Ultrasound promoted catalytic liquid-phase dehydrogenation of isopropanol for Isopropanol–Acetone–Hydrogen chemical heat pump

Ultrasonics Sonochemistry 23 (2015) 66–74

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Ultrasound promoted catalytic liquid-phase dehydrogenation of isopropanol for Isopropanol–Acetone–Hydrogen chemical heat pump Min Xu a,⇑, Fang Xin a,b, Xunfeng Li a, Xiulan Huai a, Hui Liu c a b c

Institute of Engineering Thermophysics, Chinese Academy of Sciences, Beijing 100190, China University of Chinese Academy of Sciences, Beijing 100049, China State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China

a r t i c l e

i n f o

Article history: Received 28 July 2014 Received in revised form 3 September 2014 Accepted 3 September 2014 Available online 11 September 2014 Keywords: Ultrasound Isopropanol dehydrogenation Chemical heat pump Power density Mathematical modeling

a b s t r a c t The apparent kinetic of the ultrasound assisted liquid-phase dehydrogenation of isopropanol over Raney nickel catalyst was determined in the temperature range of 346–353 K. Comparison of the effects of ultrasound and mechanical agitation on the isopropanol dehydrogenation was investigated. The ultrasound assisted dehydrogenation rate was significantly improved when relatively high power density was used. Moreover, the Isopropanol–Acetone–Hydrogen chemical heat pump (IAH-CHP) with ultrasound irradiation, in which the endothermic reaction is exposure to ultrasound, was proposed. A mathematical model was established to evaluate its energy performance in term of the coefficient of performance (COP) and the exergy efficiency, into which the apparent kinetic obtained in this work was incorporated. The operating performances between IAH-CHP with ultrasound and mechanical agitation were compared. The results indicated that the superiority of the IAH-CHP system with ultrasound was present even if more than 50% of the power of the ultrasound equipment was lost. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Large amounts of low-temperature waste heat (<100 °C), which are usually released to the environment, are produced in industrial processes [1]. The recovery of low-temperature waste heat is becoming a promising way to meet energy requirement. An effective method is upgrading the waste heat to greater than 200 °C by using the heat pump system. The upgraded heat can be easily used for refrigeration, desiccation and electrical generation [2]. To achieve this purpose, the isopropanol–acetone–hydrogen chemical heat pump (IAH-CHP) is a promising alternative because of its advantages such as high upgrading temperature, possibility of energy storage, and low hazard [3]. Fig. 1 shows the schematic diagram of the IAH-CHP system which consists of an endothermic reactor, an exothermic reactor, and a distillation column. The dehydrogenation of isopropanol occurs at the temperature TL in the endothermic reactor (reboiler in Fig. 1) to yield acetone and hydrogen. The reaction equation is shown as follows:

ðCH3 Þ2 CHOH $ ðCH3 Þ2 CO þ H2

⇑ Corresponding author. Tel.: +86 10 82543035. E-mail address: [email protected] (M. Xu). http://dx.doi.org/10.1016/j.ultsonch.2014.09.003 1350-4177/Ó 2014 Elsevier B.V. All rights reserved.

ð1Þ

The heat QL is introduced to drive the reaction and subsequently to separate of acetone and isopropanol in the distillation column. The acetone and hydrogen as the distillate at condensing temperature TC is compressed by the compressor, and then heated in the recuperator. The acetone and hydrogen are then fed into the exothermic reactor after being heated up to the temperature TH in the heater. Acetone hydrogenation occurs at the temperature TH in the exothermic reactor. The reaction equation is shown as follows:

ðCH3 Þ2 CO þ H2 $ ðCH3 Þ2 CHOH

ð2Þ

The high-temperature heat QH in the exothermic reactor is produced and released. The effluent is cooled in the recuperator and is then fed back to close the cycle. Isopropanol dehydrogenation can take place in the gas phase (TL > 82.4 °C) or liquid phase (TL 6 82.4 °C) for different temperature of heat source. The IAH-CHP system implementing gas-phase dehydrogenation of isopropanol, which was first proposed by Prevost and Bugare [4], has been studied in these decades [5–8]. A demonstration unit was also built by KlinSoda and Piumsomboon [9]. The IAH-CHP system implementing liquid-phase dehydrogenation of isopropanol has also been studied, which is considered to have remarkable advantages: (i) lower heat source

M. Xu et al. / Ultrasonics Sonochemistry 23 (2015) 66–74

67

Nomenclature C CP COP Ea kapp

concentration (mol m3) heat capacity used in Eq. (6) (J mol1 K1) coefficient of performance activation energy(kJ mol1) 1 apparent dehydrogenation rate constant (mol g1 ) cat h

k0 NH m mcat P Pdiss QC QH QL rd R t

1 pre-exponential factor of rate constant(mol g1 ) cat h the number of mole of the hydrogen (mol) weight of reactant used in Eq. (6) (g) weight of catalyst (g) pressure (Mpa) power dissipated (W) heat of the condenser(kW) heat released from the exothermic reactor(kW) heat consumed by the endothermic reactor (kW) 1 dehydrogenation rate (mol g1 ) cat h 1 1 gas constant (J mol K ) reaction time (min)

T T0 V Win x y

temperature (K) environment temperature used in Eq. (11) (K) volume (m3) power input (kW) mole fraction in liquid phase mole fraction in gas phase

Greek symbols g exergy efficiency Subscripts ace acetone cat catalyst H high temperature iso isopropanol L low temperature

Fig. 1. Schematic diagram of the Isopropanol–Acetone–Hydrogen chemical heat pump.

temperature and (ii) absence of the limitation of chemical equilibrium. Saito et al. [10] evaluated the energy efficiency of continuous and storage-type IAH-CHP with liquid-phase dehydrogenation of isopropanol. Gandia and Montes [11] established a mathematical model to estimate the optimal design parameters. Kim et al. [3] obtained the rate equation of isopropanol dehydrogenation at boiling point on Raney nickel and then evaluated the energy efficiency of the system. However, there are some disadvantages for liquidphase dehydrogenation of isopropanol: (i) lower conversion at low temperature and (ii) inhibitory effect of the produced acetone in the liquid phase. Many studies have been done to enhance the performance of the liquid-phase dehydrogenation of isopropanol by using reactive distillation column [12–15]. Unfortunately, however, these efforts are still not enough to meet the requirement of the IAH-CHP system. Ultrasound irradiation is a novel and interesting technique for process intensification in homogeneous or heterogeneous system. The physical and chemical effect brought from the acoustic cavitation, such as extraordinary high local temperature and pressure, strong acoustic streaming, high shear stress near the bubble wall, microjets near the solid surface, and decomposition of molecules in bubbles, results in the enhancement of the heat mass transfer

and reaction process. Ultrasound has been successfully used in several industrial processes including extraction [16,17], emulsification [18,19], material production [20,21], cleaning and water treatment [22,23]. Some experimental studies have also been reported to improve the heterogeneous catalytic reactions by using ultrasound irradiation [24,25]. These investigations give us the inspiration for possibility of enhancing the catalytic dehydrogenation of isopropanol by using ultrasonic. When the ultrasound is used, however, there is a question should be confirmed, i.e., how the amounts of ultrasonic power input is related to the energy performance of the IAH-CHP system. In our previous work [15], we carried out intrinsic kinetic experiments regarding liquid-phase dehydrogenation of isopropanol over Raney nickel catalyst. On the basis of experimentally determined kinetic data, a rigorous mathematic model was established to simulate the IAH-CHP system with reactive distillation column and to investigate the application of reactive distillation to enhance the thermodynamic performance of the IAH-CHP system. Based on the previous work, the aims of this work are to: (i) compare the performance of catalytic isopropanol dehydrogenation with ultrasound irradiation and mechanical stirrer experimentally; (ii) identify the quantitative relationship between the

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M. Xu et al. / Ultrasonics Sonochemistry 23 (2015) 66–74

dehydrogenation rate and ultrasonic power density; and (iii) evaluate the suitable ultrasonic power input in terms of coefficient of performance and exergy efficiency based on our previous built mathematic model.

