Pressures above atmospheric in the reboiler can significantly reduce lean glycol concentration and dehydration efficiency. The still column should be adequately vented and the packing replaced periodically to prevent excess back pressure on the reboiler.
At pressures below atmospheric the boiling temperature of the rich glycol/water mixture decreases, and a greater lean glycol concentration is possible at the same reboiler temperature. Reboilers are rarely operated at a vacuum in field gas installations, because of the added complexity and the fact that any air leaks will result in glycol degradation. In addition, it is normally less expensive to use stripping gas. However, if lean glycol concentrations in the range of 99.5% are required, consider using a reboiler pressure of 500 mm Hg absolute (approximately 10 psia) as well as using stripping gas. Sometimes the addition of a vacuum will help extend the range of an existing glycol system