Investigation of Natural Gas Storage through Activated Carbon

Article pubs.acs.org/jced

Investigation of Natural Gas Storage through Activated Carbon Ibrahim I. El-Sharkawy,*,†,§,∥ Mohamed H. Mansour,† Mostafa M. Awad,† and Rehan El-Ashry‡ †

Mechanical Power Engineering Department, Faculty of Engineering, Mansoura University, El-Mansoura 35516, Egypt Gulf of Suez Petroleum Company (Gupco), Cairo 11511, Egypt § Faculty of Engineering Sciences, Kyushu University, Kasuga-koen 6-1, Kasuga-shi, Fukuoka 816-8580, Japan ∥ International Institute for Carbon-Neutral Energy Research (WPI-I2CNER), Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ‡

ABSTRACT: This paper presents an experimental investigation of adsorption equilibrium characteristics of natural gas onto granular activated carbon (GAC). Experiments have been conducted at adsorption temperatures ranging from 20 °C to 50 °C and pressures up to 10 bar. The measurements are based on measuring the adsorption uptake difference between two equilibrium adsorption states. Experimental data have been correlated using four adsorption isotherm models, namely, Langmuir, Tóth, Dubinin−Astakhov (D−A), with and without volume correction. The proposed method reduces the time required to conduct the experiments significantly. It is found that 1 kg of GAC can adsorb about 77 g of natural gas at adsorption temperature of 20 °C and equilibrium pressure of 10 bar. Isosteric heat of adsorption of natural gas/GAC pair has been estimated using Clausius−Clapeyron equation. Comparison between storage capacities of adsorbed natural gas and compressed natural gas has also been discussed. Results extracted from the present study are useful for developing efficient adsorbed natural gas storage systems.

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INTRODUCTION In recent years, natural gas as alternative of conventional fuels has attracted a great attention due to the instability of oil market and increasing strict environmental regulations. Moreover, it is abundant and provides lower carbon emission compared with the conventional petroleum-based fuels. However, efficient natural gas storage systems are essential due to the fluctuation in energy demand. Natural gas is commonly stored in a compressed form (CNG) or in a liquefied state (LNG). However, CNG needs a high storage pressure that ranges from 200 to 300 bar and LNG needs to be stored at about 112 K which requires special equipment.1 Adsorbed natural gas (ANG) is a technology in which natural gas is stored on the surface of porous material at relatively low pressures.2 ANG has attractive advantages: (i) it allows a flexible design of the storage tank, (ii) it has a low cost, and (iii) it provides safe operation especially when it is used in the transportation applications. Adsorption characteristics of natural gas onto porous adsorbents in terms of adsorption capacity, adsorption kinetics, and heat of adsorption are crucial for designing an efficient adsorption gas storage system. Extensive research efforts have been devoted to investigate storage of natural gas onto various types of adsorbents. Following are some representative examples. Alcañiz-Monge et al.3 investigated methane storage onto two series of activated carbon fibers (ACFs). Experiments have been conducted using gravimetric method up to 4 MPa. The authors also analyzed several correlations between parameters related with the porous © XXXX American Chemical Society

texture of adsorbents and methane storage capacity. Adsorption equilibrium of methane, ethane, ethylene, hydrogen, and nitrogen onto activated carbon adsorbent have been measured using static volumetric method at temperatures of 293.15 K, 303.15 K, and 313.15 K and pressures up to 2 MPa.4 Langmuir−Freundlich equation has been used to correlate adsorption isotherm data. Saha et al.5 investigated adsorption parameters of methane onto Maxsorb III using desorption method based on measuring the amount desorbed between two equilibrium adsorption states. The authors used a gas flow meter to measure the amount of desorbed gas. Isosteric heat of adsorption of the studied pair has also been estimated. Luo et al.6 prepared activated carbon from the low cost anthracite by KOH activation. Adsorption characteristics of methane onto the developed activated carbon were measured at 298 K and pressures up to 3.5 MPa using a volumetric method. The authors reported that better methane uptake is dependent on larger micropore volume and specific surface area of adsorbent. If two samples have similar micropore volume and surface area, the activated carbon possessing a narrower micropore size distribution performed higher methane uptake. Rios et al.7 investigated experimentally the charge and discharge cycles of natural gas using a prototype storage tank filled with activated carbon. The effect of natural gas composition on the adsorption Received: May 20, 2015 Accepted: October 19, 2015

