Process for the treatment of a composition comprising thermoplastics

Abstract

The present invention relates to a process for the treatment of a composition comprising thermoplastics comprising introducing the composition into a reactor under reduction of oxygen content of the atmosphere, heating the composition in the presence of a solvent to liquefy the thermoplastics, separating insoluble fractions and recovering the liquefied thermoplastics which process is conducted in one reactor.

Claims

1. Process for the treatment of a composition comprising thermoplastics, the process comprising the steps of a) introducing the composition comprising thermoplastics into a reactor under reduction of oxygen content of the atmosphere, wherein the oxygen content is reduced to below 10 vol % of gas phase, b) heating the composition to a temperature in the range of 150 C. to 450 C. in the presence of a solvent, wherein the solvent has a boiling point of between 50 C. and 150 C. and wherein the amount of solvent is 0.1 kg to 10 kg per kg of thermoplastics, to obtain a drop in viscosity below 10.sup.4 mPas of at least 50% by weight of the thermoplastics, based on the total content of the thermoplastics, c) separating solid and/or gaseous fractions of the composition at the surface of the mixture and/or the bottom of the reactor, and d) recovering liquefied thermoplastics from the reactor, wherein the process steps a) to c) are conducted in one reactor.

2. Process according to claim 1, wherein the process steps a) to c) are conducted continuously.

3. Process according to claim 1, wherein the solvent used in step b) is an aliphatic hydrocarbon.

4. Process according to claim 1, wherein the solvent used in step b) is a C.sub.6 to C.sub.8 alkane or a mixture of C.sub.6 to C.sub.8 alkanes.

5. Process according to claim 1, wherein the solvent forms an azeotropic mixture with water.

6. Process according to claim 1, wherein in step b) the composition is heated in the presence of the solvent to a temperature in the range of above 250 C. to 400 C.

7. Process according to claim 1, wherein the composition comprises at least 80% polyethylene, polypropylene and polystyrene as thermoplastics.

8. Process according to claim 1, wherein steps a) to d) are followed by a step e) of cracking the liquefied thermoplastics.

9. Process according to claim 8, wherein the cracking is thermal or catalytic cracking.

10. Process according to claim 1, wherein the composition comprises waste plastic.

11. Process according to claim 1, wherein steps a) to d) are preceded by a step of reduction of the particle size of the thermoplastics.

12. Process according to claim 2, wherein the process steps a) to c) are conducted simultaneously in the reactor.

Description

(1) FIG. 1 shows the product of melting at 280 C. a sample of HDPE and PP under N.sub.2 atmosphere (replacing air by N.sub.2) and

(2) FIG. 2 shows the product of melting at 280]C. a sample of HDPE and PP under air atmosphere.

(3) FIG. 3 shows a system for generating melt from mixed waste plastic.

(4) According to the invention, the system for generating melt from mixed waste plastic as shown in FIG. 3 comprises an enclosure 1 equipped with an indirect heating device 2 and 2 where heat transfer medium is introduced through 3 and recovered through 4. The separation device 6 divides the enclosure 1 into two interconnected zones 7 and 8. A mixed zone 7 is equipped with a mixing device 5 and a decantation zone 8. The mixed plastic feed with reduced oxygen content is introduced in the mixed zone 7 by the pipe 9 and the solvent is introduced by the pipe 10. The plastic is mixed with the solvent and heated by the heating devices in the zone 7 and is transferred by gravity to zone 8 where most of the decantation takes place. Gaseous impurities are recovered from the top of the enclosure 1, extracted by the pipe 11 and directed to a further treatment not shown in the FIG. 3. Light insoluble fraction is recovered through the device 13 is evacuated by the pipe 12 and directed to a further treatment. The heavy insoluble fraction is recovered by the device 15, evacuated through the pipe 14 and directed to a further treatment. The liquefied thermoplastics are recovered from the zone 8 through the pipe 16 and directed to a further treatment. The invention is further illustrated by the following examples.

