Bimodal acetic acid manufacture

Details Bimodal acetic acid manufacture Bimodal acetic acid manufacture

2000-12-29

2003-03-11

Padmanabhan, Sreeni (Department: 1621)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carboxylic acids and salts thereof

C562S519000, C560S206000, C518S713000, C518S702000, C518S700000

Reexamination Certificate

active

06531630

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed generally to a process for making acetic acid from carbon monoxide and methanol, and more particularly to a bimodally operable plant wherein the carbon monoxide is obtained from syngas, or synthesis gas, made by reforming a hydrocarbon and synthesizing the methanol from the synthesis gas in a first mode of operation, or by reforming a lower alkanol and importing methanol for reaction with the carbon monoxide to form the acetic acid in a second mode of operation.
BACKGROUND OF THE INVENTION
The manufacture of acetic acid from carbon monoxide and methanol using a carbonylation catalyst is well known in the art. Representative references disclosing this and similar processes include U.S. Pat. No. 1,961,736 to Carlin et al (Tennessee Products); U.S. Pat. No. 3,769,329 to Paulik et al (Monsanto); U.S. Pat. No. 5,155,261 to Marston et al (Reilly Industries); U.S. Pat. No. 5,672,743 to Garland et al (BP Chemicals); U.S. Pat. No. 5,728,871 to Joensen et al (Haldor Topsoe); U.S. Pat. No. 5,773,642 289 to Denis et al (Acetex Chimie); U.S. Pat. No. 5,817,869 to Hinnenkamp et al (Quantum Chemical Corporation); U.S. Pat. No. 5,877,347 and U.S. Pat. No. 5,877,348 to Ditzel et al (BP Chemicals); U.S. Pat. No. 5,883,289 to Denis et al (Acetex Chimie); and U.S. Pat. No. 5,883,295 to Sunley et al (BP Chemicals), each of which is hereby incorporated herein by reference.
The primary raw materials for acetic acid manufacture are, of course, carbon monoxide and methanol. In the typical acetic acid plant, methanol is imported and carbon monoxide, because of difficulties associated with the transport and storage thereof, is generated in situ, usually by reforming natural gas or another hydrocarbon with steam and/or carbon dioxide. A significant expense for new acetic acid production capacity is the capital cost of the equipment necessary for the carbon monoxide generation. It would be extremely desirable if this capital cost could be largely eliminated or significantly reduced.
Market conditions, from time to time in various localities, can result in relatively low methanol prices (an oversupply) and/or high natural gas prices (a shortage) that can make methanol manufacture unprofitable. Operators of existing methanol manufacturing facilities can be faced with the decision of whether or not to continue the unprofitable manufacture of methanol in the hope that product prices will eventually rebound and/or raw material prices will drop to profitable levels. The present invention addresses a way of modifying an existing unprofitable methanol plant to make it more profitable when methanol prices are low and/or natural gas prices are high. The present invention also addresses a way of building a new plant with two modes of operation—one with a hydrocarbon feed and the other with an imported methanol feed.
As far as applicant is aware, there is no disclosure in the prior art for modifying existing methanol plants, including methanol/ammonia plants, to supply stoichiometric methanol and CO for manufacturing acetic acid, for example, that can be a more valuable product than methanol. Further, as far as applicant is aware, there is no disclosure in the prior art for modifying existing methanol plants, particularly the steam reformers thereof to reform either a hydrocarbon or a lower alkanol, e.g. methanol, using a hydrocarbon reforming catalyst with the optional presence of carbon dioxide, steam or both.
SUMMARY OF THE INVENTION
The present invention involves the discovery that the large capital costs associated with CO generation for a new acetic acid plant can be significantly reduced or largely eliminated by converting an existing methanol or methanol/ammonia plant to make acetic acid. The present invention is equally applicable to a new plant wherein the syngas producing portion of the plant accepts either a hydrocarbon feed, e.g., natural gas, or a lower alkanol feed, e.g., a methanol feed. The steam reformer is built or modified to accept either a natural gas feed or an imported methanol feed and to optionally have a carbon dioxide input, a steam input or both. The reformation takes place in the presence of a hydrocarbon reformation catalyst. Further, all or part of the syngas can be diverted from the methanol synthesis loop and supplied instead to a separator unit to recover CO
2
, CO and hydrogen, which are advantageously used in various novel ways to produce acetic acid. When the steam reformer is operated with a lower alkanol feed, the methanol synthesis loop is shut down and isolated from the rest of the plant. In this case, all of the synthesis gas will be diverted from the methanol synthesis loop to the separation unit. The recovered CO
2
can be supplied to the reformer to enhance CO production, or to the methanol synthesis loop to make methanol. The recovered CO is usually supplied to the acetic acid reactor with the methanol to make the acetic acid. When a lower alkanol feed, e.g., methanol feed, is used for the reformer, methanol from an imported source is also supplied to the acetic acid reactor. The recovered hydrogen can be supplied to the methanol synthesis loop (when in use) for methanol production, used for the manufacture of ammonia or other products, burned as a fuel, or exported, since the hydrogen is normally produced in excess of the requirements for methanol synthesis in the present invention.
The carbon dioxide can be fed into a steam reformer to which (1) natural gas or methanol and (2) optionally steam (water) are fed. Syngas is formed in the reformer wherein both (1) the natural gas or methanol and (2) the carbon dioxide are reformed to produce syngas with a large proportion of carbon monoxide relative to reforming without added carbon dioxide. Alternatively or additionally, the CO
2
can be supplied to the methanol synthesis loop (when in operation), with additional CO from the synthesis gas and/or additional imported CO
2
, for catalytic reaction with hydrogen to make methanol.
In the mode when the methanol synthesis loop is in operation, natural gas is preferably used as the hydrocarbon feed to the steam reformer. The syngas can be split into a first part and a second part. The first syngas part is converted to methanol in a conventional methanol synthesis loop that is operated at about half of the design capacity of the original plant since less syngas is supplied to it. The second syngas part can be processed to separate out carbon dioxide and carbon monoxide, and the separated carbon dioxide can be fed back into the feed to the reformer to enhance carbon monoxide formation, and/or fed to the methanol synthesis loop to make methanol. The separated carbon monoxide can then be reacted with the methanol to produce acetic acid or an acetic acid precursor by a conventional process.
In the mode wherein the methanol synthesis loop is shut down and isolated from the rest of the plant, imported lower alkanol, e.g., methanol, is used as a feed to the steam reformer and imported methanol is used as a feed to the acetic acid reactor. The syngas is processed to separate out carbon dioxide and carbon monoxide, and the separated carbon dioxide can be fed back into the feed to the reformer to enhance carbon monoxide formation. The separated carbon monoxide can then be reacted with the imported methanol to produce acetic acid or an acetic acid precursor by a conventional process.
In the mode wherein natural gas is used as a feed to the steam reformer, the method comprises the steps of: (a) diverting a portion of the syngas stream from at least one steam reformer to a separation unit; (b) operating the methanol synthesis loop with a feed comprising the remaining syngas stream to produce less methanol than the original methanol plant; (c) operating the separation unit to separate the diverted syngas into at least a carbon monoxide-rich stream and a hydrogen-rich stream, preferably wherein the quantity of hydrogen in the hydrogen-rich stream is greater than any net hydrogen production of the original methanol plant; and (d) reacting t

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