Note: Descriptions are shown in the official language in which they were submitted.
Swiss
-1 -
Conversion of Alikeness to unsaturated ~ldehydes
Prior Art
The invention relates generally to the prepare-
lion of unsaturated aldehydes, acids and their esters. In
one aspect, the invention relates to the preparation of
methacrolein, which is a precursor of methacrylic acid in
a tiptop process for manufacture of methacrylic acid
from isobutylene or tertiary bottle alcohol. In typical
processes of the prior art isobutylene or tertiary bottle
alcohol it reacted in the vapor phase with molecular
oxygen over a catalyst to produce methacrolein. The
methacrolein is then separated and reacted with molecular
oxygen in the vapor phase over a different catalyst to
form methacrylic acid. The methacrylic acid may then be
reacted with a suitable alcohol to form a methacrylate
ester.
n another specific aspect, the invention
relates to the preparation of acrolein, or acrylic acid,
from propane in a process similar to the one described for
methacrolein or methacrylic acid.
Generally, the feed for this process has been the
unsaturated olefin or its equivalent alcohol. Any
saturated hydrocarbon present was considered to be
essentially an inert since little, if any, oxidation
occurred. However, an economic incentive exists for the
use of alikeness as feed stocks for the preparation of
unsaturated aldehydes and acids. For example, it is known
to dehydrogenate iæobutane to form isobutylene for use in
its many applications, such as preparing of tertiary bottle
alcohol, methyl tertiary bottle ether, and bottle rubber.
Such dehydrogenation processes could be used to prepare
isobutylene as a feed stock for the known methods of
1~29-B
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preparing methacrolein. However, if the isobutylene were
separated and purified this would be an uneconomic way of
preparing methacrolein and methacrylic acid. the same
comment applies to the conversion of propane to acrolein
or acrylic acid.
In general, integrated processes are not
generally used because the by-products of dehydrogenation
must be separated, since they cannot be present in the
subsequent use of the olefin, without causing contamina-
lion of the ultimate end product. The present process relates to an integrated process whereby the dehydrogena-
lion of isobutane is combined with the oxidation of the
resulting isobutylene to methacrolein in a continuous
manner and without intermediate separation of the
isobutylene. A corresponding process can be carried out
with propane as feed and producing acrolein or acrylic
acid.
Processes of background interest include one
shown in U.S. Patent 3,470,239 in which isobutane is the
feed stock to a process for methacrylic acid or methyl
methacrylate via a tertiary bottle hydroperoxide inter-
mediate. Isobutane is oxidized to a hydroperoxide and
then used to oxidize methacrolein to methacrylic acid. In
that oxidation tertiary bottle alcohol is a by-product
which then can be used as a feed stock to prepare methacro-
loin in a conventional oxidation process. Consequently,
isobutane only serves indirectly as a feed stock to the
preparation of methacrolein.
In British Patent 1,340,891 isobutane and oxygen
are reacted to prepare isobutylene and/or methacrolein
over a variety of base metal oxide catalysts. Since the
conversion of isobutane is quite low, high concentrations
of isobutylene are used so that the net amount of
isobutylene or methacrolein produced is sufficient to
result in an practical process. Propane is also shown to
provide a parallel reaction.
1229-B
I ~Z~75~2
recent U.S. patent 4,260,822 discloses a
process for direct oxidation of isobutane to methacrylic
acid in a single step, again using large amounts of
isobutane in order to overcome the relatively low
conversion of isobutane to the product. Again, propane is
disclosed to be oxidized to acrylic acid in the same
manner.
The above one-step processes are not economic,
since the conversions are quite low and require handling
of substantial amounts of unrequited feed with recycling in
order to produce a high overall conversion of isobutane.
Also, the catalysts typically do not have the long useful
life necessary for satisfactory commercial operations.
Since isobutylene has a number of uses other
than the preparation of methacrylic acid, a number of
processes have been developed for converting isobutane to
isobutylene. In U.S. Patent 3,784,483 a cobalt, iron, and
phosphorus catalyst is used for the oxidative dehydrogena-
lion of isobutane to isobutylene or propane to propylene.
The process of British Patent 1,340,891 is similar except
that generally high ratios of isobutane to oxygen (about
4/1) are used. In U.S. 3,479,416, a process operating in
the absence of oxygen employs a base metal catalyst,
particularly one containing chromium, molybdenum, and
vanadium. In a group of patents typified by U.S.
4,083,883 a precious metal combined with promoter metals
on a support is used for the dehydrogenation of paraffins,
particularly normal paraffins.
Another approach is taken in U.S. Patents
3,692,701, 4,005,9~5 and 4,041,099. In these processes
large quantities of steam are used to dehydrogenate
butanes over a catalyst of platinum-tin on zinc acuminate
with a high selectivity to the corresponding butane, or
propane to propylene. Relatively high conversions are
achieved. Dehydrogenation of paraffins is also shown over
zinc titan ate catalysts in U.S. 4,144,277 and 4,176,140.
1229-B
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on a recent published European Patent Application 42252
of Dick, isobutylene is prepared by dehydroisomerizing
n-butane over a supported catalyst containing a Group IIIA
element or compound, especially gallium.
Many patents have disclosed processes for
oxidation of isobutylene to methacrolein. Of particular
interest with respect to the present process is that
disclosed in British Patent Application AYE, in
which methacrolein along with oxygen and steam is passed
over a molybdenum-based catalyst, providing a high
conversion and selectivity to methacrolein.