EXAMPLE 1

(5) A 2 liter closed tank is externally heated at a temperature in the range of 150-300 C. The waste plastic composition used has the following composition (in g/kg solids):

(6) TABLE-US-00001 Component g/kg solids (dry and air free) HDPE 442 PP 326 PS 81 PET 119 PVC 4 Foreign materials (1) 28 Water 100 Air 20 (1) Metal, dust, stone, wool, cardboard, paper and the like
Said composition comprising the mixture of plastics of which the air has been evacuated by a vacuum pump up to a residual pressure of 10 mbar and replaced by nitrogen is continuously fed into the reactor at the rate of 0.4 kg/h along with 0.92 kg/h of docosane as solvent. The heating is conducted by an external electrical heat jacket. Pressure in the reactor is in the range of 1.1 to 1.5 bar absolute, the residence time of the plastic is approximately 30 min. The tank is divided by a separation screen in two zones, one be mixed with a steering impeller where the incoming load of thermoplastic particles is fed in over-feed within the tank to come in contact with the solvent. In the second zone, which is non-mixed, the melted liquid is decanted, both by sedimentation of the heavy undesired solids and by upwards flotation of light undesired substances. Heavy undesired solids comprised such component as metal, dust, stone or paper. Light undesired substances comprise components as wool or cardboard. Gaseous substances are recovered from the top of the tank, and send to a condenser to condense and recover condensable fraction for a separate treatment process.

(7) The recovered liquefacted thermoplastics from the reactor show a residual impurity content of below 2% by weight and therefore could be excellently used for thermal cracking following process step.

EXAMPLE 2

(8) A sample of HDPE and PP post-consumer plastic is introduced in a laboratory test tube equipped with a stopper. Air is evacuated by a vacuum pump up to a residual pressure of 10 mbar and replaced by nitrogen. The residual O.sub.2 content is estimated to 0.2 to 0.3% volume. The tube is heated externally up to 280 C. The thermoplastic mixture melts and stays clear as seen in FIG. 1.

EXAMPLE 3

(9) The same procedure as in Example 2 is repeated but without replacing the air contained in the test tube by inert atmosphere. The O.sub.2 content in the gas phase is estimated at 21 to 22 vol %. The tube is heated externally up to 280 C. The plastic does not melt but carbonizes as seen in FIG. 2.

EXAMPLE 4 to 7

(10) 2 g of HDPE, 0.5 g of PP and 25 mg of PS small pieces of different commercial plastic items and 10 g of solvent were introduced in a glass test tube of 30 ml equipped with a magnetic agitator at room temperature. The glass tubes were then flushed with argon to remove most of the O2 atmosphere and the tubes were sealed with a conventional stopper. The tubes were heated to 200 C. in an oil bath in four steps (120, 150, 180 and 200 C., respectively) under agitation and the mixture was observed.

(11) 4 different tubes were prepared with the following results:

(12) TABLE-US-00002 Example 4 5 6 7 Solvent Octane Docosane Hexatriacontane 40% Octane + 40% Octadecane + 20% Hexa-triacontane Boiling 125 C. 369 C. 265 C. under 1 mmHg Not determined point of the solvent Melting Not 45 C. 76 C. Not determined point of the determined solvent Room Liquid Solid solvent Solid solvent Liquid + solid solvent temperature solvent + and plastic and plastic and solid plastic solid plastic 120 C. Solvent: Solvent: Solvent: liquid Solvent: liquid liquid liquid Plastic: solid Plastic: solid Plastic: Plastic: solid solid 150 C. Solvent: Solvent: Solvent: liquid Solvent: liquid liquid liquid Plastic: solid Plastic: solid Plastic Plastic partially partially solubilized solubilized 180 C. Not totally Soluble Not totally Soluble soluble soluble 200 C. Soluble Soluble Soluble Soluble

(13) Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it may render a term unclear, the present description shall take precedence.