Kinetic Hydrate Inhibition

However, there is a limit to the effectiveness of the inhibitors, commonly taken to be a subcooling (AT = temperature below the equilibrium temperature) 

FIGURE 8.10 Hydrate kinetic inhibitors. Unless indicated, every angle is the location of a carbon atom and the appropriate number of hydrogens. 

The definitive hydrate kinetic inhibition mechanism is not yet available. Some work suggests that the mechanism is to prevent hydrate nucleation (Kelland, 2006). However, a significant amount of evidence suggests that hydrate kinetic inhibitors inhibit the growth (Larsen et al., 1996). However, this apparent conflict is due to the definition of the size at which crystal nucleation stops and growth begins. To resolve this confusion, one may consider growth to occur after the critical nucleus size is achieved. 

It should be noted here that, while PVP was one of the first kinetic inhibitors discovered, it is one of the weakest kinetic inhibitors available. 

In his review of LDHIs, Kelland shows that kinetic inhibitors are well-established tools for hydrate prevention, with the following three points  

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N. Daraboina, Ch. Malmos, N. von Solms, 2013b. Investigation of kinetic hydrate inhibition using a high pressure micro differential scanning calorimeter. Energy Fuels 27, 5779-5786. 

Chua PC, KeUand MA, Ishitake K, Satoh K, Kamigaito M, Okamoto Y. Kinetic hydrate inhibition ofpoly(Af-isopropylmethacrylamide)s with different tacticities. Energy Fuels. 2012 26 3577-3585. 

Cyclopropanone hydrate has been reported to be a potent but reversible inhibitor of a number of enzymes which are sensitive to thiol reagents The kinetics of inhibition... 

Studies with other glycosidases " " showed, however, that effective inhibition by glycals is not a general phenomenon, and that inhibition does not correlate well with hydration to 2-deoxy-o-hexoses. Based on kinetic considerations, the interaction of glycosidases with D-glycals (A) can be described by the following scheme ... 

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. 

Both thermodynamic and kinetic factors affect the inhibition of hydrate deposits. 

CO3 species was formed and the X-ray structure solved. It is thought that the carbonate species forms on reaction with water, which was problematic in the selected strategy, as water was produced in the formation of the dialkyl carbonates. Other problems included compound solubility and the stability of the monoalkyl carbonate complex. Van Eldik and co-workers also carried out a detailed kinetic study of the hydration of carbon dioxide and the dehydration of bicarbonate both in the presence and absence of the zinc complex of 1,5,9-triazacyclododecane (12[ane]N3). The zinc hydroxo form is shown to catalyze the hydration reaction and only the aquo complex catalyzes the dehydration of bicarbonate. Kinetic data including second order rate constants were discussed in reference to other model systems and the enzyme carbonic anhy-drase.459 The zinc complex of the tetraamine 1,4,7,10-tetraazacyclododecane (cyclen) was also studied as a catalyst for these reactions in aqueous solution and comparison of activity suggests formation of a bidentate bicarbonate intermediate inhibits the catalytic activity. Van Eldik concludes that a unidentate bicarbonate intermediate is most likely to the active species in the enzyme carbonic anhydrase.460... 

Pronounced discrepancies between observed composition and the calculated equilibrium composition illustrate that the formation of the solid phase, for example, the nucleation of dolomite and calcite in seawater, is often kinetically inhibited, and the formation of phosphates, hydrated clay and pyrite is kinetically controlled. 

Research in this field is ongoing aiming to understand the mechanism of action of kinetic inhibitors. Lee and Englezos (2005) showed that inclusion of polyethylene oxide (PEO) to a kinetic inhibitor solution was found to enhance by an order of magnitude the performance of the hydrate inhibitor. Binding of inhibitor molecules to the surface of hydrate crystals was considered to be the key aspect of the mechanism of kinetic inhibition (Anderson et al.,... 

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. 

The special, and somewhat complex, role played by a substituent in the 4-position was first observed in the quinazoline series. The carbon atom that received the —OH group during hydration was located by preparing all six C-methylquinazolines. Only when inserted in the 4-position did a methyl group prevent the addition of water.41,23 This inhibition has both an inductive (+1) and a steric component. A kinetic study of quinazoline-4-carboxamide showed that the carbamoyl substituent, because of its steric effect, slowed the attainment of hydration from less than a second to 2-4 hours. However, this substituent has the... 

Long, J.P., Gas Hydrate Formation Mechanism and Kinetic Inhibition, Ph.D. Thesis, Colorado School of Mines, Golden, CO (1994). 

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. 

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. 

Low molecular weight PVCap-based products with added synergists were the best kinetic inhibitors for structure II hydrates on the market in 2005, and these inhibitors can provide 48 h of inhibition at a subcooling of 13°C. 

Equation (9.15) describes a reversible reaction, whereby the reaction can proceed to the right as well as to the left. Not all kinetic reactions are reversible. For example, radioactive decay, many oxidation reactions, and organic matter degradation proceed, for all practical purposes, in only one direction until the reaction is inhibited or the reactant is effectively exhausted. Reversible reactions, such as CO2 hydration, other acid-base relationships and some precipitation-dissolution reactions will attain, at some point, a steady state in which both the forward and reverse reactions occur at the same rate and the concentrations of both reactants and products no longer change. This is the state of chemical equilibrium at which the product of the reaction products raised to the exponent of their stoichiometric coefficients divided by a similar arrangement for the reactants is equal to the apparent equilibrium constant, K (see Chapter 3) ... 

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. 

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