Flash calculations defined

Since K-vslue is defined as the ratio of mol fraction of a component in the vapor to mol fraction of that component in the equilibrium liquid, when K-values arc applied in a conventional equilibrium flash calculation the condition which will give the greatest proportion of a component in the liquid phase is the condition where its K-value is lowest. 

K Values, VLE, and Flash Calculations A measure of the distribution of a chemical species between liquid and vapor phases is the K value, defined as the equilibrium ratio ... 

The computer-aided procedure, unless automated by the program, requires running a series of liquid-liquid equilibrium calculations (the equivalent of vapor-hquid flash calculations) at constant temperature and pressure. The composition is varied around the equilibrium curve, and the transition points from one phase to two, or vice versa, are noted. As many points as needed are obtained this way to generate the entire equilibrium curve. Also, each time an equilibrium calculation is done in the two-phase region, the compositions of the two phases are recorded. Each pair of data points thus obtained defines a tie line. The data obtained at one temperature and pressure generate one triangular diagram. If so desired, the procedure is repeated at other temperatures and pressures to determine the effect of these variables. 

To find fhe equilibrium-phase compositions, it is convenient to form the distribution coefficient and perform flash calculations similar to the vie calculations described in Section 4.4.2. For liquid-liquid equilibrium (lie), the distribution coefficient is defined as... 

An example of the application of abstract interfaces can be found in the Flash module. Figure 2 shows the class layout for the Flash engine , an interface to a flash calculation. The Flash engine takes two defined phases (equation of state objects) plus a database that provides properties for the components of the phase. 

One of the most powerful aspects of the HCToolkit is its extensibility in Perl. This allows rapid development of new applications for specific tasks. For example, an included extension quickly calculates and plots a pressure-temperature phase diagram at user-specified conditions (PT-PhaseDiagram.pl, found in the /apps subdirectory of the distribution tree). In fact, a scripting application that does some sort of equilibrium calculation needs only do four things create a database connection, use that connection to load a pre-defined fluid, initialize the flash object and do the flash calculation. These steps are shown in Figure 4. 

But when the same g -model is used to obtain y and yf, numerical procedures for solving (10.1.6) converge erratically, if at all. We therefore seek a procedure that is more reliable than a direct attack on (10.1.6). For example, note that if the system temperature and pressure are known (as they usually are for LLE situations), then the problem can be posed as an analogy to isothermal flash calculations. In such an approach, we take the known quantities to be T, P, and the set of overall system mole fractions z. These last are defined by... 

The motivation for posing the LLE problem in this way is that it allows us to take advantage of the Rachford-Rice procedure [11], which is a robust algorithm traditionally applied to isothermal flash calculations. To develop that procedure, we introduce a distribution coefficient Q for each component this quantity is defined by... 

The liquid and vapor flow rates in the stripping section (Ls and Vs) can be calculated if the thermal condition of the feed is known. Since the temperature, pressure, and composition of the feed are given, the fraction of the feed that is liquid can be calculated from an isothermal flash calculation. This fraction is defined as the variable q" Knowing q, the liquid and vapor flow rates in the stripping section can be calculated. If the feed is saturated liquid, is 1. If the feed is saturated vapor, q is 0. 

The flash is the most widely used unit operation block in process simulation. A flash means that a stream is transformed to a certain equilibrium state defined by two of the variables P, T, enthalpy h, or vapor fraction q. There are several kinds of equipment which can be represented by a flash calculation. 

The results of the high pressure separator flash calculations are tabulated in Table III. For ease in comparing results, K-values predicted by the Hudson correlation as defined by Table I are used as a common basis. The range of K-values and amount of liquid knock-down is summarized as follows ... 

Often in plant operations condensate at high pressures are let down to lower pressures. In such situations some low-pressure flash steam is produced, and the low-pressure condensate is either sent to a power plant or is cascaded to a lower pressure level. The following analysis solves the mass and heat balances that describe such a system, and can be used as an approximate calculation procedure. Refer to Figure 2 for a simplified view of the system and the basis for developing the mass and energy balances. We consider the condensate to be at pressure Pj and temperature tj, from whence it is let down to pressure 2. The saturation temperature at pressure Pj is tj. The vapor flow is defined as V Ibs/hr, and the condensate quality is defined as L Ibs/hr. The mass balance derived from Figure 2 is ... 

The optical absorption spectra of sulfonyl radicals have been measured by using modulation spectroscopy s, flash photolysis and pulse radiolysis s techniques. These spectra show broad absorption bands in the 280-600 nm region, with well-defined maxima at ca. 340 nm. All the available data are summarized in Table 3. Multiple Scattering X, calculations s successfully reproduce the experimental UV-visible spectra of MeSO 2 and PhSO 2 radicals, indicating that the most important transition observed in this region is due to transfer of electrons from the lone pair orbitals of the oxygen atoms to... 

