CA1098072A - Process for vacuum pyrolysis removal of polymers from various objects - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Process for pyrolysis of polymeric organic compounds to remove or retrieve them from objects or materials that are substantially unaffected by the applied heat. The steps include heating in a first vacuum chamber to render flowable such polymers as are fusible at a temperature below that at which appreciable degrad-ation is initiated. The fused polymer drops by free fall from the first chamber to a second vacuum chamber in space communicating relation therewith but maintained at a substantially lower temp-erature, where the polymer resolidifies. Upon further heating, polymer remaining in the first chamber is volatilized and withdrawn from the chamber to a water mist scrubber where it is largely trapped. Remaining vapors in the scrubber are drawn through a vapor separator to a vent stack. After the combustible volatiles are removed from the first chamber, air is admitted into the chamber to oxidize any residual carbon ash, leaving only the inorganic residue which is easily removable from the objects or materials that were not affected by the pyrolysis action.

Description

BRIEF SUMMAR~ OE THE INV NTION
~ his invention relates generally to methods for pyro-lytic separation of polymeric organic compounds from objects or substances that are sub~tantially unaffected by the applied heat.
Such methods are useful ~n the polymer related industries to remove a variety of plastics from such objects as dies, blow molding heads, breaker plates, screen packs and filters, spinner-ettes, extruder screws, pumps, nozzles and other such tooling.
The parts so cleaned may then be reused The process is also useful for reclaiming metals by separation from such polymers, and for reclai~ling certain constituents of the polymers.
More specificall~ the invention relates to improvements in the fusing of such pol~mers as are ~lowable at temperatures below those at which appreciable degradation is initiated, and improvements in the removal of plastics having an ash o~ high carbon content after pyrolysis has removed the volatile consti -~tuents.
There are numerous methods for removal of deposits of plastlc from metal parts. Except for the methods employing solvents, ultrasonic cleaning or a combin~tion of these, the methods generally involve heating to a temperature usually in the range from 800 degrees to 900 degrees F within which the plastic degrades.
During degradation volatile constituents which are generally combus~ihle are evolved, leaving a residue that is frequently composed largely of carbon. If oxygen is present oxidization occurs, leaving an i.norganic dust or powder residuum.
Prior methods of cleaning include, for example, covering the parts with molten salt; or heating the parts in an oven in an atmosphere of air~ inert gas or steam; or immersing the parts in a hed of heated aluminum oxide; or heating the parts with a _ _ ~

7~
1 blow torch or upon a hot plate. Each of these methods has one or more disadvantages such as the need for periodic replacement of salt or other medium of immersion, atmospheric cont~mination by smoke or vapor, lengthy t:lme periods for completion of the cleani.ng, annealing or distortion of parts from which the plastic is to be removed, danger to operators from salt eruption, spillage or other hazards, residues such as salt or aluminum oxide left on parts requir.ing further cleani.ng operations, and limitations either to speciic xesins, ~o limited amounts of resin, or. to the cleaning of parts that do not have complicated shapes.
Many of the foregoing disadvantages have been overcome by a particular process of vacu~n pyrolysis described in an article by Don Biklen, entitled "Now: Remove Plastic Deposits From Extruder Parts the Modern Way", published in the SPE Journal for July, 1973, Volume 29, page 25. According to this method, plastic - coated parts are placed in a chamber fitted with radiant heating ~lements and with a vacuum pump leadi:ng to a waste line. Under vacuum the parts are heated. When the temperature reaches 800 to 900 degrees F the plastic decomposes, certain constituents vaporize ~9 and the vapors are pulled off throug:h a condensate trap where -- most of the solids condense upon contact with water. Vapors and water leaving the trap enter the vacuum pump. The water and vapor move through the pump to a separator from which the water passes to a drain ox recirculates to the vacuum pump and the vapor is e~hausted to atmosphere through a vent stack either directly or through a gas-fired afterburner. ..
A refinement of this process also ha~ been in use ~y the assignee of the present application, involving a second vacuum chamber or collection receptacle installed beneath the first vacuum chamber and in space communication therewith through a --2~

