Patent Publication Number: US-2017350648-A1

Title: Process for liquefying carbon dioxide resulting from a natural gas stream

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of priority under 35 U.S.C. §119 (a) and (b) to French Patent Application No. 1654996 filed Jun. 2, 2016, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     The present invention relates to a process for liquefying a stream of hydrocarbons, such as natural gas, in particular in a process for producing liquefied natural gas and liquid CO 2 . 
     In typical natural gas liquefaction plants, refrigerant streams are used to produce the refrigeration at various levels of a main heat exchanger by vaporizing against the stream of hydrocarbons to be liquefied (typically natural gas). 
     It is desirable to liquefy natural gas for a certain number of reasons. By way of example, natural gas may be stored and transported over long distances more easily in the liquid state than in the gas form, since it occupies a much smaller volume for a given mass and does not need to be stored at a high pressure. 
     Typically, natural gas contains hydrocarbons and CO 2  (0.5 mol % to 5 mol % approximately). In order to prevent the CO 2  from freezing during the liquefaction of the natural gas, it is advisable to remove it. One means for removing the CO 2  from the natural gas stream is for example amine scrubbing upstream of a liquefaction cycle. 
     Amine scrubbing separates the CO 2  from the feed gas by scrubbing the natural gas stream with a solution of amines in an absorption column. The CO 2 -enriched amine solution is recovered at the bottom of this absorption column and is regenerated at low pressure in a distillation (or stripping) column for regenerating the amine. 
     At the top of this distillation column, a CO 2 -rich acid gas is released. Thus, the amine scrubbing treatment of the natural gas stream releases a CO 2 -concentrated “acid gas” stream, usually emitted directly into the atmosphere. 
     In natural gas liquefiers (50 000 tonnes per year to 10 million tonnes per year), the amount of CO 2  emitted is sufficient (amount of CO 2  emitted possibly ranging up to 200 tonnes per day) and it is possible to purify this CO 2 -rich “acid gas” to give food grade CO 2 . 
     Specifically, in the food field, in accordance with the current legislation, in order to be able to be sold, the CO 2  produced must meet strict specifications in terms of quality and purity. Thus, for example, any trace of hydrocarbons or of sulphur derivatives must be eliminated (content typically less than 1 ppm by volume). 
     This purification is carried out by means of a dedicated CO 2  purification unit requiring the installation of a dedicated refrigeration cycle (typically a refrigeration system operating with ammonia for example). 
     The operation of the “refrigeration unit” refrigeration cycle consists in providing the refrigeration necessary for the CO 2  purification/liquefaction process. 
     Typically, a standard CO 2  unit contains the following steps: 
     Step 1: Compression of the impure CO 2  to a pressure between 15 and 50 bar abs. 
     Step 2: Purification of the CO 2  for example by processes that use regenerative adsorbents, absorbents or catalyst to eliminate any presence of water, mercury, hydrocarbons and sulphur derivatives (non-exhaustive list of impurities). 
     Step 3: Distillation of the noncondensable gases in order to separate in particular oxygen and nitrogen from the CO 2  produced. 
     Thus, conventionally in a CO 2  purification/liquefaction unit, it is necessary to provide refrigeration at three temperature levels: 
     1. Refrigeration at −20° C./−30° C. used for step 3 described in the paragraph above. 
     2. Refrigeration at 5° C. used for step 2. 
     3. Refrigeration at ambient temperature for cooling the impure CO 2  in step 1. 
     The condenser of the distillation column used in step 3 represents around 50% of the total of the refrigeration requirements. This refrigeration may be provided via a dedicated refrigeration cycle (typically an ammonia or propane refrigeration cycle) optionally coupled with a water cooling system. 
     The system for producing frigories represents a high cost of the CO 2  purification and liquefaction unit and adds complexity of implementation to the site for implementing the process which represents a constraint. 
     One existing solution consists in separating the two (natural gas liquefaction and CO 2  purification) units which requires the installation of two systems for producing frigories, one for the natural gas liquefaction unit and one for the CO 2  purification unit. 
     SUMMARY 
     The present invention relates in particular to a process of thermal integration between a natural gas liquefaction unit and CO 2  purification/liquefaction unit. 
     The inventors of the present invention have then developed a solution that makes it possible to solve the problem raised above, namely to minimize the investment in a system for producing frigories in the CO 2  purification/liquefaction unit and therefore to optimize the investment expenditure while retaining an optimal efficiency for the liquefaction of the natural gas in the liquefaction unit. 
     The subject of the present invention is a process for producing liquefied natural gas and liquid carbon dioxide (CO 2 ) comprising at least the following steps:
         Step a): separating a natural gas feed gas, containing hydrocarbons and carbon dioxide in a treatment unit, into a CO 2 -enriched gas stream and a CO 2 -depleted natural gas stream;   Step b): liquefying the CO 2 -depleted natural gas stream resulting from step a) in a natural gas liquefaction unit comprising at least a main heat exchanger and a system for producing frigories, said natural gas liquefaction unit comprising at least one refrigeration cycle fed by a refrigerant stream;   Step c): simultaneous liquefying of the CO 2 -enriched gas stream resulting from step a) in a CO 2  liquefaction unit;
 
