Slide 1

When is flash distillation used?flash distillation a single equilibrium stage

when very crude separation is needede.g., oil/water separation in crude oil refining

when volatilities of components in the mixture are very differente.g., water desalination (4000 plants worldwide, producing 3.4 billion gallons potable H2O daily)http://www.edwardsmoving.com/Flash%20Drum.html

Flash drum 126' x 12' @ 179,000 lbs., being moved into position on 4 self propelled steerable dollies at a major oil refinery.

10x 5.9 MIGD MSF units in Saudi ArabiaDESALINATION OF SEA WATER BY MULTISTAGE FLASH DISTILLATION

There are several consecutive stages (flash drums) maintained at decreasing pressures from the first stage (hot) to the last stage (cold). Sea-water flows through the tubes of the heat exchangers, where it is warmed by condensation of the vapor produced in each stage. Its temperature increases from sea temperature to inlet temperature of the brine heater. The sea water then flows through the brine heater where it receives the heat necessary for the process (generally by condensing steam). At the outlet of the brine heater, when entering the first flash drum, sea water is overheated compared to the temperature and pressure of stage 1. Thus it will immediately "flash, i.e., release heat, and thus vapor, to reach equilibrium with stage conditions. The vapor is condensed into fresh water on the tubular exchanger at the top of the stage. The process takes place again when the water is introduced into the following stage, and so on until the last and coldest stage. The fresh water accumulates as the distillate, which is extracted from the last (coldest) stage. Sea water becomes slightly concentrated from stage to stage and becomes the brine flow, which is also extracted from the last stage.

http://www.sidem-desalination.com/en/process/MSF/

BIODIESEL MANUFACTURING

Biodiesel is made by transesterification of triglycerides with methanol, using NaOH as the catalyst:

The products are glycerol and fatty acid methyl esters (FAMEs). The latter are biodiesel.

The glycerol is much denser than the biodiesel, and separates by gravity in a settler.

The biodiesel is purified by separating it from methanol/water using simple flash distillation.

http://www.enerclean.biz/Equipment/Biodiesel/BiodieselFlash/BiodieselFlash.html

GLYCOL DEHYDRATION UNIT

The DPS Delta glycol dehydration unit removes water vapor from a gas stream to allow further treatment and transportation without risk of hydrate formation or corrosion in the presence of H2S or CO2, using ethylene glycol as a liquid desiccant.

http://www.dps-delta.com/portfolio/glycol.htm

GLYCOL DEHYDRATION PROCESS

Wet gas containing HC droplets enters the integral scrubber section of the contactor tower where free liquid is removed. Saturated gas flows up through the mass transfer section of the tower mixing with the downward flowing lean glycol. Dry gas will exit the tower via a demister pad and the rich glycol goes to a coil within the still column mounted on the reboiler. The condensing vapors provide reflux for the still column.

The warmed rich glycol flows via the lean/ rich glycol exchanger to the Flash Drum to remove entrained gas and separate HC liquid from the rich glycol. The rich glycol then passes through a solids filter to remove particulates and a carbon filter to remove traces of aromatic compounds. After filtering the rich glycol is heated by the lean glycol from the Surge Vessel .Lean glycol flows from an integral gas stripping column via the Surge Drum to the Lean/ Rich Glycol Exchanger before flowing to the Lean Glycol Pump which sends the glycol under high pressure to the Glycol Contactor via the Lean Glycol Cooler. Highlight path6Flash drum schematic

feed easiest to pump if liquid if necessary, increase pressure to keep it liquid (TF < Tbp) to make feed partially vaporize, need Tdrum > Tbp, and Pdrum < PF flash is usually adiabatic (no heat transfer across the system boundary) partial condensor operates in the same way, with hot vapor feed which partially condenses when cooled.Case 1: specified Tdrum, PdrumTMB:F = L + VCMB:F zi = L xi + V yiVLE:Ki = yi / xisolve for xi:

can also express in terms of yi: knowns: zi, Ki (Tdrum) unknowns: xi (or yi), V/F

oralso need to use:where V/F is the fraction of feed vaporized, 0 V/F 1

Rachford-Rice Solutiontrial-and-error method (except binary); easy to program

iterate until convergence then calculate xi, yi values then use TMB to calculate V, Lpossible solution:faster convergence:Rachford-Rice equationand

Finding V/F

algorithm for Newtonian convergence (Wankat, p. 37-8):

pick a value for V/F evaluate f(V/F) if f(V/F) 0, choose a new value for V/F

where

Case 2: specify Pdrum and V/FGuess TdrumCalculate Ki valuesEvaluate V/F using R-R equationIf f(V/F) 0, guess new Tdrum (recall Tbp calculation)Case 3: specify Pdrum and one xi (or yi) valueGuess TdrumCalculate a Ki valueCalculate V/F using

