A Study of Methane Adsorption and Accumulation on ... - Springer Link

ISSN 2070-2051, Protection of Metals and Physical Chemistry of Surfaces, 2016, Vol. 52, No. 5, pp. 762–770. © Pleiades Publishing, Ltd., 2016. Original Russian Text © A.A. Fomkin, A.Yu. Tsivadze, A.V. Shkolin, I.E. Men’shchikov, A.L. Pulin, 2016, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2016, Vol. 52, No. 5, pp. 456–464.

PHYSICOCHEMICAL PROCESSES AT THE INTERFACES

A Study of Methane Adsorption and Accumulation on Microporous Carbon Adsorbent in a Wide Temperature Range A. A. Fomkin, A. Yu. Tsivadze, A. V. Shkolin*, I. E. Men’shchikov, and A. L. Pulin Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences, Leninskii pr. 31, bld. 4, Moscow, 119072 Russia *e-mail: shkolin@bk.ru Received November 27, 2015

Abstract—This work studies the adsorption properties of microporous activated carbon AU-2 to determine the efficiency of methane accumulation in a wide temperature range, particularly in the low temperature range. Absolute adsorption isotherms of methane are measured in the pressure range of 20 Pa to 25 MPa and temperature range of 178–260 K. It is shown that the adsorbent accumulates up to 130 m3(ntp, CH4)/m3 at 7 MPa and 298 K. A decrease in the temperature by 55° allows reaching the value of 180 m3(ntp, CH4)/m3. The experimental data are used to plot methane adsorption isosteres that are well approximated by straight lines in the coordinates of lnp = f(1/T)a. The values of differential and integral adsorption heats of methane on the adsorbents are calculated on the basis of the experimental isotherms and are used to calculate an increase in the adsorber temperature as a result of adsorption. DOI: 10.1134/S2070205116050075

INTRODUCTION Using petroleum refinery products as fuel for cars and power setups in large cities results in serious environmental problems. In addition to greenhouse gas emissions, one problem that is caused by combustion is depletion of natural oil reserves, which is prompting a search for less readily accessible deposits and, accordingly, new energy-intensive power recovery techniques [1]. For this reason, in recent years, the problems of alternative and competitive fuel types have been becoming ever more topical. The most promising fuel types at present are believed to be gas fuels—hydrogen and methane—as their use in existing generators and engines is technically feasible with a relatively high performance and the world resources of methane, in particular, can provide for the needs of humanity for at least another 250 years, according to the data of IHS Cambridge Energy Research Associates [2]. It also appears important that using gas fuel has significant environmental advantages as compared to the conventional fuel types based on oil products. For example, the advantages of using gas in the case of methane combustion are as follows [3]: methane requires no additives based on lead (e.g., tetraethyl lead) or other heavy metals to increase the octane number, which results in the absence of their atmospheric emission; there are no solid particles characteristic for additives and carbon blacks in exhaust gases that cause development of human respiratory and car-

diovascular diseases as a result of inhalation; and there are no sulfur compounds and therefore there is no emission of sulfur dioxide, a typical exhaust gas component. Natural gas (methane) combustion produces up to 76% less CO, 75% less NOx, 88% fewer hydrocarbons, and 30% less CO2. The main problem restricting extensive development of gas fuel application for remote sources is absence of energy-saving and fire- and explosionproof storage and transportation systems with high accumulated gas density. For this reason, studying the regularities of methane adsorption accumulation is important and topical. OBJECTS OF RESEARCH The requirements for adsorbents used in methane accumulation systems, apart from such adsorption requirements as bulk density of accumulated gas at 298 K and 35 bar being at least 180 [4] or 263 m3(ntp)/m3 according to the latest requirements [5]1; gravimetric density (at least 0.5 kg/kg); optimum micropore size for facilitation of gas adsorption–desorption; high adsorbent bulk density to decrease the gas phase volume in the storage system; and relatively low adsorption heat at high heat capacity of the porous material to decrease thermal vibrations, also include a number of technological and economic requirements. It is 1 m3(ntp)/m3:

762

m3 of methane (at the normal temperature of 273.15 K and pressure of 100 kPa) per m3 of the storage system.

