SYSTEM FOR ELIMINATING BAD-SMELLING EMISSIONS FROM INDUSTRIAL PROCESSES

Abstract

A process is described, as well as a plant, for treating a raw vent gas (4,4′) containing bitumen vapours and released by a piece of equipment (1) of a polymer-bitumen membranes production line, in which operations are carried out involving a filler powder (3), such as an operation of mixing the filler powder (3) with the bitumen (2), during which the raw vent gas (4,4′) is changed from a substantially powder-free raw vent gas (4), into a raw vent gas (4′) containing the filler powder (3). The process includes steps of first conveying the raw vent gas (4,4) into a gas-washing device (20) along with a solution (9) of a surfactant; contacting the raw vent gas (4,4) with the solution (9) and removing the powder from the powder-containing gas (4′), releasing a purified vent gas (5) that is substantially free from the filler powder; conveying the purified vent gas (5) into a boiler (40) and burning the bitumen vapours. In a preferred exemplary embodiment, it is conveyed in the gas-washing device only the powder-containing gas (4′) produced during the operations of the piece of equipment (1) that involve the filler powder (3), while in the remainder steps the substantially powder-free raw vent gas (4) is directly conveyed into the boiler (40) by a direct vent line (50) that can be automatically selected. In a preferred exemplary embodiment, the gas-washing device comprises a tank (25) configured to form inside a predetermined head of the washing solution (9) and having an inlet port for the raw vent gas arranged below the liquid head. The process prevents the powder from quickly reaching the boiler (40) making the burner and the heat-exchange surfaces ineffective.

Claims

1. A process for treating a raw vent gas (4,4′) containing bitumen vapours released by at least one piece of equipment (1,1′) configured to carry out a step of treating a filler powder (3), wherein said piece of equipment (1,1′) is supplied with said filler powder (3) and/or performs a mixing operation of said filler powder (3) with said bitumen (2), during said step of treating said filler powder (3), said raw vent gas (4,4′) turning from a substantially powder-free normal raw vent gas (4), into a powder-containing raw vent gas (4′) that also contains an amount of said filler powder (3), said process comprising the steps of: prearranging a gas-washing device (20) configured to supply a washing solution (9) containing a surfactant; prearranging a boiler (40); first conveying said raw vent gas (4,4′) containing an amount of said filler powder into said gas-washing device (20); bringing said raw vent gas (4,4′) containing an amount of said filler powder into contact with said washing solution (9) containing a surfactant; removing said amount of filler powder from said powder-containing raw vent gas; releasing, by said gas-washing device (20) a purified vent gas (5) substantially free from said filler powder; second conveying said purified vent gas (5) into said boiler (40) and burning said purified vent gas (5), thus obtaining a combustion gas (8)

2. The process according to claim 1, wherein said piece of equipment (1,1′) is a part of a polymer-bitumen membranes production line.

3. The process according to claim 1, wherein said step of prearranging a gas-washing device (20) provides the steps of: prearranging a tank (25) having an inlet mouth (21) for said raw vent gas (4,4′) and an outlet mouth (23) for said purified vent gas (5); filling said tank (25) with said washing solution (9) up to a predetermined level (H) over a bottom (29) of said tank (25), said level (H) higher than a height (h) of said inlet mouth (21) over said bottom, so that said inlet mouth (21) is submerged by said washing solution (9); wherein said step of first conveying comprises a step of introducing said raw vent gas (4,4′) into said tank (25) through said inlet mouth (21), and bubbling said raw vent gas (4,4′) through said washing solution (9) contained in said tank (25), in order to carry out said step of bringing.

4. The process according to claim 1, wherein said step of prearranging a gas-washing device provides a step of prearranging a washing tower (125) comprising: a first inlet mouth (121) for said raw vent gas (4,4′) and a first outlet mouth (123) for said purified vent gas (5) arranged at opposite end portions of said washing tower (125); a second inlet mouth (122) and a second outlet mouth (125) for said washing solution (9) arranged at opposite end portions of said washing tower (125); wherein said second inlet mouth (122) is arranged in one of said end portions selected from the group consisting of: the end portion at which said first outlet mouth (123) is arranged, the end portion at which said first inlet mouth (121) is arranged; a step of conveying said washing solution (9) through said washing tower (125) between said second inlet mouth (122) and said second outlet mouth (125), wherein said step of first conveying comprises a step of conveying said raw vent gas (4,4′) through said washing tower (125) between said first inlet mouth (121) and said first outlet mouth (123), in order to provide said step of contacting as a counter-current contacting step or as an equicurrent contacting step, respectively, of said washing solution (9) and of said raw vent gas (4,4′) turning into said purified vent gas (5).

