JPS5835496B2 - Transesterification reaction method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an improved transesterification method for obtaining bis-β-hydroxyethyl terephthalate and/or its low polymer in a batch process from dimethyl terephthalate and ethylene glycol. It is.

Terephthalic acid dimethyl ester (hereinafter referred to as DMT)
and ethylene glycol (hereinafter referred to as EG) to bis-
The transesterification reaction to obtain β-hydroxyethyl terephthalate and/or its low polymer (hereinafter referred to as BHT) is a reversible equilibrium reaction, and the equilibrium constant is smaller than that of a general condensation reaction.

However, a problem when carrying out the transesterification reaction on an industrial scale is that the reaction time is long.

One of the reasons for this is that the equilibrium constant mentioned above is small,
This has the drawback that the contribution of the reverse reaction becomes large in the reaction stage near the equilibrium state, inhibiting the progress of the forward reaction and prolonging the reaction time.

In order to improve these problems, for example,
No. 26498 proposes a method in which two external heat exchangers are installed in the reactor instead of a rectifying column to increase the EG fraction in the reflux liquid and to efficiently distill the methanol produced.

However, in a batch transesterification reaction, the composition of the reaction mixture, the composition of the generated steam, and the amount of steam change over time, so it is difficult to control the operating conditions of the external heat exchanger accordingly.

Furthermore, in the transesterification reaction, there is a problem that a sufficient reaction rate cannot be obtained simply by efficiently distilling off methanol and shifting the reaction equilibrium.

Furthermore, in order to increase the transesterification reaction rate, a method has been proposed, for example, in JP-A-48-439, in which the reaction mixture is reacted at high temperature and under pressure.

However, this method has the disadvantage that since the high temperature is maintained at a stage where there is a large amount of free EG, the side reaction that produces diethylene glycol (hereinafter referred to as DEC) becomes significant, and the quality of the polymer obtained by polymerizing the obtained BHT is reduced. .

Furthermore, in the method of conducting the reaction at high temperature and normal pressure, an abnormal increase in the reaction pressure and a bumping phenomenon of the reaction mixture occur.

That is, if the mixture is heated to a high temperature at a stage when there is a large amount of unreacted DMT, a large amount of DMT vapor will be generated, resulting in condensation and solidification of DMT in the methanol distillation circuit such as the rectification column and condenser.

As the solidification of DMT progresses, the distillation circuit becomes clogged, causing an abnormal rise in the pressure of the reactor and a bumping phenomenon of the reaction liquid, causing the reaction liquid to overflow into the distillation circuit.

Therefore, unless the evaporation-condensation-solidification phenomenon of DMT is eliminated, the temperature increase rate at normal pressure cannot be increased.

The present inventors have discovered the disadvantages of the conventional transesterification method of DMT and EG, especially when the transesterification reaction of DMT and EG is carried out under normal pressure using a reactor equipped with a rectification column. The present invention was achieved by studying ways to improve the problem in which the methanol distillation circuit was blocked due to condensation and solidification of DMT, making it impossible to increase the reaction rate.

That is, the first object of the present invention is to provide a production method for stably obtaining BHT with high productivity in the transesterification reaction rate, and the second object is to provide a production method with a low DEC content and excellent The object of the present invention is to provide HT, which is useful for obtaining polyester mainly composed of polyethylene terephthalate (hereinafter referred to as PET), which does not cause whitening or decrease in softening point.

In the present invention, when reacting DMT and EG in a batchwise manner, the amount of reflux liquid present in a rectification column attached to a reactor is adjusted according to changes in reaction liquid composition, vapor composition, and vapor amount over time. This is a transesterification reaction method characterized by:

The method of the present invention will be explained with reference to the drawings.

The figure is a schematic diagram of an apparatus suitable for carrying out the method of the invention,
First, DMT and EG are supplied from each supply lower, 8 to reactor 1.
The catalyst is added together with the EG or through a separate feed port (not shown).

The reaction mixture is heated by a heating device installed in reactor 1, and the methanol produced at this time passes through a partial condenser 2 and a rectification column 3, where it is separated from other vaporized components such as EG and DMT. It is condensed in condenser 4.

