ITMI980581A1 - PROCESS AND PLANT FOR THE CONTROLLED PYROLYSIS OF PLASTIC MATERIALS - Google Patents

Description

DESCRIPTION

The present invention relates to the realization of a process and a plant for the controlled pyrolysis of plastic materials.

The growing demand for clean energy and the need to dispose of waste of difficult natural degradation such as plastic materials from hospital, urban and industrial waste, is completely displacing the problem of disposal that can no longer be considered landfill, more or less controlled, hitherto adopted.

On the other hand, thermal destruction technologies have proved unsuitable and dangerous for the fumes generated in combustion. Several pyrolytic plants have been started up in recent years, in an attempt to recover energy products from plastics; all tending to convert, through the emission of heat, the polymer into a series of products to be used both in combustion, for the production of electricity, and to eliminate the acid residues that are the cause of toxic fumes.

The technologies currently in place in this sector work with high temperatures (500-800 ° C) in static and / or rotary ovens with very long contact times (3-4 min.); this entails very high plant costs without having the right economic performance.

There are several disadvantages:

1. combustion carried out directly involves the formation of carcinogenic products such as dioxins and furfurans;

2. the attempt to burn the gasification products in place of the residual sorting of municipal solid waste (dry part consisting of paper, rags and plastics) in direct form does not solve the problem of the introduction of the aforementioned carcinogenic furai and gasification contributes the increase in management costs;

3. the presence of plastics such as PVC results in severe corrosion on the combustion equipment which leads to the impossibility of using high temperature boilers due to the inevitable corrosion of the tube bundle of the same.

For the cracking treatment of organic components with medium molecular weight, thermal cracking and catalytic cracking have long been used in refineries. Thermodynamic mechanisms have been described in numerous texts, among which Gates, B.C. et al., Chemistry of Catalytic Processes. McGraw-Hill, Inc. (1979) and Pines, H., The Chemistry of Catalytic Hydrocarbon Conversions. Academic Press (1981).

To achieve the intended purposes (elimination of environmental impact problems, recovery of thermal energies and minimum production of surpluses) the system proposed according to the present invention is articulated according to the cracking process.

FIG. 1 Flow diagram for a cracking process to obtain hydrocarbons from PVC-free plastic material.

FIG. 2 Flow diagram for a cracking process to obtain hydrocarbons from P, VC and halogenated plastics.

FIG. 3 Flow diagram for a cracking process to obtain hydrocarbons from linear plastics.

In its broadest aspect, the present invention relates to a process and plant for cracking a polymeric material comprising a reactor which contains a molten catalyst bed and means for introducing said polymeric material into said catalyst bed.

As will be apparent from the following detailed description of the invention, the catalyst employed is typically a metal or a mixture of metals and optionally contains an acid component.

The process according to the invention is useful for converting a variety of polymers, branched-chain plastics, straight-chain plastics and halogenated plastics such as PVC into hydrocarbon products which are useful for energy production and yet avoid the associated pollution problems with the direct combustion of plastic materials. The process according to the invention can also be used for the cracking of mixtures of polymeric materials derived from the sorting of solid urban waste.

In the cracking treatment of high molecular weight organic components, such as plastics, it can be specified that the treatment is a combination of two technologies, that is, between thermal cracking and catalytic cracking.

According to the present invention, the materials that can be cracked are distinguished between (1) branched plastics (methyl acrylate, polyurethanes, furan resins) where it is possible to obtain the starting monomer for the restoration of new polymers, - (2) plastics linear (PVC, PET, PP) alone or in a mixture, from which hydrocarbons destined for combustion for the production of electricity are obtained, and (3) a mixture of plastic materials derived from the mechanized sorting of urban solid residues which consist of a very heterogeneous mixture of products ranging from plastics, rubbers, paper, etc.

When it comes to a cracking system of non-homogeneous plastic material (mixture of different types of plastics), not only aliphatic hydrocarbons are obtained but instead a mixture of hydrocarbons is obtained. If you are faced with pure plastics (of only one type), it is not convenient to use them for the production of hydrocarbons but to work to obtain new monomers or the direct reuse of plastics.

