One of the goals is to enable entrepreneurs, project sponsors, investors and the public in the industry to better understand the rapidly developing circular economy. At the same time, this series of articles should also be a stimulus for discussion.
In the current article, we have solved the problem of how to combine the advantages of classic batch reactor process design with the advantages of continuous process mode. We refer to a large amount of scientific literature and the findings of successful companies in the industry.
Approximately 3 billion tires are produced worldwide every year [YADAV, 2020]. These tires reach their service life after a relatively short period of use, so the amount of waste tires produced every year far exceeds 20 million tons. The complexity of tire material composition (metals, textiles, vulcanized rubber, etc.) is mainly a challenge facing the recycling industry, unless tires are burned unsustainably under the heading “energy recovery” in industrial ovens.
In the sustainable and environmentally friendly treatment of waste tires, thermochemical conversion technologies have become more and more important in recent years, some of which have proven their technological and economic maturity.
Pyrolysis is a thermochemical way of processing vulcanized rubber to recover valuable products. It involves the decomposition of rubber in the absence of oxygen at high temperatures (400-900°C). The main products of pyrolysis are the solid part, usually raw carbon black (according to ASTM D8178); the liquid fraction composed of light oil, heavy oil and tar; and the gas part. [Ramirez-Canon and others. Et al. 2018]
The thermochemical decomposition process (pyrolysis) takes place in the reactor (reaction zone). There is a difference between batch and continuous filling and processing modes, which in turn significantly determines the design of the reactor.
First of all, it should be mentioned that there is not only an optimal reactor design and/or process mode, but also different options can be considered according to the situation. Nevertheless, there are still some aspects that can make the batch pyrolysis process a major intermediate link, especially for the thermochemical treatment of end-of-life tires (ELT).
The typical feature of the batch process is a completed loading process, followed by a completed processing and subsequent unloading process. Plot this sequence on the time axis, with different states at different times (because the material concentration in the reactor changes with time). In contrast, in continuous process design, materials and processes are “constantly changing.”
This (definition) also leads to the “disadvantages” often mentioned in batch processes: for example, due to heating and cooling phases, downtime (for loading and unloading) and low energy efficiency. The latter may require many hours in industrial applications.
However, from the perspective of modern industrial processes as a whole, these “shortcomings” can be overcome by (at least) two batch reactors in parallel, which are alternately filled and discharged, while maintaining the reaction temperature in the reactor (without cooling and Heating stage). Of course, flushing the system with non-oxidizing gas during or after loading and unloading also presents challenges, but these can be solved by complex but simple techniques.
Batch pyrolysis systems are usually equivalent to the cheapest, environmental and healthy “Asian systems”, usually with open air and unfiltered heat sources. In any case, these are banned in most countries in the world and are not the subject of our discussion.
In fact, there have been several successfully commercialized ELT pyrolysis concepts based on batch reactors, which realize an almost continuous overall process through the intelligent coordination of each completed process. Therefore, these concepts can also be called semi-continuous (as a whole, the emphasis is more on “continuous”).
As an example and reference: The existing and commercially successful design consists of two batch reactors, which are automatically filled and discharged alternately, and are always kept at the process temperature. The agitator vigorously mixes the feed, thereby increasing the heat transfer rate and minimizing the temperature difference within the reactor. These aspects can save energy, time and reduce pressure on the reactor wall at the same time. Otherwise, in this way, only two reactors are needed and 20 to 22 complete cycles can be completed in 24 hours. At more than 24 tons per day (10,000 tons of tires per year), the system seems to be no less inferior to more complex but continuously operating rotary kiln or screw screw systems.
Give a batch concept that is not recommended as a reference (the company recently filed for bankruptcy in the United States and Germany): The project consists of 20 batch reactors (!), each of which has a height of about 5 m and has An inner diameter is about 2 m (equivalent to the volume of each reactor is about 16 m3). The annual production capacity should be 40,000 tons of waste tires. This means that the capacity of each of these huge reactors is only 5 to 6 tons per day. One of the reasons for the low productivity is the need to manually load (!) the raw material basket and the extremely long heating, reaction and cooling time (up to 14 hours in total). If, due to the existence of the current media, it does not indirectly negatively affect the modern and successful semi-continuous concept, we will not pay further attention to this expensive and failed concept.
In oil refineries and the chemical industry, batch-based processes have been used for a long time. There are good reasons to pay more attention to the thermochemical treatment of scrap tires.
