Technoeconomic and Environmental Assessment for Design and

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Technoeconomic and Environmental Assessment for Design and Optimization of Tetraethyl Orthosilicate Synthesis Process Thuy Thi Hong Nguyen, Norihisa Fukaya, Kazuhiko Sato, Jun-Chul Choi, and Sho Kataoka Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03895 • Publication Date (Web): 19 Jan 2018 Downloaded from http://pubs.acs.org on January 19, 2018

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Industrial & Engineering Chemistry Research

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Technoeconomic and Environmental Assessment for Design and Optimization of Tetraethyl

2

Orthosilicate Synthesis Process

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Thuy T. H. Nguyena*, Norihisa Fukayab, Kazuhiko Satob, Jun-Chul Choib, Sho Kataokaa*

4

a

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Catalytic Chemistry, National Institute of Advanced Industrial Science and Technology, 1-1-1

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Higashi, Tsukuba, Ibaraki 305-8565, Japan

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[email protected], [email protected]

Research Institute for Chemical Process Technology and bInterdisciplinary Research Center for

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Abstract

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Biogenic silica has been considered as a potential feedstock for producing a variety of

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silicon-containing products. A new synthesis pathway of tetraethyl orthosilicate from silica that is

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contained in rice hull ash has been recently proposed. Requiring only one-step processing, it is

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expected to offer advantages over conventional routes, which are derived from silicon metal. This

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study aims to investigate the optimal synthesis conditions for which this new technology can

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sustainably replace the conventional method. Experiments employing different reaction conditions

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are performed to provide the necessary information for a conceptual process design and an economic

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and environmental viability evaluation. The results obtained by comparison with a selected

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conventional process show that, under the optimal synthesis conditions, the new process can

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decrease the production cost and markedly reduce the high greenhouse gas emissions. The 1

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competitiveness of the new process is further examined with a sensitivity analysis considering

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fluctuations in key feedstock and utility prices and alternative sources of electricity supplied to the

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conventional process. The new synthesis technology shows its high potential as a sustainable

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alternative to the conventional one.

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Keywords: ethyl orthosilicate, biogenic silica, rice hull ash, optimal synthesis condition, conceptual

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process design, economic and environmental viability, competitiveness.

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Introduction

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Tetraethyl orthosilicate or tetraethoxysilane (TEOS) has wide applications as an insulator in the

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electronic industry, as a crosslinking agent and binder in coating and painting industries, and as an

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intermediate substance in the production of diverse catalysts and products in the chemical industry.

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Conventionally, TEOS is produced via two routes: the reaction of tetrachlorosilane with ethanol and

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that of metallurgical silicon (Simg) with ethanol. Both routes utilize the same raw material, Simg. In

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the former route, tetrachlorosilane is first produced from Simg, and then it requires extensive

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energy-consuming purification steps before being used in TEOS synthesis.

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hydrogen chloride causes many problems related to equipment corrosion, occupational health risk

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exposure, and environmental impact. Since the latter route involves only one synthesis step, it is

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much simpler and greener than the former.3-6 In these two conventional routes, the input feedstock

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Simg is the main factor that determines the economic and environmental performance of the entire 2


1,2

The release of

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production process. In practice, this is primarily produced via carbon-thermal reduction of

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silica-containing sources, which requires a large amount of electricity (approximately up to 13,000

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kWh/ton product7,8), thus producing excessive greenhouse gas (GHG) emissions.

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Rice hull ash (RHA) contains a large fraction of silica (more than 95 wt.%). The direct combustion,

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gasification or pyrolysis of rice hull, which is widely available at considerably large amounts9-10,

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results in RHA. It can be obtained from Utilizing RHA for producing silicon-based materials has

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attracted increasing attention.11-16 Recently, our group synthesized TEOS by directly reacting RHA

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with ethanol.17-18 This study revealed that the high-energy-consuming step of the thermal reduction

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of silica to silicon can be eliminated. With this substantial improvement, it is expected that TEOS can

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be widely produced without requiring large quantities of electricity. Experiments have shown the

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strong impact of the ethanol volume on the product yield. The use of excess ethanol promotes a high

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yield of TEOS but requires a large amount of energy for ethanol recovery. Therefore, to promote a

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greener TEOS synthesis route, the production process must be well designed and rigorously

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evaluated.