The hydrocarbons obtainable from a cracking process must have the characteristics of (1) high content of low molecular weight products that must burn without excessive carbon residues, (2) absolute absence of halogens and products of acid origin and (3 ) low content of unsaturated compounds to avoid condensation in the storage phases.

These and still other objects, which will appear more clearly in the following, are achieved by a plant comprising a reactor which contains a bed of catalyst in the molten state.

The catalytic cracking of plastic polymers can be considered as a macromolecular splitting operation into free radicals whose composition depends on the following factors: (1) type of catalyst involved in the process, (2) temperature and time of contact with the catalyst and (3 ) quantity of plastic material involved in the unit of time and in relation to the volume of the catalyst.

The characteristics that catalysts must have are that they possess an electronegative power capable of splitting a polymeric compound into smaller molecular fractions; their validity has been experimentally determined and the most valid ones will be described below.

The addition of an acidic component such as a silicate or a carbonate is always experimentally determined; normally the addition of silicate or another compound takes place when the cracking processes occur with the formation of non-condensable gases, i.e. when the catalyst acts so rapidly that it breaks the polymer into non-condensable gaseous units at the usual temperatures (H2, CH3 <+>, CH4, etc.)

The optimal temperature of the catalyst was calculated through a thermodynamic analysis coupled to the infrared spectrum, analyzing the peaks in the infrared spectrum of the gasification products of plastic materials with the variation of time and temperature.

Typically, the catalyst has a melting point of 430 ° C or less, preferably 400 ° C or less. In the process, the catalyst is maintained at a temperature of 460-550 ° C, preferably 480-530 ° C. The temperature of the catalyst in the reactor is typically at least 60 ° C above its melting point, preferably at least 70 ° C above its melting point.

As will be seen from the examples, the catalyst is typically a metal or a mixture of metals. Preferably, said catalyst is selected from the group consisting of lead, tin, zinc, antimony and their mixtures, optionally together with other metals. Examples of metals and metal mixtures that can be used in the process of the invention are lead, a lead-zinc mixture, a lead-tin mixture, a zinc-tin mixture, a piorabo-zinc-tin mixture, a zinc-antimony mixture. , or a lead-copper mixture.

In one example of a useful catalyst mixture comprising lead and zinc, zinc is present in the amount of 15-25% by weight, preferably in the amount of about 20% by weight. In one example of a useful catalyst mixture comprising lead and tin, tin is present in the amount of 5-15% by weight, preferably in the amount of about 10% by weight.

The molten catalyst can also advantageously contain an acid component. Examples of an acid component are metal silicates, metal carbonates and their mixtures. More particularly, the acid component can be aluminum silicate or lead carbonate. In this respect, 2inc, tin and antimony silicates are effective even though they gave lower yields in the initial tests. It should be noted that the above salts are present in the reactor in the molten state and not as a heterogeneous mixture.

As a rule, the catalyst bed is stirred continuously; the agitation has the task of homogenizing the temperatures and maintaining an intimate contact between the catalysts and the reaction products. The material to be cracked is introduced into the molten bed at a depth of generally 2-15 cm. Excellent results occurred when the material was introduced to a depth of 4-5 cm. The depth depends on the polymer to be treated and on the predetermined temperature and contact times.

From the experiences gained in the pilot phase it can be seen that the quantity of plastic material in kilograms converted per liter of catalyst per unit of time (24 hours) have reached values between 12 and 15. Obviously the process can be optimized and higher yields are certainly at the reach of the person skilled in the art.

Although one does not wish to be limited by theory, the reactions that can be hypothesized are the following:

(1) in the first stage (immersion of the polymers in the melt bed of the catalyst) the polymer breaks up in the C-C bonds and the radicals obtained are fixed to the catalyst, in this phase a series of electrons are released with consequent formation of aliphatic compounds.

