JPS6143369B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for optimizing liquid level control in the continuous production of synthetic linear polyesters, particularly homogeneous polyethylene terephthalate or copolymers thereof. A method for continuously producing highly polymerized polyethylene terephthalate from monomers including bis-(β-hydroxyethyl) terephthalate or its low polymer by a melt polycondensation method is already known. For example, a mixture of high-purity terephthalic acid and ethylene glycol with predetermined amounts of additives and catalysts is first continuously fed into an initial polymerization vessel.
Ethylene glycol is discharged from the reaction solution and becomes an intermediate polymer. This intermediate polymer is sent to the next reactor for further polycondensation, and if necessary, further
It is sent to the next reactor and finished into polyethylene terephthalate with a high degree of polymerization, resulting in a polymer that can form films or fibers, and then sent continuously to a chip manufacturing device or a direct spinning device. Adoption of such a continuous polymerization method has brought into focus the problems of residence time and flow rate control in the polymerization process. Particularly in the high viscosity region, variations in the liquid level in the reactor at the final stage directly manifest as variations in the quality of the polymer, so controlling the liquid level becomes important. Usually, the amount of catalyst, reaction temperature,
There are various factors such as stirring speed, residence time, degree of vacuum, etc. Among these, the degree of polymerization has a quick response to changes in degree of vacuum and has a large effect, so control of degree of vacuum is generally widely used as a control means in industry. There is. That is,
The melt viscosity of the polymer at the reactor outlet is continuously measured online, and the degree of vacuum is controlled in a cascade to keep this value constant. Such methods are based on the assumption that other factors remain constant, namely reaction temperature, catalyst amount, stirring speed, and residence time. It is industrially possible to control the reaction temperature, catalyst amount, and stirring speed to be approximately constant, and since each is an independent control system and there is almost no mutual interference, it is safe to assume that they are constant. However, regarding the residence time, industrially, the amount continuously taken out from the reactor is not necessarily constant. For example, even when manufacturing chips directly from a reactor, failures and maintenance of chip manufacturing equipment,
Discharge amount may vary due to production planning. In particular, when using the direct spinning method, fluctuations in the output amount can be avoided in industrial production due to problems such as replacing the spinneret or spinning gear pump at regular intervals in a multi-spindle spinning device. It's tough. Furthermore, a change in the discharge amount is unavoidable due to a change in spinning conditions, such as a change in spinning speed or a change in denier. In order to cope with these changes in the discharge amount, a method is used in which a constant amount of polymer is constantly discharged from between the final reactor and the spinning device by a method such as chipping, and system fluctuations are absorbed by this method. There is also. For industrial production equipment, this method of making the final reactor system constant by discharging it outside the system is not preferred because it causes a large loss from both the raw material and management standpoints. The present inventors have made the present invention as a result of intensive research into a control system that can efficiently produce polyethylene terephthalate of a constant quality by keeping the liquid level constant in the final reactor that processes especially highly viscous reactants. Ivy. That is, the present invention aims to maintain a constant liquid level in a reaction apparatus when continuously polycondensing a polyester monomer mainly consisting of bis(β-hydroxyethyl) terephthalate or a low polymer thereof in a molten state. In order to do this, the signals from one or more liquid level gauges installed in the reactor and the signals from the melt viscometer installed at the outlet of the reactor are input into a computer, the liquid level is calculated, and the liquid level is calculated at regular intervals. In order to control the liquid level, the loop that controls the rotation speed of the metering pump installed in the reaction device and the change in the rotation speed of the metering pump installed at the outlet of the reaction device are input into a computer, and the amount of change in the metering pump is calculated. The liquid level in the reactor is controlled at a constant level using a computer using two loops: a loop that predicts and controls the amount of feed sent to the reactor by multiplying by a contribution coefficient, and a loop that controls the rotation speed of the metering pump. It is characterized by: The final reactor of polyethylene terephthalate is
In order to proceed with the reaction efficiently, stirrers of various shapes are installed to constantly renew the polymer surface or form a thin film, and the polymer liquid level in the reactor changes depending on the viscosity of the polymer. It is something to do. Therefore, it cannot be measured simply like measuring a horizontal surface, but requires measurement and averaging at certain time intervals.Also, the liquid level at the inlet of the final reactor and the liquid level at the outlet are different. Since the liquid level varies depending on the progress of the reaction, the true liquid level can be calculated by measuring the liquid level at both the inlet and the outlet and calculating the level using that. Furthermore, since the temporal response of this liquid level fluctuation does not respond immediately based on changes in throughput, it is necessary to predict changes in throughput. Therefore, the present inventors developed a metering pump (metering pump B) to directly feed the polymer to the spinning process.
