Permeability and thermal conductivity of host compressed natural graphite for consolidated activated carbon adsorbent

Abstract

Permeability and thermal conductivity test units were set up to study the heat and mass transfer performance of the host material, i.e. expanded natural graphite (ENG), for consolidated activated carbon (AC) adsorbent. The permeability was tested with nitrogen as the gas source, and the thermal conductivity was studied using steady-state heat source method. The results showed that the values of permeability and thermal conductivity were 10^-15 to 10^-12 m² and 1.7 to 3.2 W/(m·K), respectively, while the density compressed expanded natural graphite (CENG) varied from 100 to 500 kg/m³. The permeability decreased with the increasing density of CENG, whereas the thermal conductivity increased with the increasing density of CENG. Then the thermal conductivity and permeability of granular AC were researched. It was discovered that the thermal conductivity of samples with different grain size almost kept constant at 0.36 W/(m·K) while the density was approximately 600 kg/m³. This means that the thermal conductivity was not related to the grain size of AC. The thermal conductivity of CENG was improved by 5 to 10 times compared with that of granular AC. Such a result showed that CENG was a promising host material for AC to improve the heat transfer performance, while the mass transfer performance should be considered in different conditions for utilization of adsorbent.

Key words： permeability; thermal conductivity; expanded nature graphite; activated carbon

Acknowledgments

This work was supported by the National Natural Science Foundation of China (Grant Nos. 50736004, 50806043), and the National 100 Outstanding PhD Thesis Foundation in China.

Notation
B	Shape factors of the samples
K	Permeability/m2
ma	Gas mass flowrate/(kg·s-1)
p1	Inlet pressure of air/Pa
p2	Outlet pressure of air/Pa
qv	Gas volume flowrate/(L·min-1)
Q	Heat flux/W
R	Gas constant/(J·kg-1·K-1)
S	The effective heating area of the central square plate heater/m2
T	Sample temperature/K
Greek letters
λ	Thermal conductivity/(W·m-1·K-1)
ΔT	Average temperature gradient/K
Δz	Thickness of the samples/m
μ	Gas viscosity/(Pa·s)
ρ	Density/(kg·m-3)
va	Axial velocity/(m·s)

References

1 Mauran S, Lebrun M, Prades P, Moreau M, Spinner B, Drapier C. Active composite and its use as reaction medium. <patent>US Patent 5283219</patent>, 1994

2 Mauran S, Coudevylle O, Lu H B. Optimization of porous reactive media for solid sorption heat pumps. In: Proceedings of the International Sorption Heat Pump Conference, Montreal, Canada, 1996, 3–8

3 Klein H-P, Groll M. Heat transfer characteristics of expanded graphite matrices in metal hydride beds. International Journal of Hydrogen Energy, 2004, 29(14): 1503–1511

4 Menard D, Py X, Mazet N. Activated carbon monolith of high thermal conductivity for adsorption processes improvement, Part A: Adsorption step. Chemical Engineering and Processing, 2005, 44(9): 1029–1038

5 Ahmet S, Ali K. Thermal conductivity and latent heat thermal energy storage characteristics of paraffin/expanded graphite composite as phase change material. Applied Thermal Engineering, 2007, 27(8,9): 1271–1277

6 Han J H, Lee K H. Gas permeability of expanded graphite-metallic salt composite. Applied Thermal Engineering, 2001, 21(4): 453–463

7 Biloe S, Mauran S. Gas flow through highly porous graphite matrices. Carbon, 2003, 41(3): 525–537

8 Lisker I S, Solovyev S V, Axcell B P, Varlow B R, Donnelly K.A transient technique for measuring the thermal conductivity of non-metals. Experimental Thermal and Fluid Science, 2001, 25(6): 377–382

9 Hirata K F Y, Fujioka K, Fujiki S. Preparation of fine particles of calcium chloride with expanded graphite for enhancement of the driving reaction for chemical heat pumps. Journal of Chemical Engineering of Japan, 2003, 36(7): 827–832

10 Solórzano E, Reglero J A, Rodríguez-Pérez M A, Lehmhus D, Wichmann M, de Saja J A.An experimental study on the thermal conductivity of aluminium foams by using the transient plane source method. International Journal of Heat and Mass Transfer, 2008, 51(25,26): 6259–6267

11 Wang K, Wu J Y, Wang R, Wang L. Effective thermal conductivity of expanded graphite–CaCl₂ composite adsorbent for chemical adsorption chillers. Energy Conversion and Management, 2006, 47(13,14): 1902–1912

12 Wang L W, Tamainot-Telto Z, Metcalf S J, Critoph R E, Wang R Z. Anisotropic thermal conductivity and permeability of compacted expanded natural graphite. Applied Thermal Engineering, 2010, 30(13): 1805–1811

13 Tamainot-Telto Z, Critoph R E. Monolithic carbon for sorption refrigeration and heat pump applications. Applied Thermal Engineering, 2001, 21(1): 37–52