Patent Application
20180022007
The present invention relates to a twin-screw extruder, and a method for monitoring and controlling such a machine.
The invention examines so-called twin-screw extruders, which comprise a sheath, generally thermo-regulated, inside which two intermeshing screws are rotated around themselves such that they drive a material to be extruded from an upstream part of the sheath to a downstream end of the sheath where the material is then forced to flow through an output device that often includes a shaping channel for the extruded material. In such an extruder, the material undergoes both a mechanical transformation, by pressurizing and shearing via the screws, and a thermal transformation, by adjusting the temperature along the sheath.
Controlling such an extruder is delicate in that the quality of the extruded product depends not only on the mastery of the thermomechanical adjustment parameters of the extruder, which are, inter alia, the rotation speed of the screws, the geometry of the screws, the temperature imposed on the sheath and the intake flow rate of the raw material in the extruder, but also the quality of the raw material introduced into the extruder: in fact, the quality of each of the ingredients of this raw material may vary, in particular due to their physicochemical variability (moisture content, fat content, particle size, etc.), their origin, or even their storage conditions, such that the quality of the extruded material is affected by these factors even with a constant adjustment of the extruder.
To provide a response to this issue, it is known to take samples of the extruded product regularly, then to analyze them a posteriori: it will be understood that this approach does not make it possible to adjust the extruder in real time.
It is also known to try to adjust the behavior of the extruder based on the so-called specific mechanical energy (SME) property. The SME corresponds to the energy supplied by the motor driving the screws of the extruder, relative to the mass unit of extruded material. Thus, the SME reflects the level of mechanical transformation of the material being extruded. The prior art teaches that by continuously measuring the SME, it is possible to regulate the extruder by adjusting, at the running part of the screws, a passage section of the material being extruded: the idea is to “loop”, in other words to slave variations of the passage section for the material being extruded, with the SME measurement. However, this solution only provides a very partial response to the issue described above. Indeed, the SME only provides a partial overview of the quality of the product leaving the extruder. In particular, the residence time of the material in the extruder is not taken into account by the SME, whereas it directly affects the quality of the extruded finished product.
In turn, US 2004/0020272 proposes to take, during the extrusion of the polymer by a twin-screw extruder, rheological measurements of the material processed by the extruder, these measurements being in line, i.e., in the stream of material inside the extruder. To that end, pressure and temperature sensors are arranged on the extruder, more specifically at an outlet channel of this machine, as well as at the space separating the downstream end of the screws and the channel. The various measurements are used to calculate, inter alia, the viscosity of the material in the extruder. More globally, these measurements are processed in real-time to monitor the quality of the extruded polymer, and to adjust the extruder, without, however, US 2004/0020272 explaining how to carry out this adjustment.
DE 44 33 593 adopts an approach similar to US 2004/2020272: an extruder incorporates, between the tip of its screw and an output channel, a viscometer that is made up of several pressure sensors following one another in the flow direction of the processed material. The information delivered jointly by these pressure sensors is representative of the viscosity of the material flowing between the screw tip and the output channel. This information is used, in real-time, to adjust the working temperature and SME of the extruder.
The aim of the present invention is to offer a more effective and more thorough response to the issue of monitoring and controlling extruders in order to obtain a final extruded material with controlled quality.
