A process and system for vaporizing heavy oil prior to thermal cracking to olefins, by flashing with steam in first mixer, superheating the vapor, and flashing in second mixer the liquid from first mixer.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to pre-heating hydrocarbon feedstock materials prior 
to cracking the feedstock. More specifically, the invention relates to the 
vaporization of heavy oils prior to pyrolysis cracking. 
2. Description of the Prior Art 
Production of olefins and, in particular, ethylene has been achieved to a 
great extent by thermally cracking hydrocarbon feedstock and rapidly 
quenching the cracked effluent to selectively produce the desirable 
olefins. Feedstock such as ethane, naphtha and similar materials have 
provided very desirable hydrocarbon feedstock for cracking to produce 
olefins . 
Recently, the use of heavy oils, such as the vacuum tower distillates from 
crude oils and hydrocracker feedstocks have been used to produce olefins 
by thermal cracking or pyrolysis. Several problems attend the cracking of 
heavy oils to produce olefins. Regardless of the system used, whether it 
be pyrolysis performed in a tubular furnace or in a reactor tube by the 
use of heated particulate solids to provide the heat necessary for the 
endothermic reaction, the pretreatment of heavy oils has become difficult 
and expensive. 
Therefore, it is desirable and, in fact, necessary as a practical matter to 
preheat the heavy oil or any liquid hydrocarbon feedstock to a reaction 
inlet temperature of 1,100.degree. F. Typically, steam is used as a 
diluent and to provide the heat necessary to vaporize the feedstock to 
achieve the proper inlet reaction temperature and phase condition. At 
present, preheating of the hydrocarbon feedstock is achieved by flashing 
the hydrocarbon feedstock with steam at a relatively high temperature; 
i.e. 1,400.degree. F. for heavy feed (1,053.degree. F. for typical light 
feeds). Conventionally, the hydrocarbon liquid is first heated in indirect 
heat exchange relationship to about 440.degree. to 500.degree. F. The 
heated liquid is then mixed with the superheated steam and flashed to 
1,100.degree. F. from the vaporization mix temperature of 720.degree. F. 
This flashing is necessary since liquid hydrocarbon can not be totally 
vaporized by indirect exchange without incurring significant fouling of 
the equipment. In the case of heavy oils, the steam to hydrocarbon ratio 
is necessarily about 1:1. Because of the nature of the heavy oil 
feedstock, a relatively high temperature (1,400.degree. F.) and a high 
steam to hydrocarbon ratio of 1:1 is required to provide the necessary 
vaporization heat. 
SUMMARY OF THE INVENTION 
It is an object of this invention to provide a system for preheating and 
vaporizing heavy oils for use as feedstock to produce olefins. 
It is another object of the present invention to provide a preheat and 
vaporization system for heavy oil which requires considerably less heat, 
energy and steam to achieve the reaction inlet temperature of 
1,100.degree. F. 
Thus, the system and process of the present invention includes a section 
for initially heating and heavy oil to temperatures of about 440.degree. 
to 700.degree. F. Thereafter, the heated heavy oil is delivered to a 
mixing section wherein superheated dilution steam is mixed with the heavy 
oil and the heavy oil-steam mixture is flashed. The flashed mixture is 
then delivered to a separator wherein the overhead is separated from the 
liquid bottoms. The overhead contains all of the steam and a portion of 
the hydrocarbon. The equilibrium temperature in a separator is generally 
the range of 700.degree. to 800.degree. F. As such, the portion of the 
hydrocarbon flashed and taken with the steam overhead ranges up to 60 
percent of the total hydrocarbon. The bottoms taken from the separator are 
then delivered to a mixer. The overhead is heated in a steam superheater 
to a temperature to about 1,100.degree. to 1,150.degree. F., the 
criticality being that the temperature must not reach cracking temperature 
since the gas phase passing through the superheater contains considerable 
hydrocarbon. The 1,100.degree. F. steam-hydrocarbon mixture and the 
700.degree. to 800.degree. F. liquid bottoms from the separator are mixed 
and flashed to provide a composite mixture of hydrocarbon and steam at 
about 1,000.degree. F. The 1,000.degree. F. mixture is delivered to the 
reactor or first passed through a knockout drum to remove any of the 
entrained liquid.

DESCRIPTION OF THE DRAWING 
The drawing depicts the system provided to vaporize heavy oil. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
The system 2 and process of the invention as shown in the drawing depicts 
the various stages at which the hydrocarbon and steam are mixed and heated 
to effect a resultant vaporized hydrocarbon feed suitable for delivery to 
a hydrocarbon cracking unit. 
The system 2 is comprised of essentially a liquid feed heater 4, a mixer 8 
for flashing steam and the heated feedstock, a separator 10 to separate 
the flashed gas and liquid, a vapor feed superheater 12, and a second 
mixer 14 for flashing. The system also preferentially includes a knock-out 
drum 16 for the preheated vapor. 
The liquid feed heater 4 is provided for heating the hydrocarbon feedstock 
such as desulfurized Kuwait HGO to initially elevate the temperature of 
the feedstock. 
The initial mixer 8 is used in the system 2 to initially flash superheated 
steam from a steam line 6 and the heated feedstock delivered from the 
liquid feed heater 4 by a line 18. 
The system separator 10 is to separate the liquid and vapor produced by 
flashing in the mixer 8. Separated gas is discharged through a line 22 
from the separator overhead and the remaining liquid is discharged through 
a line 26. 
