Methanol plant retrofit

Details Methanol plant retrofit Methanol plant retrofit

1999-11-01

2001-08-14

Geist, Gary (Department: 1623)

Chemical apparatus and process disinfecting, deodorizing, preser

Chemical reactor

Ammonia synthesizer

C562S893000, C562S894000, C562S519000, C560S206000, C560S232000, C560S233000, C518S713000

Reexamination Certificate

active

06274096

ABSTRACT:

FIELD OF THE INVENTION
The present invention is directed generally to a process for making synthesis gas from which streams of carbon monoxide and methanol can be obtained in approximately stoichiometric proportions suitable for the manufacture of acetic acid, and more particularly to the retrofit of a methanol plant to divert a portion of the syngas from the existing methanol synthesis loop to a carbon monoxide separator and to react the methanol from the methanol synthesis loop with the carbon monoxide from the separator in approximately stoichiometric proportions to directly or indirectly make acetic acid.
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 (PB 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. Nos. 5,877,347 and 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 Sunlev 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 gas prices are high.
Mayland (U.S. Pat. No. 2,622,089) discloses a method of reforming natural gas to produce hydrogen and carbon monoxide synthesis gas. At column 1, lines 10-36, there is a disclosure that carbon dioxide is combined with natural gas to and steam in a reforming reaction to produce hydrogen and carbon monoxide. The specific method is said to obtain larger quantities of hydrogen-carbon monoxide synthesis gas mixtures with a given quantity of carbon dioxide in the feed to the hydrocarbon reforming unit (column 1, line 48 to column 2, line 10). The molar ratio of hydrogen to carbon monoxide in the syngas is said to be 2.0 (for methanol production). The molar ratio of steam to methane in the reformer feed ranges from 1.5 to 1.8, and the ratio of carbon dioxide to methane in the feed ranges from 0.64 to 0.91.
Moe (U.S. Pat. No. 3,859,230) discloses that naphtha and steam are reformed in a two-stage reforming process to make CO and H
2
. A minor portion (15-30%) of the effluent from the first reformer stage is subjected to absorption/stripping to remove CO
2
which is fed to the second reformer stage with the major portion of the effluent from the first stage. The effluent from the second reformer is used as syngas for alcohol/aldehyde production. If desired, the effluent from the second reformer stage can also be treated to remove CO
2
that can be recycled to the second reformer stage with the CO
2
recovered from the first stage effluent. At column 1, beginning at line 28, it is disclosed that the prior art obtained CO
2
from the effluent of the second reformer stage, or from the effluent from the combustion gases used to heat the reformer.
Joensen et al (mentioned above) disclose making acetic acid from CO and H
2
by using a dehydration catalyst (Cu/Zn/Al) that produces methanol (MeOH), dimethyl ether (DME) and CO
2
. The methanol and DME are then separated from the CO
2
and reacted with CO to make acetic acid. The CO is said to be made from steam-reformed CH
4
. A portion of the syngas from the reformer (2<H
2
/CO<3) is diverted from the feed to the MeOH/DME reactor and then membrane or cryogenically treated to recover the CO for feed to the acetic acid reactor. The process is said to be a parallel production of MeOH and CO, avoiding MeOH import as required in prior art processes.
Steinberg et al (U.S. Pat. No. 5,767,165) disclose that CH
4
is autothermally decomposed to make carbon black and hydrogen. In
FIG. 3
, a portion of the CH
4
feed is reformed with CO
2
(without steam) to make CO. The CO/H
2
from the CH
4
/CO
2
reformer is then reacted with the additional H
2
from the autothermal reformer to make MeOH. The CO
2
is said to be obtained by fossil fuel combustion. Excess H
2
from the syngas can be burned to supply energy for the CH
4
decomposition.
Park et al (U.S. Pat. No. 5,855,815) disclose making syngas for Fischer-Tropsch synthesis. CO
2
and CH
4
are reformed with 0-10% O
2
and 0-10% H
2
O in the presence of Ni catalyst containing an alkali metal on a silica support at 600-1000° C. and a space velocity of 1000-500,000 hr
−1
to make CO, H
2
and H
2
O. The effluent is said to have an H
2
/CO ratio less than 3, compared to an H
2
/CO ratio in the prior art of 0.5-2 with conventional CO
2
reforming.
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 MeOH and CO for manufacturing acetic acid, for example, that can be a more valuable product than MeOH.
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 retrofitting an existing methanol or methanol/ammonia plant to make acetic acid. More specifically, carbon dioxide can be fed into a reformer to which natural gas and steam (water) are fed. Syngas is formed in the reformer wherein both the natural gas and the carbon dioxide are reformed to produce syngas with a large proportion of carbon monoxide relative to reforming without added carbon dioxide.
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. The separated carbon monoxide can then be reacted with the methanol to produce acetic acid or an acetic acid precursor by a conventional process.
Separated hydrogen can also be reacted with nitrogen, in a conventional manner, to produce ammonia. Also, a portion of acetic acid that is produced can be reacted in a conventional manner with oxygen and ethylene to form vinyl acetate monomer. The nitrogen for the ammonia process (especially for any added ammonia capacity in a retrofit of an original methanol plant comprising an ammonia synthesis loop) and the oxygen for the vinyl acetate monomer process, can be obtained from a conventional air separation unit.
Broadly, the present invention p

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