Page 117 - Handbook Of Multiphase Flow Assurance

Hydrate of natural gas 113

‐ less soluble in liquid hydrocarbons than the other glycols.
‐ freezing temperature of its water solution is lower than for the other glycols.
Thermodynamic inhibitor chemicals including methanol, glycols, salts inhibit hydrates
above and below 0 °C for any length of time. However, these chemicals are required in large
quantities, methanol is toxic, and glycols need to be recovered from produced fluids stream
to be economic. Kinetic inhibitors are not toxic and can work at any water cut (up to 100%).
However, these work only during production, for a limited time, with a limited subcooling
or overpressurization, and accelerate hydrates growth when they eventually stop working.
Properties of glycol mixtures with water are important for a preliminary conceptual de-
sign of hydrate inhibitor delivery systems. One set of such data set (Union Carbide, 1978),
updated for brevity, for density and viscosity may be used to check software correlations'
predicted values. However, the best, most cost-effective and most reliable validation of a
chemical property such as density or viscosity is by a laboratory measurement. Laboratory
measured values should be used for a detailed project design.
• Low-dosage inhibitors
Low dosage inhibitors typically require 0.5–3 vol% dosage in water which is much lower
than that for thermodynamic inhibitor chemicals. Cost of treatment though remains the same
due to higher cost of specialty components used to formulate the LDHI. Operator may save
cost on logistics of chemical delivery as less chemical volume needs to be transported. While
it is not uncommon to see a separate pipeline designed to deliver MEG from shore to the
production platforms, LDHI is nearly always delivered in chemical tanks also called totes to
the platform.
Low dosage inhibitors fall in two sub-categories: kinetic and anti-agglomerant types.
Kinetic inhibitors work by adsorbing on the critical size nuclei of hydrate as shown in Fig. 5.14
and preventing their further growth, as well as by blocking methane and other gas molecules
which are like building blocks from reaching the growing hydrate surface. Altering surface
energy of hydrate nucleus and steric blockage of diffusion of guest molecules are the mecha-
nisms of KHI. Eventually the chemical dissolved in water runs out and the crystals proceed
to form. Effectively KHI chemicals delay the growth of hydrate solids.
Kinetic hydrate inhibitors or KHIs provide a delay on the order of hours to tens of hours, de-
pending on pressure. The further into the hydrate stability region on a pressure-temperature
hydrate stability diagram is the operating point, the shorter is the KHI protection time.
There are two ways to measure how far is the operating point from the hydrate stabil-
ity curve: overpressurization or the difference between the operating pressure and hydrate

FIG. 5.14 KHI polyvinylpyrrolidone (PVP) polymers adsorb on hydrate nuclei and disrupt crystal growth for
some time.