Patent Application
20250101312
This invention relates to a method and apparatus for the pyrolysis of polymers, and in particular to a method and apparatus for converting polymers into one or more liquid and gaseous hydrocarbon products.
Plastic production is set to quadruple by the year 2050 which is predicted to result in plastic's share of global oil consumption increasing from 6% in 2012 to 20% in 2050 and plastic's share of the carbon budget increasing from 1% in 2012 to 15% in 2050. If the current waste plastic production and recycling rates remain the same, the ratio of plastic waste to fish by weight in the ocean in 2050 will be 1:1.
Plastic pyrolysis is a thermochemical decomposition process that involves the heating of plastic in the absence of oxygen to produce hydrocarbon liquids and gases. Polyolefin (PO) and polystyrene (PS) plastics contain only carbon and hydrogen atoms. In theory if PO and PS plastic is separated from other plastic types and is used as a feedstock for pyrolysis the products should only contain carbon and hydrogen. This however is not the case as polymer manufacturers integrate high amounts of additives which contain components that reduce the quality of the oils, like halogens, silicones, phosphorus, nitrogen, sulphur, oxygen and metal compounds. In recent years plastic pyrolysis has received much attention as a potential method of recycling low value PO and PS plastics, such as films that mechanical recycling struggles to process. Chemical or advanced recycling means the products of plastic pyrolysis are used as a feedstock to make new plastics (so replacing feedstocks derived from fossil fuels). This would result in a substantial reduction of carbon dioxide emitted during plastic manufacturing. This reduction incentivises the recycling of plastics that are currently landfilled or incinerated such as films and other low-quality plastics. There is a growing demand from major consumer brands for food grade recycled plastic to be used in their products, and chemically recycled plastic is an ideal source for food grade recycled plastic. There have been many attempts at chemical recycling on a large scale but they have encountered several technical hurdles.
The primary problem encountered within the existing art is that the hydrocarbon products of typical waste plastic pyrolysis plants are of low quality and feature high concentrations of contaminants, mainly halogens, silicon and phosphorus, olefins, diolefins, nitrogen, oxygen, sulphur and metal compounds. The main method that the petrochemical industry uses to produce plastic monomers is by using a steam cracker that cracks hydrocarbon feedstocks at high temperatures and pressures into plastic monomers. The high temperatures used in steam crackers makes them very susceptible to corrosion from contaminants even in very low concentrations, and steam crackers are also very susceptible to coking. This results in the feedstocks for steam crackers having very stringent specifications for contaminant concentrations, see below table:
Oils from PO plastic pyrolysis typically contain contaminants that are an order of magnitude over the required specification. What makes the high levels of contamination worse is that plastic pyrolysis oils contain high levels of contaminants that are not common in fossil fuels, like metals, phosphorus, silicon and halogens like chlorine. Hydrotreaters are commonly used to remove contaminants but are typically only configured to remove contaminants that are common in fossil fuels mainly sulphur, nitrogen, oxygen, and olefins.
The presence of metals in feeds for steam crackers cause coking in the steam cracker furnace. Consequently there are very low tolerances for metals in steam cracker feeds, typically, there is a limit of 1 ppm on metals in the feedstock to avoid issues with coking, whereas plastic pyrolysis oils typically have a metals content of 25 ppm to 50 ppm. Given plastic pyrolysis oil features contaminant levels in excess of the required specifications it makes it unsuitable to be used as a feedstock for steam crackers. Historically, the levels of contamination were so high that blending even small amounts of plastic pyrolysis oil with fossil fuel derived feedstocks was extremely challenging, resulting in plastic pyrolysis failing to achieve chemical recycling status. Accordingly, there is a need within the art to produce hydrocarbon products from plastic that have low levels of contamination allowing the products to be used as a feedstock to make new plastics enabling it to reach the status of chemical recycling.
One method to remove contamination from the hydrocarbon feedstock is hydrotreating. The hydrotreating process involves treating the feed stock at high pressure and temperature with hydrogen which is not only expensive but is also extremely carbon intensive. Most hydrogen is produced by steam methane reforming, and using this method every tonne of hydrogen produced results in 10 tonnes of CO2 being emitted to the atmosphere. Moreover, petrochemical and refinery hydrotreaters are primarily designed to remove heteroatoms common in crude oil such as sulphur, oxygen and nitrogen, and these hydrotreaters are not designed for the removal of halogens, metals and nonmetals such as, silicon, phosphorus etc. which are extremely damaging to the hydrotreating catalysts and other catalysts downstream. Another drawback of using hydrotreaters for the upgrading of plastic pyrolysis oils is that the catalysts used in the hydrotreating process are susceptible to poisoning by contaminants typically found in plastic pyrolysis oil such as silicon, halogens, metals and phosphorus. There is a need within the art for a method to remove contaminants that are not typically found in fossil fuels.
There have been many attempts to remove halogens from the plastic feed but these have had very limited or no success on an industrial scale.
With the recent focus on global warming and rising atmospheric CO2 there has been a drive to reduce the emissions of steam crackers, using lighter feedstocks such as ethane, propane and butane instead of heavier feedstock such as naphtha or gas oil. This drastically reduces the energy consumption require to produce the most valuable products, ethylene and propylene, as less cracking is required. As the only saleable products of most known plastic pyrolysis processes are naphtha and gas oil which are far heavier than ethane, propane and butane they require far higher amounts of energy to convert to ethylene and propylene. The shale gas revolution precipitated a great increase in global supplies of ethane, propane and butane. As a result, steam crackers all around the world have increased their use of these feedstocks especially ethane over naphtha and gas oil. This has also resulted in a global drop in propylene and aromatics production. Accordingly, there are fewer steam crackers able to process naphtha and far fewer processing gas oil worldwide. Typically, gas oil is the largest fraction by volume produced from a plastic pyrolysis process, typically produced in three times greater quantity than naphtha. The lack of available petrochemical facilities that accept gas oil makes it harder to find a market, imposing commercial difficulties on chemical recycling plants. There is a need within the art for a pyrolysis plant that produces larger ratios of lighter products from the pyrolysis of waste plastics to meet the market demand and reduce the energy consumption of the steam cracking process.
Steam cracking hydrocarbons to produce monomers is one of the most energy intensive processes in the petrochemical industry and when it is coupled with the highly energy intensive hydrotreating of highly contaminated plastic pyrolysis oils it results in questioning whether chemical recycling is beneficial for the environment.
Accordingly, there is a need within the art for a pyrolysis process that converts the waste plastic directly into short chained monomers such as ethylene and propylene bypassing the energy intensive hydrotreating and steam cracking steps. The conversion of waste plastic directly into plastic monomers is known as monomer recycling.
