Cracking catalysts used as fluidized bed material in the Hamburg pyrolysis process

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Abstract

The effects of fluid catalytic cracking (FCC) catalysts as fluidized bed material on the pyrolysis of polystyrene and polyethylene were investigated at temperatures between 370 and 515°C in a scale of ∼1 kgh−1. The results were compared qualitatively with pyrolysis-GC-MS-experiments (200 μg). The catalytic influence on the product distribution is easily seen by the comparison to non-catalytic experiments with simple quartz sand. In the catalytic polystyrene pyrolysis, the product distribution is changed from the usual main product styrene (61 wt.%) to basic chemicals (18–26 wt.% ethylbenzene, 9–22 wt.% benzene, 1–7 wt.% styrene, 3–5 % toluene) and soot (15–23 wt.%). Surprisingly, it was shown that in the catalytic pyrolysis of polyethylene even at low temperatures (450–515°C) gases (48–52 wt.%) and low boiling aliphatic oils (38–39 wt.%) were received instead of waxes (∼90 wt.%) in non-catalytic experiments. A further success of the catalytic pyrolysis is a dramatic reduction of the process temperature. Pyrolysis-GC-MS-experiments confirmed these results qualitatively.

Introduction

Previous non-catalytic investigations dealt with the recycling of polymers and waste plastics by the Hamburg pyrolysis process by use of quartz sand as a fluidized bed material. Depending on the polymer, the corresponding monomers were gained in different quantities. For example, the production of ∼97 wt.% MMA from PMMA [1], of ∼75 wt.% styrene from polystyrene [2] and of ∼40 wt.% ethylene and propylene from polyolefins [3], [4] should be mentioned.

The main reasons for the catalytic research in the Hamburg pyrolysis process are

  • lowering the pyrolysis temperature and energy demand of the process,

  • the production of basic chemicals from polymers and waste plastics; and

  • the reuse of waste FCC-catalysts from refineries.

In non-catalytic experiments, the variation of the gas residence times in the fluidized bed shows that residence time effects are negligible in the pyrolysis of polystyrene, at least if the residence time is in the range 3–12 s. Pyrolysis-GC-MS-experiments serve as a screening method to find catalysts that can be used as fluidized bed material. These low-scale experiments are a useful method for qualitative testing catalytic effects on the product distribution.

Section snippets

Experimental

A laboratory-scale plant at the University of Hamburg was used for the experiments. A flow scheme of the Hamburg pyrolysis process is presented in previous papers [3], [4], [5]. The fluidized bed reactor consists of a steel tube that contains the fluidized bed (height 330 mm, 154 mm free diameter). The gas distributor was a porous steel plate. The fluidized bed, consisting either of FCC-catalysts or of quartz sand, was fed by two screw conveyors, one for determining the feed rate of ∼1 kg h−1

Results and discussion

The use of FCC-catalysts as a fluidized bed material in the Hamburg pyrolysis process dramatically reduces the process temperature. Polystyrene and polyethylene can be pyrolyzed far below 515°C, the temperature of non-catalytic experiments (Table 2, Table 3). Therefore, energy consumption and process costs can be reduced this way. The catalytic pyrolysis product spectra differ completely from those of non-catalytic processes [2], [3], [4], [7].

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