Patent Description:
As well known, the polymer-bitumen membranes are manufactured by hot impregnation of a polymer support armature with bitumen. The bitumen is normally provided at a temperature of about <NUM>-<NUM>, so that it is fluid enough to be easily conveyed and easily used for impregnation. Due the temperature and the vapor pressure of bitumen, relatively important amounts of bad-smelling and potentially harmful bitumen vapours are present in the vent gas from the pieces of equipment in which the bitumen is stored and processed. These vapours must therefore be removed from the vent gas before it is emitted into atmosphere.

<CIT> discloses a plant and a method for treating bitumen vapours, in which systems are provided for extracting the vapours and for mixing hem with external air, for feeding the obtained vapour/air mixture into a boiler as primary air, and for adjusting the vapour/external air proportion based on the composition of the combustion gas produced by burning the mixture.

Such invention makes it possible to reduce the amount of pollutants emitted into atmosphere along with the combustion gas within the limits set by the regulations and, less successfully, makes it possible to reduce the emitted bad-smelling substances (volatile hydrocarbons) contained therein within acceptability limits. This is an improvement of previous techniques for treating vapours of bitumen, which involve steps of sucking and conveying the vapours through active coal filters, washing towers and other devices. The solution of <CIT> is particularly well-suited for vapours coming from bitumen storage tanks.

However, as well known, before the impregnation, a very fine and impalpable mineral filler powder is added to the bitumen, for instance talc is added, which has normally a maximum of the grain size distribution curve at about <NUM>,<NUM>. This powder is mainly introduced as a release agent. In fact, the manufactured membranes are wound to form rolls to be more easily transported and sold, and such rolls could not be easily unwound when the membrane is used.

Unavoidably, a certain amount of the filler powder is sucked through the gas vent ducts from the pieces of equipment of the production line along with the vapours sent to combustion. Particularly critical are the piece of equipment known as "melting tank", which are stirred tanks in which the filler is mixed to the bitumen. The powder suction mainly occurs when the filler is fed into such these pieces of equipment, and until it is not fully incorporated into the bitumen by stirring.

In a plant like the one described in <CIT>, the powder settles in the burner of the boiler and on the surfaces of the heat-exchange equipment for recovering heat from the combustion gas, which progressively decrease their effectiveness. For this reason, the manufacture must be often discontinued for cleaning and maintaining the burner and this equipment, which limits the production rate of the polymer-bitumen membranes manufacturing lines.

Moreover, the filler that is present in the vent gas from the melting tanks can reach the chimney and a fine powder emission occurs, which is harmful for health and environment.

<CIT> relates to a system for making an HMA-type asphalt (Hot Mix Asphalt) in which a so-called "pug mill" is used to mix fine and coarse asphalt particles, and in which a combination of a dryer, a cyclone separator and.

<CIT> discloses a process to make a pavement material based on a conglomerate comprising solid particles embedded in such a binding material as bitumen, wherein steps are provided of heating at least one part of the solid particles up to a temperature higher than <NUM>, of collecting the vapours formed therefrom, and of separating these vapours into a fraction of suspended materials and a gaseous fraction, in particular, by bringing the vapours into contact with a washing liquid such as water, and possibly a step of settling the suspended materials. The process aims at preventing vapours containing harmful gaseous substances or fine particulate from being emitted into atmosphere while making the pavement material.

In <CIT> crushed stone, pitch and mineral powder are poured into a mixing tank; the pitch smoke dust produced in the mixing tank is introduced into a waste gas pipe by suction of a fan, and the waste gas pipe is communicated with a scrubbing tower; the pitch smoke dust passing the scrubbing tower passes a purifier, and tail gas is obtained; residual combustible materials are burnt by a burner.

<CIT> relates to an improvement of previously known recycling processes of asphalt-aggregate compositions, including a step of directly exposing the latter to hot combustion gas, typically within elongated rotating drums. From such processes, gaseous mixtures containing volatile hydrocarbon and mineral particulate are obtained, which are sent into a treatment chamber along with a flame in order to be burnt, for atmospheric pollution control purposes.

It is therefore an object of the present invention to provide a plant for treating by incineration bad-smelling gases released by pieces of equipment of a plant, such as a polymer-bitumen membranes production line, said gases containing bitumen vapours and thin powder, which avoids that the powder settles on the burner and on possible heat-exchange surfaces of heat recovery equipment, so as to perform cleaning/maintenance operations less often.

