Patent Publication Number: US-11378332-B2

Title: Mixing and heat integration of melt tray liquids in a cryogenic distillation tower

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of U.S. Provisional Patent Application No. 62/691,679 filed Jun. 29, 2018, entitled MIXING AND HEAT INTEGRATION OF MELT TRAY LIQUIDS IN A CRYOGENIC DISTILLATION TOWER. 
     This application is related to but does not claim priority to U.S. Provisional patent application Nos. 61/912,975 Filed Dec. 6, 2013 and titled METHOD AND SYSTEM FOR SEPARATING A FEED STREAM WITH A FEED STREAM DISTRIBUTION MECHANISM; 61/912,957 filed on Dec. 6, 2013 and titled METHOD AND DEVICE FOR SEPARATING HYDROCARBONS AND CONTAMINANTS WITH A SPRAY ASSEMBLY; 62/044,770 filed on Sep. 2, 2014 and titled METHOD AND DEVICE FOR SEPARATING HYDROCARBONS AND CONTAMINANTS WITH A SPRAY ASSEMBLY; 61/912,959 filed on Dec. 6, 2013 and titled METHOD AND SYSTEM OF MAINTAINING A LIQUID LEVEL IN A DISTILLATION TOWER; 61/912,964 filed on Dec. 6, 2013 and titled METHOD AND DEVICE FOR SEPARATING A FEED STREAM USING RADIATION DETECTORS; 61/912,970 filed on Dec. 6, 2013 and titled METHOD AND SYSTEM OF DEHYDRATING A FEED STREAM PROCESSED IN A DISTILLATION TOWER; 61/912,978 filed on Dec. 6, 2013 and titled METHOD AND SYSTEM FOR PREVENTING ACCUMULATION OF SOLIDS IN A DISTILLATION TOWER; 61/912,983 filed on Dec. 6, 2013 and titled METHOD OF REMOVING SOLIDS BY MODIFYING A LIQUID LEVEL IN A DISTILLATION TOWER; 61/912,984 filed on Dec. 6, 2013 and titled METHOD AND SYSTEM OF MODIFYING A LIQUID LEVEL DURING START-UP OPERATIONS; 61/912,986 filed on Dec. 6, 2013 and titled METHOD AND DEVICE FOR SEPARATING HYDROCARBONS AND CONTAMINANTS WITH A HEATING MECHANISM TO DESTABILIZE AND/OR PREVENT ADHESION OF SOLIDS; and 61/912,987 filed on Dec. 6, 2013 and titled METHOD AND DEVICE FOR SEPARATING HYDROCARBONS AND CONTAMINANTS WITH A SURFACE TREATMENT MECHANISM. 
     This application is also related to U.S. Application No. 62/691,676, titled HYBRID TRAY FOR INTRODUCING A LOW CO2 FEED STREAM INTO A DISTILLATION TOWER, having common inventors with this application, and filed on the same date as this application. 
    
