urea

Applied Energy xxx (2017) xxx–xxx

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Applied Energy journal homepage: www.elsevier.com/locate/apenergy

Modeling, simulation and evaluation of biogas upgrading using aqueous choline chloride/urea Chunyan Ma a,b, Yujiao Xie b, Xiaoyan Ji b,⇑, Chang Liu a,⇑, Xiaohua Lu a a b

College of Chemical Engineering, Nanjing Tech University, Nanjing 210009, China Energy Engineering, Division of Energy Science, Luleå University of Technology, Luleå 97187, Sweden

h i g h l i g h t s
Thermophysical properties of dry/aqueous ChCl/Urea were surveyed and evaluated.
The database of ChCl/Urea and its aqueous were implemented into Aspen Plus.
The biogas upgrading using aqueous ChCl/Urea was simulated and evaluated.
Performances of aqueous ChCl/Urea scrubbing were compared with water scrubbing.

a r t i c l e

i n f o

Article history: Received 7 November 2016 Received in revised form 12 February 2017 Accepted 10 March 2017 Available online xxxx Keywords: Aqueous choline chloride/urea Thermodynamic modeling Process simulation Biogas upgrading

a b s t r a c t Biogas has been considered as an alternative renewable energy, and raw biogas needs to be upgraded in order to be used as vehicle fuels or injected into the natural gas grid. In this work, the conceptual process for biogas upgrading using aqueous choline chloride (ChCl)/urea (1:2 on a molar basis) was developed, simulated and evaluated based on the commercialized software Aspen Plus. Reliable thermophysical properties and phase equilibria are prerequisite for carrying out process simulation. In order to carry out the process simulation, the thermophysical properties of ChCl/Urea (1:2) and its aqueous solutions as well as the phase equilibria of gas-ChCl/Urea (1:2), ChCl/Urea (1:2)-H2O and gas-ChCl/Urea (1:2)-H2O were surveyed and evaluated. After evaluation, the consistent experimental data of these thermophysical properties were fitted to the models embedded in Aspen Plus. The properties needed but without available experimental results were predicted theoretically. The Non-Random Two-Liquid model and the Redlich-Kwong equation (NRTL-RK) model were used to describe the phase equilibria. The equilibrium approach was used for process simulation. Sensitivity analysis was conducted to determine the reasonable operating parameters. With a set of reasonable operating conditions, the effects of ChCl/Urea (1:2) content on the total energy utilization, the diameters and pressure drops of absorber and desorber as well as the environmental assessment of the process were studied. The simulation results showed that, with the addition of ChCl/Urea (1:2), the total energy utilization decreased by 16% compared to the process with pure water, and the diameters of both absorber and desorber decreased with increasing content of ChCl/Urea (1:2). The process using aqueous ChCl/Urea (1:2) was more environmentally benign than that with pure water. Therefore, aqueous ChCl/Urea (1:2) is a promising solvent for biogas upgrading. Ó 2017 Published by Elsevier Ltd.

1. Introduction Biogas produced by anaerobic digestion of biological wastes has been considered as an alternative renewable energy resource [1–4]. Generally, biogas contains 53–70% (by volume) methane (CH4), 30–47% carbon dioxide (CO2), small amounts of hydrogen

⇑ Corresponding authors.

sulfide (H2S, 0–10,000 ppm) and other compounds [5]. CO2, as the main impurity of biogas, needs to be removed for producing what is commonly called ‘‘biomethane” which can substitute natural gas as vehicle feeding or inject into natural gas grid [4,6,7]. Technologies have been developed and commercialized for biogas upgrading (i.e. CO2 removal from raw biogas), for example, high pressure water scrubbing (HPWS), pressure swing adsorption (PSA), chemical absorption (CA), organic physical scrubbing (OPS), membrane separation (MB), cryogenic separation (CS), and so on

E-mail addresses: [email protected] (X. Ji), [email protected] (C. Liu). http://dx.doi.org/10.1016/j.apenergy.2017.03.059 0306-2619/Ó 2017 Published by Elsevier Ltd.

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C. Ma et al. / Applied Energy xxx (2017) xxx–xxx

Nomenclature Abbreviations Tb normal boiling point, K critical temperature, K Tc pc critical pressure, MPa Vc critical volume, m3kmol1 V1 infinite dilution partial volume of gas i, m3kmol1 i BO Vi characteristic volume for gas i, m3kmol1 BO Vs characteristic volume for solvent s, m3kmol1 Zc critical compression factor M molecular weight, gmol1 w acentric factor, dimensionless l viscosity, Pas r surface tension, kgs2 q density, kgm3 g acceleration due to gravity, ms2 R gas constant, 8.314 Jmol1K1 S selectivity X solubility of gas in solvent, molmol1 STP standard temperature and pressure pabsorber pressure of absorber, MPa pdesorber pressure of desorber, MPa pressure of flash, MPa pflash Tabsorber temperature of absorber, K Tdesorber temperature of desorber, K Dp/z pressure drop, Pam1 L/G the ratio of the volumetric flow rate of the absorbent to that of raw biogas, m3m3 AFR/G the ratio of the volumetric flow rate of the airflow to that of raw biogas, m3m3 RECO2 the CO2 removal efficiency

[5–10]. Among them, absorption is one of the most widely used technologies, for example, HPWS, OPS and CA are shared 40, 6 and 25%, respectively, of all biomethane plants in the European region [3,6,11]. According to the information published by IEA Bioenergy Task 37, HPWS is one of the simplest, most-efficient and widely-used upgrading techniques [3,6,11]. HPWS has the advantage of low CH4 loss, and no pretreatment of H2S is required. However, the low CO2 solubility and the low mass-transfer rate (e.g. mol CO2 (mol H2O)1s1) in water lead to a large amount of solvent required in the process. Besides, the bacterial growth is easily formed on the packing materials [12,13]. Other absorption technologies including OPS and CA are used in commercial CO2 capture processes due to high absorption capacity and rate. The drawbacks of CA include the high thermal energy demand for solvent regeneration, the corrosion to equipment as well as the significant solvent degradation and loss [13]. Although the thermal energy demand of OPS is lower than CA, the solubility of CH4 in organic physical solvent is comparatively high, leading to a high loss rate of CH4 [14]. Besides, the volatility of OPS results in solvent loss and air pollution. Since Blanchard et al. [15] firstly reported the CO2 solubility in a physical ionic liquids (ILs), ILs have been paid more attention in the field of CO2 separation due to the favorable properties of nonvolatility, thermal stability and high acid gas solubility [16]. These unique properties of ILs provide the feasibility to reduce the thermal energy demand for solvent regeneration and to avoid environmental pollutions [17]. Process simulation and assessment have been carried out for the CO2 capture with ILs as liquid absorbents based on process simulation software such as Aspen Plus. For example, Shiflett et al. [18] simulated a CO2 capture process using pure IL and claimed 16% reduction in energy utilization and 11%

gCO2 Nab YCH4,PG YCH4,SG W Q ECH4 loss SE D GD DGDP

the CO2 regeneration degree of the absorbent the number of theoretical stages of the absorber CH4 volumetric concentration in the stream left from absorber CH4 volumetric concentration in the stream left from desorber electrical power heat duty the energy utilization related to the CH4 loss the specific energy utilizations per m3 gas diameter, m green degree, gdkg1 or gdkJ1 green degree of the process, gd

Superscript Material-Out materials out of the process Material-In materials into the process Energy-In energy into the process Subscript i j w th e RG PG ab de

component i, dimensionless component j, dimensionless component of water, dimensionless thermal duty electricity raw biogas upgraded biogas absorber desorber

reduction in investment compared to aqueous amine scrubbing. Basha et al. [19,20] developed a conceptual process for selective capture of CO2 from fuel gas streams using different ILs. Huang et al. [21] simulated the CO2 capture process using IL-amine hybrid solvents and concluded that IL-based solvent combined with process modification could realize an energy-efficient and costeffective carbon capture. However, the high production cost, the high viscosity and the potential toxicity for most of the synthesized ILs limit their industrial applications. Recently, deep eutectic solvents (DESs) have received much more attention and been considered as a new type of ILs [22–26]. DESs maintain most of the favorable properties of ILs but avoid the economic and environmental problems [24]. Among the synthesized DESs, choline-based ILs (or DESs) are considered as a type of most promising solvents to achieve large-scale applications because of their low production cost, low toxicity, biodegradability and easy synthesis. In particular, choline chloride/urea (ChCl/Urea) is made of urea (a common fertilizer) and choline chloride (a vitamin B4 precursor), and both of them are relatively benign in terms of bio-toxicity. Moreover, the price of ChCl/ Urea is relatively cheap, which is only 4–9% of the conventional ILs [27]. Research has been carried out to measure the CO2 solubility in ChCl/Urea [28–30], revealing ChCl/Urea a promising solvent for CO2 absorbing. However, the viscosity of ChCl/Urea is very high. It has been reported that the addition of water can significantly decrease the viscosity of ChCl/Urea [31–33], but the CO2 solubility also decreases. The low viscosity of the aqueous ChCl/Urea will enhance the mass transfer of CO2 and decrease the pumping costs due to the reducing of friction losses [34], but the decrease of CO2 solubility (capacity) will lead to an increase of the amount

