ES2759939A1 - PROCEDURE TO CONVERT PLASTIC WASTE INTO LIQUID PRODUCTS USEFUL IN THE PETROCHEMICAL INDUSTRY (Machine-translation by Google Translate, not legally binding) - Google Patents

Abstract

The invention discloses a method for converting plastic waste into liquid products useful in the petrochemical industry. The procedure comprises obtaining a pyrolysis liquid through the steps of a) continuously melting and homogenizing the plastic waste raw material; b) performing thermal cracking of the molten raw material from step a) in a thermal cracking reactor; and c) subjecting volatile products from step b) to rapid cooling. Thanks to the process of the invention, an exit flow formed by pyrolysis liquid useful in the petrochemical industry as well as permanent gases of high calorific value is obtained at the output of stage c). (Machine-translation by Google Translate, not legally binding)

Description

DESCRIPTION

Procedure to convert plastic waste into useful liquid products in the petrochemical industry

Field of the Invention

The present invention relates generally to the field of waste recycling, and more specifically to a process for obtaining pyrolysis liquids of high added value for use as a raw material in the petrochemical industry.

Background of the Invention

The increasing consumption of plastic materials has caused an increase in the volume of its waste. Its management is a cause of growing social concern due to the exhaustion of landfill space, the loss of potential raw materials and the inevitable environmental contamination. The magnitude of the problem can be roughly estimated from the figures of consumption of plastics, since many of the applications of plastic materials are characterized by having a short useful life (Lopez et al., 2017; Panda et al., 2010).

Since the second half of the 20th century, the production of plastic materials has grown exponentially, as it has driven the development of pioneering innovations that have improved the quality of life and well-being of society (Al-Salem et al., 2009 ). Today, they are present in a large number of industrial and consumer products, making it difficult to imagine daily life without them (Hestin et al., 2015; Lopez et al., 2017; Park et al., 2019; Wang et al., 2015). According to the latest data published by Plastics Europe (Plastics - the Facts 2018), the global production of plastics went from 1.5 million tons (Tm) in 1950 to 348 Tm in 2017, of which only 64.4 Tm were produced in Europe (18.5%). It is calculated that in the event that the current situation is maintained, the world production of plastics could triple by 2050. During 2017, polyethylene and polypropylene continued to be the most popular polymers, with the packaging industry being the main user. end of this type of polymers (39.7%; 2017) and, therefore, this sector is also responsible for most of the flow of plastic waste generated (62% of the total flow of post-consumer plastic waste in Europe). This is also due to the fact that packaging plastics have a much shorter life cycle compared to other types of plastic products (Achilias et al., 2007).

According to the latest data published by Plastics Europe (Plastics - the Facts 2018), 27.1 Mt of plastic waste were generated in the European Union in 2016. Of this total, 27.3% (7.4 Mt) was dumped in landfills, 41.6% (11.3 Tm) was recycled energy and 31.1% (8.4 Tm) was recycled (mainly by mechanical recycling). The spill rate in Spain (46.4%; 2.3 Tm) is above the European average (27.3%). Analogously to what happens in Europe, the highest amount of plastic waste collected in Spain in 2016 also corresponded to packaging waste (1.5 tons), of which 38.2% had the landfill as final destination. .

In this context, therefore, the need arises to evolve towards a much more efficient in the use of resources, promoting prevention, recycling and recovery actions. From the point of view of the efficient use of resources, it is especially important to avoid the disposal of plastic waste in landfills, since it represents an obvious loss of resources. However, as previously mentioned, the percentage of plastic waste deposited in Spanish landfills remains high due to the lack of suitable alternatives. In conclusion, plastic waste, due to its composition and its origin derived from petroleum and, therefore, from an exhaustible raw material, is a valuable resource. In accordance with European environmental policies, the necessary measures must be promoted to develop treatment methods that allow obtaining new products or converting them into an important source of energy. Progress must be made towards a circular economy, low in carbon and efficient in the use of resources.

