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BACKGROUND OF THE INVENTION
This invention relates to the retorting of oil shale under conditions to
recover shale oil. More particularly, this invention relates to the
retorting of oil shale under conditions such that the reaction products
are not contaminated with products of combustion. Further, the retorting
takes place under conditions such that energy is conserved and no outside
fuel source is required.
The location of oil shale deposits is well known, and the processing of
these deposits to recover valuable products is becoming more important in
modern technology. The solid organic portion of oil shale is known as
kerogen. When the shale is heated to an elevated temperature, the kerogen
is decomposed by pyrolysis to give shale oil, light hydrocarbon gases and
a carbonaceous residue. The technology of shale oil is well covered in
Vol. 18 of "Kirk-Othmer Encyclopedia of Chemical Technology"
(Interscience-John Wiley-New York).
A typical system for processing oil shale concerns a vertical retort having
several sections, such as (a) a pre-heating section in which fresh shale
is heated to retort temperature by any of several means, such as
intermixing with hot spent shale or by direct heat transfer with hot
combustion gases, (b) a retorting section in which the kerogen is
pyrolyzed to give various products, (c) a combustion section in which
combustible material, such as carbonaceous residue on the shale or the
light hydrocarbon gases from the pyrolysis of the shale, are burned to
furnish heat for the system, and (d) a spent shale section wherein some of
the sensible heat is recovered from the hot spent shale.
Two of the major problems in this process concern the material handling of
large amounts of shale and the exchange of heat between hot spent shale
and fresh shale to give the necessary pyrolysis temperature. The prior art
literature offers many proposed solutions for these problems, as discussed
in the Kirk-Othmer article. However, these prior art methods typically
have two disadvantages: (a) some of the kerogen material is used as fuel
to provide heat for the pyrolysis, thus diminishing the yield of product
per ton of shale, and (b) when the carbonaceous residue on the spent shale
is burned to furnish heat for the process, the gaseous products of
combustion and of carbonate decomposition are mixed with the pyrolysis
products to give a low heating value (BTU/SCF) (kg. cal./m.sup.3) product.
These disadvantages are overcome in my process for the retorting of
hydrocarbons from oil shale granules in a retorting vessel in which the
oil shale is subjected to thermal decomposition of kerogen contained
therein by the addition of heat, in the substantial absence of oxygen, and
from which distillation reaction products are obtained substantially free
of gaseous products of combustion and of carbonate decomposition, during
the downward passage of shale through a plurality of physically separated
zones in the vessel, the process comprising the steps of:
a. mixing fresh oil shale and hot spent shale in a hopper zone,
b. removing gases from the shale mixture in a first low pressure zone,
c. subjecting the shale mixture to an inert purge stream in a first purge
zone,
d. forming reaction products by thermal decomposition of kerogen in a
reaction zone and removing and recovering said products, incidentally
forming a carbonaceous residue on the shale,
e. subjecting the retorted shale mixture to an inert purge stream in a
second purge zone,
f. removing gases from the retorted shale mixture in a second low pressure
zone, and
g. heating the retorted shale mixture by combustion of its carbonaceous
residue.
My invention also includes the apparatus containing the above-mentioned
zones in the retorting vessel.
As a result of my invention, several advantages are noted:
a. heat values are recovered from the hot spent shale,
b. shale fines can be used in the process,
c. the reaction products (from the reaction section) are undiluted by
gaseous products of combustion and have an improved heating (BTU/SCF)
(hg.cal./m.sup.3) value,
d. the product from the reaction section is produced in more than 100%
yield based on the Fischer assay,
e. the fuel requirement per barrel of product is significantly negligible,
f. the temperature of the reaction section is controlled so as to minimize
carbonate decomposition in the shale,
g. steam, used in the purge sections, is an essentially inert vapor and is
easily condensible and removable from the other products,
h. the net product yield per ton of shale is significantly higher than
prior art processes, and
i. the shale requirements per barrel of product are lower than prior art
processes.
These advantages are obtained by maintaining spent shale temperatures below
1400.degree.F. to prevent agglomeration of particles and to minimize use
of expensive materials of construction.