5. The process according to claim 1, comprising a step of: directly conveying said normal raw vent gas (4) into said boiler (40), wherein said step of directly conveying is discontinued and said steps of first and second conveying are carried out while said at least one piece of equipment (1) performs said step of treating said filler powder.

6. The process according to claim 5, wherein said powder-containing raw vent gas (4′) is released by a number of at least one piece of equipment (1′) of a plurality of pieces of equipment (1,1′) of said production line during respective steps of treatment of said filler powder (3), and said steps of first and second conveying said powder-containing raw vent gas (4′) are selectively carried out starting from said pieces of equipment (1′) of said number of pieces of equipment, while said step of directly conveying said normal raw vent gas (4) is carried out starting from the remainder piece(s) of equipment (1) of said plurality of pieces of equipment (1,1′).

7. An apparatus for making polymer-bitumen membranes including at least one piece of equipment (1,1′) of a polymer-bitumen membrane production line, said piece of equipment configured to carry out a step of mixing a filler powder (3) with bitumen (2), in which step said piece of equipment (1,1′) is supplied by said filler powder (3) and/or performs a mixing operation of said filler powder (3) with said bitumen (2), during said step of treating said filler powder (3) said raw vent gas (4,4′) turning from a substantially powder-free normal raw vent gas (4), into a powder-containing raw vent gas (4′) that also contains said filler powder (3), a plant for treating a raw vent gas (4,4′) containing bitumen vapours released by said least one piece of equipment (1,1′), comprising: a gas-washing device (20) comprising an inlet duct to receive a washing solution (9) containing a surfactant, said gas-washing device (20) configured to bring said raw vent gas (4,4′) containing an amount of said filler powder into contact with said washing solution (9) containing a surfactant; a boiler (40); a first vent gas duct (10) between said at least one piece of equipment (1) and said gas-washing device (20), said first vent gas duct (10) arranged to convey said raw vent gas (4,4′) containing an amount of said filler powder into said gas-washing device (20); a second vent gas duct (30) between said gas-washing device (20) and said boiler (40) arranged to release from said gas-washing device (20) directly to said boiler a purified vent gas (5) substantially free from said filler powder; a suction device (11) arranged to cause said raw vent gas (4,4′) to be conveyed through said first vent gas duct (10) and said purified vent gas (5) to exit through said second vent gas duct (30) to reach said boiler (40).

8. The apparatus according to claim 7, wherein said gas-washing device (20) comprises a tank (25) having an inlet mouth (21) for said raw vent gas (4,4′) and an outlet mouth (23) for said purified vent gas (5), wherein said inlet mouth (21) and said outlet mouth (23) are connected to said first and to said second vent gas ducts (10,30), respectively, said tank (25) configured to form a head of said washing solution (9) at a height (H) above a bottom (29) of said tank (25) higher than a height (h) of said inlet mouth (21), so that said inlet mouth (21) is submerged by said washing solution (9), such that, by introducing said raw vent gas (4,4′) into said tank (25) through said inlet mouth (21), said raw vent gas (4,4′) bubbles through said washing solution (9).

9. The apparatus according to claim 7, wherein said gas-washing device (20) includes a washing tower (125) comprising: a first inlet mouth (121) for said raw vent gas (4,4′) and a first outlet mouth (123) for said purified vent gas (5) arranged at opposite end portions of said washing tower (125); a second inlet mouth (122) and a second outlet mouth (124) for said washing solution (9) arranged at opposite end portions (121′,123′) of said washing tower (125); wherein said first inlet mouth (121) and said first outlet mouth (123) are connected to said first and to said second vent gas ducts (10,30), respectively, wherein said second inlet mouth (122) is arranged at one of said end portions (121′,123′) selected between: the end portion (123′) at which said first outlet mouth (123) is arranged, the end portion (121′) at which said first inlet mouth (121) is arranged; in order to provide in said washing tower (125) a countercurrent flow or an equicurrent flow, respectively, of said washing solution (9) and of said raw vent gas (4,4′) turning into said purified vent gas (5).

