Refrigeration – Cryogenic treatment of gas or gas mixture – Separation of gas mixture
Reexamination Certificate
1999-02-22
2001-02-06
Doerrler, William (Department: 3744)
Refrigeration
Cryogenic treatment of gas or gas mixture
Separation of gas mixture
C062S630000
Reexamination Certificate
active
06182468
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to the separation of hydrocarbon gases into components of differing boiling points. The invention relates more specifically to a method and an apparatus especially suited for separating propane, methane or ethane from natural gas.
The applicant's prior U.S. Pat. No. 4,770,683, issued Sep. 13, 1988, describes a process and an apparatus for distillation of two materials of differing boiling points. A process for distillation of two materials of differing boiling points particularly propane, ethane or carbon dioxide from natural gas is described in which the conventional distillation tower is divided into a first tower at higher pressure than a conventional tower and a second tower at lower pressure. Liquid drawn from the first is expanded to the lower pressure through two or more stages with cool extracted at each stage and used to cool gas withdrawn from the top of the first tower to keep the top tray at a required temperature. Gas withdrawn from the second tower is compressed and cooled for return to the first tower as a reflux. The use of the cool from the expanded liquid and the use of the two towers provides an improved thermodynamic efficiency and avoids the use of costly turbo-expanders.
In addition, a further arrangement by the present applicant is disclosed in PCT published application WO95/10011 of Apr. 13, 1995. This discloses an improvement to the above patent in which efficiency is enhanced by the provision of a third tower and an arrangement by which additional cool is supplied to the top of the high pressure tower as a reflux.
Traditionally natural gas at less than 100 psig has been ignored for lpg recovery. Whenever such gas is processed, it is first compressed to above 300 psig before processing. However the process of the present invention, used for separation of various materials of close boiling points generally uses a distillation tower arrangement.
This invention is particularly concerned with separation of heavier components from natural gas.
Ethane recovery is similar to lpg recovery in concept except that more energy is required for refrigeration and reflux compression. This process also applies to situations where the low pressure gas is sold at higher pressures but the benefits compared to other processes are much less than that described in the first paragraph where, essentially there are no other processes that are ever considered unless the desired residue gas pressure for the sales pipeline is above 200 psig. The use of this technology for the recovery of ethylene in ethylene plants, will reduce the power requirements and capital cost of the Demethanizer portion of these plants. The above U.S. patent of the applicant was described as being very applicable to the separation of ethane and ethylene. That patent could also be used for the Demethanizer in an Ethylene Plant but it is believed that this patent will be an improvement when combined with that patent.
This invention relates to distillation processes for the separation of close boiling point materials. Such a process is used in the extraction of various materials generally using a distillation tower. Examples of such separations are:
1. Recovering ethane from natural gas
2. Recovering propane from natural gas
3. Recovering carbon dioxide from natural gas
4. Recovering helium from natural gas
5. Rejecting nitrogen from natural gas
6. Recovering ethylene in ethylene plants.
This patent has optimal advantage when utilised in conjunction with a two tower or multi-tower process described in the above United States patent. It may also be used to advantage with other distillation patents for example the various arrangements described in patents held by the Ortloff Corporation.
SUMMARY OF THE INVENTION
It is one object of the present invention to provide an improved method for separation of residue gases from natural gas which provides improved efficiencies particularly for processing gases where the residue gases are supplied at low pressure.
It is one object of the present invention, therefore, to provide an improved distillation process which obtains as good or better separation recoveries but with an improved thermodynamic efficiency and in many cases reduced equipment cost. It is expected that this patent will improve the Demethanizer portion of ethylene production plants.
It is another object of the present invention to provide economical means of recovering ethane and/or lpg from low pressure natural gas that is otherwise ignored for liquid recovery.
According to the first aspect of the invention it is provided a method of separating heavier components from natural gas comprising:
providing a distillation tower arrangement for separating the heavier components discharged at the bottom of the tower arrangement from lighter gas discharged at the top of the tower arrangement;
providing a feed gas containing the components under a first pressure sufficient to supply the gas to the distillation tower arrangement;
separating the feed gas into a first proportion and a second proportion, where neither proportion is zero
feeding the first proportion to the distillation tower arrangement at a feed position thereon between the top and bottom thereof for separation within the distillation tower arrangement;
compressing the second proportion to a pressure higher than the first pressure, extracting heat from the compressed second proportion to effect condensation thereof, sub-cooling the compressed condensed second proportion, expanding the compressed condensed second proportion to the first pressure and supplying the second proportion after expansion to the distillation tower arrangement at a position thereon adjacent the top of the distillation tower arrangement so as to cause cooling of the materials in the distillation tower arrangement.
The lighter gas discharged at the top of the tower arrangement is supplied at a pressure which is selected depending the requirement of the supply. In many cases this is a high pressure requirement greater than 100 psig and often of the order of 500 psig. This invention however has particular applicability and advantage when the supply pressure is less than 100 psig thus leading to a low operating pressure.
Preferably the second proportion is sub-cooled by cool from the lighter gas.
Preferably the second proportion is further sub-cooled by a refrigerant.
Preferably the feed gas is dehydrated prior to separation.
Preferably the feed gas is dehydrated by a molecular sieve.
Preferably the first proportion is cooled by cool from a re-boiler of the tower arrangement.
In one example the tower arrangement includes at least two separate towers at different pressures such that the heavier product from the higher pressure tower is expanded and fed to the lower pressure tower. In one arrangement of this example, the lighter gas from the top of the lower pressure tower is fed back to the feed gas for reprocessing. In an alternative arrangement, the gas is flared.
Preferably the lighter gas from the top of the tower arrangement is supplied to a low pressure pipe line system having a supply pressure of the order of 75 psi.
Preferably the lighter gas from the top of the lower pressure tower is added to the second proportion for processing therewith and supply to the top of the higher pressure tower.
Preferably the supply gas is separated from a crude oil supply and wherein the separated heavier components are returned to the crude oil as a supplement thereto.
Preferably the separated lighter gas is flared.
According to a second aspect of the invention there is provided an apparatus for separating heavier components from natural gas comprising:
a distillation tower arrangement for separating the heavier components discharged at the bottom of the tower arrangement from lighter gas discharged at the top of the tower arrangement;
a feed gas supply line for a feed gas containing the components under a first pressure sufficient to supply the gas to the distillation tower arrangement;
means for separating the feed gas into a firs
Battison Adrian D.
Doerrler William
Thrift Murray E.
Ultimate Process Technology
Williams Michael R.
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