Dehydration Utility Systems

1. System Overview

A glycol dehydration unit consists of the contactor (absorber) tower where wet gas contacts lean glycol, and a regeneration system where rich glycol is heated to drive off absorbed water and return lean glycol to the contactor. The regeneration system and its associated utility equipment are collectively referred to as the dehydration utility systems. These utilities consume the majority of the energy input and require careful sizing to achieve the target dewpoint depression reliably.

Regeneration Loop Components

The glycol regeneration loop includes the following major components, each covered in detail in subsequent sections:

Glycol Purity Targets

The lean glycol concentration determines the outlet gas water dewpoint. A standard reboiler operating at 400°F can achieve approximately 98.5–98.9 wt% TEG purity. Achieving higher purities requires stripping gas injection, vacuum operation, or the use of a Drizo or other enhanced regeneration process.

2. Reboiler & Fuel Gas

The reboiler is the primary energy consumer in the glycol regeneration system. It heats the rich glycol to the regeneration temperature (typically 380–400°F for TEG) to drive off absorbed water. The reboiler is usually a direct-fired firetube design, though waste heat, hot oil, and electric reboilers are also used.

Reboiler Heat Duty

The total reboiler heat duty is the sum of several components:

For a typical TEG unit processing 50 MMSCFD of natural gas at 1000 psig, the reboiler duty ranges from 200,000 to 500,000 Btu/hr depending on the glycol circulation rate and water loading. A common rule of thumb is 900–1,100 Btu per gallon of glycol circulated.

Fuel Gas Consumption

For direct-fired reboilers, the fuel gas consumption is calculated by dividing the total heat duty by the burner efficiency and the fuel heating value:

Maximum Film Temperature

The firetube surface temperature must be controlled to prevent thermal degradation of the glycol. TEG begins to decompose at approximately 404°F, so the maximum recommended reboiler temperature is 400°F. However, the firetube film temperature (the temperature of the glycol in direct contact with the hot tube surface) can be significantly higher than the bulk glycol temperature. The maximum allowable firetube heat flux is typically limited to 6,000–8,000 Btu/hr·ft² to keep the film temperature below the glycol decomposition threshold.

Reboiler Types

3. Glycol Pump Sizing

The glycol circulation pump is responsible for moving lean glycol from the atmospheric-pressure surge tank to the high-pressure contactor tower. The pump must overcome the contactor operating pressure, the frictional pressure drop through the glycol/glycol heat exchanger and piping, and the hydrostatic head difference between the surge tank and the contactor inlet.

Flow Rate Determination

The glycol circulation rate is set by the required dewpoint depression and is typically expressed in gallons of TEG per pound of water removed from the gas. Common circulation rates range from 2 to 5 gallons of TEG per pound of water for standard applications, with higher rates (up to 7–8 gal/lb) occasionally used for deep dewpoint requirements or units with fewer contactor trays.

Differential Pressure

The total differential pressure the pump must develop includes the contactor operating pressure (typically 300–1500 psig), pressure drops through the glycol/glycol exchanger (5–15 psi), piping friction losses (10–30 psi), and the hydrostatic head from the surge tank to the contactor inlet nozzle. The total differential pressure typically ranges from 400 to 1600 psi.

Pump Types

Pump Sizing Considerations

• NPSH: The net positive suction head available from the surge tank must exceed the pump's NPSH required. The surge tank is typically elevated 6–10 feet above the pump suction to provide adequate NPSH.
• Glycol temperature: Hot lean glycol from the reboiler has lower viscosity and density than cold glycol, affecting both pump capacity and NPSH. The glycol/glycol exchanger cools the lean glycol before it reaches the pump.
• Turndown: The pump must handle the minimum and maximum glycol circulation rates required by seasonal gas flow variations. Kimray pumps have limited turndown; motor-driven pumps with variable speed drives offer wider operating ranges.
• Sparing: A spare pump (or the ability to quickly install one) is recommended for continuous operations to avoid shutting down the dehy unit for pump maintenance.

4. Reflux Condenser

The reflux condenser is mounted on top of the still column and serves two critical functions: it condenses glycol vapor that would otherwise escape with the water vapor overhead, and it provides reflux liquid that improves the separation efficiency of the still column. Without adequate reflux, glycol losses from the still column can be significant.

