Gas separation: processes – Liquid contacting – And degasification of a liquid
Reexamination Certificate
1999-07-28
2001-03-20
Smith, Duane (Department: 1724)
Gas separation: processes
Liquid contacting
And degasification of a liquid
C095S176000, C095S177000, C095S203000, C095S204000, C095S235000, C095S236000
Reexamination Certificate
active
06203599
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention provides a process for removing gas contaminants, such as hydrogen sulfide and carbon dioxide, from gas mixtures which include these contaminants. More particularly, the invention provides a process which utilizes a solvent comprising dialkyl ethers of polyethylene glycols and water to remove gas contaminants from product gas streams, such as natural, synthesis or other product gas streams.
It is known in the art to use solvents comprising mixtures of dialkyl ethers of polyalkylene glycols to remove gas contaminants from valuable product gases. These contaminants are removed by contacting the contaminated product gas with fresh solvent in an absorber or other specialized equipment operated under conditions of high pressure and/or low temperature which are favorable for absorption. Once the contaminants are removed, the decontaminated gas is ready for sale or for additional downstream conditioning, depending on the product stream specifications. The solvent is regenerated for reuse by driving off the absorbed contaminants under low pressure and/or high temperature conditions favorable for desorption. Flash tanks and/or stripper columns are typically used to effect this separation.
The use of dialkyl ethers of polyethylene glycols to remove gas contaminants commonly results in the co-absorption of valuable product gas, methane for example. The co-absorbed gases generally emerge from the solvent during regeneration and generally have little or no value because of the high concentration of undesirable contaminants. The loss of valuable co-absorbed gas is typically reduced by adding a high pressure recycle loop to the system design. The high pressure recycle loop allows partial regeneration of the rich solvent, and the gases liberated in the recycle loop, a mixture of valuable product gas and undesirable gas contaminants, are re-compressed, cooled and recycled back to the absorber where the liberated product gas is recovered. As is well understood in the art, the term “high pressure” is used to characterize the loop because the flash tank included within the loop operates at a pressure below the absorber pressure but above the pressure at which the solvent is finally regenerated.
While a high pressure recycle loop increases product gas revenue, capital and operating expenses for systems including a recycle loop are also increased. The high pressure flash tank, compression equipment and heat exchangers required for the recycle loop all have associated costs. In addition, increased gas flow through the absorber often increases the absorber circulation requirement, which necessitates larger high pressure pumps, heat exchangers, piping and other components throughout the entire system.
Economic optimization between increased product gas recovery and increased capital and operating costs is commonly attempted by adjusting the recycle flash tank pressure. High flash tank pressures result in low recycle gas volumes, low product gas recovery and low additional costs. Low flash tank pressures result in high recycle gas volumes, high product gas recovery and high additional costs. However, properly balancing these parameters has proven to be quite difficult, and adjusting the flash tank pressure has generally not provided an effective means of offsetting the increased capital and operating expenditures associated with systems which include a high pressure recycle loop and the need to maximize product gas recovery.
SUMMARY OF THE INVENTION
The present invention provides a process for removing gas contaminants from a product gas using a solvent comprising dialkyl ethers of polyethylene glycol and water. The process includes a high pressure recycle loop to reduce the loss of co-absorbed product gas. However, because the solvent also comprises sufficient water, the capital and operating costs typically associated with a high pressure recycle loop are significantly reduced.
The process is particularly useful for removing gas contaminants such as hydrogen sulfide and carbon dioxide from product gases such as natural gas and synthesis gas. It should be understood, however, that the invention is in no way limited in this regard and that the process may be used to remove diverse sulfur compounds, carbonyl sulfide and other gas contaminants from a variety of commercially valuable product gases.
According to the invention, a product gas containing gas contaminants is contacted with the solvent in at least one absorption stage. A specified amount of the gas contaminants are removed from the product gas by the solvent, and decontaminated product gas is discharged from the absorption stage. The solvent, which contains dissolved gas contaminants and co-absorbed product gas, is then partially regenerated in a high pressure recycle loop, where a portion of the dissolved contaminants and co-absorbed product gas is released from the solvent as a recycle gas. The recycle gas is re-compressed, cooled and then returned to the absorption stage. Regeneration of the solvent is completed by removing a residual portion of the dissolved acid gas contaminants and co-absorbed product gas remaining in the solvent. The co-absorbed product gas and gas contaminants removed from the solvent in this regeneration stage are released as a discharge gas. The fully regenerated solvent is then returned to the absorption stage.
The solvent is provided with a sufficient amount of water to increase recovery of co-absorbed product gas, while at the same time providing a reduced circulation rate requirement for the solvent and reduced re-compression and cooling requirements for the recycle gas. Accordingly, the use of the solvent provides a reduction in the capital and operating costs typically associated with processes which utilize a high pressure recycle loop, most significantly the costs associated with re-compression and cooling requirements for the recycle gas, while simultaneously providing increased recovery of product gas.
In the preferred embodiment of the invention, the solvent comprises a mixture of dimethyl ethers of polyethylene glycols and sufficient water to provide the solvent with from about 2 wt % to about 10 wt % water on a gas free basis. Most preferably, the solvent includes from about 5 wt % to about 8 wt % water on a gas free basis.
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Echt William I.
Schubert Craig Norman
Greene Jason M.
Kentoffio J.
Smith Duane
Union Carbide Chemicals & Plastics Technology Corporation
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