Page 211 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10F 5/30/2001 4:35 PM Page 174

174 Applied Process Design for Chemical and Petrochemical Plants

Note: If sensible heat, Q s , is required to bring the fluid up Heat Flux, Boiling, Ratio of Shell Diameter
2
to the boiling point, this must be calculated separately, and Btu/(ft ) (hr) to Tube Bundle Diameter
the area of heat transfer must be added to that determined
20,000 1.9 to 2.5
for the fluid boiling requirement, Q b .
15,000 1.8 to 2.1
12,000 1.5 to 1.7
1. Evaluate the heat load for the unit, Q b . 8,000 1.3 to 1.6
2. Determine the LMTD. Less than 8,000 1.2 to 1.5
3. Assume or estimate a unit size (number and size of
tubes, shell, etc.).
4. Determine the tube-side film coefficient for convection
or condensation as required, by methods previously Figure 10-104 is Palen and Small’s guide to selecting the
90
described. kettle “larger” diameter for the design of horizontal kettle
5. Determine the shell-side coefficient units.
a. Evaluate tube wall temperature
b. Evaluate boiling coefficient from Equation 10-139
or Figure 10-103. Horizontal Kettle Reboiler Disengaging Space 90
Note: the use of Figure 10-103 is considered conservative.
90
Many organic chemical and light hydrocarbon units have Palen suggests that the distance from the centerline of
been successfully designed using it; however, it is not the uppermost tube in a horizontal bundle to the top of the
known whether these units are oversized or by how much. shell should not be less than 40% of the kettle shell diame-
6. Calculate the required area, based on the film coeffi- ter. To size the kettle shell: 90
cient of steps 4 and 5 together with fouling and tube Allowable vapor load,
wall resistances; A Q/U t.
7. If the assumed unit does not have sufficient area, select 0.5
a large size unit and repeat the preceding procedure 1VL2 2,290 v c 5 d , lb/hr-ft 3 (10-160)
6.861102 1 L v 2
until the unit is satisfactory (say 10—20% excess area).
8. Determine the tube side pressure drop. 3
Vapor space, S V/(VL), ft
9. Determine the shell-side pressure drop; however, it is V actual reboiler vapors, rate, lb/hr
usually insignificant. It can be evaluated as previously surface tension, lb (force)/ft
described for unbaffled shells. Dome segment area (SA) S/L, ft 2
L average bundle length, ft
Kettle Reboiler Horizontal Shells

See Figure 10-1F.
In order to properly handle the boiling-bubbling in a ket- Kettel Horizontal Reboilers, Alternate Designs
tle unit, there must be disengaging space, and the velocities
must be calculated to be low to reduce liquid droplets being Referring to the procedure of Palen and Small 90 and
91
carried out of the unit. Generally, no less than 12 in. of space Palen and Taborek, this is an alternate check on the previ-
should be above the liquid boiling surface to the top cen- ously suggested procedure. This technique should generally
terline of the reboiler shell. When vacuum operations are be restricted to single fluids or mixtures with narrow boiling
involved, the height should be greater than 12 in. Vapor out- ranges (wide boiling range gives too optimistic results).
let nozzle velocities must be selected to be low to essentially Mean temperature difference between the bulk boiling liq-
eliminate entrainment. The liquid boiling surface should uid temperature and tube wall should be greater than 8°F;
not be greater than 2 in. above the top horizontal tube, and the ratio of pitch to tube diameter should be 1.25 to 2.0;
in order to reduce entrainment, it is often advisable to leave tube bundle diameters should be greater than 1.0 ft and less
one or two horizontal rows of tubes exposed, i.e., above the than 4 ft; and boiling must be less than 400 psia. Vacuum
liquid. This will tend to ensure that the liquid mist/droplets operations may give optimistic results and are not to be used
are vaporized and thereby reduce entrainment. As a guide to on polar compounds.
the relationship between tube bundle diameter and kettle Due to the development of the data, the method requires
shell diameter, the following can be helpful. Also, often the the use of a single tube boiling film coefficient. Using this to
tube bundle is not completely circular; that is, the upper reach the overall bundle transfer: The overall U value is
portions of circular tubes are omitted to leave a flat or hori- determined for a theoretical boiling coefficient of an
zontal tube row at the top of the bundle, at which level the unfouled tube (single) (this is an iterative procedure). See
liquid is often set. reference 90 also.