Abstract:
The invention relates to a method for treating plastic material, especially polyethylene terephthalate, wherein the relatively low temperature material is initially crystallized by heating before subjecting said material to heating or condensation in the solid phase. The material is then exposed to a hot treatment gas for at least 10 minutes in at least two chambers ( 2 ) of an apparatus and crystallized at a temperature above 135° C., e.g. 140-180° C. The is subsequently heated in a preheating chamber ( 3 ) having at least one to eight stages at a temperature of at least 185° C., preferably at least 200° C. and more preferably around 220° C.

Description:
This is a continuation of international application Ser. No. PCT/CH00/00007, filed Jan. 4, 2000, the entire disclosure of which is hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a device for crystallizing plastic material as well as to a method for treating plastic material. 
     2. Description of Related Art and Summary of the Invention 
     Such a method has for example been disclosed in EP-A-0 712 703. When such a method is implemented in practical applications, the finished product has some significant deficiencies: 
     the product is hydrolysed, i.e. partly decomposed and thus its quality is insufficient; 
     the finished product contains a relatively high dust fraction which not only means loss of material but also increased cost per unit of weight of the end product; and 
     the degree of crystallisation of the finished product is quite variable and the amorphous fraction is relatively high. 
     It is thus the object of the invention to create a product, e.g. bottle granulate made of PET or polyester material for tire cord etc., of improved quality with a low dust fraction and improved crystallinity. According to the invention this is achieved by the characterising portions of claim 1 or of claim 8. 
     The invention is based on the recognition that up to now, the production parameters have been selected in a somewhat carefree manner, with a bias to quick production. It has been found for example that slower heating during crystallisation with lower temperatures allows more gentle and economical production where in some cases even air can be used as a treatment medium instead of the normally used nitrogen. 
     It has also been found that the relatively high gas temperatures exceeding 195° C. hitherto applied not only lead to product decomposition and thus to a drop in quality, but also cause excessive heat loss in the apparatus hitherto used. It is for example known from U.S. Pat. No. 5,119,570 to carry out precrystallisation and crystallisation in separate apparatus. However, separate apparatus also means a relatively high surface-to-volume ratio which promotes heat loss. While in the above-mentioned EP-A, precrystallisation and crystallisation are carried out in a mutual device, this does not result in optimisation either. 
     From these findings, the device of the invention was developed which results in a compact and economical design which at the same time results in minimal heat loss. The small heat losses also result in the conditions of crystallisation being better able to be kept under control, so that from this point of view too, there is no compulsion to apply high treatment temperatures. There are above all advantages in that the gas throughput for the compartments from a single gas source can be smaller than was the case so far and in that the overall height of the device itself can be kept lower, thus resulting in savings of space and cost. While a crystalliser with a rotation-symmetrical housing is known from CH-A-665 473, it is not designed or suitable for carrying out precrystallisation and crystallisation, i.e. it requires an additional device for a separate crystallisation step. 
     In the above mentioned EP-A the path of the plastic material flowing through the device is such that a high throughput of air is required so as to generate an aggregate fluidisation, and that the material is thrown above a free space situated above a partition wall, into the next compartment. By contrast, the invention preferably provides for the path to meander by arranging free space or free spaces and discharge aperture(s) at various levels in longitudinal section through the treatment space. While the known throwing-over results in significant differences in dwell times of the individual material components, with the measure according to the invention there is better control over the dwell time. It has been shown that in this way an excellent crystallisation degree with negligible amorphous fraction can be achieved. By contrast, in the case of a meandering path, the quantity of air flowing through can be reduced. In addition, in the case of free spaces or apertures located at the bottom, there is a certain division of the treated material according to its density even in a fluidised bed, with material of increased relative density being more likely to be in the bottom region. While the differences in density between amorphous and crystallised material in the fluidised bed are not very large, the fact that the denser material near the bottom is already crystallised out to a larger degree than the material fluidising further up, does have some significance. Consequently, from the lower free spaces, predominantly the material which has undergone certain crystallisation progress, is conveyed to the next compartment (or to the discharge aperture). 
