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Distillation Technology and Modelling Techniques

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Distillation Technology and Modelling Techniques ● DISTILLATION Distillation Technology and Modelling Techniques provide graphical demonstrations and Part 2: Shortcut distillation design algebraic expressions that explain how column performance is dictated by methods these various constraints. McCabe-Thiele In this, the second part of their review, Konrad Miller and This shortcut method constructs Katherine Shing demonstrate some shortcut distillation ‘operating lines’, an expression of the material balance within the column, design methods such as the McCabe Thiele method and the on an X-Y diagram. Figure 1 shows Fenske Underwood Gilliland algorithm. Part 1 (February a ChemCAD generated X-Y diagram 2016) offered a conceptual framework of distillation and in of the Ethanol-Water system at 1 atmosphere (14.7 psia) using the Non- the forthcoming fi nal part three, Konrad Miller will apply Random Two Liquid (NRTL) model for these principles to the production processes of a modern solutions. In this diagram, the x-axis is brandy distillery. the mole fraction of ethanol in the liquid mixture, where 0≤χEtOH≤1. The efore the digital revolution, distil- benefi t is there to learning more ar- y-axis is the mole fraction of ethanol Blation columns, from petroleum chaic, shortcut methods? Why not just in the vapour, also bounded from 0 fractionators to bourbon stills, where learn to use simulation software only? to 1. The red line is the ‘equilibrium designed by hand calculation. Since Of what benefi t are these tools to the curve’, representing the vapour-liquid coupled differential equations for heat modern spirits distiller? equilibrium. For example, a liquid transfer, mass transfer, and vapour- The answer is that an understand- with a 0.2 mole fraction ethanol is in liquid equilibrium are extremely ing of the shortcut methods is the equilibrium with a vapour of 0.54 mole diffi cult and tedious to solve by hand, quickest way to develop a physical fraction ethanol; while a liquid with several brilliant ‘shortcut’ methods intuition of the quantitative opera- an ethanol mole fraction of 0.6 is in have been developed to rapidly analyse tion of a column. Many operational equilibrium with a vapour of 0.7 mole and size distillation columns. questions can be answered with this fraction ethanol. The blue line simply Today, distillation columns are approach, such as understanding how serves as a guide for a 1:1 line. designed almost entirely via software a distillation will respond to a higher There are several important simulation packages such as ASPEN, refl ux ratio, a colder feed, or a vari- things to note about this system: ChemCAD, and HYSIS. What possible able stage count. Shortcut methods the vapour is consistently richer in 32 z Brewer and Distiller International April 2016 www.ibd.org.uk DISTILLATION l Ethanol/water at 14.70 psia by NRTL xDD 1 0.9 0.8 0.7 0.6 0.5 0.4 0.3 Ethanol vapour mole fraction Ethanol vapour 0.2 0.1 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Ethanol liquid mole fraction Yn+1Vn+1 xnLn Figure 1: Ethanol-Water X-Y diagram ethanol than the liquid which makes Eqn 1a: Figure 2: Material balance around the top part of a column purification via distillation possible; Vn+1 = Ln+D and as the concentration of ethanol tion 2 can be extended throughout the increases, the marginal increase Eqn 1b: top section of the column to produce from liquid to vapour decreases until yn+1 Vn+1 = xn Ln + χDD Equation 3: around 0.9 mole fraction ethanol, where the liquid and vapour concen- We want to relate this mass balance Eqn 3: trations converge. This is known as to a correlation between x and y and L D an ‘azeotrope’- a point in a solution superimpose it on the X-Y diagram. y = x + x V V D where the liquid and vapour concen- Solving for Yn+1 yields Equation 2: trations are identical. Equation 3 is known as the ‘Operat- Highly non-ideal solutions, such Eqn 2: ing Line’ for the ‘rectifying’ section. as ethanol-water, form azeotropes. The rectifying section of a distillation Ln D This makes the production of 100% y = x + x column is generally above the feed and n+1 V n D (also known as absolute) ethanol n+1 Vn+1 serves to purify the LK component in impossible by distillation at 1 at- This equation