Abstract:
Method and apparatus for drying granular resin material by drawing vacuum over heating resin material in a vessel, while periodically purging the vessel with the material therein with dry air and bathing the vacuum dried material with dry air until furnished to a processing machine.

Description:
CROSS-REFERENCE TO RELATED PATENT APPLICATION 
       [0001]    This patent application claims the priority under 35 USC 119 and 120 of pending United States provisional application Ser. No. 61/986,266 entitled “Vacuum Dryer for Granular Plastic Resin Material” filed 30 Apr. 2014 in the name of Stephen B. Maguire. 
     
    
     DESCRIPTION OF THE PRIOR ART 
       [0002]    Vacuum dryers for drying granular plastic resin material, prior to that material being molded or extruded into a finished product, are known. One commercially successful vacuum dryer, as disclosed in U.S. Pat. No. 6,154,980, uses a powered, rotating carousel to move granular plastic resin material among three stations, at which the granular plastic resin material is heated, dried by vacuum, and stored. Another approach to vacuum drying of granular plastic resin material is disclosed and claimed in U.S. Pat. No. 8,776,392. 
       SUMMARY OF THE INVENTION 
       [0003]    This invention uses gravity to move granular plastic resin material in a vacuum dryer. The granular plastic resin material preferably is heated in a top heating hopper. The granular plastic resin material is then preferably dropped into a vacuum chamber. From the vacuum chamber the granular plastic resin material is preferably dropped into a retention hopper. 
         [0004]    A plastic product manufacturing process, either molding or extrusion, can preferably draw dry granular plastic resin material from the retention chamber as required, while the heating hopper and the vacuum chamber preferably continuously prepare subsequent batches of granular plastic resin material. The preferable straight down processing and drying of granular plastic resin material results in a much lower cost dry granular plastic resin material as compared to granular plastic resin material dried using known vacuum dryers. 
         [0005]    In a preferred embodiment of this invention, preferably at least one slide gate allows and blocks granular plastic resin material downward flow from part of the dryer to another. Costs are reduced by about forty percent and drying capacity is actually higher in the advantageously small footprint dryer embodying this invention. The small footprint afforded by the vertical, “stacked” configuration of this dryer is advantageous in that based in a plastic manufacturing processing plant, whether an extrusion operation or a molding operation, is often at a premium. 
         [0006]    The vacuum chamber of the granular plastic resin material dryer is preferably closed with at least one slide gate having a vacuum tight seal. The slide gate that preferably closes and seals against an o-ring to provide a vacuum tight seal. Use of the slide gate avoids vacuum leakage that could occur from the contamination that is present everywhere in a plastic molding or extrusion facility. With the slide gate, plastic dust, flakes, and pellets of granular plastic resin material do not interfere with the vacuum tight seal. 
         [0007]    The invention introduces dry air into the vacuum chamber periodically. As moisture is released from the granular plastic resin material while under vacuum, a vacuum pump preferably continues to pull the resulting air-water vapor mixture from the vacuum chamber. Over several minutes, this mixture changes to become a very high percentage of water vapor relative to the air remaining in the chamber. 
         [0008]    If the moisture in the form of water vapor is not purged, when vacuum is released from the vacuum chamber, the resulting “thin” but moisture-laden air would reenter the pellets of granular plastic resin material resident within the chamber and reverse the effect of the drying that has occurred. To prevent this, the invention preferably purges the vacuum chamber of moisture several times while vacuum is present. The invention preferably permits very dry purge air to enter the vacuum chamber and then draws the resulting mix of the very dry air and the water vapor-laden air, laden with moisture drawn out of the resin pellets, out of the chamber. 
         [0009]    When drying polyethyleneterephthalate (“PET”), used conventionally for beverage bottles, it is essential that moist ambient air not enter the vacuum chamber at the end of a vacuum cycle. The dry air purge allows effective drying of PET pellets. 
