NO168099B - PROCEDURE FOR NITROGEN REMOVAL FROM NATURAL GASES - Google Patents

Description

The present invention relates to a method for removing nitrogen from natural gases and is particularly applicable for use in separating a flow from a well into methane, natural gas liquids and nitrogen.

As the hydrocarbons become rarer and more difficult to extract, secondary recycling becomes increasingly widespread. Such secondary recovery is usually called enhanced oil recovery (EOR) and enhanced gas recovery (EGR). One such secondary recovery technique involves injecting gas that does not support combustion into a reservoir to raise the reservoir pressure to remove hydrocarbons that cannot be removed from the reservoir due to the natural reservoir pressure. A commonly used gas for this is nitrogen because it is relatively abundantly available and inexpensive, and can be produced in large quantities at the reservoir site.

Over time, nitrogen injected into the reservoir will begin to be removed along with the hydrocarbons. This requires the removal of nitrogen from the hydrocarbon stream to satisfy the minimum heat content requirements or maximum inert content requirements for product fuel gas.

Conventional processes for removing nitrogen from natural gases, often called nitrogen removal processes, separate the reservoir stream into methane, nitrogen, and hydrocarbons with two or more carbon atoms commonly called natural gas liquids. Conventional processes accomplish this separation by first separating the reservoir stream into a liquid stream containing primarily natural gas liquids and a vapor stream containing primarily nitrogen and methane. The liquid stream is recovered and the vapor stream is cryogenically separated in one or more rectification columns into nitrogen and methane. When a simple rectification column is used to carry out the cryogenic rectification, the column is often driven by means of a heat pump with recirculating fluid. The fluid for this heat pump is generally methane because this gas can withstand the reboiler and condenser temperatures necessary at reasonable pressure levels.

A problem with such conventional processes is that some of the natural gas liquids are not separated during the first separation but remain together with and are recovered together with methane. This is disadvantageous because the natural gas liquids with a better yield can be used in other applications than as fuel together with methane. Thus, it would be desirable to recover more natural gas liquids separately from methane than is possible with conventional nitrogen extraction processes.

Another problem with such conventional processes is that the use of methane as heat pump fluid for cryogenic nitrogen-methane separation columns either requires the column to work at relatively high pressures or requires the heat pump circuit to work under vacuum conditions on the low pressure side. Vacuum conditions in the methane circuit are undesirable, firstly because methane is inherently an unsafe operating condition due to possible air intrusion into the circuit, secondly because vacuum conditions cause a relatively high energy supply because the required pumping energy is substantial. However, operation of the nitrogen-methane cryogenic distillation column at high pressure to avoid heat pump circuit vacuum conditions is disadvantageous because the higher pressures place operating limitations on the column in the form of increased separation steps, increased reflux liquid requirements, or a combination thereof. It would therefore be desirable to operate the nitrogen-methane distillation column at lower pressure without the need for the heat pump circuit to work under vacuum conditions.

An object of the invention is therefore to provide an improved method for removing nitrogen from natural gases.

Another object of the invention is to provide an improved method for removing nitrogen from natural gases where a larger amount of natural gas liquids is recovered separately from methane than is possible with conventional nitrogen separation processes.

Furthermore, the object of the invention is to provide an improved method for removing nitrogen from natural gas where a nitrogen-methane-cryogenic separation column is operated at lower than conventional pressures while at the same time avoiding the need to operate a heat pump under vacuum conditions.

DE-A-2 932 561 describes a method for the rectification of a raw gas mixture under superatmospheric pressure and containing at least three components, for example natural gas where the gas mixture is fractionated in a first rectification column to a top product essentially freed of at least one higher-boiling component and in a bottoms substantially freed of at least one lower boiling component and comprising compressing the liquid bottoms from the first rectification column; passing the resulting compressed bottoms product in indirect heat exchange with the gas mixture to at least partially vaporize the bottoms products; machine expansion of resulting vaporized compressed bottoms to form a liquid which is then refluxed into a second rectification column; and fractionating resulting expanded bottoms in the second rectification column to form a top product containing lower boiling components and a bottoms product containing higher boiling components.

