Separation Criteria for Atmospheric Tower Products | |

Separation |
(5-95) Gap, °F |

Light naphtha to heavy naphtha Heavy naphtha to light distillate Light distillate to heavy distillate Heavy distillate to atmospheric gas oil |
+ 20 to + 30 + 25 to-t- 50 0 to + 10 0 to + 10 |

2. TBP cut point— the whole crude TBP temperature corresponding to the TBP cut volume.

3. TBP overlap = (TBP end point of light fraction)-(TBP initial point of heavy fraction).

Also, note that

This latter principle has been established by analysis of operating data which has shown that, for well stripped side-streams, the volume interchanges between two streams around the TBP cut point are equal.

The owner defines the material balance himself by specifying the volumetric yields on crude for all products down through heavy distillate. The designer will determine the yield of atmospheric gas oil. This type of specification will almost always include the anticipated ASTM distillations and other key physical properties of the streams. This data should be checked for accuracy and any discrepancies resolved before proceeding with the design.

The owner specifies the ASTM distillations for the side-streams down through heavy distillate and the ASTM end point of the overhead product. He may also specify the ASTM initial point of the atmospheric gas oil, but, if not, this shall be determined by the designer. The designer must also determine the TBP cut point between the overhead and the lightest sidestream. These specifications are used to estimate yields by the following procedure.

1. For the sidestreams, convert the ASTM to TBP distillations by Edmister's techniques.

2. The ASTM end point of the overhead is converted to a TBP end point by the correlation of Figure 2.15 (de,-rived by author).

3. If the ASTM initial point of the atmospheric gas oil has been given, convert it to a TBP initial point using Figure 2.15. If this ASTM temperature has not been given, assume a TBP overlap of 80 to 100 degrees F between heavy distillate and atmospheric gas oil.

4. f Calculate the TBP cut points between fractions and I determine the volumetric yields of products.

The owner desires that the design be based on production of one particular stream and that the other products be defined in terms relative to but subordinate to the key stream. Usually, this specification will give detailed ASTM temperatures for the key stream. The 0, 5, 10, 50, 90, 95 and 100 volume percent ASTM temperatures are the ones most often used along with (5-95) Gap specifications between the key stream and adjacent streams.

General Specifications

In the unlikely event that work is to be undertaken without having specific instructions from the owner, the following procedure is recommended.

1. As discussed earlier, the material balance should be based on two representative crudes, one light and one heavy. The material balances will then be based on alternately maximizing naphtha, light distillate and heavy distillate production. Total distillate yield is based on a maximum oil temperature leaving the furnace of 700 degrees F. Suggested ASTM boiling ranges for these cases are given in Table 2.2.

2. Recommended separation criteria, i.e., ASTM (5-95) Gap, are given in Table 2.3.

Figure 2.16 may be used to estimate TBP overlap for a given ASTM (5-95) Gap between the indicated fractions.

To this point, the volumetric yields of all products and the TBP and ASTM distillations for all distillates have been estimated. In order to complete the estimate of the material balance, it is necessary to define various other properties of the materials. The following steps will accomplish this.

1. Plot the ASTM curves of the distillates and calculate the (5-95) Gaps.

2. Calculate the 14.7 psia EFV curves for the distillates. Plot these curves and extrapolate to minus 20 volume percent vaporized in order to approximate the effect of the equilibrium solubility of the lighter crude oil components in the liquid leaving the draw tray.

3. Set stripping steam to the atmospheric gas oil stripper at 10 pounds per barrel stripped product and estimate the stripout from Figure 2.13. Since the other side-streams are to be reboiled, set their vaporizations equivalent to the amount which would be stripped out by steam at a rate of 10 pounds per barrel stripped product. Plot these stripout points, i.e., minus vaporization, on the product EFV curves. These are the 14.7 psia bubble points of the unstripped sidestream products and will be used later to calculate draw tray temperatures.

4. From the crude assay data, calculate the gravities and molecular weights of all products.

5. Calculate the vapor-liquid separation of the gross overhead product at the conditions of temperature and hydrocarbon partial pressure existing in the reflux drum. These calculations are detailed in the design example. The detailed calculation procedure is as follows.

a. The light ends analysis and the partial TBP curve of the gross overhead is combined into a total TBP curve.

b. The TBP curve is broken up into pseudocompo-nents which are tabulated as volume, weight and molal quantities. The use of n-alkane physical properties is acceptable. 3. Since free water will exist in the reflux drum, the vapor phase will be water saturated at the condenser exit temperature, Tp^. Calculate the hydrocarbon partial pressure of the distillate vapor stream as P^ç-

d. At conditions T^- and P^jç, make a flash calculation on the gross overhead stream, thereby defining the vapor-liquid separation. From this, define the composition of the vapor and the liquid.

e. Synthesize a TBP curve for the distillate liquid and convert it to an ASTM distillation. This step is omitted in the example calculation.

f. Calculate and tabulate the distribution of the steam to the process as vapor leaving with the distillate vapor and liquid leaving as free water.

g. Assume the number of trays for the various separation sections in the tower and define the total number of trays and draw tray locations. Plot the gravity and molecular weights of the liquids leaving trays by assuming that the properties of liquids one tray above and one tray below draw trays are the same as the draw tray liquid. Plant test work has indicated that this is true. Assume a linear change in properties across the remaining trays in each section. This plot is used only in the calculation of Type A systems.

The material balance and composition information which has been developed should now be tabulated into a process design basis for ready reference in later calculations.

Heat and Material Balance Calculations for Type U Towers

A complete Type U tower is shown in Figure 2.17. This drawing illustrates the basic process and its essential auxiliaries as well as the external hçat and material balance quantities. Note that the product draw trays are all shown as partial draws so that the reflux flows internally from the draw tray to the tray below it. Figure 2.17 will be the basis for discussing the heat and material balance calculations in this section. In the introductory remarks to this chapter, it was stated that a Type U system is not practical from an industrial viewpoint because of the obvious lack of thermal efficiency. However, it is the easiest system to calculate and to understand. Thus, the fundamentals of heat and material balance calculations will be illustrated in terms of a Type U system. Later, the methods for obtaining and calculating heat removals from the tower will be given. A secondary reason for calculating a Type U system is that, as will be

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