Abstract:
A dehumidification apparatus including an outer tubular container ( 2 ) closed at its ends, a first ( 5 ) and a second ( 9 ) transverse grids located inside the tubular container ( 2 ), one grid element being spaced from the other and from a respective adjacent end of the tubular container ( 2 ) and delimiting a receiving space therewith, a mass of molecular sieves (MS) located in the receiving space, heating elements ( 19 ) in the tubular container ( 2 ), and first and second openings ( 4, 16 ) for communication of the interior of the container with the outside, characterised in that it comprises an inner tubular jacket ( 6 ) extending from at least the first grid ( 5 ) for a distance greater than that between the first ( 5 ) and the second ( 9 ) grid, thereby shaping the receiving space for the sieves ( 8 ) as an annular gap and providing a projecting section ( 6   a ) extending from the second grid ( 9 ); and an annular partition ( 17 ) between the outer container ( 2 ) and the inner jacket ( 6 ) at the projecting section ( 6   a ) of the inner jacket ( 6 ), thereby delimiting an annular manifold space ( 7 ) with the second grid element ( 9 ) communicating with the outside through the first opening ( 4 ), and an end space ( 7   a ) adjacent to the second end of the external container ( 2 ) communicating with the interior ( 18 ) of the inner jacket ( 6 ) and with the outside through the second opening ( 16 ).

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention regards a molecular filter dehumidification apparatus and plant.  
         BACKGROUND OF THE INVENTION  
         [0002]    As is known in the transformation of plastic materials, a very important treatment is the process of dehumidification of the plastic material granules, in other words the removal of the water contained in the granules of so-called hygroscopic plastics.  
           [0003]    The elimination of the humidity content of the granules is necessary because during the high temperature fusion of the material the water penetrates into the molecular chain of the polymer granulate and breaks it, which results in a reduction of mechanical performance and generates bubbles, pitting and uneven colouring of the product.  
           [0004]    Traditional equipment for dehumidifying or separating and eliminating the water from the plastic granules are commonly known as dehumidifiers. In the majority of cases, a dehumidifier is a machine employing the principle of molecular filters. In practice, molecular filters constitute the principle component of the machine, which exploits their capacity for almost total adsorption at room temperature of the humidity of the air.  
           [0005]    The dehumidified and thus dry air is then heated and passed through the granular material in a treatment container employing molecular filters. The granular material gradually yields up its humidity to the dry air passing through it. The duration of the process depends on the residual humidity of the material, and the temperature and flow of dry air.  
           [0006]    The adsorption capacity of the filters is however quantitatively limited and after a certain time saturation is reached. At this point, the dehumidification process is interrupted and may be made to continue using a second container of filters, while the saturated filter container undergoes regeneration.  
           [0007]    The majority of dehumidification plants therefore requires two containers of molecular filters operating in alternation.  
           [0008]    Two kinds of filter containers are known, as shown in FIGS. 1 and 2.  
           [0009]    [0009]FIG. 1 shows a filter container or tower C of cartridge type. In this configuration the air enters via a lower pipe I driven by a blower or compressor (not shown) and passes across an electrical resistance R which is inactive during process. The air then passes through a lower limiting grid GI beyond which it comes into contact with the mass of molecular filters MS.  
           [0010]    Molecular filters capture and retain water molecules contained in the air being processed which thus becomes dried and after passing through the mass of filters MS crosses an upper limiting grid GS and enters an upper pipe S to be conveyed to a hopper (not shown) containing plastic granular material to be treated. The dried air flows through the granular material and is then recycled through the process.  
           [0011]    In this process molecular filters MS of the first layer, i.e. the lowest filters first to be met by the incoming air, become quickly saturated so that humidity adsorption function is gradually taken over by the next following upper layer. When also this layer becomes saturated humidity adsorption takes place in the next following higher layer and so on, until the entire cartridge is saturated, as shown in FIG. 1 a.    
