Patent Application
20250207037
The present disclosure relates to a method for producing lower hydrocarbons, preferably hydrocarbons having between 2 and 3 carbon atoms (C2-C3 hydrocarbons), by a thermal cracking of mixed waste plastic (“MWP”) under pressure and in the presence of methane and hydrogen. The present disclosure further relates to a reactor apparatus for producing the lower hydrocarbons by the said method.
Globally, the production of plastic has increased steadily in the last decades. Recycling routes are an option to minimize plastic waste while producing valuable petroleum products. However, the use of 100% MWP pyrolysis oil in catalytic cracking processes has limitations due to high amounts of coke formation in the feed nozzles during preheating of the feed. Further, the formation of heavier products (e.g., fuel oil and tar) is high with 100% waste plastic oil as a cracker feed. Integration of MWP pyrolysis oil into refinery fluid catalytic cracking unit feed streams has been previously investigated. Catalytic cracking or thermal cracking of a pyrolysis oil derived from material comprising biomass has also been disclosed in prior art. However, the majority of products produced by such processes include naphtha range components, such as products having five or more carbon atoms (C5+ products) instead of lower hydrocarbons.
Thermal cracking is used mostly for production of liquid fuels and rarely for gaseous hydrocarbons. Further, thermal cracking of plastics to low molecular weight materials has a major drawback in that a very broad product range is obtained. Other methods for waste polymer cracking comprises application of thermal cracking in the presence of hydrogen. This enables cracking waste plastic to lower hydrocarbons and reduces formation of coke. However, use of high amount of hydrogen requires separation of a large amount of non-converted hydrogen from the products stream, which leads to the additional separation cost. Furthermore, valuable olefins produced by the reactions are deeply hydrogenated by the hydrogen resulting in the conversion into aliphatic hydrocarbons, such as methane, ethane and propane which are less valuable. The ethane and propane can easily be converted to ethylene and propylene by steam cracking, but it is difficult to convert methane into an olefin by economical means, and the production of the methane also involves the consumption of the valuable hydrogen. Another problem, which results from the production of the methane by the hydrogenation of the olefins, is that the hydrogenation reaction is highly exothermic and the reaction temperature is accordingly raised. Since the hydrogenation reaction progresses vigorously at high temperature, the production of the methane is accelerated more and more, and as a result, a runaway reaction may potentially occur. In consequence, the yield of ethylene is reduced and the production of methane increased. Therefore it is difficult to maintain the yield of the olefins at a high level. This occurs even at atmospheric pressure but is particularly noticeable in a high pressure, hydrogen rich environment. As a result, the aforesaid advantage based on the employment of a high pressure is offset.
U.S. Pat. Nos. 4,527,002 and 4,599,478 disclose methods of manufacturing olefins by thermally cracking hydrocarbons, wherein the hydrocarbon is burnt with oxygen in the presence of steam to generate a high-temperature gas containing steam as a heat source for thermal cracking purposes, and methane and hydrogen in amounts required for reaction are supplied into said high-temperature gas containing said steam so that the hydrocarbon can be thermally cracked in the presence of methane, hydrogen and steam.
U.S. Pat. No. 4,520,224 discloses a method for producing olefins, particularly ethylene and propylene by a thermal cracking of hydrocarbons under pressure and in the presence of methane and hydrogen.
One of the ways to reduce methane formation is the reduction of contact time of the reaction. However, reduction of contact time or the residence time reduces conversion. Therefore, in order to obtain the desired yield of olefins by shortening the residence time, it is necessary to set the reaction temperature to an ultra-high temperature of 1200° C. or more. As a result of such a temperature rise, the production of methane is only slightly reduced, which leads to additional problems, such as decomposition of olefins at high temperatures and abnormal variation in the yield of the products.
There has been reports in some prior art documents which show that in the presence of methane and hydrogen in the feed, the formation of methane could be reduced significantly through controlling of methyl radicals, which produces C2 hydrocarbons. However, in these documents, the feed is heavy residue or heavies, which is inherently hydrogen deficient, i.e., the hydrogen to carbon (H/C) mol ratio for the feed<1. The above-mentioned concerns have not been adequately addressed in the art, when MWP is added as a feed, wherein the H/C mol ratio for the feed inherently is >1. Direct conversion of MWP to lower hydrocarbons, preferably hydrocarbons having between 2 and 3 carbon atoms (C2-C3 hydrocarbons), is a relatively less investigated area of research due to the very low yield of these gaseous hydrocarbons. Conventional processes for the conversion of MWP to C2-C3 olefins involves multiple steps, such as conversion of plastics to pyrolysis oil, dechlorination of pyrolysis oil and hydro treatment of pyrolysis oil to liquid cracker feed. Because of the requirement for so many steps, the economic viability of operating such processes is, thus, limited.
