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

Multistage Distillation Consider a process in which a benzene-toluene mixture is to be separated into benzene and toluene 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 121oC and P= 1 atm Feed zB=0.5 zT=0.5 yB xB steam ChE 334: Separation Processes Dr Saad Al-Shahrani

Multistage Distillation Feed zB=0.5 zT=0.5 yB=0.691 xB steam One way to obtain a richer benzene product is to condense a portion of vapor generated in the first stage and re-flashed again Feed zB=0.5 zT=0.5 yB=0.691 xB y`B=0.838 x`B steam water ChE 334: Separation Processes Dr Saad Al-Shahrani

Multistage Distillation By extending this concept, we can obtain the benzene product of the required purity Feed VO LO V1 L1 steam water Vn-2 Vn-1 Ln-1 Vn Ln Process for production of pure benzene ChE 334: Separation Processes Dr Saad Al-Shahrani

Multistage Distillation Example: Benzene- Toluene mixture Concentration of more volatile component in vapor phase Feed zB=0.55 zT=0.45 B-T=7.0 V1=50=F/2 L2= 25 y1 y2 L1= 50 V2=25=V1/2 V3=8.3=L3/2 L3= 16.7 y3 ChE 334: Separation Processes Dr Saad Al-Shahrani

Multistage Distillation Plot y1 Vs x1 1.0 Slope =-1 Stage#1 y1 =0.78 y X1=0.33 y1= 0.78, x1=0.33 z=0.55 x 1.0 ChE 334: Separation Processes Dr Saad Al-Shahrani

Multistage Distillation x y X1=0.33 y1 =0.78 z=0.55 1 2 X2=0.64 y2=0.92 Slope =-1 3 y1 V1 V2 y2= 0.92, x2=0.64 ChE 334: Separation Processes Dr Saad Al-Shahrani

Multistage Distillation V L xB yB xT yT 1 50 0.33 0.78 0.67 0.22 2 25 0.64 0.92 0.36 0.08 3 8.3 16.7 0.9 0.98 0.1 0.02 ChE 334: Separation Processes Dr Saad Al-Shahrani

Distillation of Binary Mixture 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. 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. 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. ChE 334: Separation Processes Dr Saad Al-Shahrani

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 20 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. ChE 334: Separation Processes Dr Saad Al-Shahrani

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. ChE 334: Separation Processes Dr Saad Al-Shahrani

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

Distillation of Binary Mixture Equipment 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. Types of trays and packing 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. ChE 334: Separation Processes Dr Saad Al-Shahrani

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. ChE 334: Separation Processes Dr Saad Al-Shahrani

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

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

Distillation of Binary Mixture 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. 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. The vapor passes up ward through openings in each plate and contacts the liquid flowing downward across the plate. 5. If the mixing of vapor and liquid on the plates were sufficient to provide equilibrium between the vapor and liquid streams leaving the plate, each plate acts as separator vessels. 6. The portion of the tower above the feed is called the rectifying or enrichment section. and The portion of the tower below the feed is called (stripping section) ChE 334: Separation Processes Dr Saad Al-Shahrani

Distillation of Binary Mixture Types of Plates (Trays) 1. Sieve tray As shown in figure (4.8, King). It is the simplest type of plate, which consists of a metal plate with 3-15 mm holes, spaced in a regular pattern. ChE 334: Separation Processes Dr Saad Al-Shahrani

Details of a contacting tray in a trayed tower. Distillation of Binary Mixture (Tray diameter) Details of a contacting tray in a trayed tower. A 2.1-m-diameter perforated (sieve) tray. The down-comer to the next tray is located on the left. ChE 334: Separation Processes Dr Saad Al-Shahrani

Distillation of Binary Mixture 2. Bubble-cap tray Bubble-cap 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 up through the tray opening into the riser, turns around and pass. ChE 334: Separation Processes Dr Saad Al-Shahrani

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 velocities the riser is fully open, while at lower vapor velocities the riser is partially or completely lowered. ChE 334: Separation Processes Dr Saad Al-Shahrani

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

Distillation of Binary Mixture Grid type packing 1.8 m in diameter and 2.4 m deep Common packing shapes ChE 334: Separation Processes Dr Saad Al-Shahrani

Distillation of Binary Mixture 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. weir Connected with each tray in order to give good mixing between phases and provide the necessary disengagement of vapor and liquid between stage. ChE 334: Separation Processes Dr Saad Al-Shahrani