           [0012]    The time required to reach saturation can be generally estimated as a function of the humidity adsorption capacity of the filters and the quantity of water to be eliminated, with an additional charge factor of about 30% to take into account local climatic conditions. Once the preset saturation time for the mass of filters MS has elapsed, the saturated filters are regenerated, i.e. they are subjected to a water removing process to be restored to their initial condition. To this end, controlled electrical resistance R is energized to heat the regeneration air from the lower pipe I to the temperature of about 300° C. required to break the electromagnetic bond binding the water molecules to the filter structure. Freed water molecules are removed by the regeneration air flowing from the bottom to the top of the device.  
           [0013]    As noted above, the tower or container C is never completely saturated, and thus the water, before being removed through the top of the structure, is captured by the upper layers of filters which have not been saturated during processing phase until the same are heated to a sufficient extent by the flow of hot air flowing through them, thereby releasing any water molecules held in them. The phenomenon is repeated and gradually moved upwards until the entire mass of filters MS below the upper limiting grid GS in the tower C has been regenerated.  
           [0014]    [0014]FIG. 2 diagrammatically shows a hollow cartridge tower CC. In order to exploit more uniformly the mass of filters MS in the tower, the an axial pipe TF extending through the filters is provided, which comprises a perforated grid to distribute air uniformly within the cartridge CC in both the lower and upper layers of the filter mass MS. The attainable results are slightly better than those achievable with the configuration of FIG. 1, the direction, course and changes in the air flows.  
           [0015]    Clearly, in order to regenerate conventional filter cartridges or towers a considerable amount of energy and time are required. Most of the energy consumption in such towers is due to production of the heat required for regenerating the sieves. A serious operating problem of conventional dehumidifiers is the fact that such configurations are “orthogonal”, i.e. air is always conveyed along preferential routes in the sieves mass MS caused by the dynamics of the fluid in the tower. This means that it is impossible to exploit entirely the totality of the molecular sieve mass and hence one does not take full advantage of the potential of the tower, although one consumes a greater amount of energy than necessary, which considerably reduces the efficiency as a whole.  
         SUMMARY OF THE INVENTION  
         [0016]    The main object of the present invention is to provide a molecular sieve dehumidification device and a plant of a new conception suitable for eliminating or considerably reducing the above mentioned limitations to be found with the dehumidification towers known in the state of the art.  
           [0017]    Another object of the present invention is to provide a safe and efficient molecular sieve dehumidification device and plant suitable for providing a high efficiency operation.  
           [0018]    A further object of the present invention is to provide a molecular filter dehumidification device and plant which is relatively simple to manufacture and install so as to be advantageous also from the economical point of view.  
           [0019]    According to a first aspect of the present invention there is provided a dehumidification apparatus including an outer tubular container closed at its ends, first and second transverse grid elements located inside said tubular container, one grid element being spaced from the other and from a respective adjacent end of said tubular container and delimiting a receiving space therewith, a mass of molecular sieves located in said receiving space, heating elements in said tubular container, and a first and a second openings for communication of the interior of said container with the outside, characterised in that it comprises an inner tubular jacket extending from at least said first grid element for a distance greater than that between said first and said second grid element, thereby shaping said receiving space for said mass sieves as an annular gap and providing a projecting section extending from said second grid element; and an annular partition between said outer container and said inner jacket at said projecting section of said inner jacket, thereby delimiting an annular manifold space with said second grid element communicating with the outside through said first opening, and an end space adjacent to said second end of said outer container communicating with the interior of said inner jacket and with the outside through said second opening.  
