Vapor-liquid Separation Process MULTICOMPONENT DISTILLATION

Transcription

1 Vapor-liquid Separation Process MULTICOMPONENT DISTILLATION

2 Outline: Introduction to multicomponent distillation Phase Equilibria in Multicomponent Distillation (Pg. 737) Bubble-point and dew-point calculation Flash Distillation of Multicomponent Mixtures (pg. 741) Fractionation of Multicomponent Mixtures (pg. 742) Key Components Minimum number of plates Minimum reflux ratio Azeotropic and Extractive Distillation (pg. 759)

3 Introduction to Multicomponent Distillation Binary Mixtures The calculation of equilibrium stages uses mass and enthalpy balances and VLE. Much simpler phase equilibria Equilibria change from stage to stage, but except with azeotropes, the more volatile component is more volatile than the other component throughout the column. Multicomponent Mixtures The calculation of equilibrium stages uses mass and enthalpy balances and VLE. Phase equilibria are much more complex because: Several components Equilibria depend on T, which changes from stage to stage. 1 component may be more volatile than the average in 1 part of the column and less volatile than the average in another part, which leads to complex concentration profiles.

4 Phase equilibria in Multicomponent Distillation The VLE for a mixture are described by distribution coefficients or K factors, where K for each component is the ratio of mole fractions in the vapor and liquid phase at equilibrium: (22.1) If Raoult s law and Dalton s law hold, values of K i can be calculated from the vapor pressure and the total pressure of the system: (22.2) (22.3) (22.4)

5 Raoult s law is a good approximation for mixtures of similar compounds. The K-factor strongly temperature-dependent because of the change in vapor pressure, but the relative value of K for 2 components change only moderately with temperature. The ratio of K factors is the same as the relative volatility of the components: / / (22.5) When Raoult s law applies, (22.6)

6 Bubble-point and Dew-point Calculation Determination of the bubble point (initial boiling point of a liquid mixture) or a dew-point (initial condensation temperature) is required for a flash distillation calculation and for each stage of a multicomponent distillation. The basic equation, for bubble-point: = 1.0(assume,T) (22.7) If the summation of K i x i >1.0, choose lower T and repeat calculation until Eq. (22.7) is satisfied. For 1.00, the composition of the vapor in equilibrium. If 1.00, the vapor composition can be determined with little error from relative contribution of each term to the summation (Eq. (22.9):

7 Bubble-point and Dew-point Calculation For dew-point = (22.8) Where N c is the number of components. (22.9) Similar procedure is used to determine the dew point of a vapor mixture abd composition of the liquid in equilibrium with this mixture.

8 Example 22.1 Find the bubble-point and dew-point temperatures and the corresponding vapor and liquid compositions for a mixture of 33 mole % n-hexane, 37 mole% n-heptane, and 30 mole% n-octaneat 1.2 atm total pressure.

9 Flash Distillation of Multicomponent Mixtures Eq. (21.1): x F =fy D + (1-f)x B,can be written for each component in a flash distillation in the form (22.10) Since the distillate and bottom streams are in equilibrium, this equation may changed to 1 (22.11) Solving Eq. (22.11) for x Bi and summing over N c components gives 1 (22.12) This equation is solved by iteration in the same manner as the dew-point calculation using Eq. (22.8), and the final values of T and K i are used to calculate the composition of the product streams.

10 Example 22.2 The mixture of Example 22.1 is subected to a falsh distillation at 1.2 atm pressure, and 60% of the feed is vaporized. (a) Find the temperature of the flash and the composition of the liquid and vapor products. (b) (b) To what temperature must the feed be heated for 60% vaporization of flashing? Solution: Page 741.

11 Fractionation of Multicomponent Mixtures Ideal plates are assumed in the design of cascades, and the number of stages is subsequently corrected for plate efficiencies. 2 limiting conditions of total reflux (minimum number of plates) and minimum reflux are also determined to help validate the design. In binary distillation, a desired separation of components is assumed, and the numbers of plates above and below the feed are calculated for chosen reflux ratio (Method 1). In multicomponent distillation, the number of plates above and below the feed is assumed, and the separation of components is calculated using assumed flows of reflux from the condenser and vapor from the reboiler.

12 Key Components Objective of distillation: to separate the feed into streams of nearly pure products. In binary distillation: The purity is usually defined by specifying x D and x B, the mole fraction of light component in the distillate and bottoms products. In multicomponent distillation: 3 or more components in the products, and specifying the concentrations of one component in each does not fully characterize these products. If the concentration of 2/3or ¾ components are specified the distillate and bottoms products, it is generally impossible to meet these specification exactly.

13 Key Components An increase in reflux ratio number or number of plates would increase the sharpness of the separation, and the desired concentration of 1 component in each product can be achieved. Often designer chooses 2 components whose concentrations or fractional recoveries in the distillate and bottoms products are a good index of the separation achieved. Once the components are identified, they are called KEY COMPONENTS. Keys must differ in volatility More volatile = Light Key (L) Less volatile = Heavy key (H)

14 Key Components Components lighter than the light key are nearly completely recovered in the distillate. Components heavier than the heavy key are usually completely recovered in the bottoms. Exceptions to distillation of close boiling materials, such as mixtures of isomers. The choice of 2 keys does not give deteminate mass balance, because not all other mole fractions are calculable by mass balance alone, and equilibrium calculations are required to calculate the concentration of dew-point vapor from the top plate and bubble-point liquid leaving the reboiler.

15 Minimum Number of Plates The Fenske equation (21.45) applies to any 2 components I and j in a conventional pant at infinite reflux ratio. In this case the equation has the form / / / (22.13) (22.14) The subscripts D, F, and B in Eq. (22.14) refer to the temperatures of the distillate, feed plate, and bottom in the column. Refer to Example 22.3.

16 Minimum Reflux Ratio The minimum reflux ratio in multicomponent distillation has similar significance as for binary distillation; at this reflux ratio, the desired separation is just barely possible, but an infinite number of plates are required. It is a guide in choosing a reasonable reflux ratio for an operating column and in estimating the number of plates needed for a given separation at certain values of the reflux ratio.

17 An alternate equation for a saturated liquid feed gives the minimum ratio of liquid rate to feed rate for a binary mixture of A and B: / / (22.15) The term in parentheses in Eq. (22.15) are the fractional recovery of A and B in the distillate product. For a multicomponent mixture, these terms would be the specified recovery of light key in the distillate. Note that the minimum value of L depends mainly on the relative volatility. Eq. (22.15) gives good approximation for multicomponent mixtures if the key components make up 90% or more of the feed.

18 Azeotropic and Extractive Distillation Separation of components that have nearly the same boiling point is difficult by simple distillation, because of azeotrope formation. Separtion can be improved by adding 3 rd component to alter the relative volatility of the original components. The 3 rd component can be: Higher-boiling liquid Solvent That is miscible with both of the key components but is chemically more similar to one of them. The key component that is more like the solvent will have a lower activity coefficient in the solution than the other component, so the separation is enhanced.

19 Azeotropic distillation = Separation of the original mixture may also be enhanced by adding a solvent that forms an azeotrope with one of the key components. Usually the material added forms a low-boiling azeotrope and is taken overhead, and such materials are called, entrainers. the azeotrope will surely contain some of all components in the feed, bit it will have much different ratio of the keys than the feed.