Multistage Distillation Multistage flash technique Reasons for devising

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Multistage Distillation Ø Multistage flash technique Ø Reasons for devising multistage process are: a)

Multistage Distillation Ø Multistage flash technique Ø Reasons for devising multistage process are: a) Increase product purity b) To reduce consumption of separating agents Ø A single stage flash distillation generally produces a vapor that is only somewhat richer in the lower-boiling constituent than the feed. Ø Further enrichment can be achieved by a series of flash distillation where the vapor from each stage is condensed then re-flashed. Ø In principle, any desired product purity could be obtained by such a multi-stage flash technique provided a suitable number of stages are employed. Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Multistage Distillation Ø In order to separate a binary mixture using distillation process, there

Multistage Distillation Ø In order to separate a binary mixture using distillation process, there must be a differences in volatilities of the components. The greater the difference, the easier it is to do so. A measure for this is termed the relative volatility. Ø We define volatility of component-i as: partial pressure of component-i divide by mole fraction component-i in liquid ØFor a binary mixture of A and B, therefore: Volatility of A = p. A / x. A Volatility of B = p. B / x. B where p. A, p. B is the partial pressure of component A, B and x is the liquid mole fraction. Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Multistage Distillation Replace with p. A = y. A PT ; p. B =

Multistage Distillation Replace with p. A = y. A PT ; p. B = ( 1 - y. A ) PT so as to express everything in MVC: In general: Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Multistage Distillation Ø When = 1. 0, no separation is possible: both component-A and

Multistage Distillation Ø When = 1. 0, no separation is possible: both component-A and component-B are equally volatile. They will vaporize together when heated. Solving the above equation for = 1. 0, we obtain: y = x. Ø The larger the value of above 1. 0, the greater the degree of separability, i. e. the easier the separation Ø If the value of is known and is constant, we can use it to obtain the equilibrium curve. This can be done by rearranging the equation for relative volatility, to obtain the function y = f(x): Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Multistage Distillation Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Multistage Distillation Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Multistage Distillation Ø Multistage Distillation. Ø Consider a process in which a benzene-toluene mixture

Multistage Distillation Ø Multistage Distillation. Ø Consider a process in which a benzene-toluene mixture is to be separated into benzene and toluene Ø Suppose, however, that the feed consists of 50 mol % benzene and 50 mol % toluene The first small amount of vapor generated will have a composition given by: B-T = 2. 25 at 121 o. C and P= 1 atm y. B Feed z. B=0. 5 z. T=0. 5 Where steam x. B is the relative volatility of benzene relative to toluene Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Multistage Distillation y. B=0. 691 Feed z. B=0. 5 z. T=0. 5 steam x.

Multistage Distillation y. B=0. 691 Feed z. B=0. 5 z. T=0. 5 steam x. B One way to obtain a richer benzene product is to condense a portion of vapor generated in the first stage and re-flashed again y`B=0. 838 y. B=0. 691 Feed z. B=0. 5 z. T=0. 5 water x`B steam Ch. E 334: Separation Processes x. B Dr Saad Al-Shahrani

Multistage Distillation By extending this concept, we can obtain the benzene product of the

Multistage Distillation By extending this concept, we can obtain the benzene product of the required purity Vn Vn-1 Vn-2 V 1 water VO water Feed steam water Ln-1 L 1 LO Process for production of pure benzene Ch. E 334: Separation Processes Dr Saad Al-Shahrani Ln

Multistage Distillation Example: Benzene- Toluene mixture Concentration of more volatile component in vapor phase

Multistage Distillation Example: Benzene- Toluene mixture Concentration of more volatile component in vapor phase y 3 V 3=8. 3=L 3/2 V 2=25=V 1/2 y 2 V 1=50=F/2 Feed L 3= 16. 7 x 3 y 1 L 2= 25 x 2 z. B=0. 55 z. T=0. 45 B-T=7. 0 Ch. E 334: Separation Processes L 1= 50 x 1 Dr Saad Al-Shahrani

Multistage Distillation Plot y Vs x 1. 0 Slope =-1 Stage#1 y 1 =0.

