Research on the Recycling of Waste Plastics (1)

In the river fields, traffic along the city streets and alleys, waste plastic waste can be seen everywhere. The windy weather drifts everywhere, or floats on the water surface, or wraps around the branches, or hangs on the wires and cables, polluting the environment and affecting the outlook. According to statistics from experts, there are 668 cities in the country, and two-thirds of them are surrounded by garbage belts. Aerial remote sensing measurements show that there are more than 7,000 rubbish heaps over 50 square meters in Beijing's suburbs. In 2001, Beijing transported 2.8 million tons of rubbish, equivalent to two half-mountain mountains. In this way, from the 1990s to the present, Beijing has been surrounded by more than 20 trash mountains that are as large as Jingshan. There are nearly 40 billion funds invested in urban waste management every year in the country, of which personnel fees account for about 70%. This huge investment is equivalent to the total amount of gold mined in the country each year. In other words, every year, China's mined gold is eaten by garbage. China's garbage disposal is still being passively received, that is to say how much, how much, how much, how much, how to clean up, to collect, to transport, to transport, to landfill (or to deal with), to be endless, endless... In addition, according to Xinhua News Agency Beijing, August 1, 2001.) It is reported that the "white trash" floating and floating on the banks of the Gezhouba Dam of the Yangtze River is sufficient for many people to stand without sinking. The garbage floating on the upper reaches of the Yangtze River is dragging down the river. It was blocked and accumulated in front of a dam of more than 2,600 meters in length. It is a white flood that poses a serious threat to ship locks and power plants, often causing downtime and huge losses. People are gradually realizing that plastics are also a threat to the living environment of humans while benefiting people.

Plastics have been widely used in various fields of industry, agriculture, national defense, and people's daily life. The plastics industry has become one of the most important industries with rapid development. The annual processing volume increases at a rate of about 10%, and the output of plastic products exceeds 20 million tons. Ranked second in the world. It is forecasted that by 2005, the national output of plastic products will reach or exceed 25 million tons, including 4.7 million tons for agriculture, 5.5 million tons for packaging, and 4.72 million tons for daily use and medical use, which together account for 59.7% of the total. With the large-scale use of plastics, the generated plastic waste is an inevitable product of the consumption of plastic products. Because plastics are light and not easily degradable, they are particularly eye-catching in the garbage, especially in disposable plastic bags and plastic lunch boxes. In particular, agricultural plastics, packaging plastics, household chemicals, and medical plastics have a relatively short life cycle. They will be discarded in about 6 to 12 months and scattered everywhere, forming “white pollution”. A considerable part of the waste plastic will eventually enter urban waste, making The proportion of waste plastic components in urban waste has risen from 1% to about 4%. Some cities have reached 8% to 10%, and the volume ratio has accounted for 1/3. If not treated, landfilling is not only a resource. The waste will also cause long-term damage to the environment. "White pollution" has received widespread attention from the society and caused controversy. Trying to “build on paper” is not necessarily desirable. It is not necessarily feasible. The correct handling of the relationship between development and the environment and the rational use of natural resources are urgent requirements of the 21st century. The experience and practices of developed countries are worth learning from. The successful strategy to solve plastic development and environmental issues is to implement the “Three R” strategy, namely, the reduction of plastic products, the reuse of Reuse, and the recycling of plastic waste (Recycle ).

The developed countries attach great importance to the research on the recycling of waste plastics and started the research and development of recycling technology and equipment. The final disposal methods for waste in the United States fall into three categories: First, recycling, which accounts for about 29%; second, landfilling, which accounts for 53%; and third, incineration (part of which is used for power generation), which accounts for 18%. The energy generated from waste is equivalent to 30 million barrels of oil per year, which is used to generate electricity. The generated electricity can be used by 2.4 million families throughout the year. Japan [4] Total waste plastics in 1996 was 9.09 million tons, of which 10.3 million tons (11%) were used for recycling, and 2.55 million tons (28%) were used as solid fuel, power generation, and heating, and the total effective utilization was 39%; Of the waste plastics that could not be used, only 3.14 million tons (24%) were incinerated and 3.37 million tons (37%) were landfilled. Japan [5] plans to recycle 65% of waste plastics in 2000 (including heat energy recovery of 50% and material recovery of 15%), and the recycling rate will reach 90% at the beginning of the 21st century (heat energy recovery 70% and material recovery 20%). That is basically entering the resource recycling society.

In recent years, as people’s awareness of environmental protection has increased, the domestic industry has increasingly attached importance to the recycling and comprehensive utilization (ie, recycling) of waste plastics. Research has become increasingly active and there have been numerous reports. At present, domestic research mainly focuses on energy recovery and chemical (or resource) recycling. Energy recycling includes oilification, co-liquefaction coking with coal, and blast furnace incineration. Chemical recovery includes the use of waste plastics for processing paints, coatings, adhesives, and building materials.

