GB2208699A - Separation of nitrogen from methane-containing gas streams - Google Patents

Abstract

A process for separating nitrogen from e.g. natural gas 2 by separating the gas in a first distillation column K1 at superatmospheric pressure to obtain an overhead vapour stream 18 having a nitrogen content greater than that of the feed gas mixture, reducing the pressure of said overhead stream e.g. in valve 20, separating it in a second distillation column K2 and recovering a nitrogen depleted product stream from one or both of the condensate streams 16 and 42 from the distillation steps, and wherein part or all of the reboil for the second distillation column is provided by effecting indirect heat exchange between the liquid column bottoms 10 and the overhead vapour stream 18, with at least partial condensation of the latter. <IMAGE>

Description

SEPARATION OF NITROGEN FROM METHANE-CONTAINING GAS STREAMS This invention relates to the separation of nitrogen from methane-containing gas streams such as natural gas.

Well-head gases recovered from natural gas wells can include significant quantities of nitrogen the presence of which may devalue the gas as a fuel or chemical feedstock. Processes have therefore been developed for separating the nitrogen from the gas stream so as to obtain a product stream whose nitrogen content does not exceed a predetermined maximum, e.g. to achieve a minimum calorific value or level of purity.

Separation by means of a single distillation column and by double column cryogenic rectification have both been proposed.

However, neither route is satisfactory for mixtures containing lower levels of nitrogen, e.g. below about 35%. At these low levels of nitrogen, the single column variant is inefficient because heat balance requirements dictate a much larger reflux than is needed to wash out the methane.

The efficiency of the double column variant suffers for a similar reason, in that reducing the level of nitrogen in the feed lowers the amount of available reflux for the top of the low pressure column and lowers methane recovery dramatically.

Some improvement in the single column variant can be achieved by the use of side condensers, and using multiple feeds, e.g. as described in EP-A-0233609, but while this increases the thermodynamic efficiency of providing the reflux, the reflux requirements remain very high.

USP 4,664,686 offers a way of overcoming the problem in relation to the double column variant by introducing a high pressure stripping column prior to the double column rectification unit. However, at low nitrogen levels, e.g. 15 mol % or less, it is necessary not only to remove a substantial proportion of the methane in the stripping step but also to substantially reduce its pressure in order to provide the requires reflux by evaporation of the methane. Therefore substantial energy is required to recompress the methane.

Accordingly, there remains a need for an alternative process for separating nitrogen from methane, especially in natural gas streams, which is less energy intensive at low levels of nitrogen concentration.

The present invention provides an improved process for rejecting nitrogen from a methane-containing gas stream, in which the separation is effected in two columns with integrated condensation of overhead first column vapour and second column reboil.

According to the present invention, there is provided a process for separating nitrogen from a feed gas containing both nitrogen and methane to obtain a product stream comprising methane and having a nitrogen content not exceeding a predetermined maximum which is less than that of the feed gas, said process including the steps of (i) in a first distillation column separating the mixture at a first superatmospheric pressure and subambient temperature into a bottoms condensate having a nitrogen content less than that of the feed gas mixture, and an overhead vapour stream containing methane and nitrogen and having a nitrogen content greater than that of the feed gas mixture;; (ii) in a second distillation column with reboil and reflux of the overhead vapour, separating the overhead stream from the first column at a second superatmospheric pressure which is below said first superatmospheric pressure and at subambient temperature into an overhead nitrogen-containing stream having a nitrogen content higher than that of the overhead stream from the first column and a bottoms methane-containing condensate stream which has a nitrogen content less than that of the feed gas mixture;; (iii) prior to reducing the pressure of the overhead vapour stream from the first distillation column for separation in the second distillation column, effecting indirect heat exchange between said overhead vapour stream and liquid column bottoms of said second distillation column, to provide part or all of the reboil of the second distillation step and at least partial condensation of said overhead vapour stream; at least one of the condensate stream from the first distillation column and the condensate stream from the second distillation column comprising said product stream.

In one preferred embodiment the condensates of the first and second columns are combined to provide the said product stream. The process may be operated to achieve nitrogen concentrations of as low as 0.5% or even lower but it is also suitable for achieving less stringent product specifications, e.g. to obtain product streams having a nitrogen content of say, 3 or even 4%.

