Note: Descriptions are shown in the official language in which they were submitted.
~Z8~;3173
IC 6325 TITLE
Use of Purge Gas in
Methanol Synthesis
DESCRIPTI
5 Technical Field
... .
The invention relates to the manufacture of
methanol and more particularly to such manufacture from
synthesis gas and the utilization of purge gas
therefrom.
Background Art
Methanol is one of the important basic chem-
ical commodities in the modern world. It is used as an
ingredient of automotive and other antifreezes. It is
also used as a constituent of solvent systems for many
processes and is a basic raw material for the manu-
facture of formaldehyde. Methanol is also used in the
manufacture of many methyl esters, methyl halides,
methyl ethers, methyl amines, methacrylates, dimethyl
terephthalate and ethylene glycol.
Though methanol can be made by a number of
different processes such as saponification of methyl
chloride, low pressure hydrogenation of carbon monoxide
and Fischer-Tropsch synthesis, by far the most widely
used route to methanol is by catalytic conversion of
synthesis gas derived from hydrocarbons such as coal,
coke, petroleum residues and light hydrocarbon-
containing streams, especially natural gas.
,_ 1
~ ' . . .
:;
..... .
: ,: ; " '
-` ~lZ8~73
In a typical process, desulfurlzed natural
gas is steam reformed at an elevated pressure over a
nickel catalyst to form a synthesis gas comprising H2
and CO in a molar ratio of about 2:1. To promote the
water-gas shift reaction, carbon dioxide is frequently
mixed with the natural gas and steam prior to reform-
ing. For example, in U.S. Patent 3,943,236 to R. V.
Green, a portion of the reformer effluent from which
the CO has been removed is admixed with CO2 in an
amount at least equivalent to the amount of ~2 present
and recycled to the reformer inlet. U.S. Patent
3,763,205 to R. V. Green describes a methanol synthe-
sis process from the aforesaid steam reformed synthe-
sis gas. In this process, the synthesis gas is com-
pressed to 137-409 atmospheres (2000-6000 psig)
and mixed with high pressure hydrogen-rich recycle
gas. The mixture is passed through a fixed bed re-
actor, normally containing a metal oxide catalyst such
as zinc, zinc-chromium, chromium-copper or copper.
The methanol reactor (converter) usually operates at
temperatures of about 285 to 400C. The hot reactor
effluent is cooled by heat exchange with incoming feed
and with water to generate steam. The cooled reactor
effluent is then passed to one or more separators in
which the normally gaseous effluent materials are
separated from the normally liquid materials. At
least the high pressure component of the effluent gas,
which contains mainly hydrogen, is recycled to the in-
let of the methanol converter where, as indicated
above, it is mixed with incoming synthesis gas and
fed to the fixed-bed reactor.
A particularly preferred reactor for meth-
anol synthesis ls the so-called "cold shot reactor"
disclosed in U.S. Patent 3,254,967 to T. O. Wentworth.
In this reactor, "cold" hydrogen gas is introduced
., , ' .~ ~.
i~28~'73
between each of a plurality of fixed catalyst beds to
obtain more thorough mixing and better temperature
control.
Reactor effluent vapor, which contains mainly
methanol, hydrogen, methane, water, CO, CO2 and N2 is
passed through a series of indirect heat exchangers to
effect cooling to 30 to 50C.
Another illustrative high pressure process
is described in U.S. Patent 3,501,516 to R. W. Parrish
(34-680 atmospheres, 500-10,000 psia) where
purge gas is removed so that the remainder of the re-
sidual gas, which is recycled to the converter feed,
has its composition maintained at a desired level.
In addition to high pressure methanol syn-
thesis processes, there are also low pressure pro-
cesses, i.e., pressures below 150 atmospheres. Illus-
trative processes are described in British Patents
1,190,071; 1,259,945 and 1,484,366. In addition, in
1,190,071, the hydrocarbon content of the purge gas
is utilized to generate additional hydrogen and car-
bon oxides. In 1,259,945, the purge gas is compressed
and fed to an additional methanol synthesis stage where
further methanol is produced despite the increased in-
ert concentration. A similar process is described in
U.S. Patent 3,615,200, issued to K. Konoki.
