Experimental determination of the adsorption capacity of synthetic Zeolite A/water pair for solar cooling applications

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1 Journal of Mechanical Engineering Research Vol. 4(4), pp , April 2012 Available online at DOI: /JMER ISSN Academic Journals Full Length Research Paper Experimental determination of the adsorption capacity of synthetic Zeolite A/water pair for solar cooling applications Ityona Amber*, Randolph Osivue Odekhe and Yinka Sofihullahi Sanusi Department of Mechanical Engineering, Ahmadu Bello University, Zaria, Nigeria. Accepted 16 January, 2012 The experimental determination of the adsorption capacity of water in synthetic Zeolite A, produced from a Nigerian clay (kankara), determined at different Zeolite temperatures and water vapor pressures for use in an adsorption cooling system is presented in this paper. 0.2 kg of synthetic Zeolite A is tested at adsorption temperatures in a range of 40 to 150 C and water vapour pressures in the range 36 to 40 kpa. Experimental result obtained show that the maximum adsorption capacity of synthetic Zeolite A prepared from kankara clay is nearly kg ad /kg w for Zeolite temperatures and water vapor pressures in the range 40 to 150 C and 36 to 40 kpa and that the adsorption capacity is a weak function of the water vapor pressure at high Zeolite temperatures. Key words: Zeolite A, adsorption capacity, adsorbate, adsorbent. INTRODUCTION Zeolite in solar cooling is economically and environmentally beneficial as adsorbent/adsorbate pair are non poisonous, non flammable, naturally available, environmentally neutral, lose water fairly continuously over a temperature range of 150 to 400 C without collapse of the framework structure and readsorb it from the atmosphere at room temperature (Meunier, 2001) and even after several thousand adsorption/desorption cycles, the structural changes of the crystals are insignificant in the process parameters such as pressure and temperature; that is, at no time does a new phase form (Alghoul, 2007). Zeolites are stable three-dimensional honeycomb structured hydrated aluminosilicates with symmetrically stacked alumina and silica tetrahedral (molecular sieves) that selectively adsorb molecules on the basis of their size, shape or electrical charge and makes them useful in many important industrial and commercial applications (Arbuznikov et al.,1998). With more than 150 synthesized Zeolites types and *Corresponding author. amberity@yahoo.com. Tel: with 40 known naturally occurring, the adsorption capacity of Zeolites varies as their selectivity to interact with water in the presence of other molecules decreases with decreasing concentration of aluminium per unit cell (Andreas et al, 1989). Zeolite A is the simplest Zeolite with a molecular ratio of one silica to one alumina to one sodium cation (1:1:1). Its synthesis produces precisely duplicated sodalite units which have 47% open space, ion exchangeable sodium, water of hydration and electronically charged pores ( Korkuna et al., 2006). Over the past few decades, many researchers have conducted experimental studies to investigate the equilibrium adsorption capacity and thermodynamic properties of different adsorbent refrigerant working pairs at various equilibrium conditions. The following is a brief review of the literature most important for the present work. The equilibrium adsorption data at several adsorbent temperatures and refrigerant pressures were determined for a wide range of pairs including natural Zeolite (Solmus et al, 2010) synthetic zeolite water (Wang and Wang, 1999), ZSM5 (Andreas et al.,1989), silica gel water (Ng et al., 1999, Afonso et al. 2005), activated carbonmethanol (Wang and Wang, 1999; Wang et al., 2006),

