CA2441136A1 - Improved thermodynamic separation of heavier components from natural gas - Google Patents
Improved thermodynamic separation of heavier components from natural gas Download PDFInfo
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- CA2441136A1 CA2441136A1 CA 2441136 CA2441136A CA2441136A1 CA 2441136 A1 CA2441136 A1 CA 2441136A1 CA 2441136 CA2441136 CA 2441136 CA 2441136 A CA2441136 A CA 2441136A CA 2441136 A1 CA2441136 A1 CA 2441136A1
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- gas
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- pressure tower
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-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D3/00—Distillation or related exchange processes in which liquids are contacted with gaseous media, e.g. stripping
- B01D3/14—Fractional distillation or use of a fractionation or rectification column
- B01D3/143—Fractional distillation or use of a fractionation or rectification column by two or more of a fractionation, separation or rectification step
- B01D3/146—Multiple effect distillation
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D3/00—Distillation or related exchange processes in which liquids are contacted with gaseous media, e.g. stripping
- B01D3/42—Regulation; Control
- B01D3/4211—Regulation; Control of columns
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B01—PHYSICAL OR CHEMICAL PROCESSES OR APPARATUS IN GENERAL
- B01D—SEPARATION
- B01D53/00—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols
- B01D53/14—Separation of gases or vapours; Recovering vapours of volatile solvents from gases; Chemical or biological purification of waste gases, e.g. engine exhaust gases, smoke, fumes, flue gases, aerosols by absorption
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
- F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
- F25J3/0204—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the feed stream
- F25J3/0209—Natural gas or substitute natural gas
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
- F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
- F25J3/0228—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream
- F25J3/0233—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream separation of CnHm with 1 carbon atom or more
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J3/00—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification
- F25J3/02—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream
- F25J3/0228—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream
- F25J3/0242—Processes or apparatus for separating the constituents of gaseous or liquefied gaseous mixtures involving the use of liquefaction or solidification by rectification, i.e. by continuous interchange of heat and material between a vapour stream and a liquid stream characterised by the separated product stream separation of CnHm with 3 carbon atoms or more
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J2200/00—Processes or apparatus using separation by rectification
- F25J2200/02—Processes or apparatus using separation by rectification in a single pressure main column system
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J2200/00—Processes or apparatus using separation by rectification
- F25J2200/04—Processes or apparatus using separation by rectification in a dual pressure main column system
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J2245/00—Processes or apparatus involving steps for recycling of process streams
- F25J2245/02—Recycle of a stream in general, e.g. a by-pass stream
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- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J2270/00—Refrigeration techniques used
- F25J2270/90—External refrigeration, e.g. conventional closed-loop mechanical refrigeration unit using Freon or NH3, unspecified external refrigeration
-
- F—MECHANICAL ENGINEERING; LIGHTING; HEATING; WEAPONS; BLASTING
- F25—REFRIGERATION OR COOLING; COMBINED HEATING AND REFRIGERATION SYSTEMS; HEAT PUMP SYSTEMS; MANUFACTURE OR STORAGE OF ICE; LIQUEFACTION SOLIDIFICATION OF GASES
- F25J—LIQUEFACTION, SOLIDIFICATION OR SEPARATION OF GASES OR GASEOUS OR LIQUEFIED GASEOUS MIXTURES BY PRESSURE AND COLD TREATMENT OR BY BRINGING THEM INTO THE SUPERCRITICAL STATE
- F25J2280/00—Control of the process or apparatus
- F25J2280/02—Control in general, load changes, different modes ("runs"), measurements
Description
IMPROVED THERMODYNAMIC SEPARATION OF IiEAVIER COMPONENTS
FROM NATURAL GAS
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.
BACKGROUND OF THE INVENTION
The applicant's prior United States Patent 4,770,683, issued September 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 in US
Patent No: 6,098,425 issued Aug 8t" 2000 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 reflex.
Ethane recovery is similar to lpg recovery in concept except that more energy is required for refrigeration and reflex 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 na 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 de-methanizer portion of these plants. The above US 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 de-methanizer in an Ethylene Plant but it is believed that this patent will be an improvement when combined with that patent.
In US Patent No: 6,182,468 issued Feb 6t" 2001 of the present Applicant is disclosed 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.