Operating Principle

The still column overhead vapor is a mixture of water vapor, glycol vapor, and absorbed hydrocarbons (including BTEX compounds). As this vapor rises through the reflux condenser, it is cooled by ambient air or by heat exchange with the incoming rich glycol. The glycol vapor condenses and flows back down the still column as reflux, while the water vapor and light hydrocarbons pass through the condenser and are vented to atmosphere or to a thermal oxidizer.

Condenser Temperature Control

The reflux condenser outlet temperature is a critical operating parameter. If the temperature is too low, excessive water condensation occurs, reducing glycol purity in the reboiler. If the temperature is too high, glycol vapor escapes with the overhead, increasing glycol losses.

The optimal condenser outlet temperature is typically 200–215°F. This allows water vapor to pass through while condensing most of the glycol vapor. The condenser should be designed for the worst-case ambient temperature conditions (winter minimum for overcondensing risk, summer maximum for glycol loss risk).

Condenser Types

Two condenser configurations are common in glycol dehydration units:

• Air-cooled condenser: A finned coil or plate exchanger using ambient air as the cooling medium. Common for field dehy units. Performance varies with ambient temperature, and winter overcondensing can be managed with louvers or bypass dampers.
• Rich glycol-cooled condenser: Uses the incoming rich glycol as the cooling medium. This recovers some heat from the still overhead and preheats the rich glycol before it enters the reboiler, improving overall thermal efficiency. This configuration is standard for units with glycol/glycol heat exchangers.

5. Stripping Gas Systems

When the reboiler alone cannot achieve the required lean glycol concentration (typically limited to 98.5–98.9 wt% TEG at 400°F and atmospheric pressure), stripping gas is used to enhance regeneration. Stripping gas reduces the partial pressure of water vapor in the reboiler/stripping column, shifting the vapor-liquid equilibrium to favor water removal from the glycol.

Stripping Gas Principle

The equilibrium glycol purity is determined by the temperature and the partial pressure of water vapor in the reboiler vapor space. At 400°F and atmospheric pressure with no stripping gas, the equilibrium TEG purity is approximately 98.9 wt%. By introducing a dry inert gas (typically natural gas) into the reboiler or a dedicated stripping column below the reboiler, the water partial pressure is diluted, allowing the glycol to achieve higher purity.

Stripping Gas Rate

The stripping gas rate is typically expressed in standard cubic feet per gallon of glycol circulated. The required rate depends on the target glycol purity:

Injection Methods

Stripping gas can be introduced at several points in the regeneration system, each with different effectiveness:

• Direct reboiler injection: Gas is bubbled directly into the glycol in the reboiler through a sparger tube. This is the simplest method but least efficient because the gas has only a single stage of contact with the glycol. Typical purity improvement is 0.5–1.5 wt% above reboiler-only operation.
• Stahl column: A short packed or trayed column is installed between the reboiler and the glycol surge tank. Stripping gas enters the bottom of this column and contacts the lean glycol flowing down from the reboiler. The multiple stages of contact in the Stahl column provide significantly better mass transfer than direct reboiler injection, achieving purities of 99.2–99.9 wt% depending on gas rate and column design.
• Lean glycol stripping: In some designs, stripping gas is injected into the lean glycol line downstream of the Stahl column for additional purification. This is less common but can provide marginal improvements for ultra-low dewpoint applications.

6. Flash Tank Sizing

The flash tank (also called the glycol flash separator) is installed in the rich glycol line between the contactor and the glycol/glycol heat exchanger. Its purpose is to remove dissolved and entrained hydrocarbons from the rich glycol before the glycol enters the regeneration system. Without adequate flash separation, hydrocarbons carry over to the reboiler, increasing fuel gas consumption, creating emissions, and potentially causing foaming and degradation.

Hydrocarbon Absorption

In the contactor, glycol absorbs not only water but also hydrocarbons from the natural gas stream. The amount of hydrocarbon absorption depends on the contactor pressure, temperature, glycol circulation rate, and gas composition. Heavier hydrocarbons (C₅+) and aromatic compounds (BTEX) are preferentially absorbed. At typical contactor conditions (800–1200 psig, 80–100°F), the rich glycol may contain 1–5 SCF of dissolved gas per gallon of glycol plus absorbed BTEX compounds.

Flash Tank Design

The flash tank operates at an intermediate pressure between the contactor and the regeneration system, typically 50–100 psig. This pressure is low enough to flash most of the dissolved light hydrocarbons out of the glycol, but high enough that the flash gas can be used as fuel or routed to a low-pressure gathering system without additional compression.