A portion of the condensed methanol is returned to the upper part of the rectification column as a reflux liquid in the distributor 5, and the remainder is led to the methanol receiving tank 6.

The reflux liquid is normally supplied to the upper part of the rectification column, and is uniformly dispersed into the rectification column by a dispersion plate having a large number of holes.

In the figure, a part of the methanol distillate is supplied as a reflux liquid, but all the distillate is led to the methanol receiver tank 6 and sent to the upper part of the rectification column from the methanol receiver tank or methanol storage tank (not shown) using a pump. It may be supplied as a reflux liquid.

Any type of rectification column attached to the reactor can be used as long as it has the ability to rectify methanol and EG, but the rectification column capacity should preferably have a theoretical plate number of 1.5 or more, and a theoretical plate number of 2 or more. .0 to 5.0 is more preferable.

A plate having a theoretical plate number of 5.0 or more is not preferable because it is too thick for the ability to separate methanol and EC and the pressure loss becomes large.

As the rectification column, a packed column, a bubble column, etc. can be used, but it is preferable to use a packed column because it provides the same capacity, has a simple device, and has high responsiveness in gas-liquid equilibrium.

Suitable objects to be filled in the packed tower are porcelain Raschig rings, stainless steel Pall rings, McMahons, etc., and the size of the packing may be uniform within the tower or may be changed depending on the filling height.

As shown in the figure, an external heat exchanger such as a partial condenser may be installed between the rectification column and the reactor to partial condense the EG.

The method of adjusting the amount of reflux liquid according to the progress of the reaction is, for example, in the case of the apparatus shown in the above figure, by changing the distribution ratio of the distributor 5, and when the pump reflux liquid is supplied from a methanol receiving tank or storage tank, Adopt a method that changes the rotation speed of the pump.

As mentioned above, in the transesterification reaction between DMT and EG, the reaction liquid composition and vapor composition change over time, so it is important to detect or estimate this and adjust the amount of reflux liquid according to the change in composition.

That is, in the first half of the reaction, when a relatively large amount of unreacted DMT exists, it is not possible to raise the temperature too high when the reaction is carried out at normal pressure, and the rate of methanol production is low.

Therefore, if most of the vapor generated from the reactor is distilled off at this stage to reduce the amount of reflux liquid, unreacted EG and DMT will rise into the rectification column in a short period of time because methanol production cannot keep up. The above-mentioned problems occur with DMT solidification in the distillation circuit.

However, if the amount of reflux liquid is increased too much at this stage, the amount of free methanol in the reaction solution will increase and the contribution of the reverse reaction will increase, which is not preferable.

Therefore, in the first half of the reaction, it is necessary to set an optimum amount of reflux liquid according to the composition of the reaction mixture so as to prevent the contribution of the reverse reaction and the condensation and solidification of DMT.

This optimal reflux ratio can be calculated by the following procedure.

In the vapor-liquid equilibrium diagram (x-y diagram) of methanol and EG, we assume raw materials with various compositions (steam entering the rectification column), and use the general stepwise drawing method (for example, "Theory and Calculation of Distillation" by Kawato). Determine the minimum reflux ratio based on the paper (co-authored by Jun and Isao Okada, published by Kogaku Tosho Co., Ltd., pp. 149-151).

In the first half of the transesterification reaction (the stage where the transesterification reaction rate is 40 to 60% or less as described later), the optimal reflux ratio is 1.7 to 2.0 times the minimum reflux ratio, and in the second half of the transesterification reaction, the optimal reflux ratio is 1.7 to 2.0 times the minimum reflux ratio. (At the stage where the reaction rate is 40 to 60% or higher as described later), the reflux ratio is set to 1.3 to 1.6 times the minimum reflux ratio.

On the other hand, in the latter half of the transesterification reaction, since the amount of unreacted DMT is small, troubles due to solidification of DMT in the methanol distillation circuit in the rectification column or partial condenser do not occur much.

However, in the latter half of the reaction when the reaction temperature is high, the amount of reflux liquid is extremely small (as a result, the EG fraction in the rectification column becomes large,
Extra time is then required to stabilize the rectifier.

On the other hand, if the amount of reflux liquid is too large, the methanol fraction in the rectification column increases, which increases the methanol content in the reaction mixture and increases the contribution of the reverse reaction, which is not preferable.

