GB2131044A - Method and apparatus for pyrolysis of atactic polypropylene - Google Patents

Abstract

The pyrolytic decomposition of polymeric materials 10 into lower molecular weight products is carried out within reactor conduits 20, 20' submerged in a fluidized bed furnace 22 operated at pyrolizing temperatures. <IMAGE>

Description

SPECIFICATION Method and apparatus for pyrolysis of atactic polypropylene This invention relates to a process and apparatus for the pyrolytic decomposition of polymeric materials, and especially to the production of fuel oils and other useful products from atactic polypropylene, and to a method of cleaning parts of the said apparatus, especially a method permitting continuous operation of the said process during cleaning.

The ever increasing production of waste polymeric materials as by-products of industrial processes, and the like, has created a well recognized need for the disposal of such materials preferably providing some economical commercial use for them.

The energy content of most polymeric waste materials makes them potentially useful as fuels.

However, many higher and intermediate molecular weight polymeric materials, for example, atactic polypropylene, are at room temperature semisolids which are difficult to feed and atomize and hence not suitable for direct burning in conventional systems. Various methods of thermally decomposing these polymeric waste materials into lower molecular weight fragments that are easy to handle and have economic value such as fuel oils and raw materials for industry have previously been proposed; for example, see United States Patents Nos. 3,829,558, 3,832,151 and 4,151,216. A major problem with previously proposed processes is accumulation of byproducts, and in particular of carbonaceous materials, on the heat transfer surfaces of the thermal reactors.Build-up of these materials on the heat transfer surfaces limits their efficiency and requires batch type operation or periodic shutdowns for cleaning. The non-uniform heating characteristics of conventional furnaces contribute to this problem by creating hot spots, which promote the accumulation of carbonaceous deposits, on heat transfer surfaces along the path of waste materials to be thermally decomposed.

None of the techniques previously proposed for dealing with this problem such as lower reaction temperatures, dispersal of accumulated carbon, and discharge of carbon-rich fractions of the reactor material have sufficiently eliminated this problem to create a commercially viable continuous process.

The invention provides apparatus for the pyrolytic decomposition of polymeric materials, which comprises means operable in use to provide a fluidized bed at pyrolysing temperatures having at least one reactor conduit disposed therein, the reactor conduit having an input opening and output opening; and, means for causing polymeric material to move through the reactor conduit from the input opening towards the output opening to expose it, in operation, to pyrolysing conditions causing it to decompose into lower molecular weight fragments.

The invention also provides a method for pyrolysing polymeric materials, which comprises: passing the polymeric material through a reactor conduit disposed in a fluidized bed, the fluidized bed being maintained at an elevated temperature so that the polymer material is exposed to pyrolysing temperatures and decomposes into lower molecular weight fragments.

The invention further provides a method for cleaning carbonaceous deposits from surfaces in thermal communication with a fluidized bed, which comprises: heating the deposits to temperatures at which they will combust when exposed to oxygen; exposing the deposits to oxygen causing their combustion; and permitting at least a portion of the heat of combustion to flow into the fluidized bed to prevent overheating of the surfaces.

In accordance with the present invention polymeric waste material, such as atactic polypropylene, may be melted in a heated tank to a viscosity at which it may be pumped at desired pressures preferably from 50-250 psig (350 to 1 750 kPa). The melted material is pumped through thermally insulated pipes to a reactor conduit, preferably comprising two or more independent reactor tubes and, more preferably, helically coiled tubes, of predetermined size wherein it is thermally decomposed by heat, that is to say, pyrolysis, to lower molecular weight fragments, relative to the molecular weight of the parent molecules on the polymeric material, in the absence of oxygen for a selected period of time.

The reaction time is determined by the dimensions of the reactor tubes and rate of flow of raw material therethrough. The reactor tubes and materials therein are uniformly heated to precise temperatures by a fluidized bed. The pyrolyzed product discharges from the reactor tubes into a separation means, for example, a flash distillation device, whereby the product polymer fragments are separated in groups substantially in accordance with their molecular weight. In the case of atactic polypropylene reacted at 8000F (4250 C) for about 10 minutes, the principal products would be No. 6 and No. 2 fuel oils as defined in American Society for Testing and Materials standard No. D 396-80 and some lighter gaseous fuels that are preferably used for fueling the heating means for the melt tank and fluidized bed.

It is an object of the present invention to provide a system for the pyrolytic decomposition of poiymeric materials to lower molecular weight fragments that produces uniform products facilitated by a precise control of temperature uniformity and level within reactor tubes.

It is a further object of the present invention to provide an efficient and economical system for producing fuel oils from polymeric materials.

It is a further object of the present invention to provide a system suitable for a substantially continuous operation wherein one or more of the reactor tubes may be cleaned as hereinafter described without influencing the operation of other tubes.

