275362 Cycle Operation of Laboratory-Scaled Adsorption Heat Pump for Regenerating Steam From Waste Water

Wednesday, October 31, 2012
Hall B (Convention Center )
Yoshiho Iwama1, Yuuki Tanaka1, Kazuya Nakashima1, Bing Xue1, Agung Tri Wijayanta2, Koichi Nakaso1 and Jun Fukai1, (1)Department of Chemical Engineering, Graduate School of Engineering, Kyushu University, Fukuoka, Japan, (2)Research and Education Center of Carbon Resources, Kyushu University, Fukuoka, Japan

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 60-90C 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 180C was generated from the feed water of 80C. The peak temperature in the packed bed reached 280C. 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 80C 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: 80C 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.

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