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Dynamic Adsorbents: Alumina for Drying: How Can Activated Alumina Be Used for Gas and

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Alumina for Drying: How Can Activated Alumina Be Used for Gas and
Liquid Dehydration?

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This will become more of an issue as the United States expands the use of its available natural gas supplies. It has been anticipated that there is a sufficient reserve of natural gas to handle much of our domestic energy needs for the next 100 years if this resource is properly extracted, stored and distributed. Maximizing our natural energy supplies will greatly improve our current budget deficit and balance of trade liability.

Trends in energy demand are encouraging natural gas to make a comeback as the fastest growing source of domestic energy production. New natural gas fields are coming on line throughout North America. As these new fields are commercially developed it is essential that the gas be transported or stored devoid of water vapor and other liquids which can corrode the transport infrastructure. A most attractive method for assuring that the liquid component is removed from natural gas is through the use of a desiccant or drying agent. In terms of cost effectiveness, the most efficient method for achieving the drying of natural gas (whether “sweet” or “sour” i.e. containing significant amounts of hydrogen sulfide or carbon dioxide) is through the use of specialized activated alumina.

An overview of using activated alumina for dehydration of gases is now provided.

Natural gases from production or storage reservoirs contain water which condenses to form solid ice like crystals called gas hydrates. These block pipeline flow and control systems. Natural gas in transit needs to be dehydrated to a controlled water content to both avoid hydrate and to minimize corrosion. The dehydration of gas must being at the source of the gas in order to protect the transmission system. Dehydration of natural gas is the removal of the water associated with natural gas in vapor form. Natural gas usually contains significant quantities of water vapor. Changes in temperature and pressure condense this vapor altering the physical state from gas to liquid to solid. Unless gases are dehydrated, liquid water may condense in pipelines and accumulate at low points along the line, reducing its flow capacity.

Dalton’s Law of Partial Pressures states that the total pressure of a gaseous mixture is equal to the sum of the partial pressures of the components. This allows the computation of the maximum volume of water vapor that natural gas can hold for a given temperature and pressure. As an example one million standard cubic feet of natural gas (MMSCF) saturated at 80 degrees F and 600 PSIG (pound per square inch gauge) will hold 49 pounds of water. At 120 degrees F and at the same pressure one million square feet of natural gas will hold 155 pounds of water. Common allowable water content of transmission gas ranges from 4 to 7 pounds per MMSCF.

The three major methods of dehydration are direct cooling, adsorption and absorption. Direct cooling is based on the fact that the saturated vapor content of natural gas decreases with increased pressure or decreased temperature. Therefore, hot gases saturated with water may be partially dehydrated by direct cooling. This method is known as Joule Thomson Expansion, and is the same principal as the removal of humidity from outside air as a result of air conditioning inside a home. Molecular sieves (sieves), silica gel and bauxite were the traditional desiccants used by the natural gas industry in adsorption processes. In absorption processes the most frequently used desiccants were diethylene and triethylene glycol. Adsorption is used for cryogenic systems to reach low moisture contents. Adsorption, or solid bed dehydration uses solid materials which can be regenerated and are used over several adsorption-desorption cycles.

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