NL8301303A - METHOD FOR CONVERTING ALKANES TO UNSATURATED ALDEHYDES - Google Patents

Description

* v

Process for converting alkanes into unsaturated aldehydes.

This invention generally relates to the preparation of unsaturated aldehydes, acids and esters thereof, and particularly to the preparation of methacroleins (which is a precursor of methacrylic acid in a two-step process for the preparation of methacrylic acid from isobutylene or t-butanol). In representative prior art processes, vapor phase isobutene or t-butanol are reacted with molecular oxygen under the action of a catalyst to form methacroleins. This is separated off and reacted with molecular oxygen to form methacrylic acid in vapor phase 10 under the influence of another catalyst. The methacrylic acid can then be converted into the ester with a suitable alcohol.

This invention likewise relates to the preparation of acrolein or acrylic acid from propane in exactly the same manner.

Generally, the feed in this process was an unsaturated olefin or the corresponding alcohol. Any accidentally present saturated hydrocarbon was considered essentially inert as little or no oxidation thereof occurred. However, there is an economic incentive to use alkanes as a starting material for the preparation of unsaturated aldehydes and acids. For example, it is known to dehydrogenate isobutane to isobutene, which has many uses, such as the preparation of t-butanol, methyl-t-butyl ether and butyl rubber. Such dehydrogenations could be used for the preparation of isobutene as a starting material for the known preparation of methacroleins. But separating and purifying isobutene represents an uneconomical way of preparing methacroleins and methacrylic acid, and the same goes for the preparation of acrolein and acrylic acid from propane.

8301303 «* 2

Generally, integrated processes are not used because the by-products of the dehydrogenation must be separated, because they cannot be present in the subsequent use of the olefin without causing contamination of the final product. This invention relates to an integrated process in which the dehydrogenation of isobutane or propane is combined with the oxidation of the resulting isobutene or propylene to methacroleins or acroles in a continuous manner, without intermediate separation of isobutylene or propylene.

Relevant background information is found in U.S. Pat. No. 3,470,239, according to which isobutane is the starting material for the preparation of methacrylic acid or methyl methacrylate via t-butyl hydroperoxide. Isobutane is oxidized to the hydroperoxide, then to methacroleins or methacrylic acid. In this oxidation, t-butanol is a by-product which can serve as a starting material in the usual way in the preparation of methacroleins. Isobutane therefore serves here only indirectly as a starting material in the preparation of methacroleins.

According to British Patent 1,340,891, isobutane and oxygen are reacted to isobutene and / or methacroleins under the influence of a variety of metal oxide catalysts. Since the conversion of the isobutane is rather low, high concentrations of isobutene are used, so that the net amount of isobutene or methacroleins formed is sufficient to arrive at a practically applicable method. With propane as a starting material, something similar is obtained.

U.S. Patent 4,260,822 discloses a process for the direct oxidation of isobutane to methacrylic acid in a single step, with large amounts of isobutane to compensate for the relatively low conversion of isobutane to product. Here, too, propane is said to be oxidized to acrylic acid in a similar manner.

The above one-step processes are not economical, since the conversions are rather low and much unchanged starting material has to be recycled by recycle 8301303 3 to obtain sufficient isobutane production. Also, the catalysts generally have too short a life for a proper technical application.

Since isobutene has a number of uses other than the preparation of methacrylic acid, a number of methods have been developed for the conversion of isobutane to isobutene. For example, in U.S. Pat. No. 3,784,483, a single cobalt, iron, and phosphorus-containing catalyst is used for the oxidative dehydrogenation of isobutane to isobutene or propane to propylene. The process of British Pat. No. 1,340,893 is similar, except that higher ratios of isobutene to oxygen (about 4: 1) are used. According to US Pat. No. 3,479,416, the operation is carried out in the absence of oxygen and with a metal catalyst, which in particular contains chromium, molybdenum and vanadium. According to a group of patents, for which U.S. Pat. No. 4,083,883 is representative, a noble metal is combined with supported promoter metals and paraffins are dehydrogenated therewith, especially the straight-chain alkanes.

20 Another approach can be found in the

U.S. Patents 3,692,701, 4,005,985, and 4,041,099. In these processes, large amounts of steam are used to dehydrogenate butanes under the influence of platinum / tin on zinc aluminate with a high selectivity for the corresponding Butene, or for the conversion of propane to propylene.

Relatively high conversions are achieved. The dehydrogenation of paraffins under the influence of zinc titanate has also been described in U.S. Pat. Nos. 4,144,277 and 4,176,140.

In a recently laid-open European patent application No. 42,252, isobutene is prepared by dehydro-isomerizing η-butane under the influence of a Group IIIA element or a compound thereof, in particular gallium, on a support.

Many patents describe methods of oxidizing isobutene to methacroleins. Of particular interest is British Patent Application No. 2,030,885A, according to 8301303 * 4, which passes methacroleins together with oxygen and steam over a molybdenum-based catalyst, which results in high conversion and good selectivity for methacroleins.

Many patents disclose methods for the oxidation of propylene to acrylic acid, including European Patent 0,000 663 and U.S. Patent 3,954,855. To achieve the best yields of acrylic acid, these processes are generally performed in two steps. In the first, propylene in the vapor phase is oxidized to acrolein under the influence of molybdenum with promoter, and in the second phase with additional oxygen over a second molybdenum catalyst with promoter to acrylic acid.

