WO2010027732A1

WO2010027732A1 - Process for producing acrolein and/or acrylic acid from propane

Info

Publication number: WO2010027732A1
Application number: PCT/US2009/054755
Authority: WO
Inventors: Anil Mehta
Original assignee: Dow Global Technologies Inc.
Priority date: 2008-09-04
Filing date: 2009-08-24
Publication date: 2010-03-11
Also published as: TW201016653A

Abstract

A process for the preparation of acrolein and or acrylic acid from propane, in which the propane is subjected, in a first reaction stage, to a partial oxydehydrogenation with molecular oxygen under homogeneous and/or heterogeneous catalysis to give propylene and the propylene-containing product gas mixture formed in the first reaction stage is then used in at least one further reaction stage for the preparation of acrolein and/or acrylic acid by gas-phase catalytic propylene oxidation. The present process for producing acrolein and/or acrylic acid from propane includes selectively removing major byproducts of the propane and propylene oxidation reactions such as carbon oxides and other inert gases such as argon and nitrogen from the gas stream recycled to the propane oxidation reactor. The present process advantageously reduces cost of producing acrolein and/or acrylic acid from propane by reducing the raw materials (propane and oxygen) requirement, by reducing the energy requirement of the process, and by significantly reducing the size of the major equipment required in the process.

Description

PROCESS FOR PRODUCING ACROLEIN AND/OR ACRYLIC ACID FROM

PROPANE FIELD OF INVENTION

The present invention relates to processes for producing acrolein and/or acrylic acid from propane.

BACKGROUND OF THE INVENTION

Acrolein and acrylic acid are important intermediates which are used, for example, in the preparation of active ingredients and polymers.

Currently, the process that is predominantly used on an industrial scale for production of acrolein and/or acrylic acid is a gas-phase catalytic oxidation of propylene, for example, as described in EP-A 575 897; the propylene being produced predominantly as a byproduct of ethylene production by steam cracking of naphtha.

For example, the process for industrial production of acrylic acid includes first converting propylene mainly into acrolein and a small amount of acrylic acid in a first reaction step by reacting a mixture of propylene, air and steam in a first reactor to produce an acrolein product. The acrolein product is then supplied to a second reactor without separation of products for a subsequent reaction of acrolein with additional air and steam to form acrylic acid. The product gas containing acrylic acid obtained from the second reactor may be introduced into a collecting apparatus to obtain acrylic acid as an aqueous solution. A part of the remaining waste gas containing unreacted propylene from the collecting apparatus is then recycled to the first reactor inlet together with the starting gas mixture of propylene, air and steam.

The use of propylene in other fields is constantly expanding, and therefore, it would be advantageous to have an industrially usable, competitive process for the preparation of acrolein and/or acrylic acid, whose raw material base is not propylene but is, for example, propane naturally occurring in large quantities as a natural gas component.

Research has picked up very significantly in the industry in the past ten years or so to replace propylene with propane in the production of acrolein and acrylic acid. The use of propane as a feed source is desired because propane is more readily available and less expensive than propylene. Using propane as the feed stock presents a very significant advantage in reduced raw material cost and it de-couples the acrolein/acrylic acid plants from very capital cost intensive ethylene/ propylene hydrocarbons plant. Processes for producing acrolein and acrylic acid from propane are known in the prior art. For example, European patent 117146 Bl teaches preparation of acrylic acid from propane via dehydrogenation of propane to produce an effluent stream comprising propylene, hydrogen, carbon oxides and unreacted propane. U.S. Patents Nos. 5,705,684; 7,388,106 B2 and 7,321,058 B2 are further examples of processes for preparing acrolein or acrylic acid from propane. The propane to acrolein and/or acrylic acid processes based on propane dehydrogenation present significant challenges in integrating with an acrolein and/or acrylic acid production process for several reasons some of which are as follows. The propane dehydrogenation to propylene is accompanied with production of hydrogen as a co-product which must be handled in the process and removed from a recycle process.

Hydrogen is highly flammable and it reduces flammability limit of the reaction gas mixture, thereby limiting oxygen content in the reaction mixture. Additionally, propane dehydrogenation is a highly endothermic reaction requiring high energy input into the process and is carried out at a temperature in the range of 500 ⁰C - 700 ⁰C which is much higher than the temperature of about 250-400⁰C required for propylene to acrolein reaction, requiring substantial cooling of the effluent from propane dehydrogenation before feeding it to the propylene to acrolein reactor. On the other hand, propane oxydehydrogenation to propylene reaction is much easier to integrate with a propylene to acrolein and/or acrylic acid processes because of many synergies between the two processes such as no production of hydrogen in the process, excess oxygen in the recycle gas can be utilized in the oxydehydrogenation reactor and lower reaction temperature in the oxydehydrogenation reaction. Also, propane oxydehydrogenation is carried out at a low conversion which provides the required propylene concentration for the propylene oxidation reaction.

U.S. Patent No. 6,492,548 teaches producing acrolein and acrylic acid from propane via propane oxydehydrogenation. Figure 1 of U.S. Patent No. 6,492,548 shows a process for producing acrolein by feeding a gaseous propane feedstream and a gaseous oxygen feedstream into a first oxydehydrogenation reactor containing a heterogeneous oxidative dehydrogenation catalyst and then feeding the effluent from the first reactor to a subsequent reactor for the production of acrolein. A recycle stream is also fed to the first reactor. Figure 2 of U.S. Patent No. 6,492,548 shows a process for producing acrylic acid from propane including incorporating a third reactor positioned after the second acrolein reactor. Other oxydehydrogenation processes to produce acrolein and/or acrylic acid from propane are taught in WO97/36848; WO97/36849; U.S. Patent No. 6,166,263 and U.S. Patent No. 6,187,963;. The processes disclose converting propane to propylene at low conversion and high selectivity, converting propylene to acrolein, recovering acrolein or converting acrolein to acrylic acid, and recycling unreacted propane to a propane oxydehydrogenation reactor. The oxydehydrogenation reaction of propane to propylene is typically less than completely selective to propylene. At least a small portion of propane and propylene are usually oxidized to carbon monoxide and carbon dioxide. This is also the case in converting propylene to acrolein. The selectivity from propane to propylene is often maximized by operating the propane oxydehydrogenation at a relatively low (e.g., 5 percent to 20 percent) conversion of propane. However, byproducts such as carbon oxides are only minimized this way and will build up in a recycle process if not removed from the process.

WO97/36848, WO97/36849; and U.S. Patent Nos. 6,166,263; and 6,492,548 teach purging recycle gas only and no separation of byproduct components from the recycle gas for a continuous recycle process for controlling build up of undesired components in the reaction gas mixture. When only purge stream is used for controlling build up of undesired byproducts in the process, a large purge from the recycle gas is required which results in several disadvantages: (1) a large purge results in a large loss of the raw material propane, and a significant loss of propylene and oxygen; and (2) a large purge is still not sufficient in achieving low concentration of byproduct carbon oxides resulting in a large reduction in the concentration of propane and propylene in the feed to the reactors, substantially reducing productivity of major equipment in the process such as the reactors, absorbers, aftercoolers, recycle compressor and the like. An even larger purge increases the concentration of reactants such as propane and propylene in the feed stream to the reactors and products such as propylene and acrolein and/or acrylic acid in the product gas mixtures but it further increases the loss of unreacted propane, propylene and oxygen in the purge because of the larger purge rate, making the overall process uneconomical.

U.S. Patent Nos. 6,388,129; 6,423,875; and 6,426,433 teach the use of oxydehydrogenation process to partially convert propane to propylene using modified (oxygen enriched) air, convert propylene to acrolein, and optionally convert acrolein to acrylic acid, recover acrolein or acrylic acid, separate nitrogen and other inerts by fractional distillation and recycle unreacted propane and propylene to the oxydehydro reactor wherein the molecular oxygen required in a first reaction zone (oxydehydrogenation zone) is added in the form of air to the reaction gas starting mixture fed to the first reaction zone.

