Process Intensification for greenhouse gas separation from biogas: More efficient process schemes based on membrane-integrated systems

International Journal of Greenhouse Gas Control 35 (2015) 18–29

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International Journal of Greenhouse Gas Control journal homepage: www.elsevier.com/locate/ijggc

Process Intensification for greenhouse gas separation from biogas: More efficient process schemes based on membrane-integrated systems Adele Brunetti a,∗ , Yu Sun b , Alessio Caravella c , Enrico Drioli a,c , Giuseppe Barbieri a a

Institute on Membrane Technology (ITM-CNR), National Research Council, Via Pietro Bucci, Cubo 17C, Rende, CS 87036, Italy Department of Materials Engineering, Hanyang University, Ansan-si, Gyeonggi-do 426-791, Republic of Korea c Department of Environmental and Chemical Engineering, The University of Calabria, Via Pietro Bucci, Cubo 44A, Rende, CS 87036, Italy b

a r t i c l e

i n f o

Article history: Received 27 October 2014 Received in revised form 22 December 2014 Accepted 6 January 2015 Keywords: Biogas Membranes Gas separation Metrics Process Intensification

a b s t r a c t The separation of biogas leads to not only recovery and sequestration of CO2 , but also to much greater purification and recovery of value-added CH4 able to be used, for example, to directly feed pipelines for domestic or small plants. In this work, an alternative approach for a preliminary design of separation process based on the use of polymeric membranes is proposed. Two different types of polymeric membranes were taken into account, Hyflon AD60 and Matrimid 5218, the first showing a higher permeability with respect to other membranes but a quite low selectivity (12.9), the second exhibiting a higher selectivity with respect to other membranes (41 and 100) even though a lower permeability. Four possible operation schemes using two different types of membranes in multistage configuration system are analysed as functions of the main design parameters, i.e., pressure ratio and permeation number. The achieved results are compared with certain targets and are also discussed in terms of process metrics, according to the Process Intensification Strategy. This latter analysis, coupled with a conventional one, provides an alternative point of view over the evaluation of the plant performance taking into account not only the final characteristics of the streams but also process efficiency, exploitation of raw material and energy, and the footprint occupied by the installation. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Biogas represents a versatile raw material, which can be used in a number of applications as an alternative to natural gas from fossil fuel source. Biogas is mainly composed of methane and carbon dioxide, besides traces of H2 S, NH3 , hydrogen, nitrogen, oxygen and steam (Alves et al., 2013). The concentration of each compound can vary depending on the type of biomass residual used during the anaerobic digestion process among animal waste, sewage treatment plants or industrial wastewater, landfills, etc. (Muradov and Smith, 2008). Basically, in a biogas mixture the methane concentration can vary from 55% to 70%, carbon dioxide from 30% to 45%, H2 S from 500 to 4000 ppm, NH3 from 100 to 800 ppm, whereas hydrogen, nitrogen, oxygen and steam can show percentage lower than 1 vol.% (Lau et al., 2011; Effendi et al., 2005).

∗ Corresponding author. Tel.: +39 0984 402012; fax: +39 0984 402103. E-mail address: [email protected] (A. Brunetti). http://dx.doi.org/10.1016/j.ijggc.2015.01.021 1750-5836/© 2015 Elsevier Ltd. All rights reserved.

In particular, the biogas can be used in a wide range of applications, as its chemical energy can be transformed into mechanical one through combustion processes (Poschl et al., 2010). It can be useful to co-generate thermal energy by producing hot water and steam through engines operated at a high temperature or can be burned to generate heat in boilers. It is also important as a direct fuel for automotive applications or in reforming processes to generate hydrogen to be further supplied to fuel cells (Herle et al., 2004; Papadias et al., 2012; Iulianelli et al., 2015). However, the presence of incombustible and acid gases like CO2 and H2 S strongly lower the fuel calorific value and, moreover, their corrosive nature of these gases reduces the possibility to compress and transport over long distances. In addition, the presence of fouling traces including, for examples, siloxanes can induce fouling in engines and turbines. The biogas upgrading is currently one of the most studied options in biogas treatment leading to the production of biomethane that can be directly supplied to natural gas grids. Activated carbon is the most used material for siloxanes removal, whereas H2 S can be usually removed by dry oxidation or absorption.

