Whole Crude Tbp Curve

Overflash Distillation Curve
Figure 2.14. Temperature relationships around the cut point between fractions.

heavy distillate and has an ASTM boiling range of approximately (350 to 675 degrees F). Marine diesel is a little heavier, having an ASTM end point in the range of 775 degrees F.

Gas Oil-all distillates heavier than heavy distillate. Gas oil yielded from the atmospheric tower will have ASTM end points of approximately 800 degrees F. Vacuum gas oils will have ASTM end points as high as 1,100 degrees F. Depending upon the operation being practiced, vacuum gas oils can have various boiling ranges. This subject is covered in detail in Chapter 3.

Note that the boiling ranges of these materials often overlap by quite a bit. This indicates that one can maximize the yield of only one product on any given operation. For example, consider maximizing the production of 325 to 525 degrees F kerosene. It is obvious that the naphtha end point cannot be 400 degrees F-corresponding to a maximum naphtha operation—nor can the heavy distillate initial point be much lower than 500 degrees F which precludes a maximum heavy distillate operation. The importance of this principle will be seen in the following material where methods for cutting the crude into products are presented.

Product Distillate Yields

In this part will be discussed methods for estimating the yields and properties of the various products which can be produced off the atmospheric tower. The general method of attack will be to determine distillation properties of the desired products and apply these to the whole crude TBP curve in order to estimate volumetric yields. Volumetric yields are then used to obtain product gravities, molecular weights and other properties from the crude assay. There are many ways to define a product slate, but most of these are based on specifying certain ASTM distillation tem

>oo 3oo 400 joo wo 700 too too «no ltoo tip tempeiatuse, *r

Figure 2.15. Relationships between astm and tbp initial (0 percent) and final {100 percent) boiling points.

Astm Distillation Tbp

>oo 3oo 400 joo wo 700 too too «no ltoo tip tempeiatuse, *r

Figure 2.15. Relationships between astm and tbp initial (0 percent) and final {100 percent) boiling points.

peratures for the products from which the remaining data will be derived.

Since the planned production rates will depend upon the crudes which are available to the refinery, the designer should base his yield studies on the heaviest (lowest API gravity) and the lightest (highest API gravity) crude which is to be run on a normal basis. Further, he should obtain specific definition of all the operations which are to be practiced. These two points are very important and are discussed in detail in the following paragraphs.

The light crude will define the design basis for the atmospheric section of the crude unit since its volume of distillates will exceed that which can be produced from the heavy crude. All equipment sizing will be based on heat and material balance data calculated for the various light crude cases. As would be expected, the heavy crude will define the facilities for processing the atmospheric tower bottoms, either a vacuum unit or, if this is not planned, the reduced crude heat exchange equipment. To further complicate the matter, the various production operations must be calculated—heat and material balance—for both crudes. This will usually entail the alternate maximization of naphtha, light distillate and heavy distillate. These studies will define the variations in the heat and material balance required to satisfy the varying yields pattern and are necessary for the optimum design of the heat exchange train, towers and furnaces.

Now, the ways in which one can define yields and, hence, the overall material balance are to be considered. It should be remembered that, to this point, the hydrocarbon vapor leaving the flash zone and the overflash have been calculated. Thus, the total distillate yield is known. In discussing the various ways for estimating yields, the following terminology will be used. Figure 2-f4 illustrates the physical significance of these terms.

1. TBP cut volume-the volumetric yield point between two fractions.

Was this article helpful?

0 0

Post a comment