Cryogenic nitrogen rejection schemes: analysis of their tolerance to CO2

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Cryogenic nitrogen rejection schemes: analysis of their tolerance to CO Giorgia De Guido, Flavia Messinetti, and Elvira Spatolisano

Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.9b02544 • Publication Date (Web): 23 Aug 2019 Downloaded from pubs.acs.org on August 29, 2019

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Industrial & Engineering Chemistry Research

Cryogenic nitrogen rejection schemes: analysis of their tolerance to CO2 Giorgia De Guido*, Flavia Messinetti, Elvira Spatolisano

Dipartimento di Chimica, Materiali e Ingegneria Chimica “G. Natta”, Politecnico di Milano, Piazza Leonardo da Vinci 32, 20133 Milano, Italy

*Corresponding author: Giorgia De Guido ([email protected]; phone: +39 02 2399 3260; fax: +39 02 2399 3280)

Abstract In the “energy transition era” we are experiencing today, natural gas grows strongly and much faster than either oil or coal, being an environmentally-friendly fuel supported by a broad-based demand and the continuing expansion of liquefied natural gas (LNG). Natural gas mainly consists of methane, but also some contaminants are present in it. Among them, nitrogen is an inert gas whose content, if too high, must be reduced to levels acceptable for producing a pipeline-quality gas or LNG. When dilution of the high-nitrogen gas stream with a low-nitrogen gas is not practical, a nitrogen removal unit (known as Nitrogen Rejection Unit or NRU) must be installed. This is becoming more and more important as we shift to lower-quality gas feedstocks. Cryogenic distillation is the only viable option for the removal of N2 on a large scale and in case of stringent specifications both in the product stream and in the rejected stream. Typically, the feed gas to the NRU has to contain methane and N2 and only very low quantities of compounds (such as CO2) that might freeze at the NRU operating temperatures and cause equipment blockage. Focusing on the production of pipeline-quality natural gas, the aim of this work is to analyse the cryogenic removal of N2 from natural gas streams that also contain CO2 since a CO2-tolerant NRU may lower capital and operating costs reducing the upstream removal of CO2. Different process configurations (i.e., the single-column, the double-column and the three-column systems) are investigated to determine the maximum allowable CO2 content in the feed gas that avoids solidification within the process and permits reaching the desired value of the Wobbe Index. Therefore, a range of applicability for each configuration is determined depending on the N2 and CO2 contents in the feed gas.

Keywords: Nitrogen Rejection Unit; Natural Gas; Carbon Dioxide; Nitrogen; Cryogenic Separation 1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1.

Introduction

One of the biggest challenges the world is facing today is the need for alternative energy sources to fossil fuels to favour the energy transition towards a low-carbon economy. In this scenario, renewable energy is the fastest-growing energy source, accounting for 40% of the increase in primary energy. However, since it would be extremely difficult to provide sufficient energy for rapid world economic growth while at the same time phasing out fossil energy for environmental reasons, natural gas is strongly increasing in importance and it is growing much faster than either oil or coal, being an environmentally-friendly fuel supported by a broad-based demand and the continuing expansion of LNG,1 which increases the availability of gas globally. Natural gas is a mixture of gases, with hydrocarbons usually being the main components (except for low-quality gas reserves). However, some non-hydrocarbon species can be also found in it, such as acid gases, water and inert gases, including nitrogen, helium, argon, etc. The N2 content in natural gas varies depending on the characteristics of the gas fields. Table 1 shows the content of naturally occurring nitrogen in some gas fields located all over the world. According to the literature,2 approximately 14% of US natural gas contains more than 4% nitrogen (i.e., the US pipeline specification for this contaminant), requiring a nitrogen removal unit when dilution with low-nitrogen gas is not practical.

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Table 1. N2 contents in some gas fields. Natural gas field/Location N2 content [mol%]

Reference

0.2

Shimekit and Mukhtar (2012)3

0.68

Kidnay and Parrish (2006)4

Ardjuna, Indonesia

1.3


Lacq, France

1.5


Canada, Alberta

3.2


>6

The Linde Group (2016)5

ca. 8

Wilkinson and Johnson (2012)6

North Field, Qatar

8.5-10.9a

Al-Kaabi (2014)7

Liverpool Bay, UK

10


Groningen, Netherlands

14.3


Southwest Kansas

14.65


Uthmaniyah, Saudi Arabia Rio Arriba County, New Mexico

Pluto and Xena, Karratha, Australia Morecambe Bay, UK’s Irish Sea

Sacramento River Delta, California

Baker and Lokhandwala

16

(2008)2

Miskar Field, Tunisia (BG Tunisia Hannibal gas

16.903

Plant, Sfax) Bhit Mountains, Pakistan Uch, Pakistan Cliffside Field, Amarillo, Texas Western Colorado Odolanów, South-western Poland

Kidnay and Parrish (2006)4, Wilkinson and Johnson (2012)6

18


25.2


25.6


26.10


42.7b

Cholast et al. (2005)8

a

Composition range on a dry basis, also including He. Original concentration, which was subject to changes in the feed gas composition over the years of operation of the KRIO nitrogen rejection plant.8

b

Moreover, nitrogen can be present in natural gas or in associated gas also as a consequence of its injection, respectively, into a gas pocket for Enhanced Gas Recovery (EGR) or into an oil reservoir for Enhanced Oil Recovery (EOR). Over time, it may happen that some of the injected nitrogen mixes 3 ACS Paragon Plus Environment


