Technoeconomic and Environmental Assessment for Design and

Article Cite This: Ind. Eng. Chem. Res. 2018, 57, 2192−2199

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Technoeconomic and Environmental Assessment for Design and Optimization of Tetraethyl Orthosilicate Synthesis Process Thuy T. H. Nguyen,*,† Norihisa Fukaya,‡ Kazuhiko Sato,‡ Jun-Chul Choi,‡ and Sho Kataoka*,† Research Institute for Chemical Process Technology and ‡Interdisciplinary Research Center for Catalytic Chemistry, National Institute of Advanced Industrial Science and Technology, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan

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†

ABSTRACT: Biogenic silica has been considered as a potential feedstock for producing a variety of silicon-containing products. A new synthesis pathway of tetraethyl orthosilicate from silica that is contained in rice hull ash has been recently proposed. Requiring only one-step processing, it is expected to offer advantages over conventional routes which are derived from silicon metal. This study aims to investigate the optimal synthesis conditions for which this new technology can sustainably replace the conventional method. Experiments employing different reaction conditions are performed to provide the necessary information for a conceptual process design and an economic and environmental viability evaluation. The results obtained by comparison with a selected conventional process show that, under the optimal synthesis conditions, the new process can decrease the production cost and markedly reduce the high greenhouse gas emissions. The competitiveness of the new process is further examined with a sensitivity analysis considering fluctuations in key feedstock and utility prices and alternative sources of electricity supplied to the conventional process. The new synthesis technology shows high potential as a sustainable alternative to the conventional one.

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attracted increasing attention.11−16 Recently, our group synthesized TEOS by directly reacting RHA with ethanol.17,18 This study revealed that the high-energy-consuming step of the thermal reduction of silica to silicon can be eliminated. With this substantial improvement, it is expected that TEOS can be widely produced without requiring large quantities of electricity. Experiments have shown the strong impact of the ethanol volume on the product yield. The use of excess ethanol promotes a high yield of TEOS but requires a large amount of energy for ethanol recovery. Therefore, to promote a greener TEOS synthesis route, the production process must be well designed and rigorously evaluated. This study focuses on the conceptual design and evaluation of the impact of synthesis conditions, with respect to the molar ratio of raw materials and the economic and environmental viability of this new synthesis route. Experiments are performed under different synthesis conditions, based on which a new synthesis process is designed. Through an in-depth assessment and comparison with a selected conventional synthesis process, considering both the production cost and the GHG emissions, optimal synthesis conditions are proposed to maximize the benefits. The competitiveness of the new process is further examined, considering fluctuations in feedstock and utility prices and different power generation systems. These analyses

INTRODUCTION Tetraethyl orthosilicate or tetraethoxysilane (TEOS) has wide applications as an insulator in the electronic industry, as a crosslinking agent and binder in coating and painting industries, and as an intermediate substance in the production of diverse catalysts and products in the chemical industry. Conventionally, TEOS is produced via two routes: the reaction of tetrachlorosilane with ethanol and that of metallurgical silicon (Simg) with ethanol. Both routes utilize the same raw material, Simg. In the former route, tetrachlorosilane is first produced from Simg, and then it requires extensive energy-consuming purification steps before being used in TEOS synthesis.1,2 The release of hydrogen chloride causes many problems related to equipment corrosion, occupational health risk exposure, and environmental impact. Because the latter route involves only one synthesis step, it is much simpler and greener than the former.3−6 In these two conventional routes, the input feedstock Simg is the main factor that determines the economic and environmental performance of the entire production process. In practice, this is primarily produced via carbonthermal reduction of silica-containing sources, which requires a large amount of electricity (approximately up to 13 000 kWh/ ton product7,8), thus producing excessive greenhouse gas (GHG) emissions. Rice hull ash (RHA) contains a large fraction of silica (more than 95 wt %). The direct combustion, gasification, or pyrolysis of rice hull, which is widely available at considerably large amounts,9,10 results in RHA. That it can be obtained from utilizing RHA for producing silicon-based materials has © 2018 American Chemical Society