The vast majority of plastic pyrolysis processes thermally crack the long plastic chains into shorter hydrocarbons molecules. Catalytic cracking would greatly reduce the energy required for plastic pyrolysis and would also improve conversion ratios and process efficiencies. Thermal cracking results in over production of diolefins and other less desirable products from the more random carbon chain scission compared to a catalytic process. The use of catalysts in the cracking of plastic is difficult and their use in a commercial plant faces several technical hurdles, mainly because the contaminants like halogens and metals that reduce the quality of the product oils also damage and deactivate the catalyst. The deactivation of the catalysts results in a high replacement rate of the catalyst being required, so the economic benefits of using catalysts are outweighed by the cost of the high replacement rate of the catalyst. There is a need within the art to develop a method of catalytically cracking waste plastic.
Accordingly, there is a need within the art for a pyrolysis process which can substantially remove problematic halogenated and heteroatomic contaminants from the waste plastic feed before they reach the reactor. Moreover, contaminants such as metals and other heteroatoms that do reach the reactor also require removal, trapping and passivation to limit poisoning and deactivation of the catalysts within the bed material and to prevent them from contaminating the products.
Additionally, there is a need within the art for a pyrolysis process which can produce monomers suitable for use in plastics and petrochemical manufacture processes that bypasses hydrotreating and steam cracking, providing large financial savings and environmental benefits.
Accordingly, in a first aspect the invention provides a plastic pyrolysis plant that comprises a reactor adapted to crack a polymeric material feedstock in an environment without oxygen to convert the polymeric material into a hydrocarbon vapor and a solid carbon product called coke, wherein the reactor cracks the polymeric material by contacting it with hot particles in a fluidised bed; an injector to introduce the polymeric material feedstock into the reactor; and comprising an extruder to melt, heat and pump the plastic feed to the injector; wherein the extruder defines inlet ports for additives in solid, liquid and/or gaseous form, and at least one vent for extracting any contamination vapors that may form, such that the extruder is configured to remove contamination from the polymeric material feedstock.
In one option the particles comprise catalyst, so the bed is a fluidised catalyst bed. In such a plant, the ratios of each constituent of the catalyst bed in the reactor can be adapted for the specific desired product or to accommodate different waste plastic feed compositions.
The injector may be arranged to spray the melted polymeric feed evenly across a hot catalyst flowing upwards in the reactor.
Preferably the extruder is adapted to reduce the viscosity of the polymer by applying shear forces, and so increasing the temperature. The extruder screw may consist of a combination of kneading and conveying blocks, the ratios of which may be configured to the desired handling properties, residence time, contamination removal requirements and the varying compositions of waste plastic feeds. The screw may also be designed to maintain the structural integrity of any solid additives, to avoid dust formation. High boiling point products produced by the pyrolysis process may also be introduced and mixed into the polymer feedstock to enhance handling properties. This enables these components to be cracked into more valuable lower boiling point components.
Preferably the extruder is adapted to pre-heat the feedstock to a temperature of approximately 200° C. to 375° C. to remove contaminants from the feedstock upstream of the reactor. The extruder features a vent to aid in the removal of the contaminants. These contaminants may be removed to a treatment system that cools and scrubs the contaminated stream to neutralise any contaminants. The resulting gas from the scrubber can then be processed by an emission abatement system without fear of it corroding any of the equipment or producing harmful emissions.
Preferably the extruder is arranged to introduce, through an inlet, a contamination removal additive that is mixed with the polymer feedstock and that reacts with any halogen atoms in the polymer feedstock; and which then breaks down as the temperature increases within the extruder, so as to release the halogen, for example as a hydrogen halide vapour. The break down and release may be aided by introducing a gas or liquid such as but not limited to steam or water into the extruder at one or more inlets downstream of the first additive inlet; this strongly enhances the extraction of contaminants via the extraction vent of the extruder, helping to strip the contaminants from the polymer. Suitable contamination removal additives include magnesium oxide, magnesium hydroxide, activated aluminium oxide (alumina), aluminium hydroxide, magnesium aluminate, calcium aluminate and standard fluidised catalytic cracker (FCC) catalysts. Activated alumina may be particularly advantageous.
The addition of reagents and additives for the removal of halogen contaminants specifically during plastic pyrolysis has been suggested previously. Within the chemical recycling industry halogen removal from products is often the main focus, especially within the context of handling mixed waste plastics, as PVC waste which has a high chlorine content is often contained in the waste plastic feed to the plants described in the prior art. Even if PVC is removed from the waste, halogens are still an issue as plastic manufacturers often integrate flame retardants into their products; these retardants are a mixture of halogens, phosphorus and silicon-based compounds. These elements may be chemically bonded to the polymer molecule and must be removed for the products to meet the stringent standards petrochemicals companies have for steam cracker feed stock, which are typically 3 ppm for halogens so the products can be used in the manufacturing of new plastics. Failure to remove contaminants like halogens, metals, nitrogen, phosphorus, silicon, sulphur and oxygen will result in products that are of relatively low quality and only suitable as fuels for burners or will require energy intensive, expensive and operationally challenging upgrading by blending into hydrotreater feed stock.
It is expensive to heat large throughputs of polyolefin plastic to high temperatures (eg. >280° C.) using an extruder. The additives enable the contamination removal reaction to happen at far lower temperatures in comparison to a purely thermal contamination removal.
Additives like calcium oxide and calcium hydroxide are effective for the removal of halogens from mixed plastic wastes. These additives very effectively bind to the halogens present in the feed where over 90% of the halogen atoms form salts such as CaCl2, CaBr2 and under 5% of the feed form HCl or HBr gases which can easily be removed through the extruder vent with the remaining 5% remaining with the feed. However the salts that are formed are carried into the reactor where they can dissociate back into their respective metal and halogen ions initially and then subsequently form compounds due to the conditions of high temperature and the presence of steam from the fluidization gas. The halogens form harmful acids like HCl and HBr and react with the products which prevents the products being suitable for use as feedstocks for steam crackers. The presence of halogens can also cause pitting and corrosion of any steel equipment which is unacceptable given the large capital investment of the equipment involved. The phosphorus present causes problems via the formation of deposits around the catalysts which results in deactivation. The silicon present adsorbs on to the metal sites on the catalysts resulting in deactivation and a curtailed lifespan which is detrimental to the economics of the plant.