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C. Ma et al. / Applied Energy xxx (2017) xxx–xxx

circulated solvent and a large absorption tower. Therefore, how the addition of water will affect the whole performance of the process is still unclear. In addition, similar to HPWS, using aqueous ChCl/ Urea for biogas-upgrading, the solvent can be regenerated easily by bubbling air, and no thermal energy is needed. Therefore, it will be highly interesting to compare these two techniques. The objective of this work was to investigate the performance of biogas upgrading using aqueous ChCl/Urea as liquid absorbents based on process simulation with Aspen Plus. Thermophysical properties and phase equilibria are crucial in conducting process simulation reliably. However, as a new absorbent, ChCl/Urea has not been studied systematically for the sake of process simulation. Therefore, the thermophysical properties of ChCl/Urea measured experimentally were surveyed, evaluated and then fitted with semi-empirical equations, and the experimental results of vaporliquid equilibria were modeled using the Non-Random TwoLiquid model and the Redlich-Kwong equation (NRTL-RK). All the fitted parameters were further implemented into Aspen Plus, making it ready for process simulation based on the equilibrium approach. After that, process simulation was conducted in order to evaluate the process performance. Briefly, the process simulation was validated based on HPWS, and then the process simulation with aqueous ChCl/Urea as CO2 absorbent was conducted. The sensitivity analysis was performed to determine a set of reasonable parameters, and the effect of ChCl/Urea content on the total energy utilization, the diameters and pressure-drops of absorber and desorber as well as the green degree of the process was studied. The performance of aqueous ChCl/Urea for biogas upgrading was further compared with HPWS. 2. Thermophysical properties and phase equilibria 2.1. Properties of pure components In the process of biogas upgrading with aqueous ChCl/Urea, the components involved can be CO2, CH4, H2O, ChCl/Urea, N2, O2 and H2S. Assuming that H2S was removed before upgrading, the content of H2S can be ignored in the simulation. The thermophysical properties of the components other than ChCl/Urea (i.e. CO2, CH4, H2O, N2, and O2) were calculated using the parameters from the property databank in Aspen Plus without further study. ChCl/Urea can be obtained by mixing various molar ratios of choline chloride to urea. Among them, the ChCl/Urea with 1:2 on a molar basis showed the highest CO2 solubility [28]. Therefore, ChCl/Urea used in this work was all based on the molar ratio of 1:2 (choline chloride: urea). For ChCl/Urea (1:2), the thermophysical properties measured experimentally were surveyed, evaluated and then fitted with semi-empirical equations. The parameter fitting was performed within Aspen Plus. For the properties that have not been determined reliably or could not be determined experimentally, theoretical estimation was conducted to satisfy the simulation requirement of Aspen Plus. The detail information was described in this section. 2.1.1. Critical properties of ChCl/Urea (1:2) The critical properties of pure components are required as one of inputs in the process simulation with Aspen Plus. For ILs, Valderrama et al. [35,36] developed an extended group contribution method to determine the critical properties, normal boiling temperatures and acentric factors of ILs, and it was stated that the estimated acentric factors were accurate enough for engineering calculations. For DESs, Emad et al. [37] estimated the critical properties of 17 kinds of DESs including choline chloride-based DESs based on the group contribution methods. Mirza et al. [38] estimated the critical properties of 39 different DESs including

3

ChCl/Urea (1:2) by a combination of the modified LydersenJoback-Reid method with the Lee-Kesler mixing rules. It was found that there was only a slight difference when different parameters were used in the extended group contribution method. Therefore, the critical properties of ChCl/Urea (1:2) used in this work were directly taken from the Ref. [38] as listed in Table 1. 2.1.2. Temperature-dependent properties of ChCl/Urea (1:2) The temperature-dependent properties such as density, viscosity, surface tension and heat capacity are needed in the process simulation. These properties have been measured experimentally and all the available sources of the experimental data for ChCl/Urea (1:2) at different temperatures were collected as listed in Table 2. The comparisons of the available experimental data in Fig. 1(a) showed that most of the experimental densities of ChCl/Urea (1:2) from different sources [32,33,39–41] were consistent with each other except the data measured by Mohsenzadeh et al. [42], and then the experimental data from [42] was excluded in the further investigation. For viscosity, as we can see from Fig. 1(b), the experimental data points from Mohsenzadeh et al. [42] and Xie et al. [32] were not consistent with other sources when the temperature was lower than 318.15 K, and they were not used in further data fitting. For the heat capacity, the experimental results from Leron et al. [43] and Chemat et al. [39] were consistent with each other. All the experimental data points from these two sources were used in the further data fitting. After the evaluation, the consistent experimental data of density, viscosity and heat capacity was fitted to the equations embedded in Aspen Plus, and all the fitting was conducted within Aspen Plus. The equations and parameters fitted in this work are listed in Table 3. For the density of ChCl/Urea (1:2), the liquid molar volume was first converted from the density, and then the IK-CAPE equation (Eq. (1) in Table 3) was used to fit the liquid molar volumes. The average relative deviation (ARD) was 0.086% in fitting for the liquid molar volume. To further illustrate the fitting accuracy, the model results were compared with the experimental data as shown in Fig. 1(a). When the temperature was lower than 348.15 K, the experimental data agreed well with each other, and the model results were consistent with the experimental data. While when the temperature was higher than 348.15 K, the experimental data showed discrepancies at 353.15 K, and the model results were within the experimental uncertainties. However, with increasing temperature, only one group of data measured by Leron et al. [40] was available, and this group of data was already lower than others at 353.15 K and obviously lower than the model results when the temperature was higher than 353.15 K. Thus, the accuracy of this group of data might be questionable. For the viscosity and heat capacity of pure ChCl/Urea (1:2), the DIPPR equations (Eqs. (2) and (3) in Table 3, respectively) were used. The fitting results of viscosity and heat capacity for ChCl/Urea (1:2) agreed well with the data points used in fitting with ARDs of 3.1% and 0.2%, respectively. The further comparison with the available experimental data was also illustrated in Fig. 1. For the surface tension of ChCl/Urea (1:2), Arnold et al. [46] reported that the surface tension of pure ChCl/Urea (1:2) was 66 ± 1 Nmm1 without reporting the temperature. The surface tension of pure ChCl/Urea (1:2) reported by D’Agostino et al. [44] and Abbott et al. [45] was 52 Nmm1 but at different temperatures (298.15 and 313.15 K). Ji et al. [30] also reported that the surface tension of ChCl/Urea (1:2) with 0.52 wt.% of water was 74.43 Nmm1, which was much higher than those reported by others. Considering the discrepancies of the experimental data [30,44–46] on the surface tension and the absence of the surface tension at different temperatures for pure ChCl/Urea (1:2), theoretical estimation was conducted. According to the work by Shahbaz et al. [47], the modified Macleod equation Eq. (5) [47] can be used

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Table 1 Critical properties together with molecular weight and acentric factor for ChCl/Urea (1:2) [38]. Property

Tb (K)

Tc (K)

pc (MPa)

Vc (m3kmol1)

M (gmol1)

w

Values

445.6

644.4

4.935

0.25437

86.58

0.661

Table 2 Sources of experimental properties. Property

Refs.

ChCl/Urea (1:2) ChCl/Urea (1:2)-H2O

Density

Viscosity

Heat capacity

Surface tension

[32,33,39–42] [31–33,40,41]

[32,33,39,42,44] [31–33]

[39,43] [43]

[44–46] [30]

Fig. 1. Densities (a), viscosities (b), heat capacity (c) and surface tension (d) of ChCl/Urea (1:2). Symbols: experimental data from literature or theoretical estimation of this work; Curves: fitting results with the equations embedded in Aspen.