Received: Revised: Accepted: Published: 2192

September 19, 2017 December 1, 2017 January 19, 2018 January 19, 2018 DOI: 10.1021/acs.iecr.7b03895 Ind. Eng. Chem. Res. 2018, 57, 2192−2199

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Industrial & Engineering Chemistry Research Table 1. Reaction Conditions Included in the Base Case and New Synthesis Routes temperature (K) key input component molar ratio key component:ethanol main byproduct conversion of key component (%) yield of TEOS (%) (key component-based) yield of byproduct (%) (key component-based)

base case6

alternative 1

alternative 2

alternative 3

alternative 4

453 silicon metal 1:23 TRES 88.0 74.0 14.0

513 biogenic silica 1:112 DS 96.9 91.3 2.8




Figure 1. TEOS synthesis process from Simg (base case). (a) Calcinator; (b) mixer; (c, f) heat exchangers; (d) heater; (e) reactor; (g) cooler; (h) gas separator; (i) ethanol recovery column; (j) TEOS purification column. ROH: anhydrous ethanol; RHA: rice hull ash; TEOS: ethyl orthosilicate; TRES: triethoxysilane.

described in detail in the following sections, and their main reactions are summarized in Table 1. Base Case: Metallurgical Silicon Utilizing Process. As mentioned above, TEOS is synthesized via two pathways: direct and indirect reactions of Simg (via tetrachlorosilane synthesis) with ethanol. The former route is free from hazardous waste and employs only one stage reaction. In the latter route, the purification of TEOS requires extensive energy consumption and releases hydrogen chloride, which make this process more costly and environmentally unfriendly.3−6As the former method is simpler, greener, and less energy-consuming, it was selected for comparison with the new synthesis route. In this conventional process, silica-containing resources such as quartz must be reduced to Simg following eq 1. This reduction is performed in an electric furnace at about 2173 K using a reducing agent containing carbon, such as coke, charcoal, or wood chips. A detailed description of Simg production is beyond the scope of this paper. More information can be found in the available literature.7,8 The Simg produced is then used for TEOS synthesis. Figure 1 shows the synthesis process of TEOS from Simg. This was designed using information obtained from the available literature.3 Simg obtained from silica reduction is heated at a high temperature (773−973 K) with cupric oxide in a calcinator (a). Stream 1 containing the activated Simg is then mixed with excess ethanol in a mixer (b). The mixed stream 2 is

can provide a good vision for the long-term implementation of new TEOS synthesis technology.

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PROCESS DESIGN AND SIMULATION MODELS OF TEOS SYNTHESIS ROUTES

The process configurations of the new and conventional synthesis routes are designed and modeled using process simulator Pro/II. Whereas the new route uses biogenic silica included in RHA, the conventional one uses metallurgical silicon as the main input feedstock. These processes consist of the main processing units used for product synthesis, byproduct separation, unreacted reactant recovery, and product purification. Data of TEOS properties and vapor−liquid equilibrium of TEOS19−23 containing output mixtures are collected and input to Pro/II library data, on the basis of which parameters necessary for process simulation are calculated using the selected thermodynamic model (NRTL). Heuristic knowledge is applied to optimize the considered processes. The parameters such as difference in temperature of the hot and cold fluids of the heat exchangers and the number of stages, reflux ratio, and feed position of the distillation columns are rigorously considered for minimizing the energy and capital costs of the main processing units. These processes are designed to produce TEOS with a purity of 99.5 wt % and capacity of 1000 tons/year. They are 2193

DOI: 10.1021/acs.iecr.7b03895 Ind. Eng. Chem. Res. 2018, 57, 2192−2199

Article

Industrial & Engineering Chemistry Research

Figure 2. TEOS synthesis process from RHA (new process). (a) Mixer; (b, e) heat exchangers; (c) heater; (d) reactor; (f) cooler; (g) water absorber; (h) ethanol recovery column; (i) TEOS purification column. ROH: anhydrous ethanol; RHA: rice hull ash; TEOS: ethyl orthosilicate; DS: hexaethyldisiloxane.

