A BITUMEN OXIDATION PROCESS.

AND APPARATUS TO PERFORM SAID PROCESS

DESCRIPTION Field of the invention

[0001] The present invention relates to process for oxidating a bitumen ob tained as a crude oil vacuum distillation residue (bitumen blowing), and to an apparatus for actuating such process.

References to the prior art - Technical problems [0002] As well known, the bitumen obtained as a crude oil vacuum distilla tion residue is largely used to make the binding materials of asphalt concretes for road pavements, or is used to obtain waterproof and/or heat insulation ma terials for buildings and civil works in general. However, a bitumen as such normally has a poor consistency, and cannot be directly used for the above- mentioned applications. Therefore, a modification of the rheological and me chanical properties is required. More in detail, the above-mentioned applica tions require a somewhat high softening temperature, conventionally evaluated by the so-called “ring and ball” test, and a somewhat low penetration rate, which can be evaluated according to EN 1426. [0003] These requirements are traditionally met by a process known as bi tumen oxidation, in which the bitumen is maintained in contact with an oxygen- containing gas stream for a predetermined time, at a treatment temperature normally set between 240°C and 280°C. This way, a partial dehydrogenation reaction occurs of certain bitumen components, along with a polymerization reaction, by which three-dimensional structures are formed. Globally, this pro cess increases the amount of asphaltenes, which improves the consistency of the bitumen, in the sense of the above-mentioned parameters.

[0004] The bitumen oxidation is normally carried out discontinuously in oxi dation reactors or columns preferably equipped with an agitator configured to disperse the air stream into bubbles of appropriate size, so as to maximize the mass transfer surface between the air and the bitumen. [0005] In particular, AT 297197 describes a process in which the speed of the agitator, and therefore the energy used to disperse the air, is in creased/reduced as the amount of air present in the reactor increas es/decreases, so as to obtain an optimum diameter of the air bubbles, regard- less the air flowrate value. Instead, WO 87/03896 relates to a process in which air turbo-dispersion is involved, in particular by an impeller with a substantially flat disc from which a plurality of teeth peripherally extends lying in the same plane as the disk.

[0006] A drawback of the prior art discontinuous methods is that they re- quire relatively long treatment times to obtain the predetermined bitumen fea tures.

[0007] Another drawback of those methods is that relatively high tempera tures are required to contain the treatment time within an industrially accepta ble limit. In fact, the process temperatures indicated above causes important amounts of volatile, low molecular weight bitumen components to be lost by evaporation. Therefore, on the one hand, the yield of the process decreases. On the other hand, a useful fluidification effect of the low molecular weight fraction on the oxidized bitumen is lost. Moreover, the evaporation losses in crease the off-gas purification costs. [0008] In order to shorten the treatment time and/or reduce the treatment temperature, some methods use catalysts. A typical catalyst is Ferric chloride, normally used in the form of an aqueous solution, which is problematic, since a water material has to be supplied to a mass at a high temperature. For in stance, a catalytic process is described in WO 95/28458, which also relates to the use an oxidation column equipped with a multiple-impeller agitator.

[0009] Continuous bitumen oxidation methods are also known, such as the ones disclosed in WO 2006/009474 and WO 2014/035262. In order to actuate the respective methods, those documents describe tubular loop reactors com prising a plurality of ducts extending serially to each other, in which static mix- ers are arranged at a mutual distance, and in which the bitumen flows along with air. The loop reactors include respective variable-flowrate recirculation pumps. Bitumen outlet and inlet nozzles are provided at the pump suction and delivery sides, respectively, which are associated to adjustable-speed bitumen extraction and supply pumps, respectively. At one reactor section, compressed air inlet is also provided for the oxidation. In a section at a height above the ducts enclosing the static mixers, a tank is provided configured to separate the liquid bitumen from exhausted air and from its own vapors. This tank has a vent outlet, at which a valve is provided to adjust the pressure in the reactor. A bitumen outlet is connected to the pump suction by a duct arranged in such a way to form a predetermined liquid head upstream of the pump, which is used to adjust the pump speed.

[0010] A drawback of continuously-operated loop reactors of the type de scribed above is that steady process conditions and therefore a constant prod uct quality are difficult to be maintained. The control of the product rheological and mechanical properties, on which the cited documents are silent, is carried out by the tests mentioned above, which cannot be real-time carried out along the reactor or along the outlet duct of the same. In fact, samples must be taken and prepared to perform such tests, in particular the samples must be cooled down to room temperature. All those steps require a considerable time. If the oxidized bitumen is out of specification and the reactor is continued to be nor mally operated during this time, further material out of specification is accumu lated, which cannot be easily treated back to acceptable quality. On the other hand, if the product is caused to recirculate within the loop reactor during this waiting time, without either feeding or withdrawing it, and without introducing air as the oxidizing agent, the reactor production rate decreases.

[0011] JP S56 82884 A relates to a process to make an asphalt from a re finery distillate taken from a tank and sequentially conveyed through a packed duct and to a reaction column, to be subsequently returned to the starting tank.

[0012] RU 2183654 C1 relates to a process for making a bitumen modified with a modifier material such as a rubber, in which bitumen at a predetermined temperature is withdrawn from the bottom portion of a vertical reactor or col umn by a pump, and conveyed first into an injector, where it receives a flow of the modifier material and an air flow, and then into a static mixer unit where a co-oxidation reaction of the bitumen and of the modifier material occurs, to be finally returned back into the vertical reactor or column.

Summary of the invention

[0013] It is therefore a feature of the present invention to provide a bitumen oxidation process and an apparatus for oxidating bitumen, by which a prede termined bitumen consistency can be reached in a shorter time than the time required by the prior art oxidation columns.

