CZ20004403A3 - Control system for continuous polyamidation process - Google Patents

Abstract

Improved polyamidation system and control system to produce polyamide from molten dicarboxylic acid monomer and molten diamine monomer. The polyamidation control system measures the molar ratio of molten dicarboxylic acid monomer to molten diamine monomer in the partially polymerized mixture. In the control system, a feed control algorithm is used to determine the ratio of the ratio of starting reactants before mixing to produce the resulting polyamide with a stoichiometrically balanced molar ratio.

Description

Control system for continuous polyamidation process

Technical field

The invention relates generally to processes for producing polyamides from dicarboxylic acid monomers and diamine monomers. More specifically, the invention relates to an improved polymerization system and its associated control system which maintains the desired mass flow of molten reactants by measuring the molar ratio of reactants after polymerization has started and adjusting the flow rate of the melt reactants.

BACKGROUND OF THE INVENTION

A major challenge in polyamide production is to ensure that the reactants are mixed sufficiently to balance the number of carboxyl and / or amine end groups in the resulting polyamide. If the reactants are mixed so that an uneven number of carboxyl and / or amine end groups is left, this may negatively affect certain properties of the resulting polyamide. E.g. It has been found that in the manufacture of nylon 6.6 from adipic acid and hexamethylenediamine, unbalanced end groups can adversely affect the colorability of nylon as well as reduce the ability to form high molecular weight 6.6 nylon. As a result, manufacturers emphasize equalizing the molar ratio of reactants during polymerization to maximize the quality of polyamides.

One prior art technique for making polyamides involves a two-step process in which a dicarboxylic acid reacts with a diamine in water to form a salt together, and then the salt is heated to cause polymerization. However, this two-stage polymerization process has the disadvantages of requiring the addition of water and the use of evaporation chambers to remove the water added during salt formation. It is also difficult to control the process to ensure the correct molar equilibrium in the final polymer because the evaporation chambers are very erratic and difficult to model.

Many different control mechanisms were used in this two-step process. One known control mechanism involves physical removal of a material sample during the end group analysis process. However, this is disadvantageous in that the embodiment takes time and tends to introduce inaccuracies, since the samples do not always adequately reflect the properties of the materials during the process.

Another known control mechanism involves performing in-line end group analysis in an attempt to ensure proper molar equilibrium in the resulting polyamide. These embedded control mechanisms are useful in avoiding the need for physical sampling of materials during the process. One technique involves indirectly determining the molar equilibrium by measuring the pH of the salt formed in the two-step process. However, this pH measurement technique is limited because pH is not an accurate indicator of end group equilibrium in the resulting polyamide.

Another technique of in-line end-group analysis involves measuring the ratio of end-groups of molten reactants during polymerization and, based on this determination, injecting an appropriate amount of reactants to the molten polymerization mixture to ensure the desired molar equilibrium in the resulting polyamide. This system is disadvantageous in that it requires expensive additional measuring devices and a system to inject additional reactants into the molten polymerization mixture. This system is also limited in that it requires additional time to mix the subsequently added reactants with the molten polymerization mixture.

Efforts have been made to produce polyamides directly from monomers without the addition of water. However, it has been shown that controlling the degree of binding of the reactants is relatively difficult since excess of one or the other reactants adversely affects the molecular weight and thus the physical properties of the product. Other problems in such direct polymerization processes include degradation of monomers and / or polymer product due to (1) maintenance at high temperatures for long periods of time (eg, several hours), (2) contact of molten monomer with oxygen and (3) exposure to trace metal impurities in materials of which the processing equipment is made.

There is a long-standing need for an improved polymerization system and its control system to overcome the aforementioned disadvantages of the prior art.

SUMMARY OF THE INVENTION

One aspect of the invention is an improved polymerization system for producing a polyamide from a dicarboxylic acid monomer and a diamine monomer. The first measuring means are equipped to measure the supply of molten dicarboxylic acid monomer. The second measuring means is equipped to measure the supply of molten diamine monomer. The first and second measuring means are coupled together so that the molten dicarboxylic acid monomer feed and the molten diamine monomer feed can be combined.

and forming a molten polymerization mixture. There is provided at least one non-vented reaction vessel to polymerize the polymerization mixture. There are means for detecting the molar ratio of molten dicarboxylic acid monomer to molten diamine monomer in the polymerization mixture. The control means are communicatively connected to the detection means and the first and second measuring means. The control means selectively adjusts the mass flow of at least one of the molten dicarboxylic acid monomer and molten diamine monomer feeds to balance the molar ratio of molten dicarboxylic acid monomer and molten diamine monomer in the polymerization mixture.

