Papers Delivered at International Conference on Cleaner Production
Beijing, China -- September 2001 -- Paper 26 of 30

The resourcization study of decomposition of waste polyester bottle into monomer DMT and EG with super critical methanol

Xiangguo Huang, Wu Wen, Xiaobing Chen, Chunyan Shao 
(Shenyang Environment Science Academy 110005)

Abstract: the paper comments the environmental pollution of the plastic waste, and compares several techniques of the comprehensive treatment of the plastic waste. Furthermore, it particularly discusses the plastic waste treatment technique with super critical liquid.

Keywords: plastic, environment, super critical liquid, reclaim and recycle, comprehensive treatment

At present, the total annual plastic output all over the world has exceeded 100,000,000 tons, and about 30 percents of which was used as packing. However, the packing material belongs to the plastic’s “short-life” application category. This packing consumed mostly becomes the municipal solid waste and enters the trash treatment system, and the other is discarded randomly. This part causes the environmental pollution, and even influences the ecological balance, harms the livestock and hydro-life. Therefore, this behavior suffers the censure of the public and the environmental protection department[1].

Super critical liquid technique began with the extraction technique, and applies in the fields of biology, food science, chemical engineering and the like[2]. But its application in the area of environmental protection is still a renascent thing. Recently, the re-resourcization technique such as decomposing the waste plastic into monomer or oligomer with super critical liquid is paid close attention. This paper will introduce the technique of efficiently decomposing the polycondensate PET into their monomer materials with super critical liquid methanol in detail.

1. The influence of the plastic waste to environment

Along with the increasing supply of the synthetic resin market, the problem of “white pollution” is increasingly prominent, and it becomes an important problem that must be solved in order to develop the synthetic resin industry in our country[3]. The meaning of “white pollution” is a sort of environmental pollution caused by the hardly degradable waste plastic. Most plastic is reclaimed as fuel. However, plastic combusts very rapidly, and it is difficult to combust completely. In addition, the combustion of plastic such as PVC can cause air pollution as it contains chlorine. Most foam such as foaming PF and foaming polystyrene is delivered to bury. Its characters of light weight, large volume and difficult degradation bring some problems to this solution, including extensive area used, short life span of the landfill.

2. The state of art of the plastic waste treatment

The universally adoptive techniques to deal with the plastic waste in our country have: heating decomposition oilization technique, heating decomposition gasification technique, high furnace raw materialization technique, cement raw materialization technique, and solid fuelization technique[4],[5],[6].

The characteristic of this five techniques are:

1. They can deal with various kinds of plastic such as plastic containing chlorine and plastic mixed with other materials.
2. Heading decomposition oilization is taken place under the condition of nitrogen gas reduction. Heading decomposition gasification and high furnace raw materialization are taken place under the high temperature. Therefore, it is impossible to produce dioxin
3. The chlorine production can be separated and reclaimed----as industrial hydrochloric acid or fertilizer.
4. The most residue of heading decomposition oilization is carbon. the heat quantity of carbon is 8000Kcal/Kg. Therefore, it can be reclaimed and recycled as fuel.
5. The regenerative production is the coarse gasoline, diesel oil, fuel oil, synthesized ammonia, synthesized methanol, solid fuel and the like. If the advanced quality of the oil is needed, than the project of catalyst must be established.

The problem existed: The above five reclamation and recycle methods can only utilize the organic components of the plastic, and only reclaim and recycle its heat energy. Since the regenerative production belongs to the low additional value’s production and has low use efficiency, and since the plastic industry is under the situation of raw material shortage and high plastic value, the five methods are only coarse plastic recycle methods.

3. The resourcization technique of waste plastic treatment with super critical liquid

Super critical liquid is the non-condensation liquid when exceeding the critical temperature and the critical pressure, and it possesses the liquid density and the gas’s molecular movement energy. Compared with the decomposition reaction in the liquid phase, the reaction velocity increases largely in the super critical liquid. Compared with the reaction in the gas phase, the reaction in the super critical liquid can reduce the production of byproduct, and increase the output rate[7],[8]. The following is the details of this technique of efficiently decomposing the polycondensate PET into their monomer materials with super critical liquid methanol.

4. Experimental

4.1 fundament

This project mainly utilizes the super critical methanol to decompose the strictly-chosen and reclaimed waste polyester bottle into the monomer of the PET’s raw material -- dimethyltryptamine (DMT) and glycol (EG), and utilizes the products again. The reaction equation (ellipsis).

4.2 materials’ name, specification and source

4.3 The pane chart of the main equipment and technique flow

4.3.1 The main equipment

Polymer smelting vessel, high pressure decomposition reaction vessel, high pressure measuring pump, crystallizing vessel, rectifying tower.

4.3.2 The pane chart of the technique flow

Figure 1: the pane chart of the technique flow of composition of PET with super critical methanol

5. Result and discuss

5.1 The relationship between experimental temperature and experimental pressure

The relationship between temperature and pressure of super critical methanol appeared linear relation. When increasing the reactor temperature, the reactor pressure was increasing accordingly.

5.2 The relationship between experimental pressure and materiel proportion

When fixed the similar highest reaction temperature, the corresponding highest reaction pressure was different as a result of the different proportion of methanol and PET. When increasing the ratio of methanol to PET, the corresponding highest reaction pressure was increasing along with it.

5.3 The influence of the experimental temperature to the output ratio

When reaction temperature was low, the remanet ratio of PET was large and the output of DMT and EG was low. When the reaction temperature was too high or the rise of the reaction temperature was too quick, the yielded DMT was subject to second decomposition, which results in the decrease of the output of DMT.

5.4 experimental result

The experimental results show that the output ratio of DMT was 96.6%, and the output of EG was 95.5%.

6. Conclusion

1. In the experiment of decomposition of polyester bottle with super critical methanol, besides there existed some un-decomposed PET in the low pressure region of 3.0 MPa, the PET can be decomposed completely in the condition of 8.0 MPa. In addition, the DMT and EG can be reclaimed almost completely as the forms of monomer or oligomer.
    

2. According to experiment, when the reaction temperature was low, the remanet ratio of PET was large, and the output quantity of DMT and EG reduced. When the reaction temperature was too high or the rise of the reaction temperature was too quick, the yielded DMT was subject to second decomposition, which resulted in the decrease of the output of DMT.
    

3. According to the material balance of reaction process, the output ratio of produced DMT to its theoretical production quantity was 96.6%, and that ratio of EG was 95.5%.
    

4. Through the experimental search and the analysis and testing of the production, the technique course of decomposing the polycondensate PET into their monomer materials with super critical liquid methanol was feasible.

Reference

[1] Guomei Zhou, the general situation of the domestic and overseas treatment technique of the waste packing, Chinese chemical engineering information, 2000 (4) 11

[2] Ziqiang Zhu, the super critical liquid technology----principle and application, Chemical industry press, 2000 (1) 14

[3] Editor: Hao Ding, Plastic industry manual, Chemical industry press 1995 (1) 19

[4] Fengzhen Qian, Plastic industry, 1999, (2) 1

[5] Environmental management (Japanese), 1999, 35 (3) 19-27

[6] Environmental management (Japanese), 1999, 35(3) 18

[7] The countermeasure to resource and environment (Japanese), 1998, 35(12) 31-37

[8] Fuel and combustion (Japanese) 1999, 66 (6) 410-420