GB2458690A

GB2458690A - Treatment of waste plastics material

Info

Publication number: GB2458690A
Application number: GB0805639A
Authority: GB
Inventors: Robert Marshall Prigmore; Steven Whitehouse; Alwyn Bowen
Original assignee: DYFODOL ENERGY Ltd
Current assignee: DYFODOL ENERGY Ltd
Priority date: 2008-03-28
Filing date: 2008-03-28
Publication date: 2009-09-30
Anticipated expiration: 2028-03-28
Also published as: GB0805639D0; EP2260088A1; WO2009118546A1; GB2458690B

Abstract

A method for the continuous treatment of waste plastic comprises; providing a plurality of pyrolysis units 10 connected in a continuous circuit, each unit having a central reaction chamber 14 in which a conveying screw 16 is mounted and a heating chamber 15 surrounding the central chamber; providing a bed 18 of substantially inert particulate heat sink material in the central chambers; operating the screws; supplying waste plastics materials to the centre chamber; and collecting gaseous product from at least one of the pyrolysis units downstream. The plastics material may be compressed in the form of a plug before addition to pyrolysis chamber. The heat sink material may be collected and separated from solid debris before reuse, may comprise a metal or metal oxide and may be in the form of powder or flakes. Apparatus for use with the method is also disclosed.

Description

Treatment of Waste Plastics Material The present invention relates to the treatment of waste plastics material and more specifically to a method of and apparatus specifically adapted to crack the hydrocarbon chains of the waste plastics material to form low molecular weight gases and liquids.

It is well known that plastics material comprising long chain hydrocarbon polymers of chain length up to 0500 can be broken down by heating into shorter chain hydrocarbons.

The hydrocarbon products obtained may range from gaseous C -05 hydrocarbons, though liquid products generally between 05 and C20 to longer chain semi-solid products, such as waxes. A range of chain lengths always results and process conditions need to be carefully controlled in order to obtain a majority of product within a narrow range of chain lengths which are liquid at ambient temperature and can be used as fuel, in other words in order to obtain a sufficient proportion of an economically useful product. A lengthy heating process, including a long period bringing the plastics material to an optimum reaction temperature of over 2000C at which breakdown of the long chain polymers occurs quite rapidly, results in a majority of less economically desirable short chain gaseous products. Rapid heating to a constant temperature (in the region of 25O) and subsequently cooling to stop the breakdown reaction are required to obtain an economic quantity of liquid hydrocarbon suitable as a fuel product.

It would be desirable to obtain liquid fuel product from waste plastics material in accordance with the above theoretical process, but in practice it is difficult to achieve on an economical scale. A batch process is known to have been used to treat about 250 kg per hour of waste plastics. Each batch is placed in a reaction vessel and heated rapidly to over 200°C. The resultant gases are collected and cooled.

An object of the present invention is to provide a continuous treatment process for waste plastics material, and plant for doing so, whereby liquid hydrocarbons for use as fuel can be recovered in economically worthwhile quantities.

According to a first aspect of the present invention a method of continuous treatment of waste plastics material comprises the steps of: providing a plurality of pyrolysis units which are connected in a continuous circuit, each unit having a central reaction chamber in which an elongate conveying screw is mounted and a heating chamber surrounding the central chamber; providing a bed of substantially inert particulate heat sink material in each central chamber; heating the respective pyrolysis units by way of their heating chambers; operating the conveying screws to move the particulate heat sink material around the circuit: supplying the waste plastics material to the central chamber of one or more of the connected pyrolysis units so that by action of the respective screws it is mixed with and conveyed with the heat sink material; and collecting gaseous and vapour products from at least one outlet of one of the pyrolysis units downstream in the circuit from the location of the waste material supply.

According to a second aspect of the present invention apparatus for the continuous treatment of waste plastics material correspondingly comprises: a plurality of pyrolysis units which are connected in a continuous circuit, each unit having a central reaction chamber in which an elongate conveying screw is mounted and a heating chamber surrounding the central chamber; a bed of substantially inert particulate heat sink material disposed in each central chamber; means for heating the respective pyrolysis units by way of their heating chambers; means for operating the conveying screws to move the particulate heat sink material around the circuit; means for supplying the waste plastics material to the central chamber of one or more of the connected pyrolysis units so that by action of the respective screws it is mixed with and conveyed with the heat sink material; and means for collection of gaseous product from at least one outlet of one of the pyrolysis units.

