A. Elsayed, R.K. AL-Dadah, S. Mahmoud,

B. Shi, A. Rezk, K. Rahbar

ADSORPTION LOW TEMPERATURE COOLING USING ACTIVATED CARBON / ETHANOL

WORKING PAIRS

SusTEM Special Sessions on

Thermal Energy Management

o Introduction

o Tested Samples

Content

o Comparison of different samples kinetics and capacity of ethanol uptake

o CFD modelling of adsorption process of plate heat exchangers

o Adsorption kinetics and isotherms prediction

o Scanning Electron Microscopy (SEM) of samples

o Dynamic vapour Sorption (DVS) Test Facility

o Conclusions

o Adsorption cooling is considered as attractive heat poweredcooling technology suitable for various applications.

o Commercially available systems use water/silica gel,water/zeolite and ammonia/ chloride salts working pairs. Thewater based pairs are limited to work above 0 °°°°C due to the waterhigh freezing temperature, while ammonia has the disadvantageof being toxic.

Introduction

o This work experimentally investigates the ethanol adsorptioncharacteristics for a wide range of activated carbon materialsusing gravimetric analysis method.

o Numerical simulation of the adsorption process was carried outto investigate the effect of adsorbent material thickness on thecycle uptake.

Introduction

Operation concept of adsorption Chiller

Tested Samples

sample Manufacture Particle

diameter

[µµµµm]

Product form Surface area

[m2/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

Scanning Electron Microscopy (SEM) of samples

Dynamic vapour Sorption (DVS) Test Facility

Drying curves

Comparison of different samples kinetics

Ethanol Uptake Maximum Capacity (Tads=25°°°°C)

0

0.2

0.4

0.6

0.8

1

1.2

1.4

Up

tak

e [

kg

/kg

ad

s]

Uptake

Stability Time/1000 [min]

Prediction of kinetics and Adsorption isotherms

o kinetics of these samples was predicted using the linear driving force adsorption model.

)exp(1/ tkxeqx LDF−−=

−

=

211,

2, 11/ln

TTk

kREa

LDF

LDF

o The activation energy was calculated from two isotherms using:

o The adsorption rate parameter KLDF could be convert to temperature independent adsorption constant Ko using:

−=

RT

Eakk LDFo exp/

−=

m

eqE

Axx expmax

−=

sP

PlnRTA

o The adsorption isotherms has been predicted using Dubinin Model

Kinetics Prediction of different Samples

Adsorption Isotherms

0

0.2

0.4

0.6

0.8

1

1.2

1.4

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Up

tak

e [k

g/k

g]

P/ps [-]

Maxsorb Eurocarb ATO

Eurocarb HDLC Chemviron SRD 12005

Isotherm of carbon samples at 25 °C

CFD Model of Plate Heat Exchanger

o

o

o

o

Governing Equations

( ) ( ) 0=•∇+∂

+∂gg

bgu

t

xρ

ρερ

( ) ( )t

xHTTuC

t

TC beffgpggeff ∂

∂∆+∇⋅∇=∇⋅+

∂

∂ρλρρ

Mass Balance

Momentum Balance (Darcy module)

Energy Balance

PK

ug

g ∇−=µ

2

32

1150 )(

dK

p

ε

ε

−=

( ) psspgbgeff C)(C)x(C ρερερρ −++= 1

sb )( ρερ −= 1

)(2

)(22

gsgs

gsgs

seffλλελλ

λλελλλλ

−++

−−+=

Physical properties

Permeability

Effective density

Effective Thermal conductivity

Effective Specific heat

Model inputs

Physical parameter Symbol Value[units]

Solid carbon density ρs 2000 [kg/m3]

Packing density ρb 300 [kg/m3]

Solid carbon conductivity λs 1.7 [W/m.K]

Intial Bed temperature T_initial 35 [°C]

Evaporating Pressure Pevap 3.93,5.232 [mbar]

Cooling water flow uwater 1[m/s]

Heat of adsorption 1053.89[kJ/kg]

Cooling water inlet Temp Tw,in 25 [°C]

Metal layer thickness t_metal 0.6 [mm]

H∆

Calculation of Bed Initial Temperature

0.01

0.1

1

10

100

1000

-15 0 15 30 45 60 75 90

Pre

ssu

re [

mb

ar]

Bed Temperature [°°°°C]

0.01

0.10.2

0.40.60.8

1

Precooling

Adsorption

Desorption

Preheating

Maxsorb

1

2 3

4

35[°C]

Adsorbent Temperature and Uptake Distribution

0.015

0.02

0.025

0.03

0.035

0.04

0.045

0 5 10 15 20U

pta

ke

[kg

/kg

ad

s]Distance from adsorbent Upper Surface [mm]

Maxsorb adsorbent Temperature and uptake in adsorbent layer after 160 sec

Uptake of initially dry bed with packing thickness

Temperature distribution of initially dry bed at various packing thickness

Comparing different samples in real chiller operation (initial uptake wmin>0.0)

Conclusions

o The ethanol / activated carbon adsorption pair can be used to achievelow temperature cooling. Simulation results showed that anevaporator temperature of -15 °°°°C can be achieved.

o 11 commercially available activated carbon materials wereinvestigated and results showed that Maxsorb outperforms otheractivated carbon materials both in terms of the equilibrium uptakeand cycle performance.

o As the adsorbent thickness increases, its temperature at the upperlayers will increase leading to lower ethanol uptake. Adsorbentthicknesses below 10mm offer better cooling and higher uptake.