[0014] It is also a feature of the invention to provide such a process and such an apparatus that allows to perform the oxidation at a temperature lower than the value required by the conventional technique, i.e., a temperature set between 240°C and 280°C, with the above-mentioned quality and process ad vantages.

[0015] Moreover, it is a feature of the invention to provide such a process and such an apparatus that makes it possible to improve quality reproducibility and stability of the oxidized bitumen.

[0016] It is also a feature of the invention to provide such process and ap paratus that allow to obtain a foam-free oxidized bitumen, as far as possible, in particular, if the oxidation includes a step of air mixing within a static mixer unit. [0017] It is another feature of the invention to provide a device to be asso ciated to a conventional including an oxidation column, operating discontinu- ously, in order to shorten the treatment time and improve the production rate accordingly, and/or in order to perform the oxidation at a lower temperature than the treatment temperatures normally used, with the advantages deducible from the above.

[0018] These and other features are attained by the process defined by claim 1 , and by the apparatus defined by claim 7. Advantageous exemplary embodiments and modifications of the process and of the apparatus are de fined by respective dependent claims. [0019] According to one aspect of the invention, a bitumen oxidation pro cess comprises the steps of: prearranging a vertical elongated equalizer tank; prearranging an amount of bitumen up to a predetermined level above a bottom portion of the equalizer tank; - heating the bitumen up to a treatment temperature set between 200°C and 240°C; prearranging an oxidation duct containing at least one static mixer unit, the oxidation duct having a predetermined capacity, excluding the volume occupied by the static mixer unit; conveying a flow of the bitumen from said bottom portion to an upper por tion of the equalizer tank through the oxidation duct, wherein said upper portion is at a height of at least one third with respect to the bitumen above the bottom portion; supplying a stream of an oxygen-containing gas into the oxidation duct;

- mixing the stream of an oxygen-containing gas with the flow of the bitu men in the static mixer unit of the oxidation duct; discontinuing the step of supplying the gas oxygen-containing stream when the bitumen achieves a predetermined target consistency value, the main feature of said process being that a predetermined treatment pres sure is maintained in the oxidation duct, in other words, a step is provided of maintaining, i.e., adjusting or controlling the treatment pressure in the oxidation duct, wherein such treatment pressure, which is maintained in the oxidation duct, is set between 2 bar(g) and 8 bar(g).

[0020] According to another aspect of the invention, a bitumen oxidation apparatus comprises: a vertical elongated equalizer tank, configured to contain an amount of bi tumen such that the equalizer tank is filled up to a predetermined level above a bottom portion thereof, and to maintain said amount of said bi tumen at a treatment temperature set between 200°C and 240°C; an oxidation duct containing at least one static mixer unit and having inlet and outlet end portions hydraulically connected to a bottom portion and to an upper portion, respectively, of the equalizer tank, the oxidation duct having a predetermined capacity, excluding the volume occupied by the static mixer unit, wherein the upper portion is at a height of at least one third with of said bitumen level above the bottom portion; a recirculation pump arranged for conveying a flow of the bitumen from the bottom portion to the upper portion of the equalizer tank through the oxidation duct; a gas supply means for supplying a stream of an oxygen-containing gas into the oxidation duct; a pressure-adjustment element located between the outlet end portion of the oxidation duct and the upper portion of the equalizer tank, the main feature of said apparatus being that the recirculation pump and the adjustment element are configured to cooperatively maintain a predetermined treatment pressure in the oxidation duct set between 2 bar(g) and 8 bar(g).

[0021] According to the method and the apparatus of the present invention, the bitumen to be treated is withdrawn from the bottom of the equalizer tank and, once it has been treated in the oxidation duct, is recycled to the equalizer tank at a higher section, significantly above the bottom portion. This way, any shortcut of the bitumen being recycled to the bitumen withdrawal point is pre vented, which increases the bitumen treatment speed. At the same time, the bitumen treated within the equalizer tank is progressively brought to the prede- termined consistency. In comparison with the prior art continuous processes, a bitumen having a predetermined quality can be obtained without any risk of treating the bitumen once again, should the analytical tests indicate that the bi tumen has failed to meet the required consistency parameters.

[0022] By treating the bitumen with the oxygen-containing gas in the static mixer unit of the oxidation duct, an intimate mixing of the gas and the bitumen is possible, which causes the formation of foam. In the present invention, the bitumen is withdrawn from the bottom portion and continuously returned as ox idized bitumen to an upper portion of the elongated vertical equalizer tank. Since the bitumen is withdrawn from the bottom, the fluid particles of the re- turned bitumen move from the upper portion to the bottom portion of the tank slowly enough to allow: the foam to accumulate and float upon the liquid, since it is much lighter than the bitumen, and since the gas entrained by the bitumen, while mov ing down, has enough time to escape from the liquid and reach the float- ing foam; the foam accumulated to split in turn into a gas, which will leave the equalizer tank through the vent, and oxidized bitumen, which will remain in the tank, which is also assisted by the treatment temperature control. [0023] For the reasons explained above, a substantially foam-free oxidized bitumen can be obtained by the process and the apparatus according to the invention.

[0024] The pressure-adjustment element is preferably controlled by a pres- sure signal taken at a location between the recirculation pump and a section of the oxidation duct.

