Heavy Distillation Curves

Dearomatized or not, lamp oils correspond to petroleum cuts between Cio and C14. Their distillation curves (less than 90% at 210°C, 65% or more at 250°C, 80% or more at 285°C) give them relatively heavy solvent properties. They are used particularly for lighting or for emergency signal lamps. These materials are similar to kerosene solvents , whose distillation curves are between 160 and 300°C and which include solvents for printing inks. 

The products could be classified as a function of various criteria physical properties (in particular, volatility), the way they are created (primary distillation or conversion). Nevertheless, the classification most relevant to this discussion is linked to the end product use LPG, premium gasoline, kerosene and diesel oil, medium and heavy fuels, specialty products like solvents, lubricants, and asphalts. Indeed, the product specifications are generally related to the end use. Traditionally, they have to do with specific properties octane number for premium gasoline, cetane number for diesel oil as well as overall physical properties such as density, distillation curves and viscosity. 

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). 

FIGURE 4.10 Typical distillation curve for (a) predominantly gasoline with trace heavy residual fuel oil and (b) predominantly gasoline with a middle distillate of diesel fuel. 

Figure 26.19 Distillation curves of light and heavy oils...

Figure 8-7 shows the characteristic pattern of distillation curves for ideal or close to ideal VLE with no azeotropes. All of the systems considered in Chapters 5. 6, and 7 follow this pattern. The y-axis (Xg = 0) represents the binary A-C separation. This starts at the reboiler (x = 0.01 is an arbitrary value) and requires only the reboiler plus 4 stages to reach a distillate value of x = 0.994. The x axis (x = 0) represents the binary B-C separation, which was started at the arbitrary value Xg = 0.01 in the reboiler. The maximum in B concentration should be familiar from the profiles shown in Chapter 5. Distillation curves at finite reflux ratios are similar but not identical to those at total reflux. Note that the entire space of the diagram can be reached by starting with concentrations near 100% C (the heavy boiler). 

The liquid phase continuity equations for the components and GOi contain the rate equations expressed by Kumar and Froment [2007] in terms of the single-event approach, already presented in Section 2.4.4 Hydrocracking of Chapter 2. Their most advanced version of the simulation model characterizes the VGO-feed by 1266 components and GOi. The current methods used for the analysis of heavy petroleum fractions do not permit to reach such detail, but methods have been developed that reconstruct their composition at the molecular level starting from global analytical results such as carbon-, hydrogen-, and sulphur-content, specific gravity, mass spectrometry, distillation curve... [Hudebine and Verstraete, 2004 Martinis and Froment, 2009 Charon-Revellin et al, 2010]. 

The new analytical developments appeared during the last decade are mostly focused on a faster determination of the distillation curve (TBP). HTSD is widely accepted as one of the best alternatives. Originally developed as an extension of conventional GC simulated distillation (ASTM D2887/D5307), it provides better information on the crude oil back ends. The technique is currently under study to become an ASTM standard, overcoming the problems related with reproducibility of front and heavy end information. 

For the purpose of establishing the distillation curves of the tower bottoms product, one can assume with only a very small possibility of error that the TBP initial point of the bottoms is equal to the nonnal boiling point of the li t key and that the TBP end point of the distillate is equal to the normal boiling point of the heavy key. From these two points, draw in the TBP curve of the bottoms product. 

The recommended distillation method for crude oils up to cutpoint 400 C (752 F) AET is Test Method D 2892. This test method can be used for heavy crude oils with initial boiling points greater than 1S0 C (302 F). However, distillation curves and fraction qualities obtain by these methods are not comparable. 

Experimental Vaporization Curves. When such complex materials as gasoline and petroleum fractions are dealt with, the application of the aforementioned equilibrium laws is cumbersome. Furthermore, the component analyses of these heavy oils cannot be easily obtained and even if such analyses are-availalUe, accurate vapor-pressure or equilibrium data for the compounds or fractions contained in them are not always available. At present most equilibrium relations are obtained by determining experimental flash-vaporization curves or by computing such curves from the empirical relationships discussed in Chap. 4. Empirical flash curves can be estimated from true-boiling-point or ASTM curves, and with less accuracy from Hempel or Saybolt distillation curves. 

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.

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