JP2681667B2 - Method for separating acetic acid / water / vinyl acetate mixture - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial application] The present invention relates to energy saving measures for a vinyl acetate production plant.

[Prior Art] Vinyl acetate is produced by reacting ethylene, acetic acid and oxygen in the gas phase with palladium or a palladium salt as a main catalyst.

In addition to vinyl acetate, the reaction product gas obtained in the reaction contains unreacted ethylene, unreacted oxygen, unreacted acetic acid, water, carbon dioxide gas and an inert gas.

To remove these impurities, the reaction product gas mixture is first cooled in multiple stages and the resulting acetic acid / water / vinyl acetate-based condensate is subjected to distillation, while the uncondensed components are fed to the reactor. Is recycled. The distillation is carried out by first distilling off the vinyl acetate / water mixture from the top and acetic acid from the bottom in the first distillation column, the mixture being further distilled and purified to pure vinyl acetate.

[Problems to be solved by the invention] Today is the era of resource saving and energy saving.
Energy saving measures have been taken up as one of the important issues in chemical plants, and vinyl acetate production plants are no exception. As such measures, various measures such as improvement of processes and improvements of various devices have been taken, but the most important energy measure is the low-temperature steam that has almost no utility value and is unavoidable because it belongs to the plant, that is, This is the effective use of waste steam.

Conventionally, as a document describing the energy saving measures of a vinyl acetate plant, for example, Japanese Patent Laid-Open No. 56-79639 discloses that when the acetic acid / water / vinyl acetate mixture is separated by distillation, the mixture having different water contents is separated. No. 58-39643 discloses a method of supplementarily adding water to the distillation column at the time of separating the mixture. The purpose is to increase the water content in the overhead distillate and to reduce the amount of heat used for water separation as compared with the case of performing the distillation according to the above, and it does not consider the use of waste steam.

[Means for Solving the Problems] The inventors of the present invention have earnestly studied utilization of waste steam in the separation distillation of an acetic acid / water / vinyl acetate mixture in a vinyl acetate production plant, and as a result, the acetic acid / water / vinyl acetate mixture was converted into a vapor column. In the separation method in which the vinyl acetate / water mixture is introduced from the top and the acetic acid is taken out from the bottom, the liquid at the stage lower than the distillation column raw material feed stage is used to evaporate the temperature of the vapor used for evaporation at the bottom of the column. The present invention has been completed by finding that the object can be achieved by evaporating by using a lower temperature steam.

INDUSTRIAL APPLICABILITY The present invention is effectively used for distillation of an acetic acid / water / vinyl acetate mixture in a vinyl acetate production method, and the high temperature steam conventionally used by utilizing waste steam can reduce a part of the usage amount. Moreover, it has a great feature that it has no effect on the separation effect of each fraction.

Generally, the composition of the acetic acid / water / vinyl acetate mixture of the present invention comprises about 25 to 50% by weight of vinyl acetate as a main component and about 5 to
It contains 15% by weight of water and 35 to 70% by weight of acetic acid, but other than that, ethyl acetate, diacetates, polymers, acetaldehyde and / or methyl acetate are added, respectively.
It may be contained in a proportion of 0.5% by weight or less.

As a distillation column for distilling the acetic acid / water / vinyl acetate mixture of the present invention, the actual stage is usually 40 to 80 stages, preferably 50
A ~ 70-stage distillation column is used. Further, the distillation is usually operated under normal pressure. At this time, generally about 125
Steam above ℃ is required.

The removal of the liquid vaporized by using a vapor (hereinafter abbreviated as low temperature vapor) having a temperature lower than that of the vapor used for vaporization of the column bottom of the present invention (hereinafter abbreviated as high temperature vapor) Any liquid can be used as long as it is a liquid below the normal raw material supply stage (hereinafter abbreviated as a charging stage), but in a 60-stage distillation column, it is 5 to 50 stages from the top of the distillation column and below the charging stage. Liquid is desirable. If the raw material charging stage is too close to the top of the column, acetic acid is mixed in the distillate from the top of the column, which does not serve the purpose of distillation. Since the difference is small and the purpose of the present application is almost lost, the extraction stage is usually 10
The range from the 40th stage is adopted.

When the raw material is divided into two or more stages of the distillation column and charged, the distillation using the low temperature steam is applied to the liquid below the uppermost charging stage.

Even if the same operation as in the present invention is performed on the liquid above the charging stage, the total amount of heat used is increased rather than the case where only high temperature steam is used, so energy saving cannot be achieved.

As a specific method for evaporating a liquid lower than the distillation column charging stage of the present invention using low-temperature steam, the liquid is extracted from the distillation column and introduced into an evaporator to evaporate a part or all of the extracted liquid. Then, there are a method of returning the distillate and the residual liquid to the distillation column again, a method of directly inserting an evaporator into the distillation column, and the like.

Next, as an example of the method of the present invention, a part of the liquid in the stage below the charging stage is extracted and introduced into an evaporator, and part or all of the extracted liquid is evaporated, and then this distillate is again extracted. The case of returning the evaporation residual liquid to the column will be described in detail with reference to the drawings, but of course the method of the present invention is not limited to this.

