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Subject: Preliminary Production Pathway of Silicon Precursor, TCS, for Solar PV Manufacturing

Cover Letter

As the solar photovoltaic (PV) cell industry grows exponentially, projected demands for silicon precursors indicate the economic opportunity for a silane production plant. Silanes are the chemical class composed of silicon precursors including silane, disilane, and trichlorosilane. Each precursor was investigated to determine the economic feasibility of producing a plant with a production capacity of 20,000 metric tons, taking into account important factors including safety, cost, and sustainability. The process design team has designed a preliminary process pathway to produce the silicon precursor, trichlorosilane, for solar PV manufacturing. Trichlorosilane was chosen as it is the least hazardous to produce, has the largest market, and requires the least processing. It also allows future flexibility to produce different precursors if that becomes economically viable. The proposed process pathway uses a two-stage reactor to react metallurgical grade silicon with hydrochloric acid to produce trichlorosilane and byproducts including silicon tetrachloride, hydrogen gas, and dichlorosilane. A fluidized bed reactor will also be considered for economic comparison. The process allows for a number of byproducts to be recycled back into the process, which reduces the raw material and energy costs. Major equipment used in the production process includes a refrigeration separation unit, two distillation columns, a chemical purification section, and a SiCl4 hydrogenation recycle stage. These steps were found to be universal in nearly all patents reviewed, however, a number of patents referenced were written before 2000, indicating the potential for additional process optimization to further increase production efficiency. The raw materials cost to produce trichlorosilane is $0.108/kg TCS (3 HCl:1 MG Si:1 TCS), and the produced trichlorosilane can be sold for $2.46/kg. Based on an economic analysis of the raw materials and sales price of the silicon precursors, TCS was determined to be the most profitable. The utility costs will be further investigated as preliminary calculations are done to determine the manufacturing costs, which will be used to determine the profitability of the design. The attached document contains the full details of the design team’s proposal. Background

As the global demand for energy grows exponentially and the availability of fossil fuels declines, the use of renewable energy is becoming increasingly important. Solar photovoltaic (PV) cells

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are an attractive alternative to conventional energy production via fossil fuels due to the clean, zero emissions process and the abundance of free solar energy. Additionally, solar panels are highly versatile and take up considerably less space than other renewable sources such as wind turbines and hydroelectric dams. The cost of solar PV cell installation has declined by 70% since 2010, and has had an annual growth rate of 60% in the last ten years.1 Due to the projected growth of the PV industry, the design team has been tasked with the preliminary plant design of a silicon precursor based on factors including safety, cost, and sustainability. Silane (SiH4), or the chemical class known as silanes, are precursors to silicon used in semiconductors, such as those found in solar photovoltaic cells. Although PV cells offer a pathway towards clean, sustainable electricity, the silicon production process is far from being environmentally benign. The precursor and production process chosen for our design must consider environmental and economic factors; therefore, a literature review was conducted investigating various silanes and production methods. The specified production capacity of the plant is 20,000 metric tons of silicon precursor per year. Silanes commonly used as precursors in semiconductors include silane, disilane, and trichlorosilane, and were the only compounds included in the scope of our research. From a chemical reactions standpoint, the production of TCS is favorable over silane as the production of silane (Eqn. 2) requires the production of TCS initially (Eqn. 1); resulting in an unnecessary step and an increase in material and equipment costs. The production of silane also produces silicon tetrachloride (SiCl4) as a byproduct, which is a highly toxic and flammable gas that cannot be easily recycled in traditional silane production processes or disposed of.

Si +3HCl → HSiCl3 + H2 (1) 4HSiCl3 → SiH4 + 3SiCl4 (2)

The production of disilane as the silicon precursor was ruled out for similar reasons. The traditional method for producing disilane requires the hydrolysis of magnesium silicide, however our process begins will metallurgical grade silicon, which requires an additional reaction to produce magnesium silicide (Eqn. 3).

2 Mg + Si→ Mg2Si (3)

Another method to producing disilane results from the pyrolysis of monosilane;2 however, this would add another step to the production process and result in a smaller yield of the desired product, whereas yields of 80% can be achieved with TCS. Various chemical compounds are produced as byproducts in TCS production, including hydrogen gas (H2), dichlorosilane (H2SiCl2), silane (SiH4), and silicon tetrachloride (SiCl4). Most

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compounds are recycled back into the process, which makes this process more economically viable and environmentally-friendly (Figure 1). Proposed Process Pathway

The production of trichlorosilane (TCS) from metallurgical grade silicon (MG Si) is a well documented process with multiple existing patents available. Upon analysis of patents and production pathways found during the literature review, the preliminary process seen in Figure 1

is to be used for use in this project.3 Figure 1: Preliminary block flow diagram of production pathway for trichlorosilane (TCS) from metallurgical grade silicon (MG Si) and gaseous hydrogen chloride (HCl). This process shown above includes major components that were universally found in existing patents considered in the literature review. The engineering team will take into consideration that the process patents found date before early 2000s, and process optimizations may be available for increased production efficiency. The process takes metallurgical silicon and reacts it with hydrochloric acid (HCl) to form trichlorosilane (TCS), dichlorosilane (DCS), silicon tetrachloride, hydrogen, and trace metallic solid contaminants. In consideration of updating the production process to be more efficient, the engineering team will investigate a fluidized bed reactor (FBR) along with the traditional Siemens (deposition) reactor, as the FBR increases efficiency with the presence of silicon quartz sand to aid in the equilibrium process.4 However, the depletion of silicon molecules and presence