The rotation speed of the metering pump (metering pump
The rotation speed of the metering pump B is changed by the amount of change of the metering pump B multiplied by the contribution coefficient, thereby providing a function to speed up the response to changes in throughput. The contribution coefficient indicates the proportion of the amount fluctuation in the direct spinning process or the chipping process that is fed forward to metering pump A, and the rest is the signal of the fluctuation of the liquid level gauge (usually several minutes to
(There is a delay of about one hour). In the final reactor, the reactants are highly viscous (2000
The viscosity increases from about 25,000 poise to about 25,000 boads)
Therefore, a liquid level gauge (for example, a Co-60 gamma ray liquid level gauge) is installed at two locations, one at the inlet and one at the outlet, and the liquid level is averaged over time, and the liquid level is used as input for each liquid level. It is desirable to calculate the true liquid level. The true liquid level represents the amount of lipolymer retained in the final reactor, and the level is calculated by multiplying the indications of the inlet level gauge and outlet level gauge by a certain coefficient, and the level is determined based on the calculated value. Level control is performed by controlling the rotation speed of the metering pump A so that the liquid level is maintained. In other words, the rotation of metering pump A is controlled based on the feedforward control that controls the rotation speed of metering pump A based on the fluctuation of metering pump B, and the level calculation value calculated from the indications of the level meters installed at two locations. The liquid level in the final reactor for polyethylene terephthalate is controlled using a control system that combines two types of control: feedback control that controls the number of reactors. The above explanation has been about liquid level control only in the final reactor, but since the polyethylene tereftate production process generally consists of multiple reactors, we have focused on controlling the amount of stagnation in the reactor preceding the final reactor. The entire reaction system can also be controlled using a similar method. However, in the low viscosity region, it is not necessarily necessary to install level gauges at two locations, one at the inlet and one at the outlet, and industrially it is sufficient to install a level gauge at one location near the outlet. Although the present invention will be explained using drawings to clarify the present invention, the present invention is not limited thereto. FIG. 1 is a schematic diagram for clarifying the liquid level and throughput control of the final reactor. 1 is the final reactor, 2 is the metering pump that sends the polymer to the final reactor, 3 is the inlet level gauge of the final reactor,
4 is a liquid level gauge at the exit of the final reactor, 5 is a metering pump for directly sending liquid to the spinning process, 6 is a control valve for vacuum control of the final reactor, 7 is a rotation speed detection device for the metering pump, and 8 is a metering pump for directly sending liquid to the spinning process. A metering pump rotation speed control device, 9 a metering pump rotation speed detection device, 10 a liquid level calculation program in the computer, 11 a liquid level control program in the computer, 12 a computer, and 13 a direct spinning process. 14 is a melt viscometer. A disk is loaded into the final reactor 1.
It rotates at about ~5 rpm, and there are Co-60 gamma ray level meters 3 and 4 at the inlet and outlet of 1, which indicate the liquid level inside 1. The liquid level is indicated by the fact that the amount of polymer lifted by the disk varies depending on the viscosity of the polymer (1), so as the viscosity increases, the apparent liquid level is indicated to have decreased. Reference numeral 5 denotes a metering pump for directly feeding the liquid to the spinning process 13, and the indication of the detection device 9 that detects the rotation speed of 5 changes depending on the fluctuation in the amount of spinning in the direct spinning process. The calculator 12 detects the fluctuation of 9,
A speed change is input to the speed control device 8 of the metering pump 2 for feeding the polymer to the metering pump 1. That is, a correction operation is input from the level control loop 11 in the computer to the level control loop ₈ in the computer such that, for example, the variation δ ₁ Kg/mm of 5 is multiplied by a contribution coefficient of 0.75 to correct it by 0.75 δ 1 Kg/mm. In this correction operation, since the time delay at 1 is large in the feed forward operation, the disturbance at 13 is quickly detected and corrected. In such an industrial system, errors in the metering pump and other disturbances in temperature and viscosity are included, so the signals from the inlet level gauge 3 and outlet level gauge 4 are input into the computer, and the According to the liquid level meter program, the actual liquid level of 1 is calculated and a correction operation, which is a feedback operation, is outputted to 8 through 11.
