BACKGROUND OF THE INVENTION

[0001] This invention relates to the production of wax.

[0002] Many waxes have traditionally been produced from fossil fuels. However, plastics such as polyethylene, polypropylene and polystyrene represent significant sources of materials from which waxes can be produced. In particular waste sources of these polymers can be recycled and depolymerised to produce hydrocarbons which are similar in nature to waxes.

[0003] The demand for waxes continues to increase whilst the supply is under pressure. For crude oil (petroleum waxes) there is significant environmental pressure on the reduction of fossil fuels, coupled with improving efficiency of refineries to produce less wax and more fuel. Natural waxes are limited in their economic feasibility and abundance. Synthetic waxes also rely on fossil fuels and carry a high carbon footprint and are under environmental pressure. Therefore recycled polymer waxes represent a significant feedstock opportunity, with a positive environmental impact.

[0004] One advantage of using waste polymers as a feedstock to produce wax is that substantial quantities of waste polymers exist. A difficulty which does arise, when using recycled feedstock, is that the final product has a reduced value due to contaminants in the material. These contaminants discolour the material and can give off unpleasant odours.

[0005] Different approaches to produce wax from plastic waste are disclosed in the prior art.

[0006] US 9080107 describes a method of producing a high value hydrocarbon product from a plastic waste. In this approach the waste is fed to an extruder and melted whereafter the melted plastics is depolymerised. Vapours from the depolymerisation are separated into hydrocarbon fractions which are refined and recovered as desired hydrocarbon products.

[0007] US 10723858 details a process which makes use of supercritical CO2 as a solvent into which contaminants from depolymerised plastic waste feedstock migrate. This approach produces a high value product with a high molecular weight and a high melting point wax.

[0008] The aforementioned techniques are however expensive and complex to implement.

[0009] It has also been proposed (WO 2017/161463) to use a selected wax as a solvent and then to leave the wax in a blend instead of fractionating the resultant product. A drawback is that the wax which is used as a solvent can materially change the properties of the final product. [0010] US 8664458 discloses the production of wax by converting mixed polyethylene waste through catalytic depolymerisation. This process uses a catalyst in a high-pressure reactor at a desired temperature.

[0011] The present invention is concerned with a method of purifying recycled depolymerised plastic waste material.

SUMMARY OF THE INVENTION

[0012] The invention is based on the realisation that a wax obtained from a recycled polymer waste material can be purified using a filtration technique. An obstacle which stands in the way of this approach is that this type of wax normally has a high viscosity which means that the wax, in molten form, cannot easily be subjected to filtration.

[0013] The applicant is aware that waxes produced by the Fischer-Tropsch process have a linear structure with a high portion of normal paraffins.

[0014] The Fischer-Tropsch waxes are synthetically produced by using gas-to-liquids (GTL) technology. The Fischer-Tropsch process is a collection of chemical reactions that converts a mixture of carbon monoxide and hydrogen, known as syngas, into liquid hydrocarbons. These reactions occur in the presence of metal catalysts such as cobalt, iron or nickel. The type of products formed are determined by the temperature and pressure of the reaction. [0015] The product stream produced by relative lower temperatures and pressure conditions consist mainly of straight chain hydrocarbons (n-alkanes) with low amounts of branching and unsaturation.

[0016] The reactions produced a range of molecules with a carbon distribution which is spread from approximately C5 to C120. The reactor product is fractionated into several different distillation cuts to produce a range of waxes with different melting points ranging from liquids at room temperature to high melting hard waxes. Some of these hard waxes are hydrogenated to convert the unsaturated molecules to saturated molecules which produce wax products that are pure white with the combination of high melting point, low viscosity and excellent hardness even at elevated temperatures. The high long chain n- alkane content produces waxes with high crystallinity.

[0017] The invention utilizes the discovery that when a high-melting point Fischer- Tropsch wax (“FT wax”) with a linear structure is blended in a suitable ratio with a wax produced from depolymerised plastic material the viscosity of the blend is lowered to a level at which a low-cost purification method can be used to clean the blend and thereby produce an enhanced product.

[0018] The FT wax has a drop melting point of from 80°C - 120°C. The wax has a low oil content (<0,05%) and a viscosity at 40°C which is less than 20mm ² /s. A benefit of adding FT wax as a solvent lies in the fact that as the properties of the FT wax are not too dissimilar from the properties of a denatured polymer wax, the overall performance of the product is not materially compromised and, in many cases, the performance can actually be elevated as the lower viscosity and crystallinity of the FT wax can complement the higher viscosity and more amorphous structure of the denatured polymer.

[0019] Modification of waxes through oxidation and subsequent saponification, esterification and other means of functionalisation increase the possible uses of the resulting product, for example in polymer additives and lubricants, and in bitumen, asphalt and wood modification. These functionalised waxes can also be used in emulsions and dispersions for a wide array of applications.

[0020] Common functionalisation techniques require a certain viscosity and molecular weight in order to be successful. By including FT wax as the solvent it is possible to achieve these objectives and thereby functionalise the denatured polymer waxes in an effective manner.

