UW-Madison CHE 430 - ChE 430 Exam 3 (4 pages)

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ChE 430 Exam 3



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ChE 430 Exam 3

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Pages:
4
School:
University of Wisconsin, Madison
Course:
Che 430 - Chemical Kinetics and Reactor Design
Chemical Kinetics and Reactor Design Documents
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ChE 430 Exam 3 Thursday April 25 2002 NAME Instructions Parts I and II of the exam are closed book and notes You may use anything you have on your laptops for both Part I and Part II Remove your wireless network card during the exam Part I does not require any numerical calculation using the laptop Part I is worth 50 points and part II is worth 50 points Write all answers on separate paper Do not write any answers on the exam itself Hand in your exam as well as your solution at the end Make sure your name is on your solution and your exam Part I Problem 1 PFR Energy Balance 25 pts The first order reaction A B is carried out in a tubular reactor Costs dictate that we must reach 90 conversion for this process to be feasible We would like to operate the reactor adiabatically but product B degrades if the reactor temperature exceeds 500 K a Is it possible to operate the reactor adiabatically with this constraint i e what is the outlet temperature The inlet flow is pure A inlet temperature is 300 K inlet pressure is 10 atm NA0 2 kmol hr MA 50 kg kmol C p 3 09 kJ kg K HR 41 900 kJ kmol Ea 37 700 kJ kmol and k0 3 27 106 1 hr Values of the gas constant include R 8 314 J mol K and R 82 06 atm cm3 mol K b If adiabatic operation is not possible assume we have a heating cooling system available to help us reach the desired conversion We now will use two reactors in series operating the first adiabatically to 500 K and the second isothermally in order to obtain the desired conversion What is the outlet conversion from the adiabatic reactor and what are the reactor volumes required for each reactor Note If finding the reactor volume requires a numerical solution set up but do not solve the equations that are needed to obtain the reactor volume How much heat must be added or removed from the isothermal reactor 1 ChE 430 Exam 3 Thursday April 25 2002 Problem 2 CSTR Energy Balance 25 pts The liquid phase reactions A B A C are each first order in the concentration of A The feed to a nonisothermal CSTR contains pure A at 45 C and 5 mol lit Table 1 lists additional information for the reactions and the reactor We want to operate this reactor such that the selectivity to B is greater than the selectivity to C i e the rate at which B forms is greater than the rate at which C forms Item VR Qf f k1 k2 HR1 HR2 C P value 1000 100 932 3 16 1014 exp 12 500 T 2 52 109 exp 8 500 T 12 000 15 000 0 22 units lit lit min g lit min 1 T in K min 1 T in K cal mol cal mol cal g K Table 1 Kinetic and reactor parameters a Defining the selectivity as S RB RC determine the rate at which heat is transferred to the surroundings to achieve a selectivity of 5 b Determine the rate at which heat is transferred to the surroundings to achieve a selectivity of 4 c Explain why the answer to part a or part b is greater 2 ChE 430 Exam 3 Thursday April 25 2002 Part II Instructions You will require your laptops for Part II of the exam Write any answers on separate paper Do not write any answers on the exam itself Hand in your exam as well as your solution at the end Make sure your name is on your solution and your exam When you have finished insert your wireless network card and email to your TA the program you have written to solve the problem and plot the solution Problem 3 Steady state multiplicity in a CSTR 50 pts Consider the liquid phase elementary reaction k A B The rate constant is temperature dependent 1 1 k T km exp E T Tm Since the chemical B is currently in great demand Badger Chemicals has decided to start producing B They have hired you as a consultant to address operational issues for their constant volume nonisothermal nonadiabatic CSTRs Heat is removed from the reactor by use of a cooling coil You may assume that the cooling medium maintains a constant temperature of Ta Also define VR Qf Table 2 contains the data for the reaction system Parameter Tf Tm Ta C P cAf km E HR Value 298 298 300 4 0 2 0 0 001 8 0 103 103 3 0 105 Units K K K kJ kg K kmol m3 min 1 K kg m3 kJ kmol Table 2 Parameter values a Write down the material and energy balance equations for this system State all assumptions 3 ChE 430 Exam 3 Thursday April 25 2002 b The available cooling coils can provide heat exchange in the range U o A VR 0 100 kJ min K m3 Generate two plots 1 the steady state conversion of A as a function of and 2 the steady state temperature as a function of comparing the steady state solutions for U o A VR values of 0 20 40 60 80 and 100 minkJ For which of these U o A VR values are multiple steady states K m3 possible c For safety reasons the reactor temperature must not exceed 350 K Accordingly Badger Chemicals decides to operate at the following reactor conditions Parameter U o A VR cA T Value 40 44 1 76 311 Units kJ min K m3 min kmol m3 K Table 3 CSTR conditions You are asked to monitor the test run of the reactor Unfortunately you fall asleep on the job When you wake up you discover that the flow rate has changed so for the past five hours 46 minutes You quickly adjust the flow rate so that 44 minutes again Now you are faced with the decision should you shut down the reactor Plot the dynamic response of the reactor temperature for twenty four hours starting when the feed flow rate first changed i e 46 minutes Assume that the reactor was initially at the steady state corresponding to the values given in Table 3 4


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