A. Brunetti et al. / International Journal of Greenhouse Gas Control 35 (2015) 18–29

In this work, we consider a purified stream where the traces of siloxanes and H2 S have been previously removed and the stream is mainly constituted of CH4 , CO2 , N2 and H2 O. Conventional separation techniques for CO2 removal mainly include chemical and physical absorption, adsorption and cryogenic. A detailed description of the main assets and hurdles of these technologies in biogas separation can be found in (Ryckebosch et al., 2011; Scholes et al., 2012; Basu et al., 2010). All these processes are energy-intensive, requiring large equipment, intensive use of solvents and adsorbents, complex control auxiliary systems. Therefore, they usually require the use of separation media that can potentially create pollution and CH4 deliver at a low pressure. Because of these hurdles, membrane-assisted gas separation can be efficiently applied thanks to its intrinsic advantages of low capital costs, high-energy efficiency, modularity and ease of control. Various types of membranes used in the past for natural gas separation can be considered suitable also for biogas processing. Among these, three main membrane types are commercially available: cellulose acetate, polyimides and perfluoro-polymers (Scholes et al., 2012). The choice of these materials is based not only on their gas permeation properties, but also on the assembly ease of membrane modules and on their mechanical, thermal and chemical resistance ensuring a long-term viability (Sridhar et al., 2007). Cellulose Acetate (CA) is used since 1980 for CO2 –CH4 separation and covers ca. 80% of the market for membranes for natural gas processing. However, the scarce resistance to plasticisation causing a strong depletion of membrane selectivity makes concerns arise on the use of this material for biogas processing. Moreover, the necessary pre-treatments stages can damage the membrane due to the presence of water and some hydrocarbon derivatives. As an alternative to CA, polyimides show interesting separation properties towards CO2 /CH4 mixtures along with a good thermal and chemical stability. As CA, polyimides are subjected to plasticisation. However, new studies have demonstrated that such an effect can be reduced by cross-linking and, more importantly, polyimides do not exhibit problems in the presence of humidified gaseous streams (Tasselli et al., 2015). The last class of polymers suitable for biogas treatment is represented by the perfluoro-polymers, which exhibit a great chemical, thermal and plasticisation resistance. In addition, they can be used in separations where a significant amount of water vapour is present, as they are hydrophobic. The main hurdles concern the high fabrication costs because of the expensive nature of the precursors. Beside these economic aspects, these membranes show quite high CO2 permeability and a low selectivity compared to CA and PI. As will be shown in the next sections, they can be used in a multistage cascade system to concentrate the stream to be purified in successive membrane steps. The main aspect that currently limits the development of biogas upgrading plants is related to the transportation costs of the material needed for digestion in large plants. As suggested by Scholz et al. (2013), the market should move on exploring solution for biogas upgrading for small upgrading plants (<100 Nm3 /h), where (a) membrane-integrated systems are particularly efficient and (b) the main assets of membrane technology (modularity, low plant size, etc.) become more and more important. In this logic, beside the membrane materials used for separation, it is also important to develop knowledge on how to use membrane modules and, thus, how to design an effective and efficient membrane separation system. This will allow to get the required targets in terms of stream purity and concentration, not only for CO2 for CCS purposes, but also for CH4 to be injected directly into the grid. In this work, the attention is paid to the use of both Matrimid 5218 and Hyflon membranes in multistage systems for biogas separation. The CA was not considered because of its sensitivity to water vapour, which makes it unsuitable for biogas treatment. Using

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previously developed performance maps (Brunetti et al., 2010b, 2014), the aim of this work is to analyse membrane-integrated systems in order to (a) identify suitable operating conditions and (b) propose possible process schemes that can reach the targets for the two streams of interest. To have a more comprehensive evaluation of the membrane systems proposed, the results obtained in terms of overall performance of the system are analysed using process metrics, which provide an alternative point of view in the evaluation of the plant performance. These metrics take into account not only the final characteristics of the streams but also the process efficiency, the exploitation of raw material and energy, and the footprint occupied by the installation. All aspects are nowadays essential points in the selection of an efficient separation process (Górak and Stankiewicz, 2012). 2. Methods In this work, a tool already developed elsewhere (Brunetti et al., 2010b) is used to carry out the calculations on both singleand multistage-membrane systems. The dimensionless model equations provide results in terms of maps of CO2 permeate concentration versus CO2 recovery. As for the biogas separation, in which the retentate stream is mainly composed of CH4 , general maps of CH4 retentate concentration versus CH4 recovery can be also developed. In dimensionless form, two terms can be distinguished: i and , which are the Permeation Number (Eq. (1)) and the Feed/Permeate Pressure Ratio (Eq. (2)), respectively. CO2 =

PermeanceCO2 AMembrane P Feed Feed Q Feed xCO

(1)

2

=

P Feed

(2)

P Permeate

The permeation number (Brunetti et al., 2010b) represents the comparison between the maximum permeating flow rate through the membrane (i.e., in the virtual case where the permeate pressure is zero) and the total flow rate flowing along the module. A high permeation number indicates a high membrane area and/or CO2 permeation with respect to the CO2 flowing along the module. The pressure ratio represents a measure of the separation driving force but is not the driving force itself. For a given feed composition, membrane properties (i.e., permeance and selectivity of the species involved in the separation process), module geometry (i.e., total installed membrane area and module length) and fixed operating conditions (i.e., feed flow rate and pressures), the solution of the system equations provides the profiles of dimensionless flow rate and composition of the species along the module on both membrane sides. The membrane module overall performance are calculated in terms of species concentration and total recovery in the permeate and retentate at the module outlets (Eqs. (3)–(6)). CO2 concentration in the permeate =

Permeate FCO 2

Permeate + F Permeate + F Permeate FCO N CH 2

2

× 100, %

(3)

× 100, %

(4)

4

CH4 concentration in the retentate =

Retentate FCH 4

Retentate + F Retentate + F Retentate FCO N CH 2

CO2 recovery =

2

4

Permeate F Permeate xCO 2

Feed F Feed xCO 2

× 100, %

(5)

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A. Brunetti et al. / International Journal of Greenhouse Gas Control 35 (2015) 18–29

Table 1 Typical biogas composition (Rasi et al., 2007).

