Process for improving product yields from delayed coking

A delayed coking process in which the coker furnace feed is free of conventional heavy recycle. Elimination of this material from the coker furnace feed produces, based on fresh feed to the process, increased liquids and decreased coke. Coker furnace feed is initially combined with a diluent hydrocarbon having a lower boiling range than conventional heavy coker recycle and then transferred to the coker furnace. The hydrocarbon diluent is much lower in coke-forming components than the heavy recycle which is normally combined with the fresh feed and fed to the coker furnace.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to delayed coking, and more particularly to a method 
of improving the product yields from a delayed coking operation. 
Delayed coking has been practiced for many years. The process broadly 
involves thermal decomposition of heavy liquid hydrocarbons to produce 
gas, liquid streams of various boiling ranges, and coke. 
Coking of resids from heavy, sour (high sulfur) crude oils is carried out 
primarily as a means of disposing of low value resids by converting part 
of the resids to more valuable liquid and gas products. The resulting coke 
is generally treated as a low value by-product. 
In the production of fuel grade delayed coke, and even to some extent in 
the production of anode or aluminum grade delayed coke, it is desirable to 
minimize the coke yield, and to maximize the liquids yield, as the liquids 
are more valuable than the coke. It is also desirable to produce a coke 
having a volatile matter content of not more than about 15 percent by 
weight, and preferably in the range of 6 to 12 percent by weight. 
The use of heavy crude oils having high metals and sulfur content is 
increasing in many refineries, and delayed coking operations are of 
increasing importance to refiners. The increasing concern for minimizing 
air pollution is a further incentive for treating resids in a delayed 
coker, as the coker produces gases and liquids having sulfur in a form 
that can be relatively easily removed. 
2. The Prior Art 
In the basic delayed coking process as practiced today, fresh feedstock is 
introduced into the lower part of a coker fractionator and the 
fractionator bottoms including heavy recycle material and fresh feedstock 
are heated to coking temperature in a coker furnace. The hot feed then 
goes to a coke drum maintained at coking conditions of temperature and 
pressure where the feed decomposes or cracks to form coke and volatile 
components. The volatile components are recovered as coker vapor and 
returned to the fractionator. Heavy gas oil from the fractionator is added 
to the flash zone of the fractionator to condense the heaviest components 
from the coker vapors. The heaviest fraction of the coke drum vapors could 
be condensed by other techniques, such as heat exchange, but in commercial 
operations it is common to contact the incoming vapors with a heavy gas 
oil in the coker fractionator. Conventional heavy recycle is comprised of 
condensed coke drum vapors and unflashed heavy gas oil. When the coke drum 
is full of coke, the feed is switched to another drum, and the full drum 
is cooled and emptied by conventional methods. 
The delayed coking process is discussed in an article by Kasch et al 
entitled "Delayed Coking," The Oil and Gas Journal, Jan. 2, 1956, pp 
89-90. 
A delayed coking process for coal tar pitches illustrating use of heavy 
recycle is shown in U.S. Pat. No. 3,563,884 to Bloomer et al. 
A delayed coking process for coal extract using a separate surge tank for 
the feed to the coker furnace is shown in U.S. Pat. No. 3,379,638 to 
Bloomer et al. 
A process for producing a soft synthetic coal having a volatile matter 
content of more than 20 percent by weight is described in U.S. Pat. No. 
4,036,736 to Ozaki et al. In that reference, a diluent gas is added to the 
coker drum to maintain a reduced partial pressure of cracked hydrocarbons, 
or the process is carried out under less than atmospheric pressure. 
A discussion of early delayed coking processes appears in an article by 
Armistead entitled "The Coking of Hydrocarbon Oils," The Oil and Gas 
Journal, Mar. 16, 1946, pp 103-111. 
U.S. Pat. No. 4,216,074 describes a dual coking process for coal 
liquefaction products wherein condensed liquids from the coke vapor stream 
and unflashed heavy gas oil are used as recycle liquid to the coker 
furnace. 
U.S. Pat. No. 4,177,133 describes a coking process in which the heavier 
material from the coke drum vapor line is combined with fresh coker feed 
as recycle and then passed to a coke drum. 
Many additional references, of which U.S. Pat. Nos. 2,380,713; 3,116,231 
and 3,472,761 are exemplary, disclose variations and modifications of the 
basic delayed coking process. 
