Process and apparatus for treating waste comprising mixed plastic waste

Abstract

A process for treating waste comprising Mixed Plastic Waste is disclosed. The process includes feeding the waste to a pyrolysis reactor, pyrolysing the waste in the pyrolysis reactor to produce a fuel and using the fuel to run a generator to produce electricity.

Claims

1. A process for treating waste comprising mixed plastic waste, the process comprising: a. feeding the waste to a pyrolysis reactor, wherein the pyrolysis reactor is a fluidised bed reactor; b. pyrolysing the waste in the pyrolysis reactor to produce a pyrolysis product; c. passing the pyrolysis product through a condenser to form a liquid fraction and a gas fraction; d. measuring an attribute of the liquid fraction, wherein the measured attribute of the liquid fraction is hydrocarbon chain length; e. adjusting the temperature and/or residence time of the pyrolysis reactor in response to the measured attribute of the liquid fraction to maintain the attribute of the liquid fraction within a desired range; and f. storing the liquid fraction as a liquid, a solid, or a mixture of a liquid and a solid in a tank.

2. The process according to claim 1, wherein the process comprises using the liquid fraction as a fuel to run a generator to produce electricity.

3. The process according to claim 2, wherein the fuel is produced continuously and the generator is run intermittently.

4. The process according to claim 1, wherein the process includes storing the waste in a vessel prior to feeding the waste to the pyrolysis reactor, wherein the waste is further mixed while stored in the vessel.

5. The process according to claim 1, wherein the temperature in the pyrolysis reactor is controlled such that the liquid fraction comprises hydrocarbons having a chain length greater than C.sub.5.

6. The process according to claim 1, wherein at least a portion of the pyrolysis product is combusted to heat a fluid, and the fluid is fed into the pyrolysis reactor to heat the pyrolysis reactor.

7. The process according to claim 1, wherein the process treats from 5,000 to 20,000 tonnes per year of waste.

8. The process according to claim 1, wherein the pyrolysis reactor contains a fluidised bed of particles and a distributor for feeding a fluidisation medium into the reactor, wherein the distributor is configured such that the particles can fall through the distributor and wherein the process comprises removing a portion of the particles that have fallen through the distributor, cleaning the particles and feeding the particles back into the reactor.

9. The process according to claim 1, wherein the waste comprises greater than 80 wt % mixed plastic waste.

10. The process according to claim 1, wherein the process additionally comprises: g. mixing the liquid fraction while it is in the tank.

11. A process for treating comingled plastics, the process comprising: a. feeding the comingled plastics to a fluidised bed pyrolysis reactor; b. pyrolysing the comingled plastics in the pyrolysis reactor to produce a pyrolysis product; c. passing the pyrolysis product through a condenser to form a liquid fraction; d. measuring an attribute of the liquid fraction, the attribute being hydrocarbon chain length; e. adjusting the temperature and/or residence time of the pyrolysis reactor in response to the measured attribute of the liquid fraction to maintain the attribute of the liquid fraction within a desired range; and f. storing the liquid fraction as a liquid, a solid, or a mixture of a liquid and a solid in a tank.

12. The process according to claim 11, wherein the step of storing the liquid fraction in a tank comprises storing the liquid fraction in a plurality of tank containers.

13. The process according to claim 11, wherein the process includes storing the comingled plastics in a vessel prior to feeding the comingled plastics to the pyrolysis reactor, wherein the comingled plastics are further mixed while stored in the vessel.

14. The process according to claim 11, wherein the process includes drying the comingled plastics in a dryer prior to feeding the comingled plastics to the pyrolysis reactor.

15. The process according to claim 11, wherein the temperature in the pyrolysis reactor is controlled such that the liquid fraction comprises hydrocarbons have having a chain length in the range C.sub.5 to C.sub.40.

16. The process according to claim 11, wherein at least a portion of the pyrolysis product is combusted to heat a fluid, and the fluid is fed into the pyrolysis reactor to heat the pyrolysis reactor.

17. The process according to claim 11, wherein the pyrolysis reactor contains a fluidised bed of particles and a distributor for feeding a fluidisation medium into the reactor, wherein the distributor is configured such that the particles can fall through the distributor and wherein the process comprises removing a portion of the particles that have fallen through the distributor, cleaning the particles and feeding the particles back into the reactor.

