Washing mechanisms for pharmaceutical filter cakes

During many chemical and pharmaceutical process operations, washing of filter cakes is necessary, as no filtration system can remove all of the mother liquor by dewatering by pressure or vacuum alone.

The need to wash the filter cake to produce a "clean cake" for subsequent processing, drying or disposal can stem from several reasons, including the removal of mother liquor, solvent exchange, dissolution of soluble components, and displacement of insoluble contaminants.

The subject of "how to more efficiently wash filter cake" is one that many process engineers grapple with when considering the various techniques available to meet operational economic, production, and quality requirements.

This article introduces pragmatism into cake filtration theory and will help guide engineers in their decision-making processes.

Filtration Equipment Selection that Produces a Filter Cake

Before discussing the washing of filter cakes, it is necessary to examine some of the equipment that can produce cakes, from thin cakes (3 mm to 25 mm) to thick cakes (up to 300 mm).

It could be suggested that any filter can produce a cake that can be washed. Given infinite time and patience, perhaps this is true; however, the real question is: "Does the process have enough time and do the operators have enough patience?"

Filter options that produce cakes and are also capable of washing the cake are well documented. These include filter presses also known as automatic pressure filters , vacuum belt filters, centrifuges, rotary drum filters, and Nutsche filters.

Whether the application is, in principle, more suitable for one over the other depends on many factors, including process requirements, cost, space availability, etc. Equipment selection should be based on the overall balance of objectives and should include bench-scale and pilot-scale testing.

However, to assess whether the selected filter will meet the cake washing requirements, there is no substitute for carrying out exhaustive laboratory tests that simulate, as closely as possible, the conditions of the pre-selected filter and then, based on the washing results, deciding if this filter remains the right choice. If not, then the Filter selection for pharmaceutical production It must be reassessed.

Technical and economic advantages of filter cake washing

The efficient washing of cakes in the pharmaceutical industry brings with it a series of technical and economic advantages that allow for the reduction or elimination of various significant costs. Below, we will analyse in detail the key aspects where substantial savings can be achieved:

• Product waste reductionThrough efficient washing of filter cakes, a more complete extraction of the desired product is achieved. This reduces waste by ensuring that as much product as possible is recovered from the filter cake. In the pharmaceutical industry, where products are often expensive and valuable, this reduction in waste translates directly into significant cost savings.
• Reduction in treatment costsEfficient washing of filter cakes also contributes to the reduction of subsequent treatment costs. By minimising the amount of impurities and unwanted by-products in the cake, subsequent purification processes such as crystallisation, evaporation or other concentration methods are simplified. This not only saves resources and time, but also reduces the costs associated with these additional processes.
• Less need for additional drying.One of the most significant benefits of a drier filter cake is the reduction in the need for further drying. Drying is an energy-intensive process and, therefore, costly in terms of energy consumption and carbon emissions. By achieving a drier filter cake from the outset, the need to apply additional heat and energy to remove residual moisture is eliminated or significantly reduced. This not only lowers operational costs but also contributes to a smaller carbon footprint, which is consistent with the sustainability and environmental responsibility goals of many pharmaceutical companies.

Preparing the Cake for Optimal Washing

It is a logical, but in most cases mistaken, assumption that the drier the filter cake, the easier it will be to wash. The reason for this apparent contradiction lies in one or both of the following causes:

1. If the filtration cake is maximally dehydrated through compression, it is almost certain the cake will be so compact that no wash liquid will be able to penetrate it. The wash liquid will seek the path of least resistance, meaning it will simply flow past the cake or find the slightest crack and disappear through it.
2. If the filter cake dewaters beyond the dry top through vacuum or the injection of pressurised gas (or steam) through it, the result will be the unavoidable formation of pockets (or cracks), which in any case will once again lead to bypass.

In general, washing of the filter cake should commence immediately after the mother liquor has been drained, leaving a well-settled and well-formed filter cake that is saturated with the mother liquor. If the surface of the filter cake (the top dry part) shows signs of drying out, then it is usually too late.

What are the mechanisms of filter cake washing?

In almost all cases, the requirement for achieving maximum washing efficiency is to develop a mechanism that dislodges the required amount of mother liquor with a minimum of wash liquid. Fundamentally, there are only two mechanisms, and in practice they are generally both present:

• Displacement of mother liquor by the washing liquid.
• Dilution of the mother liquor by mixing with wash liquids.