Many patents have disclosed processes for
oxidation of propylene to acrylic acid, such as EN 0000663 of
Oust and U.S. 3,~54,855. These pnK~sses are generally idea
out in two steps in order to obtain the best yields of
acrylic acid. In the first step, propylene is oxidized to
acrolein in the vapor-phase over a promoted lybdenum
catalyst and the effluent from the reaction is passed with
added oxygen over a second promoted molybdenum catalyst to
produce acrylic acid.
Based on the prior art discussed above, one might
assemble a multi-step process whereby an Al Kane was
dehydrogenated with or without the presence of oxygen to
produce the corresponding olefins which would then be
separated and purified and fed to a second step for the
oxidation of the olefin to the corresponding unsaturated
alluded. In this way, a combined process could be
operated which would convert substantially all of the
Al Kane feed to the unsaturated alluded by merely
combining known processes. This may not be an economic
way to produce the alluded. As will be seen, the present
invention pertains to an integrated process by which an
Al Kane may be converted to the corresponding unsaturated
alluded without first separating the olefin.
1229-B
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Dehydrogenation of an Al Kane produces substantial
amounts of hydrogen and small amounts of lower molecular
weight hydrocarbons, which in the prior art processes
would be removed from the product, see for example U.S.
3,479,416, but which in an integrated process must be
accommodated in the oxidation of the Al Kane to the
alluded. The hydrogen and by-products should not have an
adverse effect on the oxidation catalyst or its perform
mange. For example, the exposure of the by-products to
oxidizing conditions cannot produce contaminants which
reduce the quality of the alluded. Also, the presence
of hydrogen should not create an explosive mixture in the
oxidation reactor.
The oxidation step produces carbon oxides as
by-products and some lower molecular weight oxygenated
compounds and at the same time introduces oxygen into the
gases which is not acceptable in the upstream dehydrogena-
lion reactor. Thus, recycling a combined effluent from
the oxidation realtor to the dehydrogenation step involves
particular problems unique to the integrated process of
the invention. The present process is able to accommo-
date problems involved with the integration of the prior
art process by methods to be disclosed hereinafter.
Summary of the Invention
Alikeness are dehydrogenated to produce the
corresponding olefins, which are then oxidized to the
unsaturated aldehydes. For example, methacrolein is
prepared from isobutane in an integrated process in which
dehydrogenation of isobutane to isobutylene is followed
immediately, i.e. without separation of isobutylene from
the dehydrogenation effluent, by addition of oxygen and
then oxidation of the isobutylene to methacrolein.
Alternatively, some of the dehydrogenation by-products may
be removed before the oxidation step, thereby concentra-
tying the isobutylene. This may be done, for example, by
1229-B
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selective oxidation of hydrogen to water or partial
condensation of the water contained in the effluent
stream. Methacrolein may be recovered by scrubbing or
quenching the oxidation reaction effluent which also
contains unrequited isobutane, isobutylene, and oxygen,
plus hydrogen, carbon oxides and miscellaneous hydrocarbon
by-products of the dehydrogenation and oxidation
reactions. The corresponding process may be carried out
with a propane feed.
In some embodiments of the invention methacrolein
is recovered and then oxygen, hydrogen, and carbon oxides
are removed from the oxidation reaction effluent by
catalytic reactions or absorption techniques and the
remaining gases containing unrequited isobutane and
isobutylene are recycled to the dehydrogenation reaction.
A preferred method of removing oxygen and hydrogen is to
react them over a suitable oxidation catalyst at condo-
lions selected to completely remove the oxygen, but having
substantially no effect upon the isobutylene and isobutane
present. Suitable catalysts include platinum or other
Group VIII noble metals on alumina or other supports.
The oxidation of hydrogen is carried out at a temperature
which permits selective oxidation of the hydrogen. With
platinum on alumina, such oxidations may be initiated at
relatively low temperatures, such as ambient. In a
preferred embodiment, the oxygen supplied to the oxidation
reaction is adjusted to provide a limited amount in the
effluent so that the hydrogen produced in the
dehydrogenation reaction consumes all of the oxygen
remaining after isobutylene has been oxidized to
methacrolein.
In an alternative embodiment, the Al Kane and the
olefin remaining in the effluent gases after the alluded
has been removed are absorbed with a suitable liquid such
as a C8 to C10 paraffin oil. The gases can be
further processed if desired to recover valuable
1229-B
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components, turned, or otherwise disposed of. In this
embodiment, the hydrogen and oxygen are contained in the
waste gases and only the uncovered hydrocarbons are
recycled to the dehydrogenation reactor.
In still another embodiment, the effluent gases
are partially separated after the dehydrogenation step,
with the concentrated C3 or C4 hydrocarbons, i.e.
proplyene/propane or isobutylene/isobutane, supplied to
the oxidation step.
The dehydrogenation of the Al Kane to the olefin
is carried out by a vapor-phase reaction over a suitable
catalyst, which may be platinum-tin on zinc acuminate or
other noble and base metal catalysts. When the catalyst
is platinum-tin on zinc acuminate the dehydrogentation will
be carried out at about 400 to 700C and up to about 10
kg/cm gauge pressure.
The oxidation of the Al Kane to the alluded may
be carried out over suitable catalysts, such as mixed base
metal oxides, especially molybdenum oxide-based catalysts, and
particularly a catalyst comprising the oxides of molybdenum,
bismuth, cobalt, iron, nickel, thallium, antimony, silica and
one or more of the alkali metals.
