How to remove impurities without wasting solvent using ANFDs

In pharmaceutical manufacturing, washing is the step in a filtration process that most teams agree is necessary and almost no one agrees how to optimise. Get it wrong in one direction and the batch can fail purity specification. Get it wrong in the other and a plant burns through unnecessary solvent, time, and utility budget. At scale, both mistakes are common, and both are avoidable with the right approach to endpoint control and the right equipment, like an Agitated Nutsche Filter Dryer (ANFD).

In theory, washing an active pharmaceutical ingredient (API) cake is straightforward: introduce solvent, displace the mother liquor and its impurities, repeat until the cake is clean. In practice, the “repeat until clean” part is where most processes lose control.

Under-washing is the more obviously costly failure. If residual mother liquor or impurities remain above specification, the batch may need reprocessing, or worse, it may be rejected outright. Neither outcome is cheap, and neither is good for a plant’s environmental footprint. A failed batch represents solvent, energy, and raw material that has already been consumed for no useful output.

Over-washing is a quieter problem, but not a smaller one. Many operators build in a wash volume safety margin, an extra pass “just in case”, because measuring the true purity endpoint mid-process has historically been difficult. That margin adds up. Across a plant running hundreds of batches a year, a single unnecessary wash cycle per batch can mean thousands of litres of solvent purchased, stored, handled, and eventually disposed of or recovered, along with the associated energy cost of distillation and the emissions tied to solvent transport and processing. Over washing also results in extended processing time, not only reducing throughput, but potentially reducing yield if the API is partially soluble in solvent.

Both failure modes share a root cause: without visibility into wash progress, operators are forced to guess, and guessing tends to be conservative in one direction or the other.

Not all washing achieves equal purity for equal solvent. A traditional displacement wash, solvent percolating through a static cake, relies on the solvent finding a path through the product to reach residual impurities. Channelling and cake cracking mean parts of the cake are washed thoroughly while others are barely touched, so the whole batch needs another pass to catch what the first one missed.

A re-slurry wash, carried out inside an Agitated Nutsche Filter Dryer, takes a different approach. The agitator ploughs solvent into the cake and fully resuspends the solids before pressure filtration recovers the liquid. Because every particle surface is exposed to solvent directly, rather than relying on percolation through channels, the contact between solid and solvent is far more even. The ANFD also enables optimisation of mixing time by adjusting the speed, direction, and height of the agitator. The practical result is that a re-slurry wash typically reaches equivalent purity with a smaller total solvent volume than repeated displacement washes, fewer passes, increased throughput, less solvent purchased, less to recover or dispose of downstream.

Reducing solvent waste isn’t only about wash method: it’s about knowing when to stop. This is where in-process monitoring earns its keep. Conductivity or inline analytical measurement of the filtrate gives an operator a real, data-backed signal that impurity levels have dropped below specification, rather than a schedule-based guess. Combined with the agitator’s ability to programme bespoke mixing profiles for each product, a single ANFD platform can be tuned wash-by-wash to the minimum solvent volume a specific product actually requires, not the volume a worst-case assumption demands.

This matters for sustainability in a very concrete way. Solvent that isn’t used doesn’t need to be manufactured, transported, stored, or recovered. Every millilitre saved at the wash stage reduces distillation energy, cuts emissions associated with solvent logistics, and lowers the volume of waste solvent a site has to manage responsibly at end of life. In a single batch, the savings look modest. Multiplied across a plant’s annual output, it becomes one of the more meaningful, lower-effort sustainability gains available in solids processing, no new chemistry required, just better use of the equipment already on the plant floor.

Over-washing and under-washing are two sides of the same problem: a lack of visibility and control over what’s actually happening inside the cake. An ANFD’s combination of re-slurry agitation, programmable mixing, and endpoint monitoring gives operators the tools to close that gap, hitting purity specification reliably, without the solvent margin built in for safety’s sake.

If your site is looking at wash cycle times, solvent consumption, or purity rejects and wondering whether there’s a better way, get in touch with the Powder Systems team. We’re always happy to talk through where solvent, and cost, might be quietly slipping out of a process.