Heretofore, it has usually been customary to
use methanol synthesis loop purgegas as fuel. How-
ever, this is now uneconomical due to the increased
costs of hydrocarbon feedstocks such as natural gas.
While the aforesaid British Patents 1,190,071 and
1,259,945 and U.S. 3,615,200 do utilize purge gas, the
process of the present invention is directed to the
utilization of the methanol reactant values and energy
of the purge gas while minimizing the effects of inerts
contained therein.
.
~128~73
Disclosure of the Invention
According to the present invention, there is
provided a process for the manufacture of methanol
using a hydrogen-containing gaseous purge stream from
a chemical manufacturing process comprising:
(a) raising the hydrogen concentration
in the purge stream in a hydrogen enrichment
zone by removal of gaseous materials there-
from which are inert with respect to a
methanol synthesis reaction;
(b) adjusting the hydrogen to carbon
oxide molar ratio of the hydrogen-enriched
gas stream to a level within the range of
about 2:1 to 12:1 by addition of a carbon
oxide thereto;
(c) feeding the carbon oxide-adjusted
hydrogen-enriched gas stream to a methanol
synthesis zone containing a methanol syn-
thesis catalyst and forming a methanol-
containing gaseous effluent, the tempera-
ture of said zone controlled to a level
below which any substantial methanation
occurs; and
:~ (d) recovering methanol from the
gaseous effluent.
According to a preferred embodiment, there is
provided a process for the manufacture of methanol by
(1) catalytic conversion of synthesis gas in a primary
methanol synthesis zone to form an effluent comprising
a mixture of crude methanol, hydrogen, methane, water,
carbon monoxide, carbon dioxide and nitrogen; (2)
separation of crude methanol and water from the efflu-
ent to form a hydrogen-rich gas stream containing car-
bon monoxide and materials which are inert with
respect to the methanol conversion reaction; (3)
, ' - , .
: - , .
~. :
8~73
recycling the hydrogen-rich gas stream to the inlet of
the primary synthesis zone by which the level of inert
gases in the hydrogen-rich gas stream therefrom is in-
creased and (4) purging a portion of the hydrogen-rich
gas stream from the process cycle to maintain the in-
ert gas level in the recycled hydrogen-rich gas stream
below preselected maximum limits, the improvement
comprising:
(a) raising the hydrogen concentration
of the hydrogen-rich purge gas stream by re-
moval of inert gaseous materials therefrom
in a hydrogen enrichment zone;
(b) adjusting the hydrogen to carbon
oxide ratio of the hydrogen-enriched purge
gas stream from which inert gases have been
removed to a level of between about 2:1 and
about 12:1 by addition of a carbon oxide~
thereto;
. (c) catalytically converting the
carbon-oxide adjusted hydrogen-enriched
gas stream in a secondary methanol syn-
thesis zone to form an effluent comprising
a mixture of methanol, water and unreacted
gases;
(d) controlling the temperature within
the secondary synthesis zone to a level be-
low which any substantial methanation occurs
by indirect transfer of heat from the secon-
dary synthesis zone effluent to the carbon
oxide-adjusted hydrogen-enriched gaseous
feed thereto; and
(e) separating methanol and water
from the secondary synthesis zone effluent.
Brief Description of the Drawings
Figure 1 is an illustrative flow diagram
Z8!~73
showing the manufacture of methanol using a hydrogen-
containlng gas stream which is a purge stream from a
chemical manufacturing process such as a methanol pro-
cess or an ammonia process.
Figure 2 is an illustrative flow diagram
showing a preferred embodiment for the manufacture of
methanol using a hydrogen-containing purge stream from
a high pressure methanol synthesis process.
