The dehydration of natural gas is an integral part of many gas processing applications. Dehydration with triethylene glycol (TEG) is effective and has been successfully employed in both onshore and offshore facilities.
The dehydration of natural gas is an integral part of many gas processing applications. Water can form hydrate crystals with hydrocarbons and acid gases (CO2 and H2S). These hydrate crystals can agglomerate and cause plugging problems. Water, as free aqueous liquid, can dissolve acid gases and cause severe corrosion problems as well.
Different gas processing applications have varying dehydration requirements. Sales gas plants must meet climate-specific dew-point requirements for pipeline transmission. Peak-shaving gas plants require extreme operational flexibility in gas throughput. Deep natural gas liquid (NGL) recovery, via cryogenic processes at low temperatures, requires dehydration to extremely low levels (parts per million). The offshore dehydration near the wellhead has to consider the presence of contaminants in the natural gas. These contaminants can cause operational issues. For non-stationary vessels, such as floating production, storage, and offloading (FPSO) units, tilt and motion conditions can further affect process performance of the dehydration.
TEG contactor column
Sulzer has the expertise to perform project-specific evaluations that result in optimized TEG contactor column designs. Of importance is the appropriate selection and design of the mass transfer and mist elimination equipment. An optimized TEG contactor design, when determined in the early project stage, will bring significant weight, space, and cost savings.
The TEG contactor is complex to design. It performs three different processes, namely: 1. mist elimination in the inlet scrubber section, 2. absorption of water in the TEG mass transfer section, and 3. mist elimination in the outlet scrubber section (Fig. 1).
The wet natural gas first enters the inlet scrubber section, where contaminants, like liquids and solids, are removed. The gas subsequently enters the mass transfer section, where water is absorbed into the TEG solvent from the gas. Finally, the gas goes through the outlet scrubber section, where any entrained TEG droplets are removed. It exits the column as dry gas. Over the years, Sulzer has come up with four design options (Fig. 4). Each option has an appropriate combination of mass transfer and mist elimination technologies.
TEG dehydration system
The TEG gas dehydration unit comprises more than just the TEG contactor, and it is essential to consider the overall dehydration flow scheme to achieve successful gas dehydration (Fig. 1). The TEG solvent goes through a closed circulation loop, and water is removed from TEG in the regeneration section. The regenerated TEG solvent is reintroduced into the contactor for water absorption. TEG regeneration has profound effects on the lean TEG purity and quality of dehydration. The TEG gas dehydration unit can experience various operational issues, including fouling and foaming. These challenges can be partly mitigated by properly designed mass transfer and mist elimination equipment.
Case study for new TEG column design
Exact definitions and calculations for the column design guarantee process reliability for Sulzer customers. The following case study compares four combinations of mass transfer and mist elimination technologies. The feed composition and conditions of the wet gas depend on the gas or oil well (Tables in Fig. 2 and 3). For comparable results of all four options, the design parameters, like lean TEG inlet stream data and column operating pressure, remain constant (Fig. 3). The dry gas specification is 4 lb. water (1.81 kg) per millions of standard cubic feet (MMSCF) gas at a dew point of –6.7 °C.
Fig. 3 Operating conditions of the TEG contactor for the case study.
Besides the technological aspects, this case study highlights the substantial savings in column weight and size with advanced Sulzer technologies, which are especially important for offshore installations. The consequent reduction in investment costs is considerable and can affect overall project costs significantly, thus warranting column optimization studies in early project phases.
Column sizing of the TEG contactor can be compared using the F-factor, which is defined as the multiple of the gas superficial velocity and the square root of gas density. The F-factor is the appropriate parameter to compare TEG contactors, which have characteristically high gas loads and low liquid loads. The higher the F-factor, the higher the capacity of the mass transfer and mist elimination equipment is. At a fixed feed gas rate, the column diameter can be reduced.
Comparing mass transfer technologies
In the following chart, Sulzer compares four different TEG process technology options that have been developed over the years (Fig. 4).
Option 1: Until the 1980s, TEG contactors were designed with bubble cap trays. They are designed with a low F-factor of ~1.8 Pa0.5, resulting in large column sizes.
Option 2: Sulzer MellapakTM structured packing (Fig. 5) allows the size of the column to be reduced with a higher F-factor of ~2.3 Pa0.5. The weight and cost of the column drop correspondingly. Moreover, Mellapak confers other process benefits, including an increased operating range and lower TEG flow rate requirements.
Option 3: The second generation of structured packings, MellapakPlusTM (Fig. 6) incorporates all the advantages of Mellapak and leads to even better F-factors. By gradually sloping the corrugation angle at the packing element ends, MellapakPlus performs at higher capacities and lower pressure drops without sacrificing separation efficiency. The F-factor is doubled and reaches ~4.2 Pa0.5. MellapakPlus offers unparalleled capacity amongst all mass transfer equipment that operates under countercurrent flow driven by gravity.
Option 4: For the ultimate capacity, the gravity limit must be broken. The Shell Swirl TubeTM Tray, part of the Sulzer portfolio, make use of centrifugal forces for gas-liquid contacting and disengagement. Shell Swirl Tube Trays are not typically used for new column designs but rather for revamps. They maximize natural gas throughput within an existing column.
Fig. 4 Comparison of the four different TEG contactor design options for the case study.
Comparing mist elimination technologies
Advances in F-factors due to improved mass transfer technologies must be coupled simultaneously with improvements in the mist elimination technologies. Otherwise, the size of the mist elimination equipment becomes the bottleneck in decreasing the column size. Notably, although both the inlet scrubber and outlet scrubber perform mist elimination, the process requirements are different, and they warrant different design philosophies. The development from conventional wire mesh to optimized KnitMeshTM technology, including the V-MISTERTM, improves liquid drainage and liquid handling capabilities and can offer capacity improvements. MKS Multi CassetteTM (Fig. 7) is a Sulzer-patented, hybrid mist eliminator. It combines the advantages of wire mesh and cyclonic mesh eliminators. MKS Multi Cassette offers outstanding separation efficiency and capacity, and it is competitive in terms of cost and space requirements. The F-factor of MKS Multi Cassette can be more than double that of wire mesh mist eliminators.
Fig. 7 Sulzer MKS Multi CassetteTM (Option 3) offers high efficiency and high capacity mist elimination.
For new columns, customers do not typically install the ultimate capacity designs. Capital expenditure and flexibility in use are also considered in today’s market. The MellapakPlus — MKS Multi Cassette option is the state-of-the-art combination. It results in an optimized column design together with a high F-factor, and it leads to cost-effective dehydration results. The Sulzer MellapakPlus and MKS Multi Cassette design is also the preferred choice for offshore TEG contactors. Cost, weight, and space savings are achieved not only on the column shell, but also on the offshore platform or FPSO itself. In addition, under tilt and motion conditions, structured packing has displayed significantly lower susceptibility to maldistribution over random packing and trays. That makes MellapakPlus the ideal solution for TEG contactors on non-stationary structures.