It is also an object of the invention to provide such a plant that can further reduce the smell of the waste matter produced by the production line, in particular, the smell of the exhaust combustion gas.

These and other objects are achieved by a process and by an apparatus as defined by the independent claims n. <NUM> and <NUM>, respectively, for treating a raw vent gas containing bitumen vapours and released by at least one piece of equipment of a plant, for example of a polymer-bitumen membranes production line, the piece of equipment configured to carry out a step of treating a filler powder, wherein the piece of equipment is supplied with the filler powder and/or performs a mixing operation of the filler powder with the bitumen, for example, for making polymer-bitumen membranes, during the step of treating the filler the raw vent gas turning from a normal raw vent gas, substantially free from powder, into a powder-containing raw vent gas the also contains an amount of said powder filler. Particular embodiments of the invention are defined by the dependent claims.

The filler powder typically consists of particles having a diameter, which can be a true diameter or an equivalent diameter, smaller than <NUM>. In particular, the grain size distribution curve of the filler powder normally has a maximum at about <NUM>, <NUM>.

A first aspect of the invention is a process according to claim <NUM>.

A second aspect of the invention is an apparatus according to claim <NUM>.

This way, only a negligible amount of filler reaches the burner of the boiler and possible heat-exchange surfaces of the boiler, which makes it possible to reduce the frequency of maintenance and/or cleaning operations, to maintain their effectiveness in the time and, in particular, to prevent them to be worn away.

It has been also observed that, surprisingly, most unburnt bad-smelling compounds that are present in the vapours are retained by this filtration system along with the filler, which are thought to travel through the burner without being burnt, and this improves the smell-removal performances with respect to the cited prior art, and further lower the bad-smelling emissions.

The surfactant that is present in the washing liquid serves to increase the wettability of the particles forming the powder, by increasing their affinity to water, so as to retain these particles in the mass of the washing solution and to prevent them from leaving the washing solution along with the purified vent gas, in the step of second conveying / through the second vent gas duct.

The surfactant can be chosen according to the nature of the powder that is present in the raw vent gas. In particular, in the case of a polymer-bitumen membranes production plant, the powder is the filler that is added to the bitumen to make the membrane, and usually essentially consists of calcium carbonate CaCOs. In this case, the surfactant is preferably a non-ionic surfactant. For instance, the non-ionic surfactant can comprise an aliphatic glycol, such as propylene glycol. As an alternative, the surfactant can be cetyltrimethylammonium chloride, also known as cetrimonium chloride or CTAC. Preferably, the concentration of the surfactant lies between <NUM>,<NUM>% and <NUM>,<NUM>% by weight, even more preferably between <NUM>,<NUM>% and <NUM>,<NUM>% by weight.

Preferably, the gas-washing device comprises a tank having an inlet mouth for the raw vent gas and an outlet mouth for the purified vent gas, wherein the inlet mouth and the outlet mouth are connected to the first and to the second vent gas duct, respectively, the tank configured to form a liquid head at a height above the bottom of the tank that is higher than the height of the inlet mouth, so that in a step of filling the tank with the washing solution, the inlet port is submerged by the washing solution and, in a step of introducing the raw vent gas into the tank through the inlet mouth, the raw vent gas performs a step of bubbling through the washing solution contained in the tank, which causes the powder to be removed from the powder-containing raw vent gas.

As an alternative, or in addition, the gas-washing device includes a washing tower comprising:.

a step is carried out of counter-current or equicurrent contacting, respectively, the washing solution and of the raw vent gas, and a removal of the powder from the powder-containing raw vent gas takes place.

Preferably, the process provides a step of directly conveying the normal raw vent gas into the boiler, wherein, while the at least one piece of equipment performs the step of treating the filler, the step of directly conveying is discontinued, and the steps of first and second conveying are carried out.

To this purpose, in an exemplary embodiment according to the second aspect of the invention, the plant also comprises:.

As described, some pieces of equipment or even a single piece of equipment of the production line involved in the polymer-bitumen membranes production cycle release(s) powder-containing gas during some filler treatment operations, typically during the steps of feeding the filler into the pieces of equipment and during an early step of mixing it with the bitumen.