Table 1 Specifications of the Raney Ni catalyst used in our work. Bulk composition (wt%)

SBET (m2/g)a

SNi (m2/g)b

Vp (cm3/g)a

dp (nm)a

Ni93.5Al6.5

98.81

15.5

0.1202

7.50

a

2. Experimental and theoretical analysis method 2.1. Experimental apparatus and procedure A schematic representation of the experimental setup is depicted in Fig. 2. The experimental setup comprises (i) a 500 ml four-necked flask placed in an oil bath at a temperature fluctuation of ±0.1 K; (ii) a 20 kHz ultrasound probe (titanium horn with diameter of 16 mm) with a maximum power output of 200 W (Hangzhou success Ultrasonic Equipment Co., Ltd.); (iii) a reflux condenser with 5 °C cooling water, in which vaporized isopropanol and produced acetone can be cooled and refluxed to the flask; (iv) an ice trap packed with dry ice, which is used to further separate acetone from gas phase and thus ensure the purity of hydrogen in the outlet; and (v) a gas flow meter (Model 5860E, Brooks Instrument, USA) with ±1% accuracy to measure hydrogen flow. In a typical experiment, 320 ml of isopropanol (99.5 wt%, Sinopharm Chemical Reagent Beijing Co., Ltd.) is charged into the flask and heated up to the selected temperature. The reaction is started by adding 5 g of Raney nickel catalyst (Aladdin) with an average diameter of 150 lm to the reactor through a funnel. Table 1 shows the characterization of the catalyst including bulk composition, BET surface area, pore volume, mean pore diameter and active nickel surface area. During the reaction, the hydrogen flow rate is recorded on-line every second. A total of 16 runs are performed at four different temperatures and four different ultrasonic power densities. In the experimental runs, when the ultrasound irradiation is started the temperature in the liquid phase increases by 1–5 °C and hold constant temperature which is a little higher than the oil bath temperature due to the combination of ultrasonic power and oil bath. Some more runs are performed by using with mechanical agitation (a simple two-bladed stirrer is used) with the stirrer speed of 500 and 650 rpm to replace the ultrasound irradiation. No significant variation of the amount of hydrogen production is observed when the stirrer speed increases to 800 rpm. 2.2. Analysis On the basis of the assumptions of no side reaction, one molecular isopropanol can produce one molecular hydrogen. Therefore, for a catalytic reaction in a constant volume batch reactor, the rate

Determined by N2 adsorption at 77 K using a Micromeritics TriStar3000 apparatus. b Determined by means of volumetric hydrogen chemisorption under the assumption that one Ni surface atom chemisorbs one hydrogen atom.

of isopropanol dehydrogenation is calculated by the number of moles of hydrogen produced per catalyst weight per unit time [37]:

rd ¼

1 dn 1 dN iso 1 dNH ¼ ¼ mcat dt mcat dt mcat dt

ð3Þ

where n is the extent of reaction, Niso is the number of moles of isopropanol consumed, NH is the number of moles of hydrogen accumulated, rd is the rate of dehydrogenation of isopropanol, mcat is catalyst weight in the reactor, and t is the reaction time. The volume of the hydrogen accumulated is obtained by the integration of hydrogen produced over time. Based on the assumption of ideal gas, the moles of hydrogen produced can be calculated simply by [26]