1 connecting pipe. The second chamber is maintained at a substan~' tially lower temperature than the first chamber. Upon heating, certain plastics in the first chamber become flowable at higher temperatures and drop to a collection,tray leading through -the connecting pipe to the second chamber, where the temperature is low enough to cause resolidification~ The second chamber has a collection pan and door for removal of the collected plastic.
This refinement permits the retrieval of a substantial portion sf ~he total plastic material without its pyrolysis.
However, in some inst~nces difficulty has been exper-ienced with this refinement because of the failure of the fus~ed plastic to flow from the heated vacuum chamber quickly enough to prevent its deyxadation as the temperature rises~ This difficulty has been causecl primaril~ because of the viscous nature of the molten plastic which impedes its flow, in many instances blocking the conne,-ting pipe~ Attempts have been made to reduce this probl~m by surroundnng the connecting pipe with a heater, but this expedient is only partially successful.
This inv~ntion compr~ses improvements upon the process XO described by Biklen and, its later refinements, by facilitating the removal of fused pol~mer ~rom the heated vacuum chamber so that it may be collected efflciently and resolidified with minimal or negligible degradation. Thereby, the remaining polymer that is subjected to pyrolys,is comprises only a fraction of the total p,olym~r to be separated from the contents of the heated chamber.
Improvements are al50 made to the process of pyrolysis following the removal of the volatile constituents, when the remaining ash is of high carbon contentO By the latter improvements, the resid-uum, frequently comprising substantially only inorganic pigments or fillers, is a dust or powder that is easil~ cleaned from the parts.

1 BRIEF DESCRIPTION OF THE DRA~ING
The drawing is a partially schematic representation of apparatus adapted to carry out the process of this invention in its preferred form and variations.
~ETAILED DESCRIPTION

.
Referring to the drawing, the reference number 12 repre-sents a chamber unit comprising a three~part vacuum enclosure. The -parts comprise a first chamber 14, a second chamber or collection receptacle 16, and a connecting section 18 which is preferably but not necessarily a vertical pipe of substantial diameter as hereinafter further described. The first chamber is preferably lined with stainless steel, insulated and fitted with one or more electrically energized radiant heating elements 20, a heating control the.rrnocouple (not shown~, a xack 22, a door 24 or insertion or withdrawal of the rack, and a collection tray 26. The tray has a central opening or discharge point 28 located on the axls of the section 18 so that molten polymer flowing from the tray falls by ~ree fall completely out of contact with the.walls of the section 18 onto a mass 30 of congealed or resolidified polymer in the chamber 160 The section 18 is of such dimensions and orientation as to i.solate the second chamber 16 thermally from the first chamber 14 so that the temperature o the chamber 16 i5 substantially below that of the chamber 14. Th~s permits solidification to occur immediately aS the polymer reaches the chamber 16. The chamber 16 has a door 32 or removal of the resolidified polymer after the process is completed.
Preferably, the chamber 14 is arranged with means (not shown) for loading polymer-laden pieces 34 on the rack 22 externally of the c~amher 14 and for inserting the loaded rack ~4--1 into the chamber. An air valve 33 is also provided for a purpose hereinafter described.
A pipe 36 leads -to a condensate trap or water mist scrubber 38 havin~ a nozzle 40 connected to a source of water and producing a water spra~ 41. The water spra~ impinges on the vapors passing to the trap 38 from the chamber 14.
A pipe 42 is connected from the trap 38 to a vacuum pump 44 of the water ring type. An arrow 46 represents a supply of water to the pump from a recirculation tank 48 described below.
A pipe 50 is connected from the pump 44 to a separator 5~. The separatox has a water trap that separates the water carrying trapped pol~ymer particles ~rom the remaining vapor t the water passing through a pipe 54 to the recirculation tank 48 and the vapor passing through a pipe 56 to a gas-fired afterburner 58, the exhaust of which leads to a stack 60. In many cases the afterburner can be omitled as the effluent is within the atmosphere safety limits of t~pical communitiesO
A source o~ makeup water is connected through a valve ~2 to-~he recirculation tank 48. The tank 48 is of conventiolnal constxuction and serves to filter out and retain the trapped polymer partic~es in the waterl discharglng clean water to the pump 44 as indicatid by the arrow 46. Suitable means are provided for periodicall~ cleaning out the tank 4B. The valve 62 is opened to provide only the makeup water required to replace a correspond-ing amount of water discharged throuyh a P-trap 64 to a drain. The rate of discharge is adjusted to maintain the temperature of the recirculated water at a value below B5 degrees F. Maintenance oE
the water temperature below the stated value has been found to improve the efficienc~ o~ the pump 44.
3~ If desired, skimming devices and fine scxeens or other ~80~dZ