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DOI: 10.1021/acs.jced.5b00430 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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capacity has also been discussed. The authors reported that, although methane is the main constituent of natural gas, the preferential adsorption of heavier hydrocarbons and CO2 should be taken into account for the evaluation of the dynamic behavior of adsorbed natural gas storage along several charging and discharging cycles. Bastos-Neto et al.8 studied the storage and delivery of methane on storage vessel filled with activated carbon up to pressure of 40 atm. Experimental data have been compared with the results obtained from the theoretical modeling of the processes. Hao et al.9 measured adsorption isotherms of methane onto surface treated coal at adsorption temperature of 303 K and a pressures up to 5.3 MPa. The authors used Langmuir and Dubinin−Astakhov (D−A) adsorption isotherm models to correlated measured data. Sun et al.10 measured adsorption uptake of methane on metal organic framework, namely HKUST-1. Experiments have been conducted within a temperature ranging from 120 K to 300 K and pressures up to 10 bar. The authors also simulated methane uptakes and its density distribution on the studied adsorbent employing grand canonical Monte Carlo simulation. Adsorption uptakes of methane onto Maxsorb III have been measured using gravimetric method.11 Experiments have been conducted within temperature ranging from 25 °C to 80 °C and pressures up to 8 MPa. The measured equilibrium uptake data have been correlated using Tóth and Langmuir adsorption isotherm models. Other relevant studies can be found elsewhere.12−28 It is obvious from the above review that adsorption capacity is commonly measured gravimetrically using a magnetic suspension balance or volumetrically using a constant-volume variable-pressure method. Although the gravimetric method is accurate, it is expensive; on the other hand, the volumetric technique is time-consuming. Moreover, most of previous studies investigated adsorption of pure methane onto various types of adsorbents and only limited studies discussed adsorption of commercialized natural gas. Therefore, the aim of the present study is to investigate adsorption characteristics of natural gas, commercially available in the Egyptian market, onto granular activated carbon based on measuring the adsorption uptake between two equilibrium adsorption states. The present technique reduces the time required to conduct the experiments significantly compared with the commonly used methods. Experimental data have been correlated using four adsorption isotherm models and isosteric heat of adsorption has also been estimated. Comparison between storage capacities of adsorbed natural gas and compressed natural gas has also been discussed.

Figure 1. Nitrogen adsorption onto the studied adsorbent (GAC).

functional theory (DFT) and the total pore volume of the studied adsorbents has been estimated to be 0.59 cm3·g−1. Set Up. Figure 2 shows a schematic diagram of the experimental set up. It mainly is composed of the adsorption cell, load cell, water bath, natural gas cylinder, vacuum pump, and a safety relief valve. The adsorption and load cells are stainless steel cylinders. Volumes of both cells including connecting tubes have been measured using pure water. Volume of adsorption cell and the connecting tubes up to valve V1 (see Figure 2) is found to be 285 cm3, whereas the volume of load cell including connecting tubes up to V1 and V2 is 4925 cm3. The inner diameter of adsorption cell is 39.3 mm and its height is 244 mm. Length of the connecting tube between adsorption cell and valve V1 is 100 mm. To ensure that natural gas reaches to all activated carbon particles, a 1/4 in. stainless steel distributer of 215 mm length with six holes of 1 mm diameter has been installed at the center of adsorption cell. Moreover, the distributer is covered with a tiny mesh to stop migration of adsorbent particles during evacuation process. A set of Swagelok fittings and 1/4 in. stainless steel plumping have been used for connections. To measure the pressure of the load cell, LOGiT pressure and temperature data logger with external sensors for pressure and temperature measurements has been used. The accuracy of the pressure sensor is ± 3 psi. Temperatures have been measured using K-type thermocouples. Procedure. Prior of conducting the experiments, activated carbon sample is dried in the oven at a temperature of 120 °C for 12 h. The mass of moisture content in the carbon sample is found to be about 5 %. A dry sample of 122 g has been packed into the adsorption cell. The experimental procedure is based on measuring the amount of adsorbed natural gas between two equilibrium adsorption states. Measured data are then correlated to estimate the unknown parameters of adsorption isotherm models. Experiments have been conducted according to the following procedure: (1) The system is evacuated using a vacuum pump at a regeneration temperature of 80 °C for more than 6 h. The evacuation process has been repeated several times to ensure that no residual gas left in the adsorbent. After that, a certain amount of natural gas is charged into adsorption cell. (2) Water bath temperature is kept constant at specified adsorption temperature.

