PROCESS FOR PRODUCING ISOBUTYLE FROM A BUTYL RUBBER FILLER

Abstract

The invention relates to a process for producing isobutylene from a feedstock, referred to as recycled feedstock, comprising at least 50% by weight of butyl rubber, including a pyrolysis step. Fig. 1

Description

Title of the invention: METHOD FOR PRODUCING ISOBUTYLENE FROM A CA CHARGE OUTCHOUC BUTYLE Technical field of the invention

[0001] The present invention relates to the field of processes for depolymerizing butyl rubber type compounds and their purification in order to produce at least one isobutylene monomer. Previous art

[0002] Pneumatic tires, and more generally rubber articles such as conveyor belts and non-pneumatic tires, are complex objects made up of a multitude of components. A pneumatic tire, for example, is made up of more than 200 different raw materials.

[0003] The pressure on renewable and fossil resources is so great that it is essential to make the best possible use of the resources represented by used rubber products. However, the recycling of materials from used rubber products, such as used tires, for the manufacture of new tires is still limited, not only because of the complexity of this recycling process, but also because of the significant impact of the components on tire performance. A degradation of this performance can therefore lead to an overall negative environmental impact from the recycling of materials.

[0004] There is therefore a strong need for processes to recover value from end-of-life rubber articles in order to produce raw materials whose use will not degrade the performance of pneumatic tires, thus leading to a lower environmental impact.

[0005] Numerous studies have been conducted in this field, particularly on the valorization of oils obtained from the pyrolysis of tire residues. For example, documents EP 0928817, WO 2013 / 170358 and JP2017 / 008214 describe the production of tire-grade carbon black from the pyrolysis of tire tire chips.

[0006] However, there is little research on the recycling of so-called "butyl" rubbers, which are copolymers of isoprene and isobutylene (or 2-methylpropene), most often halogenated. These rubbers are used to manufacture layers that are airtight against inflation gases, such as the inner linings of pneumatic tires and inner tubes, as well as articles such as the curing membranes for pneumatic tires.

[0007] The article "Thermal degradation and pyrolysis study of brominated butyl rubber-based damping material", Polymers for Advanced Technologies, Vol. 31 (2), pp. 204-213 examines the possibility of thermally degrading a halogenated butyl rubber. Following pyrolysis, carried out at temperatures ranging from 500°C to 1000°C for residence times on the order of a few seconds, numerous compounds are produced, and in particular, predominantly (i.e., more than 50% by weight), isobutylene dimers and trimers.

[0008] Isobutylene is currently produced from petroleum resources. This monomer can find numerous applications such as, for example, and not exhaustively, the synthesis of methyl tert-butyl ether (MTBE), ethyl tert-butyl ether (ETBE), these two compounds representing about three-quarters of the world's consumption of isobutylene, tert-butyl alcohol, and butyl rubber.

[0009] However, petroleum fractions centered around 4-carbon compounds (known as "C4 fractions") also contain significant amounts of 1-butene, whose boiling point is very close to that of isobutylene. But 1-butene is a particularly undesirable impurity, especially in the production of butyl rubber, because it can degrade the sealing performance of blow-gas-tight layers. Consequently, the purification of isobutylene for use in the synthesis of butyl rubber is particularly complex.

[0010] An object of the present invention is to produce isobutylene from a recycled feedstock, thus enabling its use as a monomer in synthesis processes, and in particular for the manufacture of butyl rubber and to limit the consumption of petroleum-based isobutylene. Detailed description of the invention

[0011] Thus, the invention relates to a process for producing isobutylene from a feedstock, referred to as recycled feedstock, comprising at least 40% by weight of butyl rubber, comprising at least the following steps:

[0012] a. A step of preparing the recycled feed so as to produce chips

[0013] b. A pyrolysis step of the charge from step a) comprising a section of pyrolysis carried out at a temperature ranging from 50°C to 475°C and an absolute pressure ranging from 0.1 bar to 2 bar and producing at least one gaseous pyrolysis stream and one liquid pyrolysis stream;

[0014] c. A separation step fed at least by the gaseous pyrolysis stream from step b) producing a concentrated isobutylene stream.