         [0010]    To supply such dry purge air, the invention preferably uses a separate dry air source. Suitable dry air can be obtained in several ways. Desirably in the practice of the invention in the preferred manner, the invention utilizes compressed air, which passes through at least one oil separator coalescing filter and a compressed air membrane dryer so that the air exiting the oil separator coalescing filter and the compressed air membrane dryer is extremely dry. This dry air is desirably heated to a desired level for introduction into the vacuum chamber. Since only a relatively small amount of dry air is required for purging the vacuum chamber, the compressed air membrane dryer can be very small and of very low capacity. 
         [0011]    In the invention, the hopper in which the granular plastic resin material is initially heated is preferably designed such that hot air enters the bottom of the hopper, passes upwardly through the granular plastic resin material resident in the hopper, and exits the hopper at the top. As the hot air is passing through the heating hopper, granular plastic resin material may be dropped from the bottom of the hopper into the vacuum chamber, while new granular plastic resin material is added at the top of the hopper. The heating hopper preferably holds sufficient granular plastic resin material to provide from three to five hours of residence time for the granular plastic resin material before exiting the bottom of the heating hopper. In this way, the granular plastic resin material is exposed to hot, dry air for from three to five hours, which is the time required for the granular plastic resin material to flow downwardly through the heating hopper. 
         [0012]    The invention does not dry the granular plastic resin material using “hot” air in the conventional sense. Hot air is used only to bring the granular plastic resin material up to a desired temperature. By carefully controlling the speed of a blower that moves the hot air, air flow is adjusted so that the invention provides the hot air at the correct rate to heat the granular plastic resin material. Viewed differently, most of the useful heat, in terms of calories or BTUs, is removed from the hot or “heating” air before the heating air arrives at the upper surface of the granular plastic resin material in the heating chamber and is allowed to escape. 
         [0013]    In the instant invention, since the invention is not concerned with heating during the drying stage, the drying stage, namely the stage during which the pellets are exposed to vacuum in the vacuum chamber, is as short as possible, and may be as little as fifteen or twenty minutes, as contrasted to three to five hours in a conventional desiccant dryer. There is no air filter for the heating air in the invention. The heating air is used only once and is vented to the atmosphere after it has been used for heating and has given up most of its heat. The hearing air is not recirculated. 
         [0014]    The single pass flow of heating air and the elimination of the need for a filter for the heating air is unique to this invention. Earlier vacuum dryer designs involved recirculation of air with filtering being required. This invention eliminates the need for a filter by having the “heating” air pass through the granular plastic resin material only once. The invention further regulates the speed of the blower forcing the air through the material to avoid, to the extent possible, loss of unused, residual heat remaining in the “heating” air leaving the heating hopper  56 . Blower speed is adjusted so that only enough heated air, at a desired temperature for the resin material prior to drying, is fed to the heating hopper at the bottom so that the bottom potion of resin in the heating hopper reaches the desired final temperature to meet the appetite of the process machine, namely the molding machine or extruder, for dry granular plastic resin material to be molded or extruded. 
         [0015]    In one of its aspects, this invention provides a method for drying granular resin material prior to processing of the granular resin material by molding or extrusion that includes heating granular resin material in a heating hopper, monitoring air temperature at the top of the heating hopper, and regulating introduction of heat to the hopper bottom based on monitored air temperature at the top of the heating hopper. 
         [0016]    The method may further proceed by releasing heated granular resin material from the heating hopper for flow downwardly into a vacuum chamber while replenishing the heating hopper from above with fresh resin material, preferably in an amount substantially equal to that released into the vacuum chamber. The method preferably proceeds by drawing vacuum in the vacuum chamber, periodically purging the vacuum chamber interior with dry air while the chamber is under vacuum, draining resin material from the vacuum chamber into a retention hopper, and blanketing dried resin material in the retention hopper with dry air so long as the material is resident therein. 
         [0017]    Heating the granular resin material preferably further includes introducing dry heating air into the heating hopper at the heating hopper bottom. 
         [0018]    In another aspect of the invention, there is provided an improved method for drying granular resin material prior to processing thereof by molding or extrusion by loading granular resin material into a heating hopper from above the hopper, introducing heated air into the hopper at the hopper bottom, monitoring the temperature of the air leaving the hopper at a position above the resin material, and regulating the rate of heated air introduction into the hopper so that monitored temperature of air leaving the hopper does not exceed a preselected level. 
