The invention relates generally to systems for vaporization and sublimation and methods associated with the use thereof. More specifically, embodiments of the invention relate to a first heat exchanger configured to vaporize a fluid including solid particles therein and a second heat exchanger configured to sublimate the solid particles. Embodiments of the invention additionally relates to the methods of heat transfer between fluids, the sublimation of solid particles within a fluid, and the conveyance of fluids.
The production of liquefied natural gas is a refrigeration process that reduces the mostly methane (CH4) gas to a liquid state. However, natural gas consists of a variety of gases in addition to methane. One of the gases contained in natural gas is carbon dioxide (CO2). Carbon dioxide is found in quantities around 1% in most of the natural gas infrastructure found in the United States, and in many places around the world the carbon content is much higher.
Carbon dioxide can cause problems in the process of natural gas liquefaction, as carbon dioxide has a freezing temperature that is higher than the liquefaction temperature of methane. The high freezing temperature of carbon dioxide relative to methane will result in solid carbon dioxide crystal formation as the natural gas cools. This problem makes it necessary to remove the carbon dioxide from the natural gas prior to the liquefaction process in traditional plants. The filtration equipment to separate the carbon dioxide from the natural gas prior to the liquefaction process may be large, may require significant amounts of energy to operate, and may be very expensive.
Small scale liquefaction systems have been developed and are becoming very popular. In most cases, these small plants are simply using a scaled down version of existing liquefaction and carbon dioxide separation processes. The Idaho National Laboratory has developed an innovative small scale liquefaction plant that eliminates the need for expensive, equipment intensive, pre-cleanup of the carbon dioxide. The carbon dioxide is processed with the natural gas stream, and during the liquefaction step the carbon dioxide is converted to a crystalline solid. The liquid/solid slurry is then transferred to a separation device which directs a clean liquid out of an overflow, and a carbon dioxide concentrated slurry out of an underflow.
The underflow slurry is then processed through a heat exchanger to sublime the carbon dioxide back into a gas. In theory this is a very simple step. However, the interaction between the solid carbon dioxide and liquid natural gas produces conditions that are very difficult to address with standard heat exchangers. In the liquid slurry, carbon dioxide is in a pure or almost pure sub-cooled state and is not soluble in the liquid. The carbon dioxide is heavy enough to quickly settle to the bottom of most flow regimes. As the settling occurs, piping and ports of the heat exchanger can become plugged as the quantity of carbon dioxide builds. In addition to collecting in undesirable locations, the carbon dioxide has a tendency to clump together making it even more difficult to flush through the system.
The ability to sublime the carbon dioxide back into a gas is contingent on getting the solids past the liquid phase of the gas and into a warmer section of a device without collecting and clumping into a plug. As the liquid natural gas is heated, it will remain at approximately a constant temperature of about −230° F. (at 50 psig) until all the liquid has passed from a two-phase gas to a single-phase gas. The solid carbon dioxide will not begin to sublime back into a gas until the surrounding gas temperatures have reached approximately −80° F. While the solid carbon dioxide is easily transported in the liquid methane, the ability to transport the solid carbon dioxide crystals to warmer parts of the heat exchanger is substantially diminished as liquid natural gas vaporizes. At a temperature when the moving, vaporized natural gas is the only way to transport the solid carbon dioxide crystals, the crystals may begin to clump together due to the tumbling interaction with each other, leading to the aforementioned plugging.
In addition to clumping, as the crystals reach warmer areas of the heat exchanger they begin to melt or sublime. If melting occurs, the surfaces of the crystals becomes sticky causing the crystals to have a tendency to stick to the walls of the heat exchanger, reducing the effectiveness of the heat exchanger and creating localized fouling. The localized fouling areas may cause the heat exchanger to become occluded and eventually plug if fluid velocities cannot dislodge the fouling.
In view of the shortcomings in the art, it would be advantageous to provide a system and associated methods that would enable the effective and efficient sublimation of solid particles found within a slurry. Additionally, it would be desirable for a system and associated methods to be able to effectively and efficiently warm and vaporize slurries of fluids containing solid particles.
