Note: Descriptions are shown in the official language in which they were submitted.
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PROCESS FOR RECYCLING HETEROGENEOUS WASTE
BACKGROUND OF THE INVENTION
Field of Invention
This invention relates generally to a process for recycling mixed waste and
more
particularly to a process for maximizing the recycling of a heterogeneous
hazardous waste
stream having an uncontrolled, fluctuating content of carbon, metal, and
minerals into
separate, non-hazardous recycled components.
Description of Related Art
There is an ongoing need to dispose of hazardous waste generated by
industries,
most particularly the chemical industry. Hazardous waste is often either
buried or burned,
either of which can be costly processes, significantly increasing production
costs for the
products produced by the relevant industry. The costs for the disposal of
hazardous waste
typically, in part reflect the excise taxes and fees which must be paid to
legally dispose of
the waste. However, such hazardous waste disposal excise taxes and fees may be
reduced
or avoided totally by recycling the hazardous waste into commercial grade
chemicals and
materials, thereby decreasing the overall costs associated with disposal of
the waste.
Gasification is one method of disposing of hazardous waste materials.
Typically,
the gasification process involves the step of "pyrolysis", which involves
heating the waste
material to a temperature wherein any water, hydrocarbons, and organic
compounds are
volatilized and the remaining mineral and metallic constituents are melted
into a molten
slag. After cooling and solidifying, the molten slag may either be disposed of
or utilized
in the production of steel. The volatilized hydrocarbons and organic compounds
are
generally disposed of by burning, and may in fact be consumed as an energy
source.
However, under current regulations, energy recovery of this sort from
hazardous waste is
still classified as disposal rather than recycling, thereby still incurnng the
full amount of
taxes and fees associated with disposing of hazardous waste.
However, rather than burning the hydrocarbons and organic compounds, if the
oxygen concentration present during the gasification process is controlled, it
is possible to
partially oxidize the vaporized hydrocarbons and organic compounds producing a
"synthesis gas" which may be further processed. Synthesis gas typically
includes
substantial quantities of hydrogen (HZ) and carbon monoxide (CO), accompanied
by lesser
quantities of carbon dioxide (COZ) and water (HBO). Synthesis gas is a raw
material
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suitable for the production of a number of commercial grade chemicals such as,
for
example and not limitation, ammonia, methanol, and dimethyl ether. Since the
use of a
synthesis gas to generate commercial products is classified as recycling under
current
regulations, the excise taxes and fees associated with hazardous waste
disposal can be
S avoided by recycling the synthesis gas in this manner.
Methanol and dimethyl ether are both typically produced from synthesis gas on
an
industrial scale by a process involving the catalytic conversion of carbon
monoxide and
hydrogen. Methanol is produced from synthesis gas in the presence of a
methanol
synthesis catalyst by the reaction (2H, + CO -> CH30H). Dimethyl ether is
produced by
the dehydration of methanol in the presence of a methanol dehydration catalyst
by the
reaction (2CH30H ~ HBO + CH30CH3). Accordingly, it is often desirable to co-
synthesize methanol and dimethyl ether in a reactor containing both a methanol
synthesis
catalyst and a methanol dehydration catalyst.
Conventional methods of forming methanol require careful balancing of the
ratio
of H~ to CO present in the synthesis gas during the catalytic synthesis of
methanol to
approximately 2:1. An excess of carbon monoxide in the synthesis gas will
result
undesirable levels of carbon dioxide and carbon in the reactor, creating an
exothermic
event that overheats and ruins the catalyst. Conversely, an excess of hydrogen
produces
undesirable amounts of waste water during the methanol synthesis reaction
which results
in economically unfeasible treatment and purification costs. Accordingly,
careful control
of the composition and flow rate of the feedstock used to produce the
synthesis gas is
necessary for production of methanol or methanol and dimethyl ether.
For example, in one conventional process, coal is gasified using a strictly
controlled feed rate of oxygen, in order to obtain a synthesis gas having a
uniform
composition and at a uniform rate. In another conventional process, methane is
converted
into a synthesis gas in a reaction with a precisely controlled amount of steam
to produce a
synthesis gas having a uniform composition at a uniform rate. In each of these
cases, the
feed material from which the synthesis gas is produced has a uniform
composition, thereby
allowing narrow control of the ratio of HZ to CO in the synthesis gas.