Additionally, basic compounds are detrimental to catalysts utilized in the reactor and in other areas of the process. Zeolite and activated alumina-based catalysts are often used in the oil refining and petrochemical industries in equipment such as FCCs, and these catalysts are poisoned by basic components which neutralize their Lewis acid sites which accelerates loss of activity. Hence, the presence of metals such as calcium or sodium is detrimental to catalysts used in other parts of the process. The halogen salts are also detrimental to the cracking catalysts that are typically used in the reactor. Accordingly, there exists a need for a method of effectively removing halogens from waste plastics that utilises an additive that is less harmful to the cracking catalyst that also is capable of removing the halogens out of the extruder vent so that they are not carried into the reactor in the form of salts.
There are other volatile contamination components other than halogens such as phosphorus and some nitrates and oxygenates for example contained in the waste plastic that are also detrimental to the hydrocarbon products. The heating of the plastic and contamination in the extruder causes these contaminants to be liberated and also to exit through the extruder vent. The addition of a stripping gas or liquid which is added to the extruder also greatly aids in the removal of these volatile contaminants. Certain fluids also react with contaminants that are not yet vaporised to cause them to form gaseous components that exit through the extruder vent. Some contamination is also separated from the plastic but remains entrained within the polymer stream and would not exit through the extruder vent, but the addition of a stripping gas such as steam enhances the liberation of this contamination from the polymer stream so the contamination exits through the extruder vent.
This invention greatly reduces the problem of more volatile contaminants like halogens and phosphorus-containing compounds, mainly originating from flame retardants, reaching the reactor using two methods. First by utilizing one or more of the following additives: magnesium oxide, magnesium hydroxide, activated γ-alumina, aluminium hydroxide, magnesium aluminate, calcium aluminate, and FCC catalyst; activated alumina may be preferred as the additive for this purpose. These additives will remove halogens, phosphorus and other volatile contaminants out of the extruder vent, as they form compounds with the contaminant atoms that are in a vapour form at the elevated temperature in the extruder.
This invention injects a stripping fluid like steam into the extruder, increasing the removal of the contaminants as gases. This is more energy efficient than adding a stripping liquid, as the extruder does not have to vaporise the liquid. The extruder allows the injection of steam at one or more points along the extruder. The stripping fluid enhances the removal of several contaminant-containing compounds through dissolution, thermal dissociation, increasing stripping efficiency and subsequent extraction through the extruder vent.
Along with volatile components like halogens there are also high amounts of non-volatile components which are mainly metallic compounds and metalloids. The main contaminants in plastic wastes are sodium, magnesium, titanium, zinc, copper, barium, iron, silicon and calcium. These contaminants are detrimental to the quality of the products with the specification for metals being 1 ppm for steam cracker feedstocks. This is difficult because plastic wastes typically contain over 8,000 ppm of metals in the feed. Metal contamination however is different from the non-metallic contamination because it is not volatile. It usually stays in the coke, however there is some carryover into the product oils which historically have comprised at least 50 ppm of metals. Fluidised beds are particularly exposed to the risk of metal contamination ending up in the oil as the addition of the fluidisation gas increases the probability of the metal contaminates being blown downstream of the reactor. The high quantities of metals in the feed deactivate the cracking catalyst, requiring fresh catalyst to be added to maintain the level of catalytic activity. This can result in the costs of the catalyst outweighing the economic benefit from using the catalyst.
Accordingly, there exists a need for a method of effectively removing, trapping or passivating the contaminants described above that are formed from the waste plastic feed and preventing them from contaminating the end products and deactivating the catalysts.
The metals that are contained in the plastic waste that cause problems are sodium, iron magnesium, silicon, zinc, calcium, barium and copper. These metals are present in relatively high concentration when compared to feedstocks derived from fossil fuels. Calcium is also present in very high concentrations in comparison to the other metals. Calcium carbonate is the most common plastic additive and is commonly present in waste plastic in the range of 5,000 to 10,000 ppm. Such metals may neutralise the acid sites on the catalyst, or may deposit on the catalyst, blocking the pores of the catalyst, which leads to more thermal cracking instead of catalytic cracking. Metals present in the reactor increase the number of dehydrogenation reactions increasing the production of hydrogen, methane and coke which are undesirable products.
Cracking catalysts typically comprise four components namely zeolites, matrix, filler and binder. The zeolites are the most catalytically active component of the catalyst, common zeolites used in catalytic cracking are Y-zeolites and ZSM5. The matrix is the other component of the catalyst that is catalytically active; it is typically made up of activated alumina which is the same as the preferred contamination removal additive. The filler is most commonly made up of clay that is catalytically inert. The binder serves as a glue that holds the zeolite, matrix and filler together; in some cases the clay may act as the binder. The Y-zeolites and ZSM5 are much more expensive than the other components and far less resistant to contaminants like halogens and metals.
In the previous art expensive FCC catalysts were used in refineries to crack long hydrocarbons like gas oils. Although polyolefin plastic is a hydrocarbon its chemistry is very different from hydrocarbons found in a refinery: gas oil for example has a carbon chain length between 15 and 30, a plastic molecule could have a carbon chain length of over 1,000.
This invention adds activated alumina as a contamination removal additive to the extruder; activated alumina is also catalytic and suited to cracking the long hydrocarbon chains of plastic molecules. Activated alumina has a more suitable porosity profile that is better suited to cracking the long hydrocarbon chains present in the plastic feed compared to traditional FCC catalysts. The larger pore size allows for better diffusion of hydrocarbons to the Lewis acid sites located on the surface of the activated alumina. When compared to traditional FCC catalyst activated alumina is cheaper, has much greater resistance to fouling, and can be used for much longer duration before requiring replacement by fresh catalyst. Typical FCC zeolite catalyst pores are not suitable for cracking of large hydrocarbon molecules as their pores are too small to allow diffusion of the large plastic molecules in the cracking sites. The activated alumina is not as efficient as the zeolites at cracking the smaller hydrocarbon molecules. This invention therefore may use both activated alumina and FCC catalyst. The activated alumina can also serve as a trap to passivate metal contaminants and also traps basic nitrogen. This protects the reaction catalyst which contains zeolite components which are not as resistant to metals as the alumina. In the prior art the use of FCC catalysts has not been successful due to their short life span when processing plastic waste: the unit economics of the plant do not allow for the use of expensive catalysts with short life spans.
In another embodiment of this invention a typical FCC catalyst that contains alumina may be used as the contamination removal additive instead of alumina, although an FCC catalyst is more expensive and less resistant to deactivation by metals so a higher quantity of the catalyst is required. An FCC equilibrium catalyst (ECAT), which is used FCC catalyst that has been removed from a separate refinery FCC unit (and so is a mixture of catalysts of different levels of activity, having been used for different lengths of time) may be used as the contamination removal additive, and is considerably cheaper than activated alumina. The use of ECAT rather than alumina allows for a far greater amount to be used as the contaminant removal additive while keeping the process economic.
