WO2005116105A1

WO2005116105A1 - Method for process control during the production of polyesters or copolyesters

Info

Publication number: WO2005116105A1
Application number: PCT/EP2005/001793
Authority: WO
Inventors: Michael Reisen
Original assignee: Zimmer Aktiengesellschaft
Priority date: 2004-05-07
Filing date: 2005-02-22
Publication date: 2005-12-08
Also published as: DE102004022653A1; TW200538492A

Abstract

The invention relates to a method for producing polyesters or copolyesters, whereby the end reactor is controlled in terms of the measurement of the temperature, the difference in pressure, and the volume flow in the main melt flow.

Description

Process control process in the manufacture of polyesters or copolyesters

The invention relates to a process for producing a polyester or copolymer ester, the viscosity of the end product being controlled by measuring the viscosity of the main stream from the end reactor.

Processes for the production of polyesters and copolyesters are known (US 3,499,873, EP 0 532 988, DE 35 44 551, US 6,582,080). Conventionally, the raw materials are first reacted in an esterification reaction to give a hydroxyalkyldicarboxylic acid monomer or oligomer mixture. This partially esterified oligomer is then subjected to a prepolycondensation, a prepolymer and condensation products and a reaction gas, essentially consisting of bifunctional alcohols and water, being obtained. The prepolymer is then subjected to polycondensation in a final reactor in order to adjust the degree of polymerization or the molecular weight of the polyester to the desired value. Suitable end reactors are known, for example, from US 3,617,225.

The desired degree of polymerization depends strongly on the intended use of the polymer and must be adhered to within narrow limits in order to ensure the production safety and uniform properties of the end products produced from the polymer melt.

Instead of molar mass or degree of polymerization, the directly accessible viscosity of the polymer melt is usually measured for the purpose of production control. These quantities are related via known relationships which are readily accessible to the person skilled in the art, such as the Mark-Houwink relationship (see, for example, Hoffmann / Krömer / Kuhn, Polymeranalytik, Stuttgart, 1977). The viscosity of the melt emerging from the final reactor is usually checked online using a viscometer. Based on the measurement result, the conditions in the final reactor, e.g. B. regulated pressure, temperature or dwell time to the required values.

Various measuring principles are used for these online viscometers:

Capillary viscometers measure the viscosity in a partial stream which branches off from the main melt stream emerging from the final reactor and is usually introduced again after the measurement. Suitable methods and devices are known for. B. from EP 0 595 276, WO 00/66993 and EP 0 769 690. The disadvantages of this method are that an expensive, additional measuring device is necessary,

- the properties of only a small partial flow are measured, the partial flow can change due to the treatment in the viscometer (strong shear, different dwell time, different temperature) and can thus falsify the measurement itself a gradual adjustment of the measuring device is not noticed immediately and thus negatively influences the reactor procedure. additional calibration and maintenance is necessary.

Measurement using an outflow viscometer is also known, for example from US Pat. No. 6,405,579. Here, a small partial flow constantly emerges from the main product line through a defined nozzle. The outflowing amount is continuous, e.g. B. detected by means of a scale. Since this amount is directly correlated with the melt viscosity under otherwise constant conditions, this can also be used to monitor the melt viscosity and regulate the final reactor. The disadvantages of this method are that only the properties of a small partial flow are measured, with the continuously operating balance also an expensive additional device is necessary, due to the emerging quantity, constant monitoring and cleaning of the balance and the nozzle is necessary,

product is constantly lost.

One method that avoids the disadvantages of the partial flows is the measurement by means of rotation or oscillation rheometers (see, for example, Hoffmann / Kromer / Kuhn, Polymeranalytik, Stuttgart, 1977) directly in the main melt flow. Here, externally driven rotating bodies are located directly in the product line, where they record the properties of the melt flowing around them. However, due to the temperatures (> 280 ° C) and pressures (> 20 bar) that occur in the product line, there are various difficulties with regard to sealing, avoiding influences from the melt flow, formation of dead zones, etc. Thermal chain degradation can also take place in the laminar layer around the rotating body, which changes the physical properties of this layer compared to that of the main melt stream and thus falsifies the measurement result. Therefore, this method is also associated with high equipment and maintenance costs and a high susceptibility to failure.