Unfortunately, most industrial and hazardous wastes do not contain a uniform
mixture of materials. Workers commonly throw a variety of undesirable items
into the
waste receptacles. Additionally, hazardous waste can contaminate the
containers within
which it is stored and transported, creating additional waste. Accordingly,
the use of
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heterogeneous industrial and hazardous waste in a conventional gasification
processes will
result in a synthesis gas having a widely varying ratio of H, to CO, Thus, it
has generally
been thought that such heterogeneous industrial and hazardous wastes are
unsuitable for
use in the production of a synthesis gas suitable for the production of
methanol and
dimethyl ether.
Accordingly, it is an object of the present invention to provide a process for
maximizing the recycling of heterogeneous waste, such as municipal solid
waste,
industrial waste and hazardous chemical waste into a plurality of commercial
grade
products and chemical compounds, thereby realizing economic gains from the
resale of the
commercial grade products avoiding excise taxes and fees associated with
disposal of the
waste.
Furthermore, it is an object of the present invention to provide a process for
converting heterogeneous waste comprising a large number of miscellaneous,
unidentified
substances into a plurality of product streams having known compositions.
It is yet another object of the present invention to provide a system for
converting
heterogeneous carbon-containing waste into a synthesis gas having a desired
composition
suitable for the synthesis of methanol and/or dimethyl ether.
SUMMARY OF THE INVENTION
The above objectives are accomplished according to the present invention by
providing a process for recycling heterogeneous waste including the initial
step of
subjecting the heterogeneous waste to pyrolysis to produce a synthesis gas
stream
comprising at least carbon monoxide and hydrogen and to produce a molten
pyrolysis
product stream having a variable composition comprising at least a mineral
material and a
metallic material. The molten pyrolysis product stream is converted to a
plurality of
commercial grade solid materials. Likewise, the synthesis gas stream is also
converted
into at least one commercial grade chemical.
BRIEF DESCRIPTION OF THE DRAWINGS
The construction and design to carry out the invention will hereinafter be
described
together with other features thereof. The invention will be more readily
understood from a
reading of the following specification and by reference to the accompanying
drawings
forming a part thereof, wherein an example of the invention is shown and
wherein:
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FIG. 1 is a block diagram illustrating the basic material flow pathways by
which
heterogeneous waste is recycled in accordance with a preferred embodiment of
the present
invention.
FIG. 2 is a schematic illustrating the basic operation of a gasifier for use
in
accordance with a preferred embodiment of the present invention.
FIG. 3 is a block diagram illustrating a methanol purification process in
accordance
with a preferred embodiment of the present invention.
FIG. 4 is a block diagram illustrating a methanol and dimethyl ether
purification
process in accordance with a preferred embodiment of the present invention.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Referring now in more detail to the drawings, the invention will now be
described
in detail. As shown in FIG 1, a heterogeneous waste recycling process A
converts a
heterogeneous mixture of waste material into a plurality of commercially
useful solid
products and chemical compounds. The term "heterogeneous waste material" as
used
herein refers to a non-homogeneous carbon-containing feedstock, the
composition of
which can vary widely over time as the result of variations in the composition
of one or
more of the feedstock components and/or variations in the relative amounts of
the
components in the feedstock. Solid and liquid carbon-containing waste
materials
containing large amounts of inorganic material are processable as
heterogeneous feedstock
according to the present invention. The carbon content of heterogeneous waste
material
AA will generally vary by more than ten weight percent over a given twenty-
four hour
period. However, in its preferred embodiment, the process of the present
invention is
capable of processing heterogeneous waste material AA having a carbon content
varying
by as much as thirty weight percent to fifty weight percent over a twenty-four
hour period.
Preferably, the net heating value of the heterogeneous feedstock is greater
than
approximately three thousand Btu/lb. Examples of the carbon-containing waste
material
that can be processed according to the present invention include municipal
solid waste and
hazardous industrial wastes such as oil-contaminated dirt, demolition debris,
respirator
masks, paint and contaminated rags.
As shown in FIG. 1, heterogeneous waste material AA is compressed in a waste
compaction press 12 and fed into a gasifier 14 wherein it is pyrolytically
converted into an
undifferentiated molten slag stream BB and a raw synthesis gas stream CC
having a
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variable ratio of H, to CO unsuitable for conversion to methanol by
conventional methods.
