EP0399864A1 - Process for adjusting the injection of a bituminous binder during the continuous production of bituminous composition for road surfacing - Google Patents

Abstract

The aggregates (7) are brought continuously into a drying and mixing installation (1) at a point at which the binder is injected. The adjustment process consists in measuring the flow in terms of weight of aggregates (T) at a weighing point (11) outside the drying and mixing installation (1). The flow of binder necessary for coating the flow of aggregates measured and the transfer time of the aggregates between the weighing point (11) and the injection point are determined. Finally, injection is effected as a function of the flow and of the transfer time determined. During the startup and shutdown stages of the installation, the determined flow of bituminous binder is injected with a staggering over time greater than the transfer time of the aggregates (7) and in a progressive manner. <IMAGE>

Description

The invention relates to a method for controlling the injection of a bituminous binder during the continuous manufacture of asphalt for road surfaces by mixing aggregates and bituminous binder.

It is known to manufacture bituminous mixes for road surfaces by mixing aggregates and binder such as liquid bitumen.

Prior to mixing with the liquid bitumen, the aggregates are dried and heated to ensure good coating with the bitumen.

It is well known to carry out the drying, heating and kneading of the aggregates in the same installation such as a drum-dryer-coater constituted by a large cylindrical envelope rotating around its axis which is slightly inclined relative to the plane horizontal.

The cold and wet aggregates are brought to one end of the drum and then poured into it by a continuous handling device, such as a conveyor.

A burner enters through one end of the drum, so that hot gases circulate in the drum, in its axial direction. The burner can be introduced through the inlet end of the drum into which the cold and wet aggregates penetrate or through the outlet end of the drum opposite the inlet end.

In the first case, the hot gases and the aggregates circulate in the same direction inside the drum, this circulation being designated as circulation with parallel currents.

In the second case, the hot gases and the aggregates circulate in different directions, this circulation being designated as counter-current circulation.

The binder generally consisting of liquid bitumen is introduced by an injection pipe, inside the drum at an injection point intermediate between the inlet end and the outlet end of the drum.

It is necessary to adjust the injection of bitumen, so that the proportion of bitumen in the mixture constituting the bituminous mixes remains constant and equal to a determined value.

For this, the weight flow of the aggregates introduced into the drum is measured and the corresponding bitumen flow which must be introduced by the injection lance is determined.

The weight flow of the aggregates introduced into the drum is determined by continuous weighing, on a weighing conveyor, of the cold and wet aggregates, at a point which can be relatively far from the inlet of the drum.

To determine the bitumen flow rate that it is necessary to add to the aggregates at a given time, the weight flow rate of dry aggregates corresponding to the flow rate of wet aggregates measured on the weighing conveyor must be calculated. For this, the average proportion of water contained in the wet aggregates is determined and a correction is made to the measurement of the weight flow rate.

In addition, the weighing point being distant from the bitumen injection point, it is necessary to take into account the time of transfer of the aggregates between the weighing point and the injection point, so that the bitumen flow rate determined according to of the aggregate flow is effectively incorporated into the aggregates whose weight flow has been measured at the weighing point.

In the current technique, for an operation of the coating plant in steady state, an average travel time of the aggregates between the weighing table of the weighing conveyor and the injection point is taken into account. The injection of bitumen with a flow rate corresponding to the flow rate determined from the weight flow rate of the aggregates is deferred over time by a duration corresponding to the average travel time of the aggregates.

This technique is entirely satisfactory in the case where the coating plant operates in steady state, as well as during changes in the pace of the installation, in the direction of an increase or a decrease. of the aggregate flow or in the case of changes in the mix composition formula, if these changes take place without interrupting the circulation of the flow of aggregates through the installation.

On the other hand, this technique for adjusting the injection of bitumen is no longer satisfactory, during transient regimes and in particular during the start-up and shutdown phases of the installation at the start or at the end of which the drum is completely empty of aggregates.

During the start-up phase of the installation, if the average travel time T0 of the aggregates between the weighing table and the bitumen injection point is set too high, we recover, at the drum outlet, at the start of the operation of the installation, a large quantity of aggregates not coated with bitumen, generally referred to as "white materials".

If the time T0 is set too low, no white materials are collected at the exit of the drum, at the start of operation of the installation but of materials overdosed in bitumen whose fluidity is normally high (phenomenon designated by specialists under the name of "soup").

When the time 70 actually corresponds to the average journey time of the aggregates, it may be found, depending on the start-up phases, in the presence of one or the other of the manufacturing defects described above.