In commonly assigned copending application Ser. No. 464,181, filed Feb. 9, 
1983 now U.S. Pat. No. 4,455,219, a delayed coking process is described in 
which a diluent hydrocarbon having a boiling range lower than the boiling 
range of heavy recycle is substituted for a part of the heavy recycle that 
is normally combined with the fresh feed in delayed coking processes. 
SUMMARY OF THE INVENTION 
According to the present invention, the feed to a coker furnace is 
essentially free of unflashed heavy coker gas oil and condensed material 
from the coke drum vapors. This is accomplished by removing from the 
process unflashed heavy coker gas oil and condensed material from coke 
drum vapors, rather than combining them with fresh coker feed as is 
conventionally done. 
A hydrocarbon diluent having a boiling range lower than that of 
conventional heavy coker recycle, and having a lower amount of 
coke-forming components than heavy coker recycle does, is combined with 
the fresh feed in an amount sufficient to effectively prevent coke 
formation in the furnace tubes. The amount of diluent needed depends on 
the quality of the feedstock, furnace temperature, furnace design and 
other factors. 
Normally, the coker feedstock is fed to the bottom of the coker 
fractionator where it inherently mixes with unflashed heavy coker gas oil 
and condensed material from the coker vapor stream. The process described 
in the aforementioned U.S. Pat. No. 4,455,219 is directed to minimizing 
the amount of heavy recycle which is combined with the fresh feed. The 
present invention is directed to the total elimination of heavy recycle 
from the coker feedstock. 
It is an object of the present invention to improve the product yields from 
a delayed coking operation. 
It is a further object to eliminate unflashed heavy coker gas oil and 
condensed coker vapors from the feed to a coker furnace. 
It is still a further object to substitute a lower boiling distillate 
hydrocarbon diluent, which is low in coke-forming components, for heavy 
recycle which is relatively much higher in coke-forming components, as 
part of the feed to a coker furnace.

DETAILED DESCRIPTION OF THE PRIOR ART PROCESS 
A conventional prior art delayed coking process is illustrated in FIG. 1. 
In that process, fresh coker feed from line 10 is preheated in heat 
exchangers 12 and then fed to the bottom of coker fractionater 14. Heavy 
coker gas oil from draw pan 16 is pumped through heat exchangers 12 and 
steam generator 18. Part of the heavy coker gas oil from steam generator 
18 is recovered as a product through line 20, part of it is passed via 
line 21 to the vapor outlets of coke drums 32 where it is used to quench 
coke drum vapors, part of it is returned via line 22 to spray nozzles 24 
in the flash zone of fractionator 14 and the remainder is returned to the 
fractionator through line 23 as internal reflux. In many coker 
fractionators, a series of baffles, sometimes referred to as a "shed 
deck," is utilized in place of spray nozzles to effect contact between gas 
oil and incoming vapors. Trays or other means may be used for this 
purpose. Heavy gas oil added to a shed deck or trays performs the function 
as the spray oil referred to herein. Coke drum vapors from line 26 enter 
the flash zone of fractionator 14 below spray nozzles 24, and the heaviest 
components in the incoming vapors are condensed by contact with heavy 
coker gas oil from spray nozzles 24. The condensed material falls into the 
bottom of the flash zone where it combines with the incoming fresh feed. 
Any heavy coker gas oil from spray nozzles 24 which is not vaporized in 
the flash zone also combines with the fresh feed in the bottom of the 
flash zone. 
The combined fresh feed, condensed vapors and unflashed heavy gas oil is 
withdrawn through line 28 and pumped to coker furnace 30 where it is 
heated to coking temperature and then passed to one of the coke drums 32. 
As is conventional, one coke drum is filled while the other is cooled and 
emptied, and when the drum being filled is full of coke the heated feed is 
switched to the empty drum. Vapors from either drum 32 pass through vapor 
line 26 to fractionator 14. A small amount of heavy coker gas oil from 
line 21 is added to the vapor exiting drum 32 to quench the vapors and 
prevent coke deposition in line 26. 
Lighter material from line 26 passes up through fractionator 14, and gases 
and naphtha exit through line 34. Naphtha is condensed out in receiver 36 
and recovered from line 38. A part of the naphtha may be refluxed back 
though line 40. Coker gases are recovered as product through line 42. An 
intermediate distillate is removed via line 44, steam stripped in stripper 
46, and recovered through distillate product line 48. 
