Patent Application
20130074542
1. Technical Field
The embodiments herein generally relate to a recovery process of natural gas liquids. The embodiments herein particularly relate to a recovery process of natural gas liquids with high level of liquids recovery and more particularly relates to a process of reducing an amount of refrigeration load in a recovery process of the natural gas liquids.
2. Description of the Related Art
Natural gas liquids (NGL) recovery refers to the process of removing and gathering heavier hydrocarbon products from natural gas. These heavier hydrocarbons must be separated from methane to be recovered as natural gas liquids. These valuable natural gas liquids consist of ethane, propane, butane, and other heavier hydrocarbons. The increase in energy resources, costs and also economic issues, resulted in more complex and more efficient recovery units for recovering the gaseous liquids. New generation of NGL units have been created based on the reduction of fixed and operating costs for a defined output.
The existing processes designated to update process configurations are defined to aim at finding the new ways for maximizing the efficiency of existing equipments and reducing the operational costs. The major costs of the NGL recovery system are related to the refrigeration required for gas refrigerating. In the low temperature configurations, the required level of refrigeration is provided through a propane refrigeration cycle. In some cases, the mixed refrigerants and cascade refrigeration systems are used. Besides a refrigeration cycle, some amount of refrigeration load is either provided by Joule-Thomson expansion or expansion through a turbo expander. In a configuration, the required level of refrigeration depends on the integration of the processes and the number of cool boxes. The combined heating and cooling curves of the heat exchangers indicate an efficiency of this type of equipments. The optimal combined curves and high efficiency of cool boxes enable to achieve more integrated configurations. The more integrated a unit is, the lesser will be the fixed and operating costs.
A gas composition highly effects an economic situation of the NGL recovery unit and a selection of the process. The gas containing higher amounts of hydrocarbons, which can turn into liquid, will generate more liquid products resulting in a higher efficiency in return for the applied equipments. The richer the gas, the more is refrigeration load and a constant investment cost and also the heat exchangers with a higher heat transfer level are required. The light gases need more specific process conditions to reach to the higher recovery levels. In the units where the gas feed is rich, the usage of an external refrigeration system for achieving the higher recovery levels would be inevitable. In the case of the light gases there is no need to use the external refrigeration systems for achieving the higher recovery levels.
There are many expansion processes, which are commonly applied for the high compression feeds for recovering the liquid hydrocarbons in the gas process industries, specifically in the recovery of ethane and propane. In many of the conventional processes of NGL expansions, the gas feeds are refrigerated near to a relatively low temperature aiming at a partial liquidation, typically through a heat exchanger with a gas stream existing out from the upper section of the distillation tower and the lateral boilers. Some of the other existing techniques use propane as a refrigerant for external refrigeration or the propane refrigeration (PR) in the recovery of the heavier hydrocarbons.
As an improvement over the conventional processes, a new process called Gas Sub cooled Process (GSP) was developed. This process modified prevalent processes in some different parts. A portion of the gas is thoroughly delivered to a converter and liquefied through the output stream on the top of the tower. Then, this stream is sent to the top section of the distillation tower as an input feed stream and also as the reflux. The required work for a compression of the residue gas is less than the work, which is needed for the prevalent expander processes. Typically, the amount of energy in horsepower is less than that of the PR process with a recovery grade of 92%.
Another novel process called Cold Residue Recycle (CRR) is the rectified model of GSP process for achieving higher recovery efficiency. The stream path of the process is similar to that of GSP, except that the stream on the top of the tower in which a portion of residue gas is present, is sent back to a distillation tower as the extra return stream. The CRR process is used in high amounts of recovery.
Another process called Recycle Split Vapor (RSV) like CRR uses a feed vapor division process for providing a portion of ethane recovery in the tower. The return stream of the methane for the tower is supplied by extracting a small portion of the residual gas, which is recompressed and liquefied. The pressure of this stream is reduced to that of an entry section, which provides a portion of the feed, required for the top of the tower. The high pressure of this methane stream allows the gas in the top of the tower to be used for the process of liquefying and refrigerating. Hence, a portion of vapor feed can directly be delivered to the tower.
Yet another process called Recycle Split Vapor with Enrichment (RSVE) is the enhancement of the RSV process. Unlike RSV process, the return stream is extracted from the compressed residual gas and is mixed with the vapor stream feed before getting refrigerated and liquefied. Hence there is no need for a separate exchanger.
The development of the other NGL process based on a turbo expander leads to the recovery of increased NGL. This process uses a portion of the stream at the bottom of the distillation tower as a mixed refrigerant. The mixed refrigerant is partially or totally evaporated to provide the required refrigeration for refrigerating the input gas, which normally covers the external refrigeration. The vapor generated through this auto-refrigeration cycle is compressed again and sent back to the bottom of the tower. This vapor is used as a residue gas.
In an open-cycle refrigeration system, which aims to improve an efficiency and an economy of NGL recovery processes, a portion of a stream which contains liquid hydrocarbons is depleted from the lower parts of the distillation tower. This side stream warmed and expanded to create a diphase system for the separation of heavy hydrocarbon fluids and also to provide a vapor phase is sent back to the tower. The warm and expanded vapor returned back to the tower increases the concentration of methane and ethane. As a result of which, the temperature profile is decreased and separation efficiency is increased.