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of waste metal compounds requires further purification of the quartz sand, which may cause the design to be unviable due to increased equipment and energy costs. Solid formed during reaction will be separated before distillation to maintain mechanical integrity of the columns and prevent build-up at the bottom. As hydrogen is the most volatile, it will be removed from the gas stream first and recycled back into the process for silicon tetrachloride hydrogenation. Further distillation and chemical purification will achieve marketable product purity with the removal of silicon tetrachloride and DCS. The silicon tetrachloride will be recycled and combined with the recycle hydrogen stream plus an additional feed hydrogen stream to ensure proper hydrogenation of silicon tetrachloride before it is introduced back into the reactor. The excess chlorine removed from the molecule during hydrogenation will be sent to a waste treatment facility if no other marketable solutions are deemed profitable. DCS formed as a byproduct from the disproportionation of TCS, via the reaction between silicon and hydrochloric acid, is to be disposed of upon removal during distillation. No additional reactions were identified that could recycle dichlorosilane back into the process. DCS can be used for semiconductor processing when reacted in vapor deposition chambers with ammonia, however, the compound must be ultra purified and concentrated to be used. The additional equipment and energy costs associated with this separation process are not worth the relatively small amounts of DCS produced, and will therefore be disposed of accordingly. The engineering team will investigate the possibility of partnering with a manufacturer of the product to lower disposal costs of the highly toxic compound. Preliminary Economic Analysis

The raw materials required for the production of TCS are extremely cheap relative to the selling price of the process product. The two main reactants in the process are hydrochloric acid (HCl) and metallurgical silicon (MG Si), which have costs of $0.136/kg6 and $0.003/kg7, respectively. The selling price of our product, trichlorosilane, is $2.46/kg.8 Another possible silicon precursor, silane, has a similar production process to TCS, however the sales price is much lower at $0.56/kg.10 Disilane has the highest selling price of the silicon precursors, however, the increased equipment and utility costs associated with producing disilane outweigh the difference in sales price. Therefore, TCS was deemed the optimal choice given the low cost of raw materials, reasonable energy requirements, a large profit margin, and a competitive selling price. TCS production requires reactor and system operation at high temperatures to keep TCS in the gaseous phase, which demands a large energy expenditure. The utility costs will be mitigated by optimizing various sections of the plant, including the two-stage reactor as well as the distillation columns. Utilizing a fluidized bed reactor (FBR) in our process will assist in the control of the exothermic reaction to provide high yield of TCS and maximum production. This energy can be

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extracted from the fluidized sand bed to heat compounds in other operations of the process. Also, using FBR will decrease the need for frequent shutdown and maintenance as the deposition buildup inside the reactor will be reduced. To lower utility and operating costs of the process, certain components will be recycled within the process, thereby conforming to the principles of green chemistry by avoidance of release of waste gases to the environment. Another principle of green chemistry that will be considered with recycle streams is the reduction of hazardous materials onsite. Rather than producing byproducts that will require storage and transportation off-site, these compounds will simply be consumed within the process, reducing the need to contain large quantities of toxic and/or flammable material. Compounds to be recycled are hydrogen, silicon tetrachloride, and possibly dichlorosilane. Recycling preheated compounds will also conserve energy and reduce costs of fuel.

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References 1. Solar Energy Industries Association (2018) Solar Industry Research Data, SEIA 2. Ring, M. et. al. (1970). Pyrolysis of Monosilane, Department of Chemistry, San Diego State College 3. Ingle, et. al. (1985). United State Patent No. 4526769 4. Chee, et. al. (2010) United State Patent Application Publication, No. US 2010/0001236 A1 5. [Hot Item] Industrial Grade of Hydrochloric Acid 32% (HCl) -Qingdao Hisea. (n.d.). Retrieved from

<https://hiseachem.en.made-in-china.com/product/lBgxFkovCYrL/China-Industrial-Grade-of- Hydrochloric-Acid-32-HCl-Qingdao-Hisea.html>

6. Hydrochloric Acid 32% - Buy Hcl,The Price Of Hydrochloric Acid,Industrial Grade Hydrochloric Acid Product on Alibaba.com. (n.d.). Retrieved from <https://www.alibaba.com/product-detail/Hydrochloric- acid-32-_60687081630.html?spm=a2700.7724857.main07.53.3437281f0NBVoV>

7. Metallurgical grade silicon price. (n.d.). Retrieved from <https://www.alibaba.com/showroom/metallurgical-grade-silicon-price.htm>

8. Trichlorosilane. (n.d.). Retrieved from <http://original.metal.com/metals/productinfo/201208220006#1https://www.google.com/search?q=rmb tousd&rlz=1C1CHBF_enUS720US721&oq=rmb&aqs=chrome.1.69i57j0l5.2749j0j1&sourceid=chrome &ie=UTF-8>

9. Chemicals. (n.d.). Retrieved from < https://www.alibaba.com/showroom/trichlorosilane.html>

10. High Purity Silane Gas,99.9999% Silane Gas,Sih4,Silane - Buy 99.9999% Silane Gas,Silane Gas,High Purity Silane Gas Product on Alibaba.com. (n.d.). Retrieved from <https://www.alibaba.com/product- detail/high-purity-silane-gas-99- 9999_60688024202.html?spm=a2700.7724838.2017115.192.3e0e7fb64kXpXR>

11. Silane Gas, Silane And Nitrogen Mixtures SiH4 Gas - Buy Silane, SiH4 Gas, SiH4 Cylinder Product on Alibaba.com. (n.d.). Retrieved from <https://www.alibaba.com/product-detail/Silane-Gas-Silane-And- Nitroigen-Mixtures_60268636029.html?spm=a2700.7724838.2017115.8.3e0e7fb64kXpXR&s=>