    
     BACKGROUND 
     Fields of Disclosure 
     The disclosure relates generally to the field of fluid separation. More specifically, the disclosure relates to the cryogenic separation of contaminants, such as acid gas, from a hydrocarbon. 
     Description of Related Art 
     This section is intended to introduce various aspects of the art, which may be associated with the present disclosure. This discussion is intended to provide a framework to facilitate a better understanding of particular aspects of the present disclosure. Accordingly, it should be understood that this section should be read in this light, and not necessarily as admissions of prior art. 
     The production of natural gas hydrocarbons, such as methane and ethane, from a reservoir oftentimes carries with it the incidental production of non-hydrocarbon gases. Such gases include contaminants, such as at least one of carbon dioxide (“CO 2 ”), hydrogen sulfide (“H 2 S”), carbonyl sulfide, carbon disulfide and various mercaptans. When a feed stream being produced from a reservoir includes these contaminants mixed with hydrocarbons, the stream is oftentimes referred to as “sour gas.” 
     Many natural gas reservoirs have relatively low percentages of hydrocarbons and relatively high percentages of contaminants. Contaminants may act as a diluent and lower the heat content of hydrocarbons. Some contaminants, like sulfur-bearing compounds, are noxious and may even be lethal. Additionally, in the presence of water some contaminants can become quite corrosive. 
     It is desirable to remove contaminants from a stream containing hydrocarbons to produce sweet and concentrated hydrocarbons. Specifications for pipeline quality natural gas typically call for a maximum of 2-4% CO 2  and ¼ grain H 2 S per 100 scf (4 ppmv) or 5 mg/Nm3 H 2 S. Specifications for lower temperature processes such as natural gas liquefaction plants or nitrogen rejection units typically require less than 50 ppm CO 2 . 
     The separation of contaminants from hydrocarbons is difficult and consequently significant work has been applied to the development of hydrocarbon/contaminant separation methods. These methods can be placed into three general classes: absorption by solvents (physical, chemical and hybrids), adsorption by solids, and distillation. 
     Separation by distillation of some mixtures can be relatively simple and, as such, is widely used in the natural gas industry. However, distillation of mixtures of natural gas hydrocarbons, primarily methane, and one of the most common contaminants in natural gas, carbon dioxide, can present significant difficulties. Conventional distillation principles and conventional distillation equipment are predicated on the presence of only vapor and liquid phases throughout the distillation tower. The separation of CO 2  from methane by distillation involves temperature and pressure conditions that result in solidification of CO 2  if a pipeline or better quality hydrocarbon product is desired. The required temperatures are cold temperatures typically referred to as cryogenic temperatures. 
     Certain cryogenic distillations can overcome the above mentioned difficulties. These cryogenic distillations provide the appropriate mechanism to handle the formation and subsequent melting of solids during the separation of solid-forming contaminants from hydrocarbons. The formation of solid contaminants in equilibrium with vapor-liquid mixtures of hydrocarbons and contaminants at particular conditions of temperature and pressure takes place in a controlled freeze zone (CFZ) section. A lower section may also help separate the contaminants from the hydrocarbons but the lower section is operated at a temperature and pressure that does not form solids. 
     In known cryogenic distillation applications using a CFZ section, a feed stream is dried and precooled to about −60° F. before introduction to the distillation tower below the CFZ section and melt tray. The vapor component of the cooled feed stream combines with the vapor rising from the stripping section of the tower and bubbles through the liquid on the melt tray. This serves several beneficial purposes, including: the rising vapor stream is cooled and a portion of the CO 2  is condensed, resulting in a cooler and cleaner gas stream entering the open portion of the CFZ spray chamber; the rising vapor stream is evenly distributed across the tower cross section as it enters the CFZ spray chamber; most of the required melt tray heat input is provided via sensible heat from cooling the vapor and latent heat from condensing a portion of the CO 2  in the gas stream; and the melt tray liquid is vigorously mixed, which facilitates melting of solid CO 2  particles falling into the melt tray with the bulk liquid temperature only 2 to 3 degrees F. above the melting point of CO 2 . 
     To achieve reliable melting of solids falling into the melt tray liquid, it is important that the melt tray liquid be maintained slightly above the melting point of CO 2  and well mixed. In a standard cryogenic distillation design having a CFZ section, the mixing is achieved via vapor bubbling through the liquid. This vapor also provides the majority of the heat input required for the melt tray. A heating coil immersed in the melt tray liquid is used for fine tune control. 
     SUMMARY 
     The present disclosure provides a device and method for separating contaminants from hydrocarbons, among other things. 
     In an aspect, a cryogenic distillation tower is provided for separating a feed stream. A distillation section permits vapor to rise upwardly therefrom. One or more lines direct the feed stream into the distillation tower. A controlled freeze zone section is situated above the distillation section. The controlled freeze zone is constructed and arranged to form a solid from the feed stream. The controlled freeze zone section includes a spray assembly in an upper section of the controlled freeze zone, and a melt tray assembly in a lower section of the controlled freeze zone. The melt tray assembly includes at least one vapor stream riser that directs the vapor from the distillation section into liquid retained by the melt tray assembly, and one or more draw-off openings positioned to permit a portion of the liquid retained by the melt tray assembly to exit the controlled freeze zone section. A heat exchanger heats the portion of the liquid through indirect heat exchange with a heating fluid. One or more return inlets return the portion of the liquid to the melt tray assembly after the portion of the liquid has been heated in the heat exchanger. 
     In another aspect, a method is provided for cryogenically separating contaminants from a feed stream in a distillation tower. The feed stream is directed into the distillation tower. Vapor is permitted to rise upwardly from a distillation section of the distillation tower. A solid is formed in a controlled freeze zone section of the distillation tower. The controlled freeze zone section is situated above the distillation section. The solid comprises contaminants in the feed stream. The vapor from the distillation section is directed into liquid retained by a melt tray assembly using at least one vapor stream riser. The solid is melted using the liquid retained by the melt tray assembly. A portion of the liquid retained by the melt tray assembly is permitted to exit the controlled freeze zone section. The portion of the liquid is heated through indirect heat exchange with a heating fluid in a heat exchanger. The portion of the liquid is returned to the melt tray assembly after the liquid has been heated in the heat exchanger. 
     The foregoing has broadly outlined the features of the present disclosure in order that the detailed description that follows may be better understood. Additional features will also be described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects and advantages of the disclosure will become apparent from the following description, appending claims and the accompanying drawings, which are briefly described below. 
         FIG. 1  is a schematic diagram of a tower with sections within a single vessel. 
         FIG. 2  is a schematic diagram of a tower with sections within multiple vessels. 
         FIG. 3  is a schematic diagram of a tower with sections within a single vessel. 
         FIG. 4  is a schematic diagram of a tower with sections within multiple vessels. 
         FIG. 5  is a side view of a middle controlled freeze zone section of a distillation tower. 
         FIG. 6  is a top plan view of a melt tray according to disclosed aspects. 
         FIG. 7  is a flowchart of a method according to disclosed aspects. 
     
    
    