with the natural gas during recovery or with the associated gas during production, increasing the nitrogen content from low levels to 40 mol% or higher,6 so that it is necessary to remove it during the treatment of the recovered gas. Specifications for nitrogen depend on the final use of the processed natural gas, either as pipelinequality or as LNG. In the former case, typically no more than 3-4% nitrogen is allowed:5 the exact value depends on local government specifications, however the purpose is increasing the heating value of the gas and meeting the Wobbe Index (WI). In the latter case, nitrogen must be removed when present in high concentrations (higher than about 5 mol%) to reduce its content to below 1 mol% in order to improve its calorific value, to avoid reducing the liquefaction efficiency (due to the additional refrigeration required per unit of produced LNG for condensing nitrogen in the feed gas), and to simplify problems associated with the management of the boil-off gas during storage and transportation,9 i.e. to avoid “auto-stratification” in the LNG storage tanks, which can be a precursor to tank “rollover”. As for the separated nitrogen stream, it can be vented to the atmosphere: in such a case, the maximum methane concentration in it is generally regulated by local authorities and can lie anywhere between 100 and 10000 vppm.5, 10 In addition to that, it can be also used in various oil and gas recovery applications.6 Excess nitrogen is removed in Nitrogen Rejection Units (NRUs). Different techniques have evolved for various plant sizes, including Pressure Swing Adsorption (PSA), membranes, lean oil absorption, and cryogenic processes.5 An example for PSA is given by the Molecular Gate® process by BASF Catalyst 11-15 that employs a proprietary adsorbent based on the synthetic titanosilicate ETS-4.16 Other two ones are the Nitrotec process, which is based on the use of carbon molecular sieves,16 and the NitrexTM process developed by UOP by using the proprietary PolybedTM PSA platform.16 As for membranes,2, 17, 18 MTR’s NitroSepTM can be mentioned, which is based on the use of proprietary membranes significantly more permeable to methane, ethane, and other hydrocarbons than to nitrogen.19 A process which is based on lean oil absorption is the one used by Advanced Extraction Technologies, Inc.20, 21 The first three non-cryogenic processes are typically limited to applications with less than 50000 Nm3/h of feed gas, since they require higher OPEX and CAPEX at higher feed gas flowrates. An exhaustive overview is given by Rufford and coworkers 22 and a summary of selection/design criteria to be considered for the choice of the most suitable NRU technology is given by Kuo et al.23 For high flowrates, nitrogen rejection is typically accomplished through cryogenic separation,5 currently also considered for the separation of CO2 from low-quality gas reserves24, 25 and from biogas,26 for the advantages it offers respect to conventional technologies,27 although this requires particular attention 4 ACS Paragon Plus Environment

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to the design of the refrigeration section.28 It consists of a low-temperature distillation process that utilises the different volatilities of methane and nitrogen (having a normal boiling point of -161.5°C and -195.8°C, respectively)29 to achieve separation. As of 1999, 26 cryogenic nitrogen removal plants were in operation in the United States.2 Different cryogenic process configurations are available: single-column, single partitioned column, double-column and double-column processes with an enrichment step.5 Their optimal design, as for most cryogenic processes, results from minimizing the energy required for refrigeration. Actually, NRUs are autothermal and do not require external refrigeration when it is provided by one or more evaporating methane streams, available at low pressure. This happens at the expense of a higher power consumption required for methane compression, so that the optimization of the overall separation system concerns both the provision of refrigeration and the sales gas compression system. Process selection for the NRU should be based on some factors, such as nitrogen content and CO2 content in the feed gas, feed pressure, flowrate, methane recovery, and contaminant levels.23,

30

Among these key factors, the tolerance to CO2 is the most important one since a process that has very little CO2 tolerance may require a costly deep CO2 removal system. Of course, the tolerance of the NRU to CO2 is determined by the coldest spot where this species tends to freeze-out, which is in turn a function of the operating pressure and nitrogen content in the feed gas. Indeed, with cryogenic NRUs, which typically operate at temperatures as low as -160°C÷-190°C in their coldest sections, it is necessary to remove impurities that might form solid phases to very low levels. Removal of CO2 to low levels (10 to 1000 ppm, according to the literature)31 upstream of the NRU is a robust solution, although process designs more tolerant to CO2 would allow avoiding/reducing the requirement for upstream removal.32 To the authors’ knowledge there are no studies dealing with nitrogen rejection considering the presence of CO2 in addition to methane and N2, which is interesting to define a CO2-tolerant nitrogen rejection process. CO2 is reported to be “a real challenge” for the scheme selection.30 Indeed, if the double-column scheme is generally reported to have low tolerance to CO2, the tolerance of the other schemes is reported to be not so trivial. The aim of this work is to study different configurations for nitrogen rejection from a natural gas stream also in the presence of CO2 in order to provide the most suitable solution for the production of a pipeline-quality natural gas as a function of the composition of the gas in relation to both the N2 and the CO2 contents. Firstly, the single-column process is taken into account. Simulations have been performed varying the nitrogen feed content from 1 to 30 mol%, for which the single-column process is reported to be suitable.5 For each case (i.e., each nitrogen content in the feed gas stream), the feed stream is 5 ACS Paragon Plus Environment


characterized in terms of admissible CO2 content, which has to be intended as the maximum molar fraction that can be accepted for avoiding its solidification in some points of the process. Secondly, the double-column process is investigated. In this case, no CO2 has been considered in the feed gas stream and the study is focused on the definition of the completely thermally coupled process scheme and of the operating conditions. In particular, the ranges of the operating pressures for the two columns are investigated to perform the thermal coupling, and the acceptable nitrogen contents in the feed gas are evaluated. Then, the use of the pre-separation column is taken into account, investigating two possible applications, respectively, aimed at concentrating a diluted nitrogen stream and at making the process more tolerant to CO2. 2.

Process description

The feed gas stream (with a molar flowrate of 5000 kmol/h) is considered to come from a dehydration unit and, thus, to be at 5 MPa and 20°C. The nitrogen content in the feed gas has been varied in a certain range (depending on the process configuration), as well as the CO2 content (in order to define a CO2-tolerant process). In all the simulations, the SRK33 property package available in Aspen HYSYS® V9.034 has been used. Before performing the simulations for the process analysis presented in this work, we carried out a thermodynamic analysis to understand if the “CO2 Freeze Out” tool35 available in Aspen HYSYS® could be used in a reliable way for a check about the CO2 freezing conditions in the different simulations. In doing that, we compared the results obtained using the “CO2 Freeze Out” tool35 (with its default parameters) with the results obtained using a home-made routine based on the approach described in previous works36, 37 with regressed parameters (for pure CO2 and for binary mixtures, i.e. kijs) that had been determined to improve the description of SLVE conditions, thus considering low/cryogenic temperatures. We founded a good agreement for the results on the P-T diagram and in terms of CO2 solubility vs. temperature for different nitrogen contents, which led us decide to use the “CO2 Freeze Out” tool35 for continuing the analysis from a process point of view. In all the simulations, pressure drops of 0.01 MPa have been specified in each heat exchanger. Moreover, for every column 10 theoretical trays have been considered and, before entering it, the inlet stream has been passed through an expansion valve to bring its pressure to the column operating one.


