Estudo da dessorção de dióxido de carbono em colunas de pratos.

Abstract

The desorption process of carbon dioxide (CO2) in an aqueous solution of monoethanolamine (MEA) in a tray column was studied. The tests were carried out using a 30 % mass aqueous solution of carbonated MEA. The desorber consisted of an 8 cm diameter glass column with 5 trays, each containing a single bubble cap, spaced 6 cm apart and with a 2 cm weir. The reboiler consisted of an electrical glass flask heating mantle, and the condenser was a coil-type heat exchanger using cooling water. The column was instrumented and the temperatures of the reboiler, each tray, and condenser were measured online. Moreover, the concentrations of total CO2 (dissolved CO2, MEACOO -, HCO3and CO3 2-) and total MEA (free MEA, MEAH + , and MEACOO -), determined by potentiometric titration, were measured in the trays and in the reboiler over time.Initially, the desorption process was studied in a semi-batch process under total liquid reflux, with only wet CO2 as an outlet. The temperatures of the five trays were close throughout the test (95 °C to 96 °C). The beginning of the process was characterized by higher desorption of CO2 and evaporation of the solvent. At the end of the process, the two trays closest to the reboiler had a small amount of MEA (< 3 %), and all trays contained water (> 95 %) and CO2.In the continuous operation of the process, the feed was carried out in the second tray from the top and withdrawal in the reboiler, at the average feed rates of 1.26 L/h and 0.55 L/h. Temperatures and total CO2 concentration reached steady state. The first tray and distillate contained mostly water (≳ 98 %) and the remaining trays had a concentration of total MEA and total CO2 lower than the feed, with the total MEA concentration increasing and total CO2 concentration decreasing as they approached the reboiler.The experimental process was simulated using Aspen Plus v10, making it possible to reproduce the same amount of CO2 desorbed and the temperatures, with small deviations, throughout the column and underestimates of CO2 concentrations in the column trays. The effects of different process variables on CO2 desorption were evaluated via simulations. The amount of gas captured was favored by the increase in the number of stages, feeding in stages closest to the top of the column, high pressures, and low reflux ratio. These same variables also contributed to lower specific energy consumption for solvent regeneration, except for the reflux ratio, which increased energy consumption.Exergy analysis of the desorption process was also performed using Aspen Plus to calculate the physical and mixture exergy and a spreadsheet to calculate the chemical exergy of the streams.The desorption column is responsible for more than 35 % of the exergy destroyed in the capture process, and the thermal loads associated with the condenser and reboiler are responsible for about 70 % of this exergy destruction. The trays contribute only 5 % of the total exergy destroyed. The exergy destruction was greater in columns with fewer stages, feeds made at the bottom...