BASIC CONCEPTS AND AN OVERVIEW - Foundations of Nanotechnology: Pore Size in Carbon-Based Nano-Adsorbents - page 113

• New polices, operating procedures, decision rules, information flows, orga-
nizational procedures, and soon can be explored without disrupting ongo-
ing operations of the real system.
• New hardware designs, physical layouts, transportation systems, and soon,
can be tested without committing resources for their acquisition.
• Hypotheses about how or why certain phenomena occur can be tested for
feasibility.
• Insight can be obtained about the interaction of variables.
• Insight can be obtained about the importance of variables to the perfor-
mance of the system.
• A simulation study can help in understanding how the system operates rath-
er than how individuals think the system operates.
• “What-if” questions can be answered. This is particularly useful in the de-
sign of new system.
• The two main families of simulation technique are molecular dynamics
(MD) and Monte Carlo (MC); additionally, there is a whole range of hybrid
techniques which combine features from both.
For example, in the recent study works about PSD calculating simulated sam-
ples, the average of numbers adsorbed molecules fluctuates during the simulation
calculating of for a range of chemical potentials enables the adsorption isotherm
to be constructed. The walls of the slit pores lie in the x - y plane. Normal periodic
boundary conditions, together with the minimum image convention, are applied
in these two directions. For low pressures, P/P 0 that smaller than 0.02, the length
of the simulation cell in the two directions parallel to the walls was maintained
at 10 nm for each of the pore widths studied, to maintain a sufficient number of
adsorbed molecules. For higher pressures where more water molecules were pres-
ent, the minimum cell length in the x and y directions was 4 nm. The average num-
ber of water molecules in the simulation cell varied from a few molecules at the
lowest pressures to a few hundred or thousands of molecules when the pores were
full; filled pores contained about 320 molecules for a pore width of 0.79 nm, 460
at 0.99 nm, 830 at 1.69 nm. Calculations were carried out on the Cornell Theory
Center IBM SP 2 .In determining the adsorption isotherm, commenced with the cell
empty; a value of the fugacity corresponding to a low pressure was chosen and
the average adsorption determined from the simulation. The final configuration
generated at each stage was used as the starting point for simulations at higher fu-
gacities. The pressure of the bulk gas corresponding to a given chemical potential
was determined from the ideal gas equation of state. Gas phase densities corre-
sponding to the range of chemical potentials studied were determined by carrying
out simulations of the bulk gas. These were found to agree with those calculated
from the ideal gas equation within the estimated errors of the simulations.