         [0019]    In still another one of its aspects, this invention provides apparatus for drying granular resin material prior to molding or extrusion processing of the material. Desirably the apparatus includes a heating hopper, a vacuum chamber positioned below the heating hopper, and a retention hopper positioned below the vacuum chamber. A blower is provided for pumping heating air upwardly through the retention hopper. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]      FIG. 1  is a schematic representation of an air purge dryer manifesting aspects of the apparatus and method portions of this invention. 
       
    
    
     DESCRIPTION OF THE INVENTION 
       [0021]    Referring to  FIG. 1 , an air purge dryer in accordance with the invention is disclosed and designated generally  10 . Air purge dryer  10  includes a heating hopper  12 , a vacuum chamber  14 , and a retention hopper  16 , with the heating hopper being positioned above the vacuum chamber and the vacuum chamber in turn being positioned above the retention hopper  16 , with the heating hopper  12 , vacuum chamber  14 , and retention hopper  16  being desirably vertically aligned, as shown in  FIG. 1 . 
         [0022]    Heating hopper  12 , vacuum chamber  14 , and retention hopper  16  are all preferably independently supported by a support frame designated  20  and shown only schematically in  FIG. 1 . Specifically, heating hopper  12  does not rest on vacuum chamber  14 . To the contrary, support frame  20  supports heating hopper  12  above vacuum chamber  14  so that none of the weight of heating hopper  12  or any resin contained within heating hopper  12  is supported by vacuum chamber  14 . Heating hopper  12  is desirably an insulated stainless steel hopper and can accommodate drying temperatures of up to 350° F. The requested heating temperature is adjusted or set on a control panel portion of controller  76 . 
         [0023]    Similarly, vacuum chamber  14  is independently supported by support frame  20  so that none of the weight of vacuum chamber  14  is transferred to or borne by retention hopper  16 . While support frame  20  has been depicted in  FIG. 1  in three sections, it is to be understood that support frame  20  can be a single structural member so long as support frame  20  provides separate weight-bearing support for heating hopper  12 , vacuum chamber  14 , and retention hopper  16 . While the support frame  20  has been illustrated in the drawing for vacuum chamber  14 , vacuum chamber  14  may also desirably be suspended from above by a suitable frame member similar to schematic frame  20  in the drawing. 
         [0024]    The vertically aligned “stacked” arrangement of heating hopper  12 , vacuum chamber  14 , and retention hopper  16 , as depicted generally in  FIG. 1 , permits gravity-induced flow of granular plastic resin from heating hopper  12  downwardly into vacuum chamber  14 , and from vacuum chamber  14  downwardly into retention hopper  16 . Desirably, retention hopper  16  is supported by support frame  20  in a manner that retention hopper  16  is somewhat above floor level in the facility in which air purge dryer  10  is located. Having retention hopper  16  above the floor permits dried granular resin material to be supplied directly out of retention hopper  16  by gravity flow to a process machine such as a molding press or an extruder, or to a vacuum-powered resin distribution system within the processing facility. Support frame  20  has been illustrated in schematic form as supporting retention hopper  16 , vacuum chamber  14  and heating hopper  12 ; desirably in addition to vacuum chamber  14  being mountable on rails and in a suspended disposition from support frame  20 , heating hopper  12  and retention hopper  16  may both also be mounted on rails to facilitate removal of heating hopper  12  and retention hopper  16  as needed for maintenance, etc. 
         [0025]    Air for heating granular plastic resin within heating hopper  12  is supplied by a centrifugal blower  22  that draws in ambient air and forces that ambient air through an air heating chamber  23 , which preferably includes a heating element  24  positioned within an open ended cylindrical housing  25 . The open ended cylindrical housing  25  is preferably a 6 inch diameter, 6 inch length stainless steel cylinder having suitable insulative material around the exterior thereof. Voltage applied to heating element  24  within cylindrical housing  25  causes heating element  24  to rise in temperature. Air passing along heating element  24 , as blown through air heating chamber  23  by centrifugal blower  22 , is heated by heating element  24  and exits air heating chamber  23  at the top of chamber  23  and travels via a hot air conduit  74  to heating hopper  12 , where the hot air enters heating hopper  12  at the bottom thereof for upward passage through granular plastic resin residing in heating hopper  12 . A variable frequency drive  30  is provided for centrifugal blower  22  to modulate the speed of blower  22  and thereby control and adjust the amount of heating air, and therefore the amount of heat, that is introduced into heating hopper  12 . 