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EXPERIMENTAL SECTION Materials. Commercialized natural gas available in the Egyptian market has been used in the present experiments. The elemental composition has been obtained from the supplier company and found to be: CH4, 92.392 %; C2H6, 1.582 %; C3H8, 5.0 %; i-C4H10, 0.776 %; n-C4H10, 0.054 %; CO2, 0.106 %; and N2, 0.09 %. Adsorbent used in this study is granular activated carbon namely CellCarb provided by Chemviron Carbon, U.K. Nitrogen adsorption onto the studied adsorbent has been measured at a temperature of 77 K using 3Flex Surface Characterization Analyzer produced by Micromeritics Corporation. Adsorption isotherm of Nitrogen onto GAC is depicted in Figure 1. The surface area has been estimated from experimental adsorption data using Brunauer−Emmett−Teller (BET) equation and found to be 1587.9 ± 3.87 m2·g−1. The pore size characterization is calculated using the density B



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Figure 2. Schematic diagram of experimental apparatus. (1) load cell; (2) adsorption cell; (2a) natural gas; (2b) adsorbent; (3) natural gas distributer; (4) isolated water bath; (5) natural gas cylinder; (6) heaters; (7) safety relief valve; P: pressure sensor; T: thermocouple.

(3) A certain amount of natural gas is charged to the load cell. Enough time is taken to ensure that there is a thermal equilibrium between the load cell and water bath. At equilibrium condition, the load cell temperature is assumed to be the same as water bath temperature. The load cell pressure and the equilibrium temperature are then recorded (Ti,1, Pi,1). (4) The valve between the load and adsorption cells is opened and natural gas flows to the adsorption cell. Adsorption process takes about 2 h to reach the equilibrium condition. The equilibrium temperature and pressure are then recorded (Tf,1, Pf,1). It should be mentioned that capacity of the isolated water bath is large enough to keep the temperature nearly constant during the experiments. Therefore, at equilibrium conditions, the value of Ti, almost equals that of Tf. (5) Mass of adsorbed natural gas at state 1 (see Figure 3) is the difference between mass of natural gas in the load cell

Vvoid = Vads,cell −

MAC − vμMAC ρs

where Vi is the load cell volume including connecting tubes up to valves V1 and V2 (see Figure 2). In eq 3, Vads,cell stands for adsorption cell volume including the connecting tube up to V1. The second term of the RHS represents the volume of activated carbon and the third term is the void correction due to the pore volume of activated carbon sample. MAC is the mass of activated carbon, and ρs is the density of solid carbon. vμ is the micropore volume of the adsorbent sample.Mass of adsorbed natural gas at state 1 can be estimated using eq 4 Δmload,1 = m i,1 − mf,1

before and after adsorption process. The calculation procedure can be explained as follows:

mf,1 = ⌊ρf (Vi + Vvoid)⌋1

(2)

(5)

where mi,2 is the mass of natural gas in the load cell before adsorption process corresponding to state 2, mf,2 stands for mass of natural gas in the load cell after adsorption process. Δmload,2 presents difference in amount of natural gas adsorbed between state 1 and state 2. (7) Similarly, adsorption uptake difference between sates 3 and 2 can be estimated. The process has been repeated until state j. (8) Natural gas inside the adsorbent cell is released to the ambient until the system pressure becomes about 2 bar to avoid any leakage prior to conducting another adsorption isotherm experiment. (9) Temperature of water bath has been set at another adsorption temperature using the heaters mounted on the water bath. Steps 3 to 8 have been repeated to

Figure 3. Simplified schematic describing experimental procedure.