While the present invention may be applied to gas mixtures having a wide range of concentrations of nitrogen, it is particularly applicable to the treatment of gases wherein the nitrogen content is significantly below 50 mol %, especially below 30 to 35 mol % and particularly 15 mol % or less, e.g. 5 to 15 mol %.

While the pressure of the second column is limited by the critical pressure of nitrogen to about 27 bar maximum, by taking a nitrogen/methane mixture overhead from the first column, rather than substantially pure nitrogen, this column can be operated at substantially higher pressures, e.g. 40 bar or more, thereby enabling a sufficient temperature difference to be obtained between the first column overhead vapour and the second column bottoms to enable second column reboil to be provided at least in part by indirect heat exchange with overhead vapour from the first column, with at least partial condensation of said vapour.

Using overhead vapour from the first column to provide at least part of second column reboil is essential for achieving satisfactory thermal efficiency for the process. With certain feed streams, it may be possible to choose the process conditions of the first column so that all the second column reboil is provided by said indirect heat exchange.

Further improvements in thermal efficiency of the process may be achieved, if desired, by providing intermediate reflux to the second distillation column at one or more locations up the column above the feed, e.g. by means of side-condensers or multiple feeds, since this raises the temperature at which some of the refrigeration is required.

Thus, while cold for the reflux requirements of the second column may, if desired, be provided by a closed heat pump cycle employing a suitable heat pump fluid such as methane, it has been found that the entire reflux duty of the second column can be satisfied by expansion and vaporisation of condensate from the second column in an open heat pump cycle, even at nitrogen levels as low as 5 mol % in the feed. The use of such an open cycle significantly reduces the amount of compression equipment required as compared with closed heat pump cycles.

Best results are obtained if the first column is operated not only to achieve the required temperature difference between the first column overhead vapour and the second column liquid bottoms to permit the required indirect heat exchange but also to retain just enough methane in the overhead vapour to provide the cold requirements for the second column in an open heat pump cycle. Removing a significant proportion of the methane in the first distillation step means that the second distillation, which is the more energy intensive because it involves a more difficult separation, has to deal with a significantly smaller feed which may be less than 50% of the feed to the first distillation step.

In general, it will be preferred to operate the first column to recover overhead at least 30% of the methane in the feed and to obtain a condensate containing not more than 5% nitrogen, and to operate the second column to recover a condensate containing not more than 5% nitrogen and an overhead nitrogen stream containing at most 1%, and preferably not more than 0.5% methane.

A particular problem with prior art nitrogen rejection plants employing condensate from the second column in an open heat pump cycle to provide the reflux requirements of the process is the presence of hydrocarbon material heavier than methane, i.e. C2+ hydrocarbons, in said condensate since such presence increases the power requirements of the process. This problem may be avoided by the present invention by rejecting substantially all the r hydrocarbons in the bottoms condensate in the first distillation step, thereby providing a second column condensate for use as the fluid in the open heat pump cycle which is substantially free of C2+ hydrocarbon. Thus, the process of the present invention is particularly advantageous for the treatment of natural gas and other streams containing C2+ hydrocarbons.By C hydrocarbons, we mean hydrocarbons containing two or more carbon atoms.

The invention will now be described in greater detail with reference to preferred embodiments and with the aid of the accompanying drawings in which: Figure 1 is a flow diagram of a process for separating nitrogen from a feed gas containing both nitrogen and methane according to one embodiment of the present invention; Figure 2 is a flow diagram of a first modification of the process of Figure 1; and Figure 3 is a flow diagram of a second modification of the process of Figure 2.

Referring to Figure 1, the nitrogen-containing feed gas,e.g.

natural gas containing nitrogen, is supplied to the process through pipeline 2 after it has first been treated, if necessary, to remove components such as water vapour and C02 that would solidify under the conditions for nitrogen separation, and compressed, if necessary.

to the desired superatmospheric operating pressure of the first distillation step.