The acetic acid / water / vinyl acetate mixture is fed via tube 1 to the distillation column 2. The distillate containing vinyl acetate / water as a main component is distilled from the column top 9 through the pipe 3 from the bottom of the column 2 through a pipe 3 to remove a bottom product containing acetic acid as a main component. Part of the bottoms is heated by the reboiler 4 using high-temperature steam and supplied to the tower 2 again, and the rest is sent from the pipe 16 to the next step. A part of the liquid in the tower is removed from a pipe 5 located below the raw material charging stage and introduced into the evaporator 6. The evaporator 6 is heated with the low-temperature steam, and the vapor obtained by evaporating the extracted liquid is returned from the pipe 7 while the residual liquid is returned through the pipe 8 to the same stage as it was extracted.

The fraction at the top of the column is condensed in a cooler 10 through a pipe 9 and introduced into a tank 11 to separate it into two layers. The lower aqueous layer portion of the condensate is withdrawn through the pipe 12. The vinyl acetate layer in the upper layer is extracted by a pipe 13 and a part is refluxed through a pipe 14,
The remainder is passed through the tube 15 to the next step and purified to pure vinyl acetate.

[Operation] In the method of the present invention, waste steam is used, and by reducing a part of the amount of high temperature steam used, separation of an acetic acid / water / vinyl acetate mixture can be efficiently carried out, which is useful for energy saving of a plant. is there.

[Example] Next, the method of the present invention will be described more specifically with reference to examples.

Example 1 Ethylene was reacted with acetic acid and oxygen in the gas phase in the presence of a palladium main catalyst supported on silica. The obtained reaction product gas was condensed to obtain a condensate having a composition of 55% by weight of acetic acid, 10% by weight of water and 35% by weight of vinyl acetate.

This condensate is supplied from the top of the cap tray distillation column with a diameter of 100 mm, a height of 6 m and a real stage of 60 stages at a rate of 10,000 g per hour at the 15th stage in the actual stage, and from the 20th stage at the top of the column to the inside of the distillation column. The liquid is extracted at a rate of 7900g / h, and the extracted liquid is partially evaporated by an evaporator, the vapor part is 5600g / h (87 ° C) and the liquid part is 2h / h.
It was returned to the same stage as when it was extracted at 300 g (87 ° C). The reflux ratio
It was set to 4.5 and distilled at normal pressure.

Mixture of vinyl acetate / water from the top of the tower (vinyl acetate 95.5
% By weight and 4.5% by weight of water) at a rate of 4400 g / hr and acetic acid (98% by weight of acetic acid, 2% by weight of water) at a rate of 5600 g / hr from the bottom of the column.

At this time, the top temperature of the distillation column was 68 ° C, the top temperature was 128 ° C, the temperature of the vapor used at the bottom was 150 ° C, and the amount used was 5430 g / hr. The temperature of the (water) steam used in the evaporator was 100 ° C, and the amount used was 1000 g / hr.

Examples 2 to 6 and Comparative Examples 1 to 3 According to the method of Example 1, an experiment was conducted under the conditions shown in Tables 1 and 2, and the results are also shown in Tables 1 and 2.

[Effect] In a separation method in which an acetic acid / water / vinyl acetate mixture is introduced into a distillation column and a vinyl acetate / water mixture is distilled from the top to distill acetic acid from the bottom, a liquid in a stage lower than a raw material supply stage of the distillation column is used. Compared with the conventional separation method in which only the liquid in the bottom of the column is heated and evaporated by using a separation method characterized in that vaporization is performed using a vapor having a temperature lower than that of the vapor used for vaporization of the bottom of the column. And the resolution is the same, and
It has the effect that it can be partially replaced by low-temperature low-temperature steam, and is useful as an energy-saving measure in industrial chemical plants.

[Brief description of the drawings]

The drawings are flow sheets showing one embodiment of the present invention. 1 ... Raw material supply pipe, 2 ... Distillation tower 3 ... Tower bottom liquid extraction pipe, 4 ... Reboiler 5 ... Side liquid extraction pipe, 6 ... Evaporator 7 ... Extracted liquid vapor supply pipe, 8 ... … Extracted liquid evaporation residual liquid supply pipe 9 …… column top vapor extraction pipe, 10 …… cooler 11 …… phase separation tank, 12 …… lower layer liquid extraction pipe 13 …… upper layer liquid extraction pipe, 14 …… reflux Tube 15 …… Vinyl acetate / water mixture distillation tube 16 …… Acetic acid distillation tube

Claims (2)

(57) [Claims] 1. A separation method in which an acetic acid / water / vinyl acetate mixture is introduced into a distillation column and a vinyl acetate / water mixture is taken out from the top and acetic acid is taken out from the bottom, respectively. A method for separating an acetic acid / water / vinyl acetate mixture, characterized in that is vaporized with a vapor having a temperature lower than that of the vapor used for vaporizing the bottom of the column. 2. A part of the liquid below the distillation column raw material supply stage is withdrawn and introduced into an evaporator to evaporate a part or all of the withdrawn liquid, and then the distillate and the evaporation residual liquid are again provided. The method according to claim 1, wherein the column is returned to the column.