In natural gas processing, dehydration is a critical step to prevent hydrate formation and corrosion in pipelines. Triethylene Glycol (TEG) is the industry standard liquid desiccant used for this purpose. The TEG Circulation Rate is the volume of glycol that must be circulated through the contactor tower to achieve a specific dew point depression.
Why is the Circulation Rate Important?
Calculating the correct circulation rate is a balance between efficiency and operating costs:
Under-circulation: If the rate is too low, the TEG becomes saturated with water too quickly at the bottom of the contactor. This results in the outlet gas failing to meet moisture specifications (e.g., 7 lb/MMSCF), leading to "wet gas" entering the sales line.
Over-circulation: If the rate is too high, it leads to excessive energy consumption in the reboiler (to regenerate the glycol) and larger pumping requirements. It can also increase hydrocarbon absorption (BTEX emissions) and glycol losses.
The Calculation Formula
The calculation is based on the mass balance of water entering and leaving the system. The fundamental steps are as follows:
Formula:
1. Water Removed (lb/day) = Gas Flow (MMSCFD) × (Inlet Water – Outlet Water)
2. TEG Rate (gal/day) = Water Removed × Circulation Ratio
3. Circulation Rate (GPM) = TEG Rate (gal/day) / 1,440 min/day
Key Input Parameters
Gas Flow Rate (MMSCFD): The volume of gas being processed in Million Standard Cubic Feet per Day.
Inlet Water Content (lb/MMSCF): The amount of water vapor in the gas stream entering the contactor. This is determined by the inlet pressure and temperature.
Outlet Water Specification (lb/MMSCF): The target maximum water content allowed in the sales gas. A common pipeline specification is 7 lb/MMSCF, though northern climates may require 4 lb/MMSCF.
Circulation Ratio (gal/lb): This is an engineering design factor. It represents how many gallons of TEG are circulated for every pound of water removed.
2.0 – 3.0 gal/lb: Often used for standard dehydration where extreme dew points are not required.
3.0 – 5.0 gal/lb: Standard industry practice for ensuring sufficient driving force for mass transfer.
Example Calculation
Let's consider a gas processing facility with the following parameters:
Gas Flow: 50 MMSCFD
Inlet Water Content: 60 lb/MMSCF (saturated at inlet conditions)