Mixing Applications in Selected Green TechnologiesHowever, as shown in Figure 2 below, the use of scrapers, which actually contact the vessel surfaces, significantly increases heat

Mixing Applications in Selected Green Technologies

A White Paper Prepared By

Charles Ross & Son Company

Ross White Paper: Mixing Applications in Selected Green Technologies Page 2 of 23

Mixing Applications in Selected Green Technologies

Abstract

This white paper presents an overview of mixing technologies employed in selected green technologies. As major developments continue to advance in the areas of renewable energy, green construction, green chemistry and many other markets, new mixing applications and processing challenges arise. The aim of this paper is to provide practical information on certain mixing requirements currently seen within selected green industries and how these production needs are being met through the efficient use of specialty mixing equipment.

Introduction

Broadly speaking, green technologies refer to an evolving group of materials and methods that help conserve natural resources and reduce harm to the environment. This set of technologies is not limited to the production of new environmentally-friendly products but encompasses any steps taken to optimize existing processes and manufacture products with greater operational efficiency. As many natural resources are currently being consumed at a faster rate than the earth can replenish them, governments and private sectors around the world are “going green,” not only to address ecological necessities but also to ensure economic and sociopolitical sustainability. Renewable Energy At present, the most important and urgent concern is energy. Green technologies are focusing on renewable energy which comes from easily replenished natural resources such as sunlight, wind, rain, tides, and geothermal heat. We see new mixing applications from the following mainstream forms of renewable energy:

http://en.wikipedia.org/wiki/Natural_resource�


Wind Power

Wind power is renewable and produces no greenhouse gases during operation. Airflow is used to run wind turbines, a device that converts kinetic energy from the wind into mechanical energy which subsequently can be used to produce electricity.

The evolution of wind energy technology in recent years has created the need for improvements in the design and construction of the structural components involved. Wind generator blades have become larger, designed to transfer more energy, faster. Consequently, advanced composites and structural adhesives are continually being developed to meet the new demands in performance and functionality. For instance, the resin composite used in the production of wind turbine blades must be stable, lightweight, strong and stiff enough to maintain structural integrity in both static and dynamic environments. Most of these materials are highly-filled, viscous formulations that require powerful and thorough mixing. Double Planetary Mixers are standard workhorses for viscous applications. They move material by rotating two identical stirrers on their own axes as they orbit on a common axis. The stirrers continuously advance along the periphery of the mix vessel, removing material from the walls and transporting it towards the interior. Traditional stirrers are of a rectangular open paddle design featuring a bottom crossbar which makes it difficult to raise or lower the blades through a highly viscous batch. The vertical flights of the rectangular stirrers also generate a power spike in high viscosity applications as the surfaces pass each other at the same time and at very close tolerances. In the past, once a new formulation undergoes a viscosity peak or arrives at a final state above 3 million cP, it demanded a complete shift to a kneader extruder (sigma blade mixer). Today, a “high viscosity” stirrer blade design (US Patent No. 6,652,137) is being offered on Ross Double Planetary Mixers, which extends operating viscosity to 8 million cP. The “HV” stirrer blades feature a precisely angled helical contour which generates a unique mixing action: the sweeping curve firmly pushes batch material forward and downward, keeping it within the mixing zone at all times. The planetary HV blades pass each other in a slicing motion, so that even at close tolerances, the spike in power experienced with rectangular stirrers is eliminated. The absence of horizontal crossbars also allows the agitators to be lifted very easily out of a viscous batch.

Ross 10-gallon Double Planetary Mixer with High Viscosity “HV” Blades and Discharge System mounted on a common raised platform.


The introduction of HV blades is a welcome development because vertical mixing on a planetary mixer has several advantages over horizontal processing on a kneader extruder. One consideration is that a sigma blade mixer relies on the product being highly viscous at all times in order to mix properly – liquid components must be added very slowly, portion by portion, or else they can act like a lubricant and reduce shearing efficiency. While this issue is also present in a vertical double planetary mixer, its blades run at higher tip speeds than sigma blades and that makes it less sensitive to liquid additions or shifts in viscosity. Other comparisons are explained in the below table.

Design advantages of Double Planetary Mixers over Kneader Extruders

Easier to clean

A vertical mixer design has no shaft seals, bearings, packing glands or stuffing boxes submerged in the product zone. In addition, the agitators are raised and lowered in/out of the mix vessel by a hydraulic lift. This allows easy access for cleaning between batches. Mix vessels are interchangeable and can be dedicated to a particular formulation and/or color. There is less concern for cross contamination from batch to batch.

Less floor space required

Footprint of the double planetary mixer is considerably less than that of a double arm / sigma blade mixer.

Energy savings

Since the double planetary mixer uses less motor horsepower to operate, everyday energy/operating costs will be less. This can be significant over time.

Semi-continuous operation

With the use of extra mix vessels, the double planetary mixer can produce material in a semi-continuous basis: one vessel is being charged while other vessels in the loop are under the mixer, being discharged, and/or cleaned.