Another aspect of the invention is a polymerization control system for producing a polyamide from a dicarboxylic acid monomer and a diamine monomer. First means for measuring the supply of molten dicarboxylic acid monomer are provided. There are second means for measuring the molten diamine monomer feed to the molten dicarboxylic acid monomer feed to form a molten polymerization mixture. There are means for detecting the molar ratio of molten dicarboxylic acid monomer to molten diamine monomer in the polymerization mixture. There is a controller communicatively connected to the detection means and to at least one of the first and second measurement means. The regulator controls at least one of the first measurement means and the second measurement means based on the molar ratio input signal from the detection means to adjust the mass flow of at least one of the molten dicarboxylic acid monomer and the molten diamine monomer to be a molar ratio of molten dicarboxylic acid monomer; of the molten diamine monomer in the polymerization mixture.

This control system operates continuously to ensure the polymerization of the reactants in an equilibrium manner with the same number of carboxyl and amine end groups in the resulting polyamide. Any modification of the flow of reactants is made prior to formation of the molten polymerization mixture. That is, the first and second measuring means are adapted to vary the mass flow of at least one of the molten carboxyl monomer and the molten diamine monomer. After mixing, no additional dicarboxylic acid monomer or diamine monomer need be added.

The control system according to the invention is particularly suitable for use in a polymerization system that produces polyamides directly from monomers. Thus, it is not necessary to add water to the dicarboxylic acid, diamine or molten polymerization mixture.

· 4 ·························································· 4 4 · ··· ·· ··

The temperature of the polymerization mixture in the at least one non-vented reaction vessel is between about 220 and about 300 ° C. The overpressure in the at least one non-vented reaction vessel is preferably 0 to 3.45 MPa, more preferably between about 0.34 to 1.72 MPa, most preferably between about 0.83 to 1.24 MPa. The residence time of the polymerization mixture in the at least one non-vented reaction vessel is preferably between about 0.01 minutes to about 30 minutes, more preferably between about 0.5 to 30 minutes, most preferably between about 1 to 5 minutes. The polymerization mixture leaving the at least one non-vented reaction vessel typically contains less than 40 wt. % of unpolymerized monomers, preferably less than 10 wt. unpolymerized polymers.

In certain embodiments, the at least one ventilated reaction vessel may optionally be used downstream of the at least one non-vented reaction vessel to remove water resulting from the polymerization process and / or for further polymerization. When the at least one ventilated reaction vessel is used, the residence time of the polymerization mixture is preferably from about 1 minute to about 60 minutes.

A waste gas recovery system may also be used to recover the gasified diamine monomer and / or dicarboxylic acid monomer in the waste gas produced by the at least one ventilated reaction vessel. This offgas stream typically comprises water vapor and a vaporized diamine monomer. The waste is contacted with the molten dicarboxylic acid monomer in the recovery column, whereby at least a portion of the vaporized diamine monomer reacts with the dicarboxylic acid monomer to form a polyamide. This serves to form a liquid effluent stream in the recovery column, wherein the stream comprises polyamide and unreacted molten dicarboxylic acid monomer. The liquid waste stream can then be mixed with the molten diamine monomer.

In one embodiment, the relative viscosity (RV) of nylon 6.6 in the polymerization mixture exiting the non-vented reaction vessel is between about 0 and about 3 and the relative viscosity of nylon 6.6 in the polymerization mixture exiting the ventilated reaction vessel is between about 3 and about 15. The relative viscosity as used herein represents a viscosity ratio (in Pa.s) of 8.4% by weight. solution of polyamide in 90% formic acid (90% formic acid and 10% water) at 25 ° C to the viscosity (in Pa.s) of 90% formic acid alone at 25 ° C.

The polyamidation process of the invention can produce its resulting product without the need to add water to the reactants and without the intermediate salt formation step. Moreover, the process according to the invention can be carried out continuously and with much shorter residence times

molten reactants and molten polymer in the high temperature compartments of the process. This significantly reduces water consumption, waste water production and process energy consumption. This also eliminates the need or reduces the required size of some process devices found in prior art processes, such as evaporators that have been used to remove process water. Furthermore, exposure of the reactants and the product to excessive temperatures is abolished.

