Fractionation reboiler problems

Problem 16. Cost of Reboiler for Alkylation Unit Heat-Pump Fractionator if Fractionation Column Operation is Assumed to be at Minimum Reflux Ratiof... 

The system represents a 4 X 4 interacting control problem since there are four product compositions to be controlled at each end of both columns. Reflux flowrrates control the distillate purities in each column. Bottoms purity in the high-pressure column is controlled by manipulating the heat input to the reboiler. Bottoms purity in the low-pressure column is controlled by manipulating the fraction of the feed that is fed into the low pressure column. 

Let s finish the problem by determining the flowrates. Once again, all the flowrates can be deduced from the equimolal overflow assumption and the reboil fraction. All the liquid flowrates correspond to the flowrate of the liquid fed to the top ... 

The BP and SR methods for vapor-liquid contacting converge only with difficulty or not at all for separations involving very nonideal liquid mixtures (e.g., in extractive distillation) or for cases where the separator is like an absorber or stripper in one section and a fractionator in another section (e.g., a reboiled absorber). Furthermore, BP and SR methods are generally restricted to the very limited specifications stated above. More general procedures capable of solving ail types of multicomponent, multistage separation problems are based on the solution of all the MESH equations, or combinations thereof, by simultaneous correction (SC) techniques. 

Nevertheless, reboilers and condensers are an integral part of a distillation system. As shown in Chap. 1, problems with reboilers and condensers account for a sizable fraction of distillation malfunctions. Distillation supervisors or troubleshooters cannot perform their duties effectively unless acquainted with the pitfalls in the operation of the column heat exchangers. There have been many instances in which the cause of an apparent poor column performance problem was traced back to the reboiler or condenser. 

D8. A distillation column with a partial condenser and a partial reboiler is separating methanol and water. Column pressure is 1.0 atm We desire 120 kmol/h of a bottoms product that is 0.0001 mole fraction methanol. Boilup ratio v/B is 1.5. Equilibrium data are in Table 2-7 (in Problem 2.D1T and other data are in Problem 3.E1. Assume that all streams entering and leaving the partial reboiler contain very little methanol. Find Qr. 

D13. A distillation column is separating a 30% methanol-70% water feed. The feed rate is 237 kmol/h and is a saturated liquid. The column has a partial reboiler and a partial condenser. We desire a distillate mole fraction of y = 0.95 and a bottoms mole fraction of Xg = 0.025. Assume CMO is valid. Data are in Table 2-7 and Problem 3.E1. a. Find N,... 

D18. We have a stripping column with two feeds separating acetone and ethanol at 1 atm Feed F is a saturated liquid and is fed into the top of column (no condenser). Flow rate of F is 100 kgmol/h, and this feed is 60 mol% acetone. Feed F2 is 40 mol% acetone, it is a two-phase feed that is 80% vapor, and flow rate is 80 kmol/h. We desire a bottoms mole fraction that is 0.04 mole fraction acetone. The column has a partial reboiler. Equilibrium data are in Problem 4.D7. 

D26. A distillation column with a partial condenser and a total reboiler is separating acetone and ethanol. There are two feeds. One feed is 50.0 mol% acetone, flows at 100.0 mol/min, and is a superheated vapor where approximately 1 mole of liquid will vaporize on the feed stage for each 20 moles of feed. The other feed is a saturated liquid, flows at 150.0 mol/min and is 35.0 mol% acetone. We desire a distillate product that is = 0.85 mole fraction acetone and a bottoms product that is Xg = 0.10 mole fraction acetone. The column has a partial condenser and a total reboiler. Boilup is returned as a saturated vapor. Column operates at a pressure of 1.0 atin. Assume CMO and use a McCabe-Thiele diagram. VLE data are given in Problem 4.D7. 

At first Figure 5-4 is a bit confusing, but it will make sense after we go through it step-by-step. Since benzene recovery in the distillate was specified as 99%, benzene is the LK. Typically, the next less volatile conponent, toluene, will be the HK. Thus, cumene is the HNK, and there is no LNK. Following the benzene curve, we see that benzene mole fraction is very low in the reboiler and increases monotonically to a high value in the total condenser. This is essentially the same behavior as that of the more volatile conponent in binary distillation (for exanple, see Figure 4-141. In this problem benzene is always most volatile, so its behavior is sinple. 

H2. [VBA required] Either write your own program or use the program in Appendix A of Chapter 5 to solve the following problem, A feed of 100 mol/h of a saturated liquid that is 25 mol% A = benzene, 35 mol% B = toluene, and 40 mol% C = cumene is fed on the optimum feed plate to a distillation column that has a total condenser and a partial reboiler. Fractional recoveries of B (toluene) in the distillate of 0.9 and of C in the bottoms of 0.97 are desired. The relative volatilities are 2.25, Ogg = 1.0, and = 0.21. Use an external reflux ratio of L/D = 0.3. Find the optimum feed stage, the total number of stages, the fractional recovery of A (benzene) in the distillate, D and B. After solving the problem, try What if simulations to explore the effects of changing the feed concentrations, the fractional recoveries, L/D, and the relative volatility a B-... 

G3. [Fairly involved problem] We wish to distill 80.0 mol/s of a saturated vapor feed at 15.0 atm The feed is 0.100 mole fraction ethane, 0.300 mole fraction propane, 0.500 mole fraction n-butane, and 0.100 mole fraction n-pentane. The column operates at 15.0 atm, has a partial condenser, and produces a vapor distillate. A kettle type reboiler is used. Our goal is to design a column using mass transfer rate analysis that will have a maximum n-butane mole fraction in die vapor distillate of yD,c4,max 0.00875 and a maximum mole fraction of propane in the liquid... 

Nominal design values of the spatial structure of the column for the synthesis of MTBE (i.e. column diameter and reboiler and condenser heat exchange areas) are firstly estimated. They are obtained by solving a steady-state optimization problem, which minimizes the total annualized cost of the RD unit in the absence of disturbances. The following constraints are included to the problem formulation (i) the column diameter is bounded by flooding conditions (ii) the heat exchange areas of the condenser and reboiler are estimated by bounded values of outlet temperatures of the hot and cold utilities, and (m) the molar fraction of MTBE at the top and bottom stream is constrained to values lower than 0.1% and 99% respectively. Thus, the statics optimization problem results in,... 

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