For hydrate inhibition

As a first approximation, the temperature depression for hydrate inhibition might be considered to be similar to the depression of the freezing point of ice by an equivalent mass fraction of the inhibitor. However, Nielsen and Bucklin (1983) derived an equation indicating that the hydrate depression temperature will always be less than the ice depression temperature by a factor equal to [(heat of fusion of ice)/(heat of hydrate dissociation)], which has a numerical value between 0.6 and 0.7 as a function of the hydrate structure. This is illustrated in Figure 4.2d, by the fact that at constant pressure, the ice depression temperature (i.e., distance between... 

Techniques for hydrate inhibition deal with the methanol concentration in the aqueous liquid in equilibrium with hydrate at a given temperature and pressure. The user also must determine the amount of methanol to be injected in the vapor. This problem was addressed first by Jacoby (1953) and then by Nielsen and Bucklin (1983), who presented a revised methanol injection calculation. The most recent data are by Ng and Chen (1995) for distribution of methanol in three phases (1) the vapor phase, (2) the aqueous phase, and (3) the liquid hydrocarbon phase. 

Comparison of Two Simple Prediction Methods for Hydrate Inhibition by Methanol... 

In addition to line burial and the addition of heat at the wellhead, insulation of exposed areas near the wellhead maintained higher pipeline temperatures, thereby reducing the amount of methanol needed for hydrate inhibition. Figure 8.6 displays the temperature increase in the buried and heated pipeline when exposed pipes were insulated. A combination of the methods causes the pipeline fluid to be outside the hydrate formation region (to the right of the curve marked 0 wt% MeOH), and methanol addition is no longer needed. 

Figure 1 Hydrate safety margin determined by the developed hydrate monitoring teehnique eould be used as traffic lights for hydrate inhibition.

For temperatures lower than about -40 F, glycol injection is impractical because of the high viscosity of glycol solutions at such low temperatures. Methanol s low viscosity and other favorable characteristics make it the fluid of choice for hydrate inhibition in very low... 

Kinetic inhibitors for hydrate formation may also be effective in preventing scale deposition [1627]. This may be understood in terms of stereospecific and nonspecific mechanisms of scale inhibition. 

Other compounds that have been tested for their inhibiting action include carboxylic acids, hydrazine hydrate, chelating compounds, aniline and related compounds and petroleum-based compounds. The influence of various chelating agents such as TEA, EDTA, DPTA, HEDTA and Chel-138 on their ability to control corrosion has been investigated [70]. All the chemicals are reported to reduce the compressive strength of concrete. The strengths were particularly low in the presence of TEA and EDTA, compared to the reference. In the presence of 0.1 N NaCl solution at pH 10... 

Stoichiometry (28) is followed under neutral or in alkaline aqueous conditions and (29) in concentrated mineral acids. In acid solution reaction (28) is powerfully inhibited and in the absence of general acids or bases the rate of hydrolysis is a function of pH. At pH >5.0 the reaction is first-order in OH but below this value there is a region where the rate of hydrolysis is largely independent of pH followed by a region where the rate falls as [H30+] increases. The kinetic data at various temperatures both with pure water and buffer solutions, the solvent isotope effect and the rate increase of the 4-chloro derivative ( 2-fold) are compatible with the interpretation of the hydrolysis in terms of two mechanisms. These are a dominant bimolecular reaction between hydroxide ion and acyl cyanide at pH >5.0 and a dominant water reaction at lower pH, the latter susceptible to general base catalysis and inhibition by acids. The data at pH <5.0 can be rationalised by a carbonyl addition intermediate and are compatible with a two-step, but not one-step, cyclic mechanism for hydration. Benzoyl cyanide is more reactive towards water than benzoyl fluoride, but less reactive than benzoyl chloride and anhydride, an unexpected result since HCN has a smaller dissociation constant than HF or RC02H. There are no grounds, however, to suspect that an ionisation mechanism is involved. 

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. 

The comparisons are broken into two categories for hydrate formation temperatures and pressures (1) uninhibited systems, and (2) thermodynamically inhibited systems. These comparisons show the accuracy that may be expected from readily available hydrate prediction programs. A detailed comparison of the accuracy of these programs is given by Ballard and Sloan (2004b). 

FIGURE 5.10 Hydrate formation T error (absolute) for all inhibited hydrates. 

Frequently, hydrates become important in natural gas storage in salt caverns for peak shaving, or seasonal or diurnal volume averaging delivery of gases. The work by deRoo et al. (1983) discusses this process, regarding hydrate formation in high salt concentration, with their data provided in Chapter 6 on methane hydrate inhibited by sodium chloride. 

