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Economic assessment of the production of subcritically dried silica-based aerogels
Journal of Non-Crystalline Solids 516 (2019) 26–34
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Economic assessment of the production of subcritically dried silica-based aerogels
T
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Rita Garridoa, José Dinis Silvestreb, , Inês Flores-Colenb, Maria de Fátima Júlioc, Marco Pedrosob a
Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal CERIS, DECivil, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal c CQFM, and IN – Institute of Nanoscience and Nanotechnology, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b
A R T I C LE I N FO
A B S T R A C T
Keywords: Silica aerogel Subcritical drying Life cycle assessment Life cycle cost Thermal insulating aggregates
Traditionally, most commercial aerogels are produced by supercritical drying methods which make their application undesirable, more expensive, and less energy efficient. This paper presents the economic assessment of silica-based aerogels synthesized via ambient pressure drying. Three different approaches were considered in this paper: the production of inorganic aerogel and of two hybrid aerogels. The synthesized aerogels will be incorporated as aggregates in wall coatings to improve their thermal performance. The application of the Life Cycle Assessment (LCA) methodology is exceedingly important since it considers all costs associated with a product, process or activity during their physical and economic life, allowing the optimization of the total costs. It was possible to evaluate the production stages of the subcritical silica aerogels that influence their final cost and their relative contribution. These results allow to identify the main aspects that influence the total cost of the synthesis process. It is important to emphasize that this study is suitable for laboratory scale activities leading to high costs. It naturally follows that an adjustment to industrial scale applications is of paramount importance and should be made to maximise the economies of scale and to achieve additional gains in efficiency.
1. Introduction The industrial sector has undergone profound changes triggered by growing concerns with sustainability coupled with more demanding energy and environment European Directives [1]. This has encouraged the development of nanomaterials allowing to reduce the environmental impact during their life cycle (production, use and destination), the incorporation of processed raw-materials as well as their energy costs [2]. Since Kistler's pioneering work in 1931 [3], silica-based aerogels have been used in a wide range of scientific and technological applications [4–6]. These applications depend on the exceptional properties of silica aerogels, such as high specific surface area (≥1000 m2.g−1), very low density (3–500 kg.m−3), small pore size (1–100 nm) and extremely high porosity (above 95%) [7]. They are among the best known thermal insulating materials (with thermal conductivity around 0.015 W.m−1.K−1 at room temperature and ambient pressure), due to the fine solid constituents and porous
structure in the mesopore range [8,9]. This paper evaluates the economic life cycle (LC) associated with three different synthesis methods to obtain subcritical dried silica-based aerogels. The Life Cycle Assessment (LCA) standardized methodology was therefore used to study: an inorganic aerogel (IA) produced in monolithic form and two hybrid aerogels produced in powder (HYB-A) and monolithic (HYB-C) forms. As far as the authors are aware, no economic study of this type of any aerogels production process is available in literature sources. The results have shown how the costs of this process can be optimized, in laboratorial scale activities, and the phases of the life cycle that contribute the most to the whole life cycle of these products. Moreover, it is also possible to compare the three different synthesis methods and conclude which one is less or more expensive. 2. Economic life cycle assessment The Life Cycle Assessment (LCA) of products and/or processes arises
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Corresponding author. E-mail addresses: [email protected] (R. Garrido), [email protected] (J.D. Silvestre), ines.fl[email protected] (I. Flores-Colen), [email protected] (M.d.F. Júlio), [email protected] (M. Pedroso). https://doi.org/10.1016/j.jnoncrysol.2019.04.016 Received 10 January 2019; Received in revised form 16 April 2019; Accepted 17 April 2019 Available online 28 April 2019 0022-3093/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Photographs of the laboratory-synthesized silica aerogels: (A) IA; (B) HYB-A; (C) HYB-C [18–20].
3. Silica aerogels
from the need to estimate the environmental, energy and economic impacts along their physical and economic life. Since the 1960's, numerous studies and methods have been developed to optimize products, to make them more sustainable and less harmful to the environment. However, it was only in 1990 that this methodology, promoted by SETAC (Society of Environmental Toxicology and Chemistry), was standardized. The Life Cycle cost approach used to evaluate a product considering both internal and external costs as well as their allocation to all stakeholders was introduced between 1980 and 1990 [10,11]. In 1997, the first International Standards (ISO 14040 - Life Cycle Assessment - Principles and framework [12], 14,044 - Life Cycle Assessment - Requirements and guidelines [13]) were released which standardized a system for assessing the environmental sustainability of products based on a life cycle approach. A Life Cycle Assessment (LCA) provides a careful analysis of the whole cycle of the product: from the extraction of the raw materials until it becomes waste, passing through all the intermediate stages (production, transportation and use), and with a main focus on the interaction between the product system and the exterior [14]. Only one study related to the environmental and energy impacts of the production of aerogels is reported in literature [15]. According to that study, a silica-based aerogel was produced in the laboratory using supercritical drying conditions, at high and low temperatures (HTSCD and LTSCD, respectively). That Life Cycle Assessment (LCA) was developed from the cradle to the factory gate, excluding the impacts of transportation of raw materials and finished products, as well as their processing at the end of life. The referred study was carried out according to ISO 14044 [13] and to ISO 14040 [12], having as its functional unit the energy expenditure (kWh) and the release of CO2 (kg CO2) in the production of one cubic meter of solid (non-granular) aerogel. The authors concluded that:
Conventionally, silica aerogels are dried above the supercritical point of the solvent, thus requiring high pressures and, depending on the solvent, high temperatures. Though once a preferred method, it involves time-consuming procedures, and it is often expensive and hazardous, rendering the process difficult for industrial use [15]. As a result, cost-effective drying alternatives are required. By using subcritical drying techniques, their costs are reduced, and their safety is increased, partially overcoming the drawbacks [16–18]. When drying aerogels at ambient pressure, the key process is surface modification: by introducing nonpolar groups (usually from silanes) at the surface of the pores, capillary tensions that develop upon solvent evaporation are reduced, thus preventing pore collapse. The aerogels described in the present work were dried under subcritical conditions and the synthesis procedures were reported in detail in previous papers by the authors [9,18–20]. As in the aerogels' production phase, the predominant cost is the acquisition of raw materials, the focus being usually to reduce it and improve economic performance against other materials [21].
3.1. Synthesis of silica aerogels The life cycle (LC) study discussed in this paper includes three different laboratory-synthesized silica aerogels, dried at atmospheric pressure: an inorganic aerogel (IA) produced in monolithic form [19] and two hybrid aerogels produced in powder (HYB-A) [20] and monolithic (HYB-C) [18,19] form (Fig. 1). Fig. 2 illustrates the life cycle of a laboratory synthesized aerogel, where three distinct steps are analysed: the production, use and end-oflife. The product stage stands out because it is the subject of this study. This stage differs according to the three types of aerogel, whereas the use and end of life stages are similar for all types of aerogel.
• the LTSCD production method had a longer environmental return, • •
with a total energy consumption of 62.6 MJ per 40 ml and a total release of 6.64 kg CO2 per 40 ml; according to the HTSCD production method, the total energy consumption and the release of CO2 was 29.3 MJ per 40 ml and 0.73 kg CO2 per 40 ml, respectively; the aging of the aerogel was the production phase where more energy was spent.
As the industrial production process is optimized, the values of energy spent and the release of CO2 decrease significantly. However, it is for the LTSCD method that more significant improvements can be achieved.
Fig. 2. Aerogel generic Life Cycle. 27
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raw materials; 2. Transport and storage; 3. Aerogel synthesis; 3.1 Hydrolysis; 3.2 Condensation; 3.3 Aging; 3.4 Washing; and 3.5 Drying.
In this study, all chemical reagents were bought from a company in the Lisbon area. A medium-sized van was used to transport them to the synthesis laboratory. The reception of the raw materials and their storage was done manually. The raw materials were put on shelves, inside cabinets, in the laboratory, until the synthesis of the aerogels takes places. Dosing, bagging and carrying were the subsequent stages. It was estimated that each package had 150 g of aerogel which corresponds to an individual plastic bag. After the aerogel was bagged up and stored at the synthesis laboratory, it was carried to the construction laboratory for the production of aerogel-based renders.
