Patent Publication Number: US-2023158713-A1

Title: Process for drying granular polymeric material and plant operating according to said process

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/605,813, filed Oct. 17, 2019, which is a § 371 National Phase of PCT/IB2018/052704 filed Apr. 19, 2018, which claims priority to Italian Application No. 102017000043004, filed Apr. 19, 2017, which are incorporated by reference as if fully set forth. 
    
    
     INVENTIVE FIELD 
     The present invention relates to a process for drying granular polymeric material having the features mentioned in the preamble of the main claim. 
     It further relates to a drying plant operating according to that process. 
     BACKGROUND 
     The invention applies particularly to industrial processes for working plastics materials in granules by means of extrusion or molding. 
     It is known that, in order to ensure an appropriate level of quality in the molded product, these operations require the plastics material introduced into the molds to be as humidity-free as possible. 
     This requirement is, however, difficult to reconcile with the high hygroscopic properties of some plastics materials that are widely used in the field, for example those based on polyethylene terephthalate (PET), polyamide (PA), polycarbonate (PC) or some copolymers such as ABS (acrylonitrile-butadiene-styrene). 
     These plastics materials must therefore, before being subjected to the extrusion or molding process, be adequately dried in suitable drying plants, where the water content of the granules is reduced to the minimum amount required by the working process. 
     In a frequently used process, the granular polymeric material is dried inside a hopper containing the material to be dried and into which a continuous flow of hot and dry air is introduced. 
     Alternative drying processes require the granular polymeric material to be subjected to a predetermined degree of depressurization (vacuum) so as to help the stripping of water from the granules at relatively low temperatures. 
     The next step of working the dried polymeric material requires the latter to be brought to a molten or semi-molten state, so that it can be introduced into a mold or extruded through a shaped head. These processes require high energy input to melt the material, which is particularly costly if it is produced inside an extruder, such that the overall cost of the working process is largely determined by the energy input. 
     As a consequence, the need to find novel solutions allowing energy consumption to be reduced as far as possible is deeply felt in the relevant technical field. 
     For this reason, as well, it is desirable to feed the working machine with granular polymeric material at the highest possible temperature. 
     If kept at high temperatures for considerable periods of time, however, for example typically two to three hours in the drying processes, the polymeric material is subject to phenomena of oxidation and degradation. In general, for each polymer a “maximum temperature at which it can be maintained in air” is defined, and this temperature must not be exceeded in the drying process. The value of this temperature depends on the specific type of polymer and is provided by the producer of the granular material to be processed. 
     A further drawback of traditional drying processes arises from the long periods of time required for production changes, which affect the overall operational flexibility of the plastics material working process. 
     In the present description and in the attached claims, the term “granular material” means a plurality of distinct solid elements, separate from each other, having suitable shapes and sizes, depending on the working process to be carried out and the polymeric material used, including powdered or flaked polymeric material. 
     The expression “maximum temperature at which it can be maintained in air” means the maximum temperature at which the granular polymeric material can be maintained in air for a significant period of time without suffering from considerable degradation phenomena. 
     In the present description and in the attached claims, the term “dehumidification” means the process by which the humidity content of the granular polymeric material is reduced by substantially eliminating the water present on the surface region of the granules. 
     By way of reference, this reduction is generally of the order of about 40-60% of the initial humidity content, with residual humidity values of around 1000 ppm (parts per million). 
     Moreover, the term “drying” means the process by which the humidity content of the granular polymeric material is reduced to the desired values for the subsequent working phase (molding or extrusion), by substantially eliminating the water present in the inner regions of the granules. 
     By way of reference, the maximum residual humidity value required by the working machine  100  can be around 50-100 ppm (parts per million). 
     The term “inert atmosphere” means a gas whose composition at the temperature and for the intended period of contact with the granular polymeric material does not give rise to appreciable degradation or oxidation phenomena. An example of an inert atmosphere is industrial nitrogen, which is substantially free of oxygen. 
     SUMMARY 
     The problem addressed by the present invention is that of providing a process for drying granular polymeric material as well as a drying plant, which are structurally and functionally designed to overcome, at least in part, one or more of the drawbacks complained of above with reference to the prior art mentioned. 