characterized in that the refrigeration necessary for the liquefaction of the CO 2 -enriched gas stream and for the liquefaction of the natural gas is supplied by said frigorie-producing system of the natural gas liquefaction unit and in that all or some of the refrigeration necessary for the liquefaction of the CO 2 -enriched gas stream originates from a portion of said refrigerant stream supplying the refrigeration cycle of said natural gas liquefaction unit. Preferably, all the refrigeration necessary for the liquefaction of the CO 2 -enriched gas stream originates from a portion of said refrigerant stream supplying the refrigeration cycle of said natural gas liquefaction unit. The object of the present invention is to thermally couple a unit for liquefying a gas rich in hydrocarbons, typically natural gas, with a unit for purifying/liquefying CO 2 .
       

     Thermal coupling is understood to mean sharing the means for producing frigories in order to ensure the thermal balance of the two units, typically refrigeration cycle compressor, and optionally a turbine/booster system in the case of a nitrogen cycle. 
     A turbine/booster system is understood to mean a turbine mechanically coupled (via a common shaft) to a single-stage compressor. The power generated through the turbine is directly transmitted to the single-stage compressor. 
     This thermal integration is realized by the sharing of any column, heat exchanger, unit or other suitable arrangement (typically a heat exchanger) where streams linked to the natural gas liquefaction process and streams linked to the CO 2  purification/liquefaction process exchange thermally. 
     The process that is the subject of the present invention makes it possible to do without the refrigeration unit initially necessary for liquefying the CO 2  and to extract the refrigeration directly from the natural gas liquefier. This thermal integration thus makes it possible to do without one piece of equipment in the CO 2  purification unit. 
     The proposed integration makes it possible to provide refrigeration at the three temperature levels needed. 
     According to other embodiments, the invention also relates to:
         A process as defined above, characterized in that the feed gas comprises from 0.1 mol % to 5 mol % of CO 2 .   A process as defined above, characterized in that the CO 2 -enriched gas stream resulting from step a) comprises at least 95 mol % of CO 2 .   A process as defined above, characterized in that, prior to step b), the natural gas stream resulting from step a) is pretreated in a pretreatment unit.   A process as defined above, characterized in that said treatment unit used in step a) is an amine scrubbing unit.   A process as defined above, characterized in that the CO 2 -enriched gas stream resulting from step a) is purified prior to step c), the refrigeration necessary for this purification being provided by said frigorie-producing system of the natural gas liquefaction unit.   A process as defined above, characterized in that the CO 2 -enriched stream thus purified comprises at least 99.5 mol % of CO 2 .   A process as defined above, characterized in that said system for producing frigories comprises at least one compressor and optionally a turbine-booster system.   A process as defined above, characterized in that the refrigerant stream supplying said at least one refrigeration cycle of said natural gas liquefaction unit contains at least one of the constituents selected from nitrogen, methane, ethylene, ethane, propane, ammonia, butane and pentane.       