The above-described process and the advantages of the invention are more
fully understood by referring to the drawings.
DESCRIPTION OF DRAWINGS
FIG. 1 shows a vertical section of the retorting vessel.
FIG. 2 shows one embodiment of the method of zone separation.
DETAILED DESCRIPTION OF THE INVENTION
Crushed oil shale is treated in a vertical retorting vessel, with fresh
shale being mixed with hot recycle shale and the mixture being fed into
the top of the vessel at a rate commensurate with the removal of hot spent
shale from the bottom of the vessel. From top to bottom, the vessel is
separated into distinct sections or zones, each having its own function
and operating conditions. Broadly, these zones can be labeled top,
intermediate and bottom zones, with fresh shale entering the top zone and
spent shale exiting the bottom zone and with various phases of the process
occurring in the intermediate zones. These zones, while contiguous, are
physically separated, one from the other, by horizontally inclined plates
containing a plurality of vertically inclined pipes or tubes that allow
the crushed or granular shale to move by gravity downwardly through the
various zones. At the same time, the intermediate zones are equipped with
horizontally inclined inlet and outlet conduits for the introduction and
removal of various fluids. The pressures in these various zones can be
super- or sub-atmospheric, as well as atmospheric, depending on the
function of that particular zone.
One embodiment of the invention is described in general fashion by
referring to FIG. 1. Crushed shale is delivered to fresh shale hopper 9,
with the fresh feed going either to zone 1 or zone 3 of the vertical
retorting vessel 34. Hot spent shale, typically recovered from the bottom
of the retorting vessel 34, is also fed from spent shale disengager 12, to
zone 1, called recycle hopper section. Here the hot spent shale, or
recycle shale, can be mixed with fresh feed shale, cooling the hot spent
shale and warming the fresh feed shale. It is emphasized that the
residence time or time of contact between the hot and feed shales is short
enough so that thermal decomposition of the feed shale is minimized and
yet long enough to give some preheating of the feed shale. The mixture
travels downward to zone 2, the first low pressure zone, from which
various gases, entrained dust and steam are removed through line 14. The
mixture next enters zone 3, the upper purge section, wherein stripping
steam, entering by line 15, is passed through the mixture of shale. In an
alternate embodiment, fresh shale is introduced in zone 3 and mixed with
the hot spent shale received from zone 2. It is recognized that any air,
free or absorbed, entering with the fresh shale, will be substantially
removed through zone 2. The steam flow exits from zone 3 partly through
zone 2, removing a majority of air and trapped (or absorbed) gases from
the shale mixture, and partly through zone 4, wherein the steam and
reaction products are removed through line 16. The operating pressure in
zone 3 is higher than the pressure in zone 2 or zone 4.
Zone 4, the reaction section, is where the majority of reactions occur
(i.e., the thermal decomposition of the kerogen) that liberate the
hydrocarbonaceous products from the feed shale and result in the formation
of a carbonaceous residue on and in the shale particles. The volatilized
gaseous products are separated and removed from the reaction section
through line 16.
The shale mixture, now substantially deleted of desirable hydrocarbonaceous
products, enters zone 5, the lower purge section. In this purge section,
as in the upper purge section, steam is introduced through line 17 in
order to segregate the desirable reaction products in zone 4 from any
undesirable gaseous products in zone 6, which is the lower ejection zone,
in the same manner in which zone 3 segregates zone 2 from zone 4. After
undesirable gases are ejected from the shale mixture in zone 6 through
line 18, the shale enters zone 7, the combustion zone wherein any residual
carbonaceous material on the shale reacts with air introduced into the
zone through line 21, resulting in an exothermic reaction which raises the
temperature of the shale mixture and produces various products of
combustion, including inert nitrogen, oxides of carbon and unreacted
oxygen. Although not shown in the drawing, the combustion zone can be
further physically separated in sub-zones, with air introduced to one or
more sub-zones for controlled combustion. This zone furnishes the heat
energy required in zone 4 to separate the desirable oil shale products
from the shale feed. The temperature of the exiting spent shale is
controlled by regulating the air rate, i.e., oxygen rate, entering zone 7.
