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Posted By: acme Member Level: Silver Posted Date: 19 Apr 2008
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2007 Jawaharlal Nehru Technological University B.Tech Chemical engineering CHEMICAL REACTION ENGINEERING-II Question paper
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Code No: RR410802 Set No. 1
IV B.Tech I Semester Regular Examinations, November 2007
CHEMICAL REACTION ENGINEERING-II
(Chemical Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
? ? ? ? ?
1. Dispersed noncoalescing droplets (CAo= 2 mol/liter) react (A!R, -rA = KC2A
, k = 0.5 liter/mol. min) as they pass through a contactor. Find the average
concentration of A remaining in the droplets leaving the contactor if their RTD is
given by the curve in Figure 1. [16]
Figure 1
2. Consider a real tubular reactor in which dispersion is occurring.
(a) For small deviations from plug flow, show that the conversion for a first-order
reaction is given approximately as
X = 1 - exp -kt +
(kt )2
Pe
(b) Show that to achieve the same conversion, the relationship between the volume
of a plug-flow reactor VP and volume of a real rector V in which dispersion
occurs is
V
VP
= 1 + kt D
UL
[8+8]
3. (a) Define the following terms:
i. Micro fluid
ii. Macro fluid
iii. Degree of segregation
iv. Earliness of mixing
(b) Explain the difference in behaviour of micro fluids and macro fluids in mixed
flow reactor. [8+8]
1 of 2
Code No: RR410802 Set No. 1
4. In a non-catalytic fluid solid reaction the diffusion through Ash layer is the con-
trolling step. Derive the overall rate expression for the reaction of particles of
unchanging size. Express the conversion interms of reaction time. [16]
5. A 200 ton solid hold up is needed in a single fluidized bed reactor for 99% conversion
of particles of unchanging size where ash diffusion controls. What would be the
hold up in two fludized beds in series for indentical conversion, feed rate and gas
environment. [16]
6. Derive a rate equation for an instantaneous reaction of any order between A and
B, fluid-fluid reaction
A(gas) + bB(liquid) ! product
And sketch the concentration profiles assuming a two-film theory. [16]
7. The oxidation of ethylene to ethylene oxide is conducted in a fixed bed catalytic
reactor. The feed gas with a composition of 8% C2H4, 19% O2 and 73% N2 has
a superficial velocity of 60 cm/sec in the reactor, with silver catalyst, the intrinsic
reaction is given to r = 0.12 PC2 H4(Po2)0.2 K moles
hr. Kg catalyst
Two competing reactions occur in the reactor
C2H4 + 1/2 O2 ! C2H4O
C2H4 + 3O2 ! 2CO2 + 2H2O
The selectivity of ethylene oxide with respect to CO2 is independent of conver-
sion and is equal to 1.5. Calculate the minimum bed height required to achieve
20% conversion of ethylene to ethylene oxide. Density of catalyst particles = 0.04
gm/C.C. The reactor is operated isothermally at 280oC and 1 atm. pressure. [16]
8. The rate law for the hydrogenation (H) of Ethylene (E) to form ethane over a
catalyst is
-rE = k PE PH
1+KEPE
where in KE is equilibrium constant of the reaction and ? is rate constant of the
reaction.
Suggest a mechanism consistent with the rate law. [16]
? ? ? ? ?
2 of 2
Code No: RR410802 Set No. 2
IV B.Tech I Semester Regular Examinations, November 2007
CHEMICAL REACTION ENGINEERING-II
(Chemical Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
? ? ? ? ?
1. Consider the non-ideal reactor characterized by the RTD data given in the table.
t C(t) E(t) tE(t) t-tm (t-tm)2 E(t)
0 0 0 0 -5.15 0
1 1 0.02 0.02 -4.15 0.34
2 5 0.10 0.20 -3.15 0.992
3 8 0.16 0.48 -2.15 0.74
4 10 0.20 0.80 -1.15 0.265
5 8 0.16 0.80 -0.15 0.004
6 6 0.12 0.72 0.85 0.087
7 4 0.08 0.56 1.85 0.274
8 3 0.06 0.48 2.85 0.487
9 2.2 0.044 0.40 3.85 0.652
10 1.5 0.03 0.30 4.85 0.706
12 0.6 0.012 0.14 6.85 0.563
14 0 0 0 8.85 0
Last two column are completed after tm is found.