One form of apparatus constructed in accordance with the present invention, and a process using it, will now be described by way of example only with reference to the accompanying drawing, the single Figure of which is a schematic diagram of a system for the thermal decomposition of atactic polypropylene. In the drawing certain fittings, valves, instruments, heaters, agitators, pumps and the like have been omitted for purposes of clarity and they may be provided in any suitable conventional manner where necessary or desirable.

Referring to the drawing, the system comprises a heated melt tank 10 connected to a pump 14 by a conduit 12. The pump 14 discharges into a conduit 1 6 which divides into separate feed lines 1 8 and 18', one for each of corresponding reactor tubes 20 and 20', and each feed line is provided with a valve 1 9 or 19', respectively. The reactor tubes 20 and 20' are preferably helical coils disposed as hereinafter described in a fluidized bed furnace 22 which comprises an enclosure 24 having a distributor plate at its lower end that divides the enclosure into a lower plenum 28 and an upper bed zone 30. The lower plenum 28 is provided with a burner system 32 for heating air to be passed upwardly through the distributor plate 26 into the bed zone 30.A solid particulate bed medium is disposed in the bed zone 30 so that in operation it becomes suspended in the hot gas passing upwardly through the distributor plate 26 creating a fluidized bed 33 and thereby ensuring that heat is transferred to the reactor tubes 20 and 20' engulfed therein. The fluidizing air discharges from the enclosure 24 through a conduit 34 into a separator 36, preferably a cycione, which removes entrained solids from the exhaust gas and discharges the gas into the atmosphere. The reactor tubes 20 and 20' discharge the product into a separator 38, for example, a flash distillation device, through conduits 37 and 37'. A conduit 42 connects the lower portion of the separator 38 with a cooler 44 which leads to a first storage tank 46 for a higher molecular weight product.A conduit 48 connects the separator 38 to a condenser device 50 having a first outlet conduit 52 for low molecular weight gaseous products and a second outlet conduit 54 for intermediate molecular weight liquid products, those outlets being connected to appropriate storage facilities, for example, second and third storage tanks 56 and 57, respectively.

In a preferred process according to the present invention, 'atactic polypropylene', that is to say, a partially crystalline material which forms a solid or semi-solid at room temperatures which is composed of a mixture of waste by-products from the commercial preparation of polypropylene, is converted by thermal decomposition into the said No. 6 and No. 2 fuel oils and other useful materials.Advantageously, the waste atactic polypropylene from a commercial polypropylene plant is collected in the melt tank 10 wherein it is heated usually to about 4000F (200"C) until it becomes liquid enough to be pumped at 50-250 psig (350 to 1750 kPa) to the reactor tubes 20 and 20' wherein it is heated to sufficient temperatures to break carbon-carbon bonds in the waste material (approximately 8000F (4250C) for a sufficient time) to produce the desired products.

These products are usually 90% by weight of liquid and 10% of gaseous fuels at about 250C, and are discharged from the reactor tubes 20 and 20' into the separator 38 wherein the liquid fractions are separated into a heavy (high viscosity) portion, and a mixture of light (low viscosity) portions and the remaining gases which is sent to the condenser 50 where the light (low viscosity) portion is condensed and the remaining gases are discharged to a suitable receptacle.

These gases are preferably used to fuel the heaters for the melt tank 10 and the fluidized bed 33.

Though the extremely precise and uniform heating by the fluidized bed substantially reduces the amounts of carbonaceous deposits formed in the reactor tubes after extended periods of operation these by-products collect on the interior surfaces of the reactor tubes causing clogging and reducing the heat transfer rate from the fluidized bed to materials in the tubes.

The reactor tubes 20 and 20' are separately supplied with polymer material, nitrogen and air so that they may be 'burnt out', i.e. cleaned, individually without interrupting the processing in the other reactor coil or coils thus providing a continuous process.

By way of example, the 'burn out' operation is effected in the described system by cutting off the flow of atactic polypropylene to the selected reactor tube 20 in the operating system by closing the feed valve 1 9 and opening a purge inlet valve 60 to admit an inert gas, preferably nitrogen, thus forcing any feed product and/or pyrolysis product in the tube 20 onward, clearing that part of the system. Shortly after that, a product discharge valve 62 is closed cutting off the flow of product and/or purge gas to the separator 38 and a purge discharge valve 64 opened to permit the nitrogen purge gas to be exhausted into the atmosphere, or into the plenum chamber 28 of the fluidized bed furnace 22 for combustion of any pyrolysis products before discharge to atmosphere. An air inlet valve 66 is then opened permitting oxygencontaining gas to enter the reactor tube 20 causing spontaneous combustion of any carbonaceous build-up remaining in the tube 20 after the nitrogen purge. The nitrogen inlet valve 60 may be closed at this point to accelerate the combustion by increasing the available oxygen.

