Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS AND APPARATUS FOR SEPARATING PITCH FROM
SLURRY HYDROCRACKED VACUUM GAS OIL AND COMPOSITION
BACKGROUND OF THE INVENTION
[0001] This invention relates to a process and apparatus for the
treatment of crude oils
and, more particularly, to the hydroconversion of heavy hydrocarbons in the
presence of
additives and catalysts to provide useable products and further prepare
feedstock for refining
conversion units such as FCC or hydrocracking.
[0002] Hydroconversion processes for the conversion of heavy
hydrocarbon oils to light
and intermediate naphthas of good quality and for reforming feedstocks, fuel
oil and gas oil
are well known. These heavy hydrocarbon oils can be such materials as
petroleum crude oil,
atmospheric tower bottoms products, vacuum tower bottoms products, heavy cycle
oils, shale
oils, coal-derived liquids, crude oil residuum, topped crude oils and the
heavy bituminous oils
produced from oil sands. Of particular interest are the oils produced from oil
sands and which
contain wide boiling range materials from naphthas through kerosene, gas oil,
pitch, etc., and
which contain a large portion of material boiling above 538 C (1000 F).
[0003] As the reserves of conventional crude oils decline, these heavy
oils must be
upgraded to meet demands. In this upgrading, the heavier materials are
converted to lighter
fractions and most of the sulfur, nitrogen and metals must be removed. Crude
oil is typically
first processed in an atmospheric crude distillation tower to provide fuel
products including
naphtha, kerosene and diesel. The atmospheric crude distillation tower bottoms
stream is
typically taken to a vacuum distillation tower to obtain vacuum gas oil (VGO)
that can be
feedstock for an FCC unit or other uses. VG0 typically boils in a range
between 300 C
(572 F) and 538 C (1000 F). The bottoms of the vacuum tower typically
comprises at least 9
wt-% hydrogen and a density of less than 1.05 g/cc on an ash-free basis
excluding inorganics.
The vacuum bottoms are usually processed in a primary upgrading unit before
being sent
further to a refinery to be processed into useable products. Primary upgrading
units known in
the art include, but are not restricted to, coking processes, such as delayed
or fluidized
coking, and hydrogen-addition processes such as ebullated bed or slurry
hydrocracking
(SHC). All of these primary upgrading technologies such as delayed coking,
ebullated bed
hydrocracking and slurry hydrocracking enable conversion of crude oil vacuum
bottoms to
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VG0 boiling in the range between 343 and 538 C (650 and 1000 F) at
atmospheric
equivalent conditions.
[0004] At the preferred conversion level of 80-95 wt-% of materials
boiling above
524 C (975 F) converting to material boiling at or below 524 C (975 F), SHC
produces a
pitch byproduct at a yield of 5-20 wt-% on an ash-free basis. By definition,
pitch is the
hydrocarbon material boiling above 538 C (1000 F) atmospheric equivalent as
determined
by any standard gas chromatographic simulated distillation method such as ASTM
D2887,
D6352 or D7169, all of which are used by the petroleum industry. These
definitions of
"conversion" and "pitch" narrow the range of converted products relative to
pitch conversion.
The pitch byproduct is solid at room temperature and has minimum pumping
temperatures in
excess of 250 C, which make it impractical to move over any great distance,
since the
pipeline would need to be jacketed with hot oil or electrically heated. It
also contains
inorganic solid material, which can settle out. Hence, tank storage requires
stirring or
circulation to prevent settling, an additional capital and operating expense.
[0005] Cohesion in solids will take place when heated into the softening
region. The
onset of sticking, or softening point, is difficult to determine and may
require time-
consuming empirical tests, for example by consolidating the solids under the
expected load in
a silo, followed by measuring the shear force required to move the solids.
Such standard tests
include ASTM D6773, using the Schulz ring-shear tester, and ASTM D6128, using
the
Jenike ring-shear tester. Pitch is not a pure compound and melts over a wide
range.
Therefore, Differential Scanning Calorimetry (DSC) will not pick up a definite
melting peak
that can be used as a rapid instrumental procedure.
[0006] The softening point of pitches has traditionally been measured
using the Ring and
Ball Softening Point Method, ASTM D36, or Mettler Softening Point Method, ASTM
D3104. Both of these methods are useful for determining the temperature at
which the
material will begin liquid flow. This can be used, among other things, to set
the minimum
temperature for pitch as a liquid in the preparation of asphalt binder for
paving, roofing and
other and industrial uses. However, this information tells nothing about the
onset of softness
and cannot be directly used to determine at what point the solid will undergo
plastic
deformation, or start to stick together.
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[0007] Solidification of pitch can be accompanied by dust generation
because pitch with
a higher onset of softening point can become brittle. However, pitch with
lower onset of
softening point can become sticky which makes handling in bulk difficult.
[0008] Better methods for processing pitch produced from SHC are
needed to provide
pitch that is more easily managed. Additionally, better methods are needed for
assessing how
easily pitch can be managed.
SUMMARY OF THE INVENTION
[0009] We have found that utilizing a second vacuum column in the
recovery of
products from SHC reactor provides pitch that is less sticky and can be
solidified more easily.