Undifferentiated molten slag stream BB is allowed to gravitationally separate
to produce a
molten mineral stream DD and a molten ferric alloy stream EE which are water
quenched
in a quenching chamber 16. Once quenched, molten mineral stream DD yields a
vitreous
material suitable for commercial use as an industrial abrasive and molten
fernc alloy
stream EE yields a ferric alloy suitable for commercial use in die casting
metal production
processes or as a shock blast material. The vitreous material and ferric allow
material are
magnetically separated by magnetic separator 17.
With respect to raw synthesis gas stream CC, the H,:CO ratio is adjusted to
render
it suitable for the production of methanol or methanol and dimethyl ether.
First, the
H,:CO ratio of raw HZ synthesis gas stream CC is detected by synthesis gas
composition
sensor 18 and, in response to the sensed value thereof, a first portion FF of
raw synthesis
gas stream CC is directed via operation of a shift bypass valve 68 to shift
reactor 22 while
the remaining portion GG of raw synthesis gas stream CC flows through a shift
reactor
bypass line 24. Within shift reactor 22, first portion FF of raw synthesis gas
stream CC is
reacted with a selected amount of steam HH, both converting the CO therein to
CO, and
producing additional HZ via the shift reaction (CO + H20 -~ COZ + HZ) to
produce a shifted
gas stream II predominantly comprising COZ and HZ. Shifted gas stream II is
then mixed
with remaining portion GG of raw synthesis gas stream CC to form a mixed
synthesis gas
stream JJ having a desired ratio of Hz to CO and including substantial
quantities of COz.
Mixed synthesis gas stream JJ is directed to a COZ removal unit 26 wherein
approximately
greater than 98% of its CO~ content is removed, producing a CO, depleted mixed
synthesis
gas stream KK and a CO~ stream LL suitable for purification into commercial
grade CO,.
COZ depleted mixed synthesis gas stream KK is then converted in a liquid phase
catalyst
reactor 28 to a useful product stream MM comprising methanol or a combination
of
methanol and dimethyl ether and subjected to separation and purification by
methanol/DME recovery unit 30.
Alternatively, raw synthesis gas stream CC may also be utilized for the
synthesis of
ammonia according to the process disclosed in U.S. Patent Application Serial
No.
09/200,150, entitled "Process for making Ammonia from Heterogeneous
Feedstock,"
which is hereby incorporated by reference in its entirety. For conversion to
ammonia, all
of raw synthesis gas stream CC is directed through a shift reactor wherein its
CO content
is completely converted to CO,. Subsequently, the CO~ is removed and the
resulting
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stream of purified hydrogen is reacted with nitrogen to produce commercially
useful
ammonia.
As illustrated in FIG. 2, according to the process of the present invention, a
quantity of a heterogeneous waste material AA is first compacted by a waste
compaction
press 12 mounted at the front of a gasifier 14 into plugs having approximately
similar
dimensions and mass. In addition to forming plugs of heterogeneous waste
material AA,
waste compaction press 12 also operates to force the compacted plugs of
heterogeneous
waste material AA into gasifier 14. In the preferred embodiment, waste
compaction press
12 includes a conventional steel press rated at a maximum capacity of 320 psi,
although
any conventional press having sufficient capacity will suffice. Each
compression cycle of
waste compaction press 12 results in the introduction of a similarly sized
plug of
heterogeneous waste material into gasifier 14. Accordingly, the feed rate of
heterogeneous
waste material into gasifier 14 may be regulated simply by altering the cycle
time of waste
compaction press 12. In the preferred embodiment, waste compaction press 12
operates at
1 S a rate on the order of approximately 20 cycles per hour.
Gasifier 14 may be any conventional gasifier. Preferably, gasifier 14 is of
the type
disclosed in U.S. Patents Nos. 5,788,723, 5,711,924, 5,282,431 and 5,707,230,
which are
incorporated herein by reference in their entireties. In the preferred
embodiment, gasifier
14 includes an externally heated gasifier vessel 40 having a feed aperture 42
located along
its midplane, a synthesis gas outlet 44 located at its upper end and a slag
outlet 46 located
at its bottom end. Feed aperture 42 serves as an open conduit to the
atmosphere through
which compacted plugs of heterogeneous waste material AA may be fed by press
12 into
gasifier vessel 40. Accordingly, gasifier 14 operates at approximately
atmospheric
pressure. Upon injection into gasifier vessel 40, heterogeneous waste material
AA is
subjected to gasification by being heated to a pyrolysis temperature generally
between
2000° C and 3000° C, sufficient to volatilize any water,
hydrocarbons, and other organic
compounds entrained therein. The mixture of volatilized gases rises to the top
of gasifier
vessel 40 where it may undergo further reaction prior to exiting gasifier
vessel 40 through
synthesis gas outlet 44.