(2) during the first phase reaction the catalyst assumes acidic characteristics; the aliphatic components migrate on this surface, take on a proton and transform into carbocations. (3) the carbocation can undergo a series of different reactions: it can split into two fragments, isomerize giving a more branched form or undergo various cyclizations; in the end, however, it loses a proton, restores the activity of the catalyst and migrates to the surface in the form of gas.

By suitably varying the temperatures of the catalyst, the contact times and the charges of plastic material in the unit of time, it is possible to obtain semi-polymers alongside the hydrocarbons. The semipolymers can be collected in a fraction which condenses at 70 ± 5 ° C while the hydrocarbons are collected in another fraction which condenses at 8 ± 2 ° C.

The separation can be carried out using a particular plate condenser where the central part of these plates, for about 1/3 of the total surface of the plate itself, is heated to a temperature of 70 ± 5 ° C and the peripheral part (2/3 of the total surface of the plate itself) cooled with chilled water at 5 ± 1 ° C.

The fraction that condenses at 70 ° C represents what has been defined as a semipolymer that contains a mixture of cyclization products of the aliphatic compounds. The fraction that condenses at 8 ° C contains short-chain aliphatic products. In fact, the gas chromatographic analyzes for the first fraction indicate a mixture of hydrocarbon semipolymers containing 70-120 carbon atoms and for the second fraction a mixture of short-chain compounds containing 5-25 carbon atoms.

The semipolymers can be used in blends with papers and rags separated from the urban solids rasidui. This possibility allows us to make a mixture with paper and rags which allows us to improve the combustion yields of the aforementioned mixture; in fact, given that the dry part of the sieving of municipal solid waste composed in principle of 45-50% of plastic material and 45-50% of paper and rags has an average composition of 45-50% of plastic material and 45-50 % of paper and rags, if the mixture of paper and rags were used directly as solid fuel, a calorific value of 2000-2500 Kcal / Kg could be exploited and a volume of combustion ash equal to 26-30% of the combustion would be obtained.

The addition of 50% of the semipolymers obtained from the cracking process would bring the caloric power of the paper-rag mixture to 5500-6000 Kcal / Kg, since these semipolymers, due to their chemical conformation, are oxygen carriers, a decrease in the ashes from the 26-30% to a maximum of 4%.

On the basis of these experimental data it was decided to hypothesize the power generation plants, which will have to operate with waste plastic material. The hydrocarbons (liquid phase of the cracking process) which are preferably largely aliphatic are sent to an electric generator (turbogas) with a power output equal to 60% of the required power; the hot combustion fumes are used as a co-generation factor to heat water. The heat exchange process takes place in a boiler enslaved by a feeding system for the combustion of solid fuels, precisely the mixture of paper, rags and samipolymers in such a way that the water is brought to a temperature of 430 ° C and the vapors used for a high pressure turbine.

Now, since high-pressure turbines are not very elastic as a possibility of variation on the potential of energy delivered, it is possible to exploit the variation of energy with turbogas, it follows that, when the need arises (night hours or plant shutdowns) reducing the use of energy can turn to gas turbines and have steam available for other industrial uses.

The cracking processes according to the invention are illustrated in the flow diagrams of Fig. 1-3.

Depending on the diversification and heterogeneity of the plastic materials derived from the sorting of urban solid residues and plastics in general, the cracking process is carried out after a careful thermodynamic-infrared analysis on the mixture to be cracked.

The thermodynamic analysis coupled to the infrared spectrum is an analytical typology well known by the analysts of the art; it is a question of analyzing the gasification of an organic substance, by means of infrared spectrum, after it has been subjected to a rapid change in temperature in a precise fraction of time. The tests allow to determine the speed of introduction of the material into the catalyst bed and the quantity of the same as a function of the difference in the plastic material used.

In order to implement the cracking process, two containment silos will be set up, one in operation and the other under analysis. The silos will have a containment capacity for 24 hours of operation and the material will be suitably ground and homogenized as it will best appear in the following description.

