Hydrate depression

The problem is a typical pipeline transporting a water saturated natural gas. The pipeline conditions are such that hydrates could form in the pipeline. Methanol will be used to depress the hydrate formation temperature to an acceptable level (a 15°C hydrate depression was used). 

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... 

To approximate the hydrate depression temperature for several inhibitors in the aqueous liquid, the natural gas industry uses the original Hammerschmidt (1939) expression to this day as a check ... 

Primary or polyfunctional alcohols such as methanol and ethylene glycol have been used as freezing point suppressants. This could be seen routinely in the antifreezing windshield wash solution used for automobiles. Typically, a 25% methanol solution decreases the freezing point of water by about 36°F to about —4°F, which is called the depressed ice point. Similarly, a typical natural gas forms hydrates in the presence of free water at about 63 °F, whereas it is depressed to 39°F (24°F hydrate depression) in the presence of a 25% methanol solution. Normal and depressed hydrate points are functions of... 

Commercial appHcations of calcium chloride and its hydrates exploit one or more of its properties with regard to aqueous solubiUty, hygroscopic nature, the heat gained or lost when one hydrated phase changes to another, and the depressed freezing point of the eutectic solution at a composition of about 30% by weight calcium chloride. 

L. D. Polderman, "The Glycols as Hydrate Poiat Depressants ia Natural Gas Systems," Proceedings of the Gas Conditioning Conference University of Oklahoma, Norman, 1958. 

There are numerous solubility data in the literature the standard reference is by Seidell (loc. cit.). Valuable as they are, they nevertheless must be used with caution because the solubihty of compounds is often influenced by pH and/or the presence of other soluble impurities which usually tend to depress the solubihty of the major constituents. While exact values for any system are frequently best determined by actual composition measurements, the difficulty of reproducing these solubility diagrams should not be underestimated. To obtain data which are readily reproducible, elaborate pains must be taken to be sure the system sampled is at equihbrium, and often this means holding a sample at constant temperature for a period of from 1 to 100 h. While the published cui ves may not be exac t for actual solutions of interest, they generally will be indicative of the shape of the solubility cui ve and will show the presence of hydrates or double salts. 

Methanol is frequently used to inhibit hydrate formation in natural gas so we have included information on the effects of methanol on liquid phase equilibria. Shariat, Moshfeghian, and Erbar have used a relatively new equation of state and extensive caleulations to produce interesting results on the effeet of methanol. Their starting assumptions are the gas composition in Table 2, the pipeline pressure/temperature profile in Table 3 and methanol concentrations sufficient to produce a 24°F hydrate-formation-temperature depression. Resulting phase concentrations are shown in Tables 4, 5, and 6. Methanol effects on CO2 and hydrocarbon solubility in liquid water are shown in Figures 3 and 4. 

Chemicals can be injected into the production stream to depress the likelihood of significant hydrate formation. 

From Table 4-1 the hydrate temperature is 74°F. The required dewpoint depression then will be 74-65 = 9 F... 

The carbon dioxide carryover dissolves in condensed steam, where it is partially hydrated to form carbonic acid (H2C03), as shown below. The increase in hydrogen ion concentration causes the pH to be depressed and generally results in a condensate with a pH of approximately 5.0 to 5.5. 

The hydrate-formation temperamre can be reduced by the addition of antifreeze agents such as methanol, glycols [1430], or brines. The depression of the freezing point is given by... 

Hydrate control is not included in this chapter, but is discussed in Chapter 13 because of the relative importance and difference in chemical mechanism. Many chemicals added to water will result in a depression of the freezing point. The practical application is restricted, however, because of some other unwanted effects, such as corrosion, destruction of rubber sealings in engine parts, or economic aspects. 

Perform "wet"-methanol-hydrocarbon flashes to estimate the liquid water plus methanol and hydrocarbon phases the methanol concentration is adjusted to satisfy the Hammerschmidt equation prediction for the desired hydrate formation temperature depression. 

All three approaches predict about the same methanol requirements to give the specific hydrate formation temperature depression. 

The soluble complexes formed by activators will desorb cation from the lime depressed pyrite surface, which will expose a fresh pyrite surface and activate pyrite flotation. Therefore, the moderately strong acids such as oxalic acid and phosphoric acid exhibit a strong activation action on lime-depressed pyrite because of their ability to decrease pulp pH and to form soluble complexes with hydrated surface cations. 

CNS effects Because of dichloralphenazone s structural similarity to chloral hydrate, there is a potential for CNS depressant effects. 

Hara, T. Hashimoto, S. Sugahara, T. Ohgaki, K., (2005). Large pressure depression of methane hydrate by adding 1,1-dimethylcyclohexane. Chem. Eng. Sci., 60, 3117-3119. 

The adverse effects of valacyclovir and acyclovir are similar. Toxicity is generally minimal, consisting largely of headache, nausea, and diarrhea. Less frequently observed are skin rash, fatigue, fever, hair loss, and depression. Reversible renal dysfunction (azotemia) and neurotoxicity (tremor, seizure, delirium) are dose-Umiting toxicides of intravenous acyclovir. Adequate hydration and slow drug infusion can minimize the risk of renal toxicity. 

The main non-chloride, non-corrosive accelerating admixtures available on the market are of two types (1) accelerating admixtures which accelerate hydration but do not depress the freezing point of water and (2) accelerating admixtures for use in sub-freezing ambient temperatures which depress the freezing point of water. The former contain salts of formates, nitrates and nitrites and are effective for set acceleration and strength development. However, their effectiveness is dependent on the ambient temperature at the time of placement. 

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