After being at least partially condensed by indirect heat exchange with warmed returning low temperature process streams, identified in more detail below, in main heat exchanger 4, it is further condensed and/or sub-cooled in the reboiler 6 for the first column to provide the column reboil requirements, cooled still further firstly by indirect heat exchange with cool returning low temperature process streams in exchanger 8 and then in the reboiler 10 of the second distillation column, and then flashed through expansion valve 12 to the first column K1 through line 14.The temperature and pressure of the feed to the first column will depend on its composition but will generally be in the range 1850C to -105 C and 30 to 45 bar absolute and typically in the range -88"e to -96"e and 36 to 41 bar absolute.

In the first column, the feed is separated to provide a substantially nitrogen-free methane condensate, e.g. containing 0.5 mol % N2, recovered through line 16, and an overhead vapour comprising a nitrogen/methane mixture which is recovered through line 18. If the feed contains hydrocarbons heaver than methane, they are substantially entirely recovered in the condensate. Ideally, the column is operated to remove as much methane as possible as condensate in line 16 while leaving just enough in the overhead stream 18 to provide the cold requirements for the second distillation step.

The vapour stream in line 18 is at least partially condensed in reboiler 10 of the second distillation column, thereby providing reboil for the second column, and is then expanded through valve 20 and fed to the second distillation colulmn K2 through line 22. The temperature and pressure of the stream in line 22 depend upon its composition but generally will be in the range -100 C to -120 C and 20 to 27 bar absolute and typically -104 c to 115 cm and 24 to 27 bar absolute.

The second column is operated to separate the feed into a methane condensate which is substantially free of nitrogen, e.g.

containing about 0.5 mol % N2, and a nitrogen vapour which is substantially free of methane, e.g. containing about 0.3 mol % methane. The vapour is recovered overhead in line 24 and expanded and partially condensed in expander 26. Recovered from the low pressure side of expander 26 in line 28, it is first passed through reflux condenser 30 of the second column where it gives up cold to assist in the provision of the overheads condenser duty, and then passed through heat exchangers 32,34,36 where it passes in indirect heat exchange with heat pump fluid in line 38 and is thus revaporised, before exiting from the plant in line 40 via exchangers 8 and 4, in that order, where it gives up residual cold to the incoming feed gas in line 2.

The condensate from the second distillation column, consisting of substantially pure methane, is recovered through line 42 and split into three streams 38, 44 and 46. The first of these forms the fluid for an open heat pump cycle which provides the reflux duty for intermediate condensers 48 -and 50 and the remaining duty for overhead condenser 30.Thus, stream 38 is first cooled in exchanger 36 by indirect heat exchange with nitrogen in stream 28 and other returning streams identified below. and then a first part is expanded through valve 52 and supplied as refrigerant to intermediate condenser 48 where it is at least partially evaporated, a second part is further cooled in exchanger 34, expanded to a lower pressure through valve 54 and supplied as refrigerant to intermediate condenser 50 where it is at least partially evaporated, and the remainder is still further cooled in exchanger 32, expanded to a still lower pressure through valve 56 and supplied as refrigerant to reflux condenser 30 where it is at least partially evaporated.The at least partially vaporised streams are recovered through lines 58, 60 and 62, and each is passed separately back through the exchanger or exchangers in which it was cooled prior to expansion, and then passed through main exchangers 8 and 4, in that order, where it gives up residual cold to the incoming feed stream in line 2. The streams are then supplied to appropriate stages of a multi-stage compressor 64 for recompression to the desired product pressure, combined with the streams in lines 46 and 45, to be described, and recovered as product gas through line 66.

Of the remaining streams divided from the condensate 42 recovered from the second distillation column K2, one is pumped to the desired product pressure in pump 68, evaporated in line 44 in indirect heat exchange with incoming feed gas in heat exchangers 8 and 4 in that order, fed to line 45, and combined with the compressed product gas recovered from compressor 64, and the other is evaporated at the pressure it is recovered from the column in heat exchangers 8 and 4 in line 46 and then recompressed in compressor 64 and combined with the product gas in line 66. It will be understood that if the first distillation column is operated to remove as much methane as possible as condensate in line 16, the condensate in lines 44 and 46 will be minimised.

The condensate recovered in line 16 from distillation column K1, and which is substantially methane together with any higher hydrocarbons in the feed gas, is pumped to product pressure in pump 70, fed to line 44 where it is combined with pumped evaporated condensate in line 45, evaporated in exchanger 4 and recovered as product gas in line 66.

