RU2514804C2 - Method of nitrogen removal - Google Patents

Abstract

FIELD: process engineering.

SUBSTANCE: invention relates to method of removal of fractions with high content of nitrogen from initial fraction containing, mainly, nitrogen and hydrocarbons. Note here that initial fraction is separated by rectification into high nitrogen fraction and high methane fraction. High-methane fraction is evaporated and overheated at maximum possible pressure to produce cold. High-nitrogen fraction is at least temporarily and/or partially contracted and fed to rectification as a backflow. In compliance with this invention, at least temporarily, at least partially one partial flow (16) of compressed (C) fraction (9') with high nitrogen content after performed condensation (E1) is expanded (f) in valve and, to produce cold, at least partially, preferably completely, is evaporated (E1).

EFFECT: uniform distribution of compressor load irrespective of nitrogen concentration in initial fraction.

7 cl, 2 dwg

Description

The invention relates to a method for removing a fraction with a high nitrogen content from an initial fraction containing mainly nitrogen and hydrocarbons, where the initial fraction is separated by rectification into a fraction with a high nitrogen content and a fraction with a high methane content, and a fraction with a high methane content in order to obtain the cold is evaporated and overheated at the highest possible pressure relative to the underlying cooled fraction, and the fraction with a high nitrogen content, at least temporarily and / or at least partially compressed and fed to the rectification in the form of a return flow.

A method of this kind for removing a high nitrogen fraction from an initial fraction containing mainly nitrogen and hydrocarbons will be explained below using the process shown in FIG.

Through the pipeline 1, the initial fraction containing mainly nitrogen and hydrocarbons, which may have been pretreated, such as sulfur removal, carbon dioxide removal, drying, and others, is led to the heat exchanger E1 and in it through process flows, which will be discussed in more detail below. , cool and partially condense. By conduit 1 ′, a partially condensed starting fraction is fed to column T1 for preliminary separation.

This pre-separation column T1, together with the low-pressure column T2, forms two T1 / T2 columns. The thermal connection of the separation columns T1 and T2 is carried out through a condenser-evaporator E3.

A liquid fraction with a high hydrocarbon content is withdrawn from the lower part of the T1 column for preliminary separation through pipeline 2, which is cooled in the heat exchanger E2 by means of process flows, which will be described in more detail below, and then through the pipe 2 'and expansion valve a are fed to the low-pressure column T2 in the lower area.

Pipeline 3 from the upper region of the column T1 for the preliminary separation of the withdrawn liquid fraction with a high nitrogen content. A partial stream of this fraction through line 3 'as a return product is fed to column T1 for preliminary separation. The high nitrogen fraction withdrawn through line 3 in the heat exchanger E2 is supercooled, and through the 3 '' line and expansion valve b, they are led to the low pressure column T2 above the feed point of the high methane fraction described above.

A gaseous fraction with a high nitrogen content is withdrawn through line 4 at the head of the low-pressure column T2. Its methane content is usually less than 1% vol. In heat exchangers E2 and E1, this high-nitrogen fraction is then heated and overheated before it is diverted via the 4 '' pipe and released to the atmosphere or, if necessary, brought to another place of use.

By pipeline 5, from the lower part of the low-pressure column T2, the liquid fraction with a high methane content, which along with methane includes higher hydrocarbons contained in the initial fraction, is withdrawn. Its nitrogen content is usually less than 5% vol. The high methane fraction is pumped by pump P to the highest possible pressure, which is usually 5 to 15 bar. In the heat exchanger E2, the liquid fraction with a high methane content is heated and, if necessary, partially evaporated. Through the pipeline 5 ', the liquid fraction is then fed to the heat exchanger E1 and in it, by means of the initial fraction to be cooled, it is completely evaporated and overheated before it is withdrawn from the process through the pipeline 5' '.

Such methods for removing a fraction with a high nitrogen content from an initial fraction containing mainly nitrogen and hydrocarbons are implemented in so-called nitrogen removal devices (NRU, Nitrogen Rejection Unit). The removal of nitrogen from nitrogen / hydrocarbon mixtures is always carried out when an increased nitrogen content interferes with the proper use of the nitrogen / hydrocarbon mixture. So, for example, a nitrogen content of more than 5 mol% exceeds the usual standards for natural gas pipelines through which a nitrogen / hydrocarbon mixture is transported. The operation of gas turbines is also possible only up to a certain nitrogen content in the combustible gas.

