Investigation of thermal and catalytic pyrolysis of polyolefins and rubbers

Master thesis in Polymerization Engineering

Abstract

In this research, thermal and catalytic pyrolysis of four types of polymers such as two heavy polyethylene plastics and polypropylene and two styrene butadiene rubber and polybutadiene rubber in a semi-continuous stirred reactor under nitrogen gas and using HZSM-5, FCC and H-Mordenite catalysts have been investigated. In this study, only the amount of liquid product obtained from pyrolysis and its composition has been considered, and the evaluation of components of gaseous product and coke has been omitted. The percentage of selected catalyst for pyrolysis of plastics is 15%, according to the optimal value obtained in previous works. During the pyrolysis of each polymer by performing several optimization steps on the reactor, the amount of liquid product increased. According to the investigations, among the three used catalysts, the FCC catalyst produces the most liquid product and the HZSM-5 catalyst releases most of the product in the form of gas due to the size of the small pores. By conducting a gas chromatography test, the percentage of liquid product components, aromatic, olefin, paraffin and naphthene in the product was obtained, and also the components of the liquid product were obtained based on the carbon number. The results of the gas chromatography test show that a significant percentage of the product in pyrolysis with FCC catalyst is olefins.

By examining the thermal and catalytic pyrolysis of polybutadiene rubber using the three mentioned catalysts, it was concluded that catalytic pyrolysis with 45% FCC catalyst gives the most liquid product. Also, according to the gas chromatography test data, with the increase in the amount of the catalyst due to the greater probability of Diels-Alder reactions, the aromatic component of the liquid product increases compared to the other components of naphthene, paraffin and olefin, and on the other hand, due to the higher breakdown, it tends to the lighter components in terms of carbon number. rubber and styrene butadiene rubber) due to the presence of many double bonds in their structure and the creation of many radicals during pyrolysis, it will be a multi-stage pyrolysis process, and unlike the pyrolysis of plastics, the hydrocarbon liquid product is removed separately and in several stages. The process of temperature changes in the pyrolysis of tires is also increasing and decreasing. It was concluded that the amount of 45% catalyst is the optimal amount of catalyst in the pyrolysis of styrene butadiene rubber. By performing a thermogravimetric test at five speeds of 5, 15, 30, 45 and 90 degrees Celsius per minute for polybutadiene rubber and examining the graphs of weight loss with temperature, it was found that the degradation of polybutadiene rubber is related to the percentage of reticulation of the rubber during the process. In fact, the difference between the mentioned charts is the result of the competition between these two mechanisms. The lower the heating rate, the chains have the opportunity to create transverse networks, and the network mechanism is dominant. As the temperature increases, the networks formed during the process are broken and the slope of weight loss increases. But at a speed of 90 degrees Celsius per minute, which is completely different from other speeds, the radicals move more easily due to their high speed and energy and enable the overall destruction of the structure.

Key words: thermal pyrolysis, catalytic pyrolysis, polyolefins, rubbers, amount of liquid product, catalyst percentage

1-1 Introduction

Using polymers, plastics, and rubbers in daily life leads to the production of millions of tons of waste, and how to deal with them is one of the important issues of urban life. There are different ways to deal with these wastes, the most important of which are: burying waste, burying waste along with taking energy from it, burning waste and recycling heat and electricity, recycling and producing compost (fertilizer), reusing and converting waste into other compounds such as petroleum liquids. The correct use of waste can become one of the main sources of energy needed by mankind in the not too distant future.

The global production of rubber and plastics is increasing day by day, and subsequently, the production of plastic and rubber waste is also on the rise. The life of this waste lasts for months (agricultural film) and years (cars, household appliances) and sometimes even a century (in some construction applications) [1].

1-2  waste statistics

According to figure 1-1, plastics and other polymer products comprise about 10-12% of urban waste in England, which due to the high volume of these materials and their non-degradation in nature for a long time, this matter becomes very important as to what dangers threaten the environment [2].

(images in the main file)

1-3 waste removal methods

In the following, a brief explanation of waste removal methods is provided:

1-3-1       Garbage disposal method

This method is the most basic and cheapest method available in It is to eliminate waste. The advantage of this method is the use of methane gas from the decomposition of organic compounds. Disadvantages of this method include the transfer of pollution or the possibility of groundwater contamination, the production of greenhouse gases, and the need for a suitable space for burial[3]. Among the advantages of this method, it can be mentioned that it produces much more energy and heat than the method of landfilling and significantly reduces the volume and mass of waste. On the other hand, this method is much more expensive than the landfill method, and there is a need to be very careful in controlling the released toxic gases. Finding a suitable place for burning is one of the other problems of this method [3].

1-3-3       Converting waste into compost

In the method of producing compost, perishable organic compounds are turned into fertilizer and make the soil fertile. One of the most important advantages of this method is that it prevents soil erosion. Another advantage of this method is that it prevents organic compounds from being deposited in landfills and prevents the release of methane gas in the environment. The very bad smell of compost that remains in the environment for a long time, the limitation of using compost in city parks and the possibility of polluting the underground water and the surrounding environment if the necessary standards are not met during the process are the disadvantages of using this method. Another point in this field is that controlling pollution during compost production will greatly increase production costs [3]. 1-3-4 Recycling Recycling and pyrolysis is one of the best ways to remove waste. Pyrolysis is the conversion of materials with high molecular mass or polymers into primary monomers, or materials with low molecular mass and with a physical state of liquid or gas, which this process is carried out in the absence of oxygen. HZSM-5, FCC and H-Mordenite catalysts. Composition and amount of liquid product was studied and according to previous works, 15% catalyst is used in plastics pyrolysis. The yield of liquid product got increased by some modifications that have been employed on the reactor. The highest yield of gas fraction obtained under the HZSM-5 because of its small pore size. Composition of liquid product is studied by the GC-mass analysis. According to results of GC-mass, valuable hydrocarbons of olefins have been obtained by FCC.

Catalytic pyrolysis of PBR with 45% catalyst produces the most yield of liquid product. And according to result of GC-mass, by the increasing of catalyst's yield, aromatic component of liquid product get increased due to diels alder reactions. The diagram of liquid product vs temperature in rubber pyrolysis shows a multi-step pyrolysis mechanism due to unsaturated bonds of PBR and SBR.