Flash at Fixed Enthalpy and Pressure

The problem will be formulated for a specified final pressure and enthalpy, and under the assumption that the enthalpies are additive (that is, with zero enthalpy of mixing) and are known functions of temperature at the given pressure. The enthalpy balance is 

Similarly, when Eq. (13.27) represents the effect of pressure, the bubblepoint pressure is found with the N-R algorithm  

At temperatures and pressures between those of the bubblepoint and dewpoint, a mixture of two phases exists whose amounts and compositions depend on the conditions that are imposed on the system. The most common sets of such conditions are fixed T and P, or fixed H and P, or fixed S and P. Fixed T and P will be considered first. 

On combining these equations and introducing j8 = V/F, the fraction vaporized, the flash condition becomes 

The temperature or pressure at which a vapor of known composition first begins to condense is given by solution of the appropriate equation, 

After /8 has been found by successive approximation, the phase compositions are obtained with 

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Single-Stage Flash Calculations 375 Bubblepoint Temperature and Pressure 376 Dewpoint Temperature and Pressure 377 Flash at Fixed Temperature and Pressure 377 Flash at Fixed Enthalpy and Pressure 377 Equilibria with Ks Dependent on Composition 377... 

The main variables associated with phase relationships include the overall composition, Z , temperature, pressure, liquid composition, X , vapor composition, F, vapor mole fraction, /, and heat transferred, Q. A process in which Z, and two other independent variables are set, and equilibrium separation of the phases is allowed to take place, is called a flash operation. A general flash operation is shown in Figure 2.4. A feed stream initially at conditions T, and P, is controlled so that its final conditions satisfy two specifications. The feed is of fixed rate and composition, F and Z . A heat duty, Q, may be added to or removed from the system as required. The feed is flashed to generate a vapor product with flow rate Ft r and a liquid product with flow rate F(1 -1 /), where / is the vapor mole fraction at flash conditions and P. In general, tj/ may be equal to zero or one or any value in between. The enthalpies of the vapor and liquid products are H2 and /Z2> respectively. The type of flash operation... 

The specified variables are the final temperature and pressure, T2 and P2- The dependent variables are the vapor fraction, t /, the liquid and vapor compositions, X, and the total enthalpy of the two phases, /Z2 + H, and the heat duty, Q. The term isothermal should not be interpreted to imply that the transition from initial conditions to final conditions is at constant temperature is, in general, different from T. It simply means that within the flash drum the temperature, as well as the pressure, is fixed. The heat duty required to bring about the final conditions is equal to the enthalpy change, Q = (Hj + 2) - i> where is the enthalpy at and P,. Isothermal flash conditions may be represented by a point ( 2, P2) on tbs phase envelope diagram. It is clearly possible that this point may fall either within the phase envelope or outside it, in which case the system would be all vapor or all liquid (or dense phase). A flash drum operating at such conditions would have a single product and no phase separation would take place. In a single-phase situation, the dependent variables are the properties of the vapor or liquid product. The liquid or vapor composition is, of course, identical to the feed or overall composition, Z,. Note that any set of temperature and pressure specifications is feasible. 

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