3.1.2. HYB-A: Hybrid aerogel synthesized by a one-step sol gel process (powder form) The sodium silicate solution D40 from Solvay Portugal (27.5 wt% SiO2, 8.5 wt% Na2O and 1.37 g cm−3) is initially diluted with deionized water to obtain a 5 wt% in SiO2 precursor, at pH 12. In order to acidify this alkaline solution, nitric acid from Sigma-Aldrich (HNO3, 65%) is added. This step is intended to activate the molecular silicates in solution to silicic acid (by partial or total protonation of Si-O− centres), resulting in a pH that can range from 1 to 2. Subsequently, hexamethyldisilazane from Sigma–Aldrich (HMDZ, 99.9%) is added under constant orbital stirring, at room temperature (24–25 °C) with the double role of increasing pH and as co-precursor [20]. NH3 is released to the gas phase and pumped. The partial neutralization with HMDZ initiates condensation reactions and gelation occurs immediately. n-Hexane (96%, from ACS Basic) is added in a fixed WG:n-hexane molar ratio of 1:0.1, to replace any water included in the pores, whose surfaces are expected to be essentially hydrophobic. This exchange proceeds for 3 h, after which the two liquid phases (excess n-hexane and displaced pore water) are removed. The silylated hydrogels are dried at ambient pressure at 60 °C for 24 h followed by 24 h at 100 °C. The samples are repeatedly washed with water (to remove the NO3− and Na+ ions) and finally dried under the above conditions, until aerogel powders are obtained. The chemical reagents were supplied to the synthesis laboratory by a company outside Lisbon. Sodium silicate is a “waste material” from a company located near this supplier. The life cycle from “cradle to gate” of HYB-A aerogel is shown in Fig. 4. The associated synthesis of this type of aerogel involves the following processes: 1. Reception of raw materials; 2. Transport and storage; 3. Aerogel synthesis; 3.1 Hydrolysis; 3.2 Condensation; 3.3 Aging; 3.4 Washing; and 3.5 Drying.
3.1.1. IA: Inorganic aerogel synthesized by a two-step sol-gel process Tetraethoxysilane from Aldrich (TEOS, 98%) was used as a silica precursor, 2-propanol p.a. from Sigma-Aldrich (2-PrOH, 99.8%) as a solvent, HCl (1 M) p.a. from CARLO ERBA and ammonia p.a. from Merck (NH4OH, 0.1 M) as catalysts. The inorganic aerogel can be synthesized by a two-step sol-gel process. It involves the acid-catalysed hydrolysis of TEOS and the polycondensation of the resulting silanol groups induced by addition of ammonia [19]. TEOS was previously diluted in 2-PrOH, and distilled water was added, dropwise, while stirring. The reaction mixture was acidified with HCl to initiate the hydrolysis process. The acidic colloidal solution was placed in a sealed container, heated at 60 °C, and stirred (at 120 rpm) for 60 min. The required amount of NH4OH was then added, and the resulting homogeneous sol was left to gel, with no further stirring. For the preparation of inorganic aerogel, the alcogel was aged for 24 h in a residual liquid and for more 24 h in an equal volume of aging solution, containing TEOS, 2-PrOH and H2O in the same proportions as used for gelation to strengthen the silica network. The pore liquid was then exchanged with 2-PrOH to completely remove any residual water or TEOS, and the gels subcritically dried under ambient pressure in a solvent-saturated atmosphere, until their weight loss became negligible. Throughout the process there was gas release, namely, 2-PrOH. The “cradle to gate” (synthesis) life cycle of the Inorganic Aerogel (IA) and the production activity associated to this type of aerogel is shown in Fig. 3 and involves the following processes: 1. Reception of
Fig. 3. Inorganic Aerogel Life Cycle. 28
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Fig. 4. Hybrid aerogel HYB-A Life Cycle.
Aging; 3.4 Washing; and 3.5 Drying.
3.1.3. HYB-C: Hybrid aerogel synthesized by a two-step sol-gel process The synthesis of HYB-C is similar to the synthesis of Inorganic Aerogel (IA), except for the aging phase [18,19]: After 24 h in the residual liquid, the aged alcogels were further hydrophobized by chemical surface modification with an aging solution of HMDZ (99.9%) in i-PrOH, and left to dry until their weight loss became negligible. The life cycle of the monolithic hybrid silica aerogel (HYB-C) is shown in Fig. 5. The associated synthesis of this type of aerogel involves the following processes: 1. Receiving raw materials; 2. Transport and storage; 3. Aerogel synthesis; 3.1 Hydrolysis; 3.2 Condensation; 3.3
3.2. Economic assessment of aerogel production According to the EPA [22], the LCA methodology can be divided into the following phases: (1) definition of the objective; (2) inventory analysis; (3) impact assessment; and (4) interpretation. In this section, the inventory analysis and the respective evaluation of the economic impacts will be carried out. A detailed economic evaluation of a production process should be as
Fig. 5. Hybrid aerogel HYB-C Life Cycle. 29
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Fig. 6. Costs of aerogels in each life cycle stage (“cradle to gate”).
Fig. 7. Incubator B.
Fig. 8. Energy consumption, kWh, after two hours.
detailed as possible and include all the relevant elements required for its precise quantification. The functional unit is a parameter that serves as a reference to describe the performance of a product (or service) when it fulfils its function. Its definition is relevant, since it guarantees that the inputs and outputs are quantified according to the same reference flow. The functional unit should be consistent with the objectives of the study, and well defined and quantified. Consequently, when several LCA are performed and compared to each other, the functional unit should be the same, or the comparison will not be possible and valid [22,23]. The functional unit (fu), in the present LCA study of inorganic aerogel (IA) and of hybrid aerogels (HYB-A and HYB-C), is defined in mass (kg) and corresponds to the production of 1 kg of aerogel. This is equivalent, in this case, to the declared unit of aerogel used as raw material for renders.
Table 1 Acquisition cost of the raw materials for the IA synthesis, per aerogel kilogram. Raw material
Costs [€/kg of aerogel]
TEOS (98%) i-PrOH (99.8%) HCl (1 M) NH4OH (0.1 M) Distilled H2O
255 60 1 120 0.40
In the following sections, an economic evaluation of each aerogel synthesis will be carried out based on different costs of various Life Cycle stages. These costs are qualitatively described In Fig. 6. At the aerogel synthesis stage, the cost associated with the energy 30
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Table 2 Energy costs of the IA. Synthesis stage
a b
Laboratory Incubator A (stirring and drying) Incubator B (drying)
Energy cost [€/kg of aerogel] 70.53a 12.17b
Incubator consumption: Stirring = 0.21 kWh, Drying = 272 W. Incubator consumption: Drying = 0.21 kWh.
Table 3 Economic assessment of the Inorganic Aerogel Life Cycle.
Cost [€/kg]
Incubator A Incubator B
Raw materials
Transport and storage
Production
Dosing, bagging and carrying
Total
436.39 436.39
– –
70.92 (stirring and drying, plus mix and aging) 12.56 (drying, plus mix and aging)
0.14 0.14
507.45 449.10
Table 4 Acquisition costs of the raw materials needed to synthetize HYB-A, per kilogram of aerogel.
Table 6 Acquisition costs of the raw materials for HYB-C, per kilogram of aerogel.
Raw materials
Costs [€/kg of aerogel]
Raw materials
Costs [€/kg of aerogel]
Sodium silicate aq. (27.5 wt% SiO2) HMDZ (99.9%) HNO3 (65%) n-hexane (96%)
100 450 40 50
TEOS (98%) i-PrOH (99.8%) HCl (1 M) NH4OH (0.1 M) HMDZ (99.9%) Distilled H2O
255 40 1 120 300 0.40
required to produce 1 kg of aerogel (Ce) is given by the following Eq. (1):
Ce = T × nh × P [€/kg of aerogel]
Table 7 Energy costs of HYB-C.
