Shift Conversion

The process gas exiting the secondary reformer contains 12-15% (dry gas base) of CO. In shift conversion, most of the CO will be converted to CO2. The performance of the shift conversion is very important for the overall efficiency of the ammonia plant, because unconverted CO will consume hydrogen and form CH4 in the methanator, reducing the feedstock efficiency and increasing the inert content in the synthesis loop.

Conventionally, the conversion reaction is conducted in two steps with heat removal steps in between. Initially, the process gas passes through a bed of iron oxide/chromium oxide catalyst at around 350-380oC (High Temperature Shift conversion) and then over a copper oxide/zinc oxide catalyst at around 200-220oC (Low Temperature Shift conversion). The outcome is process gas with a residual CO content of 0.2-0.4% (dry gas base). New developments can employ an isothermal shift one-step conversion. The process gas exiting the low temperature shift converter is cooled and after most of the steam is condensed and removed it passes through the CO2 removal section. Heat released during cooling and condensation can be used for other purposes such as the regeneration of the CO2 scrubbing unit (IPTS/EC, 2007).

The steam consumption in the shift conversion and the CO2 removal processes is about 0.8-1.2 GJ/tonne and the electricity consumption is about 0.2 GJ/tonne of ammonia (56 kWh/tonne) (Worrell et al., 2000 p.21).

Shift ConversionTechnologies & Measures

Technology or MeasureEnergy Savings PotentialCO2 Emission Reduction Potential Based on LiteratureCostsDevelopment Status
Isothermal CO Converter

The investment cost for a facility with a 100,000 tonne yearly production is 500 million yen. The payback period is 0.2 years (A2A Toolkit). 

Commercial
Axial-radial Flow ConverterCommercial
Improved Catalysts for Shift ConversionCommercial
Low Pressure Drop Design for Shift Converter

Not available. 

Commercial
Low Temperature Shift (LTS) Guard with Waste Heat Recovery

Indian flag For a 1 500 tpd plant, the necessary investment is in the range of US $1 million [2005 values] (FAI, 2013).

Commercial
Selectoxo Process for Final Purification

Not available. 

Commercial

Shift Conversion Reference Documents

Reference Document on Best Available Techniques for the Manufacture of Large Volume Inorganic Chemicals - Ammonia, Acids and Fertilisers

Prepared by the Institute for Prospective Technical Studies of European Commision, this document provides detalied information on Best Available Technologies applicable to Ammonia production – as well as on the production of Acids and Fertilizers.  

Page Number: 

41