           [0020]    According to another aspect, the present invention there is provided a dehumidification plant for granular material, the plant including at least one treatment silo container for said granular material having one air outlet end communicating with an intake of at least one blower and its other air intake end communicating with a dehumidified air outlet of at least one molecular sieve dehumidification apparatus referred to above and at least one deviation valve between delivery of said at least one blower and said at least one dehumidification device. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    Further aspects and advantages of the present invention will appear more clearly from the following detailed description of a number of currently preferred embodiments, given by way of non limiting examples in FIGS.  3  to  8  of the accompanying drawings, in which:  
         [0022]    [0022]FIG. 1 is a schematic sectional view taken along a vertical plane of a filter dehumidification container or tower employing conventional cartridge filters;  
         [0023]    [0023]FIG. 1 a  shows a diagram of saturation level of the mass of filters in the dehumidification tower shown in FIG. 1;  
         [0024]    [0024]FIG. 2 is a diagrammatic section view taken along a vertical plane of a sieve dehumidification tower of conventional hollow cartridge type;  
         [0025]    [0025]FIG. 3 illustrates a longitudinal diametrical section view of a sieve dehumidification tower during a processing or air dehumidification phase according to a first embodiment of the present invention;  
         [0026]    [0026]FIG. 4 is a view similar to that of FIG. 3, illustrating a molecular sieve dehumidification phase;  
         [0027]    [0027]FIG. 5 shows a detail in section and on an enlarged scale of the dehumidification tower in FIGS. 3 and 4;  
         [0028]    [0028]FIG. 6 illustrates a diagrammatic view partly in section of a plant having two molecular sieve dehumidification towers according to the present invention;  
         [0029]    [0029]FIG. 7 shows a sectional view of a valve structure suitable for being used in a plant according to the present invention; and  
         [0030]    [0030]FIG. 8 is a diagrammatic partly sectional view of a plant with more than two sieve dehumidification towers according to the present invention.  
         [0031]    In FIGS.  3  to  8  of the accompanying drawings, similar parts or components have been identified with the same reference numerals. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0032]    With reference first to FIG. 3, it will be seen that a dehumidification tower  1  according to the present invention comprises an outer container or jacket  2  having a bottom  2   a , preferably cylindrical in shape and made of stainless steel. In the vicinity of its upper end  3  which is preferably curled as will be further explained below, the outer jacket or container  2  is equipped with an air intake/outlet opening or duct  4 , as explained below.  
         [0033]    At a short distance from the bottom  2   a  within the container  2  there is fixed, preferably by welding, a first or lower grid  5 , which extends at two different levels, i.e. it has a central portion  5   a  substantially parallel to the bottom  2   a  and a peripheral (circular) crown portion  5   b  higher than the central portion  5   a  so as to delimit an inner vertical axial shoulder  5   c.    
         [0034]    An inner tubular metal sleeve  6 , preferably made of stainless steel, is inserted into shoulder  5   c , and extends in the longitudinal axial direction. Between outer container or jacket  2  and inner jacket  6  an annular gap  7  is delimited, in which a mass of molecular sieves or filters  8  is located. The mass of molecular sieves  8  is maintained compressed by an upper annular grid  9 , which extends throughout the surface area of the annular gap  7  and is slidably mounted between the inner and the outer jackets  6  and  2 , respectively.  
         [0035]    The mobile grid  9  is also resiliently loaded by a plurality of helical springs  10  arranged angularly spaced from each other along the upper grid  9  and each having one end thereof resting against the grid  9  and its other end reacting against a respective washer  11  fitted onto one threaded end of a respective tie-rod  12  and abutting against a setting nut  13 . The other end of each tie-rod  12  extends out from section  5   b  of the lower grid  5  and is equipped with a head or abutment nut  14 . With such a structure, on the one hand by acting on nuts  13  it is possible to control the degree of compression of the mass of molecular sieves  8 , while on the other any variation in the volume of the sieve mass  8  is automatically compensated during operation of the apparatus  1 .  
         [0036]    The outer container or jacket  2  supports at its top a cap  15  with a securing flange  15   a , extends above and at a distance from the inner jacket  6 , and is formed with an opening or duct for air intake/outlet  16 . Between cap  15  and the upper end  3  of the outer container  2  a sealing device  17  is provided which is shaped as an annular partition as further described with reference to FIG. 5 and delimits an inner space  7   a  with cap  15 .  
         [0037]    The inner tubular jacket  6  extends, in turn, with its end section  6   a  beyond the upper grid  9  and the upper edge  3  of the outer container  2  so as to reach and communicate with the internal space  7   a . Inside the internal tubular jacket  6  an axial light or space  18  is delimited, in which a heat source or heating device is fitted, preferentially comprising one or more electrical resistors  19 , e.g. supported and held in position by cap  15 , through which they extend to be connected to a suitable electrical power source in any suitable manner.  