Based on the prior art discussed above, one could design a multi-step process in which an alkane with or without oxygen is dehydrogenated to the corresponding olefin, which is then separated and purified and passed in a second step for the oxidation from the olefin to the corresponding unsaturated aldehyde. In this way, it would be possible, just by combining known methods, to transfer substantially all of the alkane used into unsaturated aldehyde. That cannot be an economical way to prepare aldehyde. As will be seen, this invention provides an integrated method by which an olefin can be converted to the corresponding unsaturated aldehyde without separating the olefin in between.

The hydrogenation of an alkane results in significant amounts of hydrogen and a small amount of lower molecular weight hydrocarbons, which are removed from the product in the prior art processes (e.g., those of U.S. Pat. No. 3,479,416), but which are subject to integrated working method. Hydrogen and by-products must not adversely affect the oxidation catalyst or its behavior. For example, exposing the by-products to the oxidizing conditions should not lead to impurities that would degrade the quality of the aldehyde. Also, the presence of 8301303 hydrogen should not lead to an explosive mixture in the oxidation reactor.

The oxidation leads to CO and CO2 as by-products, as well as some lower molecular weight oxygen-containing compounds, with some oxygen also entering the gases, which is not acceptable in the upstream dehydrogenation. Thus, recirculating such a reaction mixture from the oxidation reactor to dehydrogenation leads to certain problems unique to the integrated process of this invention. But with the present invention to be described, these problems have been overcome.

Alkanes are dehydrogenated to the corresponding olefins, which are then oxidized to the unsaturated aldehydes. For example, in the integrated preparation of methacrolein from isobutane, dehydrogenation of isobutane to isobutene is direct, i.e., without separation of isobutene from the reaction mixture, followed by addition of oxygen and oxidation of the isobutene to methacrolein. Otherwise, some by-products of the dehydrogenation ring may be removed before oxidation, thereby concentrating the isobutene. This can be done, for example, by selective oxidation or by partial condensation of the water contained in that reaction mixture. "Methacrolein can be separated by washing or quenching the reaction mixture, which also contains unreacted isobutane, iso-butene, oxygen, hydrogen, CO and CO2 and various by-product hydrocarbons. The same process can also be done with propane as the starting material .

In some embodiments of this invention, methacrolein is separated and oxygen, hydrogen, CO and CO 2 are removed from the reaction mixture by catalytic reactions or absorption techniques and then the residual gas mixture containing unreacted isobutane and isobutene is returned to the dehydrogenation. A preferred way to remove oxygen and hydrogen is to react them under the influence of a suitable catalyst under conditions that completely remove the oxygen but without effect on the isobutene and isobutane present. Suitable catalysts are platinum and other 8-group noble metals on alumina or other supports. The oxidation of hydrogen takes place at a temperature which permits its selective oxidation. With platinum on, such oxidations can start at relatively low temperatures, even at room temperature. In a preferred embodiment, so much oxygen is introduced into the oxidation reaction that the reaction mixture contains only little afterwards, and the hydrogen formed in dehydrogenation consumes all the oxygen remaining after isobutene has been oxidized to methacrolein.

In another embodiment, the alkane and the olefin remaining after the aldehyde has been removed are absorbed with a suitable liquid such as a Cg-Pg paraffin oil. If desired, the gases can be further processed to recover valuable components from them, but they can also be burned or otherwise lost. In this embodiment, hydrogen and oxygen are contained in the waste gases and only the unreacted hydrocarbons need to be recycled to the dehydrogenation.

In yet another embodiment, the reaction mixture to the dehydrogenation is partially separated and the C 1 or (1 hydrocarbons-containing concentrate (propylene / propane or isobutylene / isohutane) is passed to the oxidation.

The dehydrogenation of the alkane to the alkene occurs in vapor phase under the influence of a suitable catalyst, which may be platinum / tin on zinc aluminate or another precious metal or ordinary metal catalyst. If the catalyst is platinum / tin on zinc aluminate, dehydrogenation can be done at 400 ° to 700 ° C and at pressures up to 10 ato.

The oxidation of the alkane to aldehyde can be effected with suitable catalysts, such as mixtures of metal oxides, in particular catalysts based on malybdenum oxide, and in particular a catalyst consisting of the oxides of molybdenum, bismuth, cobalt , iron, nickel, thallium, antimony, silicic acid and one or more alkali metals.

In the accompanying drawings, Figure 1 is a block diagram of the process of the invention, Figures 2 and 3 are simplified flow charts for the preparation of methacrolein from isobutane, and Figure 4 is a block diagram for the preparation of acrylic acid from propane.

Thus, this invention provides an integrated process in which dehydrogenation of an alkane (especially propane or isobutane) is combined with subsequent oxidation of the olefin to acrolein or methacrolein without intermediate purification of the propylene or isobutylene. The aldehyde is separated for further use and after removal of the by-products, the unreacted olefin and alkane can be recycled to dehydrogenation if desired. A diagram of such a complete working method is shown in figure 1.

Although this is described in detail for isobutane, all this of course also applies to propane.