[0015] Preferably, the recycled filler comprises at least 50% by weight of rubber butyl, preferably at least 60% by weight of butyl rubber, preferably at least 70% by weight of butyl rubber, preferably at least 80% by weight of butyl rubber, most preferably at least 90% by weight of butyl rubber and most preferably at least 95% by weight of butyl rubber.

[0016] Preferably, the butyl rubber is a copolymer based on isobutylene and isoprene, preferably a copolymer based on isobutylene and halogenated isoprene.

[0017] Preferably, during step a), the recycled feed is reduced into granules of an average size ranging from 10 pm to 2 cm.

[0018] Preferably, the pyrolysis section is a microwave thermally assisted pyrolysis section.

[0019] Preferably, the power delivered by the microwave device is between 0.1 and 10 kW / kg of load, preferably between 0.1 and 5 kW / kg of load, preferably between 0.5 and 4 kW / kg of load and preferably between 1 and 4 kW / kg of load.

[0020] Preferably, the pyrolysis step is carried out under an inert atmosphere, preferably under a nitrogen atmosphere, at a pressure ranging from 0.1 bar to 2 bar, preferably ranging from 0.5 bar to 1.5 bar and very preferably from 0.8 bar to 1.2 bar.

[0021] Preferably, the separation step c) includes at least one boiling point gas-liquid separation section comprising 1 to 30 theoretical stages, referred to as "concentration" stage, operated at a pressure ranging from 1 to 20 bar.

[0022] Preferably, the separation step c) includes a separation section, referred to as the "finishing" section, fed by the concentrated isobutylene stream from the concentration section and producing an isobutylene stream.

[0023] Preferably, the separation step c) includes as the sole heat input to the reboiler the heat supplied by the gaseous pyrolysis stream from step b) feeding said concentration section.

[0024] Preferably, when the process according to the invention includes a concentration section, the concentration section is implemented by distillation at a pressure ranging from 1 to 20 bar, comprising from 1 to 30 theoretical stages, the overhead condenser being operated at a temperature ranging from -40°C to -20°C, preferably ranging from -35°C to -25°C, the gaseous pyrolysis stream from step b) being fed into the bottom of the column.

[0025] Preferably, when the process according to the invention includes a concentration section, the concentration section is implemented by distillation at a pressure of 3 to 10 bar, comprising 10 to 20 theoretical stages, the overhead condenser being operated at a temperature of -60°C to -40°C, preferably of -55°C to -45°C, the gaseous pyrolysis stream from step b) being fed to the bottom of the column. Definitions

[0026] The carbon-containing compounds mentioned in the description may be of fossil origin or bio-based. In the latter case, they may be, partially or totally, derived from biomass or obtained from renewable raw materials derived from biomass.

[0027] Any interval of values ​​designated by the expression "between a and b" represents the domain of values ​​going from more than a to less than b (i.e. excluding bounds a and b) while any interval of values ​​designated by the expression "from a to b" means the domain of values ​​going from a to b (i.e. including the strict bounds a and b).

[0028] By "major compound", it is understood that the compound is present at more than 50% by mass in the whole considered. Process load

[0029] The process according to the invention is a process for producing isobutylene from a feedstock, referred to as recycled feedstock, comprising at least 40% by weight, preferably at least 50% by weight, of butyl rubber. Butyl rubber is understood to mean a homopolymer of isobutylene (or 2-methylpropene) or a copolymer based on isobutylene and isoprene. Isobutylene-isoprene copolymers preferably comprise from 0.1 to 5% by mol, preferably from 1 to 3% by mol, of isoprene units. The butyl rubber may optionally be halogenated. When halogenated, the butyl rubber used in the process according to the invention comprises from more than 0% by weight to 3% by weight of halogenated compounds, preferably from 0.5% by weight to 1.5% by weight of halogenated compounds. Halogenated compounds are preferably chosen from bromine and chlorine.

[0030] Butyl rubber can be derived from any source comprising butyl rubber such as inner tubes, curing membranes of rubber articles (in particular tires), sealing rubbers of pneumatic tires as well as non-crosslinked butyl rubbers, for example from a butyl rubber synthesis process and not incorporated into a rubber article.