Preferably, at a given instant, the steps of first and second conveying the powder-containing raw vent gas are carried out from only a group of pieces of equipment that are currently engaged in such a step of treating the filler, while the step of directly conveying the normal raw vent gas is carried out from the remainder piece(s) of equipment of the plurality, which are not engaged is any filler treatment operation.

To this purpose, in an exemplary embodiment according to the second aspect of the invention, the first vent gas duct and the direct vent gas duct have respective connection branches to each piece of equipment of the plurality of pieces of equipment, and the above mentioned valve group is arranged on each of these connection branches to selectively open/close each connection branch between the respective piece of equipment and the first vent gas duct or the direct vent gas duct. Each valve group can comprise two two-way valves, one arranged along the connection branch to the first vent gas duct and the other arranged along the connection branch to the direct vent gas duct, or can comprise a diverting three-way valve with an outlet mouth arranged at the root of the connection branch to the direct vent gas duct and the other outlet mouth arranged at the root of the connection branch to the first vent gas duct.

In this case, the operation of the valve group for diverting the flow of the raw vent gas from a predetermined piece of equipment from the direct vent gas duct to the first vent gas duct, or vice-versa, is carried out automatically when the step of treating the filler starts or ends, respectively, in this piece of equipment.

To this purpose, the plant preferably comprises a control unit configured to:.

The ongoing filler treatment signal can be generated in a way known to the skilled person, for instance, it can be a signal coming from a limit switch device system that detects the open or the closed state of a filler inlet valve of each piece of equipment that can perform a step of treating the filler, such system for example cooperating with a timer switch that switches once a predetermined time has elapsed since the opening and/or the closing of such an inlet valve. As an alternative, this ongoing filler treatment signal can be automatically generated when an opening command is sent to the inlet valve, or when an actuation command is sent to a different means for feeding the filler into the piece of equipment, or the signal can be even triggered by a powder presence detector arranged to detect the presence of powder above the bitumen that is present in the piece of equipment.

Preferably, the process provides a step of mixing the raw vent gas or the purified or normal raw vent gas with external air, preferably withdrawn at a temperature set between <NUM> and <NUM>, such that a mixture external air / raw vent gas is obtained. To this purpose, the plant comprises a means for mixing external air with the raw, purified or normal raw vent gas, preferably on a branch arranged for feeding boiler primary air, to which the direct vent gas duct and the second vent gas duct converge.

Advantageously, the step of second conveying of the purified vent gas and/or the step of directly conveying are carried out in such a way to keep the purified vent gas within the boiler for a residence time of at least <NUM> seconds. To this purpose, the size of the combustion chamber of the boiler and the features of the suction device, which is typically a fan, are selected in a coherent way to provide a predetermined gas flowrate through the combustion chamber to provide residence time longer than this minimum value.

Preferably, the step of second conveying and/or the step of directly conveying are carried out in such a way to feed the mixture into the boiler as primary air. To this purpose, the second vent gas duct and/or the direct vent gas duct is pneumatically connected with a primary air feeding means of the boiler.

Preferably, the process provides steps of:.

and the step of mixing with external air provides a step of adjusting an external air proportion in the mixture, in such a way to maintain the oxygen and/or carbon monoxide content in the combustion gas above and below the predetermined limit values, respectively.

To this purpose, the plant also comprises:.

The above described vent gas treatment process, aiming at reducing bad-smelling emissions, relates to a vent gas that contains bitumen vapours and a filler powder, however, it can be also used to treat industrial vent gas in general, in which there is a problem of reducing the smells and at the same time powder solids are present in a vent gas stream, as it is the case with the vent gas manifolds in some distillation plants, filtration plants in which filters are provided that require preparing a fossil flour pre-panel, and the like.

It falls within the scope of the invention also a process for making polymer-bitumen membranes, comprising a step of mixing a filler powder with bitumen, thus obtaining a polymer-bitumen membrane, and obtaining a raw vent gas that contains bitumen vapours and a residue of the filler powder, and also comprises a process for treating the raw vent gas in which at least one of the above described features is provided.

It falls within the scope of the invention also an apparatus for making polymer-bitumen membranes, configured to carry out a step of mixing a filler powder with bitumen, thus obtaining a polymer-bitumen membrane, and obtaining a raw vent gas that contains bitumen vapours and a residue of the filler powder, and also comprising a plant for treating said raw vent gas including at least one of the above described features.

