Question

In H2O, the 31P NMR spectrum of the compound gives 2 lines shifted from each other by about 1ppm, corresponding to the hydrate and ketone in the ratio of 60:40.

You want to measure the rates of interconversion between them in the range of 0.1-3.0 per second using a cross relaxation method. Both the hydrate and ketone have T1's of 5.0 second and linewidths, delta v1/2 of 2 hz. The experiments are performed in a field such that the 31P resonant frequency is 125.000 Mhz.

a.) How will the system respond to a non selective inversion recovery sequence?

b.) How can you generate a state in which the hydrate line has been inverted and the ketone line has been de-excited using 2 non-selective pulses similar to those used for jump-return water supression?

c.) How will the system recover if you impose the conditions of b. on the system at t=0.

d.) Describe how you will measure the chemical interconversion rate from the data in c.

Answer 1

The most accurate predictions of hydrate formation conditions are made using commercial phase equilibria computer programs. These programs are of two types:

• Those which enable the prediction of the pressure and temperature at which hydrates begin to form (incipient hydrate formation programs)
• Those which predict all phases and amounts at higher pressures and lower temperatures than the incipient hydrate formation point (flash programs, or Gibbs energy minimization programs)

Of these two program types, the flash/Gibbs type is gaining pre-eminence because its predictions are available in the phase diagram interior (where many systems operate), whereas the incipient type provides the pressure/temperature (P/T) points of hydrate initiation. State-of-the-art programs are transitioning to the flash/Gibbs free-energy type.

Not all hydrate conditions are calculable by hand. The sections below give hydrate formation hand calculations along the three-phase (L_W-H-V) system and for three-phase (L_W-H-V) hydrate formation on wet gas expansion, as through a valve.

The other three-phase regions (e.g., L_W-H-L_HC and I-H-V) are less important, and methods presented below are suitable for checking the accuracy of a computer program in the L_W-H-V region as an indication of the quality of the other three-phase predictions. Four-phase (L_W-H-V-L_HC) hand calculation methods are not available, and one generally must rely on computer methods for this most common flow assurance hydrate concern.