A method for reducing catalyst consumption and energy consumption in microwave pyrolysis of waste polymers by melt pretreatment

Abstract

The present application relates to a method for reducing catalyst consumption and energy consumption in the process of microwave pyrolysis of waste polymers by melt pretreatment, belonging to the technical field of microwave catalytic pyrolysis recycling. It is well known that the microwave absorption ability of polymers is poor, and when using efficient and clean microwave pyrolysis, a sufficient amount of catalyst with strong microwave absorption ability is needed to start the reaction. Usually, the mass ratio of catalyst to polymer is close to 1:1, or even higher. The present application proposes that after mixing the waste polymers with the catalyst, melt pretreatment is carried out to reduce the bulk volume of the polymers, so that the catalyst is more concentrated, thereby improving the response of the mixture to microwaves. At the same time, the catalyst consumption is reduced, and the time for microwave to start pyrolysis is also shortened. This melt pretreatment method can significantly reduce the catalyst consumption in the pyrolysis process while ensuring high microwave heating efficiency, and also indirectly increases the single processing capacity of pyrolysis, which has a significant industrialization prospect.

Description

TECHNICAL FIELD

[0001] The present application belongs to the field of material recycling, and particularly relates to a method for reducing catalyst consumption and energy consumption in the process of microwave pyrolysis of waste polymers by means of melting pretreatment. BACKGROUND

[0002] Plastics have become an indispensable material in modern society due to their low cost, light weight, and durability. According to a public report, the global annual production of plastics continued to grow to 335 million tons in 2016, of which about 40% was used for packaging. Unfortunately, the short-term use of plastic products is unsustainable, resulting in a large amount of plastic waste. As of 2015, an estimated 6.3 billion tons of plastic waste had been generated since the 1950s, of which about 79% was discarded in landfills or the natural environment. Although the proportion of recycling and energy recovery has increased over time, the current situation is far from satisfactory, with more than half of the plastic waste discarded in 2015. Plastic waste causes serious problems to the land and marine environment when disposed of. Pyrolysis, as one of the advanced technologies for recycling renewable resources from organic polymers, has been applied to the treatment of biomass and combustible waste for a long time. Microwave heating is a clean and efficient heating method that selectively transfers microwave energy to substances with strong microwave absorption ability through energy dissipation inside the material and rapidly converts it into heat, thereby achieving rapid heating of the substance from the inside out. Microwave-assisted pyrolysis has attracted widespread attention as a promising method for accelerating chemical reactions and improving energy efficiency. Compared with traditional electric heating, microwave heating has the advantages of rapid and efficient heating, selective heating, higher energy conversion efficiency, etc. Therefore, this technology has been applied to the pyrolysis of various materials. The microwave absorption capacity of the material determines the microwave heating conditions. For example, the microwave absorption capacity of many waste polymers is weak at an industrial frequency of 2.45 GHz and cannot basically absorb microwave energy in a microwave field. Therefore, it is necessary to add appropriate microwave initiators to improve the microwave heating efficiency. However, whether carbon-based or metal-based wave absorbers, a higher proportion is required to transfer the absorbed microwave energy to the polymer and drive the waste polymer to rapidly heat to achieve pyrolysis. However, a higher proportion of catalysts hinders the further development of microwave pyrolysis and even industrial application. SUMMARY

[0003] The present application aims at the fact that there is no suitable process in the prior art to reduce the proportion of catalyst used and energy consumption in the microwave pyrolysis process, and proposes a method for reducing catalyst consumption and energy consumption in the microwave pyrolysis process of waste polymers by melting pretreatment. The waste polymers are mixed with the catalyst and then pretreated by melting, which reduces the bulk volume of the polymers and also allows the catalyst to be more concentrated to improve the response of the mixture to microwaves, thereby reducing the proportion of catalyst used and shortening the time for the microwave to start pyrolysis, and also indirectly increasing the single processing capacity of pyrolysis, which has a significant industrial prospect.

[0004] A method for reducing catalyst consumption and energy consumption in the microwave pyrolysis process of waste polymers by melting pretreatment, the specific steps are as follows:

[0005] (1) Mix the waste polymer fragments and the catalyst in a certain proportion.

[0006] (2) Place the mixture in a heating reactor and heat it to the melting temperature and keep it for a period of time, and determine the appropriate heating atmosphere according to the properties of the waste polymers. After cooling, it is transferred to a microwave pyrolysis reactor to complete the microwave pyrolysis.

[0007] The catalyst in step (1) should have good microwave absorption capacity, and the mixing ratio of waste polymers to catalyst is 2:1-10:1.

