Recent studies at Lehigh University have demonstrated the possibility of generating useful energy from acid-base neutralization reactions involving association of hydrogen and hydroxyl ions. Many industrial processes and natural biogeochemical process (e.g., acid mine drainage) produce acidic (or basic) wastewater streams and require neutralization prior to disposal. The acid-base reaction is thermodynamically very favorable with generation of significant amount of thermal energy. Since the waste acid solution undergoing treatment is often very dilute, the significant amount of thermal energy generated in the neutralization reaction causes a miniscule increase in the temperature of the bulk aqueous phase. As a result, any energy recovery is nearly impossible and has not been reported to date. For this project, it is proposed that it is possible to recover significant amount of useful energy by carrying out the neutralization reaction inside an appropriate pH-sensitive hydrophilic polymer (or biopolymer) containing covalently attached weak-acid or weak-base functional groups. Scientifically, the phenomenon of Donnan osmosis drives the process leading to energy generation and no external membrane is necessary. When contacted with acid or base, the variation in pH causes the functional groups to reversibly acquire and lose its ionic character. This change gives rise to on-off pattern of generation of osmotic pressure inside the polymer phase causing it to reversibly swell (osmosis) and shrink (deosmosis) due to the movement of water in and out of the polymer phase. Thus, acid-base neutralization is accompanied by generation of usable mechanical energy. It is noteworthy that the process is completely carbon neutral and no greenhouse gas is produced. The proposed process has the potential to generate energy globally from waste acid and waste alkali streams using reusable pH-sensitive polymers. The global annual production of sulfuric acid is estimated to be approximately 200 million tons. Assuming 1% of this acid appears in the waste stream that is neutralized by the proposed energy recovery process, reduction in carbon dioxide emission in comparison with a plant producing the same amount of energy with fossil fuel will be over 60 million tons.
OVERVIEW: WASTE DISPOSAL AND ENERGY CREATION Acid and alkali are used in most industrial processes. To protect the environment, the effluent from plants is highly regulated depending on the size and type of effluent. To meet pH requirements, waste acid neutralization is the most common industrial pollution control process practiced globally. The acid neutralization reaction creates lots of heat and proceeds very quickly. According to the data available in 2010, the global sulfuric acid production capacity was over 200 million tonnes annually and that of phosphoric acid was about 40 million tonnes. If only 1% of this acid appears in the waste stream to be neutralized, there is an annual energy generation potential of 1100 x 106 kWh. When harnessed, this energy will produce much work. But, the waste acid solutions undergoing treatment are dilute and the significant amount of heat generated in the reaction doesn’t cause a significant increase in the solution temperature. As a result, any energy recovery from waste acid was not been reported to date prior to our work. We wanted to make use of this energy by intelligently exploiting scientific principles. The two things we carefully investigated were generating mechanical energy and desalinating water. Based on our research carried out under this project, the following findings are significant: 1) Mechanical work was created through waste acid neutralization by using polymer beads, e.g., weak acid cation exchange resins. 2) Brackish water was desalinated to permissible drinking standards concurrently with waste acid neutralization by intelligent arrangement of polymer membranes, e.g., ion exchange membranes. These findings have been disseminated through two peer-reviewed journal publications: 1) German et al., "Hydrogen Ion (H+) in Waste Acid as a Driver for Environmentally Sustainable Processes: Opportunities and Challenges". ES&T-Feature Article. January 2013. 2) Sarkar et al., "Energy Recovery from Acid-Base Neutralization Process through pH-Sensitive Polymeric Ion Exchangers". I&ECR. September 2011. OPPORTUNITIES Making Energy Ion exchange resins are small polymers (plastics) that are filled with charged functional groups. These fixed charges repel each other, like magnets, and expand the polymer to create more space between the charges. Weak ion exchange resins can have their charged turned "on" and "off" by passing weak acid or weak base solutions through the resins. When the charge is "off", the resin returns to its normal smaller size. Desalinating Water Electrodialysis (ED) is a widely practiced brackish water desalination process that uses electricity for separating salts from water by cation and anion exchange membranes. Continuous supply of electricity is the primary energy requirement for ED. Conceptually, the process of desalination by ED- removal of sodium and chloride from the feed solution – can be achieved by replacing the entire DC power supply with waste acid and a neutralizing base. From the middle of the feed chamber, sodium and chloride were continuously dissipated through ion exchange membranes in exchange for acid from the waste acid chamber and bases from the base chamber, respectively. Neutralization in the middle was very fast and pushed the desalination near to completion. The capacity of such a desalination system could be conveniently expanded or scaled up by repeating multiple stacks with a similar arrangement of cation and anion exchange membranes. RESULTS Waste to Energy Weak cation exchange resin beads can swell to 200% of their original volume after a short period of time by adding high pH solutions, i.e., waste alkali. This swelling is reversible by passing low pH solutions, i.e., waste acid. This expansion-contraction can be cycled by controlling the flow of acids and bases, similar in nature to conventional engines. To visually demonstrate this, a balloon with a weight on top was inflated and deflated while passing base and acid through the resins. This small-scale system can be envisioned as a large scale motor moving pistons. Filtering Water Replacing electrodes in ED systems with chambers of acid and alkali solutions created an acid-base driven desalination system. The speed and extent of desalination was highly dependent on the initial quality or waste concentration in the "waste acid". Solutions at waste acid pH levels were effective at lowering brackish water salinity from 5000ppm to 1000 ppm. Low cost, readily available waste citric acid showed great promise for desalination. FUTURE RESEARCH FOR COMMERCIALIZATION Real waste acids will require efficient pre-treatment; novel salt exclusion processes are under review. Efficiency in energy production was limited by material performance during initial demonstrations because the off-the-shelf polymers weren’t optimized for the new applications. The aim of future funds is to move waste pre-treatment and polymer performance to the stage for commercial energy production and treated water production.
