Thermodynamics sour-water

The conference was subdivided into four sessions, and chapters within this text are arranged according to these categories. Papers included in the first section (Thermodynamics of Electrolytes for Pollution Control) provide the reader with insights into the practical aspects of pollution control, as well as an overall appreciation of applied electrolyte phase equilibria. Other chapters include detailed descriptions of thermodynamic models that recently have been developed to describe important industrial pollution control processes with emphasis on acid gas absorption/sour water stripping and flue gas desulfurization. 

As an introduction to the technical aspects of the conference, the results of some studies conducted by the writer on two relevant subjects are presented below. The first commentary is concerned with the design of sour-water strippers and the effects of thermodynamic data on these designs the second commentary is concerned with the calculation of enthalpies of steam-containing mixtures, essential to the design of coal processing and related plants. 

Until recently the ability to predict the vapor-liquid equilibrium of electrolyte systems was limited and only empirical or approximate methods using experimental data, such as that by Van Krevelen (7) for the ammonia-hydrogen sulfide-water system, were used to design sour water strippers. Recently several advances in the prediction and correlation of thermodynamic properties of electrolyte systems have been published by Pitzer (5), Meissner (4), and Bromley ). Edwards, Newman, and Prausnitz (2) established a similar framework for weak electrolyte systems. 

The phase equilibrium equations for the interface may also need to be modified for the influence of additional species on the thermodynamic properties at the interface. A case in point is sour water stripping, in which reactions in the liquid phase create additional species (including ions), which affect the interfadal equilibrium. Enhancement factors have been derived for many cases and there is no sin-... 

The term speciation is used to describe the reactions that take place when an electrolyte is dissolved in water. Water dissociates, sour gases hydrolyze, some ions dissociate, and other ions associate until thermodynamic equilibrium is attained. The liquid phase of the ternary H2O-NH3-CO2 system contains at least the following nine species HjO, NH3(aq), COjiaq), H", OH, NH4, HCOj, COj , and NHjCOO. (aq) indicates that the species is in aqueous solution to avoid ambiguity. In order to adequately model this system, interaction parameters for the interaction between each pair of species need to be determined thus, speciation calculations are performed simultaneously with the parameter estimation, and the calculated amount of each species is compared with experimental data. Some models also require ternary parameters and consequently an additional amount of data to determine these parameters. 

The formaldehyde + water + methanol system is just one example for many technically important systems, which are intrinsically chemically reactive. Other examples include aqueous solutions of weak electrolytes, such as amine solutions used to scrub carbon dioxide and other sour gases from gaseous streams, or the solutions containing bases used for chemical extraction of acids from aqueous streams. In many of these cases, the key to the development of predictive thermodynamic models is a quantitative model of the often complex chemical reactions in those mixtures. The necessary information often only can be obtained using spectroscopic methods [25]. 

Modern refineries deal with a multitude of complex systems that may require different thermodynamic models for each refinery plant and its associated process model. For example, we cannot model the sour gas units that deal with acid gases and water with the same thermodynamic model that we use for the crude fractionation system. In fact, reasonable thermodynamic models form the heart of any process model. Chen et al. [7] have documented the variety of thermodynamic models available for frequently encoxmtered chemical and physical systems. Agarwal et. al [18] present a detailed account about the pitfalls of choosing a poor thermodynamic system for process models and the undesired consequences of using these poor models to modify plant operations. Process model developers and users must be aware of the underlying thermodynamics and its limitations. 

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