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This study focuses on the conceptual design and evaluation of the impact of synthesis conditions,

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with respect to the molar ratio of raw materials and the economic and environmental viability of this

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new synthesis route. Experiments are performed under different synthesis conditions, based on which

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a new synthesis process is designed. Through an in-depth assessment and comparison with a selected

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conventional synthesis process, considering both the production cost and the GHG emissions, 3



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optimal synthesis conditions are proposed to maximize the benefits. The competitiveness of the new

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process is further examined, considering fluctuations in feedstock and utility prices and different

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power generation systems. These analyses can provide a good vision for the long-term

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implementation of new TEOS synthesis technology.

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Process Design and Simulation Models of TEOS Synthesis Routes

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The process configurations of the new and conventional synthesis routes are designed and modeled

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using process simulator Pro/II. While the new route uses biogenic silica included in RHA, the

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conventional one uses metallurgical silicon as the main input feedstock. These processes consist of

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the main processing units used for product synthesis, by-product separation, unreacted reactant

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recovery, and product purification. Data of TEOS properties and vapor-liquid equilibrium of

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TEOS19-23 containing output mixtures are collected and input to Pro/II library data, on the basis of

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which parameters necessary for process simulation are calculated using the selected thermodynamic

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model (NRTL). Heuristic knowledge is applied to optimize the considered processes. The parameters

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such as difference in temperature of the hot and cold fluids of the heat exchangers and the number of

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stages, reflux ratio and feed position of the distillation columns are rigorously considered for

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minimizing the energy and capital costs of the main processing units.

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These processes are designed to produce TEOS with a purity of 99.5 wt.% and capacity of 1,000

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tons/year. They are described in detail in the following sections, and their main reactions are

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summarized in Table 1. 4


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Base Case: Metallurgical Silicon Utilizing Process

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As mentioned above, TEOS is synthesized via two pathways: direct and indirect reactions of Simg

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(via tetrachlorosilane synthesis) with ethanol. The former route is free from hazardous waste and

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employs only one stage reaction. In the latter route, the purification of TEOS requires extensive

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energy consumption and releases hydrogen chloride, which make this process more costly and

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environmentally unfriendly. 3-6As the former method is simpler, greener, and less energy-consuming,

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it was selected for comparison with the new synthesis route.

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In this conventional process, silica-containing resources such as quartz must be reduced to Simg

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following Eq. 1. This reduction is performed in an electric furnace at about 2,173 K using a reducing

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agent containing carbon, such as coke, charcoal, or wood chips. A detailed description of Simg

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production is beyond the scope of this paper. More information can be found in the available

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literature.7,8 The Simg produced is then used for TEOS synthesis.

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Figure 1 shows the synthesis process of TEOS from Simg. This was designed using information

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obtained from the available literature.3 Simg obtained from silica reduction is heated at a high

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temperature (773–973 K) with cupric oxide in a calcinator (a). Stream 1 containing the activated Simg

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is then mixed with excess ethanol in a mixer (b). The mixed stream 2 is directed to a heat exchanger

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(c), where it receives heat transferred from the stream 5 output from reactor (e). Output stream 3 is

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further heated to the desired temperature in heater (d). The resulting stream 4 is directed to a TEOS

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synthesis reactor (e) operated at an elevated temperature (443–463 K) and autogenous pressure. The 5



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product stream 5 output from the reactor is a mixture of TEOS and the by-products triethoxysilane

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(TRES) and hydrogen produced via Eqs. 2 and 3. Through heat exchangers (c) and (f), this transfers

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heat to streams 2 and 9. It is further cooled after passing through cooler (g). Output stream 8 is then

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directed to gas separator (h) for hydrogen removal. The output from heat exchanger (f), stream 10,

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had an elevated temperature. This is finally directed to a separation system, which consists of

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unconverted ethanol recovery and product purification distillation columns. Unconverted ethanol

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obtained from the top of column (i) is fed back into the process. This is mixed with the fresh input

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raw materials for starting a new production. In column (j), TRES is obtained at the top and can be

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used as an intermediate chemical for manufacturing other products. TEOS at the desired purity is

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obtained at the bottom of the column.