Unlike the first half of the reaction, where there is a large amount of unreacted DMT, this situation is very close to the equilibrium state in the second half of the reaction, so it has an even greater effect, so it is necessary to adjust the amount of reflux liquid.

As detailed above, it is clear that optimizing the amount of reflux liquid present in the rectification column in both the first and second half of the transesterification reaction is advantageous in increasing the reaction rate, and as a result, the transesterification reaction time can be reduced. significantly shortened.

Note that the first and second half of the transesterification reaction are the reactor,
Although it differs slightly depending on the reaction raw material composition, catalyst, temperature increase rate, etc., the stage where the reaction rate is 40 to 60% or less is defined as the first half of the reaction, and the stage with a reaction rate higher than that is defined as the second half of the reaction.

Examples of specific means for detecting changes in the composition of the reaction mixture and vapor composition over time during the transesterification reaction include the following.

The composition of the reaction mixture, especially unreacted DMT, monomethyl hydroxyethyl terephthalate, and EGl. If the correlation between the methanol distillation amount and the methanol distillation amount at various temperatures is determined in advance, it is possible to roughly measure the methanol distillation amount and the reaction temperature at that point. It can be estimated by

The vapor composition, particularly DMT, EG, and methanol, can be estimated by measuring the temperature of the gas phase if the vapor-liquid equilibrium relationship (boiling point diagram at constant pressure: t-X diagram) is determined in advance.

In this case, the temperature of the gas phase specifically refers to the temperature of the gas phase in the reactor, the part connecting the reactor and the rectification column, and any position within the rectification column.

In order to accurately predict the vapor composition, it is sufficient to measure the temperature at one location among the above-mentioned gas phase parts, but it is more preferable to measure the temperature at two or more locations.

When the number of measurement points is small, it is effective to estimate the vapor composition in relation to changes in the composition of the reaction mixture. It is good to obtain the temperature-composition correlation for the two components.

In addition, if the number of measurement points is small, in the above-mentioned gas phase part, especially near the bottom of the rectification tower, specifically, in the connection circuit between the rectification tower and the reactor, near the tower, or inside the rectification tower with packing. In this case, it is preferable to measure at a point from the bottom to the filling height %.

In an apparatus in which a fractionator is installed below a rectification column, the preferred measurement location is inside the fractionator or at the connection between the fractionation column and the fractionation column.

In other words, the temperature of the gas phase, especially the temperature at the bottom of the rectification column and/or
Alternatively, the reaction rate at a certain temperature can be determined by detecting the temperature distribution inside the rectification column, relating it to the composition of the reaction mixture estimated from the amount of distilled methanol and the reaction temperature, and adjusting the amount of reflux liquid to the rectification column. This allows the temperature increase rate of the reaction system to be maximized.

As a more specific operation, for example, the temperature at the bottom of the rectification column is set in advance according to the progress of the reaction, and the reflux ratio of the rectification column is adjusted so that the temperature at the bottom of the rectification column is controlled to the set temperature. can be mentioned.

The above-mentioned adjustment of the reflux liquid is performed by processing the temperatures of multiple gas phase parts using an online computer, determining the optimal reflux ratio to achieve the temperature distribution in the gas phase that is optimal for the reaction situation, and distributing the liquid in devices with a distributor. The ratio can be implemented in devices using feed pumps by automatically adjusting the pump speed.

In addition, if the temperature of the gas phase is to be detected at a relatively small number of points, the amount of reflux liquid should be changed so that the temperature at one point, for example, the lower part of the rectification column described above, is the optimum temperature for the given reaction situation. In order to avoid sudden abnormalities in the reaction situation, for example, when the remaining detection points deviate from the limit range considered from the given reaction situation, a flooding phenomenon occurs in the rectification column and the temperature distribution becomes abnormal. or if EG in the rectification tower
It is preferable to quickly change the amount of reflux liquid when the fraction becomes temporarily high so as to be able to operate for the purpose of eliminating the abnormality.

