mass transfer

CHE 411 OSU Chemical, Biological and Environmental E ngineering Fall 201 7 Mass Transfer Operations Page 1 of 3 Name: __________________________________ OSU ID #: ______________ Homework Assignment #3 Due Monday , October 23 at 11:00 AM in class. Problem 1. A process waste stream consists of water which is contaminated with 17 mole% of a volatile organic solvent (species “A”). It is desired to transfer the organic solvent from the liquid stream into air, using a tray tower. The outlet air is to be sent to a flare stack to burn the solvent vapor. Some of the air leaving the tower must be recycled, so the inlet air contains 1.5 mole% of species A. The liquid stream enters the tower at a total molar flow rate of 25 kgmole/hr, while the air stream enters the to wer at a total molar flow rate of 57 kgmole/hr. The outlet liquid stream must contain no more than 4 mole% of the organic solvent. a) What are the molar flow rates and composition of the liquid and vapor streams leaving the unit? (ans: Va ≈ 60 kgmole/hr ) b) How many real trays should be installed in the tower, if the average tray efficiency is 60%? c) What is the minimum possible flow rate of vapor into the tower, Vb,min that would achieve the required separation? What would be the corresponding mole fraction of A in the vapor phase, ya,min ? (ans: ya,min = 0.135) d) Would decreasing the liquid outlet concentration to 1 mole% be possible? Briefly explain your answer with a few sentences and/or a diagram. e) If you could increase the liquid outlet concentration to 5 mole%, how would this affect the maximum possible concentration in the vapor ? Equilibrium solubility data for species A at the operating temperature and pressure: x 0.000 0.03 0.06 0.11 0.17 0.21 y 0.000 0.0 15 0.038 0.080 0.151 0.221 Problem 2 . A counter -current gas absorption tray tower with three trays will be used to remove HCl gas from a mixture of HCl gas in air. Recycled w ater , which contains some dissolved HCl , will be used as the scrubbing solvent. In the present process, a mixture of air and HCl gas, containing 8.0 mole% HCl, is fed into the tower at a total molar flowrate of 38 .0 lbmole/hr. The liquid stream leaving the tower contains 32.4 mole% HCl dissolv ed in water, and its total molar flowrate is 22.0 lbmole/hr . The tower is operated at a constant temperature of 20°C. You may ignore the heat of solution for the acid vapor, and assume ideal equilibrium stage operation for the trays . Is the system dilu te? CHE 411 OSU Chemical, Biological and Environmental E ngineering Fall 201 7 Mass Transfer Operations Page 2 of 3 a) Create a well -designed spreadsheet to plot the operating line and equilibrium curve. Format the plot and spreadsheet to optimize readability, and use named cells to ensure accurate stage -to-stage calculations. b) What is the mole fraction of HCl in the gas exiting the tower, and liquid entering the tower? (ans: xa ≈ 0.2 3) Experimental equilibrium solubility data for HCl in water, 1.0 atm, 20°C: x 0.000 0.201 0.243 0.287 0.330 0.353 0.375 0.400 0.425 y 0.000 0.002 0.010 0.022 0.052 0.085 0.135 0.203 0.322 Problem 3. “Steam stripping” is often used for the treatment of contaminated waste water, in situations where the dissolved organic contaminant is appreciably volatile only at high temperatures. The figure below shows a steam stripping pro cess for the removal of dissolved dichloromethane ( DCM , CH 2Cl 2) from a waste water stream. In this process, 1500 lbmole/hr of waste water at 25°C containing 1.25 x10 -2 mole% DCM is preheated with steam to 100°C, and then the hot liquid stream is fed into the top of a tray tower. Saturated steam at 1.0 atm is injected into the bottom of the tower , at a mass flowrate of 225 lbm/hr. The waste water leaving the tower may contain no more than 1.0x10 -3 mole% of dichloromethane. The trays used in thi s tower have an average efficiency of 65%. Heat losses from the tower itself can be neglected , as the tower shell is adiabatic (insulated ). T he tower is approximately isobaric at 1.0 atm total pressure. Equilibrium data for dilute , dissolved CH 2Cl 2 in water at or near 100°C is given on the next page. CH 2Cl 2 /steam condensate (to solvent recovery) Saturated steam @ 1.0 atm, 225 lb m/hr Waste water & CH 2Cl 2 1500 lbmole /hr , 25 °C (xa= 1.25 x 10 -2mole % CH 2Cl 2) Condenser Feed preheater Water & residual CH 2Cl 2 (xb ≤ 1.0 x 10 -3mole % CH 2Cl 2) Saturated liquid @ 1.0 atm / 100 °C CHE 411 OSU Chemical, Biological and Environmental E ngineering Fall 201 7 Mass Transfer Operations Page 3 of 3 x 0.000 1.2E -05 5.0E -05 8.0E -05 2.0E -04 y 0.000 0.002 0.008 0.013 0.035 a) Using the McCabe -Thiele graphical method, estimate the number of ideal and real stages required for the process. (ans: Nreal = 7). b) Estimate the amount of energy ( in kJ/hr) required to preheat the inlet water stream from 25°C to the operating temperature of 1 00 °C. (ans: ~3.9x10 6 kJ/hr) c) Sketch a modification to the process equipment which woul d greatly reduce the total amount of input energy required by the process. Problem 4 (Extra Credit) . Look through company Web sites, other text books , product literature or trade magazines (e.g. Chemical Engineering Progress) for a picture of a packed bed or staged tower being used for a gas absorption or liquid stripping process . Include a clear c opy of the picture (ideally in color) , and summarize the following : a) What is t he c ompany name and/or location of the equipment ? What type of plant is it? b) What type of process is being performed: gas absorption, liquid stripping , or something else? c) W hat are the carrier gas, liquid solvent, and transferring solute ? Why were they chosen? d) For what reason is this process being performed ? Is a final product or a process intermediat e being produced, or is the tower used to clean up an outlet stream? e) Provide as much detail as you can on the e quipment design (type of packing or trays used , tower height and diameter, tower shell material (s) , operating temperature and pressure, etc.) f) Also include any other interesting or unusual facts about the process or tower . Were any major design innovations or tradeoffs necessary to accomplish the required processing? You must cite a reference or website for the picture to receive any credit. N eatness and grammar count ! No credit for incomplete (see above) or vague/trivial answers.