U.S. patent number RE32,082 [Application Number 06/734,433] was granted by the patent office on 1986-02-11 for conversion of isobutane to methacrolein.
This patent grant is currently assigned to The Halcon SD Group, Inc.. Invention is credited to Sargis Khoobiar.
United States Patent |
RE32,082 |
Khoobiar |
February 11, 1986 |
Conversion of isobutane to methacrolein
Abstract
Isobutane is converted to methacrolein in an integrated two-step
process wherein isobutane is dehydrogenated in a first step to
isobutylene, hydrogen, and by-products and the reaction effluent is
passed directly into a second step where isobutylene is oxidized to
methacrolein without significant oxidation of the hydrogen and
by-products. The methacrolein and by-products may be separated and
the unreacted isobutylene and isobutane recycled to the first step.
Alternatively, the effluent from the second step may be used as
feed to a further oxidation step for conversion of methacrolein to
methacrylic acid. In one embodiment, the hydrogen produced in the
first step is oxidized using the excess oxygen from the second step
under conditions selected to avoid loss of the C.sub.4 components.
In an alternative embodiment, the unreacted isobutane and
isobutylene are absorbed and separated from the remaining
components before being recycled to the first step.
Inventors: |
Khoobiar; Sargis (Kinnelon,
NJ) |
Assignee: |
The Halcon SD Group, Inc. (New
York, NY)
|
Family
ID: |
27004082 |
Appl.
No.: |
06/734,433 |
Filed: |
May 16, 1985 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
368196 |
Apr 14, 1982 |
04413147 |
Nov 1, 1983 |
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Current U.S.
Class: |
568/476; 568/475;
585/654 |
Current CPC
Class: |
B01J
27/192 (20130101); B01J 27/199 (20130101); C07C
5/3332 (20130101); C07C 5/3337 (20130101); C07C
45/33 (20130101); C07C 45/35 (20130101); C07C
45/37 (20130101); C07C 51/252 (20130101); C07C
5/3332 (20130101); C07C 11/02 (20130101); C07C
5/3337 (20130101); C07C 11/02 (20130101); C07C
45/33 (20130101); C07C 47/22 (20130101); C07C
45/35 (20130101); C07C 47/22 (20130101); C07C
45/37 (20130101); C07C 47/22 (20130101); C07C
51/252 (20130101); C07C 57/04 (20130101) |
Current International
Class: |
B01J
27/199 (20060101); B01J 27/14 (20060101); B01J
27/192 (20060101); C07C 5/333 (20060101); C07C
45/00 (20060101); C07C 51/16 (20060101); C07C
5/00 (20060101); C07C 51/25 (20060101); C07C
45/35 (20060101); C07C 45/37 (20060101); C07C
45/33 (20060101); C07C 045/35 () |
Field of
Search: |
;568/476,475
;585/440,654,616 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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623383 |
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Jul 1961 |
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CA |
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1165570 |
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Mar 1964 |
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DE |
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48-806 |
|
Jan 1973 |
|
JP |
|
49-28725 |
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Jul 1974 |
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JP |
|
950686 |
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Feb 1964 |
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GB |
|
1340891 |
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Nov 1970 |
|
GB |
|
2030885 |
|
Apr 1980 |
|
GB |
|
Primary Examiner: Lone; Werren B.
Attorney, Agent or Firm: Long; William C. Stewart; Riggs T.
Wells; Harold N.
Claims
What is claimed is:
1. A process for the preparation of methacrolein from isobutane
comprising:
(a) dehydrogenating isobutane to isobutylene.[., optionally.]. in
the presence of steam, over a dehydrogenation catalyst comprising a
Group VIII noble metal .[.or base metal oxide.]. on a support at a
temperature in the range of about 300.degree.-700.degree. C. and up
to about 10 kg/cm.sup.2 gauge pressure to form an effluent stream
comprising isobutylene, hydrogen, carbon oxides, and unreacted
isobutane;
(b) mixing oxygen and optionally steam with said effluent stream of
(a) and passing the mixture over a molybdenum-based oxidation
catalyst at conditions selected to produce methacrolein and
producing an effluent stream comprising methacrolein, unreacted
isobutylene and isobutane, .[.and.]. oxygen, .Iadd.steam,
.Iaddend.and carbon oxides;
(c) recovering the methacrolein from said effluent of
(b).Iadd.;
(d) separating from said effluent of (b) after recovery of
methacrolein therefrom the hydrogen produced in (a), the carbon
oxides produced in (a) and (b); and
(e) returning as feed to (a) the effluent of (b) after recovering
methacrolein, hydrogen, oxygen, and carbon oxides therefrom
.Iaddend.. .[.2. The process of claim 1 further comprising the
steps of:
(d) separating from said effluent of (b) after recovery of
methacrolein therefrom the hydrogen produced in (a), the carbon
oxides produced in (a) and (b); and
(e) returning as feed to (a) the effluent of (b) after
recovering
methacrolein, hydrogen, oxygen, and carbon oxides therefrom..]. 3.
