Rate modeling of CO 2 stripping from potassium carbonate promoted by piperazine § Babatunde A. Oyenekan, Gary T. Rochelle * Department of Chemical Engineering, The University of Texas at Austin, Austin, TX 78712, United States 1. Introduction Absorption/stripping with aqueous amines is an important technological option for CO 2 capture from combustion gas. K 2 CO 3 /piperazine (PZ) possesses absorption rates that are one to three times faster than 7 m monoethanolamine (MEA). These solvents also have equivalent or higher capacities than 7 m MEA (Cullinane, 2005). As such these solvents are potential alter- natives to 7 m MEA. Quantitative models based on our under- standing of the vapor–liquid equilibrium and mass transfer rates can provide optimal design of economic processes. In aqueous absorption/stripping (Fig. 1). CO 2 is absorbed into the solvent in a countercurrent contactor. The rich solution leaving the absorber is cross-exchanged with the lean solution from the stripper. CO 2 is removed from the rich solvent in the reboiled stripper. Stripping occurs in three regions (Fig. 2): (a) at the stripper inlet where flashing can occur if the sum of the equilibrium partial pressures of CO 2 and water is greater than the operating pressure of the stripper; (b) within a section of trays or packing due to normal mass transfer; (c) in the reboiler under boiling conditions. Optimal stripper design is critical because the stripping energy requirement accounts for 80% of the operating cost of an absorption/stripping system. Modeling will provide a detailed understanding of the stripper operation and mass transfer with chemical reaction at stripper conditions. Table 1 summarizes previous studies that involve stripper modeling for both gas purification and CO 2 capture. A number of studies include international journal of greenhouse gas control 3 (2009) 121–132 article info Article history: Received 23 February 2007 Received in revised form 19 May 2008 Accepted 5 June 2008 Published on line 24 July 2008 Keywords: Rate-based modeling Stripper Carbon dioxide Simulation Piperazine Potassium carbonate Mass transfer mechanisms abstract This work presents results from a rate-based model of strippers at normal pressure (160 kPa) and vacuum (30 kPa) in Aspen Custom Modeler 1 (ACM) for the desorption of CO 2 from 5 m K + /2.5 m piperazine (PZ). The model solves the material, equilibrium, summation and enthalpy (MESH) equations, the heat and mass transfer rate equations, and computes the reboiler duty and equivalent work for the stripping process. Simulations were performed with IMTP #40 random packing and a temperature approach on the hot side of the cross- exchanger of 5 8C and 10 8C. A ‘‘short and fat’’ stripper requires 7–15% less total equivalent work than a ‘‘tall and skinny’’ one because of the reduced pressure drop. The vacuum and normal pressure strippers require 230 s and 115 s of liquid retention time to get an equiva- lent work 4% greater than the minimum work. Stripping at 30 kPa was controlled by mass transfer with reaction in the boundary layer and diffusion of reactants and products (88% resistance at the rich end and 71% resistance at the lean end). Stripping at 160 kPa was controlled by mass transfer with equilibrium reactions (84% resistance at the rich end and 74% resistance at the lean end) at 80% flood. The typical predicted energy requirement for stripping and compression to 10 MPa to achieve 90% CO 2 removal was 37 kJ/gmol CO 2 . This is about 25% of the net output of a 500 MW power plant with 90% CO 2 removal. # 2008 Elsevier Ltd. All rights reserved. § Any opinions, findings, conclusions, or recommendations expressed herein are those of the authors and do not necessarily reflect the views of the DOE. * Corresponding author. Tel.: +1 512 471 7230; fax: +1 512 475 7824. E-mail address: gtr@che.utexas.edu (G.T. Rochelle). available at www.sciencedirect.com journal homepage: www.elsevier.com/locate/ijggc 1750-5836/$ – see front matter # 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.ijggc.2008.06.010