where its temperature is increased by the heat transferred from the stream 4 output from reactor (d). In heater (c), the temperature of stream 2 is further increased. The resulting stream 3 is then directed to reactor (d). In this reactor, TEOS and byproducts are synthesized following eqs 4 and 5 in the presence of a suitable amount of potassium hydroxide at 513 K.

directed to a heat exchanger (c), where it receives heat transferred from the stream 5 output from reactor (e). Output stream 3 is further heated to the desired temperature in heater (d). The resulting stream 4 is directed to a TEOS synthesis reactor (e) operated at an elevated temperature (443−463 K) and autogenous pressure. The product stream 5 output from the reactor is a mixture of TEOS and the byproducts triethoxysilane (TRES) and hydrogen produced via eqs 2 and 3. Through heat exchangers (c) and (f), this transfers heat to streams 2 and 9. It is further cooled after passing through cooler (g). Output stream 8 is then directed to gas separator (h) for hydrogen removal. The output from heat exchanger (f), stream 10, has an elevated temperature. This is finally directed to a separation system, which consists of unconverted ethanol recovery and product purification distillation columns. Unconverted ethanol obtained from the top of column (i) is fed back into the process. This is mixed with the fresh input raw materials for starting a new production. In column (j), TRES is obtained at the top and can be used as an intermediate chemical for manufacturing other products. TEOS at the desired purity is obtained at the bottom of the column. SiO2 + C → Si + CO2

(1)

Si + 4C2H5OH → Si(OC2H5)4 + 2H 2

(2)

Si + 3C2H5OH → HSi(OC2H5)3 + H 2

(3)

SiO2 + 4C2H5OH ↔ Si(OC2H5)4 + 2H 2O

(4)

2Si(OC2H5)4 + H 2O ↔ Si 2O(OC2H5)6 + 2C2H5OH (5)

As the reactions involved in this process are reversible, a high input volume of ethanol and continuous removal of water are required to attain a high product yield. Experiments with different ratios of biogenic silica to ethanol were performed to examine the impact of this ratio on the product yield. Table 1 shows alternative ethanol input volumes, which result in considerable differences in the yields of TEOS and the main byproduct. To remove water, molecular sieve (MS) 3A is used.18 As this cannot absorb water effectively at high temperature, the stream 4 output from the reactor is cooled to 333 K after passing through heat exchangers (b) and (e) and cooler (f). Output stream 7 is then passed through absorber (g), which is packed with MS 3A for water removal. Stream 8, which is free of water, is warmed after passing through heat exchanger (e). Output stream 9 is then directed to a separation system, in which unconverted ethanol is recovered as the major product of column (h) and fed back into the process. In column (i), TEOS is obtained as the major product.

New Process: Biogenic Silica (RHA) Utilizing Process. Experimental procedures reported in our previous studies17,18 are mimicked to obtain the reaction information used for process design and simulation. Different reaction conditions (molar ratios of input raw materials) are employed to investigate their impact on the process performance. The initial feedstock used in this process is RHA, which is obtained from the incineration of rice hull at about 773 K,18 the energy obtained from this process can be used for the generation of electricity. Figure 2 shows the synthesis process of TEOS using RHA. In mixer (a), RHA is mixed with excess anhydrous ethanol. Output stream 1 is then passed through heat exchanger (b),

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EVALUATION INDICATORS The above TEOS synthesis processes are evaluated considering the following economic and environmental indicators. Production Cost. The PC is calculated as described in ref 24. It is the sum of the following costs: PC = C investment + Coperation + Cothers 2194

(6)