Other controlling factors useful in maintaining the desired spent shale
temperature include utilization of heat exchange coils 24 in one or more
sub-zones and/or varying the spent shale residence time by changing the
flow or movement rate of the descending solids. As seen from the drawing,
steam is made by introducing boiler feed water into exchanger coils
located in zone 7. The hot spent shale exits zone 7, for further
disposition. Those skilled in the art will recognize that the spent shale
cooler 31, located below zone 7, is a heat exchanger embodiment by which
at least some of the residual heat is recovered from the spent shale. It
is also recognized that the gaseous combustion products from zone 7 can be
utilized to preheat the fresh shale and incoming air, such as in air
preheater 19 and that further heat can be obtained from the waste gas
stream by more complete combustion of, for example, the carbon monoxide
component of the stream.
I prefer to call my process a "Non-Combustion" retorting process, since no
outside fuel is used as a source of heat, essentially none of the
desirable reaction products from the thermal decomposition of the shale is
combusted to form heat, and no gaseous products of combustion are mixed
with the desirable reaction products obtained in the reaction zone. It is
well known in the art that typical retorting of the shale results in a
deposit of coke-like material on the retorted shale. The controlled
combustion of this carbonaceous material can and does furnish the heat
necessary for the decomposition of kerogen. Thus, heat is transferred from
the combustion zone 7 to the reaction zone 4, without being accompanied by
combustion gases. This step simplifies the ultimate purification of the
reaction products from the reaction zone. Another facet of this process is
the low energy requirement relative to prior art versus combustion
processes. The theoretical energy required to raise the shale to a
retorting temperature of 900.degree.F (482.degree.C.) from ambient is
about 640,000 BTU/ton (178 g.cal./g.). At retorting temperatures above
about 1100.degree.F. (593.degree.C.), mineral carbonates begin to
decompose. This decomposition not only absorbs a large amount of heat,
approximately 1300 BTU/lb (723 g.cal./g.) of CO.sub.2 liberated, but also
dilutes the retort gas. The mineral CO.sub.2 content of oil shale ranges
from 300 to 400 lbs/ton (150-200 g/kg), and thus additional energy
requirements due to carbonate decomposition range from 400,000 to 500,000
BTU/ton (111-139 g.cal./g). Since the temperature of my process is well
below the carbonate decomposition temperature, CO.sub.2 will range from 0
to 20 lbs/ton (0-10 g/kg.) shale. This is well below the amount expected
for other combustion type processes wherein CO.sub.2 is 200 to 400 lbs/ton
(100-200 g./kg.) shale. Furthermore, no CO.sub.2 from combustion dilutes
the retort products in my process, which is in contrast to combustion type
processes. Thus, the non-combustion process requires the separation and
disposal of much less CO.sub.2 than does the combustion process or other
shale processes.
The above disclosure presents a general description of the overall process,
while more detailed parameters are described below.
As used in my process, the shale is crushed to pieces about 1/4 inch (0.63
cm.). Thus, the overall particle range used in this process is that
obtained by typical crushing to obtain about 1/4 inch (0.63 cm.) feed
materials, without rejection of a fines cut. It will be appreciated by one
skilled in the art that an excessive amount of fines will cause
distribution problems in a retorting vessel and will also result in an
excessive pressure drop. At the same time, additional energy is required
to crush the larger particles to the fine size. Thus, while my process can
accommodate fines, it is generally desirable that the percentage of fines
in the fresh feed by minimized. The crushed shale is moved to the fresh
shale hopper by any of several convenient and well-known methods, such as
by pneumatic or mechanical lifting. FIG. 1 illustrates the use of a gas
lift leg, in which hot spent shale exits the base of retort 34, is carried
to a spent shale lift engager 11 through line 10, is lifted to disengager
12 by gas from line 23, and then moves from disengager 12 by gravity
through line 27 to enter zone 1. The lift gas is separated from fines and
dust in cyclone 13, reducing air pollution.
