US3531943A

US3531943A - Cryogenic process for separation of a natural gas with a high nitrogen content

Info

Publication number: US3531943A
Application number: US732010A
Authority: US
Inventors: John Parnag; John W Menzies; Frederick W Kirkpatrick Jr; Robert B Ritter; John L Sullwold
Original assignee: Aerojet General Corp
Current assignee: Aerojet Rocketdyne Inc
Priority date: 1965-10-23
Filing date: 1968-05-22
Publication date: 1970-10-06
Anticipated expiration: 1987-10-06

Description

Oct. 6, J PARNAG ETAL CRYOGENIG PROCESS FOR SEPARATION OF A NATURAL GAS WITH A HIGH NITROGEN CONTENT Original Flled Oct. 23. 1965 United States Patent Office 3,531,943 Patented Oct. 6, 1970 3,531,943 CRYOGENIC PROCESS FOR SEPARATION OF A NATURAL GAS WITH A HIGH NITROGEN CONTENT John Parnag, Covina, John W. Menzies, San Gabriel, Frederick W. Kirkpatrick, Jr., Covina, Robert B. Ritter, San Gabriel, and John L. Sullwold, La Canada, Calif., assignors to Aerojet-General Corporation, El Monte, Calif., a corporation of Ohio Continuation of application Ser. No. 503,382, Oct. 23, 1965. This application May 22, 1968, Ser. No. 732,010 Int. Cl. F25j 3/02, 3/08 U.S. Cl. 62-28 10 Claims ABSTRACT OF THE DISCLOSURE This patent describes a process for the reduction of the amount of inerts in a mixture comprising a combustible hydrocarbon and inert gases including nitrogen which comprises: cooling a stream of high pressure gas containing combustible hydrocarbons and inert gases including nitrogen; expanding the cooled stream under substantially isentropic conditions to cool the gas and to remove work therefrom; passing the expanded stream into a separation zone operated at suiciently high pres-- sure to permit the use of methane at its boiling point to remove heat from the top of said zone; forming said methane at its boiling point by the steps comprising cooling and condensing propane with water, condensing and sub-cooling propane with cold product streams and recycled low pressure propane refrigerant vapor, cooling and condensing ethylene with water, cold product streams, and vaporizing propane, sub-cooling ethylene with cold product streams and recycled low pressure ethylene refrigerant vapor, cooling methane with Water, cold product streams, and recycled 'low pressure methane refrigerant vapor, condensing methane with evaporating ethylene and evaporating hydrocarbon product and sub-cooling methane with cold product streams and recycled low pressure methane refrigerant vapor; passing said methane to the top of said zone, and removing substantially pure nitrogen from the top of said zone and substantially pure methane from the bottom of said zone.

This application is a continuation of application S.N. 503,382 led Oct. 23, 1965 and now abandoned.

This invention pertains to the removal of inert gases such as nitrogen from gas streams containing combustible hydrocarbons. More particularly the invention is concerned with the removal of inert gases from natural gas to increase its heating value and provide for the recovery of the valuable inert gases.

Hydrocarbon rich gases such as natural gas are widely used for heating purposes. In general, in this country, a commercially acceptable material must have a heating value of about 1000 B.t.u. per standard cubic foot. However, there exists in many places, particularly in the Great Basin area of the United States, vast reserves of hydrocarbon-containing gases which are diluted with substantial amounts of inert gases such as nitrogen, carbon dioxide, helium, argon, and the like. The presence `of large amounts of inert gases in these materials renders them practically useless as fuel because of their low heating value. Accordingly, various attempts have been made to remove the inert gas from these gas reserves to increase the heating value.

Certain of the inert gases present in the low utility hydrocarbon-containing gases may be readily removed. For example, carbon dioxide can be effectively eliminated from the gas by treatment with hot potassium carbonate. Similar procedures are effective for the removal of hydrogen sulfide. However, these techniques are ineffectual in removing nitrogen which is often the principal inert material in the low utility gases. The nitrogen, as a practical matter, must be removed by a combination of refrigerative and distillative processes. Generally, these separation techniques involve the separation of nitrogen (normal boiling point -320.5 F.) from methane (normal boiling point -258.7 F.), the principal hydrocarbon in the 10W utility gases. The separation of these gases on large scale in an economical fashion has been a long standing problem in the art, restricting the commercial exploitation and utilization off the existing gas reserves containing inerts. The economics of the separation is a function of the total amount of work required to compress and cool the gases to a liquid state where separation by distillation is possible. In the typical process of the prior art as shown in U.S. Pat. No. 2,583,090, the cooled high pressure feed gas is throttled into a rectification tower where essentially pure methane is recovered at low pressure. This involves an isenthalpic or Joule-Thomson expansion. However, this technique does not produce any work from the expansion step. Moreover, the process employing the isenthalpic expansion is characterized by other inefficiencies, since for example, it is not possible to recover valuable liquid nitrogen from the tower without the benefit of costly lower level refrigeration in the process.

It has also been proposed to isentropically expand the high pressure feed gas before cooling and throttling into a rectification column. We have found that this procedure results in less eiiicient heat transfer in cooling of the gas yand also requires costly low level refrigeration in order to recover liquid nitrogen from the rectification column.

According to this invention, it has been found that substantially increased efliciencyis obtained by cooling the high pressure feed gas, isentropically expanding into a separation column operated at a pressure suciently high to condense the separated nitrogen by heat transfer with boiling methane at a pressure at least as high as atmospheric, and recovering essentially pure nitrogen and methane. In this way, good separations are achieved and both the liquid nitrogen and methane are obtained with the least amount of refrigeration and other compression work.

Accordingly, it is a principal object of this invention to provide a process for the removal of inert gases from hydrocarbon-containing gases to provide a saleable high heating value gas in a more efficient and economical manner.

More particularly, it is an object of this invention to provide for the separation of hydrocarbons from inerts in low utility gases employing an isentropic expansion step, and boiling hydrocarbon at a pressure of at least one atmosphere as the refrigerant in the separation zone.

Still another object of the present invention is to provide a purication process for low utility gases which yields an acceptable fuel gas having a heating value of about 1000 B.t.u. per cubic foot, and additionally provides for the commercially feasible recovery of the separated essentially pure nitrogen in liquid or compressed form.

It is also an object of our invention to separate methane Aand nitrogen in a more eicient manner.