The second vacuum column further separates VG0 from pitch and the VG0 may be
recycled
to the slurry hydrocracking reactor. A portion of the pitch from the first
vacuum column may
be recycled to the slurry hydrocracking reactor. Use of the second vacuum
column allows for
lower temperatures in both of the vacuum columns which reduces coking and
cracking
concerns. Pitch byproduct may then be formed into solid particles that are
free-flowing bulk
solids that can be more easily managed at expected transportation
temperatures. Use of two
vacuum columns also enables lower pitch temperature to avoid coking in heating
apparatuses.
Pitch with VG0 concentrations under 14 wt-% do not become sticky in their
solid form when
subjected to anticipated transportation temperatures.
BRIEF DESCRIPTION OF THE DRAWINGS
[0010] For a better understanding of the invention, reference is made to
the
accompanying drawings.
[0011] FIG. 1 is a schematic flow scheme showing a process and
apparatus of the
present invention.
[0012] FIG. 2 is a schematic flow scheme showing an alternate process
and apparatus of
the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0013] The process and apparatus of this invention is capable of
converting a wide range
of heavy hydrocarbon feed stocks into lighter hydrocarbon products. It can
process aromatic
feedstocks, as well as feedstocks which have traditionally been very difficult
to hydroprocess,
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e.g. vacuum bottoms, visbroken vacuum residue, deasphalted bottom materials,
off-
specification asphalt, sediment from the bottom of oil storage tanks, etc.
Suitable feeds
include atmospheric residue boiling at or above 343 C (650 F), heavy vacuum
gas oil (VGO)
and vacuum residue boiling at or above 426 C (800 F) and vacuum residue
boiling above
510 C (950 F). Throughout this specification, the boiling temperatures are
understood to be
the atmospheric equivalent boiling point (AEBP) as calculated from the
observed boiling
temperature and the distillation pressure, as calculated using the equations
furnished in
ASTM D1160 appendix A7 entitled "Practice for Converting Observed Vapor
Temperatures
to Atmospheric Equivalent Temperatures". Furthermore, the term "pitch" is
understood to
refer to vacuum residue, or material having an AEBP of greater than 538 C
(1000 F).
[0014] The apparatus comprises a slurry hydrocracking reactor 20, a
first vacuum
column 90 and a second vacuum column 100. A fractionation column 50 may
prepare slurry
hydrocracked product for the first vacuum column 90 and a granulating machine
130 may
solidify pitch into solid particles.
[0015] In the SHC process as shown in FIG. 1, a coke-inhibiting additive or
catalyst of
particulate material in line 6 is mixed together with a heavy hydrocarbon
recycle such as
recycled heavy VG0 (HVGO) and/or pitch in line 8 in a feed tank 10 to form a
well-mixed
homogenous slurry. A variety of solid catalyst particles can be used as the
particulate
material, in an aspect, provided these solids are able to survive the
hydrocracking process and
remain effective as part of the recycle. Particularly useful catalyst
particles are those
described in US 4,963,247. Thus, the particles are typically ferrous sulfate
having particle
sizes less than 45 pm and with a major portion, i.e. at least 50% by weight,
in an aspect,
having particle sizes of less than 10 Rm. Iron sulfate monohydrate is the
preferred catalyst.
Bauxite catalyst may also be preferred. In an aspect, 0.01 to 4.0 wt-% of coke-
inhibiting
catalyst particles based on fresh feedstock are added to the feed mixture. Oil
soluble coke-
inhibiting additives may be used alternatively or additionally. Oil soluble
additives include
metal naphthenate or metal octanoate, in the range of 50-1000 wppm based on
fresh
feedstock with molybdenum, tungsten, ruthenium, nickel, cobalt or iron.
[0016] This slurry from feed tank 10 and heavy hydrocarbon feed in line
12 are pumped
into a fired heater 14 via line 16. The combined feed is heated in the heater
14 and pumped
through an inlet line 18 into an inlet in the bottom of a tubular SHC reactor
20. In the heater
14, iron-based catalyst particles newly added from line 6 typically thermally
decompose to
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smaller ferrous sulfide which is catalytically active. Some of the
decomposition will take
place in the SHC reactor 20. For example, iron sulfate monohydrate will
convert to ferrous
sulfide and have a particle size less than 0.1 or even 0.01 [an upon leaving
heater 14. The
SHC reactor 20 may take the form of a three-phase (solid-liquid-gas) reactor
without a
stationary solid bed through which catalyst, hydrogen and oil feed are moving
in a net
upward motion with some degree of backmixing.
[0017] Many mixing and pumping arrangements may be suitable. For
example, the feed
in line 12 may be mixed with catalyst from line 6 in the tank 10 instead of or
in addition to
the heavy oil recycle in line 8. It is also contemplated that feed streams 8
and 12 may be
added separately to the SHC reactor 20 instead of being mixed together.
[0018] Recycled hydrogen and make up hydrogen in line 22 are fed into
the SHC reactor
through line 24 after undergoing heating in heater 26. The hydrogen in line 24
may be
added at a location above the feed entry location in line 18. Both feed from
line 18 and
hydrogen in line 24 may be distributed in the SHC reactor 20 with an
appropriate distributor.
15 Additionally, hydrogen in line 23 may be added to the feed in line 16
before it is heated in
heater 14 and delivered to the SHC reactor in line 18 as shown. It is also
contemplated that a
single heater 14 could potentially be used to heat a combined stream of gas,
feed, and catalyst
to produce the feed stream in line 18, in which case, heater 26 and line 24
can be omitted.