In all cases, the use of the prior art results in the production of asphalt of unsatisfactory quality during the start-up phases of the installation. This results in material and energy losses and therefore in financial losses.

Similarly, during the shutdown phase of the installation, if the time T0 is evaluated at too high a value, the coated products present the phenomenon of "soup" and if the time T0 is evaluated at a too low value, we produces a significant amount of white material.

In addition, if the average travel time T0 is set to an inaccurate value, the installation is likely to produce asphalt of poor quality during changes of pace or changes of formula "on the fly" that is ie without interrupting the flow of aggregates in the drum.

The object of the invention is therefore to propose a method for adjusting the injection of a bituminous binder during the continuous manufacture of asphalt for road surfaces by mixing aggregates and bituminous binder, the aggregates being fed continuously. in a drying and kneading installation at a point from which the binder is injected and the adjustment process consisting in measuring the weight flow of aggregates at a weighing point outside the drying and kneading installation, in determining the flow of binder necessary to coat the measured flow of aggregates and the transfer time of the aggregates between the weighing point and the injection point and to carry out the injection as a function of the flow rate and transfer time, this process making it possible to obtain a correct dosage of the binder in the asphalt mixes, in steady state, during changes pace or formula as well as during the start and stop phases of the installation.

For this purpose, during the start-up and shutdown phases of the installation at least, the determined flow rate of bituminous binder is injected with a time shift greater than the aggregate transfer time and in a progressive manner.

• La figure 1 est une représentation schémati­que et fonctionnelle du dispositif de réglage de l'in­jection de bitume dans un tambour-sécheur-enrobeur.
• La figure 2 est un diagramme représentatif des variations du débit d'agrégats et du débit de bi­tume introduits dans un tambour-sécheur-enrobeur, pen­dant le fonctionnement de ce tambour en régime perma­nent.
• La figure 3 est un diagramme représentatif des variations du débit d'agrégats et du débit de bi­tume introduits dans un tambour-sécheur-enrobeur, pen­dant le démarrage de ce tambour.
• La figure 4 est un diagramme représentatif des variations du débit d'agrégats et du débit de bi­tume introduits dans un tambour-sécheur-enrobeur, pen­dant une phase d'arrêt de ce tambour.

In order to clearly understand the invention, a description will now be given, by way of nonlimiting example, with reference to the appended figures, the implementation of the method according to the invention, in the case of a work station. coating comprising a drum-coating mixer with parallel current circulation.

• Figure 1 is a schematic and functional representation of the device for adjusting the injection of bitumen in a drum-dryer-coating.
• FIG. 2 is a diagram representing the variations in the flow of aggregates and in the flow of bitumen introduced into a drum-drier-coating, during the operation of this drum in steady state.
• FIG. 3 is a diagram representing the variations in the flow of aggregates and in the flow of bitumen introduced into a drum-drier-coating, during the starting of this drum.
• FIG. 4 is a diagram representing the variations in the flow of aggregates and in the flow of bitumen introduced into a drum-drier-coating, during a phase of stopping this drum.

In FIG. 1, we can see a drum-dryer-wrapper 1 shown diagrammatically and comprising a first zone 2 in which the drying and heating of the aggregates is carried out and a second zone 3 in which the kneading of the dried aggregates is carried out and heated with bitumen introduced by a lance 4.

The flame 5 of a burner enters the first drying and heating zone through the inlet end of the drum through which the aggregates 7 are introduced.

The generally cylindrical drum is rotated about its axis during the operation of the installation. The aggregates 7 are raised inside the drum, by blades integral with the inner surface of the cylindrical envelope of this drum, until forming a curtain occupying the entire section of the drum and separating the drying zone 2 from the coating zone 3. The aggregates 7 are thus exposed to hot gases coming from the burner and circulating inside the drum in its axial direction, between its inlet end and its outlet end which penetrates inside a hopper 8 for recovering the mixes produced in the drum to which is connected a chimney for discharging the gases having passed through the drum.

The cold and wet aggregates are brought to the inlet end of the drum by conveyors 9 and 10 ensuring a continuous supply of the drum with aggregates.

A weighing table 11 is associated with the conveyor 9 in order to determine the weight of aggregates carried by the conveyor at the weighing table. The conveyor 9 also includes a speed measuring device 12, the indications of which, combined with those of the weighing table, make it possible to determine the weight flow rate of cold and wet aggregates transported by the conveyor 9.

The bitumen injection lance 4 is connected, via a valve 13, to a bitumen supply circuit 14.

