Solvent evaporation enthalpy

The hydration enthalpy of the Al3+ ion is enormous (-4690k) mol-1), and there are some interesting effects produced as a result. When NaCl is dissolved in water and the solvent evaporated, the solid NaCl can be recovered. If A1C13 is dissolved in water, evaporation of the water does not yield the solid A1C13. The Al3+ ion is so strongly solvated that other reactions become energetically more favorable than removing the solvent. This can be shown as follows. 

The high thermal conductivity, the high specific heat capacity, and the high evaporation enthalpy of water make it suitable as solvent and heat removing fluid... 

In Equations 2.9 and 2.10, the enthalpy of evaporation used is the specific enthalpy of evaporation, expressed in kjkg 1. These expressions only give the amount of solvent evaporated, which is a static parameter. They give no information about the vapor flow rate, which is related to the dynamics of the process, that is, the reaction rate (see Section 9.4). This aspect is discussed in the chapter on technical aspects of reactor safety. 

Since a solvent is only an auxiliary medium, which has to be removed after the separation step, it needs to feature a low boiling point in order to avoid thermal degradation, the formation of off-flavours and the loss of top notes. Low values of the evaporation enthalpy and specific heat, physical properties which determine the energy consumption during solvent recovery are of similar importance. 

The ideal solvent should not be flammable or at least should have a high flash point and the narrowest possible explosion interval of mixtures with air. This again is contrary to the requirement for boiling points and evaporation enthalpy. Combustible solvents not only require additional flame and explosion proofing, but bear the imminent risk of hazardous reactions if safety guidelines are not strictly observed. 

Adsorption is an exothermic process. The adsorption enthalpy, decreases as the load of adsorbed molecules increases. In activated carbon adsorption systems for solvent recovery, the liberated adsorption enthalpy normally amounts to 1.5 times the evaporation enthalpy at the standard working capacities which can result in a 20 K or more temperature increase. In the process, exothermic adsorption mechanisms may coincide with endothermic desorption mechanisms. ... 

A detailed comparison of ejqierimental techniques and the equipment used to evaluate evaporation enthalpy can be found elsewhere. Values of solubihty parameters of solvents are presented elsewhere. " ... 

Volatility Solvent volatility helps in estimation of the solvent evaporation rate at temperatures below its boiling point The Knudsen, Henry, Cox, Antoine, and Clausius-Clapeyron equations are used to estimate the vapor pressure of a solvent over a liquid, its evaporation rate, and the composition of the atmosphere over the solvent. The boiling point of a solvent gives an indication of its evaporation rate but it is insufficient for its accurate estimation because of the influence of the molar enthalpy of evaporation. 

For measurements 2-5 mg of the sample were weighed into small aluminum pans. In case of lyotropic liquid crystals, pans with a wall thickness of 0.25 pm were used, instead of the conventionally used pans with a wall thickness of 0.1 pm. This ensured that the pans do not blow up in the unfavorable event of an increasing pressure due to solvent evaporation. The sealed pan as well as a second empty pan which served as reference, were then placed into two separate microfurnaces. The furnaces were heated with a constant heat rate while the temperatures of sample and reference were measured with two independent thermo-sensors. If a phase transition occurs in the sample, a temperature difference arises between sample and reference due to the transformation enthalpy. This temperature difference was compensated by an increased heating power in the colder sample chamber. The difference in heating power was recorded as measurement signal versus time. As time and temperature are related to each other via the heat rate, the resulting thermogram was depicted as heat flow versus tempaature. 

Figure 10.4 State of solvent in processes shown in Figure 10.1 and 10.2. Extraction is with marginally subcritical CO2 at 25°C, 65 bar (point A). Paths ABCDFA and AbcdFA are for solvent evaporation at 60 and 50 bar respectively points B and b represent feed entering the separator while C and c represent the outlet. D and d are the compressor outlet conditions. E gives the outlet from exchanger H following 50 bar evaporation and F is a point during condensation. QABR, VCW and XDZ are lines of constant enthalpy...

DSC instruments measure the heat flow into a sample as the temperature is ramped, in comparison to a reference standard. The melting temperature and enthalpy of fusion are quantified. The technique is not suitable for a significant proportion of pharmaceutical compounds because they decompose at the same time as melting. In solvates and hydrates the solvent will evaporate prior to melting which also limits the methods value. Sample size is typically 10 mg. 

---