Meanwhile, the solid portions of heterogeneous waste material AA, including
non-
volatile organic compounds, metals, minerals and metallic oxides, fall to the
bottom of
gasifier vessel 40 forming a gasifier feed pile NN which is eventually melted
into an
undifferentiated molten slag stream BB. A selected amount of oxygen is
injected into the
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lower portion of the gasifier vessel 40 to react with carbon and non-volatile
organic
compounds in gasifier feed pile CC, liberating additional CO, CO~ and H20 into
the
gasifier vessel headspace. The remainder of undifferentiated molten slag
stream BB flows
through slag outlet 46 to an elongated separation chamber 54.
An excess carbon inventory should be maintained within the interior of
gasifier
vessel 40 in order to prevent an exothermic reaction of oxygen with the carbon
monoxide
in the pyrolysis gas, which can result in damage to gasifier vessel 40 and
potentially an
explosive exothermic event. In the preferred embodiment, this carbon excess is
ensured
by monitoring the height of gasifier feed pile NN and adjusting feed rate of
heterogeneous
waste material AA to maintain gasifier feed pile NN above a minimum height
which
assures that an excess carbon inventory is present within oxygen to gasifier
vessel 40. The
height of gasifier feed pile NN is preferably sensed by a gamma ray
attenuation detector
50, which measures the attenuation of gamma radiation emitted from a source 52
having a
known intensity diminution as it passes through gasifier feed pile NN.
In the preferred embodiment, undifferentiated molten slag stream BB flows
through slag outlet 46 to separation chamber 54 wherein the components of the
slag
gravitationally separate into a layer of mineral material floating on top of a
layer of molten
ferric alloy. Separation chamber 54 includes weir 56 over which the upper
mineral layer
and lower ferric alloy layer alternatively flow. The molten mineral stream and
molten
fernc alloy stream are then quenched in a quenching chamber 16, which includes
a 30 inch
diameter tube through which the molten material falls while being sprayed with
jets of
water which breakup the molten material into small particles while quenching
it. The
quenched particles then fall to the bottom of quenching chamber 16.
Upon quenching. the molten mineral stream DD, solidifies into particles of a
vitreous material having a specific gravity of approximately 2.25. The
vitreous material is
generally useful as an airblast abrasive when pulverized to an appropriate
size. This is a
particularly useful commercial product since a market exists for approximately
one million
tons of such airblast abrasive per year and the current primary sources for
this material,
coal fired power plants, are currently being phased out. Upon quenching the
molten fernc
alloy stream EE, solidifies into particles of a steel shot, wherein the
majority of
environmental metals are alloyed into the steel. This steel shot is generally
suitable for use
as feedstock into die casting metal production processes or, after tempering,
as a shock
blast material.
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Following quenching, the mixed particles of vitreous material and steel shot
are
transferred by a bucket elevator from the bottom of quenching chamber 16 to a
magnetic
separator 17. Magnetic separator 17 operates to separate the particles into a
steel shot
product stream comprising particles of the ferrous alloy and a vitreous stream
comprising
particles of the vitreous material.
The temperature of the pyrolysis gas in gasifier vessel 40 is preferably
maintained
at a value of at least 2100 degrees C, sufficient to crack entrained
hydrocarbons to form a
carbon soot and HZ and to drive the endothermic reactions necessary for the
production of
a synthesis gas rich in H, and CO. Steam produced by the heat of fusion
liberated by the
quench reactions of the mineral and fernc slag streams flows as a counter
current back into
the gasifier vessel 40 allowing for the conversion of carbon from soot and
entrained
hydrocarbons to CO in the pyrolysis gas via the endothermic reaction (C + HZO -
~ CO +
HZ), further increasing the concentrations of CO and HZ in the synthesis gas.
Additionally,
a substantial amount of the carbon and COZ in the pyrolysis gas is also
converted to CO
via the Boudard reaction (COz + C -~ 2 CO).
