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Effects of Nitrogen Co-Feeding in Fischer Tropsch Synthesis At Low CO Conversion

Dr. Adolph Muleja
University of the Witwatersrand
Johannesburg, South Africa

November 11, 2013 at 10:00am
Ben Bandy Conference Center
UK Center for Applied Energy Research

Adolph Muleja1, David Glasser1,2, Diane Hildebrandt2, Christian Lumu3, Kalala Jalama3 and Bhekie Mamba4

  1. Chemical Engineering, University of the Witwatersrand, Johannesburg, South Africa
  2. Material and Process Synthesis, University of South Africa, Johannesburg, South Africa
  3. Faculty of Engineering, University of Johannesburg, Johannesburg, South Africa
  4. Faculty of Science, University of Johannesburg, Johannesburg, South Africa

Fischer-Tropsch (FT) synthesis is a catalyzed chemical reaction in which syngas is converted to liquid fuels over a metal catalyst. Cobalt based FT catalysts have high activity for hydrogenation and tend to produce linear alkanes. Despite being studied for over decades the reaction is still not that well understood. The reaction results are often complex and contradictory. Perhaps the complexity is not just due to the reaction mechanism?

The complexity might be caused by a combination of simple phenomena such as reaction kinetics, vapor-liquid equilibrium, and product accumulation inside the reactor or preferential stripping of lighter products and reactants. The individual phenomena are quite simple; however, the interaction of these phenomena could cause quite complex behavior. Vapor-liquid equilibrium (VLE) modeling shows that the FT reaction may be either kinetically limited or limited by the product stripping rate. The preferential hold-up of heavier products in the reactor affects VLE modeling [1] and the preferential stripping of lighter hydrocarbons is an important part of FT modeling.

To investigate the effect of stripping in the FT reactor we co-fed an inert gas (N2) with the syngas to the reactor. In order to keep the "reaction kinetics" the same in all runs, we kept the molar flow rate of syngas as well partial pressure of the CO and H2 constant while varying the molar flow rate of N2 and the reactor pressure. Two sets of experiments labeled reaction with syngas (RWS) for catalyst testing and reaction with diluted syngas (RWDS) for stripping effect were conducted. Preliminary results showed that diluting syngas with N2 has a positive effect on the activity and selectivity of products even at low CO conversion.