The object of the present invention is therefore to provide a method for controlling the final reactor which avoids the disadvantages mentioned above. This object is achieved according to the invention by a method in which the end reactor is controlled on the basis of the measurement of at least one temperature, at least one pressure difference and the volume flow in the main melt flow. The main melt stream is the melt stream that flows between the end reactor of the polycondensation and the deformation unit in which molded articles are formed from the polymer melt by solidification. Fibers, films and foils as well as strands that are subsequently granulated and all other objects that can be formed from the polymer melt can be considered as shaped bodies. Correspondingly, for example, perforated nozzles, flat or round slot nozzles, film casters or casting heads of granulators are used as deformation units. It is also conceivable to use injection molding units for the production of any shaped articles.

The melt is discharged from the final reactor by means of a discharge pump, usually a large gear pump. Gear pumps work volumetrically, i. H. The volume they deliver can be determined independently of the viscosity of the melt being pumped.

The main melt stream is passed through a polymer melt filter to remove impurities.

According to the invention, the viscosity is calculated from the measured values of two pressure transducers arranged one behind the other in the main melt flow, the temperature of the melt between these two pressure transducers and the volume flow of the melt known from the gear pump and, if this viscosity deviates from the target value, the final reactor is controlled so that the target value is restored is achieved. For example, if the viscosity drops below the setpoint, the pressure in the reactor will be reduced and if the viscosity rises above the setpoint, the pressure will be increased. However, further intervention options are not excluded. In analogy to the Hagen Poisseuiile equation, the pressure loss Dp in laminar flow is proportional to the dynamic viscosity multiplied by the volume flow V _t . The proportionality constant K _η can be derived approximately from the geometry (for a tube with an inner diameter D and length L, for example, it is 128 * L / (7 ^* D ⁴ )). However, it is better to determine the constant at the start of the system from measured values in the same way that the device constant is determined with the capillary viscometer. Since many polymers have a shear-dependent viscosity, an additional constant Kscher may need to be adjusted if the known flow law is known. η = K _η * y * ficher (K _shear , η, _r , ...)

A. In the simple case of an unbranched section after the discharge pump (or before a booster pump), the viscosity can be calculated with sufficient accuracy by measuring the differential pressure, an average temperature and knowing the volume flow from the pump speed.

B. In the case of the branching of the known main volume flow onto two or more parallel partial flows, for example to continuous casters connected in parallel, the viscosity can be calculated by measuring the pressures and the temperatures in front of the continuous casters, after one has considered the route parameters K ,, and Ksc _h er of the individual parallel routes determined. An equation system for determining the viscosity and the partial flows results from the pressure loss relationships for the individual parallel lines and the summation of the partial flows to form the known main flow. The precise knowledge of the partial flows can be used, for example, to better control the individual take-off speeds, in order to produce more uniform granules. Surprisingly, it is not necessary for the quality of the reactor monitoring according to the invention that the pressure and temperature measurements have to be carried out directly at the product outlet of the final reactor or directly after the discharge pump. The changes that can be determined in the later course of the main melt stream are also still suitable for monitoring the final reactor, taking into account the extended control dead times.

Today's polyester plants use final reactors with a production output of up to 1000 t polyester per day. It is therefore possible and customary to supply several deformation units with the polymer melt from an end reactor. Special melt distributors are used for this. The division into several partial streams can be carried out by means of a single melt distributor or a branching arrangement of several melt distributors. Suitable melt distributors are e.g. B. described in EP 615 006.

For larger systems, e.g. B. in direct spinning mills, in which the polymer melt is spun directly from the final reactor to fibers or filaments without intermediate granulate stage, at least one booster pump is necessary between the discharge pump and the deformation unit due to the large pressure loss during melt transport and for the safe supply of all deformation units. This can be arranged before or after the melt distributor. In a preferred embodiment, the pressure difference between the polymer filter and the booster pump is measured.