The temperature and gas flow rate of gasifier vessel 40 are controllable to
desired
values as follows. The temperature of the pyrolysis gas in the upper portion
of gasifier
vessel 40 is controllable by adjusting the rate at which oxygen is injected
into the upper
portion of gasifier vessel 40 to exothermically react with carbon, CO and H,
therein.
Accordingly, temperature of the pyrolysis gas is increased by increasing the
amount of
oxygen injected into the upper portion of gasifier vessel 40 and decreased by
decreasing
amount of oxygen injected into the upper portion of gasifier vessel 40. In the
preferred
embodiment, a small amount of methane may also be injected with the oxygen
into the
upper portion of gasifier vessel 40 to avoid quenching the pyrolysis gas prior
to injection
of the oxygen.
The flow rate of gas exiting gasifier vessel 40 is controllable by altering
the rate at
which oxygen is injected into the bottom portion of gasifier vessel 40. To
increase the
flow rate of gas leaving gasifier vessel 14, the flow rate of oxygen into the
bottom portion
of gasifier vessel 40 is increased, driving the exothermic gasification of
carbon and non-
volatile organic constituents of gasifier feed pile NN into CO, H, and CO,,
thereby
increasing the gas flow rate leaving gasifier vessel 40. Increasing the flow
of oxygen into
the bottom portion of gasifier vessel 40 may also necessitate increasing the
feed rate of
heterogeneous waste material AA into gasifier vessel 40 to compensate for the
additional
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consumption of material from gasifier feed pile NN. Of course, increasing the
feed rate of
heterogeneous waste material AA into gasifier vessel 40 will increase the
amount of steam
flowing back into gasifier vessel from quenching the resulting increased flows
of molten
mineral material DD and molten fernc alloy EE, which may also serve to
increase the gas
flow rate since the steam may react with carbon components in gasifier feed
pile NN to
produce CO, H, and CO,. Conversely, to reduce the gas flow rate exiting
gasifier vessel
40, the oxygen injection rate into the bottom portion of gasifier vessel 40 is
simply
decreased.
The raw synthesis gas CC exiting gasifier vessel 40 through synthesis gas
outlet 44
has a variable composition, typically comprising at least carbon monoxide
(CO), carbon
dioxide (CO,) and hydrogen (Hz). The raw synthesis gas CC preferably has a
composition
of approximately twenty volume percent to approximately fifty-four volume
percent of
both CO and HZ, with the amount of CO exceeding the amount of H~.
Additionally, raw
synthesis gas CC will usually include approximately twenty volume percent to
thirty
volume percent CO2. The raw synthesis gas CC exiting gasifier vessel 40 also
may
include approximately 1 % carbon soot and trace amounts of sulfur, halogens,
and volatile
metals.
Next, the raw synthesis gas CC is directed to gas treatment system 58 wherein
it is
quenched with water at a 25:1 water/gas ratio. The quench reaction occurs
through a
complex reversing flow which shock cools raw synthesis gas stream CC to
approximately
156°F. The use of high speed shock cooling ensures that there is no
time for the de novo
synthesis of dioxins and dibenzofurans during the quench. In addition to
cooling raw
synthesis gas stream CC, the quench water serves to remove the majority of
contaminating
soot from raw synthesis gas stream CC. The quench water also absorbs halogens
and the
majority of contaminating sulfur. This lowers the pH of the quench water to
approximately two, increasing the quench water's solubility for metals and
thereby
allowing it to also dissolve the majority of contaminating metals from raw
synthesis gas
stream CC. In the preferred embodiment, the quench water is subjected to a
series of
conventional water filtration and precipitation treatment steps, as would be
known to one
of ordinary skill in the art, wherein the majority of these contaminants are
separated and
recycled.
Following the water quench, raw synthesis gas stream CC preferably undergoes a
series of wash steps to further remove contaminants. First, raw synthesis gas
stream CC is
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subjected to an alkaline wash to remove any acid radicals. Next, raw synthesis
gas stream
CC is subjected to a glycerin wash to remove hydrophobic carbon particles.
Next, raw
synthesis gas stream CC is subjected to a sulfur chealating wash, using a
sulfur chealating
agent which removes H,S, COS and other contaminating sulfur compounds. Next,
raw
synthesis gas stream CC is subjected to a chilled water wash at approximately
45°F to
condense mercury. Finally, raw synthesis gas stream CC is reheated to
approximately
113°F and passed through an activated carbon filter for a final polish.