(1)

(4)

It is worth mentioning that, Δmload,1 does not represent the total adsorption capacity at state 1 as some of natural gas is already existing in the adsorbent before conducting adsorption process. (6) An amount of natural gas is charged to the load cell once again. After reaching the equilibrium condition (Ti,2 , Pi,2), the valve between adsorption and load cells is opened and adsorption process starts. Once the equilibrium condition is achieved, the pressure and temperature are recorded (Tf,2, Pf,2). The difference between mass of adsorbed natural gas at state 1 and state 2 can be estimated using eq 5 below Δmload,2 = m i,2 − mf,2

m i,1 = (ρi Vi )1

(3)

C



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Figure 4. Plots of RHS versus LHS of eqs 12 to 15 considering natural gas as a methane gas: (a) Langmuir ; (b) Tóth; (c) D−A equation without volume correction; (d) D−A equation with volume correction α = 0.0025; (e) (D−A) with volume correction α = 1/T.

Langmuir and Tóth isotherm models can be given by the following equations:29,30

measure adsorption uptake differences between several equilibrium states (see Figure 3). It should be mentioned that no regeneration is required before each adsorption isotherm as the measurements is based on the equilibrium uptake differences between two equilibrium states, which significantly saves experimental time. Experiments have been conducted within adsorption temperature ranging from 20 °C to 50 °C. It should be highlighted that experiments have been conducted during the winter season, where the water bath temperature was found to be less than 20 °C. To conduct experements at adsorption temperatures are higher than ambient conditions, heaters mounted in the water bath have been used. Data Reduction. In the present study, expermental data have been fitted using four adsorption isotherm models, namely, Langmuir, Tóth, Dubinin−Astakhov (D−A) without volume correction, and D−A with volume correction.

C = Co

C = Co

k 0e qst / RT P 1 + k 0e qst / RT P

(6)

k 0e qst / RT P (1 + (k 0e qst / RT P)t )1/ t

(7)

where C/kg·kg−1 is adsorption uptake, Co/kg·kg−1 is the saturated amount adsorbed, qst/J·mol−1 presents isoteric heat of adsorption, R/J·mol−1·K−1 is the universal gas constant, and P/ kPa is the equilibrium pressure. The parameter t chractrizes the hetrogenety of adsorbent−adsorbate pair, T/K stands for adsorption temperature, and ko is an equilibrium constant. The Dubinin−Astakhov (D−A) without volume correction can be expressed by eq 8 D


Journal of Chemical & Engineering Data ⎡ ⎛ RT ⎛ P ⎞⎞n⎤ C = C0exp⎢ −⎜ ln⎜ s ⎟⎟ ⎥ ⎝ P ⎠⎠ ⎦ ⎝ E ⎣

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⎧ n Wo ⎪ ⎡⎢ ⎛ RTf ⎛ Ps ⎞⎞ ⎤⎥ ⎨exp −⎜⎜ ln⎜ ⎟⎟⎟ ΔCj , j − 1 = va ⎪ ⎢⎣ ⎝ E ⎝ Pf ⎠⎠ ⎥⎦ ⎩ j

(8)

where E/J·mol−1 is an adsorption characteristics parameter and Ps is the saturation pressure. As the main constituent of natural gas is methane, the pseudosaturation pressure Ps at a given temperature is calculated using the following equation31 ⎛ T ⎞2 Ps = ⎜ ⎟ Pc ⎝ Tc ⎠

⎡ ⎛ RT ⎛ P ⎞⎞n⎤ − exp⎢ −⎜⎜ f ln⎜ s ⎟⎟⎟ ⎥ ⎢⎣ ⎝ E ⎝ Pf ⎠⎠ ⎥⎦

In eq 9, the subscript “c” presents the critical condition. The Dubinin−Astakhov (D−A) with volume correction is given by the following equation:

(10)

where

W = Cva

(11a)

Specific volume of adsorbed phase, va, can be estimated using eq 11b.32 va = v bexp[α(T − Tb)]

(15)