[0019] During the SHC reaction, it is important to minimize the
formation of coke or
20 other material which tends to precipitate liquid, solid or semi-solid
phases from the bulk
material in the reactor. This can cause fouling of the reactor or downstream
equipment.
Adding a relatively polar aromatic oil to the feedstock is one means of
minimizing coke or
other precipitate. HVGO is a polar aromatic oil. In an aspect, recycled HVGO
in line 8 makes
up in the range of 0 to 50 wt-% of the feedstock to the SHC reactor 20,
depending upon the
quality of the feedstock and the once-through conversion level. The feed
entering the SHC
reactor 20 comprises three phases, solid catalyst, liquid hydrocarbons and
gaseous hydrogen
and vaporized hydrocarbon.
[0020] The process of this invention can be operated at quite moderate
pressure, in an
aspect, in the range of 3.5 to 24 MPa, without coke formation in the SHC
reactor 20. The
reactor temperature is typically in the range of 350 to 600 C with a
temperature of 400 to
500 C being preferred. The LHSV is typically below 4 h-1 on a fresh feed
basis, with a range
of 0.1 to 3 hr-1 being preferred and a range of 0.2 to 1 hr-1 being
particularly preferred. The
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per-pass pitch conversion may be between 50 and 95 wt-%. The hydrogen feed
rate is 674 to
3370 Nm3/m3 (4000 to 20,000 SCF/bbl) oil. Although SHC can be carried out in a
variety of
known reactors of either up or downflow, it is particularly well suited to a
tubular reactor
through which feed and gas move upwardly. Hence, the outlet from SHC reactor
20 is above
the inlet. Although only one is shown in the FIG. 1, one or more SHC reactors
20 may be
utilized in parallel or in series. Because the liquid feed is converted to
vaporous product,
foaming tends to occur in the SHC reactor 20. An antifoaming agent may also be
added to the
SHC reactor 20, in an aspect, to the top thereof, to reduce the tendency to
generate foam.
Suitable antifoaming agents include silicones as disclosed in US 4,969,988.
Additionally,
hydrogen quench from line 27 may be injected into the top of the reactor to
cool the slurry
hydrocracked product. It is also contemplated that the quench line could
alternatively
comprise a VG0, diesel or other hydrocarbon stream.
[0021] A hydrocracked stream comprising a gas-liquid mixture is
withdrawn from the
top of the SHC reactor 20 through line 28. Slurry hydrocracking cleaves
aliphatic groups
from the aromatic rings but leaves the aromatic rings resulting in a slurry
hydrocracked
product comprising a hydrogen concentration of 8 wt-% or less, suitably 6 wt-%
or less and
typically at least 4 wt-% on an ash-free basis excluding inorganics. The
slurry hydrocracked
product may have a density of at least 1.1 g/cc, suitably at least 1.15 g/cc
and typically no
more than 1.3 g/cc on an ash-free basis excluding inorganics. The slurry
hydrocracked
product also contains 1 to 10 wt-% toluene insoluble organic residue (TIOR).
"TIOR"
represents non-catalytic solids in a portion of the slurry hydrocracked
product boiling over
524 C (975 F).
[0022] The hydrocracked stream from the top of the SHC reactor 20 is a
vapor-liquid
mixture consisting of several products including VG0 and pitch that can be
separated in a
number of different ways. The hydrocracked effluent from the top of the SHC
reactor 20 is in
an aspect, separated in a hot, high-pressure separator 30 kept at a separation
temperature
between 200 and 470 C (392 and 878 F), and in an aspect, at the pressure of
the SHC
reaction. The optional quench in line 27 may assist in quenching the reaction
products to the
desired temperature in the hot high-pressure separator 30. In the hot high
pressure separator
30, the effluent from the SHC reactor 20 in line 28 is separated into a
gaseous stream 32 and
a liquid stream 34. The gaseous stream is the flash vaporization product at
the temperature
and pressure of the hot high pressure separator 30 and comprises between 35
and 80 vol-% of
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the hydrocarbon product from the SHC reactor 20, preferably between 50 and 70
vol-%.
Likewise, the liquid stream is the flash liquid at the temperature and
pressure of the hot high
pressure separator 30. The gaseous stream is removed overhead from the hot
high pressure
separator 30 through line 32 while the liquid fraction is withdrawn at the
bottom of the hot
high pressure separator 30 through line 34.
[0023] The liquid fraction in line 34 is delivered to a hot flash drum
36 at the same
temperature as in the hot high pressure separator 30 but at a pressure of 690
to 3,447 kPa
(100 to 500 psig). The vapor overhead in line 38 is cooled in cooler 39 and
joins line 42
which is the liquid bottoms from a cold high pressure separator in line 42 to
make line 52. A
liquid fraction leaves the hot flash drum in line 40.