[0025] In an exemplary preferred embodiment, the above mentioned upper portion of the equalizer tank, into which the bitumen flow is returned, is a sub merged portion, i.e. a portion where the bitumen is present, and is preferably located at a height set between half the level of the bitumen and the level of the bitumen. The advantages arising therefrom are described hereinafter. [0026] Firstly, the foam of the bitumen flow returning from the oxidation duct contains a high amount of bitumen volatile components, as well as air. By introducing the returning flow into a submerged portion of the equalizer tank, the volatile fractions can be recovered into the mass of the bitumen. As al ready said, in addition to improving the yield of the process, the recovered vol atile fractions have a useful fluidification effect on the oxidized bitumen. On the contrary, by feeding the returning flow above the liquid level or in any case too close to the vapor vent of the equalizer tank, a relevant amount of the volatile fraction would be loss, and would leave the equalizer tank along with other va por matter through the vent. Moreover, the high temperature of the equalizer tank would promote this loss. In a modification of the process, described here inafter, this loss would be also promoted by supplying further oxygen- containing gas directly to the equalizer tank, in addition to the gas supplied to the oxidation duct.

[0027] Moreover, the foam, which is unavoidably present in the flow return ing from the oxidation duct, can contain an important amount of finely dis persed unreacted oxygen, i.e., bubbles that can have size as small as a few micrometers. This is the case, in particular, if a considerable excess oxygen is required in the oxidation duct, i.e., if a high air-to-bitumen ratio is required in the oxidation duct feedstock. Moreover, the small bubbles of the excess oxy gen can have such a high temperature as about 260°C, due to the reaction heat released in the oxidation duct. The fine dispersion and the high tempera ture of this oxygen promotes the reaction with the bitumen that the small bub- bles meet in their upwards movement within the equalizer tank, once they have been fed back to it, from their feed point towards the free surface of the bitumen. Briefly, by feeding back the bitumen below the free surface of the liq uid, the use of the supplied oxygen can be optimized, i.e., it is possible to take advantage of the excess oxygen, and to perform a further true oxidation step in the submerged portion of the equalizer tank.

[0028] Moreover, by feeding the bitumen back below the free surface of the liquid, a heat exchange occurs between the higher-temperature gas portion of the flow returning from the oxidation duct, and the lower-temperature bitumen contained in the equalizer tank. This heat exchange further contributes to speed up the reactions taking place between the excess oxygen and the bitu men contained in the equalizer tank.

[0029] Moreover, since the bitumen is normally fed back below the free surface of the liquid at a high pressure, somewhat turbulent conditions are es- tablished within the equalizer tank, which also contributes to speed up the above-mentioned reactions.

[0030] As an alternative, the upper portion, to which the flow of bitumen is fed back, is at a height above the bottom higher than the bitumen level in the equalizer tank. In other words, the returning bitumen flow is bed back into the equalizer tank above the bitumen head, i.e., the bitumen liquid surface. In this case, advantages are obtained that differs from those described above. In par ticular, the foam removal from the recirculating bitumen is maximized. This is preferable, for instance, if the gas supplied to the oxidation duct only contains a small excess of oxygen. In this case, accordingly, the flow of the bitumen re- circulating through the oxidation duct would contains a small amount of unre acted oxygen, which would not appreciably contribute to the overall bitumen oxidation process.

[0031] Preferably, the treatment pressure maintained in the oxidation duct is set between 3 bar(g) and 7 bar(g). This way, even more favorable conditions are created for an intimate mixing of the oxygen-containing gas and the air, and for the oxygen transfer from the gas phase to the liquid phase, such condi tions being never attained in the conventional oxidation columns.

[0032] In particular, the oxygen-containing gas supplied into the oxidation duct is air, and the amount of air supplied to the oxidation duct is preferably set between 0.5 liters of air for Kg of bitumen, measured at a pressure of 2 bar(g), and 10 liters of air for Kg of bitumen, measured at a pressure of 8 bar(g). More in general, the amount of oxygen supplied into the oxidation duct with the oxy gen-containing gas per Kg of bitumen is set between an amount corresponding to the oxygen contained in 0.5 liters of air at the pressure of 2 bar(g), and an amount corresponding to the oxygen contained in 10 liters of air at the pres sure of 8 bar(g). The association of the static mixer unit and of the oxygen- containing gas pressure makes it possible to select the amount of gas, i.e. air or oxygen, supplied per Kg of product, within a range remarkably wider than what is allowed when feeding the conventional oxidation columns.

[0033] The “consistency value” of the bitumen to be achieved through the oxidation process can be determined by viscosity assessments made during the oxidation step. In fact, the viscosity of a bitumen in which such a treatment is taking place turned out to be a univocal measure of the extent of the reac tion. In other words, the viscosity increases noticeably and monotonously as the reaction itself proceeds.

[0034] To this purpose, the method can provide a step of prearranging a viscosity assessment device arranged to come into contact with a mixed por tion of the amount of bitumen contained in the equalizer tank, and to continu ously generate a viscosity signal related to the viscosity of said mixed portion.

[0035] More in detail, the viscosity assessment device can be configured to move an own measurement body at a predetermined speed in the bitumen mixed portion, and to measure a force or a torque required to maintain the predetermined speed of the measurement body, or to measure a power re quired to generate such a force or torque. An indirect measure of this power can be supplied by measuring a current absorption of an electric motor ar ranged to move the measurement body.

[0036] In particular, the viscosity assessment device can comprise a stir ring device, such as an agitator, including a drive unit, in particular an electric motor, configured to cause the measurement body, i.e. a stirring portion of the agitator comprising, for instance, an impeller consisting of a plurality of blades, to rotate at a predetermined speed.

The apparatus can further comprise: a viscosity-detection device for detecting a viscosity-related value, i.e., the value of a quantity related to the viscosity, selected from among a force or a torque applied to maintain the speed of the measurement body, and an adsorbed power required to generate said force or torque; - a display means for displaying said viscosity-related value.

