Background Currently, there is considerable concern about materials “running out,” and a renewed intent in conserving natural resources and increasing recycling. Also as landfill space runs out, recycling becomes increasingly attractive. This project concerns the recycling of polyethylene terephthalate (PET) bottles, typically used in the soda andbottle water industries. PET is the main constituent in a variety of consumer and industrial products including plastic fibers, videotape, audiotape, film, engineered resin, food containers, and beverage bottles. In 1997, approximately 2.5 billion pounds of PET were available for recycling (1). Only 22.7%, or 580 million pounds, of the available PET is reclaimed yearly, thus allowing for potentialmarket growth. This process is based on a patent for recycling of film consisting largely of PET (2). The goal was to determine if this process could be applied to PET bottles and be profitable. Environmental Significance Environmentally-friendly process that recycles a used product Process Description The BFD (Figure 1) and three PFDs (Figures 2-4) show a process to recycle PET. Unit 100 --Production of TPA and Ethylene Glycol The process flow diagram (PFD) for Unit 100 is presented in Figure 2. The purpose of Unit 100 is to reduce the size of the PET feed to that suitable for the following reaction to occur while recovering the products.
(C10H8O4)x + 2x KOH → x K2C8H4O4 + xC2H6O2
PET Di-potassium terephthalate Ethylene Glycol
Baled PET is crushed to loose scrap in balebreaker SR-101 at a rate of one to two bales per hour. The loose PET is shredded in SR-102, the PET shredder, to a size of approximately 76mm before passing through paramagnetic metal remover M-101, which eliminates any ferrous or non-ferrous metals from the feed. This protects the remaining size reduction equipment from excessive wear. The PET particles are then reduced to 3mm pieces in granulatorSR-103, and then to 0.841 mm (20 mesh) in pulverizer SR-104 where they enter the process as Stream 1. The feed to the remainder of Unit 100 is regulated by feed hopper HP-101 A/B and loaded to screw conveyor SF-101 to feed the rotary calciner reactor, R-101 (Stream 2). Additionally, potassium hydroxide is recycled from Unit 300 and combined with Stream 35 to feed to reactor, R-101. Ethylene glycoland water are vaporized in R-101 and forced out of R-101 as Stream 8, using a sweep gas consisting of air. The ethylene glycol and water are condensed in E-101 and separated from the sweep gas in air separator V-102. The vapor in Stream 9 consisting of air, carbon dioxide, and small amounts of ethylene glycol is vented to flare. The ethylene glycol bottoms is purified from the water in ethyleneglycol column T-101 and recovered in Stream 11. Unit 200 -- Potassium Hydroxide Recovery using Caustic Lime Process. The PFD for Unit 200 is shown in Figure 3 (3). In Unit 200, the di-potassium terephthalate solution from Unit 100 (Stream 7) is fed to the di-potassium terephthalate reactor, R-201, along with the di-potassium terephthalate produced by R-202. Carbon dioxide, produced in R-203 and R-205,is bubbled through R-201 where the following
reaction to convert di-potassium terephthalate into mono-potassium terephthalate and potassium bicarbonate,occurs, C8O4 K 2 H 4 + CO2 + H 2O C8O4 KH 5 + KHCO3 →
Di-Potassium Terephthalate Mono-Potassium Terephthalate Potassium Carbonate
The mono-potassium terephthalate precipitates, is removed by F-201, and is recovered in Stream 15.The cake is then sent to R-202, the mono-potassium terephthalate reactor. In R-202, mono-potassium terephthalate is suspended in an 80-wt% water solution where it is hydrolyzed by the following reaction. 2 C8O4 KH 5 C8O4 K 2 H 4 + C8O4 H 6 →
Mono-potassium Tterephthalate Potassium Terephthalate Terephthalic Acid
Because of TPA’s low solubility, it precipitates and is removed by filter...