     It should be noted that the figures are merely examples and no limitations on the scope of the present disclosure are intended thereby. Further, the figures are generally not drawn to scale, but are drafted for purposes of convenience and clarity in illustrating various aspects of the disclosure. 
     DETAILED DESCRIPTION 
     For the purpose of promoting an understanding of the principles of the disclosure, reference will now be made to the features illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the disclosure is thereby intended. Any alterations and further modifications, and any further applications of the principles of the disclosure as described herein are contemplated as would normally occur to one skilled in the art to which the disclosure relates. It will be apparent to those skilled in the relevant art that some features that are not relevant to the present disclosure may not be shown in the drawings for the sake of clarity. 
     As referenced in this application, the terms “stream,” “gas stream,” “vapor stream,” and “liquid stream” refer to different stages of a feed stream as the feed stream is processed in a distillation tower that separates methane, the primary hydrocarbon in natural gas, from contaminants. Although the phrases “gas stream,” “vapor stream,” and “liquid stream,” refer to situations where a gas, vapor, and liquid is mainly present in the stream, respectively, there may be other phases also present within the stream. For example, a gas may also be present in a “liquid stream.” In some instances, the terms “gas stream” and “vapor stream” may be used interchangeably. 
     As utilized herein, the terms “approximately,” “about,” “substantially,” and similar terms are intended to have a broad meaning in harmony with the common and accepted usage by those of ordinary skill in the art to which the subject matter of this disclosure pertains. It should be understood by those of skill in the art who review this disclosure that these terms are intended to allow a description of certain features described and claimed without restricting the scope of these features to the precise numeral ranges provided. Accordingly, these terms should be interpreted as indicating that insubstantial or inconsequential modifications or alterations of the subject matter described are considered to be within the scope of the disclosure. 
     The articles “the”, “a” and “an” are not necessarily limited to mean only one, but rather are inclusive and open ended so as to include, optionally, multiple such elements. 
     The disclosure relates to a system and method for separating a feed stream in a distillation tower. The system and method helps optimally match where the feed stream should enter the distillation tower based on the concentrations of components in the feed stream so as to improve energy efficiency and/or optimally size the distillation tower. The system and method may also help prevent the undesired accumulation of solids in the controlled freeze zone section of the distillation tower.  FIGS. 1-7  of the disclosure display various aspects of the system and method. 
     The system and method may separate a feed stream having methane and contaminants. The system may comprise a distillation tower  104 ,  204  ( FIGS. 1-2 ). The distillation tower  104 ,  204  may separate the contaminants from the methane. 
     The distillation tower  104 ,  204  may be separated into three functional sections: a lower section  106 , a middle controlled freeze zone section  108  and an upper section  110 . The distillation tower  104 ,  204  may incorporate three functional sections when the upper section  110  is needed and/or desired. 
     The distillation tower  104 ,  204  may incorporate only two functional sections when the upper section  110  is not needed and/or desired. When the distillation tower does not include an upper section  110 , a portion of vapor leaving the middle controlled freeze zone section  108  may be condensed in a condenser  122  and returned as a liquid stream via a spray assembly  129 . Moreover, lines  18  and  20  may be eliminated, elements  124  and  126  may be one and the same, and elements  150  and  128  may be one and the same. The stream in line  14 , now taking the vapors leaving the middle controlled freeze section  108 , directs these vapors to the condenser  122 . 
     The lower section  106  may also be referred to as a stripper section. The middle controlled freeze zone section  108  may also be referred to as a controlled freeze zone section. The upper section  110  may also be referred to as a rectifier section. 
     The sections of the distillation tower  104  may be housed within a single vessel ( FIGS. 1 and 3 ). For example, the lower section  106 , the middle controlled freeze zone section  108 , and the upper section  110  may be housed within a single vessel  164 . 
     The sections of the distillation tower  204  may be housed within a plurality of vessels to form a split-tower configuration ( FIGS. 2 and 4 ). Each of the vessels may be separate from the other vessels. Piping and/or another suitable mechanism may connect one vessel to another vessel. In this instance, the lower section  106 , middle controlled freeze zone section  108  and upper section  110  may be housed within two or more vessels. For example, as shown in  FIGS. 2 and 4 , the upper section  110  may be housed within a single vessel  254  and the lower and middle controlled freeze zone sections  106 ,  108  may be housed within a single vessel  264 . When this is the case, a liquid stream exiting the upper section  110 , may exit through a liquid outlet bottom  260 . The liquid outlet bottom  260  is at the bottom of the upper section  110 . Although not shown, each of the sections may be housed within its own separate vessel, or one or more section may be housed within separate vessels, or the upper and middle controlled freeze zone sections may be housed within a single vessel and the lower section may be housed within a single vessel, etc. When sections of the distillation tower are housed within vessels, the vessels may be side-by-side along a horizontal line and/or above each other along a vertical line. 
     The split-tower configuration may be beneficial in situations where the height of the distillation tower, motion considerations, and/or transportation issues, such as for remote locations, need to be considered. This split-tower configuration allows for the independent operation of one or more sections. For example, when the upper section is housed within a single vessel and the lower and middle controlled freeze zone sections are housed within a single vessel, independent generation of reflux liquids using a substantially contaminant-free, largely hydrocarbon stream from a packed gas pipeline or an adjacent hydrocarbon line, may occur in the upper section. And the reflux may be used to cool the upper section, establish an appropriate temperature profile in the upper section, and/or build up liquid inventory at the bottom of the upper section to serve as an initial source of spray liquids for the middle controlled freeze zone section. Moreover, the middle controlled freeze zone and lower sections may be independently prepared by chilling the feed stream, feeding it to the optimal location be that in the lower section or in the middle controlled freeze zone section, generating liquids for the lower and the middle controlled freeze zone sections, and disposing the vapors off the middle controlled freeze zone section while they are off specification with too high a contaminant content. Also, liquid from the upper section may be intermittently or continuously sprayed, building up liquid level in the bottom of the middle controlled freeze zone section and bringing the contaminant content in the middle controlled freeze zone section down and near steady state level so that the two vessels may be connected to send the vapor stream from the middle controlled freeze zone section to the upper section, continuously spraying liquid from the bottom of the upper section into the middle controlled freeze zone section and stabilizing operations into steady state conditions. The split tower configuration may utilize a sump of the upper section as a liquid receiver for the pump  128 , therefore obviating the need for a liquid receiver  126  in  FIGS. 1 and 3 . 
     The system may also include a heat exchanger  100  ( FIGS. 1-4 ). The feed stream  10  may enter the heat exchanger  100  before entering the distillation tower  104 ,  204 . The feed stream  10  may be cooled within the heat exchanger  100 . The heat exchanger  100  helps drop the temperature of the feed stream  10  to a level suitable for introduction into the distillation tower  104 ,  204 . 