In this presently
preferred system, following the wash steps, raw synthesis gas stream CC is
extremely
pure, having approximately 50 parts per billion sulfur, 50 parts per billion
halogens and 10
parts per billion of heavy metals.
As previously mentioned, the stoichiometric ratio of Hz to CO required for the
synthesis of methanol is 2:1. Conventional processes for methanol synthesis
are run in a
reactor at an excess of HZ, for example having HZ:CO ratios of 2.1-2.2:1, in
order to avoid
overheating the catalyst or Booting up the catalyst through the deposition of
carbon upon
the methanol synthesis catalyst. Unfortunately, the excess hydrogen in the
process results
in the production of greater amounts of waste water from the reaction. As this
waste water
contains detectable percentages of methanol and other byproducts, it must be
treated prior
to release. Therefore, minimization of the amount of waste water produced
during the
reaction would be desirable.
In the present process, therefore, the synthesis gas fed into the methanol
synthesis
reaction is desired to have at most no more than the stoichiometric amount of
hydrogen,
and preferably less than the stoichiometric amount of hydrogen, thereby
minimizing the
generation of waste water. For example, in the preferred embodiment, the ratio
of HZ:CO
of the synthesis gas fed to the reactor should preferably be CO rich, between
1.95-2.0:1
inclusive, for the synthesis of methanol. Accordingly, it is necessary to
adjust the ratio of
HZ:CO of the synthesis gas to the desired range.
As shown in FIG. l, after washing, the ratio of H~:CO in raw synthesis gas
stream
CC is sensed by a raw synthesis gas composition sensor 18. In the preferred
embodiment,
raw synthesis gas composition sensor 18 includes an infrared
spectrophotometric sensor 62
for sensing the amounts of CO and CO, in raw synthesis gas stream CC and a
specific heat
sensor 64 for sensing the amount of HZ in raw synthesis gas stream CC.
Infrared
spectrophotometric sensor 62 operates generally by measuring the absorption,
at
wavelengths specific for CO and CO, respectively, of a beam of infrared light
passing
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through raw synthesis gas CC. Specific heat sensor 64 is an online gas
analyzer which
operates by diverting a sample flow from raw synthesis gas stream CC, heating
the sample
stream to a known temperature, and then adding a standard energy input, such
as from a
heating filament, and measuring the change in temperature of the sample stream
to
determine the specific heat of raw synthesis gas stream CC. Since the specific
heat of H
is approximately an order of magnitude higher than the specific heats of CO
and CO,
respectively, the specific heat of raw synthesis gas stream CC directly
relates to its
approximate hydrogen content.
Raw synthesis gas stream CC is compressed to a pressure of, for example, about
160 psi or more, using raw synthesis gas compressor 60 in order to drive it
through
subsequent process steps. The ratio of H,:CO of the raw synthesis gas CC is
adjusted by
directing a first portion FF of raw synthesis gas stream CC into a shift
reactor 22, while
the remaining portion GG of raw synthesis gas stream CC flows through a shift
reactor
bypass line 24. The percentage of raw synthesis gas stream CC diverted into
shift reactor
22 is controlled by regulating the position of shift bypass valve 68 in
response to the
sensed composition of raw synthesis gas stream CC. In the preferred
embodiment, the
percentage of raw synthesis gas stream CC to be diverted is controlled in
response to the
measured ratio of HZ:CO of raw synthesis gas stream CC.
Within shift reactor 22, a selected amount of water HH, in the form of steam,
is
mixed with first portion FF of raw synthesis gas stream CC. The steam and CO
in the first
portion FF of raw synthesis gas stream CC react via the shift reaction (CO +
H20 ~ COZ
+ HZ) to produce a shifted gas stream II containing primarily COZ and H2. In
the preferred
embodiment, substantially all (approximately ninety eight percent) of the CO
content of
first portion FF of raw synthesis gas stream CC is converted into CO2. The
amount of
steam injected into shift reactor 22 is selected to approximately correspond
to the CO
content of first portion FF of raw synthesis gas stream CC based upon the
sensed HZ:CO
ratio of raw synthesis gas stream CC, thereby maximizing conversion of CO to
COZ and
minimizing the water content of shifted gas stream II.
