(247x) Monosilane Production: Analysis and Control of a Reactive Distillation Column to Reduce Cooling Utilities

Alternative fuels and renewable energy have been subjects of interest in recent years. With the depletion of fossil fuels, several investigations have focus their attention to solar energy. Polysilicon is the principal raw material by the solar photovoltaic and electronics industry. The evolution of applications in these areas has demanded an increase in polysilicon production.

One route to obtain ultra-high purity of polysilicon is achieved first by the preparation of a volatile silicon hydride i.e. silane and its subsequently purification, generally using fractional distillation. This is followed by the decomposition of this hydride to hyper-pure elemental silicon by reductive pyrolysis or chemical vapor deposition. The preparation of the volatile Si compound involves external reactants and its decomposition generates by-products, which need to be recycled.

The Union Carbide Corporation (UCC) process used to produce polysilicon is based on the thermal decomposition of monosilane on a heated silicon rod or filament placed inside a deposition chamber. This process requires two reactors and more distillation columns for producing monosilane. Unreacted reactants need to be separated and recycled back for further reaction, therefore utility and capital costs are extremely large.

Alternative processes to produce monosilane have been investigated and patented. Reactive distillation is a promising alternative to conventional reactor-distillation processes, so that capital and operating costs can be reduced [1]. Reactive distillation was first proposed by Bakay [2], since then, several research have been conducted to improve the process and reduce costs. One of the drawbacks of using reactive distillation is the expensive refrigeration services needed in the top of the column for silane production. Moreover, it is important to avoid thermal decomposition of the catalyst in the reactive trays. Block [3] proposed to use a two column scheme to increase temperature at the top of the reactive tower and reduce the temperature in the dome of the auxiliary purification tower by increasing operating pressure generating lower operation cost.

This work focuses on the analysis and control of such scheme and the comparison to the conventional reactive distillation column regarding economic and dynamic performance. Steady state of the configurations is simulated in AspenPlus. Simulations are then exported to AspenDynamics and temperature controllers are configured by means of a sensitivity analysis. The steady state results show that the two configuration scheme reduces operating cost but capital cost is increased. The dynamic responses are analyzed and compared under feed disturbances. The results show that the closed loop response for the two column configuration is slower, however it brings the controlled variables back to their nominal values.

[1] Muller, D., Ronge, G., Fer, J. S., & Leimkuhler, H. J. (2002). Development and economic evaluation of a reactive distillation process for silane production. Distillation and Adsorption: Integrated Processes, Bayer AG, D-51368 Leverkuse.

[2] Bakay, C. J. (1976). U.S. Patent No. 3,968,199. Washington, DC: U.S. Patent and Trademark Office.

[3] Block, H. D., Leimkühler, H. J., Müller, D., Schäfer, J. P., & Ronge, G. (2005). U.S. Patent No. 6,905,576. Washington, DC: U.S. Patent and Trademark Office.