Abstract:
A process for the separation of methane and nitrogen in a cryogenic distillation column driven by a closed loop recirculating fluid heat pump wherein additional column refrigeration is generated by rejecting process heat into a product stream thereby reducing the need for expansion of process streams and enabling recovery of product at relatively high pressure.

Description:
TECHNICAL FIELD 
     This invention relates to the separation of methane and nitrogen and is particularly applicable for use to process a feed stream from a petroleum reservoir which has undergone secondary recovery by nitrogen injection. 
     BACKGROUND ART 
     Often it is desirable to separate a mixture of nitrogen and methane into nitrogen-rich and methane-rich components. One such situation is when a stream from an oil or gas reservoir contains nitrogen in other than insignificant concentrations. The nitrogen could be naturally occurring and/or could have been injected into the reservoir as part of an enhanced oil recovery (EOR) or enhanced gas recovery (EGR) operation. Generally the stream from the reservoir will undergo initial processing wherein heavier components, such as natural gas liquids (NGL), are removed and then the remaining stream containing primarily nitrogen and methane is separated cryogenically in one or more rectification columns. When a single rectification column is used to make the cryogenic separation, the column is often driven by a recirculating fluid heat pump. A recent significant advancement in such a process is described in U.S. Pat. No. 4,501,600--Pahade. 
     An effective cryogenic separation process requires refrigeration to carry out the separation and to compensate for thermal inefficiencies such as ambient heat leak into the cold equipment. Further, the available refrigeration must be at the proper temperature levels in order to maintain the cold temperatures required for the cryogenic separation process. Of course, refrigeration may be provided to a process from an external source but this is costly. 
     Process refrigeration may be generated internally by the pressure level reduction or expansion of incoming feed or outgoing methane or nitrogen but such a procedure may have limited usefulness. For example, it may be desirable to keep the feed stream pressure relatively high in order to reduce equipment sizes or to maintain desired process conditions such as column temperature levels. Methane product may be desired at elevated pressure in order to keep pumping to pipeline pressure requirements low. Nitrogen may be required at elevated pressure to facilitate injection into the petroleum reservoir for EGR or EOR operations. Thus it may be desired that no expansion, or only a limited amount of expansion, of the feed, methane or nitrogen streams be carried out. 
     As mentioned, a single column cryogenic rectification process is often driven by a recirculating fluid heat pump. Such an arrangement does not add refrigeration to the column but rather transfers refrigeration within the column. It would be desirable to have a cryogenic separation process employing a recirculating fluid heat pump wherein added refrigeration is supplied to the column at needed temperature levels without need for significant amounts of outside added refrigeration or large expansion of process streams. 
     It is therefore an object of this invention to provide an improved process for the separation of methane and nitrogen. 
     It is a further object of this invention to provide an improved single column cryogenic distillation process for the separation of methane and nitrogen driven by a recirculating fluid heat pump wherein added refrigeration is provided to the process at the required temperature levels without need for significant amounts of outside added refrigeration or large expansion of process streams. 
     SUMMARY OF THE INVENTION 
     The above and other objects which will become apparent to one skilled in the art upon a reading of this disclosure are attained by the process of this invention which is: 
     A process for the separation of methane and nitrogen comprising: 
     (a) introducing a feed comprising methane and nitrogen into a rectification column operating at a pressure in the range of from 200 to 450 psia; 
     (b) separating the feed in said column into a nitrogen-enriched vapor and a methane-enriched liquid; 
     (c) partially condensing nitrogen-enriched vapor by indirect heat exchange with heat pump fluid to warm the heat pump fluid; 
     (d) employing at least some of the resulting liquid of step (c) as reflux liquid for the column; 
     (e) partially vaporizing methane-enriched liquid by indirect heat exchange with warm heat pump fluid; 
     (f) employing at least some of the resulting vapor of step (e) as reflux vapor for the column; 
     (g) warming remaining methane-enriched liquid of step (e) by indirect heat exchange with warm heat pump fluid wherein not more than 75 percent of said remaining methane-enriched liquid is vaporized; 
     (h) further warming the warmed methane-enriched fluid of step (g) by indirect heat exchange with feed prior to the introduction of said feed into the column; and 
     (i) recovering resulting methane-enriched fluid as product methane, thereby providing refrigeration to the column at or below column temperature levels. 