     This problem is resolved by the present invention by means of a process and a plant realized according to the appended claims. 
     In a first aspect thereof, the invention is aimed at a process for drying granular polymeric material. 
     Thanks to the process and the plant of the invention, the granular polymeric material is dried efficiently from the energy point of view and allows the degree of flexibility of the plant to be increased considerably. 
     In fact, the humidity content of the granules is reduced in two distinct, successive steps, a first step (dehumidification) in which the water fraction present on the surface region of the granules is substantially eliminated and a second step (drying) in which the water fraction present inside the granules is substantially reduced. 
     The dehumidification step can advantageously be carried out at a first, relatively low temperature, below the maximum temperature at which it can be maintained in air, but sufficient to cause the surface fraction of humidity to evaporate from the granules, while the granular material is further heated only before being subjected to the drying step. 
     The latter takes place under vacuum conditions so as to achieve particularly high drying levels in the granular material without the need to increase the temperature further. 
     By subdividing the process into various steps, it is also possible to make advantageous use of relatively small hoppers, which allows the granular polymeric material being processed to be changed in a short time, therefore increasing, while maintaining the same production capacity, the operating flexibility of the plant. 
     In addition, thanks to the provision of the filling and discharge units it is possible to obtain very high vacuum levels, reaching an absolute pressure of less than 30 mbar, for example an absolute pressure of about 10 mbar. 
     In at least one of the aspects mentioned above, the present invention can also have one or more of the preferred characteristics set out below. 
     Preferably, the first flow of gas is air taken from the environment and not recirculated air. 
     In particular, the first flow of gas is therefore preferably formed by air drawn from the environment, simply heated, put into contact with the granular polymeric material so as to dehumidify it and, at the end, returned to the environment. 
     This makes it possible to avoid costly dehumidification and recirculation treatments without significantly wasting energy, thanks to the relatively low temperature to which the gas is heated. 
     Preferably, said first temperature is between 100° C. and 150° C. 
     Preferably, the first flow of gas is heated using a heat pump. 
     The energy input required to heat the first air flow to the first temperature is thus minimized. 
     Preferably, the granular polymeric material is heated to the second temperature by means of a second flow of gas introduced into said granular polymeric material by means of a recirculation circuit. 
     In this way, the input of thermal energy to the gas introduced into the heating hopper is mostly recovered. On the other hand, taking account of the fact that a large part of the water content present in the granulate has been removed in the previous dehumidification stage, the gas leaving the heating hopper does not have high humidity values, and can therefore be reintroduced into the hopper (after further heating) without prior dehumidification treatment. 
     Preferably, the second flow of gas is formed of air. 
     Preferably, the second temperature to which the granular polymeric material is heated after having been dehumidified corresponds to the maximum temperature at which the granular polymeric material itself can be maintained in air. 
     In this way, the granular material is prepared in the subsequent vacuum drying step under the highest possible temperature conditions. 
     Preferably, the dehumidification step and the heating step are carried out in different hoppers, placed in series, between which the granular polymeric material is moved. 
     Preferably, the granular polymeric material is transported from the heating hopper to the drying hopper by means of the second gas flow. 
     In one embodiment of the invention, the granular polymeric material is subjected to a post-heating step during or after the drying step. 
     Thanks to this step, the granular polymeric material can be kept at or brought to a high temperature, preferably the maximum temperature at which it can be maintained in air, so as to be ready for use in the working machine. 
     Preferably, the plant further comprises at least one feed hopper, positioned downstream of the drying hopper and upstream of a machine for working the granular polymeric material. 
     Preferably, the dried granular polymeric material is transferred to a feed hopper of a machine for working the dried granular polymeric material. 
     In one embodiment of the invention, the granular polymeric material is post-heated by irradiation with microwaves. 
     Conveniently, the microwave irradiation step takes place by means of a suitable irradiation unit associated with the drying hopper, and occurs during the vacuum drying step. 
     In another embodiment of the invention, alternatively or in addition to the previous one, the post-heating step is performed in the feed hopper. 