The process of claim 1 wherein said dehydrogenation step (a) is fed
with isobutane and steam in a ratio of about 1/1 to 1/10 and a
pressure about
2-10 kg/cm.sup.2 gauge. 4. The process of claim 1 wherein said
oxidation step (b) is carried out at 300.degree.-400.degree. C. and
1-8 kg/cm.sup.2
gauge. 5. The process of claim 1 wherein said oxidation catalyst of
(b) comprises the elements molybdenum, bismuth, cobalt, iron,
nickel,
thallium, antimony, and one or more alkali metals. 6. The process
of claim 1 wherein said oxygen and hydrogen are separated from said
effluent of (b) after recovery of methacrolein therefrom by
reaction to form water over an oxidation catalyst under conditions
selected to oxidize said hydrogen
while leaving said isobutylene and isobutane substantially
unoxidized. 7. The process of claim 6 wherein said oxidation
catalyst is platinum or
palladium on alumina. 8. The process of claim 7 wherein said
oxidation
reaction is initiated at a temperature up to about 400.degree. C.
9. The process of claim 6 wherein the amount of oxygen added in
step (b) is adjusted to provide an amount in said effluent of (b)
substantially
equivalent to the hydrogen produced in (a). 10. .[.The process of
claim 1 further comprising the steps of:.]. .Iadd.A process for the
preparation of methacrolein from isobutane comprising:
(a) dehydrogenating isobutane to isobutylene in the presence of
steam, over a dehydrogenation catalyst comprising a Group VIII
noble metal on a support at a temperature in the range of about
300.degree.-700.degree. C. and up to about 10 kg/cm.sup.2 gauge
pressure to form an effluent stream comprising isobutylene,
hydrogen, carbon oxides, steam and unreacted isobutane;
(b) mixing oxygen and optionally steam with said effluent stream of
(a) and passing the mixture over a molybdenum-based oxidation
catalyst at conditions selected to produce methacrolein and
producing an effluent stream comprising methacrolein, unreacted
isobutylene and isobutane, oxygen, steam, and carbon oxides;
(c) recovering the methacrolein from said effluent of (b);
.Iaddend.
(d) separating from said effluent of (b) after recovery of
methacrolein therefrom the unreacted isobutane and isobutylene by
absorption in a liquid and thereafter stripping said absorbed
isobutane and isobutylene from said liquid; and
(e) returning as feed to (a) the stripped isobutane and isobutylene
from
(d). 11. The process of claim 10 wherein said liquid is a
C.sub.8
-C.sub.10 paraffin oil. 12. The process of claim 1 wherein the
effluent of (a) is mixed with molecular oxygen and passed over an
oxidation catalyst under conditions selected to selectively oxidize
said hydrogen while leaving said isobutylene and isobutane
substantially unoxidized and
thereafter carrying out step (b). 13. The process of claim 12
wherein said
oxidation catalyst is platinum or palladium on alumina. 14. The
process of claim 13 wherein said selective oxidation reaction is
initiated at a temperature up to about 400.degree. C.
Description
PRIOR ART
The invention relates generally to the preparation of methacrylic
acid or methyl methacrylate. More specifically the invention
relates to the preparation of methacrolein, which is a precursor of
methacrylic acid in a two-step process for manufacture of
methacrylic acid from isobutylene or tertiary butyl alcohol. In
typical processes of the prior art isobutylene or tertiary butyl
alcohol is 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.
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 isobutane as a feedstock for the preparation of
methacrolein. It is known to dehydrogenate isobutane to form
isobutylene for use in its many applications, such as preparing of
tertiary butyl alcohol, methyl tertiary butyl ether, and butyl
rubber. Such dehydrogenation processes could be used to prepare
isobutylene as a feedstock for the known methods of preparing
methacrolein. However, this would be an uneconomic way of preparing
methacrolein and methacrylic acid.
In general, integrated processes are not generally used because the
by-products of dehydrogenation must be separated, since they canot
be present in the subsequent use of isobutylene, without causing
contamination of the ultimate end product. The present process
relates to an integrated process whereby the dehydrogenation of
isobutane is combined with the oxidation of the resulting
isobutylene to methacrolein in a continuous manner and without
intermediate separation of the isobutylene.