It is found that four configurations (GSP, CRR, RSV and RSVE) are used to recover ethane for more than 90 percents. But the feed composition based on which CRR, RSV, RSVE were introduced contained 92.5% methane. In other words all the prior processes worked on lighter gases. The cited prior processes gave less emphasis on rich gases.
In the light of the foregoing discussions, there exists a need to provide a process for recovering natural gas liquids, which gives emphasis on rich gases. There also exists a need to provide a process for recovering the natural gas liquids to reduce the equipment cost and operational cost and increase in recovery efficiency. Furthermore there exists a need to provide a process for recovering the natural gas liquids and for proving an efficient refrigeration system resulting in a reduction of overall cost of the process.
The abovementioned shortcomings, disadvantages and problems are addressed herein and which will be understood by reading and studying the following specification.
The primary object of the embodiments herein is to provide a method and a system for providing a required refrigeration load in a recovery of natural gas liquid so that the energy required by the system is reduced drastically.
Another object of the embodiments herein is to provide a method and a system for providing a required refrigeration load in a natural gas liquid recovery using an open-closed self-refrigeration system.
Yet another object of the embodiments herein is to provide a system and method for analyzing the configuration capability under circumstances in which the composition percentage is not stable and varied
Yet another object of the embodiments herein is to provide a method and a system for providing a required refrigeration load in a natural gas liquid recovery to reduce the required energy of the overall system.
Yet another object of the embodiments herein is to provide a method and system for providing a refrigeration load, which is proportional to the variations of feed composition percentage.
Yet another object of the embodiments herein is to provide a method and a system for providing a required refrigeration load in a natural gas liquid recovery using least number of multi-stream converters, thereby leading to an integration of a process operation.
Yet another object of the embodiments herein is to provide a method and a system for a natural gas liquid recovery by providing a configuration, which performs on rich gases with high levels of liquid recovery.
Yet another object of the embodiments herein is to provide a method and a system for natural gas liquid recovery to reduce an energy consumption of the system.
Yet another object of the present invention is to provide a method and a system for natural gas liquid recovery to provide an inherent integrity resulting in reduction of numbers and volume of the applied process equipments.
Yet another object of the embodiments herein is to provide a method and a system for natural gas liquid recovery in which a distillation tower is used in the separation system thereby reducing the number of subsidiary systems and the refrigeration load to provide an optimal functioning of the configuration.
Yet another object of the embodiments herein is to provide a method and a system for natural gas liquid recovery in which the energy consumption is less than that of similar processes and systems.
These and other objects and advantages of the embodiments herein will become readily apparent from the following summary and the detailed description taken in conjunction with the accompanying drawings.
The various embodiments herein provide a method and system for a process configuration with internal refrigeration for light gas and a process configuration with external refrigeration system for rich gas. The two processes are employed with a demethanizer column. The system and method provides a novel configuration which employs an-open closed self refrigeration system to provide a required refrigeration load. The system and method detects for a need for an external refrigeration unit or internal refrigeration unit based on the variations in the feed composition percentage. The system and method further provides an external refrigeration cycle using propane as a refrigerant. The configuration with internal refrigeration system utilizes a slip stream from or near the bottom of the demethanizer as a mixed refrigerant.
According to an embodiment, an internal refrigeration method for separating a residue gas from an input gas to recover a natural liquid gas through a self-refrigeration process comprising the steps of feeding an input gas into an internal refrigeration system. The input gas is condensed in a first multi-flow heat exchanger. An output of the first multi-flow heat exchanger is separated into a fluid stream and a gas stream using a first drum separator. The separated gas stream obtained from the first drum separator is divided into a first gas part and a second gas part. The separated fluid stream obtained from the first drum separator is divided into a first fluid part and a second fluid part. The first fluid part obtained from the first drum separator is passed to a middle section of a demethanizer column through a Joule-Thomson expansion valve for further fractionating. The second fluid part obtained from the first drum separator is passed to a top rectifying section in the demethanizer column through a second multi-flow heat exchanger and a first expansion valve to form a residue gas. The first gas part is passed to a work expander device. A gas stream output of the work expander device is passed to an area below a top rectifying section in the demethanizer column as an expander discharge. The second gas part is passed through the second multi-flow heat exchanger and a second expansion valve to the top rectifying section in the demethanizer column to form a reflux liquid. The residue gas from the top rectifying section of the demethanizer column is passed through the second multi-flow heat exchanger and the first multi-flow heat exchanger to a first compressor for further compression and wherein the first compressor compresses the residue gas using a work extracted from the expander device. A compressed residue gas output from the first compressor is passed to a second compressor for further compressing the residue gas. The compressed residue gas is passing further through a first air cooler. An output of the first cooler is forwarded as a gas refrigerant. The volatile components are stripped off from a liquid collected at a bottom portion of the demethanizer column by circulating the liquids from four liquid draw trays provided in the bottom portion of the demethanizer column through the first multi-flow heat exchanger and the second multi-flow heat exchanger and collecting back the liquids at the demethanizer column. The liquids are recovered from a bottom portion of the demethanizer column.