(1)
T - Cost of 1 kWh of electricity in Portugal, for buildings requiring medium voltage and phase-to-phase voltage whose efficiency is > 1 kV and equal to or < 45 kV (€/kWh), (0.12 €/kWh [24]); nh - number of incubator operating hours [hour]; P - Incubator power [W]. In this study, two incubators (A and B) were used: the incubator A for stirring and drying and the incubator B only for drying. The power of the incubator A is 600 W whereas when the drying process is performed in the incubator B the maximum power is 1700 W, as shown in Fig. 7. During the stirring and drying of the aerogels and to characterize the energy consumptions of these processes in a better way, an energy meter (PEREL E305EM6, 230 VAC - 16 A) was used in both equipment. This measurement resulted in a value of 0.30 kWh and 0.21 kWh, respectively, as shown in Fig. 8. The value of 0.12 €/kWh [24] was obtained through the arithmetic average of the active energy prices for long durations, comprising the peak hours and full hours. In the dosing, bagging and carrying processes there was no energy expenditure because the aerogel is produced in small quantities and there is no need for dosing. At this stage, there are only expenditures on consumables, for example plastic bags for bagging. Transportation inside the laboratory is done manually, without the use of any trucks or energy.
Synthesis stage
a b
Incubator A (stirring and drying) A (stirring) + B (drying)
Energy costs [€/kg of aerogel] 70.53a 12.17b
Incubator consumption: Stirring = 600 W, Drying = 272 W. Incubator consumption: Drying = 0.21 kWh.
Since the economic evaluation is the main objective, the cost was the most important parameter to be analysed. The first cost associated with the production of aerogel is the acquisition of the raw materials: TEOS, i-PrOH, HCl and NH4OH. Table 1 shows the cost (€/kg of aerogel) of raw materials, excluding VAT and considering that the transportation to the synthesis laboratory is the supplier's responsibility. The acquisition of the raw materials is done through a supplier company, located in the Lisbon region. The information on how the different raw materials are carried to Portugal is unknown, as there are several industries in different countries. One of the reagents used in the production of aerogel is distilled water, which is distilled at the synthesis laboratory. The distiller is always in operation for all other laboratories of this research centre and the container used for filling is always the same, thus no packaging is generated as waste. Therefore, the real cost associated with this water can't be estimated. However, to get as close as possible to the actual cost, it was decided to carry out a market search, and the value of the distilled water was estimated in case it is necessary to buy it. The estimated value was 0.20 €/litre. The only acquisition cost related to the purchase of containers/reactors is approximately 120 €/kg of aerogel. However, it should be noted that these containers are only part of an initial investment, since
3.2.1. Inorganic aerogel The approach used in the LCA of the inorganic aerogel (IA) was from the “cradle to the gate” of the laboratory, considering that the analysis comprises all the production procedures at the synthesis laboratory until it is delivered at the construction laboratory to later serve as aggregate in thermal renders [25]. Table 5 Economic assessment of the HYB-A Life Cycle.
Costs [€/kg]
Raw materials
Transport and storage
Production
Dosing, bagging and carrying
Total
640.00
–
3.16 (stirring and drying)
0.14
643.30
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Table 8 Economic assessment of the HYB-C Life Cycle.
Cost [€/kg]
Incubator A Incubator B
Raw materials
Production
Dosing, bagging and carrying
Total [€/kg]
716.39 716.39
70.92 (stirring and drying, plus mix and aging) 12.56 (drying, plus mix and aging)
0.14 0.14
787.45 729.10
Note: The transportation and storage from the raw materials to the production stage is manual and there is no cost.
Table 3 presents the economic evaluation of each stage of the inorganic aerogel (IA) Life Cycle.
Table 9 Costs of the distinct stages of the aerogels Life Cycles described in this study. Cost [€/kg of aerogel]
IA HYB-A HYB-C
Raw materials
Production
Dosing, bagging and carrying
Total
436.39 640.00 716.39
12.56 3.16 12.56
0.14 0.14 0.14
449.10 643.30 729.10
3.2.2. Hybrid silica aerogel HYB-A The approach used for the Life Cycle Assessment of this type of aerogel (HYB-A) was similar to the one used in the Life Cycle Assessment of Inorganic Aerogel as it consisted in analysing all the production as far as the entrance in the construction laboratory. The cost associated with the acquisition of the various raw materials (sodium silicate solution, HMDZ, HNO3 and n-hexane), excluding VAT and considering the transportation to the synthesis laboratory the supplier's responsibility, is detailed in Table 4. As mentioned before, the information on how the different raw materials are carried to Portugal is unknown. The sodium silicate solution, considered a “waste material”, is available free of charge from a company (Solvay Portugal), also located outside of Lisbon. However, if it is necessary to buy it, this product will cost about 27 €/litre, thus increasing the purchase cost to around 100€/kg of aerogel. The consumables to synthetize HYB-A (recipients/reactors) cost approximately 50 €/kg of aerogel, but for this aerogel they are different from those used to synthetize Inorganic Aerogel (IA), Again, it is important to note that these containers are only an initial investment, given that they are subsequently reused, and therefore their costs weren't considered in the economic assessment of the HYB-A life cycle. The same plastic bags are used to bag HYB-A so this cost amounts to 0.14€/kg of aerogel. The energy cost is calculated according to Eq. (1), and the number of hours of operation of the incubator corresponds to 4 days - 7 h in the stirring phase, and the remaining time in the drying process. This represents a cost of 3.16€ per kg of aerogel, because of a consumption of 0.3 kWh during the stirring process and of 272 W during the drying process. The HYB-A can only be produced and dried using incubator A, and optimization is not possible using the incubator B. Table 5 presents the economic assessment of each stage of the HYBA aerogel Life Cycle.
Note: Transportation and storage from the raw materials to the production stage is manual and there is no cost.
Fig. 9. Total cost of the various stages of each aerogel Life Cycle ( Raw materials; Production; Dosing, bagging and carrying).
they are subsequently reused. So it was decided not to take into account the costs of these consumables in the economic evaluation of the synthesis process. To bag the aerogel, after dosing and before carrying it, it was necessary to buy plastic bags, which cost € 0.14/kg of aerogel. There are energy-related costs for the aerogel synthesis in every step of its Life Cycle. The energy cost is calculated according to Eq. (1) and, for the IA, the number of operating hours of the incubator corresponds to 7 h for the agitation phase and 2153 h for the drying process, representing a total of 90 days as shown in Table 2. The 90 days are considered when using incubator A, but it is possible to perform the drying process using incubator B, which is larger, reducing to 20 days the time needed to produce one kilogram of aerogel (using more reactors at a time). Thus, this incubator has an energy expenditure of 0.21 kWh during these 20 days, of which 7 h are in the incubator A (while stirring). The energy cost of the Inorganic Aerogel (IA), if it is dried in incubator B, is 12.17 €/kg of IA, as shown in Table 2.
3.2.3. Hybrid silica aerogel HYB-C Table 6 summarizes the costs of raw materials used for HYB-C's synthesis (€/kg of aerogel) without VAT and being the transport to the synthesis laboratory a responsibility of the supplier, which is the company that supplies the raw materials for the remaining aerogels. As in the IA, the production of the HYB-C aerogel uses distilled water, distilled in the laboratory. Given the explanation already stated, the estimated cost of this water is 0.20 €/litre. The cost of consumables and containers/reactors for HYB-C is similar to IA (approximately 120€/kg aerogel).
Table 10 Energy consumption, per aerogel kilogram, at each production (synthesis) stages. Production [kWh/kg]
IA HYB-A HYB-C
Mixing
Heating and stirring (Hydrolysis)
Condensation
Aging
Washing
Drying
Total
0.0 0.0 0.0
2.1 2.1 2.1
0.0 0.0 0.0
0.0 0.0 0.0
0.0 0.0 0.0
99.3 24.2 99.3
101.4 26.3 101.4
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Table 11 Energy consumption per kg of aerogel, for aerogels obtained through different drying processes. Subcritical drying
Supercritical drying
Incubator A IA kWh/kg
587.72
Incubator B HYB-A
(1)
26.31
(2)
HYB-C 587.72
IA (3)
101.43
Dowson et al. [15] HYB-C
(1)
101.43
(3)
HTSCD
LTSCD
138.86 (4) 79.67 (5)
460.57 264.26
(4) (5)
Note: aerogel density = (1) 209.37 kg/m3; (2) 100–200 kg/m3; (3) 305.58 kg/m3; (4) 175 kg/m3; (5) 305 kg/m3; Aerogels dried supercritically at HTSCD – High temperature, LTSCD - low temperature.
