PROCESS FOR CONVERSION OF WASTE PLASTICS INTO HYDROCARBONS

Abstract

A process for production of chemical feedstocks from waste plastics, the process comprising providing a process configuration comprising a fractionation tower (1), a furnace (2), one or more coke drum(s) (3), and a pre-reactor (4), configured so that a bottoms stream (A) from the fractionation tower is mixed with an oligomeric stream (L) and supplied to the furnace, the product stream from the furnace (C) is supplied to a coke drum, and an overhead stream (D) from the coke drum is supplied back to the fractionation tower; wherein the oligomeric stream (L) is obtained as product stream from conversion of a waste plastics stream (B) in the pre-reactor, and has a weight average molecular weight of between 5,000 and 10,000 g/mol. Such process allows for the conversion of a wide variety of waste plastics into valuable chemical products.

Description

BACKGROUND

The present invention relates to a process for conversion of waste plastics into hydrocarbons, such as hydrocarbon that may be used to form new plastics. In particular, the invention relates to a process for conversion of waste plastics into hydrocarbons via delayed coking.

To the background of the present global developments to reduce energy and material footprint in amongst others the manufacturing of materials such as polymers and chemicals, there is a clear driver to seek reduction of energy and raw materials that are used in manufacturing of such materials. Particularly, there is a driver to enhance circularity of use of materials, after having arrived at end-of-life for a certain application, thereby reducing the use of virgin raw materials in the manufacturing of polymers and chemicals, which typically are fossil-feed based raw materials, obtained from crude oil or natural gas feedstocks. By increasing the circularity, the use of virgin raw materials is reduced, and thereby the materials footprint associated with the manufacturing of polymers and chemicals.

A promising way of increasing the circularity is by re-using end-of-life plastics, collected and made available as waste plastics streams. Such waste plastics streams can be obtained from consumer waste collection, from industrial waste collection, of by collection of waste as littered in the environment, either as aquatic littering or as land-based littering. Typically, such waste plastics streams are mixtures of various types and qualities of plastics. Through sorting, certain streams may be obtained that qualify for re-use as thermoplastics, either by directly subjecting them to thermal shaping processes or via blending them with high-quality virgin-type plastic materials to compensate for loss of properties. Such way of re-use of material however is only appropriate for a limited fraction of waste plastics that can be sorted out of the mixed waste streams as provided from waste collection so that the obtained stream has high uniformity of material composition.

Still, typically a significant portion of the waste plastics as provided by collection, if not a major portion, is not suitable for such direct re-use as polymer. Such mixed plastic waste commonly is discarded of by processes like waste incineration. In order to increase material circularity, there is a desire to develop alternative processing methods for such mixed waste plastic streams. One route for doing so is by means of chemical recycling, wherein the polymer materials that constitute the waste plastics streams are depolymerised to provide hydrocarbon materials that, directly or indirectly, can by once again be converted into polymers through polymerisation processes.

Such chemical conversion processes provide certain benefits, amongst others in that they may be operated using waste plastic compositions of varying nature, including waste plastic compositions that show particularly large variation in batch-to-batch composition.

It is particularly desirable for one to be able to utilise existing chemical conversion process technologies for the purpose of converting waste plastic streams into valuable hydrocarbon materials. Therefore, where a technology would be made available via which existing petrochemical assets would be rendered suitable for use in conversion of waste plastics streams into hydrocarbons, such would be broadly desirable.

SUMMARY

One such opportunity now is presented by certain refinery assets. In view of reduced hydrocarbon consumption for energy and transport, which is a current and expectedly further developing trend, a certain fraction of the ubiquitously present refinery assets may well become underutilised or even idled, and therefore available for alternative uses. One such use may well be the conversion of waste plastics into hydrocarbons that can serve as feedstocks for making chemical and/or polymer products, thereby creating a circular economy of plastics materials and reducing the material footprint.

In accordance with the present invention, this has now been provided by a process for production of chemical feedstocks from waste plastics, the process comprising providing a process configuration comprising a fractionation tower (1), a furnace (2), one or more coke drum(s) (3), and a pre-reactor (4), configured so that a bottoms stream (A) from the fractionation tower is mixed with a stream originating from waste plastics (L), and supplied to the furnace, the product stream from the furnace (C) is supplied to a coke drum, and an overhead stream (D) from the coke drum is supplied back to the fractionation tower;

• • wherein the stream (L) is obtained as product stream from conversion of a waste plastics stream (B) in the pre-reactor;
  • wherein the stream (L) is supplied in molten condition; and preferably wherein stream (L) has a weight average molecular weight of less than 100,000 g/mol; more preferably
  • wherein the stream (L) is an oligomeric stream having a weight average molecular weight of between 5,000 and 50,000 g/mol, even more preferably between 5,000 and 10,000 g/mol.

