JPH0532385B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for esterifying terephthalic acid or a dicarboxylic acid mainly composed of terephthalic acid and ethylene glycol or a glycol mainly composed of terephthalic acid. Polyester, which is used industrially today,
In particular, polyethylene terephthalate has a high degree of crystallinity and a high softening point, and has excellent strength, chemical resistance, heat resistance,
Because it has excellent properties in terms of weather resistance and electrical insulation, it is widely used industrially as fibers, films, bottles, and other molded products. Polyethylene terephthalate is produced by obtaining bis(β-hydroxyethyl) terephthalate (including its low polymers) through a transesterification reaction between dimethyl terephthalate and ethylene glycol or a direct esterification reaction between terephthalic acid and ethylene glycol. is produced by polycondensation at high temperature and high vacuum in the presence of a catalyst, but in recent years direct esterification has been widely used due to its advantages in ease of continuous operation, reduction in raw material consumption, and energy savings. It is starting to be adopted. By the way, when producing polyester by the direct esterification method, in order to obtain high quality polyester, it is necessary to control the esterification reaction rate when carrying out the esterification reaction. The esterification reaction rate can be determined by sampling the reactants and measuring the acid value and saponification value, but such a manual analysis method is not compatible with process automation. I can't. As a method to solve this problem, there is a method of controlling the esterification reaction rate by measuring the electrical conductivity (hereinafter referred to as "conductivity") of the esterification reaction mixture (Japanese Patent Application Laid-open No. 103537/1983), and A method has been proposed in which esterification is carried out while maintaining a constant concentration (Japanese Patent Application Laid-open No. 19634/1983). However, such a method of controlling the electrical conductivity of the esterification reactant has the following problems, and has not been put into full-scale practical use. (1) Electrical conductivity in the actual operating range is extremely low and falls within the measurement error range of existing instruments, so disturbances (noise) due to temperature, pressure, air bubbles, etc. and signals due to changes in the esterification reaction rate are It cannot be put to practical use because it is difficult to identify and a method for preventing disturbance factors has not been established. (2) In the case of polyethylene terephthalate, unless the average degree of polymerization is 10 or more, preferably 15 or more, the relationship between esterification reaction rate and electrical conductivity cannot be formulated due to the effects of ethylene glycol and water produced by the reaction. , the variation due to measurement becomes large. Therefore, although it is good for controlling the degree of polymerization of polymers, it is not practical for products with an average degree of polymerization of 10 or less, such as esterification products. (3) It is extremely difficult to make corrections based on temperature, pressure, the amount of ethylene glycol added, etc., and it is impossible to make corrections when various conditions must be changed in the actual manufacturing process. The present inventors have completed the present invention as a result of intensive research on a method for controlling the esterification reaction rate that does not have such problems, that is, a practical method for controlling the esterification reaction rate that is not affected by disturbances or operating factors. That is, in the present invention, when esterifying terephthalic acid (TPA) or a dicarboxylic acid mainly composed of terephthalic acid and ethylene glycol (EG) or a glycol mainly composed of terephthalic acid, a part of the esterified product is discharged outside the reactor. Under pressure, the conductivity is measured by a measuring device connected to a microcomputer.
The gist of this is an esterification method characterized by measuring at a frequency of 100 times per second or more for 5 to 600 seconds and controlling the esterification reaction rate based on the average value. The esterification method in the present invention usually involves continuously feeding a slurry consisting of TPA and EG to a reaction tank in which bis(β-hydroxyethyl) terephthalate (including its low polymer) (BHET) is present. A method of esterification is used. This BHET may partially contain components other than TPA and EG residues, and BHET may be obtained by any known method; It is preferable to use the product as it is. Slurry consisting of TPA and EG
The molar ratio of EG/TPA is usually 1.2 to 2.0, preferably
It is preferably between 1.4 and 1.8, most preferably between 1.5 and 1.7.
This slurry may of course contain in part other acid components such as isophthalic acid, 5-sodium sulfoisophthalic acid, adipic acid, sebacic acid, naphthalene dicarboxylic acid, diphenylsulfone dicarboxylic acid, etc. or other glycol components such as tetramethylene glycol. , neopentyl glycol, 1,
4-cyclohexanedimethanol and the like may be contained to an extent not exceeding 30 mol%. Further, in order to suppress the diethylene glycol (DEG) concentration, the esterification reaction is usually carried out at a gauge pressure of 0.5 Kg/cm ² or less, preferably 0.15 Kg/cm ² or less. On the other hand, the temperature of esterification reaction is usually 220-270
℃, preferably 230-260℃, optimally 240-260℃
