Apparatus and method for controlled drying of plastic pellets

Abstract: Apparatus and a method for the controlled drying of polymer plastic pellets within a dryer hopper are provided. A sensor tree having a plurality of temperature sensors spaced vertically on the tree is positioned vertically within the dryer hopper so that the individual sensors are each located at varying vertical distances from the bottom to the top of the dryer hopper. A target temperature for a particular type of polymer plastic pellet, when maintained for a specified residence time, indicates that the pellet is thoroughly dry for use in a manufacturing process. By determining the vertical level at which the target temperature has been achieved for the desired residence time. the amount of dry material within the hopper that is ready to be input to the manufacturing process is determined. A signal processing unit operating a software program automatically controls the throughput of the dryer hopper so that only dry polymer plastic pellets leave the hopper.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to an apparatus and method for the controlled drying of plastic pellets that are ultimately melted and utilized to form plastic articles of manufacture. The controlled drying of the plastic pellets provides energy savings in the drying process and the properly dried pellet produces a better article of manufacture. 2. Description of the Prior Art Dryers and dryer hoppers have been utilized in the plastics industry for many years. These dryers provide heated, dehumidified air to a dryer hopper. Plastic pellets within the dryer hopper are exposed to the heated. dehumidified air to remove moisture from the pellets. The basic purpose of drying the plastic pellets is to remove moisture from the surface and interior of the pellets before the pellets reach the melt phase as they pass through the screw area of an injection or extrusion machine if the moisture is not removed, processing problems may occur or the quality of the finished parts may be inferior to parts which were made with properly dried material. Some properties of the parts which may be affected by inadequate drying are: Tensile strength; impact strength; surface blemishes; and degradation of barrier properties. Moisture may be either on the surface of the plastic pellet or in the polymer chain itself. Hygroscopic resin absorbs moisture into the polymer chain itself. Other resins hold only surface moisture. Proper heating and dehumidifying of the pellet causes the moisture to be driven out of hygroscopic resins onto the surface. Surface moisture is removed by the heated dehumidified air. Figure 1, which is prior art, shows a typical dryer and dryer hopper arrangement for drying plastic pellets. An insulated dryer hopper 10 is provided that has a cylindrical body 12 with a frusto conical lower body portion 14. An outlet 16 is provided at the bottom of the frusto conical lower body. At the top of the dryer hopper 10, a pellet loader 18 is typically provided to intermittently load pellets into the dryer hopper 10. The pellets move downwardly through the dryer hopper 10 in the direction of the arrows shown within hopper 10. The dryer hopper 10 has a spreader cone 20, a heated air inlet 22 and an air outlet 24. The heated air within hopper 10 passes upwardly through the bed of pellets from inlet 22 to outlet 24 in a direction opposite the movement of pellets within the hopper. Air from air outlet 24 typically passes through an after cooler 26 to cool the air after it leaves the dryer hopper 10. The cooled air then enters a process filter 28 to remove any dust and fines that may be entrained in the air system. Air from the process filter 28 enters blower 30 which provides a constant air flow through the drying circuit. The air then travels to the desiccant section 32 where moisture is removed from the air. The typical desiccant section 32 contains molecular sieves usually formed of Alumino-Silicates which remove the moisture from the air entering the desiccant section 32. The air then travels through the process heater 34 where it is heated to a temperature sufficient to dry pellets within the dryer hopper 10. A valve 35 controls the flow of heated air from the heater 34 to the dryer hopper 10. The process filter 28, blower 30, desiccant section 32 and process heater 34 are typically located within a dryer unit 36 that is positioned adjacent to the dryer hopper 10. As the heated air enters the dryer hopper 10 through heated air inlet 22, it passes upwardly through the bed of pellets and the heat from the air is transferred to the pellets causing the release of moisture which is then carried to the top of the hopper with the air to air outlet 24 and the cycle is continued. This closed loop cycle continues throughout the drying process. Efforts have been made to control the drying process within the dryer hopper 10 by controlling the temperature of the exhaust air from the hopper. United States patent 4,413,426 is an example of the such an arrangement. Another method of controlling the drying process is disclosed in DE-A2-4234696 which uses two sensors to monitor the temperature at the top and bottom of the hopper 10 and using this data controls the hot air inlets and internal heating element. The preamble of claims 1 and 8 is based on DE-A2-4234696. The present invention is directed to modification of the dryer hopper 10 which will provide vertically spaced temperature sensors throughout the dryer hopper from the bottom to the top of the hopper and provide control of the drying process utilizing data obtained from the vertically spaced temperature sensors. SUMMARY OF THE INVENTION In accordance with the present invention, there is provided in a dryer hopper for drying plastic pellets having a heated air inlet at the bottom of the hopper and an air outlet at the top of the hopper an improvement which includes a plurality of temperature sensors positioned at vertical incremental distances from the bottom of the hopper to the top of the hopper whereby the temperature of pellets at the vertical level of each of the plurality of temperature sensors may be measured to provide an indication