Patent Description:
End of life plastic chemical recycling technology is generally designed to recycle mixed waste-plastics into a variety of liquid hydrocarbon products. The waste plastics for use in such a process may, for example, include low density polyethylene (LDPE), high density polyethylene (HDPE), polystyrene (PS), and/or polypropylene (PP). These waste plastics are converted into the liquid hydrocarbon products by extruding and pumping the plastic feed in molten form into reactor vessels. The reactor vessels are externally heated by furnace systems to a temperature in excess of <NUM>. This produces rich saturated hydrocarbon vapour from the molten plastic. This flows out of the reactor vessels through contactor vessels and will condense the heavier vapour fractions to maintain a target outlet temperature set point which is determined by the end-product specification. This is then distilled at near-atmospheric pressures in a downstream atmospheric distillation column.

Each reactor vessel is fitted with an internal rotating agitator assembly. This agitator assembly assists with the mixing of the contents of the reactor vessel. Such an illustrative assembly will be described in more detail in relation to <FIG>.

These agitators are installed within the reactor vessel in order to improve a number of functions of the system. In particular, the agitators may increase the thermal homogenisation of the molten plastic mixture. This may reduce the reaction time by maximising the heat transfer from reactor vessel skin and preventing cold spots from forming. This thermal homogenisation may further prevent vapour bubbles from forming within the plastic mass of more volatile hydrocarbon chains. This may then reduce the risk of subsequent pressure and/or temperature spikes. The agitators may remove problematic by-products (material known as "char") which lead to coking forming on the inner surface of the reactor vessel wall. An excess build-up of char may inhibit thermal conduction from the outer furnace to the molten plastic. The agitator may also help char drying by continuously mixing it and bringing it into contact with hotter portions of the reactor vessel, such as the vessel skin. Finally, the agitator may improve char by-product removal by forcing the char from the reactor vessel.

Char is a fine carbon-based powder which is abrasive in nature. The working parts inside the reactor vessel, such as the agitator, are constantly exposed to this char. This char may be packed into the interstitial space between the outer diameter defined by rotation the agitator and the inner diameter of the reaction vessel. Over time the char may be compacted in this space as it is forced there by the agitator. Additionally, exposure of molten plastic to the comparatively higher temperature on the inner surface of the vessel may lead to a build-up of fouling. This may lead to the formation of a hard, brittle carbon-based coking layer which can have negative effects on heating performance, batch processing times, system efficiency and longevity.

There is therefore a need for an improved agitator assembly design which may reduce the effect of coking build-up on a reaction process.

<CIT> discloses a pyrolysis reactor system comprising a reactor and a contactor mounted above the reactor.

<CIT> discloses a polymerization reactor scraper assembly and method for its use.

<CIT> discloses a reactor, a process for producing a prepolymerization catalyst for olefin polymerization, and a process for producing an olefin polymer.

<CIT> discloses an agitator for treating mineral oils.

<CIT> discloses a support lever which is attached to a central shaft of a stirring blade support shaft of a rotary blade type stirring machine provided in a shaft type cylindrical tank so as to be crossed at right angles.

<CIT> discloses a central part of a scraper which is connected to a frame, with two scraping blades arranged symmetrically around the central part.

<CIT> discloses a scraping device intended to be mounted on a rotary agitator of a mixer.

<CIT> discloses an agitator wiper for a tubular chemical vessel in which agitation of vessel contents is achieved by a blade positioned helically around a central rotating shaft and wiping of vessel walls is achieved by segmented plates positioned along the blade and constructed to provide wiper tips which wipe all portions of the vessel walls.

<CIT> discloses an inner surface of a processing tank which is composed of a cylindrical part, a nearly spherical bottom and a curved part connecting the cylindrical part and the bottom.

<CIT> discloses a device for the removal of substances on inner walls of agitated reaction vessels which includes scrapers.

A reactor assembly for heating plastic material according to the present invention is provided according to claim <NUM>.

The agitator is mounted within the reactor vessel, but is not necessarily entirely within the reactor vessel. A portion of the agitator may extend outside the reactor vessel.

This reactor assembly allows the build-up of char to be removed without damage to the agitator itself or to the reactor vessel. Instead, the wearing parts engage the char.

