Terrence Delp, chief designer at Delp and Associates, argues that mechanical seals are a technology of the past, and new methods offer more promise in fixing old problems
"My responses are limited, detective, you must ask the right question".
So goes a well known line from a popular movie.
It seems that this is a common problem.
Researchers in the chemical industry are not asking the right questions about what equipment to use for laboratory mixing at the bench and pilot plant levels and the mixer manufacturers' responses have been limited indeed.
For decades the limited responses have been to use some form of dynamic or mechanical seal to retain pressure or vacuum.
The problems attendant with the variety of mixers using some form of dynamic seal are common and well known.
Because this equipment relies on some form of seal, typically Teflon, that is tightened radially upon the mixer shaft, no meaningful pressure or vacuum can be maintained and still leave the shaft free enough to actually spin.
The problems encountered include the unwanted introduction of spalled Teflon into the chemistry or the lack of a seal altogether.
Mechanical seals have been another limited response, but have their own set of problems.
These are seals that depend upon two precisely machined faces lubricated and separated by a barrier fluid that is applied under pressure greater than the pressure within the mixing vessel and is then forced into the chemistry being done.
When the product is pharmaceutical or some other high purity material, these problems are not only annoying, they are unacceptable.
In order to deal with these unwanted additions; the research, or production, product must be filtered and/or polished.
This additional step adds expense to the process and additional opportunities for other contamination or product loss.
The questions that should be asked are; what technology is available that could address all of these concerns and still provide efficient and safe mixing? How can a reactor system be sealed to prevent these types of problems without creating new ones? The problem solving technology is nearly as old as the dynamic seals that have become so problematic.
Magnetically coupled pumps are commonplace in the chemical industry for the movement of materials through piping systems.
Can this technology be used for driving a mixer shaft in a sealed vessel? Yes.
A magnetically coupled mixer is the only technology that addresses and solves all of the described problems.
Magnetically coupled mixers have no dynamic seals in them at all.
In fact, there is only one seal in a magnetically coupled mixer and it is a simple O-ring.
For the uninitiated a magnetically coupled mixer works like this: to the motor end of the mixer shaft is attached a permanent magnet which is encapsulated in a chemically inert molded thermoplastic and usually covered with a stainless steel case to protect the plastic from abrasion.
Over the magnet end of the shaft is fitted a stainless steel 'magnet cup'.
This cup has an outwardly formed flange on its open end.
When the mixer body is assembled to the motor or drive housing, this flange is forced into an O-ring in a groove machined into the top of the mixer body.
This is the only seal required.
The rotational drive is then supplied by either a cup-shaped driver magnet which surrounds the magnet cup and is rotated by an attached motor or by the coils of a brushless DC motor.
In the latter case, the mixer shaft with its attached magnet becomes the armature of the motor and is the only moving part of the mixer.
Properly designed, these mixers can easily exceed the pressure or vacuum capabilities of the laboratory glassware used and do not in any way become the limiting factor for containment or clean and safe chemistry.
To mount these mixers, either mount adapters or dedicated mixer bodies are used.
The mount adapters are designed to conform to standard entry ports of the vessels used (tapers, threads or flanges).
When adapters are used, a second O-ring is used between the mixer body and the adapter body.
In the case of dedicated mixer bodies, the body itself is machined with a taper, a thread or a flange which mates to the vessel used.
Again, just a simple O-ring is used.
"Is it time for the demise of the dynamic seal in chemical mixing?".
"That, detective, is the right question."