Providing a bed of particulate heat sink material within each of the reaction chambers of the apparatus, mixing it with the incoming waste plastics material, and moving it around the circuit of the apparatus enables very rapid heating of the incoming plastics material because of its intimate mixing with the hot particles. The temperature of the particulate material is maintained close to an optimum constant temperature by virtue of its movement around the circuit, continually receiving heat input to compensate for heat transmitted to the plastics material. The particulate material needs to be inert in the sense that it does not react chemically with the plastics material and is not consumed during the process. It also functions to prevent the plastics material, upon initial melting, sticking to the screw conveyor or the walls of the reaction chamber.

A preferred material for the particulate heat sink material is a metal and/or metal oxide which is a good heat conductor as heat may then be transmitted to the plastics material by conduction from the hot particles as well as by radiation from the walls of the reaction chamber and convection from gas within the reaction vessel. This enhances the rapidity of the heating of the plastics material.

A particularly advantageous material for the heat sink material has been found to be a mixture of iron and any oxide of iron. These materials are readily available in particulate form as magnetite, itself a waste product in the form of a powder and/or flakes. The relatively high density of iron and ferrous/ferric oxide as well as their good heat conductivity make them good heat sink materials.

They are non-reactive at the temperatures required for the breakdown of plastics (200-3OOC).

A bed of finely divided particulate material is, of course, desirable as the heat sink, as small particle size optimises the transfer of heat within the mixture in the reaction chamber, resulting in extremely rapid heating of the incoming waste plastics material.

It is desirable to minimise the amount of air inside each of the reaction chambers. Firstly, ingress of air with the waste plastics material may lead to cooling which is undesirable.

Secondly, a slight negative pressure within the reaction chambers stimulates vapourisation and assists rapid take off of the gaseous reaction products of the breakdown of the plastics material to small carbon chain units.

In order to minimize entry of air it is advantageous to compact the waste plastics to the form of a plug as it is being fed into the circuit of reaction chambers. Thus, the waste plastics is supplied to a hopper of conventional form, with a tapering base, but at the base it falls into an inlet passage in which it is compacted by action of an auger. The passage is also provided with a valve arrangement which allows air to flow out as items of plastics waste are compressed and crushed together, but prevents any ingress of air.

The resulting compacted plug of plastics material from which the air has been substantially removed is then fed into the circuit.

A further feature of the process and apparatus of the invention is that it is capable of dealing with plastics waste where some of the plastics is laminated with aluminium, as is the case with certain food and drink packaging.

When the plastics material is vapourised, the aluminium and any other non-volatile contaminants will remain in the bed of particulate material which is being moved around by the respective screw conveyors. At any suitable point in the system, namely the outlet end of any or all of the pyrolysis units (each having a central reaction chamber) the particulate bed can be fed into a separator, such as a drum filter. The mesh size of the filter is selected to allow the particulate material to pass through and continue to be fed back into the circuit, without losing too much heat, while any larger material, such as aluminium previously laminated and other debris will be retained and led to a collection vessel.

The form of the apparatus, which enables the process to be continuous, also allows considerable flexibility as to the position and number of locations for input of the plastics waste material, for take off of gaseous reaction products, and for take off of the particulate bed material for separation of other material, i.e. cleaning and recycling.

Thus, plastics material can be fed in from respective hoppers at the input (upstream) end of any or all of the pyrolysis units. The particulate bed material may be taken off and returned after separation of non-volatile larger size particles and debris at the output (downstream) end of any or all of the pyrolysis units. Gaseous product may be taken off at any selected one or more of a number of outlets along the length of each pyrolysis unit. It will be determined by trial which of these outlets is optimum for take off of vapour for condensation to provide the maximum amount of liquid hydrocarbon within the desired range of carbon chain length.

