Example

The 95% point of heavy naphtha is 315°F and the 5% ASTM distillation point of kerosene is 370°F. The flash point of kerosene is 127.2°F. Calculate the deviation from actual fractionation between heavy naphtha and kerosene for the steam-stripped kerosene fraction and the number of plates and reflux required for separation.

The actual 5% ASTM distillation point of a fraction can be correlated from its flash point (known), by following relation:

Flash point (°F) = 0.77 x (ASTM 5% point, °F) - 150

The actual 5% point on the ASTM distillation curve of kerosene, by this correlation, equals 360°F, which is 10° less than ideal. Since kerosene is to be steam stripped, 95% of heavy naphtha will be 325°F. Therefore,

Deviation from ideal fractionation = (55 — 35), or 20°F

From the Packie's correlation, an F factor of 11.5 is required. CHARACTERIZATION OF UNIT FRACTIONATION

In commercial atmospheric and vacuum units, the distillation is not perfect. For example, a kerosene fraction with a TBP cut of 300-400°F will have material (referred to as tails) that boils below 300°F and other material that boils above 400°F. Because of these tails, the yield of the required product must be reduced to stay within the desired product quality limits.

The size and shape of the tails of each product depends on the characteristics of the unit from which it was produced. The factors affecting the fractionation are the number of trays between the product draw trays, tray efficiency, reflux ratio, operating pressure, and boiling ranges of the products.

Several approaches are possible to characterize fractionation in an operating unit. One approach is to characterize the light tail at the front end of a stream in terms of two factors:

Vi is the volume boiling below the cut point, expressed as LV% of crude.

Tf is the temperature difference between the cut point and the TBP initial boiling point (1 LV% distilled) of the stream.

Consider the TBP distillation of products from an atmospheric distillation column (Figure 1-1). The front-end tail of kerosene (TBP cut 300-400) contains 1.5% material on crude boiling below 300°F (see Table 1-3); therefore, Vf = 1.5.

The initial boiling point of kerosene cut (1 LV% distilled) is 240°F and the temperature difference between the cut point (300°F) and IBP is 60°F; therefore, 7> = 60.

The shape of the front tail can be developed using these two parameters on a probability plot. Having established these parameters, the same values are used for the front end tails of kerosenes on this unit for different cut-point temperatures (e.g., for different flash-point kerosenes).

A similar approach is used for back end tail; in the preceding example, the lighter heavy straight-run (HSR) naphtha cut is before kerosene. The

Table 1-3

Front and Back Tail Characterization of a Typical Atmospheric

Crude Unit

FRONT END TAIL BACK END TAIL

Table 1-3

Front and Back Tail Characterization of a Typical Atmospheric

Crude Unit

FRONT END TAIL BACK END TAIL

STREAM

VF LV%

TfAT

VB LV%

Tg AT

c4

0.0

0.0

LSR

1.0

35.0

HSR

1.0

40.0

1.5

50.0

KEROSENE

1.5

60.0

2.0

65.0

LIGHT DIESEL

2.0

70.0

3.5

120.0

RESID

3.5

160.0

KEROSENE VF = HSR VB. LIGHT DIESEL VF = KEROSENE V, RESID VF = LIGHT DIESEL VB.

NOTE:

KEROSENE VF = HSR VB. LIGHT DIESEL VF = KEROSENE V, RESID VF = LIGHT DIESEL VB.

volume of HSR material boiling above the kerosene cut point of 300°F must be 1.5 LV% (on crude), equal to the front end tail volume on kerosene. Let us call it VB; therefore,

The HSR end point (99% LV distilled) is 250°F and the cut point is 300°F. Therefore,

The shape of the back end tail can be estimated using a probability paper. Similarly the shape of front and back end tails for all cuts on vacuum units can also be determined (Table 1-4).

Having established these parameters, the same values are used, for example, for all kerosene cuts on this unit at different front end cut temperatures. This is an excellent approximation, provided the changes in cut point and boiling range are not too large.

Having established the appropriate unit fractionation parameters, the individual product distillations can be established based on selected TBP cut temperatures. These are defined by the points where the produced yield cuts the crude TBP curve. For example, referring to Figure 1-1, the yield of a product lighter than kerosene is 20.4 LV%; hence, the kerosene

Table 1-4

Front and Back Tail Characterization of a Typical Vacuum Unit

Table 1-4

Front and Back Tail Characterization of a Typical Vacuum Unit

STREAM

FRONT END TAIL

BACK END TAIL

VF LV%

TpAT

VB LV%

7b AT

WET GAS OIL

_

_

_

DRY GAS OIL

1.0

32.0

HEAVY DIESEL

1.0

60.0

2.2

108.0

VACUUM RESID

2.2

100.0

HEAVY DIESEL VF = DRY GAS OIL VB. VACUUM RESID VF = HEAVY DIESEL V, RESID VF = LIGHT DIESEL VB.

NOTE:

HEAVY DIESEL VF = DRY GAS OIL VB. VACUUM RESID VF = HEAVY DIESEL V, RESID VF = LIGHT DIESEL VB.

initial cut point is 300°F where the crude volume percent distilled is 20.4. The kerosene back end TBP cut point is 448°F where the crude volume percent distilled is 36.8, giving the required kerosene yield of 16.4 LV% on the crude.

The product volume and product qualities can be determined by breaking the distillation into narrow cuts, called pseudocomponents, and blending the qualities of these using the properties of the narrow cuts from the crude assay data.

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    What is refinery distillation tail?
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