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main utilities are primarily obtained from the JLCA database.30 The input raw material RHA of the new process is the waste obtained from the combustion of rice hull, from which bioenergy can be generated and used in replacement of the fossil-based. This helps mitigate a large amount of GHG emissions, approximately 0.953 kg CO2 per kilo hull used.31 Rice hull is produced associated with rice grains and disposed during the rice milling process. It has markedly low economic value. On the basis of the economic allocation method proposed by Kasmaprapruet et al.32, rice hull production shares relatively small fraction (2%) of total GHG emissions of rice production (2.93 kg CO2/kg rice). This amount is much smaller than the amount it can help mitigate. Thus, it can be considered that the preparation of RHA has a negligible contribution to the global warming potential. The market prices of key raw material (RHA) and utilities have strong impact on the economic performance of the new process. Thus, a sensitivity analysis on the changes of these prices is performed to examine the economic competitiveness of the new process for a long-term projection. The production of the input raw material of the conventional process, Simg, requires a markedly large amount of electricity. This electricity supply, which can be generated using both renewable and fossilbased resources, has strong impact on the market price and GHG emissions profile of Simg. Therefore, the impact of the changes of Simg market price and electricity derived resources are also considered for comparing the economic and environmental performances of the conventional process with the new one.

The investment cost (Cinvestment) includes the costs of all the main processing units involved in the synthesis processes, such as reactors, heaters, heat exchangers, and distillation columns. This equipment is assessed using the data on mass and heat balances obtained from the process simulation. Its costs are estimated using Guthrie correlation,25 and adjusted using the Chemical Engineering Plant Index 2016.26 The investment cost is depreciated over a period of 10 years using the straight line method. The operating cost (Coperation) includes the costs of raw materials and utilities input to the alternative processes. This is calculated based on the market prices and the energy and mass balance data. The entire PC also accounts for other types of costs (Cothers), such as maintenance, labor, and property tax costs, the total of which is assumed to be 20% of the total operating cost. Greenhouse Gas (GHG) Emissions. Greenhouse gas emissions (CO2 equivalent) are calculated considering all sources of carbon dioxide, methane, and nitrous oxide. These mainly originate from the production of raw materials and utilities used by the considered processes. Thus, high consumption of raw materials and utilities results in high GHG emissions. CO2 = CO2raw material + CO2utilities

(7)

A cradle-to-gate life-cycle assessment method is applied for assessing GHG emissions, considering both the key raw material production and the target product synthesis stages. Basic parameters used for evaluating economic and environmental performances are displayed in Tables 2 and 3. For the

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RESULTS AND DISCUSSION The considered process flowsheets shown in Figures 1 and 2 are simulated. The obtained data of mass and energy balance are shown in Table 4. On the basis of the basic parameters and data of market prices and GHG emissions shown in Tables 2 and 3, the production cost and GHG emissions of the considered processes are evaluated. As stated above, in an attempt to optimize the reaction conditions of the new process, different experiments were performed under different reaction conditions. As shown in Table 1, alternatives 1, 2, 3, and 4, which are differentiated by the input molar ratio of biogenic silica and ethanol, give different yields of main and byproducts. These alternatives are evaluated and compared with the base case to determine the optimal synthesis conditions of the new process. Production Cost. Figure 3 compares the PCs of the base case and the new process. Figure 4 depicts the contribution of each cost element to the total operation and the investment costs of these processes. The evaluation results clearly show that the raw material’s cost makes the dominant contribution to the total operating cost of both processes. This is commonly found in commodity chemical production processes. Primarily, the raw material’s cost is influenced by the market prices and conversion ratios of the main and byproducts of the feedstocks. This includes the costs of ethanol and key feedstocks, Simg and RHA, used in the base case and the new process, respectively. In Table 1, the base case shows a high conversion to the main product but also a high loss to TRES of Simg. The high price of Simg means that the base case has a markedly high raw material cost. Due to utilizing the cheap raw material RHA, all alternatives of the new process have much lower raw material costs than the base case.

