High purity is required in a number of mixing applications, especially in the production of pharmaceuticals, food, beverages, cosmetics, and electronics. Various standards have been set by the FDA, other governmental agencies, USP, and relevant industry trade groups that specify the proper equipment and conditions for maintenance of sanitary conditions and prevention of contamination. Mixing tanks must be constructed so that process materials fully drain from the system and so that all areas may be completely cleaned and sanitized. Also, the metals and polymers comprising mixing tanks must not leach harmful substances.
As part of an interdependent system including a mixer, valves, support structures, and often also incorporating ports and spray balls, a high-purity mixing tank must be specified considering these and other components as well as the process conditions.
Tank Selection Options
In specifying a mixing tank, you have many variables to consider. These variables are usually specified by the application although some processes may allow different choices. These include;
- Tank Size
- Materials of Construction
- Mixing Elements
- Access Ports
- Process Seals
Each of these options will be discussed below and many are dependent on decisions made in other areas of the design.
Typical tank sizes are from 1-liter to 5,000-gallons capacities, although much large sizes are available. Tanks come in various shapes and sizes. Several parameters go into the determination of the tank size and shape such as;
- Maximum operating capacity
- Minimum batch volume
- Height and Footprint limits
- Headroom required (top entry mixers for example)
Materials of Construction
Materials of construction are often determined by the mixing application. Common materials of construction for sanitary mix tanks include;
- 304 or 316 stainless steel (SS)
- Other corrosion resistant metals such as Hastelloy® C22 and C276
- FDA approved linear polyethylene (PE)
- USP VI polypropylene (PP)
- Fluroelastomers (PTFE) or other polymers with a PTFE barrier lining
There are several factors to consider in selecting a material of construction for your mixing tank.
- Abrasive nature of the process materials
- Contamination concerns
- Chemical resistance
- Pressure and Temperature of the operation
- Density of the process fluid
- Sanitary requirements
If the process contains abrasive components such as silica or hard crystalline minerals, then consideration needs to be given to the material of the mix tank which will minimize the impact on the final product. Abrasive products will wear Stainless Steel tanks and contaminate the product with both materials from the tank and impart a grey color to white products. Polymeric Tanks will shed a small amount of material but if the material is an approved substance such as polypropylene, this may be a good choice.
Pressure and Temperature
Polymeric tanks are often utilized in low pressure and moderate temperature processes. Steel tanks are better suited for high pressure or vacuum and extreme temperatures. Tanks which may be suited for more extreme conditions yet offer some of the benefits of polymeric tanks are steel tanks coated with a polymer such as PTFE.
Stainless Steel tanks tend to be more susceptible to chemical corrosion issues. Some of these issues maybe improved with coatings or electropolishing, but as soon as the thin protective layer is breached, then pitting or cracking can occur, leading to contamination and tank failure problems. Different polymeric materials have resistance to different substances, so it is important to investigate all of the compatibility issues in selecting the proper material. Some testing can be done, if there isn’t enough data to make this decision.
Density of the Mixing Fluid
Higher density fluids may require stronger tank walls which may lead to an increase in the design wall thickness or a different material of construction.
Mixing can depend on many different elements, which include;
- Mixing blade(s)
- Wall Mounted Baffles
- Vortex Breaker
- Mixer Mounting
There are a variety of mixing blades to choose from both in size and design. The selection process depends on the amount of shear required and pumping rate needed. Efficient design may require multiple blades, multiple shafts, and off-center placement. The height of each blade needs to be determined by minimum and maximum mixing volumes, the ability to lift settling particles from the bottom of the tank, and ensure appropriate radial mixing to eliminate dead zones, particularly on the sides of the tank.
Wall Mounted Baffles
Wall mounted baffles are necessary in some applications to ensure enough turbulence is generated for good dispersion. Some fluids can spin inside the tank and lower the amount of radial mixing. Baffles may cause more spaces for material to hold up so this option needs to be considered with the cleanability requirements of the system.
Vortex Breakers are mounted on the bottom of the tank and disturb the natural vortex which can develop and cause cavitation and air entrainment into the mixture. Like Wall Mounted Baffles, the cleanability requirements need to be considered as well.
Mixers can mount in several different ways; Top, Side, and Bottom. Top Mounting is typical for a majority of systems as it is inexpensive although sometimes a process seal is required (see Process Seals below). Side Mounting may be required due to space limitations. The inability to remove a mixer with a full tank and the sideways load on the bearings are disadvantages of the side-mount. Bottom mount mixers with a magnetic drive are used in some applications as they remove the need for a process seal, although they can be expensive and difficult to install. The tank to the left shows an example of a top mounted mixer.
All tanks require some sort of support and mechanism for moving. Some large polymeric tanks may require metal bands to prevent bowing of the sidewalls. Mixer supports are designed with a floor to outlet port dimensions to ensure the tank matches up to the connectors required. Tanks can either have casters such as the one to the left or have fork lift channels or lifting lugs for cranes.
Some processes utilize load cells to measure the weight of the tank as materials are charged to the tank. Load cells are typically incorporated into the support stand. The materials of construction need to be addressed as some areas may require stainless steel or plastic materials only.
There are different ways to connect process lines to the mixing tank. These include;
- Tri-clamp® fitting (a.k.a. Triclamp fitting, TC fitting, sanitary clamp, or S clamp)
- Sanitary ferrule
- ANSI flange
- Tank bottom flange
- Vessel Tri-clamp® ferrule (Triclamp ferrule, TC ferrule)
- NovAseptic® connector (NA connector)
Considerations for selecting ports are hold up elimination, cleanability, tool-less access, elimination or reduction of deadlegs in valves. The ports may need to fit instrumentation for measuring process parameters such as temperature, pressure, viscosity, and pH.
Contamination concerns usually require some sort of process seal around the rotating shaft of a mixer. Typical seals are lip seals and mechanical seals. Lip seals are less expensive but have a lower threshold for pressure. Mechanical seals are better as sealing pressure and can be specified as double seals with a pressurized barrier fluid in some demanding applications. See the WMP whitepaper Introduction to Mechanical Seals for Rotating Equipment for more information.
Finally, cleanability is a major concern for most high purity mix tanks. There are many standards which determine the specifics on tank manufacture. The degree of polish and treatment of welds are given as well as the materials of construction which are approved. Some specifications require spray balls which usually require a riboflavin test for complete cleaning.
Most high purity mix operations require documentation of the materials of construction, calibration of test equipment, and operator training certifications. Quality Control Certificates are also required to ensure all of the components have undergone cGMP processes during their manufacture.
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