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2.1 Single-column NRU process The basic single-column NRU process consists of one distillation column, which separates light components including N2 as the overhead product from the hydrocarbon-rich bottom stream. Whereas the double-column process is the preferred choice for concentrations above 30%, the single-column process is favourable if the nitrogen content in the feed gas is below 30%. For each case (i.e., each nitrogen content in the feed gas stream), the feed stream is characterized in terms of admissible CO2 content that has to be intended as the maximum molar fraction that can be accepted for avoiding its solidification in some points of the process, as better explained in the following (section 3.1). The single-column process scheme simulated in Aspen HYSYS® V9.034 is shown in Figure 1. The purified feed gas (FeedGas) coming from upstream treatment is cooled down in LNG-100, depressurized in VLV-100 and fed to the distillation column (T-100) that operates at 2.7-3.0 MPa. The nitrogen-rich stream (N2 Product) is withdrawn from the top of the column, while purified methane (CH4 Product) exits from the bottom. This stream is, then, flashed to low pressure in VLV-101 (however, not too low to avoid CO2 freezing), revaporized, and warmed in LNG-100 against the incoming feed and the rejected nitrogen for cold recovery. After that, the purified natural gas stream (Gas to Comp), ready to be transported through pipelines, is compressed in K-100.

Figure 1. Single-column process scheme simulated in Aspen HYSYS® V9.0.

Table 2 summarizes the main features and the process specifications that have been assigned in the simulations of the single-column NRU process (“Case 1” in Table 2).



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Table 2. Summary of main features and specifications assigned in the simulations of the different NRU process configurations taken into account in this work: “Case 1” refers to the single-column configuration, “Case 2” refers to the double-column configuration, “Case 3” refers to the use of the pre-separation column upstream of the double-column process for concentrating a diluted nitrogen stream (“Case 3a”) or for making the process more tolerant to CO2 (“Case 3b”). Case 3

Case 1

Case 2

1-30 mol%

30-70 mol%

a 5-25 mol%

b 1-30 mol%

CO2 in feed

Yes

No

No

Yes

Aim of the analysis

To determine the max admissible CO2 content in the feed stream to avoid solidification

To determine suitable operating conditions for thermal coupling

Qreb, Qcond

None (autothermal)

None (autothermal)

Qreb, Qcond

P of T-100 = 2.7-3.0 MPa

T2: adjusted so that Qreb_HP_coupled = Qreb_HP

TF_precooled: adjusted to provide the required duty to column reboiler

TFeedGas_lowT = -90°C

TNitrogen_Prod = 5°C

TNitrogen_Prod = -118°C

TF_cool: adjusted to provide the required duty to column reboiler TCH4_double: set to allow adequate heat exchange

TN2 Vent = 18°C

T8 = -170°C

TCH4_double = 15°C

N2 feed content

External duty requirements

CH4 in top product = 0.5 mol% N2 in bottom product: adjusted to avoid solidification and to meet the Wobbe Index specification

T3: set to have a reasonable temperature approach in E-100 CH4 in top product = 0.5 mol%

Simulation specifications

To study the range of N2 To determine the max feed contents in which admissible CO2 content the pre-separation in the feed stream to column is applicable avoid solidification

TN2_out = 15°C TF_lp: set to allow an autothermal operation and a good separation

TN2_out: set to allow an adequate heat exchange CO2 in top product = 10 vppm (less than the 50 ppm limit4 to be conservative) to enable a finishing treatment in the double-column

PCH4_LP: adjusted to provide condenser duty

P of VLV-101: minimum allowable to avoid solidification

N2 in bottom product < 4 mol%

PUpgraded Natural Gas = 3.0 MPa

T5: specified to achieve a good separation and have an autothermal operation Vapour fraction in N2_inLP = 0.5

TCH4_hot: set to have a reasonable temperature approach in the condenser CH4 in top product = 50 mol%

N2 in bottom product: 1-3 mol%, to meet the Wobbe Index specification


N2 in bottom product = 0.1-0.5 mol%

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2.2 Double-column NRU process The double-column process uses two distillation columns operating at different pressures that are thermally linked, where the condenser of the high-pressure column is used to reboil the low-pressure column. In a typical double-column NRU process, the low-temperature conditions reached in the lowpressure column require carbon dioxide to be removed from the feed gas down to a few ppm for avoiding freezing in the tower. This configuration is reported to be able to more easily handle large changes in the N2 feed content.4 As an example, the double-column system was selected in the framework of the nitrogen removal project related to an installation at Odolanów in Poland,8 designed to process 250 million ft3/d feed gas containing 45 mol% N2 in two trains.6 The process scheme of the double-column NRU configuration simulated in Aspen HYSYS® V9.034 is shown in Figure 2.

Figure 2. Double-column NRU process scheme simulated in Aspen HYSYS® V9.0. The double-column process consists of two thermally coupled columns working, respectively, at high pressure (HP_Column) and at low pressure (LP_Column). The feed stream (Feed) is firstly precooled against the two product streams in LNG-100. Exiting from the process-process heat exchanger, it is used to provide duty to the reboiler of the HP_Column, so that it is further cooled down. It is, then, depressurized to the operating pressure of the distillation tower in the valve VLV-100 and fed to the HP_Column, equipped with a partial reboiler and a total condenser. Here, the feed stream is separated into a bottom stream (CH4_HP), richer in methane, and a top stream (N2_HP), richer in nitrogen. Both products are fed, after depressurization, to the LP_Column, the former being the feed (CH4_inLP) and the latter providing the reflux (N2_inLP). In the LP_Column, separation is completed and essentially pure nitrogen and methane are withdrawn at the top (N2_prod) and at the 9 ACS Paragon Plus Environment


bottom (CH4_prod). These two streams are used to perform cold recovery: the methane bottom stream is heated against the feed to the LP_Column in heat exchanger LNG-101, while the nitrogen top product cools the N2_HP stream in LNG-102. Purified methane exiting from LNG-101 is further heated up in LNG-100, to provide additional refrigeration duty, and it is finally ready for compression and gas grid distribution. The nitrogen product, after passing through LNG-102, LNG-101 and LNG100, can be vented to the atmosphere, being its methane concentration consistent with the emission limits. Table 2 summarizes the main features and the process specifications that have been assigned in the simulations of the double-column NRU process (“Case 2” in Table 2). The simulations for the double-column configuration have been performed with the binary mixture methane-nitrogen, not taking CO2 into account, since in real applications the inlet CO2 limits are extremely low due to the fact that the bottom product of the HP_Column has to be fed to the LP_Column, where any trace of CO2 would freeze. Thus, for this configuration a feasibility analysis aimed at establishing the most suitable operating conditions has been carried out, varying the nitrogen content in the feed gas from 30 to 70 mol%. The feed trays of the columns, as well as the temperatures of the column inlet streams and the two purity specifications of the HP_Column have been chosen so that the duty needed by the coupled reboiler-condenser leads to a high purity nitrogen distillate (approximately 99.5 mol% N2) in all the simulations, and to a pipeline-quality bottom product, with a CH4 content greater than 96 mol%. In this way, the nitrogen stream has a maximum CH4 residual content of 1 mol%, enabling the atmospheric discharge of the stream without environmental issues and product losses, while the methane product reaches the minimum allowable Wobbe Index of 48 MJ/Sm3 and is ready to be sent to the compression station and into the pipeline system.