A STUDY OF METHANE ADSORPTION AND ACCUMULATION

important that the adsorbent should be hydrophobic and characterized by relatively high stiffness and relatively low cost, i.e., cheap raw materials and a comparatively simple method of synthesis. All technological and economic requirements are best met by microporous carbon adsorbents. Experimental studies of methane adsorption on active carbons [6, 7] and also numeric simulation of adsorption based on the methods of molecular dynamics [8] were used to determine the optimum structure–energy parameters for methane accumulation. It was shown that, in the case of real adsorbents, the adsorbate density in micropores is close to the maximum values if the effective micropore width according to Dubinin is 1.0–1.2 nm. In this connection, the adsorption accumulator used in the work was microporous carbon adsorbent AU-2 with an effective pore size close to the optimum value. Microporous Carbon Adsorbent AU-2. The adsorbent is obtained on the basis of plant raw materials using the chemical activation technique. The structure–energy characteristics of the adsorbent were determined on the basis of the adsorption isotherm of the standard benzene vapors at 293 K using the micropore volume filling theory of Dubinin [9]. The results of analysis of the adsorption isotherm of benzene in the coordinates of the Dubinin–Radushkevich equation allowed determining the following carbon structural characteristics: specific micropore volume W0 = 0.58 cm3/g, characteristic adsorption energy of benzene E0 = 19.1 kJ/mol, and average effective micropore halfwidth X0 = 1.26 nm. The bulk density of the regenerated adsorbent was ρbulk = 0.465 g/cm3. Methane. The adsorbate used in the work was methane (especially pure grade) with a purity of 99.995%. In accordance with [10], methane has the following physico-chemical characteristics: molecular weight M = 16.043, critical temperature Tcr = 190.55 K, critical pressure pcr = 4.641 MPa, critical density ρcr = 162.3 kg/m3, boiling point Tb = 111.42 K, and triple point temperature Tt.t. = 90.66 K. ADSORPTION RESEARCH TECHNIQUE Equilibrium values of methane absolute adsorption on AU-2 were measured using three original setups developed at the Frumkin Institute of Physical Chemistry and Electrochemistry, Russian Academy of Sciences. 1. In the pressure range of from 0.1 Pa to 0.1 MPa and temperature range of 177–360 K, methane adsorption was measured using a semiautomated adsorption weight vacuum unit [11]. The maximum absolute measurement error determined according to the technique in [12] was ±0.01 mmol/g with a confidence level of 95%.

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2. Absolute adsorption of methane in the pressure range of 0.1–6 MPa and temperature range of 177– 360 K was studied using a universal adsorption– dilatometric setup with the help of a volumetric technique [13]. The maximum absolute measurement error was ±0.05 mmol/g with a confidence factor of 95%. 3. In the pressure range of 0.2–25 MPa and temperature range of 300–360 K, the equilibrium values of adsorption were measured using the volume– weight method on an original setup [14]. The maximum absolute measurement error was ±0.11 mmol/g with a confidence factor of 95%. This setup was also used to determine kinetic adsorption characteristics according to an original technique, including effective diffusion coefficients in AU-2 pores. The temperature was regulated using an LOIP LT-411 liquid circulation thermostat for adsorbent sample thermal conditioning in the temperature range of 293– 360 K with an error of ±0.01 K. An LOIP FT-311-80 liquid ultralow-temperature cryothermostat with a precision of 0.01° was used for thermal control in the range of lower temperatures. RESULTS AND DISCUSSION Adsorption Isotherms In all cases of practical application of adsorption technologies, equilibrium adsorption isotherms are the main comparative characteristic of different adsorbent types and determine the choice of the optimum working conditions of the technological procedure. Simultaneously, the adsorption isotherm is a source of information on the adsorbent structure, adsorption heat effects, and a number of other physico-chemical and technological parameters. Isotherms of methane absolute adsorption on activated carbon AU-2 at the pressures of from 20 Pa to 25 MPa and in the temperature range of 178–360 K are shown in Fig. 1. The adsorption isotherms of methane on carbon AU-2 in the a,ln P-coordinates are S-shaped (Fig. 1). Isotherms of absolute adsorption retain the same character of dependence at a transition into the supercritical temperature range: they manifest a gradual increase and undergo no abrupt changes characteristic for condensation-type first-order transitions. This indicates the special state of methane adsorbed in the activated carbon micropores. Calculation of the limiting adsorption of methane [9] at the boiling point of 111.42 K yields 13.2 mmol/g. Adsorption Isosteres Experimentally obtained isotherms of absolute adsorption of methane on carbon AU-2 were used to plot adsorption isosteres (a = const) (Fig. 2). Adsorption isosteres are well approximated by straight lines in