[0031] Preferably, the charge of the process according to the invention comprises at least 60% by weight of butyl rubber, preferably at least 70% by weight of butyl rubber, preferably at least 80% by weight of butyl rubber, most preferably at least 90% by weight of butyl rubber and most preferably at least 95% by weight of butyl rubber. Step a) of preparation

[0032] The process according to the invention includes a step of preparing the recycled feed. During this step, the recycled feed is conditioned to be able to supply step b) of pyrolysis. This preparation step makes it possible to shape the recycled charge in the form of chips, preferably chips of average size less than or equal to 20 cm, preferably less than or equal to 10 cm, preferably less than or equal to 5 cm, preferably ranging from 10 pm to 5 cm.

[0033] By chip, we mean a fragment of charge, this chip being able to have any shape, for example substantially parallelepiped, substantially spherical or any other shape generated by the fragmentation of a solid element.

[0034] The size reduction can be achieved by any means known to a person skilled in the art, for example by cutting with knives, cutting grids, or by grinding.

[0035] When the filler includes non-crosslinked butyl rubber, anti-sticking agents and alcoholic solutions can be used to facilitate the fragmentation of the filler, as non-crosslinked butyl is very sticky.

[0036] When very small average sizes are sought, less than 500 pm, cryogenic grinding techniques can be used. Step b) of pyrolysis

[0037] The process according to the invention includes a pyrolysis step of the charge from step a) comprising a pyrolysis section operated at a temperature between 50 and 475°C and producing at least a first gaseous pyrolysis stream and a first solid pyrolysis stream.

[0038] Pyrolysis means the thermal decomposition of compounds in an inert atmosphere or an oxygen-poor atmosphere, that is to say, comprising less than 0.01%, preferably less than 0.001% and more preferably less than 0.0001% by volume of oxygen.

[0039] The pyrolysis step is preferably carried out under an inert atmosphere, preferably under a nitrogen atmosphere. It is carried out at an absolute pressure ranging from 0.1 bar to 2 bar, preferably ranging from 0.5 bar to 1.5 bar and very preferably from 0.8 bar to 1.2 bar.

[0040] The pyrolysis section is preferably operated at a temperature ranging from 50°C to 475°C, preferably ranging from 50°C to 440°C and preferably ranging from 50°C to 425°C.

[0041] The conditioned charge from step a) feeds a pyrolysis section.

[0042] In a preferred arrangement, the pyrolysis section is a pyrolysis section by Microwave thermal assistance. Such microwave thermal assistance pyrolysis usable in the process according to the invention is described for example in document WO 2020 / 202089.

[0043] The microwaves used in the microwave thermally assisted pyrolysis section have a frequency of 2.45 GHz for the lowest values ​​and 915 MHz for the highest values. The power delivered by the microwave device is preferably between 0.1 and 10 kW / kg of load, preferably between 0.1 and 5 kW / kg of load, preferably between 0.5 and 4 kW / kg of load and preferably between 1 and 4 kW / kg of load.

[0044] The use of a microwave thermally assisted pyrolysis section improves heat transfer and provides more homogeneous temperatures in the core of the material, which promotes chain-end cleavage reactions and minimizes oligomer formation.

[0045] To improve heat transfer, silicon carbide granules can be fed along with the feed. When the feed of the process according to the invention includes carbon black, as is the case when the feed includes rubber articles such as cooking membranes, inner tubes, airtight rubber residues from used tires, it is not necessary to add heat-conducting particles such as silicon carbide granules, as the carbon black is present in sufficient quantity to ensure good heat conduction.

[0046] The pyrolysis section produces a gaseous pyrolysis stream and a solid pyrolysis stream. The gaseous pyrolysis stream includes isobutylene, other molecules produced in the pyrolysis section such as isoprene, 1-butene, n-butane, cis-2-butene, and higher molecular weight compounds such as isobutylene dimers.

[0047] The solid pyrolysis stream comprises solid elements, predominantly non-pyrolyzed or partially pyrolyzed polymers, and the reinforcing filler, for example carbon black or silica, present in the feed of the process according to the invention.