The apparatus can also comprise a computation unit configured to turn the above mentioned viscosity-related value into a viscosity signal related to the viscosity of the bitumen contained in the equalizer tank, so as to display a bi tumen viscosity change. [0037] This way, an operator in charge of monitoring the bitumen oxidation process can real-time know the bitumen viscosity, without taking and analyzing any samples. This is an advantage for many reasons. Firstly, the samples could not be representative of the conditions of the whole amount of bitumen. Secondly, the analysis of samples and the related preliminary operations re- quire time, therefore the results of the measurements can differ from the actual conditions of the bitumen at the end of the measurement themselves. Not last, taking and handling the samples can expose the operator to potentially dan gerous chemicals. By the invention, the operator is enabled to stop the oxida tion step and to obtain a bitumen having a predetermined viscosity and oxida- tion degree, corresponding to the features the bitumen is desired to have when in use.

[0038] As an alternative, the bitumen “consistency value” to be obtained by the oxidation process can in any case be determined by measuring the hard ness and/or some rheological properties of samples, as well known in the art. More in detail, the target “consistency value” of the bitumen can be determined by measuring its softening temperature, for instance, by the so-called “ring and ball test”, and/or by measuring its penetration rate, for instance, according to EN 1426. These measurements can be periodically carried out on bitumen samples withdrawn as the oxidation proceeds, at a rate established according to the initial properties of the bitumen and according to the actual process con ditions.

[0039] Preferably, the ratio between the volume of the amount of bitumen and the net capacity of the oxidation duct is at least 10. For instance, in the case of an equalizer tank having the size of a common oxidation column, i.e., about 30 m ³ , and of several oxidation ducts successfully used in the process, this ratio was set between 20 and 80. In fact, it has been observed that below these limits, the foam removal effect tends to become unsatisfactory. Moreo ver, if the ratio is at least 10, the viscosity assessment, if any, by means of power or current absorption of the motor of an agitator installed in the equaliz er tank, can be considered representative of the viscosity of the whole amount of the treated bitumen.

[0040] Preferably, the oxidation duct is arranged along a recycle circuit outside of the equalizer tank, wherein the inlet and outlet end portions of the oxidation duct are hydraulically connected with an outlet mouth and with a re turn inlet mouth, respectively, arranged at the bottom portion and at the upper portion, respectively, of the equalizer tank. This arrangement advantageously assists inspection and maintenance of the apparatus and of the instruments of the circuit of the oxidation duct. [0041] In this case, the suction mouth of the recirculation pump, or the inlet end of a suction duct connected thereto, is arranged at the bottom portion of the equalizer tank, whereas the outlet end of the oxidation duct is arranged at the upper portion of the tank, or can be adjusted responsive to the position of the head of the liquid that is actually present in the tank. [0042] As an alternative, the oxidation duct, and possibly even the pump, can be arranged within the equalizer tank, in which case the pump is a sub merged pump. The specific advantage of such an arrangement is to limit the overall heat loss of the apparatus.

[0043] In a modification of the process, during the steps of conveying the flow of the bitumen and of supplying the stream of an oxygen-containing gas into the oxidation duct, a step is further provided of supplying a second stream of an oxygen-containing gas to the equalizer tank, so that the oxygen- containing gas preferably passes through substantially the whole mass of the bitumen that is present in the equalizer tank, maintaining the treatment tem- perature. To this purpose, in an exemplary embodiment of the apparatus, the equalizer tank comprises a second gas supply means arranged in such a way that, by supplying a second stream of an oxygen-containing gas to the equal izer tank, this further stream preferably passes through the whole mass of the bitumen. [0044] The contemporaneous supplying of an oxygen-containing gas into both the oxidation duct and the equalizer tank, besides contributing to bitumen oxidation, also assists the above-described process allowing the production of a substantially foam-free oxidized bitumen. In fact, the upward flow of the fur ther oxygen-containing gas stream, which is supplied to the bottom portion of the equalizer tank in counter-current with respect to the downward flow of the bitumen caused by the recirculation pump, enhances the foam surface accu mulation and breakdown, as described above.

[0045] In particular, the equalizer tank is configured as an oxidation column of the type commonly used in the conventional bitumen oxidation methods. Such an oxidation column can be of any type between those known in the art.

[0046] Moreover, it falls within the scope of the invention a device for im proving the performances of a bitumen oxidation plant, the device configured to receive bitumen from an oxidation column containing such an amount of bi tumen as to fill the bitumen oxidation column up to a predetermined level above a bottom portion thereof, and to maintain the amount of bitumen at a treatment temperature set between 200°C and 240°C, and equipped with a gas supply means for supplying a gas thereto, such that, by supplying a stream of an oxygen-containing gas to the bitumen oxidation column, this oxy gen-containing gas stream passes through the bitumen; the device comprising: an oxidation duct configured to be arranged outside of the bitumen oxida tion column and containing at least one static mixer unit, the oxidation duct having inlet and outlet end portions arranged to be hydraulically connected to the bottom portion and to an upper portion, respectively, of the bitumen oxidation column; wherein the upper portion is at a height of at least one third of the level above the bottom portion; a recirculation pump arranged for conveying a flow of the bitumen from the bottom portion to the upper portion of the bitumen oxidation column through the oxidation duct; a further gas supply means for supplying a further stream of an oxygen- containing gas to the oxidation duct; a pressure-adjustment element located between the outlet end of the oxi dation duct and the upper portion of the bitumen oxidation column, the main feature of the device being that the recirculation pump and the ad justment element are configured to cooperatively maintain a predetermined treatment pressure within the oxidation duct set between 2 bar(g) and 8 bar(g).