of the dryness of the pellets at the vertical level of each of the plurality of temperature sensors. Further in accordance with the present invention, there is provided a dryer for drying plastic pellets which includes a hopper having a cylindrical body with a frusto conical lower body portion, a spreader cone positioned within the hopper body, a heated air inlet at the bottom of the hopper and an outlet at the top of the hopper. A pellet inlet port at the top of the hopper admits pellets into the hopper and a pellet discharge port at the bottom of the hopper frusto conical lower body portion selectively discharges pellets from the hopper. A plurality of temperature sensors that provide an electrical temperature responsive signal are positioned at vertical incremental distances from the bottom of the hopper to the top of the hopper whereby the temperature of pellets at the vertical level of each of the plurality of temperature sensors may be measured to provide an indication of the dryness of the pellets. Electrical circuit means connected to the plurality of temperature sensors and to a microprocessor provide temperature data from the hopper to the microprocessor. The microprocessor is programmed with the characteristics of the hopper and the location of the temperature sensors within the hopper and is programmable to receive information on the type of plastic pellets to be dried in the hopper. The microprocessor is arranged to process the temperature data from the temperature sensors to determine when pellets at a particular vertical level within the hopper have been at a target temperature for a sufficiently long residence time to be sufficiently dry to utilize in a manufacturing process. Still further in accordance with the present invention, there is provided a method of determining when a preselected amount of plastic pellets that are being dried by passing heated air upwardly through them within a dryer hopper are properly dry for use in a manufacturing process which includes placing a plurality of temperature sensors within the hopper in contact with the pellets at varying vertical distances from the bottom to the top of the hopper. Thereafter, determining when each of the temperature sensors reaches a specified target temperature. Thereafter, measuring the residence time at which the pellets in contact with each of the temperature sensors remain at the target temperature. Thereafter, calculating the amount of pellets in the hopper below the vertically highest positioned temperature sensor that has been maintained at the target temperature for the desired residence time to determine if the amount of pellets is equal to or greater than the preselected amount. Accordingly, a principal object of the present invention is to provide apparatus and a method for controlled drying of plastic pellets. Another object of the present invention is to provide apparatus for controlled drying of plastic pellets which may be built into new dryer hoppers or which may be retrofitted to dryer hoppers that are already in use. Another object of the present invention is to provide controlled drying of pellets so that the articles manufactured from the pellets are of better quality. Another object of the present invention is to provide for the efficient drying of plastic pellets so that the minimum amount of energy is utilized consistent with proper drying. These and other objects of the present invention will become apparent as this description proceeds in conjunction with the following specification, attached drawings and appended claims. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a schematic representation of a prior art dryer hopper and dryer unit. Figure 2 is a partially cut away prospective view of a dryer hopper of the present invention with temperature sensors installed therein. Figure 3 is a schematic representation of the dryer hopper of the present invention with the signal processing unit utilized in conjunction with it. Figure 4 is a graphic representation of the test results of drying plastic pellets within the dryer hopper of the present invention showing the data from individual sensors. Figures 5 and 5A are, together, a software flow chart for the signal processing unit of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, and particularly to Figure 2, there is shown a conventional dryer hopper 10 having an insulated cylindrical body 12 and a frusto conical lower body 14 with a valved outlet 16. A spreader cone 20 is positioned near the bottom of the cylindrical body 12. A heated air inlet 22 admits heated air at the frusto conical lower body 14 and an air outlet 24 at the top of the cylindrical body 12 removes air from the dryer hopper 10. The dryer hopper 10 described to this point is conventional. In accordance with the present invention, there is provided a sensor tree 40 which extends vertically from top to bottom of the dryer hopper 10. The sensor tree 40 is attached at the top to a support ring 38. The sensor tree 40 has individual temperature sensors numbered, from bottom to top of the dryer hopper 10 as 41, 42, 43, 44, 45, 46 and 47, respectively. The sensors 41 through 47, inclusive, are preferably conventional thermocouple sensors which produce an electrical signal indicative of the temperature of the sensor. There may be a greater or lesser number of sensors than shown in Figure 2 depending upon the particular process application. As seen in Figure 3, the temperature sensors 41 through 47, inclusive, are individually electrically connected to a signal processing unit which contains a multiplexer, an analog to digital converter, a microprocessor and a user interface. Referring to Figure 4, there is shown a graph providing test results of a typical dryer hopper operation equipped with sensors as shown in Figures 2 and 3. In Figure 4, sensor No. 1 was at the bottom of the dryer hopper and sensor No. 9 was ambient air temperature outside the hopper 10. Sensors 1 through 8, inclusive, of Figure 4 were from the bottom to the top of the dryer hopper at approximately 6 inch (15 cm) intervals although the spacing may be greater or less than 6 inches (15 cm) depending upon the process