Figure 2.8. Product and reflux volumetric flow rates.
criterion is not met, Packie's curves for the case of insufficient steam stripping may be found in Nelson, pp. 515-516.
Packie defined the degree of difficulty of separation as the difference between the ASTM 50 volume percent temperatures of the fractions under study. For Figure 2.6, this difference is between the ASTM 50 percent temperatures of the first (highest) sidestream product and the overhead fraction. For Figure 2.7, this difference is between the ASTM 50 percent temperatures of the lower sidestream and the total remaining lighter distillates.
He then defined the separation capability of the system as the product of the reflux-to-feed ratio at the upper draw tray as calculated on a volumetric basis and the number of actual trays in the section. This product is designated as the F-factor. In sections where pumparound heat removal systems are used, trays in this service are considered to be only one-third of an actual fractionating tray.
Thus, Packie defines degree of separation attainable and the (5-95) Gap as functions of the separation capability of the system (F-Factor) with parameters of degree of diffi culty of separation, Atso ASTM. To illustrate, consider three examples.
These examples are based on Figure 2.8 which illustrates the internal and external product rates and the internal reflux rates as expressed on a common volume/time basis and on Figure 2.9 which shows the volumetric product yield overlaid on the crude TBP curve. Figure 2.10, derived from Edmister's correlations (3), is used to convert the 50 volume percent TBP temperatures to ASTM temper-tures.
The following conditions apply to Figure 2.8.
1. Vapor rates are products only measured as liquids at 60 degrees F.
2. Internal liquids and products are measured as liquids at 60 degrees F.
3. All flow rates are on a common volume/time basis.
4. Sidestream product strippers are not shown.
5. Separation Tray Numbers in Number of Trays
Example 1
(tso) TBP = 703 degrees F - (Point A of Figure 2.9).
Converting these 50 percent temperatures from TBP to ASTM by using Figure 2.10,
Therefore, from Figure 2.7, (5 - 95) Gap = + 6 degrees F. Example 2
Figure 2.9. Product yield overlay on whole crude TBP curve.
(t50) TBP - 197 degrees F - (Point D) Converting TBP to ASTM,
Therefore, from Figure 2.7, (5-95) Gap = +37 degrees F. Example 3
For D5,
(ts o)TBP = 159 degrees F - (Point F) Converting TBP to ASTM,
Therefore, from Figure 2.6, (5-95) Gap = +22 degrees F.
These computations will be encountered later in the example calculation but are introduced at this point to illustrate the principles of petroleum fractionation and its nomenclature. By inspection of Figures 2.6 and 2.7, it is clear that lowering the F-Factor, either by reducing the number of trays or the reflux ratio by altering the heat balance, will reduce the (5-95) Gap which is the separation. At this point, the similarity between discrete-component distillation and crude petroleum fractionation should be less unclear.
As more operating data has become available, Packie's work is now generally considered to be on the conservative sidef that is, his procedure usually predicts a smaller gap than is actually attained in practice. The author believes that the degree of conservatism inherent in Packie is not excessive and recommends its use for design work. It is almost always true that crude distillation units are required to perform different operations than that for which they were designed. For this reason, the design "fat" contained in Packie becomes quite useful to the owner in later years.
The feed to the atmospheric tower is the crude oil to be processed into the various products required. Before the design material balances can be developed, it will first be necessary to derive some basic physical property data for the crude. From this, one can then estimate the total distillate production and product distribution in the atmospheric tower.
In definitive process design work and/or in refinery operations analysis, the engineer usually has access to a complete crude assay. Indeed, he should not undertake a definitive design without one. On the other hand, feasibility studies and/or order-of-magnitude work is often performed with little more than a whole crude gravity and TBP curve. In either case, the first step is to characterize the crude in order to facilitate later calculations. The minimum information required is (a) whole-crude TBP curve, (b) whole-crude API gravity and (c) whole-crude light ends analysis.
Additional information which is highly desirable is an API gravity and molecular weight study of narrow cuts of the whole crude. From this can be derived a plot of volume percent versus weight percent and mid-volume percent versus molecular weight. If this data is not available experimentally, it can be calculated.