The above 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
According to the invention there is provided a method for separating a supply material into two materials of different boiling points comprising:
providing two separate vessels connected to provide transfer of materials from one vessel to the other;
feeding supply material to one of the vessels arranged to form a mixed gas and liquid phase such that one material in the vessels separates into said liquid phase and the other material separates into said gas phase;
extracting the separated gas from the top of one vessel and the separated liquid from the bottom of the other vessel;
arranging the supply and the vessels such that the pressure in one of said vessels is higher than the other;
and injecting reflux into the top of one of the towers;
wherein the 'reflex' flow rate enters the top of one of the towers under flow control so that there is a constant flow rate of flashed liquid into the top of the tower.
Preferably the flow is measured when the overhead fluid is a gas prior to cooling.
Preferably said one of the towers is the high pressure tower.
Preferably the high pressure tower is controlled by maintaining a constant ratio between this 'reflex' flow and the flow rate of dehydrated gas feed to the process.
Preferably, in order to smooth the variation of low pressure tower overhead gas, a small amount of dehydrated gas feed is added to the 'reflex' gas in order to make the desired ratio of 'reflex' flow.
Preferably, in situations, where there is an excess of overhead gas from the low pressure tower, the surplus is processed with the main dehydrated feed gas which is cooled, chilled, partially condensed arid fed to the bottom of the high pressure tower.
Preferably all of the low pressure tower overhead is fed as 'reflex' and there is added some dehydrated feed to achieve a constant 'reflex' flow rate.
Preferably a constant ratio is maintained of condensed reflex liquid to the dehydrated inlet gas or a constant mass ratio (rather than volume ar molar ratio).
Preferably there is provided a near constant flow of liquid to the top tray of the high pressure tower.
The new invention is a process control scheme to use with all of the above distillation patents. It also applies to the ~rtloff Two Tower Process and a patent by some Southern U.S. inventors who sold their idea to a company in Houston. These processes are described below.
The present invention therefare includes all of the features disclosed in the above patents to which reference should be made for further details and the subject matter of which is incorporated herein by reference.
All of my patents involve dividing a distillation tower into two or more 5 towers having different operating pressures. When the concept is utilized for recovering propane from natural gas, all of the sweetened (usually, if required), dehydrated natural gas is cooled, chilled and contacted in a high pressure tower and liquid from this tower is processed in a lower pressure tower(s). Recycle from the low pressure tower is compressed, cooled, chilled, condensed, and sub-cooled at a high pressure, typically between 700 and 1300 psia and flashed into the top tray (usually) in the high pressure tower.
The operation of the low pressure tower (known as the de-ethanizer in propane recovery situations) may vary resulting ire a variation in flow rate of its overhead vapour. The overhead vapour is compressed, cooled, condensed, sub-cooled and used as 'reflex' for the high pressure tower.
The object of this invention is to dampen variations in flow of this reflex to result in a more stable operation and a higher average propane recovery.
The 'reflex' flow rate enters the top of the high pressure tower, known as the gas fractionator (or de-methanizer) under flow control so that there is a constant flow rate of flashed liquid into the top of the de-methanizer. The flow can be measured when the overhead fluid is a gas prior to cooling. The de-methanizer is controlled by maintaining a constant ratio between this 'reflex' flow and the flow rate of dehydrated gas feed to the process.
In order to smooth the variation of de-ethanizer overhead gas, a small amount of dehydrated gas feed is added to the 'reflex' gas in order to make the desired ratio of 'reflex' flow. In situations, where there is an excess of de-ethanizer overhead gas, the surplus could be processed with the main dehydrated feed gas which is cooled, chilled, partially condensed and fed to the bottom of the de-methanizer. (Normally, it would be preferred to design to utilize all of the de-ethanizer overhead as 'reflex' and add some dehydrated teed to achieve a constant 'reflex' flow rate.) Variations of this concept could be to maintain a constant ratio of condensed reflex liquid to the dehydrated inlet gas ~or a constant mass ratio (rather than volume or molar ratio).
This concept works by providing a near constant flow of liquid to the top tray. When more make-up to the reflex stream is required from the main dehydrated gas stream that enters the bottom of the de-methanizer, there is a higher methane content in the reflex, so more flashing on the top tray and slightly less liquid leaving the top tray. There is also slightly less vapour entering the bottom tray so the ratio of liquid enter the top to vapour entering the bottom remains closer to constant.