This method is in contrast to the prior art in that the two reactions are carried out such that the effluent from dehydrogenation reactor 10 can be fed directly into the oxidation reactor 12 to convert isobutylene into methacroleins. One skilled in the art would expect the isobutene to be separated from the dehydrogenation effluent before being oxidized. Since dehydrogenation of isobutane involves the formation of significant amounts of hydrogen, and also of small amounts of lower molecular weight hydrocarbons, the isobutene must be oxidized in the presence of significant amounts of hydrogen and by-product, without the oxidation of the isobutene to methacrolein notably influences whether oxidation of the hydrogen entails. It has been found that isobutene can still be oxidized to methacrolein in the presence of hydrogen and by-products of dehydrogenation without any problem.

8301303 8

In another embodiment, partial separation (11) can be accomplished by absorption or distillation techniques that will be known to those skilled in the art such that a concentrated C 2 feed is obtained for the oxidation (12). Depending on the composition of the effluent from the dehydrogenation, it may be desirable to remove all or part of hydrogen, the light hydrocarbons and / or water. The advantages of the invention are then preserved at least in part, since an impure feed is used in the oxidation. This concentration is most easily accomplished by oxidizing the hydrogen to water and / or condensing some of the water into the dehydrogenation effluent.

According to an aspect of the invention, after the methacrolein has been separated from the effluent from the oxidation reactor 15 (14), a gas containing unreacted isobutene and isobutane is recycled to the dehydrogenation. Since excess oxygen is used to oxidize isobutene to methacrolein, this recycle stream will contain significant amounts of oxygen which must not be present in the dehydrogenation.

All of the oxygen present would react at the operating temperature, leading to loss of C 1 -hydrocarbons.

Very typically, in processes using oxygen for dehydrogenation, only minor amounts of isohutane are present. Thus, in order to arrive at a process in which isohutane is only converted into methacrolein (with minor amounts of by-products), it is necessary to remove both the hydrogen formed in the dehydrogenation and the excess oxygen after the oxidation, as well as CO and CO2 and others. by-products. The permissible content of 3Q in the gas is adjusted as required for each component.

In a preferred embodiment, CO2 is washed away and by-products are occasionally vented, after which hydrogen is catalytically oxidized to water under conditions where no loss of C 2 components occurs. In another embodiment, the isohutane and isobutene are washed from the effluent, recovered and returned to the dehydrogenation while the gases are discarded.

Dehydrogenation of isobutane results in 1 mole of hydrogen per mole of isobutene and additional hydrogen through the formation of lower molecular weight by-products.

If isobutene is not separated prior to oxidation, as in this process, the hydrogen and other by-products also enter the subsequent oxidation. Oxidization of the by-products to compounds which contaminate the methacrolein should be avoided. Oxidizing the hydrogen along with the isobutene is undesirable since it consumes oxygen and would interfere with the oxidation of the isobutene. Also, the oxidation of hydrogen would generate undesirable heat, causing hot spots in the reactor tubes and decreasing the productivity for methacroleins. But it was found that the oxidation of isobutene in the presence of hydrogen

can be carried out with practically no consumption of that hydrogen, as the following example will show. yoorheeld I

Oxidation of isobutane in the presence of hydrogen.

A feed simulating the effluent expected in the dehydrogenation of isobutane in the presence of steam was mixed and passed into an oxidation reactor for the preparation of methacrolaen. The composition of the gas was 7% by volume t-butanol, 15% by volume steam, 15% by volume oxygen, 3.2% by volume hydrogen and the balance nitrogen.

Note that the t-Butanol dehydrates to isobutene, and the two are generally considered equivalent. This gas was passed at a space velocity of 2300 per hour and a pressure of 1.6 atmospheres over 160 ml of catalyst with 3 mm diameter beads lying in a 12.5 mm diameter tubular reactor. The heat of reaction was removed and the temperature adjusted by circulation of molten salt, as is customary in such reactions. The oxidation catalyst had the gross formula of 35 Mbj2BliFe3Co4N: L2T10} 5SbQ3 ^ SiyO ^.

The results of two tests are shown in the table below; 8301303 αα Test 1 happened in the presence of hydrogen and Test 2 was what happened after the hydrogen supply was turned off.

Table A

Reactor Conversion Test _% Selectivity for_ 5 No. Temp. v.h. t-BuOH m

° C MCHQU 'MAZU' H0AcWCO W

_ _ ________ _ _ __ x 1 346 85.2% 83.4 3.9 2.5 8.9 2 346 86.7% 82.1 2.9 2.0 10.3 10 (1) MCHO - methacontrol (2) MAZ - methacrylic acid (3) HOAc - acetic acid (4) CO - CO and CO, 15 The two tests show differences which are considered not to be significant but are within the normal variations of tests. The amount of hydrogen consumed was not measured precisely because of the small amounts involved, but it was clear from the analysis of the effluent that little or no hydrogen should be oxidized. In any case, the figures show that it had little or no influence on the oxidation of isobutene to methacrolein.

Addition of small amounts of other carbon dehydrogenation by-products, such as n-butenes, also appears to have little or no effect on oxidation to methacroleins.

The separation of the methacrolein (J4) can be done by methods known in the art. See, for example, U.S. Patent 4,234,519. This can be done by cooling and condensing the aqueous methacrolein from the effluent, followed by washing the residual gas with a circulating water stream to complete separation of the methacrolein. The resulting methacrolein solutions can be stripped at the same time to obtain a methacrolein-containing vapor which can then be utilized. Other options, such as solvent extraction and the like, can also be used.

