Equilibrium ratio atmospheric distillation

Several new problems are encountered when one conducts fractional distillations under reduced pressure. First, as the pressure is decreased, the boiling points of the constituents usually approach each other. This makes it necessary to use a column with a higher number of theoretical plates than would be required at atmospheric pressure. Second, as the pressure is decreased, the vapor velocity is increased. For example, at 25 mm, the vapor velocity for a given boil-up rate would be increased by a factor of 30 over that for atmospheric pressure. At still lower pressures, this ratio increases rapidly. Thus, to obtain comparable vapor-liquid equilibrium, the boil-up rate must be lower at reduced pressures (perhaps by a factor of the square root of the ratio of pressures). 

The olefin metathesis system is used, with the physical properties and reaction kinetics being taken from the literature (Okasinski and Doherty 1998). The reaction is considered only to occur in the liquid phase with a negligible heat of reaction and ideal vapor-liquid equilibrium behavior at atmospheric pressure. The specifications for column operation are taken from Hoffmaster and Hauan (2006). The goal is to convert a pure pentene feed into product streams of butene and hexene with a purity of at least 98 mole percent using a feed flow of 2 kmol/h and a distillate to feed ratio of 0.5. 

A batch distillation column with four theoretical stages (first stage is the still pot) is charged with 100 kmol of a 50 mol% benzene in toluene mixture at atmospheric pressure. At a constant reflux ratio R = 1.5, how many moles of the charge must be distilled if an average product composition of 84 mol% benzene is required If the boil-up ratio is 15 kmol/h, calculate the distillation time. The equilibrium distribution curve at column pressure is given in Example 6.4. 

D6. We are separating a mixture of acetone and ethanol in a distillation column operating at one atmosphere pressure. The column has a total condenser and a partial reboiler. The distillate is 90 mol% acetone, and the bottoms is 10 mol% acetone. The reflux is returned as a saturated liquid. Use a boilup ratio of V/B = 2.0. Two feeds are fed to the column. The first feed has a flow rate of 75 kmol/h, it is a saturated liquid, and it is 60 mol% acetone. The second feed has a flow rate of 100 kmol/h, it is a two-phase mixture that is 60% vapor, and it is 40 mol% acetone. Use the optimum feed location for each feed. Assume CMO. Equilibrium data are in Problem 4.D7. 

D21. A distillation column is separating water fromn-butanol at 1 atmosphere pressure. Equilibrium data are in Table 8-2. The distillation system is similar to Figure 8-3A and has a partial reboiler, a total condenser and a liquid-liquid settler. The bottom layer from the settler (rich in water with x = 0.975) is taken as the distillate product. The top layer (x = 0.573) is returned to the column as a saturated liquid reflux. The feed is 40.0 mol% water, is a saturated vapor and flows at 500.0 kmol/h. The bottoms is 0.04 mole frac water. Use a boilup ratio of V/B = 0.5. Assume CMO is valid. Step off stages from the bottom up. Find the optimum feed stage location and the total number of equilibrium stages needed. 

D7. We will use a batch distillation s5/stem with a still pot and one equilibrium stage (2 equilibrium contacts total) to distill a feed that is 10 mol% water and 90 mol% n-butanol (see Table 8-2 for VLE data). Pressure is one atmosphere. The charge is 4.0 kmoles. We desire a final still concentration that is 2.0 mol% water. The system has a total condenser and the reflux is returned as a saturated liquid. The reflux ratio L7D = 1/2. Find the final number of moles in the still and the average concentration of the distillate. 

D15. We wish to batch distill 100 kmol of a mixture of n-butanol and water. The system consists of a batch still pot plus 1 equilibrium stage. The system is at one atmosphere. The feed is 48 mol% water and 52 mol% butanol. The distillate vapor is condensed and sent to a liquid-liquid settler. The water rich product (0.975 mole fraction water) is taken as the distillate product and the butanol rich layer (0.573 mole fraction water) is refluxed to the column. We desire a final still pot mole fraction of 0.08 water. Energy is added at a constant rate to the still pot thus, V = constant. Note that the distillate product is a constant mole fraction. The reflux ratio increases as the distillate vapor mole fraction decreases during the course of the batch distillation. Equilibrium data are given in Table 8-2. 

Operation is at 1.0 atmosphere. The batch distillation system consists of a still pot plus a column with the equivalent of 9 equilibrium contacts and a total condenser. We operate at a constant external reflux ratio of 2/3. The initial charge to the still pot is 1000.0 kg. We desire a final still pot concentration of 0.004 mole fraction ethanol. Equilibrium data are in Table 2-1. Convert the amount of feed to kmole using an average molecular weight. Find Wfjnai and D otai in kmole, and... 

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