     The system may include an expander device  102  ( FIGS. 1-4 ). The feed stream  10  may enter the expander device  102  before entering the distillation tower  104 ,  204 . The feed stream  10  may be expanded in the expander device  102  after exiting the heat exchanger  100 . The expander device  102  helps drop the temperature of the feed stream  10  to a level suitable for introduction into the distillation tower  104 ,  204 . The expander device  102  may be any suitable device, such as a valve. If the expander device  102  is a valve, the valve may be any suitable valve that may aid in cooling the feed stream  10  before it enters the distillation tower  104 ,  204 . For example, the valve  102  may comprise a Joule-Thompson (J-T) valve. 
     The system may include a feed separator  103  ( FIGS. 3-4 ). The feed stream may enter the feed separator before entering the distillation tower  104 ,  204 . The feed separator may separate a feed stream having a mixed liquid and vapor stream into a liquid stream and a vapor stream. Lines  12  may extend from the feed separator to the distillation tower  104 ,  204 . One of the lines  12  may receive the vapor stream from the feed separator. Another one of the lines  12  may receive the liquid stream from the feed separator. Each of the lines  12  may extend to the same and/or different sections (i.e. middle controlled freeze zone, and lower sections) of the distillation tower  104 ,  204 . The expander device  102  may or may not be downstream of the feed separator  103 . The expander device  102  may comprise a plurality of expander devices  102  such that each line  12  has an expander device  102 . 
     The system may include a dehydration unit  261  ( FIGS. 1-4 ). The feed stream  10  may enter the dehydration unit  261  before entering the distillation tower  104 ,  204 . The feed stream  10  enters the dehydration unit  261  before entering the heat exchanger  100  and/or the expander device  102 . The dehydration unit  261  removes water from the feed stream  10  to prevent water from later presenting a problem in the heat exchanger  100 , expander device  102 , feed separator  103 , or distillation tower  104 ,  204 . The water can present a problem by forming a separate water phase (i.e., ice and/or hydrate) that plugs lines, equipment or negatively affects the distillation process. The dehydration unit  261  dehydrates the feed stream to a dew point sufficiently low to ensure a separate water phase does not form at any point downstream during the rest of the process. The dehydration unit may be any suitable dehydration mechanism, such as a molecular sieve or a glycol dehydration unit. 
     The system may include a filtering unit (not shown). The feed stream  10  may enter the filtering unit before entering the distillation tower  104 ,  204 . The filtering unit may remove undesirable contaminants from the feed stream before the feed stream enters the distillation tower  104 ,  204 . Depending on what contaminants are to be removed, the filtering unit may be before or after the dehydration unit  261  and/or before or after the heat exchanger  100 . 
     The system may include lines  12 . Each of the lines may be referred to as an inlet channel  12 . The feed stream is introduced into the distillation tower  104 ,  204  through one of the lines  12 . One or more lines  12  may extend to the lower section  106  or the middle controlled freeze zone section  108  of the distillation tower  104 ,  204  to another of the lines  12 . For example, the line  12  may extend to the lower section  106  such that the feed stream  10  may enter the lower section  106  of the distillation tower  104 ,  204  ( FIGS. 1-4 ). Each line  12  may directly or indirectly extend to the lower section  106  or the middle controlled freeze zone section  108 . Each line  12  may extend to an outer surface of the distillation tower  104 ,  204  before entering the distillation tower. 
     If the system includes the feed separator  103  ( FIGS. 3-4 ), the line  12  may comprise a plurality of lines  12 . Each line may be the same line as one of the lines that extends from the feed separator to a specific portion of the distillation tower  104 ,  204 . 
     Before entering the distillation tower  104 ,  204 , a sample of the feed stream  10  may enter an analyzer (not shown). The sample of the feed stream  10  may be a small sample of the feed stream  10 . The feed stream  10  may comprise feed from multiple feed sources or feed from a single feed source. Each feed source may comprise, for example, a separate reservoir, one or more wellbores within one or more reservoirs, etc. The analyzer may determine the percentage of CO 2  in the sample of the feed stream  10  and, therefore, the content of CO 2  in the feed stream  10 . The analyzer may connect to multiple lines  12  so that the feed stream  10  can be sent to one or more sections  106 ,  108  of the distillation tower  104 ,  204  after the sample of the feed stream  10  exits the analyzer. If the analyze determines that the percentage of CO 2  is greater than about 20% or greater than 20%, the analyzer may direct the feed stream to the line  12  extending from the lower section  106 . If the analyzer determines that the percentage of CO 2  is less than about 20% or less than 20%, the analyzer may direct the feed stream to the line  12  extending from the middle controlled freeze zone section  108 . The analyzer may be any suitable analyzer. For example, the analyzer may be a gas chromatograph or an IR analyzer. The analyzer may be positioned before the feed stream  10  enters the heat exchanger  100 . The feed stream  10  entering the analyzer may be a single phase. 
     While the feed stream  10  may be introduced into any section of the distillation tower  104 ,  204  regardless of the percentage of CO 2  in the feed stream  10 , it is more efficient to introduce the feed stream  10  into the section of the distillation tower  104 ,  204  that will employ the best use of energy. For this reason, it is preferable to introduce the feed stream to the lower section  106  when the percentage of CO 2  in the feed stream is greater than any percentage about 20% or greater than 20% and to the middle controlled freeze zone section  108  when the percentage of CO 2  in the feed stream is any percentage less than about 20% or less than 20%. 
     The feed stream may be directly or indirectly fed to one of the sections  106 ,  108 . Thus, for the best use of energy it is best to introduce the feed stream into the distillation tower  104 ,  204  at the point in the distillation process of the distillation tower  104 ,  204  that matches the relevant percentage or content of CO 2  in the feed stream. 
     The feed stream  10  may enter a feed separator  103 . The feed separator  103  separates a feed stream vapor portion from a feed stream liquid portion before the feed stream is introduced into the distillation tower  104 ,  204 . The feed stream vapor portion may be fed to a different section or portion within a section of the distillation tower  104 ,  204  than the feed stream liquid portion. For example, the feed stream vapor portion may be fed to an upper controlled freeze zone section  39  of the middle controlled freeze zone section  108  and/or the feed stream liquid portion may be fed to a lower controlled freeze zone section  40  of the middle controlled freeze zone section  108  or to the lower section  106  of the distillation tower. 
     The lower section  106  is constructed and arranged to separate the feed stream  10  into an enriched contaminant bottom liquid stream (i.e., liquid stream) and a freezing zone vapor stream (i.e., vapor stream). The lower section  106  separates the feed stream at a temperature and pressure at which no solids form. The liquid stream may comprise a greater quantity of contaminants than of methane. The vapor stream may comprise a greater quantity of methane than of contaminants. In any case, the vapor stream is lighter than the liquid stream. As a result, the vapor stream rises from the lower section  106  and the liquid stream falls to the bottom of the lower section  106 . 
     The lower section  106  may include and/or connect to equipment that separates the feed stream. The equipment may comprise any suitable equipment for separating methane from contaminants, such as one or more packed sections  181 , or one or more distillation trays with perforations downcomers and weirs ( FIGS. 1-4 ). 
