Patent Application
20240132692
The present invention relates to a process and related system for treating tyres and for their chemical conversion into products of high commercial value.
One of the main problems for the circular economy is the recovery of plastic waste (plasmix). Plasmix is a complex mixture of polymers of different nature and origin, mainly linear-branched such as Polyethylene (PE) and Polypropylene (PP) and with a reduced content of aromatic polymers such as Polystyrene (PS) and Polyethylene Terephthalate (PET). Otherwise, plasmix can comprise chlorinated plastics (PolyVinylChloride, PVC) or PolyCarbonates (PC) or PolyAmides (for example PolyMethylMetAcrylate, PMMA), as well as other types of plastics and waste of different kinds such as metals, sand or stones. An example of a possible composition is given in the following table
Such plastics are currently recovered mainly by mechanical means, which however reduces their value and limits their use. For these reasons, some chemical processes are also known that involve pyrolysis operations, carried out in the absence of oxygen, or gasification, carried out in the absence of oxygen. However, such chemical recovery technologies have limitations for different areas of application.
In particular, the known processes do not allow tyres to be chemically recovered in a sustainable, economical and efficient manner. The reasons lie in the significant presence of sulphur, used in the tyre vulcanization process. In thermal chemical conversion processes such as pyrolysis and gasification, but also in catalytic processes, sulphur leaks from the vulcanized matrices mainly in the form of H2S. Therefore, the known treatment technologies firstly have safety problems for the operators and for the environment since H2S inhibits the nasal sensors already in reduced quantities (of the order of 100 ppm) and in hardly higher quantities it becomes lethal for humans. Secondly, such processes involve management problems of the H2S that is formed, which is energetically intensive; it is released in the form of gas by reaction with a considerable amount of hydrogen according to the following reaction mechanism:
[—CH2—]n-S—[—CH2—]m+H2=[—CH2—]n*+[—CH2—]m*+H2S
[—CH2—]11*[—CH2-]M*+H2=[—CH2-]n[—CH2-]m
The need is therefore felt to have a process for the treatment of plastic polymeric material deriving from tyre waste which is economically convenient and at the same time allows to minimize unwanted products as well as to convert this polymeric material into commercial products of high value, favouring the circular economy and without any expense in terms of hydrogen consumption.
Examples of known processes related to devulcanization processes are described in documents US 2006116431 A1 and US 2020181372 A1. Specifically, document US4200611631 envisages the addition, after grinding and prior to the devulcanization step, of a swelling solvent and a precursor of the devulcanization catalyst of the rubber.
With respect to US372, this describes a further process for the devulcanization of thermosetting rubber obtained by mixing pulverized rubber and a thermoplastic polymer, a main chain backbone protecting antidegradant (in other words, sterically hindered phenols, quinones, alkylphenylamines, dialkylphenylenediamines, alkylarylphenylenediamines, and polymerized trimethylquinoline, which prevent further degradation of the polymers), a green strength enhancer, and a devulcanization promoter, i.e., an oxide of a metal supported on an inorganic material together with endcapping reagents.
In order to overcome the aforesaid problems, a process and a related system have been designed that allow the chemical recovery of tyres, allowing their high-efficiency conversion into high value-added chemicals in addition to reducing the waste produced and maintaining a high degree of safety.
The object of the present invention is therefore a process for the chemical treatment of scrap tyres, comprising the steps of:
R1:[—CH2—]n-S—[—CH2]m+H2=[—CH2—]n*[—CH2—]m*H2S
where m and n indicate non-identical lengths of the macromolecules in terms of carbon atoms and the asterisk indicates the possible presence of at least one olefin unsaturation, at a temperature comprised between 300 and 400° C., at a pressure comprised between 10 and 150 bar,
said reaction R1 being possibly carried out in the presence of the saturation reaction R2 of said possible at least one olefin unsaturation:
R2:[—CH2—]n*+[—CH2—]m*+H2=[—CH2—]n+[—CH2]m
This process is characterized in that:
step (c)
R3:H2S═H2+1/X Sx
The products of commercial value obtained in step d) are selected from:
For the purposes of the present invention, the definition “process comprising” does not exclude the presence of additional steps beyond the step expressly mentioned after such a definition.
The definition “Process constituted” and “consisting of” excludes the presence of further steps in addition to those expressly mentioned.