         [0026]    Vacuum chamber  14  is mounted on support frame  20  with one or more load cells  36  between vacuum chamber  14  and support frame  20 . Load cell  36  provides data to controller  76  as to the weight of vacuum chamber  20  and any granular plastic resin material being dried therein. 
         [0027]    Similarly, retention hopper  16  is mounted on support frame  20  using one or more load cells  38  to provide data to controller  76  as to the weight of dried granular plastic resin material resident within retention hopper  38 . 
         [0028]    Temperature sensors are provided to monitor air temperature at the inlet connecting conduit  74  to heating hopper  12  and at the top of heating hopper  12 , where the heated air, having given up most of its heat, is exhausted. The temperature sensor at the hot air inlet to heating hopper  12  is designated  44  in the drawings, while the temperature sensor at the outlet, at the top of heating hopper  12  where heated ambient air is exhausted, is designated  46 . 
         [0029]    A material level sensor  42  is provided in heating hopper  12 . Level sensor  42  provides a signal indicating excessively low level of material in heating hopper  12 . Controller  76  receives a signal from heating hopper level sensor  42  and in response to a low material level signal, controller  76  either actuates apparatus to provide granular resin material for replenishing heating hopper  12  or if no material is available, controller  76  shuts down the air purge dryer  10 . 
         [0030]    A temperature sensor  56  within retention hopper  16  senses the temperature of the dry purge air with which dried granular resin in retention hopper  16  is blanketed. A granular resin material temperature sensor  58  may be provided at the bottom, close to the material outlet from retention hopper  16 , to sense the temperature of the resin material being supplied from retention hopper  16 . 
         [0031]    Controller  76  desirably has two display screens. The upper screen  82 , which desirably has a red background, shows actual temperatures and set point temperatures. The lower screen  84 , which desirably has a blue background, shows various running mode information, set up information, and dryer configuration information, as selected by the operator by touch controls that are a part of controller  76  and are associated with the two screens. 
         [0032]    One or more oil separator coalescing filters  32  are provided to remove entrained oil and some moisture from the compressed air supply. A compressed air membrane dryer  34  further dries the air and provides very dry purge air for vacuum chamber  14  and a dry air blanket for maintenance of dry conditions for granular resin material in retention hopper  16 . 
         [0033]    As operation of the air purge dryer begins, material in heating hopper  12  is brought up to temperature. The time for preheating is determined by a specified preheat time, which may be entered by an operator into controller  76 , or by an automatic set-up option in controller  76  which establishes an inlet-to-outlet temperature difference for the air input to and exhausted by heating hopper  12 , and a minimum preheat time. Once resin material in heating hopper  12  is up to temperature, as determined by the inlet-to-outlet temperature difference as measured by temperature sensors  44  and  46 , and the temperature difference is supplied to controller  76 , approximately one-third of the resin material in heating hopper  12  is dispensed into vacuum chamber  14 . Once this occurs, a first vacuum cycle begins. Each vacuum cycle, namely the time a batch of resin material remains in vacuum chamber  14  under vacuum, has a minimum time that the material is material under vacuum. This time may be set by an operator using the inputs available on controller  76  or a default time of 20 minutes may be used. 
         [0034]    During normal operation, vacuum in vacuum chamber  14  is brought to a level of about 700 mm Hg and held to about a plus or minus 20 mm Hg differential for the vacuum cycle time. A typical vacuum cycle lasts from 15 to 20 minutes, depending on the material being dried. 