Although the scheme shows the separation of methacroleins for further use (oxidation to methacrylic acid or otherwise), it is also possible, provided that the catalyst is resistant to the various constituents present, that the effluent from the oxidation (32) is directly other oxidation to convert methacrolein to methacrylic acid. It would be preferable to conduct the oxidation (12) such that all the isobutene is converted to methacrolein before it is oxidized to methacrylic acid.

In a broad form, the invention includes the combination of dehydrogenation with oxidation to convert isobutane to methacrolein. Such a combination process without intermediate separation of isobutene has now been found to be possible. If the resulting by-product stream contains enough isobutene and isobutane to be used for other purposes, recirculation of the gas is not necessary.

In many cases it will be desirable to recycle the unreacted butene and isobutane so that the integrated process essentially converts all the isobutane into methacroleins with only minor amounts of by-products, and without ever leading to a stream of substantially pure isobutene.

To recycle gas with large amounts of isobutane and isobutene, it will be necessary to remove the hydrogen formed after dehydrogenation and the excess oxygen after oxidation, and in both cases the CO and CO2 formed. In addition, a leakage current of lower and higher boiling by-products (with contaminants of the food) must be set. CO2 removal is generally effected by washing out the circulating flow in whole or in part with a carbonate or amine solution, which is known in the art, with which the content of CO2 can be kept at the desired low level. CO, the dehydrogenation is converted into CO2. Since the presence of these substances is neither very important in the dehydrogenation nor in the oxidation, one can afford to allow those constituents to accumulate to a level at which the removal takes place easily and cheaply. Since the lower and higher boiling hydrocarbon by-products, such as methane, ethane, ethylene, propane, propylene, pentane and pentene, boil at temperatures significantly different from those of methacrolein and the 5-hydrocarbons, they can be separated by distillation or by occasionally dropping off streams concentrated to these by-products Removal of the hydrogen formed in dehydrogenation can be accomplished in various ways, such as catalytic oxidation, liquid or gas phase absorption. In this invention, hydrogen and oxygen are lost together by reacting them in vapor phase with a suitable oxidation catalyst to water under conditions such that essentially all hydrogen and oxygen are removed ... As will be seen, this can be done without significant losses of C 2 constituents occur.

If unreacted isobutane is to be recycled, the hydrogen formed as a by-product in dehydrogenation must be removed. As mentioned earlier, the oxidation of isobutene occurs such that little or no hydrogen is consumed. In a preferred embodiment of the invention, the amount of oxygen in the feed of the isobutene oxidation is adjusted such that the effluent contains no more oxygen than with the present hydrogen can react. Of course, no significant amounts of oxygen or hydrogen are allowed to be present in the dehydrogenation reactor, for maximum yield, only isobutane and steam are introduced therein. Oxygen removal is more important, but hydrogen may be allowed in minor amounts. It has been found that such selection of oxidation of hydrogen can occur without loss of valuable isobutane or isobutene, as shown in the following example.

Example II

35 Selective oxidation of hydrogen.

A feed simulating 8301303 13 recirculation gas after the methacrolein was removed therefrom was introduced into a 12.5 mm diameter tubular reactor containing 75 ml of 3 mm pellets with a specific surface area of 150 m 2 / g and impregnated with 0.3 wt% platinum.

The gas was supplied at a space velocity of 2000 per hour and at approximately atmospheric pressure. The composition was 30% by volume of isobutane, 4% by volume of hydrogen, 3% by volume of oxygen and 63% by volume of nitrogen. The reaction took place essentially at room temperature, where all hydrogen was found to be consumed, J0 but practically none of the isobutane.

Although it is preferred to carry out the selective oxidation of hydrogen in the recycle gas, it is possible to carry out this same substance after the dehydrogenation of the isobutane and before the oxidation to isobutene.

Although that would cause a concentration of isobutene, other measures must then be taken to prevent excess oxygen from returning to the dehydrogenation.

Example III

Dehydrogenation of propane.

A catalyst was prepared by impregnating zinc aluminate with 0.4 wt% Ft and 1 wt% In. A 50 ml sample as extrudate with 3 mm diameter was placed in a tubular reactor with 25.4 mm internal diameter.

Another 250 ml of zinc aluminate (the support) was placed on top of the catalyst bed. A gas stream containing 2 moles of steam per mole of propane was passed through the reactor at a space velocity of 4000 per hour and a pressure of 3.5 atoms. The catalyst bed inlet was kept at a certain temperature, and the outlet temperature revealed the endothermic nature of the dehydrogenation. The results of this are shown in Table B.

Table B

Test Inlet Conversion Selectivity for temp. v.h. C-Hft propene _ ° C j 0 35 1 548 23.6% 95.8% 2 523 29.7% 96.5% 8301303 14

The dehydrogenation of propane gives one mole of hydrogen per mole of propene and even more hydrogen by the formation of somewhat lower molecular by-products. If propylene is not separated for the prior oxidation, as is the case here, hydrogen and the other by-products will pass to the subsequent oxidation. Oxidation of the by-products to compounds contaminating the acrolein should be avoided. That hydrogen would be oxidized together with the propylene is undesirable since it would consume oxygen and interfere with the oxidation of propylene. Also, the oxidation of hydrogen would lead to an undesirable heat, which can cause hot spots on the catalyst and decrease the acrolein productivity. However, it has been found that the oxidation of propylene in the presence of hydrogen can be carried out practically without any consumption of hydrogen, as the following example will show. Also, the presence of propane which was not dehydrogenated does not appear to have a significant effect on the oxidation of propylene.

