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Density or specific gravity, total sulfur, total nitrogen, aniline point, viscosity, acid number, cloud and pour points, and carbon residue would normally be determined on these frac- tions. Cetane index would be determined on the LVGO if the material is to be used as diesel fuel oil blending stock. If the HVGO is to be used as catalytic cracker feedstock, asphal- tenes, nickel and vanadium, and hydrogen and carbon con- tents would also be determined. Lube stock evaluations would include determination of wax content. Basic nitrogen is also typically determined on these fractions. However, in general, the ratio of basic to total nitrogen is on the order of  0.3:1 for many crude oils and virgin stocks [34]. The types of  nitrogen compounds present in various crude oils appear to be essentially the same, although the actual amounts may vary considerably [94]. Consequently, for most assays it is sufficient to determine total nitrogen and to assume that approximately one third is basic in character.

RESIDUUM

Tests of the residuum fractions usually include density or spe- cific gravity, total sulfur, total and basic nitrogen, acid num- ber, viscosity, trace metals, asphaltenes, and carbon residue. Hydrogen/carbon ratio and pour point determinations on the atmospheric (long) and vacuum (short) residua are also important. Refiners are increasingly minimizing production of material that formerly went into heavy fuels such as No. 6 and bunker oil as markets for these products diminish.

Determination of the properties of asphalt, such as pen- etration (ASTM D5: Test Method for Penetration of Bitumi- nous Materials), softening point [ASTM D36: Test Method for Softening Point of Bitumen (Ring-and-Ball Apparatus)], and viscosity would also be included in some assays. How- ever, penetration and softening point tests were developed in an era of less traffic and significantly lower pavement loadings and are no longer suitable for evaluating asphalt to be used as a binder. Newer “Performance Grade” tests such as Dynamic Shear Rheometer, Rolling Thin Film Oven, and Direct Tension Test have been developed [95].

In the preceding discussion, the properties listed for each fraction and for the whole crude oil assay are not exhaustive but are illustrative of those used to evaluate quality. As noted earlier, refiners tailor their analytical scheme to their particu- lar crude oil and product slates, although one refiner is reported to have said, “The best crude oil assay is a 100,000 bbl run through my refinery” [96]. Although this opinion carries some validity, the assay tests presented here provide data that are sufficient for most refiners to evaluate crude oil streams.

With the proliferation of computer LP models and their associated assay libraries [9], many refiners no longer need to perform comprehensive assays as frequently as in the past. Often, an inspection assay is all that is required for them to plan for changes in processing that will be necessitated by varying levels of impurities or small changes in crude oil com- position resulting from changes in the production com- mingled to make up the crude oil stream. Most importantly, an inspection assay can be completed much more rapidly than a comprehensive assay and requires considerably less material. Collectively, these can result in savings in analysts’ time and shipping and handling expenses.

The quality of cargoes arriving at refineries may not conform to what is known or expected for that stream. This can be the result, among other causes, of new production coming on-stream and being commingled with existing pro- duction or field maintenance. The analyst or refiner can use the so-called “Flash Assay Tool” [97] to update an existing comprehensive assay on the stream in question with a mini- mum of data. The tool uses HTSD and various whole crude properties such as American Petroleum Institute (API) grav- ity or density and total sulfur content to adjust the older assay. The “new” assay, together with a refinery LP model, can then be used to make quick decisions such as purchas- ing and running an opportunity (challenging) crude oil. The flash assay can also be used to indicate when another full assay should be run. With the proliferation of new streams and with new production frequently being added into exist- ing streams, a flash assay can result in considerable cost and time savings. With the relatively limited data incorporated into flash assay updates, it must be kept in mind that preci- sion and accuracy are not optimum.

Moreover, today’s trend is toward automated, real-time anal- ysis using on-line detectors to the extent permitted by available instrumentation. Some of this instrumentation has the capability to provide data suitable for custody transfer determination of  crude oil properties, but this is currently quite limited. However, it seems likely that significant advances in this area will be accomplished in the near future, further removing the laboratory analyst from routine characterization of crude oil feedstocks.

SUMMARY

Crude oils, unlike refined products, do not have specifications. As such, it is fundamentally important that assays provide suf- ficient and accurate data so that the producers, traders, refin- ers, and other users of the data can achieve several objectives. Moreover, the analytical test methods and procedures used must be defined so that different laboratories can be reason- ably expected to obtain comparable data on replicate samples. From the time a crude oil stream is produced until it reaches a refinery and is processed, several factors can affect its quality. Depressurization and cooling at the well- head can result in flocculation of asphaltenes and crystalliza- tion of waxes. Production making up a defined stream varies over time because of several factors, including well mainte- nance and workovers, reservoir aging, and commingling of  new production. During subsequent marine transportation, two or more crude oils of somewhat different quality may be commingled on the same vessel.

In common carrier pipeline systems, crude oils of differ- ent quality are commonly batched. Interfacial mixing will take place between the head and tail of sequential batches until they reach a breakout terminal. At the terminal, the crude oil may be stored in a tank containing a “heel” of dif- ferent quality.

Assays provide assurance that the crude oil received is what is expected and help to define the changes that have taken place during transportation and storage. Refinery oper- ation personnel use assays to assess handling and processing characteristics of a new stream or one that has changed in quality. Assays provide important information on the pres- ence and concentration of impurities and contaminants that may affect refinery process systems. Moreover, they are important to the producer because they provide the data needed to demonstrate acceptability of a stream in a common

carrier pipeline system. Furthermore, assays are used in vari- ous ways in ascertaining compatibility of different streams.

Assay data are input to refinery LP models to optimize crude oil blends and predict product yields. These data are directionally correct, but they do not replicate refinery con- ditions. Assays do not indicate how the fraction material will behave in process units such as reformers, catalytic crackers, or cokers or how the process streams will interact with the many and complex catalysts used today. Moreover, they do not provide data on the storage stability of refined products.

However, the data available are seldom from recent ship- ments or cargoes.

Finally, detailed assays are costly and can take weeks to complete. The data they provide may not be representative of the shipment recently received, especially if it is an oppor- tunity crude oil. Assays do not account for variations between cargoes and may not be representative of current production quality. Furthermore, and perhaps most impor- tant of all, assays are fundamentally dependent on the care and attention paid to sampling and handling.

CHAPTER 4 n COMPREHENSIVE ASSAYS AND FRACTION EVALUATIONS 31

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