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a, mmol/g

12 11 10 9 8 7 6 5 4 3 2 1 0

5 3

1

8

2

7 6 4 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 ln P, Pa

Fig. 1. Isotherms of methane adsorption on microporous carbon adsorbent AU-2 at temperatures of, K, (1) 178, (2) 216, (3) 243, (4) 273.15, (5) 305, (6) 320, (7) 340, and (8) 360. Symbols: experimental data; lines: spline approximation.

the whole studied range of variation of adsorption equilibrium parameters. The characteristic isostere linearity has already been pointed out, e.g., for methane [11, 15–17] and inert gases [19, 20]. It is of interest that the property of linearity of absolute adsorption isosteres also extends to the region of the liquid state of the adsoptive [21]. The property of isostere linearity preserved in a wide range of temperature and pressure variation points to the special adsorbate state in micropores. As opposed to the equilibrium phase, adsorbed molecules in micropores remain entirely within the adsorption field of the solid, in a strongly dispersed state, up to 10–15 molecules per individual micropore [9]. Such an adsorbed substance state prevents the formation of liquid phase nuclei and, therefore, occurrence of phase transitions. This property is extremely important in practice, as it allows determining the values of adsorption in a wide range of parameters a, P, and T without additional measurements at an extremely high confidence level. This simplifies calculations using the minimum information on adsorption. Adsorption Kinetics The values of kinetic parameters of adsorbents are one of the main criteria for choosing the adsorption material for gas accumulation problems owing to the accumulation system parameters being influenced by the adsorption capacity (loading) completion rate and the rate of gas desorption from the vessel with the adsorbent. The kinetic adsorption curve represents the dependence of the adsorbed substance amount on the duration of the experiment. The kinetic curves of methane adsorption of the AU-2 adsorbent were experimentally obtained using the method of fast gas

18 17 16 ln Ps 15 14 14 13 13 12 12 11 11 10 10 9 9 8 8 7 7 6 6 5 4 5 1 3 4 2 3 0.0025 0.0035 0.0045 0.0055 0.0030 0.0040 0.0050 0.0060 1/T, 1/K 1/Tcr

ln P, Pa

764

Fig. 2. Isosteres of absolute methane adsorption on microporous carbon adsorbent AU-2 at adsorption a, mmol/g: (1) 0.3, (2) 0.5, (3) 0.7, (4) 1.0, (5) 1.5, (6) 2.0, (7) 3.0, (8) 4.0, (9) 5.0, (10) 6.0, (11) 7.0, (12) 8.0, (13) 9.0, and (14) 10.0. Symbols: experimental data; lines: approximating lines. Ps: line of saturated vapor pressure; Tcr: critical temperature.

inflow into the adsorber from zero to the given pressure. The mass of the adsorbent–adsorbate adsorption system was registered in time until equilibrium was established. Figs. 3a and 3b show kinetic methane adsorption–desorption curves on the AU-2 adsorbent at the pressures of 7.0 and 20.0 MPa. Desorption curves were obtained by decreasing the pressure and maintaining the equilibrium phase pressure at the level of 0.1 MPa. The process of adsorbate penetration into the granules of the microporous material is limited by diffusion. The parameter characterizing this process is effective diffusion coefficient calculated according to [22]. 2 (1) D = Kr , 2 π τ 0.5 where r is the granule radius, m; K is the coefficient depending on the granule shape; and τ0.5 is half the time of adsorption capacity discharge, s. Experimental data on adsorption kinetics were approximated by exponential dependences, and then the values of half-period τ0.5 of adsorption capacity discharge and effective diffusion coefficient D were calculated. The calculation results are presented in Table 1.