EP-A-0 068 587 describes a method which effectively removes nitrogen from natural gas over a wide spectrum of nitrogen concentrations and where the natural gas can also contain high concentrations of heavy hydrocarbons. The process described there is particularly advantageous for cleaning a natural gas stream that is recovered from a petroleum area where nitrogen injection is used as an improved extraction technology.

As appears from the description below, the method according to the invention differs from that in EP-A-0 068 587 in several ways, namely: 1. The present invention requires that the stripping column is between a first separation and the rectification columns. The first separation separates the raw material into liquid heavy hydrocarbons and gaseous nitrogen-methane. In contrast, EP-A-0 068 587 teaches the use of a stripping column

upstream a rectification column.

2. The present invention requires that the vapor from a first separation be partially condensed and cooled to a minimal degree and that only the resulting liquid portion be introduced to a stripping column. EP-A-0 068 587 introduces a

two-phase flow to its stripping column 70.

3. The present invention requires that the bottom liquid in a stripping column be recovered, while EP-A-0 068 587 leads

the bottoms to another stripping column.

4. The present invention requires that the steam portion of partial condensation bypasses a stripping column and is fed directly to the rectification columns.

The present invention is thus to improve the known technique by a method for removing nitrogen from natural gases where a nitrogen-containing hydrocarbon stream is first separated into a liquid containing primarily hydrocarbons with two or more carbon atoms and into a vapor containing primarily nitrogen and methane, and where the vapor stream is further separated in one or more rectification columns in nitrogen and methane, this process being characterized by: (A) cooling the vapor (12) after the first separation by at least 10"C to partially condense it; (B) introducing the condensed part (62 ) to the top of a stripping column (43) as it constitutes the only one

feeding to the stripping column, and separating it in the stripping column 1 a liquid (11) containing primarily hydrocarbons with two or more carbon atoms

and in a vapor (66) containing primary methane;

(C) recovery of the hydrocarbon liquid (11) and the methane vapor

(66) in step (B); and

(D) introducing the non-condensed portion to the rectification column(s) (45) for separation in nitrogen and

methane.

The invention further relates to a method of the type described above comprising maintaining the rectification column(s) at pressure below conventional pressures, but not under vacuum conditions in a heat pump circuit, and this embodiment is characterized by the steps: (a) introduction of the non-condensed portion as nitrogenous natural gas feedstock to a rectification column (45) operating at a pressure within the range

1379 x IO<3> to 3103 x IO<3> Pa;

(b) separation of the feedstock in the column to a nitrogen-rich

overhead vapor (70) and a methane-rich bottom liquid (79);

(c) cooling the overhead vapor (70) by indirect heat exchange (40) with evaporating heat pump fluid comprising from 0.5 to 60 mol-# nitrogen and 99.5 to 40 mol-% methane to partially condense it

top steam;

(d) returning the condensed portion of the overhead vapor to the column as liquid reflux (72) and removing from the process the non-condensed portion (73) of

the top vapor as nitrogen;

(e) heating the bottom fluid by indirect heat exchange (51) with condensing heat pump fluid to partially

evaporate the bottom liquid; and

(f) returning the vaporized portion (40) of the bottom liquid to the column (45) as vapor reflux and

recovery of the non-evaporated part (45) which

product natural gas,

where the heat pump fluid is at or above ambient pressure at all times.

The term: "column" is used here to indicate a distillation or fractionation column, that is a contact column or zone where liquid or vapor phases countercurrently come into contact to effect separation of a fluid mixture such as by contact between vapor - and the liquid basins of a series of vertically arranged bowls or plates arranged in the columns or alternatively, on packing elements with which the column is filled.