The circuit 14 comprises a main branch on which are disposed a bitumen pump 15 and a meter 18 as well as a branch in bypass 19. The two branches of the bitumen circuit 14 are connected to a tank not shown in which the bitumen is maintained in temperature. The valve 13 makes it possible to put the main branch of the circuit 14 into communication, either with the injection lance 4, or with the branch 19 in bypass ensuring the return of bitumen to the tank.

The bitumen pump 15 is driven by a variable speed motor 16 controlled by a variator 17.

The installation further comprises a regulator computer unit 20 made up of several calculation and comparison modules.

A first calculation module 21 receives in the form of signals the information coming from the weighing table 11 and from the speed measuring device 12 and makes it possible to determine, from this information the weight flow rate of wet aggregates transported by the conveyor 9 .

A second calculation module 22 receives the output signal from the first module 21 as well as a signal from a humidity measurement probe 23 representative of the water content of the wet aggregates transported by the conveyor 9,

The calculation module 22 determines the flow of dry aggregates corresponding to the weight flow of wet aggregates measured on the conveyor 9.

A third calculation module 24 determines from the flow of dry aggregates from the calculation module 22 and a signal from an input module 25 representative of the proportion of bitumen to be introduced into the aggregates, the weight flow bitumen which must be injected by the lance 4.

It should be noted that the weighing point corresponding to the weighing table 11 is located at a relatively large distance from the injection point corresponding to the end of the lance 4 opening out inside the drum 1.

The aggregates 7 travel this distance in a time T0 which can be evaluated from the average speed of the aggregates between the weighing table 11 and the injection point inside the drum.

A shift register 28 receives a signal representative of the transfer time T0, from an input module 26. The register 28 also receives a signal representative of the bitumen flow rate calculated by the module 24.

Register 28 provides at output, to a comparator 30, a value of the bitumen weight flow corresponding to the weight flow of the aggregates, taking into account a time shift equal to T0. This bitumen weight flow theoretically corresponds to the weight flow of aggregates arriving at the point of injection into the drum.

The bitumen counter 18 makes it possible to send to the module 31 of the regulator computer unit 20, a signal produced in a module 32 and representative of the instantaneous value of the bitumen flow rate injected by the lance 4.

This value of the instantaneous bitumen flow rate is sent to the comparator 30 which generates an output signal as a function of the two input signals representative of the weight bitumen flow rate which must be injected by the lance 4 and of the actual flow rate measured respectively.

The output signal from the comparator 30 controls the variator 17 and the motor 16 so as to adjust the value of the flow rate injected by the lance 4 to the value produced by the calculation module 28.

In FIG. 2, the variations over time of the weight flow rate of bitumen injected into the drum have been shown, during the operation of the installation and the variation during the same time of the weight flow rate of aggregates introduced into the drum.

At an instant t1, the weight flow rate of aggregates introduced into the drum and measured by the weighing table 11 is increased by an amount ΔQ.

The corresponding bitumen flow rate remains constant until an instant t2 = t1 + T0, to take account of the transfer time of the aggregates between the weighing table 11 and the bitumen injection point.

At time t2, the bitumen bit rate is increased by an amount Δ′Q corresponding to the increase ΔQ in the aggregate rate at time t1.

In the case of continuous drum operation, if the transfer time T0 of the aggregates between the weighing point and the injection point tion is correctly determined, the variations in the bitumen flow corresponding to the variations in the aggregate flow are implemented at the desired time so that the proportion of bitumen in the mixes remains constant.

In the case of an operation of the drum in such a way that only changes in shape take place, resulting in a variation in the flow rate of aggregates entering the drum, the technique of the prior art consisting of differing from a time T0, the variations in the flow rate of liquid bitumen corresponding to the variations in the flow rate of aggregates leads to entirely satisfactory results with regard to maintaining a constant proportion of bitumen in the coated products.

FIGS. 3 and 4 show the variations in the weight flow rate of the aggregates entering the drum and in the corresponding weight flow rate of the bitumen injected, during a start-up phase and during a stop phase of the drum-drier-coating machine, using the process according to the invention.

In FIG. 3, it can be seen that the flow rate of the aggregates measured at the level of the weighing table 11 changes at time t1, from a value 0 to a value Qn corresponding to the feed flow rate of the drum under normal conditions of operation. The instant t1 corresponds to the arrival of the flow of wet aggregates, on the conveyor 9, at the weighing table 11.

After the instant t1, the aggregate flow is maintained at its value Qn during the entire period of operation of the installation.

According to the invention, the bitumen flow rate Q ′ is maintained at a zero value until a time t3 = t2 + Tg, the time t2 being itself equal to t1 + T0.

