Elsevier

Desalination

Volume 308, 2 January 2013, Pages 209-218
Desalination

Numerical simulation and performance investigation of an advanced adsorption desalination cycle

https://doi.org/10.1016/j.desal.2012.04.021Get rights and content

Abstract

Low temperature waste heat-driven adsorption desalination (AD) cycles offer high potential as one of the most economically viable and environmental-friendly desalination methods. This article presents the development of an advanced adsorption desalination cycle that employs internal heat recovery between the evaporator and the condenser, utilizing an encapsulated evaporator–condenser unit for effective heat transfer. A simulation model has been developed based on the actual sorption characteristics of the adsorbent–adsorbate pair, energy and mass balances applied to the components of the AD cycle. With an integrated design, the temperature in the evaporator and the vapor pressurization of the adsorber are raised due to the direct heat recovery from the condenser, resulting in the higher water production rates, typically improved by as much as three folds of the conventional AD cycle. In addition, the integrated design eliminates two pumps, namely, the condenser cooling water and the chilled water pumps, lowering the overall electricity consumption. The performance of the cycle is analyzed at assorted heat source and cooling water temperatures, and different cycle times as well as the transient heat transfer coefficients of the evaporation and condensation.

Highlights

► An AD cycle with internal heat recovery between condenser and evaporator. ► A P–T–C diagram provides an insight to cycle and performance evaluation. ► It gives a three-fold improvement in water production rate. ► It consumes only 1.38 kWh/m3, and it is twice that of thermodynamic limit. ► At 55 C (85 C) heat input, it yields 8.1(26) m3 per ton of adsorbent per day.

Section snippets

Nomenclature

ρdensitykg m 3
q"heat fluxW m 2
m˙mass flow ratekg s 1
γflag which governs mode of operation
θflag which governs mode of operation
μviscositykg m 1 s 1
σsurface tensionN m 1
τnumber of cycles per day
Aaream2
cuptake by the adsorbent materialkg kg 1
cthe equilibrium uptakekg kg 1
c0the limiting uptakekg kg 1
COPthe Coefficient of Performance
cpspecific heat capacityJ kg 1 K 1
Csfa constant of Rohsenow correlation
Dsoa kinetic constant for the silica gel water systemm2 s 1
Eaactivation energy of surface

Description of the advanced AD cycle

Fig. 2 shows the schematic diagram of an advanced AD cycle with an evaporator–condenser heat recovery scheme. The major components of the cycle are (i) the feed water tank, (ii) two adsorber/desorber beds, (iii) the evaporator–condenser device, (iv) potable water collection tank and (v) the brine discharge tank. Each adsorber bed contains adsorbent materials (silica gels) packed around the tube-fin heat exchangers. The reactor beds could be communicated to the evaporator or the condenser during

Mathematical modeling

A mathematical model on an advanced AD cycle that employs an integrated evaporator–condenser design for the internal heat recovery process was developed to access the performance of the cycle. Isotherm and kinetics properties of the silica gel–water pair are used to predict the uptake of the water vapor by the silica gels at specific temperature and pressure conditions. Mass and energy balances of the components involved in the cycle are further employed to evaluate the model. Type A++ silica

Results and discussion

The advanced AD cycle with internal heat recovery between the condenser and the evaporator with an evaporator–condenser device is investigated at various operation conditions such as different cycle times, hot and cooling water temperatures, different hot and cooling water flow rates. Fig. 3 shows the temperature–time history of the adsorber, desorber, evaporator and condenser of the advanced AD cycle at the cyclic-steady-state conditions. It is noted that the present AD cycle consists of four

Conclusions

We have successfully modeled and predicted the performance of an advanced adsorption desalination cycle with condenser–evaporator heat recovery scheme. The modeling techniques incorporated realistic isotherms and kinetics, heat and mass transfer resistances in the solid–vapor uptake, the evaporative boiling and condensation processes. The cycle performances are examined for assorted heat source and cooling water temperatures.

For the same adsorbent inventory in the cycle, the advanced cycle

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