Prevention of hydrate formation

Notz (1994) noted that almost all of Texaco s efforts concerning natural gas hydrates dealt with the prevention of hydrate formation in production and transportation systems. He presented Figure 8.1 from Texaco s hydrate prevention program in a 50 mile deepwater gas pipeline, using a phase diagram similar to those discussed with Figure 4.2. 

Nucleation of hydrate in all commonly encountered situations is a heterogeneous process. Hence the prevention of hydrate formation, including nucleation, growth and elimination of memory effects, should be considered as involving impurities. 

Methods used for prevention of hydrate formation are dictated by physical and chemical nature of a process. Since equilibrium parameters of hydrates formation depend on partial pressure of water vapor in hydrating medium, any action lowering such pressure reduces the temperature of hydrate formation. In practice, the two ways are used dehydration of gas from moisture and the input into gas flow of various water-absorbing substances called inhibitors. 

For prevention of hydrate formation, an inhibitor is injected into natural gas -the high-concentration water solution of methanol in amount no more than 1 kg on 1000 m of gas at normal conditions. It corresponds to the volume content of liquid W < 5 10 m /m at characteristic pressure 5 MPa. Such small value W allows to neglect the influence of drops on dynamic parameters of gas and to consider dynamics of drop ensemble under action of the flow undisturbed by disperse phase. 

Separation processes of gas-liquid (gas-condensate) mixtures are considered in Section VI. The following processes are described formation of a liquid phase in a gas flow within a pipe coalescence of drops in a turbulent gas flow, condensation of liquid in throttles, heat-exchangers, and turboexpanders the phenomena related to surface tension efficiency of division of the gas-liquid mixtures in gas separators separation efficiency of gasseparators equipped with spray-catcher nozzles of various designs - louver, centrifugal, string, and mesh nozzles absorbtive extraction of moisture and heavy hydrocarbons from gas prevention of hydrate formation in natural gas. 

Under certain conditions of temperature and pressure, and in the presence of free water, hydrocarbon gases can form hydrates, which are a solid formed by the combination of water molecules and the methane, ethane, propane or butane. Hydrates look like compacted snow, and can form blockages in pipelines and other vessels. Process engineers use correlation techniques and process simulation to predict the possibility of hydrate formation, and prevent its formation by either drying the gas or adding a chemical (such as tri-ethylene glycol), or a combination of both. This is further discussed in SectionlO.1. 

Hydrate formation can be prevented by drying a gas to such an extent that no condensate can be formed. This method is the preferable one, but inhibition of hydrate formation from the liquid phase can be achieved. 

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 five studies of hydrate formation given in Section 8.1 are of two types. The first three case studies show thermodynamic (time-independent) methods to prevent plug formation. However, the second type provides a closer, mechanistic look at the physical kinetics (time-dependent) hydrate formation and agglomeration. A goal of this section is to show how these two methods provide two different methods of plug prevention. 

The discovery in 1934 of hydrates blocking energy flowlines led to a much more intensive period of hydrate research. Without knowledge of the hydrate structures, researchers generated methods to predict and to inhibit the conditions of hydrate formation in flowlines and to prevent them. Six major efforts, driven by pipeline discovery, hallmark this period. 

Prediction of the temperatures and pressures of hydrate formation for an infinite variety of mixtures of the above eight components. This enables specification of flowline insulation or heating, to prevent fluids from entering the hydrate formation region. 

It would be ideal to operate the pipeline outside the hydrate formation envelop. However, as mentioned above, the high pressures and low temperatures associated to less accessible reserves leave the pipelines within the hydrate formation region [2]. Therefore, the ability to predict formation of hydrates in the field will play a vital role in exploiting these reserves. The aim of this study is to develop a combined state and parameter estimator for this process as a means for the prediction of hydrate formation towards preventive feedforward control. 

Gas hydrate should be considered as a material which directly impacts our planet s population, in many ways as yet to an unknown extent. Although hydrates and their structures have been studied for many years, the understanding of hydrate formation processes is still in its infancy but is of critical importance today for a variety of reasons understanding the nature of natural gas hydrate deposits and how they accumulate", the prevention and control of pipeline hydrates, and the development of hydrate-based processes such as gas storage. ... 

Hydrate formed in pipelines has been identified as a problem by the gas and oil industry since the 1930 s, there being both hazards and economic factors in actual pipeline blockage, and the prevention and control of hydrate formation in pipelines is of major importance. Although emphasis has been placed on finding practical solutions in this area by inhibiting hydrate formation with various additives it is clear that modem practice requires better alternatives to those in common use today that are safe and environmentally benign even for the ever more... 

