METHODE DE PRODUCTION DU METHANOL

PROCESS FOR PRODUCTION OF METHANOL

Abrégé anglais

PROCESS FOR P~ODUC~IQN OF ~ETNANO~
A~STRACT
An improved process for the production of a
met~anol is provided. A purge gas is removed from the
synthesis reaction stage at an elevated pressure and is expanded
prior to combustion in a gas turbine combustor. Exhaust gases are
expanded and then sent to the re~orming stage for either pre-
heating reformer feed streams or providing h~at ne~essary for the
reforming of a feed stream with steam.

Revendications

The embodiments of the invention in which an exclusive
property or privilege is claimed are defined as follows:
1. In a process for the production of methanol
including reforming, energy recovery, compression and synthesis
reaction stages, wherein a purged gas at an elevated pressure
is removed from said synthesis reaction stage, the improvement
comprising the steps of
(a) passing said purged gas through an expander,
said expander being operatively connected to
a compressor, so as to recover energy from said
purged gasline pressure and to use said energy
to drive said compressor,
(b) combusting said expanded purged gas with an oxygen-
containing gas to obtain a heated exhaust gas,
said heated exhaust gas including oxygen,
(c) expanding said heated exhaust gas to recover
energy therefrom, and
(d) passing said expanded heated exhaust gas to said
reformed stage, a portion of said heated exhaust
gas containing oxygen being combusted with a
fuel in said reforming stage to generate heat
for use in said reforming stage.
2. The improvement of Claim 1 further comprising
the step of passing a portion of said expanded heated exhaust
gas through an indirect heat exchanger for preheating said
oxygen containing gas prior to combustion of said oxygen containing
gas with said purge gas.
3. The improvement of Claim 1 further comprising
the step of passing said expanded heated exhaust gas through
a waste heat boiler prior to the passage of said gas to said
reforming stage.
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4. The improvement of Claim 1 wherein said step
of passing said expanded heated exhaust gases to said reforming
stage comprises passing one portion of said gases through
a reformer convection section for preheating reforming stage
feed streams, and passing the remainder of said gases into
a reformer furnace section for combustion of said portion
of oxygen contained in said gas.
5. A process for the production of a crude reaction
product comprising the steps of
a) preheating a stream including natural gas and
steam,
b) passing said preheated stream through a plurality
of catalyst line reforming tubes disposed in a furnace
section of a reformer,
c) combusting a fuel with an oxygen containing
gas within said furnace section of said reformer to heat
said stream passing through said plurality of tubes,
a portion of said natural gas reacting with a portion
of said steam within said tubes to form carbon monoxide,
carbon dioxide and hydrogen gases,
d) removing a gaseous stream including carbon
monoxide, carbon dioxide, hydrogen, natural gas and steam
from said reforming tubes at an elevated temperature,
e) passing said gaseous stream through heat exchange
means to reduce the temperature of said stream and
recover heat energy therefrom,
f) compressing said gaseous stream,
g) passing said compressed gaseous stream into
a synthesis reactor, said carbon monoxide reacting with
said hydrogen, and said carbon dioxide reacting with
said hydrogen to yield methanol,
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h) removing a resultant gaseous product stream from
said synthesis reactor, said product including methanol, steam,
carbon dioxide, carbon monoxide, hydrogen and natural gas,
i) cooling said resultant gaseous product stream such
that a major portion of said methanol condenses to a liquid,
j) separating said condensed methanol from the gaseous
remainder of said cooled resultant product stream,
k) recirculating a major portion of said gaseous
remainder for addition to said gaseous stream prior to the
introduction of said gaseous stream to said synthesis reactor,
l) passing the remaining minor portion of said gaseous
remainder through an expander, said expander being operatively
connected to a compressor so as to recover energy from said
minor portion of said gaseous remainder and to use said energy
to drive said compressor, the pressure of said remainder being
reduced as it passes therethrough,
m) combusting said expanded gaseous remainder with
an oxygen containing gas to yield a heated exhaust gas,
n) expanding said heated exhaust gas, and
o) passing said expanded heated exhaust gas into said
reformer.