It is, therefore, an object of various embodiments of the disclosure to provide a method for producing lower hydrocarbons, such as C2-C3 hydrocarbons, and particularly ethylene and propylene, by thermal cracking of MWP in a single step process.
It is another object of various embodiments of the disclosure to provide a method for producing lower hydrocarbons such as C2-C3 hydrocarbons, and particularly ethylene and propylene, by thermal cracking of MWP wherein the desired C2-C3 yield is higher than the currently available thermal cracking processes.
The present inventors have found that the above-stated objectives can be achieved by thermal co-conversion of MWP and methane in the presence of hydrogen. Thermal cracking of MWP at high temperatures has been found to generate intermediate radicals, which by chain reaction leads to activation of methane and formation of lower hydrocarbons. Furthermore, the present inventors have found that by controlling the methane to hydrogen mol ratio, methane can act as an initial heat source. This reduces hydrogen consumption which helps to improve the yield of gaseous C2-C3 hydrocarbons.
Accordingly, the present disclosure relates to a method for producing lower hydrocarbons by a thermal cracking reaction from MWP comprising:
By application of the disclosure, the foregoing objectives are met, at least in part. The disclosure will now be described in more details.
The present disclosure provides a thermal cracking reaction for producing lower hydrocarbons from MWP which comprises the steps of adding methane to hydrogen to form a mixed gas feed, wherein the methane/hydrogen (CH4/H2) mol ratio in the mixed gas is from 0.1 to 5, preferably from 0.5 to 4. The mixed gas is then combined with the MWP to form a reaction mixture which is reacted at pressure of 1 to 40 bars, a temperature of 800° C. to 1200° C. and a residence time of 10 to 300 milliseconds in the reactor.
In the present disclosure, methane is added to hydrogen within a preferred methane/hydrogen mol ratio under pressure, whereby:
The above and other features, aspects and advantages of the present disclosure are better understood when the following detailed description of the disclosure is read with reference to the accompanying drawings, in which:
According to the present disclosure, the generation of aliphatic hydrocarbons from olefins owing to deep hydrogenation is restrained, thereby enabling an optimum selection of reaction temperature and a residence time for the thermal cracking reaction so as to obtain a maximum yield of the lower hydrocarbons, preferably C2-C3 hydrocarbons. Preferably, the temperature of the thermal cracking reaction is from 850° C. to 1100° C., and the residence time in the reactor is from 50 to 150 milliseconds in the reactor.
The thermal cracking reaction of MWP is an exothermic reaction. Therefore, it does not need application of external heat. However, external heat is needed for heating of reaction mixture to the reaction temperature before feeding to the reactor. The initial heat required for the thermal cracking reaction can be generated by a heat carrier circulated within the reactor. The mixed gas comprising methane and hydrogen may also act as a heat carrier in the present disclosure. The initial heat may be generated by combustion of some amount of methane with oxygen to heat the remaining amount of the reaction mixture comprising methane, hydrogen and MWP. Therefore, a stoichiometric excess amount of methane is taken, such that the CH4/H2 mol ratio from 0.1 to 5, preferably from 0.5 to 4. After the reaction, the reaction heat may be used for heating of the feed through exchange of heat in the quenching process.
Alternatively the heat carrier can be selected from one or more of oxide and carbonate salts of alkali-earth metals. Use of such solid thermo-contact material help to carry the heat via circulation of the thermo-contact material within the reaction zone and regenerator where the combustion of coke generates heat. Examples of thermo-contact material may be, but not limited to CaO, MgO, CaCO3, MgCO3 and mixtures thereof.
MWP, in the present disclosure indicates “recycled waste,” “waste stream,” and “recycled waste stream”, which can be used interchangeably to mean any type of plastic-containing waste. The recycled waste stream is a flow or accumulation of recycled waste from industrial and consumer sources that is at least partially recovered. A recycled waste stream includes materials, products, and articles (collectively “material(s)” when used alone). Recycled waste materials can be solid or liquid. In the present disclosure MWP preferably comprises waste plastics, waste textiles, waste modified cellulose, waste biomass, post-industrial waste streams, intermediate industrial waste streams, or combinations thereof.
