Water Removal

    Moisture must be removed from natural gas to reduce corrosion problems and to prevent hydrate formation. Hydrates are solid white compounds formed from a physical-chemical reaction between hydrocarbons and water under the high pressures and low temperatures used to transport natural gas via pipeline. Hydrates reduce pipeline efficiency.

To prevent hydrate formation, natural gas may be treated with glycols,  which dissolve water efficiently. Ethylene glycol (EG), diethylene glycol  (DEG), and triethylene glycol (TEG) typical solvents for water removal.Triethylene glycol is preferable in vapor phase processes because of its low vapor pressure, which results in less glycol loss. The TEG absorber normally contains 6 to 12 bubble-cap trays to accomplish the water absorption. However, more contact stages may be required to reach dew points below –40°F. Calculations to determine the number of trays or feet of packing, the required glycol concentration, or the glycol circulation rate require vapor-liquid equilibrium data. Predicting the interaction between TEG and water vapor in natural gas over a broad range allows the designs for ultra-low dew point applications to be made.

A computer program was developed by Grandhidsan et al.¹, to estimate the number of trays and the circulation rate of lean TEG needed to dry natural gas. It was found that more accurate predictions of the rate could be achieved using this program than using hand calculation.

Figure 1 shows the Dehydrate process where EG, DEG, or TEG could be used as an absorbent. One alternative to using bubble-cap trays is structural packing, which improves control of mass transfer. Flow passages direct the gas and liquid flows countercurrent to each other. The use of structural packing in TEG operations has been reviewed by Kean et al².

Another way to dehydrate natural gas is by injecting methanol into gas lines to lower the hydrate-formation temperature below ambient. Water can also be reduced or removed from natural gas by using solid adsorbents such as molecular sieves or silica gel. 

Figure 1ز Flow diagram of the Dehydrate process8: (1) absorption column,

(2) glycol sill, (3) vacuum drum.

References 

1. Gandhidasan, P., Al-Farayedhi, A., and Al-Mubarak, A. “A review of types of dessicant dehydrates, solid and liquid,” Oil and Gas Journal, June 21, 1999, pp. 36–40.

2. Kean, J. A., Turner, H. M., and Price, B. C., “How Packing Works in Dehydrators,” Hydrocarbon Processing, Vol. 70, No. 4, 1991, pp. 47–52.