Background

[0001] Desiccant dryers are used to produce dehumidified air. A desiccant dryer may include a bed or wheel containing desiccant material. A stream of moisture-laden air is passed over, or through the desiccant material. The desiccant material absorbs moisture from the air, so that the air leaves the dryer in a state of low humidity, e.g., with a dew point of -40°F or lower.

[0002] Desiccant dryers commonly are used in the plastics industry. For example, desiccant dryers may be used in drying systems that remove moisture from plastic resin granulates, such as resin pellets, before the resin pellets are molded or otherwise processed to produce plastic products. Such moisture, if present in the resin pellets during processing, can result in cracks, voids, and other flaws in the finished plastic product. Moisture is removed from the resin pellets by flowing dry, heated process air over the pellets while the pellets reside in a drying chamber. The process air absorbs moisture from within the resin pellets. The resin pellets are exposed to the process air for a time period sufficient to ensure that the moisture level within the pellets has been reduced to an acceptable level. Upon leaving the drying chamber, the process air is laden with moisture absorbed from the resin pellets. The moisture-laden air is directed back to the desiccant dryer, where the process air is dried. Upon exiting the desiccant dryer, the dried process air is heated, and recirculated to the drying chamber.

[0003] The desiccant within the desiccant dryer eventually will become saturated with the moisture that it has removed from the process air, and will lose its ability to absorb additional moisture from the process air. At or before this point, the desiccant is subjected to a regeneration process under which high- temperature air is directed over, or through the desiccant material. The high-temperature air removes moisture from the desiccant material, and typically is discharged to the ambient environment after passing over and drying the desiccant material. [0004] The desiccant dryer is isolated from the resin drying chamber during the desiccantregeneration process, and is not available to provide dry process air to the drying chamber. Thus, the need to regenerate the desiccant material can cause interruptions in the drying process for the resin pellets, which in turn can impact the downstream production operations that rely on a continuous supply of the dried pellets. Also, the energy required to heat the regeneration air supplied to the desiccant dryer can be a significant operating expense that increases the production costs of the products made from the dried resin pellets.

Summary

[0005] In one aspect of the disclosed technology, a system for drying plastic resin includes a drying chamber having an outer pressure vessel, and a liner located within an internal volume of the outer pressure vessel and configured to hold the plastic resin. The system also includes at least one of: a source of heated air in fluid communication with the internal volume of the drying chamber and configured to, during operation, provide heated air to the internal volume of the drying chamber; and a first vacuum source in fluid communication with the internal volume of the drying chamber and configured to, during operation, generate a vacuum in the internal volume of the drying chamber.

[0006] The system further includes a first and a second bed each comprising a vacuum chamber, and a desiccant located within the vacuum chamber. The vacuum chambers of the first and second beds are in fluid communication with the source of heated air and the internal volume of the drying chamber on a selective basis so that the dryer is configured to, during operation, remove moisture from air exiting the internal volume of the drying chamber.

[0007] The system also includes a second vacuum source in fluid communication with the vacuum chambers of the first and second beds on a selective basis. The second vacuum source is configured to, during operation, draw a vacuum within the internal volumes of the vacuum chambers of the first and second beds on a selective basis to remove moisture from the desiccant in the first and second beds.

[0008] In another aspect of the disclosed technology, the drying chamber is a first drying chamber, and the system further includes a second drying chamber. The source of heated air is in fluid communication with the internal volumes of the first and second drying chambers on a selective basis and is configured to, during operation, provide heated air to the internal volumes of the first and second drying chambers on a selective basis.

[0009] In another aspect of the disclosed technology, the drying chamber is a first drying chamber, and the system further comprises a second drying chamber. The first vacuum source is in fluid communication with the internal volumes of the first and second drying chambers on a selective basis and is configured to, during operation, generate a vacuum in the internal volumes of the first and second drying chambers on a selective basis.

[0010] In another aspect of the disclosed technology, a method for regenerating desiccant includes providing a drying chamber comprising an outer pressure vessel, and a liner located within an internal volume of the outer pressure vessel and configured to hold plastic resin. Th method also includes providing at least one of: a source of heated air in fluid communication with the internal volume of the drying chamber and configured to, during operation, provide heated air to the internal volume of the drying chamber; and a first vacuum source in fluid communication with the internal volume of the drying chamber and configured to, during operation, generate a vacuum in the internal volume of the drying chamber.