2.3 Three-column NRU process As a rule of thumb, gas with a N2 concentration below 25-30 vol%10 cannot usually be processed successfully in a basic double-column, but requires further processing steps, including a nitrogen enrichment column in the so-called three-column nitrogen rejection process. This pre-separation column can also remove heavy hydrocarbons contained in the feed gas,5 which is important to ensure a certain ratio between hydrocarbons and nitrogen in the low-pressure column of the double-column configuration, since this limits the purity of the N2 product. Such a ratio can be adjusted removing part of the hydrocarbons in a pre-separation column. Since no N2 is lost via the bottom of this column, the N2 concentration in the overhead product then fed to the double-column system increases and improves the reflux ratio at the top of the low-pressure column of such a system. The pre-separation column can remove the bulk of methane (or heavier hydrocarbons) and CO2 content from the bottom, 10 ACS Paragon Plus Environment

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which results in a lower CO2 content to the cold section. This makes the process more CO2-tolerant and suitable to be operated with a feed gas containing up to 1.5 mol% CO2, according to the literature.9 The use of a high-pressure pre-separation column can be applied also to the single-column configuration (leading to the so called two-column nitrogen rejection process) for reducing the CO2 content in the feed gas to the downstream low-pressure column, making it more CO2-tolerant. The use of a pre-separation column was patented by Costain38-40 and implemented in a number of nitrogen rejection facilities. The purpose was to process gases with a relatively low nitrogen content, incorporating a column upstream of the main distillation system to avoid high energy requirements where the feed gas is relatively dilute in one desired product.6 As stated in the introductory section, two possible applications of the pre-separation column have been studied: they are, respectively, aimed at concentrating a diluted nitrogen stream and at making the process more tolerant to CO2. The corresponding process flowsheets are illustrated in Figure 3 and in Figure 4, respectively.

Figure 3. Process scheme of the pre-separation section simulated in Aspen HYSYS® V9.0 for concentrating a nitrogen gas stream neglecting the presence of CO2 in it. Product streams are sent to the double-column process shown in Figure 2.



Figure 4. Process scheme of the pre-separation section simulated in Aspen HYSYS® V9.0 for investigating the process tolerance to CO2, thus considering the presence of CO2 in the feed stream. Product streams are sent to the double-column process shown in Figure 2. Referring to Figure 3, the feed gas stream (Feed_to_pre) enters heat exchanger LNG-100, in which it is cooled down by all the product streams. The pre-cooled feed stream (F_precooled) is, then, used as the heating medium in the column reboiler, simulated as E-100 from the utility side. The cooled feed stream is depressurized to the column operating pressure in VLV-100, before it enters the preseparation column Preseparator. A concentrated nitrogen stream leaves as a vapour from the top of the column (V_to_double), while a pipeline-quality gas is withdrawn from the bottom. This last stream (CH4_from_pre), after depressurization in VLV-101, can be used as coolant in the full-reflux condenser thanks to the Joule-Thomson effect. This pressure reduction and subsequent heat recovery enables the pre-separation and, therefore, the three-column process to be autothermal. The pressure reduction does not allow any CO2 to be present in the stream, since it would solidify due to the low temperatures reached. Therefore, simulations have been performed considering a feed gas stream that consists of CH4 and N2 only, and the N2 molar fraction has been varied in the range 5-25 mol% to evaluate the flexibility and range of applicability of the pre-separation column for N2 concentration. Above 25 mol% N2 the process simulation does not reach convergence, since the flowrate of the bottom methane stream, CH4_from_pre, decreases and, upon depressurization, is not able to provide enough cooling duty to the column condenser. Therefore, the process ceases to be autothermal above an inlet nitrogen content of 25 mol%. In this case, the feed gas should be directly sent to the doublecolumn process, bypassing the pre-separation section. The top vapour product from the pre-separation column is, then, fed to the double-column process (shown in Figure 2) to complete the separation. Table 2 summarizes the main features and the process specifications that have been assigned in the simulations of the process scheme of the pre-separation section performed in Aspen HYSYS® 12 ACS Paragon Plus Environment

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V9.034 for concentrating a nitrogen gas stream neglecting the presence of CO2 in it (“Case 3a” in Table 2). Referring to Figure 4, the feed stream (Feed_to_pre) is used as heating medium in the reboiler of the pre-separation column, simulated as the external heat exchanger E-100. After further cooling against all the product streams in LNG-100, it is depressurized to the column pressure and sent to the preseparation column T-100. Here, it is fractionated into a concentrated nitrogen vapour stream and a methane-rich bottom stream. The vapour product (V_to_double) is sent to the double-column process to complete the separation. The methane bottom stream, after depressurization in VLV-101, requires heating to avoid the formation of solid CO2 and can be mixed with the double-column product CH4_double, to increase its Wobbe Index value. Table 2 summarizes the main features and the process specifications that have been assigned in the simulation of the process scheme of the pre-separation section performed in Aspen HYSYS® V9.034 for investigating the process tolerance to CO2, thus considering the presence of CO2 in the feed stream (“Case 3b” in Table 2).

3.

Results and discussion

In the following, the results for the three process configurations investigated in this work are presented and discussed.