The step of stripping off the volatile components from the liquid collected at the bottom portion of the demethanizer column comprises passing a liquid drawn from a first liquid tray through the second multi-flow heat exchanger before being collected back at the demethanizer. A liquid drawing from a second liquid tray is passed through the second multi-flow heat exchanger before being collected back at the demethanizer Wherein a direction of flow of the liquid from the first liquid tray through the second multi-flow heat exchanger and a direction of flow of the liquid from the second liquid tray through the second multi-flow heat exchanger are mutually opposite. A liquid drawn from a third liquid tray is passed through the first multi-flow heat exchanger before being collected back at the demethanizer. A liquid drawn from a fourth liquid tray is passed through the first multi-flow heat exchanger before being collected back at the demethanizer. Wherein a direction of flow of the liquid from the third liquid tray through the first multi-flow heat exchanger and a direction of flow of the liquid from the fourth liquid tray through the first multi-flow heat exchanger are mutually opposite.
The step of drawing liquid from the fourth liquid tray and passing through the multi-flow heat exchanger before being collected at the demethanizer column comprises passing a liquid drawn from the fourth tray through the first multi-flow heat exchanger. An output of the first multi-flow heat exchanger is passed to a second drum separator through a third expansion valve. A vapor output from the second drum is fed to a first stage of a third compressor. An output of the third compressor is passed to a second air cooler through a third multi-flow heat exchanger. An output of the second air cooler is forwarded to a third drum separator for dividing the output of the second air cooler into a vapor part and a liquid part. The liquid part obtained from the second air cooler is divided into a third liquid part and a fourth liquid part. The vapor part and the third liquid part obtained from the third drum separator are combined and forwarded to the demethanizer. The fourth liquid part obtained from the third drum separator is passed to the third multi-flow heat exchanger through a fourth expansion valve. An output of the third multi-flow heat exchanger is divided into a fifth liquid part and a sixth liquid part. The fifth liquid part from the output of the third multi-flow heat exchanger is passed to the second drum separator. The sixth liquid part from the output of the third multi-flow heat exchanger is fed to a fourth drum separator to obtain a vaporized liquid and a non vaporized liquid. The vaporized liquid obtained from the fourth drum separator is forwarded to a second stage of the third compressor. The non-vaporized liquid from the fourth drum separator and the non-vaporized liquid from the second drum separator are combined and forwarded to the demethanizer. The input gas is a light gas containing a low amount of hydrocarbons that are converted into a natural gas liquid. The natural gas liquid has a lower volatility degree as compared to that of the residue gas.
According to an embodiment herein, a method of external refrigeration for separating a residue gas from an input gas to recover a natural liquid gas through a propane-refrigeration process comprising steps of feeding an input gas into an internal refrigeration system. The input gas is condensed in a first multi-flow heat exchanger. An output of the first multi-flow heat exchanger is separated into a fluid stream and a gas stream using a first drum separator. The separated gas stream obtained from the first drum separator is divided into a first gas part and a second gas part. The separated fluid stream obtained from the first drum separator is divided into a first fluid part and a second fluid part. The first fluid part obtained from the first drum separator is passed to a middle section of a demethanizer column through a Joule-Thomson expansion valve for further fractionating. The second fluid part obtained from the first drum separator is passed to a top rectifying section in the demethanizer column through a second multi-flow heat exchanger and a first expansion valve to form a residue gas. The first gas part is passed to a work expander device. A gas stream output of the work expander device is passed to an area below a top rectifying section in the demethanizer column as an expander discharge. The second gas part is passed through the second multi-flow heat exchanger and a second expansion valve to the top rectifying section in the demethanizer column to form a reflux liquid. The residue gas from the top rectifying section of the demethanizer column is passed through the second multi-flow heat exchanger and the first multi-flow heat exchanger to a first compressor for further compression and wherein the first compressor compresses the residue gas using a work extracted from the expander device. A compressed residue gas output from the first compressor is passed to a second compressor for further compressing the residue gas. The compressed residue gas is passing further through a first air cooler. An output of the first cooler is forwarded as a gas refrigerant. The volatile components are stripped off from a liquid collected at a bottom portion of the demethanizer column by circulating the liquids from four liquid draw trays provided in the bottom portion of the demethanizer column through the first multi-flow heat exchanger and the second multi-flow heat exchanger and collecting back the liquids at the demethanizer column. The liquids are recovered from a bottom portion of the demethanizer column. An external refrigeration process is provided using liquid propane.
The step of stripping off the volatile components from the liquid collected at the bottom portion of the demethanizer column comprises passing a liquid drawn from a first liquid tray through the second multi-flow heat exchanger before being collected back at the demethanizer. A liquid drawing from a second liquid tray is passed through the second multi-flow heat exchanger before being collected back at the demethanizer Wherein a direction of flow of the liquid from the first liquid tray through the second multi-flow heat exchanger and a direction of flow of the liquid from the second liquid tray through the second multi-flow heat exchanger are mutually opposite. A liquid drawn from a third liquid tray is passed through the first multi-flow heat exchanger before being collected back at the demethanizer. A liquid drawn from a fourth liquid tray is passed through the first multi-flow heat exchanger before being collected back at the demethanizer. Wherein a direction of flow of the liquid from the third liquid tray through the first multi-flow heat exchanger and a direction of flow of the liquid from the fourth liquid tray through the first multi-flow heat exchanger are mutually opposite.