In one aspect of the disclosure, a post-industrial material is one which has been created and has not been used for its intended application, or has not been sold to the end use customer, or discarded by a manufacturer or any other entity engaged in the sale of the material. Examples of post-industrial materials include rework, regrind, scrap, trim, out of specification materials, and finished materials transferred from a manufacturer to any downstream customer (e.g. manufacturer to wholesaler to distributor) but not yet used or sold to the end use customer.
In another aspect of the disclosure, the MWP can include one or more post-consumer waste plastic such as, for example, high density polyethylene, low density polyethylene, polypropylene, other polyolefins, polystyrene, polyvinyl chloride (PVC), polyvinylidene chloride (PVDC), polyethylene terephthalate, polyamides, poly(methyl methacrylate), polytetrafluoroethylene, or combinations thereof. As used herein, “post-consumer” refers to non-virgin plastics that have been previously introduced into the consumer market. Preferably the MWP feed may include high density polyethylene, low density polyethylene, polypropylene, other polyolefins, polystyrene, polyvinyl chloride (PVC), polyvinylidene chloride (PVDC), polyethylene terephthalate, polyamides, poly(methyl methacrylate), polytetrafluoroethylene, or combinations thereof. More preferably, the MWP feed consist of high density polyethylene, low density polyethylene, polypropylene, or combinations thereof. The MWP feed can comprise 30 to 99 wt. percent, preferably 70 to 99 wt. percent of at least one of high density polyethylene, low density polyethylene, and polypropylene.
The form of the MWP, which can be fed to the thermal cracking reactor, is not limited, and can include any of the forms of articles, products, materials, or portions thereof. A portion of an article can take the form of sheets, extruded shapes, moldings, films, laminates, foam pieces, chips, flakes, particles, fibers, agglomerates, briquettes, powder, shredded pieces, long strips, or randomly shaped pieces having a wide variety of shapes, or any other form other than the original form of the article and adapted to feed in a thermal cracking reactor of the present disclosure. Preferably, the MWP is melted before feeding it to the reactor.
Without willing to be bound by it, the present inventors believe that the feature of the present disclosure resides in the thermal co-conversion of MWP and methane in the presence of hydrogen at high temperature and pressure. For example, in the preparation of ethylene from methane, the following reactions occur. The initiation of the reaction for methane activation can be through decomposition of the MWP (for example, primarily polyethylene or polypropylene) with formation of olefin monomer, such as —CH2—CH2— (reaction formula 1), which may decompose with formation of intermediate radicals like vinyl radicals and hydrogen radical (reaction formula 2). The intermediate radicals, by chain reaction leads to activation of methane and formation of C2 hydrocarbons (reaction formulae 3-5). Application of methane as a hydrogen resource reduces hydrogen consumption and improves gaseous C2-C3 hydrocarbons yield.
n(—CH2—CH2—)→—CH2—CH2—+(n−1)-CH2—CH2— (1)
—CH2—CH2→CH2—CH—.+H. (2)
H.+CH4CH3.+H2 (3)
2CH3.→C2H6 (4)
C2H6→C2H4+H2 (5)
Therefore, as is clear from reaction formula (3), in the presence of a sufficient amount of methane, the reaction proceeds in the forward direction, and thus the hydrogen radicals change into molecular hydrogen, so that the concentration of the hydrogen radicals decreases and instead the concentration of the methyl radicals increases. In this manner methane behaves as an absorber of hydrogen radicals, thereby preventing the hydrogenation reaction of olefins due to the hydrogen radicals. Further, a dehydrogenation reaction is facilitated, so that the production of olefins is accelerated.
Simultaneously methane is converted into ethane and ethylene by the recombination of the formed methyl radicals, as mentioned above. Accordingly, methane not only serves as a diluent but also contributes to the increase in the yield of ethylene by a reactive mechanism as described above.
This is different in mechanism and effect from the conventional method, where the reactions are exothermic due to thermal cracking in the pressurized atmosphere of hydrogen or the presence of a large supply of hydrogen. In contrast, the thermal cracking method according to the present disclosure employs a reaction atmosphere including co-conversion of MWP and methane simultaneously in the presence of hydrogen. Therefore the reactions involving methyl radicals predominate, and these are endothermic. The reaction mixture is thus quenched during the progress of the reactions, so that no runaway reactions occur.
