Code No: 07A608 07A60803 03
R07
Set No. 2
III B.Tech II Semester Examinations,December-January, 2011-2012 CHEMICAL REACTION ENGINEERING-II Chemical Engineering Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks
1. (a) Discuss Discuss about about axial axial dispersi dispersion on and the disper dispersio sion n model. model. What are are its limit limitaations and applications? (b) Water is drawn from a lake, flows through a pump and passes down a long pipe in turbulen turbulentt flow. flow. A slug of tracer (not an ideal ideal pulse input) input) enters enters the intake intake line at the lake, lake, and is recorded downstream downstream at two two locations in the pipe L meters apart. The mean residence time of fluid between recording points is 100sec, and variance of the two recorded signals is σ12 = 800 sec2 σ22 = 900 sec2 What would be the spread of an ideal pulse response for a section of this pipe, free from end effects and of length L/5? [8+8] 2. What is catalyst deactivation? How and why it occurs? What are the factors that are resp onsible for deactivating a porous catalyst pellet? [16] 3. (a) Give Give a brie brieff accoun accountt of E, the the exit exit age age distr distrib ibuti ution on and the experim experimen ental tal methods for finding E curve. (b) A pulse input input to a vesse vessell gives gives the results results shown shown in figure figure 3 i. Check Check the material balance balance with the tracer curve to see whether the results results are consistent. consistent. ii. If the result result is consi consisten stent, t, determi determine ne t, V and sketc sketch h the E curve. curve. [8+8] [8+8]
Figure 3 4. What is surface diffusion. Explain its mechanism. Discuss the temperature dependence of the surface diffusion. [16] 5. (a) For a first-ord first-order er react reaction ion with with ε = 0 show that the performance equations for a macrofluid in plug flow and a microfluid in plug flow are identical. (b) Discuss about mixing of two miscible fluids.
[10+6]
6. The true density of the solid material in an activated alumina was reported as 3675 kg/m3 . The density of the particle obtained from mercury displacement technique 1
Code No: 07A60803
R07
Set No. 2
were 1547 kg/m3 and surface area as 175 m2 . Determine the pore volume /gm, the porosity and mean pore radius of the particle. [16] 7. (a) Assuming plug flow we calculate that a tubular reactor 12m long would give 96% conversion of A for the second order reaction A → R. However the fluid is very viscous, and flow will be strongly laminar, thus we expect the convection model, not the plug flow model, to closely represent the flow. How long should we make the reactor to insure 96% conversion of A? (b) Give a brief account of chemical conversion in laminar flow reactors. [10+6] 8. Show that t/τ = 1- rc /R = 1- (1 - XB )1/3 when the overall rate is controlled by a chemical reaction List out the assumptions. [16]
2
Code No: 07A60803
R07
Set No. 4
III B.Tech II Semester Examinations,December-January, 2011-2012 CHEMICAL REACTION ENGINEERING-II Chemical Engineering Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks
1. (a) Explain the difference in behavior of microfluids and macrofluids in a mixed flow reactor. (b) Derive the expression for a zero order reaction of a macrofluid in a mixed flow Reactor. [8+8] 2. Write briefly on the following: (a) External diffusion (b) Internal diffusion.
[16]
3. How the activity of a catalyst pellet at any time is defined? Write a deactivation rate equation and explain the order of deactivation. [16] 4. (a) A small diameter pipe 32m long runs from the fermentation room of a winery to the bottle filling cellar. Sometimes red wine is pumped through the pipe, sometimes white, and whenever the switch is made from one to the other a small amount of “house blend” rose is produced( 8 bottles). Because of some construction in the winery the pipeline length will have to be increased to 50m. For the same flow rate of wine, how many bottles of rose may we now expect to get each time we switch the flow? (b) Discuss briefly about the tanks-in-series model.
[8+8]
5. (a) Discuss about fitting the dispersion model for large deviation from plug flow. (b) A packed bed reactor is injected with a tracer. The pulse is injected into the bed about 4 particle diameters from the entrance, which is not perfect. The variance in the injection is σ 2 = 15 sec2 . The variance at the measuring point 3 meters downstream is σ 2 = 50 sec2. The superficial velocity is 0.01 m/s. i. Calculate the dispersion coefficient. ii. Calculate the conversion for a first order reaction in which k = 0.02 sec - 1 . [8+8] 6. Hydrogen sulfide is removed from coal gas by contact with a moving bed of iron oxide particles which convert to the sulfide as follows: Fe2O3 → FeS In our reactor the fraction of oxide converted in any particle is determined by its residence time t and the time needed for complete conversion of the particle τ and this is given by t 3 1 − X = 1 − τ when t < 1 hr and with τ = 1 hr and X = 1 when t ≥1 hr. 3
Code No: 07A60803
R07
Set No. 4
Find the conversion of iron oxide to sulfide if the RTD of solids in the contactor is approximated by the curve shown in the figure 6. [16]
Figure 6 7. The reaction P + Q → R is carried out in the pressure of a catalyst and proceeds according to the following mechanism. P + M → PM Q + M → QM PM + QM → RM + M RM → R+M PM, QM, RM are the intermediate adsorbed composites at the catalyst activated sites. M indicates active catalyst. If the reaction step is the rate controlling parameter, suggest a suitable rate expression for the reaction. [16] 8. Cylindrical solids and fluid are continuously fed into a mixed-flow reactor. The mechanism of the reaction is visualized to be the shrinking core model and the reaction step is the rate controlling. Develop the performance expression for this operation as a function of pertinent parameters. Assume that there is no elutriation. [16]
4
Code No: 07A60803
R07
Set No. 1
III B.Tech II Semester Examinations,December-January, 2011-2012 CHEMICAL REACTION ENGINEERING-II Chemical Engineering Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks
1. Discuss the spectrum of kinetic regimes in a porous catalytic reaction system and explain the factors that influence the rate of reaction of particles. [16] 2. A reactor with a number of dividing baffles is to be used to run the reaction A → R with -rA = 0.05CA mol/liter.min. A pulse rate test gives the following curve. Time, min 0 10 20 30 40 50 60 70 Concentration 35 38 40 40 39 37 36 35 Reading (a) How many tanks in series is this vessel equivalent to? (b) Calculate XA assuming the tanks-in-series model (c) Calculate XA assuming plug flow.