When reaction conditions are selected such that the temperature distribution of the rectifying column remains stable within a desirable range, the amount of reflux liquid is adjusted in several stages according to the reaction rate, and each stage is controlled under certain conditions, such as distillation. The object of the present invention can be easily achieved by refluxing a certain ratio of the amount of methanol output or by providing a certain amount of reflux liquid.

In this case, the preferred reflux ratio is 0.1 to 2.0, but it can also be set to O or (1) if necessary.

The molar ratio of EG/DMT during transesterification is preferably 1.2 to 2.5, more preferably 1.45 to 1.
95 is more preferred.

If the molar ratio is less than 1.2, the reaction will not proceed sufficiently unless the reaction temperature is kept high, and the adverse effects of solidified DMT will become significant.As a result, the temperature increase rate during the process cannot be increased, and the reaction time will become longer, resulting in poor quality. Good BHT cannot be obtained.

On the other hand, when the molar ratio exceeds 2.5, free EG in the reaction mixture
Since a large amount of is always present, the reaction temperature cannot be raised and therefore a sufficient reaction rate cannot be obtained.

Moreover, since there is a large amount of free EG and the reaction time is long, the formation of undesirable DEC becomes significant.

Known compounds can be used as transesterification catalysts, but compounds that are active in both reactions between hydroxyl groups and methyl ester groups and between hydroxyethyl groups and methyl ester groups, especially zinc acetate, manganese acetate, manganese chloride, and acetic acid. Cobalt, cobalt chloride, magnesium acetate, calcium acetate, lithium acetate, titanium oxalate, and the like are preferably used alone or in combination.

Further, compounds other than those mentioned above may be used in combination.

It is preferable to add these catalysts before the reaction, but some of them may be added during the reaction.

The transesterification reaction temperature in the present invention is 130 to 270
°C is preferred.

In addition, in the method of the present invention, the reaction rate corresponding to the reaction temperature can be maintained higher than in the case of conventionally known methods, so the temperature increase rate can be increased, and the preferable temperature increase rate is that of methanol. In the first half of the reaction after the start of distillation, the BHT obtained by the method of the present invention has a low DEC content and no decrease in softening point due to the subsequent polycondensation reaction, and can be used as fiber,
It can be made into polyethylene terephthalate which has excellent quality for film and resin molding.

In this case, BHT includes known anti-fouling agents such as phosphoric acid, phosphoric acid esters, ring phosphoric acids, and phosphorous esters, polycondensation catalysts such as antimony oxide and germanium oxide, and titanium oxide, kaolinite, talc, Erosil, etc. The following known additives can be added, but other than the above-mentioned adhesion prevention agent may be added to the system before the transesterification reaction.

The method of the present invention involves replacing a part of DMT with other conventionally known monofunctional, difunctional, or polyfunctional carboxylic acids having ester-forming ability, such as dimethyl isophthalate, 5-sulfosodium dimethyl isophthalate, and trimethyl trimellitate. and/or when a part of EG is replaced with other conventionally known glycols such as 1,3-propanediol, 1,4-butanediol, 1,4-cyclohexanedimetatool, polyethylene glycol, etc. It can also be applied to transesterification reactions.

Also, in place of DMT, known bifunctional dicarboxylic acids such as dimethyl 2,6-naphthalate are used for transesterification reactions with EG, or in place of EGO, the above glycols are used for reaction with DMT. Applicable.

The method of the present invention exhibits the following effects by adjusting the amount of reflux liquid to the rectification column according to changes in the reaction mixture composition and vapor composition within the transesterification reaction system over time.

(1) Since the rate of temperature rise of the reaction mixture can be significantly increased, the reaction time can be significantly shortened.

In other words, in the conventional technology that maintains a constant reflux ratio throughout the reaction area, the temperature increase rate during the first half of the reaction up to approximately 200°C is 15~200°C.
b The temperature increase rate above approximately 200℃ is 25-35'c/hr
However, in the method of the present invention, it is possible to make the first half of the reaction 20-b and the second half 30-b.

(2) Even if the reaction temperature is raised, clogging of the rectification column due to solidified DMT can be avoided, allowing stable operational management.

(3) Since the reaction time can be shortened, DEG by-products can be prevented and high-quality polyester can be obtained.

The present invention will be explained in detail with reference to Examples below.

