Flash Evaporation

General Description

This operation is used to simulate a single-stage flash process.

Unit Procedure Availability

●      Flash Evaporation Procedure

Flash Evaporation: Modeling Calculations

A flash is a single-equilibrium-stage distillation in which a feed is partially vaporized (or condensed) to give a vapor richer (or a liquid poorer) than the feed in the more volatile components. A flash separation is based on the principle that when two or more phases are in thermodynamic equilibrium, the pressure, temperature and partial fugacity of a component is the same in each phase while its composition is different.

Material and Energy Balances

The single-stage flash process is described by the following mathematical problem. A feed stream of known conditions (F moles at pressure P_F temperature T_F and component molar fractions z_i) is allowed to flash into a vapor stream, V, and a liquid stream, L, with or without the addition (or removal) of energy Q.

Given that thermodynamic equilibrium imposes equal vapor and liquid phase temperature T and pressure P, the following equations apply for the single-stage flash of the figure above.

where V, L are the molar flowrates of the vapor and liquid stream and y_i, x_i denote the molar fraction of component i in the vapor and liquid stream, respectively. The phase equilibrium ratios, K_i, can be evaluated by one of the following thermodynamic models:

●      Raoult’s Law

●      Hybrid Raoult’s law/Custom vapor fraction model

●      Modified Raoult’s Law

●      Equation of State (EOS)

●      Gamma-Phi Model

For a detailed description of the available options for the calculation of the K-value of a mixture please see Phase Equilibrium Ratio (K-Value) in Phase Equilibrium Ratio (K-Value).

For more details on the fundamental theory and numerical solution of the flash problem, see The Single-Stage Flash in Appendix D .

Vessel Sizing

In Design Mode, the program calculates the volume of the flash drum by the following equation:

where L the molar flowrate of the liquid stream, τ_r is the residence time and β is the working-to-vessel volume ratio.

In Rating Mode, the user specifies the vessel volume, the number of units and (from the Volumes tab) either the residence time, τ_r, or the working-to-vessel volume ratio, β. When the τ_r is specified, the program calculates β by eq. (A.262) volume ratio by equations and validates that the calculated value is between the allowable range. Similarly, when β is specified the program calculates the residence time, τ_r.

If this unit operates in batch mode, the feed flowrate F is calculated by dividing the volume of material that needs to be processed per cycle by the process time.

Equipment Purchase Cost

Equipment purchase cost is based on the total vessel volume and material of construction.

References

1.   Seader J.D., Henley E.J. and Roper D.K., Separation Process Principles. Chemical and Biochemical Operations, John Wiley & Sons (2010).

Flash Evaporation: Interface

The interface of this operation has the following tabs:

●      Oper. Cond’s, see Flash Evaporation: Oper. Conds Tab

●      Volumes, see Continuous Vessel Operations (Design Mode): Volumes Tab and Continuous Vessel Operations (Rating Mode): Volumes Tab

●      Toolbox Options, see Flash Evaporation: Toolbox Options Tab

●      Numerics, see Flash Evaporation: Numerics Tab

●      Labor, etc, see Operations Dialog: Labor etc. Tab

●      Description, see Operations Dialog: Description Tab

●      Batch Sheet, see Operations Dialog: Batch Sheet Tab

●      Scheduling, see Operations Dialog: Scheduling Tab