application. Figure 4 shows the temperature of the sensors over time as heated air was passed upwardly through the dryer hopper and through the bed of pellets contained therein. As expected, the lowermost sensor rises in temperature most rapidly since it is first exposed to the warmest air. As the air progresses upwardly through the bed of pellets, sensors located at a higher level increase in temperature but as the air nears the top of the dryer hopper, it loses some of its effectiveness in heating the pellets so that the more highly positioned sensors 7 and 8 do not attain high temperature levels. Depending upon the type of polymer plastic pellet that is to be dried in the dryer hopper 10, the target temperature of the pellets to be dried together with a specified residence time at that target temperature will indicate whether the pellets have achieved the proper degree of dryness for use in a manufacturing process. For example, referring to Figures 2 and 3, if sensor 43 achieves the desired target temperature for the desired residence time for the type of polymer pellet being dried, it can safely be concluded that all pellets below the level of sensor 43 within the dryer hopper 10 are sufficiently dry to be utilized in the manufacturing process. Sensor 43 of Figures 2 and 3 equates to sensor 3 of Figure 4. Without more, the sensor tree 40 of Figure 2 having the temperature sensors positioned thereon can be utilized to monitor the temperature of pellets at various vertical distances from the bottom of the dryer hopper 10 so that if the temperature at a particular vertical level of a sensor has achieved the target temperature for the type of pellets being dried and remains at that temperature for the desired residence time, the operator of the dryer hopper knows that all pellets below the level of that particular temperature sensor are sufficiently dried to be utilized in the manufacturing process. With the temperature sensors connected to the signal processing unit 50 as shown in Figure 3, a software program has been developed so that the microprocessor can automatically control drying within the dryer hopper 10. Figures 5 and 5A, taken together, show the software flow chart for automatically controlling drying within the dryer hopper 10. As seen in Figure 5, there are one-time setup parameters programmed into the microprocessor. These parameters include the number of temperature sensors or probes, the spacing of the temperature sensors or probes vertically one to the other, the hopper size volume and the inside diameter of the hopper size. These items remain constant for any particular dryer hopper 10 with the sensor tree 40 installed therein. Utilizing the user interface, other definable parameters are introduced into the microprocessor including delivery air set point, residence time set point. throughput set point, and bulk density. The delivery air set point varies as the type of polymer plastic pellet varies. The residence time set point also varies as the type of pellet varies determining how long the residence time must persist for a particular target temperature. The throughput set point indicates the rate at which it is desired to feed pellets from the dryer hopper 10 to the manufacturing process. The bulk density is that of the pellets so that the throughput of the pellets can be calculated by weight. Once the fixed parameters and the variable parameters are introduced into the microprocessor. the microprocessor then calls in the hopper monitor subroutine and takes the data from each temperature sensor 41 through 47. The microprocessor then determines how many temperature sensors or probes are up to target temperature. If a sufficient number of probes are up to target temperature so that the material below the highest vertically positioned temperature sensor is sufficient to achieve the desired throughput set point, the program continues. If there is not sufficient material below the highest vertically positioned sensor to achieve the desired throughput, a residence alert alarm flag becomes visible to the operator or the operation of the machine utilizing the pellets will be prevented. The program will then stay on hold until a sufficient amount of pellets are at the target temperature to achieve the desired throughput. Once this occurs. the microprocessor calculates the weight of the material currently at target temperature. The microprocessor then determines whether the material at the highest vertical sensor (probe 0) has been at target temperature for the desired residence time for that particular type of plastic pellet. If the material has. the microprocessor then determines whether there is available throughput of pellets equal to or greater than the throughput set point. If the response to that inquiry by the microprocessor is in the affirmative, the program continues for so long as all probe temperatures remain within the acceptable range. This automatic control of the temperature within the dryer hopper 10 permits efficient use of the dryer unit 36 (Figure 1) since if a relatively small throughput is required, the heated air does not have to be as hot or the volume of air need not be as great as that which would be required for a higher throughput. In like manner, if a particular type of plastic pellet requires a lower target temperature or a shorter residence time at that target temperature for effective drying, the heated air entering heated air inlet 22 need not be at as a high a temperature as might be necessary for pellets that required a higher target temperature or a longer residence time. Similarly, the volume of hot air entering the inlet 22 might be reduced by controlling valve 35 (Figure 1) to reduce the flow. It will be appreciated that because many different types of polymer plastic pellets will be dried in the apparatus, and by the method, of the present invention, it is impossible to state fixed parameters for all the various types of plastic pellets. This information will be available from the supplier of the pellets and the system described herein can be adjusted accordingly to provide for efficient drying of the particular type of pellet involved. In some instances, the