Reactors

The low thermal conductivity of the molten polymers and their extremely high viscosity are the major problems for the catalytic cracking reactor design. The most widely used reactor systems have been:

  • batch/semi-batch;
  • fixed bed;
  • fluidized bed;
  • spouted bed;
  • screw kiln.

Apart from these reactors, thermogravimetric techniques (TG) have been widely applied for the study of the thermal and catalytic cracking of several plastics over different acid solids, determining relative activity of the catalysts and kinetic parameters under both isothermal and dynamic conditions [45, 66-69]. Thus, Figure 3.9 illustrates the TG curves corresponding to the degradation of a commercial EVA copolymer by just thermal treatment or when mixed with a MCM-41 catalyst at two different heating rates [45]. It is observed that the catalytic cracking starts at lower temperatures than the thermal process. In fact for a heating rate of 10°C/min, the catalytic cracking has gone to completion at 450°C, whereas the thermal degradation requires a temperature of around 490°C to obtain total plastic conversion. TG experiments have been shown to be a fast method

Sample 1

O 10°C/min-8.90% MCM-41 o 10°C/min-absence of MCM-41 a 40°C/min-9.10% of MCM-41 □ 40°C/min-absence of MCM-41

650 700

Temperature (K)

Figure 3.9 TG analyses of EVA polymer degradation by both thermal treatment and catalytic cracking over a MCM-41 material [45]. (Reproduced with permission from Elsevier)

for evaluating the activity of different catalysts or to study the catalyst deactivation, although the results must be carefully interpreted as they are not carried out under realistic conditions, whereas no information about the product distribution is usually derived.

3.1 BATCH/SEMI-BATCH REACTORS

The literature is full of studies that use either a batch or a semi-batch reactor for the catalytic cracking of plastics provided (but not always) with a stirring device [28, 38, 41, 55, 70]. The main reason is the ease of their design and operation. However, there is a significant difference between them. The semi-batch reactor is swept by a continuous flow of an inert gas (usually nitrogen) that removes the volatile products from the medium at the reacting temperature. The removal of the volatile compounds causes the secondary reactions of the primary cracking products (e.g. oligomerization, cyclization and aroma-tization) to take place only to a little extent. This does not occur in batch reactors where secondary reactions are supposed to be promoted [54].

One recent example of the use of a stirred semi-batch reactor for the catalytic degradation of plastics wastes (HDPE, LDPE, PP and PS) over spent FCC catalysts has been reported by Lee at al. [70]. The yields of liquid products at 400° C obtained with the different plastics follows the order: PS > PP > PE (HDPE, LDPE), with values in all cases above 80%. The amount of solids deposited over the catalysts was below 1 wt% except for PS (about 5%). In the last case, small cyclic intermediate precursors are first formed, which subsequently polymerize leading towards the observed solid residues. These authors also studied the composition of the liquids coming from the cracking of the different plastics by PONA analysis (paraffins, olefins, naphthenes, aromatics). Figure 3.10(a, b) compares the composition of the liquids obtained in the cracking of HDPE and LDPE, respectively. Olefins were the main components of the liquids obtained with both polyolefins, especially in the case of HDPE (> 80%), while this share dropped with LDPE below 60%. The observed high selectivity towards olefins (primary products) and the lower one towards the remaining products agrees well with the initially reported features of this stirred semi-batch reactor.

3.2 FIXED-BEDREACTORS

The fixed bed is likely the most classical catalytic reactor. However, its usage with plastics as feed is not straightforward since the high viscosity and low thermal conductivity of plastics pose serious problems for being loaded into the reactor. In some systems, the molten polymer is introduced into the reactor through a capillary tube from a pressurized tank [71]. The most usual technical solution is to carry out a previous thermal cracking of the plastic. Then, the liquid or gaseous compounds resulting from the thermal process can be fed easily into the fixed bed [72-75].