For an extended discussion of fractionating columns, reference should be made to "Chemical Engineer's Handbook", 5th edition, published by R.H.Perry and C.H.Chilton, McGraw-Hill Book Company, New York, Part 13, "Distillation", B.D.Smith et al, p 13-3, "The Continuous Distillation Process."

The term "stripping column" is used here to denote a column in which the liquid feedstock is fed to the top of the column and the more volatile components are removed or stripped from the falling liquid due to the rising steam flow.

The term "natural gas" and "natural gases" are used here to denote a methane-containing fluid which is usually extracted from natural gas wells or oil reservoirs.

The terms "nitrogen-containing natural gas stream" and "nitrogen-containing hydrocarbon stream" are used herein to denote a stream with a nitrogen concentration from 1 to 99 K>.

The terms "natural gas liquid" and "higher hydrocarbons" are used herein to denote hydrocarbons with two or more carbon atoms.

The term "ambient pressure" is used here to indicate the pressure on the outside of the heat pump circuit piping system. In general, the ambient pressure is the atmospheric pressure.

For a closer understanding of the invention, reference should be made to fig. 1 which is a flowchart for a preferred embodiment of the method of the invention.

With reference to fig. 1, the stream 12 is a gaseous stream from a first separation of a raw material from a well or a reservoir. The original raw material contained nitrogen, methane and higher hydrocarbons and in the first separation the main quantity of higher hydrocarbons was separated out as liquid. The gas stream 12 is the second part from the first separation and contains nitrogen, methane and some higher hydrocarbons. Characteristically, the nitrogen concentration in stream 12 is from 3 to 90 H>, the methane concentration from 10 to 97 <t>, the concentration of higher hydrocarbons from 1 to 15 H> and the temperature in stream 12 from -13 3 to -43°C.

The stream 12 is cooled in the heat exchanger 40 against return streams by at least 10 and preferably at least 20, most preferably at least 30"c to give a partially condensed stream 60 which is fed to the phase separator 42. The vapor part 61 from the phase separator 42 containing essentially nitrogen and methane is cooled further towards return flows in the heat exchanger 41, is expanded through the valve 64 and introduced as raw material 65 to the rectification column 45 which operates at a pressure from 1379 x IO<3> to 1303 x IO<3> Pa. The liquid part 62 from the phase separator 42 containing essentially methane and higher hydrocarbons are expanded through the valve 63 and fed to the stripping column 43 at the top of the column. Preferably, the liquid portion 62 is expanded to a pressure of at least 345 x IO<3> Pa lower through the valve 63. In other words, the stripping column 43 is preferably run at a lower pressure of at least 345 x IO<3 >Pa lower than the pressure in the gas flow 12 from the first separation.

In the str.ippe column 43, liquid falls against the rising steam that is formed from reboiled liquid at the bottom of the column. The column bottom liquid is reboiled in the bottom reboiler 44 which can obtain heat from any suitable source such as a heat exchanger 40. The falling liquid and rising vapor come into countercurrent contact to separate the incoming liquid into a vapor which is removed from the column as a stream 66 containing primarily methane and in a liquid removed from the column as stream 11 containing primarily higher hydrocarbons. As shown in figure II, the flow 66 can be heated in the heat exchanger 40 and emerge as a flow 17.

Thus, by using phase separation and further separation of liquid flow from the phase separation in a stripping column, one may be able to recover further higher hydrocarbons, shown in Figure 1 as stream 11 separately from methane. Without the use of phase separation and stripping column separation, these additional higher hydrocarbons would be recovered as a component of a meta&tproduct from column 45. Recovery of these higher hydrocarbons separately from methane is advantageous because the higher hydrocarbons have higher economic value as chemicals or chemical- raw materials than as fuel and recovery of higher hydrocarbons together with methane would necessitate their use as fuel instead of more profitable applications.