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SiO2 + C → Si + CO2

(1)

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Si + 4C2H5OH → Si(OC2H5)4 + 2H2

(2)

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Si + 3C2H5OH → HSi(OC2H5)3 + H2

(3)

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New Process: Biogenic Silica (RHA) Utilizing Process

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Experimental procedures reported in our previous studies17,18 are mimicked to obtain the reaction

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information used for process design and simulation. Different reaction conditions (molar ratios of

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input raw materials) are employed to investigate their impact on the process performance. The initial

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feedstock used in this process is RHA, which is obtained from the incineration of rice hull at about

6


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773 K18, the energy obtained from this process can be used for the generation of electricity.

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Figure 2 shows the synthesis process of TEOS using RHA. In mixer (a), RHA is mixed with

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excess anhydrous ethanol. Output stream 1 is then passed through heat exchanger (b), where its

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temperature is increased by the heat transferred from the stream 4 output from reactor (d). In heater

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(c), the temperature of stream 2 is further increased. The resulting stream 3 is then directed to reactor

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(d). In this reactor, TEOS and by-products are synthesized following Eqs. 4 and 5 in the presence of a

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suitable amount of potassium hydroxide at 513 K.

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SiO2 + 4C2H5OH ↔ Si(OC2H5)4 + 2H2O

(4)

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2Si(OC2H5)4 + H2O ↔ Si2O(OC2H5)6 + 2C2H5OH

(5)

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As the reactions involved in this process are reversible, a high input volume of ethanol and

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continuous removal of water are required to attain a high product yield. Experiments with different

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ratios of biogenic silica to ethanol were performed to examine the impact of this ratio on the product

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yield. Table 1 shows alternative ethanol input volumes, which result in considerable differences in

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the yields of TEOS and the main by-product.

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To remove water, molecular sieve (MS) 3A is used.18 As this cannot absorb water effectively at high

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temperature, the stream 4 output from the reactor is cooled to 333 K after passing through heat

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exchangers (b) and (e) and cooler (f). Output stream 7 is then passed through absorber (g), which is

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packed with MS 3A for water removal. Stream 8, which is free of water, is warmed after passing

7



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through heat exchanger (e). Output stream 9 is then directed to a separation system, in which

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unconverted ethanol is recovered as the major product of column (h) and fed back into the process.

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In column (i), TEOS is obtained as the major product.

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Evaluation Indicators

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The above TEOS synthesis processes are evaluated considering the following economic and

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environmental indicators.

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Production Cost

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The PC is calculated as [24]. It is the sum of the following costs: PC = + + .

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(6)

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The investment cost (Cinvestment) includes the costs of all the main processing units involved in the

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synthesis processes, such as reactors, heaters, heat exchangers, and distillation columns. This

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equipment is assessed using the data on mass and heat balances obtained from the process simulation.

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Its costs are estimated using Guthrie correlation25, and adjusted using the Chemical Engineering

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Plant Index 2016.26 The investment cost is depreciated over a period of 10 years using the straight

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line method.

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The operating cost (Coperation) includes the costs of raw materials and utilities input to the alternative

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processes. This is calculated based on the market prices and the energy and mass balance data.

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The entire PC also accounts for other types of costs (Cothers), such as maintenance, and labor and

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property tax costs, the total of which is assumed to be 20% of the total operating cost. 8


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Greenhouse Gas (GHG) Emissions

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Greenhouse gas emissions (CO2 equivalent) are calculated considering all sources of carbon dioxide,

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methane, and nitrous oxide. These mainly originate from the production of raw materials and utilities

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used by the considered processes. Thus, high consumption of raw materials and utilities results in

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high GHG emissions.

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CO = CO + CO .

(7)

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A cradle-to-gate life-cycle assessment method is applied for assessing GHG emissions, considering

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both the key raw material production and the target product synthesis stages.

