Tube-Wall Reactor (TWR) is a versatile reactor to study Fischer-Tropsch (F-T) synthesis on account of excellent temperature control and negligible pressure drop. In the present work, tube wall reactor surfaces were prepared by plasma-spraying five catalysts, namely, Fe, 75Fe/25Co, 50Fe/50Co, 25Fe/75Co and Co (wt. % basis) and the catalyst surface was characterized in depth by using BET surface area measurements, chemisorption measurements, x-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe micro-analysis (EPMA). Despite the low BET surface areas for 'plasma-sprayed' catalysts, the hydrogen and carbon monoxide uptakes were found to be quite high. XRD studies showed that the various catalyst phases present on the surface were stable up to 350 °C and also that the surface consists of three types of particles, namely, Fe and Co oxides and cemented particles of CoO.Fe₂O₃. EPMA measurements show that, usually, the plasma-sprayed catalyst surface possesses a higher concentration of iron particles than cobalt particles suggesting that iron tends to migrate to the surface at the expense of cobalt.

In the literature, studies have been reported on F-T synthesis in a TWR using plasma-sprayed iron catalysts only at low pressures. In the present work, the effect of temperature (250-275 °C) , pressure (0.1-1.03 MPa) and exposure velocity (0.5-2.6 J, J = m³/h of syn gas at STP/m² of catalyst geometric area) with feed composition (H₂/CO ratio of 2) on the product yield and selectivity using plasma-sprayed cobalt and Co-Fe bimetallic (Co/Fe = 50/50, wt. basis) catalysts have been examined. Also, the catalyst activity and product selectivity were studied using plasma-sprayed iron catalyst up to a pressure 1.52 MPa. The study indicated that an isothermal operation of the tube-wall reactor was possible without any recycle of the product gas. The catalyst activity and total hydrocarbon yield were higher for Co-Fe bimetallic catalyst compared to iron and cobalt catalysts under similar process parameters. However, iron catalyst was more selective to C₄-C₁₁ (C₄-204 °C cut) range hydrocarbons compared to cobalt and Co-Fe bimetallic catalysts. For all catalysts, the product distribution shifted to higher hydrocarbons at high pressure, low temperature and low exposure velocity. On the other hand, high pressure, high temperature and low exposure velocity resulted in lowering olefin production (i.e. C₂ and C₃ olefins). The study also showed that the production of C₂ and C₃ olefins was maximum for cobalt catalyst and minimum for Co-Fe bimetallic catalyst. In all cases, the product distribution, followed the Schultz-Flory relationship and the chain growth probability (a) decreased with decrease in pressure and increase in temperature. However, a had two values for Co-Fe bimetallic catalyst, whereas it had single values for both cobalt and iron catalysts.

A mathematical model, developed earlier in our laboratory, was extended to predict the performance of Fischer-Tropsch Synthesis over cobalt and Co-Fe catalysts in a tube-wall reactor. The agreement between model predictions and experimental results was fairly good.