By way of example of the invention, a natural gas stream, the details of which are given below in Table 1, was treated in accordance with the process described above with reference to the drawing to give a product gas stream and waste nitrogen stream whose details are also given in Table 1. Details of the pressure, temperature and flow rate of various of the streams in the process are shown in Table 2.

The separation was effected such that the condensate from column K1 contained 0.5 mol % nitrogen and the overhead vapour contained 17 mol % nitrogen. Column K2 was operated to provide a condensate containing 0.5 mol % nitrogen and overhead vapour containing 0.3 mol % methane. The total power requirement of the process was 6,560 kW for the main compressor 4 and a total of 475 kW for the pumps 68 and 70, making a grand total of 7,035 kW.

By way of comparison, treatment of the same feed gas, provided at the same temperature and pressure, to provide substantially the same product gas stream at the same pressure by the conventional single column process requires 12,370 kW if an open heat pump is used to provide the reflux requirements. If a closed heat pump is used with a pure methane cycle fluid, the power requirements total 9,148 kW, and in addition the plant is more complicated.

TABLE 1 Stream 2 6 40 Name Natural Gas Feed Product Gas Waste N2 Temperature C 20 25 14 Pressure bar.a 72.0 71.6 1.6 Molar flow kg mol/hr 8802 8182.5 619.5 Helium 0.0002 0.0000 0.0028 Hydrogen 0.0009 0.0000 0.0128 Nitrogen 0.0737 0.0050 0.9814 CO 0.0000 0.0000 0.0000 Methane 0.8650 0.9302 0.0030 Ethane 0.0418 0.0450 0.0000 Propane 0.0101 0.0109 0.0000 n-Butane 0.0051 0.0055 0.0000 n-Pentane 0.0023 0.0025 0.0000 n-Hexane 0.0007 0.0008 0.0000 n-Heptane 0.0002 0.0002 0.0000 TABLE 2 Stream 14 16 18 Name HP COL FEED HP COL BTMS HP COL OHDS Temperature CC - 92.3 - 82.5 - 91.9 Pressure bar.a 40.0 40.0 40.0 Molar flow kg mol/hr 8802 5058.8 3743.2 Stream 22 24 28 Name MP COL FEED MP COL OHDS EXPDR OUTLET Temperature CC - 108.3 - 152.5 - 188.6 Pressure bar.a 26.6 26.6 22.0 Molar flow kg mol/hr 3743.2 619.5 619.5 Stream 40 42 44 Name HP COL OHDS MP COL BTMS HP CH4 Temperature C 14.6 - 100.0 - 91.2 Pressure bar.a 1.6 26.6 71.7 Molar flow kg mol/hr 619.5 3123.7 290.0 Stream 45 46 58 before h/exch. 36 Name HP CH4 MP CH4 LP CH4 Temperature C 14.7 14.7 - 117.5 Pressure bar.a 71.5 26.4 9.9 Molar flow kg mol/hr 5348.8 493.7 1639.5 Stream 58 after 60 before 60 after h/exch. 4 h/exch. 34 h/exch. 4 Name LP CH4 LP CH4 LP CH4 Temperature 0C 14.7 - 140.0 14.7 Pressure bar.a 9.6 3.85 3.45 Molar flow kg mol/hr 1639.5 498.4 498.4 Stream 62 before 62 after h/exch. 32 h/exch. 4 Name LP CH4 LP CH4 Temperature CC - 154.2 - 14.7 Pressure bar.a 1.7 1.2 Molar flow kg mol/hr 202.0 202.0 Modifications of the process illustrated above are possible without departing from the scope of the invention. For example, all the duty for the overhead condenser and side condensers of the second distillation column could be provided by expanding and vaporising methane condensate recovered from the column thereby obviating the need for the expander 26. However, this is less efficient. The expansion could also be replaced by Joule-Thomson expansion.

The expander could, if desired, be relocated to a warmer part of the process, e.g. to provide cold to exchangers 4 and/or 8.

After partial condensation, the feed to the high pressure column could be separated into two or more streams which could be fed to the column at different levels. Likewise, the overhead vapour from the high pressure column could be split into two or more streams each of which is cooled to a different temperature by heat exchange with bottoms condensate from the low pressure column, prior to feeding to the low pressure column.