In order to distribute the melt evenly over the deformation units, it has previously been customary to provide pressure sensors behind the melt distributor, with the aid of whose measured values the melt distributor is controlled. In plants in which polyester granulate is produced, these pressure transducers are usually installed in the granulating devices. The temperature of the melt is measured in front of the melt distributor. Measurements of the melt temperature after the melt distributor are also possible. In a further preferred embodiment of the invention, the measured values of all pressure sensors, the melt temperature and the known volume flow of the discharge pump are used to determine the IV. If this IV deviates from a target value, the final reactor is readjusted accordingly.

Two possible embodiments of the invention are described in more detail below, but without restricting the invention to these two configurations:

Fig. 1 shows a plant for the production of polyester granules, which can be used for example for the production of beverage bottles. The melt is passed from the end reactor 1 by means of a discharge pump 2 through a polymer melt filter 3 past a temperature sensor Ti installed in the pipeline, which measures the temperature T, to a melt distributor 4. The melt throughput V is calculated from the known delivery volume per revolution and the speed of the discharge pump 2. In the melt distributor 4, the melt is divided into several (four are shown here) granulators 5. The pressure transducers P1, P2, P3 and P4, which deliver the pressure measurement values pi - p ₄ , sit directly in the parts of these granulators known as continuous casters and measure the differential pressure via the casting head to the atmosphere. The measured values are fed into the process control system in which the current IV is calculated. Depending on the difference of this value from the specified target value of the IV, intervention is now made in the reactor control, for example by changing the pressure in the reactor space via the vacuum control PC.

Fig. 2 shows a direct spinning. This corresponds to the polymer melt filter 3 of the plant shown in FIG. 1. After filter 3, however, there is the difference pressure measuring section between the two pressure sensors P5 and P6, which supply the pressure measured values. The temperature of the main melt flow is measured between the two pressure sensors using the temperature sensor Ti1. After this pressure measuring section there is a pressure booster pump 6, which takes over the pressure build-up required for the distribution of the main melt flow via the melt distributor 4 to the individual spinning beams 7. The measured values are fed into the process control system in which the current IV is calculated. Depending on the difference between this value and the specified target value of the IV, it is now also possible to intervene in the reactor control.

In many spinning mills, there is a possibility to discharge polymer melt in front of the melt distributor. This is usually a granulator 8, which produces chips from polymer that is not currently required in the spinning mill, which can be melted again later and used, for example, for fiber, film or bottle production. In this case, the discharge takes place, for example, between the polymer filter 3 and the first pressure sensor P5. This melt flow is determined via the difference between the pump currents and, using the pressure sensor P7 and the temperature sensor Ti2, also allows the viscosity to be calculated using the process control system.

The mode of operation of the method according to the invention is illustrated below using examples.

example 1

In a plant for the production of PBT with only one pelletizer, the viscosity of the discharge pump, the

Temperature in the discharge line and the pressure in front of the pouring head net. 3 shows the good agreement of the two values over several days after adjustment of the proportionality constant K _SC .

Example 2

In a plant for the production of bottle PET with two granulators, the viscosity was calculated from the speed of the discharge pump, the temperature in the discharge line and the pressures in front of the casting heads and used for viscosity control. It turned out that the quality of this regulation was at least as good as with a conventional viscometer. The target viscosity of 0.625 dl / g could be maintained with a standard deviation of 0.0015 dl / g (± 0.004 dl / g absolute).

Claims

claims:

1. A process for producing a polyester or copolyester, in which the final reactor is controlled on the basis of the measurement of at least one temperature, at least one pressure difference and the volume flow in the main melt flow.

2. The method according to claim 1, wherein the viscosity is calculated on the basis of the pressures and the temperatures before one or more continuous casters.

3. The method according to claim 1 and 2, wherein the calculated partial flows are used to control the withdrawal speed of the individual granulators.

4. The method of claim 1, wherein the pressure difference and the temperature is measured before a booster pump.