Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
1999-12-01
2001-07-24
O'Sullivan, Peter (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C159S005000, C159S006100, C159S006200, C159S011200, C159S011300, C568S866000, C568S867000, C568S869000
Reexamination Certificate
active
06265625
ABSTRACT:
The invention relates to a process for preparing and isolating glycols and to a thin-film evaporator which is used in carrying out the process.
Ethylene oxide (EO hereinafter) and ethylene glycol are important intermediates in industrial chemistry. Important secondary products of EO are, in particular, ethylene glycol, glycol ether, ethanolamines, ethylene carbonate and polyethoxylates. Ethylene glycol is predominantly used for preparing polyesters, polyurethanes, dioxane, plasticizers and as antifreeze.
EO is currently prepared on a large scale generally by direct oxidation of ethylene with molecular oxygen or air in the presence of silver oxide catalysts. A by-product of this highly exothermic reaction is carbon dioxide which is chiefly formed by total oxidation.
EO preparation processes which are carried out industrially are described, for example, in Ullmanns Encyclopedia of Industrial Chemistry, Fifth Edition, Carl Hanser Verlag (Munich), Volume A 10, pages 117ff. According to this, a currently standard EO reactor contains the fixed-bed catalyst in a tube bundle which has several thousand tubes through which the reaction mixture is conducted in a recirculated gas stream. As heat carrier, a boiling liquid, for example water, circulates between the tubes. Ethylene and oxygen are introduced in the recirculated gas stream which is passed through the reactor and comprises, in addition to the reactants, inert gases and the by-product of total oxidation of ethylene, carbon dioxide. The cooled reactor discharge is if appropriate first quenched with an NaOH solution in order to scrub out organic acids, in particular formic acid and acetic acid, and high-boiling minor components. These components can be discharged from the recirculated gas system via a “quench bleed”. In an EO absorber, which is generally operated at approximately 16 bar and approximately from 25 to 40° C., the EO is scrubbed virtually completely out of the recirculated gas by a large scrubbing water stream (approximately 35 metric t of scrubbing water are required for the absorption of 1 metric t of EO from 25 metric t of recirculated gas). The pressure in the absorber is given here by the pressure in the recirculated gas system. Typically from 5 to 100 ppm of EO remain in the recirculated gas. After a potash scrubbing to remove carbon dioxide, the recirculated gas is again enriched with ethylene and oxygen and fed to the EO reactor. The aqueous solution produced in the absorber, in addition to approximately 3% by weight of EO, also comprises traces of the other components present in the recirculated gas. The EO is then desorbed from the loaded scrubbing water in an EO desorber (EO stripper). The resultant stripper vapors comprise approximately 50% by weight of water and approximately 50% by weight of EO and also the gases present in the loaded scrubbing water. The depleted scrubbing water, after cooling, is recycled to the EO desorber as cycle water. Since the EO is partly hydrolyzed to glycols in the EO desorber, these are discharged from the EO desorber via the glycol bleed. The desorber vapors are first condensed. The dissolved gases are then stripped from the condensate in a light-end tower and recycled to the recirculated gas system. The resultant gas-free approximately 50% strength aqueous EO solution is partly distilled in a distillation column to give pure EO or, after addition of water, is reacted in a glycol reactor to form an aqueous glycol solution.
The hydrolysis reactor is usually operated at from 120 to 250° C. and pressures of from 30 to 40 bar. The hydrolysis product is first dewatered (to a residual water content of from 100 to 200 ppm) and then separated into the various glycols in pure form. During the hydrolysis, monoethylene glycol is first predominantly formed, which can then in part further react with EO to form di-, tri- and polyethylene glycols. Since monoethylene glycol (for simplicity called ethylene glycol hereinafter) can in principle be the only glycol occurring in the entire process, the word glycols (in the plural) hereinafter can also mean solely monoethylene glycol (ethylene glycol).
The dewatering is generally performed in a cascade of towers of different pressure stages with decreasing pressure. For reasons of thermal integration, generally only the bottom reboiler of the first pressure tower is heated with external steam, and all other pressure towers are, in contrast, heated with the vapors of the respectively preceding tower. Depending on the water content of the hydrolysis reactor discharge and the pressure/temperature level of the external steam used in the bottom reboiler of the first tower, the pressure dewatering cascade consists of from 2 to 7 towers. A vacuum dewatering follows the pressure dewatering. The dewatered glycol mixture is fractionated into the pure substances in a plurality of towers. The products monoethylene glycol, di-, tri- and possibly tetraethylene glycol are each withdrawn as overhead product, and all other higher glycols are produced as bottom product of the last tower in the form of a mixture termed polyethylene glycols. The main product of this work-up is generally monoethylene glycol (ethylene glycol).
The EO-containing quench bleed cannot be disposed of as it is, because of the toxicity of EO. Furthermore, it is of economic interest to recover the valuable materials EO and glycol present. The EO can be desorbed in a quench bleed stripper, for example, and fed to the EO desorber. Alternatively, the EO in the quench bleed can be converted into glycols in a hydrolysis reactor at from 160 to 230° C. and pressures of from 20 to 35 bar. The resultant aqueous glycol solution can be disposed of without problem.
Alternatively, according to U.S. Pat. No. 4,822,926, the glycols can be recovered after concentrating them in a flasher and separating off salt in a centrifuge. A disadvantage of this method is the production of a solid which is generally more difficult to handle than liquids.
Since in the EO absorber-desorber water circuit the EO is in part hydrolyzed to glycols, a partial stream of the bottoms outflow of the EO desorber is discharged and concentrated, in which case the resultant vapors can be recycled to the EO desorber (described in DD-A-235 635). The concentrated glycol solution, the glycol bleed, is fed to a flasher where some of the glycols are recovered. The salt- and glycol-containing bottom stream is disposed of.
It is an object of the present invention to provide a process in which the glycols formed during the process are recovered as pure substances. In the process, the corresponding mixtures which, in addition to glycols, generally also comprise salts and water, are to be separated so that products of high quality are obtained. In addition, the process is to ensure that as far as possible all glycols are recovered, so that the residues which arise in the process and are to be disposed of are decreased. The method is to be effective and inexpensive.
We have found that this object is achieved by the process for isolating glycols from a liquid mixture comprising glycols, water and salts by
(a) evaporating at least a partial amount of the water and glycols which are present in the mixture, separating off a liquid or gaseous medium which is freed from salt and comprises glycols and water,
(b) dewatering the medium and
(c) isolating the glycols from the dewatered medium.
The process of the invention comprises using a thin-film evaporator for carrying out stage (a).
In a preferred embodiment, the liquid mixture comprising glycols, water and salts arises during the work-up of a reaction mixture which results from the catalytic reaction of ethylene with oxygen and comprises EO, in which case, during the work-up, before the liquid mixture is formed, at least a partial amount of the EO is hydrolyzed to form ethylene glycol. According to the invention, a process is also provided for preparing glycols. In this process, by catalytically reacting ethylene with oxygen, an EO-containing reaction mixture is produced which is then worked up. In this
de Hert Jozef
Köffer Dieter
Terjung Winfried
Theis Gerhard
Vansant Frans
BASF - Aktiengesellschaft
Keil & Weinkauf
O'Sullivan Peter
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