To Nutsche or to Centrifuge—what’s the right choice?

The agitated nutsche filter dryer (ANFD, or simply ‘Nutsche’ for short) is a commonly used filter/dryer in many biopharmaceutical and chemical operations, as also the filtering centrifuge. So much so, that in a number of applications, both these expensive pieces of equipment could seemingly equally well be used instead of the other. The right choice between the two can make a huge difference in the process efficiency, containment, and potentially millions in lost productivity over the 20-30 year life of these machines. We are often asked for guidelines or ‘rules of thumb’ to choose between these two machine types. This blog endeavors to explain some of the main points to consider while selecting door 'A' vs.'B':

1. Know your process: a thorough understanding of your own process parameters is critical to getting on the right track. Know that both the pressure nutsche and the filtering centrifuge are ‘cake filters’, meaning they depend upon the formation of a thick solids ‘cake’ on the surface of a filter media to be able to do their work. So, the characteristics of the slurry to be filtered must be known as to the behavior of both (a) the suspended solids, and (b) the liquid, under conditions of high pressure or ‘G-force’ often at elevated temperatures coming from the reactor or fermentor. The filterability of the solids depends to a large extent on their nature (crystalline or amorphous), their size (in microns, usually the particles are not all of the same size but have a particle size distribution, or psd, curve), their shape (spherical, cubical, needle-shaped, etc.) and the compressibility of the cake. Larger size particles usually allow the formation of a thicker cake quicker, but a compressible cake tends to inhibit filtration. Another aspect of the solid particles that is often missed and not properly understood, is their tendency to come together by similar size (or form ‘strata’) via a phenomenon known in solids mixing as 'size segregation'. This plays an important role inside a high-speed filtering device when, due to size segregation effects, the particles tend to stratify, greatly impacting subsequent filtration rates and wash efficiencies. Also, are the solids sticky or free flowing at various levels of moisture in them, or are they pyrophoric when exposed to air. So also, as far as the liquid part of the slurry is concerned, what are it’s density and viscosity at the temperature at which it comes into the nutsche or centrifuge. Generally, the heavier the liquid, the slower the filtration. The liquid’s vapor pressure or volatility and corrosiveness also play a role. And finally, the concentration of the solids in the liquid (usually as a volume or weight percent) must be known before any decision can be made on machine selection.

2. Know what each type of machine is capable of: the ANFD consists of a vertical cylindrical jacketed pressure vessel with a central agitator with 2 or 3 arms that can rotate as well as reciprocate, and a flat bottom lined with a filter media, the surface filtering area of which can be from as small as 0.1 m2 all the way up to 10 m2 or more. Obviously, with the increase in surface area, the working volume increases as well, from 10 liters or less, to 10,000 liters or more. The ANFD is unique in the sense that it can combine several operations (such as reaction, filtration, drying) into a single unit. As such, it has a huge advantage where complete containment (as may be required in a OEB 5 application) is required. But, that comes at a price—the ANFD is much more expensive than a centrifuge of comparable throughput, everything else being the same. Also, the ANFD depends upon the formation of a thick cake (6” or more) at about 3-4 bar pressure, to be able to be justified for an application, but of course there are many caveats to this statement. The other major advantage of an ANFD is its ability to carry out either a static (or plug) or a dynamic (or reslurry) wash of the solid particles, something that is often required in many biopharma applications. On the other hand, the filtering centrifuge is a high-speed rotating basket (kind of like your clothes washer at home) that is good at one thing, and one thing alone—and that is, to filter out the solids and produce a wet cake (which may then go into a separate dryer, again, much like your domestic clothes dryer at home). While the centrifuge can be contained and made fully air-tight, it is nowhere near what an OEB 5 would require but, the saving grace is that since the cake is wet, the chance of dry powder floating around is somewhat reduced. Also, where a dryer cake discharge may be beneficial in reducing subsequent drying time, the use of certain optional features can bring down moisture levels in the discharged cake to almost the level of dryness of the solids from an ANFD. Also, in a filtering centrifuge the washing is always static. Optional features to enable dynamic or reslurry wash are available, but they are pricey and aren’t for all applications. Couple other factors that impact productivity in a high-speed filtering centrifuge are vibration levels and filter media (more on these in future blogs).

All in all, for most applications, the choice is usually clear between the ANFD or the centrifuge. But there are some applications where they tend to overlap (meaning, on the surface, both can do the job equally well). When that happens, it is imperative that you dig deeper, because the right machine choice will usually emerge at that second level of differentiation.

Reach out to us if you have a centrifuge or pressure nutsche question, we’ll be happy to assist.