Three-Phase Flash

Figure 1 shows water content of lean, sweet natural gas. It can be used also for gases that have as much as 10% CO2 and/or HiS if the pressure is below 500psia. Above 500psia, acid gases must be accounted for by rigorous three-phase flash calculations or approximation methods. ... 

Flash Models Two Phase Flash Three Phase Flash General Flash... 

The feasibility of the above setup can be evaluated by simulation with Aspen Plus [19]. The RD column is built-up as a reboiled stripper followed by a condenser and a three-phase flash, with organic phase refluxed to column. The result is that only 3 to 5 reactive equilibrium stages are necessary to achieve over 99% conversion. The stripping zone may be limited at 2-3 stages, while the rectification zone has 1-2 stages. 

Three Phase Flash Calculation under High Pressure Using Continuous Thermodynamics Method+... 

Keywords three phase flash, continuous thermodynamics, high pressure, petroleum fraction... 

Table 1 Three Phase Flash-Calculation Results for a Semicontinuous Mixture at 49 C and 164.8bar...

Schuil, J.A. and Bool, K.K. Three-phase flash and distillation. Computers and Chemical Engineering, 9(3), 295, 1985. 

The predictions of three-phase equilibria considered so far were done as two separate two-phase calculations. Although applicable to the examples here, such a procedure cannot easily be followed in a three-phase flash calculation in which the temperature or pressure of a mixture of two or more components is changed so that three phases are formed. In this case the equilibrium relations and mass balance equations for all three phases must be solved simultaneously to find the compositions of the three coexisting phases. It is left to you (Problem 11.3-7) to develop the algorithm for such a calculation. 

Decomposition in elementary simulation blocks. Example an azeotropic distillation column may be decomposed in reboiled stripping column, heat exchanger, three-phase flash separator and reflux splitter. 

Three-phase flash vapour/liquid (l)/liquid (2). 

These equations are restricted to two equilibrium phases. For a treatment of three-phase flash calculations, see E. J. Henley and E. M. Rosen, Material and Energy Balance Computations, J. Wiley Sons, Inc., 1968, Chapter 8. 

With the reasonable assumption that the phases in a heterogeneous mixture are in phase (physical) equilibrium for a given reactor effluent composition at the temperature and pressure to which the effluent is brought, process simulators can readily estimate the amounts and compositions of the phases in equilibrium by an isothermal (two-phase)-flash calculation, provided that solids are not present. When the possibility of two liquid phases exists, it is necessary to employ a three-phase flash model, rather than the usual two-phase flash model. The three-phase model considers the possibility that a vapor phase may also be present, together with two liquid phases. 

In this case, the vapor is 94 mol% H2, for which a vapor separation section may not be needed. The organic-rich liquid phase (LI) is sent to a liquid separation section to recover a combined methanol and toluene stream for recycle to the reactor, ethylbenzene as a byproduct, and styrene as the main product. The water-rich liquid phase (L2) is sent to another liquid separation section to recover methanol for recycle to the reactor and water, which is sent to wastewater treatment to remove small quantities of soluble organic components. It is important to note that a two-phase flash calculation would produce erroneous results. If in doubt, perform a three-phase flash calculation, rather than a two-phase flash calculation. 

This effluent is cooled to 38°C and enters a flash-decanter vessel at 278 kPa. Three phases leave that vessel. The vapor phase (hydrogen rich) is sent to the vapor separation system. The aqueous phase (mostly water, with some methanol) is sent to the aqueous stream separation system. The organic-rich phase is sent to the organic stream separation system, which you will design. To obtain the composition of the feed to your section, use a simulator with the UNIFAC method to perform a three-phase flash for the above conditions. If the resulting organic liquid stream contains small amounts of hydrogen and water, assume they can be completely removed at no cost before your stream enters your separation section. 

We can now follow exacdy the same steps as were used to derive the Rachford-Rice equation [Eqs. (2z 37 to (2-42) to derive two equations for the three-phase flash. 

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