Table 3 Equations for temperature-dependent properties of ChCl/Urea (1:2). Property

Liquid molar volume Viscosity Heat capacity Surface tension

Equations

No.

Unit

Parameters C1

C2

C3

(1)

Vl/m3kmol1, T/K

0.06358

2.427  105

1.624  108

g/Pas, T/K

Cp ¼ C1 þ C2 T þ C3 T 2

(2) (3)

Cp/Jmol1K1, T/K

443.7 117.3

2.667  104 0.2085

62.14 1.679  1012

r ¼ C 1 ð1  T=T C ÞC2 þC3 T=T C

(4)

r/Nm1, T/K

0.09244

0.6043

0

V l ¼ C1 þ C2 T þ C3 T 2 ln g ¼ C 1 þ C 2 =T þ C 3 ln T

to estimate the surface tension of DESs reliably. Therefore, in this work, this method was used. In estimation, the parameter PParachor was calculated based on the molecular structure of the constituting

components with a value of 203.39 for ChCl/Urea (1:2). The estimated surface tensions at different temperatures for ChCl/Urea (1:2) were listed in Table 4. The estimated surface tension was

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C. Ma et al. / Applied Energy xxx (2017) xxx–xxx Table 4 Surface tension estimated theoretically in this work. T (K) Surface tension (Nmm1)

298.15 63.76

303.15 63.19

308.15 62.61

around 64 Nmm1 at 298.15 K, and it agreed well with the experimental measurements reported by Arnold et al. [46].

Pparachor ¼

M  r1=4

ð5Þ

q

where PParachor is a parameter linked the surface tension r with the density q. The estimated surface tension was further used to fit the parameters of the DIPPR equation (Eq. (4) in Table 3) for conducting process simulation, and the fitting results agreed well with the estimated data points with an ARD of 0.033%. 2.2. Properties of aqueous ChCl/Urea (1:2) solutions The density, viscosity, surface tension and heat capacity of aqueous ChCl/Urea (1:2) solutions have been measured experimentally and all the available sources of the experimental data were collected as listed in Table 2. The experimental data was illustrated in Fig. 2 for comparison with each other. As shown in Fig. 2, the available experimental densities were consistent with each other for those from the sources of [32,33,40,41], while the experimental data from Shah et al. [31] was higher than others. For viscosity, the available experimental data was consistent with each other for those from the sources of [32,33], while the experimental

Fig. 2. Viscosity and density of ChCl/Urea (1:2)-H2O system at 303.15 K. Symbols, experimental data from literatures; Curves, model results.

313.15 62.04

318.15 61.48

323.15 60.92

328.15 60.36

333.15 59.81

data from Shah et al. [31] was higher than others when the mass fraction of ChCl/Urea (1:2) was higher than 0.8. Therefore, the experimental data by Shah et al. [31] was questionable, and the corresponding experimental data was excluded for the further study. Therefore, only the consistent data [32,33,40,41] was used to obtain the interaction parameters of either Rackett liquid molar volume model or Andrade liquid viscosity model. In the software of Aspen Plus, the properties of density, viscosity and surface tension are independent, while the heat capacity is related to the NRTL model parameters. Therefore, in this work, the density, viscosity and surface tension of aqueous ChCl/Urea (1:2) were studied independently, while the heat capacity was used as the part of inputs to obtain the binary interaction parameters in NRTL model and the detailed discussion was described in Section 2.3.2.3. The preliminary study showed that ideal mixing rules without adjustable parameters cannot be used to represent the properties of aqueous ChCl/Urea (1:2) well, and the excess properties should be considered [32]. Therefore, the Rackett liquid molar volume model was used to describe the density of the mixture in which the interaction parameters were obtained from the fitting of the consistent density data [32,33,40,41]. The ARDs between all of the available experimental data [32,33,40,41] and modeling results for density are listed in Table 5 with a value less than 1%. Compared the modeling results with the data from Shah et al. [31], the ARD is 7.6%, which is much higher than 1%. The Andrade liquid viscosity model was used to describe the viscosity of the mixture with the interaction parameters fitted to the consistent experimental data. The ARDs for the viscosity compared to Yadav et al. [33] and Xie et al. [27] were 6.43 and 11.2%, respectively. While compared to Shah et al. [31], the ARD was 40.6%. Fig. 2 further illustrated the comparison of the experimental density and viscosity with the calculated results at 303.15 K as one example. As shown in Fig. 2, the viscosity changed greatly when the mass fraction of ChCl/Urea (1:2) changed from 0.7 to 1, and the experimental data from different sources showed relatively high discrepancies in this region. For the surface tension of aqueous ChCl/Urea (1:2), it was calculated from the mole-fraction average of liquid surface tension of each pure component. Only one group of data measured by Ji et al. [30] was available, and the model prediction provided an ARD of 9.97% as listed in Table 5. As we discussed above that the surface tension of ChCl/Urea (1:2) with a small amount of water (0.52 wt.%) reported by Ji et al. [30] was higher than those reported

Table 5 Comparison between the experimental data and model results for viscosity, density, heat capacity and surface tension of aqueous ChCl/Urea (1:2). Property

T (K)

Mass fraction of ChCl/Urea (1:2)

ARD/%

Refs.

Density

303.15 298.15–323.15 293.15–363.15 298.15–333.15 293.15–353.15

0–1 0.348–1 0–1 0–1 0–0.994

7.61 0.675 1.01 0.746 1.00

Shah et al. [31] Leron et al. [40] Yadav et al. [33] Xie et al. [32] Su et al. [41]

Viscosity

303.15 293.15–363.15 298.15–333.15

0–1 0–1 0–1

40.6 6.43 11.2

Shah et al. [31] Yadav et al. [33] Xie et al. [32]

Heat capacity

303.15–353.15

0.348–1

3.74

Leron et al. [43]

Surface tension

307.9–337.7

0.521–0.995

9.97

Ji et al. [30]

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by others, thus the deviation might be from the experimental data itself.

The Henry’s constant of gas i in the mixed solvent (Hi,mix) was calculated from those in the pure solvents [48]:

2.3. Phase equilibria

ln

2.3.1. Theory The solubility of gas (i) in a solvent can be expressed as:

ð6Þ

ln ci ¼ ln ci  lim lnci ¼ ln ci  ln c1 i

ð7Þ

where p is the system pressure, yi is the mole fraction in the vapor phase, uvi is the fugacity coefficient in the vapor phase, Hi is the Henry’s constant of gas i in the liquid phase, xi is the mole fraction in the liquid phase, ci is the activity coefficient in the liquid phase, c⁄i is the unsymmetric activity coefficient in the liquid phase, and c1 i is the infinite dilution activity coefficient in the liquid phase. In this work, the Redlich-Kwong (RK) equation of state [32] was used to calculate uvi for the gaseous components with the parameters taken from Aspen databank.

ln /vi ¼ Z  1  lnðZ  bp=RTÞ 

! a=R2 T 2:5 lnð1 þ bp=ZRTÞ b=RT

8 RT a  T 0:5 VðVþbÞ p ¼ Vb > > > > > > Z ¼ pV=RT > > < XX 0:5 R2 T 2:5 a¼ yi yj ðai aj Þ ; ai ¼ 0:42748 pcici > > i j > > X > > RT ci > > b ¼ y i bi ; bi ¼ 0:08664 pci :

ð8Þ

ð9Þ

11 m X xr srj Grj CC B m B X B xj Gij B CC j¼1 B CC Bsij  r¼1 ln ci ¼ m þ m m BX CC B X X AA @ j¼1 @ Gli xl Glj xl Glj xl l¼1

0

0

l¼1

l¼1

Gij ¼ expðcij sij Þ; sij ¼ ðg ij  g ji Þ=RT ¼ aij þ bij =T; cij ¼ cji ði–jÞ ð10Þ where m is the number of components and Gij is a dimensionless interaction parameter depending on the energy interaction parameter (gij) and the non-randomness factor (cij). cij was fixed to be 0.2 in this work. The Henry’s constant for gas i in the pure solvent j was calculated by the following equations:

Hij ðT; pÞ ¼ Hij ðTÞ exp

ln Hij ðTÞ ¼ Aij þ

 1  V ij p RT

Bij þ C ij lnðTÞ þ Dij T T

X Hij ðTÞ wj ln 1 j

2=3 xj ðV 1 Þ X ij 1 2=3 xk ðV ik Þ k

!

