Patent Publication Number: US-2013239608-A1

Title: System and method for separating components in a gas stream

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 13/302,131, filed Nov. 22, 2011, and which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Natural gas (NG) at the well head typically comes in mixtures with other hydrocarbons; principally ethane, propane, butane, and pentanes. In addition, raw natural gas contains water vapor, hydrogen sulfide (H 2 S), carbon dioxide (CO 2 ), helium, nitrogen, and other compounds. The ethane, propane, butane, and pentanes are removed from NG, because they are currently more valuable than NG and can be sold separately. In addition, H 2 S currently is removed from NG, because it creates a corrosion problem when NG is transported in a pipeline. Carbon dioxide is an inert component and is removed to increase the energy content of the gas and decrease the energy penalty for NG transportation. 
     The most common technology to remove H 2 S and CO 2  from NG is amine scrubbing. Aside from using chemicals and solvents, this technology requires steam for the amine regeneration and has relatively high energy requirements. The heavy hydrocarbons are removed in a low-temperature distillation process that uses a refrigeration system, adding to the energy requirements. 
     While the current technology for separating components of natural gas and other gas streams work, there is a need for more “green” gas separation technologies. 
     SUMMARY 
     Briefly, the present invention satisfies the need for separating one or more components from a gas stream without using chemicals or solvents as with amine scrubbing technology, by using multiple stages of expansion of compressed gas to rapidly reduce the pressure and corresponding temperature, resulting in a phase change to enable separation. The back end cooled gas stream can also be fed back to the incoming stream for pre-cooling. 
     The present invention provides, in a first aspect, a system for separating components from a compressed gas stream. The system includes a first expansion stage, including an expander configured to receive a compressed gas stream, the expander further configured to solidify and/or liquefy at least one first component of the compressed gas stream and to remove solids, the expander having a first expansion output. The system further includes a second expansion stage coupled to the first expansion stage. The second expansion stage includes another expander configured to receive a portion of the first expansion output, the another expander further configured to solidify and/or liquefy at least one second component different from the first component and having a second expansion output. 
     The present invention provides, in a second aspect, a method of separating one or more components from a compressed gas stream. The method includes providing an input gas stream, the input gas stream being compressed and including a plurality of components, wherein it is desired to separate at least one component of the plurality of components. The method further includes expanding the input gas stream via an expander to decrease a pressure and a temperature thereof, in order to solidify and/or liquefy at least one of the plurality of components, separating by the expander the solidified at least one of the plurality of components from the expanded gas stream, further expanding the expanded gas stream via another expander after the separating in order to solidify and/or liquefy at least one other component of the plurality of components different from the at least one component, and separating the solidified and/or liquefied at least one other component from the further expanded gas stream to leave a remaining gas stream. 
     These, and other objects, features and advantages of this invention will become apparent from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a block diagram of one example of a system for separating components from a gas stream, in accordance with one or more aspects of the present invention. 
         FIG. 2  is a simplified block diagram of one example of an expander for solid and/or liquid separation from a gas stream, in accordance with one or more aspects of the present invention. 
         FIG. 3  is a schematic of a cross-sectional view of one example of the expander of  FIG. 2 , in accordance with one or more aspects of the present invention. 
         FIG. 4  is a schematic of a cross-sectional view of one example of a multi-stage expander for solid and/or liquid separation from a gas stream, in accordance with one or more aspects of the present invention. 
         FIG. 5  is a schematic of one example of an expander for solid and/or liquid separation with separation channels, in accordance with one or more aspects of the present invention. 
         FIG. 6  is a schematic of one example of a heated blade, in accordance with one or more aspects of the present invention. 
         FIG. 7  is a schematic of another example of a heated blade, in accordance with one or more aspects of the present invention. 
         FIG. 8  is a schematic of one example of flow path of heating gas in the heated blade of  FIG. 7 , in accordance with one or more aspects of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Aspects of the present invention and certain features, advantages, and details thereof, are explained more fully below with reference to the non-limiting examples illustrated in the accompanying drawings. Descriptions of some well-known materials, components, processing techniques, etc., may be omitted so as not to unnecessarily obscure the present invention in detail. It should be understood, however, that the detailed description and the specific examples, while indicating aspects of the present invention, are given by way of illustration only, and are not by way of limitation. Various substitutions, modifications, additions, and/or arrangements, within the spirit and/or scope of the underlying inventive concepts will be apparent to those skilled in the art from this disclosure. 
     Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about,” is not limited to the precise value specified. In some instances, the approximating language may correspond to the precision of an instrument for measuring the value. 
     The terminology used herein is for the purpose of describing particular examples only and is not intended to be limiting of the present invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprise” (and any form of comprise, such as “comprises” and “comprising”), “have” (and any form of have, such as “has” and “having”), “include (and any form of include, such as “includes” and “including”), and “contain” (and any form of contain, such as “contains” and “containing”) are open-ended linking verbs. As a result, a method or device that “comprises,” “has,” “includes” or “contains” one or more steps or elements possesses those one or more steps or elements, but is not limited to possessing only those one or more steps or elements. Likewise, a step of a method or an element of a device that “comprises,” “has,” “includes” or “contains” one or more features possesses those one or more features, but is not limited to possessing only those one or more features. Furthermore, a device or structure that is configured in a certain way is configured in at least that way, but may also be configured in ways that are not listed. 
     As used herein, the terms “may” and “may be” indicate a possibility of an occurrence within a set of circumstances; a possession of a specified property, characteristic or function; and/or qualify another verb by expressing one or more of an ability, capability, or possibility associated with the qualified verb. Accordingly, usage of “may” and “may be” indicates that a modified term is apparently appropriate, capable, or suitable for an indicated capacity, function, or usage, while taking into account that in some circumstances the modified term may sometimes not be appropriate, capable or suitable. For example, in some circumstances, an event or capacity can be expected, while in other circumstances the event or capacity cannot occur—this distinction is captured by the terms “may” and “may be.” 
     Reference is made below to the drawings, which are not drawn to scale for ease of understanding, some of which include high-level components that omit some detail, again, for ease of understanding. In some cases, those details are provided subsequently, or, if well known, they may be omitted. 
     At the location of its production, natural gas (NG) typically comes out of the ground in mixtures of methane with other hydrocarbons, water vapor, hydrogen sulfide, carbon dioxide, helium, nitrogen, and other compounds. For example, methane is a desired component of natural gas, as it can be used for fuel for heating, cooking and other purposes. The present invention describes, in one aspect, a method of separating other components from NG to leave the methane. Removal of hydrocarbons, CO 2 , and H 2 S is achieved by cooling NG via one or more compression and expansion cycles. At low temperatures, heavy hydrocarbons, CO 2  and H 2 S, undergo phase transformation and are converted from a gas to a solid or liquid phase for physical separation or filtering, while methane remains in the gas or liquid phase. Cold methane gas resulting can also be fed back to pre-cool the incoming NG after compression and before expansion. 
       FIG. 1  depicts one example of a system  100  for separating one or more components from a gas stream. In the present example, one or more components in a compressed natural gas stream are removed, in part, using multiple stages of liquid, gas and solid separation at various temperatures achieved by cooling, compression and expansion. In this particular example, carbon dioxide, hydrogen sulfide and heavy hydrocarbons are removed, yielding nearly pure methane. In addition, the methane gas, which is at a low temperature, may be fed back to the incoming natural gas stream for use in a heat exchanger to pre-cool the incoming gas stream. 
     Returning now to the example of  FIG. 1 , an incoming dry natural gas stream  102  is fed to a heat exchanger  104 , which may be a conventional heat exchanger, for pre-cooling before expansion. The incoming natural gas stream is preferably compressed, but if not, a conventional compression stage (not shown) can be added before or after heat exchanger  104 , and prior to flash  106 . Typically, the incoming natural gas will be from a pipeline at a temperature of about 50° F. to about 80° F. and a pressure of about 100 psi to about 900 psi. The pre-cooled and compressed natural gas stream  108  is then fed to flash  106 , which may be a conventional gas/liquid separator familiar to those skilled in the art. Due primarily to the pre-cooling and high pressure, the stream  108  out of the heat exchanger is a mixture of gas and liquid, though a vast majority thereof remains in a gaseous state. Although the incoming natural gas in this example is from a pipeline, it could instead be directly from a well head, which may not require any compression, as it comes out of the ground naturally compressed to varying degrees, depending on the particular location. 
     As noted, flash  106  separates compressed and pre-cooled stream  108  into gas  110  and liquid  112 . Separated gas  110  from the stream is fed to a first expansion stage including expander  114 , which will subsequently be described in detail, but the purpose thereof is to cool the gas by rapid expansion, such that one or more components are solidified and/or liquefied, and are removed by the expander. In practice, however, it will be understood that there may be residual solids and/or liquids in the output of the expander. The term “expander” as used herein refers to a radial, axial, or mixed flow turbo-machine through which a gas or gas mixture is expanded to produce work. Relatedly, the term “expansion stage” refers to an expander that may be coupled with one or more other elements to enhance or compliment separation of one or more components of a stream. Details of the expander are provided below after the description of  FIG. 1 . It will be understood that any expander of the type described in detail below also includes an outlet for separated solids and/or liquids. Such outlets are omitted in  FIG. 1  for clarity. 
