a mixture each Processing methods 1. Single-stage process (batchwise or continuously) 2. Multiple-stage process (batchwise or continuously) process.

Transcription

1 Introduction to Separation Processes What are separation processes? those operations which transform a mixture of substances into two or more products which differ from each other in composition. Two important questions in separation processes 1. How much separating agent is required? 2. How fast can the separation be achieved? The keys to these questions are the mass balance analysis. A general view of separation processes Processing methods 1. Single-stage process (batchwise or continuously) 2. Multiple-stage process (batchwise or continuously) Equilibrium relations between phases 1. Phase rule and equilibrium 2. Gas-liquid equilibrium Figure 1. General separation process Figure 1 shows a simple schematic of a separation process. It comprises four sections: the feed, the separation device, the separation agent and the products. The feed may consist of one stream of matter or of several streams, which enters the separation device and is forced to be separated into at least two product streams of different composition, by a separation agent. The separation agent can be a stream of matter or energy and generally will cause the formation of a second phase of matter. 5 6

2 Application of Separation Processes 7 8

3 Distilled Water Types of Separation Processes (1) Distillation processes: * most popular separation process, * capable of producing pure substance from mixture, * requires two phases: liquid and vapor, * uses energy to accomplish separation. Production processes using distillation * oil production, * air separation, * distillation of wine to cognac and spirits. 9 10

4 (2) Absorption/stripping processes: * popular in environmental applications, * transfer gas component from gas to liquid or vice versa, * requires two phases: liquid and gas, * uses the different affinity gas component for gas and liquid. Production processes using absorber * scrubbing smokestacks, * removal of ammonia from refinery, * removee CO 2 from air. (3) Liquid-liquiused for processes that requires low temperature operation, * transfer soluble component from one liquid to extraction: * another, * requires two phases: two immiscible or partially miscible liquids, * uses the different affinity components for separation. Production processes using extraction * food processing, * pharmaceutical separation, * oil purification

5 (4) Solid-liquid extraction: * used liquid to extract component from a solid matrix, * transfer soluble component from solid to liquid, * requires two phases: solid and liquid, * uses the solubility of the component for separation. Phase Transformation Phase Diagram of Water Production processes using leaching * mining, * food processing, * pharmaceutical

6 Phase Equilibrium Equilibrium-Staged Operation Single Staged Unit 15 16

7 Equilibrium-Staged Operation Multiple Staged Unit Different Representation of VLE Data (1) Ideal solution & Raoult's Law: Raoult's Law states that, for an ideal solution, the equilibrium partial pressure of a component at a fixed temperature T equals the product of its vapor pressure (when it is pure) and its mole fraction in the liquid: For component A, P A = Y A P T = P A *X A Y A = (P A */P T )X A where P A = equilibrium partial pressure of component-a in the gas at temperature T P A * = vapor pressure of pure liquid A at temperature T X A = liquid-phase mole fraction of component-a at temperature T Y A = gas-phase mole fraction of component-a at temperature T P T = system total pressure Note: The vapor pressure is a constant at constant temperature. Hence, from the equation, we see that Raoult's Law predicts a linear equilibrium relationship between P and X

8 Vapor pressures of pure liquid can be estimated using Antoine Equation: ln P / where A, B and C are constants. Compound Benzene Ethanol n-heptane Toluene Water A B C The constant-temperature phase diagram for an ideal solution is shown in the Figure below. For a binary mixture of A and B; P A = P A *X A ; P B = P B *X B = P B *(1 - X A ) The partial pressures P A and P B vary linearly with X A. This is shown as P A vs. X and P B vs. X respectively in the Figure. For ideal gas mixture, the total pressure is the sum of the partial pressures. Total pressure P T = P A + P B Replacing for the partial pressures and re-arrange, we have: P T = ( P A * - P B * ) X A + P B * The total pressure varies linearly with X A. This is shown as P T vs. X in the Figure. In addition, P A = Y A P T thus P A *X A = Y A P T, and we have: Substitute into earlier equation to remove X A : 19 20