Displacement of mother liquor by washing liquid

Displacement is by far the most efficient mechanism and, under ideal conditions, would cause the wash liquid to act like a piston, expelling the mother liquor from the cake without any dilution.

This is clearly a utopia, although a high degree of "piston or plug flow effect" can be achieved with careful preparation of the cake and careful application of the wash liquid. The trick lies, above all, in the careful application of the wash liquid onto the saturated face of the filter cake.

In open filters such as belt filters or rotary drum filters, it is visually possible to observe the extent to which the wash application disturbs the cake surface and therefore prevents good displacement. In closed filters, this is more difficult and a degree of guesswork, along with verification of the results of these guesses, is unavoidable.

Yes, if the wash liquid has a lower viscosity than the mother liquor, as is very common, care must be taken not to apply too much pressure (positive or negative) because the lower viscosity wash liquid will generally find any small cracks or "easy" pathways between particles rather than pushing the mother liquor.

Finally, washing the filter cake will almost always alter the cake's structure (change in polarity, change in viscosity, change in temperature, physical disturbance from the introduction of the wash liquid, etc.). As a result, a point is reached where displacement washing effectively stops and diffusion begins.

Dilution of mother liquor by mixing with wash liquids

Dilution can occur in three different scenarios:

• If the liquids are readily miscible and the cake liquor (at this point a mixture of mother liquor and wash liquor) is not too hidden within the particles, their cracks or their clumps, it should be a fairly straightforward dilution process with a fairly predictable curve.
• If the liquids are reasonably miscible and the mother liquor does not have a great affinity for the solid particles, but is trapped within the cracks of the particles or particle groups, then some degree of agitation can help to "flip" the cake a little and remove residual mother liquor from the particles. Obviously, this agitation should only begin after the "free" cake liquor has been washed away and when only the hidden liquor trapped in the pores of the particles or between the particles remains. This agitation action is, of course, very difficult to envisage, for example, in filter presses or centrifuges. Two exceptions to this would be a multi-stage pusher centrifuge which "flips" the cake as a matter of fact or a filter press with vibrating plates instead of rotating plates.
• If the mother liquor has a high affinity for solids and is retained within the pores of the solid particles, generally the easiest and most effective washing system is to remove the filter cake, resuspend it in clean wash liquid, allow it time for the desired mixing/diffusion to take place, and then refilter the slurry.

An alternative final wash scheme: Cake Compression versus Dilution

Given that dilution is proportional to volume ratios, the efficiency in terms of residual mother liquor removal drastically decreases as the cake becomes cleaner (less mother liquor).

Assuming a wash volume equal to the cake liquor volume, at best a 50%reduction can be expected. Initially, this may be quite high, but when the cake only retains 0.1%of mother liquor, subsequent washing would only remove a further 0.05%. With the current trend towards producing purer final products, there may come a point where extremely large amounts of wash liquid and time are required to dilute the last traces and meet those purity levels. In these cases, there is considerable merit in investigating the compression of this (nearly pure) cake, with or without gas blowing, and mechanically forcing some of the remaining diluted liquor out of the cake.

Application of Filter Cake Wash Liquids and Obstacles to Avoid

The objective of washing is to subject the entire surface area of the cake to a uniform and evenly pressurised liquid wash. Some filters offer better flexibility than others in achieving this effect. Most importantly, "channeling" both through and over the cake surface must be avoided. (Dye tests will show this clearly, see "laboratory tests" section). If channeling occurs, a different filter type may need to be investigated.

It should be clear, but is often overlooked, that washing a filter cake at too high a differential pressure only leads to poor efficiency. Too high a pressure almost always causes bypass. Although the ideal cake has a uniform structure, no holes, no cracks, and is firmly supported against the retaining walls, we do not live in an ideal world and, to some extent, all cakes will have these flaws. Forcing liquid through the cake at unnecessarily high pressures only makes matters worse. A hole will become a large hole, a crack a ravine, and if the cake does not sit perfectly, the wash liquid will turn the gap into a highway. Furthermore, high-pressure differentials reduce contact time. In all cases, the optimum differential pressure is the lowest possible.

In horizontal vacuum filters (continuous indexing), overflow weirs can be used to gently discharge the liquid over the cake and prevent disruption of the same.

In all filters, after an initial displacement wash, subsequent diffusion/dilution washes may benefit from mist sprayers which often provide a better and/or more uniform "wetting" effect. The problem here is, however, that fine sprays can have a tendency to blind the cake structure, whilst coarser sprays may tend to disturb it.