The processes disclosed in U.S. Patent Nos. 6,388,129; 6,423,875; and 6,426,433 use air to supply oxygen to the oxydehydrogenation reactor. This results in a low concentration of propane in the reactor because of large amount of nitrogen present in the air, reducing a beneficial effect of high heat capacity of propane in the reaction. Also, the reaction requires a greater conversion of propane to propylene per pass to achieve sufficient concentration of propylene in the feed to the propylene to acrolein reactor to achieve sufficient productivity in the reactor. In the propane oxydehydrogenation reaction, the greater propane conversion required in such a process results in a lower selectivity to propylene and further increases the formation of byproducts such as carbon oxides. Also, fractional distillation described in the above patents must be carried out at cryogenic conditions which makes the overall process very expensive.

U.S. Patent No. 6,541,664 Bl describes a propane to acrolein process by oxidation over two catalyst beds in series and separating propane and propene from the reaction product mixture. U.S. Patent No. 6,541,664 Bl does not describe how propane and propene are separated from the product mixture or does not even mention formation of byproducts such as carbon oxides in the process.

The prior art processes for converting propane to acrolein and/or acrylic acid provide a low efficiency of propane and high capital and energy costs. Therefore, it is desired to provide an economical process for producing acrolein and/or acrylic acid from propane via an oxydehydrogenation process with a reduced total cost.

There is still a need in the industry for further enhancements directed to the use of propane as a feed source for producing acrolein and/or acrylic acid. It would be desirable if propane could be simultaneously utilized to enhance the reaction efficiency of such processes in addition to being a feed source. SUMMARY OF THE INVENTION

The present invention provides improved continuous processes for the conversion of propane to acrolein and/or acrylic acid. In the processes of the present invention, propane is first partially converted to propylene in a first reaction zone, an oxydehydrogenation reaction zone with oxygen, and then the propylene containing product gas stream from the first reaction zone is fed to a further reaction zone, a propylene oxidation zone, where propylene contained in the gas stream is converted to acrolein and/or acrylic acid.

The present invention is directed to a process for producing acrolein and/or acrylic acid from propane. The process of the present invention provides a process step and a means for selectively removing major byproducts such as carbon oxides, produced in the propane oxydehydrogenation and propylene oxidation reaction steps, and optionally removing other inert gases such as argon and nitrogen from the gas recycled to the propane oxydehydrogenation reactor. The selective removal of the byproducts greatly reduces the loss of raw material propane, propylene as well as oxygen, through a purge stream, thereby increasing the raw materials (propane and oxygen) yield. This improvement also results in a significant reduction in size of major equipment used in the process such as reactors, heat exchangers, absorbers, recycle compressor and the like, significantly reducing the overall capital cost of the process.

It is an object of the present invention to advantageously reduce the cost of producing acrolein and/or acrylic acid from propane by reducing the raw materials (propane and oxygen) requirement of the process, reducing energy requirement of the process and significantly reducing the size of major equipment used in the process. It is also an object of the present invention to selectively remove major byproducts of the propane oxydehydrogenation and propylene oxidation reactions such as carbon oxides from the gas recycled to the propane oxydehydrogenation reactor.

It is an object of the present invention to provide a process for the preparation of acrolein and/or acrylic acid from propane, in which propane is subjected, in a first reaction stage, to a partial oxydehydrogenation reaction with molecular oxygen under catalysis to give propylene. The propylene product in the gas mixture formed in the first reaction stage is then used in at least one further reaction stage for the preparation of acrolein and/or acrylic acid by gas-phase catalytic propylene oxidation; wherein at least a portion of the byproducts formed in the propane oxydehydrogenation and propylene oxidation reaction stages is removed or reduced before recycling the gas to the reaction. The processes of the present invention advantageously have a selective byproducts removal step to remove gaseous byproducts of the propane oxydehydrogenation step and propylene oxidation step. The byproducts in the process may include for example carbon dioxide and carbon monoxide; and optionally, inert gases such as argon, nitrogen and the like. Although the invention herein is described mainly for converting propane to acrolein and/ or acrylic acid, it will be clear to those skilled in the art that the present invention can also be applied to converting butane to methacrolein and/or methacrylic acid. BRIEF DESCRIPTION OF THE DRAWINGS For the purpose of illustrating the present invention, the drawings show a form of the present invention which is presently preferred. However, it should be understood that the present invention is not limited to the precise arrangements and instrumentation shown in the drawings. In the accompanying drawings, like reference numerals are used to denote like parts throughout the drawings. Figure 1 is a simplified process block flow diagram showing one embodiment of a process for converting propane to acrolein in accordance with the present invention.

Figure 2 is a simplified process block flow diagram showing one embodiment of a process for converting propane to acrylic acid in accordance with the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In general, the process of the present invention for preparation of acrolein and/or acrylic acid from propane, includes a first reaction stage or zone wherein propane is subjected to a partial oxidative dehydrogenation reaction ("oxydehydrogenation') with molecular oxygen under catalysis to give propylene. The catalyst used in the first reaction zone may be homogeneous and/or heterogeneous. A propylene-containing product gas mixture is formed in the first reaction zone; and, this propylene-containing product gas mixture (i.e., the effluent from the first reaction zone) is then used in at least one further reaction zone for producing acrolein and/or acrylic acid by a gas-phase catalytic propylene oxidation reaction. A recycle gaseous stream containing unreacted propane is preferably added to the first reaction zone along with the make-up propane and oxygen feedstreams to form the gas starting mixture of the first reaction zone. Prior to being added to the starting gas mixture in the first reaction stage, the recycle gas stream is treated to remove certain undesirable gaseous byproducts such as carbon dioxide. The source of the propane feed for use in the process of the present invention is not critical and may be obtained from various sources including for example propane naturally occurring from natural gas production in the oilfield or an off gas from a refinery unit. The purity of the propane feed used in the process of the present invention is not particularly limited. For example, the propane feed stream may contain a lower alkane such as ethane, methane, air or carbon dioxide, as impurities. Typically, the propane feed will comprise at least 30 mole percent, preferably at least 50 mole percent, more preferably at least 80 mole percent propane, and most preferably at least 90 mole percent propane. The oxygen source of the oxygen-containing gas feed stream for use in the processes of the present invention (both for propane oxydehydrogenation and propylene oxidation) is not critical and may be obtained from various oxygen producers. Preferably, the oxygen source comprises at least about 90 mole percent and more preferably at least about 95 mole percent oxygen. It is preferred that the oxygen source contain a low amount (e.g. less than 5 percent) of other inert gases such as nitrogen. Thus, the use of a low oxygen -containing gas stream such as air is not preferred because the nitrogen content in the air reduces concentration of propane in the reaction gas mixture, reducing beneficial effect of high propane concentration in the reactors as well adversely affecting the ability to recycle the unreacted propane, propylene and oxygen. The modified recycle gas mixture stream for use in the process of the present invention comprises a recycle gas mixture stream containing unreacted propane, propylene, oxygen, other inert gases such as nitrogen, argon and byproduct gases such as carbon monoxide, carbon dioxide which have been produced in the process of the present invention and the concentration of which in the recycle gas have been reduced by removing at least a part of these byproducts from the recycle gas mixture. In accordance with the present invention, the process of the present invention requires the removal of at least a part of the byproducts contained in the recycle gas mixture stream to form the modified recycle gas mixture stream before the modified recycle gas mixture stream is combined with the gaseous feed streams (propane and oxygen) used in the present invention for preparation of acrolein and/or acrylic acid.

The components contained in the recycle gas mixture may include several components, such as for example, propane, CO₂, CO, H₂O, N₂, O₂, propylene, ethane, ethene, methane, and other components in trace amounts. The gaseous byproducts that are preferably to be removed from or reduced in the recycle gas mixture to form the modified recycle gas mixture stream are carbon oxides, and specifically for example carbon dioxide.

The resulting modified recycle gas mixture stream preferably contains a lower concentration of byproducts, for example carbon dioxide, than the initial recycle gas mixture after modification from its original content. For example, the modified recycle gas mixture stream to be used according to the present invention may contain components usually present in small amounts in air, such as noble gases, carbon dioxide, water vapor and the like.

The first reaction stage or zone of the process of the present invention is an "oxydehydrogenation reaction zone" or a "oxidative dehydrogenation zone" wherein a portion of propane in the feed is converted to propylene (also "propene") and the rest of the propane as well as other essentially inert gases serve as diluent for the reaction.

