Introduction to Chemical Engineering for Lecture …

1 Introduction to Chemical Engineeringfor Lecture 7: Flash DistillationMartin Iggland, Marco Mazzotti ETH Zurich, Institute of Process Engineering , Sonneggstrasse 3, CH-8092 Zurich,Switzerland1. Flash distillationFlash evaporation is one of the simplest separation processes. A liquidstream containing several components is partially vaporised in a flash drum ata certain pressure and temperature. This results in two phases: a vapor phase,enriched in the more volatile components, and a liquid phase, enriched in theless volatile fluid is pressurized and heated and is then passed through a throttlingvalve or nozzle into the flash drum.

2 Because of the large drop in pressure, partof the fluid vaporizes. The vapor is taken off overhead, while the liquid drainsto the bottom of the drum, where it is withdrawn. The system is called flash distillation because the vaporization is extremely rapid after the feed enters thedrum. Because of the intimate contact between liquid and vapor, the systemin the flash chamber is very close to an equilibrium stage. Figure 1 shows aschematic drawing of a flash long as the feed consists of only 2 components, we have will consider this case first, then we will see how to calculate bubble- anddew-points of binary mixtures.

3 Further reading for this part of the Lecture can Corresponding author: phone +41 44 632 2456; fax +41 44 632 11 Mazzotti)Preprint submitted to ScriptSS 2015 Figure 1: Schematic drawing of a flash distillation unitbe found in chapter 2 of Wankat s Separation Process Engineering Binary flashIn a binary flash there are two components, 1 and 2. Unless stated otherwise,the compositionsx, yandzrefer to the molar fraction of the more volatilecomponent. The variablexis normally used for the liquid phase,yfor thevapor phase andzfor the feed instead ofx1, y1, z1as used in lectures 1-4. Thecomposition of the less volatile component is easily obtained from this example, if 2 is the less volatile component,x1= 1 x,y1= 1 , Separation Process Engineering , second edition; Prentice Hall, 2006.

4 This isthe revised edition of Wankat, P. C. Equilibrium staged separations; Separations in ChemicalEngineering; Prentice Hall, 1988. In the old edition, the corresponding chapters are (binaryflash), (bubble- and dew-point calculation), and 2 (vapor-liquid equilibria) Phase diagramsIn Figure 2(a), aT xydiagram for a flash distillation is shown. In thisexample, the feed is a liquid with a compositionz, at a temperatureTF. Whenthe feed enters the flash drum, its temperature is increased to the temperature ofthe flash unit,T. At this point, the feed is out of equilibrium. Therefore, it willsplit into a vapor and a liquid phase, whose compositions are given by the twogreen equilibrium lines.

5 The point on the left represents the liquid phase, withthe compositionx, while the point on the right represents the vapor phase, witha compositiony. From this diagram it can be seen that if the temperature ofthe flash is increased,xandywill decrease, and if the temperature is decreased,xandywill 2(b) shows aP xydiagram for the case of a flash distillation at aspecified temperature. The feed enters at a pressurePFand compositionz, inthis case as a liquid. In the flash drum, the pressure is decreased to a new valueP, and the feed enters the two-phase region, leading to a separation. The newliquid phase has compositionx, and the new vapor phase has compositiony.

6 (a)T xy(b)P xyFigure 2: Phase diagrams for a flash Design of flash unitsThe designer of a flash system needs to know the pressure and temperatureof the flash drum, the size of the drum and the liquid and vapor compositionsand flow rates. Which of these variables are specified and which need to bechosen depend on the application2. Normally, the feed is specified, ie. the flowrateF, compositionz, pressurePFand enthalpyhFare known3. The othervariables are the vapor flow rateV, vapor compositiony, liquid flow rateL,liquid compositionx, temperatureTand pressurePin the flash unit and heatinputQ. These can not be chosen freely.

7 To see how they are related to eachother, we write material and energy balances for the system. The boundary ofthe system is shown by the dashed, red line in Figure stated above, the two phases in the flash drum are at equilibrium (isofugacity condition). Therefore, we can write the following equations for thephase equilibria4(we assume an liquid and vapor phase):fV i=fL0ii= 1,2yP=xPv1(T)fori= 1(1 y)P= (1 x)Pv2(T)These equations can also be written using equilibrium constants:y=K1(P, T)x(1)(1 y) =K2(P, T)(1 x)(2)2 Flash drums are operated continuously. Therefore, we will only consider the enthalpy is a function of the temperature and composition.

8 In this section, we assumethis functional dependence is to the first chapters of the Lecture script for details on where these equations comefrom4 Furthermore, we have material balances for the whole flash unit, and for one ofthe components:F=V+L(3)F z=V y+Lx(4)and an energy balance based on the enthalpies (hF, hL, HV) of the variousstreams:F hF+Q=LhL+V HV(5)We have 7 unknowns (V, L, Q, x, y, P, T) and 5 equations (eqs. 1-5), whichmeans that we have 2 degrees of freedom. Often, the pressure is given. Thisleaves a few possibilities for setting the design specifications of a flash unit:a)xb)yc)V /ForL/Fd)Te)V y/F z: split factorf)Q, eq.

9 Adiabatic flash withQ= cases a)-e), we can apply a sequential solution procedure, which meansthat we first solve the phase equilibria and material balances, and then calculateQthrough an energy balance. As an example, we will consider the cases wherethe vapor or liqiud fraction (V /ForL/F, case c) is specified, and the casewhere the temperature is given (case d).c)V /ForL/Fis /Fis referred to as the vapor fraction, whileL/Fis the liquid fraction. From the overall material balance eq. 3 we see that when5one is given, the other is also set:LF= 1 VF(6)From the material balance for component 1 (eq. 4) we obtain the so-calledworking line:y= LVx+FVz(7)For a graphical solution of this problem, we plot the working line along withthe equilibrium curve in thex-ydiagram, as shown in Figure 3.

10 Note that theslope of the working line is -L/V, and that it intersects the diagonalx=ylineaty=z. At the intersection between the working line and the equilibrium line,we find the values ofxandy. Once these are known, we solve the equilibriumcondition (eq. 1) numerically to find the corresponding 3:x-ydiagram with equilibrium curve and working line6d)P, Tis the pressure and temperature are known, we can easily findyandxby solving the set of equations y=K1(P, T)x1 y=K2(P, T)(1 x)(8)f )P, Qis given: simultaneous the heat input is given, eg. asfor an adiabatic flash whereQ= 0, all the equations are coupled and we mustapply the simultaneous solution procedure.