Olefins pyrolysis

Waste plastics potentially can also be processed in hydrocracking process as an additional feed stream in mixture with vacuum gas oil or crude oil residues. Careful plastic segregation is then necessary since inorganic additives and impurities of plastics can foul the hydrocracking catalyst. Noncatalytic high-temperature olefin pyrolysis (700-800°C) and coking are insensitive to fouling. 

It has been assumed that the remaining products are formed by some sort of free radical chain mechanism, but no generalized mechanism like that of Rice s for paraffin pyrolysis has been proposed. Tanaka et al. have been able to simulate product distributions for shorter olefins—up to hexene (10). We shall describe a model for higher alpha-olefin pyrolysis and use it to account for the products from the cracking of several olefins. 

Table II. Product Distributions from Alpha-Olefin Pyrolysis...

Moeller, G. E, and Warren, C. W., Survey of Tube Experience in Ethylene and Olefins Pyrolysis Furnaces, Paper 237, presented at Coirosion/81, National Association of Corrosion Engineers, Houston, TX. 

Primary nitroparaffins react with two moles of formaldehyde and two moles of amines to yield 2-nitro-l,3-propanediamines. With excess formaldehyde, Mannich bases from primary nitroparaffins and primary amines can react further to give nitro-substituted cycHc derivatives, such as tetrahydro-l,3-oxa2iaes or hexahydropyrimidines (38,39). Pyrolysis of salts of Mannich bases, particularly of the boron trifluoride complex (40), yields nitro olefins by loss of the amine moiety. Closely related to the Mannich reaction is the formation of sodium 2-nitrobutane-1-sulfonate [76794-27-9] by warming 1-nitropropane with formaldehyde and sodium sulfite (41). 

The yield of this reaction for R=R =C2H is 73% (15). Reagents are less toxic than the corresponding sulfates and less volatile than the orthoformates. Esters of phosphoms oxo-acids having beta hydrogens undergo olefin elimination upon pyrolysis, usually beginning at 160—200°C. 

However, when the temperature is increased to 120°C, the principal reaction is the elimination to olefin. The thermal decomposition of dimethyl dodecyl amine oxide at 125°C in a sealed system, as opposed to a vacuum used by Cope and others, produces 2-methyl-5-decyhsoxa2ohdine, dimethyl dodecyl amine, and olefin (23). The amine oxide oxidi2es XW-diaLkylhydroxylainine to the nitrone during the pyrolysis and is reduced to a tertiary amine in the process. 

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. 

The separation train of the plant is designed to recover important constituents present in the furnace effluent. The modem olefin plant must be designed to accommodate various feedstocks, ie, it usually is designed for feedstock flexibiUty in both the pyrolysis furnaces and the separation system (52). For example, a plant may crack feedstocks ranging from ethane to naphtha or naphtha to gas oils. 

Petroleum-derived benzene is commercially produced by reforming and separation, thermal or catalytic dealkylation of toluene, and disproportionation. Benzene is also obtained from pyrolysis gasoline formed ia the steam cracking of olefins (35). 

Superffex C t lytic Crocking. A new process called Superflex is being commercialized to produce predorninantiy propylene and butylenes from low valued hydrocarbon streams from an olefins complex (74). In this process, raffinates (from the aromatics recovery unit and the B—B stream after the recovery of isobutylene) and pyrolysis gasoline (after the removal of the C —Cg aromatics fraction) are catalyticaHy cracked to produce propylene, isobutylene, and a cmde C —Cg aromatics fraction. AH other by-products are recycled to extinction. 

Tetracyanoethylene is colorless but forms intensely colored complexes with olefins or aromatic hydrocarbons, eg, benzene solutions are yellow, xylene solutions are orange, and mesitylene solutions are red. The colors arise from complexes of a Lewis acid—base type, with partial transfer of a TT-electron from the aromatic hydrocarbon to TCNE (8). TCNE is conveniendy prepared in the laboratory from malononitrile [109-77-3] (1) by debromination of dibromoma1 ononitrile [1855-23-0] (2) with copper powder (9). The debromination can also be done by pyrolysis at ca 500°C (10). 

Pyrolysis. The pyrolysis of simple esters of the formula RCOOCR R CHR 2 to form the free acid and an alkene is a general reaction that is used for producing olefins ... 

The combination of low residence time and low partial pressure produces high selectivity to olefins at a constant feed conversion. In the 1960s, the residence time was 0.5 to 0.8 seconds, whereas in the late 1980s, residence time was typically 0.1 to 0.15 seconds. Typical pyrolysis heater characteristics are given in Table 4. Temperature, pressure, conversion, and residence time profiles across the reactor for naphtha cracking are illustrated in Figure 2. 

Catalytic Pyrolysis. This should not be confused with fluid catalytic cracking, which is used in petroleum refining (see Catalysts, regeneration). Catalytic pyrolysis is aimed at producing primarily ethylene. There are many patents and research articles covering the last 20 years (84—89). Catalytic research until 1988 has been summarized (86). Almost all catalysts produce higher amounts of CO and CO2 than normally obtained with conventional pyrolysis. This indicates that the water gas reaction is also very active with these catalysts, and usually this leads to some deterioration of the olefin yield. Significant amounts of coke have been found in these catalysts, and thus there is a further reduction in olefin yield with on-stream time. Most of these catalysts are based on low surface area alumina catalysts (86). A notable exception is the catalyst developed in the former USSR (89). This catalyst primarily contains vanadium as the active material on pumice (89), and is claimed to produce low levels of carbon oxides. 

Cracking temperatures are somewhat less than those observed with thermal pyrolysis. Most of these catalysts affect the initiation of pyrolysis reactions and increase the overall reaction rate of feed decomposition (85). AppHcabiUty of this process to ethane cracking is questionable since equiUbrium of ethane to ethylene and hydrogen is not altered by a catalyst, and hence selectivity to olefins at lower catalyst temperatures may be inferior to that of conventional thermal cracking. SuitabiUty of this process for heavy feeds like condensates and gas oils has yet to be demonstrated. 

Olefin formation (preferentially less substituted) from alcohols via xanthate pyrolysis. 

---