To that end, the invention relates to a method for monitoring and controlling an extruder, the extruder comprising two intermeshing screws for driving a material to be extruded, the method comprising, while a material to be extruded is processed by the extruder:
One of the ideas at the base of the invention is to use the measurement for the viscosity of the material in the extruder, the viscosity being a relevant marker of the rheological variations of the material being extruded. This measurement is provided to be in-line, i.e., in the stream of the material inside the extruder: indeed, to provide real-time adjustment, the evolution of the viscosity must be assessed continuously for the material being extruded. Furthermore, the viscosity is the resultant of shearing, pressure, residence time, temperature, etc. effects applied to the extruded material. Indeed, the viscosity corresponds to the flow resistance of a material under the influence of at least one stress such as shearing, pressure, gravity, etc.: when the viscosity increases, the capacity of the fluid to flow decreases. The invention thus proposes continuously measuring the viscosity of the material being extruded and using this continuous measurement to adjust, in real-time, a passage section for the material leaving the extruder, i.e., for the material in this machine downstream from its screws: by acting on this passage section of the extruder while all other things are equal, one modifies the filling level of the machine with the material being extruded accordingly, more specifically its filling level along its intermeshing screws that cause the material to advance within the machine, which causes the viscosity of the material being extruded to vary accordingly. In particular, it will be understood that, for example, by reducing the passage section downstream from the intermeshing screws, one increases the quantity of material upstream from the restriction, i.e., at the screws; this amounts to saying that one increases the filling level of the extruder along these screws; and, under the action of these screws, the material is better sheared, which decreases its viscosity. Thus, by having an image beforehand of the viscosity value for the product to be obtained at the output of the extruder, one is capable, when the viscosity measurement deviates from the desired viscosity value, of returning to this desired viscosity value by adjusting this passage section and thus compensating, with a corresponding actuator for controlling the extruder, for the effects of the aforementioned irregularities of the raw material. One then keeps the quality of the extruded material substantially constant, in terms of density, characteristic dimensions, degree of cooking, texture, etc. More generally, by mastering the viscosity of the material during extrusion, one has an overall picture of the quality of the final extruded product. Advantageously, one is thus even capable, subject to this adjustment of the passage section downstream from the screws, slaved to the in-line viscosity measurement, of compensating the effects of the wear of the extruder, in particular the effects of the evolution of the shearing rate resulting from this wear.
More generally, the control method according to the invention makes it possible, inter alia, to:
According to a preferred embodiment of the method according to the invention, the extruder further comprises:
In that way, in order to adjust the extruder, the material filling level in the sheath is adjusted by adjusting a passage section of the flow channel.
In practice, the viscosity may be measured for the material leaving the sheath, in the flow channel of the output device.
According to an advantageous optional arrangement, the extruder is adjusted by further adjusting, based on the material viscosity measurement results, at least one behavior parameter of the extruder chosen from among:
In that way, one or several actuators for controlling the extruder, other than the actuator modifying the passage section of the material in the extruder downstream from the screws of the latter, can be implemented in combination with the actuation for adjusting this passage section, still from the in-line viscosity measurement. The monitoring and control performance of the extruder are improved as a result.
In practice, the nature of the extruded material is irrelevant: the invention relates to the extrusion of agro-food and non-agro-food materials, such as plastic, chemical, pharmaceutical, etc. materials. As a result, the method according to the invention is particularly advantageously applicable when the material to be extruded is non-Newtonian, in particular food. A fluid is considered here to be non-Newtonian when its viscosity depends on the shear rate. It will be understood that for such a non-Newtonian material, no linear laws exist relative to its viscosity: the invention makes it possible, however, to effectively adjust the twin-screw extruder by acting on its filling level, such that, all along the screws, the material is sheared, or more generally, stressed, therefore affecting the viscosity of all of the material, except for the marginal, and therefore negligible, portions of the latter.
The invention also relates to an extruder, comprising:
According to additional advantageous features of the extruder according to the invention:
Of course, the invention applies, without limitation, to various twin-screw extruders, whether the two screws of the latter are contra-rotating or co-rotating.