cij

ð13Þ

where c1i,mix is the infinite dilution activity coefficient of gas i in the mixed solvent, c1i,j is the infinite dilution activity coefficient of gas i in the pure solvent j, and wj is the weighting factor. V1 ij was calculated from the Brelvi-O’Connell model [49] with BO the characteristic volumes for the gas i (VBO i ) and solvent j (Vj ), respectively. The general form of the Brelvi-O’Connell model is:

  BO BO l V1 ij ¼ fcn V i ; V j ; V j

ð14Þ

where Vlj is the liquid molar volume of solvent j. For the components other than ChCl/Urea (1:2), the characteristic volumes were taken from the Aspen databank as listed in Table 6 in this work. For ChCl/Urea (1:2), as recommended in Aspen Plus, its critical volume was used as the characteristic volume. Due to the extremely low vapor pressure of ChCl/Urea (1:2), it can be assumed that ChCl/Urea (1:2) only exists in the liquid phase. Therefore, for the system studied in this work, H2O is the only solvent involved in vapor-liquid phase equilibrium. The vapor liquid phase equilibrium for H2O can be expressed as:

ð15Þ

psw

i

sji Gji xj

¼

pyw uvw ¼ psw xw cw

where R is the gas constant, Z is the compressibility factor of a gas, T is the absolute temperature, V is the molar volume, a is a constant of a gas mixture that corrects for attractive potential of molecules, b is a constant of a gas mixture that corrects for volume, Tci and pci are the critical temperatures and pressures for gas i. The Non-Random Two-Liquid (NRTL) model was used to calculate the activity coefficient ci in the liquid phase by the following equation: m X

!

c1 i;mix

wj ¼

pyi uvi ¼ Hi xi ci xi !0

Hi;mix

ð11Þ

ð12Þ

where Hij (T,p) is the Henry’s constant of gas i in the pure solvent j at the system temperature and pressure, Hij (T) is the Henry’s constant of gas i in the pure solvent j at zero pressure, V1 ij is the infinite dilution partial volume of gas i in the pure solvent j, and Aij, Bij, Cij and Dij are parameters.

where is the vapor pressure of water, and the subscript w represents the component of H2O. 2.3.2. Parameters for calculating phase equilibrium The parameters for calculating (a) the phase equilibrium of pure water, (b) the fugacity coefficient with RK equation and (c) the solubility of CO2, CH4, N2, or O2 in pure water were taken from the Aspen databank or estimated using the Property Constant Estimation System (PCES) embedded in Aspen Plus. Due to the low solubility of N2 or O2 in ChCl/Urea (1:2) and their trace contents in the raw biogas, their solubilities in ChCl/Urea (1:2) were assumed to be zero [21]. For other systems, the model parameters for calculating the Henry’s constant and the binary interaction parameters in NRTL for calculating the activity coefficient were obtained from the fitting of the experimental data, and the parameter fitting was conducted in Aspen Plus. The fitting results and discussion were briefly described in the following sub-sections. 2.3.2.1. Solubility of CO2 in ChCl/Urea (1:2). The CO2 solubility in ChCl/Urea (1:2) has been determined by several research groups. Li et al. [28] reported the CO2 solubility in ChCl/Urea (1:2) from 1 to 13 MPa. Leron et al. [29] measured the CO2 solubility in ChCl/ Urea (1:2) in a wide temperature and pressure range. Mirza et al. [50] also reported the CO2 solubility in ChCl/Urea (1:2) at very low pressures (0.04–0.15 MPa). The experimental data from different sources were compared, and it showed that the experimental results from Li et al. [28] and Leron et al. [29] were consistent with each other while those from Mirza et al. [50] were slightly lower maybe because of the high moisture content of the deep eutectic solvents. Therefore, in this work, the solubility data of Li et al. [28] and Leron et al. [29] was used in parameter fitting. In parameter fitting, the parameters for Henry’s constants in Eq. (12) and the NRTL binary interaction parameters in Eq. (10) were fitted simultaneously. The fitted parameters for estimating the

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C. Ma et al. / Applied Energy xxx (2017) xxx–xxx Table 6 Characteristic volume for the Brelvi-O’Connell model. Parameter V a

BO

ChCl/Urea (1:2)a

H2O

CO2

CH4

N2

O2

0.263

0.0464

0.0939

0.0992

0.0896

0.0734

The critical volume was used as the characteristic volume for ChCl/Urea (1:2).

Table 7 Parameters for calculating the Henry’s Constant in Eq. (12) (in bars). Solute i Solvent j

CO2 ChCl/Urea (1:2)

CH4 ChCl/Urea (1:2)

CO2 H2O

CH4 H2O

N2 H2O

O2 H2O

Aij Bij Cij Dij

242.95 3055.05 47.00 0.13

16.16 1438.44 4.42 0.03

159.87 8741.55 21.69 0.00110

183.78 9111.67 25.04 0.000143

164.99 8432.77 21.558 0.00844

144.408 7775.06 18.40 0.00944

Table 8 The NRTL binary interaction parameters in Eq. (10). Component i

Component j

aij

aji

bij

bji

cij = cji

CO2 CH4 H2O CO2 CH4 N2 O2 CO2 CO2 CO2 CH4 CH4 N2

ChCl/Urea (1:2) ChCl/Urea (1:2) ChCl/Urea (1:2) H2O H2O H2O H2O CH4 N2 O2 N2 O2 O2

0.50 39.50 1.61 10.064 0 0 0 0 0 0 0 0 0

4.48 0.00 0.48 10.064 0 0 0 0 0 0 0 0 0

1236.14 10000.00 538.95 3268.14 1948.87 143.351 182.272 14.263 89.041 131.978 133.685 189.157 5.121

3032.59 0.00 134.90 3268.14 562.113 36.5619 107.244 43.035 176.383 245.817 122.821 168.446 4.268

0.20 0.20 0.20 0.2 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3

Henry’s constants with Eq. (12) are summarized in Table 7, and the NRTL binary interaction parameters of CO2-ChCl/Urea (1:2) in Eq. (10) are summarized in Table 8. The ARDs for the CO2 solubility compared to Li et al. [28] and Leron et al. [29] were 1.08 and 0.61%, respectively. To further illustrate the model performance, the modeling results were compared with the experimental data as shown in the P-T-x diagram (Fig. 3). This diagram showed that the model

Fig. 3. CO2 partial pressure of ChCl/Urea (1:2)-CO2 system. Symbols, experimental data; Curves, model correlation.

represented the experimental data reliably throughout the investigated temperature and pressure range. 2.3.2.2. Solubility of CH4 in ChCl/Urea (1:2). Considering CH4 is one of the main components of the biogas, the NRTL binary interaction parameters between CH4 and ChCl/Urea (1:2) and the parameters for estimating the Henry’s constant were obtained from the fitting of the solubility data of CH4 in ChCl/Urea (1:2). The solubility of CH4 in ChCl/Urea (1:2) was only measured in our previous work [51]. This group of data was used without further evaluation in order to obtain the model parameters. The method for parameter fitting is the same as that for CO2 in ChCl/Urea (1:2). The fitted parameters are summarized in Tables 7 and 8, respectively, and the ARD was 0.88% compared to the experimental data [51]. The comparison of the modeling results of the CH4 solubility in ChCl/Urea (1:2) with the experimental data at different pressures and temperatures was depicted in Fig. 4. Compared to the CO2 solubility in ChCl/Urea (1:2), the CH4 solubility linearly increased with increasing pressure, while the effect of the pressure on the CO2 solubility was more pronounced at relatively higher pressures. The model can represent the experimental data well based on the results illustrated in Figs. 3 and 4. 2.3.2.3. Properties and vapor-liquid equilibrium (VLE) of ChCl/Urea (1:2)-H2O binary system. According to the work of Zhang et al. [48], VLE data, excess enthalpy and heat capacity can be used to determine the NRTL binary parameters including their temperature dependencies. No excess enthalpy of the ChCl/Urea (1:2)-H2O binary system is available, and thus their VLE data [52] and heat capacity [43] were used to obtain the NRTL binary parameters. The fitted NRTL parameters are summarized in Table 8. The ARDs

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Fig. 4. CH4 partial pressure of ChCl/Urea (1:2)-CH4 system. Symbols, experimental data (Xie et al. [51]); Curves, model correlation.

Fig. 6. Molar heat capacity of ChCl/Urea (1:2)-H2O system. x1 is the mole fraction of ChCl/Urea (1:2). Symbols, experimental data (Leron et al. [43]); Curves, model correlation.