Table 5 Operating conditions used in the calculations.

Species

Farm

Sewage digester

Landfill

Condition

Value

CH4 , % CO2 , % N2 , % O2 , % H2 S, ppm H2 O, % Aromatic hydrocarbons

55–58 37–38 1–3 Trace <200 4–7 Trace

61–65 34–38 1–3 Trace <100 4–7 Trace

47–57 37–41 1–17 0–2 <500 4–7 Trace

Feed composition, mol% Temperature, ◦ C Pressure ratio Permeation number

CH4 :CO2 :N2 = 60:35:5 25–35 2.5; 5; 10; 25; 50 0.5; 1; 2.5; 5; 10; 20

Table 2 Typical biogas conditions. PFeed , bar TFeed , ◦ C Feed flow rate, Nm3 /h

1.2 25–30 1000

1. Hyflon AD60X, as it shows a higher permeability with respect to other membranes (but a lower selectivity). This type of membrane is intended to be used in a one-stage separation system to concentrate the CO2 /CH4 stream. 2. Matrimid 5218, as it shows a higher selectivity with respect to other membranes (but a lower permeability). This type of membrane is intended to be used both in one- and in a two-stage separation system. In addition, the case in which the Matrimid 5218 would exhibit the same fixed permeability but a higher selectivity (=100) is also analysed.

Table 3 Typical biogas target specifications (Ryckebosch et al., 2011). Requirement

Target value

CH4 purity, % CH4 recovery, % CO2 content, % PCH4 outlet, bar

≥95 90 <2% (US target) 20–40

CH4 recovery =

Retentate F Retentate xCH 4

Feed F Feed xCH

Table 4 lists the permeation properties of the most used membranes for biogas separation (also at a scale different from the laboratory one). Based on these indications, two types of membranes are taken into account in this work:

× 100, %

(6)

4

The investigation in the whole range of operating conditions (feed pressure and flow rate) produces global maps showing all the possible solutions for the considered gas separation membrane system, which are expressed as parametric curves varying the pressure ratio and permeation number. Each point in a map corresponds to the membrane module performance for a given set of operating conditions. Therefore, the map can supply two important pieces of information: (1) the maximum performance achievable by a membrane unit for certain operating conditions, and, dually, (2) operating conditions and membrane area (or membrane type) required to produce a stream with certain target values of recovery and permeate concentration (Ryckebosch et al., 2011). Tables 1 and 2 list some examples of typical characteristics of biogas to treat coming out from different sources (Rasi et al., 2007), whereas Table 3 provides the final targets to make the purified CH4 stream suitable for a direct feed to the distribution pipeline (Favre et al., 2009). Considering the high added value of methane and the fact that the purified methane has to be pumped in the pipeline at 40–50 bar, it makes affordable to operate also with a high-pressure ratios in the membrane unit to reach the targets. Of course, the compression of the CO2 contained in the source gas requires more energy than that required for the compression of CH4 only. However, the achievements of such a CH4 -rich stream would require in any case a separation process that need itself energy duty. These aspects have to be taken into account in the analysis of an overall large plant, which, however, is not the scope of this work.

The operating conditions and membrane properties used for calculation are reported in Table 5. Most polymeric membranes exhibit a selectivity in mixture – sometimes improperly called Separation Factor, which should refer more properly to the performance of the entire module – that differs from that calculated by pure gas measures. Let us remark that, for a correct evaluation of the performance of a membrane separation unit, the mixture selectivity should be used instead of the single-gas one. In this work both the selectivity measured in pure gas and in mixture conditions (Table 4) is used. As aforementioned, the performance maps proposed in our previous work (Brunetti et al., 2010b) provide a tool for a preliminary design of a membrane unit to be used in CO2 capture from various streams at different CO2 contents. In that work, it is clearly shown that the application of membranes to flue gas separation has some limitations due to the low carbon dioxide content (typically 5–25%) and the stream low pressure. In addition, other factors like the large streams to be treated and the valueless retentate stream (mainly N2 and O2 , usually discharged in the atmosphere) do not add profit to the process, limiting the availability of extra-pressure supplied to the stream by means of compressors. Differently, in biogas separation, several positive factors like (a) the recovery of the separated product, (b) CO2 concentrated in the permeate, (c) the added value of the methane obtained already concentrated and compressed in the retentate, (d) the lower flow rates and (e) the fact that the purified methane has to be pumped in the pipeline at 40–50 bar, make it affordable to operate at high pressure ratios in the membrane unit. Fig. 1 shows the separation performance achievable by a singlestage membrane unit at different pressure ratios, considering a membrane with a not so high CO2 /i-species selectivity.