The invention will be better understood upon reading the following description, provided solely as an example and done in reference to the drawings, in which:
This extruder 1 comprises an elongate sheath 10, which extends along and is centered on geometric axis X-X. Inside the sheath 10, two screws 20 extend parallel to the axis X-X, while being received in a complementary longitudinal bore of the sheath, centered on the axis X-X. These two screws 20 extend on either side of the axis X-X, while being intermeshing, the aforementioned bore of the sheath 10 having a bilobed transverse profile. Each screw 20 is rotated around itself, around its central axis, by a motor unit, not shown in
The screws 20 of the extruder 1 are designed, due to their threaded profile, to drive the material to be extruded along the axis X-X, from an upstream part of the sheath 10, in which the ingredient(s) of this material are introduced into the aforementioned bore of the sheath, to the downstream end of the sheath 10, the terms “upstream” and “downstream” being oriented in the direction of advance of the material inside the sheath under the action of the screws 20, this direction of advance being from right to left in
The sheath 10 comprises several modular elements following one another along the axis X-X, here five of them, respectively referenced 11 to 15 from upstream to downstream. Each of the elements 11 to 15 inwardly defines a corresponding part of the central longitudinal bore of the sheath 10, these bore parts being in the extension of one another, along the axis X-X, in the assembled state of the elements 11 to 15, like in
In the example embodiment considered in
The extruder 1 also comprises an output device 40, which is arranged at the downstream end of the sheath 10. The material leaving the sheath 10 is, under the action of the screws 20, forced to flow through the output device 40, from which the extruded material emerges outside the machine. In the example embodiment of the figures, the output device 40 comprises three separate modular parts, namely:
The plate 41 is securely attached, for example by a locking collar 50, to the downstream end of the element 15, furthest downstream, of the sheath 10. In a manner known in itself, this plate 41 inwardly defines a through bore, which is centered on the axis X-X, which extends in the extension, along this axis, of the central bore part of the element 15, and inside which the free downstream end of each screw 20 is received. The central bore of this plate 41 channels the material leaving the sheath 10 pushed in the downstream direction by the screws 20. Non-limitingly, in the example considered in
The module 43, which will be described in more detail below, comprises a primary case 43.1, which is inserted, in the direction of the axis X-X, between the downstream end of the plate 41 and the upstream end of the die 42, while being fastened thereto by respective collars 44 and 45, and which inwardly defines a material flow channel 43A from the downstream end of the plate 41 to the upstream end of the die 42. This flow channel 43A traverses the case 43.1 axially all the way through, connecting the opposite axial ends thereof, while being substantially centered on the axis X-X and while thus extending in the axial extension of the central bore of the plate 41 and the central longitudinal bore of the sheath 10, as clearly shown in
In a manner known in itself, the die 42 is provided to shape the material extruded by the extruder 1, this material being forced, under the action of the screws 20, to pass through the outlet orifices 42.1 in the downstream direction, inwardly defined by the die 42. The embodiment of the die 42 is not limiting with respect to the invention: in particular, the number, arrangement, and more generally, characteristics of the outlet orifices 42.1 are indifferent. Likewise, non-limitingly, in the example embodiment considered here, the die 42 is equipped, at its upstream end, with a diffuser 42.2 that distributes the material, entering the die, between its outlet orifices 42.1, the inner volume, diverging in the downstream direction, of this diffuser 42.2 being, upstream, connected to the downstream end of the flow channel 43A of the module 43, and downstream, connected to the upstream end of the outlet orifices 42.1.
As clearly shown in
In practice, the integration of the sensor 43.2 and its installation in the flow channel 43A satisfy hygiene, flow and measuring constraints for the material in question to be extruded. In the example embodiment considered in the figures, the sensor 43 is securely carried by a base 43.3, which is attached, securely and sealably, in a dedicated complementary housing 43B defined by the case 43 transversely to the axis X-X. The aforementioned wired connection is, in a manner not shown in the figures, provided to join the outside of the module 43 via this base 43.3.
The module 43 also comprises a flap 43.4 for variable closing off of the flow channel 43A, which is arranged through this channel 43A so as to pivot around a geometric axis Z-Z perpendicular to the axis X-X, and therefore perpendicular to the flow direction of the material in the channel 43A. By pivoting around the axis Z-Z, the flap 43.4 makes it possible to vary the passage section of the flow channel 43A, in other words the passage section for the material being extruded in the extruder 1. In the figures, the flap 43.4 occupies a pivoting position intermediate between, on the one hand, an extreme position, not shown, with maximal closing off and therefore minimal opening, in which the plane of the flap extends globally at or close to the perpendicular to the axis X-X, and on the other hand, an extreme position, not shown, with minimal closing off and therefore maximal opening, in which the plane of the flap extends globally parallel or quasi-parallel to the geometric plane containing the axes X-X and Z-Z. Thus, by pivoting between the two aforementioned extreme positions, the passage section of the flow channel 43A varies between a maximum and a minimum, this variation of the passage section being adjustable based on the pivoted position of flap 43.4 around the axis Z-Z. It will be understood that by modifying the passage section of the flow channel 43A, one modifies the filling level of the extruder 1 accordingly upstream from the flap 43.4, and in this way, in particular the shear rate applied to the material by the screws 20 and therefore the viscosity of this material. In practice, for safety reasons, it is preferable, even in the extreme maximal closing off position, for the flap 43.3 not to completely close off the flow channel 43A in order to avoid any overpressure of the extruder 1.
