**2.3 Experiment with pre-industrial prototype for scale-up of MAP**

The purpose of this initial phase was the validation of the data reported on the laboratory scale, both in qualitative and quantitative terms, from the pyrolysis of

*Microwave-Assisted Pyrolysis Process: From a Laboratory Scale to an Industrial Plant DOI: http://dx.doi.org/10.5772/intechopen.104925*

**Figure 4.** *Pre-industrial prototype.*

plastic wastes. It also aimed to determine the operating parameters necessary to treat flows of various types of plastic materials at the end of their life cycle, and PS was chosen as the first example. Polystyrene, like all plastics, is unable to absorb microwave energy and convert it into heat. It must therefore be blended with a microwave absorber to realize the pyrolysis. The materials used as a microwave absorber are commonly carbon or iron powder.

The experiments were carried out using expanded polystyrene (EPS) as starting material and focused on analyzing the ratio between the amount of polystyrene processed and the microwave power used. Therefore, an energy density close to that foreseen in the prototype was used to acquire the information necessary for the design. The process was also studied to correlate the overall yield of the process and the composition of the products to the operating parameters. The data obtained are reported in **Table 3**.

In all experiments, polystyrene was mostly converted into a liquid using mainly iron powder as a microwave absorber. Working at low power, the energy is not enough to start the pyrolysis process (EXP 1–3). Even by preheating the pyrolysis chamber through electrical resistances, the liquid yield is low (EXP 4). Instead, it can be noted that under the same reaction conditions, but using carbon as an absorber, the liquid increases considerably (EXP 5) confirming carbon as a better MW absorber. To obtain good yields of the liquid, it was necessary to increase the power (EXP 6–12), working with a ratio microwave power/ PS of approx. 4.8–9.6 kW/kg. Analyses carried out on the liquid samples of the most significant tests showed aromatic hydrocarbons as the main compounds, among which singlering aromatic compounds such as styrene, toluene, ethylbenzene, and α-methylstyrene were present in very high amounts. Styrene was the compound present in the highest percentage (approx. 55–70%). The results confirmed the previous studies carried out at the laboratory level [8–10], albeit working with a considerably lower MW power than that one previuosly employed in laboratory experiments (4.8–9.6 vs. 30 kW/kg), and made it possible to develop the application of the MAP process of PS on an industrial scale.

#### **2.4 The industrial prototype plant**

The experiments with the pre-industrial prototype were run to collect process information for the realization of the industrial plant. Thanks to the collaboration between Cognito Engineering srl and the Department of Chemistry of the University of Florence, in 2019, Techwave built its first experimental industrial prototype following the scheme reported in **Figure 5**. The plant was installed in its factory in Massa (Italy) (**Figures 6–8**).


**Table 3.**

 *MAP of PS: operating parameters.* *Microwave-Assisted Pyrolysis Process: From a Laboratory Scale to an Industrial Plant DOI: http://dx.doi.org/10.5772/intechopen.104925*

**Figure 5.** *Flow diagram of MAP process.*

**Figure 6.** *Industrial prototype plant.*

#### **Figure 7.** *Plastic container located in the upper part of the prototype.*

#### *Microwave-Assisted Pyrolysis Process: From a Laboratory Scale to an Industrial Plant DOI: http://dx.doi.org/10.5772/intechopen.104925*

The prototype was realized considering its possible introduction in two standard containers for easy shipping, even if its dimension may be strongly scaled up if required. Taking into account the dimension, it may be installed on one small ship. Waste plastic materials are largely present in the sea [23], and they may be collected and immediately disposed of through this plant or another plant close to this. The products formed may be employed to produce the energy required, while the excess may be sold on the market. Furthermore, a plant of this dimension may be installed in a municipal collecting and selection center of waste plastics, where it may be employed to pyrolyze the mixed plastics collected, avoiding their sending to a disposal plant. The prototype may be useful also for a large hospital to dispose of the contaminated waste plastics. The pyrolysis products do not contain dangerous contaminants because the biological products are destroyed during the process as reported in the literature [24, 25] while the chemicals formed may be sold for their commercial uses. In 2020, the plant started testing operations using EPS, ABS, or PP as plastic materials. Tests were carried out for 1 year, improving the results step by step, both in terms of plant efficiency and the quality of the secondary raw materials produced. The description of the tests carried out and the results obtained are reported in the following paragraphs.