10. The apparatus according to claim 7, comprising furthermore: a direct vent gas duct (50) between said at least one piece of equipment (1) and said boiler (40); a valve group (13,53) that is connected to said first vent gas duct (10) and to said direct vent gas duct (50) and that is arranged to selectively deviate said normal raw vent gas (4) from said first vent gas duct (10) to said direct vent gas duct (50) and said powder-containing raw vent gas (4′) from said direct vent gas duct (50) to said first vent gas duct (10).

11. The apparatus according to claim 10, wherein said first vent gas duct (10) and said direct vent gas duct (50) have respective connection branches (12,52) to each piece of equipment (1,1′) of a plurality of pieces of equipment (1,1′) of said production line, wherein said valve group (13,53) is arranged along each of said connection branches (12,52) and is configured to selectively open/close a connection branch (12,52) of said connection branches between a respective piece of equipment and said first vent gas duct (10) or said direct vent gas duct (50), wherein the plant also comprises a control unit configured to receive an ongoing filler treatment signal (61) indicating that a filler treatment step is being carried out in said piece of equipment (1,1′), and transfer a raw vent gas redirect signal to said valve group to redirect said raw vent gas (4,4′) from said direct raw gas duct (50) to said first raw gas duct (10) according to whether said piece of equipment (1,1′) is carrying out/is not carrying out said step of treating said filler powder.

12. A process for making polymer-bitumen membranes comprising a step of mixing a filler powder (3) with bitumen (2), thus obtaining a polymer-bitumen membrane, and obtaining a raw vent gas (4,4′) that contains bitumen vapours and a residue of said filler powder (3), characterized in that it comprises a process for treating said raw vent gas (4,4′) as defined in claims from 1 to 6.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0072] 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:

[0073] FIG. 1 shows a process flow diagram of a plant for treating bitumen vapours containing a fine powder according to the invention;

[0074] FIG. 2 shows a process flow diagram of a plant for treating such vapours according to an exemplary embodiment of the invention, in which the gas-washing device comprises a tank containing a washing solution, and the raw vent gas to be purified are fed below the head of this washing solution;

[0075] FIG. 3 shows a process flow diagram of a plant for treating such vapours according to an exemplary embodiment of the invention, in which the gas-washing device comprises a washing tower arranged to form a countercurrent flow of the raw vent gas to be purified and of the washing solution;

[0076] FIG. 4 shows a process flow diagram of a plant for treating such vapours according to an exemplary embodiment of the invention, in which a direct vent gas duct to the oven and a valve group for selectively deviating the raw vent gas from the gas-washing device to the direct duct and vice-versa, are also provided;

[0077] FIG. 5 shows a process flow diagram of a plant for treating such vapours according to an exemplary embodiment of the invention which is suitable for a production line that comprises a plurality of pieces of equipment configured to treat bitumen and filler powder, in which a direct vent gas duct is provided as shown in FIG. 4, and a plurality of connection branches are provided to the direct vent gas duct and to the duct through the gas-washing device;

[0078] FIG. 6 shows a process flow diagram of plant similar to that of FIG. 5, configured to allow an automatic switch of each piece of equipment between the direct vent gas duct and the duct through the gas-washing device;

[0079] FIG. 7 shows a process flow diagram of plant similar to that of FIG. 5, in which a means is provided for mixing external air with the raw vent gas fed to the boiler;

[0080] FIG. 8 diagrammatically shows a modification of the flow diagram of FIG. 8;

[0081] FIG. 9 shows a process flow diagram of a plant for treating also vent gas that are always free from powder in the oven as a separate feed.

DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS

[0082] 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 0.7 μm.