An experimental determination of the EFV curve, at least at atmospheric pressure and preferably at one or two higher pressures covering the anticipated range of operations is also desirable. Maxwell (4), Nelson and Edmister have all published procedures for converting a whole crude TBP curve to an atmospheric EFV curve and for estimating the EFV temperatures at pressures above atmospheric. In the petroleum industry, there is a good deal of argument about how to estimate EFV behavior of crudes since the total vaporization in the tower is the sum of the equilibrium flash vapor plus some stripout from the equilibrium flash liquid. Stripout versus stripping steam correlations are largely empirical. In an operating tower, one can calculate stripout by making a heat balance around the bottoms stripping section, but this is not possible in a design situation. For these reasons, the author strongly recommends obtaining experimental data, particularly when working with unfamiliar crudes. The cost of obtaining accurate equilibrium data is trivial when compared to the possible penalties involved in incorrectly estimating yields or mis-sizing equipment.
However, if one is limited to the minimum data, the required information can be derived by using the following procedures for crude oil characterization.
Calculation Procedure for Characterizing Crude Oils
Given:
Whole-crude atmospheric TBP curve
Whole-crude gravity
Front end analysis for light ends
Calculate:
Volume percent versus weight percent Mid-volume precent versus molecular weight
1. Calculate the characterization factor, K, for the whole crude using the techniques of Maxwell, Section 2.
a. Calculate the volume average boiling point (VABP) using the 20, 50 and 80 volume percent TBP temperatures.
b. Calculate the 10 to 70 slope of the whole-crude TBP curve.
c. Using the proper correction factor, convert VABP to mean ABP.
d. K is found as a function of mean ABP and API gravity by use of Winns'nomogram (5).
2. Assuming that K remains constant, calculate the molecular weight and API gravity for various boiling-range cuts.
a. For the TBP range (initial boiling point to 200 degrees F), calculate mean ABP, API gravity and molecular weight. Based on 100 barrels of whole crude, calculate and tabulate:
barrels vapor pounds vapor moles vapor molecular weight of vapor b. For the TBP range (200 to 300 degrees F), repeat Step 2a.
c. For the TBP range (300 to 400 degrees F), repeat Step 2a, and so on until the entire crude range has been covered.
d. From these calculations, draw the following curves.
1. Volume percent over versus weight percent over.
2. Mid-volume percent versus molecular weight of vapor.
3. Calculate the atmospheric pressure EFV curve for the • whole crude using Packie's procedure. Plot this curve : on the same chart as the whole-crude TBP curve.
As is the case in any process-design problem, the definition of the materia! balance is the first and most important step. In this portion of the discussion, various methods for estimating product yields from the crude petroleum feed will be explored.
The design material balance is determined by the product characteristics required by the owner and by the amount of crude vaporization which will occur at the conditions of temperature and pressure existing in the flash zone.
Since most crude distillation units contain both atmospheric and vacuum towers, economic considerations usually favor maximizing distillate yield from the atmospheric section in order to minimize the load on the vacuum section. This may not always be true in an operating situation where the vacuum tower might have unused capacity when the atmospheric tower is operated for maximum distillate production. In that case, one would balance atmospheric and vacuum distillate production in such a way as to maximize crude throughput. In designing atmospheric towers where the bo-ttoms liquid is sold directly as fuel oil, the most economical approach in this special case is to vaporize only the required distillate products and to allow the remaining potential distillate to be yielded, unvaporized, with the bottoms. This situation arises often in foreign refineries, usually in the producing areas, which run crude primarily for local fuels requirements and for bunkering tankers. Thus, there is just no economic driving force for yielding anything heavier than diesel fuel as a distillate.
In practice, the question of optimizing the relative distillate yield between the atmospheric and vacuum towers will be settled on an economic basis and must be resolved prior to commencing definitive design work. This type of analysis will depend upon economic factors within the particular company and/or plant site under study and is outside the scope of this work.
Total Distillate Yield
The total distillate yield is found by calculating the vaporization of the crude which will occur at the conditions
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