Distillation assisted heat pump in a trichlorosilane purification process

Chemical Engineering and Processing 69 (2013) 70–76

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Chemical Engineering and Processing: Process Intensification journal homepage: www.elsevier.com/locate/cep

Distillation assisted heat pump in a trichlorosilane purification process Eduardo Díez ∗ , Araceli Rodríguez, José Ma . Gómez, Marta Olmos Grupo de Catálisis y Procesos de Separación (CyPS), Departamento de Ingeniería Química, Facultad de C.C. Químicas, Universidad Complutense de Madrid Avda. Complutense s/n, 28040 Madrid, Spain

a r t i c l e

i n f o

Article history: Received 6 June 2012 Received in revised form 13 February 2013 Accepted 14 February 2013 Available online 24 February 2013 Keywords: Solar grade silicon Trichlorosilane Heat pump Distillation Process simulation

a b s t r a c t The objective of this paper is to design and optimize a trichlorosilane distillation system with the aim of obtaining this product with a purity higher than 0.99999 mole fraction, so that it can employed as a source of solar-grade silicon. A conventional process with at least two columns is capable of separating the trichlorosilane from a mixture of this compound with dichlorosilane and silicon tetrachloride; however, due to the high purity required, large reflux ratios are needed. For this reason, a vapour recompression heat pump was considered for both columns but also for the second column only. All the alternatives were simulated with HYSYS® software platform, in order to determine economically the best one. The economic analysis indicates that, although in both heat pump assisted systems, the initial investment should be much larger than in the conventional process, the annual savings (29% for the Double Heat Pump assisted system and 4% for the Single Heat Pump assisted system), justify the use of heat pumps instead of traditional reboiler–condenser columns. © 2013 Elsevier B.V. All rights reserved.

1. Introduction 1.1. Background In the last few years, the solar photovoltaic energy is becoming more and more important, especially because the petroleum reserves are known to be depleting [1,2]; as a consequence, the demand of raw materials for this industry is also increasing. Traditionally, the photovoltaic industry has employed as raw materials, what is left from the electronic industry, so that it could manufacture solar cells at relatively low cost [3]. Nevertheless, the increasing demand of solar-grade silicon (off-spec materials) makes it impossible to obtain solar-grade silicon by using exclusively electronic-grade silicon. For this reason, new technologies to manufacture the solar cells are being developed, having the majority of them in common the employment, as starting point, of metallurgical-grade silicon [4]. The metallurgical-grade silicon is usually manufactured by reduction of silica with carbon, in Electric Arc Furnaces (EAF); because it has a purity of 98.5–99%, it has only applications in metallurgical industry, to obtain special alloys. However, this material

∗ Corresponding author. Tel.: +34 91 394 8509. E-mail address: [email protected] (E. Díez). 0255-2701/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cep.2013.02.006

can be further purified with both physical and chemical procedures, to obtain solar-grade silicon. The majority of the purification processes are chemically-based ones [5,6]. In all of them the first step is always the generation of a stream containing silicon-derived volatile compounds; later on, this stream is distilled to a one single compound (which can be either trichlorosilane, SiHCl3 , or silane, SiH4 ), and finally this compound is decomposed to pure silicon by reduction or pyrolysis to obtain, as final product, solar-grade silicon. These days, the most employed processes are the Siemens, along with Union Carbide and Ethyl Corporation ones, and combinations of all of them [7]. In all these processes, one of the key points is the distillation step, mainly due to the fact that reaching the high purity required (more than 0.99999) involves large energy consumptions as a consequence of the high reflux ratios which are to be used. For this reason, it is crucial to adequately design and optimize this step, in order to save as much energy as possible. To achieve remarkable energy savings, one reliable alternative could be a distillation assisted heat pump [8]; with this system, the energy of the vapour top stream of a distillation column, is employed as energy supply for partially vaporize the liquid hot bottom stream of the column. Basically, there are two types of heat pumps that can be attached to a distillation column: mechanical and absorption ones. In a mechanical heat pump [9], instead of employing a condenser and a reboiler attached to the column, the top product can be compressed,

E. Díez et al. / Chemical Engineering and Processing 69 (2013) 70–76

or the bottom product can be flashed (in order to make their energy content “more useful”), so that they can interchange heat between them. In absorption heat pumps [10], a separate closed loop fluid system (ammonia/water or lithium bromide/water are the most commonly employed) is used to transfer the heat up the temperature scale by means of heat of mixing. In these systems, the salt is used as the refrigerant and water as the absorbent. In literature, the most employed heat pumps attached to distillation columns have been vapour recompression mechanical heat pumps. In fact, these systems have been implemented in columns which develop difficult separations, such as C4 -splitters [11], propane/propylene [12,13], or ethylbenzene/xylene and ethylbenzene/styrene [9]. For this reason, the integration of a mechanical heat pump in a chlorosilanes purification system is expected to be economically positive. On the other hand, absorption heat pumps have been mainly used in desalination processes [14].