Dowson et al. [15] presented a Life Cycle Assessment (LCA) which compared aerogels dried supercritically at high and low temperatures. In that study, the authors obtained the same pattern for high and low temperatures (HTSCD and LTSCD), where the highest energy consumption was by far associated with the drying process when compared with their preparation or aging. Those results agree with the results of the present study, despite the subcritical drying of the aerogels. However, subcritical drying may be advantageous in terms of energy and consequently economically [26]. To confirm this expectation, the energy consumption of the aerogels production was compared using both drying processes as shown in Table 11 and Fig. 10. Due to the lack of information available, it was necessary to assume values for the aerogels' density that can be found in the bibliography. Since the subcritical drying process provides energy gains, it can reduce the production cost of the aerogels resulting in an economic improvement of the process. It should be noted that both studies were performed in the laboratory but using an industrial scale the results shall be different.
Fig. 10. Energy consumption, kWh/(kg of aerogel), of the aerogels described in both studies.
The acquisition cost of the plastic bags is the same as for the previous aerogels and around 0.14 €/kg of aerogel. The cost of energy is calculated according to Eq. (1) and, for the HYB-C Aerogel, the number of operating hours of the incubator is 90 or 20 days, when produced in the incubator A or in the incubator B, respectively, as shown in Table 7. It is possible to observe the economic evaluation of each stage of the HYB-C aerogel Life Cycle in Table 8.
4. Conclusion The Economic Life Cycle Assessment (LCA) methodology allowed the estimation of the economic impact of the production of subcritical aerogels (one inorganic and two hybrids with different forms: powder and monolithic). It was observed that the economic optimization of this process suitable for laboratorial scale activities, should be done by minimizing the time and/or the energy consumptions of the drying stage. With regard to the distinct Life Cycle stages, it was concluded that, for all types of aerogel, the acquisition of raw materials phase is the most relevant in economic terms. At the synthesis stage, the major difference is that inorganic and hybrid HYB-C aerogels require about 20 days for drying, while hybrid HYB-A aerogel is produced in only 4 days, representing only 0.49% of its total cost. In the drying process, the costs between the aerogels described in this study and those currently commercialized differ significantly. The most common commercial aerogels are supercritically dried, while those described in this paper are synthesized and dried at atmospheric pressure, which makes their production less expensive due to less energy demands. Subcritical drying is safer and more energy-efficient. However, since this study was carried out at a laboratory scale, the costs are higher. Therefore, it is important to highlight that once an industrial scale is used the production costs will decrease and will be more competitive. In fact, industrial supercritically dried aerogels available in the market present a cost six times lower than the ones presented in this study. Therefore, a reduction of cost of this magnitude might be expected when these aerogels synthesized and dried at atmospheric pressure start to be produced at an industrial scale.
3.3. Analysis of the economic evaluation of the synthesis of the aerogels After addressing the individual economic evaluation of the different aerogels produced in the laboratory, it is now appropriate to compare their different costs. Table 9 and Fig. 9 provide an overview of the costs associated with the various stages of the aerogels Life Cycles and clearly HYB-C aerogel is the most expensive. The main difference is related to the acquisition costs of raw materials. Regarding the distinct phases of the life cycle for all aerogels, the raw materials acquisition stage is the most relevant in economic terms. This stage corresponds to around 97.2%, 99.5%, and 98.3% of the total costs for IA, HYB-A and HYB-C, respectively. At the production stage and only for the IA and HYB-C aerogels, the expenses associated with energy consumption are significant. In the dosing, bagging and carrying processes, the three types of aerogel have the same cost, per kilogram, related to buying plastic bags. In the production of the aerogels, the main difference is that for the synthesis of IA and HYB-C is required about 20 days of drying (considering the incubator B), while HYB-A is synthesized and dried in only 4 days, representing only 0.49% of the total cost. Therefore, the drying stage is the most relevant synthesis step in terms of energy consuming for all aerogels as shown in Table 10. As mentioned before, the drying of IA and HYB-C aerogels is performed at the incubator B due to its capacity and power, optimizing the synthesis time. Although HYB-A is the only aerogel that is not dried in this incubator, it remains however the one that needs less energy to be produced.
Acknowledgments The authors gratefully acknowledge the support of CERIS - IST, 33
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Universidade de Lisboa, and FCT, Foundation for Science and Technology, since this work was developed in the scope of the project FCT PTDC/ECM/11826/2010 - Nanorender. Maria de Fátima Júlio acknowledges FCT and Saint-Gobain Weber Portugal S.A. for PhD grant SFRH/BDE/112796/2015. Marco Pedroso acknowledges FCT for PhD grant FCT SFRH/BD/132239/2017.
date: 11 September 2017. [12] CEN, ISO 14040:2006 - life cycle assessment – Principles and framework, Switzerland, (2006). [13] ISO, ISO 14044:2006 - Environmental management - Life cycle assessment - requirements and guidelines, Switzerland, (2006). [14] CEN, ISO 15686-5: 2008 Buildings and construction assets - service life planning part 5: life-cycle costing, (2008). [15] M. Dowson, M. Grogan, T. Birks, D. Harrison, S. Craig, Streamlined life cycle assessment of transparent silica aerogel made by supercritical drying, Appl. Energy 97 (2012) 396–404, https://doi.org/10.1016/j.apenergy.2011.11.047. [16] S.B. Riffat, G. Qiu, A review of state-of-the-art aerogel applications in buildings, Int. J. Low-Carbon Technol. 8 (2013) 1–6, https://doi.org/10.1093/ijlct/cts001. [17] J. Wang, Y. Wei, W. He, X. Zhang, A versatile ambient pressure drying approach to synthesize silica-based composite aerogels, RSC Adv. 4 (2014) 51146–51155. [18] M.F. Júlio, L.M. Ilharco, Ambient pressure hybrid silica monoliths with hexamethyldisilazane: from vitreous hydrophilic xerogels to superhydrophobic aerogels, ACS Omega 2 (2017) 5060–5070. [19] M. de F. Júlio, A. Soares, L.M. Ilharco, I. Flores-Colen, J. de Brito, Silica-based aerogels as aggregates for cement-based thermal renders, Cem. Concr. Compos. 72 (2016) 309–318, https://doi.org/10.1016/j.cemconcomp.2016.06.013. [20] M. de F. Júlio, L.M. Ilharco, Superhydrophobic hybrid aerogel powders from waterglass with distinctive applications, Microporous Mesoporous Mater. 199 (2014) 29–39, https://doi.org/10.1016/j.micromeso.2014.07.056. [21] G. Carlson, D. Lewis, K. McKinley, J. Richardson, T. Tillotson, Aerogel commercialization: technology, markets and costs, J. Non-Cryst. Solids 186 (1995) 372–379, https://doi.org/10.1016/0022-3093(95)00069-0. [22] EPA, EPA/600/R-06/060: Life cycle assessment: principles and practice, USA, (2006). [23] N. Pargana, M.D. Pinheiro, J.D. Silvestre, J. de Brito, Comparative environmental life cycle assessment of thermal insulation materials of buildings, Energy Build. 82 (2014) 466–481, https://doi.org/10.1016/j.enbuild.2014.05.057. [24] EDP, EDP electricity costs (in Portuguese) (n.d.), http://www.edpsu.pt/pt/ EDPDocs/TarifasTransitóriasjaneiro2015.pdf, (2019) , Accessed date: 5 September 2017. [25] R. Garrido, J.D. Silvestre, I. Flores-Colen, Economic and energy life cycle assessment of aerogel-based thermal renders, J. Clean. Prod. 151 (2017) 537–545, https://doi.org/10.1016/j.jclepro.2017.02.194. [26] I. Pinto, J.D. Silvestre, J. de Brito, M. de F. Júlio, Environmental Impact of the Subcritical Production of Silica-Based Aerogels (in Portuguese: Impacte Ambiental da produção subcrítica de aerogéis de sílica), Eng. Civ. da Univ. do Minho, 2016, pp. 29–42.
