Patent Application
20170038137
The present invention generally relates to a method and apparatus for efficiently producing liquefied natural gas (“LNG”).
Small scale nitrogen generators (e.g., typically 100 mtd to 400 mtd) are commonly needed at remote locations where the cost of an air separation unit producing O2 and N2 is not required or justified. The nitrogen can be (1) low purity gaseous for utility purposes (e.g., ethane cracker), (2) liquid form for fracking purposes, or (3) higher purity merchant product, particularly in locations such as North Dakota or Alberta. These sites can also have a demand for small quantities of liquefied natural gas, which is often trucked in at a high price. Alternatively, small scale LNG plants could be installed but also result in a high cost of product, thereby making these options uneconomical.
Therefore, it would be advantageous to provide a method and apparatus that could produce gaseous nitrogen, and/or LIN, while also producing LNG in a more efficient manner.
The present invention is directed to a method and apparatus that satisfies at least one of these needs. In certain embodiments, the invention can provide a lower cost, more efficient and flexible method to produce LNG. For example, in certain embodiments, the invention can include integration of a nitrogen production facility with an LNG production facility.
In one embodiment, refrigeration energy from the nitrogen production unit can be partially diverted to be used to liquefy natural gas. In one embodiment, the incoming air and natural gas are cooled in the same heat exchanger. In another embodiment, the liquefaction of the natural gas occurs in a separate heat exchanger. In embodiments in which the liquefaction of natural gas occurs in a second heat exchanger, the invention can also include an additional set of turbine boosters configured to provide additional refrigeration for the LNG heat exchanger. In one embodiment, the discharge pressure of the turbine for the LNG heat exchanger is approximate the column reboiler condensing pressure.
In one embodiment, a method for the production of liquefied natural gas (“LNG”) and nitrogen is provided. In this embodiment, the method can include the steps of: a) providing a nitrogen production facility, wherein nitrogen production facility comprises: a main heat exchanger, an air separation unit, a nitrogen recycle compressor, a first nitrogen refrigeration supply configured to provide refrigeration to the main heat exchanger for cooling a main air feed; b) providing a secondary refrigeration supply; c) liquefying a natural gas stream using refrigeration from the secondary refrigeration supply to form an LNG product stream; wherein the secondary refrigeration supply is configured to compress and expand a refrigerant to produce refrigeration, wherein the refrigerant of the secondary refrigeration supply is shared with refrigerant of the first nitrogen refrigeration supply.
In optional embodiments of the method for the production of LNG and nitrogen:
These and other features, aspects, and advantages of the present invention will become better understood with regard to the following description, claims, and accompanying drawings. It is to be noted, however, that the drawings illustrate only several embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it can admit to other equally effective embodiments.
While the invention will be described in connection with several embodiments, it will be understood that it is not intended to limit the invention to those embodiments. On the contrary, it is intended to cover all the alternatives, modifications and equivalence as may be included within the spirit and scope of the invention defined by the appended claims.
In one embodiment, the apparatus can include a Main Air Compressor (MAC), an air purification system, heat exchange means, a distillation column, and a nitrogen refrigeration cycle having a nitrogen recycle compressor and warm and cold turbine boosters. In one embodiment, the nitrogen refrigeration cycle provides compression energy for pressurized gaseous nitrogen product as well as refrigeration for both cooling down the incoming air and liquefy nitrogen, as well as providing refrigeration to liquefy the natural gas to LNG. In one embodiment, the warm turbine booster flow can be split to provide refrigeration to both the warm section of the nitrogen portion and the warm section of the LNG portion. In one embodiment, this warm turbine load can serve to supply two refrigeration loads, since both the nitrogen and LNG portion heat exchange cooling curves have similar pressures and temperatures in this warm section.
In one embodiment, the temperature at the cold end of the nitrogen portion of the heat exchanger can be about −178° C., and the cold end of the LNG portion is about −164° C. In one embodiment, different expanded nitrogen streams at different pressures and coming from different turbines are used to liquefy the LNG as compared to cool the incoming air and liquefy nitrogen. This is preferably done since the temperature differences and cooling curve differences make it less efficient to use the same turbine for both nitrogen and LNG exchangers. Also, if the separate cold turbine for the LNG portion is simply warmed up and uses the same pressure profile (for integration in to suction of the same Recycle compressor) then the process is not optimized.
In one embodiment, warming the cold end of the LNG exchanger is achieved by reducing the turbine pressure ratio. In one embodiment, the turbine discharge pressure can be increased to ˜10 bar to match the pressure for condensing in the column reboiler and the intermediate stage of the recycle compressor. This significantly reduces the flow rate at the first stage of the recycle compressor, and also improves the efficiency of the natural gas liquefaction, therefore both reducing power and cost.