The time t2 theoretically corresponds to the arrival of the first aggregates in the injection area, these aggregates having reached the weighing table at time t1

In reality, as can be seen in FIG. 1, the front part of the drum 1 with parallel current circulation in which the drying zone 2 is located has an increased diameter compared to the remaining part of the drum and its filling with aggregates. delays the arrival of the aggregates at the injection point by a duration Tg. The time Tg corresponds to the filling time of the large-diameter head portion of the drum-dryer-wrapper.

It is obvious that the time Tg depends on the morphology and the dimensional characteristics of the drum used.

From time t3 (t3 = t2 + Tg), the bitumen flow rate Q ′ changes from the value 0 to the value Q′n which represents the weight flow rate of bitumen in the installation in normal operation. This bitumen flow Q′n is calculated as a function of the aggregate flow rate Qn, so that the proportion of bitumen in the mixes produced by the drum-dryer-mixer is fixed at a predetermined value.

As can be seen in FIG. 3, from time t3 to time t4, the bitumen flow rate Q′n is established in successive stages during each of which the bitumen flow rate varies linearly as a function of time. This linear variation mode in successive stages makes it possible to adjust the bitumen in a relatively simple manner, while allowing a gradual and modulated increase in the bitumen flow rate.

This drum operating phase with a duration Tm = t4 - t3 corresponds to a ramp-up phase of the injection of bitumen.

This ramp-up is carried out in three successive stages, each resulting in a linear variation of the bitumen flow as a function of time, with a different proportionality factor.

This gradual increase in the bitumen flow injected is necessary to take account of the progressive constitution of the flow of materials in circulation in the injection zone, after complete filling of the head part of the drum.

This gradual establishment of the normal bitumen injection rate can result in any number of successive phases during which the bitumen rate varies linearly as a function of time.

In reality, the gradual establishment of the bitumen flow depends on the characteristics of the drum-dryer-coating and the variation of the bitumen flow injected as a function of time can be represented by a different curve from a succession of straight lines, as shown. in figure 3.

The process according to the invention makes it possible to add to the aggregates, at the desired time, the quantity of bitumen necessary to obtain bituminous mixes having a proportion of bitumen perfectly constant and corresponding to the desired value.

The delay time Tg due to the filling of the head part of the drum can be determined from the mass of aggregates necessary to ensure the filling of this head part and the flow rate of aggregates in the installation. The time Tg is equal to the ratio of these two parameters.

The time Tm of ramping up of the installation and the rate of variation of the flow rate of aggregates at the level of the injection zone, after filling of the head part of the drum can be determined experimentally at the time of setting up. drum-drier-wrapper service.

FIG. 4 shows the variations in the aggregate flow rate measured at the weighing table 11 of the installation and the corresponding variations in the bitumen flow rate at the injection point during a phase of drum stop followed by complete emptying thereof.

At time t1, the aggregate flow rate measured at the weighing table 11 goes from the value Qn corresponding to the flow rate during normal operation of the drum to the value 0, the conveyor 9 no longer being supplied with aggregates.

Stopping the injection of bitumen at time t1 would result in the production of a large quantity of white materials at the outlet of the drum, that is to say uncoated aggregates.

The bitumen flow rate Q′n corresponding to the aggregate flow rate Qn is therefore maintained until time t2 such that tz = t1 + T0, T0 representing the average time for transfer of the aggregates between the weighing table and the point injection.

However, as can be seen in FIG. 4, the bitumen flow rate Q′n is brought to the value 0 only gradually from time t2 to time t3. This phase, with a duration Td equal to t3 - t2, corresponds to a fall in speed during which the aggregate flow rate at the injection point gradually decreases.

This phase of descent in duration regime Td corresponds to the emptying of the large diameter head portion of the drum, this emptying occurring gradually over time, which requires a gradual decrease in the bitumen flow rate Q′n.

In FIG. 4, it can be seen that this progressive reduction can be represented by a curve constituted by a succession of straight line segments of different slopes. This curve can be determined experimentally and include for example three successive phases with linear variation.

It should be noted that the time Td of emptying the large-diameter head portion of the drum is substantially greater than the time Tg of filling this head portion. This results from the fact that the emptying of the large diameter head portion is not influenced by the thrust of other materials, unlike the filling.

After stopping the installation by interrupting the supply of aggregates, the aggregates remaining in the drum have, after a certain time, a disparate grain size and are not very usable for manufacturing asphalt mixes; on the other hand, their transit time in the drum is extremely long.