relates the composition the distillate. Note that L/V cannot be mosphere. Note that while pressure between the liquid and vapour of each greater than 1 as this corresponds to is fixed in the above diagram, tem- stage to the liquid, vapour, distillate ‘total reflux’, where all the condensed perature is not. The liquid/vapour molar flow rates, and the composi- vapour is return back to the column solutions will be at 100°C (the boiling tions of each. If certain assumptions rather than drawn away as distillate point of water) at the limiting case are made, such as constant molar (recall reflux is liquid returned from where the mole fraction ethanol is overflow throughout the column, the condenser to the column). We 0, and at 78.4°C (the boiling point of negligible enthalpic mixing effects, now see why increasing the amount of ethanol) at the limiting case where well insulated column, then Equa- liquid refluxed increases the purity of the mole fraction ethanol is 1. We can now begin to construct the Ethanol/water at 14.70 psia by NRTL McCabe-Thiele diagram on the above 1 X-Y plot. First, consider a mass bal- 0.9 ance between the top of the column, 0.8 and an arbitrary tray ‘n’ some length down the column, where trays are 0.7 numbered ‘1’ (top tray) down to ‘N’ 0.6 (bottom tray). For this system, vapour 0.5 enters from tray n+1 into tray n, liquid leaves from tray n down to tray 0.4 n+1, and distillate leaves the system 0.3 from the top of the column. Equation mole fraction Ethanol vapour 0.2 1a gives the mass balance around this section and 1b gives the mate- 0.1 rial balance around ethanol, the LK 0 (Light Key) component, while Figure 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 2 highlights the area of interest. Note Ethanol liquid mole fraction that ‘V’ refers to vapour flow, ‘L’ to liquid flow, ‘D’ to distillate, and ‘B’ to Figure 3: Rectifying Line superimposed on X-Y diagram. Note that the line intersects the ‘45 1 bottoms. Degree Line’ where x is specified. The slope is given by ( RR ) , and the y intercept is( )x D RR+1 RR+1 D www.ibd.org.uk Brewer and Distiller International April 2016 z 33 l DISTILLATION the LK in the distillate: L/V increases, increasing ‘y’, the concentration of Ethanol/water at 14.70 psia by NRTL 1 ethanol leaving the top of the column. There is one last useful modifica- 0.9 tion to the Rectifying Line equation: 0.8 Let the ‘Reflux Ratio’ (RR) be defined 0.7 as ratio of liquid reflux, L, returned to the column from the condenser 0.6 to the amount of liquid distillate, D, 0.5 drawn away from the condenser as 0.4 product, or RR = L/D. At the limiting 0.3 case of RR = L/D → ∞, all material going to the condenser is returned mole fraction Ethanol vapour 0.2 as reflux (Total Reflux), and L/V=1. 0.1 The opposite case is where no reflux 0 is returned: RR = L/D → 0, L/V = 0. If we substitute this into Equation 3, we 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Ethanol liquid mole fraction can specify the entire rectifying sec- tion operation by RR and xD, as seen in Equation 4: Figure 4: Stripping and Rectifying Lines superimposed on X-Y diagram Eqn 4: Ethanol/water at 14.70 psia by NRTL RR 1 1 y = ( ) x + ( ) x RR+1 RR+1 D 0.9 With this in mind, we can superim- 0.8 pose the Rectifying Line over the X-Y 0.7 diagram, with a distillate of 0.65 mole 0.6 fraction ethanol and a reflux ratio of 1.3. Figure 3 shows the results. 0.5 We can now apply the same analy- 0.4 sis to the area below the column, or 0.3 the “Stripping Section”. The Strip- Ethanol vapour mole fraction Ethanol vapour 0.2 ping section of a distillation column is generally below the feed, and serves 0.1 to purify the HK component in the bot- 0 toms. If we define the ‘Boilup Ratio’ 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 (BR), to be equal the ratio of vapour Ethanol liquid mole fraction driven up the column (boilup, V) and the rate of liquid product drawn from Figure 5: Rectifying, Stripping, and Q-Lines on an X-Y diagram the column (Bottoms, B), then we can derive the Operating line for the Strip- the composition and phase of the feed. shows the Rectifying Line, Stripping ping Section, Equation 5: The q parameter used in the equation Line, and Q-Line on an X-Y diagram: is a measure of the amount of heat Note how all three lines converge Eqn 5: needed to fully vaporize the feed, and at one point.
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