In the design and operation of a delayed coker, the furnace is the most 
critical piece of equipment. The furnace must be able to heat the 
feedstock to coking temperatures without causing coke formation on the 
furnace tubes. When the furnace tubes become coked, the operation must be 
shut down and the furnace cleaned out. In some cases, steam is injected 
into the furnace tubes to increase the tube velocity and to create 
turbulence as a means of retarding coke deposits. However, steam injection 
is not energy efficient and can adversely affect coke quality, and 
therefore is preferably minimized. It is, however, important to have steam 
injection capability to blow out the furnace tubes in the event of furnace 
fuel pump failure. Properly designed and operated coker furnaces can now 
operate for many months without being shut down for tube cleanout. 
It is conventional in the production of fuel grade or anode grade coke to 
recycle from about 0.05 to about 0.7 volumes of heavy recycle material for 
each volume of fresh coker feed. This recycle material improves the coker 
furnace operation and also provides a solvent effect which aids in 
preventing coke deposits on the furnace tubes. Conventional heavy recycle 
material, as mentioned previously, is a combination of condensed material 
from the coke drum vapor line and unflashed heavy coker gas oil, generally 
having a boiling rage of from about 750.degree. to 950.degree. F. or 
higher, although small amounts of components boiling below 750.degree. F. 
may be present. The operation of a coker as described above, where 
condensed vapors and unflashed heavy gas oil are combined with fresh feed 
in the bottom of the coker fractionator, inherently results in at least a 
minimum amount of heavy recycle material being combined with the fresh 
feed. This minimum amount is about 0.05 volumes of recycle for each volume 
of fresh feed. 
In cases where the feedstock is of lower quality, such as a very low 
gravity resid, it may be necessary to have as much as 0.3 to 0.7 volumes 
of recycle for each volume of fresh feed in order to prevent coke 
formation in the furnace. The use of these higher recycle rates is 
undesirable in that it affects the production capacity of the coker, and 
more importantly, it increases the coke yield measured as a percentage of 
the fresh feed. The increase in the coke yield from using high recycle 
rates of heavy recycle material is a result of coke formation from the 
recycle material itself. This is undesirable because the coke is the least 
valuable product from the coking operation. 
The process described in U.S. Pat. No. 4,455,219 mentioned previously 
represents an improvement wherein the amount of heavy recycle used is 
minimized, and a lighter distillate material is added to the fresh feed to 
provide part of the necessary diluent to prevent coking in the furnace 
tubes. This process is represented in FIG. 1 where distillate from line 48 
is withdrawn and passed through line 50 to be combined with fresh feed 
before it is preheated. That process is particularly useful when the coker 
feedstock is such that more than about 0.05 volumes of recycle per volume 
of fresh feed is required for proper furnace operation. 
DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention is an improvement over the prior art processes 
described above in that it entirely eliminates the use of heavy recycle 
material in the production of fuel grade or anode grade coke, thus 
resulting in improved product yields including a reduced coke yield and an 
increased liquids yield. As pointed out previously, the preferred product 
yields include the lowest possible amount of coke, as the other products 
from a coking operation are of greater value than the coke. 
The preferred embodiment of the invention is illustrated in FIG. 2, where 
like numbers are used for those items which are common in FIG. 1. The main 
difference between the preferred embodiment of the invention and the prior 
art is the total elimination of heavy recycle material from the feedstock, 
even in those cases where a high amount of diluent is necessary to provide 
furnace operation. 
The elimination of heavy recycle material from the feed is accomplished by 
routing fresh coker feed to a feed surge drum (FIG. 2) instead of to the 
bottom of fractionator 14 as is done in prior art processes (FIG. 1). 
Fresh feed from surge drum 60 is then passed directly, without any 
addition of heavy recycle, to coker furnace 30. In lieu of the heavy 
recycle normally used to prevent coke deposition in the furnace tubes, an 
amount of coker distillate sufficient to effectively prevent coke 
deposition on the furnace tubes is added to the fresh feed via line 50 
before it is passed to the coker furnace. 
In the embodiment of this invention illustrated in FIG. 2, heavy gas oil is 
added to the flash zone of fractionator 14 to condense heavy coke drum 
vapors and to clean up the material entering the flash zone from vapor 
line 26. However, condensed coke drum vapors and unflashed heavy gas oil 
from the bottom of fractionator 14 are removed from the process via line 
64, and do not contribute to the overall coke yield as they would in the 
prior art processes. The material from the bottom of fractionator 14 may 
be passed to a vacuum distillation unit where the distillable portion 
thereof is recovered as overhead, or the material may be hydrodesulfurized 
and/or used as feed to another refinery unit such as a fluidized bed 
catalytic cracking unit. 