With the process of the invention it is thus possible to obtain products of value, intended as:
The process according to the present invention makes it possible to remove the sulphur from vulcanized plastics, whether they come from tyres or not, to prepare the desulphurized waste plastics for chemical recovery treatments and to use the hydrogen obtained by thermally splitting the hydrogen sulphide formed in the desulphurization reaction
The process in accordance with the present invention also allows an energy recovery to self-sustain the initial pre-treatment steps of the vulcanized plastic (melting and dehalogenation).
For the purposes of the present invention, scrap tyres can fall within the definition of a plasmix with a relevant sulphur content. An example of the chemical composition of the tyres is shown in the table below.
In other words, the tyres are representable at the elementary level as a mixture of carbon, hydrogen and sulphur (C/H/S as indicated in
The process comprises a first step a) of tyre grinding, referred to as COMMINATION in
The process comprises a subsequent step b) of melting the material coming from step a) specifically the vulcanized plastic ground and cleaned of inorganic residues, indicated with MELTING in the figures. The plastic from step a) is supplied to a melter capable of melting it at temperatures above 200° C., preferably above 250° C.
The process comprises a subsequent step c) of devulcanizing the molten material from step b). Such devulcanization takes place according to the reaction R1:
R1:[—CH2—]n-S—[—CH2—]m+H2=[—CH2—]n*[—CH2—]m*+H2S
where m and n indicate non-identical lengths of the macromolecules in terms of carbon atoms and the asterisk indicates the possible presence of at least one olefin unsaturation. Possibly the R1 reaction is associated with the saturation reaction R2 of the at least one olefin unsaturation:
R2:[—CH2—]n*+[—CH2—]m*+H2=[—CH2—]n+[—CH2]m
Preferably, in step c) the slurry reagent comprising plastics and added with catalyst is reacted with hydrogen.
The catalyst used shall withstand poisoning by sulphur and sulphurized organic compounds and shall withstand the presence of light and heavy, paraffinic, naphthenic or aromatic hydrocarbons.
The catalysts used in step c) are selected from cobalt oxide or molybdenum oxide possibly supported on alumina.
According to a preferred embodiment, step c) is conducted at a temperature comprised between 300 and 350° C., between 50 and 100 bar. The above temperature range was chosen to promote the activation of the catalysts while avoiding significant degradation of the polymer chains (and to prevent the formation of high concentrations of hydrocarbons together with the leakage of H2S). On the other hand, regarding the pressure ranges, these favour hydrogenation as a function of the increase in pressure itself.
Step c) is carried out in a homogeneous or heterogeneous fixed or mobile bed reactor (fluidized or dragged).
Thus, in the devulcanization of step c) H2S is produced which, according to the present invention, is transformed into sulphur and hydrogen and the latter is recycled to the same devulcanization step c).
This step is also called splitting, which follows the reaction pattern R3
R3:H2S═H2+1/xSx
shown in
The products thus obtained can be reused. Specifically, the formed hydrogen is recycled to step c). Instead, the sulphur obtained can be treated and used in other plants (e.g., new vulcanizations, sulphuric acid production).
According to a preferred embodiment, the elemental sulphur can be separated from the hydrogen by condensation.
In the case where the unreacted H2S is present from the splitting reaction, this can be separated by softening processes and the hydrogen is recycled to step c).
Furthermore, the unreacted H2S can also be recycled internally to the hydrogen recovery unit for complete recovery. In case of small quantities of H2S present in the hydrogen stream leaving the splitting step, it is possible to recycle the same stream at step c). It should be noted that low percentages of input H2S do not significantly affect yields or related operations.
Therefore, after adequate initial feeding of hydrogen to step c), for example by the use of cylinders, the invention is thus able to self-sustain, solving one of the main problems of chemical devulcanization, namely the consumption of large amounts of hydrogen.
According to a preferred embodiment, the splitting reaction R3 is thermal or can be catalysed. It should be noted that the splitting reaction R3 can also be carried out by electrochemical means.
Specifically, the present invention has several methods of implementation as shown in the figures, in which the step of splitting and relative recovery of hydrogen is carried out by the following means of technology:
In the case where the splitting reaction R3 is of the thermal SATS type (
The oven 1 is arranged so that the first process flow A enters the oven 1 from the convective zone 3 and passing through the first set of tubes 4 exits the oven from the radiant zone 2. Alternatively, the first process flow A enters the oven 1 from the radiant zone 2 and, passing through the first set of tubes 4, exits the radiant zone 2. Instead, the second process flow B enters the oven 1 from the convective zone 3 or the radiant zone 2 and, passing through the second set of tubes 5, exits from said oven 1 from the radiant zone 3. In detail, the second flow B entering the oven comprises hydrogen sulphide passing through said second set of tubes 5, where the reaction R3 (SATS) occurs at the radiant zone 2, while the flow A comprising methane and water, passes through said first set of tubes 4, said first set of tubes comprising a tube bundle filled with catalyst and arranged at the radiant zone 2, where the reaction R4 (SMR: Steam Methane Reformer) is carried out.