2.3.2.4. Gas solubility and selectivity in aqueous ChCl/Urea (1:2). The solubility of pure gas in aqueous ChCl/Urea (1:2) is greatly affected by the binary interaction between H2O and ChCl/Urea (1:2). Among the gas studied, only the solubility of CO2 in aqueous ChCl/Urea (1:2) was measured experimentally. Su et al. [41] determined the CO2 solubility in aqueous ChCl/Urea (1:2) at different temperatures and water contents at 0.1 MPa. Xie et al. [32] determined the CO2 solubility in aqueous ChCl/Urea (1:2) with low water content at different temperatures and pressures. Hsu et al. [53] determined the CO2 solubility in aqueous ChCl/Urea (1:2) with a large percent-

age of H2O (30, 40 and 50 wt.%) at different temperatures and pressures. The experimental data from different sources showed inconsistent, and the CO2 solubility data from Su et al. [41] is much lower than those from the other two sources. The fitted parameters of the NRTL model and those for estimating the Henry’s constants of CO2-ChCl/Urea (1:2) together with the parameters existed in the Aspen databank were used to predict the CO2 solubility in ChCl/Urea (1:2)-H2O. The comparison of the predicted CO2 solubility in ChCl/Urea (1:2)-H2O with the experimental data from different sources is depicted in Fig. 7. The average deviations (ADs) on the CO2 solubility between the calculated and experimental results by Xie et al. [32], Hsu et al. [53] and Su et al. [41] were 0.016, 0.011 and 0.009, respectively, while the corresponding ARDs were 16%, 71% and 332%, respectively. As the experimental CO2 solubility in ChCl/Urea (1:2)-H2O from Xie et al. [27] agreed well with the model prediction based on both AD and ARD, the fitted parameters listed in Tables 7 and 8 can be used to predict the CO2 solubility in ChCl/Urea (1:2)-H2O for conducting process simulation. For the gas mixture, no additional adjustable parameters were used. The NRTL binary parameters between them in the liquid phase (listed in Tables 8) were estimated from the Property Con-

Fig. 5. Vapor pressure of ChCl/Urea (1:2)-H2O system. xChCl/Urea (1:2) is the mole fraction of ChCl/Urea (1:2). Symbols, experimental data (Wu et al. [52]); Curves, model correlation.

Fig. 7. Parity plot for ChCl/Urea-H2O-CO2 system CO2 mole fraction: experiment vs. model. (Xie et al. [32], Hsu et al. [53] and Su et al. [41]).

for the vapor pressure and heat capacity were 5.1% and 3.7%, respectively. The comparisons of the vapor pressure and the heat capacity were illustrated in Figs. 5 and 6. The calculated vapor pressure agreed with the experimental data very well, and the deviation increased slightly with increasing temperature and increasing mole fraction of ChCl/Urea (1:2). The model also provided satisfactory representation of the heat capacity, and the deviation became slightly larger with increasing mole fraction of water. It showed that the model with the fitted parameters and the mixing rules in Aspen Plus can be used to represent these properties reliably.

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stant Estimation System (PCES) embedded in Aspen Plus. PCES estimated these binary parameters based on the infinite-dilution activity coefficients predicted by UNIFAC methods. Base on the obtained parameters from the above section, the phase equilibria for CO2-CH4- ChCl/Urea (1:2)-H2O system can be predicted. Considering that the solubility of CO2 and selectivity for CO2/CH4 are two main factors for evaluating the process performance, we studied the effect of water content on the solubility of CO2 and selectivity for CO2 and CH4. In the investigation, the solubilities of CO2 and CH4 for the gas mixture (54% CH4, 46% CO2) in the aqueous ChCl/Urea (1:2) at 293.15 K and 0.8 MPa (i.e. the operational conditions of absorption used in Section 3) were calculated. Subsequently, the selectivity (S) for CH4 and CO2 was calculated with Eq. (16).

S ¼ X CO2 ðmol CO2 =mol solventÞ =X CH4 ðmol CH4 =mol solventÞ

ð16Þ

where XCO2 and XCH4 are the solubilities of CO2 and CH4 for the gas mixture in aqueous ChCl/Urea (1:2), respectively, at 293.15 K and 0.8 MPa. As we can see from Fig. 8, with the increasing content of ChCl/ Urea (1:2), the selectivity of aqueous ChCl/Urea (1:2) for CO2 and CH4 decreased slightly, while the solubility increased dramatically. For example, when the content of ChCl/Urea (1:2) increased from 10 to 50 wt.%, i.e., the water content from 90 to 50 wt.%, the solubility of CO2 increased to about 4 times, while the selectivity for CO2 and CH4 decreased by about 4%. This makes it unclear

Fig. 8. Selectivity between CO2 and CH4, and the CO2 solubility for the gas mixture in ChCl/Urea (1:2)-H2O system at 293.15 K.

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how the water content will affect the process performance, and the evaluation based on process simulation is needed. 3. Process simulation and evaluation for biogas upgrading 3.1. Process description for biogas upgrading The conceptual process for biogas upgrading using aqueous ChCl/Urea or H2O as CO2 absorbents is shown in Fig. 9, consisting of three parts including CO2 absorption, flash and solvent regeneration. (1) CO2 absorption. The biogas is pressurized to 0.8 MPa with the multistage-compressor (blocks COMP1 and COMP2) and injected into the bottom of the scrubber (block ABSORBER), while the liquid absorbent (pure water or aqueous ChCl/Urea solution) is sprayed from the top of the scrubber. The temperature of the absorbent was assumed to be 293.15 K in the simulation conducted in this work. The intermediate pressure and inter-cooling temperature of the multistage compressor were assumed to be 0.3 MPa and 293.15 K, respectively, to minimize the compression power. The gas leaving at the top of the scrubber is around 97% (vole) CH4, which can be dried and used in other applications. As the process of biogas compression is exothermic and the temperature of the gas after each stage of the compressor can be higher than 373.15 K, the heat exchangers are used (blocks H1 and H2) to maintain the temperature of raw biogas entered to the absorber at 293.15 K. (2) Flash. The CO2-enriched solvent (leaving from the bottom of the absorber) enters a flash tank (block FLASH) where the pressure is decreased in order to improve the recovery of methane (CH4). The vapor phase (stream GAS-REC) released from the flash tank is mixed with the raw biogas and recirculated to the second stage of the multistage-compressor (block COMP2). The temperature of the flash tank was assumed to be 293.15 K. The pressure of the flash tank was determined according to the sensitive analysis. Besides, the temperature and pressure of heat exchanger (H3) after flash were assumed to be 293.15 K and 0.1 MPa, which were taken from the literature [54]. (3) Solvent regeneration. The liquid phase leaving from the flash tank is sent to the desorption column (block DESORBER) and regenerated by decreasing the pressure to 0.1 MPa with an aeration of air using a blower, which brought N2 and O2 into this system.

Fig. 9. Schematic of the biogas upgrading process.

