ether fractionation to isolate resins, methyl ethyl ketone-benzene dewaxing on the deasphalted-deresinified material to separate wax, and an adsorption fractionation to provide cuts from which the yield-viscosity index relationship for dewaxed, solvent-refined oil is obtained. 

Vegetable oils fijom crops, with a total lipid content of at least 20% by weight, such as soy bean, rape seed and sunflower, have been studied and cultivated in plantations as sources of feed stock for biodiesel production in the USA and in European countries such as France, Austria and Italy. For Kenya and Afiica at large, where there is a dire need for food, the use of such edible oil crops for fuel would be ridiculous. Jatropha curcas (family Euphorbiaceae), which constitutes 53-57% non-edible oils, has therefore been evaluated as a source of oil for biodiesel production. 

Peroxide value is used to analyze primary oxidation products. It is important to evaluate the oxidative status of oils by both primary oxidation products as well as secondary oxidation products. If an oil initially has a high peroxide value, keeping the oil in stock for a long time... 

Example 4-8. Evaluation of Paraffin-base Cmde Oil. The following sets of yields of raw stocks can be computed from the property curves of Fig. 4-26 ... 

The reduced crude is then flashed under vacuum, distilling a clear lubricating oil stock and leaving solid tar. The lubricating oil stock can then be evaluated in the true-boiling apparatus. A true-boiling-point analysis of the crude oil is also necessary if the light stocks are to be evaluated. 

Organic sulfur compounds (e.g., mercaptans, sulfides, polysulfides, thiophenes) are present in petroleum products to a greater or lesser extent depending on the crude oil origin and the refinery treatment. The sulfur content of fuel oil (ASTM D-396) can be determined by a variety of methods (ASTM D-129,ASTM D-1552, ASTM D-2622, ASTM D-4294, IP 61, IP 63), with mercaptan sulfur in cracked stocks being particularly necessary for evaluation (ASTM D-3227, IP 342). 

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