Example IV

Oxidation of propylene in the presence of hydrogen and propane.

In the manner described in European Patent 663, a catalyst was prepared containing a mixture of 80 wt.% Kg ^ Co ^ ^ Fe ^ BijPg 5Moi2 ° x with% S1O2. Of this, 220 ml was placed as 4.7 mm pellets in a one-tube reactor with an internal diameter of 12.7 mm. A feed containing 5 vol% propylene, 20 yol% steam and 12% oxygen was passed over the catalyst at a space velocity of 1200 per hour; the pressure in the catalyst was about 1.76 ato. When analyzed by gas chromatography, the effluents of the tests conducted therewith were found to have compositions Nos 3-6 of Table C.

8301303> τΤ 15

Table G

Taste Time, Temp. Conversion of Selectivity for no hour ° C the C ^ Hg acrolein and acrylic acid 3 78.0 310 91.1% 87.4% 5 4 81.0 310 90.6 86.3 5 93.5 310 90.5 87 .7 6 99.5 310 90.1 89.7 7 102 310 94.2% 86.6% 8 115 309 92.6 88.7 10 9 124 309 92.5 87.9 10 139 309 89.3 87 , 2 11 147 309 91.4% 85.6% 12 156 310 93.7 86.7 13 180 309 92.9 86.9 15

After the behavior of the propylene-only catalyst in the feed was established, hydrogen was also added thereto, to mimic the conditions that will occur when the oxidation is fed with propylene created by dehydrogenation of propane. An additional 4% by volume of hydrogen was added to the previously used mixture.

The compositions of the now occurring effluents are nos. 7 to 10 of Table C. It can be seen that essentially the same results were obtained as in Experiments 3-6. The conclusion is therefore that essentially no hydrogen was oxidized, judging by the operating conditions in the reactor.

The hydrogen supply was interrupted and the oxidation continued with the addition of 4% by volume of propane.

The results found are those of Nos. 11-13 of Table C.

Again, essentially the same results as in Runs 3-6 were obtained, indicating that propane does not affect the oxidation of propene and is not itself oxidized.

These results show that propylene can be oxidized in the presence of hydrogen and propane, the two major by-products of dehydrogenation from propane to propylene. It is concluded that the integration of dehydrogenation and oxidation is possible.

Example V

Oxidation to acroles in the presence of hydrogen and propane.

To illustrate the oxidation of acrolein to acrylic acid and the effect of hydrogen and propane, tests were carried out according to that of Example I.

In accordance with Example 1 of U.S. Patent 3,954,855, a catalyst of the same composition as mentioned there was prepared. That composition was M0 2 gSrQ 2 ^ 2 4 u2 2 ^ x ^ ^ This catalyst, 73 ml as 4.7 mm pellets was placed in a tubular reactor with 32.7 mm internal diameter. This reactor was surrounded by a themostated bath which provided a certain uniform temperature. The feed was initially 6-8 vol.% Acrolein, 53 vol.% Air and 45 vol.% Steam. The space velocity was 3000 per hour and the average pressure in the reactor slightly above that of the outside air. The results are shown in table D.

Table D

20 Sample Time, Temp. Conversion of Selectivity for no. Hour ° C the acrolein acrylic acid_ 14 126 254 92.8% 89.1% 15 126 251 84.5 89.2 16 41 252 86.8 88.4 25 ---- 17 36 251 80 , 7 87

As in Example IV, 4% by volume of hydrogen was first added and this hydrogen was then replaced with pro-3Q pane. These were Runs Nos. 16 and 17 of Table D.

Although some difference in the conversion of the acrolein occurs, it is clear from Runs 14 and 35 that the run is very temperature sensitive and the differences between Runs 35, 36 and 17 are not considered significant. The selectivity for acrylic acid remains essentially constant.

Figure 2 is a simplified flow chart 8301303 J ... '... 11 11 of a preferred embodiment of the invention with a complete process in which isobutane is primarily converted to methacroles. A feed of fresh isobutane (20) is evaporated in heat exchanger (221) and combined with a stream recycled via 5 (24) containing significant amounts of iso-butane and isobutene The combined stream (26) is then heated in heat exchanger (28) temperature that the mixture in combination with steam supplied via (30) will have just the temperature suitable for the dehydrogenation of isobutane to isobutene The amount of steam must also be suitable for the subsequent oxidation of the isobutene to methacrolein As indicated here a feed of fresh steam was used, when this happens the water formed in the reactions is finally discharged from the system under the methacrolein stripper (46) Since water is produced in this process it can be used as steam in this process.

In any case, a feed containing isobutane and steam in a molar ratio between 1: 1 and 1:10, preferably between 1: 2 and 1: 5, with a temperature between 400 ° and 70 ° C, preferably about 650 ° C and with a pressure between 2 and 10 ato, preferably about 3 ato, to reactor 32 in which the isobutane is converted 45-50% with a selectivity of 90% or better for isobutene.