The aspect of the invention relating to the continuous melting of a dicarboxylic acid, such as adipic acid, provides a practical and economical method of continuously supplying molten dicarboxylic acid for use in a polyamidation process or for other uses. The process provides high quality molten acid without discoloration or other thermal degradation. The production of pure molten acid facilitates the production of high quality polyamide.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a block diagram illustrating an improved polyamidation assembly according to the invention; Figure 2 is a block diagram showing a polymerization control system according to the invention; Figure 3 is a second block diagram showing a polymerization control system of the invention as shown in Figure 2; and Figure 4 is a block diagram illustrating another possible polymerization control system according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

The polymerization system and the polymerization control system of the invention can be used to produce various polyamides from diacid and diamine monomers. These systems are particularly useful for producing nylon 6.6 from adipic acid and hexamethylenediamine.

Figure 1 shows a technological flow diagram of one embodiment of the method. Molten hexamethylenediamine (HMD) is supplied from a reservoir 20 of molten HMD. There are several suitable methods of delivering molten HMD. One is to place the polyamidation process apparatus next to the apparatus where the HMD is produced so that the molten HMD stream can be routed directly through the conduit to the reservoir 20. Another way would be to prepare an aqueous HMD solution, evaporate water and melt the HMD.

• · · · ·· ··

Optionally, heat can be applied to this container 20, for example by means of a heat transfer jacket around the container 20. The temperature in this container is preferably about 70 ° C. The molten HMD is then pumped through the HMD metering system 22, which accurately controls the amount of HMD delivered to the following equipment.

Adipic acid, usually in the form of dry crystals, is supplied from the reservoir 24. Adipic acid flows from the reservoir to the pressure tank 26 to eliminate oxygen. In this tank 26 the air is removed. Preferably, air is removed from the tank 26 by cycling the vacuum and displacing nitrogen in a batch mode. Vacuum can be induced by a vacuum pump 28. The cycle rate between vacuum and nitrogen pressure can be adjusted to achieve the desired degree of oxygen removal.

The oxygen removal pressure tank 26 preferably comprises a pressure vessel, the bottom of which forms a hopper of decreasing diameter towards the bottom. Preferably, the sides of the oxygen removal tank of the pressure tank form an angle of at least 70 ° with the horizontal to facilitate flow out of the bottom of the tank.

The adipic acid crystals, which are predominantly deoxygenated, then flow (preferably by gravity, using nitrogen pressure in a pressure tank to remove oxygen) from the pressure tank 26 to remove oxygen to the adipic acid melting vessel 30. The melting vessel 30 is preferably a continuously stirred jacket vessel that operates slightly pressurized with nitrogen at a temperature slightly above the melting point of adipic acid (i.e., above 153 ° C). The adipic acid crystals entering the top of the vessel quickly melt on the surface of the molten adipic acid present therein. Thus, this process can continuously melt adipic acid. Preferably, the melting vessel 30 has an inverted conical inlet orifice to reduce the current resistance. It is also preferred that the melting vessel 30 is made of a metal alloy containing little or no impurities that would adversely affect the molten monomer. Suitable materials are Hastolloy C and 316 stainless steel.

It may be useful to include additional measures to further remove oxygen from the melting vessel to minimize the possibility of thermal degradation. One way to do this is to supply the molten adipic acid in the melting vessel 30 with vibrational energy, e.g., by means of an ultrasonic device. The vibration energy can facilitate the escape of absorbed air from the molten acid by causing air bubbles to escape to the surface of the molten acid.

9 9 ·

9 9 9

9 9 ·

9 9 9

9 99

The residence time of the molten adipic acid in the melting vessel 30 is preferably minimized in order to limit the exposure of the reactant to heat. Preferably, the residence time is less than three hours, more preferably between about 1 to 2 hours. The molten adipic acid exits from the bottom of the melting vessel 30 and is pumped to the adipic acid measuring system 32, which accurately controls the amount of adipic acid fed to the subsequent apparatus.

The combination of the oxygen removal tank 26 and the adipic acid melting vessel 30 allows continuous melting of the adipic acid crystals without thermal degradation or color change.

The HMD measurement system 22 and the adipic acid measurement system 32 supply the molten monomers in stoichiometric amounts so that the molten HMD and the molten adipic acid are mixed in a Y-shaped joint 38 to form a "polymerization mixture" as will be referred to below. In other words, the molten monomers are mixed and mixed in a Y-shaped joint to initiate the polymerization process. The polymerization mixture proceeds through a further pumping section 40 and into an unvented mixer 42, which is preferably a static mixer.