Although the secondary methane remover would normally be used to increase the recovery of natural gas liquids, for some purposes it may be more advantageous to use the dual methane remover arrangement to reduce the process energy requirement. For example, process calculations have suggested that process energy savings of several percent can be achieved with the dual methane removal arrangement for equivalent natural gas liquid recovery.

In the rectification column 45, the nitrogen-containing natural gas raw material is separated into a nitrogen-rich overhead vapor and a methane-rich bottom liquid. The bottom liquid is withdrawn from the column 45 as the stream 79 and heated in the heat exchanger 51 by indirect heat exchange against condensing heat pump fluid to obtain a partially vaporized stream 8 which is introduced to the phase separator 52 and returned to the column 45 as reflux steam. The liquid part 95 from the phase separator 52 is expanded through the valve 56 and the liquid stream 67 is heated in the heat exchanger 41 to state 68, is further heated in the heat exchanger 20 and emerges as a methane product in the form of the stream 20. The top steam is withdrawn from the column 45 as the stream 70 and is cooled in the heat exchanger 49 by indirect heat exchange with evaporating heat pump fluid and gives a partially condensed stream 71 which is introduced to the phase separator 53. The liquid part 72 from the phase separator 53 is returned to the column 45 as reflux liquid. The vapor portion 73 from the phase separator 53 is heated in the heat exchanger 48 to state 74, further heated in heat exchanger 47 to state 75, further heated in heat exchanger 41 to state 76 and heated in heat exchanger 40 from where it is released as nitrogen stream 23. This stream can be released to the atmosphere or preferably recovered and used again for injection into a well or reservoir for EGR or EOR processes.

The column 45 is operated by a heat pump circuit which takes heat from the top of the column and pumps it to the bottom of the column. Preferably, the heat pump also removes some heat from an intermediate point in the column. The heating circuit is a closed loop and mass independent from the column 45. The heat pump circuit uses a heat pump fluid which is a mixture of nitrogen and methane. The mixture comprises from 0.5 to 60 mol-# nitrogen and 99.5 to 40 mol-% methane, preferably from 1 to 30 rool-# nitrogen and 99 to 70 mol-% methane and most preferably from 5 to 20 mol-# nitrogen and 95 to 80 mol-# methane.

The embodiment shown in Figure 1 uses a two-stage or dual-pressure heat pump circuit which is a preferred embodiment and which will now be described in detail. The heat pump fluid under pressure and at ambient temperature at 89 is cooled in heat exchanger 46 against heating recirculating heat pump fluid to cooled pressure state 90. Cooled heat pump fluid under pressure is then condensed in heat exchanger 51 to partially evaporate bottom liquid from column 45. Condensed heat pump fluid 91 is then subcooled in heat exchanger 47 against heating recirculating heat pump fluid and preferably the heated nitrogen return flow.

The subcooled heat pump fluid from the heat exchanger 47 is then divided into two parts. The first portion 92 is further subcooled in the heat exchanger 48 against the heating recirculating and most preferably the heating nitrogen return flow. The first part of further subcooled heat pump fluid from the heat exchanger 48 is expanded through the valve 54 to a pressure at least equal to the ambient pressure of the flow 97 and is passed through the heat exchanger 49 to partially condense the overhead steam from the column 45. The first part of the heat pump fluid exits the heat exchanger 49 as the stream 82 is heated by passing through the heat exchanger 48 to state 83, is further heated in the heat exchanger 47 to state 84, is further heated in the heat exchanger 46 to state 85 and is passed through the compressor 57 where it is compressed to an intermediate pressure. The second part of the subcooled heat pump fluid is expanded through the valve 55 to an intermediate pressure, higher than the pressure to which the first part is expanded, as the flow 96 and is passed through the heat exchanger 50 where it is heated to at least partially condense the flow 77 which is a steam flow taken from an intermediate point in column 45. The at least partially condensed stream 78 is then returned to column 45 as further reflux. The second part of the heat pump fluid exits the heat exchanger 50 as stream 86, is heated by passing through the heat exchanger 46 to state 88 and is combined with the first part at intermediate pressure. The combined stream is then passed through the compressor 58 where it is compressed and from where it exits as stream 89 and begins the recirculation again. It is then seen that the pressure in the heat pump circuit is always equal to or exceeds the ambient pressure. Thus, vacuum conditions in the circuit are avoided and there is no ingress of air into the circuit.