The first reaction zone of the novel process of the present invention may be designed to use a homogeneous oxydehydrogenation or a heterogeneous oxydehydrogenation catalyst. Preferably, a heterogeneous oxydehydrogenation catalyst is used in the present invention.

Any catalyst effective for conversion of propane to propylene by oxydehydrogenation is suitable for use in the present invention. Typical catalysts for oxydehydrogenation of propane are transition metal oxides, such as vanadium or molybdenum oxide or mixed metal oxides. The catalysts particularly suitable for oxydehydrogenation of propane according to the present invention are described, for example, in U.S. Patent Nos. 4,148,757; 4,212,766; 4,260,822; 5,198,580; and 5,380,933; all of which are incorporated herein by reference. An example of a suitable catalyst for use in accordance with the present invention is a catalyst containing mixed metal oxides comprising Mo, V, Te and the like.

The catalysts for use in the processes of the present invention may be in the form of pellets, beads, or rings containing a through hole or otherwise in a form having catalytic components deposited on a refractory carrier.

The reaction conditions for the oxydehydrogenation reaction zone are as follows: The temperature of the oxydehydrogenation reaction is generally from about

200 ⁰C to about 600 ⁰C, preferably from about 250 ⁰C to about 500 ⁰C, and more preferably from about 350 ⁰C to about 450 ⁰C.

For the oxydehydrogenation reaction, the operating pressure of the reaction is generally from about 1 bar to about 10 bar, preferably from about 2 bar to about 6 bar, and more preferably from about 2 bar to about 4 bar. The reactor pressure often depends on pressure drop in the entire recycle gas loop and the operating pressures in the rest of the equipment in the recycle loop. The gas hourly space velocity in the vapor-phase oxydehydrogenation reaction is usually within a range of from about 100 hr^"1 to about 10,000 hr^"1, preferably from about 300 hr^"1 to about 6000 hr^"1, and more preferably from about 1000 hr^"1 to about 4000 hr^"1. As used herein, "gas hourly space velocity" means the volume of reactant gas at standard conditions (0 ⁰C. and 1 atm pressure) passed over the catalyst in one hour divided by the total volume occupied by the catalyst. In the first reaction stage, i.e., the propane oxydehydrogenation step or the propane-to-propylene reaction, the molar ratio of propane to oxygen varies with the desired conversion and the selectivity of the catalyst. In general, the molar ratio of propane to oxygen in the gaseous starting mixture is advantageous in the range of from about 3:1 to about 40:1 and preferably from about 5:1 to about 20:1 and more preferably in the range of about 5:1 to about 10:1. An additional criterion in selecting the molar ratio is that the combined gas mixture feed stream to the reactor is preferably outside the flammable region to make the process more safe to operate.

The reaction gas starting mixture can also comprise further, essentially inert components, such as H₂O, CO₂, CO, noble gases and unconverted propylene. The unconverted propylene refers to the propylene left over from incomplete conversion of propylene to acrolein and/or acrylic acid in the propylene oxidation zone. The recycle gas mixture resulting after the propylene oxidation step and following the product separation and gaseous byproducts removal zones; and recycled to the oxydehydrogenation reaction are referred to in here generally as modified recycle gas. The present invention maximizes concentration of more desired propane and minimizes the concentration of less desired and essentially inert gases such as carbon oxides, nitrogen and argon in the reaction gas mixture. Concentration of propane in the feed to the oxydehydrogenation is at least about 40 mole percent, preferably at least about 50 mole percent, more preferably at least about 60 mole percent and most preferably at least about 70 mole percent. High propane concentration in the reaction is advantageous to the reaction as it increases the heat capacity of the reaction gas mixture. The increased heat capacity increases flammability limit of the reaction gas mixture allowing higher oxygen concentration in the reaction gas mixture as well as higher selectivity to the desired product by moderating the reaction. Concentration of inert gases such as N₂, CO, CO₂, water and noble gases such as argon in the gas mixture feed to the oxydehydrogenation reaction zone may be up to 40 mol percent, preferably less than 30 mol percent, more preferably less than 20 mol percent and it could be as low as 5 mol percent or less. Water is a co-product of the propane oxydehydrogenation as well as propylene to acrolein and/or acrylic acid reactions. A portion of this water can be present in the recycle gas stream and the modified recycle gas stream depending on the exact nature of the acrolein and/or acrylic acid product recovery step and the method employed for removal of gaseous byproducts from the recycle gas to produce the modified recycle gas. Concentration of water in the modified recycle gas can be from about 1 mole percent to about 10 mole percent, preferably from 1 mole percent to about 5 mole percent.

In the propane-to-propylene reaction, or propane oxydehydrogenation reaction, the propylene selectivity decreases with increasing propane conversion. Preferably, the propane oxydehydrogenation reaction is conducted to provide for relatively low conversions of propane with high selectivities to propylene. For example, it is preferred that the conversion of propane be from about 5 percent to about 40 percent and more preferably from about 10 percent to about 20 percent. As used herein, the term "propane conversion" means the percentage of propane in the reactor feed which is reacted. It is preferred that the selectivity of the conversion of propane to propylene be from about 70 to about 98 percent, more preferably from about 80 to about 98 percent and most preferably from about 90 percent to about 98 percent. As used herein, the term "propylene selectivity" means the moles of propylene produced per mole of propane reacted expressed as a percentage. Water is a co-product of the reaction. A number of byproducts are also formed including carbon monoxide, carbon dioxide, hydrogen, methane, ethane and others. The primary byproducts formed are carbon monoxide and carbon dioxide. The oxidative dehydrogenation of propane to propylene may be carried out in any suitable equipment, such as a single reactor or two or more reactors in series or in parallel, known to those skilled in the art. Any suitable reactor sequence known to those skilled in the art may be used for the propane-to-propylene reaction. For example, the reaction can be conducted in a single stage, or can be conducted in two or more stages with oxygen introduction between the stages. The reactor used in the present invention may be, for example, a shell containing a tube bundle with catalyst packed in the tubes that facilitates heat removal or a fluidized bed reactor.

Once the propylene product is produced in the first reaction zone, the effluent gas mixture from the first reaction zone is passed to the second reaction zone, where the propylene contained in the effluent gas mixture from the first zone, is reacted with oxygen over a catalyst in a second reaction zone to produce acrolein and/or acrylic acid. This second reaction stage or zone is a propylene oxidation reaction zone to form acrolein and/or acrylic acid. While the two stages or zones are described with regard to the process of the present invention, it may be possible to conduct both the propane-to-propylene and propylene-to-acrolein and/or acrylic acid reactions in a single reactor with one or more stages; or in multiple reactors for the process of the present invention.

Also, the gas-phase catalytic oxidative conversion of the propylene contained in the propane oxydehydrogenation product gas mixture to acrolein and/or acrylic acid may be carried out, in one subsequent oxidation stage or in two subsequent oxidation stages. In general, where acrylic acid is the desired product, two gas-phase catalytic oxidation stages follow, although one- stage gas-phase catalytic oxidations of propylene to acrylic acid may also be used. Generally, if acrolein is the desired product, one gas-phase catalytic oxidation stage follows.

For the process of the present invention, the propylene-to-acrolein reaction is not dependent upon any particular catalyst and any catalysts effective for the conversion of propylene to acrolein may be used. Typical catalysts include the mixed-metal-oxide catalysts, such as, for example, those disclosed in U.S. Patent Nos. 3,825,600; 3,649,930; 4,339, 355; 5,077,434; or 5,218,146; all of which are incorporated herein by reference. An example of a catalyst suitable for the propylene-to-acrolein reaction is an oxide catalyst containing Mo, Fe, and Bi.