Multistage Distillation Plot y Vs x 1. 0 Slope =-1 Stage#1 y 1 =0. 78 y X 1=0. 33 y 1= 0. 78, x 1=0. 33 Ch. E 334: Separation Processes 0 0 x z=0. 55 Dr Saad Al-Shahrani 1. 0

Multistage Distillation Stage # 2 Slope =-1 3 y 2=0. 92 V 1 y

Multistage Distillation Stage # 2 Slope =-1 3 y 2=0. 92 V 1 y 1 2 y 1 =0. 78 1 = V 2 y X 2=0. 64 X 1=0. 33 y 2= 0. 92, x 2=0. 64 Ch. E 334: Separation Processes z=0. 55 z 2=0. 78 x z 3=0. 92 Dr Saad Al-Shahrani

Multistage Distillation stage 1 2 3 V 50 25 8. 3 Ch. E 334:

Multistage Distillation stage 1 2 3 V 50 25 8. 3 Ch. E 334: Separation Processes L 50 25 16. 7 x. B 0. 33 0. 64 0. 9 y. B 0. 78 0. 92 0. 98 x. T 0. 67 0. 36 0. 1 y. T 0. 22 0. 08 0. 02 Dr Saad Al-Shahrani

Multistage Distillation Process for production of pure benzene and toluene Ch. E 334: Separation

Multistage Distillation Process for production of pure benzene and toluene Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Multistage Distillation Recycling of intermediate products Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Multistage Distillation Recycling of intermediate products Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Distillation of Binary Mixture Ø In distillation (fractionation), a feed mixture of two or

Distillation of Binary Mixture Ø In distillation (fractionation), a feed mixture of two or more components is separated into two or more products, including, and often limited to, an overhead distillate and a bottoms whose compositions differ from that of the feed. Ø The feed is a liquid or a vapor-liquid mixture. The bottoms product is almost always a liquid, but the distillate may be a liquid or a vapor or both. Ø The separation requires that: (1) a second phase be formed so that both liquid and vapor phases are present and can contact each other on each stage within a separation column. Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Distillation of Binary Mixture (2) The components have different volatilities so that they will

Distillation of Binary Mixture (2) The components have different volatilities so that they will partition between the two phases to different extents. (3) The two phases can be separated by gravity or other mechanical means Example: Distillation of a binary mixture of benzene and toluene. The purpose of the 25 sieve-tray (equivalent to 24 theoretical stages plus a partial re-boiler that acts as an additional theoretical stage) distillation column is to separate the feed into a liquid distillate of 99 mol% benzene and a liquid bottoms product of 98 mol% toluene. Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Distillation of Binary Mixture Δ P = 0. 1 psia / tray The pressure

Distillation of Binary Mixture Δ P = 0. 1 psia / tray The pressure in the reboiler = 18 + 0. 1 (25) =20. 5 psia In this range of pressure, benzene and toluene form nearly ideal mixture with a relative volatility: B-T = 2. 26 at the bottom. T-B = 2. 52 at the top tray. Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Distillation of Binary Mixture Table: shows a list of the more commonly binary distillation

Distillation of Binary Mixture Table: shows a list of the more commonly binary distillation operations in decreasing order of difficulty of separation. Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Distillation of Binary Mixture Ø EQUIPMENTS AND DESIGN CONSIDERATIONS Industrial distillation operations are most

Distillation of Binary Mixture Ø EQUIPMENTS AND DESIGN CONSIDERATIONS Industrial distillation operations are most commonly conducted in trayed tower, but packed columns are finding increasing use. Occasionally, distillation columns contain both trays and packing. Ø Factors that influence the design or analysis of a binary distillation operation include: 1. Feed flow rate, composition, temperature, pressure, and phase condition. 2. Desired degree of separation between two components. 3. Operating pressure (which must be below the critical pressure of the mixture. 4. Vapor pressure drop, particularly for vacuum operation. 5. Minimum reflux ratio and actual reflux ratio. Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Distillation of Binary Mixture 6. Minimum number of equilibrium stages and actual number of