Such NRUs, as a rule, have a design similar to a device for decomposition of air, with two columns, as, for example, described in figure 1, as a central processing unit.

If now the nitrogen concentration in the initial fraction drops below the limit value, which, depending on the problem statement, is from 20 to 30% vol., Then a sufficiently fine purification (<1% vol. Methane) withdrawn through pipeline 4 from the low-pressure column T2 a gaseous fraction with a high nitrogen content is no longer possible, since the creation of a return product for columns T1 and T2 has reached thermodynamic limits. In particular, in processes with increasing nitrogen content in the initial fraction over time, for example, in the case of oil production while maintaining the pressure by means of nitrogen (EOR = Enhanced Oil Recovery), in which the nitrogen content in associated petroleum gas increases more and more over the years, part a fraction (product) with a high nitrogen content to be withdrawn from the process through a 4 '' pipeline is therefore used as a return product medium.

To do this, at least temporarily, a partial stream of a high nitrogen fraction, which is fed via line 9 to a single or multi-stage compressor C, is compressed at least to the pressure of the column T1 for preliminary separation, i.e. to a pressure of 20 to 50 bar. The compressed partial stream of the high nitrogen fraction through the pipelines 9 'and 9' 'is directed through the heat exchangers E1 and E2, and they are cooled and, if necessary, partially or completely condensed.

Through conduit 10 and expansion valve e and / or conduits 11/12 and expansion valve d, the compressed partial stream of the high nitrogen fraction can be supplied to the pre-separation column T1 and / or the low-pressure column T2 as a return flow. Alternatively, the compressed partial stream through conduit 13, at least in part, can be added directly to the high nitrogen fraction (product). Through these actions, the working area of the two T1 / T2 columns can be significantly expanded with respect to the nitrogen content in the original fraction towards a lower nitrogen content.

Compressor C is still used solely to maintain the cleanliness of the gaseous fraction with a high nitrogen content discharged through line 4 from the low-pressure column T2 at a varying nitrogen content in the starting fraction. A low nitrogen content in the starting fraction requires a higher compressor power than the average nitrogen content. Starting with a specific nitrogen limit in the starting fraction, the use of compressor C is no longer required. A typical formulation of the problem is to process the initial fraction with a nitrogen content that grows with time. This leads to the fact that compressor C must first develop its full capacity. With a growing nitrogen content in the starting fraction, the compressor power may gradually decrease. Starting with a certain concentration of nitrogen in the initial fraction, the compressor ceases to function.

It is an object of the present invention to provide a method for removing a high nitrogen fraction from an initial fraction containing mainly nitrogen and hydrocarbons, which allows the load on the compressor to be distributed evenly regardless of the nitrogen concentration in the initial fraction, in order to absorb significant investment costs associated with the compressor.

To solve this problem, a method is proposed for removing a fraction with a high nitrogen content from an initial fraction containing mainly nitrogen and hydrocarbons, which differs in that at least temporarily at least one partial stream of a compressed fraction with a high nitrogen content after the condensation carried out is expanded and evaporated to at least partially, preferably completely, to obtain a cold.

In this case, the fraction with a high nitrogen content is preferably compressed to a pressure of from 20 to 80 bar and, after the condensation has been carried out, expanded to a pressure of from 1 to 20 bar.

In accordance with one preferred embodiment of the method of the invention, the at least temporarily at least one partial stream of the high nitrogen compressed fraction is expanded after cooling, producing cold, and in order to obtain cold, at least partially preferably completely, evaporated.

In accordance with the invention, the compressor C described above is now used non-exclusively for the described purpose of creating one or, accordingly, several return flows, and in addition it is used to produce cold.

The refrigerating capacity obtained in accordance with the invention is preferably used in order to enable the withdrawal of fractions obtained in the form of liquid products by rectification.