The main constituents of post-consumer plastic waste are polyolefins: high-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), and linear low-density polyethylene (LLDPE). These four polymers represent 67% of the total plastics present in urban solid waste (MSW). Other existing plastics in the RSU are polystyrene (PS, 13.3%), poly (vinyl chloride) (PVC, 10.3%) and poly (ethylene terephthalate) (PET, 5.3%). Currently, the main recycling pathway to recover the intrinsic value of these polymers is mechanical recycling (Ellen MacArthur Foundation, 2016; European Parliamentary Research Service Blog, 2017; Ragaert et al., 2017), which consists of washing, crushing and melting plastic waste to re-generate pellets of recovered plastic, which is later transformed by molding or injection into new recycled plastic objects. Despite being the main recycling route, there are technical and economic barriers to the application of mechanical recycling (Dahlbo et al., 2018). The recovered plastic pellets must meet the health and environmental safety requirements depending on the application for which it is intended. Due to the low compatibility of different types of plastic, the properties of mechanically recycled plastics are often inferior to those of virgin polymers. Furthermore, mechanical recycling is only applicable to thermoplastic materials (PE, PP, etc.) and requires the separation of the different types of plastics to obtain a high-purity plastic waste, that is, that is not contaminated with other fractions of plastics or other materials to guarantee the quality of the final product. In many cases this degree of separation is not technically possible due to the composition of the plastic waste stream, color or associated contamination. On the other hand, it is important to note that mechanical recycling cannot be carried out indefinitely, since after successive stages of heating and molding the plastic, it gradually loses its properties. From an economic point of view, mechanical recycling is not always economically viable, since the cost of the recycling process is not covered by the benefits obtained from the sale of recycled plastic. In general, the average cost of mechanical recycling is around € 450-800 per ton of plastic processed. During pretreatment and sorting operations, a material waste of 35% usually occurs. The mechanical recycling performance of the remaining material is approximately 75%. Therefore, taking all of the above into account, for each ton of plastic waste processed, less than 500 kg of recycled plastic will be obtained. The market price of recycled plastics varies around € 650-1000 per ton, a value that, taking into account the performance of the process, becomes € 350-500 per ton of processed plastic waste (Hestin et al., 2015 ). Therefore, on many occasions the costs of mechanical recycling are greater than the benefits obtained from the sale of recycled plastic.

Furthermore, the lack of a clear framework to apply the end of waste condition also represents a barrier to the commercialization of recycled plastic materials. Considering the barriers of mechanical recycling, currently only two types of polymers from MSW are mechanically recycled: HDPE and PET. Mechanical recycling of other types of plastic waste is not economically feasible due to the lack of quantity (constant production) and / or quality (homogeneity), and it is necessary to consider other sustainable management alternatives. This is especially important, in the case of plastic film (mainly low-density polyethylene), since it is the most abundant polymer in post-consumer plastic waste (Dahlbo et al., 2018; Diaz-Silvarrey and Phan, 2016; Horodytska et al., 2018; Kunwar et al., 2016).

The main alternative to mechanical recycling is chemical recycling, a more complex and expensive process but one that allows treating highly degraded or heterogeneous plastics. The objective of chemical or tertiary recycling is the transformation of plastic waste into hydrocarbon chains that can be used as raw materials for the petrochemical industry or as fuels. The methods can be chemical or thermochemical, depending on the type of polymer, and can be classified into five main groups: chemical depolymerization or solvolysis (hydrolysis, alcohololysis, glycolysis, aminolysis, etc.), gasification with oxygen / water vapor mixtures to obtaining synthesis gas (CO + H2) that can be used as a petrochemical raw material or for energy production, thermal cracking, catalytic cracking and hydrocracking.

