Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
2001-04-12
2002-09-24
Doerrler, William C. (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S643000
Reexamination Certificate
active
06453698
ABSTRACT:
FIELD OF INVENTION
The present invention is directed towards methods for more efficient and economical separation of hydrocarbon gas constituents and recovery of both light gaseous hydrocarbons and the heavier hydrocarbon liquids. The present invention provides methods for achieving essentially complete separation and recovery of heavier hydrocarbon liquids. More particularly, the methods of present invention more efficiently and more economically separate ethane, propane, propylene and heavier hydrocarbons liquids from any hydrocarbon gas stream i.e. from natural gas or from gases from refinery or petroleum plants. Additionally, the present invention utilizes a process scheme which can be used for high ethane recovery during ethane recovery mode and high propane recovery during ethane rejection mode of plant operation. Thus, the process scheme proposed under the present invention provides additional flexibility to the plant operator to adjust to market conditions without requiring major process scheme changes.
DESCRIPTION OF THE BACKGROUND
In addition to methane, natural gas includes some heavier hydrocarbons with impurities e.g. carbon dioxide, nitrogen, helium, water and non-hydrocarbon acid gases. After compression and separation of these impurities, natural gas is further processed to separate and recover natural gas liquids (NGL). In fact, natural gas may include up to about fifty percent by volume of heavier hydrocarbons recovered as NGL. These heavier hydrocarbons must be separated from methane to be recovered as natural gas liquids. These valuable natural gas liquid comprises of ethane, propane, butane and other heavier hydrocarbons. In addition to these NGL components, other gases, including hydrogen, ethylene and propylene may be contained in gas streams obtained from refinery or from petrochemical plants.
Processes for separating hydrocarbon gas components are well known in the art. C. Collins, R. J. Chen, and D. G. Elliot have provided an excellent general overview of NGL recovery methods in a paper presented at Gas Tech LNG/LPG Conference 84. This paper, entitled “Trends in NGL recovery for natural and associated gases” was published by Gas Tech, Ltd. of Rickmansworth, England, in the transactions of the conference on pages 287-303. The pre-purified natural gas is treated by well known methods including absorption, refrigerated absorption, adsorption and condensation at cryogenic temperatures down to −175 F. Separation of the lower hydrocarbons is achieved in one or more distillation towers. The columns are often referred to as de-methanizer or de-ethanizer columns. Processes employing a de-methanizer column separate methane and other volatile components from ethane and less volatile components in the purified natural gas liquids. The methane fraction is recovered as purified gas for pipeline delivery. The ethane and less volatile components, including propane, are recovered as natural gas liquid. In some applications, however, it is desirable to minimize the ethane content of the NGL. In those applications ethane and more volatile components are separated from propane and less. volatile components in a column generally called the de-ethanizer column.
Arn NGL, recovery plant design is highly dependent upon the operating pressure of the distillation column(s). At medium to low pressures i.e. 400 Psia or lower, the recompression horsepower requirement (to compress the residue gas to pipeline pressure) will be so high that the process becomes uneconomical. However, at higher pressures the recovery level of the hydrocarbons will be significantly reduced due to the less favorable separating conditions i.e. lower relative volatility inside the distillation column. Prior art has concentrated on operating the distillation columns at a higher pressure i.e. 400 Psia or higher while maintaining the high recovery of liquid hydrocarbons.
Many patents have been directed to methods for improving this separation technology. U.S. Pat. Nos. 4,596,588, 4,171,964, 4,278,457, 4,687,499, 4,851,020 describe relevant processes.
While prior art has been capable of recovering more than 98% of propane, propylene and heavier hydrocarbons during the ethane recovery mode, most of these processes fail to maintain the same recovery when ethane is unwanted i.e. in the ethane rejection mode. In order to achieve these goals for high propane recovery with ethane rejection, some systems have included two towers one operating at a higher pressure and one at a lower pressure.
A significant cost in the NGL recovery processes is related to the refrigeration required to chill the inlet gas. Refrigeration for these low temperature schemes is generally provided by using propane or ethane as refrigerants. Refrigeration is also provided by turboexpander processes as described below. For richer gases, containing a significant quantity of heavy hydrocarbons, a combined turbo-expander and external refrigeration process is the best approach.
In order to achieve the high propane recovery, with ethane rejection, some processes proposed in the prior art involve two tower processes i.e. these processes involve two distillation columns with one (de-methanizer) operating at a lower pressure than the other (de-ethanizer). Traditionally, for such processes four approaches to increase propane recovery have been proposed. The operating pressure of the de-ethanizer may be reduced. This approach often includes a two stage expander design to accommodate the high expansion ratio more efficiently. An alternative approach proposed in U.S. Pat. No. 4,251,249 is to install a separator downstream of the expander. This separator will separate the liquid and vapor from the partially condensed product from expander discharge. The vapor is combined with the residue gas from the de-methanizer and the liquid is sent to the de-ethanizer. However, the propane lost with the vapor from the separator reduces the recovery to 90% only. U.S. Pat. Nos. 4,657,571 and 4,690,702 propose processes which involve utilizing the overhead vapor from the de-ethanizer as a reflux in the packed top section of the de-methanizer. While high propane recovery of the order of 98% is achievable with this system, the increased recycle of methane and ethane increases the size of the de-ethanizer and both condenser and reboiler duties. In a related approach, U.S. Pat. No. 5,568,737 suggests recycling the residue gas stream from residue gas compressor discharge to be used as lean reflux in the upper portion of the de-methanizer.
In a typical cryogenic turbo expansion recovery process for high propane recovery with ethane rejection, a feed stream under pressure is cooled by heat exchange with other streams of the process and / or external sources of refrigeration such as propane compression—refrigeration system. As the gas is cooled, liquid may be condensed and collected in one or more separators as high pressure liquid containing some of the desired C
3
+ components. Depending upon the richness of the gas and the amount of liquid formed, the high pressure liquid may be expanded to a lower pressure and fractionated. The vaporization occurring during expansion causes a further cooling of the stream. Under certain circumstances pre-cooling the high pressure liquid prior to expansion may be desirable in order to further lower the temperature obtained after expansion. The expanded stream, comprising a mixture of liquid and vapor is fractionated in a distillation column (de-ethanizer). In the column, the expansion cooled stream is distilled to separate residual methane, ethane, nitrogen and other volatile components as overhead vapor product from the C
3
components and heavier hydrocarbons obtained as bottom liquid product.
The vapor resulting from the partial condensation can be passed through a work expansion machine and expanded to a lower pressure. At this lower pressure further liquid will be generated from the gas due to partial condensation due to the cooling during expansion process. The expanded stream then enters a lower part of an absorber which ope
Chen Jong Juh
Elliot Douglas G.
Jain Pallav
Lee Rong-Jywn
Yao Jame
Doerrler William C.
Drake Malik N.
IPSI LLC
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