Modeling Reactive Absorption

Reactive absorption — i.e., the absorption of gases in liquid solutions accompanied by chemical reactions — is an important industrial operation for the production of basic chemicals (e.g., sulfuric or nitric acid) and for the removal of harmful substances (e.g., H2S) from gas streams. In recent decades, this process has become especially important for the purification of gases to high purities. Unlike physical absorption (without reactions), reactive absorption is able to provide high throughput at moderate partial pressures and without requiring large amounts of solvent. The advantages of combining chemical reactions with absorption are realized only in the region of low gas-phase concentrations due to the liquid-load limitations imposed by the reaction stoichiometry or equilibrium. Other factors that may limit the efficiency of reactive absorption are the heat liberated by exothermal reactions and the difficulty of solvent regeneration. Most reactive absorption processes are steady-state operations involving reactions in the liquid phase, although some applications involve both liquidphase and gas-phase reactions. Reactive absorption is a complex rate-controlled process that occurs far from thermodynamic equilibrium. Therefore, the equilibrium concept is often insufficient to describe it, and instead, accurate and reliable models involving the process kinetics (rate-based models) are required. The effectiveness of online model-based applications, such as process control and optimization, depends strongly on the quality of the available model predictions. Equilibrium stage model Modeling and design of reactive absorption processes are usually based on the equilibrium stage model, which assumes that each gas stream leaving a tray or a packing segment (stage) is in thermodynamic equilibrium with the corresponding liquid stream leaving the same tray or segment. For reactive absorption, the chemical reaction must also be taken into account. With very fast reactions, the reactive separation process can be satisfactorily described assuming reaction equilibrium. A proper modeling approach is based on the nonreactive equilibrium stage model, which is extended by simultaneously considering the chemical equilibrium relationship and the tray or stage efficiency. If the reaction rate is slower than the mass-transfer rate, the influence of the reaction kinetics increases and becomes a dominating factor. This tendency is taken into account by integrating the reaction kinetics into the mass and energy balances. This approach is widely used today. In real reactive absorption processes, thermodynamic equilibrium is seldom reached. Therefore, correlation parameters such as tray efficiencies or height equivalent to a theoretical stage (HETS) values are introduced to adjust the equilibrium-based theoretical description to real column conditions. However, reactive absorption occurs in multicomponent mixtures, for which this simplified concept often fails (1). The acceleration of mass transfer due to chemical reacReactions and Separations