[0048] Surprisingly, the implementation of pyrolysis under the conditions of the invention makes it possible, unlike a conventional pyrolysis carried out at high temperature, to obtain a gas stream comprising a significant proportion of isobutylene, greater than 50% by weight, and very little 1-butene, at a content of less than 1% by weight.

[0049] Thus, the pyrolysis step of the process according to the invention makes it possible to obtain a gaseous pyrolysis stream comprising at least 50% by weight of isobutylene and at most 1% by weight of 1-butene preferably at most 0.5% by weight of butene and preferably at most 0.1% by weight of 1-butene.

[0050] The gaseous pyrolysis stream from the pyrolysis step is at a temperature ranging from 50°C to 450°C, preferably ranging from 300°C to 425°C, the temperature of the gaseous pyrolysis stream depending on the power implemented in the microwave-assisted pyrolysis section. Step c) of separation

[0051] The process according to the invention comprises a separation step fed at least by the gaseous pyrolysis stream from step b) and producing a concentrated iso- stream butylene.

[0052] Step c) of separation can be carried out according to methods known to those skilled in the art. Preferably, step c) of separation comprises at least one pressure separation section, referred to as a "concentration" section, operated at a pressure ranging from 1 to 20 bar.

[0053] Step c) of separation may optionally include a separation section, referred to as "finishing", fed by the concentrated isobutylene stream from the concentration section and producing an isobutylene stream.

[0054] The concentration section brings the gaseous pyrolysis stream from step b) into contact with a cold liquid, so as to prevent this gaseous pyrolysis stream from coming into contact with the colder walls of the separation equipment. Indeed, since the pyrolysis gases contain high molecular weight molecules, these would deposit on the walls and lead to fouling and therefore a loss of heat transfer.

[0055] Preferably, the concentration section includes as its sole heat input to the reboiler the heat supplied by the gaseous pyrolysis stream from step b) feeding said concentration section.

[0056] When the operating pressure of the pyrolysis section is lower than the operating pressure of the concentration section, the gaseous pyrolysis flow from step b) will be compressed before feeding the concentration section.

[0057] Preferably, the concentration section is a boiling point gas-liquid separation section comprising from 1 to 30 theoretical separation stages, preferably from 1 to 20 theoretical stages and preferably from 10 to 20 theoretical stages.

[0058] In a first preferred arrangement, the concentration section is implemented by distillation at a pressure ranging from 10 to 20 bar, comprising 10 to 20 theoretical stages, the upper condenser being operated at a temperature ranging from -40°C to -20°C, preferably ranging from -35°C to -25°C. The gaseous pyrolysis stream from step b) is fed to the bottom of the column, where it is brought into contact with the cold liquid flowing down the column. The concentrated isobutylene stream is obtained by lateral withdrawal from the upper part of the lower third of the column. Thus, for example, if the column comprises 15 theoretical stages, the concentrated isobutylene stream can be obtained by lateral withdrawal from the 9th or 10th theoretical stage, the stages being numbered from top (coldest part of the column) to bottom (hottest part of the column).

[0059] At the top and bottom of the column, respectively, a stream of light compounds comprising mainly Ci-C3 compounds and a raffinate comprising heavy compounds are drawn off.

[0060] In a second preferred arrangement, the concentration section is implemented by distillation at a pressure ranging from 3 to 10 bar, comprising 10 to 20 theoretical stages, the overhead condenser being operated at a temperature ranging from -60°C to -40°C, preferably from -55°C to -45°C. The gaseous pyrolysis stream from step b) is fed to the bottom of the column, where it is brought into contact with the cold liquid flowing down the column. The concentrated isobutylene stream is obtained by lateral withdrawal from the upper part of the lower third of the column. Thus, for example, if the column comprises 15 theoretical stages, the concentrated isobutylene stream can be obtained by lateral withdrawal from the 9th or 10th theoretical stage, the stages being numbered from top (coldest part of the column) to bottom (hottest part of the column).The column includes at least two intercoolers, one in the lower third of the column and the other in the upper half (upper being understood as the colder, top part of the column and lower as the warmer, bottom part of the column). These intercoolers reduce the cooling capacity at the top of the column and operate at higher temperatures than the main condenser. The intercooler in the lower third cools the liquid fraction to a temperature of 15°C to 30°C, preferably from 20°C to 25°C. The intercooler in the upper half cools the liquid fraction to a temperature of -10°C to 5°C, preferably from -5°C to 0°C.