     The term &#34;indirect heat exchange&#34;, as used in the present specification and claims, means the bringing of two fluid streams into heat exchange relation without any physical contact or intermixing of the fluids with each other. 
     The term, &#34;column&#34;, as used in the present specification and claims, means a distillation or fractionation column or zone, i.e., a contacting column or zone wherein liquid and vapor phases are countercurrently contacted to effect separation of a fluid mixture, as for example, by contacting of the vapor and liquid phases on a series of vertically spaced trays or plates mounted within the column or alternatively, on packing elements with which the column is filled. For a further discussion of distillation columns see the Chemical Engineers&#39; Handbook, Fifth Edition, edited by R. H. Perry and C. H. Chilton, McGraw-Hill Book Company, New York, Section 13, &#34;Distillation&#34; B. D. Smith et al, page 13-3, The Continuous Distillation Process. 
     Rectification, or continuous distillation, is the separation process that combines successive partial vaporizations and condensations as obtained by a countercurrent treatment of the vapor and liquid phase. The countercurrent contacting of the vapor and liquid phases is adiabatic and can include integral or differential contact between the phases. Separation process arrangements that utilize the principle of rectification to separate mixtures are often interchangeably termed rectification columns, distillation columns, or fractionation columns. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic flow diagram of one preferred embodiment of the process of this invention. 
     FIG. 2 is a schematic flow diagram of another preferred embodiment of the process of this invention. 
     FIG. 3 is a schematic flow diagram of a third preferred embodiment of the process of this invention. 
    
    
     DETAILED DESCRIPTION 
     The invention will be described in detail with reference to the drawings. The feed stream processed by this invention may be taken from a petroleum reservoir, and as such, typically contains water, carbon dioxide, hydrogen sulfide, natural gas liquids (NGL), i.e. hydrocarbons having two or more carbon atoms, methane and nitrogen. Pretreatment of this feed stream is performed to dry the stream and remove carbon dioxide and sulfur. Then the feed stream is further processed to recover all or most of the natural gas liquids for use as liquid fuels or chemical feedstocks. The remaining stream is then processed to separate the nitrogen from the methane. The nitrogen-methane separation is often referred to as the nitrogen rejection unit (NRU) and this invention is concerned with an improvement in the nitrogen rejection process. Thus, the process description is limited to the nitrogen rejection section. Referring now to FIG. 1, feed stream 311, comprising methane and nitrogen is cooled by passage through heat exchanger 300 and the cooled feed 314 may be expanded through valve 315 prior to being introduced as stream 316 into rectification column 301 which is operating at a pressure in the range of from 200  to 450 psia, preferably from 250 to 400 psia. In addition to nitrogen and methane, the feed stream may contain minor amounts, up to a maximum of about five percent, of remaining natural gas liquids that were not recovered in the NGL treatment section. For certain situations heavier hydrocarbons including those having two or three carbon atoms may be present in the feed in excess of five percent. Feed stream 316 preferably enters column 301 as a combined vapor and liquid stream. 
     Within the column, the feed is separated by cryogenic rectification into a nitrogen-enriched top vapor and a methane-enriched bottom liquid. Nitrogen-enriched top vapor is partially condensed by indirect heat exchange with heat pump fluid to warm the heat pump fluid. At least a portion of the resulting condensed nitrogen-enriched fluid is employed as reflux liquid for the column. The partial condensation of the nitrogen-enriched top vapor may take place within or outside the column. FIG. 1 illustrates the case where the nitrogen-enriched top vapor is partially condensed outside the column. 
     Referring back to FIG. 1, nitrogen-enriched vapor 317 is removed from column 301 and partially condensed by passage through heat exchanger 302. The resulting partially condensed stream 318 is passed to phase separator 303 and the liquid 319 from phase separator 303 is returned to column 301 as reflux. The vapor 320 from phase separator 303 is warmed by passage through heat exchanger 307 to condition 321, further warmed by passage through heat exchanger 308 to condition 322, and then still further warmed by passage through heat exchanger 300 wherein it serves to cool the feed. 