This concept will also work for the Ortloff Two Tower process which is a variation of my first patent except that their second tower operates at a higher pressure than their first tower. To achieve this higher pressure, they must pump liquid with a cryogenic pump from the first tower to the second tower. This process is not as thermo-dynamically efficient and it requires a cryogenic pump, but does not require a compressor to recycle de-ethanizer overhead vapour for condensing and feeding to the top of the de-methanizer. In a similar manner to that described in relation to my own prior patents, the flow rate of 'reflex gas' in the Ortloff type process in accordance with my invention to the top of the de-methanizer can be smoothed with a slip-stream of dehydrated feed gas.
Also this concept will work with the mast recent of the above patents that is another variation of my first patent. This patent recycles de-ethanizer overhead product (ethane) which is cooled, condensed, and sub-cooled then fed to the top of the de-methanizer. In their process, the de-ethanizer is usually a refluxed tower, whereas I usually have a quench-fed de-ethanizer. In this design, the reflex flow would be maintained at a constant ratio to the inlet feed by using make up from the inlet dehydrated gas.
Variations of this concept could be to maintain a constant ratio of 'reflex flow' to residue gas flow leaving the top to the de-methanizer.
Another variation of this concept is to take the make-up "reflex" flow from the vapour space in the bottom of the de-methanizer. This vapour could be letdown through an expansion valve to the inlet to the top tray of the de-ethanizer. Sometimes, retrograde effects will result in this letdown vapour having a small liquid phase which would add to the reflex of the de-ethanizer. This stream also tends to load level the recycle compressor so that it has a constant inlet flow gate thus a constant inlet pressure.
Another variation of this concept utilizes low temperature separator overhead vapour as make-up. In this concept, there is a low temperature separator in the stream of partially condensed dehydrated feed which sometimes is letdown through a turbo-expander into the inlet of the de-methanizer.
Another variation would be to inflate the flow of make-up gas by a certain fraction so that it would take less make-up gas to make up for a deficiency of de-ethanizer overhead gas. Of course, in some situations, it may be better to deflate the flow of make-up gas, rather than inflate it.
In addition to recovering propane from natural gas, this concept also works for recovering ethane and heavier liquids from natural gas or recovering carbon dioxide and heavier liquids from natural gas. It also works for separating closed boiling point liquids and other separations discussed in my prior patents.
BRIEF CDESCRIPTION OF TFiE DRAV1PINGS
Figure 1 is a schematic illustration of the elements of a first process according to the present invention using the two tower system of the above prior patent which is particularly but not exclusively designed for supplying the residue gas at a low pressure.
Figure 2 is a schematic illustration of the elements of a second process according to the present invention using the single tower.
Both figures are substantially as shown in DS patent no: 6,182,468 above but have been modified to show the present invention. The following description has also been primarily taken from the above patent and includes the elements with which the present invention is concerned.
DETAILED DESCRIPTION
Turning firstly to Figure 1 there is shown an arrangement for separating Ipg+
products from a feed of natural gas leaving a residue sales gas for sale at low pressure that is less than 100 psig.
The arrangement provides a feed supply line 10 which feeds to an inlet separator 11 which acts to separate gas from any incoming liquid. The liquid can be handled in a number of different ways including supply to the free water knock out system of the crude oil processing plant in arrangements where such is available.
Alternatively, the liquid can be passed through a dehydrator and fed to the de-ethanizer.
The inlet gas from the inlet separator 11 is supplied to an inlet compressor having an after-cooler 13. The gas is compressed to a sufficient pressure in the compressor 12 so that after the compressor the gas can be dehydrated in a molecular sieve 14 and processed in the Ipg recovery plant and then has sufficient pressure for the gas entering the sales pipeline 15.
Prior to entering the dehydrator 14 in the form of a molecular sieve, a further liquid separator 16 is provided for recycling a liquid through a return fine 17 having a let down valve 18.
As stated above, the arrangement described herein is particularly designed for low pressure residue gas. However if the desired pipeline pressure in the residue gas is intended to be above 600 psig, it is preferred that compression be added to the residue gas downstream of the recovery plant so that the tower assembly described hereinafter can operate at approximately 400 psig.
After the inlet gas is compressed, after-cooled and the liquids extracted in the separator 16, the gas is dehydrated in the dehydrator 14 which is preferably a molecular sieve as described above or can possibly be a "Dryso" T"" process which is a tri-ethylene glycol process. In such an arrangement a sophisticated regeneration system as shown can be provided using extractive distillation to reduce the water content of ethylene glycol. The extracted material from the regeneration system is returned to the feed as indicated in the supply line 20.
