Abstract:
A process for liquefying and subcooling a hydrocarbon-rich fraction, particularly natural gas, is described wherein, once cooled down, the fraction is subjected to a partial condensation to remove heavy hydrocarbons, particularly benzene, by the steps of: a) the liquefied hydrocarbon-rich fraction is subcooled in a separate heat exchanger (normal mode), b) the supply of the liquefied hydrocarbon-rich fraction to the heat exchanger is interrupted at the latest when a defined solid deposition value in the heat exchanger is reached (cleaning mode), c) the solid in the heat exchanger is melted with a defrost gas and drawn off from the heat exchanger and d) the liquefied hydrocarbon-rich fraction is subsequently returned to the heat exchanger.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority from German Patent Application DE 102014005936.7 filed on Apr. 24, 2014. 
       BACKGROUND OF THE INVENTION 
       [0002]    The invention relates to a process for liquefying and subcooling a hydrocarbon-rich fraction, particularly of natural gas, wherein, once cooled down, the fraction is subjected to a partial condensation to remove heavy hydrocarbons, particularly benzene. 
         [0003]    Liquefaction and subcooling of a hydrocarbon-rich fraction is typically achieved against at least one refrigerant cycle and/or at least one mixed refrigerant cycle. 
         [0004]    Preventing outages caused by freezing-out of certain components of the fraction to be liquefied is of great importance in the liquefaction of hydrocarbon-rich fractions, particularly natural gas. Water and carbon dioxide are typically removed at the beginning of the process at ambient temperature by chemical scrubbing (e.g. amine scrubbing) and/or adsorptive processes to such an extent that they do not cause undesired solid formation during liquefaction of the hydrocarbon-rich fraction. 
         [0005]    Freezing-prone heavy hydrocarbons (HH) (hereinbelow the term “heavy hydrocarbons” is to encompass C 6+  hydrocarbons), benzene in particular, can be removed under ambient conditions from the fraction to be liquefied only at great cost and inconvenience. Hence it is common practice to subject the feed gas to a slight partial condensation and then draw off an HH-rich liquid fraction in a separator to sufficiently reduce the risk that the gas phase exiting this separator will freeze during subsequent liquefaction and subcooling. 
         [0006]    However, partial condensation generally only ensures that the gas phase is sufficiently depleted in HHs, particularly benzene, when the gas mixture to be liquefied comprises components having a middle boiling range, for example propane, butane and/or pentane, which during cooling-down of the feed gas undergo liquefaction in sufficient amounts before the HHs and thus act as solvent for said HHs. 
         [0007]    When an insufficient concentration of middle boilers—this is referred to as so-called lean gas—in the composition of the feed gas does not allow sufficient depletion in benzene (typically to &lt;1 ppmv) by partial condensation and subsequent removal of the HH-rich liquid, unwanted freezing-out can still occur. 
         [0008]    It is an object of the present invention to specify a process of the type in question for liquefying and subcooling a hydrocarbon-rich fraction, particularly of natural gas, which achieves reliable and economical removal of heavy hydrocarbons even under these conditions. 
       SUMMARY OF THE INVENTION 
       [0009]    This object is achieved by a process for liquefying and subcooling a hydrocarbon-rich fraction, particularly natural gas, said process being characterized in that 
         [0000]    a) the liquefied hydrocarbon-rich fraction is subcooled in a separate heat exchanger (normal mode),
 
b) the supply of the liquefied hydrocarbon-rich fraction to the heat exchanger is interrupted at the latest when a defined solid deposition value in the heat exchanger is reached (cleaning mode),
 
c) the solid in the heat exchanger is melted with a defrost gas and drawn off from the heat exchanger and
 
d) the liquefied hydrocarbon-rich fraction is subsequently returned to the heat exchanger.
 
         [0010]    According to the invention, the already liquefied hydrocarbon-rich fraction is now subcooled in a separate heat exchanger (subcooler) in which freezing-out or deposition of solid is deliberately permitted. The process thus intentionally seeks to achieve solid formation of the heavy hydrocarbons at a temperature of below −70° C., preferably below −80° C., in the subcooler in the liquefaction of natural gas. When a defined solid deposition value in this separate heat exchanger has been reached, normal mode is interrupted and the process switches to cleaning mode. To achieve this, the supply to the subcooler of the liquefied hydrocarbon-rich fraction to be subcooled is interrupted and the liquefied fraction is immediately sent for further use and/or to intermediate storage. The aforementioned defined solid deposition value may, for example, be determined by an increased pressure drop of the hydrocarbon-rich fraction to be subcooled during passage through the subcooler. According to the invention, cleaning mode comprises melting the solid using a suitable amount of defrost gas at a suitable temperature and subsequently drawing off the resulting melt from the separate heat exchanger at a suitable point, preferably at a/the conduit low point(s), and in concentrated form and generally sending said melted solid outside the plant boundary. The amount and/or temperature of the defrost gas are to be chosen such that at least 50%, preferably at least 70%, of the amount of solid can be melted and removed. A development of the process according to the invention proposes that once the solid in the separate heat exchanger has been melted at least the heat exchanger passages of the separate heat exchanger in which solid formation can occur are purged with a gaseous or liquid purging medium. This purging melts and removes remaining solids in the separate heat exchanger. Particularly suitable purging media are dry nitrogen and a boil-off gas fraction generated during intermediate storage of the liquefied and subcooled hydrocarbon-rich fraction. 