In one embodiment, the method can include integrating a natural gas letdown system with a closed loop nitrogen refrigeration cycle. In this embodiment, the natural gas letdown essentially provides “free” refrigeration energy since the natural gas would have been alternatively letdown across a valve (i.e., the resulting drop in temperature of the natural gas would have been absorbed by the surroundings and would not have been recovered in any meaningful way). With the addition of a natural gas turbine booster and dryer (i.e., purification unit), LNG can be co-produced with a significant power savings, while also reducing the size of the nitrogen refrigeration cycle.
Referring to
Gaseous nitrogen 28 is also withdrawn from air separation unit 19 and passed through the cold side of heat exchanger 10 to provide additional cooling. However, instead of venting to the atmosphere, gaseous nitrogen 28 can be recycled in the process. Nitrogen recycle 38 exits main heat exchanger 10 and is introduced to recycle compressor 40 and compressed to form compressed nitrogen recycle 46. In one embodiment, if gaseous nitrogen is desired, a portion of compressed nitrogen recycle 46 can be removed and collected as gaseous nitrogen product 47. Compressed nitrogen recycle 46 is then cooled in second aftercooler 43 before being boosted in first booster 50 and cooled in third aftercooler 51 to form boosted nitrogen 52. Boosted nitrogen 52 is then introduced to second booster 53 in order to further compress boosted nitrogen 52 before being cooled in fourth aftercooler 55 to form fully boosted nitrogen 56. In one embodiment fully boosted nitrogen 56 can be at ambient temperature and a pressure of about 45 to about 65 bar prior to entering heat exchanger 10.
Fully boosted nitrogen 56 is then introduced to heat exchanger 10 for cooling. In one embodiment, one portion of fully boosted nitrogen 56 is fully cooled in heat exchanger 10 to form liquefied nitrogen 58, which is subsequently introduced to liquid/gas separator 60 by flashing via valve 59. In another embodiment, another portion of fully boosted nitrogen 56 is only partially cooled in heat exchanger 10 to form partially cooled boosted nitrogen 78. In one embodiment, partially cooled boosted nitrogen 78 is at or above its super critical pressure. Partially cooled boosted nitrogen 78 is then introduced into second turbine 80 in order to expand partially cooled boosted nitrogen 78 to form second expanded nitrogen 82. In one embodiment, second expanded nitrogen 82 can have a temperature that is near or below its dew point and a pressure of about 5 to about 6 bar. In one embodiment, second expanded nitrogen 82 is a two phase fluid consisting of gas and liquid phases. In a preferred embodiment, second expanded nitrogen 82 is introduced to liquid/gas separator 60 in order to separate any gaseous nitrogen from liquid nitrogen. Recovered liquid nitrogen 62 is withdrawn from liquid/gas separator 60 and collected as product. In one embodiment, gaseous nitrogen 68 is withdrawn from a top portion of liquid/gas separator 60 and combined with gaseous nitrogen 28 before introduction to the cold side of heat exchanger 10 and subsequently recycled.
In one embodiment, fraction of compressed nitrogen recycle 48 is withdrawn from compressed nitrogen recycle 46 and fed to the warm end of heat exchanger 10, where fraction of compressed nitrogen recycle 48 is partially cooled before being expanded in first turbine 70 to form first expanded nitrogen 72. In one embodiment, first expanded nitrogen 72 is reintroduced to heat exchanger 10, preferably at an intermediate point, and combined with gaseous nitrogen 28 and subsequently recycled. In one embodiment, first turbine 70 is connected by a common shaft with first booster 50 and helps to provide the energy needed for first booster 50 to compress compressed nitrogen recycle 46. Likewise, second turbine 80 is connected by a common shaft with second booster 53 and helps to provide the energy needed for second booster 53 to compress boosted nitrogen 52. In one embodiment, first turbine 70 and second turbine 80 provide substantially all of the refrigeration needs for the process.
First turbine 70 and second turbine 80 produce refrigeration by work expansion. Their respective boosters, first booster 50 and second booster 53, utilize the produced work to further compress their respective nitrogen streams.
Fraction of partially compressed nitrogen recycle 44 can be withdrawn from partially compressed nitrogen recycle 42 and, along with third expanded nitrogen 182 is fed to the warm end of heat exchanger 10. After exiting the cold end of heat exchanger 10, fraction of partially compressed nitrogen recycle 44 is used to provide heat to bottom boiler 21 before being introduced via valve 93 near a top portion of single column 20 (see
In the embodiment shown, portion of fully boosted nitrogen 57 is compressed in third booster 153, cooled in fifth aftercooler 155 to produce supplemental compressed nitrogen 156, which is then partially cooled in LNG heat exchanger 120 to form partially cooled nitrogen 178. Partially cooled nitrogen 178 is then expanded in third turbine 180 to form third expanded nitrogen 182, which is then warmed in LNG heat exchanger 120 against natural gas in order to form LNG. Production of LNG can be controlled by LNG control valve 122. After warming, third expanded nitrogen 182 is then combined with fraction of partially compressed nitrogen recycle 44, which is then used as a reboiling fluid within the distillation column (see
In one embodiment, stream 57 can also be derived from boosted nitrogen 52 or from stream 46, depending on the splits of power from turbines 70, 80 and 180, since these turbines set the pressure ratios of the boosted nitrogen streams. In one embodiment, it is preferred to have the pressures of streams 56 and 156 be elevated, while at the limit of the design pressure of secondary exchanger.