The invention will be now shown with the description of its exemplary embodiments, exemplifying but not limitative, with reference to the attached drawings, in which:.

With reference to the attached drawings, a process and a plant are described for treating a raw vent gas released by the pieces of equipment of a plant, typically a polymer-bitumen membranes production line. The described technique can be used, in particular, for treating vent gas coming from the melting tanks of a polymer-bitumen membranes production line, typically in the steps of loading and early mixing a filler powder such as fine talc in which, for instance, most particles have a dimension about a maximum of the grain size distribution curve at about <NUM>,<NUM>.

<FIG> diagrammatically shows a melting tank <NUM> of a plant such as a polymer-bitumen membranes production line, i.e. a piece of equipment used as a mixer of bitumen <NUM> with a filler powder <NUM>, as well as a temporary storage and supply unit of bitumen <NUM>. Melting tank <NUM> typically comprises a tank <NUM>, a mixing means <NUM> for mixing bitumen <NUM> and filler powder <NUM>, in this case a mechanical agitator <NUM>, as well as a filler powder feeding means <NUM>, in this case, for instance, a hopper <NUM> and/or of a valve <NUM> to be opened when performing the steps of loading filler powder <NUM> into tank <NUM>. The means for feeding filler powder <NUM> can also comprise a means for conveying filler powder <NUM>, not shown, from a storage of filler powder <NUM> to melting tank <NUM>, for example a pneumatic conveyor, a bucket conveyor or of any other known for solid matter conveyor, comprising an electromechanical actuation system. Melting tank <NUM> is also equipped with a discharging line <NUM> and a corresponding discharge valve in order to transfer the bitumen-filler mixture to a subsequent operation, typically to a polymer-bitumen membrane production step.

Melting tank <NUM> is maintained under suction by a gas vent line <NUM>,<NUM> and a suction device <NUM>, as described more in detail hereinafter, the main reason of which is to prevent bad-smelling vapours from diffusing into the working areas. Due to the suction, a gas stream of raw vent gas <NUM>,<NUM>' is steadily released by piece of equipment <NUM> during each step of working cycle in which it is involved.

As anticipated, the step of treating filler powder <NUM> in melting tank <NUM> essentially includes the operations of feeding filler powder <NUM> into tank <NUM> and of mixing filler powder <NUM> with bitumen <NUM>. In other steps of the working cycle in which melting tank <NUM> is involved, i.e. when only bitumen <NUM> is present within tank <NUM>, or once filler powder <NUM> has been fully incorporated into bitumen <NUM>, the raw vent gas is a normal raw vent gas <NUM> that is substantially free from filler powder <NUM>. Instead, during the step of treating filler powder <NUM>, including the above operations, the raw vent gas is turned from substantially powder-free normal raw vent gas <NUM> into a powder-containing raw vent gas <NUM>', i.e. a vent gas that contains an amount of said filler powder <NUM> beside the bitumen vapours.

The process according to the invention provides prearranging a gas-washing device <NUM>, in addition to boiler <NUM> having for instance the features described in <CIT>. Gas-washing device <NUM> and boiler <NUM> are arranged in such a way that raw vent gas <NUM>,<NUM>', in particular powder-containing raw vent gas <NUM>' is first conveyed into gas-washing device <NUM> and then into boiler <NUM>, as a purified, substantially powder-free vent gas <NUM>, or directly as normal raw vent gas <NUM>.

To this purpose the plant comprises, in addition to gas-washing device <NUM> and boiler <NUM>, a first vent gas duct <NUM> between piece of equipment <NUM> and gas-washing device <NUM>, and a second vent gas duct <NUM> between gas-washing device <NUM> and boiler <NUM>. The plant also comprises a suction device <NUM> arranged to convey raw vent gas <NUM>,<NUM>', in particular powder-containing raw vent gas <NUM>', and purified vent gas <NUM> through first vent gas duct <NUM> and second vent gas duct <NUM>, respectively. In a preferred exemplary embodiment, the suction device is a fan <NUM>, and is arranged along first vent gas duct <NUM>, i.e. between piece of equipment <NUM> and gas-washing device <NUM>.