[8+4+4]
3. Why carbon gets deposited on a catalyst in a cracking reaction. Is it possible to estimate the weight percent of carbon formed on the catalyst surface. Explain the phenomenon. [16] 4. Calculate the time needed to burn to completion particles of graphite (Ro = 5mm, ρB = 2.2gm/cc, K S = 20 cm/s.) in an 8% oxygen stream. For high gas velocity used assume that film diffusion doesnot offer any resistance to transfer and reaction. Reaction temperature is 9000 C. [16] 5. Derive the equation for the conversion of a first-order reaction using a macrofluid when the RTD is equivalent to (a) an ideal PFR (b) an ideal CSTR (c) Compare these conversions with those obtained from ideal reactors using micro fluid. [6+6+4] 6. In a plug flow reactor a catalytic reaction A→4R is carried out with initial concentration of A (CAo) as 0.12mol/lit. The packed bed contains 0.025kg of catalyst. The feed consists of partially converted product of 45 lit/hr of pure unreacted A Data available: Run 1 2 3 4 CA in, mol/lit 0.12 0.096 0.072 0.048 CA out mol/lit 0.1008 0.084 0.066 0.0456 Determine the rate equation to represent this reaction. 5
[16]
Code No: 07A60803
R07
Set No. 1
7. A pulse test on a piece of reaction equipment gave the following results: The output concentrations rose linearly from zero to 0.5 µmol/dm3 in 5 min, then fell linearly to zero in 10 min after reaching the maximum value. (a) Calculate in tabular form the values of E(t) and F(t) at 1 min intervals (b) What is the mean residence time? If the flow rate were 150 gal/min, what would be the total reactor volume? A second order reaction with kCA0 = 1.2 min - 1 is carried out in the system. [16] 8. Tubular reactors for thermal cracking are designed on the assumption of plug flow. On the suspicion that nonideal flow may be an important factor now being ignored, let us make a rough estimate of its role. For this assume isothermal operations in a 2.5-cm ID tubular reactor, using a Reynolds number of 10,000 for flowing fluid. The cracking reaction is approximately first order. If calculations show that 99% decomposition can be obtained in a plug flow reactor 3 m long, how much longer must the real reactor be if nonideal flow is taken into account? [16]
6
Code No: 07A60803
R07
Set No. 3
III B.Tech II Semester Examinations,December-January, 2011-2012 CHEMICAL REACTION ENGINEERING-II Chemical Engineering Time: 3 hours Max Marks: 80 Answer any FIVE Questions All Questions carry equal marks
1. Compare and contract the porous and non porous catalyst materials. Explain the catalytic potential of porous catalysts. [16] 2. (a) Write about dispersion model and mention its applications and limitations. (b) An injected slug of tracer material flows with its carrier fluid down a long, straight pipe in dispersed plug flow. At point A in the pipe the spread of tracer is 16m. At point B, 1 kilometer downstream from A, its spread is 32m. What do you estimate its spread to be at a point C, which is 2 kilometers downstream from point A? [8+8] 3. (a) Give a brief account of E, the exit age distribution and the experimental methods for finding E curve. (b) A liquid macrofluid reacts according to A → R as it flows through a vessel. Find the conversion of A for the flow pattern shown in the figure 3, with the given data. C A0 = 2 mol/liter, -rA = k C A2 , k = 2 liter/mol.min [8+8]
Figure 3 4. (a) Dispersed noncoalescing droplets containing reactant A pass through 3 ideal stirred tanks in series. The mean holding time in each tank is 1.5hr and the rate constant for the first-order decay reaction is 0.1 min - 1 . Find the fractional conversion of A in the exit stream from the three reactors. (b) Consider a single reacting macrofluid being processed in turn in batch, plug flow and mixed flow reactors. How does the degree of segregation affect the conversion in each case? [10+6] 5. Explain the method to find the first order decay in a continuous flow back mix reactor using integral method.Take a general first order equation and explain. [16] 6. A gas-solid non-catalytic reaction is investigated by measuring the time required for complete conversion of solid B as a function of particle diameter. The results are as follows: 7
Code No: 07A60803
R07
Particle dia(mm) 0.063 Time for complete conversion(min) 5
Set No. 3 0.125 10
0.250 20
If the diffusion resistance in the gas phase around the particle is negligible, what mechanism controls the rate of reaction? [16] 7. Develop the overall rate equation for the reaction A account:
⇔B
taking steps below into
(a) Adsorption rate of A r = K 1 (P a θυ − θa /K 2 ) (b) Desorption rate of B r = K 3 (θb /K 4 − P t θυ ) (c) Surface reaction rate r = K5 (θa −θb /K6 ) θa and θb are the fractions of the surface covered by species A and B. θυ is uncovered surface (θυ = 1 - θa - θb ). [16] 8. Aqueous A (CAO = 1 mol/m3 ) with physical properties close to water ( ρ = 1000 kg/m3 , D = 10 - 9 m2 /s) reacts by a second-order reaction ( k = 10 - 3 m3 /mol.s) as it flows at 10 mm/s through a tubular reactor (dt = 10 mm, L = 20 m). Find the conversion of reactant A from this reactor. [16]
8
