GaziUniversity January 6, 2009
Department of Chemical Engineering
Che 341 Chemical Reaction Engineering
2008-2009 Fall Semester
Prof.Dr. Çiğdem GÜLDÜR
Res.Asst.D.Dolunay ESLEK KOYUNCU
Problem Set-6
(Energy Balance)
1)Suppose the liquid phase decomposition of A takes place according to the following kinetics with rate laws as indicated;
AB+E rB= k1CA
AD+E rD= k2CA
Reaction is carried out isothermally in a batch reactor with only A present initially at a concentration CA0= 4 mol/lt in an inert solvent. At t=1200 s, CA=1.2 mol/lt, CB= 0.84 mol/lt. Calculate,
(a)the values of k1 and k2
(b)the values of CD and CE at t= 1200 s
2)The following liquid phase reactions were carried out in a CSTR at 325 K.
(1) 3A B+C -r1A= k1ACA k1A=0.7min-1
(2) 2C+A 3D r2D=k2DCC2CA k2D=0.3dm6/(mol2min)
(3) 4D+3C 3E r3E=k3ECDCC k3E=0.2 dm3/(mol.min)
The concentrations measured inside the reactor were CA= 0.10, CB= 0.93, CC=0.51 and CD= 0.049 all in mol/dm3.
a) What are r1A, r2A and r3A ?
b) What are r1B, r2B and r3B ?
c) What are r1C, r2C and r3C ?
d) What are r1D, r2D and r3D ?
e) What are r1E, r2E and r3E ?
f) The entering volumetric flow rate is 100 dm3/ min and the entering concentration of A is 3 M . What is the CSTR reactor volume?
3)Often a desired reaction is accompanied by a variety of undesired side reactions, some of higher order, some of lower order. To see which type of operation gives the best product distribution consider the simplest typical case, the parallel decompositions of A, CA0= 2 mol/lt,
R rR=1
A S rS=2CA
T rT=CA2
Find the maximum expected CS for isothermal operations(a) in a mixed reactor (b) in a plug flow reactor.
4)With a particular catalyst and a given temperature, the oxidation of naphthalene to phthalic anhydride proceeds as follows:
R
13 4
AS T
2
A=Naphthalene k1=0.21s-1
B= Naphthaquinone k2=0.20s-1
S=phthalic anhydride k3=4.2s-1
T= oxidation products k4=0.004s-1
Obtain an expression for the concentrations of R&S at the exit of a CSTR.
5)The reaction
is carried out in a batch reactor in which there is pure initially A.
(a) Derive an equation for the concentration of A as a function of time. If k1= 0.001 s -1, what is the ratio CA/CA0 after 1.5 min?
(b) Derive an equation that gives the concentration of B as a function of time.
If k2 = 0.00 3 s -1 , k3 = 0.002 s -1 , and CA0 = 0.2 g mol/dm 3 , what is the concentration of B after 2 min?
(c) What is the concentration of C after 1 min? 2 min?
6)Consider the following reaction network and associated networks:
kA1
A B rB=kA1 (T)CA m =A1 exp(EA1/RT) CA m
kA2
AC rC=kA2 (T)CA n =A2 exp(EA2/RT) CA n
B is desired product and C is not.
a) Should the reaction be carried out at low or high CA?
b)Should the reaction be carried out at low or high T?
c) State general conclusions arising from (a) and (b).
d) What are the implications for size and type of reactor?
7)For the set of first order consecutive reactions
k1 k2
ABC
Determine the optimum space time in a plug flow reactor in terms of maximizing the concentration of the intermediate B. What will the maximum concentration be in each case? It may be assumed that only species A is present initially.
8)(a) what reaction shemes and conditions would you use to maximize the instanteneous selectivity, s for the following parallel reactions:
A+C D rD=800e(-2000/T)CA0.5CC
A+C U rU=10e(-300/T)CACC
Where D is the desired product and U id the undesired product?
(b) State how your answer to part (a) would change if C were to react with D to form another undesired product,
D+CX rX=106e(-8000/T)CCCD
At what temperature should the reactor be operated if the concentrations of A and D in the reactor were 1 mol/dm3?
9) Consider the following system of gas-phase reaction:
AX rX=k1 k1=0.002 mol/dm3.min
AB rB=k2CA k2=0.06min-1
AYrY=k3CA2 k3=0.3dm3/mol.min
B is the desired product and X and Y are foul pollutants that are expensive to get rid of. The specific reaction rates are at 270C. the reaction system is to be operated at 270C and 4 atm. Pure A enters the system at a volumetric flow rate of 10dm3/min.
a) Sketch the instantaneous selectivities(sBX, sBY and sB/XY=rB/(rX+rY) as a function of the concentration of CA.
b) Consider a series of reactors . What should be the volume of the first reactor?
c) What are the effluent concentrations of A,B,X and Y from the first reactor?
d) What is the conversion of A in the first reactor?
e)If 90% conversion of A is desired, what reaction sheme and reactor sizes should you use?
f) Suppose that E1= 10000cal/mol, E2= 20000 cal/mol, and E3= 30000 cal/mol. What temperature would you recommend for a single CSTR with a space time of 10 min and an entering concentration of A of 0.1 mol/dm3?
10)The elementary liquid phase series reaction
k1 k2
ABC
is carried out in a 500lt batch reactor. The initial concentration of A is 6 mol/lt. The desired product is B and separation of undesired product C is very difficult and costly. Because the reaction is carried out at a relatively high temperature , the reaction is easily quenched.
Additional information :
k1= 0.4 h-1, k2=0.01 hr-1, Ea1=10000cal/mol; Ea2=20000cal/mol
a) Assuming that each reaction is irreversible , plot the concentrations of A,B and C as a function of time.
b) For a CSTR space time of 0.5 h, what temperature would you recommend to maximize B?
11)SolveProblems 8.2, 8.13
(The textbook:Levenspiel, O., ”Chemical Reaction Enginering”, 3rd ed.,John Wiley&Sons,1999)
HW-6- Pr-1 –Due to 16.01.2009