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.