71

1,0 0,9 0,8 0,7

ySiH Cl 2 2

0,6 0,5 0,4 0,3 0,2 0,1 0,0 0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

XSiH Cl 2 2

1.2. Objective

Fig. 1. x–y diagram for SiH2 Cl2 –SiHCl3 mixture at P = 7 bar.

The objective of this work is to analyze the viability of incorporating a mechanical heat pump in a trichlorosilane (SiHCl3 ) purification system, from a stream containing trichlorosilane (SiHCl3 ), as well as dichlorosilane (SiH2 Cl2 ) and silicon tetrachloride (SiCl4 ). Although some minor impurities can be present in this kind of processes, only these three compounds were taken into account since, from the point of view of heat transfer, this approximation is adequate. To achieve the proposed objective, a conventional two column distillation process was compared with a distillation assisted heat pump process, with the aim of determining the amount of energy saving that can be achieved. A vapour recompression mechanical heat pump was considered. All the simulations were undertaken with the HYSYS model version 7.3 [15] under license from Aspentech® . The implementation of a heat pump system has never been taken into account in a chlorosilanes distillation system. 2. Design of the conventional distillation process 2.1. Thermodynamic study The starting point, but not the less important, to design a distillation process, is to select the thermodynamic model which can adequately represent the vapour–liquid equilibrium of the involved binary mixtures. According to literature, the most adequate model to reproduce the vapour–liquid equilibrium of mixtures of chlorosilanes is the NRTL [16]. This model is based on calculating the activity coefficient of the liquid phase, while considering vapour phase to be ideal. This last assumption was checked by calculating the vapour phase fugacities by means of the Peng–Robinson equation of state [17], and in all cases these values were close to 1. Once the thermodynamic model had been selected, the next point to be considered was how to determine the binary interaction

parameters which are necessary to reproduce the vapour–liquid equilibrium with the NRTL model. Usually these parameters are taken from a literature database, but in this case it was impossible to find out any reliable set of parameters. For this reason, we obtained then by directly fitting the equilibrium data which can be found in literature [18]. The isobaric vapour–liquid equilibrium data of the mixtures involved are shown in Tables 1 and 2. The goodness of the equilibrium data was assessed by means of the Herington–Redlich–Kister consistency test [19–21]; the SiHCl3 –SiCl4 system was also analyzed by means of the Wisniak L/W test [22]. The results of the tests can be checked in literature and indicates that, according to them, both systems are thermodynamically consistent [23]. The fitting to NRTL model was performed by minimizing the objective function shown in Eq. (1), where  i indicates the activity coefficient of a compound i, yi indicates the vapour fraction of a compound i, and the subscripts exp and cal indicates experimentally determined and calculated values respectively. According to NRTL model, the binary interaction parameters were allowed to be temperature dependant, as shown in Eq. (2). The adjustments were carried out with an algorithm implemented in EXCEL® software [24]. O.F. =



exp

(i

2

− icalc ) +



i

exp

(yi

− yicalc )

2

(1)

i

Gij = exp(−˛ij ij ) ij = aij +

bij T

˛ij = cij

(2)

Figs. 1 and 2 show the x–y diagrams for SiH2 Cl2 –SiHCl3 and SiHCl3 –SiCl4 systems while Tables 3 and 4 show the obtained binary interaction parameters. The quality of the adjustments was checked by means of the root-mean-square deviations (RMSD) of

Table 1 T-xy equilibrium data of SiH2 Cl2 (1)–SiHCl3 system. P = 7 bar

P = 14 bar

P = 21 bar

T (K)

x1

y1

T (K)

x1

y1

T (K)

x1

y1

375.70 367.45 361.75 358.75 356.75 348.45 346.85 344.75

0.0000 0.1976 0.3372 0.3969 0.5021 0.7993 0.9000 1.0000

0.0000 0.3280 0.4990 0.5610 0.6560 0.8710 0.9350 1.0000

410.00 399.75 394.35 392.35 388.05 380.25 377.75 375.80

0.0000 0.1973 0.3368 0.3964 0.5016 0.7991 0.8999 1.0000

0.0000 0.3020 0.4700 0.5320 0.6310 0.8590 0.9300 1.0000

432.95 423.25 416.65 414.25 410.75 401.45 398.75 396.75

0.0000 0.1977 0.3372 0.3968 0.5020 0.7992 0.8999 1.0000

0.0000 0.2840 0.4490 0.5110 0.6110 0.8500 0.9250 1.0000

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Table 2 T-xy equilibrium data of SiHCl3 (1)–SiCl4 system. P = 0.987 bar

