Development of Chemical Recycling Process for
Post-Consumer PET Bottles by Methanolysis in Supercritical Methanol

1. Introduction

The production of PET bottles has been on the rise mainly for beverage bottle use these past several years
(330 000 tons in 1999, 360 000 tons in 2000) and is expected to increase further because of their convenience. On the other hand, since the Containers and Packing Recycling Law came into effect in 1997, the collection rate of post-consumer PET bottles by local municipalities has increased dramatically (22.8% in 1999, 34.5% in 2000) with the possibility of exceeding 50% in 2005.

Sales volume of products recycled from post-consumer PET bottles is also expanding to a large extent for fiber and sheet products (40 000 ton in 1999, 69 000 ton in 2000). However, the sales volume of recycled fiber and sheet products is coming to be saturated, so that there is concern that these products will be unable to cope effectively with the increase in the collection of post-consumer PET bottles in near future.

Taking into consideration the above tendency, MHI began development of a chemical recycling process for depolymerizing post-consumer PET bottles into monomers for use as feed stocks for manufacturing PET resin (2), utilizing supercritical methanol (methanolysis) (1).

MHI intends to establish an ideal recycling system consisting of a recycle flow from PET bottles to PET bottles [so called B to B (Bottle to Bottle) recycling] while minimizing the consumption of natural resources and the generation of wastes by making this process practicable.

MHI is currently undergoing the development and verification test of Mitsubishi process at a MHI pilot plant. This report presents an introductory overview of the Mitsubishi process from the viewpoint of the feasibility of the “B to B” recycling of PET bottles.

In order to commercialize the B to B recycling business, the following three challenges need to be addressed.

1. Technical aspect: The quality of the recycled monomers  must be equivalent to the virgin monomers produced from petroleum.

2. Economic aspect: Economical feasibility of the business must be established to ensure profitability.

3. Aspects unique to recycling business: Post-consumer PET bottles must be available in an amount necessary to productively operate a recycling plant.

3.1 Recycling flow sheet

Fig. 1 shows the flow sheet of a PET recycling system, with a comparison of the Mitsubishi process using supercritical methanol and a conventional process.

Fig. 2 shows an outline flow sheet of the Mitsubishi process. It can be seen that post-consumer PET bottles are flaked by a PET bottle shredding section, after which the flakes of the PET bottles are sent on to the depolymerization (with supercritical methanol) section.

The model formula of the reaction is shown in Fig. 3. The mixture of depolymerized DMT, EG, and excess methanol is then sent on to a lower-boiling product separation section to be separated into DMT and EG/excess methanol, which in turn are sent to the respective purification section to be purified by distillation. The purified DMT monomer is further converted into PTA (highly purified terephthalic acid) in the hydrolysis section (4). EG is methanol in the EG/methanol purification section. Finally, the converted PTA and purified EG are then delivered to existing PET resin production plants, thereby forming an ideal cyclical type recycling system.

3.2 Features of the Mitsubishi process

The Mitsubishi process is characterized by a combination of a newly developed high-reaction-velocity PET-depolymerizing process by supercritical methanol and an existing DMT hydrolysis process for converting PET into PTA, which is a feed stock monomer for producing PET. Another feature of this process is that it is provided with a process where impurities such as foreign plastics that might contaminate the collected post-consumer PET bottles are eliminated in accordance with the properties of the impurities.

The advantages of using supercritical methanol are as follows:

(1) Utilization of the high reactivity of supercritical fluid makes it possible to complete the reaction quickly, so that the both the depolymerization section and the reactor can be made compact. Table 1 shows a comparison of both processes of methanolysis with supercritical methanol and the conventional depolymerization. As can be seen in Table 1, the Mitsubishi process with supercritical methanol can complete the depolymerizing reaction in the shortest period of time.

(2) The Mitsubishi process does not require any catalyst for the depolymerization, so that the operation of the reaction is simplified and the separation of catalyst is not necessary.

(3) In conventional depolymerization with EG (Glycolysis process), special purification operation must be taken because the depolymerization product contains components with high boiling points exceeding 400 ⁰C, such as BHET (bis-hydroxyethyl terephthalate). In the Mitsubishi process, however, normal distillation can be applied because the depolymerize products consist of DMT (boiling point: 288 ⁰C) and EG (boiling point: 198 ⁰C). The lower boiling points and absence of any need for any special steps means that both purification sections can be simplified.

The advantages of conversion to PTA can be summarized as follows.

(1) Converted PTA is usable as a feed stock for producing PET resin, so that it can be sold to existing PET resin manufacturers.

(2) Since only the PET resin manufactured from PTA is used as the raw material of PET bottles in Japan at present, the bottle to bottle recycling system can be readily operated using existing domestic manufacturing and distributing systems.

3.3 Qualities of recovered monomers

Trial production of DMT and EG was performed using the continuous depolymerization apparatus and continuous distillation apparatus shown in Figs.  4 and 5.

The Containers and Packing Recycling Law implemented in 1997 states that a recovered product commercializing enterprise can gain a disposal commission corresponding to the amount of recycled products in the bidding from The Japan Container and Packaging Recycling Association (Fig. 6). Since the results of the feasibility study came within the scope of the application of this law, it has been shown that a “B to B” recycler using the Mitsubishi process would become feasible in a plant that has a capacity of 20 000 to 40 000 tons/year of post-consumer PET bottles.

Research and development work undertaken by MHI into PET bottle-recycling technology showed that high purity monomers equivalent to the virgin monomers would be recovered. And it was shown that our process would be feasible. MHI is planning to acquire the plant operation data for designing a commercial plant at the pilot plant, and to build the commercialization scheme for PET recycling.

(1) Sueoka, Y. et al., Chemical Recycling Techniques for Plastics, Mitsubishi Juko Giho Vol.36 No.3 (1999) pp.146-149

(2) Kondo, Y., Genta, M., Development of PET depolymerization into monomer process by Supercritical Methanol, the 9th Symposium on Material Recycling Technology (2001) pp.27-30

(3) Goto, M. et al., Plant & Process 41-2 (1999) pp.47-51

(4) Anton Schoengen, JP Patent S57-53332

(5) Sako, T., Chemical Process Technology using Supercritical fluid, The Seminar on Most-Advanced Technology by NEDO (1998-11)

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