| Summary: | 博士 === 國立臺灣大學 === 化學工程學系研究所 === 85 === The process development of depolymerizing polyethylene terephthalate(PET)
waste was studied in this dissertation. The reactions of hydrolytic
depolymerization, alkaline hydrolysis, and glycolytic depolymerization were
investigated using a batch pressurized reactor and thermal analysis techniques.
The characterization of depolymerization reactions, the
examination of catalytic
activities, and the discussion of kinetic mechanisms were presented. Besides,
the process design and economic evaluation of PET depolymerization were
performed, and the profits of capital investment on the reclaiming plant was
estimated.
The hydrolytic depolymerization of PET catalyzed by metal acetates was
carried out in a stirred batch reactor at 220-265 oC under autogenous pressure.
The solid products mainly composed of terephthalic(TPA) were obtained in the
experiments with high conversions of PET hydrolysis. The liquid products were
mostly ethylene glycol(EG) and its dimer. A second-order model including
forward and backward reactions for the PET hydrolysis was found to well fit the
experimental data. The kinetic model also suggests an autocatalytic mechanism
which indicates that some of the hydrolytic depolymerization of PET was
catalyzed by the carboxyl groups produced during the reaction. The yield of
ethylene glycol largely increased as the conversion of PET
hydrolysis approached
the complete level. It suggest a random-scission pattern for the hydrolysis of
ester linkage. Tin(II) acetate, among the catalysts studied,
showed the highest
activity for the PET hydrolysis. The catalyst effectively
enhanced the reaction
rate and lowered the activation energy of PET hydrolysis from the value of 123
kJ/mol to 90-100 kJ/mol.
The results of thermal analysis showed that potassium
hydroxide in its solid
state possessed hydrolytic activity of depolymerizing PET into small molecules;
in contrast, sodium hydroxide did not. However, the results of
reactions in the
agitated solutions showed that potassium hydroxide and sodium hydroxide had the
comparatively similar activities for the alkaline hydrolysis of PET. The
products of terephthalic acid and ethylene glycol were obtained in all
experiments and no oligomeric products were found. It suggests
a model of chain
end scission for the alkaline hydrolysis of PET linkage. The result of kinetic
analysis showed that the hydrolysis rate was the first order in alkaline
hydroxide and the half order in PET. This indicates that the PET linkages may
sequentially react with the hydroxides on the surface of solid flakes. The
Arrhenius plot gave the activation energy of 84 kJ/mol for the alkaline
hydrolysis of PET, which are in agreement with the result obtained by the
kinetic study of thermal analysis.
The glycolysis of PET resin with excess amount of ethylene glycol, using
metal acetates as catalysts, was examined by differential scanning calorimetry
(DSC) and in an agitated batch reactor. The DSC experiments were carried out
under a nitrogen atmosphere of 600 psi. The efficiency of glycolysis was
measured from peak temperature associated reaction endotherm. Zinc acetate
among the catalysts studied was confirmed to be the most effective one for the
PET glycolysis. Two isoconversion methods of kinetic analysis were applied for
estimating the activation energy of PET glycolysis. The apparent activation
energy was lowered either by adding zinc acetate or by decreasing resin size.
The experimental results obtained in batch reactor with efficient agitation
showed that adding zinc acetate as catalyst can effectively enhance the
glycolysis rate. The complete depolymerization and monomeric products of bis-
hydroxyethyl terephthalate(BHET) were then obtained in the presence of the
catalyst.
Based on the economic scale of reclaiming 8640 tons of PET per year, the
plant design and cost estimation were made for three kinds of processes of PET
hydrolysis. High reclaiming cost was required for the process catalyzed by
sulfuric acid or ammonium hydroxide; therefore, the process of PET hydrolysis
at high temperatures showed the most interests. The capital
investiment of this
process is about $2.3 million, and the annual cost of operating
is $1.7 million.
The economic evaluation of this process showed that the pretax return of this
reclaiming plant was 28% when the average products price of TPA
and EG were $400
/ton and the cost of PET feed was $200/ton.
The main achievement and contributions of this work including the following
aspects: newly efficient catalysts in the application of PET depolymerization
were developed and the invention was asked for a patent; the kinetic data and
mathematic models of PET hydrolysis were first obtained and the results were
applied in the process design of reclaiming plants; the techniques of thermal
analysis were first applied in the measuring the catalytic activities of PET
depolymerization; the economic evaluation of PET depolymerization was first
performed, and the commercial profits and the critical points of this process
were judged.
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