(57e) Process Modeling and Optimization of a Novel Membrane-Assisted Chilled Ammonia Process for CO2 Capture

Recent focus on climate change and clean energy has led to a large research focus into the area of post-combustion carbon capture. According to the Intergovernmental Panel on Climate Change, fossil fuels are a leading cause on climate change¹. Monoethanolamine (MEA) has been the standard of solvents used for carbon capture but there has been recent interest in ammonia. Ammonia is an attractive substitute due to its low energy requirements for regeneration, high CO₂ loading capabilities, and its resistance to oxidative and thermal degradation². One drawback of ammonia is the solvent loss due to the high vapor pressure of ammonia. Two methods, implemented separately or concurrently, are currently being employed to reduce absorber slip for the ammonia system; operating the absorber at low temperatures as done in what is known as the chilled ammonia process (CAP) to lower ammonia vapor pressure, and adding a wash system to remove ammonia from the flue gas before it is released to the atmosphere. This work focuses on developing a full process model of the CAP coupled with a water wash section to reduce the ammonia loss. To reduce the energy penalty in the stripper for the water-wash section as well as CO₂ regenerator, a novel membrane-based water separation technique is investigated.

The electrolyte NRTL model is used to calculate liquid properties and PC-SAFT equation of state to calculate vapor properties. A rate-based model of the towers with two-film characterization and true components is developed. Model validation for the CAP process has been done in the literature but is relatively sparse. Due to the extreme non-ideality of these systems, parameter regression can greatly improve the performance of these models. Simultaneous regression of mass transfer coefficients, interfacial area, diffusivity, and reaction kinetics are carried out by using wetted wall column (WWC) and pilot plant data together. This results in a model that is valid over a large range of operating conditions and equipment sizes. Furthermore, a model of a reverse osmosis membrane is developed to be used in the water wash and CO₂ desorber sections. The membrane model is developed for multi-component separation following the solution diffusion mechanism. Finally, the capture system is optimized to minimize the annuitized cost by considering tradeoffs between the capital cost of the membranes and the energy benefits that are achieved by using them.

REFERENCES:

1 IPCC, 2014: Climate Change 2014: Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp.

2 Puxty,G., Rowland, R., Attalla, M., Comparison of the rate of CO2 absorption into aqueous ammonia and monoethanolamine. Chemical Engineering Science. 2010; 65, 915-922