Processes of background interest include one shown in U.S. Pat. No.
3,470,239 in which isobutane is the feedstock to a process for
methacrylic acid or methyl methacrylate via a tertiary butyl
hydroperoxide intermediate. Isobutane is oxidized to a
hydroperoxide and then used to oxidize methacrolein to methacrylic
acid. In that oxidation tertiary butyl alcohol is a by-product
which then can be used as a feedstock to prepare methacrolein in a
conventional oxidation process. Consequently, isobutane only serves
indirectly as a feedstock to the preparation of methacrolein.
In British Patent No. 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 isobutane are used so that the net
amount of isobutylene or methacrolein produced is sufficient to
result in an practical process.
A recent U.S. Pat. No. 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.
The above one-step processes are not economic, since the
conversions are quite low and require handling of substantial
amounts of unreacted 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. Pat. No. 3,784,483 a
cobalt, iron, and phosphorus catalyst is used for the oxidative
dehydrogenation of isobutane to isobutylene. The process of British
Pat. No. 1,340,891 is similar except that generally high ratios of
isobutane to oxygen (about 4/1) are used. In U.S. Pat. No.
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.
Pat. No. 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. Pat. Nos. 3,692,701, 4,005,985
and 4,041,099. In these processes large quantities of steam are
used to dehydrogenate butanes over a catalyst of platinum-tin on
zinc aluminate with a high selectivity to the corresponding butene.
Relatively high conversions are achieved. Dehydrogenation of
paraffins is also shown over zinc titanate catalysts in U.S. Pat.
Nos. 4,144,277 and 4,176,140.
In a recent published European patent application No. 42252,
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 No.
2,030,885A, in which isobutylene along with oxygen and steam is
passed over a molybdenum-based catalyst, providing a high
conversion and selectivity to methacrolein.
Based on the prior art discussed above, one might assemble a
multi-step process whereby isobutane was dehydrogenated with or
without the presence of oxygen to produce isobutylene which would
then be separated and purified and fed to a second step for the
oxidation of isobutylene to methacrolein. In this way, a combined
process could be operated which would convert substantially all of
the isobutane feed to the methacrolein by merely combining known
processes. This may not be an economic way to produce methacrolein.
As will be seen, the present invention pertains to an integrated
process by which isobutane may be converted to methacrolein without
first separating isobutylene.
Dehydrogenation of isobutane 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. Pat. No. 3,479,416, but which in an integrated
process must be accommodated in the oxidation of isobutylene to
methacrolein. The hydrogen and by-products should not have an
adverse effect on the oxidation catalyst or its performance. For
example, the exposure of the by-products to oxidizing conditions
cannot produce contaminants which reduce the quality of the
methacrolein. 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 dehydrogenation reactor. Thus, recycling a combined
effluent from methacrolein oxidation reactor to dehydrogenation
step involves particular problems unique to the integrated process
of the invention. The present process is able to accommodate
problems involved with the integration of the prior art process by
methods to be disclosed hereinafter.
SUMMARY OF THE INVENTION
Methacrolein is prepared from isobutane in an integrated process in
which dehydrogenation of isobutane to isobutylene .Iadd.in the
presence of steam .Iaddend.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. Methacrolein may be recovered by scrubbing or
quenching the oxidation reaction effluent which also contains
unreacted isobutane, isobutylene, and oxygen, plus hydrogen, carbon
oxides.Iadd., steam .Iaddend.and miscellaneous hydrocarbon
by-products of the dehydrogenation and oxidation reactions.
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 unreacted 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 conditions 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, both isobutane and isobutylene
present in the effluent gases after methacrolein has been removed
are absorbed with a suitable liquid such as a C.sub.8 to C.sub.10
paraffin oil. The gases can be further processed if desired to
recover valuable components, burned, or otherwise disposed of. In
this embodiment, the hydrogen and oxygen are contained in the waste
gases and only the uncoverted C.sub.4 's are recycled to the
dehydrogenation reactor.
Under suitable conditions, the effluent of the oxidation step might
be fed directly to another oxidation reactor, where the
methacrolein is oxidized to methacrylic acid.
The dehydrogenation of isobutane to isobutylene is carried out by a
vapor-phase reaction over a suitable catalyst, which may be
platinum-tin on zinc aluminate or other noble .[.and base.]. metal
catalysts. When the catalyst is platinum-tin on zinc aluminate the
dehydrogenation will be carried out at about 300.degree. to
700.degree. C. and up to about 10 kg/cm.sup.2 gauge pressure.