[0024] The overhead stream from the hot high pressure separator 30 in
line 32 is cooled
in one or more coolers represented by cooler 44 to a lower temperature. A
water wash (not
shown) on line 32 is typically used to wash out salts such as ammonium
bisulfide or
ammonium chloride. The water wash would remove almost all of the ammonia and
some of
the hydrogen sulfide from the stream 32. The stream 32 is transported to a
cold high pressure
separator 46. In an aspect, the cold high pressure separator is operated at
lower temperature
than the hot high pressure separator 30 but at the same pressure. The cold
high pressure
separator 46 is kept at a separation temperature between 100 and 93 C (50 and
200 F), and
in an aspect, at the pressure of the SHC reaction. In the cold high pressure
separator 46, the
overhead of the hot high pressure separator 30 is separated into a gaseous
stream 48 and a
liquid stream 42. The gaseous stream is the flash vaporization fraction at the
temperature and
pressure of the cold high pressure separator 46. Likewise, the liquid stream
is the flash liquid
product at the temperature and pressure of the cold high pressure separator 46
and comprises
between 20 and 65 vol-% of the hydrocarbon product from the SHC reactor 20,
preferably
between 30 and 50 vol-%. By using this type of separator, the outlet gaseous
stream obtained
contains mostly hydrogen with some impurities such as hydrogen sulfide,
ammonia and light
hydrocarbon gases.
[0025] The hydrogen-rich stream in line 48 may be passed through a
packed scrubbing
tower 54 where it is scrubbed by means of a scrubbing liquid in line 56 to
remove hydrogen
sulfide and ammonia. The spent scrubbing liquid in line 58 may be regenerated
and recycled
and is usually an amine. The scrubbed hydrogen-rich stream emerges from the
scrubber via
line 60 and is combined with fresh make-up hydrogen added through line 62 and
recycled
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through recycle gas compressor 64 and line 22 back to the SHC reactor 20. Make-
up
hydrogen may be added upstream or downstream of the compressor 64, but if a
quench is
used, make-up line 62 should be downstream of the quench line 27.
[0026] The liquid fraction in line 42 carries liquid product to adjoin
cooled hot flash
drum overhead in line 38 leaving cooler 39 to produce line 52 which feeds a
cold flash drum
66 at the same temperature as in the cold high pressure separator 46 and a
lower pressure of
690 to 3,447 kPa (100 to 500 psig) as in the hot flash drum 36. The overhead
gas in line 68
may be a fuel gas comprising C4- material that may be recovered and utilized.
The liquid
bottoms in line 70 and the bottoms line 40 from the hot flash drum 36 each
flow into the
fractionation section 50.
[0027] The fractionation section is in downstream communication with
the SHC reactor
20. "Downstream communication" means that at least a portion of material
flowing to the
component in downstream communication may operatively flow from the component
with
which it communicates. "Communication" means that material flow is operatively
permitted
between enumerated components. "Upstream communication" means that at least a
portion of
the material flowing from the component in upstream communication may
operatively flow
to the component with which it communicates. The fractionation section 50 may
comprise
one or several vessels although it is shown only as one vessel in FIG. 1. The
fractionation
section 50 may comprise a stripper vessel and an atmospheric column but in an
aspect is just
a single column. Inert gas such as medium pressure steam may be fed near the
bottom of the
fractionation section 50 in line 72 to strip lighter components from heavier
components. The
fractionation section 50 produces an overhead gas product in line 74, a
naphtha product
stream in side cut line 76, a diesel product stream in side cut line 78, an
optional atmospheric
gasoil (AGO) stream in side cut line 80 and a VG0 and pitch stream in bottoms
line 82.
[0028] Line 82 introduces a portion of the hydrocracked effluent in the
bottoms stream
from the fractionation section 50 to a fired heater 84 and delivers the heated
bottom stream to
a first vacuum column 90 maintained at a pressure between 1 and 10 kPa (7 and
75 torr),
preferably between 1 and 7 kPa (10 and 53 torr) and at a vacuum distillation
temperature
resulting in an atmospheric equivalent cut point between light VG0 (LVGO) and
HVGO of
between 371 and 482 C (700 and 900 F), preferably between 398 and 454 C
(750 and
850 F) and most preferably between 413 and 441 C (775 and 825 F). The first
vacuum
column is in downstream communication with fractionation section 50 and the
SHC reactor
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20. The first vacuum column is in an aspect, a distillation column with a
three-stage eductor
at the overhead to provide the vacuum in the column. Each stage of the eductor
is co-fed with
a gas stream such as steam to pull a vacuum upstream of the eductor in the
vacuum column.
Pressure is greater on the downstream side of each eductor stage, causing the
overhead
stream to condense in an accumulator to liquid products that can be recovered.
Light gases
leaving the third eductor stage can be recovered and in an aspect used as fuel
in the fired
heater 84. Other types of equipment for pulling the vacuum may be suitable. In
an aspect,
steam stripping may be used in the first vacuum column. Steam is delivered by
line 99 to the
first vacuum column 90 from a steam header 104.