In accordance with one embodiment of the invention, a method for vaporizing and sublimating a fluid including solid particles is provided. The method includes feeding a slurry comprising solid particles suspended in a first liquid to a first heat exchanger, vaporizing the first fluid in the first heat exchanger to form a first gas, feeding the first gas and the solid particles to a second heat exchanger, and sublimating the solid particles in the second heat exchanger to form a second gas.
In accordance with another embodiment of the invention, a method is provided for continuously vaporizing a slurry of liquid methane and solid carbon dioxide particles. The method includes feeding the slurry of liquid methane and solid carbon dioxide particles to a first heat exchanger, vaporizing the liquid methane in the first heat exchanger to form a mixture of solid carbon dioxide particles and gaseous methane, feeding the mixture of solid carbon dioxide particles and gaseous methane to a second heat exchanger, and sublimating the solid carbon dioxide particles in the second heat exchanger.
In accordance with a further embodiment of the invention, a system for vaporizing and sublimating a fluid including solid particles is provided. The system includes a first heat exchanger configured to receive the fluid including solid particles and to vaporize the fluid and a second heat exchanger configured to receive the vaporized fluid and solid particles and to sublimate the solid particles.
While the specification concludes with claims particularly pointing out and distinctly claiming that which is regarded as the present invention, advantages of this invention may be more readily ascertained from the following detailed description when read in conjunction with the accompanying drawings in which:
Some of the illustrations presented herein are not meant to be actual views of any particular material, device, or system, but are merely idealized representations which are employed to describe the present invention. Additionally, elements common between figures may retain the same numerical designation.
The term “fluid” as used herein means any substance that may be caused to flow through a conduit and includes but is not limited to gases, two-phase gases, liquids, gels, plasmas, slurries, solid particles, and any combination thereof
As shown in
The vaporization chamber 102 may be configured to vaporize the first fluid in the slurry 112 without altering the physical state of the solid particles within the slurry 112. One embodiment of such a vaporization chamber is described in detail in previously referenced U.S. patent application Ser. No. 12/XXX,XXX((2939-10080US) entitled “Vaporization Chamber and Associated Methods,” and filed on even date herewith. Briefly, the vaporization chamber 102 may include a first chamber 140 surrounding a second chamber, which may also be characterized as a mixer 120. The second fluid 118 enters the first chamber 140 of the vaporization chamber 102 and envelops the mixer 120. Heat may be transferred from the second fluid 118 to the mixer 120 heating an outer surface of the mixer 120. The second fluid 118 also enters the mixer 120 and mixes with the slurry 112 as shown in broken lines within the vaporization chamber 102. In some embodiments, the mixer 120 may comprise a plurality of ports (not shown) that allow the second fluid 118 to enter the mixer 120 and promotes mixing of the second fluid 118 and the slurry 112. In additional embodiments, a wall of the mixer 120 may comprise a porous material which allows a portion of the second fluid 118 to enter the mixer 120 through the porous wall. In some embodiments, another portion of the second fluid 118′ may exit the first chamber 140 of the vaporization chamber 102 and be directed to the sublimation chamber 104. Alternatively, in some embodiments, the portion of the second fluid 118′ may be directed to the sublimation chamber 104 before entering the vaporization chamber 102 as shown in broken lines.
As shown in
The sublimation chamber 104 may be configured to sublimate the solid particles in the first gas and the solid particles 114 without allowing the particles to melt and stick together, fouling the system 100. One example of such a sublimation chamber 104 is described in detail in previously referenced U.S. patent application Ser. No. 12/XXX,XXX ((2939-10081US), entitled “Heat Exchanger and Related Methods,” and filed on even date herewith. Briefly, the sublimation chamber 104 may include a first portion 134 and a second portion 136. The first gas and the solid particles 114 may be fed into the first portion 134 of the sublimation chamber 104, and the portion of the second fluid 118′ may be fed into the second portion 136 of the sublimation chamber 104. A cone-shaped member 138 may separate the second portion 136 from the first portion 134. At an apex of the cone-shaped member 138 is an opening or a nozzle 132 for directing the portion the second fluid 118′ from the second portion 136 to the first portion 134 of the sublimation chamber 104. The nozzle 132 may comprise, for example, a changeable orifice or valve which may be sized to achieve a column of the second fluid 118″ having a desired velocity extending through the first portion 134 of the sublimation chamber 104.
