Organic compounds -- part of the class 532-570 series – Organic compounds – Carbonate esters
Utility Patent
1999-06-15
2001-01-02
Ambrose, Michael G. (Department: 1613)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carbonate esters
C558S270000, C558S271000, C558S277000
Utility Patent
active
06169197
ABSTRACT:
BACKGROUND OF THE INVENTION
1) Field of the Invention
The present invention relates to a process for separation between an organic carbonate and an organic carbamate and specifically, to an efficient process for separation between dialkyl carbonate and alkyl carbamate in a mixture containing dialkyl carbonate and alkyl carbamate. Alkyl carbonate having an alkyl group of 3 to 6 carbon atoms is readily synthesized from urea and useful as a raw material of other carbonates, particularly, aromatic carbonates including diphenyl carbonate.
2) Prior Art
Processes for producing dialkyl carbonate using urea or alkyl carbamate as a raw material have been already known. For example, U.S. Pat. No. 2,834,799 discloses to produce both alkyl carbamate and dialkyl carbonate from both urea and alcohol in the presence of at least stoichiometric amount of BF
3
, but a proportion of dialkyl carbonate in the products is low and it fails to describe separation of dialkyl carbonate.
Japanese Patent Kokai (Laid-open) No. 57-26645 describes to produce dibutyl carbonate from both butyl carbamate and butanol in Example 14 and to separate dibutyl carbonate from the reaction liquid containing butyl carbamate by distillation under 17 mbar, but there is no description concerning formation of azeotrope. However, in fact, dibutyl carbonate forms an azeotrope with butyl carbamate in a wide pressure range, so that it is impossible to perform perfectly separation between dibutyl carbonate and butyl carbamate by conventional distillation processes.
When dialkyl carbonate is continuously produced from urea or alkyl carbamate, it is difficult to allow completely progress of the reaction. Therefore, in industrial production of dialkyl carbonate, it is necessary to separate dialkyl carbonate as the product from unreacted alkyl carbamate to recycle unreacted alkyl carbamate as the raw material.
However, dialkyl carbonate having an alkyl group of 3 to 6 carbon atoms, which can be easily synthesized from urea and is excellent as raw material of aromatic carbonates, has a boiling point close to that of alkyl carbamate. As described above, dibutyl carbonate forms an azeotrope with butyl carbamate, so that it is difficult to perform perfectly separation between dibutyl carbonate and butyl carbamate by conventional distillation processes.
The inventors, as a process to solve above-mentioned problems, suggested to use two distillation columns different in operation pressure to each other in Japanese Patent Application No.9-286913. Separation between dibutyl carbonate and butyl carbamate can be perfectly performed by application of above-mentioned process. However, in above-mentioned process, in order to make small an amount of an azeotropic mixture to be recycled from second distillation column to first distillation column and to increase the distillation efficiency, it is necessary to make large a pressure difference between two distillation columns. Thus, when an operating pressure of second distillation column was made higher, there occurred a problem that a distillation temperature was raised, so that butyl carbamate was partially decomposed.
Further, in order to produce continuously dialkyl carbonate, it is necessary to feed continuously urea as the raw material into a reactor. As methods for feeding a powder or particles such as urea to a reactor, usually, gravity feeders such as rotary feeder, table feeder, etc., thrust feeders such as belt feeder, vibrating feeder, shaking feeder, screw feeder, etc., air force feeder such as injection feeder, air slide, etc., or other feeder combined mechanisms thereof are used.
However, in the reaction for production of dialkyl carbonate, when urea was continuously fed to a reactor using a gravity feeder or a thrust feeder, there occurred a problem that urea was coagulated in the vicinity of discharge opening of a feeder and adhered to the discharge opening since urea contacted vapors containing ammonia and alkyl alcohol as main components in the reactor, so that urea could not be steadily fed. Further, when an air force feeder was used, contact of urea with vapors in the reactor could be avoided by use of a large amount of an air current. However, also in such case, there occurred a problem that vapors also in the reactor were released during exhaust of used air current from the reactor.
Japanese Patent Kokai (laid-open) Nos. 10-152455 and 10-152456 suggest a process which comprises producing dialkyl carbonate from urea and an alkyl alcohol having an alkyl group of 3 to 6 carbon atoms and then producing diaryl carbonate from dialkyl carbonate and an aromatic hydroxy compound. In this process, diaryl carbonate can be produced from low-priced urea and aromatic hydroxy compound by using again by-produced alkyl alcohol as a raw material of dialkyl carbonate. Thus, this process is excellent.
Japanese Patent Kokai (Laid-open) No. 10-259166 and Japanese Patent Application No. 9-221563 perform the reaction for production of dialkyl carbonate from urea in a high boiling point solvent, which makes it possible to perform the reaction under about atmospheric pressure. However, since ammonium carbamate was produced as a by-product in above-mentioned reaction, there occurred the same problem that total yield of intended dialkyl carbonate and alkyl carbamate as the intermediate product was about 80 to 90% and thus selectivity was low. Alkyl carbamate produced as the intermediate product can be used again as the raw material for production of dialkyl carbonate. However, also in case where alkyl carbamate was used as the raw material for production of dialkyl carbonate, there occurred the problem that selectivity was low.
SUMMARY OF THE INVENTION
An object of the present invention, in order to solve above-mentioned prior art problems, is to provide an efficient process for separation between dialkyl carbonate and alkyl carbamate in a mixture containing both dialkyl carbonate and alkyl carbamate, depressing decomposition of alkyl carbamate.
In order to solve above-mentioned prior art problems, as a result of detailed studies concerning vapor-liquid equilibrium of a mixture containing both dialkyl carbonate and alkyl carbamate in the presence of third component, the inventors have found that separation between dialkyl carbonate and alkyl carbamate becomes easy in the presence of an aromatic hydroxy compound such as phenol.
Thus, the inventors have found that when a liquid comprising dialkyl carbonate and alkyl carbamate is distilled in the presence of an aromatic hydroxy compound in a distillation column, separation between dialkyl carbonate and alkyl carbamate is easy without causing decomposition of alkyl carbamate and a mixture of dialkyl carbonate and the aromatic hydroxy compound from a top section of the distillation column and alkyl carbamate from a bottom section of the distillation column are obtained, respectively, and have accomplished the present invention.
That is, the present invention provides a process for separation between dialkyl carbonate and alkyl carbamate which comprises:
adding an aromatic hydroxy compound to a liquid comprising alkyl carbamate represented by the following general formula (1) and dialkyl carbonate represented by the following general formula (2) to obtain a mixed liquid, and
distilling the mixed liquid thus obtained in a distillation column to obtain a mixture comprising the dialkyl carbonate and the aromatic hydroxy compound from a top section of the distillation column and a liquid comprising the alkyl carbamate from a bottom section of the distillation column;
RO—CO—NH
2
(1)
RO—CO—OR (2)
wherein R is an alkyl group having 3 to 6 carbon atoms.
Further, the present invention provides a process for separation between dialkyl carbonate and alkyl carbamate which comprises:
separating a catalyst from a reaction mixture produced by reaction of an alkyl alcohol having 3 to 6 carbon atoms with urea and/or alkyl carbamate represented by the following general formula (1) in the presence of the catalyst,
adding an aromatic hydroxy compound to
Arai Yoshihisa
Mizukami Masamichi
Ohgi Hiroaki
Ohshida Takuo
Ambrose Michael G.
Mitsubishi Gas Chemical Company Inc.
Sughrue Mion Zinn Macpeak & Seas, PLLC
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