[0029] Three fractions may be separated in the first vacuum column: an
overhead
fraction of diesel and lighter hydrocarbons in an overhead line 92, an LVGO
stream boiling at
no higher than 482 C (900 F) and typically above 300 C (572 F) from a side cut
in line 94, a
HVGO stream boiling above 371 C (700 F) in side cut line 96 and a pitch stream
obtained in
a bottoms line 98 which boils above 450 C (842 F). Much of the HVGO in line 96
is
typically recycled to the SHC reactor 20. The unrecycled portion of the HVGO
is typically
recovered as product for further conversion in other refinery operations. To
minimize vapor
generation which requires greater energy to pull the vacuum, a portion of the
LVGO stream
in line 94 is cooled by heat exchange and pumped back to the column in line 95
to condense
as much condensable material as possible. A further side cut of slop wax in
line 97, taken
below the HVGO side cut line 96 and above the bottoms line 98 carrying the
first pitch
stream, may be recycled to the SHC reactor 20 which is in downstream
communication with
slop wax side cut line 97. In this case most or all of stream 96 would be
recovered as HVGO
product. By taking the side cut in line 97, less feed is sent to the second
vacuum column 100
requiring it to have less capacity and the quality of the HVGO in line 96 is
improved. The
slop wax stream in line 97 will typically have an end boiling point below 621
C (1150 F) and
preferably below 607 C (1125 F). VG0 streams may also be recycled upstream to
enhance
separation operations.
[0030] The first pitch stream in line 98 is delivered to the second
vacuum column 100 in
line 98 which is in downstream communication with the first vacuum column 90,
the
fractionation column 50 and the SHC reactor 20. The first pitch stream in line
98 is
unsuitable for bulk flow as a granular solid. It is thermally unstable in that
it begins to crack
at temperatures as low as 300 C if subjected to this temperature for
sufficient time. The pitch
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in line 98 may have inorganic solids content which can be in the range as high
as 6 to 10
wt-%. The high solids content could make the fired heater 84 prone to fouling
by coke
formation. The temperature required in the vacuum bottoms can be reduced by
adding steam
to reduce the hydrocarbon partial pressure or by reducing the vacuum pressure
further which
are both expensive. The temperature in the vacuum bottoms must be high to lift
sufficient
HVGO from the pitch. We have found that solidification of pitch comprising at
least 14 wt-%
HVGO provides sticky particles that are not easily handled in bulk. An outlet
of the fired
heater 84 at a temperature of 385 C (725 F) will enable the first vacuum
column 90 to
produce pitch with only 10 wt-% HVGO content, but may subject the heater 84 to
excessive
coking.
[0031] The present invention utilizes a second vacuum distillation
column 100 to further
lift HVGO from the pitch. In an aspect, the second vacuum distillation column
is operated at
a lower pressure than in the first vacuum column to obtain the lift of VG0
necessary to
produce pitch that can be formed into particles that are bulk manageable. The
use of the
second vacuum column 100 provides for a lower temperature in the fired heater
84 upstream
of the first vacuum column 90 at or below 377 C (710 F) and in an aspect at or
below 370 C
(698 F), so fouling from coking is less likely. With steam stripping in the
first vacuum
column 90, the first pitch stream in line 98 may be delivered to the second
vacuum column
100 at 315 to 350 C (600 to 662 F). In an aspect, the first pitch stream in
line 98 may be
directly delivered to the second vacuum column 100 without being subjected to
heating or
cooling equipment. In other words, line 98 may be devoid of heating or cooling
equipment
until it feeds the second vacuum column 100. However, some heating or cooling
may be
necessary. Alternatively, in an aspect, heat is added to the second vacuum
column 100 via hot
oil or steam. Consequently, the entry temperature of the first pitch stream 98
to the second
vacuum column 100 is in an aspect, not more than 50 C greater or smaller than
the exit
temperature of the first pitch stream 98 from the bottoms of the first vacuum
column 90.
[0032] The second vacuum column 100 is in downstream communication
with the
bottoms of the first vacuum column 90. The second vacuum column 100 is
maintained at a
pressure between 0.1 and 3.0 kPa (1 and 23 torr), preferably between 0.2 and
1.0 kPa (1.5 and
7.5 torr) and at a vacuum distillation temperature of 300 to 370 C (572 to
698 F) resulting
in an atmospheric equivalent cut point between HVGO and pitch of between 454
and 593 C
(850 and 1100 F), preferably between 482 and 579 C (900 and 1075 F), and
most
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preferably between 510 and 552 C (950 and 1025 F). The second vacuum column
100 is in
downstream communication with the first vacuum column 90, the fractionation
section 50
and the SHC reactor 20.
[0033] The second vacuum column 100 may be a conventional vacuum
column or it may
have special functionality for driving the VG0 from the pitch by generating a
film of pitch
for facilitating evaporation of lower boiling components from the pitch.
Special film
generating evaporators are able to promote evaporation of VG0 sufficiently
quickly to avoid
coking. Film generating evaporators may include an evaporator stripper, a thin
film
evaporator, a wiped film evaporator, a falling film evaporator, a rising film
evaporator and a
scraped surface evaporator. Some of these film generating evaporators may
include a moving
part for renewing the surface of the pitch in the second vacuum column 100.
Other types of
thin film generating evaporators may be suitable. For example, a thin film
evaporator (TFE)
heats up the pitch on an internal surface of a heated tube until the VG0
starts to evaporate.
The pitch is maintained as a thin film on the internal surface of the tube by
a rotating blade
with a fixed clearance. The VG0 vapors are then liquefied on the cooler tubes
of a condenser.
A wiped film evaporator (WFE) is different from a TFE in that it uses a hinged
blade with
minimal clearance from the internal surface to agitate the flowing pitch to
effect separation.