The step of providing external refrigeration using propane comprising the steps of feeding compressed liquid propane from a third compressor into a second air cooler. An output of the second air cooler is forwarded to a second drum separator through a third expansion valve. A liquid propane output from the second drum separator is passed through a fourth expansion valve. A vapor gas output of liquid propane from the second drum separator forwarded to the third stage of a third compressor. An output liquid propane is passed from the second drum separator passed through the fourth expansion valve. An output liquid propane passed through the fourth expansion valve is divided into a first liquid propane part and a liquid propane part. The first liquid propane part output from the fourth expansion valve is fed into a first multi-flow heat exchanger for providing required refrigeration for cooling the feed gas. The first liquid propane part output from the first multi-flow heat exchanger is forwarded to a third drum separator. The second liquid propane part output from the fourth expansion valve is fed to the third drum separator directly through a sixth expansion valve. A vapor gas output of the third drum separator is forwarded into a second stage of the third compressor. A non vaporized liquid output from the third drum separator is fed to a fourth drum separator after passing the non vaporized liquid output from the third drum separator through a seventh expansion valve, the second multi-flow heat exchanger and the first multi-flow heat exchanger. A vaporized output from the fourth drum separator is passed to a first stage of the third compressor. A liquid propane output from the fourth drum separator is forwarded as an external refrigerant.
According to an embodiment herein, a system for separating a residue gas from an input gas to recover a natural liquid gas comprising a self refrigeration unit comprising an open-closed cycle of refrigeration and an external refrigeration unit. Wherein the self-refrigeration unit comprising an open-closed cycle of refrigeration provides a required refrigeration load and the external refrigeration unit provides a refrigeration load corresponding to variations in a feed composition percentage.
According to an embodiment herein, a process of internal refrigeration for separating a residue gas from an input gas to recover a natural liquid gas through self-refrigeration comprises steps of: feeding the input gas into the internal refrigeration system, condensing the input gas in a first multi-flow heat exchanger, separating a fluid stream and a gas stream by a first separator, dividing the gas stream as a first gas part and a second gas part and the fluid stream as a first fluid part and a second fluid part by the first separator, forming the residue gas by combining first gas part and second fluid part, forming a liquid reflux by condensing the second gas part, producing the natural gas liquid by condensing the first fluid part,
According to an embodiment herein, the step of producing the natural gas liquid by condensing the first fluid part includes: decreasing temperature of the first fluid stream by an expansion valve, sending the first fluid stream to middle section of a distillation tower, increasing temperature of stream from middle section of the distillation tower by a second multi-flow heat exchanger, sending output stream of the expansion valve to the lower section of the distillation tower, producing a mixed refrigerant by combining one of the streams from a lower section of the distillation tower with a open cycle refrigerant, increasing the temperature of the mixed refrigerant by the first multi-flow heat exchanger by exchanging heat with the residue gas, decreasing temperature of output stream from the first multi-flow heat exchanger by a pressure breaker valve, increasing temperature of output stream from the pressure breaker valve by the first multi-flow heat exchanger by exchanging heat with the residue gas, separating unvaporized liquid from output gas from the first multi-flow heat exchanger by a first two phase separator, sending vaporized gas stream from the first two phase separator to first stage of a first two phase compressor, decreasing temperature of output stream of the first two phase compressor by a third multi-flow heat exchanger, partial conversion of output stream from the third multi-flow heat exchanger into liquid by a first air cooler, separating liquid from output stream of the first air cooler by a second two stage separator, dividing output gas stream from the second two stage separator as a first gas stream and a second a second gas stream, increasing temperature of the first gas stream by the third multi-flow heat exchanger, dividing output gas stream from the third multi-flow heat exchanger as a first gas portion and a second gas portion, sending the first gas portion to second stage of the first two phase compressor, dividing the second gas portion into a gas stream and a fluid stream by a third two phase separator, compressing the gas stream from the third two phase separator by the second stage of the first two phase compressor, producing a vaporized refrigerant by combining the fluid stream from the third two phase separator with the liquid from the first two phase separator and producing the natural gas liquid by combining the vaporized refrigerant with the liquid from the second two phase separator in lower section of the distillation tower.
According to an embodiment herein, the step of forming a liquid reflux by condensing the second gas part includes: sub cooling the second gas part by the second multi-flow heat exchanger, reducing pressure and temperature of output of the second multi-flow heat exchanger by an expander and sending output of the expander to top of the distillation tower. The output of the expander is the liquid reflux.
According to an embodiment herein, the step of forming the residue gas by combining first gas part and second fluid part includes: reducing pressure and temperature of first gas part and second fluid part by an expander, sending output of the expander to the upper section of the distillation tower, increasing the temperature of output of the first expander through a second multi-flow heat exchanger and first multi-flow heat exchanger by exchanging heat with the input gas, increasing temperature and pressure of the output gas from the first multi-flow heat exchanger by compressing in a first compressor and reducing the temperature of the output gas from the first compressor by a second air cooler to produce a residue gas, wherein the residue gas is the closed cycle refrigerant.
According to an embodiment herein, the input gas is light gas containing less amounts of hydrocarbons and is turned into natural gas liquid.
According to an embodiment herein, the natural gas liquid is having lower volatility degree as compared to the residue gas.
According to an embodiment herein, the distillation tower is equipped with four liquid draw trays in the lower section to provide heat to the tower for stripping volatile components off from the liquid product.
