Method and Apparatus for Liquefying a CO2-Rich Gas

Abstract

This invention relates to a method for liquefying a gas containing at least 60 mol % of CO2, in order to produce at least one liquid product, wherein the gas is cooled in order to form a fluid flow, at least a portion of the liquid or supercritical flow is cooled in a heat exchanger in order to form a cycle fluid having a cycle pressure, the cycle fluid is divided into at least two fractions including an auxiliary fraction, one of the fractions being expanded up to a first pressure in a valve in order to form a biphasic mixture, and then sent to a phase separator. The liquid fraction of the phase separator is vaporized so as to form a vaporized gas in the exchanger, the vaporized gas then being expanded from the first pressure to a second pressure in an expansion means, and then compressed in the cycle compressor and mixed with the first feed gas.

1 - 14 . (canceled) 
     
     
         15 . A process for liquefying a gas containing at least 60 mol % of carbon dioxide and at least one impurity lighter than carbon dioxide, in order to produce at least one liquid product wherein a first feed gas containing at least 60 mol % of carbon dioxide and at least one light impurity at a feed pressure is cooled in order to form a liquid or supercritical flow, at least a portion of the liquid or supercritical flow is cooled in a heat exchanger to form a cycle fluid having a cycle pressure, the cycle fluid is divided into at least two fractions comprising a first fraction and an auxiliary fraction, the first fraction being expanded down to a first pressure in a valve in order to form a biphasic mixture and then sent to a phase separator, the liquid fraction of the phase separator is vaporized so as to form a vaporized gas in the exchanger, the vaporized gas then being expanded from the first pressure to a second pressure in an expansion means and then compressed in a cycle compressor and mixed with the first feed gas, the auxiliary fraction either comprising the liquid product or one of the liquid products or being treated by separation at sub-ambient temperature in at least one separation means to form the liquid product or one of the liquid products, and wherein the first pressure is varied as a function of the amount of impurities lighter than the carbon dioxide contained in the carbon dioxide-rich gas, wherein the first pressure is varied so that the higher the concentration of light impurities is, the higher the first pressure is. 
     
     
         16 . The process of  claim 15 , wherein the first feed gas is cooled so as to form a liquid or supercritical flow, after having been compressed in a cycle compressor. 
     
     
         17 . The process of  claim 15 , wherein the second pressure is substantially constant and independent of the composition of the carbon dioxide-rich gas. 
     
     
         18 . The process of  claim 15 , wherein the second pressure is equal to the inlet pressure of the cycle compressor and the gas expanded in the expansion means is sent directly from the expansion means to the inlet of the cycle compressor. 
     
     
         19 . The process of  claim 15 , wherein the first pressure is chosen so as to be below the pressure at which the carbon dioxide-rich gas would solidify in the phase separator or in the valve. 
     
     
         20 . The process of  claim 15 , wherein the expansion rate in the expansion means is regulated as a function of the composition of the gas to be expanded in the expansion means and/or as a function of the composition of the fluid expanded in the expansion valve. 
     
     
         21 . The process of  claim 15 , wherein the expansion rate in the expansion means is regulated as a function of the amount of light impurities in the gas to be expanded in the expansion means and/or as a function of the composition of the fluid expanded in the expansion valve. 
     
     
         22 . An apparatus for liquefying a gas containing at least 60 mol % of CO 2  and at least one impurity lighter than carbon dioxide to produce at least one liquid product, comprising a heat exchanger, a cycle compressor, a cooling means, a conduit for sending a feed gas containing at least 60 mol % of CO 2  and at least one light impurity to the cooling means in order to form a liquid or a supercritical fluid, a conduit for sending the liquid or supercritical fluid to the exchanger in order to form a cooled fluid, a conduit for sending a portion of the cooled fluid as an auxiliary fraction to a client or to a treatment, an expansion valve, a conduit for sending a portion of the cooled fluid to the expansion valve, a phase separator, a conduit for sending a fluid expanded in the expansion valve to the phase separator, a conduit for sending the liquid from the phase separator to the exchanger, expansion means, a conduit for sending the liquid vaporized in the exchanger to the expansion means to form an expanded gas, a conduit for sending the expanded gas to a cycle compressor, means for mixing the expanded gas with the gas containing at least 60 mol % of CO 2  upstream of, downstream of, or in the cycle compressor and means for varying the outlet pressure of the expansion valve as a function of the composition of the feed gas, wherein the expansion means is a turbine. 
     
     
         23 . The apparatus of  claim 22 , comprising a treatment unit connected to the conduit in order to send a portion of the cooled fluid as an auxiliary fraction to a treatment, the treatment unit being a liquefaction unit or a separation unit at sub-ambient temperature. 
     
     
         24 . The apparatus of  claim 22 , wherein the turbine is coupled to the cycle compressor. 
     
     
         25 . The apparatus of  claim 22 , wherein the treatment unit comprises a compressor and the turbine is coupled to the compressor of the treatment unit. 
     
     
         26 . The apparatus of  claim 22 , comprising a means for detecting the concentration of the gas sent to the expansion means and/or the fluid sent to the expansion valve and a means for varying the expansion rate of the expansion means as a function of this concentration. 
     
     
         27 . The apparatus of  claim 22 , comprising a means for detecting the pressure of the gas sent to the expansion means and a means for varying the expansion rate in the expansion means as a function of this pressure. 
     
     
         28 . The apparatus of  claim 22 , wherein the exchanger is arranged so that the latent heat from vaporization of the liquid of the phase separator is transferred to the liquid or to the supercritical fluid that is cooled through the exchanger.