This invention continuously adds fresh contamination removal additive and catalyst to the reactor and continuously removes used catalyst from the reactor to remove the metal contamination preventing it from ending up in the products.
Although the activated alumina is more tolerant to metal contamination in comparison to the reaction catalyst which contains Y-zeolites and ZSM5 it does still get deactivated by the metals. If the pores of the matrix get blocked by the metals, it will stop the hydrocarbons entering the pores of the catalyst preventing access to Lewis acid sites. The design of the activated alumina and the catalyst is important to make the catalyst more resistant to the metal poisoning by optimizing the surface porosity: larger pores are less susceptible to blockage.
The activated alumina is also considerably cheaper than the reaction catalyst. The fact that the activated alumina is catalytic results in less of the reaction catalyst being required, this allows for a much higher replacement rate. The high amount of alumina results in the overall bed material being much more resistant to typical catalyst poisons found in pyrolysis feedstocks.
Another method of removing metal contamination is by adding a solvent such as but not limited to dimethylsulphoxide (DMSO), tetrahydrofuran (THF), or dimethylformamide (DMF) to the extruder with the aim of increasing contamination removal efficiency. The addition of an acid such as but not limited to citric acid may also reduce the level of metals and other contaminants, by ionising the metals which increases the amount that dissolves in water which is separated in the condensing system. The acid also converts insoluble contamination into compounds that are soluble in water.
Any remaining metals in the product oil concentrate in the high boiling point components at the bottom of the distillation column. This invention may send this bottoms stream to a separation device which separates the metals from the high boiling hydrocarbons. The metals are sent for disposal and the high boiling hydrocarbon are recycled to either the extruder or the reactor. Any metals that are not separated by the separation device are recycled and this increases the contact time between these metals and the catalyst allowing more time for these metals to be trapped. Other volatile contaminants especially phosphorus also concentrate in the high boiling point components which also gets trapped by the catalyst. The recycling of all the high boiling point components greatly reduces the metals in the product. The recycled high boiling point components get cracked into lighter components like naphtha, syngas and char. As this invention utilises the char as a fuel for regenerating the catalyst and converts most of the syngas to a saleable product, recycling the high boiling point components is not detrimental to the economics of the plant.
It is desirable that as much of the chlorides in the plastic waste including the chlorides in the salts are removed before they interact with the catalyst and are not allowed to enter the reactor. So the contamination removal additive is preferably added early to the extruder, whereas the catalyst is added after the stripping fluid is added to the extruder and after the extruder vent where most of the contamination including the chlorides leave, as by this stage most of the metals will already have been trapped by the activated alumina. The addition of the activated alumina to the extruder decreases the temperature at which the contaminants are liberated as vapours from the main polymer stream, reducing the electricity consumption of the extruder. If an FCC catalyst or ECAT is used as the contamination removal additive the detrimental effect of the chloride contaminant will be significantly reduced by the addition of stripping fluid.
Another method of passivating or trapping any metals that reach the reactor and preventing them from deactivating the catalyst is through the addition of a trapping compound to the activated alumina, specifically to trap metals. A lot of the common halogen removal additives are also effective metal trapping additives, for example magnesium oxide or magnesium aluminate. The metal trapping additives can be impregnated onto the activated alumina or added as spheres to the bed material. The metals present in the feed will preferentially bind to these particles, which prolongs the life of the catalysts present. Moreover, the trapping of the metals will reduce the amount of any metal contamination getting entrained in the product vapour and contaminating the products further downstream.
In another embodiment of this invention a more basic compound such as but not limited to magnesium oxide can be impregnated onto the activated alumina or reaction catalyst to increase the contamination removal efficiency especially of halogens. As the most effective halogen contamination removal additives are basic compounds, they neutralise the Lewis acid sites which subsequently deactivates the activated alumina or reaction catalyst. This invention uses activated alumina as the halogen removal additive which does not damage the reaction catalyst however it is not as basic as other additives that are harmful to the catalyst like other metal oxides or hydroxides so is not as effective as removing the contamination from the plastic. However, a balance must be struck between its efficiency of removing the contamination from the plastic waste and how easily the additive releases the contaminant so that it can exit the extruder vent. The activated alumina can be impregnated with a basic compound like magnesium oxide which increases its ability to remove halogens and certain metals and because it is impregnated onto the catalyst the basic compound is trapped and cannot move to deactivate the catalyst. The below table illustrates the bond dissociation temperatures, in the presence of steam, of salt derivatives resulting from the dehalogenation process.
As can be seen from the table above supplying the stripping fluid to the extruder would be less effective if using strong bases. Aluminium chloride has a very vigorous reaction with water even at ambient temperatures producing hydrogen chloride gas and has extremely good dissociation at low plastic temperatures.
In a further alternative sand or any other inert heat carrying material may be used instead of a catalytically active bed material in the reactor's catalyst bed. This has some disadvantages, for example requiring a higher reaction energy and so a higher reactor temperature, but it eliminates the problem of deactivation of the bed material from metals or basic halogen removal additives. This enables the use of a basic halogen removal additive such as but not limited to magnesium oxide or hydroxide. The magnesium oxide or hydroxide will form a salt like magnesium chloride which will dissociate into HCl and magnesium oxide when the stripping fluid is supplied to the extruder, which will enable the HCl to be extracted from the extruder vent.
The design of the extruder is a critical factor in the contamination removal efficiency of the extruder. The residence time of standard twin screw extruders is approximately 30 seconds which is not long enough for the various contamination removal reactions to occur. This invention increases the residence by increasing the length of the extruder; typical twin-screw extruders have a length to diameter ratio (L/D) between 20 to 40, this invention utilises extruders with L/D ratio over 40. The screw design is another important factor that can increase the residence time, as certain screw elements can hold the plastic in the extruder for longer like left-handed elements for example. The time for the plastic to get up to the required contamination removal reaction temperature is significant, as the shorter the time to reach this temperature the longer the contamination removal reaction residence time. The location of the vent on the extruder affects the residence time of the contamination removal reaction as the further down the extruder the vent is positioned the longer the residence time.