The distal blade(s) may be generally helical.

Helical blade(s) are particularly effective for ensuring consistent mixing in the reactor vessel.

The wearing part(s) may be detachably mounted to the blade(s).

This allows the wearing part(s) to be replaced as they are worn down, instead of the traditional replacement of the agitator blades.

The wearing part(s) may be adjustable for adjusting the radial distance of the wearing parts from the central axis.

This allows the distance between the wearing part(s) and the reactor vessel inner wall to be adjusted such that the wearing part(s) may then be re-positioned as they wear down in order to maintain a preferred distance between the part(s) and the wall. In particular, this distance may define a maximum allowable coking layer thickness.

The wearing part(s) or the blade(s) may comprise an elongate slot, and the wearing parts may be mounted to the arm(s) via one or more bolts passing through the slot.

This is a simple method to achieve the adjustment.

The assembly may further comprise one or more sprung washer(s) and the one or more bolts may pass through the sprung washer(s) to engage the wearing part(s).

These sprung washers maintain a strong attachment between the blade and the wearing part(s) even if the bolted connection loosens during use.

The agitator may comprise a plurality of blades, preferably two blades.

This is a preferable arrangement for consistent mixing.

Each blade may comprise a plurality of wearing parts distributed along its length such that generally a plane orthogonal to the central axis at any point along the central axis in the region of the blades passes through at least one wearing part, preferably only one wearing part.

This can ensure that the entire inner surface is contacted by at least one wearable part.

The wearing part(s) may be formed of steel, preferably austenitic stainless steel, more preferably AISI <NUM>, 316Ti, <NUM>, <NUM>, <NUM> and/or <NUM>.

A method of maintaining a maximum thickness of coking on an internal surface of a reactor vessel according to the present invention is provided according to claim <NUM>.

Maintaining the coking thickness below this set level increases the heat transfer from the furnace to the molten plastic and hence minimises the cycle time of the system.

The maximum thickness may be no more than <NUM>, preferably no more than <NUM>.

This thickness generally allows sufficient heat transfer for the system to operate normally.

The present invention will now be described with reference to the following Figures in which:.

The illustrative system, or reactor assembly, <NUM> as shown in <FIG> comprises a reactor vessel <NUM> in which is rotatably mounted an agitator <NUM>. The agitator <NUM> includes a central shaft <NUM> which generally extends longitudinally in the reactor vessel <NUM>. The agitator <NUM> is mounted such that it is rotatable about an axis X. The axis X is preferably generally coincident with the central shaft <NUM> and centre-axis of upper and lower openings of the reactor vessel <NUM>. A plurality of horizontal support bars <NUM> may extend from the central shaft <NUM> of the agitator <NUM>. Attached to the horizontal bars <NUM> are a plurality of agitator blades <NUM>. In alternative embodiments, the agitator <NUM> may not include any horizontal support bars <NUM> and the agitator blades <NUM> may directly attach to the central shaft <NUM> or may terminate at a free end.

While the present embodiment includes a plurality of agitator blades <NUM> it is anticipated that the agitator <NUM> may be designed in any suitable manner. In particular, the agitator <NUM> may include one agitator blade <NUM> or three or more agitator blades <NUM> in alternative embodiments. The agitator blades <NUM> are generally helical such that they are distally spaced from the central shaft <NUM> by generally constant distance. As such, the outer edge of the agitator blades <NUM> are generally spaced a constant distance from an inner surface of the reactor vessel <NUM>. The agitator blades <NUM> may comprise at their lower end (either as a separate part or integral therewith) an agitator base portion <NUM>. The base portion <NUM> generally conforms to a lower curved surface of the reactor vessel <NUM>. The base portion <NUM> may be a separate component which is attached to a main portion of the agitator blades <NUM>, or the base portion <NUM> may be integrally formed therewith.

In use, plastic is fed into the reactor vessel <NUM>, preferably in the form of extruded melted plastic. The agitator <NUM> is driven by a motor and gearbox assembly to rotate about axis X. The agitator blades <NUM> then rotate in the plastic and ensure mixing of this plastic throughout the reactor vessel <NUM>.