[0047] By such a device, the advantages of the invention can be extended to existing bitumen oxidation plants, including discontinuously operating oxida tion columns. In particular, the device makes it possible to reduce the time and/or the working temperature, and to improve the oxidized bitumen yield/quality, thus increasing the production rate of those plants. Moreover, the device makes it possible to reduce the gas emission purification costs, since it is possible to oxidize the bitumen at lower temperatures, besides the above- mentioned further advantages.

[0048] Advantageously, such a device for improving the performances of a bitumen oxidation plant is mounted to a skid.

[0049] Preferably, the pressure-adjustment element downstream of the ox idation duct is a pressure control valve. As an alternative, the pressure- adjustment element can comprise a positive-displacement pump configured to deliver a flowrate of bitumen lower than the recirculation pump, provided the latter is a positive-displacement pump as well.

[0050] In a modification of the process, the step of supplying the oxygen- containing gas stream to the oxidation duct can be discontinued once a bitu men volume equal to a multiple of the amount of bitumen contained in the equalizer tank has been conveyed from the bottom portion to the upper portion of the equalizer tank through the oxidation duct, for instance, once a volume equal to that amount of bitumen has been conveyed. This can be useful to es tablish a standard duration of the treatment, and to reduce the analytical con trols during the oxidation step.

Brief description of the drawings [0051] The invention will be now shown with the description of some ex emplary embodiments thereof, exemplifying but not limitative, with reference to the attached drawings, in which like reference characters designate the same or similar parts, throughout the figures of which: Fig. 1 is a block diagram of the process according to the invention;

Fig. 2 is a flow chart of an apparatus according to the invention;

Fig. 3 is a flow chart of an apparatus according to a modification of the apparatus of Fig. 2, in which the equalizer tank is equipped with an agita- tor having a motor to which a torque measurement means is associated;

Fig. 4 shows more in detail a possible instrumentation for a recycle circuit that including the oxidation duct;

Fig. 5 is a block diagram of a modification of the process of Fig. 1, in which a step is provided of supplying oxygen-containing gas also to the equalizer tank;

Fig. 6 is a block diagram of the apparatus of Fig. 2, wherein a gas supply means is provided to supply an oxygen-containing gas also to the equal izer tank;

Fig. 7 is a diagrammatical perspective view of a skid-mounted device for improving the performances of an existing bitumen oxidation plant;

Figs. 8 and 9 are block diagrams related to two possible modifications of the step of maintaining, i.e., controlling the pressure within the oxidation duct.

Description of preferred exemplary embodiments [0052] With reference to Figs. 1 and 2, a bitumen oxidation process 100 is described to obtain an oxidized bitumen 9 having properties suitable for use as a binding material in asphalt concretes for road pavements and the like, or for use as a waterproof and/or heat insulation material. Bitumen 1 to be treated is obtained as a crude oil vacuum distillation residue. Flowever, the process can be used to oxidize also natural bitumen and similar materials.

[0053] Process 100 is a discontinuous process and comprises, firstly, a step 101 of prearranging an elongated and vertical equalizer tank 10, having for instance a height-to-diameter ratio of at least 1.8, preferably of at least 2.2, where the term “height” is the useful height, i.e., the height up to which the bi- tumen can be present. It is further provided a preliminary step 111 of prear ranging an amount of bitumen 1 to be oxidized in an oxidation batch, as well as a step 120 of feeding bitumen 1 into equalizer tank 10, up to a predeter mined level L above a bottom portion 15 thereof. [0054] A preliminary step 112 or 122 is also provided of heating bitumen 1 up to a treatment temperature T that, in the process according to the invention, is advantageously set between 200°C and 240°C. Heating 112 can be carried out before feeding 120 of bitumen 1 into equalizer tank 10 by a conventional heat exchanger, not shown in the drawings. As an alternative, or in addition thereto, heating 122 can be carried out within equalizer tank 10 that, in this case, is equipped with a conventional diathermic oil or steam heating circuit, not shown.

[0055] According to the invention, a preliminary step 131 is also provided of prearranging an oxidation duct 20 containing a static mixer unit 25.

[0056] Once the amount of bitumen 1 has been introduced into equalizer tank 10 and brought to treatment temperature T, a step 150 of oxidating the bi tumen can take place by contact with an oxygen-containing gas. In the present description, as indicated in Fig. 2, the oxygen-containing gas is typically air, which can be supplied by a compressor, not shown. However, the use of an industrial gas mixture containing a significant oxygen amount, or even the use of substantially pure oxygen, is not excluded.

[0057] According to the invention, step 150 of oxidating bitumen 1 com prises a step 161 of conveying a flow 2 of bitumen withdrawn from bottom 15 of equalizer tank 10 through oxidation duct 20, and of recycling flow 2 of bitu men from oxidation duct 20 back to an upper portion 16 of equalizer tank 10, at a height H equal to at least 1/3 of level L of the bitumen charged in equalizer tank 10.

[0058] To this purpose, an apparatus 200 comprises a recirculation circuit 30 connecting an outlet mouth 13 of equalizer tank 10, advantageously coinci dent with the discharge mouth of bottom 15, with a return inlet mouth 14 that defines above-mentioned upper portion 16 of equalizer tank 10.

[0059] In Fig. 2, a solid line indicates a possible return inlet mouth at a height lower than level L above bottom portion 15, in other words, inlet mouth 14 is arranged to introduce flow 2 of bitumen into a submerged upper portion 16 of equalizer tank 10. A dashed line qualitatively indicates two further possi ble alternative positions of inlet mouth 14, one of which is at a height close to a one-half level L of the bitumen contained in the tank, and the other is at a height longer than bitumen level L above bottom portion 15, in which latter case the inlet mouth is located above the bitumen level.