P = 0.987 bar

T (K)

x1

y1

T (K)

x1

y1

329.47 328.90 327.15 326.60 321.45 320.00 317.95

0.0000 0.0070 0.0490 0.0500 0.1720 0.2140 0.3080

0.0000 0.0200 0.1190 0.1220 0.3500 0.4160 0.5110

314.05 311.35 309.75 306.25 305.65 304.31

0.4630 0.5900 0.6930 0.8330 0.8940 1.0000

0.6680 0.7580 0.8260 0.9080 0.9430 1.0000

   (y1,exp − y1,calc )2 

1,0 0,9

RMSD (y1 ) =

0,8

i

(4)

N

0,7

ySiHCl

3

0,6 0,5

2.2. Simulation: conventional process system

0,4 0,3 0,2 0,1 0,0 0,0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1,0

XSiHCl 3 Fig. 2. x–y diagram for SiHCl3 –SiCl4 mixture at P = 0.987 bar. Table 3 Binary interaction parameters of SiH2 Cl2 –SiHCl3 and SiHCl3 –SiCl4 systems.

b12 (1/K) b21 (1/K) ˛12 = ˛21

SiH2 Cl2 (1)–SiHCl3 (2)

SiHCl3 (1)–SiCl4 (2)

−220.28 362.02 0.3

−48.34 46.31 0.3

Table 4  1 and y1 RMSD for all the studied mixtures.

RMSD ( 1 ) RMSD (y1 )

SiH2 Cl2 (1)–SiHCl3 (2)

SiHCl3 (1)–SiCl4 (2)

0.013 0.010

0.040 0.009

the activity coefficient ( 1 ) and the vapour fraction (y1 ) of the more volatile compound, calculated according to Eqs. (3) and (4) respectively, where N is the number of data points. The greater deviation is less than 0.05, so the validity of the model to reproduce the vapour–liquid equilibrium of both mixtures is confirmed.

   (1,exp − 1,calc )2 

RMSD (1 ) =

i

(3)

N

The schematic flow diagram of the proposed process is shown in Fig. 3. The starting up is a mixture of SiCl4 , SiHCl3 and SiH2 Cl2 , which has to be purified so that the pure trichlorosilane, obtained in the bottom product outlet stream, of the second column, can be employed to obtain solar-grade silicon. With the aim of comparing the advantages of introducing a heat pump into a conventional distillation columns process, 200 kmol/h of a mixture of SiCl4 , SiHCl3 and SiH2 Cl2 (with molar fractions equal to 0.700, 0.280 and 0.02 respectively) were fed to a two-column system, as saturated liquid at 361 kPa. These conditions are typical values of this kind of processes, which were found in literature [23,25,26]. Initially, the inlet stream is supplied to the first column, which separates silicon tetrachloride (as bottoms) from the other two chlorosilanes. This column operates with a head pressure of 360 kPa. Afterwards an expansion valve is needed, to decrease the pressure to 340 kPa, as well as a small heat exchanger, to reduce the temperature to 25 ◦ C, before this stream is fed to the second column. This second column separates a top stream, which contains dichlorosilane along with trichlorosilane, and a bottom stream which contains trichlorosilane with the adequate specifications (mole fraction higher than 0.99999). Both columns were initially set up with the short-cut design facility provided by HYSYS, to obtain an estimation of the number of theoretical stages, the feed stage, and reflux ratio required. Secondly, both columns were simulated with the rigorous column facility of HYSYS, which converged successfully. The final values obtained for the number of theoretical stages, the feed stage and the reflux ratio are in agreement with previous literature values [23,26]. The specifications of the inlet stream, as well as the ones of the outlet streams are summarized in Table 5. The main characteristics of the columns are detailed in Table 6. As it can be observed in Table 6, the molar reflux ratio is really high, mainly due to the necessity of obtaining ultrapure trichlorosi-

Table 5 Specifications of the main streams in the conventional process. Variable

Temperature (◦ C) Pressure (kPa) Mole flow (kmol/h) SiCl4 mole fraction SiHCl3 mole fraction SiH2 Cl2 mole fraction