Table 4 Permeability and selectivity of some of the most used membranes for CO2 /CH4 separation (Scholes et al., 2012). Membrane type

Cellulose Acetate Matrimid 5218 Hyflon AD60X Cytop

Permeability, m2 h−1 bar−1 CO2

CH4

1.2E−08 3.1E−08 2.4E−08 39E−08 9.5E−08

0.05E−8 0.06E−8 0.06E−8 3.0E−08 0.54E−08

Selectivity (single gas or mixture)/separation factor

Temperature, ◦ C

25.4 (mixture) 49 (pure) 41 (mixture) 12.9 (mixture) 18 (pure)

35 35 35 25 25

A. Brunetti et al. / International Journal of Greenhouse Gas Control 35 (2015) 18–29

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Fig. 2. Map of CO2 permeate concentration as function of CO2 recovery.

Fig. 1. CO2 permeate concentration and CO2 recovery as a function of CO2 feed concentration at various pressure ratio.

It appears evident that the possibility of operating at a highpressure ratio allows a significant increase of the recovery in the permeate and, meanwhile, the concentration of the stream in the permeate even when the CO2 content in the feed is not so high. Hence, the potentialities that a membrane separation unit can offer in the biogas treatment appear evident. Fig. 2 provides an example of how to use the maps for convenience of the reader. The depicted map is obtained considering a general membrane with a selectivity of 50. As previously mentioned, for a set value of selectivity and a fixed CO2 concentration in the feed, a typical map is represented by a parametric curve of CO2 concentration in the permeate versus CO2 recovery expressed at fixed values of pressure ratio and permeation number. Moving over the map, it is possible to identify the pressure ratio and permeation number required to obtain a permeate stream with certain target characteristics. Once these parameters were identified, the feed and permeate pressure are defined and the permeation number can be used to calculate, for example, the membrane area needed for the membrane unit. In cases of defined dimensions of membrane module, the permeation number (Eq. (1)) can be used either to determine the value of the flow rate to treat or to identify the permeance target value. Once the map analysis is setup, four process schemes are deduced, which are basically distinguished for the membrane type

used at each stage, the operating conditions and, thus, the required membrane area to get the target performance. To evaluate the performance of the whole cascade system from a point of view of the Process Intensification Strategy, the results are analysed in terms of Process Intensification metrics: Mass Intensity, Energy Intensity and Footprint/Productivity ratio (Curzons et al., 2001; Martins et al., 2007; Brunetti et al., 2010a). These indexes show their usefulness when applied to valuable streams like CO2 and CH4 . In fact, the Mass Intensity takes into account the CO2 or CH4 recovered as a valuable product from the overall cascade system with respect to the total mass fed (Eq. (7)). Analogously, the Energy Intensity is a measure of the power required by the system (Eq. (8)) over the products recovered. Higher values of these indicators are related to an intensified process. The main energy input terms in the process is the power required for compression. The energy duty needed to heat up the membrane modules is not significant since the performance of the modules considered in this work are achieved in a temperature range of 35–50 ◦ C. This temperature range is achieved by already taking into account the Joule-Thompson effect due to the compression and, thus, any further energy duty is not required. The Footprint/Productivity ratio (Eq. (9)) calculated for membrane technology is very interesting for the Process Intensification objectives. Considering the same spatial area occupied by the units, this metric identifies the most productive scheme to get the targets. CO2 totally recovered , Total inlet mass

kgCO2 /s

CH4 totally recovered , = Total inlet mass

kgCH4 /s

Mass IntensityCO2 = Mass IntensityCH4

kg/s

(7)

kg/s

Energy IntensityCO2 =

CO2 totally recovered , Total power required

gCO2 /s

Energy IntensityCH4 =

CH4 totally recovered , Total power required

gCH4 /s

kJ/s

(8)

kJ/s

Productivity CO2 totally recovered = , FootprintCO2 Footprint

kgCO2 /h

Productivity CH4 totally recovered = , FootprintCH4 Footprint

kgCH4 /h

m2

(9)

m2

3. Results and discussion 3.1. Single-stage separation systems As aforementioned, two different commercial types of membranes are compared, the first is the Hyflon AD60X exhibiting

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A. Brunetti et al. / International Journal of Greenhouse Gas Control 35 (2015) 18–29

Fig. 3. Stage 1 – Maps of CH4 and CO2 concentration in retentate and permeate streams, respectively, as a function of correspondent recovery at various values of pressure ratio and permeation number. Membrane: Hyflon ADX60. CO2 /CH4 selectivity = 12.9.

Fig. 4. Stage 1 – Maps of CH4 and CO2 concentration in retentate and permeate streams, respectively, as a function of correspondent recovery at various values of pressure ratio and permeation number. Membrane: Matrimid 5218 (CO2 /CH4 selectivity = 41).

