Gas Field Processing Part 2 , Wet Gas Dehydration

Water is the most common contaminant of hydrocarbons. It is always present in the gas–oil mixtures produced from wells. The sale contracts of natural gas specify either its dew point (the temperature at which water vapor condenses from the gas stream) or the maximum amount of water vapor present. Natural gas dehydration is therefore came to remove water vapor from the gas stream to lower the dew point of that gas. There are three basic reasons for the dehydration of natural gas streams:

To prevent hydrate formation. Hydrates are solids formed by the physical combination of water and other small molecules of hydrocarbons. Hydrates grow as crystal and can build up in orifice plates, valves, and other areas not subjected to full flow. Thus, hydrates can plug lines and retard the flow of gaseous hydrocarbon streams.

To avoid corrosion problems.

Corrosion often occurs when liquid water is present along with acidic gases, which tend to dissolve and disassociate in the water phase, forming acidic solutions. The acidic solutions can be extremely corrosive, especially for carbon steel, which is typically used in the construction of most hydrocarbon processing facilities.

Downstream processing requirements. In most commercial hydrocarbon processes, the presence of water may cause side reactions, foaming, or catalyst deactivation. Consequently, purchasers typically require that gas and liquid petroleum gas (LPG) feedstocks meet certain specifications for maximum water content. This ensures that water-based problems will not hamper downstream operations. The most common dehydration methods used for natural gas processing are as follows:

• Absorption, using the liquid desiccants (e.g., glycols and methanol)
• Adsorption, using solid desiccants (e.g., alumina and silica gel)
• Cooling/condensation below the dew point, by expansion and/or refrigeration
• ABSORPTION (GLYCOL DEHYDRATION PROCESS)

 In this process, a hygroscopic liquid is used to contact the gas to remove water vapor from it, Triethylene glycol (TEG) is the most common solvent used. The wet natural gas enters the absorption column (glycol contactor) near its bottom and flows upward through the bottom tray to the top tray and out at the top of the column. Lean (dry) glycol is fed at the top of the column and it flows down from tray to tray, absorbing water vapor from the natural gas. The rich (wet) glycol leaves from the bottom of the column to the glycol regeneration unit. The dry natural gas passes through mist mesh to the sales line. The glycol regeneration unit is composed of a reboiler where steam is generated from the water in the glycol. The steam is circulated through the packed section to strip the water from glycol. Stripped water and any lost hydrocarbons are vented at the top of the stripping column. The hydrocarbon losses are usually benzene, toluene, xylene, and ethyl benzene (BTXE) and it is important to minimize these emissions. The rich glycol is preheated in heat exchangers, using the hot lean glycol, before it enters the still column of the glycol reboiler. This cools down the lean glycol to the desired temperature and saves the energy required for heating the rich glycol in the reboiler. From practical experience and operational best practices , the dehydration process is more favorable at a lower temperature and higher pressure as the higher-pressure gas will contain less water vapor as compared to a lower-pressure gas at the same temperature. At the same time, pressure should not be too high, as this increases the pressure rating of the column and, consequently, increases its cost. A high operating pressure would also require high glycol pumping power. On the other hand, if the gas pressure is too low, the column size would be too large. On the other side, glycol regeneration is better achieved at lower pressures. Usually, the glycol regeneration takes place at atmospheric pressure. In some cases, the process takes place under vacuum to achieve higher lean glycol concentrations; this, however, makes the system too complicated and very expensive. The inlet gas temperature should not be too low in order to avoid condensation of water vapor and hydrocarbons. Also, a low gas temperature means a low glycol temperature. At low temperatures (below 10 °C), glycol becomes too viscous and more difficult to pump. Also, at low temperatures (below 15–21°C), glycol can form a stable emulsion with the hydrocarbon in the gas and may also cause foaming. On the other hand, high gas temperatures increase the gas volume, thus requiring a large-size column, and increase the water vapor content of the gas. Also, a high gas temperature results in high glycol losses. The inlet glycol temperature should not be lower than the gas temperature in order to avoid condensation of water and hydrocarbons. Normally, the gas temperature is maintained between 26.5 °C and 43.5°C. The inlet glycol temperature is normally kept at about 5 °C above the exiting gas temperature. In the glycol regenerator, the glycol temperature is normally raised up to between 188 °C and 199 °C. This results in a lean glycol concentration of about 98.5–98.9%. A higher temperature will cause degradation of the glycol. To achieve higher glycol concentrations, stripping gas may be used .

• ADSORPTION: SOLID-BED DEHYDRATION

When very low dew points are required, solid-bed dehydration becomes the logical choice. It is based on fixed-bed adsorption of water vapor by a selected desiccant. A number of solid desiccants could be used such as silica gel, activated alumina, or molecular sieves. . The selection of these solids depends on economics. The most important property is the capacity of the desiccant, which determines the loading design expressed as the percentage of water to be adsorbed by the bed. The capacity decreases as temperature increases .Molecular sieves is the most effective and versatile adsorption solution.

The system may consist of two-bed or three-bed. In the three-bed operation, if two beds are loading at different stages, the third one would be regenerated. The feed gas in entering the bed from the top and the upper zone becomes saturated first (each bed has three zones ).

The second zone is the mass transfer zone and is being loaded.The third zone is still not used and active.The different saturation progress and representation of different zones.While the bed is in operation, the outlet concentration has very low water concentration. At a certain point, the outlet watercontent rises to the point that is equivalent to the initial wet gas content as if bed is not present. Thus, the beginning of this period is called the breakthrough period.The operation of the process is controlled by opening valve and closing valve . After the bed has been used and loaded with water, then it is regenerated by hot gas (say 6 h, as heating time H) and then cooled by switching to cold gas (say for 2 h , C) .

The sequences are repeated for next cycles and dry gas is directed to next process for recovery of natural gas liquids , NGL , LPG If any.