     The equipment may include components that apply heat to the stream to form the vapor stream and the liquid stream. For example, the equipment may comprise a first reboiler  112  that applies heat to the stream. The first reboiler  112  may be located outside of the distillation tower  104 ,  204 . The equipment may also comprise a second reboiler  172  that applies heat to the stream. The second reboiler  172  may be located outside of the distillation tower  104 ,  204 . Line  117  may lead from the distillation tower to the second reboiler  172 . Line  17  may lead from the second reboiler  172  to the distillation tower. Additional reboilers, set up similarly to the second reboiler described above, may also be used. 
     The first reboiler  112  may apply heat to the liquid stream that exits the lower section  106  through a liquid outlet  160  of the lower section  106 . The liquid stream may travel from the liquid outlet  160  through line  28  to reach the first reboiler  112  ( FIGS. 1-4 ). The amount of heat applied to the liquid stream by the first reboiler  112  can be increased to separate more methane from contaminants. The more heat applied by the reboiler  112  to the stream, the more methane separated from the liquid contaminants, though more contaminants will also be vaporized. 
     The first reboiler  112  may apply heat to the stream within the distillation tower  104 ,  204 . Specifically, the heat applied by the first reboiler  112  warms up the lower section  106 . This heat travels up the lower section  106  and supplies heat to warm solids entering a melt tray assembly  139  ( FIGS. 1-4 ) of the middle controlled freeze zone section  108  so that the solids form a liquid and/or slurry mix. 
     The second reboiler  172  applies heat to the stream within the lower section  106 . This heat is applied closer to the middle controlled freeze zone section  108  than the heat applied by the first reboiler  112 . As a result, the heat applied by the second reboiler  172  reaches the middle controlled freeze zone section  108  faster than the heat applied by the first reboiler  112 . The second reboiler  172  also helps with energy integration. 
     The equipment may include one or more chimney assemblies  135  ( FIGS. 1-4 ). While falling to the bottom of the lower section  106 , the liquid stream may encounter one or more of the chimney assemblies  135 . 
     Each chimney assembly  135  includes a chimney tray  131  that collects the liquid stream within the lower section  106 . The liquid stream that collects on the chimney tray  131  may be fed to the second reboiler  172 . After the liquid stream is heated in the second reboiler  172 , the stream may return to the middle controlled freeze zone section  108  to supply heat to the middle controlled freeze zone section  108  and/or the melt tray assembly  139 . Unvaporized stream exiting the second reboiler  172  may be fed back to the distillation tower  104 ,  204  below the chimney tray  131 . Vapor stream exiting the second reboiler  172  may be routed under or above the chimney tray  131  when the vapor stream enters the distillation tower  104 ,  204 . 
     The chimney tray  131  may include one or more chimneys  137 . The chimney  137  serves as a channel that the vapor stream in the lower section  106  traverses. The vapor stream travels through an opening in the chimney tray  131  at the bottom of the chimney  137  to the top of the chimney  137 . The opening is closer to the bottom of the lower section  106  than it is to the bottom of the middle controlled freeze zone section  108 . The top is closer to the bottom of the middle controlled freeze zone section  108  than it is to the bottom of the lower section  106 . 
     Each chimney  137  has attached to it a chimney cap  133 . The chimney cap  133  covers a chimney top opening  138  of the chimney  137 . The chimney cap  133  prevents the liquid stream from entering the chimney  137 . The vapor stream exits the chimney assembly  135  via the chimney top opening  138 . 
     After falling to the bottom of the lower section  106 , the liquid stream exits the distillation tower  104 ,  204  through the liquid outlet  160 . The liquid outlet  160  is within the lower section  106  ( FIGS. 1-4 ). The liquid outlet  160  may be located at the bottom of the lower section  106 . 
     After exiting through the liquid outlet  160 , the feed stream may travel via line  28  to the first reboiler  112 . The feed stream may be heated by the first reboiler  112  and vapor may then re-enter the lower section  106  through line  30 . Unvaporized liquid may continue out of the distillation process via line  24 . 
     The systems may include an expander device  114  ( FIGS. 1-4 ). After entering line  24 , the heated liquid stream may be expanded in the expander device  114 . The expander device  114  may be any suitable device, such as a valve. The valve  114  may be any suitable valve, such as a J-T valve. 
     The system may include a heat exchanger  116  ( FIGS. 1-4 ). The liquid stream heated by the first reboiler  112  may be cooled or heated by the heat exchanger  116 . The heat exchanger  116  may be a direct heat exchanger or an indirect heat exchanger. The heat exchanger  116  may comprise any suitable heat exchanger. After exiting the heat exchanger  116 , the liquid stream exits the distillation process via line  26 . 
     The vapor stream in the lower section  106  rises from the lower section  106  to the middle controlled freeze zone section  108 . The middle controlled freeze zone section  108  is constructed and arranged to separate the feed stream  10  introduced into the middle controlled freeze zone section, or into the top of lower section  106 , into a solid and a vapor stream. The middle controlled freeze zone section  108  forms a solid, which may comprise more of contaminants than of methane. The vapor stream (i.e., methane-enriched vapor stream) may comprise more methane than contaminants. 
     The middle controlled freeze zone section  108  includes a lower section  40  and an upper section  39 . The lower section  40  is below the upper section  39 . The lower section  40  directly abuts the upper section  39 . The lower section  40  is primarily but not exclusively a heating section of the middle controlled freeze zone section  108 . The upper section  39  is primarily but not exclusively a cooling section of the middle controlled freeze zone section  108 . The temperature and pressure of the upper section  39  are chosen so that the solid can form in the middle controlled freeze zone section  108 . 
     The middle controlled freeze zone section  108  may comprise a melt tray assembly  139  that is maintained in the middle controlled freeze zone section  108  ( FIGS. 1-4 ). The melt tray assembly  139  is within the lower section  40  of the middle controlled freeze zone section  108 . The melt tray assembly  139  is not within the upper section  39  of the middle controlled freeze zone section  108 . 
     The melt tray assembly  139  is constructed and arranged to melt solids formed in the middle controlled freeze zone section  108 . When the warm vapor stream rises from the lower section  106  to the middle controlled freeze zone section  108 , the vapor stream immediately encounters the melt tray assembly  139  and supplies heat to melt the solids. As shown in  FIGS. 1-4  and more particularly in  FIGS. 5-6 , the melt tray assembly  139  may comprise at least one of a melt tray  118 , a bubble cap  132 , a liquid  130 , one or more draw-off openings  130   a , one or more return inlets  146 , and optionally may include a heat mechanism(s)  134 . 
     The melt tray  118  may collect a liquid and/or slurry mix. The melt tray  118  divides at least a portion of the middle controlled freeze zone section  108  from the lower section  106 . The melt tray  118  is at the bottom  45  of the middle controlled freeze zone section  108 . 
     One or more bubble caps  132  may act as a channel for the vapor stream rising from the lower section  106  to the middle controlled freeze zone section  108 . The bubble cap  132  may provide a path for the vapor stream up the riser  140  and then down and around the riser  140  to the melt tray  118 . The riser  140  is covered by a cap  141 . The cap  141  prevents the liquid  130  from travelling into the riser and it also helps prevent solids from travelling into the riser  140 . The vapor stream&#39;s traversal through the bubble cap  132  allows the vapor stream to transfer heat to the liquid  130  within the melt tray assembly  139 . 
