FR2697528A1 - Cracking polyethylene waste into wax or hydrocarbon(s) - by radiant heating of waste held in retaining grid from refractory wall, heated by burning gas from process - Google Patents

Abstract

Thermal cracking of polyethylene (PE) waste, for prodn. of artificial wax or other hydrocarbons, is effected using the heat of combustion of hydrocarbons formed in the cracking itself. The waste is gathered and pressed against a retaining grid (RG) of refractory metal, having mesh size between a few mm and a few cm. RG is completely enclosed and sealed, on the side away from the retained PE waste, by a radiating wall (RW). RW is of refractory material, and is spaced from RG by a few cm, with the common periphery sealed. The periphery of RW is provided with outlets extended by valve conduits. RW is itself completely enclosed by a second wall of refractory material, forming a combustion chamber having air inlets, and smoke outlets. The chamber is fed by gas burners so that RW (having thickness not more than 20 cm) radiates heat towards the PE waste for cracking. The cracking prods. evacuated via the outlets in RW are recovered and sepd. Part of the non-condensable or highly volatile prods. obtd. by fractionating the cracking vapours is supplied to the burners feeding the combustion chamber, so that the energy required for cracking is obtd. without external supply. The same process may be applied to waste of polypropylene or other polyolefins. Superheated steam at above 400 deg.C is produced using heat exchangers, by extraction of heat from the fumes leaving the combustion chamber. The steam is then continuously blown between RG and RW to equilibrate the cracking zone, before escaping towards the rear with the cracking vapours. Esp. PE waste is pressed into a cylindrical RG of dia. 0.3-2 (esp. 1) m, then cracked by radiant heat from a 1-20 mm thick plate of refractory alloy which surrounds RG on all sides at a spacing of a few cm. The plate is provided with outlets for cracked prod. The plate is heated to ca. 1000 deg.C in a combustion chamber heated by gas burners fed by part of the vapour from cracking. Superheated steam at above 400 deg.C is supplied between the plate and the grid as described above. USE/ADVANTAGE - PE waste is converted into wax (used e.g. as brighter, lubricant for PVC, hot melt adhesive or insulating mass) and/or aliphatic hydrocarbons rich in alpha-olefins (used as bases for biodegradable detergents and plasticisers or, after fractionation, as lubricants, heavy or light solvents or fuels). Polypropylene waste can also be converted into wax. RG retains the waste but allows easy passage of cracked prods. Vapours are not burned in direct contact with PE. Since PE readily absorbs IR, radiation from RW rapidly heats the PE to at least 900 deg.C without localised hot-spots. Loss of hydrocarbon prod. by burning, discolouration of wax prod. by tar formation and soot deposition in the appts. are minimised. The process can be adjusted to favour wax prodn. (e.g. from relatively pure PE) or hydrocarbon prod. (e.g. from PE with high impurity content).

Description

 The present invention relates to a thermal cracking process for polyethylene or other polyolefin waste, using the heat of combustion of part of this waste, or the cracking vapors which result therefrom, to maintain the cracking itself, with a view to the production of artificial waxes or other hydrocarbons.

 French Patent No. 1,457,575 of September 18, 1965 indicates on pages 1 and 2 that the manufacture of polyethylene waxes by thermal decomposition of polyethylene was already known in 1965. This patent describes the cracking of high molecular weight polyethylene granule at within a polyethylene wax between 3000C and 6000C. But to our knowledge, only one process, described in French patent n087 04926 of April 1987, claims the thermal conversion of waste of works in polyethylene into artificial wax by controlled combustion of this waste.

 As our own process is an extension and an improvement of the process described in the aforementioned patent n087 04926, we will describe it in more detail. We have verified that this process provides, compared to the quantity of clean waste treated, more than 80% of wax of excellent commercial quality, hard, crystalline, very linear and with a melting point of around 1100C. Its technical characteristics are very constant whatever the sources of the polyethylene waste used, which theory easily explains, taking into account the rather particular conditions of this cracking.

 This is a thermal decomposition of polyethylene waste under the action of the heat created by the combustion of the polyethylene itself. This process, known as controlled combustion, technically consists of cracking at medium severity of the polyethylene simultaneously producing combustible vapors which are burned off as they are formed by cracking and polyethylene wax which is collected. Let us emphasize that the expression "combustion of polyethylene" is a simplified way of expressing oneself. In reality in this case, the combustion, which is a lively reaction with the oxygen in the air, takes place only between the air and the combustible vapors produced during the thermal decomposition of the polyethylene under the effect of heat from the combustion itself.

