Patent Publication Number: US-2010115990-A1

Title: Method for liquefying a hydrocarbon-rich flow

Description:
The invention relates to a method for liquefying a hydrocarbon-rich flow, in particular a natural gas flow, via indirect heat exchange with the refrigerant mixture of a refrigerant mixture circuit. 
     A generic method for liquefying a hydrocarbon-rich flow is known for example from German patent 197 22 490. By citing this document the disclosure thereof shall be incorporated into the disclosure of the present patent application in its entirety. 
     Such methods for liquefying a hydrocarbon-rich flow, as described in the abovementioned patent, are generally designated as “Single-Flow Mixed Refrigerant Cycle”. A common element here is that the refrigerant mixture is fully vaporised prior to (again) being fed to the circuit compressor and is superheated by ca, 10 K above its dew point. 
     In the operating of generic methods for liquefying a hydrocarbon-rich flow it has been shown however that this heating of the refrigerant mixture over and above the dew point can be highly problematic, since thermal fluctuations arise in the region of the dew point, which can cause breakages in the pipes of lapped heat exchangers, preferably in the outer pipe layers. The consequence of such pipe breakage is generally unwanted interruption of the liquefaction process. 
     Other generic methods for liquefying a hydrocarbon-rich flow are for example the so-called “Parallel Mixed Refrigerant” method, as described for example in U.S. Pat. No. 6,389,844; the so-called “Double Mixed Refrigerant” method, such as described for example in U.S. Pat. No. 6,370,910; or other refrigerant mixture methods with pre-cooling circuit, such as for example a “Propane-Mixed Refrigerant” method. 
    
    
     The aim of the present invention is to provide a generic method for liquefying a hydrocarbon-rich flow, by which the abovementioned considerable disadvantage can be avoided. 
     To solve this problem a generic method for liquefying a hydrocarbon-rich flow is proposed, whereby the refrigerant mixture is compressed two-stage or multistage, is separated into at least one lower-boiling and at least one higher-boiling refrigerant mixture fraction and the refrigerant mixture fractions are vaporised at different temperature levels against the hydrocarbon-rich flow to be cooled and liquefied, and then unified prior to recompression, and whereby the refrigerant mixture fractions are not fully vaporised. 
     A further advantageous accomplishment is that the refrigerant mixture fractions are not superheated, in particular they are not superheated above their dew point, and more particularly are not superheated by ca. 10 K above their dew point. 
     In further advantageous configurations of the method according to the invention for liquefying a hydrocarbon-rich flow, which constitute the subject matter of the dependent claims, it is proposed that independently or in combination
         at least 1 to 10% by weight, preferably at least 2 to 5 by weight of the total quantity of the refrigerant mixture is not vaporised;   incomplete vaporising of the refrigerant mixture is achieved by the refrigerant mixture having a sufficiently large proportion of heavier hydrocarbons, preferably C 4 H 10 — and C 5 H 12 — hydrocarbons, depending on the selected method parameters, such as vaporising temperature and pressure;   the liquid accruing from incomplete vaporising of the refrigerant mixture fractions is separated off; and   this separated liquid is forwarded to a suitable supply point inside the refrigerant mixture circuit, preferably by means of a pump.       

     In contrast to the previously used methods from now on uninterruptedly in normal operation there is no complete vaporising of the refrigerant mixture and its superheating above the dew point. 
     The quantity of liquid or fraction resp. resulting from incomplete vaporising of the refrigerant mixture prior to feeding the refrigerant mixture to the circuit compressor is preferably separated off, since it can otherwise result in damage to the circuit compressor. It is also possible to provide at least one extra pump, by means of which the separated quantity of liquid or fraction resp. can be pumped to a suitable site inside the refrigerant mixture circuit. Referring for example to FIG. 1 of the abovementioned German patent 197 22 490, the quantity of liquid or fraction resp. accruing in a separator D 2  upstream of the compressor V is fed to the separator D 3 , preferably using a pump. 
     Apart from the advantage of safe operation, which is associated with the method according to the invention for liquefying a hydrocarbon-rich flow, a not inconsiderable thermodynamic advantage can also be realised, Due to so-called “wet operation”, i.e. non-attainment of the dew point inside the heat exchanger, the sharp knee at the dew point in the enthalpy temperature diagram of the refrigerant mixture is also avoided. The result of this is that the heating and cooling curves can be better matched, again resulting in an improvement in efficiency of the overall process. 
     The method can also be employed to advantage in other refrigerant mixture methods for liquefying hydrocarbon-rich flow, in addition to the “Single-Flow Mixed Refrigerant Cycle method”.