In the most preferred embodiment of the invention, the heavy recycle is 
replaced by a distillate material from the coker fractionator. This 
preferred distillate recycle material has a boiling range lower than that 
of heavy recycle, and most preferably is taken from distillate product 
line 48 through distillate recycle line 50 and combined with fresh feed in 
line 10. 
The distillate recycle or diluent in accordance with the invention should 
be a hydrocarbon material having a boiling range of from about 335.degree. 
to about 850.degree. F., preferably from about 450.degree. to about 
750.degree. F., and most preferably from about 510.degree. to about 
650.degree. F. Generally the diluent will come from the coker 
fractionator, but diluents from other sources might be used in special 
instances. 
The amount of diluent required is that amount needed to provide good 
furnace operation. This amount may be as much as 0.7 volumes diluent per 
volume fresh feed for those feeds which have a very high tendency to coke 
up on the furnace tubes. This amount is also a function of furnace design 
and furnace operating conditions, and generally must be determined for 
each feedstock and each coker furnace. The preferred amount of diluent is 
the minimum amount which enaables operation without significant furnace 
tube coking. Use of more than the minimum amount which prevents 
significant furnace tube coking is not particularly bad, but may affect 
capacity and efficiency of the operation. 
Suitable feedstocks for the process of the invention include any 
conventional delayed coking feedstock. The most common feedstock for fuel 
grade or anode grade coke is petroleum residuum. Usually the residuum is a 
vacuum resid from a crude oil vacuum distillation unit, but occasionally 
an atmospheric resid from a crude oil atmospheric distillation unit is 
used. In some instances feedstocks other than petroleum residuum are 
coked. These feedstocks include, but are not limited to, coal tar pitch, 
tar sands bitumen, pyrolysis tar, slurry oil or decant oil from a fluid 
bed cracking unit, and shale oil. Mixtures of any of the above may also be 
used. 
The coking operating conditions applicable to the process of the invention 
are those conditions which provide a product coke having a volatile matter 
content of not more than about 15 percent by weight, and preferably from 6 
to 12 percent by weight. Such conditions, as is known in the art, include 
coker furnace outlet temperatures of from about 875.degree. to 950.degree. 
F., preferably 925.degree. to 930.degree. F., coke drum outlet vapor 
temperatures of 775.degree. to 850.degree. F., preferably about 
835.degree. F., and coker drum pressures of from 5 to 75 psig, preferably 
about 15 to 20 psig. 
The use of subatmospheric coker drum pressure is not acceptable for several 
reasons. The economics of the process deteriorate rapidly as coker drum 
pressures approach atmospheric, and operation of a coker drum at 
subatmospheric pressure is very hazardous due to the likelihood of oxygen 
(air) leakage into the drum which contains hydrocarbons at +900.degree. F. 
temperatures. Also, as pointed out in the Ozaki et al reference discussed 
previously, the use of atmospheric or subatmospheric coker drum pressures 
produces a product which is more in the nature of a pitch than a coke. For 
example, all of the examples in the Ozaki et al reference, carried out at 
atmospheric or subatmospheric drum pressure, produced a soft pitch type 
product having a volatile matter content of well above 20 percent by 
weight. The coke product from the present invention has a volatile content 
of not more than about 15 percent by weight, preferably 6 to 12 percent by 
weight. 
To illustrate the coke yield potential from combining conventional heavy 
recycle with fresh coker feedstock, the contributions to coke yield from 
various fractions of a heavy coker gas oil were determined. Several 
boiling range fractions of heavy coker gas oil were coked individually, 
and the weight percent coke yield as well as the amount of each fraction 
was determined. The results are shown below: 
TABLE 1 
______________________________________ 
CONTRIBUTIONS OF EACH FRACTION TO THE 
WHOLE FEEDSTOCK COKE YIELD 
Fractional 
(B) Contribution 
(A) Batch to 
Heavy Fraction Coke Entire 
Coker Gas of Entire 
Yield A .times. B, 
Coke 
Oil Fraction 
Feed Wt % Wt % Yield 
______________________________________ 
550-650.degree. F. 
0.103 1.3 0.13 0.8 
650-750.degree. F. 
0.221 4.5 0.99 6.3 
750-850.degree. F. 
0.335 12.8 4.28 27.5 
850.sup.+ .degree. F. 