The importance of this precision will become apparent when we present our own process.

 One embodiment of this controlled combustion process uses a "burner" consisting of a vertical tube at the bottom of which polyethylene waste is introduced and then compressed upwards where it is retained by a grid against which it is "ignited" ". The wax formed trickles down the grid and is collected in a gutter provided at its periphery, thus being quickly withdrawn from the cracking zone.

 The manufacture of wax by the "combustion" process does not require expensive equipment and the cost price of these waxes hardly exceeds that of the waste from which they come. If we add to this the considerable potential outlets (brightening products, lubricants for PVC, "hot-melt" adhesives, insulation masses, etc.) as well as the fact of recovering valuable molecules from waste, which regulation tends to encourage more and more, this process should be promised a great success.

 We also verified that this process applied to all types of polyethylene waste, even colored and without prior cleaning, provided that the level of impurities does not exceed 5%, but it then becomes necessary to purify by means the wax obtained.

However, even when the polyethylene waste is clean and clear, the wax obtained by the combustion process is quite colored, which is annoying for certain applications and may hinder its diffusion. This coloring is due to three reasons:
1 / A locally too high temperature generating tar.

 2 / The presence of soot from unburnt products.

 3 / The local presence of excess air oxidizing the wax and generating tar.

There are two other drawbacks to the "combustion" process, which are the counterpart of the method used and the equipment described.
- All the vapors formed during cracking are destroyed by combustion.

 - It is impossible to treat polyethylene waste with more than 5% of impurities because the wax obtained becomes too difficult to purify.

 The present invention proposes to remedy these drawbacks by retaining from the combustion process only that which is essential for the cracking of polyethylene while excluding that which in this process is detrimental to the quality and quantity of the cracking products obtained, the range is not limited to artificial waxes, but extends to a whole series of lighter aliphatic and olefinic hydrocarbons formed during cracking.

 The prior art mentions a grid against which the polyethylene waste is pressed by any means before being ignited.

 We will retain in the present invention the use of this refractory metal grid as the most effective means of controlling the progressive cracking of the waste which is accumulated behind it. Its function will be to retain the waste, but to allow the cracked products to pass freely.

 During the implementation of the prior art, mention is made of the combustion of polyethylene, but we have clarified that this is an abuse of language, admissible when the combustion of the vapors resulting from the decomposition takes place at its direct contact only.

 According to the present invention, the polyethylene cracking vapors will not be ignited on direct contact with the polyethylene, but will be collected in full as soon as they are formed and only the part necessary for the supply of the energy essential for the cracking will be burned according to the methods described in the following discussion.

 On the other hand1 observing that polyethylene is very absorbent in a wide range of infrared, we found that the thermal radiation emitted by any refractory body maintained at a temperature at least equal to 9000C caused very rapid heating of the polyethylene in the mass, without the drawbacks of local overheating due to the transmission of heat by direct contact of combustion gases such as flames for example, or solid bodies at high temperature, such as heating walls for example, both generators of products In addition, as we have pointed out when listing the drawbacks of the prior art, any direct contact of the polyethylene with the gases in combustion causes, irrespective of possible overheating, undesirable browning of the cracking products by soot deposition or by oxidation due to excess air.

 However, a gas at high temperature, technically free of impurities and not reactive with cracking products at this temperature, can advantageously provide its own calories to contribute to cracking, either by scanning the polyethylene, or by thermal radiation if it is emissive in the infrared, and its action can be combined with profit to that of a radiant refractory body. Among these gases, water vapor for example, at temperatures above 4000 ° C., is particularly advantageous by its purity and its ease of use.

Ultimately, the means implemented by the present invention are intended
1 / Burn only the minimum of vapors from cracking and recover the rest as well as the wax form.

 2 / To use only the effect of thermal radiation emitted by refractory bodies heated above 9000C and / or water vapor heated above 4000C to cause cracking of polyethylene waste.

 3 / Avoid any contact between polyethylene or cracked products on the one hand and combustion gases or excess air on the other hand.

 According to the present invention, polyethylene waste is accumulated and kept pressed by any known means, behind a refractory metal grid intended to retain it, and whose meshes are between a few millimeters and a few centimeters in width, depending on the size of the waste. This grid will be called "retaining grid" throughout the text.

 A wall of conventional refractory materials hermetically surrounds on all sides, a few centimeters apart, the "retaining grid" on the side opposite the waste and joins it along their common periphery! sealingly, sealing obtained by conventional means. This wall is provided on its periphery with outlets allowing the evacuation, by conventional conduits less of valves, of the gases and liquids which may form between the wall and the "retaining grid". This wall will be referred to as "radiant wall" throughout the text.