In a simplified exemplary embodiment, the percentage of raw synthesis gas
stream
CC which must be diverted into shift reactor 22 is generally determined under
the
relationship x = (2y-z)/3y, wherein x is the percent of raw synthesis gas
stream CC to be
shifted, y is the initial percentage of raw synthesis gas stream CC which is
CO and z is the
initial percentage of raw synthesis gas stream CC which is HZ. This
relationship takes into
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account both the decrease in CO and the increase in H= which result from the
shift
reaction. For example, to shift a raw synthesis gas stream CC having a
composition of 40
percent CO, 30 percent H~, and 30 percent CO, to a desired H,:CO ratio of 2:1
we find that
x = (2(0.4)-.3)/3(.4) which simplifies to x = .4166 or 41.66%. Therefore, to
shift a raw
synthesis gas stream CC having a composition of 40 percent CO, 30 percent H,,
and 30
percent COZ to a desired H,:CO ratio of 2:1, approximately 41.66% of the flow
of raw
synthesis gas stream CC must be directed to shift reactor 22.
Shifted gas stream II is then mixed with remaining portion GG of raw synthesis
gas
stream CC to form a mixed synthesis gas stream JJ having the approximate
desired HZ:CO
ratio (approximately 1.95 to 2.0) and including substantial quantities of CO,.
The actual
ratio of H,:CO in mixed synthesis gas stream JJ is sensed by mixed synthesis
gas
composition sensor 70 and may be used as a trim signal to make minor
adjustments to the
bypass flow rate and steam flow rate to shift reactor 22. Mixed synthesis gas
composition
sensor 70 may be any means of sensing the composition a raw synthesis gas
stream CC
which would be known to one of ordinary skill in the art. However, in the
preferred
embodiment, mixed synthesis gas composition sensor 70 includes an infrared
spectrophotometric sensor 72 and a specific heat sensor 74 similar to those of
raw
synthesis gas composition sensor 18.
In the preferred embodiment, mixed synthesis gas stream JJ is subsequently
directed to a COZ removal unit 26 wherein approximately greater than 98% of
its COZ
content is removed, producing a CO~ depleted mixed synthesis gas stream KK
having the
desired ratio of HZ:CO and a COZ stream LL suitable for purification into
commercial
grade COz. In the preferred embodiment, CO, removal unit 26 operates by
passing mixed
synthesis gas stream JJ through an aqueous solution of an amine base which
capable of
binding the carbonic acid form of CO,. Of course, one of ordinary skill in the
art will
recognize that COZ removal unit 26 may be selected from a number of other
conventional
COZ removal systems. Also, in alternative embodiments, the carbon dioxide from
the shift
reaction can be left in the synthesis gas and removed following the formation
of the
methanol and/or dimethyl ether, if desired, without adversely affecting the
reaction except
by increasing the amount of waste water in the product.
The COz depleted mixed synthesis gas stream KK is next preferably compressed
to
approximately 950 psia by mixed synthesis gas compressor 78 and directed into
liquid
phase catalyst reactor 28 for conversion into a useful product stream MM
comprising
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either methanol or a mixture of methanol and dimethyl ether. Liquid phase
catalyst reactor
28 is preferably of the type developed by Air Products and Chemicals, Inc., as
disclosed in
U.S. Patents No. 5,179,129, 5,218,003, 4,910,227, 4,766,154, 5,284,878 and
4,628,066
which are incorporated herein in their entireties by reference. As described
in these
references, the liquid phase reactor may be selectively operated at from
approximately 750
to 1500 psia to produce a product stream MM comprising either methanol or a
mixture of
methanol and dimethyl ether from a CO rich synthesis gas stream such as CO,
depleted
mixed synthesis gas stream KK. Methanol is produced from synthesis gas in the
presence
of a methanol synthesis catalyst by the reaction (2H2 + CO --~ CH~OH).
Dimethyl ether is
produced by the dehydration of methanol in the presence of a methanol
dehydration
catalyst by the reaction (2CH30H --~ H,O + CH30CH3)
In the preferred embodiment, liquid phase catalyst reactor 28 operates at a
temperature of about 200°C to 250°C and a pressure of about 950
psia. Liquid phase
catalyst reactor 28 includes at least one methanol synthesis catalyst, such as
a conventional
copper-containing catalyst, suspended in an inert liquid. The liquid for the
liquid phase
reactor may be any suitable liquid described in the foregoing references
incorporated
herein by reference, including, for example, hydrocarbons, alcohols, ethers,
polyethers,
etc. For producing both methanol and dimethyl ether, the liquid phase reactor
should
contain not only the methanol synthesis catalyst, but should also contain a
methanol
dehydration catalyst. The methanol dehydration catalyst can be any
conventional catalyst
known in the art for this purpose including, for example, alumina, silica-
alumina, zeolites,
solid acids such as boric acid, solid acid ion exchange resins such as
perfluorinated
sulfonic acid, etc.