         [0011]    After cleaning, the supply of the defrost gas and/or the purging medium is terminated and the process switches to normal mode by returning the liquefied hydrocarbon-rich fraction to be subcooled to the separate heat exchanger. 
         [0012]    When, in normal mode, the liquefied hydrocarbon-rich fraction is subcooled in a separate heat exchanger against at least one refrigerant stream and/or at least one mixed refrigerant stream, one advantageous embodiment of the process according to the invention for liquefying and subcooling a hydrocarbon-rich fraction is characterized in that in cleaning mode this refrigerant stream and/or mixed refrigerant stream are used to cool the hydrocarbon-rich fraction to be liquefied. 
         [0013]    Owing to the above-described rerouting of the refrigerant stream and/or mixed refrigerant stream in cleaning mode, the heat exchanger or heat exchanger zone disposed upstream of the separate heat exchanger assumes, at least to an extent, the subcooling function of the separate heat exchanger. This regime efficaciously avoids the situation where the liquefied hydrocarbon-rich fraction exiting the liquefaction zone in cleaning mode is distinctly warmer than the subcooled fraction exiting the separate heat exchanger in normal mode. Hence even in cleaning mode the liquefied hydrocarbon-rich fraction drawn off at the cold end of the process is at a temperature no more than 30° C., preferably no more than 20° C., higher than the temperature of the subcooled hydrocarbon-rich fraction in normal mode. 
         [0014]    When the hydrocarbon-rich fraction to be liquefied is liquefied and subcooled against at least one refrigeration cycle, a further advantageous embodiment of the process according to the invention provides that the defrost gas required for cleaning mode is a substream of the refrigerant circulating in the refrigeration cycle. When this refrigeration cycle comprises, for example, a two-stage compressor unit, the refrigerant substream serving as defrost gas may be drawn off from the suction side of the second compressor stage, expanded to a suitable pressure and optionally heated, passed through the separate heat exchanger and subsequently sent to the suction side of the first compressor stage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The process according to the invention for liquefying and subcooling a hydrocarbon-rich fraction and also further advantageous embodiments thereof are more particularly elucidated hereinbelow with reference to the working examples shown in  FIGS. 1 and 2 . 
           [0016]      FIG. 1  shows a regime where the hydrocarbon-rich fraction is liquefied and subcooled against a mixed cycle while the regime shown in  FIG. 2  employs a two-stage nitrogen expander cycle. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0017]    Hydrocarbon-rich feed fraction  1  to be liquefied, for example so-called lean natural gas, is sent, prior to actual liquefaction, to removal unit A in which a chemical scrub and/or an adsorptive process are used to remove water and carbon dioxide which are drawn off via line  2 . The thus prepurified feed fraction  3  is sent to first heat exchanger or heat exchanger zone E 1  in which it is cooled down and partially condensed. Partially condensed fraction  4  is then sent to separator D 1  and separated into heavy hydrocarbons-containing liquid fraction  5  and hydrocarbon-rich gas fraction  6 . While the former is drawn off from the bottom of separator D 1  via control valve V 6 , gaseous fraction  6  is liquefied in second heat exchanger or heat exchanger zone E 2 . According to the invention, liquefied hydrocarbon-rich fraction  7  is subcooled in separate heat exchanger or subcooler E 3 . Subcooled hydrocarbon-rich fraction  8 —in the case of natural gas the LNG product fraction—is sent for further use and/or intermediate storage via valve V 4 . Heat exchangers E 1  to E 3  described above may be helically coiled heat exchangers and/or welded plate exchangers. 
         [0018]    In the regime shown in  FIG. 1 , cooling-down, liquefaction and subcooling of the hydrocarbon-rich fraction are achieved against a mixed cycle comprising two-stage compressor unit C 1 . The refrigerant vaporized and warmed in heat exchangers E 1  to E 3  is sent via line  20  to vessel D 2  disposed upstream of the first stage of compressor unit C 1 . Gas fraction  21  accumulating in said vessel is compressed to an intermediate pressure in the first compressor stage of compressor unit C 1 , cooled down and partially condensed in intermediate cooler E 4  and sent via line  22  to second separator D 3 . Gas fraction  23  accumulating in said second separator is compressed to the desired final cycle pressure in the second compressor stage of compressor unit C 1  and sent to third separator D 4  via line  27  in which aftercooler E 5  is disposed. 
         [0019]    Liquid fraction  25  drawn off from the bottom of second separator D 3  is cooled down in heat exchanger E 1 . This fraction is subsequently subjected to refrigerating expansion in valve V 1  and passed, countercurrently to hydrocarbon-rich feed fraction  3  to be cooled down, through heat exchanger E 1  via line  26 . While liquid fraction  28  accumulating in third separator D 4  is recycled to a point upstream of second separator D 3  via control valve V 5 , gas fraction  29  accumulating in third separator D 4  is likewise cooled down and partially condensed in heat exchanger E 1  and then separated into liquid fraction  30  and gas fraction  32  in separator D 5 . 
