Challenges in Mixing of High Viscosity Materials
Materials such as rubber, polymer and putties have viscosity exceeding 10 Pa-s. These materials are classified as high viscosity materials, and exhibit characteristic such as resistance to flow, elasticity and non-Newtonian behaviour. Unlike in the case of liquid mixing, flow currents do not get formed during mixing of viscous materials. Mixing is therefore achieved by mechanical action, shear force or elongation of matrix facilitated by the mixing equipment. Mixing of viscous material necessitates dispersive mixing, distributive mixing and convective mixing mechanisms to occur in a system. Design and selection of mixing equipment for high viscosity applications, poses several challenges such as low flowability of material, poor heat transfer, scale up difficulties, high power and torque requirements. Mixers for high viscosity applications are built to overcome these challenges and therefore have some common characteristics. There are several different types of high viscosity mixers that are available for batch and / or continuous operations. The challenges associated with mixing of high viscosity materials underscores the importance of selection and design of mixing equipment best suited for any given application.
The following are some of the challenges associated with mixing of viscous materials.
Equipment Design – The mixing challenges are greater when there is a change in the state of material during the process. For example, solution or homopolymerization, may start out with watery thin liquids into which a small amount of catalyst is to uniformly distributed. As the polymerization proceeds, viscosity increases to the magnitude of 10 to 50 Pa.s. To control the polymerization, the mixing system must allow for rapid blending of low viscosity reflux into the viscous polymer matrix [Paul et al, 2001]. Likewise, mixers that need to perform more than one function, pose design and operational challenges. For example, a mixer producing a uniform rubber based solution must perform the dual function of masticating the rubber bale, followed by homogenizing the mix formulation. In several cases, the formation of high viscosity material takes place within the mixing equipment. The mixing equipment for such applications are therefore required to be versatile and should be efficient in over a high range of viscosity. The mixing element must be designed and manufactured specifically to accommodate the dense materials and high-torque requirements.
Power Requirement – Mixing of heavy plastic masses and pastes requires large amounts of mechanical energy for shearing of material, and facilitating convective mixing by continuous folding over, dividing and recombining the material. The power required for mixing of high viscosity pastes and dough is many times greater than that required for mixing of free flowing solids or liquids [Tekchandaney, 2012]. Mixer drive systems must be designed to deliver the required energy for mixing of high-viscosity materials and should provide constant torque throughout the speed range, even at very low rotational speeds.
Heat Transfer – Heat transfer is generally poor in viscous materials. Materials with low flowability fail to carry heat away from high-shear zones efficiently. Besides, mixing of high viscosity materials is generally characterized by heat dissipation which occurs due to high shear and friction generated by the action of the mixing blades. Heat-sensitive materials can therefore slow down the mixing process. As a process requirement, it may be necessary to control and maintain the product temperature at a desired level. Mixers for high viscosity materials therefore need to be designed for promoting efficient heat transfer.
Equipment Scale Up – Because of the viscosity and temperature changes that occur during the mixing process, it is difficult to model the system. Considering the myriad of industrial applications, which involve high viscosity materials, the many variables in each application, there is no equation or empirical correlation that can uniformly applied. Maintaining a constant specific energy (kW/kg) is a frequently adopted scale-up method [Tekchandaney, 2012]. Temperature control may be the most difficult task on scale-up [Paul et al, 2001].
- Tekchandaney, J.R. (Author), Holloway, M.D., Nwaoha, C., Onyeweuenyi, O.A. (Editors), (2012), Process Plant Equipment, Wiley.
- Paul, E. L., Atiemo-Obeng, V. A., and Kresta, S. M. (Editors), (2004), Handbook of Industrial Mixing, Wiley.
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