It is. If the temperature is below 220°C, the esterification reaction will not substantially proceed, while if it exceeds 270°C, the DEG concentration will increase, both of which are not preferred. The reaction rate of BHET supplied to the next step, the polycondensation step, is usually 90% or more, preferably 90 to 98%, and optimally 94 to 96%, but the reaction rate of the obtained BHET may vary. As a result, the polycondensation reaction rate in the polycondensation process fluctuates and the quality of the resulting polyester varies, so it is essential to stably produce BHET with a constant esterification reaction rate in order to obtain high-quality polyester. Become. Now, regarding the relationship between the reaction rate and conductivity of the esterification product, which is the gist of the present invention, as the esterification reaction rate increases, the amount of unreacted TPA and the carboxyl terminal group of the partially esterified product decreases. As the temperature increases, the conductivity is expected to decrease linearly. However, when we actually plot the reaction rate and conductivity calculated from the acid value and saponification value of the esterified product measured according to the conventional method, we get the result shown in Figure 1, which shows a simple linear relationship. Must not be. Furthermore, the conductivity of the esterified product varies considerably due to the amount of TPA dissolved in BEHT and the effects of water and EG produced as a result of the reaction. Therefore, in order to detect a slight difference in reaction rate, it is necessary to use a considerably large number of samples and take the average value. One means of achieving this is control technology, including microcomputers. Recent advances in microcomputers have been remarkable, and there are many examples of microcomputers being put into practical use to monitor and control reactions online. The method of the present invention also employs a system using a microcomputer for the above-mentioned reasons.
It is desirable to have a system like the one shown in the figure. In FIG. 2, the electrode section and the detection section (electrometer) must of course be installed on site, but the rest can be installed in the control room. However, if the operational amplifier section is installed in the control room, the electrical signal from the detection section is weak and noise is likely to enter when the field and control room are far apart, so it is preferable to install the operational amplifier section in the field. A signal is amplified by the operational amplifier section by inputting a command from the input section, is converted into digital data by an A/D converter, is inputted to a microcomputer, is subjected to necessary calculations, and is displayed on a display section such as a CRT. In the present invention, the average value of the conductivity is used when determining the reaction rate from the conductivity, but the number of measurements, which is the number of samples, must be 100 times per second or more for 5 to 600 seconds. If the number of measurements is less than 500, noise cannot be removed sufficiently, resulting in poor accuracy, which is not preferable. On the other hand, if the number of measurements exceeds 60,000, the time delay becomes large, which is not preferable. Furthermore, since the electrical conductivity of the esterification product is affected by temperature, pressure, air bubbles, etc., it is preferable to keep the temperature and pressure constant in order to increase the accuracy of measuring the reaction rate. Therefore, when using the method of the present invention, it is preferable not to use temperature or pressure as a control factor for the reaction rate, but to use the amount of EG supplied into the reaction tank. In order to prevent bubbles from forming,
Conductivity measurements must be performed under pressure;
It is desirable that the pressure during conductivity measurement is usually 5 kg/cm ² or more, preferably 10 kg/cm ² or more. Further, the current passed between the electrodes may be either direct current or alternating current, but in the case of direct current, a slight polarization effect may occur, causing variations, and undesirable phenomena may occur. The voltage applied between the electrodes is determined by the properties of the BHET to be manufactured, such as the constituent raw materials and the desired reaction rate, and is not a one-size-fits-all task, so it is desirable to make it variable, but it is usually 25 to 1000 V,
Preferably 50 to 500V is suitable. When carrying out the method of the present invention, it is preferable to use an electrode section having a structure as shown in FIG. In Figure 3, 1 is a transfer channel for the esterification product, 2 is a heat medium channel, 3 and 4 are valves, 5 and 6 are insulators, 7 is a + electrode, 8 is a boost pump, and 9 is a temperature detection The end, 10, indicates a pressure detection end. A portion of the esterification product flowing through the flow path 1 passes through the valve 3 and flows into the branch pipe, is pressurized by the booster pump 8, and conductivity is measured by the electrode 7. The electrical conductivity is determined by the following formula. Conductivity = I・l/V・S [V: applied voltage, I: current, l: distance between electrodes,
S: Electrode surface area] Note that the conductivity of the esterification product is extremely low, and ionic compounds such as alkali metals and alkaline earth metals such as sodium, potassium, calcium, and magnesium acetates can be used as esterification reaction catalysts. When adding salt etc.
Since these exhibit significantly high electrical conductivity, it is necessary to make corrections depending on the amount of these added. As explained above, the reaction rate control method of the present invention not only enables rapid online continuous control, but also provides accurate reaction rates that are superior to conventional manual analysis methods. Therefore, if the present invention is utilized for process automation, it will not only improve accuracy and quality but also have a tremendous effect in reducing man-hours and labor. Hereinafter, the present invention will be specifically explained with reference to Examples. (“Parts” indicate parts by weight.) Example 1 and Comparative Example 1 A slurry with a TPA/EG molar ratio of 1/1.6 was continuously supplied at a rate of 110 parts/hr to an esterification tank containing BHET, and the reaction temperature was Esterification was carried out at 250° C., at a pressure of 0.05 Kg/cm ² G, and at an average residence time of 9 hours. that time,
Using the system shown in Figure 2, measure the electrical conductivity of the esterification product at a pressure of 6 kg/cm ² as shown in Figure 3, and read the measured value using a microcomputer at the frequency and time shown in Table 1. The esterification reaction rate was determined from the average value of , and the EG addition rate was manipulated as a control factor.
The reaction rate was controlled to be 88%. 3 from the average value, standard deviation and target value of esterification reaction rate for 10 days
Table 1 shows the number of times the results were off by % or more. Note that Reference Example 1 has been added, but this is an example in which the reaction rate was determined and controlled by measuring the acid value and saponification value once every hour.