     In a preferred embodiment, the granular polymeric material is post-heated in an inert atmosphere to a temperature above the maximum temperature at which it can be maintained in air. 
     Preferably, the inert atmosphere is maintained within the feed hopper by means of an inert gas delivery circuit. 
     This allows a granular polymeric material to be introduced, downstream, into the working machine at the closest temperature to melting point. In this way, the energy input required by the working machine is lower and, in particular, the Applicant has verified that heating the granular polymeric material in the drying plant leads to an overall energy balance that is lower than heating the granular polymeric material in the working machine. This advantage is even more evident when the granular polymeric material is melted inside an extruder, where the increase of the temperature is mainly obtained from the friction developed on the granules by the action of the screw that pushes them against the inner wall. 
     The presence of an inert atmosphere inside the feed hopper prevents any phenomena of oxidation and degradation of the granular polymeric material, notwithstanding the high temperatures to which it is heated. 
     Preferably, the temperature to which the granular polymeric material is post-heated in an inert atmosphere is lower than the melting point thereof by a value of less than 50° C. 
     In this way it is possible to raise the temperature of the polymeric material to be introduced into the working machine to the highest possible value without, however, melting the material in the hopper. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The characteristics and advantages of the invention will be more clearly apparent from the detailed description of a preferred exemplary embodiment thereof, illustrated by way of example and non-restrictively, with reference to the attached drawing, in which  FIG.  1    is a schematic view of a plant for drying granular polymeric material realized so as to operate according to the process of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     With reference to  FIG.  1   , the reference numeral  1  is an overall indication of a plant for drying granular polymeric material, operating according to the process of the present invention. 
     The plant  1  is designed to dry any granular polymeric material, for example polyamide, polycarbonate or ABS copolymer, even though, in the specific example described here, the material treated is formed of PET (polyethylene terephthalate) granules. 
     PET has a melting point of around 260° C. and a maximum temperature at which it can be maintained in air, as generally provided by the producers, of around 180° C. 
     The plant  1  is designed to supply a working machine  100  of the granular polymeric material, which machine, in the specific example, comprises a mold  101  fed by an extruder  102  that injects the polymeric material into the mold  101  in the molten state. 
     The plant  1  comprises a dehumidification hopper  10 , a heating hopper  20 , a drying hopper  30  and a feed hopper  40 , all positioned in series with one another. The working machine  100  is placed downstream of the feed hopper  40 . 
     In the example described here, a single hopper is provided for each step of the drying process; however, two or more hoppers can also be provided in parallel for one or more of said steps. 
     By way of example only, for a production capacity of the plant  1  of around 1000 kg/h, the dehumidification and heating hoppers  10 ,  20  may have a volume of between 1000 and 1500 liters, and the drying and feed hoppers  30 ,  40  may have a volume of between 500 and 1000 liters. 
     The plant  1  comprises a filling unit  2  provided for transferring the granular material from one or more bags  3  of untreated material into the dehumidification hopper  10 , via a filling line  4 . The bags  3  can contain the same material, or different polymeric materials. 
     The filling unit  2  comprises an extractor  5  connected to the filling line  4 , and a separation cyclone  6 , placed at the top of the dehumidification hopper  10 , at which point the granules of polymeric material separate from the transport air flow and are introduced into the hopper. 
     The dehumidification hopper  10  is connected to a dehumidification line  11 , through which a first flow of gas for dehumidifying the granular polymeric material contained in the dehumidification hopper  10  is introduced. 
     The first gas flow is formed by ambient air drawn along the dehumidification line  11  by the action of a fan  12  placed on an outlet pipe  13  of the dehumidification hopper  10 . 
     A heat pump  14  is provided on the dehumidification line  11 , for heating the first flow of gas to a first temperature preferably between 120° C. and 130° C., before feeding it into the dehumidification hopper  10 . The first gas flow is distributed into the mass of granular polymeric material to be dehumidified thanks to a diffuser  15  placed inside the dehumidification hopper  10  and, once it leaves the dehumidification hopper  10  by being extracted by the fan  12 , is returned to the atmosphere without being recirculated. 
