Читать книгу Plastics Process Analysis, Instrumentation, and Control - Группа авторов - Страница 49

A number of processes have been described for converting plastic waste into products for further processing such as catalytic or thermal processes, hydrocracking processes, extrusion processes, etc. For example, a process has been described where plastic waste is converted into lower hydrocarbons (92). This entails the plastic waste being reacted in a fluidized bed apparatus at about 300°C to 630°C. The resulting lower hydrocarbons, such as paraffins or waxes, can be converted by means of steam cracking into olefins. Plastic waste can be converted by the combination of fluidized bed apparatus and a known steam cracking process into these olefins.

An evident disadvantage of this process is that for the steam cracking process it is necessary to add naphtha to the feed materials obtained, i.e., it is not possible to convert the plastic waste into cracked products such as ethylene, propylene, etc., without adding traditional feed materials. Furthermore, the handling of the solids in the fluidized bed always proves to be a disadvantage. It is also problematic to scale up a process of this type to a large-scale industrial operation.

So, a process which can be used industrially on a large scale where plastic waste is converted into high-value feed materials for a steam cracker without the addition of naphtha, LPG and gas oil has been developed (93). Here, a melt obtained from plastic waste is converted into products at from 400°C to 550°C, and a distillate fraction is separated off from the products at from 180°C to 280°C. Then this material is fed into a steam cracker. A schematic diagram of such a process is shown in Figure 1.6.

Here, a dry and comminuted plastic waste 2, e.g., a blow molded fraction, is fed via a conveyor 1, e.g., a conveying screw, from a storage container 3 into a stirring container 4 which is equipped with a heating jacket. In this stirring container the plastic waste is converted at about 300°C into an easily pumped melt. During this process a dehydrohalogenation may take place if some PVC has inadvertently slipped through the sorting of the plastic waste. Any HCl 5 which is produced is converted with water by known processes, which are not relevant to the invention, into aqueous HCl which can be fed to other production processes or neutralized with NaOH. The above melt is fed by means of a forced circulation pump to a steam cracker 6. In this cracker the polymers are converted, without the addition of hydrogen, vapor, catalysts, solvents or diluents, into products which can be vaporized and cracked in the steam cracker in a conventional way. This involves a thermal liquid cracking at about 420°C and, furthermore, any remaining dehydrohalogenation takes place in the cracker. The required heat is supplied from outside, e.g., by oil or gas heating. The liquid/vapor mixture leaving the cracker is fed directly to a column 7, e.g., an enriching column. The bottom product removed at about 350°C comprises the higher boiling products which have not been converted into short-chain hydrocarbons.

Figure 1.6 Steam cracking process (93).

This is, on the one hand, returned directly to the cracker and, on the other hand, passed as heat transfer agent through the melt in the stirring container and through the heating jacket of the stirring container and finally returned to the cracker. Residues and solids 8 are removed, e.g., by means of a hydrocyclone 9, from the bottom product after it has left the column. The vapor mixture leaving the top of the column at about 240°C is fed, after a partial condensation, to another column 10, e.g., a packed column, at about 110°C. The liquid/gas mixture entering the packed column is washed with water or aqueous NaOH 11 in countercurrent; any HCl still present in the gas is removed as aqueous HCl or aqueous NaCl solution with the liquid mixture at the bottom. The liquid mixture emerging at the bottom (organic liquid/aqueous HCl or aqueous NaCl solution) is separated in a downstream phase separating vessel 12. The lighter organic phase is, on the one hand, removed from the process as feed material A for the steam cracker and, on the other hand, returned to the column. The heavier aqueous phase, possibly enriched with HCl or NaCl 13, is removed from the process. The HCl-free gas mixture emerging at the top of the packed column is likewise fed to the steam cracker as feed material B.

A blow molded fraction (from Duales System Deutschland GmbH, Bonn, Germany), whose plastic content essentially consists of poly(ethylene) (PE) and PP, including any adherent soiling, sticky label materials, fillers, residual contents, and other materials, was processed in a plant, as shown in Figure 1.6. The resulting feed materials A (liquid mixture) and B (gas mixture) for the steam cracker have the compositions shown in Tables 1.5 and 1.6. Here, the following abbreviations are used in the Tables: HC = hydrocarbons, NA = non-aromatics, and EB = ethylbenzene.

The cracked products obtainable from the steam cracking process have the compositions shown in Tables 1.7 and 1.8. Here, for comparison, also, the compositions of the cracked products if the steam cracker is operated with the traditional feed material naphtha are shown.

A comparison of Tables 1.7 and 1.8 shows that the yield of ethylene and propylene is higher if the steam cracker is operated with the feed materials obtained from the blow molded fraction than if the steam cracker is operated with naphtha.