The oxidation of isobutylene to methacrolein 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.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram showing the process of the invention.
FIG. 2 is a simplified flowsheet showing one embodiment of the
invention.
FIG. 3 is a simplified flowsheet showing a second embodiment of the
invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the principal aspect, the invention is an integrated process
combining the dehydrogenation of isobutane to isobutylene with the
oxidation of isobutylene to methacrolein, but without separation of
the isobutylene between the two reactions. The product methacrolein
is separated for further use and after separation of by-products
the unreacted isobutylene and isobutane may be recycled to the
dehydrogenation step if desired. A schematic view of such a
complete process is shown in FIG. 1.
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 quantities 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.
In one aspect of the invention, after methacrolein is separated
(14) from the oxidation reactor effluent a gas containing unreacted
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 C.sub.4 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. 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 C.sub.4 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 mol of hydrogen for
each mol 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 by-products 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 of hydrogen, as will be seen in the
following example.
EXAMPLE 1
A feed gas simulating the effluent expected to result from the
dehydrogenation of isobutane in the presence of steam was blended
and fed to an oxidation reactor for production of methacrolein. The
gas composition was 7 vol % tertiary butyl alcohol, 15 vol % steam,
15 vol % oxygen, 3.2 vol % hydrogen and the balance nitrogen. Note
that tertiary butyl 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.sup.-1 and a pressure of about 1.6
kg/cm.sup.2 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 Mo.sub.12 Bi.sub.1
Fe.sub.3 Co.sub.4 Ni.sub.2 Tl.sub.0.5 Sb.sub.0.3 K.sub.0.3
Cs.sub.0.3 Si.sub.7 Ox.
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 ______________________________________ Re- actor Conver-
Test Temp. sion of % Selectivity to No. .degree.C. TBA %
MCHO.sup.(1) MAA.sup.(2) HOAC.sup.(3) CO.sub.x.sup.(4)
______________________________________ 1 346 85.2 83.4 3.9 2.5 8.9
2 346 86.7 82.1 2.9 2.6 10.3 ______________________________________
.sup.(1) MCHO methacrolein .sup.(2) MAA methacrylic acid .sup.(3)
HOAC acetic acid .sup.(4) CO.sub.x carbon oxides
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 C.sub.4 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. Pat. No. 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 methacrylic acid.
It would be preferred to operate oxidation step 12 to convert
substantially all of the isobutylene to methacrolein before
carrying out the oxidation to methacrylic acid.
In its broadest form, the invention includes the combination of
dehydrogenation and oxidation steps whereby the isobutane is
converted to methacrolein. Such a combination process without an
intermediate separation of isobutylene has been shown to be
possible. If the resulting by-product stream containing significant
quantities 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 unreacted 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,
ethane, ethylene, propane, propylene, pentane, and pentene boil at
temperatures significantly different from those of methacrolein or
the C.sub.4 hydrocarbons, they may be separated by distillation or
purging of streams containing 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 substantially all of the hydrogen and oxygen present are
removed. This can be accomplished without significant loss of the
C.sub.4 components as will be seen.
Hydrogen, as a by-product of the dehydrogenation of isobutane, must
be disposed of if unreacted 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
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 75 cc of 1/8" alumina pellets having a surface area of
about 150 m.sup.2 /gm and impregnated with 0.3 wt % platinum. The
gas was fed at a GHSV of 2000 hr.sup.-1 and about atmospheric
pressure. Its composition was 30 vol % isobutane, 4 vol % hydrogen,
3 vol % oxygen, and 63 vol % nitrogen. The reaction was carried out
at essentially ambient 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 some advantages would be
obtained, it is considered a less economically attractive and
alternative means must be provided to prevent excess oxygen from
returning to the dehydrogenation step.
FIG. 2 is a simplified flowsheet 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
containing 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 isobutylene. The amount of steam used must also be
suitable for the subsequent oxidation of the isobutylene to
methacrolein. 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 300.degree.-700.degree. C. preferably
about 650.degree. C. and at pressures between about 2-10
kg/cm.sup.2 gauge, preferably about 3 kg/cm.sup.2 gauge to reactor
32 where conversion of about 40-50% of the isobutane is converted
to isobutylene with a selectivity of 90% or more.
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.
Pat. No. 4,005,985 specifically for this process. Although platinum
and tin disposed on a zinc aluminate support provides good
performance, other catalysts which have been found effective
include platinum and rhenium supported on zinc aluminate. 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 aluminates, and rare earth aluminates 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. Pat. Nos.
3,479,416 and 3,784,483 or the zinc titanate of U.S. Pat. Nos.