Description

DESCRIPTION OF THE DRAWINGS

(1) Embodiments of the present invention will now be described by way of example only with reference to the accompanying schematic drawings of which:

(2) FIG. 1 is a schematic view of an apparatus according to a first embodiment of the invention;

(3) FIG. 2 is a schematic view of a distributor in the pyrolysis reactor of FIG. 1; and

(4) FIG. 3 is a schematic view of a control system for the apparatus of FIG. 1.

DETAILED DESCRIPTION

(5) In FIG. 1 an apparatus 1 for treating waste comprising Mixed Plastic Waste has a loading conveyer 3. The loading conveyer 3 feeds a de-water press 5, the outlet of which is arranged above a conveyer 7 to a shredder 9. The outlet of the shredder 9 is directed at a conveyer 11 to a filter 13, which includes a ferrous and non-ferrous filter. The outlet of the filter 13 is connected to a dryer 15 and the outlet of the dryer 15 is connected to a storage tank 17. The storage tank 17 is fed by conveyer 19 and comprises a blending system. The outlet of storage tank 17 is connected via line 21 to the inlet of a fluidised bed pyrolysis reactor 23. The reactor 23 contains sand, which forms the fluidised bed. The bottom of the reactor 23 has a valve connected, via line 25, to a cleaner 27, which in turn is connected, via line 29 to a hopper 31. The hopper 31 feeds into the reactor 23. The outlet from the top of the reactor 23 is connected to hot gas filter 33, the outlet of which is connected, via line 35, to condenser 37.

(6) From line 35, a branch 39 is connected, via pump 41, to heat exchanger 43. From heat exchanger 43 a line 45 feeds into reactor 23. Fuel supply line 49 runs from line 35 to burner 53. Fuel supply line 47 runs from the top of condenser 37 to burner 51. Burners 51 and 53 are mounted on heat exchanger 43.

(7) Condenser 37 is on top of, and feeds into, buffer tank 55. An outlet 57 from the buffer tank 55 is connected, via pump 59, to engine 61, which is attached to generator 63. Engine 61 and generator 63 together form a generator to produce electricity and heat. Cooling water loop 65 runs through engine 61 and heat exchanger 67. Water line 69 passes through heat exchanger 67 and on to another part of the plant. The exhaust 71 from the engine 61 passes through heat exchanger 73 and then filter 75 before being exhausted to the atmosphere. Air line 77 passes through heat exchanger 73 and on to another part of the plant.

(8) In FIG. 2, the lower part of the reactor 23 has a distributor 79. The distributor 79 is at the bottom of the fluidised bed 85. The distributor 79 comprises an array of ducts 81a-e with orifices 89a-e in their upper surface. The orifices comprise nozzles 83a-e, which sit on top of the orifices. The ducts 81a-e are spaced apart such that the gaps between the ducts are large enough for particles from the fluidised bed 85 to fall through. At the bottom of the reactor 23 there is a valve 87 leading to line 25.

(9) In FIG. 3 a solvent monitor 91 is mounted on line 35. The solvent monitor 91 is in communication with reactor management system 93. Temperature monitor 99 is in reactor 23 and is also in communication with reactor management system 93. Reactor management system 93 is in communication with gas burner control 95, which controls burners 51 and 53, and material feed control 97, which controls the feed rate from line 21 into reactor 23.

(10) In use, 7000 tonnes per year of waste, in this embodiment Mixed Plastic Waste, is loaded continuously onto the loading conveyer 3. The waste travels up the loading conveyer 3 and drops into the de-water press 5, where the pressing action forces water out of the waste. The dried waste, which has a moisture content of about 15% by weight exits the de-water press 5 and is conveyed along conveyer 7 and into the shredder 9, where it is shredded. The shredded waste exits shredder 9 and travels along conveyer 11 to filter 13. Filter 13 comprises ferrous and non-ferrous filters and removes metallic contaminants from the waste. The de-watered, shredded and filtered waste then passes into dryer 15, where the water content is reduced to about 2-3 wt %. The dryer 15 is powered by heat from hot water line 69 or hot air line 77, or both. On exiting the dryer 15, the dry, shredded, filtered waste is stored in storage tank 17. Whilst in the storage tank 17, the waste is constantly blended by withdrawing a portion of the waste from the bottom of one end of storage tank 17 and recirculating it to conveyer 19 to be redistributed across the top of storage tank 17. With waste also being continuously added to and withdrawn from storage tank 17 to feed the process, the effect of the blending recirculation is to smooth variations in the composition of the waste over time.

