The Due Diligence Deep Dive: Top 10 Questions to Ask Before Buying a Glycol Dehydrator
In the world of natural gas processing, removing water vapor is not just a regulatory requirement—it’s a fundamental necessity for preventing corrosion, inhibiting the formation of dangerous gas hydrates, and ensuring the gas meets pipeline quality specifications typically 7 pounds of water per million standard cubic feet, or lb/MMscf. The Triethylene Glycol (TEG) dehydration unit is the industry’s workhorse for this task.
Investing in a glycol dehydrator is a significant capital expenditure. Getting the design specifications wrong can lead to operational headaches, excessive emissions, high operating costs (OPEX), and, critically, off-spec gas. This comprehensive guide outlines the top 10 most critical questions you must ask a vendor or engineering firm before signing the purchase order for your new TEG dehydration unit.
This master checklist outlines the Top 10 essential questions you need to ask every potential vendor.
1. Process Conditions: What is the Full Range of Gas and Water Data?
A dehydrator is only as good as the design data it’s based on. The old adage “garbage in, garbage out” is profoundly true here. You must provide (and the vendor must request and validate) a complete data set that covers the full operating envelope—not just a single point in time.
- Inlet Gas Flow Rate: What are the maximum, minimum (turndown), and average gas flow rates in MMscf/d? An oversized unit wastes fuel and glycol; an undersized one fails to meet specifications. The vendor should provide a guaranteed turndown ratio 5:1 for trayed contactors, 10:1 for packed columns.
- Inlet Gas Pressure and Temperature: What is the full range of operating pressures and temperatures? These parameters critically determine the water content of the rich gas and the final dew point. Lowering pressure or increasing temperature will increase the water load on the unit.
- Inlet Water Content: Is the inlet gas assumed to be fully saturated with water vapor? If not, what is the actual or calculated water content? A conservative design assumes saturation, but knowing the true range is essential for optimizing the design.
- Gas Composition (Including Impurities): What is the full mole percent composition, especially of CO2, and heavier hydrocarbons C6+?
- Acid Gases H2S & CO2: These can cause corrosion. The unit’s metallurgy and materials of construction must be specified to handle these contaminants.
- Heavy Hydrocarbons (BTEX): Benzene, Toluene, Ethylbenzene, and Xylenes BTEX are soluble in TEG and will be stripped from the reboiler vent, leading to higher regulated air emissions and potential foaming issues. The unit may require a BTEX elimination or destruction unit to meet air quality regulations.
2. Dry Gas Specification: What is the Required Outlet Water Dew Point or Content?
The entire purpose of the unit is to meet a sales gas specification. This is typically expressed as a maximum water content (e.g., 7 lb/MMscf or a maximum water dew point temperature 32F at a specific pressure).
- Final Destination Requirement: What is the exact, non-negotiable requirement for your downstream pipeline or processing facility?
- Safety Margin and Hydrate Formation: Is the design based on the water dew point or the hydrate formation temperature? For critical or low-temperature applications, sizing based on the water content in equilibrium with hydrates (which is lower than with liquid water) is necessary for a conservative and reliable design.
- Lean Glycol Purity: This requirement directly ties to the necessary lean TEG concentration entering the contactor. Meeting 7 lb/MMscf often requires a lean TEG concentration of approx 99.0%, which, in turn, dictates the regeneration system design (Question #3).
3. Regeneration System Design: How is High Lean Glycol Purity Achieved?
The lean TEG purity is the most crucial factor in achieving a low dew point. An atmospheric reboiler alone can typically achieve only 98.6 % TEG purity (due to the azeotrope of TEG and water). To meet the common 7 lb/MMscf specification, higher purity is required.
- Stripping Gas Rate: What is the maximum stripping gas rate required typically expressed as scf of gas per gallon of circulating TEG? This directly impacts your operating costs and emissions.
- Stripping Gas: Will the unit include a stripping gas column? Stripping gas (usually a small side stream of the dried gas) is sparged through the hot glycol to physically remove additional water vapor, significantly raising the purity from 98.6 % up to 99.9 % or higher. This is often essential for meeting modern pipeline specs.
- Reboiler Temperature: What is the maximum guaranteed reboiler temperature? TEG begins to thermally degrade at about 404^F. The design should ideally operate slightly below this, typically 380F to 400F, to maximize regeneration while minimizing decomposition.
4. Emissions Control: What is the Plan for VOC and HAP Emissions?
Glycol dehydrators are a source of Methane, VOCs (Volatile Organic Compounds), and HAPs (Hazardous Air Pollutants) like BTEX, which are absorbed with the water and vented from the still column. Environmental regulations (such as EPA’s NSPS OOOOa or NESHAP Subpart HH in the US) often mandate controls.
- BTEX/VOC Control Method: Does the unit include a mandated control device? Common methods include:
- Still Vent to Firebox/Reboiler Burner: Venting the still gas directly into the reboiler’s burner flame for destruction. This is a common, cost-effective solution but requires a reliable ignition source and burner design.
- BTEX Condenser/Eliminator: A closed-loop system that condenses and separates the BTEX and VOCs, which can then be rerouted to the main gas line or a separate storage tank. This reduces emissions and recovers valuable hydrocarbons.
- Vapor Recovery Unit VRU: For larger facilities, a VRU can capture and compress the flash gas/still vent gas.
- Flash Tank Separator: Does the unit include a flash separator vessel between the contactor and the regenerator? Flashing the rich glycol to a lower pressure (e.g., 50 psig removes the majority of the dissolved methane and a portion of the VOCs. This gas can be used as fuel or sent to a flare, which significantly reduces the gas and VOC load to the still column, thereby lowering total emissions and fuel consumption in the reboiler.