Of the aforementioned processes, the thermal cracking process (also called pyrolysis) is emerging as one of the most promising alternatives for the treatment of plastic waste (mainly LDPE) (Czajcynska et al., 2017; Diaz-Silvarrey and Phan, 2016 ; Khoo, 2019; Lopez et al., 2017, Sharuddin et al., 2016; Singh et al., 2019; Sriningsih et al., 2014; Syamsiro et al., 2014; Park, 2019). In order to obtain high-quality pyrolysis liquids, which can be used as automotive fuels and / or as raw materials for obtaining new products, the use of catalysts has been used to decrease the operating temperature and residence time. , improving the quality of the liquid product obtained (Santos et al., 2018). However, direct catalytic cracking processes also have important limitations, such as the difficulty of recovering the catalyst after use, which increases operating costs (Santos et al., 2018), or the high olefin content of the products obtained (Escola et al., 2011). In addition, the direct contact of the catalyst with the plastic waste causes its rapid deactivation by deposition of coke and poisoning due to the presence of elements, such as chlorine, sulfur, nitrogen and metals contained in some of the additives used to improve the properties of the polymers (Escola et al, 2011; López et al, 2011).

As an alternative to the direct catalytic cracking process, some authors have proposed carrying out a two-stage process (Artetxe et al., 2012; Escola et al., 2014; Park et al., 2019; Syamsiro et al., 2014) , a first stage of thermal cracking, followed by a second stage of catalytic improvement of the quality of pyrolysis liquids. This two-stage process allows solving the problem of the rapid deactivation of the catalyst discussed above, however, it increases the complexity and operational costs of the process.

ES2706283T3 refers to a process for the conversion of plastic material into diesel fuel. Heterogeneous plastics are used as raw material to carry out its pyrolysis and the gases obtained are subsequently transferred to a catalytic converter medium to alter their molecular structure.

US2018010050A1 refers to a method for recovering hydrocarbons from plastic waste, in particular polyolefin-rich waste, by means of thermolytic cracking which comprises melting the plastic waste in two heating devices, in which a recirculation stream derived from the reactor of Cracked and purified in a separator system mixes with the molten plastic residue from the heating device. The mixed plastic stream is further heated in the second heating device, and from there it is guided into the cracking reactor, where the plastic materials are cracked, and by subsequent distillation they are separated into components of low boiling point and diesel.

Therefore, there is still a need in the art for an alternative procedure that allows converting plastic waste (mainly LDPE), which currently is mainly deposited in landfill, into high-quality, value-added pyrolysis liquids that can be used as material. Premium for the petrochemical industry for the production of new plastics or other products (solvents, extractants, hydrocarbons, etc.), without using any type of catalyst. It is further desirable that the process have improved performance and reduced cost compared to alternative procedures known in the state of the art.

Summary of the invention

To solve the above technical problem, the present invention discloses a procedure for converting plastic waste into liquid products useful in the petrochemical industry, which comprises obtaining a pyrolysis liquid by the steps of:

a) carry out the fusion and homogenization of raw material of plastic waste continuously;

b) carrying out thermal cracking of the molten raw material from step a) in a thermal cracking reactor; and

c) subjecting volatile products from step b) to rapid cooling;

whereby at the output of stage c) an outflow formed by pyrolysis liquid useful in the petrochemical industry as well as permanent gases with high calorific value is obtained.

The following provides a detailed description of some preferred embodiments of the process according to the present invention. However, such detailed description is not intended in any way to limit the scope of protection sought by the present invention, defined solely by the appended claims and the technical equivalents thereof.

Detailed description of the preferred embodiments

In accordance with a preferred embodiment, the present invention relates to a continuous, catalystless pyrolysis process for converting plastic waste, preferably low-density polyethylene and polypropylene film, into high value-added liquid products useful for use as a raw material in the petrochemical industry to obtain new products, preferably new plastics.

The procedure comprises obtaining a pyrolysis liquid through the steps of:

a) carry out the fusion and homogenization of raw material of plastic waste continuously;

b) carrying out thermal cracking of the molten raw material from step a) in a thermal cracking reactor; and

c) subjecting volatile products from step b) to rapid cooling;

whereby at the output of stage c) an outflow formed by pyrolysis liquid useful in the petrochemical industry as well as permanent gases with high calorific value is obtained.