Table 2. Basic Parameters Used for the Evaluation of the PC project lifetime

10 years

working capital labor maintenance supplies property tax depreciation

15% process capital cost 10% operating cost 6% process capital cost 2% process capital cost 3% process capital cost straight line

Table 3. Price and GHG Emissions of Main Utilities and Raw Materials price ($/kg) ethanol silicon metal rice hull ash steam fuel oil cooling water (recycled) a

GHG emissions (kg CO2/kg)

Raw Material 1.6730 0.7a 2.029 depending on electricity sources 0.10a 0.00 Utility 0.02a 0.1730 0.35a 3.4630 0.00 0.00

Average estimated price as of 2016.

base case process, the life-cycle inventory data and average market price (2$/kg as of 2016) of silicon metal are updated for evaluation using the available literature.27−29 RHA is widely available in many Asian countries and can be purchased quite cheaply, on average 0.1$/kg (estimated as of 2016). In both processes, fossil-based ethanol is used, which is produced by the hydration of ethylene obtained from crude oil cracking processes. Life-cycle GHG emissions data for ethanol and its 2195


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Industrial & Engineering Chemistry Research Table 4. Main Input (kg/kg Product) of Base Case and New Process new process raw material/utility

base case

silicon metal rice hull ash ethanol

0.16

steam cooling water fuel oil

3.32 176.50 0.006

alternative 1

alternative 2

alternative 3

alternative 4

Raw material (kg/kg TEOS)

1.09

0.34 0.36 0.93 0.97 Utility (kg/kg TEOS) 13.30 8.95 637.95 429.97 0.19 0.12

0.47 1.19

0.51 1.27

6.86 329.02 0.11

6.47 309.59 0.10

in determining the utility cost of the entire process. Supplied with a much lower input volume of ethanol, the base case has much lower energy consumption for ethanol recovery than all alternatives of the new process. As shown in Table 1, it has higher conversion to byproduct, resulting in higher energy costs for byproduct separation or product purification. In the new process, the operating cost demonstrates a considerable change along with changes in the RHA and ethanol molar ratios employed. In these alternatives, excess ethanol is input to create a preferable environment stimulating the forward and backward reactions of eqs 4 and 5, respectively. The amount of ethanol used for TEOS synthesis follows the stoichiometric rules of these reactions. Thus, the higher the input amount of ethanol, the higher the amount of energy required for its recovery. As shown in Table 1, Alt One has the highest yield and the lowest loss of TEOS due to a high ethanol input flow rate. Thus, Alt One has a lower raw material cost than the other alternatives. However, this reduction is dominated by a marked increase in the ethanol recovery cost. Thus, Alt One has the highest operating cost. Alt Two utilizes a higher input amount of ethanol; thus, its energy costs for ethanol recovery are higher than for Alts Three and Four. Because of the high yield of TEOS and the low loss of raw materials, Alt Two requires lower amounts of RHA and ethanol, resulting in a lower raw material cost. To sum up, Alt Two has the lowest operating cost among the considered alternatives of the new process. The breakdown of the investment cost into three main sections clearly shows the contributions of the product synthesis, ethanol recovery, and product purification sections to the total costs of these processes. Detailed investment costs of the main equipment included in these sections are shown in Table 5. Besides the reactor, the product synthesis section includes the main equipment used for preparing the raw materials such as heater and heat exchanger. The sizes of these pieces of equipment primarily depend on their heat duty, while that of the reactor depends on the flow volume of the raw material and residence time.25 The volume of the unconverted

Figure 3. Comparison of economic performances of the base case and the new process (Alt: alternative molar ratios of RHA and ethanol of the new process; these are described in detail in Table 1).

Figure 4. Contribution of cost elements to the total operation and investment costs (left and right columns indicate the contribution of cost elements to the operation and investment costs, respectively).