In another aspect, it is an object of the invention to separate and recover hydrocarbons and inert material in a process utilizing a novel cascade refrigeration system.

from natural gas, as well as the recovery of hydrogen from manufactured or synthetic gases.

These and other objects of the invention will be apparent from the detailed description which follows, taken in conjunction with the accompanying drawing.

Briefly, the present invention comprises a process for the reduction of the amount of inerts in a mixture comprising a combustible hydrocarbon and inert gases including nitrogen, which comprises cooling a stream of high pressure gas containing combustible hydrocarbons and inert gases including nitrogen, expanding the cooled stream under substantially isentropic conditions to remove work therefrom, passing the expanded stream into a separation zone operated at sufliciently high pressure to permit the use of methane at its boiling point under a pressure, preferably at least as high as one atmosphere, to remove heat from the top of said zone, separating said expanded stream in said column into sub pure nitrogen and sub pure methane, and removing liquid and/or gaseous substantially pure nitrogen from the top of said zone and liquid and/ or gaseous substantially pure methane from the bottom of said zone. The invention includes the further steps of providing at least part of the methane for removal of heat from the top of the separation zone by methane liquefaction in a novel cascade refrigeration system utilizing cold product streams. In another aspect, our invention comprises the recovery of high purity argon and/or helium from the nitrogen removed from the top of the separation zone. In yet another aspect, the invention comprises the recovery of hydrogen from said expanded and separated stream.

The process of the present invention can be better understood by the accompanying drawing depicting an illustrative embodiment of the invention.

As shown on the drawing, any hydrocarbon-bearing gas containing inerts and non-hydrocarbons such as nitrogen, carbon dioxide, sulfur compounds and helium is gathered from gas wells and transferred to the plant. The gas is first treated to remove carbon dioxide and hydrogen sulfide by any conventional means such as monoethanolamine scrubbing. The treated gas at point 100 has any liquid water removed in a conventional entrainment separator 101, and is then dehydrated by conventional dessicants in vessel 102, connected by line 103 to vessel 101 to a dew point of about 100 F. or lower. The gas at this point is at ambient temperature and is passed via line 104 to heat exchanger 105 wherein the gas gives up some heat in separate passes to a counter iowing cold methane-rich product. The cold dried gas stream is then passed from exchanger 105 into line 106,

boiler for distillation tower 109. Heat gained by the methane and heavier hydrocarbons in the bottom of tower 109 causes vaporization of the liquids and helps in the distillation of the natural gas. The cold natural gas feed from exchanger 108 flows through line 110 through isentropic expansion engine 111, with any additional condensate being iirst removed via separator 112. Alternatively, when none of the condensed liquids are withdrawn via seperator 112 or via separator 107, it is necessary that all liquids in the feed be by-passed around the expansion engine 111 via line 113. Separation of the liquid from the gas takes place in separator 11311. This is to prevent damage to expander 111 by the introduction of excess liquid particles. The feed gas then discharges from expander 111 at reduced pressure with work having been removed. The discharge from expander 111 contains a substantial portion of liquid on the order of up to about mole percent. The vapor-liquid mixture flows then through line 114 into the feed partial condenser 115 where further liquefaction occurs as the result of a further lowering of the temperature. This cooling increases the amount of liquid up to about 30 to 40 mole percent. This feed mixture then flows into distillation tower 109 via line 116 for separation into a nitrogen rich overhead product and a hydrocarbon rich bottoms product. v.In conventional manner by proper manipulation of reboiler heat into the bottom of the tower and heat extraction from the top of the tower, as well as employment of an appropriate number of distillation stages, any desired degree of separation is achieved.

A portion of the bottoms liquid methane product is diverted into line 117 and throttled through `valve 118 in the extension of line 117 into the evaporator side of partial feed condenser 115. The vapor-liquid hydrocarbon mixture formed on throttling to a reduced pressure provides refrigeration to the natural gas feed expander efuent. The totally vaporized methane next flows through line 119 to a refrigerant methane subcooler 120 to provide refrigeration to the liquid methane refrigerant. In like manner and sequentially, the low pressure methane product flows from exchanger 120 through line 121, methane chiller 122, line 123, ethylene chiller 124 and line 125 giving up refrigeration to refrigerant fluids in heat exchange and itself becoming warmer. Beyond line 125 the flow is divided, part continuing in line 126- to ethylene cooler 127 and the remainder continuing via line 128 through propane subcooler 129. The split streams in

lines

130 and 131 at ambient temperature and normally about 5 F. colder than the incoming warm streams in heat exchange, combine and leave the system in line 132. The product from line 132 may be utilized for fuel for gas engines driving compressors as well as for the production, for example, of hydrogen for an ammonia plant.

Turning to line 133 at the bottom of tower 109, that portion of the liquid methane not diverted to line 117 ows by way of line 134 to the evaporator side of refrigerant methane condenser 135. Throttling occurs across valve 136 in line 134, the methane product is reduced in pressure in passing from line 133 to the evaporator side of unit as Well as undergoing a reduction in temperature. Evaporation of the methane provides a portion of the condensation duty of the methane refrigerant as hereinafter explained. Emerging as saturated vapor from unit 135, the methane at reduced pressure ows via line 137 to methane refrigerant chiller 122 and gains heat from cooling methane refrigerant. Sequentially, the methane product flows from unit 122 through line 138 and natural gas feed chiller 105. Finally it emerges from the system in line 139 at ambient temperature within about 5 F. of the feed gas temperature. This methane product is normally at an elevated pressure and may be utilized as reformer feed in the production of hydrogen or ammonia. The product may also be utilized for other purposes such as gas pipeline feed.

The overhead products from tower 109 are liquid and vapor nitrogen. In the case where liquid nitrogen product is desired, the liquid nitrogen is withdrawn from tower 109 via line 140 and subcooler 141. It is then throttled across valve 142 in line 143 into the nitrogen ash drum 144. A portion remains as liquid and is subsequently taken to storage via line 145. The nitrogen flash vapor from drum 144 via line 145a is used to su'bcool liquid nitrogen in exchanger 141 prior to throttling into ash drum 144. This low pressure vapor nitrogen is then warmed to ambient temperature which is about 5 F. cooler than the warm high pressure ethylene refrigerant entering exchanger 127 by passage through line 146, methane subcooler 120, line 147, methane chiller 122, line 148, ethylene chiller 124, and line 149, where it enters ethylene cooler 127. The gaseous nitrogen at about one atmosphere is then ejected from the plant to the atmosphere via line 150. Some portion of the Waste nitrogen may be conveniently used as a purge gas for adsorbent regeneration or as an inert gas for keeping insulation around the cold equipment dry. This nitrogen can also be compressed into commercial gas for sale.

