JPS5833856B2 - Processing method for liquid containing dimethyl terephthalate - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention provides dimethyl terephthalate (hereinafter referred to as DMT) from a dimethyl terephthalate-containing composition.

) is vacuum distilled using a steam ejector (hereinafter referred to as ejector).

) from the DMT-containing liquid that is sucked and discharged.
By removing T, the biochemical oxygen demand (BO
It's called D.

).

BACKGROUND ART Conventionally, when a specific component in a composition is distilled out as vapor to be separated, recovered, or purified by distillation, it has been widely practiced to accelerate the distillation of the target component by evacuating the distillation apparatus.

In such cases, vacuum generators that create a vacuum in the distillation equipment include mechanical pumps such as rotary pumps and reciprocating pumps, and steam injection pumps such as ejectors. Ejectors are mainly used because the presence of such components degrades performance, makes maintenance difficult due to corrosion caused by corrosive components, and has many industrial disadvantages in terms of equipment costs and the like.

However, in the distillation of DMT compositions, components mainly consisting of DMT, which are easily condensed and solidified, are sucked, so even if an ejector is used, there are problems in condensation of the driving steam of the ejector and subsequent wastewater treatment.

In other words, if a component that is easily condensed and solidified, such as DMT, is included in the components sucked into the ejector from the distillation device, if a condenser with indirect cooling such as a surface condenser is used, DMT will be absorbed into the condenser's cooling pipe. There is a problem in that the condensate adheres to the surface of the condensate discharge pipe and not only reduces the condensing capacity of the condenser, but also significantly reduces the vacuum capacity of the ejector.

On the other hand, in condensers that are directly cooled with a large amount of cooling water, such as barometric condensers, a large amount of cooling water is discharged together with the condensate, which prevents DMT from sticking. The suctioned condensed components are diluted and difficult to remove, which poses a problem in that an extremely large load is placed on wastewater treatment such as activated sludge treatment.

The present invention was developed as a result of intensive studies aimed at solving the problems in vacuum distillation of DMT-containing compositions at once.

That is, the present invention provides a method for vacuum distilling dimethyl terephthalate from a dimethyl terephthalate-containing composition using a steam ejector in the vacuum generating device of a distillation apparatus, in which the exhaust pipe of the steam ejector is heated in the presence of a liquid adjusted to pH 8 or higher. It is inserted under the liquid level of the well to condense the dimethyl terephthalate-containing components and ejector-driven steam drawn from the distillation device, and at the same time decompose the dimethyl terephthalate with alkali, and then drain the hot well into (1) methanol. This is a method for treating a dimethyl terephthalate-containing liquid, which is characterized by sequentially performing the following steps: rectifying, (2) acidifying terephthalic acid, and (3) filtering terephthalic acid.

In order to efficiently condense the DMT-containing fraction components and ejector-driven steam sucked from the distillation apparatus in the present invention, and to perform alkaline decomposition of DMT, it is necessary to insert the ejector exhaust pipe below the liquid level in the hot well. The length of the exhaust pipe inserted below the liquid level is preferably 50 to 1000 m.
It is m.

The hot well of the present invention is a container in which the exhaust pipe from the ejector is internally piped into the hot well liquid to condense and collect the substance (DMT in the present invention) sucked in by the ejector from the distillation apparatus and the driving steam of the ejector. The iron solution is continuously supplied and discharged while maintaining a constant water level in the hot well.

Further, the pH of the liquid in the hot well must be adjusted to 8 or higher, preferably 8 to 10.

If the pH is less than 8, DMT cannot be alkali decomposed and the object of the present invention cannot be achieved.

Compounds that make the liquid in the hot well alkaline include, for example, sodium hydroxide, potassium hydroxide, etc., but sodium hydroxide is most preferred.

It goes without saying that the ejector used as a vacuum generator is a multistage ejector in which a plurality of single ejectors are directly connected.

The present invention will be explained below with reference to the figures.

The figure is a schematic diagram showing a preferred embodiment of the invention.

In the figure, a DMT distillation apparatus 1 is evacuated by a two-stage ejector 3 directly connected to a single ejector driven by steam supplied from a pipe 2 as well.

At this time, the DMT-containing component sucked from the distillation device 1 is discharged from the exhaust pipe 4 of the ejector 3 together with the driving steam of the ejector 3, but it is condensed by the alkaline liquid present in the hot well 5, and at the same time, the DMT-containing component is It is decomposed and dissolved into water-soluble terephthalate and methanol by the action of

Condensable components that are not condensed in the alkaline liquid in the hot well 5 pass through a vapor pipe 6 and are condensed by a vent condenser 8 that is indirectly cooled by cooling water 7, thereby making the internal pressure of the hot well 5 constant.