#### **2.5 Experiment with industrial prototype plant**

The description shows how the tests for the MAP process were run in the industrial prototype with the plant working in semi-batch mode. The amount of plastic and carbon black for a single test was taken from the storage area. The carbon black required for the absorption of the microwave was manually introduced into the pyrolysis reactor, while the plastic material was added through the plastic loading system.

Experiments were carried out in an inert atmosphere (nitrogen), realized through various vacuum/nitrogen cycles. The carrier gas was not used to avoid the dilution of the uncondensable gas with the carrier gas. At the end of the purge operations, the plastic material was loaded into the pyrolysis reactor by a screw conveyor and the electrical resistances were switched on to preheat the reactor. Then, they were switched off, and the microwave generators were switched on.

The plastic thermal degradation formed hydrocarbon vapors conveyed to a cooling system. The higher boiling fraction was condensed and collected in the bottom of the cooling system, while the low boiling fraction, together with uncondensable gas, was sent to a torch. During the pyrolysis, further amounts of plastic material were added to the reactor constantly. At the end of the process, the microwave generators were switched off. When the plant was returned to room temperature, vacuum/nitrogen cycles were repeated as described above. At the end of this operation, the solid fraction was collected from the bottom of the reactor. The process was completed, and a new pyrolysis cycle could be started.

In the experiment, the microwave launch system was studied and modified several times until it reached the optimal configuration. The experiments let to identify and refine some operational parameters of the MAP process for improving the yields and quality of pyrolysis products. **Table 4** reports the operating parameters of the most significant tests. Microwave power is critical in MAP as it must provide enough energy to break the polymer bonds and start the thermal degradation process. Comparing the data in **Table 4**, an increase in MW power allowed to treat a double amount of plastic with the same reaction time (PS1, PS3, and PS4).


*a PS: Polystyrene; ABS: Acrylonitrile/Butadiene/Styrene rubber; PP: Polypropylene;*

*b Absorber 1.8 kg (in all tests);*

*c 6 KW for 210 min then 12 KW for 15 min.*

#### **Table 4.**

*Operating parameters of the most significant tests.*

*From top to bottom: temperatures log, microwave power log, and level liquid log produced in MAP of ABS2.*

#### *Microwave-Assisted Pyrolysis Process: From a Laboratory Scale to an Industrial Plant DOI: http://dx.doi.org/10.5772/intechopen.104925*

Once the microwave launch system was fine-tuned, the design of experiments (DOE) was planned to optimize the plant productivity, using various types of plastic materials (PS, ABS or PP, **Table 4**). By way of example, the logs of temperatures, microwave power, and liquid level produced during the MAP of experiment ABS2 are reported in **Figure 9** to describe the evolution of the pyrolysis process. Although the temperature plays an important role, this parameter is detected with high uncertainty in microwave pyrolysis [26]. For this reason, the process was followed by monitoring the temperatures recorded by the probes located in several areas of the plant.

The probes from TT101 to TT105 were arranged on the reactor from top to bottom: on the bottom of the reactor was the TT110 probe. The temperature of the vapors generated during pyrolysis was recorded by the TT106 on the condensation pipeline, located after the exit from the reactor. The TT120 probe monitors the electrical resistances of the preheating reactor. All the probe temperatures were referenced to the TT001 corresponding to the ambient temperature.

When the reactor was heated at the prefixed temperature by the electrical resistances, these resistances were switched off while the microwave generators were turned on. Following the curves of the temperature probes on the reactor, it was possible to see how the temperature rose suddenly. During this phase, the carbon black absorbed the microwaves and transferred the heat to the plastic material, which starts to melt. As the process went on, the temperature curves of the reactor reached a value corresponding to the start of the thermal degradation of plastics and consequent generation of hydrocarbon vapors, as could be seen from the TT106 curve. At the same time, the LT101 level curve increased due to the condensation of the vapors and liquid was collected.

When the plant was in full operation, the TT101–106 curves remained constant, and the level LT101 of the liquid in the container continuously grew until the microwave generators were switched off, so the system was cooled, and all temperatures started to decrease when the industrial plant works in the best operating conditions, let to treat 33 kg/h of waste plastic.