Differential Molar Isosteric Adsorption Heat of Methane Physical adsorption of gases in microporous adsorbents is always accompanied by evolution of adsorption heat. The efficiency of adsorption accumulation


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msp, g/g 0.18

adsorbent, the most correct approach to estimation of the molar isosteric differential heat of absolute adsorption is the thermodynamic approach of Bakaev [24]. In this case, adsorption heat is calculated on the basis of (2):

(а)

0.15 0.12 0.09 0.06

8

16

24

32

msp, g/g 0.35

40 48 T, s

56

64

72

80

(b)

0.28 0.21 0.14 0.07 0

8

16 24

32 40 48 56 T, s

64 72 80

Fig. 3. Kinetic curves of methane adsorption and desorption on carbon adsorbent AU-2 at temperature T = 303 K and pressure of gas inflow of (A) 7 and (B) 20 MPa. Squares and triangles: relative adsorbent mass variation msp under adsorption and desorption, respectively; dashed lines: approximating curves.

depends considerably on the structure–energy parameters of the adsorbent. The adsorption heat of methane in the range of high pressures can be determined experimentally using a calorimeter [23], but such measurements are extremely difficult and require unique equipment. Methane adsorption heat can also be estimated to a good approximation degree using the dependence of differential molar isosteric adsorption heat determined on the basis of adsorption isotherms of methane. In a wide range of temperatures and pressures, when it is very important to take into account nonideality of the gas phase and noninertness of the Table 1. Values of half discharge time of adsorption capacity and effective diffusion coefficients under methane adsorption and desorption on the AU-2 adsorbent Process

Parameter

7 MPa

20 MPa

Adsorption

τ0.5, s

2.04

1.95

D × 10 4, сm2/s

3.44

3.59

τ0.5, s

3.32

2.22

D ×10 4, сm2/s

2.11

3.17

Desorption

( ) ( )

⎛ ∂(ln p) ⎞ ⎛ ∂ν(a) ν g ⎞⎟ − q st = −RZ ⎜ ⎟ ⎜1 − ∂a T ⎠ ⎝ ∂(1 T ) ⎠ a ⎝ (2) ∂p ⎛ ∂ν(a) ⎞ − ⎜ ν(a) − T ⎟, ∂a T ⎝ ∂T a ⎠ where Z is the compressibility factor of the equilibrium gas phase at pressure p, temperature T, and specific volume of the gas phase νg; R is the universal gas constant; ν(a) = V o mo is the specific reduced volume of the adsorbent/adsorbate adsorption system; mo is the regenerated adsorbent mass; and a is the adsorption value at the given pressure and temperature. As follows from (2), differential molar isosteric heat of absolute adsorption depends only on adsorption system parameters (a, p, T) and particularly on the slope of adsorption isosteres (∂lnp/∂(1/T))a, gas phase nonideality Z and νg, and adsorption deformation of the system (isothermal (∂ν(a)/∂a)T and isosteric (∂ν(a)/∂T)a, as well as isotherm slope (∂p/∂a)T). Estimation of the maximum value of derivative (∂ν/∂T)a according to the data of [25] showed that T(∂ν/∂T)a was much lower than ν(a) in the studied range of pressures and temperatures. Therefore, the term of T(∂ν/∂T)a in Eq. (2) was not taken into account in calculations. Estimates of the effect of adsorption–stimulated deformation of a rigid microporous carbon adsorbent under adsorption of methane on adsorption heats in [26] showed that the contribution of deformation member (∂ν/∂a)T to adsorption heat for the adsorbent with similar strength characteristics did not exceed 2– 3% in the pressure and temperature ranges under consideration. Therefore, the energy contribution of adsorption deformation was neglected in these calculations. Finally, we obtained from (2) that

( )

0.03 0

765

( )

⎛ ∂(ln p) ⎞ ∂p (3) q st = −RZ ⎜ ν(a). ⎟ − ⎝ ∂(1 T ) ⎠ a ∂ a T Equation (3) was used to calculate differential molar isosteric adsorption heat of methane on microporous carbon adsorbent AU-2 in the temperature range of 178 to 360 K in the pressure range of 20 Pa to 25 MPa. The calculation results are presented in Fig. 4. As follows from Fig. 4, the heat decreases at an increase in adsorption in the whole temperature range. In the range of low adsorption values (0–2 mmol/g), the heat decreases relatively quickly from 22.5 to 16 kJ/mol. The decrease in differential heat suggests the presence


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of high-energy adsorption centers in the porous structure of carbon that are gradually filled with an increase in methane adsorption. Taking into account the nonideal character of the gas phase results in appearance of the temperature dependence of differential adsorption heats in the range of medium and high macropore filling values (adsorption of 2–11 mmol/g). The thermal dependence of adsorption deformation becomes more pronounced at an increase in the temperature, so that differential molar adsorption heat at the temperature of 393 K and adsorption of 8 mmol/g drops to 5.5 kJ/mol, by more than 2.5 times as compared to the heat at 178 K. To account for nonisothermicity of the methane adsorption process, it is important to determine the integral adsorption heat and then to calculate variation of the adsorbent temperature under adsorption: a

∫

Q = q st (a)da.

