Condensers enthalpy balances

To obtain a relation between Ln and Lm, it is necessary to make an enthalpy balance over the feed plate, and to consider what happens when the feed enters the column. If the feed is all in the form of liquid at its boiling point, the reflux Lm overflowing to the plate below will be Ln + F. If however the feed is a liquid at a temperature Tf, that is less than the boiling point, some vapour rising from the plate below will condense to provide sufficient heat to bring the feed liquor to the boiling point. 

Prepare the condensing curve, a plot of the vapor temperature Tg against the amount of heat removed Q, by a series of isothermal flashes and enthalpy balances. 

The system equations for the multi-jet condenser are composed of the enthalpy balance and the equation which approximates the performance data in the Chemical Engineering Handbook (8). ... 

In Chapter 1 it was shown that the number of independent variables for any problem is equal to the difference between the total number of variables and the number of linking equations and other relationships. Examples of the application of this formal procedure for determining the number of independent variables in separation process calculations are given by Gilliland and Reed (1942) and Kwauk (1956). For a multistage, multicomponent column, there will be a set of material and enthalpy balance equations and equilibrium relationships for each stage (the MESH equations) and for the reboiler and condenser, for each component. 

Multi-stage separation calculations require heat balances involving transition between the phases. The enthalpy balance equations must, therefore, include heats of vaporization and condensation. Usually these heats are not calculated separately but are implied by using appropriate methods for calculating liquid enthalpies and vapor enthalpies. Consider, for instance, a system where a vapor stream and a liquid stream enter with enthalpies and // respectively, and leave as a mixed phase stream with vapor enthalpy and liquid enthalpy h2. These are total stream enthalpy rates with units such as kJ/h. If no heat is added to or removed from the system, an energy balance is written as... 

The implication of these assumptions is that for each mole of liquid vaporized, one mole of vapor is condensed, so that liquid and vapor molar flows from tray to tray are constant within each section of the column. As in the case of the column section, the component and enthalpy balances reduce to operating line equations, which must be solved in conjunction with the vapor-liquid equilibrium equation or data. Each section within a column is represented by a different operating line. 

To illustrate the formulation of the enthalpy balance functions from the energy balances, the function corresponding to the enclosure of the condenser-accumulator of column 1 is developed. The enthalpy balance enclosing this section of column 1 is given by... 

Enthalpy-balance function for the enclosure of the condenser-accumulator of column 1... 

The enthalpy balances are stated in the form of the constant-composition method in a manner analogous to that demonstrated in Chap. 2 for conventional distillation columns with a finite number of stages. The balance enclosing the condenser-accumulator section may be stated in the form... 

The enthalpy function is formulated by use of the enthalpy balance enclosing the top of the column and the rectifying pinch [see the first expression given by Eq. (11-19)]. This expression for Lr contains the condenser duty Qc. However since dj, D, and V2 are known, the top of the column is uniquely determined at the outset (Note v2i = lu 4- d,), and Qc may be computed by use of Eq. (11-18). Use of Eq. (11-23) permits the enthalpy balance enclosing the rectifying pinch to be restated in the following functional form... 

ENTHALPY BALANCES FOR SINGLE-EFFECT EVAPORATOR. In a single-effect evaporator the latent heat of condensation of the steam is transferred through a heating surface to vaporize water from a boiling solution. Two enthalpy balances are needed, one for the steam and one for the vapor or liquid side. 

Under these assumptions the difference between the enthalpy of the steam and that of the condensate is simply the latent heat of condensation of the steam. The enthalpy balance for the steam side is... 

A dew-point calculation on (y,), gives 26.1°F (—3.3°C) as the enricher top-stage temperature. The condenser duty is determined as 4,779,000 Btu/hr from an enthalpy balance around the partial condenser stage... 

Distillation The permeate from PDMS PV moduleisfed toaplatecolumn.Operating (L/D)=1.5 x(L/D),nin obtained from Fenske s method. Product composition was fixed at 95 wt% EtOH. Gilliland s equation was used to estimate the number of theoretical plates required and capital cost. An enthalpy balance gave the heat load, reboiler, and condenser capital costs. 

For example, in the case of mass balance, pumps can be neglected because they have only one input and one output and their mass balance is a trivial one. Even in the case of enthalpy balance, pumps are often neglected as the shaft work exerted on the pump is often negligible in comparison with the enthalpy changes of streams. Similarly, if only the multicomponent balance is set up around a distillation column, the column proper is usually merged with the condenser, boiler, or other supporting equipment. 

The concentration of vapour at the foot of the tank is estimated by assuming thermodynamic equilibrium. Given total liquid flow rates and air entrainment rates (and the temperatures of both) the fihal temperature and vapour concentration can be calculated straight fonvardly. Examples of results of such a calculation for a winter grade petrol are given in Annex 2. Water vapour condensation should be included in the enthalpy balance but only makes a substantial difference if the humidity and ambient temperatures are high. 

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