Gas-washing device <NUM> is configured to receive a washing solution <NUM> containing a surfactant, and to bring raw vent gas <NUM>,<NUM>' into contact with washing solution <NUM>, in order to remove the amount of filler powder <NUM> that is present in powder-containing raw vent gas <NUM>', and to obtain purified vent gas <NUM> from gas-washing device <NUM>, substantially free from filler powder <NUM>.

Subsequently, purified vent gas enters into boiler <NUM>, in which the combustion of the bitumen vapours contained therein occurs, thus obtaining a combustion gas <NUM>, for example in the way described in <CIT>.

The surfactant present in washing solution <NUM> can be selected among well-known surfactant, at a concentration that can depend on the selected surfactant type and on the type of washing device <NUM> used.

The surfactant present in washing solution <NUM> can be also chosen according to the nature of the powder that is present in powder-containing raw vent gas <NUM>'. In particular, in the case of a plants for making polymer-bitumen membranes, the powder is the filler that is added to the bitumen to manufacture the membrane, which is usually essentially consist of calcium carbonate CaCOs. In this case, the surfactant is preferably a non-ionic surfactant. For example, the non-ionic surfactant can comprise an aliphatic glycol, in particular propylene glycol. As an alternative, the surfactant can be cetyltrimethylammonium chloride, also known as cetrimonium chloride or CTAC. Preferably, the concentration of such surfactant in washing solution <NUM> is set between <NUM>,<NUM>% and <NUM>,<NUM>% by weight on the amount of washing solution <NUM>, more preferably it is set between <NUM>,<NUM>% and <NUM>,<NUM>% by weight on the amount of washing solution <NUM>. Preferably, gas-washing device <NUM> has one of the forms shown in <FIG> and <FIG>.

In particular, with reference to <FIG>, gas-washing device <NUM> comprises a tank <NUM> with an inlet mouth <NUM> and an outlet mouth <NUM> for raw vent gas <NUM>,<NUM>', in particular for powder-containing raw vent gas <NUM>', and for purified vent gas <NUM>, respectively, said inlet and outlet mouths connected to first vent gas duct <NUM> and to second vent gas duct <NUM>, respectively. Inlet mouth <NUM> of raw vent gas <NUM>,<NUM>' is arranged at a predetermined height h from a bottom <NUM> of tank <NUM>. For instance, as shown in the figure, inlet mouth <NUM> can comprise an open end of a portion <NUM> of first vent gas duct <NUM> that is arranged as a dip tube within tank <NUM>, but can also comprise a nozzle, not shown, that is arranged on the shell of tank <NUM> at distance h from bottom <NUM>.

Tank <NUM> is configured to form a liquid head at a height H from bottom <NUM> above inlet mouth <NUM>, so that the latter is submerged by washing solution <NUM>. More in detail, tank <NUM> is configured to be filled with washing solution <NUM> through a duct <NUM> and a liquid inlet nozzle <NUM> up to a predetermined level H over bottom <NUM> higher than height h of inlet mouth <NUM> above bottom <NUM>, so that inlet mouth <NUM> of raw vent gas <NUM>,<NUM>' is submerged by washing solution <NUM>. This way, by introducing raw vent gas <NUM>,<NUM>' into tank <NUM> through inlet mouth <NUM>, it bubbles through washing solution <NUM> contained therein, which allows removing filler powder <NUM> that is present in the gas, in the case of powder-containing raw vent gas <NUM>'. A liquid discharge nozzle <NUM> is also provided that can be used in order to periodically change solution <NUM> of gas-washing device <NUM>, or to allow a continuous flow, a gooseneck tube portion being preferably provided vertically extending up to height H above bottom <NUM>, in order to maintain the liquid head.