Downstream of the dehydrator 14, there is provided a supply line 21 which is divided into two supply lines 22 and 23 acting to effect a proportional division of the feed in the supply line 21. Each line includes a flow control valve 22A and 23A which are controlled 5 using conventional flow control systems well known to ore skilled in the art to maintain the required proportions depending upon the measurement of various parameters of the process.
The process further includes a processing tower arrangement generally indicated at 30 including a high pressure tower 31 and a low pressure tower 32. These two 10 towers are generally as described in the above United States Patent and the disclosure of that document is incorporated herein by reference. The two towers each comprise a distillation tower section for effecting separation of the components in the feed so that the high pressure tower section 31 discharges lighter gas components at an upper discharge 33 and heavier components at a lower discharge 34. The low pressure tower 32 has an upper discharge 35 and a lower discharge 36. The upper gas discharge 33 provides the residue sales gas 15 while the bottom discharge 36 of the low pressure tower provides the heavier Ipg+ product 37.
The first portion of the feed gas divided into the supply line 23 is supplied as a feed to the lower part of the high pressure tower 31. Prior to supply to the tower arrangement, the gas in the supply line 23 is passed through a heat exchanger R which includes a component 38A on the fine 23 and second component 38B forming a reboiler for material at the bottom of the low pressure tower component 32. Thus the heat exchanger R
extracts cool into the component 38A to cool the supply on the line 23 and applies heat to the component 38B acting as a reboiler to return the material as a side feed to the lower part of the lower pressure component 32.
The supply on the line 23 is further passed through a second heat exchanger S having a first component 39A and a second component 39B which again acts to extract cool for the material in the line 23 and acts as a heat supply for a side reboiler on the lower pressure tower component 32.
Gas from the top discharge 35 of the low pressure tower 32 is returned to the feed through a supply duct 40. Prior to return to the feed, cool is extracted from the return gas in a further heat exchanger 41 and that cool is applied to the feed on the line 23.
Finally a refrigerator unit 42 is used to apply external cooling to the feed prior to injection into the high pressure tower component 31 at a feed position 43.
The second proportion on the line 22 is passed to a compressor system 44 including a compressor 45 and a heat extractor 46. The second proportion of the gas is compressed to a pressure in the range 500 to 1400 psig so that it can be cooled and condensed and used for injecting into the tower arrangement as a cooling top supply.
The prior patent and the prior published application of the present inventor disclose the use of liquid injection at the top of the high pressure tower for maintaining a cool temperature in the high pressure tower'. In the prior application this is termed as "reflux". However in the present invention the compressed material includes a component of the original supply from the feed 10 and in addition includes a component from the discharge gas from the discharge outlet 35 of the low pressure tower.
The second proportion is thus compressed in the compressor system 44 and cooled by the cooling arrangement 46. It is then passed through a heat exchanger 47 which extracts cool from the residue gas and supply line 48. Further cooling is effected in a further heat exchanger L which includes first component 49~ on the line 22 and a second component 49B extracting cool from the product 37. Further refrigeration cooler 50 is provided using external refrigerant. ~ownstream of the refrigerator 50 is provided a further heat exchanger 51 extracting cool from the residue gas on the supply line 48.
After the passage through the heat exchangers, the second proportion of the feed is usually totally condensed and sub-cool is provided by the heat exchanger 51. The second proportion of the feed is then passed through a let down valve 52 before injection into the high pressure tower 31 at a feed entry 53.
The compression of the second proportion only provides significant advantages in economical recoveries. In the past, all processes considered compressing all of the inlet gas to the high pressure before processing. In the present invention only the proportion in the line 22 is compressed thus avoiding the necessary power requirements for compression and also reducing capital cost.
In some situations a phase envelope of the second proportion gas has to be considered so that an optimum pressure is chosen which provides optimum cool recovery by the gaslliquid and thus the most economical system. The above optimum cool recovery is usually at a pressure that is close to the maximum cool recovery.
Turning now to Figure 2 there is shown substantially the same arrangement having the same first and second proportions divided into the first and second feed systems.
In this arrangement, however, the two tower process is replaced by a more conventional single tower process as indicated in the single tower 55 as is well known from the processes of Ortloff.
In Figure 1, the speed of compressor 45 is adjusted to obtain the flow through the Bine following Aerial cooler 46. Valve 52 is a back pressure control valve. The difference between FR 54 8~ FR55 indicates flow of make-up gas in line 22. Desired "reflux ratio" is FR 54 I Flow in line 21.
