Transport Phenomena in Catalytic Hydrocracking of Polystyrene in Solution

Abstract

Mass transfer analysis in any experimental system is fundamental before studying its kinetics or optimizing a catalyst. The effect of several variables, such as stirring rate, catalyst concentration and particle size, hydrogen pressure, reaction temperature, polystyrene concentration and average molecular weight, both from the experimental and the theoretical point of view, have been studied in the catalytic hydrocracking of polystyrene dissolved in decahydronaphthalene carried out in a stirred tank with hydrogen and catalyst (0.58 wt % Pt/H-β) in a slurry. The results show that the variables could be adequately selected to prevent both gas–liquid and liquid–solid mass transfer control. However, severe internal diffusion control occurred as a consequence of polymer molecular dimensions that, in practice, limits the reaction to the outer surface area of the catalyst. Thus, catalyst optimization for this application, and catalytic processes with polymer reactants in general, should be focused on maximizing external surface area.

Kinetic Modeling of Hydrocracking of Low-Density Polyethylene in a Batch Reactor

“…Gas and liquid volumes are assumed to be constant during the reaction time, as reported in similar studies. 11,32,33 The gas produced occupies the contained volume and replaces consumed hydrogen; however, the reaction is not isobaric. In addition, the change in bond sizes between the cracked and the produced components is theoretically insignificant.…”

“…8−10 Catalysis, however, may help to address the issue by lowering the required temperature and also enhancing the product distribution. 11 Previous studies have proven the high activity of bifunctional zeolite as a promising catalyst for hydrocracking of polymers under mild conditions. 2,12 A full conversion of polyethylene to fluid products has been reported in the literature in the temperature range of 270−800 °C, H 2 pressure of 55−71 bar, and relatively short reaction time over metals such as Ni, Pt, Co, and Mo impregnated on an acidic support.…”

“…19 Fuentes-Ordońẽz et al studied the role of mass transfer in the hydrocracking of polystyrene (PS) dissolved in decahydronaphthalene over a bifunctional Pt/H-β catalyst. 11 The authors concluded that the gas−liquid and liquid−solid mass transfer steps could be minimized by optimizing the operation conditions, and that diffusion could be the limiting step of the hydrocracking reaction of the polymer due to the structure of the catalyst.…”

“…With the increasing concerns of polymer accumulation and adverse effects on the environment, the most widespread approach to recycling the polymer chemically is pyrolysis (or cracking). Thermal cracking of polymer waste has been extensively studied but still has limitations in terms of efficiency and high-temperature demands. − Catalysis, however, may help to address the issue by lowering the required temperature and also enhancing the product distribution . Previous studies have proven the high activity of bifunctional zeolite as a promising catalyst for hydrocracking of polymers under mild conditions. , A full conversion of polyethylene to fluid products has been reported in the literature in the temperature range of 270–800 °C, H 2 pressure of 55–71 bar, and relatively short reaction time over metals such as Ni, Pt, Co, and Mo impregnated on an acidic support. − The product stream of LDPE hydrocracking is dominated by gases (C 1 –C 4 ) and gasoline fractions …”

“…In comparison, Schubert et al obtained higher activation energies in the range between 147 and 313 kJ/mol by studying the kinetics of continuous pyrolysis of LDPE and a heavy petroleum fraction mixture based on a four-lump model . Fuentes-Ordóñez et al studied the role of mass transfer in the hydrocracking of polystyrene (PS) dissolved in decahydronaphthalene over a bifunctional Pt/H-β catalyst . The authors concluded that the gas–liquid and liquid–solid mass transfer steps could be minimized by optimizing the operation conditions, and that diffusion could be the limiting step of the hydrocracking reaction of the polymer due to the structure of the catalyst.…”