(666f) Experimental and Numerical Comparison of Structured Packings with a Randomly Packed Bed for Cocurrent Gas-Liquid Flow – Implications for Fischer-Tropsch Synthesis

The structuring of the multiphase reactors introduces extra degrees of freedom allowing decoupling of conflicting design objectives, such as high mass transfer versus low pressure drop in a packed bed. In this way, structuring facilitates reaching goals such as process intensification and more sustainable operation. At Delft University of Technology, much effort has been put over the past years in the research and development of structured reactors, such as structured gas solid-fluidized beds, structured slurry bubble columns, and structured fixed bed reactors. In this paper, we will focus on the latter category.

Improvements in catalyst activity make the heat transport in fixed bed reactors increasingly important. Structured internals can play a very important role in improving heat transport. Structured packings, which are widely used in distillation columns, can also be used in fixed bed reactors to replace randomly dumped catalyst particles in order to drastically improve the heat transfer. The packing size and shape can be tailored such that an optimal combination of reaction kinetics, pressure drop and heat transfer characteristics is obtained. However, heat transfer data on structured packings is scarce. In this work structured packings such as OCFS (Open Cross Flow Structures), CCFS (Closed Cross Flow Structures), knitted wire, and foam were characterised with respect to the heat transfer performance. A dedicated set-up was designed and built enabling the determination of heat transport rates in two-phase flow at ambient pressure in the absence of reaction. Benchmarking and set-up validation was carried out using glass beads. The structured packings ? especially OCFS and CCFS ? exhibit heat transfer coefficients that are superior over those of glass beads, and at lower energy dissipation.

In addition to the experimental work, a 2D reactor model for Fischer-Tropsch synthesis, based on heat and mass balances including gas-liquid, liquid-solid and intraparticle diffusion transport, is used to compare the performance of a tubular fixed bed reactor either packed with spherical particles or equipped with the OCFS packing. The simulations show that a reactor with a structured packing can be operated with at least twice the reactor diameter compared to the packed bed reactor without any significant loss in C5+ space time yield.

Literature

K.V.Pangarkar, T.J.Schildhauer, J.R.van Ommen, J.Nijenhuis, F.Kapteijn and J.A.Moulijn, Structured packings for multiphase catalytic reactors, Ind.Eng.Chem.Res. 47 (2008) 3720-3751.

K.V.Pangarkar, T.J.Schildhauer, J.R.van Ommen, J.Nijenhuis, J.A.Moulijn, and F.Kapteijn, Heat Transport in Structured Packings with Co-current Downflow of Gas and Liquid, Chem. Engng. Sci. (2009) accepted.