References [1] Directive 2012/27/EU of the European Parliament and of the European Council of 25th October 2012 on energy efficiency, Off. J. Eur. Union 315 (October, 2012) 1–55. [2] M.R. Wiesner, J.-Y. Bottero, Environmental Nanotechnology: Applications and Impacts of Nanomaterials, 1st ed., The McGraw-Hill Companies, New York - USA, 2007. [3] S.S. KISTLER, Coherent expanded aerogels and jellies, Nature 127 (1931), https:// doi.org/10.1038/127741a0 741–741. [4] C.A. Pierre, M.G. Pajonk, Chemistry of aerogels and their applications, Chem. Rev. 102 (2002) 4243–4265. [5] S. Zhao, Aerogels, in: D. Levy, M. Zayat (Eds.), Sol-Gel Handbook: Synthesis, Characterization and Applications, Wiley VCH, Weinheim, 2015, pp. 519–−574. [6] H. Maleki, L. Durães, C.A. García-González, P. del Gaudio, A. Portugal, M. Mahmoudi, Synthesis and biomedical applications of aerogels: possibilities and challenges, Adv. Colloid Interf. Sci. 236 (2016) 1–27. [7] A.C. Pierre, A. Rigacci, SiO2 Aerogels, in: M.A. Aegerter, N. Leventis, M.M. Koebel (Eds.), Aerogels Handb, Springer Science+Business Media, New York, NY, 2011, pp. 21–45. [8] M. Koebel, A. Rigacci, P. Achard, Aerogel-based thermal superinsulation: an overview, J. Sol-Gel Sci. Technol. 63 (2012) 315–339, https://doi.org/10.1007/s10971012-2792-9. [9] M. de Fátima Júlio, A. Soares, L.M. Ilharco, I. Flores-Colen, J. de Brito, Aerogelbased renders with lightweight aggregates: correlation between molecular/pore structure and performance, Constr. Build. Mater. 124 (2016) 485–495, https://doi. org/10.1016/j.conbuildmat.2016.07.103. [10] L. Arroja, P. Quinteiro, A.C. Dias, The Past, Present and Future of Life Cycle Assessment – The Life Cycle Sustainability Assessment, I Conferência Nac. Sustentabilidade na Reabil. Urbana - O Novo Paradig. Do Merc, Da Construção, Lisboa - Portugal, 2011, pp. 99–106. [11] European Comission, EPLCA, http://eplca.jrc.ec.europa.eu/, (2017) , Accessed
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Contents lists available at ScienceDirect
Journal of Non-Crystalline Solids journal homepage: www.elsevier.com/locate/jnoncrysol
Economic assessment of the production of subcritically dried silica-based aerogels
T
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Rita Garridoa, José Dinis Silvestreb, , Inês Flores-Colenb, Maria de Fátima Júlioc, Marco Pedrosob a
Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal CERIS, DECivil, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal c CQFM, and IN – Institute of Nanoscience and Nanotechnology, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, 1049-001 Lisboa, Portugal b
A R T I C LE I N FO
A B S T R A C T
Keywords: Silica aerogel Subcritical drying Life cycle assessment Life cycle cost Thermal insulating aggregates
Traditionally, most commercial aerogels are produced by supercritical drying methods which make their application undesirable, more expensive, and less energy efficient. This paper presents the economic assessment of silica-based aerogels synthesized via ambient pressure drying. Three different approaches were considered in this paper: the production of inorganic aerogel and of two hybrid aerogels. The synthesized aerogels will be incorporated as aggregates in wall coatings to improve their thermal performance. The application of the Life Cycle Assessment (LCA) methodology is exceedingly important since it considers all costs associated with a product, process or activity during their physical and economic life, allowing the optimization of the total costs. It was possible to evaluate the production stages of the subcritical silica aerogels that influence their final cost and their relative contribution. These results allow to identify the main aspects that influence the total cost of the synthesis process. It is important to emphasize that this study is suitable for laboratory scale activities leading to high costs. It naturally follows that an adjustment to industrial scale applications is of paramount importance and should be made to maximise the economies of scale and to achieve additional gains in efficiency.
1. Introduction The industrial sector has undergone profound changes triggered by growing concerns with sustainability coupled with more demanding energy and environment European Directives [1]. This has encouraged the development of nanomaterials allowing to reduce the environmental impact during their life cycle (production, use and destination), the incorporation of processed raw-materials as well as their energy costs [2]. Since Kistler's pioneering work in 1931 [3], silica-based aerogels have been used in a wide range of scientific and technological applications [4–6]. These applications depend on the exceptional properties of silica aerogels, such as high specific surface area (≥1000 m2.g−1), very low density (3–500 kg.m−3), small pore size (1–100 nm) and extremely high porosity (above 95%) [7]. They are among the best known thermal insulating materials (with thermal conductivity around 0.015 W.m−1.K−1 at room temperature and ambient pressure), due to the fine solid constituents and porous
structure in the mesopore range [8,9]. This paper evaluates the economic life cycle (LC) associated with three different synthesis methods to obtain subcritical dried silica-based aerogels. The Life Cycle Assessment (LCA) standardized methodology was therefore used to study: an inorganic aerogel (IA) produced in monolithic form and two hybrid aerogels produced in powder (HYB-A) and monolithic (HYB-C) forms. As far as the authors are aware, no economic study of this type of any aerogels production process is available in literature sources. The results have shown how the costs of this process can be optimized, in laboratorial scale activities, and the phases of the life cycle that contribute the most to the whole life cycle of these products. Moreover, it is also possible to compare the three different synthesis methods and conclude which one is less or more expensive. 2. Economic life cycle assessment The Life Cycle Assessment (LCA) of products and/or processes arises
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Corresponding author. E-mail addresses: [email protected] (R. Garrido), [email protected] (J.D. Silvestre), ines.fl[email protected] (I. Flores-Colen), [email protected] (M.d.F. Júlio), [email protected] (M. Pedroso). https://doi.org/10.1016/j.jnoncrysol.2019.04.016 Received 10 January 2019; Received in revised form 16 April 2019; Accepted 17 April 2019 Available online 28 April 2019 0022-3093/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Photographs of the laboratory-synthesized silica aerogels: (A) IA; (B) HYB-A; (C) HYB-C [18–20].
3. Silica aerogels
from the need to estimate the environmental, energy and economic impacts along their physical and economic life. Since the 1960's, numerous studies and methods have been developed to optimize products, to make them more sustainable and less harmful to the environment. However, it was only in 1990 that this methodology, promoted by SETAC (Society of Environmental Toxicology and Chemistry), was standardized. The Life Cycle cost approach used to evaluate a product considering both internal and external costs as well as their allocation to all stakeholders was introduced between 1980 and 1990 [10,11]. In 1997, the first International Standards (ISO 14040 - Life Cycle Assessment - Principles and framework [12], 14,044 - Life Cycle Assessment - Requirements and guidelines [13]) were released which standardized a system for assessing the environmental sustainability of products based on a life cycle approach. A Life Cycle Assessment (LCA) provides a careful analysis of the whole cycle of the product: from the extraction of the raw materials until it becomes waste, passing through all the intermediate stages (production, transportation and use), and with a main focus on the interaction between the product system and the exterior [14]. Only one study related to the environmental and energy impacts of the production of aerogels is reported in literature [15]. According to that study, a silica-based aerogel was produced in the laboratory using supercritical drying conditions, at high and low temperatures (HTSCD and LTSCD, respectively). That Life Cycle Assessment (LCA) was developed from the cradle to the factory gate, excluding the impacts of transportation of raw materials and finished products, as well as their processing at the end of life. The referred study was carried out according to ISO 14044 [13] and to ISO 14040 [12], having as its functional unit the energy expenditure (kWh) and the release of CO2 (kg CO2) in the production of one cubic meter of solid (non-granular) aerogel. The authors concluded that:
Conventionally, silica aerogels are dried above the supercritical point of the solvent, thus requiring high pressures and, depending on the solvent, high temperatures. Though once a preferred method, it involves time-consuming procedures, and it is often expensive and hazardous, rendering the process difficult for industrial use [15]. As a result, cost-effective drying alternatives are required. By using subcritical drying techniques, their costs are reduced, and their safety is increased, partially overcoming the drawbacks [16–18]. When drying aerogels at ambient pressure, the key process is surface modification: by introducing nonpolar groups (usually from silanes) at the surface of the pores, capillary tensions that develop upon solvent evaporation are reduced, thus preventing pore collapse. The aerogels described in the present work were dried under subcritical conditions and the synthesis procedures were reported in detail in previous papers by the authors [9,18–20]. As in the aerogels' production phase, the predominant cost is the acquisition of raw materials, the focus being usually to reduce it and improve economic performance against other materials [21].
