(602aa) Cycle Operation of Laboratory-Scaled Adsorption Heat Pump for Regenerating Steam From Waste Water

The efforts for
energy conservation are requested because of the limitation of fossil fuel and
the environmental issue of global warming. The petrochemical and steel
industries are especially the largest energy-consuming manufacturing
industries. While large amount of waste water with the range of 60-90°C is
released from these industrial processes, steam is demanded and generally
provided from large quantities of fossil fuel. The system to generate steam
from waste heat is therefore required for the effective utilization of energy.
In this study, a novel steam generation process using a direct heat exchange
system with adsorbent-water pair is proposed. Contacting water and adsorbent
makes excess water evaporate due to the release of adsorption heat from adsorbent.
Because this system does not require any heat exchangers, increment in packing
density of adsorbent particles in the reactor is expected. The purpose of this
study is to investigate the novel steam generation system using water-adsorbent
pair. As a basic study, cycle operation consists of steam generation process
and regeneration process of adsorbent is experimentally studied under
atmospheric pressure. Steam generation at high pressure is also studied.

The schematic of
the experimental apparatus is shown in Fig.1. It consisted of a cylindrical
steam generator, a feeding pump of water, a condenser with a cooling jacket to
condense steam generated, an electric balance to measure mass of steam
generated, and an air drier for the regeneration of adsorbent. The generator
was made of a stainless steel with the height of 100 mm and the inner diameter
of 76 mm. As the adsorbent, zeolite pellet was used and packed into the
generator for 0.26kg.

Fig.1 Experimental
apparatus

In the steam
generation process, water at 80 °C
was introduced to the generator by the feeding pump. Steam was then generated
by adsorption heat. The mass of steam was measured by an electric balance as
condensed water. The steam generation
process was terminated when water interface reached the top of packed bed. After the steam generation process,
remained water was drained from the bottom
of the steam generator. Dried air at 120 °C and
0.62 Nm/s was then introduced to the generator for a given period of time in
the regeneration process. The temperature in the packed bed was measured by
thermocouples inserted in the packed bed. Both steam generation process and
regeneration process was cycled 5 times. As the basic study, the cycle experiment
was carried out under atmospheric pressure.

As a result, superheated steam at 180°C was generated from the feed
water of 80°C. The peak temperature in the packed bed reached 280°C. The mass of steam generated per
unit mass of zeolite was 0.11kg-steam/kg-zeolite for the regeneration time of
3600 s and 0.058kg-steam/kg-zeolite for regeneration of 1200 s. Although these
values were more than 90% of the theoretical mass of steam obtained from heat
and mass balances, mass of steam generated depended on the regeneration time.
Progress of the regeneration process strongly affected the initial condition of
the next steam generation process.

Based on the
cycle experiment, effect of the regeneration time on the mass of steam
generated was numerically studied. Mathematical model considering heat and mass
transfer was developed and solved to estimate local water content in the
adsorbent and local temperature at the end of the regeneration process. Mass of
steam was then calculated by the mass balance equation taking the local water
content and the temperature distribution into account. The regeneration
conditions such as the inlet air temperature were evaluated by the average
steam generation rate, which was defined by the ratio of mass of steam
generated to the mass of zeolite per the cycle time. As a result, there was the
peak in the average steam generation rate for each flow velocity and
temperature of dry air. For example, 8.73x10^-5kg-steam/kg-zeolite·s
was estimated as the peak value when dry air at 80°C and
1.77Nm/s is used for the regeneration process (see Fig.2).

Fig.2
Relationship between the average steam generation rate and the regeneration
time (Steam:1atm, Dry air: 80°C and
1.77Nm/s)

Experiment of
steam generation at higher pressure than atmospheric pressure was carried out
to show the possibility to apply the system to the various industrial
processes. The pressure inside the generator was maintained at 2 and 4 atm by
the relief valve. As a result, the generated steam was about 0.039 and 0.0056
kg-water/kg-zeolite at 2 and 4atm, respectively. These values were about 65-70%
of the theoretical value of heat obtained from mass balance equation.

Moreover, an
additional preheat process was studied to increase mass of steam. To preheat
packed bed effectively, water vapor at low pressure was introduced from the
bottom of the steam generator before the steam generation process. As a result,
the rise of the packed bed temperature in preheating was not uniform because
introduction of water vapor was not uniform and some vapor was adsorbed in the
lower part of the packed bed. Although the mass of steam generated increased 21
and 54 % at 2 and 4atm, respectively, these values were about 50% of
theoretical value. The effect of preheat process on the mass of steam generated
would be numerically studied.