         [0020]    The latter is condensed and subcooled in heat exchangers E 2  and E 3 , subjected to refrigerating expansion in valve V 3  and is passed via line  33  through separate heat exchanger E 3  to provide the peak refrigeration required therein. This fraction is subsequently admixed via control valve V 7  and line  34  with liquid fraction  30  cooled down in heat exchanger E 2 . Said liquid fraction is subjected to refrigerating expansion in expansion valve V 2  and subsequently passed, countercurrently to hydrocarbon-rich feed fraction 3/6 which is to be cooled down and liquefied, through heat exchangers E 2  and E 3  via line  31 . 
         [0021]    According to the invention, heat exchanger or subcooler E 3  is a discrete apparatus. Said apparatus is connected to heat exchangers E 1  and E 2  only via conduits. Now, when a defined solid deposition value in heat exchanger E 3  is reached, the process switches from normal mode to cleaning mode. This is achieved by closing valve V 4  and opening valve V 9 , so liquefied hydrocarbon-rich fraction  7  bypasses heat exchanger E 3  via line  9 , In a simultaneous operation valves V 3  and V 7  are closed and valve V 8  is opened, so gas fraction  32  drawn off from separator D 5  is now passed exclusively through heat exchanger E 2 . Due to this rerouting of refrigerant fraction  32 , heat exchanger E 2  assumes, at least to an extent, the subcooling of the liquefied hydrocarbon-rich fraction which in normal mode is effected in separate heat exchanger E 3 . 
         [0022]    Simultaneously with the above-described opening and closing of valves V 3 , V 4  and V 7  to V 9 , and with valves V 10  and V 11  open, a suitable amount of defrost gas at a suitable temperature is passed via line  10  through heat exchanger E 3  and drawn off via line  11 . Heat exchanger E 6  provided in line  10  heats this defrost gas. Now, rather than refrigerant fraction  32  which flows through heat exchanger E 3  in normal mode, defrost gas  10  serves as heat-transfer medium and melts the solids deposited in heat exchanger E. Said solids can be drawn off in concentrated form at a suitable point between heat exchangers E 2  and E 3 , for example at the conduit low points, via appropriate shutoff valves which, for clarity, are not shown. 
         [0023]    In the regime shown in  FIG. 2 , cooling-down, liquefaction and subcooling of the hydrocarbon-rich feed fraction are achieved via a two-stage nitrogen expander cycle. Since the regime for the hydrocarbon-rich feed fraction to be liquefied and subcooled here is identical to that of  FIG. 1 , it will not be discussed further in what follows; hence what follows describes only the nitrogen expander cycle. 
         [0024]    Nitrogen-rich refrigerant  40  warmed in heat exchangers E 1  to E 3  is compressed to an intermediate pressure in the first compressor stage of compressor unit C 1 ′, cooled down in intermediate cooler E 4 ′ and sent via line  41  to the second compressor stage of compressor unit C 1 ′. Refrigerant  42  compressed to the cycle end pressure is cooled down in aftercooler E 5 ° and cooled down in heat exchangers E 1  and E 2 . A first substream  43  of the cooled-down refrigerant is sent to a first expander X 1 , subjected to refrigerating and work-performing expansion therein and passed, countercurrently to hydrocarbon-rich   feed fraction  3  which is to be liquefied, through heat exchangers E 2  and E 1  via line  44 . The second refrigerant substream  45  is sent to second expander X 2  to likewise undergo refrigerating and work-performing expansion, passed, countercurrently to the hydrocarbon-rich fraction  7  which is to be subcooled, through separate heat exchanger E 3  via line  46  and subsequently admixed via valve V′ with the above-described refrigerant substream  44 . 
         [0025]    When the defined solid deposition value in heat exchanger X 3  is reached, second expander X 2  is taken off stream. In a simultaneous operation valve V 7 ′ is closed and valves V 8 ′, V 10 ′ and V 11 ′ are opened. With valve V 8 ′ open, second refrigerant substream  45 , hitherto sent to second expander X 2 , is now sent via line  52 , shown dashed in the figure, to a point upstream of first expander X 1 . With valve V 10 ′ open—said valve is used for adjustment of the desired defrost gas pressure—a substream of the refrigerant drawn off upstream of the second compressor stage is sent as defrost gas to heat exchanger E 3  via line  50  shown with a dotted line in the figure. Heat exchanger E 6 ′ is used for any defrost gas heating required. Having passed through heat exchanger E 3 , and with valve V 11 ′ open, the defrost gas is recycled via line  51 , shown with a dotted line in the figure, to a point upstream of the first compressor stage of compressor unit C 1 ′. 
         [0026]    The process according to the invention for liquefying and subcooling a hydrocarbon-rich fraction, particularly of natural gas, achieves reliable and economical removal of heavy hydrocarbons, particularly of benzene, even when a so-called lean gas is used. The implementation of the concept according to the invention is independent of the chosen type of liquefaction and subcooling of the hydrocarbon-rich fraction.