(11) Waste is withdrawn from the storage tank 17 along line 21 and fed to the fluidised bed pyrolysis reactor 23. On entering the reactor 23, the waste is heated to around 400 to 600 C. The heating is achieved by feeding a hot stream into the reactor 23 along line 45. That hot stream comprises pyrolysis product drawn from line 35, along line 39, and heated indirectly by combustion of a portion of the pyrolysis product, which is also drawn from line 35, along line 47 or 49. The portion of the pyrolysis product combusted is normally drawn along line 47 from the top of the condenser 37 and comprises the gas fraction of the fuel output stream 35 from the pyrolysis reactor 23 that does not condense in the condenser 37. When extra fuel is required, it is drawn directly from the fuel output stream in line 35 along line 49. In that case, some of the fuel product that would in normal circumstances be used to run the engine 61 is being used to heat the pyrolysis reactor 23.

(12) The heated waste undergoes a pyrolysis reaction that decreases the hydrocarbon chain lengths to around C.sub.5 to C.sub.100. The process is carried out in a fluidised bed 85 of sand, which results in good mixing and even temperature across the reactor 23. The sand becomes contaminated with by-products over time. To prevent excessive build-up, a portion of the sand is continuously withdrawn from the bottom of reactor 23 along line 25 and cleaned in cleaner 27. The cleaned sand is reheated and fed back into the reactor 23 via line 29 and hopper 31.

(13) The products of the pyrolysis reaction exit the top of the reactor 23 and pass through hot gas filter 33. The filter 33 removes chemical contaminants such as chlorides (resulting from PVC in the Mixed Plastic Waste) and sulphates, resulting in a clean fuel gas which flows along line 35 and is condensed into buffer tank 55 by condenser 37.

(14) The quality of the fuel in line 35 is monitored continuously by solvent monitor 91. Solvent monitor 91 measures a flame temperature resulting from burning a sample of the fuel in a hydrogen flame. The temperature of the flame can be related to the heat of combustion of the fuel. Solvent monitor 91 communicates the flame temperature to reactor management system 93 by means of an electronic signal from a thermocouple in solvent monitor 91. Reactor management system 93 also receives a signal from temperature monitor 99 in the pyrolysis reactor 23. Reactor management system 93 responds to changes in the flame temperature of solvent monitor 91 by adjusting the operation of burners 51 and 53 and the feed rate from line 21 into pyrolysis reactor 23 by means of gas burner control 95 and material feed control 97. In that way, reactor management system 93 can adjust the temperature and/or the residence time of the pyrolysis reactor 23. Thus, if solvent monitor 91 detects that the flame temperature is falling, indicating that the quality of the fuel is falling, the reactor management system 93 increases the temperature in the reactor 23, or increases the residence time in the reactor 23, or both. The average chain length of the fuel in the output from the reactor 23 should then decrease and the quality of the fuel increase. Conversely, if the solvent monitor 91 indicates that the heat of combustion of the fuel is rising, which could cause problems in engine 61, the reactor management system 93 can reduce the temperature or residence time or both of the reactor 23 so as to decrease the pyrolysis of the waste and maintain the flame temperature of the solvent monitor 91 within its desired range.

(15) The level of fuel in buffer tank 55 can be allowed to increase and decrease. Thus the reactor 23 can be run continuously at a constant steady state, but the fuel can be used in a discontinuous way or at a varying rate. At night, when the demand for electricity is low, the level of fuel in the buffer tank 55 rises, whilst at times of peak demand the level of fuel in the buffer tank 55 can be allowed to decrease.

(16) The fuel in buffer tank 55 is continuously recirculated so as to mix the fuel and smooth temporal variations in the quality of the fuel entering the tank 55 from condenser 37. The recirculation also helps to smooth spikes in contaminant concentrations that could otherwise lead to undesirable short term emissions levels.

(17) The fuel from buffer tank 55 is used to run engine 61, which is connected to generator 63. Together, engine 61 and generator 63 form a generator that is run on the fuel to produce electricity. Engine 61 is a marine diesel engine designed to run on bunker fuel and the temperature in reactor 23 is controlled by monitoring the fuel entering condenser 37 so as to achieve the correct fuel specifications for engine 61. By combining the fuel generation process with the electricity generation in a single process, the fuel specification can be relaxed. Engine 61 can be selected based in part on its ability to handle fuel of varying specification with the result that the acceptable specification for the fuel in buffer tank 55 can be broader than if the fuel was to be sold as commercial fuel.