NH ¼

PV RT

ð4Þ

where P is the pressure, V is the volume of the hydrogen accumulated over time, R is the gas constant, and T is the outlet temperature of gas flow meter. In our previous work, the intrinsic kinetics of liquid-phase isopropanol dehydrogenation over Raney catalyst was investigated and incorporated to the model. Only 0.3–0.8 g catalyst was used in 300 ml liquid and the internal and external mass transfer resistance was excluded. But in the industrial sonochemistry process large amounts of catalysts are added and internal and external mass transfer resistance is present. Therefore, in this study the apparent rate coefficient of isopropanol dehydrogenation kapp is used and given by:

kapp ¼ r d =C iso

ð5Þ

where Ciso is the isopropanol concentrations in the reactor, which is seen as a constant during the experimental runs and calculated by the number of moles and the volume of isopropanol used. We assume that the total volume in the flask is constant during all experimental runs due to the little conversion of isopropanol and the return-flow of the vaporized isopropanol. It is also noted that

Fig. 2. Experimental set-up of isopropanol liquid-phase dehydrogenation. 1: Oil bath; 2: 4-necked-flask; 3: ultrasound horn; 4: transducer; 5: ultrasound generator; 6: condenser; 7: ice trap.

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M. Xu et al. / Ultrasonics Sonochemistry 23 (2015) 66–74

the products of both gas phase and liquid phase are detected by the combination of gas chromatography and mass spectrum (GC–MS), and only the reaction given in Eq. (1) takes place with the products acetone and hydrogen, and no side-reaction is expected. The ultrasound power dissipated (Pdiss) in the reactor are experimentally determined by calorimetry [27,28], and the procedure is carried out as follows: 320 ml of isopropanol is charged into the same flask shown in Fig. 2 equipped with a thermocouple and the ultrasound probe. This flask is placed in the atmosphere with temperature of 25 °C. When the ultrasound irradiation occurs, the temperature in the flask is recorded on-line every second. The total ultrasonic power actually entered into the system in a unit time can be calculated by following equation:

Pdiss

  dT  mC p ¼ dt

ð6Þ

where dT/dt is the initial rate of the temperature rise when the liquid is exposed to ultrasonic irradiation, and m and Cp are the mass and heat capacity of the solvent, respectively. The power of the ultrasound is set the 50%, 60%, 75% and 100% of the maximum power output. The calculated actual power dissipated in the given liquid is 14.22 W, 16.46 W, 26.87 W, and 40.16 W, respectively. 2.3. Mathematical model of the IAH-CHP In this section, the mathematical models of the IAH-CHP with ultrasound and stirrer are both established. The rigorous equilibrium stage model [29,30] RadFrac, which is a rigorous model for simulating all types of multistage vapor–liquid fractionation operations, is used to simulate the distillation column in the commercial simulation software Aspen Plus. The dehydrogenation reaction specifically occurs in the reboiler. The apparent kinetic with power-law type is used to depict the experimentally-determined reaction rate model, and then incorporated in the simulation model. The applicability of the kinetic equation can be claimed due to the fact that the actual temperatures in reboiler obtained in the model are all in the range of the experimental temperatures and the isopropanol concentration is absolutely dominant in reboiler as shown in Figs. 9 and 10, which will be given in next section. The Compr module with isentropic type, which can model a compressor or turbine, is used for the compressor in our work, and a simple model called HeatX, which can model a wide variety of shell and tube heat exchanger, is used for the recuperator. For the exothermic reactor, the RGIBBS module with specified temperatures and pressures, which uses Gibbs free energy minimization with phase splitting to calculate equilibrium, is used. The predictive UNIFAC model [31], which is an activity coefficient model based on group contributions, is used to account for liquid-phase non-idealities in the mixture during the whole process. The specifications and operating conditions of the IAH-CHP system are listed in Table 2. The distillation column consists of 15 stages. The stages are numbered from top to bottom, with stage 1 as the condenser and stage 15 as the reboiler. 300 g catalyst is loaded in the reboiler. A condenser with partial-vapor type is selected. The temperature in condenser and the reflux ratio of the distillation column in the simulation are determined by using the Design Spec of Aspen Plus with the target 3% isopropanol mole content. The other optimized conditions are confirmed according to our previous work [15]. The performances of the IAH-CHP system with four ultrasound power density and two different stirring rates (500 and 650 rpm) used in the above experiments are calculated. Two thermodynamic parameters are used to investigate the effect of the operating variables of the IAH-CHP system. First, to