A regression fitting between the LHS and the RHS of eqs 12 to 15 have been conducted to estimate the parameters of studied adsorption isotherm models. Solver of the Microsoft Excel has been used to optimize these parameters that minimize the percentage of error between measured and fitted adsorption uptake differences. In eqs 12 to15, the LHS present the concentration differences between two sequential states, which have been experimentally measured. In the RHS of the same equations, equilibrium pressure and temperatures are also measured. Assessment of Overall Uncertainty. Uncertainty in the experimental results is mainly associated with the measurements of equilibrium temperature, equilibrium pressure and cylinders/connecting tubes’ volume. The volume measurement error has been found to be less than ± 0.2 % where the void volume correction has been considered in the calculations. The error in pressure measurements is ranging from ± 1.9 % to ± 5.6 % while the error in the temperature measurements is ranging from ± 2 % to ± 5 %.

(9)

⎡ ⎛ RT ⎛ P ⎞⎞n⎤ W = W0exp⎢ −⎜ ln⎜ s ⎟⎟ ⎥ ⎝ P ⎠⎠ ⎦ ⎝ E ⎣

⎫ ⎪ ⎬ ⎪ j − 1⎭

■

(11b)

where W/cm3·g−1 is volumetric adsorption uptake, Wo/cm3·g−1 is the maximum volumetric adsorption capacity. The parameter vb is the specific volume of methane at boiling condition, Tb stands for boiling temperature, and α is the isosteric coefficient expansion of the adsorbed gas that is considered as 0.0025 or 1/T as given by Himeno et al.33 and Saha et al.5 Equations 6 to 11 can be transferred as the difference of adsorption uptake capacity between two sequential states as follows:5

RESULTS AND DISCUSSION Considering Natural Gas as a Methane Gas. Figure 4a to e show the plot of the RHS versus the LHS of eqs 12 to 15 for natural gas/GAC pair. As the main constituent of natural gas is methane, adsorption isotherms of natural gas onto GAC have also been estimated assuming that natural gas is a methane gas. The density data for methane are taken from NIST Standard Reference Data.34 The experimental raw data have been furnished in Table 1. It can be seen from Figure 4 that Rsquared (R2) of the regression fitting ranging from 0.93 to 0.98 for all studied adsorption isotherm models, which reflects the

⎧⎛ ⎛ k e qst / RTf P ⎞ ⎫ ⎪ ⎪ k 0e qst / RTf Pf ⎞ 0 f ⎜ ⎟ ⎜ ⎟ ⎬ ⎨ ΔCj , j − 1 = C0 ⎜ − ⎟ ⎜ ⎟ qst / RTf ⎪⎝ 1 + k 0e qst / RTf Pf ⎠ ⎪ 1 k e P + ⎝ ⎠ 0 f ⎩ j j − 1⎭

Table 1. Experimental Raw Data Considering Natural Gas as a Methane Gas

(12)

⎧⎛ ⎞ ⎪ k 0e qst / RTf Pf ⎟⎟ ΔCj , j − 1 = C0⎨⎜⎜ ⎪⎝ (1 + (k 0e qst / RTf Pf )t )1/ t ⎠ ⎩ j ⎛ ⎞ ⎫ ⎪ k 0e qst / RTf Pf ⎟ ⎬ − ⎜⎜ qst / RTf t 1/ t ⎟ Pf ) ) ⎠ ⎪ ⎝ (1 + (k 0e j − 1⎭

(13)

⎧ ⎡ ⎛ RT ⎛ P ⎞⎞n⎤ ⎪ ΔCj , j − 1 = Co⎨exp⎢ −⎜⎜ f ln⎜ s ⎟⎟⎟ ⎥ ⎪ ⎢⎣ ⎝ E ⎝ Pf ⎠⎠ ⎥⎦ ⎩ j ⎡ ⎛ RT ⎛ P ⎞⎞n⎤ − exp⎢ −⎜⎜ f ln⎜ s ⎟⎟⎟ ⎥ ⎢⎣ ⎝ E ⎝ Pf ⎠⎠ ⎥⎦

