**3. Methodology**

A sustainable alternative was essential in the design due to the issues with nonbiodegradable plastic waste. In the last few decades, the recycling of PET as building material has been gaining ground, the properties of the plastic make it most suitable [5]. The goal of the study was to simplify existing technologies for value addition pyrolysis. Three different options were considered. Careful consideration and analysis were conducted in order to determine which of the three concepts provided the best solution to the Product Design Specification (PDS). The plant's primary selection criteria were the heat exchanger and the source of heat. Use of the binary dominance matrix was essential because it allowed for simultaneous analysis of all concepts. SolidWorks was used to model three design possibilities, as illustrated

*Conceptualization and Design of a Small Pyrolysis Plant for the Sustainable Production… DOI: http://dx.doi.org/10.5772/intechopen.100053*

in **Figure 3**. The heat exchanger in Concept (a), the Double Pipe, was laminar for fluid flow hence the rate of heat flow was lower than in the other concepts. Since Liquefied Petroleum Gas (LPG) is used as a heat source in Concept (b), the Shell and Tube, the target market without electricity may choose LPG. Furthermore, a Shell and Tube heat exchanger could be more expensive than a helical coil heat exchanger. Due to its helical coiled heat exchanger and coiled heater on the reactor, Concept (c), the Helical Pipe Heat Exchanger, dominated the weighted objectives. The selection was heavily influenced by the ease of assembly and fabrication. The different concepts are shown in **Figure 4**.

Due to its great strength, formability, and corrosion resistance, stainless steel was chosen over mild steel. **Table 3** shows the binary dominance matrix that was used to choose the most optimal concept based on functionality, efficiency, ease of maintenance, reliability, simplicity, cost, quality, and ergonomics. Concept (c) was chosen as a result.

Due to the helical heater on the pyrolysis tank there is even heating and the molten PET is then combined with the bituminous asphaltic concrete (BAC). This mixture is then put into casting bricks and also used in making concrete [8]. Recycling of PET and using it in construction will be a great stride since the PET is non-biodegradable and its chemical properties will further make the BAC durable.

**Figure 3.**

*Stages of radical degradation reaction.*

**Figure 4.** *Concepts (a) double pipe; (b) shell and tube; (c) helical pipe heat exchanger.*

### *Recent Perspectives in Pyrolysis Research*


### **Table 3.**

*Concept selection using the binary dominance matrix.*

### **3.1 Experimental design development**

The pulverized plastic wastes used in this work were collected from disposed plastic waste used for packaging water and fruit juices at Petreco Zimbabwe. The PET waste (**Figure 5**) was collected and cleaned with tap water to remove any form of contaminants and deleterious materials before sun drying for a minimum of three days.

The waste PET was added into the pyrolysis tank with a helical heating coil for even heat distribution. Oxygen was eliminated from the chamber by use of oxygen scavengers. Weight batching was used to be able to eliminate errors due to size and void in the material. Whilst molten the PET was tapped out of the pyrolysis tank into the brick cast mold. Mechanical tests were key to ascertain the properties of the brick. Hence compression test, water absorption test and flexural strength test were carried out. The different mechanical tests are described below.

### *3.1.1 Compressive strength test*

Compressive strength is a mechanical test that measures the amount of compressive load a material can sustain before failure. It is the most accurate indicator for measuring the engineering properties of a building material because a higher compressive strength indicates a good and strong material. Compressive strength in bricks and flexible pavements greatly depends on the amount of compaction, heat treatment and the concrete mix ratio.

### *3.1.2 Water absorption test*

Absorption of water is a critical factor for the durability of bricks and flexible pavements. When water penetrates brick, it decreases the strength of bricks. Hence the brick internal structure must be adequately dense to avoid the leaking of water. To increase density and decrease water absorption of clay bricks, the firing temperature must be raised [7]. Generally, the water absorption values decreased with

*Conceptualization and Design of a Small Pyrolysis Plant for the Sustainable Production… DOI: http://dx.doi.org/10.5772/intechopen.100053*

**Figure 5.** *Sample pulverized PET.*

increasing temperature, and decreased with increasing amounts of waste plastic in the mixtures. Hence PET plastic would reduce the water absorption of the pavements and bricks.