Distillation of Binary Mixture 6. Minimum number of equilibrium stages and actual number of equilibrium (stage efficiency) 7. Type of condenser (total, partial, or mixed) 8. Degrees of su-bcooling, if any, of the liquid reflux 9. Type of re-boiler (partial or total) 10. Type of contacting (trays or packing or both) 11. Height of the column 12. Feed entry stage 13. Diameter of the column 14. Column internals. Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Distillation of Binary Mixture Trayed tower Ch. E 334: Separation Processes Packed tower Dr

Distillation of Binary Mixture Trayed tower Ch. E 334: Separation Processes Packed tower Dr Saad Al-Shahrani

Distillation of Binary Mixture Ø Plate Towers The most common single-vessel device for carrying

Distillation of Binary Mixture Ø Plate Towers The most common single-vessel device for carrying out distillation is plate tower. Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Distillation of Binary Mixture Ø In the Plate Tower: 1. plates, or trays, on

Distillation of Binary Mixture Ø In the Plate Tower: 1. plates, or trays, on each of which vapor and liquid are contacted. 2. The liquid flows down the tower under the force of gravity. 3. the vapor flows upward under the force of as light pressure drop from plate to plate. The highest pressure is produced by the boiling in the bottom steam heater, called the re-boiler. 4. vapor liquid The vapor passes up ward through openings in each plate and contacts the liquid flowing downward across the plate. 5. equilibrium between the vapor and liquid streams leaving the plate, each plate acts as separator vessels. 6. section. and The portion of the tower below the feed is called (stripping section) Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Distillation of Binary Mixture Ø Types of Plates (Trays) 1. Sieve tray It is

Distillation of Binary Mixture Ø Types of Plates (Trays) 1. Sieve tray It is the simplest type of plate, which consists of a metal plate with 3 -15 mm holes, spaced in a regular pattern. Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Distillation of Binary Mixture (Tray diameter) Details of a contacting tray in a trayed

Distillation of Binary Mixture (Tray diameter) Details of a contacting tray in a trayed tower. Ch. E 334: Separation Processes A 2. 1 -m-diameter perforated (sieve) tray. The down-comer to the next tray is located on the left. Dr Saad Al-Shahrani

Distillation of Binary Mixture 2. Bubble-cap tray A bubble cap tray has bubble caps

Distillation of Binary Mixture 2. Bubble-cap tray A bubble cap tray has bubble caps that consists of a fixed cap, 3 – 6 inches in diameter, mounted over and above a riser of 2 – 3 inches in diameter. The cap has rectangular or triangular slots cut around its side cap. The vapor flows up through the tray opening into the riser, turns around and pass. Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Distillation of Binary Mixture 3. valve-cap tray The riser of the valve cap is

Distillation of Binary Mixture 3. valve-cap tray The riser of the valve cap is supported by the momentum of the up following vapor. At the higher velocities the riser is fully open, while at lower vapor velocities the riser is partially or completely lowered. Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Distillation of Binary Mixture Valve cap Bubble cap Perforation (sieve) Ch. E 334: Separation

Distillation of Binary Mixture Valve cap Bubble cap Perforation (sieve) Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Distillation of Binary Mixture Grid type packing 1. 8 m in diameter and 2.

Distillation of Binary Mixture Grid type packing 1. 8 m in diameter and 2. 4 m deep Ch. E 334: Separation Processes Packed column Dr Saad Al-Shahrani

Distillation of Binary Mixture Common packing shapes Ch. E 334: Separation Processes Dr Saad

Distillation of Binary Mixture Common packing shapes Ch. E 334: Separation Processes Dr Saad Al-Shahrani

Distillation of Binary Mixture Ø Down-comer A separate compartment provide sufficient volume and a

Distillation of Binary Mixture Ø Down-comer A separate compartment provide sufficient volume and a long enough residence time for the liquid to be freed of entrained vapor before reaching the bottom and entering the next plate. Ø weir Connected with each tray in order to give good mixing between phases and provide the necessary disengagement of vapor and liquid between stage. Ch. E 334: Separation Processes Dr Saad Al-Shahrani