Mixed plastics catalytic cracking

An alternative process based on two sequential catalytic cracking stages aimed at obtaining gasoline and diesel from waste plastics or heavy oil/waste plastics mixtures is shown in Figure 3.16 [99]. The catalyst employed in the first step is made up of powder alumina, waterglass and HZSM-5 zeolite and is mixed up directly with the waste plastics in a screw reactor preferably at 600-700°C. The second catalytic step consists in a fixed... 

Another approach for overcoming the problems posed by conventional cracking catalysts has been disclosed recently by Reverse et al. [101]. In this case, direct cracking is performed by using as catalyst a molten bed of pure metal or a metal mixture (mainly lead, zinc, tin) at a temperature of 460-550°C wherein the waste polymer is loaded inside the reactor at a certain depth. The authors point out that the products are indeed a result of the combination of both thermal and catalytic cracking. The catalyst composition may also include some acidic component such as metal silicates, metal carbonates and their mixtures. The process can be applied to pure and mixed polymers (PE, PET, PP, PVC), as well as to the plastic fraction of municipal solid wastes. 

The structure of the hydrocarbons produced can be modified by the use of catalyst. Catalytic cracking consumes less energy than the noncatalytic process and results in formation of more branch-chain hydrocarbons. On the other hand the addition of the catalyst can be troublesome, and the catalyst accumulates in the residue or coke. There are two ways to contact the melted polymer and catalysts the polymer and catalyst can be mixed first, then melted, or the molten plastics can be fed continuously over a fluidized catalyst bed. The usually employed catalysts are US-Y, and H-ZSM-5. Catalyst activity and product structure have been reported [7-11]. It was found that the H-ZSM-5 and ECC catalysts provided the best possibility to yield hydrocarbons in the boiling range of gasoline. 

Reentech Limited (Korea) has developed a patented catalytic cracking process (Figure 15.14) which converts mixed plastics (e.g. PE, PP, PS) into gasoline, kerosene and diesel fuel [27-31]. 

There are all kinds of processes and reactors for pyrolysis of PVC-containing mixed plastics which can be basically divided into three classes thermal cracking, catalytic cracking and hydrogenation. The main products are gasoline, diesel oil, fuel gas and HCl. 

Good resnlts were also achieved by catalytic cracking in the Fnji and BASF [92] processes, in which mixed plastics containing PVC were pyrolyzed over ZHSM-5 [93]. 

Pyrolysis is a tertiary or feedstock recycling technique capable of converting plastic waste into fuels, monomers, or other valuable materials by thermal and catalytic cracking processes. This method can be applied to transform both thermoplastics and thermosets in high-quality fuels and chemicals. Moreover it allows the treatment of mixed, unwashed plastic wastes. 

The Blowdec depolymerization process from Slovakia converts waste plastics into low-sulphur diesel fuel [35]. The main principle is the processing of waste plastics in a hot whirling bed of hot sand in the BLOWDEC reactor (Figure 15.19). The plastic is heated to 430°C. The process allows for simultaneous cracking of hydrocarbons and inhibition of coke formation. The fluidized sand bed products three types of cracking reactions mechano-activation thermal and catalytic (Si02/aluminosilicates). The process is covered under the US Patent 6,165,349. The process enables the economical conversion of mixed waste plastics into liquid hydrocarbons, mainly low-sulphur (25 ppm) diesel... 

SPORT [Sustainable Polymers-to-Olefins Recycling Technology] Also called the BP Polymer Cracking Process. A catalytic process for converting waste polymers to hydrocarbon mixtures, which can be used to make new polymers. The mixed waste plastics are introduced into a fluidized bed of sand in an inert atmosphere, heated externally to about 500°C. The plastics melt and coat the sand particles and then undergo pyrolysis to lighter hydrocarbons, which leave the reactor with the inert gas. The resulting gas... 

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