1 filters may ~e employed in the s~stem, ~re~erabl~ withln the closed loop between the pwmp 44 and the tank 48.
~ s an altexnative, the system can ~lso be operated by discharging the separator 52 directl~ to the drain, thus eliminat-lng the recirculation tank 48, in sltuatlons where the illustrated arran~ement is not re~uired.
The apparatus described aboYe is adapted for an~ of sev~ral selected modes of operation which are next described. The selected mode is dependent upon the type and ~uantity o~ plastic 1~ materlal to be subjected to pyrolysis~ The types of plastics gen- .
erall~ used dl~er in two important respects, namely, the property of becoming ~usible at a temperature below that at which appreciable degradation Qccurs, and the presence of an ash o high carbon content following the evolution o~ substantiall~ all of the vola-tile constituent~. E'or example~ vinyl and rubber compounds.are not fusible to an extent rendering them flowable at temperatures below tha-t at which these compounds begin to degrade. In contrast, most of the other compounds axe flowable at temperatures below those at which the~ begin to degrade, for example pol~styxenes, pol~olefins,

2~ polycarbonates9 nylons, polyesters and polypropylenes, to mention common examples. Some pol~mers including pol~oleins and polysty-renes do not h~ve an ash o~ high carbon content a~ter the e~olu~ion o~ volatila components J while otller polymers such as polyesters, nylons, vin~ls, pol~carbonates and rubber compounds do have such an ash.
~ n use, the above-described apparatus may be controlled manuall~ but is pre~erabl~ controlled in an automatic c~cle by a timer lnot shown) adjusted to accommodate the characteristics o$
the particular pol~.mers and the ~uantities thereof to be removed

3~ ~rom the parts 34. The c~cle is ~enerall~ o~ 6b to 90 minutes duration, but in some cases it can be lon~er.

l The following description is generali~ed to include the various type~ of plastics commonly used. In the case of particular plastics the dura-tion of each step, the level of vacuum maintained and the temperatures of the respective chambers are appropriakely adjusted for maxim~un efficiency, cleanliness o effluents and rapidity of pol~mer removal.
Polymer-laden or coated parts 34 are first placed in the chamber 14 on the rack 22, and the doors 24 and 32 are sealed. The air valve 33 is closed. The pump 44 is then turned on to evacuate the system comprising the entire enclosure 12~the pipe 36 and the trap 38. Typically, the vacuum is maintained at 25 to 27 inches of mercury~ At the same t~me the radiant heating element 20 or a plurality of such heaters are energized to cause the temperature within the chamber 14 to increase.
~ t a given temperature, certain of the above-stated polymer types fuse and flow from the parts 34, dropping to the inclined collection tray 26 and draining thererom through the opening 28. This material then drops through the section 18 to the substantia]ly cooler chamber 16 where it resolidifies in the mass 30. The chc~bers 14 and 16 and the connecting section 18 are so arranged that the chamber 16 preferably does not exceed a maximum tempera-ture of 150 degrees F during the cycle, whereas the temperature of the chamber 14 rises substantially higher as here-inafter described. It will be noted that since the tray 26 is situated within the heated chamber 14, the fused polymer that falls to the tray remains at substantially the same temperature until it passes through and falls from the opening 28. A~ter passing from the opening the polymer does not contact any wal] surface of the connecting section 18 and falls freely by gravity through this section ~o the cooler chamber 16.

The result is rapid elimination of the fused polymer that drops from the parts 34 from the chamber 14, thereby avoiding the clogging of pipes or passages and the possibility that khis polymer will be retained within the chamber 14 long enough to begin to degrade as the temperature of that chamber continues to increase.
The heating element 20 con-tinues to be energized and to increase the temperature within the chamber 14 until the polymer remaining on the parts 34 begins to degrade by Pvolution of its volatile components. This degradation is not accompanied by burning because of the lack o~ oxygen within the chamber. Ordinarily, the temperatures required to cause such vaporization are in the range between 800 degrees and 900 degrees E, although the apparatus is preferably d~signed for opexating temperatures within the chamber 14 up to about 1,000 degrees F.
The vapors evolved from the parts 34 during de~radation pass ~hrough the pipe 36 to the trap 38 where they are projected into the path of the water spray 41~ Here, most of the solids entrained in the vapors ar~ trapped in the waterO The water laden wlth solids, combined with the remaining vapor, passes through the pump 44 to the separator 52. From the separator~ substantiàlly all of the vapors pass thrcugh the pipe 56 to the afterburner 58, where any combustible components ma~ be burned. In practice, the afterburner 58 is usually not required because the vapors in the pipe 56 are sufficiently clean for exhausting directly to the a~mosphere.
The water containing solids passes through the pipe 54 to khe separator 48r which filters out substantially all o~ the solids and recirculates water to the pump 44.
The rate of flow of water through the trap 64 substan-tially equals that through the nozæle 40 and ma]ceup valve 62, and ~8~