high permeability but low CO2 /CH4 selectivity and the Matrimid 5218 offering lower permeability but greater selectivity. The performance of a single stage separation unit is analysed using the performance maps shown in Figs. 3–5. These are drawn as double maps, where recovery and purity of both retentate (rich in CH4 ) and permeate (rich in CO2 ) are indicated. The targets for delivering CH4 as a product are (a) gas purity higher than 95% and (b) recovery higher than 90% (Table 3). Most important, since the CO2 presence not only reduces the heating value but also can increase the pipeline corrosion during transportation and distribution, its content has to be lower than 2% to be suitable feed the stream directly in the distribution pipeline. As can be seen in Fig. 3, if the Hyflon membrane is used in the single stage system, it is not possible to get both targets of purity and recovery. In particular, a purity of methane-rich stream close to 95% and with a CO2 content below 2% can be achieved with very high values of permeation number, or, in other words, high membrane area (for fixed membrane type, feed compositions and flow rate) and pressure ratio (ca. 40) but with a low CH4 recovery (ca. 30%). An analogous approach is used to analyse the performance of a single stage operation considering other two membranes, i.e., Matrimid 5218 both as it is and with higher selectivity. The details on their characteristics are reported in Table 4. Figs. 4 and 5 show the performance maps for the first gas separation stage with Matrimid 5218 with selectivity of 41 and 100, respectively. The latter is a selectivity value really achievable with opportune modifications of the membrane matrix (fluorination, etc.

(Zhang et al., 2013)). Also with these membranes, the targets for delivering CH4 as a product cannot be reached by a single stage. With the Matrimid 5218 as it is, the maximum CH4 purity achievable by a single stage is 90% with also a relative interesting values of recovery. However, the purity target of CO2 in the permeate could be obtained only with very low recovery values (less than 15%). Similarly, the use of a Matrimid 5218 membrane with improved selectivity (Fig. 5) does not allow the achievement of a high-purity CH4 -rich stream. However, the CO2 permeate stream can get the targets for storage with interesting values of recovery (ca. 80%) for a pressure ratio of 40 and a permeation number of 10. Based on the aforementioned results, making a selection among the three different types of membranes considered, the Matrimid 5218 membranes with improved values of selectivity can be used in a single-stage operation when the separation goal is to get the targets for CO2 capture and, at the same time, the purity target for the retentate stream is not so high has. Hyflon membranes can be used instead for concentrating the retentate and permeate streams. 3.2. Double-stage separation schemes Fig. 6 shows a possible scheme operating for treatment and purification of the CH4 rich-stream. As aforementioned, the first stage consists of Hyflon membranes to concentrate retentate and permeate streams, which will be then purified in the successive stage, constituted, instead, by the high-selective Matrimid 5218 membrane in order to obtain highly pure streams.

A. Brunetti et al. / International Journal of Greenhouse Gas Control 35 (2015) 18–29

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Table 6 Retentate and permeate streams characteristics at the first stage. Permeation number = 5; Pressure ratio = 40. Retentate

Concentration, % Recovery, %

Fig. 5. Stage 1 – Maps of CH4 and CO2 concentration in retentate and permeate streams, respectively, as a function of correspondent recovery at various values of pressure ratio and permeation number. Membrane: Matrimid 5218 (CO2 /CH4 selectivity = 100).

Since it is realistic to operate at high feed pressures on the first stage, such a pressure can be exploited as well in the second stage foreseen for the treatment of the retentate. For this reason, the pressure ratio in the second stage is assumed to be 40. A fibre

Permeate

CH4

CO2

CH4

CO2

86.41 83.1

7.15 –

23.94 –

73.02 88.2

membrane module with the following characteristics has been considered in the calculations: OD fibres = 1 mm, fibre length = 50 cm, Module external diameter = 8 cm and fibre packing density of 80%. With these characteristics, the pressure drop along the first stage is below 1% and, therefore, even considering the pressure drops along the tube lines, etc., they can be assumed as negligible in any case. The addition of the second separation stage allows the selection of a permeation number at the first stage, which would imply a good compromise between the retentate and permeate stream characteristics. This in the logic of adding also a third stage for the treatment of the permeate. A high recovery of the reference species and a concentration sufficiently high to assure a satisfactory permeation driving force in the other membrane stage are well desired for both retentate and permeate stream. Consequently, a permeation number equal to 5 is chosen for the first stage (Fig. 7) along with a pressure ratio of 40, which correspond to the streams characteristics reported in Table 6. Both streams offer interesting values of purity and recovery for CH4 and CO2 . Therefore, considering these conditions as a starting point to analyse the performance of the multistage configuration, Fig. 7 shows the strategy to be used in choosing the operating conditions for the other membrane separation stages. The blue curves represent the performance of the first stage for a pressure ratio of 40. The red performance curve refers to the second stage for the treatment of the retentate stream at the same pressure ratio. From the blue curve, for a permeation number of 5, two arrows identify two possible performance targets in the second stage. Each target is characterised by a certain value of permeation number. Since the retentate stream coming out from the first membrane stage already exhibits high CH4 molar fractions, the treatment of this stream in the second stage with a high selective membrane towards CO2 further improves the stream purity, getting values close and even higher than 90%, also at low values of permeation number.

Fig. 6. Double stage module configuration.

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A. Brunetti et al. / International Journal of Greenhouse Gas Control 35 (2015) 18–29 Table 7 Characteristics of retentate and permeate streams at second stage.