[0011] The method further includes providing a dryer having a first and a second bed each having a vacuum chamber, and a desiccant located within the vacuum chamber. The vacuum chambers of the first and second beds are fluid communication with the source of heated air and the internal volume of the drying chamber on a selective basis so that the dryer is configured to, during operation, remove moisture from air exiting the internal volume of the drying chamber.

[0012] The method further includes subjecting the vacuum chamber of at least one of the first and second beds to a vacuum for a sufficient period of time to remove moisture from the desiccant in the at least one of the first and second beds.

[0013] In another aspect of the disclosed technology, a system for drying plastic resin includes a drying chamber defining an internal volume configured to hold the plastic resin. The system also includes at least one of: a source of heated air in fluid communication with the internal volume of the drying chamber and configured to, during operation, provide heated air to the internal volume of the drying chamber; and a first vacuum source in fluid communication with the internal volume of the drying chamber and configured to, during operation, generate a vacuum in the internal volume of the drying chamber.

[0014] The system further includes a dryer having a vacuum chamber and a desiccant located within the vacuum chamber. The internal volume of the vacuum chamber is fluid communication with the internal volume of the drying chamber so that the dryer is configured to, during operation, remove moisture from air exiting the internal volume of the drying chamber.

[0015] The system also includes a second vacuum source in fluid communication with the vacuum chamber, the second vacuum source being configured to, during operation, draw a vacuum within the vacuum chamber to remove moisture from the desiccant in the vacuum chamber.

Brief Description of the Drawings

[0016] The following drawings are illustrative of particular embodiments of the present disclosure and do not limit the scope of the present disclosure. The drawings are not to scale and are intended for use in conjunction with the explanations provided herein. Embodiments of the present disclosure will hereinafter be described in conjunction with the appended drawings.

[0017] FIG. 1 is a side view of a first and a second drying station of a resin drying system.

[0018] FIG 2 is a side view of an interior of the first drying station shown in FIG, 1.

[0019] FIG. 3 is a side view of the first drying station shown in FIG. 2.

[0020] FIG. 4 is a top perspective view of the first drying station shown in FIGS. 2 and 3.

[0021] FIG. 5 is a top view of the drying station shown in FIGS. 1-4.

[0022] FIG. 6 is a chart depicting an operating sequence of the resin drying system shown in FIGS.

1-5.

[0023] FIG. 7 is a chart depicting an operating sequence of an alternative embodiment of the resin drying system shown in FIGS. 1-5. [0024] FIGS. 7A and 7B are a tabular depiction of the operating sequence depicted in FIG. 6.

[0025] FIG. 8 is a diagrammatic illustration of a heater, a dryer, and related components of the drying system shown in FIGS. 1-5.

Written Description

[0026] The inventive concepts are described with reference to the attached figures, wherein like reference numerals represent like parts and assemblies throughout the several views. The figures are not drawn to scale and are provided merely to illustrate the instant inventive concepts. The figures do not limit the scope of the present disclosure or the appended claims. Several aspects of the inventive concepts are described below with reference to example applications for illustration. It should be understood that numerous specific details, relationships, and methods are set forth to provide a full understanding of the inventive concepts. One having ordinary skill in the relevant art, however, will readily recognize that the inventive concepts can be practiced without one or more of the specific details or with other methods. In other instances, well-known structures or operation are not shown in detail to avoid obscuring the inventive concepts.

[0027] The figures depict a resin drying system 10. The system 10 configured to dry plastic resin granulates such as plastic resin pellets. The plastic resin pellets can be, for example, PET resin. The system 10 comprises a first station 12a, a substantially identical second station 12b, and a controller 13. The first and second stations 12a, 12b each are configured to operate on a drying cycle having the phases depicted in FIGS. 6A, 7A, and 7B. The first and second stations 12a, 12b are not limited to this particular drying cycle; the first and second stations 12a, 12b can be configured to operate on drying cycles having other characteristics.

[0028] The first station 12a and the second station 12b each include a drying chamber 15, as shown in FIGS. 1 and 5. Referring to FIG. 2, the drying chamber 15 comprises an outer pressure vessel 16. The outer pressure vessel 16 includes a cylindrical tank body 17, an upper tank head 18, and a lower tank head 20. The upper tank head 18 and the lower tank head 20 are connected to the respective upper and lower ends of the tank body 17 by a suitable means such as fasteners. The outer pressure vessel 16 is supported by four legs 21. A load cell 22 is mounted on each leg 21. The load cells 22 are communicatively coupled to the controller 13. The controller 13 is configured to determine the quantity of resin pellets within the first station 12a, by weight, based on the outputs of the load cells 22.