A number of catalysts for this process have been mentioned in the prior art, as well as by conditions under which that reaction is carried out, depending on the catalyst selected. For this process, the catalyst of U.S. Pat. No. 4,005,985 which contains platinum is useful. While platinum and tin supported by zinc alumina do well, other catalysts have also been found to be effective, including platinum and rhenium or indium on zinc aluminate.

Other noble metals from the 8 group, alone or in combination with various carriers known in the art, may also find use in dehydrogenating isobutane to isobutene.

Other possible carriers may include aluminum oxide 8301303 18, other alkaline earth metal aluminates and the rare earth metal aluminates including lanthanum. Promoters such as tin, lead, antimony and thallium can be used. Suitable metal oxides, such as those of chromium, zirconium, titanium, magnesium and vanadium, are described in U.S. Pat. Nos. 3,479,416 and 3,784,483, and the zircon titanate of U.S. Pat. Nos. 4,176,140 and 4,144,277 may also be used. The invention is not limited to any particular catalyst.

It will be apparent to those skilled in the art that this type of process involves rapid deactivation of the catalyst, and generally the process is run with multiple reactors to allow frequent regeneration. However, the details of such operations are not considered to be part of this invention. Dehydrogenation is endothermic and the temperature at the outlet of reactor 32 will thus be 100-200 ° C lower than the inlet temperature. This is influenced by the amount of steam used, the condition of the catalyst and the flexibility of the selected reaction conditions.

The effluent from the dehydrogenation is cooled in heat exchanger (341) to a suitable temperature for the inlet to the oxidation reactor (381 and combined with a stream of oxygen (361) to provide a suitable feed for the oxidation of isobutene to methacrolein. Essentially 25 pure oxygen is preferred, although less pure oxygen can also be used when releasing the additionally introduced inert gases which are then present.

The reaction takes place under normal conditions for this technique, i.e. at temperatures between 300 ° and 40 ° C, pressures between 3CL J and 8 ato and space velocities from 2000 to 3000 per hour. A suitable oxidation catalyst is used, generally some metal oxide, in particular a molybdenum-based catalyst, and even more particularly a catalyst containing the elements molybdenum, bismuth, cobalt, iron, nickel, thallium, antimony and contains one or more alkali metals. The reactor will generally be a tubular reactor with catalyst pellets inside tubes surrounded by a heat transfer fluid to dissipate the heat of reaction.

Generally, 60-95% of the isobutene supplied is converted to methacrolein, along with minor amounts of methacrylic acid, acetic acid and even smaller amounts of lower and higher boiling by-products. A certain amount of isobutene is burned to CO and CO ^ and Ή. ^ 0. If the reactor operates in such a way that practically all the isobutene is oxidized, it may be possible to connect the effluent of this reactor directly to a second oxidation for the further oxidation of methacrolein to methacrylic acid.

The resulting gas mixture can be cooled and passed to an absorption tower (42) in which the methacrolein is absorbed in a circulating stream of water (44) at a temperature between 35 and 20 ° C. Substantially all of the methacrolein is thus obtained as a solution with up to about 2 mol% of methacrolein. This solution can be stored for further use, or it can go directly to a methaerolein stripper (46), where the methacrolein is separated from the water at a lower pressure and under heat supply and from which it exits as a vapor stream. The blown out water returns (44) to the top of the methacrolein absorber (42) for reuse. The water formed in the process is drained (49) for disposal or recirculation as steam in dehydrogenation. The light gases that go above the absorption tower (42) contain large amounts of isobutene and isobutane, along with smaller amounts of CO and CO2, and ^ 2 and lighter impurities. These gases are compressed in (48) if they are to be recycled for a further conversion of the C 2 hydrocarbons to methacroleins.

The stream may be washed in whole or in part with water to remove CO2, as schematically indicated by (50), since this is a technique known to those skilled in the art. For example, an amine or hot carbonate solution can be used for that washout. To prevent the accumulation of light impurities such as methane, ethane, ethylene, propane, propane and propylene, some gas is continuously released from the recirculation stream (51).

The gas · Contains important amounts of hydrogen which is formed during the dehydrogenation of isobutane, and also over oxygen which was introduced into the oxidation reactor. These Both must be removed. It is an aspect of an embodiment of the invention to adjust the amount of oxygen supplied to reactor (38) such that no more remains in the effluent than can react with the hydrogen formed in the dehydrogenation. Such oxidation in reactor (52) with a catalyst capable of oxidizing hydrogen to water at relatively low temperatures, so that the (-hydrocarbons remain practically unaffected, is discussed in Example 2. Various catalysts, such as precious metals or metal oxides, can serve for this purpose. In particular, platinum and palladium on A ^ Qg have proven to be particularly suitable for this reaction at room temperature, but any suitable temperature up to 40 ° C may be used, and platinum may also be used on a zeolite which does not allow hydrocarbons into its cavities. 20. Such catalysts can oxidize hydrogen completely to water without affecting the (β-constituents. Thus, the recycle stream passes over the selective oxidation catalyst in (52) and then for further utilization to the dehydrogenation reactor (321.

A representative example for the practical application of the flow chart according to figure 2 is the following.