(4)

0

Integral adsorption heats were calculated in the adsorption range of zero to the value corresponding to technically required pressures of 3.5, 7, 10, 15, and 20 MPa [4]. The calculation results are presented in Table 2. Integral adsorption heat at the temperatures below 216 K is practically constant for the whole studied pressure range, which implies that the porous sample reaches the limiting pore filling in the pressure range of up to 3.5 MPa. One must point out that integral adsorption heat practically stops increasing at 7 MPa at a temperature of 243 K, at 10 MPa at 273 K, and at 15 MPa at higher temperatures. One can conclude on the basis of the data of the dependences that the larger fraction of adsorption heat at any of the studied temperatures is develops in the initial adsorption range at pressures of up to 3.5 MPa, while the intensity of heat evolution somewhat decreases at an increase in temperature. Variation of adsorbent temperature ∆T can be estimated on the basis of heat capacity C0 of the adsor-

Q, kJ/mol

766

24 22 20 18 16 14 12 10 8 6 4 2 0

1

4

2

6

1

2

3

4

7

5

8

5 6 7 а, mmol/g

8

9

3

10 11

Fig. 4. Dependence of the differential molar isosteric adsorption heat of methane on the AU-2 adsorbent on the adsorption value at temperatures of, K, (1) 178, (2) 216, (3) 243, (4) 273.15, (5) 305, (6) 320, (7) 340, and (8) 360. Symbols: experimental data; solid lines: approximation.

bent–adsorbate system. Here, variation of the temperature of the adsorber with the adsorbent under adiabatic conditions depends also on heat capacity of the adsorber itself Ca and of the gas phase C pg at the given P and T. ∆T = Qi/(Co+ Ca + C pg ),

(5)

Thus, it is enough to determine the constant of effective heat capacity of the system, Ce = Co + Ca+ C pg , in the calibration experiment to determine the adsorbent heating temperature. The adsorption system used was a methane storage model of 40-L steel pressure vessels with the average mass of 60 kg that were filled by carbon adsorbent with a bulk density of 720 kg/m3; the system loading time was 1 h. Room temperature gas was supplied into the storage system (298 K).

Table 2. Integral molar isosteric adsorption heat of methane on the AU-2 adsorbent Q, kJ/kg T, K p = 3.5 MPa 178 216 243 273 305 320 340 360

170.86 157.46 142.36 124.77 107.06 99.27 89.06 79.60

p = 7.0 MPa 170.88 159.81 150.67 137.05 121.83 115.32 106.57 97.90

p = 10 MPa 170.88 159.81 151.30 140.82 127.28 121.38 113.60 106.66

p = 15 MPa 170.88 159.81 151.30 141.72 130.94 127.06 118.89 112.22


p = 20 MPa 170.88 159.81 151.30 141.72 131.46 127.06 120.92 114.72

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Table 3. Temperature variation when the adsorption system is filled with methane up to standard pressures. First loading (regenerated storage system) ∆T, K

T, K 216 243 273 305 320

p = 35 bar

p = 70 bar

p = 100 bar

p = 150 bar

p = 200 bar

49.12 45.61 41.31 36.43 34.28

44.24 45.22 43.26 40.14 38.38

35.06 41.70 42.09 40.33 39.16

29.79 34.86 38.18 38.38 37.99

28.42 31.74 34.47 35.84 35.84

Table 4. Temperature variation when the adsorption system is filled with methane up to standard pressures. Typical loading (the initial pressure in the storage system is 0.1 MPa) ∆T, K