     According to the above-mentioned findings, the method according to the invention starts off at relatively low temperatures which treat the material gently. While at first this requires somewhat more time, it does however bring about the preconditions for material of improved quality. The time lost in comparison to that of the state of the art can be saved by the subsequent shorter treatment time during preheating or precondensation. Preferably a fluidised bed design is used, so that there is no need for any agitator devices within the treatment space (compare U.S. Pat. Nos. 4,064,112 and 4,161,578), for it has been shown that said agitator devices cause significant losses due to strong dust formation. It must be pointed out that separation of condensation into precondensation and postcondensation with different treatment conditions has already been disclosed in U.S. Pat. No. 3,756,990 and that it is also the preferred way of implementing the method according to the present invention. This is also reflected in the final temperature of the material of at least 185° C., preferably at least 200° C., in particular approx. 220° C. 
     In the aforesaid, among other things, the unevenness of the quality of the finished product has been criticised in the state of the art according to EP-A-0 712 703. Obviously the shortcomings of the crystallisation device that has been used, with random distribution of the dwell time, have been recognised in the state of the art which provides for subsequent heating in a container comprising rotating circulation devices. As has been found, it is precisely these circulation devices which are responsible for generating an excessive dust fraction. For this reason, with a view to a more gentle and controlled treatment of the material, the invention uses a different approach in that the crystallised material is brought into the shape of a rectangular bulk material stream of four-sided, in particular rectangular, cross-section of essentially even bulking across the cross-section; with treatment gas flowing from one side of the four-sided shape through said crystallised material. This means that as a result of this cross-sectional shape, the same conditions prevail for the gas along the entire inflow side, with the essentially even bulk density contributing its part. This can be improved still further in that the ratio of the rectangular sides of the cross-section of the bulk material stream is approximately 1:2 to 1:15, preferably ranging from 1:3 to 1:10, with the treatment gas being conducted through the bulk material stream from the larger side of the rectangle. In this way the method according to the invention differs from all those methods where the thicknesses of the bulk material stream and/or the gas flow conditions along the cross-section are different. 
     For gentle treatment, precrystallisation and crystallisation advantageously require a duration of between 10 and 80 minutes, preferably between 15 and 40 minutes, in particular between approx. 20 and 30 minutes. As mentioned above, according to the invention, subsequent heating up can be made more efficient by shortening this treatment step. According to the present invention this preferably takes place in that the heating following crystallisation, including precondensation, is carried out within a duration of 60 to 120 minutes, in particular approx. 90 minutes. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further details of the invention are provided by means of a preferred embodiment, diagrammatically shown in the drawing, as follows: 
     FIG. 1 is a flow chart of the method according to the invention; 
     FIG. 2 shows a longitudinal section of an embodiment of a crystalliser for precrystallisation and postcrystallisation, according to the invention. 
     FIG. 3 shows a cross-section along the line III—III of FIG. 2; 
     FIG. 4 is a diagrammatic drawing with a longitudinal section through a device preferably used according to the invention, for preheating and condensing as well as cooling; and 
     FIG. 5 shows a cross-section of the above-mentioned device according to line V—V of FIG.  4 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     According to FIG. 1 in a preliminary stage  1  of the method according to the invention, amorphous plastic granulate, in particular polyethylene terephthalate, is made, i.e. essentially extruded and cut into pellets. In the first stage of the process according to the invention, the amorphous plastic material produced in this way is fed to a crystalliser  2  which advantageously comprises precrystallisation and postcrystallisation, as is known from the state of the art but as will be explained below in FIGS. 2 and 3 by means of an embodiment according to the invention. 
     The plastic material is at approximately ambient temperature when it enters stage  2 . Within stage  2 , gas treatment with a hot inert gas takes place, such as nitrogen which is fed into a gas inlet  2 ′ at a relatively low temperature (when compared to the state of the art) of approx. 165° C. to 185° C., e.g. 170° C. to 180° C. so as to achieve gentle treatment of the plastic material. Accordingly, it is better according to the invention if the dwell time in the crystalliser  2  is somewhat longer than is proposed in the state of the art, namely 10 to 80 minutes, preferably 20 to 40 minutes, in particular approx. 30 minutes. With the relatively low temperature and the relatively long dwell time an even and gentle treatment is achieved and it has been shown that with this method, practically complete crystallisation of the material can be achieved. Tests have shown that of the plastic material leaving stage  2 , at the most 1% has remained amorphous, but generally less than that. Subsequently the final temperature of the plastic material is approx. 135° C. to 180° C. when it leaves stage  2 . 