         [0038]    With a dehumidification tower  1  structured as described above, it is possible to eliminate the problem of formation of preferential routes within the mass of molecular sieves  8  as a flow of air is caused in the tower which is uniformly distributed in the axial direction of the cartridge of molecular sieves.  
         [0039]    As a matter of fact, during an air dehumidification phase, also called a processing phase, air is taken in via pipe  4 , is uniformly distributed throughout the gap  7  above the mobile upper grid  9  and then flows through the compression grid  9  loaded by the springs  10 . The air flow is thus uniformly distributed among the mass of sieves  8  and exits from section  5   b  to enter the space  18  in the inner jacket  6  where the resistor  19  is located. Resistor  19  heats the air flowing through the space  18  and discharged through the exhaust pipe  16  provided in the cap  14  to a temperature suitable for the type of plastics material to be dried. From opening or pipe  16  the heated air is forwarded to a plastics material treatment hopper (not shown).  
         [0040]    Once the mass of sieves  8  has reached saturation, they must be regenerated, as shown in FIG. 4. Through suitable shunting pipes outside the tower  1 , as further illustrated below, the air flow is reversed by setting the opening or pipe  4  into communication with an exhaust duct and blowing air into pipe  16 . The electrical resistor  19  is energized again, but this time it is controlled by a temperature sensor T 1  located between the bottom of the container  2  and the lower grid  5 , so that air flowing through the space  18  is heated by the resistor  19  and reaches a temperature of approximately 300° C. at the bottom of the container  2 .  
         [0041]    Air thus heated then flows through the annular section  5   b  of the lower grid  5  and rises again through the mass of molecular sieves  8 , thereby heating them starting from the bottom to the top before being discharged loaded with humidity through the upper annular grid  9 . The humid air collected in the space above the grid  9  is evacuated into the atmosphere via pipe  4 .  
         [0042]    With this specific configuration, which provided for the air being heated in an area, i.e. duct or space  18 , that is surrounded by the mass of sieves  8 , heat irradiated by the resistance is also partly absorbed by the metal wall of the inner jacket  6 , which transmits it by conduction to the mass of molecular sieves  8  contained in it. In this way, most of the heat emitted by the resistor  19  is exploited for heating the sieves  8 , at least those in the vicinity of the inner jacket  6  containing them.  
         [0043]    Moreover, the provision of the metal sealing device  17  which also acts as a partition located in direct contact with both the intake and outlet air flows makes it possible to obtain an efficient heat exchanger whose potential can vary and depends on its configuration. More specifically, the metal sealing device  17  has the advantage of transmitting heat and thus preheating air and vapours coming in through pipe  16  and at the same time cooling the vapours being discharged via pipe  4 , which results in saving regeneration energy.  
         [0044]    It will be noted that the flows in the processing and regeneration phases are inverted, i.e. regeneration takes place from below upwards while the processing phase takes place from above downwards. Thus, most humidity contained in the air is captured starting from the upper portion of the molecular sieves  8  in the processing phase (FIG. 3), so that the water content in the air decreases towards the bottom of the tower. During the regeneration phase (FIG. 4), instead, owing to the fact that the air flow has been reversed, hot and dry air first meet the layers of molecular sieves  8  containing a lower amount of adsorbed water, which water content is rapidly taken away, and subsequently the layers of sieves containing a higher water content. In this way, any water in the sieves is eliminated in a much shorter time than with conventional methods.  
         [0045]    Practical tests have shown that, compared with conventional solutions illustrated in FIGS. 1 and 2, with an apparatus according to the present invention the efficiency of the sieves  8  in the processing phase is considerably higher, and the time required for the regeneration phase is substantially reduced at the same time.  
         [0046]    One especially advantageous characteristic of the regeneration tower  1  is undoubtedly due to its structure with co-axial components, which does not give rise to preferential air routes and maintains a constant efficiency.  