Best Mode Includlng Examples
Referring to Figure 1, a hydrogen-containing
purge stream from a chemical manufacturing process,
such as a methanol synthesis process, an ammonia syn-
thesis process or a process for the reduction of nitro-
benzene with hydrogen, is passed through valve 10 in
line 11 to jet compressor 12 where the energy of the
purge gas is used to compress another hydrogen-
containing purge stream at a lower pressure in line 13
to some intermediate level. The combined, compressed
gas stream is fed to hydrogen enricher 14 through line
15. In the hydrogen enricher, gaseous materia~s which
are inert with respect to a methanol synthesis reaction
are removed from the purge gas, such as by use of a
permeator, absorption, adsorption, or through cryogen-
ics, to raise the hydrogen concentration therein. The
removed inert gases (predominantly methane for a meth-
anol purge gas and methane and nitrogen for an ammonia
purge gas) leave enricher 14 through- line 16 for burn-
ing or for recycling to a synthesis gas reformer.
Hydrogen enrichment by absorption is des-
cribed in U.S. Patent 3,064,029, issued to T. C. White
on November 13, 1962. Methane can be removed from a
gas mixture by absorption as described in U.S. Patent
2,820,071, issued ~anuary 14, 1958 to N. H. Ceaglske.
A preferred method of hydrogen enrichment is pressure
swing adsorption (PSA) such as described by H. A.
.. ..
73
Steward and J. L. Heck, "Hydrogen Purification by
Pressure Swing Adsorption," Chemical Engineering Prog-
ress, p. 78, September, 1969. Hydrogen enrichment by
cryogenics is described by Wolfgang Forg, "Purification
of Hydrogen by Means of Low Temperatures," Linde Re-
ports on Science and Technology, 15/1970. Hydrogen
enrichment by the use of permeators is described by
R. I. Gardner et al., "Hollow Fiber Permeators for
Separating Gases," Chemical Engineering Progress, Vol.
73, No. 1, pp. 76-78, October, 1977.
The hydrogen-enriched gas stream leaves the
hydrogen enricher through line 17 where it is then
joined in a preferred embodiment with gaseous effluent
in line 18 which is being recycled from a methanol con-
verter. The hydrogen-enriched gas stream as it leaves
the enricher is predominantly hydrogen with minor
amounts of other gases such as Cx and nitrogen. This
gas stream will generally have a composition on a
molar basis of about 70-100~ hydrogen and up to
about 30~ of Cx with minor amounts of other gases.
As would be expected, the composition will be dependent
upon the route taken for hydrogen enrichment and the
source of the purge gas.
The hydrogen-enriched gas stream is com-
pressed by compressor 19, if needed, passes throughline 20 and is then joined in converter feed line 21
with carbon dioxide which is fed through compressor
22 and line 23. Carbon dioxide is used to adjust the
hydrogen to càrbon oxide molar ratio of the gaseous
feed to methanol converter 24 to a level in the range
of about 2:1 to 12:1, preferably about 2:1 to 8:1,
along with optional carbon monoxide which is fed into
the carbon-adjusted hydrogen-enriched gas stream in
line _ by line 25. The optional carbon monoxide is
used to optimize conversion and to supplement the
8~73
carbon monoxide in recycle stream 18 to aid in temper-
ature control of converter 24 and affect a lower by-
product water concentration incrude methanol (line 33).
The hydrogen to carbon oxide molar ratio as
used herein is defined as ~ + C2 = 2:1 to 12:1. In
general the converter feed in line 21 is predominantly
hydrogen which contains about 2-8 mole perCQnt carbon
monoxide, about 3-28 mole percent carbon dioxide and
less than 3.5 mole percent of.methane, nitrogen and
other inert gases.
The carbon oxide-adjusted hydrogen-enriched
converter feed stream 21 is passed to converter 24
through a heat exchanger 26, which mav be integral
with 24, where the feed is preheated to reaction tem-
perature by transfer of heat with the methanol-
containing gaseous effluent in line 27 from converter
24. A particularly useful converter is described in
British Patent 1,389,709. Feed temperature is con-
trolled to maximize methanol manufacture and to keep
it to a level below which any substantial methanation
occurs by varying the amount of heat transfer in ex-
changer 26 with a bypass line 28 controlled by valve
29. Temperature control is also accomplished by ad-
jusing the carbon dioxide to carbon monoxide ratio in
the feed stream to take advantage of the lower heat
evolution of the carbon dioxide to methanol reaction
(C2 + 3H2 ~ 3 CH30H + H20) versus the carbon
monoxide to methanol reaction (C0 + 2H2 > CH30H).