     The heating hopper  20  is placed directly under the dehumidification hopper  10 , so that the dehumidified granular material can be transferred into the heating hopper  20  by falling directly into it. 
     The dehumidification hopper  20  is provided with a heating unit  21 , for heating the granular polymeric material to a second temperature, higher than the temperature achieved in the dehumidification hopper  10 , for example around 180° C. 
     The heating unit  21  comprises a recirculation circuit  22 , through which a second flow of gas is fed, also in this case formed of ambient air. 
     The recirculation circuit  22  comprises a heating line  23 , along which a heater  24  is provided, which heating line enters the heating hopper  20  and emerges into a diffuser  25 , conveniently positioned close to the bottom of the heating hopper  20 . 
     The recirculation circuit  22  further comprises a recovery line  26  leaving the heating hopper  20  and a fan  27  that drives the second gas flow back along the heating line  23 . 
     A transfer line  28  branches off from the heating line  23  before the heater  24 , the transfer line being connected to the bottom of the heating hopper  20  and designed to pneumatically convey the granular polymeric material leaving the heating hopper  20  to an intermediate holding hopper  29 , from which a return line  28   a  starts, carrying the second gas flow back to the fan  27 . 
     The intermediate holding hopper  29  acts as a small buffer tank from which the drying hopper  30  is fed. 
     The drying hopper  30  is connected to a depressurization circuit  31  capable of producing and maintaining a predefined vacuum level inside the drying hopper  30 , for example so as to reach a pressure of less than 30 mbar, preferably around 10 mbar. 
     The depressurization circuit  31  comprises a vacuum pump  32 , connected to a depressurization line  33  in which a pair of filters  34  and a protective condenser  35  are provided. 
     Upstream and downstream of the drying hopper  30 , a filling unit and a discharge unit for the hopper are provided respectively. 
     The filling unit of the drying hopper  30  comprises a small tank  36   a , blocked upstream and downstream by respective shut-off valves  36   b  and  36   c , which function overall as pressure-sealing elements. 
     Similarly, the discharge unit of the hopper comprises a small tank  37   a , blocked upstream and downstream by respective shut-off valves  37   b  and  37   c , which are also designed overall to operate as pressure-sealing elements. 
     Such high vacuum levels, equal to an absolute pressure of around 10 mbar, can be achieved thanks to the provision, upstream and downstream of the drying hopper  30 , of the tanks  36   a  and  37   a  that are in turn sealed by pairs of shut-off valves  36   b ,  36   c  and  37   b ,  37   c.    
     In the embodiment described here, a microwave irradiation unit  38  is provided in the drying hopper  30 , capable of heating the granular polymeric material contained therein. 
     Preferably, the microwave irradiation unit  38  comprises one or more Magnetron-type sources that are sufficiently powerful to keep the temperature of the granular polymeric material at the maximum temperature at which it can be maintained in air, for example, in the case of PET, at around 180° C. 
     The feed hopper  40  is connected to a circuit for delivering inert gas  41 , provided with a fan  42 , mounted on an intake line  43  that enters the feed hopper  40 , emerging into a distributor  44 , and a return line  45  that returns the inert gas leaving the feed hopper  40  to the fan  42 . 
     A heater  46  is positioned on the feed line  43 . 
     The feed hopper  40  is connected to the processing machine  100  by means of a discharge pipe  47  fixed to the bottom of the feed hopper  40  by means of a metering valve  48 . 
     A metering device  49  is also connected to the discharge pipe  47  in order to measure out, if required, any additives to the granular polymeric material that are fed into the processing machine  100 . 
     The plant  1  operates according to the process described below. 
     The granular polymeric material, for example PET, is fed into the dehumidification hopper  10  by means of the filling unit  2 , where it is dehumidified by contact with the first flow of air introduced into the dehumidification hopper  10  via the dehumidification line  11 . 
     The temperature of the first air flow introduced into the dehumidification hopper  10  is around 120-130° C. Once it leaves the dehumidification hopper  10 , the first air flow is returned to the environment. 
