Use of Enhanced CO2 Removal Solvents

The CO₂ formed in the gasification process and the shift conversion process is generally removed by scrubbing with a solvent. Conventional CO₂ removal applies a chemical absorption method using an absorbing liquid. The solvents used in chemical absorption processes are mainly aqueous amine solutions, such as monoethanolamine (MEA), or hot potassium carbonate solutions. The capture of CO₂ with solvents requires mechanical energy to circulate the solvent and heat to regenerate the solution. An almost pure CO₂ stream is recovered (1.3-1.4 t CO₂/t NH₃), which is typically vented or can be used in other (industrial) processes, such as urea production (UNEP, 1998 p.12). The energy consumption of the CO₂ removal system depends on the solvent used, the way the system is incorporated in the ammonia plant, and is affected by syngas purity and CO₂ recovery.
The solvent monoethanolamine (MEA) has been widely used in the ammonia production process, but has some significant drawbacks including the considerable amount of energy required for regeneration. Improved solvents, which require less energy for regeneration of the solution, include the Benfield process (HiPure and LoHeat) and BASF’s two-stage activated diethanolamine (aMDEA) (EFMA, 2000 p.13; UNEP, 1998 p.12).
A potential additional advantage is that the reduction in energy consumption, by using improved solvents in the CO₂ capture process, may allow a reduction in the steam-carbon ratio in the primary reforming section. Lower reboiler duty also allows for an increase in the plant capacity without any modification of equipment (CEAMAG, 2009 (link is external)).
Conventional CO₂ removal applies a chemical absorption method, however, CO₂ can also be removed using physical absorption. Physical absorption processes use an organic solvent, which absorbs CO₂ as a function of the partial pressure. The high CO₂ loadings result in low circulation rates and less utility costs for these type of processes (Kunjunny et al. (link is external), unknown date). Physical absorption solvents typically used in ammonia production processes include glycol dimethylethers (Selexol) and propylene carbonate (IPTS/EC, 2007 p.41; EFMA, 2000 p.13). Regeneration of the solution is performed by vacuum flashing and air stripping and consumes significantly less energy than in chemical absorption.
improved solvents for CO₂ removal can be applied to all new and existing ammonia plants. The type of removal process depends on the lay-out of the ammonia plant and on the requirements posed on the CO₂, e.g. purity (IPTS/EC, 2007 p.75; de Beer et al., 2001 (link is external) p.22).