(18) The engine 61 requires cooling and generates hot exhaust gases. The heat from those streams can be captured and used elsewhere in the host facility. In this embodiment, the heat is used in dryer 15 and also in other processes in the host facility. Thus the process provides combined heat and power to the facility. Cooling water circulates through the engine in cooling water line 65. The cooling water cools the engine 61, and is heated in that process. The cooling water then passes to heat exchanger 67, where it is cooled by indirect contact with cool water entering heat exchanger 67 along line 69. The cooled cooling water exits heat exchanger 67 and returns to the engine 61 to repeat the cycle. The water heated in the heat exchanger 67 exits along line 69 and is used to provide heat to the dryer 15 and also to other processes in the host facility. The hot exhaust from the engine 61 is cooled by indirect contact with an air stream in heat exchanger 73. The exhaust gases exit the engine 61 along exhaust line 71 and pass through the heat exchanger 73. Air stream 77 also passes through heat exchanger 73 and heat is passed from the exhaust to the air stream. The exhaust gases then continue along exhaust line 71, through filter 75 to remove contaminants and particulates and are vented to the atmosphere. The air stream 77 that has been heated in heat exchanger 73 is directed to the dryer 15, where the heat in the stream is used to dry the incoming waste, and also to other processes in the host facility that use heat.

(19) Whilst the present invention has been described and illustrated with reference to particular embodiments, it will be appreciated by those of ordinary skill in the art that the invention lends itself to many different variations not specifically illustrated herein. By way of example only, certain possible variations will now be described.

(20) Engine 61 and generator 63 may be replaced by other generator systems. For example, in some embodiments a turbine may be used. In some embodiments two generators are provided. A small generator runs continuously to provide a base level of electrical power to run the remainder of the recycling facility in which the apparatus is installed. A large generator is turned on when the national electricity grid requires short-term supplies of electricity. The large generator is selected so as to have a quick start-up cycle so as to benefit from the higher price that national grids are willing to pay for electrical generating capacity that is available at short notice.

(21) In some embodiments the de-watering press 5, shredder 9 and filter 13 are provided in a different order. In some embodiments the waste may be shredded first and then de-watered and filtered. In other embodiments the three steps may be performed in other orders.

(22) In some embodiments the de-watering press 5 is replaced by another de-watering system such as a de-watering centrifuge.

(23) In some embodiments, the fuel gas that is heated to be fed back into the reactor 23 via line 45 in order to heat the incoming waste is drawn from upstream of the hot gas filter 33. The volume of gas passing through the hot gas filter 33 in such embodiments is reduced as a result.

(24) In some embodiments the waste fed to the process comprises Mixed Plastic Waste, but also comprises organic material. In some embodiments the waste is Municipal Solid Waste.

(25) In some embodiments the fuel burners 51 and 53 are replaced or supplemented by burners designed to burn the char separated from the fluidised bed sand in cleaner 27. In that way the char by-product of the pyrolysis process is used to heat the reactor 23.

(26) In some embodiments the buffer tank is a plurality of intermodal 20 ft tank containers. In an example process, 1 tonne (1000 kg) per hour of waste may be fed to the process and 850 kg per hour of fuel produced by the reactor. A single 20 ft tank container provides enough storage for around 24 hours of operations with the generator off, for example if the generator is undergoing maintenance.

(27) In another example embodiment the fluidised bed reactor has a 1.5 m diameter and a 1:1 aspect ratio (diameter to height ratio). The reactor contains 3 tonnes of sand.

(28) Where in the foregoing description, integers or elements are mentioned which have known, obvious or foreseeable equivalents, then such equivalents are herein incorporated as if individually set forth. Reference should be made to the claims for determining the true scope of the present invention, which should be construed so as to encompass any such equivalents. It will also be appreciated by the reader that integers or features of the invention that are described as preferable, advantageous, convenient or the like are optional and do not limit the scope of the independent claims. Moreover, it is to be understood that such optional integers or features, whilst of possible benefit in some embodiments of the invention, may not be desirable, and may therefore be absent, in other embodiments.