Process for conversion of plastics into fuel

Abstract

A process for thermal cracking of a feedstock of plastic materials, in particular waste materials, includes the steps of melting the feedstock, conveying melted feedstock in a pyrolysis chamber where the melted feedstock is heated in a substantially oxygen purged environment, to convert it into pyrolysis gases, the process further comprising the steps of: driving pyrolysis gases from the pyrolysis chamber into a tray reflux column comprising a partial condenser at its upper extremity, returning pyrolysis gases condensed in the tray reflux column into the pyrolysis chamber, distilling pyrolysis gases exiting the partial condenser of the reflux column, to provide one or more fuel products.

Claims

1. A process for thermal cracking of a feedstock of plastic materials in an apparatus, the apparatus comprising: a mixing and melting device; a pyrolysis chamber; and a reflux column connected to a gas exit of the pyrolysis chamber, wherein the reflux column comprises an upper extremity and a plurality of crossflow trays, the plurality of crossflow trays including an upper tray disposed above a lower tray, the reflux column comprises a partial condenser disposed in the upper extremity of the reflux column, wherein the plurality of crossflow trays are valve trays, the valve trays having a plurality of valves, each valve of the plurality of valves having a corresponding cap disposed thereupon; and wherein the process comprises the steps of melting the feedstock in the mixing and melting device, conveying melted feedstock in the pyrolysis chamber, where said melted feedstock is heated in a substantially oxygen purged environment to convert it into pyrolysis gases, said process further comprising the steps of: driving pyrolysis gases from the pyrolysis chamber into the reflux column, returning pyrolysis gases condensed in the reflux column into the pyrolysis chamber, distilling pyrolysis gases exiting the partial condenser of the reflux column, to provide one or more fuel products; wherein distilling pyrolysis gases exiting the partial condenser is done by flash distillation in a flash distillation column, the flash distillation column having an upper portion disposed above a lower portion; and wherein the flash distillation step comprises partial recirculation of liquid heavy hydrocarbons obtained at the bottom of the flash distillation column in the lower portion of the flash distillation column by spraying said heavy hydrocarbons.

2. The process according to claim 1, wherein the reflux column comprises a demister pad between the upper tray and the partial condenser.

3. The process according to claim 1, wherein the partial condenser of the reflux column is a tube heat exchanger.

4. The process according to claim 1, comprising the step of injecting liquid water or liquid hydrocarbons at the bottom of the reflux column.

5. The process according to claim 1, comprising the step of condensing pyrolysis gases exiting the partial condenser of the reflux column in a condenser before distilling said gases.

6. The process according to claim 1, wherein the flash distillation step comprises condensation of the vapor phase at the head of the flash distillation column and partial recirculation of condensed light hydrocarbons in the upper portion of the flash distillation column.

7. The process according to claim 1, wherein the crossflow trays comprise down comers.

8. The process according to claim 1, wherein the upper tray of the reflux column is a chimney tray.

9. The process according to claim 1, further comprising an extruder configured to heat the feedstock.

10. The process according to claim 9, wherein the extruder is configured to extrude the feedstock into the pyrolysis chamber.

11. The process according to claim 1, comprising a flash distillation column downstream of the partial condenser of the tray reflux column.

12. The process according to claim 11, comprising a condenser positioned between the partial condenser of the tray reflux column and the flash distillation column.

13. The process according to claim 11, wherein the flash distillation column comprises a condenser and a reflux drum at a top end of the flash distillation column, and a recirculation loop for recirculating condensed light hydrocarbons of the reflux drum in the upper part of the flash distillation column.

14. The process according to claim 11, wherein the flash distillation column comprises a recirculation loop for liquid heavy hydrocarbons obtained at the bottom of the flash distillation column, said recirculation loop being equipped with spray nozzles for spraying the liquid in the flash distillation column.

Description

DESCRIPTION OF THE DRAWINGS-EXAMPLE

(1) The invention will be further illustrated from the following description of some embodiments, given by way of example to the drawings in which:

(2) FIG. 1 represents a process diagram for thermal cracking of plastics according to the invention,

(3) FIG. 2 represents a tray reflux column according to the invention,

(4) FIG. 3 is a detailed view of a valve in a valve tray of the reflux column, and the effects on vapor/liquid/solid flows around it.

(5) FIG. 4 is a plot diagram of the fuel product boiling point ranges obtained after flash distillation in the process according to the invention (left curve), as compared to those obtained with state of the art processes (right curve).

(6) FIG. 5 represents a flash distillation column according to the invention.