T, K 216 243 273 305 320

p = 35 bar

p = 70 bar

p = 100 bar

p = 150 bar

p = 200 bar

24.32 27.83 29.20 28.61 27.83

21.39 28.61 31.74 34.47 32.32

15.53 26.27 30.96 33.11 33.11

13.57 21.39 28.03 31.54 32.32

13.18 19.43 25.10 29.40 30.57

Data on gas adsorption heats on an identical adsorbent obtained in wide temperature and pressure ranges were used for calibration of heat capacity of the adsorbent. To simplify the problem, the calibration conditions were standardized and the pressure values of 3.5, 7, 10, 15, and 20 MPa and experimental temperatures were set. In the work, data on adsorption heats for the AUK–methane adsorption system at the pressures of 3.5, 7, 10, 15, and 20 MPa were used [27]. The adsorbent mass was similar in the experiment and in calibration. As specific heat capacity Co of carbon adsorbents little depends on porosity, the maximum temperature variation was measured experimentally at an increase in the pressure of methane from zero to the given pressure. The calculation results are presented in Tables 3 and 4. Calculations were carried out for two types of initial conditions: the first loading conditions, when the storage system is regenerated and is under a reduced pressure of about 1 mbar, and the conditions of typical (repeated) loading, when the system is under a residual pressure of 0.1 MPa after supplying methane to the consumer. As follows from Tables 3 and 4, the storage system loading by methane to 3.5 MPa results in an increase in the storage system temperature by 30–35°; here, the system heating temperature for the typical (repeated) loading is lower by 20% than that of the first one. It must be pointed out that the difference in the heating temperatures of the storage system in the case of the

first loading and further loading to the pressure of 3.5 MPa can reach 100% at low temperatures, which is determined by large methane accumulation volume in this range accompanied by the corresponding heat evolution. An increase in the loading pressure at low temperatures results in the further, relatively small increase in the difference between the heating temperatures, as dependent on the loading type. In the case of low-temperature loading, both for the first and further loadings, the system heating temperature somewhat decreases, which is determined, on the one hand, by the duration of loading allowing to remove the system heat into the atmosphere through the storage system walls and, on the other hand, by the decrease in the amount of adsorbed gas in the system and, therefore, a decrease in the amount of evolved heat. At room temperatures and above, superposition of these two factors results in the appearance of a heating maximum at the pressure of about 7–10 MPa. Adsorption Concentration of Methane Efficiency of methane adsorption concentration as compared to the amount of gas in the adsorbent-free system is related to the a–p–T parameters of the adsorption system and is characterized by absolute and relative adsorption efficiency. The excess in the amount of methane accumulated in volume with adsorbent Va due to adsorption on the microporous carbon adsorbent, as compared to equiv-


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200 180 160 140 120 100 80 60 40 20 0

6 5

3

2

1

7

V, m3 (ntp)/m3

V, m3 (ntp)/m3

768

4

9

8

1

2

3

4

5 6 7 Р, MPa

8

9 10 11 12

Fig. 5. Dependence of the amount of accumulated methane on microporous carbon adsorbent AU-2 on the pressure at temperatures of, K, (1) 178, (2) 216, (3) 243, (4) 273.15, (5) 305, (6) 320, (7) 340, and (8) 360. (9) Compressed gas at 293 K.

Specific accumulated methane amount (m3(ntp)/m3) was determined using (7)

(6) Va

aρ M + ερ g (7) V a = p CH 4 , ρn where a is the adsorption capacity at pressure p and temperature T; ρp is the bulk density of the adsorbent; M CH4 is the molar mass of methane; ε is the porosity or the fraction of free space for the gas phase (8); ρg and ρn are the gas phase density at the given temperature and pressure and under normal conditions, respectively; and ε is the porosity. ρp (8) − ρ pW 0, ρc where ρc ≈ 2000 kg/m3 is the “carbon skeleton” density (“helium” density) [28] and W0 is the micropore specific volume. Porosity is determined by subtracting the volume occupied by pores and carbon skeleton from the overall volume. Figure 5 shows the dependences of the amount of accumulated methane on AU-2 on pressure at different temperatures. As follows from Fig. 5, the amount of accumulated methane depends significantly on temperature and pressure. The maximum amount of accumulated methane (~180 m3(ntp)/m3) at 7 MPa is reached at 216 K. To increase the volume of accumulated methane, one has either to change the thermodynamic parameters of the methane storage system, pressure, and temε =1−

0

1 2 3 4

6

5 8

7 1

2

3

4

5

6 7 Р, MPa

8

9 10 11 12

Fig. 6. Dependence of absolute efficiency of adsorption concentration of methane on microporous carbon adsorbent AU-2 on pressure at temperatures of, K, (1) 178, (2) 216, (3) 243, (4) 273.15, (5) 305, (6) 320, (7) 340, and (8) 360.

alent adsorbent-free volume Vg, determines the absolute efficiency of adsorption concentration:

ΔV = Va − V g .