Stream number 1

2

3

4

5

6

7

63.0 361 200 0.7000 0.2800 0.0200

47.0 360 58.66 0 0.9319 0.0681

102.2 361 141.34 0.9905 0.0094 0

47.0 341 58.66 0 0.9319 0.0681

25 341 58.66 0 0.9319 0.0681

56.7 340 8.00 0 0.5005 0.4995

71.9 341 50.66 0 >0.99999 0

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73

Fig. 3. HYSYS process flow diagram for the conventional column.

lane. This is in this kind of systems where the heat pump systems have been demonstrated to be more efficient. To further determine the costs of the distillation column it is first necessary to define the characteristics of T-100 and T-101 columns. According to literature the most adequate columns for this separation are packed ones [23] so, for this simulation it was assumed that both columns are filled up with a Mellapack structured-type 752Y packing. The height equivalent to the theoretical stage was determined from Sulzer website [27] being, in the operation conditions of both columns, equal to 0.18 m. Finally, taking into account the number of theoretical stages of both columns, their heights resulted to be 11.3 m for T-100 and 5.6 m for T-101. The diameters of the columns were calculated on the basis of assuming a value of the flooding factor (vapour ascending velocity/vapour ascending velocity in flooding conditions) of 0.8 [28]. The vapour ascending velocity in flooding conditions was calculated with Eq. (5), where L and V are the liquid and vapour densities, respectively, and Cs is an empirical parameter which can be determined with the generalized correlation of Eckert–Stringle, later modified by Stringle [29]. uflooding = Cs

  −  0.5 L V

(5)

V

The final diameter results were 2.15 m for T-100 column, and 0.76 m for T-101 column. On the other hand, pressure drop along the column (from top to bottom stages) was assumed to be 1.2 Pa/m, and the building material was considered to be Stainless Steel 316L. 2.3. Heat pump assisted system The schematic diagram of the proposed process appears in Fig. 4. Initially the heat pump system was applied to both conventional columns, as shown in Figs. 4 and 5, although the possibility of including the heat pump only in the second column was also considered. According to the scheme of Fig. 4, the top column T-100 outlet stream is compressed with compressor K-100 to raise its temperature and promoting its energy content to be “more usable”. In these conditions, the temperature increases from 72.7 ◦ C to 149.6 ◦ C Table 6 Main characteristics of the two columns. Variable

T-100

T-101

Number of theoretical stages Feed stage Molar reflux ratio Head pressure (kPa) Feed temperature (◦ C) Top column temperature (◦ C) Bottom column temperature (◦ C) Reboiler duty (kW) Condenser duty (kW)

63 60 19 360 63,00 47.00 102.15 9124 8912

31 4 19 340 25 56.66 71.91 1162 1061

Fig. 4. HYSYS process flow diagram for the vapour recompression heat pump: 2nd column.

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air-cooler (AC-100), and expanded again to 360 kPa, previously to be recycled to the column. After being air-cooled and expanded, the top stream is divided in two fractions in TEE-100. One of them is the final top product while the other one is recycled back to the column as a liquid reflux. On the other hand, after being partially boiled up, the bottom stream is fed to a flash drum (V-100). The liquid outlet stream of this flash constitutes the final bottom product, while the vapour outlet stream is fed again to the column as vapour reboil. A similar scheme was applied to the second column of the conventional process (Fig. 5). In both conventional and heat pumpassisted processes, the number of theoretical stages, the feed stage, as well as the head pressure and pressure drop along the columns were kept in the same values. The specifications of the main streams of the heat pump-assisted process are detailed in Table 7. 3. Economic assessment 3.1. Economic assessment procedure The economic assessment of each alternative was performed in terms of the fixed (FC ) and operational (CV ) costs [31]. The fixed costs are related mainly to the equipment costs, while the operational costs are related to the utility consumption per year. All the economic data necessary to estimate the equipment’s costs were taken from literature sources [32–34], and the calculated prices were updated according to the Chemical Engineering Plant Cost Index (CEPCI) whose value for the year 2011 was 546 [35]. The cost of the columns was estimated as a function of their height and diameter. The cost of all heat exchangers was estimated as a function of the necessary exchange area (A), which was calculated as the ratio between UA (taken directly from HYSYS simulation) and U (overall heat transfer coefficient). Typical U values were taken from literature [30]. The cost of the compressors was estimated as a function of the required power, assuming a polytropic efficieny of 75%. The annual operational costs were calculated from the values of the individual utility costs, which are shown in Table 8. The annual electricity consumption of the compressor was calculated from the necessary power (taken directly from HYSYS simulations.). The electricity consumption of the air-coolers was calculated by considering that the base power fan of an air-cooler is 0.595 kW/m2 [36]. Finally, the water and steam consumptions were calculated from condensers and reboilers duties, respectively (values taken directly from HYSYS simulations). All monetary values are given in 2011 Euro. 3.2. Economic comparison of the two alternatives