In a preferred embodiment, the molten adipic acid stream 36 is at a temperature of about 170 ° C and the molten HMD stream 34 is at a temperature of about 70 ° C and the overpressure at the Y-shaped joint 38 is about 1.03 MPa. The in-line static mixer is preferably a 24-element Kenics static mixer. The walls of the Y-shaped joint and the intermediate mixer 42 are preferably maintained at a temperature of about 268 ° C. The residence time of the monomers in the mixer 42 is preferably between about 1 to 30 seconds, more preferably about 3 seconds. The polymerization mixture exiting the mixer 42 passes into an unvented conduit, allowing, for example, an additional 10 to 60 seconds reaction time at 260 ° C and an overpressure of 1.03 MPa.

Although the process of the present invention can operate without water in the reactants, it is not required that the reactants be completely anhydrous. E.g. the feed HMD could contain up to about 5 wt. water and the adipic acid stream could contain up to about 2 wt. water and the process should still run properly. Reactant streams with such a low water content are referred to herein as "substantially dry".

Part of the reaction of HMD and adipic acid takes place from the time of their contact in the Y-shaped joint 38 and continues until they enter the heat exchanger 44. The temperature and residence time used in this part of the process can be selected to cause complete polymerization to this point; to prevent complete polymerization from this point. In the second of these two situations, the product of the partial reaction is formed by contact

Up ·· • · · * · ·

The flfl monomers are referred to herein as "prepolymer". The prepolymer mass in the line downstream of the mixer 42 is typically 60-90% converted to nylon 6.6. There should be no clogging as the conditions used prevent crystallization of the low melting point intermediates. For optimum operation, it is important that the conduit 40 and mixer 42 be unvented and that there is a relatively low overpressure, e.g. between about 0 to 3.45 MPa, very preferably about 1.03 MPa.

In the embodiment shown in Fig. 1, the prepolymer further passes through the heat exchanger 44 and into the ventilated prepolymer reactor 46 It is not critical whether a heat exchanger is used herein. Instead, any desired heat can be provided by the internal heating coils in the reactor 46 or by the jacket around the reactor. The heated prepolymer exiting the heat exchanger 44 preferably enters the reactor 46 at a point below the surface of the liquid material contained therein. Further polymerization can take place in this reactor 46, which is preferably a continuously stirred tank reactor. The reactor bottom stream 48 may optionally be divided into a recycle stream 50 and a second stream 52, which is discharged for further processing. When recycling is used, the flow is preferably at least 15 times greater than the flow of fresh prepolymer feed to reactor 46. The reactor 46 is preferably operated about 50% filled with liquid material to provide a large vapor / liquid release surface.

In this process, it is highly desirable to provide back-mixing of the polymer end groups, generation of a large interface surface area that facilitates molten material ejection, and high heat transfer rates where these properties can rapidly increase the temperature of the molten material. These advantages can be achieved, for example, by using a continuously stirred tank reactor or by using a closed flow reactor together with recycling the product stream.

The overhead stream 54 from reactor 46 is a stream containing steam (i.e., vaporized water produced by the polycondensation reaction) and usually some HMD. The upper stream 54 passes to the HMD recovery column 56 to which water is also supplied. The condensate stream 60 containing some HMD and water is returned to the reactor 46 while the remaining steam is cooled by the heat exchanger 62 and removed as part of the off-gas stream 64.

In one embodiment, the prepolymer is heated to about 260 ° C in a heat exchanger 44 and the reactor 46 is operated at a temperature of about 260 ° C and an overpressure of 1.03 MPa. As an example of suitable relative flows, when the fresh prepolymer is fed to the reactor 46 at a rate of 45.4 kg per hour and the recycle flow from the bottom of the reactor is preferably about 907.2 kg per hour. A reactor 46 operating under these conditions can provide more than 95%; Conversion of monomers to nylon 6.6 with a water concentration of 3% by weight. after a residence time in reactor 46 of 20 minutes.

