PYROLYSIS OF PLASTICS

 PYROLYSIS

Pyrolysis is the thermal degradation of waste in an oxygen-starved environment in which the oxygen content is low for gasification to take place. Pyrolysis

liquefaction is a non-combustion heat treatment that catalytically (chemically) decomposes waste material by applying heat, directly or indirectly to the waste material in an oxygen free environment. It is an endothermic reaction which

requires an input of energy that is typically applied indirectly through the walls of the reactor in which the waste material is fed into. Pyrolysis liquefaction occurs under pressure and at operating temperatures above 430oC.

At the present time, pyrolysis is used as an effective recycling method. It has been employed to convert waste plastic into useful products such as fine chemicals, transportation fuels, and lubricant oils. Pyrolysis is also classified as the chemical and energy recovery system known as cracking, gasification, and chemolysis methods. There are various forms of the thermolysis methods including thermal cracking (pyrolysis), catalytic cracking, and hydrocracking. The pyrolysis process uses elevated temperatures to crack down high molar mass materials into smaller molecules. The plastics in this process decompose into three phases of matter: gas (condensable and non-condensable mixture), liquid, and solid. In this manner, chemical recycling of the stored energies within plastic wastes take place with the environmental advantage of minimizing plastic pollution.

TYPES OF PYROLYSIS

1.      Thermal depolymerization is a process using hydrous pyrolysis for the reduction of complex organic materials (usually waste product of various sorts, often known as biomass and plastic) into light crude oil. It mimic the natural geological processes thought to be involved in the production of fossil fuels. Under pressure and heat, long chain polymers of hydrogen, 0xygen, and carbon decompose into short-chain petroleum hydrocarbons with a maximum length of around 18 carbons . TDP can mean conversion of biomass to oil using superheated water, although it  more usually is applied to fuel production via pyrolysis.

2.      Hydrothermal liquifaction: Direct hydrothermal liquefaction involve converting biomass to an oily liquid by contacting the biomass with water at elevated temperatures(300-350*c) with sufficient pressure to maintain the water primarily in the liquid phase (12-20mpa) for resistance time up to 30 minutes.

     Hydro pyrolysis; refer to the thermal decomposition  which take place when organic compound are heated to light temperature in the process of water.

3.      Anhydrous pyrolysis; sample heating without water, anhydrous pyrolysis has long been considered to take place naturally in the earth `s crust. It is the process in which the organic material contain in rock is broking down to release the oil and fossil fuels. Some pyrolysis method which create hydrocarbons through depolymerisation use dry materials (or anhydrous pyrolysis) which requires expending a lot of energy to remove water.

 Pyrolysis of Plastics

Pyrolysis is a thermal cracking reaction of the large molecular weight polymer carbon chains under an oxygen free environment and produces small molecular weight molecules. Traditional treatments for post-consumed plastics were landfills or incineration. However, landfill of the post-consumed plastics has potential problems because of limited land resource and high durability of plastics. Incomplete incineration may generate poisonous substances and causes serious health problems. Other methods like gasification and bioconversion are mainly used for organic materials (Demirbas 2004).

HDPE, LDPE, PP and PS are all hydrocarbons consisting entirely of carbon and hydrogen, which are similar to hydrocarbon fuels such as liquefied petroleum gas

(LPG), petrol and diesel (Baines 1993, Kittle 1993). Plastics are derived from petroleum and have calorific values in a similar range as those of LPG, petrol and diesel.

The system consists of continuous plastics in feed system, pyrolysis gasification chamber, catalytic converter, condensers, gas scrubber, centrifuge, oil recovery line, off gas cleaning. Waste plastics are loaded via a hot-melt infeed system directly into main pyrolysis chamber.

Plastic waste is continuously treated in a cylindrical chamber and the pyrolytic gases condensed in a specially designed condenser system to yield a hydrocarbon distillate comprising straight and branched chain aliphatics, cyclic aliphatics and aromatic hydrocarbons. The resulting mixture is essentially equivalent to petroleum distillate. The plastic is pyrolised at 370ºC- 420ºC and the pyrolysis gases are condensed through a distillation tower to produce the distillate.

 Some commercial plastic pyrolysis plants have been in operation in which all types of post-consumed plastics accepted need to be treated using hydrochloride scrubber which is for PVC cracking and is not preferable in the fuel product because chloride is not desirable in the fuels (Scheirs 2006, Zadgaonkar 2006). Those plants are sophisticated and not suitable for relatively small scale production (Zadgaonkar 2006). In these plants, catalysts are also used to improve the quality of pyrolysis products in many existing equipments. Those equipments with catalysts have some weakness in terms of long material resistance time, undesired contact between plastics and catalysts, required high heat transfer rate, and cost of the catalysts (Masuda and Tago 2006).

In order to understand and optimise the pyrolysis of waste plastic and to investigate the impacts of different types of the plastics, extensive research has been conducted in the past decade. Kaminsky, Scheirs and their colleagues investigated the effects of reaction conditions on the pyrolysis product. Williams studied the products from pyrolysis of different individual and mixed plastics. Aguado et.al.,2000, investigated the effect of catalysts on the pyrolysis reactions.  In these studies, the lab-scale pyrolysis reactors were either batch type or semi-batch type rather than continuous type. Most studies focused on the effects of operation temperature, heating rate, and catalysts on the product yield. In addition, the final products are also very complicated. There could be over a hundred of components in the hydrocarbon products including paraffin, olefin and their isomers. Normally, the PONA system, which is an abbreviation for paraffin, olefin, naphthene and aromatic compounds, is used to describe those petroleum hydrocarbons (Scheirs 2006). Paraffins are saturated hydrocarbons with straight or branched carbon chain, which are also called “alkane”. Olefins have similar chain as paraffins, but they have one or more multiple bonds between carbon atoms in their chains. Naphthenes are saturated hydrocarbons like paraffins but their chains merge to a ring in their structure. Aromatics contain a benzene ring in the structure.Another common way to describe the hydrocarbons is based on the carbon numbers in their molecule structure. It is particularly applied to petroleum fuels. The complex pyrolysis products may also be grouped as petroleum gases, petrol, kerosene, diesel and wax. The above fuels contain hydrocarbon group with different carbon chain lengths is given below (Lee, 2006).There are also other ways to describe the hydrocarbons such as boiling range, phase of products at room temperature etc.

Table1: Hydrocarbon range in commercial fuels

Fuels	LPG	Petrol	Kerosine	Diesel	Heavy fuel oil
Hydrocarbons	C3 to C4	C4 to C12	C12 to C15	C12  to C24	C12 to C70

  Some examples of household  plastics waste for the ThermoFuel Process are:

1. plastic packaging scrap from material recovery/sorting

facilities

2. oil and detergent bottles

3. used sachet water bags,

4. mixed post-consumer plastics (ice cream cups and takeaway plates from eatery)

5. caps/labels/rejected bottles from bottle recycling operation.

The essential step in the pyrolysis of plastics involves:

1. Purging oxygen from pyrolysis chamber.

2. Evenly heating the plastic to a narrow temperature range without excessive temperature variations.

3. Pyrolising the plastics.

4. Catalytic conversion of the gases to specific carbon chain lengths.

5. Managing the carbonaceous char by-product before it acts as a thermal insulator and lowers the heat transfer to the plastic.

6. Careful condensation and fractionation of the pyrolysis vapours to produce fuels of excellent quality and consistency.

7. Removal of sulphurs and residual contaminants.