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In process simulation, the absorber and desorber were modeled with the Radfrac model without condenser and reboiler. There are two main approaches to modeling absorber and desorber in Aspen Plus: equilibrium and rate-based approaches. In this work, the equilibrium approach was used. For aqueous ChCl/Urea (1:2) process, the models and their parameters were taken from the work in Section 2. For HPWS process, the model parameters were taken directly from Aspen databank. 3.2. Validation of process simulation based on HPWS Before conducting the process simulation with aqueous ChCl/ Urea (1:2) as the absorbent, HPWS was simulated in order to validate the simulation conducted in this work and provide part of the simulation parameters. The verification was performed by comparing with the simulation results from others. HPWS process has been simulated with Aspen Plus [14,54–56]. Based on the literature survey, the capacity of a biogas plant is in a range of 150–2000 m3h1 (STP) [54]. Gotz et al. [56] was the first to simulate this process and the plant capacity was set to be 242.3 m3h1 (STP), while Cozma et al. [54] provided the most detail information about this simulation where the plant capacity was set to be 10.854 kmolh1 (243.14 m3h1 (STP)). Therefore, in this work, the capacity of the biogas plant was set to be 242.3 m3h1 (STP) and the simulation results from both of them were used to validate the results of this work based on equilibrium approach. In simulation, the number of theoretical stages for absorber was set to be 11 on the basis of the previous work [54]. The number of theoretical stages for desorber was set to be 2 in this work. The effective composition of raw biogas and the operating conditions were set to be those used by Gotz et al. [56]. Based on the above mentioned conditions, the simulation was conducted. It showed that the gas leaving the scrubber contains: 96.58% CH4, 0.385% CO2, 0.0000904 ppm H2S, 2.09% N2, 0.56% O2, 0.091% H2, and 0.30% H2O, which was almost the same as those from literature (96.5–96.8% CH4, 0.397–0.47% CO2, 0.000094 ppm H2S, 2.09– 2.10% N2, 0.56–0.563% O2, 0.0902% H2, and 0.319–0.32% H2O) [54,56]. Meanwhile, the gas leaving the desorber contains: 0.0621% CH4, 17.1% CO2, 38.4 ppm H2S, 63.6% N2, 16.9% O2, 0.165 ppm H2, and 2.34% H2O, which was also almost the same as those from literature (0.0677–0.07% CH4, 17.1–17.5% CO2, 38– 39.5 ppm H2S, 63.5–65.2% N2, 16.8–17.3% O2, 0.193 ppm H2, and 2.31–2.34% H2O) [54,56]. Therefore, we can conclude that the simulation in this work is reliable. 3.3. Process simulation of biogas upgrading with aqueous ChCl/Urea (1:2) To simulate the conceptual process for biogas upgrading using aqueous ChCl/Urea (1:2) solutions as shown in Fig. 9, the plant capacity, temperature and working pressure were set as those from literature [54–56] as listed in Table 9. Considering H2S existed in biogas may affect the performance of ChCl/Urea (1:2), it was

Table 9 The parameters for process simulation. Parameter

Units

Biogas

CH4/CO2 N2/O2

mol% mol%

53.69/45.19 0.93/0.19

Operating conditions pabsorber pdesorber Tabsorber/Tdesorber Plant capacity (raw biogas feed)

MPa MPa K m3h1

0.8 0.1 293.15/293.15 242.3 (STP)

assumed to be removed completely before entering the biogas upgrading process. In addition, H2 was also not considered in the following study due to the low content in raw biogas. Therefore, the biogas fed to the studied upgrading process is free of H2S and H2, and it only contains CH4, CO2, N2, and O2. For a better comparison of the simulation results from both HPWS and aqueous ChCl/ Urea processes, the effective composition and most of the operating conditions were set to be the same for both processes as listed in Table 9. Generally, the solvent with high viscosity requires high pumping costs and leads to high pressure drops in the absorber. The addition of water can overcome the problems caused by the high viscosity. As described in the first part of the work (Section 2), when the water content is up to 30 wt.%, the viscosity of the solvent decreases greatly from 513 to 6 mPas at 303.15 K, making the solvent suitable for the process operation in practice. Therefore, the solvent considered in this work for process simulation was the aqueous solutions with the water content higher than 30 wt.%, i.e. the aqueous ChCl/Urea (1:2) solvents with 30, 50, and 70 wt.% ChCl/Urea (1:2). The operating parameters other than those listed in Table 9 for HPWS were taken from [56] as summarized in Table 10. The mixing of the water and ChCl/Urea (1:2) will affect both the CO2 solubility and selectivity. Based on the results in Section 2, the CO2 solubility will decrease dramatically while the selectivity for CO2 and CH4 will increase slightly with increasing content of H2O. The increase of CO2 solubility will decrease the amount of absorbent and the number of theoretical stages of absorber when upgrading the same capacity of raw biogas, while the decrease of selectivity will increase the loss ratio of CH4. Meanwhile, the slight change of selectivity might affect the pressure of flash tank. Therefore, it was unclear whether the air flow rate and the operating conditions used for HPWS were still suitable for these new solvents. Therefore, the sensitivity analysis was conducted to find the reasonable operating conditions including the ratio of the volumetric flow rate of the absorbent to that of raw biogas (L/G, m3m3), the volumetric flow rate of the airflow to that of raw biogas (AFR/G, m3m3), the number of theoretical stages of absorber (Nab), and the pressure of flash tank (pflash) to achieve the requirements of biogas upgrading. After sensitivity analysis, the final operating conditions were obtained, and then total energy utilization, environmental effect, and the diameters and pressure drops of absorber and desorber were discussed in detail. 3.3.1. Sensitivity analysis for finding reasonable operating conditions The sensitivity analysis was conducted to find the reasonable operating conditions to achieve the requirements of biogas upgrading, i.e. the CH4 contents in the product gas (YCH4,PG (vol %)) and off gas (YCH4,SG (vol%)), the CO2 removal efficiency (RECO2 (%)), the CO2 regeneration degree of the absorbents (gCO2) and the loss ratio of CH4. Specifically, YCH4,PG is the CH4 volumetric concentration in the stream left from absorber (GAS-OUT in Fig. 9). YCH4,SG is the CH4 volumetric concentration in the stream left from desorber (OFFGAS in Fig. 9). RECO2 was defined as the ratio of the CO2 amount in the product gas (GAS-OUT in Fig. 9) to that in the biogas stream (BIOGAS in Fig. 9). gCO2 was defined as the ratio of the mole flow rate of CO2 in the off gas left from the desorber (OFFGAS in Fig. 9) to that in the rich solvent entered into the desorber (DESOR-IN in Fig. 9), and it reflected the CO2 regeneration degree of the absorbent. The loss of CH4 was quantified as (1-n0 CH4/nCH4), where n0 CH4 was the CH4 amount in the product gas (GAS-OUT in Fig. 9), and nCH4 was the CH4 amount in the biogas stream (BIOGAS in Fig. 9). In the sensitivity analysis, the electricity consumption included power, heating and cooling duty from the equipment used in biogas upgrading system. The heating or cooling duty required in the

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C. Ma et al. / Applied Energy xxx (2017) xxx–xxx Table 10 Operating conditions and simulation results based on plant capacity (raw biogas feed) of 242.3 m3h1 (STP). Parameter L/G Mass flow rate of absorbent Nab pflash AFR/G

gCO2 YCH4, PG YCH4, SG Loss ratio of CH4 RECO2

Units 3

3

m m kgh1

MPa m3m3 % % % % %

H2O

30 wt.% ChCl/Urea (1:2)

50 wt.% ChCl/Urea (1:2)

70 wt.% ChCl/Urea (1:2)

0.195 47075.7 11 0.3 2.1 99.53 96.67 0.062 0.304 99.52

0.0999 25515.0 8 0.3 2.1 99.45 96.67 0.0848 0.415 99.05

0.0545 14476.3 7 0.3 2.1 99.20 96.67 0.103 0.503 98.82

0.0285 7896.5 6 0.3 2.1 97.54 96.67 0.173 0.838 98.66

process was converted to electricity for comparing. For the heating duty, the conversion efficiency of heat to electricity was assumed to be 0.3 [57]. For the cooling duty, it was converted to the electricity consumption of a refrigerator, and the efficiency of a refrigerator called the coefficient of performance (COP) was assumed to be 4 [58]. 3.3.1.1. L/G and Nab. In this work, the target was set to obtain the product gas with 96.7% CH4 and at the same time the loss of CH4 is lower than 1%. The loss of CH4 is slightly different for each aqueous solvent in order to reach the same value for the target gas (96.7%). The ratio of the volumetric flow rate of the absorbent to that of raw biogas (L/G, m3m3) is related to the number of theoretical stages of the absorber (Nab). Sensitivity analysis was first carried out to get the reasonable values for L/G and Nab. It should be mentioned that during conducting this sensitivity analysis, other operating parameters listed in Table 9 were fixed as constants, and the value of YCH4,PG was set to be 96.7%. Based on the results illustrated in Fig. 10, we can see that the value of L/G first decreased greatly with the increase of Nab and then reduced slightly. For example, for 50 wt.% ChCl/Urea (1:2), the value of L/G almost kept a constant when Nab was larger than 7. Considering that the further increase of Nab led to a higher investment cost and did not affect the amount of absorbent (i.e. the value of L/G), the turning point is a better choice. It means that the reasonable Nab should be 7 for 50 wt.% ChCl/Urea (1:2), and the corresponding L/G can also be determined. The Nab and L/G for different aqueous ChCl/Urea (1:2) were determined with the same method. It was found that L/G and Nab were strongly related to the concentration of ChCl/Urea (1:2). With increasing concentration of ChCl/Urea (1:2), both Nab and L/G decrease. This is because

Fig. 10. L/G for various numbers of theoretical stages of the absorber achieving 97% CH4 in product gas using aqueous ChCl/Urea (1:2).