High pressure gaseous nitrogen may also be produced simultaneously with the liquid nitrogen. A portion of vapor nitrogen is withdrawn from the head of the tower 109 via line 151 and is then passed through methane subcooler 120, line 152 and methane chiller 122, providing in parallel with other cold gaseous streams previously described, refrigeration needed to cool methane refrigerant. Beyond exchanger 122, line 153 divides into two parts, lines 154 and 15S. Part of the nitrogen is diverted via line 155 to the ethylene refrigeration circuit. Here it ows through ethylene chiller 124, line 156 and ethylene cooler 127 ending as a warm gas in line 157. The second portion of nitrogen flows through line 154 and methane cooler 158 ending as warm gas in line 159. The split streams of high pressure nitrogen are both warmed to a point about 5 F. cooler than the warm refrigerant streams being cooled. The two nitrogen flows are reunited in line 160 whence the single flow leaves the systern. This high pressure gas may conveniently be used in ammonia synthesis in unit 160:1, or further compressed into bottles for sale.

To keep the method of this invention in heat balance, an additional amount of refrigeration exclusive of ,expander refrigeration and auto-refrigeration by-product streams, must be provided by external means. This additional refrigeration is provided by a unique form of cascade refrigeration. Starting with the lowest temperature or methane leg of the cascade, the sequence of steps involves rst compressing methane in a multistage compressor 161. With the heat of compression being removed in intercoolers and aftercooler 162, the gaseous methane then ows via line 163 to methane cooler 158 where it is cooled by heat exchange with cold product nitrogen and cold recycled methane refrigerant. The methane refrigerant is further transferred from unit 158 via line 164 to methane chiller 122 and there cooled to a state providing a saturated vapor. Recycling methane refrigerant plus several cold product streams provide cooling at this stage. Saturated methane vapor leaves heat exchanger 122 by line 165 and then divides into branch lines 166 and 167 to the methane condensers 135 and 168. The split of refrigerant methane between

units

135 and 136 will depend upon the amount of product liquid methane available at this point for condensing refrigerant in unit 135. In general, the amount of heat taken up by evaporation of product is inadequate to totally condense methane refrigerant. For this reason the remainder of the condensate load in unit 168 is carried by the ethylene loop of the cascade refrigerant system. The saturated liquid methane refrigerant from condensers 168 and 135 flows from the respective units by

lines

169 and 170 which joins into line 171. Line 171 conveys the liquid to methane subcooler 120 where the fluid is subcooled again by itself in a cold low pressure gaseous state as well as by various cold product streams. The subcooling lowers the enthalpy of the unthrottled refrigerant to maximize the amount of liquid in the condenser 172 after Joule-Thomson expansion.

The subcooled methane refrigerant flows from subcooler 120 by line 173 to condenser 172 in the top of tower 109. Just prior to entry into the evaporative side of the condenser, throttling occurs across valve 174 in line 173. The liquid formed by throttling through valve 174 is constantly vaporized in the condenser by heat exchange with rising vapors in tower 109, which in turn condense and provide both liquid nitrogen tower overhead product and internal reflux to effect separation by distillation. The combined lash vapors and evaporated liquid methane leave the condenser via line 175 and ow to the suction of the methane compressor 161 imparting refrigeration to itself in the high pressure state sequentially through methane subcooler 120, line 177, methane chiller 122, line 178, methane cooler 158 and finally via line 179 to the compressor 161.

CII

As auto-refrigeration and cold product streams together do not provide adequate refrigeration to liquefy and subcool methane refrigerant, additional outside refrigeration is provided by the propane and ethylene loops of the cascade. Starting at ethylene compressor 180, ethylene gas is compressed and inter and aftercooled, discharging into line 181. The ethylene is next cooled to saturated vapor in ethylene cooler 127 against cold product streams. From cooler 127, the ethylene vapor flows via line 182 to ethylene condenser 183. Total condensation of ethylene occurs against vaporizing propane. Saturated liquid ethylene from condenser 183 ows through line 184 to ethylene chiller 124 in which subcooling occurs by means of heat exchange with itself as a recycled cold low pressure gas, plus cold product streams. From chiller 124, the subcooled uid goes to a second subcooler 185 connected to chiller 124 by line 186. This second stage of subcooling is accomplished by self-refrigeration of the high pressure liquid ethylene by itself as expanded returning material in the cold low pressure state. The subcooled ethylene travels in line 187 from subcooler 185 to ethylene condenser 168. Before entering condenser 168, it is expanded isenthalpically across valve 188 in line 187. Acquiring heat from condensing methane, the ethylene is vaporized. This cold low pressure ethylene is returned to compressor 180 giving up refrigeration and being warmed by passage through line 189, ethylene subcooler 185, line 190, chiller 124, and iinally line 191 to the suction of the compressor 180.

As the ethylene leg of the cascade refrigeration system is required to condense methane, similarly the propane leg or loop is necessary to condense the ethylene. Propane itself is condensed by heat exchange with water. As shown, propane is compressed in propane compressor 192 and essentially completely condensed in the aftercooler-condenser 193, preceeding line 194. Line 194 conveys the virtually liquid propane to propane subcooler 129. Residual vapor condensation and subcooling of total liquid propane takes place in unit 129. Heat removal from propane is brought about by both auto-refrigeration and cold product. The subcooled propane from subcooler 129 in line 195 flashes through valve 196 1n the evaporator side of ethylene condenser 183. The evaporated cold low pressure vapor propane flows Via line 197, subcooler 129, and line 198 back to compressor 192 suction.

The following example is presented solely to illustrate the invention and hence should not be regarded as limiting in any way. In the example, the parts, percentages and ratios are by weight, and pressures are absolute, unless otherwise indicated.