Furthermore, according to the preferred embodiment of the present invention, the process comprises a previous stage of conditioning the plastic waste raw material, before step a), to eliminate unwanted compounds and homogenize the raw material.

The previous stage of conditioning the plastic waste raw material (preferably plastic film bales from urban solid waste treatment plants) is a key point to improve the efficiency and operational stability of the procedure and obtain high-quality pyrolysis liquids and constant composition, factors necessary to guarantee a final market for the product. According to a preferred embodiment of the present invention, the preconditioning step comprises a selected stage of optical separation, crushing, screening, magnetic and / or Foucault separation, centrifugation, density separation, agglomeration and any combination thereof. In this way, unwanted compounds, such as organic fraction, paper, cardboard, metal, glass, etc., are removed from the raw material.

The preconditioning step according to the preferred embodiment also comprises agglomeration or densification of the raw material to a density of 200-450 kg / m3.

The conditioned product obtained after the previous conditioning stage, and which will serve as raw material for stage a) as described in more detail hereinafter, is an agglomerate containing more than 90% by weight of polyethylene of low density, a size between 10 and 30 mm and a maximum moisture content of 15% by weight. The rest of the material can comprise high-density polyethylene and polypropylene (the sum of both polymers being less than 10% by weight), less than 5% by weight of polystyrene and less than 2% by weight of other plastics, such as poly ( ethylene terephthalate) and poly (vinyl chloride).

Furthermore, the conditioned product preferably comprises a lower calorific value (PCI) greater than 9500 kcal / kg, a carbon content greater than 80% by weight and a hydrogen content greater than 13% by weight.

After conditioning in the previous stage described above, the plastic waste raw material (in this case the conditioned product) is subjected to the conversion process itself according to the present invention. As mentioned above, said procedure comprises three stages.

In the first stage (stage a) the melting and homogenization of the plastic waste raw material is carried out continuously, using an extruder that introduces the raw material cyclically in alternating sequence in two previously inerted transfer chambers (preferably with nitrogen) , equipped with stirring means, in order to obtain a continuous process. Thus, stage a) is preferably divided into the following sub-stages:

- firstly, fill a first transfer chamber;

- when said first transfer chamber is full, stop filling the first transfer chamber and start filling a second transfer chamber;

- simultaneously with filling the second transfer chamber, heating the raw material in the first transfer chamber and pressurizing with nitrogen;

- after reaching the appropriate temperature and pressure, feed the molten raw material to the cracking reactor of step b); and

- Cyclically repeat the previous steps, alternating the filling, heating and pressurization of the first and second transfer chambers.

Preferably, during step a), the raw material is heated in the transfer chambers until reaching a temperature of between 270-310 ° C, with the aim of reducing the viscosity of the material, and it is pressurized with nitrogen until reaching a pressure value 1-5 barg above the operating pressure of the thermal cracking reactor of step b), described below.

Once the working conditions of the first transfer chamber are reached, the molten raw material is fed from this transfer chamber to step b) of thermal cracking by pressure gradient. During the conditioning time of the raw material of the first transfer chamber and feed to the thermal cracking reactor, the filling of the second transfer chamber is carried out and the procedure starts again.

The use of the double transfer chamber system allows the operation to be homogenized and stabilized, since having a double chamber can buffer possible failures or changes in the conditions of the material that is fed from the extruder. Furthermore, this system allows the use of a pressure gradient to carry out the transfer of the molten polymer, disregarding mechanical elements such as worm screws, etc.

According to an embodiment of the invention, the chambers consist of cylindrical tanks made of steel with a klopper bottom and the heating system for these tanks can be carried out using hot gases from the burner (used for the energy use of the gas generated during the thermal cracking process ) or by means of a thermal oil jacket.