 

Case 1

Case 2

40 100

40 50

Retentate CH4 purity, % CH4 recovery, % CO2 Content, %

93.5 81.69 0.8

92.4 91.0 1.3

Permeate CO2 purity, % CO2 recovery, %

26.8 92.1

40.4 84.2

promoted and, thus, CO2 concentration in the permeate decreases, as well as it does the CH4 recovery in the retentate. 3.3. Multi-stage separation scheme

Fig. 7. Stage 1 and Stage 2: Maps of CH4 and CO2 concentration in retentate and permeate streams, respectively, as a function of correspondent recovery at various values of permeation number. Pressure ratio = 40.

It has to be pointed out that low values of permeation number imply low membrane area, which represent a favourable condition in terms of footprint occupied by the installation, as well as in terms of costs. However, even looking at the retentate stream characteristics coming out from the second stage (red curve), the choice of a permeation number of 10–20 appears plausible. It assures high values of CH4 recovery and the variation in terms of purity that can be achieved at higher values of theta is not significant. On the other hand, the choice of the permeation number has to take into account also the characteristics of the permeate stream in terms of CO2 concentration and purity. To this reasoning, low values of theta necessarily implies low CO2 recovery but higher molar concentration. Also in this case, the choice of the operating conditions for the second stage was done seeking for a compromise between the retentate and permeate characteristics. The two options selected consider a Permeation Number of 100 and 50, respectively. Lower values of permeation number would imply low CO2 recoveries in the permeate and a lower CH4 concentration on retentate side, whereas higher theta would tend to lower CH4 recoveries and, thus, a significant reduction of CO2 concentration in the permeate. The retentate and permeate characteristics obtained in these conditions are summarised in Table 7. The main differences of these two cases are in the characteristics of the permeate stream and in the CH4 recovery. A high permeation number, that in other terms means, e.g., a larger membrane area, indicates firstly a greater permeation of CO2 and, thus, higher recovery. However, also the permeation of the less permeable species is

Looking at the results achieved with a two-stages configuration, it arises the need to further treat the permeate streams from the two stages to recover as much as possible the exiting methane, increasing in the meantime the CO2 concentration. Indeed, a third stage is added where the permeate streams of first and second stages are fed after mixing. As for the other separation stages, also this one is operated at a pressure ratio of 40. The choice is done analogously to the other membrane stages, with the logic to feed the recovered methane directly into the grid, where the pressure range is between 30 and 50 bar. As already done for the other two membrane stages, also in this case the performance maps are generated to identify the performance of the unit as a function of the permeation number (with a set pressure ratio of 40) (Fig. 8). The feed stream of the third stage consists of a flow rate corresponding to the sum of the permeate flow rates of first and second stage and a composition coming out from the mixing of the two streams. Consequently, changing the operating parameters of the second stage, also the characteristics of the feed stream of the third stage are affected. Indeed, in Fig. 8, each map shows two curves, each one corresponding to the case of operation at the stage II (Table 7). The Case 1 corresponds to high concentration of CH4 in the retentate (93.5%) but low CO2 concentration in permeate (26.8%). In the Case 2, the CO2 concentration in permeate was higher (40.4%) than that in the Case 1. As a result, the performances at the stage III show a greater purity of the methane-rich stream operating in the conditions of Case I and a greater purity of CO2 permeate stream when the conditions of Case II are considered. From the map in Fig. 8, four points corresponding to four different performance values are chosen. This choice is done based on the recovery and purity CH4 and CO2 , particularly focusing on the targets required for both streams. Table 8 summarises the outlet conditions at each stage, depending on the permeation number chosen for each one, in terms of purity and recovery of CH4 and CO2 in retentate and permeate stream, respectively. The same results are schematised in Figs. 9–12. First of all, it appears evident from the analysis of the results that it is not possible to reach at the same time the target of purity and that of the recovery for CO2 and CH4 in any of the schemes considered. Even though the molar fraction of CO2 in retentate stream is lower than 2% in all schemes except that in the Scheme 3, nevertheless the target purity required for CH4 stream can be achieved only under conditions of the Scheme 2, although a recovery of ca. 70% is obtained.

>90 >80 73.7 97.1 95.9 92.4

73.0 88.2 40.4 84.2 73.0 88.2

40.4 84.2

>95 (CO2 <2%) >90

26.8 92.1 73.0 88.2

67.8 98.2

93.5 81.7 86.4 83.1

For what concerns CO2 , the stream targets are well fitted by both Scheme 1 and Scheme 3. In particular, the first provides a higher recovery (97.5%) with a purity close to target, whereas the Scheme 3 allows achieving a very pure stream (95.9%) with a slightly lower recovery. It has to be highlighted that the study of the performance of the whole system is not in the aim of this work. However, in the study of the whole system an alternative use for methane contained in the CO2 stream like, for example, burning for energy production can be foreseen.

73.0 88.2 CO2 purity, % CO2 recovery, %

26.8 92.1

90.5 97.2

3.4. Process Intensification metrics

86.4 83.1

93.5 81.7

94.1 (1.3% CO2 ) 89.7

89.4 (3.6% CO2 ) 92.8

86.4 83.1 92.4 91.0 86.4 83.1 95.1 (0.77% CO2 ) 70.1

Stage III Stage II Stage II Stage I Stage III Stage II Stage I

Scheme 1

25

Fig. 8. Stage 3 – Maps of CH4 and CO2 concentration in retentate and permeate streams, respectively, as a function of correspondent recovery at various values of permeation number. Pressure ratio = 40.