         [0035]    As vacuum chamber  14  receives the heated granular resin material through first conduit  102  through operation of material flow control gates  60  and  62  and the vacuum cycle begins, a suitable loader, either human or mechanical, loads heating hopper  12  with new replenishment material, desirably concurrently with the start of the vacuum cycle. Granular resin material loaded into heating hopper  12  remains in heating hopper  12  for a minimum of the time for a vacuum cycle in vacuum chamber  14 . After a vacuum cycle in vacuum chamber  14 , granular resin material that has been dried in vacuum chamber  14  is dispensed downwardly through second conduit  104 , via operation of material flow control gates  64  and  66 , into retention hopper  16  and is ready for use. Dried granular resin material residing in retention hopper  16  and not immediately removed therefrom for molding or extrusion is blanketed with dry air so long as that granular resin material remains in retention hopper  16 . The dry air blanketing the dried granular resin material remaining in retention hopper  16  is maintained under positive pressure and is desirably slightly heated so as to be warm. 
         [0036]    The rate of consumption of dried granular resin material from retention hopper  16  dictates the time granular resin material will be heated in heating hopper  12  and dried under vacuum in vacuum chamber  14 . For example, if thirty (30) minutes are required to deplete retention hopper  16 , the vacuum cycle in vacuum chamber  14  will run past the normal twenty (20) minute set point and will last thirty (30) minutes. This is normal operation and does not in any way degrade the granular plastic resin that has been dried in vacuum chamber  14 . However, if retention hopper  16  is depleted in fifteen (15) minutes and the time for a vacuum cycle in vacuum chamber  14  has been set to twenty (20) minutes, a five (5) minute window will result when no granular resin material is available. This indicates that the throughput capacity of the dryer has been exceeded for the particular granular resin material being dried. Upon such occurrence, controller  76  senses that retention hopper  16  is empty, that vacuum chamber  14  is still drying material, and with no material being available in retention hopper  16 , controller  76  sounds an alarm. 
         [0037]    Vacuum chamber load cell(s)  36  and retention hopper load cell(s)  38  allow controller  76  to always have in memory the current weight of material in the vacuum chamber and the current weight of material in the retention hopper. This permits calculation by controller  76  of throughput of granular resin material in pounds of resin material per hour. 
         [0038]    Venturi vacuum generator  28  requires an operating air pressure of about 80 psi. The pressurized air is desirably supplied by an in-house air system. 
         [0039]    A purge air inlet temperature sensor  56  is provided in retention hopper  16 . A granular resin material outlet temperature sensor  58  is provided at the bottom of retention hopper  16 . Both sensor  56  and sensor  58  provide temperature data to controller  76 . 
         [0040]    The desired temperature of air being outlet from the top of heating hopper  12  may be set in controller  76  such that once the temperature of air escaping from the top of heating hopper  12  reaches a desired level, centrifugal blower  22  and heating element  24  will shut down for a predetermined time period specified by an operator and programmed into controller  76  or until a vacuum cycle, which is under way, ends, whichever event comes first. 
         [0041]    The fill and the fill rate for vacuum chamber  14  are controlled and may be adjusted by material flow control gates  60  and  62  above vacuum chamber  14  as actuated and controlled by controller  76 . Similarly, material dump and material dump rate from vacuum chamber  14  can be controlled and adjusted by material flow control gates  64  and  66  below vacuum chamber  14  as actuated and controlled by controller  76 . These parameters, namely vacuum chamber fill and fill rate and vacuum chamber dump and dump rate are programmable into controller  76 . Similarly, the timing by which dry purge air is introduced into vacuum chamber  14  is desirably adjusted and controlled by controller  76 . Typically during a twenty (20) minute vacuum cycle, purge air will be introduced into vacuum chamber  14  six (6) times. 
         [0042]    Controller  76  controls and allows adjustment to the heat output provided to heating hopper  12 . While the vacuum dryer of the invention produces dried material in batches, the dryer is a continuous supplier of suitably dry material for molding or extrusion. Dry material may be withdrawn from retention hopper  16  on a continuous basis. Vacuum chamber  14  processes one batch of material every 20 minutes, which is sufficient to keep retention hopper  16  and any process machine being fed by retention hopper  16  supplied on a continuous basis. 
         [0043]    The vacuum dryer of the invention uses fresh air without recycling any air in the dryer. The air coming into the dryer is used once and goes out of the dryer; there is no recycling of air. 