3.1 Single-column NRU process For a binary nitrogen-methane distillation process, the operating flexibility of a single-column NRU is limited by the critical pressure of the mixture. For the three-component mixture, simulations have been performed in the range 2.7-3.0 MPa. Depending on the feed stream composition, the optimal operating pressure is chosen as the minimum value that allows avoiding solidification downstream the J-T valve VLV-100 in the scheme shown in Figure 1. As the pressure increases, the separation gets worse, since the nitrogen critical pressure is approached. The feed tray has been chosen as the first useful to avoid CO2 freezing. To accomplish that, a first feed tray has been tried, at assigned number of theoretical trays (10), methane content in the top stream (i.e., 0.5 mol%) and nitrogen content in the bottom stream and fixing the operating pressure at 2.8 MPa. The simulation has, then, been run and the absence of solid CO2 formation has been checked within the column, while simultaneously varying the specification on the N2 content in the bottom product, so that the content of both contaminants in such a stream allows to meet the desired Wobbe Index. 13 ACS Paragon Plus Environment


As for the check on the absence of solid CO2 within the column, particular attention was devoted to the bottom trays, considering that CO2 is the heaviest species of the three-component mixture and, thus, a significant CO2 molar fraction is expected in the bottom section of the distillation column. To prove that, a typical CO2 molar fraction profile is reported in Figure 5 for a feed stream containing 9 mol% N2 and 2 mol% CO2, a column operating pressure of 2.8 MPa, a number of theoretical trays equal to 10 and the feed tray equal to the 9th from the top. Even though it is a specific case, some general conclusions can be drawn: the CO2 concentration shows a monotonic increasing profile from top to bottom, with the top section essentially free of CO2. Therefore, the problem of CO2 solidification is confined to the last trays in the bottom section, starting from the feed tray vicinity. 9.E-03 Liquid

8.E-03

CO2 molar fraction [-]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Vapour

7.E-03 6.E-03 5.E-03 4.E-03 3.E-03 2.E-03 1.E-03 0.E+00 0

1

2

3

4

5 6 7 Tray Number

8

9

10

11

Figure 5. Example of CO2 molar fraction profile within the single-column simulated for the NRU (Figure 1). Results refer to a feed stream composition of 9 mol% N2 and 2 mol% CO2, a column operating pressure of 2.8 MPa, a number of theoretical trays equal to 10 and the feed tray equal to the 9th from the top. Where the criticalities were detected, that is where the CO2 molar fraction resulted to be higher than about 50 ppm, the freezing temperatures were evaluated for all the liquid and vapour streams entering and exiting each tray, and they were compared with the tray temperature. The freezing check was performed using the “CO2 Freeze Out” tool35 available in Aspen HYSYS® V9.0.34 For the cases characterized by a low N2 content in the feed stream, it was also investigated if it were possible to operate the distillation column at a pressure lower than 2.8 MPa (i.e., at 2.7 MPa) without incurring the formation of dry ice. All the simulated conditions have been grouped into three different classes, according to the nitrogen content in the feed stream. Three of them have been chosen as representative of each class: 5 mol% N2 for the low-nitrogen content, 15 mol% N2 for the medium-nitrogen content and 25 mol%


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N2 for the high-nitrogen content. The corresponding temperature profiles are illustrated in Figure 6,

-90

-90

-100

-100

-110

-110 Temperature [°C]

Temperature [°C]

for the sake of clearness.

-120 -130 -140 -150

-120 -130 -140 -150

-160

-160

-170

-170

-180

-180 0

1

2

3

4

5 6 Tray Number

7

8

9

10

11

0

1

2

3

4

5 6 7 Tray Number

8

9

10

11

a)

0

1

2

3

4

5 6 7 Tray Number

8

9

10

11

b)

-90 -100 -110 Temperature [°C]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-120 -130 -140 -150 -160 -170 -180

c)

Figure 6. Temperature profiles in the simulated single-column (Figure 1) for: a) a low-N2 content; b) a medium-N2 content; c) a high-N2 content. The black line refers to the tray temperature, the green line to the freezing temperature from the liquid stream leaving each tray, and the red line to the freezing temperature from the vapour stream leaving each tray. Since the column temperature profile illustrated in Figure 6 is always above (by about 30°C) the calculated SVE and SLE temperatures, there is no risk of solid CO2 formation within the column. The vapour and liquid freezing temperatures do not intersect each other, meaning that SLVE conditions do not establish. The results of all performed simulations are summarized in Table 3 in terms of inlet and outlet streams composition, as well as of column operating pressure.



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Table 3. Summary of simulation results for different N2 contents in the feed gas for the single-column NRU configuration illustrated in Figure 1. N2 in CO2 in N2 in CO2 in N2+CO2 in Wobbe Pressure feed feed bottom bottom bottom Index [MPa] [mol%] [mol%] [mol%] [mol%] [mol%] [MJ/Sm3] 1

2.75

0.15

2.77

2.92

48.07

2.7

3

2.75

0.15

2.83

2.98

48.02

2.7

5

2.5

0.35

2.63

2.98

48.07

2.8

7

2.25

0.55

2.41

2.96

48.13

2.8

9

2

0.8

2.18

2.98

48.16

2.8

11

1.75

1.2

1.95

3.15

48.11

2.8

13

1.5

1.5

1.7

3.20

48.13

2.8

15

1.25

1.8

1.45

3.25

48.16

2.8

17

1

2.2

1.18

3.38

48.13

2.8

20

0.75

2.5

0.92

3.42

48.16

2.8

22

0.65

2.6

0.81

3.41

48.19

2.8

24

0.55

2.7

0.71

3.41

48.22

2.8

26

0.47

2.8

0.62

3.42

48.23

2.8

28

0.4

2.9

0.54

3.44

48.23

2.8

30

0.4

3

0.49

3.49

48.22

2.8

As illustrated in Figure 7, at increasing nitrogen content in the feed gas stream, the maximum inlet CO2 content that the single-column configuration can withstand has to decrease in order to avoid solidification issues. Several trends can be observed, depending on the inlet nitrogen molar fraction. Up to 3 mol% N2, the corresponding CO2 feed content keeps constant, without solidification problems. For N2 contents up to 20 mol%, a linear trend can be observed: an increase in the nitrogen content of 2% corresponds to a decrease in the CO2 molar fraction of 0.25%. Above 20 mol% N2, the trend in Figure 7 becomes flat: the maximum CO2 admissible concentration in the feed stream decreases slower at increasing N2 content, reaching a sort of asymptotic behavior at 0.4 mol% CO2. The curve in Figure 7 can be used as reference to establish how deep the CO2 removal upstream of the NRU should be if the single-column configuration is used for nitrogen rejection.