The locations where the contamination removal additive and where the reaction catalyst are added are also important as the activated alumina and reaction catalyst are manufactured to have a specific particle diameter, typically 25 μm to 300 μm in diameter, so that the particles stay in the reactor and do not flow into the condensation system. If the activated alumina gets added too early in the extruder before the kneading blocks its physical integrity and structure will get damaged and it will form dust which will flow to the condensation system. The metals contained in the plastic waste will form dusts that will flow to the product oil if not trapped. The activated alumina along with removing the halogens and phosphorus will also trap the metals, which will stop them being carried into the product oil and will also stop the metals from deactivating the reaction catalyst.
Although the activated alumina traps most of the metals contained in the plastic waste it will not trap every metal. Small proportions of the alumina and of the reaction catalyst will also break up into dust. This invention may have a filter or a guard bed on the reaction vapour line to the condensation system to remove any solids that may have got past the cyclones, which reduces the amount of metals in the product oils.
The stripping fluid avoids the need for a vacuum pump to pull the contaminants out of the extruder as the stripping fluid drives the contamination from the extruder. One of the considerations for the extruder is to keep an inert atmosphere in the extruder vent to avoid the safety risks of a flammable atmosphere. If the stripping fluid is an inert fluid like steam or nitrogen then it can keep an inert atmosphere in the vent.
This invention thus overcomes the problems of the prior art by using additives which selectively react with the halogens and other heteroatoms present in the waste plastic feed to form compounds which then proceed to substantially dissociate inside the extruder and exit via the extruder vent. The contamination can therefore be routed to a contaminant vapor management system. This will allow the products of the process to be blended with fossil fuel derived oil for use in the petrochemical industry, which is one of the most lucrative markets for pyrolysis oil.
Preferably the extruder pumps the molten polymer to an injection device located at the bottom of the reactor section of a dual fluidised bed; the injection device is designed to atomise or spray the polymer by combining it with high pressure steam or other gases. For example steam may be added after the extruder vent in order to make the polymer spray when it exits the injection device in the reactor.
Preferably the dual fluidised bed is configured to crack the long hydrocarbon chains of the plastic feedstock into shorter chain hydrocarbon products. The reactor does this by contacting the molten plastic stream with hot catalyst in the reactor in the absence of oxygen. The reactor blows the hot catalyst up the reactor starting from below the location of the injection device, for example with a flow of steam. The hot catalyst causes the plastic to crack into a hydrocarbon vapour and a solid carbonous product called coke or char. The mixture of char, catalyst and hydrocarbon vapours from the reactor will flow to a cyclone which separates the hydrocarbon vapor from the catalyst and coke. The catalyst and coke will fall down into a steam stripping tower (or “steam stripper”) which will remove the entrained hydrocarbon products in the catalyst. This will prevent the loss of product that would otherwise be burnt in the regenerator. If too many hydrocarbons reach the regenerator this could result in the temperature range going outside of the optimal range and require a reduction in throughput to maintain control of the process. A loss of catalyst activity can occur if excessive temperatures are present in the regenerator, as such temperatures can damage the catalyst's crystalline structure. The steam stripper may be adapted to supply 1-10 kg of steam for every 1,000 kg of circulating catalyst. The metal contamination on the catalyst will block the pores; this reduces the ability of the hydrocarbons to enter the catalyst pores but it also makes it harder for the hydrocarbon to exit the catalyst. It is important that the catalyst has an optimised porosity to avoid blocking by metals depositing on the catalyst so that the hydrocarbons can be effectively stripped out of the catalyst.
This invention significantly reduces the energy required to produce light olefins and aromatics by catalytically cracking instead of thermally cracking the hydrocarbon molecules. This reduces the energy required for the process and provides much higher conversion ratios. Certain catalysts can produce larger fractions of the desired products such as propylene and naphtha, moreover they also reduce the undesirable products like coke methane, hydrogen and long chain olefins and diolefins.
As indicated above, a dual fluidised bed type reactor is used. One side of the dual fluidised bed is the reactor which pyrolyzes the polymer waste, and the other side is the regenerator which combusts the coke on the catalyst, so raising its temperature, and the catalyst is recirculated back to the bottom of the reactor.
The hot flue gas produced from the regenerator flows through a heat exchanger which produces steam which can be used in the process as an atomisation and lift gas in the reactor. The excess steam in this process can also be used to drive equipment or sent to a steam turbine which produces electricity.
Preferably the hydrocarbon vapour that emerges from the steam stripper flows to a filter or a guard bed to remove any remaining solids before flowing to the condensation system or distillation column that separates the hydrocarbon vapour into three streams: the overhead product (or syngas), heavy naphtha and gas oil. The heavy naphtha is a saleable stream as it is a common feedstock for steam crackers. The gas oil product is not a common steam cracker feedstock and can be recycled back to the extruder to be pyrolysed again, eventually forming naphtha, syngas and coke.
The reactor used in this invention also cracks and liberates organic sulphur, nitrogen and oxygen compounds. Cracking of organic nitrogen compounds creates hydrogen cyanide (HCN), ammonia (NH3), and other nitrogen compounds. Cracking of organic sulphur compounds produces hydrogen sulphide (H2S), mercaptan (R—SH) and other sulphur compounds. Cracking of organic oxygen produces water (H2O), carbon dioxide (CO2) and other oxygen compounds. Any of the halogens that may not have been removed by the extruder will be carried into the reactor in the form of salts and will dissociate into acid compounds like hydrochloric acid (HCl). Many of the aforementioned compounds are corrosive and lower the product oil's quality and will knock the oil out of the strict specifications of the petrochemical industries. This invention may therefore employ a continuous caustic wash system to remove these components so that they do not end up in the products of the process. The caustic reacts with contaminants to form aqueous solutions or suspensions that can easily be removed, like but not limited to the following reactions
CO2+2NaOH→Na2CO3+H2O
H2S+2NaOH→Na2S+H2O
HCl+NaOH→NaCl+H2O
HBr+NaOH→NaBr+H2O
The other contaminants are soluble in water and are separated in the caustic system which is able to separate the immiscible oil and water phases by decanting.
Typically, syngas represents anywhere from 10 to 25% of the total product of a plant targeting liquid products. As only 5% of the products of pyrolysis are required to heat the reaction and for heat loss, it would be wasteful to burn the syngas. As this invention utilises the char product to fuel the process, the efficiency of the dual fluidised bed at heating the polymer feedstock, the catalyst used in the reaction greatly reducing the energy required for the reaction, the preheating of the feedstock in the extruder, and the efficient design result in this invention only needing to burn a small amount of the syngas product to run the process. The syngas product produced contains the following molecules:
Although the proportions of the relevant molecules change when reaction parameters and feedstocks are changed, the composition of the syngas is very similar to the product stream from the pyrolysis section of a conventional steam cracker.