[0029] The drying chamber 15 also comprises an inner liner 23 positioned within the interior volume of the outer pressure vessel 16. The inner liner 23 receives pellets of plastic resin, such as PET, to be dried; and holds the resin pellets during the drying process. The inner liner 23 can be formed, for example, from 304 stainless steel. The inner liner 23 is suspended from the upper tank head 18 of the outer pressure vessel 16. The inner liner 23 has a substantially cylindrical upper portion 24, and a substantially conical lower portion 26 connected to the upper portion 24 by a suitable means such as welding. An outlet 28 is formed at the bottom of the lower portion 26. The outlet 28 extends through the lower tank head 20 of the outer pressure vessel 16, and provides an exit for the resin pellets from the drying chamber 15.

[0030] Referring to FIGS. 1-5, the drying chamber 15 further includes a material valve 32. The material valve 32 is mounted on the lower tank head 20, and is fluid communication with the outlet 28 and the interior volume of the inner liner 23. The material valve 32 is communicatively coupled to the controller 13, and can be opened and closed in response to inputs generated by, and received from the controller 13. The material valve 32, when opened, permits the resin pellets to exit the drying chamber 15. When closed, the material valve 32 prevents the resin pellets from exiting the drying chamber 15, and forms an airtight seal between the interior and exterior of the drying chamber 15.

[0031] The drying chamber 15 also includes a hot air inlet tube 34; and a hot air inlet valve 35. The hot air inlet valve 35 is mounted on the upper end, or inlet, of the hot air inlet tube 34, and is communicatively coupled to the controller 13. The hot air valve 35 is connected to a source of heated, low-moisture -content air (discussed below) by hosing or ducting 36. The ducting 36 is depicted in FIG. 8. The hot air inlet tube 34 is suspended from, and extends through a port formed in the upper tank head 18. A hot air diffuser cone 36 is connected to a lower end of the hot air inlet tube 34. The hot air inlet tube 34 extends downward through the upper portion 24 of the inner liner 23; the hot air diffuser cone 36 is located within the lower portion 26 of the inner liner 23, as shown in FIG. 2.

[0032] During heating of the resin pellets, the hot air valve 35 is opened, and remains open, in response to inputs from the controller 13. This allows heated, low-moisture-content air from the heated air source to enter, and travel downward though the hot air inlet tube 34. The heated air enters the hot air diffuser cone 36, and then flows into the interior volume of the inner liner 23. The heated air subsequently flows upward through the inner liner 23, heating the resin pellets resident in the inner liner 23. The heating of the resin pellets removes moisture from the resin pellets. When the hot air valve 35 is closed, the interior volume of the drying chamber 14 is isolated from the source of heated air.

[0033] The drying chamber 15 further includes two return air valves 38 mounted on the upper tank head 18 and communicatively coupled to the controller 13, as shown in FIGS. 1-5. Each return air valve 38 is connected to hosing or ducting 39, shown in FIG. 8. Each return air valve 38 is in fluid communication with the interior volume of the drying chamber 15, via a corresponding port on the upper tank head 18. During heating of the resin pellets, the return air valves 38 are opened, and remain open, in response to inputs from the controller 13. This allows the air that has passed over and heated the resin pellets to exit the interior volume of the drying chamber 15. After exiting the drying chamber 15 by way of the return air valves 38, the air enters the ducting 39 and is dehumidified and re-heated as discussed below before being recirculated to the drying chamber 15. The drying chamber 15 includes perforated screens 41, shown in FIG. 2, that filter fines and other contaminants from the air exiting the drying chamber 15.