According to the invention, the control system is adapted to adjust the feed rate or mass flow rate of at least one of the molten adipic acid monomer and the molten diamine monomer to ensure the correct molar ratio. In a preferred embodiment, the mass flow of at least one of the reactants is adjusted depending on the balance of the carboxyl end groups and the amine end groups of the reactants in the polymerization mixture. The intermediate end group measurement can be performed at any point after the Y-shaped joint 38. In the illustrated embodiment, the end group equilibrium measurement in stream 52 exiting reactor 46. A preferred method of measuring the carboxyl and amine end groups equilibrium of molten monomers in the polymerization mixture is photospectrometry. . In a preferred embodiment, the near infrared (NIR) analyzer 66 detects the number of carboxyl and amine end groups in the polymerization mixture by determining the spectral photometric content of the monomers contained therein. .

In one embodiment, the control system of the invention includes a near infrared (NIR) device 66, a controller 67 that receives an input signal from the NIR analyzer 66, a HMD measurement system 22, and a molten adipic acid measurement system 32.

The NIR analyzer 66 is, by way of example and not limitation, a device for determining the molar ratio of molten dicarboxylic acid monomer to molten diamine monomer in the polymerization mixture. The NIR analyzer 66 accomplishes this by continuously detecting the number of carboxyl and amine end groups in the partially polymerized material exiting the reactor 46. Although the use of the NIR analyzer 66 is preferred, any number of molar ratio means may be used in the contemplated control system of the invention. molar equilibrium of molten adipic acid and molten diamine during polymerization.

The NIR analyzer 66 generates an input signal to the controller 67 indicating the balance of the carboxyl and amine groups of the molten dicarboxylic acid monomer and the molten diamine monomer in the polymerization mixture. Using this signal, the controller 67 can adjust the mass flow rate of the molten dicarboxylic acid monomer and / or the molten diamine monomer so that the polyamide formed by polymerizing the polymerization mixture has the desired molar ratio. In a preferred embodiment, the controller 67 uses a feed control algorithm that varies the feed rate of the molten diamine monomer depending on the input signal from the NIR analyzer 66. Using this feed control algorithm, the ratio of molten dicarboxylic acid monomer to molten diamine monomer can be controlled to produce a polymerized end product having a predetermined molar ratio. In a preferred embodiment, this is achieved by adjusting the feed rate of the molten diamine monomer with the HMD measuring system 22. The control and operation of the HMD measuring system 22 and the molten adipic acid measuring system 32 will be discussed in more detail below with reference to Figures 2 to 4.

Note that although the NIR analyzer 66 is shown positioned at stream 52, it is contemplated that it may be located at any point beyond the Y-shaped joint 38. For example, the NIR analyzer 66 may be located within the reactor 46 at a point below the liquid level. mixer 42 and reactor 46, within the static mixer 42 or between the static mixer 42 and the Y-shaped joint 38.

Although the material is polymerized at this point in the process, in some embodiments of the process, the extent of polymerization, and thus the molecular weight and relative viscosity (RV) of the polymer, will not be as high as required for the end product. Therefore, the partially polymerized material may pass through the Benson boiler 68 to provide additional heat, and then to the second reactor 70. The purpose of the second reactor 70 is to allow further polymerization and thus increase the molecular weight and RV of the product. The polymer product in stream 72 downstream of the second reactor should have the desired molecular weight. Preferably, the temperature in the second reactor 70 is between about 260 and about 280 ° C and the pressure is atmospheric.

The HMD steam generated in the second reactor 70 is removed in the upper stream 74 that enters the scrubber 76. A water stream 78 is also fed to the scrubber so that the steam condenses and can be removed as a waste water stream 80. The remaining steam leaves the scrubber 76 in the upper stream 82 and becomes part of the off-gas stream 64.

The polymer product can either be sent through the granulating machine 84 or it can be passed through the conduit 86. When the product passes through the granulating machine, the polymer granules then pass to the dryer 88. The nitrogen gas inlet 90, the nitrogen compressor 92 and the nitrogen heater 94 are used. to supply nitrogen gas to vessel 88, which dries the polymer granules. The dried granules emerging from the bottom of the dryer 88 pass through a water spray cooler 96, a sorter 98, and are moved to the product storage space 102 by a fan 100.

FIG. 2 is a block diagram of a control system of a preferred embodiment of the invention generally designated 120 for use in the improved polymerization system shown in FIG. 1. The control system 120 includes a molten diamine (HMD) measuring system 22, a measuring system 32 molten adipic acid, regulator 67, and NIR analyzer 66. The control system 120 serves to control the amount of molten adipic acid that is mixed with the molten diamine at the Y-shaped joint 38 to form a polymerization mixture that enters the static mixer 42 along the way to the prepolymer reaction. containers 46.