Such process allows for the conversion of a wide variety of waste plastics into valuable chemical products. Furthermore, such process allows for the increase in production of naphtha-range hydrocarbon products that may be used for production of for example new polymer products via conversion in steam cracker facilities.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an embodiment in accordance with the present disclosure; and

FIG. 2 shows an embodiment in accordance with the present disclosure.

DETAILED DESCRIPTION

The process in accordance with the present invention is further elucidated by the FIG. 1. In FIG. 1, the unit (1) represents the fractionation tower. The furnace is represented by unit (2). The coke drum(s) are represented by units (3). A pre-reactor is represented by unit (4). This configuration presents a representative embodiment of the invention, but does not limit the invention thereto. The process according to the present invention may comprise one single coke drum (3), or multiple coke drums. FIG. 1 presents a particular embodiment of the invention, wherein two coke drums are present. Such configuration allows for switching operations between the coke drums so that one drum can be used in operation whilst the other drum can be cleaned out. From the fractionation tower, a bottoms stream (A) is provided to feed the furnace. The bottoms stream (A) may for example be supplied at a temperature of ≥300 and ≤400° C. Out of the furnace, a furnace product stream (C) is provided to coke drum(s). Feed (B) is a waste plastics stream, which may be first reacted in the pre-reactor (4) to reduce the molecular weight and by so form an oligomer (L) which is then mixed with the bottoms stream (A) and fed into the furnace. Out of the coke drums, a cracked overhead product (D) is obtained that is to be supplied to the fractionation tower for fractionation, with a fraction residual oil (K), into typically a top gaseous stream (F), a naphtha-range stream (G), a light gas oil stream (J), a heavy gas oil stream (H), and a bottoms stream (A). Further, out of the coke drums, a coke stream (E) is obtained, which accumulates in the drum, to be evacuated therefrom upon discontinuation of the operation of that drum.

The furnace (2) that may be used in the process according to the present invention may for example be a furnace in which multiple feed tubes are passed through a heating chamber, also referred to as a firebox, so that the feed is heated by external heating. The tubes may be passed through the firebox multiple times, for example two or four times. The heat may for example be provided by burners placed below the tubes. The burners may be controlled in such way to provide the required heating of the feed in the tubes to obtain the desired temperature of the feed exiting the furnace. In order to ensure that the coke formation of the feed does not occur in the furnace tubes, but is delayed until the feed materials reach the coke drum, the mass velocity of the feed through the furnace is preferably greater than 1800 kg/s/m². A quantity of steam may be added to the feed tubes, such as for example between 0.1 and 2.0 wt % of steam with regard to the total weight of the feed. Such addition of steam contributes to increase of velocity in the tubes. The combined feed (A) and (B) may be heated in the furnace by passing the feed through heating tubes and subjecting it to external heat energy to obtain a furnace product stream (C) having a temperature of ≥450° C. and ≤550° C., preferably of ≥475° C. and ≤500° C.

The feed exits the furnace at a temperature of preferably ≥450° C. and ≤550° C., more preferably of ≥475° C. and ≤500° C. Upon exiting the furnace, the heated furnace product stream (C) is transported via a transfer line into a coke drum. It is desirable that the residence time in the transfer line is kept as short as possible, to avoid occurrence of coking prior to reaching the coke drum. Accordingly, it is desirable to keep the transfer line as short as possible. Furthermore, as typically a furnace is connected to multiple coke drums to ensure continuous operations, a switch valve may be present in the transfer line, to allow directing the feed to a desired coke drum.

A typical coke drum that may be used in the process according to the present invention may have a diameter of between 4 and 9 m., and a length of between 20 and 30 m. The drum typically is positioned vertically. The drum may be operated at a pressure of between 100 and 600 kPa, such as between 200 and 300 kPa.

A typical configuration may involve two or more, often two, coke drums, so that one drum may be in operation whilst the other drum(s) may be subjected to coke removal and cleaning before switching back in operation again. As coke is deposited into the drum, the drum needs to be evacuated from time to time in a batch operation.

As a result of the temperature in the product exiting the furnace, a cracking process occurs that in the coke drum results in a top product that is continuously removed from the drum as overhead stream (D), and a bottoms product, being the coke, that is removed as stream (E) when the feed to the drum is discontinued. The overhead stream (D) may be removed from the drum at a temperature of below 500° C., such as between 475° C. and 500° C., to avoid coke formation in the transport line.

The overhead stream (D) is supplied to the fractionation tower (1). In the fractionation tower, a separation process is performed so that a gaseous stream (F), a naphtha-range stream (G), a light gas-oil stream (J), and a heavy gas-oil steam (H) are obtained. Furthermore, a bottoms steam (A) is obtained that is recycled to the furnace (2). The fractionation tower is further fed with fresh residual oil (K), which preferably is fed to the bottom part of the fractionation tower to avoid condensation of vapours in the upper parts of the tower. The fractionation tower may for example be operated so that the temperature in the bottom section of the tower is between 340° C. and 385° C.