     One or more heat mechanisms  134  may further heat up the liquid  130  to facilitate melting of the solids into a liquid and/or slurry mix. The heat mechanism(s)  134  may be located anywhere within the melt tray assembly  139 . For example, as shown in  FIGS. 1-4 , a heat mechanism  134  may be located around bubble caps  132 . The heat mechanism  134  may be any suitable mechanism, such as a heat coil. The heat source of the heat mechanism  134  may be any suitable heat source. 
     The liquid  130  in the melt tray assembly is heated by the vapor stream. The liquid  130  may also be heated by the one or more heat mechanisms  134 . The liquid  130  helps melt the solids formed in the middle controlled freeze zone section  108  into a liquid and/or slurry mix. Specifically, the heat transferred by the vapor stream heats up the liquid, thereby enabling the heat to melt the solids. The liquid  130  is at a level sufficient to melt the solids. 
     According to an aspect of the disclosure, the liquid  130  may also be heated by drawing off part of the liquid, heating the liquid through heat exchange with one or more heat sources external to the controlled freeze zone section  108 , and returning the heated liquid to the remainder of the liquid retained by the melt tray assembly  139 . As shown in  FIG. 5 , one or more draw-off openings  130   a  may be included to draw off part of the liquid  130 . The draw-off openings  130   a  may be positioned at or near the bottom of the melt tray  118 . At least part of the liquid retained by the melt tray exits the draw-off openings  130   a  and is pumped, using pump  142 , to one or more heat exchangers to heat or warm the liquid  130 . The heat exchangers may include heat exchanger  100 , which operates to drop the temperature of the feed stream  10  as previously described. The drawn-off liquid  143  may alternatively or additionally pass through one or more additional heat exchangers (depicted and described herein as additional heat exchanger  144 ) and be heated by a heating fluid  145 . The heating fluid may be ethane, propane, or another suitable fluid, and is returned to heating source (not shown) after passing through the additional heat exchanger  144 . Once heated by the heat exchanger  100  and/or the additional heat exchanger  144 , the heated liquid  143   a  is returned to the melt tray assembly  139  through the one or more return inlets  146 . The heated liquid  143   a  mixes with the liquid  130  and provides additional heat to melt solids retained in the liquid  130 . 
     The use of heat exchanger  100  to heat liquid  130  may eliminate the necessity of heating element  134 . Such elimination of the heating element  134  may significantly reduce the congestion and complexity of the melt tray assembly  139 . Also, heating the liquid  130  in the heat exchanger  100 , combined with recycling the heated liquid  143   a  back to the melt tray assembly  139 , provides a more even heating operation to the liquid  130  across a wide range of operating conditions. In contrast, the heating element  134  could provide undesirable non-uniform heating, particularly at turn-down conditions where the temperature pinches out well before the end of the heating element. 
     The heat exchanger  100  and/or the additional heat exchanger  144  may be advantageously used with applications in which low CO 2  feed gas (i.e., feed gas having a composition of less than 30 mol % CO 2 , or less than 25 mol % CO 2 , or less than 22 mol % CO 2 , or less than 20 mol % CO 2 ) is introduced above the melt tray assembly  139 , as is described, for example, in United States patent application titled “Hybrid Tray for Introducing a Low CO 2  Feed Stream into a Distillation Tower,” being commonly owned and filed on an even date herewith, the disclosure of which is incorporated herein by reference in its entirety. In such a situation, the heat exchangers may provide beneficial heat integration between the relatively warmer feed gas (about −60° F., which is the lowest achievable temperature using a conventional propane-based refrigeration system plus Joule-Thompson cooling via valve  102 ) and the relatively colder melt tray liquid (about −75° F. to −80° F.). This would provide a means to cool the feed gas while maintaining its low CO 2  content. The heated liquid  143   a  provides a portion of the heat required to melt solids in the melt tray assembly  139 , while the additional cooling of the feed gas reduces the load on the condenser  122  and associated equipment while the additional heating of the melt tray liquid reduces the heat input needed from other sources—such as the heating element  134 . 
     According to aspects of the disclosure, the pumping rate of pump  142  may advantageously be established based on design parameters of the distillation tower  104 ,  204 , such as tower size, sizes and locations of draw-off openings  130   a  and return inlets  146 , and the like. In an aspect there should be no need to throttle or control the pumping rate during operation. Additionally, heat input to the additional heat exchanger  144 , if present, should be controlled on the side of the heating fluid  145 . In a preferred aspect the heat input may be adjusted to maintain a target temperature for the heated liquid  143   a , drawn-off liquid, the liquid  130 , or another location. 
     The duty for heat exchanger  100  should be maximized to provide the most efficient operation. As a precaution, a feed gas bypass line  147  and a bypass valve  148  may be used to permit the feed gas  10  to bypass the heat exchanger  100 , thereby increasing the temperature of the feed gas. This option may be used if feed gas risers, which introduce feed gas above the liquid level of liquid  130 , experience fouling from solid CO 2  in a low CO 2  environment. 
     The middle controlled freeze zone section  108  may also comprise a spray assembly  129 . The spray assembly  129  cools the vapor stream that rises from the lower section  40 . The spray assembly  129  sprays liquid, which is cooler than the vapor stream, on the vapor stream to cool the vapor stream. The spray assembly  129  is within the upper section  39 . The spray assembly  129  is not within the lower section  40 . The spray assembly  129  is above the melt tray assembly  139 . In other words, the melt tray assembly  139  is below the spray assembly  129 . 
     The spray assembly  129  includes one or more spray nozzles  120  ( FIGS. 1-4 ). Each spray nozzle  120  sprays liquid on the vapor stream. The spray assembly  129  may also include a spray pump  128  ( FIGS. 1-4 ) that pumps the liquid. Instead of a spray pump  128 , gravity may induce flow in the liquid. 
     The liquid sprayed by the spray assembly  129  contacts the vapor stream at a temperature and pressure at which solids form. Solids, containing mainly contaminants, form when the sprayed liquid contacts the vapor stream. The solids fall toward the melt tray assembly  139 . 
     The temperature in the middle controlled freeze zone section  108  cools down as the vapor stream travels from the bottom of the middle controlled freeze zone section  108  to the top of the middle controlled freeze zone section  108 . The methane in the vapor stream rises from the middle controlled freeze zone section  108  to the upper section  110 . Some contaminants may remain in the methane and also rise. The contaminants in the vapor stream tend to condense or solidify with the colder temperatures and fall to the bottom of the middle controlled freeze zone section  108 . 
     The solids form the liquid and/or slurry mix when in the liquid  130 . The liquid and/or slurry mix flows from the middle controlled freeze zone section  108  to the lower distillation section  106 . At least part of the liquid and/or slurry mix flows from the bottom of the middle controlled freeze zone section  108  to the top of the lower section  106  via a line  22  ( FIGS. 1-4 ). The line  22  may be an exterior line. The line  22  may extend from the distillation tower  104 ,  204 . The line  22  may extend from the middle controlled freeze zone section  108 . The line may extend to the lower section  106 . The line  22  may extend from an outer surface of the distillation tower  104 ,  204 . 
     As shown in  FIGS. 1-2 , the vapor stream that rises in the middle controlled freeze zone section  108  and does not form solids or otherwise fall to the bottom of the middle controlled freeze zone section  108 , rises to the upper section  110 . The upper section  110  operates at a temperature and pressure and contaminant concentration at which no solid forms. The upper section  110  is constructed and arranged to cool the vapor stream to separate the methane from the contaminants. Reflux in the upper section  110  cools the vapor stream. The reflux is introduced into the upper section  110  via line  18 . Line  18  may extend to the upper section  110 . Line  18  may extend from an outer surface of the distillation tower  104 ,  204 . 