6. The process of Claim 5 wherein said step of
passing said expanded heated exhaust gas into said reformer
comprises passing said exhaust gas into said furnace section
for combustion with a fuel therein.
7. The process of Claim 5 wherein said step of
passing said expanded heated exhaust gas into said reformer
comprises passing said exhaust gas into a convection section
of said reformer.
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Description

1~9~3~4
P~OCESS FOR PRODUCTION OF METHANOL
BACKGROUND OF THE INVENTION
This invention relates to an improvement in a process
for the production of a crude reaction product such as liquid
phase methanol. More specifically, this invention provides
a method of utiliæing purge gas commonly associated with the
synthesis stage of the overall process so as to improve the
efficiency of the overall process.
A typical methanol synthesis process includes
reforming, energy recovery, compression and synthesis reaction
stages. A purge gas is normally removed from the synthesis
reaction stage and then used as a fuel in the reforming stage.
According to the present invention, the purge gas is initially
passed through an expander to provide mechanical energy, then
used as a fuel for production of electrical energy and
ultimately used as a source of heat to generate steam, preheat
feed streams, or heat reactants in the reforming stage. Since
this method uses the potential energy associated with
pressurized purge gas as well as the chemical energy of the
purge gas, more efficient use of the total energy associated
with the purge gas is achieved.
SUMMARY OF THE INVENTION
In accordance with an illustrative embodiment
demonstrating features and advantages of the present invention
there is provided an improvement in a process for the
production of methanol which process includes reforming, energy
recovery, compression and syn-thesis reaction stages.
A purge gas at an elevated pressure is removed from
the synthesis reaction stage and is passed through an expander
to recover energy from the purge gas line pressure. The
expanded gas is combusted to yield a heated exhaust gas which
is expanded and thereafter sent to the reforming stage.
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The invention, in :Its broadest aspectJ contempla-tes
a process for the production of methanol including reforming,
energy reco~ery, compression and synthesis reaction stages,
wherein a purged gas.at:an elevated pressure is removed
from the synthesis reaction stage. The improvement comprises
the steps of passing the purged gas through an expander,
with the expander beingoperatl~èly connected to a compressor,
so as to recover energy from the purged gasline pressure
and to use the energy to drive the compressor. The expanded
purged gas is combusted with.an oxygen-containing gas
to obtain a heated exhaust gas which includes oxygen.
The heated exhaust gas is expanded to recover energy therefrom,
and the expanded heated ~xhaust gas is passed to the reformed
stage with a portion ofthe heatea exhaust gas containing
oxygen being combusted with a fuel in the reforming stage
to generate heat for use ln the reforming stage.
BRIEF DESCRIPTION OF THE DRAWINGS
The above brief description as well.as further
ob~ects, features and adYantages of the present invention
will ~e more fully appreciated by.xeferenc:e to the following
description of presently preferred but nonetheless illustrative
embodiments in accordance with the present invention when
taken in connection with the.accompanying drawings, wherein:
FIG. 1 is a schematic representation of a methanol
synthesis process incorporating the present invention;
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FIG. 2 is a schematic representation of an alternative
arrangement of the present invention.
BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT
Referr~ng to FIG. 1, there is shown.a schematic
diagram illustrative of.a typical process 10 for production
of methanol. In the preerred embodiment methanol is
produced from natural gas. However, it is to be understood
that other feedstock~ such as naptha can be used,.and
other crude reaction products such as.ammonia can be obtained.
