Page 195 - Applied Process Design For Chemical And Petrochemical Plants Volume II

184 Applied Process Design for Chemical and Petrochemical Plants

venting liquid from passing through the holes and bypass- using the given volumetric weeping liquid rate, and
ing the overflow weir and downcomer. total hole area.
This point is generally considered the lower point of (d) Calculate J&
operation for the tray while maintaining acceptable effi- JE1/2 + m JE1/2 = C (8-273)
ciency. Some systems are known to operate at only slight
reduction in efficiency while vapor velocities are well JE=C
below the weep point values. It is impossible to predict this
behavior at present. Weeping is usually the limiting condi- (e) Calculate vapor rate, VG, based on value of J*G in
tion in design for low vapor rate, high liquid rate systems. (d) above using:
Some factors affecting the weep point of any system are
described in the following sections. (8 - 274)
Weeping and dumping are types of drainage that occur
during tray operation, and are more sensitive in the opera-
tion/control of sieve trays than for valve or bubble cap trays. (f) Using V, from (e) above based on superficial gas
on
based
Lessi [194] presents an analysis of these conditions. Weep rate calculate VG,~~~~ total perforated
ing simply means that the gas/vapor volume passing hole area only, ft/sec. That is:
upward through the tray is not sufficient to prevent liquid ft3/sec vapor - (vc, ft/sec) (AH, ft2)
on the tray from running back down to the tray below,
thereby affecting the tray efficiencies. Dumping is a term The weep point for sieve or valve trays is the vapor rate
more associated with sieve trays than the others; however, in at which the liquid weeping rate is diminished to zero.
concept it represents a large or excessive amount of liquid Thus, J*L approaches zero asJ*G is increased [210]. For a
draining off the tray, greater than weeping, and could be vapor rate that leads to J*G higher than the weep point
considered a type or forerunner of flooding of the column. value, then there should be no weeping.
Hsieh and McNulty [210] developed a new correlation Windm > 0, for no weeping
for weeping of sieve and valve trays based on experimental
research and published data. For sieve trays the estimation Windex = J*G - J*~(wee~ pt.)
of the weeping rate and weep point is recommended using
a two-phase countercurrent flow limitation model, CCFL. The higher the value of Windex, the more confidence
The procedure [210] for weeping calculation and that there will be no weeping [210]. At a constant weep
determination of vapor rate that will result in a certain point, J*G then, the higher the percentage opening of the
weeping rate (used by permission, Reference 210 Chemi- tray, and the higher will be the vapor volumetric flow
cal Engr: Progress, all rights reserved) : required to satisfy the weep point criteria.
To calculate the weep point, useJ*G = 0.74 and calculate
(a) Calculate Z Z from (a) above, then calculate VG from (e) above.
The author’s [210] report that the test results show that
z = h,1.5/12 ~H0.j (8 - 271)
below the weep point for the Type-T and TypeA valve
(b) Use values of m and C as determined by Reference trays, a consistently low weeping rate can be maintained,
210 while for sieve trays the weeping rate increases rapidly at
For sieve trays low gas flow. For similar operating conditions, the weeping
rate for a valve tray can be an order of magnitude lower
m = 2.01 than the corresponding weeping rate for a sieve tray with
C = 0.74
the same open area. The tests assume uniform weeping
For type T-valve style (Koch) across the entire tray deck; however, recent tests [210]
indicate that for a larger &ft diameter (versus 3ft) tray,
m = 2.87
C = 0.74 weeping occurs preferentially along the periphery of the
bubbling area, indicating that for the larger diameter the
For type A-valve style (Koch) actual weeping rate can be lower by more than 30% when
m = 2.01 referenced to the present models prediction.
C = 0.74 The equivalent hole diameter for use in the equation
for Z when considering the two types of valve trays studied
(c) Calculate J*L here is given by:

(8 - 272) Dm = 2 [1/(2.331) (Fp + F3) (R~)ll’~, (8-275)
(equivalent hole diameter, in.), see Equation 8-277