User:Milton Beychok/Sandbox: Difference between revisions

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[[Image:Vapor-Liquid Separator.png|frame|right|246px||{{#ifexist:Template:Vapor-Liquid Separator.png/credit|{{Vapor-Liquid Separator.png/credit}}<br/>|}}Typical flash drum]]


'''Flash evaporation''' is the partial [[vaporization]] that occurs when a [[Boiling point|saturated liquid]] stream undergoes a reduction in [[pressure]] by passing through a throttling valve or other throttling device. This process is one of the simplest [[unit operation]]s. 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 [[Vapor-liquid separator|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 (i.e., evaporates). Both the vapor and the residual liquid are cooled to the [[Boiling point|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]]), a part of the liquid will also immediately flash into a vapor and the flashed vapor will be richer in the more [[volatility (chemistry)|volatile]] components than is the remaining liquid.<br><br>
==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 process|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.<br><br>
:'''''X = 100 ( H<sub>u</sub><sup>L</sup>  – H<sub>d</sub><sup>L</sup> ) &divide; ( H<sub>d</sub><sup>V</sup>  – H<sub>d</sub><sup>L</sup> )'''''
:{| border="0" cellpadding="2"
|-
|align=right|where:
|&nbsp;
|-
!align=right|''X''
|align=left|= &nbsp;'''weight percent vaporized'''
|-
!align=right|''H<sub>u</sub><sup>L</sup>''
|align=left|= &nbsp;'''upstream liquid enthalpy  at upstream [[temperature]] and pressure, J/kg'''
|-
!align=right|''H<sub>d</sub><sup>V</sup>''<br>&nbsp;
|align=left|= &nbsp;'''flashed vapor enthalpy at downstream pressure and corresponding saturation<br> &nbsp; &nbsp; temperature, J/kg'''
|-
!align=right|''H<sub>d</sub><sup>L</sup>''<br>&nbsp;
|align=left|= &nbsp;'''residual liquid enthalpy at downstream pressure and corresponding saturation<br> &nbsp; &nbsp; temperature, J/kg'''
|}
If the enthalpy data required for the above equation is unavailable, then the following equation may be used.<br><br>
:'''''X = 100 &middot; c<sub>p</sub> ( T<sub>u</sub> – T<sub>d</sub> ) &divide; H<sub>v</sub>'''''
:{| border="0" cellpadding="2"
|-
|align=right|where:
|&nbsp;
|-
!align=right|''X''
|align=left|= &nbsp;'''weight percent vaporized'''
|-
!align=right|''c<sub>p</sub>''
|align=left|= &nbsp;'''liquid [[specific heat capacity|specific heat]] at upstream temperature and pressure, J/(kg &middot; °C)'''
|-
!align=right|''T<sub>u</sub>''
|align=left|= &nbsp;'''upstream liquid temperature, °C'''
|-
!align=right|''T<sub>d</sub>''
|align=left|= &nbsp;'''liquid saturation temperature corresponding to the downstream pressure, °C'''
|-
!align=right|''H<sub>v</sub>''<br>&nbsp;
|align=left|= &nbsp;'''liquid [[heat of vaporization]] at downstream pressure and corresponding<br> &nbsp; &nbsp; 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 [[desalination]] of brackish water or ocean water by ''[[Multi-Stage Flash Distillation]]''. The water is heated and then routed into a reduced-pressure flash evaporation "stage" where some of the water flashes into steam. This steam 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. 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 world's installed desalination capacity uses multi-stage flash distillation. Typically such plants have 24 or more sequential stages of flash evaporation.
==Equilibrium flash of a multi-component liquid==
The '''equilibrium flash''' of a multi-component liquid is also an enthalpic process and 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 method|iterative]] solution. Such a calculation is commonly referred to as an ''equilibrium flash calculation''. It involves solving the following '''Rachford-Rice equation''':<ref>
[http://faculty.mcneese.edu/pdauten/lec5.ppt McNeese University lecture] (scroll down to Rachford-Rice Equation)</ref><ref>[http://pubs.acs.org/cgi-bin/abstract.cgi/iecred/1993/32/i07/f-pdf/f_ie00019a029.pdf?sessid=6006l3 ''Polynomial Objective Functions for Flash Calculations''], John H. Warrent and Michael A. Adewumi, Ind. Eng. Chem. Res. 1993, Vol.32, pages 1528-1530</ref><ref>[http://ascend.cheme.cmu.edu/ftp/pdfPapersRptsSlides/tech_complementarity.pdf "Complementarity Formulation for the Representation of Algebraic Systems Containing Conditional Equations", V. Rico-Ramírez and A. W. Westerberg] Institute for Complex Engineered Systems,