Page 17 of 37

3.0 2.5

CO2 feed content [mol%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.0 1.5 1.0 0.5 0.0 0

2

4

6

8

10 12 14 16 18 20 22 24 26 28 30 32 N2 feed content [mol%]

Figure 7. Admissible CO2 content as a function of the N2 content in the feed gas to the single-column NRU configuration illustrated in Figure 1. In Table 3, the composition of the bottom product stream is reported, in terms of N2 and CO2 molar fractions, as well as of their total content (on a molar basis). For the different feed compositions analysed, the specification on the content of N2 in the bottom product has been varied in order to ensure a minimum WI of 48 MJ/Sm3 (the value of the Wobbe Index reported in Table 3 is the one associated to a material stream as provided by the Aspen HYSYS® V9.034 process simulator). To understand the simulation results, it is important to recall the definition of the Wobbe Index (Eq. (1)). (1)

=

In Eq. (1), HCV stands for Higher Calorific Value and Crel denotes the relative density, which is given by Eq. (2). (

=

)

(2)

=

Carbon dioxide influences negatively the WI, being its density significantly high. On the contrary, nitrogen is less dense and, therefore, it does not lower the product value as much as carbon dioxide does. For this reason, where the admissible CO2 fraction in the feed stream is high, the bottom product is richer in CO2 and this reduces its WI. In this case, a lower N2 content can be accepted to meet commercial specifications. As a result, the impurity content of the bottom product, intended as the summation of CO2 and N2, mainly consists of CO2. On the contrary, as the N2 content in the feed gas increases and the maximum allowable content of CO2 decreases, the CH4 bottom product becomes not only richer in nitrogen, but also in total impurities (Table 3).



The results in Table 3 are consistent with the information available in the literature,10 which states that, according to US pipeline operators, the total concentration of inerts (N2 and CO2) is limited to a maximum value of 4 vol%, with an upper threshold of 3 vol% N2. 3.2 Double-column NRU process At first, the pressure ranges at which the double-column process can be operated have been evaluated considering the need for performing the heat exchange between the methane stream withdrawn from the bottom of the LP_Column (i.e., CH4_prod in Figure 2) and the nitrogen stream withdrawn from the top of the HP_Column (i.e., N2_HP in Figure 2). The theoretical limits for achieving the thermal coupling can be established considering the properties of pure methane and nitrogen. The critical temperature of nitrogen is -146.96°C, corresponding to a critical pressure of 3.4 MPa. Clearly, the highest temperature for the actual nitrogen condensation must be lower than the critical one, and has been set at -148°C to be conservative, with the corresponding calculated vapour pressure of 3.2 MPa. As for methane, its lowest acceptable temperature is -161.37°C, which is the boiling-point temperature at atmospheric pressure, since no vacuum is applied in the LP_Column. To take a safety margin, the minimum methane temperature has been set at -160°C. Considering a minimum temperature approach between the two streams of 4°C, the temperature ranges for the methane and nitrogen streams are, respectively, -160÷-152°C and -156÷-148°C. From these values, it is possible to determine the corresponding operating pressure ranges, that are 0.11-0.21 MPa and 2.19-3.24 MPa, respectively, for methane and nitrogen, and consequently for the LP_Column and the HP_Column. These conclusions hold for pure CH4 and N2 only, but show the principle of the heat integration via a combined reboiler/condenser. More accurate evaluations have been performed considering the actual compositions of these two streams. The outlet stream from the bottom of the LP_Column is purified methane. Therefore, the allowable compositions are those with a WI value above the minimum one of 48 MJ/Sm3, which corresponds to a methane molar fraction above 96.03 mol%. However, the preliminary pressure range considerations are performed with a higher methane product stream purity, allowing a maximum residual nitrogen content of 3 mol%. The stream exiting the condenser of the HP_Column (i.e., N2_HP) constitutes the reflux for both the HP_Column and the LP_Column and needs to be suitably rich in nitrogen to provide an acceptable reflux in order to perform the N2-CH4 separation. In this respect, compositions between 92 mol% and 100 mol% N2 are considered. For these ranges of composition and fixing an arbitrary temperature within the maximum identified admissible range, the corresponding stream pressure has been evaluated using Aspen HYSYS® V9.0.34 In addition to the temperature and the composition, a vapour 18 ACS Paragon Plus Environment

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phase fraction of 0 has been fixed for the methane product, that is at its bubble point, while a value of 1 has been assigned to the nitrogen product, being at its dew point. The calculated pressures are summarized in Figure 8: it is possible to observe that impurities of methane in the nitrogen stream from the top of the HP_Column lower its bubble-point pressure and, consequently, narrow the pressure difference between the two columns. Considering the methane-rich stream withdrawn from the bottom of the LP_Column, it is possible to observe that the P-T curves shift towards those related to the condenser at increasing nitrogen molar fractions. Therefore, the presence of a suitable temperature difference between the condensing and reboiling streams has been verified in every simulation, and results are shown in the following. 10

1

P0 [MPa]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


100% N2

0.1

99% N2 97% N2 95% N2 93% N2

0.01

92% N2 100% CH4 99% CH4 97% CH4

0.001 -190

-180

-170 -160 Temperature [°C]

-150

-140

Figure 8. VLE equilibrium pressures for the methane-rich stream withdrawn from the bottom of the LP_Column (red curves, bubble points) and for the nitrogen-rich stream withdrawn from the top of the HP_Column (blue curves, dew points) in the double-column NRU process (Figure 2). The curves differ because of the composition as specified in the legend. Several simulations have been performed varying the nitrogen molar fraction in the feed stream. Both the HP_Column and the LP_Column are influenced by the feed composition, since both the reflux, N2_inLP, and the feed stream, CH4_inLP, change in composition. However, the pressure of the LP_Column is maintained constant at 0.15 MPa regardless of the feed composition, while the HP_Column requires changes in the operating pressure (as summarized in Table 4), depending on the feed stream composition to keep the autothermal behavior and to meet purity requirements.



Table 4. Operating pressure of the HP_Column in the double-column NRU process (Figure 2) as a function of the nitrogen content in the feed gas. N2 in feed Pressure in [mol%]

HP _Column [MPa]

30

2.40

35

2.80

40

2.80

50

2.70

60

2.70

70

2.70

The range of N2 contents in the feed stream taken into account is 30-70 mol%. Indeed, no nitrogen contents above 70 mol% have been considered, since such a high nitrogen content is unlikely to occur in practice. On the contrary, molar fractions up to 70 mol% might be encountered in gases from fields in which enhanced recovery with nitrogen is performed or from pre-concentration treatments. The lower limit of 30 mol% N2 has been obtained from the simulations since, below this value, there is not enough nitrogen flowrate in the process to provide reflux to both columns and, thus, it is no longer possible to couple them and to obtain high purity products. The dependence of the purity of the HP_Column products on the nitrogen feed content is shown in Figure 9.