3.1. Synthesis of silica aerogels The life cycle (LC) study discussed in this paper includes three different laboratory-synthesized silica aerogels, dried at atmospheric pressure: an inorganic aerogel (IA) produced in monolithic form [19] and two hybrid aerogels produced in powder (HYB-A) [20] and monolithic (HYB-C) [18,19] form (Fig. 1). Fig. 2 illustrates the life cycle of a laboratory synthesized aerogel, where three distinct steps are analysed: the production, use and end-oflife. The product stage stands out because it is the subject of this study. This stage differs according to the three types of aerogel, whereas the use and end of life stages are similar for all types of aerogel.
• the LTSCD production method had a longer environmental return, • •
with a total energy consumption of 62.6 MJ per 40 ml and a total release of 6.64 kg CO2 per 40 ml; according to the HTSCD production method, the total energy consumption and the release of CO2 was 29.3 MJ per 40 ml and 0.73 kg CO2 per 40 ml, respectively; the aging of the aerogel was the production phase where more energy was spent.
As the industrial production process is optimized, the values of energy spent and the release of CO2 decrease significantly. However, it is for the LTSCD method that more significant improvements can be achieved.
Fig. 2. Aerogel generic Life Cycle. 27
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raw materials; 2. Transport and storage; 3. Aerogel synthesis; 3.1 Hydrolysis; 3.2 Condensation; 3.3 Aging; 3.4 Washing; and 3.5 Drying.
In this study, all chemical reagents were bought from a company in the Lisbon area. A medium-sized van was used to transport them to the synthesis laboratory. The reception of the raw materials and their storage was done manually. The raw materials were put on shelves, inside cabinets, in the laboratory, until the synthesis of the aerogels takes places. Dosing, bagging and carrying were the subsequent stages. It was estimated that each package had 150 g of aerogel which corresponds to an individual plastic bag. After the aerogel was bagged up and stored at the synthesis laboratory, it was carried to the construction laboratory for the production of aerogel-based renders.
3.1.2. HYB-A: Hybrid aerogel synthesized by a one-step sol gel process (powder form) The sodium silicate solution D40 from Solvay Portugal (27.5 wt% SiO2, 8.5 wt% Na2O and 1.37 g cm−3) is initially diluted with deionized water to obtain a 5 wt% in SiO2 precursor, at pH 12. In order to acidify this alkaline solution, nitric acid from Sigma-Aldrich (HNO3, 65%) is added. This step is intended to activate the molecular silicates in solution to silicic acid (by partial or total protonation of Si-O− centres), resulting in a pH that can range from 1 to 2. Subsequently, hexamethyldisilazane from Sigma–Aldrich (HMDZ, 99.9%) is added under constant orbital stirring, at room temperature (24–25 °C) with the double role of increasing pH and as co-precursor [20]. NH3 is released to the gas phase and pumped. The partial neutralization with HMDZ initiates condensation reactions and gelation occurs immediately. n-Hexane (96%, from ACS Basic) is added in a fixed WG:n-hexane molar ratio of 1:0.1, to replace any water included in the pores, whose surfaces are expected to be essentially hydrophobic. This exchange proceeds for 3 h, after which the two liquid phases (excess n-hexane and displaced pore water) are removed. The silylated hydrogels are dried at ambient pressure at 60 °C for 24 h followed by 24 h at 100 °C. The samples are repeatedly washed with water (to remove the NO3− and Na+ ions) and finally dried under the above conditions, until aerogel powders are obtained. The chemical reagents were supplied to the synthesis laboratory by a company outside Lisbon. Sodium silicate is a “waste material” from a company located near this supplier. The life cycle from “cradle to gate” of HYB-A aerogel is shown in Fig. 4. The associated synthesis of this type of aerogel involves the following processes: 1. Reception of raw materials; 2. Transport and storage; 3. Aerogel synthesis; 3.1 Hydrolysis; 3.2 Condensation; 3.3 Aging; 3.4 Washing; and 3.5 Drying.
3.1.1. IA: Inorganic aerogel synthesized by a two-step sol-gel process Tetraethoxysilane from Aldrich (TEOS, 98%) was used as a silica precursor, 2-propanol p.a. from Sigma-Aldrich (2-PrOH, 99.8%) as a solvent, HCl (1 M) p.a. from CARLO ERBA and ammonia p.a. from Merck (NH4OH, 0.1 M) as catalysts. The inorganic aerogel can be synthesized by a two-step sol-gel process. It involves the acid-catalysed hydrolysis of TEOS and the polycondensation of the resulting silanol groups induced by addition of ammonia [19]. TEOS was previously diluted in 2-PrOH, and distilled water was added, dropwise, while stirring. The reaction mixture was acidified with HCl to initiate the hydrolysis process. The acidic colloidal solution was placed in a sealed container, heated at 60 °C, and stirred (at 120 rpm) for 60 min. The required amount of NH4OH was then added, and the resulting homogeneous sol was left to gel, with no further stirring. For the preparation of inorganic aerogel, the alcogel was aged for 24 h in a residual liquid and for more 24 h in an equal volume of aging solution, containing TEOS, 2-PrOH and H2O in the same proportions as used for gelation to strengthen the silica network. The pore liquid was then exchanged with 2-PrOH to completely remove any residual water or TEOS, and the gels subcritically dried under ambient pressure in a solvent-saturated atmosphere, until their weight loss became negligible. Throughout the process there was gas release, namely, 2-PrOH. The “cradle to gate” (synthesis) life cycle of the Inorganic Aerogel (IA) and the production activity associated to this type of aerogel is shown in Fig. 3 and involves the following processes: 1. Reception of
Fig. 3. Inorganic Aerogel Life Cycle. 28
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Fig. 4. Hybrid aerogel HYB-A Life Cycle.
Aging; 3.4 Washing; and 3.5 Drying.
3.1.3. HYB-C: Hybrid aerogel synthesized by a two-step sol-gel process The synthesis of HYB-C is similar to the synthesis of Inorganic Aerogel (IA), except for the aging phase [18,19]: After 24 h in the residual liquid, the aged alcogels were further hydrophobized by chemical surface modification with an aging solution of HMDZ (99.9%) in i-PrOH, and left to dry until their weight loss became negligible. The life cycle of the monolithic hybrid silica aerogel (HYB-C) is shown in Fig. 5. The associated synthesis of this type of aerogel involves the following processes: 1. Receiving raw materials; 2. Transport and storage; 3. Aerogel synthesis; 3.1 Hydrolysis; 3.2 Condensation; 3.3
3.2. Economic assessment of aerogel production According to the EPA [22], the LCA methodology can be divided into the following phases: (1) definition of the objective; (2) inventory analysis; (3) impact assessment; and (4) interpretation. In this section, the inventory analysis and the respective evaluation of the economic impacts will be carried out. A detailed economic evaluation of a production process should be as
Fig. 5. Hybrid aerogel HYB-C Life Cycle. 29
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Fig. 6. Costs of aerogels in each life cycle stage (“cradle to gate”).