REFERENCE NUMBERS

(7) 1. Extruder with molten plastic feed to 2 reactors 2. Pyrolysis reactor 3. Tray reflux column 3a. Partial condenser 4. Diesel, light oil and syngas vapors (pyrolysis gases) to flash column 5. Char conveying screw 6. Char cooling screw 7. Flash distillation column 7a. Condenser 7b. Vapor phase at the head of the distillation column 7c. Heavy liquid hydrocarbons obtained at the bottom of the distillation column 7d. Tray for extracting liquid hydrocarbons (light oil tray) 7e. Spray nozzle 7f. Packing elements 8. Diesel offtake 9. Diesel tank 10. Light oil offtake-reflux drum 10a. Condenser 10b. Condensed light hydrocarbons 11. Light oil tank 12. Syngas to scrubber/furnaces 13. Partial condenser (tube heat exchanger) 14. Demister 15. Chimney tray 16. Valve trays 17. Down comer 18. Column 19. Water/liquid hydrocarbon injection 20. Vapor flow upwards through valve 21. Liquid flow horizontally across trays 22. Char solids flow downwards due to eddies in vapour flow 23. Fixed valve with cap 24. Tray 25. Vapor rising upwards hitting underside of cap-causing eddies in flow 26. Char solids flow from washing out of vapor

(8) Waste plastic material, typically mainly containing polyolefins, typically between 70% to 90% (weight percentage) of low density polyethylene (LDPE) from 5 to 10% of polypropylene (PP), with minor amounts (typically less than 5% for each) of styrene polymers, polyvinylchloride and minor amounts of other plastics that maybe polyurethane, polyester, polyamide, is dry cleaned and processes into granular or flake form. It is then heated in an extruder (1) at a temperature around 300, and the molten plastic is then fed into the pyrolysis chamber (2). This is done under nitrogen purge, ensuring that no oxygen enters the system. The molten plastic is maintained at a temperature between 300° C. to 400° C. until the pyrolysis chamber is completely fed.

(9) In the pyrolysis chamber (2), the plastic material is heated up to a temperature between 380° C. and 450° C. in a nitrogen purged system under agitation, and this temperature is maintained during the pyrolysis. The melted plastic material is thermally cracked into pyrolysis gases and char formation occurs simultaneously in the pyrolysis chamber.

(10) An industrial installation implementing the process according to the invention will typically contain at least two pyrolysis chambers (2), since the process is a semi batch process. As one pyrolysis chamber is active, char cooking and removal takes place in the other one.

(11) As pyrolysis of the melted plastic is being completed, the load on the agitator increases, showing that char drying is taking place and batch is ending. A cook off cycle of the char may then take place, with further heating of the char above 450° C., to complete drying and remove remaining gases from it.

(12) For safety reasons, dried char then has to be cooled and removed from the pyrolysis chamber in a gas tight environment. Char may for example be carried from the pyrolysis chamber to a screw conveyor unit (5) through air tight valves, said screw conveyor being optionally cooled with an external cooling jacket (6).

(13) While the pyrolysis reaction is ongoing, pyrolysis gases are continuously extracted from the pyrolysis chamber as they rise through the tray reflux column (3) and the partial condenser (3a). Pressure in the pyrolysis chamber is maintained around 270 millibars by controlling the exit valve of the pyrolysis gases, connecting the pyrolysis chamber (2) to the tray reflux column (3).

(14) Part of the pyrolysis gases are condensed in the partial condenser (3a), which is for example tube heat exchanger using chilled water at around 5° C. as a cooling media. These condensed gases fall back into the tray reflux column, first though a chimney tray (15) designed to distribute these condensed liquid product down to the other trays of the column that are valve trays (16)). Liquid transfer from one valve tray to the valve tray below is made through downcomers (17). Valve trays are designed to ensure good contact and heat transfer between condensed liquid falling down and rising pyrolysis gases, so that longer chain hydrocarbon components are efficiently sent back to the bottom of the reflux column, and then back to the pyrolysis chamber for further cracking.

(15) Trays are typically stainless steel trays, in which valves or apertures covered with caps (23), have been provided. These are very effective not only for liquid gas transfer, but also for scrubbing of char particles entrained by the gases by the down flowing liquid. Vapor rising from underneath the tray hit the underside of the cap (23), causing eddies (25) in flow and char to remain on said underside of the cap, where they will be washed off by liquid flowing horizontally (21) across the tray and downward (22). The presence of downcomers (17) ensuring good liquid hold up on the trays also improves efficiency of char scrubbing.

(16) Final retention of the last char particle is made through a demister (14) composed of stainless steel mesh, and placed above the chimney tray of the reflux column. Said demister is ideally designed to be easily removed for cleaning.