[0083] FIG. 1 diagrammatically shows a melting tank 1 of a plant such as a polymer-bitumen membranes production line, i.e. a piece of equipment used as a mixer of bitumen 2 with a filler powder 3, as well as a temporary storage and supply unit of bitumen 2. Melting tank 1 typically comprises a tank 90, a mixing means 91 for mixing bitumen 2 and filler powder 3, in this case a mechanical agitator 91, as well as a filler powder feeding means 3, in this case, for instance, a hopper 92 and/or of a valve 93 to be opened when performing the steps of loading filler powder 3 into tank 90. The means for feeding filler powder 3 can also comprise a means for conveying filler powder 3, not shown, from a storage of filler powder 3 to melting tank 1, for example a pneumatic conveyor, a bucket conveyor or of any other known for solid matter conveyor, comprising an electromechanical actuation system. Melting tank 1 is also equipped with a discharging line 19 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.

[0084] Melting tank 1 is maintained under suction by a gas vent line 10,30 and a suction device 11, 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 4,4′ is steadily released by piece of equipment 1 during each step of working cycle in which it is involved.

[0085] As anticipated, the step of treating filler powder 3 in melting tank 1 essentially includes the operations of feeding filler powder 3 into tank 90 and of mixing filler powder 3 with bitumen 2. In other steps of the working cycle in which melting tank 1 is involved, i.e. when only bitumen 2 is present within tank 90, or once filler powder 3 has been fully incorporated into bitumen 2, the raw vent gas is a normal raw vent gas 4 that is substantially free from filler powder 3. Instead, during the step of treating filler powder 3, including the above operations, the raw vent gas is turned from substantially powder-free normal raw vent gas 4 into a powder-containing raw vent gas 4′, i.e. a vent gas that contains an amount of said filler powder 3 beside the bitumen vapours.

[0086] The process according to the invention provides prearranging a gas-washing device 20, in addition to boiler 40 having for instance the features described in EP 3018410 A1. Gas-washing device 20 and boiler 40 are arranged in such a way that raw vent gas 4,4′, in particular powder-containing raw vent gas 4′ is first conveyed into gas-washing device 20 and then into boiler 40, as a purified, substantially powder-free vent gas 5, or directly as normal raw vent gas 4.

[0087] To this purpose the plant comprises, in addition to gas-washing device 20 and boiler 40, a first vent gas duct 10 between piece of equipment 1 and gas-washing device 20, and a second vent gas duct 30 between gas-washing device 20 and boiler 40. The plant also comprises a suction device 11 arranged to convey raw vent gas 4,4′, in particular powder-containing raw vent gas 4′, and purified vent gas 5 through first vent gas duct 10 and second vent gas duct 30, respectively. In a preferred exemplary embodiment, the suction device is a fan 11, and is arranged along first vent gas duct 10, i.e. between piece of equipment 1 and gas-washing device 20.

[0088] Gas-washing device 20 is configured to receive a washing solution 9 containing a surfactant, and to bring raw vent gas 4,4′ into contact with washing solution 9, in order to remove the amount of filler powder 3 that is present in powder-containing raw vent gas 4′, and to obtain purified vent gas 5 from gas-washing device 20, substantially free from filler powder 3.

[0089] Subsequently, purified vent gas enters into boiler 40, in which the combustion of the bitumen vapours contained therein occurs, thus obtaining a combustion gas 8, for example in the way described in EP 3018410 A1.

[0090] The surfactant present in washing solution 9 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 20 used.

[0091] The surfactant present in washing solution 9 can be also chosen according to the nature of the powder that is present in powder-containing raw vent gas 4′. 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 CaCO.sub.3. 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 9 is set between 0.1% and 1.0% by weight on the amount of washing solution 9, more preferably it is set between 0.2% and 0.4% by weight on the amount of washing solution 9. Preferably, gas-washing device 20 has one of the forms shown in FIGS. 2 and 3.

[0092] In particular, with reference to FIG. 2, gas-washing device 20 comprises a tank 25 with an inlet mouth 21 and an outlet mouth 23 for raw vent gas 4,4′, in particular for powder-containing raw vent gas 4′, and for purified vent gas 5, respectively, said inlet and outlet mouths connected to first vent gas duct 10 and to second vent gas duct 30, respectively. Inlet mouth 21 of raw vent gas 4,4′ is arranged at a predetermined height h from a bottom 29 of tank 25. For instance, as shown in the figure, inlet mouth 21 can comprise an open end of a portion 15 of first vent gas duct 10 that is arranged as a dip tube within tank 25, but can also comprise a nozzle, not shown, that is arranged on the shell of tank 25 at distance h from bottom 29.