1~
Figure 2 is a modification of the old single tower process. In a similar manner to Figure 1, desired flow in FR 54 can be achieved by adjusting speed of compressor 45. ~esired "flow ratio" is FR54 / FR55.
FROM NATURAL GAS
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.
BACKGROUND OF THE INVENTION
The applicant's prior United States Patent 4,770,683, issued September 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 in US
Patent No: 6,098,425 issued Aug 8t" 2000 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 reflex.
Ethane recovery is similar to lpg recovery in concept except that more energy is required for refrigeration and reflex 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 na 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 de-methanizer portion of these plants. The above US 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 de-methanizer in an Ethylene Plant but it is believed that this patent will be an improvement when combined with that patent.
In US Patent No: 6,182,468 issued Feb 6t" 2001 of the present Applicant is disclosed 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.
The above 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
According to the invention there is provided a method for separating a supply material into two materials of different boiling points comprising:
providing two separate vessels connected to provide transfer of materials from one vessel to the other;
feeding supply material to one of the vessels arranged to form a mixed gas and liquid phase such that one material in the vessels separates into said liquid phase and the other material separates into said gas phase;
extracting the separated gas from the top of one vessel and the separated liquid from the bottom of the other vessel;
arranging the supply and the vessels such that the pressure in one of said vessels is higher than the other;
and injecting reflux into the top of one of the towers;
wherein the 'reflex' flow rate enters the top of one of the towers under flow control so that there is a constant flow rate of flashed liquid into the top of the tower.
Preferably the flow is measured when the overhead fluid is a gas prior to cooling.
Preferably said one of the towers is the high pressure tower.
Preferably the high pressure tower is controlled by maintaining a constant ratio between this 'reflex' flow and the flow rate of dehydrated gas feed to the process.
Preferably, in order to smooth the variation of low pressure tower overhead gas, a small amount of dehydrated gas feed is added to the 'reflex' gas in order to make the desired ratio of 'reflex' flow.
Preferably, in situations, where there is an excess of overhead gas from the low pressure tower, the surplus is processed with the main dehydrated feed gas which is cooled, chilled, partially condensed arid fed to the bottom of the high pressure tower.
Preferably all of the low pressure tower overhead is fed as 'reflex' and there is added some dehydrated feed to achieve a constant 'reflex' flow rate.
Preferably a constant ratio is maintained of condensed reflex liquid to the dehydrated inlet gas or a constant mass ratio (rather than volume ar molar ratio).
Preferably there is provided a near constant flow of liquid to the top tray of the high pressure tower.
The new invention is a process control scheme to use with all of the above distillation patents. It also applies to the ~rtloff Two Tower Process and a patent by some Southern U.S. inventors who sold their idea to a company in Houston. These processes are described below.
The present invention therefare includes all of the features disclosed in the above patents to which reference should be made for further details and the subject matter of which is incorporated herein by reference.
All of my patents involve dividing a distillation tower into two or more 5 towers having different operating pressures. When the concept is utilized for recovering propane from natural gas, all of the sweetened (usually, if required), dehydrated natural gas is cooled, chilled and contacted in a high pressure tower and liquid from this tower is processed in a lower pressure tower(s). Recycle from the low pressure tower is compressed, cooled, chilled, condensed, and sub-cooled at a high pressure, typically between 700 and 1300 psia and flashed into the top tray (usually) in the high pressure tower.
The operation of the low pressure tower (known as the de-ethanizer in propane recovery situations) may vary resulting ire a variation in flow rate of its overhead vapour. The overhead vapour is compressed, cooled, condensed, sub-cooled and used as 'reflex' for the high pressure tower.
The object of this invention is to dampen variations in flow of this reflex to result in a more stable operation and a higher average propane recovery.
The 'reflex' flow rate enters the top of the high pressure tower, known as the gas fractionator (or de-methanizer) under flow control so that there is a constant flow rate of flashed liquid into the top of the de-methanizer. The flow can be measured when the overhead fluid is a gas prior to cooling. The de-methanizer is controlled by maintaining a constant ratio between this 'reflex' flow and the flow rate of dehydrated gas feed to the process.