     The dehumidification step lasts around 120 minutes, at the end of which the granular polymeric material has a humidity content of around 1000 ppm and a temperature of around 120-130° C. 
     The dehumidified granular polymeric material is then discharged by gravity into the heating hopper  20 , where it is brought to the maximum temperature at which it can be maintained in air, equal to around 180° C., thanks to contact with the second air flow fed via the recirculation circuit  22 . 
     The air introduced into the heating hopper is recirculated without being dried, and thereby the action of dehumidifying the granular polymeric material is overall less comprehensive than the previous dehumidification step. 
     At the end of the heating step, the dehumidified and heated granular polymeric material is gradually transferred to the drying hopper  30 , using the pneumatic transport provided by the transfer line  28  to the intermediate holding hopper  29 . 
     From this, the material passes to the filling unit of the drying hopper, the shut-off valve  36   b  placed upstream of the tank  36   a  being opened, while the shut-off valve  36   c  placed downstream of the tank  36   a  is kept shut. 
     The tank  36   a  is small, for example around 30-50 liters, and the material it contains is transferred to the drying hopper by opening the shut-off valve  36   c  after having closed the shut-off valve  36   b.    
     The material is then transferred to the drying hopper  30  a little at a time, to avoid excessive variations in the vacuum level inside the drying hopper  30 . 
     In the drying hopper  30 , the residual pressure is less than 30 mbar, preferably around 10 mbar and this, together with the high temperature, results in effective deabsorption of the humidity present inside the granules. 
     After a suitable treatment period, for example around 40-50 minutes, the granular polymeric material has a residual humidity content of less than about 30 ppm. 
     During the drying step, the granular polymeric material is post-heated by the microwave irradiation unit  38 , to keep the temperature of the material at the temperature of 180° C. 
     The dried material is then transferred to the feed hopper  40 , passing through the discharge unit and the tank  37   a  after alternate closing and opening of the shut-off valves  37   b  and  37   c.    
     In the feed hopper  40 , the dried material can be further post-heated by a flow of inert gas, for example nitrogen, introduced into the feed hopper  40  via the delivery circuit  41 . 
     The inert gas is introduced at a temperature of around 220-230° C., higher than the maximum temperature at which it can be maintained in air (180° C.) and about 30-40° C. below the melting point of PET (260° C.). 
     The granular polymeric material is then transferred to the processing machine  100  through the discharge pipe, actuating the metering valve  48 . 
     The plant of the present invention can be produced in variations differing from the preferred example described above. 
     In a first variant, provision is made not to provide the feed hopper  40  of the delivery circuit with inert gas  41 . 
     In this case, the granular polymeric material is fed into the processing machine at the maximum temperature at which it can be maintained in air, which the granular polymeric material already has when it reaches the feed hopper  40 , thanks to the post-heating carried out by the microwave radiation unit  38 . 
     In a second variant, provision is made for the delivery circuit  41  to be supplied with air instead of inert gas. 
     In this case too, the granular polymeric material is fed into the processing machine at the maximum temperature at which it can be maintained in air. 
     In this case, it is possible to heat the granular polymeric material contained in the feed hopper if its temperature tends to fall during its dwell time or if it is not sufficiently heated in the drying hopper, so as to supplement the heating of the microwave irradiation. 
     In a third variant, provision is made to eliminate the microwave irradiation unit  38 . 
     In this case, the post-heating phase takes place only in the feed hopper  40 , where it can be carried out with air or inert gas depending on the desired final temperatures. 
     Thanks to the process and plant of the present invention, it is possible to obtain excellent results in terms of drying the granular polymeric material while optimizing the energy efficiency of the process. 
     Moreover, the plant can change production in a very short period of time, around two hours as against the six hours required in traditional drying plants (with the same production capacity). 
     A further important advantage results from the fact that, when the processing machine is fed with a granular polymeric material at a temperature above the maximum temperature at which it can be maintained in air, the energy efficiency of the processing machine is increased. 
     Moreover, if the granular polymeric material is fed into an extruder, the latter can be given dimensions with a smaller footprint and power, so as to further improve the layout of the plant as well as the energy efficiency.