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

The last two cases of boundary conditions (5 and 6) are devoted to lower inlet
hot water temperature of 65 ºC at higher water mass flow rates and low and high
air mass flow rates. The results clearly reveal the importance of maintaining high
inlet water temperature for the system productivity regardless of the packing
type. However a minor role of conductive packing can be noticed, in dependency
on the packing height due to difference in the specific interfacial surface areas,
and number of layers active in the MEHH process.
It can be said that at a constant thermal conductivity of the packing medium, the
influence of the conductive packing on the system performance is strongly
dependent on the magnitude of the binary solid-gas and solid-liquid interactions,
which is governed by the specific dry/wet areas on the outer surface of beads,
quality of the inlet hot water temperature to the system, packing height or number
of active packing layers in the MEHH process, and associated heat and mass
transport coefficients or mass flow rates.
5.5.3 Effect of packing specific area
Further questions arise whether the PCM thermal conductivity and stratification
effects, which induce the MEHH phenomenon, would have a higher impact on
the system performance or enhancing the direct contact diffusion at the liquidgas
interface, using e.g. smaller packing elements would lead to same or better
performance.
In order to follow this question, results are furthermore compared between PCM
and Hiflow Rings, which is a commercially packing material being used in
industrial stripping columns. Extra six experimentation runs were conducted
using Hiflow packing elements. Four experiments were conducted under the
same first four sets of boundary conditions that are listed in table (6.2) and
presented in figure 5.10. The last two experiments were conducted under natural
draft operation at high inlet hot water temperature and different inlet hot water
mass flow rates of 250 and 500 l/h to get more insight on the effect of the MEHH.
The Hiflow Rings with size 20 mm that have been used, have a specific surface
area of 291 (m 2 /m 3 ), and porosity of 0.75 compared to 46.6 (m 2 /m 3 ) and 0.42 for
those of the PCM spheres respectively. For a packed height of 0.39m, the total
surface areas available for heat and mass transfer (i.e. wetted area and dry
patches) are 2.26 and 13.9 m 2 for the PCM spheres and Hiflow rings,
respectively (i.e. the Hiflow rings area is 6 times larger than that of the PCM
spheres).
Figure (5.11) exhibits the total distillate output through 2 hours of steady state
operation for the three packing types under different boundary conditions and
natural draft air flow. It can be seen from the first case that when the high mass
flow rate of hot water is combined with high air velocity in the system, increasing
the specific surface area of packing (i.e. Hi-flow Rings) gives strong rise to direct
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