The irreversible gas phase non elementary reaction
A + B ! C+ D
is first order in A and second order in B and is to be carried out isothermally.
Calculate the conversion for
(a) A PFR, a laminar flow reactor with complete segregation and CSTR.
(b) The cases of complete segregation and maximum mixedness. [8+8]
2. A stream of fully suspended fine solids (v= 1m3/min) passes through two mixed
flow reactors in series, each containing 1 m3 of slurry. As soon as a particle enters
the reactors, conversion to product begins and is complete after two minutes in
the reactors. When a particle leaves the reactors, reaction stops. What fraction of
particles is completely converted to product in this system? [16]
3. (a) Define the following terms:
i. Micro fluid
ii. Macro fluid
1 of 2
Code No: RR410802 Set No. 2
iii. Degree of segregation
iv. Earliness of mixing
(b) Explain the difference in behaviour of micro fluids and macro fluids in mixed
flow reactor. [8+8]
4. Particles react with gas of given composition and at given temperature to give a
solid product what can you say about the kinetics of the reaction if the rate of
reaction per gram of solids is:
(a) proportional to diameter of particles
(b) Proportional to the square of the particle diameter
(c) Independent of particle size. [16]
5. Iron sulfide were roasted and it was found that time for complete conversion is
related with the particle size as T proportional to R1.6 most of the particles were
found to form hard mass. If a fluidized bed reactor is planned to convert this ore to
various oxides with mean residence time of 90 min. with a feed of uniform size cor-
responding to space time of 25 min. Determine the fraction reminded unconverted.
[16]
6. Derive a rate equation for fast reaction with a second-order rate between A and B,
fluid-fluid reaction
A(gas) + bB(liquid) ! product
And sketch the concentration profiles assuming a two film theory. [16]
7. Write short notes on:
(a) Catalyst poisoning
(b) Thiele parameter
(c) Catalyst selectivity
(d) Catalyst fouling [4×4]
8. For the solid catalyzed reaction A + B !
R + S, develop an expression for rate
equation, if desorption of R is controlling the overall reaction. All components are
adsorbed. [16]
? ? ? ? ?
2 of 2
Code No: RR410802 Set No. 3
IV B.Tech I Semester Regular Examinations, November 2007
CHEMICAL REACTION ENGINEERING-II
(Chemical Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
? ? ? ? ?
1. The following readings represent a continuous response to a delta function input
into a closed vessel to be used as a reactor. Plot E(?) Vs ? and estimate the variance
: [16]
Time, min. Con. of tracer g/lit
0 0
5 3
10 5
15 5
20 4
25 2
30 1
35 0
2. Carry out the liquid phase, second order dimerization
2A! B; -rA = kC2 A
for which k = 0.01 m3/mol-min at the reaction temperature. The feed is pure A
with CA0 = 8 mol/m3. The reactor is non ideal and perhaps could be modeled as
two CSTRs with interchange. The reactor volume is 1000 m3, and the feed rate of
dimerization 25 m3/min. A tracer test is run on this reactor and the results are
given in column.
Columns 3 to 5 are calculated from 1 and 2.
The bounds on the conversion for different possible degrees of micro mixing for the
RTD of this reactor are to be known. What are they ? [16]
3. (a) Derive the general equation for determining conversion for macro fluid in mixed
reactors.
(b) Deduce the above equation for a first order reaction and show that the degree
of segregation has no effect on conversion for first order system. [8+8]
4. Discuss the various models available to deal with non-catalytic fluid-solid reaction.
Compare and contrast the important differences between them. Which of them are
practically important and why? [16]
5. A batch of solids of uniform size is treated by gas in a uniform environment solid
is converted give a non flaking product according to the shrinking core model con-
version is about 7/8 for a reaction time of one hour, conversion is complete in two
hours. What mechanism is rate controlling. [16]
1 of 2
Code No: RR410802 Set No. 3
1 2 3 4 5
t(min) C(mg/m3) E(t)(min-1) 1-F(t) E(t) / (1-F(t))(min-1)
0 112 0.0280 1.000 0.0280
5 95.8 0.0240 0.871 0.0276
10 82.2 0.0206 0.760 0.0271
15 70.6 0.0177 0.663 0.0267
20 60.9 0.0152 0.584 0.0260
30 45.6 0.0144 0.472 0.0242
40 34.5 0.00863 0.353 0.0244
50 26.3 0.00658 0.278 0.0237
70 15.7 0.00393 0.174 0.0226
100 7.67 0.00192 0.087 0.0221
150 2.55 0.000638 0.024 0.0266
200 0.90 0.000225 0.003 0.075
6. Derive a rate equation for fast reaction with a second-order rate between A and B,
fluid-fluid reaction
A(gas) + bB(liquid) ! product
And sketch the concentration profiles assuming a two film theory. [16]
7. Write short notes on:
(a) Experimental methods of finding rates of solid catalyzed reactions
(b) Experimental determination of pore size and surface area of catalyst. [8+8]
8. The rate law for the hydrogenation (H) of Ethylene (E) to form ethane over a
catalyst is
-rE = k PE PH
1+KEPE
where in KE is equilibrium constant of the reaction and ? is rate constant of the
reaction.