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.degree.-200.degree. C.
lower than the inlet temperature. This will be affected by the
amount of steam employed and the severity of the reaction
conditions chosen.
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 isobutylene 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.degree.-400.degree. C., pressures of about 1-8
kg/cm.sup.2 gauge, and gas hourly space velocities on the order of
2000-3000 hrs.sup.-1. A suitable oxidation catalyst will be used,
typically a mixture of base metal oxides, especially those which
are molybdenum-based, particularly a catalyst comprising the
elements molybdenum, bismith, 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 80-95% 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.degree.-20.degree. C. Substantially all of the methacrolein is
recovered in a solution containing up to about 2 mol% 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 application 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 (18) if they are to be returned as a
recycle stream for further conversion of the C.sub.4 '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, ethane,
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 hydrogen made in
the dehydrogenation of isobutane and excess oxygen supplied to the
oxidation reactor. 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 relatively low temperatures so that
the C.sub.4 components are substantially unaffected, as has been
shown in Example 2 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 400.degree. C. might be employed. Alternately, platinum on a
zeolite support sized to exclude C.sub.4 hydrocarbons could be
chosen. Such catalysts are capable of completely oxidizing hydrogen
to water without oxidizing C.sub.4 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 FIG. 2 is as follows.
EXAMPLE 3
One hundred mols/hr of a feed stream containing 95% isobutane is
vaporized and fed to the dehydrogenation reactor, along with 378.3
mols/hr of a recycle gas stream containing 24.9% isobutane and
19.6% isobutylene, 27% water, and essentially no oxygen or
hydrogen. The combined streams 26 are heated to about 750.degree.
C. (38) and mixed with 323 mol/hr 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
dehydrogenation reactor 32 at about 650.degree. C., where about 45%
of the isobutane fed is converted to isobutylene. Leaving the
reactor at about 520.degree. C., the effluent stream is cooled to
about 130.degree. C. in exchanger 34 and mixed with 171 mol/hr 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 342.degree. C. and 1.4 kg/cm.sup.2
gauge the effluent gases are cooled to about 150.degree. C. 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 mol % methacrolein. This solution is
then stripped in a reboiled stripper 46 to produce a vapor
sidestream product containing 69.7 mol/hr methacrolein, 6 mol/hr
methacrylic acid and 9.4 mol/hr 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 dehydrogenation reactor 32 (about 3.9
kg/cm.sup.2 gauge. The 429 mol/hr 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.degree. C. and the effluent
leaves at about 175.degree. C. owing to the heat of combustion. The
gas is returned (24) to the freshly vaporized isobutane feed (20)
to complete the cycle.
An alternative embodiment is illustrated in FIG. 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 FIG. 2. In this embodiment, all gases are purged and only
unreacted isobutane and isobutylene are recycled for further
production of methacrolein. This may be accomplished by absorbing
the C.sub.4 hydrocarbons by suitable liquid solvents such as
paraffin or aromatic hydrocarbons, or solid materials such as
molecular sieves. The method of FIG. 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 C.sub.8 -C.sub.10
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 C.sub.4
-rich liquid is withdrawn from the bottom of the column 4 and
passed (60) to stripping column 62 where the C.sub.4 's are
stripped out. The C.sub.4 -lean 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 slipstream may
be taken from the C.sub. 4 -lean 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 FIG. 3 is as follows:
EXAMPLE 4
One hundred mols/hr of a feed stream containing 95% isobutane is
vaporized and fed (26) to the dehydrogenation reactor 32, along
with 146 mols/hr of a recycle gas stream (24) containing 62%
isobutane and 38% isobutylene. The combined streams are heated and
mixed with 461 mol/hr 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 650.degree. C., where about 45% of the isobutane fed is
converted to isobutylene. Leaving the reactor at about 520.degree.
C., the effluent stream is cooled to about 130.degree. C. in
exchanger 34 and mixed with 141 mol/hr 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 342.degree. C. and 1.4 kg/cm.sup.2 gauge the effluent gases
are cooled to about 150.degree. C. in 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 mol% methacrolein (45). This solution
is then stripped in reboiled stripper 46 to produce a vapor
sidestream product containing 69.7 mol/hr methacrolein, 6 mol/hr
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/cm.sup.2 gauge). The 302 mol/hr 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 mol/hr of C.sub.8
-C.sub.10 solvent. The C.sub.4 -rich liquid (60) passes to the
stripping column 62 where the C.sub.4 's are stripped and are
returned (24) to the dehydrogenation reactor 32 to complete the
cycle.
As with the selective oxidation step previously discussed, the
absorption of C.sub.4 '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.
* * * * *