Experimental and Numerical Analysis of a PCM-Supported ...

At flooding, essentially all the voids are filled with liquid. The flooding point can be
described in a number of ways, but the clearest definition [39] is the point where at a
certain gas flow, flow reversal of the liquid occurs and the liquid is unable to flow
through the packing and collects on the top of the bed. The pressure drop per unit
height increases rapidly due to this liquid accumulation. This point is the operating
limit for counter-current flow of gas and liquid through solid packing. Flooding is a
kind of hydrodynamic phenomenon to be prevented in the operation of packed
columns, which are extensively used in absorption and distillation, and in empty
tubes mainly used in reflux condensers, where the condensate moves downward
counter-currently with respect to the uprising vapor. Kister and Gill [32] applied the
principle that the packing pressure drop at flooding point increases as the packing
capacity increases to derive a simple flood point correlation.
0.7
PFl
0.115Fp
(4.58)
Equation (4.58) expresses the pressure drop at the flood point as a function of the
packing factor (F p ) alone. It is recommended [32] to use this correlation for flood
point prediction if pressure drop data are available or when pressure drop can be
reliably predicted.
Flooding is not often of significant importance for small schemes with relatively low
gas flow rates, such as small scale HDH system proposed in the present study. On
contrary, as the HDH systems are usually operated at relatively low liquid loads, the
minimum wetting rate becomes significantly more important than the flooding rate.
The minimum wetting rate (MWR) is the lower stability limit of packing. It is the liquid
load below which the falling liquid film breaks up, and the liquid storage causes
dewetting of the packing surface [32]. Kister [32] presented various correlations and
commonly known rules of thumb for estimating the MWR and recommends a simple
correlation/rule of thumb:
L MW
60 a 0. 5
L
(4.59)
where L is a reference minimum wetting rate based on an average value of a about
60 gpm.ft -2 . Values of L for different packing materials are tabulated in Kister [32], for
PVC and Polypropylene L is 1.4 and 1.6 gpm.ft -2 (~ 950.5 – 1086.3 l.m -2 .h -1 )
respectively.
4.3.2 Column Sizing
Three different techniques are being used for tower sizing; the flooding point, the
maximum operation capacity (MOC), and the maximum pressure drop criteria. In
flooding point criterion, packed towers are usually designed at 70 to 80 percent of
the flood point flow velocity. This practice provides sufficient margin to allow for
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