According to an embodiment herein, a process of external refrigeration for separating a residue gas from an input gas to recover a natural liquid gas through propane-refrigeration comprising the steps of: distributing the propane refrigerant for the first multi-flow heat exchanger, feeding the input gas into the external refrigeration system, condensing the input gas in a first multi-flow heat exchanger, separating a fluid stream and a gas stream by a first separator, dividing the gas stream as a first gas part and a second gas part and the fluid stream as a first fluid part and a second fluid part by the first separator, forming the residue gas by combining first gas part and second fluid part, forming a liquid reflux by condensing the second gas part, producing the natural gas liquid by condensing the first fluid part,
According to an embodiment herein, the step of producing the natural gas liquid by condensing the first fluid part includes: decreasing temperature of the first fluid stream by an expansion valve, sending the first fluid stream to middle section of a distillation tower, increasing temperature of stream from middle section of the distillation tower by a second multi-flow heat exchanger by exchanging heat with the propane refrigerant, sending output stream of the expansion valve to the lower section of the distillation tower, increasing the temperature of the stream from lower section of the distillation tower by the first multi-flow heat exchanger by exchanging heat with the propane refrigerant, decreasing temperature of output stream from the first multi-flow heat exchanger by a pressure breaker valve, increasing temperature of output stream from the pressure breaker valve by the first multi-flow heat exchanger by exchanging heat with the propane refrigerant, separating unvaporized liquid from output gas from the first multi-flow heat exchanger by a first two phase separator, sending vaporized gas stream from the first two phase separator to first stage of a first two phase compressor, decreasing temperature of output stream of the first two phase compressor by a third multi-flow heat exchanger by exchanging heat with the propane refrigerant, partial conversion of output stream from the third multi-flow heat exchanger into liquid by a first air cooler, separating liquid from output stream of the first air cooler by a second two stage separator, dividing output gas stream from the second two stage separator as a first gas stream and a second a second gas stream, increasing temperature of the first gas stream by the third multi-flow heat exchanger by exchanging heat with the propane refrigerant, dividing output gas stream from the third multi-flow heat exchanger as a first gas portion and a second gas portion, sending the first gas portion to second stage of the first two phase compressor, dividing the second gas portion into a gas stream and a fluid stream by a third two phase separator, compressing the gas stream from the third two phase separator by second stage of the first two phase compressor, producing a vaporized refrigerant by combining the fluid stream from the third two phase separator with the liquid from the first two phase separator and producing the natural gas liquid by combining the vaporized refrigerant with the liquid from the second two phase separator in lower section of the distillation tower.
According to an embodiment herein, the step of forming a liquid reflux by condensing the second gas part includes: sub cooling the second gas part by the second multi-flow heat exchanger, reducing pressure and temperature of output of the second multi-flow heat exchanger by an expander and sending output of the expander to top of the distillation tower. The output of the expander is the liquid reflux.
According to an embodiment herein, the step of forming the residue gas by combining first gas part and second fluid part includes: reducing pressure and temperature of first gas part and second fluid part by an expander, sending an output of the expander to the upper section of the distillation tower, increasing the temperature of output of the first expander through a second multi-flow heat exchanger and first multi-flow heat exchanger by exchanging heat with the input gas, increasing the temperature and the pressure of the output gas from the first multi-flow heat exchanger by compressing in a first compressor and reducing the temperature of the output gas from the first compressor by a second air cooler to produce a residue gas, wherein the residue gas is the closed cycle refrigerant.
These and other aspects of the embodiments herein will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. It should be understood, however, that the following descriptions, while indicating preferred embodiments and numerous specific details thereof, are given by way of illustration and not of limitation. Many changes and modifications may be made within the scope of the embodiments herein without departing from the spirit thereof, and the embodiments herein include all such modifications.
The other objects, features and advantages will occur to those skilled in the art from the following description of the preferred embodiment and the accompanying drawings in which:
Although the specific features of the embodiments herein are shown in some drawings and not in others. This is done for convenience only as each feature may be combined with any or all of the other features in accordance with the embodiments herein.
In the following detailed description, a reference is made to the accompanying drawings that form a part hereof, and in which the specific embodiments that may be practiced is shown by way of illustration. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments and it is to be understood that the logical, mechanical and other changes may be made without departing from the scope of the embodiments. The following detailed description is therefore not to be taken in a limiting sense.
The various embodiments herein provide a method and system for a process configuration with internal refrigeration for a light gas and a process configuration with external refrigeration system for a rich gas. The two processes are employed with the separation methods using a demethanizer column. The configuration with internal refrigeration system utilizes a slip stream from or near the bottom of the demethanizer as a mixed refrigerant.