Another problem in using the products of plastic pyrolysis is that they contain a high concentration of olefins. Although the purpose of a conventional steam cracker is to convert paraffins into olefins, having a high concentration of olefins in the feed causes coking in the steam cracker. The steam cracker converts paraffins into olefins which are the building blocks of many plastics. Steam cracking for the production of light olefins, such as ethylene and propylene, is the single most energy-consuming process in the chemical industry. It is found that the pyrolysis section of steam crackers alone consumes approximately 65% of the total process energy and approximately 75% of the total energy loss. Bypassing the pyrolysis section of the steam cracker would save the energy required to crack the paraffins into olefins. Bypassing the pyrolysis section of the steam cracker also avoids the issue of olefins in the product stream, and in fact they are the most valuable components of the stream. The output product stream from the pyrolysis section of a conventional steam cracker is very similar to the syngas stream leaving the top of the distillation column. This invention aims to solve this problem by piping low boiling products to the downstream of the pyrolysis section of a conventional steam cracker, where the syngas which contains a mixture of paraffins and olefins ranging in carbon chain length C1 to C4 can be separated into its individual components.
Locating the plastic pyrolysis plant next to a steam cracker plant may not be practical because the low bulk densities of plastic feedstock may make the logistics of transporting the vast quantities of plastic feedstocks to the steam cracker site not practical. It makes more sense to locate the plastic pyrolysis plants in dispersed areas, and to transport the syngas to the steam cracker sites. It is not practical to pipe this syngas via pipeline over any substantial distance. To transport this syngas, it would need to be liquified.
In a second aspect of the present invention, the valuable short-chain hydrocarbons of a syngas stream produced by pyrolysis of plastics are separated so they can be transported to be fed downstream of the pyrolysis section of a steam cracker, where they can be separated into the individual components. The short-chain hydrocarbons can be separated by either cooling, compression or absorption or by a combination of cooling, compression and absorption. The separated short-chain hydrocarbons can also be fed to the downstream of a Fischer-Tropsch synthesis or methanol-to-olefins reactor or any other similar process to separate the various components. The short-chain hydrocarbons may be C2 to C4, or C2 to C5, or C3 to C4.
For example the syngas stream produced by pyrolysis of plastics may be treated to condense and liquify short-chain hydrocarbons (e.g. C2 to C4), or alternatively may be treated so as to absorb C3 to C4 hydrocarbons into a hydrocarbon liquid, to form monomer rich liquid (MRL). To achieve this the syngas may be pressurised to a pressure of 10 to 25 bar (g) and cooled to a temperature of 0 to 20° C. The hydrocarbon liquid may be light naphtha. This process requires much less energy than liquifying all the syngas components. In practice a proportion of the C2 components and any C5 components are also absorbed in the MRL. As the hydrogen, carbon monoxide and methane components are low value products that are not desired by the steam cracker process it is not worth the shipping cost to transport them to the petrochemical plant. The C2 components especially the ethylene component is highly valuable, but ethylene separation would require much higher pressures over 80 bar (g) and lower temperature under −5° C. which would require refrigeration, which may not be economical. Propylene on the other hand does not require refrigeration, as it can be liquified just by compression.
This invention thus captures a high proportion of the C5, C4 and C3 components and a small proportion of the C2 components of the syngas stream by condensing it using pressurisation, cooling, pressure reduction and absorption which greatly increases the yield of the plant. As another alternative the C2 to C5 products can be separated using a membrane or pressure swing adsorption unit.
As the propylene is a lot easier to separate than ethylene, it is desirable if the pyrolysis can be carried out so as to increase the propylene to ethylene ratio of the syngas product. This invention therefore increases the proportion of C3 components to C2 components by using catalysts as the bed material for pyrolysis. The contamination removal additive which is typically activated alumina is also catalytic and as it is delivered to the reactor with the plastic it ends up as the reactor bed material. A reaction catalyst can also be added to the reactor.
In the previous art sand was used as the bed material in the fluidised bed, and as sand is inert this resulted in thermal cracking. With thermal cracking the propylene/ethylene (P/E) ratio is approximately 0.5. The use of catalytic material can increase this ratio anywhere from 1 to 2.5 depending on reaction conditions and the catalyst used. The increase of C3 components which are far easier to condense and separate over C2 components greatly increases the yield of the process.
This invention has a higher yield than any process described in the prior art. This results in this process being able to recycle a far greater amount of its products to the reactor while still being economic. As there is a higher demand for the low boiling points components one very effective method is to increase the recycle rate of the higher boiling point products; for example, the gas oil stream can be recycled to be converted into lighter more valuable products like naphtha and syngas.
This invention recycles the gas oil products to make only two saleable products, MRL and a heavy naphtha product. The two products of the process have very low amounts of contaminants like halogens, phosphorus, sulphur, nitrogen, oxygen and metals. Although these two products can be directly fed into a suitable hydrotreater in a refinery or petrochemical facility they cannot be fed directly into a steam cracker because of the high quantity of olefins.
As the plastic pyrolysis reaction is a cracking reaction it produces a high quantity of olefins similar to common streams of cracked hydrocarbons like cracked naphtha produced from equipment like fluid catalytic cracking, delayed cokers and visbreaking units. It is common for these streams to be used as feedstocks for steam crackers. These streams must first be hydrotreated before they are fed to steam crackers to reduce the olefin, diolefin and other contaminants (mainly sulphur, nitrogen and oxygen components). These hydrotreaters would be an ideal location to feed in the products of plastic pyrolysis to leverage existing assets of refineries and petrochemical facilities.
This invention seeks to remove or trap the majority of halogens, metals and other contaminants and so produce oils from which all contamination that is not typical in crude oil has been removed. Thus the plastic pyrolysis product oil coming from the pyrolysis reactor can be fed into a suitable existing hydrotreater in a refinery or petrochemical facility of which the output is fed to a steam cracker; this will guarantee the plant achieves a classification of chemical recycling as opposed to waste to energy.
An apparatus for the pyrolysis of waste plastic in accordance with an embodiment of the present invention will now be described, by way of example only, with reference to:
An embodiment of a waste plastic pyrolysis system, in accordance with a preferred overall embodiment of the present invention is schematically illustrated in
The apparatus is adapted to heat a feedstock of waste plastic to an elevated temperature in the absence of oxygen, breaking the long-chain polymers into shorter hydrocarbon chains to produce a stream which can be subsequently separated to produce products, such as gas oil, naphtha, syngas and a carbonous solid material called coke. Once produced these products are separated from each other. The waste plastic feedstock 1, which preferably contains only polyolefin plastic types, is initially processed so that it can be delivered to an extruder 2 in a form which is readily manageable such as crumb, pellet or flake.