In an embodiment according to the invention, in the fractionation tower a fractionation of a mixture comprising the overhead stream (D) and a residual oil (K) is performed to result in a gaseous output stream (F) obtained as overhead stream from the fractionation tower, a naphtha-range stream (G), a light gas oil stream (J), a heavy gas oil stream (H), and a bottoms stream (A).

As feed (K) to the fractionation tower, a residual oil stream obtained from refinery operations may be used. The residual oil may be a residual oil from atmospheric distillation (ADR), or may be a residual oil from vacuum distillation in a refinery (VDR). Preferably, the residual oil is a residual oil from vacuum distillation. In the context of the present invention, ADR is to be understood to be the fraction of crude oil having an initial boiling point of above 340° C. In the context of the present invention, VDR is to be understood to be the fraction of crude oil having an initial boiling point of above 535° C.

The stream (L) may for example be provided at a temperature of ≥300 and ≤450° C., preferably of ≥300 and ≤400° C. The stream (L) may be prepared by converting solid waste plastics into a molten stream via one of more melt extruder(s) as unit (4). The waste plastics stream may for example by supplied by a melt extruder that is connected to the feed line to the furnace.

In certain embodiments of the invention, the pre-reactor (4), preferably being a melt extruder, is operated under such conditions that the oligomer stream (L) has a weight average molecular weight of less than 100,000 g/mol, preferably between 5,000 and 50,000 g/mol, more preferably between 5,000 and 10,000 g/mol. This involves operating the pre-reactor, preferably being a melt extruder, under such conditions that certain degradation of the waste plastic material that is supplied to the extruder occurs during the melt extrusion operation. During the melt extrusion operation, in such circumstances, the waste plastic material may be subjected to each or both of high temperature and high shear which result in chain fission. Such chain fission leads to reduction of the weight average molecular weight of the plastic material, such that the oligomer stream (L) that, upon exiting the melt extruder, is supplied as feed to the furnace (2) has a reduced weight average molecular weight, such as a weight average molecular weight of less than 100,000 g/mol, preferably between 5,000 and 50,000 g/mol, more preferably between 5,000 and 10,000 g/mol. In the context of the present invention, the weight average molecular weight of the stream (L) may for example be determined according to the method of ASTM D6474-12.

For example, the pre-reactor (4) may be operated at a temperature of ≥350 and ≤450° C. Preferably, the pre-reactor (4), preferably a melt extruder, is operated at a temperature of ≥400 and ≤450° C. In order to adequately perform the chain fission process, it is desirable that the waste plastic is subjected to a certain lengthy residence time in the pre-reactor (4). For example, the waste plastic may be subjected to a residence time of ≥10 min, preferably of ≥10 and ≤30 min.

In one embodiment of the invention, as shown in FIG. 1, the pre-reactor (4), which may for example be a melt extruder such as a twin-strew melt extruder, is operated under such conditions as to effect the removal of chlorine from the waste plastics stream (B). In this operation, the solid mixed waste plastics stream (B), which may for example comprise 80-90 wt % of polyolefins, and 1-5 wt % of chlorine-containing polymers such as polyvinylchloride (PVC), is heated to such temperature where the PVC is molten and decomposes to form a gaseous HCl product that is removed as stream (M). Stream (M) may further be contacted with an aqueous stream (R) containing a base such as NaOH to neutralise the HCl and thereby produce an aqueous stream (S) containing NaCl. The removal of the gaseous HCl stream (M) from the pre-reactor (4) may be further enhanced by applying a vacuum to the pre-reactor (4), or by use of an inert sweep gas that may be injected to pre-reactor (4) as stream (N). The contact time in the pre-reactor (4) preferably is sufficient to remove >95%, preferably >99%, of the chlorine from stream (L).

In another embodiment of the invention, as shown in FIG. 2, the pre-reactor (4) comprises a first pre-reactor (4A) and a second pre-reactor (4B). In the first pre-reactor (4A), which may for example be a melt extruder such as a twin-screw melt extruder, the solid mixed waste plastics stream (B), which may for example comprise 80-90 wt % of polyolefins, and 1-5 wt % of chlorine-containing polymers such as polyvinylchloride (PVC), is heated to such temperature, preferably ≥300° C. and ≤350° C., where the PVC is molten and decomposes to form a gaseous HCl product that is removed as stream (M). Stream (M) may further be contacted with an aqueous stream (R) containing a base such as NaOH to neutralise the HCl and thereby produce an aqueous stream (S) containing NaCl. The removal of the gaseous HCl stream (M) from the first pre-reactor (4A) may be further enhanced by applying a vacuum to the first pre-reactor (4A), or by use of an inert sweep gas that may be injected to first pre-reactor (4A) as stream (N). The contact time in the first pre-reactor (4A) preferably is sufficient to remove >95%, preferably >99%, of the chlorine, to form a molten plastic stream (B′). Stream (B′) may then be supplied to a second pre-reactor (4B), which may for example be a melt extruder such as a twin-screw melt extruder, wherein stream (B′) is processed at a temperature of ≥350 and ≤450° C., preferably ≥400 and ≤450° C., during a residence time of ≥10 min, preferably of >10 and ≤30 min, to form the oligomeric stream (L). Under such conditions, the chain fission process may be adequately performed.