According to an embodiment herein, a process of internal refrigeration for separating a residue gas from an input gas to recover a natural liquid gas through a self-refrigeration comprising the steps of: feeding an input gas into the internal refrigeration system, condensing the input gas in a first multi-flow heat exchanger, separating a fluid stream and a gas stream by a first separator, dividing the gas stream as a first gas part and a second gas part and the fluid stream as a first fluid part and a second fluid part by the first separator, forming the residue gas by combining first gas part and second fluid part, forming a liquid reflux by condensing the second gas part, producing the natural gas liquid by condensing the first fluid part,
According to an embodiment herein, the step of producing the natural gas liquid by condensing the first fluid part includes: decreasing a temperature of the first fluid stream by an expansion valve, sending the first fluid stream to a middle section of a distillation tower, increasing a temperature of a stream from a middle section of the distillation tower by a second multi-flow heat exchanger, sending an output stream of the expansion valve to the lower section of the distillation tower, producing a mixed refrigerant by combining one of the streams from a lower section of the distillation tower with a open cycle refrigerant, increasing the temperature of the mixed refrigerant by the first multi-flow heat exchanger by exchanging the heat with the residue gas, decreasing a temperature of an output stream from the first multi-flow heat exchanger by a pressure breaker valve, increasing a temperature of an output stream from the pressure breaker valve by the first multi-flow heat exchanger by exchanging the heat with the residue gas, separating non-vaporized liquid from an output gas from the first multi-flow heat exchanger by a first two phase separator, sending a vaporized gas stream from the first two phase separator to a first stage of a first two phase compressor, decreasing a temperature of an output stream of the first two phase compressor by a third multi-flow heat exchanger, partial conversion of an output stream from the third multi-flow heat exchanger into a liquid by a first air cooler, separating a liquid from an output stream of the first air cooler by a second two stage separator, dividing an output gas stream from the second two stage separator as a first gas stream and a second a second gas stream, increasing a temperature of the first gas stream by the third multi-flow heat exchanger, dividing an output gas stream from the third multi-flow heat exchanger as a first gas portion and a second gas portion, sending the first gas portion to a second stage of the first two phase compressor, dividing the second gas portion into a gas stream and a fluid stream by a third two phase separator, compressing the gas stream from the third two phase separator by a second stage of the first two phase compressor, producing a vaporized refrigerant by combining the fluid stream from the third two phase separator with the liquid from the first two phase separator and producing the natural gas liquid by combining the vaporized refrigerant with the liquid from the second two phase separator in lower section of the distillation tower.
According to an embodiment herein, the step of forming a liquid reflux by condensing the second gas part includes: sub cooling the second gas part by the second multi-flow heat exchanger, reducing a pressure and a temperature of an output of the second multi-flow heat exchanger by an expander and sending an output of the expander to a top portion of the distillation tower. The output of the expander is a reflux liquid.
According to an embodiment herein, the step of forming the residue gas by combining a first gas part and a second fluid part includes: reducing a pressure and a temperature of a first gas part and a second fluid part by an expander, sending an output of the expander to the upper section of the distillation tower, increasing the temperature of output of the first expander through a second multi-flow heat exchanger and first multi-flow heat exchanger by exchanging heat with the input gas, increasing a temperature and a pressure of the output gas from the first multi-flow heat exchanger by compressing in a first compressor and reducing the temperature of the output gas from the first compressor by a second air cooler to produce a residue gas, wherein the residue gas is the closed cycle refrigerant.
According to an embodiment herein, the input gas is light gas containing less amounts of hydrocarbons that are turned into natural gas liquid.
According to an embodiment herein, the natural gas liquid is having lower volatility degree as compared to the residue gas.
According to an embodiment herein, the distillation tower is equipped with four liquid draw trays in a lower section to provide a heat to the tower for stripping off the volatile components from the liquid product.
According to an embodiment herein, a process of external refrigeration for separating a residue gas from an input gas to recover a natural liquid gas through a propane-refrigeration comprising the steps of: distributing the propane refrigerant for the first multi-flow heat exchanger, feeding the input gas into the external refrigeration system, condensing the input gas in a first multi-flow heat exchanger, separating a fluid stream and a gas stream by a first separator, dividing the gas stream as a first gas part and a second gas part and the fluid stream as a first fluid part and a second fluid part by the first separator, forming the residue gas by combining first gas part and second fluid part, forming a liquid reflux by condensing the second gas part, producing the natural gas liquid by condensing the first fluid part,
According to an embodiment herein, the step of producing the natural gas liquid by condensing the first fluid part includes: decreasing a temperature of the first fluid stream by an expansion valve, sending the first fluid stream to a middle section of a distillation tower, increasing a temperature of a stream from the middle section of the distillation tower by a second multi-flow heat exchanger by exchanging the heat with the propane refrigerant, sending an output stream of the expansion valve to the lower section of the distillation tower, increasing the temperature of the stream from a lower section of the distillation tower by the first multi-flow heat exchanger by exchanging the heat with the propane refrigerant, decreasing a temperature of an output stream from the first multi-flow heat exchanger by a pressure breaker valve, increasing a temperature of an output stream from the pressure breaker valve by the first multi-flow heat exchanger by exchanging the heat with the propane refrigerant, separating non-vaporized liquid from an output gas from the first multi-flow heat exchanger by a first two phase separator, sending a vaporized gas stream from the first two phase separator to a first stage of a first two phase compressor, decreasing a temperature of an output stream of the first two phase compressor by a third multi-flow heat exchanger by exchanging the heat with the propane refrigerant, partial conversion of an output stream from the third multi-flow heat exchanger into a liquid by a first air cooler, separating a liquid from an output stream of the first air cooler by a second two stage separator, dividing an output gas stream from the second two stage separator as a first gas stream and a second a second gas stream, increasing a temperature of the first gas stream by the third multi-flow heat exchanger by exchanging the heat with the propane refrigerant, dividing an output gas stream from the third multi-flow heat exchanger as a first gas portion and a second gas portion, sending the first gas portion to a second stage of the first two phase compressor, dividing the second gas portion into a gas stream and a fluid stream by a third two phase separator, compressing the gas stream from the third two phase separator by a second stage of the first two phase compressor, producing a vaporized refrigerant by combining the fluid stream from the third two phase separator with the liquid from the first two phase separator and producing the natural gas liquid by combining the vaporized refrigerant with the liquid from the second two phase separator in lower section of the distillation tower.