     Although the temperature and pressure decrease in expander  114  has resulted in conversion of some components to solid and liquid phases, still other constituents of the stream remain in the gas stage. Thus, the stream  116  out of expander  114  includes liquid and gas, and is fed to flash  118  for separation thereof. Also fed to flash  118  is the separated liquid stream  112  from flash  106 , preferably after running the same through a Joule-Thompson valve  120  or similar. As one skilled in the art will know, such a valve reduces the pressure of the stream  112 , resulting in a slight cooling of the same. Additional inputs to flash  118  will subsequently be described. 
     A separated gas stream  122  from flash  118  is fed to a second expansion stage including expander  124 . Expander  124  may be of the same type as expander  114 , or, alternatively, may be of a type lacking the feature of solid/liquid removal. This type of expander simply provides expansion. Where additional solid removal is desired, the type of expander  114  would be warranted. As with the first expander, the stream coming into the expander is rapidly expanded in the expander, lowering the pressure and corresponding temperature thereof such that one or more components of the stream change phase to allow separation thereof. In the present example, the temperature is such that methane remains in a gas phase, and carbon dioxide is in a solid phase, while hydrogen sulfide and one or more heavy hydrocarbons (e.g., ethane, propane and butane) are in a liquid phase. The output stream  126  from the second expander  124  is fed to a third flash  128  to separate liquid and any solid present from gas. A vast majority of the separated output gas  130  includes methane, and is fed back to heat exchanger  104 . Being at a much lower temperature from the two expanders than the incoming natural gas, the relatively cold methane gas may be used to pre-cool the incoming natural gas prior to expansion. Conventional refrigeration could instead be used for pre-cooling, but is not as energy efficient. After cooling the incoming natural gas stream, the final output gas  130  may be, for example, compressed via conventional compressor  150 , prior to collection for end or further use. 
     As noted, the separated gas  130  includes mostly methane, however, some methane is also present in the separated liquid stream  132  out of flash  128 . The purpose of section  134  of system  100  is to recover the liquid methane. A heat exchanger  136 , which may be a conventional heat exchanger, is used to warm the separated liquid  132 . Warming may cause some components of the separated liquid to revert back to gas. The warmer separated liquid  138  is preferably fed through another Joule-Thompson valve  140 , prior to reaching a fourth flash  142  for separating gas from liquid. The separated gas  144  is fed back to the second flash  118 , as is the separated liquid  146  out of flash  118  itself. Although constituting a minority portion of the separated liquid  148  from flash  142 , the goal of recovering the remaining methane (in a liquid state) is achieved. 
     Gas temperatures and pressures after expanders  114  and  124  depend on the initial pressure of incoming stream  102 , and are typically in the range of about 1 atm to about 5 atm and about −110° C. to about −150° C. after expander  114 , and about 0.4 atm to about 1 atm and about −180° C. to about −185° C. after expander  125 . 
       FIG. 2  is a simplified block diagram of one example of an expander  200 , in accordance with one or more aspects of the present invention.  FIG. 3  is one example of a cross-sectional view of expander  200 , in accordance with one or more aspects of the present invention. The description below commonly refers to the expander in both  FIGS. 2 and 3 . 
     As indicated in the example of  FIG. 3 , the expander  200  includes a housing  214 . As indicated in  FIG. 3 , the expander  200  may further include at least one rotating component or a rotor  215 . The expander  200  may further include at least one stationary component  216 . The stationary component may include a stator or a nozzle. As indicated in  FIG. 3 , the expander  200  may further include one or more seals  217 . As indicated in  FIG. 3 , the expander  200  may further include one or more blades  219 / 222 , which may be one or more stationary blades  219  and one or more rotor blades  222 , respectively. 