The molten diamine measuring system 22 includes a diamine measuring pump 124 and a diamine flow meter assembly 126. In a preferred embodiment, the pump diamine 124 is a positive displacement pump with a main drive motor 128, a plurality of main pump heads 130-134 and an alignment head 136. The main drive motor 128 includes a drive shaft 138 that extends into each of the main pump heads 130-134 and into the alignment head 136. The individual pistons (not shown) are located in the main pump heads 130 to 134 and the alignment head 136. The pistons (not shown) are coupled to the drive shaft 138 to provide bulk pumping of molten diamine monomer from the molten diamine vessel 20. through the flow meter assembly 126 and further into a Y-shaped joint 38 to pass to the static mixer 42.

The main drive motor 128 also includes a speed encoder 140 and a speed regulator 142 that collectively form a closed-loop feedback system to control the speed of the main drive motor 128. The speed encoder 140 monitors the speed of the main drive motor 128 and transmits the motor speed signal to the controller 67. Speed Control 142 receives an input signal from controller 67 to control the speed of the main drive motor 128.

The main pump heads 130-134 are equipped with servomotors 144-148, stroke position encoders 152-156, and stroke position controllers 160-164. The servomotors 144 to 148 are connected to the pistons (not shown) located in respective main pump heads 130 to 134. The stroke position encoders 152 to 156 monitor the shaft position of each servomotor 144 to 148 and transmit signals indicating a displacement (0 to 100%) to The stroke position regulators 160 to 164 receive input signals from the regulator 67 to control the actuator shaft position 144 to 148 to provide a predetermined stroke volume (0 to 100%) within the main pump heads 130-134. 134 are preferably capable of providing flow rates suitable to supply a sufficient volume of molten diamine to the system depending on the scale of application.

*) · T) ············································································

Equalizing head 136 is similarly equipped with servo motor 150, stroke position encoder 158 and stroke position regulator 166. The servomotor 150, the stroke position encoder 158 and the stroke position regulator 166 cooperate in the same manner as those of the main pump heads 130-134. The main difference is that the alignment head 136 has a significantly slower flow capacity than the main pump heads 130-34. 134. This is because the alignment head 136 is used to deliver the molten diamine at a relatively slow flow rate to higher flow rates from the main pump heads 130 to 134 to fine tune the total molten diamine feed to the static mixer 42. As will be described in greater detail below, this The property is important in that it allows the control system 120 of the invention to vary the ratio of the initial reactants (molten adipic acid and molten diamine) before mixing, so that the resulting polyamide has a stoichiometric equilibrium molar ratio.

The flow meter assembly 126 of the molten diamine measuring system 22 includes a flow meter 168 and a current transmitter 170. The current transmitter 170 is configured to monitor the flow of molten diamine monomer as detected by the flow meter 168 and simultaneously output an output signal to the controller 67 representing the diamine flow to the static mixer. 42. The flow meter 168 and the flow transmitter 170 may consist of any of a variety of commercially available flow meters and flow transmitters. Collectively, the flow meter assembly 126 and the diamine pump 124 cooperate with the regulator 67 to form a closed-loop arrangement to selectively adjust the flow of molten diamine to the static mixer 42.

The molten adipic acid measuring system 32 includes an adipic acid measuring pump 172 and a flow meter assembly 174. In a preferred embodiment, the metering acid pump 172 is an adip volumetric pump with a main drive motor 176 and multiple heads 178-182. The main drive motor 176 has a single drive shaft 184 extending into each of the heads 178-182. Individual pistons (not shown) are located within the heads. 178-182 and are coupled to the drive shaft 184 to provide bulk pumping of molten adipic acid from the adipic acid melting vessel 30 through the flow meter assembly 174 and further into a Y-shaped joint 38 and then into a static mixer 42.

The main drive motor 176 also includes a speed encoder 186 and a speed controller 188 that collectively form a closed-loop feedback system to control the speed of the main drive motor 176. The speed encoder 186 monitors the speed of the main drive motor 176 and transmits the motor speed signal to the controller 67. The speed regulator 188 receives an input signal from the regulator 67 and regulates the speeds of the main drive motor 176. The manual stroke regulators 190 to 194 are provided to adjust the stroke volume of the pistons within the heads 178 to 182, preferably between 0 to

100%.

