Simulação e controle de uma coluna de absorção para purificação de biogás: avaliação da viabilidade técnica e econômica do processo

Abstract

In the last few decades, there has being an effort in searching alternative energy sources such as biogas, a gas mixture which presents an improved energetic potential after a process of purification. In order to do so, one of the most commonly used techniques is the hydraulic water wash in absorption columns for carbon dioxide (CO2) and hydrogen sulfide (H2S). However, given that there is a variation in the biogas CO2 concentration, it is important to apply a control system. An example is the PID (Proportional Integral Derivative) controller, which operation depends on parameters (the and constants) that may generally be adjusted through a trial and error process. In order for the adjustment to occur more precisely, there can be used techniques that assist to choose an interval that contain the optimal values for the parameters. The Central Composite Rotatable Design (CCRD) is one of the techniques that could contribute to the control of systems of this nature. Therefore, the purpose of this paper was to model, simulate and control an absorption column process for CO2 removal from biogas, using experimental design techniques in the PID controller tuning, considering the economical feasibility of the system. The economical evaluation was carried out with the Internal Rate of Return method, considering a project 10 years long. The column sizing and project were done through mass balances and criteria obtained from Spellman e Whiting (2004). The column modeling was carried out according to the dynamic model proposed by Maia (1994), whereas the simulation and control were executed with the software MATLAB 2013b. It was applied a feedback controller to the regulatory problem, and its performance was measured using the ITAE (Integral of time-weight absolute error) index. The CCRD carried out was a 23 factorial design, with eight factorial trials, six axial points, plus three replications in the central point, and the real values for the levels were previously defined based upon a preliminary estimate. The absorption column projected has 5 trays, 4,04m of height and diameter of 17cm. It was possible to obtain a quadratic model of the statistically significant variables, which was only . The model was proven valid through the ANOVA test, allowing to generate surface response and response profiles graphs, as well as to obtain a optimal region for the variables. It was carried out a validation test of the optimal conditions found through the model. These optimal values, obtained through the CCRD, kept the system stable even when it was inserted a number of disturbances, therefore proving this method’s efficiency in the controller tuning. The system proved to be economically feasible, with a IRR value of 13%, superior to the Minimum Attractiveness Rate, and a payback time of less than 6 years.