As an alternative thereto, as shown in <FIG>, or in addition thereto, in a modification not shown, gas-washing device <NUM> still comprises a tank <NUM>, but in the form of a washing tower <NUM>, i.e. an elongated tank with an inlet mouth <NUM> and an outlet mouth <NUM> for raw vent gas <NUM>,<NUM>', in particular for powder-containing raw vent gas <NUM>', and for purified vent gas <NUM>, respectively, and connected to first vent gas duct <NUM> and to second vent gas duct <NUM>, respectively. Washing tower <NUM> also comprises a second inlet mouth <NUM> and a second outlet mouth <NUM> for washing solution <NUM>, which are also arranged at opposite end portions of washing tower <NUM>. In the exemplary embodiment as depicted, second inlet mouth <NUM> is arranged at an end portion <NUM>' at which first outlet mouth <NUM> is arranged, in such a way that, by conveying raw vent gas <NUM>,<NUM>' through washing tower <NUM> between first inlet mouth <NUM> and first outlet mouth <NUM>, and by conveying washing solution <NUM> between second inlet mouth <NUM> and second outlet mouth <NUM> a step is performed of countercurrent contacting raw vent gas <NUM>,<NUM>' and washing solution <NUM>. In an exemplary embodiment, not shown, second inlet mouth <NUM> is arranged at an end portion <NUM>' at which first inlet mouth <NUM> is arranged, in such a way that, by conveying raw vent gas <NUM>,<NUM>' through washing tower <NUM> between first inlet mouth <NUM> and first outlet mouth <NUM> and by conveying washing solution <NUM> between second inlet mouth <NUM> and second outlet mouth <NUM> a step is performed of equicurrent contacting the raw vent gas and washing solution <NUM>.

Such step of countercurrent or equicurrent contacting allows, in the case of powder-containing raw vent gas <NUM>', removing of filler powder <NUM> present therein.

In the exemplary embodiment of <FIG>, which is a preferred embodiment, washing tower <NUM> is arranged vertically, even if in not shown exemplary embodiments a horizontal arrangement of washing tower <NUM> is possible, or in any case an arrangement at an angle with respect to the vertical direction. Moreover, in the exemplary embodiment of <FIG>, washing tower <NUM> is arranged with end portion <NUM>' as a lower end portion, and therefore also with first vent gas inlet mouth <NUM> as a lower mouth, which it is preferred, even if, in not shown exemplary embodiments an arrangement of washing tower <NUM> is possible with end portion <NUM>' as an upper end portion and therefore with first vent gas inlet mouth <NUM> as an upper mouth.

However, the invention is not limited by the above-described forms of gas-washing device <NUM>.

In an exemplary embodiment, the method provides conveying only powder-containing raw vent gas <NUM>' through gas-washing device <NUM>, powder-containing raw vent gas being released by piece of equipment <NUM> only during the steps of treating filler powder <NUM>, as specified, and provides conveying substantially powder-free normal raw vent gas <NUM> directly into the boiler, normal raw vent gas being released in the other steps of the manufacture cycle in which piece of equipment <NUM> is involved.

<FIG> it relates to an exemplary embodiment of the plant configured to this purpose, in which a direct vent gas duct <NUM> between piece of equipment <NUM> and boiler <NUM>, as well as a valve group <NUM>,<NUM> connected to first vent gas duct <NUM> and to direct vent gas duct <NUM> are also provided, so as to selectively deviate normal raw vent gas <NUM> from first vent gas duct <NUM> to direct vent gas duct <NUM>, at the end of each step of treating filler powder <NUM>, and deviate powder-containing raw vent gas <NUM>' from direct vent gas duct <NUM> to first vent gas duct <NUM>, at the beginning each step of treating filler powder <NUM> by piece of equipment <NUM>. In the exemplary embodiment of <FIG>, the direct vent gas duct has a suction device <NUM> for normal raw vent gas, typically a fan <NUM>, different from suction device <NUM> for powder-containing raw vent gas.

Actually, a polymer-bitumen membranes production line or unit can comprise a plurality of melting tanks <NUM>,<NUM>'. At a given instant, one or some melting tanks <NUM>', can be performing a step of treating filler powder <NUM> and of releasing therefore powder-containing raw vent gas <NUM>', while the other melting tanks <NUM> are carrying out a step before the step of treating filler powder <NUM>, in particular a step of loading/storing bitumen <NUM>, before receiving filler powder <NUM>, or a step after the step of treating filler powder <NUM>, in particular a step of late mixing bitumen <NUM> and filler powder <NUM>, or a step of feeding bitumen <NUM> containing filler powder <NUM> to the impregnation basins; these latter melting tanks <NUM> release therefore normal raw vent gas <NUM>, which is substantially free from filler powder <NUM>.

