If I understand correctly, shale oil is generally made by pyrolysis of kerogen from the shale. It therefore should contain virtually no ash, i.e. no non-volatile inorganic matter originally present as dissolved in the "oil". If it does contain "ash", that would likely be dust from the shale rather than "ash" per se. Filtering 1000 ppm out of it is going to be quite costly because the particles could be very fine indeed.

in my knowledge:
- HFO + gasoline: no clear relation with shale oil;
- ash is generated thru the combustion of fuel oil by the salts (mainly chlorides of Na, Mg, Ca....) and metals (organometallic compounds with Fe, V, Ni....) originally included in the oil. you can't filter them. salts, but not metals, can be reduced by a desalting process (also removes sediments, if any).

questions:
- have you an analysis of the ashes (and salts, and BS&W, and metals)?
- which is the scope for reducing ashes?

Thanks for your reply

@ moltenmetal :It comes out as suspended solids and as you say could be dust. Was wondering a solution between filter cartridges or a high speed centrifuge(disc stack type), which should be better option.

@poli60 - I do not have the analysis. The reduction is to achieve 50 ppm from 1000 ppm

Try using a 2 step operation: Step 1 - hydrocyclones to knock out the bigger particles and most of the SS. Step 2 - a finer filter, Pall / Peco or similar filter cartridge.
A 2 step operation will extend the life of the stage 2 filters.
The reject stream from the hydrocyclones containing the suspended solids would need to disposed off somehow.

Thanks George
Would a high speed centrifuge ( disc stack centrifuge ) work after the hydrocyclone instead of filter cartridge ?

Dont think you need to go as far as a centrifuge. Expensive and high maintenance. To extend the life of the filters, would suggest setting up 2 stages of hydrocyclones. Talk to hydrocyclone vendors who can help you with this. Agreed, if the particles are too small, the hydrocyclones may not work as intended.