As noted previously, the fresh shale feed can be delivered to either zone 1
or zone 3. At the same time, the fresh shale feed can be pre-heated by
vent gases 8, the combustion products from zone 7, as a heat recovery
device. It is recognized that, depending upon the amount of preheating,
the temperature of the fresh shale feed will vary between ambient and
200.degree.-300.degree.F. (93.degree.-149.degree.C.). While the fresh
shale feed is entering the retorting vessel, spent shale, meaning shale
substantially free of kerogen and carbonaceous residue, is taken from the
bottom of the retorting vessel through line 10 and, by well-known means,
such as a lift engager, recycled to the hopper section, zone 1. Depending
on the combustion temperatures, the amount of steam formed, and other heat
losses, the spent shale leaving the bottom of the retorting vessel can
have a temperature of from approximately 1100.degree. to about
1400.degree.F. (593.degree.-760.degree.C.).
This spent shale is lifted by a lift engager 11 to a spent shale disengager
12, from whence it enters the recycle hopper section 1. The pressure in
this section is approximately atmospheric, while the average temperature,
depending on the previously mentioned factors, can vary from about
800.degree.-950.degree.F. (426.degree.-510.degree.C.), but it is
understood that the colder fresh shale particles do not reach this average
temperature immediately. Some of the gases used in the lift system are
vented via cyclone 13, and the solids are returned to the spent shale
disengager to avoid air pollution. The shale moves, by gravity, to the
next zone by way of vertically aligned pipes or tubes which are arranged
in a horizontally oriented plate that physically isolates the various
zones, as shown in FIG. 1. FIG. 2 shows one arrangement of this separating
device. The number, spacing and dimensions of these pipes, tubes or
conduits are generally not critical, as long as sufficient structural
strength is retained in the horizontal plate to resist buckling by the
load above, to afford sufficient passage of the shale to provide the
desired throughput of the retorting vessel, and to form a sufficient bed
to provide proper distribution of moving solids and allow passage of
gases, vapors, etc. Typically, in a retorting vessel having a 10 foot
(3.05 m.) diameter, there will be approximately 400 pipes, approximately 3
inches I.D. (7.6 cm.) and about 2 to 4 feet (0.61-1.2m.) in length, spaced
throughout the horizontal plate so as to provide proper downflow and to
avoid any channeling. Alternatively, the number of pipes can be
substantially less, with each pipe having a larger diameter.
In zone 2, the first low pressure zone, air, other gases absorbed on or in
the shale, and steam formed by volatilized water, are removed from the
shale by pipe 14. The pressure in zone 2 is lower than the pressures in
adjoining zones 1 and 3, with this lower pressure (approximately 2-10
inches of water below atmospheric) (50.8 - 254 kg./m.sup.2 below
atmospheric) being achieved, for example, by an ejector system, not shown.
The hot deaerated shale then passes into zone 3, the upper purge zone, in
which steam is introduced, via line 15 and a suitable distributor, to act
as a purging medium to assist in the physical separation of the gaseous
material in zone 2 from the desirable reaction products of zone 4. The
pressure in zone 3, due to the steam, is kept at super-atmospheric
pressure, such as about 10 to 50 inches of water (254-1270 kg./m.sup.2)
above atmospheric, pressure a pressure in excess of the pressures found in
zone 2 and zone 4. The temperatures found in zones 2 and 3 are not
critical but, due to heat losses, will be approximately the same as or
slightly below the temperature found in zone 1, especially if some or all
fresh shale is added to zone 3, as alternative to the shale addition to
zone 1.
In zone 4, the reaction zone, the desirable reaction products resulting
from the thermal decomposition of kerogen are separated from the shale and
removed from the reaction zone via line 16, typically leaving a
carbonaceous residue on the retorted shale. The temperature of zone 4
typically varies from about 800.degree. to 950.degree.F.
