Efficient membrane systems for natural gas dehydration
Applications of membrane technology for reducing water content in natural gas to meet pipeline specifications.
MICHAEL MITARITEN, Air Liquide Advanced Separations
SAEID MOKHATAB, Gas Processing Consultant
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Membranes offer an attractive option for cases in which drying is required to meet pipeline gas specifications and conventional glycol dehydration technologies are considered unfeasible due to the process’s equipment complexity. Benefits are expected to compare favourably to conventional technologies due to reduced energy consumption, no chemicals being required in the process, and reduced emissions. With a very compact footprint and low weight, membrane systems are well suited for offshore and unmanned applications. However, more commercial demonstration is needed to gain a broader industrial acceptance. The mechanism of membrane based natural gas dehydration is reviewed using Air Liquide’s PEEK-Sep membranes, and recent practical applications are reviewed.
Natural gas streams from production wells are saturated with water vapour, which will condense or can form gas hydrates if the gas temperature is cooled below its hydrate formation temperature. Gas hydrates are solids which can agglomerate and plug pipelines and equipment, interrupting operations and stopping gas production. This can create an unsafe condition, especially if a significant pressure differential occurs across the hydrate plug. Condensed water in pipelines leads to erosion and corrosion. Water accumulation in the pipelines can lead to blockages and reduction in the pipeline flow capacity. To avoid these potential problems, the gas stream needs to be dried to lower its water dewpoint.1
To meet the pipeline specifications, natural gas must be dehydrated to meet water levels in the 4-10 lb/MMscf range (typically 7 lb/MMscf). These values provide protection against water condensation and hydrate formation during winter.1 Traditionally, triethylene glycol (TEG) units have been used for this purpose. Though widely used, glycol dehydration faces increasing environmental restrictions since the units can emit volatile organic compounds (VOCs) such as benzene, toluene, ethylbenzene and xylene (BTEX), hazardous air pollutants, and nitrogen oxide (NOx) from the fired regeneration reboiler. Handling of chemicals and maintenance requirements can make operation of glycol units in remote locations challenging. Glycol evaporation and condensation in downstream pipelines have been reported to lead to corrosion and can cause foaming problems in downstream processing units. Membrane technology is an attractive alternative to conventional glycol dehydrators for natural gas dehydration. Where feed or recycle compression is used, the membrane system removes water as a liquid with essentially zero air emissions in contrast to the glycol absorption which removes water as a vapour.
Membrane systems dehydrate the gas by passing a high pressure feed over a selective gas permeable membrane with the permeate side of the membrane maintained at a lower pressure as compared to the feed side. As gas flows over the membrane, the highly permeable gas components, typically contaminants to be removed, permeate selectively to the low pressure side and are concentrated in the permeate gas stream. Water vapour is one of the most permeable gas components. The high permeability of the water vapour as compared to other feed gas components allows membrane units to permeate the water vapour selectively and thus reduce the water content of the resulting high pressure product gas to low levels required by pipeline specifications.
Membrane units are designed to operate at pressures of up to 1200 psig with the feed gas containing 500-2000 vppm of water. The product gas stream typically contains 80-150 vppm of water and is available at near feed pressure. The volume of low pressure (1-60 psig) permeate gas is about 3-5% of the feed gas volume, though high volumes can be required to achieve low dew points.2 The small permeate flow will contain some portion of feed hydrocarbons, typically at least 3% and higher amounts depending on the permeate pressure and process conditions, along with some acid gases dependent upon the feed concentration and process conditions. This stream is best used as local fuel if the demand exists. Where the feed gas is compressed, recirculating the permeate to the feed compressor eliminates any losses, and the water is removed as a condensed liquid.
The modular nature of membrane systems, their light weight, large turndown ratio, and low maintenance make them competitive with glycol units in some situations. Based on commercial experience of field units installed and several studies, membranes are most economically attractive for dehydration of gas when flow rates are small, for example less than 10 MMscfd.3 Membrane units are also competitive with TEG dehydrators on offshore platforms at larger flow rates due to the low weight and footprint, leading to considerable platform capex savings as a result. Certainly, the reliability and simplicity of membranes make them attractive for offshore and remote site applications, provided the low pressure permeate gas is used effectively. The added benefit, compared to TEG units, is the absence of BTEX emissions with membrane systems.
Conventional membranes require additional costly and complex processing steps to protect membranes from irreversible damage by heavy hydrocarbons, liquids and chemical solvents. The inlet gas must be free of solids and droplets larger than 3µ. The inlet gas temperature should be at least 20°F above the dew point of water to avoid condensation in the membrane.2 In general, membranes require the following characteristics to meet market needs:4
• Robust, durable, chemically resistant with no costly and complex pretreatment
• Highly efficient thermodynamics (counter-current flow required – can be met only by hollow fibre devices). Spiral wound membranes operate in less efficient cross-flow mode
• Methane loss similar to or better than glycol absorption.
Membrane process limitations in natural gas dehydration
Membranes have been widely adopted for over 30 years as a process unit in gas separations. However, the use of membranes for natural gas dehydration began only 10 years ago, and these systems are still in the early commercialisation stage. For the time being, there are just a few natural gas dehydration installations, and the information available comes from experimental data and from the few small installed units.
Cellulose acetate membranes are widely used for acid gas (primarily CO2) removal from natural gas. Since water permeates cellulose acetate membranes, they can provide dehydration. However, these membranes are damaged by condensed water. For this reason, dehydration prior to the acid gas membrane is common. Other membranes such as polyimides can recover from condensed water contact, though there is still a decrease in performance until the membrane is dried.
PEEK-Sep membrane technology
PEEK-Sep membrane technology takes advantage of the exceptional chemical durability of polyether ether ketone (PEEK) polymer, which exhibits ‘best in class’ thermo- mechanical properties and chemical resistance.
The porous PEEK hollow fibre substrate is formed by a melt extrusion process. It is subsequently coated with a separation layer polymer to form the target composite gas separation membrane. The separation layer material is tailored for the target gas separation application. Targets for natural gas product purity can be achieved using either conventional or non-conventional gas permeation hierarchy, or a combination of both.
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