⎫ ⎪ ⎬ ⎪ j − 1⎭

(14) E

Ti

Tf

Pi

Pf

mi

mf

ΔC

°C

°C

bar

bar

g

g

g·kg−1

19.50 19.80 19.80 20.50 29.20 29.50 29.80 29.30 40.20 40.60 39.70 40.20 49.80 50.50 49.60 49.20

19.50 20.00 18.80 19.40 30.30 31.60 30.50 31.20 40.30 40.50 40.30 39.70 51.20 50.20 49.50 49.10

5.06 7.16 9.08 10.92 5.01 6.01 8.01 10.01 5.09 7.08 9.10 10.90 4.50 6.05 7.98 10.12

3.38 6.27 8.40 10.29 3.40 5.40 7.40 9.42 3.60 6.27 8.44 10.28 3.25 5.35 7.38 9.45

16.59 23.55 29.96 36.06 15.89 19.07 25.47 31.98 15.55 21.66 28.01 33.58 13.32 17.91 23.75 30.22

11.39 21.21 28.65 35.15 11.05 17.51 24.19 30.82 11.33 19.79 26.74 32.71 9.87 16.34 22.65 29.11

42.58 19.20 10.69 7.49 39.62 12.75 10.45 9.51 34.63 15.35 10.39 7.11 28.28 12.81 9.01 9.11



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Table 2. Adsorption Parameters of the Studied Adsorption Isotherm Models Considering Natural Gas as a Methane Gas Isotherm parameters

Tóth

Langmuir

D−A equation without volume correction

Co/kg·kg−1 (qst/R)/K ko/kPa−1 E/J·mol−1 n or t/− Wo/cm3·kg−1 ARE/%

0.183 1250 1.28 × 10−5

0.14 1150 2.18 × 10−5

4

5

with volume correction α = 0.0025

7722.88 1.81 0.598 5.5

7331.6 1.73 0.586 3.6

0.175

6722.4 1.42

0.83

with volume correction α = 1/T

6.5

goodness of fitted data. Parameters of the studied adsorption isotherm models have been optimized to minimize the percentage of deviation between experimental and predicted adsorption uptake differences. Numerical values of these parameters have been given in Table 2. The average relative error (ARE) between the experimental and predicted adsorption uptake difference of the studied adsorption isotherm models have been estimated according to eq 16 below and presented in Table 2. ARE% =

100 Nexp

∑

|ΔCexp − ΔCcal| ΔCexp

(16)

It can be seen from Table 2 that the percentage of ARE ranging from 3.6 % to 6.5 %, which falls within the accepted experimental error. Figure 5 shows the adsorption isotherms of

Figure 6. Adsorption isotherms at 30 °C of natural gas onto GAC using the studied adsorption isotherm models (natural gas is considered as a methane gas). Legend: red ■, Langmuir; red ▲, Tóth; blue ◆, D−A equation without volume correction; fuschia ×, D−A equation with volume correction α = 0.0025; *, D−A equation with volume correction α = 1/T.

comparison between measured and predicted adsorption uptake differences for all adsorption isotherm models has been depicted in Figure 7. It can be seen from Figure 7 that the maximum percentage of deviation is not more than ± 10 %.

Figure 5. Adsorption isotherms of natural gas onto GAC using D−A equation without volume correction where natural gas is considered as a methane. Legend: blue ■, 20 °C; red ▲, 30 °C; fuschia ●, 40 °C; blue □, 50 °C; red △, 60 °C; fuschia ○, 70 °C.

natural gas onto GAC within adsorption temperature ranging from 20 °C to 70 °C and pressures up to 15 bar using D−A equation without volume correction. It can be seen from Figure 5 that adsorption uptake of natural gas/GAC pair is about 72 g/ kg at adsorption temperature of 30 °C and equilibrium pressure of 10 bar and it increases up to 77 g/kg at adsorption temperature of 20 °C and the same pressure. Therefore, it is important to keep the adsorbent temperature as low as possible during the charging process in practical applications. External cooling of adsorbent cylinder could be an effective method to increase adsorption uptake capacity, which is one of our future research interests. Adsorption uptake has been estimated using all studied adsorption isotherm models at adsorption temperature 30 °C and presented in Figure 6. It can be noticed that there is a good agreement between adsorption uptakes predicted using all of studied adsorption isotherm models. A

Figure 7. Comparison of measured and predicted adsorption uptake differences for all adsorption isotherm models. Legend: red ■, Langmuir; red ▲, Tóth; blue ◆, D−A equation without volume correction; fuschia ×, D−A equation with volume correction α = 0.0025; blue *, D−A equation with volume correction α = 1/T.