Lower cost

Depending on specifications, a double planetary mixer is generally 1/2 – 1/3 the cost of a comparably sized new sigma blade / double arm mixer.

Certain formulations have fillers that are shear-sensitive, such as high viscosity composites filled with hollow microspheres, which require gentle and thorough mixing to reduce fractures. The Ross double planetary mixer with high viscosity blades is also ideal for this requirement. The gentle and thorough folding action imparted by the orbiting blades carefully mixes fragile ingredients into a viscous batch.


Solar Energy Solar energy is one of the most promising renewable energies on the brink of widespread commercialization. Not surprisingly, the drive to convert more solar power into electricity is high on the political agenda of many countries. Currently, commercial photovoltaic (PV) cells are predominantly based on crystalline silicon technologies. Silicon is a stable semiconductor with a well-balanced set of electronic, chemical and physical characteristics. Its prolific use in microelectronics has created an enormous market where the economies of scale directly benefit the emerging photovoltaic industry.

Figure 1: Anatomy of a crystalline silicon PV cell Image courtesy of www.advanced-energy.com

Mixing applications include conductive inks and pastes that are screen-printed over the antireflection coating of the front contact layer (cathode) and on the back contact layer (anode) of the PV cell. A typical paste consists of metal and glass particles uniformly dispersed in a viscous organic medium which serves as a temporary carrier. Once the paste is applied onto the silicon wafer, the organic compounds (solvent, cellulosic resins, wetting agents and other additives) are evaporated and burned out during a drying/firing step, which leaves the inorganic compounds behind, forming a grid of stripes or fingers. The metal and glass components in the paste each serve a unique function. For instance, silver powder provides conductivity and glass helps penetrate the anti-reflective layers, allowing the metal contacts to bond to the underlying silicon. The back contact of a cell (the side away from incoming sunlight) usually consists of a layer of aluminum or molybdenum metal which help to improve electrical performance.

Solar Panel


Paste quality and performance depends on the chemistry of the formulation, the purity/quality of raw materials and the preparation method. The desired features include good adhesion, printability, fine line and high aspect ratio (since metal stripes or fingers must be thick enough to conduct well with low resistance, but thin enough not to block too much of the incoming sunlight). Efficient mixing is required to produce a uniform blend of the paste components and ensure that there are no agglomerates or air voids in the product. Agglomerates of metal particles can affect conductivity and cause uneven or thick lines on the grid. Air bubbles in the mixture leads to inconsistencies, defects and voids in the dispensed paste. Depending on viscosity, mixing of the paste components may be accomplished in a Dual-Shaft Mixer or a Hybrid Planetary Mixer.

A Dual-Shaft Mixer is composed of two independently-driven agitators working in tandem. A saw-tooth disperser blade runs at high speed (typically in the range of 5,000 fpm) and generates a vortex which enables fast powder wet-out and dispersion, while a slow-speed anchor agitator supplies a steady exchange of materials from different parts of the vessel, essentially “feeding” the high-speed blade. On its own, the disperser blade can produce acceptable flow patterns for products up to around 50,000 centipoise. When used with an anchor, it can process formulations that are several hundred thousand centipoise.

Ross 50-gallon Dual-Shaft Mixer The anchor also provides a means for agitating product near the vessel surface. Its horizontal and vertical wings are designed to run at close proximity to the vessel walls. This, in itself, helps to constantly remove product from the sidewalls and bottom so that fresh material can fill those areas. However, as shown in Figure 2 below, the use of scrapers, which actually contact the vessel surfaces, significantly increases heat transfer efficiency (especially in cooling operations).


Figure 2: Wall and bottom scrapers improve heating and cooling efficiencies in a multi-shaft mixer. This graph shows that using scrapers reduced the heating time of a viscous material by approximately 33%, while the cooling cycle was reduced by approximately 60%. The larger improvement in cooling time can be explained by the tendency of most products to increase in viscosity when cooled – materials near the jacketed sidewalls thicken up, and in the absence of scrapers, further inhibit product circulation and heat exchange.

As viscosity rises, a dual-shaft mixer arrangement will eventually fail to produce sufficient flow. For instance, localized heating can occur in the areas closest to the high speed disperser blade or the anchor agitator may simply carve a path through the viscous batch as agitation close to the axis of rotation becomes more and more limited.

When dealing with viscous applications in the range of 100,000 to 2 million cP, creating a fine dispersion is more efficiently done on a Hybrid Planetary Mixer. This design consists of a planetary stirrer and a high speed disperser blade that revolve on their own axes while orbiting the vessel on a common axis. By constantly advancing the agitators into the batch, both blades contact fresh product all the time. This mechanism applies intense mixing action and uniform heat distribution. Solids are quickly incorporated into the viscous bulk material and stubborn agglomerates are dispersed regardless of product flow characteristics. Shear levels and flow patterns are easily fine-tuned because the agitators are individually-controlled.