Particles from the first gas and the solid particles 114 may be entrained and suspended within the column of the second fluid 118″. As the particles are suspended in the column of the second fluid 118″, the column of the second fluid 118″ heats the particles and causes the particles to sublimate, forming the second gas. The cone-shaped member 138 helps direct the solid particles into the column of the second fluid 118″.
The system 100 may be controlled using at least one valve and at least one temperature sensor. For example, as shown in
In one embodiment, the system 100 may be used as part of a liquefaction process for natural gas. For example, the present invention may be used in conjunction with an apparatus for the liquefaction of natural gas and methods relating to the same, such as is described in U.S. Pat. No. 6,962,061 to Wilding et al., the disclosure of which is incorporated herein in its entirety by reference. The methods of liquefaction of natural gas disclosed in the Wilding patent include cooling at least a portion of a mass of natural gas to form a slurry which comprises at least liquid natural gas and solid carbon dioxide. The slurry is flowed into a hydrocyclone (i.e., the separator 108 as shown in
In this embodiment of the invention, the slurry 112 comprises a continuous flow of liquid natural gas and solid carbon dioxide particles as might be produced in a method according to the Wilding patent, as it is conveyed into the vaporization chamber 102. As the slurry 112 enters the mixer 120 within the vaporization chamber 102, the second fluid 118, which comprises a continuous flow of heated gas in this example (such as heated natural gas or heated methane), enters the vaporization chamber 102. The second fluid 118 heats the outside of mixer 120 and also enters the mixer 120, as desired. The heat from the second fluid 118 causes the liquid natural gas in the slurry 112 to vaporize. The temperature and pressure within the vaporization chamber 102 may be controlled such that the liquid natural gas in the slurry 112 vaporizes but that the solid carbon dioxide particles do not melt or sublimate. The second fluid 118 and the slurry 112 may be fed to the vaporization chamber 102 in about equal ratios. For example, in one embodiment, the mass flow rate of the second fluid 118 to the vaporization chamber 102 may be about one (1.0) to about one and a half (1.5) times greater than the mass flow rate of the slurry 112 to the vaporization chamber 102. In one embodiment, the mass flow rate of the second fluid 118 to the vaporization chamber 102 is about one and three tenths (1.3) times greater than the mass flow rate of the slurry 112 to the vaporization chamber.
As the slurry 112 is conveyed through the vaporization chamber 102, the initial heat energy provided by the second fluid 118 may be used to facilitate a phase change of the liquid methane of the slurry 112 to gaseous methane. As this transition occurs, the temperature of the slurry 112 may remain at about −230° F. (this temperature may vary depending upon the pressure of the fluid) until all of the liquid methane of the slurry 112 is converted to gaseous methane. At this point, the solid carbon dioxide particles of the slurry 112 may now be suspended in the combined gaseous methane from the slurry 112 and second fluid 118 which exits the vaporization chamber 102 as the first gas and the solid particles 114. The temperature of the first gas and solid particles, determined by the second temperature sensor 128, may be controlled via the first valve 122 and the second valve 124 so that the temperature at the second temperature sensor 128 is higher than the vaporization temperature of the methane but colder than the sublimation temperature of the solid carbon dioxide particles. This ensures that the solid carbon dioxide particles do not begin to melt and become sticky within the vaporization chamber 102, preventing fouling of the vaporization chamber 102.
