Organic compounds -- part of the class 532-570 series – Organic compounds – Heterocyclic carbon compounds containing a hetero ring...
Reexamination Certificate
2000-08-14
2001-12-11
Killos, Paul J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Heterocyclic carbon compounds containing a hetero ring...
C560S190000
Reexamination Certificate
active
06329532
ABSTRACT:
The present invention relates to a process for the separation of maleic anhydride from a gas stream containing maleic anhydride, to a process for the preparation of maleic esters, and to a process for the preparation of hydrogenation products of maleic acid derivatives which include the separation of maleic anhydride from a gas stream containing maleic anhydride.
Maleic anhydride is produced on a industrial scale by catalytic oxidation of hydrocarbons, such as benzene, butenes or butane, using air. Besides maleic anhydride, the resultant gaseous reaction mixture contains principally water, carbon monoxide and carbon dioxide.
Maleic anhydride is usually washed out of maleic anhydride-containing reactor offgases from the oxidation of hydrocarbons by quenching with high-boiling aliphatic alcohols. A process of this type is described, for example, in DE-A-31 06 819. This process has the disadvantage that, in the quenching process, some of the alcohol is lost with the offgas. These losses cause high costs.
The maleic anhydride obtainable from the reactor offgases from the oxidation of hydrocarbons and the maleic acid derivatives, such as maleic esters, obtained from maleic anhydride by reaction with liquid absorbents are frequently subjected to subsequent hydrogenation to butanediol, tetrahydrofuran or &ggr;-butyrolactone. WO 97/43234 describes a process for the preparation of &ggr;-butyrolactone, butane-1,4-diol and tetrahydrofuran in which maleic anhydride is washed out of the maleic anhydride-containing reactor offgases from the oxidation of hydrocarbons using a high-boiling, inert, organic solvent as absorbent, and maleic anhydride is separated from the resultant absorption product by stripping using a stream of hydrogen gas. The stripped-out maleic anhydride is then passed to a gas-phase hydrogenation for the preparation of butanediol, tetrahydrofuran and &ggr;-butyrolactone. This process has the disadvantage of the formation of free maleic acid or fumaric acid. In the presence of even small amounts of water, as are frequently present in the reactor offgases from the oxidation of hydrocarbons, maleic anhydride forms free maleic acid, which is corrosive. Furthermore, free maleic acid tends to isomerize into fumaric acid, which has only low solubility and can cause considerable problems in the subsequent hydrogenation step due to deposition on the hydrogenation catalyst. Furthermore, maleic or fumaric acid forms succinic acid in the hydrogenation step. This has extremely low volatility, which can sooner or later result in caking of the catalyst.
It is an object of the present invention to provide a process for the separation of maleic anhydride from maleic anhydride-containing reactor offgases from the oxidation of hydrocarbons which is suitable for the preparation of maleic acid derivatives or hydrogenation products thereof and avoids the abovementioned disadvantages.
We have found that this object is achieved by a process for the separation of maleic anhydride from a maleic anhydride-containing gas stream in which the maleic anhydride-containing gas stream is brought into contact with a liquid absorbent phase containing at least one high-boiling, inert absorbent for maleic anhydride, and maleic anhydride is separated from the resultant liquid absorbate phase by vapor stripping, wherein the stripping agent used is an alcohol, at least some of the maleic anhydride reacting with the alcohol.
For the purposes of the present invention, the term absorbent phase is taken to mean the liquid mixture of absorbents. The absorbate phase is the absorbent phase loaded with the absorptive, here maleic anhydride or reaction products thereof.
In the presence of alcohol as stripping agent, at least partial alcoholysis of the maleic anhydride takes place with formation of the monoester, and in addition further esterification takes place with formation of the diester. The hydrolysis of the anhydride to free maleic acid is thus suppressed by traces of water absorbed from the gas stream in favor of ester formation. Any maleic acid formed by hydrolysis then reacts to give maleic monoesters or diesters. The proportion of free acid in the absorbate phase is consequently low.
The liquid absorbent or absorbate phase contains at least one high-boiling, inert absorbent for maleic anhydride. The inert, high-boiling absorbent generally has a boiling point which is at least 30° C., preferably at least 50° C., particularly preferably at least 70° C., higher than the boiling point of maleic anhydride.
Suitable absorbents are described, for example, in WO 97/43234. Examples are high-boiling esters of phthalic acid, terephthalic acid or maleic acid, such as dimethyl, diethyl or dibutyl phthalates, dimethyl terephthalate or dibutyl maleate, aromatic hydrocarbons, such as dibenzylbenzene, esters of cycloaliphatic acids, such as dibutyl hexahydro-phthalate, furthermore polymethylbenzophenones, ethylene glycol ethers and polysiloxane ethers. Preferred absorbents are esters of aromatic and cycloaliphatic dicarboxylic acids, particularly preferably esters of phthalic acid and terephthalic acid. The high-boiling absorbent is generally present in excess in the absorbate phase. The weight ratio between the absorbent and maleic anhydride is generally from 1:1 to 100:1, preferably from 2:1 to 100:1, particularly preferably from 4:1 to 100:1.
Maleic anhydride is separated from the absorbate phase by vapor stripping using an alcohol as stripping agent. The term stripping is taken to mean the removal of the absorptive present in the absorbate phase with the aid of a desorption aid (stripping agent), with the absorptive accumulating in the stripping agent. The concentration of the absorptive in the absorbate phase is correspondingly reduced, and the absorbate phase is regenerated in the process. In addition to mass transfer from the absorbate phase, the present process also includes at least partial reaction of the absorptive with the stripping agent.
Suitable stripping agents for the novel process are in principle all alcohols which are volatile under the process conditions (stripping alcohols). Preference is given to methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, sec-butanol, n-pentanol and isopentanol, particularly preferably methanol, ethanol and n-butanol. The molar ratio between alcohol and maleic anhydride is generally from 1:1 to 300:1, preferably from 1.5 to 200:1, particularly preferably from 3:1 to 50:1.
The stripping is generally carried out in countercurrent, in which the maleic anhydride-loaded absorbate phase moves in the opposite direction to the stripping agent in vapor form, with intensive mass and heat exchange taking place between the falling, liquid absorbate phase and the rising vapor phase. The liquid absorbate phase is gradually depleted in maleic anhydride, while the vapor phase is correspondingly enriched in maleic anhydride or reaction products thereof. The stripping is preferably carried out in countercurrent in a column, preferably a packed column or bubble-cap column. The alcohol used as stripping agent is preferably introduced into the lower section of the column, and the maleic anhydride-loaded absorbate phase is preferably introduced into the upper section of the column. The maleic anhydride-depleted absorbate phase is obtained at the bottom of the column, and the alcohol employed as stripping agent together with the maleic acid derivatives are obtained at the head of the column.
For the purposes of the present invention, the term maleic acid derivatives is taken to mean maleic anhydride, maleic acid, maleic monoesters, maleic diesters, fumaric acid, fumaric monoesters and fumaric diesters, the esters being based on the alcohols used for the stripping.
It is also possible to carry out the separation of maleic anhydride from the absorbate phase in a hold tank with attached distillation head, with it being possible for the alcohol to be transported into the tank together with or separately from the absorbate phase. The maleic acid derivatives pass over the distillation head t
Pinkos Rolf
Rahn Ralf-Thomas
BASF - Aktiengesellschaft
Calve John Npo
Keil & Weinkauf
Killos Paul J.
LandOfFree
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