Flash evaporation

Equilibrium flash of a multi-component liquid

The equilibrium flash of a multi-component liquid may be visualized as a simple distillation process using a single equilibrium stage. It is very different and more complex than the flash evaporation of single-component liquid. For a multi-component liquid, calculating the amounts of flashed vapor and residual liquid in equilibrium with each other at a given temperature and pressure requires a trial-and-error iterative solution. Such a calculation is commonly referred to as an equilibrium flash calculation. It involves solving the Rachford-Rice equation:

${\displaystyle \sum _i\frac{z_i(K_i-1)}{1+\beta (K_i-1)}=0}$

where:

• z_i is the mole fraction of component i in the feed liquid (assumed to be known);
• β is the fraction of feed that is vaporised;
• K_i is the equilibrium constant of component i.

The equilibrium constants K_i are in general functions of many parameters, though the most important is arguably temperature; they are defined as:

${\displaystyle y_i=K_i{x}_i}$

where:

• x_i is the mole fraction of component i in liquid phase;
• y_i is the mole fraction of component i in gas phase.

Once the Rachford-Rice equation has been solved for β, the compositions x_i and y_i can be immediately calculated as:

${\displaystyle \begin{array}{cc}{x}_i& =\frac{z_i}{1+\beta (K_i-1)}\\ y_i& =K_i{x}_i.\end{array}}$

The Rachford-Rice equation can have multiple solutions for β, at most one of which guarantees that all x_i and y_i will be positive. In particular, if there is only one β for which:

${\displaystyle \frac{1}{1-K_{\text{max}}}={\beta }_{\text{min}}<\beta <{\beta }_{\text{max}}=\frac{1}{1-K_{\text{min}}}}$

then that β is the solution; if there are multiple such β's, it means that either K_max<1 or K_min>1, indicating respectively that no gas phase can be sustained (and therefore β=0) or conversely that no liquid phase can exist (and therefore β=1).

It is possible to use Newton's method for solving the above water equation, but there is a risk of converging to the wrong value of β; it is important to initialise the solver to a sensible initial value, such as (β_max+β_min)/2 (which is however not sufficient: Newton's method makes no guarantees on stability), or, alternatively, use a bracketing solver such as the bisection method or the Brent method, which are guaranteed to converge but can be slower.

The equilibrium flash of multi-component liquids is very widely utilized in petroleum refineries, petrochemical and chemical plants and natural gas processing plants.

Contrast with spray drying

Spray drying is sometimes seen as a form of flash evaporation. However, although it is a form of liquid evaporation, it is quite different from flash evaporation.

In spray drying, a slurry of very small solids is rapidly dried by suspension in a hot gas. The slurry is first atomized into very small liquid droplets which are then sprayed into a stream of hot dry air. The liquid rapidly evaporates leaving behind dry powder or dry solid granules. The dry powder or solid granules are recovered from the exhaust air by using cyclones, bag filters or electrostatic precipitators.

Natural flash evaporation

Natural flash vaporization or flash deposition may occur during earthquakes, resulting in deposition of minerals held in supersaturated solutions, sometimes including valuable ore in the case of auriferous (gold-bearing) waters. This results when blocks of rock are rapidly pulled and pushed away from each other by jog faults.