When calculating the liquid level, the liquid level indication, especially the indication 4, changes depending on the viscosity of the polymer in 1, so the viscosity term detected by the viscometer 14 is reflected in the liquid level calculation. As an example, to quantitatively illustrate the contents of FIG. 1, the following constants and formulas are used. That is, signals 3 and 4 are taken in at intervals of 5 seconds for liquid level calculation, and are A/D converted and input into the computer. The input signal is smoothed and the liquid level is calculated every minute using the following calculation formula. Furthermore, the viscosity signal from 14 is also A/D converted and input to the computer. LK=αL ₁ +(1-α)L ₂ +βV+C LK: Calculated liquid level α: Contribution rate of inlet level gauge and outlet level gauge β: Contribution rate of viscosity term L ₁ : Signal of inlet level gauge L ₂ : Normalized signal from the outlet level gauge V: Normalized signal from the viscometer C: Constant term Normally α has a value of 0.50 to 0.80, and β has a value of 0.05 to 0.10. Although used, it is not particularly limited to this value. The liquid level calculation value calculated every minute is sent to 11 as the input of the liquid level control program, and the liquid level control program sends the operation output to 8 every 10 minutes to control the speed of metering pump 2. . These operations keep the liquid level constant through feedback operations, but
For a variation of 13, an operation delay of about 60 minutes occurs. The rotational speed of the metering pump No. 5 is input into the computer every minute, and a level control program is run every minute according to the fluctuation of the rotational speed, and is output as a signal to the speed control No. 8. That is, 11 LC-
The output △Pn of 1 is △Pn ₁ = Kp (LKn _-1 -LKn) +K _I (TV - LKn) +K _D (2LKn _-1 -LKn _-2 -LKn) LKn: Input from 3, 4 and 14 Liquid level calculated based on liquid level signal. n is the current calculated value, n-1 is the value from the previous time (10 minutes ago in this case), and n-2 is the value from the time before last (20 minutes ago in this case).
indicates the value of TV: Target value of liquid level Kp: Ratio constant K _I : Integral constant K _d : Differential constant Feedback control signal △Pn ₁ every 10 minutes and feedback control signal △Pn ₂ every 1 minute are output. . △Pn ₂ = K (Sn _-1 -Sn) S: Rotation speed of metering pump 5, n is the current value n
-1 is the previous value (in this case, one minute ago). K: Constant of proportionality Usually this value is around 0.75 and is 5
Approximately 75% of the variation in the rotation speed of 2 is immediately reflected in the rotation speed of the metering pump 2. Therefore, the control output is the sum of the feedback output for liquid level control every 10 minutes and the feedback output based on the rotational speed fluctuation of the metering pump for directly sending the polymer to the spinning process every minute. Output. △Pn = △Pn ₁ + △Pn ₂ A schematic time chart of the operating status of the control output is shown in Fig. 3. That is, the liquid level in the reactor 1 is kept constant by outputting the feedback output signal ○a every minute and the feedback output signal ro every 10 minutes. FIG. 2 is a schematic diagram for explaining liquid level and throughput control in which a plurality of reactors (final reactor 1 and pre-stage reactor 26) are connected. 1 to 14 are the same as shown in FIG. 1, 15 is a rotation control device for the metering pump 5, 16 is a pressure control device in the direct spinning process liquid feed pipe, and 17 is for controlling the viscosity of the polymer coming out from 1. A control device, 18 is a control device for predicting and controlling the viscosity of the polymer sent to 1, 19 is a melt viscometer, 20 is a control device for controlling the viscosity of the polymer coming out from 26, 2
1 is the vacuum control valve of 26, 22 is the rotation speed detection device of the metering pump, 23 is the liquid level gauge of 26, and 24 is the 2
metering pump for feeding the polymer to 6, 25
is a metering pump rotation speed control device. 16 due to load fluctuations in the direct spinning process 13.
The speed of the metering pump 5 of SC-2B of 15 is controlled by the pressure control of PC-3.The number of revolutions of metering pump 5 is detected by Si-2B of 9,
The rotational speed of the metering pump 2 is controlled by a control system similar to that described in the figure, and the throughput of the system of the pre-stage reactor 26 is similarly controlled by variations in the metering pump 2. In this example, the Co-60 gamma ray level gauge is in the front reactor 2.
6, it is installed only at the exit, and 23 Li
-1 provides feedback to SC-1A of the speed control 25 of the metering pump 24. It is exactly the same as in reactor 1 that the feed forward to 24 is done based on the variations of course. In this example, a melt viscometer 14 is installed at the outlet of each reactor.
19 are installed, feedback control of reactor 26 is performed in VC-1 of 20, and VC-1 of 18 is installed.
