Membrane and Desalination Technologies - TCE Moodle Website

Adsorption Desalination: A Novel Method 393
Table 9.1
Energy costs comparison for various methods of desalination
Method of
desalination
Multistage flash
(MSF)
Multieffect
distillation (MED)
Vapour compression
(VC)
Reverse osmosis
(RO) – single pass
Reverse osmosis
(RO) – double pass
Adsorption
desalination (highgrade
water)
Thermal energy
consumed
kWh/m 3 (A)
1.1. Description of Sorption Processes
The rubric “sorption” represents both adsorption and absorption processes. Adsorption
(solid–vapor) phenomenon concerns the separation of a substance (adsorbate) from one
phase, accompanied by its accumulation or concentration of another (adsorbent). The adsorbing
substance is the adsorbent and the material adsorbed on that substance is the adsorbate.
On the contrary, absorption process is a process in which material transferred from one phase
to another (e.g., liquid) interpenetrates the second phase to form a solution (4).
Adsorption processes can be classified into physical adsorption (physisorption) or chemical
adsorption (chemisorption). Physical adsorption involves only relatively weak intermolecular
forces which are caused by Van der Waals forces, while chemical adsorption involves
essentially the formation of a chemical bond between the sorbate molecule and the surface of
adsorbent (5, 6). It should be mentioned that the adsorptive actions is physical for almost all
of solid adsorbents that are commonly used in adsorption cooling systems.
1.2. Adsorption Equilibrium
Electric energy
kWh/m 3
consumed (B)
Primary fuel input
kWh/m 3
C =(A/ b + B/ c)
19.4 5.2 37.9 0.647
16.4 3.8 30.5 0.520
– 11.1 29.2 0.497
– 8.2 21.5 0.366
– 9.0 23.7 0.403
Energy from
waste heat
5.63 a
Author’s data
(3)
14.8 0.25
Energy cost of
water US$/m 3 =
5(C 3.6)/1,055
All data are extracted from Seawater Desalination in California, California coastal commission Chapter 1:
Energy Use section, http://www.coastal.ca.gov/index.html. The conversion units of 1 AF = 1,345 m 3 , one
million BTU (1.055 GJ) of Natural gas costs US$5 (adopted from Singapore’s natural gas prices in 2005). The
electricity conversion efficiency, c, of power plants is 38% and the efficiency of boiler is 80%.
a The authors believe that the parasitic pumping energy could be further reduced (about 1.2 kWh/m 3 ) in a fullscale
plant where the pumping operation is much more efficient.
When an adsorbent is in direct contact with a surrounding fluid (usually vapor), adsorption
phenomenon takes place and after a sufficient long time, both the adsorbent and the