Since the COZ depleted mixed synthesis gas stream KK has a HZ:CO ratio
slightly
lower than the stoichiometric value of 2.0 for the methanol synthesis
reaction, the
production of waste water in liquid phase catalyst reactor 28 is minimized. In
fact,
compared to conventional gas phase methanol synthesis processes that operate
on the
hydrogen rich side of the stoichiometric value, the amount of waste water
produced in the
present process is reduced on the order of ten times or more.
Liquid phase catalyst reactor 28 is effective to convert approximately 40% of
the
CO and HZ in COZ depleted mixed synthesis gas stream KK to methanol per pass
through
liquid phase catalyst reactor 28. Accordingly, product stream MM also includes
the
remaining unreacted 60% of the initial CO and HZ from COz depleted mixed
synthesis gas
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stream KK. Therefore, product stream MM is next directed to condenser 80
wherein the
products methanol and dimethyl ether and any contaminating water and CO, are
condensed to form a liquid product stream 00. The remainder of product stream
MM,
comprising mostly CO and Hz is then recycled back into liquid phase catalyst
reactor 28
for subsequent reaction into methanol or methanol and dimethyl ether. By
continually
recycling CO and H~ from product stream MM, near complete conversion of the CO
and
HZ to methanol or methanol and dimethyl ether may be achieved. However, small
waste
gas stream PP comprising approximately two percent of the flow of product
stream MM is
removed from liquid phase catalyst reactor 28 via a purge line to prevent the
buildup of
any contaminating non-reactive gases therein.
Liquid product stream 00 is fed to methanol/DME recovery unit 30 to separate
the
methanol and,dimethyl ether products from carbon dioxide and water. As seen in
FIG. 3,
when liquid phase catalyst reactor 28 is operating in methanol only mode,
methanol/DME
recovery unit 30 includes a first distillation column 86 through which liquid
product
stream 00 is passed for separating out any carbon dioxide which may be
dissolved
therein. Liquid product stream 00 then passes through a second distillation
column 88
which separates out any contaminating water, producing a high purity methanol
stream
RR.
As seen in FIG. 4, when liquid phase catalyst reactor 28 is operating in
methanol/dimethyl ether mode, methanol/DME recovery unit 30 includes a first
distillation column 90 through which liquid product stream 00 is passed to
separate it into
a methanol/water stream SS and a dimethyl ether/carbon dioxide stream TT.
Methanol/water stream SS is then passed through a second distillation column
92 which
separates out any contaminating water to produce high purity methanol stream
RR.
Dimethyl ether/carbon dioxide stream TT is passed through a third distillation
column 94
which separates out any contaminating carbon dioxide to produce high purity
dimethyl
ether stream UU.
Thus, it may be seen, that an advantageous process to maximize the recycling
of
heterogeneous waste is provided according to the present invention. The
recycling of
heterogeneous waste may be maximized by pyrolytically converting it into a
plurality of
useful solid components and a synthesis gas having a variable composition,
including CO
and H2. By removing a portion of CO from the synthesis gas, the composition of
the
synthesis gas may be adjusted to render it suitable for the production of
methanol and/or
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dimethyl ether. Furthermore, by using a liquid catalyst reactor, the
production of methanol
and/or dimethyl ether may be accomplished while minimizing the generation of
waste
water. Accordingly, by utilizing the present invention, a heterogeneous waste
material
having a variable and unknown composition is convertible into a plurality of
useful
material streams having known compositions, including a ferric alloy stream, a
vitreous
material stream, a methanol stream, a dimethyl ether stream, a carbon dioxide
stream, and
miscellaneous streams containing sulfur and salts, thereby realizing economic
gains from
the resale of the commercial grade products in addition to receiving excise
tax and fee
benefits.
It thus will be appreciated that the objects of this invention have been fully
and
effectively accomplished. It will be realized, however, that the foregoing
preferred
specific embodiment has been shown and described for the purpose of this
invention and is
subject to change without departure from such principles. Therefore, this
invention
includes all modifications encompassed within the spirit and scope of the
following
claims.
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