Ross PowerMix Hybrid Planetary Mixer (US Patent No. 4,697,929)


Certain solar pastes, thick film inks and other viscous applications – such as silicone sealants (used for sealing PV cells into their aluminum or stainless steel frame), encapsulation compounds and conductive adhesives – are also prepared on Double Planetary Mixers. Although a relatively low speed mixing device, a Double Planetary Mixer can impart significant shear when product viscosity is artificially raised in the early stage of mixing by withholding a portion of the liquid vehicle. The higher the viscosity, the greater the energy produced from particle-to-particle, particle-to-blade, and particle-to-vessel interactions. To achieve a finer dispersion, the mixture may also be further processed or ‘polished’ on a Three Roll Mill. A Three Roll Mill is composed of three horizontally positioned rolls rotating at opposite directions and different speeds. The material to be milled is placed between the feed and center rolls and gets transferred from the center roll to the apron roll by adhesion. Dispersion is achieved by the shear forces generated between adjacent rolls. Milled material is removed from the apron roll by a knife that runs against the roll. The cycle can be repeated to improve dispersion until equilibrium is reached. The Three Roll Mill is an old technology with inherently low throughput and requires a skilled operator but it remains to be one of the best methods for preparing very fine dispersions. Gaps are set relatively tight, often 0.001” or less.

Ross Three Roll Mill. Each adjacent roll rotates at progressively higher speeds. For example, the feed roll may rotate at 30 rpm, the center roll at 90 rpm and the apron roll at 270 rpm.

Aside from crystalline silicon PV cells, other concepts and materials are continually being tested and developed in the effort to make solar technologies competitive on the large-scale energy market. These include organic polymer-based PV cells which show much promise due to their low cost, light weight, and mechanical flexibility. However, to date, low efficiencies, stability and strength inhibit the application of these devices.


In a bulk heterojunction cell, so far one of the most successful organic solar cell structures, the conductive polymer layer is a blend of two materials which act as electron donor and acceptor. Another polymer layer, Poly(3,4-ethylenedioxythiophene):poly(styrene sulfonic acid (PEDOT:PSS), is coated onto the indium tin oxide electrode to help decrease the density of pinholes and reduce current leakage. Figure 3: Architecture of an organic photovoltaic device. The negative electrode is aluminum, indium tin oxide (ITO) is a common transparent electrode, and the substrate is glass. The schematic depicts a bulk heterojunction (BHJ) active layer where the donor and acceptor blend forms phase segregated domains within the active layer. The structure of the BHJ is critical to the performance of the solar device. Source: http://www-ssrl.slac.stanford.edu Horizontal Ribbon Blenders and Vertical Cone Screw Blenders are widely used for dry mixing powders, pellets or granules of different polymers. The resulting resin blend combines the properties of each polymer and sometimes synergistically increases their physical characteristics. Various additives, also known as processing aids or polymers stabilizers, are added to improve mechanical performance and impart specific properties to the mixture.

A Ribbon Blender consists of a U-shaped horizontal trough and an agitator made up of inner and outer helical ribbons that are pitched to move material axially in opposite directions, as well as radially. The ribbons rotate with tip speeds of approximately 300 fpm. This blender design is very efficient and cost-effective for dry mixing processes. Certain applications involve coating a liquid component onto the bulk material. In these cases, liquid addition is best accomplished through the use of spray nozzles installed in a spray bar located just above the ribbon agitator.

A well-designed blender holds close tolerances – in the range of 1/8 to 3/16 in. – between the outer ribbon and the trough to prevent ‘dead zones’ in the batch. The blender may be cleaned with a brush, steam, air, or water spray. In most applications, there is no need to remove the agitator when cleaning or disassembling the seals for maintenance. But for processes that involve highly sensitive formulations, the risk of contamination is mitigated by rigorous cleaning in between cycles with the ribbon agitator removed for total access to the trough. To accommodate this requirement, simple customizations can be made on the standard blender model, including match marks on the shaft flanges to ensure

http://www-ssrl.slac.stanford.edu/�


proper realignment. However, if the clean-up protocol requires the removal of the agitator at more frequent intervals, say after every batch, a more elaborate modification is needed. The use of a flat-flange, clamshell coupling will allow operators to raise the ribbon agitator out of the blender without moving the end shafts, bearings or seals. This is a more expensive design but one that will save hours between batches.

Ross Ribbon Blender with removable ribbon agitator. The ribbon is hoisted out of the blender using an overhead crane so operators can quickly and easily clean both the ribbon agitator and the blender’s trough. The blender is used for batching different blends of polymer additives. After blending, the additives are further processed into a pellet or other compacted form for shipment to the customer.