Outlet conditions CH4 purity, % CH4 recovery, %

Scheme 2 100 5 Permeation number

92.5 (1.4% CO2 ) 77.1

Stage III Stage I Stage I Stage III

Stage II Scheme 4 Scheme 3

5 20 100 5 5

Stage II Stage I

Scheme 2

Stage III Stage II Stage I

Scheme 1

Table 8 Performances of membrane stages for the different process schemes.

92.4 91.0

Targets

15 50

Stage II Stage I

5 2.5

Scheme 4

Stage III Stage II Stage I Stage III

Scheme 3

50

Stage III

A. Brunetti et al. / International Journal of Greenhouse Gas Control 35 (2015) 18–29

Quantities like resources exploitation, emission, effluents and waste related to the production, are a measure of the progress towards sustainability. Therefore, new environmental indicators of the industrial process are defined along with more traditional performance analysis indexes. Most of these indicators (or metrics), are calculated in the form of appropriate ratios, which can provide a measure of the environmental impact independently of the scale of the operation, or a measure of costs against benefits. In some cases, they can allow the comparison between different operations. Recently, new metrics have been used for membrane operations taking into account size, weight and yield of the plants. Among these, the so-defined Mass Intensity, Energy Intensity and Productivity/Footprint – all desired as higher as possible – are chosen to be used in this work (Eqs. (7)–(9)). Since both CH4 and CO2 can be considered as products of interest, the three metrics are defined

26

A. Brunetti et al. / International Journal of Greenhouse Gas Control 35 (2015) 18–29

Fig. 9. Three stages modules configuration (Scheme 1).

Fig. 10. Three stages modules configuration (Scheme 2).

assuming both gases as references, respectively, and are evaluated considering the whole system. As can be seen in Table 9, no significant differences can be observed for the Mass and Energy Intensities of the four schemes. This means that the exploitation of energy and mass of the various systems with respect to the final recovery of CH4 and CO2 is comparable among the various schemes. The difference is much

more relevant in terms of Productivity/Footprint. As aforementioned, such index indicates the productivity of the system over the footprint occupied by the installation. A high value of the index means an intensified/compact process with a high productivity in a small installation. For both the products of interest, the Scheme 3 is found to be the most intensified, showing a Productivity/Footprint

Fig. 11. Three stages modules configuration (Scheme 3).

A. Brunetti et al. / International Journal of Greenhouse Gas Control 35 (2015) 18–29

27

Fig. 12. Three stages modules configuration (Scheme 4).

index significantly higher than those achieved in the other schemes. This means that the area of installation occupied by the membrane units to achieve a set productivity is significantly lower than the membrane units required by the other scheme solutions.

Summarising the results in terms of both targets achievement for the two streams and metrics, it can be concluded that: 1. The Scheme 3 allows getting the CO2 target requirements, even though it is far from the purity target for CH4 stream and the CO2

Fig. 13. Summary of performance achieved by the four membrane systems with respect to targets.

28

A. Brunetti et al. / International Journal of Greenhouse Gas Control 35 (2015) 18–29

Table 9 Process Intensification metrics for the four schemes. Scheme 1

Process Intensification metrics Mass intensity, kg kg−1 Energy intensity, g kJ−1 Productivity/footprint, kg m−2 h−1