that the CO2 solubility increases remarkably with increasing ChCl/ Urea (1:2) content. 3.3.1.2. Pflash. The flash tank was used to minimize the methane loss, and the pressure of the flash tank (pflash) affected YCH4,PG, RECO2, the loss ratio of CH4 and the electricity consumption. Based on values of Nab and L/G listed in Table 10, the sensitivity analysis was further conducted on pflash. Results for different aqueous ChCl/ Urea (1:2) solutions are similar, and Fig. 11 illustrates the results for 50 wt.% ChCl/Urea (1:2) as one example. As shown in Fig. 11, when pflash was higher than 0.3 MPa, YCH4,PG, RECO2 and the electricity consumption almost maintained as constant, but the loss ratio of CH4 increased greatly. It can be explained with the following text. On one hand, when pflash increased, the amount of gas released from the top of flash tank decreased, that is, the amount of CH4 recirculated to the absorber decreased, which led to an increase of CH4 loss. On the other hand, when pflash decreased, the amount of gas released from the top of flash tank increased, the L/G determined in the first round of sensitivity analysis was not enough to treat the gas, and thus YCH4,PG and RECO2 decreased. To maintain the loss of CH4 be lower than 1%, pflash should be in a range of 0.3–0.34 MPa using 50 wt.% ChCl/Urea (1:2) as absorbent. While for water, 30 wt.% ChCl/Urea (1:2), and 70 wt.% ChCl/ Urea (1:2), the ranges were 0.3 MPa  pflash  0.36 MPa, 0.3 MPa  pflash  0.35 MPa and 0.3 MPa  pflash  0.31 MPa, respectively. Based on the analysis, it was proper to set pflash to be 0.3 MPa for all of these absorbents in order to achieve the target of CH4 loss (<1%). 3.3.1.3. AFR/G. In desorption, the ratio of the volumetric flow rate of the airflow to that of raw biogas (AFR/G, m3m3) affected gCO2, RECO2, YCH4,PG, the loss ratio of CH4 and the electricity consumption. Based on the assumed values of Nab, L/G and pflash for each solvent, the effect of AFR/G on the process performance was studied. In general, with the increase of AFR/G, YCH4,PG, RECO2 and gCO2 changed dramatically at first and then slightly when AFR/G was higher than 2.1 as shown in Fig. 12. The reason can be explained as the following text. On one hand, with the increase of AFR/G, the desorbed CO2 from the liquid absorbent increased, that is, less CO2 was recirculated and finally RECO2 and YCH4,PG increased. When AFR/G was higher than 2.1, the amount of the CO2 in liquid phase was low. This means that the absorbent regeneration reached equilibrium, and then the amount of the recirculated CO2 changed slowly. In this case, the increase of AFR/G gave slight effect on YCH4,PG, RECO2 and gCO2. On the other hand, the electricity consumption and the loss ratio of CH4 increased linearly with the increase of AFR/G. The increase of the volumetric flow rate of the airflow led to the increase of electricity consumption of blower and then the overall electricity consumption. The regeneration degree of the absorbent increased with increasing AFR/G. This led to an increase of the

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Fig. 11. Effect of pressure in flash tank on (a) YCH4,PG and loss ratio of CH4; (b) Electricity consumption and RECO2 using 50 wt.% ChCl/Urea (1:2) as absorbent.

Fig. 12. Effect of AFR/G on (a) YCH4,PG and loss ratio of CH4; (b) Electricity consumption, RECO2 and gCO2 using 50 wt.% ChCl/Urea (1:2) as absorbent.

amount of CH4 absorbed by the absorbent, resulting in an increase of the loss ratio of CH4. For the solvents with 30 and 70 wt.% ChCl/Urea (1:2), the results were similar to those with 50 wt.% ChCl/Urea (1:2). Therefore, AFR/ G was set to be 2.1 for all of these absorbents in order to achieve the target of the CH4 purity in product and CH4 loss and to use less electricity. As we can see from Table 10, when AFR/G was set to be 2.1 for these absorbents, gCO2 changed slightly when the content of ChCl/Urea (1:2) was changed from 0 to 50 wt.%. While for 70 wt.% ChCl/Urea (1:2), the gCO2 changed greatly compared with other solvents due to the difficulty to desorb CO2. After sensitivity analysis, the final operating conditions were summarized in Table 10 for each new absorbent. Based on the results in Table 10, the L/G and mass flow rate of absorbent decreased by 85% when the content of ChCl/Urea (1:2) increased to 70 wt.%. This means that the absorption capacity of the solvent increased 7 times. Meanwhile, the number of theoretical stages reduced from 11 to 6, which directly affected the height of absorber. Besides, the L/G, AFR/G, Nab and pflash listed in Table 10 can be used as a reference in process design even when the plant capacity (raw biogas feed) is not 242.3 m3h1 (STP). 3.3.2. Energy utilization Based on the assumed values for Nab, L/G, AFR/G and pflash for each solvent listed in Table 10, the energy utilization was studied. It should be mentioned that the energy utilization caused by the CH4 loss in biogas upgrading system was also included in this part besides power, heating and cooling duty. For the heat duty, the

negative value means that cooling duty is required; conversely, heating duty is required. For the biogas upgrading process with different absorbents, the energy requirements from different parts were listed in Table 11. The total electrical power included the power for operating the solvent pump (WP), compressor (WC) and blower (WB). In the simulation, isentropic efficiency of 75% was assumed for the gas compression [59], while the mechanical efficiency due to the losses from the seals and valves in the compressor was assumed to be 85%. The efficiency of the pump was assumed to be 80% [8], while the driver efficiency was assumed to be 85%. In addition, the heat duty for cooling the compressed biogas (QC), the heat duty of the flash tank (QF) and the duty used for transferring the solution from the flash tank (QH) to the desorber were also considered in estimating the total energy utilization. The ‘‘kWth” and ‘‘kWe” are the units of heat duty and electricity power, respectively. For a better comparison, when calculating the total energy utilization, the contributions from the heat duty and the CH4 loss were converted to electricity. The heat duty required in the process was converted to electricity as described in Section 3.3.1. For accounting the energy utilization caused by the CH4 loss (ECH4 loss), the electricity equivalent of 1 tonne of natural gas was assumed to be 12,547 kWeh [60] in calculating ECH4 loss. The results showed that for the biogas plant with the capacity of 242.3 m3h1(STP), the total energy utilization for biogas upgrading with water was 60.1 kWe. As we can see from Fig. 13, the addition of ChCl/Urea (1:2) into water made the total energy utilization decrease first and then increase. For example, with 50 wt.% ChCl/

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13

C. Ma et al. / Applied Energy xxx (2017) xxx–xxx Table 11 Summary of energy requirements for biogas upgrading using different absorbents. Energy requirements

Units

Water

30 wt.% ChCl/Urea (1:2)

50 wt.% ChCl/Urea (1:2)

70 wt.% ChCl/Urea (1:2)

WC WP WB Total

kWe kWe kWe kWe

30.1 13.5 4.6 48.2

29.5 6.9 4.6 41

29.1 3.8 4.6 37.5

28.6 2.0 4.6 35.2

QC QF QH Total

kWth kWth kWth kWth kWe

26.2 5.3 1.9 33.4 8.4

25.7 2.2 1.0 24.5 6.1

25.4 2.2 0.6 28.2 7.1

24.9 4.8 0.3 30 7.5

ECH4 loss Total energy utilization SERG SEPG

kWe kWe kWhm3 kWhm3

3.5 60.1 0.25 0.44

4.8 51.9 0.21 0.39

5.9 50.5 0.21 0.37

9.8 52.5 0.22 0.39

including water scrubbing. The gathered SEPG were relatively consistent with a range of 0.20–0.43 kWhm3. In this work, the calculated values of SERG and SEPG for HPWS were 0.25 and 0.44 kWhm3, respectively. These values are consistent with the values reported by others, and thus it further confirmed that the simulation results obtained in this work are reliable. Compared with other CO2 capture process such as amine scrubbing (0.56– 0.646 kWhm3) [9], the total energy utilization of HPWS process and the process using aqueous ChCl/Urea (1:2) was lower because of no thermal energy demand for solvent regeneration.