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Basic parameters used for evaluating economic and environmental performances are displayed in

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Tables 2 and 3. For the base case process, the life-cycle inventory data and average market price

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(2$/kg as of 2016) of silicon metal are updated for evaluation using the available literature.27-29 It is

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widely available in many Asian countries and can be purchased quite cheaply, on average 0.1$/kg

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(estimated as of 2016). In both processes, fossil-based ethanol is used, which is produced by the

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hydration of ethylene obtained from crude oil cracking processes. Life-cycle GHG emissions data for

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ethanol and its main utilities are primarily obtained from the JLCA database.30 The input raw

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material RHA of the new process is the waste obtained from the combustion of rice hull, from which

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bioenergy can be generated an used in replacement of the fossil-based. This helps mitigate a large

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amount of GHG emissions, approximately 0.953 kg CO2 per kilo hull used.31 Rice hull is produced

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associated with rice grains and disposed during the rice milling process. It has markedly low 9



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economic value. Based on the economic allocation method proposed by Kasmaprapruet et al.(32),

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rice hull production shares relatively small fraction (2%) of total GHG emissions of rice production

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(2.93 kg CO2/kg rice). This amount is much smaller than the amount it can help mitigate. Thus, it can

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be considered that the preparation of RHA has a negligible contribution to the global warming

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potential.

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The market prices of key raw material (RHA) and utilities have strong impact on the economic

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performance of the new process. Thus, a sensitivity analysis on the changes of these prices is

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performed to examine the economic competitiveness of the new process for a long-term of projection.

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The production of the input raw material of the conventional process, Simg, requires a markedly large

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amount of electricity. This electricity supply, which can be generated using both renewable and

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fossil-based resources, has strong impact on the market price and GHG emissions profile of Simg.

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Therefore, the impact of the changes of Simg market price and electricity derived resources are also

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considered for comparing the economic and environmental performances of the conventional process

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with the new one.

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Results and Discussion

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The considered process flowsheets shown in Figures 1 and 2 are simulated. The obtained data of

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mass and energy balance are shown in Table 4. Based on the basic parameters and data of market

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prices and GHG emissions shown in Tables 2 and 3, the production cost and GHG emissions of the

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considered processes are evaluated. As stated above, in an attempt to optimize the reaction conditions 10


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of the new process, different experiments were performed under different reaction conditions. As

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shown in Table 1, alternatives (Alts.) 1, 2, 3, and 4, which are differentiated by the input molar ratio

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of biogenic silica and ethanol, give different yields of main and by-products. These alternatives are

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evaluated and compared with the base case to determine the optimal synthesis conditions of the new

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process.

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Production Cost

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Figure 3 compares the PCs of the base case and the new process. Figure 4 depicts the contribution of

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each cost element to the total operation and the investment costs of these processes.

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The evaluation results clearly show that the raw material’s cost makes the dominant contribution to

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the total operating cost of both processes. This is commonly found in commodity chemical

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production processes. Primarily, the raw material’s cost is influenced by the market prices and

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conversion ratios of the main and by-products of the feedstocks. This includes the costs of ethanol

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and key feedstocks, Simg and RHA, used in the base case and the new process, respectively. In Table

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1, the base case shows a high conversion to the main product but also a high loss to TRES of Simg.

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The high price of Simg means that the base case has a markedly high raw material cost. Due to

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utilizing the cheap raw material RHA, all alternatives of the new process have much lower raw

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material costs than the base case.

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The utility cost is mainly spent on product synthesis, ethanol recovery, and product purification. This

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is the cost of the energy used to heat the input raw material mixture, recover unconverted reactant, 11



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and purify the target product to the desired degree. Thus, besides the conversion ratio of key

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feedstocks to main and by-products, the flow mass of the material employed in the processes is also

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an important factor in determining the utility cost of the entire process. Supplied with a much lower

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input volume of ethanol, the base case has much lower energy consumption for ethanol recovery than

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all alternatives of the new process. As shown in Table 1, it has higher conversion to by-product,

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resulting in higher energy costs for by-product separation or product purification.

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In the new process, the operating cost demonstrates a considerable change along with changes in the

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RHA and ethanol molar ratios employed. In these alternatives, excess ethanol is input to create a

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preferable environment stimulating the forward and backward reactions of Eqs. 4 and 5, respectively.

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The amount of ethanol used for TEOS synthesis follows the stoichiometric rules of these reactions.

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Thus, the higher the input amount of ethanol, the higher the amount of energy required for its

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recovery. As shown in Table 1, Alt. 1 has the highest yield and the lowest loss of TEOS due to a high

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ethanol input flow rate. Thus, Alt. 1 has a lower raw material cost than the other alternatives.