EXAMPLE Natural gas containing about 60% by volume nitrogen and about 39% by Volume of hydrocarbons, mostly methane, together with small amounts of carbon dioxide, helium, argon, and hydrogen sulfide is fed at a pressure of 2000 p.s.i. to a purification treatment utilizing monoethanolamine to remove carbon dioxide and hydrogen sulfide. The treated gas at point 100 in the accompanying drawing is then dried by passage through entrainment separator 101 and further dried in vessel 102 to a dew point of 100 F. The gas at this point is at a temperature of about 90 F. At this point the gas composition is about 60.61% by volume nitrogen, 38.86% by volume methane, 0.29% by volume ethane, 0.08% by volume propane, 0.04% by volume butane, and 0.12% by volume pentane. The dehydrated treated gas from dryer 102 flows through line 104 to heat exchanger 105 where the gas gives yup some heat to the counter-flowing methane product. The temperature of the feed following erchanger is about +49 F. Any condensed hydrocarbons flowing from exchanger 105 into line 106 are removed from the system in separator 107. The chilled feed gas flows from exchanger 105 through line 106 to a heat exchanger 108 (the reboiler for tower 109) and is therein further cooled to about 96 F. Heat gained by the methane and heavier hydrocarbon liquids in the bottom of tower 109 causes vaporization of the liquids and aids in the distillation of the natural gas. The cold natural gas flows from exchanger 108 through line 110 to expansion engine 111, with any liquids being separated and Withdrawn via separator 112. The natural gas discharges from expander 111 at a pressure of 400 p.s.i., a temperature of 197 F. and containing 15 mole percent liquids. The vapor-liquid mixture ows through line 114 into the feed partial condenser 115 where further liquefaction occurs, the temperature dropping to about 203 F. and the amount of liquid increasing to about 36 mole percent. This feed mixture flows into tower 109 via line 116 for separation into a nitrogen rich overhead product and a hydrocarbon rich bottoms product. The bottoms liquid hydrocarbon product from tower 109 is removed via line 133 and contains only 0.3 by volume nitrogen. A portion of the bottoms liquid methane is diverted to line 117 and throttled through valve 118 into the evaporator side of feed partial condenser 115. The vapor-liquid hydrocarbon mixture formed on throttling to about 55 p.s.i. provides refrigeration to the natural gas feed expander effluent. The totally vaporized methane then flows through line 119 to refrigerant methane subcooler 120 through line 121, methane chiller 122, line 123, ethylene chiller 124 and line 125 giving up refrigeration to refrigerant fluids in heat exchange and iitself becoming warmer. From line 125 the flow is divided, part continuing in line 126 through ethylene cooler 127 and the remainder continuing via line 128 through propane subcooler 129. The split streams in

lines

130 and 131, as a result of heat exchange in

exchangers

129 and 127, respectively, emerge at a temperature about F. colder than the incoming warm streams, combine, and leave the system in line 132. At line 133 at the bottom of tower 109, that portion of the liquid methane not diverted to line 117 flows by way of line 134 to the evaporator side of refrigerant methane condenser 135. Throttling occurs across valve 136 in line 134, the methane product dropping from line 133 pressure of about 400 p.s.i. to about 255 p.s.i. in the evaporator side of unit 135, and from about 141 F. to 167 F. Evaporation of the methane provides a portion of the condensation duty of the methane refrigerant. Emerging as saturated vapor from unit 135, the 255 p.s.i. methane flows via line 137 to methane refrigerant chiller 122 and gains heat from cooling methane refrigerant. Sequentially, the methane product flows from unit 122 through line 138 and natural gas feed chiller 105. Finally it emerges from the system in line 139 at ambient temperature within about 5 'F, of the feed gas temperature. The overhead products from tower 109 are liquid and vapor nitrogen having a purity of 99.9997% by volume nitrogen, the remaining three parts per million being principally methane. Liquid nitrogen at about 254 F. and 397 p.s.i. is withdrawn from the top of tower 109 via line 140 and subcooler 141. It is then throttled across valve 142 in line 143 into the nitrogen flash drum 144 at about 17.2 p.s.i. A portion remains as liquid and is subsequently taken to a storage vessel by line 145. The nitrogen flash vapor from drum 144 via line 1-45a is used to subcool tower draw-off liquid nitrogen in exchanger 141 prior to throttling into flash drum 144. The low pressure vapor nitrogen routed through line 146, methane subcooler 120, line 174, methane chiller 122, line 148, ethylene chiller 124, line 149 and ethylene cooler 127 is warmed to ambient temperature about 5 F. colder than the warm high pressure ethylene refrigerant entering exchanger 127. At this point, line 150, the gaseous nitrogen at one atmosphere pressure is ejected to the atmosphere. A portion of high pressure vapor nitrogen is withdrawn from the head of tower 109 via line 151 and passed in order through methane subcooler 120, line 152, and methane chiller 122, providing in parallel with other cold gaseous streams refrigeration needed to cool methane refrigerant. Beyond exchanger 122, line 153 divides into two parts, lines 154 and 155. Part of the nitrogen is diverted via line 155 to the ethylene chiller 124, line 156, and ethylene cooler 127. It leaves cooler 127 as warm gas line 157. The second portion of nitrogen ows through line 154 and methane cooler 158 from which it leaves as warm gas in line 159. The split streams of high pressure (about 395 p.s.i.) nitrogen are both warmed to a point about 5 F. colder than the warm refrigerant streams being cooled. The two nitrogen ows are reunited in line whence the single flow leaves the system. In the cascade refrigeration system, the methane is iirst compressed in multistage compressor 161 to develop a discharge pressure of about 285 p.s.i. The heat of compression is removed by water in aftercooler 162 preceding line 163. The gaseous methane now at about +90 F. flows via line 163 to methane cooler 158 and is there cooled to about 125 F. by heat exchange with cold product nitrogen and cold recycled methane refrigerant. The methane refrigerant is further transferred from unit 158 via line 164 to methane chiller 122 and there cooled to about 162.5" F. at 282 p.s.i. (saturated vapor). The saturated methane vapor leaves heat exchanger 122 by line 165 and then divides into branch lines 166 and 167 to the methane condensers 135 and 168. Saturated liquid methane refrigerant at about 162.5. F. from condensers 135 and 168 ows from the respective units by

