Elena Hájeková

Separation and Characterization of Products from Thermal Cracking of Individual and Mixed Polyalkenes

Low-density polyethylene (LDPE), polypropylene (PP), and their mixture in the mass ratio of 1: 1 (LDPE/PP) were thermally decomposed in a batch reactor at 450°C. The formed gaseous and oil/wax products were separated and analyzed by gas chromatography. The oils/waxes underwent both atmospheric and vacuum distillation. Densities, molar masses and bromine numbers of liquid distillates and distillation residues were determined. The first distillate fraction from the thermally decomposed LDPE contained mostly linear alkanes and alk-1-enes ranging from C6 to C13 (boiling point up to 180°C). The second distillate fraction was composed mostly of hydrocarbons C11 to C22 (boiling point up to 330°C). 2,4-Dimethylhept-1-ene was the major component of the first distillate fraction obtained from the product of PP decomposition, while in the 2nd distillate fraction it was 2,4,6,8-tetramethylundec-1-ene. The yields of some gaseous or liquid hydrocarbons obtained by distillation from thermally degraded LDPE/PP differed from the values corresponding to the decomposition of individual plastics due to the mutual influence of polyalkenes during their thermal cracking. Similarly, the yields of propene and methylpropene in the gaseous phase were higher in the case of mixture. Whereas the content of C9 to C17 alkanes and alkenes in the distillates separated from the liquid mixture obtained by the decomposition of LDPE/PP decreased, the formation of 2,4,6,8,10,12-hexamethylpentadec-1-ene remained unchanged. The corresponding mechanisms of thermal cracking were discussed.

Kinetics and modelling of heptane steam-cracking

The kinetics and product distribution during the cracking of heptane in the presence of steam were investigated. The experiments were performed in a flow reactor under atmospheric pressure in a temperature range of 680–760°C with a mass ratio of steam to heptane of 3: 1. The overall decomposition of heptane is represented by a first-order reaction with activation energy of 249.1 kJ mol−1 and a frequency factor of 3.13 × 1013 s−1. The reaction products were analysed using gas chromatography, the main product being ethylene. The molecular reaction scheme, which consists of a primary reaction and 24 secondary reactions between primary products, was used for modelling the experimental product yields. The yields of ethylene and hydrogen were in good agreement; however the experimental yields of propylene were higher than the predicted yields.

Fuels obtained by thermal cracking of individual and mixed polymers

Utilization of oils/waxes obtained from thermal cracking of individual LDPE (low density polyethylene), HDPE (high density polyethylene), LLDPE (linear low density polyethylene), PP (polypropylene), or cracking of mixed polymers PP/LDPE (1: 1 mass ratio), HDPE/LDPE/PP (1: 1: 1 mass ratio), HDPE/LDPE/LLDPE/PP (1: 1: 1: 1 mass ratio) for the production of automotive gasolines and diesel fuels is overviewed. Thermal cracking was carried out in a batch reactor at 450°C in the presence of nitrogen. The principal process products, gaseous and liquid hydrocarbon fractions, are similar to the refinery cracking products. Liquid cracking products are unstable due to the olefins content and their chemical composition and their properties strongly depend on the feed composition. Naphtha and diesel fractions were hydrogenated over a Pd/C catalyst. Bromine numbers of hydrogenated fractions decreased to values from 0.02 g to 6.9 g of Br2 per 100 g of the sample. Research octane numbers (RON) before the hydrogenation of naphtha fractions were in the range from 80.5 to 93.4. After the hydrogenation of naphtha fractions, RON decreased to values from 61.0 to 93.6. Diesel indexes (DI) for diesel fractions were in the range from 73.7 to 75.6. After the hydrogenation of diesel fractions, DI increased up to 104.9.