Solvents account for 8% of methanol demand and include process uses such as extracting, washing, drying and crystallizing. Miscellaneous uses of methanol include the production of glycol methyl ethers, methyl acrylate and methyl acetate. Other uses include antifreeze, gasoline deicer, windshield washer fluid and hydrate inhibition in natural gas. 

Contemporary clinical medicine pays careful attention to the hydration state of the extracellular space, but not enough to cellular hydration probably because of the lack of routinely applicable techniques for the assessment of cell volume in patients. However, it should be kept in mind that cell hydration is determined primarily by the activity of ion and substrate transporting systems in the plasma membrane, and, to a minor extent, by the hydration state of the extracellular space. The role of cell hydration in regulating protein turnover is an important one, partly because it has a direct bearing on the problem of the pathogenesis of protein-catabolic states in the severely ill. As emphasized earlier, a decrease in cell hydration inhibits protein... 

Gas hydrate inhibitors. Gas hydrates, solid water clathrates containing small hydrocarbons, are problematic for oil and gas production because they can precipitate and cause line blockage. Simple cationic surfactants containing at least two butyl groups were previously developed to inhibit formation of gas hydrate precipitates in gas production lines [87]. However, similar to the situation with cationic drag reduction additives, poor toxicity profiles prevent widespread commercial acceptance. Ester quaternaries with structures somewhat similar to those used in fabric care have been claimed as hydrate inhibitors [88 ]. Additionally, certain alkylether quaternary compounds, e.g. C12-C14 alkyl polyethoxy oxypropyl tributyl ammonium bromide, were shown to have hydrate inhibition properties [89]. 

The most widely used synthetic and natural enhanced oil recovery polymers, such as partially hydrolyzed polyacrylamide, carboxymethyl(ethyl) cellulose, polysaccharides, or xanthan gums, are not suitable for high-temperature reservoirs (> 90 °C) with high-density brine fluid due to excessive hydrolysis and precipitation [277]. The main advantages of polymeric betaines over the mentioned standard polymers are (1) thermostability (up to 120 °C) (2) brine compatibility and (3) viscosification in brine solution [278]. Carbobetaines grafted onto hydroxyethyl cellulose were tested as a drilling-mud additive for clay hydration inhibition and mud rheological control [279]. An increase in the content of carbobetaine moieties resulted in an enhanced inhibitive abiUty, especially for sahne mud. 

SELECTION FOR ICE RESISTANCE , ANTIFREEZE PROTEINS AND ICE OR HYDRATE INHIBITION... 

One has to distinguish our studies here from the more popular studies on the adsorption of AFPs or LDHIs on ice or gas hydrate since the former focuses on the effect of AFP/LDHI on heterogeneous nucleation of ice/gas hydrate and the later focuses on the effect on the effects on the ice/hydrate growth. Thus, the studies represented in this study deals with impurity surfaces but not ice/hydrate surface. The authors think that this is an important aspect for the understanding of good ice/hydrate inhibition since the nucleation process is heterogeneous and must involve impurity surfaces. 

The individual Kj values in Scheme lean be determined as follows k is the Kfl term for anionic inhibition of CO2 hydration activity at limiting low pH. k is the term under the same conditions. 

Equation (2) is consistent with the observed diminution of inhibitor power of anions towards HCO3 dehydration as the pH is increased. An important prediction evident from a comparison of Scheme 1 and 2 is that the term referred to in both schemes as K- represents exactly the same dissociation equilibrium. If these schemes do in fact hold, then the values determined at low pH for the inhibition of both CO2 hydration and HCO3 dehydration should be identical. Inspection of Table III bears this out completely. 

For mixtures of hydrocarbons and alcohols, often involved in hydrate inhibition, the cubic plus association equation of state has been used to estimate the (vapour + liquid) and (liquid-b Uquid) equilibria. In both cases no combining rules are needed for the association parameters and for (alcohol + hydrocarbon). 

A method for gas hydrate inhibiting with quaternary ammonium compounds has been developed by Klomp et al. [190,191]. New legislation caused a change to esterquats and good results have been obtained. Today huge amounts of up to 30% of methanol are used in the gas exploration industry to prevent gas hydrates. Small amounts of esterquat have a similar effect and offer the possibility to explore deeper gas fields. Buijs et al. developed a process to manufacture these esterquats [192]. 

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