Suggest a mechanism consistent with the rate law. [16]
? ? ? ? ?
2 of 2
Code No: RR410802 Set No. 4
IV B.Tech I Semester Regular Examinations, November 2007
CHEMICAL REACTION ENGINEERING-II
(Chemical Engineering)
Time: 3 hours Max Marks: 80
Answer any FIVE Questions
All Questions carry equal marks
? ? ? ? ?
1. The following tracer data have seen obtained for liquid decomposing with rate
Time t, min Tracer Output Concentration, Cpulse
gm/liter fluid
0 0
5 3
10 5
15 5
20 4
25 2
30 1
35 0
rA = kCA, k = 0.307 min-1
Find the fraction of reactant unconverted in the real reactor and compare this with
the fraction unconverted in a plug flow reactor of the same size. [16]
2. Hydrogen sulfide is removed from coal gas 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 t , and
this is given by figure2
1 - x = ??1 - t
when t < 1 hr, andwith t = 1 hr
and X =1 when t 1 hr
Figure 2
Find the conversion of iron oxide to sulfide if the RTD of solids in the contactor is
approximated by the above given curve. [16]
3. (a) Define the following terms:
1 of 3
Code No: RR410802 Set No. 4
i. Micro fluid
ii. Macro fluid
iii. Degree of segregation
iv. Earliness of mixing
(b) Explain the difference in behaviour of micro fluids and macro fluids in mixed
flow reactor. [8+8]
4. For a particle which reacts under “Ash diffusion” control, what is the ratio of the
time required for the radius of unreacted solid to be reduced to R/2 to that required
for complete reaction. Sketch the concentration profiles. [16]
5. Spherical particles of zinc blende of radius 1 mm are roasted in an 8% oxygen
stream at 900C and 1 atm. The reaction is
2ZnS + 3O2 ! 2Zno + 2SO2
Assuming that the reaction proceeds by shrinking core model and neglecting the
film resistance.
Calculate the time needed for complete conversion of a particle and the relative
resistance of ash layer during this operation.
Data:Density of solid = 4.13Kg/m3
Rate constant = 0.02 m/s
Effective diffusivity = 0.08Cm2/S. [16]
6. The concentration of an undesirable impurity A in air is to be reduced from 0.1%
to 0.02% by absorption in pure water. Find the height of the tower required for
counter current operations, using the following data.
Data: for packing: KAga = 32, 000mol/hr.m2.atm.
KAia = 0.1/hr.
Flow rates : L1 = L = 7 ! 105mol/hr.m2
G1 = G = 1 ! 105mol/hr.m2
p = 1atm.
Henry’s coefficient HA = 125 × 10-6 HA = 125x10-6 atm. m3
mol
Molar density of liquid under all conditions
CT = 56, 000mol/m3. [16]
7. The use of a differential reactor to study the formation of methane from hydrogen
and carbon monoxide over a nickel catalyst gave the following:
rCH4 =
0.0183 P
1
/2
H2
PCO
1+1.5 PH2
It is desired to to produce 20 tons/day of CH4. Calculate the catalyst weight
necessary to achieve 80% conversion in a fixed bed reactor. The feed consist of 75%
hydrogen and 25% carbon monoxide at a temperature of 300 C and a pressure of
10 atm. [16]
8. For the solid catalyzed reaction A + B ! R + S, develop an expression for rate
equation, if desorption of R is controlling the overall reaction. All components are
adsorbed. [16]
2 of 3
Code No: RR410802 Set No. 4
? ? ? ? ?
3 of 3
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