[0021] Thus, according to the present invention, a wax produced from depolymerised plastic is blended with a Fischer-Tropsch wax which has a linear structure and a high crystallinity and the blend is then subjected to a filtration process to remove contaminants.

[0022] The filtration process may be at an elevated pressure e.g. above 1 atmosphere.

[0023] Suitable filter media include diatomaceous earth and activated carbon. These media are exemplary and non-limiting.

[0024] After filtration the molten product is solidified. [0025] The filter which is used may be a filter press, a leaf filter or a relevant high-solids content filter.

[0026] The Fischer-Tropsch wax is added to the depolymerised or denatured polymer wax so that the Fischer-T ropsch wax comprises from 25% to 75%, by weight, of the blend.

BRIEF DESCRIPTION OF THE DRAWING

[0027] The invention is further described by way of example with reference to the accompanying drawing which is a flow diagram representation depicting a way in which the method of the invention is implemented.

DESCRIPTION OF PREFERRED EMBODIMENT

[0028] The accompanying drawing illustrates in block diagram form a method 10, according to the invention, of producing a high value wax 32 from recycled polymers 12 such as polypropylene, polyethylene and polystyrene.

[0029] In the method of the invention waste polymer feedstock 12 is processed in a reaction vessel 14, which preferably comprises a bubble column reactor, to produce a denatured polymer wax 16. The wax 16, invariably, contains a variety of contaminants which impart to the wax a dark colour and which can give off unpleasant odours. Solvents in the form of hydrocarbon solvents or supercritical carbon dioxide (by way of example) have been used in the prior art to upgrade this type of wax. [0030] In accordance with the present invention wax 18 obtained from a Fischer-Tropsch process is blended in a step 20 with the wax 16. The Fischer-Tropsch wax 18 has a drop melting point of from 80°C to 120°C, and is chosen to have a linear structure with a high portion of paraffins. The Fischer-Tropsch wax has a very low viscosity when molten and due to the high paraffin content has a high crystallinity when solid. Blending takes place at a temperature which is above the melting point temperatures of both waxes with a satisfactory result being achieved by working at a temperature in the range of 140°C to 180°C.

[0031] The blending process 20 produces a blended wax product 22 of the two waxes. It has been found that the viscosity of the blended wax 22 is reduced to 100 cps at 140°C (ASTMD445) which is a level at which a suitable low cost purification method (e.g. filtration) can be used to enhance the value of the wax blend. In particular the viscosity of the wax blend is significantly lower than the viscosity of the depolymerised wax 16 which typically is of the order of 225 cps at 140°C (ASTMD445).

[0032] The molten blended wax 22 at a temperature in the range of 140°C to 180°C is subjected to a pressurised filtration process 26, typically at a pressure in excess of 1 atmosphere, using filter media 28 such as diatomaceous earth and activated carbon. A filter press used in the process 26 may be a filter press, a leaf filter or a high-solids content filter.

[0033] The product 32 of the filtration process 26, upon solidification, comprises a high quality wax blend with a high molecular weight and a high melting point. [0034] The yield of the filtration process, which ranges from 75% to 95%, depends on the quality of the feedstock 12 i.e. its dirt content and the extent to which it was degraded. Waste 34, produced by the filtration step, has a low value but due to its combustible nature the waste can be used in appropriate applications.

[0035] The quantity of the Fischer-Tropsch wax 18 which is added to the denatured wax 16 can be varied according to requirement. The wax blend 22 can include from 25% to 75% of the Fischer-Tropsch wax 18. By increasing the relative quantity of Fischer- Tropsch wax in the wax blend 22 the viscosity of the blend 22 can be reduced to a value which is sufficiently low to allow for chemical modification and functionalisation techniques, such as oxidation, to be carried out successfully, an aspect which is enhanced due to the use of the bubble column reactor 14.

[0036] In one example of the invention Ceranovus polypropylene wax A155 in an amount of 75% was combined with 25% of Fischer-Tropsch wax produced by Sasol (identified as wax C105). Pressurised filtration 26 was carried out in combination with filter media comprising diatomaceous earth present in the amount of 2.5%, activated carbon present in the amount of 2.5%, and a 5% mineral filter aid.

[0037] The resulting product was a wax with a white colour measured by means of a Saybolt chronometer with a Saybolt +10 reading. The wax had a dropping point of about 140°C and a congealing point of ± 100°C. The wax had no odour and was usable in high value products, e.g. as a feedstock for jet-milled micronized waxes, in hotmelt adhesives, emulsions, asphalt modification and polymer processing, and in chemical oxidation processes and similar specialised applications. [0038] A principal benefit lies in the fact that the Fischer-Tropsch wax is used, effectively, as a solvent for upgrading the recycled polymer wax in a way which does not subsequently require the Fischer-Tropsch wax to be distilled or fractionated out, as is the case with a traditional solvent. It is believed that this is because the Fischer-Tropsch wax has complementary qualities that do not reduce the properties of the polymer waxes. This aspect allows for an effective and inexpensive way to upgrade recycled polymer waxes which have a higher market value, and which are suited for use in a range of applications.