9 P T P A * = (P A * - P B * ) ( Y A P T ) + P A * P B * P T P A * - (P A * - P B * ) ( Y A P T ) = P A * P B * Phase Diagram (pressuree is constant): The total pressure varies nonlinearly with Y A. This is shown as P T vs. Y in the Figure. Also, since P A * X A = Y A P T ; we have, All the above equations are useful in determining the equilibrium variables X and Y. Boiling Points of Mixture of Liquids Even at constant pressure, depending on relative concentrations of each component in the liquid, many boiling point temperatures are possible for mixture of liquids (solutions). For mixture, we use the term bubble point when the liquid starts to boil, and dew point when the vapor starts to condense. For liquid solution, bubble points need not be the same as dew points. Indeed, boiling of a liquid mixture takes place over a range of boiling points. Likewise, condensation of a vapor mixture takes place over a range of condensation points

10 Ideal Solution: benzene and toluene Example 1: Determine the T-x-y relationship for ethanol and water using Antoine Equation assuming that ethanol-water form an ideal solution. Plot the resulting data. Please use ambient pressure. Solution: Ambient pressure = kpa Take an example of o C: /( )= P E * = lnp W *= /( )= P W * = X W = 1- X E = = Y W = 1- Y E = = Repeat this process for other temperatures, we have the following table: 23 24

11 Example 2: For the system of ethanol-water mixture at ambient pressure, determine 1) What is the boiling point of pure ethanol and water? 2) What is the bubble point temperature of a mixture containing 0.25 mole fraction of ethanol? What is its dew point temperature? 3) What are the bubble and dew point temperatures of a solution containing 30 wt.% water? The T-X-Y plot is as below: Solution: Ambient pressure = kpa 1) For pure ethanol, = /(T ) The normal boiling point of ethanol is T = 78.3 o C For pure water, = /(T ) The normal boiling point of water is T = 100 o C 2) Bubble point at X E = 0.25: from the Figure, T B = 92 o C, the equilibrium Y E = 0.46 Dew point at Y E = 0.25: from the Figure, T D = 96 o C, 25 26

12 3) 30% w/ww water = 30 g water + 70 g ethanol: x W = 0.3, convert to moles 30w% water =30/ 18 moles water + 70/46 moles ethanol = 1.67 moles water moles ethanol Total moles = 3.19 X W = 1.67/3.19 = X E = 1 - X W = Bubble point at X E = : T B = o C, the equilibrium Y E = 0.68 Dew point at Y E = : T D = 91.8 o C (2) Tabulated data: X(ethanol) X(water) Y(ethanol) Y(water) T(C) The experimental VLE data are usually obtained using special stills where the temperature, pressure and concentrations of the components in liquid and vapor phases could be accurately determined. X, Y (mole fraction) x, y (mass fraction) for binary mixture (2-components): x = X*MW A /(X *MW A + (1-X) *MW B ) ** derive the equations for n-component and for x X 27 28

13 (3) Graphical representation: T-x-y diagram Example 3: Determine the concentration of an alcohol solution if its bubble point temperature is 92.5 o C? What should be the ethanol contentt of the vapor at equilibrium with this solution? What is the bubble point temperature of 78 and 95 wt% alcohol? Solution: At T B = o C: x A = 0.04, y A = 0.28 When x A = 0.78 : T B = 78.3 o C When x A = 0.95 : T B = 78.3 o C 29 30

14 (3) Graphical representation: X-Y or McCabe-Thiele diagram Determine the composition of the vapor at equilibrium with an 30, 60, 80 and 90 mol.% ethanol solution

15 (4) Relative Volatility (α AB ) α AB = (Y A X B )/(Y B X A ) Y A = α AB X A /(1 + (α AB -1)X A ) T(C) Kic4 KC6 alpha Example 4: Plot X-Y diagram for isobutane-isopentane to be mixture if the relative volatility is known 1.7 Xic Y ic