Process and apparatus for heavy hydrocarbon removal from lean natural gas before liquefaction

Abstract

A process is described herein for removing high freeze point hydrocarbons, including benzene compounds, from a mixed feed gas stream. The process involves cooling process streams in one or more heat exchangers and separating condensed compounds in multiple separators to form a methane-rich product gas stream. Select solvent streams from a fractionation train and/or separate solvent streams are employed to lower the freeze point of one or more streams that contain high freeze point hydrocarbons. A corresponding system also is disclosed.

What is claimed is: 
     
         1 . A process for removing high freeze point hydrocarbons, including benzene compounds, from a mixed feed gas stream, comprising:
 cooling the mixed feed gas stream in a first heat exchanger to condense at least a portion of the C3, C4 and C5 components and high freeze point hydrocarbons,   separating the condensed C3, C4, C5 components and high freeze point hydrocarbons in a first separator to form a first liquid stream and a first gas stream,   cooling the first gas stream in a second heat exchanger to condense at least a portion of the first gas stream,   separating the condensed portion of the first gas stream in a second separator to form a methane-rich second gas stream as a top stream and a second liquid stream,   feeding the first and second liquid streams to a first fractionator, and removing methane gas in a top stream and a third liquid stream as a bottom stream,   removing a methane-rich product gas stream downstream from the top of the second separator,   fractionating the third liquid stream in a fractionation train to obtain a recycle stream comprising at least one of C3 and components and C4 components, and a high freeze point hydrocarbon stream, and   feeding the recycle stream comprising at least one of C3 components and C4 components to the process at a location upstream from the first fractionator to lower the freeze point of the stream at the location where the recycle stream is introduced.   
     
     
         2 . The process of  claim 1 , wherein the recycle stream is combined with the feed gas stream upstream from the first separator. 
     
     
         3 . The process of  claim 1 , wherein the recycle stream is combined with the first liquid stream. 
     
     
         4 . The process of  claim 1 , wherein the recycle stream is combined with the second liquid stream. 
     
     
         5 . The process of  claim 1 , wherein the recycle stream is combined with the first gas stream. 
     
     
         6 . The process of  claim 1 , wherein the first separator comprises a warm separator and the first liquid stream obtained from the first separator contains C2 components. 
     
     
         7 . The process of  claim 1 , wherein the second separator comprises a cold separator. 
     
     
         8 . The process of  claim 1 , further comprising a third separator downstream from the second separator, wherein the bottom stream from the third separator is fed to the first fractionator and the top stream comprises at least a portion of the methane-rich product gas stream. 
     
     
         9 . The process of  claim 7 , wherein the recycle stream is mixed with the bottom stream from the third separator. 
     
     
         10 . The process of  claim 8 , wherein the top stream from the second separator is expanded before being fed to the third separator. 
     
     
         11 . The process of  claim 10 , wherein the methane-rich product gas stream has a methane content of at least 80% molar methane. 
     
     
         12 . The process of  claim 1 , further comprising mixing a portion of the methane-rich product gas stream with the mixed feed gas stream during plant start-up. 
     
     
         13 . The process of  claim 1  wherein a stream of non-freezing components is added to the mixed feed gas stream from an external source to enable initial start-up and cool-down of the system without freezing until temperatures are achieved which allow adequate condensation of non-freezing components from the feed gas to be made available for recycle. 
     
     
         14 . The process of  claim 1 , further comprising using an expander to auto-refrigerate the methane-rich second gas stream after separation of the majority of the most high freeze point components, wherein the auto-refrigerated methane-rich second gas stream is used to cool the feed gas. 
     
     
         15 . The process of  claim 1  wherein the recycle of a portion of recovered non-freezing components to the feed gas changes the composition so that the composition of the feed gas is similar to richer feed gases that require less or no recycle to avoid freezing. 
     
     
         16 . The process of  claim 15  wherein the recycle of a portion of recovered non-freezing components increases total condensation of feed gas components to approximate the conditions in a plant when the methane content of the feed gas is higher, resulting in all plant equipment being operated at similar conditions whether or not the feed gas is methane-rich. 
     