lines

169 and 170 which join into line 171. Line 171 conveys the liquid to methane subcooler 120 wherein the luid is subcoled to about 241.8 F. 'Ihe subcooled methane refrigerant flows from subcooler 120 by line 173 to condenser 172 in the top of tower 109. Just prior to entry into the evaporative side of the condenser, throttling occurs across valve 174 in line 173. Condenser conditions are about 254.7" F. and 17.6 p.s.i. The liquid formed from throttling across valve 174 is constantly vaporized in the condenser by heat exchange with rising vapors in the tower which in turn condense and provide both liquid nitrogen overhead product and internal reflux. The combined ash vapors and evaporated liquid methane leave the condenser via line 175 and flow to compressor 161 imparting refrigeration to itself in the hot high pressure state sequentially through methane subcooler 120, line 177, methane chiller 122, line 178, methane cooler 158 and nally via line 179 to compressor 161. In the ethylene loop, ethylene gas at 50 F. and about 7.6 p.s.i. is compressed by ethylene compressor 180, aftercooled and discharging at +90 F. and 197 p.s.i. into line 181. The ethylene is next cooled to 45.5 F. with saturated vapor in ethylene cooler 127 against cold product streams. From cooler 127 the vapor ethylene ows via line 182 to ethylene condenser 183. Total condensation of etheylene occurs against vaporizing propane. Saturated liquid ethylene from condenser 183 flows through line 184 to ethylene chiller 124 in which subcooling occurs by means of heat exchange with itself plus cold product streams. From chiller 124, the subcooled fluid goes via line 186 to a second subcooler 185. This second stage of subcooling is accomplished by self-refrigeration of the high pressure liquid ethylene lby heat exchange with itself as expanded returning material in the cold low pressure vapor state. The subcooled ethylene travels in line 187 from subcooler 185 to methane condenser 168. Before entering condenser 168, it is expanded isenthalpically across valve 188 in line 187. Acquiring heat from condensing methane, the ethylene is vaporized. This cold low pressure ethylene at 9.6 p.s.i. is returned to compressor giving up refrigeration and being warmed to about 50 F. while passing through line 189, ethylene subcooler 185, line 190, ethylene chiller 124, and line 191 to the suction of the com-pressor 180. Propane is compressed in propane compressor 192 and essentially completely condensed iby heat exchange with water in aftercooler 193 preceeding line 194 to a temperature of |90 F. and a pressure of 170 p.s.. By line 194 the propane, which is virtually 100% liquid, is conveyed to propane subcooler 129. Residual vapor condensation and subcooling of total liquid propane takes place in unit 129. Heat removal from propane is brought about by both auto-refrigeration and cold products. The subcooled propane from unit 129 and line 195 flashes through valve 196 into the evaporative side of ethylene condenser 183. The evaporated cold low pressure vapor propane from unit 183 ows via line 197, subcooler 129 and line 198 back to compressor 192 suction.

While the foregoing discussion has been with particular reference to the upgrading of natural gases, the present invention is in no Way limited to natural gas. There exist many synthetic gas mixtures formed as primary raw material sources or by-products of various coking, cracking, reforming and partial oxidation processes. The raw gas mixtures must be further refined to retain desired valuable constituents. The described invention with little or no modification lends itself quite readily to separating such synthetic gas mixtures. The present invention possesses the flexibility to efficiently handle mixtures of varying composition as well as of varying volume. It is also possible to combine the eluent from several adjoining reneries and/or chemical plants, with the combined eflluent being piped to a single gas separation unit having the design of the present invention.

As indicated above, the location of the expansion engine 111 in this invention, that is, after the cooling of the gas -but prior to injection into the column 109, forms an essential feature of the process. If the expansion engine 111 were located at or near 100, the removal of work by the expansion engine would be increased, however, the subsequent removal of heat from the incoming gas would be less et'rcient because the gas would be at a lower pressure, and hence, the heat transfer coelicients would be less favorable.

The use of expansion engine 111 to reduce the pressure of the cold gas possesses many other advantages unattainable by the use of isenthalpic throttling or by the use of an expansion engine located at some other point in the process. According to the process of the present invention, the cold feed is isentropically reduced in pressure across expansion machine 111. It will be understood that while this expansion is ideally isentropic, as a practical matter, the isentropic efficiency of expander 111 is usually on the order of about 70%. The advantages obtained by isentropic expansion of cold feed gas will be -better understood upon analysis of the thermodynamic considerations associated with expansion under various conditions. For example, for a 60 mole percent nitrogen 40 mole percent methane mixture, if the feed temperature to the expander 111 is at about 25 F. and 2000 p.s.., upon expansion to 400 p.s.. the final gas temperature would be relatively high (about 165 F.) with 710.5 B.t.u./lb. mole of gas being obtained as Work (assuming expander eiciency of 70%). `On the other hand, when the gas is precooled to 96 F. at 2000 p.s.., and then isentropically expanded at 70% efficiency to 400p.s.i., the discharge temperature is low (about 197 F.) and the work produced is 384.9 B.t.u./lh. mole of gas.

If the precooled feed gas were throttled into a distillation tower by isenthalpic expansion, no work would be produced by such expansion. However, by the practice of this invention work isproduced at the shaft of expander 111 for use in driving other machinery in the operation, such as, refrigerant and product compressors. Isenthalpic expansion has other disadvantages since the least level of refrigeration attained upon expansion of the gas is about 198 F. including cooling of the feed in exchanger 115. On the other hand, the use of expander 111 gives a gas discharge temperature (for a 60:40 nitrogen-methane mixture) of about 197 F. which followed by cooling in exchanger 115 reduces the gas temperature to about 203 F. This is due to the fact that While isenthalpic expansion can sometimes approach the cooling effect obtained by isentropic expansion, it can never reach this cooling level. It will be understood that the foregoing discussion is with reference to a mixture containing 60% nitrogen and 40% methane. Other proportions of the nitrogen and methane necessitate other temperatures, although the advantages of isentropic expansion of the cold feed gas would be the same.

The pressure of the feed gas at point is preferably from about 2000 to about 6000 p.s.. The feed gas pressure is based on several factors including: (l) higher gas pressure reduces the equilibrium amount of water vapor retained in the gas, and the greater the amount of liquid water removed in the gas, and the greater the amount of liquid water removed in separator 101, the lesser the amount of water vapor to be removed in vessel 102 and hence the smaller and less expensive vessel 102 becomes; (2) a gas at high pressure is denser and for this reason its thermodynamic and physical properties are better suited to heat transfer. By taking advantage of this phenomenon and holding the gas at high pressure until it is cooled in the process to the least possible temperature before expansion, better heat transfer coefficients are obtained which permits the use of smaller and cheaper heat transfer exchangers; (3) the feed gas pressure influences the operation of the expansion engine 111. Thus, the greater the pressure expansion ratio across expander 111 the greater the temperature drop and the greater the work available.