Fig. 5. HYSYS process flow diagram for the vapour recompression heat pump: 2nd column.

and the pressure passes from 360 kPa to 1639 kPa. The compressor polytropic efficiency was assumed to be 75%. Next to the compressor, the heat exchanger E-101 transfers the energy from this top stream (which is condensed and further cooled from to 149.6 ◦ C to 107.7 ◦ C) to the bottom column outlet stream, which is partially boiled up. An HYSYS adjust unit was employed to calculate what the outlet compressor pressure should be to obtain a minimum approach [30] of 5 ◦ C. This is a typical design value of these kinds of shell and tube heat exchangers. However, the amount of energy transferred from the hot top stream to the cold bottom stream is not enough. For this reason, the top stream must be further cooled from 107.7 ◦ C to 71.6 ◦ C in an

The obtained results of equipment, utility costs and economic potential for all the evaluated alternatives are summarized in Table 9. It has been studied the possibility of including the heat pump cycle in the two columns (Double Heat Pump system), but also the possibility of including the heat pump only in the second column (Single Heat Pump system). As it can be observed, in both heat pump cases, capital costs are higher than in the conventional distillation column because of additional items, namely an expensive compressor and an aircooler. Nevertheless, the operational costs are lower in the heat pump assisted systems, so that with the Single Heat Pump system an energy saving of 4% is achieved while, with the Double Heat Pump system, the energy saving that can be obtained is the 29%. Another important parameter to consider is the “Simple Rate of Return” (SRR) [9], which is the ratio between the obtained profit and the investment (capital costs) so that the higher this value is, the better the investment is. The SRR calculated values are 13.4 for

E. Díez et al. / Chemical Engineering and Processing 69 (2013) 70–76

75

Table 7 Specifications of the main streams in the heat pump-assisted system. Variable

Stream number 1

3

4

5

6

7

Temperature (◦ C) Pressure (kPa) Mole flow (kmol/h) SiCl4 mole fraction SiHCl3 mole fraction SiH2 Cl2 mole fraction

63.0 361 200.00 0.7000 0.2800 0.0200

72.7 360 1174.84 2.4E−05 0.9330 0.0670

149.6 1639 1174.84 2.4E−05 0.9330 0.0670

107.7 1639 1174.84 2.4E−05 0.9330 0.0670

71.6 1639 1174.84 2.4E−05 0.9330 0.0670

71.7 360 1174.84 2.4E−05 0.9330 0.0670

Variable

Stream number

◦

Temperature ( C) Pressure (kPa) Mole flow (kmol/h) SiCl4 mole fraction SiHCl3 mole fraction SiH2 Cl2 mole fraction

8

9 = 10

11

12

13 = 14

15

71.7 360 58.67 2.4E−05 0.9330 0.0670

71.7 360 1116.17 2.4E−05 0.9330 0.0670

102.6 361 1298.16 0.9970 0.0029 5.0E−05

102.7 361 1298.16 0.9970 0.0029 5.0E−05

102.7 361 1156.83 0.9968 0.0031 5.4E−05

102.7 361 141.33 0.9988 0.0012 1.5E−05

Stream number

Variable

16

17

18

19

20

21

Temperature ( C) Pressure (kPa) Mole flow (kmol/h) SiCl4 mole fraction SiHCl3 mole fraction SiH2 Cl2 mole fraction

69.6 341 58.67 2.4E−05 0.9330 0.0670

25.0 341 58.67 2.4E−05 0.9330 0.0670

61.5 340 160.04 0 0.5088 0.4912

129.5 1228 160.04 0 0.5088 0.4912

76.9 1228 160.04 0 0.5088 0.4912

56.7 1228 160.04 0 0.5088 0.4912

Variable

Stream number

◦

Temperature (◦ C) Pressure (kPa) Mole flow (kmol/h) SiCl4 mole fraction SiHCl3 mole fraction SiH2 Cl2 mole fraction Variable