In the second stage (stage b), thermal cracking of the molten raw material from stage a) is carried out in a thermal cracking reactor. Thermal cracking is preferably carried out under temperature conditions of between 375-525 ° C, at a pressure of between 1-20 barg and with a residence time of between 0.5 and 5 h (defined as the amount of material in the inside of the reactor in kg divided by the feed rate in kgh-1).

More preferably, the thermal cracking conditions are a temperature of between 400-450 ° C, a pressure of between 1-10 barg and a residence time of between 0.5-2 h.

For carrying out the process according to the preferred embodiment of the present invention, the reactor of the thermal cracking stage is a continuous stirred tank reactor (called complete mixing reactor). Cracking is exclusively thermal, in the absence of any catalyst, so that the risk associated with its deactivation is avoided (by deposition of coke and poisoning due to the presence of elements, such as chlorine, sulfur, nitrogen and metals contained in some of the additives used to improve the properties of the polymers), consumption, recovery and waste management. This further allows to reduce the costs associated with the process of the present invention.

According to the preferred embodiment of the present invention, to carry out the thermal cracking step, the use of any inert stripping gas is not necessary. The pressure that is achieved in the reactor autogenously, with the gases themselves formed during the degradation of the polymeric material, is sufficient to drive the volatile products produced until the next stage c) of the process.

To carry out the process of the present invention by means of a continuous process, the process further comprises continuously extracting a solid generated during the thermal cracking of step b). This extraction can be carried out by means of mechanical devices, such as, for example, a centrifuge or a hot filtration system. The liquid cracked polymer, free of solids, is recirculated again to the thermal cracking reactor of stage b), keeping the filling level constant at all times.

According to an embodiment of the invention, the solid extraction system is specifically designed for a pressure reactor allowing to separate the solid from the liquid phase and return the solid-free product to the reactor keeping the level constant. This system consists of a heat exchanger, a lung tank, a safety filter, an automatic filter with a mesh of 1 - 40 microns, a pumping system to the reactor, a second heat exchanger and an auxiliary cleaning and maintenance system.

In the third stage of the process (stage c), rapid cooling of volatile products from stage b) of thermal cracking is carried out. Rapid cooling is carried out, for example, through the use of shell and tube exchangers (using as a cooling agent a mixture of water and monoethylene glycol at 30% by volume) or by direct cooling or "quenching" using the pyrolysis liquids generated in the process itself. As a result of the cooling of step c), an outflow is obtained at a temperature of less than 80 ° C, more preferably less than 50 ° C, consisting of pyrolysis liquids and permanent gases. Pyrolysis liquids are collected in a tank equipped with a mechanical agitation system, where they are stored in an inert atmosphere and at a temperature above the fog point to avoid freezing of the paraffins.

As a result of thermal cracking under the ideal conditions, high quality pyrolysis liquids are obtained formed by a mixture of hydrocarbons with a low bromine index of between 20-35 g Br2 / 100 g sample, which indicates that the concentration of olefins is low and the impact on oxidation stability is positive, as well as a low content of heteroatoms (N, S, O, Cl, Br) and metals, which is particularly positive for use in subsequent catalytic processes in the petrochemical industry .

As a result of thermal cracking, in addition to pyrolysis liquids, permanent gases formed by H2 and C1-C6 hydrocarbons, (mainly gas rich in C4 compounds), with high calorific value, similar to that of commercial butane, are obtained.

According to a preferred embodiment of the present invention, the permanent gases are brought to a burner used to cover the energy requirements of the procedure itself. Due to the quantity and quality of the gas generated during thermal cracking, the procedure is self-sufficient from an energy point of view and may even become surplus, in which case, the surplus gas not used to cover the energy requirements of the procedure will be carried out to an alternative internal combustion engine for electricity production.

The following describes a practical example of implementation of the method according to the present invention, provided only by way of illustrative example and in no way limiting the method according to the present invention.

