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Removing salts and contaminants in glycol regeneration

A new process combining candle filters and pressure plate filters enables more glycol to be safely recovered from natural gas than by conventional methods

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Article Summary
For the transport of liquefied natural gas (LNG) in pipelines, monoethylene glycol (MEG) is added as corrosion inhibitor and to reduce the freezing point of the water in the LNG. The MEG is then reclaimed.

The rich MEG (high water content up to 70% and containing dissolved salts) is run through a MEG reclamation unit and the lean MEG (water content reduced to 10%) is produced by evaporating the majority of the water. The MEG is recycled and reused in the system, leading to an increase of salts concentration during the evaporation process. Although the majority of the salts are monovalent salts such as sodium chloride, there are also divalent salts, such as carbonates (calcium, sodium) and hydroxides (magnesium, iron). These salts along with pipeline debris and oxidation products end up in the heat exchangers and jeopardise their performance.

As a solution to the above, the reclamation unit normally contains a pH adjustment step, allowing the divalent salts to precipitate after adding sodium hydroxide and seeding crystals. The salts and other contaminants can be removed from the rich MEG prior to entering the thermal regeneration section.

This article discusses the use of a combination system for solid-liquid filtration as an alternative to static thickeners, centrifuges, filter presses and cartridges. In the first stage, the low concentrated slurry is pre-thickened with the use of candle filters to achieve a solids-free filtrate and a concentrated sludge of approximately 4-5% solids. In the second stage, the sludge is then washed to produce an MEG-free cake for disposal and then dried for non-hazardous disposal.

This article begins with the bench-top laboratory tests for pressure, filter media, precoat material and similar process parameters. The article concludes with a case history and installation details including maintenance and reliability. 

Typically, MEG is added to natural gas for transportation in pipelines to absorb the water contained in the gas and thus lower the freezing point. At the same time, it serves as a corrosion inhibitor. The glycol is then extracted from the gas in ‘MEG reclamation units’ and regenerated.

Primarily, this involves removal of the previously absorbed water, which is loaded with various salts, depending on the location of the production well. Beyond this, MEG contains oxides and hydroxides due to piping abrasion (as rust particles) as well as other impurities. The resulting deposits clog the heat exchangers, piping and other plant components and lead to high maintenance costs.

Due to increasing environmental awareness and rising operating costs, the treatment and recovery of glycol is gaining importance. The goal of many operations is for the dried solids for disposal to contain no more than 10% glycol. Particularly for offshore applications, compact systems are required that operate automatically as well as have low maintenance requirements.

Overview of MEG regeneration
Initially for MEG regeneration, the divalent salts form a precipitate and the solids are subsequently extracted from the glycol. Depending on the location of the treatment plant, various systems are used for this solid-liquid separation.

For onshore regeneration plants, for example, static thickeners (settling tanks) are often used. 
They are cost-efficient, but the settled sludge still contains 
large amounts of glycol. Additional separation systems for example, such as high-speed separators 
and decanter centrifuges, are subject to considerable wear due to the abrasive properties of the solids.

In offshore MEG regeneration plants, maintenance of separators and centrifuges is a significant cost factor since it is frequent and requires trained specialists. This has often led to the use of back-flush or cartridge filters equipped with deep-bed filtration elements. These produce a sludge that contains large amounts of glycol and must, therefore, be processed further. The deep bed filter elements also only have limited self-cleaning properties and require time-consuming disposal. This in turn results in high operating costs as salt content increases.
Conventional single-step precoat filtration
Candle filters, as an alternative to separators and centrifuges, have proven to be particularly suitable for both onshore and offshore use. They achieve high-quality filtration, operate fully automatically and are low maintenance.

A filter cake forms in the filters and subsequently pre-dries. It is then automatically discharged into containers. Since the MEG suspension only contains a small amount of very fine solids, a precoat material, usually perlite, is applied. It ensures a high filtration rate regardless of water quality and operating state. This precoat layer prevents the filter medium from plugging with fine solids (hydroxides, dust or abrasive particles) and allows the medium to be less impacted by the hydrocarbon constituents in the MEG.

The use of candle filters results in recovering significantly more glycol than by conventional separation and has proven reliable under an extremely wide range of operating conditions. The BHS candle filter systems with precoat has been installed for these applications. A typical installation is shown in Figure 1.

While there are benefits to the candle filters with precoat, there are also increased operating costs. The precoat material is delivered in large bags and must be suspended into a precoat slurry tank with water or recirculated filtrate. A typical system operates with three candle filters, divided into 3 x 50%. The filters are offset in operation since each filter is precoated and filled anew about every eight to 12 hours.

Use of the precoat material, however, increases the amount of solids for disposal. The precoating also leads to increased costs for handling, storage and disposal costs. While this type of filtration leaves a small amount of glycol in the filter cake, there is a need to further reduce the residual glycol in the cake and reduce the amount of precoat material used.

BHS combination process
In order to further reduce the residual amount of glycol and simultaneously offer compact and low-maintenance plants, BHS has developed a new combination filtration and washing-drying process that does not use precoating or chemicals. It reduces glycol loss by displacing the residual glycol in the filter cake with water. BHS’s lab and pilot plant testing developed the process and operating parameters.
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