Energy Recovery from Acid–Base Neutralization Process through pH-Sensitive Polymeric Ion Exchangers

Abstract

Globally speaking, waste acid neutralization is a widely used industrial pollution control process. In principle, acidÀbase neutralization is a thermodynamically favorable reaction that involves the association of hydrogen and hydroxyl ions to form neutral water molecules with the generation of a significant amount of thermal energy (ΔH = À55.84 kJ/mol). However, it is not possible to recover the energy, because the increase in temperature of the bulk solution phase is often miniscule, because of the diluted nature of the waste acid solution and the high specific heat of water. Here, we demonstrate a methodology to recover a significant amount of useful energy by carrying out the neutralization reaction inside a pH-sensitive hydrophilic polymer (or biopolymer) phase containing covalently attached weak-acid or weak-base functional groups. When contacted with an acid or a base, the variation of pH causes the functional groups to reversibly acquire and lose its ionic character. This gives rise to an onÀoff pattern regarding the generation of osmotic pressure inside the polymer phase, causing it to reversibly swell and shrink, because of the movement of water into and out of the polymer phase. Laboratory experiments validate the reversibility of the swellingÀshrinking pattern of a commercial weak-acid ion-exchange resin for multiple numbers of cycles with a concomitant generation of mechanical energy during the neutralization reaction. The process can be enhanced by tailoring the cross-linking and hydrophilicity of the polymer phase. The energy generated is free of carbon emission.

Salinity Gradient Energy from Expansion and Contraction of Poly(allylamine hydrochloride) Hydrogels

“…The recovered energy ( E ) was calculated from the work conducted by lifting the external load W 1 and the work related to the upward movement of the gravity center of the hydrogel W 2 : , where g 0 (9.81 m/s 2 ) is the gravitational constant, h 1 is the displacement of the plunger calculated based on the expanding volume and the diameter of the syringe ( D ), h 2 (= h 1 /2) is the displacement of the center of gravity of the hydrogel, m 1 is the external load, and m 2 is the average weight of the swollen and deswollen hydrogels calculated according to volumes, assuming a hydrogel density ( d ) in water of 1000 kg/m 3 .…”

“…Recovered Energy. The recovered energy (E) was calculated from the work conducted by lifting the external load W 1 and the work related to the upward movement of the gravity center of the hydrogel W 2 : 17,31…”

Energy Harvesting by Waste Acid/Base Neutralization via Bipolar Membrane Reverse Electrodialysis

“…EMF dC HCl (11) and it is~24 kWh m −3 at HCl and NaOH concentrations equal to 1 M. At a current density of 29 A m −2 the efficiencies were estimated to be~19% and~24% either with or without the background salt, respectively. This 5% decrease in the efficiency when salt is added is mainly due to the reduction of the membrane selectivity, as shown by the reduction of OCV reported in Section 3.3.…”

“…For this purpose, several approaches have been proposed over the years. A solution consisted in harvesting mechanical energy by carrying out the neutralization reaction, with waste acid or base, inside an ion-exchange polymer causing a reversible swelling and shrinking of the polymer phase because of the water transport inwards and outwards [11][12][13]. Another approach involved the energy storage derived by the proton insertion/de-insertion cycle in a so-called neutralization pseudocapacitor, where energy storage was derived from the partial entropy change related to the difference in the acid concentration of the electrolytic solution [14].…”

Preparation and properties of modified magnesium oxysulfate cement derived from waste sulfuric acid

“…Consequently, hundreds of thousands of tonnes of residual free sulfuric acid solutions are produced annually (Fa, 1999;Liu, 2002;Sarkar et al, 2011). Without proper treatment and recycling, this overproduction of sulfuric acid solutions could result in environmental problems and waste of resources.…”