The invention relates to an essentially horizontally arranged rotary drum furnace for thermal cracking of plastic, comprising a heatable furnace housing defining a furnace chamber, the furnace chamber incorporating an axially extending tubular infeed device, the drum being rotatable in relation to the infeed device, the infeed device comprising, in the furnace chamber, a plurality of exit openings spaced axially at a distance from each other, the furnace comprising an outfeed opening at an axial end thereof, and to a method for thermoplastic treatment of plastic.

State of the art

More and more plastic waste is generated, becoming a major problem for the world. In many countries, plastic waste is processed separately, but the processing industries have difficulties to produce something meaningful with it, because recycling such a mixture is not easy. Therefore, recycling is only possible to a very limited extent.

The usual processing is simple incineration, but this amounts to a waste of raw materials.

In recent years, many companies around the world have been investigating chemical recycling, wherein the molecular chains are only separated to produce reusable chemicals, which are CO ₂ neutral on the one hand and can reduce oil consumption on the other.

Problems

The problems related to thermal cracking, i.e. the separation of molecular chains by heat action, are manifold.

In order to obtain products of equal quality, it is very important that the plastic is continuously subjected to the same heat. Depending on the construction of the reactors, this is virtually impossible to guarantee. In addition, due to the low IR absorption of most plastics, the use of radiant heat is not possible. Convection is not a solution either, as herein, heat transfer is very low, especially during a short residence time. The only secure heat transfer takes place through contact heat, but for this, on the one hand, there must be sufficient surface area, and, on the other hand, the heat should also reach the plastic quickly and easily.

The thermal cracking of plastic is known from NL2015089. Herein, a horizontally installed rotary drum furnace is described, the drum of which is rotatable around a horizontal axis, an inner wall of the drum being heatable. The drum extends axially through a tubular lance with exit openings, through which lance particulate material, such as plastic material, can be fed into the drum. It is intended that when the oven is in operation, the particles are blown through the lance and then enter the heated furnace chamber through the exit openings and are thermally cracked by contact with the wall. However, blowing in solid particulate material causes problems that make the furnace operation more difficult. For example, the particles can stick to the wall, or, when the particulate material is converted into a gaseous state by contact with the wall, the flow of particles from the lance towards the drum wall can be prevented due to the rising gases. The introduction of plastic material in particulate form, in particular, remains problematic. One must constantly make sure that the plastic is prepared in such a way that the plastic flakes quickly reach the surface. Due to their low density, the flakes also have to overcome the resistance of the gases formed on the surface.

Solution of the problem

The invention envisages to eliminate the abovementioned problem and provides a rotary drum furnace of the abovementioned type, being characterized in that means are located in the tubular infeed device to displace plastic in molten state in the direction of the outfeed opening therethrough.

It has been found that the introduction of the plastic in liquid form into the furnace chamber does not, or to a significantly lesser extent, give rise to the above-mentioned problems above, and that the liquid plastic already evaporates in the furnace chamber before coming into contact with the furnace wall. Moreover, it is no longer necessary to sort the plastic carefully according to its composition and size beforehand.

According to the invention, the plastic is melted prior to being introduced into the rotary drum furnace and the melted plastic is then displaced by the infeed device towards the outfeed opening of the rotary drum furnace to be released through the exit openings into the furnace chamber. Such displacement devices are conveniently arranged as an extrusion device in the furnace chamber through which the molten plastic is introduced into the furnace chamber.

For performing the method of the present invention, the extrusion device cannot only be heated, e.g. to at least 300°C, but can be brought to a higher temperature in the direction of the outfeed end, advantageously of up to 500°C, therewith separating the molecular chains already towards paraffin.

As a result, the viscosity of the molten plastic decreases, which considerably facilitates the infeed.

Heating of the plastic to melt can be advantageously carried out prior to the introduction of the plastic into the infeed device, such as in a melting furnace positioned upstream of the rotary drum furnace. It is important that at such an elevated temperature, the pressure is kept under control and that inlet of air is preferably prevented. Thereto, the rotary drum furnace is preferably airtight. It is also possible that the plastic has not yet melted at the beginning of the feed device, but that it is melted during displacement through the infeed device, wherein the plastic is melted by the time it reaches the first exit opening.

With a single or double screw extruder, the melt can be pressed into the reactor and subsequently be introduced in various ways. This can be done by means of a nozzle, wherein the plastic droplets, or already paraffinized droplets are sprayed into the reactor chamber. The exit openings are therefore preferably designed to allow the plastic melt to pass through dropwise.

A significantly improved infeed takes place over the entire length of the furnace chamber when the tubular infeed device extends over at least half the length of the furnace chamber, preferably over at least three quarters of the length, and most preferably over substantially the entire length of the furnace chamber. In the case of plastics, this may be just a tube comprising openings along its length which may also get larger the further away from the infeed. These openings may have the form of slits, wherein each slit can be designed as a slit-shaped cluster of round holes wherethrough the molten plastic enters the furnace chamber.

Alternatively, a slit slightly deviating over the entire length is also possible. At the end, such a tube can be closed so that sufficient transport pressure can be built up. It is also advantageous if such a slit is composed of round openings, the diameter of which increases as the distance to the infeed increases. In a preferred embodiment, the tubular infeed device is encased by several layers, allowing thermal oil to flow therethrough in order to avoid overheating. It is also possible for an insulating layer to be present, which can comprise an aluminum sheath towards the hot side, therewith reflecting heat radiation. Such is advantageously covered by a light insulating layer to reduce or avoid contact with the hot outer jacket.

Optimally, the extrusion unit can also support the transport of the viscous mass by means of pumping jolts and accelerate the exiting droplets or veil so that these can spread better over the inner surface of the reactor when colliding therewith.

The invention also relates to a method for thermal cracking of plastic in melted form using the above device.

Description

Figures 1 to 4 show the device according to the invention to perform the process of to the invention, showing the following details:

Figure 1 shows the heating and transport of molten plastic. Plastic flakes pass through funnel 1 into the heating container 2 having heatable walls 3. Herefrom, the melt is transported through screws 4 of the extrusion device 5 in the direction of outfeed 7. The extrusion unit 5 is surrounded by heaters 6, so that the plastic remains liquid or is made even more liquid by higher temperatures towards the outfeed 7. The cracked plastic, or the product 8, in particular gaseous hydrocarbons and/or paraffin, is then released through the outfeed 7.

Figure 2 shows the material infeed into the rotary drum reactor 12. The plastic is pressed into the inlet 9 of tube 10, which is fixed in the upright cover 1 1 of the reactor. The wall 13 of the reactor 12 is set to the correct temperature for the desired cracking process. The infeed tube 10 has at the bottom a plurality of openings 14 from which the melt can drip into the furnace chamber 13. At the end 15 the infeed tube is closed.

Figure 3 shows the inlet tube 10, with here clearly separate openings 15 that increase towards the end of the tube.

Figure 4 shows again an infeed tube 10, but now with a long continuous opening 17 that deviates more and more towards the end of the tube.