Carnegie Mellon University, Pittsburgh, Pennsylvania, Technical Report ICES 06-243-98</ref><ref>[http://www.infochemuk.com/publicat/poster.pdf Automatic Plotting of Multiple Phase Boundaries and Flash Calculations], Infochem Computer Services, [[United Kingdom]]</ref><ref>[http://www.mathworks.com/matlabcentral/fileexchange/loadFile.do?objectId=9341&objectType=file Flash Calculations using the Soave-Redlich-Kwong equation of state] (view full-size image)</ref>
:<math>\sum_i\frac{z_i\, (K_i - 1)}{1 + a\, (K_i - 1)}=0</math>
:Overall material balance equation :
:<math>F = V + L \,</math>
:Material balance equation for any component <math>i</math> :
:<math>F\!\cdot\! z_i = V\!\cdot\! y_i + L\!\cdot\! x_i</math>
:Equation defining the [[vapor-liquid equilibrium]] constant <math>K_i</math> :
:<math>K_i = y_i / x_i \,</math>
:{| border="0" cellpadding="2"
|-
|align=right|where:
|&nbsp;
|-
!align=right|<math>F \,</math>
|align=left|= '''mols of total feed liquid'''
|-
!align=right|<math>V \,</math>
|align=left|= '''mols of flashed vapor'''
|-
!align=right|<font style="vertical-align:-10%;"><math>a \,</math></font>
|align=left|= '''fraction of feed that is vaporized''' = <math>V/F</math>
|-
!align=right|<math>L \,</math>
|align=left|= '''moles of residual liquid'''
|-
!align=right|<font style="vertical-align:-15%;"><math>K_i \,</math></font>
|align=left|= '''vapor-liquid equilibrium constant'''
|-
!align=right|<font style="vertical-align:-30%;"><math>z_i \,</math></font>
|align=left|= '''mol fraction of component''' <math>i</math> '''in the feed liquid'''
|-
!align=right|<font style="vertical-align:-30%;"><math>y_i \,</math></font>
|align=left|= '''mol fraction of component''' <math>i</math> '''in the flashed vapor'''
|-
!align=right|<font style="vertical-align:-30%;"><math>x_i \,</math></font>
|align=left|= '''mol fraction of component''' <math>i</math> '''in the residual liquid'''
|}
The mole fraction of a component in a multi-component mixture is the relative proportion of [[mole (unit)|moles]] belonging to the component to the total moles of liquid or vapor. It is one way of measuring concentration. For each component <math>i</math>, the mole fraction <font style=vertical-align:-15%;><math>x_i</math></font> is the number of [[mole (unit)|moles]] <font style=vertical-align:-15%;><math>n_i</math></font> divided by the total number of moles in the mixture <font style=vertical-align:-5%;><math>n</math></font>:
:<math> x_i = \frac{n_i}{n}</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 [[Microsoft Excel|Excel]] spreadsheet and the Excel Solver function can be used.
The equilibrium flash of multi-component liquids is very widely utilized in [[Petroleum refining processes|petroleum refineries]], [[petrochemical]] and [[chemical plant]]s and [[natural gas processing]] plants.
==Spray drying==
[[Spray drying]] is the rapid drying of a [[slurry]] of very small solids suspended in a liquid. The slurry is first [[atomization|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 [[cyclone (industry)|cyclones]], [[Filtration|bag filters]] or [[electrostatic precipitator]]s.
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.
==References==
{{reflist}}
==See also==
*[[Evaporation]]
*[[Evaporator]]
*[[Vapor-liquid separator]]
==External links==
*[http://www.tlv.com/global/US/steam-theory/flash-steam-and-vapor.html Vapor and Flash Steam] Animation, photos and technical explanation of the difference between Flash Steam and Vapor.
*[http://www.spiraxsarco.com/resources/steam-engineering-tutorials/condensate-recovery/flash-steam.asp Flash Steam Tutorial] The benefits of recovering flash steam, how it is done and typical applications.
*[http://www.escwa.org.lb/information/publications/edit/upload/tech-01-3-e.pdf Water Desalination Technologies] in the Middle East and Western Asia
*[http://www.niro.com/ndk_website/niro/cmsresources.nsf/filenames/spray-drying.pdf/$file/spray-drying.pdf Discussion of spray drying]
*[http://petrochemical.gronerth.com/flash_distillation_program_explorer.htm Flash evaporation program online] Flash distillation of the hydrocarbon compounds.

Revision as of 12:53, 4 July 2008

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