         [0044]    The load cells, together with controller  76 , facilitates tracking throughput of granular resin material by the vacuum dryer of the invention, permitting optimization of manufacturing parameters in the plastic molding or extrusion facility in which the dryer of the invention is located. 
         [0045]    During the course of operation of the invention, vacuum is drawn by Venturi vacuum generator  28  from vacuum chamber  14  via vacuum drawing conduit  90 . 
         [0046]    Incoming compressed air from the plastics molding or extrusion facility is supplied to pressure regulator  100  as indicated in the drawing. This regulated pressurized air, with pressure regulated to a required level, is then supplied via regulated pressure air line  106 , which splits as illustrated in  FIG. 1  with one portion of line  106  leading to oil separating coalescing filter  32  and the other portion of line  106  leading to Venturi vacuum generator  28 . An exhaust line  92  leads from Venturi vacuum generator  28  to ambient air. 
         [0047]    Purge air is provided via purge air supply line  94  which exits compressed air membrane dryer  34  and supplies purge air in very dry form after exiting dryer  34  to both retention hopper  16  and to vacuum chamber  14 . Introduction of purge air to retention hopper  16  is controlled by valve  96 , which in turn is actuated by controller  76 . Introduction of purge air to vacuum chamber  14  is controlled by vacuum chamber purge air valve  98 , which in turn is also controlled by controller  76 . The wiring for connection of valves  96 ,  98  and the other components to controller  76  is not illustrated in the drawing to enhance the drawing clarity. 
         [0048]    Flow of granular plastic resin material downwardly from heating hopper  12  to vacuum chamber  14  is desirably through a first conduit  102 . Flow of dried granular resin material from vacuum chamber  14  to retention hopper  16  is desirably through a second conduit  104 . Conduits  102 ,  104  are respectively mechanically connected, preferably substantially air tightly, respectively to heating hopper  12 , vacuum chamber  12  and retention hopper  16 . 
         [0049]    Gates  60 ,  62 ,  64 , and  66  have been illustrated positioned respectively in the bottom of heating hopper  60 , at the top and at the bottom of vacuum chamber  14 , and at the top of retention hopper  16 . These gates may desirably be positioned in respective first and second conduits  102 ,  104  according to the manner of selected construction for the flow through vacuum dryer. 
         [0050]    It is desirable to have two gates, such as gates  60 ,  62 , above vacuum chamber  14  to control downward flow of resin from heating hopper  12 , with an upper gate  60  providing gross, course control and a lower gate  62  providing air tight vacuum sealing of the vacuum chamber. Use of the two gates,  60 ,  62 , with course control afforded by upper gate  60 , minimizes the possibility of resin material becoming stuck in gate  62  and thereby precluding gate  62  from making the vacuum tight seal required for effective operation of vacuum chamber  14  during the drying phase. Desirably, gate  62  is a slide gate providing vacuum tight seal using a rubber gasket with the movable slide portion of the gate closing against the rubber gasket and moving first in a direction laterally across with respect to the direction of downward flow of resin and then vertically parallel with the direction of downward flow of resin, with such horizontal and then vertical movement of the gate effectuated by the shape of the slot in which the slide gate moves. 
         [0051]    Material gate  64  may similarly be a slide gate or may be a pivoting gasket-equipped gate actuated by an air cylinder with the gate pivoting downwardly to effectuate downward flow of dried plastic resin material out of vacuum chamber  14  upon the conclusion of the vacuum cycle. Use of a pivoting-type gate at gate  64  reduces cost over the cost of a slide gate since gravity will carry any residual granules of plastic resin material downwardly through second conduit  104  into retention hopper  16 . Gates  60  and  66  may be of any suitable type, desirably actuated by air cylinders controlled by controller  76 . 
         [0052]    All components illustrated in  FIG. 1  are controlled by controller  76 . This includes the drive  30  for centrifugal blower  22 , heating element  23 , the various gates that control the flow of resin downwardly through the dryer, the load cells that detect weight thereby allowing the computation of amount of material flowthrough, and the like. Controller  76  controls all aspects of the operation of the dryer and once the dryer is started, human intervention is not necessary. Of course, controls provided on controller  76  to allow human intervention if desired.