         [0047]    To obtain such a particularly simple co-axial construction, the delivery and return streams are kept separated, a compartmenting that must be air tight both between both the delivery and return streams and inside and outside of the tower  1  even at the relatively high processing temperatures. Such a construction must also allow components to be disassembled whenever required.  
         [0048]    This has been achieved according to the present invention by means of the sealing device  17  illustrated in detail in FIG. 5. Such device is located between the upper edge  3 , which is preferably curled, of the outer container  1  and the lower edge  15   a , preferably outwardly flanged, of the cap  15 , i.e. between the delivery/return ducts  4  and  16 .  
         [0049]    Sealing device  17  comprises a annular flanged plate-like element  20  having a flared rim  20   a , which delimits the internal light of the flanged plate-like element, and extends, in use, downwards, and an annular counter-flanged plate-like element  21  having a reduced width respect to flange  20  and a flared rim  21   a , which delimits the inner light of the counter-flanged element, and extends upwards, in use. Thus, once the counter-flange  21  has been located on flange  20  with its flared rim extending upwards, it delimits with flange  20  an annular end seat  22  against the inner jacket  6  for receiving a resilient elastic gasket  23  that can withstand temperatures of about 350° C.  
         [0050]    Flange  20  and counter-flange  21  can be fixed together by means of any suitable means, e.g. a plurality of angularly spaced screws  24 , which while locking together the two flanges compress the gasket  23  and push it towards the inner jacket  6  surrounding the heating element  19  in such a way as to ensure tight sealing between the upper inner space of the container  2  delimited by cap  15  and the inner space delimited by container  2 .  
         [0051]    The outer rim of the annular flange  20  is surrounded by an annular silicon gasket  25  having a U-shaped profile and being inserted onto flange  20 . The gasket  25 , in use, is located between the curling of the upper rim  3  of container  2  and the flange  15   a  of cap  15  to prevent air leaks. The curled rim  3  and flange  15   a  are tightened together by a surrounding clamp  26  having a channel-shaped cross-section with inclined side walls, e.g. inclined at an angle of 15° to 30°, and equipped with a lever- or screw-locking device designed to press against the sides of the clamp  26  so as to compress together the flange  15   a , the silicon gasket  25 , the outer rim of flange  20 , and the curled rim  3  of container  2 , thereby obtaining a tight seal.  
         [0052]    A dehumidification tower or apparatus as described above is intended to be part of a dehumidification plant, generally called a “dryer”, —see IT-A-1263362 issued to applicant of the present invention—that can include one or several towers, as illustrated in FIGS.  6  to  8 .  
         [0053]    [0053]FIG. 6 shows a plant comprising two molecular sieve towers  1  connected in such a way that, while one tower is dehumidifying processing air and hence the plastic granules, the other is being regenerated. In such a plant configuration, one of the towers is always available to ensure continuity for the dehumidification process of the granular material being treated.  
         [0054]    The dehumidification plant according to the present invention, shown in FIG. 6 includes a blower  27 , a four way slide valve  28  for reversing the air flow, the valve being fed by a delivery duct  29  on the blower  27  and designed to control the flow through three ducts  30 ,  31  and  32 , respectively, two towers  1   a  and  1   b  and a return filter  33 . Ducts  30  and  31  communicate with the pipe  4  of towers  1   b  and  1   a , respectively, while duct  32  discharges air into the atmosphere via a preferably adjustable calibrated throttling  32   a.    
         [0055]    Pipes  16  of the towers  1   a  and  1   b  are in communication with one another through a common duct  34  from which an exit duct  35  branches off to convey dry air to one or more treatment hoppers  36  containing granular material to be dehumidified. Humidified air that has crossed the granular material contained in the hopper or hoppers  36  is conveyed via a duct or ducts  37  to the intake of the return filter  33 . The dehumidification plant thus operates in a closed circuit.  
