The Selection and Use of Laboratory and Pilot Plant
Fluidized Bed Plow Mixers
The selection and use of laboratory mixers involves several criteria. How well you match these criteria can be the difference between a seldom-used lab device and a fully utilized lab mixer.
The selection and sizing is dictated by the initial and eventual needs of this equipment. There are four (4) typical categories for this equipment:
One of the more recent trends is for companies to standardize on a type of testing mixer for all its facilities, then replicate this at all of its facilities. This replication is very detailed; even agitator clearances are kept as identical as possible to avoid any differences in one facility’s results as compared with another.
Types: The typical types of mixers seen in most laboratory or pilot plant facilities are ribbon blenders, sigma blade mixers, paddle blenders, and plow mixers. Ribbon blenders are the most common; with 1 to 5 cubic foot being the most typical sizes. The problems that ribbon blenders give are quality of mix, clean-ability, blend time as well as a potential to eventually "UN-mix" the product. They do, however, require less horsepower than other types of mixing equipment.
Tumble Blenders come in two types; the "Double Cone" style and the "twin shell" style. These mixers mix by rotating the vessel, thereby tumbling the product back and forth on itself. The twin shell style actually divides the flow between the two shells as it tumbles. These blenders are of a very low intensity and therefore take longer and provide a lower quality mix than others. They are, however easily cleaned and are very good with friable products.
Sigma blade mixers come with two style blades, overlapping and tangential. They actually knead the mix and are very good for high viscosity pastes. The complete bowl is usually tilted to discharge the mix. The agitator usually has to run to discharge the mix, making this a dangerous implement for a laboratory environment. It also is a very difficult machine to clean.
The mix action is one of the most precise mix actions in that it provides three dimensional fluidized bed mixing. All particles are in constant motion, generating precise mixing with to ratios of up to 1:20,000.
Fluidized bed plow mixers blend the mix using flat or plow shaped elements rotating around one or two tangential shafts. These are usually medium intensity, medium horsepower, and have a blend action much superior to any of the above mentioned mixers. They do however, require more horsepower.
Sizes: Laboratory mixers and blenders usually range in size from 4 to 140 liters. Usually, batch sizes tend to be smaller, so that expensive ingredients are not wasted on multiple tests. Pilot plant mixers are sized around what volume that the pilot plant anticipates producing or what batch size is required for premixing or postmixing processes.
An important requirement for sizing a mixer is knowing the maximum and minimum working volumes. Mixers have several volume designations and many times are designated by a size noted as "total volume". This is actually inconsequential, as what is really important is the maximum and minimum working volumes. That is, most mixers require some freeboard space to move the material. Also, most mixers have a minimum amount that they can effectively mix. Sizing a mixer for either laboratory or pilot plant operations require careful thought as to what size batches will be run both now and in the future. This minimum to maximum range should handle all your anticipated future needs.
Size also impacts the amount of power required. This becomes a consideration depending on the utilities available.
Construction Requirements: There are several different questions that need to be answered prior to ordering a laboratory or pilot plant mixer. The correct specification of this mixer will direct how it performs over its useful life as well as how useful it becomes towards future needs.
Material of construction: The choices of product side material vary from carbon steel to 300 series stainless steels and high nickel alloys. Carbon steel, while a viable alternative to large processes that do not involve corrosive materials, should not be a material of choice in a small laboratory unit due to the marginal increased costs and limitations on future usage. The usually material of choice is a 300 series stainless steel with 304 Stainless being the standard and 316 stainless steel used for lower pH processes. Using a low carbon grade (304L or 316L stainless) precludes carbon precipitation in welded parts and accompanying loss of corrosion resistance. Specialty stainless steels and high nickel alloys are used for extremely corrosive products; you should consult a metallurgical engineer for the best material for your product.
The outside material of construction should also receive consideration. While painted surfaces look good in the beginning, what do you want this machine to look like after years of rough service? This question also needs to be asked when considering a finish (stainless). While mill finishes of glass bead surfaces suffice, polishes such as a No 3 or No 4 are more readily cleanible. For interior surfaces, nothing less than a No. 4 finish should be considered.
Drives: These mixers are usually powered through gearboxes and/or belt drives. Depending on the environment, these motors can be either TEFC (Totally Enclosed Fan Cooled), TENV (Totally Enclosed Non Ventilated), or Explosion Proof. If explosion proof, it is important to discern what Class, Division, and Group you application falls into. The Class and Group differ with the product being run. In almost all circumstances, mixer drives are considered Division 1 because of the proximity to the flammable materials.
Shaft Seals: On mixers with moving shafts, decisions must be made on what type of sealing mechanism to use. Stuffing boxes have typically been the mainstay, with a mechanically compressed packing sealing in the product. The problems with this design have been the erosion of the shaft, contamination of the seal area with product, and continual need to adjust and repack.
