Q7.1. An instantaneous reversible liquid phase reaction A<=>B is to be carried out in a tubular vessel.
Data:
Feed Temperature
120°C
Coolant Temperature
10°C
Feed
600 lit/hr
Feed Concentration
12 lit
Specific Heat
1000 cal/lit°C
Heat of Reaction
-15000 cal/mol
Equilibrium Constant
0.02 at 120°C
Heat transfer Coefficient
500 kcal/m^2 °C hr
A reactor volume of 20 lit is proposed. Tube sizes of 5, 10, 15 cm are available. Estimate the conversion attainable in each case and outlet temperature also.
X = 0.22, 0.104, 0.072 respectively
Q7.2. Vinyl Chloride monomer is to be produced by hydro chlorination of acetylene. This gas phase reaction takes place over a temperature range 75-250°C over mercuric chloride catalyst supported on activated carbon. 10% mercuric chloride is generally used. Catalyst deactivates rapidly at temperatures above 200°C.
C2H2 (B) + HCl (A) = C2H3Cl (C)
Reaction is irreversible and no side products are formed. Reaction is carried out using excess acetylene so that kinetics of reaction can be assumed to be first order with respect to HCl gas.
Reaction is exothermic. Reactor is enclosed in a jacket through which large quantity of coolant circulates. Experimental pseudo-homogeneous rate constants are as below:
Table Q7.2 dat on Rate Constants versus temperature
T (C)100
100
120
140
160
180
200
220
240
260
k(s-1)
0.27
0.54
1.0
--
1.24
--
1.5
1.6
Q7.2.1. In a bench scale laboratory experiment of 5 cm diameter tube a hot spot temperature of 180°C is indicated. What is the conversion at this point?
Q7.2.2. Estimate conversion at hot spot if 3 cm diameter tube is used.
Q7.2.3. What is the volume needed to obtain 99% conversion in an adiabatic reactor ? Assume catalyst deactivation to be unimportant during the period of operation.
CA0
0.0016 kmol/m3
T0
100°C
CP
0.2 kcal/m^3°C
FA0
12 gmol/hr
CB0/CA0
9
h
10 kcal/m2 h °C
∆H
-25000 kcal/kmol
Coolant temperature TC
100°C
Q7.3. Can Hot spot temperature be used for process Control ? Hot spot temperatures observed in experiments are as below:
SN
T(C)
CA0 (gmol/cum)
Tube size CM
Observed Conversion, X
1
180
3.2
5
0.59
2
220
9.6
2.5
0.84
3
240
6.4
5
0.75
Based on the data above can we design using hot spot as a means of controlling the processes ?
Q7.4. How to design for Isothermal operation of a fixed Bed Reactor ?
A packed bed catalytic reactor (diameter D0 is to be used for carrying out first order reaction A ⇔ B. The reaction is exothermic with rate function r A = - k CA where k (T) is rate constant, α activity of catalyst, CA reactant concentration. Catalyst manufacturer desires that the reactor be operated isothermally at T. Accordingly manufacturer advises that catalyst be diluted with inert and so packed in the bed that reaction heat generation (q) be absorbed by wall cooling fluid circulating at constant temperature TC with wall to coolant heat transfer coefficient h. Volumetric flow at inlet is vO at T
Q7.4.1.Show that catalyst loading profile α(V) to achieve this isothermal operation can be given as below:
Q7.4.2. In operating process temperature of reaction, fee concentration, feed flow could all change significantly. How then would you run your process at constant T ?
Q7.4.3. Rework the problem for the case of reversible reaction with equilibrium constant K
Q7.5. Why the need for multi staging for reversible exothermic reactions ?
The elementary liquid phase reaction A ⇔ B is to be carried out in a tubular reactor system. Suitable choices of number of stages, adiabatic operation interstage cooling are to be made. Overall conversion of 95 percent is desired. Specify viz. feed temperature, tube size, interstage cooling, number of stages assuming that 95 percent equilibrium approach in each stage.
Item
ΔH
Density
Ke at 25 oC
k1 at 21C
TO
Values
-20000
0.91
12.2
12
10
unit
cal/mol
g/mL
--
1/hr
C
Item
CAO
CP
TO
E1
VO
Values
1.6
0.7
21
25000
5
unit
mol/cum
cal/kg.C
C
cal/mol
cum/hr
Q7.6.What do we learn from the reactor designs of the following industrial processes?
Prepare a sketch of reactor system used for
(a) NH3 oxidation
(b) SO2 oxidation
(c) Steam reformation
(d) Shift conversion.
See Reactor Design for Process Plants , Rase Vol.II
Q7.7. Design of Vinyl Chloride Reactor System
Reaction of Q7.2 is to conducted . Feed gas enters at 35 C. Since deactivation sets in temperatures not to exceed 140 C. Accordingly design uses adiabatic reactors with interstage cooling of reactor exit gases to 100 C with large supply of cooling water in jacket.
Unreacted HCl is scrubbed out and unreacted acetylene is recycled to storage.
Q7.7.1. Specify number of stages to reach 99.5% HCL conversion. Show T-X plot for each stage and also estimate reactor volume for each stage.
Data
Item
FAO
FBO
FCO
rA
k
Values
1.0
9.0
0.0
- k CA
exp(13.7-5300/T)
unit
mol/h
mol/h
mol/h
1/hr
Item
h
TO
TC
P
CP
ΔH
Values
5
35
22
1
1.5
25000
unit
Kcal/m2.hr
C
C
atm
Kcal/m3.hr
cal/mol
Q7.8. Comment on the following
Q7.8.1. In a CSTR (chemostat) cell growth can occur even if feed is sterile why ?
7.8.2. A first order rate constant with k =1.0/min and second order reaction with kCA0 = 1.0 /min is carried out in a reactor. What might be expected differences in performance ?
Q7.8.3. Radial flow reactors are said to be superior to conventional packed bed reactors ?
Q7.8.4. Combustion is an instantaneous reaction so it is used in propulsion. Is this true?
Q7.8.5. Commercial polymerization is carried out in CSTRs. Is it true?
Q7.8.6. Biological reactions are controlled by heat transfer. Is it correct ?
Q7.8.7. Free energy of chemical reaction cannot be recovered as work.Is it true ?