Page 279 - Handbook Of Multiphase Flow Assurance

278 10. Research methods in flow assurance

ring should enhance the inhibiting properties of the polymer. The name PVCA in the title of
this paragraph stands for poly(vinyl cyanuric acid) because the structure of the proposed
monomer is close to the structure of cyanuric acid. Structure of the proposed monomer was
shown in Fig. 10.36.
For the comparability of results, the same weight concentration of polymer of 0.7% was
used. A single monomer of PVCA was solvated in 1273 water molecules to produce the
desired concentration. Atoms in the monomer had the charges computed by the Pullman
method and SPC water molecules were used. In this simulation the dielectric function was
scaled by a factor of 1.535. Simulation temperature was set to 203 K (equivalent to 277 K) and
the potential energy of the system was minimized. The simulation was run in an NVT ensem-
ble for 100,000 fs.
The number of hydrogen bonded rings oscillated in a fashion similar to the pure water. The
values of hydrogen bonded ring distributions averaged over the last ten timesteps of the run
were given in Table 10.11. The total number of hydrogen bonded polygons in PVCA solution
decreased by 36.2% compared to pure water at the same conditions. The number of hydrogen
bonds was oscillating about the average value of 86.7% bonding within ±3%.
A shell of irregularly hydrogen bonded water molecules was found around the PVCA
monomer, similarly to PVP and PVCap. Four water molecules were hydrogen bonded to the
three carbonyl oxygens of the inhibitor. PVCA is a rather strong local hydrogen bond maker,
since it has three times as many active carbonyl groups as other inhibitors. On an overall ba-
sis, a slight decrease in the number of hydrogen bonds was noticed.

Overview of inhibitor simulation
A novel method of analyzing the structure of water was used in this research. The perfor-
mance of simulated hydrate inhibitors was similar to performance in physical experiments,
thus showing the validity of computer simulation as a tool for designing new chemicals.
The following suggestions can be made based on this study. A common feature shared by
all these polymers is a presence of a lactam ring in side groups. The group common to all
lactam rings is the >CO (carbonyl) group. The high electronegativity of the carbonyl oxy-
gen may affect the surrounding water molecules and disrupt the hydrogen bonded network
between them.
In order to increase the inhibiting effect of the polymer, the number of carbonyl groups can
be increased. The nitrogen atom present in the lactam ring next to the carbonyl group plays an
important role of enhancing the charge on oxygen. If nitrogen in the lactam ring is replaced by
CH, the negative charge on the carbonyl oxygen calculated in SYBYL® increases from −0.388
to −0.349, an 11% change. If a second nitrogen atom is introduced into the lactam ring next
to the carbonyl group, the negative charge on oxygen decreases to −0.418. These calculations
were made using SYBYL® by building the polymer structure and letting the program assign
charges to the atoms using Pullman method (Berthod and Pullman, 1965).
Irregularly hydrogen bonded water molecules were found to form a solvation shell around
the inhibitor molecules. Some of the water molecules were hydrogen bonded to the carbonyl
oxygen. Three water molecules were found hydrogen bonded to the oxygen of PVP in the fi-
nal timestep of the simulation. One water molecule was hydrogen bonded to PVCap oxygen.
Four water molecules were hydrogen bonded to PVCA having three carbonyl groups on the
ring. The diameter of the solvation shell around the inhibitor is the van der Waals diameter of
the polymer groups plus van der Waals diameter of hydrogen in water. No apparent structure
was noticed in these solvation shells.