Experimental and mathematical modeling of a novel humidification-dehumidification desalination system
J.Moumouh1*, M.Tahiri1, M.Salouhi, L.Balli
1Group of Thermodynamics Applied, Mohammadia School of Engineering
1Mohammed V University-Agdal
Rabat, Morocco
*.
Abstract— In this paper, a desalination system based on air humidification-dehumidification is investigated experimentally and theoretically.To evaluate the productivity of the proposed humidification-dehumidification desalination unit, a mathematical model has been developed for the design of a unit, based on mass and energy balances as well as thermodynamic analysis of cooling tower and condenser that arethe main constituents of the unit. A set ofnon linear differential equations is obtained then solved to predict water production of the system. The mathematical model is tested at different operating conditions. The results show that highest fresh water productivity is obtained when saline water temperature is 85 °C.
The results obtained from the theoretical model are in very good agreement with experimental data and show evidence that the proposed model is valid and may be used under different appropriate boundary conditions.
Keywords-Humidification, Dehumidification, Simulation; Open-air, open-water (OAOW).
Numerical simulation, Experimental investigation, Water productivity
Introduction
Remote areas in arid regions in Morocco especially in the north and in the south, luck access tosafe clean water supply.Seawater or brackish water desalination may be the only solution to the shortage of fresh water. If solar energy use isprobably the simplest in seawater distillation systems it however produces small quantities of water and becomes non economical to cover all daily needs [1].
Humidification– dehumidification (HD) desalination system is a promising technique for the production of fresh water in remote regions [2]. Thisdesalinationprocess is based principally on the capacity of air to be mixed with large quantities of water vapor [2].HD technique has been subjected to many studies in recent years due to its low-temperature renewable energy use (such as the solar), its simplicity, its low installation cost and its ease of operation [3].Moreover HD desalination systems work at atmospheric pressure; hence they do not need additional large mechanical energy [4]. These kinds of systems do not require sophisticated high-technologies so that they can be easily implemented in rural regions[4]. Therefore, their design, construction and operation are easy. The system is modularso that the capacity can beincreased by additional solar collectors and HDH cycles [5].
1.Description of the physical model
Process description of open-air, open-water with water heating
The principle of this desalination process is based on the evaporation of water and the condensation of steam from humid air. The humid air flows in an open circuit driven by natural convection between a cooling tower and a condenser.
In the cooling tower the hot saline water heated by a heating source, is then distributed on a large contact area packing to allow the air. The air moves in a counter-current flow to the brine through the evaporator and becomes water saturated.
In the condenser water vapor from the humid air condenses on heat exchanger. The distillate runs down and collected in a basin.
2.Experimental setup
An experimental prototype is built to study thermodynamics as well as heat and mass transfers inherent to the system to evaluate fresh water productivity.
Fig.1is an image of desalination station composed of the heating source, the humidifying tower and the dehumidifier.
The dimensions of the condensation tower are 72 cm height, 33 cm length, and 33 cm width. A copper tube coil is used in the condenser, it has 50 m in length and 1.27 cm as outer diameter. The total heat exchange area Acond is 20m2.
The other compartment of the desalination unit is the humidifying tower. Its dimensions are 170 cm height, 33 cm length, and 30 cm width. A packing material is fixed inside the humidifier; the specific area of metal mesh is 250m2/m3, presenting 34 m2 as total area for evaporation, when assuming complete wetting.
Water is sprayed on the packing material using a hydraulic grid.
A 200watts electric fan is used to blow the air through the unit. The fan is supported at the bottom of the tower. Water circulates throughout a 500 watts centrifugal pump. A filter isplaced at the suction side to separate any impurities coming from the elevated water feeding tank.
A flow meter is connected on the delivery side of the pump to measure the water flow rate. A group of valves are used to separate each part of the saline water loop from the system when necessary. The external heat is provided by two electrical resistances (2400 watts). Temperatures of air are measured with wet thermometers. The air flow is measured by Ex-tech digital anemometer. The rate of distillation is measured with a 1000ml graduated test tube and a chronometer.
Fig.1Experimental prototype: left is the humidifier and right is the dehumidifier.
The saline groundwater used is from Meknassa which is a rural village, situated in Taza region in northernMorocco;it is characterized by a salinity of6g/l.
Fig.2 Location of Meknassa in the map
The composition of Meknassa saline water is represented below.
Table 1 Analysis of saline groundwater
Expérimental procedure
The experimental procedure is as follows:
Adjusting the air flow rate.
Adjusting the saline water flow rate.
Adjusting the heating temperature.
Calibrating the measuring instrument before stating and installing them in their right position.
Operating system till steady operation occurs and measuring the required parameters
Experimental results
Fig.3 show the fresh water productivity obtained experimentally for two different flow rates of air and saline water as function of heating temperature of the saline water at inlet of the humidifier.
