Flash Unit Operation - Technical Information
The Flash unit operation calculates the thermodynamic state of any stream when two specifications (e.g. temperature and pressure) are given. Once the phase equilibrium is determined, the phases may be separated into distinct product streams. The duty required to achieve this state is reported for all Flash Types, except adiabatic flash.
Calculation Method
The Flash unit requires exactly two degrees of freedom to determine the exact thermodynamic state of a stream. The parameters that must be specified depend upon the type of flash calculation desired.
The isothermal flash forms the basis for many flash calculations. An iterative procedure with damping is employed to carry out the convergence in composition. All other Flash Types iterate the appropriate variables using the isothermal flash to achieve convergence.
PRO/II can handle the following three general classes of flash calculations:
(a) VLE Calculations: Two phase calculations containing one vapor and one liquid phase. If water is present in the system, a pure water phase may be decanted as a second liquid phase and it is not considered further in the equilibrium calculations. See VLE Model for more details.
(b) VLLE Calculations: Three phase calculations containing one vapor and two liquid phases. See VLLE Model for more details.
(c) Electrolytic Calculations: Calculating the equilibrium for aqueous systems. See Electrolyte Models for more details.
Feed and Product Streams
The flash unit can have any number of feed streams. The inlet pressure is taken to be the lowest pressure of all the feed streams.
The flash unit can have up to four product streams with different phases in each stream. The possible product phases are vapor, liquid, decanted water / second liquid phase, a mixture of vapor and liquid, and solids. If there are multiple product streams leaving the Flash unit, the phase condition for each stream must be specified using the Product Phases Window. For the Dew point (vapor phase only) and Bubble point (liquid phase only) flash types, you can specify an imaginary product stream containing other non-existent phases called Pseudo Streams.
The Flash unit does not perform any equilibrium calculations for the solid phase, but they can be removed as a separate product stream if desired.
Phase Assignment
When a flash is performed in the super-critical region, the fluid actually is in a so-called "dense" phase that is neither liquid nor vapor. The PRO/II flash drum uses heuristic rules to declare the product phase as liquid or vapor. Beginning in version 8.1, PRO/II allows users to override the default phase selection. Refer to "Phase Assignment" in the Flash Main Window topic.
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