The utility cost is mainly spent on product synthesis, ethanol recovery, and product purification. This is the cost of the energy used to heat the input raw material mixture, recover unconverted reactant, and purify the target product to the desired degree. Thus, besides the conversion ratio of key feedstocks to main and byproducts, the flow mass of the material employed in the processes is also an important factor

Table 5. Estimated Capital Costs of Main Equipment of the Base Case and New Process (in Dollars) new process main equipment

base case

alternative 1

alternative 2

alternative 3

alternative 4

heat exchangers heater cooler reactor distillation columns total

73,676 4,586 26,029 89,053 416,216 609,560

197,637 18,557 20,628 828,207 1,138,851 2,203,880

153,136 18,378 15,044 653,902 928,617 1,769,077

129,928 20,004 12,365 562,193 797,170 1,521,660

125,397 20,473 11,863 546,725 769,492 1,473,950

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Industrial & Engineering Chemistry Research reactant and byproduct formed after the reaction, as well as the complexity of separating these substances from the target product, strongly impacts the investment cost of the ethanol recovery and product purification columns. As the result clearly shows, the investment costs of both processes are mainly determined by the ethanol recovery and product synthesis sections. The higher the ethanol input volume, the larger the equipment required for processing. Thus, the base case has a smaller investment cost than all alternatives of the new process. In the new process, the rank of the alternatives is clearly reflected by the ethanol volume used, with Alt One as the highest and Alt Four as the lowest. The total costs of operation and investment and other costs (i.e., labor cost, maintenance cost, and property tax) clearly indicate the high economic competitiveness of the new process compared with the base case. Among the considered alternatives, Alt Two brings the highest returns. Thus, its molar ratios of RHA and ethanol can be proposed as the most suitable conditions for a direct synthesis of TEOS. Analysis of the Economic Competitiveness of the New Process for Long-Term Projection. The economic competitiveness of the best alternative of the new process (Alt Two) is further examined by considering the changes in the market prices of the key feedstocks and utilities. The price of Simg used in the base case is the main factor that determines the economic performance of this process. It is primarily influenced by the balance of supply and demand of this feedstock. Besides, the source of electricity supply also has strong impact on the market price of Simg. Historical databases33 have shown wild fluctuations in the price of Simg, with a decrease from more than 3$/kg in 2014 to 2$/kg in 2016. This drop led to a considerable decrease in the TEOS production cost, which influenced the economic competitiveness of the new process. The key RHA feedstock used in the new process is obtained as the byproduct of a rice-hull-fired power plant. It is available in large quantities in many countries and it can be used as a type of fertilizer or as an additive in the cement and steel industries. Such competitive utilization can influence the market price of RHA. In Asia, RHA can be purchased for 0.05−0.15$/kg. As Figure 4 clearly shows, the entire consumption of utilities (mainly steam and heavy fuel oil used for product synthesis, ethanol recovery, and product purification) contributes significantly to the total PC of the new process (approximately 22%). In the base case, this represents quite a small fraction. Therefore, the economic performance of the new process is expected to be more sensitive to changes in the price of utilities, which are strongly correlated with the price of crude oil determined by oil-exporting countries. Figure 5 shows the influence of changes in key feedstock and utilities prices on the economic performance of the base case and the new process. A minus value indicates a drop in the current prices of feedstocks and utilities supplied to the two processes, and vice versa. The new process can economically substitute for the conventional one, even if the current price of RHA rises by more than 150% or that of Simg drops by less than 30%. The new process becomes uneconomical if the Simg price drops by more than 40%. The new process promises to bring higher returns even if the current price of utilities increases by up to 50%. As this requires a much higher energy supply than the base case, it cannot economically substitute for the base case if the price of utilities rises to 75%.

Figure 5. Impact of the key raw materials and utilities prices on the economic competitiveness of the new process. Solid and dotted lines indicate the impacts of key raw material (Simg for conventional and RHA for new processes) and utilities prices, respectively. Circles and triangles refer to conventional and new processes, respectively.