Suitable catalysts that can be used for the propylene-to-acrolein oxidation stage are, for example, those described in DE-A 29 09 592; incorporated herein by reference. Alternatively, multimetal oxide catalysts described in DE-A 1 97 53 817, incorporated herein by reference, may also be used. The catalysts may be in the form of unsupported hollow cylinder catalysts as described in EP-A 575 897 incorporated herein by reference. Bi-, Mo- and Fe-containing multimetal oxide catalyst such as ACF-2 from Nippon Shokubai can also be used in the propylene oxidation stage. The catalysts for use in the processes of the present invention may be in the form of pellets, beads, or rings containing a through hole or otherwise in a form having catalytic components deposited on a refractory carrier. Suitable propylene-to-acrolein catalysts are commercially available, for example, ACF-4, ACF-7 from Nippon Shokubai; as well as from Nippon Kayaku and Mitsubishi. In the propylene-to-acrolein reaction zone, the gas composition of the feedstream, a mixture of the effluent coming from the first reaction zone and oxygen that may be added, generally has a propylene content in the range of from about 5 mole percent to about 30 mole percent, and preferably from about 6 mole percent to about 15 mole percent based on the total moles of the feedstream; an oxygen content in the range of from about 8 mole percent to about 30 mole percent, and preferably from about 10 mole percent to about 15 mole percent based on the total moles of the feedstream; a propane content is at least about 40 mole percent preferably at least about 50 mole percent based on the total moles of the feedstream; and a carbon oxides content of less than about 30 mole percent, preferably less than about 20 mole percent, and more preferably less than about 10 mole percent based on the total moles of the feedstream.

Reducing carbon oxides concentration in the gas composition of the feedstream increases propane concentration in the feed stream to the reaction. High concentration of propane in the feed stream for the propylene to acrolein reaction has a beneficial effect of moderating the reactor temperatures and improving acrolein selectivity due to increased heat capacity of the reaction gas mixture. The steam concentration in the gas composition of the feedstream depends on the extent of conversion in the first reaction zone because water in the form of steam is a co-product of the reactions in the first reaction zone. Water is a co-product of the reaction in the second reaction zone also. The presence of at least some steam is believed to have a beneficial effect on the propylene-to-acrolein reaction.

The molar ratio of oxygen to propylene in the propylene oxidation reaction in the second reaction zone, is in the range of from about 1.1 to about 2.5, and preferably from about 1.2 to about 1.8. To achieve the desired propylene to oxygen ratio, it may be necessary to add oxygen in to the oxydehydrogenation product gas effluent stream before the product gas enters the propylene oxidation reaction zone.

The general reaction conditions of the propylene-to-acrolein reaction are as follows: The propylene-to-acrolein reaction operates at temperatures of from about 250 ⁰C to about 450 ⁰C, and preferably from about 275 ⁰C to about 400 ⁰C. Propylene to acrolein oxidation temperatures are usually lower than the propane oxydehydrogenation temperature. Therefore, an after-cooler may be provided after the oxydehydrogenation zone to cool the effluent from the oxydehydrogenation reaction zone to a lower temperature before forwarding it to the propylene oxidation zone. Generally, the temperature of the effluent exiting the after-cooler is from about 100 ⁰C to about 350 ⁰C, preferably from about 100 ⁰C to about 250 ⁰C. A portion of the water contained in the effluent gas may condense as a result of cooling the effluent gas depending on the temperature, pressure and composition of the gas. The condensed water can be easily removed from the process. The operating pressure of the propylene to acrolein reaction is from about

1 bar to about 4 bar, although subatmospheric, atmospheric, or superatmo spheric pressures may be used. Preferably the pressure is from about 2 bar to about 3 bar. The operating pressure in this reaction zone will be slightly less than the operating pressure in the oxydehydrogenation reaction zone by the amount of pressure drop in each of the reaction zones. Pressures in each of the zones are influenced by pressure drop in the whole system.

Contact time in the propylene to acrolein reaction is in the range of from about 0.2 second to about 2 seconds and preferably from about 0.5 second to about

2 seconds. As used herein, "contact time" is defined as the ratio of the open volume in the catalyst bed to the process volumetric flow at the process conditions.

Conversion of propylene is preferably at least about 70 percent, and more preferably at least about 80 percent. In the propylene-to acrolein reaction the selectivity to acrolein is from about 80 percent to about 99 percent and more preferably from about 90 percent to about 99 percent. As used herein, the term "conversion" means the moles propylene reacted per mole propylene fed to the reactor expressed as a percent and the term "selectivity" means moles acrolein produced per mole of propylene reacted expressed as a percent. The propylene to acrolein selectivity is improved when propane concentration in the feed stream to the reaction is increased as explained earlier. The present invention describes how to increase propane concentration in the reaction feed stream by selectively removing the undesired gaseous byproducts of the reaction. A number of byproducts such as carbon dioxide, carbon monoxide, acetaldehyde, formaldehyde, acetic acid, allyl acetate are also formed to a small extent in the propylene to acrolein reaction.

The type of reactor used in the conversion of propylene to acrolein is not critical and may be, for example, a fixed-bed, tubular-flow reactor with liquid coolant passed through the shell. Fluidized bed reactors may also be employed. Further details of suitable reactors are known to those skilled in the art.

The effluent gas mixture from the propylene oxidation stage is a mixture of acrolein and other components such as propane, unreacted propylene, water, carbon oxides, excess oxygen etc. The acrolein product can be separated form other components in a manner known to those skilled in the art. If acrolein is the desired product of the process, then the acrolein containing product gas effluent is fed to the acrolein recovery zone. The type of equipment used to recover the acrolein product from the reaction effluent is known to those skilled in the art. The acrolein separated in this manner can be used as an intermediate for synthesis of various end products.

In another embodiment of the present invention, instead of acrolein being recovered as a product, the acrolein containing product gas effluent , may be fed into an acrolein-to-acrylic acid reaction zone to produce acrylic acid. The acrolein containing product gas effluent may be used for the gas-phase catalytic oxidation for preparation of acrylic acid. When the acrolein is used for the preparation of acrylic acid in a further gas-phase catalytic oxidation stage, the acrolein-containing reaction gases of the propylene oxidation stage are generally transferred to this further oxidation stage without separation from secondary components.

If necessary, the acrolein containing effluent gas may be cooled before the gas effluent enters the acrolein oxidation reactor to convert acrolein to acrylic acid. Also, it may be necessary to add oxygen to the acrolein product gas effluent stream before it is fed to the acrolein oxidation reactor to produce acrylic acid. The oxidation stage of the acrolein-containing reaction gases is realized in an expedient manner, for example, in a fixed-bed reactor having multiple catalyst tubes, as described, for example, in DE-A 44 31 949, DE-A 44 42 346, DE-A 1 97 36 105 or EP-A 731 082; all of which are incorporated herein by reference.

The catalyst for use in the acrolein-to-acrylic acid reaction can be any catalyst suitable for the conversion of acrolein to acrylic acid and may be the same or different from the catalyst used to oxidize propylene to acrolein. For example, the catalysts useful in this reaction may be those described in U.S. Patent No. 5,218,146, incorporated herein by reference. Other catalysts for conversion of acrolein to acrylic acid are described, for example, in U.S. Patent Nos. 4,892,856; 5, 077,434; 5,198,580 and 5,380,933; all of which are incorporated herein by reference. Suitable acrolein to acrylic acid catalysts are commercially available, for example, from Nippon Shokubai, Japan.

The conditions for the acrolein oxidation zone are as follows: The acrolein oxidation reaction is carried out at a temperature in the range of from about 180 ⁰C to about 350 ⁰C, and preferably from about 200 ⁰C to about 320 ⁰C. The residence time of the reaction mixture in the acrolein oxidation zone is in the range of about 1 second to about 7 seconds, and preferably from about 1.5 second to about 6 seconds.

The operating pressure of the acrolein-to-acrylic acid reaction is essentially similar to the pressure required for the propane to propylene reaction and the propylene to acrolein reaction. The operating pressure in the acrylic acid reaction zone is less than the operating pressures in the acrolein reaction zone and the propane oxydehydrogenation zones by the amount of pressure drop in each of the preceding reaction zones.

The conversion of acrolein to acrylic acid is generally at least about 90 percent or greater; and preferably from about 95 percent to about 99 percent or greater. Preferably, the overall conversion of propylene to acrylic acid over a two- stage operation per pass is not less than about 70 mol %, and preferably not less than 80 mol %.