[0060] Oxidation duct 20 is arranged along recirculation circuit 30, and has an inlet end 21 that can be selectively brought into hydraulic connection with outlet mouth 13, and an outlet end 22 that is hydraulically connected with re- turn inlet mouth 14 of equalizer tank 10. A recirculation pump 31 is installed upstream of oxidation duct 20.

[0061] Still according to the invention, step 150 of oxidating bitumen 1 also comprises a step 162 of supplying a stream 4 of an oxygen-containing gas, preferably air, into oxidation duct 20, as shown in Fig. 2. To this purpose, a gas supply means 29 is provided along recirculation circuit 30, upstream of oxida tion duct 20 for supplying the oxygen-containing gas to oxidation duct 20, said supply means comprising an inlet port for stream 4 of the oxygen-containing gas.

[0062] Step 161 of conveying and recycling the bitumen is carried out in such a way that a predetermined treatment pressure Pt is maintained within oxidation duct 20 set between 2 bar(g) and 8 bar(g), in particular, between 3 bar(g) and 7 bar(g). In other words, step 150 of oxidating bitumen 1 includes a step 165 of maintaining, i.e., controlling the pressure between the above- mentioned values. To this purpose, apparatus 200 comprises a pressure sen- sor, not shown, preferably installed upstream of oxidation duct 20 or even on oxidation duct 20, and a pressure transmitter 24 associated with this sensor. Still to this purpose, apparatus 200 comprises a pressure-adjustment element mounted between outlet end 22 of oxidation duct 20 and return inlet mouth 14 of equalizer tank 10. In particular, as shown in Fig. 2, this adjustment element can be a control valve 33 whose opening is controlled by a control signal 42 generated by a processor means 80 responsive to a pressure signal 41 com ing from pressure transmitter 24.

[0063] As an alternative, if a positive-displacement recirculation pump 31 is used, the pressure-adjustment element in the oxidation duct can be a further positive-displacement control pump, not shown, wherein recirculation pump 31 is configured to deliver a flowrate larger than said control pump. In this case, a flowrate decrease imposed to the control pump causes the pressure to in crease, since the second pump handles a two-phase, globally compressible mixture consisting of the bitumen and the residual gas after the treatment per- formed within the oxidation duct, whereas the first pump handles “pure”, i.e., gas-free bitumen, which is substantially incompressible. The speed of the con trol pump, in the case of a rotary positive-displacement pump, or the frequen cy/length of a piston stroke, in the case of a reciprocating positive- displacement pump, is controlled by control signal 42 that processor means 80 generates responsive to pressure signal 41 coming from pressure transmitter 24.

[0064] Figs. 8 and 9 are block diagrams showing step 165 of maintaining, i.e., controlling treatment pressure Pt in oxidation duct 20, in two alternative modifications of the method. In both cases, step 165 of controlling the pressure includes a preliminary step 164 of setting a target pressure value, i.e., a pres sure set-point P ^* , typically carried out by processor means 80. During step 150 of oxidating the bitumen, step 165 of controlling the pressure comprises cyclic steps 167 and 168 of detecting pressure Pt within oxidation duct 20 and of comparing detected value Pt with target pressure value P ^* , respectively.

[0065] In the case of Fig. 8, if detected pressure Pt is higher than target pressure P ^* , a further cycle 167,168 is carried out of detecting/comparing the detected pressure. On the contrary, if detected pressure Pt is lower than target pressure P ^* , a step is carried out of actuating pressure-adjustment element 33, typically through control signal 42 that processor means 80 generates respon sive to pressure signal 41 coming from pressure transmitter 24, and then a fur ther cycle 167,168 is carried out of detecting/comparing the detected pressure.

[0066] In the case of a pressure-adjustment element comprising a control valve, actuating the adjustment element means changing the opening of the valve. Instead, in the case of a pressure-adjustment element comprising a second rotary positive-displacement pump, actuating the adjustment element means changing the speed of the rotor of the pump itself.

[0067] In the case of Fig. 9, if the absolute value of the difference between detected pressure Pt and target pressure P ^* is lower than a reference value e, a further cycle 167,168 is carried out of detecting/comparing the detected pressure. On the contrary, if the above absolute value is larger than reference value e, a step is carried out of actuating pressure-adjustment element 33, typ ically through control signal 42 that processor means 80 generates responsive to pressure signal 41 coming from pressure transmitter 24, and then a further cycle 167,168 is carried out of detecting/comparing the detected pressure. [0068] Apparatus 200 advantageously also includes a means, not shown, for adjusting the flowrate of the oxygen-containing gas. In particular, if air is used as the oxygen-containing gas, the air supply means and the flowrate- adjustment means are configured to supply a given air amount to oxidation duct 20, said air amount set between 0.5 liters of air per Kg of bitumen, meas ured at a pressure of 2 bar(g), and 10 liters of air per Kg of bitumen, measured at a pressure of 8 bar(g).

[0069] In such conditions, a step 163 of mixing stream 4 of oxygen- containing gas with flow 2 of bitumen 1 takes place along static mixer unit 25 of oxidation duct 20, which assists the oxidation reaction, thus yielding oxi dized bitumen 9 at the end of step 150 of oxidizing the bitumen.

[0070] Due to static mixer unit 25, as the oxidation proceeds in oxidation duct 20, bitumen 2 and oxygen-containing gas 4 form a foam, i.e., a more or less steady gas/bitumen dispersion, which is conveyed into upper portion 16 of equalizer tank 10. The elongated shape of equalizer tank 10 and the with drawal of the bitumen from bottom 15 thereof cause bitumen to flow down along equalizer tank 10 at a low and substantially plug-flow speed, with small turbulence and poor mixing effect. This way, the foam tends to form a layer 6 that floats on bitumen 1 ,2 contained in equalizer tank 10, and that tends to re lease the gas included therein. A vent stream 5 formed by this released gas and bitumen vapors is extracted from equalizer tank 10 through a vent opening 17 by a conventional suction means, not shown.

