Improving TEG gas dehydration operations

Glycol dehydration is by far the most commonly used process for removing water vapor from natural gas, whether sweet, sour or aromatic. The use of all-welded plate heat exchangers in place of shell & tube units can save space and lengthen cycles between maintenance.

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Plate HEs, with their high U values and close temperature approach, offer the potential for exchanger consolidation from individual units into two-in-one, multiple-core units sharing one common channel. With a review of the dehydration process, the potential for improved operation becomes clear.

The dryness of the gas is measured in terms of residual water content or of dew point temperature at a defined pressure. Removing water improves the heating value of the gas, prevents hydrate formation, corrosion and maximizes pipeline efficiency. Additionally, sales gas contracts and/or piping specifications have a maximum water content (0.11 kg/km3).

Triethylene glycol (TEG) is the most common glycol used for closed loop dehydration and regeneration. The regeneration of water-rich, spent TEG is generally obtained by reboiling at atmospheric pressure. The highest reboiling temperature allowed is limited by the initial thermal degradation of glycol. For this reason at atmospheric pressure the highest purity obtainable for TEG is of 98.8 wt%.

Why TEG?
Glycols are liquid desiccants, or fluids that have an affinity for water. Depending on process requirements three different types of glycol desiccants are used: ethylene glycol (EG), diethylene glycol (DEG) or triethylene glycol (TEG). TEG is the most widely selected for its:
• Easy regeneration
• Relatively high decomposition temperature of 206ºC (DEG is 164ºC).
• Low volatility and high thermal stability, enabling TEG to achieve very low dew points
• Lower vapor losses than EG or DEG
• Relatively low viscosity above 21ºC, reducing pumping load

TEG dehydration is also cost effective vs. solid desiccants, exhibiting a lower pressure drop and the ability to operate in a continuous rather than a batch mode.

The Process
Figure 1 presents a typical flow diagram for the TEG dehydration process. The water-rich natural gas is passed through an absorber, where TEG comes in contact with the gas. The absorber or contactor is a column either with bubble cap trays or random structured packing. Under high pressure and low temperature (~30-55°C), the water is absorbed by the TEG in a countercurrent flow. The dehydrated gas leaves at the top, while a water-rich TEG phase is withdrawn at the bottom of the column. The rich TEG is then fed to a stripping column where the water is evaporated and then returned back to the absorber in a continuous loop.

Fig. 1: TEG gas dehydration flow scheme showing two-in-one lean/rich interchanger.

Often the gas to be dehydrated is sour or rich in aromatic compounds. TEG absorbs not only water but also other polar components such as CO2, H2S, benzene, toluene and xylene (BTX) and other volatile organic compounds (VOCs). These volatiles are then released during the regeneration of the water-rich TEG and must be processed before discharge to the atmosphere.

The TEG gas dehydration process is employed at the production site (including offshore platforms) as well as in refineries. The equipment can be incorporated into pre-engineered modules or skids (Fig. 2). To attain target dehydration with consistency, TEG must be kept free from contamination and decomposition by excessive heat.

Fig. 2: Tranter ULTRAMAX all-welded plate heat exchanger in lean TEG/rich TEG service on a gas dehydration skid.

The Advantages of Plate Heat Exchangers
Until recently, heat exchangers in elevated pressure/temperature or corrosive media applications were by necessity shell & tube (S&T) units. Gasketed plate heat exchangers could not withstand the process conditions. Use of S&T units involved tradeoffs in thermal efficiency, initial cost, material mass and excessive physical footprint.

All-welded plate heat exchangers, introduced in recent years, have presented solutions to some of the traditional S&T limitations. All-welded units have proven to withstand challenging process conditions with liquids, gases, steam and two-phase mixtures, including aggressive media, organic solvents and steam (SIDEBAR). Compact, these all-welded plate units require only 30–50% of the space of equivalent S&T exchangers (Table I).

Table I. PHE Comparative Footprints For Largest Size Standard Units.

All-welded plate units offer the distinct advantages of high plate heat transfer efficiency, due in large measure to the turbulent flow created by the corrugated patterns of their plates. This same turbulent flow keeps the plates free from scaling and fouling longer than does the laminar flow seen in S&T units. 

Because of their high efficiency, all-welded plate units can handle temperature approaches of less than 1°C. Their small hold-up volume provides fast start-ups and close following of process changes. With a much smaller mass of expensive alloys, all-welded units cost less and can be fabricated faster than S&T units. This advantage is particularly important in offshore operations using sea water as a coolant, where titanium is the alloy of choice. All-welded plate units enjoy a significant price advantage, especially in larger-duty sizes.

When it is time to open the exchanger for cleaning, all-welded units offer advantages, also. The S&T unit must have a service space equal to its length to pull the tube bundle. Because of its thermal efficiency, the Tranter SUPERMAX® Shell & Plate unit, for example, is about seven times shorter than an equivalent S&T unit. Thus, the core pulling length is also seven times less. This can be important on space- and static load-limited offshore platforms.

TEG Dehydration HE Applications
Gas dehydration involves the use of several heat exchangers, which traditionally have been S&T units. All of these applications have proven to be compatible with all-welded plate units.

• Hot Lean/Rich TEG Exchanger. An all-welded plate heat exchanger preheats the rich TEG to 150–175°C prior to entering the stripper using the hot (193–204°C), lean TEG leaving the reboiler (Fig. 3; HE-1). This unit improves the thermal efficiency of the process, minimizing fuel required to operate the reboiler.
• Cold Lean/Rich TEG Exchanger. Another all-welded plate HE (Fig.3 HE-2) takes the ~80°C lean TEG from the HE-1 outlet to heat the rich TEG from the reflux condenser to at least 65°C at 3.5–7 bar. This temperature and pressure is optimal for skimmer/flash tank operation, which separates the rich TEG from hydrocarbon condensate and gas.
• Combined, Two-In-One Lean/Rich TEG Interchanger. A recent refinement in TEG heat exchange is employment of a two-in-one (double section), all-welded glycol/glycol plate heat exchanger (Fig. 1, HE-1/2). The unit combines both the hot and cold lean/rich exchangers in a single, compact unit. This execution saves further footprint and piping requirements. The all-welded plate interchanger operates with U values around 600-800 W/M2•K – several times higher than a S&T unit, which helps make this performance possible.
• Two-in-one TEG/TEG applications have used both the ULTRAMAX® all-welded unit and the SUPERMAX® all-welded shell & plate unit. A significant requirement for the two-in-one application is a low pressure drop from the reboiler, because feed is by gravity only. The SUPERMAX with the hot channel on its shell side is particularly suited to meet the pressure drop requirement.
• Lean TEG cooler. An all-welded plate heat exchanger (HE-3) after the HE-2 lean/rich TEG unit cools the lean TEG to approximately 85°C to protect the pump that lifts the lean TEG to the inlet of the wet gas absorber. Seawater or fresh water is normally used as the cooling medium.
• Gas/Glycol exchanger. Depending on actual process conditions, a gas/glycol exchanger (HE-3a) can be used between the lean TEG pump and the wet gas absorber tower inlet. The dry gas from the tower provides the cooling medium.

Advantages Becoming Known
Gas refining operations are beginning to enjoy the advantages of all-welded plate HEs. With their easier servicing and cleaning, particularly regarding the lean TEG cooler and the vent condenser, they are reducing maintenance burdens. As their advantages become more known, all-welded plate HEs are likely to take on additional duties in the refinery and on the platform.

Fig. 3: Detail showing alternate plate HEs and hook-ups.