The effluent gas mixture from the propylene oxidation to acrylic acid is a mixture of acrylic acid and other components such as propane, unreacted propylene, water, carbon oxides, excess oxygen, acrolein etc. The acrylic acid containing effluent gas mixture is then fed to product recovery zone for recovering acrylic acid from the gas mixture.

The product recovery stage or zone of the process of the present invention includes treating the acrolein and/or acrylic acid-containing product gas mixture stream formed in the second reaction zone in at least one further zone to separate a product stream and a recycle gas mixture stream from the acrolein and/or an acrylic acid-containing product gas mixture stream. A wide variety of product separation and refining schemes known to those skilled in the art may be used in the present invention. For example, partial condensation, absorption in a solvent including water and fractionation may be employed to separate acrolein from the first effluent stream. The acrolein may be recovered, for example, as described in the U.S. Patent No. 5,198,578 and 6,492,548; both which are incorporated herein by reference.

Similarly, a wide variety of product separation and refining schemes known to those skilled in the art such as, for example, partial condensation, absorption and fractionation may be employed to separate acrylic acid from acrylic acid containing effluent stream from the second reaction zone. The acrylic acid produced in the process of the present invention may be recovered, for example, by absorption with a solvent (cf. also DE-A 43 08 087) or by absorption with water or by partial condensation or fractionation, or further processes as disclosed in U.S. Patent No. 4,999,452, incorporated herein by reference. The various known methods for separating acrylic acid are summarized also in, for example, DE-A 1 96 00 955, incorporated herein by reference.

A common feature in all separation processes employed is that the remaining gas is essentially free of acrolein and/or acrylic acid, other condensable byproducts of the acrolein and/or acrylic acid producing reactions such as acetaldehyde, acetic acid, allyl alcohol and the water content in the gas is substantially reduced or is essentially free of water. The remaining gas, called recycle gas, comprises primarily propane and smaller amounts of other components such as unconverted propylene, oxygen, water, carbon dioxide, carbon monoxide and other essentially inert components such as nitrogen, argon etc. A preferred feature of any separation scheme is that it avoids contamination of the recycle gas stream with potential catalyst poisons that can poison either the oxydehydrogenation catalyst, acrolein catalyst or acrylic acid catalyst. The presence of poisons for either the oxydehydrogenation catalyst or the acrolein/acrylic acid catalyst would prevent recycle of the unreacted gases such as propane, propylene and oxygen to the reaction sequence.

Temperature and pressure of the effluent product stream leaving the product recovery zone depends up on the specific acrolein or acrylic acid recovery process used but will typically be in the range of from about 30 ⁰C to about 70 ⁰C and from about 1 bar to about 2 bar, respectively. Separation of at least a portion of the byproducts, such as carbon dioxide, contained in the recycle gas stream is required in the present invention before the said recycle gas stream may be used as a modified recycle gas stream in the oxydehydrogenation reaction zone. Carbon oxides such as carbon monoxide and carbon dioxide are significant byproducts in the propane oxydehydrogenation reaction as well as in the propylene to acrolein and/or acrylic acid reaction. If these byproducts are not removed from the process, concentration of these components will build up in a recycle process resulting in decreased concentration of more desired diluent gas for the reaction such as propane. Decreased propane concentration in the feed to the oxydehydrogenation reaction requires greater conversion of propane to propylene to achieve the same propylene concentration in the feed to the propylene oxidation reaction which lowers the oxydehydrogenation reaction selectivity to propylene and produces even more byproducts in the reaction. Similarly, higher propane concentration is also beneficial and desired in the propylene to acrolein and/or acrylic acid reactions where the higher propane concentration and the resulting higher heat capacity of the reaction gas mixture moderates the reaction and increases yield of desired products such as acrolein and/or acrylic acid. Therefore, at least a portion of the carbon dioxide contained in the recycle gas is separated before recycling the gas to the first reaction zone. At least a portion of carbon monoxide contained in the modified recycle gas gets oxidized to carbon dioxide in the propane oxydehydrogenation and the subsequent propylene oxidation reactions. Therefore, separate removal of carbon monoxide is not necessary for the process of this invention.

There are many methods known in the art and in commercial use for separating carbon dioxide from a gas stream as described in the Kirk-Othmer Encyclopedia of Chemical Technology, Volume 4 and Volume 15, incorporated herein by reference. The removal of at least a part of the byproduct carbon dioxide contained in the recycle gas mixture to form the modified recycle gas mixture stream can be carried out, using any method and equipment known in the art suitable for this process. For example, removal of carbon dioxide from the recycle gas stream can be accomplished, by passing at least a portion of the recycle gas stream through a sorption unit such as an absorption/stripper unit using a solvent such as, for example, a hot aqueous solution comprising potassium carbonate or sodium carbonate as described in Kirk-Othmer Encyclopedia of Chemical Technology, Volume 4, incorporated herein by reference.

Alternatively, at least a portion of the recycle gas can be passed through a filtration unit, such as one containing a membrane unit which selectively removes at least a portion of the carbon dioxide and preferably a portion of other inert gases such as nitrogen and/or argon, as described in Kirk-Othmer Encyclopedia of Chemical Technology, Volume 15, incorporated herein by reference. In the case when a membrane unit is used for separating at least a portion of carbon dioxide, the membrane selected for the application has high selectivity for separating carbon dioxide and other gases to be removed from the recycle gas. For example, cellulose acetate membranes are used in the industry for removal of carbon dioxide from natural gas.

In the case when an absorber/stripper unit is used to separate carbon dioxide from the recycle gas, it is preferred to use an aqueous absorbent such as one comprising potassium carbonate. Absorption processes using organic compounds such as ethanolamine solutions are not preferred for the process of this invention to prevent contamination of the process with such components. Many variations of the process using aqueous carbonate solutions are commercially available and practiced. For example, there are at least three commercial processes that use a hot aqueous carbonate solution; these are Benfield process (owned by UOP), the Catacarb process, and the Giamarcco-Ventrocke process as described in the Kirk-Othmer Encyclopedia of Chemical Technology, incorporated herein by reference. Various additives are often added to the potassium carbonate solutions to improve the carbon dioxide absorption efficiency. In an embodiment of a process using a hot aqueous potassium carbonate solution for removing a portion of carbon dioxide, at least a portion of the recycle gas stream containing carbon dioxide is passed through an absorber, countercurrent to the flow of a hot aqueous solution comprising potassium carbonate. At least a portion of carbon dioxide contained in the gas stream is absorbed in the potassium carbonate solution resulting in a gas leaving the absorber lean in carbon dioxide. The carbon dioxide laden aqueous solution is sent to a stripper where the carbon dioxide is stripped from the solution and the resulting lean solution is returned to the absorber for reuse.

Efficiency of the selective byproducts removal such as carbon dioxide separation from the recycle gas increases with increased pressure in the byproducts separation unit. However, increasing pressure in the selective byproducts removal unit beyond that required for operating the rest of the process can result in a less economical process overall. Therefore, the selective byproducts removal step is best operated at the pressure normally required for the rest of the process such as at a pressure that is slightly higher than the pressure required for the oxydehydrogenation reaction.

Preferably the selective byproducts removal step is sequenced immediately down stream of the recycle gas pressurization step and upstream of the oxydehydrogenation step to maximize the operating pressure in the byproducts removal unit and thereby, efficiency of the byproduct removal step by taking advantage of the highest pressure in the recycle gas loop. The operating pressure for the oxydehydrogenation zone in the process of the present invention is described in the earlier section. In one embodiment, the operating pressure in the byproducts removal from the recycle gas stream is within 2 bar of operating pressure of the oxydehydrogenation reaction zone.