Thus, in accordance with the present teachings,
a process is provided for the preparation of an unsaturated
alluded from the corresponding Al Kane which comprises:
pa) dehydrogenating the Al Kane to the
corresponding olefin in the presence of steam, over a
dehydrogenation catalyst comprising the group VIII noble
metal on a support to form an effluent stream comprising
the olefin, hydrogen, carbon oxides, steam and unrequited
Al Kane,
(b) mixing oxygen and optionally steam with
the effluent stream of (a) and passing the mixture over
a molybdenum-based oxidation catalyst at conditions
selected to produce the unsaturated alluded and producing
an effluent stream comprising the unsaturated alluded,
unrequited Al Kane and olefin, oxygen, hydrogen, steam and
-pa-
750~
carbon oxides;
(e) recovering the aloud from the effluent
of (b);
(d) separating from the effluent of ( ) after
the recovery of the alluded therefrom the hydrogen
produced in (a), the carbon dioxide produced in (a) and
(b), and the unrequited oxygen of (b) to produce a
depleted effluent and;
(e) returning as feed to (a the depleted
effluent of (d).
Description of the Drawings
Figure 1 is a block diagram showing the process
of the invention.
Figure 2 is a simplified flow sheet showing
production of methaerolein from isobutane according to
one embodiment of the invention.
Figure 3 is a simplified flow sheet showing
production of methacrolein from isobutane according to a
second embodiment of the invention.
Figure 4 is a block diagram showing the
production of acrylic acid from propane according to the
invention.
-8- l Z 7 So
Description of the Preferred Embodiments
In one aspect, the invention is an integrated
process combining the dehydrogenation of an Al Kane,
particularly propane or isobutane, to the corresponding
olefin, i.e. propylene or isobutylene, with the subsequent
oxidation of the propylene or isobutylene to acrolein or
methacrolein, but without purification of the propylene or
isobutylene between the two reactions. The product
alluded is separated for further use and after separation
of by-products the unrequited olefin and Al Kane may be
recycled to the dehydrogenation step if desired. A
schematic view of such a complete process is shown in
Figure 1.
Although discussed specifically as it relates to
isobutane, it should be understood that the process may be
applied also propane.
The process contrasts with those of the prior art
in that the two reactions are operated so that the
effluent from the dehydrogenation reaction 10 may be fed
directly to the oxidation reaction 12 for conversion of
isobutylene to methacrolein. One familiar with the prior
art would expect that isobutylene would be separated from
the effluent of the dehydrogenation step before feeding it
to the oxidation step. Since the dehydrogenation of
isobutane involves the production of significant quanta-
ties of hydrogen, as well as small amounts of lower
molecular weight hydrocarbons, the isobutylene must be
oxidized in the presence of significance quantities of
hydrogen and by-products, without significantly affecting
the oxidation of isobutylene to methacrolein or causing
oxidation of the hydrogen. I have found that isobutylene
may be oxidized to methacrolein in the presence of
hydrogen and by-products of the dehydrogenation step
while not adversely affecting the oxidation process.
1229-B
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In an alternative embodiment, a partial
separation (11) may be made by absorption or distillation
techniques familiar to those skilled in the art to provide
a concentrated C4 hydrocarbon feed to the oxidation
step (12). Depending upon the dehydrogenation effluent
composition, it may be desirable to remove any or all of
the following: hydrogen, light hydrocarbons, and water,
in whole or in part. The benefits of the invention are
still retained at least in part, since an impure feed is
suppled to the oxidation step. Most conveniently this
concentration may be accomplished by oxidizing the
hydrogen to water and/or condensing a portion of the water
contained in the effluent from dehydrogenation.
In one aspect of the invention, after methacro-
loin is separated (14) from the oxidation reactor effluent
a gas containing unrequited isobutylene and isobutane is
returned to the dehydrogenation step. Since excess oxygen
is employed to oxidize isobutylene to methacrolein, this
recycle stream contains substantial quantities of oxygen
which may not enter the dehydrogenation step. Any oxygen
present would be reacted at the operating temperature
causing a loss of the C4 components. Processes which
employ oxygen in dehydrogenation characteristically use
only minor amounts relative to the isobutane. Thus, in
order to assemble a complete process wherein isobutane is
converted only to methacrolein (plus minor amounts of
by-products), it is necessary to remove (16) both the
hydrogen formed during the dehydrogenation step and excess
oxygen remaining after the oxidation step plus carbon
dioxides and other by-products. The allowable level of
each component in the gas will be adjusted as necessary.
1229-B
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In a preferred embodiment, carbon dioxide is scrubbed out
and by-products are purged, after which hydrogen is
oxidized to water over an oxidation catalyst under
conditions such that substantially no loss of the C4
components occurs. In another embodiment, the isobutane
and isobutylene are scrubbed from the effluent gases and
recovered for recycle to the dehydrogenation step while
the gases are discarded.
The dehydrogenation of isobutane produces one mow
of hydrogen for each mow of isobutylene and additional
hydrogen from the formation of lower molecular weight
by-products. If isobutylene is not separated before
subsequent oxidation, as in this process, the hydrogen and
other by-products are carried into the subsequent
oxidation reaction. Oxidation of the byproduct to
compounds which contaminate methacrolein must be avoided.
Oxidizing the hydrogen along with isobutylene would be
undesirable since it would consume oxygen and interfere
with the oxidation of isobutylene. Also, oxidation of
hydrogen would generate undesirable heat and could create
hot-spots in the reactor tubes and lower the productivity
for methacrolein. However, it has been found that
oxidation of isobutylene can be carried out in the
presence of hydrogen with substantially no consumption of5 hydrogen, as will be seen in the following example.