     
         17 . The process of  claim 1  wherein the recycle of a stream comprising at least one of C3 components and C4 components increases the volume percent liquid in the stream entering the first and second separators and dilutes the concentration of high freeze point hydrocarbons in the liquid, thereby reducing the amount of high freeze point hydrocarbons that leave the separator with the vapor stream due to incomplete liquid recovery. 
     
     
         18 . The process of  claim 1 , wherein the recycle stream further comprises at least one of C2 components and C5 components. 
     
     
         19 . A process for removing high freeze point hydrocarbons, including benzene compounds, from a mixed feed gas stream, comprising:
 cooling the mixed feed gas stream in a first heat exchanger to condense at least a portion of the C3, C4 and C5 components and high freeze point hydrocarbons,   separating the condensed C3, C4, C5 components and high freeze point hydrocarbons in a first separator to form a first liquid stream and a first gas stream,   cooling the first gas stream in a second heat exchanger to condense at least a portion of the first gas stream,   separating the condensed portion of the first gas stream in a second separator to form a methane-rich second gas stream as a top stream and a second liquid stream,   feeding the first and second liquid streams to a first fractionator, and removing methane gas in a top stream and to remove a third liquid stream as a bottom stream,   removing a methane-rich product gas stream downstream from the top of the second separator,   fractionating the third liquid stream in a fractionation train to obtain hydrocarbon product streams, and   feeding a solvent stream comprising at least one of C3 components and C4 components to the process at a location upstream from the first fractionator to lower the freeze point of the stream at the location where the solvent stream is introduced, thereby enabling lower process temperatures to be used.   
     
     
         20 . A system for pre-treatment of a mixed feed gas stream containing methane and benzene components to remove the benzene components, comprising:
 a first heat exchanger for partially condensing the mixed feed gas,   a first separator configured to separate the mixed feed gas to form a first liquid hydrocarbon stream containing C3+ components from a first methane-containing gas stream,   a second heat exchanger configured to at least partially condense the first methane-rich gas stream,   a second separator configured to separate a second methane-containing gas stream from a second liquid hydrocarbon stream,   a fractionator configured to remove methane from the first liquid hydrocarbon stream and the second liquid hydrocarbon stream, and   a solvent inlet configured to feed a solvent stream comprising at least one of C3 components and C4 components to the system, the solvent inlet being positioned upstream from the first or second separator, or downstream from the second separator and upstream from the fractionator.   
     
     
         21 . The system of  claim 20 , further comprising an expander positioned downstream from the second separator, a third separator positioned downstream from the expander and upstream from the demethanizer, and a line configured to feed a bottom stream from the third separator to the fractionator. 
     
     
         22 . A process for removing high freeze point hydrocarbons, including benzene compounds, from a mixed hydrocarbon feed gas stream, comprising:
 cooling the mixed feed gas stream in a first heat exchanger to condense at least a portion of the C3, C4 and C5 components and high freeze point hydrocarbons,   separating the condensed C3, C4, C5 components and high freeze point hydrocarbons in a first separator to form a first liquid stream and a first gas stream,   partially condensing the first gas stream by cooling the first gas stream in a second heat exchanger or reducing the pressure of the first gas stream,   separating the condensed portion of the first gas stream in a second separator to form a methane-rich second gas stream, and a second liquid stream,   removing a methane-rich product gas stream downstream from the top of the second separator, feeding the first liquid stream to a fractionation train, and fractionating the first liquid stream to obtain hydrocarbon product streams and a high freeze point hydrocarbon stream comprising benzene components, and   withdrawing at least a portion of the second liquid stream, increasing the pressure of the withdrawn portion, and recycling at least some of the withdrawn and compressed portion to the process to a location upstream from, or at, the first separator to prevent freezing of process streams and process components.   
     
     
         23 . The process of  claim 22 , wherein at least some of the withdrawn and compressed portion of the second liquid stream is combined with the mixed feed gas stream upstream from the first heat exchanger. 
     