2 Amber et al. 143 ACF(A-15)-ethanol and ACF (A-20)-ethanol (El-Sharkawy II, 2006), activated carbon ammonia (Wang et al., 2006), composite adsorbent-ammonia (Wang et al., 2006), NAwater and NB-ethanol (Cui et al., 2005), Maxsorb III-nbutane (Saham et al., 2008), and MaxsorbIII- R134a (Saha et al., 2009). Some researchers also calculated the isosteric heat of adsorption from the equilibrium adsorption data (Ng et al., 1999; Saham et al., 2008; Saha et al., 2009). Moreover, a review of the types, characteristics, advantages and disadvantages of different adsorbents, refrigerants and working pairs together with their models are presented by Wang et al. (2009). The objective of this work is to experimentally determine the maximum adsorption capacity (or loading) of synthetic Zeolite A, prepared from a Nigerian (kankara) clay at different Zeolite and water temperatures using a simple test rig developed at the Department of Mechanical Engineering, Ahmadu Bello University, Zaria, Nigeria, to be used in solar adsorption cooling applications. MATERIALS AND METHODS The goal for the experimental procedure is to obtain adsorption data relating the equilibrium adsorption capacity of the Zeolite to the Zeolite temperature and the water vapor pressure that is ( Pires, et.al, 1997, Ramos et al 2003): X = f (P,T ad) (1) These data are important as they define the thermodynamic operation of an ideal cycle and therefore set limits with respect to performance and system size. In a closed vessel filled with Zeolite, the pressure of water vapor (P w) at equilibrium is a function of the temperature (T z(k)) and the adsorption capacity (X) of the Zeolite is calculated as given by Equation 1: mass of water m w adsorbed by the mass of Zeolite m z is: If m w is evaporated, the refrigerant is cooled down by the cooling energy: Where h = enthalpy of vaporization Zeolite synthesis Zeolite A was produced from Kankara Kaolin (clay), sourced from Katsina State, Nigeria. 2 kg of kaolin was mixed with 2500 cm 3 of distilled water and the resulting mixture was allowed to soak for four days with intermittent stirring at predetermined intervals. On the fourth day, the mixture was thoroughly stirred and sieved using a 75 μ mesh. The filtrate was then allowed to settle and the column of water decanted. The clay part was centrifuged to remove excessive water from it. On completion of the centrifuge stage, the kaolin clay was dried in an oven at a temperature of 50 C for 24 h, after which it was pulverized in a mortar and sieved further with a 53 µ mesh to give an extra fine texture in readiness for calcinations. The crystalline clay was converted to amorphous metakaolin by thermal treatment in a furnace under controlled conditions at a temperature of 600 C for 8 h. 600 g of metakaolin was mixed thoroughly with 518 g of sodium hydroxide pellets dissolved in cm 3 of distilled water. The mixture was allowed to age for three days at room temperature in a polypropylene bottle and then placed in an autoclave (WS , model-7101) set at 100 C, for 8 h. After the synthesis, the samples were then allowed to cool, washed with distilled water to remove excess sodium hydroxide, dried at 150 C for 3 h and subsequently calcined at 300 C. The samples were characterized x-ray powder diffraction (XRD), differential thermal analysis (DTA) and thermogravimetric analysis (TGA). Procedures are as outlined in Howell (1963). (4) (5) When the desorption starts, the Zeolite is nearly saturated with high water load c h. The desorbed water condenses in the condenser. There, the vapor pressure is equal to the saturation vapor pressure of water P s at ambient temperature T a. The process stops when equilibrium is reached. Then, the low load c 1 of the Zeolite causes a vapor pressure in the adsorber which is equal to the vapor pressure in the condenser. During cooling, the water vapor is absorbed by the Zeolite while at ambient temperature T a. The adsorption stops when the Zeolite is saturated containing the high water content c h. At equilibrium, the pressure in the adsorber is equal to the saturation vapor pressure of the refrigerant at the cold temperature T c: From Equation 3, c h can be calculated. In the adsorption cycle, the (2) (3) Experimental procedure An overview of the rig components, sizing, and functions, are presented in Table 1. The experimental rig used is similar to the one outlined in (Solmus et al, 2010) whose schematic is shown in Figure 1 and experimental setup is presented in Figure kg of Zeolite A of 53 µm particle size is loaded into the Zeolite canister (Figure 1) and then sealed. This canister is put inside the oven which is heated by the electrical stove and connected to the rest of the experimental apparatus. All valves