         [0056]    In the operating situation illustrated in FIG. 6, while in the tower  1   a  processing air is being dehumidified, in the tower  1   b  sieves  8  are regenerated. The fan or blower  27  circulates the processing and regeneration air. Pressurised air is delivered to control valve  28  which directs the air flow selectively either to tower  1   a  or tower  1   b . In FIG. 6 the slide valve  28  is shown while diverting the processing air towards tower  1   a . Thus, the air enters pipe  4  of the tower and flows downwards through the mass of molecular sieves  8  and removes any water molecules loaded in them. From sieves  8  the air flows through the space  18  and comes into contact with the heating element  19 , thus becoming heated before being conveyed into the common duct  34 . Most of the heated air is directed towards the outlet  35  connected to the delivery to the hoppers  36 .  
         [0057]    In hopper delivery pipe  35  a thermoregulation probe  35   a  is located which controls the power supplied to the electrical resistor  19  which heats the processing air, thereby maintaining the processing air at a constant temperature. The air delivered to the treatment hopper or hoppers  36  then flows through the granular material to be treated and is discharged into return pipe  37  and filtered by filter  33  before being sucked into the suction circuit by the blower  27 .  
         [0058]    A small part of the air coming from tower  1   a  and flowing through duct  34  also reaches tower  1   b  via duct  16  and comes into contact with its electrical resistor  19  controlled by a probe T 1  whereby the air is heated to about 300° C. The air at this temperature is fed from below to the sieves  8  to extract water trapped in the sieves and conveyed into the exhaust duct  30  that, via the slide valve  28 , communicates with the exhaust duct  32 , to be discharged into the atmosphere. The amount of air flowing through tower  1   b  for regeneration purposes is controlled by the calibrated and preferably adjustable throttling  32   a.    
         [0059]    This condition is maintained for a preset time until tower  1   b  is completely regenerated. A cooling phase then starts, which is obtained by switching off resistor  19  to lower the temperature of tower  1   b  to about 80° C which is a preferred operating temperature interval.  
         [0060]    When the slide valve  28 , which is actuated by a hydraulic unit including a piston and cylinder  28   a  or any other suitable driving means reverses the air flow, the towers commutate their functions and that tower which was previously regenerating its sieves  8 , e.g. tower  1   b , is now set to perform air dehumidification, whereas tower  1   a  that was previously effecting air dehumidification is now set to regenerate its sieves  8 . As already noted above, the air flow in the processing tower is directed downwards, whereas the air flow in the tower undergoing sieve regeneration moves upwards to promote removal of any water retained in the sieves. Towers  1   a  and  1   b  thus operate in cyclically opposite direction. In this way, the regeneration resistors  19  are also employed for heating the processing air, thus the provision of a heating chamber being unnecessary. The air exhausted during the regeneration phase can be replaced by providing an inlet hole in the return filter  35  to balance the whole air circuit.  
         [0061]    The entire plant is advantageously programme controlled, e.g. by a programmable controller, generically indicated by CC in FIG. 6  
         [0062]    It should also be noted that the regeneration takes place with dry air and thus with a higher water absorption capacity, whereas during air cooling the sieves  8  are not polluted with ambient humidity.  
         [0063]    In contrast with sieve regeneration circuits known in the art, no coolers for return pipe  37  are required in order to protect the blower  27  against too high temperatures which could detrimental for the blower.  
         [0064]    The dehumidification plant described above has shown to be a very efficient two-tower molecular sieve dryer, very simple in construction with co-axial structure towers operating with dry regeneration air in reverse flows, and with no heating chamber or coolers.  
         [0065]    High performance dryer plants can be obtained by using an extremely simple circuit layout, and more than two independently controlled towers. Such a circuit layout is obtained by employing a valve structure  45  as shown in FIG. 7 to be provided in connection with each tower.  
         [0066]    Valve  45  comprises a main valve body  46  having two internal chambers  47  and  48  separated by an internal partition  49 . Chamber  47  is in direct communication with two openings or ducts  50  and  51 . In the partition  49  an opening  52  is formed which is controlled by a valve or shutter  53 . The shutter  53  supports a smaller shutter  54  on its side away from opening  52 . The two shutters  53  and  54  are controlled by a common rod  55  that is actuated by a linear actuator, e.g. an air actuated cylinder and piston unit  56 .  