[0093] Tank 25 is configured to form a liquid head at a height H from bottom 29 above inlet mouth 21, so that the latter is submerged by washing solution 9. More in detail, tank 25 is configured to be filled with washing solution 9 through a duct 39 and a liquid inlet nozzle 22 up to a predetermined level H over bottom 29 higher than height h of inlet mouth 21 above bottom 29, so that inlet mouth 21 of raw vent gas 4,4′ is submerged by washing solution 9. This way, by introducing raw vent gas 4,4′ into tank 25 through inlet mouth 21, it bubbles through washing solution 9 contained therein, which allows removing filler powder 3 that is present in the gas, in the case of powder-containing raw vent gas 4′. A liquid discharge nozzle 24 is also provided that can be used in order to periodically change solution 9 of gas-washing device 20, or to allow a continuous flow, a gooseneck tube portion being preferably provided vertically extending up to height H above bottom 29, in order to maintain the liquid head.

[0094] As an alternative thereto, as shown in FIG. 3, or in addition thereto, in a modification not shown, gas-washing device 20 still comprises a tank 125, but in the form of a washing tower 125, i.e. an elongated tank with an inlet mouth 121 and an outlet mouth 123 for raw vent gas 4,4′, in particular for powder-containing raw vent gas 4′, and for purified vent gas 5, respectively, and connected to first vent gas duct 10 and to second vent gas duct 30, respectively. Washing tower 125 also comprises a second inlet mouth 122 and a second outlet mouth 124 for washing solution 9, which are also arranged at opposite end portions of washing tower 125. In the exemplary embodiment as depicted, second inlet mouth 122 is arranged at an end portion 123′ at which first outlet mouth 123 is arranged, in such a way that, by conveying raw vent gas 4,4′ through washing tower 125 between first inlet mouth 121 and first outlet mouth 123, and by conveying washing solution 9 between second inlet mouth 122 and second outlet mouth 124 a step is performed of countercurrent contacting raw vent gas 4,4′ and washing solution 9. In an exemplary embodiment, not shown, second inlet mouth 122 is arranged at an end portion 121′ at which first inlet mouth 121 is arranged, in such a way that, by conveying raw vent gas 4,4′ through washing tower 125 between first inlet mouth 121 and first outlet mouth 123 and by conveying washing solution 9 between second inlet mouth 122 and second outlet mouth 124 a step is performed of equicurrent contacting the raw vent gas and washing solution 9.

[0095] Such step of countercurrent or equicurrent contacting allows, in the case of powder-containing raw vent gas 4′, removing of filler powder 3 present therein.

[0096] In the exemplary embodiment of FIG. 3, which is a preferred embodiment, washing tower 125 is arranged vertically, even if in not shown exemplary embodiments a horizontal arrangement of washing tower 125 is possible, or in any case an arrangement at an angle with respect to the vertical direction. Moreover, in the exemplary embodiment of FIG. 3, washing tower 125 is arranged with end portion 121′ as a lower end portion, and therefore also with first vent gas inlet mouth 121 as a lower mouth, which it is preferred, even if, in not shown exemplary embodiments an arrangement of washing tower 125 is possible with end portion 121′ as an upper end portion and therefore with first vent gas inlet mouth 121 as an upper mouth.

[0097] However, the invention is not limited by the above-described forms of gas-washing device 20.

[0098] In an exemplary embodiment, the method provides conveying only powder-containing raw vent gas 4′ through gas-washing device 20, powder-containing raw vent gas being released by piece of equipment 1 only during the steps of treating filler powder 3, as specified, and provides conveying substantially powder-free normal raw vent gas 4 directly into the boiler, normal raw vent gas being released in the other steps of the manufacture cycle in which piece of equipment 1 is involved.