except V 2 and V 4 are open. The oven temperature is set to 150 C and the Zeolite canister is evacuated until the pressure inside the Zeolite canister is less than 0.1 kpa. Valve V 1 is closed and valve V 2 is opened, and the rest of the system is completely evacuated. After the measured residual gas pressure is less than approximately 0.1 kpa, the system is assumed to be evacuated. Subsequently, valve V 3 is closed and the vacuum pump is turned off. The water bath temperature is set to 5 C and valve V 2 is closed. The water intake valve V 4 is then slowly cracked and de-ionized water is allowed to flow into the evaporator/condenser. The pressure in the evaporator/condenser is compared with the

3 144 J. Mech. Eng. Res. Table 1. Components of test rig. Component The Zeolite canister The evaporator/ condenser The water bath The electric stove Function and size The Zeolite canister consists of a 140 mm diameter and 250 mm tube with top and bottom covers. Eight heat transfer fins, a circular galvanized sheet and a fine mesh are used. The fins are 5 mm thick, 65 mm wide and 150 mm long. The galvanized sheet and fine wire mesh are used to separate the Zeolite from a 100 mm vapor gap at the top of the canister. Tightly spaced 3 mm mass transfer holes were drilled throughout the circular galvanized sheet to allow mass transfer between the vapor gap and the packed Zeolite. Mass transfer to the lower levels of Zeolite is enhanced through eight mass transfer tubes, each 10 mm in diameter and 167 mm in height. The evaporator condenser consists of an 80mm inner diameter galvanized steel tube that is 200 mm long, with top and bottom covers. The weight of liquid water indicates the change in mass of liquid water inside the evaporator/condenser, and therefore the change in water adsorbed on the Zeolite. The interior volume of the evaporator is 0.88 L and it is submersed in a temperature controlled water bath The water bath for the evaporator/condenser contains two 1.5 kw electrical heaters. By varying the temperature of the water bath, the vapor pressure inside the Zeolite canister is controlled. One thermocouple measures and controls the temperature of the water bath. An electrically heated stove in which the Zeolite canister is placed in such a way as to allow the surrounding air to enhance heat transfer between the oven air and the Zeolite canister, is used One thermocouple measures and control the temperature of the oven. Piping system A piping system connects the Zeolite canister to the evaporator/condenser and links them to the vacuum pump. The piping system consists of galvanized steel pipes with appropriate fitting such as sockets and valves to control the flow of air and water vapor. Note that condensation will occur on any inside surface exposed to the water vapor and at a lower temperature than the saturated water in the evaporator/condenser. Therefore, the whole piping system is maintained at an elevated temperature by properly lagging the pipe with fiber glass held around the pipe with an electrical insulation tape. The vacuum pump The vacuum pump is used to initially evacuate the system before introducing the refrigerant (water) into the system E1 Electric stove E2 Zeolite caister E3 Vaccum pump E4 Feed water E5 Water bath E6 Electric water heater E7 Evaporator/condenser V1-4 Vacuum valves 1-1,1-2 Pressure gauges Figure 1. Schematic of the experimental rig. Fig 1 Schematic of the experimental rig

4 Amber et al. 145 C D E B A F G H A=Electric Stove; B=Zeolite canister; C=Vacuum gauges; D=Vacuum valves; E=Electric heater; F=Evaporator/Condenser; G=Vacuum pump; H=Water bath; I=Digital thermometer (multimeter). I Figure 2. Experimental set-up. Table 2. Adsorption data T ad = 150 C water s saturation pressure at the bath temperature. If the measured and saturation pressure differ by more than 0.3 kpa, the vacuum pump is turned on and valves V 3 and V 2 are opened. Once the measured and saturation pressures are approximately equal, valves V 3 is closed and the vacuum pump is turned off. The mass of the water vapor pumped out of the system is assumed negligible. Finally, valves V 1 and V 2 is slowly cracked allowing water vapor to flow to the Zeolite canister. Valves V 1 and V 2 are closed when the