     
         24 . The process of  claim 22 , wherein at least some of the withdrawn and compressed portion of the second liquid stream is combined with the cooled mixed feed gas stream upstream from, or in, the first separator. 
     
     
         25 . The process of  claim 22 , wherein at least some of the withdrawn and compressed portion of the second liquid stream is combined with the cooled mixed feed gas stream downstream from the first separator and upstream from the second heat exchanger. 
     
     
         26 . The process of  claim 24 , wherein some of the withdrawn and compressed portion of the second liquid stream is combined with the cooled mixed feed gas stream downstream from the first separator and upstream from the second heat exchanger. 
     
     
         27 . The process of  claim 24 , wherein the pressure of the first gas stream is reduced in an expander. 
     
     
         28 . The process of  claim 22 , wherein the first gas stream is partially condensed in a heat exchanger, further comprising removing a methane-rich product gas stream from the top of a third separator positioned downstream from the second separator. 
     
     
         29 . The process of  claim 28 , further comprising sending the bottoms stream from the third separator to the fractionation train. 
     
     
         30 . The process of  claim 22 , wherein the first gas stream is partially condensed by reducing the pressure of the first gas stream in at least one of a Joule Thompson valve or an expander. 
     
     
         31 . The process of  claim 30 , further comprising removing a methane rich product gas stream from the top of the second separator. 
     
     
         32 . The process of  claim 22 , wherein the first separator comprises a multistage fractionation tower, and the recycle stream is fed to the tower above the feed point of the cooled feed gas stream. 
     
     
         33 . The process of  claim 22 , wherein the second separator comprises a multistate fractionation tower, and the recycle stream is fed to the tower above the feed point of the cooled first gas stream  10 . 
     
     
         34 . The process of  claim 22 , further comprising using a second separator inlet reduction valve to reduce pressure at the inlet to the first separator in order to reduce the likelihood of freezing. 
     
     
         35 . The process of  claim 32 , wherein the valve is a Joule-Thompson valve. 
     
     
         36 . The process of  claim 22 , wherein the withdrawn and compressed portion of the second liquid stream is heated in one of the heat exchangers before entering a separator. 
     
     
         37 . The process of  claim 28 , wherein the first, second and third separators are arranged within a single vertical shell structure. 
     
     
         38 . The process of  claim 28 , further comprising withdrawing at least a portion of a bottom stream from the third separator, compressing the withdrawn stream, and recycling at least some of the withdrawn and compressed stream to a location upstream from the second heat exchanger. 
     
     
         39 . The process of  claim 38 , wherein the withdrawn and compressed stream comprises low freeze point hydrocarbons. 
     
     
         40 . A system for pre-treatment of a mixed feed gas stream containing methane and benzene components to remove the benzene components, comprising:
 a first heat exchanger for cooling and partially condensing the mixed feed gas,   a first separator configured to separate the cooled and partially condensed mixed feed gas stream to form a first liquid hydrocarbon stream containing C3+ components and a first methane-containing gas stream,   an expander configured to expand and partially condense the first methane-containing gas stream,   a second separator configured to separate the first methane-containing gas stream to form a second methane-containing gas stream and a second liquid hydrocarbon stream,   a pressure-increasing device configured to increase the pressure of at least one of the first liquid hydrocarbon stream and the second liquid hydrocarbon stream, and   a recycle inlet configured to feed a recycled portion of at least one of the first liquid hydrocarbon stream and the second liquid hydrocarbon stream back into the system at a location upstream from, or at, the first separator.   
     
     
         41 . The system of  claim 40 , wherein the pressure-increasing device comprises a pump. 
     
     
         42 . The system of  claim 40 , wherein the recycle inlet is configured to combine the recycled portion of the second liquid hydrocarbon stream with the mixed feed gas stream. 
     
     
         43 . The system of  claim 40 , wherein the recycle inlet is configured to combined the recycled portion of the second liquid hydrocarbon stream with the cooled and partially condensed mixed feed gas.