Effect of Thermodynamic Inhibitors on Hydrate Formation

Several means of hydrate prevention and dissociation are discussed in detail in Chapter 8. In the present section we consider the lowering of the three-phase (Lw-H-V) temperature or the increase of the Lw-H-V pressure via an inhibitor. In this section we consider only thermodynamic inhibitors such as alcohols, glycols, or salts. For kinetic inhibition using LDHIs, such as KIs or A As, the reader is referred to Chapter 8. 

By Gibbs Phase Rule illustrated in this chapter s introduction, a second intensive variable is needed (in addition to either temperature or pressure) to specify the three-phase binary system with an inhibitor (F = 3 — 3 + 2). Typically, the concentration of the inhibitor in the free water phase is specified as the second intensive variable. Substances that have considerable solubility in the aqueous phase, such as alcohols, glycols, and salts, normally act as inhibitors to hydrate formation. The colligative mechanism of formation inhibition is aided by increased competition for water molecules by the dissolved inhibitor molecule or ion through hydrogen bonding for alcohols or glycols, or via Coulombic forces (for salt ions). 

2 The translation of Makogon s work indicates hydrocarbon vapor, but the condensed hydrocarbon phase is clearly required by the pressure and temperature conditions. 

Several types of inhibitors have been tried, but the glycols and alcohols have proved to be the most successful. As an example, ammonia was initially determined to be twice as effective an inhibitor as methanol however, over long periods, ammonia reacts with carbon dioxide and water to form ammonium carbonate and 

The solid ammonium carbonate and carbamate are much more difficult to remove than the hydrates (Townsend and Reid, 1978, p. 100). The natural gas industry has opted for methanol and glycols, which may be injected into pipelines and processes without undesirable side reactions. 

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