     Another subject of the present invention is a device for producing liquefied natural gas and liquefied CO 2  comprising a feed gas treatment unit, producing at least a CO 2 -enriched gas stream and a CO 2 -depleted natural gas stream, and a natural gas liquefaction unit, said natural gas liquefaction unit comprising at least a main heat exchanger and a system for producing frigories, characterized in that the system for producing frigories is capable of and designed for liquefying both the CO 2 -enriched stream resulting from the treatment unit and the CO 2 -depleted natural gas stream circulating in the natural gas liquefaction unit, said natural gas liquefaction unit comprising at least one refrigeration cycle supplied by a refrigerant stream resulting from the main exchanger. 
     According to other embodiments, the present invention also relates to:
         A device as defined above, characterized in that said system for producing frigories comprises at least one compressor and optionally a turbine-booster system.   A device as defined above, characterized in that said system for producing frigories comprises a refrigeration cycle, comprising a compressor driven by a heat engine, a circuit for circulating a refrigerant fluid.   A device as defined above, characterized in that it comprises a CO 2 -enriched gas purification and liquefaction unit comprising at least one compression means, one purification means and at least one distillation column, characterized in that said device is designed so that the refrigeration necessary for the use of the CO 2 -enriched gas purification and liquefaction unit originates from the refrigerant stream supplying the refrigeration cycle of said natural gas liquefaction unit.       

     Since the refrigeration requirement of a natural gas liquefaction unit is generally greater than the refrigeration requirement of a CO 2  purification/liquefaction unit, it is relevant to benefit from the available capacity of the machines (compressors and/or turbine/boosters) of the natural gas liquefaction unit in order to provide, fully or at least partially, the refrigeration requirement of the CO 2  purification/liquefaction unit and in particular to limit the investment in machinery of the CO 2  purification/liquefaction unit. 
     In particular, the incremental investment for increasing the liquefaction capacity of a hydrocarbon liquefier is much lower than the incremental investment for increasing the liquid production capacity of a CO 2  purification/liquefaction unit. 
     Moreover, other intermediate treatment steps between the hydrocarbon stream/CO 2  separation and the liquefaction of the hydrocarbons may be carried out. The stream of hydrocarbons to be liquefied is generally a stream of natural gas obtained from natural gas fields, oil reservoirs or a domestic gas network distributed via pipelines. 
     Customarily, the natural gas stream is essentially composed of methane. Preferably, the feed stream comprises at least 80 mol % of methane. Depending on the source, the natural gas contains quantities of hydrocarbons heavier than methane, such as for example ethane, propane, butane and pentane and also certain aromatic hydrocarbons. The natural gas stream also contains non-hydrocarbon products such as H 2 O, N 2 , CO 2 , H 2 S and other sulphur-containing compounds, mercury and others. 
     The feed stream containing the natural gas is therefore pretreated before being introduced into the heat exchanger. 
     This pretreatment comprises the reduction and/or the elimination of the undesirable components such as the CO 2  and the H 2 S, or other steps such as precooling and/or pressurization. Given that these measures are well known to a person skilled in the art, they are not described in further detail here. 
     In the process that is the subject of the present invention, it is essential to pretreat the natural gas stream in order to extract a CO 2 -enriched stream that will itself be liquefied by means of the frigorie-producing system of the natural gas liquefaction unit. 
     The expression “natural gas” as used in the present application relates to any composition containing hydrocarbons including at least methane. This includes a “crude” composition (prior to any treatment or scrubbing), and also any composition that has been partially, substantially or completely treated for the reduction and/or elimination of one or more compounds, including, but without being limited thereto, sulphur, carbon dioxide, water, mercury and certain heavy and aromatic hydrocarbons. 
     The heat exchanger may be any heat exchanger, any unit or other arrangement suitable for allowing the passage of a certain number of streams, and thus enabling a direct or indirect heat exchange between one or more refrigerant fluid lines, and one or more feed streams. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
       For a further understanding of the nature and objects for the present invention, reference should be made to the following detailed description, taken in conjunction with the accompanying drawing, in which like elements are given the same or analogous reference numbers and wherein: 
         FIG. 1  illustrates a schematic representation of one embodiment of the present invention. 
     