The above analyses clearly show that the economic performance of the new process is more sensitive to the utility cost than the cost of the RHA raw material. Its performance is expected to be substantially improved by coproduction of TEOS and electricity using a much cheaper raw material, rice hull, which costs on average 0.03$/kg. In this integrated production process, RHA can be directly obtained from rice hull incineration, which produces energy and can be used to generate electricity or steam. The generated electricity can be sold to the national grid as a surplus, while the produced steam can be fed back into the production process. Therefore, the total PC can be remarkably reduced. Greenhouse Gas (GHG) Emissions. As with the PC, GHG emissions are calculated based on mass and heat balances obtained from the simulation of the processes. This indicator intrinsically correlates with the amount of raw materials and utilities used in the processes, which was determined by the conversion ratio of the key feedstocks to the main and byproducts, the mass flow rates of materials, and the effort of purifying the target product from the unconverted reactant and byproducts. As mentioned above, Simg is the main factor that determines both the economic and the environmental performances of the base case. The production of Simg requires a vast supply of electricity, approximately 13 kWh/kg. In Europe (i.e., Norway), hydropower is the main source.27 In other parts of the world where modest quantities of renewable energy are available, fossil fuels are still the main resources. In Asia, a large proportion of Simg production originates from China, which has an electric power industry that is primarily reliant on coal. Therefore, the contribution of the base case to the global warming potential relies heavily on the source of electricity used to produce its key feedstock. In the new process, as a source of biogenic silica, RHA is a byproduct obtained from the burning of rice hull for electricity generation. As stated above, GHG emissions from RHA production can be considered negligible. In this study, the environmental performance of the new process is compared with that of the base case by considering the sources of electricity supplied to the latter. Life-cycle GHG emissions from different power generation systems are obtained from the available literature.34 Life-cycle inventory data on Simg are used to calculate GHG emissions from Simg production using different sources of electricity. 2197


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investigate suitable reaction conditions for optimizing this conversion.

Figure 6 compares the GHG emissions of the base case and the new process. Clearly, the raw material consumption makes

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Thuy T. H. Nguyen: 0000-0001-7194-6627 Jun-Chul Choi: 0000-0002-7049-5032 Sho Kataoka: 0000-0003-3954-7292 Notes

The authors declare no competing financial interest.

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Figure 6. Comparison of GHG emissions. CP: coal power; FP: fuel oil power; GP: liquefied natural gas power; NP: nuclear power, HP: hydropower; WP: wind power; SP: solar-photovoltaic power.

ACKNOWLEDGMENTS We express special thanks to New Energy and Industrial Technology Development Organization (NEDO) for providing financial support.

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a dominant contribution to the total CO2 emission of the base case. The new process (except Alt One) can substantially reduce the GHG emissions by 34%, 26%, and 21% when electricity derived from coal, fuel oil, and liquefied natural gas is used to produce Simg, respectively. If these resources are replaced by renewable energy such as hydropower, wind power, and photovoltaic power, the base case contributes slightly less to the global warming potential than the new process. In the new process, utility consumption is the major contributor in Alts One and Two, in which ethanol is used in large volumes. In Alts Three and Four, the low ethanol input volumes result in lower energy requirements but high raw material consumption due to the high yield of byproduct. For these alternatives, raw material (mainly ethanol) consumption represents a major fraction of the total CO2 emission. Among these alternatives, Alt Three produces the lowest GHG emissions.

REFERENCES

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CONCLUSION In this study, a direct synthesis of TEOS from biogenic silica contained in RHA was designed and evaluated considering both economic and environmental indicators. Experiments were performed to examine the impact of the molar ratios of the key raw material and ethanol on the yields of the main and byproducts. A large input volume of ethanol creates a favorable environment for stimulating the target reactions but results in high energy and high equipment costs. A small input volume of ethanol reduces these costs but increases the loss of raw materials due to the formation of large amounts of byproduct. By assessing the PCs and GHG emissions, the most sustainable conditions for TEOS synthesis from biogenic silica could be determined. Under current market conditions, the new process at the optimal conditions of Alt Two attains higher economic returns and much lower contribution to global warming potential. Approximately 7% production cost and 34% GHG emissions can be reduced by substituting the new process for the conventional one. The economic and environmental performances of the new process can be further increased upon an effective use of byproduct, e.g., converting DS to the target product TEOS following the backward reaction of eq 5. This can substantially reduce raw material loss, and thus entire production cost. Further experiments should be performed to 2198


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