Ideal Flow Reactors - Bioprocess Engineering: Kinetics, Biosystems, Sustainability and Reactor Design - page 245

PROBLEMS

5.1. Ethylene is the monomer for polyethylene production. Polyethylene is an important
commodity plastics widely used today. As a replacement for petroleum products,
ethanol is being developed to manufacture ethylene as ethanol can be produced from
biomass. The following reaction:

C 2 H 5 OH ð A Þ / C 2 H 4 ð B Þþ H 2 O ð C Þ

is being considered to be carried out in a PFR. 1) The feed is pure ethanol at 1 kg/s,

kC A
1 þ C A =K A

0.1 min 1 500 K, and 2 bar; 2) the kinetics is given by r ¼ , k ¼ and

K A ¼ 1 mol/L; 3) the reactor operates isothermally with negligible pressure drop;

4) the final conversion at the outlet of the reactor f Ae ¼

0.4. Compute

(a) The reactor volume V ;
(b) The space time ;

(c) And the dilution rate, D.
5.2. At your favorite fast food joint, the french fries are made by filling a basket with
potatoes and dipping them in hot animal or vegetable fat for 4 min and then draining
them for 4 min. Every hour the small pieces that fell out of the basket are scooped out
because they burn and give a bad taste. At the end of the 16 h day, the fat is drained and
sent out for disposal because at longer times the oil has decomposed sufficiently to give
a bad taste. Approximately 2 pounds of potatoes are used in 10 gal of oil.
(a) Why is a batch process usually preferred in a restaurant?
(b) Design a continuous process to make 1 ton/day of french fries, keeping exactly
the same conditions as above so they will taste the same. Describe the residence
times and desired flow patterns in solid and liquid phases. Include the oil
recycling loop.
(c) Howmight you modify the process to double the production rate from that specified
for the same apparatus? What experiments would you have to do to test its
feasibility?
(d) How would you design this continuous process to handle varying load demands?
5.3. An irreversible first-order reaction gave 95% conversion in a batch reactor in 20 min.
(a) What would be the conversion of this reaction in a CSTR with a 20 min residence
time?
(b) What residence time would be required for 95% conversion in a CSTR?
(c) What residence time would be required for 99% conversion in a CSTR?
(d) What residence time would be required for 95% conversion in a PFR?
(e) What residence time would be required for 99% conversion in a PFR?
5.4. Calculate the ratio of residence times in CSTR and PFR for the n th-order irreversible
reaction for conversions of 50, 90, 99, and 99.9% for n

s

¼ 0, 1/2, 1, 2, and 1 for

1.0 mol/L.
5.5. Calculate the reactor volumes required to process 100 L/min of 3 mol A in the aqueous
reaction A

C A0 ¼

/ 2B for PFR and CSTR reactors.