Waxes thermal cracking

Thermal cracking of wax. From thermal cracking a thermodynamic mixture might have been expected, but the wax-cracker product contains a high proportion of 1-alkenes, the kinetically controlled product. Still, the mixture contains some internal alkenes as well. For several applications this mixture is not suitable. In polymerisation reactions only the 1-alkenes react and in most cases the internal alkenes are inert and remain unreacted. For the cobalt catalysed hydroformylation the nature of the alkene mixture is not relevant, but for other derivatisations the isomer composition is pivotal to the quality of the product. 

Thermal cracking is not very selective and produces a mixture of products including n-olefin, i-olefin, di-olefin, aromatics, and paraffin wax. The major challenge associated with the cracking process is separation of the desired olefins from the other byproducts. 

A thermal cracking unit for waxes consists of a furnace, a primary separation column, a stabilization column and a distillation section. The feedstock is vaporized, mixed with steam to 40 per cent weight, and enters a tubular furnace in which the residence time is a few seconds (2 to 10 s) at 500 to 600°C. Once-tbrougb cbnversion is relatively low (15 to 30 per cent) to avoid side reactions. Operation is at atmospheric pressure or ghtly above. Direct quench, or quench with a heat transfer fluid, generates steam. Primary fiactionation allows the recycling of the unconverted part of the feedstock. 

As an example, the results from catalytic cracking of three types of waxes from thermal cracking of PE are given in Table 8.2 [22], The catalyst used, the equilibrium FCC catalyst. 

Table 11.3 shows that, in most cases, the three polyalkene plastics prodnced very similar product yields, with high yields of wax, and hydrocarbon gas and negligible char yields. Higher temperatnres of pyrolysis result in thermal cracking of the oil/wax to prodnce increased concentrations of gas. For example, Kaminsky et al. [9] reported an oil/wax yield of 92.3 wt% and 7.6 wt% gas, at a pyrolysis temperature of 530°C in a flnidized bed. However at the higher temperature of 760°C, the oil/wax was thermally degraded to produce 42.4 wt% oil/wax and 55.8 wt% gas. 

The components of products from thermal and catalytic cracking of HDPE, LDPE, LP, PP, PS were analyzed [48], and the results are shown in Table 28.2 and Table 28.3. The products from thermal cracking of HDPE, LDPE and LP (linear polyethylene) are mainly wax-like substances at normal temperamre. The fraction under 200°C recovered from HDPE accounts for 16% of the total cracking products, while that from LP accounts for 23%. Compared with tlie products of PE, PP produces less solid residue, but more liquid components, and PS produces the highest proportion of liquid fraction, which is 99.17% by thermal cracking and 99.56% by catalytic cracking. 

Development of the process in Germany was expedited when Ruhrchemie and I.G. Farbenindustrie pooled their facilities about 1940. Results of laboratoiy- and bench-scale operations led to the construction of a demonstration unit at Leuna employing a catalyst slurry in a continuous two-stage process with an output of metric tons of alcohds per day. The olefin feed was obtained by mild thermal cracking of soft paraffin wax from the Fischer-Tropsch synthesis. The product, a mixture of alcohols, was readily sulfonated to detergents, which were in great demand in... 

As we learned more about the hydrocarbon reactions, some similarities to reactions catalyzed by strong acids at much lower temperatures became evident. An additional and different impetus to understanding came from an interlude of thermal cracking studies. At the time we were interested in the thermal cracking of normal paraffins (waxes) for production of alpha olefins. Thermal cracking of n-hexadecane gave products in... 

Dr Dry from Sasol proposed in 1982 to use slurry phase reactors to ultimately produce mainly diesel with naphtha as a significant co-product by using a scheme in which the reactor wax is hydrocracked (19, 20). It was further proposed that this naphtha is a good feedstock for thermal cracking to produce ethylene. Gulf Oil in 1985 proposed the use of a slurry reactor with a modem precipitated cobalt catalyst to produce mainly diesel as a final product (21). The advent of still more active cobalt catalysts has now resulted in the ability to consider gas velocities for the LTFT reactors that are in line with those used for the HTFT fluid bed reactors. 

Usually, Polyolefin s thermal cracking at high temperatures (700 °C) produces an olefin mixture of C1-C4 gases and aromatic compounds (benzene, toluene, and xylene). Pyrolysis of polyolefins at low temperatures (400-500 °C) yields high calorific value gases, condensable hydrocarbon oils, and waxes [46]. 

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