Table 2 Specifications and operation conditions of the IAH chemical heat pump in our simulations. Contents RadFrac(reactive distillation column) Total stages Feed stage Column pressure (MPa) Distillate flow rate (mol s1) Liquid holdup in reboiler (ml) Catalyst density in reboiler (kg/m3) Isopropanol mole fraction in distillate Rgibbs (exothermic reactor) Temperature (K) Pressure (Mpa) Hydrogen to Acetone mole ratio, CH/Cace HeatX (recuperator) Feed temperature of the distillation column, Tfeed (K) Compr (compressor) Isentropic efficiency Mechanical efficiency

Specifications and conditions 15 4 0.11 0.3 3000 100 0.03 473 0.12 1 338

0.72 1

study the effect of the ultrasound power input on the energy performance, the coefficient of performance (COP) is defined as follows which is different from the conventionally used enthalpy efficiency (as shown in Eq. (10) in Ref. [15]).

COP ¼

QH Q L þ W in

ð7Þ

where QH is the high-temperature heat released from the exothermic reactor, QL is the low-temperature heat input in reboiler, and Win is the work supplied to the ultrasound generator, the compressor and the heater. The ultrasonic power input is first calculated by multiplying ultrasound power density by liquid holdup in reboiler we used in the mathematic model, which is only the power amplitude on the reboiler. The whole power loss of electric to ultrasonic, i.e. power drawn from the power panel for the entire ultrasound equipment, is also simply considered by the method that the power amplitude on the liquid was multiplied by two and three. Another parameter is exergy efficiency g which is defined as follows [9]

g¼

Q H ð1  T 0 =T H Þ Q L ð1  T 0 =T L Þ þ W in

ð8Þ

where T0 is the environmental temperature (298 K in our work). COP pays more attention to the heat released from the exothermic reactor, while the exergy efficiency takes into consideration of both quantity and quality of the heat released. 3. Results and discussion 3.1. Apparent rate and kinetic of isopropanol dehydrogenation Fig. 3(a)–(d) shows the experimental hydrogen production over time for isopropanol dehydrogenation with different ultrasonic power at four different temperatures. Fig. 3(a) shows the results with different ultrasonic power at 353 K. As seen, for this fixed reaction temperature the hydrogen production increases with the increasing ultrasonic power dissipated in the reactor. The result indicates that power dissipated is one of the important parameters for enhancing the isopropanol dehydrogenation. In addition, Fig. 3(b) shows the results with different temperature at a fixed sonication power of 40.16 W. The result indicates that the hydrogen production increases as the reaction temperature increases. Fig. 3(c) and (d) shows the comparisons of the results

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(a) 1800

1500

40.16 W 26.87 W 16.46 W 14.22 W 650 rpm 500 rpm

1200

H2 Production (mmol)

1500

H2 Production (mmol)

(c)

40.16 W 26.87 W 16.46 W 14.22 W

1200

900

600

900

600

300

300

T=353 K

T=351 K

0 0

5

10

15

0

20

0

5

10

Time (min)

(b) 1800

(d)

400

40.16 W 26.87 W 16.46 W 14.22 W 650 rpm 500 rpm

300

H2 Production (mmol)

H2 Production (mmol)

20

Time (min)

353 K 351 K 349 K 346 K

1500

15

1200

900

600

200

100

300

Pdiss=40.16 W

T=346 K

0 0

5

10

15

0

20

0

3

6

Time (min)

9

12

15

Time (min)

Fig. 3. Accumulation of hydrogen with different ultrasonic power and stirring rate at different temperatures.

