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Off-Gas Recovery & Recycling
Vent Gas Recovery and Recycle Process Technology Package
Poly Plant Project (PPP) provides a Vent Gas Recovery & Recycling package that details the chemical processes, equipment, materials of construction, operating parameters, mass and energy balance, and control systems for recovery and recycle of the vent gases from the CVD reactor and from the STC-to-TCS thermal conversion reactors. This package from Poly Plant Project includes start-up and operating technology, passivation, commissioning of all processes and equipment, storage methods, quality control techniques, and risk analysis and safety recommendations.
The vent gas from the CVD reactor consists of hydrogen, unreacted trichlorosilane, silicon tetrachloride, other chlorosilanes, and hydrogen chloride. Less than 20% of the trichlorosilane gas is converted to polysilicon, so for the TCS deposition process to be cost-effective, the vent gases must be collected, separated and recycled.
The recycle system is designed to recover the hydrogen, hydrogen chloride, silicon tetrachloride and other chlorosilanes from the CVD reactor vent line and from the STC-to-TCS hydrogenation converter vent line. After recovery, the gases from the CVD reactor are separated and recycled, with the STC being sent to the hydrogenation reactor to convert the STC-to-TCS. The hydrogen is recovered and returned to a storage tank for reuse. The HCl is recovered and returned to the HCl storage tank for reuse. The unreacted TCS is collected and sent for reuse in the deposition reactors.
The gases collection and recovery system is designed to accept the peak flow rates and components of the exhaust during the different cycles in the CVD reactor. The Vent Gas Recovery Unit accepts the mixed vents from the reactors and converters and separates the mixed gases into a mixed chlorosilanes liquid, recovered HCl, and recovered hydrogen gas.
The warm vent gases are cooled and then chilled in heat exchangers to condense chlorosilane liquids. The HCl and hydrogen are then warmed and piped to compressors. The compressors are one stage reciprocating compressors designed to increase the pressure to the required hydrogen return pressure. The compressed gases are chilled and piped to the HCl Absorber Unit, where the HCl is absorbed in a chilled liquid chlorosilane stream. The concentrated chlorosilanes are warmed and piped to the HCl distillation tower, where HCl is removed as a gas.
The gas from the HCl Absorber Unit is piped to Carbon Adsorber Towers where any remaining impurities are removed. Clean hydrogen is recycled to the CVD reactors. The Carbon Adsorber Towers are regenerated by thermal cycling. The regeneration purge contains chlorosilane impurities, so this is either recycled to TCS distillation columns, or collected for disposal.