In a multitubular fixed bed reactor, the catalyst particles are packed into narrow tubes, grouped in bundles and enclosed in an outer shell (see Fig. 19.4). The tube bundles are immersed in water, which abstracts the heat and converts to high

pressure steam. The use of narrow tubes, high syngas velocities and large catalyst particles ensure rapid heat exchange and minimize exothermic temperature rise (Dry, 1996). The increased particle size of the catalyst is also necessary in order to avoid large pressure drops (Sie and Krishna, 1999), a problem encountered with this reactor type. Still, catalytic particles with a large diameter reduce the effectiveness of the material and the overall reaction rate due to intra-particle diffusion limitation.

Overall, the fixed bed reactor choice is easy to operate and scale-up. It can be used over a wide temperature range and the liquid/catalyst separation can be performed easily and at low costs, rendering this reactor type suitable for LTFT. Moreover, in case of syngas contamination with H2S, the H2S is absorbed by the top catalyst layer and does not affect the rest of the bed; thus, no serious loss of activity occurs (Dry, 1996). On the down side, fixed bed reactors are expensive to construct and the high gas velocities required translate to high gas compression costs for the recycled gas feed. Moreover, it is maintenance and labour intensive and has a long down time due to the costly and time-consuming process of periodical catalyst replacement (Tijmensen et al., 2002).

Recent advances in this type of reactor are the multitubular fixed bed reactors applied in the SMDS process for the conversion of syngas from methane in a heavy, waxy FT product (Eilers et al., 1990; Sie et al, 1991). Shell operates such reactors in its GTL plant in Bintulu, with a capacity of approximately 3000 bbl/ day per reactor. This capacity has an order of magnitude higher than previous fixed bed reactors, developed by Lurgi and Ruhrchemie, and is attained due to the specially developed Shell catalyst formulation and reactor design (Geerlings et al, 1999; Sie and Krishna, 1999).