Delayed coking process with split fresh feed

A delayed coking process is provided in which the fresh hydrocarbonaceous oil feed is divided into at least two streams. One stream is introduced directly into the preheating zone of the coking zone and one stream is introduced into the coker product fractionator. The fractionator bottoms fraction is recycled to the preheating zone. The given fresh feed splitting configuration permits decreasing the recycle rate of the heavy coker product and increasing liquid yield.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an improvement in a delayed coking process. 
2. Description of the Prior Art 
Delayed coking is a well-known process in which a hydrocarbonaceous oil is 
heated to a coking temperature and the preheated oil is introduced into a 
coking drum to produce a vapor phase product, including normally liquid 
hydrocarbons and coke. The drum is decoked by hydraulic means or by 
mechanical means. In most configurations of the delayed coking process, 
the fresh hydrocarbonaceous coker feed is introduced into the coker 
product fractionator, usually for heat exchange purposes, where it 
combines with the heavy coker products that are recycled to the coker 
heater. See Hydrocarbon Processing, September, 1980, pages 153. 
It is known that decreasing the recycle ratio of the fractionator bottoms 
fraction that is recycled to the coking preheater will increase the 
hydrocarbon liquid yield and decrease the coke yield of the delayed coker. 
See, for example, "Delayed Coking. Latest Trends" in Hydrocarbon 
Processing, May 1982, pages 99 to 104, where the effect of recycle ratio 
to coke yield is shown. As recycle decreases, the cut point of the recycle 
increases. 
All boiling points referred to herein are atmospheric pressure boiling 
points unless otherwise specified. 
The effect of recycle ratio of heavy coker hydrocarbon product and cut 
point of the fractionator bottoms recycle on coker product yields is shown 
in Table I, which summarizes the results of delayed coking run A and 
delayed coking run B, in which the same feed, namely, a vacuum residuum 
was utilized. 
TABLE I 
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Effect of Cut Point of Bottoms Recycle 
on Yields in Delayed Coking 
Run A B 
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Feed - Vacuum Residuum 
Gravity, .degree.API at 60.degree. F. 
8.6 7.4 
Conradson Carbon wt. % 
17.5 18.3 
Sulfur, wt. % 3.2 3.1 
Ash, wt. % 0.035 0.019 
Operating Conditions 
Coil outlet 930 
temperature .degree.F. 
Coke drum pressure, 33 
psig 
.sup.(1) Recycle, wt. % on 
14.8 9.8 
fresh feed 
Cut point between Heavy 
848 895 
Gas Oil and Recycle, .degree.F. 
Yields on Fresh Feed 
C.sub.1 -C.sub.2 gas, wt. % 
4.92 4.56 
C.sub.3 -C.sub.4 gas, vol. % 
8.31 8.20 
C.sub.5 -cut point 70.91 72.18 
liquids, vol. % 
Coke, wt. % 31.7 31.0 
Coke (corrected, 31.7 29.6 
wt. %.sup.(2)) 
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.sup.(1) Recycled heavy coker product. 
.sup.(2) Coke yield on run B corrected to run A Conradson carbon (i.e., 
17.5 wt. %) using the average carbon producing factor, that is, coke yiel 
to feed Conradson Carbon Residue. 
In run A, the cut point of the fractionator bottoms recycle was 848.degree. 
F. In run B, the cut point was 895.degree. F. As can be seen from table I, 
run B produced 29.6 weight percent coke, whereas run A produced 31.7 
weight percent coke. Thus, when it is desired to minimize coke production 
in delayed coking and increase the hydrocarbon liquid yield, it is 
desirable to increase the cut point of the fractionator bottoms recycle 
stream. 
U.S. Pat. No. 2,159,502 discloses a coking process in which a portion of 
the coking feed is sent to a product fractionator and a portion is sent 
directly to a coke chamber. 
U.S. Pat. No. 4,066,532 discloses a coker feedstock introduced directly 
into a furnace in an admixture with product fractionator bottoms. 
It has now been found that the cut point of the fractionator bottoms 
fraction that is withdrawn from the fractionator and recycled to the coker 
preheating zone can be increased when the fresh oil coker feed is split 
into at least two streams and that these streams are introduced at 
specified locations in the process. 
SUMMARY OF THE INVENTION 
In accordance with the invention, there is provided, in a delayed coking 
process which comprises the steps of: 
(a) preheating a hydrocarbonaceous oil chargestock to a coking temperature 
in a preheating zone; 
(b) introducing the resulting preheated oil chargestock into a coking zone 
at delayed coking conditions to form coke and a vapor phase product, 
including heavy and light hydrocarbon products; 
(c) introducing said vapor phase product and a fresh hydrocarbonaceous oil 
into a separation zone; 
(d) withdrawing a heavy bottoms fraction, including at least a portion of 
said heavy hydrocarbon products, from said separation zone, and 
(e) recycling at least a portion of said withdrawn bottoms fraction to said 
preheating zone of step (a), 
the improvement which comprises: 
introducing a first portion of a fresh hydrocarbonaceous oil directly into 
said preheating zone of step (a), and introducing a second portion of said 
fresh hydrocarbonaceous oil into said separation zone of step (c).