If desired. methane may be separated from the condensate from the high pressure column to. leave a natural gas liquids stream which may be recovered as a further product of the process where the feed gas contains C2+ hydrocarbons. The flow sheet for one method of achieving this is illustrated in Figure 2 where pipelines and equipment common with the arrangement of Figure 1 are accorded the same reference numerals, plus one hundred. In this process, a third column, K103, is provided for recovering natural gas liquid from condensate recovered from the bottom of the first column.

Referring to Figure 2, the feed gas entering the plant in line 102 is compressed in compressor 170 and cooled in after-cooler 172 and prior to feeding to exchanger 104, a part of the feed is diverted through line 174 to provide at least a part of the reboil for the third column K103, the arrangement of which is described in more detail below. After providing this reboil requirement, the contents of line 174 are emptied back into line 102 downstream of heat exchanger 104 after passing through heat exchanger 176 where they give up heat to other streams to be described below.Thereafter, the feed is cooled, condensed and sub-cooled by passage through reboiler 106, heat exchanger 108 and reboiler 110, in that order, prior to being expanded and fed to the high pressure column K101 where it is separated in the same manner as described for the process illustrated in Figure 1, with condensate recovered in line 116 and overhead vapour in line 118.

However, in this case only part of the overhead vapour is led onward to provide reboil for column K102 and subsequent separation in K102; and the remainder of the vapour is emptied via line 178 into line 144 carrying condensate from column K102 after it has been pumped in pump 168 and warmed in heat exchanger 108.

The operation of column K102 is similar to that of the low pressure column of the process of Figure 1 except that it is operated to recover an additional stream comprising crude helium. To this end, the waste nitrogen in line 124 is recovered as a side stream from the column below the top, and a separate overhead stream is recovered in line 180. Part of this overhead stream is condensed in condenser 182 by indirect heat exchange with the nitrogen stream in line 124 after said stream has been expanded through a valve 184. Condensate is returned to the column as reflux through line 186 and the uncondensed vapour, comprising crude helium is recovered in line 188, passing back through heat exchangers 132, 134, 136, 108 and 104, in that order, where it is warmed and exiting the plant through line 190.

Reverting to high pressure colum K101, the condensate after being pumped in pump 170 is fed to column K103 via line 192 instead of being recovered as product gas. By means of line 192, it is fed first to heat exchanger 176 where it is warmed and partially evaporated, and then to gas/liquid separator 194 from which the liquid is recovered in line 196, expanded through valve 198, passed again through heat exchanger 176 for further warming and partial evaporation and then supplied to distillation colum K3 for separation into NGL which is recovered from the bottom in line 200 and a methane-containing vapour stream recovered overhead in line 202.Part of the overhead vapour is condensed in reflux condenser 204 and returned to the column as reflux; the remainder is passed via line 206 where it is warmed in exchanger 104, compressed to product gas pressure in the compressor side of compander 208 and combined with the stream in lines 144 and 172 to be recovered as product gas.

The gas recovered from gas/liquid separator 194 in line 210 is expanded and partially condensed in the expander side of compander 208 and the condensate is separated from the uncondensed gas in gas/liquid separator 212 and fed to column K103 through line 214 to recover any C2+ hydrocarbon values.

The uncondensed gas from gas/liquid separator 212 is recovered overhead in line 216, supplied as refrigerant to the reflux condenser 204 and then combined with the overhead vapour in line 206 to be recovered ultimately in the product gas.

Figure 3 illustrates application of the process of the invention where the low pressure column forms the high pressure part of a double column K302. In Figure 3, the pipelines and equipment common to Figure 1 are accorded the same reference numerals plus three hundred.

As in the process of Figure 1, feed gas in line 302 is cooled.

condensed and sub-cooled by passage through exchanger 304, reboiler 306 of column 301, heat exchanger 308 and reboiler 310 prior to expansion through valve 312 and supply to first distillation column K301. The condensate from column K301 is recovered in line 316, pumped through pump 370 to product gas pressure and exits the plant as product gas in line 345/366 after being warmed in exchanger 304. Also as in Figure 1, overhead vapour from K301 is recovered in line 318, passed through reboiler 310 and then expanded through valve 320.