Source: Powder and Bulk Engineering, December 2005

Ribbon Blenders made of abrasion-resistant steel are suitable for handling abrasive slurries such as those used in the wire saw cutting process in solar cell production. (To produce the thin silicon wafers that actually go into each cell, bricks of silicon are gradually lowered into a "web" of fast moving, ultra-thin wires. The cutting action is created by pouring an abrasive slurry on the wire web, which is actually a single wire being fed from one spool to another. After this slicing step, the wafers are cleaned in a series of chemical baths to remove any residual slurry.)

Another blender configuration, the Vertical Blender, is a more popular choice for abrasive applications in general. A slow turning auger screw orbits a conical vessel wall while it turns and gently lifts material upward. As materials reach the upper most level of the batch, they cascade slowly back down in regions opposite the moving auger screw.


The gentle blending action of the Vertical Blender makes it ideal for abrasive and delicate applications. For added protection against wear, product-contacting moving parts can be coated or hard-chrome plated. An important design feature that must be considered when selecting a Vertical Blender is a fully top-supported screw. The presence of a bottom support bearing requires more intense cleaning and increases contamination risks.

Vertical Blender Unsupported screw design Apart from abrasive application, vertical blenders are also commonly used for vacuum drying polymers or blends of polymers. Requiring only low heat to drive off moisture or solvents, vacuum drying is an excellent method for drying heat-sensitive products without fear of thermal degradation. A third common blender configuration is the Tumble Blender which is characterized by a rotating vessel that usually comes in a double-cone or V-shaped configuration. Asymmetric vessels designed to reduce blend times and improve uniformity are also available. Generally,

tumble blenders operate at a speed of 5 to 25 revolutions per minute. Materials cascade and intermix as the vessel rotates. Mixing is very low-impact. Like Ribbon Blenders and Vertical Blenders, Tumble Blenders are used for preparing glass and mineral blends, mixing polymers and dry additives prior to molding, and other applications.

Tumble Blenders with Double Cone Vessel (left) and V-Cone Vessel

How a vertical blender/dryer system works

Heat is applied to the jacketed vessel of the vertical blender. Once vacuum is established, the combination of low absolute pressure, gentle heat and slow agitation quickly forces moisture from the bulk material. The vapors generated in the process pass through a filter and into a condenser. A receiver collects the condensed liquids for re-use or proper waste disposal. At the end of the drying cycle, the finished product is discharged completely through the bottom valve of the vertical blender/dryer.


Biofuels Biofuels are fuels derived in some way from biomass. The term covers solid biomass, liquid fuels and various biogases. In response to uncertain fuel supplies and the call to reduce carbon dioxide emissions, bioethanol and biodiesel have become two of the most popular biofuels worldwide. In the United States and Brazil, bioethanol is widely used as an additive to gasoline to enhance octane rating and improve vehicle emissions. It can also be used in its pure form as the main fuel for vehicles. Bioethanol (or simply ethanol) is an alcohol traditionally made from starch crops (like corn and wheat) and from sugar crops (like sugar cane and sugar beet). The development of lingo-cellulosic technology has opened the door to using woody biomass in addition to the high energy content starch and sugar crops. Trees, grasses and other waste residues from forestry can now be used as feedstock for ethanol production. Bioethanol is seen as a good alternative to fossil-based fuels mainly because its source crops can be grown renewably in most climates around the world.

Source Crops for Bioethanol Production

Source: European Biomass Industry Association


Biodiesel, on the other hand, is a fuel composed of mono-alkyl esters of long chain fatty acids derived from vegetable oils or animal fats. Common plant feedstocks include oil from palm, jatropha, coconut, and soybeans. Waste cooking oil can also be used as raw material. Like bioethanol it can be used as a fuel for vehicles in its pure form (designated “B100”) but is now currently being used as a petroleum diesel additive to reduce levels of particulates, carbon monoxide and unburned hydrocarbons. It also lowers sulfur oxides and sulfates (major components of acid rain) in exhaust emissions. “B20”, a blend of 20% biodiesel with 80% petroleum diesel, has demonstrated significant environmental benefits with a minimum increase in cost for consumers (biodiesel blends of up to 20% work in any diesel engine with no modifications to the engine or the fuel system). Biodiesel is the most common biofuel in Europe.

In a chemical process called transesterification, the triglycerides in oil or fat are reacted with an alcohol in the presence of a catalyst, usually a strong alkaline like sodium hydroxide. This reaction yields alkyl esters (biodiesel) and glycerin (a valuable byproduct used in the manufacture of soaps, cosmetics, lubricants, pharmaceuticals and other products).