The flow meter assembly 174 of the molten adipic acid measuring system 32 includes a flow meter 196 and a flow transmitter 198. The flow transmitter 198 is configured to monitor the flow of molten adipic acid monomer as detected by the flow meter 196 and simultaneously transmit an output signal to the flow rate regulator 67. Adipic acid to static mixer 42. Flow meter 196 and flow transmitter 198 may consist of any of a variety of commercially available flow meters and flow transmitters.

The controller 67 is adapted to receive different input signals and output different control signals to coordinate the operation of the control system 120. The controller 67 should preferably be programmed to operate according to the feed control algorithm. In such a control scheme, the number of carboxyl and amine end groups is measured to determine the end group equilibrium of unreacted molten monomers in the polymerization mixture. From this measurement, the regulator 67 can use a look-up table to determine to what extent the ratio of the starting reactants must be changed before mixing to give the resulting polyamide a stoichiometric equalized molar ratio.

The NIR analyzer 66 includes the analyzer 200 and the analyzer 202. The analyzer 200 is coupled directly to the prepolymer reactor 46 to detect the number of carboxyl and diamine end groups of unreacted monomers within the polymerization mixture as they exit the prepolymer reactor 46. The analyzer 202 is coupled between the analyzer The NIR analyzer 66 may be any number of commercially available near-infrared analyzers capable of determining diamine end groups in the resulting polyamide.

The NIR analyzer 66 transmits to the controller 67 an output signal indicating the molar equilibrium of unreacted monomers within the polymerization mixture. The controller 67 uses a feed control algorithm that varies the feed rate of the molten diamine monomer depending on the output signal from the NIR analyzer 66. With this feed control algorithm, the ratio of molten dicarboxylic acid monomer to molten diamine monomer input can be controlled to have a polymerized resultant. product stoichiometrically balanced molar ratio.

In one embodiment, this is accomplished by shortening the feed rate of molten diamine monomer through the diamine measuring system 22. The control and operation of the diamine measuring system 22 and the adipic acid measuring system 32 will be discussed in more detail below with reference to Figures 2 to 4.

Referring to Fig. 3, the controller 67 may be any number of commercially available programmable controllers, including, but not limited to, a DCS, a programmable logic unit, or a microprocessor-based personal computer. The feed control arrangement used in the controller 67 is possible because the components of the improved polyamidation system shown in Figure 1 are predictable in terms of their properties and effect on the polyamidation process. In particular, by omitting the salt formation step by adding water, the present invention eliminates the need for evaporators in the polyamidation process. Evaporative vessels are unpredictable in terms of their effect on polyamidation and can exhibit extensive changes during manufacture.

Operation of control system 120 will now be described with reference to Figs. 2 and 3. Referring first to Fig. 3, the initial step in control system 120 includes that the operator inserts a set value (capacity SP) into controller 67. 67 shown in FIG. 3 including a setpoint input terminal 67a and a forward modeling controller 67b to control operation of the control system 120. After the user enters the desired setpoint value (SP capacity), the computer terminal 67a transmits this information to the forward modeling controller. regulator 67b.

The advanced modeling controller 67b then adjusts the engine speed control of the speed controller 188 for the main drive motor 176 of the adipic acid meter 172. In a preferred embodiment, the flow rate of the adipic acid measuring system 32 is further controlled by manually adjusting the stroke controllers 190 to 194 so that the molten adipic acid flows into the static mixer 42 at a predetermined flow rate corresponding to a set value selected by the operator. Once the flow rate is set, it should preferably not be changed by changing the control signal from regulator 67 or adjusting the manual stroke regulators 190 to 194.

The forward modeling controller 67b also adjusts the engine speed control for the speed controller 142 associated with the main drive motor 128 of the diamine measuring pump 124. The forward modeling controller 67b continuously monitors the output signal from the NIR analyzer 66 to achieve a molar ratio of molten dicarboxylic acid monomer to molten diamine monomer in the polymerization mixture. In one important aspect, the forward modeling controller 67b uses this NIR output

4 · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · · A signal for generating and transmitting the equalization setpoint signal to the stroke position controller 166 of the servomotor 150 coupled to the equalization head 136. The equalization setpoint signal transmitted to the stroke position controller 166 is generated based on the feed control algorithm contained in the controller 67b. This feed control algorithm may be in the form of a memory lookup table containing data representing the degree to which the reactant ratio (molten adipic acid and molten diamine) must be changed to achieve the desired setpoint capability selected by the operator based on the NIR analyzer input. 66.

