Mathematical Modelling for Evaluation of Multi-Stage Flash Evaporator for Concentrating Na ₂SO ₄, H ₂SO ₄, ZnSO ₄ and H ₂O Mixture

Abstract

Production of regenerated cellulosic fibres by viscose process generates water and Na₂SO₄ in the Spin-Bath (SB, comprising of H₂SO₄, Na₂SO₄ ,ZnSO₄ and water) during the spinning process(Woodings 2001). This water and Na2SO4 is a by-product of the reaction between H₂SO₄ in the SB and caustic soda in viscose dope. To maintain the concentration of SB, generated water and Na₂SO₄ must be removed from the dilute SB. For removal of water, Multi Stage Flash Evaporator (MSFE) is widely used and crystallization method is used for Na₂SO₄ removal followed by MSFE. MSFE has been a widely studied topic for its application in desalination process. Although the equipment serves the same purpose in both desalination and viscose SB recovery process, the liquids that are subjected to evaporation have different thermodynamic and physical properties. Moreover, there is meagre literature available that reports and attempts at mathematical modelling, simulation & optimization of MSFE system for SB. Steady-state modelling using first principles has been done that takes into consideration, the parameters that majorly affect the evaporation of water from SB. The model is validated by comparison with operating plant data and optimal design is determined by varying the number of flash vessel stages, vacuum levels in each flash vessel & heat exchanger approach temperature.

Keywords: MSF Evaporation, Viscose, Mathematical Modelling, Concentrating Salt Solution