The mixture of metal chlorides and molten polymer enters zone 3 and is further heated and combined with steam which enters through a nozzle in zone 3. The high temperature and the presence of steam causes the metal chlorides to dissociate leading to the formation of the respective metal cation and HCl or HBr. The metal chlorides will begin to dissociate at a temperature of approximately 150° C. depending on the additive used. This mixture of steam, hydrogen halides, molten polymer and additive compounds which will now have been converted back to a metal oxide or hydroxide is delivered to zone 4. A vent 5 is present in zone 4 and this allows the vapour in the extruder 2 to be liberated, so HCl, steam and any other vapour will exit the vent 5 in zone 4. The majority of halogens in the feed will be liberated via this vent 5 from the extruder 2. A solvent 133 may also be added in zone 3 before the steam to increase the removal efficiency of the contamination and to passivate any metals that would be harmful to the catalyst including the contamination removal additive. The final zone, zone 5, in the extruder 2 has two nozzles which allow for the addition of recycled high boiling point components (portion 25 as shown in
The extruder 2 feeds molten plastic into a reactor 14 which contains an up flowing stream of hot catalyst, as described below. The lowering of viscosity of the feedstock 1 aids in the injection and atomization processes downstream. The extruder 2 decreases the chances of agglomeration of molten plastic in the reactor 14 which operates by circulating a catalytic material. (Agglomeration of the waste plastic would happen if plastic, coke and catalyst clump together inside the reactor bed 14 subsequently causing a reduction of the movement of the bed material, resulting in the defluidisation of the bed material in the reactor 14 and a significant reduction in heat transfer between the bed material and the plastic feed). The extruder 2 gives the plastic excellent fluid dynamic and handling properties that enables the plastic and catalyst to circulate in the reactor 14 without agglomeration, transfer heat efficiently and reduces reactor downtime required for maintenance.
Referring to
The plastic feedstock exits the extruder 2 as a liquid with a low viscosity and at a temperature low enough that bulk pyrolysis has not occurred and enters the injection nozzle 4. The injection device 4 first thins the polymer out into a thin stream and orientates the polymer at least partially upwards in the reactor 14 to aid with the fluid dynamics of the reactor. The injection nozzle 4 combines the molten polymer with high velocity, high pressure and high temperature steam which is a by-product of the process. The injection nozzle 4 atomizes or sprays the polymer feed into the reactor 14 which contains an up flowing stream or fluidised bed of hot catalyst particles. The atomization of the plastic feedstock will provide a larger surface for the particle heat carrier to crack the plastic feedstock 1, and also inhibits clumping or agglomeration of the molten plastic feed and bed material.
Steam is utilized as the atomizing gas for the nozzle 4 and lift gas for reactor 14. The lift gas will enter the bottom of reactor 14 and blow the particles of catalyst which are at a temperature between 400° C. and 1000° C. up the reactor riser. The injection nozzle 4 is located towards the bottom of the reactor riser and sprays the polymer feed into the up-flowing stream of hot catalyst particles. The plastic is thereby cracked into shorter carbon chain products. The catalyst and hydrocarbon vapour produced from cracking the plastic are blown up through the reactor 14 and into a cyclone 15 located at the top of the reactor 14.
The cyclone 15 separates the solids, such as catalysts and coke or any solid residue, from the hydrocarbon vapour stream. The vapour stream leaving the top of the cyclone 15 then enters the top of a stripper 16, while the solid particles stream exits at the bottom of the cyclone 15 and enters the bottom of the stripper 16.
Steam enters the bottom of the stripper 16, and the steam permeates through the catalyst which gathers at the bottom of the stripper 16. The steam desorbs any hydrocarbon products that have been adsorbed in the catalyst. The steam additionally allows the escape of any trapped hydrocarbon vapours entrained in the solid particles that have gathered at the bottom of the stripper 16. The trapped vapours exit through the top of stripper 16 where they enter a cyclone 17 designed to separate finer particles from the vapour, and the exiting vapour flows to a filter or guard bed 140 which captures any solids that may have got through the cyclone 17. A guard bed can be used which contains a material which captures and traps contamination such as metals and halogens. The vapour then flows to a distillation column 21. The solid particles that gather at the bottom of stripper 16 exit at the bottom of the unit through a valve which maintains a barrier between the stripper 16 and a regenerator 18. The stripper 16 may be located at an elevation higher than the regenerator 18 and the catalyst may fall by gravity from the stripper 16 to the regenerator 18.
The regenerator 18 combusts the solid carbon or coke produced from cracking the plastic, by introducing air 20 to the regenerator using an air blower (not shown). The regenerator 18 also may be supplemented by a fuel product produced by the process. The combustion of the coke on the catalyst results in the reheating of the catalyst back up to the required temperature. The exhaust exiting the top of regenerator 18 enters a cyclone 19 which separates any solid particles from the flue gas. The resulting flue gas stream exiting the top of the cyclone 19 flows into a waste heat recovery which transfers the heat of the flue gas to the steam cycle. The cooled flue gas flows to the emissions treatment system (not shown). In one embodiment carbon monoxide that is present in the flue gas may be combusted to recover further heat from the gas. The reheated catalyst subsequently exits the bottom of regenerator 18 and will pass through a cooler, not shown, to set the catalyst particle temperature to the required temperature prior to re-entering the bottom of the reactor 14.
The bed material in the reactor is a combination of the contamination removal additive 3 which in this example is activated alumina (which is catalytically active) and the reaction catalyst which is an FCC catalyst which may principally comprise a matrix which is activated alumina, zeolite such as Y-zeolite or ZSM-5, clay and a binder material. The bed material may also include smaller proportions of sand and/or other catalytic materials.
The bed material in the reactor contains a higher percentage of the activated alumina component than a conventional catalytic cracking unit. This is to crack the long hydrocarbon chains of the plastic feedstock. The activated alumina is much more resistant to metal contamination and acts a trap to protect the more expensive zeolite and ZSM5 catalyst in the reaction catalyst. The lower cost of the alumina allows for a higher replacement rate which enables the contamination to be flushed out of the process. The contamination removal additive 3 and reaction catalyst have an optimised porosity to make them more resistant to metals poisoning which blocks the pores.
By way of example the catalyst particles may be of diameter in the range 25 μm to 300 μm, for example 100 μm to 200 μm, with a composition 5 to 25% ZSM5, 20 to 40% zeolite, 20 to 50% alumina (proportions by weight) with the remainder being clay and binder. In one example the composition is 7% ZSM5, 25% zeolite, 40% alumina; and another example has the composition 15% ZSM5, 22% zeolite, 45% alumina.
