ADSORPTION LOW TEMPERATURE COOLING USING ACTIVATED CARBON / ETHANOL WORKING PAIRS

Transcription

1 SusTEM Special Sessions on Thermal Energy Management ADSORPTION LOW TEMPERATURE COOLING USING ACTIVATED CARBON / ETHANOL WORKING PAIRS A. Elsayed, R.K. AL-Dadah, S. Mahmoud, B. Shi, A. Rezk, K. Rahbar

2 Content o Introduction o Tested Samples o Scanning Electron Microscopy(SEM) of samples o Dynamic vapour Sorption (DVS) Test Facility o Comparison of different samples kinetics and capacity of ethanol uptake o Adsorption kinetics and isotherms prediction o CFD modelling of adsorption process of plate heat exchangers o Conclusions

3 Introduction o Adsorption cooling is considered as attractive heat powered cooling technology suitable for various applications. o Commercially available systems use water/silica gel, water/zeolite and ammonia/ chloride salts working pairs. The waterbasedpairsarelimitedtoworkabove0 Cduetothewater high freezing temperature, while ammonia has the disadvantage of being toxic. o This work experimentally investigates the ethanol adsorption characteristics for a wide range of activated carbon materials using gravimetric analysis method. o Numerical simulation of the adsorption process was carried out to investigate the effect of adsorbent material thickness on the cycle uptake.

4 Introduction Operation concept of adsorption Chiller

5 Tested Samples sample Manufacture Particle diameter [µm] Product form Surface area [m 2 /g] Maxsorb Kansai coke 72 Powder 3000 RX1 Norit 947 Pellet 1450 RX3 Norit 2570 Pellet 1370 HR5 Eurocarb 630 Granular 1050 YAO Eurocarb 700 Granular ATO Eurocarb 650 Granular HDLC Eurocarb 50 powder SRD12004 Chemviron 20 powder SRD12005 Chemviron 40 powder SRD12006 Chemviron 50 powder SRD12009 Chemviron 1000 Granular

6 Scanning Electron Microscopy (SEM) of samples

7 Dynamic vapour Sorption (DVS) Test Facility

8 Drying curves

9 Comparison of different samples kinetics

10 Ethanol Uptake Maximum Capacity (Tads=25 C) 1.4 Uptake [kg/kg ads ] Uptake Stability Time/1000 [min]

11 Prediction of kinetics and Adsorption isotherms o kinetics of these samples was predicted using the linear driving force adsorption model. x / xeq = 1 exp( k t) k Ea = R ln k LDF,1 LDF o The activation energy was calculated from two isotherms using: 1 / T1 T LDF, o The adsorption rate parameter K LDF could be convert to temperature independent adsorption constant K o using: k o = k LDF / exp Ea RT o The adsorption isotherms has been predicted using Dubinin Model x eq = x max A exp E m P A = RT ln P s

12 Kinetics Prediction of different Samples

13 Adsorption Isotherms Maxsorb Eurocarb ATO Eurocarb HDLC Chemviron SRD Uptake [kg/kg] Isotherm of carbon samples at 25 C P/p s [-]

14 CFD Model of Plate Heat Exchanger

15 Governing Equations ( ερ + ρ x) g t b + ( ρ u ) = 0 T ( ρc) + ρ C u T = ( λ T ) eff t g pg g g g eff + ρb H x t Mass Balance Energy Balance ug K = µ g P Momentum Balance (Darcy module) Physical properties o o o o λ eff d 2 3 pε K = ( 1 ε ) b = ( 1 ) ρs ρ ε 2λs + λg 2ε ( λs λg ) = λs 2λ + λ + ε ( λ λ ) ( ρc) = ( ερ g + ρbx )C pg + ( 1 ε ) ρsc ps eff s g s g Permeability Effective density Effective Thermal conductivity Effective Specific heat

16 Model inputs Physical parameter Symbol Value[units] Solid carbon density ρ s 2000 [kg/m 3 ] Packing density ρ b 300 [kg/m 3 ] Solid carbon conductivity λ s 1.7 [W/m.K] Intial Bed temperature T_initial 35 [ C] Evaporating Pressure P evap 3.93,5.232 [mbar] Cooling water flow u water 1[m/s] Heat of adsorption H [kJ/kg] Cooling water inlet Temp T w,in 25 [ C] Metal layer thickness t_metal 0.6 [mm]

17 Calculation of Bed Initial Temperature Pressure [mbar] Preheating 1 4 Adsorption Precooling Desorption [ C] Maxsorb Bed Temperature [ C]

18 Adsorbent Temperature and Uptake Distribution Uptake [kg/kg ads ] Distance from adsorbent Upper Surface [mm] Maxsorb adsorbent Temperature and uptake in adsorbent layer after 160 sec

19 Uptake of initially dry bed with packing thickness

20 Temperature distribution of initially dry bed at various packing thickness

21 Comparing different samples in real chiller operation (initial uptake w min >0.0)

22 Conclusions o Theethanol/activatedcarbonadsorptionpaircanbeusedtoachieve low temperature cooling. Simulation results showed that an evaporator temperature of-15 C can be achieved. o 11 commercially available activated carbon materials were investigated and results showed that Maxsorb outperforms other activated carbon materials both in terms of the equilibrium uptake and cycle performance. o As the adsorbent thickness increases, its temperature at the upper layers will increase leading to lower ethanol uptake. Adsorbent thicknesses below 10mm offer better cooling and higher uptake.

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