The pivoted position of the closing off flap 43.4 is commanded from outside the module 43. In the example embodiment considered in the figures, the flap 43.4 is secured to a rod 43.5 driving the rotation around the axis Z-Z, which is substantially centered on this axis Z-Z and one longitudinal end of which emerges outside the module 43, i.e., upward in the figures. This rod 43.5 is mounted rotating in a complementary support 43.6, in turn received, fixedly and sealably, in a dedicated complementary housing 43C defined by the case 43.1 of the module 43. For precise control of the angular position of the rod 43.5 around the axis Z-Z, and therefore the pivoted position of the flap 43.4, this rod 43.5 is advantageously provided with an outer thread, screwed into a complementary tapping defined by the support 43.6. Furthermore, in order to reinforce the stability of the flap 43.4, the latter is, opposite the rod 43.5 along the axis Z-Z, secured to a pin 43.7, centered on the axis Z-Z and rotatably received around its axis in a complementary support 43.8, in turn attached, fixedly and sealably, in a dedicated complementary housing 43D defined by the case 43.1 of the module 43.
Of course, the embodiment of the flap 43.4 and the associated parts 43.5 to 43.8 is not limiting with respect to the invention: more generally, the module 43 is equipped with a closure member for its flow channel 43A, the variable closure action of which, resulting from its mobility in the flow channel, is adjustable, in particular from outside this module.
Furthermore, it will be noted that, in the embodiment considered in the figures, the closure flap 43.4 or, more generally, a variable closure member of the flow channel 43A is advantageously placed downstream from the viscosity measuring sensor 43.2. Although it is possible to consider, as an alternative that is not shown, the viscosity measuring sensor being downstream from the closure flap 43.4 or, more generally, a variable closure member of the flow channel 43A, the arrangement shown in the figures is preferred because in this way, the viscosity measured by the sensor 43.2 is precisely that of the material exclusively having undergone shearing by the screws 20, and not that of the material also having crossed the flap 43.4 or the closure member. In all cases, the module 43 is advantageously inserted particularly compactly between the plates 41 of the sheath 10 and the channel 42.
As shown schematically in
A method for controlling the extruder 1 described thus far with respect to
Rather than implementing a slaved loop like with the extruder 1 of
Taking the above considerations into account, one option of the method for controlling an extruder similar to the machine 1 consists of using one or several behavior parameters of this extruder in addition to that related to the passage section of the flow channel 43A. Indeed, the real-time measurement of the viscosity of the material being extruded, in the flow of this material inside the extruder, provides viscosity measurement results based on which this extruder can be adjusted, subject both to the adjustment of the passage section for the material being extruded downstream from the screws 20 and the adjustment of one or several additional behavior parameters of this extruder, this or these additional parameters being chosen from among:
Furthermore, one alternative of the method described thus far consists of the in-line viscosity measurement not at the output device 40, but at one of the elements 11 to 15 of the sheath 10, in particular the element furthest downstream 15. In other words, the continuously measured viscosity is no longer that of the material leaving the sheath, but that of the material in the sheath, in particular in the downstream part of this sheath. Of course, in this case, the extruder is, in a manner not shown in the figures, arranged accordingly: for example, a viscosity sensor, similar to the sensor 43.2, is arranged in the bore of the sheath, in particular at a zone of the screws 20, in which its or their thread is locally reduced or even eliminated in favor of a substantially smooth surface. It will thus be understood that in general, the extruder according the invention comprises in-line viscosity measuring means, i.e., means making it possible to measure the viscosity of this material in the flow of the material flowing in its sheath or in its output device, or even both, for example for measuring safety or extruder adaptability reasons.
Various arrangements and options for the extruders described thus far, as well as the control method, can also be considered. As examples:
| Number | Date | Country | Kind |
|---|---|---|---|
| 1550835 | Feb 2015 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2016/052139 | 2/2/2016 | WO | 00 |