This part of the materials whose coating is not of great interest can be used to carry out a cleaning of the drum.

However, it is undesirable to stop the injection of bitumen too abruptly after the supply of aggregates to the drum has stopped, to avoid producing too much white material.

The scheduled and gradual shutdown as shown in Figure 4 avoids this drawback.

In general, the method according to the invention makes it possible to obtain a satisfactory bituminous binder dosage, both under steady conditions and during changes in gait or changes of formula "on the fly" during the operation of the installation. coating. This dosage is obtained by using the average travel time of the aggregates T0 between the weighing table and the binder injection point, as a basis for determining the delay time of bitumen injection.

In addition, the process according to the invention makes it possible to carry out the starting and stopping phases of the coating installation without loss of materials and while maintaining a correct dosage of bitumen in the mixes produced, at all times.

Finally, the method according to the invention makes it possible to take into account the morphology of the drum dryer-coater. For example, in the case of a drum-dryer-wrapper with parallel streams having a large diameter head portion in which the drying and heating of the aggregates are carried out, this shape is taken into account when establishing the required bitumen flow. , at the start of the installation with an additional delay time due to the filling of the large diameter head.

The invention is not limited to the embodiment which has been described.

It is thus possible to envisage variations in the bitumen flow over time during the start-up and shutdown phases of the installation of a form different from those which have been described and re presented. These variations depend in particular on the morphology of the drum-dryer-wrapper, its operating conditions and the nature of the aggregates.

It is possible to apply the adjustment method according to the invention in the case of a dryer drum and enrober with counter-current circulation. In this case, the burner enters through the outlet of the drum and optionally includes an elongated body whose end from which the flame develops is distant from the ends of the drum. Such a drum generally does not have a zone for entering the aggregates with a large diameter and the terminal zone for the drum in which the mixing of the aggregates and the bitumen can on the contrary have an enlarged diameter.

In this case, at the start of the installation, the bitumen flow rate is established progressively without the injection being delayed by a duration corresponding to a filling time of the inlet part of the drum. The gradual establishment of the bitumen flow rate takes account of the progressive constitution of a material flow at a constant flow rate, in the mixing zone.

The bitumen flow rate can be established according to any law of variation as a function of time.

The invention applies to any installation for the continuous production of bituminous mixes by mixing aggregates and binder.

Claims (5)

1.- Method for adjusting the injection of a bituminous binder during the continuous manufacture of asphalt for road surfaces by mixing aggregates (7) and bituminous binder, the aggregates (7) being continuously fed into a installation (1) for drying and kneading at a point from which a binder injection is carried out and the adjustment method consisting in measuring the weight flow of aggregates (7) at a weighing point (11) outside the installation drying and kneading (1), to determine the flow rate of binder necessary to coat the flow rate of aggregates (7) measured and the transfer time of the aggregates between the weighing point (11) and the injection point and at perform the injection as a function of the flow rate and transfer time, characterized in that, at least during the start-up and shutdown phases of the installation, the determined flow rate of bituminous binder is injected with an offset in time greater than transfer time T0 of the aggregates and gradually. 2.- adjustment method according to claim 1, in the case of a drying and kneading installation consisting of a drum-dryer-coating (1) having a head portion (2) with large diameter, characterized by the fact that during the start-up phase of the drum-drier-wrapper (1), the injection of bitumen into the drum (1) is delayed for a time greater than the transfer time T0 of the aggregates between the weighing point (11) and the injection point, of a duration Tg corresponding to the duration necessary for the filling of the head portion (2) with large diameter of the drum (1) by the aggregates. 3.- adjustment method according to claim 2, characterized in that the bitumen flow Q′n corresponding to the normal operation of the drum supplied with aggregates with a flow Qn is established gradually, in successive phases during which the bitumen flow varies linearly over time. 4. Adjustment method according to any one of claims 1 to 3, in the case where the drying and kneading installation is constituted by a drum-dryer-coater (1) comprising a head part (2) to large diameter, characterized in that during the shutdown phases of the drum-dryer-wrapper, the bitumen flow Q′n corresponding to the normal operation of the installation in which a flow of aggregates Qn circulates is brought about progressively from the value Q ′ n to the value 0, after a time at least equal to the time of transfer of the aggregates between the weighing point (11) and the injection point, from the time t1 where the weight flow of the aggregates at the weighing point (11) take a zero value. 5.- Adjustment method according to claim 4, characterized in that the bitumen bit rate changes from Qn to 0, progressively, in successive phases during which the bitumen rate varies linearly by in relation to time.

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