<FIG> relates to an exemplary embodiment of such a plant that comprises a plurality of melting tanks <NUM>,<NUM>' configured to conveying only powder-containing raw vent gas <NUM>' coming from piece of equipment <NUM>' during a step of treating filler powder <NUM> through gas-washing device <NUM>. Three pieces of equipment <NUM>,<NUM>' are shown, one piece of equipment <NUM>' of which is engaged in a step of treating filler powder <NUM> and releases a powder-containing raw vent gas <NUM>', while the other two pieces of equipment <NUM> are not engaged with this step of treatment and release a normal raw vent gas <NUM>. In this case, first vent gas duct <NUM> and direct vent gas duct <NUM> have respective connection branches <NUM>,<NUM> to each piece of equipment <NUM>,<NUM>', and on each these connection branches <NUM>,<NUM> a valve group <NUM>,<NUM> is arranged that is configured to selectively open/close each connection branch <NUM>,<NUM> between the corresponding piece of equipment <NUM>,<NUM>' and first vent gas duct <NUM> or direct vent gas duct <NUM>, respectively. Even if the valve group of <FIG> comprises two two-way valves <NUM>,<NUM>, one of which is arranged along connection branch <NUM> to first vent gas duct <NUM> and the other is arranged along connection branch <NUM> to direct vent gas duct <NUM>, in an exemplary embodiment not shown, this valve group can comprise a diverting three-way valve that has an outlet mouth arranged at the root of connection branch <NUM> to direct vent gas duct <NUM> and has the other outlet mouth arranged at the root of connection branch <NUM> to first vent gas duct <NUM>.

Preferably, valve group <NUM>,<NUM> has servo-actuated actuators <NUM>, <NUM> in order to be automatically operated when the corresponding piece of equipment <NUM>,<NUM>' starts or ends the step of treating filler powder <NUM>. <FIG> relates to an exemplary embodiment of the plant that is configured to this purpose, and that comprises a plurality of pieces of equipment or melting tanks <NUM>,<NUM>'. This exemplary embodiment is valid also in the case of a production line comprising a single melting tank <NUM> as shown in <FIG>. In this case, a control unit <NUM> is provided that is configured to receive a start/stop signal <NUM> of a step of treating filler powder <NUM> in piece of equipment <NUM>'/<NUM>, and also configured to transfer to valve group <NUM>,<NUM>, i.e. in this case to actuators <NUM>, <NUM>, signals <NUM> of.

In the exemplary embodiment of <FIG>, start/stop signal <NUM> of a step of treating filler powder <NUM> is generated by a limit switch device system <NUM> that is associated with the inlet valve <NUM> of filler powder <NUM> in order detect the open/closed status indicating whether valve <NUM> of filler powder <NUM> is open and/or closed. Preferably, a timer switch device is provided for delaying the generation and the transmission of signal <NUM> of diverting raw vent gas <NUM>,<NUM>' with respect to the instant at which valve <NUM> is closed, based on a standard mixing time within which it is believed that filler powder <NUM> and bitumen <NUM> are fully mixed. For simplicity, only signals <NUM>, <NUM> are shown that relate to piece of equipment <NUM>', but it is understood that a similar configuration is used for shown pieces of equipment <NUM>. In an exemplary embodiment, not shown, signal <NUM> of diverting normal raw vent gas <NUM> can be automatically generated when an opening command is sent to the inlet valve <NUM> of the filler, or when an actuation command is sent to a different means, if present, for conveying filler <NUM> as specified above. As an alternative, signal <NUM> of diverting normal raw vent gas <NUM> can be triggered by a powder presence detector, not shown, arranged above the head of bitumen <NUM> in piece of equipment <NUM>, and configured to emit a signal indicating the presence and/or absence of powder in the space above bitumen <NUM>.

In a modification, the process provides mixing normal raw vent gas <NUM> or purified vent gas <NUM> with external air <NUM>, preferably withdrawn at a temperature set between <NUM> and <NUM>, such that a mixture <NUM> of external air and purified vent gas <NUM> or normal raw vent gas <NUM> is obtained.

<FIG> relates to an exemplary embodiment of the plant that is configured to this purpose, wherein a mixing means <NUM> is provided for mixing external air <NUM> with the raw vent gas fed to the boiler, which can be purified vent gas <NUM> coming from second vent gas duct <NUM> or normal raw vent gas <NUM> coming from direct vent gas duct <NUM>. Preferably, as shown in <FIG>, mixing means <NUM> are arranged along a feed branch <NUM> of primary air to boiler <NUM> to which second vent gas duct <NUM> and direct vent gas duct <NUM> converge.