system reaches thermodynamic equilibrium as thus. The Zeolite canister is assumed to reach thermodynamic equilibrium when the thermocouple on the canister is within 5 C of the oven temperature. The evaporator/condenser is assumed to reach thermodynamic equilibrium when the temperature of the vapor and that of the liquid in the evaporator/ condenser agree within 1 C as measured using a thermocouple. A final check for system equilibrium is that the vapor pressure is stable. Adsorption data X (that is, the difference in the weight of water in the evaporator condenser) are obtained at several T ad and P. While holding T ad constant at 150 C, adsorption data are obtained for P, corresponding to water bath temperatures of approximately 5, 10, 20, 30 and 40 C. This process is repeated for oven temperature of 120,100 and 40 C. RESULTS Adsorption data The adsorption data obtained for P, corresponding to water bath temperature of approximately 5, 10, and 40 C while holding T ad constant are tabulated in Tables 2 to 5: Mass of Zeolite (m) = 0.2 kg Mass of empty evaporator (m 1 ) = 2.0 kg Mass of evaporator + water (before adsorption) = m 2 = 2.6 kg Mass of evaporator + water (after adsorption) = m 3 (kg) DISCUSSION Three intensive parameters, water vapour pressure (P), temperature (T), and water adsorption capacity (n) define the Zeolite -water vapour system. The arbitrary choice of two of the three intensive parameters that define the zeolite-water vapour system leads to a series of continuous equilibrium curves that are isobaric (n,t) p, isothermic (n,p) T, or isosteric (P,T) n. The adsorption data for the synthetic Zeolite A, at four adsorption temperatures (T ad ) 40 to 150 C are given in Tables 2 to 5. Within the range of Zeolite temperatures and water vapor pressures studied, the adsorption

5 146 J. Mech. Eng. Res. Table 3. Adsorption data T ad = 120 C Table 4. Adsorption data T ad = 100 C Table 5. Adsorption data T ad = 40 C capacity ranged between 0.2 to 0.25 with the highest obtained being about kg ad /kg w which was obtained at T ad =100 C. At T ad = 150 C, the maximum capacity was 0.2 kg ad /kg w. The general trend observed showed that the adsorption capacity of the Zeolite A increased with increasing water vapor pressure and decreasing Zeolite temperature. The isobaric adsorption isotherm (n,t) p plot of the various adsorption capacities of the synthetic Zeolite against the temperature range 5 to 40 C gives the maximum adsorption capacity of kg ad /kg w at T ev =16 C. The cyclic adsorption capacity swing for different condenser, evaporator and adsorbent temperatures compared with that for Zeolite 13X water shows that the maximum adsorption capacity of natural Zeolite is approximately twice (0.12 kg ad /kg w ) over the same range of zeolite temperatures and water vapor pressures. Subsequently, the same set of Zeolite temperatures and water vapor pressures are revisited in the reverse direction such that desorption occurs between each equilibrium state to check for both repeatability and hysteresis. The adsorption capacities measured for adsorption processes are compared at low Zeolite temperature. Little hysteresis is found in the form if: i. The condensation surface area in the condenser/evaporator is not large enough, at low water vapor pressure the condensation process takes a long time to reach equilibrium conditions. ii. The pressure of non-condensable gases on condensation surface adversely affects the condensation rate. iii. The experiment at any given conditions may have been terminated before equilibrium was reached in the condenser/evaporator. Consequently, the adsorption capacity measured during adsorption/desorption process is in close agreement to the result of Miguel et al. (2003) which reports the maximal adsorption capacity of Zeolite A/water pair at 30 C ambient temperature to be 0.3 kg absorbate per kilogram of adsorbent. Also, the adsorption capacity is a weak function of the water vapor pressure at high Zeolite temperatures, but its dependency on the vapor pressure is seen to be a little stronger than for a natural Zeolite (clinoptilolite).

Conclusion The experimental determination of the adsorption capacity of synthetic Zeolite A water pair has been presented in this paper.