120 110 100 90 80 70 60 50 40 30 20 10

perature, or to increase bulk density of the microporous material. Specific amount Vg of the accumulated gas in the absence of any adsorbent (in m3 (ntp)/m3) is calculated using (9):

ρg (9) . ρn Figure 6 shows dependences of absolute efficiency of adsorption concentration of methane in AU-2 on pressure in the temperature range of 178 to 360 K. In the whole studied temperature range (Fig. 6), the absolute efficiency of accumulation grows at an increase in pressure, passes a maximum, and decreases in the range of higher pressures. At an increase in the temperature, the maximum of absolute efficiency is shifted into the range of higher pressures and it is in the range of 40–50 MPa at room temperatures. Also, the maximums of absolute efficiency become less pronounced at an increase in the temperature. Such a behavior of curves is due to the properties of the adsorption system and gas phase becoming more similar at an increase in the pressure. Moreover, an important technical characteristic of gas accumulation systems is relative efficiency of adsorption concentration reflecting the ratio between the amounts of accumulated methane per unit volume in the presence and in the absence of adsorbent (10): Vg =

η = Va V g .

(10)

Figure 7 shows dependences of absolute effectiveness of methane adsorption concentration in AU-2 on pressure in the temperature range of 178 to 360 K. These functions are decreasing and have no extrema. At pressures above 5 MPa and temperatures below 320 K,


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1 2 3 4 5 6

η

10 9 8 7 6 5 4 3 2 1 0

7 8

1

2

3

4

5 6 7 Р, MPa

8

9 10 11 12

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3. Isosteric adsorption heats in the range of the nonideal gas phase depend significantly on temperature. The initial adsorption heat of methane is ~22 kJ/mol. 4. The maximum amount of accumulated methane (~180 m3(ntp)/m3) is reached at 216 K and 7 MPa. 5. The absolute efficiency of adsorption accumulation of methane on the AU-2 adsorbent grows at an increase in pressure, passes a maximum in the range of 3.0–7.0 MPa in the studied temperature range, and decreases in the range of higher pressures. 6. The relative efficiency decreases at an increase in pressure from 10 to 2 in the pressure range of 4.0–7.0 MPa and at room temperatures above 240 K. ACKNOWLEDGMENTS

Fig. 7. Dependence of relative efficiency of adsorption concentration of methane on microporous carbon adsorbent AU-2 on pressure at temperatures of, K, (1) 178, (2) 216, (3) 243, (4) 273.15, (5) 305, (6) 320, (7) 340, and (8) 360. The dashed line corresponds to methane content in the adsorbent-free volume.

This work was financially supported by the Ministry of Education and Science of the Russian Federation, agreement no. 14.607.21.0079. The unique identifier of the agreement: RFMEFI60714X0079. REFERENCES

the curves of relative efficiency converge and practically coincide. In the range of lower pressures, relative efficiency grows abruptly at a decrease in pressure and decreases at an increase in temperature. Thus, at pressures of 40–50 MPa, the amount of accumulated methane in the storage system will be twice as high in the whole studied temperature range as in an adsorbent-free vessel. Considering Figs. 6, 7 together allows it to be concluded that the maximum methane accumulation efficiency on the AU-2 adsorbent is reached in the region of 10–70 bar. When methane–AU-2 accumulation systems are used at room temperatures, the maximum efficiency can be obtained at a pressure of about 40– 70 bar. Also, as follows from the data in Fig. 6, the absolute efficiency of adsorption concentration at the temperatures of 178–320 K drops to 1.3 at 12 MPa, which indicates the necessity of using storage systems based on AU2 in the pressure range of up to 7 MPa, where the relative accumulation efficiency retains a value above 2. CONCLUSIONS 1. Adsorption isotherms of methane on microporous carbon sorbent upon a transition from the subcritical to supercritical temperature range do not undergo any drastic changes. The maximum values of methane adsorption reach 13 mmol/g. 2. Adsorption isosteres are linear to a good degree of approximation in the whole range of pressures and temperatures, and their slope remains unchanged at a transition into the supercritical range.

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Translated by M. Ehrenburg

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