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     In  FIG. 1 , a natural gas feed stream  1  (flow rate considered: 500 000 tonnes per year, i.e. around 60 tonnes per hour) containing CO 2  is introduced into a treatment unit  2  in which said stream  1  is separated into at least two gas streams  3  and  4 . The natural gas feed stream  1  contains for example from 0.1 mol % to 5 mol % of CO 2 . 
     The first stream  3  is a CO 2 -depleted natural gas stream. The second stream  4  is a CO 2 -enriched stream. 
     The treatment unit  2  is a unit that separates the CO 2  from the natural gas stream, for example a chemical absorption unit, in particular an amine (of MDEA, MEA, etc. type) scrubbing unit that makes it possible to produce pure (or concentrated) CO 2  at low pressure (typically slightly greater than atmospheric pressure). Pure CO 2  is understood to mean a stream containing more than 95 mol % of CO 2  on a dry basis. 
     After possible pre-treatment steps to eliminate all traces of mercury, water or sulphur derivatives for example (pre-treatment in unit  7 ), the CO 2 -depleted natural gas stream  3  is introduced into the main exchanger  8  of a natural gas liquefaction unit  5  in order to be liquefied. 
     The pressure of this gas stream is for example between 25 and 60 bar absolute. Typically, the gas stream  3  contains between 30 ppm by volume and 500 ppm by volume of benzene, usually less than 100 ppm by volume. The gas stream  3  is cooled by heat exchange in the heat exchanger  8  in contact with a refrigerant. The heat exchanger  8  is supplied by at least one refrigerant stream  8 . 
     For example, this stream may be composed of a nitrogen or mixed refrigerant stream that provides the refrigeration necessary for the liquefaction of the natural gas stream. The refrigerant stream is sent into the exchanger at high pressure (typically from 30 to 60 bar) and sent back at low pressure (from 1 to 10 bar). The recompression energy necessary for the operation of the refrigeration cycle is provided by a cycle compressor (optionally supplemented by a turbine/booster system within the context of a nitrogen cycle). 
     The CO 2 -depleted natural gas stream  3  introduced into the main exchanger  8  of a natural gas liquefaction unit  5  is for example liquefied according to the process described in the following lines. 
     The natural gas stream cooled to a temperature between −20° C. and −70° C., typically between −35° C. and −40° C. at the outlet of the exchanger  8  is introduced into a unit  11  for separating the heavy hydrocarbons from the natural gas stream, for example a scrubbing column in which the heavy products 10 are separated from the natural gas. Heavy products are understood to mean hydrocarbons having more than four carbon atoms and aromatic compounds including in particular benzene. 
     A liquid stream  10  containing all the hydrocarbons that it is desired to extract from the natural gas stream, such as benzene, (from the initial gas stream  1 ), is discharged at the bottom of the scrubbing column. 
     At the top of the column, a gas stream that no longer presents a risk of freezing due to the presence of heavy hydrocarbons or aromatic derivatives (typically comprising less than 1 ppm by volume of benzene) is recovered in order to be introduced into a second section of the heat exchanger  8 . Via heat exchange, it is cooled to the desired temperature (typically −160° C.) in order to be sent to a liquefied natural gas storage means 14. 
     The mixed refrigerant stream recovered at the outlet of the heat exchanger  8  is introduced into a phase separator vessel that produces a gas stream containing the light elements of the refrigerant at the top of the vessel and a liquid stream  13  containing the heavy elements of the refrigerant at the bottom of the vessel. The refrigerant stream circulates in a closed cycle in the heat exchanger  8  in order to provide the refrigeration necessary for liquefying said natural gas stream  3 . 
     In particular, the liquefaction cycle 9 uses a refrigerant that may be a mixture of refrigerants typically selected from nitrogen, methane, ethane, ethylene, propane, butane and pentane. It may be a cycle based on a refrigeration cycle consisting of a refrigerant or a mixture of several refrigerants. 
     A refrigerant stream is introduced into the 9 rigories-producing system 9 of the liquefaction unit  5  via a compressor (and optionally via a compressor/booster system). 
     The second CO 2 -enriched gas stream  4  resulting from the treatment unit  2  is compressed to medium pressure (typically 25 bar abs), cooled, purified (elimination of any trace of H 2 O, hydrocarbons, sulphur derivatives in particular) then sent back to a distillation column (stripping column) that separates the noncondensable gases at the top from the concentrated liquid CO 2    15  recovered at the bottom. 
     In order to provide the refrigeration necessary for the correct operation of the purification/liquefaction unit  6 , a portion of the liquid stream  13  containing the heavy elements of the refrigerant is extracted and is sent to circulate between the CO 2  purification/liquefaction unit  6  and the natural gas liquefaction unit  5 . Thus, owing to this thermal integration, a refrigeration cycle dedicated to the CO 2  purification/liquefaction unit  6  is avoided by increasing the power of the cycle dedicated to the natural gas liquefaction (typically by the order of 5%). 
     It will be understood that many additional changes in the details, materials, steps and arrangement of parts, which have been herein described in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims. Thus, the present invention is not intended to be limited to the specific embodiments in the examples given above.