With reference to the flow drawing illustrated in Fig. 1, the cracking is carried out as follows:

In the pre-treatment phase, the grinding is carried out in two distinct phases divided between a coarse grinding and a final grinding. The coarse grinding is carried out by a tearing mill capable of reducing the material into pieces not exceeding 12 mm, preferably not exceeding 10 mm, while the final grinding is carried out by means of a knife mill capable of reducing the pieces to non-superior sizes 3 mm, preferably not exceeding 2 mm.

Between the first and second grinding the material undergoes an immersion screening process and a drying according to glove described below. In coarse grinding, the sorted material, coming from the accumulation pit, is subjected to grinding by means of a tearing mill whose grids are articulated to allow a size not exceeding 12 mm, preferably not exceeding 10 mm. In immersion screening, the material subjected to coarse grinding is sent to a vibrating screen by immersion in water. By exploiting the difference between the specific weight of the halogenated plastics and the others, a clear separation is obtained and the halogenated plastics are collected at the bottom of the sieve while the lighter ones are dragged by the motion of the water and recovered on the upper part of the sieve. The screened material is dried with hot air by means of a cyclone or rotary dryer (not shown in Fig. 1) depending on the amount of plastic involved. To obtain a fine grinding, the dried material is sent to the grinding by means of a knife mill whose grids must be articulated in such a way as to obtain a size not exceeding 3 mm, preferably not exceeding 2 mm.

The material thus prepared is stored in fiberglass silos (not shown in Fig. 1) whose capacity must be at least 24 hours of work and for this purpose at least 3 containment silos for the mixture to be cracked are set up to always have the opportunity to set reaction temperatures and times as described herein.

In the cracking phase, the dosage of the material is determined by a terraodynamic / infrared analysis which establishes, as a function of the contact time / operating temperature, the quantity over time to be introduced into the reactor. This data is extremely important since it allows to obtain high yields without producing large quantities of pyrolytic carbons, furthermore in the analysis the quantities of simpler and higher caloric power carbon / hydrogen products are identified.

The thermodynamic analysis is carried out on a mixture of samples taken during the fine grinding phase and sample taken will be used located on the conveyor belt that feeds the storage silos.

Cracking follows a preheating phase in which the dried material is taken by means of an extractor screw and sent to an extruder connected directly to the cracking reactor (each reactor will have extruders that will work in a well-defined synchronism depending precisely on the above analysis) ; the extruders are fed through a screw / piston valve which has the task of accurately dosing the quantity of product to be introduced into the reactor. The extruders will bring the material to a state of plasticity such that it can be injected into the reactor without difficulty and the raising of the temperature allows us to reduce the contact times to the advantage of the quality of the aliphatic hydrocarbons obtainable.

Cracking takes place in a stainless steel reactor, for example 18/8/2, which must be kept at a temperature that avoids unwanted condensation under the top of the reactor (i.e. the part not occupied by the catalyst; about 1/3 of the reactor volume ). The reactor temperature is maintained by forced circulation of diathermic oil or with other heating sources but avoiding the use of flame heating to avoid the dangers of fires. As for the reactor free-top temperature, it is usually maintained at a temperature of 320 - 430 ° C.

The reactor is filled, 2/3 of its volume, with a catalyst maintained in the molten state, for example by forced circulation of diathermic oil heated in a special control unit.

The plastic material is injected into the catalyst bed and the extruded plastic material in contact with the catalyst gasifies in a few seconds and the cracking gases are captured under suction (the whole system works in slight depression) by a cyclone not shown in Fig. 1) which has the purpose of separating the traces of distilled metals from the cracking gas; these metals are recovered from the bottom of the cyclone and sent for recovery.

The gases, separated from the metal parts, undergo two distinct condensation processes that depend on the type of plastic being cracked.

As regards the plastics with halogens (see Fig. 2), in a first phase the gases coming from the cracking reactor are washed in a square tower with circulation of a solution of 15% by weight of NaOH. The head part of the column is cooled by a coil with forced circulation of brine at a temperature of -15 ° C. The eluate, which contains the salified halogens, all the water-soluble products (methyl alcohol, amyl alcohol, formaldehyde, etc.) and the insoluble hydrocarbons, is collected in an agitated lung and kept cool by the brine.

In a second phase, the eluates are sent to a liquid / liquid centrifuge of the ALFA LAVAL class or similar. Centrifugation separates the aqueous fraction from that of the cracking hydrocarbons (largely aliphatic). The hydrocarbons are collected in a steel tank equipped with cooling and stirring to be used as fuel for the production of electricity. The aqueous part undergoes a fractional distillation process in order to separate the halogenated salts from the combustible products. The combustible products collected at the top of the distillation column (a perforated plate column is used) are sent, after passing through a shell and tube condenser, to the hydrocarbon collection tank.

In the case of halogen-free plastics (see Fig. 1), the gases coming from the cracking reactor pass directly to the condensation column and are then condensed with the same cracking hydrocarbons kept in cooling by means of a brine heat exchanger and stored in a stirred and cooled tank.

The entire system must operate in slight depression (10 - 15 mm of water column), from the extruders to the condensation column. The plant must be saturated with nitrogen and at the end of the condensation column there must be a capture of nitrogen which, before being released into the environment, must be burned in order to eliminate the traces of non-condensable substances that inevitably form during the process.

With reference to Fig. 3, the plastic material, generally collected after separation of solid urban waste, therefore a mixture of plastics, is stored in a silos with a capacity of 24 hours of operation. The material undergoes a first grinding by means of a knife mill, then a screening to separate the pieces greater than 2 mm. The part above 2 mm is returned to the grind. The fine-sized material (2 mm) is stored in a silo with the same capacity as the first silos (24 hours). The final storage operation is preceded by drying carried out with hot air obtained in the cooling of the separator cyclone whose task is to separate the cracked coal entrained from the cracking gases. The quantity of plastic material destined for the cracking process is taken from the fine material storage silos (2 mm) by means of a metering valve or volumetric valve; this quantity is always determined by the thermodynamic-infrared analysis described above. The gases produced in the cracking process are sent to the cyclone to separate the carbon produced and then pass to a water-cooled condenser. Two products are formed in this condenser: an incondensable gas and a liquid consisting of a very heterogeneous mixture of hydrocarbons. The non-condensables are sent to an absorption column in countercurrent to an alkaline solution of NaOH at 15%; this column retains, by chemical reaction between halogens and NaOH, the halogens themselves with formation of the respective salts which are recovered by crystallization. The hydrocarbons undergo fractional distillation from which 3 different types of products are obtained and precisely: (1) a low boiling fraction consisting of a mixture of alcohols and low molecular weight hydrocarbons (C3-C8) which are used to maintain the heating of the reactors , 2) a medium fraction (C5-C25) which is stored for use in turbo gas generators, and (3) a high boiling fraction (semipolymers) which is used as an additive for papers and rags exactly as described above.

Although it is possible to hypothesize the formation reactions of hydrocarbons and small molecules only after a careful analysis of the products obtained, it can be said that the incondensable gases are the following:

H2S; CH4; N2; NH3; HCl; HF; traces of C02, and by entrainment C2H5OH and low boiling components will be present which can be completely blocked if rapid cooling and low temperatures (5-7 ° C) are carried out.

The gases listed above must therefore first undergo absorption in the abatement tower in an alkaline environment (caustic soda or caustic potash). The gases H2S, NH3, HCl, HF and C02 will be transformed into Na2S, NH4OH, NaCl, NaF and Na2C03, respectively.

The other components, ie CH4, C2H5OH and the entrainment products, as they are flammable, will be passed to a torch and transformed into C02 and H20.

From the experiences gained in the pilot stage, catalysts particularly suitable for the cracking of plastic materials are listed in the following table. In Examples 1-6, the percentage composition of the metal catalysts and the percentage composition of the plastic materials are expressed by weight.

Methyl methyl acrylate and polyphenolic resins

2

Low density polyethylene

3 Polyethyl tetraftalate

4

Polypropylene

5

Mixture of plastic materials derived from the dry part of the

screening of municipal solid waste composed of roughly 45 50% of

plastic material and 45-50% paper and rags.

