In the presence of a sink, a gas stream with high solute concentration can undergo large decreases in pressure and flow rate because of the transfer of solute. This behavior has been observed during the loading of concentrated gas adsorbers. In this paper we introduce a dynamic tray model capable of predicting gas-phase transients during the start-up of concentrated absorbers. The model accounts for gas holdup and pressure, solvent evaporation, weeping, entrainment, and downcomer holdup. The main assumptions in the model are equilibrium trays, well-mixed gas and liquid phases, and ideal gas behavior. Simulation results for the absorption of concentrated HCl gas in water and concentrated NH3 gas in water are discussed. Predictions with the model reveal severe flow and pressure transients capable of inducing excessive weeping. An alternate start-up procedure to reduce transient weeping is discussed.

A dynamic sieve-tray model is used to investigate the effect of suddenly placing a concentrated absorber on standby by replacing the feed gas with inert gas. Model predictions show large desorption rates during the transient, resulting in a dramatic increase in gas flow rates, which leads to extremely high entrainment. The model also predicts a large pulse of solute in the vent gas and a large rise in the downcomer liquid level. The results indicate a strong possibility of design failure during the transient. We discuss methods to reduce the severity of the transient behavior.

A dynamic, nonisothermal, nonequilibrium model is developed to predict detailed transient behavior during startup and transition to standby for concentrated random-packed absorbers. The model accounts for solvent evaporation and transient hydraulic behavior and is used to simulate the aqueous absorption of gaseous HCl. During startup, the simulations show a large decrease in the gas flow rate followed by a recovery due to high rates of solvent evaporation. The model predicts a spike in the solute composition in the gas during the startup transient. At low mass-transfer rates, this spike can cause unacceptable solute levels in the vent. During the transition to standby operation (where feed gas is replaced with carrier gas), there is an appreciable increase in the gas flow rate and pressure in the column. The peak transient gas flow rates are about 3 times greater than the feed rate, and the peak pressure drops are about 20 times greater than those at flooding. These spikes may be too brief to flood the column, but mechanical damage is possible. The transient also shows solute being desorbed from the bottom part of the column and re-absorbed in the upper part of the column, which causes a burp of solute in the vent gas. In less concentrated columns, the burp can result in unacceptable solute levels in the vent. These problems can be addressed by initially reducing the carrier gas flow rate. The model also applies to the use of scrubbers for the control of toxic vent emissions.