Note: Descriptions are shown in the official language in which they were submitted.
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IMPROVED FLUID COKING PROCESS
FIELD OF THE INVENTION
The present invention relates to an improved coking process
wherein a heavy hydrocarbonaceous chargestock is partially converted, or
cracked in a first conversion zone, at a temperature below the coking
temperature in a fired heater. The partially converted chargestock,
with the conversion products, is then fed to the scrubbing zone of a
fluid coker whereupon the products are stripped off and the unconverted
material is fed to the coking zone.
BACKGROUND_OF THE INVENTION
A substantial amount of work has been done over the years to
convert heavy hydrocarbonaceous materials to more valuable lighter
boiling products. Various thermal processes which have resulted from
such work include visbreaking; hydroconversion, in both a slurry and
ebullating bed; fluid coking; and delayed coking.
Of particular interest in the practice of the present
invention is improved fluid coking. In fluid coking, a heavy
hydrocarbonaceous chargestock, such as a vacuum residuum, is fed to a
coking zone comprised of a fluidized bed of hot solid particles, usually
coke particles, sometimes also referred to as seed coke. The heavy
hydrocarbonaceous material is reacted in the coking zone resulting in
conversion products which include a vapor fraction and coke, which coke
is deposited on the surface of the seed particles. A portion of the
coked-seed particles is sent to a heating zone which is maintained at a
temperature higher than that of the coking zone, wherein some of the
coke is burned off. Hot seed particles from the heating zone are
returned to the coking zone as regenerated seed material which serves as
the primary heat source for the coking zone. In an extension of a fluid
coking process, a portion of hot coke from the heating zone is
circulated back and forth to a gasification zone which is maintained at
a temperature higher than that of the heating zone. In the gasifier,
substantially all of the remaining coke on the seed material is burned,
or gasified, off. Examples of U.S. Patents which teach fluid coking,
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with or without an integrated gasification zone, are U.S. Patent Nos.
3,725,791; 4,203,759; 4,213,848; and 4,269,696; all of which are
incorporated herein by reference.
U.S. Patent No. 2,927,073 to Moser et al. teaches a fluid
coking process wherein the feedstreams is preheated to a temperature
from about 260 to 370C. That is, the feedstream is merely brought to
a relatively high temperature prior to being introduced into the fluid
coking zone. There is no suggestion of a first conversion zone where
only cracking occurs and a second conversion zone where coking occurs.
Notwithstanding any advantages the foregoing processes may have, there
is a need in the art to be able to operate fluid coking to increase the
conversion of the feed to more desirable liquid products at the expense
of less desirable products such as gas and coke.
SUMMARY OF THE INVENTION
In accordance with the present invention, there is provided
a fluid coking process, for converting a heavy hydrocarbonaceous
chargestock to lower boiling products, comprising the steps of:
(a) partially converting a hydrocarbonaceous chargestock
having a Conradson carbon residue of at least 5 wt% in a first
conversion zone at a temperature in the range of 400-500C, but below
the coking temperature of the chargestock;
(b) introducing the partially converted hydrocarbonaceous
chargestock, and any resulting conversion products to a second
conversion zone comprised of at least a coking zone and a scrubbing
zone, the coking zone comprised of a bed of fluidized refractory solids
maintained at fluid conditions, including a temperature from about
450C to about 650C and a total pressure of up to about 150 psig, said
partially converted hydrocarbonaceous chargestock and any conversion
products from said first conversion zone being introduced into said
scrubbing zone wherein there is produced: a vapor phase product,
including normally liquid hydrocarbons; and coke, the coke depositing on
the fluidized solids;
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(c) introducing a portion of said solids, with coke
deposited thereon, into a heating zone comprised of a fluidized bed of
refractory solid particles and operated at a temperature from about 35-
200C higher than said coking zone and in which the fluidizing gas is
sellected from steam and a mixture of steam and an oxygen-containing gas;
(d) recycling a portion of the solids from said heating
zone to said coking zone to maintain the temperature of the coking zone
between about 450C and 650C; and
(e) passing the vapor phase product of step (b) from the
coking zone to a scrubbing zone where a lower boiling fraction is
collected and a higher boiling fraction is recycled to the coking zone.
