Thermodynamic inhibitors methanol

Thermodynamic inhibitors Antinucleants Growth modifiers Slurry additives Anti-agglomerates Methanol or glycol modify stability range of hydrates. Prevent nucleation of hydrate crystals. Control the growth of hydrate crystals. Limit the droplet size available for hydrate formation. Dispersants that remove hydrates. 

The usual practice for avoiding the plugging of production facilities by hydrates is to add thermodynamic inhibitors, such as methanol or glycol. A newer concept is the injection of low-dosage additives either kinetic inhibitors, which delay nucleation or prevent the growth of hydrate crystals, or hydrate dispersants, which prevent the agglomeration of hydrate particles and allow them to be transported within the flow [880,1387]. Hydrate control is discussed extensively in Chapter 13. Classes of hydrate control agents are shown in Table 11-9, and additives are shown in Table 11-10. 

The inhibition of three-phase hydrate formation is discussed in Section 4.4. These predictions enable answers to such questions as, How much methanol (or other inhibitor) is required in the free water phase to prevent hydrates at the pressures and temperatures of operation Classical empirical techniques such as that of Hammerschmidt (1934) are suitable for hand calculation and provide a qualitative understanding of inhibitor effects. It should be noted that only thermodynamic inhibitors are considered here. The new low-dosage hydrate inhibitors [LDHIs, such as kinetic inhibitors (KIs) or antiagglomerants (AAs)] do not significantly affect the thermodynamics but the kinetics of hydrate formation LDHIs are considered in Chapter 8. 

The pressures and temperatures of the Lw-H-V and the I-H-V lines mark the limits to hydrate formation. At higher temperatures or lower pressures of both lines, hydrate cannot form and the system will contain only aqueous and hydrocarbon fluid phases, while hydrate formation can occur to the left of Lw-H-V and I-H-V. Since ice and hydrates both cause flow problems, a gas pipeline rule of thumb is to keep the system temperature above the ice point and to the right of the Lw-H-V and the I-H-V lines, or to displace the Lw-H-V line below the pipeline operating conditions by injection of a thermodynamic inhibitor such as methanol. 

In Figure 4.2d, the presence of a thermodynamic inhibitor (e.g., methanol) shifts the upper two-thirds of the line Q1Q2B to the left, approximately parallel (on a semilogarithmic plot of In P versus T) to the uninhibited line. With inhibitor, however, the transition temperature from water to ice (Qi) is decreased, so that the inhibited Lw-H-V line intersects the I-H-V at a lower point (labeled Qj for 10 wt% methanol and Qj for 20 wt% methanol). The inhibited three parallel lines represent Lw-H-V or Lw-H-Lhc equilibrium at methanol concentrations (marked 0%, 10%, and 20% MeOH) in the free water phase. 

The methods have yet to be extended to common thermodynamic inhibitors such as methanol or monoethylene glycol. In principal the extension is not a function of the ab initio methods since the thermodynamic inhibitors affect the water activity yet this extension has not been quantified. 

While the past methods of preventing hydrate plugs have been to use avoidance with thermodynamic inhibitors such as methanol or glycols, our new understanding of how plugs form, allows us to propose economic risk management (kinetics) to avoid hydrate formation. These concepts differ in type for oil-dominated and gas-dominated systems. 

Note that regular methanol (or monoethylene glycol) injection is used only with gas-dominated systems. In oil-dominated systems the higher liquid heat capacity allows the system to retain reservoir heat, so that insulation maintains sufficient temperatures to prevent hydrate formation. Thermodynamic inhibitor is normally only injected for planned shutdowns in oil-dominated systems. 

While accurate thermodynamic predictions enable avoidance via use of thermodynamic inhibitors such as methanol or glycol, hydrates risk management is enabled by experience in the form of experiments, both in the field and in the laboratory. This is because, as indicated in Chapter 3, there is no comprehensive, predictive hydrate kinetic theory that can be accurately invoked at high hydrate... 

Quantification of the effect of thermodynamic inhibitors (such as methanol, glycol, or salts) on the operation of processes. This determines the amount of inhibitor to be inserted into the free water phase to prevent hydrate formation from vapor and free water. 

Over the last decade or so, many research efforts have been focused on developing what are termed low-dosage hydrate inhibitors , or LDHIs, that potentially can kinetically inhibit hydrate formation/ LDHIs operate via a much different mechanism than thermodynamic inhibitors such as methanol. They are often effective at concentrations as low as 0.5 wt% and act by delaying the onset of hydrate formation, while thermodynamic inhibitors are effective only at much higher concentrations and act by changing the conditions of hydrate thermodynamic stability, thus shifting the phase diagram. 

Of the above-mentioned techniques, thermodynamic inhibitors, which include alcohols, salts, and glycols, are by far the most prevalent. For example, adding methanol to a natural gas will shift the equilibrium conditions so that a higher pressure is required to form hydrates, at a given temperature, as illustrated for methane in Fig. 3. Methods for estimating the saturation water content of natural gases and amounts of methanol or glycol required to suppress hydrate formation are discussed by Katz, Sloan,and Campbell. Current practice for the estimations is to use computer software based on phase equilibrium calculations. ... 

Thermodynamic inhibitors are chemicals such as methanol or ethylene glycol (ethane-l,2-diol). These materials shift the hydrate formation phase boundary away from fhe femperafure and pressure conditions of fhe gas-fransportation process by increasing the driving force required for fhe formation. Thermodynamic inhibitors are used at very high concentrations, as much as 60 % methanol can be used in deep water extractions, which proves to be a very costly way of preventing the build-up of hydrates. 

While accurate thermodynamic predictions (as in Chapters 4 and 5) enable avoidance via use of inhibitors such as methanol, risk management is enabled by operating experience and by kinetic predictions. Hydrate thermodynamic predictions can provide avoidance techniques, but kinetic predictions are required to provide techniques of risk management. 

By now an extensive experience has been accumulated in using of methanol in gas industry. The big volume of field and laboratory experiments together with theoretical works on thermodynamics of hydrocarbon systems have allowed to elucidate the influence of an inhibitor on the natural gas-water system [61, GIGS]. 

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