Fig.3 Experimental fresh water productivity
3.The mathematical model
Humidifier modeling
The mathematical model for the cooling tower (humidifier) is formulated by applying mass and energy balances on an elementary volume as shown in Fig.3
Fig.4 An element of the humidifier.
Energy balance for saline water:
Shell Energy balance, between hight z and z+Δz for saline water is:
(1)
Using Taylor series expansion we get:
(2)
hence:
(3)
Energy balance for humid air:
Air gaines heat by convection from water in addition to the heat from the vapor that comes to get mixed to it, making its mass and enthalpy increase. The direction of air flow is opposite to that of water flow. Change in total enthalpy is given by:
(4)
The enthalpy of humid air is:
(5)
(6)
(7)
(8)
Mass balance for water
(9)
The mass of air changes due to the amount of vapor that gets suspended in it as it flows inside the humidifier. Since the rate of dry air is constant, the mass balance is expressed as:
(10)
Absolute humidity of saturated air, Ws, is computed from the following common expression [3]:
(11)
The vapor pressure of water, Ps, is calculated as a function of temperature with the following equation [4]:
(12)
The mass transfer coefficient (Dh) and the heat transfer coefficients (hw,ha) as function of air flow rate (G) and water flow rate L are [3]:
(13)
(14)
(15)
The main mathematic model for the humidifier is expressed by Eqs. (3),(6),(9)and(10),Eqs. (5),(7),(8),(11),(12),(13),(14),(15) are used in calculations and do not add any new unknowns.
Dehumidifier modeling
Inthecondenser we assume that heat and mass transfers are homogenous. Global energy balances are suitable to determine air temperature, humidity at exit condenser.
At steady state, the energy balance of the condenser can be written as follows:
(16)
This can be rearranged as:
(17)
Humid air enthalpy is calculated using Eq. (5).
In the condenser, the heat transfer rate for cross-flow is written as:
(18)
(19)
The system of equations for condenser are Eqs (17),(18) and(19).
The purpose of the model is to calculate the flow rate ofcondensed water with Eq. (19).
The next parameters are considered to be known
Flow rates: G and L;
Temperatures:Tamb,TwintC, Twmoy;
Properties: Cpw,Cpa,Cpv,ΔH,Patm;
Geometrie: Ah, AC, Acond, a;
Transfert coefficient: Ucond, ULC;
These parameters are used to calculate the distillate rate using a matlab program.
- Results and discussion
ThePresent mathematical model is validated by comparing theoretical results, obtained in the present work with the results obtained from the experiment.
Fig. 5 and 6shows the production rate of distillate predictedwith Eq. (19), as compared, with the actual production obtained in the experiment.
Data of Fig. 5 have a coefficient of determination R2 = 0,96157 and those of Fig.6 have a coefficient of determination R2= 0,97568.
Fig. 5 Calculated distillation rate vs. Measured distillation rate.
(G=0,036kg/s , L=0,04kg/s)
Fig. 6 Calculated distillation rate vs. Measured distillation rate.
(G=0,022kg/s ,L=0,02kg/s)
Conclusion
A desalination system working under the air humidification and dehumidification principle and its thermal study were presented. The mathematical model works very well to simulate the air temperatures and air humidity of the system at steady state. It works reasonably well also to calculate the rate of distillate production
Nomenclature
S: Salinity(g/l).
L: Saline water flow rate.
G: Air flow rate.
Tw:Water temperature (°C).
:Rate of heat transfer to evaporate water in humidifier (W).
:Rate of heat transfer from water to air in humidifier (W).
Ah: Surface area of humidifier (m2).
AC: Surface area of condenser (m2).
Acond:condenser area of heat transfer, ( m2).
Cpa: Specific heat at constant pressure for air (J/kg K).
Cpv: Specific heat at constant pressure for water vapor (J/kg K).
Cpw: Specific heat of saline water (J/kg K).
Dh: Mass transfer coefficient in humidifier (kg/m2 s).
hw:Water heat transfer coefficient at the air–water interface (W/m2 K).
ha: Air heat transfer coefficient in the cooling tower (W/m2 K).
UC:Overall heat transfer coefficient in condenser (J/m2.s K).
ULC:overall heat loss coefficient of condenser (J/m2.s K).
Ha:Enthalpy of air (J/kg).
Ta:Air temperature (°C)
Tamb:Mean temperature of air (°C).
Twm: Mean temperature of water.
TavgC: average surface temperature of condenser, (°C).
:Air temperature at point i, (°C).
a:Specific area of evaporator (m2/m3).
Wamb:Mean humidity ratio of air (kgv/kga).
Wa: humidity ratio of air (kgv/kga).
Ws: Saturation humidity in the humidifier (kgv/kga).
ΔH: Latent heat of evaporation of water at zero °C (J/kg).
Ps:Saturation pressure (Pa).
:Height of element (m).
D: Distillate kg/min.
Symbol
intC: Condenser inlet
exitC:Condenser exit
References
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