Temperature at which methanation becomes significant
is dependent upon pressure and catalyst; however, the
methanol synthesis temperature will generally be in
the range of about 200 to 400C, and preferably will
be about 250 to 350C for converters other than iso-
thermal. For an isothermal converter, the upper re-
action temperature is about 300C.
l~Z1~73
Pressure in converter 24 will generally bein the range of about 40-155 atm (about 600-2250 psig),
preferably about 45-155 atm (about 700-2250 psig). The
pressure ranges in atmospheres as used herein are only
approximate values with respect to pressure values in
pounds per square inch gauge (psig).
The methanol synthesis catalyst contained in
converter 24 can be any of those known in the art which
are operable under specific converter pressures.
Typically, methanol synthesis catalysts are copper-
containing. Examples of useful catalyst compositions
(reduced form) and their typical temperatures and
pressures of operation are as follows:
Temper-
Pressure-Atm ature
Catalyst (psig) (C)
Cu/Zno/A1203 273 (4000) 270
ZnO/Cr203 273-341 (4000-5000) 350-400
Cu/ZnO/V oxides44 (630) 230
Cu/ZnO/A120 2 51 (735) 250
20 Cu/ZnO/Cr203 3103 (1500) 260
Cu/ZnO/mixture of 4
rare earth oxides 53 (770) 270
CutZnO/Cr O 5 and ~
Cu/ZnO/~1203 ~ B 130 (1900) 240
25 ~~~~~~~~~~
1 - U.S. Patent 3,897,471
2 - U.S. Patents 3,923,694 and 3,850,850
3 - Canadian Patent 925,069
4 - U.K. Patent 1,364,096
5 - U.S. Patent 3,840,478
6 - German Patent 2,449,493
The gaseous effluent from converter 24 is
passed through condenser 30 and then via line 31 to
methanol recovery. Separator 32 operating at a pres-
35 sure o~ 600-2000 psig (about 40-135 atm) separates
,, ~
~Z8~'3
methanol and water (with dissolved hydrogen and Cx
therein) from unreacted and inert gases and sends this
crude methanol through line 33 to further separation
and a conventional methanol refining unit. The energy
in streams 33 and 18 may be reclaimed by using
expanders in place of valves if economically justified.
The off-gas in line 18 from separator 32 is basically
hydrogen, Cx and a little (less than 5 mole percent)
nitrogen. It is preferred to recycle the off-gas to
the hydrogen-enriched gas stream via line 18 in order
to recover the carbon oxide values therein (about 25
mole percent); however, alternatively, the gas can be
burned or, if hydrogen enrichment occurs via a
permeator where carbon monoxide is passed through,
secondary synthesis may not need a recycle of unreacted
gases. In this latter case, unreacted gases wou-ld be
recovered via lines 33 and 13.
In a preferred embodiment as illustrated in
Fig. 2, the hydrogen-containing purge gas in line 11
fed through jet compressor 12 to hydrogen enricher 14
is derived from a primary methanol synthesis process,
preferably one operated at high pressure, i.e., about
1000-5000 pounds per square inch gauge, (about 70-340
atm) and preferably about 4000-5000 pounds per square
inch gauge (about 270-340 atm), so that the energy of
the purge stream can be used to compress hydrogen-
containing purge stream 13 which is at a lower pressure,
e.g., about 100-2000 pounds per square inch gauge
(about 8-135 atm), and preferably 100-600 pounds per
square inch gauge (about 8-40 atm). While high
pressure synthesis processes are preferred, lower
pressure processes, i.e., about 600-750 psig (about
40-50 atm), can also be used. In this case, purge
stream _ is at a lower pressure, e.g., about 50 psig
(4.5 atm), and supplemental pressure has to be used
to get converter 24 up to a useful pressure.