Example 1
Oxidation of Isobutane in the Presence of Hydrogen
A feed gas simulatillg the effluent expected to
result from the dehydrogenation of isobutane in the
presence of steam was blended and fed to an oxidation
1229-B
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reactor for production of methacrolein. The gas compost-
lion was 7 vow % tertiary bottle alcohol, 15 vow % steam,
15 vow % oxygen, 3.2 vow % hydrogen and the balance
nitrogen. Note that tertiary bottle alcohol dehydrates to
form isobutylene, for which it is generally considered to
be an equivalent in the oxidation reaction. The gas was
passed at a gas hourly space velocity (GHSV) of 2300
hr~1 and a pressure of about 1.6 kg/cm2 gauge over
160 cc of a catalyst having a size of 1/8" diameter and
disposed in a 0. 5" I'd . tubular reactor. The heat of
reaction was removed and the temperature adjusted by a
circulating molten salt in the usual manner of carrying
out such reactions. The oxidation catalyst had the
nominal formula
Mo12Bi1Fe3co4Ni2Tlo.ssbo.3 Kiwi Chihuahuas 7x
The results of two tests are compared in the table below:
test 1 shows the results with hydrogen present and test 2
shows the results after the hydrogen supply was cut off.
Table A
Reactor
Test Temp. Conversion % Selectivity to
No. C of TUBA % MACHO MA Hoax I
1 346 85.2 83.4 3.9 2.5 8.9
2 346 86.7 82.1 2.9 2. 10.3
(1) MACHO - methacrolein
(2) MA - methacrylic acid
(3) HOAX - acetic acid
(4) Coy - carbon oxides
1229-B
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The tests show differences not considered
significant, but within the normal variation of the test
measurements. The amount of hydrogen consumed was not
accurately measured because of the small quantities
involved, but it was clear from analysis of the reactor
effluent that little of the hydrogen must have been
oxidized. However, the data show that it had little or no
effect in the oxidation of isobutylene to methacrolein.
Additions of small amounts of other C4
by-products of the dehydrogenation reaction, such as
n-butenes also appear to have little or no effect on the
oxidation to methacrolein.
Separation of methacrolein (14) may be done by
methods known to the prior art. See for example, U.S.
4,234,519. This may be done by cooling and condensing
water-containing methacrolein from the isobutylene
reactor effluent followed by scrubbing of the resulting
gas with a recirculating water stream in order to complete
the removal of methacrolein. The resulting methacrolein
solutions may be stripped at a convenient time to produce
a methacrolein-containing vapor for subsequent use.
Other alternatives, such as solvent extraction and the
like may be used.
Although separation of methacrolein for
subsequent oxidation to methacrylic acid (or other use)
is shown, it is possible, provided that the catalyst is
resistant to the various compounds present, to feed the
effluent of the oxidation step 12 directly to another
oxidation step for conversion of methacrolein to methacry-
fig acid. It would be preferred to operate oxidation stop to convert substantially all of the isobutylene to
methacrolein before carrying out the oxidation to
methacrylic acid.
1229-B
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In its broad form, the invention includes the
combination of dehydrogenation and oxidation steps whereby
isobutane is converted to methacrolein. Such a combine-
lion process without an intermediate separation of
isobutylene has been shown to be possible. If the
resulting by-product stream containing significant qua-
lilies of isobutylene and isobutane can be used for other
purposes, then recycling of the gas is not required. In
many cases, it will be desirable to recycle unrequited
isobutylene and isobutane so that the integrated process
substantially converts isobutane to methacrolein with only
minor amounts of by-products and without ever producing a
substantially pure isobutylene stream. In order to
recycle gas containing large amounts of isobutane and
isobutylene, it will be necessary to remove hydrogen
produced in the dehydrogenation step, excess oxygen from
the oxidation step, and carbon oxides formed in both
steps. In addition, a purge of light and heavy
by-products and feed impurities will be taken. Removal of
carbon dioxide typically would be done by scrubbing all or
a portion of the recycle stream with a carbonate or amine
solution known in the art in order to maintain the desired
level of carbon dioxide. Carbon monoxide will be
converted to carbon dioxide in the dehydrogenation
reactor. Since the presence of these materials is not
critical to either the dehydrogenation or oxidation steps,
they may be economically permitted to build up in the
recycle stream to a level in which they may be
conveniently and economically removed. Since the light
and heavy hydrocarbon by-products, such as methane,
ethanes ethylene, propane, propylene, pontoon, and pontoon
boil at temperatures significantly different from those of
methacrolein or the C4 hydrocarbons, they may be
separated by distillation or purging of streams containing
122~-B
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the by-products in concentrated amounts. Removal of the
hydrogen formed by the dehydrogenation of isobutane might
be done by various methods such as catalytic oxidation,
liquid phase absorption or gas phase adsorption. In a
preferred embodiment of the invention, both the hydrogen
and oxygen are disposed of at the same time by reacting
them to form water in a vapor phase over a suitable
oxidation catalyst under conditions such that sub Stan-
tidally all of the hydrogen and oxygen present are removed.
This can be accomplished without significant lows of the
C4 components as will be seen.
Hydrogen, as a by-product ox the dehydrogenation
of isobu~ane, must be disposed of if unrequited isobutane
is to be recycled. As previously seen, the oxidation of
isobutylene is carried out in such a manner that little,
if any, hydrogen is consumed. According to a preferred
embodiment of the invention, the amount of oxygen in the
feed to the oxidation of isobutylene is adjusted so that
the effluent contains no more oxygen than can react with
the hydrogen present. Obviously, neither oxygen or
hydrogen should be present in significant quantities in
the dehydrogenation reactor, which should operate with
only isobutane and steam as feeds for maximum yield.
Removal of oxygen is more important, but hydrogen could be
permitted in minor amounts. It has been found that such
selective oxidation of hydrogen can be carried out without
loss of the valuable isobutane or isobutylene, as will be
seen in the following example.