     The nitrogen is removed from the nitrogen rejection process as stream 313. It should be noted that there are options to rewarming the nitrogen stream 320. Preferably all of the stream is rewarmed in heat exchanger 307 versus heat pump fluids. Then the stream can bypass heat exchanger 308 and be utilized directly in heat exchanger 300 to cool incoming feed. Alternatively stream 321 can be subdivided and a portion used in heat exchanger 307 and the remainder can be further rewarmed in heat exchanger 308. In that case the portion rewarmed in heat exchanger 308 would bypass heat exchanger 300 and proceed directly to warm level heat exchangers in the NGL section or even be combined with other nitrogen streams at the warm end of the process. Following further warming versus incoming feed in the NGL process section, the nitrogen may simply be released to the atmosphere or may be gainfully employed such as for reinjection into a reservoir for EOR or EGR operations. As can be appreciated from the drawing and the description, there is no need for pressure reduction of the nitrogen stream and this is particularly advantageous if further use of the nitrogen is desired as this reduces the compression requirements and thus the cost of such further use of the nitrogen. 
     Methane-enriched bottom liquid is partially vaporized by indirect heat exchange with warm heat pump fluid. At least a portion of the resulting vaporized methane-enriched fluid is employed as reflux vapor for the column. The partial vaporization of the methane-enriched bottom liquid may take place within or outside the column. FIG. 1 illustrates the case where the methane-enriched bottom liquid is partially vaporized outside the column. 
     Referring back to FIG. 1, methane-enriched bottom liquid 325 is removed from column 301 and partially vaporized by passage through heat exchanger 305. The resulting partially vaporized stream 326 is passed to phase separator 306 and the vapor 327 from phase separator 305 is returned to column 301 as vapor reflux. 
     The process of this invention employs a closed loop recirculating fluid heat pump whereby heat is pumped to the bottom of the column to supply refrigeration at the top and at an intermediate point of the column. This heat pump circuit will now be described. 
     Warm heat pump fluid 332 is cooled and condensed by passage through heat exchanger 305 to condition 334. The heat pump fluid may be methane but preferably is a mixture of methane and nitrogen wherein nitrogen may comprise from 0.5 to 60 mole percent of the heat pump fluid with the remainder methane, preferably nitrogen may comprise from 1 to 30 mole percent and most preferably from 5 to 20 mole percent of the heat pump fluid. The liquid 334 is further cooled by passage through heat exchanger 308 to condition 335. All or part 385 of stream 335 is further cooled by passage through heat exchanger 307 to condition 339, expanded to a lower pressure through valve 340 and vaporized by indirect heat exchange in heat exchanger 302 against partially condensing nitrogen-enriched top vapor. The resulting heat pump vapor 341 is then warmed by passage through heat exchanger 307 to condition 342, further warmed by passage through heat exchanger 308 to condition 343, further warmed by passage through heat exchanger 309 to condition 344 and compressed in compressor 350 to condition 351. 
     FIG. 1 illustrates a preferred heat pump loop wherein refrigeration is also supplied to an intermediate point in the column. In this preferred arrangement a portion 365 of the liquid at condition 335 is expanded through valve 336 to an intermediate pressure which is greater than the pressure to which the liquid is expanded through valve 340. Portion 365 may be from zero to about 50 percent of the liquid at condition 335. The resulting intermediate pressure liquid 337 is vaporized by indirect heat exchange in heat exchanger 304 against a vaporous nitrogen-methane stream 323 taken from an intermediate point in the column. The nitrogen-methane stream is at least partially condensed and as stream 324 returned to column 301 as additional reflux. The resulting vaporized heat pump fluid 338 is warmed by passage through heat exchanger 308 to condition 345, further warmed by passage through heat exchanger 309 to condition 346, and combined with stream 351 to form stream 352 which is compressed in compressor 310 to form high pressure compressed fluid 331. This fluid is cooled by passage through heat exchanger 309 from which it emerges as warm heat pump fluid 332. Although not shown, the high pressure heat pump fluid 331 may be cooled against cooling water prior to further cooling against heat pump streams. 
     The process of this invention comprises a process improvement wherein not only is heat pumped from the intermediate and top of the column to the bottom of the column, but also net refrigeration is added to the column without need for significant pressure reduction of the process streams. In accord with the improved process of this invention, methane-enriched liquid resulting from the partial vaporization of the methane-enriched column bottoms is further processed in a way so as to remove heat from the column. 
