The higher the concentration of the lean glycol the greater the dewpoint depression for a given glycol circulation rate and number of trays.
Figure 8-9 shows the equilibrium water dew point at different temperatures for gases in contact with various concentrations of glycol. At 100°F contact temperature there is an equilibrium water dew point of 25°F for 98% glycol and 10°F for 99% glycol. Actual dew points of gas leaving the contactor will be 10°F to 20°F higher than equilibrium.
Figure 8-10 shows that increasing the lean glycol concentration can have a much greater effect on dew-point depression than increasing the circulation rate. To obtain a 70°F dew-point depression a circulation rate of 6.2 gaVlb at 99.95%, 8.2 gal/lb at 99.5% or in excess of 12 gal/lb at 99% is required.
The lean glycol concentration is determined by the temperature of the reboiler. the gas stripping rate, and the pressure of the reboiler. Glycol concentrations between 98 and 99% are common for most field gas units.