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Presentation of Reactor Types for Thermo-Catalytic Thermal Cracking

Conference Paper · January 2017

DOI: 10.26649/musci.2017.066

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PRESENTATION OF REACTOR TYPES FOR THERMO- CATALYTIC THERMAL CRACKING

Andor Zsemberi1, Zoltán Károly Siménfalvi2, Árpád Bence Palotás3

1PhD student, 2associate professor, 3professor 1,2Institute of Energy Engineering and Chemical Machinery, University of Miskolc 3Institute of Energy and Quality Affairs, Department of Combustion Technology,

University of Miskolc ABSTRACT Thermo-catalytic thermal cracking processes refer to processes of the chemical industry whose input raw material are solid, liquid or gas state hydrocarbons. The purpose of technology is to produce a liquid and gas state hydrocarbon fraction with higher value. As a starting point in our research, we examined thermal cracking of solid and/or rubber waste by means of a fixed and fluid-bed (semi-batch) complex reactor system using a catalyst at 450 °C. Common features of these operations of the chemical industry are the inertised atmosphere, temperatures between 400 and 450 °C and a pressure range close to the atmosphere. However, the selected reactor design may imply a considerable difference as it largely determines the distribution and quality parameters of the valuable products

• formed. Our publication discusses the optimisation possibilities of reactor

constructions of diverse types based on the values measured with the equipment constructed by us and experiences.

• 1. PRESENTATION OF THERMAL CRACKING PROCESSES

Recycling various plastic and rubber waste sorts thermo-catalytically has been drawing more and more attention in the last years as valuable raw materials for the chemical industry and energy carriers can be generated this way. Moreover, it is worth noting that considerable environmental and waste-treatment issues can become resolvable, too. One of the most important operational units of the process is the reactor, in which the degradation, more precisely the chemical conversion of the solid raw material occurs with the impact of a catalyst and inert fluid mixer. However, it is a relevant fact that solid raw material-processing thermal cracking technologies face a number of important challenges. The design and operation of catalytic cracking reactors is difficult due to the poor thermal conductivity factor and extremely high viscosity of molten polymer. Types used in a broad range applications are the following:  batch/semi-batch reactors;  fixed-bed reactors;  fluid-bed reactors;

MultiScience - XXXI. microCAD International Multidisciplinary Scientific Conference University of Miskolc, Hungary, 20-21 April 2017 ISBN 978-963-358-132-2

DOI: 10.26649/musci.2017.066

 streamed-bed reactors;  screw press (extruder) reactors. Our work leveraged a combined-design laboratory-scale (capacity: 40 g/h) semi- batch operated fluid and fixed-bed reactor system by means of which experimental measurements were performed under thermal and thermo-catalytic circumstances at 450 °C.

• 2. ANALYSIS OF REACTOR TYPES OF THE THERMAL AND THERMO-

CATALYTIC THERMAL CRACKING PROCESSES The selection of reactor type(s) appropriate for the solid raw material to be processed is key in the process as it affects directly both the quality and quantity of the product formed. 2.1. Batch/semi-batch reactors Many studies on thermo-catalytic plastics cracking in mixed (not in each case) batch

• r semi-batch reactor are available in the professional literature [1]. The main reason

for this is principally the easy designability and operability. In the case of semi-batch reactors, a stream of continuous inert gas (nitrogen in general) is generated, which removes the volatile components from the vapour space at the temperature of

• reaction. The removal of volatile products minimises the possibility of the secondary

cracking (e.g. via oligomerisation, cyclisation and aromatization) of primary cracking