In both TFE and WFE's pitch enters the unit tangentially above a heated
internal tube and is
distributed evenly over an inner circumference of the tube by the rotating
blade. Pitch spirals
down the wall while bow waves developed by rotor blades generate highly
turbulent flow and
optimum heat flux. VG0 evaporates rapidly and vapors can flow either co-
currently or
countercurrently against the pitch. In a simple TFE and WFE design, VG0 may be
condensed
in a condenser located outside but as close to the evaporator as possible. A
short path
distillation unit is another kind of TFE or a WFE that has an internal
condenser. A scraped
surface evaporator (SSE) operates similarly to the principle of the WFE.
However, an SSE
does not endeavor to maintain only a thin film on the internal heated surface
but endeavors to
keep a film of pitch on the heated surface from overheating by frequent
removal by a scraper.
[0034] In a falling film evaporator (FFE), the pitch enters the
evaporator at the head and
is evenly distributed into heating tubes. A thin film enters the heating tubes
and flows
downwardly at boiling temperature and is partially evaporated. Inert gas, such
as steam, may
be used for heating the tubes by contact with the outside of the tubes. The
pitch and the VG0
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vapor both flow downwardly in the tubes into a lower separator in which the
vaporous VG0
is separated from the pitch.
[0035] A rising film evaporator (RFE) operates on a thermo-siphon
principle. Pitch
enters a bottom of heating tubes heated by steam provided on the outside of
the tubes. As the
pitch heats, vapor VG0 begins to form and ascend. The ascending force of this
vaporized
VG0 causes liquid and vapors to flow upwardly in parallel flow. At the same
time the
production of VG0 vapor increases and the pitch is pressed as a thin film on
the walls of the
tubes while ascending. The co-current upward movement against gravity has the
beneficial
effect of creating a high degree of turbulence in the pitch which promotes
heat transfer and
coke inhibition.
[0036] In an aspect, the special second vacuum column 100 for
generating a thin film
may be an evaporator stripper available from Artisan Industries of Waltham,
Maryland. The
second vacuum column 100 is shown to be an evaporator stripper in FIG. 1. The
first pitch
stream 98 may pass through an optional pre-evaporator 102 which may be an RFE
to
evaporate the bulk of the VG0 from the pitch. An evaporator stripper may
operate without
the pre-evaporator 102. Steam or other inert gas enters an upper end of the
pre-evaporator
102 from a steam header 104 and condensate exits at a lower end. Pitch and VG0
enter an
enlarged diameter flash section 108 of the evaporator stripper 100 via line
106. Vaporous
VG0 exits the top of the evaporator stripper perhaps through an entrainment
separator such
as a demister to knockout condensables. The vapor exits in line 110 and enters
a condenser
112 and perhaps an accumulator 114. The vacuum is pulled from the condenser
112, perhaps
by staged eductors or other suitable device. Line 116 takes VG0, in an aspect,
primarily
HVGO, to be recycled to the SHC reactor 20 in line 8. Accordingly, the SHC
reactor 20 is in
downstream communication with an overhead of the second vacuum column 100. A
portion
of the HVGO in line 116 may be recovered issued as a net product in line 124.
Pitch in the
evaporator stripper 100 cascades downwardly over heated or unheated trays,
such as tube-
and-disc trays, while the remaining volatiles are stripped by the rising
vapor. The trays
provide a fresh liquid thin film at each stage, renewing the surface of the
pitch film for
evaporation and stripping. In an aspect, the trays may define interior
cavities in
communication with a heating fluid from line 126 for indirectly heating the
pitch traveling
over the trays. Heating fluid exits the second vacuum column 100 in line 128
for reheating.
Inert gas, such as steam or nitrogen, may be sparged into the column from line
118 to strip
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the pitch and further enhance mass transfer. A second pitch stream is removed
from the
second vacuum column 100 in line 120 and comprises less than 14 wt-% VG0 and
preferably
no more than 13 wt-% VG0. In this context, less than 14 wt-%, in an aspect no
more than 13
wt-% and preferably no more than 10 wt-% of the second pitch stream in line
120 from the
second vacuum bottoms boils at or below 538 C (1000 F). Furthermore, less than
14 wt-%,
in an aspect no more than 13 wt-% and preferably no more than 10 wt-% of the
second pitch
stream in line 120 boils in a range between 300 C (572 F) and 538 C (1000 F).
In an aspect,
at least 1 wt-% of the second pitch stream in line 120 is VG0 that boils at or
less than 538 C
(1000 F). The second pitch stream in line 120 also comprises a hydrogen
concentration of 8
wt-% or less, suitably 6 wt-% or less and typically at least 4 wt-% on an ash-
free basis
excluding inorganics. The second pitch stream may have a density of at least
1.1 g/cc,
suitably at least 1.15 g/cc and typically no more than 1.3 g/cc on an ash-free
bases excluding
inorganics. The second pitch stream may also contain 1 to 10 wt-% toluene
insoluble organic
residue (TIOR). The second vacuum column 100 is able to recover as much as 15
wt-% VG0
from the pitch. This recovered VG0 leaves from vacuum column 100 in the
overhead line
110 which may be recycled in lines 116, 8, 16 and 18 back to the SHC reactor
20.