Fig. 7. Incubator B.
Fig. 8. Energy consumption, kWh, after two hours.
detailed as possible and include all the relevant elements required for its precise quantification. The functional unit is a parameter that serves as a reference to describe the performance of a product (or service) when it fulfils its function. Its definition is relevant, since it guarantees that the inputs and outputs are quantified according to the same reference flow. The functional unit should be consistent with the objectives of the study, and well defined and quantified. Consequently, when several LCA are performed and compared to each other, the functional unit should be the same, or the comparison will not be possible and valid [22,23]. The functional unit (fu), in the present LCA study of inorganic aerogel (IA) and of hybrid aerogels (HYB-A and HYB-C), is defined in mass (kg) and corresponds to the production of 1 kg of aerogel. This is equivalent, in this case, to the declared unit of aerogel used as raw material for renders.
Table 1 Acquisition cost of the raw materials for the IA synthesis, per aerogel kilogram. Raw material
Costs [€/kg of aerogel]
TEOS (98%) i-PrOH (99.8%) HCl (1 M) NH4OH (0.1 M) Distilled H2O
255 60 1 120 0.40
In the following sections, an economic evaluation of each aerogel synthesis will be carried out based on different costs of various Life Cycle stages. These costs are qualitatively described In Fig. 6. At the aerogel synthesis stage, the cost associated with the energy 30
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Table 2 Energy costs of the IA. Synthesis stage
a b
Laboratory Incubator A (stirring and drying) Incubator B (drying)
Energy cost [€/kg of aerogel] 70.53a 12.17b
Incubator consumption: Stirring = 0.21 kWh, Drying = 272 W. Incubator consumption: Drying = 0.21 kWh.
Table 3 Economic assessment of the Inorganic Aerogel Life Cycle.
Cost [€/kg]
Incubator A Incubator B
Raw materials
Transport and storage
Production
Dosing, bagging and carrying
Total
436.39 436.39
– –
70.92 (stirring and drying, plus mix and aging) 12.56 (drying, plus mix and aging)
0.14 0.14
507.45 449.10
Table 4 Acquisition costs of the raw materials needed to synthetize HYB-A, per kilogram of aerogel.
Table 6 Acquisition costs of the raw materials for HYB-C, per kilogram of aerogel.
Raw materials
Costs [€/kg of aerogel]
Raw materials
Costs [€/kg of aerogel]
Sodium silicate aq. (27.5 wt% SiO2) HMDZ (99.9%) HNO3 (65%) n-hexane (96%)
100 450 40 50
TEOS (98%) i-PrOH (99.8%) HCl (1 M) NH4OH (0.1 M) HMDZ (99.9%) Distilled H2O
255 40 1 120 300 0.40
required to produce 1 kg of aerogel (Ce) is given by the following Eq. (1):
Ce = T × nh × P [€/kg of aerogel]
Table 7 Energy costs of HYB-C.
(1)
T - Cost of 1 kWh of electricity in Portugal, for buildings requiring medium voltage and phase-to-phase voltage whose efficiency is > 1 kV and equal to or < 45 kV (€/kWh), (0.12 €/kWh [24]); nh - number of incubator operating hours [hour]; P - Incubator power [W]. In this study, two incubators (A and B) were used: the incubator A for stirring and drying and the incubator B only for drying. The power of the incubator A is 600 W whereas when the drying process is performed in the incubator B the maximum power is 1700 W, as shown in Fig. 7. During the stirring and drying of the aerogels and to characterize the energy consumptions of these processes in a better way, an energy meter (PEREL E305EM6, 230 VAC - 16 A) was used in both equipment. This measurement resulted in a value of 0.30 kWh and 0.21 kWh, respectively, as shown in Fig. 8. The value of 0.12 €/kWh [24] was obtained through the arithmetic average of the active energy prices for long durations, comprising the peak hours and full hours. In the dosing, bagging and carrying processes there was no energy expenditure because the aerogel is produced in small quantities and there is no need for dosing. At this stage, there are only expenditures on consumables, for example plastic bags for bagging. Transportation inside the laboratory is done manually, without the use of any trucks or energy.
Synthesis stage
a b
Incubator A (stirring and drying) A (stirring) + B (drying)
Energy costs [€/kg of aerogel] 70.53a 12.17b
Incubator consumption: Stirring = 600 W, Drying = 272 W. Incubator consumption: Drying = 0.21 kWh.
Since the economic evaluation is the main objective, the cost was the most important parameter to be analysed. The first cost associated with the production of aerogel is the acquisition of the raw materials: TEOS, i-PrOH, HCl and NH4OH. Table 1 shows the cost (€/kg of aerogel) of raw materials, excluding VAT and considering that the transportation to the synthesis laboratory is the supplier's responsibility. The acquisition of the raw materials is done through a supplier company, located in the Lisbon region. The information on how the different raw materials are carried to Portugal is unknown, as there are several industries in different countries. One of the reagents used in the production of aerogel is distilled water, which is distilled at the synthesis laboratory. The distiller is always in operation for all other laboratories of this research centre and the container used for filling is always the same, thus no packaging is generated as waste. Therefore, the real cost associated with this water can't be estimated. However, to get as close as possible to the actual cost, it was decided to carry out a market search, and the value of the distilled water was estimated in case it is necessary to buy it. The estimated value was 0.20 €/litre. The only acquisition cost related to the purchase of containers/reactors is approximately 120 €/kg of aerogel. However, it should be noted that these containers are only part of an initial investment, since
3.2.1. Inorganic aerogel The approach used in the LCA of the inorganic aerogel (IA) was from the “cradle to the gate” of the laboratory, considering that the analysis comprises all the production procedures at the synthesis laboratory until it is delivered at the construction laboratory to later serve as aggregate in thermal renders [25]. Table 5 Economic assessment of the HYB-A Life Cycle.
Costs [€/kg]
Raw materials
Transport and storage
Production
Dosing, bagging and carrying
Total
640.00
–
3.16 (stirring and drying)
0.14
643.30
31
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Table 8 Economic assessment of the HYB-C Life Cycle.
Cost [€/kg]
Incubator A Incubator B
Raw materials
Production
Dosing, bagging and carrying
Total [€/kg]
716.39 716.39
70.92 (stirring and drying, plus mix and aging) 12.56 (drying, plus mix and aging)
0.14 0.14
787.45 729.10
Note: The transportation and storage from the raw materials to the production stage is manual and there is no cost.
Table 3 presents the economic evaluation of each stage of the inorganic aerogel (IA) Life Cycle.
Table 9 Costs of the distinct stages of the aerogels Life Cycles described in this study. Cost [€/kg of aerogel]
IA HYB-A HYB-C
Raw materials
Production
Dosing, bagging and carrying
Total
436.39 640.00 716.39
12.56 3.16 12.56
0.14 0.14 0.14
449.10 643.30 729.10
3.2.2. Hybrid silica aerogel HYB-A The approach used for the Life Cycle Assessment of this type of aerogel (HYB-A) was similar to the one used in the Life Cycle Assessment of Inorganic Aerogel as it consisted in analysing all the production as far as the entrance in the construction laboratory. The cost associated with the acquisition of the various raw materials (sodium silicate solution, HMDZ, HNO3 and n-hexane), excluding VAT and considering the transportation to the synthesis laboratory the supplier's responsibility, is detailed in Table 4. As mentioned before, the information on how the different raw materials are carried to Portugal is unknown. The sodium silicate solution, considered a “waste material”, is available free of charge from a company (Solvay Portugal), also located outside of Lisbon. However, if it is necessary to buy it, this product will cost about 27 €/litre, thus increasing the purchase cost to around 100€/kg of aerogel. The consumables to synthetize HYB-A (recipients/reactors) cost approximately 50 €/kg of aerogel, but for this aerogel they are different from those used to synthetize Inorganic Aerogel (IA), Again, it is important to note that these containers are only an initial investment, given that they are subsequently reused, and therefore their costs weren't considered in the economic assessment of the HYB-A life cycle. The same plastic bags are used to bag HYB-A so this cost amounts to 0.14€/kg of aerogel. The energy cost is calculated according to Eq. (1), and the number of hours of operation of the incubator corresponds to 4 days - 7 h in the stirring phase, and the remaining time in the drying process. This represents a cost of 3.16€ per kg of aerogel, because of a consumption of 0.3 kWh during the stirring process and of 272 W during the drying process. The HYB-A can only be produced and dried using incubator A, and optimization is not possible using the incubator B. Table 5 presents the economic assessment of each stage of the HYBA aerogel Life Cycle.