     The material heated to max. 180° C. must now continue to be heated so as to trigger a condensation reaction. Advantageously, a preheater  3  can be provided for this, followed by the actual reactor  4 . As far as the gas circuit  5  (e.g. nitrogen) is concerned, the two stages are interconnected, with a gas purification stage  6  being inserted, before the gas which has passed through the circuit  5  once is again fed to the preheater  3 . 
     The two-stages of the condensation method shown here are normal and advantageous, but not absolutely required. The diagram shown in FIG. 1 merely reflects the fact that it is advantageous if two devices  3  and  4 , which are arranged in line, are provided. However, in practical application, preheating where the plastic material is heated to approx. 180° C. (if the material enters at a lower temperature than this) may be carried out separately in a heating device, with the condensation being carried out subsequently in one or several steps. For reasons of efficiency it is however more favourable if preheating and precondensation, and if required also condensation, are combined in one device. For example it would be imaginable to combine preheater or precondenser and condenser (reactor) in a single device. In this case it is advantageous to provide several gas inlets and outlets at various heights in the vertical reactor so as to divide said reactor into individual zones of different gas temperatures and/or gas quantities and/or speeds. 
     While the reactor  4  will be designed in the conventional way as a wall bed reactor or packed bed reactor with a tube, through which the plastic material passes at a controlled speed, a preferred embodiment of the preheater  3  will be described later by means of FIGS. 4 and 5. The “controlled speed” within reactor  4  mentioned, can be achieved by installing roof-like devices extending across the longitudinal axis. Such roof-like devices exert a braking effect on the material, thus preventing too rapid a flowthrough, but by their roof shape which converges to a point at the top, they promote separation of the individual particles which per se can have a definite tendency to stick together. 
     While the preheater  3  can be designed so as to be in one stage, it preferably comprises at least two stages, if necessary up to eight stages, with the gas temperature advantageously increasing from stage to stage. Depending on the design, at the end of the preheater  3 , a material temperature of 190 to 235° C. will result. In the embodiment shown, which comprises the two stages  3  and  4 , a material temperature of approx. 220° C. will be usual. 
     Subsequently, the reaction should be terminated as quickly as possible. For this purpose a cooling apparatus  7  is connected at the outlet side, with the postcondensed PET material subsequently issuing from said cooling apparatus  7 . However, the material could also be a polyolefin, PEN or PA. If precooling takes place already at the end of the reactor  4 , so that the plastic material issues at a temperature clearly below 185° C., for example at approx. 160° C., then it will no longer be necessary for the cooling apparatus  7  to be using inert gas; cooling with air will be possible. 
     FIG. 2 shows a crystalliser  10  according to the invention which comprises a rotation-symmetrical, in particular cylindrical treatment space  12  (see FIG.  3 ), surrounded by walls  11 , for accommodating the plastic material in the shape of pieces or pellets which issues from the preliminary stage  1  (FIG.  1 ). For control of precrystallisation with external drying-on of the pellets and crystallisation, said treatment space  12  is divided into at least two treatment compartments  12 ′ and  12 ″. This division is by a partition wall  13 . FIG. 3 illustrates that the division is approximately sector-shaped, with the first treatment compartment  12 ′ taking up at least approx. 50% of the cross-sectional area, preferably ⅔ to ¾ of the cross-section. By contrast, compartment  12 ″ takes up the remainder of the available space. However, division per se (when seen in top view or in sectional view corresponding to FIG. 3) could also take place by means of radial division walls around the longitudinal axis A of the crystallisation apparatus  10 . While the embodiment shown only shows two compartments  12 ′,  12 ″, if required, more than two compartments can be provided by at least one further partition wall. 