• products. This process ‘takes a back seat’ in a batch reactor as there the secondary

cracking is ‘brought to the fore’ [2]. Lee et al. examined the catalytic degradation of plastic waste ((HDPE high density polyethylene), LDPE (low density polyethylene), PP (polypropylene) and PS (polystyrene) on FCC (fluid catalytic cracking) catalyst in a semi-batch mixed reactor [3]. The yield of liquid products depending on the plastics of various types at 400 °C was as follows: PS > PP > PE (HDPE, LDPE). The quantity of liquid products was over 60 % in each case. In the catalytic case, the solid remains were below 1 m/m% except for PS, where it was about 5 m/m%. To avoid the mentioned secondary cracking, we decided to operate the vertically- positioned fluid-bed tube reactor in our work in a semi-batch manner with the continuous stream-in of nitrogen. 2.2. Fixed-bed reactors Fixed-bed reactors are probably listed among the most classic reactors. However, there use with plastics is not easy as these materials have an elevated viscosity and low thermal conductivity factor, due to which extremely serious issues can arise even at the feed-in. In specific cases, the molten polymer fraction is introduced to the reactor [4] via a capillary tube from the tank under overpressure. The most frequent technical solution is to perform a so-called preventive thermal cracking. Next, the liquid or gas components – originating from the thermal cracking – can be simply transferred onto the fixed-bed [5, 6] in a simple fashion.

In our experiments, the role of the fixed-bed outlined above was played by the horizontal tube reactor operated at 300 °C as the product vapours formed coming from the vertically-positioned operational unit were transferred directly in here, which can actually be regarded as a preventive thermo-catalytic cracking as well. 2.3. Fluid-bed reactors Fluid-bed reactors are characterised by homogeneity in terms of temperature and product composition. This is a remarkable advantage in the cracking of polymers as a thermal gradient develops due to the generally low thermal conductivity factor and high viscosity, leading to developing other reaction systems as heat does not transfer uniformly everywhere. One of the most known processes for the pyrolysis of plastic waste is the Hamburg process developed by Kaminsky et al. [7]. The reactor was heated with a heating filament with a power of 5 kW. The raw material was in-fed by two screw conveyor belts. The products produced in this way underwent a thorough separation, composed of a cyclone, coolers and electrostatic separators, too. Initially this system was used for thermal cracking of waste polymers exclusively, thereby using nitrogen or pre-heated vapour as fluidising agent [7]. At a conversion of 90 m/m% oil and waxy products formed, whereas oil had 40% BTX (benzene, toluene, xylene) at higher temperatures (690 to 735 °C) [7]. In our work, we noticed that the conversion was almost 90% already at 450 °C with the reactor combination selected by us as we mixed 10 w% catalyst into the raw material and the NiO-coated metal mesh, which was also responsible for the establishment of the homogeneous temperature field. The oil produced in our case contained 40% BTX fraction, too (Table 1) because the product vapours could be in direct contact with the catalyst attachment in the horizontal tube reactor. 2.4. Streamed-bed reactors One of the first proposals to recycle plastic waste was to crack them directly together with standard FCC raw materials in a combined material flow in FCC refining units. In this line of thinking, a new reaction system is established in a streamed-bed. The in-feeding contains normally mixed plastic (PE, PP, PS) at 5-10 m/m%, light oil (LCO), vacuum gas oil (VGO) or benzene [8]. The key element of the process is the internal recirculation reactor, which can ensure little contact time (1 to 10 s) and can operate at a ratio of C/O = 6 (catalyst/oil ratio). The catalyst is placed in a basket and gases circulate through the basket via a streaming enforced by a turbine. At time 0, in-feeding takes place by injection, when the reaction is finished, the valve opens and the products flow into a vacuum chamber [8]. Cracking PE/LCO and PP/LCO mixtures on a HZSM-5 catalyst resulted in the formation of mainly C5-C12 hydrocarbon and aromatic-type product as well as small quantity of C1-C2 gas and chark at 450 °C in the ascending simulator [8].

2.5. Screw press (extruder) reactors Screw press (extruder) reactors provide assistance for thermo-catalytic cracking of plastic and plastic-oil mixture by opening up a novel reaction system [9-11]. What is interesting about the process is that the design of the reactor reminds of extruders, which are also used for a broad range of polymer-processing applications. The equipment is shown in Figure 1 in a schematic form. Figure 1 Schematic of a screw press reactor [10] The reactor has one hopper where the plastics and plastic-oil mixtures can be in-

• fed. The externally heated furnace operates at 250 to 300 °C in a nitrogen atmosphere

