Modeling and simulation of the transient operation of concentrated absorbers
Abstract
The transfer of solute from a concentrated gas stream causes significant decreases in the flowrate and pressure. In this work, we develop dynamic models for tray and packed absorbers that predict the transient flowrate and pressure during concentrated absorption. The models have separate material balances for each phase and are non-isothermal, account for solvent evaporation, and predict transient hydraulic behavior. We study the aqueous absorption of gaseous HCl and NH3. The equilibrium sieve-tray model predicts large decreases in the gas flowrate during startup which triggers excessive transient weeping. Steady-state weeping is suppressed by solvent evaporation in adiabatic columns with hot liquid feed, but manifests in colder columns. Using two half-steps in succession instead of one large concentration step reduces transient weeping. During the transition to standby operation, the influx of inert gas causes solute desorption and solvent evaporation which result in large spikes in the gas flowrates and pressure. Switching from pure HCl to pure N2 results in a peak exit gas flowrate that is five times the inlet flowrate, causing excessive entrainment. Peak solute loss in the vent gas is five times the acceptable limit. Decreasing the inert flowrate after the step improves transient behavior. The non-equilibrium packed absorber model also predicts large decreases in the gas flowrate during startup for both random and structured packing. However, packed columns are stable at low flowrates and do not weep or dump liquid like tray columns, and therefore, perform better at startup. The model predicts solute accumulation in the bulk gas and delayed solvent evaporation during startup, causing a small spike in the gas compositions. At low mass-transfer rates, the spike causes excessive solute losses in the vent. During the transition to standby operation, the peak gas flowrates are 3–5 times the feed gas rate, and the peak pressure drops are 20–30 times the flooding values. The spikes are brief and may not cause flooding, but mechanical damage is possible and hold-down plates are recommended. Colder columns may fail due to a large solute spike in the vent gas. Smaller steps in the inert gas flowrate greatly improve transient behavior.
Degree
Ph.D.
Advisors
Wankat, Purdue University.
Subject Area
Chemical engineering
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