In order to smooth the variation of de-ethanizer overhead gas, a small amount of dehydrated gas feed is added to the 'reflex' gas in order to make the desired ratio of 'reflex' flow. In situations, where there is an excess of de-ethanizer overhead gas, the surplus could be processed with the main dehydrated feed gas which is cooled, chilled, partially condensed and fed to the bottom of the de-methanizer. (Normally, it would be preferred to design to utilize all of the de-ethanizer overhead as 'reflex' and add some dehydrated teed to achieve a constant 'reflex' flow rate.) Variations of this concept could be to maintain a constant ratio of condensed reflex liquid to the dehydrated inlet gas ~or a constant mass ratio (rather than volume or molar ratio).
This concept works by providing a near constant flow of liquid to the top tray. When more make-up to the reflex stream is required from the main dehydrated gas stream that enters the bottom of the de-methanizer, there is a higher methane content in the reflex, so more flashing on the top tray and slightly less liquid leaving the top tray. There is also slightly less vapour entering the bottom tray so the ratio of liquid enter the top to vapour entering the bottom remains closer to constant.
This concept will also work for the Ortloff Two Tower process which is a variation of my first patent except that their second tower operates at a higher pressure than their first tower. To achieve this higher pressure, they must pump liquid with a cryogenic pump from the first tower to the second tower. This process is not as thermo-dynamically efficient and it requires a cryogenic pump, but does not require a compressor to recycle de-ethanizer overhead vapour for condensing and feeding to the top of the de-methanizer. In a similar manner to that described in relation to my own prior patents, the flow rate of 'reflex gas' in the Ortloff type process in accordance with my invention to the top of the de-methanizer can be smoothed with a slip-stream of dehydrated feed gas.
Also this concept will work with the mast recent of the above patents that is another variation of my first patent. This patent recycles de-ethanizer overhead product (ethane) which is cooled, condensed, and sub-cooled then fed to the top of the de-methanizer. In their process, the de-ethanizer is usually a refluxed tower, whereas I usually have a quench-fed de-ethanizer. In this design, the reflex flow would be maintained at a constant ratio to the inlet feed by using make up from the inlet dehydrated gas.
Variations of this concept could be to maintain a constant ratio of 'reflex flow' to residue gas flow leaving the top to the de-methanizer.
Another variation of this concept is to take the make-up "reflex" flow from the vapour space in the bottom of the de-methanizer. This vapour could be letdown through an expansion valve to the inlet to the top tray of the de-ethanizer. Sometimes, retrograde effects will result in this letdown vapour having a small liquid phase which would add to the reflex of the de-ethanizer. This stream also tends to load level the recycle compressor so that it has a constant inlet flow gate thus a constant inlet pressure.
Another variation of this concept utilizes low temperature separator overhead vapour as make-up. In this concept, there is a low temperature separator in the stream of partially condensed dehydrated feed which sometimes is letdown through a turbo-expander into the inlet of the de-methanizer.
Another variation would be to inflate the flow of make-up gas by a certain fraction so that it would take less make-up gas to make up for a deficiency of de-ethanizer overhead gas. Of course, in some situations, it may be better to deflate the flow of make-up gas, rather than inflate it.
In addition to recovering propane from natural gas, this concept also works for recovering ethane and heavier liquids from natural gas or recovering carbon dioxide and heavier liquids from natural gas. It also works for separating closed boiling point liquids and other separations discussed in my prior patents.
BRIEF CDESCRIPTION OF TFiE DRAV1PINGS
Figure 1 is a schematic illustration of the elements of a first process according to the present invention using the two tower system of the above prior patent which is particularly but not exclusively designed for supplying the residue gas at a low pressure.
Figure 2 is a schematic illustration of the elements of a second process according to the present invention using the single tower.
Both figures are substantially as shown in DS patent no: 6,182,468 above but have been modified to show the present invention. The following description has also been primarily taken from the above patent and includes the elements with which the present invention is concerned.
DETAILED DESCRIPTION
Turning firstly to Figure 1 there is shown an arrangement for separating Ipg+
products from a feed of natural gas leaving a residue sales gas for sale at low pressure that is less than 100 psig.
The arrangement provides a feed supply line 10 which feeds to an inlet separator 11 which acts to separate gas from any incoming liquid. The liquid can be handled in a number of different ways including supply to the free water knock out system of the crude oil processing plant in arrangements where such is available.
Alternatively, the liquid can be passed through a dehydrator and fed to the de-ethanizer.
The inlet gas from the inlet separator 11 is supplied to an inlet compressor having an after-cooler 13. The gas is compressed to a sufficient pressure in the compressor 12 so that after the compressor the gas can be dehydrated in a molecular sieve 14 and processed in the Ipg recovery plant and then has sufficient pressure for the gas entering the sales pipeline 15.
