Crude assays

Crude assay a procedure for determining the general distillation characteristics (e.g., distillation profile, q.v.) and other quality information of crude oil. 

The operation of the preprocessor is shown schematically in Figure 1. There are five basic steps o Select crude assay data o Build and report input tables o Generate crude data o Generate process data o Build and access LP data tables In the first step, the preprocessor accesses the disc file which contains all of Sun Petroleum Products Company s crude assays. The preprocessor extracts assay data for those crudes which the user has identified by card input as part of the base crude mix or to be made available to the LP model as an incremental refinery feed. The user can identify up to ten crudes any five of which can be designated as incremental. 

In the second step, the preprocessor builds and prints out tables showing the extracted crude assay data. The preprocessor also prints out in tabular form all information which the user has specified by card input for the following process units o Crude distillation unit o Propane deasphalter o FCC... 

The feed to the hydrocracker consists of the heavy naphtha cut from the base crude mix and light cycle oil from the base FCC operation. The hydrocracker simulator requires a description of each of these feeds in terms of the hydrocarbon components shown in Table I. Since these components are not directly measured in the crude assay nor are they predicted by the FCC simulator, special techniques were developed to estimate them from available data. 

From each crude assay, the density, molecular weight, and average aromatic and cycloparaffin rings per molecule are obtained for both the saturate and aromatic fraction from each of the two gas oil cuts distilled in the laboratory. 

The reformer simulator also requires a description of the feed in terms of hydrocarbon components. These are shown in Table II. Fortunately, most of these components are measured in the crude assay and are predicted by the hydrocracker simulator. 

Once the crude slate is established, the supply of 1,000+ material is determined from our database of crude assays. 1,000+ material is that part of the barrel of crude oil which has a boiling point of 1,000°F or greater. This is typically the feed material to a coker. 

A crude assay for the presence of avidin-specific antibodies may be performed by adding increasing amounts (0.0, 0.1, 0.2, 0.3, or 0.4 ml) of serum to 30ju.liters of an avidin solution containing 2mg/ml. These components are mixed and allowed to incubate overnight at 4°C. If the serum contains antibodies a precipitate forms under these conditions. Very small amounts of precipitate may be detected only as an opalescent appearance of the test reaction mixture compared to that of the control from which avidin was omitted. A positive reaction to this test may be expected 3 to 5 weeks after the initial injection (step 8-7). 

A more recent test method (ASTM D-5236) is seeing increasing use and appears to be the method of choice for crude assay vacuum distillations. Wiped-wall or thin-fihn molecular stills can also be used to separate the higher-boiUng fractions under conditions that minimize cracking. In these units, however, cut points cannot be directly selected because vapor temperature in the distillation column cannot be measured accurately under... 

The convention now is to measure base stock viscosities at 40°C and 100°C. Waxy intermediates usually have their viscosities measured at 65°C instead of at 40°C, a temperature at which many of these will have solid wax present. These measurements are made according to ASTM D4456 or its equivalent, and when performed correctly give results that are sufficiently accurate to determine the viscosity index (VI)7 of the sample. Historically, particularly when viscosities were measured in SSU units, the temperatures employed were 100°F and 210°F. It should be mentioned that it is not uncommon to find that viscosities measured for other purposes (e.g., for vacuum gas oil fractions in crude assays) are not sufficiently accurate to provide good waxy Vis (which are useful for assessing crudes for lube use). 

A lube crude assay is a laboratory process to measure the lube processing response from crude to base oil. It is an important step in a manufacturer s lube crude selection. A crude assay will include process yields for desired base oils at their quality specifications. The manufacturer can use the assay data to predict the process response for their refinery and to assess the desirability of purchasing particular crude. The assay results may be used to calculate the impact on profitability. 

Assays of the crude types B and C will need frequent revisions in the crude oil assay libraries, but the nature will be different. With crude oils from group B there is a continuous need to update the crude assay in the databank, which can be platmed following two possible criteria on a fixed delta time basis or on a delta quaUty basis, which is the usual procedure. 

Crude oils Ifom group C present a more serious problem. A considerable effort in updating crude assays can be made without any improvement, due to the random quality variations. It is very difficult to define the appropriate moment to update the assay, and most of the times different qualities are ineluded in the erude oil library to represent different scenarios... 

Some authors suggest an existing tradeoff between improving LP predictions with rigorous models and the cost associated to continuously update the crude assay libraries. A conventional crude oil assay evaluation is... 

Some recent references have demonstrated the power of statistical techniques applied to crude assay prediction. The availability of large crude assay databases integrated with advanced statistical methods allows the prediction of updated crude oil assays or isolated physical properties. 

With the basis of the mentioned requirements and the shortage of commercially available tools, a new technology was developed with the objective to update/complete crude oil assays with limited experimental data using rigorous models. This new technology, registered under the name repCAPT (Repsol-YPF Crude Assay Prediction Tool) was intended to complement conventional evaluation procedures. ... 

The center of this technology is an expert system for crude assay generation. This expert system, which includes a set of mathematical algorithms based in powerful data-mining techniques and first-principle models, may generate a complete crude oil evaluation from variable levels of information available. 

The system is integrated with unified libraries, updated through the intranet, compiling all the crude oil assay information available. Instant access is provided to crude assay updates, and software interfaces assure integration with other supply chain tools available in planning and operation areas (Figure 7). 

Villalanti, D Maynard, J. Raia, J. Arias, A., Yield Correlations Between Crude Assay Distillation and High Temperature Simulated Distillation (HTDS) , AlChE Spring National Meeting, Houston, TX, March 9-13, 1997. 

Needs for Rapid and Accurate Prediction of Crude Assay Properties... 

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