Crude oil distillation

A novel crude oil fractional distillation process, characterized by improved regulation of reflux in the de-entrainment section of a fractional distillation column, is disclosed.

BACKGROUND OF THE INVENTION 
The invention relates to an improvement in the distillation of crude oil. 
Briefly, the distillation of crude oil is normally carried out in multiple 
stage fractional distillation columns having a number of side raw product 
streams. In such columns, the liquid is vaporized or flashed in a lower 
flash zone, and flash vapor is fractionated in the upper portion of the 
column. The section of trays between the flash zone and the lowest 
sidedraw is called the de-entrainment section. The primary function of the 
de-entrainment section is to remove drops of liquid entrained in the vapor 
leaving the flash zone. Liquid from the bottom tray of the de-entrainment 
section, the distillate portion of which is sometimes called overflash, is 
either drained to the flash zone or recycled back to the crude heater. 
The process designer is faced with two conflicting design requirements for 
the de-entrainment section. First, there must be sufficient reflux liquid 
on the trays to obtain effective contacting of liquid and vapor, and 
consequent removal of black oil drops entrained from the flash zone, and, 
second, for maximum distillate yield and thermal efficiency, overflash 
should be as low as possible. In the typical design, a compromise must be 
made between these two requirements. Accordingly, a need has existed for 
an improved process for distillation of crude oil which overcomes these 
limitations. The invention is such a process. 
SUMMARY OF THE INVENTION 
Briefly, then, the invention relates to a process for the fractional 
distillation of crude oil wherein the crude oil is vaporized or flashed in 
the lower or flash zone of a multiple tray fractional distillation column 
to produce a vapor which is fractionated in the upper or condenser 
sections of the column. In particular, the invention provides for the 
removal of liquid from the tray just above the flash zone and return of 
the liquid to the column at a zone or location higher than said tray, but 
below the lowest sidedraw tray, preferably at least 2 trays above the 
point of removal. Concomitantly, liquid is removed from the same tray (the 
first tray above the flash zone) and returned to the flash zone or to the 
feedstock entry. Ideally, this liquid is simply purged from the line 
containing liquid for return higher in the column. The term "tray", as 
used herein, includes bubble cap, valve type, or perforated plate trays, 
or modifications thereof, as well as equivalent structures. Again, the 
term "column" is understood to include one structure, or two or more such 
units in series, the designations of "higher" and "upper" merely referring 
to cooler temperature areas of such unit or units. 
In a preferred embodiment of the invention, the flow of recirculated 
de-entrainment liquid is held constant at a rate that results in good 
vapor/liquid contacting on the trays in the circulation loop, the purge 
liquid flow is controlled in response to the level of liquid on the tray 
just above the flash zone of the column, and the supply of liquid to the 
circulation loop is controlled to provide a sufficient purge flow to 
maintain the concentration of de-entrained black oil at an acceptable 
level.

DETAILED DESCRIPTION OF THE INVENTION 
In order to describe the invention with greater detail, reference is made 
to the drawing. FIGS. 1 and 2 represent schematically the preferred manner 
of operation of the invention and a preferred manner of control of the 
liquid flows involved, respectively. 
More particularly, preheated crude oil enters fractional distillation 
column (1) via line (2) where it is flashed in flash zone (3) to produce a 
vapor which is fractionated in the upper portion of the column. As shown, 
the trays below the lowest sidedraw tray (4) function as the 
de-entrainment section. In accordance with the invention, a withdrawal 
line (5) is provided at the first tray (6) above the flash zone (3). 
Liquid from tray (6) is drawn off in line 5 through pump 7 and returned to 
column 1 via line 8 to a location three trays higher in the column. The 
circulation rate is adjusted to provide adequate tray liquid for effective 
contacting and de-entrainment. At the same time, a small purge stream is 
taken from line 8 via line 9 to remove de-entrained black oil. The purge 
may be returned to the flash zone, as shown, or sent to the crude feed 
heater (dotted line). The upper two or more de-entrainment section trays 
are not in the circulating oil loop, and they function to remove 
entrainment from the circulating oil, which contains some black oil. 
(There is normally a small amount of liquid entrainment between trays in a 
distillation column.) Liquid flow on these trays is adequate for good tray 
action without recirculation. 
Utilization of the invention has a number of distinct advantages. To begin 
with, distillate yield is increased. In the embodiment shown, the 
circulation loop purge is only 10-20% of the liquid flow required from the 
bottom tray of the de-entrainment section not having a circulation loop. 
The difference in these two flows is the increase in distillate yield 
(decrease in overflash) due to the circulation loop. As a result of the 
invention, distillate quality is improved. The lowest sidedraw product 
will contain less heavy ends (entrained black oil) by employing the 
circulation loop of the invention. Moreover, coke formation should be 
reduced in the de-entrainment section. There is a tendency for coke 
formation to occur on hot metal surfaces in the de-entrainment section, 
especially at the lower (hotter) trays in the section. The high liquid 
flows possible with a circulation loop reduce coke formation by more 
effective washing of the metal surfaces. 
The invention provides improved thermal efficiency. The higher distillate 
yield means, conversely, a lower straight run residue yield. If the 
residue is further distilled in a vacuum flasher, as is the usual case, 
less flasher heater fuel will be required. Finally, if the straight run 
residue goes to a vacuum flasher, its operation will be improved in 
addition to the fuel saving mentioned since it will have a lower feed 
rate, less light ends in the feed, higher temperature of the topmost heat 
removal stream so heat recovery potential is improved, improved vacuum 
system operation, and improved flash distillate yield in flashers 
operating at a capacity constraint. 
The preferred instrumentation for the circulating oil system is shown in 
FIG. 2. Similar numbers refer to corresponding elements. 
In this discussion, it is assumed that operation of column 1, generally, is 
similar to that described above in relation to FIG. 1. To provide 
appropriate control, two flow rate controllers and a level control (or 
their computer analogues) are used. As shown, a sensor in line (8), in 
response to the flow rate in line (8), produces a signal which is compared 
with a pre-determined value in flow rate controller (10), and flow rate 
controller (10) operates valve (11) to maintain a desired rate of flow to 
column (1). 
Simultaneously, since it similarly desirable to maintain a steady rate of 
flow in line 9, a sensor-flow rate controller-valve combination may be 
employed. Preferably, however, the flow of liquid to flash zone (3) is 
controlled in response to the liquid inventory on tray (6). More 
particularly, liquid level controller (12), in response to the level on 
tray (6), regulates valve (13), and thus the rate of flow to flash zone 
(3). The purge flow is controlled by adjusting the amount of liquid 
supplied to the de-entrainment zone, i.e., the liquid overflowing draw 
tray (4). Thus, in response to the rate of flow in line (9), flow rate 
controller (14) regulates valve (15), increasing or decreasing the flow of 
sidedraw liquid, and thereby decreasing or increasing the amount of liquid 
overflowing tray (4) into the de-entrainment zone. 
As is apparent, while the invention has been illustrated with respect to 
particular apparatus, those skilled in the art will appreciate that other 
equivalent or analogous structures may be employed. Again, all pumps, 
valves, entry and exit lines, etc., have not been illustrated, as such 
expedients can readily be supplied by the skill of the art.