A vapor feed superheater 12 heats the gaseous overhead from the line 22 to 
a high temperature and discharges the heated vapor through a line 24. 
The second mixer 14 is provided to flash the vaporized gaseous discharge 
from the vapor feed superheater 12 and the liquid bottoms from the 
separator 10, thereby vaporizing the composite steam and feed initially 
delivered to the system 2. 
A knock-out drum 16 is employed to remove any liquid from the flashed vapor 
discharged from the second mixer 14 through the line 28. The liquid-free 
vapor is delivered to a reactor through the line 30. 
In the process, the heavy oil liquid hydrocarbon feedstock is first heated 
in the liquid feed heater 4 to a temperature of about 440.degree. to 
700.degree. F. The heated heavy oil hydrocarbon feedstock is then 
delivered through the line 18 to the mixer 8. Superheated steam from the 
line 6 is mixed with the heated heavy oil hydrocarbon feedstock in the 
mixer 8 and the steam-heavy oil mixture is flashed to about 700.degree. to 
800.degree. F. For lighter feedstock the flashing temperature will be 
about 500.degree. to 600.degree. F., and for heavier feedstock the 
flashing temperature will be about 700.degree. to 900.degree. F. 
The flashed mixture of steam and hydrocarbon is sent to the system 
separator 10 wherein the vapor or gas is taken overhead through the line 
22 and the liquid is discharged through the line 26. Both the overhead 
vapor and liquid bottoms are in the temperature range of about 700.degree. 
to 800.degree. F. The temperature level and percent of hydrocarbon 
vaporized are determined within the limits of equipment fouling criteria. 
The vapor stream in the line 22 is comprised of essentially all of the 
steam delivered to the system 2 and a large portion of the heavy oil 
hydrocarbon feedstock. Between 30% and 70% of the heavy oil hydrocarbon 
feedstock supplied to the system will be contained in the overhead leaving 
the separator 10 through the line 22. 
The steam-hydrocarbon vapor in the line 22 is delivered to the system vapor 
feed superheater 12 wherein it is heated to about 1,030.degree. F. The 
heated vapor is taken from the vapor feed superheater 12 through the line 
24 and sent to the second mixer 14. Liquid bottoms from the separator 10 
is also delivered to the second mixer 14 and the vapor-liquid mix is 
flashed in the mixer 14 to a temperature of about 1,000.degree. F. 
The flashed vapor is then sent downstream through the line 28 to the 
knock-out drum 16 for removal of any liquid from the vapor. Finally, the 
vaporized hydrocarbon feed is sent through the line 30 to a reactor. 
An illustration of the system pre-heat process is seen in the following 
example. 
A Nigerian Heavy Gas Oil is preheated and vaporized in the system 2 prior 
to delivery to a reactor. The Nigerian Heavy Gas Oil has the following 
composition and properties: 
______________________________________ 
Elemental 
Analysis, Wt. % 
Properties 
______________________________________ 
Carbon 86.69 Flash Point, F. 230.0 
Hydrogen 
12.69 Viscosity, SUS 210 F. 
44.2 
Sulfur .10 Pour Point, F. +90.0 
Nitrogen 
.047 Carbon Residue, Ramsbottom 
.09 
Nickel .10 Aniline Point, C. 87.0 
Vanadium 
.10 
Distillation 
Vol. % 
IBP 
10 669.2 
30 755.6 
50 820.4 
70 874.4 
90 944.6 
EP 1,005.8 
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3,108 pounds per hour of the Nigerian Heavy Gas Oil is heated to 
750.degree. F. in the liquid feed heater 4 and delivered at a pressure of 
150 psia to the mixer 8. 622 pounds per hour of superheated steam at 
1,100.degree. F. is simultaneously delivered to the mixer 8. The pressure 
in the mixer is 50 psia. 
The superheated steam and Heavy Gas Oil are flashed in the mixer 8 to a 
temperature of 760.degree. F. wherein 60.degree. of the Heavy Gas Oil is 
vaporized. 
The vapor and liquid from the mixer 8 are separated in the separator 10. 
622 pounds per hour of steam and 1,864.8 pounds per hour of hydrocarbon 
are taken in line 22 as overhead vapor. 1,243.2 pounds per hour of 
hydrocarbon are discharged through the line 26 as liquid and sent to the 
mixer 14. 
The mixture of 622 pounds per hour of steam and 1,864.8 pounds per hour of 
hydrocarbon are superheated in the vapor superheater 12 to 1,139.degree. 
F. and delivered through line 24 to the mixer 14. The mixer 14 is 
maintained at 45 psia. 
The 1,243.2 pounds per hour of liquid at 760.degree. F. and the vaporous 
mixture of 622 pounds per hour of steam and 1,864.8 pounds per hour of 
hydrocarbon are flashed in the mixer 14 to 990.degree. F. 
The vaporization of the hydrocarbon is effected with a steam to hydrocarbon 
ratio of 0.2. The heat necessary to vaporize the hydrocarbon and generate 
the necessary steam is 2.924 MM BTU/hr. 
The same 3,108 pounds per hour of Nigerian Heavy Gas Oil feedstock 
vaporized by a conventional flashing operation requires steam in a 1:1 
ratio to maintain a steam temperature of 1,434.degree. F. The composite 
heat to vaporize the hydrocarbon and generate the necessary steam is 6.541 
MM BTU/hr. In order to reduce the input energy in the conventional process 
to the same level as the present invention, a steam temperature of 
3,208.degree. F. is required, which temperature is effectively beyond 
design limitations.