The lean glycol concentration leaving the reboiler can be lowered by contacting the glycol with stripping gas. Often, wet gas that is saturated with water vapor at ambient temperature and 25 to 100 psig is used. At 25 psig and 100°F this gas is saturated with 1,500 Ib/MMscf of water vapor. At atmospheric pressure and the temperatures in the reboiler the gas can absorb over 100,000 Ib/MMscf.
In most situations the additional fuel gas required to heat the reboiler to increase lean glycol concentration is less than the stripping gas required for the same effect. Thus, it is normally desirable to use stripping gas only to increase lean glycol concentration above 98.5 to 98.9%, which can be reached with normal reboiler temperatures and normal back pressure on the still column. If the glycol circulation rate must be increased above design on an existing unit and the reboiler cannot reach desired temperature, it is often possible to use stripping gas to achieve the desired lean glycol concentration.
Figure 8-12 shows the effects on the glycol purity of stripping gas flow rate for various reboiler temperatures, assuming the gas is injected directly into the reboiler. Greater purities are possible if stripping gas contacts the lean glycol in a column containing one or more stages of packing before entering the reboiler.