Mixing of High Viscosity Materials
Mixing of High Viscosity Materials – Part 1 Presentation
Mixing of High Viscosity Materials:
Presented at International Conference on Innovation in Technology for Processing of Rubber & Elastomers – 26th and 27th October at Mumbai
Abstract – 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.
Introduction to Mixing of High Viscosity Materials
Mixing is the process wherein the non-uniformity within the mixture is reduced.
Materials such as polymer, putties, pastes, grease, chewing gum, solid propellant have viscosities exceeding 10 Pa-s (10,000 centipoise) and are categorized as viscous materials.
The viscosities of materials to be processed are constantly on the rise, as there is an urgent need to cut levels of volatile organic compounds in most parts of the chemical process industry. As viscosity rises, we cross a threshold into a different realm of mixing technology.
Viscous materials generally exhibit characteristics such as resistance to flow, non-Newtonian behaviour and elasticity. Unlike liquid mixing where mixing takes place due to generation of flow currents; in viscous materials there are no turbulent eddies, or flow currents to help distribute the components. Mixing in viscous systems can therefore be achieved only by mechanical action or by the forced shear or by elongation flow of the matrix.
Challenges in Mixing of High Viscosity Materials
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].
Mixing of High Viscosity Materials – Part 2 Presentation
Mixing Mechanisms for High Viscosity Materials
Mixing of viscous material requires deformation of the mixture ensuring both lateral and transverse motion of material. The geometry of the mixing vessel and the design of the mixing element have significant impact on the mixing process. The relative motion between mixing element and the internal walls of the mixing vessel creates both shear and bulk motion. The shear effectively creates thinner layers of non-uniform material, which diminishes striations or breaks agglomerates to increase homogeneity. Bulk motion redistributes the effects of the stretching processes throughout the mixing vessel
Mixing of viscous material necessitates dispersive mixing, distributive mixing and convective mixing mechanisms to occur in a system.
Dispersive mixing: Dispersive mixing is defined as the breakup of agglomerates or lumps to the desired ultimate grain size of the solid particulates or the domain size (drops) of other immiscible fluids [Paul et al, 2001].
Distributive mixing: Distributive mixing is defined as providing spatial uniformity of all the components and is determined by the history of deformation imparted to the material [Tekchandaney, 2012].
Convective mixing: Convective mixing in the laminar regime is effected by shear, kneading and stretching of material and results in reorientation of the dispersed elements [Tekchandaney, 2012].
These mechanisms are to be facilitated by the mixing equipment.
Mixer Characteristics for Viscous Materials
Most of the high viscosity mixing equipment have the following characteristics.
- Mixing element operates within all parts of the mixing vessel.
- Low clearances between the mixing element and the mixing container.
- High connected power per unit volume.
- As the forces generated during the mixing process are high, these mixers are rigid in construction.
- The heat evolved during mixing is high. The mixing vessels are therefore provided with external jackets for circulation of cooling media.
- The mixing elements may comprise of intermeshing blades that prevent the material from cylindering along with the rotating mixing element.
- Most mixers are provided with close-clearance blades and / or scraper devices to move stagnant material away from heat-transfer surfaces.
- High viscosity mixers operate at low speeds, require high power and therefore need high torque.
- Discharge of materials after mixing may be difficult and may require special arrangements.
High Viscosity Mixing Equipment
High viscosity mixers can be designed for batch as well as continuous operation. The commonly used high viscosity equipments are as under -
Single Stirred Mixers
- Anchor Mixer
- Helical Ribbon Mixer
Change Can Mixers
- Single Planetary Mixer
- Double Planetary Mixer,
- Other Change Can Mixers
Double Arm Kneader Mixers
Kneader Mixer Extruders
- Banbury Mixers,
- High Intensity Mixers
- Roll Mills
Pan Muller Mixers (High Shear Mixers)
Single Screw Extruders
Twin Screw Extruders
Selection of Mixing Equipment
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. Unfortunately, there are many variables in each application and as a result no standard set of rules pinpoints which type of mixer is best suited for an application.
The following are the basic criteria to be considered for the selection of mixers [Tekchandaney, 2012].
- Material characteristics
- Process setup
- Mixer operating parameters
- Mixing accuracy
- Mixer cleanability
- Equipment costs
After short-listing the equipment options, based on the basic criteria; the next step is to test the equipment for the process. Test results can useful data with respect to selection of mixer capacity, power requirements, heat transfer and above all the performance of the mixing process. Selection and scale-up of high viscosity mixers should be carried out in consultation with experts.
- 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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