[0008] The atmosphere for melting treatment in step (2) should be selected according to the properties of the polymers, if the linear polyethylene content in the polymers exceeds 50wt.%, in order to prevent crosslinking with oxygen in the air, an inert atmosphere should be selected. If the polyethylene content is less than 50wt.%, the pretreatment can be carried out in air.

[0009] The temperature for melting treatment in step (2) depends on the melting point of the main polymer in the polymer and ensures that the mixture is melted, and the holding time is 0.1-1h.

[0010] The beneficial effects of the present application are:

[0011] (1) The mixture of polymers and catalyst is pretreated by melting, which reduces the bulk volume of the polymers and also allows the catalyst to be more concentrated to ensure a large enough microwave response, thereby reducing the proportion of catalyst used and also shortening the time for the microwave to start pyrolysis.

[0012] (2) The method of the present application has simple equipment requirements and is easy to realize continuous and large-scale operation mode. BRIEF DESCRIPTION OF DRAWINGS

[0013] Figure 1The microwave heating conditions of the mixture before and after the melt pretreatment of the catalyst and waste plastics (waste disposable medical masks) mixed at a mass ratio of 1:0.5 in Example 1. Specific embodiments

[0014] The application will be further described in connection with the specific embodiments, but the scope of the application is not limited to the content described.

[0015] Example 1: A method for reducing catalyst consumption and energy consumption in the microwave pyrolysis process of waste polymers by melt pretreatment, the specific steps are as follows:

[0016] (1) The waste polymer (disposable medical mask) fragments and the catalyst with strong microwave absorption capacity were mixed at a ratio of 1:0.3.

[0017] (2) The mixture was placed in a heating reactor and heated to 200°C under air atmosphere and kept for 10 min. After cooling, it was transferred to a microwave pyrolysis reactor and the pyrolysis was completed.

[0018] In this example, the mixture of waste polymers (disposable medical masks) and the prepared catalyst mixed at a ratio of 1:0.3 was heated to 200°C under air atmosphere and kept for 10 min. The mixture melted and collapsed, and the volume was significantly reduced. The average heating rate of the mixture in the first 13 min in a 1000W microwave field was 58.3°C·min-1. While the mixture without melt pretreatment was heated under the same microwave heating conditions, it only rose to 253°C in 13 min, as shown in Figure 1 The results show that melt pretreatment can not only shorten the time of microwave start-up pyrolysis, but also significantly reduce the consumption of catalyst, as shown in Figure 1 .

[0019] Example 2: A method for reducing catalyst consumption and energy consumption in the microwave pyrolysis process of waste polymers by melt pretreatment, the specific steps are as follows:

[0020] (1) The waste polymer (polyethylene) fragments and the catalyst with strong microwave absorption capacity were mixed at a ratio of 1:0.2.

[0021] (2) The mixture was placed in a heating reactor and heated to 250°C under argon atmosphere and kept for 30 min. After cooling, it was transferred to a microwave pyrolysis reactor and the pyrolysis was completed.

[0022] Example 3: A method for reducing catalyst consumption and energy consumption in the microwave pyrolysis process of waste polymers by melt pretreatment, the specific steps are as follows:

[0023] (1) The waste polymer (polypropylene) fragments and the catalyst with strong microwave absorption capacity were mixed at a ratio of 1:0.5.

[0024] (2) Place the mixture in a heating reactor, heat it to 190°C in an air atmosphere and hold it for 10 minutes. After cooling, transfer it to a microwave pyrolysis reactor to complete the pyrolysis.

[0025] The specific embodiments of the present invention have been described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A method for reducing catalyst consumption and energy consumption during microwave pyrolysis of waste polymers through melt pretreatment, characterized in that, The specific steps are as follows: (1) Mix waste polymer fragments and catalyst in a certain ratio; the mixing ratio of waste polymer to catalyst is 2:1-10:1; (2) Place the mixture in a heating reactor and heat it to the melting temperature and keep it at that temperature for a period of time. Determine the appropriate heating atmosphere according to the properties of the waste polymer. After cooling, transfer it to a microwave pyrolysis reactor to complete microwave pyrolysis. The atmosphere for melting treatment should be selected according to the properties of the polymer. If the linear polyethylene content in the polymer exceeds 50 wt.%, an inert atmosphere should be selected to prevent cross-linking with oxygen in the air. If the polyethylene content is less than 50 wt.%, pretreatment in air should be selected.

2. The method for reducing catalyst consumption and energy consumption during microwave pyrolysis of waste polymers through melt pretreatment according to claim 1, characterized in that: The melting temperature in step (2) depends on the melting point of the main polymer in the polymer and ensures that the mixture melts and is kept at that temperature for 0.1-1h.

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