Enthalpy change composite process

Solution In this example, it is assumed that we add a solute to a large enough volume of solution so that the composition of the mixture does not change. The enthalpy change for this process is referred to as a differential enthalpy of solution. We can represent this process by... 

Each temperature is a fixed value on the vertical axis, and enthalpy change rates are relative quantities. We estimate the enthalpy changes rather than absolute enthalpies, and the horizontal location of a composite line on the diagram is arbitrarily fixed. The location of ATmm on the composite diagram is where the hot and cold curves most closely approach each other in temperature in a vertical direction. We move one of the two curves horizontally until the distance of the closest vertical approach matches the selected ATmin. The overshoot of the hot composite curve represents the minimum cold utility (qc mm) required, and the overshoot of the cold composite curve represents the minimum hot utility (gh mm) required for the process. 

Measured values of A/f for desolvation derived from DSC are dependent on sample particle size and heating rate. These factors have been discussed elsewhere [61]. The enthalpy change is a composite quantity taking into account all of the steps involved in the desolvation mechanism. The latter has been thoroughly reviewed by Bym et al. [62] who considered the role of atmosphere, crystal packing and crystal defects in the desolvation process, as well as the nature of the final product. Desolvation of a crystalline solvate may involve a major or a minor change in crystal structure, or produce an amorphous form. This is assessed using PXRD,... 

Suppose we heat a sample of the solid compound to its melting point to form a liquid mixture of the same composition as the solid. The molar enthalpy change of the fusion process is the molar enthalpy of fusion of the solid compound, Af sH, a positive quantity. When the liquid has the same composition as the solid, the dissolution and fusion processes are identical under these conditions, A o H is equal to AfusT/ and is positive. 

Throughout the preceding discussion, simplified expressions of stream-specific enthalpy as a function of temperature are used. They have to be updated during process operation to consider changes in steady-state compositions. 

Thus we have established that the specific (of a unit mass) enthalpy is constant throughout the space occupied by the gas phase, from x = 0 to x — oo near x = 0 the temperature, composition and density of the gas vary rapidly due to transport processes and the chemical reaction. Our result establishes a relation between changes in the temperature and composition in the gas. At the same time we have clarified the assumptions under which this relation holds. 

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