Flash Tank Sizing Parameters

Flash Gas Utilization

The flash gas from the flash tank typically has a heating value of 900–1,050 Btu/SCF and is often used as reboiler fuel gas, instrument gas supply, or routed to a low-pressure gathering system. Venting flash gas to atmosphere is increasingly restricted by environmental regulations. In sour gas service, the flash gas may contain H₂S and require treatment or incineration.

7. Glycol Filtration

Glycol filtration is essential for maintaining glycol quality and preventing operational problems including foaming, fouling, corrosion, and equipment damage. Contaminants in the glycol system come from multiple sources: well production (formation fines, scale, corrosion products), process upsets (hydrocarbons, compressor oil), and glycol degradation products (organic acids, sludge).

Particulate Filtration

A particulate filter (typically a cartridge filter or sock filter) removes solid particles from the glycol stream. The filter is installed in the lean glycol line after the glycol/glycol exchanger and before the contactor, or in the rich glycol line before the reboiler. Common filter ratings are 5–10 microns for primary filtration, with some units using a finer 1–2 micron polishing filter downstream.

Activated Carbon Filtration

An activated carbon bed is installed downstream of the particulate filter to adsorb dissolved hydrocarbons, surfactants, compressor oils, and other organic contaminants that cause foaming and glycol degradation. The carbon bed also removes color bodies and odor-causing compounds. The carbon bed is typically sized for 15–20 minutes of contact time at the glycol flow rate and is replaced or regenerated when breakthrough is detected (usually indicated by increasing foaming tendency or darkening glycol color).

Glycol Makeup Rate

Glycol is lost from the system through several mechanisms: carryover from the contactor (glycol mist and vapor), losses from the still column overhead, flash tank venting, filter change-outs, and leaks. The total glycol loss determines the makeup rate required to maintain the glycol inventory.

Glycol losses exceeding 0.10 gal/MMSCF indicate a problem that should be investigated. Common causes of excessive losses include damaged or missing mist eliminator pads in the contactor, reflux condenser malfunction, high glycol temperatures in the contactor, contaminated glycol causing foaming, and mechanical leaks in pumps, valves, or piping.

8. BTEX Emissions

Benzene, toluene, ethylbenzene, and xylenes (BTEX) are volatile organic compounds (VOCs) present in natural gas that are preferentially absorbed by TEG in the contactor. When the rich glycol is regenerated in the reboiler and still column, these absorbed BTEX compounds are released with the water vapor and overhead gases from the still column. BTEX emissions are a significant environmental and regulatory concern for glycol dehydration facilities.

BTEX Absorption in the Contactor

The amount of BTEX absorbed by glycol depends on the BTEX concentration in the inlet gas, contactor pressure and temperature, glycol circulation rate, and number of contactor trays. Higher pressures, lower temperatures, and higher glycol rates increase BTEX absorption. For a typical contactor processing gas with 100–500 ppmv total BTEX at 800–1200 psig, the rich glycol may contain 0.5–3.0 lb of BTEX per MMSCF of gas processed.

BTEX Emission Sources

BTEX Emission Estimation

BTEX emissions from glycol dehydration units can be estimated using several methods:

• GRI-GLYCalc: An industry-standard computer model developed by the Gas Research Institute that simulates the glycol dehydration process and estimates BTEX and VOC emissions based on inlet gas composition and operating parameters.
• AP-42 emission factors: EPA AP-42, Chapter 5.3 provides emission factors for glycol dehydration units based on throughput and operating conditions. These are used for emissions inventory and permitting purposes.
• Direct measurement: Stack testing or continuous emissions monitoring (CEMS) of the still column overhead provides site-specific emission data for compliance demonstration.

BTEX Control Technologies

Several technologies are available to reduce BTEX emissions from glycol dehydration still column vents:

References

1. GPSA Engineering Data Book, Chapter 20 — Dehydration
2. API 12GDU — Specification for Glycol-Type Gas Dehydration Units
3. Campbell, J.M. — Gas Conditioning and Processing, Volume 2
4. EPA AP-42, Chapter 5.3 — Natural Gas Processing (Glycol Dehydration)
5. 40 CFR Part 63, Subpart HH — NESHAPs for Oil and Natural Gas Production
6. GRI-GLYCalc — Gas Research Institute Glycol Dehydration Emissions Model
7. Kidnay, A.J. & Parrish, W.R. — Fundamentals of Natural Gas Processing, 2nd Edition

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