R4:CH4+H2O=CO+3H2
Preferably, the reaction R3 is in the ranges of preferred temperatures and pressures contemplated in the aforesaid WO2020/234708 A1.
In the case in which the splitting reaction R3 is of catalytic SACS type (
The oven 1′ is arranged so that the first process flow A′ enters the oven 1′ from the convective zone 3′ and, passing through the first set of tubes 4′, exits said oven from the radiant zone 2′. Alternatively, the first process flow A′ enters the oven 1′ from the radiant zone 2′ and, passing through the first set of tubes 4′, exits the oven from the radiant zone 2′. Instead, the process flow B′, enters said oven 1′ from the convective zone 3′ passing through said second set of tubes 5′ and exits from the oven 1′ from the convective zone 3′. It should be noted that the second flow B′ comprising H2S passes into the second set of tubes 5′, where, at the convective zone 3′, the reaction R3 (SACS) is carried out. Instead, the flow A′ comprising methane and water, passes through the first set of tubes 4′, comprising a tube bundle filled with catalyst and arranged near the radiant zone 2′. In the first set of tubes 4′, the reaction R4 (SMR) is carried out
R4:CH4+H2O=CO+3H2
Preferably, the reaction R3 is carried out in the preferred pressure and temperature ranges contemplated in WO 2020/234709 A1.
For the purposes of the present invention, it should be noted that the oven 1, 1′ comprises an upper convective zone, 3′ where the heat exchange takes place by convection. The lower part, defined radiant zone, 2′, comprises a firebox with one or more vertical and/or horizontal burners, configured to radiate the sets of tubes. The convective zone in which the process flow enters the oven is heated by convection through the exhaust gases produced in the radiant zone by combustion of combustible gases. Thereby, the inlet gas process flow undergoes a preheating step.
According to a preferred embodiment, if there is a marked presence (a few percent) of PVC contained in the tyre compounds or in the devulcanizing plastic material, the process can comprise a de-halogenation step b′), indicated in the figures with DE-HALOGENATION (
R5:2HCl=H2+Cl2
Such a step is carried out if it is desired to chemically convert the tyre rubber with the conventional-type waste plastic (plasmix).
In accordance with a preferred embodiment, step b′) is preferably of a homogeneous non-catalytic type.
Preferably, said step b′) is carried out at a temperature comprised between 300 and 350° C. at residence times greater than 3 minutes, preferably comprised between 5 and 10 minutes. Thereby, using a temperature range similar to that of step c), the marked thermal degradation of the plastic macromolecules is avoided while allowing the HCl to escape as a gas from the liquid phase without excessive dragging of hydrocarbons.
More preferably, the pressure at step b′) can be significantly lower than the pressure expected for devulcanization step c) and is at 1 bar or a little higher or at a pressure of a few bar. For these reasons, downstream of the de-halogenator in which step b′) occurs, it is therefore necessary to provide a pumping system for melted and de-halogenated plastics to bring the fluid to the operating pressures of the devulcanizer (10-150 bar) in which step c) occurs. The operating temperatures of step b′) as well as of the previous and subsequent steps make the molten plastics very fluid and not very viscous, allowing to significantly save the operating costs of pumping. It should be noted that temperatures closer to 350° C. are preferable.
During step b′) no marked sulphur and sulphurized compound release phenomena occur due to the absence of a specific catalyst as well as hydrogen. There are also no marked thermal degradations of the polymer chains. The release of gaseous HCl occurs according to the following reaction:
[—CH2—CHCl-]n=[—CH═CH-]n+HCl
The recovery of HCl in a single unit allows to use precious materials limited to such a unit in the construction of the equipment, with significant savings. The HCl removed from the plastics and separated by phase is then purified to the quality required by the market. Although with modest market value, it represents an additional process product.
