Process intensification for the iron-catalysed slurry-phase Fischer-Tropsch Reactor System

Master Thesis


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University of Cape Town

A set of operating conditions was identified with the potential to enable improved slurryphase reactor productivity for hydrocarbon production using Fischer-Tropsch synthesis. Compared to the most relevant prior art publication, this requires operation at higher gas velocity, higher catalyst concentration and at higher temperature and/or pressure. The closest prior art proposal was published by Van der Laan et al. (1999) and a target was set to improve the reactor productivity by at least 50 %, relative to this reference, while also ensuring stable catalyst performance. Prediction of gas holdup in the reactor is essential to determine the reactor productivity and previous correlations used to predict gas holdup are potentially unreliable for extrapolation to the new proposed conditions. A new approach is adapted, from previous theoretical approaches, to provide a more fundamental and reliable basis for gas holdup prediction. Referred to as the ¡®adapted two-phase theory¡¯ it predicts the gas holdup at any slurry solids concentration using data from a representative solids-free liquid. This approach is shown to provide accurate predictions for paraffinic liquids using data covering a wide range of solids concentrations. Two laboratory reactor experiments were performed, at 260 and 270 ¢ªC, to characterise the selected catalyst performance at conditions relevant to the newly proposed operating regime. An achievable reactor performance was calculated corresponding to the catalyst performance from the experiment at 270 ¢ªC and using the new approach to predict gas holdup. Compared to the proposal by Van der Laan et al. (1999), a reactor with a given diameter is able to produce almost double the amount of product (94 % more with a lower slurry bed height). This is achievable by using higher catalyst concentrations and, most importantly, using a higher operating temperature. The undesirable methane selectivity, at or below 4 %, is still acceptable when operating at 270 ¢ªC. In spite of the higher reactor productivity with increasing temperature, the optimum operating temperature, in the range from 250 to 270 ¢ªC, may depend on the selectivity to the desired hydrocarbon products. The scope for further potential reactor productivity improvement is described. More work is needed to accurately quantify the selected iron catalyst selectivity performance, in the proposed temperature range, but the hydrocarbon selectivity was found to be insensitive to other operating conditions (i.e. pressure and gas composition). It is now possible to better quantify the reactor productivity in the trade-offs which are made with the selectivity performance and the overall plant design configuration which requires recycle of carbon dioxide to the methane reformers to adjust feed gas H2/CO ratio for natural gas applications. The carbon dioxide selectivity for the selected catalyst at the conditions tested was found to be too high for gas-to-liquid (GTL) applications using a natural gas feed.

Includes bibliographical references.