The liquid condensed in the vent condenser 8 is returned to the hot well 5 through a seal pipe 9.

Note that non-condensable components that are not condensed even in the vent condenser 8 are discharged into the atmosphere from the vent 10.

The liquid in the hot well 5 is quantitatively extracted by a pump 11 and supplied to a methanol rectification column 12;
2 may be one in which a methanol composition is supplied from a pipe 13 to rectify methanol, and in this case, it is preferable to supply the methanol composition to an appropriate number of stages.

The liquid (bottoms liquid) from which methanol has been removed in the rectification column 12 is temporarily stored in a receiving tank 14, and a portion is sent to the hot well 5 via piping 16 by a pump 15, where the pH is adjusted and the alkali of DMT is condensed. Subjected to decomposition.

The other part is sent to the acid precipitation tank 17, where an acid (for example, sulfuric acid) is added and dissolved to precipitate L-terephthalate as terephthalic acid.Then, the terephthalic acid is removed by a filtering device 18, and the activated sludge treatment device (not shown).

The present invention has an excellent feature in that ejector exhaust pipes 4' from a plurality of distillation apparatuses are led to a single hot well 5, as shown by the chain lines, and condensation and alkali decomposition can be carried out collectively.

The method of the present invention is based on the D
Because MT is alkaline decomposed in the hot well and decomposed into water-soluble terephthalate and methanol, there is no sticking of DMT to the hot well or piping, and the amount of liquid discharged from the hot well is smaller than the surface condenser. It can be kept to a small amount in exactly the same way as when using it.

Therefore, treatments such as rectification and filtration are extremely easy, and BOD in wastewater can be reduced, resulting in excellent environmental conservation effects.

Furthermore, since the removed methanol and terephthalic acid can be reused as raw materials for polyester production, it has an excellent effect in terms of resource saving.

The present invention will be specifically described below with reference to Examples.

Example 1 A two-stage ejector was installed in each of four rectification columns for distillation purification of DMT, which is a raw material for polyester, and four exhaust pipes of the ejector were inserted below the liquid level in a single hot well. Steam pressure 10kg/cnG pressure (Total amount of steam used 2
DMT was distilled by driving the ejector at a pressure of -700 mmHg G pressure in the rectification column.

The pH of the liquid in the hot well was adjusted to 9 with sodium hydroxide, and the liquid level was kept constant by the amount of extraction and the amount of bottoms supplied from the methanol rectification column.

The liquid extracted from the hot well is supplied to a methanol rectification column to recover methanol, and then a portion of the bottoms is supplied to the hot well and sent to another acid precipitation tank where sulfuric acid is added and neutralized. Terephthalic acid was precipitated and collected by filtration.

As a result, the amount of wastewater was 60 tons/day, and the BOD load in the wastewater was 352 kg/day.

On the other hand, when DMT was purified by rectification using a conventional barometric condenser under exactly the same rectification conditions, the amount of wastewater was 264 tons/day, and the BOD load in the wastewater was 79.
The amount was 2 kg/day.

In other words, the method of the present invention reduces the amount of wastewater to 204 tons.
/day, the BOD load in wastewater decreased by 440 kg/day.

[Brief explanation of the drawing]

The figure is a schematic diagram showing a preferred embodiment of the invention. 1: Distillation device, 2: Ejector-driven steam piping, 3 Nisteam ejector, 5: Hot well 8: Vent condenser, 11: Extraction pump, 12: Rectification column, 14: Receiving tank, 15: Feed pump, 17: Acid precipitation Tank, 18: Filtration device.

Claims (1)

[Claims] 1. When vacuum distilling dimethyl terephthalate from a dimethyl terephthalate-containing composition using a steam ejector in a vacuum generator of a distillation apparatus, a solution adjusted to pH 8 or higher is present in the exhaust pipe of the steam ejector. The dimethyl terephthalate-containing components and the ejector-driven steam sucked from the distillation device are condensed, and at the same time dimethyl terephthalate is decomposed with alkali, and then the hot well is drained (1). 1. A method for treating a dimethyl terephthalate-containing liquid, comprising sequentially performing the steps of rectifying methanol, (2) acidifying terephthalic acid, and (3) filtering terephthalic acid.