Static mixers and inline high shear rotor/stator mixers are used in biofuel production in conjunction with stirred reactors. For example, a pre-reaction mixture of the oil, alcohol and catalyst can be pumped through a static mixer to improve yield and pre-heat the reactants. The mixer can also be installed in a recirculation loop, for continuous mixing of the components during reaction. A static mixer is a unique device in that it has no moving parts and it relies on external pumps to move fluids through it. An array of static mixer elements is placed inside a pipe and the mixing operation is based on splitting and diverting input streams. Various designs are available for selection based on flow regime (laminar or turbulent), viscosity, allowable pressure drop, solubility and other factors. The static mixer elements can also be placed inside a heat exchanger for simultaneous mixing and heating. Ross Interfacial Surface Generator (ISG) Static Mixer (left) and Low Pressure Drop (LPD) Static Mixer (right). The ISG is ideal for blending materials with large viscosity ratios, as high as 250,000:1 or more. When operated under sufficient pressure, it is capable of producing oil-in-water emulsions with extremely small droplet sizes. The LPD is an exceptionally efficient static mixer designed for easy installation in new and existing pipelines where allowable pressure drop is limited.


Rotor/stator mixers are also used to recirculate product through a reactor. Rigorous mixing and emulsification of immiscible phases such as methanol and soybean oil, for instance, helps to expose more surface area or “reaction sites”, increasing yield and reducing reaction time. An inline high shear mixer consists of a rotor/stator assembly installed in a housing with inlet and outlet connections. The rotor is driven by a shaft that is directly or belt connected to the motor. A mechanical seal is utilized on the rotating shaft to isolate the mix chamber from the environment. The inline rotor/stator mixer behaves like a centrifugal pumping device. It is not self-priming and thus requires static pressure (gravity-feeding) or positive pressure (pump-feeding) to introduce materials into the mix chamber. Because an inline mixer is typically positioned on the floor or on a platform below the liquid level, gravity usually feeds the product into the mix chamber. Here the product is subjected to high shear as the rotor turns at tip speeds ranging from 3,000 to 4,000 ft/min and expels the mixture out of the chamber through the holes of the stationary stator. Schematic of an Inline High Shear Mixer (left); View of Four-Blade Rotor and Stator Assembly (right) Multi-stage rotor/stators and ultra-high shear mixer designs with rotors that run up to 11,000 ft/min or higher are also available and these impart greater energy into the emulsion therefore producing smaller droplet sizes. These designs are discussed in further detail on Page 19. In downstream processes, static mixers and rotor/stator mixers are also used to blend different fuel types, such as pure biodiesel and petroleum diesel or pure ethanol and gasoline along with other liquid additives.

Mixture out

Raw materials in


Green Building

Aside from tapping new energy sources, green technologies also give attention to reducing energy consumption. In the construction industry, efforts at “sustainable building” include the design of structures that efficiently use energy, water, and other resources. New technologies are also constantly being developed to reduce the overall impact of the built environment on human health and curb waste generation.

Below are two examples of such technologies and the new mixing applications within these markets.

Cool Roofs

In industrial and commercial buildings, a cool roof is a roofing system that reflects and emits the sun's heat back to the sky instead of transferring it to the building below. In the summer sun, for example, cool roofs stay 50 to 60 degrees F cooler, thereby reducing energy costs, improving occupant comfort, cutting maintenance costs and increasing the life cycle of the roof. Most cool roofs are white or other light colors.

Existing roofs are transformed into cool roofs using single-ply materials and/or coatings. Single-ply materials are large sheets of pre-made roofing that are mechanically fastened over an existing roof and sealed at the seams. Coatings are applied using rollers, sprays, or brushes leak-free roof surfaces. Acrylic coatings and polyurethane sealants utilized for this purpose are being made on a Ross Dual-Shaft Mixer equipped with a three-wing anchor and a high speed disperser. The anchor features helical flights in between the wings which produce better top-to-bottom mixing. Vacuum is pulled during mixing to fully deaerate the mixture. After the mix cycle, the vessel is rolled out of the mixer and moved under a heavy-duty discharge system. The platen-style hydraulic discharge system improves speed, efficiency and cleanliness of the discharge operation. As the platen is lowered hydraulically into the vessel, a specially-fitted O-ring rides against the wall, literally wiping it clean. Product is forced out through a valve in the bottom of the vessel, or through the top of the platen. The discharge system eliminates wasted hours of scraping heavy or sticky materials out of the mix vessel.


Ross Dual-Shaft Mixer and Discharge System for making roofing sealants and coatings. Formaldehyde-free Soy-based Adhesives For many years, hardwood plywood used for cabinetry, fine furniture and commercial fixtures utilized urea formaldehyde (UF) based adhesives. UF is a thermosetting resin derived from natural gas and its attributes of high tensile strength, high surface hardness and low water absorption make it a practical resin choice for adhesives, finishes and molded products. Starting in the 1980s, UF-based products attracted criticism because it was found to be a source of formaldehyde off-gassing in homes. The adhesive in hardwood plywood release free formaldehyde immediately after product installation. Emission rates would gradually decline over time, but they continue long afterward. High concentrations of free formaldehyde in household air triggers watery eyes, nose irritations, wheezing and coughing, fatigue, skin rash, severe allergic reactions, burning sensations in the eyes and throat, nausea, and difficulty in breathing in some people. In 2004, the International Agency for Cancer Research classified formaldehyde as a known carcinogen. Today, one of the most successful alternatives to UF-based materials is an adhesive comprised of cost-effective soy proteins and an amino acid that serves as a crosslinking agent and wet-strength resin. This new soy-based adhesive offers fast curing and high bond strength even when wet. This combination of properties has enabled manufacturers of hardwood plywood and other products to make a complete switch from UF adhesives.