0.07 0.06 0.05 -1

kapp(L (gcat h) )

of isopropanol dehydrogenation with ultrasound irradiation and stirrer. It can be seen that the dehydrogenation rate with the stirrer speed of 650 rpm is higher than that with sonication power of 16.46 W, but it is lower than that with sonication power of 26.87 W. Moreover, the dehydrogenation rate with the stirrer speed of 500 rpm is higher than that with sonication power of 14.22 W at 351 K, whereas it is lower at 346 K. Detailed discussion will be carried out later by calculating the apparent rate constant and activation energy. According to Eq. (3)–(5), the apparent rate constants for different conditions are calculated from the initial slopes of hydrogen production versus time and shown in Fig. 4. Besides the results with ultrasound irradiation, the data obtained from the experiments with mechanical agitation is also shown in Fig. 4. As seen, the apparent rate constants increase with the increasing ultrasonic power and temperature. Also, at a selected temperature almost all apparent rate constants other than two data with Pdiss of 14.22 W are higher than that when using stirrer with stirring speed of 500 rpm. When the ultrasonic power dissipated is equal with or larger than 26.87 W, the apparent dehydrogenation rates with ultrasound irradiation are higher than that with agitation even if the used stirring rate is 650 rpm. The rate enhancement by a factor of 1.5–7 is observed at different temperatures when using ultrasound irradiation with Pdiss of 40.16 W. It is also interesting that the superiority of ultrasound is more obvious for the lower reaction temperature. This phenomenon may due to the fact that

0.04

Pdiss=40.16 W Pdiss=26.86 W Pdiss=16.46 W Pdiss=14.22 W Stirring rate= 650 rpm Stirring rate= 500 rpm

0.03 0.02 0.01 0.00 345

346

347

348

349

350

351

352

353

354

T (K) Fig. 4. Comparison of apparent rate constant of isopropanol dehydrogenation with ultrasound irradiation and stirrer.

ultrasound varies the reaction activation energy, which will be explained later. In our opinion, good macro- and micromixing characteristic caused by acoustic streaming [32,33] should be mainly contributed

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M. Xu et al. / Ultrasonics Sonochemistry 23 (2015) 66–74

-2.4

kapp ¼ k0 exp ðEa=RTÞ

-2.8

The experimental data of the rate constant in Fig. 4 are used to obtain the linear regression of ln kapp versus 1/T at different temperatures. The Arrhenius plot with ultrasound irradiation and stirrer are also provided and shown in Figs. 5 and 6, respectively. The activation energy Ea and pre-exponential factor of rate constant k0 are obtained from the slope of an Arrhenius plot at different conditions, which are shown in Table 3. It can be seen that the activation energy is reduced with the increase of the ultrasonic power dissipated, and the value is smaller than that when using mechanical agitation. These results suggest that the ultrasonic power input can reduce the activation energy of this dehydrogenation reaction of isopropanol. Based the data in Table 3, the correlations of activation energy, pre-exponential factor and ultrasonic power density are obtained as shown in Fig. 7. And the following expressions are also given:

-3.2 -3.6

lnkapp

-4.0 -4.4

40.16 W 26.87 W 16.46 W 14.22 W

-4.8 -5.2 -5.6 -6.0 2.82

2.83

2.84

2.85

2.86 -1

2.87 3

2.88

2.89

2.90

-1

T *10 (K ) Fig. 5. Arrhenius’s plot s as a function of temperature for different ultrasonic power.

ð9Þ

Ea ¼ 19:978  ðPdiss =VÞ0:939 with R2 ¼ 0:931

ð10Þ

ln k0 ¼ 845:66  ðP diss =VÞ þ 149:91 with R2 ¼ 0:891

ð11Þ

where Pdiss/V is the amount of power dissipated per ml of the given reactant, W/ml, and R2 is the coefficient of determination for the expressions.

