There are many considerations in selecting and specifying a sanitary mixer, including the mixing process, materials to be mixed, and the sanitation requirements of the industry. This article provides a general overview to help educate you on the various parameters and choices in selecting a mixer for your process.
Different industries and processes are subject to different needs regarding “sanitary,” and a mixer may be required to comply with various rules and regulations from agencies such as the FDA and other entities such as U.S. Pharmacopeia (USP). A primary step in selection of the proper mixer is to determine which rules and regulations will apply.
Common applications that require sanitary mixers:
- Laboratory applications
SANITARY MIXERS REQUIREMENTS
A sanitary mixer is one designed, constructed, and documented to meet the needs of mixing applications in which sanitation and cleanability is paramount.
Typical sanitary mixer requirements:
- Adequate mixing of process materials
- No leaching of harmful materials
- Sealed from outside contamination
- Documentation of materials of construction and testing
These requirements will be discussed below and many are dependent on decisions made in other areas of the design.
MIXING IMPELLER DESIGN
The primary purpose of a sanitary mixer is to properly mix process materials. The best way to properly design a mixing impeller system is to have testing done on the actual process materials. There are many variables which affect the design of a mixing element.
- Viscosity – Higher viscosity materials may require multiple shafts or sweeper blades.
- Shear effects – Some materials change properties as shear is applied.
- Batch Size - Minimum and maximum mixing volumes.
- Tank Design – Taller tanks usually require multiple mixing blades on the shaft.
- Abrasive Products – Polymeric blades or coatings may be required.
- Temperature – Extreme temperatures may prevent certain polymeric materials for being used.
- Location – Impeller size may be limited by proximity to the tank wall or instrumentation.
- Mounting – Top, bottom or side mount will have different mixer designs.
- Cleanability – some processes require smooth surfaces which require welding and polishing for metallic elements or special machining for polymeric materials.
- Power Requirements – difficult to mix materials may require high horsepower motors which would affect shaft diameter and impeller mounting style.
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 materials with a PTFE coating
There are several factors to consider in selecting a material of construction for your mixing element.
- Abrasive nature of the process materials
- Contamination concerns
- Chemical resistance
- 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 mixer which will minimize the impact on the final product. Abrasive products will wear stainless steel impellers and contaminate the product with both materials and impart a grey color to white products. Polymeric blades 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 mixing elements are often utilized in moderate temperature processes. Steel blades are better suited for extreme temperatures. Mixers which may be suited for more extreme conditions yet offer some of the benefits of polymeric elements are stainless steel coated with a polymer such as PTFE.
Stainless steel mixing elements 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/or mixer mechanical failure. 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.
Now let’s take a look at the parameters that must be considered in specifying a mixer which are related to its use in conditions requiring a high level of sanitation.Clean in Place (CIP)
This refers to the ability of the mixer to be thoroughly cleaned without having to be removed from the vessel or disassembled. CIP design enhances efficiency of the overall operation by reducing cleaning time requirements.
Clean out of Place (COP)
The mixer must be removed from the vessel and separately cleaned in, for instance a COP tank.
Sterilize in Place (SIP)
This refers to the ability of the mixer to be completely sterilized without being removed from the vessel, also increasing operational efficiency.
Electro-polishing is a means of using electrical current to dissolve away microscopic surface high spots on metal parts, reducing roughness to a degree unattainable by mechanical polishing. This roughness measured by a profilometer, an instrument that measures roughness by moving a diamond stylus across a surface.
Mixer components must resist process chemicals and conditions, including cleaning chemicals and conditions. Parts must not corrode, weaken, or become brittle–all of which introduce the risk of introducing contaminants into the process. Common chemical-resistant materials include stainless steel, Hastelloy® C, and various polymer materials such as polypropylene, Teflon®, and high-density polyethylene.
Polymer materials must of course be able to withstand any sterilization temperatures to which they must be subjected. Heat-resistant polymers are polypropylene (PP) and Kynar® polyvinylidine fluoride (PVDF).
Sanitary Mechanical Seals
The mechanical seals used must of course be designed for sanitation and there is a wide range of choices available, from single seals to double seals and from cartridge designs to designs based on individual components (component seals). Each has advantages and disadvantages that must be thoroughly assessed and considered for the application.
In applications in which a seal is not required (non-pressurized vessels, for instance) a sanitary connector may be used to attach the mixer to the vessel. Again, several choices are available and must be thoroughly assessed. Among the more common sanitary connector options are: Tri-clamp® (TC), NovAseptic® (NA), and WMP-Flushmount. These allow optimum drainage of process materials and are fully cleanable/sterilizable.
Mixers are mounted with several different methods. These include:
- Sanitary Fitting
- Mounting Plate
- Mounting Rails
The weight of the mixer and motor should be considered when deciding on a mounting style. Large mixers mounted with a sanitary fitting may require additional supports to prevent movement of the mixer assembly which can lead to tank or shaft failure.
The mixer motor delivers rotational force to the drive or shaft to spin the mixing shaft. The motor needs to have enough torque to begin rotating a shaft from a stopped condition. High viscosity or heavy solids require a higher starting torque than other thinner mixtures. The power rating, usually in Horsepower (HP) or kilowatts (kW), should be high enough to maintain the desired mixer speed without exceeding the maximum motor current. Motors have a variety of options for their enclosure, power input, speed output, and current load.
- Electric (typically 110 VAC, 230 VAC, or 460 VAC for U.S. based installations)
- Stainless steel
- Washdown Rated
- Motor Speed (typically 1800 or 3600 RPM for US installations)
- Direct Drive – Motor shaft couples directly with mixer shaft.
- Gear Drive – Can be Inline or right angle and may require lubrication.
- Gear Reducer –Most commonly a gear reducer is necessary to decrease output speed.
- Variable Frequency Drive – VFD scale the frequency (Hz) with voltage to change the speed of electric motors. Inverter Duty or Inverter Ready motors need to be selected depending on the environment class ratings.
- Digital readouts – Most VFDs will display Frequency (Hz), Speed (RPM), Voltage (V), Motor Load (A), or Percent Full Load (%)
- Air control valves – Necessary for pneumatic motor speed control.
- Tachometers - Measure the actual rotational speed of the shaft instead of relying on the VFD readout, which only scales RPM to frequency. This can detect a problem with the drive system.
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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