As discussed above, as the system <NUM> is operated, a layer of coking begins to form on an inner surface of the reactor vessel <NUM>. This is shown in <FIG> which depict a representative close-up view of a cross-section of the wall of the reactor vessel <NUM>. <FIG> shows a system <NUM> early on in the life cycle of the agitator <NUM>. As can be seen, the agitator blade <NUM> extends to a distally outmost point which is spaced a first distance from the inner surface of the reactor vessel <NUM>. This distance allows a build-up of coking <NUM> to form between the outermost point of the agitator blade <NUM> and the inner surface of the reactor vessel <NUM> as the agitator <NUM> will define an outer diameter as it rotates at this distally outer-most point. This amount of coking will have a thickness <NUM> which matches the distance between the new agitator blade <NUM> and the inner surface of the reactor vessel <NUM>. In preferred embodiments, this initial distance <NUM> may be less than or equal to <NUM>.

As the system <NUM> is operated over a number of cycles the agitator blade <NUM> is worn down by the constant contact with the coking <NUM>. As a result, the outermost point of the agitator blade <NUM> will no longer extend the same distance radially from the axis X as when it was initially installed. As a result, an additional build-up of coking 5A is able to form with a corresponding increase in coking thickness <NUM>. This increase in coking thickness <NUM> may reduce the efficiency of the reactor vessel <NUM>. In particular, the coking <NUM> may reduce the heat transfer to the molten plastics within the reactor vessel <NUM>.

In order to compensate for this reduced heat transfer, the reactor vessel <NUM> may be exposed to higher furnace temperatures. These higher temperatures may increase the wear on the reactor vessel <NUM>, and thereby shorten the service life of the reactor vessel <NUM>. In particular, creep deformation and resultant stress rates may be increased by the greater heat input. The greater heat input may also result in accelerated metal ageing, oxidation/scale spalling and/or embrittlement/hardening due to lattice grain growth. As a result of this, the reactor vessel <NUM> may need to be replaced more regularly than would otherwise be necessary. This reduced heat transfer may also require an extension of the batch processing time which slows down the overall process. The variability in batch times due to poor temperature control may then lead to further difficulties in downstream processes and variations in quality of output product.

An improved system, or reactor assembly, <NUM> according to the present invention is shown in <FIG>. This system <NUM> likewise includes a reactor vessel <NUM> and agitator <NUM> rotatably mounted therein. The agitator <NUM> is generally similar to that disclosed with respect to the illustrative system <NUM>. However, additional wearing parts <NUM> are mounted to the blades <NUM> of the agitator <NUM>. The wearing parts <NUM> are mounted on the blades <NUM>. The wearing parts <NUM> may be mounted to generally align with the outermost edge of the blades <NUM>. As the blades <NUM> wear down in use, the wearing parts <NUM> will then extend past the outermost edge of the blades <NUM>. Alternatively, the wearing parts <NUM> may be mounted such that they extend distally further from the central shaft <NUM> than the outermost point of the agitator blades <NUM>.

As a result, during rotation of the agitator <NUM> the wearing part <NUM> will preferentially contact the coking <NUM> forming on the inner surface of the reactor vessel <NUM>.

While a single wearing part <NUM> could be provided along a length of each agitator blade <NUM> this is not preferable. Instead, a plurality of wearing parts <NUM> is preferably provided distributed along the agitator blades <NUM>. Preferably, the wearing parts <NUM> are distributed without a gap therebetween. However, they may be spaced apart on each blade <NUM> with wearing parts <NUM> from different blades <NUM> overlapping (for example, axially) so that the wearing parts <NUM> are distributed along the agitator blades <NUM> such that a plane orthogonal to the axis X (i.e. a plane extending into the page in <FIG>) can be defined along the portion of the axis X corresponding to the agitator blades <NUM> such that for any position along the axis in this region the plane passes through at least one wearing part <NUM>. Preferably, the plane passes through only one wearing part <NUM> at any point. There may be a negligible amount of overlap between wearing parts <NUM> in order to ensure a continuous provision of wearing parts <NUM> over the entire region of interest.

The wearing parts <NUM> may be detachably fixed to the blade <NUM> such that they can be removed from the blade <NUM> in a non-destructive manner. That is, the wearing parts <NUM> can be removed from the blade <NUM> without any damage being done to the blade <NUM>. This allows the wearing parts <NUM>, which will wear at a greater rate than the rest of agitator <NUM>, to be replaced with new wearing parts <NUM> without a total replacement of the agitator <NUM> or originally provided components such as the blades <NUM> or base portions <NUM>. As such, the life of the agitator <NUM> may be extended.