Scheme 2

Scheme 3

Scheme 4

CO2

CH4

CO2

CH4

CO2

CH4

CO2

CH4

0.55 0.77 0.37

0.34 0.48 0.60

0.55 0.78 0.19

0.34 0.38 0.09

0.52 0.74 1.55

0.36 0.50 2.28

0.52 0.74 0.38

0.30 0.42 0.21

content exceeds the limit for distribution in pipeline. Indeed, it is found to be the most intensified process, showing the highest productivity/footprint metrics. 2. The Scheme 1 represents a good compromise, as it allows the achievements of the target characteristics for CO2 and, meanwhile, the CO2 content in CH4 rich stream is below the limit. Moreover, the recovery and purity are really close to targets and this scheme shows the best PI indexes among the Schemes 1, 2 and 4. Further work on the optimisation of the operating conditions could lead to the fitting of all the targets. 3. The Scheme 2 allows the achievements of the own target purity of CH4 , whereas the CH4 recovery is significantly below the target of 90%. Similarly, the CO2 stream fits the targets of recovery but the purity is not enough. 4. The Scheme 4 gets only the target recovery of CO2 , but the achieved purity is quite low. The CH4 stream characteristics are far from the targets in terms of both purity and recovery, even though the CO2 content is below the pipeline limit. In terms of metrics, it is the worst among the four cases considered (Fig. 13). 4. Conclusions In this work, an alternative approach for a preliminary design of a membrane-integrated biogas separation system was proposed. The analysis was performed by means of performance maps aiming at providing a tool for the design of the separation process based on the use of some types of polymeric membranes, defining operating conditions and performance limits of each configuration. Four possible schemes of operation were investigated considering Hyflon ADX60 and Matrimid 5218 membranes, respectively. The differences among the schemes were mainly based on the permeation number, which can be intended, e.g., as a measure of the membrane area required to carry out the separation and of the operating pressure ratio. The latter parameter was increased up to 40 for each stage, considering the high added value of the methane stream and the fact that the purified methane can be pumped into the pipeline at 40–50 bar. Both single-stage and multi-stage configuration systems were analysed as functions of the main design parameters, i.e., the pressure ratio and the permeation number. The Hyflon membranes were not found to be suitable for a single stage configuration system, as it was not possible to reach the targets of purity and recovery either for permeate or for retentate stream even for high values of the pressure ratio (40). On the contrary, the Matrimid 5218 membranes were found to fit the targets for CO2 storage with interesting values of recovery ca. 80% for a pressure ratio of 40 and a permeation number of 10, even though such membranes did not allow the achievement of a sufficiently pure CH4 -rich stream. Differently, the three-stage solution was found to be the most suitable to achieve the targets for both streams. In particular, the Scheme 1 allowed the achievements of the target characteristics for CO2 and, at the same time, the CO2 content in the CH4 -rich stream was below the limit. For this scheme, recovery and purity were really close to targets. As a complementary support, the achieved results were analysed also in terms of

process metrics, i.e., Mass Intensity, Energy Intensity and Productivity/Footprint Ratio, used for both streams. No significant differences were observed in the Mass and Energy Intensity indexes for the four schemes considered, this indicating that the exploitation of energy and mass of the various systems with respect to the final recovery of CH4 and CO2 was comparable among them. It was shown that the main difference among the four process schemes regards the Productivity/Footprint Ratio, which was found to be significantly higher for the Scheme 3. This means that the area of installation occupied by the membrane units for achieving a set productivity was significantly lower than those required by the other schemes. This result pushes the attention to the different and additional indications provided by this type of analysis which is not always in agreement with the conventional one but can provide important indications useful to provide guidelines to choose the most convenient solution based on the required process target.

List of symbols molar fraction x Q flow rate, m3 (STP) h−1 z membrane module axial coordinate, m membrane module length, m L Permeance membrane permeance, m3 (STP) m−2 h−1 bar−1 AMembrane membrane area, m2 P pressure, bar

Superscripts feed phase referred to Feed Retentate retentate phase referred to Permeate permeate phase referred to

Greek letters  pressure ratio membrane selectivity ˛ ϕ dimensionless flow rate permeation number   dimensionless axial coordinate

Acknowledgements The “Ministero degli Affari Esteri e della Cooperazione Internazionale, Direzione Generale per la Promozione e la Cooperazione Culturale” of Italy is gratefully acknowledged for the financial support of project “New highly innovative membrane operations for CO2 separation (capture) at a medium and high temperature: Experimental preparation and characterization, theoretical study on elementary transport mechanisms and separation design” cofunded in the framework of a bilateral agreement between MAE (Italy) and MOST (South Korea). Alessio CARAVELLA acknowledges the “Programma Per Giovani Ricercatori ‘Rita Levi Montalcini”’ funded by the “Ministero dell’Istruzione, dell’Università e della Ricerca, MIUR”.

A. Brunetti et al. / International Journal of Greenhouse Gas Control 35 (2015) 18–29

29

Appendix A.

References

A dimensionless 1D mathematical model for the multi-species steady-state permeation in no sweep mode and co-current configuration was used for the calculations. In the case of ternary mixtures (CO2 –CH4 ), the model consists of a system of three ordinary differentials (for the retentate side) and three algebraic (for the permeate side) equations (A.1)–(A.6) Feed/retentate side

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Retentate dϕCO 2

d Retentate dϕCH 4

d

=−

=−

1 Retentate Permeate − xCO ) CO2 ( xCO 2 2  Feed xCO

1 1 2 Retentate Permeate CO2 ( xCH − xCH ) Feed ˛ 4 4 xCH CO2 /CH4  4

Retentate dϕO 2

d

=−

(A.1)

(A.2)

Feed xCO 2

1 1 Retentate Permeate CO2 ( xO − xO ) Feed ˛ 2 2 xO CO2 /O2 

(A.3)

2

Permeate side Permeate Feed Retentate ϕCO () = ϕCO − ϕCO ()

(A.4)

Permeate Feed Retentate ϕCH () = ϕCH − ϕCH ()

(A.5)

Permeate Feed Retentate ϕO () = ϕO − ϕO ()

(A.6)

2

2

4

2

4

2

4

2

2

In the equations, ϕCO2 , ϕCH4 and ϕO2 are the dimensionless molar flow rates for CO2 , CH4 and O2 , respectively, and ␨ is the dimensionless module length. ϕi = =

Qi

(A.7)

QiFeed z L

(A.8)

i and  are parameters affecting the performance of a onestage membrane system, the permeation number and the feed to permeate pressures ratio, respectively. CO2 =

PermeanceCO2 AMembrane P Feed Feed Q Feed xCO

(A.9)

2

=

P Feed P Permeate

(A.10)