Advantageously, purified vent gas <NUM> and/or normal raw vent gas <NUM> are conveyed, in such a way to keep the raw vent gas in the combustion chamber of boiler <NUM> for a residence time of at least two seconds. To this purpose, the size of the combustion chamber and the features of suction device <NUM>, typically a fan, are selected cooperatively.

Preferably, the process provides measuring the oxygen and/or carbon monoxide content in the combustion gas <NUM>, downstream of boiler <NUM>, and comparing this detected contents with a predetermined O<NUM> content lower limit value and with a predetermined CO content upper limit value, and the step of mixing with external air <NUM> includes a step of adjusting a proportion of external air <NUM> in the mixture <NUM>, in such a way to maintain the content of oxygen and/or carbon monoxide in the combustion gas <NUM> above and below the predetermined limit values, respectively.

To this purpose, as shown still in <FIG>, the plant also comprises a device <NUM> for measuring the content of oxygen and/or carbon monoxide in the combustion gas <NUM>, configured to emit a corresponding O<NUM> and/or CO concentration signal <NUM> in the combustion gas, and also comprises a logic unit <NUM>' configured to receive the concentration signal and/or the concentration signals <NUM>, in order to compare the latter with a predetermined lower O<NUM> content limit value and with a predetermined upper CO limit value, respectively, and to generate and transfer to mixing means <NUM> an adjustment signal <NUM> for adjusting a proportion of external air <NUM> in the mixture <NUM>, so as to keep the content of oxygen and/or carbon monoxide respectively above and below these values limit.

<FIG> diagrammatically shows an exemplary embodiment of the plant, in which a further direct vent gas duct <NUM> is provided to which converge suction branches <NUM> coming from respective extractor hoods <NUM> of filler powder-free vapours. This duct has an own suction device <NUM>, typically a fan <NUM>. Moreover, first vent gas duct <NUM>, direct vent gas duct <NUM> and the further direct vent gas duct <NUM> converge to a manifold <NUM> along which shut-off valves <NUM>,<NUM> are provided, possibly equipped with respective actuators, not shown, for remote control. In its own normal position, valve <NUM> is closed in order to separate first vent gas duct <NUM>, through which powder-containing raw vent gas <NUM>' flows towards gas-washing device <NUM>, from direct vent gas ducts <NUM> and <NUM>, through which substantially powder-free raw vent gas flows directly into boiler <NUM>. Valve <NUM>, instead, is normally open. This structure of manifold <NUM> makes it possible to use each of fans <NUM>, <NUM>, and <NUM> as a spare fan for any other of them, in case of out-of-service.

Claim 1:
A process for treating a raw vent gas (<NUM>,<NUM>') containing bitumen vapours released by at least one piece of equipment (<NUM>,<NUM>') configured to carry out a step of treating a filler powder (<NUM>), wherein said piece of equipment (<NUM>,<NUM>') is supplied with said filler powder (<NUM>) and/or performs a mixing operation of said filler powder (<NUM>) with said bitumen (<NUM>), during said step of treating said filler powder (<NUM>), said raw vent gas (<NUM>,<NUM>') turning from a substantially powder-free normal raw vent gas (<NUM>), into a powder-containing raw vent gas (<NUM>') that also contains an amount of said filler powder (<NUM>), said process comprising the steps of:
- prearranging a gas-washing device (<NUM>) configured to supply a washing solution (<NUM>) containing a surfactant;
- prearranging a boiler (<NUM>);
- first conveying said raw vent gas (<NUM>,<NUM>') containing an amount of said filler powder into said gas-washing device (<NUM>);
- bringing said raw vent gas (<NUM>,<NUM>') containing an amount of said filler powder into contact with said washing solution (<NUM>) containing a surfactant;
- removing said amount of filler powder from said powder-containing raw vent gas;
- releasing, by said gas-washing device (<NUM>) a purified vent gas (<NUM>) substantially free from said filler powder;
- second conveying said purified vent gas (<NUM>) into said boiler (<NUM>) and burning said purified vent gas (<NUM>), thus obtaining a combustion gas (<NUM>).