     The expander  200  may further include a plurality of outlets  212  and  213 , as indicated in  FIGS. 2 and 3 . As indicated in  FIGS. 2 and 3 , the expander  200  may further include at least one first outlet  212  configured to discharge a stream  202  of solids and/or liquids after phase change from expansion temperature drop. The housing  214  includes one or more separation channels  218  in fluid communication with the at least one first outlet  212 , and configured to separate stream  202  from incoming stream  201 . The expander  200  may further include a volute/housing  214  including the one or more separation channels  218  configured to discharge stream  202 , as indicated in  FIGS. 3-5 . The at least one first outlet  212  may be configured to discharge stream  202  through the separation channels  218  present in the housing/volute  214 , as indicated in  FIGS. 3-5 . As indicated in  FIGS. 3 and 4 , the at least one first outlet  212  may be disposed upstream of the rotating component. The term “upstream” as used herein refers to a location between the stationary component  216  and the rotating component  215 . As indicated in  FIG. 5 , the first outlet  212  may be disposed in the housing  214  at a location between the stationary component  216  and the rotating component  215 . Where a multi-stage expander is included, as in  FIG. 4 , the first outlet may be located upstream of at least one rotating component. Where a plurality of first outlets  212  is included, a first outlet  212  may be located upstream of at least one rotating component  215 , as indicated in  FIG. 4 . 
     The flow field within the expander  200  may be utilized to aid in separation of solids and/or liquid from gas by incorporating the one or more separation channels  218  into the expander housing  214 . In addition, the separation channels  218  may be designed such that the solid and/or liquid particles enter due to centrifugal force, and may be precluded from re-entering the expander flow path by a deflector. 
     Where configured to separate solids, the incoming stream may include or the separated stream may be made to include one or more carrier gases. For example, where the incoming stream includes CO 2 , which is frozen to a solid, the incoming or separated stream may further include, for example, one or more of nitrogen gas, oxygen gas, or carbon dioxide gas, as a carrier gas that may be transported to the first outlet  212  along with the solid CO 2  by centrifugal force. 
     In one example, at least one component of the expander  200  includes a coating configured to substantially reduce or preclude adhesion of one or more solids to a surface of the expander component. One or more of the housing  214 , the rotating component  215 , or the stationary component  216 , may include such a coating. For example,  FIG. 3  depicts rotating component  215  in the expander  200  including such a coating  220  on a surface  221  of the rotating component  215 . In one example, the coating  220  includes a non-stick material capable of precluding adhesion of the one or more solids to the surface  221  of the rotating component  215 . 
     In another example, the expander  200  includes at least one heated component configured to preclude adhesion of one or more solids to a surface of the expander component. For example, one or more of the housing  214 , the rotating component  215 , or the stationary component  216 , may include a heated component to preclude adhesion of one or more solids to a surface of the expander component. For example, in  FIG. 2 , stationary component  216  in the expander  200  may be heated to preclude adhesion of one or more solids to a surface  223  of the stationary component  216 . 
     As still another example, one or more of the stationary blades may be heated by using electrical heating elements.  FIG. 6  illustrates one example of an electrically heated blade  219 . The blade  219 , as indicated in  FIG. 6 , further includes heated elements  224  disposed in holes of the blade  219 . As yet another example, one or more components of the expander  200  may be heated by circulating air or gas.  FIG. 7  illustrates one example of the present invention, including a blade  219  heated by circulating gas. The blade  219  may further include gas flow channels  225 , such as, for example, Z-shaped channels, as indicated in  FIG. 7 . The gas flow channels may have any suitable shape, such as, for example, U-shape, E-shape, and the like.  FIG. 8  further shows an illustrative flow path of the heating gas in the heated blade  219 . 
     As indicated in  FIGS. 2 and 3 , the expander  200  further includes at least one second outlet  213  configured to discharge a remaining stream  203  after separation from incoming stream  201  of any solids and/or liquids in stream  202 . As indicated in  FIGS. 3 and 4 , the second outlet may be disposed downstream of the rotating component. Where a multi-stage expander is present, as indicated in  FIG. 4 , the second outlet  213  may be located downstream of at least one rotating component  214 . For example, the second outlet  213  may be located downstream of the last rotating component  214  in the expander  200 . 
     As previously noted, the expander  100  for separating one or more solids and/or liquids from a gas stream  201  may include a single-stage expander, as illustrated in  FIG. 3  or a multi-stage expander  200 , as illustrated in  FIG. 4 . The multi-stage expander may include a plurality of stationary components  216  and a plurality of rotating components  215 . Further, the multi-stage expander  200  may be configured to include a plurality of first outlets  212 . As indicated in  FIG. 4 , the plurality of first outlets  212  may be configured to discharge a plurality of solid and/or liquid streams  202  at different stages of the multi-stage expander  200 . The multi-stage expander  200  may further include at least one second outlet  213 , as indicated in  FIG. 4 . 