Example VI

Of a feed containing 0.5 Z propane, 3 Z 30 n-butane and 1.5 Z pentanes and 95 Z of isobutane (20), evaporated and combined with 378.3 mol / hour recirculation flow containing 30 Z isobutane, 37 Z isobutene and 16 Z water but practically no oxygen or hydrogen. The combined streams (26) were heated in (28) to about 750 ° C and mixed with 323 mol / hour steam (30) obtained by recirculation and evaporation from a methacrole stripper (46) 8301303 21 drained water ( 49). The combined stream went at a temperature of about 650 ° C to the dehydrogenation reactor (32) in which about 45% of the supplied isobutane was converted to isobutene. The effluent exiting the reactor at about 520 ° C was cooled to about 130 ° C in heat exchanger (34) and mixed with 171 mol / hour oxygen (36) before entering oxidation reactor (38} containing about 82 % of the isobutene was converted to methacrolein The effluent, which came with about 342 ° C and 1.4 atom reactor (38) was cooled to about 150 ° C in 10 (40) and then went to absorbers (42 ), in which the methacrolein was separated by absorption in pumped water circulating enough to give a solution containing 1-2 mol.Z of methacrolein, this solution was stripped in a column (46) with a boiling pot, giving a vapor flow of 69 per hour 69 .7 moles of methacroleins, 6 moles of methacrylic acid and 9.4 moles of various by-products (acetone, aeroleins, water).

The raw gas exiting the absorption tower (42) was compressed enough (about to 3.9 ato) to combine it with fresh feed for the dehydrogenation reactor (32). This 20,429 mol / hour vapor contained 23.8% hydrogen, 11.9% oxygen, 21.9% isobutane and 17.2 Z isobutene. The gas also contained CO and CO2, the content of which was increased to the extent that it became worthwhile to wash out (50), which happened after each cycle. That scrubbed gas passed through the selective reactor (52) in which practically all hydrogen and oxygen were combined to water. That reactor was fed with gas of about 25 ° C, but leaving the combustion value at a temperature of 75 ° C. That gas (24) was then combined with freshly supplied evaporated isobutane (20) to complete the loop.

An alternative embodiment is found in Figure 3. The dehydrogenation of isobutane to isobutene in reactor (32), followed by the oxidation of isobutene to methacrolines in reactor (38) and subsequent separation of the methacroleins are the same as in the scheme of figure 2. However, in this embodiment, all gases are vented and only unreacted isobutane and isobutene are recycled for further methacrolein production. This can be done by absorbing the C 1 hydrocarbons in a suitable solvent such as paraffin or aromatic hydrocarbons, or in solid materials such as molecular sieves. Figure 2 illustrates the use of a liquid solvent for effective separation of isobutane and isobutene from other gases. An economical solvent is paraffin oil with C 2 -C 3 hydrocarbons. The gas to be recycled containing CO and CO2, O2 and 10 light by-products goes to absorption tower (54) in which the isobutane, isobutene and the higher boiling materials are absorbed by flow of liquid (56), leaving the lighter gases in the vapor phase which are dropped from the top of the column (58). The C 2 -rich liquid which is drained from the bottom of the absorption column (54) goes via (60) to the stripper column (62) in which the C 2 hydrocarbons are blown out (stripped). The low-carbon liquid is drained from the bottom of column (62), cooled in heat exchanger (64) and returned via (56) to absorption column (54) for repeated use. Evaporated iso-20 butane and isobutene (24) return to dehydrogenation reactor (32). A leakage current for the Charm solvent can be adjusted in line (56) to distill out higher boiling materials in the usual manner (not covered here).

A representative example of the practical application of the flow chart according to figure 3 is the following. Example VII

From a feed containing 95% isobutane, 100 mol / hour was evaporated (22) and passed (26) to dehydrogenation reactor (32), along with 146 mol / hour recycle gas (24) 30, which is 62% isobutane and 38% isobutene. The combined streams were heated and mixed with 461 mol / hour steam (30) obtained by evaporation of waste water from the methacrolein stripper (46) but could also have been fresh steam. The combined stream (29) entered the dehydrogenation reactor (32) at a temperature of about 650 ° C in which about 45% of the supplied isobutane was converted to isobutene. The effluent, 8301303 23 leaving the reactor at about 520 ° C, was cooled to about 130 ° C in heat exchanger (34) and mixed with 141 mol / hour of oxygen (36) before entering the oxidation reactor (38), into which about 82 Z of the isobutene was converted into methacro-5 slates. The effluent, which left reactor (38) at about 342 ° C and 1.4 ato, was cooled in heat exchanger (40) to about 150 ° C and passed to an absorption column (42) in which the methacrolein was separated by absorption circulated in enough water (44) to give a solution with 1-2 mole% of methacrolein (45). This solution was blown into stripper column (46) to give a vapor stream containing 69.7 mole methacrolein, 6 mole / hour methacrylic acid per hour and various by-products (including acetic acid, acrolein and acetone). The crude gas exiting (43) from absorption column (42) was compressed (48) (to about 3.9 ato) enough to combine it with fresh feed for dehydrogenation reactor (32). This 302 mol / hour vapor contained about 28 Z hydrogen, 7 Z oxygen, 30 Z isobutane, 39 Z isobutene and 11 Z CO and CO 2. This gas was passed through absorption tower (54) in which practically all the isobutane and isobutene were washed with 150 mol / hour of Cg-C 1 -C 2 solvent (56). The C 2 -rich solution (60) went to stripper column (62) in which the C 2 hydrocarbons were blown out, and then it was returned (24) to dehydrogenation reactor (32) to complete the cycle.