In a preferred embodiment of the present invention, the
fluidizing gas for the heating zone is only steam and a portion of the
solids from the heating zone is passed to a gasification zone which also
contains a fluidized bed of refrac~ory solid particles and which is
maintained at a temperature greater than that of the heating zone. The
fluidizing gas for the gasification zone is a mixture of steam and an
oxygen-containing gas.
BRIEF DESCRIPTION OF THE nRAWING
The sole figure hereof is a schematic flow plan of a
preferred embodiment of the present invention showing a fired heater for
the first conversion zone, and a fluid coking process unit comprised of
a coking zone, a heating zone, a gasification zone, and a scrubbing zone
for the second conversion zone.
DETAILED DESCRIPTION OF THE INVENTION
Any heavy hydrocarbonaceous material which is typically used
as a feed for fluidized coking can be used in the practice of the
present invention. Generally, the heavy hydrocarbonaceous material will
have a Conradson carbon residue of about 5 to 40 wt.% and be comprised
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of hydrocarbon components, the majority of which boil above about
525C. Suitable hydrocarbonaceous materials include heavy and reduced
petroleum crudes, petroleum atmospheric distillation bottoms, petroleum
vacuum distillation bottoms, pitch, asphalt, bitumen, liquid products
derived from coal liquefaction processes, including coal liquefaction
bottoms, and mixtures thereof.
A typical petroleum chargestock suitable for the practice of
the present invention will have composition and properties within the
ranges set forth below.
Conradson Carbon 5 to 40 wt.%
Sulfur 1.5 to 8 wt.%
Hydrogen 9 to 11 wt.%
Nitrogen 0.2 to 2 wt.%
Carbon 80 to 86 wt.%
Metals 1 to 2000 wppm
Boiling Point 345C+ to 650C+
Specific Gravity -10 to 35 API
The heavy hydrocarbonaceous chargestock is partially
converted in a first conversion zone, by feeding it to a fired heater
having an exit boil temperature high enough so that cracking of the
chargestock occurs~ but not so high that coking occurs. By a
temperature where cracking occurs we mean at least that minimum
temperature wherein the cracking of higher boiling fractions to lower
boiling fractions is significant. Cracking of such feedstocks will
typically start at about 400C. Generally, the temperature, as
measured at the exit coil of a fired heater will be from about 400C to
500C, preferably from about 425C to 500C, more preferably from about
445C to 485C, and most preferably from about 450C to 475C. The
chargestock is converted to the extent that coking does not occur in the
first conversion zone. When the partially converted chargestock, along
with any conversion products, is fed to the scrubbing zone of the fluid
coking unit, the lighter, or cracked products, are flashed and drawn off
overhead thereby preventing the products from undergoing secondary
reactions in the second conversion zone which would lead to destruction
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of the valuable liquid products to gas and coke. The remaining
unconverted chargestock is withdrawn from the scrubbing zone and fed to
the coking zone.
Reference is now made to the Figure hereof, which shows
an integrated fired heater/coking/gasification unit. The fluid coking
reaction vessels shown in the figure are similar to those in a
conventional fluid coking unit except for the gasification vessel. A
heavy hydrocarbonaceous chargestock is passed via line 10 to fired
heater 11 where it is partially converted to lower boiling products to
the extent that coking reactions do not occur in the fired heater. That
is, it undergoes cracking reactions, but not coking. The exit coil of
the fired heater is operated at a temperature as set forth above. The
partially converted chargestock is fed via line 13 to scrubbing zone 25
where the products of conversion are drawn off overhead via line 28. By
producing cracked, but not coked, products in the fired heater, and
drawing them off overhead from the scrubbing zone, serves to prevent
such products from undergoing undesirable secondary reactions. This
preserves valuable gas oil and naphtha from being degraded to gas and
coke. The remaining heavier products are fed via line 26 to coking zone
12, in which is maintained a fluidized bed of refractory solids having
an upper level indicated by 14. Although it is preferred that the
solids, or seed material, be coke particles, they may also be any other
suitable refractory materials. Non-limiting examples of such materials
are those selected from the group consisting of silica, alumina,
zirconia, magnesia, alumdum or mullite, synthetically prepared or
naturally occurring material such as pumice, clay, kieselguhr,
diatomaceous earth, bauxite, and the like. The solids will have an
average particle size of about 40 to 1000 microns, preferably from about
40 to 400 microns. For purpose of the description of the process of the
Figure hereof, the refractory solid particles are coke particles.