Additionally, it is very difficult for large reactor vessels <NUM> to be accurately cylindrical. For example, its diameter may vary along its length. The wearing parts <NUM> can be mounted on the blades <NUM> in order to conform to the particular non-cylindrical shape of the vessel <NUM>. As a result, the agitator <NUM> with wearing parts <NUM> may more closely contact the inner surface of the vessel <NUM> and thereby minimise the thickness of the coking layer <NUM>.

Furthermore, the wearing parts <NUM> may be attached to the blade <NUM> such that they are adjustable. In particular, it may be possible to adjust the radial extension of the wearing parts <NUM> from the blade <NUM>. In order to achieve this, the wearing parts <NUM> may be provided with an elongate slot <NUM> formed therein. A nut and bolt connection <NUM> passes through this slot <NUM> and a corresponding hole in the blade <NUM>. As such, the wearing part <NUM> can be slid radially by the bolt <NUM> sliding in the slot <NUM>. The bolt <NUM> may be tightened to hold the wearing part <NUM> in place. This is shown in <FIG>. As a result of this, when the wearing part <NUM> wears down and the coking <NUM> may begin to increase in thickness <NUM>, the system <NUM> can be stopped and the wearing plate <NUM> adjusted to maintain the initial coking thickness <NUM>. Alternatively, the elongate slot <NUM> may be formed in the blade <NUM> and the hole can be formed in the wearing part <NUM>. This alternative allows operation in the same manner.

In preferred embodiments, the coking thickness <NUM> can be maintained to be less than <NUM>. Spring washers <NUM> can be provided with the nut and bolt assembly <NUM>. The spring washers <NUM> store elastic potential energy and thereby provide a clamping force to clamp the wearing parts <NUM> to the blade <NUM>. As a result, even if the bolt assembly <NUM> relaxes in tightness during use the wearing part <NUM> is held rigidly to the blade <NUM>. Additionally, the wearing part <NUM> may be spot welded <NUM> to the blade <NUM>. This may prove particularly useful in agitators <NUM> which are designed to run at a high angular velocity and/or with an increased rotating mass. This arrangement may also assist with agitators <NUM> designed to be used in a high thermally-cyclic service. Such spot welds <NUM> can be easily removed in the event of readjustment or replacement as discussed above.

The agitator <NUM> of the improved system <NUM> can be provided as a kit of parts including an agitator <NUM> and the wearing parts <NUM> for attachment thereto. Alternatively, the wearing parts <NUM> may be provided separately and are retro-fittable onto an existing agitator <NUM>.

The wearing parts <NUM> may be made of any suitable material. However, in particular embodiments the wearing parts <NUM> may be formed of a steel. This steel may preferably be an austenitic stainless steel. More preferably this steel may be AISI <NUM>, 316Ti, <NUM>, <NUM>, <NUM> and/or <NUM>. These steels have been shown to provide a good balance of high temperature strength, hardness, corrosion resistance and availability. Such steels also do not have any requirement for post-fabrication heat treatment. As there may be risk of the wearing part <NUM> contacting the inner surface of the reactor vessel <NUM> during operation it is inadvisable to use very high hardness steels as this can introduce additional wear on the wall of the reactor vessel <NUM>.

Claim 1:
A reactor assembly for heating plastic material comprising:
a reactor vessel (<NUM>) comprising a central axis (X);
a furnace arranged to heat the reactor vessel (<NUM>) to a temperature in excess of 350ºC; and
an agitator (<NUM>) mounted within the reactor vessel (<NUM>), the agitator (<NUM>) comprising:
one or more blade(s) (<NUM>) distal from the central axis (X) for mixing contents of the reactor vessel (<NUM>) in use; and
one or more wearing part(s) (<NUM>) mounted to the blade(s) (<NUM>) to extend from the blade(s) (<NUM>) for engaging with and removing char from the reactor vessel (<NUM>), wherein the wearing part(s) (<NUM>) are spaced at a first distance from an internal surface of the reactor vessel (<NUM>).