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Abstract In designing dehydration units for natural gas, several critical parameters exist which can be varied to achieve a specified dew point depression. This paper studies the effects of varying the glycol flow rate, number of stages in the contactor,. The presence of heavy ends (C7+).
Water and hydrocarbons are natural companions. Hydrocarbons are formed in water environment and are in equilibrium with water. The water content of a gas depends on system pressure and temperature and the composition of the water containig gas. The presence of heavy ends (C7+) effects the water capacity of gas. In this study the effects of glycol circulation rate, glycol concentration and C7+ mol fraction are evaluated in a dehydration system.
This paper presents optimization of dehydration units. The results provide an analysis of the dehydration effectiveness at a variety of common operating variables for a typical dehydration facility. Next, the effect of c7+ in the plant feed will be presented.
Introduction A common method to remove water from natural gas is glycol dehydration. In this process, triethylene glycol (TEG) or diethylene glycol (DEG) is used to remove the presence of water in the gas stream. Water vapor can cause hydrate formation at low temperatures and high pressures or corrosion when it is in contact with hydrogen sulfide (H2S) or carbon dioxide (CO2), components regularly present in the gas stream. Glycol dehydration units are typically represented by a contactor, a flash tank, heat exchangers, and a regenerator, as shown in Figure 1. The glycol, usually TEG, enters at the top of the contactor and absorbs water as It progresses toward the bottom of the column. A dry gas exits at the top of the contactor and may be used for cooling the incoming lean glycol.
The rich stream flows to a separator or flash tank where gaseoushydrocarbons that were absorbed along with some of the water in the contractor are liberated and used as fuel. Finally, the glycol flows to the stripper where it is regenerated by boiling off the water and returned to the contractor. For processes requiring gas with very low water dew points, a stripping vapor will most likely be needed to aid the regeneration process. The region enclosed in the dotted line in Figure1 illustrates this technique. For maximum stripping, this vapor is normally injected into a short column at the bottom of the reboiler. However, the gas may also be introduced directly into the reboiler.
All calculations provided in this paper are based upon calculations made the base data are shown in Table 1, The program has heavy ends/crude characterization, complex heat exchanger, tray rating and a variety of utility calculational operations. In order to provide the results needed for this paper, a base set of operating parameters was selected as presented in Table I. Except when noted, these parameters are held constant throughout the analyses presented here. The process flow scheme is basically that presented in Figure 1 except for cases where stripping gas was not utilized.
Design of Dehydration Unit When optimizing the design of dehydration facilities, the impact of the following parameters should normally be considered:Number of trays in the contactor
Glycol circulation rate through the contactor
Temperature of the reboiler in the regenerator
Amount of stripping gas used, if any
Operating pressure of the regenerator
Of the above parameters, only the first four are normally considered as variable parameters. The first two parameters affect the approach to equilibrium at the top of the absorber while the third and fourth dictate the value of the equilibrium water content by limiting the purity of the lean glycol to the absorber. The last parameter affects the lean glycol purity in a manner similar to reboiler temperature. However, the vast majority of units are vented to the atmosphere so this parameter is beyond control.
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