1. Waste plastic energy recovery research
Waste plastic oil, liquefaction or coking technology is the use of pyrolysis or catalytic cracking of waste plastics or with coal at high temperatures to obtain higher value liquid fuels. Coking is the coking of waste plastics and coking coal at low temperatures to save coking coal, increase the yield of coking coal coking products, reduce pyrolysis moisture, and increase oil yield. Oilification and co-coking are important methods for the recycling of waste plastics and have received extensive attention and research. The research progress at home and abroad in recent years is mainly in the following aspects:

1.1 Pyrolysis/Catalytic Pyrolysis
Jiao Shiyun, Jia Yu, et al. studied and computerized the best process conditions during the thermal cracking conversion of liquid waste plastics in down circulating fluidized beds. The results of computer simulations show that under the optimal process conditions, the down circulating fluidized bed can reach a relatively high medium component yield of 0.5477 within a short time (<1.2 s=), and this component is ideal. High value-added products. The optimum process conditions are as follows: the initial temperature of the solid heating medium is 645°C, the initial temperature range of the waste plastic gas entering the reactor is 450-500°C, and Xue Susheng tests the catalytic pyrolysis of waste plastics to recover fuel oil and chemicals. The test results show that the catalyst has an obvious effect on the output value and output of waste plastic pyrolysis products. In particular, zinc oxide and titanium oxide have the best effect on improving the output value and yield of pyrolysis products.

Li Wenhong and Li Yuhong selected waste polyethylene, polypropylene, and polystyrene mixtures as samples, and systematically studied the influence of catalysts on product distribution and yield during catalytic cracking and catalytic upgrading. The relationship between different catalysts and oil yield and gasoline octane number was obtained.

Lu Jiangyin, Wu Haitao, Li Ru and others studied the catalytic pyrolysis recovery of waste polystyrene plastics (PS). At the same time, the pyrolysis products were analyzed and their comprehensive utilization was studied and discussed. The chemical recycling method is simple and practical, and has practical significance for the actual environmental protection.

Wang Min and others investigated the production and production of waste plastics for gasoline and diesel. Combined with several years of trial and development, the company designed a set of waste plastics from small and medium-sized cities as raw materials, preheated with solvent, heated by tube furnace, and automatically cycled. The slag pyrolysis furnace adopts self-made high-efficiency catalysts, and the production cost of new gasoline and diesel production lines is RMB 165,000 per unit. It annually processes 150 tons of waste plastics, annually produces 100 tons of gasoline and diesel, and has an annual profit and tax of RMB 150,000. It is suitable for the production and development of suburban township small enterprises.

Lu Jiangyin established a laboratory device for the catalytic cracking of waste plastics, and discussed the factors that affect the catalytic cracking of gasoline into gasoline, such as temperature, pressure, catalyst, and raw materials. 80% - 90% of decomposed fuel oil is obtained under optimized conditions, with a gasoline content of 50% - 70%. The catalytic cracking mechanism of hydrocarbon polymer plastics was preliminary discussed, and the application value and significance of the process under industrial scale-up were predicted.

Cheng Shuiyuan, Jin Ye, Hao Ruixia, etc. studied the cracking of the mixture of polyethylene and polypropylene into raw oil. At a temperature of 440°C, different ratios of polyethylene and polypropylene were cracked. It was found that polypropylene has the highest liquid recovery rate. Several catalysts were applied to the cracking of polypropylene. By comparing the catalytic results of several different catalysts, it was found that the composite catalyst has a better catalytic effect. It provides a new and useful method for the comprehensive utilization of waste plastics in the future.

Liu Yirong and Qi Xing selected several typical waste plastics for thermal degradation and catalytic degradation, tested their degradation phenomena, product distribution and characteristics, and recommended various types of waste plastic processing. Waste PS, PP, PE thermal degradation products have high liquid yields and thus have high utility value. PS thermal degradation products can produce styrene monomer or high octane gasoline blending components; PP, PE thermal degradation can obtain wax or fuel oil, if you want to produce high quality gasoline, diesel, you must carry out catalytic upgrading .

Japan's Saito Kikuchi, Matsubara Tsuyoshi, etc. [Patent Publication Nos. 98108883, 97114906] invented a method for continuously and rapidly degrading and converting thermoplastics, cross-linked plastics, thermosetting plastics or mixtures thereof into oil without Need to distinguish between various types of waste plastics. Powdered plastics obtained by grinding thermoplastics, cross-linked plastics, thermosetting plastics, or mixtures thereof are mixed with water to form a slurry, and a dispersant such as a water-absorbent resin, a water-soluble polymer, or a surfactant is added to the slurry. The resulting mixed slurry is fed to a tubular continuous reactor in which the powdered plastic is degraded with water at or near its supercritical region, and finally the oil is recovered from the reaction product.

At present, the recovered oils are mainly tars and diesel oils, with lower numbers and higher costs. It is generally believed that the oiling technology is only suitable for polyolefin plastics. The focus of the study is to find efficient, long-lived catalysts, increase oil yields, reduce costs, and overcome secondary pollution to the environment during processing.

The four ministries and commissions of the State Economic and Trade Commission issued the “Circular on Strengthening the Management of Gasoline and Diesel Fuel Production from Waste Plastics” (National Economic, Trade and Technology [2001] No. 440) on June 12, 2001, according to the State Economic and Trade Commission and the relevant authorities. In some areas, special investigations were conducted on the production of gasoline and diesel fuel from waste plastics. After organizing relevant experts to conduct research, it was concluded that the current use of waste plastics to produce gasoline and diesel has only achieved laboratory results, and industrial production technology is not yet mature. The main problems are as follows: First, the company's production equipment is simple, the process is incomplete, and the necessary product testing methods are lacking. Some companies have never even conducted formal product inspections; the second is that some samples of gasoline and diesel are sampled A number of indicators failed to meet the standards of No. 90 gasoline and No. 0 diesel, and unqualified oil products had already flown into the market. Third, non-condensable gases produced in the oil refining process were directly discharged and there was obvious secondary pollution. Necessary security measures have security risks. For this reason, the notice pointed out that it is necessary to regulate the research, development and management of the use of waste plastics in the production of gasoline and diesel fuel.