[0034] The heated, low-moisture content air is supplied to the hot air inlet tube 34 via the hot air valve 35, until the controller 13 determines that a heat time setpoint has been reached. When the heat time setpoint is reached, the controller 13 generates inputs that cause the hot air valve 35 and the return air valves 38 to close, isolating the interior volume of the drying chamber 15 from the ducting 36, 39. [0035] Referring to FIGS. 1-5, the drying chamber 15 also includes a suction port 42 located on the upper tank head 18. The suction port 42 is connected to a vacuum pump 44 via tubing 45 and a vacuum check valve 46. The vacuum-drying phase of drying cycle follows the heating phase, and is conducted without any heated air being supplied to the drying camber 15. At the start of the vacuum-drying phase, the vacuum check valve 46 is opened, and the vacuum pump 44 is activated in response to inputs from the controller 13. The vacuum pump 44 subsequently draws a vacuum in the interior volume of the drying chamber 15 by way of the suction port 42, and remains activated until the controller 13 determines that a vacuum time setpoint has been reached. When the vacuum time setpoint is reached, the controller 13 generates inputs that cause the vacuum pump 44 to deactivate and the vacuum check valve 46 to close. Subjecting the resin pellets to a vacuum during the vacuum drying phase causes additional moisture to be removed from the resin pellets.

[0036] At the start of the drying cycle, the “wet” resin pellets initially are conveyed to the drying chamber 15 via a wet material line 48 connected to a port on the upper tank head 18, a convey suction valve, and a convey pump (not shown). At the start of the loading process, the convey suction valve 50 is opened, and the convey pump is activated in response to inputs from the controller 13. This causes the wet resin pellets to be drawn from a hopper or other storage reservoir, conveyed to the interior of the inner liner 23 via the wet material line 48, and deposited into the inner liner 23. The convey pump remains activated, and the convey suction valve remains open until the controller 13 determines that the resin pellets have reached a predetermined “high” level in the drying chamber 15. The controller 13 determines when the resin pellets reach the high level by monitoring the overall weight of the drying chamber 15 and its contents based on the outputs of the load cells 22. When the controller 13 determines that the resin pellets have reached the high level, the controller 13 generates inputs that cause the convey pump to deactivate and the convey suction valve to close. The first or second station 12a, 12b at this point is fully loaded, and is ready to begin the heating phase of the drying cycle.

[0037] The drying chamber 15 further includes a vacuum valve 50, shown in FIG. 2, communicatively coupled to the controller 13 and fluidly coupled to a vacuum source (not shown). The vacuum valve 50 is located directly downstream of the material valve 32. After the vacuum drying phase of the drying cycle has been completed, the dried resin pellets are dispensed from the drying chamber 15 via the material valve 32 and the vacuum valve 50. Specifically, at the start of the dispense process, the vacuum valve 50 is opened, followed by the opening of the material valve 32, in response to inputs from the controller 13. The vacuum valve 50 and the material valve 32 are maintained in the open state for a sufficient period of time to permit all of the resin pellets to be discharged from the drying chamber 15 by way of the material valve 32. Once the controller 13 determines that the discharge period has elapsed, i.e., once the resin pellets have reached a “low” level in the drying chamber 15 based on the amount of time that has elapsed after the opening of the vacuum valve 50 and the material valve 32, the controller 13 generates inputs that cause the vacuum valve 50 and the material valve 32 to close. Alternatively, the controller 13 can monitor the weight of the drying chamber 15 and its contents, and can determine the end of the discharge cycle based on the measured change in weight of the drying chamber 15 and its contents. At this point, the discharge phase is complete and the first station 12a or the second station 12b is ready to begin another drying cycle, beginning with the loading of a new batch of “wet” resin particles into the drying chamber 15.

[0038] The first and second stations 12a, 12b can be operated independently of each other. Alternatively, the first and second stations 12a, 12b can be operated simultaneously, in a coordinated manner, such that the loading, heating, vacuum-drying, and discharge phases of the first station 12a do not overlap with the respective loading, heating, vacuum-drying, and discharge phases of the second station 12b. Thus, one source of heated air, one vacuum pump, and one convey pump can be used to simultaneously support the operation of both the first station 12 and the second station 12b. FIGS. 6A, 7A, and 7B depict the sequence and timing of the various phases of the drying cycles for the first station 12a (“Station A”) and the second station 12b (“Station B”) when the first and second stations 12a, 12b are operated in such a simultaneous, coordinated manner. FIG. 6B depicts the sequence and timing of the various phases of the drying cycles when three identical stations (“Station A,” “Station B,” and “Station C”) are used in a simultaneous, coordinated manner.