The heat of combustion would normally cause excessive temperatures damaging or destroying the reactor tube in conventional systems. In the present apparatus, the temperature of the reactor tubes is kept at a safe level by the fluidized bed which efficiently carries the excess heat away preventing damage from overheating caused by the heat combustion of the carbonaceous deposits.

When all the carbonaceous material has been burnt out of the tube 20, the tube is returned to service by closing the air inlet valve 66 and purging the reactor tube 20 with nitrogen until all oxygen is exhausted. The discharge valve 64 and the nitrogen inlet valve 60 are then closed and the feed valve 1 9 is reopened permitting polymeric material to flow into the tube. Finally, the product discharge valve 62 is reopened restoring tube 20 to full operation.

As is noted above, the other reactor tube 20' remains in operation unaffected by the burn-out of the tube 20. When the reactor tube 20 is returned to full service, the tube 20' may be burnt out without affecting tube 20 by following the same procedure outlined above on the corresponding valves for that tube (some of which have been omitted from the drawing in the interests of clarity). It will be appreciated that systems having a plurality of reactor tubes, preferably two to six, are contemplated for the present invention and that one of these tubes may be 'burnt out' at one time by obvious modification of the method described above.

Claims (22)

1. Apparatus for the pyrolytic decomposition of polymeric materials, which comprises means operable in use to provide a fluidized bed at pyrolysing temperatures having at least one reactor conduit disposed therein, the reactor conduit having an input opening and output opening; and, means for causing polymeric material to move through the reactor conduit from the input opening towards the output opening to expose it, in operation, to pyrolysing conditions causing it to decompose into lower molecular weight fragments.

2. An apparatus as claimed in claim 1, which comprises a tank with means for heating the tank for melting the polymeric material and discharging it into the reactor conduit.

3. Apparatus as claimed in claim 1 or claim 2, wherein the means for causing the polymeric material to move through the reactor conduit is a pump.

4. Apparatus as claimed in any one of claims 1 to 3, wherein there are provided a plurality of reactor conduits and means for regulating the supply of polymeric material to the input openings of the difference conduits independently of each other.

5. Apparatus as claimed in claim 4, which comprises means for providing a regulated supply of inert gas to the input opening of each reactor conduit; and means for providing a regulated supply of oxygen-containing gas to the input opening of each reactor conduit.

6. Apparatus as claimed in any one of claims 1 to 5, comprising separator means for receiving pyrolysed polymeric material discharged from the reactor conduit output opening and separating it into fractions.

7. Apparatus as claimed in claim 6, wherein the separator means is a flash distillation device.

8. Apparatus for the pyrolytic decomposition of polymeric materials, substantially as hereinbefore described with reference to, and as shown in, the accompanying drawing.

9. A method for pyrolysing polymeric materials, which comprises: passing the polymeric material through a reactor conduit disposed in a fluidized bed, the fluidized bed being maintained at an elevated temperature so that the polymeric material is exposed to pyrolysing temperatures and decomposes into lower molecular weight fragments.

10. A method as claimed in claim 9, comprising the step of melting the polymeric material prior to passing it through the reactor tube.

11. A method as claimed in claim 9 or claim 10, comprising the step of separating the polymeric material fragments by sizes when they emerge from the reactor tube.

12. A method as claimed in any one of claims 9 to 11, comprising the step of cleaning carbonaceous deposits from the reactor conduit by exposing the deposits to oxygen at a sufficiently high temperature to cause their combustion and at least a portion of the heat of combustion flowing into the fluidized bed so preventing overheating of the conduit.

13. A method as claimed in claim 12 using a plurality of reactor conduits, wherein the cleaning step is carried out in at least one reactor conduit while polymeric material is pyrolytically decomposed in at least one other reaction conduit so that the apparatus continuously produces the lower molecular weight fragments.

14. A method as claimed in claim 12 or claim 13, comprising the step of purging the reactor conduit or conduits with an inert gas before exposing the carbonaceous deposits therein to oxygen.

1 5. A method as claimed in claim 14, wherein the inert gas is nitrogen.

1 6. Method as claimed in any one of claims 12 to 15, comprising the step of purging the reactor conduit or conduits with an inert gas after carbonaceous deposits have been combusted therein.

17. A method as claimed in claim 16, wherein the said inert gas is nitrogen.

18. A method as claimed in any one of claims 12 to 17, wherein the oxygen is supplied by introducing air into the reactor conduit or conduits.

19. A method for pyrolysing polymeric materials, substantially as hereinbefore described with reference to the accompanying drawing.

20. A method for cleaning carbonaceous deposits from surfaces in thermal communication with a fluidized bed, which comprises: heating the deposits to temperatures at which they will combust when exposed to oxygen; exposing the deposits to oxygen causing their combustion; and permitting at least a portion of the heat of combustion to flow into the fluidized bed to prevent overheating of the surface.

21. A method for cleaning carbonaceous deposits from surfaces substantially as hereinbefore described with reference to the accompanying drawing.

22. Any novel feature or combination of features herein disclosed.