22

23

24 = 25

26

27

28

56.7 340 160.04 0 0.5088 0.4912

56.7 340 8.00 0 0.5088 0.4912

56.7 340 152.04 0 0.5088 0.4912

71.9 341 225.57 6.3E−06 1.0000 1.2E−06

71.9 341 225.57 6.3E−06 1.0000 1.2E−06

71.9 341 50.66 9.8E−06 1.0000 0

Stream number 29 = 30

Temperature (◦ C) Pressure (kPa) Mole flow (kmol/h) SiCl4 mole fraction SiHCl3 mole fraction SiH2 Cl2 mole fraction

71.9 341 174.91 5.2E−06 1.0000 1.4E−06

the Double Heat Pump system and 3.0 for the Single Heat Pump system. This means that, although, a Double Heat Pump system involves a large initial investment, in the long run this process is a really profitable one. As a final remark, is important to point out that, due to the specific characteristics of the chlorosilane-type compounds, adding a Heat Pump system (single or double) in a trichlorosilane purification process imply a large equipment cost. However there are two clear advantages that justify the fact of incorporating these systems: on one hand, the amount of energy savings that can be achieved is considerable; on the other, by adding a Heat Pump Table 8 Utility costs in 2011 Euro [34]. Utility description

Price

Cooling water (D /m3 ) Low pressure steam (D /ton) Electricity (D /kW h)

0.052 15.2 0.091

Table 9 Economic comparison of the different alternatives (D ). Conventional process

Double Heat-Pump system

Single Heat Pump system

571,100 146,500 – – 727,000 886,000 1,430,00 2,473,600

571,100 146,500 315,000 142,500 99,000 247,000 32,000 1,553,100

826,000 826,000

873,000 155,000 82,500 1,110,500

Capital costs

Columns Packing Condensers Reboilers Compressors Heat exchangers Air-coolers Overall value

571,100 146,500 381,500 164,500 – 7000 – 1,270,600

Operational costs

Steam Cooling water Electricity Overall value

985,000 174,000 – 1,159,000

system, the employment of mineral oils (highly toxics), which are

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frequently used as refrigerants in the condensers of the columns, can be avoided. 4. Conclusion This work compares three alternatives to develop a trichlorosilane (SiHCl3 ) purification system (a conventional two-column distillation process, a single heat pump assisted process and a double heat pump assisted process) with the aim of selecting the most profitable system. An economic assessment shows that, although in both heat pump assisted systems, the initial investment should be much larger than in the conventional process, the annual savings (29% for the Double Heat Pump assisted system and 4% for the Single Heat Pump assisted system), justify the use of heat pumps instead of traditional reboiler–condenser columns. The calculated values of the “Simple Rate of Return” (13.4 for the Double Heat Pump system and 3.0 for the Single Heat Pump system) show that in the long run a Double Heat Pump system will be more profitable than a Single Heat Pump system. Acknowledgement All the simulations were performed with HYSYS® software from Aspentech. References [1] H. Takiguchi, K. Morita, Sustainibility of silicon feedstock for a low-carbon society, Sustainability Science 4 (2009) 117–131. [2] A. Dong, L. Zhang, L.N.W. Damoah, Beneficial and technological analysis for the recycling of solar grade silicon wastes, Journal of the Minerals Metals & Materials Society 63 (2011) 23–27. [3] J. Fernández-Anta, La tecnología solar fotovoltaica, Anales de Mecánica y Electricidad 83 (2006) 28–34. [4] S. Pizzini, Bulk solar grade silicon: how chemistry and physics play to get a benevolent microstructured material, Applied Physics A 96 (2009) 171–188. [5] A. Goetzberger, V.U. Hoffmann, Photovoltaic Solar Energy Generation, first ed., Springer-Verlag, Berlin Heidelberg, 2005. [6] W.C. O’Mara, R.B. Herring, L.P. Hunt, Handbook of Semiconductor Silicon Technology, first ed., Noyes Publications, New Jersey, 1990. [7] B. Ceccaroli, O. Lohne, Solar grade silicon feedstock, in: A. Luque, S. Hegedus (Eds.), Handbook of Photovoltaic Science and Technology, John Wiley and Sons Ltd., Chichester, 2003, pp. 153–204. [8] N.J. Hewit, Heat pumps and energy storage – the challenges of implementation, Applied Energy 89 (2012) 37–44. [9] J.A. Ferre, F. Castells, J. Flores, Optimization of a distillation column with direct vapor recompression heat pump, Industrial & Engineering Chemistry Process Design and Development 24 (1985) 128–132.

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