     Referring back to FIG. 1, remaining methane-enriched liquid 328 from phase separator 306 is warmed by indirect heat exchange with warm heat pump fluid in heat exchanger 305. The methane-enriched liquid may be partially vaporized by this heat exchange but such partial vaporization should not exceed 75 percent of the liquid; preferably the partial vaporization of methane-enriched liquid 328 in heat exchanger 305 does not exceed 50 percent and most preferably does not exceed 25 percent. Excess vaporization is detrimental from an energy efficiency standpoint. 
     The warmed methane-enriched fluid 330 from heat exchanger 305 can then be further warmed by indirect heat exchange with the feed in heat exchanger 300 and recovered as methane product 312. It should be noted that dependent on the degree of warming of stream 328 in heat exchanger 305, the temperature of stream 330 may be such that the stream can bypass the rewarming step in heat exchanger 300. Instead, the stream may be passed directly to high temperature heat exchangers in the NGL processing section for warming against incoming feed. Thus, heat from the column is passed to the methane product stream resulting in a net refrigeration gain for the column. As can be appreciated from the drawing and the description, these advantages are accomplished without need for pressure reduction of the methane product stream and this is particularly advantageous if the methane is further used at elevated pressure, such as in a pipeline, as this reduces the compression requirements and hence the cost of such further use. The methane, as is also the case with the nitrogen, may be recovered at up to the pressure at which the rectification column operates minus whatever pressure drop occurs through the necessary piping. 
     As is known by those skilled in the art, a column heat pump circuit does not generate net refrigeration for the column but instead removes heat from a column at lower temperatures at the top or intermediate level of the column and adds that heat to the column at higher temperatures at the column bottom. Normally, a heat pump circuit is used only to generate liquid and vapor reflux flows necessary for a column separation and does not influence column feed or return stream conditions. Usually, a column heat balance, or thermal condition of the process streams, is dependent on the thermal condition of the column feed stream. That is, the withdrawal of product methane as liquid requires that the feed stream be introduced into the column with sufficient liquid fraction to enable the liquid withdrawal. One means of doing this is to reduce the pressure of the feed in order to generate liquid (refrigeration) or to reduce the pressure of the return methane product in order to generate colder liquid (refrigeration) and use it to cool the incoming feed stream. As previously noted, this method is not advantageous because of the pressure reduction of the process streams. 
     The process of this invention provides another method of providing the necessary liquid (refrigeration) to the column, by advantageously pumping heat from the column. The process of this invention comprises pumping some additional heat from the column, and thus at temperature levels corresponding to the column, and this allows liquid generation (refrigeration) in the column without the need for pressure reduction of process streams. The generated liquid (refrigeration) is then available not only to allow the desired liquid withdrawal but also can be used to compensate process thermal inefficiencies such as heat leak into the cold equipment. 
     The extent of the additional heat pumping is dependent on the degree of vaporization of the return methane liquid product 328. It is highly desirable to minimize that vaporization to no more than 75 percent, preferably no more than 50 percent, and most preferably no more than 25 percent, since any added vaporization is reflected in added heat pumping and added compression in the heat pump compressors 310 and 350. It is energy efficient for the heat pump modification to supply only incremental added refrigeration to the column. Most of the column refrigeration is still supplied by process stream pressure expansion. The addition of this incremental heat pumping to the system that already has the heat pump circuit to drive the column separation results in a very energy efficient process and one which has marked advantages from an equipment standpoint. 
     FIG. 2 illustrates another preferred embodiment of the process of this invention wherein the remaining methane-enriched liquid is expanded prior to warming by the warm heat pump fluid. The numerals for FIG. 2 are identical to those of FIG. 1 for the common elements and these common elements will not be specifically described. Referring now to FIG. 2, methane-enriched liquid 328 from phase separator 306 is expanded through valve 329 to condition 347. Stream 347 is then warmed by passage through heat exchanger 305 by indirect heat transfer with warm heat pump fluid. This heat exchange may result in vaporization of up to 75 percent percent of stream 347, preferably not more than 50 percent and most preferably not more than 25 percent. Resulting warm stream 330 is further warmed by indirect heat exchange with feed in heat exchanger 300 and recovered as product 312, thereby supplying refrigeration to the column. 
     The heat exchanger 305 arangement of FIG. 2 is an option to the FIG. 1 arrangement. The FIG. 2 arrangement allows easy control of the degree of heating of the product liquid methane 347, since its maximum temperature will be the same as the temperature of the column liquid bottoms stream 325. 
