Page 444 - Standard Handbook Petroleum Natural Gas Engineering VOLUME2

406 Production

(to assure that the mixture is in two-phase region) cannot be conveniently used
in most computer equation-of-state-based flash calculations because the K values
for a given system are not known until the final solution has been reached.
Consequently, the flash calculation (and it convergence algorithm) must be
capable of performing "flash calculation" on single phase systems (subcooled
liquids, superheated vapors and dense gas systems) as well as reliably predicting
the amount of vapor and liquid present in a two-phase system. When the above
flash equation/convergence algorithm is used on single phase systems, the final
predicted value of L/F will usually be outside the interval described by Equa-
tion 6-25 unless the material balance constraint is enforced. Should a value of
(L/F)"+' outside the limits defined by Equation 6-25 be detected in an inter-
action, we recommend that the value of L/F predicted by Equation 6-24a be
replaced by the appropriate value described by the following equations:
if (L/F),+' < 0.0, (l/F)"+l = (l/F)"/2.0

or
if (L/F)"*l 1.0, (L/F)'"+l = [l + (L/F)"I/2.0

This procedure eliminates most of the problems associated with flash calcula-
tions in single-phase regions and yields excellent results in relatively few
iterations inside the two-phase region. Some problems still occur when attempt-
ing flash calculations in the dense gas regions.
Initial estimates of the phase composition must be made to initiate the flash
calculation. Several procedures are available. It was found that a combination
of the bubble point/dew point initial phase estimation procedures works quite
well [12]. Set the vapor phase mole fraction of the highest component in the
system to 1.0 and the liquid phase mole fraction of the heaviest component in
the system at 1.0. All other mole fractions are set to loa. This procedure is
believed to be superior to the technique of basing the initial assumption of the
phase composition on some noncomposition dependent K value estimation
procedure, particularly when a wide range of temperatures, pressures, component
types, composition ranges, etc., is to be considered.
The estimated vapor and liquid phase compositions must be compared with
the calculated phase compositions. Equations 6-19 and 6-23 describe this
checking procedure. If the restraints described by these equations for any
component (in either phase) are not satisfied, the calculations must be repeated
even though an acceptable value for L/F has been determined. Some feel that
this detailed checking procedure is unnecessary. It probably is unnecessary for
most problems involving moderate temperature-pressure-composition conditions.
However, at extreme conditions of temperature, pressure and composition (low-
temperatures, high-pressure, high-acid-gas compositions) failure to perform these
composition checks will lead to results that are completely incorrect (poor
estimates of the phase compositions and incorrect L/F ratios). Unfortunately,
the boundary changes in temperature, pressure of composition can completely
alter the difficulty of a given problem. Consequently, carehl application of these
checks in all calculations is strongly recommended since one can never be sure
that a particular problem will not fall into the area of extreme conditions.
A logic diagram illustrating the basic flash calculation is shown in Figure 6-28.
All the necessary features described earlier are embodied in this diagram.
Flash calculations at fixed L/F and temperature or pressure are frequently
necessary. In these calculations, the dependent variable becomes pressure or