~ ~ '
:.
: ~
28~)73
11
A methanol synthesis process normally
includes, in addition to the synthesis section, a
synthesis gas generating section in which a carbonaceous
feedstock is converted to carbon oxides and hydrogen by
a high temperature reactlon with steam, optionally with
carbon dioxide such as described in U.S. Patent
3,943,236. This synthesis process has many forms which
are well known and the process used will depend upon
the carbonaceous feedstock used.
In the present process, it is preferred to
start with natural gas and generate synthesis gas by
"steam reforming" in reformer section 34. The exiting
gas in line 35 is at a pressure typically up to about
20 atmospheres absolute and usually has to be com-
pressed before feeding it to the methanol synthesis.
Thus, the synthesis gas in line 35 is compressed by
compressor 36 and is fed through heat exchanger 38 via
line 37 where the gas is preheated to reaction temper-
ature by heat transfer with the methanol-containing
effluent in line 39 from the primary methanol converter
40. The construction of converter 40 and the catalyst
contained therein can be any of those described in the
art and can be the same as or different from secondary
converter 24. Typical catalysts were mentioned pre-
viously. The reaction temperature in converter 40depends upon the catalyst used and operating pressure.
At low pressure, i.e., about lO0 psig (70 atm), the
temperature will usually be about 250-350C, whereas
at the higher end of the pressure range, i.e., about
4000-5000 psig (270-340 atm), the temperature will
usually be about 350 to 400C.
A portion of the synthesis gas can also be
used as a source of carbon monoxide which can be used
to adjust the carbon dioxide to carbon monoxide and
hydrogen to carbon ratios in the feed to the secondary
28~73
12
converter. Thus, synthesis gas can be passed through
optional valved line 35a to secondary converter feed in
line 21.
The methanol-containing gaseous effluent
(also containing the unreacted gases hydrogen and
carbon oxides, water, and gases inert to the methanol
reaction, i.e., methane, nitrogen and minor amounts of
other materials) from converter 40 is passed through
condenser 41 and then to methanol recovery and purge
gas utilization.
Condensed effluent is separated from gaseous
effluent in high pressure separator 42 operating at
essentially the same pressure as converter 40. Crude
methanol and water with unreacted gases dissolved
therein is passed via line 43 and valve 44 to inter-
mediate pressure separator 45 operating at about 100-
600 psig (about 8-40 atm) and normally about 300 psig
(about 20 atm), wherein a portion of the dissolved
gases is released. The hydrogen-containing high pres-
sure gas stream from separator 42 is primarily recycledvia lines 46 and 47 and recycle compressor 48 (if
necessary) to the feed to primary converter 40.
As practiced heretofore, when the level of
inert gases in the recycle gas reaches a predetermined
maximum level, a purge of the gas is taken via line 49
and valves 50 and 51 which is then burned. In the
present process, this purge stream for burning
approaches zero due to the fact that it now is used to
produce additional methanol in a process as shown in
Fig. 1.
The hydrogen-containing off-gas from inter-
mediate pressure separator 45 passes via line 13 and
13a to jet compressor 12 where it is compressed by the
energy from the hydrogen-containing purge gas from high
pressure separator 42. In the preferred embodiment,
~'
.. : . :
128~73
separator 45 is also used to receive the crude methanol
with unreacted gases dissolved therein produced by the
secondary converter 24. This crude methanol in line
33 is from higher pressure separator 32 as described
previously.
Methanol and water with remaining unreacted
gases dissolved therein is passed from separator 45 to
low pressure separator 52 via line 53 and valve 54.
This separator is only slightly above atmospheric pres-
sure. Crude methanol is passed through line 55 and valve
_ to a conventional methanol refining unit. The remain-
ing unreacted and inert gases along with any low-boiling
impurities are passed through line 57 and valve 58 for
burning.