-2.4 -2.8 -3.2

3.2. Performances of the IAH-CHP system

-3.6

lnkapp

-4.0 -4.4 -4.8

650 rpm 500 rpm

-5.2 -5.6 -6.0 2.82

2.83

2.84

2.85

2.86 -1

2.87 3

2.88

2.89

2.90

-1

T *10 (K ) Fig. 6. Arrhenius’s plot s as a function of temperature for different stirring rate.

to the enhanced performance observed with sonication, which means that the mechanical effects of acoustic cavitation make the more uniform dispersed of catalyst particles in the liquid reactant than stirrer. As discussed by Monnier et al. [34] and Parvizian et al. [35], increasing the ultrasound power and decreasing the reactor volume is capable to improving mixing. These phenomenon can explain the experimental observations that the increase of ultrasound power results in the increase of the dehydrogenation rate. In addition, very high local temperature brought from implosive collapse of cavitation bubbles may favor the dehydrogenation of isopropanol, and microjets near the solid catalyst surface also improve the desorption of produced acetone and hydrogen. The temperature dependence of rate constant can be expressed by the Arrhenius equation:

Fig. 8 shows the variations of the COP and exergy efficiency of the IAH-CHP system with the ultrasonic power density. COP and g increases as the ultrasonic power density increases. The enhanced rate of isopropanol dehydrogenation in reboiler when using high-intensity ultrasound irradiation should be contributed to the results, which suggest that the introduction of the ultrasonic power can improve the energy performances of the IAH-CHP system. In the scope of the ultrasonic power density we used, for the IAH-CHP system, the benefits of the ultrasound can compensate for the ultrasonic power input. There may be an optimized value of ultrasonic power density, which should be large than 0.137 W/ml, to give the maximum COP or exergy efficiency for the IAH-CHP system. Because the ultrasound generator and transducer cannot transfer the electrical power to ultrasonic power with no energy loss, the performances of the IAH-CHP system with the actual ultrasonic power input of two and three fold are also calculated according to Eq. (7) and (8). The results are also given in Fig. 8. As shown, COP decreases slightly and g decreases remarkably with the increase of ultrasonic power used in our calculations, which is due to more high-level power input. COP and exergy efficiency of the IAH-CHP system with stirring rate of 500 and 650 rpm are also calculated and shown in Fig. 8. It should be noted that we assume that the mechanical agitation need no power input which makes the COP and exergy efficiency be overestimated. It can be seen that the higher COP and g is observed with ultrasound than agitation when Pdiss/V is larger than 0.089 W/ml. When 2-fold Pdiss is used, the predominance of sonication can also be observed as long as power density is higher than 0.089 W/ml. Moreover, the COP and exergy efficiency with 3-fold actual ultrasound power is a little lower than that with stirrer. Considering the overestimated

Table 3 Activation energy and pre-exponential factor of rate constant for isopropanol dehydrogenation with ultrasound irradiation and stirrer. Parameters

Ea/kJ mol1 1 k0/mol g1 cat h

Pdiss/W

Stirrer speed/rpm

40.16

26.86

16.46

14.22

650

500

144.24 1.66  1020

162.93 7.52  1022

333.62 1.92  1048

345.04 4.32  1049

374.29 1.32  1054

424.24 2.75  1061

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M. Xu et al. / Ultrasonics Sonochemistry 23 (2015) 66–74 360

(a)

350

350

340

250

T (K)

Ea (kJ/mol)

300

330

320

200

withultrasound irradiation, Pdiss/V=0.137 W/ml

310

with stirrer, stirrer speed=650rpm

150 0.04

0.06

0.08

0.10

0.12

0.14

Pdiss /V (W/ml)

300

0

2

4

6

8

10

12

14

16

Stage no.

