Cork Institute of Technology. Summer 2005 CE3.6 Reactor Design and Biochemical Engineering (Time: 3 Hours) Section A

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1 Cork Institute of Technology Bachelor of Engineering (Honours) in Chemical & Process Engineering Stage 3 (Bachelor of Engineering in Chemical and Process Engineering Stage 3) (NFQ Level 8) Summer 005 CE3.6 Reactor Design and Biochemical Engineering (Time: 3 Hours) Instructions Answer any FOUR questions. Use separate answer books for each Section. All questions carry equal marks. Section A Examiners: Mr. I. O Sullivan Dr. S. M. Raissian Prof. R. Ocone Mr. D. O Connor Q1 (a) Ethanol is to be used as a substrate for single-cell protein production in a chemostat. The available equipment can achieve an oxygen transfer rate of 10 go /l.h. Assume the kinetics of cell growth on ethanol is of the Monod type, with µ m = 0.5 h -1, K S = 30 mg/l, Y X/S = 0.5, Y O/S =. We wish to operate the chemostat with an ethanol concentration in the feed of g/l. We also wish to maximise the biomass productivity and minimise the loss of ethanol in the effluent. Determine the required dilution rate and whether sufficient oxygen can be provided. Explain your reasoning and state any assumptions made. (10 marks) (b) A strain of mold was grown in a batch culture on glucose and the following data was obtained t (mins) X (g/l) S (g/l)

2 Calculate the net specific growth rate. (8 marks) Calculate the maximum specific growth rate. (1 marks) Calculate the apparent growth yield (1 marks) Calculate the biomass doubling time. ( marks) What maximum cell concentration could one expect if 150 g of glucose were used with the same size inoculum. (1 marks) What are primary and secondary metabolites? ( marks) Q (a) The air supply to a fermenter was turned off for a short period of time and then restarted. A value for C* of 7.3 mg/l has been determined for the operating conditions. Use the tabulated measurements of dissolved oxygen (DO) values to estimate the oxygen uptake rate and k L a for the system using the dynamic method. (14 Marks) If you only had the Air on data suggest an alternative method for determining the oxygen uptake rate and k L a for the system. (4 Marks) Time (mins) DO (mg/l) Air off Air on Note: See Figure Q attached

3 Q (b) The following BOD results were obtained from a sample of untreated wastewater at 0 C. t,days BOD t, mg/l Show that the reaction rate constant, k and the ultimate BOD can be determined graphically using this data, given that dbod dt r = k BOD 1 r Hint 1: It is not necessary to use the least squares method Hint : It will be necessary to plot dbod t /dt Vs BOD t (7 marks) Q3 A complete mix activated sludge system with recycle is used to treat municipal wastewater after primary sedimentation. The characteristics of the primary effluent are: flow = 1000 m 3 /d, bscod = 19 g/m 3, nbvss = 30 g/m 3, and inert organics = 10 g/m 3. The aeration tank MLVSS = 500 g/m 3. Using this data and the kinetic coefficients given below, design a system with a 6-d SRT and determine the following (i) What is the effluent bscod concentration? (ii) What value of τ should be used so that the MLVSS concentration is 500 g/m 3? (iii) What is the daily sludge production in kg/day as VSS and TSS? (iv) What is the fraction of biomass in the MLVSS? (v) What is the observed solids yield in g TSS/g bscod? (vi) What is the oxygen requirement in kg/d? Kinetic Coefficients k = 1.5 g COD/g VSSd K s = 10 g COD/m 3 Y = 0.4 g VSS/g COD used f d = 1.15 g VSS/g VSS k d = 0.1 g VSS/g VSSd Biomass VSS/TSS = 0.85 Useful Equations 3

4 P QY(S S) (f )(k )YQ(S S)SRT o d d o X,VSS = (k d)srt 1 + (k d)srt QX o,i K[1 + (k)srt] S d S = [SRT(Yk k d )] 1 ( o ) ( ) YS SSRT X = 1+ kd SRT τ ( ) R = Q S S 1.4P S o o X,bio ( + ) K D k S d = µ m D kd DP = q X p µ= 1dX X dt dc dt l ( ) = kac* C q X L l O D=µ m 1 KS K + S s o µ ms µ= K + S S Section B 4

5 Q4. (a) Derive, from first principles, the performance equations (both general and constant density) for mixed flow reactor. Define any term that is used and give its units. (8 Marks) (b) A homogeneous liquid phase reaction A R -r A = k C A takes place with 50% conversion in a mixed reactor. (i) What will be the conversion if this reactor is replaced by one six times as large all else remaining unchanged? (8 Marks) (ii) What will be the conversion if the original reactor is replaced by a plug flow reactor of equal size all else remaining unchanged? (9 Marks) Data: dx x = (1 x) 1 x. Q5. (a) Discuss fully, with the aid of sketches, diagrams, etc., how to find outlet conversion from a given series of unequal size mixed reactors. Assume constant density. (1 Marks) (b) A first order reaction is to be treated in a series of two mixed reactors. Show that the total volume of the two reactors is minimum when the reactors are equal in size. Assume constant density reaction. (13 Marks) 5

6 Q6. The reversible first order reaction A k 1 k R is to be carried out at 300K and 400K in a reactor. Calculate the equilibrium conversions at these two temperatures and hence deduce if 50% conversion of reactant is possible at any of these two temperatures. Data: k 1 = 10 3 exp [-4800/RT] Cp = Cp R Cp A = 0. Hr = Cal/mol at 300K K = k k 1 = 10 at 300K Feed consists of pure A and total pressure remains constant. R = Cal/mol K. (5 Marks) 6