A stream of natural gas is passed through line
12 into.a preheater 14 disposed within the convection
section 16 of primary reformer lB. Thereafter the preheated
nat~ral gas is passed through llne 20 to desulurizer
22. A stream of hydrogen gas is passed through line 24
and added to -the preheated natural gas at a point 26 upstream
of desulfuri~er 22. The combined stream of natural gas
and hydrogen is removed from desulfurizer 22, and.added
to a stream of steam flowing through line 27~ The combined
stream of natural gas,. hydrogen,.and steam is then passed into
another preheater 28 disposed in convection section 16. The
preheated combined stream is then passed via line 29 through a
series of reormer tubes 30 filled with catalyst, such as a
nickel-containing material, within the furnace section 32 of
primary reformer 18. As the combination stream passes through
the tubes 30 the natural gas, hydrogen a~d steam, react to
yl~d a synthesis gas stream including carbon monoxide,
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~lg634~
carbon dioxide, and hydrogen gas. A portion of the natural gas
usually does not react with a portion of the ~team and therefore
the gas stream obtained from the reforming stage will include
carbon monoxide, carbon dioxide, hydrogen, ste~m and unreacted
natural gas. This gas exits reformer 18 at a temperature of
approximately 1500 degrees F. and is passed through line 34 into
a waste heat boiler 36. In boiler 36 ~ome of the heat is removed
from the syn~hesis gas stream and i5 absorbed by water circulated
through boiler 36, thereby generating steam which is removed ~rom
boiler 36 through line 38. A portion of the steam removed through
line 38 is sent through line 27 for combination with the preheated
natural gas and hydro~en stream. The remainder of the steam is
passed through line 40 to an expander 42; after expansion the
steam is sent thxough line 44, and supplemented by make-up water
introduced through line 46. Thereafter the expanded steam and
make-up water will pass through a feedwater heater 48 and then
be recirculated to boiler 36 via line 49.
After passing through waste hea~ boiler 36 tha synthesis
gas is removed at a temperature of approximately 450 degrees F.
and then passes through feedwater heater 48 wherein it gives off
another portion of its heat to the boiler feedwater passing in an
indirect heat exchange relation therethrough. Upon exiting heater
48 the temperature of the synthesis ga8 is approximately 425 degrees
F. The synthesis gas is then pas~ed via line 50 through a second
waste heat boiler 52 for additional heat recovery. Altbough not
shown, it is to be understood that this boiler could generate
steam that would later be used for purification of the
methanol. , The synthesis gas is removed from the second waste
heat boiler 52 at a temperature of approximately 340 degrees F. The
synthesis gas is next sent via line 53 through a cooler 54 to fur-
ther reduce the temperature of the synthesis gas to approximately
110 degrees F. It should be understood that a fan could also be
used to cool the synthesis gas to this level, in which case the fan
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could ~e disposed upstream or do~nstream of cooler 54.
The synthesis gas removed from cooler 54 at approxi-
mately 110 degrees F. is at a pressure of approximately 400 psi.
This gas is then sent via line 55 to compressor 56 in order to
raise the pressure of the synthesis gas to approximately 1500 psi.
Some hydrogen is bled from the gas stream at a point downstream
of compressor 56, and is sen~ via line 24 to poin~ 26 for addition
to the feed straam. Thereafter the synthesis gas is passed through
line 5a to a second stage of compression within compressor 60.
The synthesis gas is compre~sed to approximately 1600 psi as it
passes through compressor 60. The compressed syn~hesis gas is then
introduced to reactor 62 wherein the carbon monoxide, carbon diox-
ide, and hydrogen gas react to form a product gas which includes
methanol. ~he gaseous phase product is removed from reactor 62
through line 64 and then passed through partial condenser 66 wherein
most of the methanol included in the product gas is condensed to a
liquid phase. Thereafter the entire stream of product is intro-
duced to separator drum 68. Within drum 68 ~he liquid methanol is
separated from the gaseous portion of the crude reaction product and
is removed through line 70. The liquid methanol is then compressed
in liquid compressor 72 and is ultimately removed as crude liquid
product through line 74. The gaseous portion of the crude reaction
product is removed from drum 68 through line 76. A portion of the
gaseous product is recirculated through line 78 and combined with
incoming compressed synthesis gas at a point 80 upstream of com-
pressor 60; the portion of the gaseous product passed through line
78 represents approximately 90% of the ga~eous product removed from
drum 68, and is commonly referred to as "recycle" gas. The remain-
ing portion of the gaseous product remo~ed from drum ~8 is passed
through line 82; this portion of the gaseous produc~ i~ commonly
referred to as "purge" gas. The purge gas comprises hydrogen,
carbon monoxide, carbon dioxide, steam~ gaseous methanol, and
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34~
unreacted natural gas. No:rmally -the purge gas is sent to
the ~urnace section of primary reformer 18, and is combustecl
as a fuel -to provide heat necessary -to promo-te the reac-tlon
of the natural gas with steam within reformer tubes 30.
