MEG reclaiming technology: From mobile usage to the world’s largest unit
The formation of hydrates in deepwater natural gas tie-back lines historically was inhibited by injecting triethylene glycol (TEG) or methanol at the subsea manifold. Gelling of the TEG at the manifold or in the delivery umbilical, and lack of recovery of the methanol, were persistent drawbacks to their continuous use.
CCR Technologies Ltd. (CCR) was instrumental in moving the offshore flow assurance mindset away from TEG and methanol, and toward monoethylene glycol (MEG) as the hydrate inhibitor-of-choice for multiphase tie-back production lines. The world’s first commercial MEG Reclaiming Unit (MRU) started up in 1999 for Shell’s Mensa WD-143 offshore project in the U.S. Gulf of Mexico.
Following an agreement between Prosernat and CCR, Prosernat is now the exclusive licensor of the MEG reclaiming proprietary process developed by CCR in the 1990s.
The technology has its origins in mobile reclaimers. CCR has directly operated its wholly-owned mobile processing units since 1988. The hands-on knowledge gained from over 700 mobile campaigns during the past 25 years has confirmed the technology, and enabled it to evolve continuously. This know-how has been incorporated into all CCR MEG Regeneration and Reclaiming Units used in the flow assurance arena. These processing systems essentially are industrial-scale laboratories on wheels, making them distinctive from the systems of other technology providers.
CCR–PATENTED VACUUM DISTILLATION
In subsea developments, the injected MEG is contaminated by salts (from formation water), water (condensed water), and other contaminants, such as pipeline scale, etc. The water has, historically, been removed by a conventional MEG regenerator (kettle reboiler style), but this leaves the other contaminants, such as salts and solids, in the regenerated lean MEG. Over time, they will build up in the lean MEG inventory to the point, where they will deposit on process equipment and cause accelerated corrosion in topside equipment and subsea pipelines.
Conventional regeneration of glycols at elevated pressures involves boiling at an appropriate temperature with a heating medium that will concentrate the MEG to its original level for reuse as the hydrate inhibitor. However, the higher temperatures required to do this regeneration can cause thermal degradation of the glycols, which, in turn, can reduce their effectiveness, as well as contribute to corrosion through the formation of acid degradation products. Eventually, the salts remaining in the glycol, increase to unmanageable levels.
Reclaiming by means of vacuum distillation has been used, in an attempt to reduce reboiler temperatures, but thermal degradation and solids deposition still occur in static reboiler systems. Although the temperature is kept low enough in the bulk fluid to avoid thermal degradation, static type reclaimers develop elevated film wall temperatures with long contact times on the tube bundles. Not only does this thermal degradation lower the solvent effectiveness leading to revenue losses, but the amount of waste generated increases, thereby increasing waste disposal costs.
To avoid the problems associated with static reclaimer designs mentioned above, Prosernat proposes a CCR-patented and proprietary vacuum distillation process with a novel heater design.
With the CCR design, elevated film wall temperatures and long residence times do not occur, and bulk fluid and skin temperatures are kept close to each other, thereby inhibiting scaling tendencies. The CCR design also operates with free salts in the heater loop that gently polish the exchanger and piping internals, further reducing any scaling tendency. The CCR technology can control the free solids content in its recirculation heater loop to within 1 weight percent (wt%), if so desired. Depending on the customer’s requirements, and type of solids disposal route selected, designs can be adjusted to provide virtually liquid-free solids to fully dissolved salt solution concentrates for final disposal.
UNIQUE DESIGN PACKAGES
Each MEG Regenerator and Reclaimer Unit design is unique. Consideration is given to the rich MEG physical and chemical make-up, projected volumes, and desired quality of lean MEG to be returned subsea, so that the most optimal system is supplied to the end-user. The typical configurations (with their benefits and drawbacks) are provided in Table 1.
By definition, configuration 1 results in the lowest capital and operating cost. This is because there are no salts or contaminants in the Rich MEG, other than the condensed water from the production. This configuration also has the lowest flexibility to react to changes in the formation water. As soon as any contaminants, such as formation salt, enter the system, they will begin to accumulate and foul the regenerator. The good news is that a reclaimer can be retrofitted into the system.
Configuration 2 can be considered, when the amount for formation salts is relatively low. This configuration includes a small reclaimer, which processes only a portion of the lean MEG exiting the regenerator. This optimized system can accommodate a small increase in formation salts in the future, so it is somewhat flexible compared to Configuration 1.
Configuration 3 is the same as Configuration 2, except that this reclaimer treats 100% of the lean MEG that exits the regenerator. This system is almost immune to changes in salt levels in the formation water. The other benefit of this configuration is that is can be operated as a slipstream unit (like Configuration 2) at any time without any changes to the equipment design.
Configuration 4 is an integrated unit, which combines the regeneration and reclamation processes into a single step. This is typically used in systems, where the flowrate of rich MEG is relatively low, compared to the level of contaminants in the feed to the unit. It should be noted that pre-treatment (removal of low soluble salts) is not needed with configuration 4, since the process is capable of removing these salts from the flow assurance loop. Pre-treatment may be considered for configurations 2 and 3.
Analyses and modeling in early stages of a flow assurance study (typically part of conceptual/FEL-1 studies) should consider utilizing the Institute for Energy Technology (IFE), the Research Institute for Energy and Nuclear Technology in Oslo, Norway, to define critical parameters around the flow assurance loop. Valuable insight is provided, not only for the design of the MRU, but by modeling the entire flow assurance loop to determine corrosion and scale inhibition programs.
APPLYING MRU TECHNOLOGY IN SAUDI ARABIA
CCR technology was selected by Saudi Aramco for the MRU package at the Wasit gas plant, the largest MEG Regeneration and Reclaiming Unit in the world.
The design of this facility required the removal of a large amount of water across three parallel trains, designed to process 66 m3/hr, each, of rich MEG. Due to the low salt content in the rich MEG, a conventional regenerator system was utilized, with the reclaimer package being included downstream of the system. On reviewing the entire flow assurance loop, it was determined that a maximum salts content of 2 wt% could be re-injected, with the lean MEG bypassing the reclaimer system. By allowing some of the salty lean MEG to be bypassed around the reclaimer, treatment for removal of the divalent cations is required.
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