The use of mixed heat pump fluid according to the invention results in a number of advantages. The use of the defined amount • of nitrogen in the heat pump fluid lowers the boiling point of the fluid and thereby allows the separation column 45 to work at a lower pressure. Operation of the separation column 45 at lower pressure reduces the condensation temperature for the column top flow 70. Correspondingly, the boiling temperature of the heat pump fluid in the heat exchanger 49 must also decrease. The use of the defined amount of nitrogen in the heat pump fluid maintains positive pressure at this point in the heat pump circuit and thereby avoids vacuum conditions in it.

Lower pressure operation of column 45 allows more efficient separation of the feed stream into its nitrogen and methane components with a reduction in either the required number of separation stages or the required amount of reflux liquid or a combination of these two factors. Furthermore, the column 45 can be operated at a lower pressure than in a conventional nitrogen-methane single-column separation process without the significant shortcomings that occur with vacuum conditions in the heat pump circuit. The single column nitrogen-methane separation process according to the invention allows the column to operate at a lower pressure than that required by a conventional single column while maintaining positive pressure at all points in the heat pump circuit. The pressure reduction will depend on the amount of nitrogen in the heat pump fluid and can reach approx. 1034 x 10<3> Pa. For those situations where the nitrogen flow is not intended for use again in a nitrogen reinjection process, no pressure energy is lost from the process when nitrogen is released at low pressure.

Another advantage of the mixed fluid heat pump loop according to the invention relates to temperature patterns in the condenser-reboiler 51 and the reflux condensers 50 and 49. For those situations where separation in the column 45 does not require pure products, the overhead steam 70 will condense over a temperature range instead of at a constant temperature and the bottom liquid 49 will evaporate over a certain temperature range instead of at a constant temperature. Similarly, the vapor from the center of the column, which is always a mixture, will condense over a range of temperatures. If according to this heat pump fluid is a mixture such that it condenses over a temperature range and evaporates over a temperature range instead of at constant temperatures, the use of countercurrent heat exchange in the reboiler 51 and the reflux condensers 50 and 49 will allow better adaptation of temperature patterns in these heat exchangers so that the pressure levels in the heat pump loop are reduced. This directly results in lower energy costs associated with the heat pump circuit, that is, lower energy requirements to accommodate a given amount of reflux liquid for column separation. The degree of energy reduction can be recognized by considering that for an application where the nitrogen column above the top may contain 5 mol-% methane, the use of a mixed nitrogen-methane heat pump fluid with approx. 5 mol-% nitrogen and 95 mol-Æ methane reduce the energy requirement by approx. 4 # compared to the energy requirement when 100% methane is the heat pump fluid.

Table 1 lists typical process conditions obtained by a computer simulation of the process according to the invention. The stream numbers correspond to those in Figure 1 and the indication C£ + indicates hydrocarbons with two or more carbon atoms. By using the method according to the invention, a feed stream containing nitrogen, methane and higher hydrocarbons can be more effectively separated into these component parts.

