Page 238 - Chemical Process Equipment - Selection and Design

208 HEAT TRANSFER AND HEAT EXCHANGERS

TABLE 8.14. A Guide to the Selection of Reboilers
Reboiler Type
Horizontal Vertical
Kettle or Shell-Side Tube-Side Forced
Process Conditions Internal Therrnosiphon Therrnosiphon Flow
~ ~
Operating pressure
Moderate E G B E
Near critical B-E R Rd E
Deep vacuum B R Rd E
Design AT
Moderate E G B E
Large B R G-Rd E
Small (mixture) F F Rd P
Very small (pure component) B F P P
Fouling
Clean G G G E
Moderate Rd G B E
Heavy P Rd B G
Very heavy P P Rd B
Mixture boiling range
Pure component G G G E
Narrow G G B E
Wide F G B E
Verv wide, with viscous liquid F-P G-Rd P B
"Category abbreviations: B, best; G, good operation; F, fair operation, but better choice is
possible; Rd, risky unless carefully designed, but could be best choice in some cases; R, risky
because of insufficient data; P, poor operation; E, operable but unnecessarily expensive.
(HEDH, 1983, 3.6.1).

weight fraction vaporized should be in the range of 0.1-0.35 for The design procedure must start with a specific geometry and
hydrocarbons and 0.02-0.10 for aqueous solutions. Circulation may heat transfer surface and a specific percentage vaporization. Then
be controlled with a valve in the supply line. The top tube sheet the heat transfer coefficient is found, and finally the required area is
often is placed at the level of the liquid in the tower. The flow area calculated. When the agreement between the assumed and
of the outlet piping commonly is made the same as that of all the calculated surfaces is not close enough, the procedure is repeated
tubes. Tube diameters of 19-25 mm diameter are used, lengths up with another assumed design. The calculations are long and tedious
to 12 ft or so, but some 20 ft tubes are used. Greater tube lengths and nowadays are done by computer.
make for less ground space but necessitate taller tower skirts. Example 8.12 summarizes the results of such calculations made
Maximum heat fluxes are lower than in kettle reboilers. on the basis of data in Heat Exchanger Design Handbook (1983).
Because of boiling point elevations imposed by static head, vertical Procedures for the design of kettle, thermosiphon and forced
thermosiphons are not suitable for low temperature difference serv- circulation reboilers also are outlined by Polley (in Chisholm, 1980,
ices. Chap. 3).
Shell side vertical thermosiphons sometimes are applied when
the heating medium cannot be placed on the shell side.
8.10. EVAPORATORS
FORCED CIRCULATION REBOILERS
Forced circulation reboilers may be either horizontal or vertical. Evaporators employ heat to concentrate solutions or to recover
Since the feed liquid is at its bubblepoint, adequate NPSH must be dissolved solids by precipitating them from saturated solutions.
assured for the pump if it is a centrifugal type. Linear velocities in They are reboilers with special provisions for separating liquid and
the tubes of 15-20ft/sec usually are adequate. The main vapor phases and for removal of solids when they are precipitated
disadvantages are the costs of pump and power, and possibly severe or crystallized out. Simple kettle-type reboilers [Fig. 8.4(d)] may be
maintenance. This mode of operation is a last resort with viscous or adequate in some applications, especially if enough freeboard is
fouling materials, or when the fraction vaporized must be kept low. provided. Some of the many specialized types of evaporators that
are in use are represented on Figure 8.16. The tubes may be
CALCULATION PROCEDURES horizontal or vertical, long or short; the liquid may be outside or
inside the tubes, circulation may be natural or forced with pumps or
Equations for boiling heat transfer coefficients and maximum heat propellers.
fluxes are Eqs. (37) through (48) of Table 8.10. Estimating values Natural circulation evaporators [Figs. 18.16(a)-(e)] are the
are in Tables 8.4-8.7. Roughly, boiling coefficients for organics are most popular. The forced circulation type of Figure 18.16(f) is most
300 Btu/(hr)(sqft)("F), or 1700 W/m2 K; and for aqueous solutions, versatile, for viscous and fouling services especially, but also the
1000 Btu/(hr)(sqft)("F), or 5700 W/m2 K. Similarly, maximum most expensive to buy and maintain. In the long tube vertical
fluxes are of the order of 20,000 Btu/(hr)(sqft), or 63,000 W/m2, for design, Figure 8.16(d), because of vaporization the liquid is in
organics; and 35,000 Btu/(hr)(sqft) or 110,000 W/m2, for aqueous annular or film flow for a substantial portion of the tube length, and
systems. accordingly is called a rising film evaporator. In falling film