 The "radiant wall" is itself surrounded on all sides by a second wall made of usual refractory materials forming a conventional combustion chamber, supplied by gas burners according to known techniques making it possible to heat the "radiant wall" to a temperature above 9000C without excessive heat loss. In accordance with current techniques, this combustion chamber is provided with adjustable air inlets and outlet for the fumes.

 The "radiant wall" has a thickness which must not exceed a few centimeters and at most twenty centimeters, so as to quickly reach its equilibrium temperature. Justifying its name then, it transmits heat by heat radiation to the polyethylene waste facing it, causing it to crack. The cracked products are, as they are produced, discharged through the openings provided for this purpose.

 The wax formed in liquid form is collected separately, and the vapors from the cracking are sent to a conventional fractionation device.

 A portion of the non-condensable or very volatile products obtained after fractionation of the cracking vapors is sent to the burners supplying the combustion chamber in order to provide them with the energy necessary for further cracking without external inputs.

 The combustion gases take away a significant amount of unused heat which can be partially recovered to produce steam at low pressure (less than 1/10 bar) but very superheated to more than 400 "C per using conventional exchangers placed on the smoke evacuation path according to a known technique. We can then perfect the cracking method that we have just described by continuously blowing during the cracking, between the "radiant wall" and the "grid of retaining ", very superheated water vapor as it has just been said. We then approach the conditions of a steam cracking. In addition, this very superheated vapor brings its calories, which accelerates the reactions and decreases all the calories necessary for cracking, and consequently the quantities of products formed during cracking to be sent to the burners. This water vapor, after sweeping the cracked zone, escapes with the vapors cracked from which it is easy to separate by condensation.

 The method according to the invention thus makes it possible to remove polyethylene waste by cracking a whole range of hydrocarbons, a large part of which was burned in pure loss with the prior techniques. The present invention also makes it possible at will to promote the production of wax, for example when the waste used contains only few impurities which are easily separable from the wax formed, or else to give preference to the production of distillable hydrocarbons, by example when the waste to be treated contains too many impurities.

 The wax obtained can be very clear if the waste comes from uncoloured polyethylene. The hydrocarbons recoverable from the condensation of vapors are absolutely free of sulfur, very rich in dzolefins, bases of biodegradable detergents and plasticizers. After conventional fractionation they can directly supply lubricants, heavy or light solvents, and of course fuels.

 Polypropylene or other commercial polyolefin wastes are subject to the same treatments as those of polyethylene and provide similar but specific products.

 Polypropylenes of various origins thus give, contrary to a current opinion probably based on a false interpretation of the theory of thermal cracking, waxes with very constant characteristics.

 According to a particular embodiment of our process, the polyethylene waste is introduced continuously at the end of a horizontal steel cylinder resting on any supports, with a diameter between 0.3 meters and 2 meters but of preferably about one meter and of length equal to about three times its diameter, from where they are pushed by any known means, such as jacks for example, to the other end. The latter is extended by a cylindrical grid, of the same diameter as the cylinder and of length equal at most to three times its diameter, consisting of refractory steel wires of a few millimeters in section forming meshes of any shape and of average diameter of 1 to 5 centimeters approximately. This retaining grid ends with a flat, curved or ogival grid of the same nature and constitution forming the bottom.

 The "retaining grid" is surrounded on all sides at a distance of approximately 8 cm to 20 cm by a sheet of refractory alloy with a thickness of between 1 and 20 millimeters, optionally covered on the outside with conventional refractory materials to a maximum thickness of 10 centimeters.

 The refractory plate, which corresponds to the radiant wall of the description, is sealed and has an approximately cylindrical shape. It rests on the refractory base of a conventional combustion chamber which surrounds it on all sides.

The refractory plate is tightly connected, by known means, for example bolted flanges, to the end of the cylinder, where it is extended by the "retaining grid"
A stainless steel basin, fitted with outlets, is located a few centimeters below the "retaining grid" all along the latter.

 This basin, which rests on the bottom of the refractory plate, makes it possible to collect by gravity the wax formed and to quickly evacuate it through its outlets which pass through the bottom of the refractory plate in a sealed manner.

 At the high level of the junction between the refractory plate and the waste supply cylinder, outlets are provided for collecting the cracked vapors which are evacuated by conventional conduits.

 The "retaining grid" lacking in rigidity, circular supports in refractory steel surround it, a few decimeters from each other, and keep it at a constant distance, a few centimeters, from the bottom of the sheet on which they rest.