Example A packed column type rectification column with a theoretical plate number of 3.0 was installed.The reaction product contained 100 parts of DMT, 65 parts of EG, and 0 manganese acetate tetrahydrate.
．． After adding 0.4 parts, heat from 150℃ to 170℃ for 2 hours.
The temperature was raised from 170°C to 200°C in 1 hour and 20 minutes, and from 200°C to 235°C in 50 minutes.

At this time, temperature detectors were inserted into the height H portion of the packed column (middle part), and directly below (lower part) and directly above (top of the column) the filled area.

The temperature of the lower part was changed from 90°C to 120°C for 20 minutes from the start of the reaction.
The temperature was set to rise from 120°C to 150°C for the next 40 minutes, and then from 150°C to 190°C for 1 hour and 30 minutes.

So that the temperature at the bottom of the rectification column changes according to the set temperature,
A temperature controller was installed and controlled.

This temperature controller detects the deviation between the set temperature and the actual bottom temperature, and when the actual temperature is high, the amount of reflux liquid is increased, and when the set temperature is high, the reflux liquid is increased. The reflux-distillate distribution ratio of the distributor is adjusted and the temperature of the lower stage of the rectification column is controlled so that the amount of distillation is reduced.

When the temperature in the middle of the rectification column exceeded 110° C., total reflux was performed and the temperature increase was temporarily interrupted to prevent EG from being mixed into the liquid distilled from the top of the column as much as possible.

Methanol was led to the receiver tank from the time when the temperature at the top of the rectification column reached 65°C, but the point at which this distillation started was 10 minutes after the start of the reaction.
The reaction temperature was 160°C.

The reaction rate determined from the distilled amount of methanol is at a reaction temperature of 170
8% at °C (20 minutes after the start of the reaction), 30% at 180 °C (47 minutes), 52% at 190 °C (1 hour and 13 minutes),
72% at 200℃ (after 1 hour and 40 minutes), 89% at 220℃ (after 2 hours and 9 minutes), 9 at 235℃ (after 2 hours and 30 minutes)
8%, and the reaction was completed when the temperature reached 235°C.

The DEC content in the obtained BHT was 0.32% by weight.
Trimethyl phosphate 0.0 as an anti-adhesion agent for HT
4 parts, 0.04 part of antimony trioxide as a polymerization catalyst, and 0.5 part of titanium dioxide as a matting agent, and polycondensation was carried out in a conventional manner.

The DEC content of the obtained polymer was 0.65% by weight, the white discoloration (L value) was 69, and the yellowing (b value) was +2.0.

Comparative Example The same conditions as in Example were used except that the rectification column temperature was set, the operation of changing the reflux liquid was stopped, and the reflux ratio throughout the reaction area was set to 1.0.

The reaction rate determined from the distilled amount of methanol is at a reaction temperature of 170
6% at °C (20 minutes after the start of the reaction), 21% at 180 °C (47 minutes), 45% at 190 °C (1 hour 35 minutes),
60% at 200°C (after 2 hours and 10 minutes), 80% at 220°C (after 2 hours and 50 minutes), 91% at 235°C (after 3 hours and 20 minutes), and 98% after maintaining at 235°C for 20 minutes. The total reaction time was 3 hours and 40 minutes.

The reason why it was not possible to follow the set temperature increase rate was because the temperature in the middle of the rectification column rose during the reaction, and the temperature increase was frequently interrupted.

The DEC content of the obtained BHT was 0.57% by weight, and the DEC content of the polymer obtained by polycondensation in the same manner as in the example was 1.02% by weight, and the white discoloration (L value) was 67. Yellowish(
b value) was +4.5.

[Brief explanation of the drawing]

The figure is a schematic representation of an example of a device suitable for carrying out the method of the invention. 1: reactor, 2: partial condenser, 3: rectification column, 4: total condenser, 5
: Distributor, 6: Methanol receiver.

Claims (1)

[Claims] 1 When dimethyl terephthalate and ethylene glycol are reacted batchwise in a reactor equipped with a rectification column to produce bis-β-hydroxyethyl terephthalate and/or its low polymer, the A transesterification reaction method characterized in that the amount of refluxed liquid to be refluxed is adjusted according to the progress of the reaction.