[0099] FIG. 4 it relates to an exemplary embodiment of the plant configured to this purpose, in which a direct vent gas duct 50 between piece of equipment 1 and boiler 40, as well as a valve group 13,53 connected to first vent gas duct 10 and to direct vent gas duct 50 are also provided, so as to selectively deviate normal raw vent gas 4 from first vent gas duct 10 to direct vent gas duct 50, at the end of each step of treating filler powder 3, and deviate powder-containing raw vent gas 4′ from direct vent gas duct 50 to first vent gas duct 10, at the beginning each step of treating filler powder 3 by piece of equipment 1. In the exemplary embodiment of FIG. 4, the direct vent gas duct has a suction device 51 for normal raw vent gas, typically a fan 51, different from suction device 11 for powder-containing raw vent gas.

[0100] Actually, a polymer-bitumen membranes production line or unit can comprise a plurality of melting tanks 1,1′. At a given instant, one or some melting tanks 1′, can be performing a step of treating filler powder 3 and of releasing therefore powder-containing raw vent gas 4′, while the other melting tanks 1 are carrying out a step before the step of treating filler powder 3, in particular a step of loading/storing bitumen 2, before receiving filler powder 3, or a step after the step of treating filler powder 3, in particular a step of late mixing bitumen 2 and filler powder 3, or a step of feeding bitumen 2 containing filler powder 3 to the impregnation basins; these latter melting tanks 1 release therefore normal raw vent gas 4, which is substantially free from filler powder 3.

[0101] FIG. 5 relates to an exemplary embodiment of such a plant that comprises a plurality of melting tanks 1,1′ configured to conveying only powder-containing raw vent gas 4′ coming from piece of equipment 1′ during a step of treating filler powder 3 through gas-washing device 20. Three pieces of equipment 1,1′ are shown, one piece of equipment 1′ of which is engaged in a step of treating filler powder 3 and releases a powder-containing raw vent gas 4′, while the other two pieces of equipment 1 are not engaged with this step of treatment and release a normal raw vent gas 4. In this case, first vent gas duct 10 and direct vent gas duct 50 have respective connection branches 12,52 to each piece of equipment 1,1′, and on each these connection branches 12,52 a valve group 13,53 is arranged that is configured to selectively open/close each connection branch 12,52 between the corresponding piece of equipment 1,1′ and first vent gas duct 10 or direct vent gas duct 50, respectively. Even if the valve group of FIG. 5 comprises two two-way valves 13,53, one of which is arranged along connection branch 12 to first vent gas duct 10 and the other is arranged along connection branch 52 to direct vent gas duct 50, 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 52 to direct vent gas duct 50 and has the other outlet mouth arranged at the root of connection branch 12 to first vent gas duct 10.

[0102] Preferably, valve group 13,53 has servo-actuated actuators 16, 56 in order to be automatically operated when the corresponding piece of equipment 1,1′ starts or ends the step of treating filler powder 3. FIG. 6 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 1,1′. This exemplary embodiment is valid also in the case of a production line comprising a single melting tank 1 as shown in FIG. 4. In this case, a control unit 60 is provided that is configured to receive a start/stop signal 61 of a step of treating filler powder 3 in piece of equipment 1′/1, and also configured to transfer to valve group 13,53, i.e. in this case to actuators 16, 56, signals 64 of [0103] diverting normal raw vent gas 4, which is being turned into powder-containing raw vent gas 4′, from direct vent gas duct 50 into first vent gas duct 10, and of [0104] diverting powder-containing raw vent gas 4′, which is being turned into normal raw vent gas 4, from first vent gas duct 10 into direct vent gas duct 50,
respectively.

[0105] In the exemplary embodiment of FIG. 6, start/stop signal 61 of a step of treating filler powder 3 is generated by a limit switch device system 66 that is associated with the inlet valve 93 of filler powder 3 in order detect the open/closed status indicating whether valve 93 of filler powder 3 is open and/or closed. Preferably, a timer switch device is provided for delaying the generation and the transmission of signal 61 of diverting raw vent gas 4,4′ with respect to the instant at which valve 93 is closed, based on a standard mixing time within which it is believed that filler powder 3 and bitumen 2 are fully mixed. For simplicity, only signals 61, 64 are shown that relate to piece of equipment 1′, but it is understood that a similar configuration is used for shown pieces of equipment 1. In an exemplary embodiment, not shown, signal 64 of diverting normal raw vent gas 4 can be automatically generated when an opening command is sent to the inlet valve 93 of the filler, or when an actuation command is sent to a different means, if present, for conveying filler 3 as specified above. As an alternative, signal 64 of diverting normal raw vent gas 4 can be triggered by a powder presence detector, not shown, arranged above the head of bitumen 2 in piece of equipment 1, and configured to emit a signal indicating the presence and/or absence of powder in the space above bitumen 2.

