Flash drum mass balance

From the mass balance around the flash drum, we have rh6 =rhl -m3 =168.15kg/s... 

As in the illustration, we choose the system for writing balance equations to be the subsystem consisting of the heat exchanger, throttle valve and flash drum (though other choices could be made). The mass and energy balances for this subsystem (since there are no heat losses to the outside or any work flows) are... 

As is true in the design of many separation techniques, the choice of specified design variables controls the choice of the design method. For the flash chamber, we can use either a sequential solution method or a simultaneous solution method. In the sequential procedure, we solve the mass balances and equilibrium relationships first and then solve the energy balances and enthalpy equations. In the simultaneous solution method, all equations must be solved at the same time. In both cases, we solve for flow rates, compositions, and temperatures before we size the flash drum. 

In the sequential solution procedure, we first solve the mass balance and equilibrium relationships, and then we solve the energy balance and enthalpy equations. In other words, the two sets of equations are uncoupled. The sequential solution procedure is applicable when the last degree of freedom is used to specify a variable that relates to the conditions in the flash drum Possible choices are ... 

Except for sizing the flash drum, which is covered later, this conpletes the sequential procedure. Note that the advantages of this procedure are that mass and energy balances are uncoupled and can be solved independenfly. Thus trial and error is not required. 

For binary flash distillation, the simultaneous procedure can be conveniently carried out on an enthalpy-composition diagram First calculate the feed enthalpy, hp, from Eq. t2-81 or Eq. (2=9b) then plot the feed point as shown on Figure 2-9 (see Problem 2-All. In the flash drum the feed separates into liquid and vapor in equilibrium Thus the isotherm through the feed point, which must be the T nun isotherm, gives the correct values for x and y. The flow rates, L and V, can be determined from the mass balances, Eqs. f2-51 and 2-61. or from a graphical mass balance. 

The heat and mass balances for the flash drum can be applied to determine the amount of flash steam based on the inlet and outlet conditions. 

E) Calculate the flow rate of stream 8 and the flow rate of stream 7. You may use the lever rule and/or mass balances. Note that the flow rate of stream 4 may be changed by the addition of the second flash drum. 

These calculations combine vapor-liquid equilibrium relationships with total mass and component balances. Material of known composition Zj is fed into a flash drum at a known rate of F mols/min. Both the temperature and the pressure in the drum are given. Variables that are unknown are liquid and vapor compositions and liquid and vapor flow rates. See Figure 2.12. 

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