The first gas and the solid particles 114 comprising the vaporized methane and solid carbon dioxide particles are then continuously fed to the sublimation chamber 104. As the first gas and solid particles 114 enters the first portion 134 of the sublimation chamber 104, the portion of the second fluid 118′, which again comprises a continuous flow of heated gas in this example (such as heated natural gas or heated methane), enters the second portion 136 of the sublimation chamber 104. The vaporized methane from the first gas and solid particles 114 exits the sublimation chamber 104 as part of the exit gas 116 while the solid carbon dioxide particles gather in the cone-shaped barrier 138. The portion of the second fluid 118′ enters the first portion 134 of the sublimation chamber 104 through the nozzle 132 at about −80° F. (this temperature may vary depending upon the pressure of the fluid environment) forming the column of the second fluid 118″. The particles of carbon dioxide are funneled into the column of the second fluid 118″ by the cone-shaped barrier 138 where the particles are suspended as they change phase from solid to vapor. All of the carbon dioxide particles may be converted to gaseous carbon dioxide. Once the gaseous carbon dioxide is formed, the gaseous carbon dioxide mixes with the gaseous methane from the first gas and solid particles 114 and the second fluid 118, 118′ and exits the sublimation chamber as the exit gas 116.
The exit stream 116 may be monitored to maintain a temperature at the third temperature sensor 130 higher than the sublimation temperature of the solid carbon dioxide. However, it may be desirable to not overheat the exit stream 116 as the exit stream 116 may be reused as a refrigerant when cooling the natural gas to form the liquid natural gas according to the Wilding patent. In one embodiment, the temperature of the exit stream 116 may be maintained at about twenty degrees higher than the sublimation temperature of the solid carbon dioxide. For example, the exit stream 116 may be kept at about −40° F. and about 250 psia. By maintaining the exit stream 116 at about twenty degrees higher than the sublimation temperature of the solid carbon dioxide, all of the solid carbon dioxide in the exit stream 116 will be vaporized while still producing a cold stream for reuse in another heat exchanger.
In one example, the slurry 112 may enter the vaporization chamber 102 at about 245 psia and about −219° F. at a mass flow rate of about 710 lbm/hr. The second fluid may enter the vaporization chamber 102 at about 250 psia and about 300° F. at a mass flow rate of about 950 lbm/hr. The combined vaporized slurry, including the first fluid and the vaporized particles, and the second fluid may exit the system as the exit stream 116 at about −41° F. and about 250 psia.
By using a separate vaporization chamber 102 and sublimation chamber 104 to form the exit gas 116, the process conditions (i.e., pressure and temperature) for each of the vaporization chamber 102 and the sublimation chamber 104 may be optimized for gasifying the liquid and solid components of the slurry 112. By splitting the gasifying process of the slurry 112 into a vaporization chamber 102 and a sublimation chamber 104, the solid particles may be continuously sublimated without fouling the vaporization chamber 102. The system 100, therefore, provides a continuous method of transforming the slurry 112 into the exit gas 116, which may be easily disposed of.
In light of the above disclosure it will be appreciated that the apparatus and methods depicted and described herein enable the effective and efficient conveyance and sublimation of solid particles within a fluid. The invention may further be useful for a variety of applications other than the specific examples provided. For example, the described system and methods may be useful for the effective and efficient mixing, heating, cooling, and/or conveyance of fluids containing solids where there is a temperature difference between the vaporization temperature of the fluid and the sublimation temperature of the solid.
While the invention may be susceptible to various modifications and alternative forms, specific embodiments of which have been shown by way of example in the drawings and have been described in detail herein, it should be understood that the invention is not intended to be limited to the particular forms disclosed. Rather, the invention includes all modifications, equivalents, and alternatives falling within the scope of the invention as defined by the following appended claims and their legal equivalents.
The present application is related to co-pending U.S. patent application Ser. No. 11/855,071 filed on Sep. 13, 2007, titled HEAT EXCHANGER AND ASSOCIATED METHODS, U.S. patent application Ser. No. 12/XXX,XXX filed on even date herewith and titled VAPORIZATION CHAMBERS AND ASSOCIATED METHODS (attorney docket number 2939-10080US (BA-494)) and copending U.S. patent application Ser. No. 12/XXX,XXX filed on even date herewith and titled HEAT EXCHANGER AND RELATED METHODS (attorney docket number 2939-10081US(BA-495)). The disclosure of each of the foregoing applications is incorporated herein by reference in its entirety.
This invention was made with government support under Contract Number DE-AC07-051D14517 awarded by the United States Department of Energy. The government has certain rights in the invention.