Page 317 - Petroleum and Gas Field Processing

tion must be designed to allow for a close approach to
equilibrium while the flow continues. Two means are provided to
accomplish this task for a countercurrent flow of the feed natural
gas and liquid (solvent or glycol):
(i) Tray column, or stage wise operation (equilibrium concept)
(ii) Packed column or continuous-contact operation (rate
concept)

This countercurrent system allows for the ‘‘wet’’ gas to enter the
bottom of the column and contact the rich glycol (high water
content) at its exit point. On the other hand, as the gas works its
way up the column, it encounters the leanest glycol (lowest water
content) before the gas leaves the column.

12.4.2 Absorption System
The absorption process is shown schematically in Figure 5. 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. Usually six to eight trays are used. 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 [4].

12.4.3 Pressure and Temperature Considerations
The absorption process improves at higher pressures because the higher-
pressure gas will contain less water vapor as compared to a lower-pressure
gas at the same temperature. The effect of pressure on the process, however,
is not significant for pressures below about 3000 psi. However, the gas

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