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However, this reduction is dominated by a marked increase in the ethanol recovery cost. Thus, Alt. 1

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has the highest operating cost. Alt. 2 utilizes a higher input amount of ethanol; thus, its energy costs

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for ethanol recovery are higher than for Alts. 3 and 4. Due to the high yield of TEOS and the low loss

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of raw materials, Alt. 2 requires lower amounts of RHA and ethanol, resulting in a lower raw

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material cost. To sum up, Alt. 2 has the lowest operating cost among the considered alternatives of

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the new process. 12


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The breakdown of the investment cost into three main sections clearly shows the contributions of the

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product synthesis, ethanol recovery, and product purification sections to the total costs of these

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processes. Detail investment cost of the main equipment included in these sections are shown in

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Table 5. Besides the reactor, the product synthesis section includes the main equipment used for

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preparing the raw materials such as heater and heat exchanger. The sizes of these pieces of

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equipment primarily depend on their heat duty, while that of the reactor depends on the flow volume

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of the raw material and residence time.25 The volume of the unconverted reactant and by-product

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formed after the reaction, as well as the complexity of separating these substances from the target

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product, strongly impacts the investment cost of the ethanol recovery and product purification

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columns. As the result clearly shows, the investment costs of both processes are mainly determined

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by the ethanol recovery and product synthesis sections. The higher the ethanol input volume, the

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larger the equipment required for processing. Thus, the base case has a smaller investment cost than

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all alternatives of the new process. In the new process, the rank of the alternatives is clearly reflected

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by the ethanol volume used, with Alt. 1 as the highest and Alt. 4 as the lowest.

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The total costs of operation and investment and other costs (i.e., labor cost, maintenance cost, and

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property tax) clearly indicate the high economic competitiveness of the new process compared with

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the base case. Among the considered alternatives, Alt. 2 brings the highest returns. Thus, its molar

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ratios of RHA and ethanol can be proposed as the most suitable conditions for a direct synthesis of

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TEOS. 13



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Analysis of the Economic Competitiveness of the New Process for Long-Term Projection

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The economic competitiveness of the best alternative of the new process (Alt. 2) is further examined

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by considering the changes in the market prices of the key feedstocks and utilities.

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The price of Simg used in the base case is the main factor that determines the economic performance

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of this process. It is primarily influenced by the balance of supply and demand of this feedstock.

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Besides, the sources of electricity supply also has strong impact on the market price of Simg.

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Historical databases33 have shown wild fluctuations in the price of Simg, with a decrease from more

253

than 3$/kg in 2014 to 2$/kg in 2016. This drop led to a considerable decrease in the TEOS

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production cost, which influenced the economic competitiveness of the new process. The key RHA

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feedstock used in the new process is obtained as the by-product of a rice-hull-fired power plant. It is

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available in large quantities in many countries and it can be used as a type of fertilizer or as an

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additive in the cement and steel industries. Such competitive utilization can influence the market

258

price of RHA. In Asia, RHA can be purchased for 0.05–0.15$/kg.

259

As Figure 4 clearly shows, the entire consumption of utilities (mainly steam and heavy fuel oil used

260

for product synthesis, ethanol recovery, and product purification) contributes significantly to the total

261

PC of the new process (approximately 22%). In the base case, this represents quite a small fraction.

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Therefore, the economic performance of the new process is expected to be more sensitive to changes

263

in the price of utilities, which are strongly correlated with the price of crude oil determined by

264

oil-exporting countries. 14


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Figure 5 shows the influence of changes in key feedstock and utilities prices on the economic

266

performance of the base case and the new process. A minus value indicates a drop in the current

267

prices of feedstocks supplied to the two processes, and vice versa. The new process can economically

268

substitute for the conventional one, even if the current price of RHA rises by more than 150% or that

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of Simg drops by less than 30%. The new process becomes uneconomical if the Simg price drops by

270

more than 40%. The new process promises to bring higher returns even if the current price of utilities

271

increases by up to 50%. As this requires a much higher energy supply than the base case, it cannot

272

economically substitute for the base case if the price of utilities rises to 75%.