The invention will be described further, by way of example, by reference to the accompanying drawings, in which: Figure 1 is a perspective view of a practical embodiment of apparatus in accordance with the second aspect of the invention for carrying out the method in accordance with the first aspect of the invention; Figure 2 is an enlarged perspective view of one of the four substantially identical modules of the apparatus shown in figure 1; Figure 3 is a diagrammatic longitudinal cross-section of one of the four pyrolysis units of the apparatus shown in figure 1; Figure 4 is an enlarged scale fragmentary cross-section showing a portion of one pyrolysis unit with greater clarity than in figure 3; and Figure 5 is a corresponding transverse cross-section along line A-A of the pyrolysis unit shown in figure 4.

With reference to figure 1 a practical embodiment of apparatus for continuous treatment of waste plastics material comprises four substantially identical modules. A single module is shown in figure 2. Each module comprises a respective pyrolysis unit 10, 20, 30, 40 mounted at an inclination of between 2O and 30g. These pyrolysis units 10, 20, 30, 40 are connected to each other in a continuous circuit, the upper end of each unit connecting to the lower end of the adjacent unit by intervening pipe work and/or filtration devices. One such filtration device is indicated at 50 in figure 2 leading to a receptacle 52 for waste material, such as ash or other non-volatile debris, or aluminium from plastics laminates.. A respective combustion chamber 11, 21, 31, 41 in the form of a substantially cylindrical vessel is mounted substantially horizontally above each pyrolysis unit 10, 20, 30, 40 upon a framework formed of upright, horizontal and oblique beams 60. Hoppers 12, 22, 32, 42 of conventional shape with tapering lower sections are mounted to supply waste plastics material to a location near the lower end of each pyrolysis unit 1 0, 20, 30, 40. It is not necessary to supply waste plastics to each unit, but it is economical to do so. Access platforms 13, 23, 33, 43 are shown in figure 1 adjacent to each hopper.

With reference to figures 3, 4 and 5, each pyrolysis unit 10, 20, 30, 40 is of cylindrical form and has a central reaction chamber 14 surrounded by a heating chamber 15, which in turn is surrounded by an insulation jacket 19. Inside each reaction chamber 14 a conveying screw 16 is mounted. It is driven by a motor 17 (see figure 2) to convey material upwards along the reaction chamber from the lower end to the upper end thereof. Therefore, the lower end is an upstream end and the upper end a downstream end. A bed of particulate material, specifically a mixture of iron and iron oxides in powder and flake form is provided in the central reaction chamber 14 of each pyrolysis unit, as indicated at 18 in figure 3. This particulate bed 18 may take up 20%-40% of the internal volume of the central chamber 14. Upon operation of the apparatus it is conveyed around the circuit of the pyrolysis units 10, 20, 30, 40 and is intimately mixed with the incoming waste plastics material.

A number of gas take off ducts are provided in each pyrolysis unit 10, 20, 30, 40. In the illustrated embodiment there are four such gas take off ducts 24, 25, 26, 27. As shown in figures 4 and 5, these outlets lead directly from the reaction chamber 14 through the heating chamber 15 and insulation sleeve 19 to the exterior where respective outlet pipes lead to condenser apparatus (not shown). Valves are provided in the respective ducts 24, 25, 26, 27 so that gas may be taken off at any selected outlet.

Each combustion chamber 11, 21, 31, 41 is provided with a respective gas burner 34 and a respective circulating fan 35 whereby hot air is supplied via connecting duct 36 to the heating chamber 15 of the respective pyrolysis unit 10, 20, 30 or 40 mounted below.

Although not apparent in the drawings, the waste plastics material is supplied from each hopper 12-42 to the respective pyrolysis unit 10-40 by way of an inlet passage where the plastics is compacted by the action of an auger so that it is in the form of a plug upon entry to the respective reaction chamber 14. A valve in the compression chamber allows outflow of air but prevents ingress of air so that minimal air passes into the reaction chamber 14 with the plastics material and the reaction chamber is at a slightly negative pressure compared to ambient atmospheric pressure.

