The filtration rate for recovering organic solids from organic liquids in the laboratory is not normally a concern. However, in the plant, slow filtration can extend an operation by several shifts. Reduced throughput in the plant costs significant money. Increasing the throughput in the laboratory can be just a matter of putting more reactants in a larger reaction flask and a larger bulk of crystals into a larger filter. Throughput in the plant is more often increased only by reducing the cycle time; one's choice of reactors and filters is small or nonexistent. Still, slow filtrations increase cycle time.
In the laboratory, the norm is water pump-vacuum filtration with a paper filter medium. However, paper filters clog more readily than filter pads. In process development laboratories filtration testing should use the identical filter pad material that is planned for usage in the plant. The material needs to be cut to size to fit the laboratory filter funnel which is usually of the Buchner variety.
Most preferably the size of the Buchner filter funnel to do the laboratory testing should be chosen so that the final cake thickness at the plant and laboratory scales are similar. If the laboratory filter cake is thicker the prediction will be even more conservative and hence dependable.
Normally the rate at which the product slurry is poured onto the filter is slower in the plant than in the laboratory.
Filtering duration in the plant is longer than in the lab because the filter cake is usually thicker.
In this discussion, I will need to make clear the boffin’s argot for this technical area. The liquid accumulating after passing through the filter is called the filtrate. The solid collected on the filer pad is called the filtrand. ’Washing’ means cleaning a filter cake. The predominant contaminants being removed are product impurities that have not precipitated out of the crystallizing solvent. This ‘washing’ is distinguished from ‘cleaning’ which refers to cleaning parts of the filter equipment itself,. Washing the equipment to remove adhering solid is instead called ‘rinsing’ (e.g., rinsing a filter screen or a filter cloth with jets of water).
A distinction is made between washing and extraction or leaching. ‘Washing ’ means the elimination of a solution of both dissolved contaminants and residual dissolved target substance (the mother liquor) from the pores between the particles/crystals of a filter cake. ‘Extraction’ or ‘leaching’ is a subtype of ‘washing’ that additionally draws soluble matter out of the interstices of the solid particles themselves.
The word ‘drying’ refers to thermal drying wherein volatile materials, predominantly solvent or a processing liquid, are removed by the application of heat and sometimes lower pressure. The reduction of liquid from the filter cake by mechanical means is called ‘deliquoring.’ In the laboratory with the standard Buchner filtration, deliquoring occurs during that brief moment between when the final portion of slurry liquid disappears into the surface of the filtrand and when, with a rushing sound, a flurry of bubbles exit the throat of the porcelain or glass filter assembly.
The operation called ‘expression’ is the separation of liquid from solid-liquid systems by increasing the pressure on the material. The liquid is pressed out from the solid.
Filtrations differ in where the filtrand particles are retained. If the particles are generally large compared to the pore sizes of the filter pad, the particles will predominate on the outer surface of the filter pad and will end up in the cake supported on the filter. This ‘cake filtration’ is most common for recovering organic crystals from recrystallizations. Filter pads very often outperform filter papers. Filter papers more frequently clog because too many small particles get trapped in the channels within the thin paper layer.
‘Depth filtration’ on the other hand, occurs when the solid is trapped inside part of the filter medium. This arises, for example, when the organic solid is filtered in admixture with a filter aid. The completely insoluble filter aid particles provide an additional surface beside the filter pad for the organic crystals to agglomerate on. This speeds the separation from mother liquors but adds a further necessary step separating the desired organic solid from the filter aid. Using a filter aid to speed up filtration can be helpful particularly if it does not significantly add time to later processing steps. The use of filter aids will be treated in a different blog article.
The pressure difference across a vacuum filter system is very limited, and the residual moisture in its filter cake is higher than with pressure filters. Pressure filters allow high-pressure differences from top to bottom of the separated cake. They are preferred when the product must be kept in a closed system for safety reasons, or if the residual moisture content is important. Access for the handling of the filter cake is obviously more difficult using a pressure filter.
Filtration by centrifugal force, which is another possibility, has the shortcoming of needing more complex capital equipment, but as a general rule, it yields solids with lower residual moisture.
Because most filtrations of crystals in organic chemical synthesis are vacuum filtrations, the pressure drop is never more than 1 atmosphere. 'Expression' from the filter cake is limited and in fact, 'deliquoring' is normally no more than a very brief passage of air or inert gas through the cake. Washing normally quickly follows this draining with gas assistance. The passage of gas through the cake can quickly start some drying action, with concomitant deposition of impurities from any residual mother liquors still on the surface of the product crystals.
Washing can start immediately after the first gas passes through the cake signaling the end of deliquoring or it can start even before any gas enters the cake. The more volatile the solvent the more quickly washing should follow filtration. Washing is typically done using solvent as cold or even colder than the slurry that was filtered. The washing liquid is usually first used to clean the vessel from which the slurry came. A major concern during washing is losing, by dissolution, any of the desired product already trapped as solid and lying on the filter pad.
Wash liquid flows through the filter cake and displaces mother liquor, a mixture of both liquids leaves the cake as a wash filtrate. If the wash liquid is collected separately from the filtrate, analysis can give a good indication of its efficacy.
The crystal bed of some substances shrinks horizontally and cracks if the solvent is completely sucked away. In such a case, it is particularly important to add the wash liquid to the crystal bed’s surface before this has a chance to happen. Whenever the crystal bed cracks, the washing will become very inefficient, and a large amount of mother liquors will likely be retained in the crystal mass and end up getting dried with the bulk of the solid product; trapping impurities. When cracking is characteristic of the filtrand, the wash should be started even before all the mother liquors have passed into the surface of the cake bed and before any deliquoring.
If shrinking during washing is a washing problem, slurrying the cake in wash liquid and filtering it again is indicated.
An acceptably small amount of washing will not remove all the mother liquors. Plug flow of the wash liquid cannot be expected through a filter cake. A filter cake contains stagnant zones or dead-end pores, which are not reached by the flow. They deliver their contaminant content by diffusion to the effluent if they do it at all. Diffusion is an asymptotic process and depends on the time elapsed, not on the quantity of liquid. Wash liquid threads its way through the filter cake in what is called ‘fingering.’ In most washing processes the wash liquid has a lower viscosity than the mother liquor. It tends to flow through the cake in finger-like streams past islets of more viscous fluid. This is a random (stochastic) process, and the local concentration in the cake and the momentary concentration in the effluent will vary randomly. Scale-up from a laboratory test filter even with only small-scale fingering can be misleading as can the results of a few small samples taken from a big cake.
Sometimes relieving the vacuum while the wash liquid is still in the cake will provide more time for impurities to diffuse out of dead zones into the bulk wash liquid so that they are removed when vacuum pressure is reapplied. The downside of this modification is that it allows the wash liquid to warm somewhat, possibly increasing the loss of the desired product.
In the laboratory, when the washing is over, air or inert gas is typically drawn through the cake for some period. Also sometimes, in the laboratory, some additional wash liquid may be expressed either by pressing down on the cake’s surface with a flat, inert tool or more efficiently using a section of dental dam that completely blocks the entry of gas into the cake.
In the plant, the surface of the cake is rarely manipulated. The only exception is when cracks arise in the cake in which case they must be closed mechanically before continuing.
Drying at scale in large drying ovens adds appreciably to the processing time for many products. When a high boiling solvent is involved, it may be cost-effective to add an additional wash with a different solvent in which the desired product is very poorly soluble but which itself is much more volatile.
This can significantly shorten the overall drying time.