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100

Purity of products from HP_Column [mol%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


90

80

70

60 %CH4 in CH4_HP %N2 in N2_HP

50 30

40

50

60

70

N2 feed content [mol%]

Figure 9. Methane content (solid line) and nitrogen content (dashed line), respectively, in the bottom and top product streams of the HP_Column vs. the nitrogen molar fraction in the feed stream (doublecolumn NRU process in Figure 2). Both top and bottom product streams show a decreasing trend, but, if the former is almost flat, achieving an asymptotic profile, the latter decreases deeply. At low nitrogen contents in the feed stream, the distillate flowrate from the HP_Column is lower and, since it serves as reflux to the LP_Column, a higher purity is required to finish the separation in the latter column. The methane content in the N2-rich product and the nitrogen content in the CH4-rich product have been tuned so that the nitrogen product stream contains less than 1 mol% methane and the methane product stream satisfies the Wobbe Index requirement of 48 MJ/Sm3. In reality, the actual results show 0.5-0.47 mol% residual methane in the top product (N2_prod), and the actual methane content varying from 96.25 mol% to 98.66 mol% in the bottom product (CH4_prod), which means a WI above the minimum specified. The composition of top and bottom products, together with their flowrates and WI, is reported in Table 5. The best separation results are obtained with an inlet nitrogen concentration of 50 mol%, for which a methane-rich product with a 98.66 mol% purity is obtained and, consequently, with the highest WI.



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Table 5. Composition and flowrates of the products of the LP_Column in the double-column configuration (Figure 2) and Wobbe Index of the sales gas product as a function of the nitrogen content in the feed gas. Flowrate of Flowrate of N2 in feed N2_prod CH4_ prod WI of CH4_ prod N2_prod CH4_prod [mol%] [mol% N2] [mol% CH4] [MJ/Sm3] [kmol/h] [kmol/h] 30

99.5

96.25

48.15

1371

3630

35

99.51

97.89

49.26

1689

3312

40

99.53

97.77

49.18

1941

3059

50

99.52

98.66

49.79

2478

2522

60

99.53

97.83

49.23

3015

1985

70

99.53

96.37

48.23

3518

1483

As the nitrogen content in the feed gas stream increases, the duties required for the separation increase as well (Table 6), as also the duties available within the process itself. Therefore, the process is balanced and can be operated in an autothermal mode.

Table 6. Duties for the reboiler of the HP_Column and for the HP_Column consenser/LP_Column reboiler in the double-column NRU configuration (Figure 2) as a function of the nitrogen content in the feed gas. N2 in feed Qreb HP_Column Qcond HP_Column = Qreb LP_Column [mol%]

[kW]

[kW]

30

854

1389

35

792

1600

40

947

1578

50

1136

2147

60

1375

2462

70

1661

2765

Nevertheless, to achieve an autothermal operation, modest values of the minimum temperature approach are registered in some cases (at 30 and 35 mol% N2 in the feed gas) in LNG-101, so that it would require a very large surface. This is not true for the reboiler of the HP_Column and for the


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HP_Column condenser/LP_Column reboiler, in which the minimum temperature approaches are, respectively, 3°C and 9.5°C, as shown in Figure 10. -110 -120

Temperature [°C]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-130

3 CH4_HP

-140

N2_HP CH4_prod

-150 -160 -170 25

35

45

55

65

75


Figure 10. Temperature of the streams coupled in the reboiler of the HP_Column (stream 3 and stream CH4_HP) and in the thermally coupled HP_Column condenser/LP_Column reboiler (i.e., stream CH4_prod and N2_HP) in the double-column NRU configuration (Figure 2). The green lines in Figure 10 refer to heat exchanger E-100 virtually coupled with the HP_Column reboiler. Profiles show that the incoming pre-cooled feed stream 3 can be exploited as the heating medium in the HP_Column reboiler to vaporize the stream entering its reboiler, since a sufficient temperature approach is always maintained. As previously mentioned, the temperature of stream 2 is adjusted through the Aspen HYSYS® Adjust tool so that the duty provided by E-100 is the same as that required by the HP_Column. Consequently, the product stream Methane_Prod leaving LNG-100 is at a temperature that varies depending on the case study (however, it is always in the range 0-15°C to guarantee the adequate pre-cooling of the feed stream). On the other hand, the blue lines in Figure 10 refer to the thermally coupled HP_Column condenser/LP_Column reboiler. Also in this case the coupling is effective, being the temperature of the heating fluid N2_HP always higher than the one of CH4_prod.

3.3 Three-column NRU process Firstly, the results obtained for the application of the pre-separation column to nitrogen concentration (Figure 3, “Case 3a” in Table 2) are discussed.



The number of theoretical trays of such a column has been fixed at 10, while the feed tray has been chosen as the one that allows the best thermal coupling between the reboiler and the condenser and the heating and cooling media. The operating conditions have been obtained as a result of a trial-and-error procedure, aimed at making the process autothermal. The pressure of the pre-separation column is higher than the pressure of the high-pressure column in the double-column process and of that in the single-column process.9 Therefore, a range of pressure between 3.2 and 3.5 MPa has been investigated. As reported in Table 7, to guarantee the autothermal operation of the process, the pre-separation column works at a pressure of 3.5 MPa up to an inlet nitrogen content of 15 mol%, while for higher nitrogen contents the pressure decreases to 3.3 MPa.

Table 7. Pre-separation column (as shown in Figure 3) pressure, and bottom product composition as a function of the nitrogen content in the feed gas. N2 in feed Pressure CH4 in CH4_ pre [mol%]

[MPa]

[mol%]

5

3.5

99.0

10

3.5

98.0

15

3.5

97.5

20

3.3

97.0

25

3.3

97.0

The process is demonstrated to be autothermal in the investigated feed gas composition range, with the thermal coupling verified as shown in Figure 11.