     FIG. 3 illustrates another preferred embodiment of the process of this invention wherein heat pump fluid is employed to cool incoming feed. The numerals for FIG. 3 are identical to those of FIG. 1 for the common elements and these common elements will not be specifically described. Referring now to FIG. 3, a portion 370 of heat pump fluid at condition 335 is expanded through valve 348 and the expanded stream passed through heat exchanger 300 to cool the incoming feed by indirect heat exchange. Portion 370 may be from zero to about 50 percent of the liquid at condition 335. In this way some of the refrigeration generated by the heat pump circuit is supplied directly to the feed. The resulting warmed heat pump fluid 349, which is preferably completely vaporized, is warmed by passage through heat exchanger 308 to condition 360, further warmed by passage through heat exchanger 309 to condition 361 and then passed to compressor 310 and returned to the major part of the heat pump fluid stream. 
     The process arrangement of FIG. 3 is an option that allows refrigeration to be supplied directly to the incoming feed stream. It should be noted that this refrigeration is still available at temperature levels corresponding to the column temperature levels and is equivalent to supplying refrigeration to the column at an intermediate temperature level between the top (coldest temperature) and the bottom (warmest temperature). However, the option can be advantageous from an equipment viewpoint, since the feed heat exchanger 300 can be utilized for the necessary heat exchange. 
     As can be appreciated by one skilled in the art, the process of this invention comprises in general, additional heat pumping between the temperature levels of the rectification column and the temperature level above that at the bottom of the column. The additional heat pumping involves some heat rejection from the column to the methane product stream so that this heat is carried out of the column resulting in net generation of refrigeration for the column. The process has been described specifically with respect to several preferred embodiments. Those skilled in the art may envision other embodiments within the scope of the invention. One such other option is the elimination of phase separator 306 and the return of stream 326 directly to the column. Product stream 328 would then be removed from column 301 and passed through heat exchanger 305. Still another option would be to subdivide heat exchanger 305 so that the column liquid bottoms 325 and product stream 328 would be heated in separate units. Another embodiment would be to incorporate side condenser 304 as a part of feed heat exchanger 300 in the process arrangements illustrated in FIGS. 1 and 2, thus eliminating the need for the use of stream 323. 
     In Table I, there is tabulated data from a computer simulation of the process of this invention. The numerals correspond to those of the drawings. The streams in Table I labelled Feed, Nitrogen, Fuel Gas and NGL Product refer to streams in the NGL section of the overall process and are included in Table I for completeness. 
     
                       TABLE I______________________________________      Overall Process NGL &amp; NRUStream       Feed      Nitrogen   Fuel Gas______________________________________Flow, lb moles/hr        1,000     769        174Pressure, psia        785       388        407Temperature, °K.        328       286        284Composition, mole %Nitrogen     77        99.6       4.0Methane      16        0.4        93.0Other Hydrocarbons        7         --         3.0______________________________________                  Heat Pump  Heat Pump        NGL       Fluid In   Fluid OutStream       Product   331        344______________________________________Flow, lb moles/hr        57        345        345Pressure, psia        403       428        29Temperature, °K.        314       314        309Composition, %Nitrogen     --        1          1Methane      0.5       99         99Other Hydrocarbons        99.5      --         --Heat Pump Heat DutyHeat Exchange Unit        302       305        305Stream       317       325        328BTU/hr       774,000   660,000    114,000Fraction     1.00      0.85       0.15______________________________________Stream       316       320        330______________________________________Flow, lb moles/hr        927       769        158Pressure, psia        407       404        410Temperature, °K.        136       122        176Composition, %Nitrogen     83.1      99.6       2.4Methane      16.4      0.4        94.1Other Hydrocarbons        0.5       --         3.5______________________________________ 
    
     As can be seen from Table I, the process of this invention would have enabled the production of a product stream containing only 2.4 percent nitrogen with a feed stream containing over 83 percent nitrogen. Furthermore, in the example of Table I, 15 percent of the heat pump heat duty is added to the product stream and this would have enabled a reduction of about 10 percent in the energy requirement for the feed separation over that required by heretofore known processes such as that disclosed in U.S. Pat. No. 4,501,600. 
     Although the product purity will vary and will depend on the concentrations in the feed, generally the process of this invention will enable the production of a methane product stream having a nitrogen content less than 5 percent to as low as 100 ppm.