65- The multi-factor swirl separator for parent and daughter separation is used in polysilicon projects to recover silicon powder from the off-gases of trichlorosilane reactors
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This post was last edited by luoli519 on 2023-10-2 at 15:05. Many polysilicon projects have been launched in China in recent years. Many companies choose to use the trichlorosilane route to produce polysilicon. Many enterprises face the problem of low recovery rates in silicon powder capture and recycling, as well as excessive consumption of silicon powder. In fact, in the trichlorosilane synthesis unit, the high-efficiency gas-solid separator is the key equipment used to capture and recover silicon powder from the reaction exhaust gases, thereby reducing the consumption of silicon powder. The selection and design of this equipment are directly related to the recovery rate of silicon powder and its consumption. Regarding the high-efficiency gas-solid separators used in similar projects at home and abroad in recent years, how should their structure be designed to ensure excellent operational performance? Let’s all discuss this together.
In recent years, polysilicon projects both domestically and internationally that use the trichlorosilane route have mostly adopted fluidized bed reactors to improve production efficiency and capacity. However, compared to traditional fixed-bed reactors, fluidized-bed reactors are more prone to causing silicon powder to escape from the reactor with the exhaust gas. Therefore, the gas-solid separator for capturing and recovering silicon powder from reaction exhaust gases has become a focus of attention for plant owners and process designers.
Many owners and designers at home and abroad have adopted an integrated structure of reactor and gas-solid separator, by installing a column section above the light-phase zone of the reactor; the cross-sectional area of this column is set to be 3 times that of the light-phase zone of the reactor, based on empirical considerations, resulting in a simple design. Actual operation has shown that the solid-phase capture efficiency in the exhaust gas is low, which increases the loading on the downstream gas-phase washing system; furthermore, the costs associated with the subsequent treatment of solids and liquids are not low.
The owner, the process package supplier, and the designer also made improvements to the combined structure of the simple tower and the reactor by using three-stage standard cyclone separators in series. The cost of the third-stage cyclone separator increases significantly, but the overall efficiency of silicon powder capture and recovery is still less than 90%.
In recent years, with the widespread use of high-temperature resistant filtering elements such as ceramic and sintered metal filters, some owners and designers prefer to use high-temperature filter elements to replace three-stage cyclone separators. The advantage is that the silicon powder capture rate is significantly improved. The drawback is that the silicon powder in the airflow clogs the filter elements too quickly, requiring frequent regeneration; this leads to the need for more equipment, as well as programming for operation and regeneration. High investment and high operating and maintenance costs.
In recent years, some foreign process packages have adopted multi-factor cyclone mother-son separators for the capture, recovery, and separation of silicon powder from the exhaust gases in the trichlorosilane reaction unit, as a means of technical upgrading to replace filters and the conventional three-stage cyclone separators. The efficiency of capturing, recovering, and separating silicon powder has been significantly improved, while the cost has decreased. The treated exhaust gas can then be sent to subsequent processing steps, which is very convenient. One multi-factor cyclone mother-son separator per production line replaces the original 3 standard cyclone separators and multiple filters; it requires fewer pieces of equipment, occupies less space and needs fewer pipelines. There is no need for spare units, nor is it necessary to replace internal components, resulting in low operational and maintenance costs.
This post was last edited by luoli519 on 2019-12-25 at 15:23. Domestic design firms have begun to follow this structural design for multi-factor cyclone mother-son separators; although the equipment is still referred to as an “exhaust gas filter,” in reality its internal components are no longer of the type that function by blocking and separating particles. The following diagram is a schematic illustration of the off-gas filter for the trichlorosilane reaction unit in a polysilicon project owned by a certain client, which was undertaken as a turnkey project by an engineering company:
This post was last edited by luoli519 on 2016-12-23 at 17:35. The owner and the designer approached a company in East China, which manufactured and supplied 8 units of equipment for them, one unit for each of the 8 production lines. It is unknown how the precise design calculations for dynamic separation are carried out. After operating for a period of time, the owner reported that silicon powder accumulated in the inlet air duct, resulting in a decrease in separation efficiency. The equipment manufacturer also turned to Novel Company for diagnosis and solutions.
This post was last edited by luoli519 on 2016-12-23 at 19:58. The operational data for the related projects are as follows: 1. Names of the media: chlorosilane, silicon powder, hydrogen, hydrogen chloride； 2. Gas flow rate: 3850 kg/h; 3. Gas density: 2.868 kg/m^3; 4. Medium viscosity: 0.027 MPa·s; 5. Gas molar mass: 29.370; 6. Solid flow rate: 1 kg/h; 7. Solid bulk density: 1250 kg/m^3; 8. Operating temperature: 450℃ ； 9. Operating pressure: 0.58 MPaG.
Technical requirements are as follows: 1. Separation requirements and efficiency: 100% removal of particles larger than 10μm； 2. Maximum allowable pressure drop: 0.03 MPaG; 3. Design flexibility: 25%-110%.
Regarding the feedback from owners that silicon powder accumulates in the inlet airflow channel of the separators manufactured and supplied by a factory in East China, everyone should take another look at the structure of the separator internals provided by the design institute; these internals use a counterflow design, and such a design choice leads to the accumulation of solid particles in the areas with low flow velocities within the internals. As a professional company in dynamic separation technology, it is necessary to inform the owners and design institutes to choose the axial-flow multi-factor cyclone parent-child separation internals, namely the G54A type.