Note: Transportation and storage from the raw materials to the production stage is manual and there is no cost.
Fig. 9. Total cost of the various stages of each aerogel Life Cycle ( Raw materials; Production; Dosing, bagging and carrying).
they are subsequently reused. So it was decided not to take into account the costs of these consumables in the economic evaluation of the synthesis process. To bag the aerogel, after dosing and before carrying it, it was necessary to buy plastic bags, which cost € 0.14/kg of aerogel. There are energy-related costs for the aerogel synthesis in every step of its Life Cycle. The energy cost is calculated according to Eq. (1) and, for the IA, the number of operating hours of the incubator corresponds to 7 h for the agitation phase and 2153 h for the drying process, representing a total of 90 days as shown in Table 2. The 90 days are considered when using incubator A, but it is possible to perform the drying process using incubator B, which is larger, reducing to 20 days the time needed to produce one kilogram of aerogel (using more reactors at a time). Thus, this incubator has an energy expenditure of 0.21 kWh during these 20 days, of which 7 h are in the incubator A (while stirring). The energy cost of the Inorganic Aerogel (IA), if it is dried in incubator B, is 12.17 €/kg of IA, as shown in Table 2.
3.2.3. Hybrid silica aerogel HYB-C Table 6 summarizes the costs of raw materials used for HYB-C's synthesis (€/kg of aerogel) without VAT and being the transport to the synthesis laboratory a responsibility of the supplier, which is the company that supplies the raw materials for the remaining aerogels. As in the IA, the production of the HYB-C aerogel uses distilled water, distilled in the laboratory. Given the explanation already stated, the estimated cost of this water is 0.20 €/litre. The cost of consumables and containers/reactors for HYB-C is similar to IA (approximately 120€/kg aerogel).
Table 10 Energy consumption, per aerogel kilogram, at each production (synthesis) stages. Production [kWh/kg]
IA HYB-A HYB-C
Mixing
Heating and stirring (Hydrolysis)
Condensation
Aging
Washing
Drying
Total
0.0 0.0 0.0
2.1 2.1 2.1
0.0 0.0 0.0
0.0 0.0 0.0
0.0 0.0 0.0
99.3 24.2 99.3
101.4 26.3 101.4
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Table 11 Energy consumption per kg of aerogel, for aerogels obtained through different drying processes. Subcritical drying
Supercritical drying
Incubator A IA kWh/kg
587.72
Incubator B HYB-A
(1)
26.31
(2)
HYB-C 587.72
IA (3)
101.43
Dowson et al. [15] HYB-C
(1)
101.43
(3)
HTSCD
LTSCD
138.86 (4) 79.67 (5)
460.57 264.26
(4) (5)
Note: aerogel density = (1) 209.37 kg/m3; (2) 100–200 kg/m3; (3) 305.58 kg/m3; (4) 175 kg/m3; (5) 305 kg/m3; Aerogels dried supercritically at HTSCD – High temperature, LTSCD - low temperature.
Dowson et al. [15] presented a Life Cycle Assessment (LCA) which compared aerogels dried supercritically at high and low temperatures. In that study, the authors obtained the same pattern for high and low temperatures (HTSCD and LTSCD), where the highest energy consumption was by far associated with the drying process when compared with their preparation or aging. Those results agree with the results of the present study, despite the subcritical drying of the aerogels. However, subcritical drying may be advantageous in terms of energy and consequently economically [26]. To confirm this expectation, the energy consumption of the aerogels production was compared using both drying processes as shown in Table 11 and Fig. 10. Due to the lack of information available, it was necessary to assume values for the aerogels' density that can be found in the bibliography. Since the subcritical drying process provides energy gains, it can reduce the production cost of the aerogels resulting in an economic improvement of the process. It should be noted that both studies were performed in the laboratory but using an industrial scale the results shall be different.
Fig. 10. Energy consumption, kWh/(kg of aerogel), of the aerogels described in both studies.
The acquisition cost of the plastic bags is the same as for the previous aerogels and around 0.14 €/kg of aerogel. The cost of energy is calculated according to Eq. (1) and, for the HYB-C Aerogel, the number of operating hours of the incubator is 90 or 20 days, when produced in the incubator A or in the incubator B, respectively, as shown in Table 7. It is possible to observe the economic evaluation of each stage of the HYB-C aerogel Life Cycle in Table 8.
4. Conclusion The Economic Life Cycle Assessment (LCA) methodology allowed the estimation of the economic impact of the production of subcritical aerogels (one inorganic and two hybrids with different forms: powder and monolithic). It was observed that the economic optimization of this process suitable for laboratorial scale activities, should be done by minimizing the time and/or the energy consumptions of the drying stage. With regard to the distinct Life Cycle stages, it was concluded that, for all types of aerogel, the acquisition of raw materials phase is the most relevant in economic terms. At the synthesis stage, the major difference is that inorganic and hybrid HYB-C aerogels require about 20 days for drying, while hybrid HYB-A aerogel is produced in only 4 days, representing only 0.49% of its total cost. In the drying process, the costs between the aerogels described in this study and those currently commercialized differ significantly. The most common commercial aerogels are supercritically dried, while those described in this paper are synthesized and dried at atmospheric pressure, which makes their production less expensive due to less energy demands. Subcritical drying is safer and more energy-efficient. However, since this study was carried out at a laboratory scale, the costs are higher. Therefore, it is important to highlight that once an industrial scale is used the production costs will decrease and will be more competitive. In fact, industrial supercritically dried aerogels available in the market present a cost six times lower than the ones presented in this study. Therefore, a reduction of cost of this magnitude might be expected when these aerogels synthesized and dried at atmospheric pressure start to be produced at an industrial scale.
3.3. Analysis of the economic evaluation of the synthesis of the aerogels After addressing the individual economic evaluation of the different aerogels produced in the laboratory, it is now appropriate to compare their different costs. Table 9 and Fig. 9 provide an overview of the costs associated with the various stages of the aerogels Life Cycles and clearly HYB-C aerogel is the most expensive. The main difference is related to the acquisition costs of raw materials. Regarding the distinct phases of the life cycle for all aerogels, the raw materials acquisition stage is the most relevant in economic terms. This stage corresponds to around 97.2%, 99.5%, and 98.3% of the total costs for IA, HYB-A and HYB-C, respectively. At the production stage and only for the IA and HYB-C aerogels, the expenses associated with energy consumption are significant. In the dosing, bagging and carrying processes, the three types of aerogel have the same cost, per kilogram, related to buying plastic bags. In the production of the aerogels, the main difference is that for the synthesis of IA and HYB-C is required about 20 days of drying (considering the incubator B), while HYB-A is synthesized and dried in only 4 days, representing only 0.49% of the total cost. Therefore, the drying stage is the most relevant synthesis step in terms of energy consuming for all aerogels as shown in Table 10. As mentioned before, the drying of IA and HYB-C aerogels is performed at the incubator B due to its capacity and power, optimizing the synthesis time. Although HYB-A is the only aerogel that is not dried in this incubator, it remains however the one that needs less energy to be produced.
Acknowledgments The authors gratefully acknowledge the support of CERIS - IST, 33
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Universidade de Lisboa, and FCT, Foundation for Science and Technology, since this work was developed in the scope of the project FCT PTDC/ECM/11826/2010 - Nanorender. Maria de Fátima Júlio acknowledges FCT and Saint-Gobain Weber Portugal S.A. for PhD grant SFRH/BDE/112796/2015. Marco Pedroso acknowledges FCT for PhD grant FCT SFRH/BD/132239/2017.
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