Cork Institute of Technology. Summer 2005 CE4.6 Chemical and Biochemical Reactors (Time: 3 Hours) Section A

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1 Cork Institute of Technology Bachelor of Engineering (Honours in Chemical and Process Engineering Award (Bachelor of Engineering in Chemical and Process Engineering Award Answer any FOUR questions. (NFQ Level 8 Summer 005 CE4.6 Chemical and Biochemical Reactors (Time: 3 Hours Use separate answer books for each Section. Examiners: Mr. I. O Sullivan r. M. Raissian Prof. R. Ocone Mr.. O Connor Section A Q1. (a A medium containing a vitamin is to be sterilized. Assume that the number of spores initially present is 10 5 /L. The values of the pre-arrhenius constant and E od for the spores are E od = 65 kcal/g-mol α = min -1 similarly for the vitamin the following constants are valid E od = 10 kcal/g-mol α = 10 4 min -1 the initial concentration of the vitamin is 30 mg/l. Compare the amount of active vitamin in the sterilized medium for 10L and 10,000L fermenters when both are sterilized at 11 C. In both cases the probability of an unsuccessful fermentation is to be Ignore the effects of the heat up and the cool down periods. (0 marks A continuous culture system is being constructed. The fermentation tank is to be 50,000 L in size and the residence time is to be h. A continuous sterilizer is to be used. The unsterilized medium contains 10 4 spores. The value of k d has been determined to be 1 min -1 at 11 C and 61 min -1 at 140 C. For each temperature, determine the required residence time in the holding section so as to ensure that 99% of the time those 4 weeks of continuous operation can be obtained without contamination. (5 marks Q. (a Consider a 1000L CSTR in which Biomass is being produced with glucose as the substrate. The microbial system follows a Monod relationship with µ m = 0.4 h -1, K S = 1.5

2 g/l and a yield factor Y X/S = 0.5 g biomass/g substrate consumed. If normal operation is with a sterile feed containing 10 g/l of glucose at a rate of 100 g/h determine (i What is the specific biomass production rate (g/l-h at steady state (ii if a recycle is used with a recycle stream of 10 l/h and a recycle biomass concentration five times as large as that at the reactor exit what would be the new specific biomass production rate (g/l-h at steady state (iii explain the differences between the values found in part (i and (ii. (10 marks Consider the following batch growth data, which was obtained in a laboratory scale batch reactor t (hrs X (g/l P (g/l dx/dt (g/lh dp/dt (g/lh A three stage chemostat system is to be utilized to produce product P and the vessels are arranged optimally in the following order, 900L, 600L and 300L. What is the product concentration from the 300L vessel? (15 marks

3 Q3. (a In a fluidized bed biofilm reactor, cells are attached on spherical plastic particles to form biofilms of average thickness (L = 0.5 mm. The bed is used to remove carbon compounds from a waste water stream. The feed flow rate and concentration of total fermentable carbon compounds in the feed are F = l/h and S = 000 mg/l. The diameter of the column is 10 cm. The kinetic constants of the microbial population are r m = 50 mgs/cm 3 h and K S = 5 mgs/cm 3. The specific surface area of the biofilm in the reactor is 5 cm /cm 3. Assuming first order reaction kinetics and an average effectiveness factor of η = 0.7 throughout the column. Perform a substrate balance on a differential element of this column and thus determine the required height of the column for effluent total carbon concentration of S o,e = 100 mg/l. The following data was obtained in a constant pressure filtration unit for filtration of a yeast suspension t (sec (l The filter has the following characteristics: A = 0.8 m C = 190 kg/m 3 µ = kg/ms α = 4 m/kg etermine the pressure drop across the system, the filter medium resistance and the size of filter required for the same pressure drop to process 4000L of cell suspension in 0 minutes. rs m rs = K + S S ( 1 P0 t = 1 1 e kt 1 0 EO,d RT ( µ= 1+α 1 C C = C e k =αe + O = kt kt d where O rm A = and αc k A = Pg C α µ c 3

4 Section B Q4. (a For first-order reactions, the performance of a real reactor which is close to plug flow (i.e. when is small, may be given by: UL 1 X A = exp kτ + ( kτ UL show that for identical conversion the ratio of length of the real reactor to plug flow reactor is given by: L L P = p = 1+ ( kτ UL Assume constant density throughout. (1 marks Tubular reactors for cracking reactions are designed on the assumption of plug flow. It is suspected that non-ideality may be an important factor which is being ignored. You are required to make an estimate of its role. For this consider 1 st order cracking in a 5.4 mm inside diameter tubular reactor. If calculations show that 99% decomposition is obtained in a plug flow reactor 3 m long what should be the length of the real reactor? Assume that intensity of dispersion ( is 0.5 for the reactor. (13 marks Ud Q5. (a iscuss the relative merits of packed and fluidised beds for solid catalysed reactions. What is the overwhelming advantage of fluidised beds over packed beds? (13 marks iscuss, with the aid of sketches, etc, the cold-shot cooling and when it can be used. (1 marks Q6. (a Show with the aid of sketches, etc, for particles of constant size, with unreacted core model, when ash-layer resistance is controlling, then, t rc rc = τ R R where t = time required for the particle to react to r c τ = time for complete conversion of particle (i.e. r c = 0 N.B. The reaction is given by: 3 A( gas + bb( solid Products (1 marks 4

5 A feed consisting: 0% of 0 µ particles 30% of 50 µ particles 30% of 100 µ particles 0% of 00 µ particles is to be reacted in a fluidised-bed steady-state reactor. The fluidising gas is the gas-phase reactant, and at the planned operating conditions the time needed for complete conversion is 4, 5, 100 and 400 minutes for the four sizes of feed. Find the conversion of solids in the reactor for a feed rate of 10 kg solids/min if the bed contains 1,000 kg solids. State clearly any assumptions that you make. For fluidised bed, assume: Rm 1 τ ( Ri 19 τ ( Ri F( Ri 1 xb = { +...} 5 t 40 t F (13 marks 5