     Example 
     In one example, using the system of  FIG. 1 , an incoming natural gas stream  102  has a temperature of about 25° C., a pressure of about 200 bar, and is included of about 81.55% (i.e., 0.8155 mole fraction) methane (CH 4 ), along with about 4.07% carbon dioxide (CO 2 ), about 2.55% hydrogen sulfide (H 2 S) and various other components as previously noted. After passing through heat exchanger  104 , the temperature of the stream has dropped to about −32.8° C., while the pressure and phase remain the same. After gas/liquid separation in flash  106 , the gas  110  has a temperature of about −74.7° C. and a pressure of about 36.5 bar. Expander  114  has a significant effect, rapidly lowering the pressure to about 3 bar, which causes the temperature to drop to about −144.7° C. In addition, while any solids (here, CO 2 ) have been removed by the expander, as described previously, the remainder of the stream  116  out of the expander is a combination of about 85.7% gas and about 14.3% liquid. This combination stream is then sent to flash  118 , along with other streams in the system. 
     The gas stream  122  separated by flash  118  has a temperature of about −135.5° C. and a pressure of about 3 bar. At this point, the gas stream  122  is about 96% methane. After the second expander  124 , the output  126  has become a mixture of gas and liquid, about 93.6% and 6.4%, respectively, due to expansion further dropping the pressure to about 1 bar and the temperature to about −161.1° C. If expander  124  is of the same type as expander  114 , then H 2 S and additional CO 2  are solidified and removed. The portion of the stream out of the second expander that is methane has increased slightly to about 97%. The combination stream is again separated in flash  128 , and the gas output  130  has increased slightly to about −141.4° C. and 1.8 bar, while the fraction of methane remains about the same. 
     The liquid  132  out of flash  128  remains at a temperature of about −161.1° C. and a pressure of about 1 bar. As noted above, the purpose of section  134  of heat exchanger  136 , valve  140  and flash  142  is to remove as much of the liquid methane as possible. Accordingly, heat exchanger  136  raises the temperature of the stream  138  to about −90° C. and a pressure of about 20 bar. Raising the temperature and pressure results in almost a complete phase change from liquid to gas, with only about 0.02% remaining as liquid. After JT valve  140 , the stream has a temperature of −112.8° C., a pressure of 3 bar, and is now fully in the gas phase. The gas  144  out of flash  142  has the same characteristics as prior to the flash, however, the liquid  148  has a temperature of about −74.7° C. and a pressure of about 36.5 bar. The liquid is about 61% methane. The gas out of flash  142  is fed into flash  118 , as is the stream exiting JT valve  120 , which has a temperature of about −126.2° C. and a pressure of about 3 bar. The stream exiting JT valve  120  is an almost equal mixture of gas and liquid. After being fed back to heat exchanger  104  for cooling the incoming stream, the final methane stream  130  has a temperature of about 10° C. and a pressure of about 1.8 bar. The methane remains at about 97% of the final stream. Optionally, the final stream may be compressed in compressor  150  (e.g., for transport) to a pressure of about 30 bar, also raising the temperature to about 25° C. 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the various embodiments without departing from their scope. While the dimensions and types of materials described herein are intended to define the parameters of the various embodiments, they are by no means limiting and are merely exemplary. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. The scope of the various embodiments should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. Further, the limitations of the following claims are not written in means-plus-function format and are not intended to be interpreted based on 35 U.S.C. §112, sixth paragraph, unless and until such claim limitations expressly use the phrase “means for” followed by a statement of function void of further structure. It is to be understood that not necessarily all such objects or advantages described above may be achieved in accordance with any particular embodiment. Thus, for example, those skilled in the art will recognize that the systems and techniques described herein may be embodied or carried out in a manner that achieves or optimizes one advantage or group of advantages as taught herein without necessarily achieving other objects or advantages as may be taught or suggested herein. 
     While the present invention has been described in detail in connection with only a limited number of embodiments, it should be readily understood that the present invention is not limited to such disclosed embodiments. Rather, the present invention can be modified to incorporate any number of variations, alterations, substitutions or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the present invention. Additionally, while various embodiments of the present invention have been described, it is to be understood that aspects of the disclosure may include only some of the described embodiments. Accordingly, the present invention is not to be seen as limited by the foregoing description, but is only limited by the scope of the appended claims. 
     This written description uses examples to disclose the present invention, including the best mode, and also to enable any person skilled in the art to practice the present invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the present invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.