calculation performed to determine whether there are sufficient dry pellets to permit the plastic injection moulding or extrusion process to proceed without alarm, so that sufficient through-put is available, may indicate that a level of pellets between the vertical level of two particular sensors is required in the hopper. In such an instance, the operator may interpolate between the readings of two contiguous sensors to determine that there are sufficient dry pellets to provide the desired through-put if the lower of the two contiguous sensors has exceeded the desired temperature and residence time but next higher sensor has not yet achieved either the desired temperature or the desired residence time. This particularly desirable where a small number of sensors are utilized in the system. Since overdrying of plastic pellets can sometime have an adverse effect on the quality of plastic manufactured articles, the present invention provides a system whereby the molding or extrusion process can be activated as soon as the desired temperature and residence time are achieved to indicate that the pellets are sufficiently dry. The temperature or volume of heated air entering the dryer hopper 10 can be adjusted by the system to prevent overdrying. The sensors of the present invention can also provide an indication of the depth to which the dryer hopper 20 is filled. The temperature of the individual sensors varies as the temperature of pellets in contact with the particular sensors varies. If the hopper 10 is partially emptied so that two or three sensors 47, 46 or 45 at the top of the hopper are no longer surrounded by pellets, those sensors will register the same temperature which will be the temperature of the hopper above the pellet level. When this occurs, the operator will know that the level of pellets within the hopper is below the lowest of the plurality of sensors that record the same temperature. It will be appreciated that the present invention can be utilized either to provide an alarm when there are not sufficient dry pellets to accommodate a specified through-put to the molding or extrusion machine or to provide for stoppage of the molding or extrusion machine when the desired through-put cannot be achieved. In either event, it will be up to the system operator to take appropriate action to provide for the desired pellet dryness.

Claims: A dryer hopper (10) for drying plastic pellets using heated air passing upwardly through pellets in the hopper to an air outlet (24) at the top of the hopper, such that a plurality of temperature sensors (41-47) are provided at vertical incremental distances between the bottom and the top of the hopper, for sensing the temperatures of the pellets in the hopper at the respective levels of the temperature sensors, and processing means (50) are provided for: monitoring the temperatures sensed by the different temperature sensors (41-47), comparing those sensed temperatures with target temperatures for the respective temperature sensors, CHARACTERIZED IN THAT the further processing means (50) provide for: measuring the time duration for which each of the sensed temperatures attains the respective target temperature, and comparing the measured time durations with determined residence times sufficient to provide dry pellets at the vertical levels of the respective temperature sensors (41-47). A dryer hopper according to claim 1, wherein the processing means (50) further comprises means for establishing the quantity of pellets within the hopper which are sufficiently dry to be utilized in a given manufacturing process. A dryer hopper according to claim 2, further comprising means for preventing discharge of the pellets from the hopper until the quantity of pellets calculated to be sufficiently dry to be utilized in the given manufacturing process is sufficient to carry out that process. A dryer hopper according to claim 2 or claim 3, further comprising means for actuating an alarm or warning until the quantity of pellets calculated to be sufficiently dry to be utilized in the given manufacturing process is sufficient to carry out that process. A dryer hopper according to any preceding claim, further comprising means for adjusting the temperature and/or the humidity and/or the flow rate of the heated air passing through the plastic pellets. A dryer hopper according to any preceding claim, wherein the temperature sensors (41-47) are at least four in number. A dryer hopper according to any preceding claim, wherein the hopper (10) has a cylindrical body (12) with a frusto-conical lower body portion (14) and a spreader cone (20) above an outlet (16); at least the lowest (41) of the temperature sensors (41-47) being positioned below the level of the top of the spreader cone (20), and at least two of the temperature sensors (41-47) being positioned above the level of the top of the spreader cone (20). A method of drying plastic pellets prior to their utilization in a manufacturing process, which comprises: passing a countercurrent of heated air vertically upwardly through the pellets in a hopper (10); monitoring the temperature of the pellets at a plurality of different vertical levels in the hopper (10) and comparing the monitored temperatures with target temperatures to be attained at the respective levels; the method being characterized by calculating the times for which the target temperatures have been attained at the respective levels; and calculating the amount of the pellets in the hopper(10) that have been dried to an extent sufficient to allow their utilization in the manufacturing process. A method according to claim 8, further comprising means for continuing the drying until the calculated amount of dried pellets reaches an amount required by the manufacturing process. A method according to claim 8 or claim 9, further comprising means for recording the temperature variation with respect to time at each of the different vertical levels during the drying process.

Inventors: DOWNIE JOHN P, DOWNIE, JOHN P.
Applicants: CONAIR GROUP INC, THE CONAIR GROUP, INC.
Application year: 1995
Main classification: B29B13
IPC codes: F26B19, F26B25, F26B9, B29B13, F26B21, B29B9