According to an embodiment herein, the step of forming a liquid reflux by condensing the second gas part includes: sub cooling the second gas part by the second multi-flow heat exchanger, reducing a pressure and a temperature of an output of the second multi-flow heat exchanger by an expander and sending an output of the expander to a top of the distillation tower. The output of the expander is the liquid reflux.
According to an embodiment herein, the step of forming the residue gas by combining a first gas part and a second fluid part includes: reducing a pressure and a temperature of the first gas part and a second fluid part by an expander, sending an output of the expander to the upper section of the distillation tower, increasing the temperature of an output of the first expander through a second multi-flow heat exchanger and a first multi-flow heat exchanger by exchanging the heat with the input gas, increasing a temperature and a pressure of the output gas from the first multi-flow heat exchanger by compressing in a first compressor and reducing the temperature of the output gas from the first compressor by a second air cooler to produce a residue gas, wherein the residue gas is the closed cycle refrigerant.
The demethanizer operated at approximately 25 bars is a distillation column containing conventional kinds of trays applied in the demethanizer towers. The demethanizer is equipped with four liquid draw trays in the lower section of the column to provide a heat to the column for stripping off the volatile components from the bottom liquid product. This is accomplished via the use of two multi stream heat exchangers MSHX-1, MSHX-2. The liquids drawn from the side of the demethanizer 14& 16, and 18& 20 are passed to the MSHX-2 and MSHX-1 at −54, −53, −36 & −16.5° C. respectively, and exit as streams 15&17, and 19 & 37 at approximately −42, −41, −24 & 25° C., respectively, prior to returning to the demethanizer
The residue gas 7 exiting the upper portion of the demethanizer is fed to the MSHX-2 exchanger, providing refrigeration for condensing/sub cooling the vapor split stream 6 and sub cooling the liquid stream 3 from the drum D-1. The residue gas 10 exiting the MSHX-2 is further warmed to near the feed gas temperature via MSHX-1. The warmed residue gas 11 leaving the MSHX-1 at approximately 34° C. is sent to the suction of the expander compressor C-1, where it is compressed to 29 bars by utilizing a work extracted from the expander Ex-1. Depending upon the needed delivery pressure, a residue gas compressor C-2 may be needed to further compress the residue gas stream 12 from the compressor C-1, followed by an air cooler AC-1, prior to its final delivery at 62 bar.
In this configuration, the refrigeration provided by the residue gas from the demethanizer, turbo expander Ex-1 and the side liquid draws is not sufficient to achieve a high level of ethane recovery. So, a self-refrigeration system is applied. Stream 20, the open cycle refrigerant, is withdrawn from the chimney tray of the demethanizer column; the resulting mixed refrigerant is preferentially fed to the MSHX-1 for sub cooling prior to being expanded through the expansion device VLV-1 at 9 bar. The expanded stream 21 is directed back to the MSHX-1 providing an indirect heat exchange with the Feed gas stream and thereafter fed to the suction knockout drum D-2 where un vaporized liquid 25, if any, is separated while the refrigerant is used to cool the inlet gas stream. The vapor stream 24 produced in the knockout drum D-2 is withdrawn from the top thereof and fed to a first stage of a two-stage recycle compressor C-3. The re-pressurized gas stream 28 exiting compressor C-3 is cooled in MSHX-3 to 89° C., and then the output 29 of the heat exchanger MSHX-3 is fed to the air cooler AC-2 resulting in a partial condensation. The partially condensed product 30 exiting the cooler AC-2 is introduced into separator D-3 where condensed liquid is separated. A portion of the output liquid stream 34 withdrawn from separator D-3 (representing closed cycle refrigerant) is used as a refrigerant in the heat exchanger MSHX-3. In fact this portion provides a part of the required refrigeration in the condensation section of open-cycle refrigeration system. The flow rate of Stream 34, pressure drop in the VLV-4 expansion device and the temperature of stream 35 are parameters, which should be adjusted, based on the open refrigeration cycle performance. Also these three parameters affect the air cooler performance and its design condition. Consequently a tradeoff between the compressors shaft work and condensation costs (fixed and operating costs related to the air cooler) will determine their optimum value.
A portion of the outlet stream 35 from MSHX-3 flows to D-4 knockout drum as the closed cycle separator. The vapor product 27 of D-4 is introduced into the second stage of the compressor C-3. After an indirect heat exchange with one or more process streams, the heated open and closed refrigerant is preferably combined for simplicity and introduced into suction knockout drum D-2 where the vaporized refrigerant is separated. The vapor stream 24 is then introduced to the first stage recycle compressor C-3. The pressure of a liquid product drawn from D-4 is increased by the P-2 pump, next this stream, 36 is mixed with stream 26 (Separator D-2 liquid product). Finally stream 37 is introduced to the bottom of the demethanizer column (stream 37). The pressure, temperature and flow rate of the process streams are presented in Table 1.
The form of combined curves is an important factor illustrating the efficiency of multi-flow heat exchangers. Pinch point in this curve illustrates the extreme proximity of the curves. The structural capabilities of the heat exchanger determine the lowest allowed temperature difference between cool and hot curves. The lowest temperature difference in this curve is considered to be 2° C.