Prior to entering the dehydrator 14 in the form of a molecular sieve, a further liquid separator 16 is provided for recycling a liquid through a return fine 17 having a let down valve 18.
As stated above, the arrangement described herein is particularly designed for low pressure residue gas. However if the desired pipeline pressure in the residue gas is intended to be above 600 psig, it is preferred that compression be added to the residue gas downstream of the recovery plant so that the tower assembly described hereinafter can operate at approximately 400 psig.
After the inlet gas is compressed, after-cooled and the liquids extracted in the separator 16, the gas is dehydrated in the dehydrator 14 which is preferably a molecular sieve as described above or can possibly be a "Dryso" T"" process which is a tri-ethylene glycol process. In such an arrangement a sophisticated regeneration system as shown can be provided using extractive distillation to reduce the water content of ethylene glycol. The extracted material from the regeneration system is returned to the feed as indicated in the supply line 20.
Downstream of the dehydrator 14, there is provided a supply line 21 which is divided into two supply lines 22 and 23 acting to effect a proportional division of the feed in the supply line 21. Each line includes a flow control valve 22A and 23A which are controlled 5 using conventional flow control systems well known to ore skilled in the art to maintain the required proportions depending upon the measurement of various parameters of the process.
The process further includes a processing tower arrangement generally indicated at 30 including a high pressure tower 31 and a low pressure tower 32. These two 10 towers are generally as described in the above United States Patent and the disclosure of that document is incorporated herein by reference. The two towers each comprise a distillation tower section for effecting separation of the components in the feed so that the high pressure tower section 31 discharges lighter gas components at an upper discharge 33 and heavier components at a lower discharge 34. The low pressure tower 32 has an upper discharge 35 and a lower discharge 36. The upper gas discharge 33 provides the residue sales gas 15 while the bottom discharge 36 of the low pressure tower provides the heavier Ipg+ product 37.
The first portion of the feed gas divided into the supply line 23 is supplied as a feed to the lower part of the high pressure tower 31. Prior to supply to the tower arrangement, the gas in the supply line 23 is passed through a heat exchanger R which includes a component 38A on the fine 23 and second component 38B forming a reboiler for material at the bottom of the low pressure tower component 32. Thus the heat exchanger R
extracts cool into the component 38A to cool the supply on the line 23 and applies heat to the component 38B acting as a reboiler to return the material as a side feed to the lower part of the lower pressure component 32.
The supply on the line 23 is further passed through a second heat exchanger S having a first component 39A and a second component 39B which again acts to extract cool for the material in the line 23 and acts as a heat supply for a side reboiler on the lower pressure tower component 32.
Gas from the top discharge 35 of the low pressure tower 32 is returned to the feed through a supply duct 40. Prior to return to the feed, cool is extracted from the return gas in a further heat exchanger 41 and that cool is applied to the feed on the line 23.
Finally a refrigerator unit 42 is used to apply external cooling to the feed prior to injection into the high pressure tower component 31 at a feed position 43.
The second proportion on the line 22 is passed to a compressor system 44 including a compressor 45 and a heat extractor 46. The second proportion of the gas is compressed to a pressure in the range 500 to 1400 psig so that it can be cooled and condensed and used for injecting into the tower arrangement as a cooling top supply.
The prior patent and the prior published application of the present inventor disclose the use of liquid injection at the top of the high pressure tower for maintaining a cool temperature in the high pressure tower'. In the prior application this is termed as "reflux". However in the present invention the compressed material includes a component of the original supply from the feed 10 and in addition includes a component from the discharge gas from the discharge outlet 35 of the low pressure tower.
The second proportion is thus compressed in the compressor system 44 and cooled by the cooling arrangement 46. It is then passed through a heat exchanger 47 which extracts cool from the residue gas and supply line 48. Further cooling is effected in a further heat exchanger L which includes first component 49~ on the line 22 and a second component 49B extracting cool from the product 37. Further refrigeration cooler 50 is provided using external refrigerant. ~ownstream of the refrigerator 50 is provided a further heat exchanger 51 extracting cool from the residue gas on the supply line 48.
After the passage through the heat exchangers, the second proportion of the feed is usually totally condensed and sub-cool is provided by the heat exchanger 51. The second proportion of the feed is then passed through a let down valve 52 before injection into the high pressure tower 31 at a feed entry 53.