Example 2
Selective Oxidation of Hydrogen
A feed gas simulating the recycle gas after the
methacrolein has been removed as product was fed to an
0.5" I'd. oxidation reactor containing 75cc of 1/8
"alumina pellets having a surface area of about 150
m2/gm and impregnated with 0.3 wit % platinum. The
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gas was fed at a GHSV of 2000 hr~1 and about atoms-
phonic pressure. Its composition was 30 vow isobutane,
4 vow hydrogen, 3 vow % oxygen, and 63 vow % nitrogen.
The reaction was carried out at essentially ambient
5 temperature, where it is found that all of the hydrogen is
consumed, but substantially none of the isobutane.
While it is preferred to carry out the selective
oxidation of hydrogen in the recycle gas, it is feasible
to introduce the equivalent step after the dehydrogenation
of isobutane and before the oxidation of isobutylene.
Although a concentration of the isobutylene would be
achieved, alternative means must be provided to prevent
excess oxygen from returning to the dehydrogenation step.
Example 3
15 Dehydrogenation of Propane
A catalyst composed of 0.4 wit % Pi and 1 wit % In
impregnated on a zinc acuminate support was prepared. A
catalyst sample of 50 cc as 3 mm extradite was charged to
a tubular reactor having 25.4 mm I'd. An additional 250
cc of the zinc acuminate support was added to the reactor
above the catalyst bed. A gas stream containing 1 mow of
propane for each 2 mows of steam was fed to the reactor at
a gas sourly space velocity of 4000 hr~1 under a
pressure of 3.5 kg/cm2 gauge. The inlet to the
catalyst bed was held at a predetermined temperature, with
the outlet temperature reflecting the endothermic nature
of the dehydroyenation reaction. The results are given
below in Table A.
Table A
_ _
Inlet Conversion Selectivity to
Test Temp.C.of Propane,% Propylene%
1 54~ 23.6 95.8
2 523 29.7 96.5
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-16-
The dehydrogenation of propane produces one mow
of hydrogen for each mow of propylene and additional
hydrogen from the formation of some lower molecular weight
by-products. If propylene is not separated before subset
quint oxidation, as in this process, the hydrogen another by-products are carried into the subsequent oxide-
lion reaction. Oxidation of the by-products to compounds
which contaminate acrolein must be avoided. Oxidizing the
hydrogen along with propylene would be undesirable since
it would consume oxygen and interfere with the oxidation
TV propylene. Also, oxidation of hydrogen would generate
undesirable heat and could create hot-spots in the
catalyst and lower the productivity for acrolein. How-
ever, it has been found that oxidation of propylene can be
carried out in the presence of hydrogen with substantially
no consumption of hydrogen, as will be seen in the follow-
in example. The presence of propane which has not been
dehydrogenated also appears to have no significant effect
on the oxidation of propylene.
Example 4
Oxidation of propylene in the Presence of
Hydrogen and Propane
A catalyst was prepared according to the method
described in EN 0000663 corresponding to Reference
Catalyst A and having a composition of 80% (wt.)
K0.1Ni2.5C4.5Fe3Bi1Po.sMo12Oxand 20% Sue. A charge
of 220 cc of catalyst as 4.7 mm pellets was placed in a
single-tube reactor having an I'd. of 12.7 mm. A feed
gas containing 5 volt propylene, 20 vow % s and 12~ oxygen
was passed at a volume hourly space velocity of 1200
hr~1 over the catalyst. The pressure in the catalyst
was about 1.76 kg/cm2 gauge. Analysis by gas
chromatography Go the effluent gases gave the results
designated in test 3-6 Table A.
1229-B
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-17-
Table A
Test Time, Temp. Conversion of Selectivity to Acrolein
No. his. C Propylene, % and Acrylic Acid, %
3 78.0 310 91.1 87.4
4 81.0 310 90.6 86.3
g3.5 310 90.5 87.7
6 99.5 31Q 90.1 89.7
7 102 310 94.2 86.6
8 115 309 92.6 88.7
9 124 309 92.5 87.9
739 309 ~9.3 87.2
11 147 309 91.4 85.6
12 156 310 93.7 86.7
13 180 309 92.9 86.9
After the performance of the catalyst had been
established with only propylene feed, hydrogen was added
to the feed gas to simulate in part the conditions which
would pertain to the oxidation reaction when supplied with
propylene produced by dehydrogenation of propane. An
additional 4 vow % of hydrogen was included with the
mixture previously fed. The results are shown as tests
7-10 in Table A. it will be seen that substantially the
same results were achieved as in tests 3-6. It was
concluded that essentially no hydrogen was being oxidized,
judging from the operating conditions in the reactor.
The hydrogen was discontinued and the oxidation
reaction continued with 4 vow propane added. The
results are shown as tests 11-13 in Table A. Again
substantially the same results were obtained as in tests
3-6, indicating that propane was not affecting the
oxidation of propylene and was not itself being oxidized.
1229-B
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-18-
These results show what propylene can be oxidized
in the presence of hydrogen and propane, the two principal
components of the dehydrogenation of propane to propylene
It may be concluded that the integration of the
dehydrogenation and oxidation reactions is feasible.
Example 5
Oxidation of Acrolein in the Presence of
Hydrogen Propane
To illustrate the oxidation of acrolein to
acrylic acid and the effect of hydrogen and propane
additions to the feed gas, experiments were carried out
which parallel those of Example 1.