Other preferred embodiments of the method of the invention for removing a high nitrogen fraction from an initial fraction containing mainly nitrogen and hydrocarbons, which are the subject of the dependent patent claims, are characterized in that

- the still not completely evaporated fraction with a high methane content is fed to the circulation tank, the liquid fraction of the high methane fraction formed exclusively in the circulation tank is partially evaporated and again fed to the circulation tank, and the completely evaporated head product of the circulation tank is overheated,

- the methane content in the fraction obtained with the rectification method with a high nitrogen content is less than 1% vol.,

- the nitrogen content in the obtained by rectification fraction with a high methane content is less than 5% vol.,

- when the initial fraction is separated by rectification in two columns consisting of a preliminary separation column and a low-pressure column, in the upper region of the preliminary separation column, preferably above the extreme upper bottom of the preliminary separation column, a fraction with a high helium content is withdrawn and expanded in a low-pressure column preferably in the head of the low-pressure column.

The method of the invention for removing a high nitrogen fraction from an initial fraction containing mainly nitrogen and hydrocarbons, as well as other preferred embodiments thereof, which are the subject of the dependent patent claims, will be explained in more detail below with reference to the embodiment shown in FIG. 2.

In the description or, respectively, explanation of the exemplary embodiment shown in FIG. 2, the applicant will not return in detail to those stages of the method that have already been explained using FIG. 1.

In contrast to the process depicted in FIG. 1, the process depicted in FIG. 2 shows that, at least temporarily, through a pipe 16, a partial stream of a compressed fraction 9 ″ with a high nitrogen content that has been condensed in the heat exchanger E1, divert, expand in the valve f and through the pipe 17 lead to the fraction with a high nitrogen content in the pipe 4 '. Together with it, the expanded partial stream in the heat exchanger E1 is evaporated at least partially, preferably completely, to obtain cold. After the high nitrogen fraction in compressor C is preferably compressed to a pressure of 20 to 80 bar, preferably expansion to a pressure of 1 to 20 bar occurs in valve f.

In addition, through pipe 15, at least temporarily, a partial stream of the high nitrogen compressed fraction after cooling in the heat exchanger E1 can be diverted and expanded to produce cold in the expansion turbine X. Then, the expanded partial stream through pipe 15 'is also supplied to fractions with a high nitrogen content in the pipe 4 'and in the heat exchanger E1 is heated to obtain cold. With this embodiment, additional cooling capacity is increased.

According to another preferred embodiment of the method according to the invention, the partial flow of the high nitrogen fraction compressed fraction withdrawn through the pipe 15 from the heat exchanger E1 can be expanded in the expansion turbine X to a higher pressure, and when separately passed through the heat exchanger E1, it is heated and then brought to the intermediate stage Compressor C. Thanks to this, the efficiency of producing cold is further improved.

By the process of the invention, it is achieved that at least one partial stream of a high nitrogen fraction withdrawn through line 5 from the bottom of the low pressure column T2 can be discharged through line 20 and valve h in liquid form. Alternatively or additionally, a partial stream of a high nitrogen fraction can be discharged in liquid form through line 18 and valve g.

Alternatively or in addition to discharging the high methane content liquid fraction through line 20, the high methane content liquid fraction withdrawn from the lower part of the low pressure column T2 may also be first supercooled in the heat exchanger E2 and discharged through line 21 and valve i.

The same is true for the high nitrogen liquid fraction discharged via line 18, which can also be first supercooled in heat exchanger E2 and discharged through line 22 and valve k.

Now the load on compressor C can be evenly distributed, regardless of the concentration of nitrogen in the initial fraction, at any time. In particular, when the nitrogen content in the initial fraction increases with time, the compressor costs are not depreciated for a long time, and the compressor performs an additional, economically useful task of complex production of LNG and / or LIN.

In the case of a low nitrogen content in the starting fraction, the possible production of LNG and / or LIN is less than with a high nitrogen content. Therefore, the installed compressor power is selected in accordance with the optimized range of products for the entire service life of the installation.

