Feedstocks for pyrolysis

High-temperatnre pyrolysis and cracking of waste thermoplastic polymers, such as polyethylene, polypropylene and polystyrene is an environmentally acceptable method of recycling. These type of processes embrace both thermal pyrolysis and cracking, catalytic cracking and hydrocracking in the presence of hydrogen. Mainly polyethylene, polypropylene and polystyrene are used as the feedstock for pyrolysis since they have no heteroatom content and the liquid products are theoretically free of sulfur. 

The composition of the plastic feedstock for pyrolysis processes has a direct bearing on the quality of the resultant fuel products, especially flash point, cetane index, low-temperature properties and heteroatom content (e.g. sulphur, chlorine and nitrogen). 

In general, halogen-containing polymers (e.g. those containing Br and Cl) are not acceptable feedstock for pyrolysis because they would necessitate the use of special alloys to prevent corrosion and pinholing of the plant components (e.g. condenser coils). 

The distribution of contaminants over volatiles and charcoal in the pyrolysis process the major contaminants present in the feedstock for pyrolysis process are chlorine, nitrogen and alkalis. Jensen et.al. (9) owed that during pyrolysis of wheat straw considerable amounts of chlorine are released during the pyrolysis between 300-400 C. Potassium was only released at temperatures above 600-700 C. 

Propjiene [115-07-17, CH2CH=CH2, is perhaps the oldest petrochemical feedstock and is one of the principal light olefins (1) (see Feedstocks). It is used widely as an alkylation (qv) or polymer—ga soline feedstock for octane improvement (see Gasoline and other motor fuels). In addition, large quantities of propylene are used ia plastics as polypropylene, and ia chemicals, eg, acrylonitrile (qv), propylene oxide (qv), 2-propanol, and cumene (qv) (see Olefin POLYMERS,polypropylene Propyl ALCOHOLS). Propylene is produced primarily as a by-product of petroleum (qv) refining and of ethylene (qv) production by steam pyrolysis. 

Thermolysis gasification, pyrolysis... to produce petrochemical feedstocks for steamcracking or alternative fuels. 

Fast pyrolysis utilizes biomass to produce a product that is used both as an energy source and a feedstock for chemical production. Considerable efforts have been made to convert wood biomass to hquid fuels and chemicals since the oil cri-... 

Methanol was first produced commercially in 1830 by the pyrolysis of wood to produce wood alcohol. Almost a century later, a process was developed in Germany by BASF to produce synthetic methanol from coal synthesis gas. The first synthetic methanol plant was introduced by BASF in 1923 and in the United States by DuPont in 1927. In the late 1940s, natural gas replaced coal synthesis gas as the primary feedstock for methanol production. In 1966, ICI announced the development of a copper-based catalyst for use in the low-pressure synthesis of methanol. 

The high temperature necessary for pyrolysis is obtained by burning fuel in excess air in a combustion chamber. Natural gas is still the fuel of choice, but other gases, e.g., coke oven gases or vaporized liquid gas, are occasionally used. Various oils including the feedstock are occasionally be used as fuel for economic reasons. Special burners, depending on the type of fuel, are used to obtain fast and complete combustion. 

Feedstocks for various industrial pyrolysis units are natural gas liquids (ethane, propane, and n-butane) and heavier petroleum materials such as naphthas, gas oils, or even whole crude oils. In the United States, ethane and propane are the favored feedstocks due, in large part, to the availability of relatively cheap natural gas in Canada and the Arctic regions of North America this natural gas contains significant amounts of ethane and propane. Europe has lesser amounts of ethane and propane naphthas obtained from petroleum crude oil are favored in much of Europe. The prices of natural gas and crude oil influence the choice of the feedstock, operating conditions, and selection of a specific pyrolysis system. 

Gas phase free radical reactions are used in industry for pyrolysis, halogenation and combustion reactions. Nowadays, and probably for a long time to come, the thermal cracking of hydrocarbons constitutes the main production route for olefins, which are the basic feedstocks of the chemical industry around the world. Hydrocarbon pyrolysis is thus of considerable economic interest, as is shown by the very large amount of effort dedicated both to fundamental and applied research in this field (see, for example, refs. 35—37). 

Oil Collection. Table I shows the temperature of the process stream as it passed through the heat exchangers, as well as the amount and moisture content of condensate collected at each location. For Run 83, this oil collection train demonstrated a wet oil recovery of 67% of the dry feedstock for a mass closure of 94%. The wet oil contained an average of 18% water of pyrolysis for a recovered yield of 55% dry pyrolysis oil. 

Feedstocks For either gaseous (ethane/propane) or liquid (C4/naphtha/ gasoil) feeds, this technology is based on Technip s proprietary Pyrolysis Furnaces and progressive separation. This method allows producing olefins at low energy consumption with particularly low environmental impact. 

Vised directly. To obtain very pure propylene (99.5 per cent weight), however, it is necessary to remove the propane in a supplementary column. Table 2.10 offers a glance at the specifications that the ethylene and propylene thus separated are required to meet The heavier hydrocarbons obtained at the bottom of the depropanizer are treaty in a debutanizer, which produces a l,3>butadiene-rich cut at the top. In accordance with the severity, Table 2.11 provides a typical example of the composition of this effluent for a naphtha feedstock. The pyrolysis gasoline drawn off at the bottom may, depending on the severity, contain 50 to S5 per cent weight of aromatic hydrocarbons, of which more than half is benzene. Its composition and treatment are discussed separately in Section 2.1.5. 

The price of oil influences the cracking economics by changing both the price of feedstock and fuel and changing the value for pyrolysis gasoline and fuel oil. The sensitivity of the unit production is illustrated in Figure 9.5. 

The statisties for a naphtha cracker integrated with polymer and BTX production are illustrated in Table 9.2. The complex is based on the CLOSED case and only ethylene, propylene and pyrolysis gasoline pass to downstream processing. For brevity it is assumed the polymers are produced at 100% yield and require no other feedstock. The pyrolysis gasoline is passed to an aromatics extraction plant and produces 298 kt/y benzene, 149 kt/y toluene and 52 kt/y xylene. The rest of the pyrolysis gasoline (246 kt/y) produees a raffinate, whieh is sold as a gasoline. 

PVC is not recommended as a feedstock material for pyrolysis. The reasons for this being that it contains about 57% chlorine by weight which will affect diesel quality and can produce chlorinated hydrocarbons, and also because it thermally decomposes to hydrochloric acid that is very corrosive and toxic. 

Conrad Industries (Chehahs, Washington) have demonstrated the pyrolysis of post-use plastics into petrochemical feedstocks. The plastic most studied was a mixture of 60% high-density polyethylene, 20% polypropylene, and 20% polystyrene. Yields of liquid products were in the range 65-75% at 482-510°C. Other studies examined the effects of PET and PVC on the liquid yields. The liquid products were determined to be suitable feedstocks for further refining, but the economics were not competitive with conventional petroleum refining in 1994 [36]. 

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. 

It is apparent that if one wishes to obtain pure chemicals by biomass pyrolysis, further processing to separate the reaction mixture is necessary. As will be shown later, this did not hinder commercial use of biomass pyrolysis for the manufacture of specific chemicals. The slow, destructive distillation of biomass was commercial technology for the production of several commodity chemicals long before fossil fuels became the preferred feedstocks. Hardwood pyrolysis once served as an important commercial source of methanol, acetic acid, ketones, and other chemicals. 

---