[0061] The concentrated isobutylene stream has an isobutylene content of at least 98% by weight. Due to the operating conditions and operations implemented in the process according to the invention, this stream contains very little 1-butene, i.e. less than 1% by weight, preferably less than 0.5% by weight, preferably less than 0.1% by weight, very preferably less than 0.01% by weight and most preferably less than 0.005% by weight of 1-butene, making this stream suitable for directly feeding a butyl rubber synthesis process, the separation of 1-butene being facilitated by the specific composition of the gaseous pyrolysis stream from step b) feeding the separation step c). This stream also includes little cis-2-butene, i.e. less than 2% by mass, preferably less than 1% by mass, and less than 0.3% by mass of butadiene.

[0062] Step c) of separation may also include a separation section, referred to as the "finishing" section, fed by the concentrated isobutylene stream from the concentration section and producing an isobutylene stream. The process according to the invention makes it possible to obtain a concentrated isobutylene stream that can directly feed a butyl rubber synthesis process. However, it may be desirable to adjust the contents of certain compounds according to the intended applications of the concentrated stream. in isobutylene, in particular the levels of propylene, butadiene or 2-butene.

[0063] This finishing section can be implemented by any means known to those skilled in the art. In particular, it can be implemented in a distillation column, preferably operated at a pressure between 3 and 20 bar, from which an isobutylene stream is drawn off as a raffinate, or alternatively as a lateral draw-off. Description of the figures

[0064] For each of the following figures, identical numbers and letters correspond to similar flows and operations.

[0065] [Fig.1] Fig.1 schematically illustrates a process for producing isobutylene from a filler comprising at least 50% by weight of butyl rubber according to the invention.

[0066] A charge comprising at least 50% by weight of butyl rubber (1) feeds a preparation step (a) in which it is reduced to chips.

[0067] The prepared charge (2) feeds a microwave thermally assisted pyrolysis step (bl) producing a gaseous pyrolysis stream (3) and a liquid pyrolysis stream (6), the liquid portion (6) being separated (b.2) into a solid portion (8) and a recyclable liquid portion (7). The solid portion can be recycled as a charge for the process of the invention, or it can be used to recover value from the products it contains, for example, the reinforcing charge.

[0068] The gaseous pyrolysis stream (3) feeds a separation stage (cl, c.2) in which it is separated in a concentration section (cl) into a concentrated isobutylene stream (4), a light compound stream (10) and a raffinate (9).

[0069] The concentrated isobutylene stream (4) feeds a finishing section (c.2) in which it is separated into an isobutylene stream (5), an extract (13) and a raffinate (12).

[0070] In the process shown [Fig. 1], the light compound streams (10), extract (13), and raffinates (11) and (11, 14) are utilized in a cogeneration unit (d) producing electricity (15) and heat (16) that can power the process. These streams could also be used as feedstocks for synthesis processes. The light compound stream may optionally undergo a treatment step designed, in particular, to remove sulfur compounds from this stream, for example, compounds such as hydrogen sulfide. Examples

[0071] Example 1 according to the invention: conventional pyrolysis

[0072] This example illustrates the production of isobutylene by thermal means.

[0073] The charge consists of gas-tight rubber residues from pneumatic tires. This charge is cut into chips with an average size of less than 1 mm and then fed into a thermally assisted pyrolysis stage.

[0074] 300 g of the mixture is introduced into a first 2 L single-necked flask. The single-necked flask is equipped with a condenser allowing the pyrolysis flux to be recovered in a second 500 mL two-necked flask cooled to -78°C. This is the unpurified condensed gas.

[0075] The 2L single-neck balloon is brought to a temperature between 350 and 500°C in inert conditions under a nitrogen flow of 0.2L / min for approximately 120min.