Also with regard to yield characteristics of the reactions, a substantially unchanged yield range (plateau range) exist in the system, irrespective of variation in reaction time and reaction temperature. In a preferred embodiment, the lower hydrocarbon product stream comprises from 40% to 50% by weight of a mixture of ethane and propane. In another preferred embodiment the lower hydrocarbon product stream further comprises from 40% to 50% by weight of a mixture of ethylene and propylene.
According to the disclosure the H/C mol ratio of the reaction mixture comprising methane, hydrogen and MWP as disclosed herein is ≥1. In a preferred embodiment, the product yields of ethane and propane obtained in accordance with the disclosure is at least 3 to 4 times higher than those obtained by thermal cracking of a reaction mixture wherein the mol ratio of hydrogen to carbon in the reaction mixture is <1. In another preferred embodiment, the product yields of ethylene and propylene obtained in accordance with the disclosure is at least 40% higher than those obtained by thermal cracking of a reaction mixture wherein the mol ratio of hydrogen to carbon in the reaction mixture is <1. For the avoidance of doubt it is noted that for comparison of yields, only the indicated feature, in this case the H/C mol ratio of the reaction mixture is different. All other materials and processes are identical.
The disclosure further relates to a reactor apparatus for producing lower hydrocarbons by a thermal cracking reaction from MWP. In a preferred embodiment, the disclosure related to a reactor apparatus for producing lower hydrocarbons by a thermal cracking reaction from MWP comprising:
Both hydrogen and methane can be combined to form a mixed gas and fed in to the reactor before the start of the reaction. The decomposition of methane is facilitated by regulating the reaction temperature and pressure as well as the methane/hydrogen mol ratio in the atmosphere, so that the added methane can be converted into more valuable lower hydrocarbons such as ethane and ethylene.
The present disclosure will now be described using the following non-limiting examples to demonstrate the practice and advantages thereof. It is understood that the examples are given by way of illustration and are not intended to limit the specification of the claims to follow in any manner.
The following examples draw on published information in the open literature, which is commonly used in the thermal cracking reactor configurations examined by kinetic modeling and simulation. To describe the reaction kinetics, a kinetic modeling simulation was done with octane as a model feed and observed the variation of reaction conditions and other parameters. Thus the reaction mixture in the present examples comprises methane, hydrogen and octane. Table 1 shows the variation of feed ratios as used for the kinetic simulation study.
The results of the kinetic simulations are shown in the following figures.
To demonstrate the advantages of adding methane in the feed in accordance with the various embodiments of the present disclosure a comparative kinetic simulation experiment was done wherein two otherwise similar feeds were compared, one containing an inert gas, nitrogen (Feed A) and a second where instead of nitrogen, methane was added (Feed B). The feed compositions for the experiment are provided in Table 2.
The results of the kinetic simulations for feed compositions A and B are shown in the following figures.
From the examples described above, the respective ranges for various embodiments of the present disclosure are as follows: First, with regard to the reaction pressure, a suitable reaction pressure could be between 1-40 bars, preferably 5 bars or more, most preferably 20 bars. With regard to the proportion of methane to be added for forming the mixed gas, when a methane/hydrogen mol ratio is less than 0.1, the effect of methane is limited, and on the other hand, when it is greater than 5, the yield of coke is great even at an optimal pressure. Therefore, a suitable methane/hydrogen mol ratio is from 0.1 to 5, preferably from 0.2 to 4, most preferably from 0.5 to 4. With regard to a residence time of reactions, for a good yield of C2-C3 hydrocarbons, it can be in the range of 10 to 300 milliseconds, preferably 50 to 100 milliseconds. The reaction temperature is preferably within 800° C. to 1200° C. According to the present disclosure, there is provided an integrated and industrially useful thermal cracking method for producing C2-C3 hydrocarbons from MWP with enhanced yield.
While embodiments of the disclosure have been shown and described, modifications thereof can be made without departing from the spirit and teachings of the disclosure. The embodiments and examples described herein are exemplary only, and are not intended to be limiting. Many variations and modifications of the disclosure disclosed herein are possible and are within the scope of the disclosure. Accordingly, the scope of protection is not limited by the description set out above but is only limited by the claims which follow, that scope including all equivalents of the subject matter of the claims. Each and every claim is incorporated into the specification as an embodiment of the present disclosure. Thus, the claims are a further description and are an addition to the detailed description of the present disclosure. The disclosures of all patents, patent applications, and publications cited herein are hereby incorporated by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| 22166402.2 | Apr 2022 | EP | regional |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2023/058128 | 3/29/2023 | WO |