Claims (13)

1. Process for removing nitrogen from natural gases where a nitrogenous hydrocarbon stream is first separated into a liquid containing primarily hydrocarbons with two or more carbon atoms and into a vapor containing primarily nitrogen and methane, and where the vapor stream is further separated in one or more rectification columns into nitrogen and methane, characterized by: (A) cooling the vapor (12) after the first separation by at least 10°C to partially condense it; (B) introducing the condensed portion (62) to the top of a stripping column (43) as it constitutes the only feed to the stripping column, and separating it in the stripping column into a liquid (11) containing primarily hydrocarbons having two or more carbon atoms and in a vapor (66) containing primary methane; (C) recovering the hydrocarbon liquid (11) and the methane vapor (66) in step (B); and (D) introducing the non-condensed portion to the rectification column(s) (45) for separation into nitrogen and methane. 2. Method according to claim 1, characterized in that the steam after the first separation is cooled by at least 20°C. 3. Method according to claim 2, characterized in that the steam after the first separation is cooled by at least 30°C. 4. Method according to any one of claims 1 to 3, characterized in that the stripping column is operated at a pressure of at least 344.8 x IO<3> Pa below the pressure in the steam from the first separation. 5. Method according to any one of claims 1 to 4, characterized in that the cooling in step (A) is carried out at least partially by indirect heat exchange of the steam after the first separation with the methane steam from the stripping column. 6. Method according to claim 1 comprising maintaining the rectification column(s) (45) at pressure below conventional pressures, but not under vacuum conditions in a heat pump circuit, characterized by the steps: (a) introducing the non-condensed portion as nitrogen-containing natural gas feedstock into a rectification column (45 ) operating at a pressure in the range 1379 x 10<3> to 3103 x 10<3> Pa; (b) separating the feedstock in the column into a nitrogen-rich overhead vapor (70) and a methane-rich bottom liquid (79); (c) cooling the overhead vapor (70) by indirect heat exchange (40) with evaporating heat pump fluid comprising from 0.5 to 60 mol% nitrogen and 99.5 to 40 mol% methane to partially condense said overhead vapor; (d) returning the condensed portion of the overhead vapor to the column as liquid reflux (72) and removing from the process the non-condensed portion (73) of the overhead vapor as nitrogen; (e) heating the bottom liquid by indirect heat exchange (51) with condensing heat pump fluid to partially vaporize the bottom liquid; and (f) returning the evaporated portion (40) of the bottom liquid to the column (45) as vapor reflux and recovering the non-evaporated portion (45) as product natural gas, where the heat pump fluid is at or above ambient pressure at all times. 7. Method according to claim 6, characterized in that a heat pump fluid is used which has a composition of from 1 to 30 mol-* nitrogen and from 99 to 70 mol-* methane. 8. Method according to claim 7, characterized in that a heat pump fluid is used which has a composition of from 5 to 20 mol-* nitrogen and from 95 to 80 mol-* methane. 9. Method according to any one of claims 6 to 8, where the heat pump fluid is recycled in a closed loop, characterized in that it comprises: (1) cooling of the heat pump fluid by indirect heat exchange with the bottom fluid; (2) dividing the cooled heat pump fluid into a first and into a second part; (3) expanding the first part to a pressure at least equal to ambient pressure and heating the expanded first part by indirect heat exchange with the overhead steam; (4) expanding the second part to an intermediate pressure above the pressure to which the first part is expanded and heating the expanded second part by indirect heat exchange with a steam stream taken from the intermediate point in the column; and (5) compressing the expanded first and second portions to form the heat pump fluid to be cooled in step (1). 10. Method according to claim 9, characterized in that the intermediate steam flow in step (4) in it the smallest is partially condensed and recycled to the column as reflux. 11. Method according to claim 9 or 10, characterized in that at least one of the heated expanded portions is heated further before compression by indirect heat exchange with the heat pump fluid before the expansion. 12. Method according to any one of claims 1 to 11, characterized in that the heat pump fluid is additionally cooled before its expansion by indirect heat exchange with the non-condensed portion of the overhead steam before removal from the process. 13. Method according to any one of claims 9 to 12, characterized in that the first part is compressed to the intermediate pressure, combined with the second part and that the combined stream is compressed to form the heat pump fluid to be cooled in step (1).