In a preferred embodiment, the flow rate of the diamine measuring system 22 will be continuously adjusted to provide a suitable ratio of reactants (molten adipic acid and molten diamine) prior to mixing so that the resulting polyamide has a stoichiometric equilibrium molar ratio.

Referring to Fig. 2, the controller 67 accomplishes this first by controlling the main drive motor 128 and at least one of the servomotors 144-148. to direct the molten diamine from the main pump heads 130 to 134 towards the static mixer 42. The regulator 67 then adjusts the alignment head 136 to fine tune the ratio of reactants transferred to the static mixer 42. The equalization adjustment is based on the flow rate of the molten adipic acid (as measured flow meter assembly 174), molten diamine flow rate (as measured by flow meter assembly 126), measuring the percent weight of each reactant in the partially polymerized mixture inside or after reactor 46 (as measured by NIR analyzer 66) and stroke volume and pump motor speed information 124 for diamine measurement (as measured by position encoders 152-158, position controllers 160-166, speed encoder 140, and speed controller 142).

It is within the scope of this invention to use a feed control algorithm wherein the mass flow rate of the molten dicarboxylic acid is adjusted based on end group equilibrium measurements. Referring to Fig. 4, the flow rate of molten diamine monomer is maintained at a constant rate, while the dicarboxylic monomer is fine tuned to produce a suitable ratio of reactants to produce the desired setpoint capability. As will be appreciated, the circuit control system of FIG. 4 is inverted to that of FIG. 2, and therefore a complete discussion of the operation of the embodiment of FIG. 4 is not necessary.

The foregoing description of individual embodiments of the invention is not intended to be a complete list of all possible embodiments of the invention. Those skilled in the art will recognize that it is possible to • ·· ·

IX · φ · · · · ·

ΙΟ * · _e « _{e e e} ......... Make modifications to the individual embodiments described herein, where such modifications would be within the scope of the invention. E.g. although adipic acid and hexamethylenediamine react in the embodiments detailed herein to form nylon 6,6, other monomers known to those skilled in the art may be used to form other polyamides.

Claims (6)

PATENT CLAIMS A control system for producing a polyamide from a dicarboxylic acid monomer and a diamine monomer, comprising: (a) first means for measuring the supply of molten dicarboxylic acid monomer; (b) second means for measuring the molten diamine monomer feed to the molten dicarboxylic acid monomer feed to form a molten polymerization mixture; (c) means for detecting the molar ratio of molten dicarboxylic acid monomer to molten diamine monomer in the polymerization mixture; and (d) a controller communicatively coupled to the detection means and at least one of the first and second measurement means, wherein the controller controls at least one of the first measurement means and the second measurement means based on the molar ratio input signal from the detection means to be treated flowing the material of at least one of the molten dicarboxylic acid monomer and the molten diamine monomer to equalize the molar ratio of the molten dicarboxylic acid monomer and the molten diamine monomer in the polymerization mixture. Control system according to claim 1, characterized in that the detection means comprise a near-infrared analyzer for measuring the number of carboxyl monomer end groups and diamine monomer end groups in the molten polymerization mixture during polymerization. The control system of claim 1, wherein the first measuring means comprises a first pump connected to the molten dicarboxylic acid monomer feed and the second measuring means comprises a second pump connected to the molten diamine monomer feed. 4. The control system of claim 1 wherein the dicarboxylic acid monomer is adipic acid, the diamine monomer is hexamethylenediamine, and the polyamide is nylon 6.6. * ·, • 5. A system for producing polyamide from a dicarboxylic acid monomer and a diamine monomer comprising: (a) first measurement means provided for feeding molten dicarboxylic acid monomer; (b) the second measuring means are provided for feeding the molten diamine monomer, the first and second measuring means coupled together so as to mix the molten dicarboxylic acid monomer feed with the molten diamine monomer feed to form a molten polymerization mixture; (c) at least one non-vented reaction vessel to polymerize the polymerization mixture; (d) means for detecting the molar ratio of molten dicarboxylic acid monomer to molten diamine monomer in the polymerization mixture; and (e) control means communicatively coupled to the detection means and first and second measurement means, wherein the control means adjust the flow of at least one of the molten dicarboxylic acid monomer and molten diamine monomer feeds in the polymerization mixture. 6. The system of claim 5, wherein the dicarboxylic acid monomer is adipic acid, the diamine monomer is hexamethylenediamine, and the polyamide is nylon 6.6.