Considering Natural Gas as a Real Gas Mixture. Natural gas is also considered as a real gas mixture, where its density has been estimated based on its elemental compositions from ref 35. The experimental raw data are given in Table 3. Parameters of Langmuir and Tóth isotherm models have been estimated using eqs 12 and 13, respectively. The numerical F



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relatively higher than those in the case where natural gas is considered as a methane gas only. This indicates that where natural gas is considered as a real gas mixture, adsorption uptake predicted is relatively higher than that where natural gas is considered as methane gas only. It is also worthy to mention that most of studies investigate adsorption characteristics of methane and assume natural gas could have similar characteristics of methane. Results of the present study show that adsorption characteristics of methane and natural gas are relatively similar; however, adsorption uptake is slightly lower in the case where natural gas is considered as pure methane. This is because the GAC has the ability to adsorb the other components of natural gas. Isosteric Heat of Adsorption. Figure 9 shows the plot of ln(p) versus (1/T) of natural gas on the studied adsorbent

Table 3. Experimental Raw Data Considering Natural Gas as a Gas Mixture Ti

Tf

Pi

Pf

mi

mf

ΔC

°C

°C

bar

bar

g

g

g·kg−1

19.50 19.80 19.80 20.50 29.20 29.50 29.80 29.30 40.20 40.60 39.70 40.20 49.80 50.50 49.60 49.20

19.50 20.00 18.80 19.40 30.30 31.60 30.50 31.20 40.30 40.50 40.30 39.70 51.20 50.20 49.50 49.10

5.06 7.16 9.08 10.92 5.01 6.01 8.01 10.01 5.09 7.08 9.10 10.90 4.50 6.05 7.98 10.12

3.38 6.27 8.40 10.29 3.40 5.40 7.40 9.42 3.60 6.27 8.44 10.28 3.25 5.35 7.38 9.45

18.72 26.62 33.90 40.85 17.93 21.53 28.78 36.19 17.55 24.46 31.66 37.99 15.03 20.21 26.82 34.16

12.85 23.95 32.41 39.80 12.46 19.77 27.33 34.86 12.77 22.33 30.22 37.00 11.13 18.43 25.58 32.90

48.16 21.85 12.19 8.58 44.79 14.44 11.87 10.88 39.14 17.46 11.82 8.12 31.97 14.53 10.21 10.37

values of these parameters have been furnished in Table 4. As can be seen from Table 4, the Tóth equation presents a better Table 4. Adsorption Parameters of Langmuir and Tóth Adsorption Isotherm Models Considering Natural Gas as a Real Gas Mixture isotherm parameters

Tóth

Langmuir

Co/kg·kg−1 (qst/R)/K ko/kPa−1 E/J·mol−1 n or t/− Wo/cm3·g−1 ARE/% R2

0.185 1300 1.3 × 10−5

0.15 1250 2.03 × 10−5

Figure 9. Plot of ln(p) versus 1/T at various adsorption uptake capacities. Legend: blue ■, 0.15; red ▲, 0.13; fuschia ●, 0.11; blue □, 0.09; red △, 0.07; fuschia ○, 0.05; black ×, 0.03 kg·kg−1.

8.5 0.941

using the D−A equation without volume correction. Isosteric heat of adsorption has been estimated using Clausius− Clapeyron equation as given by eq 17 below;

0.82 6.3 0.976

qst(c = constant) =

−R ∂ln P ∂(1/T )

fitting where R-squared (R2) of the regression fitting is 0.98, which is relatively higher than that of Langmuir fitting. Figure 8 shows adsorption isotherms of natural gas onto GAC using Tóth equation. It can be seen from Table 4 that adsorption isotherm parameters of Tóth and Langmuir equations are

The plot of isosteric heat of adsorption versus the fractional adsorption uptake using eq 17 along with eq 6 have been depicted in Figure 10. Isosteric heat of adsorption as estimated using Langmuir and Tóth adsorption models are also

Figure 8. Adsorption isotherms of natural gas onto GAC using Tóth equation considering natural gas as real gas mixture. Legend: blue ■, 20 °C; red ▲, 30 °C; fuschia ●, 40 °C; blue □, 50 °C; red △, 60 °C; fuschia ○, 70 °C.

