Naphtha distillation curve

A plot of boiling temperatures (°F) vs. cumulative percent volume removed from the sample is referred to as a distillation curve. The boiling temperatures for various products range from high to low divided into the following product types residue, heavy gas-oil, light gas-oil, kerosene, naphtha, gasoline, and butanes (Table 4.4). 

Although pure hydrocarbons such as pentane, hexane, heptane, benzene, toluene, and xylene, which are now largely of petroleum origin, may be characterized by a fixed boiling point, naphtha is a mixture of many hydrocarbons and cannot be so identified. The distillation test does, however, give a useful indication of their volatility. The data obtained should include the initial and final temperatures of distillation together with sufficient temperature and volume observations to permit a characteristic distillation curve to be drawn. 

As the supply and demand of global crude changes, heavier crudes become more attractive to process. However, many existing columns cannot produce cuts that meet distillation product specifications. There are many process changes that could improve the distillation curve of a given product However, it may be unclear what the side effects of a given change could be. In this case study, we look at how we can improve the distillation curve (5%) of the heavy naphtha and kerosene cut One option is to draw more or less of a particular cut to force the distillation curve to shift However, this will affect other product draws as well. 

Figure 2.53 Change in distillation curves as a function of heavy naphtha stripping steam rate.

Table 2.14 contains the distillation curve and specific gravity (or density) from each cut If a complete distillation curve is not available, we recommend using the beta distribution fitting method to identify missing values (Section 1.4). The residue distillation curve may not be available routinely. We use the simple correlation outlined by Kaes [3] to identify key points on the distillation curve as a function of residue density. We can then use the same beta distribution fit to complete the entire required distillation curve. Finally, we also require the light gas composition (C1-C5) composition leaving with the naphtha and overhead products. 

We use a total of 233 different data points containing laboratory measured chemical composition and bulk property information (distillation curve, density, refractive index and viscosity) for light naphtha, heavy naphtha, kerosene, diesel and VGO. These data points come from various plant measurements made over the six-month period of this study and a variety of light and heavy crude assay data (spanning several years) available to the refinery. 

Figure 4.83 shows the entry window for the Heavy Liquid section of the Prod Meas. Tab. The measures required for the Naphtha and LCO cuts are routine measurement data. The distillation curve, density, concarbon, sulfur content and nitrogen content are required for all the heavy liquid cuts. In addition, the Olefins, Naphthenes and Aromatics content are required for at least one of the cuts. We must also enter Cloud Point for all LCO type cuts. In most cases, we cannot obtain the distillation curve of the bottoms cut (Routinely not measured or only partial measurement available). Kaes [51] provides a simple correlation to estimate the TBP curve of a bottoms cut as a function of density only. In general, we do not require accurate values for the TBP curve of the bottoms since it is typically not a significant product... 

Figure 5.54 shows the Feed Data tab from the Reformer sub-model. The Feed Type is a basic set of relationships and initial values for the all kinetic lumps in the reactor model. Aspen HYSYS uses bulk property information such as density, distillation curves and total PNA content in conjunction with the feed type to predict the composition of feed lumps to the model. The Default type is sufficient for hght-to-heavy naphtha. However, there is no guarantee that a particular feed type represents the actual feed accurately. Aspen HYSYS will attempt to manipulate the feed composition to satisfy bulk property measures given. In general, we advise users to develop a few sets of compositional analysis to verify the kinetics lumps calculated by Aspen HYSYS. We discuss a process to verify these lumps later. 

Composition, particularly C3 and C4, is the most important indicator to evaluate the quality of the LPG product Figure 6.59 represents selected model predictions on LPG composition with AAD of each component. For the most important components, C3 and C4, the model shows only 0.021 and 0.058 AADs, respectively, in molar fraction predictions. For other liquid products, the distillation curve is the most popular analysis to indicate the vaporization temperature after a certain amount of oil fraction vaporized. Figures 6.60 and 6.61 illustrate selected model predictions on distillation curves of light naphtha, heavy naphtha, jet fuel, and residue oil. 

The breakeven curve for light (atomospheric) gas oil is given in Figure 5. If light gas oil can be disposed of at prevailing middle distillate prices of about 1.1 /lb, naphtha would have to sell at 1.4 to 1.5 /lb for the gas oil to become attractive. Therefore, pyrolysis of atmospheric gas oil will not be ordinarily attractive. 

Although much dependence is placed on the assessment of volatility by distillation methods, some specifications include measurement of drying time by evaporation from a filter paper or dish. Laboratory measurements are expressed as evaporation rate either by reference to a pure compound evaporated under conditions similar to those for the sample under test or by constructing a time-weight loss curve under standard conditions. Although the results obtained on the naphtha provide a useful guide, it is better, wherever possible, to carry out a performance test on the final product when assessing formulations. 

In addition, a method of petroleum classification has been developed that is based on other properties as well as the density of selected fractions. The method consists of a preliminary examination of the aromatic content of the fraction boiling up to 145°C as well as that of the asphaltene content, followed by more detailed examination of the chemical composition of the naphtha (b.p. <200°C). For this examination, a graph (a composite of curves expressing the relation between percentage distillate from the naphtha, the aniline point, refractive index, specific gravity, and the boiling point) is used. The aniline point after acid extraction is included in order to estimate the paraffin-naphthene ratio. 

Figure 11.1 gives typical boiling curves for a light naphtha stream. The curve on the left is a TBP curve and that on the right is an ASTM D-86 curve. The abscissa is volume percent distilled. The ordinate is temperature. Note that the initial and final parts of the curves are quite different because of the fractionation that occurs in the TBP distillation. The 50% boiling point is almost the same (249 and 243 °F). Table 11.1 compares the results of these two methods. 

The design methods considered for multicomponent mixtures in Chap. 9 were based on a limited number of definitely known components. In some cases, the mixtures are so complex that the composition with reference to the pure component is not known. This is particularly true of the petroleum naphthas and oils which are mixtures of many series of hydrocarbons, many of the substances present having boiling points so close together that it is practically impossible to separate them into the pure components by fractional distillation or any other means. Even if it were possible to determine the composition of the mixture exactly, there are so many components present that the methods of Chap. 9 would be too laborious. It has become customary to characterize such mixtures by methods other than the amount of the individual components they contain, such as simple distillation or true-boiling-point curves, density, aromaticity (or some other factor related to types of compounds), refractive index, etc. 

Bright stock solution, like pressed distillate, must be distilled with as little discoloration as possible. Several two-flash systems have been built. In the first flash, the naphtha is removed. The residue is then further heated in a part of the pipestill, and gas oil and a neutral oil are vaporized in the second fla. The tw6-flash system holds no important advantages over the single-flash system, but as an illustration the two-flash method of operation is shown in Fig. 7-18. Evaluation curves are shown in Fig. 7-19> The steam required in a pipestill rerun system is often less than one-fifth the amount required by shellstill redistillation.. Modem practice is tending toward single- rather than two-flash systems. 

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