EXAMPLE 1

In this example, the process of the invention was carried out in a pilot plant comprising a 250 l capacity stainless steel continuous reactor heated by electrical resistors and equipped with mechanical stirring. As raw material, low-density polyethylene in pellet format was used, prepared according to the procedure described in this document from white plastic film bales from the entire line of the urban solid waste treatment plant (RSU) in Zaragoza ( recovered by manual triage in bulky cabin). 500 kg of raw material were melted in an inert atmosphere and heated to 275 ° C. The melting chamber was pressurized with nitrogen and the material was transferred to the thermal cracking reactor by pressure gradient at a speed 40 kg / h, keeping the reactor fill level constant throughout the procedure. Cracking was carried out at approximately 420 ° C and at an autogenous pressure of 6 bar, with stirring at 50 rpm, at steady state for several hours. The liquid product resulting from thermal cracking is a mixture of hydrocarbons with a low olefin content (bromine index: 31 g / 100 g of sample) and contaminants (see Table 1). The water content is less than 130 ppm and the content of aromatic compounds is approximately 32% by weight.

Table 1. Contaminants of the pyrolysis liquids obtained by thermal cracking according to Example 1.

The thermal cracking gas obtained according to the embodiment of Example 1 is formed mainly by cis-2-butene (approximately 30% by vol.) And has a calorific value similar to that of commercial butane (11,000 kcal / kg). The gas density is approximately 1.8 kg / Nm3 and the average molecular weight is approximately 41 g / mol.

Therefore, the process according to the present invention provides several advantages over the known alternatives of the prior art. A main advantage is the possibility of carrying out the pyrolysis procedure without the need for an auxiliary drag with nitrogen gas and without the need for an external energy source. The pressure that is achieved in the cracking reactor autogenously, with the gases themselves that are generated by supplying heat, is sufficient to push the gaseous product to stage c) of cooling (in the case of the condensable part) or to a burner or internal combustion engine in the case of permanent gases. In the rest of the pyrolysis procedures of the prior art, which are carried out at atmospheric pressure or under vacuum in the main reactor, this entrainment is achieved with nitrogen, which supposes greater expense and also reduces the calorific value of the gas, making it difficult to use as fuel. . In the case of the proposed procedure, this gas has a calorific value similar to that of commercial butane, which, added to the fact that it is generated in a significant quantity (8 - 15% by weight of the feed), favors the procedure to be self-sufficient from the energy point of view and can even become surplus, in which case the excess gas will be burned in an alternative internal combustion engine to produce electricity.

Another advantage offered by the process of the present invention is that it does not need a catalyst for its operation, with which the risk of its deactivation, consumption and waste management is avoided. Furthermore, the process conditions lead to obtaining high added value pyrolysis liquids with a low bromine index, which indicates that the concentration of olefins is low and the impact on oxidation stability is positive.

Another advantage of the process according to the present invention is a very low content of heteroatoms (N, S, O, Cl, Br) of the obtained product, which is particularly positive for its use in subsequent catalytic processes in the petrochemical industry. This is due to the combination of two factors, the previous conditioning stage carried out to ensure that the feed material meets the requirements of the procedure and the way in which thermal cracking is carried out, with which it is achieved that many of unwanted substances that have not been removed in the previous conditioning stage are retained in the solid.

Although the present invention has been described with reference to preferred embodiments thereof, one skilled in the art will understand that modifications and variations can be applied to such embodiments without thereby departing from the scope of protection conferred by the following claims.

Claims (26)