MSXH-1 exchanger decreases the temperature of the feed to 300 C.
The outlet vapor stream 2 from D-1 drum is divided into two portions, the main portion 2 and the remaining portion 6. The main portion 2 which is about 42%, is expanded through a work-expansion turbine EX-1 prior to entering the demethanizer right below the top rectifying section as expander discharge 8. The remaining vapor portion 6 is cooled to substantially condensation, and in most cases sub-cooling, to approximately −71.5° C. via MSHX-2. This sub-cooled liquid stream 9 is expanded through the expansion valve VLV-2 to top of the demethanizer as a reflux liquid.
The demethanizer operated at approximately 23 bar is a distillation column containing conventional kinds of trays applied in the demethanizer towers. It is equipped with four liquid draw trays in the lower section of the column to provide heat to the column for stripping off volatile components from the bottom liquid product. This is accomplished via the use of two multi stream heat exchangers MSHX-1, MSHX-2. The side draw liquids 14&16 and 18&20 to enter the MSHX-2 and MSHX-1 at −55.29, −51.88, −31.34 & −7.70° C. respectively, and exit as streams 15, 17, 19 & 21 at approximately −43, −38, −20 & 35° C., respectively, prior to returning to the demethanizer
The residue gas 7 exiting the upper portion of the demethanizer is fed to the MSHX-2 exchanger, providing refrigeration for condensing/sub-cooling the vapor split stream 6 and sub-cooling the liquid stream 3 from the drum D-1. The residue gas 10 exiting the MSHX-2 is further warmed to near the feed gas temperature via MSHX-1. The warmed residue gas 11 leaving the MSHX-1 at approximately 10° C. is sent to the suction of the expander compressor C-1, where it is compressed to 26.6 bar by utilizing a work extracted from the expander Ex-1. Depending upon the delivery pressure needed, a residue gas compressor C-2 may be needed to further compress the residue gas stream 12 followed by an air cooler AC-1, prior to its final delivery at 62 bar.
In this configuration the refrigeration provided by the residue gas from the demethanizer, turbo expander EX-1 and side liquid draws is not sufficient to achieve high levels of ethane recovery. So, a three-stage propane refrigeration cycle is applied in order to compare the performance of the process with the configuration explained in the section 2.1. It should be noted that based on the potential of the proposed configuration, distribution of the propane refrigerant in the evaporators (MSHX-1 and MSHX-2) was done in the manner that the maximum cold recovery is gained. Also integrity of the process was considered though using the external refrigeration system.
The outlet stream 37 from the air cooler AC-2 (Condenser of the cycle) in the temperature of 37.7° C. and pressure of 13 bar follows to VLV-4 and its pressure is decreased to 6.5 bar. Stream 38 output from the VLV-4 is sent to the flash drum D-2. Gas product of the D-2 enters the third stage of a three-stage compressor C-3. The outlet liquid propane 24 from D-2 is split into two parts after passing an expansion device VLV-5. A portion 26 of it, 85% follows to MSHX-1 in and provides the required refrigeration for cooling the feed gas. Outlet stream 35 from MSHX-1 enters flash drum D-4 at −18° C. and 2.61 bars. Another portion, 28 follows to D-4 after passing expansion valve VLV-7. The gas product 33 of D-4 enters the second stage of compressor C-3 and the liquid product 29 which is the first stage refrigerant of the cycle follows to the MSHX-2 through the expansion valve VLV-6 at −38.55° C. and 1.18 bars. The outlet stream, 31 is not vaporized totally in this heat exchanger MSHX-2 so it is sent to the MSHX-1 as second evaporator. Finally the vaporized refrigerant 32 with minimum pressure enters the D-3 before following to the 1st stage of compressor C-3 at −15.65° C. and 1.17 bar. The pressure, temperature and flow rate of the process streams are presented in Table 2.
Tables 4, 5 and 6, indicate the required refrigeration (whether external or internal) for feeds under investigation.
As it was expected, the heavier is the gas, the more refrigeration does it need. Also, changes in specifications of the process as the combination of the feed varies can be followed through these two tables. Therefore the types of configurations can be compared accurately (according to
The embodiments herein provide a method and a system for providing required refrigeration load in a natural gas liquid recovery. The method and system provides required refrigeration load in a natural gas liquid recovery using an open-closed self-refrigeration system. The system and method for analyzing the configuration capability under circumstances in which the composition percentage is not stable and varies is provided. The process configuration provides required refrigeration load in a natural gas liquid recovery to reduce the required energy of the overall system. The method and system investigates the need of the unit to refrigeration proportional to the variations of feed composition percentage. The process configuration uses least number of multi-stream converters, further leading to the integration of the process operation. The process configuration here in performs on rich gases with high levels of liquid recovery. The presented process configuration reduces over all consumed energy of the system. The method and system for natural gas liquid recovery provides inherent integrity resulting in reduction of numbers and volume of the applied process equipments.
The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the appended claims.
Although the embodiments herein are described with various specific embodiments, it will be obvious for a person skilled in the art to practice the embodiments herein with modifications. However, all such modifications are deemed to be within the scope of the claims.
It is also to be understood that the following claims are intended to cover all of the generic and specific features of the embodiments described herein and all the statements of the scope of the embodiments, which as a matter of language might be said to fall there between.
