Page 375 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

but at high pressures slimmer proportions are economical.
3
4. Power input to a homogeneous reaction stirred tank is 0.1–0.3 kW/m (0.5–1.5 hp/1000 gal), but
three times this amount when heat is to be transferred.
5. Ideal CSTR (continuous stirred tank reactor) behavior is approached when the mean residence
time is 5 to 10 times the length needed to achieve homogeneity, which is accomplished with
500–2000 revolutions of a properly designed stirrer.

6. Batch reactions are conducted in stirred tanks for small daily production rates or when the
reaction times are long or when some condition such as feed rate or temperature must be
programmed in some way.
7. Relatively slow reactions of liquids and slurries are conducted in continuous stirred tanks. A
battery of four or five in series is most economical.
8. Tubular flow reactors are suited to high production rates at short residence times (seconds or
minutes) and when substantial heat transfer is needed. Embedded tubes or shell-and-tube
construction then is used.
9. In granular catalyst packed reactors, the residence time distribution is often no better than that of
a five-stage CSTR battery.
10. For conversion less than about 95% of equilibrium, the performance of a five-stage CSTR
battery approaches plug flow.

11. The effect of temperature on chemical reaction rate is to double the rate every 10°C.
12. The rate of reaction in a heterogeneous system is more often controlled by the rate of heat or
mass transfer than by the chemical reaction kinetics.
13. The value of a catalyst may be to improve selectivity more than to improve the overall reaction
rate.

(Adapted from S. M. Walas, Chemical Process Equipment: Selection and Design, Stoneham, MA:
Butterworth, 1988. Copyright © 1988 by Butterworth Publishers, adapted by permission of Butterworth
Publishers, Stoneham, MA. All rights reserved)

Table 11.18 Heuristics for Refrigeration and Utility Specifications

1. A ton of refrigeration is the removal of 12,700 kJ/h (12,000 Btu/hr) of heat.
2. At various temperature levels: –18 to –10°C (0 to 50°F), chilled brine and glycol solutions; –45
to –10°C (–50 to –40°F), ammonia, freon, butane; –100 to –45°C (–150 to –50°F) ethane or
propane.
3. Compression refrigeration with 38°C (100°F) condenser requires kW/tonne (hp/ton) at various
temperature levels; 0.93 (1.24) at –7°C (20°F); 1.31 (1.75) at –18°C (0°F); 2.3 (3.1) at –40°C
(–40°F); 3.9 (5.2) at –62°C (–80°F).
4. At less than –62°C (–80°F), cascades of two or three refrigerants are used.
5. In single-stage compression, the compression ratio is limited to 4.
6. In multistage compression, economy is improved with interstage flashing and recycling, so-
called economizer operation.

7. Absorption refrigeration: ammonia to –34°C (–30°F), lithium bromide to 7°C (+45°F) is
economical when waste steam is available at 0.9 barg (12 psig).
8. Steam: 1–2 barg (15–30 psig), 121–135°C (250–275°F); 10 barg (150 psig), 186°C (366°F);
27.6 barg (400 psig), 231°C (448°F); 41.3 barg (600 psig), 252°C (488°F) or with 55–85°C
(100–150°F) superheat.
9. Cooling water: For design of cooling tower use supply at 27–32°C (80–90°F) from cooling