273

The above analyses clearly show that the economic performance of the new process is more sensitive

274

to the utility cost than the cost of the RHA raw material. Its performance is expected to be

275

substantially improved by co-production of TEOS and electricity using a much cheaper raw material,

276

rice hull, which costs on average 0.03$/kg. In this integrated production process, RHA can be

277

directly obtained from rice hull incineration, which produces energy and can be used to generate

278

electricity or steam. The generated electricity can be sold to the national grid as a surplus, while the

279

produced steam can be fed back into the production process. Therefore, the total PC can be

280

remarkably reduced.

281

Greenhouse Gas (GHG) Emissions

282

As with the PC, GHG emissions are calculated based on mass and heat balances obtained from the

283

simulation of the processes. This indicator intrinsically correlated with the amount of raw materials 15



284

and utilities used in the processes, which was determined by the conversion ratio of the key

285

feedstocks to the main and by-products, the mass flow rates of materials, and the effort of purifying

286

the target product from the unconverted reactant and by-products.

287

As mentioned above, Simg is the main factor that determines both the economic and the

288

environmental performances of the base case. The production of Simg requires a vast supply of

289

electricity, approximately 13 kWh/kg. In Europe (i.e., Norway), hydropower is the main source27. In

290

other parts of the world where modest quantities of renewable energy are available, fossil fuels are

291

still the main resources. In Asia, a large proportion of Simg production originates from China, which

292

has an electric power industry that is primarily reliant on coal. Therefore, the contribution of the base

293

case to the global warming potential relies heavily on the source of electricity used to produce its key

294

feedstock. In the new process, as a source of biogenic silica, RHA is a by-product obtained from the

295

burning of rice hull for electricity generation. As stated above, GHG emissions from RHA production

296

can be considered negligible.

297

In this study, the environmental performance of the new process is compared with that of the base

298

case by considering the sources of electricity supplied to the latter. Life-cycle GHG emissions from

299

different power generation systems are obtained from the available literature34. Life-cycle inventory

300

data on Simg are used to calculate GHG emissions from Simg production using different sources of

301

electricity.

302

Figure 6 compares the GHG emissions of the base case and the new process. Clearly, the raw 16


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material consumption makes a dominant contribution to the total CO2 emission of the base case. The

304

new process (except Alt. 1) can substantially reduce the GHG emissions by 34%, 26%, and 21%

305

when electricity derived from coal, fuel oil, and liquefied natural gas is used to produce Simg,

306

respectively. If these resources are replaced by renewable energy such as hydropower, wind power,

307

and photovoltaic power, the base case contributes slightly less to the global warming potential than

308

the new process.

309

In the new process, utility consumption is the major contributor in Alts. 1 and 2, in which ethanol

310

is used in large volumes. In Alts. 3 and 4, the low ethanol input volumes result in lower energy

311

requirements but high raw material consumption due to the high yield of by-product. For these

312

alternatives, raw material (mainly ethanol) consumption represents a major fraction of the total CO2

313

emission. Among these alternatives, Alt. 3 produces the lowest GHG emissions.

314

Conclusion

315

In this study, a direct synthesis of TEOS from biogenic silica contained in RHA was designed and

316

evaluated considering both economic and environmental indicators. Experiments were performed to

317

examine the impact of the molar ratios of the key raw material and ethanol on the yields of the main

318

and by-products. A large input volume of ethanol creates a favorable environment for stimulating the

319

target reactions but results in high energy as well as high equipment costs. A small input volume of

320

ethanol reduces these costs but increases the loss of raw materials due to the formation of large

321

amounts of by-product. By assessing the PCs and GHG emissions, the most sustainable conditions 17



322

for TEOS synthesis from biogenic silica could be determined.

323

Under current market conditions, the new process at the optimal conditions as of Alt. 2 attains

324

higher economic returns and much lower contribution to global warming potential. Approximately,

325

7% production cost and 34% GHG emissions can be reduced by substituting the new process for the

326

conventional one. The economic and environmental performances of the new process can be further

327

increased upon an effective use of by-product, e.g., converting DS to the target product TEOS

328

following the backward reaction of Eq. 5. This can substantially reduce raw material loss, and thus

329

entire production cost. Further experiments should be performed to investigate suitable reaction

330

conditions for optimizing this conversion.

331 332

Acknowledgements

333

The authors express special thanks to New Energy and Industrial Technology Development

334

Organization (NEDO) for providing financial support.