The amount of carbon dioxide and other inert gases to be separated from the recycle gas stream according to the invention can range from about 5% to about 99% of the carbon dioxide and other inert gases contained in the recycle gas. Preferably, at least about 20%, more preferably at least about 30%, most preferably at least about 40% and even more preferably about 50 %; and up to about 99%, preferably up to about 90%, more preferably up to about 80% and most preferably up to about 60% of the carbon dioxide and other inert gases are separated from the recycle gas. Presence of some amount of carbon dioxide and other inert gases in the recycle gas is not harmful to the process. Therefore, it is not necessary to achieve very low levels of carbon oxides and other inert gases in the modified recycle gas. Allowing some build up of carbon dioxide in the recycle gas, improves efficiency of the carbon dioxide removal. For example, the amount of carbon oxides contained in the modified recycle gas mixture stream is generally from about 1 mole percent to about 30 mole percent, preferably from about 2 mole percent to about 20 mole percent and more preferably from about 5 mole percent to about 10 mole percent based on the total amount of the modified recycle gas mixture. A small amount of the modified recycle gas stream is usually removed from the process as purge to control build up of other inert gases such as nitrogen, argon which may not be removed in the selective byproducts removal step.

The present invention is hereafter described with reference to Figure 1 and Figure 2 which are not intended to limit the scope of the claims that follow. Figure 1 represents the process of the present invention configured to produce primarily acrolein; and Figure 2 represents the process of the present invention configured to produce primarily acrylic acid.

With reference to Figure 1, there is shown a process used to produce acrolein, generally indicated by numeral 10, comprising a propane oxydehydrogenation reactor 30 with aftercooler 40, an acrolein reactor 50 with aftercooler 60, an acrolein recovery system 70, a recycle gas compressor unit 80 and a byproducts removal system 90. In the process shown in Figure 1, small amounts of acrylic acid may also be made and may be recovered as a co-product, if desired.

With reference to Figure 1 again, there is shown one embodiment of the present invention wherein a gaseous propane feedstream 31 comprising about 99 mol % propane and about 1 mol % other hydrocarbons such as propylene and a gaseous oxygen feedstream 32 are fed to reactor 30, i.e., alkene reaction zone, containing a heterogeneous oxidative dehydrogenation catalyst, i.e., an alkene reaction catalyst, such as the preferred catalysts described herein. The oxygen feed is about 99.5 mol%, the balance being inert gases such as argon. A modified recycle stream 92 is also fed to reactor 30. The modified recycle gas stream 92 contains unconverted propane and oxygen which passed through the process without conversion at an earlier time. The modified recycle gas stream 92 also contains propylene, water and various noncondensable gases which are essentially not reactive in the process. The essentially non-reactive gases would include, but not be limited to, carbon dioxide and carbon monoxide, and nitrogen.

All feedstreams are preheated to approximately an operating temperature of reactor 30, which may operate at a temperature of between about 400 ⁰C and about 500 ⁰C. The pressure of the feedstreams may be slightly greater than the reactor pressure, which may be between about 2 bars and about 3 bars. The gaseous species and the solid catalyst are contacted effectively in the reactor, which may have various designs including fixed or fluidized catalyst beds. The propane conversion to propylene may be in the range of from about 10 % to about 20 %. The gas product stream 35 contains the propylene product, unreacted propane, oxygen, water, small amounts of by-products, and the nonreactive feed species.

The first effluent from the propane oxydehydrogenation reactor 30 is a crude propylene product stream 35 which is first cooled in an after-cooler 40 to about 300 ⁰C (with introduction of a water stream 41 into the aftercooler 40 and an exiting steam stream 42 from the aftercooler 40), then mixed with additional oxygen stream 36 to achieve the desired oxygen to propylene ratio, and then sent via stream 37 to a propylene oxidation reactor 50, i.e., the propylene reaction zone, where the propylene contained in the effluent is oxidized to acrolein. The reactor 50 contains a heterogeneous catalyst for the oxidation of propylene, i.e., acrolein reaction catalyst, such as the preferred catalysts described herein. The gaseous reactant and solid catalyst are contacted effectively in the reactor 50, which may have various designs including fixed or fluidized catalyst beds. The reactor 50 may be operated in the temperature range of from about 300 ⁰C to about 400 ⁰C and a pressure range of from about 2 to about 3 bars. The conversion of the contained propylene is approximately 90%, but may be in the range of from about 70 % to about 100%. The principal product from the reactor 50 is acrolein with acrylic acid being a minor co-product. The effluent stream from the reactor 50 is immediately cooled to approximately 250° C in after cooler 60 (with introduction of a deionized water stream 61 into the aftercooler 60 and an exiting steam stream 62 from the aftercooler 60), forming the effluent stream 51. Stream 51 has a pressure of approximately 2 bars, but it can range from about 1 bar to about 3 bars.

A wide variety of recovery and refining schemes known to those skilled in the art, such as for example, absorption and fractionation, may be employed to separate acrolein from the other components in the effluent stream 51, using a recovery system or unit 70. The recovered acrolein is removed from a separation unit 70 in stream 71 and the remaining gases leave the unit in a stream 72. The temperature and pressure of the stream 72 depend upon the specific acrolein separation process used, but may typically be in the range of from about 30 ⁰C to about 70 ⁰C and from about 1 bar to about 2 bars absolute, respectively. Stream 72 is composed primarily of propane, propylene, oxygen and various non reactive gases noted previously. Recycle stream 72 is then compressed to a working pressure of the selective byproducts removal unit 90 which is above the working pressure of reactor 30. The recycle stream 81 from the compressor 80 is then modified by selectively removing the byproduct carbon dioxide from the recycle stream 81 by passing at least a portion of the gas stream through a selective byproducts removal unit 90 to form a modified recycle stream 91 exiting the removal unit 90.

The byproducts removal unit 90 may consist of for example an absorber/stripper system using an aqueous solvent comprising potassium carbonate solution and absorbs about 50% of carbon dioxide contained in the recycle gas. Stream 91 is then divided into a modified recycle stream 92, which contains the majority of the flow, and a small purge stream 93. The magnitude of purge stream 93 is selected to achieve the desired level of other minor inert gases in the modified recycle gas. The modified recycle stream 92 is then mixed with feedstreams 31 and 32 to make up the feed stream 34 to the propane oxydehydrogentation reactor 30. Another embodiment of the present invention is shown in Figure 2 which represents the process configured to produce acrylic acid, generally indicated by numeral 20. As shown in Figure 2, similar equipment is used as in Figure 1 except that a third reactor 100 is incorporated into the process and positioned after the acrolein reactor 50 and before an acrylic acid recovery unit 120. Operation of the oxydehydrogenation reactor 30 is the same as described above with reference to Figure 1. Operation of the acrolein reactor 50 of Figure 2 is very similar to the operation of reactor 50 as described with reference to Figure 1, with a possible exception that the temperature, pressure and/or oxygen content may be shifted modestly to favor the formation of acrylic acid over acrolein. Also, optionally, the reactor 50 of Figure 2 may or may not be connected to an aftercooler as shown in Figure 1. As shown in Figure 2, the effluent stream 51 from reactor 50 is not cooled but rather is combined with additional oxygen from stream 52 to form feedstream 53, which enters the acrylic acid reactor 100, i.e., the acrylic acid reaction zone.

Reactor 100 contains a heterogeneous catalyst for the conversion of acrolein to acrylic acid, such as the preferred catalysts described herein. Reactor 100 is designed to contact effectively the catalyst and reactant gases. The conversion of acrolein to acrylic acid is high, in the range of from about 70 % to about 100 %. The effluent gases from reactor 100 are cooled in after cooler 110 (with introduction of a deionized water stream 111 into the aftercooler 110 and an exiting steam stream 112 from the aftercooler 110), and routed in stream 101 to the acrylic acid recovery unit 120.

Many possible recovery schemes known to those skilled in the art are possible for separating acrylic acid product stream 121 from the residual reactants, gaseous by-products and diluent gases in stream 122. The recovered acrylic acid is removed from the separation unit 120 in stream 121 and the unreacted gases leave the unit in stream 122.

The temperature and pressure of stream 122 depend upon the specific acrylic acid separation process used, but may typically be in the range of from about 30 ⁰C to about 70 ⁰C. and from about 1 bar to about 2 bars absolute. Stream 122 is composed primarily of propane, propylene, oxygen and various essentially inert gases such as carbon monoxide, carbon dioxide, and, nitrogen.