Figure 10. Plots of isosteric heat of adsorption versus fractional adsorption uptake. Legend: red △, Clausius−Clapeyron, solid line is fitting of eq 18; dotted line is Tóth, long dashed line is Langmuir. G

(17)



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superimposed on the same figure. It can be seen that isosteric heat of adsorption decreases with the increase of adsorption uptake. The isosteric heat of adsorption can be simply described as a function of fractional uptake using eq 18. The trend of isosteric heat of adsorption agrees well with the studies of Saha et al.5 and El-Sharkawy et al.36 ⎛C⎞ qst = 6538.6 − 503ln⎜ ⎟ ⎝ Co ⎠

where natural gas is considered as a real gas mixture. It can be seen from Figure 12 that storage capacity of the studied adsorbent per unit volume superior that of Maxsorb III because of the high packing density of GAC. It also can be noticed that the capacity of ANG storage is superior that of CNG storage especially at low pressure range. However, charging and discharging processes are essential parameters that need to be addressed in future studies.

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CONCLUSIONS Adsorption isotherms of natural gas onto granular activated carbon (GAC) have been experimentally investigated within adsorption temperatures ranging from 20 to 50 °C and pressures up to 10 bar. The main remarkable points of the present study can be summarized as follows. (i) Adsorption equilibrium data of natural gas onto GAC can be fairly fitted with four popular adsorption isotherm models, namely, Langmuir, Tót h, and Dubinin− Astakhov (D−A) with and without volume correction. (ii) Results of the present study show that adsorption characteristics of methane and commercialized natural gas are relatively similar; however, adsorption uptake is slightly lower for pure methane. (iii) Isosteric heat of adsorption has been estimated using Clausius−Clapeyron along with Dubinin−Astakhov equations. Isosteric heat of adsorption has been correlated as a function of fractional uptake. (iv) It is found that the storage capacity of the studied adsorbent per unit volume superior that of activated carbon powder adsorbent because of the high packing density of GAC.

Storage Capacity of Adsorbed Natural Gas. Figure 11 shows the plot of adsorption equilibrium uptake of methane at

Figure 11. Comparison between adsorption uptake capacity of methane onto various adsorbents at 298 K. Legend: red ▲, Himeno et al.;33 fuschia ◆, Saha et al.;5 blue ●, present study using Tóth equation.

adsorption temperature of 298 K onto the studied adsorbent and other types of activated carbon powders that reported by Himeno et al.33 and Saha et al.5 It can be seen from Figure 11 that adsorption uptake of GAC per unit mass of adsorbent (kg· kg−1) is slightly lower than that of activated carbon powders (Maxsorb). However, the packing density of adsorbent has a significant effect on the storage capacity of natural gas per unit volume. Figure 12 shows the natural gas storage capacity in one cubic meter tank filled with GAC or activated carbon powder (Maxsorb III). The packing density of the studied GAC is found to be 461 kg·m−3, whereas the packing density of Maxsorb III is considered to be 310 kg m−3 as reported by Saha et al.5 For the sake of comparison, the natural gas storage capacity in the same volume of empty tank is also calculated

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] or [email protected]. kyushu-u.ac.jp. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors express their gratitude to Chemviron Carbon, U.K., for providing adsorbent samples. The authors also thank GASTEC, Egypt, for their cooperation & technical support, and Gasco for providing natural gas.

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REFERENCES

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Figure 12. Comparison between ANG and CNG storage capacities at 298 K. Legend: Solid line, present study; long dashed line, Saha et al.;5 dotted line, CNG. H



Article

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Investigation of Natural Gas Storage through Activated Carbon

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