Page 235 - Applied Process Design For Chemical And Petrochemical Plants Volume III

66131_Ludwig_CH10G 5/30/2001 4:37 PM Page 198

198 Applied Process Design for Chemical and Petrochemical Plants

For clean conditions, the combined tube wall and steam- where L length of reboiler tube, ft
3
v o specific volume of fluid at outlet of reboiler, ft /lb
side resistance is calculated as 0.00090, corrected to inside
3
v i specific volume of fluid at inlet to reboiler, ft /lb
dimensions. Thus, the calculated U is
log log base 10
1 Friction resistance to flow inside tubes, flow rate into tubes (per
2
U 282 Btu>1hr2 1ft 21°F2
1 tube):
0.00090
377
This is close to the assumed value of U. It does not, how-
W W114421n2
ever, consider fouling in service. Additional trial designs may , lb/hr-ft cross-section (10-190)
2
a
be used, or the temperature difference may be adjusted N t a i
upward. n 1 tube pass
where W total flow rate, lb/hr into tubes
N t total number of tubes
Kern’s Method Stepwise 70 a i cross-sectional flow area per tube, in 2
D i tube I.D., ft
In vacuum applications, use this method with caution and Re DG/
, per tube
compare it with other methods. G flow into tubes, lb/hr (ft cross-section) with
2
evaluated at the boiling temperature for the liquid.
1. Determine the heat requirements or duty for any
sensible heat as well as the latent heat. Read friction factor, f, from Figure 10-137.
2. Assume a unit size, number and size of tubes, and area. Calculate mean specific gravity in tube as average of
3. Evaluate sensible heat transfer inside tubes as previ- inlet liquid and outlet vapor-liquid mixture.
ously outlined for in-tube transfer. Determine the area
2
required. fG Ln
pressure drop p t (10-191)
4. Evaluate LMTD for isothermal boiling. 10
5.22 10 D i s t
5. Trial 1: Estimate area, A, for maximum flux condition,
Let t 1.0
limiting Q/A to 12,000 Btu/hr-ft surface for organic
2
materials. Experience has shown that a value of 6,000-
11. Total resistance to flow:
8,000 is a good starting value for Q/A for organics.
Q (static pressure of reboiler leg) (pressure drop through
A tubes) (frictional resistance of inlet piping) (frictional
Q>A
resistance of outlet piping) (expansion loss) (10-192)
6. Re-estimate the unit size assumed in Step 2, making the
area the value of Step 5. Note that for preliminary calculations, the frictional
7. Evaluate an operating overall coefficient: resistances in piping can be neglected but should be
included in final calculations, particularly at high recir-
Q culation ratios.
U D (10-188)
A1 t2 12. Driving force
t LMTD
x 2 L
, psi (10-193)
8. Assume a recirculation ratio of 4:1. 144
9. Determine the material balance around unit
where x 2 height of liquid level in column above reboiler
Total weight of recirculated liquid (4) (desired vapor rate, V) bottom tubesheet, ft
Vapor Desired vapor rate, V L density of liquid, lb/ft 3
Liquid 4 (desired vapor rate, V)
Total 5 (desired vapor rate, V) W
13. If the driving force, Step 12, does not equal or slightly
10. Pressure balance across reboiler
exceed the total of resistances in Step 11, the unit
should be rebalanced; that is, shorter tubes used to give
Static pressure of boiler leg
less pressure drop, lower recirculation ratio used to give
L 1avg.2 2.3L v o
log , psi (10-189) less pressure drop, or a larger number of total tubes
144 1441v o v i 2 v i
used to give less pressure drop. The liquid can be raised