Recycle stream 122 is compressed to a working pressure of the selective byproducts removal unit 90 which is above the working pressure of reactor 30 using a compressor 80. The recycle stream 81 from the compressor 80 is then modified by removing the byproduct carbon dioxide from the recycle stream 81 in removal unit 90 to form a modified recycle stream 91. Stream 91 is divided into a recycle stream 92, which contains the majority of the flow, and a small purge stream 93. The magnitude of purge stream 93 is selected to prevent the slow accumulation of minor, inert gases. Stream 92 is then mixed with feed streams 31 and 32. In order to provide a better understanding of the present invention including representative advantages thereof, the following examples are offered. The following examples are provided for illustrative purposes only and are not intended to limit the scope of the claims which follow.

EXAMPLES In the following examples, computer modeling experiments were performed to show the effect of selective removal of carbon dioxide from a recycle gas stream fed into a propane oxyhydrogenation reactor.

Example 1

In a computer simulated experiment using commercial process modeling software and physical properties, to prepare acrolein from propane, 823 mol/h of propane,

602 mol/h of oxygen and 6454 mol/h of modified recycle gas of the following composition: 74.9 % by mol propane 1.7 % by mol propylene

4.3 % by mol water

5.1 % by mol CO₂ 1.6 % by mol CO

4.5 % by mol O₂

1.5 % by mol N₂; and

6.4 % by mol other components; were combined to give the following composition for the reactor feed: 71.8 % by mol propane

1.4 % by mol propylene

3.6 % by mol water

4.2 % by mol CO₂ 1.3 % by mol CO 11.3 % by mol O₂

1.2 % by mol N₂; and

6.5 % by mol other components.

The above reactor feed was fed to a gas phase catalytic propane oxydehydrogenation reactor. The reactor exit temperature was controlled at 450⁰C. Conversion of propane was 13 % and selectivity to propylene was 90 %. The primary byproducts of the reaction were carbon dioxide and carbon monoxide. Some of the carbon monoxide present in the feed to the reactor was converted to carbon dioxide in the reactor.

The reactor effluent was cooled to 300 ⁰C in an after-cooler and combined with 921 mol/h of additional oxygen before feeding the combined stream to a catalytic propylene to acrolein reactor to give the following composition of the combined feed gas to the reactor:

53.6 % by mol propane

8.4 % by mol propylene 13.5 % by mol water

6.3 % by mol CO₂ 0.8 % by mol CO 11.9 % by mol O₂; and

5.5 % by mol other components. The peak reactor temperature was controlled at about 300 ⁰C. Conversion of propylene was about 85% and selectivity to acrolein was about 92%. The primary byproducts of the reaction were carbon dioxide and carbon monoxide with other byproducts typically found in the propylene to acrolein oxidation reaction. The composition of the acrolein reaction zone product gas was as follows: 53.2 % by mol propane

1.2 % by mol propylene 6.6 % by mol acrolein 21.5 % by mol water 7.2 % by mol CO₂

1.1 % by mol CO 3.2 % by mol O₂; and 6.0 % by mol other components. The reactor product gas stream was immediately cooled in an aftercooler to 250⁰C and sent to an absorber for recovering acrolein out of the gas mixture stream where the gas was scrubbed with water to absorb essentially all of the acrolein and small amounts of acrylic acid as well as other water soluble byproducts in the water. Acrolein in desired purity can be recovered from the acrolein solution thus obtained by those skilled in the art. The gas leaving the absorber, referred to herein as "recycle gas," was at a temperature of 4O⁰C and was essentially free of acrolein, acrylic acid and other water soluble components. The composition of the recycle gas was as follows:

71.1 % by mol propane 1.6 % by mol propylene

4.3 % by mol water 9.8 % by mol CO₂

1.5 % by mol CO

4.3 % by mol O₂; and

7.4 % by mol other components.

The recycle gas was pressurized to 2 bar gauge pressure before sending the gas to the selective byproducts removal unit. 50% of carbon dioxide present in the recycle gas was removed from the recycle gas by scrubbing it with an aqueous solution comprising potassium carbonate. Carbon dioxide was stripped from the rich potassium carbonate solution and the lean solution was recycled back to the absorber. The gas leaving the carbon dioxide absorber was lean in carbon dioxide and is referred to herein as "modified recycle gas." The modified recycle gas was split into two parts, a very small part (less than 2 % of the total) and the remaining larger part. The small part was removed from the process as purge to remove other inert gases such as nitrogen, argon etc. from the process. The remaining large portion of the modified recycle gas mixture stream was recycled to the oxydehydrogenation reactor. The results of Example 1 are shown in Table I. Comparative Example A

A computer simulated experiment, similar to the experiment in Example 1, was carried out except without the use of a selective byproducts removal unit. Purge rate for the recycle gas was increased from 0.12 kg/ kg acrolein in Example 1 to 0.71 kg/ kg acrolein. The results of Comparative Example A are shown in Table I. Comparative Example B

In yet another computer simulated experiment, an experiment similar to the experiment in Comparative Example A was carried out without the selective byproducts removal unit. Purge rate for the recycle gas was increased further from 0.71 kg/ kg acrolein in Comparative Example A to 1.16 kg/ kg acrolein. The results of Comparative Example B are shown in Table I.

Table I

*The prior art processes in the above example are taught in WO97/36848, and U.S. Patent Nos. 6,166,263 and 6,492,548. The results in Table I show that concentration of carbon oxides in the propane oxydehydrogenation (propylene) reactor feed in the improved process is only about 6 mol % compared with about 30 to 50 mol % concentration of carbon oxides in the prior art processes in spite of a much smaller gas purge rate from the improved process. Likewise, the concentration of propane in the feed is about 72 mol % in the improved process of the present invention compared with about 38 to 56 mol % concentration of propane in the prior art processes. The high propane concentration in the feed of the improved process of the present invention to the propane oxydehydrogenation reactor offers many benefits such as it increases heat capacity of the reaction mixture. The high heat capacity moderates the reaction, allowing better control of the exotherm and improving efficiency of the propane to propylene reaction. The high propane concentration also allows lower conversion of propane per pass which further increases selectivity to propylene and increases raw material yield to the desired products

The results in Table I show that acrolein reactor feed composition is also similarly improved. Concentration of carbon oxides in the acrolein reactor feed in the improved process is only about 7 mol % compared with concentration of about 28 to 48 mol % carbon oxides in the prior art processes in spite of a much smaller gas purge rate from the improved process. Likewise, the concentration of propane is about 54 mol % in the improved process of the present invention compared with concentration of 30 to 43 mol % propane in the prior art processes. The high propane concentration in the reactor feed in the process of present invention is also advantageous in this reactor.

Replacing carbon oxides with propane as a reactor diluent has beneficial effects in both propylene and acrolein reactors, moderating the temperature rise and increasing reaction efficiencies. The results in Table I also show that the concentration of propylene in the acrolein reactor feed is 8.4 mol % in the improved process of the present invention compared with 4.7 mol % to 6.7 mol % in the prior art processes for the same propane to propylene conversion in the oxydehydrogenation reactor. This has the effect of increasing productivity in the acrolein reactor and the rest of the process. Concentration of acrolein in the reactor effluent gas is 6.4 mol % in the process of present invention compared with 3.6 mol % to 5.1 mol % concentration in the prior art processes, demonstrating increased productivity in the improved process. The improved process of the present invention provides the following additional benefits as shown in Table 1 :

(1) The process of the present invention requires 15% less propane (1.08 Ib propane compared to 1.25 Ib propane) per Ib acrolein product offering a significant saving in the raw material propane cost.

(2) Recycle gas flowrate in the process of the present invention is 11 Ib / Ib acrolein compared with 20 lb/lb acrolein in the prior process. The reduced gas flow required in the process of the present invention, nearly half when compared with results of the prior art process shown in Comparative Example A, reduces size of major equipment in the entire recycle gas loop such as propane oxydehydrogenation reactor, propylene oxidation reactor, reactor after-coolers, acrolein absorber, recycle gas compressor etc. very significantly.

(3) In the acrolein absorber, the process of the present invention requires 46 Ib water/lb of acrolein compared to 76 Ib water/lb acrolein in the prior art process, reducing size of equipment in the cycle water loop such as the acrolein absorber and acrolein stripper by a proportion similar to the reduction in the water ratio.

