Total heat content

Binary minimum reflux so calculated implies feed enthalpy just equal to the above started vapor V and liquid L. Any increase or decrease in that enthalpy must be matehed by inerease or decrease in total heat content of overhead reflux. Note that the Underwood" binary reflux equation essentially computes the flash versus specifi-eation composition relationship along with enthalpy correction. 

The total heat content of a body. The sum of sensible heat and latent heat. Entrainment ... 

ALL CHANGES IN PHASE involve a release or absorption of calories. One reason for this is that each solid has its own heat capacity. That is, there is a characteristic heat content for each material which depends upon the atoms composing the solid, the nature of the lattice vibrations within it, and its structure. The total heat content, or enthalpy, of each solid is defined by ... 

The heat absorbed in a process at constant pressure is equal to AH, the increase in the enthalpy of the system. It can thus be said that the heat change accompanying a chemical reaction is equal to the difference between the total heat content of the products and that of the reactants, at constant pressure and temperature conditions. This quantity is called the heat of reaction, AH, and can be expressed as follows... 

Solution The conditions for the inlet steam are fixed, but the conditions of the outlet steam will depend on the performance of the turbine. First, estimate the steam flowrate from the process heating duty. A good approximation is that the sum of the heat content of the superheat and latent heat is constant from inlet to outlet. At the expander inlet, the heat content of the superheat is higher than that of the outlet, but the latent heat is lower in the inlet than in the outlet. The two trends tend to cancel each other out, with the total heat content of superheat and latent heat being approximately constant across the turbine. From steam properties, the enthalpy of the superheated steam HSup, enthalpy of the saturated steam HSat and enthalpy of the saturated condensate Hi at 100 barg are ... 

Therefore, the total heat content, Q, of the crystal can be expressed as the sum of the number of particles populating each level times the energy of that level. 

If an evaporator, fed with steam at 399 K with a total heat of 2714 kJ/kg, is evaporating water at 373 K, then each kilogram of water vapour produced will have a total heat content of 2675 kJ. If this heat is allowed to go to waste, by condensing it in a tubular condenser or by direct contact in a jet condenser for example, such a system makes very poor use of steam. The vapour produced is, however, suitable for passing to the calandria of a similar unit, provided the boiling temperature in the second unit is reduced so that an adequate temperature difference is maintained. This, as discussed in Section 14.2.4, can be effected by applying a vacuum to the second effect in order to reduce the boiling point of the liquor. This is the principle reached in the multiple effect systems which were introduced by Rillieux in about 1830. 

The term specific heat refers to the sensible-heat content of either vapor or liquid. The specific heat is the amount of heat needed to raise the temperature on one pound of the vapor or liquid by 1°F. The term latent heat refers to the heat of vaporization, or the heat of condensation, needed to vaporize or condense one pound of liquid or vapor at constant temperature. Note that the heat of condensation is equal to the heat of vaporization. Each is referred to as the latent heat. The sum of the sensible heat, plus the latent heat, is called the total heat content, or enthalpy. 

Enthalpy—Total heat content the sum of the sensible heat of the air and water vapor and of the latent heat of vaporization. 

After the discovery of the thermometer it became possible to study the interchange of heat between hot and cold bodies. The results of this investigation are as follows AVhen I kg. of water at 100° C. is mixed with an equal quantity of water at 0° C., the resulting mixture assumes a temperature of 50° C. If. 2 kg. of water at 100° C. are mixed with I kg. at 0°, we obtain a temperature of 66f° C., if 9 kg. at 100° are mixed with I kg. at zero, we obtain water at 90°, and so on. It appeared plausible therefore to regard the heat substance, like other material substances, as indestructible, and, therefore, to apply to it a law of conservation. From this it followed that the total heat content of a system would be unchanged by the equalisation of the temperature of its parts, and therefore that the heat w hich is absorbed by the cold bodies in the process of heating up is exactly equal to the heat which is set free by the coohng of the hot bodies. 

Enthalpy and enthalpy changes The total amount of energy a substance contains depends upon many factors, some of which are not totally understood today. Therefore, it s impossible to know the total heat content of a substance. Fortunately, chemists are usually more interested in changes in energy during reactions than in the absolute amounts of energy contained in the reactants and products. 

J. Effect of Temperature on Heat of Reaction The Elirchhoff Equation.— An expression for the variation of the heat of reaction with temperature can be derived in a simple manner. If Ha is the total heat content of... 

The total heat content H of the mixture is thus equal to the sum of individual heat contents of the pure liquid constituents hence, there is no heat change upon mixing the components of an ideal solution. In this respect, therefore, an ideal liquid solution resembles an ideal gas mixture ( 30d). 

For a nonideal solution exhibiting positive deviations fi/ft for each component is greater than its mole fraction N -. It is an experimental fact that as the temperature is increased most liquid solutions tend toward ideal behavior. This means that for a system of given composition for which the deviations from Raoult s law are positive, the ratio / // usually decreases with increasing temperature. According to equation (34.4), therefore, which holds for solution of all types, the numerator Jff — tti must be negative thus Hi is greater than The total heat content of the solution, con-... 

The quantity niSi + nJSa may be taken as the reference value of the total heat content of the solution, so that the left-hand side of equation (44.5) gives the total relative heat content, for which the symbol L is used. It follows, therefore, that... 

The significance of A f for the reaction may be seen in the following manner. The total heat content of the products (right-hand side) in their standard states is 1 + + and this is equivalent to the standard... 

The total heat content of the feed regardless of state is denoted by H. 

As the heat-transfer area varies during the evaporation process, the overall heat-transfer coefficient is best defined in relation to the initial drop area. By calculating the overall resistance to heat transfer directly from the temperature driving force, the total evaporation time and the total heat content of the drop, Sideman, Hirsch, and Gat (SI la) obtained a relationship between the average overall heat-transfer coefficient and the initial diameter. For single pentane drops evaporating in sea water,... 

Enthaipy - A thermodynamic property of a substance, defined as the sum of its internal energy plus the pressure of the substance times its volume, divided by the mechanical equivalent of heat. The total heat content of air the sum of the enthalpies of dry air and water vapor, per unit weight of dry air measured in Btu per pound (or calories per kilogram). 

Enthalpy is a measure of the energy that something contains, and it s defined as the total heat content of a system. In terms of chemical reactions, we are most often interested in the change in enthalpy (denoted H) associated with a reaction. The H for a reaction is defined as the enthalpy of the products minus the enthalpy of the reactants, and this is typically measured via changes in temperature that take place during the reaction. 

A calorimeter does not allow one to find the total heat content (H) of a system in a single measurement, such as one can for other extensive quantities like volume by dilatometry in Sect. 4.1 or mass by thermogravimetry in Sect 4.6. A calorimeter is thus not a total-heat meter. Heat must always be determined in steps as AH, and then summed from a chosen reference temperature. The two common reference temperatures are 0 K and 298.15 K (25°). 

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