In general, the presently available single stage expanders are limited by mechanical characteristics to a pressure ratio range of about 15/1 maximum to about 5/1 minimum. Since the pressure after passage of the gas stream through expander 111 corresponds to the operating pressure of column 109, and since this column is operated at a pressure sufficiently high to permit the use of methane evaporating at about atmospheric pressure as the refrigerant in the nitrogen condenser 172 (normally about 400 p.s..), it follows that an expansion ratio range of 5/1 to 15/1 would tix the inlet gas pressures at 100 from about 2000 to about 6000 p.s.. Should the gas be available at pressures greater than 6000 p.s.., it would be used to advantage without any need for booster compressors. For example, it would be possible to use dual stage expanders in lieu of the single stage expander 111 to utilize inlet pressures greater than 6000 p.s.. However, the design of such a dual stage compressor at the present time would be uneconomical. On the other hand, if the gas pressure at point 100 is substantially less than 2000 p.s.., it becomes desirable to compress the gas to at least about 2000 p.s.. This is due to the fact that expander 111, when operated at a ratio of less than 5/1, will not remove an optimum amount of work from the gas, nor will cooling of the gas before introduction into column 109 be achieved to the desired extent. If the feed gas were isentropically expanded at ambient temperature, the greatest possible work of expansion Would be obtained. However, it would be necessary to provide additional costly lower level refrigeration somewhere in the process to compensate for not having done so by expansion machine. According to the present invention, while the Work of expansion obtained is somewhat less, the lowest possible level of refrigeration by expansion engine is achieved.

The pressure of the feed gas at point 100 limits the degree of expansion across expander 111 since, according to this invention, the discharge pressure from the expansion machine 111 must be sufficiently high to permit methane refrigeration by boiling at about one atmosphere to condense nitrogen in condenser 172. We have found substantially lower pressures to be unsatisfactory. For example, it has been suggested that methane and nitrogen be separated in a column operated at p.s.. This pressure indicates a saturation temperature for pure nitrogen of 275 F. Under this condition should it be desired to recover pure nitrogen it is necessary to provide high pressure nitrogen refrigeration in order to achieve a condenser temperature of at least as low as 275 F. According to the present invention the valuable nitrogen is readily recovered without the need for nitrogen or other low level refrigeration. The required heat transfer at the top of the column 109 is achieved by the use of methane. In the operation of the column 109 all refrigeration to keep the column in heat balance and provide down tlowing reflux liquids to effect gas separation is easily provided by a form of cascade refrigeration system, the lowest level of which is methane at about one atmosphere and at about -258.7 F. (saturated vapor). This is suicient to condense the nitrogen in condenser 172 since at about 400 p.s.i. nitrogen boils at -240.4 F. If the column were operated at pressures substantially below 400 p.s.i., the boiling point of the nitrogen at the top of the column would be lower than the boiling point of methane at one atmosphere, that is at -258.7 F. Thus, it would be necessary to use some colder material to provide refrigeration on condenser 172. The provision of such a refrigerant would necessitate additional compression lwork. This compression work is eliminated by the practice of this invention.

The nitrogen produced by our invention from the top of column 109 may be used in ammonia synthesis. Since the nitrogen produced by the present invention is already at a high pressure, less compression is required to raise the nitrogen to the pressure of ammonia synthesis. Thus, by using nitrogen from the process of this invention, the eciency of ammonia production is increased. The nitrogen from separation may also be readily liquitied at pressure prior to flashing to a saleable liquid product and held in storage prior to delivery.

The limiting distillation tower pressure in column 109 would be determined both by the critical properties of any given feed nitrogen-hydrocarbon composition, and the desired product purities. Accordingly, the present invention is not limited to the use of any specific pressure in column 109, so long as the nitrogen can be condensed by boiling methane. In the present invention, the removal of work in expansion of the feed gas is the maximum believed to be attainable, consistent with the minimizing of the amount of refrigeration work required to condense the nitrogen at the top of column 109. Under the conditions in expander 111, up to about 15 mole percent liquid is formed during the expansion. This percentage of liquids formed is considered the practical limit for mechanical reasons with the presently available expanders. However, the invention is not limited to operating expander 111 in the liquid range. The expander 111 could just as well be operated with discharged gas as saturated vapor or even superheated vapor.

In addition to nitrogen, various other inerts occur in natural gases including helium, and argon. Both of these gases are valuable in pure form. According to the present invention it is possible to obtain argon and helium in substantially pure form. For example, the argon because of its relative volatility preferentially tends to separate out with the nitrogen product and to accumulate in the liquid nitrogen from the distillation tower 109. Thus, as an alternated embodiment of the invention, an argon rich side stream may be withdrawn at some intermediate point in the distillation tower 109, generally above the feed point, said stream being treated by absorption, distillation or other appropriate means to concentrate and purify argon and return the nitrogen and methane to tower 109. The argon might also be recovered from overhead nitrogen product from tower 109 by appropriate means such as adsorption or distillation. It is also possible to recover argon directly from the liquid nitrogen product in drum 144. Again, means such as adsorption or distillation can be utilized.

Helium concentrations in natural gas range from O to as much as 8.5 volume percent. Generally, a helium containing natural gas will contain about as much nitrogen as helium or more likely more nitrogen than helium.

A natural gas containing helium can be treated in the same manner as any other natural -gas in tower 109. In the tower, by virtue of its high volatility, the helium tends to collect overhead with the nitrogen. To accomplish separation of the helium from the nitrogen, a staged condenser or dephlegmator is added above condenser 172. The capacity of the cascade refrigeration system would have to be increased and a nitrogen loop added to provide the necessary low temperature refrigeration required for condensing the nitrogen from the helium. To hold helium losses to a minimum, condensation of the nitrogen should occur at lower pressures. Therefore, the liquid nitrogen contatining helium is throttled into the dephlegmator to lower pressure than in tower 109. Part of the totally condensed nitrogen is used to refrigerate the dephlegmator. The portion of liquid nitrogen from the tower in line 140 so utilized is a function of the demand for liquid nitrogen product at line 145. The vapor helium from the dephlegmator would be further puried by conventional means. After extraction the cold helium can be incorporated in the cascade refrigeration system in a separate pass to take advantage of its refrigeration value.

