Simulation of Flow in a Helical Device Used for Irradiating Biological Fluids

Plasma based therapeutic products must undergo viral inactivation steps in order to minimize the risk of infection. Non-enveloped viruses such as hepatitis A and parvovirus are more recently being inactivated by using Ultra Violet light C (UV-C) irradiation techniques. A commonly used form of UV-C irradiation involves passing the protein solution through a helical tube that wraps around an irradiation source. The helical nature of the tube allows the formation of two counter-rotating Dean vortices along the cross-section of the helix. These ensure controlled mixing of the fluid leading to uniform exposure to UV-C irradiation. A detailed assessment of Dean vortices in a helical tube is performed in this paper using the Smoothed Particle Hydrodynamics (SPH) method. SPH being a Lagrangian Computational Fluid Dynamic (CFD) method is particularly well suited for evaluating history dependent properties such as residence time distribution of the fluid as it transits such a device. The simulations provided a basis for setting flow rate limits for the laboratory scale apparatus. Experimentally recorded UV inactivation rates using chemical actinometry led to a very good match with simulations. Results indicate that for the laboratory scale equipment the Dean vortex structure is absent for flow rates above 10 l/hr leading to reduced efficiency of UV-C irradiation. It is shown that the operating range of the laboratory scale device can be extended to at least until 25 l/hr by altering the crosssectional shape of the tube. Simulations were also performed for the process scale device to investigate the effect of scaling up the size of the device on the Dean vortex pattern. Dean vortices were present in this case for the entire flow rate range from 5 to 40 l/hr.