Fig. 13. The total energy utilization and ECH4 ChCl/Urea (1:2).

loss

of biogas upgrading with aqueous

Urea (1:2), the energy utilization decreased down to 50.5 kWe, that is, the total energy utilization decreased dramatically by 16% compared to water. Further increasing the content of ChCl/Urea (1:2) led to a slight increase of energy utilization, that is, when the content of ChCl/Urea (1:2) increased from 50 to 70 wt.%, the energy utilization increased by 4%. While the energy utilization excluding ECH4 loss always decreased. The main reason is that at first the ECH4 loss did not contribute too much on the total energy utilization compared to other parts. With the addition of ChCl/Urea (1:2) up to 50 wt.%, ECH4 loss contributed more than 10%, which led to the change of the trend of total energy utilization. For the electrical power from the equipment, it includes the power for operating the pump, compressor and blower. As we can see from Table 11, it decreased down to 35.2 kWe from 48.2 kWe, when the content of ChCl/Urea (1:2) increased from 0 to 70 wt.%. This corresponds to a dramatic decrease by 27% with the increasing content of ChCl/Urea (1:2). For the electrical power converted from heat duty, it affected slightly on the total energy utilization, because it contributes a small part to the total energy utilization and changes slightly. To compare with other biogas plant with different capacity, the specific energy utilizations per m3 raw biogas (SERG) and per m3 upgraded biogas (SEPG) were calculated, and the corresponding results are listed in Table 11. According to the report of Bauer at al. [8], for a practical HPWS process with the plant capacity of 250 m3h1 (STP), the value of SERG was around 0.3 kWhm3. Patterson et al. [9] gathered SEPG from a range of industrial and academic literatures for relatively well-established technologies

3.3.3. Diameters and pressure drop of the absorber and desorber In this work, the packing material was a plastic pall ring with the diameter of 25 mm, and 62% of the flooding velocity (maximum capacity) as recommended in Aspen Plus was used when calculating the diameters. As listed Table 12, the diameters for both absorber and desorber decreased with increasing ChCl/Urea (1:2) content. When the content of ChCl/Urea (1:2) increased from 0 to 70 wt.%, the diameter of the absorber decreased from 0.59 to 0.33 m and the diameter of the desorber decreased from 0.73 to 0.49 m. The average values of pressure drop/height (Dp/z) in the absorber and desorber are listed in Table 12 for different absorbents. The Dp/z in the absorber changed from 21.4 to 151.6 Pa/m while that in desorber changed from 238.7 to 274.2 Pam1 when the content of ChCl/Urea (1:2) increased from 0 to 70 wt.%. The Dp/z increased obviously in the absorber with increasing content of ChCl/Urea (1:2) but still within the recommended value for the absorber (150–500 Pam1) [61]. Owing to the lack of the mass transfer rate for the studied absorbents, the heights and volumes of absorber and desorber were not considered in this work. 3.3.4. Environmental assessment The effect of the process on the environment was also considered in this work. A variety of quantitative methods for the design of environmentally-friendly chemical processes have been proposed [62]. Recently, Zhang et al. [63] developed a new assessing method (Green degree, GD), in which an integrated index including nine environmental impact categories (including global, air, water, and toxicological effects) was used. This method has been applied to analyze and assess the chemical processes [63,64] as well as biogas upgrading process [57]. In this work, the GD method was used to evaluate the environmental influence of the biogas upgrading process with aqueous ChCl/Urea (1:2). For the biogas upgrading, neither new material nor energy is produced, and the green degree change of a process can be simplified as the following formula [64]:

DGDP ¼

X

GDMaterial-Out 

X

GDMaterial-In 

X

GDEnergy

ð17Þ

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C. Ma et al. / Applied Energy xxx (2017) xxx–xxx

Table 12 Summary of parameters for absorber and desorber with different absorbents. Parameters

Units

Water

30 wt.% ChCl/Urea (1:2)

50 wt.% ChCl/Urea (1:2)

70 wt.% ChCl/Urea (1:2)

Dab Dde Dp/z in absorber Dp/z in desorber

m m Pam1 Pam1

0.59 0.73 21.4 238.7

0.46 0.61 60.0 239.8

0.38 0.53 109.0 248.4

0.32 0.47 151.6 274.2

where DGDP is the green degree of the process, GDMaterial-Out and GDMaterial-In represent the GD values of the materials into and out of the process, respectively, and GDEnergy-In represents the GD value of the energy into the process. The unit values of GD for the materials other than ChCl/Urea (1:2) and biogas were taken from the work by Zhang et al. [63]. For calculating the green degree of this process, the biogas produced via digestion from the low grade biomass was considered as sustainable resources, the water is also sustainable resources, and thus the GDs of biogas and water were set to be zero. The loss of ChCl/Urea (1:2) in this process was negligible due to its extremely low vapor pressure and thermal stability. Because the process was to produce biomethane as the product that can be used to replace fossil fuels, the GD of the process was always higher than zero. To verify the calculation, DGDP for the HPWS process was studied. Xu et al. [57] estimated the DGDP for the HPWS process with a plant capacity of 500 m3/h (STP) and reported a value about 1200 gd/h, that is, the specific DGDP per m3 raw biogas was about 2.40 gd/m3. In this work, the DGDP for the HPWS process with the same operating parameters of Xu et al. [57] was calculated, and the specific DGDP per m3 raw biogas was 2.397 gd/m3, which was almost the same with the literature. Based on this, we can conclude that our calculation is reliable. Therefore, DGDP was calculated for the biogas upgrading using different absorbents. The results are illustrated in Fig. 14. In general, DGDP was higher than zero for the process using aqueous ChCl/Urea (1:2) as absorbents, and it reached the highest value (512 gd/h) for the process with the absorbent of 50 wt.% ChCl/Urea (1:2), indicating that this process is the most environmentally benign. In addition, compared to the water scrubbing process (i.e. ChCl/Urea (1:2) = 0), DGDP for the process with aqueous ChCl/Urea (1:2) was always higher, which means that the process with aqueous ChCl/Urea (1:2) is more environmentally benign than HPWS.

4. Conclusion A conceptual process for biogas upgrading was developed in which aqueous ChCl/Urea (1:2) was chosen as the liquid absorbent. To perform process simulation based on Aspen Plus, the experimental thermophysical properties and vapor-liquid equilibria related to ChCl/Urea (1:2) were surveyed, evaluated and then fitted to the empirical equations or models embedded in the software Aspen Plus. The consistent experimental data of the thermophysical properties were fitted to the models embedded in Aspen Plus. Based on the fitted parameters in Aspen Plus, the properties of vapor pressure, density, viscosity and molar heat capacity of binary system H2O-ChCl/Urea (1:2) were predicted, which showed a good agreement with the experimental data. The NRTL-RK model was used to describe the phase equilibria for the related systems with the assumption that ChCl/Urea was only in the liquid phase. The model parameters were further implemented into Aspen Plus, making it ready for process simulation. The performance was evaluated by conducting process simulation based on Aspen Plus and compared with HWPS. Specifically, process simulation of biogas upgrading was conducted and verified with HWPS. The sensitivity analysis was used to determine the reasonable L/G, Nab, pflash and AFR/G for each absorbent. The effects of ChCl/Urea (1:2) content on the total energy utilization, the green degree of process (DGDP), and the diameters and pressure drop of absorber and desorber were further studied. Based on the systematic analysis, aqueous ChCl/Urea (1:2) is a promising absorbent. The total energy utilization was decreased by 16% where the total electrical power was decreased by 27%. The process using 50 wt.% ChCl/Urea (1:2) showed the lowest energy utilization. Using aqueous ChCl/Urea (1:2) as absorbents is more environment-friendly compared to HPWS and the content of 50 wt.% ChCl/Urea (1:2) showed the highest DGDP. The diameters of both absorber and desorber decreased with the increasing content of ChCl/Urea (1:2). The pressure drop was clearly increased in the absorber but still within the recommended value. In general, aqueous ChCl/Urea (1:2) is a promising solvent for the biogas upgrading process in a biogas plant. In order to be employed in biogas plant, the rate base simulation and pilot test will be conducted in the future study. Acknowledgements This work was supported by the National Basic Research Program of China (No. 2013CB733501), the National Natural Science Foundation of China (No. 21136004, 21476106), the Project of Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) and the Swedish Energy Agency. References

Fig. 14. The green degree of biogas upgrading process (DGDP) with aqueous ChCl/ Urea (1:2).

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Please cite this article in press as: Ma C et al. Modeling, simulation and evaluation of biogas upgrading using aqueous choline chloride/urea. Appl Energy (2017), http://dx.doi.org/10.1016/j.apenergy.2017.03.059