As with the selective oxidation discussed above, the absorption of the C 2 components by solvent can be adjusted to allow partial separation of the effluent from the dehydrogenation, so that only isobutene and unconverted isobutane to the 3Q oxidation reactor to go. As in the past, this offers some advantages, but also much more costs, and it still remains necessary to remove oxygen and CO and CO2 nit the effluent from the oxidation reactor.

The above discussions regarding the manner in which isobutane is converted to methacrolein can generally be applied analogously to converting propane to acroleins. Essentially the same technique is used, although one skilled in the art will understand that the operating conditions and equipment need to be changed somewhat to obtain the best results, but that is within the scope of this invention.

The method used in practice generally does not make acrolein, but acrylic acid. The present invention can also be applied to the preparation of acrylic acid, as schematically illustrated by the block diagram of Figure 4.

The process is in contrast to that of the prior art in that the effluent from the dehydrogenation (105 goes directly to the oxidation reactor (12) to convert propene into acroleins. Those familiar with the prior art would expect that the propylene would be separated from the effluent of the dehydrogenation (105 before being introduced into the oxidation (121). Since the dehydrogenation of propene involves the formation of Significant amounts of hydrogen, and also of small amounts of lower molecular weight hydrocarbons, the propylene must be oxidized in the presence of significant amounts of hydrogen, propane and by-products without adversely affecting the oxidation of propene to acroleins or co-oxidation of the hydrogen. hydrogen, propane and dehydrogenation by-products can be oxidized to acroleins without causing unfavorable has an effect on the oxidation.

After the propylene has been oxidized to acrolein, the whole effluent from the first oxidation (12) can go directly to a second oxidation (14), in which the acrolein is oxidized to acrylic acid. Then the effluent from oxidation (14) is partially condensed and separated (16). Pure acrylic acid is the product while the lower boiling components are separated (18). Off-gases and higher boiling by-products are discarded while unreacted propane is returned to (10) and (14).

8301303 before "25

The effluent from the first oxidation (12) can be separated so that acroleins are recovered therefrom, which then goes to the second oxidation (14) together with air or oxygen.

This separation can be done by absorption in suitable liquids such as water or the lower carboxylic acids. Unreacted propane would then be returned to dehydrogenation (10) as indicated. If a significant amount of acroleins does not remain converted, this can optionally be separated and, as recovered, recycled to oxidation (34).

10 8301303

Claims (9)

1. A process for the preparation of an unsaturated aldehyde from the corresponding alkane under the influence of an appropriate catalyst, optionally in the presence of steam, to dehydrogenate the corresponding olefin to give an effluent containing the olefin, hydrogen, CO and CO2 and contains unreacted alkane, characterized in that this effluent, optionally mixed with steam, is passed over an oxidation catalyst under conditions suitable for the formation of the unsaturated aldehyde to give an effluent which contains unsaturated aldehyde, unreacted alkane and olefin, oxygen and contains CO and CO2, after which the aldehyde is separated from that second effluent. 2. Process according to claim 1, wherein the alkane is propane and the aldehyde acrolein, characterized in that a mixture of propane and steam with a molar ratio between 2: 1 and 1:10 at a temperature between 400 ° and 700 ° C and a pressure of up to 10 atoms over a catalyst which has a noble metal or a metal oxide as the active ingredient. Process according to claim 1, wherein the alkane isobutane and the aldehyde is methacrolein, characterized in that a mixture of isobutane and steam with a molar ratio between 1: 1 and 1:10 and a temperature between 300 ° and 700 ° C and a pressure between 2 and 10 ato passes over a catalyst 25 which has a precious metal or a metal oxide as the active ingredient. 4. Process according to any one of the preceding claims, characterized in that after separating the aldehyde from the oxidation effluent, the hydrogen formed during dehydrogenation, the oxygen not consumed during the oxidation and the CO and CO2 formed in both stages are separated from that effluent, and the residual gas mixture is recycled as feed to the dehydrogenation. 5. Process according to claim 4, characterized in that oxygen and hydrogen are separated from the oxidation effluent by being subjected to oxidation. 27 react with each other under conditions where the hydrogen is selectively oxidized but the alkane and olefin present are substantially unaffected. 6. Process according to claim 4, characterized in that the alkane and the olefin are separated from the effluent of the oxidation by absorption in a liquid, which is subsequently stripped, and that the alkane and olefin are recycled as feed to the dehydrogenation . Process according to any one of Claims 10 to 3, characterized in that the effluent from the dehydrogenic ring is mixed with molecular oxygen and passed over an oxidation catalyst under conditions where the hydrogen is selectively oxidized while alkane, olefin and other hydrocarbons are substantially unaffected, after which the oxidation to aldehyde is carried out. Process according to any one of claims 1 to 4, characterized in that the olefin is concentrated from the effluent of the dehydrogenation. 9. Method mainly according to description and / or examples. 8301303