Thermodynamic Process Assessment

Adiabatic flame temperatures agree with values measured by optical techniques, when the combustion is essentially complete and when losses are known to be relatively small. Calculated temperatures and gas compositions are thus extremely useful and essential for assessing the combustion process and predicting the effects of variations in process parameters (4). Advances in computational techniques have made flame temperature and equifibrium gas composition calculations, and the prediction of thermodynamic properties, routine for any fuel-oxidizer system for which the enthalpies and heats of formation are available or can be estimated. 

The process we have followed Is Identical with the one we used previously for the uranium/oxygen (U/0) system (1-2) and Is summarized by the procedure that Is shown In Figure 1. Thermodynamic functions for the gas-phase molecules were obtained previously (3) from experimental spectroscopic data and estimates of molecular parameters. The functions for the condensed phase have been calculated from an assessment of the available data, Including the heat capacity as a function of temperature (4). The oxygen potential Is found from extension Into the liquid phase of a model that was derived for the solid phase. Thus, we have all the Information needed to apply the procedure outlined In Figure 1. 

However, making an even small step to qualitative assessment of availability of active particles on the surface under regular thermodynamic conditions is difficult. This is especially difficult if we are faced with the problem of quantitative evaluation of particles origin and role in specific heterogeneous processes. 

Assessments of the sustainability of processes and systems, and efforts to improve sustainability, should be based in part upon thermodynamic principles, and especially the insights revealed through exergy analysis. 

To prevent or at least minimize such problems, we must better understand the environment at all levels, including the fundamental chemical processes that affect it. We have learned the lesson that when assessing the fate of new products in the environment, we should not underestimate the potential of these to appear in unexpected places. The recognition, avoidance, or solution of complex environmental problems requires the expertise of a variety of science and engineering disciplines. Only then will it be possible to produce realistic evaluations of how new compounds will be distributed and will act in the ecosystem. In addition to chemistry and biochemistry, fields such as solution thermodynamics and transport phenomena in which many chemical engineers work, as well as earth sciences and environmental engineering, have crucial contributions to make. 

In most circumstances, it can be assumed that the gas-solid reaction proceeds more rapidly than the gaseous transport, and therefore that local equilibrium exists between the solid and gaseous components at the source and sink. This implies that the extent and direction of the transport reaction at each end of the temperature gradient may be assessed solely from thermodynamic data, and that the rate of transport across the interface between the gas and the solid phases, at both reactant and product sites, is not rate-determining. Transport of the gaseous species between the source of atoms and the sink where deposition takes place is the rate-determining process. 

As for many other materials, phase diagrams are roadmaps not only for the description of these substances and of their reactions, but also for their processing and for research and development planning. The systematic experimental determination of phase diagrams, their assessment and compilation and their thermodynamic optimization and calculation are the essential steps in the development of materials science and make up one of the bases of the intermetallic disciplines. 

Non-statistical successive binding of O2 and CO to the four heme centers of hemoglobin ( cooperativity ) has been thoroughly documented. It is difficult to test for a similar effect for NO since the equilibrium constants are very large ( 10 M ) and therefore difficult to measure accurately. It is found that the four successive formation rate constants for binding NO to hemoglobin are identical. In contrast, the rate constant for dissociation of the first NO from Hb(NO)4 is at least 80 times less than that for removal of NO from the singly bound entity Hb(NO). This demonstrates cooperativity for the system, and shows that it resides in the dissociation process. The thermodynamic implications of any kinetic data should therefore always be assessed. 

It should be clear that any partially direct approach (e.g., the use of predetermined lists of compounds) is not consistent with the complete and comprehensive requirements of either the direct or the phased environmental assessment philosophy and therefore is not an alternative to either approach. Similarly, a priori judgments based on process chemistry, thermodynamics, etc. are not acceptable practices in this context. 

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