NGL Recovery Methods and Configurations

Abstract

Contemplated plants and methods for NGL recovery from feed gases having a carbon dioxide content equal or greater than about 2% employ temperature control configurations that allow high-level and flexible recovery of ethane and heavier components while avoiding freezing of the carbon dioxide in the process. Where the feed gas has a significant fraction of C3+ components and moderate carbon dioxide content, a single column configuration with an intermediate reflux condenser may be used, while two-column configurations may be used for feed gases with high carbon dioxide content and relatively low C3+ component concentration.

1 . A method of operating a plant for NGL recovery from a feed gas, comprising:
 separating the feed gas in a refluxed column to thereby produce a residue gas, and using a portion of the residue gas after cooling as a first reflux;   expanding a portion of the feed gas upstream of the column to thereby form a second reflux; and   controlling temperature of the column by using at least one of (1) a control circuit that controls a temperature of an expander discharge stream that is fed to the column and (2) an intermediate reflux condenser disposed between an upper section and a lower section of the column that maintains temperature of the lower section above a temperature sufficient to prevent carbon dioxide freezing.   
   
   
       2 . The method of  claim 1  wherein the feed gas comprises at least 2% carbon dioxide, and wherein the column is operated such that at least 90% of ethane is recovered from the feed gas. 
   
   
       3 . The method of  claim 1  wherein the step of controlling temperature of the column is performed using the intermediate reflux condenser, and wherein the upper section of the column generates a liquid product that is used to cool the feed gas. 
   
   
       4 . The method of  claim 1  further comprising a step of cooling the feed gas and separating the cooled feed gas in a separator into a vapor portion and a liquid portion, wherein a portion of the vapor is further cooled and expanded to form the second reflux. 
   
   
       5 . The method of  claim 1  wherein step of controlling temperature of the column is performed using the control circuit, and wherein the column is a demethanizer that produces a bottom product. 
   
   
       6 . The method of  claim 5  wherein the bottom product is fed to a second column that is operated at a lower pressure, and that produces a natural gas liquids bottom product and an ethane overhead product. 
   
   
       7 . The method of  claim 6  wherein at least part of the ethane overhead product is compressed and routed back to the demethanizer. 
   
   
       8 . The method of  claim 1  further comprising a step of splitting the feed gas into three streams, wherein the first stream is cooled to a first temperature before entering the column, wherein the second stream is cooled and expanded in a turbo expander before entering the column at a second temperature, and wherein the third stream bypasses the feed exchanger and is used for temperature control by the control circuit. 
   
   
       9 . A plant comprising:
 a column having an intermediate reflux condenser that is configured to operate at a temperature of between about −20° F. and about −40° F. and that is located between a fractionation section and a rectification section of the column, and wherein the column is further configured to receive a first and a second reflux stream and to produce an overhead product;   a circuit coupled to the column such that a liquid stream is fed from the fractionation section to the rectification section via a heat exchanger that is configured such that the liquid stream is heated in the heat exchanger; and   a recycle circuit that is configured to provide a portion of the overhead product as the first reflux stream to the column.   
   
   
       10 . The plant of  claim 9  wherein the heat exchanger is a feed gas exchanger, and wherein the feed gas exchanger is configured to heat the liquid stream to a temperature that is suitable for at least partial removal of methane and ethane from C3+ components in the rectification section. 
   
   
       11 . The plant of  claim 10  wherein the feed gas exchanger is further configured to cool the portion of the overhead product. 
   
   
       12 . The plant of  claim 9  further comprising a second heat exchanger that is configured to cool a vapor portion of a feed gas using refrigeration cold of the overhead product to thereby produce a cooled vapor portion. 
   
   
       13 . The plant of  claim 12  further comprising an expansion device that is configured to reduce a temperature of the cooled vapor portion to thereby form the second reflux stream. 
   
   
       14 . The plant of  claim 9  wherein the column is further configured to provide an ethane product stream and a natural gas liquids product stream. 
   
   
       15 . A plant comprising:
 a first column configured to receive a first and a second reflux stream and further configured to receive an expanded feed gas stream;   a temperature control unit thermally coupled to the first column and configured to determine a temperature in the first column;   a heat exchanger that is configured to cool a first and a second portion of a feed gas to thereby form the second reflux stream and a cooled second portion of the feed gas, respectively;   a bypass valve configured to control flow volume of a third portion of the feed gas to the cooled second portion of the feed gas; and   a control system that is configured to adjust the flow volume of the third portion of the feed gas as a function of the temperature in the first column.   
   
   
       16 . The plant of  claim 15  further comprising a recycle circuit that is configured to provide a portion of an overhead product of the first column back to the first column as the first reflux stream. 
   
   
       17 . The plant of  claim 15  further comprising a second column and an expansion device, wherein the first column is configured to produce a bottom product, wherein the expansion device is configured to receive expanded bottom product, and wherein the second column is configured to produce a natural gas liquids bottom product and an ethane overhead product. 
   
   
       18 . The plant of  claim 17  wherein the expansion device is configured to reduce pressure of the first column bottom product by at least 50 psi. 
   
   
       19 . The plant of  claim 17  further comprising a circuit that provides at least a portion of the ethane overhead product back to the first column. 
   
   
       20 . The plant of  claim 15  further comprising a turbo expander that is configured to receive the cooled second portion and the third portion of the feed gas, and that is further configured to provide expanded feed gas to the first column.