Large Volume Inorganic Chemicals - Ammonia ... - ammk-rks.net

Chapter 22.4.14 Isothermal shift conversionDescriptionLower temperatures favour the strong exothermic shift conversion. Therefore, heat has to beremoved to achieve a low residual CO concentration. In conventional plants, this is performedin two stages: high temperature shift conversion (330 – 440 °C) and low temperature shiftconversion (200 – 250 °C), using different types of catalysts. The gas is cooled down betweenthe two steps.Alternatively, a single step system can be used to the two-step approach. Here, the conversion isperformed isothermally by continuously removing the heat from the catalyst bed, by usingcooling tubes. In this way, the need for a conventional high temperature shift catalyst containingchromium is avoided. As the isothermal shift occurs without a chromium-based catalyst, theFisher-Tropsch reaction does not take place in the shift reactor, thus allowing a lower steam tocarbon ratio. The Fischer-Tropsch synthesis is the reaction of hydrogen with carbon monoxide,carbon dioxide or mixtures of these carbon oxides (synthesis) to yield one or more carboncompounds, e.g. hydrocarbons, alcohols, esters, acids, ketones and aldehydes.Achieved environmental benefits• energy saving• no necessity for the disposal of a chromium spent catalyst• startup time of the one-step isothermal shift is reduced compared to the conventional twostepshift systems, resulting in lower emissions.Cross-media effectsNone believed likely.Operational dataNo information provided.ApplicabilityThis integrated technique is mainly applicable in new autothermal heat exchange reformingplants.EconomicsNo information provided.Driving force for implementationEnergy saving.References to literature and example plants[3, European Commission, 1997]78 Large Volume Inorganic Chemicals – Ammonia, Acids and Fertilisers