According to this concept, Masuda et al. [75] studied the catalytic cracking of the oil coming from a previous thermal pyrolysis step of polyethylene at 450°C in the bench-scale fixed-bed reactor shown in Figure 3.11. The catalysts employed were different zeolite types: REY (rare earth exchanged zeolite Y), Ni-REY (nickel and rare earth

Polyethylene Temperature Degradation

50 100 150

50 100 150

0 50 100 150 200 250 300 Lapse time (min.) (b)

Figure 3.10 Fractions of paraffins, olefins, naphthene and aromatic products from the catalytic degradation of plastics over spent FCC catalyst in a semi-batch reactor (400°C, P/C = 10, N = 200 rpm): (a) HDPE; (b) LDPE [70]. (Reproduced with permission from Elsevier)

Ldpe Reactor Internal
Figure 3.11 Bench-scale fixed-bed reactor used for the catalytic reforming of products coming from the thermal degradation of polyethylene [75]. (Reproduced with permission from Elsevier)

metal exchanged Y-type zeolite) and HZSM-5. The reactions were carried out under a steam or nitrogen atmosphere at 400°C. Nickel was included in the catalyst formulation in order to promote the transport of hydrogen atoms by spillover from steam to adsorbed olefinic hydrocarbons over the acid sites. When steam is used as the carrier, the strong acid sites of the catalysts are partially covered by hydrogen. As these strong acid sites are responsible for the formation of coke, catalyst activity is maintained for a longer time. Optimum nickel level in Ni-REY zeolite was about 0.5%. Conversions above 70% of the heavy oil and selectivities towards gasoline close to 80% were attained and maintained constant over Ni-REY after five sequences of reaction and regeneration. When HZSM-5 zeolite was used as catalyst, a fast deactivation was observed as a consequence of a dealumination process caused by the steam presence.

3.3 FLUIDIZED-BED REACTORS

Fluidized-bed reactors are featured by presenting both temperature and composition homogeneity. This is a remarkable advantage for the cracking of polymers due to their low thermal conductivity and high viscosity that usually lead to the appearance of temperature

Figure 3.12 Flow scheme of a fluidized-bed reactor (Hamburg process) [76]. (Reproduced by permission of Wiley VCH)

gradients in other reaction systems wherein heat is not as properly transferred. One of the best known process for pyrolysis of plastic wastes is the Hamburg process developed by Kaminsky et al. [76] which is depicted in Figure 3.12. The fluidized bed consists of a 154 mm diameter, 670 mm height tube of stainless steel filled with 9 kg of sand and a gas distributor at the bottom formed by a steel plate with 108 tubes. The reactor is heated by 5 kW filaments and the input material is fed into the reactor by two screw conveyors. The products obtained are separated in several stages comprising a cyclone, coolers and electrostatic separators. Initially, this system was exclusively applied for the thermal cracking of polymer wastes using as fluidizing agent nitrogen or preheated steam. Thus, by pyrolysing at 510°C a mixed plastic waste mainly formed by polyolefins, a 90 wt% conversion towards oil and waxy products was attained while at higher temperatures (690-735°C) an oil consisting of 40% of BTX (benzene, toluene and xylenes), was obtained [77]. When PS and PMMA were used as feed, actual depolymerization towards the constituting monomers took place since they were obtained with high selectivity (61% of styrene and 97% of methyl methacrylate). On the other hand, if steam was used as fluidizing agent at 700-750°C, high yields of light olefins such as ethylene (21-29%), propylene (16-21%) and butadiene (5.6-6.6%) are achieved [17].

Mertinkat et al. [78] modified the Hamburg process for performing the catalytic cracking of the polymers by using as fluidizing medium an FCC catalyst instead of the inert sand or quartz employed in the thermal process. The amount of catalyst and plastic fed to the reactor was roughly 1 kgh-1 each and the residence time was varied within 3-12 s. Catalytic cracking of polyethylene at 450-515°C yielded gases (50%) and aliphatic oils (40%), while for polystyrene degradation, the main products were ethylbenzene (18-26%) and benzene (9-22%), with a significant reduction in styrene share (1-7%). Other researchers have also studied the catalytic pyrolysis of PS over HZSM-5 zeolite [79] and of different polyolefins over several acid solids using fluidized beds [80-82].