(426.degree.-510.degree.C) while the pressure is slightly more than
atmospheric (about 5-40 inches of water) (127-1016 kg/m.sup.2), sufficient
to maintain a flow of the reaction products. The presence of steam in zone
4 is non-deleterious since it is a minor amount of the volatile fluids
present and is readily condensable, in contrast to the hydrocarbonaceous
products obtained from the shale. Thus, downstream treatment of the
effluent from zone 4 in line 16 can be handled by typical equipment used
by those skilled in the art. The atmosphere in zone 4 is substantially
free of air or oxygen.
The retorted shale mixture, essentially free of desirable reaction products
and typically containing carbonaceous residue, moves to the lower purge
zone 5 wherein the temperature is in the range of about
800.degree.-950.degree.F. (426.degree.-510.degree.C.) and the pressure,
due to purge steam 17 introduced in zone 5 through a suitable distributor
is about 10 to 50 inches of water above atmosphereic pressure (254-1270
kg./m.sup.2). Here again, due to the zone pressure that is higher than
that found in adjacent zones, this purge zone acts to isolate the reaction
zone from the second low pressure zone, thus assuring a barrier to the
flow of combustion gases from the combustion zone 7. At the same time,
steam flowing from the lower purge section 5 to the reaction section 4, by
its sweeping action assures the removal of reaction products from the
reaction zone.
The mixture next enters zone 6, the second low pressure zone, in which, due
to the ejection system used, the pressure is below atmospheric (about 2-10
inches of water below atmospheric) (50.8-254 kg/m.sup.2 below
atmospheric). This ejector system, combined with higher pressure steam
from the lower purge zone, acts to remove any stray or residual gases from
the shale mixture via line 18, including combustion gases that are not
removed from the combustion zone 7. The temperature in this zone is
approximately 800.degree.-950.degree.F. (426.degree.-510.degree.C.).
In zone 7, the combustion zone, the descending shale encounters air,
preheated in 19, introduced by line 21, and distributed by distributor 20,
which results in controlled combustion of the carbonaceous residue, thus
producing heat and combustion gases. Though not shown, air can be
introduced into zone 7 by more than 1 pipe. The combustion of the residue
is controlled by regulating the conditions in zone 7. By varying the
retorting temperature in zone 4, the amount of carbonaceous residue can be
varied. It is realized that too much residue indicates improper retorting,
with accompanying loss of desirable reaction products. On the other hand,
a certain amount of residue is necessary to furnish the heat required in
other portions of the process. Thus, by varying the amount of residue
formed on and in the shale mixture and the amount of air (oxygen), and its
temperature, entering the combustion zone through line 21, the temperature
of the combustion zone, and thus the temperature of the hot spent shale,
can be controlled. Decomposition of carbonates in the shale is endothermic
in nature, so the combustion zone temperature is maintained to give a
desirable balance between minimal carbonate decomposition and enough heat
output for the rest of the process. Uncontrolled combustion in this zone
could lead to high temperatures that would require more expensive
materials of construction and would result in inefficient heat
utilization. Depending on the air flow, the combustion temperature, the
amount of carbonaceous residue, etc., the majority of the combustion gases
is composed of CO.sub.2, CO and N.sub.2, with a smaller amount of
unreacted O.sub.2. Also, depending on any sulfur or nitrogen compounds
found in the carbonaceous residue, there may also be sulfur and nitrogen
compounds in the vent gases, along with some dust. Typically, the
combustion gases, leaving zone 7 through line 8, can be vented through
line 22, utilized in the lift engager through line 23 and a thermal
compressor, used in air preheater 19, or run through another combustion
zone to oxidize the CO to CO.sub.2, or mixed with the fresh shale to
pre-heat the shale. The disposition of these waste gases through line 8 is
not critical to the invention; further utilization of the gases improves
the efficiency of the process.
