Composed of the fossilised remains of aquatic algae, their inert, siliceous and highly microporous structures make them unique for providing depth filtration to submicron level.
However, diatomite demonstrates limited adsorption properties, so producers tend to use bleaching earth adsorbents to remove soluble components.
These ‘activated’ or ‘surface-modified’ bleaching clays are used extensively in oleofuel feedstock pretreatment applications for removing colour pigments in vegetable oils or used cooking oils, as well as phosphorus, calcium, magnesium and various trace metals which can affect catalyst life in downstream processes such as hydrogenation.
Although good adsorbents, bleaching clays lack diatomite’s highly porous structure, so demonstrate poor filtration properties and tend to form ‘tight’ filter cakes, which can block the filters.
New applications
Rising use of hydrogenated vegetable oil (HVO) and sustainable aviation fuel (SAF), coupled with environmental pressure to adopt low-quality or waste-based feedstocks, is generating various challenges.
These feedstocks often contain higher levels of soluble or insoluble contaminants requiring the use of high doses of adsorbents, restricting filtration capacity even further.
To improve the filtration properties of bleaching earth, diatomite filter aids are applied as precoats over filter screens, or blended with the bleaching earth during the bodyfeed - or both.
Imerys trials with a used cooking oil feedstock confirmed the improvement in filtration cycle length/blockage when a diatomite based filter aid such as Imerys’ Celite® BF grade is used (Figure 1).
Further Imerys studies have shown a significant reduction in unwanted soluble and insoluble contaminants can be achieved by filtration alone without the need for bleaching clay (Figure 2).
A simple filter sheet will remove a significant proportion of these components, and incorporating Celite® BF filter aids result in a further reduction, the extent of which depends on filter sheet micron rating and Darcy, where a higher Darcy equates to a more ‘open’ filter cake and vice versa.
Filter aids
When selecting the most suitable filter aid for any application, there will always be a compromise between filtration throughput (filter cycle duration) and solid removal efficacy - the classic filtration ‘tightrope’.
Higher permeability filter aids (higher Darcy) generate more open filter cakes thus longer filter run lengths, but retain less contaminants. Conversely, tighter cakes remove more, but restrict filter run length (Figure 3).
Figure 3 is based on the addition of a single mineral. It has already been suggested that optimal filtration and adsorption during pre-treatment could be achieved using a blend of both Celite BF and adsorbent.
Here, the selection of the ‘correct’ Celite BF grade is critical due to ‘particle packing’.
Using an adsorbent (which gives a ‘tight’ filtration cake), with a high permeability (hence more ‘open’) filter aid can be counterproductive, since the smaller adsorbent particles can migrate through the filter cake and block the filter screen.
Better filtration performance is achieved when combining similar size particles, (Figure 4, columns A - D).
When combined with conventional bleaching earths, optimal filtration is achieved using a Celite BF grade of approximately 1.2 Darcy. Superior particle removal efficiency is apparent based on the lower turbidity of the UCO post filtration.
Taking this one step further, Imerys has developed a proprietary range of adsorbents, known as Innocycle®, designed to be mixed with Celite BF grades. Innocycle provides similar contaminant removal to bleaching earth, but demonstrates superior filtration performance (Figure 4, columns E - F, and Figure 5).
Conclusion
In summary, in waste feedstock pretreatment applications, Celite BF diatomite-based filter aids are ideal for removing insoluble contaminants that can have a detrimental effect on downstream catalyst life.
Unlike bleaching earth adsorbents, Celite BF filter aids minimise filter blockage and extend filter cycle life.
Moreover, a more efficient reduction in the total amount of contaminants enables producers to lower adsorbent rates or to use a less activated adsorbent agent such as Imerys’ Innocycle. This can improve pretreatment throughput whilst reducing operating costs.
For more information: Visit imerys.com
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