Ross Dual-Shaft Mixers are being used at Columbia Forest Products for batching their soy-based wood adhesive formulation with a viscosity of >200,000cP. The dual-blade disperser provides intense shear to mix the soy flour (more than 30% loading) with water and other minor additions. The two-wing anchor promotes bulk flow and scrapes material from the bottom and sidewalls which aids in the mixing and discharging processes. The independently driven agitators enable the operator to fine-tune flow patterns and shear levels.

Source: Chemical Engineering, October 2010

Green Chemistry According to the US Environmental Protection Agency, green chemistry is the design of chemical products and processes that reduce or eliminate the use or generation of hazardous substances. These environmentally friendly, sustainable chemicals and processes result in less waste, safer outputs, and reduced or eliminated. Green chemistry relies on the Life Cycle Assessment (LCA) approach, a way of analyzing all the impacts that a particular product can have on the environment, from its manufacture and use, to its ultimate resting place and decomposition. For products that are currently being made using older technologies, each step of their life span can be examined for opportunities to make better choices for the environment. Generally speaking, green chemistry goes hand in hand with any form of green technology. Its principles (see box on Page 18) are incorporated in the development of better solar cells, wind turbines, biofuels, construction materials, etc. In particular, two branches, namely nanotechnology and advanced batteries, are responsible for the most number of new mixing applications that we have encountered so far.


The 12 Principles of Green Chemistry

1. Prevention It is better to prevent waste than to treat or clean up waste after it has been created.

2. Atom Economy Synthetic methods should be designed to maximize the incorporation of all materials used in the process into the final product. There should be few, if any, wasted atoms.

3. Less Hazardous Chemical Syntheses Wherever practicable, synthetic methods should be designed to use and generate substances that possess little or no toxicity to human health and the environment.

4. Designing Safer Chemicals Chemical products should be designed to effect their desired function while minimizing their toxicity.

5. Safer Solvents and Auxiliaries The use of auxiliary substances (e.g., solvents, separation agents, etc.) should be made unnecessary wherever possible and innocuous when used.

6. Design for Energy Efficiency Energy requirements of chemical processes should be minimized. If possible, synthetic methods should be conducted at ambient temperature and pressure.

7. Use of Renewable Feedstocks A raw material or feedstock should be renewable rather than depleting whenever technically and economically practicable.

8. Reduce Derivatives Unnecessary derivatization (use of blocking groups, protection/ deprotection, temporary modification of physical/chemical processes) should be minimized or avoided if possible, because such steps require additional reagents and can generate waste.

9. Catalysis Catalytic reagents (as selective as possible) are superior to stoichiometric reagents. Catalysts are used in small amounts and can carry out a single reaction many times. Stoichiometric reagents are used in excess and work only once.

10. Design for Degradation Chemical products should be designed so that at the end of their function they break down into innocuous degradation products and do not persist in the environment.

11. Real-time analysis for Pollution Prevention Analytical methodologies need to be further developed to allow for real-time, in-process monitoring and control prior to the formation of hazardous substances.

12. Inherently Safer Chemistry for Accident Prevention Choose substances and their form (solid, liquid, gas) that minimize the potential for chemical accidents, including releases, explosions, and fires.

Source: http://www.epa.gov/gcc/pubs/principles.html Adapted from Anastas, P. T.; Warner, J. C.; Green Chemistry: Theory and Practice, Oxford University Press: New York, 1998.

Nanotechnology A growing number of consumer products – from computer chips and cosmetics to fabrics and surfboards – are incorporating small amounts of nanomaterials to enhance existing properties or provide new functionalities. Nanomaterials are structures the size of 100 nanometers or smaller in at least one dimension. To put that scale into context, consider that human hair is approximately 80,000 nanometers in diameter. It is due to their size that nanomaterials possess their novel quantum mechanical properties. For instance, an increase in the ratio of surface area to volume translates to an exponential increase in the portion of constituent atoms at or near the surface, creating more sites for bonding or reaction with surrounding materials. This results in improved properties such as increased strength and greater chemical or heat resistance. In the production of coatings, nanopigments increase transparency, gloss, smoothness, as well as resistance to scratching, corrosion and ultraviolet radiation. Carbon nanotubes added to aircraft components, electronics and sports equipment give rise to enhanced electrical and thermal conductivity, as well as excellent mechanical load bearing capacity. Compared to the traditionally used filler, carbon black, which are relatively spherical particles, carbon nanotubes have a higher aspect ratio (ratio of its longer dimension to its shorter dimension) and can provide the same conductivity in much lower loadings. High carbon black loadings in a polymer tend to make it brittle and this issue is averted with the use of carbon nanotubes.