It should be noted that de-halogenation step b′) becomes necessary where it is desired to chemically convert the tyre plastic together with the classic plasmix (
According to an alternative embodiment, step b′) and c) with the related de-halogenators and devulcanizers can be envisaged on parallel lines and flow downstream thereof in a single premixed flow before conversion step d) described in detail below. In the case where the mixture of unvulcanized plastics does not have halogens in significant quantities, it is possible to supply the flow just upstream of the unit where conversion step d) occurs, after melting in an additional melter. Such a solution allows to reduce the volumes involved in step c) as well as in the devulcanization unit.
According to a preferred embodiment, the HCl, preferably extracted at step b′) is subjected to electrolysis to obtain hydrogen which is added in step c) and Cl2 (
2HCl═H2±Cl2
The electrolysis unit is known to be particularly energetic, although to a lesser extent for chlorine with respect to water, but the usually small amount of HCl with respect to the process flow rates allow an easy energy integration with renewable systems (e.g., photovoltaic and wind). Thereby, the process allows to obtain good yields of the products at low costs.
As anticipated, the process according to the present invention comprises a step d) of converting plastics from step c) into products of commercial value. The chemical conversion carried out in step d) involves receiving the molten plastic flow cleaned from the sulphur fraction and suitably freed from the devulcanization catalyst. Such a molten mixture is further heated to temperatures greater than 350° C., preferably to temperatures greater than 400° C. and, more preferably, especially to reduce residence times and operating volumes, to temperatures greater than 450° C. According to alternative embodiments, the temperatures used can be greater than 450° C. in the case of oxygen gasification (oxy-gasification) or gasification systems, for which an oxygen stream is included, or a mixture of oxygen (e.g., air or enriched air) or oxygenated molecules (e.g., CO2, steam).
The products exiting step d) and the relevant conversion unit comprise products of commercial value such as:
Furthermore, it should be noted that, depending on the output temperatures of the products, the hot stream can be used as an energy integration in the previous steps of melting, de-halogenation and in some cases also for devulcanization. Specifically, the energy integration occurs in the steps where heating is required.
The conversion step can be performed in accordance with different operating modes which can be combined with the aforesaid SATS and SACS operating modes. Specifically, step d) can be accomplished by:
It should be noted that while gasification requires oxygen, pyrolysis is carried out without an initial supply of oxygen. A further difference between the two types of conversion is the outlet temperature of the products. Specifically, in the case of pyrolysis, the temperatures of 400-500° C., but also up to 700° C. for gas phase pyrolysis systems, allow an energy recovery in melting step b) using the thermal energy of the effluents to melt the plastic. Regarding the case of gasification, the temperatures of the products being in a wider temperature range (750-1100° C.) allow a greater energy recovery by passing the flow of hot products, for example, to the splitting unit and then to the unit where the melting occurs or directly to the unit where the melting occurs. Furthermore, in the case of the gasification being the product flow with greater enthalpy, this can also be used for the de-halogenation unit as well as for the previous units.
According to a preferred embodiment illustrated in
R6:[—CH2—]+H2O=CO+2H2
R7:[—CH2—]n+H2=CnH(2n+2);
R4: CH4+H2O=CO+3H2
R8:CH4+CO2=2CO+2H2
Specifically, such steps are carried out in a plant 10, 20, 30, 40, 50 illustrated in
R9:[—CH2—]+1,5O2=CO2+H2O
R10:CH4+2O2═CO2+2H2O
When in the process according to the present invention in step d) it is desired to obtain a chemical with high added value, syngas is used as the starting material which is converted into methanol according to the following reaction R11:
R11:2H2+CO→CH3OH
Subsequently if dimethyl ether is to be obtained, the methanol is converted to dimethyl ether according to the reaction R12
R12:2CH3OH→CH3OCH3+H2O
Dimethyl ether can also be obtained by direct synthesis from syngas and not only from methanol dehydration.
If acetic acid is to be obtained, it is obtained according to the reaction R13:
R13:CH3OH+CO→CH3COOH
In accordance with a preferred embodiment, illustrated in
Preferably, the reactor where step d) is conducted in accordance with the present embodiment is a tubular reactor provided with multiple multi-step tubes.
It is a further object of the present invention a system configured for carrying out the process described above. As mentioned during the description, each step of the process is carried out in a relative unit provided with structural features able to withstand the temperatures of the steps and the relative release of the products of the different reactions. Each unit is in fluid communication with the unit at which the next step of the process occurs. In detail, the plant comprises:
| Number | Date | Country | Kind |
|---|---|---|---|
| 102021000004901 | Mar 2021 | IT | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/IB2022/051799 | 3/1/2022 | WO |
