Static mixers provide a means of achieving homogenization of gases, liquids and viscous materials without the use of moving mechanical parts. In its simplest form, the materials are passed through a fixed geometric structure which repeatedly splits the stream of material into numerous parts as it passes through and then reunifies them with a different part of the stream. The mixer housing is generally pipe-shaped and is mostly supplied flanged so that it can be easily installed in line as part of a continuous process.
A simple static mixer offers many advantages:
Static mixers do not require a separate energy supply. The pumps or blowers, delivering the materials to be mixed, supply all the energy required.
Pressure drop is small so energy consumption is low.
They have no moving parts, so they require little maintenance and down time is minimized.
They require minimal and operational cost investment.
Performance is predictable, uniform and consistent. Homogeneity, expressed as a deviation from the mean, is quantifiable.
They are compact and require little space.
Shear forces set up in static mixers are generally small, so the product is treated gently during processing.
Sealing problems are eliminated.
They are suitable for quick response on-line proportional control dosing systems to provide representative samples.
Differences in concentration, temperature and velocity are equalized over the cross section of the flow.
Since 1970 basic static mixers have undergone a lot of development which has led to a very wide range of applications in different forms. They are no longer used only for simple blending, but also where heat and mass transfer operations or chemical reactions are involved. Such processes take advantage of the ability to provide not only good blending, but also good heat transfer, uniform residence time, and where two or more phases are involved intimate dispersal and contact.
Specialist forms of static mixers are now used in many industrial sectors: petroleum, natural gas and refineries; petrochemicals; chemicals; polymer production and plastics processing, pulp and paper; cosmetics and detergents; foods; water and wastewater treatment; energy and environmental protection.
There are now at least 20 manufacturers, some of whom produce a limited variety of fairly basic designs, but the market leaders have now developed a wide range of specialist designs, based on the original concept, but adapted and refined to suit particular specialist purposes. Each manufacturer adopts a different nomenclature for his own designs. The following relate to the products of the Sulzer-Chemtech Division of Sulzer (UK) Ltd at Farnborough, Hampshire, but similar products can be found from other manufacturers.
The Sulzer range starts with two types of the basic static mixer for turbulent and laminar flow, the SMV and SMX, respectively. The SMV mixing elements are made in the form of corrugated plates which form open intersecting channels. Apart from their use for mixing low-viscosity liquids, gases and dispersing insoluble liquids, they are used for contacting gases with liquids and as a mass transfer device. The SMX is similar but has mixing elements in the form of a lattice of intermeshing and intersecting bars and is used for higher viscosity material.
Where static mixers are used for the physical absorption of gases into liquids and for gas/liquid chemical reactions they give safer operation and reduce the inventory of material. The reaction takes place quickly. An example is the dissolution of chlorine into water and its subsequent reaction with alkenes.
Another product based upon the same static mixing principle is mixer packing (SMVP) for use in bubble extraction and reaction columns. This has proved to have good hydrodynamic properties with strong cross mixing, little back mixing and high capacity.
Static mixers have also been developed in various ways to make them suitable for use as heat exchangers for viscous liquids. The simplest mixer-heat exchanger is the monotube version in which the tube containing the geometric mixing structure and product is enclosed within another pipe, so that the heating or cooling medium is fed into the space between the inner and outer pipes. This device generally remains limited to the transfer of small amounts of heat or the throughput of small amounts of product, otherwise the pipe lengths or pressure drop become too great.
For larger throughputs an alternative is multitube heat exchangers. Here the product is divided into parallel streams and mixing occurs only within the part streams; there is no radial mixing.
A more advanced development is a mixer in which the mixing elements are formed from hollow tubes, which contain the heat transfer medium. This then becomes a mixer-reactor (SMR) in which high heat transfer coefficients are obtained with a large internal heat transfer surface. This allows highly exothermic reactions to be closely controlled at the correct temperature. Equally it can be used for temperature control of viscous products, giving very quick response.
The SMR is designed for use in continuous processes and there are many possible configurations. One is the simple "once-through" product flow (plug-flow) suitable for applications such as cooling solutions, increasing viscosity of thermoplastic melts, adjusting melt temperature, continuous heat treatment and reaction control.
The SMR can also be configured to operate with product recirculation (loop). This is suitable for continuous high rate exo- or endothermic chemical reactions, either single or multiphase.
A third common configuration is a combination of loop and plug flow in order to achieve a rapid preliminary or main reaction, perhaps with further conversion down stream by admixed additives.
The SMR and SMXL (mixer reactor for laminar flow) are suitable for temperature-sensitive materials, where simultaneous reactions are required, or where temperature-controlled reactions require a uniform residence time and long mixing lengths. Static mixers are available in a wide variety of materials for different applications: stainless steel; exotic metals for corrosive materials; fiberglass reinforced plastic; polypropylene and so on. Mixing elements in various plastics such as PVDF, PP and ETFE are available installed in glass pipes (NPS, 40,25,15 mm) for laboratory experiments. Other designs are available in sizes up to eight meters or more in diameter.
Pilot tests are often necessary when a continuous process is being developed, for example, carrying out temperature controlled reactions. Scale up to large production rates is without risk if the results of such test results are available. Most suppliers of static mixers are able to undertake such tests in their own pilot plants. Alternatively, a small range of mixers are generally available from many manufacturers for trial purposes. On this basis, the suppliers are generally able to provide performance guarantees.
Figure 1. Schematic diagram of the SMR mixer-reactor. The product flows in the square channel, in which the mixing elements are arranged, constructed from tubing through which the heat transfer medium flows.
Figure 2. An SMR mixer-reactor module, showing the mixing partly drawn from its casing. A series of modules can be assembled in loops or other configurations according to the process requirement.
Gerstenberg, H., Schuhr, P., and Steiner, U. R. (1982), Rührkessel-Reaktoren für Polymer-Synthesen. Chemie-Ingenieur-Technik 54 6, pp. 541-553.
Heierle, A. (1980): Chemie-Tcchnik 9 pp. 83-85.
Lynn, S. and Oldershaw, C. F. (1984): Analysis and Design of a Viscous-Flow Cooler, Heat Transfer Engineering 5 1-2, pp. 85-92.
Müller, W. (1982): Chemie-Ingenieur-Technik 54 6, pp. 610-11.
Schneider, G. (1990): Institution of Chemical Engineers Symposium Series 121, paper 8, pp. 109-119. Static Mixers as Gas/Liquid reactors.
Streiff, F. A. (1986): Wärmeubertragung bei der Kunstsoffaufbereitung. VDI-Verlag, pp. 241-275.
Sulzer Mixing Processes, Prospectus 23.27.06.20.