H O, as a solvent with very high dielectric permeability (81.0), separates cations and anions and prevents their interaction between themselves. For this reason in very diluted water solutions charged ions are sufficiently remote from one another and interact mostly with dipole H O. Previously we reviewed the effect of hydrate formation on the structure of water solution. However, the very process of their formation actively affects also the behaviour of dissolved components themselves and plays an important role in the formation and composition of underground waters. 

Removal of water vapors from the gas. This process is called gas dewatering (dehydration). Since dehydration causes a decrease in the threshold temperature of hydrate formation, this procedure often includes additional steps intended to prevent the formation of hydrates. 

Install low point drains to remove water, thereby reducing the chance of hydrate formation. The water should be drained on a regular basis. Also, use antifreeze agents, such as methanol or ethylene glycol, to prevent hydrate formation. 

Dehydration can be performed by a number of methods cooling, absorption and adsorption. Water removal by cooling is simply a condensation process at lower temperatures the gas can hold less water vapour. This method of dehydration is often used when gas has to be cooled to recover heavy hydrocarbons. Inhibitors such as glycol may have to be injected upstream of the chillers to prevent hydrate formation. 

To meet sales specifications, gas produced at the wellheads must be free of water and hydrocarbon liquids. Twin turboexpanders are a key component in this process, providing dewpoint control with optimal efficiency. Initial processing takes place at the wellhead platforms, where methanol is injected to inhibit hydrate formation. A corrosion inhibitor is also added to prevent gas from damaging downstream equipment. 

In a typical gas oil design, the lighter products overhead from the quench tower/primary fractionator are compressed to 210 psi, and cooled to about 100°F. Some Q plus material is recovered from the compressor knockout drums. The gases are ethanolamine and caustic washed to remove acid gases sulfur compounds and carbon dioxide, and then desiccant dried to remove last traces of water. This is to prevent ice and hydrate formation in the low temperamre section downstream. 

Methods of preventing hydrate formation include adding heat to assure that the temperature is always above the hydrate formation temperature, lowering the hydrate formation temperature with chemical inhibition, or dehydrating the gas so that water vapor will not condense into free water. It is also feasible to design the process so that if hydrates form they can be melted before they plug equipment. 

Knowledge of the temperature and pressure of a gas stream at the wellhead is important for determining whether hydrate formation can be expected when the gas is expanded into the flow lines. The temperature at the wellhead can change as the reservoir conditions or production rate changes over the producing life of the well. Thus, wells that initially flowed at conditions at which hydrate formation in downstream equipment was not expected may eventually require hydrate prevention, or vice versa. 

Chapter 4 discussed the need to prevent hydrates, the techniques necessary to predict hydrate formation, and the use of chemical inhibitors. This chapter covers two types of equipment for handling hydrates. 

In order to adequately describe the size of a heater, the heat duty, the size of the fire tubes, the coil diameters and wall thicknesses, and the cor lengths must be specified. To determine the heat duty required, the maximum amounts of gas, water, and oil or condensate expected in the heater and the pressures and temperatures of the heater inlet and outlet must be known. Since the purpose of the heater is to prevent hydrates from forming downstream of the heater, the outlet temperature will depend on the hydrate formation temperature of the gas. The coil size of a heater depeiuLs on the volume of fluid flowing through the coil and the required heat duty. 

Stoppage of natural gas-water streams due to the formation of gas hydrates is prevented by incorporation of a surface-active agent in such streams, e.g., a 15% aqueous solution of hydroxylamine phosphate, which inhibits the formation of gas hydrates and the agglomeration of hydrate crystallites into large crystalline masses [255],... 

Polyethercyclicpolyols possess enhanced molecular properties and characteristics and permit the preparation of enhanced drilling fluids that inhibit the formation of gas hydrates prevent shale dispersion and reduce the swelling of the formation to enhance wellbore stability, reduce fluid loss, and reduce filter-cake thickness. Drilling muds incorporating the polyethercyclicpolyols are substitutes for oil-based muds in many applications [195-197,1906,1907]. Polyethercyclicpolyols are prepared by thermally condensing a polyol, for example, glycerol to oligomers and cyclic ethers. 

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. 

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. 

As mentioned previously, the classic additive to prevent hydrate formation is alcohol. Traditional hydrate inhibitors such as methanol and glycols have been in use for many years, but demand for cheaper methods of inhibition is great. Therefore the development of alternative, cost-effective, and environmentally acceptable hydrate inhibitors is a technologic challenge for the oil and gas production industry [947]. 

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