[0106] In a modification, the process provides mixing normal raw vent gas 4 or purified vent gas 5 with external air 6, preferably withdrawn at a temperature set between 5° C. and 40° C., such that a mixture 7 of external air and purified vent gas 5 or normal raw vent gas 4 is obtained.

[0107] FIG. 7 relates to an exemplary embodiment of the plant that is configured to this purpose, wherein a mixing means 54 is provided for mixing external air 6 with the raw vent gas fed to the boiler, which can be purified vent gas 5 coming from second vent gas duct 30 or normal raw vent gas 4 coming from direct vent gas duct 30. Preferably, as shown in FIG. 8, mixing means 54 are arranged along a feed branch 55 of primary air to boiler 40 to which second vent gas duct 30 and direct vent gas duct 50 converge.

[0108] Advantageously, purified vent gas 5 and/or normal raw vent gas 4 are conveyed, in such a way to keep the raw vent gas in the combustion chamber of boiler 40 for a residence time of at least two seconds. To this purpose, the size of the combustion chamber and the features of suction device 11, typically a fan, are selected cooperatively.

[0109] Preferably, the process provides measuring the oxygen and/or carbon monoxide content in the combustion gas 8, downstream of boiler 40, and comparing this detected contents with a predetermined O.sub.2 content lower limit value and with a predetermined CO content upper limit value, and the step of mixing with external air 6 includes a step of adjusting a proportion of external air 6 in the mixture 7, in such a way to maintain the content of oxygen and/or carbon monoxide in the combustion gas 8 above and below the predetermined limit values, respectively.

[0110] To this purpose, as shown still in FIG. 7, the plant also comprises a device 80 for measuring the content of oxygen and/or carbon monoxide in the combustion gas 8, configured to emit a corresponding O.sub.2 and/or CO concentration signal 81 in the combustion gas, and also comprises a logic unit 60′ configured to receive the concentration signal and/or the concentration signals 81, in order to compare the latter with a predetermined lower O.sub.2 content limit value and with a predetermined upper CO limit value, respectively, and to generate and transfer to mixing means 54 an adjustment signal 82 for adjusting a proportion of external air 6 in the mixture 7, so as to keep the content of oxygen and/or carbon monoxide respectively above and below these values limit.

[0111] FIG. 9 diagrammatically shows an exemplary embodiment of the plant, in which a further direct vent gas duct 70 is provided to which converge suction branches 76 coming from respective extractor hoods 75 of filler powder-free vapours. This duct has an own suction device 71, typically a fan 71. Moreover, first vent gas duct 10, direct vent gas duct 50 and the further direct vent gas duct 70 converge to a manifold 99 along which shut-off valves 97,98 are provided, possibly equipped with respective actuators, not shown, for remote control. In its own normal position, valve 97 is closed in order to separate first vent gas duct 10, through which powder-containing raw vent gas 4′ flows towards gas-washing device 20, from direct vent gas ducts 50 and 70, through which substantially powder-free raw vent gas flows directly into boiler 40. Valve 98, instead, is normally open. This structure of manifold 99 makes it possible to use each of fans 11, 51, and 71 as a spare fan for any other of them, in case of out-of-service.

[0112] The foregoing description of exemplary specific embodiments of the invention will so fully reveal the invention according to the conceptual point of view, so that others, by applying current knowledge, will be able to modify and/or adapt for various applications such embodiments without further research and without parting from the invention, and it is therefore to be understood that such adaptations and modifications will have to be considered as equivalent to the specific embodiments. The means and the materials to perform the different functions described herein could have a different nature without, for this reason, departing from the field of invention. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation.