WO1998009997A1

WO1998009997A1 - Process for degrading plastics

Info

Publication number: WO1998009997A1
Application number: PCT/NO1997/000236
Authority: WO
Inventors: Trond Myrstad; Svenn Utengen; Fredrik Steineke
Original assignee: Den Norske Stats Oljeselskap AS
Current assignee: Equinor ASA
Priority date: 1996-09-06
Filing date: 1997-09-04
Publication date: 1998-03-12
Anticipated expiration: 1999-03-06
Also published as: NO304383B1; NO963736D0; AU4226597A; NO963736L

Abstract

A process for degrading a feedstock consisting of plastics and/or other polymer material to liquid hydrocarbons. The feedstock is mixed with solid particles and optionally with oil and/or water, and is cracked in a hammer mill type of apparatus, in which the heat required for the cracking is supplied mechanically. The treated mixture is then subjected to a separation to separate out at least a substantial part of its content of solid particles. The cracking is preferably carried out in a hammer mill at or near the atmospheric pressure, at a temperature in the range of 200 °C to 500 °C, and with a peripheral speed of the hammers of the hammer mill in the range of 15 to 75 m/s.

Description

Process for degrading plastics.

The present invention relates to a process for degrading plastics to liquid hydrocarbons which may be used as a feedstock, for example in refineries.

Plastics waste has been referred to as "resources astray". Much effort has been laid down in attempts to recover these resources . One alternative has been to recover the plastic itself by purifying and processing it for production of second generation plastic articles which in some cases will be sufficient for certain purposes, e.g. as fence posts, park benches, pallets, etc. However, the market for such products is not sufficiently extensive to absorb all the plastics which may be recovered in this way. Another alternative is to utilize the chemical energy contained in the plastics, by means of combustion of the plastics waste. Plastics contain as much energy per kilo as usual fuels, and by using adequately dimensioned combustion plants it is possible to recover the energy from the plastics in the form of heat.

A third option for utilization of the resources in plastics waste consists in treating the plastics waste in such manner as to remake the starting materials which were used for pro- ducing the plastics, or to form other relatively low molecular compounds which can be processed to form adequate industrial feedstocks and products. Methods for producing such starting materials and related products from the plastics waste may be chemical decomposition, thermal decomposition and degradation of the plastics, as well as catalytic degradation processes.

EP-A 208525 describes a process for degrading olefin polymers to products of lower molecular weight suitable as additives in the lubricant industry. The degradation of the polymer is carried out in an inert atmosphere in a rubber masticator, in the presence of a free radical initiator, especially a peroxide, under mechanical agitation, preferably at temperatures of 120 °C to 250 °C. It is preferred to perform the degradation of the polymers under the influence of shear forces, which allegedly increases the degradation speed. The principle on which this process is based is chemical degradation.

EP-A 502618 describes a process for degradation of a particu- late polymer waste, e.g. plastic packaging, especially made of polyolefins, to lower hydrocarbons by evaporation of the polymer waste in a fluidized bed of a particulate material, such as quartz sand, in the presence of a fluidizing gas at temperatures in the range of 300 °C to 690 °C. The products obtai- ned by the process are suitabie as a feedstock for a cracking plant. Prior to the pyrolysis in the reactor, the polymer waste may be mixed with a paraffin oil, such as naphtha or a gas oil (boiling point 250-340 °C). This process is based on pyrolysis only.

GB-A 2 058 086 describes a process for thermal cracking of polyolefins at temperatures in the range of 250 °C to 350 °C in an inert atmosphere. The cracking is carried out under vigorous agitation of a solution of the polyolefins in a mineral oil, preferably in the presence of minor amounts of a free radical scavenger to avoid crosslinking reactions. The polyolefins are cracked to form a viscosity index improving additive for lubricants. No solid particles are added to the polyolefins before the cracking.

It has now been found that when a plastic and/or a polymer material in particulate form or finely divided form, optionally in liquid form, is/are blended with solid particles, such as foundry sand, and optionally with oil and/or water, and the obtained mixture is subjected to high mechanical shear in a hammer mill type of apparatus, a particularly efficient cracking of the plastic and/or polymer material can be achieved. The particularly high effect achieved with such process is presumed to be due to a combination of the substantial con- tents of added solid particles in the mixture and the vigorous grinding and beating effects which can be achieved in a hammer mill type of apparatus. To use a hammer mill type of equipment for degradation of hydrocarbons is known per se. US Patent No. 3,282,826 (1966) describes a process for depolymerising bituminous coal and products derived therefrom in a hammer mill, ball mill or s similar milling and grinding equipment. In the hammer mill or the like, a solid-solid reaction is carried out between coal particles and the particles of an friable solid metal having a good affinity to oxygen and sulphur atoms. The metal reacts with oxygen and sulphur bridges in the macromolecules of the o coal, whereby these molecules are broken down to smaller molecules, with the result that the coal is converted to preponderantly liquid products. The process can be carried out in the presence of a low-viscous solvent and a catalyst may optionally be used for the depolymerization reactions. The milling s in the hammer mill or the like generates the required heat for the depolymerization reactions and the process is normally carried out at temperatures below 400 °C and at pressures from 1 to 10 ata. The process requires a pre-drying of the coal to the lowest possible moisture content, as moisture will inter- o fere with the reaction between the metal and the oxygen and sulphur atoms. The metal which is used, and which may be scrap iron powder, aluminum powder or zinc dust, is regenerated for renewed use after having been through the process.

5 Norwegian Patent No. 175,847 describes a process for selectively and/or unselectively evaporating and/or decomposing liquid hydrocarbon compounds in a hammer mill type of apparatus. In embodiments of the process, a very high specific energy supply can be achieved per unit of area. In addition to 0 bringing about evaporation of the liquid or components of the liquid, said treatment causes, above a given peripheral velocity, a decomposition of the hydrocarbons to lighter fractions at ambient temperatures in the reactor up to 50 % lower than the temperatures required in a purely thermal decomposition 5 process. The "thermomechanical decomposition" achieved by the process of the patent is allegedly enabling the process to be used for decomposition of hydrocarbons in petroleum products, so that the process would make possible a direct refining and/or pretreatment of oil or distillation residues from oil refineries.

Norwegian Patent Application No. 943367 describes a process for thermo echanical cracking and hydrogenation of chemical substances such as hydrocarbons. The chemical substances to be treated may be hydrocarbons in liquid or solid form, carbonates, oil shale, oil sand, tar sand, refinery feedstocks, oil residues from refineries and rest products in crude oil tanks, petroleum residues, plastics and the like, and the treatment may suitably be carried out in a hammer mill. The cracking of the hydrocarbons takes place in a fluidized bed of finely divided solids in the hammer mill and is believed to be induced by local temperature jumps of short duration caused by friction forces between i.a. the fluidized solid particles and the hammers of the hammer mill. The hydrocarbons are thereby heated very quickly to high temperatures suitable for cracking and they are then quickly cooled to the lower ambient temperatures in the hammer mill. Thus, the local cracking reactions have a very short duration.

The present invention now provides a process for cracking a feedstock consisting of plastics and/or other polymer material to liquid hydrocarbons, wherein the feedstock, optionally mixed with an oil, is subjected to a heating under agitation. The process is characterized thereby that the supplied feedstock is mixed with solid particles and optionally with oil and/or water, and the feedstock in the obtained mixture is cracked in a hammer mill type of apparatus, in which the heat required for the cracking is supplied mechanically, whereupon the treated mixture is subjected to a separation to separate out at least a substantial part of its content of solid particles .

In preferred embodiments the cracking is carried out in a hammer mill, at or near the atmospheric pressure, at temperatures in the range of 200 °C to 500 °C, preferably from 250 °C to 400 °C, and with a peripheral speed of the hammers of the hammer mill in the range of 15 to 75 m/s. Plastics and/or polymer materials which are suitable for treatment by the process of the invention are all the usual commercial grades of such materials, e.g. polyolefins, poly- amides, polystyrenes, and the like. A certain care must be 5 taken in using materials containing certain inorganic elements, e.g. PVC (polyvinyl chloride), because toxic compounds, such as dioxines, may be formed during the degradation of such materials.

o The solid particles which are mixed with the plastics and/or polymer materials in the process of the invention are primarily mineral particles, such as quarts sand, silica, ceramic particles, aluminosilicates, and the like, but other solid particles, such as metal particles, may be used in specific s cases. It has been reported (K. C. Khulbe et al . , Fuel Processing Technology 41, 1, 1994) that sand has no catalytic effect in cracking of bitumen. Since no other catalytic materials need be present in the reactor in order to achieve the desired cracking in the present process for degrading plastics, the o process appears to be a temperature controlled process and not a catalytic process.

The oil which it may be desirable to add to and to mix with the plastics or polymer raw material in the process of the 5 invention may typically be a gas oil or heavier oil.

The presence of water in the process seems to stabilize the process. Therefore, it may be desirable to add water in quantities of 1 to 20 % by weight, preferably 5 to 15 % by o weight, calculated on the amount of plastics or polymer material .

A plausible explanation of the good cracking effect is the generation on friction surfaces of local temperatures which 5 are substantially higher than the temperatures in the bulk mass in the reactor. On said surfaces, the temperature can be sufficiently high for a thermal cracking to occur. The reaction time is short, because the local reaction media and the formed reaction products are quickly cooled down in the much less hot, and strongly turbulent medium of hydrocarbons, sand and steam in the reactor. It is reported in the literature that water may be decomposed under such conditions, with formation of hydrogen and hydroxyl radicals, cf. Joseph Haginn, "Water effect on petroleum formation studied", Chemical and Engineering New, September 1992. The hydrogen radicals may participate in cracking and dealkylation reactions and thus degrade the hydrocarbon chains. Active reaction sites may also be formed under special conditions, as a result of the grinding of sand, cf. W.A. Radtsig, Ki et. Katal., 1979, Vol. 20, pp. 445-458.

The principle behind the functioning of the reactor justifies a description of the reactor as a "quench reactor", wherein reactions and subsequent quenching take place in a microscale.

Some embodiments of the process of the invention are described in more detail below with reference to the appended drawings, wherein: Fig. 1 is a perspective view of a hammer mill suitable as a reactor for effecting the process of the invention, Fig. 2 shows the inner main parts of the hammer mill shown in Fig. 1, and

Fig. 3 shows a simplified block diagram of an e bodi- ment of the process of the invention.

Fig. 1 is a perspective view of a reactor 20 designed as a hammer mill. The reactor comprises a cylindrical reaction chamber 21 having an inlet 22 for feeding a mixture of plastic or polymer material to be degraded, added solid particles, optional oil and optional water. The inlet 22 is shown located at the top of the reaction chamber, but other locations are also possible. Solid particles are withdrawn through an outlet 23 located at one end of the reaction chamber 21. Other loca- tions are also possible. Cracked products are withdrawn through an outlet 24, which in the depicted embodiment is located on top of the reaction chamber 21. The reaction chamber 21 may optionally be equipped with inner longitudinal ribs (not visible in Fig. 1). The reactor 20 is further equipped with a through shaft 25 having hammers mounted thereon (not visible in Fig. 1). The shaft 25 is passing through an end part 26 having a bearing 27, at each end of the reaction chamber 21.

Fig. 3 shows the inner main parts in the reactor 20 depicted in Fig. 1, comprising a through shaft 25 having a number of hammers 30. In the depicted embodiment the hammers are assembled in groups of four hammers, so that the hammers of each group extend radially from the shaft 25 as an equal-armed cross. It is to be understood that the number of hammers in each group may vary. In the embodiment shown, having several groups of four assembled hammers, arranged adjacent to one another along the shaft, the various groups of hammers are not displaced in relation to one another. However, in other embodiments, each group of assembled hammers may be displaced a certain angle, for example 45°, in relation to the preceding group. A circular plate 31 is mounted near the end of the shaft 25, between the outlets 23 and 24 shown in Fig. 1 for solid particles and cracked products, respectively. The plate 31 may be secured either to the shaft 25 or to the reaction chamber 21 (Fig. 1), preferably to the reaction chamber 21. Cracked products pass the plate 31 through a slit between said plate 31 and the shaft 25.

In the process plant schematized in Fig. 3, pellets of a low density polyethylene (PEL) are introduced at 1 and are mixed in a mixing unit 5 with an oil supplied via line 3, and with a foundry sand, to which water may optionally have been added, supplied via a conveyor screw 4, whereupon the obtained mixture is introduced via a conveyor screw 6 into a reactor 7 designed as a hammer mill having a rotating shaft with hammers mounted thereon. Such hammer mills are well known for instance from the mining industry.

The reactor 7 has been started beforehand and has been made ready for the process, viz. by supplying foundry sand to the reactor and putting the shaft with hammers into rotation. The hammers are beating up the sand so that it is dispersed as a fluidized bed along the reactor wall. The friction between the sand and the rotating hammers brings about an increase in the temperature of the fluidized sand bed. When the temperature has reached the desired reaction temperature for the process, 5 the feeding of the mixture of polyethylene pellets, oil and sand via the conveyor screw 6 is started. The supplied mixture is dispersed in the fluidized bed of sand particles in the reactor 7 and the polyethylene is cracked to lighter hydrocarbons. 0

The reactor 7 may be operated at atmospheric pressure or at a somewhat higher pressure, e.g. at a pressure of up to 10 ata, and at temperatures which are preferably in the range of 250 °C to 400 °C. The peripheral speed of the hammers is suitably s in the range of 15 to 75 m/s.

Concurrently with the supply of the mixture of polyethylene, oil, sand and optionally water to the reactor 7, sand is withdrawn from the reactor through an outlet 12, so as to 0 maintain a constant level of sand in the reactor. Thus, the sand supplied with the polyethylene will eventually replace the original "starter sand" in the fluidized bed in the reactor.

5 The cracked polyethylene leaves the reactor 7 as a mixture of gas, oil mist and vapor containing minor amounts of sand particles and is passed via a line 8 to a cyclone 9 or other separation means for separation of the sand particles. The hydrocarbon-containing stream is condensed in a condensator 2 o and is then passed to a storage tank 11. The separated sand particles may be returned to the mixing unit 5 either directly (not shown) or via a regeneration unit 13.

The ratio of hydrogen to carbon (the H/C ratio) is higher in 5 the products obtained by the process than in the feedstock. This indicates that a hydrogenation has taken place. The source of the hydrogen required for such hydrogenation may either be thermal cracking of the feedstock and formation of coke having a low H/C ratio, or decomposition of H₂0. During the cracking reactions taking place in the reactor 7, the sand particles will be coated by a certain amount of coke and non-vaporized hydrocarbons. The sand particles are withdrawn from the bottom of the reactor 7 and are passed via a line 12 either directly back (not shown) to the mixing unit 5, or to the regeneration unit 13, wherein the coke is burned off in a gas consisting of air, oxygen-enriched air or air diluted with nitrogen. The sand particles, which are now substantially free from carbon deposition, may be returned to the mixing unit 5 to be mixed with additional amounts of feedstock. Optionally, the sand particles may be disposed of (15).

As mentioned, the used sand particles that have been treated in the regeneration unit 13 are practically free from hydro- carbons and if they are not to be reused in the process they may be utilized for example as a land fill, in cement production, or in asphalt production. The rest content of hydrocarbons on the sand particles after the treatment is < 500 ppm. Today's strictest maximum limit for admixture with soil (prac- tised by the Netherlands) is 800 ppm.

The invention is illustrated in the example below.

Example Tests were carried out in a hammer mill type of reactor to study the degradation of a polyethylene (PEL) in pellet form to liquid hydrocarbons. Three tests 1, 2 and 3 were carried out, in which tests the reactor was supplied with a mixture consisting of moist foundry sand, oil and varying amounts of polyethylene, viz. 0% by weight, 5% by weight and 15% by weight of polyethylene, respectively, calculated on the amount of sand. The oil was a crude oil which had been separated from cuttings. The mixing ratio of crude oil to moistened sand was in each test 1:10 on a weight basis. In all the tests the load on the engine operating the shaft with hammers was 90A, while the peripheral speed of the shaft was 37 m/s, and the bulk temperature was maintained at 250 °C. Neither in the test using 5% of admixed polyethylene, nor in the test using 15% of admixed polyethylene, could there be observed any residues of plastics in the reactor or in the liquid product obtained. Also, no increase in the hydrocarbon content on the sand particles could be observed, compared to the tests carried out with no addition of polyethylene.

In each test, the main part of the obtained liquid product belonged to the gas oil range and consisted of hydrocarbons in the C₁₂-C₁₈ range. Analyses of^~the liquid product by gas chromatography and subsequent mass spectro etry (GC-MS) indicated the following compositions:

Table

Tests

Amount of PEL in the mixture (% by weight of 0 15 the sand )

Composition of the liquid product ( % by weight ) :

Aliphates: 79.8 84.4 82.4

Mono-aromates : 7.0 4.0 4.8

Di-aromates: 2.9 2.7 2.2

Tri-aromates : 0.6 0.9 0.2

Others^*: 9.7 8.2 10.4

Components not identified by the analysis method and components outside the C₁₂-C_lβ range.

It can be seen from the Table that the composition of the liquid product changes when polyethylene (PEL) is added to the mixture of crude oil and sand, which indicates that the polyethylene has been cracked.

Claims

Patent claims

1. A process for degrading a feedstock consisting of plastics and/or other polymer material to liquid hydrocarbons, wherein the feedstock, optionally mixed with an oil, is subjected to a heating under agitation, characterized thereby that the supplied feedstock is mixed with solid particles and optionally with oil and/or water, and the feedstock in the obtained mixture is cracked in a hammer mill type of apparatus, in which the heat required for the cracking is supplied mechanically, whereupon the treated mixture is subjected to a separation to separate out at least a substantial part of its content of solid particles. 5

2. A process according to claim 1, characterized in that the added solid particles are mineral particles.

3. A process according to claim 2, characterized in that o sand is used as said mineral particles.

4. A process according to any of claims 1 to 3, characterized in that the feedstock is cracked in mixture with an oil. 5

5. A process according to claim 4, characterized in that the oil is a gas oil or heavier oil.

6. A process according to any of claims 1 to 5, 0 characterized in that the feedstock is cracked in mixture with water.

7. A process according to claim 6, characterized in that the feedstock is mixed with water in such amounts as to obtain 5 from 1 to 20% by weight, preferably from 5 to 15% by weight, of water in the mixture, calculated on the amount of the feedstock.

8. A process according to any of claims 1 to 7, characterized in that the treatment is carried out in a hammer mill at or near the atmospheric pressure, at a temperature in the range of 200 °C to 500 °C, and with a peripheral speed of the hammers of the hammer mill in the range of 15 to 75 m/s.

9. A process according to claim 8, characterized in that the treatment is carried out a temperature in the range of

250 °C to 400 °C.

10. A process according to any of claims 1 to 9, characterized in that the mixture is preheated before being introduced into the hammer mill.

11. A process according to any of claims 1 to 10, characterized in that the cracking is carried out in the presence of a free radical initiator, especially a peroxide.