     After contacting the reflux in the upper section  110 , the feed stream forms a vapor stream and a liquid stream. The vapor stream mainly comprises methane. The liquid stream comprises relatively more contaminants. The vapor stream rises in the upper section  110  and the liquid falls to a bottom of the upper section  110 . 
     To facilitate separation of the methane from the contaminants when the stream contacts the reflux, the upper section  110  may include one or more mass transfer devices  176 . Each mass transfer device  176  helps separate the methane from the contaminants. Each mass transfer device  176  may comprise any suitable separation device, such as a tray with perforations, or a section of random or structured packing to facilitate contact of the vapor and liquid phases. 
     After rising, the vapor stream may exit the distillation tower  104 ,  204  through line  14 . The line  14  may emanate from an upper part of the upper section  110 . The line  14  may extend from an outer surface of the upper section  110 . From line  14 , the vapor stream may enter a condenser  122 . The condenser  122  cools the vapor stream to form a cooled stream. The condenser  122  at least partially condenses the stream. After exiting the condenser  122 , the cooled stream may enter a separator  124 . The separator  124  separates the vapor stream into liquid and vapor streams. The separator may be any suitable separator that can separate a stream into liquid and vapor streams, such as a reflux drum. Once separated, the vapor stream may exit the separator  124  as sales product. The sales product may travel through line  16  for subsequent sale to a pipeline and/or condensation to be liquefied natural gas. Once separated, the liquid stream may return to the upper section  110  through line  18  as the reflux. The reflux may travel to the upper section  110  via any suitable mechanism, such as a reflux pump  150  ( FIGS. 1 and 3 ) or gravity ( FIGS. 2 and 4 ). 
     The liquid stream (i.e., freezing zone liquid stream) that falls to the bottom of the upper section  110  collects at the bottom of the upper section  110 . The liquid may collect on tray  183  ( FIGS. 1 and 3 ) or at the bottommost portion of the upper section  110  ( FIGS. 2 and 4 ). The collected liquid may exit the distillation tower  104 ,  204  through line  20  ( FIGS. 1 and 3 ) or outlet  260  ( FIGS. 2 and 4 ). The line  20  may emanate from the upper section  110 . The line  20  may emanate from a bottom end of the upper section  110 . The line  20  may extend from an outer surface of the upper section  110 . 
     The line  20  and/or outlet  260  connect to a line  41 . The line  41  leads to the spray assembly  129  in the middle controlled freeze zone section  108 . The line  41  emanates from the holding vessel  126 . The line  41  may extend to an outer surface of the middle controlled freeze zone section  108 . 
     The line  20  and/or outlet  260  may directly or indirectly ( FIGS. 1-4 ) connect to the line  41 . When the line  20  and/or outlet  260  directly connect to the line  41 , the liquid spray may be pumped to the spray nozzle(s)  120  via any suitable mechanism, such as the spray pump  128  or gravity. When the line  20  and/or outlet  260  indirectly connect to the line  41 , the lines  20 ,  41  and/or outlet  260  and line  41  may directly connect to a holding vessel  126  ( FIGS. 1 and 3 ). The holding vessel  126  may house at least some of the liquid spray before it is sprayed by the nozzle(s). The liquid spray may be pumped from the holding vessel  126  to the spray nozzle(s)  120  via any suitable mechanism, such as the spray pump  128  ( FIGS. 1-2 ) or gravity. The holding vessel  126  may be needed when there is not a sufficient amount of liquid stream at the bottom of the upper section  110  to feed the spray nozzles  120 . 
       FIG. 7  is a flowchart depicting a method  700  of cryogenically separating contaminants from a feed stream in a distillation tower according to disclosed aspects. At block  702  the feed stream is directed into the distillation tower. At block  704  vapor is permitted to rise upwardly from a distillation section of the distillation tower. At block  706  a solid is formed in a controlled freeze zone section of the distillation tower. The controlled freeze zone section is situated above the distillation section. The solid comprises contaminants in the feed stream. At block  708  the vapor from the distillation section into is directed into liquid retained by a melt tray assembly using at least one vapor stream riser. At block  710  the solid is melted using the liquid retained by the melt tray assembly. At block  712  a portion of the liquid retained by the melt tray assembly is permitted to exit the controlled freeze zone section. At block  714  the portion of the liquid is heated through indirect heat exchange with a heating fluid in a heat exchanger. At block  716  the portion of the liquid is returned to the melt tray assembly after the liquid has been heated in the heat exchanger. 
     It is important to note that the steps depicted in  FIG. 7  are provided for illustrative purposes only and a particular step may not be required to perform the inventive methodology. The claims, and only the claims, define the inventive system and methodology. 
     Disclosed aspects may be used in hydrocarbon management activities. As used herein, “hydrocarbon management” or “managing hydrocarbons” includes hydrocarbon extraction, hydrocarbon production, hydrocarbon exploration, identifying potential hydrocarbon resources, identifying well locations, determining well injection and/or extraction rates, identifying reservoir connectivity, acquiring, disposing of and/or abandoning hydrocarbon resources, reviewing prior hydrocarbon management decisions, and any other hydrocarbon-related acts or activities. The term “hydrocarbon management” is also used for the injection or storage of hydrocarbons or CO 2 , for example the sequestration of CO 2 , such as reservoir evaluation, development planning, and reservoir management. The disclosed methodologies and techniques may be used to produce hydrocarbons in a feed stream extracted from, for example, a subsurface region. The feed stream extracted may be processed in the distillation tower  104 ,  204  and separated into hydrocarbons and contaminants. The separated hydrocarbons exit the middle controlled freeze zone section  108  or the upper section  110  of the distillation tower. Some or all of the hydrocarbons that exit are produced. Hydrocarbon extraction may be conducted to remove the feed stream from for example, the subsurface region, which may be accomplished by drilling a well using oil well drilling equipment. The equipment and techniques used to drill a well and/or extract the hydrocarbons are well known by those skilled in the relevant art. Other hydrocarbon extraction activities and, more generally, other hydrocarbon management activities, may be performed according to known principles. 
     Aspects of the disclosure may include any combinations of the methods and systems shown in the following numbered paragraphs. This is not to be considered a complete listing of all possible aspects, as any number of variations can be envisioned from the description above. 
     1. A cryogenic distillation tower for separating a feed stream, the distillation tower comprising: 
     a distillation section permitting vapor to rise upwardly therefrom; 
     one or more lines for directing the feed stream into the distillation tower; 
     a controlled freeze zone section situated above the distillation section, the controlled freeze zone constructed and arranged to form a solid from the feed stream, the controlled freeze zone section including
         a spray assembly in an upper section of the controlled freeze zone, and   a melt tray assembly in a lower section of the controlled freeze zone, wherein the melt tray assembly includes
           at least one vapor stream riser that directs the vapor from the distillation section into liquid retained by the melt tray assembly, and   one or more draw-off openings positioned to permit a portion of the liquid retained by the melt tray assembly to exit the controlled freeze zone section;   
               

