Plastics feeding

Results obtained for two mixed plastics are summarized in Table 4. A balance exists between process temperature, plastics feed rate, and product yields (67). For example, lower temperatures increase wax formation due to incomplete depolymerization. Slower feed rates and increased residence times reduce wax formation and increase the yield of Hquids. The data summarized in Table 4 illustrate that the addition of PET to a HDPE PP PS mixture changes the performance of the Conrad process. Compared to the reference HDPE PP PS mixture, increased amounts of soHds ate formed. These are 95% terephthahc acid and 5% mono- and bis-hydroxyethyl esters. At higher temperatures, apparentiy enough water remains to promote decarboxylation. 

Pellet Mills Pellet mills operate on the principle shown in Fig. 20-92. Moist, plastic feed is pushed through holes in dies of various shapes. The friction of the material in the die holes supplies the resistance necessary for compaction. Adjustable knives shear the rodlike extrudates into pellets of the desired length. Although several designs are in use, the most commonly used pellet mills operate by applying power to the die and rotating it around a freely turning roller with Fixed horizontal or vertical axis. 

Some preparation of the waste plastics feed is required before pyrolysis, including size reduction and removal of most nonplastics. This feed is charged into the heated fluidized bed reactor, operating at 500°C, in the absence of air. The plastics thermally crack to hydrocarbons, which leave the bed together with the fluidizing gas. Solid impurities and some coke either accumulate in the bed or are carried out as fine particles and captured by cyclones. 

The plastic feed may contain chlorine in the form of PVC or PVDC. Preferably, a substantial portion of any chlorine in the feed is removed by the addition of a chlorine scavenger compound to the feed, for example, sodium carbonate or slaked lime (calcium hydroxide) to the feed. It reacts in the pyrolysis zone with the hydrogen chloride to form sodium chloride or calcium chloride which becomes part of the residue at the bottom of the pyrolysis vessel. Preferably, the chlorine content should be removed to less that about 5 ppm. 

ER = equivalent ratio Q = plastic feed rate T = bed temperature U = fluidizing velocity... 

Tables 17.2-17.5 show some detailed product composition of different plastic feeds, pyrolyzed in a laboratory or pilot plant [11, 12].

Figure 17.6 Scheme of the industrial pilot plant for the pyrolysis of 5000 t/yr plastics or tires. Z cyclone Q quench cooler K cooler DW high-pressure quench cooler KM compressor WT heat exchanger F flare G excess gas R carbon black P plastic feed K calcium oxide, LS low-boiling oil HS high-boiling oil... 

According to Meszaros [9], the feed preparation is simplified in comparison to other processes, which require size reduction, washing and removal of nonplastic contaminants. Furthermore the inorganic contaminants in the plastic feed do not disrupt the process and exit the unit with unreacted material and carbonaceous products. 

In the second phase of the parametric study the removal of halogens from the reaction products was examined. Two different halogen removal methods were studied. The first method used a calcium oxide fixed bed placed between the reactor and condenser to remove organochloride vapours. However, it was found that the calcium oxide bed would plug up very rapidly if PET was present in the feed mixture and would become ineffective. This is because the PET would depolymerize into terephthalic acid and CO2 and would react with calcium oxide and cause it to plug up. The second method that was used for removal of chlorine was to add calcium oxide or hydroxide directly into the reactor with the plastic feed. It was found that this approach is far more effective than the previous method. Through trial and error it was found that calcium hydroxide feed of 10 wt% would remove the highest amount of chlorine (Table 19.7) [9]. 

Serrano et al. [11] studied the use of a laboratory-scale screw kiln reactor to transform low-density polyethylene (LDPE) into petrochemical feedstock. In this process, pyrolysis was carried out at reaction temperatures of 400-550°C and screw speeds of 3-20 rpm (Figure 19.6). In this process the plastic feed is initially heated in a feed hopper until the feed is melted. The melted plastic is then fed into the screw conveyor where it is depolymerized into gas, liquid and solid. The hopper is equipped with a stirrer to mix the feed plastic. Nitrogen is also used to provide an inert medium for pyrolysis. 

Conventional coal briquetting uses a thermoplastic binder and a well-plasticized feed. Referring again to Figure 403, vapors emanating from the blend are removed in a special horizontal screw conveyor (15) before the material enters the feeder of the roller press (16). 

Oil or Plastics/Oil Feed Plastics Feed Steam Lines Cooling Water (C W.) Main Process Lines... 

The products obtained from thermal cracking of plastics depend on the type of plastics, feeding arrangement, residence time, temperatures employed, reactor type, and condensation arrangement [42]. Reaction temperature and residence time have strong influence on the yield of pyrolysis products and the distribution of their components for plastic samples. Jude et al. conducted smdies on thermal cracking of LDPE in a batch reactor resulted in the production of a broad range of hydrocarbon compounds where the yield of aromatics and aliphatics (olefins and paraffins) deeply depended on the pyrolysis temperature and residence time. 

Experimental Results The mean values of operating variables monitored during the experimental tests include fiimace temperature, exhaust temperature, waste feed rate, air flow rate, and carbon monoxide concentration and are given in Table 15.10. The data show that 100% plastic feed rate is less than 100% nonplastic for the same inlet air and furnace temperature (compare samples 1... 

Jaw crushers are sensitive to moist and plastic feed material and tend to choke if there is a substantial proportion of fine particles in the feed. The attainable reduction ratio is between about 6 1 and 8 1. For obtaining a product of favourable size for feeding to grinding mills it is generally necessary to apply secondary crushing in another type of crusher. 

Like the jaw crusher, the gyratory crusher is sensitive to moist and plastic feed material and it tends to choke if the material has a high fines content. 

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