[0076] The pyrolysis effluent, at a temperature above 350°C and at atmospheric pressure, is separated into a gaseous pyrolysis effluent, which represents between 40 and 60% by weight of the pyrolysis effluent, and an effluent comprising the solid residues (mostly carbon black), which represents at least 40% by weight of the pyrolysis effluent.

[0077] The results in isobutylene content are detailed in the table below.

[0078] [Tables] Test Temperature (°C) % weight of condensed gas % weight of i-C4 in condensed gas i-C4 efficiency 1.1 500 28.6 58 32 1.2 450 35.1 46 31 1.3 400 31.7 78 47 1.4 350 17.6 85 28 1.5 400 35.7 76 52

[0079] Example 2 according to the invention: microwave pyrolysis

[0080] This example illustrates the production of isobutylene by microwave according to the scheme in [Fig.1].

[0081] A charge consisting of gas-tight rubber residues from pneumatic tires is cut into chips of average size less than 0.5 mm and then feeds a microwave thermally assisted pyrolysis step where it is subjected to radiation of the order of 1 kW under a nitrogen atmosphere.

[0082] A first 2L single-necked flask is filled with 500 g of the charge. The single-necked flask is equipped with a condenser that allows the pyrolysis flux to be recovered in a second 500 mL two-necked flask cooled to -78°C. This is the unpurified condensed gas.

[0083] The 2L single-neck balloon is heated to a microwave power of 1000W in inert conditions under a nitrogen flow of 0.5L / min for approximately 40min.

[0084] The pyrolysis effluent, at atmospheric pressure, is separated into a gaseous pyrolysis effluent, which represents 56% by weight of the pyrolysis effluent, and an effluent comprising the solid residues (mostly carbon black), which represents 44% by weight of the pyrolysis effluent.

[0085] The condensed gas fraction represents 32% by weight of the pyrolysis effluent with an isobutylene purity of 89%. The isobutylene recovery yield is 55%.

[0086] The composition of the gaseous pyrolysis effluent is shown in Table 1.

[0087] The gaseous pyrolysis stream is compressed to 15 bar and feeds from the bottom into a distillation column comprising 15 theoretical stages. The only heat input to this column is the heat from the gaseous pyrolysis stream.

[0088] The column is operated at a pressure of 15 bar (pressure at the bottom of the column, at the level of the gaseous pyrolysis flow feed). The column condenser is at a temperature of -30°C.

[0089] A stream of light compounds is drawn off at the top of the column and a raffinate, at a temperature of 180°C, is drawn off at the bottom. The isobutylene-concentrated stream is drawn off at the theoretical 9th ​​stage, the stages being numbered from the condenser. The temperature of this stream is 82°C. Its composition is shown in Table 2. It can be seen that this stream no longer contains 1-butene.

[0090] The composition of the concentrated isobutylene stream is shown in Table 2.

[0091] The concentrated isobutylene stream then feeds a finishing section, which in this case removes the last traces of propylene. The finishing section is a distillation unit comprising 15 theoretical stages, fed by the concentrated isobutylene stream at the 4th theoretical stage, the stages being numbered from the condenser to the reboiler. The isobutylene stream is withdrawn as raffinate at the 15th theoretical stage. The condenser temperature is 84°C and the reboiler temperature is 90°C. The composition of the resulting isobutylene stream is shown in Table 2.

[0092] [Tables2] Components: Gaseous pyrolysis effluent (% wt.) Isobutylene concentrated flux (% wt.) Isobutylene flux (% wt.) Hydrogen <1% 0% 0% Nitrogen <1% 0% 0% Carbon monoxide 0% 0% 0% Oxygen <1% 0% 0% Methane 6% <0.1% 0% Ethylene 2% <0.1% 0% Ethane <1% 0% 0% Acetylene <1% 0% 0% Carbon dioxide <1% 0% 0% Propylene 3% 0.4% 0% Propane <1% 0.1% 0% Isobutane 0% 0% 0% Isobutylene 65% 98.6% 99.2% 1-Butene <1% 0% 0% 1,3-Butadiene <1% 0.5% 0.4% N-Butane <1% 0.2% 0.2% trans-2-Butene 0% 0% 0% c7s-2-butene <1% 0.1% 0.1% Isoprene <1% <0.1% <0.1% Benzene <1% 0% 0% Water 0% 0% 0% Unknown (aromatic equivalent ethylbenzene) 19% 0% 0% Styrene <1% 0% 0% Limonene <1% 0% 0% Indene <1% 0% 0%

[0093] Thus, the process according to the invention makes it possible to obtain, from a recycled feedstock, isobutylene of sufficient purity to be used as a monomer in a synthesis process, in particular for the synthesis of butyl rubber, without implementing complex separation.