A catalyst was prepared corresponding to the
composition of Example 1 of U.S. 3,954,855 and according
Jo the procedure described there. The catalyst compost-
lion was Mol2v4.gsro.sw2~4cu2.2ox-
A 73 cc sample of the catalyst as 4.7 mm pellets was
placed in a tubular reactor having 12.7 mm I'd. The
reactor was surrounded by a bath thermostatically
providing a uniform predetermined temperature. The feed
to the reactor was initially 6-8 vow % acrolein,4s-50 vow
air, and 43-44 vow % steam. the volumetric space velocity
was 3000 hr-1 and the average reactor pressure
slightly above atmospheric. The results are shown in
Z5 Table B.
Tall e B
Test Time, Temp. Conversion of Selectivity to
No. his. ~CAcrolein, % Acrylic Acid, %
14 126 254 92.8 89.1
3015 126 251 84.5 89.2
16 41 252 86.8 88.4
17 36 251 80.7 87
1229-~
121~
As was done in Example 1, first 4 vow % hydrogen
was added and then the hydrogen was replaced with propane.
The results are shown in Table B as Test 16 (Ho) and
17 (propane). Although some difference in the conversion
of acrolein appears, it is clear from Test 14 and 15 that
conversion is very sensitive to temperature and the
differences among Tests 15t 16, and 17 are not considered
significant. The selectivity to acrylic acid is
substantially the same.
Figure 2 is a simplified flow sheet showing a
preferred embodiment of the invention with a complete
process whereby isobutane is converted substantially to
methacrolein. Fresh isobutane feed 20 is vaporized in
exchanger 22 and combined with recycle stream 24 con-
twining substantial amounts of isobutane and isobutylene.
The combined stream 26 is then heated in exchanger 28 to a
temperature such that when combined with the required
amount of steam (30) the temperature obtained will be that
suited for the dehydrogenation of isobutane to isobuty-
tone. The amount of steam used must also be suitable forth subsequent oxidation of the isobutylene to methacro-
loin. As shown here, steam is supplied as a fresh feed
stream. If this is done, then water produced in the
reactions is eventually purged from the system from the
bottom of the methacrolein stripper 46. Since the process
is a net producer of water this water may be used to
provide the steam for the process. In either case, a feed
stream containing isobutane and steam in molar ratios
between 1/1 and 1/10, preferably 1/2 to 1/5, is fed at
temperatures between about 400-700C preferably about
~50C and at pressures between about 2-10 kg/cm2
gauge, preferably about 3 kg/cm2 gauge to reactor 32
where conversion of about 40 - 50% of the isobutane is
converted to isobutylene with a selectivity of 90% or
more.
1229-8
issue
-20-
A number of catalysts have been disclosed in the
prior art for use in this process and the conditions under
which the reaction is carried out will depend on the
catalyst selected. Of particular usefulness is a
platinum-based catalyst of the type shown in U.S.
4,005,985 specifically for this process. Although
platinum and tin disposed on a zinc acuminate support
provides good performance, other catalysts which have been
found effective include platinum and rhenium or indium
supported on zinc acuminate. Other Group VIII noble
metals, alone or in combination on various supports known
to the art may have application in the dehydrogenation of
isobutane to isobutylene.
Other potential supports would include alumina,
other alkaline earth metal acuminates, and rare earth
acuminates including lanthanum. Promoters such as tin,
lead, antimony, and thallium may be used. Base metal
catalysts such as the chromium, zirconium, titanium,
magnesium and vanadium oxides as shown in U.S. 3,479,416
and 3,784,483 or the zinc titan ate of U.S. 4,176,140 and
4,144,277 also might be used. The invention is not
considered to be limited to specific catalyst
formulations.
It will be understood by those skilled in the art
that this type of process involves a rapid deactivation of
the catalyst and typically the process will be operated
with multiple reactors so that frequent regeneration is
possible. The details of such operations are, however,
not considered part of the invention. The dehydrogenation
reaction is endothermic and the temperature leaving the
reactor 32 will be on the order of 100-200C lower than
the inlet temperature. This will be affected by the
amount of steam employed, the condition of the catalyst,
and the severity of the reaction conditions chosen.
1229-B
-21- lo SO
The dehydrogenation reactor effluent is cooled in
exchanger 34 to a suitable temperature for inlet to the
oxidation reactor 38 and joined with an oxygen stream 36
to provide a suitable feed for the oxidation of isobuty-
tone to methacrolein. Substantially pure oxygen is preferred, although less pure oxygen could be used if
means were provided for purge of the additional inert
gases that would normally be present. The reaction would
be carried out under conditions typical of the art, that
is, temperatures in the range of about 300-400C,
pressures of about 1-8 kg/cm2 gauge, and gas hourly
space velocities on the order of 2000-3000 hrs~1. A
suitable oxidation catalyst will be used, typically a
mixture or base metal oxides, especially those which are
molybdenum-based, particularly a catalyst comprising the
elements molybdenum, bismuth, cobalt, iron, nickel,
thallium, antimony, and one or more of the alkali metals.
The reactor typically will be of the tubular type where
the pelleted catalyst is placed inside tubes which are
surrounded by a heat transfer fluid for the removal of the
heat of reaction. Typically 60-95% of the isobutylene
feed to the reactor will be converted to methacrolein,
along with minor amounts of methacrylic acid, acetic acid,
and less significant quantities of lighter and heavier
by-products. A certain amount of the isobutylene is
burned to carbon oxides and water. If the reactor is
operated to oxidize substantially all of the isobutylene
then it may be possible to feed the reactor effluent
directly to a second oxidation step for the further
oxidation of methacrolein to methacrylic acid.