is controlled by the valve 62 to maintain the tempera-ture of the recirculating water below approximately 85 degrees F.
After an interval of time, substantially all of the vapori~ab:Le constituents of the polymer, which are frequen-tly combustible, are removed from the chamber 14 leaving an ash upon the parts 34. In certain cases this ash has a high carbon content.
For example, the ash of polyesters is approximately 86 percent carbonO While maintaining the chamber 14 at a temperature in the ~ange of 800 degrees to 900 degrees F and.continuing to operate the pump 44, the vacuum is gradually reduced to a lower value or zero by introduction of air through the valve 33 to react with the carbon, thereby producing carbon dioxide and carbon monoxide. To facilitate this reaction the air introduced through the valve 33 may be preheated, and means may be provided to distribute it uniformly ovex the parts 34. These gasses are withdrawn through the pipe 36 to the trap 38 The remai:ning residue on the parts 34 after this combustion is completed is generally in the form of dust or powder and mos-tly comprises inorganic pigments and fillers.
~t this stage the heater 20 and the pump 44 are de-en~rgi7ed, and the door 24 may b~ opened to remove the parts 34~
It wîll be understood that the abo~e-described step of introducing air through the valve 33 ma~ be omitted in certain instances, for example whexe the polymer is a polyvinyl chloride compound and it is desired to avoid the release of chlorine gas by combustlon.
~he final cleanup of the parts 34 may be accomplished by any of the known technique5 such as air-jet cleaning, wiping or bead dusting at low pressure with glass, woodflour, lime or soda particles~
~lthough the apparatus described above employs a three- :

_g_

4~2 1 part vacuum enclosure 12 including a vertical connecting pipe section 18, other forms of construction can be used to carry out the process. For example, a single enclosure may be provided with a heat baffle dividing the internal space into two reyions, one above the other. The upper region may contain the rack, the inclined tray for molten polymer and the radiant heating means. The heat baffle may have an aperture through which the molten polymer falls by free fall to the lower, cooler region of the enclosure. The heat baffle ma~ be provided with reflective surfaces to reflect the radiant heat upwardly toward the articles on the rack.
The process described above has been employed success-fully to remove pol~mers ~rom parts having complicated shapes. It has also accomplished the removal of polymers from parts wikhout distorting them or materially changing their surface finish.
This process has been further demonstrated to be safe - for workers, for waste water disposal and for control of atmospheric pollution.

~1 ~10-

Claims (2)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. The process of removal of organic material coating an object, comprising the steps of evacuating air from a first space immediately surrounding the object, heating said space to cause the temperature thereof to rise into a first range sufficient to fuse said material but insufficient for substantial degradation thereof, said heating causing heat to penetrate said coating in a direction toward said object, whereby a portion of said material in non-degraded form falls from the object, while collecting said falling portion and concentrating and directing its flow by gravity to a drain located within said first space, said portion being permitted to fall from said drain through a conduit connecting said first space to a second space, said conduit being of sufficient orientation and dimensions to substantially thermally isolate said second space from said first space, said second space being maintained at a temperature permitting said portion to resolidify therein, said collected portion being directed to fall from the first space into the second space through the connecting conduit without contacting any intervening structure, and thereafter further heating said first space to cause the temperature thereof to rise into a second range between 800 degrees F. and 1,000 degrees F.,said second range being sufficient to degrade the material remaining on the object includ-ing first withdrawing volatile products produced by said degradation from the first space to leave a resulting ash and thereafter introducing air into the first space while maintaining
1. Claim 1 continued....
   
   the temperature thereof within said second range, whereby carbon in said ash is combined with oxygen to produce gaseous oxides, including withdrawing said gaseous oxides from the first space to leave a substantially inorganic residue.
2. 2. The process according to claim 1, further including trapping in water particulate matter carried by said volatile products of degradation as the latter are withdrawn from the first space.