Effects of carbon dioxide hydration kinetics and evaporative convection on pH profile development during interfacial mass transfer of ammonia and carbon dioxide

Abstract

Interfacial mass transfer of $\hbox {NH}_3$ and $\hbox {CO}_2$ are important in processes as diverse as $\hbox {NH}_3$ emission from animal manure and gas scrubbing for removal of carbon dioxide. Predicting transfer rates is complicated by bidirectional interactions between solution pH and emission rates, which may be affected by physical, chemical, and biological processes. We studied the effects of $\hbox {CO}_2$ hydration kinetics and evaporative convection on the development of pH profiles in solutions undergoing simultaneous emission of $\hbox {NH}_3$ and $\hbox {CO}_2$. Profiles of pH were measured at a 0.1 mm resolution over 15 h, and interpreted using a reaction-transport model. Under high humidity, surface pH increased quickly (>0.2 units in 8 min) and an increase gradually extended to deeper depths. An increase in $\hbox {CO}_2$ hydration and carbonic acid dehydration rates by addition of carbonic anhydrase increased the elevation of surface pH and the depth to which an increase extended, due to an increase in $\hbox {CO}_2$ emission. Results show that unless carbonic anhydrase is present, the equilibrium approach typically used for modeling interfacial transport of $\hbox {CO}_2$ and $\hbox {NH}_3$ will be inaccurate. Evaporation and resulting convection greatly increased mass transfer rates below an apparent surface film about 1 mm thick. Emission or absorption of $\hbox {CO}_2$ can produce steep gradients in pH over small distances (<0.5 to >20 mm) in systems with and without convective mixing, and the resulting surface pH, in turn, strongly affects $\hbox {NH}_3$ transfer. Both convection and the rate of hydration/dehydration reactions are likely to affect pH profile development and rates of $\hbox {NH}_3$ and $\hbox {CO}_2$ transfer in many systems. Accurately predicting mass transfer rates for these systems will require an understanding of these processes in the systems.