A simplified and general approach to absorption and stripping with parallel streams

Abstract

Abstract This work presents a simplified and general approach for calculating the number of ideal stages and the corresponding concentrations in absorption and stripping with parallel streams. Vapor (gas)/liquid separation processes with parallel streams involve the division of either the vapor phase or the liquid phase in a selected and feasible number of sub-streams and their contact, in alternate trays, with the entire stream of the other phase, i.e. the liquid or vapor phases, respectively. The division of the vapor phase (para-absorption or stripping) allows the possible allocation of a larger number of trays in the same column height and can be an alternative for processes conducted under low pressure and/or involving thermosensitive compounds. On the other hand, the partition of the liquid phase (meta-absorption or stripping) permits a better liquid phase flowing, especially in the case of large liquid loads. The mass balance equations are formulated for mixtures with a unique component being transferred and for two or more partitions of the vapor or liquid phases. Based on the obtained operation lines, the proposed approach allows the calculation of the number of ideal stages for para- and meta-absorption/stripping with any integer number of divisions of the corresponding stream. Furthermore, the additional assumption of a linear equilibrium behavior makes possible the formulation of recurrence equations representing the change of the vapor (para-separation) or liquid (meta-separation) concentrations along the column. In fact, a linear β th -order difference equation, with β corresponding to the number of divisions of the selected stream, can be formulated in each case and the exact solutions for β equal to 2, 3 and 4 are presented. The approach was tested in the calculation of para- and meta-absorption/stripping columns of two different mixtures of interest for bio-refineries, namely those mixtures related to the recovery of bioethanol lost during sugar fermentation and to the desorption of free fatty acids from vegetable oils. Based on the calculated results, the advantages of absorption/stripping with parallel streams are discussed. The new approach can be used as a reliable shortcut procedure in the optimization and intensification of complex chemical processes involving absorption and/or stripping steps, as well as a learning tool for teaching the design of downstream equipment.