After processing the plastic waste, the catalyst's activity begins to decrease below the required level. This is from the catalyst contaminants contained in the plastic waste, as the catalyst used in this invention traps a lot of these contaminants so they do not end up in the products. To maintain the required activity and to remove the contamination from the system a portion 113 of the catalyst circulating in the reactor 14 is removed from the regenerator 18 using an auger 112 and fresh alumina 3 and reaction catalyst 6 is added to either the extruder 2 or to the reactor 14. Lower cost equilibrium catalyst may also be added to increase amount of bed material being replaced. In another embodiment of this invention just equilibrium catalyst may be used as both the contamination removal additive 3 and the reaction catalyst.
The main fractionating column 21 fractionates the vapour exiting the stripper 16 in a continuous process. The vapour enters the column 21 near the middle, higher carbon chain oil products such as gas oils 22 are expected to exit the bottom of the column 21 while heavy naphtha product 23 will exit the middle of column 21. A portion of the gas oil 22 exiting the bottom of column 21 will be pumped to a cooler 24 which cools the gas oil 22 which is recirculated back to the column 21 to cool the reactor vapour. The cooler 24 is tied into the site steam cycle so that the heat from the reactor vapour does not go to waste. The metal contaminants concentrate in the gas oil product. A portion 25 of the gas oil 22 exiting the bottom of the column 21 will not be sent to the cooler 24 but is sent to a separator 131 which will remove most of the metals and other fine particles 132 that are in stream 22 and may also be further purified using a solvent washing step (not shown) and is then recycled back to the extruder 2 to be further cracked in the reactor 14.
The non-condensed vapour 26 exiting the top of column 21 will be combined with a caustic solution from separator 37 and is cooled further using the overhead condenser 28. The further cooling condenses a fraction of the stream, with most of the water and much of the C4 to C6 light naphtha components being condensed. The caustic solution from separator 37 neutralises contaminants and stops unwanted reactions. This partially condensed stream enters a three-phase separator 29 which separates the incoming water, caustic, naphtha and syngas into three separate streams. The caustic and water stream 30 is sent to the caustic wash treatment unit 7. A portion of the light naphtha is pumped using pump 31 to the column 21 to act as a reflux 34 while the other portion 35 is sent to an adsorption column 36. The middle product 23 from the column 21 which is heavy naphtha is pumped to a heavy naphtha storage tank 48 and a portion is also sent to a second absorption column 49.
Uncondensed vapour exiting the top of three-phase separator 29 is compressed by a compressor 32 to approximately 6 bar (g); this compression will result in the temperature of the gas and fluid increasing, therefore a cooler 33 is required to lower the temperature to condense the product stream further. The mixture of gas and liquid will enter a caustic wash scrubber/separator 37, which separates the gas from the liquid. The pressurized hydrocarbon vapour will flow upwards through a down-coming caustic solution which permeates through the packed bed in the scrubber/separator 37 resulting in any remaining contamination like unremoved HCl, NH3 etc. being removed. The caustic wash scrubber/separator 37 acts as a three-phase separator, separating the oil product from the caustic wash, while the uncondensed vapor will exit the top of the unit. This vapor is compressed further to a pressure of approximately 20 bar by a compressor 38; this will cause the temperature to rise therefore a chiller 39 is needed to counteract this temperature rise and to chill the vapour to approximately 10° C., and additionally the liquid oil products from the caustic wash scrubber/separator 37 are combined with the compressed stream prior to chiller 39. This cooling and pressurisation will result in much of the propylene product being condensed. The combined liquid and vapor stream then enters another caustic wash scrubber/separator unit 41. A fresh stream of caustic 40 is added to the top of the scrubbing column on unit 41; this will again scrub the vapor liquid, and unit 41 will separate the caustic wash, liquid oil products and uncondensed hydrocarbon vapor. The separated caustic solution from unit 41 is pumped to the top of the scrubbing column on unit 37. The propylene rich oil stream 42 leaving the separator 41 is pumped via pump 43 to the MRL storage tank 44. The uncondensed stream 45 leaving separator 41 still contains a high proportion of valuable propylene, and to remove this valuable monomer, stream 45 is sent to either a Joule Thomson valve 141 or an expansion turbine where the pressure of the stream is reduced, which cools the stream 45, and after the valve 141 the stream 45 is routed to the bottom of an absorption column 36 where it is contacted with the light naphtha stream. The propylene absorbs into the light naphtha, and the stream 46 with the absorbed propylene leaves the absorber 36 and is pumped to the MRL storage tank 44. The uncondensed gas stream 47 leaving the primary absorber column 36 is fed to the secondary absorber 49 where the heavy naphtha from stream 23 is used to absorb the remaining propylene which is recycled back to the column 21 by stream 50. The uncondensed gas 51 leaving the top of the secondary absorber 49 is routed to either the regenerator 18 or to the waste heat boiler to be combusted.
The plant shown in
The majority of the C2 components may alternatively be captured by condensing them. This is done by increasing the pressure the syngas stream is compressed to and by cooling this stream to a much lower temperature than that required for the C3 components.
As the output streams of heavy naphtha and MRL are both liquids, they can readily be transported to a petrochemical plant, for example as feedstocks for producing fresh polymers. At such a plant the C2 to C4 hydrocarbons can readily be desorbed from the light naphtha using conventional equipment and processed separately. Contaminants that were initially present in the plastic feedstock 1 are removed firstly within the extruder 2, then by adsorption onto the catalytic bed material in the reactor 14, and finally by the caustic wash scrubber/separators 37 and 41, so the levels of remaining contaminants are low enough to comply with the requirements in conventional petrochemical plants.
The MRL product will be transported via tanker to a petrochemical facility that includes a gas separation plant, for example downstream of a steam cracker; such a gas separation plant can be used to separate the components of the MRL. The MRL product from multiple decentralized plants can be transported to a single petrochemical plant; this overcomes the problem of transporting large quantities of low volumetric density plastic to a single location. At the petrochemical facility the C2 to C4 compounds can be fed downstream of the pyrolysis section of a steam cracker in order for them to be separated into individual components. Referring now to
However, while the arrangement depicted in
Referring to
Hence, in the same way as described above in relation to
| Number | Date | Country | Kind |
|---|---|---|---|
| 2200332.1 | Jan 2022 | GB | national |
| 2215821.6 | Oct 2022 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2023/050445 | 1/10/2023 | WO |