The resulting gaseous mixture may be cooled and
fed to an absorber tower 42 where methacrolein is absorbed
in a recirculating water stream 44 at temperatures in the
range of about 15-20C. Substantially all of the Matthew-
crolein is recovered in a solution containing up to about
1229-B
5~Z
~22-
2 molt methacrolein. This solution may be stored for
further use or may be immediately sent to a methacrolein
stripper 46 where, at lower pressure and with the apply-
cation of heat, methacrolein is stripped from the water
and withdrawn as the vapor side stream. The stripped
water is returned ~44) to the top of the methacrolein
absorber 42 for further use. Water produced in the
process is removed (49) for disposal or recirculation as
steam in the dehydrogenation step. The light gases leaving
the top of the methacrolein absorber 42 contain large
quantities of isobutylene and isobutane along with lesser
amounts of carbon oxides, hydrogen, oxygen, and light
impurities. These gases are compressed ~48) if they are
to be returned as a recycle stream for further conversion
of the C4's to methacrolein. All or part of the
- stream may be scrubbed for removal of carbon dioxide as is
shown only schematically (50), since it represents a
technique familiar to those skilled in the art. For
example, amine or hot carbonate scrubbing may be employed.
In order to prevent build up of light impurities such as
methane, ethanes ethylene, propane and propylene, a purge
stream 51 may be taken continuously or intermittently
from the recycle stream as shown.
The gas still contains significant quantities of
I hydrogen made in the dehydrogenation of isobutane and
excess oxygen supplied to the oxidation reactor r Both of
these must be removed. It is a feature of one embodiment
of the invention to adjust the amount of oxygen supplied
to the reactor 38 so that no more remains in the effluent
than can be reacted with the hydrogen produced in the
dehydrogenation reaction. Such an oxidation is shown
being carried out in oxidation reactor 52 employing a
catalyst capable of oxidizing hydrogen to water at rota-
lively low temperatures so that the C4 components are
substantially unaffected, as has been shown in Example 2
, .
1229-B
lZ17~Z
--23--
above. Various oxidizing catalysts may be used for this
purpose, such as noble metals or base metals. In
particular, platinum or palladium on alumina has been
found particularly useful since the reaction can be
initiated at near ambient temperature. however, any
convenient temperature up to about 400C might be
employed. Alternately, platinum on a zealot support
sized to exclude C4 hydrocarbons could be chosen.
Such catalysts are capable of completely oxidizing
hydrogen to water without oxidizing C4 components.
Thus, the recycle stream is passed over the selective
oxidation catalyst ~52) for removal of both hydrogen and
oxygen and, thereafter, passed to the dehydrogenation
reactor 32 for further processing.
A typical example of the practical operation of
the flow sheet shown in Figure 2 is as follows:
Example 6
One hundred ls/hr of a feed stream containing
95~ isobutane, I n-butane, 1.5% pontoons, and 0.5%
propane is vaporized and fed to the dehydrogenation
reactor, along with 378.3 mols/hr of a recycle gas stream
containing 30% isobutane and 37% isobutylene, 16% water,
and essentially no oxygen or hydrogen. The combined
streams I are heated to about 750C (28) and mixed with
323 Mueller of steam (30) which may be provided by
recycling and vaporizing stream 49 from the methacrolein
stripper 46. The combined stream is fed to the dodder-
genation reactor 32 at about 650C, where about 45% of the
isobutane fed is converted to isobutylene. Leaving the
reactor at about 520~C, the effluent stream is cooled to
about 130C in exchanger 34 and mixed with 171 Mueller of
oxygen (36) before being supplied to the oxidation reactor
38, where about 82% of the isobutylene is converted to
methacrolein. Leaving the reactor 38 at about 342C
1229-B
~Z17~2
-24-
and 1.4 kg/cm2 gauge the effluent gases are cooled to
about 150C and passed to the absorber 42 where the
methacrolein is recovered by a recirculating water stream
sufficient to produce an aqueous solution containing about
1-2 mow % methacrolein. This solution is then stripped in
a reboiled stripper 46 to produce a vapor side stream
product containing 69.7 Mueller methacrolein, 6 Mueller
methacrylic acid and 9.4 Mueller various by-products, such
as acetone, acrolein, and water. The crude recycle gas
leaving the top of the absorber 42 is compressed
sufficiently to permit the gas to rejoin the fresh feed to
the dehydroge~ation reactor 32 (about 3.9 kg/cm2
gauge. The 429 Mueller vapor contains about 23.8%
hydrogen, 11.9% oxygen, 21.9% isobutane, and 17.2%
isobutylene. The gas also contains carbon oxides, which
are allowed to accumulate to a desired level and then
maintained at that level by scrubbing out the net make of
carbon dioxide on each pass (50). The gas is passed
through the selective oxidation reactor 52 where
substantially all of the hydrogen and the oxygen are
combined to produce water. The reactor is fed with the
gas at about 25~C and the effluent leaves at about 175C
owing to the heat of combustion. The yes is returned (24
to the freshly vaporized isobutane feed (20) to complete
the cycle.
An alternative embodiment is illustrated in
Figure 3. The dehydrogenation of isobutane to isobutylene
in reactor 32, followed by the oxidation of isobutylene to
methacrolein in reactor 38 and the subsequent recovery of
methacrolein are the same as on the flow sheet of Figure
2. In this embodiment, all gases are purged and only
unrequited isobutane and isobutylene are recycled for
further production of methacrolein. This may be
1229-B
25-- lZ175~
accomplished by absorbing the C4 hydrocarbons by
suitable liquid solvents such as paraffin or aromatic
hydrocarbons, or solid materials such as molecular sieves.