http://www.epa.gov/gcc/pubs/principles.html�


It is obvious to see that the technical breakthroughs of nanomaterials apply to different markets and can impact many more as research and development efforts continue. It is worth noting that nanomaterials also hold the potential to unlock advances in various green technologies. Already, nanomaterials are driving improvements in the design of solar cells, wind turbines, fuel cells and water filtration systems. One example is the nanocomposite coating sprayed on to the windmill blades to improve their durability. Dispersion of nanoparticles into a low-viscosity vehicle typically involves a pre-mix stage to combine the raw materials. This is done with the use of low speed propellers, turbines or simple agitators. Due to attractive forces between the individual nanoparticles, “wetting out” or combining them with the liquid vehicle only really disperses agglomerates of the nanoparticles. In addition to utilizing specialty surfactants, high shear forces are necessary to break up groups of these agglomerates. How aggressive those shear forces need to be vary from one formulation to another. The premix can be fed into a high pressure homogenizer, a media mill or an ultra-high shear mixer (UHSM) for creating the final dispersion. Of the three, ultra-high shear mixers are the newest answer to nanoprocessing. The interchangeable mixing heads of UHSM’s are discussed in the below table.

X-Series

The X-Series head (US Patent No. 5,632,596) consists of concentric rows of intermeshing teeth. The product enters at the center of the stator and moves outward through radial channels in the rotor/stator teeth. The combination of extremely close tolerances and very high tip speeds (11,300 fpm or higher) subjects the product to intense shear in every pass through the rotor/stator. The gap between adjacent surfaces of the rotor and stator are adjustable from 0.010” to 0.180” for fine-tuning shear levels and flow rates.

QuadSlot

The QuadSlot mixing head is a multi-stage rotor/stator with a fixed clearance. It produces higher pumping rates and requires higher horsepower compared to a similar size X-Series rotor/stator set.

MegaShear

The MegaShear head (US Patent No. 6,241,472) operates at the same tip speed as the X-Series and QuadSlot heads, but is even more aggressive in terms of shear and throughput levels. It consists of parallel semi-cylindrical grooves in the rotor and stator towards which product is forced by high velocity pumping vanes. Different streams are induced within the grooves and collide at high frequency before exiting the mix chamber.


In certain applications, ultra-high shear mixers effectively replace high pressure homogenizers and colloid mills. Manufacturers that find this to be true for their particular formulations welcome the change because high pressure homogenizers and colloid mills are high maintenance machines; during crossovers of different batches, the clean-up procedure is labor-intensive. Also, throughput rates of a similarly-powered ultra-high shear mixer are far greater when compared to a high pressure homogenizer or colloid mill. Lastly, ultra-high shear mixers cost less upfront. A sample application proven to be successfully processed in an ultra high shear mixer is polyol filled with carbon nanotubes. In their dry form, nanotubes appear pearl-like and not very dusty. The “pearls” are actually bundles of nanotubes and the mixing objective is to detangle and disperse the individual strands throughout the liquid vehicle. Exposing as much surface of the nanotubes as possible reveals their full functionality. In one lab test, the resin-nanotube premix was recirculated through the MegaShear. The mixture progressively became more viscous and glossier in appearance. Rise in temperature was another evidence of the intense shear imparted to the product. Microscope analysis of the mixed sample showed virtually complete de-bundling of the nanotube strands. At this point, it is important to note that not all nanoparticles require extreme shear to go into dispersion. Some materials need only mild stirring but may involve elevated temperatures or several hours of gentle agitation. One example is nanoclay in caprolactam, a raw material in the production of synthetic fibers. At relatively low levels (less than 5%), nanoclay is reported to improve mechanical, thermal and gas barrier properties of the final polymer product. Caprolactam monomer, a solid in room temperature, is melted and nanoclay is dispersed into the liquid phase prior to the polymerization step. The nanoclay particles “swell” as the monomer enters into the spaces between silicate layers. Monomer intrusion is time dependent and aided by temperature. As the distance between silicate layers increase, their mutual attraction decreases and the level of shear required to disperse the nanoclay drops accordingly. A mixing trial was performed to find a fast and efficient way of introducing nanoclays into caprolactam. 155-lbs of melted caprolactam was recirculated through an inline high shear mixer with powder induction capability, a technology called SLIM (short for Solids/Liquid Injection System). The mixer features a specially designed rotor/stator assembly designed to generate a powerful vacuum that can be used to draw powders right into the mix chamber. 13.5 lbs of nanoclay was charged in 15 seconds. The mixture was continuously being heated through the jacketed recirculation vessel and through the shear imparted by the mixer. As swelling of the nanoclay progresses, the nanocomposite mixture gains viscosity but with the application of shear, it is easily recirculated by the inline mixer and just as easily pumped downstream to the reactor. Overall processing is streamlined with the use of a powder injection system. Compared to traditional slow speed agitation systems, the SLIM allows for easier handling of raw materials, quicker dispersion, shorter cycle time, reduced dusting and improved operator safety.