Claims

Claims

1. Process for producing isobutylene from a feedstock, called recycled feedstock, comprising at least 40% by weight of butyl rubber comprising at least the following steps: a. A step of preparing the recycled feedstock so as to produce chips; b. A step of pyrolysis of the feedstock resulting from step a) comprising a pyrolysis section operated at a temperature ranging from 50°C to 475°C and an absolute pressure ranging from 0.1 bar to 2 bar and producing at least one gaseous pyrolysis stream and one liquid pyrolysis stream; c. A separation step supplied at least by the gaseous pyrolysis stream resulting from step b) producing a stream concentrated in isobutylene.

2. A process for producing isobutylene according to the preceding claim wherein the recycled feedstock comprises at least 50% by weight of butyl rubber, preferably at least 60% by weight of butyl rubber, preferably at least 70% by weight of butyl rubber, preferably at least 80% by weight of butyl rubber, very preferably at least 90% by weight of butyl rubber and very preferably at least 95% by weight of butyl rubber.

3. A process for producing isobutylene according to any preceding claim wherein the butyl rubber is a copolymer based on isobutylene and isoprene, preferably a copolymer based on isobutylene and halogenated isoprene.

4. A process for producing isobutylene according to any preceding claim wherein, in step a), the recycled feedstock is reduced to granules with an average size ranging from 10 pm to 2 cm.

5. A method of producing isobutylene according to any preceding claim wherein the pyrolysis section is a microwave heat-assisted pyrolysis section.

6. Process for producing isobutylene according to the preceding claim in which the power delivered by the microwave device is between 0.1 and 10 kW / kg of load, preferably between 0.1 and 5 kW / kg of load, preferably between 0.5 and 4 kW / kg of load and preferably between 1 and 4 kW / kg of load.

7. Process for producing isobutylene according to any one of the preceding claims, in which the pyrolysis step is carried out under an inert atmosphere, preferably under a nitrogen atmosphere, at a pressure ranging from 0.1 bar to 2 bar, preferably ranging from 0.5 bar to 1.5 bar and very preferably from 0.8 bar to 1.2 bar.

8. Process for producing isobutylene according to any one of the preceding claims in which the separation step c) comprises at least one gas-liquid separation section by boiling point comprising from 1 to 30 theoretical stages, called "concentration section", operated at a pressure ranging from 1 to 20 bar.

9. Process for producing isobutylene according to any one of the preceding claims in which the separation step c) comprises a separation section, called a "finishing" section, supplied with the concentrated isobutylene stream from the concentration section and producing an isobutylene stream.

10. Process for producing isobutylene according to any one of the preceding claims, in which the separation step c) comprises, as the only heat input to the reboiler, the heat input by the gaseous pyrolysis stream from step b) feeding said concentration section.

11. Process for the production of isobutylene according to any one of claims 8 and 9 to 10 when they depend on claim 8 in which the concentration section is carried out by distillation at a pressure ranging from 1 to 20 bar, comprising from 1 to 30 theoretical stages, the head condenser being operated at a temperature ranging from -40°C to -20°C, preferably ranging from -35°C to -25°C, the gaseous pyrolysis stream from step b) being fed to the bottom of the column.

12. A process for producing isobutylene according to any one of claims 8, claims 9 to 10 when dependent on claim 8, and claim 11, in which the concentration section is carried out by distillation at a pressure ranging from 3 to 10 bar, comprising from 10 to 20 theoretical stages, the head condenser being operated at a temperature ranging from -60°C to -40°C, preferably ranging from -55°C to -45°C, the gaseous pyrolysis stream from step b) being fed to the bottom of the column.

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