The method of Figure 3 employs a liquid solvent selected
to efficiently separate and recover isobutane and
isobutylene from the other gaseous components. One
economical solvent is a paraffin oil containing
C8-C10 hydrocarbons. The recycle gas containing
carbon oxides, hydrogen, oxygen, and light hydrocarbon
by-products is passed into an absorber tower 54 where the
isobutane, isobutylene, and heavier materials are absorbed
by a liquid stream 56, leaving in the vapor phase the
light gases which are withdrawn (58) from the top of the
column for recovery of useful components or disposal. The
Croatia liquid is withdrawn from the bottom of the
column 54 and passed (60) to stripping column 62 where the
C4's are stripped out. The Clown liquid is
withdrawn from the bottom of the column 62, cooled in
exchanger 64 and returned (56) to the absorber 54 for
reuse. The vaporized isobutane and isobutylene are
recycled (24) to the dehydrogenation reactor 32. A slip-
stream may be taken from the Clown solvent and
distilled to separate high-boiling materials in a
conventional manner (not shown).
A typical example of the practical operation of
the flow sheet shown in Figure 3 is as follows:
Example 8
One hundred mols/hr of a feed stream containing
95% isobutane is vaporized and fed (26) to the dehydrogen-
anion reactor 32, along with 146 mols/hr of a recycle gas
1229-B
I 7 5~J
stream (24) containing 62% isobutane and 38% isobutylene.
The combined streams are heated and mixed with 461 Mueller
of steam (30) either as fresh steam or recycled from the
methacrolein stripper 46 and vaporized. The combined
stream 29 is fed to the dehydrogenation reactor 32 at
about 650C, where about 45% of the isobutane fed is
converted to isobutylene. Leaving the reactor at about
520C, the effluent stream is cooled to about 130C in
exchanger 34 and mixed with 141 Mueller of oxygen (36)
before being supplied to the oxidation reactor 38, where
about 82~ of the isobutylene is converted to methacrolein.
Leaving the reactor 38 at about 342C and 1.4 kg/cm2
gauge the effluent gases are cooled to about 150C it
exchanger 40 and passed to the absorber 42 where the
methacrolein is recovered by a recirculating water stream
(44) sufficient to produce an aqueous solution containing
about 1-2 molt methacrolein (45). This solution is then
stripped in a reboiled stripper 46 tug produce a vapor
side stream product containing 69.7 Mueller methacrolein, 6
Mueller methacrylic acid and various by-products including
acetic acid, acrolein, and acetone. The crude recycle gas
(43) leaving the top of the absorber 42 is compressed (48)
sufficiently to permit the gas to rejoin the fresh feed to
the dehydrogenation reactor 32 (about 3.9 kg~cm2
gauge). The 302 Mueller vapor contains about 28~ hydrogen,
7% oxygen, 30% isobutane, 19% isobutylene, and 11% carbon
oxides. The gas is passed through the absorber tower 54
where substantially all of the isobutane and isobutylene
are scrubbed out by stream 56 containing 150 Mueller of
Cg-C10 solvent. The Croatia liquid (60) passes
to the stripping column 62 where the C4's are stripped
and are returned (24) to the dehydrogenation reactor 32 to
complete the cycle.
1229-B
121750~Z
-27-
As with the selective oxidation step previously
discussed, the absorption of C4's by a solvent could
be adapted to provide a partial separation of the effluent
from the dehydrogenation step so that only isobutylene and
unconverted isobutane are fed to the oxidation reactor.
As before, some advantages are obtained, but at additional
cost and it still remains necessary to remove oxygen and
carbon oxides from the gaseous effluent.
The discussion above with respect to processes
by which isobutane may be converted to methacrolein also
may be generally applied to an analogous process by which
propane is converted to acrolein. Substantially the same
technique would be employed, although one swilled in the
art will understand that the operating conditions and the
equipment will be modified to provide the best results,
but without departing from the invention in its broader
aspects.
The usual commercially-practiced process
generally produces acrylic acid, rather than acrolein.
The present invention may be applied to the production of
acrylic acid as shown schematically in the block diagram
of Figure 4.
The process contrasts with those of the prior art
in that the effluent from the dehydrogenation reaction 10
may be fed directly to the oxidation reaction 12 for
conversion of propylene to acrolein. One familiar with
the prior art would expect that propylene would be
separated from the effluent of the dehydrogenation step 10
before feeding it to the oxidation step 12. Since the
dehydrogenation of propane involves the production of
significant quantities of hydrogen, as well as small
amounts of lower molecular weight hydrocarbons r the
propylene must be oxidized in the presence of significance
1229-B
~z~su~
-28-
quantities of hydrogen, propane, and by-products, without
significantly affecting the oxidation of propylene to
acrolein or causing oxidation of the hydrogen. We have
shown in the examples that propylene may be oxidized to
acrolein in the presence of hydrogen, propane and
by-products of the dehydrogenation step, while not
adversely affecting the oxidation process.
after the propylene has been oxidized to
acrolein, the entire effluent from the first oxidation
step 12 may be sent directly to a second oxidation step 14
where the acrolein is oxidized to acrylic acid. There-
after, the effluent of oxidation step 14 is partially
condensed and separated at 16. Purified acrylic acid is
the product while the lower-boiling components are
separated at 18~ Waste gases and heavy by-products are
discharged while unrequited propane is recycled to steps 10
and 14.
The effluent from the first oxidation step 12 may
be separated to recover acrolein, which is then fed, along
with air or oxygen to the second oxidation step 14.
Recovery may be made by absorption in suitable liquids,
such as water and the lower carboxylic acids. Unrequited
propane would be returned to the dehydrogenation step 10
as shown. If any significant amount of acrolein remains
unconverted, it optionally may be separated and recycled
to the oxidation stage 14 as shown.
1229-B