Obviously, the pyrolysis process is very sensitive to application conditions such as raw material composition and temperature (T), heating rate (HR), mass flow (FR) or load. The thermal degradation behavior and kinetic parameters vary with the waste tire resources, depending on the manufacturer and its proprietary chemical composition. [QUEK et al. 2012]
Tires are composed of approximately 60% m/m of decomposable materials. These materials are synthetic rubber (CBR, SBR and BR), natural rubber (NR), lubricants, antioxidants and plasticizers. The remaining 40% m/m are non-decomposable compounds, such as carbon black (CB), zinc oxide (ZnO), calcium carbonate (CaCO3), silicon dioxide (SiO2), steel and other small amounts of additives, all of which are recovered in solid form .
Degradation (pyrolysis) mainly occurs on rubber compounds. These processes interfere with each other and overlap. Otherwise, the pyrolysis process of all rubber in the tire is carried out in parallel at the same time. [Gonzales et al., 2001]
The direct product of this degradation is a gas, which in most cases will condense into oil with a wide range of fuel characteristics [WILLIAMS et.al., 2010] or used as a raw material for the chemical industry.
This clearly shows how important it is to accurately and quantitatively determine the rubber in a tire. However, since the exact chemical composition of the delivery of waste tires cannot be easily determined in practice, it is necessary to determine the time point of completion of the thermal decomposition as accurately as possible during the ongoing pyrolysis process.
This point in time has been reached when all volatile components have been extracted from the solid part. Simplification: When the raw carbon black is “dried”. If this is not the case, the pyrolysis process is incomplete and requires a longer reaction time or higher temperature. [Lopez et al. Et al. 2010]
A relatively simple way to determine the correct point in time is to measure the gas flow: if no more gas escapes from the material, the pyrolysis process can be considered complete. However, this simple criterion cannot be applied to a continuously charged reactor model because the material flow in the reactor is uninterrupted and gas is always generated at a constant reaction temperature (without large amplitude).
On the other hand, in the case of automatic batch filling of the reactor, this indicator can be relatively easily used to determine the end of the pyrolysis process, because this is a closed process with a certain amount of material (batch).
Of course, the correct process parameters (such as residence time) can also be checked by analyzing the feedstock carbon black of the continuous feeding reactor-but only periodically and afterwards.
This aspect shows that modern batch-type semi-continuous reactors can be very flexible in terms of precise process control, which is necessary due to the given inhomogeneity of the material composition of scrap tires. In addition, the correct process design with multiple reactors will result in a semi-continuous overall process sequence, which is by no means more technically complex (less flexible in terms of raw material quality) process variants that have the disadvantage of continuous feed reactors.
For decades, continuously operating reactors (such as rotary reactors and Auger reactors) have covered a wide range of industrial applications. They have been tried and tested and are the most advanced. This also applies to batch-based reactor concepts.
However, there are construction-related differences that affect service life, maintenance cycles (and costs), and operational safety.
In simple terms, all common reactor designs (unless they are electrically heated) consist of two cylindrical tubes. The pyrolysis process takes place in the inner tube. The flue gas flows in the space between the inner tube and the outer tube and heats the reaction zone in the inner tube to the process temperature.
Since these two tubes are fixed in the spiral reactor and the batch reactor, the sealing effect (to prevent the outflow of flue gas) is relatively low compared with the rotary kiln design. In the latter case, the inner tube weighing several tons will rotate, thus creating a significantly higher and challenging sealing task between the moving and non-moving parts.
Pyrolysis occurs in an oxygen-free atmosphere (otherwise it is combustion). This means that the inner tube (the actual reactor) must be reliably sealed to prevent oxygen from entering from the outside. In order to prevent oxygen from entering in the event of a seal leak, the inside of the reactor can be operated under positive pressure. This has the disadvantage that toxic pyrolysis gas escapes into the environment in the event of an accident. Alternatively, the reactor can also be operated under a slight negative pressure, which can prevent the pyrolysis gas from escaping, but will cause the oxygen in the atmosphere to be inhaled and cause an explosive atmosphere in the reactor. Here, the structural advantage of a reactor design with as few moving parts as possible becomes obvious, which in turn clearly illustrates the advantages of fixed screw or batch reactors (in both cases, there is only one rotating shaft Must be sealed: for screw or agitator).
It can therefore be assumed that, from the standpoint of easier sealing, the operation of spiral or batch reactors can in principle be operated more safely than rotating reactors.
Although modern steel alloys can withstand extreme conditions, in the medium term, fluctuating temperatures are disadvantageous and weaken the material. Due to the continuous feeding of the reactor (rotary kiln and auger), except for maintenance and/or emergency shutdown, the process temperature is always maintained, which will be an advantage in terms of service life, as there are no (multiple) daily heating and cooling phases .
Commercially successful semi-continuous systems based on batch reactors clearly demonstrate that batch reactors can also be operated without these material stress downtimes while permanently maintaining the process temperature.
In short, it can be said that a batch-based semi-continuous tire pyrolysis system can take advantage of the “best of both worlds” (or even more) if
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