5. Contactor Column Internals: Trays vs. Structured Packing?
The contactor column is where the wet gas and lean glycol meet. The choice of internal structure directly affects efficiency, size, and turndown capability.
- Recommendation: Based on your specific gas flow, contaminant load, and required dew point, which internal configuration is being quoted, and why? What is the guaranteed efficiency for the selected internal type?
- Trays (e.g., Bubble Cap or Valve Trays): Generally lower cost, simpler, and more robust against fouling and liquid slugging. However, they typically have a smaller operating range (turndown) and may require a taller vessel for the same performance.
- Structured Packing: Offers superior mass transfer efficiency, resulting in a smaller/shorter column, lower glycol circulation rate, and higher turndown ratio approx 10:1. However, they are more sensitive to fouling and debris.
6. Glycol Circulation Pump: What Type, and How is it Powered?
The glycol circulation pump is the heart of the unit and a primary point of maintenance and energy consumption.
- Pump Type: Is the pump a gas-powered piston pump (like a Kimray or Williams) or an electric/solar powered diaphragm or piston pump?
- Gas-Powered Pumps: Simple and highly reliable, but they use a side stream of wet gas to operate, which adds to the total gas/methane emissions from the unit (the pump exhaust is vented).
- Electric Pumps: Eliminate the emissions associated with the pneumatic drive gas and offer more precise control over the circulation rate. They are a much more environmentally conscious choice but require a reliable source of electric power or a dedicated solar/battery system.
- Glycol-to-Water Ratio: What is the design glycol circulation rate? This is typically measured as the ratio of gallons of TEG circulated per pound of water removed gal TEG lb H2O. A standard rule-of-thumb is 3 gal/lb, but a well-designed unit should target a lower, more efficient ratio (e.g., 1.5 to 2.5 gal/lb to minimize operational costs and emissions.
7. Filtration and Maintenance: What Pre- and Post-Filtration is Included?
Contaminants are the enemy of an efficient dehydrator. Hydrocarbon liquids, corrosion products, and particulates can lead to foaming, fouling, corrosion, and poor dehydration performance.
- Inlet Gas Scrubber: Is a highly efficient, properly sized inlet scrubber/separator included immediately upstream of the contactor to remove free liquid hydrocarbons and water droplets?
- Glycol Filtration: What filtration is provided in the lean glycol loop?
- Particulate Filter: A cartridge filter (typically 5 micron) to remove solids. What is the filter housing size and material?
- Activated Carbon Filter: An essential component for removing dissolved hydrocarbon liquids and degradation products that cause foaming. Is it sized for continuous or batch use? What is the recommended change-out frequency?
- Metallurgy and Corrosion: Given your gas composition (especially H2S or CO2), is the unit’s metallurgy (e.g., carbon steel, stainless steel) appropriate, and does the vendor recommend a corrosion inhibitor injection package?
8. Utilities and Heat Source: How is the Reboiler Heated?
The reboiler requires a significant amount of heat to boil off the water.
- Heat Source Type: Is the unit direct-fired (a burner heats the reboiler shell directly), or does it use an indirect heat medium (hot oil, steam, or electric immersion)?
- Direct-Fired: Most common and simplest, but poor heat distribution can lead to localized “hot spots” that degrade the glycol faster, reducing its life and efficiency.
- Indirect/Electric: Provides more uniform, controlled heating, which prolongs glycol life and is essential for remote or unmanned operations, though the CAPEX is often higher.
- Fuel Gas: How much fuel gas (MMBTU/hr) is required to operate the reboiler at maximum water load? This is a core OPEX consideration. Is the fuel gas taken from the main gas stream, or is an auxiliary fuel source required?
9. Design Codes, Documentation, and Certification: Is the Unit Code Compliant?
Ensuring the unit is built to recognized industry standards is critical for safety, regulatory compliance, and insurance.
- Design Codes: Will the pressure vessels (contactor, flash separator) be stamped and registered in accordance with the ASME Boiler and Pressure Vessel Code, Section VIII, Division 1?
- Instrumentation and Safety: Does the unit comply with industry standards (e.g., API, ISA), and does it include a comprehensive set of safety systems (Pressure Safety Valves (PSVs), High-Temperature Shutdowns, Low-Level Shutdowns, etc.)?
- Documentation: What level of documentation is provided? This should include certified Piping and Instrumentation Diagrams (P&IDs), mechanical design drawings, an exhaustive equipment list, material certifications, and a detailed, site-specific operating and maintenance manual.
10. Vendor Experience, Support, and Warranty: Who is Building Your System?
The best equipment is only as good as the company that stands behind it.
- Experience with Specific Conditions: Can the vendor provide references for similar units operating in environments with comparable gas compositions, flow rates, and climate conditions (e.g., extremely cold winters or high ambient temperatures)?
- After-Sales Support: What is the geographical reach and response time for field service, troubleshooting, and spare parts? Is the vendor just a fabricator, or do they offer full lifecycle support, including startup assistance, optimization, and training?
- Warranty: What are the terms of the equipment and performance warranties? For example, is there a guarantee on the final achieved dry gas water content under the specified operating conditions?
Conclusion: Due Diligence Drives Efficiency
Purchasing a glycol dehydrator is a long-term operational commitment. By thoroughly asking and evaluating the answers to these ten critical questions, you move beyond a simple price comparison and into a detailed assessment of the unit’s long-term performance, reliability, and total cost of ownership (TCO). A well-specified unit is an efficient unit—one that consistently meets pipeline specifications, minimizes emissions, and maximizes the profitability of your natural gas stream.
Contact CROFT today!
Contact us today, call our office to talk to a sales representative or email [email protected]