[Table] Examples 2 to 4 and Comparative Examples 2 to 3 The ester product obtained in Example 1 was continuously supplied to the esterification tank containing BHET, and the reaction temperature was 260°C.
The reaction was carried out in the same manner as in Example 1, except that the reaction was carried out at a temperature of 0.05 Kg/cm ² G, a pressure of 0.05 Kg/cm 2 G, and an average residence time of 2 hours, and the esterification reaction rate was controlled to be 96%. Table 2 shows the average value of the esterification reaction rate for 10 days, the standard deviation, and the number of times it deviated from the target value by 1% or more. Note that Reference Example 2 is an example in which the reaction rate was determined and controlled by measuring the acid value and saponification value once every hour.

【table】 [Brief explanation of the drawing]

Figure 1 is a diagram showing the relationship between the reaction rate and conductivity of the esterification product, Figure 2 is a block diagram showing an example of a reaction rate control system for esterification reaction, and Figure 3 is a diagram showing the relationship between the reaction rate and conductivity of the esterification product. It is a figure showing an example of the composition of the electrode part of. 1... Esterification product transfer channel, 7... Electrode.

Claims (1)

[Claims] 1. When esterifying terephthalic acid or a dicarboxylic acid mainly composed of terephthalic acid and ethylene glycol or a glycol mainly composed of terephthalic acid, a part of the esterified product is led out of the reactor and its electrical conductivity is evaluated under pressure. An esterification method characterized by measuring 100 times per second or more for 5 to 600 seconds using a measuring device connected to a microcomputer, and controlling the esterification reaction rate based on the average value.