In operation the combustion chambers 11 to 41 heat the respective reaction chambers 14 to a temperature in the region of 25OC. The particulate heat sink material 18 is raised to the same temperature. As waste plastics material is fed in it is heated almost instantaneously to 250C. Thus, the hydrocarbon chains are broken and the smaller chain fragments vapourised. The vapour is collected via selected take off outlets 24-27 around the circuit and rapidly cooled in the condenser apparatus. This results in a high yield of the desirable liquid fuel hydrocarbons.

The foregoing is illustrative and not limitative of the scope of the invention. Variations in construction details of the apparatus and in processing conditions of the method are possible in other embodiments. In particular, any number of modules, any number of pyrolysis units may be connected in the continuous circuit. A minimum number would be three. Four units as in the illustrated embodiment is most convenient for practical purposes. However, more units than this may be connected if required.

Claims

CLAIMS1. A method of continuous treatment of waste plastics material comprising the steps of: providing a plurality of pyrolysis units which are connected in a continuous circuit, each unit having a central reaction chamber in which an elongate conveying screw is mounted and a heating chamber surrounding the central chamber; providing a bed of substantially inert particulate heat sink material in each central chamber; heating the respective pyrolysis units by way of their heating chambers; operating the conveying screws to move the particulate heat sink material around the circuit; supplying the waste plastics material to the central chamber of one or more of the connected pyrolysis units so that by action of the respective screws it is mixed with and conveyed with the heat sink material; and collecting gaseous product from at least one outlet of one of the pyrolysis units downstream in the circuit from the location of the waste material supply.
2. A method according to claim 1 including the further step of compressing the plastics waste material to the form of a plug prior to supplying same to the central chamber of one or more of the connected pyrolysis units to minimise the ingress of air to the or each chamber.
3. A method according to claim 1 or claim 2 including the further step of collecting the particulate heat sink material from at least one outlet of one of the pyrolysis units, separating same from any solid debris in the waste material being treated and returning same to one of the pyrolysis units.
4. A method according to claims 1, 2 or 3 wherein the particulate heat sink material is a metal and/or metal oxide.
5. A method according to claims 1, 2 or 3 wherein the particulate heat sink material is an iron and/or iron oxide.
6. A method according to any preceding claim, wherein the particulate heat sink material is in the form of powder or flakes.
7. Apparatus for the continuous treatment of waste plastics material comprising: a plurality of pyrolysis units which are connected in a continuous circuit, each unit having a central reaction chamber in which an elongate conveying screw is mounted and a heating chamber surrounding the central chamber; a bed of substantially inert particulate heat sink material disposed in each central chamber; means for heating the respective pyrolysis units by way of their heating chambers; means for operating the conveying screws to move the particulate heat sink material around the circuit; means for supplying the waste plastics material to the central chamber of one or more of the connected pyrolysis units so that by action of the respective screws it is mixed with and conveyed with the heat sink material; and means for collection of gaseous product from at least one outlet of one of the pyrolysis units.
8. Apparatus according to claim 7 wherein the central chamber of each pyrolysis unit extends at an inclination and the conveying screw therein is mounted to convey the matenal upwardly along said chamber.
9. Apparatus according to claim 7 or claim 8 wherein the means for supplying the waste plastics material to the central chamber of one or more of the connected pyrolysis units comprises a hopper leading into an inlet passage and an auger mounted in the passage so as to compress the waste plastics material to the form of a plug prior to feeding same into the central chamber of the respective pyrolysis unit.
10. Apparatus according to any of claims 7 to 9 further including at least one filtration device mounted between downstream and upstream ends of the reaction chambers of adjoining pyrolysis units in order to separate and remove any solid debris from the particulate heat sink material.
11. Apparatus according to any of claims 7 to 10 wherein the particulate heat sink material is a metal and/or metal oxide.
12. Apparatus according to any of claims 7 to 10 wherein the particulate heat sink material is an iron and/or iron oxide.
13. Apparatus according to any of claims 7 to 10 wherein the particulate heat sink material is in the form of powder or flakes.
14. Apparatus for the continuous treatment of waste plastics material substantially as hereinbefore described with reference to and as illustrated by the accompanying drawings.