However, in this case reboiler 310 is the reboiler for the high pressure section 380 of a double column K302 and after expansion as aforesaid through valve 320 the overhead from column K301 is supplied to this high pressure section where it is separated into a substantially methane condensate and a nitrogen-rich vapour which is recovered overhead.

After recovery in line 342, the condensate is split into three streams. One is pumped to product gas pressure in pump 368 and then passed back through heat exchangers 308 and 304, in that order, through line 344 and exits the plant in product gas stream 366. The second is passed back through exchangers 308 and 304 in line 346 at column pressure, compressed to product gas pressure in a high pressure stage 9f multi-stage compressor 364 and exits the plant in product gas stream 366. The third forms the fluid for an open heat pump cycle which provides reflux duty for intermediate condenser 382. Thus, it is recovered through line 338, cooled in exchanger 384 in indirect heat exchange with streams identified below, expanded through valve 352 and supplied as refrigerant to intermediate condenser 382 where it is at least partially evaporated.Recovered in line 358, it is passed back through heat exchangers 384, 308 and 304 in that order where it is completely evaporated and warmed. It is then recompressed in multistage compresser 364 and combined with other streams to form the product gas in line 366.

Overhead vapour from the high pressure section 380 is recovered in line 386 at least partially condensed in reboiler 388 of the low pressure section 390 of the double column, thereby providing reboil for said section, and then a first part is returned as reflux to the high pressure column through line 392 and the balance is cooled in heat exchanger 394, expanded through valve 396 to the pressure of the low pressure section 390 and supplied to that section through line 398.

A side stream recovered from high pressure section 380 via line 400 is cooled in heat exchanger 402, expanded through valve 404 to the pressure of the low pressure section and supplied to that section through line 406. A stream consisting substantially of methane is recovered from the low pressure section through line 408, passed back through exchangers 402, 384, 308 and 304 in that order, where it is evaporated and warmed, compressed to product gas pressure in multistage compresser 364 and recovered in the product gas stream. The overhead from the low pressure section of the double column, consisting essentially of nitrogen, is recovered overhead in line 410, and warmed in exchangers 394, 402, 384, 308 and 304 in that order.

The pressures of the first column 301 and the high pressure section of the double column will typically be similar to those of columns K1 and K2 of the arrangement described with reference to Figure 1 The low pressure section 390 of the double column typically will operate at close to atmospheric pressure.

In general the use of the arrangement of Figure 3 is favoured over that of the arrangement of Figure 1 for treating feed streams with the higher nitrogen content.

By way of further example, a feed gas stream, the details of which are given below in Table 3, was treated to separate nitrogen using the arrangement of Figure 3 to give a product gas stream and waste nitrogen stream whose details are also given in Table 3. Details of the pressure, temperature and flow rate of various of the process streams are given in Table 4.

The separation was effected such that the condensate from column K301 contained in the order of 0.45 mol% nitrogen and the overhead vapour contained primarily only nitrogen and methane, the nitrogen being in the order of 30 mol%. Column K302 was operated to provide a condensate containing in the order of 0.45 mol% nitrogen and overhead vapour containing 0.29 mol% methane.

The total power requirement of this process was 10,020 KW. By way of comparison, treatment of the same feed gas, provided at the same temperature and pressure1 by an arrangement similar to that illustrated in Figure 3, but without reboil for the high pressure section of the double column being provided by the overhead stream from the first column requires over 11,700 KW.