A modification of this, called a gas purge seal, introduces a gas such as air or nitrogen into the seal. This gas pressurizes the seal, helping prevent product from entering the seal area. The problem with this arrangement is that the mixer must be well ventilated to prevent pressure build-up, as well as the cancellation of vacuum when deareating of vacuum drying.
The mechanical seal is the most efficient method of sealing, using two sets of very flat seal rings running together with a barrier fluid injected between them to aid in cooling and lubrication. This seal is very tight, and will allow the attainment of very high vacuums as well as the protection of the shaft surface. They do require support systems (pumps, filters, and coolers) to handle the lubrication of the seal. Furthermore, the choice of barrier fluid should be compatible with the product.
Heating/cooling Jackets: Does your process need to be heated or cooled? If so, then some type of media jacket will be required. Different style jackets are supplied by different manufacturers, including mechanically attached platecoils, dimpled jackets, and fully enclosed labrinyth jackets. The attached platecoils, while inexpensive, are less effective due to the separation of the heat transfer surfaces and definitely should not be used for scale up purposes. While dimpled jackets are more effective, the best heat transfer can be obtained using a labrinyth full jacket.
The temperature to be achieved needs to be considered, especially when using steam as a media. Most mixers are limited to 350-400 degrees F (175-200 Deg C) because of the elastomers used on sealing the ports, access doors, and discharge. Generally, ASME (American Society of Mechanical Engineers) Boiler and pressure vessel (section VIII) code stamps are not required due to the small size of these vessels. This should, however, be discussed with your particular underwriter as some facilities insist on code stamps for all vessels. When using steam as a heating media, you need to establish the design pressure requirements for the product temperature to be reached. Even with water, relief valves and design pressures should be scrutinized to avoid collapsing the jacket.
When deciding on a jacket pressure, be sure to include the effect of vacuum should you anticipate pulling vacuum during your process. For example: A jacket designed for 50 PSIG should stand up to a 50 PSIG media pressure under atmospheric conditions. However, under a high internal vacuum the inner jacket wall could see 64.5 or more PSI, causing an internal collapse.
For most applications, a stainless jacket is recommended. This lessens the stresses set up by welding dissimilar metals together and prevents corrosion inside the jacket.
Size, mobility, types of discharge all need to be investigated. How is the mixer charged/discharged? Is the machine safeguarded with limit switches to prevent accidents? Even small 4 liter mixers and cause grave injury if not protected properly.
The characteristics of the products to be mixed also need to be investigated. Solvents and some powders need explosion proof motors and controls. One variation for the controls is to use a nitrogen purged control panel. These type systems require changes of the nitrogen inside the dryer prior to allowing power to the control panel. This allows you to use a stainless Nema 4 electrical box instead of a heavy cast box in an explosive environment.
Cleanability of the unit is also important. For drug and food applications, this is obvious. However, for precise product development or quality work, it is necessary to insure that no previous batch contaminants jeopardize the test results. Many mixers have removable agitators and even drums for assisting in cleaning and decontamination. Sanitary connections such as I-Line and S-Clamp fittings also aid in cleaning.
Instrumentation is also important. At minimum some type of ammeter or wattmeter needs to be included. Other variables measured include product temperature, jacket inlet temperature, jacket outlet temperature, vapor temperature, batch time and agitator speed. Chart recorders and process software are becoming increasingly more popular in these environments.
Other uses: Can this machine also be used to vacuum dry or react. Is any internal pressure required? Do nozzles need to be added to inject liquids? Planning for these potential applications again develop a machine that stays useful through its lifespan.
The Processall Tilt-A-Mix: The Processall Tilt-A-Mix laboratory mixer is unique in that it combines many of the features mentioned in a small mobile package amenable to many lab environments. The Tilt-A-Mix comes in 4, 10, 20, 40, 60 and 90 liter sizes and all have mobile stainless steel bases.
All have the following features:
The patented Tilt-A-Mix feature allows the mixer to be rotated to a vertical position to facilitate loading through either an open front end or the discharge port. After loading the machine is tilted to a horizontal position for processing. After processing, the drum then tilts down 20 or more degrees for discharge. The large units (20 liter and above) can be equipped with a power tilt to assist in movement. The smaller units (1,4,10 liter) have detachable drums that can be taken to a sink and cleaned. All units are equipped with 75 PSIG stainless steel jackets for heating or cooling. Standard product surfaces are 316L stainless steel with all other stainless and nickel alloys available as options. All types of instrumentation and control packages are available.
In summary, the purchase decision on a small laboratory or pilot plant mixer needs as much attention if not more than that of a production sized mixer. A correct purchase in this area can effect the quality of your products as well as future products that your company may develop.