As has been indicated above, the incoming gas stream from point is normally dried rst by passage through separator 101 and vessel 102. This is to prevent formation of ice in the gas as the temperature is lowered in the process. Similarly, the heavy hydrocarbons are removed frorn the system by separator 107 via separator 112. This prevents possible plugging due to the accumulation of heavier hydrocarbons in the system. However, the ice and heavier hydrocarbons may be removed by other techniques, for example, by the use of adsorbent materials or by chemical treatment. Similarly the removal of residual carbon dioxide and sulfur compounds may be accomplished by any other chemical or physical treatment. These procedures are Well-known to those skilled in the art and will not be further described here.

While the foregoing discussion has been with particular reference to the separation of a gas stream containing about 60 volume percent nitrogen, the process is not limited thereto. Any percentage of nitrogen from about 0 mole percent to 100 mole percent can be separated by the process of this invention although obviously separation would not ordinarily be necessary where the feed gas is essentially pure.

As shown in the drawing, the cooling effect for heat exchanger is supplied by counterflowing cold methane rich product. However, the cooling effect for this heat exchanger need not be supplied in this fashion. For example, some of the ethylene or propane refrigerant streams may be utilized to provide the necessary cooling a this point. Accordingly, such variations are contemplated by the present invention.

The degree of separation achieved in tower 109 may be varied according to the desired purity of the nitrogen and methane products. This may be accomplished by varying the number of separation stages in the column, as well as by altering the amount of heat re-y moved from the column via condenser 172. Similarly, the ratio of gas to liquid in the nitrogen products removed frorn the top of column 109 may be varied according to whether the one type of product or the other type of product is preferred.

The cascade system employed to provide additional refrigeration in the system is unique in incorporating cold vaporizing and gaseous product streams, including waste products, together with cold low pressure refrigerant streams as cooling, liquefying and subcooling media for the Warm compressed refrigerants. While the foregoing discussion of the cascade system has been with particular reference to propane, ethylene and methane, other refrigerants such as ammonia, other hydrocarbons, halocarbons and the like could be used equally as well. The selection of the particular refrigerants in any situa- 13 tion is dictated by availability, cost and efliciency. For any given system, those skilled in the art will be readily able to select the refrigerants to be employed, based on the criteria set forth above.

The number of cold streams employed in the cascade refrigeration process is not fixed. For example, in methane chiller 122 five cold streams are shown. However, the number and composition of the cold streams utilized may be varied. For example, they could be interchanged for other streams elsewhere in the total system or interchanged for different product splits than are specifically illustrated in the drawing. Accordingly, such variations n the cascade system are included within the scope of the present invention.

As shown in the drawing, the methane refrigerant is pumped by compressor 161 through nitrogen condenser 172, methane subcooler 120, line 177, methane chiller 122, line 178, methane cooler 158 and finally via line 179 to the compressor 161. As an alternative, methane refrigerant can be partially cooled by passing it through a second reboiler parallel to, and mechanically dependent of, the natural gas reboiler 108 in the base of the tower 109. In such case, cold streams above-described as imparting refrigeration to methane, can be shifted to natural gas feed and/or propane and ethylene refrigerant streams. Obviously, whatever the arrangement of the heat exchanger system, it must be in heat balance. A second alternative would be to effect all tower reboiling by methane refrigerant and all natural gas precooling against product streams.

In yet another embodiment of the invention hydrogen from natural gas or from a synthetic gas may be recovered. It is also possible to incoroprate liquid nitrogen scrubbing of impure hydrogen in conjunction with the gas separation process. In such a case additional refrigeration would be provided by the cascade system to form more liquid nitrogen from tower 109 at line 140. This extra liquid nitrogen, subcooled, and then downowed in a separate tower scrubs upowing cold impure hydrogen free of all higher boiling materials except nitrogen and a trace of carbon oxides. The hydrogennitrogen vapor from the top of such a scrubbing tower is used in a separate pass in the cascade refrigeration system for its refrigeration value and then utilized elsewhere, as for example, in the synthesis of ammonia. the liquid, nitrogen rich, bottoms from the scrubbing tower can be further treated to remove undesirable components such as carbon oxides and the remaining material recycled to tower 109 as an intermediate feed or reux at some point between feed line 116 and the liquid nitrogen overhead, line 140. In this way liquid nitrogen and minor components such as hydrogen and methane from the bottom of the hydrogen scrubber are continuously recycled and reclaimed. In addition, the presence of an intermediate feed in distillation tower 109 relieves the separation by acting as additional reflux to decrease the number of separation stages required and/or to provide increased separation of products.

Many other modifications of the invention will be apparent to those skilled in the art from the foregoing disclosure. Accordingly, the invention is not -to be limited to the details described, but rather is of the full scope of the appended claims.

We claim:

1. A process for the reduction of the amount of inerts in a mixture comprising a combustible hydrocarbon including methane and inert gases including nitrogen, which comprises:

(a) cooling a stream of high pressure gas containing combustible hydrocarbons and inert gases including nitrogen;

(b) expanding the cooled stream under substantially isentropic conditions to cool the gas and remove work therefrom;

(c) passing the expanded stream into a separation zone operated at sufliciently high pressure to permit the use of a methane refrigerant at its boiling point to remove heat from the top of said zone;

(d) forming said methane refrigerant at its boiling point by the steps comprising:

cooling and condensing a propane refrigerant with water, condensing and subcooling said propane with cold product streams and recycled low pressure propane refrigerant vapor;

cooling and condensing an ethylene refrigerant with water, cold product streams, and vaporizing propane;

subcooling said ethylene with cold product streams and recycled low pressure ethylene refrigerant vapor;

cooling said methane refrigerant with water, cold product streams, and recycled low pressure methane refrigerant Vapor;

condensing methane with evaporating ethylene and evaporating hydrocarbon product; and

subcooling methane with cold product streams and recycled low pressure methane refrigerant vapor;

(e) passing said methane to the top of said zone; and

(f) removing substantially pure nitrogen product stream from the top of said zone and substantially pure methane product stream from the bottom of saidzone.