[0039] Alternative embodiments of the system 10 can be configured with only one of the first or second stations 12a, 12b. [0040] Referring to FIG. 8, the system 10 also includes a process heater 70, and a dual bed dryer 72 for supplying and recycling the air supplied to the drying chamber 15 during the heating phase of the drying cycle. This air is referred to below as “process air.” The dryer 72 and the heater 70 dehumidify, i.e., reduce the moisture content of, and re-heat the process air after the process air has left the drying chamber 15, so that the process air can be recycled to the drying chamber 15 during the heat-drying phase of the drying cycle. Recycling the process air in this manner can save energy that otherwise would be needed to heat ambient air to the temperature required for the heat-drying phase of the drying cycle.

[0041] The dryer 72 receives the process air from the respective drying chambers 15 of the first and second stations 12a, 12b by way of a valve 74, shown in FIG. 8. The valve 74 is connected to the return air valves 38 of the drying chambers 15 by way of the ducting 39. The process air exiting the drying chambers 15 has heated and removed moisture from the resin pellets in the drying chamber 15. The process air exiting the drying chambers 15 and entering the dryer 72 thus has a relatively low temperature and a relatively high moisture content.

[0042] The valve 74 is communicatively coupled to the controller 13. The valve 74, in response to inputs from the controller 13, directs process air from the drying chamber 15 of the first station 12a to the dryer 72 only when the first station 12a is operating in the heat-drying phase. Similarly, the valve 74, in response to inputs from the controller 13, selectively directs process air from the drying chamber 15 of the second station 12b to the dryer 72 only when the second station 12b is operating in the heat-drying phase. [0043] After the process air has been dehumidified in the dryer 72, the process air is directed the process heater 70. The process heater 70 raises the temperature of the process air to a level sufficient for heating the resin in the drying chamber 15 during the heat-drying phase, so that the now high- temperature, low-moisture-content process air can be recycled to the drying chambers 15.

[0044] Referring to FIG. 8, the process heater 70 is in fluid communication with a valve 75 located downstream of the process heater 70. The valve 75 is in fluid communication with the ducting 36 and the hot air inlet valves 35 of the respective drying chambers 15 of the first and second stations 12a, 12b. [0045] The valve 75 is communicatively coupled to the controller 13. The valve 75, in response to inputs from the controller 13, directs the process air from the process heater 70 to the hot air inlet valve 35 of the first station 12a only when the first station 12a is operating in the heat-drying phase. Similarly, the valve 75, in response to inputs from the controller 13, selectively directs the process air from the process heater 70 to the hot air inlet valve 35 of the second station 12b only when the second station 12b is operating in the heat-drying phase.

[0046] The dryer 72 comprises a controller 77. The controller 77 is communicatively coupled to the controller 13. Referring further to FIG. 8, the dryer 72 also includes two towers, or beds 78a, 78b of desiccant material. During operation, one of the beds 78a, 78b is used to remove moisture from the process air directed to the dryer 72 by way of the valve 74. While one of the beds 78a, 78b is performing the dehumidification process, the desiccant in the other bed 78a, 78b can undergo regeneration to remove moisture from the desiccant. This moisture had been absorbed by the desiccant while the desiccant previously was used to remove moisture from the process air flowing through the dryer 72. The regeneration process, as discussed below, is performed exclusively by vacuum drying, i.e., by subjecting the desiccant to a vacuum.

[0047] The beds 78a, 78b are substantially identical. Each bed 78a, 78b incudes a chamber 80, and beads of desiccant material located within the internal volume of the chamber 80. The desiccant is a suitable hygroscopic material such as, but not limited to, type 13X molecular sieve desiccant.

[0048] Referring to FIG. 8, the dryer 72 includes two inlet valves 82 communicatively coupled to the controller 77. One of the inlet valves 82 is located directly upstream of the first bed 78a, and controls the flow of process air to the first bed 78a. The other inlet valve 82 is located directly upstream of the second bed 78b, and controls the flow of process air to the second bed 78b. Each inlet valve 82 is in fluid communication with the valve 74 that selectively directs the low-temperature, high-moisture-content air to the dryer 72 from the drying chambers 15 of the first and second stations 12a, 12b.

[0049] The dryer 72 includes two outlet valves 84 communicatively coupled to the controller 77.

One of the outlet valves 87 is located directly downstream of the first bed 78a, and controls the flow of process air from the first bed 78a. The other outlet valve 84 is located directly downstream of the second bed 78b, and controls the flow of process air from the second bed 78b. Both of the outlet valves 84 are located upstream of the process heater 70.

