ABO AKADEMI KEMISK-TEKNISKA FACULTY OF FAKULTETEN CHEMICAL ENGINEERING Forbranningskemiska Combustion Chemistry forskargruppen Research Group REPORT 97-2 Solubility of Flue Gas Components in NaOH Based Scrubber Solutions Kristoffer Sandelin, Rainer Backman m&TBmtm of im document s unumtth) Lemminkainengatan 14-18 B FIN-20520 Abo, Finland Internet: http://rost.abo.fi/ccrg/ ISSN 0785-5052 ISBN 952-12-0003-0 Abo Akademis tryckeri Abo, Finland, 1997 DISCLAIMER Portions of this document may be illegible in electronic image products. Images are produced from the best available original document Preface There is a growing interest for the chemical and the power generation industry to effectively recover possible harmful gases from their effluent streams. It is therefore of interest to predict the solubility of gases into aqueous solutions. The purpose of this study was to develop a general thermodynamic model for calculating the solubility of flue gas components in sodium hydroxide based scrubber solutions. Simultaneous absorption of gaseous SO2, H2SO4 , CO2, and NH3 was of special interest but N20, NO, and H2S were also studied. The work presented here is part of the energy and environmental technology research programme, SIHTI 2. The Finnish Technology Development Centre (Tekes) is gratefully acknowledged for the funding of this work. The authors want to thank TkL Bengt Skrifvars for valuable discussions and comments on the manuscript. Mr. Sture Bostrom is acknowledged for his help with the software. Abo, May 6, 1997 Kristoffer Sandelin Rainer Backman Abstract The work reported here is a thermodynamic study on the solubility of flue gas components in aqueous solutions containing sodium salts. The result of the work is an equilibrium model, based on the expressions developed by Pitzer and co-workers. The model developed here is based on a previous study by Sandelin (1996), where the temperature effect on the solubility of S02, NH3, CO2, H2S, HCN, N2, 02, N20, and NO was investigated. The model presented here includes sodium hydroxide and sodium salts that makes it possible to study simultaneous absorption of flue gas components in alkaline scrubber solutions. The model is in this report applied on the absorption of a flue gas into a NaOH scrubber solution. The calculations show that it is possible to simultaneously absorb sulfur dioxide, sulfuric acid, and ammonia without carbon dioxide co-absorption. The calculations also show that gaseous NO and N20 cannot be scrubbed unless they are oxidized to nitrate or reduced to ammonia. Key words: alkaline, absorption, equilibrium, gases, Pitzer, scrubber, sodium hydroxide solubility, thermodynamic Contents Preface Abstract Contents Introduction 1 Thermodynamic background 2 Method 5 Metastable equilibrium 7 Solubility data for nitrous oxide and nitric oxide 8 Results 12 Application of the model 15 Discussion 19 Conclusions 20 Literature 21 Introduction There is a growing interest for the chemical and the power generation industry to effectively recover possible harmful gases from their effluent streams. Flue-gas cleaning by using aqueous solutions to remove sulfur dioxide is in power plants already established technology, but since the 80’s development has also focused on nitrogen compounds. Special interest of the nitrogen compounds have arisen in the krafr recovery cycle. It has been shown that nitrogen compounds may be released from the krafr recovery process and could be a source of harmful nitrogen emissions. The recovery of weak electrolytes from the effluent streams in the petroleum industries is also of great importance. It is towards that end important to study the vapor-liquid equilibrium of gases in aqueous solutions. Dalton showed in 1801 that the solubility of a gas in a mixture of gases is proportional to the partial pressure of the individual gas, and that the solubility of each gas is nearly independent of the presence of an other gas. The solubility of gases in water is however affected by the addition of other solutes, particularly electrolytes. With an increasing salt concentration, the gas solubility is nearly always decreasing (Wiesenberger and Schumpe, 1996). The extent of the “salting out” effect varies with different salts, but with a given salt, the relative decrease in the solubility of a gas is nearly the same. Thermodynamic studies on vapor-liquid equilibria, in multi-component aqueous solutions has previously been done by many authors. The work by Edwards et al. (1978) gives an extensive and detailed description on the calculation of vapor- liquid equilibrium in aqueous solutions containing one or more volatile electrolytes: ammonia, carbon dioxide, hydrogen sulfide, sulfur dioxide, and hydrogen cyanide. An essentially identical model has been presented by Bautier and Renon (1978). This model in contrast to the model developed by Edwards et al. (1978) also includes ternary data (two-solute systems). Results on simultaneous absorption of ammonia and carbon dioxide in sodium salt solutions has more recently