1. Procedure to convert plastic waste into liquid products useful in the petrochemical industry, the procedure comprising obtaining a pyrolysis liquid through the following steps: 2. a) carry out the fusion and homogenization of raw material of plastic waste continuously; b) carrying out thermal cracking of the molten raw material from step a) in a thermal cracking reactor; and c) subjecting volatile products from step b) to rapid cooling; whereby at the output of stage c) an outflow formed by pyrolysis liquid useful in the petrochemical industry as well as permanent gases with high calorific value is obtained. 3. Procedure according to claim 1, characterized in that step a) comprises introducing raw material cyclically in alternating sequence in two transfer chambers, comprising: Four. - firstly, fill a first transfer chamber; - when said first transfer chamber is full, stop filling the first transfer chamber and start filling a second transfer chamber; - simultaneously with filling the second transfer chamber, heating the raw material in the first transfer chamber and pressurizing with nitrogen; - after reaching the appropriate temperature and pressure, feed the molten raw material to the cracking reactor of step b); and - Cyclically repeat the previous steps, alternating the filling, heating and pressurization of the first and second transfer chambers. 5. Process according to claim 2, characterized in that the raw material is heated to 270-310 ° C. 6. Process according to any of claims 2 and 3, characterized in that the raw material is pressurized up to 1-5 barg above the operating pressure of the thermal cracking reactor of step b). 7. Method according to any of claims 2 to 4, characterized in that the transfer chambers are previously inerted. 8. Process according to claim 5, characterized in that the transfer chambers are previously inerted with nitrogen. 9. Method according to any of claims 2 to 6, characterized in that the transfer chambers comprise stirring means. 10. Process according to any of the preceding claims, characterized in that step b) of thermal cracking is carried out at a temperature of 375-525 ° C, at a pressure of 1-20 barg and with a residence time of between 0, 5 and 5 h. 11. Process according to claim 8, characterized in that step b) of thermal cracking is carried out at a temperature of 400-450 ° C, at a pressure of 1-10 barg and with a residence time of between 0.5 and 2 h. 12. Process according to any of the preceding claims, characterized in that step b) of thermal cracking is carried out without a catalyst. 13. Process according to any of the preceding claims, characterized in that it further comprises continuously extracting a solid generated during step b) of thermal cracking. 14. Method according to claim 11, characterized in that it further comprises recirculating a liquid from thermal cracking, free of solids, back to step b) of thermal cracking. 15. Method according to any of the preceding claims, characterized in that the temperature of the outflow from step c) is less than 80 ° C. 16. Method according to claim 13, characterized in that the temperature of the outflow from step c) is less than 50 ° C. 17. Process according to any of the preceding claims, characterized in that the outflow pyrolysis liquid is composed of a mixture of hydrocarbons with a low bromine number of between 25-35 g Br2 / 100 g of sample, as well as a low content in heteroatoms and metals. 18. Process according to any of the preceding claims, characterized in that the permanent gases of the outflow are formed by H2 and C1-C6 hydrocarbons of high calorific value. 19. Procedure according to any of the preceding claims, characterized in that the permanent gases of the outflow are led to a burner to meet the energy requirements of the procedure itself. 20. Method according to any of the preceding claims, characterized in that it comprises a previous stage of conditioning the raw material of plastic waste, before stage a), to eliminate unwanted compounds and homogenize the raw material. 21. Method according to claim 18, characterized in that the previous stage comprises a selected stage of optical separation, crushing, screening, magnetic and / or Foucault separation, centrifugation, densimetric separation, agglomeration and any combination thereof. 22. Process according to any of claims 18 and 19, characterized in that the previous stage comprises densifying the raw material to a density of 200-450 kg / m3 23. Process according to any of claims 18 to 20, characterized in that after the previous conditioning stage, a conditioned product is obtained that will serve as raw material for stage a), and which is an agglomerate with more than 90% by weight of low-density polyethylene, with a size between 10 and 30 mm and a maximum moisture content of 15% by weight. 24. Process according to claim 21, characterized in that the conditioned product comprises less than 10% by weight in total of high-density polyethylene and propylene, less than 5% by weight of polystyrene and less than 2% by weight of other plastics. 25. Process according to any of claims 21 and 22, characterized in that the conditioned product further comprises a lower calorific value greater than 9500 kcal / kg, a carbon content greater than 80% by weight and a hydrogen content greater than 13% in weigh. 26. Process according to any of the preceding claims, characterized in that the plastic waste raw material comprises low-density polyethylene and polypropylene film.