A fluidizing gas e.g. steam, is admitted at the base of
coker reactor 1, through line 16, in an amount sufficient to obtain
superficial fluidizing velocity. Such a velocity is typically in the
range of about 0.5 to 5 ft/sec. Coke at a temperature above the coking
temperature, for example, at a temperature from about 35 to 200C,
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preferably from about 65 to 175C, and more preferably about 65C to
120C in excess of the actual operating temperature of the coking zone
is admitted to reactor 1 by line 42 from heating zone 2, in an amount
sufficient to maintain the coking temperature in the range of about
450 to 650C. The pressure in the coking zone is maintained in the
range of about 0 to 1035 kPa, preferably in the range of about 5 to 45
psig. The lower portion of the coking reactor serves as a stripping
zone to remove occluded hydrocarbons from the coke. A strea~ of coke is
withdrawn from the stripping zone by line 18 and circulated to heater 2.
Conversion products from the coker are passed through cyclone 20 to
remove entrained solids which are returned to coking zone through dipleg
22. The vapors leave the cyclone through line 24, and pass into a
scrubber 25 mounted on the coking reactor. A stream of heavy materials
condensed in the scrubbing zone is recycled along with the unconverted
chargestock fed from line 13 to the coking reactor via line 26. The
coker conversion products, as well as the conversion products from the
fired heater, are removed from the scrubbing zone 25 via line 28 for
fractionation in a conventional manner.
In heater 2, stripped coke from coking reactor 1 (cold
coke) is introduced by line 1~ to a fluid bed of hot coke having an
upper level indicated by 30. The bed is partially heated by passing a
fuel gas into the heater by line 32. Supplementary heat is supplied to
the heater by coke circulating from gasifier 3 through line 34. The
gaseous effluent of the heater, including entrained solids, passes
through a cyclone which may be a first cyclone 36 and a second cyclone
38 wherein the separation of the larger entrained solids occur. The
separated larger solids are returned to the heater bed via the
respective cyclone diplegs 39. The heated gaseous effluent which
contains entrained solids is removed from heater 2 via line 40. The
fluidizing gas in the heating zone will be steam. In a fluid coking
process wherein a gasification zone is not present, the fluidizing gas
in the heating zone will be a mixture of steam and an oxygen-containing
gas.
Hot coke is removed from the fluidized bed in heater 2
and recycled to coking reactor by line 42 to supply heat thereto.
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Another portion of coke is removed from heater 2 and passed by line 44
to a gasification zone 46 in gasifier 3 in which is maintained a bed of
fluidized coke having a level indicated at 48. If desired, a purged
stream of coke may be removed from heater 2 by line 50.
The gasification zone is maintained at a temperature
ranging from about 870 to 1095C at a pressure ranging from about 0 to
150 psig, preferably at a pressure ranging from about 25 to about 45
psig. Steam by line 52, and a molecular oxygen-containing gas, such as
air, commercial oxygen, or air enriched with oxygen by line 54 pass via
line 56 into gasifier 3. The reaction of the coke particles in the
gasification zone with the steam and the oxygen-containing gas produces
a hydrogen and carbon monoxide-containing fuel gas. The gasified
product gas, which may further contain some entrained solids, is removed
overhead from gasifier 3 by line 32 and introduced into heater 2 to
provide a portion of ~he required heat as previously described.
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