(4) Higher acrolein concentration in the acrolein solution (6.4% compared to 3.6% in the prior process) from the acrolein absorber results in more efficient acrolein recovery in the acrolein stripper column, requiring less energy in the acrolein stripper reboiler.

Thus the process of the present invention provides a very significant raw material, operating cost and capital cost saving over the prior art processes. This improvement can be used in producing acrolein or acrylic acid as acrylic acid production involves further oxidation of acrolein to acrylic acid. Although the present invention has been described with respect to specific aspects, those skilled in the art will recognize that other aspects are intended to be within the scope of the claims that follow. For example, other alternative embodiments of the present invention may include using the process of the present invention to produce methacrolein or methacrylic acid from isobutane.

Claims

WHAT IS CLAIMED IS:

1. A process for preparation of acrolein and/or acrylic acid from propane, comprising feeding a reaction gas starting mixture to a first reaction zone; wherein the reaction gas starting mixture comprises propane, molecular oxygen and a modified recycle gas mixture stream containing at least propane; wherein the propane contained in the reaction gas starting mixture is subjected to partial oxydehydrogenation with the molecular oxygen under a catalyst, in the first reaction zone, to give a propylene-containing product gas mixtureas a first effluent; wherein the propylene-containing product gas mixture formed in the first reaction zone, the first effluent, is then used in at least one further reaction zone for preparation of acrolein and/or acrylic acid by gas-phase catalytic oxidation of propylene such that an acrolein and/or an acrylic acid-containing product gas mixture stream is formed; wherein the acrolein and/or an acrylic acid-containing product gas mixture stream formed in the at least one further reaction zone is used in at least one separation zone to separate a product stream containing acrolein and/or acrylic acid and a recycle gas mixture stream containing propane and an amount of byproducts, from the acrolein and/or an acrylic acid-containing product gas mixture stream; wherein at least a portion of the byproducts contained in the recycle gas mixture stream is separated from said recycle gas mixture stream to form the modified recycle gas mixture stream, and wherein the modified recycle gas mixture stream is added to the reaction gas starting mixture of the first reaction zone.

2. A process for producing acrolein from propane comprising the steps of:

(i) passing a feedstream comprising (a) propane, (b) oxygen and (c) a modified recycle gas stream comprising propane to a first reaction zone, wherein the feedstream is contacted with a propane reaction catalyst at conditions effective to promote partial oxidative dehydrogenation or oxydehydrogenation of propane to provide a first effluent stream comprising propylene, unreacted propane, water, and carbon oxides;

(ii) passing the first effluent stream to a second reaction zone wherein the first effluent stream is contacted with an acrolein reaction catalyst at conditions effective to promote the conversion of propylene contained in the gas stream to acrolein to provide a second effluent stream comprising propane, acrolein, and carbon oxides; (iii) separating acrolein from the second effluent stream to provide (a) a product stream comprising acrolein and (b) a recycle gas stream comprising propane, and carbon oxides;

(iv) selectively removing at least a portion of byproduct carbon dioxide from the recycle gas stream to form a modified recycle gas stream comprising propane; and

(v) recycling at least a portion of the modified recycle gas stream from step (iv) to the first reaction zone to comprise a portion of said feedstream.

3. A process for producing acrylic acid from propane comprising:

(i) passing a feedstream comprising (a) propane, (b) oxygen and (c) a recycle gas comprising propane to a first reaction zone wherein the feedstream is contacted with a propane reaction catalyst at conditions effective to promote partial oxidative dehydrogenation or oxydehydrogenation of propane to provide a first effluent stream comprising propylene, unreacted propane water; and carbon oxides;

(ii) passing the first effluent stream to second reaction zone wherein the first effluent stream is contacted with an acrolein reaction catalyst at conditions effective to promote the conversion of propylene contained in the gas stream to acrolein to provide a second effluent stream comprising propane, acrolein and carbon oxides;

(iii) passing the second effluent stream to acrylic acid reaction zone wherein the second effluent stream is contacted with an acrylic acid reaction catalyst at conditions effective to promote the conversion of acrolein contained in the gas stream to acrylic acid to provide a third effluent stream comprising propane, acrylic acid and carbon oxides;

(iv) separating acrylic acid from the third effluent stream to provide (a) a product stream comprising acrylic acid and (b) a recycle gas stream comprising propane, and carbon oxides;

(v) selectively removing at least a portion of byproduct carbon dioxide from the recycle gas stream to form a modified recycle gas stream comprising propane; and

(vi) recycling at least a portion of the modified recycle gas stream to the first reaction zone to comprise a portion of said feedstream.

4. The process of Claims 1-3, wherein a portion of the byproducts contained in the recycle gas mixture stream is removed from the recycle gas mixture stream by a sorption process, a filtration process or a combination thereof to form the modified recycle gas mixture stream.

5. The process of Claims 1-3, wherein a small portion of the modified recycle gas mixture stream containing inert gases is removed from the process as purge stream to remove at least a portion of inert gases from the process.

6. The process of Claims 1-3, wherein any of the effluents from the reaction zones, the first effluent, the second effluent or the third effluent are optionally cooled to a temperature lower than the initial temperature of the effluent.

7. The process of Claims 1-3, wherein the recycle gas pressurization is done upstream of the selective byproducts removal step such that the pressure in the byproducts removal unit is the highest in the entire recycle gas loop.

8. The process of Claims l-3,wherein the byproducts removed selectively from the recycle gas stream comprises carbon oxide and/or carbon dixoide.

9. The process of Claims l-3,wherein carbon dioxide is removed from the recycle gas stream by an absorption process utilizing a solvent solution; and wherein the solvent solution comprises an aqueous potassium carbonate solution or sodium carbonate solution.

10. The process of Claims 1-3, wherein carbon oxide is removed from the recycle gas stream by a filtration process utilizing a separation material; and wherein the separation material comprises a polymeric membrane.

11. The process of Claims 1-3, wherein from about 10 percent to about 99 percent of byproducts including carbon oxides and/or carbon dioxide are removed from the recycle gas stream per pass to form the modified recycle gas stream.

12. The process of Claims 1-3, wherein about 50 percent of carbon oxide and/or carbon dioxide is removed from the recycle gas stream per pass to form the modified recycle gas stream.

13. The process of Claims 1-3, wherein the concentration of carbon oxides in the modified recycle stream is from about 1 mole percent to about 30 mole percent based on the total moles in the modified recycle stream.

14. The process of Claims 1-3, wherein the reaction gas starting mixture or the feedstream comprises at least 50 mole percent propane based on total moles in the feedstream.

15. The process of Claims 1-3, wherein the concentration of inert gases including carbon oxides in the reaction gas starting mixture or the feed stream is less than 40 mole percent based on the total moles in the feed stream.

16. The process of Claims 1-3, wherein said propane and oxygen in the reaction gas starting mixture or the feedstream are present in a molar ratio of from about

5 to about 10 moles of propane per mole of oxygen.

17. The process of Claims 1-3, wherein the conversion of propane is from about 5 percent to about 40 percent and the selectivity of the conversion of propane to propylene is from about 70 percent to about 99 percent.

18. The process of Claims 1-3, wherein the concentration of propane in the feed stream to the propylene oxidation zone is at least about 40 mole percent based on the total moles in the first effluent stream.

19. The process of Claims 1-3, wherein the concentration of propylene in the feed stream to the propylene oxidation zone is from about 5 mole percent to about

20 mole percent based on the total moles in the feed stream.

20. The process of Claims 1-3, wherein in the propylene-to-acrolein reaction, the conversion of propylene to acrolein is from about 70 percent to about

99 percent and the selectivity of the conversion of propylene to acrolein is from about 80 percent to about 99 percent

21. The process of Claims 1-3, wherein the concentration of acrolein in the second effluent stream is at least 5 mole percent based on the total moles in the second effluent stream.

22. The process of Claims 1-3, wherein the unit ratio of propane to acrolein (mass flowrate of propane in the feed stream to the mass flowrate of acrolein product in the acrolein product stream) is less than about 1.2.

23. The process of Claims 1-3, wherein less than about 5% of the modified recycle gas (mass flowrate of purge gas stream to the mass flowrate of modified recycle gas stream) is purged from the process.