Plastics, Rubber, and Paper Recycling - American Chemical Society

Chapter 14

A Review of Advanced Recycling Technology

Downloaded by UCSF LIB CKM RSCS MGMT on November 20, 2014 | http://pubs.acs.org Publication Date: May 5, 1995 | doi: 10.1021/bk-1995-0609.ch014

George Mackey Granville Research Laboratories, Dow Chemical Company, P.O. Box 515, Granville, OH 43023-0515

This paper is intended as a review of the various processes put forth to convert mixed and unwashed waste plastic to a product with some economic value. Conventional mechanical or melt recycling of plastics works well when the processes can acquire large quantities of reasonably clean, single polymer articles such as PET soda bottles or natural HDPE milk bottles. The remaining rigid or flexible plastics in the waste streams are often heavily contaminated, multilayered, heavily pigmented, and difficult to sort into single polymer streams. This portion of the waste plastic stream, nicknamed the third bale, is best recycled using a thermal process to convert the mix to a liquid product which is suitable as a refinery feedstock. The three major process subgroups used are 1) pyrolysis, 2) gasification, and 3) hydrogenation. Pyrolysis converts the material to a liquid in the absence of oxygen while gasification converts the plastic to a mix of carbon monoxide and hydrogen in a limited oxygen atmosphere. Hydrogenation is a variant in the gasification process whereby hydrogen is added during the polymer cracking phase. An overview of the advantages and disadvantages of each process is described in this paper.

The recovery of monomers or oil from waste plastic by a depolymerization process is called tertiary recycling. Reprocessing scrap as part of a product production is defined as primary recycling, while melt recycling is considered secondary recycling, and burning with energy recovery is considered quaternary recycling. 1

0097-6156/95/0609-0161$12.00/0 © 1995 American Chemical Society In Plastics, Rubber, and Paper Recycling; Rader, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.


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Chemical vs Thermal There are two types of tertiary recycling, chemical and thermal as shown in Figure 1. Depolymerization of the plastic by chemical means is called solvolysis, and the process produces a monomer or oligomers. Condensation polymers such as polyethylene terephthalate (PET), polyurethane (PUR), polycarbonate (PC), and polyamide (nylon) contain specific functional groups which allow reversal of the original polymerization reaction. These depolymerization reactions are known as methanolysis, glycolysis, hydrolysis, aminolysis, acidolysis, etc. and each process requires a single pure polymer stream for good results. These processes are widely used at the manufacturing point of these polymers in order to recycle plant scrap, off-grade material or post-consumer waste. The decomposition of polymers by heat is called thermolysis. If the process is done in the absence of air, it is called pyrolysis or if done with a controlled amount of oxygen, it is called gasification. Pyrolysis will produce a liquid fraction which is a synthetic crude oil and should be suitable as a refinery feedstock. The non-condensable fraction created during pyrolysis is normally used to provide process heat and any excess is flared. Gasification of plastic takes place at a higher temperature than pyrolysis and with controlled oxygen addition. The result is a syngas that is composed primarily of carbon monoxide and hydrogen. As a mixture, the syngas is valued only as a fuel. But if the gases are separated, the carbon monoxide and the hydrogen are valued as chemical intermediates, which can have 2 to 3 times the fuel value of the mixture. A third form of thermolysis is hydrogenation, which is a modification of the oldest catalytic process for refining petroleum. Here the plastic is both depolymerized by heat and exposed to an excess of hydrogen at a pressure of over 100 atmospheres. The cracking and hydrogenation are complementary, with the cracking reaction being endothermic and the hydrogenation reaction being exothermic. The surplus of heat normally encountered is handled by using cold hydrogen as a quench for this reaction. Hydrotreating can remove many heteroatoms. The resultant product is usually a liquid fuel like gasoline or diesel fuel. Thermolysis is a much more versatile and forgiving technology for tertiary recycling than solvolysis. It can handle mixed polymer waste streams along with some level of non-plastic contaminants. Solvolysis will require a relatively pure polymer stream and has little tolerance for contaminants; therefore the raw material preparation costs are larger. Thermolytic processes can be used for mixed polymers streams from municipal solid waste, auto shredder residue, medical waste, and mixtures of rubber and plastic. Some pre-treatment for sizing or certain contaminant removal may be needed, but it will be much less than that required for solvolysis. 2

Markets Each year the United States produces about 60 billion pounds of plastic of which about 75% are addition polymers and 25% are condensation polymers. The distribution of addition polymers produced in the US and appear in the waste stream are as shown on Table I. The breakdown of condensation polymers is

In Plastics, Rubber, and Paper Recycling; Rader, C., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1995.

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shown on Table II. Only about 9% of condensation polymers (5% polyurethane and 4% polyethylene terephthalate) are suitable for existing solvolysis processes. Therefore, if one wanted to make the largest impact on the solid waste stream with some form of tertiary recycling, it would be with thermolysis. This decision is reinforced by examining the available raw material streams. Raw Material Streams for Plastic Recycling Municipal Solid Waste 20.0 (70% Olefin) Carpet/Textile 2.5 (75% mixed) Automotive 2.0 (mixed) Wire and Cable 0.5 (60% Olefin)


3

Based on a DOE sponsored study at the Polymer Processing Institute at Stevens Institute of Technology, Hoboken, NJ. These statistics would support a decision to focus on thermolysis as the process most likely to address the largest part of the problem. Another reason for focusing on thermolysis becomes apparent when we look at the proposed legislative mandates for recycled plastics. Material recycling seems to have viable markets for certain recycled plastics at recycling rates of 10% to 15%. If the industry is forced to go to higher rates we must deal with a more contaminated material. These contaminated mixed polymer streams become very costly to separate, and there are few markets for the resin produced. Many initiatives propose national recycling rates for plastic packaging of 25% to 40%. The tolerance of thermolysis to polymer mixtures and contaminants makes it a good candidate process to meet the higher percentage rates. However, there is still some argument that converting plastic back to crude oil should not be credited toward meeting the recycling rates. If the crude oil produced is used as a fuel, the argument is that tertiary recycling becomes a form of incineration and should be classed as energy recovery. 4

Processes If we survey the processes most commonly discussed for thermolysis of waste plastic, they would include those shown on Figure 2. For the most part, these processes have been well developed by utilizing other raw materials, and only recently have they been addressing waste plastic. The following is an attempt to make some generalizations about the applicability of these processes to the thermolysis of waste plastic. Kilns & Retorts As a group, they all primarily produce a liquid product whose yield is dependent on the time/temperature process conditions shown on Table III. The noncondensable fraction produced is normally used for process heating and any excess is wasted. Because the product produced is normally intended for a refinery customer, any heteroatoms present can be a problem. Halogens are


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TABLE I ADDITION POLYMERS FOR PYROLYTIC P R O C E S S RESIN LDPE HDPE PVC PP PS ABS Acrylics

% OF U.S. SALES 20.0 15.1 15.0 13.4 10.0 1.8 tJL 76.4

T A B L E II CONDENSATION POLYMERS FOR SOLVOLYSIS P R O C E S S RESIN PUR PET Phenolics* U/F M/F* Epoxies*

% U.S. SALES 5.0 4.0 4.0 2.5

Plastics, Rubber, and Paper Recycling - American Chemical Society

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