The pressure in combustion zone 7 is approximately atmospheric, (2-5 inches
of water above atmospheric) (50.8-127 kg./m.sup.2) while the temperature
typically varies from about 900.degree.-1400.degree.F.,
(482.degree.-760.degree.C.), the maximum found in the process. In one
embodiment, a heat exchanging device, such as heat exchanger coils 24, can
be used in this zone to change boiler feed water 25 to steam 26, thus
utilizing a portion of the available heat. The combustion zone is the
primary heat source for the process. Controlled combustion of the
carbonaceous residue on and in the retorted shale mixture furnishes heat
to raise the temperature of the shale exiting this zone (hot spent shale)
to about 900.degree.-1400.degree.F. (482.degree.-760.degree.C.). At least
a portion of this hot spent shale is moved via hopper section 1, where it
is typically mixed with fresh feed shale 28. The mixture then proceeds to
the reaction section 4, in which the conditions of temperature and
residence time are sufficient to cause thermal decomposition of the
kerogen of the shale, thus liberating the desirable reaction products. As
mentioned previously, the transport of the hot spent shale to the recycle
hopper section is well understood by those skilled in the art.
Since the output of a retorting vessel is typically calculated in barrels
of desirable reaction products per day, this means that a certain number
of tons of fresh shale 29 must be retorted each day. Since a portion of
the hot spent shale is recycled, this means that a certain portion of the
spent shale is discarded through line 30. The overall efficiency of the
process is improved if recoverable heat is extracted from this discarded
spent shale, such as by a heat exchanger 31 in which boiler feed water 32
is converted to steam 33.
As an example of this process, fresh shale assaying 30 gal./ton, (129
l./metric ton) by Fischer assay, is used as a basis for calculating inputs
and outputs.
For every ton (0.9 metric ton) of fresh shale, C.sub.3.sup.- gas product is
obtained in the amount of about 700 SCF (19.8m.sup.3) (dry basis) having a
heating value of approximately 775 BTU/SCF (6900 kg. cal./m.sup.3). This
is approximately 48 lbs./ton (24g./kg.). A liquid C.sub.4.sup.+ product is
obtained in the amount of about 31.7 gal./ton (132 l./metric ton), having
about 0.8 wt.% S, 2 wt.% N and 24.degree. API. This is approximately 240
lbs./ton (120 g./kg.).
The retorted shale (from zone 4) is approximately 1712 lbs. (777 kg.),
based on 1 ton (907 kg.) of fresh shale, of which approximately 62 lbs. of
carbon/ton (31 g./kg.) of fresh shale are burned off under controlled
combustion, leaving approximately 1650 lbs. (750 kg.) of hot spent shale
discharged from zone 7 to the spent shale cooler per ton (907 kg.) of
fresh shale introduced. This amount of carbon is approximate and is based
on the reaction of carbon being oxidized to CO.sub.2, giving the
equivalent heat required to raise the temperature of one ton (907 kg.) of
fresh shale from ambient to reactor temperature. A greater quantity of
carbon will be burned if CO is an appreciable amount of the combustion
gas.
Recycle shale, at an approximate temperature of about 1200.degree.F.
(649.degree.C.), is withdrawn and used at about 5600 lbs. per ton (2.8
g./kg.) of fresh shale. This amount of recycle shale can vary, depending
on the amount of preheating done on the fresh shale.
The apparatus used for the above-described process is typically a
vertically oriented cylindrical retorting vessel, having the described
zones placed in the proper order and physically separated, one from the
other, as described. The physical dimensions of the vessel are naturally
dependent upon the desired throughput. The L/D ratio for the retort is not
critical but is typically in the range of about 8. Typical materials of
construction are used, since the maximum operating temperature is about
1400.degree.F (760.degree.C.). A 20 ft. (6.1m.) diameter retort would have
the capacity for producing 25,000 BPD (3980 m.sup.3 /day) of shale oil.
The relative sizes or proportions of the various zones, based on the total
vessel size, are approximately 25-30% for the reaction section, 35-40% for
the combustion zone, 5-10% for the recycle hopper zone and each of the
first and second low pressure zones, and 10-15% for each of the purge
zones. More importantly, the reaction zone is sufficient in volume to
maintain a fresh shale residence time of from about 2 to about 6 minutes.
While the present invention has been described herein with reference to
particular embodiments thereof, it will be appreciated by those skilled in
the art that various changes and modifications can be made without
departing from the scope of the invention as set forth.
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