     a heat exchanger arranged to heat the portion of the liquid through indirect heat exchange with a heating fluid; and 
     one or more return inlets that return the portion of the liquid to the melt tray assembly after the portion of the liquid has been heated in the heat exchanger. 
     2. The cryogenic distillation tower of claim  1 , wherein the heating fluid is the feed stream prior to the feed stream being directed into the distillation tower. 
     3. The cryogenic distillation tower of claim  1  or claim  2 , wherein the heat exchanger is a first heat exchanger and the heating fluid is a first heating fluid, and further comprising a second heat exchanger arranged to heat the portion of the liquid through indirect heat exchange with a second heating fluid.
 
4. The cryogenic distillation tower of paragraph 3, wherein the second heating fluid comprises one or more of ethane and propane.
 
5. The cryogenic distillation tower of any one of paragraphs 1-4, further comprising a pump for pumping the portion of the liquid exiting the one or more draw-off openings.
 
6. The cryogenic distillation tower of any one of paragraphs 1-5, further comprising a bypass line configured to selectively permit the feed stream to bypass the heat exchanger.
 
7. The cryogenic distillation tower of any one of paragraphs 1-6, wherein the one or more return inlets comprise two or more return inlets that are evenly distributed about a perimeter of the cryogenic distillation tower.
 
8. The cryogenic distillation tower of any one of paragraphs 1-7, wherein the melt tray assembly includes a heating element to heat the liquid retained in the melt tray assembly.
 
9. The cryogenic distillation tower of any one of paragraphs 1-8, wherein the feed gas comprises less than 30 mol % carbon dioxide.
 
10. The cryogenic distillation tower of any one of paragraphs 1-9, wherein the solid comprises carbon dioxide.
 
11. A method of cryogenically separating contaminants from a feed stream in a distillation tower, the method comprising:
 
     directing the feed stream into the distillation tower; 
     permitting vapor to rise upwardly from a distillation section of the distillation tower; 
     forming a solid in a controlled freeze zone section of the distillation tower, the controlled freeze zone section being situated above the distillation section, wherein the solid comprises contaminants in the feed stream; 
     directing the vapor from the distillation section into liquid retained by a melt tray assembly using at least one vapor stream riser; 
     melting the solid using the liquid retained by the melt tray assembly; 
     permitting a portion of the liquid retained by the melt tray assembly to exit the controlled freeze zone section; 
     heating the portion of the liquid through indirect heat exchange with a heating fluid in a heat exchanger; and 
     returning the portion of the liquid to the melt tray assembly after the liquid has been heated in the heat exchanger. 
     12. The cryogenic distillation tower of paragraph 11, wherein the heating fluid is the feed stream prior to the feed stream being directed into the distillation tower. 
     13. The method of paragraph 11 or paragraph 12, wherein the heat exchanger is a first heat exchanger and the heating fluid is a first heating fluid, and further comprising: 
     heating the portion of the liquid through indirect heat exchange with a second heating fluid in a second heat exchanger. 
     14. The method of paragraph 13, wherein the second heating fluid comprises one or more of ethane and propane. 
     15. The method of any one of paragraphs 11-14, further comprising: 
     pumping the portion of the liquid exiting the one or more draw-off openings. 
     16. The method of any one of paragraphs 11-15, further comprising: 
     selectively permitting the feed stream to bypass the heat exchanger. 
     17. The method of any one of paragraphs 11-16, wherein the one or more return inlets comprise two or more return inlets that are evenly distributed about a perimeter of the tower. 
     18. The method of any one of paragraphs 11-17, further comprising: heating the liquid retained in the melt tray assembly using a heating element. 
     19. The method of any one of paragraphs 11-18, wherein the feed gas comprises less than 30 mol % carbon dioxide. 
     20. The method of any one of paragraphs 11-19, wherein the solid comprises carbon dioxide. 
     It should be understood that numerous changes, modifications, and alternatives to the preceding disclosure can be made without departing from the scope of the disclosure. The preceding description, therefore, is not meant to limit the scope of the disclosure. Rather, the scope of the disclosure is to be determined only by the appended claims and their equivalents. It is also contemplated that structures and features in the present examples can be altered, rearranged, substituted, deleted, duplicated, combined, or added to each other.