(at a pressure somewhat higher than the atmospheric one) [9-11]. The molten reaction mixture is then forwarded into the reactor body, which means essentially a stainless steel tube meaning the current reaction zone at the same time. The tube is heated externally by two tube furnaces, whose temperatures can be regulated irrespective of each other, thus, the reactor can be split-up into two heating sections (marking T1/T2). The temperature can be continually regulated with a series

• f integrated thermoelements to avoid the probability of formation of cold/colder

points that would lead to the hardening of plastic. The revolution of the screw press can be varied between 0.5 and 25 rpm, as a result of which the residence time of melt can be defined, too. Due to the small screw diameter, the radial temperature profile is technically negligible. The catalyst is mixed with the plastic in the hopper as this ensures the homogeneous reaction mixture and consequently the identical stream image in the entire system. The catalyst can be recovered at the output point with a simple filtering [9-11]. Unlike in other reaction systems, no viscosity issue arises in this reactor as the extrudes squeezes out the material, therefore, the plastic phase can be fed-in the hopper continually without any flow issue. This is a significant benefit in contrast to

conventional fixed-bed reactors where plastic moves simply gravitationally or the feed-in is provided from a previous thermal cracking. In comparison to conventional batch reactors, the application of screw press reactors entail less gas fraction, as well as reduces the probability of cracking of heavier fractions as well [9-11].

• 3. PRESENTATION OF THE RAW MATERIALS USED AND THE

RESULTS 3.1. Presentation of the raw materials used The raw materials of combined cracking experimentation in the current phase of

• ur research were: polystyrene (waste fraction) and rubber waste.

The plastics of various sorts constitute about 10 w% of the whole solid waste

• fraction. Figure 2 illustrates the distribution of household plastic waste broken down

into sorts. Figure 2 Distribution of household plastic waste broken down into sorts [12] Figure 2 shows that the main constituents are polyolefins:  HDPE (high density polyethylene);  LDPE (low density polyethylene);  LLDPE (linear low density polyethylene);  HDPE (high density polyethylene);  PP (polypropylene). These plastics make up 66.9 % of the entire volume. Other important constituents: PCV (polyvinyl chloride), PS (polystyrene), PET (polyethylene terephthalate).

3.1.1. Justification of the selection of PS Polystyrene is a plastic sort used at a relevant scale and large quantities because of its favourable properties all over the world. We observed in our earlier experiments that by increasing the quantity of polystyrene added to rubber waste the valuable aromatic (BTX) hydrocarbon concentration rises in the liquid fraction, in addition, it promotes the formation of paraffin during the process due to its significant hydrogen content resulting in a qualitative improvement. Table 1 lists the composition of the liquid hydrocarbon fraction produced from pure PS. The use of PS is justified by the fact as well that the activation energy in polyolefins is lower compared to various synthetic caoutchoucs. Table 1 reveals that the values

• f the valuable toluene and o xylene fraction were 47.3 and 296 g/l, respectively,

which can be considered a remarkable result. Table 1 Composition of liquid products produced thermo-catalytically Component Measure ment 1 mg/l Measure ment 2 mg/l Measure ment 3 mg/l Benzene 974 961 1 010 Toluene 45 000 46 200 47 300 Ethyl benzene 30 900 30 920 29 033 m+p xylene 20 100 19 300 21 100

• xylene

293 000 291 000 296 000 In total 389 974 388 381 394 443 It can be stated based on the results of Table 1 that the aromatic hydrocarbon fraction in the oil stemming from PS cracking is noteworthy, which can boost the economical potential of the process significantly as well. 3.1.2. Justification of selection of rubber waste Annually several hundreds of millions of rubber tyre become waste globally causing considerable load to the environment. Its recycling is highly complicated as it contains many different heterogeneous components:  SBR (styrene butadiene synthetic caoutchouc);  BR (butadiene synthetic caoutchouc);  IIR (butyl caoutchouc);  NR (natural caoutchouc);  carbon black;  sulphur;

