CL 152 Introduction to Chemical Engineering

Transcription

1 CL 152 Introduction to Chemical Engineering 1. A mixture of roane and butane is burned with excess air. Partial analysis of the stack gas roduces the following dry basis volume ercentages: % C 3 H 8, % C 4 H 10, 1.48% CO, and 7.12% CO 2. The stack gas is at an absolute ressure of 780 mm Hg and dew oint of the gas is 46.5 o C. Antoine equation for calculating vaour ressure of water is: log10 = o, where T is in T ( C ) o C and is in mm Hg Calculate the following: a) Molar comosition of the fuel. b) Theoretical air and ercentage excess air. 2. This question consists of three subarts. a) A vessel contains a liquid hase that is in equilibrium with vaour hase at a certain temerature and a ressure of 1 atmoshere. The vaour hase contains 30.0 mole% benzene and 41.6 mole% toluene and the balance is nitrogen. Vaour ressures of benzene and toluene at the temerature of the vessel are mm Hg and mm Hg, resectively. Using Raoult s law, determine the mole fractions of benzene and toluene in the liquid hase? b) C 2 H 4 is sulied to a reactor at 110 o C and 137 atm where it olymerizes to olyethylene. One of your friends thinks that C 2 H 4 could be considered as ideal gas because of high temerature while another friend believes that it is non ideal because of high ressure. Which of them do you suort? Justify your suort. T c = K and P c = 50.5 atm. c) 125 SCMH (standard cubic meters er hour) of a gas containing 70.0 mole% CH 4 and 30.0 mole% H 2 at 40 o C and 1.1 atm is burned with excess air in a furnace. What is the molar flow rate of H 2 in the gas entering the furnace? 3. Answer the following briefly a) What are i) adiabatic flame temerature, ii) higher heating value of a fuel and iii) lower heating value of a fuel? b) What is the benefit of reheating air fed to a furnace? c) Exlain the following seemingly contradictory argument: In an exothermic reaction, heat of reaction is negative, which means that the total enthaly of roducts is less than the total enthaly of reactants. But, if heat is not removed, roducts are at a higher temerature than reactants. d) Exlain in your own words the meaning of wet bulb temerature.

2 e) Draw Langmuir adsortion isotherm and exlain how it could be generated. 4. Air at a dry bulb temerature of 35 o C and a wet bulb temerature of 30 o C is cooled at constant ressure to 20 o C. With the hel of sychrometric charts rovided on age 4 of the exam, answer the following for a basis of 1 kg dry air. i. What is the ercentage relative humidity of air at 35 o C? ( ii. What is the dew oint temerature ( o C)? ( iii. Calculate the amount of water condensed in kg? iv. Estimate heat to be removed for condensation in kj. v. What is the humid volume of the air at 35 o C in m 3? 5. The endothermic liquid hase reaction A + B 2 C roceeds to comletion in a reactor. The liquid mixture in the reactor is vigorously agitated with the hel of an agitator driven by a motor. Agitation ensures good mixing and, the outlet conditions of the reactor are same as the conditions in the reactor. That is, comosition and temerature of the exiting stream are same as those in the reactor. The heat necessary for the reaction is rovided through a jacket surrounding the reactor. Saturated steam at 11.2 bar and o C enters annular sace between the reactor and the jacket and condensate leaves the annular sace at same ressure and temerature (11.2 bar and o C). The heat transfer rate from the condensing steam to the reaction mixture can be estimated by the following equation: Q (kw) = 0.79(T s T) Ts is temerature of condensing steam and T is temerature of reaction mixture, both in o C. Following data are given: Agitator shaft horseower: 25 h (1 h = 746 W) Standard heat of reaction = kj/gmol A B C Feed (gmol/h)

3 Feed Temerature( o C) Secific heat, kj/(mol o C) i) Preare and fill an inlet outlet enthaly table; some of the entries could be in terms of the reactor temerature. ii) Using the table in art i), find the reactor temerature by making an energy balance. iii) Determine the steam flow rate if enthalies of saturated steam and water at 11.2 bar are 2780 kj/kg and 785 kj/kg, resectively. 6. Weather reort in a newsaer indicates that the maximum temerature of air during the day is 32 o C and the relative humidity is 50%. a. Two thermometers are fixed to a board on a wall. The bulb of one of them is wraed with cotton and is artly died in a ool of liquid to kee the cotton moist. What is the temerature read by the thermometers? b. You collect 1 kg of the air at the conditions given in the roblem. To what temerature must it be cooled to condense 75% of water? 7. An equimolar liquid mixture of n hexane and n octane is flowing at a ressure P and 140 o C. Its ressure is reduced to 1 atm and the temerature to 80 o C to roduce a vaour hase and a liquid hase that are in equilibrium with each other. a. What is the minimum value of P in mm Hg? What haens if the ressure is lower than the minimum value? Vaour ressures of n hexane and n octane at 140 o C are 4602 mm Hg and 1113 mm Hg, resectively. b. What fraction of feed liquid is vaourized? Vaour ressures of n hexane and n octane at 80 o C are 1068 mm Hg and 175 mm Hg, resectively. 8. An air stream containing 2.0 mole% carbon tetrachloride (CCl 4 ) is flowing continuously at a rate of 10.0 mole/min through a bed of 1 kg activated carbon. CCl 4 adsorbs on the carbon. The adsortion equilibrium can be redicted an isotherm given by X = 0.076, where X is g CCl 4 / g carbon and is artial ressure of CCl 4 in mm Hg. Predict breakthrough time for the bed. It is the time at which equilibrium is attained and the concentration of CCl 4 in the outlet air steam is same as its inlet concentration. a. Consider a multicomonent mixture consisting of n comonents with x i and y i being mole fractions of i th comonent in liquid and vaour hases, resectively. P is vaour ressure of i th comonent. The mixture is at a ressure P. Exlain i

4 how the bubble oint and the dew oint temeratures of the mixture are estimated. Draw T x y diagram clearly indicating variables on x and y axes and bubble oint and dew oint curves. b. A mixture consisting of 50 mole% benzene and 50 mole% toluene is to be searated by assing it through a valve across which the ressure falls to P (mm Hg). The outlet stream from the valve enters a chamber at the reduced ressure P; the chamber is maintained at a temerature of 70 o C. Estimate the range of P, that is, its lower and uer limits. What haens to searation if the chamber is oerated at ressures beyond the estimated range? Vaour ressures of benzene and toluene at 70 o C are 551 mm Hg and 204 mm Hg, resectively. c. Define distribution coefficient for extraction of a solute (A) from a solvent (B) using another solvent (C). The two solvents, B and C, are mutually immiscible. d. A mixture containing water, acetone and methyl isobutyl ketone (MIBK) slits into two hases sontaneously at 25 o C. One of the hases contains 75 wt% MIBK. Determine the comositions of each hase with the hel of triangular hase diagram rovided. Consult Felder s textbook for ternary equilibrium diagram. e. Draw a Langmuir adsortion isotherm for a solute adsorbing on an adsorbent clearly indicating the variables on x and y axes. Would the amount of the solute adsorbed rise or fall if the temerature of adsortion is raised? f. The equilibrium adsortivity of water on silica gel is given by the exression X (kg water/100 kg silica gel) = 12.5 water water water is artial ressure of water and water is vaour ressure of water at the system temerature. What is the maximum amount of water (kg) that can be adsorbed in 10 kg of the gel? 9. Define Raoult s and Henry s laws. Under what conditions the above laws hold good? Give one examle of a system for each law.

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