[0050] The controller 77 is configured to generate inputs that cause the inlet valve 82 and the outlet valve 84 associated with the first bed 78a to open, and remain open while the first bed 78a is being used to reduce the moisture content of the process air being supplied to dryer 72. The opening of the inlet valve 82 and the outlet valve 84 associated with the first bed 78a permits process air to flow through the interior volume of the first bed 78a. More specifically, when the inlet valve 82 associated with the first bed 78a is open, the process air is directed to the interior volume of the chamber 80 of the first bed 78a by way of a lower port formed at the bottom of the chamber 80. When the outlet valve 84 associated with the first bed 78a is open, the process air within the interior volume of the chamber 80 of the first bed 78a can exit the first bed 78a by way of an upper port formed at the top of the chamber 80.

[0051] Upon entering the chamber 80 of the first bed 78a via the lower port of the chamber 80, the process air flows upward and passes over the desiccant beads within the chamber 80. After passing over the desiccant beads, the process air exits the chamber 80 by way of the upper port of the chamber 80. The desiccant beads absorb moisture from the process air as the process air passes over and contacts the resin beads. The chamber 80 is sized to hold an amount of desiccant sufficient to achieve a required or otherwise desired reduction in the moisture content of the process air. For example, and without limitation, the chamber 80 can be configured so that the process air, upon exiting the chamber 80, has a dewpoint of about - 40° C. The process air, after exiting the chamber 80 and passing though the associated outlet valve 84, is directed to the process heater 70 for re-heating prior to being directed back to the drying chamber 15 of the first or second station 12a, 12b for use in the heat-drying phase of the drying cycle.

[0052] The inlet valve 82 and the outlet valve 84 associated with the second bed 78b remain closed while the first bed 78a is configured to dehumidify the process air, i.e., while the inlet valve 82 and the outlet valve 84 associated with the first bed 78a are open. The second bed 78b thus remains isolated from the process air, the first and second stations 12a, 12b, the first bed 78a, and the process heater 70 when the process air is flowing through the first bed 78a. A first vacuum valve 90a, discussed below, also remains closed while the first bed 78a is configured to dehumidify the process air.

[0053] Likewise, when the second bed 78b is used to dehumidify the process air, the controller 77 causes the inlet valve 82 and the outlet valve 84 associated with the second bed 78b to open, and to remain open. The controller 77 also causes the inlet valve 82 and the outlet valve 84 associated with the first bed 78a to remain closed. Under these conditions, the process air flows through the chamber 80 of the second bed 78b and is dehumidified, in the manner discussed above in relation to the first bed 78a; and the first bed 78a remains isolated from the process air, the first and second stations 12a, 12b, the second bed 78a, and the process heater 70. A second vacuum valve 90b, discussed below, also remains closed while the second bed 78b is configured to dehumidify the process air.

[0054] The controller 77 is configured to monitor the moisture content of the process air exiting the first bed 78a and the second bed 78b. The humidity level can be sensed, for example, by a dewpoint sensor 86 communicatively coupled to the controller 77. The dewpoint sensor 86 can be located downstream of the outlet valves 84 and upstream of the process heater 70, as shown in FIG. 8. The controller 77 recognizes a humidity level at or above a predetermined level as an indication that the desiccant in the active bed, i.e., whichever of the first or second beds 78a, 78b is receiving the process air at the time, has become saturated and is approaching a condition at which the desiccant no longer is able to reduce the moisture content of the process air to an acceptable level. At this point, the controller 77 can reconfigure the inlet valves 82 and the outlet valves 84 so that the process air is redirected to the inactive bed, and the active bed becomes isolated.

[0055] For example, if the first bed 78a is active and the readings from the dewpoint sensor 86 indicate that the humidity level of the process air is above the acceptable level, the controller 77 issues inputs that cause the inlet valve 82 and the outlet valve 84 associated with the second bed 78b to open; and that cause the inlet valve 82 and the outlet valve 84 associated with the first bed 78a to close. These actions divert the process air to, and through the second bed 78b; and isolate the first bed 78a from the process air, the first and second stations 12a, 12b, the second bed 78b, and the process heater 70.

Conversely, if the second bed 78b is active and the readings from the dewpoint sensor 86 indicate that the humidity level of the process air is above the predetermined threshold, the controller 77 issues inputs that cause the inlet valve 82 and the outlet valve 84 associated with the first bed 78a to open; and that cause the inlet valve 82 and the outlet valve 84 associated with the second bed 78b to close.