3.4 SPOUTED-BED REACTORS

One of the firstly proposed approach for the feedstock recycling of plastic waste was to merge them with standard FCC feedstocks and to submit them to cracking directly in FCC refinery units. According to this idea, a new reaction system denoted as spouted bed or riser simulator was proposed that allows for reproducing the conditions existing in an actual FCC unit [83-85]. The feed is usually a blend of 5-10 wt% plastic (PE, PP, PS) in an oil such as light cycle oil (LCO), vacuum gas oil (VGO) or even pure benzene. The scheme of the reactor is shown in Figure 3.13. It is an internal recycle reactor which can operate at low contact times (1-10 s) and with a suitable catalyst/oil ratio (e.g. C/O = 6). As inferred from Figure 3.13, the catalyst is placed in a basket and the gases are impelled

Injection

Ideal riser

Riser simulator

Ideal riser

Riser simulator

Injection

Shaft Seal

Cooling jacket

Impeller Gasket

Heater cartridge Catalyst Porous plate

Shaft Seal

Cooling jacket

Impeller Gasket

Heater cartridge Catalyst Porous plate

Figure 3.13 Scheme of a riser simulator reactor for plastic conversion [83]. (Reproduced by permission of Javier Bilbao)

by a turbine located in the upper part, circulating through the basket. At zero time, the feed is injected and when the reaction is completed, a valve is open, releasing the products into a vacuum chamber, being subsequently analysed.

The cracking of PE/LCO and PP/LCO blends over HZSM-5 zeolite catalysts in the riser simulator at 450°C led towards mainly a C5-C12 hydrocarbon fraction of aromatic nature and a low yield of C1-C2 gases and coke.

3.5 SCREWKILNREACTORS

Recently, a new reaction system has been designed for the thermal and/or catalytic degradation of plastics and plastics-oil mixtures [86-88]. The reactor is named screw kiln reactor and its design reminds that of the extruders widely applied for polymer processing. The scheme and a photograph of this system are shown in Figure 3.14. Basically, the reactor is provided with a hopper wherein the plastics or plastics-oil mixtures are fed and heated at temperatures within 250-300°C by two external furnaces under a nitrogen atmosphere (at slightly higher pressure than atmospheric). The melted reacting mixture is subsequently fed into the reactor by means of a screw located inside a 52-cm-long stainless steel tube with 2 cm internal diameter, which is the actual reaction zone. The tube is heated externally by two furnaces whose temperature can be adjusted independently, the reactor being divided effectively into two heating sections (denoted as T1/T2). Temperature is continuously controlled by a set of built-in thermocouples through the reactor in order to avoid the appearance of cold spots and the subsequent plastics solidification. Screw speed can be varied within 0.5-25 rpm, thereby changing the residence time of the plastics. The small diameter of the screw assures that the radial temperature profiles are practically negligible. The catalyst is mixed up with the plastic at the hopper in order to attain a homogeneous reacting mixture and it follows the same flow as the polymer through the whole system, being recovered at the outlet simply by filtration.

As an example, catalytic cracking of pure LDPE was carried out in the screw kiln reactor over mesoporous Al-MCM-41 (plastic/catalyst mass ratio of 50) at T1/T2 = 400/450°C, respectively. The reported yields of hydrocarbons within the gasoline range (C5-C12) amounts to 80% with a high content of C7 -C8 hydrocarbons, stemming likely from the occurrence of secondary catalytic oligomerization reactions affecting to the initially formed C3 and C4 fractions.

Unlike other reacting systems, this reactor is not bound by the usual viscosity problems of polymers as the extruder is used to displace the plastics, so pure plastics can be fed without any flow trouble. This is a significant advantage with regard to conventional fixed-bed reactors as in most of them the plastics just fall by gravity or the feed is indeed the product coming from a previous thermal pyrolysis, not the polymer itself. Compared with a conventional batch reactor, the screw kiln reactor leads to a lower formation of gaseous products and reduces the overcracking of the heavier fractions. These results were ascribed to secondary reactions occurring in the reactor due to the intimate contact of the primary cracking products (mostly light gases) and the partially cracked plastics, since no selective removal of the volatile hydrocarbons takes place inside the reactor, as it occurred in the semi-batch reactors. Moreover, all the hydrocarbon fractions present similar residence times within the screw kiln reactor, which also leads to a narrow product distribution.

Load of LDPE Nitrogen feed

Load of LDPE Nitrogen feed

Screw Kiln ReactorScrew Kiln
Figure 3.14 Screw kiln reactor for the conversion of plastic and plastic-oil mixtures [87]: (a) scheme; (b) photograph of a laboratory reactor. (Reproduced by permission from Elsevier)

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