been reported by Kurz et al (1996a ), Kurz et al. (1996b), and Bieling et al. (1995). Practical studies on the chemical equilibria in flue-gas cleaning systems has been reported in Hudson and Rochelle (1982). A study on the topic has also been carried out by Luckas et al. (1994). Buzek and Jaschik (1995) have studied the partial pressure of sulfur dioxide over aqueous sodium -bisulfite, -sulfite and -sulfate solutions. In their work they give a semi empirical formula for calculating this system. The absorption of SO2 into NazSC^ and H2SO4 solutions has also been studied thoroughly by Hunger et al. (1990). There has also arisen an interest in predicting absorption of hydrogen sulfide in aqueous multi-component solutions. A study on the absorption of H2S in aqueous solutions including organic basis (triethanolamine, metyldiethanolamine, and monoethanolamine) has previously been reported in Li and Mather (1996). The focus of this study is to develop a general thermodynamic model that describes the solubility of flue gas components in sodium based scrubber solutions. Chemical equilibrium 1 has in this study been calculated based on the non stoichiometric formulation (Smith and Missen, 1982). The benefit of the non stoichiometric approach is, in contrast to the stoichiometric approach, that no constants for the specific chemical reactions are needed. Thermodynamic background Chemical equilibrium in a system is given by the minimum of the total Gibbs free energy. For a pure species, the free energy, is dependent on the pressure and the temperature. It is usually determined from standard enthalpy (AFFWk), entropy (S°298°k ) and the heat capacity (C° p or C°v) as a function of the temperature and/or pressure. The total Gibbs free energy is the sum of Gibbs energies for all species in the system. The minimization of Gibbs free energy is therefore done for all species in all phases. The minimization problem may be written as: Minimize G = (1) / where ju, is the chemical potential of species i and n, is the number of moles of species i. In order to carry out the minimization one must have an expression for the chemical potential. The expression for the chemical potential is given as: A =ju° +RTln(ai) (2) where //° is the standard chemical potential for species i. R is the gas constant and T is the absolute temperature, a, is the activity of species i. The activity of a single species is affected by the relative amount (concentration) and the composition (other solutes) due to interactions between the species of the system. For aqueous solutions the activity of a species is expressed as the product of the activity coefficient, y, and an the molality, m . 4 = (3) There are many models for predicting the activity coefficients in aqueous solutions. The cornerstone for the more resent models: Bromley ’s- (1973) Chen’s- (1982), Davie ’s- (1938 and 1962), Guggenheim’s- (1935 and 1955), Meissner’s- (1972), Pitzer’s- (1973), and Sander ’s (1984) equations is the Debye-Huckel equation (Debye and Htickel 1923a and 1923b). A model based on the equations by Pitzer (1973) as referred to by Harvie et al. (1984) have been used in this study. Consequently, the total Gibbs free energy is not only dependent on the temperature and pressure but also on the chemical composition. Determination of the composition of a multiphase multi-component system is practically done with an computer program using a 2 suitable algorithm for the minimizing process (Eriksson, 1975). The computer program ChemSage (Eriksson and Elack, 1990) was used in this study. Gibbs free energy for a species can be calculated from the enthalpy and the entropy at a given temperature and pressure. At a constant pressure Gibbs free energy is given by: 7%; (4) where H°T is the enthalpy and S°T the entropy at the temperature T. Enthalpy and entropy values are in thermodynamic handbooks and databases given as standard enthalpies and entropies at 298K and 1 bar pressure. In order to calculate the enthalpy and entropy at an other temperature than 298K we need to know the heat capacity function, since the enthalpy and entropy is calculated from: + (5) $t = ^298 + J (6) 298 * The simplest method for calculating thermodynamic properties at other temperatures than 298K is to assume constant heat capacity. The heat capacity for aqueous species is however not found in most thermodynamic handbooks even at 298K (25°C). The heat capacity can however be calculated from the entropy. In 1964, Criss and Cobble proposed the principle of such a method (Criss and Cobble, 1964a and 1964b). The method has been successful in predicting the standard molal heat capacities for many aqueous species when experimental data is not available. The model gives however a rather poor fit for many experimental solubilities. In addition the Criss and Cobble method does not consider the temperature dependency of uncharged aqueous species. The heat capacity of aqueous species can also be predicted by the Helgeson-Kirkham-Flowers model, HKF-model (Helgeson et al., 1981).