120

(b)

Fig. 9. Profile of temperature in the distillation column with ultrasound and stirrer in reboiler: the stage no. is counted from the top to bottom including the stages of condenser and reboiler.

lnk0

100

80

60

40 0.04

0.06

0.08

0.10

0.12

0.14

Pdiss/V (W/ml) Fig. 7. Connection between Activation energy (a), pre-exponential factor (b) for isopropanol dehydrogenation and ultrasound irradiation power density.

COP and g for mechanical agitation, the superiority of the IAH-CHP system with ultrasound should be also claimed even if the 67% of the power of the ultrasound equipment is lost. Due to the extended ultrasonic horn, the highest power efficiency of our used ultrasound equipment is about 20%. However, the energy loss of less than 67% is easy to obtain according to the references [27,36]. Therefore, the IAH-CHP system with ultrasound should be superior to that with stirrer by using proper ultrasound equipment. The reason of the enhancement is clear if we look the profiles of temperature in the distillation column with ultrasound and stirrer in reboiler as shown in Fig. 9. The temperature in the most of stages in the column with ultrasound is lower than that with stirrer. It may be due to the enhanced rate of isopropanol dehydrogenation in reboiler, and hence the required low-temperature heat QL is reduced. Fig. 10 also illustrates the profile of liquid composition in the distillation column with ultrasound and stirrer.

Fig. 8. Comparison of the performances of the IAH-CHP system with ultrasound irradiation and stirrer, w denote the condition that the stirrer is used.

M. Xu et al. / Ultrasonics Sonochemistry 23 (2015) 66–74

Mole fraction in liquid phase

1.0

0.8

xiso, ultrasound, 0.137 W/ml

0.6

xace, ultrasound, 0.137 W/ml xiso, stirrer, 650 rpm

0.4

xace, stirrer, 650 rpm 0.2

0.0 0

2

4

6

8

10

12

14

16

18

Stage no. Fig. 10. Profile of liquid composition in the distillation column with ultrasound and stirrer: the stage no. is counted from the top to bottom including the stages of condenser and reboiler.

4. Conclusions We investigated the application of ultrasound irradiation to enhance catalytic dehydrogenation rate of isopropanol for the IAH-CHP system. First, apparent kinetic experiments regarding liquid-phase dehydrogenation of isopropanol with different ultrasound power were carried out at 346–353 K over Raney nickel catalyst. The dehydrogenation rate of isopropanol with ultrasound and stirrer is also compared. Based on experimentally determined kinetic data, a rigorous mathematic model was established to simulate the IAH-CHP system. The operating performances of the IAHCHP system with ultrasound and stirrer were compared in terms of COP and exergy efficiency. The following major conclusions can be drawn. (1) Dehydrogenation of isopropanol strongly depends on ultrasound power input and reaction temperature. When the ultrasonic power dissipated is larger than 26.87 W for reactant of 300 ml, the apparent dehydrogenation rates with ultrasound irradiation are significantly improved in comparison to that with agitation. (2) The activation energy Ea and pre-exponential factor of rate constant k0 are both obtained. The activation energy when using ultrasound is smaller than that when using stirrer, and it decreases with the increase of the ultrasonic power dissipated. The quantitative relationship between the dehydrogenation rate and ultrasonic power density is identified. (3) When ultrasound power density is larger than 0.089 W/ml, COP and exergy efficiency of an IAH-CHP system with ultrasound is higher than that with agitation. The superiority of the IAH-CHP system with ultrasound is claimed even if more than 50% of the power of the ultrasound equipment is lost. Acknowledgments This research is supported by the National Natural Science Foundation of China (21306192, 51276181) and the National Basic Research Program of China (2011CB710705). References [1] A.N. Ajah, A. Mesbah, J. Grievink, P.M. Herder, P.W. Falcao, S. Wennekes, On the robustness, effectiveness and reliability of chemical and mechanical heat pumps for low-temperature heat source district heating: a comparative simulation-based analysis and evaluation, Energy 33 (2008) 908–929.

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