According to the present invention -the purge gas
is sen-t through a line 82 to an expander 84 wherein the purge
gas is expanded from a pressure of approximately 1400 pii
to a pressure of approximately 210 psi. Expander 84 drives
one or more compressor services, such as compressor 86
associated with gas turbine 88, The expanded purge gas is
then sent to the gas turbine combuster 90 wherein it is burned
with an oxygen-containing gas such as air. A heated exhaust
gas is removed from gas turbine combustor 90 through line
92 and is expanded in gas turbine expander 94 which drives
electrogenerator 96, thereby producing electrical energy.
Expanded exhaust gas is removed from expander 94 through line
98. Some or all of the exhaust gases can be sent through
line 100 into prehea-ter 102. Within preheater 102 the exhaust
gases come in indirect heat exchange contact with the air
charge which has been compressed in compressor 86, and sent
through line 104 to combustor 90, thereby preheating the air
charge before its introduction -to the combustor. The exhaust
gases are then routed through line 106 back into line 98 where
they combine with the remainder of the exhaust gases passing
through line 98.
The exhaust gases can thereafter be sent directly
to the primary reformer, or all or a por-tion of the gases
can first be passed -through a waste heat boiler 108. When
the boiler 108 is used, the exhaust gases give up some of
their heat to generate steam within boiler 108 the steam
is removed through line 110 and then used to drive a steam
expander 112. Steam expander 112 drives an electrogenera-tor
114 which generates electrical energy. The expanded steam
is then recirculated through line 116 back into boiler
108. The exhaust gases, if not passed through waste heat
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boiler 108, or after being passed through boiler 108 are -then
routed through line 118 to the reforming stage of the overall
methanol process. A branch line 119 connects between line
118 and the furnace section of primary reformer 18. Since
the exhaust gases are rich in oxygen, all or a portion of
the exhaust gas can ~e used as a source of preheated oxygen
for use in the furnace section of reformer 18 for combustion
therein with a fuel introduced through line 120. The remaining
portion of the exhaust gases flowing through line 118 are
introduced into the convection section 16 of reformer 18 in
order that they may give up additlonal heat to the streams flow-
ing through preheaters 14, 28. The exhaust gas is ultimately
removed from reformer 18 through line 122.
~ eferring to FIG. 2, an alternative embodiment of
the invention is shown, wherein expander 84 drives liquid compressor
72 rather than compressor 86. It is to be understood that the
expander 84 can be used to drive other compressor services, bearing
in mind that the princ~pal aspect of the invention relates t~
the use of the potential enargy of the pressurizea purge gas as
well as its chemical energy so as to improve the efficiency of
the overall process.
In order to illustrate the advantages of the present
invention, the following example is provided:
EXAMPLE
In this example the purge gas is removed from drum
68 at a pressure o~ approximately l400 psig, passed through line
82toexpander 84 wherein it is reducad to a pressure of approxi-
mately 210 psig. In the case of a 500 MT/day methanol plant,
the purge gas has a net heating value of approximately 310 BTU
per standard cu. ft. This gas may be burned in gas turbine 88
to produce approximately 40 megawatts of electricity at electro-
generator 96 after expansion through expander 94. The gas turbine
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exhaust will be removed through llne 98 at a temperature of
approximately 1000 degrees F. When the exhaust is used -to hea-t
water in boiler 1~8, additional electrical energy can be generated
at electrogenerator 114.
Since gas turbines are usually supplied with excess
air in order to keep the temperature of the exhaust gases below
an acceptable upper limit imposed by materials from which gas
turbine components are made (approximately 2000 degrees F.) the
exhaust gas is rich in oxygen. Th~s oxygen rich preheated stream
can be introduced through line 11~ to the furnace section 32 for
combustion with a fuel provided via line 120, or can be passed
into con~ection section 16 for hea~lngfeed streams passing through
preheaters 14, 28.
A latitude of mod$fication, change and substitution
is intended in the foregoing disclosure and in some instances
some features of the invention will be employed without a corres-
ponding use of other features. Accordingly, it is appropriate
that the appended claims be construed broadly and in a manner
consistent with the spirit and scope of the invention herein~
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