     The rotation-symmetrical design ensures a high volume-to-surface ratio, so that not only are heat losses minimal, but also an even temperature in the treatment room  12  is more easily assured. Plastic material can be fed to the treatment room  12  via at least one filling aperture  14 . Within the filling aperture, a conventional rotor R for distributing the plastic material during entry in the treatment space can be provided via an upper floor  16 ′. Due to the relatively large volume of the compartment  12 ′, or for space reasons, the wall  13  below preferably comprises a funnel-shaped deflection section  13 ′ which also imparts increased rigidity to the wall. Thus, the plastic material slides down (to the left in FIG. 2) along section  13 ′, into the first compartment or precrystallisation compartment  12 ″. 
     Hot inert treatment gas such as nitrogen, flows through the treatment space  12 . For this purpose a gas inlet nozzle  15  has been provided. The gas then flows upward according to the arrows  17 , through a perforated floor  16 . However, it is understood that as part of the invention it is quite possible to provide for at least two gas inlets  15  such that one of them admits gas only to compartment  12 ′, and the other only to compartment  12 ″. Such a divided gas supply makes it possible if necessary to treat the plastic material in the two compartments  12 ′,  12 ″ with differing gas quantities and/or gas speeds and/or gas temperatures. For example it might be advantageous to provide for a higher gas speed in compartment  12 ′, to achieve faster drying of the surface of the plastic pellets. By contrast, in compartment  12 ″ perhaps a lower speed but a higher temperature of the treatment gas is provided. 
     By providing for a relatively high flow rate of the gas, and for the treatment space  12  to become enlarged in an upper section  11 ′ towards the top, both through the funnel section  13 ′ and through an increase in diameter of the outer wall  11 , the conditions for a so-called aggregate fluidisation bed are created in which the plastic material is very strongly fluidised. In this way, gas flows around the pellets on all sides and the surface of the pellets is dried. Amorphous pellets are somewhat lighter than completely crystallised pellets, and although the difference is not very great, it contributes to carrying out a selection of still amorphous and already partially crystallised material, if the transition from compartment  12 ′ to compartment  12 ″ is selected at the underside of the partition wall  13  where a passage  18  is shown in the drawing. It is understood that it can for example be advantageous if the partition wall  13  is connected to the floor  16  by means of downward protruding stays and if said partition wall  13  is supported by said floor  16 , so that a number of such free spaces  18  are formed. 
     The height of the passage depends on the total volume of the treatment space  12 , the degree to which said treatment space  12  is filled, and the type of material to be treated. It is thus quite possible within the scope of the invention, to provide for an adjustment device for the height of this free space  18 , for example a slide gate which can be approximately slot shaped, delimiting the lower end of the partition wall  13 . If necessary, the vertical position of the wall  13  is secured by radial spokes (not shown), in particular in the upper part of the treatment space, if need be also in the lower part. Generally, however, it has been shown that there is no need for an adjustment device for adjusting the height of the free space  18 , and that this free space can be fixed. It has been shown that a gap height for the free space  18 , of 3 to 8 cm, in particular around 5 cm is generally advantageous. 
     If it is to be possible to operate the crystalliser  10  continuously, as is preferred, an open discharge aperture  19  must always be provided, said discharge aperture  19  which according to FIG. 2 is located at the top of the crystallisation compartment  12 ″ being provided in the end section of a discharge tube  20  which at its lower end discharges the material to the exterior. The location of the free space  18  below the partition wall  13 , and the discharge aperture at the top, results in a somewhat meandering or switchback-like path according to the arrows  21  along which the plastic material must flow to travel from the feed aperture  14  to the discharge aperture  19 . This not only increases the dwell time in the compartments but it also prevents any “shortcut” which would result if the material were able to pass directly from the free space  18  to a discharge aperture below. As the weight fraction of amorphous material has already dropped below half in compartment  12 ′ (precrystallisation), now only a relatively short treatment in the second compartment  12 ″ (postcrystallisation) is required in order to obtain almost 100% crystallisation. Therefore, this compartment  12 ″ can be relatively small. A path according to arrows  21  will however also be advantageous if more than one partition wall  13  has been provided, in which case the material after the upward movement in compartment  12 ″, could carry out a downward movement in a third compartment so that the discharge opening could advantageously be located in the floor region. 