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-85 -90 -95

Temperature [°C]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-100 F_cold

-105

CH4_from_pre V_to_double

-110

CH4_hot

-115 -120 -125 0

5

10

15

20

25

30


Figure 11. Temperature of the streams coupled in the reboiler (streams F_cold and CH4_from_pre in Figure 3) and in the condenser (streams CH4_hot and V_to_double in Figure 3) of the preseparation column used for nitrogen concentration vs. the N2 content in the feed gas. The green lines in Figure 11 refer to the pre-separation column reboiler. The trends show that the minimum temperature difference between the stream F_cold and the stream CH4_from_pre is always around 2°C, for each nitrogen content in the feed gas. On the other hand, the blue lines in Figure 11 show the temperatures of the streams involved in the column condenser. Also in this case, a minimum temperature difference of around 2°C is registered between the vapour distillate (V_to_double) and the cold stream CH4_hot. It follows that, in both cases, the thermal coupling in the process scheme is effective and thermal integration can be accomplished. Table 8 reports the pressure of the bottom product stream after depressurization (CH4_LP). As the feed gets richer in nitrogen, the nitrogen flowrate from the top of the column increases, while the methane flowrate from the bottom decreases. Since there is a smaller methane flowrate that can be depressurized, a higher pressure drop is needed across the valve, as shown in Table 8.



Table 8. Pressure of the bottom product of the pre-separation column applied to N2 concentration (Figure 3, “Case 3a” in Table 2) after depressurization for different nitrogen contents in the feed gas. N2 in feed Pressure of CH4_LP [mol%]

[MPa]

5

1.358

10

1.376

15

1.380

20

0.775

25

0.130

In the following, the results obtained for the application of the pre-separation column to CO2 removal (Figure 4, “Case 3b” in Table 2) are discussed. Such a column has 10 theoretical trays, with the feed tray fixed at the fifth one. It operates at 3.5 MPa and it has a partial reboiler and a full-reflux condenser, with a vapour stream leaving the column from the top. The performances of the preseparation section have been evaluated varying the inlet feed composition. The investigated composition range follows the one analysed in the single-column process, which starts from a very low nitrogen content and reaches 30 mol%. The CO2 content is the maximum allowable in both processes. The main constraints of the process are the avoidance of freezing conditions in the whole scheme, for which a check through the “CO2 Freeze Out” tool35 of Aspen HYSYS® V9.034 has been performed, and the achievement of a final WI of at least 48 MJ/Sm3. The dashed black line in Figure 12 provides the maximum theoretical CO2 concentration that is acceptable to achieve the desired WI and does not take into account the freezing issues that may occur. Starting from this, it is possible to define the feasible range of inlet CO2 contents for the pre-separation process, corresponding to the maximum allowable CO2 molar fraction for a given nitrogen content, in compliance with the absence of solid carbon dioxide and with the achievement of a pipeline-quality gas. This result is shown by the solid black line in Figure 12, which allows identifying a feasibility range for the whole process, corresponding to the area under the solid black line. This shows that, for low N2 contents (i.e., lower than about 25 mol%) in the feed gas, the process can be operated also in the presence of a CO2 content higher than 1.5 mol% (i.e., the value reported in the literature)9.


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3.5 3.0

CO2 feed content [mol%]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


2.5 2.0 1.5 1.0 Theoretical maximum

0.5

Three-column

0.0 0

5

10 15 20 25 N2 feed content [mol%]

30

35

Figure 12. Maximum allowable content of CO2 in the feed gas vs. the N2 content for the scheme shown in Figure 4 involving the pre-separation column upstream of the double-column. The pre-separation process scheme with the inlet feed containing carbon dioxide is not autothermal; there is no process stream that can provide enough cooling duty to the condenser, since the methanerich bottom stream contains a non-negligible fraction of CO2 and, therefore, it cannot be depressurized to provide the necessary cooling duty. However, to minimize external duty requirements, the feed is proposed to be the heating fluid in the reboiler, being the approach temperature in the different cases higher than 6°C (i.e., the minimum value that is obtained for a N2 content in the feed stream of 5 mol%), as shown in Figure 13.



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separation in the double-column, as previously discussed in section 3.2 (according to the results reported in Table 5). The methane-rich stream leaving from the bottom of the LP_Column of the double-column system is composed of almost pure methane and can be mixed with the stream separated from the bottom of the pre-separation column. As the nitrogen content in the feed gas to the pre-separation column decreases, the flowrate of the bottom stream from this tower increases, while the flowrate of the nearly pure methane stream from the bottom of the LP_Column in the downstream double-column process decreases. Therefore, when the mixing between the bottom products of the pre-separation column (CH4_from_pre in Figure 4) and of the LP_Column in the downstream double-column process (CH4_prod in Figure 2) occurs, the effect on the WI is more pronounced for the higher inlet nitrogen contents (Table 10).

Table 10. Composition of the streams resulting from the mixing of the bottom products of the preseparation column (CH4_from_pre in Figure 4) and of the double-column (CH4_prod in Figure 2) at different N2 contents in the feed gas. N2 in feed CH4 N2 CO2 Wobbe Index [mol%]

[mol%]

[mol%]

[mol%]

[MJ/Sm3]

1

97.08

0.10

2.83

48.06

3

97.02

0.10

2.88

48.00

5

97.17

0.10

2.73

48.14

10

97.00

0.46

2.54

48.07

15

96.99

0.43

2.58

48.05

20

97.28

0.48

2.24

48.32

25

97.49

0.38

2.13

48.49

30

97.97

0.32

1.71

48.91

3.4 Overall results Examining the overall graph reported in Figure 14, that summarizes the results presented in the previous sections 3.1-3.3, two areas can be identified corresponding to: low-nitrogen inlet contents (below 30 mol%), and high-nitrogen inlet contents (above 30 mol%). In order to process feed streams with a high nitrogen content, the double-column process scheme is certainly the most advantageous solution. Unfortunately, the double-column configuration can be operated with an inlet CO2 molar fraction of a few ppm, so that a deep acid gas removal is required upstream of the NRU. Nevertheless, the configuration is completely autothermal and does not need any external duty.



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Page 31 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and operating costs by reducing the upstream removal of CO2. A range of applicability for each considered configuration (i.e., the single-column, the double-column and the three-column systems) has been determined depending on the N2 and CO2 contents in the feed gas. As a result, it is possible to state that at high N2 contents (>30 mol%) the double-column process scheme is the most advantageous solution, though it requires a deep CO2 removal upstream of the NRU. On the contrary, at low N2 contents (

Cryogenic nitrogen rejection schemes: analysis of their tolerance to CO2

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