Coupling between Mass Transfer and Chemical Reactions during the Absorption of CO2 in a NaHCO3-Na2CO3 Brine : Experimental and Theoretical Study

Abstract

A key step of the refined sodium bicarbonate (BIR ) production by the Solvay process consists in the gas-liquid mass transfer of carbonic gas CO 2 from a gaseous mixture of CO 2 and air to a sodium carbonate Na 2 CO 3 and bicarbonate NaHCO 3 brine. This transfer takes place in 20-m high and 2.5-m wide bubble columns (the BIR columns). This large size induces a large contact time between gas and liquid, in order to increase the amount of transferred CO 2 . Nevertheless, the gas phase leaving the columns contains an important quantity of CO 2 (only half of the CO 2 is transferred). It causes a huge CO 2 emission to the atmosphere (the equivalent of 150 Smart driving at 100 km/h, for a single column). More, the CO 2 is produced by lime calcination. This process requires a large amount of energy. It represents the major part of the energy consumption to produce BIR. In the past decades, several optimizations of the BIR process were performed by an empiric approach. There are today some limits to this approach for applications requiring high levels of purity and a well-defined granulometry. Moreover, the ecologic pressure becomes more and more constraining for the greenhouse effect gas emission. Accordingly, Solvay is seeking for a more fundamental approach. Currently, several studies are realized in the chemical engineering department at the ULB. The final goal is to create a complete model of a BIR bubble column, taking into account all the phenomena taking place in it. This model will be used in order to optimize the CO 2 transfer rate in the column. As a direct consequence of this optimization, the CO 2 emission and the energy consumption of the BIR production will hopefully be reduced.The objective of the work presented in this paper is to develop a model of the mean CO 2 flux density, expressed by unit of time and interfacial area, in a BIR bubble column, that will be integrated in the future complete model of a BIR column. No solid phase is considered in this paper.The CO 2 transfer rate from a bubble of gas to the brine is controlled by the physico-chemical phenomena occurring in the thin layer of liquid near the interface. After the CO 2 absorption in this liquid layer, the CO 2 is transferred by diffusion and is involved in several chemical reactions. These reactions modify (accelerate) the gas-liquid mass transfer rate.The liquid phase flow around the bubbles, rising up in the columns, influences the mass transfer rate too. The scale of this phenomenon is different from the physico-chemical phenomena. A multiscale approach is then followed.A mathematical modelling of the coupling between mass transfer and chemical reaction is first developed. The equations of the model are solved numerically, using COMSOL Multiphysics. In order to validate experimentally this model, a Mach-Zehnder interferometer is set up. The scale-up of this model to a bubble, in a second time, is carried out by completing the model of the coupling with a representation of the liquid phase flow around a bubble. It is called the bubble-li...

Density functional theory studies on the electronic, structural, phonon dynamical and thermo-stability properties of bicarbonates MHCO3, M = Li, Na, K

“…The structural transition of KHCO 3 from an ordered P2 1 /a structure to a disordered C2/m structure at 340 K has been observed [22]. Lee and Kim [31] observed thermal decomposition of KHCO 3 to K 2 CO 3 around 459 K. Wylock et al investigated carbonic gas absorption in a brine of NaHCO 3 and Na 2 CO 3 and found that the CO 2 absorption is coupled with several chemical reactions taking place in the liquid phase [32]. It has been noted that MHCO 3 , M = Li, Na, K, can exist in a molten ternary eutectic mixture at high temperature (>560 • C) [33].…”

Dynamics of a Reactive Thin Film

“…including additional reactants, which lead to improved efficiency. For example [20] included additional reactions involving sodium carbonate and bicarbonate, but the emphasis of this study was on buoyancy instabilities. …”

Compartmental Modeling of an Industrial Column

“…For the conditions in the BIR columns, the reaction (3) is very fast compared to the reaction (2) and the pH is close to 10, in the entire columns. It has been demonstrated in a previous work (Wylock et al, 2008a) that, in the BIR columns, the CO 2 consumption rate in the liquid phase can be accurately modeled by the following pseudo first order expression: The Higbie approach is used. In this approach, the liquid phase in the vicinity of a bubble-liquid interface is seen as a mosaic of liquid elements continuously renewed (Coulson and Rirchardson, 1999).…”

“…An accurate estimation is then required. Experimental studies are currently realized, using a Mach-Zehnder interferometer (Wylock et al, 2008a), to validate the correlations. The method is not further developed here.…”

“…The bubble-liquid CO 2 transfer rate is quantified by an expression validated for the BIR columns by Wylock et al (2008a) and presented in section 2.3. The solid phase is modeled by a classical Population Balance Equation (PBE) (Randolph and Larson, 1971).…”