-2 performs feedback control of reactor 1, and VC-3 of 17 performs feedback control of reactor 1. Example Bis-β-hydroxyethyl terephthalate was continuously supplied to the first reactor at a rate of about 100 kg per hour, the reaction temperature was controlled at 265°C, the degree of vacuum was 20 mmHg, the residence time was 2 hours, and the intrinsic viscosity was 0.30 was obtained and continuously supplied to the next second reactor, the reaction temperature was controlled at 275℃, the degree of vacuum was 3mmHg, the residence time was 2.5 hours, and the intrinsic viscosity was 0.58.
was obtained and continuously supplied to the next third reactor (final reactor), the reaction temperature was controlled at 275°C, the degree of vacuum was 0.5 mmHg, the residence time was 2.5 hours, and a product with an intrinsic viscosity of 0.94 was obtained. . Intrinsic viscosity [η] is dissolved in a mixed solvent of phenol:tetrachloroethane=1:1 using an Ubbelohde viscometer at a ratio of 0.5 g/100 c.c.
This is a value calculated from the solution viscosity ηrel obtained by measurement at 20°C using the following formula. [η] = 1.458ηrel - 1.375 Instead of measuring the intrinsic viscosity, a melt viscometer is installed at the outlet of the reactor, and the degree of vacuum inside the reactor is controlled in a cascade to keep the melt viscosity constant. In addition, a thin film forming disk is installed inside the reactor to promote the polycondensation reaction, and it rotates at approximately 2 rpm. A metering pump of 100 c.c./rev is installed between each reactor, and the third reactor is connected to a direct spinning process with six spindles. 100 millicuries of Co-6 to measure the outlet liquid level of each reactor
A 0 level gauge is installed, and only the third reactor has a level gauge installed at the inlet. By controlling the liquid level according to the present invention, the operating method shown in Fig. 4 was carried out during 3 days of continuous operation, and the melt viscosity at the outlet of the second and third reactors and the intrinsic viscosity of the yarn were adjusted for one hour. Table 1 shows the results of each sampling and measurement. In FIG. 4, the idle time for changing the spinneret is 7 minutes per 11 times. During that time, the processing amount is 1/
This will result in a decrease of 6 to 2/6. ( indicates a reduction of 1/6 when one spindle was replaced, and 2/6 when two spindles were replaced at the same time.) The throughput was 80Kg/hr for the first time, 65Kg/hr for the second time,
The third time, the polymer was directly fed to the spinning process at 72 kg/hr. Comparative Example 1 The results were obtained by operating under the same conditions as in Example 1, and when the throughput fluctuated without liquid level control, the rotation speed of the metering pump was manually changed and checked in the same manner as in Example 1. are shown in Table 1. Comparative Example 2 The operation was carried out under the same conditions as in Example 1, and the throughput of the polymerization reaction system was kept constant by discharging the polymer from between the final reactor and the direct spinning process to the outside of the system using a tipping device. The results are shown in Table 1. Comparative Example 3 Table 1 shows the results of operation under the same conditions as in Example 1, using only feedback control without performing the feed forward control of the present invention. 【table】

[Brief explanation of the drawing]

FIGS. 1 and 2 are schematic diagrams explaining the control method of the present invention, FIG. 3 is a schematic diagram of control output, and FIG.
The figure is a diagram showing fluctuations in the throughput of the reaction system in Examples. 1: Final reactor, 26: First stage reactor, 3, 4,
23: Liquid level gauge, 14, 19: Melt viscometer, 12:
calculator.

Claims (1)

[Claims] 1. To maintain a constant liquid level in a reaction device when polycondensing polyester monomers mainly consisting of bis(β-hydroxyel) terephthalate or their low polymers in a molten state. In order to do this, the signals from one or more liquid level gauges installed in the reactor and the signals from the melt viscometer installed at the outlet of the reactor are input into a computer, the liquid level is calculated, and the liquid level is measured at regular intervals. In order to perform surface control, a loop that controls the rotation speed of the metering pump installed in the reaction device and changes in the rotation speed of the metering pump installed at the outlet of the reaction device are input into a computer, and the amount of change in the metering pump is calculated. The liquid level in the reactor is controlled to be constant using a computer using two loops: a loop that predicts and controls the amount of supply sent to the reactor by multiplying by a parasitic coefficient, and a loop that controls the rotation speed of the metering pump. A continuous polyester polymerization method characterized by the following. 2. The method according to claim 1, wherein liquid level gauges are installed in the final reactor at two locations, one at the inlet and one at the outlet, and the liquid level in the reactor is controlled using a liquid level calculation value according to the following formula. LK=αL ₁ +(1-α)L ₂ +βV+C LK: Calculated liquid level α: Contribution rate of inlet level gauge and outlet level gauge β: Contribution rate of viscosity term L ₁ : Signal of inlet level gauge L ₂ : Normalized signal from the outlet level meter V: Standardized signal from the viscometer C: Constant term