Distillation: processes – separatory – Distilling to separate or remove only water – From organic compound
Reexamination Certificate
1994-10-05
2002-04-23
Bhat, Nina (Department: 1761)
Distillation: processes, separatory
Distilling to separate or remove only water
From organic compound
C203S026000, C159S047100
Reexamination Certificate
active
06375806
ABSTRACT:
TECHNICAL FIELD
This invention relates to reconcentration of desiccants used in dehydration of gases. More particularly, the invention relates to reconcentration of such desiccants as glycols used in the dehydration of fuel gases such as natural gas, while controlling or preventing discharge of pollutants such as VOCs (volatile organic compounds), H
2
S (hydrogen sulfide) gas and other gaseous (including vaporous) pollutants into the atmosphere.
BACKGROUND
Heretofore, drying fuel gases such as NG with liquid desiccants such as TEG was widely acknowledged as both acceptable and effective. Many commercial units have been constructed to practice this method.
Water absorbed from the gas by the desiccant in such installations could be removed from the “wet” desiccant in reconcentrating equipment. Thus freed of most of the water, the resulting “lean” desiccant could be recycled for use in drying additional gas.
A common type of reconcentrating device included a reboiler and an associated column having a reflux section. These components cooperated to vaporize most of the water and a relatively small proportion of the desiccant, ejecting most of the water as vapor while returning most of the desiccant to the liquid phase.
An extensive patent literature has developed concerning this mode of processing, including for example U.S. Pat. No. 3,105,748 (Stahl), U.S. Pat. No. 3,347,019 (Barnhart), U.S. Pat. No. 3,370,636 (Francis, Jr., et al), U.S. Pat. No. 3,450,603 (Meyers et al), U.S. Pat. No. 3,451,897 (Welch), 3,736,725 (Alleman), U.S. Pat. No. 3,824,177 (Honerkamp et al), U.S. Pat. No. 3,841,382 (Gravis III et al), U.S. Pat. No. 4,010,065 (Alleman), U.S. Pat. No. 4,026,681 (Roskelley), U.S. Pat. No. 4,070,231 (Alleman), U.S. Pat. No. 4,182,659 (Anwer et al), U.S. Pat. No. 4,280,867 (Hodgson), U.S. Pat. No. 4,322,265 (Wood), U.S. Pat. No. 4,460,383 (Valerius) and U.S. Pat. No. 5,084,074 (Beer et al).
TEG and other popular desiccants are subject to thermal decomposition, which occurs to a greater or lesser extent depending upon the time/temperature history of the desiccant. Decomposition, if excessive, unduly increases operating costs by requiring removal of undue amounts of tars or chars from the system while also creating a corresponding need to purchase makeup desiccant. As a consequence, reboiler operating temperature has been limited to about 400° F. or less when reconcentrating TEG.
Those practicing in this art have long known that, other factors remaining equal, desiccant decomposition can be reduced through operating at lower desiccant temperatures while maintaining the total vapor pressure in the gas spaces of the reboiler and reflux sections of the reconcentrator at relatively low levels. Thus, it was quite common to operate such equipment at sub-atmospheric or at substantially atmospheric pressure. For example, see U.S. Pat. No. 4,322,265 to Wood.
In U.S. Pat. No. 5,084,074, to exclude entry of atmospheric oxygen into their system, Beer et al suggested using a slightly positive pressure in an accumulator unit indirectly connected with their reboiler. Other workers in the art have recommended beginning the water removal at super-atmospheric pressures, but completed the removal of water in a chamber at sub-atmospheric pressure without a heating device. See for example U.S. Pat. No. 3,824,177 to Honerkamp et al and U.S. Pat. No.4,182,659 to Anwer et al.
Customarily, the water ejected from the desiccant during its reconcentration was released into the atmosphere. Unfortunately, water vapor that was so released carried with it a variety of pollutants.
Because these pollutants are typically present in natural gas and other fuels, and because typical desiccants such as TEG and other glycols have an affinity for such pollutants, the pollutants were present in the used desiccant when the latter was processed in the above-described reboilers and their reflux sections. The character of the pollutants and of the conditions within the reconcentrating units were such that substantial amounts of these pollutants were released into the atmosphere in admixture with the water vapor.
Examples of such pollutants include the above-mentioned VOCs and H
2
S. The VOCs are composed primarily of aromatic compounds, such as benzene, toluene an xylene, known carcinogens. (H
2
S) is a poisonous gas. It is unfortunate that TEG, being one of the most popular desiccants for use in such processes, has a tremendous affinity for aromatics and ejects large quantities of same along with the water vapor. TEG also absorbs enough (H
2
S) to cause similar difficulties.
In response to these difficulties, persons skilled in the art began the practice of cooling and condensing the water and pollutants released from the reflux sections of reconcentrating units. Condensed VOCs were generally pumped back into the system.
Depending on the amounts of VOCs present and the amount of cooling applied to the vapors, such cooling was not always able to recover sufficient VOCs to meet environmental standards, but, when used, increased the capital and/or operating costs of the reconcentrating unit. Moreover, such measures provided only marginal control of H
2
S at best, and in many cases the fuel and system characteristics were such that H
2
S could not be adequately controlled to meet environmental requirements.
There was also some use of vapor compressors to assist in the collection of H
2
S gas, non-condensed VOCs, and other non-condensed gases after discharge at atmospheric pressure or near atmospheric pressure from the reflux section. However, due to the relatively high investment and operating costs along with additional problems associated with compressors having a suction pressure at or near atmospheric pressure, this method has not proven to be popular. This method typically required investment in a compressor dedicated to the recovery of the vapors, which would otherwise not be required in the process. In addition, small deviations in the control system controlling the vapor compressor suction pressure at or near atmospheric pressure creates upsets in the dehydration process and when the deviation allows the suction pressure to drop below atmospheric pressure, air can be introduced in the system. Air, when introduced into the dehydration system causes severe corrosion and degradation of the desiccant and, in addition, creates the possible hazard of an explosive mixture being developed in the dehydration and compression system.
A rather widely used apparatus for removing the last traces of water from partially reconcentrated desiccant was a gas stripper, as described in U.S. Pat. No. 3,105,748 to Stahl. The stripper received the partially reconcentrated liquid desiccant from the reboiler and subjected it, under conditions promotive of mass transfer, to countercurrent contact with a stripping gas, usually NG, which had an affinity for water and thus absorbed water from the desiccant. The “wet” NG was then returned to the reboiler and was eventually discharged therefrom in admixture with water vapor, VOCs, H
2
S and possibly other gases. However, from time to time, escalating NG prices have exerted economic pressure on the use of stripping, and have supported a trend in the art toward finding ways to minimize or avoid the use of stripping gas. These included, among others, vacuum distillation, atmospheric distillation in the 425-430° F. temperature range, and azeotropic distillation.
As a result of these circumstances, a need has arisen for environmentally acceptable gas drying methods which are practical and economical. The present invention was developed to meet this need.
SUMMARY OF THE INVENTION
The invention has a number of novel and non-obvious aspects, enumerated below, which may be employed singly or in any combination with one another and/or in combination with conventional reconcentration techniques:
1. reboiler gas space is maintained at total vapor pressure of about 25 to about 125 psia
2. reflux gas space is maintained at total vapor pressure of about 25 to about 125 psia
3. overheads from reflux pass to compres
Allen Process Systems, L.L.C.
Bhat Nina
Hall Priddy Myers & Vande Sande
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