Page 336 - Handbook Of Multiphase Flow Assurance

Computer modeling of interaction between a hydrate surface and an inhibitor 335

Simulations for methane adsorption on hydrate surface indicate that methane condenses
at a temperature of 100 K. Vapor pressure calculation was performed for methane using the
empirical equation obtained from the reference (Reid et al., 1987). Vapor pressure of 5 atm is
exerted at 90.1 K which is close to the value of 100 K obtained in the simulation. However, the
freezing temperature of methane is 90.7 K which means that instead of condensing, methane
freezes at this temperature. The model closely reflects the methane phase transition at 100 K
as compared to the empirical value of ca. 90 K.
Simulation of methane adsorption
Simulations were performed for a range of temperatures from 100 K to 400 K in 50 K incre-
ments. Length of simulations included 50,000 equilibration cycles and 50,000 run cycles. Each
simulation was repeated 4–8 times, and the results were averaged.
It was found that the adsorption of methane in hydrate cavities had changed with the
presence of a polymer on the hydrate surface (Fig. 10.86). Different polymers had different
effects on the methane adsorption. The largest decrease in methane adsorption was caused by
PVCap, followed by PVP and PVA.

1.00
no polymer
pvcap
Fraction of simulation CH4 spent in cavities
0.80 pvp
pva

0.60

0.40

0.20

0.00

100.00 200.00 300.00 400.00
Temperature, K

FIG. 10.86 Adsorption of methane on bare sII {111} hydrate surface and with octamers of PVCap, PVP and PVA.