Cut points

To extend the applicability of the characterization factor to the complex mixtures of hydrocarbons found in petroleum fractions, it was necessary to introduce the concept of a mean average boiling point temperature to a petroleum cut. This is calculated from the distillation curves, either ASTM or TBP. The volume average boiling point (VABP) is derived from the cut point temperatures for 10, 20, 50, 80 or 90% for the sample in question. In the above formula, VABP replaces the boiling point for the pure component. 

This is the most common method. It is used for gasolines, kerosenes, gas oiis and similar products. The test is conducted at atmospheric pressure and is not recommended for gasolines having high dissolved gas contents or solvents whose cut points are close together. 

Each petroleum cut obtained by mixing the TBP distilled fractions (and thus characterized by the TBP cut points) is described by a coiiection of properties including the viscosity at two temperatures. 

We consider the problem for the bar with the cut and give another presentation of the solution as compared with the previous one. For convenience the dependence of the functions obtained on the cut point y is indicated. 

Cut point, °C Athabasca, wt % distilled NW Asphalt Ridge, wt % distiUed" P.R. Springs, wt % distilled Tar Sand Triangle, wt % distilled ... 

Petroleum asphalts, compared to native asphalts, are organic with only trace amounts of inorganic materials. They derive their characteristics from the nature of their cmde origins with some variation possible by choice of manufacturing process. Although there are a number of refineries or refinery units whose prime function is to produce asphalt, petroleum asphalt is primarily a product of integrated refineries (Fig. 1). Cmdes may be selected for these refineries for a variety of other product requirements and the asphalt (or residuum) produced may vary somewhat in characteristics from one refinery-cmde system to another and even by cut-point (Table 2) and asphalt content (Fig. 2) (5,6). The approximate asphalt yields (%) from various cmde oils are as follows ... 

Fig. 2. Different cmde oils contain different amounts of residua and the properties of specific "cut point" residua may be different, eg, for the 565 °C...

Remembering that the properties of residua vary with cut-point (Table 2), ie, the vol % of the cmde oil (Fig. 13) helps the refiner produce asphalt of a specific type or property. There are several properties that are usually controlled in asphalt specifications ... 

TBP cut point between the heavy distillate and the bottoms = 650°F. Percent overflash = 2 volume percent of feed. 

From the produc t specifications, distillate yields are computed as follows From Fig. 13-86 and the ASTM D 86 50 percent temperatures, TBP 50 percent temperatures of the three intermediate cuts are obtained as 155, 236, and 316°C (311, 456, and 600°F) for the HN, LD, and HD respectively. The TBP cut points, corresponding volume fractions of crude oil, and flow rates of the four distillates are readily obtained by stai-ting from the specified 343°C (650°F) cut point as follows, where CP is tne cut point and T is the TBP temperature (°F) ... 

These cut points are shown as vertical lines on the crude-oil TBP plot of Fig. 13-91, from which the following volume fractions and flow rates of product cuts are readily obtained. ... 

General. With simple instrumentation discussed here, it is not possible to satisfactorily control the temperature at both ends of a fractionation column. Therefore, the temperature is controlled either in the top or bottom section, depending upon which product specification is the most important. For refinery or gas plant distillation where extremely sharp cut points are probably not required, the temperature on the top of the column or the bottom is often controlled. For high purity operation, the temperature will possibly be controlled at an intermediate point in the column. The point where AT/AC is maximum is generally the best place to control temperature. Here, AT/AC is the rate of change of temperature with concentration of a key component. Control of temperature or vapor pressure is essentially the same. Manual set point adjustments are then made to hold the product at the other end of the column within a desired purity range. The technology does exist, however, to automatically control the purity of both products. 

Another important use of correlations is the optimization of existing unit operations. Cat cracking correlations can provide the refiner with valuable information for optimizing reactor temperature level, gasoline/distillate cut point, and feed and recycle rates. The practical application of this information can mean increased profitability for the cat cracking operation. 

Fig. 3-9. Preparative HPLC of 100 mg of the test racemate 8 in a single 2 mL injection using a 250 x 4.6 mm i.d. column containing (5)-Glu-(5)-Leu-DNB CSP. Conditions mobile phase ethyl acetate, flowrate 2.0 mL min , UV detection at 380 nm. Injection 2 mL of 50 mg mL racemate solution. Fractions collected before and after the indicated cut point were 98.4 % ee and 97 % ee pure, respectively. (Reprinted with permission from ref. [86]. Copyright 1999, American Chemical Society.)...

Flydrodemetallization reduces the amount of nickel and, to a lesser extent, vanadium in FCC feeds. Nickel dehydrogenates feed to molecular hydrogen and aromatics. Removing these metals allows heavier gas oil cut points. 

Gasoline and LCO rates will be adjusted to standard cut points. 

The reactor yield is then determined by performing a component balance. The amount of C5+ in the gasoline boiling range is calculated by subtracting the C4 and lighter components from the total gas plant products. Example 5-4 shows the step-by-step calculation of the component yields. The summary of the results, normalized but unadjusted for the cut points is shown in Table 5-4. 

Table 5-6 shows the normalized FCC weight balance with the adjusted cut points. 

Damaged or plugged feed nozzle(s) and/or damaged stripping steam distributor(s) are the common causes of mechanical failures that affect true conversion. Note that the apparent conversion, as discussed in Chapter 5, is affected by the distillation cut point and main column operations. 

Normally, the economic incentive is to maximize the fresh feed rate and/or conversion, sacrificing the bottoms cut-point and rate. Increasing conversion by 1.5% (through increasing the riser top temperature by 10°F), provides an incremental profit even though LCO is lost to bottoms. 

The difference between adjacent TBP cut points can be determined by the following equation ... 

A,B = constants varying for cut points ranges, shown in the following table ... 

The performance of an ideal screen in terms of the screen analysis of the feed is shown in Figure 2.13 (A). The cut point is the point C in the curve. Fraction A comprises all particles bigger than Dpc, and fraction B comprises all particles smaller than Dpc. The fractions A and B are the overflow and underflow respectively. Screen analyses of the ideal fractions A... 

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