Thermodynamic Assessment of Ion Exchange Technology for Phosphorus (P) Recovery from Waste: Entropy Generation (Sgen) as Sustainability Indicator

Developing circular economy will require recovering valuable products from waste streams or converting waste into value added products which necessitates developing new technologies. To ensure sustainability of circular... [ view full abstract ]

Developing circular economy will require recovering valuable products from waste streams or converting waste into value added products which necessitates developing new technologies. To ensure sustainability of circular economy, both economic and environmental impact assessment of emerging technologies is crucial before adoption at industrial scale. Thermodynamic efficiency is one of the key environmental criterion for sustainable technologies to ensure that the energy required for production is minimized. Many thermodynamic methods/metrics such as energy-analysis, cumulative exergy-consumption analysis (CExC), emergy analysis (Odum 1996, Bakshi 2004) and entropy-generation (Sgen) are being used to measure sustainability (Bakshi, Gutowski & Sekulic 2011). While energy and exergy analysis are already quite established in their own terms these do not provide insights into the theoretical minimum energy required for the process which can be captured by Sgen. However, Sgen has not been widely used and requires more research to interpret and apply for industrial adoption.  

In this work, we focus on insights provided by Sgen for thermodynamic performance of an emerging technology for Phosphorus (P) recovery from waste. Agricultural, urban and industrial waste contains a large amount of nutrients such as P. Since, P is a critical element for agricultural production and reserves are limited (Cordell et al, 2009), it is crucial to implement P recovery technologies at large scale to increase the recovery yield substantially. Some of the established industrial scale technologies are AirPrex, Phosnix and, Crystalactor with recovery efficiencies 98%, 90%, and 75% respectively and with daily recovery capacities of 2500, 550, and 4500 Kgs respectively. All these established technologies recover P as struvite and/or calcium phosphate and depend extensively on large quantities of chemicals for processes like acid leaching and precipitation (Nieminen, 2010). Recently an Ion-Exchange technology has been proposed that is independent of large quantities of additional chemicals; potentially being more sustainable relative to current technologies (Petruzzelli et al, 2003). However, ion-exchange may result in higher energy requirements, for which we use Sgen as the metric.Based on the Gouy-Stodala theorem, Sgen can be used to quantify the minimum work required for overall functioning of a process that generates the required nutrient recovery. Thus, using Sgen as a thermodynamic metric can help in identifying the most sustainable technologies (least energy requirement) for scale up among several opportunities. 

Our approach involves a rigorous analysis of each of the sub-processes in the ion-exchange based nutrient recovery system using the process modeling approach to quantify total Sgen. The proposed model relies on two unit operations: Ion-exchange and anaerobic-digestion. ASPEN custom modeling was used since there are no standard unit operations representing these processes in ASPEN plus. Results show that both these unit operations involve an entropy generation of 34.5 kCal/K and-478 kCal/mole-K respectively. These values will be compared with a standard energy analysis for the system to demonstrate the advantage of using Sgen as a thermodynamic sustainability metric.