Evaluate f(V/F) for this TdrumIf f(V/F) 0, guess new Tdrum

or

Ready! To design the flash drummeaning, how large?Consider the flash drum as a vertical cylinder:important size parameters:diameter Dheight hplacement of feed nozzlematerials issues:high temperature?high pressure?corrosive feed?DhFVLIf the flash drum is too small:bubble entrainment in Lliquid entrainment in V (demister helps)Separation is compromised

If the flash drum is too large:- wasted expenseIf the liquid volume is large:Use a horizontal drum instead

12Vertical drum size1. Calculate maximum permissible vapor velocity, uperm (m/s)

where Kdrum is an empirical constant, typically 0.1 Kdrum 0.352. Calculate cross-sectional area, Ac (m)

where V is vapor flow rate and MWV is the vapor average molecular weight3. Calculate drum diameter, D, and height, h

rule-of-thumb: 3 h/D 5 Lecture 4 ends here13Graphical solutionfor binary mixturesCMB:F zi = L xi + V yisolve for yi:yi = -(L/V) xi + (F/V) ziequation of a straight linethe operating line

slope = -(L/V) solution to the flash drum problem must simultaneously satisfy CMB (operating line) and VLE (equilibrium line)

for a binary mixture, we can represent both on a 2D graph McCabe-Thiele diagramFrom Separation Process Engineering, Third Edition by Phillip C. Wankat (ISBN: 0131382276) Copyright 2012 Pearson Education, Inc. All rights reserved.Figure 2-2 McCabe-Thiele diagram for ethanol-water

VLE lineyE = -(L/V) xE + (F/V) zEyExEFind Tdrum?

yE/xE = KE (Tdrum)slope = -(L/V)operating line simultaneous solution of CMB and VLEDrawing the operating lineFrom Separation Process Engineering, Third Edition by Phillip C. Wankat (ISBN: 0131382276) Copyright 2012 Pearson Education, Inc. All rights reserved.Figure 2-2 McCabe-Thiele diagram for ethanol-water

y=xyi = -(L/V) xi + (F/V) ziyint = (F/V)zEoperating linefor specified feed (zi), Pdrum and V/F:

yint = (F/V) zi

-(L/V) = -(F-V)/V = 1 - (F/V)VLE linewhen yi = xi,

yi = -(L/V) yi + (F/V) zi

yi (1 + L/V) = (F/V) zi

yi (V + L)/V = (F/V) zi

i.e.,yi = xi = zizEall operating lines must pass through this pointLimits of separationFrom Separation Process Engineering, Third Edition by Phillip C. Wankat (ISBN: 0131382276) Copyright 2012 Pearson Education, Inc. All rights reserved.

y=xyi = -(L/V) xi + (F/V) zifamily ofoperating linesfor a given feed (zi), 0 (V/F) 1VLE line(V/F) = 1 (vaporize all the feed)

(L/V) = (F-V)/V = (F/V) 1 = 0

operating line is horizontalcorresponds to xminV/F = 0(V/F) = 0 (vaporize no feed)

(L/V) = (F/V) 1 =

operating line is verticalcorresponds to ymaxV/F = 1xE,minyE,maxzEFigure 2-2 McCabe-Thiele diagram for ethanol-water as x xmin, or y ymax, product yield 0 to achieve finite yield, must have less separationEnergy balance (EB)EB:FhF + Qflash = VHv + LhLwhere hF, hL are liquid enthalpiesHV is vapor enthalpyall are functions of temperature, composition0assume flash drum is well-insulated, flash is adiabaticfor ideal mixtures and arbitrary Tref:

for a particular component i, CP,L is molar heat capacity of pure liquid CP,V is molar heat capacity of pure vapor l is latent heat of vaporization at T = TrefFeed temperatureEB:FhF = VHv + LhLWhat temperature must the feed be to cause the flash?Case 1: for specified TF, value of hF is knownthis determines values of HV, hL necessary to satisfy EBi.e., Tdrum is determinedCase 2: for specified Tdrum, values of HV, hL are knownthis determines value of hF necessary to satisfy EBi.e., TF is determinedHeating the feedif the feed is not already at temperature TF, how much heat must be supplied?energy balance on heater:

F h1(T1) + Qh = F hF(TF)Qh = F (hF h1)

(dictates size of heater required)F, zi, T1F, zi, TF20