B41oa oil and Gas Processing Section a flow Assurance Heriot-Watt University

1.6.2 Low-Dosage Inhibitors 
Due to the various problems of thermodynamic inhibitors already discussed, 
the oil and gas industries have dedicated much effort (and large sums of 
money) over the last 25 years into identifying new inhibitors that can be used 
in concentrations below 1 wt.%. 
This type of inhibitor is termed “low dosage” and it provides a very much more 
viable alternative, in terms of cost and environmental issues, to the 
thermodynamic inhibitors. 
Such low-dosage inhibitors will obviously have limited dilution effect on the 
water phase making them rather unsuitable as thermodynamic inhibitors. Their 
use is however not intended to affect the equilibrium conditions, but rather 
reduce the rate of formation of hydrate (kinetic inhibitors), or to prevent 
individual hydrate crystals from sticking together (anti-agglomerants). 
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They operate either by delaying the formation of critical nuclei 
(prolonging the induction time), or by retarding crystal growth, possibly 
by interfering with the preferred direction of crystal growth. 
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Low dosage inhibitors would be effective in the field if the formation 
and growth of hydrates could be precluded, for periods of time that 
exceed the residence time of the fluids in the pipeline. 
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Therefore, one of the main objectives of low dosage inhibitors is to 
extend the period prior to catastrophic growth, in order to surpass the 
free water residence time in a pipeline. 
Low dosage inhibitors can also be sub-divided into two further groups: polymer 
inhibitors and small molecule inhibitors. The polymer type has pendant groups, 
such as a lactam ring, containing an amide group (NCO) at the top of each 
ring adjacent to the polymer backbone. They are generally water-soluble. 

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TOPIC 1: Gas Hydrates 
 
 
 
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The first polymer inhibitor was polyvinylpyrrolidone (PVP). The repeating unit 
for PVP is shown in Figure 17. There are wide ranges of different molecular 
weight PVP molecules, with the molecular weight 360,000 being the most 
effective towards hydrate inhibition. 
The PVP polymer systems are thought of as the ‘first generation’ of kinetic 
inhibitors – the monomer unit for PVP is shown below: 
Further development of these polymer inhibitors involved the synthesis of 
molecules with different types and/or combinations of lactam rings. VC-713, a 
terpolymer, possesses both five- and seven-membered lactam rings, see 
Figure 18. 
Figure 17 
Figure 18 

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TOPIC 1: Gas Hydrates 
 
 
 
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These are representative of the second generation of kinetic inhibitors. Patents 
for various kinetic inhibitors have been awarded with increasing frequency 
each year. The first patent granted was to Shell for the use of alkylarylsulfonic 
acid and its salts. Amongst others awarded included tyrosine derivatives and 
different surfactants. A good summary of low dosage inhibitors can be found in 
Kelland (2006).