 rubber accelerators;  activators;  silica;  zinc and other metal oxides;  antioxidants;  antiozonants;  steel;  Kevlar;  synthetic fibre textile material. The biggest issue with rubber tyre waste is that it is not bio-degradable, therefore it accumulates in landfills. We observed in our earlier research that by mixing PS to rubber waste in the thermo-catalytic process a significant qualitative improvement can be achieved for the produced liquid fraction. This is due, in particular, to the fact that a considerable amount of hydrogen forms in the vertical reactor at 450 °C (Table 2), which facilitates the hydrogenation of olefin fraction in the vapour phase. Table 2 Composition of the thermo-catalytically produced gas fraction Components Quantity [V/V%] CO2 2.4 C2H4 9.1 C2H6 1.9 H2 45.15 O2 0.2 N2 1.2 CH4 30.85 CO 9.2 3.2. Presentation of the mechanism of the process By revealing the mechanisms, the operational parameters of chemical industry processes by cracking in combined material flow can be enhanced, too, as a result of which the more valuable liquid and gas products can be produced more economically. Figure 3 illustrates the degradation of the solid hydrocarbon fraction. The goal within our work was to produce as much aromatic component-containing liquid products as possible from PS and rubber waste (solid hydrocarbon fraction).

Figure 3 The mechanism of thermo-catalytic cracking of the solid hydrocarbon fraction on acidic catalyst [13] It is well visible that reaching our goal is a somewhat complicated optimisation task as not solely the ratio of the two heterogeneous components is necessary to be

• ptimised. The combination of the employed reactor system is key as well as that way

the degree of the formation of aromatic hydrocarbon can be viable via two routes. 3.3. Presentation of the experimental equipment Figure 4 shows the photograph taken of the equipment used for the measurement, which features two main units:

• 1. vertically-positioned semi-batch fluid-bed reactor;
• 2. horizontally-positioned fixed-bed reactor.

Figure 4 The vertical semi-batch fluid-bed reactor as well as the horizontal fixed-bed reactor system Experiments were performed in oxygen-free (inert) atmosphere and closed reactor system at moderate temperatures in each case. Depolymerisation reactions are endothermic processes therefore the power used for heating must be provided from an external source [14], which was realised with the tube furnace shown in the figure. The reactor space was inertised by introducing a controlled quantity of nitrogen. The so-called fluid-bed was ensured by the stream-in of nitrogen during the process,

thus, it has a two-fold purpose. Furthermore, it is important to mention that by regulating the flow velocity the residence time of hydrocarbons in the vapour space can be influenced as well, which prevents secondary cracking. We filled 40 g raw material in the vertically-positioned tube reactor in each case, accompanied by 10 w% zeolite-type catalyst. The duration of the operation was 60 minutes at 450 °C. A NiO-coated steel mesh was placed in the vertical tube reactor, which catalysed hydrocarbons in the vapour phase so that it served as a so-called catalyst attachment during the operation. The temperature of the experiment was 300 °C. 3.3.1. Product separation We condensed the hydrocarbon vapours of almost 400 °C down to a temperature of 20 °C by means of a water exchanger, after which the liquid product (Figure 5) was transferred in the collector. The gas fraction formed was flared off. Figure 5 Photograph taken of the liquid product On average, 26 g liquid, 10 g gas as well as 4 g solid fraction were obtained during

• measurements. The average quantity of the liquid was 65 w%, which was also in line

with values of professional literature.

• 4. CONCLUSIONS AND DETERMINATION OF RESEARCH GOALS

Cracking processes take place usually in five types of reactors in which the reaction mechanism of components varying both in space and time differ in each case. As a matter of fact, this is what determines product composition, distribution as well as qualitative parameters, too. By determining the complex reaction mechanism and reaction rate constants, an opportunity opens up to model the process mathematically, by means of which also the process technological issues derived from the examined circumstances can be addressed in an exact fashion.

We managed to create a reactor combination in our experimental work that can reduce the degree of secondary cracking to a sufficient level. We noticed that it was possible to reach a conversion of 90% of the in-fed raw material already at 450 °C, due to which the BTX content of the oil fraction formed was 40%. The selected PS and rubber waste proved to be an optimum raw material combination as a considerable qualitative improvement can be attained in the liquid phase during combined cracking that is key from the aspect of the process.

• 5. REFERENCES

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