2. A process for the reduction of the amount of inerts in a mixture comprising a combustible hydrocarbon and inert gases including nitrogen, which comprises:

(c) passing the expanded stream into a separation zone operated at sufficiently high pressure to permit the use of methane at its boiling point to remove heat from the top of said zone;

(d) forming said methane at its boiling point by the steps comprising:

condensing propane with water, condensing and subcooling propane with cold product streams and recycled low pressure propane refrigerant vapor;

condensing ethylene with water, cold product streams, and vaporizing propane;

subcooling ethylene with cold product streams, and recycled low pressure ethylene refrigerant vapor;

cooling methane with water, cold product streams, and recycled low pressure methane refrigerant vapor;

condensing methane with evaporating ethylene;

and

(e) passing said methane to the top of said zone; and

(f) removing substantially pure nitrogen from the top of said zone and substantially pure methane from the bottom of said zone.

3. A process for the reduction of the amount of inerts in a mixture comprising a combustible hydrocarbon and inert gases including nitrogen, which comprises:

(c) passing the expanded stream into a separation zone operated at sufficiently high pressure' to permit the use of methane at its boiling point to remove heat from the top of said zone;

(d) forming said methane at its boiling point at a pressure of at least one atmosphere in a separate pass by the steps comprising:

condensing propane with water, condensing and subcooling propane with cold product streams and recycled 10W pressure propane refrigerant vapor;

condensing ethylene with water, cold product streams, and vaporizing propane;

subcooling ethylene with cold product streams and recycled low pressure ethylene refrigerant vapor;

condensing methane with evaporating ethylene; and

(e) passing said methane to the top of said zone; and

4. A process for the reduction of the amount of inserts in natural gas comprising a combustible hydrocarbon and inert gases including nitrogen, which comprises:

(c) passing the expanded stream into a separation zone operated at sufliciently high pressure to permit the use of methane at its boiling point to remove heat from the top of said zone;

(d) forming said methane at its boiling point by the steps comprising:

`condensing ethylene With water, cold product streams, and vaporizing propane;

subcooling ethylene -with cold product streams and recycled low pressure ethylene refrigerant vapor;

condensing methane with evaporating ethylene; and subcooling methane with cold product streams and recycled low pressure methane refrigerant vapor;

(e) passing said methane to the top of said zone; and

5. A process for the reduction of the amount of inerts in a mixture comprising a combustible hydrocarbon and inert gases including nitrogen, which comprises:

(a) cooling a stream of high pressure gas at about 2000 to 6000 p.s.i.a. containing combustible hydrocarbons and inert gases including nitrogen;

(d) forming said methane at its boiling point by the steps comprising:

condensing ethylene with water, cold product streams and vaporizing propane;

condensing methane With evaporating ethylene;

and

(e) passing said methane to the top of said zone; and

6. A process for the reduction of the amount of inerts in a mixture comprising a combustible hydrocarbon and inert gases including nitrogen, which comprises:

(b) expanding the cooled stream under substantially isentropic conditions to about 400 p.s.i.a. to cool the gas and remove work therefrom;

(c) passing the expanded stream into a separation zone operated at suiciently high pressure to permit the use of methane at its boiling point to remove heat from the top of said zone;

(d) forming said methane at its boiling point by the steps comprising:

condensing ethylene with Water, cold product streams, and vaporizing propane;

cooling methane -With Water, cold product streams, and recycled low pressure methane refrigerant vapor;

(e) passing said methane to the top of said zone; and

7. A process for the reduction of the amount of inerts in a mixture comprising a combustible hydrocarbon and inert gases including nitrogen, which comprises:

(d) forming said methane at its boiling point by the steps comprising:

Acondensing propane with Water, condensing and subcooling propane with cold product streams and recycled low pressure propane refrigerant vapor;

condensing ethylene with Water, cold product streams, and vaporizing propane;

cooling methane with water, cold product streams,

17 and recycled low pressure methane refrigerant vapor; condensing methane with evaporating ethylene;

and

(e) passing said methane to the top of said zone;

(f) removing substantially pure nitrogen from the top of said zone and substantially pure methane from the bottom of said zone; and

(g) converting said nitrogen to ammonia.

8. A process for the reduction of the amount of inerts in a mixture comprising a combustible hydrocarbon and inert gases including nitrogen, which comprises:

(d) forming said methane at its boiling point by the steps comprising:

condensing ethylene with water, cold product streams, and vaporizing propane;

condensing methane With evaporating ethylene;

and

(e) passing said methane to the top of said zone;

(g) separating argon from said nitrogen.

9. A process for the reduction of the amount of inerts in a mixture comprising about 40 mole percent methane and about 60 mole percent nitrogen, which comprises:

(b) expanding the cooled stream under substantially isentropic conditions to about 400 p.s.i.a. to cool the gas to about 230 F. and remove work therefrom;

(d) forming said methane at its boiling point by the steps comprising:

condensing ethylene with water, cold product streams, and vaporizing propane;

condensing methane with evaporating ethylene;

and

subcooling methane with cold product streams and recycled low pressure methane refrigerant vapor,

(e) passing said methane to the top of said zone; and

10. A process for the reduction of the amount of inerts in a mixture comprising a combustible hydrocarbon and inert gases including nitrogen, which comprises:

- (a) cooling a stream of high pressure gas at about 2000 to 6000 p.s.i.a. containing combustible hydrocarbons and inert gases including nitrogen;

(d) forming said methane at its boiling point by the steps comprising:

condensing propane with water, condensing and subcooling propane lwith cold product streams and recycled low pressure propane refrigerant vapor;

condensing ethylene with Water, cold product streams, and vaporizing propane;

condensing methane with evaporating ethylene; and

(e) passing said methane to the top of said zone; and

References Cited UNITED STATES PATENTS 2,355,589 8/1944 Brandt 62-31 XR 2,556,850 6/1951 Ogorzaly 62-40 XR 2,557,171 6/1951 Bodle 62-31 XR 2,716,332 8/1955 Haynes 62-32 XR 2,823,528 2/1958 Ea'kin 62--26 2,960,837 11/ 1960 Swenson 62-40 XR 2,990,690 7/ 1961 Martin 62-27 XR 3,020,723 2/ 1962 De Lury 62-40 XR 3,315,477 4/1967 Carr 62-40 XR NORMAN YUDKOFF, Primary Examiner A. F. PURCELL, Assistant ExaminerA U.S. Cl. XR. 62-40, 22, 38