TABLE 3 410 After Stream 302 366 h/exch 304 Name Natural Gas Feed Product Gas Waste N2 TemperatureoC 20 15.5 15.5 Pressure bar.a 72 71.5 1.1 Molar flow kg mol/hr 8802 8179 623 Helium 0.0002 0.0000 0.0028 Hydrogen 0.0009 0.0000 0.0127 Nitrogen 0.0737 0.0045 0.9816 C02 0.0000 0.0001 0.0000 Methane 0.8649 0.9306 0.0029 Ethane 0.0418 0.0450 0.0000 Propane 0.0101 0.0109 0.0000 i-Butane 0.0051 0.0055 0.0000 i-Pentane 0.0023 0.0025 0.0000 n-Hexane 0.0007 0.0008 0.0000 n-Heptane 0.0002 0.0002 0.0000 TABLE 4 Stream 314 316 318 Name HP COL FEED HP COL BTMS HP COL OHDS Temperature CC - 92.3 - 82.5 - 91.9 Pressure bar.a 40.0 40.0 40.0 Molar flow kg mol/hr 8802.0 5054.7 3747.3 Stream 322 410 342 Name MP COL FEED LP COL OHDS MP COL BTMS Temperature CC - 108.4 - 191.4 - 101.5 Pressure bar.a 26.6 1.6 25.0 Molar flow kg mol/hr 3747.3 623.0 2367.9 Stream 346 before Name 344 h/exch 308 HP CH4 MP CH4 Temperature C - 93.1 101.5 Pressure bar.a 71.7 25.0 Molar flow kg mol/hr 264.0 637.4 Stream 346 after 358 before 358 before h/exch 304 h/exch 384 h/exch 304 Name MP CH4 LP CH4 LP CH4 Temperature CC 15.5 - 131.8 15.5 Pressure bar.a 24.8 3.0 2.7 Molar flow kg mol/hr 637.4 1466.5 1466.5 Stream 408 before 408 after 406 after h/exch 402 h/exch 304 valve 404 Name LP CH4 LP CH4 LP COL FEED Temperature CC - 155.7 15.5 - 172.0 Pressure bar.a 1.6 1.2 1.6 Molar flow kg mol/hr 756.4 756.4 1056.6 Stream 398 after 302 after h/exch 394 h/exch 308 Name N2 REFLUX COOLED FEED Temperature CC - 193.8 - 87.6 Pressure bar.a 1.6 71.5 Molar flow kg mol/hr 322.8 8802.0

Claims (9)

1. A process for separating nitrogen from a feed gas containing both nitrogen and methane to obtain a product stream comprising methane and having a nitrogen content not exceeding a predetermined maximum which is less than that of the feed gas, said process including the steps of (i) in a first distillation column separating the mixture at a first superatmospheric pressure and subambient temperature into a bottoms condensate having a nitrogen content less than that of the feed gas mixture, and an overhead vapour stream containing methane and nitrogen and having a nitrogen content greater than that of the feed gas mixture;; (ii) in a second distillation column with reboil and reflux of the overhead vapour, separating the overhead stream from the first column at a second superatmospheric pressure which is below said first superatmospheric pressure and at subambient temperature into an overhead nitrogen-containing stream having a nitrogen content higher than that of the overhead stream from the first column and a bottoms methane-containing condensate stream which has a nitrogen content less than that of the feed gas mixture;; (iii) prior to reducing the pressure of the overhead vapour stream from the first distillation column for separation in the second distillation column, effecting indirect heat exchange between said overhead vapour stream and liquid column bottoms of said second distillation column, to provide part or all of the reboil of the second distillation step and at least partial condensation of said overhead vapour stream; at least one of the condensate stream from the first distillation column and the condensate stream from the second distillation column comprising said product stream.

2. A process as claimed in claim 1 wherein intermediate reflux is provided for the second distillation at one or more locations up the column above the feed.

3. A process as claimed in claim 1 or claim 2 wherein cold for the reflux requirements of the second distillation step is provided by evaporating expanded methane-containing condensate from the bottom of the column in an open heat pump cycle.

4. A process as claimed in any one of claims 1 to 3 in which the first column is operated to provide just sufficient methane in the overhead vapour stream to provide the cold requirements for the second column by means of an open heat pump cycle in which the heat pump cycle fluid comprises condensate from said second column.

5. A process as claimed in any one of claims 1 to 4 in which the feed gas contains hydrocarbon material containing two or more carbon atoms and this is substantially completely recovered in the condensate from the first distillation step.

6. A process as claimed in any one of claims 1 to 5 in which the feed gas contains less than 35 mol % nitrogen.

7. A process as claimed in any one of claims 1 to 6 in which the feed gas contains from 5 to 15 mol %.

8. A process as claimed in any one of claims 1 to 7 in which the second column comprises the high pressure column of a double column.

9. A process as claimed in any one of claims I to 8 in which the condensate from the first column is further distilled to obtain natural gas liquid.