0.327 31.3 10.2 65.4 
Sum 15.6 100.0 
______________________________________ 
As seen in Table 1, the potential coke yield from heavy coker gas oil is 
significant. It is also apparent that the bulk of the coke from the heavy 
gas oil comes from the highest boiling fraction. It is thus especially 
important to eliminate the heaviest condensible material in the coker 
vapors and the heaviest material in the heavy coker gas oil from the feed 
to the coker furnace. By substituting a distillate hydrocarbon material 
boiling from about 335.degree. to about 850.degree. F. for the heavy 
recycle normally used, the coke yield as a percent of fresh feed is 
significantly reduced, and the more desirable liquid product yield is 
increased. 
Coker fractionators are not intended to make "clean" separations, and heavy 
coker gas oil may contain small amounts of material boiling as low as 
550.degree. F., while coker distillate streams may have small amounts of 
material boiling as high as 750.degree. F., and in some cases possibly as 
high as 850.degree. F. However, the amount of this high boiling material 
in coker distillate (such as from line 44 in FIG. 2) is very low, and the 
contribution to overall coke yield from this small amount of high boiling 
material is not significant. On the other hand, condensed coke drum vapors 
and unflashed heavy coker gas oil are relatively high in +850.degree. F. 
material, and contribute significantly to overall coke yield if they are 
combined with fresh feed as in the prior art process. 
The essence of this invention is the total elimination from coker furnace 
feed of material from the bottom of the flash zone of the coker 
fractionator in a delayed coking operation operated at conditions which 
produce a fuel grade or anode grade delayed coke product having a volatile 
matter content of less than about 15 percent by weight. This is 
accomplished by removing from the process the materials normally combined 
with fresh feed as recycle, and substituting therefor in an amount 
sufficient to effectively prevent coke deposition on the coker furnace 
tubes a hydrocarbon diluent having a boiling range lower than the boiling 
range of conventional heavy recycle. 
Expressed another way, the condensed coke drum vapors which fall to the 
bottom of the flash zone in the fractionator and the unflashed portion of 
the heavy gas oil which is added to the flash zone are collected and 
removed from the process rather than being combined with fresh feed as 
recycle, and a lower boiling hydrocarbon distillate is substituted 
therefor. 
EXAMPLE 
The improved product yields provided by this invention are demonstrated in 
the following simulated example derived from a highly developed coker 
design program. In this example, two runs were made using identical 
feedstocks and coking conditions, except in one case conventional heavy 
recycle (20 parts by volume for each 100 parts by volume fresh feed) was 
used for the recycle, and in the other case a hydrocarbon distillate 
material having a boiling range of from 510.degree. to 650.degree. F. (20 
parts by volume for each 100 parts by volume fresh feed) was used for the 
recycle. 
In both runs, a 1000.degree. F.+ Bachaquero vacuum resid having an API 
gravity of 4.3, a Conradson carbon value of 23.5 weight percent, a UOP 
characterization factor "K" of 11.5 and a sulfur content of 3.5 weight 
percent was coked at a coke drum pressure of 20 psig and a coke drum top 
temperature of 835.degree. F. The product distribution for the two runs is 
tabulated below: 
______________________________________ 
YIELDS - WEIGHT PERCENT 
Conventional 
Heavy Distillate 
Component Recycle Recycle 
______________________________________ 
H.sub.2 S 1.00 1.00 
Hydrogen 0.09 0.09 
Methane 3.65 3.53 
Total C.sub.2 1.32 1.16 
Total C.sub.3 1.58 1.32 
Total C.sub.4 1.71 1.54 
Liquids (C.sub.5 +) 
55.99 58.84 
Green Coke 34.66 32.53 
Green Coke Volatile 
9.8 9.4 
Matter 
______________________________________ 
As seen in the above Table, a reduction in coke yield of over 6 percent 
(34.66 versus 32.53) is obtained when a distillate hydrocarbon having a 
boiling range of 510.degree. to 650.degree. F. is used as recycle in place 
of conventional heavy coker recycle. A corresponding increase of almost 5 
percent in C.sub.5 + liquids is obtained (58.84 versus 55.99). Similar 
decreases in coke yield and increases in liquids yield are obtained with 
different feedstocks at the same or different coking conditions, thereby 
demonstrating the value of removing from the process the material normally 
used as recycle. 
The foregoing description of the preferred embodiments of the invention is 
intended to be illustrative rather than limiting the invention, which is 
defined by the appended claims.