 Distributed over the entire surface of the retaining grid and fixed on the aforementioned circular supports of this grid, a few centimeters from this rear end and inside the refractory plate, open outlets of superheated steam, via conduits passing through the refractory sheet in a leaktight manner at any point and coming from exchangers of conventional types located on the smoke evacuation path, at the outlet of the combustion chamber.

 Conventional types of gas burners are distributed at the entrances to the combustion chamber according to known techniques in order to obtain an approximately uniform temperature of approximately 1000 "C.

 On cold start, the burners are supplied with combustible gases stored for this purpose, until the thermal equilibrium temperature of the combustion chamber is reached. The superheated water vapor sweeps through the cracking zone and under its influence, combined with that of the thermal radiation from the refractory sheet at high temperature, the polyethylene waste plated behind the retaining grid undergoes a crack that generates wax which flows in the receiving basin, and cracking vapors which evacuate themselves by the high exits under the influence of the slight overpressure due to their formation and that of the blowing of water vapor. The cracked vapors then undergo a fractionation accompanied by the condensation of the majority of the water vapor and the heavier fractions. The incondensables, and if necessary a portion of the light fractions still in the form of vapors, are directed to the burners where they take over from the combustible gases used for starting, with optional passage through a conventional regulating buffer system constituting a reserve.

If the quality of the cracked waste allows it1, the wax formed is evacuated as it forms to its further refining. If the waste is too much
dirty or too colored for the refining of the wax to be profitable, this wax is left on the spot and undergoes a cracking in turn then leading only to distillable hydrocarbons.

 Waste polypropylene or other polyolefin can be treated according to this particular embodiment by providing specific types of waxes and / or distillable hydrocarbons.

Claims (4)

 1 / Process for the thermal cracking of polyethylene waste, for the production of artificial waxes or other hydrocarbons, using the effect of the heat given off by the combustion of hydrocarbon vapors from the cracking itself, characterized by the fact that the polyethylene waste is accumulated and kept pressed by known means behind a refractory metal grid called "retaining grid" intended to retain them and whose meshes are between a few millimeters and a few centimeters in width, and by the fact that the "retaining grid" is surrounded on all sides, hermetically, on the side opposite to the waste it retains, and a few centimeters away, by a wall of conventional refractory materials which joins it along their common periphery in a sealed manner according to known conventional means. This wall, called "radiant wall", is provided on its periphery with exits extended by co conventional products fitted with valves. Still further characterized by the fact that the "radiant wall" is itself surrounded on all sides by a second wall made of usual refractory materials forming a conventional combustion chamber provided with air inlets and smoke outlets and supplied by gas burners according to known techniques making it possible to heat the "radiant wall", the thickness of which must not exceed approximately 20 centimeters and which then radiates the heat which is transmitted to it towards the polyethylene waste which faces it, producing their cracking. finally by the fact that the cracking products evacuated by the exits practiced in the "radiant wall" for this purpose are collected and separated and that a portion of the non-condensable or very volatile products obtained after fractionation of the cracking vapors is sent to the burners supplying the combustion chamber in order to provide them with the energy necessary to continue cracking without external inputs urs.  2 / A method according to claim 1 characterized in that steam overheated to more than 4000C is produced by conventional heat exchangers by borrowing heat from the fumes leaving the combustion chamber according to known techniques and continuously blown into during the cracking, between the "radiant wall" and the "retaining grid", so as to sweep the cracked area before escaping towards the outside with the cracking vapors  3 / A method according to claims 1 or 2 characterized in that the polypropylene or other commercial polyolefin waste can be treated in the same way as that of polyethylene by giving similar but specific products.  4 / Particular embodiment according to claim 2 characterized in that the waste is accumulated and kept pressed inside a horizontal cylindrical grid with a diameter between 0.3 meters and 2 meters, but preferably 1 meter, then subjected to cracking under the influence of thermal radiation from a sheet of refractory alloy with a thickness of between 1 and 20 millimeters which hermetically surrounds it on all sides a few centimeters apart and which is provided with exits for the evacuation of the cracked products formed. This refractory sheet is brought to a temperature in the region of 10000C by means of a combustion chamber in which it is placed and which is heated by gas burners supplied with part of the vapors from the cracking. The combustion chamber and the burners are of known technique. Steam superheated above 400 "C and coming from conventional heat exchangers heated by the fumes leaving the combustion chamber, sweeps the cracking zone between the refractory plate and the" retaining grid "and s' escapes with cracked vapors.