The waste plastics stream (B) that is subjected to the process of the present invention preferably comprises a major quantity of polyolefin plastics. For example, the waste plastics stream may comprise ≥60.0 wt % of polyolefin plastics, preferably ≥75.0 wt %.

Claims

1. A process for production of chemical feedstocks from waste plastics, the process comprising providing a process configuration comprising a fractionation tower (1), a furnace (2), one or more coke drum(s) (3), and a pre-reactor (4), configured so that a bottoms stream (A) from the fractionation tower is mixed with a stream originating from waste plastics (L), and supplied to the furnace, the product stream from the furnace (C) is supplied to a coke drum, and an overhead stream (D) from the coke drum is supplied back to the fractionation tower; wherein the stream (L) is obtained as product stream from conversion of a waste plastics stream (B) in the pre-reactor;wherein the stream (L) is supplied in molten condition.

2. The process according to claim 1, wherein in the fractionation tower a fractionation of a mixture comprising the overhead stream (D) and a residual oil (K) is performed to result in a gaseous output stream (F) obtained as overhead stream from the fractionation tower, a naphtha-range stream (G), a light gas oil stream (J), a heavy gas oil stream (H), and a bottoms stream (A).

3. The process according to claim 1, wherein the stream (L) is supplied at a temperature of ≥300 and ≤450° C.

4. The process according to claim 1, wherein the bottoms stream (A) is supplied at a temperature of ≥300 and ≤400° C.

5. The process according to claim 1, wherein the combined feed (A) and (B) are heated in the furnace by passing the feed through heating tubes and subjecting it to external heat energy to obtain a furnace product stream (C) having a temperature of >450° C. and ≤550° C.

6. The process according to claim 1, wherein the bottoms stream (A) has a boiling point of ≥400° C.

7. The process according to claim 1, wherein the pre-reactor (4) is a melt extruder.

8. The process according to claim 1, wherein the pre-reactor (4) is operated at a temperature of ≥350 and ≤450° C., and/or wherein the waste plastic is subjected to a residence time in the pre-reactor of ≥10 min.

9. The process according to claim 2, wherein the residual oil (K) is a residual oil obtained from atmospheric distillation of crude oil, or a residual oil obtained from vacuum distillation of the residual oil obtained from atmospheric distillation of crude oil.

10. The process according to claim 1, wherein the coke drum is operated at a pressure of between 100 and 600 kPa.

11. The process according to claim 1, wherein the fractionation tower operated so that the temperature in the bottom section of the tower is between 340° C. and 385° C.

12. The process according to claim 1, wherein the furnace product stream (C) comprises ≥0.1 and ≤50.0 wt % of the stream (L), with regard to the total weight of the stream (C).

13. The process according to claim 1, wherein a gaseous stream (M) comprising HCl that is formed from decomposition of chlorine-containing polymers, is removed from the pre-reactor (4), wherein the pre-reactor (4) is operated at a temperature where decomposition of the chlorine-containing polymers that are present in the waste plastics stream (B) occurs.

14. The process according to claim 1, wherein the pre-reactor (4) comprises a first pre-reactor (4A) and a second pre-reactor (4B), wherein in the first pre-reactor (4A), which may for example be a melt extruder such as a twin-screw melt extruder, the solid mixed waste plastics stream (B), which may for example comprise 80-90 wt % of polyolefins, and 1-5 wt % of chlorine-containing polymers such as polyvinylchloride (PVC), is heated to such temperature, where the chlorine-containing polymers are molten and decompose to form a gaseous HCl product that is removed as stream (M), so that a molten, dechlorinated plastic stream (B′) is formed, which then is supplied to a second pre-reactor (4B), which may for example be a melt extruder such as a twin-screw melt extruder, wherein stream (B′) is processed at a temperature of ≥350 and ≤450° C., during a residence time of >10 min to form the oligomeric stream (L).

15. The process according to claim 13, wherein the stream (M) is further contacted with an aqueous stream (R) containing a base to neutralise the HCl and thereby produce an aqueous stream (S).