In this regard, engineers can custom-design a highly effective spray system when attempting to optimise a washing technique. A particular case that may be of interest is the design of a unique spray system for a continuously indexing vacuum belt filter. The system used a square spray pattern, angled at approximately 45º and positioned so that the jets lost their velocity just at the point where washing was to take place, and thus a gentle shower of wash liquid fell onto the cake. This system is by far both a very economical and an efficient and gentle wash liquid applicator, and can equally be used on rotary drum filters, belt filters, and rotary paddle filters, etc.

Washing of filter cakes and final product quality in pharmaceutical production

The point "Washing and final product quality" is essential in pharmaceutical production, as the quality and purity of the products are fundamental to ensuring patient safety and regulatory compliance. When it comes to filtration systems using filter presses and Vacuum band filters in the pharmaceutical industry, the cake washing process plays a critical role in obtaining high-quality final products. Here's an expansion on this point:

The importance of quality in pharmaceutical production

The pharmaceutical industry is governed by extremely high quality standards. Pharmaceutical products, such as active pharmaceutical ingredients (APIs), vitamins, and antibiotics, must meet precise specifications to ensure their efficacy and safety. Any contamination or impurity in these products can have serious consequences for patient health and can result in significant regulatory penalties for companies.

Influence of the filtration process

In the pharmaceutical production process, both filter presses and vacuum belt filters are used to separate solids from liquid suspensions. After the filtration phase, the filter cakes, which are the retained solids, must undergo a washing process to remove any unwanted impurities or residues. This washing step is critical to ensure the purity of the filter cake and, ultimately, the quality of the final product.

Removal of pollutants and waste

The primary aim of cake washing is to remove any unwanted substances that may have become trapped in the solids during filtration. This can include residues of chemicals, fine particles, impurities, or contaminants that might have been present in the original slurry. Efficient cake washing ensures that these contaminants are completely removed, contributing to product purity.

Washing parameter control

In both filter presses and vacuum belt filters, a number of parameters can be controlled during the washing process. This includes the choice of washing liquid, the washing speed and pressure, the number of washing stages, and the duration of each stage. Optimising these parameters is essential to ensure that all contaminants are effectively removed without compromising the integrity of the filter cake.

Regulatory compliance

In pharmaceutical production, compliance with regulations is fundamental. Washing processes must be validated and adequately documented to demonstrate that all necessary measures have been taken to ensure the quality and purity of the final product. This involves rigorous quality control and the implementation of procedures that comply with regulations, such as Good Manufacturing Practices (GMP) and Food and Drug Administration (FDA) regulations.

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Tips and strategies for optimising cake washing processes

Here are 10 tips to optimise cake washing processes in the pharmaceutical industry and reduce production costs:

• Know your products and processesComprehend the pharmaceutical products you are filtering and the processes involved in depth. This will allow you to design a specific and efficient washing process.
• Selection of suitable equipmentSelect press filters or vacuum belt filters that suit your specific washing needs and production capacity.
• Optimisation of cake formation timeAdjust the cake formation time to ensure efficient compaction, which will facilitate subsequent washing.
• Counter-current washingUse countercurrent washing whenever possible, as it is usually more effective for removing impurities.
• Slurry viscosity controlMonitor and adjust the slurry viscosity at process temperature, as this can significantly affect filtration and washing rates.
• Use suitable solvents: If you are working with solvent-based products, select the most suitable solvents and ensure they are compatible with your process and your equipment.
• Consider the use of filter aidsEvaluate whether it is necessary to use filter aids such as diatomaceous earth, perlite, or cellulose to improve wash efficiency.
• Minimise residual moistureAdjust the washing and drying parameters to achieve the lowest possible residual moisture in the filter cake.
• Preventative maintenanceImplement a preventative maintenance programme to ensure your filtration equipment is in optimal working order.
• Staff trainingEnsure your staff are properly trained in washing procedures and equipment handling to maximise efficiency and safety.

These tips can help you optimise your cake washing processes in the pharmaceutical industry and reduce production costs while improving the quality of your final products.

Don't wait any longer to take your filtration process to the next level! Discover how the Metso filters They can optimise your pharmaceutical industry operations. Get detailed insights and personalised advice today. Contact us for a no-obligation consultation! Your efficiency and quality deserve excellence in filtration.