[0037] The second pitch stream in vacuum bottoms line 120 may be
discharged directly
to a granulation machine 130. In an aspect, the temperature of the pitch in
line 120 does not
need to be adjusted by heat exchange to prepare the pitch for granulation. A
particularly
useful granulation machine 130 is a pastillation device called a Rotoformer
provided by
Sandvik Process Systems of Sandviken, Sweden which produces a half-spherical
particle
called a pastille. Other granulation machines can be melt strand granulators,
underwater melt
cutters, extruders with die plates, prilling systems, spray driers and the
like. The granules
produced should have a rounded or semi-rounded aspect which allows them to
move freely in
bulk handling and transfer systems. Rounded or semi-rounded granules are less
likely to stick
together because they have fewer points of contact and are less prone to dust
formation
because they lack sharp edges of flaked material.
[0038] A granulation machine 130 of the pastillation type comprises a
heated cylindrical
stator 134 which is supplied with molten pitch from the second pitch stream
120 or a storage
tank 132. The granulation machine 130 is in downstream communication with the
bottoms of
the second vacuum column 100 via line 120. A rotating perforated cylindrical
wall 136 turns
concentrically around the stator 134 to form particles or pastilles of pitch
by emission through
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openings in the perforated wall 136. The pastilles are deposited across the
whole operating
width of a metal conveyor belt 138 which is in an aspect, stainless steel.
Heat released during
solidification and cooling of the dropped pastilles is transferred through the
belt 138 which is
cooled by indirect heat exchange with cooling media such as water sprayed
underneath the
belt from line 140. The sprayed cooling water is collected in tanks and
returned in line 142 to
a water chilling system without contacting the pitch particles. A heated re-
feed bar may force
excess pitch remaining in the openings of the rotating cylindrical wall 136
into a position
from which it is re-dropped onto the belt 138. The belt 138 conveys the
pastilles into a
collector 144. The pitch pastilles can now be easily handled in bulk and
transported for
consumption. The pitch pastilles may now be stored or transported without need
of further
intentional cooling. The pastilles will not stick together because sufficient
VG0 has been
separated from the pitch to raise the onset of softening point temperature to
above the highest
anticipated transportation temperature. The highest anticipated temperature in
transportation
will necessarily depend on the climate of the route and type of container. A
credible global
maximum of 66 C (150 F) can be estimated from data of the International Safe
Transit
Association, OCEAN CONTAINER TEMPERATURE AND HUMIDITY STUDY, Preshipment
Testing
Newsletter ( 2d Quarter 2006).
[0039] FIG. 2 depicts an alternative flow scheme of the present
invention in which pitch
recycle in line 150 from the first pitch stream in line 98 is recycled to the
SHC reactor 20.
FIG. 2 is the same as FIG. 1 with the exception of a pitch recycle line 150
that diverts a
portion of the first pitch stream 98 regulated by a control valve 142 to
bypass the second
vacuum column 100 to join line 116 to feed line 8. Accordingly, the SHC
reactor 20 is in
downstream communication with a bottoms of the first vacuum column 100. All
other aspects
of the embodiment of FIG. 2 are the same as FIG. 1. At least a portion of the
first pitch
stream may optionally be recycled as a portion of the feed to the SHC reactor
20 in line 8.
Remaining catalyst particles from SHC reactor 20 in the SHC effluent in line
28 will be
present in the first pitch stream 98. A portion of the catalyst can be
conveniently recycled
back to the SHC reactor 20 along with a portion of the first pitch stream.
This alternative will
conserve SHC catalyst. The remaining portion of the first pitch stream in line
98 is delivered
to the second vacuum column 100 in line 146. In this alternative aspect, the
first vacuum
column 90 may be flash column with no heat input or cooling.
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EXAMPLE
[0040] To determine which pitch materials can be solidified and
transported 66 C
(150 F) was taken as a highest temperature to which pitch materials would be
exposed during
transportation, considering an acceptable safe operating margin. Pitch
materials would have
to be transportable up to this maximum temperature without beginning to stick
together. That
is, the onset of softening temperature of the pitch must be greater than 66 C
(150 F).
[0041] A procedure for using a thermomechanical analyzer (TMA) is
similar to a
procedure reported for measuring densities of powdered molding polymer by
McNally, G.
and McCourt, M., DENSITY MEASUREMENT OF THERMOPLASTIC POWDERS DURING HEATING
AND COOLING CYCLES USING THERMAL MECHANICAL ANALYSIS, ANTEC 2002 Conference
Proceedings, 1956-1960. A TMA Model Q400 from TA Instruments of New Castle,
Delaware was used to measure the melting onset temperature and the fusion
temperature.
10 mg of hand-ground, unsized pitch powder was introduced in a 7 mm aluminum
pan. The
layer of powder is covered with an aluminum cover plate. A quartz plunger on
the lid
measures the position of the lid. A load of 5 grams is imposed on the powder
and the powder
is heated 5 C per minute. The pitch softens and collapses as the temperature
is raised. The
tabular data of position vs. temperature is collected and the first derivative
of change in
deflection vs. change in temperature at 5 C intervals is plotted as a function
of temperature.
The melting or fusion point is the temperature of maximum negative
displacement, when the
rate of thermal expansion overtakes the rate of powder collapse and is seen as
a distinct sharp
valley on a rate plot. This valley is manifest because the powdered sample,
after collapsing,
begins now to expand as temperature is raised when it is in the liquid state.
The onset of
melting is defined as detectable deviation of 1% of the first derivative
relative to the valley.