The compression of the second proportion only provides significant advantages in economical recoveries. In the past, all processes considered compressing all of the inlet gas to the high pressure before processing. In the present invention only the proportion in the line 22 is compressed thus avoiding the necessary power requirements for compression and also reducing capital cost.
In some situations a phase envelope of the second proportion gas has to be considered so that an optimum pressure is chosen which provides optimum cool recovery by the gaslliquid and thus the most economical system. The above optimum cool recovery is usually at a pressure that is close to the maximum cool recovery.
Turning now to Figure 2 there is shown substantially the same arrangement having the same first and second proportions divided into the first and second feed systems.
In this arrangement, however, the two tower process is replaced by a more conventional single tower process as indicated in the single tower 55 as is well known from the processes of Ortloff.
In Figure 1, the speed of compressor 45 is adjusted to obtain the flow through the Bine following Aerial cooler 46. Valve 52 is a back pressure control valve. The difference between FR 54 8~ FR55 indicates flow of make-up gas in line 22. Desired "reflux ratio" is FR 54 I Flow in line 21.
1~
Figure 2 is a modification of the old single tower process. In a similar manner to Figure 1, desired flow in FR 54 can be achieved by adjusting speed of compressor 45. ~esired "flow ratio" is FR54 / FR55.
Claims (9)
1. A method for separating a supply material into two materials of different boiling points comprising:
providing two separate vessels connected to provide transfer of materials from one vessel to the other;
feeding supply material to one of the vessels arranged to form a mixed gas and liquid phase such that one material in the vessels separates into said liquid phase and the other material separates into said gas phase;
extracting the separated gas from the top of one vessel and the separated liquid from the bottom of the other vessel;
arranging the supply and the vessels such that the pressure in one of said vessels is higher than the other;
and injecting reflex into the top of one of the towers;
wherein the 'reflex' flow rate enters the top of one of the towers under flow control so that there is a constant flow rate of flashed liquid into the top of the tower.
providing two separate vessels connected to provide transfer of materials from one vessel to the other;
feeding supply material to one of the vessels arranged to form a mixed gas and liquid phase such that one material in the vessels separates into said liquid phase and the other material separates into said gas phase;
extracting the separated gas from the top of one vessel and the separated liquid from the bottom of the other vessel;
arranging the supply and the vessels such that the pressure in one of said vessels is higher than the other;
and injecting reflex into the top of one of the towers;
wherein the 'reflex' flow rate enters the top of one of the towers under flow control so that there is a constant flow rate of flashed liquid into the top of the tower.
2. The method according to Claim 1 wherein the flow is measured when the overhead fluid is a gas prior to cooling.
3. The method according to Claim 1 wherein said one of the towers is the high pressure tower.
4. The method according to Claim 2 wherein the high pressure tower is controlled by maintaining a constant ratio between this 'reflex' flow and the flow rate of dehydrated gas feed to the process.
5. The method according to Claim 1 wherein, in order to smooth the variation of low pressure tower overhead gas, a small amount of dehydrated gas feed is added to the 'reflex' gas in order to make the desired ratio of 'reflex' flow.
6. The method according to Claim 1 wherein, in situations, where there is an excess of overhead gas from the low pressure tower, the surplus is processed with the main dehydrated feed gas which is cooled, chilled, partially condensed and fed to the bottom of the high pressure tower.
7. The method according to Claim 1 wherein all of the low pressure tower overhead is fed as 'reflex' and there is added some dehydrated feed to achieve a constant 'reflex' flow rate.
8. The method according to Claim 1 wherein a constant ratio is maintained of condensed reflex liquid to the dehydrated inlet gas or a constant mass ratio (rather than volume or molar ratio).
9. The method according to Claim 1 wherein there is provided a near constant flow of liquid to the top tray of the high pressure tower.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2441136 CA2441136A1 (en) | 2003-09-17 | 2003-09-17 | Improved thermodynamic separation of heavier components from natural gas |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2441136 CA2441136A1 (en) | 2003-09-17 | 2003-09-17 | Improved thermodynamic separation of heavier components from natural gas |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2441136A1 true CA2441136A1 (en) | 2005-03-17 |
Family
ID=34318740
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2441136 Abandoned CA2441136A1 (en) | 2003-09-17 | 2003-09-17 | Improved thermodynamic separation of heavier components from natural gas |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2441136A1 (en) |
-
2003
- 2003-09-17 CA CA 2441136 patent/CA2441136A1/en not_active Abandoned
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