Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
1999-12-22
2001-11-20
Killos, Paul J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
Reexamination Certificate
active
06320076
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the preparation of 3-(methylthio)propanal, and more particularly to a continuous process for the direct manufacture of 3-(methylthio)propanal in a gas/liquid reaction system.
3-(Methylthio)propanal (hereinafter “MMP”) is an intermediate for the manufacture of both d,l-methionine and 2-hydroxy-4-(methylthio)butanoic acid (“HMBA”). Methionine is an essential amino acid in which components of the animal feed compositions are commonly deficient. HMBA provides a source of methionine, and is widely used as a methionine supplement in animal feed formulations. MMP relatively free of impurities is typically required for the manufacture of HMBA or methionine.
MMP is produced by reaction of acrolein with methyl mercaptan. In a conventional process for the preparation of MMP, liquid acrolein and methyl mercaptan are introduced into a reactor containing liquid phase MMP product. Reaction takes place in the liquid phase. In order to produce MMP of desired quality, refined acrolein is used in the process, and/or the MMP product is distilled before use in the manufacture of either HMBA or methionine.
Acrolein is a highly toxic and flammable material. It is conventionally prepared by vapor phase oxidation of propylene over a solid phase catalyst, producing a crude gaseous reaction product which contains water vapor, acrylic acid, acetaldehyde, and other organic by-products.
Typically, the gas is treated to remove acrylic acid, then contacted with refrigerated water for absorption of acrolein. The resultant aqueous solution is distilled to recover the absorbed acrolein and other organic components. The crude acrolein is then refined to reject lower boiling impurities such as acetaldehyde, producing a purified liquid acrolein product. The refined liquid acrolein is stored for use in the manufacture of MMP.
Storage of liquid acrolein involves significant toxicity, fire and explosion hazards. High capital and operating costs are consequently incurred in providing for the safe handling of this material. The cost of handling acrolein could be substantially reduced if gas phase acrolein were transferred directly and continuously from the acrolein manufacturing process to the MMP reactor without storage or condensation. However, since the conventional commercial processes for the preparation of MMP involve liquid phase reactions, the need to condense the gaseous acrolein product has been considered unavoidable. Moreover, because the conventional process typically uses a batch reaction system, condensation and in-process storage of liquid acrolein is necessary as a surge buffer between operation of the acrolein process and the MMP reactor.
Netherlands patent No. 6809647 describes a process in which acrolein is produced by catalytic oxidation of propylene and the acrolein-containing reaction gas mixture is passed to a vertical reaction column in which MMP is formed. MMP is circulated through the reaction column and both the acrolein-containing gas and methyl mercaptan are added near the bottom. MMP exiting the column contains a separate aqueous phase which is removed in a separator. MMP from the separator is partially recycled to the reaction column. A sodium bicarbonate solution is supplied to the circulating MMP. MMP product removed from the circulating reaction system is distilled at a pressure of 100 mmHg.
U.S. Pat. No. 4,225,516 describes a continuous process for the manufacture of MMP from the acrolein product gas obtained in the catalytic oxidation of propylene. In this process, the gas is first treated for removal of acrylic acid, then cooled to condense water vapor. To reduce the water vapor content to a level acceptable in the MMP reaction, the final condensation temperature is 0° to −5° C. The treated and cooled acrolein gas stream is contacted with a stream of liquid MMP in a countercurrent absorption tower, resulting in absorption of acrolein in the MMP. The MMP liquid stream containing dissolved acrolein is circulated to an MMP reactor where methyl mercaptan is added. The process proceeds by reaction of methyl mercaptan with MMP to form the hemimercaptal of MMP, and the hemimercaptal in turn reacts with acrolein in the liquid phase to produce additional MMP. Thus, the process requires the presence of up to 1% by weight of the hemimercaptal in the reaction mixture. MMP product is withdrawn from the system at a rate equivalent to MMP production in the reactor, while the bulk of the MMP stream is recirculated to the acrolein absorber.
To provide for quantitative absorption of acrolein in MMP, the '516 patent requires cooling the circulating MMP to a temperature 0° to −15° C. before it enters the absorber. The refrigeration required for condensing water vapor at 0° to −5° C. and cooling MMP to as low as −15° C. contributes substantially to the capital and operating expense of the '516 patent process. Moreover, because the reaction proceeds through formation of the hemimercaptal, the kinetics of the conversion reaction are relatively slow, resulting in less than desirable productivity and thus further adding to the cost of operation of the process.
Although sub-zero absorption increases acrolein recovery at equilibrium, it also increases the absorption of impurities, such as acetaldehyde, in the MMP product. Moreover, since the scrubber is separate from the reactor, acrolein absorbed in the scrubber is not consumed immediately in the absorption zone. As a consequence, acrolein tends to accumulate in the liquid phase, which decreases the driving force for mass transfer. The high concentration of acrolein in MMP liquid also increases the possibility of forming by-products from reactions between acrolein and MMP.
SUMMARY OF THE INVENTION
Among the several objects of the present invention are the provision of an improved process for the preparation of MMP; the provision of such a process which can be operated in a continuous mode; the provision of such a process which can be operated with high productivity; the provision of such a process which can be operated with a relatively crude acrolein raw material; the provision of such a process which does not require refrigeration for absorption or condensation of acrolein; the provision of such a process which eliminates the need for storage of liquid acrolein, in particular, the provision of such a process which can be operated using a gaseous acrolein feed obtained directly from the continuous oxidation of propylene or other suitable hydrocarbon; the provision of such a process which can be operated without formation of a separate aqueous phase in the MMP reaction mixture; and the provision of such a process which can produce high quality MMP for direct use in the preparation of methionine or HMBA without the need for further purification.
Briefly, the invention is directed to a process for the continuous preparation of MMP in which a liquid reaction medium is contacted with a gaseous acrolein feed stream in a gas/liquid contact zone. The reaction medium contains MMP, methyl mercaptan and a catalyst for the reaction between methyl mercaptan and acrolein. The gaseous acrolein feed stream comprises acrolein vapor and non-condensable gas. The relative proportions of acrolein and methyl mercaptan entering the contact zone are substantially stoichiometrically equivalent. Acrolein is transferred from the feed stream to the reaction medium and reacts directly with methyl mercaptan in the medium, without substantial formation of the intermediate hemi(methylthio)acetal of MMP, to produce a liquid reaction product containing MMP Non-condensable gas is separated from the liquid reaction product, the reaction product is divided into a product fraction and a circulating fraction, and the circulating fraction is recycled to the gas/liquid contact zone. Acrolein and methyl mercaptan react in the liquid medium in a reaction zone that comprises the gas/liquid contact zone and a circulation zone into which the liquid reaction product is discharged from the gas/liquid contact zone an
Hsu Yung C.
Ruest Dennis A.
Killos Paul J.
Novus International, Inc.
Senniger Powers Leavitt & Roedel
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