Flash evaporation

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The flash (or partial) evaporation is one of the simplest unit operations. When a saturated liquid stream undergoes a reduction in pressure by passing through a throttling valve or other throttling device, it is partially vaporized. If the throttling valve or device is located at the entry into a pressure vessel so that the flash evaporation occurs within the vessel, then the vessel is often referred to as a "flash drum".

If the saturated liquid is a single-component liquid (for example, liquid propane or liquid ammonia), a part of the liquid immediately "flashes" into vapor. Both the vapor and the residual liquid are cooled to the saturation temperature of the liquid at the reduced pressure. This is often referred to as "auto-refrigeration" and is the basis of most conventional vapor compression refrigeration systems.

If the saturated liquid is a multi-component liquid (for example, a mixture of propane, isobutane and normal butane), the flashed vapor is richer in the more volatile components than is the remaining liquid.

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Flash evaporation of a single-component liquid

The flash evaporation of a single-component liquid is an isenthalpic (i.e., constant enthalpy) process and is often referred to as an "adiabatic flash". The following equation, derived from a simple heat balance around the throttling valve or device, is used to predict how much of a single-component liquid is vaporized.

X = 100 ( HuL – HdL ) ÷ ( HdV – HdL )
where:  
X =  weight percent vaporized
HuL =  upstream liquid enthalpy at upstream temperature and pressure, J/kg
HdV
  		=  flashed vapor enthalpy at downstream pressure and corresponding saturation
    temperature, J/kg
HdL
  		=  residual liquid enthalpy at downstream pressure and corresponding saturation
    temperature, J/kg

If the enthalpy data required for the above equation is unavailable, then the following equation may be used.

X = 100 · cp ( Tu – Td ) ÷ Hv
where:  
X =  weight percent vaporized
cp =  liquid specific heat at upstream temperature and pressure, J/kg/°C
Tu =  upstream liquid temperature, °C
Td =  liquid saturation temperature corresponding to the downstream pressure, °C
Hv
  		=  liquid heat of vaporization at downstream pressure and and corresponding saturation
    temperature, J/kg

( Note: The words "upstream" and "downstream" refer to before and after the liquid passes through the throttling valve or device.)

This type of flash evaporation is used in the "Multi-Stage Flash Distillation" desalination of ocean water or brackish water. The water is heated and then routed into a reduced pressure flash evaporation "stage" where some of the water flashes into steam which is subsequently condensed into salt-free water. The residual salty liquid from that first stage is introduced into a second flash evaporation stage at a pressure lower than the first stage pressure where more water is flashed into steam which is also subsequently condensed into more salt-free water. This sequential use of multiple flash evaporation stages is continued until the design objectives of the system are met. A large part of the installed desalination capacity worldwide uses Multi-Stage Flash Distillation and typically such plants have up to 24 or more stages of flash evaporation.

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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, the calculation to determine 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 and it involves solving the following equation known as the Rachford Rice equation:

<math>\sum_i\frac{z_i\, (1-K_i)}{1 + \frac{V}{F}\, (K_i - 1)}</math>

Overall material balance equation:
<math>F = V + L</math>
Material balance equation for any component i:
<math>F\!\cdot\! z_i = V\!\cdot\! y_i + L\!\cdot\! x_i</math>
Equation defining the vapor-liquid equilibrium constant <math>K</math>:
<math>K = y_i / x_i</math>
where:  
<math>F</math> <math>= moles\; of\; total\; feed\; liquid</math>
<math>V</math> <math>= moles\; of\; flashed\; vapor</math>
<math>L</math> <math>= moles\; of\; residual\; liquid</math>
<math>K</math> <math>= equilibrium\;constant</math>
<math>z_i</math> <math>= mole\; fraction\; of\; component\; i\; in\; the\; feed\; liquid</math>
<math>y_i</math> <math>= mole\; fraction\; of\; component\; i\; in\; the\; flashed\; vapor</math>
<math>x_i</math> <math>= mole\; fraction\; of\; component\; i\; in\; the\; residual\; liquid</math>

Newton's method (also known as the Newton-Raphson method) is an efficient iterative algorithm for solving the above Rachford Rice equation. Alternatively, an Excel spread sheet and the Excel Solver function can be used.

Those who may be interested in the derivation of the Rachford Rice equation will find complete details by following the the external links listed at the end of this article.

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

(See mole fraction for a definition of that terminology).

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Spray drying

Spray drying is the rapid drying of a slurry of very small solids suspended in a liquid so as to produce dry powders or solid granules. The slurry is first atomized into very small liquid droplets which are then sprayed into a stream of hot dry air which rapidly evaporates the liquid leaving behind a 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.

A brief explanation of spray drying has been included here because some readers may consider spray drying to be a form of flash evaporation. However, although it is a form of liquid evaporation, it is quite different from flash evaporation.

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See also

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External Links

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