[0071] If the bitumen is fed at a submerged position, the foam of flow 2 of the bitumen returning from oxidation duct 20, and therefore also the micromet ric gas bubbles dispersed therein, moves upwards between upper portion 16 and level L of bitumen 1 contained in equalizer tank 10. If air is used in oxida tion duct 20 as oxygen-containing gas 4, the upwards moving gas contains ni trogen, unreacted oxygen and an organic portion coming from the most volatile fraction of the bitumen.

[0072] During the upward movement of the foam between return inlet mouth 14 and level L of bitumen 1 , a heat transfer occurs from the foam to bi tumen 1 , since the temperature of flow 2 of oxidized bitumen is normally higher than the temperature of the bitumen contained in equalizer tank 10, due to the reaction heat released in oxidation duct 20. Moreover, during the upward movement, one part of the volatile components of the bitumen can condense and join the mass of the bitumen 1 itself.

[0073] Moreover, a more or less important amount of unreacted oxygen is normally present in the returning bitumen flow, due to the excess oxygen of stream 4 of oxygen-containing gas supplied to oxidation duct 20. This oxygen amount can cause a secondary oxidation of bitumen 1 that is present between upper portion 16 of equalizer tank 10 and level L of the bitumen itself. The high temperature of the foam of returning flow 2 promotes this secondary oxidation. [0074] In the case of apparatus 200 of Fig. 2, a three-way valve 18 or, as an alternative, a couple of two-ways valves, not shown, is mounted at outlet mouth 13 in order to selectively connect bottom 15 of equalizer tank 10 with recirculation circuit 30 or with a discharge duct 99 to a storage tank, not shown, or to a user device. [0075] Supplying 162 of stream 4 of oxygen-containing gas into oxidation duct 20, flow 2 of bitumen being oxidated within recirculation circuit 30, as well as treatment temperature T and pressure Pt, are maintained until bitumen 1 ,2 being oxidated reaches the predetermined consistency. Once this condition has been achieved, a step 180 of discontinuing oxidation step 150 takes place. In other words, firstly, supplying 162 of oxygen-containing gas, and therefore its mixing 163 with already oxidized bitumen 2, is discontinued. Optionally, conveying 163 of the bitumen through recirculation circuit 30 and oxidation duct 20 can also be discontinued. A step 190 follows of discharging oxidized bitumen 9 from equalizer tank 10, and of transferring the oxidized bitumen, for instance, into a storage tank, not shown, which allows starting a new oxidation batch.

[0076] With reference to Fig. 3, an apparatus 201 for oxidating bitumen, according to a modification of the exemplary embodiment of Fig. 2, comprises an agitator 34 in which a stirring portion 35 is rotatably arranged within equal- izer tank 10, in such a way to be submerged by bitumen 1 , and also comprises a drive unit 36 configured to cause stirring portion 35 to rotate at a predeter mined speed.

[0077] Fig. 3 also shows an optional viscosity assessment device 40, con figured to continuously measure the viscosity of bitumen 1 ,2 contained within equalizer tank 10, wherein said viscosity measuring device is in contact with a mixed portion of bitumen 1 ,2, and is also configured to continuously generate a viscosity signal, not shown, related to said mixed portion.

[0078] Viscosity assessment device 40 comprises a viscosity-detection de vice 38 for detecting a viscosity-related value 37, i.e. the value 37 of a quantity related to the viscosity of bitumen 1 , typically the torque that drive unit 36 must generate in order to cause stirring portion 35 to rotate at the predetermined speed, or the power required to generate said torque, or the corresponding current absorption. In other words, movable portion 35 of agitator 34 is config ured as a measurement body 35 of a device 34 for measuring the viscosity of bitumen 1 ,2.

[0079] Drive unit 36 can be an electric motor or also a hydraulic motor con trolled by a control unit, not shown, and including a pump for circulating a working fluid, which is normally actuated by an electric motor. Advantageously, a display means 39 is also provided to display the torque value T or the value of the related power absorption, or a corresponding viscosity value m. Actually, viscosity assessment device 40 can include a computation unit 45 configured to turn torque value T or power value into a viscosity value m, and display means 39 is arranged to display viscosity value m.

[0080] Equalizer tank 10 of apparatuses 200 and 201 of Figs. 2 and 3 is preferably configured to contain a volume of bitumen 1,2 that is at least 10 times greater than capacity of oxidation duct 20. This way, the viscosity value continuously made available by viscosity measurement device 40 can be con sidered representative of the whole amount of bitumen 1 ,2 being oxidated. [0081] As an alternative, the consistency of the bitumen can be evaluated by testing bitumen samples, for instance by measuring the ring and ball soften ing point and/or the penetration rate according to EN 1426, or by equivalent measurements.

[0082] However, the inventors also realized that rheological and mechani cal properties suitable for conventional uses of the bitumen are obtained once a predetermined multiple of the amount of bitumen 1 initially prearranged has been conveyed through oxidation duct 20.

[0083] The diagram of Fig. 4 shows more in detail piping and instrumenta- tion of recirculation circuit 30, according to an exemplary embodiment.

[0084] Piping 60 of recirculation circuit 30, including the shut-off and con trol valves, as well as recirculation pump 31 , comprises a temperature control means, in this case a jacket 61 , configured to receive a flow of a heat ex change fluid, such as diathermic oil, in order to maintain the bitumen at a tem perature at which the same is fluid enough to be easily conveyed.