     FIG. 3 shows a few geometric aspects of the design of the crystalliser  10 . In top view the gas inlet  15  and a manhole  22  located opposite, result in a transverse axis T along which the gas outlet  23  is situated, as shown in a dot-dash line (in FIG. 2 situated above the sectional plane III—III). So as to provide access via a manhole  22  to both compartments  12 ′ and  12 ″, the compartment  12 ″ is offset by a specified angle α in relation to the transverse axis T through the manhole  22 . Said angle can range between 30° and 60° but is essentially not critical. For example 30° has proven advantageous because in such a way a geometric arrangement can be achieved in which the lower section  13 ″ of the partition wall  13  in FIG. 3 is located approximately in the middle region of the manhole  22 . Since the desired final condition of the plastic material is to be achieved in compartment  12 ″, it is preferably associated with at least one monitoring device, for example a spectrometer “looking” into compartment  12 ″, said spectrometer detecting the crystallisation state of the plastic by means of the spectrogram determined. But at the very least such a monitoring device can be realised in the shape of an inspection glass  24 . Of course, such a monitoring device can already be provided in the preceding compartment, should this be desired. 
     After passing through the crystalliser (stage  2  in FIG.  1 ), as shown in the diagram, the plastic material reaches a preheater  3 , a preferred embodiment of which is shown in FIGS. 4 and 5. The arrow  10 ′ in FIG. 4 indicates that at this stage the material emanating from the crystalliser  10  enters the feed aperture of a shaft  31  of the preheater (precondenser)  30 . Within an exterior wall  30 ′ approximately circular in shape, the shaft  31  is of rectangular cross-section, as can be seen in particular in FIG.  3 . The ratio of length L to width B is for example approximately 1:3 to 1:10, preferably 1:5 to 1:8, in particular around 1:6. The shaft  31  is delimited on both sides by means of perforated plates, sieves or similar  32 . This ensures even flow conditions and treatment conditions along the entire length L. In addition, the vertical arrangement of the shaft  31  leads to even density of the material across the entire cross-section of the shaft. 
     As is shown, the device  30  which can generally be called a heat exchanger is divided into several stages with different functions. If for example N 2  is used as an inert gas for treatment, said gas can enter at  33  at a relatively low temperature in order to bring the treated goods in a compartment  34  separated by an upper wall  35 , to a lower temperature or to expel volatile compounds which were released in one of the upper compartments. 
     After the nitrogen in compartment  34  has streamed through the sieve walls  32  of the shaft  31 , it leaves this cooling compartment  34  and is subsequently brought to a higher temperature by an electrical heating device E. In this, a temperature for example of at least 185° C. is reached which serves as a holding temperature for precondensation in a compartment  36 . Here again the gas streams in a zigzag through the shaft  31 , i.e. in the opposite direction to the flow in compartment  34  below, so as to subsequently again be further heated in a heating device E and to pass through a precondensation compartment  37  where the plastic material is for example treated at a gas temperature between 200° C. and 240° C., if required up to 260° C. Again, the direction of flow in this compartment  37  is opposite to that in the adjacent compartment  36 . Subsequently, in the uppermost compartment  38  after renewed heating, if required the highest temperature is reached for preheating the plastic material which is still at a lower temperature. However, this is not absolutely essential, for the temperature management can also be selected such that in the last, uppermost heating device E, only the temperature losses in the compartment  36  situated below are compensated for. In this way the treatment gas essentially flows upwards, while the plastic material sinks in the shaft  31 , thus moving in reverse flow. The rate of fall in the shaft  31  can be controlled by controlled discharge at the lower end, for example by means of a cellular wheel sluice  39  with variable speed or e.g. by way of temperature sensors at the end of the shaft  31 . It is understood that if required more than four compartments, e.g. six or eight or fewer, or even only one compartment can be provided, depending on the tasks assigned to the heat exchanger  30 , such as only preheating, precondensation, condensation etc. Nor is it necessary for the gas to be led in a zigzag path, but instead, separate gas inlets may be provided for individual compartments, for example to vary the quantity of flow, the type of gas and/or the gas speed in the individual compartments  34  to  38 .