Figure 2. Non-steady state variation of Ot pressure with time during flashing of a stable polycrystalline Pd substrate. Initial OJCO = 1.5. The origin of the ordinate is defined as the Ot pressure at 300 K. Key O, observed 02 V, calculated on the...

Pantelides [27, 28], in his work with Sargent, defined the index in a manner that exposes its potential to cause problems in initialization as well as in the integration error. They too showed that the index problem can be eliminated by differentiation. Noting that only some of the equations need to be differentiated, they use a method based on the structural properties of the equations to discover these equations. They cite several examples in which the index problem is almost certain to occur in setting up and solving dynamic simulation models, e.g., calculations of flash dynamics and problems in which the trajectory of a state variable is specified. 

In this method, a mixed A -value is defined as the ratio of the mole fraction of a component in the vapor to its mole fraction in the mixed liquid phase (Schuil and Bool, 1985). Applied to an equilibrium stage or a flash drum, the phase equilibrium is solved using the mixed /f-valucs instead of the usual vapor-liquid X-values to determine the flow rates and compositions of the vapor and the total liquid. The liquid phase split is then calculated on the basis of A -values for each liquid phase to determine the flow rates and compositions of the two liquid phases. An energy balance may also be included to determine the temperature or the heat transfer for the unit. 

The stream defined below is heated to 100°C to be partially vaporized in a flash drum before entering a distillation column. The fraction vaporized is controlled by the flash drum pressure. Calculate the required pressure at 100°C to have 20% mole vaporization, assuming Raoult s law applies. What are the products flow rates and compositions The constants for the Antoine Equation 2.19 are given for each component, with the pressure in kPa and the temperature in K. 

To begin the calculations the column variables must be first initialized to some estimated values. Simple methods can be used for this purpose, based on the column specifications and possibly supplemented by shortcut methods. The column temperature profile may be assumed linear, interpolated between estimated condenser and reboiler temperatures. The values for Lj and Vj may be based on estimated reflux ratio and product rates, assisted by the assumption of constant internal flows within each column section. The compositions Xj- and T, may be assumed uniform throughout the column, set equal to the compositions of the liquid and vapor obtained by flashing the combined feeds at average column temperature and pressure. The other variables to be initialized are Rf,Rj, and Sj, which are calculated from their defining equations. The values for Qj may either be fixed at given values (zero on most stages) or estimated. 

Flash points of mixtures of oxygenated and hydrocarbon solvents cannot be predicted simply. A computer based method is proposed which exhibits satisfactory prediction of such Tag Open Cup flash points. Individual solvent flash point indexes are defined as an inverse function of the component s heat of combustion and vapor pressure at its flash point. Mixture flash points are then computed by trial and error as the temperature at which the sum of weighted component indexes equals 1.0. Solution nonidealities are accounted for by component activity coefficients calculated by a multicomponent extension of the Van Laar equations. Flash points predicted by the proposed method are compared with experimental data for 60 solvent mixtures. Confidence limits of 95% for differences between experimental and predicted flash points are +8.0-+3.0°F. 

The flash point of a multicomponent system cannot be determined by summing a simple fraction of the flash points of the individual components. If, however, the component flash points were defined by some temperature dependent property which might then be calculated for a multicomponent system, the flash point of that system might be predicted. 

As a first approach to utilizing this flashing index calculation technique, component flashing indexes were defined as the reciprocal of the component vapor pressure at the flash point. Mixture flash points were then computed by trial-and-error as the temperature at which... 

In flash separations, a mixture of known overall composition is brought into the two-phase region and the resulting liquid and vapor are separated into two streams whose composition is given by the tie line that corresponds to the temperature and pressure in the separator. Calculations are streamlined by making use of the K factors. These were defined in Chanter 8 as K, =yi/Xi, and in the case of ideal solution are given by... 

Aspen HYSYS used the concept of the fluid package to contain all necessary information for performing flash and physical property calculations. This approach allows you to define all information (property package, components, hypothetical components, interaction parameters, reactions, tabular data, etc.) inside a single entity. 

A. Define. We want to calculate y, Xj, V, and L for the equilibrium flash chamber shown in the diagram. 

Various degrees of effort can be applied in process simulation. A simple split balance can give a first overview of the process without introducing any physical relationships into the calculation. The user just defines split factors to decide which way the particular components take. In a medium level of complexity, shortcut methods are used to characterize the various process operations. The rigorous simulation with its full complexity can be considered as the most common case. The particular unit operations (reactors, columns, heat exchangers, flash vessels, compressors, valves, pumps, etc.) are represented with their correct physical background and a model for the thermophysical properties. 

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