Regarding
In
The vapor from the top of condenser 23 is the waste stream of the air distillation process and can be withdrawn through the plant equipment and sent to atmospheric vent. The pressure at the condenser 23 can be 1.3-1.5 bara. This pressure and the composition of this waste stream yields a vaporizing temperature of about −179° C. in condenser 23 which is thermally linked to the condensing N2 of stream 27, resulting in a pressure in column 20 (which is approximately 5 bara). Reboiler 21 thermally links the vaporization of the oxygen rich liquid with a condensing nitrogen stream. This vaporizer operates by condensing nitrogen at approximately −170° C., thereby setting the pressure of the condensing nitrogen at approximately 9.3 bara.
Matching the discharge pressure of the third turbine 180 to the pressure of the reboiler can help allow the LNG section and N2 section to operate more efficiently (i.e., that is with a specific power equal to that of separate independent liquefiers, as well as a significant capex savings (shared recycle compressor and warm turbine booster)). As mentioned before, if the third expanded nitrogen pressure is at a higher level than the suction pressure of the nitrogen recycle compressor, the heat exchange efficiency of the natural gas liquefaction process can be improved. Operating at the ˜9 bara (or ˜8 to 11 bar range) allows the LNG liquefier to operate at its optimum point. But this third expanded nitrogen pressure is limited by the possible condensation of nitrogen at the turbine discharge, facilitated at higher pressure
The term “ambient temperature” if used herein refers to the temperature of the air surrounding an object. Typically the outdoor ambient temperature is generally between about 0 to 110° F. (−18 to 43° C.).
The term “cryogenic gas” if used herein refers to a substance which is normally a gas at ambient temperature that can be converted to a liquid by pressure and/or cooling. A cryogenic gas typically has a boiling point of equal to or less than about −130° F. (−90° C.) at atmospheric pressure.
The terms “liquefied natural gas” or “LNG” as used herein refers to natural gas that is reduced to a liquefied state at or near atmospheric pressure.
The term “natural gas” as used herein refers to raw natural gas or treated natural gas. Raw natural gas is primarily comprised of light hydrocarbons such as methane, ethane, propane, butanes, pentanes, hexanes and impurities like benzene, but may also contain small amounts of non-hydrocarbon impurities, such as nitrogen, hydrogen sulfide, carbon dioxide, and traces of helium, carbonyl sulfide, various mercaptans or water. Treated natural gas is primarily comprised of methane and ethane, but may also contain small percentages of heavier hydrocarbons, such as propane, butanes and pentanes, as well as small percentages of nitrogen and carbon dioxide.
While the invention has been described in conjunction with specific embodiments thereof, it is evident that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the foregoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations as fall within the spirit and broad scope of the appended claims. The present invention may suitably comprise, consist or consist essentially of the elements disclosed and may be practiced in the absence of an element not disclosed. Furthermore, if there is language referring to order, such as first and second, it should be understood in an exemplary sense and not in a limiting sense. For example, it can be recognized by those skilled in the art that certain steps can be combined into a single step.
The singular forms “a”, “an” and “the” include plural referents, unless the context clearly dictates otherwise.
“Comprising” in a claim is an open transitional term which means the subsequently identified claim elements are a nonexclusive listing (i.e., anything else may be additionally included and remain within the scope of “comprising”). “Comprising” as used herein may be replaced by the more limited transitional terms “consisting essentially of” and “consisting of” unless otherwise indicated herein.
“Providing” in a claim is defined to mean furnishing, supplying, making available, or preparing something. The step may be performed by any actor in the absence of express language in the claim to the contrary.
Optional or optionally means that the subsequently described event or circumstances may or may not occur. The description includes instances where the event or circumstance occurs and instances where it does not occur.
Ranges may be expressed herein as from about one particular value, and/or to about another particular value. When such a range is expressed, it is to be understood that another embodiment is from the one particular value and/or to the other particular value, along with all combinations within said range.
All references identified herein are each hereby incorporated by reference into this application in their entireties, as well as for the specific information for which each is cited.
This application claims priority to U.S. Provisional Application Ser. No. 62/201,947 filed on Aug. 6, 2015, U.S. Provisional Application Ser. No. 62/305,381 filed on Mar. 8, 2016, and U.S. Provisional Application Ser. No. 62/370,953 filed on Aug. 4, 2016, all of which are hereby incorporated by reference in their entireties.
| Number | Date | Country | |
|---|---|---|---|
| 62201947 | Aug 2015 | US | |
| 62305381 | Mar 2016 | US | |
| 62370953 | Aug 2016 | US |