[0056] The dryer 72 is configured to regenerate the desiccant in the inactive chamber after that particular chamber has been taken off line and isolated. The controller 77 can be configured to begin the regeneration process automatically. The dryer 72 regenerates the desiccant by subjecting the desiccant to a vacuum. Heat is not used to regenerate the desiccant. It is believed that regenerating the desiccant using a vacuum, as opposed to heating the desiccant, can result in significant energy savings. In alternative embodiments, the vacuum drying process can be preceded or followed by a heating phase to further remove moisture from the desiccant. The regeneration process removes moisture from the desiccant that the desiccant previously had absorbed from the process air, to render the desiccant suitable once again for dehumidifying the process air.

[0057] The dryer 72 further comprises a vacuum source, in the form of a vacuum pump 88, to provide the vacuum needed to regenerate the desiccant. The vacuum pump 88 can be placed in fluid communication with the first and second beds 78a, 78b on a selective basis, using the following configuration of valving and ducting. Alternative embodiments can use other configurations for placing the vacuum pump 88 in selective fluid communication with the first and second beds 78a, 78b.

[0058] Referring further to FIG. 8, the dryer 72 also includes a first vacuum valve 90a, and a second vacuum valve 90b. The first and second vacuum valves 90a, 90b are in fluid communication with the vacuum pump 88 via ducting 92, and are configured to selectively place the vacuum pump 88 in fluid communication with the respective first and second beds 78a, 78b. More specifically, the first vacuum valve 90a is in connected to the chamber 80 of the first bed 78a by ducting 93a that fluidly communicates with the output port of that chamber 80, upstream of the outlet valve 84 associated with the first bed 78a.

The second vacuum valve 90b likewise is connected to the chamber 80 of the second bed 78b by ducting 93b that fluidly communicates with the output port of that chamber 80, upstream of the outlet valve 84 associated with the second bed 78b.

[0059] The chamber 80 of the first bed 78a can be subjected to a vacuum as follows. The controller 77 issues inputs that cause the vacuum pump 88 to start, and the first vacuum valve 90a to open. The second vacuum valve 90b remains closed, so that the second bed 78b remains isolated from the vacuum pump 88 and can be used to dry process air received from the first or second stations 12a, 12b as the desiccant in the first bed 78a is regenerated. Also, inlet valve 82 and the outlet valve 84 associated with the first bed 78a remain closed, so that the first bed 78a remains isolated from the process air, the first and second stations 12a, 12b, the second bed 78b, and the process heater 70. Because the first bed 78a is isolated from airflow from all components except the vacuum pump 88, the vacuum pump 88 draws a vacuum in the chamber 80 of the first bed 78a.

[0060] The controller 77 can regulate the speed of the vacuum pump 88 to maintain a required or otherwise desired vacuum level within the chamber 80 of the first bed 78a. A pressure sensor (not shown) can be mounted in or near the chamber 80, and can provide the controller 77 with an indication of the vacuum level in the chamber 80.

[0061] Subjecting the desiccant beads within the first bed 78a to a vacuum causes moisture in the desiccant beads to be drawn out of the desiccant beads, and eliminated from the first bed 78a by the suction provided by the vacuum pump 88. The air drawn from the first bed 78a in this manner can be vented to the atmosphere. The vacuum within the first bed 78a can be maintained for a predetermined period of time deemed sufficient to remove enough moisture from the desiccant beads so that the desiccant beads once again can be used to dry the process air generated by the first and second stations 12a, 12b. Alternatively, a humidity sensor positioned in the ducting 92 can be used to monitor the moisture content of the air being drawn from the first bed 78a; and the controller 77 can be configured to end the vacuum cycle when the moisture content has decreased to an acceptable level. [0062] The chamber 80 of the second bed 78b can be subjected to a vacuum, and the desiccant beads in the second bed 78b can be dehumidified in a similar manner to that described above in relation to the first bed 78a.

[0063] Alternative embodiments of the dryer 72 can be configured with only one of the beds 78a, 78b. Also, the dryer 72, and alternative embodiments thereof, can be used in conjunction with drying stations other than the first and second stations 12a, 12b, including conventional drying hoppers that do not subject the resin pellets to a vacuum during the drying process.