It is preferred that the process be operated
under such conditions that the off-gas in line 18 admixed
with the primary synthesis zone effluent is about 20 to
90 mole percent of unreacted gases from the secondary
synthesis zone effluent in line 27.
The present invention has the advantage of
preparing additional methanol from hydrogen-containing
purge streams which have previously been burned. Energy
utilization is high because (1) the methane separated from
the hydrogen enricher will yield about 70~ of the heating
value now derived from burning the hydrogen-containing
purge stream, and (2) energy in the form of pressure in
the purge stream is used to compress a lower pressure
purge stream.
The invention can be further understood by the
following example in which parts and percentages are on a
molar basis unless otherwise indicated.
EXAMPLE
A methanol synthesis process based on the supply
of purge gas from a high pressure methanol synthesis process
is illustrated. The compositions, flow rates, pressures
and temperatures of the process effluents are set forth in
Table I with line references to Fig. 1 and Fig. 2.
The primary reactor used is of the cold shot
type similar to that described in U.S. 3,254,967 and
13
~lZ13~73
14
the secondary reactor is a tubular isothermal reactor
similar to that described in U.K. 1,389,709. The
catalyst in the primary reactor is zinc chromite
(ZnO/Cr2O3) and the catalyst in the secondary reactor
is Cu/ZnO/V oxides.
Hydrogen enrichment is conducted by pressure
swing adsorption such as described in H. A. Stewart and
J. L. Heck, "Hydrogen Purification by Pressure Swing
Adsoroption," Chemical Engineering Progress, p. 78,
September, 1969.
14
~ , -''~ .
~128~)73
TABLE I
Composition (Mole ~)
Line Press. Temp- H COC02 CH4
11 H.P. 305 4073.6 5.31.9 17.1
Purge
13 L.P. 19 4047.1 2.810.1 35.8
Purge
15 H2 43 4061.6 4.25.6 25.6
Enricher
Feed
16 Inerts- 0.7 3218.1 6.413.2 60.2
Fuel
17 H2-rich 40 4695.6 2.6
Stream
20 H2-rich +135 5487.1 3.58.3
Recycle
23 C02 135 54 - -100
Feed
21 Conv. 135 20070.1 2.826.2
Feed
27 Conv. 125 26539.3 4.416.6
Efflu.
13a L.P. 19 40 49 2.910.4 33.3
Purge
18 Recycle 40 4073.9 4.921.2
33 Crude 19 4012.6 3.913.1
CH30H(2)
43 Crude 19 40 3.6 0.21.1 3.1
CH30H(l)
53 Crude 19 40 0.15 0.26 1.19 0.04
CH~OH to
L.p. Sep.
49 Excess 19 4056.9 3.412.1 22.4
Purge to
Fuel
.
: '
il;2~3~73
16
TABLE I (cont.)
Composition (M! e %)
Line _ N2 H20 CH OH 0th Flow Rate
11 H.P. 1.7 0.1 0.2 0.1 6,322
Purge
13 L.P. 2.2 0.2 1.6 0.2 9,666
Purge
15 H2 1 9 0.1 0.8 0.115,988
Enricher
Feed
16 Inerts- 2.2 - ~ ~ 12,545
Fuel
17 H2-rich 1.8 - _ _ 2,891
Stream
20 H2-rich + 1.1 - - - 10,051
Recycle
23 C02 - - - 15,954
Feed
21 Conv. 0.9 - - - 26,005
Feed
27 Conv. 1.219.5 19 - 26,005
Efflu.
13a L.P. 2.3 0.2 1.6 0.311,738
Purge
18 Recycle (1) - _ _ 7,160
33 Crude 2.234.5 33.7 - 18,842.5
CH30H(2)
43 Crude 0.227.3 64.o 0.5265,522
CH30H(l)
53 Crude 0.1330.3867.36 0.49272,627
CH~OH to
L.P. Sep.
49 Excess 2.9 0.2 1.8 0.3 2,072
Purge to
Fuel
(1) up to ~2.0~ N2.
16
:~ :