Claims (30)

CLAIMS 1. A process for cracking a polymeric material, comprising introducing said material into a molten catalyst bed. 2. Process according to claim 1, characterized in that said catalyst is maintained at a temperature of 460-550 ° C, preferably 480-530 ° C. 3. Process according to claim 1, characterized in that said catalyst melts at a temperature of 430 ° C or less, preferably at a temperature of 400 ° C or less. 4. Process according to claim 1, characterized in that said catalyst is maintained at at least 60 ° C above its melting point, preferably at least 70 ° C above its melting point. 5. Process according to any one of claims 1-4, characterized in that said catalyst is a metal or a mixture of metals, optionally mixed with an acid component. Process according to any one of claims 1-5, characterized in that said catalyst is selected from the group consisting of lead, tin, zinc, antimony and their mixtures, optionally in admixture with other metals and / or an acid component. 7. Process according to claim 6, characterized in that said catalyst and lead, a lead-zinc mixture, a lead-tin mixture, a lead-zinc-tin mixture, a zinc-antimony mixture or a lead-copper mixture, optionally mixed with an acid component. 8. Process according to claim 6, characterized in that said catalyst is lead. 9. Process according to claim 6, characterized in that said catalyst is a mixture of metals comprising lead, and moreover comprising zinc in the amount of 15-25% by weight, preferably in the amount of about 20% by weight. 10. Process according to claim 6, characterized in that said catalyst is a mixture of metals comprising lead, and further comprising tin in the amount of 5-15% by weight, preferably in the amount of about 10% by weight. 11. Process according to claim 5 or 6, characterized in that said acid component is selected from metal silicates, metal carbonates and their mixtures. 12. Process according to claim 1, characterized in that said acid component is an aluminum silicate or a lead carbonate. 13. Process according to claim 1, characterized in that said polymeric material is introduced into the catalyst bed at a depth of 2-15 cm, preferably at a depth of 4-5 cm. 14. Process according to claim 1, characterized in that said molten catalyst bed is continuously stirred. 15. Process according to claim 1, characterized in that said polymeric material is a branched-chain plastic. 16. Process according to claim 1, characterized in that said polymeric material is a straight-chain plastic or a mixture of straight-chain plastics. 17. Process according to claim 1, characterized in that said polymeric material is PVC and / or other halogenated plastics. 18. Process according to claim 1, characterized in that said polymeric material is a mixture of polymeric materials derived from the sorting of solid urban waste. 19. Plant for cracking a polymeric material, comprising a reactor containing a bed of molten catalyst and means for introducing said polymeric material into said catalyst bed. 20. Plant according to claim 19, characterized in that said means are an extruder. 21. Implant according to claim 12, characterized in that said means are suitable for introducing said polymeric material at a depth of 2-15 cm, preferably at a depth of 4-5 cm. 22. Plant according to claim 12, characterized in that said reactor is a stirred reactor. 23. Plant according to claim 19, characterized in that said catalyst is a metal or a mixture of metals, optionally mixed with an acid component. 24. Plant according to claim 12 or 23, characterized in that said catalyst is selected from the group consisting of lead, tin, zinc, antimony and their mixtures, optionally in admixture with other metals and / or an acid component. 25. Plant according to claim 24, characterized in that said catalyst is lead, a lead-zinc mixture, a lead-tin mixture, a lead-zinc-tin mixture, a zinc-antimony mixture or a lead-copper mixture, optionally mixed with an acid component. 26. Plant according to claim 24, characterized in that said catalyst is lead. 27. Plant according to claim 24, characterized in that said catalyst is a mixture of metals comprising lead, and moreover comprising zinc in the amount of 15-25% by weight, preferably in the amount of about 20% by weight. 28. Plant according to claim 24, characterized in that said catalyst is a mixture of metals comprising lead, and moreover comprising tin in the amount of 5-15% by weight, preferably in the amount of about 10% by weight. 29. Plant according to any one of claims 23-25, characterized in that said acid component is selected from metal silicates, metal carbonates and their mixtures. 30. Plant according to claim 29, characterized in that said acid component is an aluminum silicate or a lead carbonate.