Page 354 - Elements of Chemical Reaction Engineering Ebook

Chap. 6 Questions and Problems 325

P6-9nI The elementary liquid-phase-series reaction

is carried out in a 500-dm3 batch reactor. The initial concentration of A. is 1.6
mol/drn3. The desired product is B and separation of the 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:
Cost of pure reactant A = $lO/mol A
Selling price of pure B = $50/mol B
Separation cost of A from B = $50/mol A
Separation cost of C from B = $30 (eoScC- 1)
k, = 0.4 h-’
k2 = 0.01 h-l at 100°C
(a) Assuming that each reaction is irreversible, plot the concentrations, of A,
B, and C as a function of time.
(b) Calculate the time the reaction should be quenched to achieve the maxi-
mum profit.
(c) For a CSTR space-time of 0.5 h, what temperature would you recom-
mend to maximize B? (E, = 10,000 cal/mol, E, = 20,000 cal/mol)
(d) Assume that the first reaction is reversible with k-l = 0.3 h-,. Plot the
concentrations of A, B, and C as a function of time.
(e) Plot the concentrations of A, B, and C as a function of time for thle case
where both reactions are reversible with k-, = 0.005 h-l.
(0 Vary kl, k2, k-,, and k-,. Explain the consequence of k, > 100 and
k2<0.1 withk-,=k-,=Oandwithk-,= l,k-,=O,andk-,=O.25.
(g) Reconsider part (a) for reactions are carried out in a packed-bed rleactor
with 100 kg of catalyst for which kl = 0.25 dm3/kg cat:min and
k2 = 0.15 dm3/kg cat:min. The flow is turbulent with uo = 10 drrP and
C,, = 1 mol/dm3. It has been suggested-to vary the particle size, keeping
W = 100 kg, in order to increase FB, YB , and SBC . If the particle size
could be varied between 2 and 0.1 cm, what particle size would you
choose? The pressure drop parameter for particles 1 cm in diameter is
OL = 0.00098 kg-l. Is there a better way to improve the selectivity?
P6-1011 You are designing a plug-flow reactor for the following gas-phase reaction:

A --+ B ~g = k,Ci k, = 15 ft3/lb mol ‘S
Unfortunately, there is also a side reaction:
A --+ C rc = k2CA k, = 0.015 s-l

C is a pollutant and costs money to dispose of; B is the desired product.
(a) What size of reactor will provide an effluent stream at the maximum dol-
lar value? B has a value of $60/lb mol; it costs $15 per Ib mol to dispose
of C. A has a value of $10 per Ib mol. (Am: V = 896 ft3.)
(b) Suppose that El = 10,000 Btuflb mol and E2 = 20,000 Btu/lb mol.
What temperature (400 to 700°F) would you recommend for a 400-ft3
CSTR?