6 Adsorption capacity (kg) Amber et al. 147 Temperature ( C) Figure 3. Adsorption capacity(x) of synthetic Zeolite A vs. temperature ( C). The the performance of zeolite/water pair for adsorption cooling systems based on a number of operating conditions is presented Liu and Leong (2005). Conclusion The experimental determination of the adsorption capacity of synthetic Zeolite A water pair has been presented in this paper. The maximum adsorption capacity of water in the synthetic Zeolite A (X max ) was found to be nearly 26% and is dependent on the water vapor pressure at high Zeolite temperatures. The amount of water adsorbed however increased with increasing water vapor pressure and decreasing Zeolite temperature (Figure 3). REFERENCES Afonso Jr MRA, Silveira V (2005). Characterization of equilibrium conditions of adsorbed silica gel/water bed according to Dubinin Astakhov and Freundlich.Therm. Eng., 4: 3 7. Alghoul MA, Sulaiman MY, Azmi BZ,Sopian K, Abd.wahab M (2007). Review of materials for adsorption refrigeration technology. Anti- Corrosion Meth.Mat., 54: Andreas ES, Warecks G, Derewinski M, Lercher JA (1989). Adsorption of water on ZSM5 Zeolites. Am. Chem. Society. J. Phys. Chem. Arbuznikov A, Vasilyev V, Goursot A (1998). Relationships between the structure of a zeolite and its adsorption properties Surface Science; 397: Cui Q, Tao G, Chen H, Guo X, Yao H (2005). Environmentally benign working pairs for adsorption refrigeration. Energy; 30: El-Sharkawy II, Kuwahara K, Saha BB, Koyama S, Ng KC (2006). Experimental investigation of activated carbon fibers/ethanol pairs for adsorption cooling system application. Appl. Therm. Eng., 26: Fundamentals of Zeolite/water Technology (2007). http// Howell PA (1963). Process for synthetic Zeolite A. united states patents office Grand Island N.Y assignor to union carbide corporation IEA. Ongoing research relevant for solar assisted air conditioning systems.technical report: IEA solar heating and cooling, pp.1-2. Korkuna O, Leboda R, Skubiszewska-zieba J, Vrublevs'ka T, Gunko VM, Ryczkowski J (2006). Structural and physicochemical properties of natural zeolites: clinoptilolite and mordenite. 87: Liu Y, Leong KC (2005). The effect of operating conditions on the performance of zeolite/water adsorption cooling systems. Appl. Therm. Eng., pp Meunier F (2001). Adsorptive cooling: a clean technology. Clean Products Process; 3: Miguel R, Espinoza RL, Manfred JH, Antonio PF (2003). Evaluation of a Zeolite water solar adsorption refrigerator. Ng KC, Chua HT, Chung CY, Loke CH, Kashiwagi T, Akisawa A (2001). Experimental investigation of the silica gel water adsorption isotherm characteristics. Appl. Therm. Eng., 21: Pires J, Brotas De Carvalho M (1997). Water adsorption in aluminium pillared clays and zeolites. J. Mater. Chem., 7: Ramos M, Espinoza RL, Horn MJ, Ferreira leite AP (2003). Evaluation of a zeolite-water solar adsorption refrigerator. In: ISES Solar World Congress, June Göteborg, Sweden. Saha BB, Habib K, El-Sharkawy II, Koyama S (2009). Adsorption characteristics and heat of adsorption measurements of R-134a on activated carbon. Int. J. Refrig. doi: /j.ijrefrig Saham BB, Chakraborty A, Koyama S, Yoon SH, Mochida I, Kumja M (2008). Isotherms and thermodynamics for the adsorption of n-butane on pitch basedactivated carbon. Int. J. Heat. Mass. Transfer., 51: Solmus I, Yamalı C, Kaftanog lu B, Baker D, Çag lara A (2010). Adsorption properties of a natural zeolite water pair for use in adsorption cooling cycles. Appl. Energy., 87: Wang LW, Wang RZ, Lu ZS, Chen CJ, Wang K, Wu JY (2006). The performance of two adsorption ice making test units using activated carbon and a carbon composite as adsorbents. Carbon; 44: Wang LW, Wang RZ, Olieveira RG (2009). A review on adsorption working pairs for refrigeration. Renew Sustain Energy. Rev; 13:

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