Available in both batch and inline (continuous) configurations, the SLIM technology is a novel solution to the challenge of mixing hard-to-disperse powders. Even with a strong vortex generated by a top-mounted mixer in an open vessel, some powders, such as fumed nanosilica, will resist wetting out and could float on the surface for hours. Using the SLIM, solids are combined with the liquid subsurface and instantly subjected to intense shear. In other words, solids and liquid meet at precisely the point where high shear is generated and where flow is most turbulent. When solids and liquids are combined and mixed simultaneously, agglomerates are prevented from forming because dispersion is virtually instantaneous. For optimal performance, liquid viscosity must remain below 20,000cP during powder induction. Moderate to high viscosity nanocomposites are batched in multi-agitator equipment, planetary mixers and kneader extruders. Advanced Batteries Advances in battery technology such as higher capacity, improved cyclability and mechanical toughness contribute to the creation of green products: longer-lasting batteries that are more stable and safer to use. Beyond this however, lies the more dramatic impact of new generation high-performance batteries on the evolution of electric, hybrid and fuel cell vehicles. Indeed, an important element of the global competition in automotive green technology is the market for advanced lithium-ion batteries and other components for electric vehicles. Analysts believe that whoever has the leading edge on manufacturing these advanced batteries will control the future of the automotive industry. Equally important is the fact that improving the chemistry, composition and structure of batteries also increases opportunities to utilize renewable resources for reliable electricity generation. Batteries and other energy storage technologies support the use of wind and solar power by enabling the energy to be stored for use at the most beneficial times.

Inline (Continuous) SLIM. The liquid stream (blue) enters the mixer and immediately encounters the powder addition. Drawn into the mixer by a powerful vacuum, the powder (yellow) is injected through the ported rotor directly into the high shear zone. The resulting dispersion (green) is expelled centrifugally through the stator openings at high velocity.


High viscosity battery pastes and gels are processed on Double Planetary Mixers. On the other side of the spectrum, low to moderate viscosity solutions and slurries are batched in Multi-Shaft Mixers, some of which are integrated with an inline Ultra-High Shear Rotor/Stator Mixer for further particle size reduction and ‘polishing’ of the mixture. For applications that belong in between (viscous battery slurries and pastes in the range of 100,000 to 2 million cP), fine dispersions are efficiently done on Hybrid Planetary Mixers. The classic design consists of a planetary stirrer and a high speed disperser blade that revolve on their own axes while orbiting the vessel on a common axis. By constantly advancing the agitators into the batch, both blades contact fresh product all the time. This mechanism applies intense mixing action and uniform heat distribution. Shear levels and flow patterns are easily fine-tuned because the agitators are individually-controlled. More demanding applications benefit from multiple planetary stirrers and disperser blades. Solids are quickly incorporated into the viscous bulk material and stubborn agglomerates are dispersed regardless of product flow characteristics. Products that undergo a viscosity peak mid-processing, even if the final viscosity is relatively manageable in a multi-shaft mixer, are also ideal candidates for the hybrid planetary mixer. So are applications that are batched in a double planetary mixer but let down and further mixed using a high speed device to achieve the desired level of dispersion. In reality, many of today’s mixing technologies overlap in use and function. By comparing capital costs and measuring efficiencies through simulation trials, one can determine the best solution for a particular mixing requirement.

Triple-Shaft Mixer equipped with a three-wing anchor with scrapers, a high speed disperser blade and a SLIM rotor/stator mixer for subsurface powder induction.

Hybrid Planetary Mixer equipped with a rectangular planetary stirrer, a dual-blade high speed disperser shaft, and a removable scraper arm.


Battery pastes with ultra-fine dispersion quality are made on this Ross Model PDDM Planetary Dual Disperser. The unit is equipped with two helical planetary stirrers, one of which includes a bottom scraper. Two saw-tooth disperser blades are installed on each high speed shaft. A scraper arm removes material from the vessel wall to ensure superior heat transfer during the mixing process.

Conclusion More and more corporations are recognizing that “going green” benefits their businesses in the long term. The new strategy is to hit the triple bottom line: economical, environmental and social. A trusted equipment manufacturer that offers long-term experience, rental and testing resources will make for a very strategic partner to any company looking to achieve their own specific green goals. It’s also the right time to explore new mixing technologies and consider process upgrades in an effort to reduce waste, conserve energy and raw materials, increase plant safety and improve overall efficiency. Technology was in large part a contributor to the problem but when applied in the right direction, it can also be part of the solution.

The 12 Principles of Green Chemistry

Documents

Mixing Applications in Selected Green TechnologiesHowever, as shown in Figure 2 below, the use of scrapers, which actually contact the vessel surfaces, significantly increases heat