[0042] The onset melting temperature of 1% deformation, represented as
T(1%), is
defined in the following way:
T(1%) is the temperature at which (Z-Zliq)/(Zo-Zliq)= 0.01 (1)
wherein
Z = position measured at temperature T;
Zo = initial position of plunger with sample at ambient temperature; and
Act= position at fusion point which is peak of the rate plot.
[0043] Seven residual pitch products were prepared from a mixture of
slurry
hydrocracker heavy product to illustrate the process required to achieve a non-
sticky, free-
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flowing pitch granule. The starting material for each residual pitch produce
was the heavy
fraction of the products obtained after 87 wt-% conversion, defined by
material boiling above
524 C (975 F) converted to material boiling below 524 C (975 F) from slurry
hydrocracking
a bitumen vacuum tower bottoms. The vacuum tower bottoms was prepared from
cold-
produced bitumen from the Peace River (Seal) formation near Slave Lake,
Alberta, Canada.
This bitumen bottoms was slurry hydrocracked at 13.79 MPa (2000 psi) in the
presence of
hydrogen using an iron sulfate-based catalyst in a stirred continuous reactor.
The
hydrocracked products leaving the reactor were flashed to remove products
lighter than
middle distillate and stripped of hydrogen and all non-condensable products.
The starting
material for further fractionation will be hereafter referred to as heavy ends
(HE).
[0044] Sample 1 was a pitch pastille prepared by subjecting HE to
conventional vacuum
fractionation. The solidified pastille of Sample 1 did not move freely and was
visibly sticky at
room temperature. The onset of deformation as measured by TMA was 44 C. Sample
1 is not
acceptable for bulk handling and transport.
[0045] Sample 2 was a clarified pitch produced from the following process:
HE was
allowed to settle in a reservoir, and the solids-free liquid was then vacuum
flashed at 380 C
and 5 torr (0.7 kPa). The clarified heavy vacuum-flashed liquid was not
subjected to further
treatment. It was not visibly sticky and had a onset of softening point of
72.5 C which is
marginally above the maximum transportation temperature. Therefore, material 2
is
marginally acceptable.
[0046] Sample 3 was a de-oiled sludge produced from the HE settling
operation that was
used to make Sample 2. The physical separation consisted of draining oil off
the vacuum
flashed liquid on a sieved tray while volatiles were allowed to evaporate off.
The de-oiled
sludge was then subjected to vacuum evaporation by a falling film evaporator
under high
vacuum of 0.3 kPa (2 ton) but not subjected to further treatment. Like Sample
1, it was
visibly sticky and also did not move freely. The onset of softening point of
52.7 C for
material 3 is not acceptable. Its VG0 content was determined by a mass balance
to be 14
wt-%.
[0047] Samples 4 and 5 were pitch samples in which HE was vacuum
fractionated in a
laboratory batch still at deep vacuum with magnetic stirring. Samples 4 and 5
are acceptable
because they have a higher onset of softening point temperature than the
maximum
transportation temperature. However, sample 5 was heated to a temperature of
320 C to drive
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off more of the VG0. At this temperature some thermal cracking occurred.
Partially
pyrolyzing a pitch material will increase its onset of softening point
temperature. However,
the pitch will be harder to manage due to its higher fluid viscosity and the
high temperature
will causing coking on heat exchange surfaces. Moreover, thermal cracking will
generate a
higher volume of gases which will quickly overcome the capacity of the vacuum
system,
especially at low absolute pressures.
[0048] Samples 6 and 7 were prepared by a first step of vacuum
fractionating the HE and
a second step of sending to a wiped film evaporator running at 300 C internal
flash
temperature and 0.1 and 0.3 kPa (0.7 and 2.5 Torr) respectively. Samples 6 and
7 were
subsequently granulated by re-melting and forming into 7 mm half-round
pastilles on a
Sandvik Rotoformer. The pastilles were non-sticky and free-flowing without any
agglomeration, even at 100 C, confirming that the granulated material could be
handled at
temperatures above any possible transportation temperature.
[0049] The Table below shows the results of the tests. VG0 fraction is
defined by the
fraction of the pitch that boils at or below 538 C (1000 F). Pitch with VG0
fractions less
than 14 wt-% had acceptable onset of softening point temperatures generally
for bulk
handling.
TABLE
Sample No. Fusion Point, Onset of Softening VG0 Fraction,
C Point, C wt-%
1 86.1 43.7 18
2 96.4 72.5 13
3 88.1 52.7 14
4 116.5 72.2 2
5 169.5 118.5 2
6 153.5 113.8 1
7 143.7 95.0 1.5
[0050] The pitch products in Samples 1-7 would be expected to have a
hydrogen
concentration of 5 wt-% and a density of 1.2 g/cc on an ash-free basis
excluding inorganics.
[0051] Without further elaboration, it is believed that one skilled in
the art can, using the
preceding description, utilize the present invention to its fullest extent.
The preceding
preferred specific embodiments are, therefore, to be construed as merely
illustrative, and not
limitative of the remainder of the disclosure in any way whatsoever.
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[0052] In the foregoing, all temperatures are set forth in degrees Celsius
and, all parts
and percentages are by weight, unless otherwise indicated.
[0053] The scope of the claims should not be limited by the preferred
embodiments
set forth in the examples, but should be given the broadest interpretation
consistent with the
description as a whole.
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