Method and apparatus for pyrolysis of atactic polypropylene

This invention relates to an apparatus and a method for pyrolytic decomposition of polymeric materials into lower molecular weight products involving the heat treatment of raw polymeric material within reactive conduits submerged in a fluidized bed furnace operated at pyrolizing temperatures.

This invention relates to a process and an apparatus for the pyrolytic 
decomposition of polymeric materials and, in particular to the production 
of fuel oils and other useful products from atactic polypropylene. 
BACKGROUND OF THE INVENTION 
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 heat content of most polymeric waste materials makes them potentially 
useful as fuels. However, many higher and intermediate molecular weight 
polymeric materials are semi-solids at room temperature, e.g. atactic 
polypropylene, 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 are known in the art, for example 
see: U.S. Pat. Nos. 3,829,558, 3,832,151 or 4,151,216. A major problem 
with the known processes is accumulation of by-products, in particular, 
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 shut downs 
for cleaning. The non-uniform heating characteristics of conventional 
furnaces contributes to this problem by creating hot spots on heat 
transfer surfaces along the path of waste materials to be thermally 
decomposed which promotes the accumulation of carbonaceous deposits. None 
of the techniques proposed in the prior art 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. 
SUMMARY OF THE INVENTION 
In the present invention polymeric waste materials, such as atactic 
polypropylene, maybe melted in a heated tank to a viscosity at which it 
may be pumped at desired pressures preferably from 50-250 psig. The melted 
material is pumped via thermally insulated pipes to a reactor conduit, 
preferably, two or more independent reactor tubes and, more preferably, 
helically coiled tubes, of predetermined size wherein it is thermally 
decomposed by heat i.e. 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 
fragments are separated in groups substantially in accordance with their 
molecular weight. In the case of atactic polypropylene reacted at 
800.degree. F. for about 10 minutes, the principal products would be No. 6 
and No. 2 fuel oils 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 polymeric 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. 
With the above and other incidental objects in view as will more fully 
appear herein, the invention intended to be protected by Letters Patent 
consists of the features of construction, the parts and combinations 
thereof, and the mode of operation as hereafter described or illustrated 
in the accompanying drawings, or their equivalents.

DETAILED DESCRIPTION OF THE INVENTION 
As shown in FIG. 1, the system comprises a heated melt tank 10 connected to 
pump 14 by conduit 12. Pump 14 discharges into conduit 16 which divides 
into separate feed lines 18 and 18' for each of the corresponding reactor 
tubes 20 and 20' and each feed lines is provided with a valve 19 and 19' 
respectively. The reactor tubes 20 and 20' are preferably helical coils 
disposed as hereinafter described in the fluidized bed furnace 22 which 
comprises an enclosure 24 having a distributor plate 26 at its lower end 
that divides the enclosure into lower plenum 28 and upper bed zone 30. The 
lower plenum 28 is provided with burner system 32 for heating air to be 
passed upwardly through the distributor plate 26 into bed zone 30. A solid 
particulate bed media is disposed in bed zone 30 so that it becomes 
suspended in the hot gas passing upwardly through the distributor plate 26 
creating a fluidized mass 33 that transfers the heat to reactor tubes 20 
and 20' engulfed therein. The fluidizing air discharges from the enclosure 
24 through conduit 34 into separator 36 preferably a cyclone, which 
removes entrained fluidized solids from the exhaust gas and discharges the 
gas into the atmosphere. The reactor tubes 2 and 20' discharge product 
into a separator 38, for example a flash distillation device, via conduits 
37 and 37'. A conduit 42 connects the lower portion of separator 38 with a 
cooler 44 which leads to a first storage tank 46 for 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 said outlets being connected to appropriate storage 
facilities, for example, second and third storage tanks 56 and 57 
respectively. 
In the preferred embodiments of the present invention `atactic 
polypropylene`, that is, 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 No. 6 and No. 2 fuel oils and 
other useful materials. Typically, the waste atactic polypropylene from a 
commercial polypropylene plant is collected in melt tank 10 wherein it is 
heated usually to about 400.degree. F. until it becomes liquid enough to 
be pumped at 50-250 psig to reactor tubes 20 and 20' wherein it is heated 
to sufficient temperatures to break carbon-carbon bonds in the waste 
material (approximately 800.degree. F. for a sufficient time) to produce 
the desired products. These products are usually 90% wt. liquid and 10% 
gaseous fuels, at about 25.degree. C., and are discharged from the reactor 
tubes into separator 38 wherin the liquid fractions are separated into a 
heavy (high viscosity) portion, and a mixture of light (low viscosity) 
portions and the remaining gases which are sent to a condensor 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 and fluidized bed. 
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 
(s) 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 feed valve 19 and 
opening purge inlet valve 60 to admit an inert gas, preferably nitrogen, 
thus forcing any feed product and/or pyrolyis product in the tube 20 
onward clearing that part of the system. Shortly thereafter, product 
discharge valve 62 is closed cutting off product/purge gas flow to the 
separator 38 and purge discharge valve 64 opened to permit the nitrogen 
purge gas to be exhausted into the atmosphere or into the plenum chamber 
of the fluid bed furnace for combustion of any pyrolysis products before 
discharge to atmosphere. Air inlet valve 66 is then opened permitting 
oxygen containing gas to enter the reactor tube 20 causing spontaneous 
combustion of any carbonaceous build-up remaining in the tube 20 after the 
nitrogen purge. 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 
invention, temperature of the reactor tubes is controlled at safe levels 
by the fluidized bed which efficiently carries the excess heat away 
preventing damage from overheating caused by the heat of combustion of the 
carbonaceous deposits. 
When all the carbonaceous material is burnt out of the tube 20 it is 
returned to service by closing the air inlet valve 66 and purging the 
reactor tube 20 with nitrogen until all oxygen is exhausted. Discharge 
valve 64 and nitrogen inlet valve 60 are then closed and feed valve 19 
reopened permitting polymeric material to flow into the tube. Finally 
product discharge valve 62 is reopened restoring tube 20 to full 
operation. 
As noted above, the other reactor tube 20' remains in operation unaffected 
by the burn out of tube 20. When reactor tube 20 is returned to full 
service 20' may be burnt out without affecting tube 20 by following the 
same procedure outlined above on the corresponding valves for that tube. 
It will be appreciated that systems having a plurality of reactor tube 
preferably two to six, are contemplated by the present invention and that 
more than one of these tubes may be `burnt out` at one time by obvious 
modification of the method described above. 
From the above description it will be apparent that there is thus provided 
a device of the character described possessing the particular features of 
advantage before enumerated as desirable, but which obviously is 
susceptible of modification in its form, proportions, detail construction 
and arrangement of parts without departing from the principle involved or 
sacrificing any of its advantages. 
While in order to comply with the statute the invention has been described 
in language more or less specific as to structural features, it is to be 
understood that the invention is not limited to the specific features 
shown, but that the means and construction herein disclosed comprise but 
one of several modes of putting the invention into effect and the 
invention is therefore claimed in any of its forms or modifications within 
the legitimate and valid scope of the appended claims.