335

18


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336

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409

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410


Table 1: Reaction conditions included in the base case and new synthesis routes Base case6

Alternative 1

Alternative 2

Alternative 3

Alternative 4

453

513

513

513

513

Key input component Molar ratio key component:ethanol

Silicon metal 1:23

Biogenic silica 1:112




Main by-product Conversion of key component (%) Yield of TEOS (%)

TRES 88.0

DS 96.9

DS 77.2

DS 82.0

DS 70.7

74.0

91.3

68.0

56.2

44.8

14.0

2.8

4.6

12.9

12.9

Process ID Temperature [K]

(key component-based) Yield of by-product (%) (key component-based) 411 412

23



413

Table 2: Basic parameters used for the evaluation of the PC Project life time

10 years

Working capital Labor

15% process capital cost 10% operating cost

Maintenance Supplies Property tax

6% process capital cost 2% process capital cost 3% process capital cost

Depreciation

Straight line

414 415

Table 3: Price and GHG emissions of main utilities and raw materials Price [$/kg]

GHG emissions [kg CO2/kg]

Ethanol

0.7*

1.6730

Silicon metal

2.0 29

Rice hull ash

0.10*

Depending on electricity sources 0.00

Steam

0.02*

0.1730

Fuel oil Cooling water (recycled)

0.35* 0.00

3.4630 0.00

Raw material

Utility

416

*Average estimated price as of 2016

417 418

24


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419


Table 4: Main input (kg/kg product) of base case and new process Raw material/ utility

Base case

New process Alternative 1

Alternative 2

Alternative 3

Alternative 4

Raw material (kg/kg TEOS) Silicon metal

0.16

-

-

-

-

Rice hull ash Ethanol

1.09

0.34 0.93

0.36 0.97

0.47 1.19

0.51 1.27

3.32

13.30

8.95

6.86

6.47

176.50 0.006

637.95 0.19

429.97 0.12

329.02 0.11

309.59 0.10

Utility (kg/kg TEOS) Steam Cooling water Fuel oil 420 421 422

Table 5: Estimated capital costs of main equipment of the base case and new process [$] Main equipment

Base case

New process Alternative 1

Alternative 2

Alternative 3

Alternative 4

Heat exchangers

73,676

197,637

153,136

129,928

125,397

Heater Cooler Reactor Distillation columns

4,586 26,029 89,053 416,216

18,557 20,628 828,207 1,138,851

18,378 15,044 653,902 928,617

20,004 12,365 562,193 797,170

20,473 11,863 546,725 769,492

Total

609,560

2,203,880

1,769,077

1,521,660

1,473,950

423 424

25



425

426

428

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)

429

TEOS purification column. ROH: anhydrous ethanol; RHA: rice hull ash; TEOS: ethyl orthosilicate;

430

TRES: triethoxysilane.

427

431

26


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432 433


434

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

435

column. ROH: anhydrous ethanol; RHA: rice hull ash; TEOS: ethyl orthosilicate; DS:

436

hexaethyldisiloxane.

437

27



438 439

Figure 3: Comparison of economic performances of the base case and the new process (Alt.:

440

alternative molar ratios of RHA and ethanol of the new process; these are described in detail in Table

441

1).

442

28


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443 444

Figure 4: Contribution of cost elements to the total operation and investment costs (left and

445

right columns indicate the contribution of cost elements to the operation and investment costs,

446

respectively).

447

29



448 449

Figure 5: Impact of the key raw materials and utilities prices on the economic competitiveness

450 451

of the new process. Solid and dotted lines indicate the impacts of key raw material (Simg for conventional and RHA for

452

new processes) and utilities prices, respectively. Circle and triangle marks refer to conventional and

453

new processes, respectively.

454 455

30


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456

457 458

Figure 6: Comparison of GHG emissions.

459

CP: coal power; FP: fuel oil power; GP: liquefied natural gas power; NP: nuclear power, HP:

460

hydropower; WP: wind power; SP: solar-photovoltaic power.

31


Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41

Lab experiment SiO2 Silica in RHA

+ C2H5OH

Design and optimization

Si(OCH2CH3)4 Tetraethyl orthosilicate

Process assessment New process


Page 32 of 32

Technoeconomic and Environmental Assessment for Design and

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