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to the FIGURE, a fresh hydrocarbonaceous oil feed carried in line 
10 is split into a first portion and a second portion. The first portion 
of the fresh oil feed is removed from line 10 by line 13 and passed by 
line 15 directly into coil 12 of coking heater 14. The second portion of 
the fresh oil feed is passed by line 11 into separation zone 28 and 
processed as will be described later. The fresh oil feed of line 10 is 
desirably split such that at least about 20 weight percent, generally from 
about 20 to 80 weight percent, preferably from about 20 to about 50 weight 
percent, based on total fresh oil feed to the entire process, is 
introduced directly into coil 12 of coking heater 14. Suitable fresh 
hydrocarbonaceous oil feeds include heavy hydrocarbonaceous oils; whole 
and reduced petroleum crude oils, including heavy crude oils; petroleum 
atmospheric distillation bottoms; petroleum vacuum distillation bottoms; 
pitch; asphalt; bitumen; other heavy hydrocarbon residues; tar sand oils; 
shale oil; liquid products derived from coal liquefaction processes, 
including coal liquefaction bottoms and mixtures thereof. Typically, such 
feeds have a Conradson carbon content of at least about 5 weight percent, 
generally from about 5 to about 50 weight percent, preferably above about 
7 weight percent (as to Conradson carbon residue, see ASTM test D-189-55). 
These oils usually have a high metals content (vanadium, iron and nickel). 
The metals content may range up to 2000 wppm metal or more. The oil is 
preheated in heater 14 to a coking temperature ranging from about 
775.degree. to about 1000.degree. F., preferably from about 875.degree. to 
about 950.degree. F. The coil outlet pressure will range suitably from 
about 10 to about 200 psig, preferably from about 50 to about 100 psig. In 
preheater 14, the oil is partially vaporized and mildly cracked. The 
preheated oil (vapor-liquid mixture) is removed from heater 14 and passed 
by line 16 into one of two coking drums, 18 and 20 (i.e., delayed coking 
zone) connected to coking heater 14. When one drum is in use, the other 
drum is being decoked. The coking drum operates at a lower temperature 
than the heater coil outlet temperature since the coking reaction is 
endothermic. The pressure in the coking drums suitably ranges from about 
20 to about 60 psig. The residence time in the coking drum is generally 
from about a half hour to about 36 hours, that is, a time sufficient to 
fill the drum with coke. The vapor phase overhead product of the coking 
drum, which includes light and heavy normally liquid hydrocarbons, is 
removed from the respective coking drums by lines 22 and 24 and passed by 
line 26 to a separation zone, such as fractionator 28, where the coker 
overhead product is separated into fractions. The second portion of fresh 
oil feed that was removed from line 10 by line 11 is introduced into 
fractionator 28 wherein it mixes with the coker vapor phase product and 
quenches the vapor. A gas is removed from fractionator 28 by line 30. A 
light fraction is removed by line 32 and an intermediate boiling fraction 
is removed by line 34 from fractionator 28. The heavier bottoms fraction, 
which includes fresh oil and a portion of the heavy hydrocarbons that were 
separated from the coker vapor product, is removed from fractionator 28 by 
line 36. The initial boiling point of the heavy bottoms fraction withdrawn 
by line 36, (i.e., fractionator bottoms fraction) will range from about 
950.degree. to about 850.degree. F., preferably from about 950.degree. to 
about 890.degree. F. At least a portion of the heavy bottoms fraction 
withdrawn by line 36 is recycled to fresh feed line 13 for introduction 
via line 15 into coil 12 of heater 14. The recycle bottoms fraction could 
be introduced separately into coil 12 instead of being mixed with the 
fresh oil feed portion of line 13. The ratio of heavy coker product 
recycled to heating coil 12 will range, suitably, from about 1 to 15 
weight percent, preferably from about 1 to 10 weight percent, based on 
total fresh feed. 
For example, if stream 10 were 30,000 barrels per day and stream 15 were 
33,000 barrels per day, the recycle rate would be 10% on total fresh feed. 
Splitting of the fresh oil feed into a portion that is introduced directly 
into the preheating zone of the coker and another portion that is 
introduced into the coker product fractionator permits decreasing the 
recycle ratio and, thereby, increasing the cut point of the heavy bottoms 
fraction which is removed from the fractionator.