In contrast to the process depicted in FIG. 1, the process depicted in FIG. 2 shows that the still not fully evaporated fraction with a high methane content, which is taken off via a pipe 5 ′ from the heat exchanger E2, is not led directly to the heat exchanger E1, but to circulation tank D. The liquid fraction of the high methane fraction formed exclusively in the circulation tank D, which is fed to the heat exchanger E1 through line 6, is partially evaporated in the heat exchanger E1 and then again fed through line 6 'to the circulation capacity D. The completely evaporated head product with a high methane content diverted through the pipe 7 at the head of the circulation tank D is then overheated in the heat exchanger E1 before it is withdrawn from the process via the pipe 7 '.

The implementation of the fraction process with a high methane content in the heat exchanger E1 is locally determined due to the fact that the path is divided into the evaporation section and the overheating section. The evaporation of the fraction with a high methane content now occurs exclusively on the site of the heat exchanger E1, which is connected through the pipe 6 to the lower part of the circulation tank D.

The described process implementation provides the possibility of reliable and stable evaporation of a fraction (product) with a high methane content even under changing operating conditions, such as, for example, a change in the amount of crude gas, the composition of the crude gas, the pressure of the crude gas, and also in the case of regulation fluctuations. These circumstances, for example, are very pronounced in oil production while maintaining pressure through nitrogen (EOR = Enhanced Oil Recovery), in which the nitrogen content in associated petroleum gas increases over the years.

According to another preferred embodiment of the method according to the invention, in the upper region of the preliminary separation column T1, preferably above the upper plate of the preliminary separation column T1, a high helium fraction 8 is withdrawn and expanded via valve c to a low-pressure column T2, preferably to the head low pressure column T2. This embodiment of the method of the invention with the initial fractions containing helium has the advantage that the inert gas helium can be removed, and the effects of production fluctuations or changes in the fraction of helium in the initial fraction during backwashing in the low-pressure column T2 are quenched and do not directly lead to contaminants (product) high nitrogen with high methane content.

Claims (7)

1. A method for removing a fraction with a high nitrogen content from an initial fraction containing mainly nitrogen and hydrocarbons, where the initial fraction is separated by rectification into a fraction with a high nitrogen content and a fraction with a high methane content, and a fraction with a high methane content in order to obtain the cold is evaporated and overheated at the highest possible pressure with respect to the starting fraction to be cooled, and the fraction with a high nitrogen content is at least temporarily and / or at least partially compressed and fed to rectification in the form of a return stream, characterized in that at least temporarily, at least one partial stream (16) of compressed (C) fraction (9 ') with a high nitrogen content after the condensation (E1) is carried out is expanded ( f) in the valve and in order to obtain cold, at least partially, preferably completely, is evaporated (E1). 2. The method according to claim 1, characterized in that the fraction (9) with a high nitrogen content is compressed (C) to a pressure of 20 to 80 bar and, after the condensation (E1) has been carried out, expanded (f) to a pressure of 1 to 5 bar. 3. The method according to claim 1 or 2, characterized in that at least temporarily at least one partial stream (15) of compressed (C) fraction (9 ') with a high nitrogen content after cooling (E1) expand (X), producing cold, preferably up to a pressure of 1 to 20 bar, and in order to obtain cold, if necessary, evaporate and heat (E1). 4. The method according to claim 1 or 2, characterized in that the still not completely evaporated fraction (5 ') with a high methane content is fed to the circulation tank (D), which is formed exclusively in the circulation tank (D), the liquid fraction of the fraction (5') with a high methane content, partially evaporated (E1) and again brought to the circulation tank (D), and the completely evaporated head product (7) of the circulation tank (D) is overheated (E1). 5. The method of claim 1 or 2, characterized in that the methane content in the fraction obtained by distillation (T1 / T2) (4-4``) with a high nitrogen content is less than 1% vol. 6. The method according to claim 1 or 2, characterized in that the nitrogen content in the fraction obtained by distillation (T1 / T2) fraction (5) with a high methane content is less than 5% vol. 7. The method according to claim 1 or 2, in which the separation is carried out by rectification of the initial fraction in two columns consisting of a preliminary separation column and a low-pressure column, characterized in that in the upper region of the column (T1) for preliminary separation, preferably above the upper plate columns for pre-separation (T1), a high helium fraction (8) is withdrawn and expanded (c) in a low-pressure column (T2), preferably in the head of the low-pressure column (T2).

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