         [0067]    Shutter  54  is designed to control an opening  57  provided in a side wall of chamber  48  in axial alignment with opening  52 . Chamber  48  also has a larger opening  58  arranged laterally to the smaller opening  57 . More particularly, opening  52  can be closed by shutter  53  and has a much larger light than opening  57  which is controlled by the smaller shutter  54 .  
         [0068]    The shunt valve  45  of FIG. 7 is used in a plant such as that shown in FIG. 8, which diagrammatically represents a dryer including more than two towers. In FIG. 8 towers  1   a ,  1   c  and  1   d  are shown as being carrying out an air dehumidification process, whereas in tower  1   b  sieves are being regenerated.  
         [0069]    More particularly, air coming from hopper  36  and filtered at filter  33  is taken in by the fan or blower  27  and delivered under pressure to shunt valves  45   a ,  45   b ,  45   c , and  45   d  which can be connected together either in series or in parallel.  
         [0070]    Valve  45 , as described above, comprises two openings  50  and  51  which freely communicate with one another via chamber  47 , opening  50  being in communication with the blower delivery duct  29 , whereas opening  51  is in communication with opening  50  of the immediately successive valve via a duct  51   a ,  51   b ,  51   c , respectively, and so on, the last valve  45  having its opening  51  closed. Chamber  47  also has its opening  52  in communication with chamber  48 , whose opening  58  communicates with pipe  4  of its respective tower.  
         [0071]    Valves  45   a ,  45   c  and  45   d  connected to the processing towers have their shutters  53  raised or in its open position to allow air to flow through it to reach opening  58  for being delivered to pipe  4  in communication with a respective tower, to flow through molecular sieves  8 , and reach space  18  to be heated by the respective resistor  19  to a set temperature for the type of granulate to be dried. Outlet  16  of each tower is communication to a manifold  34  having branches  35  which supply an air flow to respective treatment hoppers or silos  36 , as shown in FIG. 6.  
         [0072]    In the illustrated embodiment tower  1   b  is the sole being regenerating its sieves. In this case shutter  53  rests on its seat closing the passage  52  and shutter  54  is away from its seat  57  thus placing opening  58  in communication with the atmosphere. When shutter  53  closes opening  52  air coming from the blower  27  is prevented from reaching tower  1   b , whereas when smaller shutter  54  is moved away from opening  57  pipe  4  of tower  1   b  is set in communication with the atmosphere. This results in a counter current air flow being generated in tower  1   b  which is in communication with manifold  34 , such counter current flow being issued from opening  57 . As the air from the manifold  34  has been processed in the towers  1   a ,  1   c  and  1   d , such air is dry, and thus it is suitable for regenerating tower  1   b  more effectively. Again regeneration phase occurs as described with reference to FIG. 4.  
         [0073]    As will be noted, compared with the solutions known in the art, the present invention proposes a very simple circuit layout while using modular components. If desired, dehumidifiers can be manufactured with any desired number of towers in order to meet various flow rate requirements.  
         [0074]    With a number of towers higher than three, regeneration occurs in rotation, in a very short time and with the great advantage that once heating is terminated there is no need to cool down the tower. As a matter of fact, once heating is terminated, the air coming from the just regenerated tower can be immediately used in a processing phase, thus obtaining a substantial energy saving owing to the recovery of already accumulated heat. Given that the regeneration temperature is about 300° C. the amount of recovered heat is significant. Moreover, the time required to conduct a regeneration cycle is reduced and constant electrical absorption and efficiency are obtained.  
         [0075]    There is then the possibility of:  
         [0076]    regenerating more than one tower at a time even in response to one saturation control signal,  
         [0077]    incorporating valve  45  in the sieve towers, thus further reducing dimensions and assembly costs,  
         [0078]    isolating one or more towers from the dehumidifier and locate them in the vicinity of the treatment hoppers or incorporate them into the treatment hoppers so as to reduce load losses, heat dispersion and costs.  
         [0079]    The invention described above is susceptible to numerous modifications and variations within the protection scope as defined by the claims.