[0085] Recirculation circuit 30 also includes temperature and pressure indi cators 62,63 for locally displaying the values of these quantities. In particular, these instruments are mounted on the delivery side of recirculation pump 31 , upstream and downstream of oxidation duct 20 and downstream of the pres sure-adjustment element of oxidation duct 20, in this case, downstream of con trol valve 33. In particular, pressure indicator 63 mounted on the delivery side of recirculation pump 31 is advantageously associated with a pressure switch PSH configured to generate a pressure threshold signal upon exceeding a predetermined pressure value, wherein said threshold signal is used to prevent the discharge pressure of recirculation pump 31 from exceeding a maximum admissible pressure in connection with the stability of the apparatus and of the piping of recirculation circuit 30. Obviously, temperature and pressure indica tors 62,63 can be associated with respective transmitters configured to emit respective signal in order to remotely display the corresponding variable val ues, in particular, at a control room.

[0086] Fig. 4 also shows the structure of the pressure control loop for ad justing the pressure in oxidation duct 20. Pressure transmitter 24 is configured to generate an electric pressure signal 41 , processor means 80, preferably a PLC, is configured to receive electric pressure signal 41 and to generate an electric control signal 42, an electric-pneumatic converter 43 including a pneumatic feed device 44 is configured to turn electric control signal 42 into a pneumatic control signal 42’. Pressure control valve 33 is equipped with an ac tuator 33’ configured to receive pneumatic control signal 42’ and to operate control valve 33 such that an opening degree is attained for adjusting the pres sure in oxidation duct 20 at a predetermined value, preferably set between 2 bar(g) and 8 bar(g), in particular between 3 bar(g) and 7 bar(g).

[0087] Moreover, Fig. 4 shows in detail the gas supply means to supply ox ygen-containing gas to oxidation duct 20. In this case, the oxygen-containing gas is compressed air. The gas supply means has the shape of a supply unit 29 including a compressor 70. A compressed air treatment unit 71 can be pro vided downstream of compressor 70. Supply means 29 further includes a compressed air pressure control valve 73, equipped with conventional pneu- matic actuation means 72. Moreover, in order to prevent the bitumen from reaching compressor 70, a coil 74, and/or a check valve 75 can be provided downstream of the latter, and/or a further flow control valve 77 can be provided configured to ensure that the compressed air is correctly introduced into oxida tion duct 20, and that bitumen 2 does not flow into the compressed air inlet duct. To this purpose, a differential pressure transmitter 28 associated with pressure sensors 78, 79 respectively arranged in the inlet duct of compressed air flow 4 and at inlet end 21 of oxidation duct 20. The differential pressure transmitter 28 is configured to generate a differential pressure signal 47 that processor means 80 is configured to receive. Processor means 80 is also con- figured to analyse a control signal 48 for operating flow control valve 77 so that compressed air pressure is always higher than the pressure of bitumen 2.

[0088] With reference to Figs. 5 and 6, a bitumen 1 oxidation process 300 is now described, according to an exemplary embodiment of the invention, wherein oxidation 150 comprises a step 151 of supplying/dispersing a further stream 3 of oxygen-containing gas through the mass of bitumen 1 contained in an equalizer tank 10. In this case, the equalizer tank can be configured as a true oxidation column, for example a conventional type oxidation column to ac tuate a discontinuous bitumen oxidation method.

[0089] More in detail, equalizer tank 10 comprises a container 11 and a first gas supply/dispersion means 19 for introducing a further stream 3 of a preferably finely dispersed oxygen-containing gas into container 11. Even in this case, in some exemplary embodiments, equalizer tank 10 has a stirring device that is preferably configured to obtain/maintain a predetermined disper sion degree of oxygen-containing gas 3, and that can synergize with gas sup- ply/dispersion means 19.

[0090] Further stream 3 of oxygen-containing gas can be a compressed air flow supplied by above-mentioned compressor 70, or by a further compressor, not shown. Even in this case, the use of a different oxygen-containing gas or also of substantially pure oxygen is not excluded. [0091] In a closed dash-dot line, Fig. 6 also shows a device 50 for improv ing the overall performance of an existing apparatus 400 based on a conven tional oxidation column 12. Device 50 comprises recirculation circuit 30 includ ing equipment 20, 31, instrumentation 24, 33 and respective connection ducts. Device 50 can also comprise a processor means 80 for controlling the device, in addition to an oxygen-containing gas supply unit, in particular compressed air, which can have the features of gas supply unit 29 previously described with reference to Fig. 4.

[0092] The components of device 50 can be advantageously mounted to a skid, as shown, for example, in Fig. 7, which can also be related to recircula tion circuit 30 of apparatuses 100 and 300 of Figs. 1 and 5.

[0093] Some of the elements shown in the diagrams of Figs. 2, 4 and 5 can be recognized in Fig. 7. In particular, a compressor unit 70’ can comprise, in addition to compressor 70, compressed air treatment unit 71 and coil protec- tion device 74. Moreover, an advantageous exemplary embodiment of oxida tion duct 20 is shown comprising in this case a plurality of piping elements or tubes 23 serially arranged along an upward path and connected to each other by U- fittings, wherein the connection portion to connect tubes 23, in this case flange portions, lay in mutually non-parallel planes. [0094] The foregoing description exemplary embodiments and exemplary embodiments of the invention will so fully reveal the invention according to the conceptual point of view, so that others, by applying stream knowledge, will be able to modify and/or adapt for various applications such embodiment without further research and without parting from the invention, and, accordingly, it is meant that such adaptations and modifications will have to be considered as equivalent to the exemplary embodiments and exemplary specific embodi ments. The means and the materials to realize the different functions de scribed herein could have a different nature without, for this reason, departing from the field of the invention. It is to be understood that the phraseology or terminology that is employed herein is for the purpose of description and not of limitation.