(17) Pyrolysis gases (4) exiting the partial condenser (3a) have a temperature between 260 and 280° C. In order to complement the cooling capacity of the partial condenser, some liquid water, typically at a temperature between 5 and 20° C., may be injected at the bottom of the reflux column (19). This will not only improve cooling, but also agglomerate char particles, which will be more easily retained in the liquid phase and in the demister of the reflux column. If water is used, it may be separated and recovered at the bottom of the reflux drum (10) of the flash column. Liquid water may be replaced by liquid hydrocarbons, for example heavy hydrocarbons obtained after distillation of pyrolysis gases exiting the partial condenser. Liquid hydrocarbons are preferred because they will have less expansion in the reflux column (3) and will cause less perturbations of pressure in the reflux column.

(18) Said liquid injection may be periodical or continuous. In the case of a periodical injection, it may be triggered when the temperature of the pyrolysis gases (4) exiting the partial condenser of the reflux column exceeds a certain threshold.

(19) Gases (4) exiting the partial condenser at a temperature between 260 and 280° C., are cooled at a temperature around between 130° C. and 150° C., Condensation and cooling of pyrolysis gases (4) may naturally occur in the piping system of the installation. Preferentially, this is done in a condenser 7a.

(20) The resulting liquid, or more exactly saturated gas liquid mixture, containing hydrocarbons comprising around 1 to 25 carbon atoms, is then fed to a flash distillation column (7) where light fractions are vaporized and the heavier fraction remains liquid.

(21) A reflux drum (10) is provided at the head of the flash vessels. Syngas or non-condensable gases (12) from the reflux drum are sent to scrubbers and furnaces. The liquid part (11) is light oil and water, especially if liquid water injection (19) has happened in the process.

(22) Heavy fractions (diesel oil) are recovered at the bottom of the flash distillation vessel in a diesel tank (11). Further separation step may be implemented on said diesel oil, for example solid separation, tar separation, and optionally a vacuum distillation step to remove lighter fraction and correct flash point.

(23) The flash distillation column is a column where the saturated liquid gas mixture fed after condensation in condenser (7a) undergoes pressure reduction. The flash distillation column is typically operated at a pressure close to atmospheric pressure.

(24) A flash distillation column contains no internals, or very few internal elements, thus reducing fouling risks. According to the present invention, the internal volume, or the major part of the internal volume of a flash distillation column is empty, free of internals (typically at least 50%, or 80% or 90% of the internal volume of the column is free of internal elements).

(25) Typically, when a flash distillation column contains internal elements, no other internal elements than those listed below are present.

(26) Starting from the top, the flash distillation column may contain a recirculation device for liquid hydrocarbons from the reflux drum (10), a liquid distribution tray for said recirculated liquid, a minimum layer of packing elements (7f) to help prevent liquid entrainment, a light oil tray (7d) for extracting liquid hydrocarbon positioned in the middle, preferentially in the top third the column, and connected to a recirculation loop for said liquid, preferentially below the extraction level, said recirculation equipped with spray nozzle to spray recirculated liquid, and, in the lower part, and a recirculation loop for heavy hydrocarbons obtained at the bottom of the column, equipped as well with spray nozzle to spray recirculated heavy hydrocarbons.

(27) Said recirculation and spraying helps to control temperature of the column and also concentrates tar in the bottom part of the column.

(28) For avoidance of doubt, a flash distillation column is not an interfacial unit, filled with internal elements designed to increase its surface area, such as packing elements.

(29) Given the efficiency of the tray reflux column in terms of refluxing long chain hydrocarbons to pyrolysis, a simple flash distillation step provides hydrocarbons suitable for use in fuel blendings, such as for example diesel blendings for heavy fractions, or for use as chemical feedstock or heating oil. This has the advantage of reducing the risks of fouling by char and tar, since flash vessels are empty vessels that do not contain internals or at least very few.

(30) FIG. 4 compares properties of products obtained after flash distillation step of the process according to the invention with products obtained after complex atmospheric distillation, in processes using a contactor as described in patent application n°WO2011077419 instead of a tray reflux column according to the invention. One can see that the process according to the invention provides hydrocarbons with lower end boiling point and more favorable boiling points distribution (left curve).

(31) Lighter compounds may further be removed, for example by sending said products to a vacuum distillation step.

(32) Also given the efficiency of the tray reflux column in terms of char scrubbing, the process according to the invention can be run with a minimum of interruption for cleaning and maintenance. Up to 100 batches have been run consecutively without such interruption, as compared to around 30 with state of the art processes, where the installation had to be stopped to remove the column packing internals of the distillation column, that were filled with char.

(33) The availability of the industrial installation, typically treating around 4000 tons of plastic per year, has been improved with the use of a tray reflux column, as compared to contactors or reflux systems of the state of the art, due to the tray design knocking char particles back into the pyrolysis reactor due to the washing of the vapour flow.

(34) Char yield per batch (calculated as the ration between kilograms of char obtained after pyrolysis and kilograms of plastic fed in the pyrolysis chamber) in the pyrolysis increased from 5% on average to an average of 10%, indicating increased retention of char.